Complete oxidation of methane at low temperature over noble metal based catalysts

Complete oxidation of methane at low temperature over noble metal based catalysts Quá trình oxy hóa của khí metan ở nhiệt độ thấp dùng các chất xúc tác kim loại quý Complete oxidation of methane at low temperature over noble metal based catalysts Quá trình oxy hóa của khí metan ở nhiệt độ thấp dùng các chất xúc tác kim loại quý

Applied Catalysis B: Environmental 39 (2002) 1–37 Complete oxidation of methane at low temperature over noble metal based catalysts: a review Patrick Gélin∗ , Michel Primet Laboratoire d’Application de la Chimie à l’Environnement, UMR CNRS 5634, Université Claude Bernard Lyon 1, Building Chevreul, 43 Boulevard du 11 Novembre 1918, 69622 Villeurbanne Cedex, France Received 29 October 2001; received in revised form 20 March 2002; accepted 25 March 2002 Abstract This review examines recent developments in the complete oxidation of methane at low temperature over noble metal based catalysts in patents and open literature. The abatement of natural gas vehicle (NGV) methane emissions is taken as one example among possible applications. The review develops current ideas about the properties of palladium and platinum catalysts supported on silica and alumina supports in the complete oxidation of methane under oxidising conditions, focusing on low-temperature reaction conditions. The influence of residual chloride ions on the catalytic activity, the kinetic aspects of the oxidation of methane over these catalysts, the nature of the active sites, the influence of metal particle size and reaction products on the activity, the observed changes in catalytic activity with reaction time and the effect of sulphur containing compounds are examined. The latest studies concerned with improved palladium and platinum supported catalysts which would exhibit enhanced and stable catalytic activity at low temperature in the presence of water and sulphur containing compounds are reported. Possible routes for preparing catalysts able to meet future regulations concerning methane emissions from lean-burn NGV vehicles are discussed. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Methane oxidation; Noble metals; Catalytic combustion; Low temperature; Lean-burn NGV; Natural gas; Emission abatement; Platinum; Palladium; Catalyst poisoning; Water inhibition; Poisoning by sulphur and chlorine containing compounds; Kinetic studies; Silica; Alumina; Zirconia; Tin dioxide; Ceria; Ceria-zirconia solid solution; Zeolite; Aluminophosphate; Mixed oxide supports; Oxide additives; Bimetallic catalysts 1. Introduction The catalytic combustion of methane has been extensively studied as an alternative to conventional thermal combustion and was reviewed [1–5]. This method was shown to be effective in producing energy in gas turbine combustors, while reducing emissions. Many studies were devoted to the design of catalytic ∗ Corresponding author. Tel.: +33-4-72-43-11-48; fax: +33-4-72-44-81-14. E-mail address: patrick.gelin@univ-lyon1.fr (P. G´elin). materials able to withstand high temperatures in atmospheres containing steam and oxygen. Another main application of catalytic total oxidation of hydrocarbons is the abatement of methane emissions from natural gas or methane combustion devices, being either catalytic or non-catalytic. This would in turn cover a wide range of applications, such as e.g. the abatement of methane emissions from lean-burn natural gas vehicles (NGVs). Programs for the use of NGVs in urban areas, especially heavy-duty vehicles, are currently being developed very rapidly in most industrial countries. 0926-3373/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 3 3 7 3 ( 0 2 ) 0 0 0 7 6 - 0 2 P. G´elin, M. Primet / Applied Catalysis B: Environmental 39 (2002) 1–37 Table 1 US federal heavy-duty emission limit values (g/hph)a (1 hph = 2.6856 × 106 J) Effective date Vehicle type CO Hydrocarbon From 1998 Diesel Urban bus Diesel Urban bus 15.5 15.5 15.5 15.5 1.3 1.3 From 2004 (proposal) a b NMHC + NOx NOx Particulate matter 4.0 4.0 0.1 0.05 0.1 0.05 2.4 or 2.5 2.4 or 2.5b Current regulations require, since 1985, emission testing over the rather complicated transient FTP engine dynamometric cycle. NMHC limit: 0.5 g/hph. The need for governments to diversify energy sources together with the huge world-wide resources of natural gas (much larger than crude oil) partially explains this fact. In addition to economical and/or political reasons, the use of compressed natural gas (CNG) for automotive applications offers significant environmental advantages over gasoline and diesel. Natural gas engines can operate under lean conditions so that the fuel efficiency can be much increased compared to stoichiometric conditions. Under lean-burn conditions, nitrogen oxides (NOx ) emissions of CNG engines are much reduced. This is due to cooler combustion resulting from the high air to fuel ratios at which the lean engines operate. Typically, NOx emissions of a European diesel bus meet the final Euro III standard (since October 2000), i.e. 5 g/kWh with either stationary cycle European stationary cycle (ESC) or European transient cycle (ETC). For a lean-burn CNG bus submitted to the same test cycles, NOx emission falls down to less than 2 g/kWh. CO2 emissions are also reduced because of the high H:C ratio of the methane molecule, which is the main component (85–95%) of natural gas. Because of the very low sulphur content of natural gas, NGVs SOx emissions are very low. In addition, the crucial advantage of NGV engines compared to diesel is definitely the very low amount Table 2 European heavy-duty diesel and gas emission limit values (g/kWh) Tier Implementation Euro II Test cyclea CO THC NMHC NOx Particulate matter ECE R-49 4.0 1.1 – 7.0 0.15 Smoke (m−1 ) IIIb October 2000 ESC/ELR ETC 2.1 5.45 0.66 1.6c – 0.78 5.0 5.0 0.10 0.16 0.8 Euro III EEVd October 1999 ESC/ELR ETC 1.5 3.0 0.25 0.65c – 0.4 2.0 2.0 0.02 0.02 0.15 Euro IV October 2005 ESC/ELR ETC 1.5 4.0 0.46 1.1c – 0.55 3.5 3.5 0.02 0.03 0.5 Euro V October 2008 ESC/ELR ETC 1.5 4.0 0.46 1.1c – 0.55 2.0 2.0 0.02 0.03 0.5 Euro 1 kWh = 3.6 × 106 J. a It can be noticed that the old steady-state engine test cycle R-49 is replaced by two new cycles since Euro III standard: a stationary cycle (ESC) and a transient cycle (ETC). Smoke opacity is measured on the European load response (ELR) test. In the Euro III standard, diesel engines can be tested on either of ESC/ELR and ETC tests while both tests will be required in Euro IV. Diesel engines equipped with after-treatment device are tested on both ESC/ELR and ETC cycles. For the first time, NGV heavy-duty vehicles must comply with emission limits and are tested on the ETC test only. The ETC test is similar to the US heavy-duty transient test, although the cycle is different. b Directive 1999/96/EC of 13 December 1999. c CH for natural gas engine only. 4 d A special low-emission vehicle class (environmentally enhanced vehicle, EEV) is defined. Tax incentives can be granted for vehicles complying with these requirements. P. G´elin, M. Primet / Applied Catalysis B: Environmental 39 (2002) 1–37 of particulates (oil derived) in the exhaust gases. For these reasons, urban CNG buses appear attractive to solve transportation problems in cities and reduce pollution. As a matter of fact, NGV programs are currently developed very rapidly in European countries, NGVs replacing diesel vehicles as a viable approach to reducing NOx and particulates in urban areas. The NGV advantages are however partially balanced by the emission of unburned methane. The hydrocarbon composition in the exhaust gases of lean-burn CNG engines reflects the composition of natural gas in methane and non-methanic hydrocarbons (NMHCs), typically 90–95% methane. Methane is a potent greenhouse gas, which is recognised to contribute more to global atmosphere warming than carbon dioxide at equivalent emission rates, all the more since its lifetime is quite long. The environmental impact of NGV methane emissions is now being taken into account in present and future regulations in many countries [6]. Tables 1 and 2 report heavy-duty emission limit values in US and in Europe. It should be noted that, besides differing limit values between different countries, engine testing modes might be also very different. This point was clearly addressed by Lampert et al. in their evaluation of palladium catalysts for methane emissions abatement from lean-burn NGVs [7]. A steady-state cycle, such as the ECE R-49 test used in Europe before 2000, was shown to result in better performance for hydrocarbons emissions abatement than a transient cycle, such as the US federal heavy-duty FTP test. This is due to higher NGV exhaust gas temperatures in steady-state cycles, which favour higher conversions. While steady-state type testing is still used in Japan for emission certification of heavy-duty engines, the situation in Europe changed recently with the new transient ETC cycle, which has been adopted for testing NGV heavy-duty engines and turns to give emissions levels similar to the US Federal heavy-duty FTP test. Without any after-treatment device, the total hydrocarbon emission from a lean-burn natural gas bus reaches typically 3 g/kWh on the ESC test cycle, which can be extrapolated to 4 g/kWh on the ETC test. These values are much higher than the limit value of 1.6 or 1.7 g/kWh in Euro III and US standards respectively for heavy-duty gas engines methane emissions. CO and NOx emissions are lower than the limit values of the present regulations, even without any 3 exhaust gases after-treatment. Therefore, emissions of methane from NGVs must be necessarily reduced and this can be achieved by catalytic after-treatment of exhaust gases which would perform the complete oxidation of methane. At least 60% methane conversion is required to meet the most stringent current regulations (Europe). As an example, a European bus-maker proposes currently NGV buses equipped with a 19 l catalytic exhaust converter, i.e. almost twice the engine capacity, loaded with 250 g/ft3 noble metal, which induces an extra cost of ca. 3000 with respect to the equivalent Euro III diesel vehicle. The approach is more difficult than for NMHCs because of the higher stability of the methane molecule. Additional obstacles arise from the reaction conditions specific to lean-burn NGV engine exhausts: • low temperatures at which the catalyst must operate (typically less than 500–550 ◦ C), • low concentrations of methane (500–1000 ppm), • large amounts of water vapour (10–15%) and CO2 (15%), • large excess of oxygen, • presence of SOx (about 1 ppm) and NOx . For low-temperature combustion applications, such as in the abatement of methane emissions from lean-burn NGV, it is clear that the thermal stability of the catalyst is out of concern. The main objective is rather to design catalytic materials exhibiting the highest activity at the lowest temperature and the best resistance to poisons present in exhaust gases. The complete oxidation of methane can be performed over either noble metals or transition metal oxides. These two families of catalysts have been extensively studied during last decades in view of developing catalytic combustion applications. The main advantage of noble metal catalysts over metal oxides is definitely their superior specific activity, which make them as the best candidates for low-temperature combustion of hydrocarbons. This is particularly true in the case of methane which is the hydrocarbon the most difficult to activate. Among noble metals, platinum and palladium are the most commonly used and studied catalysts. They can be obtained in a high degree of dispersion when deposited on conventional supports with a high specific area like silica or alumina. The increase of the metal dispersion allows to improve the catalytic activity. 4 P. G´elin, M. Primet / Applied Catalysis B: Environmental 39 (2002) 1–37 The aim of the present contribution is to review the recent developments in the complete oxidation of methane over noble metal based catalysts at low temperature. The abatement of lean-burn NGV methane emissions is considered as one example. It should be noted that, however, very few references actually concern NGV emission conditions. Interestingly, contributions found both in the patents and the open literature provide useful material to understand the catalytic behaviour and provide ideas to find a good, durable formulation for NGVs but also more generally for all low-temperature applications. First, we will develop the current ideas about the properties of palladium and platinum catalysts supported on silica and alumina supports in the complete oxidation of methane mainly under oxidising conditions. In this section, we will examine successively the influence of residual chloride ions on the catalytic activity, the kinetic aspects of the oxidation of methane over these catalysts, the nature of the active sites, the influence of the metal particle size and the reaction products on the activity, the observed changes in catalytic activity with reaction time and the effect of sulphur containing compounds. All the aspects concerning the application of these catalysts to combustion reactions at temperatures exceeding 600 ◦ C will be discarded. In Section 2, we will review the latest studies concerning improved palladium and platinum supported catalysts which would exhibit enhanced and stable catalytic activity at low temperature. We will endeavour to compare the catalytic properties of the ‘improved’ catalysts to that of the ‘reference’ catalysts described in this section. 2. Silica- and alumina-supported Pt and Pd catalysts 2.1. Poisoning effect of chlorine The inhibiting effect of halogenated compounds on the catalytic activity of supported palladium and platinum catalysts with respect to the total oxidation of methane was first reported by Cullis and Willatt [8]. The reaction was carried out in a pulse-flow reactor under stoichiometric O2 :CH4 conditions. After the injection of a pulse containing 1.8 ␮mol CH4 , 3.6 ␮mol O2 and 0.14 ␮mol halogenated compound at 377 ◦ C, the activity decreased in all cases and was restored or not, depending on the nature of the metal and/or the support, by flowing in helium and pulses of the CH4 :O2 mixture free of halogen compound. Rich mixtures were found to be more effective to restore the activity. Pd catalysts appeared more sensitive to poisoning than Pt ones. Moreover, when supported on alumina, the catalytic activity of Pt was fully restored while the deactivation of Pd was irreversible. No change of the particle size was observed during these experiments. No clear explanation for the decrease of the catalytic activity was given. Auger electron spectroscopy indicated that after exposure to dichloromethane, chlorine and carbon were found where palladium was present and palladium was no more in the form of PdO. XPS showed a reduction of palladium to the metallic state and the partial recovery of activity was associated with a partial re-oxidation of Pd. Supported Pd and Pt catalysts are usually prepared by impregnation of the support with Cl-containing metal precursors. It turns out that conventional activation treatments (calcination in oxygen followed by reduction in hydrogen) do not allow the complete removal of chloride ions originating from the metal precursor. Commercially available supports might also contain significant of amounts of chloride themselves. Therefore, the question of whether residual chlorine still present on Pd and Pt catalysts after conventional activation could inhibit their catalytic activity for oxidation reactions was of concern [9]. The effects of chloride originating from the precursor salts or other impurities from the alumina on the activity of Pd/Al2 O3 for the complete oxidation of methane were first addressed by Simone et al. [10]. A commercial ␥-alumina containing 700 ppm Cl was used. It was clearly evidenced that Pd(NO3 )2 led to catalysts having a much better activity than PdCl2 , although the PdCl2 prepared catalyst exhibited higher dispersion (from CO chemisorption) and smaller crystallite size (from TEM). The presence of chloride in the latter catalyst was revealed by chemical analysis and XPS, thus explaining its low activity (vide infra). Moreover, the catalyst prepared from PdCl2 showed significant improvements in performance after long-term ageing at 600 ◦ C in air. This was related to the removal of chloride ions as revealed by the chemical analysis of the aged catalyst. Various treatments were studied in order to improve the catalytic performance of the Pd catalysts prepared with Cl-containing P. G´elin, M. Primet / Applied Catalysis B: Environmental 39 (2002) 1–37 salts. In the same paper [10], a loss of Pd after ageing in air at high temperature via the formation of Pdx Oy Clz complexes was proposed. Very recently, the inhibiting effect of residual chlorine ions either originating from chlorinated precursor or subsequent impregnation of Cl-free catalysts with HCl on the catalytic activity of 2 wt.% Pd/Al2 O3 was proved unambiguously [11]. Experiments were carried out with Cl-free and Cl-containing catalysts having constant and equal dispersions (ca. 10%) so that sintering effects could be ruled out. As previously observed, the catalyst prepared from chlorinated precursor strongly activated with time on stream at constant temperature (475 ◦ C, 1 vol.% methane, 4 vol.% oxygen in nitrogen/helium carrier). The key point of the experiment was the detection of HCl at the reactor outlet and the in situ direct measurement of HCl departure could be correlated with the progressive increase of catalytic activity (Fig. 1). The activity of the Cl-free Fig. 1. In situ measurements of Cl departure and catalytic activity in methane oxidation over a fresh Cl-containing 2 wt.% Pd/Al2 O3 catalyst (prepared from H2 PdCl4 , treated in O2 at 500 ◦ C and reduced in H2 at 300 ◦ C). The variations of HCl, CH4 and H2 O concentrations at the reactor outlet are plotted as a function of time on stream at 475 and 600 ◦ C. The feed (1 vol.% CH4 , 4 vol.% O2 , He balance; GHSV = 15,000 h−1 ) was first introduced on the catalyst (200 mg) preheated in He at 475 ◦ C. The dash line indicates the time at which the catalyst was heated up to 600 ◦ C. (Reproduced from Fig. 3 of [11], with permission from Elsevier Science.) 5 catalyst was finally reached when chloride ions were completely removed from the catalyst surface. After subsequent impregnation of the Cl-free catalyst with HCl, the activity was strongly decreased to the same level as the catalyst prepared with the chlorinated precursor. Similar to palladium, chlorine was proposed to strongly inhibit the catalytic activity of platinum particles supported on alumina supports in the complete oxidation of methane under oxidising conditions (1 vol.% CH4 :4 vol.% O2 ) [12,13]. Samples prepared from H2 PtCl6 still retained 0.3–0.8 wt.% Cl after reduction in H2 at 350 or 500 ◦ C. The Cl-containing Pt catalysts exhibited a strong activation under reactants in spite of the decrease of the number of Pt surface sites due to particle sintering. After reaching the steady-state activity, residual chlorine was completely removed from the catalysts and Pt dispersion did not change any more. On the other hand, Cl-free Pt catalysts exhibited a progressive deactivation with time on stream, which was related to the sintering of metal particles. The authors concluded that residual chlorine originating from Cl-containing Pt precursor is responsible for a strong inhibition in the complete oxidation of methane, this effect being larger than the decrease of the activity due to particle sintering. The catalysts prepared from H2 PtCl6 and stabilised under reactants at 600 ◦ C were found to be more active than the ones prepared from Cl-free precursors, probably because of a higher dispersion. The mechanism by which Cl ions would act as strong inhibitors on catalytic activity is not yet clearly established. In the case of palladium catalysts, it was proposed [11] that Cl ions, regardless of the way they were introduced, would be mainly localised on the support after activation treatments, since satisfactory agreement between dispersions measured by electron microscopy and chemisorption was obtained. During catalytic reaction, Cl ions desorb in the form of HCl and evolved HCl was proposed to compete with reactants on the metal active sites, blocking these sites and inhibiting the activity. For Pt/Al2 O3 prepared with chlorinated Pt precursor, Marceau et al. [13] suggested that, in addition to chloride ions siting on the support, a second type of chlorine species would exist, being located at the platinum–alumina interface possibly as bridging species between Pt and Al, and directly influencing the adsorptive and catalytic properties of the 6 P. G´elin, M. Primet / Applied Catalysis B: Environmental 39 (2002) 1–37 metal. Although being in a small amount, this second type of species would predominate, masking the influence of chloride ions siting on the alumina alone. Electronic effects affecting the surface properties of the metal are put forward to explain the inhibition of the activity by Cl. Even if it is now clear that Cl ions play an inhibiting role in the total oxidation of methane on platinum and palladium supported catalysts, more experimental work is needed to fully understand how it affects the catalytic properties of the metal. In the case of platinum catalysts, recognised to easily sinter under oxidising conditions, it would be worthwhile to check whether the presence of chloride ions originating from metal precursor play a role in the sintering process. It must be kept in mind for the next sections that a proper comparison of the catalytic behaviour measured with varying catalysts will require to know whether some residual chlorine is present or not at the surface of the studied catalysts. Unfortunately, this aspect was not considered in many of the studies and this will be mentioned whenever possible. 2.2. Kinetic studies The catalytic activity of alumina-supported Pt and Pd catalysts was shown to depend on the O2 :CH4 molar ratio. In an oxidising CH4 :O2 feedstream (O2 :CH4 molar ratio greater than 2), Pd/Al2 O3 is more active than Pt/Al2 O3 [14,15]. Under these conditions, the oxidation of methane is complete and carbon dioxide is the only carbon-containing product. Typically, in a He flow containing 0.2 vol.% CH4 and 1 vol.% O2 with a space velocity of 52,000 h−1 , the temperature at half conversion (T50 ) of a 0.2 wt.% Pt/Al2 O3 catalyst was found more than 100 ◦ C higher than that of a 0.16 wt.% Pd Al2 O3 catalyst. The presence of CO in the feed had no effect on the catalytic activity, which is expected in view of the reactivity of CO towards O2 much higher than that of CH4 . Several kinetic studies of the complete oxidation of methane over Pt and Pd supported catalysts were reported in the literature [16–21]. Except for reference [21], catalysts were prepared from Cl-containing precursors and the evolution of the Cl content during the study was not addressed. Another difficulty arises from the fact that, in some cases, reaction conditions were changed from oxidising to reducing (lean to rich), which might also affect the nature of the active sites (vide infra). When performed on Pd/Al2 O3 catalysts under oxidising or stoichiometric conditions, it can be stated that the reaction is first order with respect to methane and 0 (or almost 0) order with respect to oxygen concentration. This was observed at very high space velocities with Pd on Si-stabilised Al2 O3 [20]. This was also recently established by van Giezen et al. [21] in their thorough kinetic study of methane oxidation over a 7.3 wt.% PdO-on-alumina catalyst. A Cl-free catalyst was prepared by impregnation of an alumina with Pd(NH3 )4 (NO3 )2 solution and further calcined in air at 450 ◦ C. The rate of the reaction was measured between 180 and 515 ◦ C, after stabilisation of the catalyst in a reaction feed consisting of 1 vol.% CH4 , 4 vol.% O2 in helium. The apparent activation energy was measured between 200 and 320 ◦ C. Special attention was paid to avoid thermal effects and diffusion limitations and it was checked that no deactivation of the catalyst occurred. While CO2 was shown to have no influence on the reaction rate (between 0 and 5 vol.% CO2 ), a strong inhibition by H2 O was observed. When operating under dry conditions (without water added except that produced by the reaction), the inhibition by H2 O was shown to depend on conversion. An apparent activation energy of 86 kJ/mol was found, very close to those ranging between 70 and 90 kJ/mol reported previously in the literature [22,23]. When adding 2 vol.% H2 O to the dry feed the water content was very slightly affected by the reaction and differential conditions were reached. An apparent activation energy of 151 ± 15 kJ/mol was found. Kinetic measurements were carried out under wet conditions. The CH4 concentration was varied between 0 and 6 vol.% and O2 between 2 and 7 vol.% while maintaining stoichiometric or lean conditions, and 2 vol.% H2 O. The orders with respect to methane and oxygen pressures were 1.0 ± 0.1 and 0.1 ± 0.1, respectively. In a study of 0.5 wt.% Pd catalysts on SiO2 , Al2 O3 and silica-alumina, Muto et al. reported slightly different values of reaction orders [18]. The catalytic activity was measured at 400 ◦ C after the activity was stabilised at 450 ◦ C for 12 h. The CH4 concentration was varied from 2 to 15 vol.% with 10 vol.% O2 and the O2 amount varied between 10 and 60 vol.% with 10 vol.% CH4 . So that experimental conditions varied from rich to lean mixtures (reducing to oxidising P. G´elin, M. Primet / Applied Catalysis B: Environmental 39 (2002) 1–37 compositions). The orders with respect to methane were 0.46, 0.53 and 0.58 for Pd/SiO2 , Pd/Al2 O3 and Pd/SiO2 -Al2 O3 respectively while the orders with respect to O2 were 0.14, 0.18 and 0, respectively. It is difficult to explain the low values of the order with respect to methane. It can be suspected that, in this case, varying conditions from rich to lean mixtures has some influence on the nature of active sites, leading to changes of the order with respect to methane. Concerning platinum based catalysts, the trend is the same as for palladium catalysts: the order of the reaction with respect to oxygen mostly tends toward zero while the order with respect to methane would be close to unity. This was first claimed by Cullis and Willatt in a pioneering study of Pt (and Pd) catalysts on various supports (Al2 O3 , TiO2 , SnO2 , ThO2 ) at temperatures in the range 300–440 ◦ C with O2 :CH4 ratios varying from lean (10:1) to rich (1:10) values [16]. The same conclusion was reached by Niwa et al. for 2 wt.% Pt/Al2 O3 under stoichiometric conditions [17]. More recently, Ma et al. [19] reported the kinetics of oxidation of light hydrocarbons (methane, ethane and propane) on Pt/␦-Al2 O3 . In this work, the catalyst was prepared by impregnation of the support by chloroplatinic acid and kinetic studies were carried out under differential conditions (less than 10% conversion) at a space velocity of ca. 35,000 h−1 . The orders with respect to methane and oxygen pressures were respectively 0.95 and −0.17 [19]. Several models were developed and the kinetics of methane oxidation was best described by a Langmuir–Hinshelwood model in which adsorbed methane molecules react with atomic oxygen atoms [19]. 2.3. Nature of the active sites It is well known that platinum and palladium exhibit very different reactivities towards oxygen. In the presence of oxygen, palladium and platinum oxidises into PdO and PtO2 respectively. PdO forms between ca. 300–400 ◦ C, being stable in air at atmospheric pressure up to about 800 ◦ C. Above this temperature, the stable species is metallic palladium. By contrast, PtO2 is highly unstable: compared to PdO, it decomposes at a much lower temperature, around 400 ◦ C. In addition, PtO2 is highly volatile and this property is often considered to explain reconstruction of platinum 7 surfaces under oxygen atmosphere by transport of Pt in the form of PtO2 over nanometric distances. This suggests that PdO forms easily in oxidising atmosphere, while Pt would mostly remain at the metallic state under the same conditions. It seems that the same ideas would apply to supported Pt and Pd catalysts. Supported Pd catalysts were shown to adsorb between 80 and 600 ◦ C much higher oxygen amounts than the corresponding Pt ones (e.g. 100 times for an alumina supported catalyst) [16]. It is also established that Pt0 forms even after calcination of supported Pt precursors in oxygen (or in air) at 500 ◦ C. These ideas are addressed in this section. 2.3.1. Palladium catalysts When supported on carriers having a high specific surface area, the thermal stability of PdO under oxygen atmosphere exhibit significant variations depending on the nature of the support used. Farrauto et al. [24] studied the thermal stability of PdO supported on alumina in air (1 bar) by thermo-gravimetric analysis (TGA). They found that a freshly prepared PdO/Al2 O3 (4 wt.% Pd sample prepared from the nitrate salt) decomposed in air between 800 and 850 ◦ C. However, a surprising feature is that, once PdO is decomposed, temperatures well below 650 ◦ C are required for its re-formation. The influence of various oxide supports other than alumina, Ta2 O3 , TiO2 , CeO2 and ZrO2 , on the thermal stability of supported PdO and the reformation of PdO from Pd was examined [25]. The temperature of PdO decomposition was found to vary from one support to another. For example, PdO on ZrO2 decomposes more than 100 ◦ C below the temperature of decomposition measured for PdO on the other supports (alumina included), all of which show only tiny variations of PdO decomposition temperatures. The existence of significant support–metal oxide interactions was proposed. On the other hand, regeneration of PdO from Pd metal in the cooling step of temperature cycles was also found to be strongly dependent on the support. TiO2 and CeO2 were thus shown to induce a sharp increase in the temperature of PdO reformation (ca. 130 ◦ C) with respect to Al2 O3 . These supports were concluded to increase the temperature domain for which PdO is stable, compared to Al2 O3 or ZrO2 . It must be pointed out that for applications involving temperatures lower than typically 600 ◦ C, which is the topic of the present 8 P. G´elin, M. Primet / Applied Catalysis B: Environmental 39 (2002) 1–37 review, PdO is stable, so that its regeneration is out of concern. In spite of these effects, it is clear that in the presence of 2–4 vol.% oxygen PdO is the thermodynamically stable phase in the temperature range where the catalyst is active, starting from 300 ◦ C up to at least 600 ◦ C. However, the kinetics of the oxidation process may limit the extent of oxidation. Cullis and Willatt [16] measured by a pulse method and the adsorption of oxygen on Pd supported on various supports (Al2 O3 being included in these supports) at various temperatures. Large amounts of oxygen were consumed, compared to platinum catalysts and this was consistent with the transformation of Pd metal into palladium oxide to some extent. The phenomenon was found to be dependent on the support and the temperature. The amount of adsorbed oxygen increased with temperature, e.g. an optimum temperature for oxygen adsorption on Pd/Al2 O3 being 600 ◦ C. The influence of dispersion on the oxygen uptake of Pd/Al2 O3 catalysts was studied by Hicks et al. [26]. The O2 adsorption was carried out at 300 ◦ C for Pd particles supported on alumina in which the dispersion varied from 3 to 80%. Bulk palladium oxidised and the extent of oxidation was found to be dependent on the dispersion: the lower the dispersion, the lower the Pd oxidation extent. The mechanism by which Pd oxidises into PdO is still unclear. For example, Chen and Ruckenstein [27] and Jacobs and Schryvers [28] have studied the reaction of oxygen with supported palladium particles by electron microscopy. At 350 ◦ C, the particle size is retained. At 500 ◦ C, extensive fragmentation and spreading of all particles were observed. Oxidised particles contained mostly fcc metal and no crystalline PdO was observed [28]. A mechanism was proposed, initiated by lattice imperfections and developing cracks and fissures. Once an oxide film is formed the reaction slows down dramatically. Metal particles are fragmented and porous, covered with a thin layer of oxide not detectable. In a more recent study, Voogt et al. [29] investigated the oxidation of palladium model catalysts by XPS analysis. Palladium particles of 5 and 8 nm supported on SiO2 /Si(1 0 0) were studied. The oxidation was modelled by a stepwise growing of the oxide layer around the core of the metal particle. The thickness of the oxide layer formed during oxidation was found to increase linearly with time. The rate of the oxidation was strongly dependent on temperature. The activa- tion energy for the oxidation would be at least equal to 100 kJ/mol. It was proposed that the rate-limiting step in the process is the lattice reconstruction needed for the formation of a new oxide layer at the oxide–metal interface. A 8 wt.% Pd/SiO2 catalyst behaved similar to the model catalysts. It is now generally agreed that, under reaction conditions, especially in oxygen-rich atmosphere, palladium oxide is formed and represents the main active phase. Several studies clearly establish the importance of PdO for methane oxidation [24,30–37]. The occurrence of a reversible PdO ↔ Pd0 transformation in the presence of oxygen was clearly established by TGA and temperature programmed decomposition–temperature programmed oxidation (TPD–TPO) experiments [24,37]. Fig. 2 shows the typical O2 concentration profile obtained in a TPD–TPO experiment upon heating (from 200 up to 900 ◦ C at a heating rate of 15 ◦ C/min in 1% O2 in He) a PdO catalyst supported on La-stabilised alumina and upon its subsequent cooling [37]. At least Fig. 2. Comparison between the catalytic activity in CH4 oxidation of an alumina-supported Pd catalyst and the O2 profile measured in a TPO experiment. The Pd catalyst was prepared by impregnation of a La-doped Al2 O3 by Pd(NO3 )2 (Pd loading = 5 wt.% Pd), calcined at 1000 ◦ C and deposited as a thin layer on an annular tube reactor. For the reaction test, the feed composition was 5000 ppm CH4 , 2 vol.% O2 , He balance, (GHSV = 1,100,000 cm3 /gcat h). The conversion curves correspond to a second temperature cycle between 200 and 900 ◦ C. The TPO profile was obtained with the same coated catalyst with 1 vol.% O2 in helium (flow rate 120 cm3 /min). (Reproduced from Fig. 3 of [37], with permission from Elsevier Science.) P. G´elin, M. Primet / Applied Catalysis B: Environmental 39 (2002) 1–37 two decomposition peaks (appearing positive) can be observed above 700 ◦ C on heating ramp while one re-formation peak below 500 ◦ C (appearing negative) is observed on cooling. This indicates that there is a temperature range in the cool-down step of the cycle where PdO is not re-formed, palladium being in the metallic state. This observation together with the assumption that metallic palladium is much less active than PdO was used to explain the catalytic behaviour during the heating-cooling cycle, as reported in Fig. 2 for comparison. A strong drop of activity during the cooling-down step is observed in the range where Pd is still at the metallic state until PdO re-forms, thus leading to a catalytic activity identical to that obtained during the heating-up step, before PdO decomposition. It is clear that, depending on the experimental conditions used for catalytic testing, the activity drop can be more or less pronounced [24,37]. Moreover, the activity and TPO curves cannot be strictly compared one to the other since being obtained in slightly different conditions (different oxygen concentrations and very different heating/cooling rates). This would explain why the activity starts to decrease before PdO decomposition in TPO on heating and, conversely, the CH4 conversion is restored before PdO re-formation on cooling. Anyway, these results remarkably support the idea that palladium in the metallic state has a much lower activity than the oxide form. Burch and Urbano compared the reactivity of oxygen chemisorbed on Pd metal to that of oxide ions for Pd/Al2 O3 catalysts [31]. A 4 wt.% Pd/Al2 O3 sample was prepared by impregnating the alumina with palladium nitrate, dried at 120 ◦ C and then submitted to a wide range of oxidation and reduction treatments at 500 ◦ C before the catalytic activity (1 vol.% CH4 in air) was measured as a function of time on stream at 300 ◦ C. Catalytic steady-state and pulse experiments on the pre-reduced samples clearly indicated that metallic palladium is not active while pre-oxidised catalyst is active. The study of the oxidation at 300 ◦ C of the pre-reduced samples revealed that the oxidation always proceeds via a very fast formation of a monolayer of oxygen followed by a slower oxidation step leading to almost complete oxidation of palladium. The question of what is the optimum state of PdOx between chemisorbed oxygen on Pd metal, a PdO skin on a Pd metal core or bulk PdO was further addressed by comparing the evolution of methane oxidation 9 Fig. 3. Comparison of the methane conversion at 300 ◦ C vs. time and the oxygen uptake at 300 ◦ C vs. time for a 4 wt.% Pd/Al2 O3 catalyst pre-reduced in H2 at 300 ◦ C. Results were obtained in separate experiments. Composition of the reaction feed: 1 vol.% CH4 in air. The oxygen uptake was measured by a pulse-flow experiment. (Reproduced from Fig. 1 of [33], with permission from Elsevier Science.) activity and oxygen uptake at the same temperature (300 ◦ C) as a function of time [32,33]. It was assumed that the same Pd surface state was reached under reactants or in oxygen atmosphere. The results are shown in Fig. 3. A chemisorbed monolayer of oxygen formed very quickly at 300 ◦ C while the activity was low. It was deduced that oxygen chemisorbed on Pd metal is poorly active. Further exposure to oxygen led to a slower oxidation of the Pd up to the almost complete oxidation to bulk PdO. The activity increased simultaneously to reach a plateau at the point where 70–75% of the complete oxidation of Pd into PdO was achieved. It was concluded that fully oxidised bulk PdO is the optimum state for methane oxidation and the intermediate state corresponding to a ‘skin’ of PdO on a Pd metal core has no greater activity than bulk PdO. For other authors, both PdO and Pd may be present on the catalysts under reaction conditions. Lyubovsky and Pfefferle [38] and Datye et al. [39] studied the decomposition of PdO into Pd in Pd catalysts supported on ␣-Al2 O3 plates and its reformation during temperature cycling. The formation of highly dispersed PdO clusters in thermodynamic equilibrium with the previously formed metallic Pd surface after pre-treatment at temperatures above 800 ◦ C was first suggested [38]. 10 P. G´elin, M. Primet / Applied Catalysis B: Environmental 39 (2002) 1–37 More recently [39], it was proposed that the PdO → Pd transformation is initiated at the surface of PdO and causes small domains of Pd metal to form on the surface of PdO. These small domains are easy to re-oxidise upon cooling, which is not the case when complete transformation into Pd metal is achieved. Strongly bound oxygen at the surface of Pd would inhibit bulk oxidation. The re-oxidation of Pd when it does occur would involve the oxide growing in patches on the metal. The re-oxidation via the growth of a thicker oxide film (shrinking core of Pd metal) was not retained as the most likely mechanism. Complete oxidation of the Pd metal would lead to the formation of polycrystalline PdO with a roughening of the particle surfaces. Re-activation of the Pd metal catalysts upon cooling could be associated to two possible mechanisms: PdO re-formation (increase in the fraction of Pd metal particles fully re-oxidised into bulk PdO) or a reaction mechanism which involves metallic Pd. It is worthwhile noticing a recent work studying the influence of the oxygen content in supported Pd/PdO particles on the activity in methane oxidation, even though the conventional alumina support was substituted in this case by a ceria-zirconia support [40]. A 3 wt.% Pd/CeO2 -ZrO2 (containing 10% ceria) was prepared by impregnation of the support with palladium nitrate. The catalyst was calcined at 500 ◦ C in O2 . Catalytic testing was performed with pulses of a 1:4 CH4 :O2 mixture in helium after cycling up to 900 ◦ C in a continuous flow of the same reaction mixture. The degree of reduction was varied by controlled chemical reduction with methane. Interestingly, slight reduction was observed to improve the catalytic activity compared to either fully oxidised or fully reduced metallic particles. In addition, the partially reduced sample was found to be easier to re-oxidise than the completely reduced one. Even though there seems to be a general agreement on the fact that PdO is formed under oxygen-rich reaction conditions, some parameters such as the pre-treatment history, the metal dispersion and the composition of the reaction mixture seem to have also some influence on the catalytic behaviour. More experimental work is still needed to fully understand the exact nature of the active sites under working conditions. Attempts to correlate the activity in methane oxidation with parameters such as the reactivity of PdOx species towards methane or the extent of oxidation were unsuccessful so far. 2.3.2. Platinum catalysts Fewer studies were devoted to platinum catalysts compared to palladium ones. For platinum catalysts, it seems that a distinction between large and small supported particles should be made. Experimental works on Pt single crystals have shown that oxygen reacts with at most the top two layers of the surface, dissociatively chemisorbing onto the platinum surface below 500 ◦ C [41]. When supported, small and large Pt particles may co-exist. Hicks et al. [26] studied alumina supported Pt catalysts prepared from Cl containing precursors. IR spectra of adsorbed carbon monoxide at saturation coverage were used to probe the metal surface before and after exposure to reaction conditions. It is noteworthy that the catalysts were reduced in H2 and evacuated in vacuo at 300 ◦ C prior to CO adsorption. Two bands were observed at ca. 2080 and 2070 cm−1 , which were attributed to CO linearly bonded to surface platinum atoms. These bands were associated to the two phases of platinum on alumina proposed in previous studies ([42] and references herein). The high frequency band would be related to a crystalline phase weakly interacting with the support and responsible for high reaction rates. The low frequency band would be due to a highly dispersed phase much less active for methane oxidation. In the absence of any in situ characterisation of the catalysts, it was proposed [26] that under reaction conditions, two types of oxidised species formed, associated to two TPR peaks: a peak observed at low temperature would be due to oxygen dissociatively chemisorbed on large crystallites while a TPR peak at higher temperatures would correspond to a dispersed PtO2 phase, less reactive towards methane oxidation. The formation of PtO2 was reported above 300 ◦ C in the case of 100% dispersed particles [43]. Recently, Hwang and Yeh studied the various Pt-Ox species formed on oxidation of reduced Pt/␥-Al2 O3 [44] and Pt/SiO2 [45] by TPR technique. Samples were prepared by impregnating the support with PtCl4 solution. Higher dispersions (100 and 65%) were obtained with alumina compared to silica supported catalysts (30 and 50%). For Pt/Al2 O3 catalysts, essentially four different forms of oxidised Pt species were proposed to form depending on the temperature P. G´elin, M. Primet / Applied Catalysis B: Environmental 39 (2002) 1–37 of oxidation: PtS –O at room temperature (PtS being a surface platinum atom), PtO at 100 ◦ C, PtO2 at 300 ◦ C and PtAl2 O4 at 600 ◦ C. On SiO2 , a decrease of the O/Pt stoichiometry was observed and attributed to the decomposition of PtO and PtO2 . X-Ray photoelectron spectroscopy confirmed the co-existence of these two oxides below 400 ◦ C. The authors mentioned a loss of Pt determined by ICP analysis upon oxidation above 400 ◦ C. This is unlikely since transport of platinum should be restricted to atomic distances because of the high volatility associated with the low stability of PtO2 . No such a loss of platinum was mentioned elsewhere in the literature. It is worthwhile to notice that these studies were carried out on highly dispersed samples which are known to sinter severely with time on stream in the conditions of the reaction. It would be more convenient to study less dispersed samples. According to Burch and Loader, the extent of oxidation of the platinum surface would be a key factor on the catalytic behaviour, a less oxidised platinum surface being more active compared to a more oxidised surface [15]. On this basis, operating under methane-rich conditions is expected to lower the oxidation extent of Pt surface. As a result, Pt/Al2 O3 exhibited higher activity than Pd/Al2 O3 catalysts in the combustion of methane under reducing conditions [15]. 2.3.3. Mechanistic considerations The different states of palladium and platinum under reaction conditions being taken into account, two different mechanisms for the oxidation of methane on these two metals were proposed, involving two different ways for the activation of the C–H bond of methane [46]. It was proposed that Pt would activate the almost non-polar C–H bonds of methane through a homolytic mechanism (dissociative adsorption of CH4 at free metal sites), oxygen species acting as an inhibitor for the reaction at full coverage. This would agree with the optimum and very high activity under rich conditions and a poor activity under lean conditions. By contrast, Pd is much more effective than Pt under lean conditions. Pd would be fully oxidised and Pd2+ O2− ion pairs at the surface of PdO would activate the C–H bonds by a heterolytic mechanism [46], similar to that proposed by Choudhary and Rane on oxide catalysts [47]. However, the metal and the metal oxide were thought to be in dynamic equilibrium 11 depending on temperature, gas composition, the transition Pd/PdO being slow. The activity of a 0.5 wt.% Pd/Al2 O3 catalyst was exposed to a reaction mixture of composition oscillating between rich and lean conditions was found to be much lower than that reached under steady-state conditions (either rich or lean). 2.4. Particle size effect Some early studies mentioned that the complete oxidation of methane over Pd and Pt catalysts supported on alumina could be structure sensitive [9,23,26,48–50] since very strong variations of the catalytic activity were observed with varying dispersions. These conclusions were drawn from the calculation of turn-over frequencies (TOFs), the number of active sites being measured by hydrogen uptake of the reduced catalysts. For example, Hicks et al. [26] measured the catalytic activity of supported Pt and Pd catalysts under slightly oxygen-rich conditions (O2 :CH4 = 2.2). TOFs were calculated using the initial dispersion determined by chemisorption of hydrogen on the reduced samples before exposure to reaction conditions. The mean steady-state TOFs at 335 ◦ C were found to vary as follows for the different catalysts: dispersed phase of platinum, TOF = 0.005 s−1 , crystalline phase of platinum; TOF = 0.08 s−1 , small particles of palladium; TOF = 0.02 s−1 , large particles of palladium; TOF = 1.3 s−1 . The structure sensitivity was related to differences in the reactivity of adsorbed oxygen. A more careful investigation of Pd catalysts seemed to confirm the structure sensitivity of Pd towards methane oxidation [9]. A series of Pd/Al2 O3 catalysts of varying Pd dispersions (2–74 nm average diameter from hydrogen chemisorption) were obtained by varying the calcination temperature and were tested under continuous-flow conditions, in the complete methane oxidation in an oxygen rich atmosphere (1 vol.% CH4 in air) [23]. Wide activity variations which could not be attributed to support effects were observed. No clear relationship between palladium particle size and reaction rate was established [23]. The ‘structure sensitivity’, proposed in these studies must be qualified, however, for two reasons. All catalysts were prepared from Cl-containing precursors. Chlorine which is known to strongly inhibit the reaction of methane oxidation could affect the reaction 12 P. G´elin, M. Primet / Applied Catalysis B: Environmental 39 (2002) 1–37 rate in a uncontrolled manner dependent on its removal from the catalyst surface. The second argument concerns the determination of the number of active sites from which TOF are calculated. The number of active sites is determined from the H2 uptake by the reduced metal particles. For platinum particles, this number is likely to be the same under working conditions, at least for large particles, since the particle core is likely to be at the metallic state. However, for palladium catalysts, the number of PdO sites accessible to methane and involved in the reaction might differ from that measured in the reduced metallic state (Section 2.3). Burch and Urbano [31] did not find any correlation between either the initial or steady-state activity and the amount of exposed palladium oxide surface, as determined by chemisorption on reduced samples. But long-term morphological changes (especially in the presence of water) are suggested to account for varying catalytic behaviour. The opposite conclusion, i.e. the structure insensitivity, was drawn from activity and dispersion measurements of other studies [15,16,51,52]. After prolonged heating (40 days) in O2 at 550 ◦ C of 2.7 wt.% Pd/␥-Al2 O3 , the sintering of particles was evidenced by electron microscopy but this did not induce any variation in the catalytic activity [16]. With Pt/Al2 O3 catalysts having particles in the range 1.4–3.7 nm diameter, Burch and Loader [15] found no evidence of a particle size effect in contrast to previous works mentioning that the complete oxidation of methane is structure sensitive [48,51,53]. In the absence of any Cl impurities originating from the Pd precursor, Hoyos et al. [51] observed a slight increase of the catalytic activity of Pd/SiO2 (2.2 wt.% Pd, 1% CH4 :4% O2 in nitrogen) in spite of the decrease of the metallic dispersion from 34 to 17% (from H2 chemisorption): the light-off temperature (temperature at half conversion, T50 ) decreasing from 309 to 304 ◦ C. For the determination of how the turn over rate varies with the structure of the catalyst, the average Pd particle size on a series of Pd catalysts supported on various supports (ZrO2 , Al2 O3 and Si-Al2 O3 ) was varied between 2 and 130 nm [22]. The turn over rate was calculated at steady state (24 h reaction) on the basis of Pd dispersion measured after reaction. The values ranged in the interval 2 × 10−2 to 8 × 10−2 s−1 . It was concluded that the reaction was ‘structure insensitive’ but this does not exclude some variation of the activity with the particle size. On two Cl-free Pt catalysts stabilised under reactants at 600 ◦ C for 60 h (1% CH4 :4% O2 ) and exhibiting 6 and 14% dispersion, respectively. Marceau et al. [12] found TOF of 0.15 and 0.08 s−1 , respectively, suggesting an increase of TOF with the metal particle size. 2.5. Influence of the reaction products on the activity As already mentioned in Section 2.2, water either being present as a product of the methane conversion or as an adduct to the reaction feed has an inhibiting effect on the methane oxidation rate over palladium catalysts. Cullis and Willatt [8] examined the catalytic activity of a 2.7 wt.% Pd/␥-Al2 O3 in a 2:1 O2 :CH4 mixture at 352 ◦ C using pulse-flow experiments. Water was added to the pulses containing 1.8 ␮mol CH4 in the range 0–55 ␮mol H2 O, i.e. 30 times the amount of CH4 . An apparent order of −0.8 with respect to H2 O was derived (Fig. 4) but within experimental uncertainties a variable negative order from −0.4 to −1.3 could be also deduced from given data. Ribeiro et al. [22] studied a 7.7 wt.% Pd/Si-Al2 O3 prepared by incipient wetness using an aqueous solution of Pd(NH3 )2 (NO2 )2 and calcined at 500 ◦ C. The dependence on H2 O was measured at 277 ◦ C by establishing excess CO2 (feed of 1% CH4 , 0.25% CO2 , balance air, with a CH4 conversion level less than 3%) and vary- Fig. 4. The effect of H2 O addition on the activity of a 2.7 wt.% Pd/Al2 O3 catalyst in CH4 oxidation performed at 352 ◦ C in a pulse-flow reactor. Composition of the reactant pulse: 1.8 ␮mol CH4 , 3.6 ␮mol O2 . (Reproduced from Fig. 1 of [8], with permission from Elsevier Science.) P. G´elin, M. Primet / Applied Catalysis B: Environmental 39 (2002) 1–37 ing the H2 O concentration in the range 0.03–0.15%. An order dependence of −0.98 was determined. The authors suggested a competition of H2 O with CH4 for surface active sites leading to the formation of Pd(OH)2 at PdO surface sites as first proposed by Cullis et al. [54]. The competition between methane and water for the active sites was also suggested by van Giezen et al. [21] in a kinetic study of the oxidation of methane over a Cl-free PdO-on-alumina catalyst. In this study, the order with respect to H2 O (within the interval 0.6–3.1% H2 O) measured in the temperature range 180–515 ◦ C was found to vary with reaction temperature in the range 0.8 ± 0.2. The influence of water concentration on the rate of oxidation of methane over Pd/Al2 O3 was also investigated by Burch et al. [52] at different temperatures. A 4 wt.% Pd/Al2 O3 was prepared from Pd(NO3 )2 and calcined at 500 ◦ C. The feed consisted of 1% CH4 in air. Prior to experiments the catalyst was held in the reaction mixture at 300 ◦ C for at least 12 h in order to be sure that steady state had been reached. An inhibiting effect decreasing with increasing temperature was observed, becoming small above about 450 ◦ C. This effect was reversible. Again, the existence of the equilibrium PdO + H2 O = Pd(OH)2 was proposed, where PdO represents the active phase and Pd(OH)2 an inactive state for methane oxidation. On this basis, it was postulated that the true rate determining step could be the loss of H2 O from Pd(OH)2 instead of the activation of the first C–H bond in methane. Pulse experiments in which small pulses of 1% CH4 /air were passed at 300 ◦ C over 5% Pd/SiO2 maintained in dried air atmosphere revealed a very high and stable activity, much higher than the steady-state activity [32,33]. At 250 ◦ C, some deactivation was observed with increasing the number of pulses. It was concluded that the surface OH groups produced either by the reaction or formed by H2 O added to the feed stream are not easily removed at 250 ◦ C (respectively 300 ◦ C). In this process, the support might have some influence. Supports with a greater affinity for H2 O would initially show higher activity because of the trapping of water by the support. But, it is also possible that at steady state the local water concentration above PdO particles could be higher resulting in a lower reaction rate. The authors indicated that the activity of Pd/Al2 O3 initially higher becomes lower than the one of Pd/SiO2 at steady state. 13 Water inhibition on the catalytic activity of zirconia supported Pd catalysts in methane oxidation was also observed [55,56]. Reaching the water adsorption/desorption equilibrium was found to be slow compared to the methane oxidation time scale, especially at low temperatures. In addition the time required for reaching the equilibrium strongly depended on temperature. This was reflected in the strong water inhibition on catalytic activity observed in pulse experiments at lower temperatures. No influence of CO2 on the catalytic activity of Pd/Al2 O3 catalysts could be observed [22,57,58]. In the study of Ribeiro et al. [22], the dependence of the activity of a 7.7 wt.% Pd/Si-Al2 O3 on CO2 was measured at 277 ◦ C in the presence of added H2 O (feed of 1 vol.% CH4 , 0.05 vol.% H2 O, balance air, with a CH4 conversion level less than 2.5%) and varying the CO2 concentration (0.012–0.82 vol.%). No effect on reaction rate was observed up to 0.5 vol.% CO2 above which a strong inhibition occurred and did not find any explanation. 2.6. Changes of activity under reactants (activation and deactivation) Early studies mentioned large enhancements of the activity of Pd/Al2 O3 catalysts with time on stream [9,23,50,59]. On the other hand, the effect, if any, was much less pronounced with Pt catalysts [48,49]. Although morphological effects were invoked to explain this phenomenon, it is now admitted [11,32,33] that most of the activation of the catalysts with time on stream has to be related to the slow removal of residual chlorine from the catalyst with time on stream (Section 2.1). It is noteworthy that chlorine contamination could be due to Cl-containing precursors of the metal, as for instance in references [9,23,50,59,60]. Burch [33] suspected that the Pd catalysts studied in early works by his group and prepared from Pd nitrate salt might have been contaminated by chlorine present as impurities in the salt. Chlorine could be originally present on the support as an impurity [51]. In this study [51], the Pd/Al2 O3 catalyst was prepared by impregnating a commercial alumina prepared by flame hydrolysis of AlCl3 (aluminium oxyd-C from Degussa) with palladium acetylacetonate in toluene. A strong enhancement of the catalytic activity under reaction conditions 14 P. G´elin, M. Primet / Applied Catalysis B: Environmental 39 (2002) 1–37 was observed, which was believed not to be due to chlorine removal. However, although the metal precursor was free of chloride, the chlorine impurities of the alumina itself should be considered and might play an inhibiting role in the catalytic activity as well. In this study, the alumina contained high amounts of chloride ions (ca. 5000 ppm). This interpretation finds further support in examining the data obtained with Pd/SiO2 prepared with Cl-free silica (Degussa Aerosil 200, Cl content less than 240 ppm) and Pd(NH3 )4 (OH)2 . In this case, no activation could be observed and no chlorine was present [51]. Although chlorine removal is likely to play the major role in the activation of catalysts under reaction conditions, this is not to claim the absence of morphological effects [22,48,49]. For Pt/Al2 O3 , a change in the reactivity of adsorbed oxygen was postulated [48]. Nanodiffraction and transmission electron studies [49] revealed the preferential formation, under reactants, of Pt(1 1 0) planes at the surface of the Pt particles, these planes developing parallel to ␥-alumina (1 1 0) planes and allowing the epitaxial growth of Pt particles on the alumina surface. The increase of reactivity was related by the authors to the preferential exposure of Pt(1 1 0) planes at the surface of Pt particles, these planes being thought to be more active towards methane oxidation. A similar study was carried out with a Pd/Al2 O3 catalyst [61]. A 1.95 wt.% Pd/Al2 O3 was prepared by impregnating a transition alumina (SCM129 from Rhˆone Poulenc) with an aqueous solution of tetrachloropalladic acid. The catalysts taken before and after catalytic reaction (1 vol.% CH4 :4 vol.% O2 in nitrogen) were subsequently reduced in order to be studied by nanodiffraction, electron spectroscopy and adsorption of CO followed by FT-IR. While the freshly reduced sample exhibits mainly Pd(1 1 1) crystal planes exposed to the reactants, less dense surface crystal planes develop at the expense of Pd(1 1 1) ones after reaction at 600 ◦ C and the dispersion of the metal decreased. This reconstruction of Pd particles was considered as the main factor explaining the sharp increase of the catalytic activity after ageing in the reaction mixture. Less dense planes were proposed to allow easier reversible transition between surface metallic palladium and surface PdO because of only slight changes of lattice parameters. Much fewer studies were devoted to the catalytic behaviour of Pd/Al2 O3 and Pt/Al2 O3 in the complete oxidation of methane under oxygen rich atmosphere with time on stream ([11,62] for Pd, and [12,13] for Pt). In the presence of 1 vol.% CH4 :4 vol.% O2 in nitrogen (200 mg of 2 wt.% Pd/Al2 O3 , 6.5 l/h), Roth et al. [11] have shown that, for reaction temperatures in the range 350–450 ◦ C, Cl free Pd catalysts exhibit slow deactivation with time on stream. This deactivation was quite severe since at 350 ◦ C the conversion decreased from initially ca. 90–66% after 3 h reaction. It has to be mention that this behaviour was observed under ‘dry’ conditions, that is in the absence of water added to the reaction feed except that produced by the reaction. Purging the catalyst in gas carrier for 15 min at the same temperature did not restore the activity. Regeneration of the catalyst could be however achieved by purging in dry carrier at temperatures above 500 ◦ C. These results were tentatively attributed to the slow conversion of the active PdO phase into a less active or inactive Pd(OH)2 phase. This behaviour was distinguished from the inhibition by water also observed when water is added to the feed, as already shown in the “kinetic studies” part. Similar observations were made by Mowery et al. [62] on 1 wt.% Pd/Al2 O3 prepared from Pd nitrate salt. Deactivation and inhibition was also observed when water was added to the feed. Irreversible deactivation of the catalyst was observed even at 520 ◦ C. Based on TPD experiments, it was proposed that water would be retained by the catalyst at this temperature. No explanation was given. This deactivating behaviour of Pd/Al2 O3 catalysts is far from being understood. Complementary experiments to elucidate this phenomenon and find out some relationship with the presence or the absence of residual Cl at the surface of the support are required. In the case of Pt/Al2 O3 catalysts, deactivation with time on stream was also sometimes observed [12,13]. According to Marceau et al. [12,13], deactivation would be due to sintering of the Pt particles under reaction mixture at 600 ◦ C. No data are given concerning the behaviour of these catalysts at intermediate temperatures. 2.7. Sulphur poisoning The influence of sulphur compounds present in the reaction mixture on the catalytic activity of Pd and Pt catalysts in the oxidation of methane was examined in the literature. Poisoning by sulphur compounds was P. G´elin, M. Primet / Applied Catalysis B: Environmental 39 (2002) 1–37 established, being more pronounced with palladium than with platinum. The influence of the reaction conditions (concentration of sulphur compounds, presence of water in the feed), or the nature of the support on the poisoning are examined below. The influence of the nature of the support on the poisoning of Pd catalysts by the addition of H2 S was examined by Hoyos et al. [51]. Alumina- and silica-supported Pd catalysts were tested at 350 ◦ C in the oxidation of methane under oxidising conditions (1 vol.% CH4 :4 vol.% O2 in nitrogen) in the absence and in the presence of 100 vpm H2 S. Both catalysts exhibited sharp deactivation in the presence of sulphur. The fact that neither the Pd particle size nor the apparent activation energy were affected by the presence of sulphur in the feed suggested to associate the deactivation to the decrease of the number active sites by sulphur contamination without change of their nature. The extent of deactivation was the same for both catalysts but the alumina support lowered the rate of deactivation. This property was attributed to the ability of the alumina support to trap sulphate species, H2 S being fully oxidised into SO2 /SO3 in the reaction conditions. In the case of Pd/SiO2 , where no surface sulphate species could form on the support, the formation of a palladium sulphate species characterised by an IR band at 1435 cm−1 was proposed. These species can be totally decomposed upon heating in vacuo or in flowing nitrogen at 600 ◦ C, as shown by the full depletion of the 1435 cm−1 band, and the surface properties of palladium, together with the catalytic activity, are fully restored. A correlation could be established between the 1435 cm−1 intensity obtained after increasing thermal regeneration of the catalyst in nitrogen and the catalytic activity, the higher the 1435 cm−1 intensity the lower the rate of methane oxidation. Tentative regeneration of the catalyst in hydrogen led to the decomposition of sulphate species at temperatures as low as 350 ◦ C. But, the catalytic activity was not regenerated in this case. The formation of highly stable surface palladium sulphide species was suggested. The catalytic performance of Pd and Pt catalysts supported on alumina or silica was also studied under NGV engine exhaust gases or model reaction mixtures corresponding to the gas composition of NGV exhausts (high redox ratios, total hydrocarbon concentrations less than 0.3 and 10 vol.% steam) [7]. Under 15 real exhaust gases, the activity of Pd based catalysts for methane oxidation declines rapidly. It was established that this phenomenon is due mainly to sulphur contained in the exhaust, even though its concentration is very low, typically 1 ppm or less. Exhaust sulphur is derived from natural gas itself, i.e. the odorizer contained in the gas, or from engine lubricating oil. Ethane, propane and CO oxidation are also inhibited by low SOx concentrations but to a lesser extent than methane. The authors developed the same ideas as in [51] concerning the mechanism by which the Pd catalyst deactivates and the role of the support in the process (Fig. 5). The deactivation curve obtained with Pd supported on a non-sulphating support (ZrO2 -SiO2 ) (110 g/ft3 Pd, 320 ◦ C, 800 ppm CH4 , 8 vol.% O2 , 200,000 h−1 , 0.1 or 0.9 ppm SO2 ) was consistent with the 1:1 selective adsorption of SOx on PdO. The deactivation is very rapid (2 h with 0.9 ppm SO2 ). In contrast, palladium on sulphating supports such as ␥-Al2 O3 , deactivates more slowly, which was attributed to the adsorption of some SOx by the support. But in this case, the regeneration of the catalyst is also more difficult than with non-sulphating supports. Sulphated catalysts have equal activation energies, suggesting identical sites irrespective of the support. In addition, the activation energy of Pd catalysts poisoned with SO2 increased significantly, which is consistent with the transformation of active PdO sites to less active PdO-SOx sites. The 0.5 eV increase of Pd 3d5/2 electron binding energies observed after low SOx exposure would indicate an increase of the Pd oxidation state, possibly responsible for a decrease in the availability of oxygen from PdO. Compared to palladium catalysts, platinum catalysts are much less active than Pd but also more resistant to deactivation by SOx . However, platinum is still less active than sulphur poisoned palladium catalyst. It is concluded from this work that NGVs equipped with palladium oxidation catalysts can meet NMHCs and particulates standards for US heavy-duty transient test but not total hydrocarbons standards which require methane abatement during cool operation. The effect of traces (20 vpm) of hydrogen sulphide and of sulphur dioxide on the catalytic activity of Pd, Pd and Rh catalysts supported on alumina in the complete oxidation was examined by Meeyoo et al. [63]. The nature of the metal salts was not indicated. The reaction feed contained 1.8 vol.% CH4 and 21 vol.% O2 16 P. G´elin, M. Primet / Applied Catalysis B: Environmental 39 (2002) 1–37 Fig. 5. Proposed mechanism for SO2 inhibition of PdO activity in methane oxidation for PdO supported on sulphating or non-sulphating materials. PdO converts SO2 to SO3 . With a sulphating support, SO3 adsorbs on both PdO and the support, retarding complete PdO poisoning. On the contrary, a non-sulphating support cannot act as a sink for SO3 which poisons PdO directly. On suppressing SO2 from the gas stream, SO3 spills over from the sulphating support to PdO, while it desorbs directly from PdO with a non-sulphating support. (Reproduced from Fig. 7 of [7], with permission from Elsevier Science.) in helium. Both gases (H2 S, SO2 ) inhibited catalytic oxidation over Pd and Rh. The similarity of the catalytic behaviour of Pd with respect to H2 S and SO2 can be explained by the fact that H2 S oxidises to sulphur oxides above ca. 350 ◦ C. And the sulphate formation is proposed again to be responsible for deactivation. The same explanation was advanced to explain results with the Rh containing catalyst. On the contrary, the activity of the Pt catalyst was found to be slightly enhanced by the pollutants. Similar promoting effect of SO2 was previously observed by Burch et al. [46] in the combustion of propane on 1 wt.% Pt/Al2 O3 but not with Pt/SiO2 . According to the results of Meeyoo et al. [63], the activity at low temperatures (below 550 ◦ C) is at least increased by a factor of 2 (with SO2 ) compared to that obtained in the absence of pollutant. The formation of aluminium sulphates resulting in an increase of acidity was thought to be responsible for the enhancement of the activity. In the case of platinum, the sulphate formation on the metal is unlikely since platinum oxide is not the favoured surface state. In the case of Pd/Al2 O3 catalysts, Yu and Shaw [64] proposed an alternative explanation to the deactivation by sulphur by formation of palladium sulphate. The catalyst (4 wt.% PdO supported on ␥-Al2 O3 , 67 m2 /g) was prepared from the nitrate salt and calcined in air at 500 ◦ C before testing in 1 vol.% methane in air (dilution of the catalyst with ␥- or ␦-alumina). The progressive inhibition of the conversion versus time by adding H2 S (80 vpm) to the feed at 400 ◦ C was again shown. Suppressing H2 S allowed the activity to be only slightly recovered. Pre-exposure of the catalyst to H2 S in air (24 h) at increasing temperatures (100–400 ◦ C) caused an increasing inhibiting effect on the activity. FT-IR data indicated the formation of aluminium sulphate (1145 cm−1 ) and to a lesser extent sulphite (1060 cm−1 ). On the basis of the decrease of the BET area due to aluminium sulphate formation, the PdO occlusion was proposed to explain the decrease of the activity instead of the formation of palladium sulphate. Another disagreement with Hoyos et al. concerns the effect of H2 S poisoning on the activation energy, which decreased upon poisoning from ca. 130 kJ/mol (without H2 S) to ca. 90 kJ/mol (with H2 S). It must be noted that the value obtained without H2 S differs significantly from those (85 ± 15 kJ/mol) usually reported under similar experimental conditions, which might possibly indicate thermal effects. The pre-exponential factor decreased by 4 orders of magnitude. The decrease of the activation energy was P. G´elin, M. Primet / Applied Catalysis B: Environmental 39 (2002) 1–37 attributed to changes in the kinetics from surface control to pore-diffusion control, the latter phenomenon being due to the build-up of sulphate (sulphite) groups at the surface of the alumina. Most of these groups were shown to be removed by H2 treatment at 600 ◦ C, the activity for methane oxidation being thus regenerated. This interpretation of the poisoning of palladium catalysts by sulphur was not further considered by other groups. The influence of sulphur poisoning (H2 S) and regeneration in reducing conditions on the activity for methane oxidation of 2 wt.% Pt, Pd or Rh on alumina was examined [65]. The catalysts were either reduced in H2 at 400 ◦ C (fresh) or exposed to 100 ppm H2 S/900 ppm H2 at 100 ◦ C and regenerated in H2 at 400 ◦ C (regenerated). The reaction feed consisted of 4 vol.% CH4 in air (6 l/h, 100 mgcat ). For Pt and Rh catalysts, regenerated samples were found to be slightly more active than the fresh ones, while the reverse is observed for Pd/Al2 O3 . For Pd/Al2 O3 , the activation energy increased after pre-poisoning of the catalyst (113 kJ/mol compared to 84 kJ/mol for the fresh sample). Based upon FT-IR of adsorbed CO, the regeneration seems to be successful for Pd/Al2 O3 . However, an increase of νCO on regenerated Pt catalyst possibly indicated the presence of residual sulphur species at the surface of Pt after regeneration. The mechanism of the deactivation of PdO/Al2 O3 catalysts by SO2 was re-examined by Mowery et al. [62] focusing on the deactivation in the presence of both water and SO2 . Particular attention was given to the ageing of the catalyst being placed in the exhaust of a lean-burn spark ignited natural gas engine. Two types of catalysts were studied: a commercial catalyst, 100 g/ft3 palladium/alumina deposited on a cordierite monolith pre-calcined at 700 ◦ C in air, and a model catalyst, 1 wt.% Pd/␥-alumina (260 m2 /g) (impregnation with palladium nitrate) calcined at 500 ◦ C in air. The catalytic activity in complete methane oxidation was measured in a microreactor flowed with 800 ppm CH4 , 6.5 vol.% O2 , N2 balance (for dry feed), 800 ppm CH4 , 16.4 vol.% O2 , 2–3 vol.% H2 O, N2 balance (for wet feed) and 800 ppm CH4 , 410 ppm CO, 340 ppm NO, 2–3 vol.% H2 O, 6 vol.% CO2 , 16.4 vol.% O2 , N2 balance (for simulated exhaust). Rapid deactivation of PdO/Al2 O3 catalysts was seen as the result of engine ageing. Phosphate (originating from lubricant additives) and sulphate deposits were 17 detected, but only sulphates forming both on PdO and alumina surfaces are considered to be the main cause of deactivation. When the conversion of methane was performed in dry feed, fresh and engine aged samples exhibited the same activity. The presence of water in the feed appeared necessary to reveal the deactivation of the catalyst. SO2 was shown to cause both inhibition (10% loss of activity with 10 ppm SO2 at 460 ◦ C) and deactivation with time on stream. Deactivation was partly (or fully) reversible depending on the temperature. The presence of both water and SO2 in the feed caused the catalyst to deactivate more rapidly and the recovery of the activity was more difficult than when either poison was added separately. On the basis of TPD experiments, it was proposed that sulphation of alumina produced a more hydrophilic surface, adsorbing water more strongly and in larger amounts. The presence of water was thought to force spillover of surface SOx species from the alumina to PdO and enhance the rate of bulk PdSO4 formation. Lee et al. [66] merely addressed the oxidation of H2 S in the presence or not of methane over Pd- and Pt-based monolith catalysts. Both catalysts consisting of ceramic monoliths coated with a washcoat of alumina and the precious metal (1.4 g/l metal) were tested in the oxidation of H2 S, methane and methane/H2 S. Over Pd based catalyst, H2 S (26 ppm) was shown to have a strong inhibiting effect on the methane conversion (the light-off temperature being ca. 200 ◦ C higher in the presence of H2 S). On the contrary, with platinum catalyst, the catalytic activity was slightly increased in the presence of H2 S. Its activity was higher than that of the Pd catalyst under the same experimental conditions. The Pt-based catalyst was already used to design an industrial converter, which operated satisfactorily for 2 years. 3. Improved catalysts 3.1. Pd or Pt supported on sol–gel silica or alumina Silica and alumina supported Pd and Pt catalysts are usually prepared by using commercial supports. However, some attempts were made to synthesise these supports by sol–gel method and the catalytic properties were compared to that of conventional ones. 18 P. G´elin, M. Primet / Applied Catalysis B: Environmental 39 (2002) 1–37 Mizushima and Hori [67] prepared alumina supported Pd and Pt catalysts following two ways. Pd and Pt were supported on alumina aerogels by mixing the metal chloride (H2 PtCl6 and PdCl2 ) precursor to the alumina sol (method A) or more conventionally impregnating the calcined aerogel by the metallic salt (method B). A commercial ␥-Al2 O3 was also used as a support for comparison. A 1 wt.% metal catalysts were thus obtained. Pt catalysts, either supported on commercial alumina or aerogel, exhibited approximately the same catalytic activity. On the contrary, Pd/Al2 O3 were found more active when aerogel aluminas were used than with the commercial ␥-Al2 O3 . Method B (impregnation of palladium after calcination of the gel at 1000 ◦ C) gave better results of activity. Unfortunately, the influence of residual chloride ions after calcination of the catalyst at 500 ◦ C was not examined. It cannot be concluded whether aerogel has a beneficial effect on the intrinsic activity of Pd/Al2 O3 in the complete oxidation of methane or Cl departure resulting from metal precursor is facilitated on the aerogel support. More recently, 0.5 wt.% Pd/SiO2 catalysts were prepared by sol–gel methods [68]. The influence of the pH of gelation and the way of Pd addition, before gelation or by conventional impregnation of the sol–gel calcined support, on the complete oxidation of methane (stoichiometric conditions) was investigated. Different states of the catalysts were studied in 1 vol.% CH4 :2 vol.% O2 (He balance) (200 mgcat , flow rate 50 cm3 /min), calcined at 450 ◦ C, reduced at 500 ◦ C or aged under reactants at 100% conversion. The metal particle size was examined carefully, being measured before catalytic testing and after reduction at 500 ◦ C by hydrogen chemisorption and TEM with a good agreement between the two techniques. For samples in which Pd was incorporated before gelation (SG samples), the particle size did not vary much (2–3.3 nm) with varying preparation conditions. Approximately, the same particle size was obtained for most of the impregnated samples. The light-off temperatures (T50 ) were systematically lower for the impregnated samples compared to the sol–gel samples independently on the activation procedure. Moreover a strong activation under reactants could be observed for sol–gel samples. The trapping of Pd particles in sol–gel samples was suggested to explain their low activity but this seems to contradict dispersion measurements. It can be concluded that the method consisting of incorporating Pd before gelation does not improve the catalytic activity. It is noteworthy however that the impregnated sample corresponding to SiO2 with the highest surface area exhibits interesting performances as indicated by a T50 equal to 300 ◦ C. A Pt/Al2 O3 catalyst was prepared by sol–gel method and calcined in air at various temperatures before reduction [69]. The calcination of the catalyst between 500 and 800 ◦ C seemed to favour metallic dispersion. A migration of platinum occluded in the alumina matrix to the surface has been proposed for the increase of the catalytic activity in propene combustion as a function of the calcination temperature. 3.2. Pd or Pt supported on ZrO2 Except silica and alumina supports, ZrO2 is probably the support of Pd and Pt catalysts which attracted the largest number of studies in the most recent past years [34–36,70–72]. But this concerns only Pd catalysts. In the patent literature, Pd or Pt supported on ZrO2 , stabilised or not by Y or La, associated or not with other refractory oxide supports, are designated as catalysts mainly used for combustion applications [73–80] and scarcely for treatment of CH4 -containing waste gases [81]. Pd/ZrO2 catalysts exhibit catalytic performances in the oxidation of methane comparable to or higher than Pd/Al2 O3 catalysts [34,70–72,82]. Fujimoto et al. [71] prepared Pd/ZrO2 catalysts by impregnating the zirconia (16 m2 /g) with solutions of Pd(NH3 )2 (NO2 )2 in HNO3 . Steady-state turnover rates (280 ◦ C, 2 vol.% CH4 , 20 vol.% O2 in He balance) were calculated as a function of crystallite size based on dispersion measured by H2 O2 titration. The maximum rate was 0.18 s−1 . An approximately linear dependence of the rate versus crystallite size (3–11 nm) was observed. The same trend was also reported by Baiker and co-workers [34,72] with Pd/ZrO2 catalysts prepared by oxidation of glassy Pd25 Zr75 alloy in air. The resulting catalysts consist of poorly crystalline palladium oxide and monoclinic and tetragonal zirconia in intimate contact. The main advantage of the preparation over other conventional methods is the very high level of purity of the catalyst. Based on the catalyst mass and Pd surface area, this catalyst was claimed to exhibit a superior catalytic activity for complete methane P. G´elin, M. Primet / Applied Catalysis B: Environmental 39 (2002) 1–37 oxidation compared to conventionally prepared catalysts. In fact, the activity is comparable to the best active Pd/ZrO2 catalysts. For example, TOF was equal to 0.17 s−1 at 327 ◦ C (1 vol.% CH4 , 4 vol.% O2 in helium) with Pd particles of mean size equal to 13 nm. A series of samples with mean Pd crystallite size varying from 6 to 13 nm (as determined by XRD line broadening) was prepared by carrying out reducing treatments in H2 at increasing temperatures (20–750 ◦ C). The activity per surface Pd atom (measured before testing) was found to strongly increase with particle size. As proposed for Pd/Al2 O3 catalysts, PdO is thought to be the active phase of Pd/ZrO2 catalysts in methane oxidation. This is based on the fact that PdO was the only phase observable by XRD after catalytic testing [34]. This is also consistent with the observed correlation between PdO reducibility by methane and catalytic activity [72]. According to the same authors, the lower activity of smaller PdO particles would be due to a stronger influence of ZrO2 stabilising palladium oxide and then reducing its catalytic performance. In other words, the interaction of ZrO2 with PdO would be detrimental on the catalytic activity [34]. Another group also agreed with PdO as being the active phase [35,36]. These authors studied 10 wt.% Pd/ZrO2 prepared by impregnation of ZrO2 with palladium nitrate. The catalytic activity for complete oxidation of methane was measured in reducing conditions (3 vol.% CH4 , 5 vol.% O2 in helium balance, ca. 300 mgcat , flow rate of 60 cm3 /min), focusing on the low-temperature region. Several oxidation–reduction cycles were found to be necessary to achieve stable activity. Oxidation was carried out in O2 at 500 ◦ C and reduction in 3 vol.% CH4 :He at 500 ◦ C. A correlation was established between the catalytic activity measured as a function of time on stream at 260 ◦ C and the oxidation state of Pd measured by TPR (after the reaction) was quenched by cooling the sample in He down to room temperature. It was observed that fully reduced Pd is inactive and that the activity increases until a plateau was reached when 30–35% of Pd was oxidised to PdO. PdO formed under reaction conditions and during oxidation in O2 revealed some differences according to TPR and Raman spectroscopy. PdO formed under reaction conditions would be largely crystalline (detected by Raman) whereas the oxide formed in pure O2 would be a mixture of amorphous and crystalline phases. The crystalline form was 19 found to reduce more readily than the amorphous one. The activity of PdO could be enhanced by partially reducing the surface of oxide to produce a small amount of metallic palladium. But this effect could not be sustained under steady-state conditions since Pd particles were oxidised back to PdO with time. Different reaction pathways were proposed to explain the reactivity of Pd/ZrO2 catalysts in the oxidation of methane. Redox or so called Mars and van Krevelen mechanism was suggested to proceed for methane oxidation over PdO/ZrO2 as generally proposed for Pd/Al2 O3 catalysts [34,72,83]. Baiker and co-workers ascertained this proposal by 18 O isotopic labelling experiments [34,72]. 18 O labelled catalysts were exposed to pulses of reaction gas mixture. Reaction products containing 18 O were detected. The amount of labelled CO2 was 20% in the first pulse and progressively decreased with increasing pulse number as a consequence of catalyst 18 O impoverishment. This rather slow process was attributed to diffusion limitations of the oxygen within the PdO phase as well as between PdO and ZrO2 . At temperatures lower than 500 ◦ C (for which CO2 scrambling was not preponderant), these results agreed with a redox mechanism involving the reaction of methane with surface oxidised Pd species followed by re-oxidation of palladium with oxygen. A scheme describing all these processes was proposed (Fig. 6). This model of reaction pathway was slightly modified by Fujimoto et al. [70,71]. The presence of oxygen vacancies on PdOx surfaces was postulated. And the activation of methane was supposed to occur on site pairs consisting of oxygen atoms (surface PdO) and oxygen vacancies (surface Pd) (Fig. 7). The inhibition of the methane oxidation by H2 O would be due to the titration of vacancies on PdO surface, the density of such site pairs being controlled by quasi-equilibrated desorption of H2 O. The same inhibiting process was thought to hold for CO2 . Finally, the varying reactivity of PdOx in methane oxidation was related to the existence of oxygen-deficient PdOx crystallites in which the Pd–O bonds strength would be higher than for stoichiometric PdO, therefore being less reactive for methane oxidation. The density of oxygen vacancies responsible for the activity would decrease with the particle size and the oxygen content of PdOx . Oxygen-deficient PdOx crystallites could form during rapid activation in O2 , by unintended local decompo- 20 P. G´elin, M. Primet / Applied Catalysis B: Environmental 39 (2002) 1–37 Fig. 6. Processes suggested to occur during the contact of PdO/ZrO2 with a CH4 :18 O2 reaction gas mixture: surface reactions leading to CO2 scrambling without contribution of a redox mechanism, O2 scrambling with the catalyst and redox mechanism. The palladium oxide was labelled with 18 O prior to the reaction with non-labelled CH4 and O2 molecules. The formation of 18 O-labelled CO2 can be visualised in shaded boxes. (Reproduced from Scheme 1 of [72], with permission from Elsevier Science.) sition of PdO to Pd due to local high temperatures, the re-oxidation being incomplete. In a very recent work, isotopic labelling experiments were again used to study the complete oxidation of methane on a PdO/ZrO2 catalyst [84]. The catalyst (containing 3 wt.% Pd) was prepared by impregnation of the support with palladium nitrate and calcined in O2 at 500 ◦ C, the average particle size being 10 nm. The experiment consisted of monitoring the distribution of oxygen isotopes in the reaction products when flowing the catalyst (PdO initially being Pd16 O) with successive pulses of a reaction mixture containing 1 vol.% CH4 and 4 vol.% 18 O2 in helium at 327 and 427 ◦ C. Interestingly, the absence of 18 O atoms in the products resulting from the first reaction pulses at each temperature confirmed previous reports suggesting that oxygen from PdO is used more efficiently than oxygen from the gas phase in the methane combustion [34,72,85]. Further increasing the number of pulses induced the formation of labelled molecules of water and CO2 . However, different distributions of labelled oxygen were found in water and carbon dioxide. The isotopic oxygen composition of water molecules was proposed to reflect that of bulk PdO while that of CO2 molecules would reflect that of the surface of PdO, as a consequence of a much shorter residence time on Fig. 7. Reaction scheme for the activation of methane on a surface Pd–PdO site pair. The methane molecule interacts with the PdO surface at an oxygen vacancy, H abstraction being produced by interaction with an adjacent PdO site. (Reproduced from Scheme 1 of [71], with permission from Elsevier Science.) P. G´elin, M. Primet / Applied Catalysis B: Environmental 39 (2002) 1–37 the surface compared to water. In addition, the support itself would contribute to the oxygen pool of the PdO phase available for the methane oxidation reaction. The activation pre-treatment of Pd/ZrO2 was found to affect the redox behaviour of the catalyst. Müller et al. [34] found that severe reducing treatments of Pd/ZrO2 prepared from PdZr alloy led not only to Pd sintering but also to a decreased amount of Pd crystallites accessible to reactants (by pore blocking). Catalysts reduced at low temperatures were faster re-oxidised than the ones reduced at high temperatures. Changes in the chemical behaviour of palladium oxide were indicated by different decomposition behaviour and reducibility with methane and hydrogen. The as-calcined palladium oxide (unreduced sample) contained a significant amount of oxide species decomposing at markedly higher temperatures than reduced and re-oxidised samples. Palladium oxide became more reactive towards methane after prereduction and oxidation. Epling and Hoflund studied the influence of activation pre-treatments (oxidising or reducing) on the activity of conventional Pd/ZrO2 catalysts in the oxidation of methane under atmosphere [83]. A commercial ZrO2 (Aldrich, 99% purity) was used as a support and Pd nitrate as a precursor. The average BET surface area of the Pd/ZrO2 catalysts was ca. 25 m2 /g. The reaction mixture consisted of 0.8 vol.% CH4 , 9.6 vol.% O2 , balance N2 (total flow rate of 31 cm3 /min, 100 mgsample ). Mild reductive (H2 , 250 ◦ C) and oxidative (O2 , 280 ◦ C) treatments slightly enhanced the catalytic activity by comparison to the sample reduced or oxidised at higher temperatures (the T50 decreasing by at most 20 ◦ C). Increasing the Pd loading from 0.1 to 10 wt.% also improved catalytic performance (see Table 3, calcination at 280 ◦ C) but higher Pd loadings yield negligible improvement. It is noteworthy to underline the surprising high activity of the support itself, which can be hardly explained except if a contamination by an active phase (Pd, . . . ) is assumed. The temperature at which ZrO2 was pre-calcined prior to its impregnation with the Pd salt was observed to have a strong influence on the catalytic behaviour of Pd/ZrO2 catalysts [86]. Commercial ZrO2 was pre-calcined at various temperatures (600–1200 ◦ C) before being impregnated with Pd(NH3 )4 (OH)2 (reduction at 300 ◦ C, calcination at 600 ◦ C). The cata- 21 Table 3 Influence of the Pd loading on the temperature at which methane conversion reaches 50% in the oxidation of CH4 over Pd/ZrO2 catalysts. Reaction conditions: 0.8 vol.% CH4 , 9.6 vol.% O2 , balance N2 , 100 mgcat , 31 cm3 /min [83] Pd (wt.%) T50 (◦ C) 0 0.1 0.5 2 5 10 20 350 338 284 270 253 234 233 lysts (ca. 0.9 wt.% Pd) were tested in a feed containing 4000 ppm CH4 , 500 ppm CO, 5 vol.% CO2 , 10 vol.% O2 , 10 vol.% H2 O, balance N2 . The catalytic activity strongly depended on the calcination temperature of ZrO2 . The highest activity was shown for the support calcined at 1000 ◦ C for 6 h and to a lesser extent the one calcined at 800 ◦ C. Changes in the Pd dispersion were proposed for taking into account the modifications of the activity. An excessive sintering of ZrO2 was put forward to explain the sharp decrease of activity but this argument is not fairly consistent with given data: the surface area only decreases from 15 to 10 m2 /g between the most active and the least active samples. The main advantage of Pd/ZrO2 over Pd/Al2 O3 catalysts in the oxidation of methane seems to be the stability of the catalytic activity with time on stream [83,86]. The catalytic activity of a 5 wt.% Pd/ZrO2 measured at 250 ◦ C as a function of time was compared with that of a 5 wt.% Pd/Al2 O3 [83]. It is worth mentioning that the chosen reaction temperature is especially low for studying the oxidation of CH4 traces. While Pd/Al2 O3 exhibits first activation and then deactivation with time on stream, the maximum conversion being 40%, the Pd/ZrO2 was more active and its activity remained fairly constant over a 50 h period. For Pd/ZrO2 catalysts prepared from commercial ZrO2 treated at 800 and 1000 ◦ C [86], no deactivation was observed with time on stream (100 h), which contrasts with the deactivation observed on Pd/Al2 O3 catalysts. For Nomura et al. [86], the hydrophobicity of the support could explain the superior behaviour of ZrO2 based catalyst, but this hypothesis was not further confirmed. In addition this seems to be contradic- 22 P. G´elin, M. Primet / Applied Catalysis B: Environmental 39 (2002) 1–37 tory with the strong inhibiting effect of water on the catalytic activity. The activity of Pd supported on ZrO2 seems also dependent on the crystalline form of the support, the monoclinic ZrO2 phase leading to slightly more active catalysts at high conversion levels than the tetragonal phase [87]. 3.3. Pt or Pd supported on carriers having redox and/or acido-basic character Some attempts were made to improve the catalytic properties of Pd and Pt by using as supports various metal oxides MOx (M = Ti, Mn, Co, Y, Zr, Nb, In, Sn), mixtures of metal oxides and mixed oxides such as solid solutions of CeO2 -ZrO2 . 3.3.1. Pd catalysts The catalytic activity of Pd catalysts supported on various commercial metal oxides MOx (M = In, Nb, Sn, Y, Zr) in the complete oxidation of methane at low temperature was investigated by Widjaja et al. [88] and compared to that of Pd/Al2 O3 . All the supports had a low specific surface area (3–6 m2 /g) compared to the alumina (109 m2 /g). The commercial supports were impregnated with a palladium nitrate solution so as to obtain 1 wt.% Pd loading and calcined at 800◦ C in air before catalytic testing (1 vol.% CH4 in air, GHSV = 48,000 h−1 ). In spite of their low surface area (ca. 6 m2 /g) Pd/ZrO2 and Pd/SnO2 exhibited higher activity than Pd/Al2 O3 at all temperatures, Pd/SnO2 being the most active. The T50 of Pd/SnO2 was equal to ca. 360 ◦ C compared to 430 ◦ C for Pd/Al2 O3 . Observations by high resolution transmission electron microscopy indicated the existence of a Pd layer uniformly covering SnO2 spherical particles. A strong interaction between Pd and SnO2 was proposed by the authors to be responsible for the improved activity. It is not clear however whether the origin of the difference of activity between Pd/SnO2 and Pd/Al2 O3 should not be attributed to the wide difference of specific areas of the two supports. It is expected that SnO2 (ca. 6 m2 /g) is much denser than Al2 O3 (137 m2 /g). Since activity measurements were carried out at constant GHSV, i.e. at constant volume of catalyst for a given flow rate, this likely implies that the catalytic activity was measured with significantly larger amounts of SnO2 based catalysts than the Al2 O3 catalyst, which could easily explain the apparent superiority of Pd/SnO2 . Nevertheless, the preparation of Pd/SnO2 catalysts was also mentioned in a patent [89], the catalysts being claimed to exhibit enhanced low-temperature ignition activity in methane combustion. Mixed oxide supports SnO2 -MOx (M = Al, Ce, Fe, Mn, Ni, Zr) were also prepared and the catalytic activity of Pd supported on these supports in the complete oxidation of methane was studied [90]. Aqueous solutions of SnCl4 and metal nitrates with the composition being adjusted so as to molar ratio of SnO2 :MOx equal to 1 were used. After evaporation of water, the solids were calcined at 800 ◦ C. Pd introduction (to obtain 1 wt.% Pd) and catalytic testing were the same as previously described. The results are reported in Table 4. The activity of Pd/SnO2 -MOx was lower than that of Pd/SnO2 . No further improvement of the activity could be obtained by the use of mixtures of oxide supports compared to commercial SnO2 . It is even surprising that the reverse behaviour is observed with Table 4 Catalytic activity in methane oxidation and BET surface area of Pd/SnO2 and Pd catalysts supported on equimolar mixed oxides SnO2 -MOx (M = Al, Ce, Fe, Mn, Ni, Zr) Catalyst Pd/SnO2 Pd/SnO2 -Al2 O3 Pd/SnO2 -CeO2 Pd/SnO2 -Fe2 O3 Pd/SnO2 -MnO Pd/SnO2 -NiO Pd/SnO2 -ZrO2 Surface area (m2 /g) 6.4 55.3 13.1 1.8 1.4 4 11.3 Catalytic activity T10 (◦ C) T30 (◦ C) T70 (◦ C) T90 (◦ C) 305 410 360 435 500 425 330 345 435 390 515 595 440 365 390 485 470 675 760 470 430 440 630 585 825 855 540 490 Pd loading: 1 wt.%. Reaction conditions: 1 vol.% CH4 in air, GHSV = 48,000. From [90]. P. G´elin, M. Primet / Applied Catalysis B: Environmental 39 (2002) 1–37 Al2 O3 , CeO2 and ZrO2 based catalysts. In this case, it can be remarked that the activity decreases when the surface area of the catalyst is improved: the highest surface area (Pd/SnO2 -Al2 O3 ) catalyst exhibits the lowest activity. Again it might be suspected that the apparent density varies from one sample to another, leading to different amounts of catalysts since GHSV was kept constant. And it cannot be concluded whether these supports promotes the catalytic behaviour of palladium in the oxidation of methane. Some data (XPS, TPD and XRD) are given to establish a more complete picture of the structure of Pd on SnO2 in addition to the TEM data already published. It was specified that fine SnO2 particles could be covered by a shell of PdO. XPS data of Pd catalysts supported on SnO2 , Al2 O3 and Al2 O3 -36 NiO before and after H2 reduction clearly indicated that PdO is much less reducible when supported on SnO2 than on Al2 O3 based supports. From these results, the authors suggested that the origin of the support effect is two-fold: (i) increasing the Pd dispersion by an egg-shell structure and (ii) stabilising PdO state responsible for catalytic activity. Roth et al. [91,92] also studied the catalytic activity of Pd/SnO2 in the oxidation of methane at low temperature. A 2 wt.% Pd/SnO2 catalyst was prepared by impregnation of a commercial SnO2 (Aldrich, 7 m2 /g) with H2 PdCl4 , calcination at 500 ◦ C in O2 and reduction in H2 at 300 ◦ C. The catalytic activity was measured with a reaction stream containing 1 vol.% CH4 , 4 vol.% O2 mixture in nitrogen (200 mgcat , 6.5 l/h). After being freshly reduced the catalyst exhibited a strong activation under reactants, similar to Pd/Al2 O3 prepared with the same Cl containing precursor. The removal of chlorine species was thought to explain this behaviour. After ageing under reaction stream at 600 ◦ C, Pd/SnO2 and Pd/Al2 O3 exhibited exactly the same activity, which clearly indicates that SnO2 does not promote the Pd activity. In the same work, SnO2 was tentatively grafted on the alumina surface by selectively reacting the alumina surface hydroxyl groups with an organometallic Sn compound. A layer of SnO2 was shown to form at the surface of the alumina. But no improvement of the catalytic activity could be obtained after ageing under reactants compared to Pd/Al2 O3 . Unfortunately, the SnO2 layer was shown to be unstable under these experimental conditions and sinter into large SnO2 aggregates. So 23 that the interface SnO2 /Pd particles is expected to be severely reduced and the promoting effect of SnO2 , if any, would no longer exist. Further attempts to obtain SnO2 with high surface area are required. While SnO2 does not seem to improve Pd activity in methane oxidation under dry conditions, SnO2 would have a beneficial effect by limiting the influence of water on the catalytic activity of Pd. The effect of water addition on the activity of Pd/SnO2 was found to be attenuated compared to Pd/Al2 O3 , and the kinetics of deactivation with time on stream was found to be slower [93]. TEM investigation has shown that the morphology of particles deposited on SnO2 and Al2 O3 is not the same. As expected, finely deposited PdO particles were observed for Pd/Al2 O3 . On the other hand, for Pd/SnO2 , two types of Pd particles were observed: amorphous-like PdO thin layer surrounding small SnO2 particles and crystalline PdO strongly interacting with large SnO2 particles. Changes in the morphology of PdO particles are probably responsible for changes in sensitivity to water. Li and Hoflund [94] investigated the catalytic properties of Pd catalysts supported over various oxide supports in the complete oxidation of methane. Pd/Co3 O4 appeared as one of the most active catalysts and optimisation of the catalyst might still lead to improved performance. Commercial Co3 O4 was impregnated with Pd nitrate solution, and calcined at 280 ◦ C [94]. Catalytic tests were performed with 1.2 vol.% CH4 , 12 vol.% O2 in N2 (100 mgsample ) at increasing temperatures without exceeding 550 ◦ C [94]. It could be observed that Co3 O4 exhibited a significant activity for the reaction. Typically, 67 % CH4 conversion was obtained at 350 ◦ C at a total flow rate of 10 cm3 /min (100 mgcat ). Loading the catalyst with 10 wt.% Pd considerably improved the catalytic activity: 72% CH4 conversion at 250 ◦ C. It must be pointed out that increasing Pd loading up to 5 wt.% progressively improved the activity but only a slight improvement was obtained between 5 and 10 wt.% Pd. The striking feature is the rate dependence with respect to methane concentration while maintaining oxidising conditions. A strong inhibiting effect of methane was observed, which is quite unusual. The activity was maximum at the lowest methane concentration (1 vol.%) and decreased progressively with increasing methane concentrations up to 7 vol.%. No further variation was then observed when further 24 P. G´elin, M. Primet / Applied Catalysis B: Environmental 39 (2002) 1–37 increasing CH4 concentration. The activity remained stable over a 90 min test period. Pd supported on nanocrystalline TiO2 , Mn3 O4 , CeO2 and ZrO2 was recently studied [95]. Nanoparticles of oxides were synthesised using the gas condensation method. In this method, metals, suboxides or oxides are evaporated by heating in He atmosphere, form nanoparticles by nucleation in the supersaturated vapour phase and collected on a liquid nitrogen-cooled stainless-steel plate. Ti metal, Mn metal, ZrO, and CeO2 were evaporated. The nanocrystalline oxides were obtained by replacing He by O2 in the synthesis chamber and finally calcined at 300 ◦ C. The supports were then impregnated with Pd nitrate and calcined at 280 or 500 ◦ C. Catalytic tests were performed in 1 vol.% CH4 :10 vol.% O2 N2 balance (100 mg, total flow rate of 30 cm3 /min). TiO2 is active but n-TiO2 is much more active than p-TiO2 probably because of much higher surface area (45 m2 /g compared to 7 m2 /g), n and p standing for nanocrystalline and polycrystalline, respectively. The addition of Pd enhanced the catalytic activity but the effect was more pronounced with the nanocrystalline support. As an example, 10% conversion was reached at ca. 250 ◦ C for 5 wt.% Pd/p-TiO2 and ca. 220 ◦ C for 5 wt.% Pd/n-TiO2 (35 nm). Increasing Pd loading led to slight improvement of the activity. The bare n-CeO2 is an excellent catalyst while the polycrystalline form is almost inactive. Adding Pd only slightly improved the catalytic activity, essentially at higher temperatures. The activity of the n-sample is again higher (shift at lower temperatures by ca. 20 ◦ C) than the one of the p-sample. Interestingly, the surface area of the n-catalysts increased drastically upon Pd impregnation. The activity of Mn3 O4 base catalysts is much lower than CeO2 based catalysts. Similar to CeO2 , Pd/ZrO2 catalysts using n-crystalline support are more active than the p-crystalline form. The activity being approximately the same as Pd/CeO2 . It was claimed that Pd on nanocrystalline supports performs better on a mass basis than Pd supported on polycrystalline supports but no explanation was given. The influence of the acid strength of the support on the catalytic properties of Pd catalysts in the oxidation of methane was not addressed. A recent study concerns the oxidation of propane over Pd catalysts supported on materials of varying acid strength [96]. It was concluded that a support material with moderate acid strength gave maximum conversion. The oxygen concentration in the reaction mixture was shown to strongly influence the catalytic activity. The oxygen concentration at which the maximum activity was obtained depended on the support: high concentration for Pd/SiO2 -Al2 O3 , moderate for Pd/Al2 O3 , and low for Pd/ZrO2 . This sequence corresponded to that of the acid strength of the support measured by Hammett indicator. XRD and XPS measurements after test run suggested to attribute these observations to changes in the oxidation state of palladium. Partially oxidised palladium, i.e. co-existing Pd and PdO phases, was proposed to be the most effective for the oxidation of propane. To this respect, acidic supports would favour the “resistibility” of palladium against oxidation, being then more suitable for the design of active catalysts under high oxygen concentrations. 3.3.2. Pt catalysts Much less studies concern Pt catalysts supported on oxides supports compared to Pd catalysts. For example, there exists no study examining the influence of different oxide supports on the total oxidation of methane over supported platinum catalysts. This point was addressed by Sugaya et al. [97], but it concerns the oxidation of propane. Although propane is much more reactive than methane towards oxygen, the authors brought interesting conclusions which are worthy to be checked in the oxidation of methane for future studies. In this work, various commercial oxides were impregnated with an aqueous solution of Cl-free Pt(II) complex salt (Pt(NH3 )2 (NO2 )2 ) so as to obtain 0.5 wt.% Pt. The catalysts were dried, calcined at 400 ◦ C and reduced in H2 at 350 ◦ C prior to catalytic testing (C3 H8 :O2 1:20 kPa, 150 cm3 /min). The activities were in the order Al2 O3 > SiO2 > TiO2 > CeO2 , Al2 O3 > ZrO2 > La2 O3 > MgO. The variations of activity were attributed mainly to variations of acid-base properties, even though the dispersion of Pt metal slightly differed from one support to another, which could partially explain differences of activity. It was therefore concluded that the activity is higher on acidic supports and lower on basic ones. An increase of the support acidic strength was also found favourable for propane oxidation by Wu et al. [98], the acidity change being achieved in this case by impregnation of an alumina support by H2 SO4 . Sugaya et al. [97] established a linear correlation between the P. G´elin, M. Primet / Applied Catalysis B: Environmental 39 (2002) 1–37 Fig. 8. Variation of the turnover frequency in propane oxidation for 0.5 wt.% Pt catalysts supported on commercial oxides as a function of the electronegativity of the metal in the oxide support. The reaction was performed with 1 vol.% C3 H8 and 20 vol.% O2 at 350 ◦ C (open circles) and 300 ◦ C (full circles). (Reproduced from Fig. 5 of [97], with permission from Elsevier Science.) electronegativity of the cation in metal oxides and the turnover frequency (Fig. 8). These authors also suggested that basic oxides tend to stabilise the oxidised form of platinum, which is less active for catalytic oxidation of propane and this could explain the deactivation with time on stream. This interpretation was supported by the fact that the activity of deactivated Pt/MgO could be restored upon reducing treatment. A strong deactivation with time on stream was observed also with La2 O3 , but interestingly not with CeO2 . Since the use of acidic supports led to the best catalytic performance, mixed oxides (SiO2 -Al2 O3 , SiO2 -TiO2 , SiO2 -ZrO2 ) and SO4 2− -doped oxides (Al2 O3 and ZrO2 ) of varying acid strength were then used as support of platinum and the catalytic activity of resulting Pt catalysts in the oxidation of propane at low temperature was studied [99] in the same conditions as in [97]. At 350 ◦ C, a good correlation between the activity and the acid strength of the support was established. At lower reaction temperature (250 ◦ C), the superiority of Pt supported on SO4 2− -doped oxides over Pt supported on mixed oxides was observed. (Zr-S Al-S > Si-Al > Si-Zr > Si-Zn). While oxidising treatment reduced the catalytic activity of Pt when supported on ZrO2 compared to a reducing treatment, no effect was observed on SO4 2− -ZrO2 . 25 A possible inhibition of the oxidation of supported platinum by acidic support was suggested to explain this difference as proposed in [97]. In the case of methane oxidation, the influence of the acidity of the support on the catalytic activity of supported Pt catalysts was not examined since the pioneering work by Niwa et al. [17] on 0.2 wt.% Pt catalysts. In a stoichiometric reaction mixture (10 vol.% CH4 , 20 vol.% O2 in nitrogen) and for similar dispersion of Pt (about 70%), Pt/silica alumina (15 % alumina) was found much more active than Pt/alumina. The higher activity of the Pt/SiO2 -Al2 O3 catalyst compared to that of the Pt/Al2 O3 one was correlated to a higher mobility of oxygen bonded to Pt as evidenced by TPR measurements. Another interesting route was explored by using reducible supports. CeO2 -ZrO2 solid solutions are known for their high oxygen storage capacity (OSC), this property being used in TWC application. The catalytic properties of Pt/CeO2 -ZrO2 catalysts in the oxidation of methane were studied by Bozo et al. [100]. CeO2 -ZrO2 mixed oxides of varying compositions were prepared by co-precipitation with ammonia of an aqueous solution of the corresponding nitrates, drying at 100 ◦ C and calcination at 700 ◦ C. Catalysts containing 1.6 wt.% Pt were prepared by impregnation of the supports with Pt(NH3 )4 (NO3 )2 , calcination in O2 at 400 ◦ C and subsequent reduction at 300 ◦ C. Purge in N2 at 300 ◦ C until the initial temperature of the reaction is reached. Catalytic testing was performed in a 1 vol.% CH4 :4 vol.% O2 (balance N2 ) reaction feed (500 mgcat , total flow rate of 6.4 l/h). Typically, the reaction rate at 250 ◦ C was 0.3 mmol/h gcat (3.6 mol/h mol Pt). The activity was considerably higher than Pt/Al2 O3 as indicated by T50 at 335 ◦ C compared to 470 ◦ C for Pt/Al2 O3 . It was remarkable that the activity of Pt/CeO2 -ZrO2 reached that of Pd/Al2 O3 (T50 = 320 ◦ C). However, a strong deactivation with time on stream could be observed at temperatures higher than 250 ◦ C. It appeared that the promoting effect of the CeO2 -ZrO2 solid solution on the catalytic property of Pt could not be sustained with time on stream. Reductive treatment (H2 , 300 ◦ C) restored almost completely the activity while oxidative treatment (O2 , 350 ◦ C) or purge in N2 at 500 ◦ C did not affect the catalytic activity. The promoting effect of the solid solution on the Pt activity and the deactivation of the catalyst as well are not yet understood. It 26 P. G´elin, M. Primet / Applied Catalysis B: Environmental 39 (2002) 1–37 was proposed that Pt could promote the activity of the support which is active itself at higher temperatures. The methane would be oxidised by mobile oxygen species of the support. The limiting step in the process would the re-oxidation of reduced surface species by oxygen from the gas phase. This step in the presence of Pt was thought to be facilitated by the easier dissociative adsorption of O2 on Pt metal particles than on reduced sites of the support. The methane oxidation rate would thus be enhanced. The deactivation of the catalyst would be therefore due to the progressive transformation of Pt metal at the surface of which oxygen could dissociate and spill over onto the support. The progressive formation of Pt oxide or Pt-O-Ce phase blocking the oxygen dissociation and spill over is proposed. Recently, Roth et al. [91,92] found a spectacular improvement of the activity of Pt in the oxidation of methane at low temperature when substituting Al2 O3 by a commercial low surface area SnO2 . The reaction mixture consisted of 1 vol.% CH4 and 4 vol.% O2 in nitrogen (200 mgcat , 6.5 l/h). After equilibration of the catalysts under reactants at 600 ◦ C, the 2 wt.% Pt/SnO2 catalyst exhibited a T50 shifted by more than 80 ◦ C toward lower temperatures compared to the 2 wt.% Pt/Al2 O3 catalyst (Fig. 9). The interesting question of knowing whether the promotional effect of SnO2 by using a low surface area support could be still enhanced by increasing the surface area was also addressed. Although Pd/Al2 O3 catalysts still exhibits the highest activity for methane at low temperature, the stability and the high activity of Pt/SnO2 catalysts offer an interesting route for novel catalysts to be used in NGV application. It is now worthwhile to investigate the influence of water on the activity and the stability of these catalysts, as well as their resistance to sulphur compounds. 3.4. Zeolitic supports In order to obtain a catalyst efficient at low temperature and thermally stable, Takashima deposited the noble metal selected from Pd, Rh and Pt onto a mordenite type carrier with a high SiO2 :Al2 O3 molar ratio, e.g. equal to 210 [101]. The catalysts was claimed to be active for the complete oxidation of traces of methane at low temperatures, being useful for purification of combustion exhaust gases containing methane. Several attempts of supporting Pd on different zeolitic structures by the ion-exchange technique were then reported: ZSM-5 [102–106], mordenite [102,103], ferrierite [102,107], SAPO [105,108,109]. Fig. 9. Catalytic activity of 2 wt.% Pt supported on Al2 O3 or SnO2 in methane oxidation as a function of reaction temperature. Catalysts were prepared by impregnating the support with the metal precursor (H2 PtCl6 for Pt/Al2 O3 and Pt(C5 H7 O2 )2 for Pt/SnO2 ) and reduced in H2 at 300 ◦ C. Reaction conditions: 1 vol.% CH4 , 4 vol.% O2 , 6.5 l/h, 200 mgcat . (A) First heating ramp under reactants after reduction in H2 at 300 ◦ C. (B) Second heating ramp after reaction at 600 ◦ C for 12 h, purge in helium and cooling down to 150 ◦ C. (Reproduced from Fig. 3 of [92], with permission from Elsevier Science.) P. G´elin, M. Primet / Applied Catalysis B: Environmental 39 (2002) 1–37 Li and Armor [102] studied palladium cation exchanged ZSM-5, mordenite and ferrierite as catalysts for methane oxidation. Na forms of ZSM-5 (Si:Al = 14) and mordenite (Si:Al = 5) were used while ferrierite (Si:Al = 8) was exchanged with ammonium ions. Palladium(II) ions were introduced by exchange with an aqueous solution of palladium nitrate. For comparison, 4.2 wt.% Pd/Al2 O3 was prepared by impregnating a ␥-alumina with the same Pd salt. Highly loaded Pd catalyst (3.4, 5.6, 7.7 and 4.2 wt.% Pd, respectively) were thus obtained and calcined in air at 450 ◦ C prior to catalytic testing (1 vol.% CH4 in air, GHSV = 30,000 h−1 ). PdO/Al2 O3 showed a light-off temperature (T50 ) of 70◦ C higher than Pd/ZSM-5 (255 ◦ C compared to 325 ◦ C for PdO/Al2 O3 ) in spite of the higher Pd loading of the Al2 O3 sample (4.2 wt.%) compared to the H-ZSM-5 sample (3.4 wt.%). Other Pd-zeolites showed in all cases much higher activities than PdO/Al2 O3 . The higher dispersion of palladium on the zeolite supports was suggested to be responsible for their improved activity compared to the alumina catalyst. TPR experiments with carbon monoxide showed a lower reduction temperature (ca. 157 ◦ C) for Pd/ZSM-5 than for PdO/Al2 O3 (225 ◦ C). Difference of palladium catalysts or different strengths of the Pd–O bond were suggested to explain dramatic differences of catalytic activity. According to the authors, extra-lattice oxygen atoms could be very active forms of oxygen involved in the complete oxidation of methane at low temperature. The main disadvantage of these catalysts was their instability toward steaming at high temperatures. A mild steaming (500 ◦ C in a 4 vol.% H2 O/He stream for 18 h) of Pd/H-ZSM-5 caused a increase of T50 by 50 ◦ C (Fig. 10). It is now well known that the sintering of palladium in zeolites is favoured by the presence of water vapour at high temperatures [110–114] and its stabilisation inside the zeolite pores against sintering under steaming conditions remains a challenge. In spite of this negative conclusion, a further study of Pd/ZSM-5 in the oxidation of methane was reported by Maeda et al. [104]. The mordenite structure was also investigated in this study. The same work was also reported in a patent [103]. Varying Al composition and varying charge-balancing cations (H+ and Na+ ) were examined. A 0.5 wt.% Pd catalysts were prepared by impregnation of PdCl2 or cationic 27 Fig. 10. Influence of a mild steaming treatment on methane combustion rate over 3.4 wt.% Pd/ZSM-5, 5.6 wt.% Pd/MOR and 4.2 wt.% Pd/Al2 O3 . The catalysts were steamed in a helium steam containing 4 vol.% H2 O at 500 ◦ C for 18 h. Reaction conditions: 1 vol.% CH4 in air, GHSV = 30,000 h−1 . Zeolite-supported catalysts deactivate after steaming while PdO/Al2 O3 is stable. (Reproduced from Fig. 4 of [102], with permission from Elsevier Science.) exchange with aqueous solutions of Pd(NH3 )4 Cl2 . The samples were all calcined at 500◦ C in nitrogen before catalytic testing. The catalytic activity was measured under steady-state conditions at 450 ◦ C as a function of time on stream (10 vol.% CH4 :20 vol.% O2 in N2 ). H-MOR-145 (silica-to-alumina molar ratio of 145) and H-ZSM-5–76 showed the highest rates (5 and 3 mmol/min, respectively) and durability. The activity was strongly increased with these two supports being loaded with Pd by the exchange method but experimental data seem to indicate a continuous deactivation with time which is not observed with impregnated samples. One might expect in this case the possible sintering of highly dispersed Pd in the course of the reaction leading to the decrease of the activity for methane oxidation. Many studies have shown that Pd ions in zeolite pores of mordenite or ZSM-5 structures are unstable and tend to aggregate into larger PdO particles in the presence of water, these particles being located outside the zeolite crystals [110–114]. In this work, no comparison of activity was made with Pd supported on SiO2 or Al2 O3 . It is however predictable that the main problem of this type of catalyst is to stabilise PdO in a high degree of dispersion inside the zeolite porosity. This is even more difficult 28 P. G´elin, M. Primet / Applied Catalysis B: Environmental 39 (2002) 1–37 Table 5 Catalytic activity of various Pd-exchanged molecular sieves Catalyst Pd/Al2 O3 Pd/HY Pd/H-SAPO-5 Pd/Na-SAPO-5 Pd loading (wt.%) Surface areaa (m2 /g) 1.00 1.00 0.96 1.05 109 411 107 166 Catalytic activityb T10 (◦ C) T30 (◦ C) T50 (◦ C) T90 (◦ C) 330 310 375 325 400 380 415 380 435 395 430 410 665 475 480 570 The catalysts were calcined in air at 800 ◦ C before reaction. Reaction conditions: 1 vol.% CH4 in air, GHSV = 100,000 h−1 . From [105]. a From BET. b Temperature at which CH conversion reached 10, 30, 50 and 90%. 4 with highly siliceous zeolites because of a reduced cation exchange capacity. Pd/SAPO catalysts were studied by Ishihara et al. [105]. These catalysts were prepared by ion-exchange of SAPOs (SAPO-5, -11 and -34) with an aqueous Pd(NH3 )4 Cl2 solution, calcination in O2 at 450 ◦ C, reduction in H2 at 450 ◦ C, calcination in air at 800 ◦ C. The reaction mixture consisted of 1 vol.% CH4 in air (GHSV = 100,000 h−1 ). The catalytic activity of Pd/SAPO catalysts was compared to Pd/Al2 O3 and other Pd/zeolites catalysts (Pd/HY, Pd/H-ZSM-5, Pd/MOR, . . . ). The data are reported in Table 5. It is noteworthy mentioning the surprisingly low activity of Pd/Al2 O3 , which is likely to be related to the severe conditions of the calcination treatment (800 ◦ C) possibly favouring extensive sintering of the Pd phase. According to the values of T10 , T30 and T50 reported, Pd/HY and Pd/Na-SAPO-5 appeared more active than Pd/Al2 O3 in the low-temperatures region. The order of activity changed when T90 was considered, which might suggest deactivation phenomena in some cases. On the basis of T90 , Pd/H-SAPO-5 was retained by the authors as the most active catalyst of the series and this was attributed to a better dispersion of Pd on this support. The activity was also found to be stable and high temperatures of calcination (1000 ◦ C) did not affect significantly the methane conversion. These results seemed to indicate that H-SAPO-5 could stabilise Pd in its pore structure and maintain a high degree of dispersion. In a further work, Takita et al. [108] confirmed previously published data showing that Pd/H-SAPO-5 is slightly less active than Pd/Al2 O3 below 420 ◦ C but becomes more active than Pd/Al2 O3 above 420 ◦ C. It must be noticed that these experiments appeared to be strongly affected by mass transfer limitations. Surprisingly these limitations were stronger for the alumina catalyst than for the microporous Pd/H-SAPO-5. The high thermal stability of the SAPO-5 structure and Pd particle size was again demonstrated at temperatures reaching 1000 ◦ C and this property was claimed to be responsible for the sustained activity. Recently, Noro and Nomura [109] reported the preparation of SAPO-based catalysts by ion exchanging protons of silicoaluminophosphate-series materials in aqueous solutions of metal salts such as Pd(NH3 )4 (OH)2 , Pt(NH3 )4 (OH)2 , . . . and sintering. The resulting catalysts were claimed to be especially useful for oxidation of unburnt methane present in exhaust gases from lean-burn gas engines. It cannot be concluded however from these studies that Pd/H-SAPO-5 is really more effective than a conventional Pd/Al2 O3 catalyst in the oxidation of methane. More experimental work is needed to ascertain this idea. Pd, Pt, Pd-Pt/H-ZSM-5 and Pd/H-ZSM-35 were prepared for the abatement of pollutants from biofuels emissions [106]. The reactants mixture contained other pollutants than methane. Typically, it consisted of naphthalene (50 ppm), methane (200 ppm), carbon monoxide (2550 ppm), water (13.5 vol.%), carbon dioxide (12 vol.%), oxygen (10 vol.%) and nitrogen as balance (GHSV = 20,000 h−1 , total flow rate 2100 cm3 /min, 3.5 gcat ). The influence of numerous parameters such as the Pd content, the addition of La or Zr, ageing under reactants at 600 ◦ C, the Na or H form of the zeolite, the steam ageing of the catalyst, the addition of 5 ppm SO2 to the reaction mixture was examined. For fresh catalysts, the best activity was obtained with the 2 wt.% Pd/H-ZSM-5: the T50 P. G´elin, M. Primet / Applied Catalysis B: Environmental 39 (2002) 1–37 was equal to 375 ◦ C. Neither Pt, La or Zr addition did improve the catalytic activity. Upon ageing at 600 ◦ C under reactants and cooling down to room temperature in air, all catalysts de-activated (T50 increased up to 420 ◦ C). None of the additives (Pt, La or Zr) was shown to significantly improve the catalytic activity compared to 2 wt.% Pd/H-ZSM-5. Steam treatment at 800 ◦ C had a strong negative effect on the activity. The influence of adding 5 ppm SO2 to the reaction mixture on the activity of the fresh Pd and Pd-Pt samples was also tested. In this case, it was claimed that the inhibition of sulphur on methane oxidation was much less than reported for PdO/Al2 O3 . In conclusion, the use of zeolitic supports did not bring any evidence of improved catalytic activity in methane oxidation of Pd catalysts. 3.5. Alumina supported Pt or Pd catalysts modified by oxide additives Instead of supporting Pt or Pd directly on oxides supports which could modify the catalytic properties of the metal by inductive effects, or could themselves participate to the oxidation reaction, some attempts were made to both introduce the metal and the potentially active oxide phase on a conventional support in order to increase the dispersion of the metal and the oxide and hopefully improve the synergistic effect, if any, between the two components. Patent literature provides many catalysts of this type but the application is generally clearly identified as being the catalytic combustion of methane or natural gas, i.e. high temperatures applications. Thus, Shimada et al. proposed catalysts based on Pd supported on heat-resistant Al2 O3 supports to which Ti, Mg or Mn was added [115]. La, Ce or Ba were also used as additive to the alumina, being itself used as the support for the Pd active phase [116]. Recently, a catalyst comprising a monolithic substrate, a porous support based on refractory inorganic oxide and an active phase formed by cerium, iron and art least one metal from the group formed by Pd and Pt, was patented for its use in catalytic combustion processes involving one or more catalytic stages [117]. Catalysts consisting of Pd, Pt and/or Rh deposited with Cu on MgO support were claimed as efficient catalysts for combustion at low and middle temperatures [118]. It is expected that the main objective to be reached for these catalysts is 29 to stabilise the active PdO and increase the temperature domain within which PdO is stable, as suggested in [119]. The influence of numerous additives (Ni, Sn, Ag, Rh, Mn, Pt, Pb, Co, Fe, Cr, Ce, and Cu) was examined by Ishihara et al. [120]. Binary Pd catalysts were prepared by co-impregnation (CI) of PdCl2 and metal nitrates on alumina (Aerosil) in a molar ratio of Pd:metal additive equal to 9 and a total metal loading of 1 wt.%. The catalysts were reduced in H2 at 500 ◦ C and calcined in air at 800 ◦ C prior to catalytic testing (1 vol.% CH4 in air, GHSV = 100,000 h−1 ). The effect of Pd dispersion on the catalytic activity was claimed to be negligible in the study. The binary Pd catalysts were always found to be more active than Pd/Al2 O3 , as indicated by temperatures at 30 and 50% conversions (T30 and T50 ) lower than for Pd/Al2 O3 (Table 6). In particular, NiO added Pd catalysts exhibited the highest activity for complete methane oxidation. The most spectacular effect of NiO was certainly to broaden the domain of stability of PdO, i.e. to strongly increase the temperature at which PdO decomposes into Pd. This is relevant to high or intermediate combustion applications. It is not sure however whether the low-temperature activity of palladium is really enhanced by the additive, the conversion of Pd and Pd/NiO catalysts starting at the same temperature. Moreover, the influence of chloride was not addressed Table 6 Catalytic activity of Pd/Al2 O3 catalysts modified by metal oxide additives. The total metal loading is 1 wt.% and the Pd:metal adduct molar ratio is 9 Catalysts T30 (◦ C) T50 (◦ C) Pd Pd/Mn3 O4 Pd/Cr2 O3 Pd/Fe2 O3 Pd/PbO Pd/CoO Pd/CeO2 Pd/Ag2 O Pd/PtO Pd/SnO2 Pd/NiO Pd/CuO Pd/RhO 463 425 420 400 390 390 385 380 375 375 350 345 330 533 450 445 460 460 435 430 410 425 430 380 465 415 The catalysts were calcined in air at 800 ◦ C before testing. Reaction conditions: 1 vol.% CH4 in air, GHSV = 100,000 h−1 . From [120]. 30 P. G´elin, M. Primet / Applied Catalysis B: Environmental 39 (2002) 1–37 in this work although Pd chloride was used as Pd precursor. In addition, the reference Pd/Al2 O3 catalyst exhibits a surprisingly low activity in CH4 oxidation, as revealed by a temperature of half conversion as high as 533 ◦ C. A 1.1 wt.% Pd/Al2 O3 -MOx (M = Co, Cr, Cu, Fe, Mn and Ni) catalysts were also prepared by impregnation of metal nitrates and Pd(NO3 )2 , calcined at 800 ◦ C in air and tested in the oxidation of methane for catalytic combustion applications [121]. It has to be noticed that calcination temperatures were high (800 ◦ C), which led to poor Pd dispersions. As an example, the Pd/PdO crystallite size for the Pd/Al2 O3 catalyst was 53 nm. The reaction mixture was 1 vol.% CH4 :20 vol.% O2 in nitrogen (GHSV = 48,000 h−1 ). Under these conditions, specific of high temperatures applications, only NiO additives were shown to improve the catalytic activity of Pd/Al2 O3 , the optimum content being 36 NiO/Al2 O3 . This was attributed to the improved dispersion of Pd (measured from XRD) on this support compared to pure alumina. This explanation might not be fully convincing since only a small decrease of the activity could be observed when the particle size for 1.1 wt.% Pd/Al2 O3 -36 NiO samples calcined at 600 and 800 ◦ C, respectively increased from 11 to 38 nm. A complementary study of Pd/m Al2 O3 -n NiO with various compositions of NiO (m and n standing for molar compositions) was performed and confirmed previous results [122]. Although the surface area decreased with increasing NiO content, the highest catalytic activity was found for the highest NiO content (Al2 O3 -36 NiO). For this catalyst, the temperatures T10 and T30 were equal to 320 and 340 ◦ C, respectively compared to 340 and 370 ◦ C for the reference Pd/Al2 O3 . This indicated a promoting effect of 36 NiO on the catalytic activity of Pd. On the contrary, supporting Pd on pure NiO resulted in a lower activity compared to Pd/Al2 O3 . Three phases contained in the mixed oxides were identified: amorphous-like alumina, NiAl2 O4 spinel and NiO. By using mechanical mixtures, it was shown that the promoting effect on catalytic activity was associated with the increased amount of the NiO phase while the presence of NiAl2 O4 would be detrimental for the catalytic activity. Finally the influence of NiO on the activity of Pd/Al2 O3 was rationalised as follows [123]. X-ray line broadening method was used to evaluate the PdO particle size. A good agreement was found with TEM at low Ni contents. The average particle size was found to progressively decrease with the increase of Ni content from 53 nm for the pure Al2 O3 support to 32 nm for Al2 O3 -36 NiO. The improved dispersion of PdO on mixed Al2 O3 -NiO was again proposed to partially explain the improved catalytic behaviour. In addition, TPD of oxygen indicated a slightly higher amount of oxygen desorbing at low temperatures (below 500 ◦ C) in Pd/Al2 O3 -36 NiO than in Pd/Al2 O3 . Moreover, the addition of NiO was found to retard the decomposition of PdO into Pd metal towards higher temperatures. Alumina-supported Pt and Pd catalysts prepared by impregnation of H2 PtCl6 or PdCl2 were promoted with ceria or cobalt oxide and studied in the low-temperature oxidation of CO and propene [124]. The catalytic activity was observed to depend on the metal and also the pre-treatment conditions of the catalyst (oxidising or reducing). Let us report the data obtained under net oxidising conditions (S = 1.17). For catalysts being pre-oxidised at 550 ◦ C in 10 vol.% O2 , ceria or cobalt oxide were found to promote the activity of Pt but not that of Pd. Pre-reduction of Pt catalysts at 550 ◦ C in 4 vol.% H2 enhanced the promotion effect, for ceria weakly and for cobalt oxide strongly. For pre-reduced Pd catalysts, ceria did not promote the catalytic activity while cobalt oxide strongly enhanced the oxidation of CO and propene. The presence of reduced cobalt-oxide sites on the surface of these pre-reduced samples were proposed to account for the improved catalytic performance. The oxidation of methane was not studied. The addition of CeO2 to alumina supported palladium catalysts was shown to have no beneficial effect on the low-temperature catalytic activity in the oxidation of methane under lean conditions [125–128]. For Groppi et al. [125], the effect, if any, would be slightly negative on the low-temperature catalytic activity. However, it markedly improved the activity at high temperature by stabilisation of the active PdO phase. In the presence of ceria, temperatures of reduction of PdO and reoxidation of PdO are both shifted 50–60 ◦ C above those observed with unpromoted samples. For Deng and Nevell [127], who prepared highly loaded Pd/Al2 O3 with CeO2 additive, the activity of 15 wt.% Pd/CeO2 -Al2 O3 (15 wt.% CeO2 ) in methane oxidation (2 vol.% CH4 in air, flow rate P. G´elin, M. Primet / Applied Catalysis B: Environmental 39 (2002) 1–37 of 100 cm3 /min) was very similar to that of 15 wt.% Pd/Al2 O3 . There exists, however, a study in which the addition of CeO2 is claimed to have a positive effect on the catalytic property of Pd/Al2 O3 [129]. This study concerns the removal of pollutants from biofuel combustion appliances [129]. It must be mentioned that the activity in the oxidation of methane was measured for a reaction mixture containing pollutants other than methane (200 ppm CH4 , 50 ppm naphtalene, 2500 ppm CO, 10 vol.% O2 , 12 vol.% CO2 , 12 vol.% H2 O, nitrogen balance). Under these conditions, it was found that a 1 wt.% Pd/0.8 wt.% Ce-Al2 O3 catalyst prepared by impregnation of the alumina with PdCl2 and Ce nitrate has the same activity in the oxidation of methane as 2 wt.% Pd/Al2 O3 . Increasing Ce loading has no effect while increasing Pd loading (6 wt.% Pd) induces a decrease of T50 , irrespective of the catalyst being fresh or aged. Activation is observed upon ageing which is ascribed to Cl removal. SO2 poisons the catalysts and its removal above 550 ◦ C restores the activity. The positive effect of ceria additive could be used to reduce the amount of precious metal, thus reducing the cost of the catalyst for the application. While ceria additive does not seem to improve the catalytic activity of Pd/Al2 O3 in the oxidation of methane under a lean CH4 :O2 reaction mixture, a synergistic effect between Pd and a non-stoichiometric cerium oxide CeO2 − x -Al2 O3 for methane oxidation was reported by Haneda et al. [128] by comparison with Pd catalysts supported on CeO2 -Al2 O3 , CeO2 -SiO2 , Al2 O3 and SiO2 . A 1 wt.% Pd catalysts were prepared by impregnating the support with palladium nitrate. Let’s briefly recall that the CeO2 − x -Al2 O3 sample was prepared by sol–gel method, then calcined at 900 ◦ C in air and finally reduced by hydrogen at the same temperature. The amorphous cerium oxide thus obtained was confirmed to be several kinds of non-stoichiometric cerium oxides expressed as CeO2 − x . Such a material is difficult to re-oxidise when in contact with O2 at room temperature. Even after reaction with methane and oxygen at 500 ◦ C, the non-stoichiometric form was kept. The catalytic activity was measured at 400 ◦ C in 1.5 kPa CH4 , 5.3 kPa O2 and 15 kPa He in a recirculating reactor. The addition of CeO2 to Pd/Al2 O3 had a negative effect on the activity for methane oxidation. On the other hand, the remark- 31 able point was the promotion of the activity by the addition of CeO2 − x to Pd/Al2 O3 , the rate being increased from 50 ␮mol/min g (9 × 10−3 mol/s mol Pd) to 120 ␮mol/min g at 400 ◦ C. In addition, the reaction rate order with respect to oxygen increased from 0 to 0.4 upon addition of non-stoichiometric ceria. A large amount of lattice oxygen atoms of CeO2 − x in Pd/CeO2 − x -Al2 O3 were found to desorb easily at 400 ◦ C, which was not the case of Pd/CeO2 -Al2 O3 . Moreover, these species were found to be chemically active for methane oxidation, being continuously supplied to react with methane. The authors proposed a mechanism in which the lattice vacancies of CeO2 − x store the spillover oxygen from Pd to maintain palladium in active state (i.e. metallic, which would allow methane activation) and the oxygen stored on CeO2 − x could migrate back to Pd and react with activated methane. The key point is the preparation of this new cerium oxide catalyst by sol–gel method followed by reduction resulting in a non-stoichiometric CeO2 − x phase stable even after reaction at 500 ◦ C under oxidising conditions. It would be worthwhile to examine the catalytic properties of such Pd catalysts under flowing lean conditions and also the catalytic properties of Pt catalysts supported on this type of material. Concerning Pt catalysts, no study of the influence of Ce on the activity of Pt in the oxidation of methane was reported. Tiernan and Finlayson studied Ce doped Pt/Al2 O3 catalysts in the oxidation of isobutane under stoichiometric conditions [130]. Alumina supported Pt catalysts with 0.5 wt.% Pt were prepared (impregnation with H2 PtCl6 ). Ce was added in various amounts by impregnation with Ce nitrate. After impregnation and calcination in air, the use of CeO2 additive had a negative effect on the activity with increasing Ce content. But, in all cases (with or without additive), the catalyst activated under reactant mixture and in some cases the presence of ceria on the reactant aged catalysts promoted the low-temperature activity, as indicated by a decrease of T10 and T50 of 25 ◦ C with respect to Pt/Al2 O3 . After subsequent reduction in H2 the presence of Ce was always beneficial for the isobutane oxidation activity. A decrease of T10 and T50 of up to 50 and 65 ◦ C, respectively compared to Ce free catalyst could be reached by Ce addition. The existence of an interaction between Ce and Pt in Pt/Ce-Al2 O3 affecting TPR profiles was associated with the improved catalytic activity after reduction. 32 P. G´elin, M. Primet / Applied Catalysis B: Environmental 39 (2002) 1–37 Qi et al. studied the catalytic properties of Pd supported on MgO/␥-Al2 O3 composite supports in the oxidation of CO [131] and methane [132]. MgO/␥-Al2 O3 composite supports were prepared by impregnation of alumina with Mg(NO3 )2 and calcination at 450 ◦ C. The MgO content was varied from 0.2 to 15 wt.%. Interestingly, 0.7 wt.% Pd/MgO-␥-Al2 O3 catalysts exhibited improved activity for the oxidation of CO (4 vol.% CO:22 vol.% O2 in N2 ) compared to Pd/␥-Al2 O3 . For example, adding 0.2 wt.% MgO decreased the temperature of complete CO oxidation by as much as 70 ◦ C. Further increasing the MgO content up to a value corresponding to the theoretical monolayer (5 wt.%) was detrimental to the catalytic activity. However, the positive effect of MgO additive on the catalytic activity of Pd/Al2 O3 was found to be much reduced in the case of the oxidation of methane under lean conditions (1 vol.% CH4 , 4 vol.% O2 in nitrogen at a flow rate of 20 ml/min, 300 mg of catalyst). While MgO/Al2 O3 supports exhibited non-negligible levels of activity in the oxidation of methane, the best activity being obtained with the 0.2 wt.% MgO content, 1.95 wt.% Pd/0.2 wt.% MgO-Al2 O3 (prepared from Pd nitrate) exhibited a conversion curve shifted by only 20–30 ◦ C toward lower temperatures compared to Pd/Al2 O3 . Conversion started at 240 ◦ C instead of 270 ◦ C for Pd/Al2 O3 . No beneficial effect on activity could be observed with the 5 wt.% MgO catalyst. When present, the effect on the catalytic activity was related to the pre-treatment of the catalyst, an oxidising pre-treatment improving the activity compared to a reduced one. 3.6. Bimetallic systems There are some patents concerned with the preparation of supported metallic catalysts in which Pd is associated with one or more elements of the platinum group. Some of these catalysts were proposed to be used for methane combustion application, with the aim of maintaining the catalytic activity to a high level. A catalyst in which Pd and Rh were associated in bimetallic oxide particles supported on a carrier was thus proposed to suppress the oscillations in the activity for methane combustion [133]. An improvement of the catalytic properties seemed to be further achieved by substituting Pd particles for Pd-Pt particles [134]. Other catalysts comprising platinum group elements other than palladium deposited onto carriers containing at least two oxides among Al2 O3 , SiO2 , TiO2 , ZrO2 and rare earth oxides were also proposed [135]. It is noteworthy that these catalysts would probably not improve catalytic properties for low-temperature applications, i.e. for temperatures lower than the PdO decomposition temperature. In order to provide a catalyst with improved low-temperature activity, for purifying methane-containing waste gas in the presence of water vapour and sulphur dioxide, a catalyst consisting of palladium and at least one metal selected from ruthenium, iridium and copper, deposited on Al2 O3 was proposed in the patent literature as an efficient catalyst [136]. A few other studies in the open literature mention the preparation and the study of the catalytic properties in complete methane oxidation of bimetallic catalysts (essentially Pd-Pt) supported mainly on alumina. Pd-Pt/Al2 O3 catalysts were prepared by conventional CI of the metal salts [127,137,138] or by controlled surface reactions [138]. In the case of 7.5 wt.% Pd-7.5 wt.% Pt/Al2 O3 catalyst [127], the catalytic activity was lower on heating than the reference catalyst, as indicated by a 20 ◦ C increase of T50 , while on decreasing back the temperature from full conversion, the Pd-Pt/Al2 O3 catalyst was found significantly more active than the reference catalyst (T50 decreased by 30 ◦ C compared to the reference catalyst). No interpretation was given on the catalytic behaviour of the bimetallic catalysts. Yamamoto and Uchida [139] studied the catalytic properties of Pt-Pd supported on alumina deposited itself on a monolith in view of the removal of methane from lean-burn natural gas engine exhaust. The same study by the same authors can also be found in a patent [140]. The Pt-Pd catalyst was compared to monometallic Pt and Pd catalysts prepared in the same way. The complete oxidation of hydrocarbons contained in the natural gas engine exhaust (3000 ppm hydrocarbon, 5 vol.% O2 , about 14 vol.% H2 O) at low temperature (385 ◦ C) was studied on high loading Pt-Pd/alumina catalysts. No detail was given on the metal salts nor their preparation. A spectacular improvement of the catalytic behaviour was observed with 5 wt.% Pt-5 wt.% Pd/Al2 O3 compared to 5 wt.% Pd/Al2 O3 and to a lesser extent 5 wt.% Pt in terms of both activity and durability. The activity of Pd/Al2 O3 and Pt-Pd/Al2 O3 were initially comparable. However, P. G´elin, M. Primet / Applied Catalysis B: Environmental 39 (2002) 1–37 both catalysts behave differently with time on stream. The Pd/Al2 O3 catalyst deactivated very rapidly at the beginning (60% loss of conversion) and then more slowly but continuously: less than 10% conversion was measured after 15:00 h. On the contrary, the Pt-Pd catalyst, after initial deactivation, kept a constant conversion equal to 50%. The Pt catalyst, although being much less active initially, showed a constant activity with time (ca. 20%). Increasing the Pd content in Pt-Pd catalysts increased the activity and the durability. The higher durability is thought to be attributed to the inhibition of Pd/PdO sintering induced by platinum. The positive effect of platinum on catalytic behaviour did not depend on the Pt content. The same improved durability of Pt-Pd catalysts compared to that of monometallic Pd catalysts was observed by Narui et al. [137] on catalysts containing much lower amounts of noble metals. Pd and Pt acetylacetonate were used as metal precursors to impregnate ␣-Al2 O3 (14 m2 /g). A 0.5 wt.% Pd/Al2 O3 and 0.5 wt.% Pd-0.1 wt.% Pt/Al2 O3 were prepared and calcined at 500 ◦ C in air before catalytic testing (0.5 vol.% CH4 in air, 18,000 h−1 , dilution of 400 mg of catalyst in alumina beads). The addition of Pt to PdO/Al2 O3 did not only prevent the catalyst deactivation (over 6 h reaction) as shown by Yamamoto but also increased the activity (from 90 to 98% conversion at 350 ◦ C) (Fig. 11). These effects were attributed to the higher dispersion of the supported particles and the suppression of the particle growth in the presence of reactants on the basis of ex situ observations of catalysts by TEM. From in situ measurements performed in the specimen chamber of the microscope under ultra high vacuum conditions, it was assessed that the supported particles of PdO/␣-Al2 O3 migrated and then coalesced with each other like dewdrops between 570 and 590 ◦ C while these phenomena were not observed on Pt-Pd sample. PdO particles were observed to contain Pt and they presented smaller contact angles than PdO free from Pt. It must be pointed out that these experiments were carried out under vacuum, that is under conditions significantly different from the test conditions. It seems therefore difficult to conclude on the role of platinum in the properties of the bimetallic Pt-Pd catalyst. Moreover, if the sintering of PdO particles would be the main cause of deactivation under reactants at 350 ◦ C, it remains to explain why sintering of PdO particles did 33 Fig. 11. Variation of the methane conversion at 350 ◦ C with time on stream over 0.5 wt.% Pd/␣-Al2 O3 (full symbols) and 0.5 wt.% Pd-0.1 wt.% Pt/␣-Al2 O3 (open symbols). Reaction conditions: 0.5 vol.% CH4 in air, GHSV = 18,000 h−1 , dilution of 400 mg of catalyst in alumina beads. The bimetallic catalyst exhibits a superior catalytic activity compared to the monometallic one and does not deactivate over a 6 h period. (Reproduced from Fig. 1 of [137], with permission from Elsevier Science.) not proceed at 500 ◦ C during the pre-treatment of the catalyst. Finally, it must be stressed that these results were obtained with ␣-Al2 O3 as a support. They must be confirmed with a more conventional support such as ␥-Al2 O3 . Bimetallic Pd-Pt/Al2 O3 catalysts were also prepared by controlled surface reactions [138]. The method was compared to a conventional CI method. The catalysts contained 0.9 wt.% Pd and 0.1–0.5 wt.% Pt deposited on ␥-Al2 O3 (60 m2 /g) by using Pd or Pt bis-acetylacetonate. Two techniques of surface redox reactions were used to introduce Pt: (1) The “refilling” method (RC): Pt2+ ions are reduced in situ by hydrogen pre-adsorbed on the Pd surface according to the reaction 2Pd/H + Pt 2+ → Pd2 Pt + 2H+ , Pt would mainly decorate the surface of Pd particles. (2) The “direct redox” method (RD), in which Pd reduces directly Pt2+ ions (the Pt2+ –Pt couple has a higher electrochemical potential than the Pd2+ –Pd couple). The Pd2+ ions produced are supposed to stay at exchange sites of the support, being not 34 P. G´elin, M. Primet / Applied Catalysis B: Environmental 39 (2002) 1–37 removed upon washing. This limits the method to low metal contents. An intimate mixing of both metals in the core of the particles is obtained. CI leads to the formation of bimetallic particles near the alloy model. The catalysts were studied in the complete oxidation of methane in oxidising feed (0.4 vol.% CH4 , 2 vol.% O2 in nitrogen), the activity being measured after reduction (fresh ample, first cycle) and after ageing at 600 ◦ C under the reaction mixture (aged sample, second cycle). Light-off temperatures (T50 ) were reported. All experiments showed the aged Pd and Pd-Pt samples being more active than the fresh ones by contrast to Pt/Al2 O3 . Concerning fresh samples, the addition of Pt by CI and RC methods tended to slightly increase the light-off temperature (less than 20 ◦ C). By contrast, RD bimetallic catalysts were more active than the parent one: a T50 decrease of 30 ◦ C was observed independently on the Pt content. For aged samples, no significant variation of T50 could be observed upon addition of Pt. From this study, it appears that bimetallic Pd-Pt catalysts where an intimate mixing of both metals is obtained could be slightly more active than monometallic Pd catalysts. This agrees with the studies of Narui et al. [137]. However, the improvement of the activity is not maintained after ageing under reactants, which would indicate a change in the morphology of the particle under reaction conditions. A segregation of metals into monometallic particles of each metal under reactants was proposed. Complementary work is needed to conclude whether Pt-Pd catalysts are more active than Pd catalysts as well as more stable with time on stream and more resistant against sulphur poisons. 4. Conclusion Supported Pd catalysts are the most active materials in the total oxidation of methane under lean-burn conditions. PdO is agreed to be the active phase. However, in spite of a substantial research effort devoted to more fundamental studies, there are still uncertainties to explain the influence of some parameters such as the pre-treatment history, the metal dispersion and the composition of the reaction mixture on the catalytic behaviour of Pd catalysts. More experimental work is needed to fully determine the nature of the active sites under working conditions. The question of how to measure the number of surface sites of oxidised active phase involved in the catalytic reaction is also of concern. Usual techniques used to estimate the number of surface sites are applied to characterise the reduced state of the catalyst. They are not appropriate to evidence possible defects in the PdO particles which could be responsible for some inner sites being accessible to reactants. Techniques such as isotopic exchange of surface oxygen with 18 O2 coupled to physical techniques probing surface species under real conditions might be more appropriate to estimate the ‘real’ surface of the catalyst compared to conventional ones. Recently, the number of surface sites of a Pd foil measured by isotopic exchange reaction after carrying the reaction of methane oxidation in excess oxygen was shown to be twice higher than the number of surface Pd atoms calculated from the initial geometric area of the foil [141,142]. The sensitivity of Pd catalysts to H2 O and sulphur containing compounds represents a serious drawback to their use as efficient catalyst for methane emissions abatement from lean-burn NGVs. Large amounts of water vapour in the exhaust gases induce a strong inhibition of catalytic activity of Pd based catalysts together with a long-term deactivation. Even more serious is the deactivation by SO2 /H2 S which leads to the transformation of active PdO into inactive PdO-SOx species, the latter sulphate groups being stable and irreversibly poisoning the catalyst because of the too low temperature range of exhaust gases. Attempts to improve the catalytic activity of Pd catalysts in the total oxidation of methane were unsuccessful so far, which seems to indicate that the intrinsic activity of PdO for this reaction cannot be overcome, as far as PdO is obtained in a sufficiently high degree of dispersion, and this is not too difficult. But improving the catalytic activity of Pd catalysts is no more the major issue of these catalysts, compared to the effect of sulphur. It is highly desirable that future studies could be devoted to searching routes for improved catalysts resistance to sulphur rather than improving catalytic activity. It turns out that no one has proposed yet Pd-based catalysts which are sulphur-resistant. Although the number of contributions involving the influence of sulphur compounds on the catalytic activity is growing up, more effort is needed. The mechanism by which Pd P. G´elin, M. Primet / Applied Catalysis B: Environmental 39 (2002) 1–37 deactivates by sulphur is established: it readily forms stable sulphate. Using sulphating supports was shown to retard Pd poisoning but not to suppress it. The question is: does it exist any route to decrease the stability of surface Pd sulphate species significantly? Another route to preparing efficient catalysts in the lean-burn oxidation of methane traces in the presence of water and sulphur containing compounds could be to improve the catalytic activity of Pt based catalysts. Supported Pt catalysts have drawn much less attention than Pd based catalysts, likely because of their much lower catalytic activity in the total oxidation of methane compared to Pd catalysts. On the contrary to palladium, platinum oxides are unstable in the condition of the reaction, which suggests a different mechanism for the methane reaction and also a different sensitivity to sulphur containing compounds. As a matter of fact, Pt catalysts are considered to be much less sensitive to sulphur containing poisons than Pd catalysts under oxidising conditions. This is particularly true when Pt is deposited on non-sulphating supports such as ZrO2 -SiO2 since Pt converts SO2 into SO3 which then can react with a sulphating support (e.g. Al2 O3 ) and cause deactivation by support sulphating. Such catalysts are commercially available for VOC abatement for stationary applications [143]. On the contrary to Pd catalysts, there exist some examples indicating that the catalytic activity of platinum in the total oxidation of methane in excess oxygen can be strongly improved by substituting conventional supports (Al2 O3 , SiO2 ) by metal oxide supports having redox properties (SnO2 , CeO2 -ZrO2 , . . . ). Attention must be focused on the preparation of these supports in order to increase the surface area and thus improve hopefully the synergistic effect between the metal phase and the support itself. However, more experimental work needs to be done to evaluate the resistance of such platinum catalysts to sintering upon mild steam ageing and their resistance to sulphur containing compounds, in order to demonstrate their superiority to Pd based catalysts. Sustained effort in preparing new phases (such as non-stoichiometric cerium oxide phases) could finally lead to new supports and/or additives exalting the catalytic activity of platinum and/or palladium. 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Difference of palladium catalysts or different strengths of the Pd–O bond were suggested to explain dramatic differences of catalytic activity According to the authors, extra-lattice oxygen atoms could be very active forms of oxygen involved in the complete oxidation of methane at low temperature The main disadvantage of these catalysts was their instability toward steaming at high temperatures A mild... after increasing thermal regeneration of the catalyst in nitrogen and the catalytic activity, the higher the 1435 cm−1 intensity the lower the rate of methane oxidation Tentative regeneration of the catalyst in hydrogen led to the decomposition of sulphate species at temperatures as low as 350 ◦ C But, the catalytic activity was not regenerated in this case The formation of highly stable surface palladium... to examine the catalytic properties of such Pd catalysts under flowing lean conditions and also the catalytic properties of Pt catalysts supported on this type of material Concerning Pt catalysts, no study of the influence of Ce on the activity of Pt in the oxidation of methane was reported Tiernan and Finlayson studied Ce doped Pt/Al2 O3 catalysts in the oxidation of isobutane under stoichiometric conditions... noteworthy that these catalysts would probably not improve catalytic properties for low- temperature applications, i.e for temperatures lower than the PdO decomposition temperature In order to provide a catalyst with improved low- temperature activity, for purifying methane- containing waste gas in the presence of water vapour and sulphur dioxide, a catalyst consisting of palladium and at least one metal selected... the stability of surface Pd sulphate species significantly? Another route to preparing efficient catalysts in the lean-burn oxidation of methane traces in the presence of water and sulphur containing compounds could be to improve the catalytic activity of Pt based catalysts Supported Pt catalysts have drawn much less attention than Pd based catalysts, likely because of their much lower catalytic activity... Nb, In, Sn), mixtures of metal oxides and mixed oxides such as solid solutions of CeO2 -ZrO2 3.3.1 Pd catalysts The catalytic activity of Pd catalysts supported on various commercial metal oxides MOx (M = In, Nb, Sn, Y, Zr) in the complete oxidation of methane at low temperature was investigated by Widjaja et al [88] and compared to that of Pd/Al2 O3 All the supports had a low specific surface area... slightly improved the catalytic activity, essentially at higher temperatures The activity of the n-sample is again higher (shift at lower temperatures by ca 20 ◦ C) than the one of the p-sample Interestingly, the surface area of the n -catalysts increased drastically upon Pd impregnation The activity of Mn3 O4 base catalysts is much lower than CeO2 based catalysts Similar to CeO2 , Pd/ZrO2 catalysts using n-crystalline... steady-state TOFs at 335 ◦ C were found to vary as follows for the different catalysts: dispersed phase of platinum, TOF = 0.005 s−1 , crystalline phase of platinum; TOF = 0.08 s−1 , small particles of palladium; TOF = 0.02 s−1 , large particles of palladium; TOF = 1.3 s−1 The structure sensitivity was related to differences in the reactivity of adsorbed oxygen A more careful investigation of Pd catalysts. .. improvement of the activity of Pt in the oxidation of methane at low temperature when substituting Al2 O3 by a commercial low surface area SnO2 The reaction mixture consisted of 1 vol.% CH4 and 4 vol.% O2 in nitrogen (200 mgcat , 6.5 l/h) After equilibration of the catalysts under reactants at 600 ◦ C, the 2 wt.% Pt/SnO2 catalyst exhibited a T50 shifted by more than 80 ◦ C toward lower temperatures compared ... intensity the lower the rate of methane oxidation Tentative regeneration of the catalyst in hydrogen led to the decomposition of sulphate species at temperatures as low as 350 ◦ C But, the catalytic... reaction temperature in the range 0.8 ± 0.2 The influence of water concentration on the rate of oxidation of methane over Pd/Al2 O3 was also investigated by Burch et al [52] at different temperatures... of CeO2 -ZrO2 3.3.1 Pd catalysts The catalytic activity of Pd catalysts supported on various commercial metal oxides MOx (M = In, Nb, Sn, Y, Zr) in the complete oxidation of methane at low temperature