Supported nanosized gold catalysi the influence of support morphology and reaction mechanism 5

Chapter Oxidation of Carbon Monoxide over Copper Oxide Supported Gold Catalysts - Catalytic Performances and Reaction Mechanism- In this chapter, CuO with different morphologies and structures were utilized to support gold nanoparticles. Au/CuO(NP) showed better catalytic activity than the Au/CuO(CB) and Au/CuO(NF) samples, whereby there is no significant differences in surface area of these three kinds of supports. More Au+ species were found on the Au/CuO(NP) sample, indicating different interactions between the Au nanoparticles and the CuO supports. Also great differences in catalytic activity for samples pre-treated at different temperature were observed. Catalysts pre-treated at 300oC showed much better catalytic activities for CO oxidation than catalysts pre-treated at 200oC. In preferential oxidation of carbon monoxide over Au/CuO catalysts testing, Au/CuO (NP) still showed the best activity in terms of CO conversion, while gold supported samples on three kinds of copper oxide supports showed similar H2 selectivity. 5.1 Introduction Due to their unique catalytic activity in various reactions, metal oxide supported nanosized gold has become one of the most popular research topics, and has attracted the attentions of researchers all over the world. Extensive works have been conducted concerning both application and fundamental academic research. Nevertheless, there are 139 still a lot of controversial and uncertain topics in this area of study. For example, the reaction mechanism and factors that can affect the reaction remain debatable. It has been generally agreed that particles size of gold and the nature of the metal oxide supports have a great influence on catalyst’s activity. Other factors such as preparation method, support surface area; pre-treatment conditions etc. can also affect their catalytic activity. Many kinds of metal oxide have been used as supports, and the most widely used supports are TiO2, Fe2O3, Al2O3, MgO, CeO2 and Co3O4 etc. Although gold supported on reducible metal oxide supports showed promising results, and copper oxide can be classified into easily reducible oxide support, not much research has been conducted using copper oxide as support for nano gold catalysis. CuO itself can catalyze CO oxidation, 1,2 and copper-based catalysts are known to be active in several industrial chemical processes, such as the methanol synthesis, the water-gas shift reaction and the catalytic oxidation of hydrocarbons. Many of these catalytic reactions involve intermediate states showing that carbon monoxide directly interacts with the copperbased catalyst. A recent study showed 100% CO conversion on Au/CuO at temperatures between 95 and 125oC.3 G. Hutchings compared nano-Au catalysts prepared by coprecipitation on CuO, CuO/ZnO and ZnO supports. Au particles on CuO and CuO/ZnO supports had large particles size and hence lower CO oxidation activity 4. But in general the study of CuO- supported Au catalysts for CO oxidations is very limited. In this chapter three CuO samples with different morphologies and structures, including bulk (commercial, denoted as CuO (CB)), nanoflake (self-made, denoted as CuO (NF)) and nanoparticles (commercial, denoted as CuO (NP)), were utilized to prepare CuOsupport gold catalysts. The catalytic activities and kinetics of carbon monoxide oxidation 140 were investigated using on-line GC. The mechanism of the low temperature CO oxidation was carefully investigated using in-situ DRIFT and in-situ XPS. 5.2 Experimental 5.2.1 Materials and catalysts preparation Commercial bulk CuO purchased from Merck (particle size~150 nm, surface area 7.9m2/g) and nanoparticle CuO from Sigma-Aldrich (particle size~80nm surface area 27.1m2/g) were used in the experiment without further treatment. CuO nano flakes used in this work were prepared by precipitation: The precipitation was performed by adding 0.5 M NaOH dropwise to the prepared 0.5M Cu(NO3)2∙2H2O (Merck, >98.5%) solution till pH 9.5. The resulting mixture was stirred at 80oC for 48 h. Then the precipitate mixture was separated by centrifuge and washed by deionized water for four times. The obtained powder was dried at room temperature for 24 h and calcined at 400oC for h in static air. The surface area (BET) of the as-prepared CuO nanoflakes is 17.8 m2/g. These three kinds of copper oxide samples were used as support for depositing gold nano particles. Colloid-based (CB) method as described in Chapter was selected as the best method for the preparation of Au/CuO catalysts. HAuCl4 (1mM) was used as a precursor, NaBH4 (0.1M) as a reducing agent and lysine as a capping agent. During the reduction period, sonication was applied. The slurry was dried at 70ºC after centrifuge four times using DI water.5-9 5.2.2 Evaluation of catalysts Catalytic evaluations were carried out at atmospheric pressure in a continuous-flow fixed-bed quartz micro-reactor (I.D. mm) packed with samples and quartz wool. Before 141 testing, the catalysts were pre-treated in-situ with a flow of air (100 ml min-1) for h at 200 and 300oC respectively. For CO oxidation reactions, the feed gas was a mixture of 90%He + 5%CO + 5%O2, which was introduced into the reactor at a gas hourly space velocity (GHSV) of 60,000 cm3 g-1 h-1. For preferential oxidation of CO in the presence of hydrogen, the feed gas was a 70%H2 + 1%CO + 2%O2 mixture, introduced into the reactor at a GHSV of 60,000 cm3 g-1 h-1. For both reactions, the reaction products were analyzed on-line using Shimadzu GC-2010 gas chromatography equipped with a thermal conductivity detector (TCD). The catalysts were evaluated for activity (in terms of CO conversion) and CO2 productivity in a temperature range of 25-300 oC. We took measurement readings after the system had stabilized for at least 15mins for every designated reaction temperature. For kinetics study, details are given on Chapter page 55-56. 5.2.3 Characterization of catalysts Powder X-ray diffraction patterns were recorded at room temperature on a Bruker D8 Advance Diffractometer using a Cu Kα radiation source. Diffraction angles were measured in steps of 0.015o at s/step in the range of 10-80o (2θ). Transition electron microscope measurements were performed on a Tecnai TF 20 S-twin instrument. Before measurement, all samples were ultrasonically dispersed in ethanol solvent and then were dried over a carbon grid. The average size of Au particles and its distributions was estimated by counting about 300 Au particles. JEOL JSM-6700F Field Emission Scanning Electron Microscope was used to observe the particle shape, size and morphology. The Au and Cu contents of prepared catalysts were determined by X-ray 142 fluorescence multi-elemental analyses on a Bruker AXS S4 Explorer. Temperature programmed reduction studies were performed in a continuous-flow fixedbed quartz micro-reactor (I.D. mm) with 50 mg samples. The catalyst was first outgassed by heating at 300oC under air flow for 60 to make sure that the samples were tested under the same condition as CO oxidation reaction. After cooling to room temperature, the feed gas was switched to 5%H2/Ar. After the baseline had stabilized, the temperature was increased to 600oC at a heating rate of 10 oC /minute. The amount of H2 consumed was measured as a function of temperature by means of a thermal conductivity detector (TCD). The in-situ Diffusion Reflectance Infrared Fourier Transform spectroscopy (DRIFTS) of CO adsorption study was carried out on a Bio Rad FTIR 3000 MX spectrometer equipped with a reaction cell (modified Harricks model HV-DR2). The CuO or CuO-supported Au sample was loaded into the DRIFT cell with 1:1 weight ratio with KBr. The spectra were acquired with a resolution of 4cm-1 typically averaging 150 scans. The sample was purged with Helium flow (20 ml min-1) for hours before exposure to reaction gas. For CO adsorption experiments, in the flow of various concentrations of CO (0.5%, 1% and 2.0% CO in He) DRIFT spectra were taken after. And as for DRIFT study on surface species during CO oxidation reaction, the spectra were taken after the introduction of 5%CO with 5%O2 in He balance. Table 5-1 summarizes the experimental procedure implemented for CO adsorption and CO oxidation DRIFT study. 143 Table 5-1 Experimental procedure for CO adsorption and oxidation DRIFT study CO adsorption CO oxidation Pre-treat catalyst in air (He) flow at 300 oC Pre-treat catalyst in air (He) flow at 300 oC (573 K) for hour and then cool down to (573 K) for hour and then cool down to RT under He flow RT under He flow   Background spectra: Catalyst in He Background spectra: Catalyst in He   0.1% CO (10 min) 0.1% CO+O2 (10 min)   Purge in He flow for at least 30 mins Purge in He flow for at least 30 mins remove gas phase CO and physisorbed CO remove gas phase CO and physisorbed CO   Take spectra of 0.1%CO adsorption Take spectra of 0.1%CO oxidation   Purge in He flow for 1hour Purge in He flow hour   0.2% CO (10 min) 0.2% CO+O2 (10 min)   Purge in He flow for at least 30 mins Purge in He flow for at least 30 mins remove gas phase CO and physisorbed CO remove gas phase CO and physisorbed CO   Take spectra of 0.2%CO adsorption Take spectra of 0.2%CO oxidation   Purge in He flow for 1hour Purge in He flow for 1hour   1% CO (10 min) 1% CO+O2 (10 min)   Purge in He flow for at least 30 mins Purge in He flow for at least 30 mins 144 remove gas phase CO and physisorbed CO remove gas phase CO and physisorbed CO   Take spectra of 1%CO adsorption Take spectra of 1%CO oxidation   Purge in He flow for 1hour Purge in He flow for 1hour   2% CO (10 min) 2% CO+O2 (10 min)   Purge in He flow for at least 30 mins Purge in He flow for at least 30 mins remove gas phase CO and physisorbed CO remove gas phase CO and physisorbed CO   Take spectra of 2%CO adsorption Take spectra o 2% oxidation X-ray photoelectron spectroscopy was performed on a VG ESCALAB XPS, ESCA MK II using Mg Kα (1254.6 eV) source under UHV better than × 10-9 torr. XPS spectra were recorded at  = 90° for the X-ray sources. The in-situ XPS experiments were performed in a UHV chamber at the SINS beamline in the Singapore synchrotron light source (SSLS) at National University of Singapore.10 XPS spectra were measured using a hemispherical electron energy analyzer (EA 125, Omicron NanoTechnology GmbH). The XPS experiments were done at normal photoelectron emission conFigureuration, with the photon energy resolution of 0.5 eV. XPS measurements were done at constant pass energy mode. Table 5.2 summarizes the experimental procedure for CO oxidation in-situ XPS study. The same scan time on each sample were maintained, because in a Au/CuO system, not only might the gold species be reduced under x-ray, but the CuO support might also be reduced under x-ray scan. 145 Table 5.2 Experimental procedure for CO oxidation in-situ XPS study CO oxidation As-prepared catalyst in pretreatment chamber, degas for 30 then transfer to analysis chamber  Wide Scan  Scan for C1s, O1s, Cu 3p and Au4f  Transfer the samples back to pre-treatment chamber and 2%CO + 2%O2 in He doses was injected into pretreatment chamber with the chamber pressure at 1*10-4 Torr for 10min  CO + O2 does was pumped out and sample was outgas for hour then transferred back to analysis chamber  Scan for C1s, O1s, Cu 3p and Au4f 5.3 Results and discussions 5.3.1 Characterization of the Au/CuO catalysts Figure 5.1 shows the SEM micrograph of three kinds of CuO and three kinds of Au/CuO samples after pre-treatment at 300 oC in air for hour. No obvious change in sample’s morphology was observed for these three kinds of CuO samples after gold deposition. 146 A B D E C F Figure 5.1 SEM micrograph for six samples. 5.1(A) CuO(CB) pre-treated in air for hour at 300oC 5.1(B) CuO(NP) pre-treated in air for hour at 300oC 5.1(C) CuO(NF) pre-treated in air for hour at 300oC 5.1(D) Au/CuO (CB) pre-treated in air for hour at 300oC 5.1(E) Au/CuO(NP) pre-treated in air for hour at 300oC 5.1(F) Au/CuO(NF) pre-treated in air for hour at 300oC Figure 5.2 displays XRD patterns of three kinds of Au/CuO samples after heating in air at 300oC for hour. All three CuO supports show crystalline structure of monoclinic Tenorite CuO. The Au diffraction at 38.2 º is not detected because of its overlapping with strong (111) (200) reflections of CuO. Gold signals may be too weak due to the small crystallite size of gold and the low concentration of gold. 147 Au (111) a:Au/CuO (CB) b:Au/CuO (NF) c:Au/CuO (NP) 5500 Counts 5000 4500 c 4000 b 3500 a 3000 10 20 30 40 50 60 70 80  Figure 5.2 XRD data for three kinds of Au/CuO catalysts. Figure 5.3 shows the TEM images of the three kinds of Au/CuO samples heated in air for hour at 300oC. Clearly Au particles are well dispersed in a few nanometer ranges. A D B C E F 148 3277 Absorbance (a.u.) 10 a: 0.1% CO b: 0.5% CO c: 1.0% CO d: 2.0% CO 2922 d 3770 c b a 2857 2664 1597 1433 827 9.2 2101 2189 d Intensity 8.8 8.4 c 8.0 b 7.6 2300 4000 a 2250 3500 2200 2150 2100 wavenumber(cm-1) 3000 2050 2500 2000 2000 1500 1000 500 -1 Wavenumber(cm ) Figure 5.17 DRIFT spectra over Au/CuO (NP) sample taken after soaking into a flow CO in He for 10 min, followed by He purge for 15 min. Before the measurements, the catalyst was pretreated in air flow (20 ml min-1) for hour at 300oC, then cooled down to room temperature a: soak in 0.1%CO for 10 minutes taken after 15 minutes of helium purge b: soak in 0.5%CO for 10 minutes taken after 15 minutes of helium purge c: soak in 1%CO for 10 minutes taken after 15 minutes of helium purge d: soak in 2%CO for 10 minutes taken after 15 minutes of helium purge Upon the introduction of CO, a band at c.a. 2181cm-1 is observed, which later shifted to 2185 cm-1 after evacuation. This band is assigned to Aun+-CO. There was another band at 2112 cm-1 after 0.1% CO introduction, which however disappeared upon evacuation. When the CO concentration was increased to 0.5%, it remained observable after evacuation. This band is assigned to Au0-CO. Cu+--CO vibration is also in this range, which however does not appear after He purging. The band at 2112 cm-1 decays more quickly on purging, indicating the Au0-CO species is more weakly bonded. If compared with CuO(CB) sample, no band at 1126 cm-1 (1126 cm-1 was assigned to the (O2-) 165 stretching in cubic Cu2O) was observed, whereas bands at 1031, 1377, 1435 and 1578 cm-1 are evident in Figure 5.16. The bands at 1377, 1435 and 1578 cm-1 are remarkable and may be assigned to bidentate carboxylate (HO-COO) bonded to Au. The 1031 cm-1 may be due to alcoholic C-O stretching. Two bands at 2923 and 2857 cm-1 were also detected. These bands are assigned as the C—H stretching vibration of HCOO- ion. This species was formed by the CO and the surface OH group: CO + --OH- → HCOO-.28 The reaction of HCOO + O2 can result in HO-COO. Note that for the CuO (CB) sample without the presence of Au, the –OH vibration in the 3480-3200 cm-1 range is very weak, whereas the Au/CuO (B) sample showed a strong and broad band contributed from surface --OH group, indicating Au/CuO (B) has more active sites on its surface than CuO (CB). For the Au/CuO(NP) sample upon the introduction of CO, a band at c.a. 2185 cm-1 is observed. This band later shifted to 2189cm-1 after evacuation. It can be assigned to Au+-CO. An Au0-CO band at 2112 cm-1 is also observable when the CO concentration was increased to 1%. Again the Au-CO bands in Figure 5.17 are much stronger as compared to Cu-CO in Figure 5.15. The –OH vibration on Au/CuO (NP) sample at 3277 and 3770 cm-1 is stronger than those of Au/CuO(CB). The IR bands related to HCOO or CO3 are also evident. However the alcoholic C-O at 1031 cm-1 is not evident on Au/CuO(NP). 5.3.3.4 CO oxidation over CuO samples A flow of reactant gas CO + O2 with 1:1 molar ratio was introduced into the DRIFT reaction cell for 10 minutes, followed by helium purge. The DRIFT spectra of the CuO (CB) sample were then taken. As shown in Figure 5.18, a weak band due to Cu0--CO is 166 observed at 2100 cm-1, which disappears after longer time feed gas flow. (Figure 5.18 inset curve d) Noticeably adsorbed CO2 band are observable around 2350 cm-1, indicating that CO oxidation can take place on CuO alone. Compared with Figure 5.14(a), the bands due to O-H (3300-3600 cm-1) and formate/carboxylate COOH/CO3 (2928, 2854, 1553, 1433, 1358 cm-1) vibrations are markedly enhanced. The band at 1125 cm-1 (1126 cm-1 was assigned to the (O2-) stretching in cubic Cu2O) is evidently observed. 1433 .0 2104 2092 1553 1358 2082 1125 0.6 d Absorbance (a.u.) c b 2140 a 2120 2100 2080 2060 2040 2928 3598 3512 0.3 4000 d c b a 3500 2969 2854 2350 a: 0.1%O2+CO b: 0.5%O2+CO c: 1.0%O2+CO d: 2.0%O2+CO 3000 2500 2000 1500 1000 500 Wavenumber(cm-1) Figure 5.18 DRIFT spectra of room temperature CuO (CB) sample taken after soaking into a flow of CO+O2 in He for 10 and purged in He for 10 min. Before the measurements, the catalyst was pretreated in air flow (20 ml min-1) for hour at 300oC, then cooled down to room temperature 167 Figure 5.19 shows the DRIFT spectra of the CO+O2 on CuO (NP) without the presence of Au, which were taken after exposure to reactant gas (CO: O2 = 1:1) for 10 minutes then purged with helium for 15 minutes. Rather weak CO band at 2120 cm-1 is observed only after longer time feed gas flow, which can be assigned to Cu+-CO vibration. There is a band at 2100cm-1 upon introduction of CO + O2 reactant and this band disappear due to purging. 0.01 a He b:0.5% CO+O2 c: 1% CO+O2 d: 2% CO+O2 2118 Absorbance (a.u.) 1285 d c b a 2200 2150 2100 2050 wave number(cm-1) d c b a 4000 3600 3200 2800 2400 2000 1600 1200 800 400 wavenumber(cm-1) Figure 5.19 DRIFT spectra of CO+O2 on CuO (NP) sample, which were taken after a flow of CO+O2 for 10 and purged in He for 10 min. Before the measurements, the catalyst was pretreated in air flow (20 ml min-1) for hour at 300oC, then cooled down to room temperature 168 5.3.3.5 CO oxidation over Au/CuO samples Figure 5.20 is the DRIFT spectra of CO + O2 on Au/CuO (CB) sample, taken at room temperature after soaking in a flow of CO + O2 (CO : O2 = 1:1) in He for 10 min, followed by He purge for 15 min. Upon the introduction of CO + O 2, a band at 2191 cm-1 was observed together with the band at 2165 cm-1. Five minutes after the evacuation and helium purge, the band at 2164 cm-1 disappeared. When CO + O2 partial pressure reached 2% (i.e. 2%CO + 2%O2), the band at 2164 cm-1 is still observable after 15 minutes of helium purge, while CO species with higher wavenumber ~2191 cm-1 are also detectable. Both bands can be assigned to Aun+--CO, but the band at 2191 cm-1 should be bonded to more positively charged Au3+. d 2191 c 2164 b Absorbance(a.u.) a 2180 1604 1439 2100 2150 2200 2250 2922 2857 d c b a 4000 3500 a: 0.1% CO+O2 b: 0.5% CO+O2 c: 1.0% CO+O2 d: 2.0% CO+O2 3000 2500 2000 1500 1000 500 wavenumber(cm-1) Figure 5.20 DRIFT spectra of room temperature on Au/CuO (CB) sample taken after soaking into a flow of CO+O2 (CO:O2=1:1) in He for 10 min, followed by He purge for 15 min. Before the measurements, the catalyst was pretreated in air flow (20 ml min-1) for hour at 300oC, then cooled down to room temperature 169 Figure 5.21 is the DRIFT spectra of Au/CuO (NP) sample taken at room temperature after soaking into a flow of CO+O2 (CO : O2=1:1) in He for 10 min, followed by He purge for 15 min. Adsorbed CO2 bands are observed around 2350 cm-1 region. Additionally the HCOO and CO3 bands at 2922, 2854, 1602 and 1450 cm-1 are evident in the Figure 5.21. a: 0.1%CO+O2 b: 0.5%CO+O2 c: 1.0%CO+O2 d: 2.0%CO+O2 1566 2922 1602 2959 2854 Absorbance (a.u.) 3665 1978 a 1450 1376 2195 b c d 4000 3500 3000 2500 2000 1500 1000 500 Wavenumber(cm-1) Figure 5.21 DRIFT spectra of room temperature on Au/CuO (NP) sample taken after soaking into a flow of CO+O2 (CO:O2=1:1) in He for 10 min, followed by He purge for 15 min. Before the measurements, the catalyst was pretreated in air flow (20 ml min-1) for hour at 300oC, then cooled down to room temperature 170 Bands at 1978, 2102 and 2195 cm-1 are observed after the introduction of CO + O2. The band at 2102 cm-1 is assigned to Au0—CO, here CO is linearly adsorbed on metallic gold species. This band disappeared after minutes of helium purge. The other two bands experienced some decrease in intensity under helium purge. The band at 1978 cm-1 can be assigned to either CO bonded with negative charged Au species, or the bridge bondage of CO with Au. After consideration of our XPS results, which would be discussed later, we assigned this band to negatively charged Au. The OH band at 3770 cm-1, which was observed over Au/CuO (NP) after CO introduction, disappeared after the introduction of CO + O2. We also noticed that the relative intensity of surface –OH groups was decreased after the introduction of CO + O2, which indicates that surface –OH groups were also involved in the reaction. This phenomenon was also observed over the Au/CuO (CB) sample, but the relative band intensity difference of Au/CuO (CB) sample is not as much as that of Au/CuO (NP) sample. Thus we can conclude that Au/CuO (NP) sample has a different surface structure from that of Au/CuO (CB) sample (Figure 5.22). Carboxylates and (bi)-carbonates are formed when the cations can be easily reduced or when non-stoichiometric oxygen exists on the surface. 29 Other than the bands at 1033, 1125 and 1285 cm-1, the CuO (NP) sample also showed bands at 1431 and 1492 cm-1. The band at 1492 cm-1 is assigned to the νas(CO2) of carbonites.30,31 The CuO (NP) DRIFT results after CO introduction seems to be unaffected by CO intensity. This means that either there are some weakly adsorbed species on CuO (NP) surfaces that were desorbed upon evacuation or all the active site on the CuO (NP) surface were taken even under the lowest CO concentration (i.e. 0.1%). If our catalytic activity results are taken into consideration (that the CuO (NP) sample exhibits the best CO oxidation activity 171 among the three kinds of CuO samples), we might able to assume that CO related species adsorbed weakly on the CuO (NP) sample surface. 3274 3769 3749 Absorbance(a.u.) c a b a:Au/CuO(NP) COA b: Au/CuO(NP) COO c: Au/CuO(CB) COA d: Au/CuO(CB) COO d 4000 3800 3600 3400 3200 3000 -1 Wavenumber(cm ) Figure 5.22 Comparison of DRIFT spectra of Au/CuO (CB) and Au/CuO (NP) sample taken after soaking into a flow of CO or CO+O2 in He for 10 and purged in He for 10 min. (a) Au/CuO (CB) sample taken after 2% CO introduction; (b) Au/CuO (CB) sample taken after 2%CO+2%O2 introduction. (c) Au/CuO (NP) sample taken after 2% CO introduction; (d) Au/CuO (NP) sample taken after 2%CO+2%O2 introduction 5.3.4 Electronic structure of CuO supported gold catalysts In-situ XPS measurements were conducted to investigate the electric structure of Au/CuO (CB) and Au/CuO (NP) samples before and after CO + O2 doses. The binding energies (BE) of XPS peaks were calibrated using Cu 3p3/2 = 78.2 eV as reference 32 in order to exclude surface charging effects. Figure 5.23 shows Au 4f7/2 XPS spectra of the Au/CuO 172 (CB) and Au/CuO(NP) samples before adsorption, while Figure 5.24 the Au4f7/2 spectra of the samples after CO+O2 doses. Detailed peak analysis data are given in Table 5.4. (b) Intensity(a.u.) Intensity(a.u.) (a) 86 85 84 83 82 85 84 BE (eV) 83 82 BE(eV) Figure 5.23 Au 4f XPS spectra of (a) Au/CuO (CB) and (b) Au/CuO(NP) samples before gas adsorptions (b) Intensity(a.u.) Intensity(a.u.) (a) 87 86 85 BE(eV) 84 83 86 85 84 83 82 BE(eV) Figure 5.24 Au 4f XPS spectra of (a) Au/CuO (CB) and (b) Au/CuO(NP) samples after CO + O2 adsorption 173 Table 5.4 Comparison of the in-situ XPS results for the Au/CuO (CB) and Au/CuO (NP) sample before and after CO + O2 doses. position Au/CuO (CB) area FWHM Au4f 7/2 position Au/CuO (NP) area FWHM before doses 83.8 85.1 609.7 (96%) 28.1 (4%) 1.0 0.8 83.3 84.0 481.4 (94%) 30.7 (6%) 0.8 0.6 After CO+O2 doses 83.6 84.7 85.4 97.4 (65%） 40.5 (27%) 12.6 (8%) 1.0 0.9 0.9 83.5 84.5 85.4 125.4 (63%) 50.5 (34%) 22.3 (3%) 1.0 1.0 0.9 Two peaks are contributed to Au 4f 7/2 for the Au/CuO (CB) sample before CO + O2 doses. The 83.8 eV peak contributes to metallic gold, while the other at 85.1 eV may be due to Au+. The peak area ratio of these two peaks is 24:1. After CO + O2 doses, the peak intensity shifts to lower BE side, and two peak components can be distinguished at 84.7 and 85.6 eV, corresponding to Au+ and Au3+ respectively. For Au/CuO(NP) initially the main Au peak is at 83.3 eV, lower than that of Au/CuO(CB). The negative shift of the peak is confirmed by measuring the energy gap between Cu3p (75.5 eV) and Au4f (83.8 eV). The value is 8.3 eV for Au/CuO(CB) while 7.8 eV for Au/CuO(NP), indicating the Au/CuO(NP) carrying more negative charges. The more charge transfer from CuO(NP) results from stronger support-metal interaction on CuO(NP) than CuO(CB). After CO+O2 adsorption more Au+ and Au3+ are detected. These XPS data are consistent to the above IR data. 5.4 CO oxidation mechanisms Though both XRD and transmission FTIR confirm the CuO structure of our three CuO samples, including bulk CuO(CB) and CuO(NP), DRIFT data show that surface Cu atoms on these CuO samples are largely in Cu+ oxidation state (Figure 5.15). O2 can 174 react with CuO, generating oxygen ion (OCu=O)- or peroxide species Cu+(O22-) with their IR. peaks between 800 and 900 cm-1. CO can be weakly adsorbed at Cu+ or Cu0 sites (surface Cu ions with low coordination numbers), giving Cu+CO or Cu0CO at 2120 and 2100 cm-1 respectively. But they are weakly adsorbed and can be purged by He gas easily. CuO samples can catalyze CO+O2 producing CO2 on CuO samples. But the activity is very low particularly on CuO(NP) sample (Figure 5.19). By depositing Au nano-particles on CuO supports, OH groups were also introduced to the CuO surfaces.(Figures 5.16 and 5.17) Au can greatly enhance the CO adsorption by Au/CuO catalysts as shown by the IR peaks at 2185 and 2112 cm-1 which can be assigned to Au+CO and Au0CO respectively. Noticeably the C-H vibrations at 2922 and 2857 cm-1 are evident together with the COO vibrations between 1300 and 1600 cm -1, indicating the formation of HCOO species. This strongly suggests that OH is involved in the CO adsorption and following conversion to CO2: OH + COad  HCOOad  HCad + CO2 When CO+O2 is introduced to Au/CuO samples the presence of O2 in the feed-gas causes the decrease of OH vibration at 3600-3700cm-1. (see Figure 5.22) This may be expressed as Step 1: O2 + OH- → HO2- Step 2: HCOO + HO2- → HCO3 + OH- Step 3: HCO3 → OH- + CO2 This mechanism is similar to that proposed by G.C. Bond or by Kung. 28,33 The above steps may involve charge transfer between adsorbate and adsorbent, which results in 175 drastic changes in Au oxidation states and broad CO adsorption band. In XPS Au -1 to +3 have been observed while CO vibrations at 1978 and 2185 cm -1 can be detected. 5.5 Conclusion The study in this chapter has demonstrated that the catalytic activity of Au/CuO catalysts could be significantly improved for CO oxidation at room temperature using nanosized supports (i.e. CuO (NF) and CuO (NP)). XRD showed that all the three CuO supports have the same crystalline structure and have very low surface area. The Au catalysts prepared by colloid-based method on the three CuO supports have similar gold particle size and size distribution, Au/CuO(NP) catalyst with larger support surface area showed better catalytic activities. The activation energy of Au/CuO (NP) was 3.8 KJ/mol in a temperature range of 25-125oC. Under the same reaction conditions, Ea of 5.8 kJ/mol was measured for the Au/CuO (CB) sample. The reaction rate was 0.22 mmol/gAu·s for Au/CuO (NP) and 0.07 mmol/ gAu·s for Au/CuO (CB). The pre-treatment temperature also affected the samples’ catalytic activity. 300oC is found to be the optimum pretreatment temperature for air pre-treatment. DRIFT results show that CuO (NP) and CuO (CB) have high oxygen vacancies on surface so that Cu ions are mainly in +1 oxidation state. CO is found to be mainly adsorbed at Au active sites. By colloids-based process OH groups were formed on Au/CuO surfaces. Surface –OH group were involved in the CO oxidation reaction for both Au/CuO samples. The interaction between OH and adsorbed CO may result in HCOO intermediates. O2 can be adsorbed by CuO forming O- or O2- species which can react with surface HCOO to release CO2 via carboxylate or carbonate intermediates. 176 During this interaction Au- and Aun+ can be formed in addition to Au0. They can adsorb and activate CO stronger. Au particles on CuO(NP) show strong support-metal interaction and then have higher CO oxidation activity. 177 References .Y. Liu, Q. Fu and M.F. Stephanopoulos, Catal. Today 241 (2004) 93 2. G. Avgouropoulos, T. Ioannides and H. Matralis, Appl. Catal. B 56 (2005) 87 3. E.Y. Ko, E. D. Park, Catal. Today 116 (2006) 377 4. G. Hutchings, Gold Bulletin 29 (1996) 123 5. Z.Zhong, S. Patskovskyy, P. Bouvrette, J.H.T. Luong and A. Gedanken, J. Phys. Chem. B 108 (2004) 4046. 6. Z. Zhong, J. Luo, J. 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Thompson, Gold Bull. 33 (2000) 41 180 [...]... conversion of CO for 72 hours Kinetics of CO Oxidation Figure 5. 8 shows that the catalytic activity of the Au/CuO (NP) is much higher than that of the Au/CuO (CB) in the temperature range of 25- 100 oC For Au/CuO (NP), the conversion of CO was ca 50 % at 37oC (T50) and reached 100% at 50 oC In contrast, in the Au/CuO (CB) sample, T50 was detected at 70oC, and 100% conversion was obtained 155 at 80 oC The Arrhenius... 15 min Adsorbed CO2 bands are observed around 2 350 cm-1 region Additionally the HCOO and CO3 bands at 2922, 2 854 , 1602 and 1 450 cm-1 are evident in the Figure 5. 21 a: 0.1%CO+O 2 b: 0 .5% CO+O 2 c: 1.0%CO+O 2 d: 2.0%CO+O 2 156 6 2922 1602 2 959 2 854 Absorbance (a.u.) 36 65 1978 a 1 450 1376 21 95 b c d 4000 350 0 3000 250 0 2000 150 0 1000 50 0 -1 Wavenumber(cm ) Figure 5. 21 DRIFT spectra of room temperature on... 16 33 70 33 1 (a) 100 80 90 3449.4 80 70 70 50 650 58 8 58 8.2 87 5 60 60 53 2 600 55 0 50 0 450 400 -1 Wavenumber(cm ) 53 6.2 09 59 % Transmittance 90 50 350 0 3000 250 0 2000 150 0 1000 50 0 -1 Wavenumber(cm ) 160 Absorpsions(a.u.) (b) (i) Cu2O (ii) CuO(CB) (iii) CuO(NP) 4000 350 0 3000 250 0 2000 150 0 1000 50 0 -1 wavenumber(cm ) Figure 5. 14 FTIR spectra of the CuO (CB) sample (a) transition mode; (b) DRIFT... distribution(%) Size distribution(% ) 25 AuCuO (NF) 40 AuCuO (CB) 30 30 25 20 30 25 20 15 15 10 10 10 5 5 5 0 1 2 3 4 5 6 Figure 5. 4 Table 5. 3 0 0 7 1 Au particle size(nm ) 2 3 4 5 6 1 7 2 3 4 5 Bar graph of three kinds of Au/CuO samples (a) Au/CuO (CB) (b) Au/CuO (NP) 7 (c) Au/CuO (NF) Au atom% in three kinds of Au/CuO samples from XRF, BET results of three kinds of CuO, and three kinds of Au/CuO samples CuO Au/CuO... s-1 of TOF at 25oC (298 K) for Au/CuO (NP) is about three times that of Au/CuO (CB) The activation energies of Au/CuO (CB) and Au/CuO (NP) samples are also lower than the values obtained from the Al2O3 results, and the reaction rate is faster than CeO2, Al2O3 and ZrO2 supported Au catalysts prepared by other groups.17-22 Selective oxidation of carbon monoxide in hydrogen Figure 5. 10 -5. 13 show the. .. 85. 4 97.4 ( 65% ） 40 .5 (27%) 12.6 (8%) 1.0 0.9 0.9 83 .5 84 .5 85. 4 1 25. 4 (63%) 50 .5 (34%) 22.3 (3%) 1.0 1.0 0.9 Two peaks are contributed to Au 4f 7/2 for the Au/CuO (CB) sample before CO + O2 doses The 83.8 eV peak contributes to metallic gold, while the other at 85. 1 eV may be due to Au+ The peak area ratio of these two peaks is 24:1 After CO + O2 doses, the peak intensity shifts to lower BE side, and. .. enhanced The band at 11 25 cm-1 (1126 cm-1 was assigned to the (O2-) stretching in cubic Cu2O) is evidently observed 1433 0 0 2 2104 2092 155 3 1 358 2082 11 25 Absorbance (a.u.) 0.6 d c b 2140 a 2120 2100 2080 2060 2040 2928 359 8 351 2 0.3 4000 d c b a 350 0 2969 2 854 2 350 a: 0.1% O +CO 2 b: 0 .5% O +CO 2 c: 1.0% O +CO 2 d: 2.0% O +CO 2 3000 250 0 2000 150 0 1000 50 0 -1 Wavenumber(cm ) Figure 5. 18 DRIFT spectra of. .. increase the life span of gold on copper oxide catalysts and at the same time, increase their catalytic activities In this chapter, our interests are gold catalyst supported on CuO CO oxidation reactions over CuO without the presence of Au were studied as reference for comparing the differences between activities of CuO and Au/CuO samples Therefore, the oxidation reaction conditions were optimized for the. .. surface structure from that of Au/CuO (CB) sample (Figure 5. 22) Carboxylates and (bi)-carbonates are formed when the cations can be easily reduced or when non-stoichiometric oxygen exists on the surface 29 Other than the bands at 1033, 11 25 and 12 85 cm-1, the CuO (NP) sample also showed bands at 1431 and 1492 cm-1 The band at 1492 cm-1 is assigned to the νas(CO2) of carbonites.30,31 The CuO (NP) DRIFT results... no band at 1126 cm-1 (1126 cm-1 was assigned to the (O2-) 1 65 stretching in cubic Cu2O) was observed, whereas bands at 1031, 1377, 14 35 and 157 8 cm-1 are evident in Figure 5. 16 The bands at 1377, 14 35 and 157 8 cm-1 are remarkable and may be assigned to bidentate carboxylate (HO-COO) bonded to Au The 1031 cm-1 may be due to alcoholic C-O stretching Two bands at 2923 and 2 857 cm-1 were also detected These . than that of the Au/CuO (CB) in the temperature range of 25- 100 o C. For Au/CuO (NP), the conversion of CO was ca. 50 % at 37 o C (T 50 ) and reached 100% at 50 o C. In contrast, in the Au/CuO. information for XRF and BET results of gold supported CuO samples are listed in Table 5. 3. The gold wt% content of these Au/CuO samples were all around 2.2-2.0 % according to the x-ray fluorescence. Pretreat NA 7.4 NA 27.4 NA 16.8 Pretreat 300 o C NA 6.8 NA 25. 1 NA 15. 9 1 2 3 4 5 6 7 0 5 10 15 20 25 30 Au particle size(nm) AuCuO (CB) Size distribution(%) 1 2 3 4 5 6 7 0 5 10 15 20 25 30 35 40 Size distribution(%) Au