Supported nanosized gold catalysi the influence of support morphology and reaction mechanism 4

Chapter Low Temperature CO Oxidation over TiO2-supported Gold Catalysts -Effect of Hydrothermal Process of TiO2 Support on Nano Gold catalysis- In this chapter, kinds of TiO2, including bulk TiO2 (from Merck), nanotubular (NT) TiO2, and other commercial TiO2 products, were utilized as the support for gold nanoparticles. The catalytic performance and kinetics of the carbon monoxide oxidation reaction over the Au/TiO2 catalysts were investigated, and roles of various factors that can influence the catalytic performances were identified. The surface area and morphology of the titanium oxide support influence the catalytic performance of the supported gold nanoparticles. The gold supported on TiO2 nanotubes, which have the largest surface area among the TiO2 supports, showed better catalytic activity than the Au/TiO2 catalysts. Also, in the Au/TiO2 system, various preparation methods and treatments play more important role than the support crystalline structure in terms of the CO oxidation catalytic performance. The catalysts calcined at 300oC showed much better catalytic activities for CO oxidation than the catalysts calcined at 200oC, and the hydrothermal processing of the TiO2 oxide supports can enhance the nanogold catalytic activity. Further studies showed that some structural factors contribute to the catalytic performances, e.g., the hydrothermal processing of the TiO2 oxide supports can introduce more oxygen vacancies and chemically bonded OH/H2O to the support/catalyst, and the OH/H2O are indeed involved in the CO oxidation reaction. Both DRIFT and XPS results confirmed the existence of Aun+ species in Au/TiO2 system during CO oxidation reaction. More Aun+ species were found in Au/TiO2 (NT) 97 sample than in Au/TiO2 (CB) sample. In preferential oxidation of carbon monoxide over the Au/TiO2 catalysts, the Au/TiO2 (NT) catalyst still showed the best activity in terms of the CO conversion, while the Au/TiO2 (NT), Au/TiO2 (CB) samples showed similar H2 selectivity. 4.1 Introduction The high activity of nano-sized gold catalysts supported on metal oxides for the carbon monoxide oxidation reaction at or below room temperature has attracted great interest and attention from scientists worldwide. Despite that various reaction routes and mechanisms have been proposed, it has been generally agreed that the size of the gold particles and the interaction between gold nanoparticles and supports are very important factors that contribute to the extraordinary catalytic performance of the supported nanogold catalysts, for example, Au particle exhibits good catalytic performances under mild conditions only when the gold particle size is smaller than 5nm; Oxide supports not only play role in supporting Au catalysts and keeping them in well-dispersed condition, but may also modify the Au electronic structure via metal-support interaction; Moreover, the catalyst supports may participate in activation of oxygen via adsorption at oxide vacancies. Therefore the structural and electronic properties of oxide supports are significantly correlated to the catalytic performance of the nanogold catalysts. The oxide catalyst supports for the gold nanoparticles are usually classified into three groups: (i) easily reducible oxides such as FeOx and CuO; (ii) less easily reducible oxides including TiO2 and CeO2 and (iii) non-reducible oxides like Al2O3 and SiO2 etc. In Chapter 3, iron oxide support was selected for studying the effect of preparation methods on the nanogold catalysts. Colloids-based (CB) method using 98 Lysine as capping agent is found to be more effective in deposition of gold onto the catalyst supports as compared to Co-Precipitation (CP) and Deposition-Precipitation (DP). Therefore, in Chapter 4, we will employ the same method for gold deposition but mainly focus on the TiO2 support, and will study how the support surface area, crystalline structure, morphology, pre-treatment conditions, presence of defects, presences of water, can affect the nanogold catalysts in low temperature oxidations. 1-6 Though Mayfair et al believe that, unlike most of traditional catalysts, the effect of support surface area might not be significant on supported gold nanoparticles. Their conclusion is based on the fact that when nanosized gold particles were supported on metal oxide with high surface area (normally > 50 m2/g) and low loading, which could also result in poor catalytic performance.8-9 Titanium oxide supported nanogold system is currently one of the most investigated systems among all the metal oxide supported gold nanocatalysts.10-17 The most widely used preparation method of Au/TiO2 is deposition–precipitation, and this method produces gold particles that are quite uniform.18 The effects of different crystalline structures of TiO2 on the activity of Au catalysts have been addressed.18 TiO2 exists in three main different crystalline forms: anatase, brookite, and rutile. W.F. Yan et al. compared supported gold nanocatalysts on anatase, brookite, rutile, and P25 polymorphs of TiO2 for catalytic oxidation of CO, and concluded that brookitesupported gold catalyst sustains the highest catalytic activity. But not many groups have concentrated on researching the effect of support morphology on the catalytic performance of Au/TiO2 catalysts, though some group had prepared Au supported TiO2 nanotube catalyst. For example, Idakiev et al. have reported the usage of TiO2 nanotube supported gold nanoparticles as catalysts for low temperature water-gas shift reaction.19 99 4.2 Experimental 4.2.1 Preparation of TiO2 Nanotubes via Hydrothermal Process Firstly, a systematic hydrothermal preparation was carried out. In a typical preparation, 1g of commercially available bulk TiO2 (Merck, anatase) was dispersed in 100ml of 10M NaOH and the slurry was put in a screw-capped autoclave containing a Teflon vessel. The autoclave was then placed in a furnace at 393K for 48 hrs. All acid washing processes were carried out with 0.1M HCl at room temperature. The TiO2-derived nanotubes were washed with copious amount of 0.1M HCl acid (2L), then pickled in fresh 500ml 0.1M HCl overnight and followed by washing with distilled water, and finally recovered by centrifugation. The acid washing was considered as a proton exchange process in our experiments. All the samples were then air dried overnight at 80oC. Subsequent air annealing was carried out in a wellventilated furnace. Figure 4.1(A) refers to the starting TiO2 materials. After 6-10 hrs of 10M NaOH hydrothermal treatment at 120oC (Figure 4.1(B), 4.1(C), 4.1(D)), step-like structures and nanosheets exfoliated from bulk TiO2 were observed. This strongly suggests that a sheet-folding process is involved. Curling and scrolling of the nano sheets can be observed in Figure 4.1(E) and Figure 4.1(F) (18hr). In Figure 4.1(I) and Figure 4.1(J) (24hr) of the hydrothermal process, formation of TiO2-derived nanotubes is observable. Figure 4.1(K) and Figure 4.1(L) show the multi-layered nanotubular structure after TiO2-derived nanotubes were washed with 0.1M HCl. 100 A B C D E F 101 G H I J K L Figure 4.1 (A-L) TEM Observation of sodium titanate nanotube Formation. 102 4.1(A): 4.1(B): 4.1(C): 4.1(D): 4.1(E): 4.1(F): 4.1(G): 4.1(H): 4.1(I): 4.1(J): 4.1(K): 4.1(L): Starting material, commercial TiO2 sample. Commercial TiO2 after 10 hours of 10 M NaOH hydrothermal processing at 120oC. Layered structure around TiO2 after 10 hours of 10 M NaOH hydrothermal processing at 120oC. TiO2 after 10 hours of 10M NaOH hydrothermal processing at 120oC layered structure peel off from the surface. TiO2 after 18 hours in 10 M Na10 hours of 10 M NaOH hydrothermal process at 120oC, curving formed. TiO2 after 18 hours of 10 M NaOH hydrothermal processing at 120oC. TiO2 after 20 hours 10 hours of 10 M NaOH hydrothermal processing at 120oC, end of tube. TiO2 after 20 hours of 10 M NaOH hydrothermal processing at 120oC. TiO2 after 24 hours of 10 M NaOH hydrothermal processing at 120oC. TiO2 after 24 hours of 10 M NaOH hydrothermal processing at 120oC. TiO2 after 48 hours of 10 M NaOH hydrothermal processing at 120oC. TiO2 after 48 hours of 10 M NaOH hydrothermal processing at 120oC. 4.2.2 Preparation of Au/TiO2 via Colloids-Impregnation Procedure Commercial TiO2 (Merck, anatase), TiO2 nanotubes, and four other TiO2 supports from ISHIHARA SANGYO KAISHA, LTD are labeled as CB, NT, MC-50, TTO-D1, TTO-S-1 and MC-150 respectively. These six kinds of titanium oxide were then used as support for the preparation of TiO2 supported gold nano particle samples using colloids-based method. HAuCl4 (1mM) is 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 centrifuged for four times using DI water.20-24 4.2.3 Evaluation of Catalytic Activity Catalytic runs were carried out at atmospheric pressure in a continuous-flow fixedbed quartz micro-reactor (I.D. mm) packed with samples and quartz wool. Before testing, the catalysts were pre-treated in situ with a flow of air (100 ml/min) for h at 200 or 300oC. For CO oxidation reactions, the feed gas was a mixture of 90%He + 5%CO + 5%O2, introduced at a gas hourly space velocity (GHSV) of 60,000 cm3g-1h1 . 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 60,000 103 cm3g-1h-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 20-300oC. Data were collected after the system had stabilized for at least 15mins at every set reaction temperature. The Conversion and Selectivity are calculated in terms of concentration: Inlet CO concentration – Outlet CO concentration CO conversion (%) = x 100% Inlet CO concentration Inlet CO concentration – Outlet CO concentration O2 selectivity (%) = x 100% x (Inlet O2 concentration – Outlet O2 concentration) For kinetics study, the catalyst was diluted with SiC powder. Absolute mass-specific reaction rates were calculated using Eq. (4.1). Detail calculation of CO Conversion, Selectivity and Kinetic data can refer to Chapter (3.2.2 ) rCO = ċCO,in XCO Vgas [moles·s-1gcat-1] Eq. (4.1) mcat 4.2.4 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 at a step rate of 0.015o/s in the range of 10-80o (2θ). Transmission electron microscope measurements were performed on a Tecnai TF 20 S-twin instrument with 104 a Lorentz lens. Before measurement, all samples were ultrasonically dispersed in ethanol solvent and then dried over a carbon grid. The average size of the Au particles and its distributions were estimated by counting about 300 Au particles. The Au and Ti contents of prepared catalysts were determined by X-ray fluorescence multielemental analyses on a Bruker AXS S4 Explorer. The in-situ Diffusion Reflectance Infrared Fourier Transform spectroscopy 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 Au/TiO2 catalyst 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 using helium flow (20 ml/min) for hrs before exposure to the reaction gas. For CO adsorption experiments, 2.0% CO were used to investigate the relative surface reaction rate. And as for DRIFT study on surface species during CO oxidation reaction, 2%CO + 2%O2 (He as balance gas) were used as the reactant gas. The detailed experimental procedure implemented for CO adsorption and CO oxidation DRIFT study is presented in Table 4.1. Table 4.1 Experimental procedure for CO adsorption and oxidation DRIFT study CO adsorption CO oxidation Pre-treat catalyst in air (He) flow at 300 o C (573 K) for hour and then cool down Pre-treat catalyst in air (He) flow at 300 o C (573 K) for hour and then cool down to RT under He flow to 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)   105 Purge in He flow for at least 30 mins Purge in He flow for at least 30 mins remove gas phase CO and physisorbed remove gas phase CO and physisorbed CO 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 remove gas phase CO and physisorbed CO 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   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 remove gas phase CO and physisorbed CO 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 (1254.6eV) source under UHV better than × 10-9 torr. The background contribution B (E) (obtained by the Shirley method) caused by inelastic processes was subtracted, while the curve-fitting was performed with a GaussianLorentzian profile, and binding energies (BEs) were calculated using the Origin 7.0 106 No carboxylates species were found in this range over Au/TiO2 (NT) sample before CO + O2 introduction. Ceasing addition of CO and O2, together with helium purging, caused the adsorbed CO and CO2 peaks to decrease simultaneously. Although the bands at 1647 and 1338 cm-1 also decreased, their absolute intensities remained similar before and after CO was introduced. Thus, these bands were fairly stable spectator species. 4.3.4 Electronic Structure of gold supported TiO2 catalysts The binding energies (BE) were calibrated using Ti2p3/2 peak in order to exclude surface charging effects. According to XRD results (Figure 4.4), the TiO2 supports in both Au/TiO2 (CB) and Au/TiO2 (NT) samples were anatase. So we set the standard binding energy of Ti2p3/2 to 459.2eV.44 To determine the surface state of Au particles on TiO2 surfaces, the XPS Au 4f spectra was carefully recorded for the fresh and spent catalysts, as presented in Figure 4.14a and Figure 4.14b. Detailed information for peak position, area and FWHM are presented in Table 4.5. BE of Au 4f7/2 is ca. 83.9 eV for the fresh catalyst, and shifts slightly to ca. 84 eV after reaction. Peak analysis generates another band at 84.8 eV for the spent sample. (a ) 123 (b) Figureure 4.14 Au 4f XPS spectra of the fresh (a) and spent (b) Au/ TiO (CB) catalysts. Figure 4.14 Au 4f XPS spectra of the fresh (a) and spent (b) Au/TiO2 (CB). Table 4.5 Comparison of Au 4f XPS results for fresh and spend samples of Au/TiO2 (CB) catalysts and Au/TiO2 (NT) catalysts. Au/TiO2(CB) position area Au/TiO2(NT) FWHM position area FWHM Au4f 7/4 fresh spent 83.9 3268.0 (100%) 0.7 84.3 5180.2 (71%) 0.8 84.7 2148.2 (29%) 0.6 83.9 2202.7 (69% 0.9 84.1 8080 (62%) 1.2 84.8 1024.4 (31%) 1.9 85.3 4980 (38%) 2.0 XPS Au 4f spectra and detailed peak position, area and FWHM information of fresh and spent sample of Au/TiO2(NT) sample are shown in Figure 4.15 (a), Figure 4.15(b) and Table 4.5. The Au 4f7/2 peak for the fresh sample is from both metallic gold (84.3 eV) and positively charged gold (84.7eV) (Figure 4.15a). After exposed to CO+O2, the high oxidation state peak increases its intensity relatively. 124 (a) (b) Figure 4.15 Au 4f XPS spectra of the (a) fresh and (b) spent Au/TiO2 (NT). 125 These results indicate the existence of Au+ even before the exposure of reactant gases. Au+ is thought to be a ‘chemical glue’ which can anchor Au particles strongly. In this sense titania nanotubes are better support for Au particles. After CO oxidation reaction more gold atoms are in oxidation state. Also, if compared with Au/TiO2 (CB) sample, more Aun+ species (38% ) were found in spend Au/TiO2 (NT) sample than on spend Au/TiO2 (CB) sample ( 31%). In-situ XPS In-situ XPS measurements were conducted to investigate the electric structure of Au/TiO2 (NT) sample after CO doses and after CO + O2 doses. Figure 4.16 (a) shows (a) 126 (b) Figure 4.16 Au 4f XPS spectra of the Au/ TiO2 (NT) catalysts (a) after CO doses and (b) after CO+O2 does Au4f XPS spectrum of the Au/TiO2 (NT) sample after CO doses. Due to CO adsorption some peak intensity shifts to lower binding energy 83.9 eV, while some shifts to higher binding energy sides at 84.9 eV (Au+) and 85.4 eV (Au3+). CO adsorption may transfer some charge density to the support (while C-O increased to >2143 cm-1) as indicated in the in-situ DRIFT study above. This would induce anionic supported Au in Figure 4.16. There exists cationic Au on titania nanotubes supports. More positively charged gold species in the Au/TiO2 (NT) sample after CO doses. Before CO doses, there were no Au3+ peak in the Au/TiO2 (NT) sample, whereas after CO doses, the peak at 85.4 eV which contributed to 9.6% of the whole Au species was detected. This peak was neither shown in the fresh Au/TiO2 (NT) nor in the spent sample. Similarly two peaks appear at low binding energy sides due to 127 CO+O2 adsorption. It is interesting to see that on more active Au catalysts more different Au oxidation states could be present (Au/TiO2 (NT) vs Au/TiO2(CB)). This agrees with what has been observed on Au/Fe2O3 prepared via colloids-based method (see Chapter 3, page 30-32). These results can be well understood using the pictorial representation proposed by Prof. G.C. Bond in his review paper. 45 Mx+ H AuIII oxidation Au0 particle Au0 oxidation Au0(OH) Active particle reduction O2- calcination, reduction Figure 4.17 Pictorial representation of supported gold catalyst indicating possible changes under conditions giving oxidation or reduction of the active gold particles. In this model the active catalyst contains both gold atoms and ions, and the latter binds the Au particle to the oxide support, thus each Au particle is decorated with a layer of Au ions. The structure is not fixed, and the Aun+/Au0 ratio may change during the process of calcinations, adsorption or reaction. The oxidation reaction may result in Au0(OH) formation. Hence Au3+ species were detected on Au/TiO2 (NT) before and after CO or CO + O2 doses, but not on spent Au/ TiO2 (NT). 128 Table 4.6 Comparison of Au 4f XPS results of Au/TiO2 (NT) catalysts with and without CO doses; and Au/TiO2 (NT) catalysts with and without CO+O2 doses. Au/TiO2 (NT) position Area FWHM position Area FWHM Au4f 7/4 before 84.3 5180.2 (71%) 0.8 before 84.1 8080 (62%) 1.2 doses fresh 84.7 2148.2 (29%) 0.6 doses spent 85.3 4980 (38%) 2.0 83.9 547 (57%) 0.9 83 23 (3%) 0.8 After CO 84.6 151 (16%) 1.0 84 626 (72%) 0.9 doses 85.5 150 (16%) 1.0 85 160.1 (18%) 0.8 86.7 97 (1%) 1.0 86.1 62.7 (7%) 0.8 After CO+O2 doses 4.4 Hydrothermal Effect on the Activity of Au/TiO2 Titania nanotubes were prepared from the hydrothermal processing of commercially available TiO2(CB). When bulk anatase TiO2 (non-layer-structured materials) is treated in 10M NaOH at 120oC, layer-structured titanates such as Na2Ti3O7 may be formed as intermediate products. During the hydrothermal process, the Na+ cations residing between the edge-shared [TiO6] octahedral layers may be replaced gradually by H2O molecules. The size of intercalated H2O molecules is larger than that of Na+, thus the interlayer distance was enlarged and the static interaction between neighboring [TiO6] octahedral sheets is weakened. Consequently, the layered titanate particles exfoliated to form nanosheets. In order to release strain energy, the nanosheets curl-up, forming TiO2 nanotubes.46 (Figure 4.18) Post-synthesis treatment of extensive acid washing was very crucial to obtain anatase titania nanotubes, which however could leave large amount of defects as well as chemically bonded water molecules in the nanotubes. The weak XRD signals of TiO2 (NT) may also mean the presence of large amount of structural disorders/defects. The TiO2 nanotubes have hollow core and open ends. The outer diameter of the tubes is uniformly distributed around 10nm while the length varies between 200nm and 1m. The specific surface 129 area determined by BET measurements is as large as 284m2/g, almost seven times that of Au/TiO2 (CB) (ca. 41 m2/g). The large surface area and nanoscaled size of oxidesupport are significantly important for CO oxidation Au catalysts. M. Comotti et al have conducted similar investigation and concluded that for the Au catalysts prepared by colloid-based methods the most important parameter is the grain size of the support.47 Figure 4.18 exfoliating-rolling model of TiO2 nanotubes formation from the layered Na2Ti3O7 particles by a hydrothermal chemical process The deposition of the colloidal gold particles as well as the oxidation activity is greatly enhanced in the case of supports composed of particles of few nanometers in size. They attributed this phenomenon to a bigger amount of defects such as oxygen vacancies. Au/MC-150 has a similar surface area as that of Au/TiO2 (NT) (Table 4.3) but the defects density of Au/MC150 is smaller. Indeed the XPS data of our 130 Au/TiO2(NT) sample show a smaller O/Ti atomic ratio for Au/TiO2(NT) (1.8) as compared to Au/MC150 (2.0), indicating more oxygen vacancies. The XPS data also revealed the presence of significant proportion of adsorbed (OH) and H2O (i.e. Ti–OH) on the TiO2 nanotube surface. As shown in Figure 4. 17, in addition to the O1s peak at 530.5 eV, corresponding to oxygen from TiO2, there appears a shoulder at high BE side up to 534 eV. It is well known that the O from H2O or Ti-OH is located at 532.6eV or 533.1eV. TiO2(NT) contains more OH/H2O as compared to TiO2(CB). Intensity(a.u.) TiO2(NT) 536 534 532 530 528 BE(eV) Intensity(a.u.) TiO2 CB 536 534 532 530 528 BE(eV) Figure 4.19 XPS O1s Spectra of (a) Au/TiO2(NT); and (b) Au/TiO2(CB) samples. Based on our DRIFT results, CO is weakly bounded on gold in both Au/TiO2 (NT) and Au/TiO2(CB). CO adsorption is less likely to be the crucial step for CO oxidation 131 over Au/TiO2 samples. The adsorbed CO species can be easily removed from both samples, but faster on Au/TiO2 (NT) than on Au/TiO2 (CB) sample. In commonly accepted mechanism45 absorbed carbon monoxide species are lineally binded to metallic gold species on Au/TiO2. Oxygen does not dissociate directly on metallic gold surfaces. Most of the theoretical studies get rather large energy barriers for the direct dissociation of O2.48 Oxygen is most likely to be activated on TiO2 support via the charge transfer with surface lattice oxygen. Gold atoms at the perimeter interface between gold and TiO2 support may transfer charges to TiO2-adsorbed O2. (Figure 4.17) Gold atoms at the perimeter interface between gold and TiO2 support may transfer charges to TiO2-adsorbed O2. (The schematic reaction mechanism below (1) to (7) from G.C. Bond 45 ) First-principles calculations49 by A. Bongiorno have shown that the coadsorption of H2O and O2 leads to formation of a hydroperoxyl (HO-O)-like complex well bound to the gold catalysts. In other words the existence of chemically adsorbed H2O can enhance the O-O activation and hence the CO oxidation activity. The barrier for desorption of the CO2 product is 0.6 eV under dry condition while it is 0.3 eV with co-adsorbed H2O. Au0 + CO → Au0…CO (1) AuIII + OHs- → AuII…OH (2) Au0…CO + AuII…OH → AuII .COOH + Au0 (3) O2 + s- → O2- .s (4) AuII…COOH + O2-…s → AuII + CO2 + HO2-…s (5) AuII…COOH + HO2-…s → AuII + CO2 + 2OHs- + s (6) AuII + s- → AuIII + s- (7) 132 According to both DRIFT and in-situ XPS results, it can be found that there are two kinds of gold species in fresh Au/TiO2 (NT) sample (after 300oC air pre-treatment). The introduction of reactant gas (CO + O2) into the system caused two kinds of charge transfer over gold species. For the positively charged gold species (Au δ+), they experience charge transfer with support and gradually transformed to Au3+ during charge transfer process. The OH groups on the TiO2 support and metal surface are supposed to produce from the hydrolysis of adsorbed water.50 Eq. (4.2) shows the detailed reaction route for the production of surface OH group. H2O(g) + O(L)+ O(v) → OHad + OHad Eq. (4.2) where O(L) represents the lattice oxygen and O(v) denotes oxygen vacancies These positively charged gold species bound with surface –OH group, carbon monoxide molecules then arrive and react via hydroxycarbonyl ions, forming CO— Aun+--OH species (n = 0~3). Most gold species in our Au/TiO2 (NT) system were metallic gold particles. This was observed not only in fresh Au/TiO2 (NT) sample, but also in spent Au/TiO2 (NT) samples, Au/TiO2 (NT) after CO doses and Au/TiO2 (NT) after CO + O2 doses. So the reaction involved with these species was the main reaction occurring on the catalyst’s surface. All these metallic gold species shifted to lower binding energy after CO or CO + O2 doses. The OH group may be bonded to Au as Au(OH) or to Ti4+ as TiO(OH). It is hypothesized that the metallic gold bonded with weakly adsorbed CO and result in slightly negative charged Auδ-. Then these CO--(Auδ-) species reacts with Au(OH) or TiO(OH) to form AuCOOH that has been detected by DRIFT on Au/TiO2(NT) catalyst as caroxyglates at 1300-1600 cm-1. According to our DRIFT spectra, caroxylate or hydroxyl carbonyl may act as an intermediate for CO oxidation. In this model chemically bonded water or OH groups play important role in CO oxidation via carboxylate intermediates, AuCOOH. TiO 133 nanotubes have high oxygen vacancies and chemically bonded water due to the hydrothermal process. Au/TiO2 (NT) therefore show higher activity compared to Au/MC150 which has similar surface area. 4.5 Conclusion In this Chapter Au supported on six different types of TiO2 are studied. It has been demonstrated that, compared to the commercial anatase TiO2(CB) and kinds of TiO2 support from ISHIHARA SANGYO KAISHA Ltd, the TiO2 nanotubes are better catalyst support, on which the Au nanoparticles exhibit significantly improved catalytic activity for the CO oxidation at room temperature. Surface area is found to be an important factor for catalysts’ activity, since higher surface area samples show higher activity. However, the Au/TiO2(NT) and Au/MC150 catalysts have similar surface areas but exhibit remarkably different activity. A deeper study shows that TiO2 nanotube is a better support because it has smaller diameter ([...]... 83.9 eV for the fresh catalyst, and shifts slightly to ca 84 eV after reaction Peak analysis generates another band at 84. 8 eV for the spent sample (a ) 123 (b) Figureure 4. 14 Au 4f XPS spectra of the fresh (a) and spent (b) Au/ TiO 2 (CB) catalysts Figure 4. 14 Au 4f XPS spectra of the fresh (a) and spent (b) Au/TiO2 (CB) Table 4. 5 Comparison of Au 4f XPS results for fresh and spend samples of Au/TiO2... Figure 4. 15(b) and Table 4. 5 The Au 4f7/2 peak for the fresh sample is from both metallic gold ( 84. 3 eV) and positively charged gold ( 84. 7eV) (Figure 4. 15a) After exposed to CO+O2, the high oxidation state peak increases its intensity relatively 1 24 (a) (b) Figure 4. 15 Au 4f XPS spectra of the (a) fresh and (b) spent Au/TiO2 (NT) 125 These results indicate the existence of Au + even before the exposure of. .. Au/TiO2 (CB) and Au/TiO2 (NT) samples were anatase So we set the standard binding energy of Ti2p3/2 to 45 9.2eV .44 To determine the surface state of Au particles on TiO2 surfaces, the XPS Au 4f spectra was carefully recorded for the fresh and spent catalysts, as presented in Figure 4. 14a and Figure 4. 14b Detailed information for peak position, area and FWHM are presented in Table 4. 5 BE of Au 4f7/2 is ca... investigation and concluded that for the Au catalysts prepared by colloid-based methods the most important parameter is the grain size of the support. 47 Figure 4. 18 exfoliating-rolling model of TiO2 nanotubes formation from the layered Na2Ti3O7 particles by a hydrothermal chemical process The deposition of the colloidal gold particles as well as the oxidation activity is greatly enhanced in the case of supports... out and sample was for 1hour then transferred back to analysis outgas for 1hour then transferred back to analysis chamber chamber Scan for C1s, O1s, Ti 2p and Au 4f Scan for C1s, O1s, Ti 2p and Au 4f 4. 3 Results and Discussions 4. 3.1 Characterization of catalysts 107 Figure 4. 2 shows the TEM micrographs of the six Au/TiO2 samples after pretreatment at 300oC in air for 1 hour The morphology of the oxide... Catalytic Activity for CO Oxidation Reaction over the NanoGold Supported on Various TiO2 110 Before the reaction, all six samples were purple-pink color powder, and no changes in color after the reactions were observed Also, no obvious changes for the Au nanoparticles were found in the TEM micrographs of these samples before and after the CO oxidation reaction The CO oxidation on the six TiO2 samples without... min-1, 14. 7 psi) for 1 hour at 300oC, then cooled down to room temperature CO adsorption did not change the OH band of the Au/TiO2 catalysts However the CO+O2 adsorption 121 results in evident observation of Ti-O-H as well as the enhanced intensity of the 3300cm-1 band, indicating the participation of the surface H2O, or the interaction of O2 with H-bonded H2O Figure 4. 13: DRIFT spectra recorded in the. .. Although the bands at 1 647 and 1338 cm-1 also decreased, their absolute intensities remained similar before and after CO was introduced Thus, these bands were fairly stable spectator species 4. 3 .4 Electronic Structure of gold supported TiO2 catalysts The binding energies (BE) were calibrated using Ti2p3/2 peak in order to exclude surface charging effects According to XRD results (Figure 4. 4), the TiO2 supports... had greater bands intensity in this range This result confirmed the assumption of non-competitive adsorption, i.e the adsorption of carbon monoxide was not affected by the presence of oxygen Though it is rather weak the IR band at 2 141 cm-1 appears in the 4 spectra, and may be due to CO molecules adsorbed at Au + A few more weak bands between 2210 and 2150 cm-1 after the introduction of CO + O2 may... and Au/TiO2 (NT) catalysts Au/TiO2(CB) position area Au/TiO2(NT) FWHM position area FWHM Au4f 7 /4 fresh 3268.0 (100%) 0.7 84. 3 5180.2 (71%) 0.8 84. 7 spent 83.9 2 148 .2 (29%) 0.6 83.9 2202.7 (69% 0.9 84. 1 8080 (62%) 1.2 84. 8 10 24. 4 (31%) 1.9 85.3 49 80 (38%) 2.0 XPS Au 4f spectra and detailed peak position, area and FWHM information of fresh and spent sample of Au/TiO2(NT) sample are shown in Figure 4. 15 . involved. Curling and scrolling of the nano sheets can be observed in Figure 4. 1(E) and Figure 4. 1(F) (18hr). In Figure 4. 1(I) and Figure 4. 1(J) (24hr) of the hydrothermal process, formation of TiO 2 -derived. and roles of various factors that can influence the catalytic performances were identified. The surface area and morphology of the titanium oxide support influence the catalytic performance of the. Au/TiO 2 (CB) and there are not many differences for the surface area of samples before and after being pre-treated. Figure 4. 4 is the XRD patterns of the six TiO 2 supports. There are two kinds of TiO 2 structures