Chapter Oxidation of Carbon Monoxide over Nanogold Catalysts supported on various iron Oxides --Effect of Preparation Conditions on Catalytic Performances-- In this chapter, four kinds of gold/iron oxide catalysts, including AuCH, AuCM , AuCP, AuDP were prepared and used for CO oxidation reactions in the absence/presence of H2 respectively. It is found that the catalytic performance of the Au/iron oxide catalysts in the CO oxidations was influenced by a number of factors such as the calcinations temperature and the pre-reduction treatment, etc. For the AuDP and AuCP samples, CO conversion decreases with increase of the calcination temperature, while for the AuCM and AuCH catalysts, the influence of the calcination was less evident. Also, for all the tested catalysts, a higher reduction temperature resulted in a lower CO conversion. The effect of the reduction treatment on catalysts activity even exceeded that of the calcination temperature. On all the catalysts, the selectivity towards CO oxidation in H2rich environment (PROX) decreased appreciably with increase of the reaction temperature; however, the dependence of this selectivity on the pretreatment temperature was negligible. On the Au/iron oxide system the PROX reaction occurs through a Marsvan Krevelen type reaction mechanism, which involves lattice oxygen of the iron oxide and CO and H2 adsorbed on gold particles. The XRD, TPR, XPS and SIMS studies show the presence of OH and COO groups on the AuCH samples. The performance of Au/iron oxide catalysts towards PROX reaction was found to be strongly affected by catalyst 50 preparation and post-treatment. The colloid-based method can better control Au particle size and distribution. 3.1 Introduction The high activity of nanoparticulate gold (Au) catalysts supported on metal oxide (iron oxide and titanium oxide in particular) at room temperature or even lower temperature may lead to a revolution in our traditional understanding of heterogeneous catalysis, because it is very rare for synthesized catalysts to work at ambient environment, which is usually the case for enzymes. Despite that various reaction routes and mechanisms have been proposed, it has been generally agreed that the size of the Au particles and the interaction between gold nanoparticles and supports are very important factors that contribute to the extraordinary catalytic performance of supported nano-gold catalysts. Au particle exhibits good catalytic performances under mild conditions only when the gold particle size is smaller than nm.1-6 It is also reported that the optimum Au particle size for catalyzing CO oxidation reaction should be 2-4 nm. 7-8 Oxide supports may also modify the Au electronic structure via metal-support interaction. Moreover they may participate in activation of oxygen via adsorption at oxide vacancies. Therefore the interaction between Au nanoparticles and metal oxide support is very complicated, and has attracted a lot of attention. Different metal oxide supports seem to interact with Au nanoparticles differently, and the mechanism of one system might not be the same as the other system. Even for the same oxide support, preparation method may affect the Au particle sizes, the oxide support morphology and structure, and the presence of impurities in the system, all of which would possibly change the catalytic activity. 51 In this chapter iron oxide is selected as the support of the Au catalysts, and the effect of preparation methods on the oxide crystalline structure and oxidation state is studied carefully. Iron oxides are often classified as easily reducible oxides. Quite clear correlation between the reducibility of the support and the activity was found. Au supported on iron oxides is highly active in CO oxidation, better than ZrO2 (less easily reducible oxide) and Al2O3 (non-reducible oxide). It is reported that the catalytic performance of the Au/iron oxide system in the CO oxidation is related both to the gold state and the iron oxide phase. 9-12 Among three different phases of iron oxides, i.e.  Fe2O3, γ-Fe2O3 and Fe3O4, Haruta has shown that co-precipitated -Fe2O3 is more active than impregnated -Fe2O3 and impregnated γ-Fe2O3 due to the smaller size of gold particles.13-15 Šmit et al. indicated that the surface –OH group plays an important role in the CO activity over gold/ iron oxide system.16 CO may react with –OH groups forming very reactive adsorbed formates, HCOO(ad), which can be oxidized to carbon dioxide and water by lattice oxygen. Thus, the catalyst preparation and post-treatment conditions may affect the catalytic activity of the gold/ iron oxide markedly due to the change in the iron oxide crystalline structure, the amount of surface OH group and oxide ion vacancies. Supported Au catalysts were generally prepared by standard methods, namely coprecipitation (CP), deposition precipitation (DP), and colloid-based method (CB).17 Coprecipitation method involves simultaneous precipitation of HAuCl4 and metal nitrate by Na2CO3 (or NH4OH). Deposition-precipitation technique requires aging of an aqueous solution of HAuCl4 at temperature 50-90oC and a fixed pH value in the range of 6-10, which is selected based on the isoelectric point of metal oxide support, to enable selective deposition of Au(OH)3 only on the surface of the metal oxide, without 52 precipitation in the liquid phase. The third method, colloid-based method, is to impregnate oxide support with mono dispersed Au colloids stabilized by organic ligands or capping agents. The Au/oxides prepared by the above methods usually undergo subsequent drying and calcinations in air to obtain gold particles dispersed on oxides. The catalyst preparation as well as post-treatment conditions, e.g. precipitation temperature/pH value/time, aging temperature/time, calcination temperature/time etc, are important factors which may change Au particle size and the contact structure between the Au particles and the support. Providing that gold particles are small enough in the Au/iron-oxide prepared by various methods to be able to activate CO and O2, the CO oxidation activity is found to remarkably depend on the iron oxide structures. The CO oxidation activity of different iron oxide species was in the order: ferrihydrite > hematite > magnetite. 3.2 Experimental 3.2.1 Materials and catalysts preparation Au/iron oxide catalysts were prepared by co-precipitation (CP) or depositionprecipitation (DP), using HAuCl4 (sigma-aldrich) and Fe(NO3)3·9H2O (sigma-aldrich) as precursors. In the case of the co-precipitation (CP) method, an aqueous mixture of the HAuCl4 and Fe(NO3)3 precursors was poured into an aqueous solution of Na2CO3 (0.25M) which was maintained at 70oC under vigorous stirring (500 rpm). The precipitate was washed, dried, and calcined in air at 110oC for 12 hrs. This co-precipitation sample is coded AuCP. In the deposition-precipitation method, Au nanoparticles were deposited on iron oxide support by keeping the pH value of the aqueous solution of HAuCl at pH = 53 using 0.1M NaOH. The Fe2O3 support was generated, prior to the DP process, from 1.0 M Fe(NO3)3 solution. Excessive amount of 1.0M NaOH solution was added to the Fe(NO3)3 solution drop-wisely till all the iron ions in the solution were deposited. Then the mixed solution was thoroughly washed using DI water by centrifugation. The slurry after centrifuge was dried in 110oC oven for 48 hours. The above prepared sample was then calcined at 500oC for hour. The as-prepared iron oxide was mainly presented in Fe2O3 phase, with small amount of γ-Fe2O3 phase detectable by XRD. This self-prepared iron oxide sample was used as the support for the AuDP catalyst (The depositionprecipitation sample is coded AuDP). Two other samples, AuCH and AuCM were prepared using colloid-based method with assistance of the ultrasound irradiation17. The support used for AuCH was commercial Fe2O3 (hematite, Sigma-Aldrich), while that for AuCM was commercial Fe3O4 (Magnetite, Sigma-Aldrech). In colloid-based method Llysine was added as a capping agent, which has better control on gold particle size compared to conventional DP method used in literature. HAuCl4 (1mM) was reduced by NaBH4 (0.1M). During the reduction period, colloid-based method was applied. The nano-Au particles were deposited on iron oxide supports. The slurry was dried at 70ºC after centrifuge four times using DI water. As chloride ions is a poison to the catalytic reaction and may affect the activity of catalyst, the addition of capping agent and reduction agent and the followed washing procedure are able to remove almost of chlorine in the solution. 3.2.2 Evaluation of catalysts 54 Catalytic runs 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 testing, the catalysts were pre-treated in situ with a flow of air (100 ml min-1) for h at 200oC 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 balanced with helium, and was 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 CO productivity in a temperature range of 25-200 oC. Measurement readings were taken after the system had been stabilized for at least 15mins for every designated reaction temperature. The Conversion and Selectivity are calculated in terms of concentration: CO conversion (%) = Inlet CO concentration – Outlet CO concentration Inlet CO concentration x 100% CO2 conversion (%) = Inlet CO concentration – Outlet CO concentration x (Inlet O2 concentration – Outlet O2 concentration) x 100% For kinetics study, the catalyst was diluted with SiC powder. Absolute mass-specific reaction rates were calculated for the average concentration of each component ċi, at the in- and outlet of the reactor; mAu, mass of Au in the reactor bed; V, total molar flow rate; XCO, conversion of CO on the basis of CO2 formation; ċCO, concentration of CO in gas 55 mixture, equal to pi/p0; pi, partial pressure of reactants; p0, total pressure in the system (3.1). The mean particle size (D) gets from (3.2). rCO = ċCO,in XCO Vgas mAu [moles·s-1gAu-1] D=∑ nidi3/∑ nidi2 3.2.3 (3.1) (3.2) Characterization of catalysts Powder X-ray diffraction (XRD) patterns were recorded at room temperature on a Bruker D8 Advance Diffractometer using a Cu Kα radiation source. Diffraction angles were o o measured in steps of 0.015 at s/step in the range of 10-80 (2θ). Transition electron microscope (TEM) measurements were performed on a Tecnai TF 20 S-twin instrument with a Lorentz lens. The samples were ultrasonically dispersed in ethanol solvent and then were dried over a carbon grid for measurements. The average size of Au particles and its distributions were estimated by counting about 300 Au particles. The Au and Fe contents of prepared catalysts were determined by X-ray fluorescence multi-elemental analyses (XRF) on a Bruker AXS S4 Explorer. Temperature programmed reduction (TPR) studies were performed in a continuous-flow fixed-bed quartz micro-reactor (I.D. mm) with 50 mg of samples. The catalyst was first heated in a flow air at 200, 300 or 400 oC for 60 min. After cooling to room temperature, the feed gas was switched to 5%H2/Ar. After the baseline had been stabilized, the temperature was increased to 600oC at a heating rate of 10 oC/minute. The amount of H2 56 consumed was measured as a function of temperature by means of a thermal conductivity detector (TCD). Thermogravimetric Analysis was conducted using TA Instruments SDT 2960 Simultaneous (DTA-TGA), under nitrogen (flow rate= 70ml/min) at a heating rate of o 20 C/min. X-ray photoelectron spectroscopy (XPS) was performed on a VG ESCALAB XPS, ESCA MK II using Mg Kα (1254.6 eV) light source under UHV better than 3×10-9 torr. The in-situ XPS experiments were performed in a UHV chamber at the SINS beamline of the Singapore synchrotron light source (SSLS) at National University of Singapore.18 XPS spectra were measured using a hemispherical electron energy analyzer (EA 125, Omicron NanoTechnology GmbH). The XPS experiments were done at normal emission, and the photon energy resolution for the experiments was about 0.5 eV. XPS measurements were done at constant pass energy mode with overall energy resolution. Table 3.1 summarizes the experimental procedure for CO oxidation in-situ XPS study. The same scan time on each sample was maintained. Table 3.1 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, Fe 2p and Au4f  Transfer the samples back to pre-treatment chamber 57 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, Fe 2p and Au4f Time-of-flight (TOF) secondary ion mass spectrometry (SIMS) analysis was performed on VG SIMSLAB incorporating a duoplasmatron ion gun and a VG M12-12 quadrupole mass spectrometer, in the mass range (m/z) from to 800. The VGX 900 software was used to control the experiments and analyze the data. During the analysis samples were neutralized by an electron flood gun of 500 eV energy with a maximum current of A. A positive bias potential of around 10 V was applied to obtain a maximum secondary ion count. Ar+ ion was used for analysis with 10keV energy and pA beam current to ensure that the operation is in static SIMS mode. The scan size was 300m and scan time 800s. 3.3 Results and Discussions 3.3.1 Crystalline structure of various Au on iron oxide samples: XRD Characterization The effect of catalyst preparation and post-treatment on the crystalline structure of Au/iron oxide samples has been studied by XRD. Figure 3.1 compares the XRD patterns of the four kinds of the Au/iron-oxide (AuCH, AuCM, AuCP and AuDP) samples after calcination at 300◦C. The AuDP, AuCH and AuCM samples show well crystallized iron 58 oxide structures after calcined at 300oC, while the co-precipitate AuCP sample remains poorly crystallized. It is noticed that AuCP is the only sample that does not exhibit the Au peak at c.a. 2=38o, and its α-Fe2O3 peaks are weak in intensity even after a calcination at 300oC. Pattern a in Figure 3.1 shows five distinct peaks which are identified as magnetite, Fe3O4, for AuCM catalyst. The AuCH (line b) and the AuDP (line d) samples contain mainly -Fe2O3 (hematite) and iron oxide hydrate (γ-FeO(OH), Lepidocrocite), The Au signal is strong for AuCH and rather weak for AuDP. To investigate the effect of calcinations to the crystalline structure of our samples, XRD patterns before and after calcinations are presented in Figure 3.2 and Figure 3.3 for AuCP and AuDP samples respectively. As can be seen from Figure 3.2, the “as-prepared” co-precipitate AuCP catalyst is poorly crystallized, while after calcinations at 400oC for hour, four peaks with quite low signal to noise ratio are visible and identified as the presence of α-Fe2O3 (hemitate). A peak that is attributed to metal gold at ca. 38 o was observed at the AuCP sample after 400oC calcinations. Note that the AuCP sample did not exhibit Au peaks even after calcined one hour at 300oC, the Au reflection was identified only after 400oC calcination. The FeO(OH) peak, Lepidocrocite, at ~64o is detectable after the calcinations, though it is not very strong. 59 (a) (b) Figure 3.19 XPS Au4f spectra of the AuCH samples before (a) and after (b) CO+O2 dose Table 3.3 XPS Au4f data of the AuCH sample before and after CO+O2 dose Position (eV) 3.3.8 AuCH FWHM (eV) Au species (%) Before dose 84.1 1.6 Au0 (100) After dose 82.1 83.7 85.2 0.9 1.55 0.87 Auδ- (5.7) Au0 (85.5) Auδ+ (8.8) TOF- SIMS Studies of O2 adsorption on AuCH To investigate the surface chemical composition of gold/ iron oxide system, TOF-SIMS was carried out over the AuCH sample, the best catalyst among the four gold /iron oxide 81 samples. Because the intensity of positive Au-containing secondary ions was rather low, only negative secondary ion spectra were displayed in Figure 3.20 to illustrate the surface species on the AuCH samples. O OH (a) 100000 Amounts O2 10000 FeO2 FeO FeO3 1000 O18 50 100 200 220 240 Mass(m/z) Au (b) Amounts 1000 AuOH 100 AuO2H AuO2 AuO AuOH*H2O 10 Au*2H2O Au*H2O 200 210 220 230 240 mass(m/z) Figure3.20 SIMS spectra for the AuCH sample 82 The strong peaks at m/z = 16, 17, 72, 108, 124, and 197 are due to O, OH, FeO, FeOO, FeOOO and Au are respectively. The abundance of 56 Fe isotope is 91.7%, much higher than other iron isotopes such as 54 Fe (5.8% only), whose peaks are ignored in Figure 3.20 (a). It is reasonable to detect the FeO clusters on Au/iron-oxide surface. But the great abundance of OH species is very important structural feature for Au/oxide surfaces. In Figure 3.20 (b) most important Au-related peaks are listed. Peaks of 213, 214, and 216 are ascribed to AuO, AuOH and AuOH3, respectively, while those at 229, 230 and 232 are due to AuO2, AuO(OH), and Au(OH)2, respectively. It can be seen that the majority of gold species on the AuCH sample surface is gold atoms while a small parts of gold are attached to oxygen (AuO or AuOO). In particular Au(OH) is also detectable at the concentration comparable to AuO, and there exist AuO(OH) and Au(OH)2. The gold atoms in direct contact with oxygen ions or OH group, i.e. AuO, AuOH and AuO in total counts for 8% of all gold species. This SIMS result is agreeable to the structure model derived from the above TPR, XRD and XPS study. As indicated in literature 31, 70% of Au particles on oxides (with Au particle size between and nm) are to layers in thickness, while only 8% of Au particles are small (2-3 nm) and 2-3 layers thick. It is the latter portion of Au particles (2-3 nm in diameter and 2-3 atomic layer in thickness) that have direct contact with oxide support and would contribute to detectable AuOx species with static SIMS. Very recently A.A. Herzing and G.J. Hutchings 32 disclosed that only those bilayer subnanometer Au clusters were active for CO low temperature oxidation whereas the monolayer clusters, individual Au atoms as well as large Au particles were inactive. The smaller particles have larger portion of low coordination number Au atoms which possess a d-band that is closer to the Fermi level so they can adsorb O2 molecules 83 more readily. Also the smaller particles have higher degree of hydroxylation, and have higher portion of non-metallic Au species. Hence they are more active in CO low temperature oxidation. The SIMS data as well as the above XPS, XRD data can well support this model. To study the O2 adsorption on iron oxide supported Au catalysts, AuCH sample was exposed to the 10% O2 in helium gas at 10-6 mbar for 10 minutes. After the exposure to O2, the AuO- and AuO2- ion clusters on the O2-exposed AuCH sample are c.a. 1.7 times of that without O2 dose, while the Au signal increases to 1.3 times after O2 dose. (Figure 3.21-Figure 3.24) On the other hand the AuOH species was only 0.8 that of the sample before O2 dose. The increase in Au signal may be caused by the oxygen on the surface. However the relative increase in AuO or AuO2 intensity after the O2 exposure is the evidence of oxygen adsorption at Au sites. Amount after O2 / Amount before O2 2.0 AuO-2 AuOAu* H2O AuO2H 1.5 AuOH*H2O Au 1.0 AuOH 0.5 197 213 214 216 229 230 232 Mass(m/z) Figure 3.21 The intensity ratio of gold related SIMS speaks before and after O2 exposure for the AuCH sample 84 2.0 Fe2O4 Amount after O2 / Amount before O2 FeO3 1.5 Fe2O3 FeO2 Fe 1.0 FeO 0.5 25 30 35 40 45 50 Mass(m/z) Figure 3.22 The intensity ratio of iron related SIMS speaks before and after O exposure for the AuCH sample O Au 100000 AuO2H 1000 Amounts OH AuO FeO2 O2 AuOH 100 AuOH*H2O Au*H2O 10 Au*2H2O 195 Amounts AuO2 200 10000 205 210 215 220 225 230 235 mass(m/z) FeO3 FeO 1000 H2O 50 100 200 mass(m/z) Figure 3.23 SIMS spectra for the AuCH sample after O2 dose 85 3.4 Colloids-based method for preparing active Au/iron-oxide catalysts is better than traditional co-precipitation and deposition-precipitation The catalytic data reported in the present chapter show that the low temperature CO oxidation over the Au/iron oxide catalysts strongly depends on both the preparation method and the thermal pretreatment of the catalyst. Different preparation method and thermal pretreatment conditions can lead to different support crystalline structure, Au particle size distribution, gold species oxidation states and iron-oxide hydroxylation etc. In our system, the support crystalline structure might be an important factor that determines the catalytic performance of the gold/ iron oxide samples. This hypothesize is based on the fact that all our samples showed similar gold particle size distribution. In terms of iron oxide support crystalline structure, Lepidocrocite (γ-FeO(OH)) > hematite (α- Fe2O3) > Magnetite (γ-Fe2O3). Also, more importantly, support with well crystallized structure is better than amorphous or poorly crystallized support. Note that the above conclusion is based on the condition of similar gold particle size distribution. The asprepared samples are not discussed in details in this chapter because both reducing agent and capping agent were added during the preparation and they must be removed by heat treatment to get stable catalysts. The fact that as-prepared AuCP sample showed much better catalytic activity than the sample after heat treatment might be due to small gold particles in the as-prepared sample, which were rather small in size and amorphous in structure, thus make it undetectable for XRD and TEM.28 30 Unfortunately the as- prepared AuCP catalysts were not stable, and deactivated much faster than AuCH and AuDP. The AuCH and AuDP samples after heat pretreatment at 200oC and 300oC have shown very good activity. In particular AuCH is rather stable for long hour operation, 86 indicating the colloid-based method is better than deposition-precipitation and coprecipitation in the present studies. G.C.Bond et al. also gave out similar conclusions.33 The CO conversion of both CO oxidation reaction and PROX reaction over AuCP and AuDP samples decreased after reaching 100% conversion, while the CO selectivity of RROX reaction remained similar as calcination temperature increases. For the AuCH and AuCM samples the calcination temperature did not have very much influence over the CO conversion of both reactions as the AuCP and AuDP sample do. Similarly, the CO selectivity of PROX reaction was not affected much by the samples’ calcinations temperature. This high temperature stability may be derived from the application of capping agent lysine. Heating treatment may decompose lysine, leaving COO- fragment on the catalyst surface. The presence of COO- species were shown in XPS O1s spectra for the AuCH and AuCM samples, with a very strong peak at 533 eV in addition to the main peak at 530 eV. The 533 eV peak was not observed on AuCP and AuDP. The COO group could anchor Au particles strongly so that the dispersion and stability of Au particles are remarkably better on AuCH than AuCP and AuDP. Characterization experiments carried out over Au/iron oxide catalysts at different calcination temperature aimed to clarify the influences induced by the thermal pretreatment on the support , gold nanoparticles and in turn the influence on the catalytic activity towards CO oxidation and PROX reaction. XRD profiles of the AuCP catalyst calcined at 300 ◦C and 400◦C have shown that the sample has a structure with a low crystallinity with the presence of some α-Fe2O3 hematite phase. As we proposed earlier, most of iron oxides in the AuCP samples (as-prepared and calcined) are present as amorphous ferrihydrite. The absence in the as-prepared and the 300◦C-calcined AuCP 87 samples of XRD patterns due to gold can be probably justified considering that the preparation method used leads to Au cation species and amorphous phase Au particles and with a mean particle size smaller than nm. At higher calcinations temperature (400 ◦ C) hematite is the prevailing crystallographic phase of the AuCP sample indicating that a phase transition of the iron oxide (from ferrihydrite to hematite) mainly occurs between 300 ◦C and 400 ◦C. It must be also noted that on the 400◦C-calcined AuCP sample a diffraction peak related to the presence of metallic gold (with a mean size of 5.3 nm from TEM) was found. On the basis of these observations it can be stated that high calcination temperature for the AuCP sample lead both to a change in the iron oxide phase and to an enlargement of gold particles. And both these effects can be claimed to explain the drop of the CO oxidation activity observed when Au/iron oxide samples were calcined at high temperatures (400 ◦C). Magnetite (γ-Fe2O3) being much less active than hematite (Fe2O3). A higher activity of hematite compared to magnetite in the CO oxidation has been already reported in the literature.14,15 By comparing the H2-TPR results of pure iron oxides (Figure 3.7) and gold/ iron oxides samples (Figure 3.8 and Figure 3.9), it can be concluded that interaction between gold and iron oxide occurs. Gold catalysts enhance the reducibility of iron oxide/hydroxides. Both the reduction of ferrihydrite to hematite and hematite to magnetite are enhanced. But the reduction of magnetite to Wüstite remains almost the same. This result might induce that the doping of gold might weaken the bonding of surface Fe–O bonds, and this will lead to higher surface lattice oxygen mobility for the gold/ iron oxides sample. Considering the results of H2-TPR, XRD and catalytic activities, the conclusion could be obtained that for the gold/ iron oxides samples with similar gold particle sizes and 88 distribution, the iron oxide support greatly influences the catalytic performance of CO oxidation and PROX reaction over gold/iron oxide systems. In terms of support crystalline structure, hematite > magnetite. Ferrihydrite is written as 5Fe2O3•9H2O or as Fe2O3•2FeOOH•2.6H2O, and only exists as a nanomaterial, showing crystals 2-4 nanometers wide for 2-line ferrihydrite and 5-6 nanometers wide for 6-line ferrihydrite. Ferrihydrite is a metastable mineral. It is known to be a precursor of more crystalline minerals like hematite and goethite. Thus, ferrihydrite is less likely to be detected by XRD. Lepidocrocite, also called hydrohematite, was detected over the AuCH and AuDP sample calcined at 300oC and 400oC. It is well known that OH group at Au surface or Au/oxide boundary plays key role in CO oxidation. This may explain why AuCH is the best catalyst, and colloids-impregnation is better preparation method than DP and CP. The in-situ TOF-SIMS experiment carried out in this chapter has confirmed the presence of large amount of OH groups on AuCH surface. The investigation also shows that both Au atoms and FeOx can be the active sites for oxygen adsorption over the AuCH sample. For the CO oxidation reaction over gold supported catalysts, the difficulty in detect oxygen active site lie in the fact that oxygen molecules are weakly bonded over gold/ metal oxide systems at room temperature, and the dissociative chemisorption of oxygen is thermodynamically prohibited. The main reason for the weak bonding of oxygen on gold is due to the Pauli repulsion between the O valence states and the metal d states.29 From the SIMS results, it can be seen that after O2 dose more AuO- and AuO2- species were detected compared with that without O2-dose. As for the supports for gold/ iron oxide support; more FeO3- and Fe2O4 species were detected after O2 dose. These data support the hypothesis that on the Au/iron oxide system the PROX reaction occurs 89 through a Mars-van Krevelen type reaction mechanism which involves active lattice oxygen species of the support reacting with CO adsorbed on gold particles and/or at the metal–support interface. This also agrees with the fact that in the absence of oxygen in the feed we observed that the reaction proceeds for a very short time and then the activity drops to zero. Iron oxide support being proposed as responsible for the activation of oxygen. It is important to stress the fact that the role of the iron oxide support is crucial, provided that gold particles are small enough to be able to easily activate CO. In fact, when the dimension of gold particle increases the activation of CO on gold becomes so far more difficult and at certain points presumably the rate determining step in the CO oxidation. Only in this case, therefore, the status of gold should be determinant in directing CO oxidation and PROX activity. In what concern PROX selectivity of investigated Au/iron oxide samples data point out that on all catalysts selectivity decreases with the reaction temperature exhibiting a trend which is similar on all samples. Moreover, at the same reaction temperature selectivity was roughly independent from the level of conversion (Figure 3.11-3.14). It is also important to stress the fact that the selectivity of different samples resulted to be almost unaffected by calcination condition, being only related to the reaction temperature employed. Considering that pretreatment conditions were found to strongly affect mainly the status of the support, the observed independence of selectivity on pretreatment conditions suggests that the activation of CO and H2 on investigated Au/iron oxide catalysts is not substantially affected by the iron oxide phase. This is quite reasonable considering that both CO and H2 are adsorbed/activated on gold particles. According to the above proposed reaction mechanism for PROX, when gold 90 particles are small enough, the rate determining step is the activation of oxygen. Therefore, an increase in the oxygen reactivity results in a higher activity in the oxidation of both CO and H2, thus being indifferent for the CO2 selectivity, which only depends from the reaction temperature. On the basis of these considerations, AuCH can be considered more suitable than AuDP and AuCP for a practical use in the PROX reaction in so as the lower temperature at which the reaction can be carried out on these more active samples leads a higher selectivity than that achievable on the less active AuDP and AuCP samples. The colloids-based preparation of oxide-supported nano-Au catalysts using Lysine (HO2CCH(NH2)(CH2)4NH2 as capping agent was invented in our lab.19 Au exists in colloids in its metallic state, and the Lysine molecules always cap the Au colloids so that it is easier to control the Au particle size in colloidal form than in supported catalysts. Furthermore the typical contamination source Cl-, which can poison the catalyst and cause agglomeration of Au particles during heat treatment, can now be separated from the colloids, and easily removed by washing. The capping molecules, lysine can be easily removed by washing too, and by the subsequent heating treatment at 300 oC. The residual fragment COO group resulting from heating can strengthen the Au-support contact, anchoring nano-Au particles strongly and prevent them from agglomeration. Moreover the COO or OH groups may participate in the CO oxidation, improving the catalytic performance of nano-Au. (a) (b) 91 Figure 3.24 (a) Capped Au Colloids with Adsorbed Anions and Capping Molecules; (b) Au colloids in solution 3.5 Conclusion The performance of Au/iron oxide catalysts towards PROX reaction was found to be strongly affected by catalyst preparation and post-treatment. Among the three methods studied in this chapter, colloid-based (CB) method is better than Co-Precipitation (CP) and Deposition-Precipitation (DP). The AuCH sample prepared by colloid-based method, shows high activity, selectivity and stability at temperatures between RT and 100oC. The AuDP sample exhibits increasing activity with increasing temperature from room temperature and reaches 100% conversion at c.a. 50oC, then experiences a drop in activity as temperature is kept increased. While the AuCP and AuCM samples are not active at low temperatures. The use of colloid-based method can better control Au particle size and distribution. Providing that gold particles are small enough in the Au/iron-oxide prepared by various methods to be able to activate CO and O2, the CO oxidation activity is found to 92 remarkably depend on the iron oxide structures. The CO oxidation activity of different iron oxide species was in the order: ferrihydrite > hematite > magnetite. The XRD, TPR, XPS and SIMS studies show the presence of OH and COO groups on the AuCH samples. Moreover non-metallic Au has been detected on the AuCH sample in addition to the metallic Au. All these would contribute to the enhanced activity of the AuCH sample. 93 Reference 1. Q. Fu, H. Saltsburg, M. 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Eng. 41 (1999) 319 96 [...]... area of the three samples AuCP, AuDP and AuCH are not distinctly different, while BET of AuCM is ~4 times lower Table 3. 2 Au wt% in three kinds of gold iron oxide samples from XRF, BET results of four kinds of gold supported iron oxide samples AuCM (Au wt%) AuCH AuDP 3. 1% 3. 9% 3. 0% 3. 4% 10 .3 XRF AuCP 267.5 33 .4 136 .9 7.4 36 .5 27.4 31 .2 6.8 29.8 25.1 26.7 As-prepared As-prepared BET Pre-treated 2 at 473K... that of H2 oxidation These results (Fig 3. 11- Fig 3. 15) indicate that the CO oxidation activity of Au/iron oxides systems is strongly affected by the iron oxide support crystalline structure and the method used to prepare the catalyst In terms of iron oxide crystalline structure, FeO(OH) + Fe2O3 is better than Fe2O3 than Fe3O4 3. 3.7 Electronic Structure of gold supported on Fe2O3 – XPS investigations The. .. The peak at 38 7 and 620 o C can be assigned to the reduction of Fe2O3 Fe3O4 and Fe3O4 FeO Fe respectively.21,22 66 H2 consumpsion 9000 a: FeO b: Fe2O3 c: Fe3O4 620 b 38 7 c 6000 508 30 00 a 0 200 400 600 Temp(oC) Figure 3. 7 TPR profiles of commercial FeO, Fe2O3 and Fe3O4 samples Figure 3. 8 displays TPR profiles of AuCP (left) and AuCH (right) samples calcined at different temperatures On the AuCP sample... Figure 3. 9, in which three TG peaks correspond to the dehydration of various OH groups at 1 73, 231 , 285 and 33 0 oC respectively 68 100 200 30 0 400 500 600 700 800 100 5 o 1 73 C 0 -5 Weight loss(%) 231 oC -10 90 285oC -15 -20 85 Heat Flow(mW/mg) 95 -25 -30 80 100 200 30 0 400 500 600 700 -35 800 Temperature(oC) Figure 3. 9 Thermogravimetry (TG) and differential thermal analysis (DTA) of the AuCP sample The. .. 573K 3. 3.4 Reducibility of iron oxide supports: H2-TPR Characterization The effect of catalyst heating treatment on the nature of Au/iron oxide samples was studied by H2-TPR As a reference, Figure 3. 7 presents the H2-TPR results of pure commercial iron oxides, FeO (curve a), Fe2O3 (curve b) and Fe3O4 (curve c) The peak at 508oC for FeO is attributed to the reduction of FeO to Fe0 20 The peak at 38 7... Au/ -Fe2O3 provided by the World Gold Council (WGC).20 The AuCH sample is the best catalyst, while AuCM is the worst The preparation method and conditions for AuCM and AuCH are identical except that -Fe2O3 is used in the preparation of the AuCH sample, whereas smaller BET surface area Fe3O4 was used as the support for the AuCM sample It can be concluded that the preparation method, and hence the iron... Fe2O3 to Fe3O4, 2Fe3+O - OH + 2H2 This is consistent with literature 19 Fe3+O + Fe2+ + 3H2O in which the TPR peak is located at 150 -35 0oC The peak at 650oC in Figure 3. 8 (left) is obviously due to the reduction of Fe3O4 FeO Fe as that happens in pure Fe2O3 sample For the AuCP which was calcined at 30 0-400oC, the 200oC peak shifts to 250 -30 0oC This is easy to understand by referring to the TG/DTA profiles... further understands the electronic structure of the gold/ iron oxide in the presence of CO+O2 Figure 3. 19 shows the XPS Au 4f spectra of the AuCH sample after exposing to 5 torr of CO+O2 mixed gas for 5 seconds Detailed peak analysis data are given in Table 3. 3 For the clean AuCH sample before CO+O2 dose, the Au 4f7/2 is located at 84.1 eV, and is attributed mainly to Au0 After the gas adsorption the. .. XRF and BET results of these for gold/ iron oxide samples are listed in Table 3. 1 The gold wt% content of these Au/iron samples are all around 3. 0 -3. 9% according to the x-ray fluorescence (XRF) results It is noted that the surface area of the four as-prepared samples is very different, decreasing in the order: AuCP > AuDP> AuCH > AuCM Nevertheless after calcination in air at 30 0 oC for 1 65 hour the. .. the AuO- and AuO2- ion clusters on the O2-exposed AuCH sample are c.a 1.7 times of that without O2 dose, while the Au signal increases to 1 .3 times after O2 dose (Figure 3. 21-Figure 3. 24) On the other hand the AuOH species was only 0.8 that of the sample before O2 dose The increase in Au signal may be caused by the oxygen on the surface However the relative increase in AuO or AuO2 intensity after the . kinds of gold supported iron oxide samples AuCM AuCP AuCH AuDP XRF (Au wt%) As-prepared 3. 1% 3. 9% 3. 0% 3. 4% BET (m 2 /g) As-prepared 10 .3 267.5 33 .4 136 .9 Pre-treated at 473K 7.4 36 .5 27.4 31 .2 Pre-treated at. type reaction mechanism, which involves lattice oxygen of the iron oxide and CO and H 2 adsorbed on gold particles. The XRD, TPR, XPS and SIMS studies show the presence of OH and COO groups on the. a: FeO b: Fe 2 O 3 c: Fe 3 O 4 a b c 38 7 508 620 Figure 3. 7 TPR profiles of commercial FeO, Fe 2 O 3 and Fe 3 O 4 samples Figure 3. 8 displays TPR profiles of AuCP (left) and AuCH (right) samples