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Low-Temperature CO Oxidation on Au/FexOy Catalysts Hsin-Yu Lin and Yu-Wen Chen* Department of Chemical and Materials Engineering, Nanocatalysis Research Center, National Central University, Chung-Li 32054, Taiwan
Supported gold catalyst has been reported to have high CO oxidation activity at low-temperature. The aim of this study was to develop a Au/FexOy catalyst that is active under ambient conditions, is low-cost, is resistant to moisture, and has high stability. Iron oxide was chosen because it is cheap and resistant to moisture. A method to prepare a FexOy support that has a higher surface area and abundant hydroxyl groups on the surface was developed. The FexOy was prepared with a very low feeding rate of FeCl3 (10 mL/min), at a constant pH of 10, was dried at 120 °C, and had a very high surface area with abundant hydroxyl groups. The surface area of FexOy was as high as 400 m2/g. It was used as a support for gold in low-temperature CO oxidation. Supported gold catalysts were prepared by deposition-precipitation (DP) using HAuCl4 as the Au precursor under various pH values and calcined at various temperatures. The catalysts were characterized by atomic absorption spectroscopy, X-ray diffraction (XRD), N2 sorption, transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). XRD results showed that gold metal had a particle size under the detection limit, which was less than 4 nm. TEM images confirmed that the particle sizes of gold for all the catalysts were less than 4 nm. XPS spectra showed that Au is in metal state Au0. The method applied in this study leads to a fairly uniform dispersion of gold nanoparticles with diameter less than 4 nm and narrow size distribution. The catalytic activity of CO oxidation under ambient conditions was measured using a fixed bed continuous flow reactor. It demonstrated that pH value and calcination temperature play key roles in creating high catalytic performance for the Au/FxOy catalyst. The catalyst prepared by the DP method at pH 9 and calcined at 180 °C gave the highest activity. In addition, this catalyst was very resistant to moisture. Full conversion was kept at ambient temperature with a gas hourly space velocity of 40 000 h-1 over 2000 h on stream. 1. Introduction Gold has long been regarded as a poorly active catalyst. A theoretical calculation has explained that the smooth surface of Au is noble in the dissociation adsorption of hydrogen.1 However, when Au is deposited as nanoparticles on metal oxides by means of coprecipitation or deposition-precipitation techniques, it exhibited surprisingly high catalytic activity for CO oxidation at a temperature as low as 200 K. This finding has motivated many scientists and engineers to investigate the catalysis of Au since the 1990s. Many excellent reviews have been reported.2-10 Date´ et al.11 reported that Au/TiO2 is a very good catalyst for CO oxidation under ambient conditions. The best performance was obtained for a catalyst calcined at 200 °C and left at room temperature for a few days. The activity for CO oxidation was greatly influenced by moisture in the reactant gas, and a maximum was observed at around 200 ppm. Tabakova et al.12 reported that the presence of gold enhanced the formation of a nonstoichiometric γ-Fe2O3 spinel phase, which manifests a higher catalytic activity in the water-gas shift reaction compared to magnetite. The amorphous material may contain many surface defects that should also strengthen Au-support interactions and, therefore, prevent gold from agglomerating into large gold particles. Nanosized gold particles have been reported extensively to be an active catalyst for CO oxidation and * To whom correspondence should be addressed. Fax: (886)3-4252296. E-mail:
[email protected].
preferential oxidation of CO in a hydrogen-rich stream at low temperature.13-36 The suitable supports are the metal oxides that can be partially reduced, such as TiO2, Fe2O3, Co3O4. Many metal oxide supports have been used for CO in CO oxidation. The best supports reported in the literature are Fe2O3, TiO2, and Co3O4. Iron oxide was chosen in this study because it is cheaper than TiO2. Although iron oxide has been extensively used as a support of gold, most of the Au/Fe2O3 catalysts reported in the literature36-40 were prepared by coprecipitation, and the surface area of iron oxide was low (200 m2/g), and abundant hydroxyl groups on the surface, it would be a good support for gold in CO oxidation. In a previous paper,16 one of the authors has reported the high activity of Au/FexOy in CO oxidation under ambient conditions. However, the preparation method to have high surface area was not reported. In addition, the effects of preparation and pretreatment conditions on the properties of Au/FexOy catalysts were not investigated. The aim of this study was to develop a method to prepare iron
10.1021/ie0491488 CCC: $30.25 © 2005 American Chemical Society Published on Web 05/18/2005
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oxide support that has high surface area, small particle size, and abundant hydroxyl groups and use it as a support for gold. The Au/FexOy catalysts were tested for CO oxidation under ambient conditions. The effects of preparation parameters and pretreatment temperature on the characteristics of Au/FexOy were investigated. Haruta et al.7,8 reported that noble metal catalysts are mostly prepared by impregnation methods, which, however, cannot produce active gold catalysts. Therefore, an impregnation method was not employed in this study. All of the Au catalysts were prepared by the deposition-precipitation method in this study. 2. Experimental Section 2.1. Chemicals. Reagents used were analytical grade. FeCl3‚6H2O and NH4OH (Showa Chemicals Co.) were used as the starting materials for the preparation of the FexOy support. HAuCl4 (Showa, 99.0%) was used as the precursor for Au. 2.2. Catalyst Preparation. 2.2.1. Preparation of Iron Oxide Support. Iron oxide was prepared by a precipitation method. In a typical procedure, an aqueous solution of the iron salt FeCl3 (0.1 mol in 200 mL of H2O) was added drop by drop (10 mL/min) to an aqueous solution of NH4OH (15%) under vigorous stirring. The pH of the mixture was maintained at a fixed value (from 7 to 12). After 2 h of aging at room temperature, the brown precipitate was filtered and washed several times with distilled water until chloride disappeared. The resultant iron hydroxide was dried at 110 °C and calcined at various temperatures between 110 and 300 °C for 4 h. 2.2.2. Preparation of Gold Catalysts. Supported gold catalysts were prepared by the deposition-precipitation (DP) method. HAuCl4 solution was poured at a rate of 10 mL/min into a solution of FexOy under vigorous stirring; the temperature of solution was maintained at 60 °C. An ammonia solution (1 M) was used to adjust the pH value. Various pH values were tested. After 2 h of aging at room temperature, the precipitate was filtered and washed with hot water until no Cl- was detected and then was dried at 110 °C to obtain gold catalyst. The cake was treated at different temperatures. Date´ et al. reported that the particle size of gold on the metal oxide supports can be controlled by the selection of a suitable temperature. The Au catalysts in this study were pretreated at low temperatures below 200 °C with a heating rate of 30 °C/min; the temperature was then maintained for 12 h. The temperature was kept below 200 °C to prevent sintering of the gold metal. The preparation procedure allowed one to obtain good reproducibility in all Au/FexOy catalysts. The catalyst is denoted as Au(x)/FexOy where x is the pH during DP method. 2.3. Characterization. The catalysts were characterized by atomic absorption spectroscopy, N2-sorption, powder X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). 2.3.1. Atomic Absorption. The Au loading was determined by atomic absorption spectroscopy (AAS) using a Perkin-Elmer 3100 with an air/acetylene flame. For that purpose, the catalysts were dissolved in hot aqua regia. Subsequently, the solution was cooled and diluted with demineralized water. 2.3.2. N2 Sorption. N2-sorption isotherms were measured at -197 °C using a Micromeritics ASAP 2010
instrument. Prior to the experiments, the samples were degassed at 100 °C until the vacuum pressure was below 0.1 Pa. The measurement of the surface areas of the samples was achieved by the Brunauer-EmmettTeller (BET) method for relative pressures in the range P/P0 ) 0.05-0.2, assuming a cross-sectional area of 0.162 nm2 for the N2 molecules. 2.3.3. XRD. The XRD experiments were performed using a Siemens D500 powder diffractometer. The XRD patterns were collected using Cu KR1 radiation (0.154 05 nm) at a voltage and current of 40 kV and 30 mA, respectively. The sample was scanned over the range 20-80° 2θ at a rate of 0.05° min-1 to identify the crystalline structure. Samples for XRD were prepared as thin layers on a sample holder. 2.3.4. TEM. The morphologies and particle sizes of the samples were determined by TEM on a JEM-1200 EX II operated at 160 kV. Initially, a small amount of sample was put into the sample tube filled with a 95% ethanol solution. After agitating under ultrasonic environment for 10 min, one drop of the dispersed slurry was dripped onto a carbon-coated copper mesh (300#) (Ted Pella Inc.) and dried in an oven at 100 °C for 1 h. Elemental analysis was carried out by means of a buildin Oxford EXL II EDS system. Au catalysts were measured in order to obtain a good statistical particle size distribution. 2.3.5. XPS. XPS spectra were recorded with a Thermo VG Scientific Sigma Probe spectrometer. The XPS patterns were collected using Al KR radiation at a voltage and current of 20 kV and 30 mA, respectively. The base pressure in the analyzing chamber was maintained on the order of 10-9 Torr. The spectrometer was operated 23.5 eV pass energy. The binding energy of XPS was corrected by contaminant carbon (C1s ) 285.0 eV) in order to facilitate the comparisons of the values among the catalysts and the standard compounds. 2.4. CO Oxidation Reaction. Catalytic activity was measured using a fixed bed continuous flow reactor. A catalyst sample was placed in a glass tube. No treatment was applied before the measurements of catalytic activity. The catalyst sample was 500 mg. The reactant gas (1 vol % CO and 0.603 vol % H2O in air) was admitted at a flow rate of 500 mL/min through the reactor. The flow rates were monitored by mass flow controllers (Brooks). The catalysts were tested at ambient temperature (20 °C) and the temperature was measured by a thermocouple placed inside the catalyst bed. Quantitative analysis of CO2, CO, and O2 was performed by gas chromatography (China Chromatography Co., model 5890A) with a thermal conductivity detector using helium as the carrier gas. A CO analyzer (Industrial Scientific Corp., model T82) was used to analyze the CO concentration up to 1 ppm. The catalytic activity is expressed as the degree of CO conversion. 3. Results and Discussion 3.1. AAS. The Au loading was determined by flame AAS. The Au loadings in the catalysts were the same as the intended loadings within experimental error (3%). In addition, the catalysts did not lose any Au after the reaction. 3.2. XRD. The preparation parameters of iron oxidesupported gold catalysts prepared by the depositionprecipitation method in this study are summarized in Table 1.
Ind. Eng. Chem. Res., Vol. 44, No. 13, 2005 4571 Table 1. Effect of pH on the Deposition-Precipitation Process and Pretreatment Temperature on the Surface Area of Au/FexOy
samplea
calcination temp (C)
iron salt
surface area (m2/g)
3% Au(9)/FexOy 3% Au(10.5)/FexOy 3% Au(9)/FexOy
120 120 180
FeCl3 FeCl3 FeCl3
406 406 406
a 3% denotes the weight percent of Au, and the number in parentheses signifies the pH during the DP process.
Figure 1. XRD patterns of 3% Au(9)/FexOy catalysts, calcined at (a) 150 °C, (b) 180 °C, (c) 200 °C, (d) 250 °C, and (e) 300 °C.
XRD patterns of the 3% Au/FexOy calcined at 150, 180, 200, 250, and 300 °C, respectively, are shown in Figure 1. No distinct peaks were observed for either iron oxide or gold, indicating that the particle sizes of both species were very small. Diffraction peaks of the crystalline phase were compared with those of standard compounds reported in the JCPDS data file. The XRD patterns in Figure 1 are similar to the prevailing crystallographic phase of magnetite (FeFe2O4). These patterns also reveal that a magnetite (FeFe2O4) f maghemite (γFe2O3) transition occurs at a temperature above 150 °C, because the intensities of the weak peaks (2θ ) 30.335° and 43.4228°) increased upon increasing the calcination temperature. XRD patterns of the 3% Au/FexOy catalyst did not give any characteristic peaks for gold species. This can be due to a lack of sufficient crystalline material present (low loading or amorphous material). However, since Au tends to crystallize easily on a metal oxide support, this is probably caused by severe line broadening, which is typical of very small particles. XRD patterns also did not show distinct peaks for iron oxide, indicating that the FexOy particles were very small, too. Li et al.36 reported that the powder XRD patterns of Fe2O3 nanoparticle (2-5 nm) catalyst revealed only broad, indistinct reflections, suggesting that the material was either amorphous or of a particle size too small for this method to resolve, in accordance with the results in this study. Minico et al.37 reported that calcining the catalyst below 300 °C did not modify the XRD profile. They also presented that, for the catalyst calcined at 450 °C, the XRD pattern showed the presence of diffraction peaks related prevalently to the hematite crystallographic phase. Neri et al.38 reached the same conclusions. Our results are in accord. Tabakova et al.12 reported that the presence of gold enhanced the formation of a nonstoichiometric γ-Fe2O3 spinel phase, which
manifests a higher catalytic activity in the water-gas shift reaction. The amorphous iron oxide material contained many surface defects that should also strengthen Au-support interactions and, therefore, prevent the gold from agglomerating into large gold particles. The authors believed that hydroxyl groups acted as a stabilizer, because if the support was heated at high temperature with fewer hydroxyl groups, the particle size of the gold became larger. 3.3. Surface Area. FexOy prepared with precipitation at constant pH value of 11 using FeCl3 and NH4OH as the starting materials with very low feeding rate (10 mL/min) gave the largest surface area. In a parallel study, the authors have found that the FexOy prepared at other pH’s and with high feeding rate (30 mL/min) had lower surface area. This material was used as a support for gold catalyst throughout this study. Incorporation of Au onto FexOy support did not change the surface area, showing that gold species was welldispersed and did not block any pore channel of support. It demonstrated that careful control of the pH value, feeding rate of FeCl3, and calcination temperature is the key point to have high surface area FexOy materials. The FexOy obtained by precipitation at pH 11 and calcined at 120 °C gave the largest surface area (406 m2g-1). To our knowledge, this is the highest surface area of iron oxide reported in the literature. Kozlova et al.5 used Fe(NO3)3 and Na2CO3 to prepare their iron oxide support. The surface area was much lower than that in this study. In this study, we have found that the FexOy prepared with FeCl3 has a higher surface area than those prepared with FeCl2 and Fe(NO3)3. The only disadvantage of using FeCl3 is the remaining chlorine. Since chlorine is detrimental to the Au catalyst, it should be removed completely. In this study, the Fe(OH)3 precipitate was washed more than 20 times to remove residual chlorine completely. Kozlova et al.39,40 studied the state and structure of the iron oxide support and gold/iron oxide catalysts. They reported that the amounts of adsorbed water and surface hydroxyls of the support on which the gold precursor was impregnate seem to be very important, because the decrease in the water and hydroxyl contents of the Fe(OH)3 by drying and calcination resulted in a lowering of the catalytic activity of the Au catalysts. The FexOy prepared in this study has a very high surface area and abundant hydroxyl groups, which is beneficial for Au catalysts in CO oxidation. The low calcination temperature not only prevents sintering but also preserves the abundant hydroxyl groups. The abundant hydroxyl groups on the surface of the support can provide more sites to deposit gold species. It is also beneficial for CO oxidation. In this study, only the FexOy prepared at pH 11 and treated at 120 °C was used as the support. 3.4. TEM. Figure 2 shows the TEM images of the 3% Au/FexOy catalysts prepared by DP method at pH 10 and calcined at various temperatures. As can be seen, small gold particles are homogeneously dispersed on FexOy particles. The TEM images clearly showed that the particle diameters of gold for all the samples are less than 4 nm, independent of calcinations temperature. This result was confirmed by the analysis of XRD. The FexOy support was amorphous and Au nanoparticles were attached on the support. The method applied in this study leads to a fairly uniform dispersion of gold nanoparticles of diameter about 2 nm and narrower size
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Figure 2. Effect of calcination temperature on the TEM images of 3% Au(9)/FexOy samples: (a) 150 °C, (b) 180 °C, (c) 200 °C, and (d) 250 °C.
distribution. The independence of particle size of gold with calcinations temperature is in agreement with the results of Date´ et al.,41 Akita et al.42 and Takavka et al.43 They reported that TEM images showed no significant change in structure after the increase in calcination temperature. Because it is relatively difficult to detect gold particles smaller than 2 nm by TEM in the heterogeneous structures, it is probable that among gold clusters, chemical and morphological changes proceed mainly at a smaller scale. It should be noted that iron oxide support was in the nanosize range (about 5 nm) as shown in the TEM. The nanosize support is beneficial for gold species to diffuse into the pore of the support during preparation. In
addition, the gold particle was confined by the size of the iron oxide support during the DP process. 3.5. XPS. The scale of the binding energies was calibrated taking, as a reference, the adventitious C 1s peak at 285 eV. Figures 3 and 4 show XPS Fe 2p and Au 4f spectra obtained for the 3% Au/FexOy catalysts calcined at 150, 200, 250, and 300 °C, respectively. The binding energies of Fe 2p and Au 4f are tabulated in Tables 2 and 3, respectively. The binding energy of Fe 2p appeared between 710.8 and 710.5 eV which shows the presence of Fe2O3 and/or FeO(OH) species.44 Figure 4 shows the XPS of Au 4f of the catalysts with various calcination temperatures. In the Au 4f region, 3% Au/FexOy catalysts calcined at 150 °C and 300 °C
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Figure 3. Effect of calcination temperature on the XPS Fe 2p spectra of 3% Au(9)/FexOy samples: (a) 150 °C, (b) 200 °C, (c) 250 °C, and (d) 300 °C.
Figure 4. Effect of calcination temperature on the XPS Au 4f spectra of 3% Au(9)/FexOy samples: (a) 150 °C, (b) 200 °C, (c) 250 °C, and (d) 300 °C. Table 2. Effect of the Calcination Temperature on the XPS Fe 2p Spectra of 3% Au/FexOy Samplesa
a
XPS (eV)
calcination temp (°C)
Fe 2p1/2
Fe 2p3/2
150 200 250 300
724.8 724.2 724.2 724.4
710.8 710.6 710.6 710.5
The pH of the DP process was 9.
Table 3. Effect of Calcination Temperature on the XPS Au 4f Spectra of 3% Au/FexOy Samplesa XPS (eV)
calcination temp (°C)
Au 4f5/2
Au 4f7/2
150 200 250 300
87.5 88.3 87.8 87.7
83.7 84.6 83.1 84.0
a The 3% Au/Fe O was prepared by the DP method using x y NH4OH and HAuCl4 and pH 9.
showed the Au 4f7/2 peaks at binding energy below 84.1 eV. The binding energy (BE) values reflect only a characteristic of metallic Au state for all the samples with various calcinations temperatures, as shown in Table 3. No oxidized form of gold was detected by XPS in all the samples. Guczi et al.44-47 reported that the
Au 4f binding shifts slightly to lower binding energy when it is supported on Fe2O3. The Au 4f binding energy of Au/Fe2O3 catalysts prepared by a coprecipitation method shift to 83.8 eV, indicating the formation of an electron-rich state of the metallic gold particles.44 The discrepancy between our results and literature data44 can be attributed to the different preparation method, which resulted in different metal-support interaction. The sample calcined at 250 °C showed the peaks at 84.1 and 87.8 eV for Au 4f7/2 and Au 4f5/2 lines, respectively. This is in good agreement with the results of Park and Lee,40 who have reported that two distinct peaks of Au/ Fe2O3 catalysts appeared at 84.1 and 87.8 eV, which are close to Au 4f binding energies of metallic Au. The nature of the active site of gold for CO oxidation is still in debate. Active catalysts always contain metallic Au particles, while oxidic Au species are not responsible for steady state high catalytic activity. However, the smooth surface of metallic Au does not adsorb CO at room temperature, indicating that CO is adsorbed only on steps, edges, and corner sites. Thus, smaller metallic Au particles are preferable. Haruta35 and Grisel et al.21 reported that metallic Au is the site to adsorb CO. In contrast, a higher activity of oxidized gold species compared to metallic Au has been reported by several authors.18,31,40,41 Margitfalvi et al.31 reported that Au/ MgO catalysts modified with ascorbic acid was active for low-temperature CO oxidation. They reported that the activation of CO requires the formation of oxidized gold ensemble sites, in which the ionic gold is involved in the activation of CO molecules. Neri et al.38 used temperature-programmed reduction (TPR) and X-ray diffraction (XRD) to study Au/iron oxide catalysts. The effect of gold on the reducibility of the iron oxides is related to an increase of the structural effects and/or of the surface hydroxyl groups. It should be noted that when the particle size of the metal reaches about 2 nm, the metallic gold species would become electron-deficient to some extent. It would be difficult to differentiate the oxidation state of gold particles. This is the reason different researchers reached different conclusions on the active state of gold. 3.6. Low-Temperature CO Oxidation. The effect of pH during the DP process on the CO conversion of iron-oxide supported gold catalysts are given in Figure 5. Initially, the CO conversion on 3% Au(10.5)/FexOy was 27%. After 90 min time on stream, the conversion of CO reached 100%. It started to decay at 100 min time on stream. In the same time on stream, 100% CO conversion was still obtained on 3% Au(9)/FexOy catalyst. It can be seen that the apparent increase in catalytic activity of the 3% Au(9)/FexOy in comparison with 3% Au(10.5)/FexOy was due to the proper pH value being used in the DP process. Active Au catalysts can be prepared using different methods. Coprecipitation, impregnation, and deposition-precipitation are the most commonly used methods. It has been reported that deposition-precipitation gives the higher activity.35 Therefore, it was adopted in this study. Various preparation variables have been studied. It has been established that the pH value plays an important role. In the DP method, HAuCl4 was used as the metal precursor in this study. The chloroauric anion hydrolyzes in solution to form Au(OH)xCl-4-x. The extent of hydrolysis depends on the pH and Au and Cl concentrations. Clwas completely removed and Au concentration was fixed in this study. The only variable that remained was the
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Figure 6. CO oxidation activity of 3% Au(9)/FexOy catalyst. The support was high surface area iron oxide. The Au catalysts were prepared by the DP method using NH4OH and HAuCl4, synthesized at pH 9 and calcined at 180 °C. Catalyst sample, 500 mg of 3 wt % Au/FexOy; reactant gas, 0.25 vol % CO and 0.603 vol % H2O in air, 500 mL/min. The reaction temperature was 20 °C.
Figure 5. Effect of pH during the DP process on the CO conversion of 3% Au/FexOy catalyst: (a) 3% Au(10.5)/FexOy and (b) 3% Au(9)/FexOy. Catalyst sample, 500 mg of 3 wt % Au/FexOy; reactant gas, 0.25 vol % CO and 0.603 vol % H2O in air, 500 mL/ min. The reaction temperature was 20 °C.
pH value. It has been found that preparation at a pH ranging from 7 to 8 is preferable, depending on the support. At this pH, the value of x is close to 3. At low pH’s, below the isoelectric point of the support, the surface is positively charged and is capable of adsorbing more of the negatively charged gold species. This results in not only a larger gold loading but also a high concentration of chloride on the surface. The presence of chloride increases the mobility of Au on the support, leading to large Au particles and some vaporization of Au from the solid upon calcination. At pH’s above the isoelectric point of the oxide support, adsorption of the negatively charged Au(OH)xCl4-x- complex decreases rapidly, resulting in a lower gold loading. However, there will also be less chloride at the catalyst surface, so small gold particles can be formed. Therefore, there is a narrow range of pH where sufficient Au can be deposited onto the support with minimal chloride in the Au complex. A systematic study of the synthesis conditions for the preparation of gold catalysts by depositionprecipitation has been reported.34 Optimization of the synthesis parameters resulted in highly active Au/TiO2, Au/Co3O4, Au/Al2O3, and Au/ZrO2 catalysts. The activity increased with increasing pH value during precipitation and decreasing temperature of calcination. Figure 6 shows the conversion of CO on 3% Au(9)/ FexOy catalyst. A calcination temperature between 120 and 180 °C did not significantly change the activity in this study. The catalytic activity of 3% Au(9)/FexOy was much greater than that of 3% Au(10.5)/FexOy, indicating that the pH value during deposition-precipitation is important and that the optimum value is 9. It was observed that 100% conversion of CO was obtained at room temperature over 90 min on stream on all catalysts. It demonstrated that the suitable selection of pH value and calcination temperature play
a key role in creating high catalytic performance for lowtemperature CO oxidation. In this study, the Au/FxOy catalyst that was prepared by the DP method, supported on high surface area iron oxide, synthesized at pH 9, and then calcined at 180 °C showed the best performance for CO oxidation. Full conversion was still achieved at ambient temperature even after 2000 h time on stream. It should be noted that the activity of this catalyst was not influenced by the concentration of moisture in the range between 20 ppm and saturated moisture. Date´ and Haruta8 reported that that moisture enhanced the CO reaction on Au/TiO2 by more than 10 times up to 200 ppm H2O, while further increases in the moisture content suppressed the reaction. Au/FexOy catalyst developed in this study did not show any suppress in CO oxidation reaction under a high concentration of moisture. The difference in this study is essentially due to the different support. Haruta6,7,9 and Haruta and Date´8 have reviewed the effect of particle size on the catalytic properties of gold catalyst. The turnover frequency increased with a decrease in the mean diameter of gold particles. The catalytic activity can be correlated with the contact structure giving the longest perimeter distance of goldsupport interface. Guczi et al.47 has reported the size effect of Au nanoparticles that demonstrated correlation between Au nanoparticle size, electron structure, and catalytic activity. The valence band density of states of gold nanoparticles changed with decreasing particle size and brought about the enhancement of activity.47 Goodman and co-workers48 reported that turnover frequency for CO oxidation reaches a maximum at a diameter of Au islands of 3.5 nm (three atoms thick), where Au partially loses its metallic nature (see the following figure). They have suggested that this transition might be related to the high catalytic activity. The catalysts in this study had gold particles around 3 nm in size; therefore, they demonstrated high catalytic activity in CO oxidation. 4. Conclusion Nanosized iron oxide was prepared by a precipitation method using FeCl3, followed by washing 20 times at
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least with distilled water until disappearance of chloride, and then calcined in a flow of air. The FexOy sample, prepared by precipitation at pH 11 with a low feeding rate of aqueous solution of FeCl3 and calcined at 120 °C, had the largest surface area (406 m2 g-1). The technique used in this study gave a high surface area, small and uniform particle size (
