Fumed SiO2 by Treatment with

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Promotion of Au(en)2Cl3-Derived Au/Fumed SiO2 by Treatment with KMnO4 Hongfeng Yin, Zhen Ma, Steven H. Overbury, and Sheng Dai* Chemical Sciences DiVision, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 ReceiVed: January 27, 2008; ReVised Manuscript ReceiVed: March 1, 2008

A new method for the removal of organic ligands used in the synthesis of nanoparticle catalysts via solutionphase chemical oxidation and subsequent pretreatment at moderate temperatures was developed. This synthesis protocol is advantageous for the preparation of highly dispersed gold catalysts with minimum sintering. Highly active Au/SiO2-based catalysts were prepared by treating Au(en)2Cl3-derived Au/fumed SiO2 in strongly oxidative KMnO4 solutions. The low-temperature activity in CO oxidation increased dramatically following the KMnO4 treatment and subsequent thermal activation (including treatment in O2-He at 300–600 °C). The influences of the pH values of KMnO4 solutions and temperatures of activation in O2-He prior to the reaction testing were investigated, and relevant characterization using XRD, TEM, TG/DTG, SEM, EDX, and ICPOES was conducted. The catalyst stability in CO oxidation was studied as a function of reaction time on stream. The origin of the promotional effect was discussed. 1. Introduction Recent advances in nanoscience have led to the development of numerous methodologies for controlled synthesis of monodispersed nanoparticles via surface stabilization by organic capping ligands.1–3 The application of these nanoparticles in catalysis and other fields often requires the removal of organic ligands. High-temperature calcination has been widely used to remove organic ligands, but its main drawback is the facile sintering of nanoparticles. Here we describe a methodology that relies on the use of a strong oxidant (aqueous KMnO4) at room temperature to partially oxidize residual organics on gold nanoparticle catalysts, followed by moderate thermal calcination. Although the preparation of gold nanocatalysts is chosen to demonstrate the basic protocol, the methodology may be extended to the preparation of other nanoparticle-based catalysts and opens up a new avenue in the promotion of surface-capped nanocatalysts. Finely divided gold nanoparticles are useful for ablating environmental pollutants, cleaning H2 streams for fuel-cell applications, and synthesizing chemicals.4–8 Compared with the most studied Au/TiO2, Au/SiO2 usually exhibits much lower activity in CO oxidation.9–14 It was hypothesized that reducible TiO2 can activate and store oxygen, whereas nonreducible SiO2 is inherently “inert”.11,14,15 However, via delicate inorganic synthesis and appropriate catalyst pretreatment, high activity of Au/SiO2 in CO oxidation can still be achieved.16–22 For instance, our group reported the preparation of Au/mesoporous SiO2 and Au/fumed SiO2 using Au(en)2Cl3 (en ) ethylenediamine) as the precursor.21,22 The Au(en)2Cl3 precursor23 was also used by other groups for synthesizing supported gold nanocatalysts, such as Au/NaHY,24 Au/TiO2,25 Au/C,26 and Au/ SiO2.27 Our Au/SiO2 catalysts need to be reduced in H2 at 150 °C and pretreated in O2-He at 400–600 °C to show high activity in CO oxidation at room temperature; in contrast, as-synthesized Au/SiO2 that contains unburned organic ligands shows no activity at reaction temperatures below 300 °C.22 * Corresponding author. Tel: 1-865-576-7307. Fax: 1-865-576-5235. E-mail: [email protected].

Attempts have been made to modify SiO2 supports before loading gold.28–35 For instance, Nieuwenhuys and co-workers reported that the T50 (reaction temperature at which 50% of CO molecules are converted to CO2) value of Au/SiO2 in CO oxidation is 240 °C, but when SiO2 is modified by CoOx, LaOx, or CeOx and gold is loaded thereafter, the T50 values decrease to 185, 135, and 115 °C, respectively, indicating the promotional effect of these additives.28 Our group29,31 and others28,30,32–35 developed Au/TiO2/SiO229–33 and Au/CoOx/SiO234,35 catalysts for CO oxidation. Nevertheless, even though the addition of certain promoters may improve the activity, the promoted catalysts are still not particularly active due to the disadvantageous starting point: the deposition-precipitation method does not work well for making active Au/SiO2 catalysts.29 Hence, the promotional effect based on a better starting point (i.e., highly active Au/SiO216–22) may be further considered, and new methodologies for the installation of appropriate promoters may be developed. Recently, the groups of Suib36 and Shi37,38 prepared MnOxcontaining mesoporous SiO2 by treating as-synthesized mesoporous SiO2 with aqueous KMnO4 followed by calcination. The role of KMnO4 is to react with organic templates in assynthesized SiO2 samples to yield MnOx that is otherwise difficult to introduce.37,38 Here we report a new method to activate Au(en)2Cl3-derived catalysts by removing organic groups surrounding supported gold nanoparticles at moderate temperatures (Figure 1). We show that the treatment of H2reduced Au/SiO2 by aqueous KMnO4 followed by thermal activation in O2-He at 300 °C leads to a catalyst that is active even below -70 °C. The catalysts were characterized by XRD, TEM, TG/DTG, SEM, EDX, and ICP-OES, and the catalyst stability was tested as a function of reaction time. 2. Experimental Section Au(en)2Cl3 precursor was synthesized using HAuCl4 · 3H2O and ethylenediamine according to our previous papers.21,22 To synthesize Au/SiO2, 0.08 g Au(en)2Cl3 was dissolved in 100 mL of H2O, the pH of the solution was adjusted to 10 with 1 M NaOH solution, and 2.0 g of fumed SiO2 (Cab-O-Sil) was added. The pH value of the solution dropped drastically after

10.1021/jp800797t CCC: $40.75  2008 American Chemical Society Published on Web 05/08/2008

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Figure 1. Schematic representation for the preparation of MnOx-loaded Au/SiO2 by treating Au(en)2Cl3-derived Au/SiO2 with KMnO4 solutions followed by thermal activation.

Figure 2. CO conversions on H2-reduced Au/SiO2 and KMnO4/Au/SiO2 (pH 7). These catalysts were either tested as prepared without pretreatment or pretreated in O2-He at 220–600 °C prior to the reaction testing.

adding SiO2 and was readjusted to approximately 10 by NaOH solution. The suspension was stirred at 60–65 °C for 2 h, filtered, thoroughly washed with H2O, and dried in a vacuum oven at 70 °C for 5 h to yield pale yellow powders (denoted by “as-

synthesized” Au/SiO2). Here we emphasize that the sample was thoroughly washed by H2O to remove any chloride in the sample. We previously showed that the residual chlorine level of the catalysts derived via the above protocol was very low

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Figure 3. CO conversions on KMnO4/Au/SiO2 (pH 3) and KMnO4/Au/SiO2 (pH 11). These catalysts were either tested as prepared without pretreatment or pretreated in O2-He at 300–600 °C prior to the reaction testing.

based on elemental analysis.21 Here we also emphasize that the sample was dried under vacuum at 70 °C for 5 h. The pale yellow color of the sample would turn into apricot and claypot, respectively, if the duration of drying was overnight and 1 week, respectively, indicating the decomposition of the organic ligand during extended heating. After drying, the as-synthesized SiO2 was then reduced in 4% H2 (balance Ar) at 130 °C for 1 h to obtain dark-red powders denoted as “H2-reduced” Au/SiO2. For the KMnO4 treatment, 0.20 g of H2-reduced Au/SiO2 was suspended in 20 mL of H2O, and 1 mL of 0.05 M KMnO4 solution was added. The pH of the suspension was adjusted to either 3.0 or 11.0 with 1 M HNO3 or 1 M NaOH or not changed by adding the same amount of H2O. The suspension was stirred at room temperature for 20 h, filtered, and thoroughly washed with H2O. The product was dried at 70 °C for 10 h, yielding brown powders denoted as KMnO4/Au/SiO2 (pH 3), KMnO4/ Au/SiO2 (pH 7), or KMnO4/Au/SiO2 (pH 11). Unless otherwise specified in the text, a 50 mg sample was packed into a U-type quartz tube (i.d. ) 4 mm) sealed by quartz wool and pretreated in flowing 8% O2 (balance He) at a specified temperature for 1 h (heating rate, 30 °C/min; flow rate, 37 cm3/ min). After cooling, a gas stream of 1% CO (balance air, < 4 ppm water) flowed through the catalyst at a rate of 37 cm3/ min, and the exiting stream was analyzed by a gas chromato-

graph equipped with a dual molecular sieve/porous polymer column and a thermal conductivity detector. The reaction temperature was varied using a furnace or by immersing the U-type tube in ice–water or liquid N2 cooled acetone. XRD data were collected on a Siemens D5005 diffractometer with Cu KR radiation, in the range of 2θ ) 20–90° at the rate of 0.01°/s. TG/DTG experiments were conducted on a TGA 2950 instrument using a heating rate of 10 °C/min under N2 or air atmosphere. TEM-EDX experiments were carried out on a Hitachi HD-2000 STEM operated at 200 kV. Elemental analysis of Au and Mn was performed using inductivity coupled plasmaoptical emission spectrometry (ICP-OES) on a Thermo IRIS Intrepid II spectrometer. Weighed samples were dissolved in HNO3-HCl-HF acids, diluted by H2O to 200 mL, and analyzed by ICP-OES along with Mn and Au solutions with known concentrations. SEM-EDX experiments were conducted on a JEOL JSM-6060 scanning electron microscope coupled with an EDX detector. 3. Results 3.1. Catalytic Activity in CO Oxidation. Figure 2A shows the CO light-off curves of Au/SiO2 synthesized using Au(en)2Cl3 as the precursor. The catalyst reduced in H2-Ar at 130 °C is not very active: the CO conversion ignites at 230 °C and


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Figure 4. XRD patterns of Au/SiO2, KMnO4/Au/SiO2 (pH 3), KMnO4/Au/SiO2 (pH 7), and KMnO4/Au/SiO2 (pH 11). The samples marked as “H2-reduced” or “as-prepared” refer to fresh samples without thermal treatment in O2-He and without reaction testing. The samples marked as “pretreated” refer to the spent catalysts collected after thermal treatment in O2-He and subsequent reaction testing.

TABLE 1: Average Gold Particle Sizes Estimated from the XRD Data in Figure 4 Average Size of Gold Particles Estimated by XRD, nm catalyst Au/SiO2 KMnO4/Au/SiO2 (pH ) 3) KMnO4/Au/SiO2 (pH ) 7) KMnO4/Au/SiO2 (pH ) 11)

H2-reduced or 300 °C500 °C600 °Cas-prepared pretreated pretreated pretreated 2.1 2.5

3.3 3.0

4.5 4.4

4.8 7.0

2.5

3.0

3.6

4.4

2.8

2.5

3.4

4.1

achieves ∼100% at 300 °C. The H2-reduced Au/SiO2 needs to be further treated in O2-He at 300–600 °C to realize complete CO conversion at room temperature, whereas the pretreatment at 220 °C is still not enough. These observations are explained as the thermal removal of organic residues derived from ethylenediamine being necessary for achieving high activity:22 reducing Au(en)2Cl3-derived Au/SiO2 in H2-Ar at 130 °C and further pretreating it in O2-He at 220 °C cannot sufficiently remove the organic moieties. More information on this point will be addressed by TG/DTG experiments in Figure 8. Figure 2B shows the CO light-off curves of KMnO4/Au/SiO2 (pH 7). As-prepared KMnO4/Au/SiO2 exhibits a significant

decrease in the catalytic ignition temperature (the CO conversion ignites at 140 °C as compared to 230 °C for H2-reduced Au/ SiO2 mentioned above) and achieves complete conversion at 170 °C (compared to 300 °C for H2-reduced Au/SiO2). The observed decrease in the ignition temperature may be attributed to the facilitated removal of organic ligands catalyzed by MnOx on catalyst surfaces. On the other hand, the ignition temperature is still as high as 140 °C (rather than around -10 °C for 300–600 °C-pretreated Au/SiO2 whose organic moieties are more sufficiently combusted, Figure 2A), implying that the organic moieties are not completely oxidized by aqueous KMnO4 to CO2 at room temperature. Our TG/DTG data to be addressed in Figure 8 provide strong evidence for these explanations. The KMnO4/Au/SiO2 (pH 7) catalyst is greatly activated after treatment in O2-He at 220–600 °C, and the optimal pretreatment temperature is 300 °C (Figure 2B). After such pretreatment, complete CO conversion is achieved at -70 °C. This corresponds to a very high specific rate of at least 1.1 mol gAu-1 h-1 at -70 °C, based on the gold loading (1.68 wt %) measured by ICP-OES. Such anomalously high conversion at very low temperature is evasive in the literature but is seen with H2reduced Au/TiO2/TiO2/SiO2 and Au/Al2O3/TiO2/SiO231 as well



Figure 5. Representative TEM images of Au/SiO2. The sample in part A refers to a fresh sample without thermal treatment in O2-He, whereas those in parts B-D refer to spent catalysts after thermal treatment and reaction testing.

as gold supported on TiO2 modified by CaO, ZnO, Y2O3, Pr2O3, Yb2O3, or Er2O3.39 Figure 3 shows the catalytic results when the H2-reduced Au/ SiO2 is treated by KMnO4 under acidic (pH 3) or basic (pH 11) conditions. Overall, treatment by aqueous KMnO4 under neutral conditions (Figure 2B) generally leads to higher activity at low reaction temperatures than treatment under acidic (Figure 3A) or basic (Figure 3B) conditions. In addition, pretreating the KMnO4/Au/SiO2 catalysts in O2-He at 300 °C generally leads to the highest activity at low reaction temperatures. However, dips on conversion curves when the reaction temperature is in the range from -10 to 0 °C are often observed, probably due to the influences of moisture, carbonate, or the interplay of competing reaction mechanisms.40,41 Although the root cause of the observed dips is not clear at the moment, we note that similar dips on conversion curves were also reported with other gold catalysts.22,39–41 This would be a very interesting topic for further in-depth research in the future. Also noted is that, when the reaction temperature is above 10 °C, the CO conversions on KMnO4/Au/SiO2 samples (Figures 2B and 3A,B) are comparable to or even lower than what is achieved on Au/ SiO2 (Figure 2A). 3.2. Catalyst Characterization. Figure 4 collects four sets of XRD patterns of Au/SiO2, KMnO4/Au/SiO2 (pH 3), KMnO4/ Au/SiO2 (pH 7), and KMnO4/Au/SiO2 (pH 11). The samples

marked in the figure as “H2-reduced” or “as-prepared” samples refer to the fresh catalysts without thermal treatment in O2-He, whereas the samples marked as “pretreated” refer to the spent catalysts collected after pretreatment in O2-He and subsequent reaction testing. In general, SiO2 shows a broad amorphous feature at 2θ ) 22°, and gold peaks appear at 2θ ) 38°, 44°, 65°, and 78°, corresponding to gold (111), (200), (220), and (311), respectively.42,43 The gold peaks become sharper with the increase of pretreatment temperature, indicating the growth of gold nanoparticles. On the other hand, SiO2 maintains its amorphous phase, and the formation of new phases is not detected. The sintering problem associated with gold nanoparticles seems to be less obvious for KMnO4/Au/SiO2 (pH 7) and KMnO4/Au/SiO2 (pH 11) than for Au/SiO2 and KMnO4/Au/ SiO2 (pH 3), as best judged by the Au(111) peak sharpness of 600 °C-pretreated samples. The gold particle sizes calculated by X-ray line-broadening method are summarized in Table 1 for quantitative comparison. To know the gold particle size ranges, we used TEM to probe different positions and recorded several images for each sample. For convenience, only representative images are shown here. Figure 5 collects typical TEM images of the Au/SiO2 (no KMnO4) following different pretreatment. The gold particle sizes of H2-reduced Au/SiO2, without undergoing reaction, are in the


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Figure 6. Representative TEM images of KMnO4/Au/SiO2 (pH 7). The sample in part A refers to a fresh sample without thermal treatment in O2-He, whereas those in parts B-D refer to spent catalysts after thermal treatment and reaction testing.

range of 2–4 nm, typically 3 nm (e.g., Figure 5A). The typical gold particle size of Au/SiO2 collected after 300 °C-pretreatment and light-off curve measurement is 3 nm (e.g., Figure 5B). The population of 4-nm gold particles grows when the pretreatment temperature is 500 °C (e.g., Figure 5C). Finally, gold nanoparticles with sizes of 3–7 nm dominate on 600 °C-pretreated Au/ SiO2 (e.g., Figure 5D). For comparison, Figure 6 collects typical TEM images of KMnO4/Au/SiO2 (pH 7). Again, several TEM images were recorded for each sample, and only representative ones are shown here. The gold particle growth in Figure 6 appears to be somewhat inhibited compared to those in Figure 5. The gold particle sizes of as-prepared KMnO4/Au/SiO2 (pH 7) are in the range of 2–5 nm, typically 3 nm (e.g., Figure 6A). The gold particle sizes are well maintained after pretreatment at 300–600 °C (e.g., Figure 6B-D): the 600 °C-pretreated catalyst still typically contains gold particles with sizes of 2–5 nm; occasionally there are big gold particles with sizes on the order of 10 nm, but in these regions small gold particles (2–3 nm) are still populated (e.g., Figure 7). Figure 8 compares TG/DTG data of Au/SiO2 and KMnO4/ Au/SiO2 (pH 7). Experiments were carried out under N2 or air to compare the trends. In general, the weight loss below 100 °C is considered as the release of adsorbed water, and those at

higher temperatures correspond to the removal of organic moieties.22 For H2-pretreated Au/SiO2 under N2, the major weight loss at elevated temperature occurs at 280–420 °C (Figure 8A). The region of high-temperature weight loss shifts to 200–340 °C when the thermal analysis is conducted under air (Figure 8B). On the other hand, the major weight loss at elevated temperature takes place at 180–300 or 120–240 °C when studying KMnO4/Au/SiO2 (pH 7) in N2 or air (Figure 8C,D). There are only minor weight losses above 300 °C (Figure 8C,D). These data mean that the thermal removal of organic moieties is facilitated as a result of treating H2-reduced Au/ SiO2 in aqueous KMnO4, even though the KMnO4-treatment does not completely oxidize organic moieties to CO2 in the aqueous solution. 3.3. Catalyst Stability on Stream. Finally, we tested the stability of 300 °C-pretreated KMnO4/Au/SiO2 (pH 7). Because loading 50 mg in the reactor would lead to 100% conversion at room temperature (Figure 2B), the catalyst load is decreased to 25 mg to make sure that the activity trend would not be biased by 100% conversion all the time.7,44 As shown in Figure 9, the initial conversion is 72%. The deactivation is more obvious during the initial 20 h on stream, and the activity is then stabilized. After testing the catalyst for 65 h, the conversion is 50%.



Figure 7. A TEM image of 600 °C-pretreated KMnO4/Au/SiO2 (pH 7) sample, showing the presence of both small and bigger gold nanoparticles. EDX analysis data of a selected region are shown in the bottom.

4. Discussion Several explanations may be proposed to explain the high activity of KMnO4-treatment of Au/SiO2 at low reaction temperature (Figure 1). The first explanation is that the treatment with KMnO4 can stabilize gold nanoparticles against sintering, and small gold nanoparticles lead to high activity in CO oxidation.4–8 In our experiments, the gold particles of 600 °Cpretreated KMnO4/Au/SiO2 (pH 7) are smaller than those of 600 °C-pretreated Au/SiO2 (compare Figures 5D and 6D), consistent with the finding that 600 °C-pretreated KMnO4/Au/ SiO2 (pH 7) is more active than 600 °C-pretreated Au/SiO2 at low temperature (compare Figurs 2A and 2B). However, this proposal cannot explain why the gold particle size of 300 °Cpretreated KMnO4/Au/SiO2 (pH 7) is comparable to that of 300 °C-pretreated Au/SiO2 (compare Figures 5B and 6B), but the former shows a much higher conversion at -70 °C (compare parts A and B of Figure 2). Therefore, the size effect of gold

nanoparticles may be an important factor when comparing the activity within the same type of gold catalyst, but it is difficult to predict whether one type of catalyst is more active than another type of catalyst solely based on their differences in particle sizes. The presence of promoters (or poisons) may increase (or decrease) the catalytic activity, even though the gold particle sizes are comparable. The second proposal is that the residual organic moieties can be removed at lower temperatures during pretreatment in O2-He if Au(en)2Cl3-derived Au/SiO2 is initially treated by aqueous KMnO4 (Figure 8). It is known that the removal of organic capping agents or organic residues on gold nanoparticles is necessary for achieving high activity.9,10,12,20,45–50 In surface chemistry, clean metal surfaces are more active than “dirty” metal surfaces passivated by organic fragments.51,52 However, the more facile removal of organic residues in the KMnO4/Au/ SiO2 system (Figure 8) cannot explain why 500 °C-pretreated


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Figure 8. TG/DTG analysis of H2-reduced Au/SiO2 and KMnO4/Au/SiO2 (pH 7) under N2 or air.

Figure 9. Stability test of 300 °C-pretreated KMnO4/Au/SiO2 (pH 7). The experimental conditions are specified in the figure.

Figure 10. Control experiments to compare the CO oxidation on four different samples.

KMnO4/Au/SiO2 (pH 7) is much more active than 500 °Cpretreated Au/SiO2 at low reaction temperatures (compare parts A and B of Figure 2), given that residual organic species on both catalysts can be all removed below 500 °C (Figure 8) and gold particle sizes are all about 2–4 nm (Figures 5C and 6C). In a third mechanism, KMnO4 may partially oxidize the organic moieties on Au/SiO2, either during the treatment of Au/ SiO2 in aqueous KMnO4 or during pretreatment in O2-He at elevated temperatures, resulting in MnOx deposited near gold nanoparticles. The deposited MnOx indeed facilitates the removal of organic fragments (Figure 8) and the stabilization of gold nanoparticles (Figures 4 and 6) during thermal pretreatment in

O2-He, but these factors are not the dominant factors that significantly improved the activity at low reaction temperatures (Figure 2B). The presence of the Au-MnOx interface as a result of the KMnO4 treatment may be the dominant factor instead. In our case, the Mn contents of KMnO4/Au/SiO2 (pH 3), KMnO4/Au/SiO2 (pH 7), and KMnO4/Au/SiO2 (pH 11) are measured by ICP-OES as 0.67, 0.49, and 0.17 wt %, respectively, lower than the maximum value (1.38 wt %) calculated on the basis of the amount of KMnO4 (1 mL, 0.05 M) and Au/ SiO2 (0.20 g) used in our synthesis. Therefore, the KMnO4 in solution is not completely consumed. In addition, our catalytic

Promotion of Au(en)2Cl3-Derived Au/Fumed SiO2 experiments indicate that thermal activation in O2-He is necessary (Figures 2B and 3A,B), and TG/DTG experiments register the weight loss of KMnO4/Au/SiO2 at elevated temperatures (Figure 8C,D). That means that the majority of organic moieties on the H2-reduced Au/SiO2 are not oxidized by aqueous KMnO4 to CO2 at room temperature. In fact, Shi and co-workers synthesized MnOx-loaded mesoporous SiO2 by treating assynthesized mesoporous SiO2 (containing organic templates) in aqueous KMnO4 at room temperature to form MnOx.37,38 However, they still calcined their samples at 510 °C under air to completely remove the organic templates.37,38 These scenarios are different from the cases in which neat carbon materials are oxidized by KMnO4 to CO2 at room temperature (4KMnO4 + 3C + 2H2O f 4MnO2 + 3CO2 + 4KOH).53–59 Although the KMnO4-treatment does not completely oxidize the organic ligands to CO2 at room temperature in the solution phase, the resulting Au-MnOx interface (formed during the subsequent thermal treatment that was significantly above room temperature) is believed to promote low-temperature CO oxidation. In the literature, Au/MnOx catalysts are known to be active for CO oxidation,60–64 and the promotional effect of MnOx in Au/MnOx/Al2O3,65 Au/MnOx/TiO2,66 and Au/MnOx/C67 for CO oxidation has been established as well. The MnOx component may promote the reaction via supplying active oxygen for CO oxidation.65–67 In our case, because the residual organics are believed to be near gold active sites through surface interaction (Figure 1), the redox reaction products (MnOx species) are expected to locate near gold nanoparticles. In addition to the detection of Mn by ICP-OES and SEM-EDX (date not shown), we also conducted TEM-EDX experiments and found the presence of Au, Mn, and Si surrounding a gold nanoparticle (Figure 7 and other figures not shown). To further validate the proposed promotional effect, we designed control experiments. In one, 500 °C-pretreated Au/ SiO2 (with all the organic ligands removed via pretreatment in O2-He at 500 °C) was soaked in a KMnO4 solution, followed by thorough washing, drying, and pretreatment in O2-He at 300 °C. In this case because KMnO4 has no sacrificing organic moiety to react with, there is little MnOx on the surface, as verified by SEM-EDX. Indeed, the activity of the sample is lower than that of 500 °C-pretreated Au/SiO2 (Figure 10). In another control experiment, 500 °C-pretreated Au/SiO2 (with the previously adsorbed organic moieties totally removed) is exposed to ethylenediamine to allow for the adsorption of this capping agent on naked gold nanoparticles. The ethylenediamine-treated Au/SiO2 was then soaked in a KMnO4 solution to allow for the deposition of MnOx by reacting the capping ligand with KMnO4, followed by thorough washing, drying, and pretreatment in O2-He at 300 °C. Only 27 mg instead of the regular load (50 mg) of sample was put in the reactor and tested, but the promotional effect of such KMnO4 treatment is obvious (Figure 10). The results from these control experiments convincingly demonstrate that the presence of a gold-MnOx interface is important for achieving high activity at low reaction temperatures (below -10 °C). 5. Conclusions A new methodology to remove organics on catalytic nanoparticles with minimum sintering is developed and successfully employed to promote Au(en)2Cl3-derived Au/SiO2 for lowtemperature CO oxidation. Au/SiO2 catalysts synthesized using Au(en)2Cl3 as the precursor are reduced in H2-Ar at 130 °C, soaked in aqueous KMnO4 solutions followed by washing, drying, and pretreatment in O2-He at elevated temperatures.

J. Phys. Chem. C, Vol. 112, No. 22, 2008 8357 The highest activity in CO oxidation at low reaction temperatures (below -10 °C) is achieved when these KMnO4-treated catalysts are further pretreated in O2-He at 300 °C. Treatment by KMnO4 under neutral conditions is preferred for catalyst activation. Under such conditions, gold particles can be stabilized upon thermal treatment, and the presence of Mn species further facilitates the removal of residual organic fragments at lower treatment temperatures. The presence of MnOx species is beneficial for achieving high activity at low reaction temperatures (below -10 °C), although no significant promotional effect is observed at higher reaction temperatures (above 20 °C). KMnO4/Au/SiO2 (pH 7) shows good stability within 65 h on stream. It would be interesting to further elucidate the surface chemistry of KMnO4 modification and the nature of active sites. Considering the fact that many supported metal catalysts are prepared by dispersing organic ligand-capped metal or oxide colloids on solid supports,2,12,20,68,69 the KMnO4-oxidation method reported here may provide an alternative methodology to activate this class of nanocatalysts. Acknowledgment. This work was supported by the Office of Basic Energy Sciences, U.S. Department of Energy. The Oak Ridge National Laboratory is managed by UT-Battelle, LLC for the U.S. DOE under Contract DE-AC05-00OR22725. This research was supported in part by the appointment of H.F.Y. and Z.M. to the ORNL Research Associates Program, administered jointly by ORNL and the Oak Ridge Associated Universities. References and Notes (1) Astruc, D.; Lu, F.; Aranzaes, J. R. Angew. Chem., Int. Ed. 2005, 44, 7852. (2) Bönnemann, H.; Nagabhushana, K. S. In Surface and Nanomolecular Catalysis; Richards, R., Ed.; Taylor & Francis (CRC Press): Boca Raton, FL, 2006; p 63. (3) Ott, L. S.; Finke, R. G. Coord. Chem. ReV. 2007, 251, 1075. (4) Haruta, M.; Daté, M. Appl. Catal., A 2001, 222, 427. (5) Choudhary, T. V.; Goodman, D. W. Top. Catal. 2002, 21, 25. (6) Hashmi, A. S. K.; Hutchings, G. J. Angew. Chem., Int. Ed. 2006, 45, 7896. (7) Bond, G. C.; Louis, C.; Thompson, D. T. Catalysis by Gold; Imperial College Press: London, 2006. (8) Kung, M. C.; Davis, R. J.; Kung, H. H. J. Phys. Chem. C 2007, 111, 11767. (9) Yuan, Y. Z.; Asakura, K.; Wan, H. L.; Tsai, K.; Iwasawa, Y. Catal. Lett. 1996, 42, 15. (10) Martra, G.; Prati, L.; Manfredotti, C.; Biella, S.; Rossi, M.; Coluccia, S. J. Phys. Chem. B 2003, 107, 5453. (11) Overbury, S. H.; Ortiz-Soto, L.; Zhu, H. G.; Lee, B.; Amiridis, M. D.; Dai, S. Catal. Lett. 2004, 95, 99. (12) Chou, J.; Franklin, N. R.; Baeck, S.-H.; Jaramillo, T. F.; McFarland, E. W. Catal. Lett. 2004, 95, 107. (13) Bore, M. T.; Pham, H. N.; Switzer, E. E.; Ward, T. L.; Fukuoka, A.; Datye, A. K. J. Phys. Chem. B 2005, 109, 2873. (14) Delannoy, L.; El Hassan, N.; Musi, A.; Le To, N. N.; Krafft, J.M.; Louis, C. J. Phys. Chem. B 2006, 110, 22471. (15) Schubert, M. M.; Hackenberg, S.; van Veen, A. C.; Muhler, M.; Plzak, V.; Behm, R. J. J. Catal. 2001, 197, 113. (16) Okumura, M.; Nakamura, S.; Tsubota, S.; Nakamura, T.; Azuma, M.; Haruta, M. Catal. Lett. 1998, 51, 53. (17) Okumura, M.; Tsubota, S.; Haruta, M. J. Mol. Catal. A 2003, 199, 73. (18) Yang, C.-M.; Kalwei, M.; Schüth, F.; Chao, K.-J. Appl. Catal., A 2003, 254, 289. (19) Chi, Y.-S.; Lin, H.-P.; Mou, C.-Y. Appl. Catal., A 2005, 284, 199. (20) Budroni, G.; Corma, A. Angew. Chem., Int. Ed. 2006, 45, 3328. (21) Zhu, H. G.; Liang, C. D.; Yan, W. F.; Overbury, S. H.; Dai, S. J. Phys. Chem. B 2006, 110, 10842. (22) Zhu, H. G.; Ma, Z.; Clark, J. C.; Pan, Z. W.; Overbury, S. H.; Dai, S. Appl. Catal., A 2007, 326, 89. (23) Block, B. P.; Bailar, J. C. J. Am. Chem. Soc. 1951, 73, 4722. (24) Guillemot, D.; Polisset-Thfoin, M.; Fraissard, J. Catal. Lett. 1996, 41, 143.

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