CO Oxidation Catalyzed by Au−Ag Bimetallic Nanoparticles Supported

Sep 18, 2009 - We report a novel Au−Ag bimetallic nanocatalyst supported on an acidic mesoporous aluminosilicate Au−Ag@APTS-MCM prepared by a two-...
0 downloads 13 Views 373KB Size
Download PDF
J. Phys. Chem. C 2009, 113, 17831–17839

17831

CO Oxidation Catalyzed by Au-Ag Bimetallic Nanoparticles Supported in Mesoporous Silica Chun-Wan Yen,† Meng-Liang Lin,† Aiqin Wang,†,‡ Shin-An Chen,§ Jin-Ming Chen,§ and Chung-Yuan Mou*,† Department of Chemistry and Center of Condensed Matter Research, National Taiwan UniVersity, Taipei, Taiwan 106, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China, and National Synchrotron Radiation Research Center, Hsinchu 30076 ReceiVed: August 18, 2008; ReVised Manuscript ReceiVed: August 19, 2009

We report a novel Au-Ag bimetallic nanocatalyst supported on an acidic mesoporous aluminosilicate Au-Ag@APTS-MCM prepared by a two-step synthesis procedure, which is very active for low-temperature CO oxidation. Its catalytic activity is still quite appreciable after 1 year of storage under room conditions. The silane APTS [H2N(CH2)3-Si(OMe)3] was used to surface functionalize mesoporous silica. The functionalized mesoporous silica was used to absorb the gold precursor AuCl4- and silver precursor AgNO3 to form gold-silver bimetallic nanoparticles inside the nanochannels after chemical reduction. The catalysts were activated by calcinations, followed with hydrogen reduction at 873 K. Using various characterization techniques, such as X-ray diffraction, UV-vis, transmission electrom microscopy, and X-ray absorption fine structure spectroscopy (EXAFS), we elucidated the structure and surface compositions. As compared with the previously reported Au-Ag@MCM, prepared by one-pot procedure, the new method yields smaller sizes of AuAg bimetallic nanoparticles (4-6 vs 20 nm). They exhibited higher activity in catalysis for low-temperature CO oxidation with high stability. Moreover, the catalyst is resistant to moisture over a long storage time. A synergetic effect in relative composition was also found. The EXAFS study shows that Ag predominantly resides on the surface of the bimetallic nanoparticle. This distribution helps to yield a catalyst that is very active in both CO and O2 neighboring sites. 1. Introduction Supported gold nanoparticles have received much recent attention due to their unique catalytic performances in a series of oxidation-reduction reactions, including CO oxidation,1–5 epoxidation of alkenes,6–9 water-gas shift reaction,10–12 hydrogenation,13,14 and C-C bond formation.15 In particular, lowtemperature CO oxidation has been extensively studied for its importance as a model reaction in fundamental research and also for its wide applications in air cleaning. It has been wellestablished that the catalytic properties of gold for CO oxidation are not only determined by the particle size but also largely dependent on the nature of the support and the preparation procedures, which give the very complex nature of gold catalysis. A clear description of the active site in gold catalyst has not yet been obtained, although it has been proposed that the active sites are at the interface between the gold and the support.16–18 Central to the problem of CO oxidation is the activation of oxygen molecules.19 While gold nanoparticles adsorb CO molecules well, they do not strongly adsorb and activate oxygen molecules.20,21 Thus, the oxide support often plays an important role in the activation of oxygen, especially those with good redox property. In fact, almost all of the active gold catalysts reported so far are a combination of an active support (TiO2, Fe2O3, etc.) and a small size for gold particles (∼3 nm). Previously, we reported an alternative way to modify the gold-based catalysts, which is to use silver that can form an alloy with gold and * To whom correspondence should be addressed. E-mail: cymou@ ntu.edu.tw. † National Taiwan University. ‡ Chinese Academy of Sciences. § National Synchrotron Radiation Research Center.

possesses stronger affinity with O2 than gold.22 That is, where two different metal atoms are in intimate proximity to each other, as in an alloy, the activated O2 can easily react with the activated CO at a neighboring gold atom to give the product CO2. With this idea, we developed a simple one-pot method in our earlier work23–25 to incorporate surfactant-protected gold-silver alloy particles into mesoporous MCM-41. After high-temperature hydrogen reduction, a strong synergetic effect between gold and silver was observed in our Au-Ag@MCM system. Especially when the molar ratio of Au/Ag is at 3/1, the activity is optimum; CO conversion was above 10% even at a temperature as low as 210 K, which is comparable to the best reported active catalyst including Au/TiO2 and Au/Fe2O3. Recently, Liu et al.26 have employed a general two-step approach27 for the synthesis of very small (∼3 nm) bimetallic gold-silver nanoparticles on inert supports such as silica and alumina. The resultant Au-Ag alloy particles are highly catalytically active for CO oxidation. A similar synergetic effect in CO oxidation was also found in the Au-Cu alloy nanoparticles system.28,29 In this work, we make Au-Ag nanocatalyst supported on an acidic support of mesoporous aluminosilicate. It is known that gold catalyst often failed to function on acidic supports;1 thus, a study of a goldbased catalyst on an acidic support would be desirable. Furthermore, we would like to give detailed characterization of Au-Ag nanocatalyst supported on the aluminosilicate MCM41 to understand the origin of the high activity. These bimetallic systems would actually offer one a unique chance to understand the catalytic action of gold nanoparticles in oxidation reactions. This is because the transport of the oxygen species is no longer a limiting factor that tends to complicate the kinetic. Also, the affinity toward CO can be finetuned in Au-Ag alloy by varying the alloy composition.

10.1021/jp9037683 CCC: $40.75  2009 American Chemical Society Published on Web 09/18/2009

17832

J. Phys. Chem. C, Vol. 113, No. 41, 2009

In this work, we prepare Au-Ag bimetallic particles on aluminosilicate MCM-41 support with a two-step method by using the surface-functionalized silica as the supporting host to confine the bimetallic nanoparticles, and the silane APTS [H2N(CH2)3-Si(OMe)3] was used in the surface modification. The surface functionalization-loading approach has been previously developed for pure Au catalyst supported on mesoporous silica.30–32 It has been found that the chemical reduction sequence for the two metals is vital to obtaining the Au-Ag bimetallic particles.27 Such Au-Ag bimetallic particles have wellcontrolled particle sizes in the range of 3-6 nm and are highly active for catalyzing CO oxidation.26 These kinds of AuAg bimetallic catalysts made by functionalized mesoporous silica are named as AuAg@ATPS-MCM to distinguish the notation AuAg@MCM we used for the previous catalyst using one-pot method. It is found this catalyst gives excellent catalytic activity for CO oxidation over an acidic aluminosilicate support. Several characterization techniques were used to study the catalyst system, including nitrogen adsorption, X-ray diffraction (XRD), UV-vis spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and especially X-ray absorption fine structure spectroscopy (EXAFS). In particular, EXAFS has been proven useful in identifying the degree of alloy of the catalyst. Using EXAFS, interatomic distances, and variation in the distances, the identification and number of the nearest neighboring atoms within the first few coordination shells of the X-ray excited atoms can be determined.33,34 Finally, we discuss the mechanism of the catalysis and the possible causes for the high activity of this Au-Ag nanocatalyst and compare them with the catalytic ability of the literature reported Au/SiO2.35 2. Experimental Section 2.1. Catalyst Preparation. 2.1.1. Nanosized MCM-41. For improving materials transport in the nanochannels, we adopt a special synthesis to make nanosized MCM-41 as previously reported by Lin et al.36 The reactant molecules could avoid a long journey through the channels and possible obstruction. A 5.5 g amount of sodium silicate with a pH 9 was added to a mixture of 1.6 g of hexadecyl trimethyl ammonium bromide (CTAB), 0.063 g of NaAlO2, and water (the Si to Al ratio is 33 to 1). A white precipitate gel solution was formed and then stirred for 1 h at 318 K. Then, the gel solution was transferred to an autoclave, undergoing hydrothermal reaction at 373 K for 24 h. Finally, filtration, washing, and drying gave the assynthesized MCM-41. 2.1.2. Gold-SilWer Bimetallic Catalysts (Denoted as AuAg@ APTS-MCM). Next, we functionalized the surface of MCM41 to prepare for metal loading. Previously,30,31 we used surfacefunctionalized mesoporous silica as the supporting host to confine the gold nanoparticles and obtained well-controlled particle sizes (2-3 nm) of gold. In such a procedure,37,38 the surfactant was removed simultaneously with the surface functionalization. Afterward, gold precursor was adsorbed on the modified surface of mesoporous silica and was reduced to gold nanoparticles by reducing agents. One outstanding advantage of this method is that the gold particles are highly confined and less susceptible to sintering, even after high-temperature pretreatment. The nanosized MCM-41 was functionalized with 3-aminopropyltrimethoxysilane (APTS). The surface-functionalized mesoporous silica (1 g) was added to the tetrachloroaurate (HAuCl4) solution to adsorb AuCl4-. The material was reduced with 0.1 M sodium boronhydride (NaBH4) aqueous solution.

Yen et al. SCHEME 1: Flowchart of the Synthesis Steps of AuAg@ATPS-MCM

After Au nanoparticles were anchored on the mesoporous silica, silver nitrate (AgNO3) solution was added and also reduced with NaBH4. The whole loading amount of metal was controlled to 5 weight percent. After calcinations at 833 K in air for 6 h, the AuAg@APTS-MCM (this notation is to distinguish from our previous AuAg@MCM made by one pot) was obtained. Monometallic Au@APTS-MCM and Ag@APTS-MCM were made in a similar way. The synthetic flowchart of AuAg@APTSMCM is summarized in Scheme 1. 2.2. Activity Measurements. The CO oxidation reaction was performed in a continuous flow fixed-bed microreactor. A 0.02 g amount of catalyst was used in each experiment. Before measurement, the catalyst was prereduced in situ in 10% H2 in N2 at 873 K for 1 h. The reactant gases were purified by 4 Å molecular sieves, then mixed, and passed into the reactor. The reactant flow consisted of a mixture of 1% CO and 99% air as balance. A total gas flow of 33.3 mL min-1 was applied corresponding to a GHSV of about 100 L g cat-1 h-1. The reactants and products were analyzed online with a HP6890 gas chromatograph equipped with a Carboxen1000 column and TCD detector. The CO conversion is defined as the amount of CO2 produced divided by the total amount of CO fed to the catalyst. 2.3. Characterizations of Catalyst. The UV-vis spectra were recorded on the Hitachi U-3010 UV-vis spectrophotometer at ambient temperature operating in the reflection mode at a resolution of 2 nm using barium sulfate as a standard for the background correction. The powder XRD patterns were collected on a Philips PW 1830 instrument operating at 45 kV of voltage and a current of 40 mA with Cu KR radiation in the 2θ range from 20° to 80°. Nitrogen adsorption-desorption isotherms were obtained at 77 K on a Micromeritics ASAP 2010 apparatus, and the pore size distribution was calculated from the nitrogen adsorption isotherm by the BJH (Barrett-Joyner-Halenda) method. SEM was taken on a JEOL-JSM-6700F field emission SEM operated at 10 kV. TEM images of the catalysts were obtained using a JEOL JEM-2010 transmission electron microscope with an operating voltage of 200 kV. Samples were dispersed in

CO Oxidation Catalyzed by Au-Ag Nanoparticles

J. Phys. Chem. C, Vol. 113, No. 41, 2009 17833

Figure 1. (A) XRD patterns of nanosized MCM-41 shown in black lines and AuAg@ATPS-MCM (Au:Ag ) 8/1) shown in red lines. The inset picture shows an SEM image of nanosized MCM-41. (B) TEM images and size histogram of AuAg@APTS-MCM (Au/Ag ) 8/1), and the average bimetallic particles size is 3.7 nm (D ) 3.7 ( 1.5 nm) after calcination.

TABLE 1: Chemical Composition and Textual Properties of Catalysts and Estimated Metal Particle Size metal particle size (nm) determined by Au/Ag (molar ratio)a 1/0 8/1 4/1 0/1

1/0 8.62 4.76 0/1

pore size (nm)

pore volume (cm3/g)b

BET surface area (m2/g)

metal loading amount (wt %)

XRDc

TEM

2.3 2.3 2.3 2.3

0.95 0.85 0.81 0.83

693 683 651 705

5.89 4.03 4.63 0.91

4.0 4.1 6.0 12.3

4.1 3.7 6.8 11.9

The first column is nominal composition, and the second column is determined by ICP-AES. b Total pore volume obtained at P/P0 ) 0.9. Particle size calculated from the Scherrer’s equation: CL ) Kλ/ β cos θ(K ) 0.9; λ ) 0.1541 nm) corresponding to the (111) plane. a

c

ethanol, and a drop of the obtained suspension was fixed on a microgrid covered with amorphous carbon film. X-ray absorption spectra at the Au L edge and the Ag K edge were recorded at the Wiggler 17C1 and superconducting wavelength shifter 01C1 beamlines of the National Synchrotron Radiation Research Center (NSRRC) in Taiwan. The electron storage ring of the NSRRC was operated at an energy of 1.5 GeV. Both beamlines employed a double Si(111)-crystal monochromator for energy selection with a resolution ∆E/E better than 1 × 10-4. All X-ray absorption spectra were recorded at room temperature in a transmission mode in which the intensities of incident and transmitted X-ray beams were measured by gasfilled ionization chambers. A reference compound for energy calibration, Ag foil or Au foil, was measured simultaneously by using the third ionization chamber. The procedures for analyzing the EXAFS data were developed at NSRRC, Taiwan, and are summarized as follows.34 First, the raw X-ray absorption spectrum in the pre-edge region was fitted to a straight line using the least-squares algorithm, and the background above the edge was fitted with a cubic spline. The EXAFS function, χ, was obtained by subtracting the postedge background from the overall absorption and then normalized with respect to the edge jump step. The normalized χ(E) was transformed from energy space to k-space. k3-weighted χ(k) data in the k-space ranging from 3.2 to 10.4 Å-1 for the Ag K edge and from 3.4 to 12.1 Å-1 for the Au L edge were Fourier transformed to r-space to distinguish the EXAFS contributions from the different coordination shells. A nonlinear least-squares algorithm was applied to the curve fitting of an EXAFS in the r-space ranging from 1.2 to 3.3 Å for Ag and from 1.4 to 3.6 Å for Au. The ranges for k and R used in fitting the EXAFS spectra for all of the samples are listed in the

Supporting Information (Table S1). To verify the validity of the fits, fittings with both k1 and k3 weightings were done for the sample Au/Ag ) 4/1* and are presented in Table S2 and Figure S5 of the Supporting Information to show that there is not an appreciable difference between the two weightings. All of the computer programs were complemented in the Ifeffit 1.2.10 package with the backscattering amplitude and the phase shift for the specific atom pairs calculated by FEFF 6.02 L code. The amplitude reduction factors, S02, for Au and Ag were deduced by analyzing the Au and Ag foil reference samples, respectively. The S02 values were found to be 0.793 and 0.827 for Au and Ag, respectively. 3. Results The first part is the structure characterization of the catalysts. Figure 1A gives the low-angle part of the XRD showing a 2D hexagonal mesostructure of MCM-41. The broadening is due to the very small particle size. After loading the metal into MCM-41, the array channels were affected by the incorporation with nanoparticles. Therefore, the intensity of XRD pattens of AuAg@APTS-MCM is lower than it of the pure MCM-41. Also in Figure 1A, the SEM image shows nanosized mesoporous silica MCM-41 at around 50 nm size; those short channels of nanosized MCM-41 can ensure easy molecular diffusion inside the channel without obstructions. The BET surface area, pore volume, and average pore size of the catalysts with different Au/Ag molar ratios are listed in Table 1. The BET surface areas of the catalysts were between 650 and 700 m2/g, and the pore size was 2.3 nm. For the nitrogen adsorption-desorption isotherms (Supporting Information, Figure S1), all samples show a capillary condensation at P/P0 around 0.35 as the typical

17834

J. Phys. Chem. C, Vol. 113, No. 41, 2009

Yen et al.

Figure 2. Wide-angle XRD patterns of the AuAg@APTS-MCM-41 with different Au/Ag ratios: (a) 1/0, (b) 8/1, (c) 4/1, and (d) 0/1.

MCM-41 mesoporous aluminosilicate. An extraordinarily large rise in adsorption at P/P0 ∼ 0.9 is ascribed to the filling of the textural mesoporosity, which resulted from the aggregation of the nanoparticles of MCM-41.36,39 Over 70% of metal nanoparticles size is below 3 nm, and only less than 10% is bigger than 5 nm (Figure 1B). From the TEM images (Figure S2 of the Supporting Information), the gold-silver nanoparticles were found somewhat restricted but not completely confined by the mesoporous channels. Although the average size (3.7 nm) of metal nanoparticles is bigger than the pore channel size (2.3 nm) of MCM-41, the pore wall of MCM-41 still limits silver-gold nanoparticles against seriously sintering when subjected to thermal treatment at temperatures up to 560 °C for 6 h. That the 560 °C calcination completely removes the organics (mainly APTS) was checked by thermal gravity analysis (Figure S3 of the Supporting Information). The loading amount of gold and silver was determined by ICP-AES. The total metal loading was controlled at around 5 wt % (see Table 1). Figure 2 shows wide-angle XRD patterns of the catalysts with different Au/Ag ratio after they were calcined at 833 K. The XRD patterns of monometallic Ag@APTS-MCM and bimetallic Au-Ag catalysts are identical with Au@APTS-MCM, which is characterized by four peaks positioned at 2θ ) 38.2, 44.3, 64.5, and 77.6° due to the (111), (200), (220), and (311) lattice planes. The XRD pattern of Ag@APTS-MCM is sharper than other catalysts, which indicates that the particle size of silver is larger than others. The particle sizes of Au-Ag bimetallic and pure Au as determined by both TEM and the half-width of XRD peaks agree with each other rather well and all of the sizes of metal nanoparticles are in the range of 4-6 nm, except for pure silver (see Table 1). It is not possible to determine Ag-Au alloy formation from XRD pattern since silver and gold has the same crystal structure (fcc) and similar lattice constant (0.409 vs 0.408 nm). Therefore, further instruments such as UV-vis spectrometry and EXAFS are needed to examine whether those Au-Ag bimetallic nanoparticles form a homogeneous alloy structure or not. The absorption bands in UV-vis spectrum (Figure 3) for the pure Au@ATPS-MCM and pure Ag@APTS-MCM are at 515 and 405 nm, which is consistent with the reported spectra.40,41 For Ag-Au nanoparticles, there is only one plasmon absorption peak in-between the absorption bands of pure Au and pure Ag in UV-vis spectrum and the maximum of the plasmon band

Figure 3. UV-vis absorption spectra of gold, silver, and gold-silver bimetallic nanoparticles with varying Au/Ag molar ratios. The Au/Ag molar ratios were (a) 1/0, (b) 8/1, (c) 4/1, and (d) 0/1.

blue shifts with increasing silver content. Those results imply that gold and silver form homogeneous alloy nanoparticles.40,41 Figure 4A shows the CO conversions over Au-Ag@APTSMCM with various compositions. One can see that in the lowtemperature range (300-380 K), pure metal catalysts (Au@APTSMCM and Ag@APTS-MCM) show much lower CO conversion than the bimetallic catalysts. There is a synergetic effect. Au-Ag@APTS-MCM owns greater activity toward CO conversion. In particular, the bimetallic catalyst with Au/Ag ) 8/1 gives the best activity; it is active even at 250 K. Although our reactions were not run under differential conditions, but to compare activities, at least semiquantitatively, over different works we gave reaction rates in Table 2. The rates in Table 2 for our cases are somewhat underestimated. As compared to other Au/SiO2 catalysts (see Table 2), the reaction rate of our catalysts, Au-Ag@APTS-MCM (Au/Ag ) 8/1) at 303 K is 0.72 molco g Au-1 h-1, which is higher than the other pure gold Au/SiO2 catalysts, and it is comparable to the other best active catalysts reported in literature, such as Au/TiO2 and Au/Fe2O3.14,15 As comparing with the previously reported AuAg@MCM, prepared by colloidal templating, we list the activities (at 30 °C) for the best performing catalysts from the two methods in Table 2. The new bimetallic catalyst (AuAg@APTS-MCM) yields higher activity in catalysis for low-temperature CO oxidation. AuAg@APTS-MCM (Au/Ag ) 8/1) even without hydrogen reduction at 873 K still owns great activity (Figure 4B). Contrary to current opinion, it seems silica can be a good catalyst support for CO oxidation. Previously, we found the SiO2-supported gold-silver catalysts needed to be activated with high-temperature hydrogen reduction to obtain good alloy formations.23–25 However, for Au-Ag@APTS-MCM, this hightemperature reduction pretreatment is not necessary. We believe it is because we used the functionalization method to prepare our catalysts, and in this way, the bimetallic particles were much smaller (just 5 nm) and formed alloy already in the step of calcination (we used EXAFS to determine this), not in the

CO Oxidation Catalyzed by Au-Ag Nanoparticles

J. Phys. Chem. C, Vol. 113, No. 41, 2009 17835

Figure 4. (A) CO conversion profile with reaction temperature over Au-Ag@APTS-MCM after calcinations and reduction with Au/Ag molar ratio: 1/0 (b), 8/1 (2), 4/1 (f), and 0/1 (9). (B) AuAg@APTS-MCM (Au/Ag ) 8/1) only with calcinations still owns activity toward CO conversion at 353 K. (C) CO conversion of AuAg@APTS-MCM (Au/Ag ) 8/1) stored for 1 year. (D) The CO oxidation conversion of moisture condition.

TABLE 2: Reaction Rates for the Oxidation of CO to CO2 with Different Au/SiO2 Catalysts at Specified Temperatures and Contact Times catalysts

T (°C)

reaction rate (mol g Au-1 h-1)

contact time (gcat h molCO-1)

particle size (nm)

source

Au@APTS-MCM AuAg@APTS-MCM (Au:Ag ) 8:1) AuAg@APTS-MCM (Au:Ag ) 8:1) Ag@APTS-MCM Au-Ag/SiO2 Au@SBA-15 (thin film) Au@MCM-41 Au@MCM-48 Au@SBA-15 AuNPs

30 30 0 30 20 80 80 80 80 30

0.245 1.097 0.720 0.093 0.92 0.246 0.625 0.350 0.475 0.480

22.6 23.4 22.6 22.6 2.2 22.6 22.8 23.6 24.2 93

4.0 4.1 4.1 12.3 3.3 1.8 5.1 6.9 5.8 3.6

this work this work this work this work ref 26 ref 24 ref 23 ref 23 ref 23 ref 33

hydrogen reduction. So it can explain why our bimetallic catalyst had such a good activity and unique character. In comparison with the reported catalysts toward CO oxidation, we note that it is not hard to find high conversion of CO oxidation catalysts; however, the most crucial point of practical application is long-term stability.16,17 We tested one Au-Ag@ APTS-MCM (Au/Ag ) 8/1), which has been stored under ambient conditions for 1 year, and found that the catalyst can still give 100% conversion (at 353 K test and pretreatment conditions the same as Figure 4A) and lasted for 72 h (figure not shown here) with normal pretreatment (in 10% H2 in N2 at 873 K for 1 h). Besides, we also stored another high-temperature hydrogen-treated catalyst and tested its activity after 1 year (composition shown in Figure 4C). With only mild drying and the catalyst under nitrogen flow for half an hour at 423 K before the CO oxidation reaction, the bimetallic catalyst shows appreciable activity at 353 K and can reach almost 90% conversion at 443 K. These results show that AuAg@APTS-

MCM can be stored in ambient conditions for a long time and its activity would not be destroyed by environmental factors, like heat, moisture, and lightness. Moisture is one of the major components in practical application environments (for example, in PROX), but most catalysts will be deactivated by a high concentration of water vapor.18,25 Therefore, we investigated the moisture effect for AuAg@APTS-MCM with 3% moisture in the reactant gas. In Figure 4D, one can see that AuAg@APTSMCM (Au/Ag ) 8/1) gave 100% conversion at 350 K under 3% moisture and AuAg@APTS-MCM (Au/Ag ) 4/1) had 95%; the CO oxidation activity was quite stable under dry or moisture conditions. AuAg@APTS-MCM is hardly to be deactivated by moisture even in considerable quantities. To understand the atomic organization of the bimetallic nanoparticles, we performed EXAFS experiments on our catalysts. Figure 5 shows k3-weighted EXAFS oscillations at the Au L3 edge and the Ag K edge of the calcined and reduced samples with different Au/Ag ratios. Figure 6 shows the radial

17836

J. Phys. Chem. C, Vol. 113, No. 41, 2009

Yen et al. 4 (Ag K edge). From the Au L3 edge in Table 3, one can see that the bond distance for Au-Au and Au-Ag is almost the same, and the total coordination number (CN) of gold is between 10 and 11 for all of the ratios of Au/Ag. Besides, there are no notable differences of local atomic structure between the calcined samples and the hydrogen reduction samples. However, they exhibit the variation in the Ag K edge (see Table 4). In the Ag K edge, the CN of silver without high-temperature hydrogen reduction samples, the sum of coordination number of Ag-Ag and Ag-Au was low. This indicates that most of the Ag atoms are on the surface, so their CN is low. For calcined sample, Ag-O bonds were also included to fit the experimental data. The calcination under air for 6 h was to make sure there will be no organic residues. Metals tend to be oxidized in such conditions and form metal-oxygen bond. Also, as one can see, with an increasing amount of silver (comparison between Au/ Ag ratio of 8/1 and 4/1), the extent of Ag-O formation increases at the same time. Silver is more easily oxidized than gold, so in the fitting of the Ag K edge, one has to take into account the Ag-O bond.43 For hydrogen-reduced samples, all oxidized silver metals should be reduced by high-temperature hydrogen. 4. Discussions

Figure 5. k3-weighted EXAFS oscillations at (A) the Au L3 edge and (B) the Ag K edge of the calcined and reduced samples with different Au/Ag ratios.

distribution function around the Au and Ag atoms obtained by taking Fourier transforms of the corresponding k3χ(k) EXAFS data at the Au L3 edge and the Ag K edge in Figure 5 for the calcined and reduced samples. As is clearly seen from Figures 5 and 6, a phase shift in χ(k) is observed, and Au-Ag in the first shell appears as a doublet. The doublet components vary with the ratio of gold to silver, and this indicates a significant amount of Ag in the nearest-neighbor around Au. In Figure 5A, oscillatory features of bimetallic nanoparticles change greatly from the oscillation of pure Au particles at k ) 6 Å-1. It is due to an interference change between Au and Ag oscillation because Au and Ag atoms have quite different backscattering phase shifts at higher k. The same results had also been observed in Ag K-space. It is worth pointing out that both calcined and reduced bimetallic samples show different oscillation from pure gold sample at k ) 6 Å-1, a dephasing of the oscillation pattern. This is a signature of alloy formation. In previous reports,42 the oscillation at k ) 6 Å-1 would be similar to pure gold if the Au-Ag did not form alloy or core-shell structures. Therefore, we conclude that AuAg@APTS-MCM is in some degree of alloying at the step of calcination, and this result is in agreement with the above observation in UV-vis spectra. From EXAFS data, one cannot only know the formation of alloy but also can study the local structural information.42 We analyzed EXAFS spectra in Figure 6 to determine the atom type and the coordination number (CN) of Au and Ag; the results of analysis are listed in Table 3 (Au L3 edge) and Table

Mesoporous materials are becoming popular as catalyst supports in recent years. The advantage is that the high surface area of the nanochannels provides a host, within which nanoparticles can be confined. Good dispersion and size control can often be achieved. Haruta’s group using CVD method44 and Dai’s groups using an DP method45 have deposited organometallic gold precursors to obtain silica-supported gold particles that were small enough to be active for CO oxidation. However, the precursors are rather expensive. We, on the other hand, used surface functionalization of mesoporous silica with strong ligands (such as aminopropyle) for a high and uniform loading of HAuCl4. After simple reduction, one obtained gold nanoparticles confined in mesoporous silica system that also gave excellent activities in CO oxidation.30,31 Bimetallic nanoparticles are generally not so straightforward to be loaded on supports.26 In this work, a synthetic approach based on surface functionalization was shown to give gold-silver bimetallic nanoparticles supported on mesoporous silica. The role of APTS is to capture gold by its interaction with amine. This point has been well-studied in previous works.26,30,31 It is particularly important for the Au and Ag alloy formation. Without APTS, the negatively charged silica surface would not adsorb AuCl4- at all. With APTS, it becomes positively charged due to -NH3+, and it attracts AuCl4-. After reduction, the Ag+ can then be loaded onto Au nanoparticle by electrodeless reduction. We chose to use the nanoparticle form of MCM-41, for reason of better transport, as the support. We note here that the method also worked for SBA-15, which in fact would give stronger confinement because of its thicker wall. MCM-41, on the other hand, is not strong enough to completely confine the Au particles within nanochannels after calcination. Nonetheless, the AuAg bimetallic nanoparticles of 4-6 nm are much smaller than the sizes obtained in our previous one-pot method.22,23 We should note that it is important to load gold and reduce it first and then to follow with the loading and reduction of silver.26,27 Any other order or simultaneous loading of both metals would not result in the desired alloy form. This method can be easily generalized to other bimetallics. For example, we recently successfully loaded Au-Cu bimetallic nanoparticles with sizes of ∼3 nm in the confined space of SBA-15 and obtained good catalytic activity in CO oxidation.28

CO Oxidation Catalyzed by Au-Ag Nanoparticles

J. Phys. Chem. C, Vol. 113, No. 41, 2009 17837

Figure 6. Fourier transforms of k3χ(k) EXAFS data at (A) the Au L3 edge and (B) the Ag K edge for the calcined and reduced samples. Solid lines denote the experimental data, and dashed lines show the numerical fittings.

TABLE 3: Curve Fitting Results of the EXAFS Data at Au L3 Edge of the Catalysts after Calcination or after Hydrogen Reduction sample Au/Ag ) 1/0 Au/Ag ) 8/1 Au/Ag ) 8/1a Au/Ag ) 4/1 Au/Ag ) 4/1a a

bond

CN

Au-Au 10.5(7) Au-Au 8.1(8) Au-Ag 2.7(3) Au-Au 7.9(10) Au-Ag 3.3(3) Au-Au 4.6(8) Au-Ag 4.9(4) Au-Au 6.0(9) Au-Ag 4.5(4)

R (Å)

σ2 (Å2)

r factor (%)

2.85(1) 2.82(1) 2.86(2) 2.81(1) 2.86(2) 2.83(2) 2.84(1) 2.82(2) 2.84(1)

0.0077(6) 0.0107(16) 0.0103(17) 0.0114(19) 0.0097(12) 0.0081(20) 0.0083(10) 0.0097(20) 0.0089(11)

0.59 0.38 0.48 0.44 0.23

With hydrogen reduction (at 600 °C).

TABLE 4: Curve Fitting Results of the EXAFS Data at Ag K Edge of the Catalysts after Calcination or after Hydrogen Reduction sample

bond

Au/Ag ) 8/1 Ag-O Ag-Ag Ag-Au Au/Ag ) 8/1a Ag-Ag Ag-Au Au/Ag ) 4/1 Ag-O Ag-Ag Ag-Au Au/Ag ) 4/1a Ag-Ag Ag-Au Au/Ag ) 0/1 Ag-O Ag-Ag a

CN

R (Å)

σ2 (Å2)

r factor (%)

1.0(17) 1.9(10) 2.9(18) 2.7(10) 3.2(16) 2.8(33) 3.7(9) 2.3(11) 5.7(5) 2.9(6) 0.7(2) 7.0(3)

2.16(15) 2.81(3) 2.86(2) 2.83(3) 2.86(2) 2.12(22) 2.83(3) 2.84(1) 2.83(1) 2.84(1) 2.38(3) 2.87(1)

0.0108(271) 0.0068(104) 0.0053(92) 0.0067(49) 0.0057(70) 0.0242(223) 0.0113(52) 0.0078(87) 0.0094(14) 0.0059(31) 0.0085(58) 0.0100(4)

0.41 0.17 0.70 0.85 0.06

With hydrogen reduction (at 600 °C).

Then, we would like to discuss the comparison with previously reported reaction rates in Table 2. We compare first the performance of AuAg@APTS-MCM with AuAg@MCM. Both samples yield strong synergetic effect in comparing with pure Au or Ag nanocatalysts. The synergy can probably be explained in the same way previously proposed; for example, CO and O2 are activated in neighboring Au and Ag sites on an alloy surface. The transfer of oxygen atom between them is thus much facilitated. Second, for the AuAg bimetallic system, there seem

to be little size effect. This is in contrast to the pure Au nanocatalyst where a strong dependence on size has been observed. Although the new catalyst with smaller size shows an apparently higher rate, in terms of per surface Au atom basis, the two samples give quite comparable reaction rates roughly. This is understandable since CO and O2 are activated in neighboring Au and Ag sites on a bimetallic surface. One then does not expect much size effect except for the surface area effect. In the moisture tolerance test, the AuAg bimetallic catalysts prepared by the two methods, however, behave very differently. For the AuAg@MCM one-pot catalyst, we previously reported that its activity toward CO oxidation is completely quenched by moisture. However, in AuAg@APTSMCM, its activity is not affected by moisture. This moistureresistance property of our catalyst is rather interesting. Generally, water at ∼100 ppm level is promotional to catalytic activity in CO oxidation.46 However, at the very level such as in our experiment, water deactivates the catalytic activity very much.47 We believe the moisture resistance of our catalysts may be due to the acidic nature of the silica support. Most of the other supports, such as Fe2O3, are basic in nature and in the combination of moisture and carbon dioxide, the chance of surface carbonate formation is increased. However, our aluminosilicate is an acidic support; it can reduce the possibility of poison by carbonate. In fact, as compared to that in a preious report,26,30,31 the acidic support in this work may have the advantage in fuel cell applications where an acidic environment is encountered. In the literature, most Au nanocatalyst for CO oxidation is less active when acidic supports are used.48 Finally, our AuAg@APTS-MCM catalyst is very stable over a storage time of 1 year. The stability was recently explained by us26 in terms of good anchorage on support. This new catalyst is thus more useful in practical applications. Finally, we observe that silica is not really a poor support provided the sizes of the nanoparticles are controlled to less than 6 nm. Our group and Corma’s group had shown (the last few rows in Table 2) that well-confined Au nanoparticle in silica can give good catalytic activity for CO oxidation. Silica was usually considered to be an ineffective support for the dispersion of gold in a catalytically active form because the weak

17838

J. Phys. Chem. C, Vol. 113, No. 41, 2009

interaction between unconfined Au and silica usually results in a much too large size upon high-temperature pretreatment. From the Au L3 edge data in Table 3, we note that the total coordination numbers of gold in all bimetallic samples are between 10 and 11, less than the bulk value of 12. This is due to the fact that the metal particle size in our AuAg@APTSMCM is rather small (4-6 nm). We also note that there is a strong enrichment of Ag on the surface of the bimetallic nanoparticles. Take the sample of Au/Ag ) 8/1 (total composition) in Table 3; the EXAFS technique gives a relative ratio of Au/Ag ) 8.1/2.7 ) 3/1 from the ratio of Au-Au and Au-Ag coordination, which is different from bulk ratio. This means that there is partial segregation of the two elements in microscopic distribution. A likely explanation is that when the catalyst is first calcined, there is partial surface oxidation as we find the Ag K edge EXAFS can be better fitted with some Ag-O coordination (Table 4). After hydrogen reduction, the EXAFS can be fitted by metallic coordination enough. However, this prior oxidation probably moves the Ag atoms to the surface. Thus, Ag is more enriched in the surface. Our previous AuAg@MCM samples also show a tendency of surface enrichment of silver after calcination and high-temperature reduction.23 However, here, the new sample AuAg@APTS-MCM, by a twostep method, shows a stronger surface Ag enrichment. The coordination ratio (near surface) at Au/Ag ) 8 of the optimized catalyst is actually close to the Au/Ag ) 3 ratio previously found for the optimized activity for the one-pot method found previously.22 From the Ag K edge results in Table 4, the tendency of surface enrichment of Ag is also observed. The surface is enriched with Ag relative to Au. This is quite reasonable as Ag is more oxidable and Ag tends to move to the surface during calcination. For fitting the Ag K edge data, we find that we have to include Ag-O binding to better fit the calcined but not reduced sample. Table 4 shows that before hightemperature reduction, the Ag-Ag coordination is partially replaced by Ag-O coordination, indicating a strong presence of surface oxygen. After reduction, one can fit the data without oxygen. It seems to revert to the alloy state. This gives one an indication that the activated catalyst is in a metal alloy state. At the low-temperature reaction condition and with the presence of oxygen, there could be some surface oxygen present to promote the reaction. A plausible explanation for this promotional effect is that the Ag sites on the cluster surfaces easily adsorb and activate oxygen, while the neighboring gold sites adsorb CO.22,23 An easy oxygen atom transfer between the neighboring CO and the oxygen thus give a high rate of conversion. In a density functional calculation, the binding energy of O2 to a Ag site in bimetallic Au25Ag30 clusters is found to be higher than those of the monometallic counter parts, Au55 and Ag55.24,49 This is attributed to electron transfer to the antibonding π* orbital of oxygen to weaken the O-O bond. Previously, we observed (from XPS) that the alloying helps to increase the tendency to lose electrons. This stronger electron-donating power would help to increase the adsorption of both CO and oxygen. The oxygen would be more easily activated by the Au-Ag ensemble on the surface. The presence of neighboring Au strengthens the tendency of electron transfer. From this observation, the synergetic effect of AuAg bimetallic should not be limited to the oxidation of CO; other catalytic oxidation reactions could well be catalyzed by the AuAg bimetallic nanocatalyst. Recently, Chaki et al.50 found enhancement of the catalytic aerobic oxidation of alcohol by Ag-doped Au nanoparticles. The

Yen et al. promotional effect of Ag was attributed to the stronger tendency of electron transfer to oxygen for its activation. In viewing the strong synergetic effect on CO oxidation activity upon silver doping of gold, recent claims of intrinsic activity of metallic gold in nanoporous gold (NPG) foams,51 which are obtained by selective leaching of silver from Au-Ag alloy, should be re-examined carefully. In fact, in ref 51a, the reported Au/Ag ratio is about 28. The high catalytic activity in NPG could well be due to the residue silver in NPG. Baumer and co-workers52,53 concluded that the surface residue silver is responsible for promoting the catalytic activity in NPG. Our EXAFS analysis indicates that silver atoms tend to migrate to the surface of AuAg alloy even when the total Ag content is low. Iizuka et al. have found that a very minor amount of Ag impurity is responsible for a promoted activity in some so-called “pure” gold nanoparticle.54,55 5. Conclusion In summary, we have developed a new two-step method for loading Au-Ag bimetallic nanoparticles confined in mesoporous silica. As compared to our previous one-pot method, the AuAg@APTS-MCM catalysts also show strong synergy in the catalytic activity of CO oxidation. Detailed structural studies by XRD, TEM, and EXAFS show that the new catalyst gives a smaller size of surface Ag-enriched bimetallic particles. Its activity is fairly stable even after long-term storage. Although the composition of the AuAg bimetallic nanoparticles may look more complex than pure gold, the mechanism of catalysis of the AuAg bimetallic nanoparticles is actually simpler than that of Au catalyst because of the weaker size effect and weaker support effect. Its study may not only shed light to a detailed understanding of CO oxidation but also lead to useful application due to its high stability. Acknowledgment. This work was supported by the Academy Excellence program of National Science Council of Taiwan. We benefited from discussions with Dr. W. C. Shih. Supporting Information Available: Figures of N2 adsorption-desorption isotherms of MCM-41, TEM images of AuAg@APTS-MCM, thermal gravity analaysis (TGA) of the as-synthesized catalyst AuAg@APTS-MCM, and TEM image of as-synthesized catalyst AuAg@APTS-MCM and tables of ∆k and ∆R used for EXAFS at the Au L3 edge and the Ag K edge and comparison between two different weightings of fittings. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Haruta, M. Chem. Rec. 2003, 3, 75. (b) Haruta, M. Gold Bull. 2004, 37, 27. (2) Kung, H. H.; Kung, M. C.; Costello, C. K. J. Catal. 2003, 216, 425. (3) Goodman, D. W. Catal. Lett. 2005, 99, 1. (4) Li, W. C.; Comotti, M.; Schu¨th, F. J. Catal. 2006, 237, 190. (5) Comotti, M.; Li, W. C.; Spliethoff, B.; Schu¨th, F. J. Am. Chem. Soc. 2006, 128, 917. (6) Uphade, B. S.; Akita, T.; Nakamura, T.; Haruta, M. J. Catal. 2002, 209, 331. (7) Yap, N.; Andress, R. P.; Delgass, W. N. J. Catal. 2004, 226, 156. (8) Sinha, A. K.; Seelan, S.; Tsubota, S.; Haruta, M. Angew. Chem., Int. Ed. 2004, 43, 1546. (9) Hughes, M. D.; Xu, Y. J.; Jenkins, P.; McMorn, P.; Landon, P.; Enache, D. I.; Carley, A. F.; Attard, G. A.; Hutchings, G. J.; King, F.; Stitt, E. H.; Johnston, P.; Griffin, K.; Kiely, C. J. Nature 2005, 437, 1132. (10) Fu, Q.; Deng, W. L.; Saltsburg, H.; Flytzani-Stephanopoulos, M. Appl. Catal., B 2005, 56, 57.

CO Oxidation Catalyzed by Au-Ag Nanoparticles (11) Silberova, B. A. A.; Mul, G.; Makkee, M.; Moulijn, J. A. J. Catal. 2006, 243, 171. (12) Andreeva, D.; Ivanova, I.; Ilieva, L.; Abrashev, M. V. Appl. Catal., A 2006, 302, 127. (13) Zhang, X.; Shi, H.; Xu, B. Q. Angew. Chem., Int. Ed. 2005, 44, 7132. (14) Corma, A.; Serna, P. Science 2006, 313, 332. (15) Carrettin, S.; Guzman, J.; Corma, A. Angew. Chem., Int. Ed. 2005, 44, 2242. (16) Schubert, M. M.; Hackenberg, S.; van Veen, A. C.; Muhler, M.; Plazak, V.; Behm, R. J. J. Catal. 2001, 197, 113. (17) Haruta, M. Catal. Today 1997, 36, 153. (18) Lopez, N.; Janssens, T. V. W.; Clausen, B. S.; Xu, Y.; Marrikakis, M.; Bligaard, T.; Norskov, J. K. J. Catal. 2004, 223, 232. (19) Schubert, M. M.; Hackenberg, S.; van Veen, A. C.; Muhler, M.; Plazak, V.; Behm, R. J. J. Catal. 2001, 197, 113. (20) Liu, H.; Kozlov, A. I.; Kozlova, A. P.; Shido, T.; Asakura, K.; Iwasawa, Y. J. Catal. 1999, 185, 252. (21) Liu, Z.-P.; Hu, P.; Alavi, A. J. Am. Chem. Soc. 2002, 124, 14770. (22) Liu, J. H.; Wang, A. Q.; Chi, Y. S.; Lin, H. P.; Mou, C. Y. J. Phys. Chem. B 2005, 109, 40. (23) Wang, A. Q.; Liu, J. H.; Lin, S. D.; Lin, T. S.; Mou, C. Y. J. Catal. 2005, 233, 186. (24) Wang, A. Q.; Chang, C. M.; Mou, C. Y. J. Phys. Chem. B 2005, 109, 18860. (25) Wang, A. Q.; Hsieh, Y. P.; Chen, Y. F.; Mou, C. Y. J. Catal. 2006, 237, 197. (26) Liu, X. Y.; Wang, A. Q.; Yang, X. F.; Zhang, T.; Mou, C. Y.; Su, D. S.; Li, J. Chem. Mater. 2009, 21, 410. (27) Yen, C. W. Master Thesis, National Taiwan University, 2005. (28) Liu, X. Y.; Wang, A. Q.; Wang, X. D.; Zhang, T.; Mou, C. Y. Chem. Commun. 2008, 3187. (29) Mozer, T. S.; Dziuba, D. A.; Vieira, C. T. P.; Passos, F. B. J. Power Sources 2009, 187, 209. (30) Chi, Y. S.; Lin, H. P.; Mou, C. Y. Appl. Catal., A 2005, 284, 199. (31) Chiang, C. W.; Wang, A. Q.; Wan, B. Z.; Mou, C. Y. J. Phys. Chem. B 2005, 109, 18042. (32) Chiang, C. W.; Wang, A. Q.; Mou, C. Y. Catal. Today 2006, 117, 220. (33) Harada, M.; Toshima, N.; Yoshida, K.; Isoda, S. J. Colloid Interface Sci. 2005, 283, 64. (34) (a) Chen, N. M.; Peng, H. C.; Liu, R. S.; Hu, S. F.; Jang, L. Y. Chem. Phys. Lett. 2006, 420, 484. (b) Liu, D. G.; Lee, J. F.; Tang, M. T. J. Mol. Catal. A: Chem. 2005, 240, 197. (35) (a) Budroni, G.; Corma, A. Angew. Chem. 2006, 45, 3328. (b) Budroni, G.; Corma, A. Angew. Chem. 2006, 45, 6244 (Corrigenda).

J. Phys. Chem. C, Vol. 113, No. 41, 2009 17839 (36) Lin, H. P.; Tsai, C. P. Chem. Lett. 2003, 32, 1092. (37) Lin, H. P.; Yang, L. Y.; Mou, C. Y.; Liu, S. B.; Lee, H. K. New J. Chem. 2000, 24, 253. (38) Liu, Y. H.; Lin, H. P.; Mou, C. Y. Langmuir 2004, 20, 3231. (39) Pauly, T. R.; Liu, Y.; Pinnavaia, J. J.; Billinge, S. J.; Rieker, T. P. J. Am. Chem. Soc. 1999, 12, 8835. (40) (a) Mallin, M. P.; Murphy, C. J. Nano Lett. 2002, 2, 1235. (b) Bohren, C. F.; Huffman, D. R. Absorption and Scattering of Light by Small Particles; Wiley: New York, 1983. (41) (a) Shi, H. Z.; Zhang, L. D.; Cai, W. P. J. Appl. Phys. 2000, 87, 1572. (b) Link, S.; Wang, Z. L.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 3529. (42) (a) Shibata, T.; Bunker, B. A.; Zhang, Z.; Meisel, D.; Vardeman, C. F.; Gezelter, J. D. J. Am. Chem. Soc. 2002, 124, 11989. (b) Hwang, B. J.; Sarma, L. S.; Chen, J.; Chen, C.; Shin, S.; Wang, G.; Lin, D.; Lee, J.; Tang, M. J. Am. Chem. Soc. 2005, 127, 11140. (43) Ghilane, J.; Fan, F. R.; Bara, A. J.; Dunwoody, N. Nano Lett. 2007, 7, 1406. (44) Okumura, M. M.; Tsubota, S.; Iwamoto, M.; Haruta, M. Chem. Lett. 1998, 4, 1998. (45) Zhu, H. G.; Liang, C. D.; Yan, W. F.; Overbury, S. H.; Dai, S. J. Phys. Chem. B 2006, 110, 10842. (46) Date´, M.; Okumura, M.; Tsubota, S.; Haruta, M. Angew. Chem., Int. Ed. 2004, 43, 2129. (47) Dobrosz-Go´mez, I.; Kocemba, I.; Rynkowski, J. M. Catal. Lett. 2009, 128, 297. (48) Okumura, M.; Tsubota, S.; Haruta, M. J. Mol. Catal. A 2003, 199, 73. (49) Chang, C. M.; Cheng, C.; Wei, C. M. J. Chem. Phys. 2008, 128, 124710. (50) Chaki, N. K.; Tsunoyama, H.; Negishi, Y.; Sakurai, H.; Tsukuda, T. J. Phys. Chem. C 2007, 111, 4885. (51) (a) Xu, C. X.; Su, J. X.; Xu, X. H.; Liu, P. P.; Zhao, H. J.; Tian, F.; Ding, Y. J. Am. Chem. Soc. 2007, 129, 42. (b) Xu, C. X.; Xu, X. H.; Su, J. X.; Ding, Y. J. Catal. 2007, 252, 243. (52) Zielask, V.; Jurgens, B.; Schulz, C.; Biener, J.; Biener, M. M.; Hamza, A. V.; Baumer, M. Angew. Chem., Int. Ed. 2006, 45, 8241. (53) Wittstock, A.; Neumann, B.; Schaefer, A.; Dumbuya, K.; Ku¨bel, C.; Biener, M. M.; Zielask, V.; Steinru¨ck, H.-P.; Gottfried, J. M.; Biener, J.; Hamza, A.; Baumer, M. J. Phys. Chem. C 2009, 113, 5593. (54) Iizuka, Y.; Kawamoto, A.; Akita, K.; Date’, M.; Tsubota, S.; Okumura, M.; Haruta, M. Catal. Lett. 2004, 97, 203. (55) Iizuka, Y.; Miyamae, T.; Miura, T.; Date’, M.; Okumura, M.; Haruta, M. J. Catal. 2009, 262, 280.

JP9037683