Synthesis and reactivity of gold nanoparticles supported on transition metal doped mesoporous silica

Microporous and Mesoporous Materials 95 (2006) 118–125 www.elsevier.com/locate/micromeso

Synthesis and reactivity of gold nanoparticles supported on transition metal doped mesoporous silica Mangesh T. Bore a, M. Peter Mokhonoana b,1, Timothy L. Ward a, Neil J. Coville b, Abhaya K. Datye a,* b

a Department of Chemical and Nuclear Engineering, University of New Mexico, Albuquerque, NM 87131, USA Molecular Sciences Institute, School of Chemistry, University of the Witwatersrand, Johannesburg 2050, South Africa

Received 10 February 2006; received in revised form 26 April 2006; accepted 5 May 2006 Available online 30 June 2006

Abstract Mixed mesoporous silica samples with Co, Al and Fe as heteroelements were synthesized. Two classes of mesoporous silica were studied: MCM-41 prepared under basic conditions to yield one dimensional (1-D) pores; and aerosol-derived silica prepared under acidic conditions to yield 1-D pores arranged within spherical particles. The silica surface was subsequently functionalized with amine groups to allow deposition–precipitation of gold nanoparticles. These gold catalysts supported on amine functionalized mixed mesoporous silica were tested for CO oxidation and were found to become active at T > 250 C. The Au particles grew in size after reactivity measurements to sizes larger than the pore diameter; however, the majority of Au particles remained within the pores. Our results indicate that the silica walls (1 nm thick) are not able to restrain the growth of Au particle size. The addition of the heteroelement did not lead to a significant improvement in the thermal stability of these catalysts; the major effect was an increase in CO oxidation activity compared to the pure silica. The highest reactivity was seen in cobalt-silica mixed mesoporous oxide prepared by the aerosol method, with a reactivity of 2.3 · 105 mol CO2/s/gcat at 250 C and activation energy of 40 kJ/mol.  2006 Elsevier Inc. All rights reserved. Keywords: Gold nanoparticles; Mesoporous silica; Aerosol silica; Sintering; CO oxidation; Cobalt-modified silica

1. Introduction Gold nanoparticles supported on oxide supports show high reactivity for CO oxidation at low temperature [1], and these gold particles are more reactive if supported on reducible metal oxides such as TiO2, Co3O4 and Fe2O3 [2]. The catalytic activity of gold strongly depends on the particle size, gold nanoparticles become highly reactive in the range of 2–5 nm. Thus it is important to be able to stabilize nanoparticles in the 2–5 nm size range. One approach to prevent the sintering of nanoparticles is to deposit them inside the pores of mesoporous silica. In our earlier work, *

Corresponding author. Tel.: +1 505 277 0477; fax: +1 505 277 5433. E-mail address: [email protected] (A.K. Datye). 1 Present address: Chemistry Department, University of Limpopo (Turfloop Campus), P/Bag X1106, Sovenga 0727, South Africa. 1387-1811/$ - see front matter  2006 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2006.05.007

we reported that sintering can be better controlled if particles are trapped inside 1-D pores with thick walls [3] (SBA15 better than MCM-41) and curved pores were found to be better than straight pores (Aerosol silica better than MCM-41) at controlling the sintering of these Au nanoparticles [4]. One limitation of mesoporous silica is that the pores collapse at temperatures above 500 C [5–7]. The introduction of heteroelements such as Al or Zr in the framework of mesoporous silica has been found to improve its hydrothermal stability [7]. It is also known that the activity of gold supported on reducible metal oxides such as Fe2O3, Co3O4 or TiO2 is greater than the activity observed on Au/silica [2]. It was therefore of interest to combine the desirable properties of mesoporous silica, such as high surface area and narrow pore size distribution, with the beneficial effects of heteroatom substitution, which can yield

M.T. Bore et al. / Microporous and Mesoporous Materials 95 (2006) 118–125

improved hydrothermal stability and improved catalytic activity. In this study, we have synthesized mixed mesoporous silica containing heteroelements Co, Al and Fe incorporated into the silica. The mixed mesoporous silica samples were prepared by a batch method to get straight 1-D pores like MCM-41 and by the aerosol method to get 1-D pores coiled up in spherical geometry. Gold nanoparticles were deposited on these samples after amine functionalization. We measured the Au nanoparticle size distributions and measured the reactivity for CO oxidation. As we show in this paper, it is the cobalt substituted mesoporous silica that was most effective at yielding high activity Au nanoparticles for CO oxidation. 2. Experimental 2.1. Synthesis of mixed mesoporous silica by the batch method The mixed MCM-41 was prepared by using cetyltrimethyl ammonium bromide (CTAB) as surfactant, sodium silicate as silica source and cobalt nitrate, aluminium nitrate and ferric nitrate as sources of Co, Al and Fe, respectively. The molar ratio of the precursor solution was 3.4SiO2:1CTAB:286H2O. The Si/heteroelement molar ratio was maintained at 30, a ratio where we observed significant improvement in hydrothermal stability [7]. The surfactant was first dissolved in water, the corresponding nitrate salt was added to the surfactant solution and the solution was thoroughly mixed. Sodium silicate was next added to this mixture and the pH of the solution was adjusted to 10 with 1 M nitric acid. The mixture was then transferred to a polypropylene bottle and kept in a water bath under static conditions for 8 h at 80 C. The mixture was then cooled to room temperature and kept at this temperature for 15 h. The powder was collected, washed with water, vacuum dried, and then calcined at 500 C for 12 h in air to remove the surfactant. 2.2. Synthesis of mixed mesoporous silica by the aerosol method The precursor solution consisted of cetyltrimethyl ammonium bromide (CTAB), water, tetraethylorthosilicate (TEOS), 1 N hydrochloric acid and cobalt nitrate, aluminum nitrate or ferric nitrate. The molar ratio of TEOS:H2O:CTAB:HCl used was 1:63:0.16:0.02. The Si/ heteroelement molar ratio used was 30:1. CTAB was first dissolved in water and stirred for 10 min. Next TEOS and HCl were added to the CTAB solution and the solution stirred for 10 min. The nitrate salt of the heteroelement was mixed with the above mixture and stirred to get a clear solution. The precursor solution then passed through an aerosol reactor. The aerosol was generated by an ultrasonic nebulizer and nitrogen (carrier gas) flow rate was maintained at 3.6 slpm to carry the droplets through the reactor. A detailed description of the aerosol reactor has been pub-

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lished in our earlier work [8]. The furnace temperature was maintained at 125 C. The powder was collected on a filter paper and was calcined at 500 C for 12 h in air to remove the surfactant. 2.3. Gold deposition on mesoporous silica We used the amine functionalization method [9,10] to deposit gold inside the pores of mixed mesoporous silica. Since the silica surface is negatively charged, functionalizing the surface with the amine facilitates adsorption of the partially hydrolyzed (HAuCl4) complex. It is possible to mix the organic amine during the silica precipitation step, but we found this led to silica which was not well ordered. Hence, we used a two step method, where mixed mesoporous silica was first synthesized and next the preformed silica was treated with 3-aminopropyltrimethoxysilane for 24 h, washed in deionized water (DI) and vacuum dried. After amine functionalization, the samples were labeled as NH2-heteroelement-mesoporous silica e.g. NH2-Co-MCM-41 or NH2-Co-Aerosol silica. The BJH pore sizes reported are based on N2 adsorption performed after the amine functionalization. This amine functionalized silica powder was then mixed with an aqueous hydrogen tetrachloroaurate hydrate solution (pH 7, adjusted with 1 M sodium hydroxide) to achieve a nominal gold loading of 5 wt%. After contacting the Au precursor for 24 h, the excess precursor solution was washed off with DI water, and then vacuum dried for 24 h. The powder was then reduced at 200 C for 2 h in flowing hydrogen, heating from room temperature at a ramp rate of 0.73 C/min. After reduction, the samples were labeled Au-NH2-heteroelement-mesoporous silica-reduced e.g. Au-NH2-Co-MCM41-Reduced or Au-NH2-Co-Aerosol silica-Reduced. 2.4. Reactivity measurements The reactivity measurements were performed using 10 mg of catalyst in a flowing stream containing 1% CO and 0.5% O2 and the balance helium at a total pressure of 670 Torr (atmospheric pressure in Albuquerque) with total flow rate of 100 ml (STP) per min (Space velocity = 600 000 ml/gcat/h). The reactivity was measured at temperatures from room temperature up to 400 C at 50 C intervals. These measurements were repeated for a total of three heating and cooling cycles indicating that the results shown here are reproducible. After reaction, the samples were removed for examination by electron microscopy. These samples were labeled Au-NH2-heteroelement-mesoporous silica-after reaction e.g. Au-NH2-Co-MCM41-after reaction or Au-NH2-Co-Aerosol silica-after reaction. 2.5. Characterization X-ray powder diffraction (XRD) patterns were obtained on a Scintag PAD-V diffractometer. Adsorption isotherms

M.T. Bore et al. / Microporous and Mesoporous Materials 95 (2006) 118–125

a.u.

of nitrogen were performed on a Micromeritics Gemini 2360 analyzer. HAADF (high angle annular dark field) scanning transmission electron microscopy (STEM) images were obtained on a JEOL 2010F, 200 kV microscope. High resolution scanning electron microscopy (SEM) images were obtained on a Hitachi S-5200 microscope at an accelerating voltage of 2 kV.

a.u.

NH2-Co-Aerosol Silica

a.u.

NH2-Al-Aerosol Silica

NH2-Fe-Aerosol Silica

a.u.

120

NH2-Aerosol Silica

3. Results and discussion

a.u.

XRD diffraction patterns of mixed mesoporous silica samples after amine functionalization are shown in Figs. 1a and 1b. The BET surface area, d100 spacing and the BJH pore size of these samples is also summarized in Table 1. The XRD patterns and the surface area of these samples indicate well ordered accessible pores, and very little influence of the added heteroatoms on the mesopore silica structure. However, the addition of the amine functional groups leads to lower surface areas (600 m2/g) compared to the aerosol silica or MCM-41 (1200–1300 m2/g) [7]. The amine functional groups are, however, essential for adsorption of the Au precursor, otherwise we get negligible uptake of the Au into the silica pore structure. Fig. 2 shows the HAADF STEM images of samples after reduction at 200 C for 2 h in 100% flowing H2. The gold nanoparticle particle size is less than 2 nm for all samples except NH2-Co-MCM-41 (Fig. 2a) and NH2-Fe-Aerosol silica (Fig. 2f). We find that mesoporous silica treated with the 3-aminopropyltrimethoxysilane allows us to obtain highly dispersed Au samples by the deposition–precipitation method. While this synthesis is reproducible, it is necessary to wash off all chloride impurities to achieve

a.u.

NH2-Co-MCM-41

a.u.

NH2-Fe-MCM-41 C

a.u.

l NH2-Al-MCM-41

NH2-MCM-41 2

3

4

5

2 Theta Fig. 1a. X-ray diffraction patterns for mixed mesoporous silica produced by the batch method after amine functionalization.

2

3

4

5

2 Theta Fig. 1b. X-ray diffraction patterns for mixed mesoporous silica produced by aerosol method after amine functionalization.

Table 1 BET surface area, interplanar spacing d100 from XRD and BJH pore size obtained from N2 adsorption for the amine functionalized mixed mesoporous oxides Sample

Surface area (m2/g)

d100 (nm)

Pore size (nm)

NH2-MCM-41 NH2-Fe-MCM-41 NH2-Co-MCM-41 NH2-Al-MCM-41 NH2-Aerosol silica NH2-Fe-Aerosol silica NH2-Co-Aerosol silica NH2-Al-Aerosol silica

479 522 457 641 602 481 484 332

3.4 3.3 3.3 3.5 3.4 3.6 3.9 3.6

2.9 3.1 2.9 3.3 2.4 2.4 2.9 1.9

small Au particle sizes. It is possible that the presence of larger Au particles in two of the six samples is a result of residual chloride that could not be removed during the washing step in these samples. These H2 reduced samples were tested for CO oxidation with 1% CO and 0.5% O2 and the balance helium for three temperature cycles from room temperature up to 400 C at 50 C intervals. Fig. 3 shows the conversion with respect to temperature for the third heating and cooling cycle for the Al and Co containing silica samples, with one pure silica sample included for comparison. Fig. 4 also shows the Arrhenius plots from the data obtained from the third heating and cooling cycle. The reactivity of the Fe doped mesoporous silica is not shown since the Fe was found to have phase segregated resulting in a separate iron oxide phase. All the other samples showed the dopant (Al or Co) to be uniformly distributed in the silica, as measured

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Fig. 2. HAADF STEM images of samples after reduction at 200 C for 2 h in flowing hydrogen. (a) Au-NH2-Co-MCM-41, (b) Au-NH2-Al-MCM-41, (c) Au-NH2-Fe-MCM-41, (d) Au-NH2-Co-Aerosol silica, (e) Au-NH2-Al-Aerosol silica and (f) Au-NH2-Fe-Aerosol silica.

60 Au-NH2-Co-Aerosol Silica

% Conversion

50 40

Au-NH2-Al-MCM-41

30

Au-NH2-Al-Aerosol Silica Au-NH2-MCM 41 Silica

20 10

Au-NH2-Co-MCM-41 0 0

100

200

300

400

500

Temperature (oC)

ln (reactivity, 10-5mol CO2/s/gcat)

Fig. 3. Conversion with respect to temperature after two heating and cooling cycles between room temperature and 400 C. Reactivity for CO oxidation was measured with 1% CO and 0.5% O2 and the balance helium.

-8 Au-NH2-Al-MCM-41 Ea = 63 kJ/mol

-9

Au-NH2-Co-Aerosol Silica Ea = 26 kJ/mol

-10 -11 -12 Au-NH2-Al-Aerosol Silica Ea = 62 kJ/mol

-13 -14 -15 1.4

Au-NH2-Co-MCM-41 Ea = 36 kJ/mol

Au-NH2-MCM-41 Ea = 57 kJ/mol 1.5

1.6

1.7

1.8

1.9

2.0

1/T x 103 (K-1) Fig. 4. Arrhenius plots for CO oxidation after two heating and cooling cycles between room temperature and 400 C. Reactivity for CO oxidation was measured with 1% CO and 0.5% O2 and the balance helium.

by EDS analysis. Table 2 summarizes the reactivity of these samples for CO oxidation at 250 C, the activation energy, the pre-exponential factors and Au average particle size after reaction. Activation energies for the Co-containing silica were 40 kJ/mol, while the other silica samples (including the Al doped sample) exhibited an activation energy 60 kJ/mol. These values are comparable to the values reported by Haruta et al. [2,11]. We measured the reactivity at temperatures from 50 C to 400 C over three cycles. Under our experimental conditions, these Au catalysts did not become active for CO oxidation till 250 C. We note that our reaction gases consist of dry helium and oxygen. Since Au catalysts in CO oxidation are sensitive to moisture content, we suspect that the use of dry gases leads to lower reactivity than observed by other investigators. Among our samples, the Au-NH2Co-Aerosol silica sample was more reactive than the other samples we studied. The Au-NH2-Al-Aerosol silica and Au-NH2-Al-MCM-41 samples show reactivity comparable to the undoped silica. The higher reactivity of the Co containing sample can be due to smaller particle size or it could be a promotion effect of the Co. To investigate this further, we studied the Au particle sizes in the catalyst after the reactivity measurements. Fig. 5 shows the HAADF STEM images of the Al and Co containing samples after reaction studies. Approximately 300 particles were counted using Digital MicrographTM software, and the particle size distributions were fit to a log normal distribution, as shown in Fig. 6. The number average particle size in these catalysts decreased in sequence, Au-NH2-Co-MCM-41 (11.1 nm), Au-NH2-Co-Aerosol (7.0 nm) silica, Au-NH2-Al-Aerosol silica (4.2 nm) and Au-NH2-Al-MCM-41 (3.9 nm). Among this group of samples, the Au-NH2-Co-MCM-41 shows the lowest reactivity with an average particle size of 11.1 nm with <20% of the particles in the 3–8 nm size range. The

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Table 2 Characterization of Au nanoparticles Sample

Au-NH2-Co-MCM-4 1 Au-NH2-Co-Aerosol silica Au-NH2-AI-MCM-4 1 Au-NH2-Al-Aerosol silica

Gold nanoparticles average particle size (after reaction) (nm)

Reactivity · 105 at 250 C (mole CO2/s/gcat)

Reactivity · 102 at 250 C (mole CO2/s/moleAu)

Activation Energy (kJ/mol)

Pre-exponential factor (molecules/s/gcat)

Number average

Surface average

11.1 s.d. 4.5 7.0 s.d. 3.2 4.2 s.d. 0.9 3.9 s.d. 1.3

14.3

0.5

2.0

36

1.3 · 1022

10.4

2.3

9.2

40

9.4 · 1022

4.6

0.3

1.3

63

4.3 · 1024

4.9

0.3

1.3

60

1.7 · 1024

The particle sizes were obtained by STEM after three reaction cycles at temperatures ranging from room temperature to 400 C. The reactivity reported at 250 C is for the third heating cycle. P P The activation energy is calculated using third heating cycle. Number average, d N ¼P ni d i =P ni . Surface average, d s ¼ ni d 3i = ni d 2i . P s.d.: standard deviation, r ¼ ð ðd i  d N Þ2 =ðN  1ÞÞ1=2 .

Au-NH2-Al-Aerosol silica and Au-NH2-Al-MCM-41 samples have similar reactivity, and both of these samples have >90% of Au particles in the size range of 3–8 nm. The most reactive sample in the group Au-NH2-Co-Aerosol silica has

average Au particle size of 7 nm with 75% of the particles in the size range of 3–8 nm. While the particle size distributions for the Al doped mesoporous silica and Co doped aerosol silica is similar, the reactivity of the Au-NH2-Co-

Fig. 5. HAADF STEM images of samples after three reaction cycles at temperatures ranging from room temperature to 400 C. (a) Au-NH2-Co-MCM41, (b) Au-NH2-Al-MCM-41, (c) Au-NH2-Co-Aerosol silica, (d) Au-NH2-Al-Aerosol silica.

M.T. Bore et al. / Microporous and Mesoporous Materials 95 (2006) 118–125 0.6 0.4 0.2 0.0

Au-NH2-Aerosol Silica

0.2

Au-NH2MCM-41

0.0

Fraction

0.2

Au-NH2-Co-Aerosol Silica 0.0 0.2

Au-NH2-Co-MCM-41

0.0 0.4

Au-NH2-Al-Aerosol Silica

0.2 0.0 0.4 0.2

Au-NH2-Al-MCM-41

0.0 0

2

4

6

8 10 12 14 16 18 20 22 24 26

Particle size (nm) Fig. 6. Au particle size distributions after three reaction cycles at temperatures ranging from room temperature to 400 C. The average pore diameter for these silica samples is 3 nm. It is clear that most of the Au particles have grown to be larger than the pore diameter in each of these silica samples.

Aerosol silica is seven times greater. The particle size distribution for the undoped silica-supported Au after similar reaction treatments also shows the majority of Au particles to be in the 3–8 nm size range, but with reactivity much lower than the doped silica [3]. These results suggest that while the presence of Au particles in the 3–8 nm size range is important to attain a high reactivity in CO oxidation, the significant reactivity differences in these catalysts must be due to the presence of reducible metal oxides that were incorporated into the silica support. Reducible metal oxides can create interfacial sites that are reactive for CO oxidation [2].

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An important question concerns the role of the pores in helping maintain the Au particles in the size range which is most effective for CO oxidation. The majority of the Au particles reach sizes greater than the pore size after the three reaction cycles (see Fig. 6). To determine whether these Au particles were still located inside the pores, these samples were imaged by a combination of high resolution SEM using a low accelerating voltage (probe size <2 nm at 2 kV), and in a STEM (probe size <0.2 nm at 200 kV). As explained below, the limited penetration of the 2 kV electrons in the SEM ensures that the signal comes from the near-surface region, while 200 kV electrons are transmitted through the sample, hence we can image all of the Au particles. The mean free path (1 nm) of the low energy SE electrons ensures that the signal comes from the near-surface region. Hence the SE image shows primarily topographic contrast, making it difficult to see the metal particles, unless they are large and stand out clearly on the surface. On the other hand, the back-scattered electron (BSE) image provides atomic number contrast, making it easier to see the Au particles, but the signal still originates from the near-surface region due to the limited penetration of the 2 kV electrons. The higher penetration of the 200 kV electrons ensures that all of the Au particles in the silica are being imaged. The comparison of the SEM images and STEM images provides clues to the location of the Au particles, confirming that a large fraction of the Au particles are still located within the mesoporous particles, and only a small fraction are located on the silica surface. Figs. 7 and 8 show images of Au-NH2-Al-MCM-41, and Au-NH2-Co-Aerosol silica samples after three reaction cycles. The HAADF image of Au-NH2-Al-MCM-41 (Fig. 7a) obtained at comparable magnification as the BSE image (Fig. 7b) shows that many more particles are visible in the STEM image. This indicates that only a small fraction of particles were located near the surface. Fig. 8 shows images of the Au-NH2-Co-Aerosol silica sample. Mesoporous silica particles prepared by aerosol method are spherical and have a smooth surface making it easy to distinguish gold particles, even in SE images (Fig. 8a). The BSE image (Fig. 8b) shows more of the Au particles due to improved contrast. When we compare these images

Fig. 7. High resolution STEM and SEM images of the Au-NH2-Al-MCM-41 sample after reaction. (a) STEM image (200 kV) and (b) BSE image (2 kV). The higher penetration of the 200 kV electrons allows the STEM to image all of the Au particles, while the BSE image only shows particles in the nearsurface region.

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Fig. 8. High resolution SEM and STEM images of the Au-NH2-Co-Aerosol silica sample after reaction. (a) SE image at 2 kV, (b) BSE image at 2 kV of the same silica particle; (c, d) STEM images of silica particles from the same sample. Numerous smaller particles can be seen in the STEM image that are not evident in the SE or BSE images. These smaller particles must be located in the interior of the mesoporous silica.

with the HAADF STEM images, we notice there are a lot more Au particles visible per spherical silica particle than in the SEM images. It is also significant that the additional particles visible in the STEM image are all much smaller than those in the near-surface region, suggesting that the pores help retain smaller sized particles. However, while the mesoporous silica is able to retain Au in smaller particles, the majority of particles have grown larger than the pore size. To investigate the influence of the Au particles on the silica pore structure, we examined these samples at high magnification. Fig. 9 shows STEM images of Au/MCM-41 silica (Fig. 9a) and Au/aerosol silica (Fig. 9b). These images

show that while the Au particles grow to sizes larger than the pore diameter, the ordered silica structure is still preserved. In order to interpret these images, we should remember that the STEM image shows a projection of the three-dimensional structure, typically 20–30 pore diameters deep. Hence, destruction of a single pore to accommodate a gold particle would not be visible in these STEM images, since the image represents a superposition of 20– 30 pores all lined up along the electron beam. No direct information on any local deformation of the silica pores can be obtained in these STEM images. Other structural techniques, such as X-ray diffraction, provide information that is averaged over the entire sample, making it even

Fig. 9. STEM image of (a) Au/MCM-41 and (b) Au/aerosol silica, showing that the pores are intact while the Au particles have all reached diameters greater than the pore size of 3 nm. The image confirms that the silica exhibits ordered pores while still accommodating Au particles within the pore structure, presumably because of local deformation of the pores.

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more difficult to study such local deformation. Images such as those presented in Fig. 9 provide the best evidence for the coexistence of Au particles in an ordered silica pore structure. The combination of SEM images (providing near surface information) and the STEM images (providing information from the entire sample) confirms that a significant fraction of the Au particles are still present within the silica pore structure. 4. Conclusions Mesoporous silica samples containing Co, Al and Fe as heteroelements were synthesized. Two kinds of pore morphologies were investigated, straight 1-D pores in an MCM-41 structure and tortuous 1-D pores in aerosol synthesized silica. In each instance, the gold nanoparticles were deposited on the silica after amine functionalization. These Au catalysts were subjected to three cycles of CO oxidation reactivity measurement at temperatures ranging from room temperature to 400 C, but were found to become active only at T > 250 C. The particle size distributions were determined after the reactivity measurements. The samples showing the highest activity had the majority of gold nanoparticles in the size range of 3–8 nm. However, despite having similar sizes, Au/Co-doped aerosol silica showed the highest reactivity for CO oxidation, suggesting that the nature of the support is very important for this reaction. We found that the Au particles in the pores had grown in size and in most cases were larger than the nominal pore diameter of the silica, suggesting that the pore walls were deformed to accommodate the Au particles. However, the particles that were on the silica surface (outside the pores) had grown to much larger sizes and consti-

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tuted the broad tail towards the larger particles sizes in the particle size distribution. This study shows that the chemistry of the support and the pore structure are both important in retaining Au particles in the size range required for high CO oxidation activity. Acknowledgments We acknowledge financial support from NSF grants CTS 01-21619, CTS 02-10835, EEC 04-36455, and the Materials Corridor Council supported by the Department of Energy. The characterization facilities used in this work are supported by NSF EPSCOR and NNIN infrastructure grants. References [1] M. Haruta, T. Kobayashi, H. Sano, N. Yamada, Chem. Lett. 2 (1987) 405. [2] M. Haruta, S. Tsubota, T. Kobayashi, H. Kageyama, M.J. Genet, B. Delmon, J. Catal. 144 (1993) 175. [3] M.T. Bore, H.N. Pham, E.E. Switzer, T.L. Ward, A. Fukuoka, A.K. Datye, J. Phys. Chem. B 109 (2005) 2873. [4] M.T. Bore, H.N. Pham, T.L. Ward, A.K. Datye, Chem. Commun. 22 (2004) 2620. [5] S.C. Shen, S. Kawi, J. Phys. Chem. B 103 (1999) 8870. [6] R. Mokaya, J. Phys. Chem. B 104 (2000) 8279. [7] M.T. Bore, T.L. Ward, R.F. Marzke, A.K. Datye, J. Mater. Chem. 15 (2005) 5022. [8] M.T. Bore, S.B. Rathod, T.L. Ward, A.K. Datye, Langmuir 19 (2003) 256. [9] A. Gosh, C.R. Patra, P. Mukherjee, M. Sastry, R. Kumar, Micropor. Mesopor. Mater. 58 (2003) 201. [10] H. Zhu, B. Lee, S. Dai, S.H. Overbury, Langmuir 19 (2003) 3974. [11] M. Okumura, S. Nakamura, S. Tsubota, T. Nakamura, M. Azuma, M. Haruta, Catal. Lett. 51 (1998) 53.