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2009, 113, 13457–13461 Published on Web 07/14/2009
Preparation of ∼1 nm Gold Clusters Confined within Mesoporous Silica and Microwave-Assisted Catalytic Application for Alcohol Oxidation Yongmei Liu,† Hironori Tsunoyama,† Tomoki Akita,‡,§ and Tatsuya Tsukuda*,†,§ Catalysis Research Center, Hokkaido UniVersity, Nishi 10, Kita 21, Sapporo 001-0021, Japan, Research Institute for Ubiquitous Energy DeVices, National Institute of AdVanced Industrial Science and Technology (AIST), 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan, and CREST, Japan Science and Technology Agency, Kawaguchi, Saitama 332-0012, Japan ReceiVed: May 19, 2009; ReVised Manuscript ReceiVed: July 8, 2009
A simple, effective method has been demonstrated to immobilize ∼1 nm Au clusters within mesoporous silicas (SBA-15, MCF, HMS) using triphenylphosphine-protected Au11 (Au11:TPP) clusters as precursors, which were deposited on the silica surface in an organic medium. A unique feature of this method is the ability to disperse Au11:TPP homogeneously over a large surface area by optimizing the solvent-mediated interaction. The Au11:TPP-silica composite was then carefully calcined to remove the protecting ligands while suppressing the aggregation of the resulting Au clusters. For SBA-supported Au clusters, the absence of the surface plasmon band in the reflectance spectrum indicated that contamination by AuNPs larger than 2 nm was negligibly small. The Au cluster size supported on SBA was estimated to be 0.8 ( 0.3 nm by HAADF-STEM observations. The SBA-supported Au clusters exhibited catalytic activity for oxidation of various alcohols by H2O2 under microwave irradiation and were found to be reusable. Oxidation over supported gold catalysts using molecular oxygen has gained much attention recently and is known to be affected by the chemical nature of the support as well as the particle size.1 Au nanoparticles (AuNPs) deposited on reducible metal oxides, such as TiO2, Fe2O3, and Co3O4, show a high degree activity for CO oxidation because the support or gold-support interface can activate the O2 molecules. Thus, AuNPs supported on insulating oxides, such as SiO2 and Al2O3, have been considered to be ineffective catalysts due to the chemical inertness of the support. In fact, AuNPs (>5 nm) on silica were found to exhibit poorer activity than on active supports. However, highly active silica-supported Au catalysts may be produced if the size of the AuNPs is reduced to below 1 nm, since small Aun clusters (n e 20) are known to activate the O2 molecule.2 Immobilization of small, monodisperse Au clusters on silica is a technical challenge, since the conventional methods such as deposition-precipitation (DP) and impregnation (IP) cannot be applied. For example, the DP method cannot be used for the preparation of silica-supported Au catalysts because under normal conditions AuCl4- cannot be deposited effectively onto a negatively charged silica surface due to electrostatic repulsion. A typical way to circumvent these difficulties is to chemically modify the silica surface with functional groups (e.g., -SH, -NH2)3-7 or metal oxides (e.g., TiO2, Al2O3).8,9 In contrast, Au catalysts on “pure” silica have been prepared using various Au precursors: [Au(en)2]3+ (en * To whom correspondence should be addressed. E-mail: tsukuda@ cat.hokudai.ac.jp. Fax: +81-11-706-9156. † Hokkaido University. ‡ National Institute of Advanced Industrial Science and Technology. § CREST.
10.1021/jp904700p CCC: $40.75
SCHEME 1: Synthesis Procedure of Sub-Nanometer-Sized Au Clusters within SBA-15 using Au11:TPP as a Precursor
) ethylenediamine) cations,10,11 Me2Au(acac) (acac ) acetylacetonate) vapor,12 and colloidal AuNPs.13-16 Colloidal AuNPs are especially attractive, since the loading and size distribution of the Au catalysts can be easily controlled. For example, monodisperse Au clusters with average diameters of 1.5 and 3.5 nm have been successfully prepared on amorphous silica using triphenylphosphine-protected Au55 compounds16 and alkanethiolate-protected Au clusters,14 respectively. The present work aims to develop a method to prepare monodisperse Au clusters smaller than 1 nm on pure mesoporous silica supports (SBA-15, HMS, MCF). As an example, the synthesis process of Au clusters within mesoporous channels of SBA-15 is shown schematically in Scheme 1. First, Au113+ 2009 American Chemical Society
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Figure 1. (A) (a) UV-vis spectrum of Au11:TPP dispersed in CH2Cl2 and (b) diffuse reflectance UV-vis spectrum of Au11:TPP-SBA composites prepared in CH2Cl2. (B) Sticking probability of Au11:TPP to SBA-15 in C2H5OH/CH2Cl2. The numbers in blue represent the Au content in wt % estimated from the sticking probability. The curve is a guide for the eye. The inset shows photographic images of the mother solution of Au11:TPP and the filtrates.
magic clusters (diameter 0.8 nm), protected by triphenylphosphine (Au11:TPP), were deposited on SBA-15 in organic media. A unique feature of this strategy is to cause Au11:TPP to be dispersed homogeneously over a large surface area of SBA-15 by optimizing the solvent-mediated interaction. The Au11: TPP-SBA composite was then calcined carefully to remove the organic ligands while suppressing the aggregation of the resulting Au clusters. The catalytic performance of the calcined composite Au11-SBA was tested for oxidation of various alcohols. It is hoped that this work will provide a platform for developing efficient Au catalysts based on the intrinsic nature of Au clusters. The preparation of SBA-15 supported Au clusters is described in more detail in the following. The Au11 precursors were first prepared, most likely in the form of [Au11(TPP)8Cl2]+, following the reported procedures.17,18 The optical absorption spectrum of Au11:TPP shows a characteristic peak at 415 nm17,18 indicating quantized electronic structure (curve a in Figure 1A). The SBA15 support was also prepared following a reported protocol,19 and characterized by small angle XRD, TEM, and nitrogen adsorption/desorption isotherms.18 The Au11:TPP clusters (4 mg) and SBA-15 (1 g) were then dispersed in mixed solvents of CH2Cl2/C2H5OH (24 mL). After the mixture was stirred for 2 h, the composite of Au11:TPP and SBA-15 was collected by filtration. The Au11:TPP in pure CH2Cl2 was found to be adsorbed completely on SBA-15, as evidenced by colorless filtrate (inset of Figure 1B). The diffuse reflectance UV-vis spectrum shows that the sharp peaks of Au11:TPP are slightly red-shifted and broadened in the Au11:TPP-SBA composites (curve b in Figure 1A). This spectral change is not associated with irreversible chemical reactions between Au11:TPP and
Letters SBA-15 but with slight modification of the electronic structure of Au11:TPP due to interaction with the support, since Au11: TPP thus adsorbed was easily eluted from the composite in the intact form by washing with C2H5OH. The sticking probability of Au11:TPP on SBA-15 gradually decreased with increasing C2H5OH content (inset of Figure 1B).18 The sticking probability was quantitatively estimated from the relative populations of Au11:TPP contained in the filtrate and the original organosol, which were calculated using their absorbance at 415 nm. Then, the Au content in the Au11:TPP-SBA composite was calculated using the sticking probability. Figure 1B plots the sticking probability and the Au content thus obtained as a function of the solvent composition. The composites thus obtained are hereafter denoted using the Au content and solvent composition; for example, the composite prepared in the solvent with C2H5OH/CH2Cl2 ) 20% is referred to as 0.16Au11: TPP-SBA(20). The question arises as to what the nature of the attractive force between Au11:TPP and silica is and how the dependence of the sticking probability on the solvent compositions can be explained? Zheng et al. proposed that efficient deposition of thiolate-protected Au clusters (>3.5 nm) on silica in an aprotic solvent is due to an attractive force between the dipoles of the OH groups on the silica surface and polarized Au cores.14 It was found that the sticking probability on SBA-15 is less than 10% for [Au25(SC2H2Ph)18]- (ref 19) and Au clusters (1-2 nm) protected by PhSH.20 Since the Au11 core is less polarizable due to its smaller size, this result suggests that the interaction between the Au core and the silica does not play a key role in the adsorption. Instead, it is believed that the presence of ligands is more important, since the TPP ligand is expected to have a larger polarizability than PhC2SH and PhSH. In this model, Au11: TPP becomes anchored to the silica through a weak attractive force between the permanent dipole of the OH groups of the silica surface and the dipole induced within the ligand layer. The attractive force between the Au11:TPP and SBA-15 becomes weaker with increasing C2H5OH content, since the permittivity of the dispersing medium becomes larger. This trend can qualitatively explain the adsorption behavior shown in Figure 1B. In the next step, the protecting ligands were removed by calcination of the Au11:TPP-SBA composites. Thermogravimetric analysis was first carried out on the Au11:TPP to determine a suitable heat treatment to remove the ligands without causing aggregation of the Au clusters. It was found that the Au11:TPP clusters calcined at 200 °C for 2 h lost 49.4% of their initial weight (Figure S1, Supporting Information), which corresponds to the organic moiety ratio of [Au11(TPP)8Cl2]+ (49.7%). Thus, this calcination condition was used for all of the Au11:TPP-SBA composites. In the following discussion, a sample obtained by calcination of 0.16Au11:TPP-SBA(20), for example, will be referred to as 0.16Au11-SBA(20). The size of the gold clusters after calcination was probed by diffuse reflectance UV-vis spectroscopy. Figure 2A shows the optical spectra obtained after calcination of a series of Au11:TPP-SBA composites (curves b-f). It can be seen that, for all samples, the sharp peaks associated with quantized electronic structures of Au11:TPP on SBA (curve a) become smeared out following calcination. The surface plasmon (SP) band is absent in curve b but is apparent when the concentration of C2H5OH is lower than 10% (curves c-f). This indicates that the Au clusters in 0.16Au11-SBA(20) are smaller than those in the other samples. The suppression of aggregation in 0.16Au11-SBA(20) cannot be ascribed to a smaller amount of loading of the Au11:TPP
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Figure 2. (A) Diffuse reflectance UV-vis spectra of (a) 0.16Au11:TPP-SBA(20), (b) 0.16Au11-SBA(20), (c) 0.17Au11-SBA(10), (d) 0.18Au11-SBA(5), (e) 0.19Au11-SBA(2.5), (f) 0.20Au11-SBA(1), and (g) 0.10Au11-SBA(0). (B) Representative HAADF-STEM image and size distribution of Au clusters of 0.16Au11-SBA(20).
precursor, since 0.10Au11-SBA(0) (curve g) exhibited a much stronger SP band than the other samples despite the fact that the Au content is smaller. Thus, it is reasonable to suppose that the suppressed cluster aggregation in 0.16Au11-SBA(20) is related to the composition of the dispersing media used for the adsorption of Au11:TPP. It is believed that aggregation is suppressed because Au11:TPP is adsorbed more homogeneously on SBA-15 as a result of the weaker attractive force in the mixed solvent with C2H5OH/CH2Cl2 ) 20% than in the other compositions. We stress here that the monodisperse, sub-nanometersized Au clusters could not be obtained using the composite prepared by simple evaporation of the dispersing medium employed by Lambert and co-workers for the adsorption of Au55: TPP on a silica.16 The structure of the composite 0.16Au11-SBA(20), which contains the smallest Au clusters, is now considered. Given that optical spectroscopy probes the electronic structure of the ensemble of Au clusters and is highly sensitive to the presence of AuNPs (>2 nm), curve b in Figure 2A indicates that the population of Au clusters larger than 2 nm is negligibly small. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) confirmed that most of the clusters in 0.16Au11-SBA(20) are ∼1 nm in size (Figure 2B). Figure 2B represents a histogram of the Au cluster size determined by measuring ∼120 particles. The average diameter is determined to be 0.8 ( 0.3 nm, although the number of particles measured is not sufficient for a full statistical analysis. This size is comparable to the diameter of the Au11 core and is the smallest so far reported for silica-supported Au clusters.3-16,21 The structure of the SBA-15 support was found to be unaffected by the calcination, as confirmed by STEM observation (Figure 2B) and small angle XRD measurements (Figure S2, Supporting Information). The BET surface area and average pore size were determined from N2 adsorption/desorption isotherms to be 866 m2/g and 8.0 nm, respectively (Figure S3 and Table S1, Supporting Information). The present approach, as illustrated in Scheme 1, was also applied to other mesoporous silicas (MCF and HMS) and amorphous silica (SiO2). The average size of the Au clusters for 0.09Au11-MCF(20) and 0.07Au11-HMS(20) was determined to be 1.1 ( 0.4 and 1.3 ( 0.5 nm, by STEM, respectively (Figures 3), whereas that of 0.10Au11-SiO2(20) was 2.4 ( 0.6 nm from the conventional TEM (Figure S4, Supporting Information). This result indicates that the large pore size (8 nm) as well as surface area (866 m2/g) of SBA-15 is crucial for suppressing aggregation of the Au clusters in 0.16Au11-SBA(20); the density of Au11:TPP adsorbed on MCF
Figure 3. Representative HAADF-STEM images and size distributions of Au clusters of (A) 0.09Au11-MCF(20) and (B) 0.07Au11-HMS(20).
and HMS is higher than that on SBA-15 because of the smaller surface area of MCF (556 m2/g) and smaller pore diameter of HMS (2 nm). The catalytic properties of the smallest Au clusters in 0.16Au11-SBA(20) were studied using the oxidation of benzyl alcohol (1) in water as a model reaction.22 In this study, microwave heating was employed and H2O2 was used as an oxidant, since the catalytic performance was not sufficiently high under aerobic conditions at ambient temperature. Gas chromatographic (GC) analysis showed that benzaldehyde (2), benzoic acid (3), and benzyl benzoate (4) were obtained as reaction products. The recovery of 1 and yields of 2-4 were determined by the external standard method and are listed in Table 1. Entries 1 and 2 confirmed that oxidation proceeds in the presence of the small Au clusters and that benzoic acid (3) is the major product. Entries 2 and 3 show that the microwave heating is more effective than the conventional oil-bath heating. The remarkable acceleration of the reaction by microwave heating is probably due to the so-called localized “superheating effect”,23 in which the Au clusters are heated directly by microwave irradiation. Importantly, the 0.16Au11-SBA(20) catalyst showed higher activity than larger AuNPs (∼10 nm; Figure S5, Supporting Information) prepared by conventional IP and DP methods (entries 4 and 5).18 The higher selectivity of0.16Au11-SBA(20)fortheformationof3than0.16Au-SBA(IP)
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TABLE 1: Oxidation of Benzyl Alcohol
recovery (%)
a
catalyst
time (min)
temp (°C)
1
2
3
4
1 2 3a 4 5 6 7 8 9
SBA-15 0.16Au11-SBA(20) 0.16Au11-SBA(20) 0.16Au-SBA(IP) 0.16Au-SBA(DP) 0.16Au11-SBA(20) 2ndb 3rdc 4thd
60 60 720 60 60 90 90 90 90
80 80 80 80 80 60 60 60 60
93 0 16 24 29 0 0 0 0
2 6 2 42 20 5 6 7 7
0 91 80 15 36 94 93 91 91
0 2 0 2 0 0 0 0 0
Under oil-bath heating. b Recovered from entry 6. c Recovered from entry 7. d Recovered from entry 8.
TABLE 2: Oxidation of Primary and Secondary Alcohols
substrate 1
entry
R
1 2 3 4 5 6 7 8 9 10 11
C 6H 5 p-O2NC6H4 p-ClC6H4 p-CH3OC6H4 p-CH3C6H4 o-CH3C6H4 m-HOC6H4 n-C5H11 C6H5 n-C4H9 1-indanol
a
yield (%)
entry
recovery (%) R
2
yield (%)
time (min)
1′
2′
3′
4′
60 90 50 50 50 50 25 120 90 120 30
0 0 0 0 0 0 0 81 2 71 0
6 4 4 4 3 16 11 17 96 28 98
91 76 81 85 83 62 86 0
2 17a 10a 0 10a 21a 0 0
H H H H H H H H CH3 CH3
Determined by 1H NMR.
and 0.16Au-SBA(DP) can be explained by the higher oxidation ability of smaller clusters. The reusability of 0.16Au11-SBA(20) was tested using the filtered catalyst in a subsequent run under the same conditions. The results indicated that the catalyst can be recycled at least four times at 60 °C without any loss of activity (entries 6-9). The substrate scope for the catalytic activity of 0.16Au11-SBA(20) was also investigated. As shown in Table 2, most of the primary benzylic alcohols were readily oxidized to produce carboxylic acids (3′) with excellent yields (entries 1-7). In the case of oxidation of o-methylbenzyl and mhydroxybenzyl alcohol (entries 6 and 7), the yields of the corresponding aldehydes (2′) were relatively high. Ester derivatives (4′) were observed as side-products except for entries 4 and 7. Contrary to the case for the benzylic alcohols, oxidation of the primary aliphatic alcohols did not complete even after an extended reaction time (entry 8). The results for oxidation of secondary alcohols are listed in entries 9-11. In each case, the corresponding ketone derivative (2′) was obtained as the sole product in combination with recovery of 1′, and no other products such as esters (Baeyer-Villiger products) were detected. The oxidation of acyclic alcohol proceeded very slowly (entry 10), while cyclic secondary alcohol was fully oxidized within 30 min (entry 11). In summary, a simple, effective method was introduced to immobilize ∼1 nm Au clusters within mesoporous silicas (SBA-
15, MCF, HMS) using Au11:TPP as a precursor. It was found to be crucial to disperse the Au11:TPP homogeneously on the silica before calcination. The areal density of the Au11:TPP could be reduced by utilizing mesoporous silicas with large surface areas. Homogeneous adsorption was achieved by adjusting the average permittivity of the dispersing media. The size of SBAsupported Au clusters after calcination was estimated to be 0.8 ( 0.3 nm by HAADF-STEM observation. The absence of the SP band in the reflectance spectrum indicated that contamination by Au NPs larger than 2 nm was negligibly small. The 0.8 nm Au clusters confined within SBA-15 were shown to exhibit catalytic activity for oxidation of various alcohols by H2O2 under microwave irradiation and were found to be reusable. These results hint at the possibility that highly active and practical gold catalysts are developed even on a chemically inert silica support if the cluster size is reduced below 1 nm. Acknowledgment. This work was financially supported by a CREST grant from JST, Japan. Supporting Information Available: Details of the synthesis and characterization for Au11:TPP, mesoporous silicas (SBA15, MCF, HMS), and the silica-supported Au cluster. 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) Hashmi, A. S. K.; Hutchings, G. J. Angew. Chem., Int. Ed. 2006, 45, 7896. (c) Corma, A.; Garcia, H. Chem. Soc. ReV. 2008, 37, 2096. (d) Pina, C. D.; Falletta, E.; Prati, L.; Rossi, M. Chem. Soc. ReV. 2008, 37, 2077. (2) (a) Kim, Y. D.; Fischer, M.; Gantefo¨r, G. Chem. Phys. Lett. 2003, 377, 170. (b) Bernhardt, T. M. Int. J. Mass Spectrom. 2005, 243, 1. (c) Yoon, B.; Ha¨kkinen, H.; Landman, U. J. Phys. Chem. A 2003, 107, 4066. (d) Molina, L. M.; Hammer, B. J. Chem. Phys. 2005, 123, 161104. (3) Bore, M. T.; Pham, H. N.; Switzer, E. E.; Ward, T. L.; Fukuoka, A.; Datye, A. K. J. Phys. Chem. B 2005, 109, 2873. (4) Yang, C. M.; Kalwei, M.; Schu¨th, F.; Chao, K. J. Appl. Catal., A 2003, 254, 289. (5) Gu, J.; Fan, W.; Shimojima, A.; Okubo, T. J. Solid State Chem. 2008, 181, 957. (6) Rombi, E.; Cutrufello, M. G.; Cannas, C.; Casu, M.; Gazzoli, D.; Occhiuzzi, M.; Monaci, R.; Ferino, I. Phys. Chem. Chem. Phys. 2009, 11, 593. (7) (a) Petkov, N.; Stock, N.; Bein, T. J. Phys. Chem. B 2005, 109, 10737. (b) Guari, Y.; Thieuleux, C.; Mehdi, A.; Rey, C.; Corriu, R. J. P.; Gomez-Gallardo, S.; Philippot, K.; Chaudret, B. Chem. Mater. 2003, 15, 2017. (8) Chiang, C. W.; Wang, A.; Wan, B. Z.; Mou, C. Y. J. Phys. Chem. B 2005, 109, 18042.
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