4D09

坦持金クラスターのサイズ選択的合成と酸化触媒活性（北大・触セ ，産総研 ，千葉大院・工 ）123劉 永梅 ，角山 寛規 ，秋田 知樹 ，一國 伸之 ，○佃 達哉
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1Oxidation 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 theparticle size. Highly active Au catalysts may be developed even on chemically inert supports,such as SiO2 and Al2O3, if the size of AuNPs can be reduced to below 1 nm, since small Aunclusters (n!20) are known to activate the O2 molecule. However, 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. The present
work aims to develop a method to prepare subnanometer-sized Au clusters on mesoporous
silicas with large surface areas (SBA-15, HMS, MCF) using phosphine-protected Au11 clusters
as precursors.
Scheme 1 shows the synthesis process of 0.8-nm Au11 clusters within mesoporous
channels of SBA-15 (pore diameter 8 nm, BET surface area 860 m2/g).
(1) First, the Au11:TPP clusters and SBA-15 were dispersed for 2 hours in mixed solvents ofCH2Cl2/C2H5OH and the composite of Au11:TPP and SBA-15 was collected by filtration. Thecomposites thus obtained are hereafter denoted using the Au content and solvent
composition; for example, the composite prepared in the solvent with C2H5OH/CH2Cl2=20% isreferred to as 0.16Au11:TPP-SBA(20).
(2) The Au11:TPP-SBA composite was then calcined at 200 oC for 2 hours to remove theorganic ligands. 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 wasprobed by diffuse reflectance UV-vis spectroscopy. Figure 1A shows the optical spectra
obtained after calcination of a series of Au11:TPP-SBA composites (curves b-f). The surface
plasmon (SP) band is absent in curve b, but isapparent when the concentration of C2H5OH islower than 10 % (curves c-f). This indicatesthat the Au clusters in 0.16Au11-SBA(20) aresmaller than those in the other samples. The
suppressionofaggregation
in0.16Au11-SBA(20) cannot be ascribed to asmaller amount of loading of the Au11:TPPprecursor (curve g), but is related to thecomposition of the dispersing media used forthe adsorption of Au11:TPP. It is believed that
aggregation is suppressed because Au11:TPP isadsorbed more homogeneously on SBA-15 as a SCHEME 1. Synthesis Procedure of
result of weaker attractive force in the mixed Subnanometer-sized Au Clusters withinsolvent with C2H5OH/CH2Cl2=20% than in the SBA-15.
other compositions.
The structure of 0.16Au11-SBA(20), which contains the smallest Au clusters, is now
considered. Given that optical spectroscopy probes the electronic structure of the ensembleof Au clusters and is highly sensitive to the presence of AuNPs (>2 nm), curve b in Figure 1A

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 thatmost of the clusters in 0.16Au11-SBA(20) are ~1 nm in size (Figure 1B). Figure 1B
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. This size is comparable to the diameterof the Au11 core and is the smallest so far reported for silica-supported Au clusters.

Figure 1. (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), g) 0.10Au11-SBA(0).
(B) Representative HAADF-STEM image and size distribution of Au clusters of 0.16Au11-SBA(20).

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. In this study, microwave
heating was employed and H2O2 was used as an oxidant. The 0.16Au11-SBA(20) catalyst
showed higher activity than larger AuNPs (~10 nm) prepared by conventional IP and DP
methods.
The higher selectivity of 0.16Au11-SBA(20) for the formation of 3 than0.16Au-SBA(IP) and 0.16Au-SBA(DP) can be explained by higher oxidation ability of smallerclusters. The 0.16Au11-SBA(20) catalyst can be recycled at least four times at 60 °C without
any loss of activity.
TABLE 1: Oxidation of Benzyl Alcohol

Entry

Catalyst

12
3

0.16Au11-SBA(20)0.16Au-SBA(IP)
0.16Au-SBA(DP)

Recovery (%)
102429264220

Yield (%)
391
1536

4220In 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 wasfound to be crucial to disperse the Au11:TPP homogeneously on the silica before calcination.
Homogeneous adsorption was achieved by adjusting the average permittivity of thedispersing media. The size of SBA-supported Au clusters after calcination was estimated tobe 0.8±0.3 nm by HAADF-STEM observation. The 0.8-nm Au clusters confined within
SBA-15 were shown to exhibit catalytic activity for oxidation of various alcohols by H2O2 undermicrowave irradiation and were found to be reusable. These results hint at the possibility
that highly active and practical gold catalysts are developed even on chemically-inert silica
support if the cluster size is reduced below 1 nm.