calcination in air or H 2 to obtain bare Au surface. Removal of the ligands is
performed at 200–400
C, which often causes aggregation of the Au clusters.
Golovko et al. reported that TiO 2 supported Au 9 (PPh 3 ) 8 (NO 3 ) 3 and
Au 101 (PPh 3 ) 21 Cl 5 was active for aerobic oxidation of benzyl alcohol, but the catalysts calcined in O 2 or followed by H 2 reduction exhibited much higher catalytic
activity, whereas the size of Au particles of the calcined catalysts was significantly
larger (2.5–5.7 nm) than those of the protected Au clusters [70]. The improved
activity was ascribed to the change of the electronic state of Au, i.e., from cationic to
metallic Au, by removal of phosphine ligands. In addition, Au 101 (PPh 3 ) 21 Cl 5 /TiO 2
exhibited higher catalytic activity than Au 9 (PPh 3 ) 8 (NO 3 ) 3 /TiO 2 in spite of larger Au
cluster size even after calcination, indicating that the nitrate poisoned the catalytic
activity of Au.
Protected Au clusters were also confined in the porous materials in order to avoid
the aggregation of Au clusters during calcination. Tuel et al. prepared Au 25 ( paminobenzenethiolate) 17 immobilized on the mesoporous silica, SBA-15, and examined the catalytic activity for benzyl alcohol oxidation [71]. The catalytic activity of
Au 25 /SBA-15 was markedly improved with calcination temperature owing to the
removal of thiolate ligands and the change of the electronic state of Au, S-Au(I)-S to
metallic Au, although the size of Au increased. Xie and Yang et al. attempted the
removal of alkyl thiolate ligands by oxidation using t-butyl hydroperoxide (TBHP)
under mild conditions [72]. The thiolate ligands of Au 25 (SC 12 H 25 ) 18 /HAP were
completely removed by TBHP at 50
C as sulfonic acid after 48 h while maintaining
the Au cluster size (1.2–1.5 nm). They demonstrated that the presence of sulfur, even
in a very small amount, significantly affected the catalytic activity of Au clusters.
Tsukuda et al. prepared Au 25 (SC 12 H 25 ) supported on carbon nanotubes (CNT)
(Au 25 :SC 12 /CNT) and examined the catalytic activity for the benzyl alcohol oxidation in the presence of O 2 and K 2 CO 3 [73]. The calcination of Au 25 :SC 12 /CNT
successfully removed the thiolate ligand without the aggregation of the Au clusters
(Au 25 /CNT). Although the Au 25 :SC 12 /CNT was inactive, Au 25 /CNT appeared to be
active to produce benzaldehyde as a major product together with benzoic acid and
benzyl benzoate. The doping of Pd into Au 25 /CNT catalyst (Pd 1 Au 25 /CNT) significantly improved the catalytic activity, which was explained by the ligand effect in
which Pd atoms modulated the electronic structure of Au by electron transfer from
Pd to Au.
2.6.4 Size Dependence
Rossi et al. first reported the size dependence “naked” colloidal Au without
protecting agents on the catalytic activity for the glucose oxidation (Scheme 4) in
the range of 3–10 nm [96]. Naked Au NPs became catalytically active from the 5 nm
of Au NPs, and the reaction rate increased with a decrease in the size of
Au. However, the TOF did not alter by the size of Au, indicating that the apparent
increase in the reaction rates was ascribed to an increase in the number of surface Au
atoms.
20
T. Ishida et al.
performed at 200–400
C, which often causes aggregation of the Au clusters.
Golovko et al. reported that TiO 2 supported Au 9 (PPh 3 ) 8 (NO 3 ) 3 and
Au 101 (PPh 3 ) 21 Cl 5 was active for aerobic oxidation of benzyl alcohol, but the catalysts calcined in O 2 or followed by H 2 reduction exhibited much higher catalytic
activity, whereas the size of Au particles of the calcined catalysts was significantly
larger (2.5–5.7 nm) than those of the protected Au clusters [70]. The improved
activity was ascribed to the change of the electronic state of Au, i.e., from cationic to
metallic Au, by removal of phosphine ligands. In addition, Au 101 (PPh 3 ) 21 Cl 5 /TiO 2
exhibited higher catalytic activity than Au 9 (PPh 3 ) 8 (NO 3 ) 3 /TiO 2 in spite of larger Au
cluster size even after calcination, indicating that the nitrate poisoned the catalytic
activity of Au.
Protected Au clusters were also confined in the porous materials in order to avoid
the aggregation of Au clusters during calcination. Tuel et al. prepared Au 25 ( paminobenzenethiolate) 17 immobilized on the mesoporous silica, SBA-15, and examined the catalytic activity for benzyl alcohol oxidation [71]. The catalytic activity of
Au 25 /SBA-15 was markedly improved with calcination temperature owing to the
removal of thiolate ligands and the change of the electronic state of Au, S-Au(I)-S to
metallic Au, although the size of Au increased. Xie and Yang et al. attempted the
removal of alkyl thiolate ligands by oxidation using t-butyl hydroperoxide (TBHP)
under mild conditions [72]. The thiolate ligands of Au 25 (SC 12 H 25 ) 18 /HAP were
completely removed by TBHP at 50
C as sulfonic acid after 48 h while maintaining
the Au cluster size (1.2–1.5 nm). They demonstrated that the presence of sulfur, even
in a very small amount, significantly affected the catalytic activity of Au clusters.
Tsukuda et al. prepared Au 25 (SC 12 H 25 ) supported on carbon nanotubes (CNT)
(Au 25 :SC 12 /CNT) and examined the catalytic activity for the benzyl alcohol oxidation in the presence of O 2 and K 2 CO 3 [73]. The calcination of Au 25 :SC 12 /CNT
successfully removed the thiolate ligand without the aggregation of the Au clusters
(Au 25 /CNT). Although the Au 25 :SC 12 /CNT was inactive, Au 25 /CNT appeared to be
active to produce benzaldehyde as a major product together with benzoic acid and
benzyl benzoate. The doping of Pd into Au 25 /CNT catalyst (Pd 1 Au 25 /CNT) significantly improved the catalytic activity, which was explained by the ligand effect in
which Pd atoms modulated the electronic structure of Au by electron transfer from
Pd to Au.
2.6.4 Size Dependence
Rossi et al. first reported the size dependence “naked” colloidal Au without
protecting agents on the catalytic activity for the glucose oxidation (Scheme 4) in
the range of 3–10 nm [96]. Naked Au NPs became catalytically active from the 5 nm
of Au NPs, and the reaction rate increased with a decrease in the size of
Au. However, the TOF did not alter by the size of Au, indicating that the apparent
increase in the reaction rates was ascribed to an increase in the number of surface Au
atoms.
20
T. Ishida et al.