462 cm
À1 corresponding to the fluorite phase was red-shifted, indicating the surface
distortion caused by Au–CeO 2 interaction. The peak-shift was pronounced for
nanorods rather than nano-octahedra and nanocube. Taking into account that the
nanorod was largely enclosed by (110) planes, the interaction between Au and CeO 2
(110) plane would be stronger than (100) and (111) planes.
Recently, supported single Au atom catalysts have attracted growing interest
because of the maximized atom efficiency. Qiao and Li et al. reported that single
Au atoms on CeO 2 (Au 1 /CeO 2 ) exhibited higher activity and selectivity for the
solvent- and base-free benzyl alcohol oxidation (16% conversion, 98% selectivity,
MTY of 3,091 h
À1 ) than Au NPs on CeO 2 (37% conversion, 72% selectivity, MTY
of 216 h
À1 ) [29].
Au/TiO 2 has also been intensively studied for alcohol oxidations [30–
37]. Hutchings et al. reported that Au/TiO 2 promoted the benzyl alcohol oxidation
under solvent- and base-free conditions to give benzaldehyde in 55% selectivity at
68% conversion with TOF of 31,900 h
À1 [30].
Solvent-free selective oxidation of benzyl alcohol to benzaldehyde is usually
reported at low conversions, and an achievement of high selectivity at high conversions is rather difficult. Corma et al. demonstrated the corresponding aldehydes was
obtained with excellent selectivity (98% conversion, 99% selectivity for
cinnamaldehyde) by the oxidation of primary alcohol oxidation over Au/TiO 2 and
Au/CeO 2 in organic solvents in the absence of base [34]. Baiker et al. screened
organic solvents for benzyl alcohol oxidation over Au/TiO 2 , and mesitylene [35] or
supercritical CO 2 was more suitable than toluene in terms of the selectivity of
benzaldehyde.
Deposition of reducible MO x onto inert supports has attempted to give highly
dispersed reducible MO x NPs, and Au NPs on reducible MO x /inert supports, such as
Au/Ce-SiO 2 [74], Au/SnO x -SiO 2 [38], and Au/CeO 2 -g-C 3 N 4 [75], were reported.
Au/SnO x -SiO 2 showed superior catalytic activity (MTY of 15,248 h
À1 ) to those of
Au/SiO 2 and Au/MoO x -SiO 2 [38]. The high catalytic activity of Au/SnO x -SiO 2 was
due to the electron transfer between SnO x and Au and the lattice distortion of Au,
leading to more low-coordinated Au NPs. DFT calculation also suggested that the
deposition of SnO x onto Au would enhance the O 2 activation. On the other hand, no
positive effect was observed for Au/MoO x -SiO 2 due to weak interaction between
MoO x and Au.
2.1.2 Size Dependence
Rossi et al. studied the size effect of Au/TiO 2 and Au/Al 2 O 3 having 3.8, 4.4, and
6.8 nm of Au NPs on the oxidation of ethylene glycol to glycolic acid and found that
the catalytic activity increased as the size of Au NPs decreased [16]. Corma et al.
examined the size effect of Au/TiO 2 having different size of Au AuNP in the range
of 5–27 nm on the oxidation of cinnamyl alcohol to cinnamaldehyde [34]. The TOFs
increased exponentially as the Au NPs became smaller (Fig. 1a). TOFs plotted as a
function of the total number of surface Au atoms showed a linear relationship
6
T. Ishida et al.
À1 corresponding to the fluorite phase was red-shifted, indicating the surface
distortion caused by Au–CeO 2 interaction. The peak-shift was pronounced for
nanorods rather than nano-octahedra and nanocube. Taking into account that the
nanorod was largely enclosed by (110) planes, the interaction between Au and CeO 2
(110) plane would be stronger than (100) and (111) planes.
Recently, supported single Au atom catalysts have attracted growing interest
because of the maximized atom efficiency. Qiao and Li et al. reported that single
Au atoms on CeO 2 (Au 1 /CeO 2 ) exhibited higher activity and selectivity for the
solvent- and base-free benzyl alcohol oxidation (16% conversion, 98% selectivity,
MTY of 3,091 h
À1 ) than Au NPs on CeO 2 (37% conversion, 72% selectivity, MTY
of 216 h
À1 ) [29].
Au/TiO 2 has also been intensively studied for alcohol oxidations [30–
37]. Hutchings et al. reported that Au/TiO 2 promoted the benzyl alcohol oxidation
under solvent- and base-free conditions to give benzaldehyde in 55% selectivity at
68% conversion with TOF of 31,900 h
À1 [30].
Solvent-free selective oxidation of benzyl alcohol to benzaldehyde is usually
reported at low conversions, and an achievement of high selectivity at high conversions is rather difficult. Corma et al. demonstrated the corresponding aldehydes was
obtained with excellent selectivity (98% conversion, 99% selectivity for
cinnamaldehyde) by the oxidation of primary alcohol oxidation over Au/TiO 2 and
Au/CeO 2 in organic solvents in the absence of base [34]. Baiker et al. screened
organic solvents for benzyl alcohol oxidation over Au/TiO 2 , and mesitylene [35] or
supercritical CO 2 was more suitable than toluene in terms of the selectivity of
benzaldehyde.
Deposition of reducible MO x onto inert supports has attempted to give highly
dispersed reducible MO x NPs, and Au NPs on reducible MO x /inert supports, such as
Au/Ce-SiO 2 [74], Au/SnO x -SiO 2 [38], and Au/CeO 2 -g-C 3 N 4 [75], were reported.
Au/SnO x -SiO 2 showed superior catalytic activity (MTY of 15,248 h
À1 ) to those of
Au/SiO 2 and Au/MoO x -SiO 2 [38]. The high catalytic activity of Au/SnO x -SiO 2 was
due to the electron transfer between SnO x and Au and the lattice distortion of Au,
leading to more low-coordinated Au NPs. DFT calculation also suggested that the
deposition of SnO x onto Au would enhance the O 2 activation. On the other hand, no
positive effect was observed for Au/MoO x -SiO 2 due to weak interaction between
MoO x and Au.
2.1.2 Size Dependence
Rossi et al. studied the size effect of Au/TiO 2 and Au/Al 2 O 3 having 3.8, 4.4, and
6.8 nm of Au NPs on the oxidation of ethylene glycol to glycolic acid and found that
the catalytic activity increased as the size of Au NPs decreased [16]. Corma et al.
examined the size effect of Au/TiO 2 having different size of Au AuNP in the range
of 5–27 nm on the oxidation of cinnamyl alcohol to cinnamaldehyde [34]. The TOFs
increased exponentially as the Au NPs became smaller (Fig. 1a). TOFs plotted as a
function of the total number of surface Au atoms showed a linear relationship
6
T. Ishida et al.