Abstract Supported gold nanoparticles (Au NPs) exhibit unique catalytic properties for the oxidation of organic compounds. The catalytic activities and the selectivities of the supported Au catalysts largely depend on the kind of support and the
particle size of Au. For oxidation of alcohols to aldehydes and ketones, the reducibility of metal oxide (MO x ) supports plays a prominent role, while the basicity of
the supports or the size of Au particles are more important factors for non-reducible
MO x , non-oxides, and other supports. The size effect is more pronounced for
dehydrogenation than aerobic oxidation because dehydrogenation takes place
mainly on the low-coordinated edge and corner Au atoms. Oxidation of alkenes to
epoxides using O 2 as a sole oxidant has been achieved by supported Au clusters
having a diameter of 2 nm or less. For oxidation of cyclohexane using O 2 as a sole
oxidant, the presence of Brønsted acid sites contributes to the production of K/A oil.
The size of Au particles also largely affects the reaction rate and product selectivity;
sub-nanometer Au clusters exhibited significantly higher catalytic activity and K/A
oil selectivity than Au NPs.
Keywords Alcohol oxidation · Alkane oxidation · Alkene oxidation · Gold
clusters · Gold nanoparticles · Oxidative coupling · Oxidative esterification
1 Introduction
Selective oxidation is of great importance in chemical industry, because oxidation
reactions occupy ca. 30% of chemical processes. In particular, aerobic oxidation using
O 2 or air as an oxidant is regarded as an environmentally benign reaction that can
avoid use of harmful stoichiometric oxidants and minimize wastes. Thus, development
of heterogeneous oxidation catalysts that have not only high catalytic activity but also
high selectivity to a specific functional group in the substrates is highly desired.
Since the discovery of catalysis by gold nanoparticles (Au NPs) in 1987 [1], Au has
attracted growing interest in the field of catalysis [2–4]. In the early studies, reducible
metal oxides (MO x ) supported Au NPs were revealed to exhibit very high catalytic
activity for CO oxidation below room temperature, while supported Pd and Pt catalysts
generally required higher reaction temperature such as 100
C [1, 5]. Au also exhibits
unique catalysis for propylene oxidation to give propylene oxide directly and selectively in the presence of H 2 and O 2 [6] and then O 2 and H 2 O [7]. Reducible MO x , such
as Fe 2 O 3 , Co 3 O 4 , NiO, and MnO 2 , are defined as that the MO x have redox properties
or have semiconducting properties. The oxygen atoms in the reducible MO x are easily
removed to form the oxygen vacancies on the surface. Because bare Au surface cannot
dissociate O 2 in contrast to Pd and Pt [8], the oxygen vacancies near the Au NPs play
an important role for O 2 activation in the gas-phase CO oxidation over Au catalysts.
Namely, CO is adsorbed on Au NPs, and O 2 is activated at the oxygen vacancies of the
reducible MO x , so that the reaction of CO and O 2 takes place at the perimeter interface.
Therefore, the catalytic activity of Au is significantly affected by the kind of supports
in terms of O 2 storage-release capacity of the supports, and Au NPs on reducible MO x
2
T. Ishida et al.
particle size of Au. For oxidation of alcohols to aldehydes and ketones, the reducibility of metal oxide (MO x ) supports plays a prominent role, while the basicity of
the supports or the size of Au particles are more important factors for non-reducible
MO x , non-oxides, and other supports. The size effect is more pronounced for
dehydrogenation than aerobic oxidation because dehydrogenation takes place
mainly on the low-coordinated edge and corner Au atoms. Oxidation of alkenes to
epoxides using O 2 as a sole oxidant has been achieved by supported Au clusters
having a diameter of 2 nm or less. For oxidation of cyclohexane using O 2 as a sole
oxidant, the presence of Brønsted acid sites contributes to the production of K/A oil.
The size of Au particles also largely affects the reaction rate and product selectivity;
sub-nanometer Au clusters exhibited significantly higher catalytic activity and K/A
oil selectivity than Au NPs.
Keywords Alcohol oxidation · Alkane oxidation · Alkene oxidation · Gold
clusters · Gold nanoparticles · Oxidative coupling · Oxidative esterification
1 Introduction
Selective oxidation is of great importance in chemical industry, because oxidation
reactions occupy ca. 30% of chemical processes. In particular, aerobic oxidation using
O 2 or air as an oxidant is regarded as an environmentally benign reaction that can
avoid use of harmful stoichiometric oxidants and minimize wastes. Thus, development
of heterogeneous oxidation catalysts that have not only high catalytic activity but also
high selectivity to a specific functional group in the substrates is highly desired.
Since the discovery of catalysis by gold nanoparticles (Au NPs) in 1987 [1], Au has
attracted growing interest in the field of catalysis [2–4]. In the early studies, reducible
metal oxides (MO x ) supported Au NPs were revealed to exhibit very high catalytic
activity for CO oxidation below room temperature, while supported Pd and Pt catalysts
generally required higher reaction temperature such as 100
C [1, 5]. Au also exhibits
unique catalysis for propylene oxidation to give propylene oxide directly and selectively in the presence of H 2 and O 2 [6] and then O 2 and H 2 O [7]. Reducible MO x , such
as Fe 2 O 3 , Co 3 O 4 , NiO, and MnO 2 , are defined as that the MO x have redox properties
or have semiconducting properties. The oxygen atoms in the reducible MO x are easily
removed to form the oxygen vacancies on the surface. Because bare Au surface cannot
dissociate O 2 in contrast to Pd and Pt [8], the oxygen vacancies near the Au NPs play
an important role for O 2 activation in the gas-phase CO oxidation over Au catalysts.
Namely, CO is adsorbed on Au NPs, and O 2 is activated at the oxygen vacancies of the
reducible MO x , so that the reaction of CO and O 2 takes place at the perimeter interface.
Therefore, the catalytic activity of Au is significantly affected by the kind of supports
in terms of O 2 storage-release capacity of the supports, and Au NPs on reducible MO x
2
T. Ishida et al.