Atomically Precise Gold Catalysis - Atomically-Precise Methods for Synthesis of Solid Catalysts - page 93

clusters, tuning of catalytic activity and selectivity is also feasible. For in-
stance, Goodman and co-workers have observed size-dependent catalytic
activity of gold clusters supported over TiO 2 for CO oxidation reaction in a
gas phase reaction. 22 In several of those studies, complex formation between
gold clusters and molecular oxygen has been reported due to the interaction
of dioxygen with the gold clusters. 23,24
The number of investigations on the use of atomically precise catalysts for
catalyzing a variety of chemical reactions continue to increase. The chemical
conversion reactions that are currently being explored using atomically
precise gold cluster catalysts were previously carried out using traditional
gold nanoparticle catalysts. A major set of catalytic reactions that are widely
investigated using atomically precise catalysts include (i) the CO oxidation
reaction, (ii) selective oxidation of styrene, and (iii) selective hydrogenation
of a,b-unsaturated ketones/aldehydes; which are discussed in detail in the
following section. In addition, catalytic reactions such as the selective
oxidation of sulfide to sulfoxide, 25 Ullmann-type homocoupling reaction of
aryl iodides 26 and electrocatalytic reduction of CO 2 27 are also being explored
by researchers.

d n 9 r 4 n g | 7

4.2.2.1 The CO Oxidation Reaction
Haruta et al. have pioneered the work on carbon monoxide oxidation cata-
lyzed by Au nanoparticles supported on oxides. 1,2 Since then, gold nanoca-
talysts have been widely investigated, particularly due to their unusually high
activity for CO oxidation even at temperatures as low as 70 1C. This re-
action has practical applications as well, e.g., eliminating CO impurities
from H 2 -rich syngas for fuel cells. 28 Despite significant progress in gold
catalysis, the fundamentally important mechanistic aspects of CO oxidation
by nanoparticles are still unclear. 29 Although conventional gold catalysts
prepared using wet impregnation methods produce active catalysts for CO
oxidation, these catalysts are not useful in understanding the mechanism
clearly. Some of the disadvantages include:

.

A broad size distribution of the gold nanoparticles (with respect to
atomic-precision), posing a major challenge in determining the active
sites (such as metallic nanoparticles or gold ions) that catalyzes the CO
oxidation.
Lack of accurate information on the active sites (such as gold surface or
gold/support interface) and the nature of support.
Inability to utilize computational tools for modeling ''atomically-
imprecise'' traditional nanoparticle catalysts.

Among several supports for gold catalysts that have been tested for CO
oxidation, TiO 2 appears to be the best in terms of catalytic activity. Detailed
investigation into CO oxidation using supported gold nanoclusters also re-
vealed that the reaction is sensitive to the size of the clusters and significant