Atomically Precise Gold Nanoclusters: Synthesis and Catalytic Application - Atomically-Precise Methods for Synthesis of Solid Catalysts - page 136

conversion than those with electron-deficient groups, and sulfides with
greater steric group showed lower conversion than those with less steric
group. The support effect was also investigated, and the order of the catalyst
activity of Au 25 (SR) 18 /oxide was found to be TiO 2 4Fe 2 O 3 4CeO 2 4MgO. The
oxide-supported Au 25 (SR) 18 catalysts showed excellent recyclability in the
sulfoxidation process. 28
The above catalytic results of the oxidation by Au n (SR) m nanoclusters
allow us to gain some valuable insight at the atomic level. In the
[Au 25 (SR) 18 ] q nanocluster, the Au 13 core possesses 8 (when q ¼ 1) or 7
(when q ¼ 0) delocalized valence electrons, which originate from Au(6s) and
are primarily distributed within the Au 13 core, 27 while the Au 12 shell bears
positive charges due to charge transfer from gold to sulfur atoms of the
thiolate ligands. The electron-rich Au 13 core should facilitate the activation
of the oxidant [O] (e.g., O 2 , TBHP, PhIO, etc.) by electron transfer to [O],
accompanied by [Au 25 (SR) 18 ] conversion to neutral [Au 25 (SR) 18 ] 0 ; of note,
the oxidation readily occurs at elevated temperatures (e.g., under catalytic
reaction conditions). For catalytic oxidation on Au 25 (SR) 18 , activation of
reactants (e.g., styrene, sulfide, CO, and alcohol, etc.) should be facilitated by
the partial positive charge (Au d1 , d ¼

d n 9 r 4 n g | 3

0.3) 38 on the triangular Au 3 motif of

B

gold atoms.

5.5.3 Catalytic Selective Hydrogenation
5.5.3.1 Selective Hydrogenation of Aldehydes and Ketones
Beside the oxidation reactions, the Au n (SR) m nanocluster catalysts were also
demonstrated to be capable of catalyzing hydrogenation reactions, such as
the chemoselective hydrogenation of a,b-unsaturated ketones to a,b-
unsaturated alcohols under mild conditions (60 1C, in mixed solvents (1 : 1
toluene/acetonitrile)) (Table 5.1). 35 The Au 25 (SR) 18 nanoclusters were found
to hydrogenate preferentially the C ¼ O bond of benzalacetone against the
C ¼ C bond, and the C ¼ O hydrogenated product (i.e., unsaturated alcohol,
UA) was obtained with 76% selectivity, whereas 14% selectivity was observed
for unsaturated ketone, and 10% for saturated alcohol. 35 The UV-visible
spectra and MS analysis of the free Au 25 (SR) 18 nanocluster catalyst con-
firmed that the Au 25 (SR) 18 nanoclusters remain intact during the hydro-
genation reaction. Further optimization of the reaction led to a nearly
complete selectivity for a,b-unsaturated alcohol in a mixed toluene/ethanol
(1 : 1) solvent at room temperature. 39 The oxide-supported catalysts (in-
cluding Au 25 (SR) 18 /Fe 2 O 3 ,Au 25 (SR) 18 /TiO 2 , and Au 25 (SR) 18 /SiO 2 ) were also
investigated (Table 5.1). It was found that the catalytic activity was improved
except the Au 25 (SR) 18 /SiO 2 catalyst, and the catalytic activity of the
Au 25 (SR) 18 /Fe 2 O 3 and Au 25 (SR) 18 /TiO 2 catalysts is almost equivalent
(Table 5.1). In all cases, the chemoselectivity was 100%. In addition,
the chemoselective hydrogenation was tested with a range of substituted

.