Atomically Precise Gold Nanoclusters: Synthesis and Catalytic Application - Atomically-Precise Methods for Synthesis of Solid Catalysts - page 133

temperature (e.g., T pre ¼ 100 1C) could lead to the same drastic enhancement
in activity. 32
In subsequent work, Nie et al. 33 further investigated the catalytic activity of
Au 38 (SR) 24 nanoclusters. The ligand-on and ligand-off CeO 2 -supported Au 38
cluster catalysts were compared. It was also found that oxygen (O 2 ) thermal
pretreatment of Au 38 (SR) 24 /CeO 2 at a temperature between 100 and 175 1C
can largely enhanced the catalytic activity. While pretreatment at higher
temperatures (4200 1C) to remove the protecting thiolate ligands instead
gave rise to a somewhat lower activity than that for the 175 1C pretreatment;
note that the ligand-off Au 38 clusters were unstable. With respect to the
water vapor effect, it was observed that the CO conversion catalyzed by
Au 38 (SR) 24 /CeO 2 in the case of wet feed-gas was appreciably higher than the
case of dry feed-gas when the reaction temperature was at 60 to 80 1C, and
interestingly the ligand-on and ligand-off Au 38 catalysts exhibited opposite
response to water vapor.
The above CO oxidation results imply that the interface between gold
nanoclusters (e.g., Au 25 (SR) 18 and Au 38 (SR) 24 ) and CeO 2 is critical and should
be the catalytic active site. 32 We speculate that O 2 should first convert to O 2
by withdrawing an electron from the Au nanocluster, then migrates to the
nanocluster/CeO 2 interface and converts to hydroperoxide species on CeO 2 ;
the activated CO should be oxidized at the perimeter sites of the catalyst
(Scheme 5.1).
To improve the high temperature (4200 1C) stability of nanoclusters, Dai
and co-workers explored the stabilization of nanoclustes using mesoporous
materials as supports (e.g., CuO-EP-FDU-12 and Co 3 O 4 -EPFDU-12). 34 The
protecting ligands on Au 25 (SCH 2 CH 2 Ph) 18 and Au 144 (SCH 2 CH 2 Ph) 60
nanoclusters can be removed via thermal treatment at 300 1C (note: the
ligand desorption onset temperature is

d n 9 r 4 n g | 3

.

200 1C) without sintering the Au 25
and Au 144 clusters because the ligand-off Au 25 and Au 144 clusters can be
effectively stabilized on mesoporous material supports. The measured size of
the ligand-off Au 144 cluster was 1.67 0.2 nm, which is consistent with the
size of the native Au 144 (SCH 2 CH 2 Ph) 60 nanoclusters and thus no aggregation
of the clusters after ligands were removed. The ligand-off Au 25 and Au 144
clusters supported on both CuO-EP-FDU-12 and Co 3 O 4 -EP-FDU-12 were

B

Scheme 5.1 Proposed CO oxidation at the perimeter sites of Au n (SR) m/ CeO 2 . Using
the Au 25 (SR) 18 /CeO 2 as model catalyst.
Adapted with permission from ref. 32.