Atomically Precise Gold Catalysis - Atomically-Precise Methods for Synthesis of Solid Catalysts - page 100

C ¼ C bond in a,b-unsaturated ketones at mild temperatures. The Au atoms
on the surface have a low-coordination (N ¼ 3) behavior, which enables the
adsorption of H 2 and later for its dissociation.
In a different line of investigation, Jin et al. have also studied the
Au 25 (SR) 18 catalysts for the stereoselective formation a single cyclic alcohol
isomer in the oxidation of bicyclic ketones. 45 The mechanism of its for-
mation was explained as follows: Similar to the case of hydrogenation of a,b-
unsaturated ketones, C ¼ O bond becomes activated by the Au 13 core and the
H 2 adsorption and dissociation happens on the surface sites of Au 12 shell.
The C ¼ O bond is then stereoselectively attacked by a H atom to yield the
desired product. The axial attack of the H atom yields exo-alcohol and
equatorial attack results in endo-alcohol. The spatial environment around
Au 25 (SR) 18 catalyst appears to strongly influence the direction of H atom
attacks and the products formed showed preference for axial attack over
equatorial attack.

d n 9 r 4 n g | 7

4.3 Overview of Electronic and Magnetic Structure
of Catalysts
Generally, gold nanoparticles (42 nm particles) possess a metal core with
their electron conduction band containing the quasi-continuous energy
levels. This leads to the situation where the average spacing between the
energy levels (d)isfarlessthank B T (where k B T is the thermal energy at the
given temperature T and k B is the Boltzmann constant). Hence, the elec-
trons in the conduction band roam freely within the metal core and when
light shines upon them, they are collectively excited to show plasmon
resonance. 46 Contrastingly, gold nanoclusters ( o 2nmparticles)donot
show any plasmon resonance due to quantization of energy levels within
their conduction band. Therefore, they exhibit discrete electronic struc-
ture with molecule-like properties with single electron HOMO-LUMO
transitions instead of collective plasmon excitations of electrons in
nanoparticles (Figure 4.9). 47 One can observe multiple optical absorption
peaks in the optical spectrum of

.

the gold nanoclusters whereas

gold nanoparticles (spherical) produce a single surface plasmon

resonance peak.
The distinctive features of the nanoclusters emerge predominantly be-
cause of their quantum size, which can be estimated using the free electron
theory. 48 The average spacing between the electronic energy levels (d)is
expressed as:

d E E F /N (1)

where, E F is the Fermi energy and N is the number of gold atoms. Since N
relates to the particle size (d) as d 3 , the spacing (d) is inversely related to the
cube of the particle size. Thus, with decreasing size, the spacing between the
energy levels increases. Considering room temperature thermal energy (k B T)