ABSTRACT
A continuing focus of gold catalyst work is attempting to correlate
catalytic activity and selectivity with a nanoparticle’s physical
properties. These properties include particle size, particle shape,
gold oxidation state, support oxidation state, particle stability, and
catalyst electronic structure. Electron microscopy studies are useful
for estimating catalyst particle sizes and how they evolve with
time/temperatures. However, they suffer from a severe limitation:
they only sample a small fraction of the gold particles. Indeed,
if one were to sample 1000 2.5 nm hemispherical particles
using transmission electron microscopy (TEM) this would only
account for around 10%–16% of the total particles in a 1 g
gold catalyst sample with 1 wt% gold loading. X-ray absorption
spectroscopy (XAS) will provide information about the oxidation
state and particle size (based on extended X-ray absorption fine
structure measurements) but requires a synchrotron radiation
source [1, 2]. These measurements have the advantage of being
able to be performed in situ through specially designed cells [3]. 197Au-Mo¨ssbauer spectroscopy is a nuclear resonance technique
suitable for identifying oxidation states and estimating chemical
environments around the isotope of interest [4-6]. In this method 197Pt is irradiated with neutrons to generate a monochromatic
gamma source. The monochromaticity derives from the nuclear
transition from gamma decay. Fitting Mo¨ssbauer data results in
two variables for gold, the isomer shift (IS) and the quadrapole
splitting (QS). The IS is a measure of electron density in the 6s orbital of gold. The QS provides an estimate of the asymmetric
distribution of electrons around the gold atom. Larger QS is due to
lower symmetry around the gold atom due to ligands. Together the
IS and QS results provide an unequivocal description of the metal
oxidation state and its local symmetry. The difficulties with Au-
Mo¨ssbauer spectroscopy are the need to perform themeasurements
at low temperatures (∼4.2 K), increase sensitivity, and produce the Pt source on a neutron reactor and the short half-life of the Pt source
(18 hours).
