An FTIR and quadrupole mass spectroscopic study of CO adsorption
and oxidation with 16O2 and
18O2 is
presented together with coadsorption experiments of CO and water on
gold supported on ZnO and TiO2.
The experimental results are closely related to those presented in
the parallel paper on copper catalysts supported
on the same oxides. The most important points are as follows:
(i) CO is found to be activated on the metallic
particles on two kinds of sites, the normal terrace sites and sites at
the borderline of the particles; (ii) there
is evidence that on the examined samples there are metallic sites which
are able to adsorb both oxygen and
carbon monoxide molecules at the same time; (iii) carbon dioxide and
carbonate-like species are formed; the
asymmetric stretching frequencies of CO2, the quadrupole
mass spectroscopic analysis, and the IR spectra
analysis reveal that with 18O2, different
isotopic molecular CO2 is formed while the carbonate-like
species
have the same frequencies of those produced in
16O2; (iv) there are two independent pathways
for CO oxidation,
a rapid direct oxidation of CO at the surface of the
metallic particles and a slow induced oxidation with
the
surface lattice oxygen species of the supports; (v) evidence is found
that CO−H2O coadsorption experiments
produce atomic hydrogen on Au/ZnO, not on
Au/TiO2.
An FTIR study of CO adsorption and oxidation on Au/TiO2 is reported. At 90 K different CO−18O2 interactions
have been performed, with preadsorbed carbon monoxide or preadsorbed isotopically labeled oxygen, with
dry or wet carbon monoxide. From the experimental results it can be inferred that (i) at 90 K carbon monoxide
and oxygen are molecularly adsorbed on gold particles, (ii) the reaction occurs if carbon monoxide is
preadsorbed (it is almost completely inhibited if oxygen is preadsorbed), and (iii) the reaction occurs more
easily in the presence of water. It is proposed that dissociation of oxygen is needed in order to observe the
oxidation reaction.
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