(571c) In-Situ Ftir and Xas Study of the Evolution of Surface Species during Transient Co Oxidation on Supported Au/Tio2

Au/TiO₂ catalyst prepared by
deposition-precipitation of HAuCl₄, is remarkably active for CO
oxidation at low temperature. The as-prepared catalyst is inactive; it is activated
by contact with flowing H₂ at room temperature. Transient
experiments involving the saturation of the catalyst either with CO or O₂,
followed by admission of O₂ or CO, are performed. In-situ FTIR and XANES spectroscopy are used to monitor the
development of surface species; the steady state CO oxidation activity of the
catalyst is measured by gas phase FTIR. Exposure of the activated catalysts to 10
mbar of CO at -60 ^oC, leads to the formation of CO adsorbed
species over only 13% of the gold. The process is verified by the development of
a composite band at 2091 cm^-1, which is mainly due to CO adsorbed on
Au⁰ step sites close to the contact perimeter with the support. These
species are very reactive toward CO₂ in the presence of oxygen; they
can also be slowly removed by He, undergoing transformation to less labile adsorbed
species as the CO coverage decreases. Purging of a CO-saturated catalyst with He,
reduces the coverage of gold to 5% after 30 min. These less labile species appear
at 2101 cm^-1; they correspond to CO adsorbed on step sites on top of
the Au⁰ particles.

Admission of 25 mbar of O₂ to the
CO-saturated catalyst induces drastic changes on the nature and population of
surface species. CO₂ is readily formed at the expense of the highly
active CO-Au species (2091 cm^-1), at a TOF of 1.4
mol/mol Au-min. The 2091 cm^-1 species are also simultaneously transformed
into CO co-adsorbed with oxygen on the gold step sites, as evidenced by the
development of a band centered at 2114 cm^-1. The contribution of the
last species to the production of CO₂ is minor; they appear to act
as a CO sink as the CO surface coverage decreases. XANES data show that the
activation of CO by Au⁰, which gives place to the CO-Au species
characterized by the 2091 cm^-1 band, is accompanied by a transfer of
charge from Au⁰ to CO; thereby, an increment in the white line in
the x-ray absorption edge of Au is observed by exposure to CO. After admittance
of O₂, gold returns to its zerovalent state at the time that CO is
oxidized.

When CO flows through a catalyst previously
saturated with O₂, a lag in the appearance of
CO₂ is observed. CO₂ production is spread over a longer
time scale, with a maximum TOF of 1 mol/mol Au-min. Whether the catalyst
is initially saturated with CO or O₂ before admitting the other
reactant, does not affect the total amount of CO₂
generated. Transformation of CO into CO₂ entails a hydroxocarbonyl
intermediate characterized by an absorption band at 1240 cm^-1. Its
rate of disappearing correlates with the rate of formation of CO₂. At
steady state, CO₂ is generated at a TOF of 1.1 mol/mol Au-min, at a
CO surface coverage of 10%, corresponding to 0.75 mol CO/mol surface Au. Oxygen
co-adsorbs with CO on 3% of the available gold. All
the results point to the main role of metallic Au as part of the active
centers.