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

Authors: 
Henao, J. D., Northwestern University
Caputo, T., Università di Napoli Federico II
Yang, J. H., Northwestern University
Kung, M. C., Northwestern University


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.