Chemisorption and Reaction of Sulfur Dioxide with Oxidized and Reduced Ceria Surfaces
Journal
of Physical Chemistry, 103 (1999) 11308.
Adsorption of SO2 on fully oxidized ceria at temperatures from 200 K to 600 K leads primarily to a single S 2p species (Fig. 1a, bottom). This species is reversibly adsorbed and its saturation coverage depends upon the temperature of adsorption as shown in Fig. 1b. Based upon previous IR data, this state is assigned to sulfite. Identical coverages appear to be obtained at a given temperature whether the sample is exposed at low temperature and annealed to a final temperature or if the sample is exposed to a saturation dose at the final temperature. No reduction of the ceria substrate occurs. After annealing to temperatures of 600 to 700 K, the sulfite coverage is very low and another low coverage state is revealed at a higher binding energy of 170.8 eV which is assigned to sulfate.
Adsorption on highly reduced
ceria yields quite different results than on fully oxidized ceria.
As in the oxidized case, a prominent species is seen at low and
intermediate temperatures with a slightly higher binding energy (Fig. 1a, top)
than is observed for the sulfite on the oxidized surface.
This species behaves like the sulfite on the oxidized surface (Fig. 1b)
and is also assigned to sulfite. However, annealing a sulfite covered surface,
produced by exposure at low temperature, to
300 K or above causes conversion of the sulfite to another state with much lower
binding energy (Fig. 1a, top). Based upon its binding energy (163.0 eV) this state
is assigned to a sulfide. Above
600 K the sulfide peak decreases in
intensity, but some remains upon the surface to temperatures as high as 1000 K.
Changes in the Ce 4d spectra indicate that a slight oxidation of the
ceria accompanies the sulfide formation, but changes in the VB spectra give
ambiguous indications of the ceria oxidation state changes. Sputter and
annealing experiments indicate that this sulfide behaves similarly to lattice
oxide and diffuses into and out of the film, partitioning between surface and
bulk sites. Sulfate formation was not seen on the reduced surface.
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