posted on 2017-01-12, 00:00authored byChristoph Gleichweit, Christian Neiss, Sven Maisel, Udo Bauer, Florian Späth, Oliver Höfert, Andreas Görling, Hans-Peter Steinrück, Christian Papp
The adsorption and
reaction of CO on a monolayer carbide and a
bulk carbide, prepared on Mo(110), was studied with synchrotron-based
XPS, TPD, and density-functional calculations using slab models. In
the experiments on the monolayer carbide, we find two CO species at
140 K, with a saturation coverage of ∼0.7 ML, while on the
bulk carbide, Mo2C, three molecular adsorption states are
found, showing a similar total coverage of ∼0.7 ML at saturation.
In addition, CO partly dissociates on both surfaces (monolayer carbide:
7%, bulk carbide: 15%). The calculations on the monolayer carbide
show that the adsorption of CO on Mo sites is most stable. At increased
coverages, several different adsorption sites on the monolayer carbide
become possible. From the core level shifts, an assignment to the
experimentally found species becomes available. Upon heating, we find
on both carbides the competing processes of desorption, interconversion
of different CO species, and dissociation of CO. The detailed quantitative
analysis of these processes shows that desorption and dissociation
to atomic oxygen and carbon is completed at ∼400 K on the monolayer
carbide and ∼450 K on the bulk carbide; in both cases, about
35% (0.25 ML) of the initially adsorbed CO decomposes upon heating.
Above 800 K, atomic carbon and oxygen desorb associatively, and at
1200 K the carbide surfaces are restored.