Our recent surface characterization studies of extended
and nanosized
PtCo alloys under hydrogen and oxygen atmospheres, indicated significant
and reversible surface segregation in response to the gas phase environment
[J. Phys. Chem. Lett.20112900]. In the present communication, an insight into the effect of the
support on the PtCo alloy stability is attempted. A model PtCo/TiO2 interface is investigated under reducing, oxidizing, and
catalytic reaction conditions using ambient pressure X-ray photoelectron
and absorption spectroscopies (APPES and NEXAFS respectively). Encapsulation
of PtCo by the TiO2 support was observed upon vacuum annealing.
Upon oxidation/reduction conditions, a mixture of CoO
y
(1 ≤ y < 1.33), TiO2, and mixed Co
x
Ti
y
O
z
phases with Pt located
in the subsurface was formed. TiO2 was found to be remarkably
stable under the temperature and pressure conditions used here (up
to 620 K, 0.2 mbar), with titanium remaining always in the Ti4+ state. The interplay between the gas atmosphere and the
surface is limited to modifications of the cobalt oxidation state.
However, in contrast to the observations on the unsupported PtCo alloy,
neither oxidation of CoO to Co3O4 in O2 nor full reduction to metallic Co under various reducing agents
(H2, CH3OH), occurred. Synchronized changes
of the binding energy position of core level photoelectron peaks in
response to the gas phase are related to the band-bending development
at the gas/solid interface. This documents the direct coupling of
the electronic properties and the gas phase chemical potential of
a chemically functional material useful as catalyst or gas sensing
device.
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