2 research outputs found
Probing Metal–Support Interaction in Reactive Environments: An in Situ Study of PtCo Bimetallic Nanoparticles Supported on TiO<sub>2</sub>
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. 2011, 2, 900]. In the present communication, an insight into the effect of the
support on the PtCo alloy stability is attempted. A model PtCo/TiO<sub>2</sub> 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 TiO<sub>2</sub> support was observed upon vacuum annealing.
Upon oxidation/reduction conditions, a mixture of CoO<sub><i>y</i></sub> (1 ≤ <i>y</i> < 1.33), TiO<sub>2</sub>, and mixed Co<sub><i>x</i></sub>Ti<sub><i>y</i></sub>O<sub><i>z</i></sub> phases with Pt located
in the subsurface was formed. TiO<sub>2</sub> 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 Ti<sup>4+</sup> 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 Co<sub>3</sub>O<sub>4</sub> in O<sub>2</sub> nor full reduction to metallic Co under various reducing agents
(H<sub>2</sub>, CH<sub>3</sub>OH), 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
Effect of the Specific Surface Sites on the Reducibility of α‑Fe<sub>2</sub>O<sub>3</sub>/Graphene Composites by Hydrogen
The
reducibility of iron oxide nanoparticles (NPs) supported over
few-layer thick graphite upon annealing in hydrogen is investigated
by near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS),
X-ray absorption spectroscopy (XAS), electron energy loss spectroscopy
(EELS), and high-resolution transmission electron microscopy (HR-TEM).
It is found that the stability of the iron oxide NPs toward reduction
is enhanced by the interaction with the graphene nanosheets as compared
to the bulk iron oxide. Postannealing TEM micrographs reveal the existence
of both core/shell and homogeneous iron oxide NPs with the latter
forming irregular trenches into the graphene sheets. EELS analysis
and TEM images clearly demonstrate that the reducibility of iron oxide
particles depends on the specific graphene site on which they are
attached. Furthermore, we show that graphene etching can be mediated
by iron oxide NPs at relatively mild reduction conditions