Probing Metal–Support Interaction in Reactive Environments: An in Situ Study of PtCo Bimetallic Nanoparticles Supported on TiO₂

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/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_<i>y</i> (1 ≤ <i>y</i> < 1.33), TiO₂, and mixed Co_<i>x</i>Ti_<i>y</i>O_<i>z</i> 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

Effect of the Specific Surface Sites on the Reducibility of α‑Fe₂O₃/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