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