XPS Study of Stability and Reactivity of Oxidized Pt Nanoparticles Supported on TiO₂

The method of X-ray photoelectron spectroscopy was used to study the interaction of the model Pt/TiO₂ catalysts with NO₂ and the following reduction of the oxidized Pt nanoparticles in vacuum, hydrogen, and methane. It was shown that, while interacting with NO₂ at room temperature, the metal Pt nanoparticles transform, first, into the phase which was tentatively assigned as particles containing subsurface/dissolved oxygen [Pt-O_sub], and then, into the PtO and PtO₂ oxides. If only the first state of platinum [Pt-O_sub] is formed, it demonstrates exclusively high reactivity toward hydrogen. For the samples containing simultaneously [Pt-O_sub], PtO, and PtO₂, the highest reaction ability was demonstrated by PtO₂; contrary to the other two oxidized states, it is reducing while kept in vacuum under X-ray irradiation. All three coexisting states of the oxidized platinum can be reduced when heated in vacuum as well as while interacting with hydrogen at room temperature. First, PtO₂ is reduced to PtO. PtO and [Pt-O_sub] begin being reduced after the complete consumption of PtO₂. We propose that, when a sample contains simultaneously all three states of oxidized platinum, the supported particles have a core–shell structure with a nucleus of perturbed platinum containing oxygen atoms, which are covered with a film of Pt oxides. It was shown that none of the oxidized states of platinum react with methane at room temperature