Catalytic activity of PtCu intermetallic compound for CO oxidation: A theoretical insight

Abstract

To understand the microscopic mechanism of the CO oxidation reaction at PtCu nanoparticles, which have unique geometric and electronic structures compared to their component metals, we present here a theoretical study, based on density functional theory calculations, of the main reaction steps of this reaction. We examine the O2 dissociation, the CO adsorption and the CO + O2 reaction at an atomic level and use the computed geometries, Bader charges, and vibrational frequencies to rationalize the role of the intermetallic nanoparticles surface structure on the experimentally observed much higher activity of these nanoparticles as catalysts of the preferential oxidation of CO. By comparing with clean Pt (111) surface and with different Cu-doped models of this same surface, our results show that, at the surface, the presence of Cu induces the segregation of CO molecules at Pt sites and of O2 molecules at Cu sites. Contrarily to Pt surfaces, the unassisted O2 dissociation has a high barrier at the intermetallic nanoparticle surface and proceeds through a CO-assisted mechanism in which the new CO bond is formed while the OO bond is broken with a kinetic barrier much lower than on either Pt (111) or in Pt-doped surfaces. The particular structure of the intermetallic surface is shown to have a significant role in the low kinetic barrier for the reaction, allowing for an easy approach of the CO to the adsorbed O2 molecule that permits an early transition state with a low energetic barrier.