Stabilization of Small Platinum Nanoparticles on Pt–CeO₂Thin Film Electrocatalysts During Methanol Oxidation

Abstract

Pt-doped CeOx thin film electrocatalysts have recently been shown to exhibit high activity and stability at the anode of proton exchange membrane fuel cells (PEM-FC). To identify the role of the Pt dopant and the origin of the high stability of Pt–CeOx films, we applied electrochemical in situ IR spectroscopy on Pt–CeOx model thin film catalysts during methanol (1 M methanol) oxidation. The model catalysts were prepared by magnetron cosputtering of Pt (9–21 atom %) and CeO2 onto clean and carbon-coated Au supports. All samples were characterized by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), and X-ray photoelectron spectroscopy (XPS) before and after reaction. At pH 1 (0.1 M HClO4) the Pt–CeOx dissolves partially during potential cycling, whereas the films are largely stable at pH 6 (0.1 M phosphate buffer). Electrochemical IR spectroscopy of the adsorbed CO shows that metallic Pt is formed on all Pt–CeOx samples during methanol oxidation. In comparison to Pt(111), Pt aggregates on Pt–CeOx show a CO on-top signal, which is red shifted by at least 25 cm–1 and suppression of the bridging CO signals. Whereas the Pt particles on Pt–CeOx films with high Pt concentration (>20 atom %) undergo rapid sintering during the potential cycling, small metallic Pt aggregates are stable under the same conditions on films with low Pt concentration (<15 atom % Pt). By means of density functional theory (DFT) calculations we analyzed the spectral shifts of adsorbed CO as a function of nanoparticle size both on free and ceria-supported Pt particles. Comparison with the experiment suggests the formation of “subnano”-particles, i.e., particles with up to 30 atoms (<1 nm particle diameter), which do not expose regular (111) facet sites. At sufficiently low Pt loading, these subnano-Pt particles are efficiently stabilized by the interaction with the ceria support under conditions of the dynamically changing electrode potential.