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

Pt-doped CeO x 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–CeO x films, we applied electrochemical in situ IR spectroscopy on Pt–CeO x model thin film catalysts during methanol (1 M methanol) oxidation. The model catalysts were prepared by magnetron cosputtering of Pt (9–21 atom %) and CeO 2 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 HClO 4 ) the Pt–CeO x 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–CeO x samples during methanol oxidation. In comparison to Pt(111), Pt aggregates on Pt–CeO x 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–CeO x 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.