Stabilization of Small Platinum Nanoparticles on Pt-CeO2 Thin 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 MHClO4) 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 lbw 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.