Stabilization and characterization of platinum nano-catalysts formed on highly ordered pyrolytic graphite

Platinum catalysts are prepared on highly ordered pyrolytic graphite (HOPG). Two different methods are used to deposit platinum onto HOPG; electrochemical deposition and impregnation followed by thermal decomposition. To increase interactions between the Pt deposits and the carbon support, HOPG is oxidized either electrochemically or with ozone gas. The combination two different deposition techniques and three different substrates (freshly cleaved, electrochemically, and ozone oxidized HOPG) results in six different Pt/carbon electrodes that are studied with respect to their particle morphology, activity towards electrochemically oxidizing adsorbed carbon monoxide (COads), and electrochemical stability. Oxidation of HOPG generated pits on the basal plane; ozone oxidation formed nano-pits, < 10 nm in diameter; electrochemical oxidation produced larger pits, > 85 nm in diameter. Regardless of the substrate pre-treatment, electrochemical deposition resulted in clusters of nanoparticles, while impregnation followed by thermal decomposition resulted in individual Pt nano-catalysts. Regardless of the deposition technique, Pt particles are confined to step edges on freshly cleaved HOPG, and dispersed over the basal plane on oxidized HOPG. When comparing all six electrodes it became evident that substrate pre-treatment did not affect the electrochemical activity of the electrodes towards the oxidation of COads, while the method of Pt deposition did significantly affect the measured activity. For example, Pt/HOPG electrodes prepared by electrochemical deposition using short deposition times, < 10 s, show slower COads electro-oxidation kinetics compared to polycrystalline Pt foil, while electrodes prepared by impregnation followed by thermal decomposition show faster kinetics compared to polycrystalline Pt foil. For a given substrate, Pt/HOPG electrodes prepared by impregnation followed by thermal decomposition proved to be more electrochemically stable than electrodes prepared by electrochemical deposition. For a given deposition technique, ozone oxidized HOPG has the greatest stabilizing effect. The chemical interaction between the Pt catalyst and the carbon substrate was studied by XPS analysis. It was found that Pt 4f spectra of all six Pt/HOPG electrodes were shifted to higher binding energies indicating that Pt was interacting with the HOPG possibly forming Pt-C bonds. The XPS results indicate that the electrochemical stability is positively influenced by the interaction between the catalyst and the carbon support