Mechanistic Differences between Electrochemical and Gas-Phase Thermal Oxidation of Platinum-Group Transition Metals As Discerned by Surface-Enhanced Raman Spectroscopy

Abstract

The oxidation of five Pt-group metalsplatinum, palladium, iridium, rhodium, and rutheniumis examined by means of surface-enhanced Raman spectroscopy (SERS) in aqueous electrochemical and gaseous dioxygen environments as a function of electrode potential and temperature, respectively, with the objective of intercomparing systematically the conditions required for surface oxide formation and discerning the reaction mechanisms involved. The SERS strategy, utilizing ultrathin Pt-group metal films electrodeposited on a gold substrate, enables monolayer-level metal oxide vibrational spectra to readily be obtained in both the electrochemical and gaseous environments. The SER spectra obtained during positive- and then negative-going potential excursions in aqueous 0.1 M HClO4 display metal−oxygen vibrational bands signaling anodic oxide formation and subsequent removal at potentials consistent with corresponding voltammetric data. The nature of the amorphous oxides (or hydroxides) formed is deduced by comparison with bulk-phase metal oxide Raman spectra. The onset potentials for surface oxide formation are comparable to the thermodynamic potentials for the bulk-phase metal oxides. In contrast, the onset of surface oxidation even in ambient-pressure dioxygen uniformly requires elevated temperatures, ≥200 °C for each metal except for iridium, where oxide formation occurs at ca. 100 °C. While spectral differences are evident, especially on palladium and ruthenium, the nature of the oxides formed in the electrochemical and gaseous systems is largely similar. The highly activated nature of the gaseous O2 oxidations is consistent with literature reports for Pt-group surfaces in ultrahigh vacuum as well as higher-pressure conditions. Likely reasons for the markedly more efficacious metal electrooxidations are discussed. Thermodynamic factors are not responsible, since the free-energy driving forces for the gaseous O2 oxidations are larger than for the electrochemical reactions at the applied potentials where surface oxidation for the latter processes proceeds at room temperature. The electrostatic driving forces for oxygen incorporation into the metal lattice (via “high-field ion transport”) are also typically more favorable for the gaseous systems, as evidenced by a comparison of the metal−solution and metal−gas surface potentials. The intrinsically more facile electrochemical processes thereby deduced are attributed to the occurrence of direct oxide production via a metal−oxygen place-exchange mechanism, expedited by interfacial solvation and therefore being energetically unfavorable in the anhydrous gas-phase environment. Other factors, such as the formation of precursor chemisorbed oxygen, are also considered.