Surface oxidation of platinum-group transition metals in ambient gaseous environments: Role of electrochemical versus chemical pathways

The effect of water vapor on the temperature-dependent surface oxidation of Pt-group metals in ambient-pressure gaseous oxygen environments is explored by means of surface-enhanced Raman spectroscopy (SERS). This exploits the ability of SERS to monitor monolayer-level oxide formation on thin Pt-group films on gold substrates in ambient gaseous as well as solution environments from the characteristic lattice vibrational (phonon) spectra. In contrast to the markedly elevated temperatures (greater than or equal to 200 degrees C) required to initiate surface oxidation on rhodium and ruthenium in dry oxygen, the presence of water vapor triggers monolayer-level oxidation of rhodium and ruthenium surfaces even at room temperature. Exposure of initially reduced rhodium surfaces to wet O-2 at different temperatures showed that this catalytic influence of water vapor is limited to ca. 50 degrees C or below, where water forms a liquid surface film. Rhodium surface oxidation is also observed upon rinsing with aerated water. Related measurements undertaken for rhodium in aqueous electrochemical environments reveal that the electrode potential-dependent formation of metal oxide from water accounts for the water-catalyzed surface oxidation observed in both gaseous and solution-phase oxygen. This follows from the observed ability of O-2 electroreduction (to water) to shift the surface potential to sufficiently high values so to trigger water electrooxidation to surface oxide under the open-circuit conditions necessarily pertaining in the gaseous system. This "electrochemical half-reaction" pathway is markedly more facile than the alternative "thermal chemical" route necessarily followed in dry O-2. Only slight (submonolayer) surface oxidation of palladium is induced at near-ambient temperatures in gaseous wet O-2, extensive oxide production only occurring above 200 degrees C, as is the case in dry oxygen. This behavior can also be understood in terms of an "electrochemical" pathway in wet gaseous O-2, the occurrence of O-2 electroreduction shifting the potential to insufficiently positive values to induce extensive water electrooxidation to oxide on palladium, due primarily to the lower thermodynamic stability of PdO compared to rhodium and ruthenium oxides. Furthermore, the inability of water to catalyze extensive palladium surface oxidation in gaseous oxygen suggests that oxide formation via a concerted metal-oxygen "place-exchange" mechanism occurs only in conjunction with the "electrochemical half-reaction" pathway.