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 explore d by means of surface-enhanced Raman spectroscopy (SERS). This exploits the ability of SERS to monitor monolayer-level oxide formation on thin Pt-grou p films on gold substrates in ambient gaseous as well as solution environme nts from the characteristic lattice vibrational (phonon) spectra. In contra st to the markedly elevated temperatures (greater than or equal to 200 degr ees C) required to initiate surface oxidation on rhodium and ruthenium in d ry oxygen, the presence of water vapor triggers monolayer-level oxidation o f rhodium and ruthenium surfaces even at room temperature. Exposure of init ially 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 oxidati on is also observed upon rinsing with aerated water. Related measurements u ndertaken for rhodium in aqueous electrochemical environments reveal that t he electrode potential-dependent formation of metal oxide from water accoun ts for the water-catalyzed surface oxidation observed in both gaseous and s olution-phase oxygen. This follows from the observed ability of O-2 electro reduction (to water) to shift the surface potential to sufficiently high va lues so to trigger water electrooxidation to surface oxide under the open-c ircuit conditions necessarily pertaining in the gaseous system. This "elect rochemical half-reaction" pathway is markedly more facile than the alternat ive "thermal chemical" route necessarily followed in dry O-2. Only slight ( submonolayer) surface oxidation of palladium is induced at near-ambient tem peratures in gaseous wet O-2, extensive oxide production only occurring abo ve 200 degrees C, as is the case in dry oxygen. This behavior can also be u nderstood 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 pal ladium, due primarily to the lower thermodynamic stability of PdO compared to rhodium and ruthenium oxides. Furthermore, the inability of water to cat alyze extensive palladium surface oxidation in gaseous oxygen suggests that oxide formation via a concerted metal-oxygen "place-exchange" mechanism oc curs only in conjunction with the "electrochemical half-reaction" pathway.