Surface Oxidation of Platinum-Group Transition Metals in Ambient Gaseous Environments: Role of Electrochemical versus Chemical Pathways

Abstract

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 (≥200 °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 O2 at different temperatures showed that this catalytic influence of water vapor is limited to ca. 50 °C or below, where water forms a liquid surface film. Rhodium surface oxidation is also observed upon rinsing with aerated water. Related measurements undertaken...