SURFACE OXIDATION OF RHODIUM AT AMBIENT PRESSURES AS PROBED BY SURFACE-ENHANCED RAMAN AND X-RAY PHOTOELECTRON SPECTROSCOPIES

Presented here is a comprehensive study of ambient-pressure thermal ox idation of rhodium surfaces utilizing surface-enhanced Raman spectrosc opy (SERS) and X-ray photoelectron spectroscopy (XPS) as complementary techniques. Real-time SERS provides details regarding surface bonding and the time-dependent evolution of the surface oxides under in situ conditions, while XPS is used to obtain quantitative information about relative amounts of the oxides and extent of oxidation. The surfaces examined are electrochemically deposited rhodium thin films on gold su bstrates, and polycrystalline rhodium foil. Surface oxidation involved heating in a flowing stream of oxygen or oxygen/argon mixtures at atm ospheric pressure, and temperatures up to 300 degrees C. Raman spectra from the oxidized thin film exhibit several features in the 200-1000 cm(-1) region that may be attributed to rhodium-oxygen vibrations of s urface oxygen and oxides. Bands at 530 and 800 cm(-1) are assigned to symmetric and asymmetric Rh-O stretches, respectively, in Rh2O3. A thi rd band at 290 cm(-1), attributed to a Rh-O bending mode, arises from a behaviorally distinct oxide species. The corresponding XPS spectra e xhibit several features that can be correlated with the SERS bands. Th e presence of Rh2O3 is inferred from the appearance of a RH(3d) peak a t 308.6 eV and a matching O(1s) peak at 530 eV. An additional O(1s) fe ature appears at 531.7 eV, which apparently correlates with the 290-cm (-1) SERS band. While a clearcut assignment of the latter has proven t o be difficult, possible species are chemisorbed oxygen along with RhO OH or RhO2. A differing reactivity of the spectrally distinct oxygen s pecies towards CO was observed. A model for the spatial oxide composit ion is proposed based on a quantitative analysis of the XPS data. (C) 1994 Academic Press,Inc.