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 oxidation of rhodium surfaces utilizing surface-enhanced Raman spectroscopy (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 substrates, and polycrystalline rhodium foil. Surface oxidation involved heating in a flowing stream of oxygen or oxygen/argon mixtures at atmospheric pressure, and temperatures up to 300°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 surface oxygen and oxides. Bands at 530 and 800 cm−1 are assigned to symmetric and asymmetric Rh-O stretches, respectively, in Rh2O3. A third band at 290 cm−1, attributed to a Rh-O bending mode, arises from a behaviorally distinct oxide species. The corresponding XPS spectra exhibit several features that can be correlated with the SERS bands. The presence of Rh2O3 is inferred from the appearance of a RH(3d) peak at 308.6 eV and a matching O(1s) peak at 530 eV. An additional O(1s) feature appears at 531.7 eV, which apparently correlates with the 290-cm−1 SERS band. While a clearcut assignment of the latter has proven to be difficult, possible species are chemisorbed oxygen along with RhOOH or RhO2. A differing reactivity of the spectrally distinct oxygen species towards CO was observed. A model for the spatial oxide composition is proposed based on a quantitative analysis of the XPS data.