A comparison of thermally and electrochemically prepared oxidation adlayers on rhodium: Chemical nature and thermal stability

Abstract

Bulk rhodium was oxidized at the surface the thermal means in O 2 atmosphere or electrolytically in 1N H 2SO 4. The resulting oxidation layers of rhodium were investigated by X-ray photoelectron spectroscopy (XPS) and cyclic voltammetry. Thermal oxidation took place at 870 K in 0.1 MPa of O 2 for 1 h. XPS binding energies were found at 530.0 and 308.4 eV for the O 1s and Rh 3d 5 2 peaks, respectively. From the XPS signal intensities an atomic ratio of O-to-Rh equal to 1.6 could be calculated. The data are consistent with a Rh 2O 3 oxide phase. Thermal decomposition in ultrahigh vacuum occurs at 470–570 K to a mixed phase of Rh 2O 3 · Rh that is stable up to about 800 K. An oxidation layer of approximately 3 nm thickness was formed on a rhodium electrode by anodic polarization at 4 V versus standard hydrogen electrode (SHE) in 1N H 2SO 4 at 300 K. This layer was quite different from the one obtained by thermal oxidation. From the chemical shift and peak shape of the Rh 3d 5 2 (E bind = 308.0 eV) and the O 1s signal ( E bind = 530.9 eV) in the XPS spectra this surface phase was identified as RhOOH. This is consistent with two reduction peaks in the cyclic voltammogram. The oxyhydroxide decomposes to the metal at 420 K.