XPS and TPD study of Rh/SnO2 system – Reversible process of substrate oxidation and reduction

Rh films were deposited onto SnO2 polycrystalline substrate prepared by spray pyrolysis method. Immediately after the Rh deposition, partial reduction of the substrate was observed, while almost no reduced Sn was found on the pure SnO2 substrate. The peak of metallic Sn increased after annealing of Rh deposited sample at about 670 K. No substrate reduction was observed after heating it up to 670 K, when the surface had not been covered by Rh deposit. These results confirm an influence of Rh deposit to the SnO2 reduction. After the Rh/SnO2 sample was treated in O2, the partial reduction of SnO2 almost disappeared. The process of SnO2 substrate reduction and oxidation was performed repeatedly. The shape of Rh 3d doublet did not change during all these procedures; it only shifted to lower binding energy when the substrate was reduced. Thermo programmed desorption spectra of CO were different on the oxidized and reduced SnO2 substrate. We concluded that the substrate oxygen influences this behaviour of Rh/SnO2 system. These effects can be important in heterogeneous catalysis and gas sensors technology.