Surface reduction of Cr–V2O3 by CO

Abstract

Interesting irreversible effects have been observed involving surface reduction of Cr-doped V2O3 by CO. Adsorption characteristics of CO on the paramagnetic insulating phase of (V0.985Cr0.015)2O3(0001) were examined by ultraviolect photoelectron spectroscopy (UPS), x-ray photoelectron spectroscopy, low-energy electron diffraction, and Auger electron spectroscopy, along with thermal desorption of the adsorbate and exposure of reduced surfaces to O2. When V2O3:Cr is exposed to CO at 273 K, little interaction is observed up to 103 Langmuir; however, higher exposures result in significant coverage of a C-containing moiety, a large amount of electron transfer to V cations at energies near EF, and a corresponding decrease in the occupation of states near the top of the O 2p band. Heating above 400 K desorbs the C-containing species, but the surface becomes further reduced, with increased charge transfer to V cations, suggesting that adsorbed CO reacts strongly with lattice oxygen and desorbs as CO2. During repeated CO adsorption/desorption cycles, the degree of surface reduction and the magnitude of O→V charge transfer continues to increase. However, no energy shift of the O 2p band is observed upon reduction. Comparison of the electronic structure of V2O3:Cr surfaces reduced by CO exposure and by inert-gas ion bombardment shows both similarities and differences. Both types of surfaces display a higher initial sticking coefficient for CO than do stoichiometric surfaces. However, while the electronic structure of ion-bombarded V2O3:Cr is largely restored to that of the stoichiometric surface by exposure to O2, that of CO-reduced V2O3:Cr is not. UPS indicates that O2, rather than reoxidizing CO-reduced V2O3:Cr, adsorbs as O− or (O2)−; the surface remains highly disordered as well. The restoration of the original surface structure during high-temperature (> 800 K) annealing is found to be independent of the adsorbed oxygen species.