Photoemission studies of H2O adsorption on pure and Cr-doped V2O3

Vanadium oxides having initial surface compositions of VO2 and V2O5 are used extensively as industrial catalysts and catalyst supports. However, the V cations generally become further reduced under reaction conditions, with V3+ a common species. We have used single-crystal V2O3 to study the chemisorption of H2O on V3+ cations; both pure and 1.5% Cr-doped V2O3(0 0 0 1) were studied, the latter of which undergoes a bulk metal–insulator transition near room temperature. Clean, stoichiometric (0 0 0 1) surfaces were prepared by annealing; the work function of Cr-doped V2O3(0 0 0 1) was found to be 0.7 eV lower than that of pure V2O3(0 0 0 1). UPS and XPS were employed to examine the interaction with H2O at temperatures ⩾180 K. Molecular H2O adsorption is observed at 180 K on pure and Cr-doped substrates (which are both metallic at this temperature), and adsorption quickly saturates at about monolayer coverage. There was no evidence of multilayer water adsorption at 180 K, and the molecular species desorb above 220 K. In contrast, large H2O exposures (⩾103 L) at 180 K result in UPS features indicative of adsorbed hydroxide, accompanied by a further decrease in the work function of 0.6 eV. H2O exposure at 273 K – where pure V2O3 is metallic, while the Cr-doped material is insulating – results only in dissociative adsorption on both substrates, with a sticking coefficient ⩽10−3. The mode of H2O adsorption on Cr-doped V2O3 is similar to that on pure V2O3 at both 180 and 273 K; however, less H2O adsorbs on Cr-doped V2O3 at both temperatures.