An XPS study of the surface oxidation states of metals in some oxide catalysts

Abstract

Calcined samples of chromia supported on Al₂O₃, ZnO, or SnO₂ show both Cr(VI) and Cr(III) on the surface, Cr(VI) being preponderant in the case of Al₂O₃-supported catalysts. The proportion of Cr(VI) decreases with increase in Cr content of the calcined catalysts. Reduction of the supported chromia catalysts in H₂ at 720 K for 1 hr gives rise to Cr(III) and Cr(V). On carrying out the dehydrogenation of cyclohexane on the chromia catalysts a higher proportion of Cr(V) is found than after treatment with hydrogen. Vanadia supported on Al₂O₃ or MoO₃ shows significant proportion of V(IV) on carrying out the oxidation of toluene on the catalysts. Calcined MoO₃ (10%)/Al₂O₃ shows only Mo(VI) on the surface at 300 K, but on heating to 670 K in vacuum shows the presence of a considerable proportion of Mo(V) which on cooling disproportionates to Mo(IV) and Mo(VI). Mo(V) is noticed on surfaces of this catalyst on reduction with hydrogen as also on carrying out dehydrogenation of cyclohexane. While Bi₂MoO₆ shows only Mo(VI) on the surface at 300 K, heating it to 670 K in vacuum changes it entirely to Mo(V) which then gives rise to Mo(IV) and Mo(VI) on cooling.