Reduction and metathesis activity of MoO 3/Al 2O 3 catalysts : I. An XPS investigation of MoO 3/AI 2O 3 catalysts

Abstract

MoO 3/Al 2O 3 catalysts (2–13 wt% MoO 3) were investigated by XPS in the oxidized form, after thermal treatment in flowing Ar (973 K), and after reduction in H 2 (673–973 K), which are conditions typically employed in the activation of these catalysts for the metathesis reaction. A new assignment of Mo 3d binding energies to Mo oxidation states was applied in the analysis of the reduced samples. During the thermal treatment in flowing Ar, part of the hexavalent Mo present in the initial samples underwent reduction to Mo(V), which could also be detected by EPR. The reduction of alumina-supported Mo(VI) in H 2 was found to produce surfaces, on which Mo(VI), Mo(V), Mo(IV), Mo(II), and, at reduction temperatures above 900 K, Mo(O), coexist. For reduction temperatures of about 800 K, distributions of these Mo states, which differ from those reported in the literature by a Mo(V) contribution not exceeding 10% of the total Mo and by the presence of Mo(II), were derived. When reduced MoO 3/Al 2O 3 catalysts were subsequently treated in flowing inert gas at 973 K a partial reoxidation of the surface was observed.