Reduction behavior and metathesis activity of [formula omitted] catalysts : I. An XPS investigation of [formula omitted] catalysts

Abstract

The reduction of WO 3 Al 2O 3 catalysts with 7–23 wt% WO 3 and of pure tungsten compounds was studied by XPS. In the pure compounds, W(VI) is reduced to the metal via W(V) and W(IV). An additional intermediate observed only in WO 3 may be W(II) or W(IV) engaged in a particular interaction. In the reduction of Al 2(WO 4) 3 a stabilization of W(VI) and W(IV) relative to Al-free samples is observed at high reduction temperatures. On the Al 2O 3 support the reduction of W(VI) starts at substantially higher temperatures than that in pure compounds. The stabilization is highest at low WO 3 content, where the severe conditions required for W(VI) reduction lead to the direct formation of metallic W. At WO 3 loadings near the theoretical monolayer capacity of the Al 2O 3 support, the lower temperatures of reduction permit the detection of W(IV) as an intermediate. Metallic W on Al 2O 3 exhibits a negative XPS shift relative to the bulk metal.