Catalytic surface oxidation by rare-earth metals: A photoemission study of Gd- and Y-promoted oxidation of W(110)

Catalytic oxidation of the substrate has been observed for rare-earth metals on refractory metal surfaces, where the mechanism responsible has been attributed to the ability of the rare earth to form oxides with more than one oxidation state. It has been postulated that, as a result, only rare earths with more than one oxidation state will exhibit catalytic activity, at least in terms of surface oxidation. We show, using high-resolution core-level photoemission, that submonolayer coverages of Gd and Y, two rare-earth metals with only one stable oxide each, promote catalytic oxidation of W(110). Initial oxygen dosing oxidizes the rare-earth overlayer, followed by oxidation of W(110) at higher rates, and with higher resulting oxidation states, than are observed for oxygen dosing on the clean surface. We suggest that the lowering of the W(110) work function, due to the presence of the rare-earth metals and/or their oxides, is responsible for the promoted oxidation. Catalytic activity of rare-earth metals is therefore not limited to those with more than one oxidation state.