The oxidation of methane on heteropolyoxometalates: III. Effect of the addition of cesium on silica-supported 12-molybdophosphoric acid, molybdena, vanadia, and iron oxide

Abstract

The effect, on the oxidation of methane with nitrous oxide, of the addition of cesium to silica-supported 12-molybdophosphoric acid (HPMo), molybdena, vanadia, and iron oxide has been studied. All four catalytic systems show similarities in results with cesium producing a poisoning effect, eventually reducing the solid to a behaviour similar to that of the support. The correspondence of deactivation with the number of available protons in supported HPMo suggests that the protons play a crucial role in the oxidation process. A mechanism is postulated in which the protons abstract oxygen from the Keggin Unit to produce anionic oxygen vacancies which are visualized as vital to the oxidation process on HPMo. The similarity of behaviour of silica-supported molybdena, vanadia, and iron oxide to that of HPMo on addition of cesium suggests the existence of isopolyoxometalate structures on these catalysts. The effect of cesium results from the blocking of the vacancy generation process (dehydration) and from an enhancement of the turnover number resulting from the unblocked sites, this latter effect presumably being electronic in nature.