Electron spin resonance studies of free and supported 12-heteropoly acids. Part 6.—The investigation of reduced H4(SiW12O40)·xH2O and Ag4(SiW12O40)·x H2O and the effects of oxygen adsorption

Abstract

A number of W⁵⁺ e.s.r. signals have been observed when reducing 12-tungstosilicic acid and its silver salt with hydrogen or methanol up to 873 K. By means of various adsorption measurements and by comparison with results of molybdenum-containing heteropoly acids, a more detailed conclusion concerning the state of the samples could be drawn. The adsorption of oxygen, leading to the formation of O^–₂ radicals, was shown to proceed on decomposed Keggin anions only. A strong interaction between the acid or silver salt and alumina was found to promote decomposition. Samples used as catalysts in the conversion of methanol showed only a narrow signal due to paramagnetic coke residues, possibly because the catalyst, partly reduced in the course of the reaction, is immediately reoxidized in air. Compared to methanol, the use of hydrogen facilitates reduction of the acid and the silver salt.