A temperature programmed reduction study of gamma-alumina supported molybdenum and tungsten oxide

Results are presented of a Temperature Programmed Reduction (TPR) study of alumina supported molybdenum and tungsten oxide. The interpretation of the TPR patterns obtained is discussed in detail. Results are used from various independent techniques, mainly X-ray Photoelectronspectroscopy (xps) and Raman spectroscopy. Also measurements of hydrodesulfurization activities are used in the interpretation. The XPS intensities show that the catalysts are highly dispersed systems. Alumina supported tungsten oxide shows at all studied coverages « 4,5 W at/nm2) one single reduction peak. The pOSition of this peak is a function of the surface coverage: it shifts to lower temperatures at increasing surface coverage. It is concluded that this peak is due to the reduction of a surface compound probably consisting of aggregates of tungstate ions with a degree of aggregation varying with the surface coverage. The structure of alumina supported molybdenum oxide is at low coverage « 0,5 Mo at/nm2) similar to that of alumina supported tungsten oxide. At higher coverages an extra peak in TPR is observed at a relatively low temperature. It is concluded that it is orobably due to the reduction of b1- and trilayer compounds at the y-alumina surface. The difference in structure of the tungsten and molybdenum catalysts is explained by the difference in active phase-support interaction