Characterization of supported molybdate catalysts during preparation using laser Raman spectroscopy

Laser Raman spectroscopy was used to investigate the formation of supported molybdate catalysts. Samples were prepared by the dry impregnation technique for the cobalt-molybdate hydrodesulfurization catalyst using γ-alumina, silica, and α-alumina supports. Variations in the preparation procedure were studied, including loadings of molybdenum and cobalt, order of impregnation, pH of impregnation solutions, and drying and calcining conditions. Spectra were recorded in situ using a rotating, controlled-atmosphere cell. Raman spectroscopy was demonstrated to be a very sensitive probe of the effect of variations in the preparation procedure. Spectra of dried samples indicated the presence of aggregated, distorted molybdenum species on all supports. Calcination of these samples resulted in the formation of "MoO₃" at higher molybdenum loadings; the presence of distorted molybdenum species showing high degrees of aggregation could be discerned for the silica and γ-alumina samples. Addition of cobalt to the molybdenum samples markedly changed the nature of the Raman spectra. The presence of "CoMoO₄" was observed; formation of this phase suppressed the formation of "MoO₃" until higher loadings were reached. Furthermore, addition of the cobalt tended to reduce the aggregation of the molybdenum surface species. Variation in the order of impregnation dramatically affected the relative amounts of surface molybdates as well as the aggregation of the surface species.