Laser Raman spectroscopy was used to characterize molybdenum on alumina catalysts using a controlled-atmosphere rotating cell. <i>In situ</i> spectra could be recorded at temperatures up to 600°C and pressures as low as 1.333 × 10<sup>−4</sup> Pa. Catalysts were examined during the drying and calcining operations; adsorption of pyridine was also studied. Spectra of the dried and calcined catalysts indicated the presence of aggregated molybdenum species. Close resemblance to the aggregation of molybdenum species in aqueous solution was apparent. At higher loadings of molybdenum, a molybdenum trioxide phase developed. Adsorbed pyridine spectra indicated that a considerable portion of the alumina surface was not covered by molybdenum even after molybdenum trioxide was formed. A new form of pyridine adsorption associated with the aggregated phase was observed.

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