| Laser Raman spectroscopy was used to characterize supported cobalt molybdate hydrodesulfurization catalysts in both the oxide and the sulfided form. Spectra of the oxide catalyst during various stages of preparation, and the transformation of the oxide to the active sulfided form were recorded. Pyridine adsorption was used as a probe of the surface of the oxide surface.; The catalysts were prepared by the dry impregnation technique. Two different catalyst supports ((gamma)-Al(,2)O(,3) and silica) and several variations in the preparation procedure were studied. Raman spectra of catalysts supported on (gamma)-Al(,2)O(,3) indicated the presence of a polymolybdate phase with molybdenum in distorted octahedral coordination and had strong interaction with the support. A MoO(,3) phase was formed when higher molybdenum loadings (> 7.5% MoO(,3)). were reached. A CoMoO(,4) phase was observed in addition to the polymolybdate phase when cobalt was added, and MoO(,3) was not formed until higher molybdenum loadings (> 10% MoO(,3)); and a Co(,3)O(,4) phase was formed when Co/Mo>>1. The pH of the impregnation solutions had little effect on the structure of the catalyst, while the sequence of impregnating cobalt and molybdenum strongly influenced the relative amounts of the polymolybdate, CoMoO(,4) and MoO(,3) phases formed. Cobalt and molybdenum interacted less strongly with silica; bulk phases of MoO(,3) and CoMoO(,4) were formed at relatively low surface loadings.; Raman spectra of pyridine adsorbed on the (gamma)-Al(,2)O(,3) catalysts indicated that molybdenum would selectively occupy the octahedral sites on the (gamma)-Al(,2)O(,3) surface. The polymolybdate phase formed exhibited Bronsted aciditiy. The Lewis acid sites on the (gamma)-Al(,2)O(,3) surface were not occupied, leaving open surfaces even after the formation of MoO(,3). Addition of cobalt caused drastic changes in the surface acidity. Cobalt would first occupy the Lewis acid sites left open by molybdenum; a site having acidity intermediate between that of Lewis and Bronsted acid site was created. A model on the structure of the Mo/(gamma)-Al(,2)O(,3) catalyst based on these results was developed.; A poorly crystalline MoS(,2) phase was rapidly formed when the Mo/(gamma)-Al(,2)O(,3) catalysts were sulfided with H(,2)S/H(,2) mixture at 400(DEGREES)C. At lower temperatures, reduction of the MoO(,3) and the polymolybdate took place before sulfidation. An intermediate oxy-sulfide species was observed. This species was unstable in air, and was converted into an oxide phase upon re-exposure to air. However, the oxide species thus formed had a different structure from that of the fresh catalyst. For the CoMo/(gamma)-Al(,2)O(,3) catalysts, the CoMoO(,4) phases remained unchanged until sulfided up to 300(DEGREES)C. The same oxy-sulfide intermediate was observed. MoS(,2) formation, however, occurred to a much lesser extent; and no mixed cobalt and molybdenum or cobalt sulfide species could be detected in the Raman spectra. The results favored the argument of the intercalation model proposed by de Beer (1976). |
