TEMPERATURE-PROGRAMMED REDUCTION AND HDS ACTIVITY OF SULFIDED TRANSITION-METAL CATALYSTS - FORMATION OF NONSTOICHIOMETRIC SULFUR

Sulfided alumina-supported transition metal catalysts have been studied using temperature-programmed reduction (TPR). The H-2 and H2S concentrations were determined simultaneously and the major processes occurring were discriminated, namely reduction of oxidic and sulfidic species and desorption of H2S. Because of the high maximum reduction temperature and the high hydrogen partial pressure applied, a complete description of all species was acquired, except for the Cr and Mn catalysts which could not be reduced below 1270 K. After sulfiding at 673 K, TPR of alumina-supported transition metals showed that several types of sulfur were present, viz. (i) sulfur species with a stoichiometry in agreement with bulk thermodynamics, indicated as stoichiometric sulfur, (ii) a nonstoichiometric sulfur species (S-x) which was present in excess of the S/Me ratio based on bulk thermodynamics (not identical to elemental sulfur since it is present after purging at 673 K), (iii) S-H groups, and (iv) adsorbed H2S. The mechanism of S-x formation is discussed. The amounts of nonstoichiometric sulfur mainly depended on the type of transition metal and the pretreatment procedure. The influence of the reductibility of the stoichiometric and nonstoichiometric sulfur species on the hydrodesulfurisation activity is discussed. The results presented indicate that chemisorption measurements may depend on the pretreatment of sulfided catalysts. (C) 1995 Academic Press, Inc.