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Raman microspectroscopy was used to study the preparation of industrial MoS2/Al2O3 extrudates. Using this technique, the influence of the impregnation solution composition on the nature and macrodistribution of Mo complexes inside these catalyst bodies was evaluated during their preparation process (impregnation, drying, and calcination). It was found that poor dispersion of the MoOx phase in the calcined samples can be brought about by the formation of bulk (NH4)3[Al(OH)6Mo6O18] during impregnation or redistribution of Mo complexes during drying. The formation of crystalline MoO3 and Al2(MoO4)3 in the oxidic precursors leads to a poorly dispersed MoS2 phase in the final catalyst and significantly lower hydrodesulfurization activity. The spatially resolved information that can be obtained, and its inherent sensitivity for the detection of crystalline phases make Raman microspectroscopy a powerful characterization technique for studying the preparation of industrial supported metal oxide catalysts.

The influence of the Si/Al ratio of the support of (Co)Mo/saponite catalysts on the performance in the hydrodesulfurisation of thiophene and hydrocracking of n-decane has been studied. The initial thiophene HDS activity of the sulfided Co catalysts increased with increasing support acidity, while with the CoMo catalysts the opposite effect was observed. The large drop in activity at lower Si/Al ratios is ascribed to the occurrence of two separate cobalt and molybdenum sulfide phases instead of the “Co‐Mo‐S” phase. In the hydrocracking of n-decane both the Co and the CoMo catalysts displayed an almost linear relationship between the first-order rate constant and the number of Bronsted acid sites. As with zeolite catalysts the Si/Al ratio was also found to affect the product selectivity. At low Si/Al ratios the relative amount of secondary cracking products increased due to a changing balance between the number of cracking and hydrogenation sites. Remarkably, it was found that in the presence of H2S between 423 and 573 K the n-decane conversion passes through a maximum. The observed relation between this “low-temperature” activity and the number of Bronsted acid sites seem to indicate the involvement of both H2S and the acid sites on the support. c ∞ 1999 Academic Press

The performance of sulfided (Co)Mo/alumina catalysts was studied in the conversion of thiophene between 423 and 773 K at atmospheric pressure. Reaction orders for thiophene, H2, and H2S were determined at several temperatures. The reaction order in thiophene increases with rising temperature, indicating a Langmuir‐ Hinshelwood mechanism. Beyond 550 K the conversion curve deviates from pseudo-first-order kinetics, demonstrating the influence of the surface coverage of thiophene on the reaction rate. Above 673 K the reaction rate rises exponentially with temperature which can be ascribed to the presence of a second type of active site that is only active at high temperatures. These catalytic data can be combined with recent structural studies using EXAFS spectroscopy to derive an activity‐structure relation. It is proposed that at low temperature the active sites are terminal sulphur vacancies on the Co-promotor atom. In addition around 673 K a second type of sites becomes active, which is believed to consist of bridging sulphur vacancies in between Co and Mo atoms on the MoS2 edges. c ∞ 1998

The metal-support interaction in oxidic and sulfided (Co)Mo/Al{2}O{3}and (Co)Mo/TiO{2}catalysts has been studied with X-ray absorption spectroscopy. Analysis of the oxidic catalysts showed that on both Mo/Al{2}O{3}and Mo/TiO{2}the molybdenum oxide particles possess a highly distorted octahedral structure with Mo-O distances ranging from 1.71 A to 1.94 A. The second shell Mo-Mo coordination number of less than 1.0 indicated that the size of the molybdenum oxide particles is very small. Upon addition of Co the particle size is increased to approximately four Mo atoms per particle. EXAFS data analysis showed the presence of second shell Al and Ti neighbours indicating a linkage of the molybdenum oxide particles to the support via Mo-O-X(X=Al or Ti) bonds. Upon sulfidation of the oxidic catalysts small MoS{2}particles are formed with second shell Mo-Mo coordination numbers ranging from 0.8 to 3.9. A Mo-O contribution at 2.0 A was found in the first coordination shell of Mo in the sulfided catalysts. Since the coordination number of this Mo-O contribution correlated with the MoS{2}particle size, deduced from the second shell Mo-Mo coordination number, this Mo-O contribution was assigned as an interfacial Mo-O{s}{u}{p}{p}{o}{r}{t}linkage. This Mo-O{s}{u}{p}{p}{o}{r}{t}linkage stabilises the small MoS{2}particles and prevents them from sintering. It is proposed that the activity of these catalysts can be controlled by optimising the molybdenum sulfide-support interaction, which consists of the observed Mo-O{s}{u}{p}{p}{o}{r}{t}bonds.