Structure and nature of the active sites in CoMo hydrotreating catalysts conversion of thiophene

摘要

The performance of sulfided (Co)Mo/alumina catalysts was studied in the conversion of thiophene between 423 and 773 K at almospheric pressure. Reaction orders for thiophene, H-2, 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 Academic Press. [References: 32]