Abstract

A series of sulfided CoMoAl2O3 catalysts with different CoMo ratios but with constant molybdenum content is investigated. The catalysts are characterized by in situ Mössbauer emission spectroscopy (MES) and investigated for their thiophene hydrodesulfurization activity. The catalytic activity shows a pronounced maximum at a CoMo ratio of about 1.0. The MES spectra reveal that cobalt may be present in three distinctly different phases: cobalt located in the alumina lattice (Co:Al2O3), cobalt in Co9S8, and cobalt located in the CoMoS surface phase discussed in the preceding paper. It is found that the relative amounts of the three phases depend strongly on the CoMo ratio. The Co:Al2O3 phase and the CoMoS phase are observed in all catalysts studied, whereas Co9S8 is observed only in catalysts with CoMo ≳ 0.4. It is shown that the presence of Co9S8 cannot explain the promoting role of cobalt in the CoMoAl2O3 catalysts. However, a linear relation between the catalytic activity and the amount of Co in the CoMoS phase leads to the conclusion that the promoting effect of cobalt is associated with the presence of this phase.