Abstract
A series of sulfided CoMo Al 2 O 3 catalysts with different Co Mo 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 Co Mo 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:Al 2O 3), cobalt in Co 9S 8, and cobalt located in the CoMoS surface phase discussed in the preceding paper. It is found that the relative amounts of the three phases depend strongly on the Co Mo ratio. The Co:Al 2O 3 phase and the CoMoS phase are observed in all catalysts studied, whereas Co 9S 8 is observed only in catalysts with Co Mo ≳ 0.4 . It is shown that the presence of Co 9S 8 cannot explain the promoting role of cobalt in the CoMo Al 2 O 3 catalysts. However, a linear relation between the catalytic activity and the amount of Co in the CoMoS phase leads to the conclusion that the promoting effect of cobalt is associated with the presence of this phase.
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