Coimpregnated and consecutively impregnated (Mo first) catalysts were prepared on γ-Al2O3 support with various mole ratios r = Co/(Co+Mo). Samples were calcined at 723K for 4h then subjected to three types of pretreatment: none, prereduction in H2 for 2h and presulfidation with H2/H2S mixture for 1h, both at 673K. They were then contacted with hydrogenthiophene (H2/T) mixture and the activity in thiophene hydrodesulfurization (HDS) and hydrogenation selectivity (HYD) determined. The surface Co and S concentrations were monitored by XPS. Any of the treatment brought about a partial sulfidation only; upon sulfidation, a surface Co-enrichment occurred. No synergistic effects were observed with catalysts not prereduced; synergy appeared with presulfided and, coimpregnated and prereduced catalysts in HDS activity. Hydrogenation selectivity decreased with increasing Co-content. A comparison with unsupported catalysts led us to the conclusion that HDS synergy is a temporary phenomenon. It can be attributed to a certain state of Co which promotes Mo reduction. As Co is segregated to the surface, synergy disappears. This may be the main reason of deactivation of unsupported catalysts; the much less significant deactivation of supported samples may be connected with surface carbonization.
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