SULFIDATION AND ACTIVITY OF CO/C CATALYSTS HAVING EXTREMELY LOW COBALT-LOADING - A MOSSBAUER EMISSION-SPECTROSCOPY AND THIOPHENE HYDRODESULFURIZATION STUDY

Up to a sulfidation temperature of 473 K, the behaviour of Co/C catalysts with extremely low cobalt-loadings (ppm range) agrees with the trend observed before in the Mossbauer emission spectroscopy (MES) spectra of Co/C catalysts with much higher cobalt-loadings (0.04-4.3 wt.-%). Interestingly, sulfidation at 573 K results in a rather well-defined very highly dispersed (most likely monatomically) 'Co-sulfide' species which shows a doublet with the extremely large value Q.S.=4.11 mm/s. In this species the cobalt atoms may be four-fold (square-planar) or five-fold (square-pyramid) coordinated by sulfur. Sulfidation at 673 K results in the disappearance of this highly dispersed ''Co-sulfide'' species and the newly formed species are again similar to the one previously found for Co/C catalysts with higher cobalt-loadings. It is worth noting that this finally formed 'Co-sulfide' species does not exhibit the ''Co-Mo-S'' MES spectrum (Q.S. between 1.0 and 1.3 mm/s) whereas its intrinsic thiophene hydrodesulfurization (HDS) activity equals that of cobalt in the ''Co-Mo-S'' phase. These observations clearly lead to the conclusion that there is no general relation between the thiophene HDS activity (measured at atmospheric pressure) and the amount of cobalt exhibiting a ''Co-Mo-S'' MES spectrum.