Sulfidation and activity of Co/C catalysts having extremely low cobalt-loading: A Moessbauer emission spectroscopy and thiophene hydrodesulfurization study

Up to a sulfiding temp. of 473 K, the behavior of Co/C catalysts with extremely low cobalt loadings (ppm range) agrees with the trend obsd. before in the Moessbauer emission spectroscopy (MES) spectra of Co/C catalysts with much higher cobalt loadings (0.04-43. wt.%). Sulfiding 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. Sulfiding 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. 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. Thus, there is no general relation between the thiophene HDS activity (measured at atm. pressure) and the amt. of cobalt exhibiting a Co-Mo-S MES spectrum. [on SciFinder (R)]