Relationship Between Sulfidation and HDS Catalytic Activity of Activated Carbon Supported Mo, Fe–Mo, Co–Mo and Ni–Mo Carbides

Abstract

Temperature-programmed sulfiding (TPS) and temperature-programmed reduction of the sulfided catalysts (TPR-S) have been employed to evaluate Mo, Fe–Mo, Co–Mo and Ni–Mo carbide catalysts supported on activated carbon (AC). The carbides were prepared by carbothermal hydrogen reduction from co-impregnated precursors, and submitted to presulfiding prior to HDS. Samples of passivated catalysts prepared from the sulfates of Fe and Co contained variable amounts of sulfur, as shown by XPS and elemental analysis, while all samples showed increased sulfur contents after HDS. The TPS traces (up to 400 °C) of the Mo and Co–Mo samples exhibited two H2S consumption peaks, the first one starting at 100 °C and the second within the 400 °C isothermal region, located immediately after a H2S production peak centered at 400 °C for Mo and at 340 °C for Co–Mo. The Fe–Mo and Ni–Mo carbides exhibited a broad H2S consumption signal between 100 and 400 °C. The low temperature signals can be attributed to reduction of Mo(VI) oxide into Mo(V) or Mo(IV) sulfides, while the broad, higher temperature band is assigned to further reduction to the fully sulfided surface species. The TPR-S spectra showed three H2S evolution peaks, where the area of the first peak (assigned to reduction of sulfur species adsorbed on coordinatively unsaturated edge/corner sites) follows the order Co–Mo ≫ Ni–Mo ≈ Mo > Fe–Mo, which fully agrees with the order of HDS activities at steady state. Elemental analysis of the pre-sulfided AC support suggest that a large part of the sulfur consumed in sulfiding the catalysts is retained as elemental sulfur within the microporous structure of the support, although HRTEM observations confirm that MoS2-like structures are present in these samples. These results reinforce the importance of sulfided surface phases in HDS on carbides of transition metal catalysts. Transmission electron microscopy shows that activated carbon supported (M-)Mo carbide catalysts (M = Fe, Co, Ni) present MoS2-like fringes on top of bimetallic carbide crystals. Temperature-programmed techniques and elemental analysis prove that a large amount of sulfur is retained in the catalyst, likely within the slit-shaped microporous structure of the support. A good correlation was established between thiophene HDS activity at steady state and the size of a TPR peak of sulfided catalyst samples.