In situ fluorescence XAFS study for hydrodesulfurization catalysts

Abstract

The chemical state analysis of Co and Ni in Co–W and Ni–W sulfide catalysts are studied by means of an in situ fluorescence XAFS technique. The structural changes of Co and Ni with the reaction atmosphere and temperature and the effects of the addition of a chelating agent on the sulfidability of Co and Ni are discussed. In the absence of a chelating agent, sulfidation of Co and Ni rapidly proceeds with increasing temperature and is almost completed at 473 K. On the other hand, the Co and Ni species of Co–W/Al₂O₃ and Ni–W/Al₂O₃ catalysts having nitrilotriacetic acid (NTA) were scarcely sulfided at room temperature. Furthermore, XANES analysis revealed that the rate of sulfidation is significantly suppressed in the presence of NTA even at a higher temperature. It is also found that the catalytic activity of Co–W/Al₂O₃ for thiophene hydrodesulfurization is improved more significantly by the addition of NTA than that of Ni–W/Al₂O₃. It is demonstrated by means of the XANES analysis that the formation of bulk Co sulfide species is considerably suppressed in the NTA-added Co–W catalyst.