Temperature-programmed sulfiding of [formula omitted] catalysts

Abstract

The conversion of oxides into sulfides has been studied by means of temperature-programmed sulfiding (TPS). In TPS the H 2S, H 2O, and H 2 concentrations are measured continuously during sulfiding with a H 2S/H 2/Ar mixture, as a function of temperature. Application of TPS to MoO 3 Al 2O 3 hydrodesulfurization catalysts leads to detailed information on the sulfiding rate and mechanism. Sulfiding of MoO 3 Al 2O 3 takes place at low temperature in comparison with bulk compounds ( MoO 3 MoO 2 ). The sulfiding mechanism is dominated by O-S exchange reactions. Elemental sulfur is formed by rupture of metal sulfide bonds and is reduced subsequently by H 2. In fact, H 2 plays only a minor role in sulfiding at low temperatures. Particularly the “H 2O content” of the catalysts influences the sulfiding rate to a large extent. “Wet” catalysts, in equilibrium with 3% H 2O Ar at room temperature, sulfide at very low temperature (typically 400–500 K). “Dry” catalysts, treated in Ar at 775 K, on the other hand, sulfide at relatively high temperature (600–700 K). This H 2O effect is explained tentatively by catalysis of OS exchange by Brønsted acid sites. Prereduction hinders sulfiding more than predrying. This suggests a minor importance of reduced intermediates in normal sulfiding procedures. An increase in the Mo content (0.5–4.5 atoms/nm 2) leads to sulfiding at lower temperature, but the influence of Mo content is not as pronounced as has been found in TPR reducibility studies. The influence of Mo content on TPR and TPS is explained by detailed consideration of the heterogeneity. Sulfiding of a 4.5 atoms/nm 2 catalyst can be completed at ca. 500 K, up to a S/Mo ratio of 1.9, provided that a sufficiently low heating rate is chosen. The fact that such a low temperature is sufficient suggests the initial formation of monolayer-type sulfide species with a S/Mo ratio near 2. These species can exist if steric factors are taken into account.