Sulfidation of MoO3/γ-Al2O3 towards a highly efficient catalyst for CH4 reforming with H2S

Abstract

The process of CH₄ reforming with H₂S is an efficient approach to H₂ production, which can not only directly convert sour natural gases without separating H₂S from CH₄ but can also make full use of waste H₂S, making it into high value-added products. In the present study, a MoO₃/γ-Al₂O₃ material was prepared and further applied to the CH₄/H₂S reforming process after in situ sulfidation treatment under an H₂S/N₂ atmosphere at variable sulfidation temperatures (400−800 °C) and times (0−7 h). The reaction conversion and the H₂ production rate of sulfided catalysts were examined under the conditions of 800 °C and 1 atm with a molar ratio for CH₄/H₂S of 1.5 : 1 and a gas hourly space velocity (GHSV) of 15 000 h⁻¹. By combining various characterization techniques, such as X-ray diffraction, X-ray photoelectron spectroscopy and transmission electron microscopy, the effects of sulfidation factors on the structure and the catalytic performance of the Mo-based catalyst were studied. The catalytic activity increased at first and then reduced along with increasing the sulfidation temperature and time, implying the existence of optimal sulfidation conditions, which were identified to be around 500 °C for 1 h. Both the sulfidation temperature and time have significant influences on the transformation of MoO₃ to MoS₂ and consequently affect the phase composition, the crystal size, and the texture, as well as the surface Mo valence states of the sulfurized MoO₃/γ-Al₂O₃ catalysts. A catalyst structural evolution mechanism during sulfidation and the subsequent reaction process were proposed, and it was speculated that two simultaneous MoS₂ formation routes, one through the MoO₂ phase and the other through the Mo⁵⁺O_xS phase, were involved. The relationship between the catalyst structure and the catalytic performance was further established, and the edge Mo atoms were assumed to cooperate with the neighboring surface Mo sites to catalyze the reaction together.