Valency and adsorption characteristics of a sulphided MoO3/γ-Al2O3 methanation catalyst

Abstract The sulphur-to-molybdenum ratio and the activity of the methanation reaction as a function of sulphiding time were measured on a sulphided MoO 3 /γ-Al 2 O 3 catalyst. The sulphur-to-molybdenum atomic ratio of catalyst samples did not increase after 2 h of sulphidation at 400°C, whereas the activity of methanation was constant after 6 h of sulphidation. X-ray photoelectron spectroscopic (XPS) measurements showed that most of the Mo 6+ was easily reduced to Mo 4+ and the valency of most of the sulphur was −2 during the sulphidation process in a CS 2 H 2 stream at 400°C. Mo 5+ was only detected by XPS under very mild sulphidation conditions and in samples with a lower content. A small amount of high-valency sulphur was detected that vanished with prolonged sulphidation. No change in the valency of molybdenum or sulphur was observed after reaction for 8 h. Low-temperature oxygen chemisorption and the adsorption and temperature-programmed desorption (TPD) of hydrogen were investigated. A linear correlation was observed between the methane production rate and oxygen uptake. The oxygen uptake with different pretreatments showed that carbon monoxide and hydrogen were not adsorbed on the same sites on the catalyst surface. Probably carbon monoxide was adsorbed on the coordinatively unsaturated molybdenum sites whereas hydrogen was adsorbed on sulphur sites. The TPD spectra of hydrogen indicated that it had two adsorption states, i.e., molecular adsorption and atomic adsorption. Only atomic adsorption was detected when the adsorption temperatures were above 300°C. The mechanism of the methanation reaction on molybdenum catalysts is discussed.