Effect of sulfidation temperature on the catalytic behavior of unsupported MoS2 catalysts for synthetic natural gas production from syngas

Abstract

Unsupported MoS2 catalysts were obtained by thermal decomposition of ammonium tetrathiomolybdate (ATM) or ammonium heptamolybdate (AHM) using sulfur powder as sulfiding agent at variable sulfidation temperature (400–550°C). The CO conversion, selectivity and yield of CH4 on the catalysts were studied for the methanation reaction. It was found that CO conversion increased with temperature rise at first, reached maximum value at sulfidation temperature of 450°C and then decreased sharply with further increase of temperature. The catalyst derived from ATM could get higher CO conversion than the catalyst from AHM because of the structural similarity between ATM and MoS2. XRD analysis demonstrated that amorphous MoS2 was favorable for the methanation reaction and a crystal transition of MoS2 nanoparticle happened during the methanation reaction. The higher the sulfidation temperature was, the more easily regular crystal structure of MoS2 formed. TEM characterization results showed that at the optimum sulfidation temperature of 450°C, the length and stacking degree of MoS2 crystallite were the highest and so more active sites could reside on the edges of MoS2 slabs for CO methanation. The crystal growth of MoS2 particle and its structure change are probably the reasons for the deactivation of the MoS2 catalysts with the increase of sulfidation temperature. In addition, it is the loss of surface sulfur that caused deactivation of the catalyst with reaction time.