Study Of The Sulfidation Of Mo-based Sulfur-resistant Methanation Catalyst

Transforming coal to synthetic nature gas (SNG) is a reasonable route inefficiently utilizing energy and protecting the environment. Methanation is a keyprocess to synthesize SNG product. Mo-based can be used in sulfur-resistantmethanation due to its sulfur tolerant capability. Generally, MoS2is the active phaseof Mo-based catalyst in sulfur-resistant methanation. Hence, studing the presulfidationof Mo-based catalyst not only favors to prepare the catalyst with high activity andselectivity, but also provides the technology support for SNG form coal. Threecatalysts (MoO3/Al2O3, CoO-MoO3/Al2O3and MoO3/CeO2-Al2O3) were researchedand charactered in this study. The reaction condition was the syngas (H2/CO=1.0)containing1.2vol.%H2S,20vol.%N2,550°C,3MPa and gas hourly space velocityof5000h-1.The sulfidation factors of MoO3/CeO2-Al2O3catalyst indicated that thesulfidation pressure and temperature had effect on catalytic performance. Increasingthe sulfidation pressure slightly improved stability of catalyst. Although increasing thesulfidation temperature decreased the catalytic activity, it enhanced the stability ofcatalyst.The catalytic activity of MoO3/Al2O3catalyst showed that it decreased with theincrease of sulfidation temperature. The characteristion results indicated that thedecrease of catalytic activity in high sulfidation temperature was due to two aspects:(1) the decrease of the BET surface area of MoO3/Al2O3catalyst;(2) thetransformation of amorphous MoS2to crystalline MoS2. For the CoO-MoO3/Al2O3catalyst, however, the effect of sulfidation temperature is different to MoO3/Al2O3catalyst. The optimizing sulfidation temperature of CoO-MoO3/Al2O3catalyst was400°C, in this sulfidation temperature, there was the most CoMoS structure in thecatalyst. When the sulfidation temperature was lower than400°C, some CoMoO4species were not sulfided. However, the CoMoS structure was over-sulfided to formCo9S8and crystalline MoS2when the sulfidation temperature was higher than400°C.The catalytic activity of MoO3/CeO2-Al2O3catalyst decreased with theincreasing of sulfidation temperature. The characteristic results revealed that the decrease of catalytic activity in high sulfidation temperature was due to:(1) thedecrease of the BET surface area of the MoO3/CeO2-Al2O3catalyst;(2) thetransformation of amorphous MoS2to crystalline MoS2;(3) the sulfidation of CeO2inthe catalyst. Additionally, it is noted that the MoO3/CeO2-Al2O3catalyst sulfided athigh sulfidation temperature exhibited better stability. This was due to the removal ofunstable CeO2which located on the surface of catalyst.Compared with the methanation activities of one-step sulfided MoO3/CeO2-Al2O3catalysts, stepwise sulfidation processes greatly enhanced not only themethanation activity but also the stability of the catalysts. The characteristic resultsdemonstrated that the stepwise sulfidation could elimitate the unstable CeO2on thesurface of catalyst, but it decreased the damage degree of CeO2on the surface.Meanwhile, it also decreased the damage degree of textural property of MoO3/CeO2-Al2O3catalyst.