Investigation Of Unsupported Catalysts For Sulfur-resistant Methanation

Methane (CH4) is the main component of natural gas. The methanation reactionprocess is the key link of producing natural gas rfom the coal，and the development ofefficient methanation catalyst is an important step for methanation reaction process. Inthis paper, sulfur powder sulfuirzation, directly thermal-decomposition method wasadopted to prepare unsupported Mo-based catalysts. The effects of different Moprecursors as well as different S/AHM ratio on the catalysts catalytic methanationperformance were studied. The results showed that the M0S2catalyst prepared rfomammonium tetrathiomolybdate (ATM) had relatively the highest methanation activitycompared with M0S3and AHM-deirved ones. However, taken the price as well asactivity into consideration，our study was focusesd on using ammoniumheptamolybdate (AHM) as an efficient precursor and studied the effect of differentS/AHM weight ratio and preparation conditions，such as treatment atmosphere andcalcination temperature on the catalytic methanation activity of as-prepared catalysts.With the increase of S/AHM weight ratio，the sulfidation degree and theamorphous degree increased correspondingly, and also the catalyst activity. WhenS/AHM was1，crystalline M0O2was observed while there was only M0S2phasewhen S/AHM>1. Particularly, when S/AHM was3，the catalytic methanation activityof the catalyst was the highest. A lower concentration of hydrogen sulfide yielded thecatalyst with more amorphous crystal structure and relatively higher activity. Differentcalcination atmosphere had a great impact on the nature of the catalyst. In an inertatmosphere (N2)，the obtained M0S2particles were the smallest and amorphousdegree was the lowest. TPR characterization showed that there existed active sulfurspecies on the catalyst surface, these sulfur species could transform to sulfurvacancies or defects onto the surface under the reaction condition，and these sulfurvacancies were the catalytic active sites. The catalytic activity of the catalyst firstincreased and then decreased as the increase of calcination temperature. Theoptimumt calcination temperat°ure was450C and a higher calcination temperaturehad a negative influence on the catalytic performance. Different calcination temperature could yield the catalyst with different crystalline and the catalyticreaction performance.In addition，we examined the effect of additives Co，Zr and AI2O3powder on thesulfur-resistant methanation activity of Mo-based catalysts. The results showed thatthe additive Co reduced the activity of unsupported Mo-based catalyst mainly becauseof the formation of C09S8phase. The stabilization effect of additive Zr was notobvious. While adding AI2O3powder could significantly improve the catalyticstability. Characteirzation analysis revealed that the sulifdation degree of the catalystand the dispersion of the active ingredient were deepened by adding AI2O3，thusreducing the agglomeration of the active component in high temperature, which wasthe reason for an improved stability.