Preliminary Study On Nickel-based Catalysts For Syngas To Methane

The catalytic conversion of syngas（CO+H₂）to methane has been extensively investigated,because its raw material is less expensive coal with abundant reserves,while its product natural gas is urgently needed in our life,meanwhile the energy utilization rate of this conversion process is high.The key point to realize industrialization of this process technology is to develop a catalyst of high activity and stability.Although several metals are active for the methanation of CO,only nickel-based catalysts appear economically feasible on an industrial scale.The important aspects of catalyst research include preparation conditions,process conditions,supports,promoters and so on.In this paper,the influences of preparation conditions,process conditions,supports and promoters on the catalytic behaviors of nickel-based catalysts in the methane synthesis were investigated.The reasons of Ni/Al₂O₃ catalyst deactivation were explored by means of XRD,TPR,BET,and TPD.Main results from this work are as follows.The optimal preparation condition is obtained when Ni loading content is 15（wt）%,impregnation time is 6 h,the pH value of impregnation solution is 4 and the calcination temperature is 400 ℃.Among these factors,the Ni loading content has the greatest effect on the catalyst performance,followed by calcination temperature,while the impregnation time and the pH value of impregnation solution have little effect.The catalytic activity increases with the increase of Ni loading content,while decreases with the increase of calcination temperature under the experimental conditions.The optimum reaction conditions of catalyst are as follows:a temperature of 350 ℃,a pressure of 5.0 MPa,a H₂/CO ratio of 2.0 and the space velocity of 10000 h^-1.Among these factors,H₂/CO has the greatest impact on the catalyst performance,while temperature,pressure and space velocity have little impact.With the increase of H₂/CO,the CO conversion and the selectivity of CH₄ increase,while the syngas（CO+ H₂）conversion decreases.Among three supports Al₂O₃-1,Al₂O₃-2 and SiO₂,the catalyst prepared by SiO₂ support possesses the highest stability during the methane synthesis reaction,followed by Al₂O₃-2,while Al₂O₃-1 is extremely poor.Characterization results shows that carbon deposition is not a key factor in determining the stability of catalysts,while structure destruction of the support and the migration and agglomeration of Ni particles during the reaction may be the main reasons for the rapid deactivation of Ni/Al₂O₃ catalyst.The addition of promoters has a great impact on the H₂ conversion and syngas（CO+H₂）conversion,while it has little impact on the selectivity of CH₄.Among these promoters,La₂O₃ has the greatest effect on the catalyst performance,followed by Na2O and Fe2O3,while the others have no obvious effect.The optimum composition of catalyst is 10%NiO,3%La₂O₃,0.5%Na2O,3%Fe2O3 and Al₂O₃ balance.