Preparation Of Metal Oxide And Composite Oxide Catalysts For CO Methanation

Due to China's abundant coal resources and lack of natural gas resources,the coal-to-natural gas technology can realize the clean transformation of coal and alleviate the contradiction between the supply and demand of natural gas in China.Methanation process and catalyst are the key points of methanation technology in the coal to natural gas process.However,the methanation process is a complex reaction system,operating conditions and feed gas components have a great influence on methanation.Moreover,currently developed methanation catalysts are mainly supported nickel-based catalysts,but Ni-based catalysts are prone to carbon deposition and sinter,which lead to catalyst deactivation.Thus,the development of catalysts with excellent performance is the key point of methanation reactions.Therefore,in view of this problem,this paper mainly through the thermodynamics analysis of methanation reaction,discusses the influence of temperature,pressure and raw gas components on methanation reaction,finds out the main influencing factors in methanation process,and determines the appropriate process operating conditions,so as to optimize methanation process.In addition,a series of transition metal cobalt oxide catalysts were prepared to investigate the CO methanation properties,and the structure of the catalysts was discussed through a series of physical and chemical characterizations.The research contents and results are as follows:Firstly,the process of methanation of syngas,coke oven gas and coal pyrolysis to produce natural gas was analyzed by thermodynamics.The results showed that it was favorable for CO methanation under low temperature and high pressure.Under high temperature conditions,CH₄ and CO₂ in the syngas led to the increase of carbon deposition in the catalyst bed.Therefore,low temperature reaction and product gas recycling process can be adopted.A large amount of CH₄ in coke oven gas led to the aggravation of carbon deposition in the catalyst bed under low pressure and high temperature conditions,while CO₂ and C₂H₄ have no effect on the reaction,so the reaction under high pressure and carbon supplement process can be used to balance the excess H₂ in the raw gas.However,the high carbon content in pyrolysis gas led to serious carbon deposition,while other carbon-containing groups led to intensified bed carbon deposition,so it was not recommended to use the product gas circulation process.According to the thermodynamic results,it was found that methanation at high temperature would produce a large amount of carbon deposition,leading to the deactivation of the catalyst.Therefore,it was crucial to develop a methanation catalyst with excellent performance at low temperature.Co₃O₄ nanoparticles with dandelion-like and spherical morphologies were prepared by homogeneous precipitation method by changing the solvents?water and ethanol?.The catalysts were texted in CO methanation and characterized by XRD,SEM,N₂-adsorption/desorption,H₂-TPR,UV-vis and DRIFT.The results showed that the Co₃O₄-W catalyst prepared by water solvent had the best catalytic activity,and the CO conversion was close to 100%at the temperature of 340?,but its stability was relatively poor,the CO conversion decreased by 18%after reaction 25 h.Through characterization,it was found that Co²⁺was the active site of CO methanation.The possible mechanism of CO methanation on Co₃O₄ catalyst was as follows:the adsorbed CO reacted with lattice oxygen to form CO₂^-species,and then reacted with the adsorbed H to generate HCOO^-intermediate,which are broken by C-O bonds and hydrogenated continuously to form CH₄.In order to improve the stability of Co₃O₄ catalyst,composite metal oxides were prepared to investigate its CO methanation performance in order to improve the stability of the catalyst.A series of composite metal oxide catalysts Co B₂O₄?B=Fe,Mo and Al?and ACo₂O₄?A=Ni,Zn,Mn and Mg?were prepared by hydrothermal method and solvent thermal method,respectively.The results showed that among Co B₂O₄ catalysts,Co Mo₂O₄ catalyst had the best activity,and the CO conversion was about 90%,but the CH₄ selectivity was only reached60%.The activity of ACo₂O₄ catalysts were significantly higher than that of Co B₂O₄ catalysts,Among them,Ni Co₂O₄catalyst had the best catalytic activity and stability,the CO conversion and CH₄ selectivity were 98%and 76%,respectively.According to H₂-TPR characterization analysis,it was found that the interaction between Ni and Co oxides in Ni Co₂O₄ catalyst is stronger,so the methanation catalytic performance of the catalyst is significantly improved.The above experimental conclusions prove that the transiton metal oxide catalysts also has perfect catalytic performance for CO methanation.In the reaction process,the catalysts do not need to undergo H₂ pretreatment and can be directly used for CO methanation reaction.