Theoretical Study On Mechanism Of Carbon Dioxide Methanation Catalyzed By Ni₄/γ-Al₂O₃ Catalyst

The global climate problem is becoming more and more serious.While actively promoting energy conservation and emission reduction,governments around the world have also noticed that the capture and utilization of carbon dioxide is of great significance to solve the problem of global warming.Widely existing carbon dioxide is also an important basic carbon source,and its conversion into energy products is very important for carbon cycle and carbon neutrality issues.Carbon monoxide is a by-product gas of refining and chemical industry,which is harmful to the environment and human body.Converting carbon monoxide into methane also has very positive significance.At present,both in the field of experiments and theoretical simulations,the optimal reaction path for carbon dioxide methanation is still very controversial.Therefore,this thesis aims to explore the optimal reaction path for carbon dioxide methanation in industrial applications,which is of great significance to the development of catalysts and the optimization of reaction conditions.In this thesis,referring to relevant experiments and theoretical simulation studies,a representative Ni₄cluster,which is widely used in industry,is selected and supported on the surface ofγ-Al₂O₃（110）with better activity.The optimal adsorption sites of reactants,products and their intermediates on the Ni₄/γ-Al₂O₃（110）surface were discussed using density functional theory（DFT）,and carbon dioxide methane was verified in detail by the transition state search of the best reaction path.The results show that:（1）Ni₄is the most stable state when it has a regular tetrahedral configuration,and it is stably adsorbed on the"groove"position on the surface ofγ-Al₂O₃（110）.One of the vertices of the cluster faces the direction of the vacuum layer.（2）The best adsorption sites for CO₂and CO are bridge sites,the best adsorption sites for CHO and others are hollow sites,the best adsorption sites for CH₄are top sites,and most of the other intermediates have the best adsorption sites of bridge site.（3）The reaction heat and reaction energy barrier of the elementary reactions that may occur in each step of the carbon dioxide methanation process were analyzed and compared with each other,and the product of the elementary reaction with the smallest activation energy in this step was used as the reactant in the next step.The reaction is continued,by selecting each step in turn,until the product CH₄is obtained,and the rate-controlling steps for the optimal reaction path are also identified.The results show that the optimal reaction path for carbon dioxide methanation is:CO₂→HCOO→HCOOH→CHO→CH₂O→CH₂OH→CH₂→CH₃→CH₄.The rate-controlling step of this reaction is HCOO+H→HCOOH,and the reaction energy barrier of this elementary reaction is 2.08e V.The optimal reaction path of carbon monoxide methanation is:CO→CHO→CH₂O→CH₂OH→CH₂→CH₃→CH₄.The rate-controlling step of this reaction is CH₂OH→CH₂+OH,and the reaction energy barrier of this elementary reaction is 0.65 e V.（4）Simultaneously,the formation of the reaction product H₂O was simulated and studied,and the optimal reaction path of H₂O was:O→OH→H₂O.