Theoretical Study On CO₂ Hydrogenation Catalyzed By Ni Based Catalyst

The extensive use of fossil fuels in human industrialization has led to a sharp increase in atmospheric CO₂ content.In modern society,there are serious energy crisis and environmental problems such as global warming and ocean acidification.Converting CO₂into high value-added chemicals can not only alleviate energy crisis but also solve environmental problems,which is one of the routes to achieve sustainable development in today’s society.Catalytic hydrogenation of CO₂ to produce methane,methanol and formic acid is a promising technology for CO₂ conversion.The key is to prepare catalysts with good activity and low cost.Catalysts with nickel as active metal have been widely studied because of its high activity and low price,but there are some problems such as poor catalytic activity at low temperature,easy sintering and agglomeration at high temperature and unclear reaction mechanism.Density functional theory（DFT）was used to calculate and study the introduction of metal Ru as an auxiliary agent to form a bimetallic Ni-Ru alloy catalyst with Ni,and nitrogen doped graphene as a carrier to support the stability of the biatomic catalyst and catalyze the hydrogenation of CO₂.In terms of theory,it provided guidance for the subsequent related experiments and reduced the experimental cost.It is expected to design catalysts that can effectively catalyze the hydrogenation of CO₂,reduce the content of CO₂ in the atmosphere,and convert it into methane,methanol,formic acid and other resources,so as to alleviate a series of environmental problems and energy crisis caused by excessive CO₂ emission.The main work is as follows:A Ni₃ Ru alloy model with a ratio of 3∶1 Ni to Ru was constructed to investigate the mechanism of CO₂ hydrogenation catalyzed by Ni₃Ru alloy.The results showed that compared with Ni（111）surface,the adsorption capacity of Ni₃Ru（111）on reactants and intermediates was improved,and the catalytic hydrogenation of CO₂ was easier to produce methanol.The optimal reaction path was CO₂→CO→HCO→H₂CO→H₃CO→H₃COH.The rate-determining step（RDS）is CO*+H*→HCO*,and the activation barrier of RDS is 1.22 e V,which is lower than that of most Ni-based catalysts for CO₂ hydrogenation.Meanwhile,Ni₃Ru alloy has good carbon deposition resistance.The Ni atom on the surface of Ni（111）was replaced by 1～3 Ru atoms,and the n Ru/Ni（n=1～3）alloy catalyst model was constructed.The stability of n Ru/Ni（n=1～3）alloy catalyst was investigated,and the most stable structure of n Ru/Ni（n=1～3）alloy catalyst was obtained.The results show that methanol is more easily produced by hydrogenation of CO₂catalyzed by n Ru/Ni（n=1～3）,and the optimum reaction path and RDS are the same as those of Ni₃Ru（111）bulk phase alloy.The RDS activation barriers of 1Ru/Ni（111）,2Ru/Ni（111）and 3Ru/Ni（111）were 1.22 e V,1.27 e V and 1.25 e V,respectively.Two transition metals M₁ and M₂（M₁,M₂=Ti,V,Cr,Mn,Fe,Ru,Co,Ni,Cu,Zn）were anchored on nitrogen-doped graphene to construct the M₁M₂-NC diatomic catalyst model.By comparing the binding energy and cohesive energy of these models and analyzing their stability,13 kinds of catalysts with stable structure were obtained,and their adsorption and activation performance of CO₂ was further studied.Two diatomic catalyst models,Ni Fe-NC and NiRu-NC,were selected to study the catalytic mechanism of CO₂ hydrogenation.The results showed that Ni Fe-NC and NiRu-NC catalyzed CO₂ hydrogenation to produce formic acid more easily,and the optimal reaction path was CO₂→COOH→HCOOH.The RDS of Ni Fe-NC is CO₂*→COOH*and the activation barrier is 0.44 e V.The RDS of NiRu-NC is COOH*→HCOOH*and the activation barrier is 0.29 e V.Therefore,the catalytic activity of NiRu-NC is better.