Theoretical Investigations On The Catalytic Reduction Of CO₂ By Cu And Doped Nanoparticle Catalyst

As a crucial component of the atmosphere,CO₂ is abundant in nature.It is mainly produced by decomposition and fermentation of organic compounds,the use of fossil fuels,and the metabolism of animals and plants.Since the first industrial revolution,people’s extensive exploitation and use of fossil fuels has led to the energy shortage in today’s society.At the same time,the large increase of CO₂ gas released by human activities sharply increased the concentration of greenhouse gas in the atmosphere.Therefore,the greenhouse effect leading to global warming and global climate problems become a severly issue that might be harmful to human beings.However,if CO₂ can be used as a"carbon source"to be restored to other clean fuels or chemical products,it can effectively solve the increasingly serious energy and environmental problems of humankind.Electrocatalytic reduction is the most widely studied CO₂ reduction method at present,and it is easily to be controlled and can achieve a higher conversion rate,which is beneficial for expanding production.As the first electrocatalyst that can directly reduce CO₂ to hydrocarbons,metal Cu has attracted much attention in recent years.Up till now,a large number of experiments have been carried out to prove that metal Cu can reduce CO₂ effectively as an electrocatalyst.In this paper,five catalyst models including Cu₃₈ nanoparticles and its Ag,Zn,Pd and Au doped Cu₃₇-X nanoparticles were studied by DMOL3 module implemented in Materials Studio 7.0.The different adsorption sites of CO₂ and H₂O molecules on a series of catalysts were studied in detail respectively,and the stable adsorption configuration was determined.The reduction mechanisms of CO₂ on a series of catalysts were further studied,and the activation energies of a series of catalytic reactions were compared to determine the activity of a series of catalysts,based on which the influence of doping effects on the catalytic activity was revealed.When H₂O and CO₂ molecules were co-adsorbed on the surface of Cu₃₈ nanoparticles,it was found that CO₂ molecules would transform from the original stable linear molecules to V-shaped molecules.The bond length of C=O increased in the same time,indicating that CO₂ molecules were activated during the adsorption process.Since CO is an important intermediate in the process of CO₂ reduction,the reduction reaction is divided into two steps in this paper.The first step is the reduction of CO₂ to CO,and the reaction mechanism consists of two steps:One of the H atom in H₂O molecule first transfers to CO₂ to form COOH*,and then COOH*is decomposed to form CO.The hydrogen transfer process has two possible reaction paths.In path I H atom transfer directly to the CO₂,path II for the H atoms migrated to the first Cu₃₈ nanoparticles to CO₂.The computational results showed that the step reaction mechanism path I give priority to.The second step of the reaction is the reduction of CO→CH₃OH.The reaction mechanism is as follows:the co-adsorbed water molecules transfer H atom to C atom or O atom on CO by steps to form CH₃OH.Because the four H atoms transfer to C or O atom on CO atom in different order,there are four feasible reaction paths.The optimal reaction path is CO→CHO*→CH₂O*→CH₃O*→CH₃OH.The control step of the whole reduction process of CO₂→CH₃OH is COOH*→CO+OH*.From the analysis of the reaction activation energy of the control step,the catalytic reaction process is feasible.According to the experimental results,Au,Ag and other metal catalysts can effectively catalyze the reduction of CO₂ to CO.After comparing the reaction activation energies of different doping catalysts,Au doping has the optimal catalytic effect in the process of reducing CO₂ to CO,which is followed by Ag doping,and finally Pd and Zn doping.The research results are consistent with the experiment.In the process of catalytic reduction of CO to CH₃OH,the activity of Cu₃₇-X catalyst is stronger than that of Cu₃₈ catalyst,and the order of catalytic activity roughly follow the order of Cu₃₇-Zn>Cu₃₇-Au≥Cu₃₇-Ag>Cu₃₇-Pd>Cu₃₈.Based on the activity of a series of catalysts in the two-stage reaction process,the reaction control step is:CO→CH₃OH process.We found that the catalytic activity of Cu nanoparticles doped with Zn,Au,Ag and Pd can all improve the CO₂ reduction to CH₃OH,and the order of the doping catalytic activity is as follows:Cu₃₇-Zn>Cu₃₇-Au≥Cu₃₇-Ag>Cu₃₇-Pd.Relatively speaking,the doping effect of Zn,Ag and Au is better.By studying the mechanism of CO₂ reduction catalyzed by Cu₃₈ and Cu₃₇-x nanoparticles,we have revealed the catalytic mechanism of bimetallic catalysis,which provides a new idea for improving the activity of the catalyst and reducing the cost of the catalyst through doping of other metals.