Kinetic Study On Electrochemical Carbon Dioxide Reduction Reaction On Transition Metal Surfaces

The large-scale application of fossil fuels has caused a series of problems such as the energy crisis and global warming.Direct reduction of CO₂ to chemicals or fuels with high added value is an important way for CO₂ conversion and utilization.The electrochemical reduction of CO₂ to hydrocarbon fuels is easy to operate in mild conditions and draws great attentions in the research of CO₂ conversion and utilization.Electrochemical CO₂ reduction reaction（CO₂RR）is a multi-proton and multi-electron transfer process.The reaction mechanism is complicated and the product selectivity is different on different metals.Currently,copper is the only elementary metal that can drive CO₂RR to produce a large number of hydrocarbons.However,CO₂RR on copper electrodes requires a high overpotential and lacks selectivity,accompanied by competing reactions such as hydrogen evolution reaction（HER）.The majority product of CO₂RR on silver and gold is CO,while on metal electrodes such as lead and tin there is a high selectivity to HCOOH/HCOO^-.At present,most theoretical studies are based on thermodynamic stability to explore the reaction mechanism of CO₂RR.However,very few researches have been performed on kinetics.In this paper,Density Functional Theory（DFT）is used to study the reaction mechanism and product selectivity of the CO₂RR process.The following two aspects are studied:1)The establishment of a kinetic model theory method for the mechanism of CO₂ reduction on copper electrodes.2)Research on the selectivity of CO₂RR products on a series of transition metal surfaces.The main contents of this article are as follows:（1）A kinetic model based on Marcus theory is established.This model is different from the previous hydrogen atom transfer（HAT）model,which reflects the authentic proton-electron transfer process of CO₂ reduction.Based on this model,the potential energy curves of the electrochemical CO₂ reduction reaction with different potentials on different single crystal surfaces of copper electrodes were calculated.The calculation results show that onset potentials and rate-determining steps for CO and CH₄ formation are determined by the first and third concerted proton-electron transfer steps C1 and C3.Then the influence of binding energy,electrode potential,and reorganization energy on the computed reaction barriers of C1 and C3 reactions is discussed.In general,the computed reaction barrier shows a quadratic relationship with the applied electrode potential.Specifically,the reaction barrier is merely determined by the reorganization energy at equilibrium potential.Among the studied four copper single crystal surfaces,Cu（211）surface exhibits the best electrocatalytic activity for CO formation and CH₄formation due to the lowest onset potential and overpotential.（2）Based on the established kinetic model,the product selectivity of CO₂RR on a series of transition metals were studied.In this chapter,we mainly discuss the effect of electrode potential on the CO₂ reduction path.CH₄ is the main product on Cu（111）surface.The elementary reaction*CO to*COH is the key step for CH₄ formation at a certain applied potential.CO is the main product on Ag（111）and Au（111）surfaces.Although the intermediate*HCOO is more stable than*COOH thermodynamically,at a certain applied potential,the activation barrier to form*COOH is lower.Thus,*COOH is the only intermediate to form CO on Ag（111）and Au（111）surfaces.For the metals Pb,In,and Tl,we mainly study the flat crystal planes of Pb（111）,In（101）,and Tl（001）,whose majority products are HCOOH/HCOO^-.The intermediate*HCOO is not only thermodynamically more stable on these three crystal surfaces,but also has a lower activation barrier compared to*COOH.Thus,*HCOO is the only intermediate for HCOOH formation,which agrees with that the selectivity of HCOOH is high on Pb,In and Tl surfaces in experiment.For metals Pt,Pd,Ni,Rh and Ir,hydrogen is the main product although CO is very easy to be produced with a very large negative value ofΔG.This is explained from electronic structure analysis of CO on these metals.The electron density of the CO molecule and the d orbital of each metal are analyzed.It is found that the d orbits of Pt,Ni and other metals coincide better with the 2?*anti-bond orbitals of CO.As a result,CO is adsorbed tightly on these metal surfaces.In this case,the electrode is poisoned by CO and the hydrogen evolution reaction becomes the main reaction.