Theoretical Investigations Of Relationship Between Reaction Mechanisms Of CO₂ Reduction And The Structures Based On Model Catalysts

The conversion/utilization of CO2 is an ideal and sustainable strategy to relieve the accumulation of CO2 and the globe energy demands.However,due to the inertness of CO2 molecule,additional energy and catalysts are needed.In traditional catalytic and electrocatalytic processes,Cu catalyst is commonly used to catalyze the CO2 reduction.However,the main obstacles currently are the structural sensitivity of this reaction,the diversity of products and the controversial reaction mechanism.At the same time,the metal N-doped carbon materials or N-doped carbon materials((M)-NC)are also used for CO2 electroreduction.Although the product is single,the active sites are still not clear due to the complex structure of the catalyst,which hinders the study of the reaction mechanism.In order to design the catalysts rationally and increase the activity and selectivity for CO2 reduction,it is important to understand the relationship between the reaction mechanism and the structure under the atomic level.The model catalysts can be used to simplify the catalyst system and provide a new perspective for study of reaction mechanism on complex structures.Therefore,Cu nanoparticles and surfaces well-defined structures or molecular catalyst with(M)-NC structures are performed as the model catalysts,combing DFT calculation and Microkinetic simulation,the relationship between mechanism and structure in CO2 reduction is explored from thermodynamics and kinetics.The details are as follows:1.Based on the controversial intermediates in the literature and combined with the microkinetic simulation,the reaction mechanism of CO2 reduction to produce CH3OH on Cu(111)and Cu(211)surfaces is studied.The reaction pathways mainly include COOH,HCOO,CO and CHO,COH and other intermediates.The kinetic simulation along these pathways shows that the catalytic activity on Cu(111)surface is much lower than that on Cu(211)surface,and CO2 dissociation is the rate-determing step of the whole reaction due to the weak CO2 adsorption.Reaction pathway analysis shows that CO2 reduce to methanol along the pathway of CO2? CO? CHO?CH2O?CH3O?CH3OH within the temperature range commonly used in industry.It was proved that HCOO and COOH are not the key intermediates while CO and CHO are the key intermediate for the reaction.2.The size effect of C02 hydrogenation to methanol on Cu catalysts is explored.Copper clusters with atomic numbers n = 13,15,19,55 and 79 are chosen to simulate Cu nanopariticles less than 2 nm and the periodic Cu(111)and Cu(211)surface are used to simulate the surface of larger nanoparticles.Structure dependence manifests itself in the form of size-dependent adsorption behavior.The activation barriers involved in methanol formation exhibit linear scaling relationship with the adsorption energies of CO and 0.Microkinetics simulations were then conducted to predict reaction rates based on these linear scaling relationships.Through the volcano curves,we can speculate about optimum Cu catalysts for CO2 hydrogenation to methanol.3.Combine DFT calculations with linear free energy models to investigate electrochemical CO2 reduction on two periodic Cu surfaces.The dependence of the activation barriers of electrochemical elementary reactions on the electrochemical potential is added to microkinetic simulation.In this work,two typical model Cu surfaces,Cu(111)and Cu(211),are considered to study the structural sensitivities of copper nanoparticles for CO2 electroreduction.Although we did not explicitly consider the electrolyte,the effect on the activation barriers of the elementary reaction step of water molecules is evaluated The results show that the current density and product distribution of CO2 reduction strongly depend on the Cu surface topology,applied potential,and the stable transition state structure through water molecules.4.A simple method that combines thermodynamic parameters(redox potential and acidity)and micro-kinetics is proposed to find the optimal experimental conditions and catalysts.It is helpful to visualize the contribution of the electronic and proton energy levels to thermodynamic overpotentials by placing both the electronic energy level and the proton energy level in a unified level diagram at the common potential scale.The activity of the PCET reaction depends on the deviation between the thermodynamic energy of the reaction and the experimental conditions,resulting in a volcanic curve.Based on this method,combined with the experimental data,the dependence of pH on CO2 reduction on cobalt porphyrin(CoP)can be understood,and the optimal experimental conditions for CO2 reduction on FePc can also also identified.The differences in the activity of CO2 reduction and hydrogen evolution are also compared.This method provides important guiding principle for finding suitable reaction conditions for experiments and rationally designing catalysts.5.The carbon-supported phthalocyanine(Pc)and iron phthalocyanine(FePc)molecules with well-defined structure are used as model catalyst to simulate(M)-NC materials.Combing with experimental results and density functional theory(DFT),the relationship between the mechanism of CO2 reduction and different sites are explored.Through the energy level diagrams,it can be seen by comparing the contribution of redox potential(Ure)and acidity(pKa)in thermodynamic overpotential that the activity of CO2 reduction on FePc molecule is much higher than that on Pc molecule,while the activity of H2 evolution on FePc molecule is lower than that on Pc molecule.Our results show that both Fe site and N site can serve as the active sites for CO2 reduction and different active sites will alter the activity and selectivity of CO and H2 production.The systematic study on the mechanism of CO2 reduction on(M)-NC catalysts not only reflects the advantage to evaluate the activity and selectivity of catalysts with energy level diagram but also provides a new reference insight for rational design of the catalysts.