The Mechanism Investigation and Catalyst Design of CO2 Electroreduction

Author:Zhao Chen Xu

Supervisor:jiang qing


Degree Year:2019





Carbon dioxide(CO2)is the ultimate product in the process of utilizing fossil fuels and causes a serious greenhouse effect.It is urgently needed to convert CO2 into valuable chemicals,while the electrochemical method is one of the most promising ways.The CO2electroreduction reaction(CRR)is impeded by the low selectivity and high limiting potential of catalysts.Clarifying the mechanism and designing catalysts with high efficiency are crucial to solve these issues.In revealing the mechanism of CRR,large numbers of researches are based on gas-phase model.To approach the real electrochemical condition,we deliberately introduce the coupling effect between solvation/cations at different potentials,which is treated as an effective correction for the gas-phase model.In designing materials,we have designed a novel single-metal catalyst base on two-dimentional InSe(2D InSe).This is treated as an effective way in breaking the intrinsic limitation of transition metals.The details of this research can be listed as follows:Herein,we have constructed a liquid-solid interface based on Pb(111)using density functional theory(DFT)calculations and further added cations and potential effects into the model.The results reveal that the effects of solvation and cations lead to different active intermediates for CO2 electroreduction to HCOOH(HCOO in gas-phase condition,both HCOO and COOH in solvent condition,and COOH in cation solution).In particular,the size and hydrophilic/hydrophobic nature of the cations are found to significantly affect the reaction activity of CO2 electroreduction at different overpotentials.The starting potential,which converts the major intermediate from COOH to HCOO,is increased with increasing size of cations in hydrophilic environment.The tendency corresponds to an increased activity(HCOO is a poisoning specie which is likely to accumulate on the electrode.Thus,HCOO is highly desired to be activated).The hydrophobic environment is more beneficial than hydrophilic conditions and K salt is thus advantageous compared with other salts.We herein design a novel type of SMC based on two-dimensional InSe and transition metal atoms(M@2DInSe)using density functional theory calculations.The CRR on transition metal(TM)surfaces has been investigated extensively.The distinct selectivity of transition metals can be understood from the descriptor proposed by Norskov et al.:the binding energies of C-species correlate with each other via a scaling relationship based on the d-band center theory.Thus,*CO binding energy(Eb[*CO])determines production distribution:Cu exhibits the most suitable Eb[*CO]towards hydrocarbons whereas Au and Ag favor the formation of CO.However,the large limiting potential(-0.8 V for Cu)still limits the practical application of transisiton metals.Remarkably,According to the descriptor,there are two general ways to improve the reactivity of a catalyst.First,Cu is close to the volcano peak among transition metals,while there is still room to design advanced catalysts to approach the optimal reactivity.Second is to break the binding correlation between*CO and C based intermediates such as*COOH and*CHO,by stabilizing these intermediates with respect to*CO.Embedding sigle metal atoms into two-dimentional substrates is an effective way to break the traditional theory.Different from the known SMCs on graphene,TiC,and TiN,the adsorption energies of*CO and*COOH intermediates on most of the M@2DInSe systems obey the linear relationship established on transition metals,with the results on few candidates deviating from the relationship significantly.Besides,the adsorption energy values of*CO/*COOH on M@2DInSe systems are closer to the values on Cu(111),compared to their bulk counterparts.This characters originate from the hybridization between InSe and d-/s-bands of metal atoms.These properties allow us to quickly screen the optimal M@2DInSe.We find that Cr@2DInSe favors the formation of CH4 with a limiting potential of-0.36 V,while Cu@2DInSe(Rh@2DInSe)prefers to produce HCOOH(H2)with a limiting potential of-0.11(-0.03)V.To improve the design of high-performance catalysts,it is essensial to investigate the mechanism of CRR catalyzed by M@2DInSe.CO adsorption can be treated as a prototype model for CRR.We thus deliberately discussed the detail of CO adsorption on M@2DInSe and calculated many properties based on DFT such as:adsorption energies,charge transfer,and electronic properties.The results reveal that the conventional Blyholder mechanism can be applicable to M@2DInSe systems,and the charge transfer between CO and M@2DInSe can serve as an effective measurement for the adsorption energy of CO.The change of conductivity induced by CO adsorption also correlate strongly with the adsorption energy of CO:Fe@2DInSe demonstrates an apparent metal-to-insulator transition,while Cu@2DInSe maintains unchanged metal property.Most M@2DInSe can capture CO with more than-1 eV adsorption energies and possess metal properties,which reals proper candidates for CRR catalysts.When CO interacts with M@2DInSe with a charge transfer of less than 0.12 e,a pristine electronic character is maintained for M@2DInSe.These findings not only rationalize several experimental observations and contribute to a thorough understanding of CO2 electroreduction.But also provide a reasonable material design scheme for further theoretical research.