CO_2在Cu表面还原成碳氢化合物的DFT计算研究

DFT Study of CO_2 Reduction to Hydrocarbons on Cu Surfaces

应用密度泛函理论(DFT)反应能计算及最小能量路径分析研究了CO2在气相和电化学环境中于Cu(111)单晶表面的还原过程.气相CO2还原为碳氢化合物的反应路径可能为:CO2(g)+H*→COOH*→(CO+OH)*→CHO*;CHO+H*→CH2O*→(CH2+O)*;CH2*+2H*→CH4或2CH2*→C2H4.整个反应由CO2(g)+H*→COOH*→(CO+OH)*,(CO+H)*→CHO*和CH2O*→(CH2+O)*等几个步骤联合控制.在-0.50 V(vs.RHE)以正的电势下,CO2在Cu(111)表面电化学还原主要形成HCOO-和CO吸附物;随着电势逐渐负移,CO2加氢解离形成CO的反应越来越容易,CO成为主要产物;随电势进一步变负,形成碳氢化合物的趋势逐渐变强.与CO2的气相化学还原不同的是,电化学环境下CO质子化形成的CHO中间体倾向于解离形成CH,而在气相中CHO中间体则倾向于进一步质子化形成CH2O中间体.

The CO2 reduction on Cu（111） single crystal surfaces was studied using DFT calculations on the reaction energies and the minimum energy paths.The results indicated that the possible reaction paths for CO2 reduction on Cu（111） surface are CO2（g）＋H＊→COOH＊→（CO＋OH）＊,（CO＋H）＊→CHO＊,CHO＋H→CH2O＊→（CH2＋O）＊,CH2＊＋2H＊→CH4 or 2CH2＊→C2H4.On Cu（111） surface,the reaction rate is controlled by steps of CH2O＊→（CH2＋O）＊,CO2（g）＋H＊→COOH→（CO＋OH）＊ and（CO＋H）＊→CHO＊.In addition,the reaction energies for various steps in the electrochemical reduction of CO2 were calculated under different electrode potentials.The results indicated that HCOO-and CO are mainly formed when the potential is more positive than-0.50 V（vs.RHE）.The hydrogenated dissociation of CO2 to form CO and the subsequent hydrogenation of CO become increasingly exothermic as the potential goes negative,so that hydrocarbons gradually become the favored products in the electrochemical reduction.Under electrochemical conditions,the CHO intermediate prefers to dissociate to form CH,rather than to form CH2O intermediate via protonation as does in gas phase reduction.