摘要
电催化二氧化碳(CO2)还原有望实现温室气体的回收,还能合成一系列有经济价值的产物,实现碳循环.从已报道的法拉第效率和电流密度方面考虑,最具经济前景的产物包括一碳产物(一氧化碳和甲酸)以及二碳产物(乙烯和乙醇)等.不同于一碳产物,二碳产物的生成涉及碳-碳偶联步骤,其机理更为复杂,因此对催化剂的设计提出了更高的要求.本文综述了近年来电催化CO2还原合成二碳产物的主要进展,简要介绍了电催化CO2还原的基本原理,阐释了目前最为广泛接受的3种碳-碳偶联反应机理,并按催化剂体系分类讨论了如何通过催化剂设计提高二碳产物的选择性.在催化剂设计方面,主要围绕Cu基催化剂展开讨论,强调了其暴露晶面、尺寸、形貌、担载密度、表面原子氧化态,与其他金属合金化和复合对产物选择性的影响.经过设计优化的催化剂可以有效提高CO中间体的局域浓度,或降低碳-碳偶联反应的活化能垒,从而促进二碳、多碳产物的生成.
The technique of electrocatalytic CO2 reduction(ECR)is emerging as a competent candidate for neutralizing the excessive anthropogenic carbon emission,and for producing a multitude of value-added chemicals.In particular,ECR is well compatible with renewable electricity-generating technologies,and can help to alleviate their innate issue of intermittency and to level the energy output of power grid.Yet ECR itself typically suffers from the chemical inertness and low solubility of CO2,multiple products and low selectivity,and the undesired parasitic hydrogen evolution reaction in aqueous electrolytes.Therefore,electrocatalysts featuring low overpotentials,high current densities,high Faradaic efficiencies and high stabilities are in constant pursuit.From the viewpoint of Faradaic efficiency and current density that have been achieved thus far,the most promising products are CO and formic acid(among C1 products),and ethylene and ethanol(among C2 products).Different from C1 products,C2 products are formed via carbon-carbon coupling(C–C coupling)following a second-order reaction kinetics,generally with more complicated reaction mechanisms involving multiple electron transfer and therefore more stringent requirements on the electrocatalysts.As such,at the center of synthesizing C2 products via ECR is the in-depth understanding on reaction mechanisms and the rational design of advanced electrocatalysts.Up to date,the ECR catalysts that can yield C2 products are primarily based on Cu-related materials,and CO has been recognized as the most important and versatile intermediate for C–C coupling.This review summarizes the reports on the relevant underlying reaction mechanisms,and elaborates on the three most widely accepted catalytic mechanisms for C–C coupling on Cu-based catalysts:CO dimerization,CO+CHO coupling,and CH2 carbene dimerization.These mechanisms proposed on the basis of theoretical calculation,take effect in different regimes of applied electric potential,and have also found substantial support from experimental data obtained particularly via in situ and operando spectroscopies.The rational design of Cu-based catalysts can effectively improve the reaction selectivity for C–C coupling.In this regard,this review discusses Cu-based catalysts in different subcategories:bulk Cu catalysts,Cu nanocatalysts,oxidederived Cu catalysts,and Cu-based bimetallic catalysts,and put major emphasis on the effects of exposed facet,particle size,shape,loading density,oxidation state of surface atoms and alloying on steering the selectivity towards C–C coupling pathways.It has been found that Cu-based catalysts that feature(100)facets,regular shapes,high loading areal densities and partially oxidized Cu atoms generally give superior performances in yielding C2 and C2+products,because their optimized surface and electronic structures can effectively elevate the local concentration of CO intermediates or lower the activation barrier for C–C coupling.A few topics other than Cu-based catalysts are also briefly discussed at the end of this review.We stress the irreplaceable importance of in situ and operando characterization techniques in probing and deciphering the catalytic mechanism of ECR,the design and optimization of electrolytes and electrodes in help to alter the selectivity for different products,and most importantly,a range of novel catalysts that have emerged in recent years,including tandem catalysts,single-atomic catalysts,non-Cu-based metal catalysts,non-metal catalysts based on graphene and nanodiamond,as well as molecular catalysts,which have witnessed a resurgence after the proposal of the idea of heterogeneous immobilization.
作者
张超
鲁统部
Chao Zhang;Tongbu Lu(Institute for New Energy Materials&Low Carbon Technologies,School of Materials Science and Engineering,Tianjin University of Technology,Tianjin 300384,China)
出处
《科学通报》
EI
CAS
CSCD
北大核心
2020年第31期3401-3417,共17页
Chinese Science Bulletin
基金
国家重点研发计划(2017YFA0700104)
国家自然科学基金重点项目(21931007)资助。
关键词
二氧化碳还原
电催化
二碳产物
铜基催化剂
碳-碳偶联
CO2 reduction
electrocatalysis
C2 products
Cu-based catalysts
carbon-carbon coupling
