利用可再生电能将CO2转化为高附加值的化学品和燃料为缓解能源和环境危机提供了一条潜在的技术路线.在电化学CO2还原反应的各种产物中,甲烷(CH4)燃烧热最高,达到56 kJ g-1,是一种合适的能源载体.在Cu催化CO2电化学还原过程中,中间体*CO可以通过加氢转化为CH4,也可以与另一分子*CO或*CHO偶联生成多种C2+产物(C2H4,CH3COOH等).在孤立催化位点上,生成C2+产物的偶联反应会受到抑制,因此,开发Cu单原子催化剂有望提高CO2还原为CH4的选择性.本文报道了一种新型含氮石墨炔衍生物材料(N2-GDY),该材料由2,3,6,7,10,11-六炔基二吡嗪并[2,3-f:2',3'-h]喹喔啉通过Glaser偶联反应制得,具有规则的周期性孔洞结构与均匀分布的N^N-双齿配位位点.基于Cu与含氮双齿配位位点之间的配位作用,通过Cu2+吸附与NaBH4还原两步过程,制备了具有低配位数Cu-N2结构的Cu单原子催化剂(Cu1.0/N2-GDY).利用球差校正透射电镜确认了Cu以单原子形式分散在石墨炔载体上,电感耦合等离子体质谱测定Cu负载量为1.0 wt％.Cu扩展边X射线吸收精细结构谱拟合以及密度泛函理论计算结果表明,Cu1.0/N2-GDY中Cu以Cu-N2配位形式存在.作为对照样品,Cu0.6/TP-GDY的基底石墨炔材料中不含N^N-双齿配位位点,Cu以纳米颗粒形式存在.电催化CO2还原实验研究表明,在相对于可逆氢电极-0.96 V时,Cu1.0/N2-GDY催化CO2转化为CH4的法拉第效率高达80.6％,分电流密度为160 mA cm-2;在总电流密度为100至300 mA cm-2时,该催化剂对CH4的选择性均高于70％.作为对照,Cu0.6/TP-GDY在总电流密度为150 mA cm-2时CH4选择性最高,达到44.1％,明显低于Cu1.0/N2-GDY.此外,在N2-GDY上制备了同时含有Cu-N2位点与Cu纳米颗粒的Cu2.1/N2-GDY与Cu4.6/N2-GDY样品(Cu负载量分别为2.1 wt％与4.6 wt％),随着Cu纳米颗粒的出现及其尺寸的增加,催化剂催化CH4生成的选择性与质量活性均下降.原位红外光谱研究表明,相比于Cu纳米颗粒,配位不饱和的Cu-N2位点可以促进CO2活化为*COOH中间体,并促进关键中间体*CHO和*OCH3的形成,进而促进甲烷的生成.综上所述,本文合成了一种新型石墨炔衍生材料N2-GDY,基于其均匀分布的N^N-双齿配位位点构建了含Cu-N2位点的Cu单原子催化剂Cu1.0/N2-GDY.在电催化CO2还原反应中,该催化剂孤立的Cu-N2位点能抑制C2H4的生成,并促进*COOH,*CHO和*OCH3的形成,从而高效催化CO2到CH4的转化.N2-GDY石墨炔衍生物丰富的N^N-双齿配位位点有望合成其它含M-N2(M为金属)结构的单原子催化剂.
Cu single-atom electrocatalyst on nitrogen-containing graphdiyne for CO2 electroreduction to CH4
Developing Cu single-atom catalysts(SACs)with well-defined active sites is highly desirable for producing CH4 in the electrochemical CO2 reduction reaction and understanding the struc-ture-property relationship.Herein,a new graphdiyne analogue with uniformly distributed N2-bidentate(note that N2-bidentate site=N^N-bidentate site;N2 ≠ dinitrogen gas in this work)sites are synthesized.Due to the strong interaction between Cu and the N2-bidentate site,a Cu SAC with isolated undercoordinated Cu-N2 sites(Cu1.0/N2-GDY)is obtained,with the Cu loading of 1.0 wt％.Cu1.0/N2-GDY exhibits the highest Faradaic efficiency(FE)of 80.6％for CH4 in electrocatalytic reduction of CO2 at-0.96 V vs.RHE,and the partial current density of CH4 is 160 mA cm-2.The selec-tivity for CH4 is maintained above 70％when the total current density is 100 to 300 mA cm-2.More remarkably,the Cu1.0/N2-GDY achieves a mass activity of 53.2 A/mgcu toward CH4 under-1.18 V vs.RHE.In situ electrochemical spectroscopic studies reveal that undercoordinated Cu-N2 sites are more favorable in generating key*COOH and*CHO intermediate than Cu nanoparticle counterparts.This work provides an effective pathway to produce SACs with undercoordinated Metal-N2 sites toward efficient electrocatalysis.
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