单原子Co位点的CO_(2)还原反应路径:局部配位环境的影响

CO_(2) reduction reaction pathways on single‐atom Co sites:Impacts of local coordination environment

在CO_(2)还原反应(CO_(2)RR)应中,单原子催化剂被认为是很有前途的电催化剂.Co-N_(4)活性位点因其优异的CO选择性和活性而受到广泛关注.然而,Co位点的局部配位环境与CO_(2)RR途径之间的相关性尚不明确.本文采用密度泛函理论(DFT)计算,研究了含1,10-菲咯啉基底的N_(4)-大环配体(Co-N_(4)-CPY)负载的CoN_(4)位点上的CO_(2)RR路径.该模型的平面空间构型使其具有高度离域的π电子轨道,可以通过π-π相互作用更好地与底物相互作用.在相对可逆氢电极(RHE)为-0.70 V时,最大CO的法拉第效率为96%,CO的转换频率为9.59 s^(–1).众所周知,单原子位点的局部配位环境可以进一步优化电催化性能.因此,本文还研究了N被O(Co-N_(3)O-CPY)和C(Co-N_(3)C-CPY)取代的单原子Co位点的局部配位环境.计算结果表明,Co-N_(4),Co-N_(3)O和Co-N_(3)C位点的键长相差不大,而且其结合能都较大,意味着它们具有良好的稳定性.Co-N_(3)C-CPY,Co-N_(3)O-CPY和Co-N_(4)-CPY的投影态密度(PDOS)、电荷密度差分和晶体轨道哈密顿布居(COHP)显示,C和O配位均打破了初始CoN_(4)配体场的对称性,并诱发了Co原子的电荷再分布.Co原子与配位原子间的相互作用由强到弱依次为Co-C键、Co-N键和Co-O键,这与电负性O>N>C相反.通过分析CO_(2)RR过程的吉布斯自由能变化,确定了Co-N_(4)-CPY的主要产物为CO.当Co-N_(4)位点的局部配位环境改变后,Co-N_(3)O位点上有利于*COOH的形成,而Co-N_(3)C位点对碳氢化合物(CH_(3)OH或CH_(4))有更高的选择性.这是由于CO_(2)在Co-N_(3)O位点上更容易质子化形成*OCHO,从而有利于HCOOH的形成.而在Co-N_(3)C位点上时,由于*CO难以脱附,*CO将进一步质子化,最终形成CH_(3)OH或CH_(4).CO_(2)质子化形成*COOH或*OCHO的PDOS显示,在Co位上吸附之前,*OCHO的自旋密度是对称的,而*COOH的自旋密度是不对称的.对于催化剂而言,只有Co-N_(3)O-CPY的自旋密度是对称的,因此容易形成*OCHO中间体.Co-3d轨道与*COOH的C-2p轨道和*OCHO的O-2p轨道的相互作用也体现了这一关系.本工作为单原子位点的局部配位环境与CO_(2)RR途径之间的关系提供了一个新的视角.

Single‐atom catalysts have been proposed as promising electrocatalysts for CO_(2) reduction reactions(CO_(2)RR).Co‐N_(4) active sites have attracted wide attention owing to their excellent CO selectivity and activity.However,the effect of the local coordination environment of Co sites on CO_(2) reduction reaction pathways is still unclear.In this study,we investigated the CO_(2) reduction reaction pathways on Co‐N_(4) sites supported on conjugated N_(4)‐macrocyclic ligands with 1,10‐phenanthroline subunits(Co‐N_(4)‐CPY)by density functional theory calculations.The local coordination environment of single‐atom Co sites with N substituted by O(Co‐N_(3)O‐CPY)and C(Co‐N_(3)C‐CPY)was studied for comparison.The calculation results revealed that both C and O coordination break the symmetry of the primary CoN_(4) ligand field and induce charge redistribution of the Co atom.For Co‐N_(4)‐CPY,CO was confirmed to be the main product of CO_(2)RR.HCOOH is the primary product of Co‐N_(3)O‐CPY because of the greatly increased energy barrier of CO_(2) to*COOH.Although the energy barrier of CO_(2) to*COOH is reduced on Co‐N_(3)C‐CPY,the desorption process of*CO becomes more difficult.CH3OH(or CH_(4))are obtained by further*CO hydrogenation reduction when using Co‐N_(3)C‐CPY.This work provides new insight into the effect of the local coordination environment of single‐atom sites on CO_(2) reduction reaction pathways.