【摘要】 在CO₂还原反应（CO₂RR）应中,单原子催化剂被认为是很有前途的电催化剂.Co-N₄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CO₂RR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N₄-大环配体（Co-N₄-CPY）负载的Co N₄位点上的CO₂RR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-N₃O-CPY）和C（Co-N₃C-CPY）取代的单原子Co位点的局部配位环境.计算结果表明, Co-N₄, Co-N₃O和Co-N₃C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-N₃C-CPY, Co-N₃O-CPY和Co-N₄-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Co N₄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CO₂RR过程的吉布斯自由能变化,确定了Co-N₄-CPY的主要产物为CO.当Co-N₄位点的局部配位环境改变后, Co-N₃O位点上有利于*COOH的形成,而Co-N₃C位点对碳氢化合物（CH3OH或CH4）有更高的选择性.这是由于CO₂在Co-N₃O位点上更容易质子化形成*OCHO,从而有利于HCOOH的形成.而在Co-N₃C位点上时,由于*CO难以脱附,*CO将进一步质子化,最终形成CH3OH或CH4.CO₂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-N₃O-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CO₂RR途径之间的关系提供了一个新的视角.更多还原

【Abstract】 Single-atom catalysts have been proposed as promising electrocatalysts for CO₂ reduction reactions（CO₂ RR）. Co-N₄ 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 CO₂ reduction reaction pathways is still unclear. In this study, we investigated the CO₂ reduction reaction pathways on Co-N₄ sites supported on conjugated N₄-macrocyclic ligands with 1,10-phenanthroline subunits（Co-N4-CPY） by density functional theory calculations. The local coordination environment of single-atom Co sites with N substituted by O（Co-N₃ O-CPY） and C（Co-N₃C-CPY） was studied for comparison. The calculation results revealed that both C and O coordination break the symmetry of the primary CoN₄ ligand field and induce charge redistribution of the Co atom. For Co-N₄-CPY, CO was confirmed to be the main product of CO₂ RR. HCOOH is the primary product of Co-N₃O-CPY because of the greatly increased energy barrier of CO₂ to *COOH. Although the energy barrier of CO₂ to *COOH is reduced on Co-N₃ C-CPY, the desorption process of *CO becomes more difficult. CH₃OH（or CH₄） are obtained by further *CO hydrogenation reduction when using Co-N₃ C-CPY. This work provides new insight into the effect of the local coordination environment of single-atom sites on CO₂ reduction reaction pathways.更多还原