Synchrotron Operando Spectroscopy for Electrochemical reduction of Single-Atom Alloy Ca talysts

著名的M-N4配位催化剂对电化学二氧化碳还原(ECR)表现出良好的催化活性。然而，由于在ECR操作条件下缺乏实验证据，中心金属中心的催化机理和作用仍然是一个激烈的争论。本文以钴酞菁(CoPc)为模型催化剂，用HERFD-XAS和Soft-XAS研究了活性Co中心的电子结构演化。(1)石墨烯负载的CoPc(CoPc/G)对H_2和CO的生成表现出势依赖行为。CO产物的选择性随着外加电压的增加而增加，在-0.8V(vs.RHE)时达到最大值~96%法拉第效率。(2)Co K HERFD-XAS分析表明，在CO_2饱和的电解液中，随着偏压的增加，7708 eV处的峰(1s→3D-4P杂化)的强度从-0.6V逐渐升高，并在没有偏压的情况下恢复到初始强度。相反，7715 eV(1s→4pz)处的峰值强度降低。这表明CoPc/G的Co位可以与中间体(*COOH/*CO)相互作用，并且在反应过程中表现出可逆的行为。(3)Co-L谱表明，在777.8 eV处的峰强度相对于CoPc面外的跃迁强度在-0.6V处减小，这归因于Co位上吸附的中间体，导致了D4h对称性的大扭曲。光谱特征相同，但峰强度不同，表明CoPc/G的Co位仍为CoII氧化态。

The well-known M-N4 coordinated catalysts show a superior catalytic activity for electrochemical CO2 reduction (ECR). However, the catalytic mechanism and the role of centered metal sites are still remaining a bitter controversy due to the lack of experimental evidence under the operating ECR conditions. Here, the cobalt phthalocyanine (CoPC) is used as a model catalyst to investigate the electronic structure evolution of reactive Co sites by HERFD-XAS and soft-XAS.(1) The graphene supported CoPc (CoPc/G) shows a potential-dependent behavior for H2 and CO production. The selectivity for CO products is increasing as the applied potentials and reaches the maximum value of ~96% Faradic efficiency at-0.8 V (vs. RHE).(2) The Co K HERFD-XAS shows that, as increasing the bias in CO2-saturated electrolyte, the peak at 7708 eV (1s→3d-4p hybridization) is gradually raising the intensity from -0.6 V and is back to the initial without the bias . Inversely, the intensity of the peak at 7715 eV (1s→4pz) is decreased. It implies that the Co site of CoPc/G could be interacted with intermediates (*COOH/*CO) and shows a reversible behavior during the reaction.(3) The Co L soft-XAS shows the intensity of the peak at 777.8 eV regarding to the transitions with components out of CoPc plane decreases at -0.6 V. It is ascribed to the intermediates absorbed on the Co site and results in a large distortion of the D4h symmetry. The spectra show same characteristic features but difference in peak intensities, indicating that the Co site of CoPc/G remains the CoII oxidation state.