Distinguishing Homogeneous from Heterogeneous Catalysis in Electrode-Driven Water Oxidation with Molecular Iridium Complexes

Molecular water-oxidation catalysts can deactivate by side reactions or decompose to secondary materials over time due to the harsh, oxidizing conditions required to drive oxygen evolution. Distinguishing electrode surface-bound heterogeneous catalysts (such as iridium oxide) from homogeneous molecular catalysts is often difficult. Using an electrochemical quartz crystal nanobalance (EQCN), we report a method for probing electrodeposition of metal oxide materials from molecular precursors. Using the previously reported [Cp*Ir(H2O)3]2+ complex, we monitor deposition of a heterogeneous water oxidation catalyst by measuring the electrode mass in real time with piezoelectric gravimetry. Conversely, we do not observe deposition for homogeneous catalysts, such as the water-soluble complex Cp*Ir(pyr-CMe2O)X reported in this work. Rotating ring-disk electrode electrochemistry and Clark-type electrode studies show that this complex is a catalyst for water oxidation with oxygen produced as the product. For the heterogeneous, surface-attached material generated from [Cp*Ir(H2O)3]2+, we can estimate the percentage of electroactive metal centers in the surface layer. We monitor electrode composition dynamically during catalytic turnover, providing new information on catalytic performance. Together, these data suggest that EQCN can directly probe the homogeneity of molecular water-oxidation catalysts over short times.

分子水氧化催化剂因驱动析氧所需的严苛氧化环境，随时间推移易发生副反应失活，或分解为次级产物。区分电极表面结合的多相催化剂（如氧化铱）与均相分子催化剂，通常存在较大难度。本研究借助电化学石英晶体微天平（electrochemical quartz crystal nanobalance, EQCN），开发了一种可探测分子前驱体中金属氧化物材料电沉积过程的方法。本研究利用此前已报道的[Cp*Ir(H2O)3]2+配合物，通过压电重量分析法实时监测电极质量变化，以此追踪多相水氧化催化剂的沉积过程。与之相反，本研究报道的水溶性配合物Cp*Ir(pyr-CMe2O)X这类均相催化剂，则未出现沉积现象。旋转环盘电极电化学测试与克拉克型电极研究结果表明，该配合物可作为水氧化催化剂，产物为氧气。对于由[Cp*Ir(H2O)3]2+生成的表面附着型多相材料，我们可估算表层内电活性金属中心的占比。我们在催化循环过程中动态监测电极组成，为催化性能研究提供了全新视角。综上，上述实验结果表明，电化学石英晶体微天平可在短时间内直接表征分子水氧化催化剂的均相特性。