Department of Physics , Freie Universität Berlin , Arnimallee 14 , 14195 Berlin , Germany.
Department of Physics , University of Sistan and Baluchestan , Zahedan , 98167-45845 , Iran.
J Am Chem Soc. 2019 Feb 20;141(7):2938-2948. doi: 10.1021/jacs.8b10002. Epub 2019 Feb 6.
Understanding the mechanism for electrochemical water oxidation is important for the development of more efficient catalysts for artificial photosynthesis. A basic step is the proton-coupled electron transfer, which enables accumulation of oxidizing equivalents without buildup of a charge. We find that substituting deuterium for hydrogen resulted in an 87% decrease in the catalytic activity for water oxidation on Co-based amorphous-oxide catalysts at neutral pH, while O-to-O substitution lead to a 10% decrease. In situ visible and quasi-in situ X-ray absorption spectroscopy reveal that the hydrogen-to-deuterium isotopic substitution induces an equilibrium isotope effect that shifts the oxidation potentials positively by approximately 60 mV for the proton coupled Co and Co electron transfer processes. Time-resolved spectroelectrochemical measurements indicate the absence of a kinetic isotope effect, implying that the precatalytic proton-coupled electron transfer happens through a stepwise mechanism in which electron transfer is rate-determining. An observed correlation between Co oxidation states and catalytic current for both isotopic conditions indicates that the applied potential has no direct effect on the catalytic rate, which instead depends exponentially on the average Co oxidation state. These combined results provide evidence that neither proton nor electron transfer is involved in the catalytic rate-determining step. We propose a mechanism with an active species composed by two adjacent Co atoms and a rate-determining step that involves oxygen-oxygen bond formation and compare it with models proposed in the literature.
理解电化学水氧化的机理对于开发更高效的人工光合作用催化剂至关重要。其中一个基本步骤是质子耦合电子转移，它可以在不积累电荷的情况下积累氧化当量。我们发现，在中性 pH 条件下，用氘取代氢会导致基于钴的非晶态氧化物催化剂上水氧化的催化活性降低 87%，而 O 到 O 的取代则导致催化活性降低 10%。原位可见和准原位 X 射线吸收光谱表明，氢到氘的同位素取代诱导了平衡同位素效应，使质子耦合的 Co 和 Co 电子转移过程的氧化电位正移约 60 mV。时间分辨光谱电化学测量表明不存在动力学同位素效应，这意味着预催化质子耦合电子转移是通过逐步机制发生的，其中电子转移是速率决定步骤。在两种同位素条件下，观察到 Co 氧化态和催化电流之间的相关性表明，施加的电势对催化速率没有直接影响，而是与平均 Co 氧化态呈指数关系。这些综合结果表明，在速率决定步骤中既没有质子转移也没有电子转移。我们提出了一种具有两个相邻 Co 原子的活性物种的机制，以及涉及氧-氧键形成的速率决定步骤，并将其与文献中提出的模型进行了比较。
