Intracluster O-O Coupling Pathway Evidenced for an Anderson-Type Single-Cobalt Polymolybdate Water Oxidation Catalyst
Intracluster O-O Coupling Pathway Evidenced for an Anderson-Type Single-Cobalt Polymolybdate Water Oxidation Catalyst
复制标题
安德森型单钴多钼酸盐水氧化催化剂的簇内 O-O 偶联途径
DOI：
10.1021/acscatal.2c05925
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发表时间：
2023
期刊：
影响因子：
12.9
通讯作者：
Sakai Ken
中科院分区：
文献类型：
--
作者：
Taira Natsuki;Yamauchi Kosei;Sakai Ken
为了建立碳中和社会，人工光合作用，用可再生能源（H2，HCOOH，CH 3OH等）取代化石燃料，近年来引起了极大的关注。这一领域最重要的问题之一是提高我们在处理水氧化（WO）催化方面的知识和技术技能，这通常被认为是整个光合过程中的瓶颈。本文研究的聚氧乙烯酸盐水氧化催化剂（WOC）属于最高活性以及最高度稳健的WOC家族之一。然而，由于它们的结构复杂性和与高含量的重元素相关的高分子量，它们的作用机制在很大程度上仍未从理论和实验的角度进行探索。为了解决这些问题，在此我们集中于最小和最轻的聚氧乙烯酸盐WOC之一，单聚氧乙烯酸钴（NH 4）3[CoMo 6 O24 H6]·5 H2O（Co-POM-Mo），其先前被证明对WO具有高度活性[Tanaka，S. 2012，48，1653 - 1655]。采用停流技术研究了Co-POM-Mo和[RuIII（bpy）3]3+混合体系的WO反应速率与温度的关系，结果表明WO反应具有正的活化熵（ΔS ε = ca. 20-40 cal mol-1 K-1），揭示了分子内O-O偶联的促进作用是通过预先安装在簇中的氧的解离活化来实现的。通过17 O NMR和18 O-标记实验，阐明了在Co-POM-Mo体系中，氧的来源，以及16 O在O-O偶联反应中的贡献。Co-POM-Mo（100%in16O）三个催化循环从富含18 O的水溶液（49%in18O）中释放的O2导致16 O2、16 O 18 O和18 O2丰度分别为46，40和14%。Monte Carlo模拟再现了观测到的丰度，假设三个连续的O-O耦合仅通过采用六个可用位点中的单个μ3-OH位点进行。我们的模拟还假设O-O耦合过程与催化活性μ3-OH氧与本体水氧的交换竞争，前者的速率比后者高两倍。密度泛函理论的结果也证实了与中心钴离子结合的μ3-OH氧原子与构成Mo 6（μ2-O）6环的相邻μ2-O氧原子之间的O-O耦合的促进作用.该研究有力地表明，预安装的氧之间的O-O耦合是唯一可用的途径，以促进WO的一个高活性的聚氧乙烯酸酯WOCs，提供了一个新的视角，氧簇催化。
Toward the establishment of a carbon-neutral society, artificial photosynthesis, replacing fossil fuels with renewable energies (H2, HCOOH, CH3OH, etc.), has attracted great attention in recent years. One of the most important issues in this area has been to advance our knowledge and technical skills in handling water oxidation (WO) catalysis often considered as the bottleneck in the overall photosynthetic processes. The polyoxometalate water oxidation catalysts (WOCs) studied herein belong to one of the most highly active as well as the most highly robust family of WOCs. Nevertheless, due to their structural complexity and the high molecular weight associated with the high content of heavy elements, their mechanism of action remains largely unexplored from both theoretical and experimental viewpoints. To solve these problems, here we focus on one of the smallest and lightest polyoxometalate WOCs, single-cobalt polyoxometalate (NH4)3[CoMo6O24H6]·5H2O (Co-POM-Mo), which was previously demonstrated to be highly active for WO [Tanaka, S. ,Chem. Commun.2012, 48, 1653−1655]. The temperature dependence of the WO rate observed by mixingCo-POM-Moand [RuIII(bpy)3]3+using a stopped-flow technique revealed a positive entropy of activation (ΔS‡= ca. 20–40 cal mol–1K–1), revealing the promotion of intramolecular O–O coupling via the dissociative activation of the oxygens preinstalled in the cluster. The17O NMR and18O-labeling experiments, conducted to understand the source of oxygens in the O2evolved, clarified the major contribution of the preinstalled16O oxygens in the O–O coupling byCo-POM-Mo. The O2evolved from the three catalysis cycles byCo-POM-Mo(100% in16O) from an18O-enriched aqueous solution (49% in18O) resulted in the16O2,16O18O, and18O2abundances of 46, 40, and 14%, respectively. The Monte Carlo simulation reproduced the observed abundances under the assumption that the three consecutive O–O coupling proceed by only adopting a single μ3-OH site among the six available sites. Our simulation also supposed the O–O coupling process competes with the exchange of the catalytically active μ3-OH oxygen with the bulk water oxygen with the former twice higher in rate than the latter. Our DFT results also agree well with the promotion of O–O coupling between the inner μ3-OH oxygen bound to the central cobalt ion and the adjacent μ2-O oxygen constructing the Mo6(μ2-O)6ring. This study strongly suggests that the O–O coupling among the preinstalled oxygens is the only available pathway to promote the WO by one of the highly active polyoxometalate WOCs, providing a new perspective on the catalysis by oxo clusters.