|Eric J. Sundstrom, Xinzheng Yang, Hemamala I. Karunadasa, Sara V. Thoi, Christopher J. Chang, Jeffrey R. Long, Martin Head-Gordon*|
J. Am. Chem. Soc., Article ASAP
Publication Date (Web): February 22, 2012
Copyright © 2012 American Chemical Society
We investigate the mechanism for the electrocatalytic generation of hydrogen from water by the molecular molybdenum-oxo complex, [(PY5Me2)MoO]2+ (PY5Me2 = 2,6-bis(1,1-bis(2-pyridyl)ethyl)pyridine). Computational and experimental evidence suggests that the electrocatalysis consists of three distinct electrochemical reductions, which precede the onset of catalysis. Cyclic voltammetry studies indicate that the first two reductions are accompanied by protonations to afford the Mo-aqua complex, [(PY5Me2)Mo(OH2)]+. Calculations support hydrogen evolution from this complex upon the third reduction, via the oxidative addition of a proton from the bound water to the metal center and finally an α-H abstraction to release hydrogen. Calculations further suggest that introducing electron-withdrawing substituents such as fluorides in the para positions of the pyridine rings can reduce the potential associated with the reductive steps, without substantially affecting the kinetics. After the third reduction, there are kinetic bottlenecks to the formation of the Mo-hydride and subsequent hydrogen release. Computational evidence also suggests an alternative to direct α-H abstraction as a mechanism for H2 release which exhibits a lower barrier. The new mechanism is one in which a water acts as an intramolecular proton relay between the protons of the hydroxide and the hydride ligands. The calculated kinetics are in reasonable agreement with experimental measurements. Additionally, we propose a mechanism for the stoichiometric reaction of [(PY5Me2)Mo(CF3SO3)]+ with water to yield hydrogen and [(PY5Me2)MoO]2+ along with the implications for the viability of an alternate catalytic cycle involving just two reductions to generate the active catalyst.
Remarks: 跟Martin Head-Gordon、 Chris Chang和Jeff Long合作的一篇文章。主要是针对Chris Chang和Jeff Long大约两年前在Nature上发表的[(PY5Me2)MoO]2+电催化分解水产氢反应的机理研究。这个工作其实从2010年就开始做，但因为需要等一些实验数据，合作人也比较多，所以文章拖到现在才发表。
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