Linear Free Energy Relationships in the Hydrogen Evolution Reaction: Kinetic Analysis of a Cobaloxime Catalyst

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• 作者：Eric S. Rountree ; Daniel J. Martin ; Brian D. McCarthy ; Jillian L. Dempsey
• 刊名：ACS Catalysis
• 出版年：2016
• 出版时间：May 6, 2016
• 年：2016
• 卷：6
• 期：5
• 页码：3326-3335
• 全文大小：549K
• 年卷期：0
• ISSN：2155-5435

文摘

Kinetic analysis of hydrogen production catalyzed by Co(dmgBF₂)₂(CH₃CN)₂ (dmgBF₂ = difluoroboryl-dimethylglyoxime) was performed in acetonitrile with a series of para-substituted anilinium acids. It was determined that the mechanism of hydrogen evolution is governed by three elementary steps; two are acid concentration and pK_a dependent, whereas the third was shown to be intrinsic to the catalyst, likely reflecting either H–H bond formation or H₂ release. The kinetics of the first proton transfer step, the protonation of the singly reduced catalyst, were evaluated using foot-of-the-wave analysis, as well as current–potential analysis for voltammograms displaying total catalysis behavior. Analysis of the total catalysis peak shift required the empirical determination of a new equation for the ECEC′ catalytic mechanism using digital simulations. The kinetics of the second proton transfer step—assigned to protonation of the doubly reduced, singly protonated species—and the acid-independent step were determined by analyzing the plateau current of the catalytic wave over a range of acid concentrations. Both proton transfer steps follow linear free energy relationships of log(k) vs acid pK_a. These linear relationships give slopes of −0.94 and −0.77 for the first and second proton transfers, respectively, indicating that both steps become faster with increasing acid strength.