Four-electron reduction of O₂ over multiple Cu^I centers: Quantum theory

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• 作者：Ellen Vayner ; Hannes Schweiger ; Alfred B. Anderson
• 关键词：Oxygen reduction ; Quantum theory ; Mechanisms ; Cu^I centers
• 刊名：Journal of Electroanalytical Chemistry
• 出版年：2007
• 期刊代码：27_00220728
• 类别：ph
• 出版时间：1 September 2007
• 卷：607
• 期：1-2
• 页码：90-100
• 文件大小：1189 K

摘要

A series of copper complexes were tested theoretically for oxygen electro reduction, beginning with bare Cu and Cu⁺ centers, with and without N-bearing ligands, and ending with one, two, and three H₂N–Cu–Imidazole Cu^I centers. Adsorption energies for reaction intermediates formed during the four one-electron reduction steps to water were calculated, using B3LYP hybrid density functional theory, and then used in a linear free energy relationship to predict the reversible potential for each step. Cu^I sites not poisoned by O(ads), OH(ads), and H₂O were sought, and so initial screening was based on the predicted reversible potentials for O(ads) reduction to OH(ads) and OH(ads) reduction to H₂O and the adsorption bond strengths of O₂ and H₂O. The Cu^I centers in H₂N–Cu–Imidazole and H₂N–Cu–Imidazole had the best properties in this screening. The former was used to model four-electron reduction by copper laccases, some of which have very little overpotential. On the basis of calculations on a model for 3-Cu^I catalytic sites composed of three H₂N–Cu–Imidazole in loose association, the following conclusions were reached: (i) the reduction potential for OH bonded to the model is higher than calculated for Pt, which is consistent with the higher observed overpotentials for Pt compared to lacasses; (ii) H₂O bonds weakly to the Cu^I centers and does not poison them; (iii) model-dependent heat losses were calculated for non-electron transfer steps and it is shown how they contribute to the overpotential for the overall four-electron reduction. Finally, it is shown that CH₃S–Cu–(Imidazole)₂ possesses the electron donor–acceptor properties that allow it to be an intermediate in electron transfer to the catalytic site. This study shows that loosely coordinated Cu^I centers present opportunities for four-electron oxygen reduction at low overpotential.