雑誌会（200115）回答_藤田

Efficient water oxidation catalyzed by homogeneous cationic cobalt porphyrins with critical roles for the buffer base

• Dong Wang and
 
• John T. Groves

PNAS 2013 110 (39) 15579-15584; published ahead of print September 9, 2013,doi:10.1073/pnas.1315383110

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Significance

Efficient and scalable catalysts that mediate the interconversion of electrical and chemical energy are essential to the future world economy. Central to this challenge are robust systems that can split water into its elements, hydrogen and oxygen, with minimal cost. In this article, we describe the discovery and characterization of an efficient, single-site homogeneous water oxidation catalyst based on a cationic cobalt-porphyrin system. Important roles of the buffer base in this catalysis are also described. Our results shed light on the design of next-generation water oxidation catalysts using earth-abundant transition metals and redox active ligand systems.

Abstract

A series of cationic cobalt porphyrins was found to catalyze electrochemical water oxidation to O₂ efficiently at room temperature in neutral aqueous solution. Co–5,10,15,20-tetrakis-(1,3-dimethylimidazolium-2-yl)porphyrin, with a highly electron-deficient meso-dimethylimidazolium porphyrin, was the most effective catalyst. The O₂ formation rate was 170 nmol⋅cm⁻²⋅min⁻¹ (k_obs = 1.4 × 10³ s⁻¹) with a Faradaic efficiency near 90%. Mechanistic investigations indicate the generation of a Co^IV-O porphyrin cation radical as the reactive oxidant, which has accumulated two oxidizing equivalents above the Co^III resting state of the catalyst. The buffer base in solution was shown to play several critical roles during the catalysis by facilitating both redox-coupled proton transfer processes leading to the reactive oxidant and subsequent O–O bond formation. More basic buffer anions led to lower catalytic onset potentials, extending below 1 V. This homogeneous cobalt-porphyrin system was shown to be robust under active catalytic conditions, showing negligible decomposition over hours of operation. Added EDTA or ion exchange resin caused no catalyst poisoning, indicating that cobalt ions were not released from the porphyrin macrocycle during catalysis. Likewise, surface analysis by energy dispersive X-ray spectroscopy of the working electrodes showed no deposition of heterogeneous cobalt films. Taken together, the results indicate that Co–5,10,15,20-tetrakis-(1,3-dimethylimidazolium-2-yl)porphyrin is an efficient, homogeneous, single-site water oxidation catalyst.

• electrocatalysis
 
• oxygen
 
• photosynthesis
 
• high valent
 
• single site