Reinvestigation of Water Oxidation Catalyzed by a Dinuclear Cobalt Polypyridine Complex: Identification of CoO_x as a Real Heterogeneous Catalyst

Abstract

Recently, a dinuclear cobalt complex, [(TPA)Co III (µ-OH)(µ-O 2 )Co III (TPA)] (ClO 4 ) 3 (1, TPA = tris(2-pyridylmethyl)amine), has been reported as a homogeneous catalyst for electrochemical and photochemical water oxidation (Wang et al. Angew. Chem. Int. Ed. 2014, 53, 14499.). During the re-investigation of the reported water oxidation catalyst (WOC) of 1, several characterizations such as EDTA and bipyridine titrations, electrochemistry, SEM, EDX, ICP-AES, TEM, XPS and UV-vis spectroscopy have revealed that the water oxidation may happen due to the formation of CoOx as a real heterogeneous WOC, and 1 itself lacks the ability to catalyze water oxidation. This manuscript presents a practical and simple procedure to clarify whether the water oxidation is catalyzed really by a molecular catalyst or not.

Recent Advances in First‐Row Transition Metal Clusters for Photocatalytic Water Splitting

“…Cobalt‐based cluster as molecular water oxidation catalysts have been studied widely because of the characterization of cheapness and easily procurable to different oxidation state of cobalt . In 2010, one molecular cobalt catalyst, [Co 4 (H 2 O) 2 (PW 9 O 34 ) 2 ] 10− , which contains two polyhedral [PW 9 O 34 ] units symmetrically attached a planar Co 4 O 16 core, was reported by Hill's group .…”

“…Cobalt-based cluster as molecular water oxidation catalysts have been studied widely because of the characterization of cheapness and easily procurable to different oxidation state of cobalt. [36][37][38][39][40][41][42] Hill's group. [43][44] It was a soluble molecular catalyst based on noble-metal-free metals.…”

Electronic π-Delocalization Boosts Catalytic Water Oxidation by Cu(II) Molecular Catalysts Heterogenized on Graphene Sheets

“…They usually get deactivated when immobilized on electrode surfaces and they suffer from instability due to hydrolytic behavior and decomposition into metal-oxides upon oxidation of the organic ligands. 3 However, from an engineering perspective, solid-state electroanodes are desired due to the simplicity of assembly for potential devices. Therefore, it is imperative to understand the influence of the anchoring functionality on the performance of the immobilized catalysts to learn how to anchor and stabilize functional molecular catalysts on electrode surfaces.…”

Homogeneous Electrocatalytic Water Oxidation by a Rigid Macrocyclic Copper(II) Complex

“…As a result, no catalytic current was observed compared to the background with a freshly polished GC electrode ( Figure S9), further demonstrating the electrocatalysis of 1 is homogeneous. [48] Additionally, adding one equivalent of bipyridine in the CV solution of 1.0 mmol•L 1 1 did not cause the current decrease, demonstrating the catalytic water oxidation is due to the complex rather than the free copper ions ( Figure S10). [48] The SEM images ( Figure S11) and EDX measurement ( Figure S12) on the GC plate after CPE revealed that only trace of copper species was found on the electrode surface.…”

“…[48] Additionally, adding one equivalent of bipyridine in the CV solution of 1.0 mmol•L 1 1 did not cause the current decrease, demonstrating the catalytic water oxidation is due to the complex rather than the free copper ions ( Figure S10). [48] The SEM images ( Figure S11) and EDX measurement ( Figure S12) on the GC plate after CPE revealed that only trace of copper species was found on the electrode surface. The Cu residue on the GC electrode might originate from trace amount of 1 absorbed on the surface of working electrode or inactive copper phosphate.…”