The design of molecular oxygen-evolution reaction (OER) catalysts requires fundamental mechanistic studies on their widely unknown mechanisms of action. To this end, copper complexes keep attracting interest as good catalysts for the OER, and metal complexes with TMC (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) stand out as active OER catalysts. A mononuclear copper complex, [Cu­(TMC)­(H2O)]­(NO3)2 (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane), combined both key features and was previously reported to be one of the most active copper-complex-based catalysts for electrocatalytic OER in neutral aqueous solutions. However, the functionalities and mechanisms of the catalyst are still not fully understood and need to be clarified with advanced analytical studies to enable further informed molecular catalyst design on a larger scale. Herein, the role of nanosized Cu oxide particles, ions, or clusters in the electrochemical OER with a mononuclear copper­(II) complex with TMC was investigated by operando methods, including in situ vis-spectroelectrochemistry, in situ electrochemical liquid transmission electron microscopy (EC-LTEM), and extended X-ray absorption fine structure (EXAFS) analysis. These combined experiments showed that Cu oxide-based nanoparticles, rather than a molecular structure, are formed at a significantly lower potential than required for OER and are candidates for being the true OER catalysts. Our results indicate that for the OER in the presence of a homogeneous metal complex-based (pre)­catalyst, careful analyses and new in situ protocols for ruling out the participation of metal oxides or clusters are critical for catalyst development. This approach could be a roadmap for progress in the field of sustainable catalysis via informed molecular catalyst design. Our combined approach of in situ TEM monitoring and a wide range of complementary spectroscopic techniques will open up new perspectives to track the transformation pathways and true active species for a wide range of molecular catalysts.

Copper(ii) complexes are very promising catalysts for water oxidation. Herein new findings on the water-oxidizing activity of a few copper(ii) complexes under water oxidation conditions are reported. Copper compounds in this study are copper(ii) phthalocyanine-3,4',4'',4'''-tetrasulfonic acid tetrasodium salt (1), the product from the hydrolysis of Cu(ii)tptz(H2O)(CH3COO)2 (tptz: 2,4,6-tris(2-pyridyl)-s-triazine) (2), Cu(ii)(phen)(CH3CN)2(ClO4)2 (3), Cu(ii)(phen)2(CH3CN)(ClO4)2 (4), and copper(ii) sulfate pentahydrate (Cu(ii) salt), which were investigated in the context of the water oxidation reaction by electrochemical and related methods. The experiments showed that among these compounds at pH = 11, only Cu(ii) salt and 3 led to immediate water oxidation. On the other hand, for stable complexes 1, 2 and 4 even after a few minutes low water oxidation rates were observed. The role of nanosized particles of Cu oxide or Cu ions in electrochemical water oxidation was investigated. Under the water oxidation conditions, the electrode, Cu(ii) complexes and Cu(ii) salt were studied and a relationship between the stability of the Cu(ii) complex and water oxidation rate was suggested. It is proposed that Cu(ii) ions or clusters, rather than the starting copper(ii) complex or copper(ii) oxide, are the true catalysts for the investigated water oxidation process in short-term amperometry. For 3 and in long-term amperometry, CuOx was detected. The experiments showed that a molecular mechanism for the water oxidation reaction in the presence of copper(ii) complexes should be carefully analyzed to verify the role of copper ions or cluster formation in the water oxidation reaction.

Water splitting is a promising reaction for storing sustainable but intermittent energies. In water splitting, water oxidation is a bottleneck, and thus different catalysts have been synthesized for water oxidation. Metal−organic frameworks (MOFs) are among the highly efficient catalysts for water oxidation, and so far, MOF-based catalysts have been divided into two categories: MOF-derived catalysts and direct MOF catalysts. In particular, a nickel/cobalt MOF is reported to be one of the best direct catalysts for water oxidation. For the first-row transition MOF structures in general, a hypothesis is that the harsh conditions of OER could cause the decomposition of organic ligands and the formation of water-oxidizing oxide-based structures. By electrochemical methods, scanning electron microscopy, transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy, and X-ray absorption spectroscopy, a nickel/cobalt MOF known to be a highly efficient catalyst for water oxidation is shown to form Ni/ Co oxide, making it a candidate catalyst for oxygen evolution. MOFs are interesting precatalysts for metal oxide water-oxidizing catalysts, but control experiments are necessary for determining whether a certain MOF or other MOFs are true catalysts for OER. Thus, finding a true and direct MOF electrocatalyst for OER is a challenge.

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The decomposition reaction for a manganese complex under water oxidation was investigated.