Innovative Insights into Water-Oxidation Mechanism: Investigating Birnessite's Reaction with Cerium(IV) Ammonium Nitrate.

Developing Mn-based water-oxidation reaction (WOR) catalysts is key for renewable energy storage, utilizing Mn's abundance, cost-effectiveness, and natural role. Cerium(IV) ammonium nitrate (CAN) has been widely utilized as a sacrificial oxidant in the exploration of WOR catalysts. In this study, advanced techniques, such as X-ray absorption spectroscopy (XAS), in situ Raman spectroscopy, and in situ electron paramagnetic resonance (EPR), to delve into the WOR facilitated by CAN and birnessite were employed. XANES analysis has demonstrated that the average oxidation states (AOSs) of Mn in birnessite, a birnessite/CAN mixture, and in the birnessite/CAN mixture postwater addition are 3.7, 3.8, and 3.9, respectively. In situ Raman spectroscopy performed in the presence of birnessite and CAN revealed a distinct peak at 784 cm-1, which is attributed to Mn(IV)═O. A shift of this peak to 769 cm-1 in H218O confirms its association with Mn(IV)═O. No change in this peak was observed in D2O, further supporting the notion that it is linked to Mn(IV)═O rather than Mn-OH (D). Furthermore, EPR spectroscopy shows the presence of Mn(IV). It is suggested that the WOR mechanism initiates with the oxidation of birnessite by CAN, which enhances the concentration of Mn(IV) sites in the birnessite structure. Under acidic conditions, birnessite, enriched in Mn(IV), facilitates oxygen evolution and subsequently transitions into a form with reduced Mn(IV) levels. This process highlights the critical function of the Mn (hydr)oxide structure, similar to its role in the water-oxidizing complex of Photosystem II, where it serves as charge storage for oxidizing equivalents from CAN, paving the way for a four-electron reaction that drives the WOR.

Role of decomposition products in the oxidation of cyclohexene using a manganese(III) complex.

Metal complexes are extensively explored as catalysts for oxidation reactions; molecular-based mechanisms are usually proposed for such reactions. However, the roles of the decomposition products of these materials in the catalytic process have yet to be considered for these reactions. Herein, the cyclohexene oxidation in the presence of manganese(III) 5,10,15,20-tetra(4-pyridyl)-21H,23H-porphine chloride tetrakis(methochloride) (1) in a heterogeneous system via loading the complex on an SBA-15 substrate is performed as a study case. A molecular-based mechanism is usually suggested for such a metal complex. Herein, 1 was selected and investigated under the oxidation reaction by iodosylbenzene or (diacetoxyiodo)benzene (PhI(OAc)2). In addition to 1, at least one of the decomposition products of 1 formed during the oxidation reaction could be considered a candidate to catalyze the reaction. First-principles calculations show that Mn dissolution is energetically feasible in the presence of iodosylbenzene and trace amounts of water.

A copper(II) coordination compound under water-oxidation reaction at neutral conditions: decomposition on the counter electrode.

In the context of energy storage, the oxygen-evolution reaction (OER, 2H2O → O2 + 4H+ + 4e-) through the water-oxidation reaction is a thermodynamically uphill reaction in overall water splitting. In recent years, copper(II) coordination compounds have been extensively used for the OER. However, challenges remain in finding the mechanism of the OER in the presence of these metal coordination compounds. Herein, the electrochemical OER activity is investigated in the presence of a copper(II) coordination compound at pH ≈ 7. While the investigations on finding true catalysts for the OER are focused on the working electrode, herein, for the first time, the focus is on the decomposition of copper(II) coordination compound (CuL3, L: 2,2'-bipyridine N,N'-dioxide) during the OER on the counter electrode toward the precipitation of copper(I) oxide and metallic Cu.

Mechanistic Understanding of Water Oxidation in the Presence of a Copper Complex by In Situ Electrochemical Liquid Transmission Electron Microscopy.

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.

Ultra-small and highly dispersive iron oxide hydroxide as an efficient catalyst for oxidation reactions: a Swiss-army-knife catalyst.

Ultra-small and highly dispersive (< 10 nm) iron oxide hydroxide is characterized by some methods. The compound is an efficient and stable catalyst for alcohol oxidation, organic sulfide oxidation, and epoxidation of alkenes in the presence of H2O2. The electrochemical oxygen-evolution reaction of the iron oxide hydroxide is also tested under acidic, neutral, and alkaline conditions. In the presence of the iron oxide hydroxide, excellent conversions (75-100%) and selectivities of substrates (92-97%), depending on the nature of the sulfide, were obtained. Benzylalcohols having electron-donating and-withdrawing substituents in the aromatic ring were oxidized to produce the corresponding aldehydes with excellent conversion (65-89%) and selectivity (96-100%) using this iron oxide hydroxide. The conversion of styrene and cyclooctene toward the epoxidation in the presence of this catalyst are 60 and 53%, respectively. Water oxidation for the catalysts was investigated at pH 2, 6.7, 12, and 14. The onset of OER at pH 14 is observed with a 475 mV overpotential. At 585 mV overpotential, a current density of more than 0.18 mA/cm2 and a turnover frequency of 1.5/h is observed. Operando high-resolution visible spectroscopy at pH 14, similar to previously reported investigations, shows that Fe(IV)=O is an intermediate for water oxidation.

Revisiting Metal-Organic Frameworks for Oxygen Evolution: A Case Study.

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.

A synthetic manganese-calcium cluster similar to the catalyst of Photosystem II: challenges for biomimetic water oxidation.

Herein, we report the synthesis, characterization, crystal structure, density functional theory calculations, and water-oxidizing activity of a pivalate Mn-Ca cluster. All of the manganese atoms in the cluster are Mn(iv) ions and have a distorted MnO6 octahedral geometry. Three Mn(iv) ions together with a Ca(ii) ion and four-oxido groups form a cubic Mn3CaO4 unit which is similar to the Mn3CaO4 cluster in the water-oxidizing complex of Photosystem II. Using scanning electron microscopy, transmission electron microscopy, energy dispersive spectrometry, extended X-ray absorption spectroscopy, chronoamperometry, and electrochemical methods, a conversion into nano-sized Mn-oxide is observed for the cluster in the water-oxidation reaction.

A manganese(ii) phthalocyanine under water-oxidation reaction: new findings.

Phthalocyanines are a promising class of ligands for manganese because of their high binding affinity. This effect is suggested to be an important factor because phthalocyanines tightly bind manganese and stabilize it under moderate conditions. The strong donor power of phthalocyanine is also suggested as a critical factor to stabilize high-valent manganese phthalocyanine. Herein, a manganese(ii) phthalocyanine, which is stable under moderate conditions, was investigated under harsh electrochemical water oxidation. By scanning electron microscopy, transmission electron microscopy, energy dispersive spectrometry, X-ray diffraction, extended X-ray absorption fine structure analysis, X-ray absorption near edge structure analysis, chronoamperometry, magnetic measurements, Fourier-transform infrared spectroscopy, and electrochemical methods, it is shown that manganese phthalocyanine, a known molecular complex showing good stability under moderate conditions, could not withstand water oxidation catalysis and ultimately is altered to form catalytic oxide particles. Such nanosized Mn oxides are the true catalyst for water oxidation. Besides, we try to go a step forward to find an answer as to how Mn oxides form on the surface of the electrode.

Correction: Water oxidation by a copper(ii) complex: new findings, questions, challenges and a new hypothesis.

Correction for 'Water oxidation by a copper(ii) complex: new findings, questions, challenges and a new hypothesis' by Mohammad Mahdi Najafpour et al., Dalton Trans., 2018, 47, 9021-9029.

Water oxidation by a copper(ii) complex: new findings, questions, challenges and a new hypothesis.

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.