Mn oxide as a kinetically dominant “true” catalyst for water oxidation

Nature uses an Mn cluster for water oxidation, and thus, water oxidation using Mn clusters is interesting when used in artificial water-splitting systems. An important question is whether an Mn cluster is a true catalyst for water oxidation or not. Herein, an Mn–K cluster was investigated for electrochemical water oxidation to find the true and the kinetically dominant catalyst using X-ray absorption spectroscopy, scanning electron microscopy, transmission electron microscopy, Fourier transform infrared spectroscopy, X-ray diffraction, and electrochemical methods. The experiments showed that conversion into nanosized Mn oxide occurred for the cluster, and the nanosized Mn oxides are the true catalyst for water oxidation

in situ tracking of redox transitions and mode of catalysis

Water oxidation by amorphous oxides is of high interest in artificial photosynthesis and other routes towards non-fossil fuels, but the mode of catalysis in these materials is insufficiently understood. We tracked mechanistically relevant oxidation-state and structural changes of an amorphous Co-based catalyst film by in situ experiments combining directly synchrotron-based X-ray absorption spectroscopy (XAS) with electrocatalysis. Unlike a classical solid-state material, the bulk material is found to undergo chemical changes. Two redox transitions at midpoint potentials of about 1.0 V (CoII0.4CoIII0.6 ↔ all-CoIII) and 1.2 V (all-CoIII ↔ CoIII0.8CoIV0.2) vs. NHE at pH 7 are coupled to structural changes. These redox transitions can be induced by variation of either electric potential or pH; they are broader than predicted by a simple Nernstian model, suggesting interacting bridged cobalt ions. Tracking reaction kinetics by UV-Vis-absorption and time-resolved mass spectroscopy reveals that accumulated oxidizing equivalents facilitate dioxygen formation. On these grounds, a new framework model of catalysis in an amorphous, hydrated and volume-active oxide is proposed: Within the oxide film, cobalt ions at the margins of Co-oxo fragments undergo CoII ↔ CoIII ↔ CoIV oxidation-state changes coupled to structural modification and deprotonation of Co-oxo bridges. By the encounter of two (or more) CoIV ions, an active site is formed at which the O–O bond-formation step can take place. The Tafel slope is determined by both the interaction between cobalt ions (width of the redox transition) and their encounter probability. Our results represent a first step toward the development of new concepts that address the solid-molecular Janus nature of the amorphous oxide. Insights and concepts described herein for the Co-based catalyst film may be of general relevance also for other amorphous oxides with water-oxidation activity

Electrosynthesis, functional, and structural characterization of a water-oxidizing manganese oxide

Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugänglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.In the sustainable production of non-fossil fuels, water oxidation is pivotal. Development of efficient catalysts based on manganese is desirable because this element is earth-abundant, inexpensive, and largely non-toxic. We report an electrodeposited Mn oxide (MnCat) that catalyzes electrochemical water oxidation at neutral pH at rates that approach the level needed for direct coupling to photoactive materials. By choice of the voltage protocol we could switch between electrodeposition of inactive Mn oxides (deposition at constant anodic potentials) and synthesis of the active MnCat (deposition by voltage-cycling protocols). Electron microscopy reveals that the MnCat consists of nanoparticles (100 nm) with complex fine-structure. X-ray spectroscopy reveals that the amorphous MnCat resembles the biological paragon, the water-splitting Mn4Ca complex of photosynthesis, with respect to mean Mn oxidation state (ca. +3.8 in the MnCat) and central structural motifs. Yet the MnCat functions without calcium or other bivalent ions. Comparing the MnCat with electrodeposited Mn oxides inactive in water oxidation, we identify characteristics that likely are crucial for catalytic activity. In both inactive Mn oxides and active ones (MnCat), extensive di-μ-oxo bridging between Mn ions is observed. However in the MnCat, the voltage-cycling protocol resulted in formation of MnIII sites and prevented formation of well-ordered and unreactive MnIVO2. Structure–function relations in Mn-based water-oxidation catalysts and strategies to design catalytically active Mn-based materials are discussed. Knowledge-guided performance optimization of the MnCat could pave the road for its technological use.DFG, EXC 314, Unifying Concepts in CatalysisEC/FP7/212508/EU/European Solar-Fuel Initiative - Renewable Hydrogen from Sun and Water. Science Linking Molecular Biomimetics and Genetics/SOLARH

Bridging-hydride influence on the electronic structure of an [FeFe] hydrogenase active-site model complex revealed by XAES-DFT

Two crystallized [FeFe] hydrogenase model complexes, 1 = (μ-pdt)[Fe(CO)2(PMe3)]2 (pdt = SC1H2C2H2C3H2S), and their bridging-hydride (Hy) derivative, [1Hy]+++ = [(μ-H)(μ-pdt)[Fe(CO)2 (PMe3)]2]+ (BF4−), were studied by Fe K-edge X-ray absorption and emission spectroscopy, supported by density functional theory. Structural changes in [1Hy]+++ compared to 1 involved small bond elongations (<0.03 Å) and more octahedral Fe geometries; the Fe–H bond at Fe1 (closer to pdt-C2) was [similar]0.03 Å longer than that at Fe2. Analyses of (1) pre-edge absorption spectra (core-to-valence transitions), (2) Kβ1,3, Kβ′, and Kβ2,5 emission spectra (valence-to-core transitions), and (3) resonant inelastic X-ray scattering data (valence-to- valence transitions) for resonant and non-resonant excitation and respective spectral simulations indicated the following: (1) the mean Fe oxidation state was similar in both complexes, due to electron density transfer from the ligands to Hy in [1Hy]+++. Fe 1s→3d transitions remained at similar energies whereas delocalization of carbonyl AOs onto Fe and significant Hy- contributions to MOs caused an [similar]0.7 eV up-shift of Fe1s→(CO)s,p transitions in [1Hy]+++. Fed-levels were delocalized over Fe1 and Fe2 and degeneracies biased to Oh–Fe1 and C4v–Fe2 states for 1, but to Oh–Fe1,2 states for [1Hy]+++. (2) Electron-pairing of formal Fe(d7) ions in low-spin states in both complexes and a higher effective spin count for [1Hy]+++ were suggested by comparison with iron reference compounds. Electronic decays from Fe d and ligand s,p MOs and spectral contributions from Hys,p→1s transitions even revealed limited site-selectivity for detection of Fe1 or Fe2 in [1Hy]+++. The HOMO/LUMO energy gap for 1 was estimated as 3.0 ± 0.5 eV. (3) For [1Hy]+++ compared to 1, increased Fed (x2 − y2) − (z2) energy differences ([similar]0.5 eV to [similar]0.9 eV) and Fed→d transition energies ([similar]2.9 eV to [similar]3.7 eV) were assigned. These results reveal the specific impact of Hy-binding on the electronic structure of diiron compounds and provide guidelines for a directed search of hydride species in hydrogenases

Electronic and molecular structures of the active-site H-cluster in [FeFe]-hydrogenase determined by site-selective X-ray spectroscopy and quantum chemical calculations

The [FeFe]-hydrogenase (HydA1) from green algae is the minimal enzyme for efficient biological hydrogen (H2) production. Its active-site six-iron center (H-cluster) consists of a cubane, [4Fe4S]H, cysteine-linked to a diiron site, [2Fe]H. We utilized the spin-polarization of the iron Kβ X-ray fluorescence emission to perform site-selective X-ray absorption experiments for spectral discrimination of the two sub-complexes. For the H-cluster in reduced HydA1 protein, XANES and EXAFS spectra, Kβ emission lines (3p → 1s transitions), and core-to-valence (pre-edge) absorption (1s → 3d) and valence-to-core (Kβ2,5) emission (3d → 1s) spectra were obtained, individually for [4Fe4S]H and [2Fe]H. Iron–ligand bond lengths and intermetal distances in [2Fe]H and [4Fe4S]H were resolved, as well as fine structure in the high-spin iron containing cubane. Density functional theory calculations reproduced the X-ray spectral features and assigned the molecular orbital configurations, emphasizing the asymmetric d-level degeneracy of the proximal (Fep) and distal (Fed) low-spin irons in [2Fe]H in the non-paramagnetic state. This yielded a specific model structure of the H-cluster with a bridging carbon monoxide ligand and an apical open coordination site at Fed in [2Fe]H. The small HOMO–LUMO gap ([similar]0.3 eV) enables oxidation and reduction of the active site at similar potentials for reversible H2 turnover by HydA1, the LUMO spread over [4Fe4S]H supports its role as an electron transfer relay, and Fed carrying the HOMO is prepared for transient hydride binding. These features and the accessibility of Fed from the bulk phase can account for regio- specific redox transitions as well as H2-formation and O2-inhibition at the H-cluster. We provide a conceptual and experimental framework for site- selective studies on catalytic mechanisms in inhomogeneous materials

Electrochemical alcohols oxidation mediated by N-hydroxyphthalimide on nickel foam surface

Alcohol to aldehyde conversion is a critical reaction in the industry. Herein, a new electrochemical method is introduced that converts 1 mmol of alcohols to aldehydes and ketones in the presence of N-hydroxyphthalimide (NHPI, 20 mol%) as a mediator; this conversion is achieved after 8.5 h at room temperature using a piece of Ni foam (1.0 cm2) and without adding an extra-base or a need for high temperature. Using this method, 10 mmol (1.08 g) of benzyl alcohol was also successfully oxidized to benzaldehyde (91%) without any by-products. This method was also used to oxidize other alcohols with high yield and selectivity. In the absence of a mediator, the surface of the nickel foam provided oxidation products at the lower yield. After the reaction was complete, nickel foam (anode) was characterized by a combination of scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS), and spectroelectrochemistry, which pointed to the formation of nickel oxide on the surface of the electrode. On the other hand, using other electrodes such as Pt, Cu, Fe, and graphite resulted in a low yield for the alcohol to aldehyde conversion

mechanistic promiscuity in hydrogen atom abstraction reactions

In addition to oxometal [Mn+[double bond, length as m-dash]O] and imidometal [Mn+[double bond, length as m-dash]NR] units, transient metal–iodosylarene [M(n−2)+–O[double bond, length as m-dash]IPh] and metal–iminoiodane [M(n−2)+–N(R)[double bond, length as m-dash]IPh] adducts are often invoked as a possible “second oxidant” responsible for the oxo and imido group transfer reactivity. Although a few metal–iodosylarene adducts have been recently isolated and/or spectroscopically characterized, metal–iminoiodane adducts have remained elusive. Herein, we provide UV-Vis, EPR, NMR, XAS and DFT evidence supporting the formation of a metal–iminoiodane complex 2 and its scandium adduct 2-Sc. 2 and 2-Sc are reactive toward substrates in the hydrogen-atom and nitrene transfer reactions, which confirm their potential as active oxidants in metal-catalyzed oxidative transformations. Oxidation of para-substituted 2,6-di-tert-butylphenols by 2 and 2-Sc can occur by both coupled and uncoupled proton and electron transfer mechanisms; the exact mechanism depends on the nature of the para substituent

new controversies and puzzles

Nature uses an Mn cluster for water oxidation, and thus, water oxidation using Mn clusters is interesting when used in artificial water-splitting systems. An important question is whether an Mn cluster is a true catalyst for water oxidation or not. Herein, an Mn–K cluster was investigated for electrochemical water oxidation to find the true and the kinetically dominant catalyst using X-ray absorption spectroscopy, scanning electron microscopy, transmission electron microscopy, Fourier transform infrared spectroscopy, X-ray diffraction, and electrochemical methods. The experiments showed that conversion into nanosized Mn oxide occurred for the cluster, and the nanosized Mn oxides are the true catalyst for water oxidation

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

Water oxidation catalysis – role of redox and structural dynamics in biological photosynthesis and inorganic manganese oxides

Water oxidation is pivotal in biological photosynthesis, where it is catalyzed by a protein-bound metal complex with a Mn4Ca-oxide core; related synthetic catalysts may become key components in non-fossil fuel technologies. Going beyond characterization of the catalyst resting state, we compare redox and structural dynamics of three representative birnessite-type Mn(Ca) oxides (catalytically active versus inactive; with/without calcium) and the biological catalyst. In the synthetic oxides, Mn oxidation was induced by increasingly positive electrode potentials and monitored by electrochemical freeze-quench and novel time-resolved in situ experiments involving detection of X-ray absorption and UV-vis transients, complemented by electrochemical impedance spectroscopy. A minority fraction of Mn(III) ions present at catalytic potentials is found to be functionally crucial; calcium ions are inessential but tune redox properties. Redox-state changes of the water- oxidizing Mn oxide are similarly fast as observed in the biological catalyst (<10 ms), but 10–100 times slower in the catalytically inactive oxide. Surprisingly similar redox dynamics of biological catalyst and water-oxidizing Mn(Ca) oxides suggest that in both catalysts, rather than direct oxidation of bound water species, oxidation equivalents are accumulated before onset of the multi-electron O–O bond formation chemistry in Mn(III)–Mn(IV) oxidation steps coupled to changes in the oxo-bridging between metal ions. Aside from the ability of the bulk oxide to undergo Mn oxidation-state changes, we identify two further, likely interrelated prerequisites for catalytic activity of the synthetic oxides: (i) the presence of Mn(III) ions at catalytic potentials preventing formation of an inert all-Mn(IV) oxide and (ii) fast rates of redox-state changes approaching the millisecond time domain