8 research outputs found

    A Functionally Stable Manganese Oxide Oxygen Evolution Catalyst in Acid

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    First-row metals have been a target for the development of oxygen evolution reaction (OER) catalysts because they comprise noncritical elements. We now report a comprehensive electrochemical characterization of manganese oxide (MnOx) over a wide pH range, and establish MnOx as a functionally stable OER catalyst owing to self-healing, is derived from MnOx redeposition that offsets catalyst dissolution during turnover. To study this process in detail, the oxygen evolution mechanism of MnOx was investigated electrokinetically over a pH range spanning acidic, neutral, and alkaline conditions. In the alkaline pH regime, a ∼60 mV/decade Tafel slope and inverse first-order dependence on proton concentration were observed, whereas the OER acidic pH regime exhibited a quasi-infinite Tafel slope and zeroth-order dependence on proton concentration. The results reflect two competing mechanisms: a one-electron one-proton PCET pathway that is dominant under alkaline conditions and a Mn<sup>3+</sup> disproportionation process, which predominates under acidic conditions. Reconciling the rate laws of these two OER pathways with that of MnOx electrodeposition elucidates the self-healing characteristics of these catalyst films. The intersection of the kinetic profile of deposition and that of water oxidation as a function of pH defines the region of kinetic stability for MnOx and importantly establishes that a non-noble metal oxide OER catalyst may be operated in acid by exploiting a self-healing process

    Nature of Activated Manganese Oxide for Oxygen Evolution

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    Electrodeposited manganese oxide films (MnOx) are promising stable oxygen evolution catalysts. They are able to catalyze the oxygen evolution reaction in acidic solutions but with only modest activity when prepared by constant anodic potential deposition. We now show that the performance of these catalysts is improved when they are “activated” by potential cycling protocols, as measured by Tafel analysis (where lower slope is better): upon activation the Tafel slope decreases from ∼120 to ∼70 mV/decade in neutral conditions and from ∼650 to ∼90 mV/decade in acidic solutions. Electrochemical, spectroscopic, and structural methods were employed to study the activation process and support a mechanism where the original birnessite-like MnOx (δ-MnO<sub>2</sub>) undergoes a phase change, induced by comproportionation with cathodically generated Mn­(OH)<sub>2</sub>, to a hausmannite-like intermediate (α-Mn<sub>3</sub>O<sub>4</sub>). Subsequent anodic conditioning from voltage cycling or water oxidation produces a disordered birnessite-like phase, which is highly active for oxygen evolution. At pH 2.5, the current density of activated MnOx (at an overpotential of 600 mV) is 2 orders of magnitude higher than that of the original MnOx and begins to approach that of Ru and Ir oxides in acid

    Photophysical Properties of β‑Substituted Free-Base Corroles

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    Corroles are an emergent class of fluorophores that are finding an application and reaction chemistry to rival their porphyrin analogues. Despite a growing interest in the synthesis, reactivity, and functionalization of these macrocycles, their excited-state chemistry remains undeveloped. A systematic study of the photophysical properties of β-substituted corroles was performed on a series of free-base β-brominated derivatives as well as a β-linked corrole dimer. The singlet and triplet electronic states of these compounds were examined with steady-state and time-resolved spectroscopic methods, which are complemented with density functional theory (DFT) and time-dependent DFT calculations to gain insight into the nature of the electronic structure. Selective bromination of a single molecular edge manifests in a splitting of the Soret band into <i>x</i> and <i>y</i> polarizations, which is a consequence of asymmetry of the molecular axes. A pronounced heavy atom effect is the primary determinant of the photophysical properties of these free-base corroles; bromination decreases the fluorescence quantum yield (from 15% to 0.47%) and lifetime (from 4 ns to 80 ps) by promoting enhanced intersystem crossing, as evidenced by a dramatic increase in <i>k</i><sub>nr</sub> with bromine substitution. The nonbrominated dimer exhibits absorption and emission features comparable to those of the tetrabrominated derivative, suggesting that oligomerization provides a means of red-shifting the spectral properties akin to bromination but without decreasing the fluorescence quantum yield

    Ag(III)···Ag(III) Argentophilic Interaction in a Cofacial Corrole Dyad

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    Metallophilic interactions between closed-shell metal centers are exemplified by d10 ions, with Au(I) aurophilic interactions as the archetype. Such an interaction extends to d8 species, and examples involving Au(III) are prevalent. Conversely, Ag(III) argentophilic interactions are uncommon. Here, we identify argentophilic interactions in silver corroles, which are authentic Ag(III) species. The crystal structure of a monomeric silver corrole is a dimer in the solid state, and the macrocycle exhibits an atypical domed conformation. In order to evaluate whether this represents an authentic metallophilic interaction or a crystal-packing artifact, the analogous cofacial or “pacman” corrole was prepared. The conformation of the monomer was recapitulated in the silver pacman corrole, exhibiting a short 3.67 Å distance between metal centers and a significant compression of the xanthene backbone. Theoretical calculations support the presence of a rare Ag(III)···Ag(III) argentophilic interaction in the pacman complex

    Solvent-Induced Spin-State Change in Copper Corroles

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    The electronic structure of copper corroles has been a topic of debate and revision since the advent of corrole chemistry. The ground state of these compounds is best described as an antiferromagnetically coupled Cu(II) corrole radical cation. In coordinating solvents, these molecules become paramagnetic, and this is often accompanied by a color change. The underlying chemistry of these solvent-induced properties is currently unknown. Here, we show that a coordinating solvent, such as pyridine, induces a change in the ground spin state from an antiferromagnetically coupled Cu(II) corrole radical cation to a ferromagnetically coupled triplet. Over time, the triplet reacts to produce a species with spectral signatures that are characteristic of the one-electron-reduced Cu(II) corrole. These observations account for the solvent-induced paramagnetism and the associated color changes that have been observed for copper corroles in coordinating solvents

    Properties of Site-Specifically Incorporated 3‑Aminotyrosine in Proteins To Study Redox-Active Tyrosines: <i>Escherichia coli</i> Ribonucleotide Reductase as a Paradigm

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    3-Aminotyrosine (NH<sub>2</sub>Y) has been a useful probe to study the role of redox active tyrosines in enzymes. This report describes properties of NH<sub>2</sub>Y of key importance for its application in mechanistic studies. By combining the tRNA/NH<sub>2</sub>Y-RS suppression technology with a model protein tailored for amino acid redox studies (α<sub>3</sub>X, X = NH<sub>2</sub>Y), the formal reduction potential of NH<sub>2</sub>Y<sub>32</sub>(O<sup>•</sup>/OH) (<i><i>E</i>°′</i> = 395 ± 7 mV at pH 7.08 ± 0.05) could be determined using protein film voltammetry. We find that the Δ<i><i>E</i>°′</i> between NH<sub>2</sub>Y<sub>32</sub>(O<sup>•</sup>/OH) and Y<sub>32</sub>(O<sup>•</sup>/OH) when measured under reversible conditions is ∼300–400 mV larger than earlier estimates based on irreversible voltammograms obtained on aqueous NH<sub>2</sub>Y and Y. We have also generated D<sub>6</sub>-NH<sub>2</sub>Y<sub>731</sub>-α2 of ribonucleotide reductase (RNR), which when incubated with β2/CDP/ATP generates the D<sub>6</sub>-NH<sub>2</sub>Y<sub>731</sub><sup>•</sup>-α2/β2 complex. By multifrequency electron paramagnetic resonance (35, 94, and 263 GHz) and 34 GHz <sup>1</sup>H ENDOR spectroscopies, we determined the hyperfine coupling (hfc) constants of the amino protons that establish RNH<sub>2</sub><sup>•</sup> planarity and thus minimal perturbation of the reduction potential by the protein environment. The amount of Y in the isolated NH<sub>2</sub>Y-RNR incorporated by infidelity of the tRNA/NH<sub>2</sub>Y-RS pair was determined by a generally useful LC-MS method. This information is essential to the utility of this NH<sub>2</sub>Y probe to study any protein of interest and is employed to address our previously reported activity associated with NH<sub>2</sub>Y-substituted RNRs

    Water Oxidation Catalysis by Co(II) Impurities in Co(III)<sub>4</sub>O<sub>4</sub> Cubanes

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    The observed water oxidation activity of the compound class Co<sub>4</sub>O<sub>4</sub>(OAc)<sub>4</sub>(Py–X)<sub>4</sub> emanates from a Co­(II) impurity. This impurity is oxidized to produce the well-known Co-OEC heterogeneous cobaltate catalyst, which is an active water oxidation catalyst. We present results from electron paramagnetic resonance spectroscopy, nuclear magnetic resonance line broadening analysis, and electrochemical titrations to establish the existence of the Co­(II) impurity as the major source of water oxidation activity that has been reported for Co<sub>4</sub>O<sub>4</sub> molecular cubanes. Differential electrochemical mass spectrometry is used to characterize the fate of glassy carbon at water oxidizing potentials and demonstrate that such electrode materials should be used with caution for the study of water oxidation catalysis

    Water Oxidation Catalysis by Co(II) Impurities in Co(III)<sub>4</sub>O<sub>4</sub> Cubanes

    No full text
    The observed water oxidation activity of the compound class Co<sub>4</sub>O<sub>4</sub>(OAc)<sub>4</sub>(Py–X)<sub>4</sub> emanates from a Co­(II) impurity. This impurity is oxidized to produce the well-known Co-OEC heterogeneous cobaltate catalyst, which is an active water oxidation catalyst. We present results from electron paramagnetic resonance spectroscopy, nuclear magnetic resonance line broadening analysis, and electrochemical titrations to establish the existence of the Co­(II) impurity as the major source of water oxidation activity that has been reported for Co<sub>4</sub>O<sub>4</sub> molecular cubanes. Differential electrochemical mass spectrometry is used to characterize the fate of glassy carbon at water oxidizing potentials and demonstrate that such electrode materials should be used with caution for the study of water oxidation catalysis
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