8 research outputs found
A Functionally Stable Manganese Oxide Oxygen Evolution Catalyst in Acid
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
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
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
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
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
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
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
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