Abstract
Iron alloying of oxidic cobaltate catalysts results in catalytic activity for oxygen evolution on par with Ni-Fe oxides in base but at much higher alloying compositions. Zero-field
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Fe ...Mössbauer spectroscopy and X-ray absorption spectroscopy (XAS) are able to clearly identify Fe
4+
in mixed-metal Co-Fe oxides. The highest Fe
4+
population is obtained in the 40–60% Fe alloying range, and XAS identifies the ion residing in an octahedral oxide ligand field. The oxygen evolution reaction (OER) activity, as reflected in Tafel analysis of CoFeO
x
films in 1 M KOH, tracks the absolute concentration of Fe
4+
. The results reported herein suggest an important role for the formation of the Fe
4+
redox state in activating cobaltate OER catalysts at high iron loadings.
Iron doping of nickel oxide films results in enhanced activity for promoting the oxygen evolution reaction (OER). Whereas this enhanced activity has been ascribed to a unique iron site within the ...nickel oxide matrix, we show here that Fe doping influences the Ni valency. The percent of Fe3+ doping promotes the formation of formal Ni4+, which in turn directly correlates with an enhanced activity of the catalyst in promoting OER. The role of Fe3+ is consistent with its behavior as a superior Lewis acid.
Photosynthetic antenna complexes capture and concentrate solar radiation by transferring the excitation to the reaction center that stores energy from the photon in chemical bonds. This process ...occurs with near-perfect quantum efficiency. Recent experiments at cryogenic temperatures have revealed that coherent energy transfer—a wave-like transfer mechanism—occurs in many photosynthetic pigment-protein complexes. Using the Fenna–Matthews–Olson antenna complex (FMO) as a model system, theoretical studies incorporating both incoherent and coherent transfer as well as thermal dephasing predict that environmentally assisted quantum transfer efficiency peaks near physiological temperature; these studies also show that this mechanism simultaneously improves the robustness of the energy transfer process. This theory requires long-lived quantum coherence at room temperature, which never has been observed in FMO. Here we present evidence that quantum coherence survives in FMO at physiological temperature for at least 300 fs, long enough to impact biological energy transport. These data prove that the wave-like energy transfer process discovered at 77 K is directly relevant to biological function. Microscopically, we attribute this long coherence lifetime to correlated motions within the protein matrix encapsulating the chromophores, and we find that the degree of protection afforded by the protein appears constant between 77 K and 277 K. The protein shapes the energy landscape and mediates an efficient energy transfer despite thermal fluctuations.
Earth-abundant oxygen evolution catalysts (OECs) with extended stability in acid can be constructed by embedding active sites within an acid-stable metal-oxide framework. Here, we report stable ...NiPbOₓ films that are able to perform oxygen evolution reaction (OER) catalysis for extended periods of operation (>20 h) in acidic solutions of pH 2.5; conversely, native NiOₓ catalyst films dissolve immediately. In situ X-ray absorption spectroscopy and ex situ X-ray photoelectron spectroscopy reveal that PbO₂ is unperturbed after addition of Ni and/or Fe into the lattice, which serves as an acid-stable, conductive framework for embedded OER active centers. The ability to perform OER in acid allows the mechanism of Fe doping on Ni catalysts to be further probed. Catalyst activity with Fe doping of oxidic Ni OEC under acid conditions, as compared to neutral or basic conditions, supports the contention that role of Fe3+ in enhancing catalytic activity in Ni oxide catalysts arises from its Lewis acid properties.
The formation of high-valent states is a key factor in making highly active transition-metal-based catalysts of the oxygen evolution reaction (OER). These high oxidation states will be strongly ...influenced by the local geometric and electronic structures of the metal ion, which are difficult to study due to spectroscopically active and complex backgrounds, short lifetimes, and limited concentrations. Here, we use a wide range of complementary X-ray spectroscopies coupled to DFT calculations to study Co(III)4O4 cubanes and their first oxidized derivatives, which provide insight into the high-valent Co(IV) centers responsible for the activity of molecular and heterogeneous OER catalysts. The combination of X-ray absorption and 1s3p resonant inelastic X-ray scattering (Kβ RIXS) allows Co(IV) to be isolated and studied against a spectroscopically active Co(III) background. Co K- and L-edge X-ray absorption data allow for a detailed characterization of the 3d-manifold of effectively localized Co(IV) centers and provide a direct handle on the t2g-based redox-active molecular orbital. Kβ RIXS is also shown to provide a powerful probe of Co(IV), and specific spectral features are sensitive to the degree of oxo-mediated metal–metal coupling across Co4O4. Guided by the data, calculations show that electron–hole delocalization can actually oppose Co(IV) formation. Computational extension of Co4O4 to CoM3O4 structures (M = redox-inactive metal) defines electronic structure contributions to Co(IV) formation. Redox activity is shown to be linearly related to covalency, and M(III) oxo inductive effects on Co(IV) oxo bonding can tune the covalency of high-valent sites over a large range and thereby tune E 0 over hundreds of millivolts. Additionally, redox-inactive metal substitution can also switch the ground state and modify metal–metal and antibonding interactions across the cluster.
We extend traditional two-dimensional (2D) electronic spectroscopy into a third Fourier dimension without the use of additional optical interactions. By acquiring a set of 2D spectra evenly spaced in ...waiting time and dividing the area of the spectra into voxels, we can eliminate population dynamics from the data and transform the waiting time dimension into frequency space. The resultant 3D spectrum resolves quantum beating signals arising from excitonic coherences along the waiting frequency dimension, thereby yielding up to 2
n-fold redundancy in the set of frequencies necessary to construct a complete set of
n excitonic transition energies. Using this technique, we have obtained, to our knowledge, the first fully experimental set of electronic eigenstates for the Fenna-Matthews-Olson (FMO) antenna complex, which can be used to improve theoretical simulations of energy transfer within this protein. Whereas the strong diagonal peaks in the 2D rephasing spectrum of the FMO complex obscure all but one of the crosspeaks at 77 K, extending into the third dimension resolves 19 individual peaks. Analysis of the independently collected nonrephasing data provides the same information, thereby verifying the calculated excitonic transition energies. These results enable one to calculate the Hamiltonian of the FMO complex in the site basis by fitting to the experimental linear absorption spectrum.
Substitution of one pyridine by pyrazine in each DPA appendage of ZP1 leads to a new zinc sensor, ZPP1, with a modified background fluorescence and zinc affinity. ZPP1 exhibits a two-step zinc ...response during fluorescence titrations, which leads to a new method for zinc quantification. The ability of ZPP1 to image and quantify zinc was demonstrated in pancreatic Min6 cells.
The nature of the photoexcited state of octabutoxy nickel(II) phthalocyanine (NiPcOBu
) with a 500 ps lifetime was investigated by X-ray transient absorption (XTA) spectroscopy. Previous optical, ...vibrational, and computational studies have suggested that this photoexcited state has a ligand-to-metal charge transfer (LMCT) nature. By using XTA, which provides unambiguous information on the local electronic and nuclear configuration around the Ni center, the nature of the excited state of NiPcOBu
was reassessed. Using X-ray probe pulses from a synchrotron source, the ground- and excited-state X-ray absorption spectra of NiPcOBu
were measured. Based on the results, we identified that the excited state exhibits spectral features that are characteristic of a Ni
(3dz2 ,3dx2-y2 ) state rather than a LMCT state with a transiently reduced Ni center. This state resembles the (d,d) state of nickel(II) tetramesitylphorphyrin. The XTA features are rationalized based on the inherent cavity sizes of the macrocycles. These results may provide useful guidance for the design of photocatalysts in the future.
Following the observation of long-lived coherences in the two-dimensional (2D) electronic spectra of the Fenna-Matthews-Olson (FMO) complex, many theoretical works suggest that coherences between ...excitons may play a role in the efficient energy transfer that occurs in photosynthetic antennae. This interpretation of the dynamics depends on the assignment of quantum beating signals to superpositions of excitons, which is complicated by the possibility of observing both electronic and vibrational coherences in 2D spectra. Here, we explore 2D spectra of bacteriochlorophyll a (BChla) in solution in an attempt to isolate vibrational beating signals in the absence of excitonic signals to identify the origin of the quantum beats in 2D spectra of FMO. Even at high laser power, our BChla spectra show strong beating only from the nonresonant response of the solvent. The beating signals that we can conclusively assign to vibrational modes of BChla are only slightly above the noise and at higher frequencies than those previously observed in spectra of FMO. Our results suggest that the beating observed in spectra of FMO is of a radically different character than the signals observed here and can therefore be attributed to electronic coherences or intermolecular degrees of freedom.
The nature of the photoexcited state of octabutoxy nickel(II) phthalocyanine (NiPcOBu8) with a 500 ps lifetime was investigated by X‐ray transient absorption (XTA) spectroscopy. Previous optical, ...vibrational, and computational studies have suggested that this photoexcited state has a ligand‐to‐metal charge transfer (LMCT) nature. By using XTA, which provides unambiguous information on the local electronic and nuclear configuration around the Ni center, the nature of the excited state of NiPcOBu8 was reassessed. Using X‐ray probe pulses from a synchrotron source, the ground‐ and excited‐state X‐ray absorption spectra of NiPcOBu8 were measured. Based on the results, we identified that the excited state exhibits spectral features that are characteristic of a Ni1, 3(3dz2
,3dx2-y2
) state rather than a LMCT state with a transiently reduced Ni center. This state resembles the (d,d) state of nickel(II) tetramesitylphorphyrin. The XTA features are rationalized based on the inherent cavity sizes of the macrocycles. These results may provide useful guidance for the design of photocatalysts in the future.
Change of state! X‐ray transient absorption (XTA) spectroscopy is used to investigate the long‐lived (500 ps) photoexcited state of octabutoxy nickel(II) phthalocyanine (NiPcOBu8). The X‐ray absorption spectrum of the excited state shows spectral features that are indicative of a Ni1, 3(3dz2
,3dx2-y2
) state rather than a ligand‐to‐metal charge transfer (LMCT) state with a transiently reduced Ni center.
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