The binding energy between the catalyst and the reactant is considered to be the primary descriptor of catalytic activity. Therefore, identifying the optimum binding energy that would yield maximum ...activity is fundamentally important for the development of efficient catalysts. Here, we show analytically how the binding energy that maximizes the activity at large reaction rates, i.e., the operating conditions of catalysis, may deviate from the traditional understanding obtained near equilibrium. This shift can be on the order of 0.5 eV, which is easily sufficient for the optimum material to change. This binding energy shift is consistent with experimental observations in the literature, suggesting that a reinvestigation of materials previously considered to be inactive may be necessary.
Efficient and low-cost electrocatalysts for the oxygen evolution reaction are essential components of renewable energy technologies, such as solar fuel synthesis and providing a hydrogen source for ...powering fuel cells. Here we report that the nitrogen-doped carbon materials function as the efficient oxygen evolution electrocatalysts. In alkaline media, the material generated a current density of 10 mA cm(-2) at the overpotential of 0.38 V, values that are comparable to those of iridium and cobalt oxide catalysts and are the best among the non-metal oxygen evolution electrocatalyst. The electrochemical and physical studies indicate that the high oxygen evolution activity of the nitrogen/carbon materials is from the pyridinic-nitrogen- or/and quaternary-nitrogen-related active sites. Our findings suggest that the non-metal catalysts will be a potential alternative to the use of transition metal-based oxygen evolution catalysts.
The development of Mn-oxide electrocatalysts for the oxidation of H2O to O2 has been the subject of intensive researches not only for their importance as components of artificial photosynthetic ...systems, but also as O2-evolving centers in photosystem II. However, limited knowledge of the mechanisms underlying this oxidation reaction hampers the ability to rationally design effective catalysts. Herein, using in situ spectroelectrochemical techniques, we demonstrate that the stabilization of surface-associated intermediate Mn3+ species relative to charge disproportionation is an effective strategy to lower the overpotential for water oxidation by MnO2. The formation of N–Mn bonds via the coordination of amine groups of poly(allylamine hydrochloride) to the surface Mn sites of MnO2 electrodes effectively stabilized the Mn3+ species, resulting in an ∼500-mV negative shift of the onset potential for the O2 evolution reaction at neutral pH.
Efficient, earth‐abundant, and acid‐stable catalysts for the oxygen evolution reaction (OER) are missing pieces for the production of hydrogen via water electrolysis. Here, we report how the ...limitations on the stability of 3d‐metal materials can be overcome by the spectroscopic identification of stable potential windows in which the OER can be catalyzed efficiently while simultaneously suppressing deactivation pathways. We demonstrate the benefits of this approach using gamma manganese oxide (γ‐MnO2), which shows no signs of deactivation even after 8000 h of electrolysis at a pH of 2. This stability is vastly superior to existing acid‐stable 3d‐metal OER catalysts, but cannot be realized if there is a deviation as small as 50‐mV from the stable potential window. A stable voltage efficiency of over 70 % in a polymer–electrolyte membrane (PEM) electrolyzer further verifies the availability of this approach and showcases how materials previously perceived to be unstable may have potential application for water electrolysis in an acidic environment.
Window of opportunity: Spectroscopic measurements allowed the identification of a stable potential window in which γ‐MnO2 is able to catalyze the oxygen evolution reaction under acidic conditions for more than 8000 hours. This shows how the limitations on the stability of 3d‐metal materials acting as electrocatalysts can be overcome.
Extracellular redox-active compounds, flavins and other quinones, have been hypothesized to play a major role in the delivery of electrons from cellular metabolic systems to extracellular insoluble ...substrates by a diffusion-based shuttling two-electron-transfer mechanism. Here we show that flavin molecules secreted by Shewanella oneidensis MR-1 enhance the ability of its outer-membrane c -type cytochromes (OM c- Cyts) to transport electrons as redox cofactors, but not free-form flavins. Whole-cell differential pulse voltammetry revealed that the redox potential of flavin was reversibly shifted more than 100 mV in a positive direction, in good agreement with increasing microbial current generation. Importantly, this flavin/OM c- Cyts interaction was found to facilitate a one-electron redox reaction via a semiquinone, resulting in a 10 ³- to 10 ⁵-fold faster reaction rate than that of free flavin. These results are not consistent with previously proposed redox-shuttling mechanisms but suggest that the flavin/OM c- Cyts interaction regulates the extent of extracellular electron transport coupled with intracellular metabolic activity.
Current high‐performance thermoelectric materials require elaborate doping and synthesis procedures, particularly in regard to the artificial structure, and the underlying thermoelectric mechanisms ...are still poorly understood. Here, we report that a natural chalcopyrite mineral, Cu1+xFe1−xS2, obtained from a deep‐sea hydrothermal vent can directly generate thermoelectricity. The resistivity displayed an excellent semiconducting character, and a large thermoelectric power and high power factor were found in the low x region. Notably, electron–magnon scattering and a large effective mass was detected in this region, thus suggesting that the strong coupling of doped carriers and antiferromagnetic spins resulted in the natural enhancement of thermoelectric properties during mineralization reactions. The present findings demonstrate the feasibility of thermoelectric energy generation and electron/hole carrier modulation with natural materials that are abundant in the Earth’s crust.
It came from the deep: A chalcopyrite mineral from a deep‐sea hydrothermal vent has been found to have a large thermoelectric power and a high power factor, in which electron–magnon scattering and a large effective mass were detected. These findings highlight the feasibility of the generation of thermoelectric energy from natural materials that are abundant in the Earth’s crust.
Abstract
Understanding how to tune enzymatic activity is important not only for biotechnological applications, but also to elucidate the basic principles guiding the design and optimization of ...biological systems in nature. So far, the Michaelis-Menten equation has provided a fundamental framework of enzymatic activity. However, there is still no concrete guideline on how the parameters should be optimized towards higher activity. Here, we demonstrate that tuning the Michaelis-Menten constant (
$${K}_{m}$$
K
m
) to the substrate concentration (
$${{{\rm{S}}}$$
S
) enhances enzymatic activity. This guideline (
$${K}_{m}={{{\rm{S}}}$$
K
m
=
S
) was obtained mathematically by assuming that thermodynamically favorable reactions have higher rate constants, and that the total driving force is fixed. Due to the generality of these thermodynamic considerations, we propose
$${K}_{m}={{{\rm{S}}}$$
K
m
=
S
as a general concept to enhance enzymatic activity. Our bioinformatic analysis reveals that the
$${K}_{m}$$
K
m
and in vivo substrate concentrations are consistent across a dataset of approximately 1000 enzymes, suggesting that even natural selection follows the principle
$${K}_{m}={{{\rm{S}}}$$
K
m
=
S
.
Nitrate is a pervasive aquatic contaminant of global environmental concern. In nature, the most effective nitrate reduction reaction (NRR) is catalyzed by nitrate reductase enzymes at neutral pH, ...using a highly‐conserved Mo center ligated mainly by oxo and thiolate groups. Mo‐based NRR catalysts mostly function in organic solvents with a low water stability. Recently, an oxo‐containing molybdenum sulfide nanoparticle that serves as an NRR catalyst at neutral pH was first reported. Herein, in a nanoparticle‐catalyzed NRR system a pentavalent MoV(=O)S4 species, an enzyme mimetic, served as an active intermediate for the NRR. Potentiometric titration analysis revealed that a redox synergy among MoV−S, S radicals, and MoV(=O)S4 likely play a key role in stabilizing MoV(=O)S4, showing the importance of secondary interactions in facilitating NRR. The first identification and characterization of an oxo‐ and thiolate‐ligated Mo intermediates pave the way to the molecular design of efficient enzyme mimetic NRR catalysts in aqueous solution.
In neutral: The nitrate reduction reaction (NRR) is essential for water remediation. In an artificial NRR catalyst, an enzyme mimetic intermediate has been identified for the first time in the activation of the NRR at neutral pH. The newly identified intermediate is stable in neutral water, and the secondary electronic interactions were found to play a key role.
Manganese oxides have been extensively investigated as model systems for the oxygen-evolving complex of photosystem II. However, most bioinspired catalysts are inefficient at neutral pH and ...functional similarity to the oxygen-evolving complex has been rarely achieved with manganese. Here we report the regulation of proton-coupled electron transfer involved in water oxidation by manganese oxides. Pyridine and its derivatives, which have pKa values intermediate to the water ligand bound to manganese(II) and manganese(III), are used as proton-coupled electron transfer induction reagents. The induction of concerted proton-coupled electron transfer is demonstrated by the detection of deuterium kinetic isotope effects and compliance of the reactions with the libido rule. Although proton-coupled electron transfer regulation is essential for the facial redox change of manganese in photosystem II, most manganese oxides impair these regulatory mechanisms. Thus, the present findings may provide a new design rationale for functional analogues of the oxygen-evolving complex for efficient water splitting at neutral pH.
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