Reduction Half-reaction Research Articles

PdSeO3 Monolayer: Promising Inorganic 2D Photocatalyst for Direct Overall Water Splitting Without Using Sacrificial Reagents and Cocatalysts.

Direct production of H2 from photocatalytic water splitting is a potential solution to environmental pollution and energy crisis, and tremendous efforts have been made to seek efficient photocatalysts that can split pure water (pH = 7) under visible light irradiation. Herein, by means of systematic density functional theory (DFT) computations, we demonstrated that the two-dimensional (2D) PdSeO3 monolayer is a promising candidate. The mechanical exfoliation of PdSeO3 monolayer from its bulk phase is experimentally feasible due to the rather small cleavage energy of ∼0.42 J/m2. Remarkably, PdSeO3 monolayer is semiconducting with a moderate indirect band gap of 2.84 eV, and its valence and conduction bands perfectly engulf the redox potentials of water. In particular, water oxidation and hydrogen reduction half reactions can both occur readily on the different active sites of PdSeO3 monolayer under the potentials solely provided by photogenerated electrons and holes. As PdSeO3 monolayer also has rather pronounced optical absorption in the visible and ultraviolet regions of the solar spectrum, it could be utilized as a highly efficient photocatalyst for splitting pure water into H2 and O2 in a stoichiometric amount of 2:1 without using sacrificial reagents or cocatalysts.

Efficient Production of Biorenewable Monomers from Paired Electrocatalytic Hydrogenation and Oxidation of 5-(Hydroxymethyl)Furfural

Electrochemical synthesis is a promising route for sustainable chemical production; however, its widespread application is hindered by low reaction rates and high energy demands. In this presentation, we report the efficient electrocatalytic conversion of 5-(hydroxymethyl)furfural (HMF) to biobased monomers by pairing HMF reduction and oxidation half-reactions in a single electrochemical cell. HMF hydrogenation to 2,5-bis(hydroxymethyl)furan (BHMF) was achieved at mild pH and ambient temperature using a self-synthesized Ag/C cathode catalyst. Meanwhile, HMF oxidation to 2,5-furandicarboxylic acid (FDCA) was facilitated with close to 100% faradaic efficiency using an inexpensive carbon felt anode together with a homogeneous nitroxyl radical catalyst. The selectivity and faradaic efficiency for Ag-catalyzed BHMF formation were sensitive to cathode potential, owing to competition from HMF hydrodimerization reactions and water reduction (hydrogen evolution). Moreover, the carbon support of Ag/C was active for HMF reduction and contributed to undesired dimer/oligomer formation at strongly cathodic potentials. As a result, high BHMF selectivity and efficiency were only achieved within a narrow potential range. Fortunately, the selectivity of nitroxyl-mediated HMF oxidation was not dependent on anode potential, thus allowing HMF hydrogenation and oxidation half-reactions to be performed together in a single cathode-potential-controlled cell. Electrocatalytic HMF conversion in the paired cell achieved high yields of BHMF and FDCA, and nearly two-fold greater overall electron efficiency compared to the unpaired cells. Future research will focus on the design of continuous-flow electrochemical reactors, in which the use of membrane electrode assemblies (MEA) may significantly lower internal resistance and further reduce total energy demands.

Half Reactions with Multiple Redox States Do Not Follow the Standard Theory: A Computational Study of Electrochemistry of C60

The standard theory of electron transfer advanced by Marcus predicts that the solvent reorganization energy of electron transfer does not depend on the redox state of the reactant. For instance, it should be the same in the reduced and oxidized states of a half reaction. This theory prediction is verifiable by measuring activation barriers of electron transfer reactions involving multiple oxidation states. We use here the opportunity offered by electrochemistry of C60, which allows charges from 0 to −4 in a sequence of reduction half reactions. We find that the activation barrier does change with altering redox state of a fullerene, which can be experimentally verified by measuring Arrhenius slopes of corresponding reaction rates. This outcome is connected to the alteration of the molecular polarizability caused by electronic transitions. Classical molecular dynamics simulations of a fullerene in water are combined here with the analytical Q-model of electron transfer involving polarizable molecules. The ...

Isothermal versus two-temperature solar thermochemical fuel synthesis: A comparative study

Metal-oxide based, two-step thermochemical cycling is a promising means of harvesting solar energy, in which water or carbon dioxide is dissociated to synthesize fuels via successive reduction and oxidation half-reactions driven by concentrated solar heat. Isothermal thermochemical cycling has recently emerged as an important special case of two-step solar TC with distinct advantages of easier heat recovery and potentially high efficiencies, and criteria for the design of oxide materials of isothermal thermochemical cycling are talked about recently. However, the pros and cons of isothermal versus two-temperature (i.e. two-T) TC are under debate. In this work, the two approaches are compared by exploring the influence of temperature, reduction pressure and thermodynamic properties of materials on solar-to-fuel efficiencies. Through the analysis, isothermal cycling is shown to work much better on CO2 splitting for easily reducible materials than two-T cycling; even for materials conventionally considered good for two-T cycling, isothermal cycling could still excel (over two-T cycling) under certain operating conditions. General principles for materials screening are also explored. There are many factors that favour isothermal thermochemical cycling over two-T cycling, such as high temperature, high heat recovery rate, small reduction enthalpy or splitting of CO2 instead of water. In addition, materials with large specific heat, which are unfavorable for two-T cycling, may be suitable for isothermal cycling. At high temperatures or with high heat recovery rates, isothermal thermochemical cycling exhibits excellent performance even for H2O splitting. The theoretical solar-to-fuel efficiency of ∼28% (for CO2 splitting at 1650°C and pO2=10−5 atm, without heat recovery) of isothermal cycling may indicate a meaningful route to effective solar thermochemical fuel production.

Sols of Silver Nanoparticles as Analytical Reagents for the Determination of Active Chlorine in Water Samples by Spectrophotometry and Photometric Titration

Sols of silver nanoparticles surface-stabilized by cetyltrimethylammonium bromide or polyvinylpyrrolidone can be used for the determination of active chlorine in water samples. As analytes modeling active chlorine in water, we used freshly prepared sodium hypochlorite solutions. Because of the high values of standard electrode potentials of reduction half-reactions, active forms of chlorine can oxidize silver nanoparticles, exhibiting the effect of surface plazmon resonance with an absorption maximum near 400 nm. The absorption intensity of silver nanoparticles correlates with the concentration of sodium hypochlorite. This effect was used for the qualitative and quantitative determination of active chlorine in water samples. The quantitative characteristics of the described method were obtained for two versions of analysis, spectrophotometry and photometric titration. The calibration graphs for both versions were similar and linear in the range of sodium hypochlorite concentrations 0.1–2.5 mM.

Proton Reduction Catalysis at a Modified Gallium Phosphide Photocathode Surface

Gallium phosphide (GaP) is a semiconductor with conduction and valence bands that straddle the potential of the proton reduction half-reaction and a bandgap of 2.26 eV, allowing for absorption of a significant portion of the solar spectrum and generation of a large photovoltage. These properties give GaP great potential as a photocathode material for the solar-driven water splitting reaction. However, a hurdle that needs to be overcome for the implementation of this material in photoelectrochemical cells is the large overpotential for the proton reduction half-reaction. This study investigates the pairing of cobalt bis(dichlorobenzenedithiolate) ((TBA)[Co(Cl2bdt)2]) – a molecular HER catalyst – with GaP photocathodes. (TBA)[Co(Cl2bdt)2] demonstrates good catalytic activity for proton reduction in acidic media and has previously been shown to strongly physisorb to graphitic surfaces through π-interactions. Herein is presented investigations into the physisorption of (TBA)[Co(Cl2bdt)2] to single crystal GaP wafers through a transparent, conductive graphitic thin film. Reduced graphene oxide (RGO) thin films are prepared on GaP through a novel method that is exceedingly gentle towards the underlying semiconducting material. Because of their graphitic character, these RGO films can participate in the π-interactions necessary for catalyst adsorption. X-ray photoelectron spectroscopy and electrochemistry are used to detect the presence of the catalyst on the RGO films. Photoelectrochemical experiments have been performed investigating the effects of an RGO thin film on homogeneous light-driven electrocatalysis at GaP photocathodes. Further experiments will delve into heterogeneous catalysis in which the catalyst is physisorbed to the GaP/RGO photocathode. Additionally, finite-element models of catalyzed photoelectrochemical reactions at a GaP-electrolyte interface will be discussed.

Artificial photosynthesis by light absorption, charge separation, and multielectron catalysis.

Our society's current energy demands are largely met by the exploitation of fossil fuels, which are unsustainable and environmentally harmful resources. However, Nature has provided us with a clean and virtually limitless alternative in the form of solar energy. This abundant resource is utilized constantly by photosynthetic organisms, which has in turn motivated decades of research in our quest to create artificial counterparts of comparable scales. In this feature article, we will highlight some of the recent novel approaches in the field of artificial photosynthesis (AP), which we define by a more general term as a process that stores energy overall by generating fuels and chemicals using light. We will particularly emphasize on the potential of a highly modular plug-and-play concept that we hope will persuade the community to explore a more inclusive variety of multielectron redox catalysis to complement the proton reduction and water oxidation half-reactions in traditional solar water splitting systems. We discuss some of the latest developments in the vital functions of light harvesting, charge separation, and multielectron reductive and oxidative catalysis, as well as their optimization, to achieve the ultimate goal of storing sunlight in chemical bonds. Specific attention is dedicated to the use of earth-abundant elements and molecular catalysts that offer greater product selectivity and more intricate control over the reactivity than heterogeneous systems. In this context, we showcase our team's contributions in presenting a unique oxidative carbon-carbon bond cleavage reaction in aliphatic alcohols and biomass model compounds, under ambient atmospheric conditions, facilitated by vanadium photocatalysts. We offer this discovery as a promising alternative to water oxidation in an integrated AP system, which would concurrently generate both solar fuels and valuable solar chemicals.

Understanding the effect of partial N3−-to-O2− substitution and H+-to-K+ exchange on photocatalytic water reduction activity of Ruddlesden–Popper layered perovskite KLaTiO4

Ruddlesden–Popper perovskites, HLaTiO4 and HLaTiO4−xNx were prepared by proton exchange of layered KLaTiO4 synthesized by a solid-state reaction and KLaTiO4−xNx obtained by nitridation, respectively. The influence of nitridation time on crystal structure, morphology, and absorption wavelength and optical band-gap energy of KLaTiO4 crystals was studied. According to the XRD and SEM results, the crystal structure and plate-like morphology of the parent oxide were roughly retained even after nitridation at 800°C for 10h. The absorption edge wavelength of the KLaTiO4 crystals was found to be at about 350nm (Eg=3.54eV), while the absorption edge wavelength of the KLaTiO4−xNx crystals (after 10h nitridation) was about 586nm (Eg=2.12eV). To investigate the effect of partial N3−-to-O2− substitution and H+-to-K+ exchange on photocatalytic water reduction activity of Ruddlesden–Popper layered perovskite KLaTiO4, the photocatalytic activity for water reduction half-reaction over Pt-photodeposited KLaTiO4, KLaTiO4−xNx, HLaTiO4, and HLaTiO4−xNx was evaluated under simulated solar light. Among all the samples, Pt-photodeposited KLaTiO4 exhibited the highest photocatalytic activity for H2 evolution. In contrast, Pt-photodeposited KLaTiO4−xNx showed a low photostability and photocatalytic activity for H2 evolution due to the negative impact of the defective layer and reduced titanium species. In addition, perovskite oxynitride [LaTiO4−xNx]− nanosheets were successfully fabricated by a mechanical exfoliation (sonication) of the KLaTiO4−xNx crystals. The colloidal suspension of the oxynitride nanosheets showed a Tyndall effect, implying their good dispersion and stability in water.

Paired Electrolysis in the Simultaneous Production of Synthetic Intermediates and Substrates.

In electrochemical processes, an oxidation half-reaction is always paired with a reduction half-reaction. Although systems for reactions such as the reduction of CO2 can be coupled to water oxidation to produce O2 at the anode, large-scale O2 production is of limited value. One may replace a low-value half-reaction with a compatible half-reaction that can produce a valuable chemical compound and operate at a lower potential. In doing so, both the anodic and cathodic half-reactions yield desirable products with a decreased energy demand. Here we demonstrate a paired electrolysis in the case of the oxidative condensation of syringaldehyde and o-phenylenediamine to give 2-(3,5-dimethoxy-4-hydroxyphenyl)benzimidazole coupled with the reduction of CO2 to CO mediated by molecular electrocatalysts. We also present general principles for evaluating current-voltage characteristics and power demands in paired electrolyzers.

Hierarchical Inorganic Assemblies for Artificial Photosynthesis.

Artificial photosynthesis is an attractive approach for renewable fuel generation because it offers the prospect of a technology suitable for deployment on highly abundant, non-arable land. Recent leaps forward in the development of efficient and durable light absorbers and catalysts for oxygen evolution and the growing attention to catalysts for carbon dioxide activation brings into focus the tasks of hierarchically integrating the components into assemblies for closing of the photosynthetic cycle. A particular challenge is the efficient coupling of the multi-electron processes of CO2 reduction and H2O oxidation. Among the most important requirements for a complete integrated system are catalytic rates that match the solar flux, efficient charge transport between the various components, and scalability of the photosynthetic assembly on the unprecedented scale of terawatts in order to have impact on fuel consumption. To address these challenges, we have developed a heterogeneous inorganic materials approach with molecularly precise control of light absorption and charge transport pathways. Oxo-bridged heterobinuclear units with metal-to-metal charge-transfer transitions absorbing deep in the visible act as single photon, single charge transfer pumps for driving multi-electron catalysts. A photodeposition method has been introduced for the spatially directed assembly of nanoparticle catalysts for selective coupling to the donor or acceptor metal of the light absorber. For CO2 reduction, a Cu oxide cluster is coupled to the Zr center of a ZrOCo light absorber, while coupling of an Ir nanoparticle catalyst for water oxidation to the Co donor affords closing of the photosynthetic cycle of CO2 conversion by H2O to CO and O2. Optical, vibrational, and X-ray spectroscopy provide detailed structural knowledge of the polynuclear assemblies. Time resolved visible and rapid-scan FT-IR studies reveal charge transfer mechanisms and transient surface intermediates under photocatalytic conditions for guiding performance improvements. Separation of the water oxidation and carbon dioxide reduction half reactions by a membrane is essential for efficient photoreduction of CO2 by H2O to liquid fuel products. A concept of a macroscale artificial photosystem consisting of arrays of Co oxide-silica core-shell nanotubes is introduced in which each tube operates as a complete, independent photosynthetic unit with built-in membrane separation. The ultrathin amorphous silica shell with embedded molecular wires functions as a proton conducting, molecule impermeable membrane. Photoelectrochemical and transient optical measurements confirm tight control of charge transport through the membrane by the orbital energetics of the wire molecules. Hierarchical arrangement of the components is accomplished by a combination of photodeposition, controlled anchoring, and atomic layer deposition methods.

ChemInform Abstract: Coupling Carbon Dioxide Reduction with Water Oxidation in Nanoscale Photocatalytic Assemblies

AbstractReview: 206 refs.

Hybrid nanocomposites with enhanced visible light photocatalytic ability for next generation of clean energy systems

Heterogenous photocatalysis is widely used for waste-water treatment and degradation of pollutants and promises to advance the science of alternative materials with visible photo-excitations abilities. However, there are still fundamental material properties and processes that need to be understood in order to increase user-tailored catalytic systems’ performance and efficiency while ensuring their optimized reactivities and large-scale development and implementation.Herein we developed graphene-based hybrid composites to be used as efficient nanocatalysts with increased ability to absorb visible light, that retain high corrosion-resistance properties when used in solution, and provide energy levels that match their reduction and oxidation half-reactions. Using both photo-deposition and photo-reduction methods, we first created platinum/tungsten trioxide conjugates with physico-chemical characteristics investigated by microscopical and spectroscopical analyses, and further decorated such conjugates onto graphene surfaces to create the hybrids. Our results demonstrate that the synthesized hybrids can degrade a model azo dye and further, show that graphene plays an important role in delaying electron transfer at its interface, with such effect being exploited for possible integration in the next generation of clean energy systems.

Improved Efficiency and Stability of Cadmium Chalcogenide Nanoparticles by Photodeposition of Co-Catalysts

ABSTRACTThe production of hydrogen by photocatalytic water splitting is a potentially clean and renewable source for hydrogen fuel. Cadmium chalcogenides are attractive photocatalysts because they have the potential to convert water into hydrogen and oxygen using photons in the visible spectrum. Cadmium sulfide rods with embedded cadmium selenide quantum dots (CdSe@CdS) are particularly attractive because of their high molar absorptivity in the UV-blue spectral region, and their energy bands can be tuned; however, two crucial drawbacks hinder the implementation of these materials in wide spread use: poor charge transfer and photochemical instability.Utilizing photochemical deposition of co-catalysts onto CdSe@CdS substrates we can address each of these weaknesses. We report how novel co-catalyst morphologies can greatly increase efficiency for the water reduction half-reaction. We also report photostability for CdSe@CdS under high intensity 455nm light (a wavelength at which photocatalytic water splitting by CdSe@CdS is possible) by growing metal oxide co-catalysts on the surface of our rods.

Perfect Photon-to-Hydrogen Conversion Efficiency.

We report a record 100% photon-to-hydrogen production efficiency, under visible light illumination, for the photocatalytic water-splitting reduction half-reaction. This result was accomplished by utilization of nanoparticle-based photocatalysts, composed of Pt-tipped CdSe@CdS rods, with a hydroxyl anion-radical redox couple operating as a shuttle to relay the holes. The implications of such record efficiency for the prospects of realizing practical over all water splitting and solar-to-fuel energy conversion are discussed.

1,2,4-Triazole-Based Approach to Noble-Metal-Free Visible-Light Driven Water Splitting over Carbon Nitrides

MoS2/Co2O3/poly(heptazine imide) composite photocatalysts which are active in both noble-metal-free water reduction and water oxidation half reactions upon irradiation with visible light were synthesized by a one-step procedure. Here, the LiCl/KCl eutectic melt was used as a high-temperature solvent; 3-amino-1,2,4-triazole-5-thiol simultaneously acts as a carbon nitride precursor, sulfur source, and reducing agent for Mo5+, while MoCl5 was added as a metal source for MoS2 nanoparticles being active centers for the water reduction reaction. Water oxidation centers could be created using the rich complexation chemistry of 1,2,4-triazoles. Namely, cobalt species were introduced into carbon nitride network using Co3[3,5-diamino-1,2,4-triazole]6 complex as a dopant, prepared in a separate step. The developed method enables one to control the cocatalysts’ loading and tune the dimensions of MoS2 NPs. The materials reported here show 10% of the efficiency of a reference Pt/mesoporous graphitic carbon nitride comp...

Ultrafine Metal Phosphide Nanocrystals in Situ Decorated on Highly Porous Heteroatom-Doped Carbons for Active Electrocatalytic Hydrogen Evolution.

In spite of being technologically feasible, electrochemical water reduction to facilitate hydrogen production is confronted with issues mainly due to the lack of affordable and efficient catalysts for the water reduction half reaction. Reported herein is the fabrication of metal phosphides nanocrystals uniformly loaded on highly porous heteroatom-modified carbons through one-step carbonization-phosphization methodology. Remarkably, the well-structured porosity and the increased electrochemically accessible active sites ensure the high catalytic efficiency for electrochemical hydrogen evolution in acidic medium in terms of small onset potentials (33 mV) and large cathodic current density (0.481 mA cm(-2)), even comparable to the state-of-the-art Pt/C benchmark. The easily prepared composite catalysts of structural and textural peculiarities may serve as promising non-noble metal catalysts for realistic hydrogen evolution.

(Invited) Electroless Deposition of Ruthenium Using Sodium Borohydride Reducing Agent: A Mechanistic Study Using Electrochemical Quartz Crystal Microbalance

Further miniaturization of interconnects and devices to keep up with Moore’s law in the 21st century requires materials innovation. Copper, as the interconnect metal has been the workhorse of the semiconductor industry for the past two decades. As wire dimensions shrink below 20 nm in future technology nodes, the ability of copper to meet the conductivity and reliability benchmarks is strained1. One metal that becomes attractive at smaller dimensions (<10 nm) is ruthenium, due to its high conductivity and electromigration resistance2. Traditional routes for metallization of ruthenium includes physical vapor (PVD), chemical vapor (CVD) and atomic layer deposition (ALD). PVD suffers from its line-of-sight, non-conformal deposition process that is not suitable for narrow, high aspect ratio geometries. CVD and ALD processes are slower and more expensive than PVD. Here we present an electroless deposition process (ELD) for ruthenium that overcomes some of the obstacles associated with the aforementioned processes. ELD is inherently selective which yields conformal coating in challenging geometries. Furthermore, ELD has lower cost of ownership due to relatively inexpensive precursors and significantly higher deposition rate than ALD and CVD. Here, sodium borohydride is used as the reducing agent for electroless deposition of Ru, and the effects of temperature, precursor and ligand concentrations on ruthenium growth rate are characterized. A linear increase in deposition rate is observed with increasing temperature and ruthenium concentration. A critical borohydride concentration is observed at ~2 g/L, where the maximum deposition rate is obtained. The mechanism of the ruthenium deposition process (reduction half-reaction) is further investigated via electrochemical quartz crystal microbalance (EQCM) analysis to probe the role of pH and complexation. Ru deposition on Cu initiates at a potential of -1.05 V vs. Ag/AgCl reference electrode, and further analysis indicates that a high pH of the electrolyte is needed to suppress any side reaction(s) and to form kinetically active ruthenium-hydroxy complexed species. Finally, some challenges to integration and different applications for an electroless deposited Ru are highlighted. Figure 1: (a) Linear potential scan of the background electrolyte and the electrolyte containing Ru precursor. Increase in current density is observed at potential <-1.05 V vs. Ag/AgCl. Coupled quartz crystal microbalance analysis (b) confirms Ru deposition at potentials more cathodic than -1.05 V vs. Ag/AgCl.

Electroless Deposition of Ferromagnetic CoxFe1-x alloys Using Metal Ion Reducing Agent

21st century computing applications such as big-data analytics, cloud-based computing, etc., require enormous memory capacity. Shuttling large amounts of information between numerous levels of interconnects and memory hierarchies that are necessary for these kind of applications incurs substantial latency and energy costs which is not suitable for today’s low-power, high performance devices. A different approach that is being considered involves inserting low-power non-volatile memory cells closer to logic/processor for faster and more efficient processing1 in so-called ‘monolithic integration’. Due to its low power and high endurance, spin-based (spin-torque transfer, spin Hall effect) 2, 3 memories have recently gained attention. In spin-torque transfer memories, information is stored in the spin orientation of a soft ferromagnetic layer, typically a CoFe or CoFeB alloy. A spin-polarized current is used to change the spin orientation in the ferromagnetic layer either ‘up’ or ‘down’, providing the binary states. This memory stack is usually formed by a subtractive process. Individual stacks are deposited by physical vapor deposition followed by reactive ion etching to obtain the memory cells. Alternatively, an additive process where the ferromagnetic layers are selectively deposited can be proposed. The additive scheme does not have some of the negative consequences associated with subtractive processes such as non-uniform profile, re-deposition of etch by-products and difficulties in scaling. Here, we demonstrate a selective deposition process for CoxFe1-x (x=0-1) ferromagnetic alloy enabled by electroless deposition. Trivalent TiCl3 is used as a reducing agent which facilitates growth of crystalline and contaminant-free (such as B or P) film even at room temperature. Electrochemical characterization of the oxidation and reduction half-reactions are presented and are used to rationally design the electrolyte. X-ray photoelectron spectroscopy (XPS) characterization indicates the presence of metallic Co and Fe in the bulk of the deposit. Finally, the magnetic properties of the CoFe film are investigated using vibrating sample magnetometer (VSM). Electroless deposited CoFe thin films demonstrate magneto-crystalline anisotropy with distinct easy and hard axes and high saturation moment. Effect of heat treatment and surface capping on magnetic properties are also discussed. Finally, some challenges involving integration of such an additive process is highlighted along with possible solutions. Figure 1: XPS characterization of the CoFe film deposited on a PVD Cu substrate. Carbon and oxygen are mainly present at the surface and not shown for clarity. Chemical state analysis indicates presence of metallic Co and Fe in the bulk film.

Hot electron-induced reduction of small molecules on photorecycling metal surfaces.

Noble metals are important photocatalysts due to their ability to convert light into chemical energy. Hot electrons, generated via the non-radiative decay of localized surface plasmons, can be transferred to reactants on the metal surface. Unfortunately, the number of hot electrons per molecule is limited due to charge–carrier recombination. In addition to the reduction half-reaction with hot electrons, also the corresponding oxidation counter-half-reaction must take place since otherwise the overall redox reaction cannot proceed. Here we report on the conceptual importance of promoting the oxidation counter-half-reaction in plasmon-mediated catalysis by photorecycling in order to overcome this general limitation. A six-electron photocatalytic reaction occurs even in the absence of conventional chemical reducing agents due to the photoinduced recycling of Ag atoms from hot holes in the oxidation half-reaction. This concept of multi-electron, counter-half-reaction-promoted photocatalysis provides exciting new opportunities for driving efficient light-to-energy conversion processes.

Adiabaticity of the proton-coupled electron-transfer step in the reduction of superoxide effected by nickel-containing superoxide dismutase metallopeptide-based mimics.

Nickel-containing superoxide dismutases (NiSODs) are bacterial metalloenzymes that catalyze the disproportionation of O2(-). These enzymes take advantage of a redox-active nickel cofactor, which cycles between the Ni(II) and Ni(III) oxidation states, to catalytically disprotorptionate O2(-). The Ni(II) center is ligated in a square planar N2S2 coordination environment, which, upon oxidation to Ni(III), becomes five-coordinate following the ligation of an axial imidazole ligand. Previous studies have suggested that metallopeptide-based mimics of NiSOD reduce O2(-) through a proton-coupled electron transfer (PCET) reaction with the electron derived from a reduced Ni(II) center and the proton from a protonated, coordinated Ni(II)-S(H(+))-Cys moiety. The current work focuses on the O2(-) reduction half-reaction of the catalytic cycle. In this study we calculate the vibronic coupling between the reactant and product diabatic surfaces using a semiclassical formalism to determine if the PCET reaction is proceeding through an adiabatic or nonadiabatic proton tunneling process. These results were then used to calculate H/D kinetic isotope effects for the PCET process. We find that as the axial imidazole ligand becomes more strongly associated with the Ni(II) center during the PCET reaction, the reaction becomes more nonadiabatic. This is reflected in the calculated H/D KIEs, which moderately increase as the reaction becomes more nonadiabatic. Furthermore, the results suggest that as the axial ligand becomes less Lewis basic the observed reaction rate constants for O2(-) reduction should become faster because the reaction becomes more adiabatic. These conclusions are in-line with experimental observations. The results thus indicate that variations in the axial donor's ability to coordinate to the nickel center of NiSOD metallopeptide-based mimics will strongly influence the fundamental nature of the O2(-) reduction process.