Standard Reduction Potentials for Oxygen and Carbon Dioxide Couples in Acetonitrile and N,N-Dimethylformamide

A directional electrode separator improves anodic biofilm current density in a well-mixed single-chamber bioelectrochemical system

In this study, a directional electrode separator (DES) was designed and incorporated into a single-chamber bioelectrochemical system (BES) to reduce migration and reoxidation of hydrogen. This issue arises when H2, generated at the cathode, travels to the anode where anodic biofilms use H2. To test the feasibility of our design, a 3D-printed BES reactor equipped with a DES was inoculated with anaerobic digestor granules and operated under fed-batch conditions using fermented corn stover effluent. The DES equipped reactor achieved significantly higher current densities (∼53 A/m²) compared to a conventional single-chamber BES without a separator (∼16 A/m²), showing a 3.3 times improvement. Control abiotic electrochemical experiments revealed that the DES exhibited significantly higher proton conductivity (456±127 µS/mm) compared to a proton exchange membrane (67±21 µS/mm) with a statistical significance of P=0.03. The DES also effectively reduced H2 migration to the anode by 21-fold relative to the control. Overall, incorporating a DES in a single-chamber BES enhanced anodic current density by reducing H2 migration to the anode.

Group 7 carbonyl complexes of a PNN-heteroscorpionate ligand

A series of rhenium and manganese carbonyl complexes of a heteroscorpionate ligand with an atypical N2P-donor set has been prepared to better understand their electronic and CO releasing properties. Thus, the ligand, pz2TTP, with an a,a-bis(pyrazol-1-yl)tolyl group decorated with an ortho-situated di(p-tolyl)phosphanyl reacts with carbonyl group 17 reagents to give [fac-(κ2NP-pz2TTP)Re(CO)3Br], 1, and [fac-(κ3N2P-pz2TTP)M(CO)3](OTf = O3SCF3), 2-M (M = Re, Mn), if care is taken during the preparation of the manganeses derivative. When heated in CH3CN, 2-Mn slowly transforms to [fac,cis-(κ3N2P-pz2TTP)Mn(CO)2(NCCH3)](OTf), 3-Mn. In contrast, the corresponding 3-Re can only be prepared from 2-Re using Me3NO; pure 3-Mn can also be prepared by this method. Experimental and density functional calculations at the M06L/Def2-TZVP/PCM(CH3CN) level show that the replacement of a carbonyl with an acetonitrile solvent decreases the oxidation potential by around 0.8 V per carbonyl released, making decarbonylated species potent reductants. At the same time, the electronic spectrum broadens and undergoes a red-shift, making dicarbonyl complexes more susceptible to photo-initiated decarbonylation reactions than tricarbonyls. When 2-Mn or 3-Mn are irradiated in with 390 nm LED light in aerated solutions, [trans-Mn(pz2TTP = O)2](OTf)2, 4, along with insoluble manganese oxides are rapidly formed.

Leveraging ligand-based proton and electron transfer for aerobic reactivity and catalysis

While O2 is an abundant, benign, and thermodynamically potent oxidant, it is also kinetically inert. This frequently limits its use in synthetic transformations. Correspondingly, direct aerobic reactivity with O2 often requires comparatively harsh or forcing conditions to overcome this kinetic barrier. Forcing conditions limit product selectivity and can lead to over oxidation. Alternatively, O2 can be activated by a catalyst to facilitate oxidative reactivity, and there are a variety of sophisticated examples where transition metal catalysts facilitate aerobic reactivity. Many efforts have focused on using metal-ligand cooperativity to facilitate the movement of protons and electrons for O2 activation. This approach is inspired by enzyme active sites, which frequently use the secondary sphere to facilitate both the activation of O2 and the oxidation of substrates. However, there has only recently been a focus on harnessing metal-ligand cooperativity for aerobic reactivity and, especially, catalysis. This perspective will discuss recent efforts to channel metal-ligand cooperativity for the activation of O2, the generation and stabilization of reactive metal-oxygen intermediates, and oxidative reactivity and catalysis. While significant progress has been made in this area, there are still challenges to overcome and opportunities for the development of efficient catalysts which leverage this biomimetic strategy.

Examining the Importance of Hydrogen Bonding and Proton Transfer in Iron Porphyrin-Mediated Carbon Dioxide Upconversion

ConspectusThe title should give a sense of the "big picture" of this Account, but what is it really about? An unexpected change in research direction? A series of courageous and creative students? A team taking on challenging problems in chemistry? The answer is a definite "yes" to all of the above. More specifically, the problem in which we are interested is the upconversion or valorization of carbon dioxide. This problem has captured the attention of a great many chemists in earnest following the gas crisis of the 1970s and more recently galvanized due to climate concerns arising from the ongoing release of anthropogenic carbon. Addressing the problem of atmospheric carbon accumulation requires effort in two very broad areas: capture and conversion. Storage is an alternative to conversion, but this eliminates the opportunity to use what might be otherwise a waste product. Our group has investigated a series of modified versions of iron(III)-5,10,15,20-tetraphenylporphyrin (FeTPP) that can convert CO2 to carbon monoxide, which is a versatile and useful precursor for other syntheses. Following pioneering work from Savéant and his colleagues in the 1990s and thereafter, we started with a simple question: how many pendent ancillary groups that can donate H-bonds or protons are needed to support efficient CO2-to-CO conversion? Using a molecule with only one 2-hydroxylphenyl group, we demonstrated that the single prepositioned -OH group gave rise to efficient turnover, but only when experiments were carried out in a weakly H-bond-accepting solvent system. In other words, the ability of a solvent to accept H-bonds can impede CO2 reduction. We followed up with a deeper investigation of the influence of H-bonding interactions with external acids in FeTPP-mediated CO2 reduction. Savéant's framework mechanism appears to be independent of solvent, and rate differences can be approximated by considering H-bonding equilibria. Following that work, we sought to better understand the minimum catalyst design requirements with respect to internal H-bond/proton donors. To that end, we produced all possible isomers of tetraarylpoprhyrins with 2,6-dihydroxyphenyl + phenyl groups. All else being equal, the complexes with a formally trans orientation of the 2,6-dihydroxyphenyl groups performed the best. Most recently, we surveyed the roles of internal and external Brønsted acids with different pKa values. Surprisingly, the best-performing catalysts have more weakly acidic internal groups. Overall, our work has demonstrated that CO2 reduction mediated by porphyrin catalysts can be improved by considering solvent H-bonding, the orientation of internal H-bonding groups, and the balance of the pKa values of internal and external acids. The future for molecular electrocatalysts is promising as more ideas emerge about how to design molecules and conditions for CO2 reduction.

Three-Way Control on Product Selectivity in Electrocatalytic CO2 Reduction Reaction Using a Single Molecular Co-NHC Catalyst

For a long time, molecular electrocatalysts have been developed to reduce CO2 efficiently to value-added products such as CO and HCO2H, along with H2; however, selectivity remained as a major issue. Recent work toward addressing this issue showed that several different catalysts could be used to achieve product selectivity. It is desirable that instead of using different catalysts for specific products, a single catalyst should be able to produce the target products by subtle tuning of the reaction conditions. Toward this objective, herein we presented the organometallic Co electrocatalyst Co-NHCU and successfully utilized it in the electrochemical CO2 reduction reaction (CO2RR) to produce CO, H2, and HCO2H with notable selectivity. The reduction of CO2 selectively produced CO with 81 ± 2% Faradaic efficiency (FE) in the presence of 5% H2O as a proton source. The selectivity was changed toward H2 with 80 ± 3% FE when 1.5 M triethylamine was added as an additive in the presence of 10% H2O as a proton source. In the presence of 1.0 M morpholine as an external additive, the CO2-saturated solution containing 10% trifluoroethanol as a proton source generates 55 ± 5% HCO2H as the predominant product, with H2 as a competitive side product. It was found that the combined effect of the proton source and the additive in association with the nature of the Co-NHCU catalyst changed the selectivity of the products in this reaction.

Pre-equilibrium reactions involving pendent relays improve CO2 reduction mediated by molecular Cr-based electrocatalysts

Homogeneous earth abundant transition-metal electrocatalysts capable of carbon dioxide (CO2) reduction to generate value-added chemical products are a possible strategy to minimize rising anthropogenic CO2 emissions. Previously, it was determined that Cr-centered bipyridine-based N2O2 complexes for CO2 reduction are kinetically limited by a proton-transfer step during C-OH bond cleavage. Therefore, it was hypothesized that the inclusion of pendent relay groups in the secondary coordination sphere of these molecular catalysts could increase their catalytic activity. Here, it is shown that the introduction of a pendent methoxy group favorably impacts a pre-equilibrium protonation prior to the catalytic resting state, resulting in a significant increase in catalytic activity without a loss of product selectivity for generating carbon monoxide (CO) from CO2. Interestingly, combining the pendent methoxy group with a cationic acid causes a positive shift of the catalytic reduction potential of the system, while maintaining increased activity and quantitative selectivity. This work suggests that tuning the secondary coordination sphere with respect to cationic proton sources can result in activity improvements by modifying the kinetic and thermodynamic aspects of proton transfer in the catalytic cycle.

Bioinspired Tetranuclear Manganese Cubane Complex as an Efficient Molecular Electrocatalyst for Two-Electron Water Oxidation Towards Hydrogen Peroxide

Stable homogeneous two-electron water oxidation electrocatalysts are highly demanded to understand the precise mechanism and reaction intermediates of electrochemical H2O2 production. Here we report a tetranuclear manganese complex with a cubane structure which can electrocatalyze water oxidation to hydrogen peroxide under alkaline and neutral conditions. Such a complex demonstrates an optimal Faradaic efficiency (FE) of 87 %, which is amongst (if not) the highest FE(H2O2) of reported homogeneous and heterogeneous electrocatalysts. In addition, active species were identified and co-catalysts were excluded through ESI-MS characterization. Furthermore, we identified water binding sites and isolated one-electron oxidation intermediate by chemical oxidation of the catalyst in the presence of water substrates. It is evident that efficient proton-accepting electrolytes avoid rapid proton building-up at electrode and substantially improve reaction rate and selectivity. Accordingly, we propose a two-electron catalytic cycle model for water oxidation to hydrogen peroxide with the bioinspired molecular electrocatalyst. The present work is expected to provide an ideal platform to elucidate the two-electron WOR mechanism at the atomic level.

Lithium-coupled electron transfer reactions of nano-confined WOx within Zr-based metal-organic framework

Interfacial charge transfer reactions involving cations and electrons are fundamental to (photo/electro) catalysis, energy storage, and beyond. Lithium-coupled electron transfer (LCET) at the electrode-electrolyte interfaces of lithium-ion batteries (LIBs) is a preeminent example to highlight the importance of charge transfer in modern-day society. The thermodynamics of LCET reactions define the minimal energy for charge/discharge of LIBs, and yet, these parameters are rarely available in the literature. Here, we demonstrate the successful incorporation of tungsten oxides (WOx) within a chemically stable Zr-based metal-organic framework (MOF), MOF-808. Cyclic voltammograms (CVs) of the composite, WOx@MOF-808, in Li+-containing acetonitrile (MeCN)-based electrolytes showed an irreversible, cathodic Faradaic feature that shifted in a Nernstian fashion with respect to the Li+ concentration, i.e., ∼59 mV/log [(Li+)]. The Nernstian dependence established 1:1 stoichiometry of Li+ and e-. Using the standard redox potential of Li+/0, the apparent free energy of lithiation of WOx@MOF-808 (ΔGapp,Li) was calculated to be -36 ± 1 kcal mol-1. ΔGapp,Li is an intrinsic parameter of WOx@MOF-808, and thus by deriving the similar reaction free energies of other metal oxides, their direct comparisons can be achieved. Implications of the reported measurements will be further contrasted to proton-coupled electron transfer (PCET) reactions on metal oxides.

Photoelectrochemical CO2 Reduction to CO Enabled by a Molecular Catalyst Attached to High-Surface-Area Porous Silicon

A high-surface-area p-type porous Si photocathode containing a covalently immobilized molecular Re catalyst is highly selective for the photoelectrochemical conversion of CO2 to CO. It gives Faradaic efficiencies of up to 90% for CO at potentials of -1.7 V (versus ferrocenium/ferrocene) under 1 sun illumination in an acetonitrile solution containing phenol. The photovoltage is approximately 300 mV based on comparisons with similar n-type porous Si cathodes in the dark. Using an estimate of the equilibrium potential for CO2 reduction to CO under optimized reaction conditions, photoelectrolysis was performed at a small overpotential, and the onset of electrocatalysis in cyclic voltammograms occurred at a modest underpotential. The porous Si photoelectrode is more stable and selective for CO production than the photoelectrode generated by attaching the same Re catalyst to a planar Si wafer. Further, facile characterization of the porous Si-based photoelectrodes using transmission mode FTIR spectroscopy leads to highly reproducible catalytic performance.

Photocatalytic CO2 reduction by topologically matched polymer-polymer heterojunction nanosheets

Conversion of solar energy into chemical fuel can be achieved through a number of routes but direct conversion, via photocatalysis, is potentially the simplest and cheapest route to the transformation of low-value substances, water and CO2, to useful chemical fuels or feedstocks such as hydrogen, formate, methanol, and syngas. 2D polymers, including carbon nitrides and COFs, have emerged as one of the most promising classes of organic photocatalysts for solar fuels production due to their energetic tunability, charge transport properties and robustness. They are, however, difficult to process and so there have been limited studies into the formation of heterojunction materials incorporating these components. In this work we use our novel templating approach to combine topologically matched imine-based donor polymers with acceptor polymers formed through Knoevenagel condensation. An efficient heterojunction interface was formed by matching the isostructural nodes and linkers that make up the D1 and A1 semiconductors and this was reflected in the increased photocatalytic activity of the heterojunction material T1. Tuning of the templating synthesis route to give heterojunctions with optimised donor : acceptor ratios, as well as the photocatalytic conditions, resulted in CO production rates that were between 1.5 and 10 times higher than those of the individual polymers. A further set of polymers A5 and D5 were developed with more optimised structures for CO2 reduction including increased overpotential for the reduction reaction and the presence of co-catalyst chelating groups. These had increased activity compared to the group 1 family and again showed higher activity for CO production by the templated heterojunction, T5, than either individual component or a physical mixture of the donor and acceptor.

Understanding the Interplay of the Brønsted Acidity of Catalyst Ancillary Groups and the Solution Components in Iron-porphyrin-Mediated Carbon Dioxide Reduction

The rapid and efficient conversion of carbon dioxide (CO2) to carbon monoxide (CO) is an ongoing challenge. Catalysts based on iron-porphyrin cores have emerged as excellent electrochemical mediators of the two proton + two electron reduction of CO2 to CO, and many of the design features that promote function are known. Of those design features, the incorporation of Brønsted acids in the second coordination sphere of the iron ion has a significant impact on catalyst turnover kinetics. The Brønsted acids are often in the form of hydroxyphenyl groups. Herein, we explore how the acidity of an ancillary 2-hydroxyphenyl group affects the performance of CO2 reduction electrocatalysts. A series of meso-5,10,15,20-tetraaryl porphyrins were prepared where only the functional group at the 5-meso position has an ionizable proton. A series of cyclic voltammetry (CV) experiments reveal that the complex with -OMe positioned para to the ionizable -OH shows the largest CO2 reduction rate constants in acetonitrile solvent. This is the least acidic -OH of the compounds surveyed. The turnover frequency of the -OMe derivative can be further improved with the addition of 4-trifluoromethylphenol to the solution. In contrast, the iron-porphyrin complex with -CF3 positioned opposite the ionizable -OH shows the smallest CO2 reduction rate constants, and its turnover frequency is less enhanced with the addition of phenols to the reaction solutions. The origin of this effect is rationalized based on kinetic isotope effect experiments and density functional calculations. We conclude that catalysts with weaker internal acids coupled with stronger external acid additives provide superior CO2 reduction kinetics.

Insight into the Role of Entropy in Promoting Electrochemical CO2 Reduction by Imidazolium Cations

The electroreduction of CO2 plays an important role in achieving a net-zero carbon economy. Imidazolium cations can be used to enhance the rate of CO2 reduction reactions, but the origin of this promotion remains poorly understood. In this work, we show that in the presence of 1-ethyl-3-methylimidazolium (EMIM+), CO2 reduction on Ag electrodes occurs with an apparent activation energy near zero, while the applied potential influences the rate through the pre-exponential factor. Our findings suggest that the CO2 reduction rate is controlled by the initial state entropy, which depends on the applied potential through the organization of cations at the electrochemical interface. Further characterization shows that the C2-proton of EMIM+ is consumed during the reaction, leading to the collapse of the cation organization and a decrease in the catalytic performance. Our results have important implications for understanding the effect of potential on reaction rates, as they indicate that the common picture based on vibrational activation of electron transfer reactions is insufficient for describing the impact of potential in complex systems, such as CO2 reduction in the presence of imidazolium cations.

The beneficial role of nano-sized Fe3O4 entrapped in ultra-stable Y zeolite for the complete mineralization of phenol by heterogeneous photo-Fenton under solar light

Highly efficient, separable, and stable magnetic iron-based-photocatalysts produced from ultra-stable Y (USY) zeolite were applied, for the first time, to the photo-Fenton removal of phenol under solar light. USY Zeolite with a Si/Al molar ratio of 385 was impregnated under vacuum with an aqueous solution of Fe2+ ions and thermally treated (500-750 °C) in a reducing atmosphere. Three catalysts, Fe-USY500°C-2h, Fe-USY600°C-2h and Fe-USY750°C-2h, containing different amounts of reduced iron species entrapped in the zeolitic matrix, were obtained. The catalysts were thoroughly characterized by absorption spectrometry, X-ray powder diffraction with synchrotron source, followed by Rietveld analysis, X-ray photoelectron spectroscopy, N2 adsorption/desorption at -196 °C, high-resolution transmission electron microscopy and magnetic measurements at room temperature. The catalytic activity was evaluated in a recirculating batch photoreactor irradiated by solar light with online analysis of evolved CO2. Photo-Fenton results showed that the catalyst obtained by thermal treatment at 500 °C for 2 h under a reducing atmosphere (FeUSY-500°C-2h) was able to completely mineralize phenol in 120 min of irradiation time at pH = 4 owing to the presence of a higher content of entrapped nano-sized magnetite particles. The latter promotes the generation of hydroxyl radicals in a more efficient way than the Fe-USY catalysts prepared at 600 and 750 °C because of the higher Fe3O4 content in ultra-stable Y zeolite treated at 500 °C. The FeUSY-500°C-2h catalyst was recovered from the treated water through magnetic separation and reused five times without any significant worsening of phenol mineralization performances. The characterization of the FeUSY-500°C-2h after the photo-Fenton process demonstrated that it was perfectly stable during the reaction. The optimized catalyst was also effective in the mineralization of phenol in tap water. Finally, a possible photo-Fenton mechanism for phenol mineralization was assessed based on experimental tests carried out in the presence of scavenger molecules, demonstrating that hydroxyl radicals play a major role.

Further Understanding the Roles of Solvent, Brønsted Acids, and Hydrogen Bonding in Iron Porphyrin-Mediated Carbon Dioxide Reduction

Improving our understanding of how molecules and materials mediate the electrochemical reduction of carbon dioxide (CO2) to upgraded products is of great interest as a means to address climate change. A leading class of molecules that can facilitate the electrochemical conversion of CO2 to carbon monoxide (CO) is iron porphyrins. These molecules can have high rate constants for CO2-to-CO conversion; they are robust, and they rely on abundant and inexpensive synthetic building blocks. Important foundational work has been conducted using chloroiron 5,10,15,20-tetraphenylporphyrin (FeTPPCl) in N,N-dimethylformamide (DMF) solvent. A related and recent report points out that the corresponding perchlorate complex, FeTPPClO4, can have superior function due to its solubility in other organic solvents. However, the importance of hydrogen bonding and solvent effects was not discussed. Herein, we present a detailed kinetic study of the triflate (CF3SO3-) complex of FeTPP in DMF and in MeCN using a range of phenol Brønsted acid additives. We also detected the formation of Fe(III)TPP-phenolate complexes using cyclic voltammetry experiments. Importantly, our new analysis of apparent rate constants with different added phenols allows for a modification to the established mechanistic model for CO2-to-CO conversion. Critically, our improved model accounts for hydrogen bonding and solvent effects by using simple hydrogen bond acidity and basicity descriptors. We use this augmented model to rationalize function in other reported porphyrin systems and to make predictions about operational conditions that can enhance the CO2 reduction chemistry.

Coupled Fe(III) reduction and phenanthrene degradation by marine-derived Kocuria oceani FXJ8.057 under aerobic condition

Diverse aerobic actinobacteria possess the capacity to degrade polycyclic aromatic hydrocarbons (PAHs) and have recently been shown to reduce Fe(III). However, the coupling of the two processes under oxic conditions remains unclear. Here, the co-metabolism of phenanthrene (PHE) and Fe(III) by marine-derived Kocuria oceani FXJ8.057 was realized under aerobic condition. In the presence of both PHE and Fe(III), the rates of PHE degradation (83.91 %) and Fe(III) reduction (50.00 %) were synchronously enhanced, compared to those with PHE (67.34 %) or Fe(III) (38.00 %) alone. Transcriptome analysis detected upregulation of PHE biodegradation and riboflavin biosynthesis in the strain cultured with both PHE and Fe(III) compared to that with PHE alone. Metabolite analysis indicated that, with the addition of Fe(III), the strain could efficiently degrade PHE via three pathways. Moreover, the strain secreted riboflavin, which acted as a shuttle to promote electron transfer from PHE to Fe(III). It also secreted organic acids that could delay Fe(II) reoxidation. Finally, H2O2 secreted by the strain caused extracellular Fenton reaction to generate •OH, which also played a minor role in the PHE degradation. These findings provide the first example of an aerobic bacterium that couples PAH degradation to Fe(III) reduction and extend our understanding of Fe(III)-reducing microorganisms.

Rapid, green disulphide bond formation in water using the corrin dicyanocobinamide

Peptide chemists seek rapid methods combined with facile purification when producing disulphide bonds post solid-phase synthesis. Current methods typically require long reaction times of up to two days, can result in side-products from over-oxidation and/or degradation, require organic solvents, and/or require challenging purification. Herein, we describe a rapid, green, and facile oxidation of a series of peptides with up to three disulphide bonds. The method was conducted in aqueous solution, in air, utilizing the biocompatible corrin ring-containing compound dicyanocobinamide, and offers reaction times under 1 hour with simple one step removal of the catalyst.

Assessment of the technological viability of photoelectrochemical devices for oxygen and fuel production on Moon and Mars

Human deep space exploration is presented with multiple challenges, such as the reliable, efficient and sustainable operation of life support systems. The production and recycling of oxygen, carbon dioxide (CO₂) and fuels are hereby key, as a resource resupply will not be possible. Photoelectrochemical (PEC) devices are investigated for the light-assisted production of hydrogen and carbon-based fuels from CO₂ within the green energy transition on Earth. Their monolithic design and the sole reliance on solar energy makes them attractive for applications in space. Here, we establish the framework to evaluate PEC device performances on Moon and Mars. We present a refined Martian solar irradiance spectrum and establish the thermodynamic and realistic efficiency limits of solar-driven lunar water-splitting and Martian carbon dioxide reduction (CO₂R) devices. Finally, we discuss the technological viability of PEC devices in space by assessing the performance combined with solar concentrator devices and explore their fabrication via in-situ resource utilization.

Electro- and Photoinduced Interfacial Charge Transfers in Nanocrystalline Mesoporous TiO2 and TiO2/Iron Porphyrin Sensitized Films under CO2 Reduction Catalysis

Electro- and photochemical CO2 reduction (CO2R) is the quintessence of modern-day sustainable research. We report our studies on the electro- and photoinduced interfacial charge transfer occurring in a nanocrystalline mesoporous TiO2 film and two TiO2/iron porphyrin hybrid films (meso-aryl- and β-pyrrole-substituted porphyrins, respectively) under CO2R conditions. We used transient absorption spectroscopy (TAS) to demonstrate that, under 355 nm laser excitation and an applied voltage bias (0 to -0.8 V vs Ag/AgCl), the TiO2 film exhibited a diminution in the transient absorption (at -0.5 V by 35%), as well as a reduction of the lifetime of the photogenerated electrons (at -0.5 V by 50%) when the experiments were conducted under a CO2 atmosphere changing from inert N2. The TiO2/iron porphyrin films showed faster charge recombination kinetics, featuring 100-fold faster transient signal decays than that of the TiO2 film. The electro-, photo-, and photoelectrochemical CO2R performance of the TiO2 and TiO2/iron porphyrin films are evaluated within the bias range of -0.5 to -1.8 V vs Ag/AgCl. The bare TiO2 film produced CO and CH4 as well as H2, depending on the applied voltage bias. In contrast, the TiO2/iron porphyrin films showed the exclusive formation of CO (100% selectivity) under identical conditions. During the CO2R, a gain in the overpotential values is obtained under light irradiation conditions. This finding was indicative of a direct transfer of the photogenerated electrons from the film to absorbed CO2 molecules and an observed decrease in the decay of the TAS signals. In the TiO2/iron porphyrin films, we identified the interfacial charge recombination processes between the oxidized iron porphyrin and the electrons of the TiO2 conduction band. These competitive processes are considered to be responsible for the diminution of direct charge transfer between the film and the adsorbed CO2 molecules, explaining the moderate performances of the hybrid films for the CO2R.

Amine Groups in the Second Sphere of Iron Porphyrins Allow for Higher and Selective 4e-/4H+ Oxygen Reduction Rates at Lower Overpotentials

Iron porphyrins with one or four tertiary amine groups in their second sphere are used to investigate the electrochemical O₂ reduction reaction (ORR) in organic (homogeneous) and aqueous (heterogeneous) conditions. Both of these complexes show selective 4e^-/4H⁺ reduction of oxygen to water at rates that are 2-3 orders of magnitude higher than those of iron tetraphenylporphyrin lacking these amines in the second sphere. In organic solvents, these amines get protonated, which leads to the lowering of overpotentials, and the rate of the ORR is enhanced almost 75,000 times relative to rates expected from the established scaling relationship for the ORR by iron porphyrins. In the aqueous medium, the same trend of higher ORR rates at a lower overpotential is observed. In situ resonance Raman data under heterogeneous aqueous conditions show that the presence of one amine group in the second sphere leads to a cleavage of the O-O bond in a Fe^III-OOH intermediate as the rate-determining step (rds). The presence of four such amine groups enhances the rate of O-O bond cleavage such that this intermediate is no longer observed during the ORR; rather, the proton-coupled reduction of the Fe^III-O₂^- intermediate with a H/D isotope effect of 10.6 is the rds. These data clearly demonstrate changes in the rds of the electrochemical ORR depending on the nature of second-sphere residues and explain their deviation from linear scaling relationships.

Interplay between β-Diimino and β-Diketiminato Ligands in Nickel Complexes Active in the Proton Reduction Reaction

Two Ni complexes are reported with κ⁴-P₂N₂ β-diimino (BDI) ligands with the general formula [Ni(XBDI)](BF₄)₂, where BDI is N-(2-(diphenylphosphaneyl)ethyl)-4-((2-(diphenylphosphaneyl)ethyl)imino)pent-2-en-2-amine and X indicates the substituent in the α-carbon intradiimine position, X = H for 1(BF₄)₂ and X = Ph for 2(BF₄)₂. Electrochemical analysis together with UV-vis and NMR spectroscopy in acetonitrile and dimethylformamide (DMF) indicates the conversion of the β-diimino complexes 1²⁺ and 2²⁺ to the negatively charged β-diketiminato (BDK) analogues (1-H)⁺ and (2-H)⁺ via deprotonation in DMF. Moreover, further electrochemical and spectroscopy evidence indicates that the one-electron-reduced derivatives 1⁺ and 2⁺ can also rapidly evolve to the BDK (1-H)⁺ and (2-H)⁺, respectively, via hydrogen gas evolution through a bimolecular homolytic pathway. Finally, both complexes are demonstrated to be active for the proton reduction reaction in DMF at E_app = -1.8 V vs Fc^+/0, being the active species the one-electron-reduced derivative 1-H and 2-H.

Highly Robust Rhenium(I) Bipyridyl Complexes Containing Dipyrromethene-BF2 Chromophores for Visible Light-Driven CO2 Reduction

New rhenium bipyridyl complexes with dipyrromethene-BF₂ chromophores (A-ReBDP-CZ, A-ReBDP₂ , ReBDP-CZ, and ReBDP₂ ) were developed for highly efficient photocatalytic carbon dioxide (CO₂ ) reduction to carbon monoxide (CO). These catalysts consisted of two moderate electron-deficient groups (dipyrromethene-BF₂ , BDP) as the visible-light-harvesting antenna as well as both electron donor (N-phenylcarbazole, CZ) and acceptor (BDP) on Re bipyridyl framework. Among ReBDP-CZ and ReBDP₂ complexes, the ReBDP₂ incorporating two electron-deficient BDP chromophores had a longer-lived photoexcited state (182.4 μs) and a twofold enhanced molar absorption coefficient (ϵ=157000 m^-1  cm^-1 ) compared with ReBDP-CZ. Thus, ReBDP₂ achieved the superior photocatalytic reactivity and stability with a CO turnover number (TON_CO ) value as high as 1323 and quantum yield (Φ_CO ) up to 55 %, which was the most excellent photocatalysis efficiency among the single-active-site Re catalysts without additional photosensitizer. Furthermore, the acetylene-bridged linker was detrimental to the photoactivity and durability of the catalyst. In brief, two BDP-based Re bipyridyl systems with outstanding catalytic performance and significant visible-light-harvesting capabilities in the solar spectrum offer a promising strategy for solar-to-fuel conversion schemes.

Molecular Catalysts with Diphosphine Ligands Containing Pendant Amines

Pendant amines play an invaluable role in chemical reactivity, especially for molecular catalysts based on earth-abundant metals. As inspired by [FeFe]-hydrogenases, which contain a pendant amine positioned for cooperative bifunctionality, synthetic catalysts have been developed to emulate this multifunctionality through incorporation of a pendant amine in the second coordination sphere. Cyclic diphosphine ligands containing two amines serve as the basis for a class of catalysts that have been extensively studied and used to demonstrate the impact of a pendant base. These 1,5-diaza-3,7-diphosphacyclooctanes, now often referred to as "P₂N₂" ligands, have profound effects on the reactivity of many catalysts. The resulting [Ni(P^R₂N^R'₂)₂]²⁺ complexes are electrocatalysts for both the oxidation and production of H₂. Achieving the optimal benefit of the pendant amine requires that it has suitable basicity and is properly positioned relative to the metal center. In addition to the catalytic efficacy demonstrated with [Ni(P^R₂N^R'₂)₂]²⁺ complexes for the oxidation and production of H₂, catalysts with diphosphine ligands containing pendant amines have also been demonstrated for several metals for many different reactions, both in solution and immobilized on surfaces. The impact of pendant amines in catalyst design continues to expand.

Upconversion nanoparticles coupled with hierarchical ZnIn2S4 nanorods as a near-infrared responsive photocatalyst for photocatalytic CO2 reduction

Developing near-infrared responsive (NIR) photocatalysts is very important for the development of solar-driven photocatalytic systems. Metal sulfide semiconductors have been extensively used as visible-light responsive photocatalysts for photocatalytic applications owing to their high chemical variety, narrow bandgap and suitable redox potentials, particularly the benchmark ZnIn₂S₄. However, their potential as NIR-responsive photocatalysts is yet to be reported. Herein, for the first time demonstrated that upconversion nanoparticles can be delicately coupled with hierarchical ZnIn₂S₄ nanorods (UCNPs/ZIS) to assemble a NIR-responsive composite photocatalyst, and as such composite is verified by ultraviolet-visible diffuse reflectance spectra and upconversion luminescence spectra. As a result, remarkable photocatalytic CO and CH₄ production rates of 1500 and 220 nmol g^-1h^-1, respectively, were detected for the UCNPs/ZIS composite under NIR-light irradiation (λ ≥ 800 nm), which is rarely reported in the literature. The remarkable photocatalytic activity of the UCNPs/ZIS composite can be understood not only because the heterojunction between UCNPs and ZIS can promote the charge separation efficiency, but also the intimate interaction of UCNPs with hierarchical ZIS nanorods can enhance the energy transfer. This finding may open a new avenue to develop more NIR-responsive photocatalysts for various solar energy conversion applications.

Rhodium-Based Metal-Organic Polyhedra Assemblies for Selective CO2 Photoreduction

Heterogenization of molecular catalysts via their immobilization within extended structures often results in a lowering of their catalytic properties due to a change in their coordination sphere. Metal-organic polyhedra (MOP) are an emerging class of well-defined hybrid compounds with a high number of accessible metal sites organized around an inner cavity, making them appealing candidates for catalytic applications. Here, we demonstrate a design strategy that enhances the catalytic properties of dirhodium paddlewheels heterogenized within MOP (Rh-MOP) and their three-dimensional assembled supramolecular structures, which proved to be very efficient catalysts for the selective photochemical reduction of carbon dioxide to formic acid. Surprisingly, the catalytic activity per Rh atom is higher in the supramolecular structures than in its molecular sub-unit Rh-MOP or in the Rh-metal-organic framework (Rh-MOF) and yields turnover frequencies of up to 60 h^-1 and production rates of approx. 76 mmole formic acid per gram of the catalyst per hour, unprecedented in heterogeneous photocatalysis. The enhanced catalytic activity is investigated by X-ray photoelectron spectroscopy and electrochemical characterization, showing that self-assembly into supramolecular polymers increases the electron density on the active site, making the overall reaction thermodynamically more favorable. The catalyst can be recycled without loss of activity and with no change of its molecular structure as shown by pair distribution function analysis. These results demonstrate the high potential of MOP as catalysts for the photoreduction of CO₂ and open a new perspective for the electronic design of discrete molecular architectures with accessible metal sites for the production of solar fuels.

A photosensitizer-polyoxometalate dyad that enables the decoupling of light and dark reactions for delayed on-demand solar hydrogen production

Decoupling the production of solar hydrogen from the diurnal cycle is a key challenge in solar energy conversion, the success of which could lead to sustainable energy schemes capable of delivering H₂ independent of the time of day. Here, we report a fully integrated photochemical molecular dyad composed of a ruthenium-complex photosensitizer covalently linked to a Dawson polyoxometalate that acts as an electron-storage site and hydrogen-evolving catalyst. Visible-light irradiation of the system in solution leads to charge separation and electron storage on the polyoxometalate, effectively resulting in a liquid fuel. In contrast to related, earlier dyads, this system enables the harvesting, storage and delayed release of solar energy. On-demand hydrogen release is possible by adding a proton donor to the dyad solution. The system is a minimal molecular model for artificial photosynthesis and enables the spatial and temporal separation of light absorption, fuel storage and hydrogen release.

Outer-coordination sphere in multi-H+/multi-e-molecular electrocatalysis

Electrocatalysis is an indispensable technique for small-molecule transformations, which are essential for the sustainability of society. Electrocatalysis utilizes electricity as an energy source for chemical reactions. Hydrogen is considered the “fuel for the future,” and designing electrocatalysts for hydrogen production has thus become critical. Furthermore, fuel cells are promising energy solutions that require robust electrocatalysts for key fuel cell reactions such as the interconversion of oxygen to water. Concerns regarding the rising concentration of atmospheric carbon dioxide have prompted the search for CO₂ conversion methods. One promising approach is the electrochemical conversion of CO₂ into commodity chemicals and/or liquid fuels, but such chemistry is highly energy demanding because of the thermodynamic stability of CO₂. All of the above-mentioned electrocatalytic processes rely on the selective input of multiple protons (H⁺) and electrons (e^–) to yield the desired products. Biological enzymes evolved in nature to perform such redox catalysis and have inspired the design of catalysts at the molecular and atomic levels. While it is synthetically challenging to mimic the exact biological environment, incorporating functional outer coordination spheres into molecular catalysts has shown promise for advancing multi-H⁺ and multi-e^– electrocatalysis. From this Perspective, herein, catalysts with outer coordination sphere(s) are selected as the inspiration for developing new catalysts, particularly for the reductive conversion of H⁺, O₂, and CO₂, which are highly relevant to sustainability. The recent progress in electrocatalysis and opportunities to explore beyond the second coordination sphere are also emphasized.

Selectivity in Electrochemical CO2 Reduction

The electrocatalytic CO₂ reduction reaction (CO₂RR) to generate fixed forms of carbons that have commercial value is a lucrative avenue to ameliorate the growing concerns about the detrimental effect of CO₂ emissions as well as to generate carbon-based feed chemicals, which are generally obtained from the petrochemical industry. The area of electrochemical CO₂RR has seen substantial activity in the past decade, and several good catalysts have been reported. While the focus was initially on the rate and overpotential of electrocatalysis, it is gradually shifting toward the more chemically challenging issue of selectivity. CO₂ can be partially reduced to produce several C₁ products like CO, HCOOH, CH₃OH, etc. before its complete 8e^-/8H⁺ reduction to CH₄. In addition to that, the low-valent electron-rich metal centers deployed to activate CO₂, a Lewis acid, are prone to reduce protons, which are a substrate for CO₂RR, leading to competing hydrogen evolution reaction (HER). Similarly, the low-valent metal is prone to oxidation by atmospheric O₂ (i.e., it can catalyze the oxygen reduction reaction, ORR), necessitating strictly anaerobic conditions for CO₂RR. Not only is the requirement of O₂-free reaction conditions impractical, but it also leads to the release of partially reduced O₂ species such as O₂^-, H₂O₂, etc., which are reactive and result in oxidative degradation of the catalyst.In this Account, mechanistic investigations of CO₂RR by detecting and, often, chemically trapping and characterizing reaction intermediates are used to understand the factors that determine the selectivity in CO₂RR. The spectroscopic data obtained from different intermediates have been identified in different CO₂RR catalysts to develop an electronic structure selectivity relationship that is deemed to be important for deciding the selectivity of 2e^-/2H⁺ CO₂RR. The roles played by the spin state, hydrogen bonding, and heterogenization in determining the rate and selectivity of CO₂RR (producing only CO, only HCOOH, or only CH₄) are discussed using examples of both iron porphyrin and non-heme bioinspired artificial mimics. In addition, strategies are demonstrated where the competition between CO₂RR and HER as well as CO₂RR and ORR could be skewed overwhelmingly in favor of CO₂RR in both cases.

Copper(II) tetrakis(pentafluorophenyl)porphyrin: highly active copper-based molecular catalysts for electrochemical CO2 reduction

We report a highly active copper-based catalyst for electrochemical CO₂ reduction. Electrochemical analysis revealed that the maximum turnover frequency for CO₂ to CO conversion reached 1 460 000 s^-1 at an overpotential (η) of 0.85 V. Surprisingly, this value is more than 1 000 000 times higher than those of other reported copper-based molecular catalysts.

Strategies for Breaking Molecular Scaling Relationships for the Electrochemical CO2 Reduction Reaction

The electrocatalytic CO2 reduction reaction (CO2RR) is a promising strategy for converting CO2 to fuels and value-added chemicals using renewable energy sources. Molecular electrocatalysts show promise for the selective conversion...

Free Energies of Proton-Coupled Electron Transfer Reagents and Their Applications

We present an update and revision to our 2010 review on the topic of proton-coupled electron transfer (PCET) reagent thermochemistry. Over the past decade, the data and thermochemical formalisms presented in that review have been of value to multiple fields. Concurrently, there have been advances in the thermochemical cycles and experimental methods used to measure these values. This Review (i) summarizes those advancements, (ii) corrects systematic errors in our prior review that shifted many of the absolute values in the tabulated data, (iii) provides updated tables of thermochemical values, and (iv) discusses new conclusions and opportunities from the assembled data and associated techniques. We advocate for updated thermochemical cycles that provide greater clarity and reduce experimental barriers to the calculation and measurement of Gibbs free energies for the conversion of X to XH_n in PCET reactions. In particular, we demonstrate the utility and generality of reporting potentials of hydrogenation, E°(V vs H₂), in almost any solvent and how these values are connected to more widely reported bond dissociation free energies (BDFEs). The tabulated data demonstrate that E°(V vs H₂) and BDFEs are generally insensitive to the nature of the solvent and, in some cases, even to the phase (gas versus solution). This Review also presents introductions to several emerging fields in PCET thermochemistry to give readers windows into the diversity of research being performed. Some of the next frontiers in this rapidly growing field are coordination-induced bond weakening, PCET in novel solvent environments, and reactions at material interfaces.

Effects of Protonation State on Electrocatalytic CO2 Reduction by a Cobalt Aminopyridine Macrocyclic Complex

A critical component in the reduction of CO₂ to CO and H₂O is the delivery of 2 equiv of protons and electrons to the CO₂ molecule. The timing and sequencing of these proton and electron transfer steps are essential factors in directing the activity and selectivity for catalytic CO₂ reduction. In previous studies, we have reported a series of macrocyclic aminopyridine cobalt complexes capable of reducing CO₂ to CO with high faradaic efficiencies. Kinetic investigations reveal a relationship between the observed rate constant (k_obs) and the number of pendant amine hydrogen bond donors minus one, suggesting the presence of a deprotonated active catalytic state. Herein, we investigate the feasibility of these proposed deprotonated complexes toward CO₂ reduction. Two deprotonated derivatives, Co(L^4-) and Co(L^2-), of the tetraamino macrocycle Co(L) were independently synthesized and structurally characterized revealing extensive delocalization of the negative charge upon deprotonation. ¹H nuclear magnetic resonance spectroscopy and ultraviolet-visible titration studies confirm that under catalytic conditions, the active form of the catalyst gradually becomes deprotonated, supporting thus the n_donor - 1 relationship with k_obs. Electrochemical studies of Co(L^4-) reveal that this deprotonated analogue is competent for electrocatalysis upon addition of an exogenous weak acid source, such as 2,2,2-trifluoroethanol, resulting in faradaic efficiencies for CO₂-to-CO conversion identical to those observed with the fully protonated derivative (>98%).

Spontaneous N2 formation by a diruthenium complex enables electrocatalytic and aerobic oxidation of ammonia

The electrochemical conversion of ammonia to dinitrogen in a direct ammonia fuel cell (DAFC) is a necessary technology for the realization of a nitrogen economy. Previous efforts to catalyse this reaction with molecular complexes required the addition of exogenous oxidizing reagents or application of potentials greater than the thermodynamic potential for the oxygen reduction reaction-the cathodic process of a DAFC. We report a stable metal-metal bonded diruthenium complex that spontaneously produces dinitrogen from ammonia under ambient conditions. The resulting reduced diruthenium material can be reoxidized with oxygen for subsequent reactions with ammonia, demonstrating its ability to spontaneously promote both half-reactions necessary for a DAFC. The diruthenium complex also acts as a redox mediator for the electrocatalytic oxidation of ammonia to dinitrogen at potentials as low as -255 mV versus Fc^0/+ and operates below the oxygen reduction reaction potential in alkaline conditions, thus achieving a thermodynamic viability relevant for the future development of DAFCs.

A Metal-Free, Photocatalytic Method for Aerobic Alkane Iodination

Halogenation is an important alkane functionalization strategy, but O₂ is widely considered the most desirable terminal oxidant. Here, the aerobic iodination of alkanes, including methane, was performed using catalytic [ⁿBu₄N]Cl and light irradiation (390 nm). Up to 10 turnovers of CH₃I were obtained from CH₄ and air, using a stop-flow microtubing system. Mechanistic studies using cyclohexane as the substrate revealed important details about the iodination reaction. Iodine (I₂) serves multiple roles in the catalysis: (1) as the alkyl radical trap, (2) as a precursor for the light absorber, and (3) as a mediator of aerobic oxidation. The alkane activation is attributed to Cl^• derived from photofragmentation of the electron donor-acceptor complex of I₂ and Cl^-. The kinetic profile of cyclohexane iodination showed that aerobic oxidation of I₃^- to produce I₂ in CH₃CN is turnover-limiting.

"How Should I Think about Voltage? What Is Overpotential?": Establishing an Organic Chemistry Intuition for Electrochemistry

Redox reactions are ubiquitous in organic synthesis and intrinsic to organic electrosynthesis. The language and concepts used to describe reactions in these domains are sufficiently different to create barriers that hinder broader adoption and understanding of electrochemical methods. To bridge these gaps, this Synopsis compares chemical and electrochemical redox reactions, including concepts of free energy, voltage, kinetic barriers, and overpotential. This discussion is intended to increase the accessibility of electrochemistry for organic chemists lacking formal training in this area.

Electrocatalytic syngas generation with a redox non-innocent cobalt 2-phosphinobenzenethiolate complex

A cobalt complex supported by the 2-(diisopropylphosphaneyl)benzenethiol ligand was synthesized and its electronic structure and reactivity were explored. X-ray diffraction studies indicate a square planar geometry around the cobalt center with a trans arrangement of the phosphine ligands. Density functional theory calculations and electronic spectroscopy measurements suggest a mixed metal-ligand orbital character, in analogy to previously studied dithiolene and diselenolene systems. Electrochemical studies in the presence of 1 atm of CO₂ and Brønsted acid additives indicate that the cobalt complex generates syngas, a mixture of H₂ and CO, with faradaic efficiencies up to >99%. The ratios of H₂ : CO generated vary based on the additive. A H₂ : CO ratio of ∼3 : 1 is generated when H₂O is used as the Brønsted acid additive. Chemical reduction of the complex indicates a distortion towards a tetrahedral geometry, which is rationalized with DFT predictions as attributable to the populations of orbitals with σ*(Co-S) character. A mechanistic scheme is proposed whereby competitive binding between a proton and CO₂ dictates selectivity. This study provides insight into the development of a catalytic system incorporating non-innocent ligands with pendant base moieties for electrochemical syngas production.

Recent Advances in Carbon Dioxide Conversion: A Circular Bioeconomy Perspective

Managing the concentration of atmospheric CO2 requires a multifaceted engineering strategy, which remains a highly challenging task. Reducing atmospheric CO2 (CO2R) by converting it to value-added chemicals in a carbon neutral footprint manner must be the ultimate goal. The latest progress in CO2R through either abiotic (artificial catalysts) or biotic (natural enzymes) processes is reviewed herein. Abiotic CO2R can be conducted in the aqueous phase that usually leads to the formation of a mixture of CO, formic acid, and hydrogen. By contrast, a wide spectrum of hydrocarbon species is often observed by abiotic CO2R in the gaseous phase. On the other hand, biotic CO2R is often conducted in the aqueous phase and a wide spectrum of value-added chemicals are obtained. Key to the success of the abiotic process is understanding the surface chemistry of catalysts, which significantly governs the reactivity and selectivity of CO2R. However, in biotic CO2R, operation conditions and reactor design are crucial to reaching a neutral carbon footprint. Future research needs to look toward neutral or even negative carbon footprint CO2R processes. Having a deep insight into the scientific and technological aspect of both abiotic and biotic CO2R would advance in designing efficient catalysts and microalgae farming systems. Integrating the abiotic and biotic CO2R such as microbial fuel cells further diversifies the spectrum of CO2R.

Enhancing the CO2-to-CO Conversion from 2D Silver Nanoprisms via Superstructure Assembly

The electrochemical reduction of CO₂ in a highly selective and efficient manner is a crucial step toward its reuse for the production of chemicals and fuels. Nanostructured Ag catalysts have been found to be effective candidates for the conversion of CO₂-to-CO. However, the ambiguous determination of the intrinsic CO₂ activity and the maximization of the density of exposed active sites have greatly limited the use of Ag toward the realization of practical electrocatalytic devices. Here, we report a superstructure design strategy prepared by the self-assembly of two-dimensional Ag nanoprisms for maximizing the exposure of active edge ribs. The vertically stacked Ag nanoprisms allow the exposure of >95% of the edge sites, resulting in an enhanced selectivity and activity toward the production of CO from CO₂ with an overpotential of 152 mV. The Ag superstructures also demonstrate a selectivity of over 90% for 100 h together with a current retention of ≈94% at -600 mV versus the reversible hydrogen electrode and a partial energy efficiency for CO production of 70.5%. Our electrochemical measurements on individual Ag nanoprisms with various edge-to-basal plane ratios and the Ag superstructures led to the identification of the edge ribs as the active sites thanks to the ≈400 mV decrease in the onset potential compared to that of the Ag (111) basal planes and a turnover frequency of 9.2 × 10^-3 ± 1.9 × 10^-3 s^-1 at 0 V overpotential.