Concurrent Ammonia Synthesis and Alcohol Oxidation Boosted by Glutathione-Capped Quantum Dots under Visible Light

Mother nature accomplishes efficient ammonia synthesis via cascade N2 oxidation by lightning strikes followed with enzyme-catalyzed nitrogen oxyanion (NOx -, x = 2,3) reduction. The protein environment of enzymatic centers for NOx --to-NH4 + process greatly inspires the design of glutathione-capped (GSH) quantum dots (QDs) for ammonia synthesis under visible light (440 nm) in tandem with plasma-enabled N2 oxidation. Mechanistic studies reveal that GSH induces positive shift of surface charge to strengthen the interaction between NOx - and QDs. Upon visible light irradiation of QDs, the balanced and rapid hole and electron transfer furnish GS·radicals for 2e-/2H+ alcohol oxidation and H·for 8e-/10H+ NO3 --to-NH4 + reduction simultaneously. For the first time, mmol-scale ammonia synthesis is realized with apparent quantum yields of 5.45% ± 0.64%, and gram-scale synthesis of value-added acetophenone and NH4Cl proceeds with 1:4 stoichiometry and stability, demonstrating promising multielectron and multiproton ammonia synthesis efficiency and sustainability with nature-inspired artificial photocatalysts.

Electrocatalytic CO2 reduction to formate by a cobalt phosphino-thiolate complex

Electrochemical conversion of CO2 to value-added products serves as an attractive method to store renewable energy as energy-dense fuels. Selectivity in this type of conversion can be limited, often leading to the formation of side products such as H2. The activity of a cobalt phosphino-thiolate complex ([Co(triphos)(bdt)]+) towards the selective reduction of CO2 to formate is explored in this report. In the presence of H2O, selective production of formate (as high as 94%) is observed at overpotentials of 750 mV, displaying negligible current degradation during long-term electrolysis experiments ranging as long as 24 hours. Chemical reduction studies of [Co(triphos)(bdt)]+ indicates deligation of the apical phosphine moiety is likely before catalysis. Computational and experimental results suggest a metal-hydride pathway, indicating an ECEC based mechanism.

Kinetic isotope effect offers selectivity in CO2 reduction

A binuclear Ni complex with N,O donors catalyzes CO2 reduction via its Ni(I) state. The product distribution when H2O is used as a proton source shows similar yields for CO, HCOOH and H2. However, when D2O is used, the product distribution shows a ∼65% selectivity for HCOOH. In situ FTIR indicates that the reaction involves a Ni-COO* and a Ni-CO intermediate. Differences in H/D KIEs on different protonation pathways determine the selectivity of CO2 reduction.

Bioinspired Binickel Catalyst for Carbon Dioxide Reduction: The Importance of Metal-ligand Cooperation

Catalyst design for the efficient CO2 reduction reaction (CO2RR) remains a crucial challenge for the conversion of CO2 to fuels. Natural Ni-Fe carbon monoxide dehydrogenase (NiFe-CODH) achieves reversible conversion of CO2 and CO at nearly thermodynamic equilibrium potential, which provides a template for developing CO2RR catalysts. However, compared with the natural enzyme, most biomimetic synthetic Ni-Fe complexes exhibit negligible CO2RR catalytic activities, which emphasizes the significance of effective bimetallic cooperation for CO2 activation. Enlightened by bimetallic synergy, we herein report a dinickel complex, NiIINiII(bphpp)(AcO)2 (where NiNi(bphpp) is derived from H2bphpp = 2,9-bis(5-tert-butyl-2-hydroxy-3-pyridylphenyl)-1,10-phenanthroline) for electrocatalytic reduction of CO2 to CO, which exhibits a remarkable reactivity approximately 5 times higher than that of the mononuclear Ni catalyst. Electrochemical and computational studies have revealed that the redox-active phenanthroline moiety effectively modulates the electron injection and transfer akin to the [Fe3S4] cluster in NiFe-CODH, and the secondary Ni site facilitates the C-O bond activation and cleavage through electron mediation and Lewis acid characteristics. Our work underscores the significant role of bimetallic cooperation in CO2 reduction catalysis and provides valuable guidance for the rational design of CO2RR catalysts.

Switching Electrocatalytic Hydrogen Evolution Pathways through Electronic Tuning of Copper Porphyrins

The electronic structure of metal complexes plays key roles in determining their catalytic features. However, controlling electronic structures to regulate reaction mechanisms is of fundamental interest but has been rarely presented. Herein, we report electronic tuning of Cu porphyrins to switch pathways of the hydrogen evolution reaction (HER). Through controllable and regioselective β-oxidation of Cu porphyrin 1, we synthesized analogues 2-4 with one or two β-lactone groups in either a cis or trans configuration. Complexes 1-4 have the same Cu-N4 core site but different electronic structures. Although β-oxidation led to large anodic shifts of reductions, 1-4 displayed similar HER activities in terms of close overpotentials. With electrochemical, chemical and theoretical results, we show that the catalytically active species switches from a CuI species for 1 to a Cu0 species for 4. This work is thus significant to present mechanism-controllable HER via electronic tuning of catalysts.

Antenna Effect in Noble Metal-Free Dye-Sensitized Photocatalytic Systems Enhances CO2 -to-CO Conversion

Dye-sensitized photocatalytic systems (DSPs) have been extensively investigated for solar-driven hydrogen (H2 ) evolution. However, their application in carbon dioxide (CO2 ) reduction remains limited. Furthermore, current solar-driven CO2 -to-CO DSPs typically employ rhenium complexes as catalysts. In this study, we have developed DSPs that incorporate noble metal-free components, specifically a zinc-porphyrin as photosensitizer (PS) and a cobalt-quaterpyridine as catalyst (CAT). Taking a significant stride forward, we have achieved an antenna effect for the first time in CO2 -to-CO DSPs by introducing a Bodipy as an additional chromophore to enhance light harvesting efficiency. The energy transfer from Bodipy to zinc porphyrin resulted in remarkable stability (turn over number (TON)=759 vs. CAT), and high CO evolution activity (42 mmol g-1  h-1 vs. CAT).

Facile electrocatalytic proton reduction by a [Fe-Fe]-hydrogenase bio-inspired synthetic model bearing a terminal CN- ligand

An azadithiolate bridged CN- bound pentacarbonyl bis-iron complex, mimicking the active site of [Fe-Fe] H2ase is synthesized. The geometric and electronic structure of this complex is elucidated using a combination of EXAFS analysis, infrared and Mössbauer spectroscopy and DFT calculations. The electrochemical investigations show that complex 1 effectively reduces H+ to H2 between pH 0-3 at diffusion-controlled rates (1011 M-1 s-1) i.e. 108 s-1 at pH 3 with an overpotential of 140 mV. Electrochemical analysis and DFT calculations suggests that a CN- ligand increases the pKa of the cluster enabling hydrogen production from its Fe(i)-Fe(0) state at pHs much higher and overpotential much lower than its precursor bis-iron hexacarbonyl model which is active in its Fe(0)-Fe(0) state. The formation of a terminal Fe-H species, evidenced by spectroelectrochemistry in organic solvent, via a rate determining proton coupled electron transfer step and protonation of the adjacent azadithiolate, lowers the kinetic barrier leading to diffusion controlled rates of H2 evolution. The stereo-electronic factors enhance its catalytic rate by 3 order of magnitude relative to a bis-iron hexacarbonyl precursor at the same pH and potential.

Mechanistic insights of electrocatalytic CO2 reduction by Mn complexes: synergistic effects of the ligands

The electrocatalytic mechanisms of CO2 reduction catalyzed by pyridine-oxazoline (pyrox)-based Mn catalysts were investigated by DFT calculations. In-depth comparative analyses of pyrox-based and bipyridine-based Mn complexes were carried out. C-OH cleavage is the rate-determining step for both the protonation-first path and the reduction-first path. The free energy of CO2 activation (ΔG1) and the electrons donated by CO ligands in this step are effective descriptors in regulating the C-OH cleavage barrier. The reduction of carboxylate complex 6 (E6) is the potential-determining step for the reduction-first path. Meanwhile, for the protonation-first path, the initial generation (E2) or the regeneration (E8) of active catalyst might be potential-determining. Hirshfeld charge and orbital contribution analysis indicate that E6 is definitely based on the heterocyclic ligand and E2 is related to both the heterocyclic ligand and three CO ligands. Therefore, replacement of the CO ligand by a stronger electron donating ligand can effectively boost the catalytic activity of CO2 reduction without increasing the overpotential in the reduction-first path. This hypothesis is supported by the mechanism calculations of the Mn complex in which the axial CO ligand is replaced by a pyridine or PMe3.

Second Coordination Sphere Effect Shifts CO2 to CO Reduction by Iron Porphyrin from Fe0 to FeI

Iron porphyrins are among the most studied molecular catalysts for carbon dioxide (CO2 ) reduction and their reactivity is constantly being enhanced through the implementation of chemical functionalities in the second coordination sphere inspired by the active sites of enzymes. In this study, we were intrigued to observe that a multipoint hydrogen bonding scheme provided by embarked urea groups could also shift the redox activation step of CO2 from the well-admitted Fe(0) to the Fe(I) state. Using EPR, resonance Raman, IR and UV-Visible spectroscopies, we underpinned a two-electron activation step of CO2 starting from the Fe(I) oxidation state to form, after protonation, an Fe(III)-COOH species. The addition of another electron and a proton to the latter species converged to the cleavage of a C-O bond with the loss of water molecule resulting in an Fe(II)-CO species. DFT analyses of these postulated intermediates are in good agreement with our collected spectroscopic data, allowing us to propose an alternative pathway in the catalytic CO2 reduction with iron porphyrin catalyst. Such a remarkable shift opens new lines of research in the design of molecular catalysts to reach low overpotentials in performing multi-electronic CO2 reduction catalysis.

Electrocatalytic Interconversions of CO2 and Formate on a Versatile Iron-Thiolate Platform

Exploring bidirectional CO2/HCO2- catalysis holds significant potential in constructing integrated (photo)electrochemical formate fuel cells for energy storage and applications. Herein, we report selective CO2/HCO2- electrochemical interconversion by exploiting the flexible coordination modes and rich redox properties of a versatile iron-thiolate platform, Cp*Fe(II)L (L = 1,2-Ph2PC6H4S-). Upon oxidation, this iron complex undergoes formate binding to generate a diferric formate complex, [(L-)2Fe(III)(μ-HCO2)Fe(III)]+, which exhibits remarkable electrocatalytic performance for the HCO2--to-CO2 transformation with a maximum turnover frequency (TOFmax) ∼103 s-1 and a Faraday efficiency (FE) ∼92(±4)%. Conversely, this iron system also allows for reduction at -1.85 V (vs Fc+/0) and exhibits an impressive FE ∼93 (±3)% for the CO2-to-HCO2- conversion. Mechanism studies revealed that the HCO2--to-CO2 electrocatalysis passes through dicationic [(L2)-•Fe(III)(μ-HCO2)Fe(III)]2+ generated by unconventional oxidation of the diferric formate species taking place at ligand L, while the CO2-to-HCO2- reduction involves a critical intermediate of [Fe(II)-H]- that was independently synthesized and structurally characterized.

Closed Synthetic Cycle for Nickel-Based Dihydrogen Formation

Dihydrogen evolution was observed in a two-step protonation reaction starting from a Ni0 precursor with a tripodal N-heterocyclic carbene (NHC) ligand. Upon the first protonation, a NiII monohydride complex was formed, which was isolated and fully characterized. Subsequent protonation yields H2 via a transient intermediate (INT) and an isolable NiII acetonitrile complex. The latter can be reduced to regenerate its Ni0 precursor. The mechanism of H2 formation was investigated by using a deuterated acid and scrutinized by 1 H NMR spectroscopy and gas chromatography. Remarkably, the second protonation forms a rare nickel dihydrogen complex, which was detected and identified in solution and characterized by 1 H NMR spectroscopy. DFT-based computational analyses were employed to propose a reaction profile and a molecular structure of the Ni-H2 complex. Thus, a dihydrogen-evolving, closed-synthetic cycle is reported with a rare Ni-H2 species as a key intermediate.

Self-Powered Biohybrid Systems Based on Organic Materials for Sustainable Biosynthesis

Sustainable energy conversion and effective biosynthesis for value-added chemicals have attracted considerable attention, but most biosynthesis systems cannot work independently without external power. In this work, a self-powered biohybrid system based on organic materials is designed and constructed successfully by integrating electroactive microorganisms with electrochemical devices. Among them, the hybrid living materials based on S. oneidensis/poly[3-(3'-N,N,N-triethylamino-1'-propyloxy)-4-methyl-2,5-thiophene chloride] (PMNT) biofilms for microbial fuel cells played a crucial role in electrocatalytic biocurrent generation by using biowaste as the only energy source. Without any external power supplies, the self-powered biohybrid systems could generate, convert, and store electrical energy for effective photosynthetic regulation and sustained chemical production. This work provides a new strategy to combine comprehensive renewable energy production with chemical manufacturing without an external power source in the future.

Electrocatalytic reduction of CO2 to CO by a series of organometallic Re(I)-tpy complexes

A series of organometallic Re(I)(L)(CO)3Br complexes with 4'-substituted terpyridine ligands (L) has been synthesised as electrocatalysts for CO2 reduction. The complexes' spectroscopic characterisation and computationally optimised geometry demonstrate a facial geometry around Re(I) with three cis COs and the terpyridine ligand coordinating in a bidentate mode. The effect of substitution on the 4'-position of terpyridine (Re1-5) on CO2 electroreduction was investigated and compared with a known Lehn-type catalyst, Re(I)(bpy)(CO)3Br (Re7). All complexes catalyse CO evolution in homogeneous organic media at moderate overpotentials (0.75-0.95 V) with faradaic yields of 62-98%. The electrochemical catalytic activity was further evaluated in the presence of three Brønsted acids to demonstrate the influence of the pKa of the proton sources. The TDDFT and ultrafast transient absorption spectroscopy (TAS) studies showed combined charge transfer bands of ILCT and MLCT. Amongst the series, the Re-complex containing a ferrocenyl-substituted terpyridine ligand (Re5) shows an additional intra-ligand charge transfer band and was probed using UV-Vis spectroelectrochemistry.

Effects of proton tunneling distance on CO2 reduction by Mn terpyridine species

Herein, we report two manganese terpyridine dicarbonyl complexes, covalently attached to a proximal (1) or distal (2) amide moiety at the ortho position of the pendent phenyl ring as a proton relay. The isomer 1 achieves a turnover frequency (TOF) of 325 s-1 with a minor overpotential of ca. 200 mV. The performance ranks it among the most efficient molecular catalysts for CO2-to-CO conversion, and it is ca.2 orders faster than isomer 2.

Main group elements in electrochemical hydrogen evolution and carbon dioxide reduction

Main-group elements are renowned for their versatile reactivities in organometallic chemistry, including CO2 insertion and H2 activation. However, electrocatalysts comprising a main-group element active site have not yet been widely developed for activating CO2 or producing H2. Recently, research has focused on main-group element-based electrocatalysts that are active in redox systems related to fuel-forming reactions. These studies have determined that the catalytic performances of heavier main-group element-based electrocatalysts are often similar to those of transition-metal-based electrocatalysts. Our group has recently reported the scope of including the main-group elements in the design of molecular catalysts and explored their applications in redox catalysis, such as the generation of H2 upon coupling of two protons (H+) and two electrons (e-). This feature article summarizes our research efforts in developing molecular electrocatalysts comprising main-group elements at their active sites. Furthermore, we highlight their influence on the rate-determining step, thereby enhancing the reaction rate and product selectivity for multi-H+/multi-e- transfer catalysis. Particularly, we focus on the performance of our recently reported molecular Sn- or Sb-centered macrocycles for electrocatalytic H2 evolution reaction (HER) and on how their mechanisms resemble those of transition-metal-based electrocatalysts. Moreover, we discuss the CO2 reduction reaction (CO2RR), another promising fuel-forming reaction, and emphasize the recent progress in including the main-group elements in the CO2RR. Although the main-group elements are found at the active sites of the molecular catalysts and are embedded in the electrode materials for studying the HER, molecular catalysts bearing main-group elements are not commonly used for CO2RR. However, the main-group elements assist the CO2RR by acting as co-catalysts. For example, alkali and alkaline earth metal ions (e.g., Li+, Na+, K+, Rb+, Cs+, Mg2+, Ca2+, and Ba2+) are known for their Lewis acidities, which influence the thermodynamic landscape of the CO2RR and product selectivity. In contrast, the elements in groups 13, 14, and 15 are primarily used as dopants in the preparation of catalytic materials. Overall, this article identifies main-group element-based molecular electrocatalysts and materials for HER and CO2RR.

Mimicking Metalloenzyme Microenvironments in the Transition Metal-Single Atom Catalysts for Electrochemical Hydrogen Peroxide Synthesis in an Acidic Medium

Electrochemical reduction of oxygen into hydrogen peroxide in an acidic medium offers an energy-efficient and green H2 O2 synthesis as an alternative to the energy-intensive anthraquinone process. Unfortunately, high overpotential, low production rates, and fierce competition from traditional four-electron reduction limit it. In this study, a metalloenzyme-like active structure is mimicked in carbon-based single-atom electrocatalysts for oxygen reduction to H2 O2 . Using a carbonization strategy, the primary electronic structure of the metal center with nitrogen and oxygen coordination is modulated, followed by epoxy oxygen functionalities close to the metal active sites. In an acidic medium, CoNOC active structures proceed with greater than 98% H2 O2 selectivity (2e- /2H+ ) rather than CoNC active sites that are selective to H2 O (4e- /4H+ ). Among all MNOC (M = Fe, Co, Mn, and Ni) single-atom electrocatalysts, the CoNOC is the most selective (> 98%) for H2 O2 production, with a mass activity of 10 A g-1 at 0.60 V vs. RHE. X-ray absorption spectroscopy is used to identify the formation of unsymmetrical MNOC active structures. Experimental results are also compared to density functional theory calculations, which revealed that the structure-activity relationship of the epoxy-surrounded CoNOC active structure reaches optimum (ΔG*OOH ) binding energies for high selectivity.

Mimicking reductive dehalogenases for efficient electrocatalytic water dechlorination

Electrochemical technology is a robust approach to removing toxic and persistent chlorinated organic pollutants from water; however, it remains a challenge to design electrocatalysts with high activity and selectivity as elaborately as natural reductive dehalogenases. Here we report the design of high-performance electrocatalysts toward water dechlorination by mimicking the binding pocket configuration and catalytic center of reductive dehalogenases. Specifically, our designed electrocatalyst is an assembled heterostructure by sandwiching a molecular catalyst into the interlayers of two-dimensional graphene oxide. The electrocatalyst exhibits excellent dechlorination performance, which enhances reduction of intermediate dichloroacetic acid by 7.8 folds against that without sandwich configuration and can selectively generate monochloro-groups from trichloro-groups. Molecular simulations suggest that the sandwiched inner space plays an essential role in tuning solvation shell, altering protonation state and facilitating carbon-chlorine bond cleavage. This work demonstrates the concept of mimicking natural reductive dehalogenases toward the sustainable treatment of organohalogen-contaminated water and wastewater.

Bimetallic Molecular Catalyst Design for Carbon Dioxide Reduction

The core challenge in developing cost-efficient catalysts for carbon dioxide (CO2 ) conversion mainly lies in controlling its complex reaction pathways. One such strategy exploits bimetallic cooperativity, which relies on the synergistic interaction between two metal centers to activate and convert the CO2 substrate. While this approach has seen an important trend in heterogeneous catalysis as a handle to control stabilities of surface intermediates, it has not often been utilized in molecular and heterogenized molecular catalytic systems. In this review, we gather general principles on how natural CO2 activating enzymes take advantage of bimetallic strategy and how phosphines, cyclams, polypyridyls, porphyrins, and cryptates-based homo- and hetero-bimetallic molecular catalysts can help understand the synergistic effect of two metal centers.

Metallocavitins as Advanced Enzyme Mimics and Promising Chemical Catalysts

The supramolecular approach is becoming increasingly dominant in biomimetics and chemical catalysis due to the expansion of the enzyme active center idea, which now includes binding cavities (hydrophobic pockets), channels and canals for transporting substrates and products. For a long time, the mimetic strategy was mainly focused on the first coordination sphere of the metal ion. Understanding that a highly organized cavity-like enzymatic pocket plays a key role in the sophisticated functionality of enzymes and that the activity and selectivity of natural metalloenzymes are due to the effects of the second coordination sphere, created by the protein framework, opens up new perspectives in biomimetic chemistry and catalysis. There are two main goals of mimicking enzymatic catalysis: (1) scientific curiosity to gain insight into the mysterious nature of enzymes, and (2) practical tasks of mankind: to learn from nature and adopt from its many years of evolutionary experience. Understanding the chemistry within the enzyme nanocavity (confinement effect) requires the use of relatively simple model systems. The performance of the transition metal catalyst increases due to its retention in molecular nanocontainers (cavitins). Given the greater potential of chemical synthesis, it is hoped that these promising bioinspired catalysts will achieve catalytic efficiency and selectivity comparable to and even superior to the creations of nature. Now it is obvious that the cavity structure of molecular nanocontainers and the real possibility of modifying their cavities provide unlimited possibilities for simulating the active centers of metalloenzymes. This review will focus on how chemical reactivity is controlled in a well-defined cavitin nanospace. The author also intends to discuss advanced metal–cavitin catalysts related to the study of the main stages of artificial photosynthesis, including energy transfer and storage, water oxidation and proton reduction, as well as highlight the current challenges of activating small molecules, such as H2O, CO2, N2, O2, H2, and CH4.

Bio-Inspired Bimetallic Cooperativity Through a Hydrogen Bonding Spacer in CO2 Reduction

At the core of carbon monoxide dehydrogenase (CODH) active site two metal ions together with hydrogen bonding scheme from amino acids orchestrate the interconversion between CO₂ and CO. We have designed a molecular catalyst implementing a bimetallic iron complex with an embarked second coordination sphere with multi-point hydrogen-bonding interactions. We found that, when immobilized on carbon paper electrode, the dinuclear catalyst enhances up to four fold the heterogeneous CO₂ reduction to CO in water with an improved selectivity and stability compared to the mononuclear analogue. Interestingly, quasi-identical catalytic performances are obtained when one of the two iron centers was replaced by a redox inactive Zn metal, questioning the cooperative action of the two metals. Snapshots of X-ray structures indicate that the two metalloporphyrin units tethered by a urea group is a good compromise between rigidity and flexibility to accommodate CO₂ capture, activation, and reduction.

Bioinspired and biomolecular catalysts for energy conversion and storage

Metalloenzymes are remarkable for facilitating challenging redox transformations with high efficiency and selectivity. In the area of alternative energy, scientists aim to capture these properties in bioinspired and engineered biomolecular catalysts for the efficient and fast production of fuels from low-energy feedstocks such as water and carbon dioxide. In this short review, efforts to mimic biological catalysts for proton reduction and carbon dioxide reduction are highlighted. Two important recurring themes are the importance of the microenvironment of the catalyst active site and the key role of proton delivery to the active site in achieving desired reactivity. Perspectives on ongoing and future challenges are also provided.

A Comprehensive Review on Graphitic Carbon Nitride for Carbon Dioxide Photoreduction

Inspired by natural photosynthesis, harnessing the wide range of natural solar energy and utilizing appropriate semiconductor-based catalysts to convert carbon dioxide into beneficial energy species, for example, CO, CH₄ , HCOOH, and CH₃ COH have been shown to be a sustainable and more environmentally friendly approach. Graphitic carbon nitride (g-C₃ N₄ ) has been regarded as a highly effective photocatalyst for the CO₂ reduction reaction, owing to its cost-effectiveness, high thermal and chemical stability, visible light absorption capability, and low toxicity. However, weaker electrical conductivity, fast recombination rate, smaller visible light absorption window, and reduced surface area make this catalytic material unsuitable for commercial photocatalytic applications. Therefore, certain procedures, including elemental doping, structural modulation, functional group adjustment of g-C₃ N₄ , the addition of metal complex motif, and others, may be used to improve its photocatalytic activity towards effective CO₂ reduction. This review has investigated the scientific community's perspectives on synthetic pathways and material optimization approaches used to increase the selectivity and efficiency of the g-C₃ N₄ -based hybrid structures, as well as their benefits and drawbacks on photocatalytic CO₂ reduction. Finally, the review concludes a comparative discussion and presents a promising picture of the future scope of the improvements.

Microalgae-derived hydrogen production towards low carbon emissions via large-scale outdoor systems

Hydrogen as a clean fuel is receiving attention because it generates only water and a small amount of nitrogen oxide upon combustion. Biohydrogen production using microalgae is considered to be a highly promising carbon-neutral technology because it can secure renewable energy while efficiently reducing CO2 emissions. However, previous studies have mainly focused on improving the biological performance of microalgae; these approaches have struggled to achieve breakthroughs in commercialization because they do not heavily consider the complexity of the entire production process with microalgae, including large-scale cultivation, biomass harvest, and biomass storage. This work presents an in-depth analysis of the state-of-the-art technologies focused on large-scale cultivation systems with efficient downstream processes. Considering the individual processes of biohydrogen production, strategies are discussed to minimize carbon emissions and improve productivity simultaneously. A comprehensive understanding of microalgae-derived biohydrogen production suggests future directions for realizing environmental and economic sustainability.

Second and Outer Coordination Sphere Effects in Nitrogenase, Hydrogenase, Formate Dehydrogenase, and CO Dehydrogenase

Gases like H₂, N₂, CO₂, and CO are increasingly recognized as critical feedstock in "green" energy conversion and as sources of nitrogen and carbon for the agricultural and chemical sectors. However, the industrial transformation of N₂, CO₂, and CO and the production of H₂ require significant energy input, which renders processes like steam reforming and the Haber-Bosch reaction economically and environmentally unviable. Nature, on the other hand, performs similar tasks efficiently at ambient temperature and pressure, exploiting gas-processing metalloenzymes (GPMs) that bind low-valent metal cofactors based on iron, nickel, molybdenum, tungsten, and sulfur. Such systems are studied to understand the biocatalytic principles of gas conversion including N₂ fixation by nitrogenase and H₂ production by hydrogenase as well as CO₂ and CO conversion by formate dehydrogenase, carbon monoxide dehydrogenase, and nitrogenase. In this review, we emphasize the importance of the cofactor/protein interface, discussing how second and outer coordination sphere effects determine, modulate, and optimize the catalytic activity of GPMs. These may comprise ionic interactions in the second coordination sphere that shape the electron density distribution across the cofactor, hydrogen bonding changes, and allosteric effects. In the outer coordination sphere, proton transfer and electron transfer are discussed, alongside the role of hydrophobic substrate channels and protein structural changes. Combining the information gained from structural biology, enzyme kinetics, and various spectroscopic techniques, we aim toward a comprehensive understanding of catalysis beyond the first coordination sphere.

Through-Space Electrostatic Effects of Positively Charged Substituents on the Hydrogen Evolution Reaction

Elucidating the effects of various structural components on energy-related small molecule activation is of fundamental and practical significance. Herein the inhibition effect of positively charged substituents on the hydrogen evolution reaction (HER) was reported. With the use of Cu porphyrins 1-5 containing different numbers and locations of positively charged substituents, it was demonstrated that their electrocatalytic HER activities significantly decreased when more cationic units were located close to the Cu ion: the i_cat /i_p (i_cat is the catalytic peak current, i_p is the one-electron reduction peak current) value decreased from 38 with zero cationic unit to 15 with four closely located cationic units. Inspired by this result, Cu porphyrin 6, with four meso-phenyl groups each bearing a negatively charged para-sulfonic substituent, was designed. With these anionic units, 6 outperformed the other Cu porphyrins for electrocatalytic HER under the same conditions.

The integration of bio-catalysis and electrocatalysis to produce fuels and chemicals from carbon dioxide

The dependence on fossil fuels has caused excessive emissions of greenhouse gases (GHGs), leading to climate changes and global warming. Even though the expansion of electricity generation will enable a wider use of electric vehicles, biotechnology represents an attractive route for producing high-density liquid transportation fuels that can reduce GHG emissions from jets, long-haul trucks and ships. Furthermore, to achieve immediate alleviation of the current environmental situation, besides reducing carbon footprint it is urgent to develop technologies that transform atmospheric CO₂ into fossil fuel replacements. The integration of bio-catalysis and electrocatalysis (bio-electrocatalysis) provides such a promising avenue to convert CO₂ into fuels and chemicals with high-chain lengths. Following an overview of different mechanisms that can be used for CO₂ fixation, we will discuss crucial factors for electrocatalysis with a special highlight on the improvement of electron-transfer kinetics, multi-dimensional electrocatalysts and their hybrids, electrolyser configurations, and the integration of electrocatalysis and bio-catalysis. Finally, we prospect key advantages and challenges of bio-electrocatalysis, and end with a discussion of future research directions.

A Pyrene-Triazacyclononane Anchor Affords High Operational Stability for CO2 RR by a CNT-Supported Histidine-Tagged CODH

An original 1‐acetato‐4‐(1‐pyrenyl)‐1,4,7‐triazacyclononane (AcPyTACN) was synthesized for the immobilization of a His‐tagged recombinant CODH from Rhodospirillum rubrum (RrCODH) on carbon‐nanotube electrodes. The strong binding of the enzyme at the Ni‐AcPyTACN complex affords a high current density of 4.9 mA cm⁻² towards electroenzymatic CO₂ reduction and a high stability of more than 6×10⁶ TON when integrated on a gas‐diffusion bioelectrode.

Recent developments in the synthesis of bio-inspired iron porphyrins for small molecule activation

Nature utilizes a diverse set of tetrapyrrole-based macrocycles (referred to as porphyrinoids) for catalyzing various biological processes. Investigation of the differences in electronic structure and reactivity in these reactions have revealed striking differences that lead to diverse reactivity from, apparently, similar looking active sites. Therefore, the role of the different heme cofactors as well as the distal superstructure in the proteins is important to understand. This article summarizes the role of a few synthetic metallo-porphyrinoids towards catalyzing several small molecule activation reactions, such as the ORR, NiRR, CO₂RR, etc. The major focus of the article is to enlighten the synthetic routes to the well-decorated active-site mimic in a tailor-made fashion pursuing a retrosynthetic approach, learning from the biosynthesis of the cofactors. Techniques and the role of the second-sphere residues on the reaction rate, selectivity, etc. are incorporated emulating the basic amino acid residues fencing the active sites. These bioinspired mimics play an important role towards understanding the role of the prosthetic groups as well as the basic residues towards any reaction occurring in Nature.

Bioinorganic Chemistry on Electrodes: Methods to Functional Modeling

One of the major goals of bioinorganic chemistry has been to mimic the function of elegant metalloenzymes. Such functional modeling has been difficult to attain in solution, in particular, for reactions that require multiple protons and multiple electrons (nH⁺/ne^-). Using a combination of heterogeneous electrochemistry, electrode and molecule design one may control both electron transfer (ET) and proton transfer (PT) of these nH⁺/ne^- reactions. Such control can allow functional modeling of hydrogenases (H⁺ + e^- → 1/2 H₂), cytochrome c oxidase (O₂ + 4 e^- + 4 H⁺ → 2 H₂O), monooxygenases (RR'CH₂ + O₂ + 2 e^- + 2 H⁺ → RR'CHOH + H₂O) and dioxygenases (S + O₂ → SO₂; S = organic substrate) in aqueous medium and at room temperatures. In addition, these heterogeneous constructs allow probing unnatural bioinspired reactions and estimation of the inner- and outer-sphere reorganization energy of small molecules and proteins.

Manganese Carbonyl Complexes as Selective Electrocatalysts for CO2 Reduction in Water and Organic Solvents

The electrochemical reduction of CO₂ provides a way to sustainably generate carbon-based fuels and feedstocks. Molecular CO₂ reduction electrocatalysts provide tunable reaction centers offering an approach to control the selectivity of catalysis. Manganese carbonyl complexes, based on [Mn(bpy)(CO)₃Br] and its derivatives (bpy = 2,2′-bipyridine), are particularly interesting due to their ease of synthesis and the use of a first-row earth-abundant transition metal. [Mn(bpy)(CO)₃Br] was first shown to be an active and selective catalyst for reducing CO₂ to CO in organic solvents in 2011. Since then, manganese carbonyl catalysts have been widely studied with numerous reports of their use as electrocatalysts and photocatalysts and studies of their mechanism.

This class of Mn catalysts only shows CO₂ reduction activity with the addition of weak Brønsted acids. Perhaps surprisingly, early reports showed increased turnover frequencies as the acid strength is increased without a loss in selectivity toward CO evolution. It may have been expected that the competing hydrogen evolution reaction could have led to lower selectivity. Inspired by these works we began to explore if the catalyst would work in protic solvents, namely, water, and to explore the pH range over which it can operate. Here we describe the early studies from our laboratory that first demonstrated the use of manganese carbonyl complexes in water and then go on to discuss wider developments on the use of these catalysts in water, highlighting their potential as catalysts for use in aqueous CO₂ electrolyzers.

Key to the excellent selectivity of these catalysts in the presence of Brønsted acids is a proton-assisted CO₂ binding mechanism, where for the acids widely studied, lower pK_a values actually favor CO₂ binding over Mn–H formation, a precursor to H₂ evolution. Here we discuss the wider literature before focusing on our own contributions in validating this previously proposed mechanism through the use of vibrational sum frequency generation (VSFG) spectroelectrochemistry. This allowed us to study [Mn(bpy)(CO)₃Br] while it is at, or near, the electrode surface, which provided a way to identify new catalytic intermediates and also confirm that proton-assisted CO₂ binding operates in both the “dimer” and primary (via [Mn(bpy)(CO)₃]⁻) pathways. Understanding the mechanism of how these highly selective catalysts operate is important as we propose that the Mn complexes will be valuable models to guide the development of new proton/acid tolerant CO₂ reduction catalysts.

Dissection of Light-Induced Charge Accumulation at a Highly Active Iron Porphyrin: Insights in the Photocatalytic CO2 Reduction

Iron porphyrins are among the best molecular catalysts for the electrocatalytic CO₂ reduction reaction. Powering these catalysts with the help of photosensitizers comes along with a couple of unsolved challenges that need to be addressed with much vigor. We have designed an iron porphyrin catalyst decorated with urea functions (UrFe) acting as a multipoint hydrogen bonding scaffold towards the CO₂ substrate. We found a spectacular photocatalytic activity reaching unreported TONs and TOFs as high as 7270 and 3720 h^-1 , respectively. While the Fe⁰ redox state has been widely accepted as the catalytically active species, we show here that the Fe^I species is already involved in the CO₂ activation, which represents the rate-determining step in the photocatalytic cycle. The urea functions help to dock the CO₂ upon photocatalysis. DFT calculations bring support to our experimental findings that constitute a new paradigm in the catalytic reduction of CO₂ .

Understanding the Role of Inter- and Intramolecular Promoters in Electro- and Photochemical CO2 Reduction Using Mn, Re, and Ru Catalysts

Recycling of carbon dioxide to fuels and chemicals is a promising strategy for renewable energy storage. Carbon dioxide conversion can be achieved by (i) artificial photosynthesis using photoinduced electrons; (ii) electrolysis using electricity produced by photovoltaics; and (iii) thermal CO₂ hydrogenation using renewable H₂. The focus of our group's research is on molecular catalysts, in particular coordination complexes of transition metals (e.g., Mn, Re, and Ru), which offer versatile platforms for mechanistic studies of photo- and electrochemical CO₂ reduction. The interactions of catalytic intermediates with Lewis or Brønsted acids, hydrogen-bonding moieties, solvents, cations, etc., that function as promoters or cofactors have become increasingly important for efficient catalysis. These interactions may have dramatic effects on selectivity and rates by stabilizing intermediates or lowering transition state barriers, but they are difficult to elucidate and challenging to predict. We have been carrying out experimental and theoretical studies of CO₂ reduction using molecular catalysts toward addressing mechanisms of efficient CO₂ reduction systems with emphasis on those containing intramolecular (or pendent) and intermolecular (solution phase) additives. This Account describes the identification of reaction intermediates produced during CO₂ reduction in the presence of triethanolamine or ionic liquids, the benefits of hydrogen-bonding interactions among intermediates or cofactors, and the complications of pendent phenolic donors/phenoxide bases under electrochemical conditions.Triethanolamine (TEOA) is a common sacrificial electron donor for photosensitizer excited state reductive quenching and has a long history of use in photocatalytic CO₂ reduction. It also functions as a Brønsted base in conjunction with more potent sacrificial electron donors, such as 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH). Deprotonation of the BIH^•+ cation radical promotes irreversible photoinduced electron transfer by preventing charge recombination. Despite its wide use, most research to date has not considered the broader reactions of TEOA, including its direct interaction with CO₂ or its influence on catalytic intermediates. We found that in acetonitrile, TEOA captures CO₂ in the form of a zwitterionic adduct without any metal catalyst. In the presence of ruthenium carbonyl catalysts bearing α-diimine ligands, it participates in metal hydride formation, accelerates hydride transfer to CO₂ to form the bound formate intermediate, and assists in the dissociation of formate anion from the catalyst ( J. Am. Chem. Soc. 2020, 142, 2413-2428).Hydrogen bonding and acid/base promoters are understood to interact with key catalytic intermediates, such as the metallocarboxylate or metallocarboxylic acid during CO₂ reduction. The former is a high energy species, and hydrogen-bonding or Lewis acid-stabilization are beneficial. We have found that imidazolium-based ionic liquid cations can stabilize the doubly reduced form of the [ReCl(bpy)(CO)₃] (bpy = 2,2'-bipyridine) electrocatalyst through both hydrogen-bonding and π-π interactions, resulting in CO₂ reduction occurring at a more positive potential with a higher catalytic current ( J. Phys. Chem. Lett. 2014, 5, 2033-2038). Hydrogen bonding interactions between Lewis basic methoxy groups in the second coordination sphere of a Mn-based catalyst and the OH group of the Mn-COOH intermediate in the presence of a Brønsted acid were also found to promote C-(OH) bond cleavage, enabling access to a low-energy protonation-first pathway for CO₂ reduction ( J. Am. Chem. Soc. 2017, 139, 2604-2618).The kinetics of forming the metallocarboxylic acid can be enhanced by internal acids, and its proton-induced C-OH bond cleavage to the metallocarbonyl and H₂O is often the rate-limiting step. Therefore, proton movement organized by pendent hydrogen-bonding networks may also accelerate this step. In contrast, during electrolysis, OH groups in the second coordination sphere are deprotonated to the oxyanions, which deter catalytic CO₂ reduction by directly binding CO₂ to form the carbonate or by making an M-O bond in competition with CO₂ binding ( Inorg. Chem. 2016, 55, 4582-4594). Our results emphasize that detailed mechanistic research is critical in discovering the design principles for improved catalysts.

Metallocavitins as Promising Industrial Catalysts: Recent Advances

The energy, material, and environmental problems of society require clean materials and impose an urgent need to develop effective chemical processes for obtaining and converting energy to ensure further sustainable development. To solve these challenges, it is necessary, first of all, to learn solar energy harvesting through the development of artificial photosynthesis. In our planet, water, carbon dioxide, and methane are such affordable and inexhaustible clean materials. Electro/photocatalytic water splitting, and also CO₂ and CH₄ transforming into valuable products, requires the search for relevant efficient and selective processes and catalysts. Of great interest is the emerging new generation of bioinspired catalysts—metallocavitins (MCs). MCs are attracting increasing attention of researchers as advanced models of metalloenzymes, whose efficiency and selectivity are well known. The primary field of MC application is fine organic synthesis and enantioselective catalysis. On the other hand, MCs demonstrate high activity for energy challenging reactions involving small gas molecules and high selectivity for converting them into valuable products. This mini-review will highlight some recent advances in the synthesis of organic substances using MCs, but its main focus will be on the rapid development of advanced catalysts for the activation of small molecules, such as H₂O, CO₂, and CH₄, and the prospects for creating related technological processes in the future.

Mechanistic Study of Tungsten Bipyridyl Tetracarbonyl Electrocatalysts for CO2 Fixation: Exploring the Roles of Explicit Proton Sources and Substituent Effects

Tungsten bipyridyl tetracarbonyl complexes were shown to reduce CO2 to CO in acetonitrile [Chem. Sci., 2014, 5, 1894-1900]. Here, we employ density functional theory (DFT) calculations to investigate the electronic structure and reactivity of a series of tungsten electrocatalysts, [W(bpy-R)(CO)4] (where R = H, CH3, tBu, OCH3, CF3, and CN), for the CO2 reduction reaction (CO2RR). Our proposed mechanism suggests that initial reduction of the starting material by two electrons is required to access the active catalyst upon CO dissociation, which is slightly endergonic, consistent with the slow product release observed experimentally. The doubly reduced species, which has a closed-shell singlet ground state, can bind CO2 via an η2-CO2 binding mode to yield the metallocarboxylate intermediate. Based on the energy span model, CO2 addition is the TOF-determining transition state (TDTS) in the presence of water as the proton source. Different substituents at the 4,4'-positions of the bipyridine ligand in [W(bpy-R)(CO)4] (R = H, CH3, tBu, OCH3, CF3, and CN) were considered to comprehend the substituent effects for CO2RR. DFT results show that electron-withdrawing substituents, such as CN and CF3, do not yield efficient CO2 reduction catalysts due to the necessity of forming high energy intermediates for the protonation steps, resulting in low TOFs and high overpotentials. Among electron-donating groups, the parent compound and tert-butyl substituted complex are the most active catalysts for CO2RR due to higher TOFs at low overpotentials. Overall, based on the energy span model and theoretical Tafel plots, our computational approach provides quantitative information for designing CO2 reduction electrocatalysts.