Bio-inspired mechanistic insights into CO2 reduction

Insights into the Electrochemical CO2RR Performance and Binding of Small Molecules on Quaternary Thiospinels Ag2FeSn3S8 and Cu2FeSn3S8

Using a mechanical synthesis method in the form of ball milling and an additional annealing step, a novel and accelerated route for the synthesis of the thiospinels toyohaite (Ag2FeSn3S8) and rhodostannite (Cu2FeSn3S8) was discovered. Both thiospinels display faradaic efficiencies of up to 73% for CO2 reduction to CO using an organic electrolyte in an H-type cell. The materials were furthermore implemented in a zero-gap electrolyzer, with toyohaite producing 22% CO and 52% H2 at 100 mA cm-2 and rhodostannite 28% CO and 37% H2. The catalytically active sites are studied using density functional theory, revealing strong CO binding interactions on both Ag and Cu, whereas Sn is found to contribute to the decomposition of Ag2FeSn3S8 and Cu2FeSn3S8 by coordination with oxygen. Postmortem analysis of the thiospinel-based electrodes by means of SEM-EDX, XRD, XPS, and Mössbauer spectroscopy showed sulfur leaching from the catalysts after applying 100 mA cm-2. These spectroscopic results-in conjunction with DFT calculations of the oxidized surfaces-suggest that the catalytically active species consists of metal oxides. As a conversion of the metal sulfides into the corresponding metallic species was observed via XRD, the decomposition pathways of both catalysts were also computed using DFT; thus, elucidating the energetically most favorable decomposition products and expanding the possible composition of the catalysts postelectrolysis.

"Catch and release" of the CpN3 ligand using cobalt: dissociation, protonation, and C-H bond thermochemistry

The coordination chemistry of an amine-rich CpN3 ligand has been explored with cobalt. We demonstrate that in the presence of NaCo(CO)4, the cationic precursor [CpN3]+ yields the complex CpN3CoI(CO)2. While 2e- oxidation generates new CoIII complexes such as [CpN3Co(NCMe)3]2+ and CpN3CoI2(CO), subsequent ligand loss is facile, generating free [CpN3]+ or the protonated dication [CpN3H]2+. We have structurally characterized both these ligand release products via single crystal X-ray diffraction and obtained thermochemical C-H bond strengths via experiment and density functional theory (DFT). Upon reversible 1e- reduction, the radical cation [CpN3H]˙+ has a weak C-H BDFE of 52 kcal mol-1 in acetonitrile. Mechanistic analysis shows that [CpN3H]˙+ undergoes radical-radical disproportionation in the absence of exogenous H-atom acceptors, which is supported by deuterium isotope labelling experiments. Structural comparison of these organic molecules shows a high degree of iminium-like electron delocalization over the C-N bonds connected to the central five-membered ring.

Carbon dioxide reduction to high-value chemicals in microbial electrosynthesis system: Biological conversion and regulation strategies

Large emissions of atmospheric carbon dioxide (CO2) are causing climatic and environmental problems. It is crucial to capture and utilize the excess CO2 through diverse methods, among which the microbial electrosynthesis (MES) system has become an attractive and promising technology to mitigate greenhouse effects while reducing CO2 to high-value chemicals. However, the biological conversion and metabolic pathways through microbial catalysis have not been clearly elucidated. This review first introduces the main acetogenic bacteria for CO2 reduction and extracellular electron transfer mechanisms in MES. It then intensively analyzes the CO2 bioconversion pathways and carbon chain elongation processes in MES, together with energy supply and utilization. The factors affecting MES performance, including physical, chemical, and biological aspects, are summarized, and the strategies to promote and regulate bioconversion in MES are explored. Finally, challenges and perspectives concerning microbial electrochemical carbon sequestration are proposed, and suggestions for future research are also provided. This review provides theoretical foundation and technical support for further development and industrial application of MES for CO2 reduction.

Visible-Light-Driven CO2 Reduction with Homobimetallic Complexes. Cooperativity between Metals and Activation of Different Pathways

Visible-light-driven reduction of CO2 to both CO and formate (HCOO-) was achieved in acetonitrile solutions using a homobimetallic Cu bisquaterpyridine complex. In the presence of a weak acid (water) as coreactant, the reaction rate was enhanced, and a total of ca. 766 TON (turnover number) was reached for the CO2 reduction, with 60% selectivity for formate and 28% selectivity for CO, using Ru(phen)32+ as a sensitizer and amines as sacrificial electron donors. Mechanistic studies revealed that with the help of cooperativity between two Cu centers, a bridging hydride is generated in the presence of a proton source (water) and further reacts with CO2 to give HCOO-. A second product, CO, was also produced in a parallel competitive pathway upon direct coordination of CO2 to the reduced complex. Mechanistic studies further allowed comparison of the observed reactivity to the monometallic Cu quaterpyridine complex, which only produced CO, and to the related homobimetallic Co bisquaterpyridine complex, that has been previously shown to generate formate following a mechanism not involving the formation of an intermediate hydride species.

Unusual Hydrogenation Reactivities of a Thiolate-Bridged Dicobalt μ-Nitride Featuring a Bent {CoIII-N-CoIII} Core

Transition metal nitrides have received considerable attention owing to their crucial roles in nitrogen fixation and nitrogen atom transfer reactions. Compared to the early and middle transition metals, it is much more challenging to access late transition metal nitrides, especially cobalt in group 9. So far, only a handful of cobalt nitrides have been reported; consequently, their hydrogenation reactivity is largely unexplored. Herein, we present a structurally and spectroscopically well-characterized thiolate-bridged dicobalt μ-nitride [Cp*CoIII(μ-SAd)(μ-N)CoIIICp*] (2) featuring a bent {CoIII(μ-N)CoIII} core. Remarkably, complex 2 can realize not only direct hydrogenation of nitride to amide but also stepwise N-H bond formation from nitride to ammonia. Specifically, 2 can facilely activate dihydrogen (H2) at mild conditions to generate a dicobalt μ-amide [Cp*CoII(μ-SAd)(μ-NH2)CoIICp*] (4) via an unusual mechanism of two-electron oxidation of H2 as proposed by computational studies; in the presence of protons (H+) and electrons, nitride 2 can convert to dicobalt μ-imide [Cp*CoIII(μ-SAd)(μ-NH)CoIIICp*][BPh4] (3[BPh4]) and to CoIICoII μ-amide 4, and finally release ammonia. In contrast to 2, the only other structurally characterized dicobalt μ-nitride Na(THF)4{[(ketguan)CoIII(N3)]2(μ-N)} (ketguan = [(tBu2CN)C(NDipp)2]-, Dipp = 2,6-diisopropylphenyl) (e) that possesses a linear {CoIII(μ-N)CoIII} moiety cannot directly react with H2 or H+. Further in-depth electronic structure analyses shed light on how the varying geometries of the {CoIII(μ-N)CoIII} moieties in 2 and e, bent vs linear, impart their disparate reactivities.

Molecularly dispersed nickel complexes on N-doped graphene for electrochemical CO2 reduction

In this work, new hybrid catalysts based on molecularly dispersed nickel complexes on N-doped graphene were developed for electrochemical CO2 reduction (ECR). Nickel(II) complexes (1-Ni, 2-Ni), and a new crystal structure ([2-Ni]Me), featuring N4-Schiff base macrocycles, were synthesized and investigated for their potential in ECR. Cyclic voltammetry (CV) in NBu4PF6/CH3CN solution demonstrated that the nickel complexes bearing N-H groups (1-Ni and 2-Ni) showed a substantial current enhancement in the presence of CO2, while the absence of N-H groups ([2-Ni]Me) resulted in an almost unchanged voltammogram. This indicated the necessity of the N-H functionality towards ECR in aprotic media. All three nickel complexes were successfully immobilized on nitrogen-doped graphene (NG) via non-covalent interactions. All three Ni@NG catalysts exhibited satisfactory CO2-to-CO reduction in aqueous NaHCO3 solution with the faradaic efficiency (FE) of 60-80% at the overpotential of 0.56 V vs. RHE. The ECR activity of [2-Ni]Me@NG also suggested that the N-H moiety from the ligand is less important in the heterogeneous aqueous system owing to viable hydrogen-bond formation and proton donors from water and bicarbonate ions. This finding could pave the way for understanding the effects of modifying the ligand framework at the N-H position toward fine tuning the reactivity of hybrid catalysts through molecular-level modulation.

Mechanism of Hydroboration of CO2 Using an Fe Catalyst: What Controls the Reactivity and Product Selectivity?

Using a combination of density functional theory (DFT) and ab initio complete active space self-consistent field (CASSCF) calculations, various elementary steps in the mechanism of the reductive hydroboration of CO₂ to two-electron-reduced boryl formate, four-electron-reduced bis(boryl)acetal, and six-electron-reduced methoxy borane by the [Fe(H)₂(dmpe)₂] catalyst were established. The replacement of hydride by oxygen ligation after the boryl formate insertion step is the rate-determining step. Our work unveils, for the first time, (i) how a substrate steers product selectivity in this reaction and (ii) the importance of configurational mixing in contracting the kinetic barrier heights. Based on the reaction mechanism established, we have further focused on the effect of other metals, such as Mn and Co, on rate-determining steps and on catalyst regeneration.

Toward a Comprehensive Understanding of Amorphous Photocatalysts: Fundamental Hypotheses and Applications in CO2 Photoreduction

In principle, photocatalytic activity can be precisely controlled with crystalline catalysts. However, an amorphous photocatalyst could be a viable candidate for CO₂ photoreduction to form value-added products. The amorphous phase is currently part of the crystalline material in several ongoing CO₂ photoreduction studies. Additionally, no study indicates the amorphous material required for overall CO₂ photoreduction. This perspective review article highlights fundamental assumptions that are necessary to gain insights and understand the effectiveness of amorphous photocatalysts for CO₂ photoreduction. We start with basic ideas and theories about these materials, including light harvesting, variable coordination number, and the interaction of CO₂ molecules with the amorphous catalytic surface. To understand the prospects of the amorphous photocatalyst, we explore machine learning with EXAFS. Furthermore, we discuss product selectivity and regeneration of photocatalysts in detail. Finally, we briefly review the work in progress on amorphous materials and compare it to that on crystalline ones.

Catalytic Hydrogenation of CO2 to Methanol: A Review

High-efficiency utilization of CO2 facilitates the reduction of CO2 concentration in the global atmosphere and hence the alleviation of the greenhouse effect. The catalytic hydrogenation of CO2 to produce value-added chemicals exhibits attractive prospects by potentially building energy recycling loops. Particularly, methanol is one of the practically important objective products, and the catalytic hydrogenation of CO2 to synthesize methanol has been extensively studied. In this review, we focus on some basic concepts on CO2 activation, the recent research advances in the catalytic hydrogenation of CO2 to methanol, the development of high-performance catalysts, and microscopic insight into the reaction mechanisms. Finally, some thinking on the present research and possible future trend is presented.

Considering the Influence of Polymer-Catalyst Interactions on the Chemical Microenvironment of Electrocatalysts for the CO2 Reduction Reaction

The electrochemical CO₂ reduction reaction (CO₂RR) is an attractive method for capturing intermittent renewable energy sources in chemical bonds, and converting waste CO₂ into value-added products with a goal of carbon neutrality. Our group has focused on developing polymer-encapsulated molecular catalysts, specifically cobalt phthalocyanine (CoPc), as active and selective electrocatalysts for the CO₂RR. When CoPc is adsorbed onto a carbon electrode and encapsulated in poly(4-vinylpyridine) (P4VP), its activity and reaction selectivity over the competitive hydrogen evolution reaction (HER) are enhanced by three synergistic effects: a primary axial coordination effect, a secondary reaction intermediate stabilization effect, and an outer-coordination proton transport effect. We have studied multiple aspects of this system using electrochemical, spectroscopic, and computational tools. Specifically, we have used X-ray absorption spectroscopy measurements to confirm that the pyridyl residues from the polymer are axially coordinated to the CoPc metal center, and we have shown that increasing the σ-donor ability of nitrogen-containing axial ligands results in increased activity for the CO₂RR. Using proton inventory studies, we showed that proton delivery in the CoPc-P4VP system is controlled via a proton relay through the polymer matrix. Additionally, we studied the effect of catalyst, polymer, and graphite powder loading on CO₂RR activity and determined best practices for incorporating carbon supports into catalyst-polymer composite films.In this Account, we describe these studies in detail, organizing our discussion by three types of microenvironmental interactions that affect the catalyst performance: ligand effects of the primary and secondary sphere, substrate transport of protons and CO₂, and charge transport from the electrode surface to the catalyst sites. Our work demonstrates that careful electroanalytical study and interpretation can be valuable in developing a robust and comprehensive understanding of catalyst performance. In addition to our work with polymer encapsulated CoPc, we provide examples of similar surface-adsorbed molecular and solid-state systems that benefit from interactions between active catalytic sites and a polymer system. We also compare the activity results from our systems to other results in the CoPc literature, and other examples of molecular CO₂RR catalysts on modified electrode surfaces. Finally, we speculate how the insights gained from studying CoPc could guide the field in designing other polymer-electrocatalyst systems. As CO₂RR technologies become commercially viable and expand into the space of flow cells and gas-diffusion electrodes, we propose that overall device efficiency may benefit from understanding and promoting synergistic polymer-encapsulation effects in the microenvironment of these catalyst systems.

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.

Electronic structure analysis of electrochemical CO2 reduction by iron-porphyrins reveals basic requirements to design catalysts bearing non-innocent ligands

Electrocatalytic CO2 reduction is a possible solution to the increasing CO2 concentration in the earth atmosphere, because it enables storage of energy while using the harmful CO2 feedstock as starting...

Impacts of the Catalyst Structures on CO2 Activation on Catalyst Surfaces

Utilizing CO2 as a sustainable carbon source to form valuable products requires activating it by active sites on catalyst surfaces. These active sites are usually in or below the nanometer scale. Some metals and metal oxides can catalyze the CO2 transformation reactions. On metal oxide-based catalysts, CO2 transformations are promoted significantly in the presence of surface oxygen vacancies or surface defect sites. Electrons transferable to the neutral CO2 molecule can be enriched on oxygen vacancies, which can also act as CO2 adsorption sites. CO2 activation is also possible without necessarily transferring electrons by tailoring catalytic sites that promote interactions at an appropriate energy level alignment of the catalyst and CO2 molecule. This review discusses CO2 activation on various catalysts, particularly the impacts of various structural factors, such as oxygen vacancies, on CO2 activation.

Cobalt-Catalyzed Hydrosilylation of Carbon Dioxide to the Formic Acid, Formaldehyde, and Methanol Level-How to Control the Catalytic Network?

The selective hydrosilylation of carbon dioxide (CO₂) to either the formic acid, formaldehyde, or methanol level using a molecular cobalt(II) triazine complex can be controlled based on reaction parameters such as temperature, CO₂ pressure, and concentration. Here, we rationalize the catalytic mechanism that enables the selective arrival at each product platform. Key reactive intermediates were prepared and spectroscopically characterized, while the catalytic mechanism and the energy profile were analyzed with density functional theory (DFT) methods and microkinetic modeling. It transpired that the stepwise reduction of CO₂ involves three consecutive catalytic cycles, including the same cobalt(I) triazine hydride complex as the active species. The increasing kinetic barriers associated with each reduction step and the competing hydride transfer steps in the three cycles corroborate the strong influence of the catalyst environment on the product selectivity. The fundamental mechanistic insights provide a consistent description of the catalytic system and rationalize, in particular, the experimentally verified opportunity to steer the reaction toward the formaldehyde product as the chemically most challenging reduction level.

CO2 reduction routes to value-added oxygenates: a review

Energy is a key attribute that is used to evaluate the economic development of any country. The demand for energy is going to rise in developing countries and will be 67% of global use by 2040. The energy surge in these rising economies will be responsible for 60-70% of the global greenhouse gas emissions. The quest for higher energy motivates technological development to curb the climate change occurring with GHG emissions. Carbon dioxide is one of the primary greenhouse gases in the atmosphere. Current work is intended to give an updated review on the different routes of carbon dioxide utilization that are catalytic route, photocatalytic route, electrocatalytic route, microwave plasma route, and biocatalytic route. These routes are capable of converting CO₂ into different valuable products such as formic acid, methanol, and di-methyl ether (DME), which are majorly derived from biomass and/or fossil fuels (coal gasification and/or natural gas). This work investigates the effect of different routes available for the production of value-added products by CO₂ reduction, discusses various challenges that come across the aforementioned routes, and shares views on future scope and research direction to pave new innovative ways of reducing CO₂ from the environment.

A Prospective Concept on the Fabrication of Blend PES/PEG/DMF/NMP Mixed Matrix Membranes with Functionalised Carbon Nanotubes for CO2/N2 Separation

With an ever-increasing global population, the combustion of fossil fuels has risen immensely to meet the demand for electricity, resulting in significant increase in carbon dioxide (CO₂) emissions. In recent years, CO₂ separation technology, such as membrane technology, has become highly desirable. Fabricated mixed matrix membranes (MMMs) have the most desirable gas separation performances, as these membranes have the ability to overcome the trade-off limitations. In this paper, blended MMMs are reviewed along with two polymers, namely polyether sulfone (PES) and polyethylene glycol (PEG). Both polymers can efficiently separate CO₂ because of their chemical properties. In addition, blended N-methyl-2-pyrrolidone (NMP) and dimethylformamide (DMF) solvents were also reviewed to understand the impact of blended MMMs’ morphology on separation of CO₂. However, the fabricated MMMs had challenges, such as filler agglomeration and void formation. To combat this, functionalised multi-walled carbon nanotube (MWCNTs-F) fillers were utilised to aid gas separation performance and polymer compatibility issues. Additionally, a summary of the different fabrication techniques was identified to further optimise the fabrication methodology. Thus, a blended MMM fabricated using PES, PEG, NMP, DMF and MWCNTs-F is believed to improve CO₂/nitrogen separation.

Recent advances in single atom catalysts for the electrochemical carbon dioxide reduction reaction

The electrochemical carbon dioxide reduction reaction (CO2RR) offers a promising solution to mitigate carbon emission and at the same time generate valuable carbonaceous chemicals/fuels. Single atom catalysts (SACs) are encouraging to catalyze the electrochemical CO2RR due to the tunable electronic structure of the central metal atoms, which can regulate the adsorption energy of reactants and reaction intermediates. Moreover, SACs form a bridge between homogeneous and heterogeneous catalysts, providing an ideal platform to explore the reaction mechanism of electrochemical reactions. In this review, we first discuss the strategies for promoting the CO2RR performance, including suppression of the hydrogen evolution reaction (HER), generation of C1 products and formation of C2+ products. Then, we summarize the recent developments in regulating the structure of SACs toward the CO2RR based on the above aspects. Finally, several issues regarding the development of SACs for the CO2RR are raised and possible solutions are provided.

From CO2 to Value-Added Products: A Review about Carbon-Based Materials for Electro-Chemical CO2 Conversion

The global warming and the dangerous climate change arising from the massive emission of CO2 from the burning of fossil fuels have motivated the search for alternative clean and sustainable energy sources. However, the industrial development and population necessities make the decoupling of economic growth from fossil fuels unimaginable and, consequently, the capture and conversion of CO2 to fuels seems to be, nowadays, one of the most promising and attractive solutions in a world with high energy demand. In this respect, the electrochemical CO2 conversion using renewable electricity provides a promising solution. However, faradaic efficiency of common electro-catalysts is low, and therefore, the design of highly selective, energy-efficient, and cost-effective electrocatalysts is critical. Carbon-based materials present some advantages such as relatively low cost and renewability, excellent electrical conductivity, and tunable textural and chemical surface, which show them as competitive materials for the electro-reduction of CO2. In this review, an overview of the recent progress of carbon-based electro-catalysts in the conversion of CO2 to valuable products is presented, focusing on the role of the different carbon properties, which provides a useful understanding for the materials design progress in this field. Development opportunities and challenges in the field are also summarized.

Biochemical and artificial pathways for the reduction of carbon dioxide, nitrite and the competing proton reduction: effect of 2nd sphere interactions in catalysis

Reduction of oxides and oxoanions of carbon and nitrogen are of great contemporary importance as they are crucial for a sustainable environment.

Selective Oxidation of H2 and CO by NiIr Catalyst in Aqueous Solution: A DFT Mechanistic Study

One of the challenges in utilizing hydrogen gas (H₂) as a sustainable fossil fuel alternative is the inhibition of H₂ oxidation by carbon monoxide (CO), which is involved in the industrial production of H₂ sources. To solve this problem, a catalyst that selectively oxidizes either CO or H₂ or one that co-oxidizes H₂ and CO is needed. Recently, a NiIr catalyst [Ni^IICl(X)Ir^IIICl(η⁵-C₅Me₅)], (X = N,N'-dimethyl-3,7-diazanonane-1,9-dithiolate), which efficiently and selectively oxidizes either H₂ or CO depending on the pH, has been developed (Angew. Chem. Int. Ed. 2017, 56, 9723-9726). In the present work, density functional theory (DFT) calculations are employed to elucidate the pH-dependent reaction mechanisms of H₂ and CO oxidation catalyzed by this NiIr catalyst. During H₂ oxidation, our calculations suggest that dihydrogen binds to the Ir center and generates an Ir(III)-dihydrogen complex, followed by subsequent isomerization to an Ir(V)-dihydride species. Then, a proton is abstracted by a buffer base, CH₃COO^-, resulting in the formation of a hydride complex. The catalytic cycle completes with electron transfer from the hydride complex to a protonated 2,6-dichlorobenzeneindophenol (DCIP) and a proton transfer from the oxidized hydride complex to a buffer base. The CO oxidation mechanism involves three distinct steps, i.e., (1) formation of a metal carbonyl complex, (2) formation of a metallocarboxylic acid, and (3) conversion of the metallocarboxylic acid to a hydride complex. The formation of the metallocarboxylic acid involves nucleophilic attack of OH^- to the carbonyl-C followed by a large structural change with concomitant cleavage of the Ir-S bond and rotation of the COOH group along the NiIr axis. During the conversion of the metallocarboxylic acid to the hydride complex, intramolecular proton transfer followed by removal of CO₂ leads to the formation of the hydride complexes. In addition, the barrier heights for the binding of small molecules (H₂, OH^-, H₂O, and CO) to Ir were calculated, and the results indicated that dissociation from Ir is a faster process than the binding of H₂O and H₂. These calculations indicate that H₂ oxidation is inhibited by CO and OH^- and thus prefers acidic conditions. In contrast, the CO oxidation reactions occur more favorably under basic conditions, as the formation of the metallocarboxylic acid involves OH^- attack to a carbonyl-C and the binding of OH^- to Ni largely stabilizes the triplet spin state of the complex. Taken together, these calculations provide a rationale for the experimentally observed pH-dependent, selective oxidations of H₂ and CO.

Recent Advances in Niobium-Based Materials for Photocatalytic Solar Fuel Production

The search for renewable and clean energy sources is a key aspect for sustainable development as energy consumption has continuously increased over the years concomitantly with environmental concerns caused by the use of fossil fuels. Semiconductor materials have great potential for acting as photocatalysts for solar fuel production, a potential energy source able to solve both energy and environmental concerns. Among the studied semiconductor materials, those based on niobium pentacation are still shallowly explored, although the number of publications and patents on Nb(V)-based photocatalysts has increased in the last years. A large variety of Nb(V)-based materials exhibit suitable electronic/morphological properties for light-driving reactions. Not only the extensive group of Nb2O5 polymorphs is explored, but also many types of layered niobates, mixed oxides, and Nb(V)-doped semiconductors. Therefore, the aim of this manuscript is to provide a review of the latest developments of niobium based photocatalysts for energy conversion into fuels, more specifically, CO2 reduction to hydrocarbons or H2 evolution from water. Additionally, the main strategies for improving the photocatalytic performance of niobium-based materials are discussed.

Computational mechanistic insights into non-noble-metal-catalysed CO2 conversion

The conversion of CO2 into liquid fuels and value-added fine chemicals is of significant interest for both the environment and the global energy demand. In this frontier article, we highlight viable methods for transforming CO2 into valuable C1 feedstocks and summarize the key mechanistic aspects obtained by in-depth computational investigations of three important pathways of two-electron CO2 reduction: (i) CO2 dissociation to CO (ii) CO2 dimerization to CO32- and CO, and (iii) CO2 hydrogenation to formate. Lastly, we present our outlook on how theoretically obtained mechanistic insights could be translated into strategies for designing efficient non-noble-metal catalysts for CO2 reduction.

Nature-inspired electrocatalysts and devices for energy conversion

A NICE approach for the design of nature-inspired electrocatalysts and electrochemical devices for energy conversion.

Semi-biological approaches to solar-to-chemical conversion

This review provides an overview of the cross-disciplinary field of semi-artificial photosynthesis, which combines strengths of biocatalysis and artificial photosynthesis to develop new concepts and approaches for solar-to-chemical conversion.

Anion Binding and Oxidative Modification at the Molybdenum Cofactor of Formate Dehydrogenase from Rhodobacter capsulatus Studied by X-ray Absorption Spectroscopy

Formate dehydrogenase (FDH) enzymes are versatile catalysts for CO₂ conversion. The FDH from Rhodobacter capsulatus contains a molybdenum cofactor with the dithiolene functions of two pyranopterin guanine dinucleotide molecules, a conserved cysteine, and a sulfido group bound at Mo(VI). In this study, we focused on metal oxidation state and coordination changes in response to exposure to O₂, inhibitory anions, and redox agents using X-ray absorption spectroscopy (XAS) at the Mo K-edge. Differences in the oxidative modification of the bis-molybdopterin guanine dinucleotide (bis-MGD) cofactor relative to samples prepared aerobically without inhibitor, such as variations in the relative numbers of sulfido (Mo═S) and oxo (Mo═O) bonds, were observed in the presence of azide (N₃^-) or cyanate (OCN^-). Azide provided best protection against O₂, resulting in a quantitatively sulfurated cofactor with a displaced cysteine ligand and optimized formate oxidation activity. Replacement of the cysteine ligand by a formate (HCO₂^-) ligand at the molybdenum in active enzyme is compatible with our XAS data. Cyanide (CN^-) inactivated the enzyme by replacing the sulfido ligand at Mo(VI) with an oxo ligand. Evidence that the sulfido group may become protonated upon molybdenum reduction was obtained. Our results emphasize the role of coordination flexibility at the molybdenum center during inhibitory and catalytic processes of FDH enzymes.

Electro- and Solar-Driven Fuel Synthesis with First Row Transition Metal Complexes

The synthesis of renewable fuels from abundant water or the greenhouse gas CO2 is a major step toward creating sustainable and scalable energy storage technologies. In the last few decades, much attention has focused on the development of nonprecious metal-based catalysts and, in more recent years, their integration in solid-state support materials and devices that operate in water. This review surveys the literature on 3d metal-based molecular catalysts and focuses on their immobilization on heterogeneous solid-state supports for electro-, photo-, and photoelectrocatalytic synthesis of fuels in aqueous media. The first sections highlight benchmark homogeneous systems using proton and CO2 reducing 3d transition metal catalysts as well as commonly employed methods for catalyst immobilization, including a discussion of supporting materials and anchoring groups. The subsequent sections elaborate on productive associations between molecular catalysts and a wide range of substrates based on carbon, quantum dots, metal oxide surfaces, and semiconductors. The molecule-material hybrid systems are organized as "dark" cathodes, colloidal photocatalysts, and photocathodes, and their figures of merit are discussed alongside system stability and catalyst integrity. The final section extends the scope of this review to prospects and challenges in targeting catalysis beyond "classical" H2 evolution and CO2 reduction to C1 products, by summarizing cases for higher-value products from N2 reduction, C x>1 products from CO2 utilization, and other reductive organic transformations.

Chemistry and Quantum Mechanics in 2019: Give Us Insight and Numbers

This Perspective revisits Charles Coulson’s famous statement from 1959 “give us insight not numbers” in which he pointed out that accurate computations and chemical understanding often do not go hand in hand. We argue that today, accurate wave function based first-principle calculations can be performed on large molecular systems, while tools are available to interpret the results of these calculations in chemical language. This leads us to modify Coulson’s statement to “give us insight and numbers”. Examples from organic, inorganic, organometallic and surface chemistry as well as molecular magnetism illustrate the points made.

Reaction mechanism of formate dehydrogenase studied by computational methods

Formate dehydrogenases (FDHs) are metalloenzymes that catalyse the reversible conversion of formate to carbon dioxide. Since such a process may be used to combat the greenhouse effect, FDHs have been extensively studied by experimental and theoretical methods. However, the reaction mechanism is still not clear; instead five putative mechanisms have been suggested. In this work, the reaction mechanism of FDH was studied by computational methods. Combined quantum mechanical and molecular mechanic (QM/MM) optimisations were performed to obtain the geometries. To get more accurate energies and obtain a detailed account of the surroundings, big-QM calculations with a very large (1121 atoms) QM region were performed. Our results indicate that the formate substrate does not coordinate directly to Mo when it enters the oxidised active site of the FDH, but instead resides in the second coordination sphere. The sulfido ligand abstracts a hydride ion from the substrate, giving a Mo(IV)-SH state and a thiocarbonate ion attached to Cys196. The latter releases CO₂ when the active site is oxidised back to the resting (Mo^VI) state. This mechanism is supported by recent experimental studies.

A photoactive semisynthetic metalloenzyme exhibits complete selectivity for CO2 reduction in water

A series of artificial metalloenzymes containing a ruthenium chromophore and [NiII(cyclam)]2+, both incorporated site-selectively, have been constructed within an azurin protein scaffold. These light-driven, semisynthetic enzymes do not evolve hydrogen, thus displaying complete selectivity for CO2 reduction to CO. Electrostatic effects rather than direct excited-state electron transfer dominate the ruthenium photophysics, suggesting that intramolecular electron transfer from photogenerated RuI to [NiII(cyclam)]2+ represents the first step in catalysis. Stern-Volmer analyses rationalize the observation that ascorbate is the only sacrificial electron donor that supports turnover. Collectively, these results highlight the important interplay of elements that must be considered when developing and characterizing molecular catalysts.

Countercations and Solvent Influence CO2 Reduction to Oxalate by Chalcogen-Bridged Tricopper Cyclophanates

One-electron reduction of Cu₃EL (L^3- = tris(β-diketiminate)cyclophane, and E = S, Se) affords [Cu₃EL]^-, which reacts with CO₂ to yield exclusively C₂O₄^2- (95% yield, TON = 24) and regenerate Cu₃EL. Stopped-flow UV/visible data support an A→B mechanism under pseudo-first-order conditions ( k_{obs, 298K} = 115(2) s^-1), which is 10⁶ larger than those for reported copper complexes. The k_obs values are dependent on the countercation and solvent (e.g., k_obs is greater for [K(18-crown-6)]⁺ vs (Ph₃P)₂N⁺, and there is a 20-fold decrease in k_obs in THF vs DMF). Our results suggest a mechanism in which cations and solvent influence the stability of the transition state.

Positional effects of second-sphere amide pendants on electrochemical CO2 reduction catalyzed by iron porphyrins

The development of catalysts for electrochemical reduction of carbon dioxide offers an attractive approach to transforming this greenhouse gas into value-added carbon products with sustainable energy input. Inspired by natural bioinorganic systems that feature precisely positioned hydrogen-bond donors in the secondary coordination sphere to direct chemical transformations occurring at redox-active metal centers, we now report the design, synthesis, and characterization of a series of iron tetraphenylporphyrin (Fe-TPP) derivatives bearing amide pendants at various positions at the periphery of the metal core. Proper positioning of the amide pendants greatly affects the electrocatalytic activity for carbon dioxide reduction to carbon monoxide. In particular, derivatives bearing proximal and distal amide pendants on the ortho position of the phenyl ring exhibit significantly larger turnover frequencies (TOF) compared to the analogous para-functionalized amide isomers or unfunctionalized Fe-TPP. Analysis of TOF as a function of catalyst standard reduction potential enables first-sphere electronic effects to be disentangled from second-sphere through-space interactions, suggesting that the ortho-functionalized porphyrins can utilize the latter second-sphere property to promote CO2 reduction. Indeed, the distally-functionalized ortho-amide isomer shows a significantly larger through-space interaction than its proximal ortho-amide analogue. These data establish that proper positioning of secondary coordination sphere groups is an effective design element for breaking electronic scaling relationships that are often observed in electrochemical CO2 reduction.

Direct CO2 Addition to a Ni(0)-CO Species Allows the Selective Generation of a Nickel(II) Carboxylate with Expulsion of CO

Addition of CO₂ to a low-valent nickel species has been explored with a newly designed ^acriPNP pincer ligand (^acriPNP^- = 4,5-bis(diisopropylphosphino)-2,7,9,9-tetramethyl-9H-acridin-10-ide). This is a crucial step in understanding biological CO₂ conversion to CO found in carbon monoxide dehydrogenase (CODH). A four-coordinate nickel(0) state was reliably accessed in the presence of a CO ligand, which can be prepared from a stepwise reduction of a cationic {(^acriPNP)Ni(II)-CO}⁺ species. All three Ni(II), Ni(I), and Ni(0) monocarbonyl species were cleanly isolated and spectroscopically characterized. Addition of electrons to the nickel(II) species significantly alters its geometry from square planar toward tetrahedral because of the filling of the d_x²-y² orbital. Accordingly, the CO ligand position changes from equatorial to axial, ∠N-Ni-C of 176.2(2)° to 129.1(4)°, allowing opening of a CO₂ binding site. Upon addition of CO₂ to a nickel(0)-CO species, a nickel(II) carboxylate species with a Ni(η¹-CO₂-κC) moiety was formed and isolated (75%). This reaction occurs with the concomitant expulsion of CO(g). This is a unique result markedly different from our previous report involving the flexible analogous PNP ligand, which revealed the formation of multiple products including a tetrameric cluster from the reaction with CO₂. Finally, the carbon dioxide conversion to CO at a single nickel center is modeled by the successful isolation of all relevant intermediates, such as Ni-CO₂, Ni-COOH, and Ni-CO.

Electronic Structure and Spin Multiplicity of Iron Tetraphenylporphyrins in Their Reduced States as Determined by a Combination of Resonance Raman Spectroscopy and Quantum Chemistry

Iron tetraphenylporphyrins are prime candidates as catalysts for CO₂ reduction. Yet, even after 40 years of research, fundamental questions about the electronic structure of their reduced states remain, in particular as to whether the reducing equivalents are stored at the iron center or at the porphyrin ligand. In this contribution, we address this question by a combination of resonance Raman spectroscopy and quantum chemistry. Analysis of the data allows for an unequivocal identification of the porphyrin as the redox active moiety. Additionally, determination of the spin state of iron is possible by comparing the characteristic shifts of spin and oxidation-state-sensitive marker bands in the Raman spectrum with calculations of planar porphyrin model structures.

Organic, Organometallic and Bioorganic Catalysts for Electrochemical Reduction of CO2

A broad review of homogeneous and heterogeneous catalytic approaches toward CO2 reduction using organic, organometallic, and bioorganic systems is provided. Electrochemical, bioelectrochemical and photoelectrochemical approaches are discussed in terms of their faradaic efficiencies, overpotentials and reaction mechanisms. Organometallic complexes as well as semiconductors and their homogeneous and heterogeneous catalytic activities are compared to enzymes. In both cases, their immobilization on electrodes is discussed and compared to homogeneous catalysts in solution.

From Enzymes to Functional Materials-Towards Activation of Small Molecules

The design of non-noble metal-containing heterogeneous catalysts for the activation of small molecules is of utmost importance for our society. While nature possesses very sophisticated machineries to perform such conversions, rationally designed catalytic materials are rare. Herein, we aim to raise the awareness of the overall common design and working principles of catalysts incorporating aspects of biology, chemistry, and material sciences.

CO2 Activation over Catalytic Surfaces

This article describes the main strategies to activate and convert carbon dioxide (CO₂ ) into valuable chemicals over catalytic surfaces. Coherent elements such as common intermediates are identified in the different strategies and concisely discussed based on the reactivity of CO₂ with the aim to understand the decisive factors for selective and efficient CO₂ conversion.

Efficient reduction of CO2 by the molybdenum-containing formate dehydrogenase from Cupriavidus necator (Ralstonia eutropha)

The ability of the FdsABG formate dehydrogenase from Cupriavidus necator (formerly known as Ralstonia eutropha) to catalyze the reverse of the physiological reaction, the reduction of CO₂ to formate utilizing NADH as electron donor, has been investigated. Contrary to previous studies of this enzyme, we demonstrate that it is in fact effective in catalyzing the reverse reaction with a k_cat of 11 ± 0.4 s^-1 We also quantify the stoichiometric accumulation of formic acid as the product of the reaction and demonstrate that the observed kinetic parameters for catalysis in the forward and reverse reactions are thermodynamically consistent, complying with the expected Haldane relationships. Finally, we demonstrate the reaction conditions necessary for gauging the ability of a given formate dehydrogenase or other CO₂-utilizing enzyme to catalyze the reverse direction to avoid false negative results. In conjunction with our earlier studies on the reaction mechanism of this enzyme and on the basis of the present work, we conclude that all molybdenum- and tungsten-containing formate dehydrogenases and related enzymes likely operate via a simple hydride transfer mechanism and are effective in catalyzing the reversible interconversion of CO₂ and formate under the appropriate experimental conditions.

Biocatalytic and Bioelectrocatalytic Approaches for the Reduction of Carbon Dioxide using Enzymes

In the recent decade, CO₂ has increasingly been regarded not only as a greenhouse gas but even more as a chemical feedstock for carbon-based materials. Different strategies have evolved to realize CO₂ utilization and conversion into fuels and chemicals. In particular, biological approaches have drawn attention, as natural CO₂ conversion serves as a model for many processes. Microorganisms and enzymes have been studied extensively for redox reactions involving CO₂. In this review, we focus on monitoring nonliving biocatalyzed reactions for the reduction of CO₂ by using enzymes. We depict the opportunities but also challenges associated with utilizing such biocatalysts. Besides the application of enzymes with co-factors, resembling natural processes, and co-factor recovery, we also discuss implementation into photochemical and electrochemical techniques.