Cobalt-porphyrin catalyzed electrochemical reduction of carbon dioxide in water. 1. A density functional study of intermediates

Proton-Coupled Electron Transfer Mechanisms for CO2 Reduction to Methanol Catalyzed by Surface-Immobilized Cobalt Phthalocyanine

Immobilized cobalt phthalocyanine (CoPc) is a highly promising architecture for the six-proton, six-electron reduction of CO2 to methanol. This electroreduction process relies on proton-coupled electron transfer (PCET) reactions that can occur by sequential or concerted mechanisms. Immobilization on a conductive support such as carbon nanotubes or graphitic flakes can fundamentally alter the PCET mechanisms. We use density functional theory (DFT) calculations of CoPc adsorbed on an explicit graphitic surface model to investigate intermediates in the electroreduction of CO2 to methanol. Our calculations show that the alignment of the CoPc and graphitic electronic states influences the reductive chemistry. These calculations also distinguish between charging the graphitic surface and reducing the CoPc and adsorbed intermediates as electrons are added to the system. This analysis allows us to identify the chemical transformations that are likely to be concerted PCET, defined for these systems as the mechanism in which protonation of a CO2 reduction intermediate is accompanied by electron abstraction from the graphitic surface to the adsorbate without thermodynamically stable intermediates. This work establishes a mechanistic pathway for methanol production that is consistent with experimental observations and provides fundamental insight into how immobilization of the CoPc impacts its CO2 reduction chemistry.

Al-Sehemi A, Alargarsamy S, Gajendran P, Ramachandran R. Development of Different Kinds of Electrocatalyst for the Electrochemical Reduction of Carbon Dioxide Reactions: An Overview

Significant advancements have been made in the development of CO2 reduction processes for applications such as electrosynthesis, energy storage, and environmental remediation. Several materials have demonstrated great potential in achieving high activity and selectivity for the desired reduction products. Nevertheless, these advancements have primarily been limited to small-scale laboratory settings, and the considerable technical obstacles associated with large-scale CO2 reduction have not received sufficient attention. Many of the researchers have been faced with persistent challenges in the catalytic process, primarily stemming from the low Faraday efficiency, high overpotential, and low limiting current density observed in the production of the desired target product. The highlighted materials possess the capability to transform CO2 into various oxygenates, including ethanol, methanol, and formates, as well as hydrocarbons such as methane and ethane. A comprehensive summary of the recent research progress on these discussed types of electrocatalysts is provided, highlighting the detailed examination of their electrocatalytic activity enhancement strategies. This serves as a valuable reference for the development of highly efficient electrocatalysts with different orientations. This review encompasses the latest developments in catalyst materials and cell designs, presenting the leading materials utilized for the conversion of CO2 into various valuable products. Corresponding designs of cells and reactors are also included to provide a comprehensive overview of the advancements in this field.

Quasi-Covalently Coupled Ni-Cu Atomic Pair for Synergistic Electroreduction of CO2

Developing highly active, selective, and stable electrocatalysts for the carbon dioxide reduction reaction (CO₂RR) is crucial to establish a CO₂ conversion system for industrial implementation and, therefore, to realize an artificially closed carbon loop. This can only be achieved through the rational material design based upon the knowledge of the operational active site at the molecular scale. Enlightened by theoretical screening, herein, we for the first time manipulate a novel Ni-Cu atomic pair configuration toward improved CO₂RR performance. Systematic characterizations and theoretical modeling reveal that the secondary Cu metal incorporation positively shifts the Ni 3d orbital energy to the Fermi level and thus accelerates the rate-determining step, *COOH formation. In addition, the intrinsic inactivity of Cu toward the competing hydrogen evolution reaction causes a considerable reaction barrier for water dissociation on the Ni-Cu moiety. Due to these attributes, the as-developed Ni/Cu-N-C catalyst exhibits excellent catalytic activity and selectivity, with a record-high turnover frequency of 20,695 h^-1 at -0.6 V (vs RHE) and a maximum Faradaic efficiency of 97.7% for CO production. Furthermore, the dynamic structure evolution monitored by operando X-ray absorption fine-structure spectroscopy unveils the interaction between the Ni center and CO₂ molecules and the synergistic effect of the Ni-Cu atomic pair on CO₂RR activity.

Atomic- and Molecular-Level Modulation of Dispersed Active Sites for Electrocatalytic CO2 Reduction

Global climate changes have been impacted by the excessive CO 2 emission, which exacerbates the environmental problems. Electrochemical CO 2 reduction (CO 2 RR) offers the solution for utilizing CO 2 as feedstocks for value-added products while potentially mitigating the negative effects. Owing to the extreme stability of CO 2 , selectivity and efficiency are crucial factors in the development of CO 2 RR electrocatalysts. Recently, single-atom catalysts have emerged as potential electrocatalysts for CO 2 reduction. They generally comprise of atomically- and molecularly dispersed active sites over conductive supports, which enable atomic-level and molecular-level modulations. In this minireview, catalyst preparations, principle of modulations, and reaction mechanisms are summarised together with related recent advances. The atomic-level modulations are first discussed, followed by the molecular-level modulations. Finally, the current challenges and future opportunities are provided as guidance for further developments regarding the discussed topics.

Calculating Entropies of Large Molecules in Aqueous Phase

Entropy benchmarking of different sized molecules in aqueous phase is carried out for known solvation models, where we compare geometry and solvation cavity packing parameters, which allows us to improve the accuracy of the obtained entropy values using empirical corrections. A comparison of solvation entropy models is conducted for a benchmarking set of 56 molecules, showing how an accurate description of cavitation entropy and its hindrance on other entropy values is important for large-sized solute molecules. Finally, we compare reaction free energies with entropies calculated using the most accurate solvation model considered, where we demonstrate a significant improvement in the accuracy relative to experimental values.

Enhancing the Reduction Kinetics of LiSF6 Batteries by Dispersed Cobalt Phthalocyanines on Porous Carbon

Reducing SF₆ (as gas cathode) in Li batteries is a promising concept for the double benefit of mildly converting greenhouse SF₆ and providing a high theoretical energy density of 3922 Wh kg^-1 . However, the reduction process is hampered by its sluggish kinetics. Here, cobalt phthalocyanine (CoPc) molecules immobilized on porous carbon matrix are, for the first time, introduced to the LiSF₆ chemistry to deliver an enhanced energy density. It is revealed that the high redox potential of Co(II)Pc/[Co(I)Pc]^- (≈2.85 V) facilitates the formation of Co(I)N₄ sites to catalyze the SF₆ electrochemical reduction. By using highly porous holey nitrogen-doped carbon nanocages as carbon matrix, the LiSF₆ cells deliver a high discharge voltage of 2.82 V at 50 mA g_C+CoPc ^-1 and an unprecedented areal capacity of 25 mAh cm^-2 at 0.1 mA cm^-2 , much superior to previous results. This work opens up new possibilities for high-efficiency conversion of SF₆ in lithium batteries.

Electrochemical Reduction of CO2: A Review of Cobalt Based Catalysts for Carbon Dioxide Conversion to Fuels

Electrochemical CO2 reduction reaction (CO2RR) provides a promising approach to curbing harmful emissions contributing to global warming. However, several challenges hinder the commercialization of this technology, including high overpotentials, electrode instability, and low Faradic efficiencies of desirable products. Several materials have been developed to overcome these challenges. This mini-review discusses the recent performance of various cobalt (Co) electrocatalysts, including Co-single atom, Co-multi metals, Co-complexes, Co-based metal-organic frameworks (MOFs), Co-based covalent organic frameworks (COFs), Co-nitrides, and Co-oxides. These materials are reviewed with respect to their stability of facilitating CO2 conversion to valuable products, and a summary of the current literature is highlighted, along with future perspectives for the development of efficient CO2RR.

Light-Driven CO2 Reduction by Co-Cytochrome b 562

The current trend in atmospheric carbon dioxide concentrations is causing increasing concerns for its environmental impacts, and spurring the developments of sustainable methods to reduce CO2 to usable molecules. We report the light-driven CO2 reduction in water in mild conditions by artificial protein catalysts based on cytochrome b 562 and incorporating cobalt protoporphyrin IX as cofactor. Incorporation into the protein scaffolds enhances the intrinsic reactivity of the cobalt porphyrin toward proton reduction and CO generation. Mutations around the binding site modulate the activity of the enzyme, pointing to the possibility of further improving catalytic activity through rational design or directed evolution.

Recent Developments in Metalloporphyrin Electrocatalysts for Reduction of Small Molecules: Strategies for Managing Electron and Proton Transfer Reactions

Porphyrins are archetypal ligands in inorganic chemistry. The last 10 years have seen important new advances in the use of metalloporphyrins as catalysts in the activation and reduction of small molecules, in particular O₂ and CO₂ . Recent developments of new molecular designs, scaling relationships, and theoretical modeling of mechanisms have rapidly advanced the utility of porphyrins as electrocatalysts. This Minireview focuses on the summary and evaluation of recent developments of metalloporphyrin O₂ and CO₂ reduction electrocatalysts, with an emphasis on contrasting homogeneous and heterogeneous electrocatalysis. Comparisons for proposed reaction mechanisms are provided for both CO₂ and O₂ reduction, and ideas are proposed about how lessons from the last decade of research can lead to the development of practical, applied porphyrin-derived catalysts.

DFT and Empirical Considerations on Electrocatalytic Water/Carbon Dioxide Reduction by CoTMPyP in Neutral Aqueous Solutions*

A combined experimental and density functional theory (DFT) investigation was employed in order to examine the mechanism of electrochemical CO₂ reduction and H₂ formation from water reduction in neutral aqueous solutions. A water soluble cobalt porphyrin, cobalt [5,10,15,20-(tetra-N-methyl-4-pyridyl)porphyrin], (CoTMPyP), was used as catalyst. The possible attachment of different axial ligands as well as their effect on the electrocatalytic cycles were examined. A cobalt porphyrin hydride is a key intermediate which is generated after the initial reduction of the catalyst. The hydride is involved in the formation of H₂ and formate and acts as an indirect proton source for the formation of CO in these H⁺ -starving conditions. The experimental results are in agreement with the computations and give new insights into electrocatalytic mechanisms involving water soluble metalloporphyrins. We conclude that in addition to the porphyrin's structure and metal ion center, the electrolyte surroundings play a key role in dictating the products of CO₂ /H₂ O reduction.

Controlling the nucleophilic properties of cobalt salen complexes for carbon dioxide capture

The nucleophilic properties of cobalt salen complexes are examined using density functional theory to investigate its carbon fixing capacity.

Reaction mechanism of the selective reduction of CO2 to CO by a tetraaza [CoIIN4H]2+ complex in the presence of protons

The tetraaza [CoIIN4H]2+ complex (1) is remarkable for its ability to selectively reduce CO2 to CO with 45% Faradaic efficiency and a CO to H2 ratio of 3 : 2. We employ density functional theory (DFT) to determine the reasons behind the unusual catalytic properties of 1 and the most likely mechanism for CO2 reduction. The selectivity for CO2 over proton reduction is explained by analyzing the catalyst's affinity for the possible ligands present under typical reaction conditions: acetonitrile, water, CO2, and bicarbonate. After reduction of the catalyst by two electrons, formation of [CoIN4H]+-CO2- is strongly favored. Based on thermodynamic and kinetic data, we establish that the only likely route for producing CO from here consists of a protonation step to yield [CoIN4H]+-CO2H, followed by reaction with CO2 to form [CoIIN4H]2+-CO and bicarbonate. This conclusion corroborates the idea of a direct role of CO2 as a Lewis acid to assist in C-O bond dissociation, a conjecture put forward by other authors to explain recent experimental observations. The pathway to formic acid is predicted to be forbidden by high activation barriers, in accordance with the products that are known to be generated by 1. Calculated physical observables such as standard reduction potentials and the turnover frequency for our proposed catalytic cycle are in agreement with available experimental data reported in the literature. The mechanism also makes a prediction that may be experimentally verified: that the rate of CO formation should increase linearly with the partial pressure of CO2.

Cobalt-porphine catalyzed CO2 electro-reduction: a novel protonation mechanism

The urgent need for artificially fixing CO₂ calls for catalysts of high efficiency. The transition metal functionalized porphyrin (TMP) is one of the most important types of organic catalysts for CO₂ reduction. However, the catalytic mechanisms of TMP in CO₂ reduction still remain controversial. Starting from the previously neglected catalyst self-protonation model, we uncover a new CO₂ reduction mechanism on cobalt-porphine, which involves an indirect proton transfer step occurring at the beginning of the reduction cycle. Based on this protonation mechanism, we demonstrate the different correlations between producing rate and pH for the formation of CO and methane, in good agreement with available experimental observations. Our results reveal how pH and potential affect the CO₂ reduction process, providing important clues and insights for further optimization of TMP catalysts.

Proton-coupled electron transfer in the electrocatalysis of CO2 reduction: prediction of sequential vs. concerted pathways using DFT

Herein we investigate computationally in detail the mechanism of the formation of the carboxylate adduct during the electroreduction of CO₂ in water catalysed by cobalt porphyrin complexes. Specifically, we address qualitatively the competition between the concerted and sequential pathways for the proton-coupled electron transfer. We use a simple methodology for accurate computation of the pK_a of the neutral and anionic carboxylate intermediates, [CoP-COOH] and [CoP-COOH]^- (where CoP is a cobalt porphine complex), based on the isodesmic proton-exchange reaction scheme. The predicted values are used as in input for a theoretical model that describes the transition between the sequential and concerted pathways. The activation of the sequential pathway (ET-PT) that leads to the formation of the neutral [CoP-COOH] intermediate at pH ≈ 3.5 (pK_a[CoP-COOH] = 3.5 ± 0.4), as predicted by the calculations, is in good agreement with the drastic increase in the faradaic efficiency of the CO₂ reduction reaction towards CO at pH = 3 compared to pH = 1, as experimentally observed. This confirms the existence of the CO₂ anionic adduct [CoP-CO₂]^- as a viable intermediate at pH = 3 and its crucial role for the pH dependence of the faradaic efficiency for the CO₂ reduction. The analysis also shows that when the pH is significantly higher than the pK_a of the neutral carboxylate adduct, the CO₂ reduction has to go through an alternative pathway with the formation of the anionic carboxylate intermediate [CoP-COOH]^-. It is formed through a concerted proton-electron transfer step from the anionic CO₂ adduct [CoP-CO₂]^- when the pH is below ∼8.6 (pK_a[CoP-COOH]^- = 8.6 ± 0.4). At pH ≈ 8.6 and above, another decoupled ET-PT is predicted to take place, leading to the formation of a dianionic CO₂ adduct [CoP-CO₂]^2-.

Catalysts and Reaction Pathways for the Electrochemical Reduction of Carbon Dioxide

The electrochemical reduction of CO2 has gained significant interest recently as it has the potential to trigger a sustainable solar-fuel-based economy. In this Perspective, we highlight several heterogeneous and molecular electrocatalysts for the reduction of CO2 and discuss the reaction pathways through which they form various products. Among those, copper is a unique catalyst as it yields hydrocarbon products, mostly methane, ethylene, and ethanol, with acceptable efficiencies. As a result, substantial effort has been invested to determine the special catalytic properties of copper and to elucidate the mechanism through which hydrocarbons are formed. These mechanistic insights, together with mechanistic insights of CO2 reduction on other metals and molecular complexes, can provide crucial guidelines for the design of future catalyst materials able to efficiently and selectively reduce CO2 to useful products.

Electrocatalytic reduction of carbon dioxide to carbon monoxide and methane at an immobilized cobalt protoporphyrin

The electrochemical conversion of carbon dioxide and water into useful products is a major challenge in facilitating a closed carbon cycle. Here we report a cobalt protoporphyrin immobilized on a pyrolytic graphite electrode that reduces carbon dioxide in an aqueous acidic solution at relatively low overpotential (0.5 V), with an efficiency and selectivity comparable to the best porphyrin-based electrocatalyst in the literature. While carbon monoxide is the main reduction product, we also observe methane as by-product. The results of our detailed pH-dependent studies are explained consistently by a mechanism in which carbon dioxide is activated by the cobalt protoporphyrin through the stabilization of a radical intermediate, which acts as Brønsted base. The basic character of this intermediate explains how the carbon dioxide reduction circumvents a concerted proton–electron transfer mechanism, in contrast to hydrogen evolution. Our results and their mechanistic interpretations suggest strategies for designing improved catalysts.

The conversion of carbon dioxide to useful products is a major challenge in energy research. Here, the authors report a cobalt protoporphyrin immobilized on graphite that is capable of the selective and efficient electrochemical reduction of carbon dioxide, primarily to carbon monoxide, in acidic media.

Heterogeneous catalytic conversion of CO2: a comprehensive theoretical review

The conversion of CO2 into fuels and useful chemicals has been intensively pursued for renewable, sustainable and green energy. However, due to the negative adiabatic electron affinity (EA) and large ionization potential (IP), the CO2 molecule is chemically inert, thus making the conversion difficult under normal conditions. Novel catalysts, which have high stability, superior efficiency and low cost, are urgently needed to facilitate the conversion. As the first step to design such catalysts, understanding the mechanisms involved in CO2 conversion is absolutely indispensable. In this review, we have summarized the recent theoretical progress in mechanistic studies based on density functional theory, kinetic Monte Carlo simulation, and microkinetics modeling. We focus on reaction channels, intermediate products, the key factors determining the conversion of CO2 in solid-gas interface thermocatalytic reduction and solid-liquid interface electrocatalytic reduction. Furthermore, we have proposed some possible strategies for improving CO2 electrocatalysis and also discussed the challenges in theory, model construction, and future research directions.

Activation of CO2by ionic liquid EMIM–BF4in the electrochemical system: a theoretical study

The electrochemical reduction of CO₂to CO by an ionic liquid EMIM–BF₄is one of the most promising CO₂reduction processes proposed so far with its high Faradaic efficiency and low overpotential.

Computational and experimental study on the electrocatalytic reduction of CO2 to CO by a new mononuclear ruthenium(ii) complex

A new mononuclear ruthenium(ii) complex, trans-[Ru(dmb)2(Cl)(EtOH)](PF6) (dmb = 4,4'-dimethyl-2,2'-bipyridine), has been prepared and characterized by elemental analysis, spectroscopic techniques and single crystal X-ray structure determination. The complex was studied as a precatalyst for the electrocatalytic reduction of CO2 to CO in an acetonitrile solution by cyclic voltammetry (CV). The catalytic mechanism was investigated by means of quantum chemical calculations to gain deeper insight into the process of CO2 reduction. The results suggest that the reaction proceeds in six steps initiating by the two sequential 1ē reductions at the dmb ligands followed by CO2 addition to give a metallocarboxylate intermediate. This intermediate undergoes further reduction and loses a CO molecule. The results reported in this paper are of great significance in providing theoretical insight into a class of electrocatalysts for reduction of CO2 to CO.

The superior catalytic CO oxidation capacity of a Cr-phthalocyanine porous sheet

Two-dimensional organometallic sheets containing regularly and separately distributed transition atoms (TMs) have received tremendous attentions due to their flexibility in synthesis, well-defined geometry and the promising applications in hydrogen storage, electronic circuits, quantum Hall effect, and spintronics. Here for the first time we present a study on the superior catalytic CO oxidation capacity of a Cr-phthalocyanine porous sheet proceeding first via Langmuir-Hinshelwood (LH) mechanism and then via Eley-Rideal (ER) mechanism. Compared to the noble metal based catalysts or graphene supported catalysts, our studied system has following unique features: without poisoning effect and clustering problem, having comparable reaction energy barrier for low-temperature oxidation, and low cost for large-scale catalytic CO oxidation in industry.

Conversion of Carbon Dioxide into Several Potential Chemical Commodities Following Different Pathways - A Review

This article reviews the literature related to the direct uses of CO2and its conversion into various value added chemicals including high energy density liquid fuels such as methanol. The increase in the direct uses of CO2and its conversion into potential chemical commodities is very important as it directly contributes to the mitigation of CO2related global warming problem. The method being followed at present in several countries to reduce the CO2associated global warming is capturing of CO2at its major outlets using monoethanolamine based solution absorption technique followed by storing it in safe places such as, oceans, depleted coal seams, etc., (i.e., carbon dioxide capturing and storing in safe places, CCS process). This is called as CO2sequestration. Although, the CCS process is the most understood and immediate option to mitigate the global warming problem, it is considerably expensive and has become a burden for those countries, which are practicing this process. The other alternative and most beneficial way of mitigating this global warming problem is to convert the captured CO2into certain value added bulk chemicals instead of disposing it. Conversion of CO2into methanol has been identified as one of such cost effective ways of mitigating global warming problem. Further, if H2is produced from exclusively water using only solar energy instead of any fossil fuel based energy, and is used to convert CO2into methanol there are three major benefits: i) it contributes greatly to the global warming mitigation problem, ii) it greatly saves fossil fuels as methanol production from CO2could be an excellent sustainable and renewable energy resource, and iii) as on today, there is no better process than this to store energy in a more convenient and highly usable form of high energy density liquid fuel. Not only methanol, several other potential chemicals and value added chemical intermediates can be produced from CO2. In this article, i) synthesis of several commodity chemicals including poly and cyclic-carbonates, sodium carbonate and dimethyl carbonate, carbamates, urea, vicinal diamines, 2-arylsuccinic acids, dimethyl ether, methanol, various hydrocarbons, acetic acid, formaldehyde, formic acid, lower alkanes, etc., from CO2, ii) the several direct uses of CO2, and iii) the importance of producing methanol from CO2using exclusively solar energy are presented, discussed and summarized by citing all the relevant and important references.

Computational approaches to the chemical conversion of carbon dioxide

The conversion of CO₂ into fuels and chemicals is viewed as an attractive route for controlling the atmospheric concentration and recycling of this greenhouse gas, but its industrial application is limited by the low selectivity and activity of the current catalysts. Theoretical modeling, in particular density functional theory (DFT) simulations, provides a powerful and effective tool to discover chemical reaction mechanisms and design new catalysts for the chemical conversion of CO₂, overcoming the repetitious and time/labor consuming trial-and-error experimental processes. In this article we give a comprehensive survey of recent advances on mechanism determination by DFT calculations for the catalytic hydrogenation of CO₂ into CO, CH₄, CH₃OH, and HCOOH, and CO₂ methanation, as well as the photo- and electrochemical reduction of CO₂. DFT-guided design procedures of new catalytic systems are also reviewed, and challenges and perspectives in this field are outlined.

Biomimetic Carbon Nanotube for Catalytic CO2 Hydrolysis: First-Principles Investigation on the Role of Oxidation State and Metal Substitution in Porphyrin

Hydrolysis of carbon dioxide is an important reaction for CO2 collection. Using accurate first-principles electronic structure calculations, we predict how the catalytic hydrolysis reaction in carbonic anhydrase (CA) can be mimicked in a metal-porphyrin carbon nanotube. The two-step catalytic process can be improved remarkably by controlling the porphyrin oxidation state via the nanotube charge state and by substituting the porphyrin metal atom. The oxidation state and the metal substitution both have profound effects on the reaction energetics for the initial hydration reaction step. For the subsequent product-release reaction step, two different reaction mechanisms could take place. These mechanisms are distinctively sensitive to either the oxidation state change or the metal substitution, but not to both. For the overall catalytic cycle, a significant dependence on the nanotube charge state at low pH and on the metal substitution at high pH is expected.

Cobalt-porphyrin catalyzed electrochemical reduction of carbon dioxide in water. 2. Mechanism from first principles

We apply first principles computational techniques to analyze the two-electron, multistep, electrochemical reduction of CO(2) to CO in water using cobalt porphyrin as a catalyst. Density functional theory calculations with hybrid functionals and dielectric continuum solvation are used to determine the steps at which electrons are added. This information is corroborated with ab initio molecular dynamics simulations in an explicit aqueous environment which reveal the critical role of water in stabilizing a key intermediate formed by CO(2) bound to cobalt. By use of potential of mean force calculations, the intermediate is found to spontaneously accept a proton to form a carboxylate acid group at pH < 9.0, and the subsequent cleavage of a C-OH bond to form CO is exothermic and associated with a small free energy barrier. These predictions suggest that the proposed reaction mechanism is viable if electron transfer to the catalyst is sufficiently fast. The variation in cobalt ion charge and spin states during bond breaking, DFT+U treatment of cobalt 3d orbitals, and the need for computing electrochemical potentials are emphasized.