Competition between Initial CO2 Electroreduction and Hydrogen Evolution Reaction on Cu Catalysts in Acidic Media: Role of Specifically Adsorbed Halide Anions

The role of halide anions and competing mechanisms between initial CO2 electroreduction pathways and hydrogen evolution reaction (HER) are systematically identified at halide anions modified Cu(111)/H2O interfaces based on density functional theory calculations in this paper. The present results show that halide anions modified Cu(111)/H2O interfaces can notably enhance electroreduction activity of CO2 into CO. Simultaneously, it is concluded that the specifically adsorbed halide anions modified Cu electrodes can inhibit HER by studying competing HER mechanisms, and thus the enhanced CO2 electroreduction activity can be ascribed to the suppressed HER. The origin of enhanced CO production activity and inhibited HER is further scrutinized. The present results show that the presence of halide anions can lead to stronger CO adsorption and the increased adsorption strength of CO can explain easier CO production based on the Sabatier principle. Interestingly, the calculated results show that the presence of halide anions does not exert an effect on H adsorption strength, which is regarded as a key descriptor of HER activity, implying that halide anions modified Cu electrodes may be not able to directly lead to the inhibited HER. However, the present results indicate that co-adsorbed CO can weaken adsorption strength between H and Cu electrodes and thus result in inhibited HER and decreased HER activity. The upshift of d-band centers of surface Cu atoms due to modification of halide anions may be a reason for stronger CO adsorption, whereas the downshift of the d-band center due to the presence of co-adsorbed CO can lead to a weakening effect on H adsorption strength. Our present insights into the role of halide anions can aid in designing an optimal electrolyte and developing electrocatalysts that are more selective toward CO2 electroreduction than HER.

Cation Effects on the Adsorbed Intermediates of CO2 Electroreduction Are Systematic and Predictable

The electrode-electrolyte interface, and in particular the nature of the cation, has considerable effects on the activity and product selectivity of the electrochemical reduction of CO2. Therefore, to improve the electrocatalysis of this challenging reaction, it is paramount to ascertain whether cation effects on adsorbed intermediates are systematic. Here, DFT calculations are used to show that the effects of K+, Na+, and Mg2+, on single carbon CO2 reduction intermediates can either be stabilizing or destabilizing depending on the metal and the adsorbate. Because systematic trends are observed, cation effects can be accurately predicted in simple terms for a wide variety of metals, cations and adsorbed species. These results are then applied to the reduction of CO2 to CO on four different catalytic surfaces (Au, Ag, Cu, Pd) and activation of weak-binding metals is consistently observed by virtue of the stabilization of the key intermediate *COOH.

Theoretical understanding of the effect of specifically adsorbed halide anions on Cu-catalyzed CO2 electroreduction activity and product selectivity

Initial CO2 electroreduction into CO and its subsequent electroreduction pathways were selected to study the effect of specifically adsorbed halide anions X- (X = F, Cl, Br, I) on CO2 electroreduction activity and product selectivity at Cu(111)/H2O interfaces via DFT calculations. The calculated results show that the presence of halide anions can exert a notable effect on the CO2 adsorption characteristics and that chemically adsorbed CO2 molecules can be formed. Furthermore, the halide-anion-modified Cu(111)/H2O interfaces could significantly enhance the initial CO2 electroreduction into CO activity, which is regarded as the rate-determining step during CO2 electroreduction at clean Cu(111)/H2O interfaces. Analysis of the initial CO2 electroreduction and Volmer reaction pathways showed that the halide-anion-modified Cu(111)/H2O interfaces could suppress the HER and thus improve the CO2 electroreduction activity and product selectivity. It is speculated that the enhanced initial CO2 electroreduction activity at the F--, Cl--, Br--, and I--modified Cu(111)/H2O interfaces may originate from the decreased work functions and anion radical ·CO2- formations. Simultaneously, we concluded that dimer OCCO formations in the presence of halide anions were more favorable than CHO during CO electroreduction according to the order of I- > Br- > Cl- > F- and could result in the production of C2 product, suggesting an improved CO2 electroreduction product selectivity. The present analyses of electronic structure may explain the more favorable OCCO formations in the order of I- > Br- > Cl- > F-. The present understanding of this effect will provide an improved scientific guideline for the control of CO2 electroreduction pathways and design of more efficient electrocatalysts.

Using micro-solvation and generalized coordination numbers to estimate the solvation energies of adsorbed hydroxyl on metal nanoparticles

Solvent-adsorbate interactions have a great impact on catalytic processes in aqueous systems. Implicit solvent calculations are inexpensive but inaccurate toward hydrogen bonds, while a full incorporation of explicit solvation is computationally demanding. Micro-solvation attempts to break this dilemma by including only those solvent molecules directly interacting with the solute and any nearby interfaces, thereby providing a compromise between accuracy and computational expenses. Here, we show that micro-solvation of *OH and its relation to adsorption sites is largely transferable across late transition metal nanoparticles. Solvation energies for *OH on nanoparticles of Ir, Pd, and Pt range from -0.63 ± 0.04 eV to -0.67 ± 0.12 eV, while those on Au and Ag are -0.75 ± 0.07 eV and -1.01 ± 0.05 eV, respectively. These results enable the use of average solvation corrections for *OH on late transition metal nanostructures.

Evaluating the stability and activity of dilute Cu-based alloys for electrochemical CO2 reduction

Cu-based catalysts currently offer the most promising route to actively and selectively produce value-added chemicals via electrochemical reduction of CO₂ (eCO₂R); yet further improvements are required for their wide-scale deployment in carbon mitigation efforts. Here, we systematically investigate a family of dilute Cu-based alloys to explore their viability as active and selective catalysts for eCO₂R through a combined theoretical-experimental approach. Using a quantum-classical modeling approach that accounts for dynamic solvation effects, we assess the stability and activity of model single-atom catalysts under eCO₂R conditions. Our calculations identify that the presence of eCO₂R intermediates, such as CO*, H*, and OH*, may dynamically influence the local catalyst surface composition. Additionally, we identify through binding energy descriptors of the CO*, CHO*, and OCCO* dimer intermediates that certain elements, such as group 13 elements (B, Al, and Ga), enhance the selectivity of C₂₊ species relative to pure Cu by facilitating CO dimerization. The theoretical work is corroborated by preliminary testing of eCO₂R activity and selectivity of candidate dilute Cu-based alloy catalyst films prepared by electron beam evaporation in a zero-gap gas diffusion electrode-based reactor. Of all studied alloys, dilute CuAl was found to be the most active and selective toward C₂₊ products like ethylene, consistent with the theoretical predictions. We attribute the improved performance of dilute CuAl alloys to more favorable dimerization reaction energetics of bound CO species relative to that on pure Cu. In a broader context, the results presented here demonstrate the power of our simulation framework in terms of rational catalyst design.

Characterization of the alkali metal oxalates (MC2O4-) and their formation by CO2 reduction via the alkali metal carbonites (MCO2-)

The reduction of carbon dioxide to oxalate has been studied by experimental Collisionally Induced Dissociation (CID) and vibrational characterization of the alkali metal oxalates, supplemented by theoretical electronic structure calculations. The critical step in the reductive process is the coordination of CO2 to an alkali metal anion, forming a metal carbonite MCO2- able to subsequently receive a second CO2 molecule. While the energetic demand for these reactions is generally low, we find that the degree of activation of CO2 in terms of charge transfer and transition state energies is the highest for lithium and systematically decreases down the group (M = Li-Cs). This is correlated to the strength of the binding interaction between the alkali metal and CO2, which can be related to the structure of the oxalate moiety within the product metal complexes evolving from a planar to a staggered conformer with increasing atomic number of the interacting metal. Similar structural changes are observed for crystalline alkali metal oxalates, although the C2O42- moiety is in general more planar in these, a fact that is attributed to the increased number of interacting alkali metal cations compared to the gas-phase ions.

Progress and Perspectives of Electrochemical CO2 Reduction on Copper in Aqueous Electrolyte

To date, copper is the only heterogeneous catalyst that has shown a propensity to produce valuable hydrocarbons and alcohols, such as ethylene and ethanol, from electrochemical CO₂ reduction (CO₂R). There are variety of factors that impact CO₂R activity and selectivity, including the catalyst surface structure, morphology, composition, the choice of electrolyte ions and pH, and the electrochemical cell design. Many of these factors are often intertwined, which can complicate catalyst discovery and design efforts. Here we take a broad and historical view of these different aspects and their complex interplay in CO₂R catalysis on Cu, with the purpose of providing new insights, critical evaluations, and guidance to the field with regard to research directions and best practices. First, we describe the various experimental probes and complementary theoretical methods that have been used to discern the mechanisms by which products are formed, and next we present our current understanding of the complex reaction networks for CO₂R on Cu. We then analyze two key methods that have been used in attempts to alter the activity and selectivity of Cu: nanostructuring and the formation of bimetallic electrodes. Finally, we offer some perspectives on the future outlook for electrochemical CO₂R.

Elementary kinetics of nitrogen electroreduction on Fe surfaces

Electrochemical ammonia synthesis could provide a sustainable and efficient alternative to the energy intensive Haber-Bosch process. Development of an active and selective N₂ electroreduction catalyst requires mechanism determination to aid in connecting the catalyst composition and structure to performance. Density functional theory (DFT) calculations are used to examine the elementary step energetics of associative N₂ reduction mechanisms on two low index Fe surfaces. Interfacial water molecules in the Heyrovsky-like mechanism help lower some of the elementary activation barriers. Electrode potential dependent barriers show that cathodic potentials below -1.5 V-RHE (reversible hydrogen electrode) are necessary to give a significant rate of N₂ electroreduction. DFT barriers suggest a larger overpotential than expected based on elementary reaction free energies. Linear Brønsted-Evans-Polanyi relationships do not hold across N-H formation steps on these surfaces, further confirming that explicit barriers should be considered in DFT studies of the nitrogen reduction reaction.

Metal-Free Boron Nitride Nanoribbon Catalysts for Electrochemical CO2 Reduction: Combining High Activity and Selectivity

Developing metal-free catalysts for reduction of CO₂ into energy-rich products is a popular yet very challenging topic. Using density functional theory calculations, we investigated the electrocatalytic performance of C-doped and line-defect (Ld)-embedded boron nitride nanoribbons (BNNRs) for CO₂ reduction reaction (CRR). Because of the presence of bare edge B atoms neighboring to C dopant and C₂ dimer as active sites, defective BNNRs exhibit high CRR catalytic activity and selectivity. The Ld-embedded BNNR structures with C₂ dimer can not only convert CO₂ into CO with very low overpotential of -0.5 V versus reversible hydrogen electrode but also ensure high selectivity in deactivating the hydrogenation channel of the desorbed CO to CH₄. The C-doped zigzag and armchair BNNRs bind strongly to the CO intermediate and thus promote the selective conversion of CO₂ to CH₄, with the lower energy cost on the armchair ribbon than the zigzag one. The presence of edge B atoms and C dopant as dual active sites in BNNRs enables effective couplings between *CH₂ and CO intermediates, leading to the formation of C₂ products including C₂H₄ and C₂H₅OH, with a high selectivity for C₂H₅OH. Importantly, unwanted hydrogen evolution reaction is suppressed during CRR catalyzed by these BNNR-based configurations. Overall, the present findings highlight a promising new class of low-cost, metal-free electrocatalysts combining high CRR activity and selectivity.

pH effects on the electrochemical reduction of CO(2) towards C2 products on stepped copper

We present a microkinetic model for CO₍₂₎ reduction (CO₍₂₎R) on Cu(211) towards C₂ products, based on energetics estimated from an explicit solvent model. We show that the differences in both Tafel slopes and pH dependence for C₁ vs C₂ activity arise from differences in their multi-step mechanisms. We find the depletion in C₂ products observed at high overpotential and high pH to arise from the 2^nd order dependence of C-C coupling on CO coverage, which decreases due to competition from the C₁ pathway. We further demonstrate that CO₍₂₎ reduction at a fixed pH yield similar activities, due to the facile kinetics for CO₂ reduction to CO on Cu, which suggests C₂ products to be favored for CO₂R under alkaline conditions. The mechanistic insights of this work elucidate how reaction conditions can lead to significant enhancements in selectivity and activity towards higher value C₂ products.

CO₂ conversion to reduced products provides a use for greenhouse gases, but reaction complexity stymies mechanistic studies. Here, authors present a microkinetic model for CO₂ and CO reduction on copper, based on ab initio simulations, to elucidate pH’s impact on competitive reaction pathways.

Adsorption of CO2 on Heterostructures of Bi2O3 Nanocluster-Modified TiO2 and the Role of Reduction in Promoting CO2 Activation

The capture and conversion of CO₂ are of significant importance in enabling the production of sustainable fuels, contributing to alleviating greenhouse gas emissions. While there are a number of key steps required to convert CO₂, the initial step of adsorption and activation by the catalyst is critical. Well-known metal oxides such as oxidized TiO₂ or CeO₂ are unable to promote this step. In addressing this difficult problem, a recent experimental work shows the potential for bismuth-containing materials to adsorb and convert CO₂, the origin of which is attributed to the role of the bismuth lone pair. In this paper, we present density functional theory (DFT) simulations of enhanced CO₂ adsorption on heterostructures composed of extended TiO₂ rutile (110) and anatase (101) surfaces modified with Bi₂O₃ nanoclusters, highlighting in particular the role of heterostructure reduction in activating CO₂. These heterostructures show low coordinated Bi sites in the nanoclusters and a valence band edge that is dominated by Bi-O states, typical of the Bi³⁺ lone pair. The reduction of Bi₂O₃-TiO₂ heterostructures can be facile and produces reduced Bi²⁺ and Ti³⁺ species. The interaction of CO₂ with this electron-rich, reduced system can produce CO directly, reoxidizing the heterostructure, or form an activated carboxyl species (CO₂ ^-) through electron transfer from the reduced heterostructure to CO₂. The oxidized Bi₂O₃-TiO₂ heterostructures can adsorb CO₂ in carbonate-like adsorption modes, with moderately strong adsorption energies. The hydrogenation of the nanocluster and migration to adsorbed CO₂ is feasible with H-migration barriers less than 0.7 eV, but this forms a stable COOH intermediate rather than breaking C-O bonds or producing formate. These results highlight that a reducible metal oxide heterostructure composed of a semiconducting metal oxide modified with suitable metal oxide nanoclusters can activate CO₂, potentially overcoming the difficulties associated with the difficult first step in CO₂ conversion.

On the origin of the elusive first intermediate of CO2 electroreduction

We resolve the long-standing controversy about the first step of the CO₂ electroreduction to fuels in aqueous electrolytes by providing direct spectroscopic evidence that the first intermediate of the CO₂ conversion to formate on copper is a carboxylate anion *CO₂ ^- coordinated to the surface through one of its C-O bonds. We identify this intermediate and gain insight into its formation, its chemical and electronic properties, as well as its dependence on the electrode potential by taking advantage of a cutting-edge methodology that includes operando surface-enhanced Raman scattering (SERS) empowered by isotope exchange and electrochemical Stark effects, reaction kinetics (Tafel) analysis, and density functional theory (DFT) simulations. The SERS spectra are measured on an operating Cu surface. These results advance the mechanistic understanding of CO₂ electroreduction and its selectivity to carbon monoxide and formate.

C2N-graphene supported single-atom catalysts for CO2 electrochemical reduction reaction: mechanistic insight and catalyst screening

Single-atom catalysts (SACs) have emerged as an excellent platform for enhancing catalytic performance. Inspired by the recent experimental synthesis of nitrogenated holey 2D graphene (C2N-h2D) (Mahmood et al., Nat. Commun., 2015, 6, 6486-6493), we report density functional theory calculations combined with computational hydrogen electrode model to show that C2N-h2D supported metal single atoms (M@C2N) are promising electrocatalysts for CO2 reduction reaction (CO2 RR). M confined at pyridinic N6 cavity promotes activation of inert O[double bond, length as m-dash]C[double bond, length as m-dash]O bonds and subsequent protonation steps, with *COOH → *CO → CHO predicted to be the primary pathway for producing methanol and methane. It is found that *CO + H+ + e- → *CHO is most likely to be the potential determining step; breaking the scaling relation of *CO and *CHO binding on M@C2N SACs may simply be a rare event that is sensitively controlled by the detailed geometry of the adsorbate. Among twelve metals screened, M@C2N SACs where M = Ti, Mn, Fe, Co, Ni, Ru were identified to be effective in catalyzing CO2 RR with lowered overpotentials (0.58 V-0.80 V).

Steric Hindrance in Sulfur Vacancy of Monolayer MoS2 Boosts Electrochemical Reduction of Carbon Monoxide to Methane

The efficient generation of methane by total electroreduction of carbon monoxide (CO) could be of benefit for a more sustainable society. However, a highly efficient and selective catalyst for this process remains to be developed. In this study, density functional theory calculations indicate that steric hindrance in monolayer molybdenum sulfide with 2 S vacancies (DV-MoS₂ ) can facilitate the conversion of CO into CH₄ with high activity and selectivity under electrochemical reduction at a low potential of -0.53 V vs. RHE and ambient conditions. The potential is a significant improvement on the state-of-the-art Cu electrode (-0.74 V vs. RHE), with less electrical energy. Moreover, the results suggest that such steric hindrance effects are important for structure-sensitive catalytic reactions.

Elucidating the active sites for CO2 electroreduction on ligand-protected Au25 nanoclusters

This work reveals the mechanism of CO₂ electrochemical reduction on ligand-protected Au nanoclusters and catalytic sites responsible for increased selectivity towards CO.

CO2electroreduction performance of a single transition metal atom supported on porphyrin-like graphene: a computational study

Single transition metal atoms supported by porpyrin-like graphene exhibit high catalytic activity for the electroreduction of CO₂.