Free-Energy Barriers and Reaction Mechanisms for the Electrochemical Reduction of CO on the Cu(100) Surface, Including Multiple Layers of Explicit Solvent at pH 0

Liquid water is a dynamic polydisperse branched polymer

We developed the RexPoN force field for water based entirely on quantum mechanics. It predicts the properties of water extremely accurately, with T _melt = 273.3 K (273.15 K) and properties at 298 K: ΔH_vap = 10.36 kcal/mol (10.52), density = 0.9965 g/cm³ (0.9965), entropy = 68.4 J/mol/K (69.9), and dielectric constant = 76.1 (78.4), where experimental values are in parentheses. Upon heating from 0.0 K (ice) to 273.0 K (still ice), the average number of strong hydrogen bonds (SHBs, r_OO ≤ 2.93 Å) decreases from 4.0 to 3.3, but upon melting at 273.5 K, the number of SHBs drops suddenly to 2.3, decreasing slowly to 2.1 at 298 K and 1.6 at 400 K. The lifetime of the SHBs is 90.3 fs at 298 K, increasing monotonically for lower temperature. These SHBs connect to form multibranched polymer chains (151 H₂O per chain at 298 K), where branch points have 3 SHBs and termination points have 1 SHB. This dynamic fluctuating branched polymer view of water provides a dramatically modified paradigm for understanding the properties of water. It may explain the 20-nm angular correlation lengths at 298 K and the critical point at 227 K in supercooled water. Indeed, the 15% jump in the SHB lifetime at 227 K suggests that the supercooled critical point may correspond to a phase transition temperature of the dynamic polymer structure. This paradigm for water could have a significant impact on the properties for protein, DNA, and other materials in aqueous media.

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 2nd 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.

Understanding CO oxidation on the Pt(111) surface based on a reaction route network

Kinetic analysis by the rate constant matrix contraction on the reaction route network of CO oxidation on the Pt(111) surface obtained by the artificial force induced reaction reveals the impact of entropic contributions arising from a variety of local minima and transition states.

Mechanistic study on Cu-catalyzed CO2 electroreduction into CH4 at simulated low overpotentials based on an improved electrochemical model

An improved CO coverage-dependent electrochemical model with explicit relaxed H2O molecules used in CO2 electroreduction is presented, which is firstly applied to Cu-catalyzed CO2 electroreduction into CH4 production at low overpotentials in this paper. The results show that the present defined CH2O and CHOH pathways via common intermediates CHO and CH2 may be able to occur parallelly at the present simulated low overpotential. The potential-limiting steps may be the formation of CO and its further electroreduction into CHO, which are considered as the origin of the observed experimentally high overpotential. The present study also explains why at electrochemical interfaces, only CH4 is observed experimentally on the Cu surface rather than CH3OH. The present results are found to be in excellent agreement with the available experimental data and partial theoretical analysis, further validating the rationality of the present employed methodology.

Towards operando computational modeling in heterogeneous catalysis

An increased synergy between experimental and theoretical investigations in heterogeneous catalysis has become apparent during the last decade. Experimental work has extended from ultra-high vacuum and low temperature towards operando conditions. These developments have motivated the computational community to move from standard descriptive computational models, based on inspection of the potential energy surface at 0 K and low reactant concentrations (0 K/UHV model), to more realistic conditions. The transition from 0 K/UHV to operando models has been backed by significant developments in computer hardware and software over the past few decades. New methodological developments, designed to overcome part of the gap between 0 K/UHV and operando conditions, include (i) global optimization techniques, (ii) ab initio constrained thermodynamics, (iii) biased molecular dynamics, (iv) microkinetic models of reaction networks and (v) machine learning approaches. The importance of the transition is highlighted by discussing how the molecular level picture of catalytic sites and the associated reaction mechanisms changes when the chemical environment, pressure and temperature effects are correctly accounted for in molecular simulations. It is the purpose of this review to discuss each method on an equal footing, and to draw connections between methods, particularly where they may be applied in combination.

The Effect of Organic Additives on the Activity and Selectivity of CO2 Electroreduction: The Role of Functional Groups

Electrochemical reduction of CO₂ (ERC) to useful chemicals is an environmentally and technologically significant process. The process is confronted with significant challenges to simultaneously enhance the catalyst activity and product selectivity. In this paper, the effects of organic additives on the ERC process were systematically investigated by using DFT to screen additives with different functional groups for enhanced activity and selectivity. In particular, the additives with -NH₃ ⁺ and -SO₃ H groups had a remarkably positive effect on the ERC activity and hydrocarbon selectivity, which were predicted to impart a positive shift on onset potential of approximately 162 and 108 mV, respectively. Importantly, the additive can accelerate the electron transfer of the intermediate and tune the electronic structure of the catalyst surface, resulting in a clear deviation from transition-metal scaling lines. Combining bonding energy of crucial intermediates with partial atomic charge analysis, we rationalized the negative effect of high concentration additives and confirmed the proposed electron transfer model. Furthermore, additive molecules containing functional groups with positive charges and maximizing the deviation from transition-metal scaling lines are meaningful strategies to design and choose organic additives to enhance activity and selectivity of ERC.

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).

Dopant-induced electron localization drives CO2 reduction to C2 hydrocarbons

The electrochemical reduction of CO₂ to multi-carbon products has attracted much attention because it provides an avenue to the synthesis of value-added carbon-based fuels and feedstocks using renewable electricity. Unfortunately, the efficiency of CO₂ conversion to C₂ products remains below that necessary for its implementation at scale. Modifying the local electronic structure of copper with positive valence sites has been predicted to boost conversion to C₂ products. Here, we use boron to tune the ratio of Cu^δ+ to Cu⁰ active sites and improve both stability and C₂-product generation. Simulations show that the ability to tune the average oxidation state of copper enables control over CO adsorption and dimerization, and makes it possible to implement a preference for the electrosynthesis of C₂ products. We report experimentally a C₂ Faradaic efficiency of 79 ± 2% on boron-doped copper catalysts and further show that boron doping leads to catalysts that are stable for in excess of ~40 hours while electrochemically reducing CO₂ to multi-carbon hydrocarbons.

Nano-designed semiconductors for electro- and photoelectro-catalytic conversion of carbon dioxide

This review describes a systematic overview on rational design of semiconductor catalysts for electro- and photoelectro-chemical CO₂ conversion.

Full atomistic reaction mechanism with kinetics for CO reduction on Cu(100) from ab initio molecular dynamics free-energy calculations at 298 K

A critical step toward the rational design of new catalysts that achieve selective and efficient reduction of CO₂ to specific hydrocarbons and oxygenates is to determine the detailed reaction mechanism including kinetics and product selectivity as a function of pH and applied potential for known systems. To accomplish this, we apply ab initio molecular metadynamics simulations (AIMμD) for the water/Cu(100) system with five layers of the explicit solvent under a potential of -0.59 V [reversible hydrogen electrode (RHE)] at pH 7 and compare with experiment. From these free-energy calculations, we determined the kinetics and pathways for major products (ethylene and methane) and minor products (ethanol, glyoxal, glycolaldehyde, ethylene glycol, acetaldehyde, ethane, and methanol). For an applied potential (U) greater than -0.6 V (RHE) ethylene, the major product, is produced via the Eley-Rideal (ER) mechanism using H₂O + e^- The rate-determining step (RDS) is C-C coupling of two CO, with ΔG^‡ = 0.69 eV. For an applied potential less than -0.60 V (RHE), the rate of ethylene formation decreases, mainly due to the loss of CO surface sites, which are replaced by H*. The reappearance of C₂H₄ along with CH₄ at U less than -0.85 V arises from *CHO formation produced via an ER process of H* with nonadsorbed CO (a unique result). This *CHO is the common intermediate for the formation of both CH₄ and C₂H₄ These results suggest that, to obtain hydrocarbon products selectively and efficiency at pH 7, we need to increase the CO concentration by changing the solvent or alloying the surface.

Mechanistic Insights into the Unique Role of Copper in CO2 Electroreduction Reactions

Cu demonstrates a unique capability towards CO₂ electroreduction that can close the anthropogenic carbon cycle; however, its reaction mechanism remains elusive, owing to the obscurity of the solid-liquid interface on Cu surfaces where electrochemical reactions occur. Using a genetic algorithm method in addition to density functional theory, we explicitly identify the configuration of a water bilayer on Cu(2 1 1) and build electrochemical models. These enable us to reveal a mechanistic picture for CO₂ electroreduction, finding the key intermediates CCO* for the C₂ H₄ pathway and CH* for the CH₄ pathway, which rationalize a series of experimental observations. Furthermore, we find that the interplay between the Cu surfaces, carbon monomers, and water network (but not the binding of CO*) essentially determine the unique capability of Cu towards CO₂ electroreduction, proposing a new and effective descriptor for exploiting optimal catalysts.

Electrochemical reduction of CO2 into CO on Cu(100): a new insight into the C–O bond breaking mechanism

Using ab initio molecular dynamics simulations and an aqueous interfacial model with explicit water molecules, we firstly identified a new C–O bond breaking mechanism in the electrochemical conversion of CO₂ to CO on Cu(100) via proton–electron transfer, which is different from the traditional surface catalytic mechanism.

Atomistic Mechanisms Underlying Selectivities in C(1) and C(2) Products from Electrochemical Reduction of CO on Cu(111)

Practical environmental and energy applications of the electrochemical reduction of CO₂ to chemicals and fuels require far more efficient and selective electrocatalysts beyond the only working material Cu, but the wealth of experimental data on Cu can serve to validate any proposed mechanisms. To provide design guidelines, we use quantum mechanics to predict the detailed atomistic mechanisms responsible for C₁ and C₂ products on Cu. Thus, we report the pH dependent routes to the major products, methane and ethylene, and identify the key intermediates where branches to methanol, ketene, ethanol, acetylene, and ethane are kinetically blocked. We discovered that surface water on Cu plays a key role in the selectivity for hydrocarbon products over the oxygen-containing alcohol products by serving as a strong proton donor for electrochemical dehydration reductions. We suggest new experiments to validate our predicted mechanisms.

Electrochemical interfacial influences on deoxygenation and hydrogenation reactions in CO reduction on a Cu(100) surface

Electroreduction of CO2 to hydrocarbons on a copper surface has attracted much attention in the last few decades for providing a sustainable way for energy storage. During the CO2 and further CO electroreduction processes, deoxygenation that is C-O bond dissociation, and hydrogenation that is C-H bond formation, are two main types of surface reactions catalyzed by the copper electrode. In this work, by performing the state-of-the-art constrained ab initio molecular dynamics simulations, we have systematically investigated deoxygenation and hydrogenation reactions involving two important intermediates, COHads and CHOads, under various conditions of (i) on a Cu(100) surface without water molecules, (ii) at the water/Cu(100) interface and (iii) at the charged water/Cu(100) interface, in order to elucidate the electrochemical interfacial influences. It has been found that the electrochemical interface can facilitate considerably the C-O bond dissociation via changing the reaction mechanisms. However, C-H bond formation has not been affected by the presence of water or electrical charge. Furthermore, the promotional roles of an aqueous environment and negative electrode potential in deoxygenation have been clarified, respectively. This fundamental study provides an atomic level insight into the significance of the electrochemical interface towards electrocatalysis, which is of general importance for understanding electrochemistry.

Identification of Possible Pathways for C-C Bond Formation during Electrochemical Reduction of CO2: New Theoretical Insights from an Improved Electrochemical Model

We have carried out a periodic Kohn-Sham density functional theory investigation of the pathways by which carbon-carbon bonds could be formed during the electrochemical reduction of CO2 on Cu(100) using a model that includes the effects of the electrochemical potential, solvent, and electrolyte. The electrochemical potential was set by relating the applied potential to the Fermi energy and then calculating the number of electrons required by the simulation cell for that specific Fermi energy. The solvent was included as a continuum dielectric, and the electrolyte was described using a linearized Poisson-Boltzmann model. The calculated potential of zero charge for a variety of surfaces agrees with experiment to within a mean average error of 0.09 V, thereby validating the assumptions of the model. Analysis of the mechanism for C-C bond formation revealed that at low-applied potential, C-C bond formation occurs through a CO dimer. However, at high applied potentials, a large activation barrier blocks this pathway; therefore, C-C bond formation occurs through reaction of adsorbed CHO and CO. Rate parameters determined from our calculations were used to simulate the kinetics of ethene formation during the electrochemical reduction of CO over a Cu(100) surface. An excellent match was observed between previously reported measurements of the partial current for ethene formation as a function of applied voltage and the variation in the partial current for C-C bond formation predicted by our microkinetic model. The electrochemical model reported here is simple, fairly easy to implement, and involves only a small increase in computational cost over calculations neglecting the effects of the electrolyte and the applied field. Therefore, it can be used to study the effects of applied potential and electrolyte composition on the energetics of surface reactions for a wide variety of electrochemical reactions.

Faraday efficiency and mechanism of electrochemical surface reactions: CO2 reduction and H2 formation on Pt(111)

An atomic scale model of the electrical double layer is used to calculate the mechanism and rate of electrochemical reduction of CO₂ as well as H₂ formation at a Pt(111) electrode. The water layer contains solvated protons and the electrode has excess electrons at the surface. Density functional theory within the generalized gradient approximation is used to describe the electronic structure while the mechanism and activation energy of the various elementary reactions is obtained by calculating minimum energy paths using the nudged elastic band method. The applied electrical potential is deduced from the calculated work function. The optimal reaction mechanism for CO₂ reduction to either methane or methanol is found and the estimated rate compared with that of the competing reaction, H₂ formation. When the free energy of only the intermediates and reactants is taken into account, not the activation energy, Pt(111) would seem to be a good electrocatalyst for CO₂ reduction, significantly better than Cu(111). This, however, contradicts experimental findings. Detailed calculations reported here show that the activation energy for CO₂ reduction is high for both Heyrovsky and Tafel mechanisms on Pt(111) in the relevant range of applied potential. The rate-limiting step of the Heyrovsky mechanism, *COOH + H⁺ + e⁻ → *CO + H₂O, is estimated to have an activation energy of 0.95 eV at −0.9 V vs. standard hydrogen electrode. Under the same conditions, the activation energy for H₂ formation is estimated to be only 0.5 eV. This explains why attempts to reduce CO₂ using platinum electrodes have produced only H₂. A comparison is made with analogous results for Cu(111) [J. Hussain et al., Procedia Comput. Sci., 2015, 51, 1865] where a reaction mechanism with low activation energy for CO₂ electroreduction to methane was identified. The difference between the two electrocatalysts is discussed.