Surface Coverage Of Intermediates Research Articles

A continuous flow reactor for tubular gas diffusion electrodes

Tubular gas diffusion electrodes (GDEs) have been mainly characterized in H-cell environments without consideration of changing flow conditions and scale-up possibilities. In this study, a continuous flow reactor for tubular GDEs is presented, which allows for different electrode configurations and manipulation of flow conditions to control mass transfer. 3D printed copper GDEs are employed for CO2 electroreduction while membrane electrode assemblies (MEAs) from stainless steel electrodes are used on the anode side. We demonstrate the impact of the gas supply in either flow-through or flow-by mode on catalyst performance. Flow-through mode yields a variety of typical products, such as CO, HCOOH, CH4 and C2H4. Flow-by mode results in mainly CH4, which we link to a higher surface coverage of reaction intermediates with diffusion as the governing transport mechanism in flow-by operation. Additionally, a hybrid porosity GDE is introduced that combines different functional domains in one electrode for improved CO2 mass transfer. Microcomputed tomography confirms the successful realization of these domains in close proximity to each other. With the presented approach, we aim to highlight the importance of the reaction environment for an effective scale-up of tubular concepts in CO2 electroreduction and many other electrochemical applications.

Time‐Resolved Product Observation for CO2 Electroreduction Using Synchronised Electrochemistry‐Mass Spectrometry with Soft Ionisation (sEC‐MS‐SI)

AbstractThe mechanistic understanding of electrochemical CO2 reduction reaction (CO2RR) requires a rapid and accurate characterisation of product distribution to unravel the activity and selectivity, which is yet hampered by the lack of advanced correlative approaches. Here, we present the time‐resolved identification of CO2RR products by using the synchronised electrochemistry‐mass spectrometry (sEC‐MS). Transients in product formation can be readily captured in relation to electrochemical conditions. Moreover, a soft ionisation (SI) strategy is developed in MS for the direct observation of CO, immune to the interference of CO2 fragments. With the sEC‐MS‐SI, the kinetic information, such as Tafel slopes and onset potentials, for a myriad of CO2RR products are revealed and we show the hysteresis seen for the evolution of some species may originate from the potential‐driven changes in surface coverage of intermediates. This work provides a real‐time picture of the dynamic formation of CO2RR products.

Time-Resolved Product Observation for CO2 Electroreduction Using Synchronised Electrochemistry-Mass Spectrometry with Soft Ionisation (sEC-MS-SI).

The mechanistic understanding of electrochemical CO2 reduction reaction (CO2 RR) requires a rapid and accurate characterisation of product distribution to unravel the activity and selectivity, which is yet hampered by the lack of advanced correlative approaches. Here, we present the time-resolved identification of CO2 RR products by using the synchronised electrochemistry-mass spectrometry (sEC-MS). Transients in product formation can be readily captured in relation to electrochemical conditions. Moreover, a soft ionisation (SI) strategy is developed in MS for the direct observation of CO, immune to the interference of CO2 fragments. With the sEC-MS-SI, the kinetic information, such as Tafel slopes and onset potentials, for a myriad of CO2 RR products are revealed and we show the hysteresis seen for the evolution of some species may originate from the potential-driven changes in surface coverage of intermediates. This work provides a real-time picture of the dynamic formation of CO2 RR products.

In-Situ-Formed Potassium-Modified Nickel-Zinc Carbide Boosts Production of Higher Alcohols beyond CH4 in CO2 Hydrogenation.

Ni-based catalysts have been widely studied in the hydrogenation of CO2 to CH4 , but selective and efficient synthesis of higher alcohols (C2+ OH) from CO2 hydrogenation over Ni-based catalyst is still challenging due to successive hydrogenation of C1 intermediates leading to methanation. Herein, we report an unprecedented synthesis of C2+ OH from CO2 hydrogenation over K-modified Ni-Zn bimetal catalyst with promising activity and selectivity. Systematic experiments (including XRD, in situ spectroscopic characterization) and computational studies reveal the in situ generation of an active K-modified Ni-Zn carbide (K-Ni3 Zn1 C0.7 ) by carburization of Zn-incorporated Ni0 , which can significantly enhance CO2 adsorption and the surface coverage of alkyl intermediates, and boost the C-C coupling to C2+ OH rather than conventional CH4 . This work opens a new catalytic avenue toward CO2 hydrogenation to C2+ OH, and also provides an insightful example for the rational design of selective and efficient Ni-based catalysts for CO2 hydrogenation to multiple carbon products.

In‐Situ‐Formed Potassium‐Modified Nickel‐Zinc Carbide Boosts Production of Higher Alcohols beyond CH4 in CO2 Hydrogenation

AbstractNi‐based catalysts have been widely studied in the hydrogenation of CO2 to CH4, but selective and efficient synthesis of higher alcohols (C2+OH) from CO2 hydrogenation over Ni‐based catalyst is still challenging due to successive hydrogenation of C1 intermediates leading to methanation. Herein, we report an unprecedented synthesis of C2+OH from CO2 hydrogenation over K‐modified Ni−Zn bimetal catalyst with promising activity and selectivity. Systematic experiments (including XRD, in situ spectroscopic characterization) and computational studies reveal the in situ generation of an active K‐modified Ni−Zn carbide (K‐Ni3Zn1C0.7) by carburization of Zn‐incorporated Ni0, which can significantly enhance CO2 adsorption and the surface coverage of alkyl intermediates, and boost the C−C coupling to C2+OH rather than conventional CH4. This work opens a new catalytic avenue toward CO2 hydrogenation to C2+OH, and also provides an insightful example for the rational design of selective and efficient Ni‐based catalysts for CO2 hydrogenation to multiple carbon products.

Impact of Catalyst Morphology on the Adsorption and Electrocatalytic Hydrogenation of Benzaldehyde

Biomass represents an untapped resource for achieving a circular carbon economy. Current processes for converting biomass are limited to centralized processing facilities. Electrochemical approaches are inherently modular and enable decentralized upgrading of pyrolysis oil to value-added chemicals and fuels (products). A primary component of the aqueous phase of such feedstocks are carbonyl functionalities, where benzaldehyde (BZ) represents a model compound. Selective reduction of BZ to benzyl alcohol (BnOH) can be used to understand the catalytic upgrading of pyrolysis oil to value-added products. Electrocatalytic hydrogenation (ECH) of BZ can be carried out at mild temperatures and pressure with the hydrogen required for BZ reduction coming from the protons in the solution. However, these protons can also combine in the hydrogen evolution reaction (HER) in a competitive process. Among the noble and base metals studied in the literature, palladium (Pd) is very active and has demonstrated a high faradaic efficiency (FE) for ECH of BZ relative to HER. Palladium also forms a thermodynamically stable hydride under these conditions. However, prior work has shown that BZ inhibits the formation of palladium hydride under reaction conditions, whereas the presence of weaker adsorbing organics does not. Together, these observations raise interesting questions whether ECH occurs via a proton coupled electron transfer (PCET) pathway or via addition of H from the catalyst surface directly (Langmuir-Hinshelwood, LH pathway). The specific aims of this work investigate the effect of catalyst morphology on the surface coverage of intermediates like H and adsorbed BZ, the relative rates of PCET and LH pathways, and their competition with HER and hydride formation. For this, we investigate high surface area Pd nanoparticle electrocatalysts and a gel network (Pd Gel) of them rich with grain boundaries and strain via cyclic voltammetry. Adsorption isotherms on these Pd catalysts show changes in hydride formation and the relative affinity of H and BZ on the Pd surface. We further study the reaction rate orders and FE as a function of applied potential to better understand the contributions of PCET and LH pathways. These findings point to catalyst morphology as a further handle to manipulate activity and FE for ECH of BZ. Furthermore, these findings will enhance the mechanistic understanding of BZ ECH kinetics and will contribute to improving the overall efficacy of biomass conversion to biofuels.

(Invited) Investigations of Electrochemical CO2 Reduction with Differential Electrochemical Mass Spectrometry

Differential electrochemical mass spectrometry (DEMS) is an analytical technique wherein an electrochemical reactor is interfaced with a mass spectrometer using a pervaporation membrane. This configuration enables volatile electrochemical reaction products to be continuously collected, identified, and quantified during steady state and dynamic polarization. The capabilities of this analytical technique are highly dependent on the design of the electrochemical reactor and how it is interfaced to the mass spectrometer. This presentation will introduce a variety of different DEMS cell designs and will compare their capabilities and limitations in terms of product sensitivity, product quantifiability, and time response. These comparisons will be illustrated through a series of vignettes investigating the electrocatalysis of CO2 reduction over Cu, Ag, and Au. This reaction is particularly difficult to investigate with DEMS since many of the reaction products yield identical mass fragments upon electron impact ionization. A general strategy for deconvoluting the extent to which a given product contributes to the observed mass ion currents will be presented. The utilization of DEMS for the direct observation of both the composition of the local reaction environment and the transient formation of intermediate reaction products will be discussed, as well as how these insights can be leveraged to guide rational electrocatalyst design. Finally, a new type of DEMS setup capable of quantifying the steady state surface coverage and surface lifetimes of electrochemical reaction intermediates will be presented. Figure 1

Organic-inorganic hybrid crystal-assisted etching of nickel foam for the collectively exhaustive electrochemical performance of oxygen evolution reaction.

An organic-inorganic hybrid crystal, violet-crystal (VC), was used to etch the nickel foam (NF) to fabricate a self-standing electrode for the water oxidation reaction. The efficacy of VC-assisted etching manifests the promising electrochemical performance towards the oxygen evolution reaction (OER), requiring only ~356 and ~376 mV overpotentials to reach 50 and 100 mA cm-2, respectively. The OER activity improvement is attributed to the collectively exhaustive effects arising from the incorporation of various elements in the NF, and the enhancement of active site density. Furthermore, the self-standing electrode is robust, exhibiting a stable OER activity after 4,000 cyclic voltammetry cycles, and ~ 50 h. The anodic transfer coefficients (aa) show that the first electron transfer step is the rate-determining step on the surface of NF-VCs-1.0 (NF etched by 1 g of VCs) electrode, while the chemical step involving dissociation following the first electron transfer step is identified as the rate-limiting step in other electrodes. The lowest Tafel slope value observed in the NF-VCs-1.0 electrode indicates the high surface coverage of oxygen intermediates and more favorable OER reaction kinetics, as confirmed by high interfacial chemical capacitance and low charge transport/interfacial resistance.

Surface tension of nanoparticle dispersions unravelled by size-dependent non-occupied sites free energy versus adsorption kinetics

The surface tension of dispersions presents many types of behaviours. Although some models, based on classical surface thermodynamics, allow partial interpretation, fundamental understanding is still lacking. This work develops a single analytical physics-based formulation experimentally validated for the surface tension of various pure nanoparticle dispersions, explaining the underlying mechanisms. Against common belief, surface tension increase of dispersions appears not to occur at low but rather at intermediate surface coverage, owed by the relatively large size of nanoparticles with respect to the fluid molecules. Surprisingly, the closed-form model shows that the main responsible mechanism for the various surface tension behaviours is not the surface chemical potential of adsorbed nanoparticles, but rather that of non-occupied sites, triggered and delicately controlled by the nanoparticles ‘at a distance’, introducing the concept of the ‘non-occupancy’ effect. The model finally invites reconsidering surface thermodynamics of dispersions and provides for criteria that allow in a succinct manner to quantitatively classify the various surface tension behaviours.

A special Bi-S motif catalyst for highly selective CO2 conversion to methanol

Carbon dioxide as raw materials chemically transformed to valuable, renewable and high-energetic carbon feedstocks, especially if driven by renewable energy, is a desirable way to mitigate the depletion of fossil fuels and allow environmentally neutral use of carbon fuels. However, it remains more exploration of electrocatalysts with high activity and superior selectivity in converting CO2 to storable carbon-based liquid fuels, like methanol. Herein, we develop an in-situ sulfurizing strategy over the Bi-based metal-organic frameworks (Bi-BTC) and disperse stable and special Bi-S motif on subsequent S-doped carbon-framework (called BS@Cx). Unlike the previous reports on Bi-based catalysts, as-prepared BS@Cx converted CO2 to methanol as major product at the whole applied potentials. Benefiting from the special Bi-S motif, it brought to the dramatic selectivity change of CO2 reduction production: dramatic decrease in HCOO− selectivity and increase in methanol selectivity. At −0.9 V vs RHE, maximum methanol faradaic efficiency (FE) and the selectivity of methanol arrived at 78.6% and 78.8%(Total-C FE was 95.5%), respectively. Some effective experiments involving of ECSA and charge transfer efficiency (Φ) certified that BS@Cx catalysts in the form of Bi-S motif exhibited higher intrinsic activity and selectivity, and overcame the energy barrier for methanol production. Coupled with PDOS of catalyst absorbed intermediates (OCHO* and CO*) and discussion of intermediates (COOH* and CH2O*), we bring forward a new possible pathway for catalyzing CO2 to methanol over BS@800. Further though microkinetic analysis with experimental Tafel slopes, we provided first-order understanding on the proposed reaction mechanisms, such as the rate-determining steps (RDS) and surface coverage (θ) of major reaction intermediates, and shed light on how rate-limiting steps can give rise to different Tafel slopes. This work may occur worthy insights into crystal structure engineering to achieve efficient electrocatalysts for selective CO2 conversion toward generation of valuable carbon-based chemicals.

Deep Learning-Assisted Investigation of Electric Field-Dipole Effects on Catalytic Ammonia Synthesis.

External electric fields can modify binding energies of reactive surface species and enhance catalytic performance of heterogeneously catalyzed reactions. In this work, we used density functional theory (DFT) calculations-assisted and accelerated by a deep learning algorithm-to investigate the extent to which ruthenium-catalyzed ammonia synthesis would benefit from application of such external electric fields. This strategy allows us to determine which electronic properties control a molecule's degree of interaction with external electric fields. Our results show that (1) field-dependent adsorption/reaction energies are closely correlated to the dipole moments of intermediates over the surface, (2) a positive field promotes ammonia synthesis by lowering the overall energetics and decreasing the activation barriers of the potential rate-limiting steps (e.g., NH2 hydrogenation) over Ru, (3) a positive field (>0.6 V/Å) favors the reaction mechanism by avoiding kinetically unfavorable N≡N bond dissociation over Ru(1013), and (4) local adsorption environments (i.e., dipole moments of the intermediates in the gas phase, surface defects, and surface coverage of intermediates) influence the resulting surface adsorbates' dipole moments and further modify field-dependent reaction energetics. The deep learning algorithm developed here accelerates field-dependent energy predictions with acceptable accuracies by five orders of magnitudes compared to DFT alone and has the capacity of transferability, which can predict field-dependent energetics of other catalytic surfaces with high-quality performance using little training data.

Tafel Analysis Guided Optimization of ZnNP-O-C Catalysts for the Selective 2-Electron Oxygen Reduction Reaction in Neutral Media.

The lack of characterizations of the adsorption capability toward intermediates during reactions causes difficulties in determining the structural optimization principle of the catalysts for the 2-electron oxygen reduction reaction (2e- ORR). Here, a Tafel-θ method is proposed to evaluate the surface coverage (θ) of important intermediates (*OOH and *OH) on the material surface and further help optimize the catalyst. With the assistance of Tafel-θ analysis, a Zn nanoparticle incorporated oxygen-doped carbon (ZnNP-O-C) catalyst with high 2e- ORR performance (onset of ∼0.57 V and selectivity of >90.4%) in neutral media was achieved. Both the theoretical calculation and characterization results are consistent with the Tafel-θ deduction, revealing that an appropriate ratio of Zn nanoparticles and bridging O can optimize the *OOH adsorption/desorption strength of the adjacent carbon site. This study not only provides an advanced ZnNP-O-C catalyst for electrochemical H2O2 production but also proposes a fast and precise method for the comprehensive assessment of future catalysts.

Methanol to hydrocarbons conversion: Why dienes and monoenes contribute differently to catalyst deactivation?

Dienes, formed at much lower concentrations than monoenes via alkylation of monoenes with formaldehyde (HCHO), have much higher propensity than monoenes to foment catalyst deactivation in methanol-to-hydrocarbons (MTH) conversion over zeolites via HCHO-mediated deactivation pathways. The mechanistic basis for why monoenes and dienes contribute differently to cause catalyst deactivation during MTH is investigated using independent kinetic studies of HCHO reactions with representative monoenes and dienes, i.e., propylene and 1,3-butadiene, over zeolites. Despite 3,6-dihydro-2H-pyran being the predominant product for both propylene/HCHO and 1,3-butadiene/HCHO reactions, a combination of transient and steady-state rate measurements and stoichiometric experiments reveals distinct persistent intermediates being involved in the addition of HCHO to different surface-bound alkyl species derived from propylene and butadiene. These persistent intermediates desorb only via HCHO- and water-mediated proton transfer steps and as such can be sequestered on the catalyst. A nonlinear mixed-effects kinetic model reveals that the intermediate formed during HCHO-diene reactions has a higher steady-state surface coverage and its desorption via HCHO- and water-mediated routes exhibit > 8× and > 3×, respectively, higher rate constants than those during HCHO-monoene reactions. These results evidence the distinct identity, surface coverage, and reactivities of surface-bound alkyl intermediates as critical factors in the much more pernicious effects of dienes compared to monoenes on catalyst lifetime during MTH.

Electrocatalytic volcano relations: surface occupation effects and rational kinetic models

Electrocatalysis plays a vital role in technologies of energy and environment relevance, such as water electrolysis, fuel cells, synthesis of carbon and nitrogen-based fuels, etc. The volcano relations (VRs) are general and standard tools for predicting and understanding the activity trends of electrocatalysts. The modern electrocatalytic VRs are generally based on the kinetic models with the maximum free energy (Δ G 0 max ) of reaction steps as the rate-determining term (RDT), in which some important factors that crucially impact the reaction kinetics are missed, for examples, the surface structures and coverages of reaction intermediates and spectators, other free energy demanding steps than that associated with the Δ G 0 max , and so on. In this perspective, we first give a brief introduction of the theoretical framework of current electrocatalytic VRs and the underlying problems in the oversimplified Δ G 0 max -based kinetic models, and then provide an account of our effort in constructing more rational VRs for electrocatalytic reactions. We introduce a new theoretical framework of electrocatalytic VRs based on kinetic model with the so-called energetic span ( δE ) serving as RDT. Since the surface-coverage effects and multiple free energy-demanding steps are considered, the VRs thus obtained show several new features such as strong potential dependence, asymmetric ascending and descending branches, relatively flat tops, and so on. The effectiveness of the δE -based VRs is verified for hydrogen and oxygen electrocatalytic reactions. Finally, research directions to further rationalize the electrocatalytic VRs are discussed. Electrocatalytic volcano relations are crucially impacted by diverse factors such as multiple energy-demanding steps, surface occupation, electrode potential, and electrolyte pH. New kinetic model indicates relative flat volcano tops.

Experimental and computational kinetics study of the liquid-phase hydrogenation of C[dbnd]C and C[dbnd]O bonds

Solvent effects on adsorption equilibrium and reaction kinetics are evaluated for hydrogenation reactions catalyzed by Pd/alumina in a series of different solvents. Three reactants – cyclohexene, benzene and benzaldehyde – and three solvents – n-heptane (HEP), methylcyclohexane (MCHA), decalin (DL) – have been investigated. Kinetic analysis of hydrogenation of cyclohexene reveals that hydrogen adsorbs on different sites from those where cyclohexene and the solvents adsorb; however, the presence of hydrogen on these separate sites affects the heats of adsorption of the hydrocarbons. When the solvent is a weakly interacting linear alkane (HEP), the rate determining step of the reaction is the first hydrogenation of adsorbed cyclohexene. This conclusion is supported by DFT calculations that show a higher enthalpy barrier for the first hydrogenation than for the second, while statistical thermodynamics analysis validates the physical significance of the entropy of adsorption parameters derived from the kinetic fitting of experimental data. By contrast, with solvents such as MCHA and DL, which interact more strongly with the metal surface and compete for active sites with the reactant and the surface intermediate, the rate limiting step seems to shift to the second hydrogenation step.One of the main conclusions of the study is that the effects observed with non-polar solvents are mostly due to competitive adsorption of the solvent, which causes a decrease in surface coverage of reactants and surface intermediates. Indeed, under identical conditions, the hydrogenation rates of CC bonds in cyclohexene (or the aromatic ring in benzene) follow the order HEP > MCHA > DL, which is consistent with the strongest solvent/metal interaction for DL and the weakest for HEP. Further, binary solvent mixtures of DL-HEP exhibit non-idealities that are clearly evident in the fitting of hydrogenation rates. It is proposed that the higher activity coefficient of liquid DL in the liquid mixture enhances its surface coverage, competing more effectively with the reactant, which causes a decrease in catalytic activity larger than that predicted assuming an ideal mixture of solvents.Likewise, large drops in benzene hydrogenation rates are observed in water-organic co-solvent mixtures due to the competitive adsorption of water on the Pd surface. Water adsorption is further enhanced by the non-ideality of the water-organic mixture. By contrast, in the hydrogenation of the CO bond of benzaldehyde, the presence of water provides a favorable alternative reaction path with lower activation barrier, resulting in higher hydrogenation activity when the reactant is a carbonyl-containing molecule, such as benzaldehyde. In this system, the H-bonded water network assists shuttling a proton from the Pd surface to the hydrophilic O atom in CO, assisted by surface charge separation. Since this H transfer pathway cannot take place with CC bonds or aromatic rings, only site inhibition occurs for these reactions.

Machine-Learned Corrections to Mean-Field Microkinetic Models at the Fast Diffusion Limit

We present a machine learning-based formalism to correct the mean-field assumption in microkinetic models to incorporate adsorbate interactions and surface inhomogeneity at the fast diffusion limit. Lattice Monte Carlo simulations are used to compute the macroscopic reaction rate in the presence of adsorbate interaction at different values of surface coverage. This dataset is then used to train an artificial neural network to compute precise reaction rates as a function of surface coverage of intermediates, and the underlying microkinetic model of the reaction system is modified by correcting the typical mean-field rate terms with these data-driven functions. An example of CO oxidation on the square ordered lattice is used to illustrate the speed, accuracy, and robustness of this approach, vis-à-vis a full-fledged kinetic Monte Carlo simulation. In particular, we show that while the traditional mean-field model completely misses the bistability of this system under certain conditions, the neural network-modified formalism correctly captures this phenomenon. We posit that this method scales well to larger reaction systems and is a cost-effective means to improve the accuracy of differential equation-based microkinetic models.

Chemical kinetics for generalized two-step reaction schemes

Chemical reaction kinetics for steady-state catalytic reactions on heterogeneous catalysts are generally governed by a limited number of elementary steps and the nature of these steps is controlled by the degree of rate control and the surface coverage of reaction intermediates. The generalized two-step reaction scheme, first proposed by Michel Boudart, is used in this paper to drive analytical expressions for the surface coverages by the Most Abundant Reactive Intermediate (MARI); the surface coverage by the Most Abundant Surface Intermediate (MASI); the fraction of the free surface; the degree of rate control for each kinetically significant step; and the overall reaction rate. The two most ubiquitous reaction mechanisms in heterogeneous catalysis, i.e., the associative reaction mechanism and the dissociative reaction mechanism are studied. The applicability of the derived expressions is demonstrated by two case studies: acetone hydrogenation for the associative case and ammonia synthesis for the dissociative case. Scaling relations and BEP correlation are incorporated in the generalized two-step reaction scheme to determine the surface properties of the optimal catalyst. Acetone hydrogenation is used as a case study to demonstrate the applicability of the derived expressions for optimal catalyst. Finally, the application of generalized two-step reaction schemes in understanding the response of a catalyst surface to transient reaction conditions is discussed. Transient reaction kinetics of acetone hydrogenation is used as a case study to demonstrate the applicability of the generalized two-step reaction approach.

Evidence of collision-induced resuspension of microscopic particles from a monolayer deposit

Even after one century of research on particle resuspension the multiphysics mechanism for it remains unclear. In this Letter we show evidence for the inter-particle collision in a monolayer deposit with an intermediate surface coverage. Previously, the inter-particle collision was neglected in monolayer deposits and was considered only in multilayer beds due to lack of evidence. This experimental work indicates that particle cluster resuspension plays an important role in the early stage of resuspension. This explains the strong bimodal character of the resuspension curve, which shows the remaining particle fraction as a function of the flow velocity.

Revealing the CO Coverage-Driven C-C Coupling Mechanism for Electrochemical CO2 Reduction on Cu2O Nanocubes via Operando Raman Spectroscopy.

Electrochemical reduction of carbon dioxide (CO2RR) is an attractive route to close the carbon cycle and potentially turn CO2 into valuable chemicals and fuels. However, the highly selective generation of multicarbon products remains a challenge, suffering from poor mechanistic understanding. Herein, we used operando Raman spectroscopy to track the potential-dependent reduction of Cu2O nanocubes and the surface coverage of reaction intermediates. In particular, we discovered that the potential-dependent intensity ratio of the Cu–CO stretching band to the CO rotation band follows a volcano trend similar to the CO2RR Faradaic efficiency for multicarbon products. By combining operando spectroscopic insights with Density Functional Theory, we proved that this ratio is determined by the CO coverage and that a direct correlation exists between the potential-dependent CO coverage, the preferred C–C coupling configuration, and the selectivity to C2+ products. Thus, operando Raman spectroscopy can serve as an effective method to quantify the coverage of surface intermediates during an electrocatalytic reaction.

Why the microkinetic modeling of experimental tafel plots requires knowledge of the reaction intermediate's binding energy

AbstractKnowledge of the relation between electrocatalytic activity and the rate‐determining step or the surface coverage of reaction intermediates is crucial to design next‐generation electrocatalysts that may contribute to the sustainability of our society. Commonly, microkinetic models making use of the quasi‐equilibrium or steady‐state assumptions are applied to correlate potential mechanistic descriptions to experimental data, which are usually depicted in the form of a Tafel plot. Yet, there is a discrepancy in the literature on the utility of the quasi‐equilibrium and steady‐state conditions, both of which are approximations per se. This inconsistency is the starting point of the present work, which compares the quasi‐equilibrium and steady‐state approaches for the analysis of a Tafel plot by the concept of free‐energy diagram for a two‐electron process. A correlation between the two frameworks is deduced and important guidelines for the application of the quasi‐equilibrium and steady‐state assumptions are obtained. While the quasi‐equilibrium approach is a suitable approximation for the analysis of a linear Tafel line without change in the Tafel slope, it may fail in the evaluation of a Tafel plot with two linear Tafel regimes. There, the binding energy of the reaction intermediate governs whether the quasi‐equilibrium model is justified, or the steady‐state approach is needed. This implies that density functional theory calculations are indispensably required as a supplement to analyze a Tafel plot with two different Tafel slopes, a situation that is particularly observed for highly active electrocatalysts.