Hydrogen bonding steers the product selectivity of electrocatalytic CO reduction

Abstract

The product selectivity of many heterogeneous electrocatalytic processes is profoundly affected by the liquid side of the electrocatalytic interface. The electrocatalytic reduction of CO to hydrocarbons on Cu electrodes is a prototypical example of such a process. However, probing the interactions of surface-bound intermediates with their liquid reaction environment poses a formidable experimental challenge. As a result, the molecular origins of the dependence of the product selectivity on the characteristics of the electrolyte are still poorly understood. Herein, we examined the chemical and electrostatic interactions of surfaceadsorbed CO with its liquid reaction environment. Using a series of quaternary alkyl ammonium cations (methyl 4 N + , ethyl 4 N + , propyl 4 N + , and butyl 4 N + ), we systematically tuned the properties of this environment. With differential electrochemical mass spectrometry (DEMS), we show that ethylene is produced in the presence of methyl 4 N + and ethyl 4 N + cations, whereas this product is not synthesized in propyl 4 N + -and butyl 4 N + -containing electrolytes. Surface-enhanced infrared absorption spectroscopy (SEIRAS) reveals that the cations do not block CO adsorption sites and that the cation-dependent interfacial electric field is too small to account for the observed changes in selectivity. However, SEIRAS shows that an intermolecular interaction between surface-adsorbed CO and interfacial water is disrupted in the presence of the two larger cations. This observation suggests that this interaction promotes the hydrogenation of surface-bound CO to ethylene. Our study provides a critical molecular-level insight into how interactions of surface species with the liquid reaction environment control the selectivity of this complex electrocatalytic process.hydrogen bonding | cation effects | electrocatalysis | carbon dioxide reduction | catalytic selectivity T he reaction environment profoundly impacts the kinetics of many chemical processes. Examples include the influence of the solvating environment on the rates of electron transfer (1), isomerization (2), peptide folding (3), and organic reactions (4), as well as the sensitivity of enzymatic catalysis to changes in the molecular structure of the active site (5). For a chemical process that can lead to multiple reaction products, solvent effects can impact the relative rates of product formation and therefore the product selectivity (6, 7). These effects, which can have complex energetic and/or dynamical origins (1,8,9), are fundamentally rooted in intermolecular interactions between the reactants and their environment. In the context of heterogeneous electrocatalysis, the reaction environment is asymmetric; i.e., reactants at the electrochemical interface are interacting with the solid electrode and the liquid electrolyte. Understanding the interactions of intermediates with their interfacial environment is essential for controlling the reaction paths of electrocatalytic processes that exhibit poor product selectivity.The reduc...

Hydroxide Is Not a Promoter of C2+ Product Formation in the Electrochemical Reduction of CO on Copper

“…This is in contradiction to the data presented in Figure c. We note that the lack of specific adsorption of C‐Na + is confirmed by the lack of Stark tuning of any bands associated with the crown ether (Figure S10). Another commonly invoked mechanism to explain the cation effect is the modification of the interfacial electric field due to the different cation sizes . However, we do not observe any detectable change in the Stark tuning rate in adsorbed CO on Cu in the Na + concentration range of 0.1 to 1 m (30–33 cm −1 V −1 ; Figure S11).…”

“…This is confirmed by the reduced Stark tuning rate of the adsorbed CO on polycrystalline Cu from 34 cm −1 V −1 in 0.1 m NaOH to 25 cm −1 V −1 in 0.1 m C‐NaOH determined with attenuated total reflectance surface enhanced infrared absorption spectroscopy (ATR‐SEIRAS; Figure S9). A lower Stark tuning rate within the Gouy‐Chapman‐Stern (GCS) model suggests that the distance between the outer Helmholtz plane (OHP) and the electrode surface is larger, presumably due to the larger cation size of C‐Na + as compared to Na + . It is worth noting that the presence of 0.5 m of C‐Na + in the presence of 0.5 m of free Na + still exerts a major impact on the reactions occurring on the electrode surface (Figure c), with j CORR being only roughly half as much in 1 m NaOH with 50 % of Na + chelated into C‐Na + as in 0.5 m NaOH (Figure c).…”

“…Modified local electric field, as suggested by Bell and co‐workers, at higher cation concentrations could lead to higher densities of “hot spots” that favor the formation of C 2+ products. Another possibility is that interfacial water structure is modified at higher concentrations of cations, which could facilitate the C−C coupling pathway by better solvating the transition state complex. We stress that these speculative rationalizations of the cation effect highlight the gap in the molecular level understanding how the nature and concentration of cations impact surface‐mediated electrocatalytic reactions.…”

Surface Chemistry Modulates CO2 Reduction Reaction Intermediates on Silver Nanoparticle Electrocatalysts

“…In the context of heterogeneous systems, CO2R performance has been successfully modulated through the use of grafted surface ligands which interact with adsorbed species or promote their transfer to the catalyst surface, [14][15][16][17][18][19] design of enzyme-mimetic catalytic pockets in porous materials, 20,21 and tuning of the electrolyte. [22][23][24] Complementary to CO2R system design is the innovation of operando techniques which provide mechanistic information on the reaction and are performed simultaneously as the reaction proceeds. [25][26][27] Within this context, operando vibrational spectroscopies such as Raman and infrared spectroscopy monitor distinct bands of CO2 and CO2R intermediate species.…”

Interaction of CO with Gold Under Gas Phase and Electrochemical Environments

“…Despite the apparent simplicity of many of these reaction processes, their mechanisms remain contentious. [5][6][7] The determination of relevant adsorbate coverages in situ, especially under electrochemical conditions, remains an open challenge. Computational mechanistic investigations have recently begun to include the impact of solvation and field, which can have a critical impact on reaction energetics.…”