Unified Mechanistic Understanding of CO2 Reduction to CO on Transition Metal and Single Atom Catalysts

Vijay, Sudarshan; Ju, Wen; Brückner, Sven; Strasser, Peter; Chan, Karen

doi:10.26434/chemrxiv.14427986.v1

Cited by 5 publications

(2 citation statements)

References 38 publications

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“…We underscore the need for experiments in acidic conditions to exlude the involvement of a proton transfer in the rate-limiting step, as e.g. performed recently for single atom catalysts 44,45 . Potential-dependent ab-initio kinetics involving grand-canonical DFT simulations 46 identify that the dimerization of *CO is favored over its initial protonation to *COH/*CHO, and the potential response of *CO dimerization alone can explain the experimentally observed Tafel slopes, which is a direct consequence of the large degree of polarization of the *OCCO dimer species.…”

Section: Introductionmentioning

confidence: 74%

Using pH dependence for understanding mechanisms in electrochemical CO reduction

Kastlunger

Wang

Govindarajan

et al. 2021

Preprint

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Utilizing electrochemical conversion of CO(2) into hydrocarbons and oxygenates is envisioned as a promising path towards closing the carbon cycle in modern technology. To this day, however, the exact reaction mechanisms towards the plethora of single and multi-carbon products on Cu electrodes are still disputed. This uncertainty even extends to the rate-limiting step of the respective reactions. Since multi-carbon products do not show a dependence on the electrolyte pH in neutral and alkaline media, CO dimerization on the Cu surface has been proposed as the rate-limiting step. However, other elementary steps would lead to the same pH dependence, namely the proton-electron transfer to *CO followed by subsequent coupling or the protonation of the *OCCO dimer. The pH dependence of methane production on the other hand suggests that the rate limiting step is located beyond the first proton-electron transfer to *CO. In order to conclusively identify the rate limiting steps in CO reduction, we analyzed the mechanisms on the basis of constant potential DFT calculations, CO reduction experiments on Cu at varying pH values (3 - 13) and fundamental rate theory. We find that, even in acidic media, the reaction rate towards multi-carbon products is nearly unchanged on an SHE potential scale, which indicates that its rate limiting step does not involve a proton donor. Hence, we deduce that the rate limiting step can indeed only consist of the coupling of two CO molecules on the surface, both in acidic and alkaline conditions. For methane, on the other hand, the rate-limiting step changes with the electrolyte pH from the first protonation step in acidic/neutral conditions to a later step in alkaline conditions. Finally, based on an in-depth kinetic analysis, we conclude that the pathway towards CH4 involving a surface combination of *CO and *H is unlikely, since it is unable to reproduce the measured current densities and Tafel slopes.

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Section: Introductionmentioning

confidence: 74%

Using pH dependence for understanding mechanisms in electrochemical CO reduction

Kastlunger

Wang

Govindarajan

et al. 2021

Preprint

Self Cite

View full text Add to dashboard Cite

show abstract

“…We underscore the need for experiments in acidic conditions to exlude the involvement of a proton transfer in the rate-limiting step, as e.g. performed recently for single atom catalysts 44,45 . Potential-dependent abinitio kinetics involving grand-canonical DFT simulations 46 identify that the dimerization of *CO is favored over its initial protonation to *COH/*CHO, and the potential response of *CO dimerization alone can explain the experimentally observed Tafel slopes, which is a direct consequence of the large degree of polarization of the *OCCO dimer species.…”

Section: Introductionmentioning

confidence: 74%

Using pH dependence to understand mechanisms in electrochemical CO reduction

Kastlunger

Wang

Govindarajan

et al. 2021

Preprint

Self Cite

View full text Add to dashboard Cite

Electrochemical conversion of CO(2) into hydrocarbons and oxygenates is envisioned as a promising path towards closing the carbon cycle in modern technology. To this day, however, the reaction mechanisms towards the plethora of products are disputed, complicating the search for novel catalyst materials. In order to conclusively identify the rate-limiting steps in CO reduction on Cu, we analyzed the mechanisms on the basis of constant potential DFT calculations and experiments at a wide range of pH values (3 - 13). We find that *CO dimerization is energetically favoured as the rate limiting step towards multi-carbon products. This finding is consistent with experiments, where the reaction rate is nearly unchanged on an SHE potential scale, even under acidic conditions. For methane, both theory and experiments indicate a change in the rate-limiting step with electrolyte pH from the first protonation step in acidic/neutral conditions to a later one in alkaline conditions. We also show, through a detailed analysis of the microkinetics, that a surface combination of *CO and *H is inconsistent with the measured current densities and Tafel slopes. Finally, we discuss the implications of our understanding for future mechanistic studies and catalyst design.

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Multicapacitor Approach to Interfacial Proton-Coupled Electron Transfer Thermodynamics at Constant Potential

et al. 2021

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Theoretical calculations of interfacial thermodynamics at constant potential enhance understanding of heterogeneous electrocatalytic reactions. Herein, a strategy is devised for computing reaction thermodynamics for electrochemical proton-coupled electron transfer (PCET), a key elementary step in a wide range of electrocatalytic processes. In this approach, Gibbs free energies obtained from constant charge periodic density functional theory calculations are transformed to grand potentials by using a grid-based mapping procedure in conjunction with a multicapacitor model that accounts for constantly charged surface adsorbates. The energetic contribution of the adsorbate capacitor to the grand potential is found to be essential for computing proton-coupled redox potentials at constant potential. This strategy is applied to graphite-conjugated catalysts, wherein organic acids are attached to carbon surfaces through a conductive phenazine bridge. In these systems, PCET occurs at both the phenazine bridges and the organic acids, which can be negatively or positively charged. For a given graphite-conjugated catalyst active site, the charge of the organic acid adsorbate remains virtually constant for the relevant range of electrode potentials. Moreover, the potential-dependent graphite surface charge density, which excludes this adsorbate charge, is consistent across all systems studied. Within this framework, the potential-dependent PCET reaction free energies are independent of cell size, thereby avoiding the need for computationally expensive cell size extrapolation techniques. The proton-coupled redox potentials computed with this strategy are in agreement with experimental data. This computational strategy, as well as the conceptual insights about the impact of charged adsorbates on electrochemical interfaces, is applicable to other materials and processes.

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Unified Mechanistic Understanding of CO2 Reduction to CO on Transition Metal and Single Atom Catalysts

Cited by 5 publications

References 38 publications

Using pH dependence for understanding mechanisms in electrochemical CO reduction

Using pH dependence for understanding mechanisms in electrochemical CO reduction

Using pH dependence to understand mechanisms in electrochemical CO reduction

Multicapacitor Approach to Interfacial Proton-Coupled Electron Transfer Thermodynamics at Constant Potential

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