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

Abstract

A critical step toward the rational design of new catalysts that achieve selective and efficient reduction of CO 2 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 H 2 O + 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 C 2 H 4 along with CH 4 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 CH 4 and C 2 H 4 . 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.reaction mechanism | electrocatalysis | copper | QM metadynamics | free-energy reaction barriers

Nature of the Active Sites for CO Reduction on Copper Nanoparticles; Suggestions for Optimizing Performance

“…Our previous QM metadynamics calculations with full solvent showed that a CO binding energy of 0.70 eV is sufficient to keep CO on the surface at potentials less negative than -0.91 V (RHE). 15 We find that this combination of binding sites reduces the *OCCOH formation energy to 0.52 eV, which is the best performance among the active surface sites investigated here.…”

“…24 In our recent work, we carried out full solvent QM based metadynamics to determine the RDS for C-C coupling, which we also found associated with the process of first electron transfer. 15,25 Consequently, we take the formation energy of *OCCOH as a descriptor to characterize the performance of surface sites toward C2 production. The experimental work of Verdaguer et al presented a correlation between strong CO binding sites and high CO reduction activity.…”

Catalyst electro-redeposition controls morphology and oxidation state for selective carbon dioxide reduction

“…Previous mechanistic studies revealed that the reaction pathways for CH 4 and C 2 H 4 differ at the bound CO* intermediate [48][49][50][51] .…”

Competition between H and CO for Active Sites Governs Copper‐Mediated Electrosynthesis of Hydrocarbon Fuels

“…[3][4][5][6] Synthesis of these products will by necessity require CÀHa nd CÀCb ond formation events.H owever,l ittle is understood about the processes taking place at the electrode surface.Specifically,in forming abond, two distinct surface-reaction mechanisms can be in play:surface-bound species can react with each other in what the heterogeneous catalysis field calls aL angmuir-Hinshelwood (LH) step or asurface-bound species can react with as pecies in solution in what is termed an Eley-Rideal (ER) step. [8] Insight into this mechanistic distinction is essential for the design of efficient CO 2 reduction catalysts.For example,inan ER mechanism leading to methane,s imply increasing the surface concentration of CO at the expense of Hadsorption is sufficient to promote higher-order product formation relative to H 2 evolution. Analogous mechanisms can be envisioned for CÀCbond formation leading to ethylene.…”

“…[11][12][13][14][15][16] However,t hese studies alone are unable to parse reaction mechanisms.I ndeed, studies of the COdependent kinetics of higher-order product formation are uniquely suited to distinguish different mechanistic possibilities because they directly probe kinetically relevant intermediates and provide reaction orders for each product. [8,17,18] However,w hile excellent studies have investigated the pH, electrolyte cation, and crystal facet dependence of the reaction, [19][20][21][22][23][24][25][26][27][28][29] investigations of the CO-dependent kinetics have thus far been hampered by the low solubility of CO in aqueous electrolytes (1.1 mm), [30] which severely restricts the concentration range over which activation-controlled CO reduction kinetics can be probed. Furthermore,t he mechanistic regimes become distinguishable at near saturation CO coverage,t herefore requiring an experimental approach that fosters strong CO binding to the Cu surface.…”