Tracking a Common Surface-Bound Intermediate during CO₂-to-Fuels Catalysis

Abstract

Rational design of selective CO2-to-fuels electrocatalysts requires direct knowledge of the electrode surface structure during turnover. Metallic Cu is the most versatile CO2-to-fuels catalyst, capable of generating a wide array of value-added products, including methane, ethylene, and ethanol. All of these products are postulated to form via a common surface-bound CO intermediate. Therefore, the kinetics and thermodynamics of CO adsorption to Cu play a central role in determining fuel-formation selectivity and efficiency, highlighting the need for direct observation of CO surface binding equilibria under catalytic conditions. Here, we synthesize nanostructured Cu films adhered to IR-transparent Si prisms, and we find that these Cu surfaces enhance IR absorption of bound molecules. Using these films as electrodes, we examine Cu-catalyzed CO2 reduction in situ via IR spectroelectrochemistry. We observe that Cu surfaces bind electrogenerated CO, derived from CO2, beginning at −0.60 V vs RHE with increasing surface population at more negative potentials. Adsorbed CO is in dynamic equilibrium with dissolved 13CO and exchanges rapidly under catalytic conditions. The CO adsorption profiles are pH independent, but adsorbed CO species undergo a reversible transformation on the surface in modestly alkaline electrolytes. These studies establish the potential, concentration, and pH dependencies of the CO surface population on Cu, which serve to maintain a pool of this vital intermediate primed for further reduction to higher order fuel products.

Reactivity of Copper Electrodes towards Functional Groups and Small Molecules in the Context of CO2 Electro-Reductions

“…Under such conditions, other mechanistic pathways on other crystal planes may prevail. This is supported by recent results provided by Surendranath [61] and coworkers, who found a pH-invariant strong rise of the CO-binding ability of copper surface for potentials more negative than −0.6 V and a saturation of CO-binding strength for potentials more negative than −0.8 V. This clearly sets up two different potential regimes. This study and the work of Ma et al [57] were both executed in a high current density and, therefore, high overpotentials regime.…”

“…CO (2) electrolysis yielded a product distribution, that was surprisingly similar to the one observed with CO 2 (1) under the same conditions. Thus, CO (2) is key intermediated to most products, which also corresponds to current literature [50,52,58,61]. The most prominent difference was the absence of formate (4) as product.…”

“…As methylene (10) dimerisation does not involve CO (2) addition, this points to methylene (10) as "SDI" for ethene (12) and ethanol (17)/acetate (14). Surendranth and coworkers have recently proven that under high current densities, CO ads is in a potential-invariant quasi equilibrium with CO free [61]. This would also support an enhancement for carbonylation reaction in presence of high CO concentrations.…”

Electrochemical CO2 Reduction over Compressively Strained CuAg Surface Alloys with Enhanced Multi-Carbon Oxygenate Selectivity

“…Thus, we hypothesized that supplying additional CO to Cu by colocating domains of Cu with those of a CO-generating metal would enhance the C 2+ product selectivity because the elevated CO concentration in the vicinity of the cathode will result in a higher steady-state coverage adsorbed on Cu (see Supporting Information, SI-3). 35 To this end, we identified the Cu−Ag system as the optimal bimetallic system to probe this hypothesis because Ag produces more CO than Cu at a given potential and because Cu and Ag are virtually immiscible in the bulk at all compositions at room temperature (see Supporting Information, SI-4). 36,37 In the balance of this paper, we report the results of our investigation of CO 2 RR over CuAg bimetallic electrodes and surface alloys.…”

Ethylene Selectivity in CO Electroreduction when using Cu Oxides: An In Situ ATR‐SEIRAS Study

“…In this regard, an intensively powerful and surface‐sensitive analysis tool is needed to investigate the electrocatalysis on the electrode. As one of the most high end equipment, in situ attenuated total reflectance‐surface enhanced infrared absorption spectroscopy (ATR‐SEIRAS) can be used to intensively measure the signals which indicate the adsorbed CO intermediate that appears during CO 2 reduction . In contrast to surface enhanced Raman spectroscopy (SERS) and external reflection Fourier transform infrared (FTIR) spectroscopies, IR beam in the ATR‐SEIRAS can approach the back side of working electrode surface, which can avoid interference of electrolyte solution and dissolved molecules…”