Subsurface oxide plays a critical role in CO ₂ activation by Cu(111) surfaces to form chemisorbed CO ₂ , the first step in reduction of CO ₂

Abstract

A national priority is to convert CO 2 into high-value chemical products such as liquid fuels. Because current electrocatalysts are not adequate, we aim to discover new catalysts by obtaining a detailed understanding of the initial steps of CO 2 electroreduction on copper surfaces, the best current catalysts. Using ambient pressure X-ray photoelectron spectroscopy interpreted with quantum mechanical prediction of the structures and free energies, we show that the presence of a thin suboxide structure below the copper surface is essential to bind the CO 2 in the physisorbed configuration at 298 K, and we show that this suboxide is essential for converting to the chemisorbed CO 2 in the presence of water as the first step toward CO 2 reduction products such as formate and CO. This optimum suboxide leads to both neutral and charged Cu surface sites, providing fresh insights into how to design improved carbon dioxide reduction catalysts. T he discovery of new electrocatalysts that can efficiently convert carbon dioxide (CO2) into liquid fuels and feedstock chemicals would provide a clear path to creating a sustainable hydrocarbon-based energy cycle (1). However, because CO2 is highly inert, the CO2 reduction reaction (CO2RR) is quite unfavorable thermodynamically. This makes identification of a suitable and scalable catalyst an important challenge for sustainable production of hydrocarbons. We consider that discovering such a catalyst will require the development of a complete atomistic understanding of the adsorption and activation mechanisms involved. Here the first step is to promote initiation of reaction steps.Copper (Cu) is the most promising CO2RR candidate among pure metals, with the unique ability to catalyze formation of valuable hydrocarbons (e.g., methane, ethylene, and ethanol) (2). However, Cu also produces hydrogen, requires too high an overpotential (>1 V) to reduce CO2, and is not selective for desirable hydrocarbon and alcohol CO2RR products (2). Despite numerous experimental and theoretical studies, there remain considerable uncertainties in understanding the role of Cu surface structure and chemistry on the initial steps of CO2RR activity and selectivity (3, 4). To reduce CO2 to valuable hydrocarbons, a source of protons is needed in the same reaction environment (2), with water (H2O) the favorite choice. Thus, H2O is often the solvent for CO2RR, representing a sustainable pathway toward solar energy storage (1). However, we lack a comprehensive understanding of how CO2 and H2O molecules adsorb on the Cu surface and interact to first dissociate the CO2 (5, 6). An overview of the various surface reactions of CO2 on Cu(111) is reported in Fig. 1, illustrating the transient carbon-based intermediate species that may initiate reactions.Previous studies using electron-based spectroscopies observed physisorption of gas-phase g-CO2 at 75 K, whereas a chemisorbed form of CO2 was stabilized by a partial negative charge induced by electron capture (CO δ− 2 ) (Fig. 1A) (7, 8). The same experiments showed th...

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

“…Recent work by Favaro et al . using AP‐XPS interpreted with DFT calculation shows that the presence of subsurface oxide below the Cu layer plays a crucial role in activation of the first step toward CO 2 reduction . For the activation of CO 2 reduction, the inert linear l ‐CO 2 molecule should be a bent b ‐CO 2 configuration.…”

Standards and Protocols for Data Acquisition and Reporting for Studies of the Electrochemical Reduction of Carbon Dioxide

“…These studies have stimulated efforts aimed at understanding the origin of the seemingly superior catalytic activity of these oxide-derived catalysts compared to polycrystalline Cu foils. [65][66][67][68][69][70][71][72] However it has not been clearly demonstrated if the enhanced activity is due to an increase in the total surface area of the catalyst or to an enhancement of the intrinsic activity. [61][62][63][64][65][66][67][68] Using the metrics discussed above, we show in Figure 10 an example of an activity comparison 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 between Cu standards (polycrystalline Cu foil and epitaxial Cu thin films) and a plasma treated Cu catalyst for which surface area measurements are available.…”

Selective CO2 Electroreduction to Ethylene and Multicarbon Alcohols via Electrolyte‐Driven Nanostructuring

“…We found a positive correlation between the production of C 2+ and the amount of Cu + species in the halide‐modified Cu catalysts in the following order: Cu_I> Cu_Br > Cu_Cl. Previous theoretical studies predicted that subsurface oxygen as well as the presence of a Cu + /Cu 0 interface plays a crucial role in CO 2 activation and CO dimerization, ultimately resulting in higher C 2+ selectivity . Interestingly, the adsorbed halides are known to bind more strongly to the oxidized Cu surface and to facilitate the formation and stabilization of the intermediates during CO 2 RR required to obtain C 2+ products.…”

“…It is well‐known that the activity and selectivity of CO 2 RR catalysts strongly depend on the precise control of their structure, such as the content of defects, subsurface oxygen or Cu + species, the specific shape of the nanocrystals, or the surface composition and atomic ordering in bimetallic nanostructures . Previous experimental and theoretical studies demonstrated that Cu(100) is the most favorable crystal orientation for the C−C coupling process .…”