An In Situ Surface-Enhanced Infrared Absorption Spectroscopy Study of Electrochemical CO₂ Reduction: Selectivity Dependence on Surface C-Bound and O-Bound Reaction Intermediates

Abstract

The CO2 electro-reduction reaction (CORR) is a promising avenue to convert greenhouse gases into high-value fuels and chemicals, in addition to being an attractive method for storing intermittent renewable energy. Although polycrystalline Cu surfaces have long known to be unique in their capabilities of catalyzing the conversion of CO2 to higher-order C1 and C2 fuels, such as hydrocarbons (CH4, C2H4 etc.) and alcohols (CH3OH, C2H5OH), product selectivity remains a challenge. Rational design of more selective catalysts would greatly benefit from a mechanistic understanding of the complex, multi-proton and multi-electron conversion of CO2. In this study, we select three metal catalysts (Pt, Au, Cu) and apply in situ surface enhanced infrared absorption spectroscopy (SEIRAS) and ambient-pressure X-ray photoelectron spectroscopy (APXPS), coupled to density-functional theory (DFT) calculations, to get insight into the reaction pathway for the CORR. We present a comprehensive reaction mechanism for the CORR, and show that the preferential reaction pathway can be rationalized in terms of metal-carbon (M-C) and metaloxygen (M-O) affinity. We show that the final products are determined by the configuration of the initial intermediates, C-bound and O-bound, which can be obtained from CO2 and (H)CO3, respectively. C1 hydrocarbons are produced via OCH3,ad intermediates obtained from O-bound CO3,ad and require a catalyst with relatively high affinity for O-bound intermediates. Additionally, C2 hydrocarbon formation is suggested to result from the C-C coupling between C-bound COad and (H)COad, which requires an optimal affinity for the C-bound species, so that (H)COad can be further reduced without poisoning the catalyst surface. It is suggested that the formation of C1 alcohols (CH3OH) is the most challenging process to optimize, as stabilization of the O-bound species would both accelerate the formation of key-intermediates (OCH3,ad) but also simultaneously inhibit their desorption from the catalyst surface. Our findings pave the way

Double layer charging driven carbon dioxide adsorption limits the rate of electrochemical carbon dioxide reduction on Gold

“…Previous studies found a 20% *CO coverage at all potentials <−0.8 V vs. SHE using ATR-SEIRAS spectroscopy 3 , which was, however, later suggested by Dunwell et al 6 to arise from Pt impurities. The latter authors could not detect any *CO coverage under reducing conditions all the way down to −1.0 V vs. SHE, which is also in line with later studies that measured down to −0.9 V vs. SHE 27 . These results together with ours thus suggest that *CO coverage is indeed negligible under CO 2 R conditions.…”

“…also Supplementary Note 1) if CO 2 adsorption were rate-limiting, the activity would show no pH dependence on an absolute potential reference scale (e.g., on the standard hydrogen electrode-SHE scale), as long as the coverage of empty sites are pH independent. On Gold, experimental and theoretical results, as well as our simulations below, suggest negligible coverages of reaction intermediates 3,26,27 , in which case the relative change of the coverage of free active sites with a change in the pH is negligibly small. In contrast to CO 2 adsorption, all other reaction steps depend on the pH either directly due to protons being reactants or indirectly via the surface coverage of reactants, suggesting that the pH dependence can be used to distinguish the ratelimiting step.…”

“…First of all, previous theoretical studies found small binding energies of all reaction intermediates 26 , suggesting negligibly small coverages. Experiments based on ATR-SEIRAS spectroscopy 3,27 have additionally not observed any higher coverages of species. These theoretical and experimental findings suggest that there is effectively no competition for active sites.…”

Ab Initio Cyclic Voltammetry on Cu(111), Cu(100) and Cu(110) in Acidic, Neutral and Alkaline Solutions

“…For Cu( hkl ) in neutral electrolytes, different H*, OH*, cation and anion configurations need to be considered. Previously we have shown simulated results for phosphate and carbonate anions on the Cu (100) facet and moreover CO species has been experimentally observed on polycrystalline Cuin KHCO 3 …”

“…This also reproduces the experimental CV with a stronger average binding. Finally, a pure OH* and H* solution could also satisfy the experimental data, but this is neglected due to the observation of on polycrystaline Cu …”