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

Katayama, Yu, Nattino, Francesco, Giordano, Livia, Hwang, Jonathan, Rao, Reshma R., Andreussi, Oliviero, Marzari, Nicola, Shao-Horn, Yang
Journal of physical chemistry 2018 v.123 no.10 pp. 5951-5963
X-ray photoelectron spectroscopy, carbon dioxide, carbonates, catalysts, catalytic activity, copper, density functional theory, desorption, electrochemistry, ethanol, ethylene, fuels, gold, greenhouse gases, infrared spectroscopy, methane, methanol, platinum, reaction mechanisms, renewable energy sources
The CO₂ electroreduction reaction (CO2RR) 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 been 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, multiproton, and multielectron 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 CO2RR. We present a comprehensive reaction mechanism for the CO2RR and show that the preferential reaction pathway can be rationalized in terms of metal–carbon (M–C) and metal–oxygen (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 toward a design strategy for CO2RR catalysts with improved selectivity, based on the experimental/theoretical reaction mechanisms that have been identified. These results also suggest that designing the electronic structure of the catalyst is not the sole determining factor to achieve highly selective CO2RR catalysis; rather, tuning additional experimental reaction conditions such as electrolyte–intermediate interactions also become critical.