Surface-Adsorbed CO as an Infrared Probe of Electrocatalytic Interfaces

Electrocatalytic interfaces enable chemical transformations of fundamental and technological significance that are challenging to achieve by other means. Examples include the selective oxidation of alcohols, the conversion of biomass, and the reduction of carbon dioxide. However, these electrocatalytic processes often show poor reaction selectivity and the catalysts are prone to deactivation. Key to improving the reaction selectivity and catalyst stability is a better molecular-level understanding of these complex interfaces. Generating these insights requires the probing of the catalytically active sites, the structure and dynamics of the electric double layer, and the evolution of these interfacial features under reaction conditions. Surface-enhanced infrared absorption spectroscopy (SEIRAS) is well-suited to provide these insights. In particular, the C≡O stretch mode of surface-adsorbed CO (COads) is sensitive to the applied potential, surface morphology, and electric double layer structure. COads is an intermediate in the reduction and oxidation of carbon monoxide and can be introduced as a spectator species during other reactions. Therefore, it is a powerful and broadly applicable infrared probe of the electrocatalytic interface. However, the complex dependence of the C≡O stretch spectra on the local environment of the COads probe renders the interpretation of the spectra a non-trivial task. On the basis of recent SEIRAS studies of C≡O stretch spectra of COads on various electrocatalysts, we discuss successful strategies for inferring key structural aspects of the interface from the spectra. We highlight pitfalls in the analysis and propose future experimental and theoretical advances that will be required to realize the full potential of this methodology. Further, we show how SEIRAS can be augmented with differential electrochemical mass spectrometry (DEMS) for the elucidation of interfacial structure-reactivity relationships.