CO Cryo‐sorption as a Surface‐sensitive Spectroscopic Probe of the Active Site Density of Single‐atom Catalysts

Quantifying the number of active sites is a crucial aspect in the performance evaluation of single metal‐atom electrocatalysts. A possible realization is using adsorbing gas molecules that selectively bind to the single‐atom transition metal and then probing their surface density using spectroscopic tools. Herein, using in situ X‐ray photoelectron (XPS) and near edge X‐ray absorption fine structure (NEXAFS) spectroscopy, we detect adsorbed CO gas molecules on a FeNC oxygen reduction single atom catalyst. Correlating XPS and NEXAFS, we develop a simple surface‐ and chemically‐sensitive protocol to accurately and quickly quantify the active site density. Density functional theory‐based X‐ray spectra simulations reaffirm the assignment of the spectroscopic fingerprints of the CO molecules adsorbed at Fe‐N4‐C sites, and provide additional unexpected structural insights about the active site needed to explain the low‐temperature CO adsorption. Our work represents an important step towards an accurate quantitative catalytic performance evaluation, and thus towards developing reliable material design principles and catalysts.