Synchrotron AP-XPS and XAS Detection of Active Sites in Single-Atom Catalysts

Research Summary

This study introduces a novel surface-sensitive analytical method for quantitatively measuring the active site density (Fe-N₄) of FeNC (single-atom Fe-N-C) catalysts used in fuel cells. Specifically, CO molecules were cryo-adsorbed at low temperature onto the catalyst surface, and their selective binding to Fe-N₄ active sites was probed using X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy.
A distinct spectroscopic peak corresponding to CO bound to Fe-N₄ was clearly identified, and its assignment was validated through density functional theory (DFT) simulations. Based on this, a simple, fast, and accurate protocol for estimating the active site density was developed. Compared to conventional electrochemical or gas-phase measurements, this method provides superior surface selectivity, is non-destructive, and enables high-throughput analysis.

Expected Outcomes

• ● Development of a surface-specific and rapid spectroscopic protocol for active site quantification
• ● Provides a reliable framework for evaluating and engineering single-atom catalysts (SACs)
• ● Applicable to a wide range of electrocatalytic systems beyond ORR (e.g., CO₂RR, NOₓRR)
• ● Enables rational catalyst design for scalable and cost-effective fuel cell technologies

Related Figures

• [Figure 1] Detection of CO Species Adsorbed on Fe-N₄ Active Sites Using XAS and DFT calculation
• [Figure 2] Schematic depiction of the experimental setup for CO cryo-sorption to Fe-N₄