Monitoring Electron Flow in Nickel Single-Atom Catalysts during Nitrogen Photofixation.

An efficient catalytic system for nitrogen (N 2 ) photofixation generally consists of light-harvesting units, active sites, and an electron-transfer bridge. In order to track photogenerated electron flow between different functional units, it is highly desired to develop in situ characterization techniques with element-specific capability, surface sensitivity, and detection of unoccupied states. In this work, we developed in situ synchrotron radiation soft X-ray absorption spectroscopy ( in situ sXAS) to probe the variation of electronic structure for a reaction system during N 2 photoreduction. Nickel single-atom and ceria nanoparticle comodified reduced graphene oxide (CeO 2 /Ni-G) was designed as a model catalyst. In situ sXAS directly reveals the dynamic interfacial charge transfer of photogenerated electrons under illumination and the consequent charge accumulation at the catalytic active sites for N 2 activation. This work provides a powerful tool to monitor the electronic structure evolution of active sites under reaction conditions for photocatalysis and beyond.