Resonant photoelectron spectroscopy: a tool for characterising the electronic properties of catalysts in operation.

Resonant photoelectron spectroscopy (RPES) combines element- and orbital-selective enhancement of valence electronic states, providing a powerful approach to probe the electronic structure of catalytic materials under operation. Despite this potential, its broader adoption beyond model surfaces in ultrahigh vacuum (UHV) conditions has remained limited. In this perspective, we review key UHV studies that established the potential of RPES for the description of catalytic materials, together with recent in situ and operando RPES studies under gas and liquid environments. We discuss the information that can be extracted during operando RPES measurements and propose new research perspectives building on the strong foundation provided by the existing literature. We then critically examine the experimental requirements for obtaining reliable and interpretable RPES data. Key parameters governing the selection of an appropriate absorption edge for RPES are discussed in the context of catalysis, where the environment differs from UHV conditions. We further discuss complications in the interpretation of RPES spectra arising from artefacts such as second-light excitation and contributions from resonant spectator photoelectrons and Auger electrons. We also highlight that combining metal- and ligand-edge RPES is essential for disentangling metal-ligand interactions, yet remains largely unexplored in operando or in situ RPES studies. Finally, this perspective discusses instrumental development that could broaden the applications of operando RPES, including increased probing depth, spatial and momentum resolution, and time-resolved measurements.