A Multitechnique\nStudy of C<sub>2</sub>H<sub>4</sub> Adsorption on a Model Single-Atom\nRh<sub>1</sub> Catalyst

Single-atom catalysts are potentially ideal model systems\nto investigate\nstructure–function relationships in catalysis if the active\nsites can be uniquely determined. In this work, we study the interaction\nof C<sub>2</sub>H<sub>4</sub> with a model Rh/Fe<sub>3</sub>O<sub>4</sub>(001) catalyst that features 2-, 5-, and 6-fold coordinated\nRh adatoms, as well as Rh clusters. Using multiple surface-sensitive\ntechniques in combination with calculations of density functional\ntheory (DFT), we follow the thermal evolution of the system and disentangle\nthe behavior of the different species. C<sub>2</sub>H<sub>4</sub> adsorption\nis strongest at the 2-fold coordinated Rh<sub>1</sub> with a DFT-determined\nadsorption energy of −2.26 eV. However, desorption occurs at\nlower temperatures than expected because the Rh migrates into substitutional\nsites within the support, where the molecule is more weakly bound.\nThe adsorption energy at the 5-fold coordinated Rh sites is predicated\nto be −1.49 eV, but the superposition of this signal with that\nfrom small Rh clusters and additional heterogeneity leads to a broad\nC<sub>2</sub>H<sub>4</sub> desorption shoulder in TPD above room temperature.