A Multitechnique Study of C₂H₄ Adsorption on a Model Single-Atom Rh₁ Catalyst

Single-atom catalysts are potentially ideal model systems to investigate structure-function relationships in catalysis if the active sites can be uniquely determined. In this work, we study the interaction of C₂H₄ with a model Rh/Fe₃O₄(001) catalyst that features 2-, 5-, and 6-fold coordinated Rh adatoms, as well as Rh clusters. Using multiple surface-sensitive techniques in combination with calculations of density functional theory (DFT), we follow the thermal evolution of the system and disentangle the behavior of the different species. C₂H₄ adsorption is strongest at the 2-fold coordinated Rh₁ with a DFT-determined adsorption energy of -2.26 eV. However, desorption occurs at lower temperatures than expected because the Rh migrates into substitutional sites within the support, where the molecule is more weakly bound. The adsorption energy at the 5-fold coordinated Rh sites is predicated to be -1.49 eV, but the superposition of this signal with that from small Rh clusters and additional heterogeneity leads to a broad C₂H₄ desorption shoulder in TPD above room temperature.