  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. Adsorption
at the 5-fold coordinated Rh sites is predicated to -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.
