  Oxidation catalysis on reducible oxide-supported small metal clusters often
involves lattice oxygen. In the present work, we trace the path of lattice
oxygen from Fe3O4(001) onto small Pt clusters during the CO oxidation, aiming
at differentiating whether the reaction takes place at the cluster/support
interface or on the cluster. While oxygen vacancies form on many other
supports, magnetite maintains its surface stoichiometry upon reduction thanks
to a high cation mobility. In order to investigate whether size-dependent
oxygen affinities play a role, we study two specific cluster sizes, Pt5 and
Pt19. By separating different reaction steps in our experiment, lattice oxygen
can be accumulated on the clusters. Temperature programmed desorption (TPD) and
sophisticated pulsed valve experiments indicate that the CO oxidation takes
place on the Pt clusters rather than at the interface. Scanning tunneling
microscopy (STM) shows a decrease in apparent height of the clusters, which
density functional theory (DFT) explains as a restructuring following lattice
oxygen reverse spillover.
