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Abstract
Single‐atom catalysts (SACs) bridge homo‐ and heterogeneous catalysis because the active site is a metal atom coordinated to surface ligands. The local binding environment of the atom should thus strongly influence how reactants adsorb. Now, atomically resolved scanning‐probe microscopy, X‐ray photoelectron spectroscopy, temperature‐programmed desorption, and DFT are used to study how CO binds at different Ir1 sites on a precisely defined Fe3O4(001) support. The two‐ and five‐fold‐coordinated Ir adatoms bind CO more strongly than metallic Ir, and adopt structures consistent with square‐planar IrI and octahedral IrIII complexes, respectively. Ir incorporates into the subsurface already at 450 K, becoming inactive for adsorption. Above 900 K, the Ir adatoms agglomerate to form nanoparticles encapsulated by iron oxide. These results demonstrate the link between SAC systems and coordination complexes, and that incorporation into the support is an important deactivation mechanism.
Highlights
The field of single-atom catalysis (SAC)[1] arose as the ultimate extension of attempts to reduce the precious-metal content of supported heterogeneous catalysts
This change in the subsurface cation ordering would manifest as an elongated, less bright feature close to the Irrelated protrusion. While this is observed in some cases, we note that Fe exchange with the bulk is already facile at this temperature,[13] so it is possible that the excess Fe diffuses into deeper layers
Our results show that the choice of metal and the coordination environment have a significant effect on adsorption properties in SAC systems
Summary
The field of single-atom catalysis (SAC)[1] arose as the ultimate extension of attempts to reduce the precious-metal content of supported heterogeneous catalysts. Theoretical screening studies suggest several Me1/ FeOx systems will outperform Pt for CO oxidation,[3] but which metal adatom yields the best performance depends on the reaction mechanism that is assumed It is not clear whether the assumed adsorption site exists in real SAC systems, which are complex, often inhomogeneous, and difficult to characterize. We study how Ir1 species bind to a precisely defined Fe3O4(001) support,[7] and correlate the different sites with the ability of the model catalyst to adsorb CO This system was selected because Ir1/FeOx catalysts have already been shown to be active for both CO oxidation[3] and the water-gas shift reaction,[8] where CO is a reactant, and because Ir-based coordination complexes are common in homogeneous catalysis.
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