(246a) Dynamic Activation of Single Atom Catalysts By Reaction Intermediates: Conversion of Formic Acid on Rh/Fe3O4(001)

Single atom catalysts have emerged as a new catalyst frontier due their unique catalytic properties and high selectivity. Yet key fundamental challenges exist regarding the understanding of how their activity and stability depend on their coordination environment. Here, we prepared distinct Rh sites on Fe₃O₄(001) and investigated their stability and reactivity using X-ray photoelectron spectroscopy (XPS), temperature programed desorption (TPD), and scanning tunneling microscopy (STM). By varying the Rh deposition and annealing temperature, we identified a series of model catalysts possessing unique Rh sites including Rh adatoms and mixed surface layers with octahedrally-coordinated Rh. Their adsorption properties and catalytic activity were characterized using formic acid as a probe molecule. On bare Fe₃O₄(001), formic acid deprotonates to form adsorbed formate and hydroxyl intermediates. At higher temperatures the formate decomposes to CO and CO₂ with CO being the dominant species (dehydration pathway) while the hydroxyls form water through oxygen abstraction from the surface. The addition of Rh (either Rh adatom or octahedral Rh) shifts the product selectivity to CO₂ (dehydrogenation pathway) which evolves at lower temperatures. Rh adatoms induce this shift with very little loading (~0.01 Rh/unit cell) demonstrating a turnover number as high as ~40 formic acid molecules per Rh site. Rh octahedral expresses a similar activity but requires ~20 times higher coverage for a similar CO₂ yield suggesting a common active site as the Rh adatoms. We demonstrate that Rh octahedral sites are destabilized in the presence of surface hydroxyls and are dynamically converted to Rh adatoms during the reaction. Desorption of all surface intermediates leads to the reformation of the octahedral Rh demonstrating the switchable nature of the catalyst between octahedral and adatom states. The destabilization of octahedral Rh illustrates how seemingly stable catalyst structures can be quite dynamic under reaction conditions.