Active Site Evolution during Formic Acid Conversion on Rh-Substituted Fe3O4(001)

Abstract

Single-atom catalysts (SACs) offer a promise of providing unique properties, superior selectivity, and maximum atomic efficiency compared to traditional nanoparticle catalysts. However, their stability under reaction conditions remains a critical challenge. This study examines the reactivity and structural evolution of a thermally stable (~ 700 K) model Rh/Fe₃O₄(001) SAC, where Rh is substituted into the surface layer. Previously, we demonstrated that water formation via the Mars-van Krevelen mechanism during formic acid conversion destabilizes in-surface octahedral Rh, yielding active Rh adatoms and clusters that dynamically re-incorporate into the Fe₃O₄ lattice at 700 K. Here, we follow the evolution of the catalyst structure and changes in the CO and CO₂ formation kinetics during multiple formic acid conversion cycles. Temperature-programmed reaction spectroscopy (TPRS) cycles to 700 K reveal that small Rh clusters formed during the first several cycles can re-incorporate into the Fe₃O₄(001) lattice. Over subsequent cycles, larger nanoparticles eventually form and persist. These effects are further accelerated when annealing is limited to only 550 K. Changes in the CO₂ formation/desorption temperature in TPRS reveal that the activity for formic acid dehydrogenation increases progressively from single atoms to clusters and nanoparticles. This study provides fundamental insights into the dynamic behavior and performance of SACs during catalytic reactions.