Single-atom catalysts have received extensive attention for reducing noble metal utilization and potential elimination of side reactions. Yet, their active sites remain highly debated, and fundamental insights are limited due to experimental challenges. Here, we introduce first-principles microkinetic modeling with CO oxidation over Pd atoms on γ-alumina as a test case to provide insights into single-atom catalysis. Contrary to widespread practice, we show that the state of the catalyst under working conditions is not described by ab initio thermodynamics without knowledge of the rate-determining step. This is especially important for single-atom catalysts whose reaction mechanism is less established and different from the one of extended surfaces. Kinetic Monte Carlo simulations indicate that the steady-state cationic view of single atoms on oxide supports is simplistic; metal atoms possess discrete, stochastic states during a catalytic cycle whose drastically different lifetimes dictate the observed, fractional, and strongly temperature-dependent catalyst oxidation state. We provide evidence that microkinetic simulations can discriminate mechanisms and sites and expose the oxidation state of a catalyst with high spatial and temporal resolution.
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