Structural Dynamics and Energy Landscape of the Forward and Reverse Water–Gas Shift Catalytic Cycle on Ceria

The water–gas shift catalytic cycle (CO + H2O ⇄ CO2 + H2), in both the forward (WGS) and reverse (RWGS) directions, is a classical reaction central to fuel and chemical synthesis that interconverts reductant-and-oxidant pairs (red + ox → ox′ + red′), i.e., in WGS, CO (red) to CO2 (ox′) and H2O (ox) to H2 (red′), and in RWGS, H2 (red) to H2O (ox′) and CO2 (ox) to CO (red′). Within this cycle, the energy landscape and kinetic relevance of the steps have not yet been fully established and reconciled, especially on oxide surfaces, simply due to their structural complexities and changing oxygen contents. Here, we establish the mechanistic framework and associated barriers for each step of the cycle, in both directions, on modeled ceria (CeO2) through kinetic interrogations under induction, steady-state, and transient regimes, each providing distinct thermodynamic and kinetic insights into the barriers of individual steps and intraparticle O-atom diffusion between surface and bulk, as the oxygen content in CeO2–x varies (x = 0–0.2) and redistributes. In RWGS, surface O atom abstraction by H2 is the sole kinetically relevant step on partially reduced CeO2–x oxides (x = 0.024–0.038) with a barrier of 180 ± 3 kJ mol–1, determined from steady-state kinetics, whereas surface reoxidation by CO2 at O-vacancies is rapid with an apparent activation energy of 51 ± 1 kJ mol–1 that decreases with increasing O-vacancy formation energy, revealed by transient kinetic studies that decouple this step from the preceding kinetic bottleneck. In WGS, the reverse reaction, both surface O atom abstraction by CO (92 ± 21 kJ mol–1) and surface reoxidation by H2O (89 ± 6 kJ mol–1) are kinetically relevant, derived from steady-state rate measurements. Integrating findings from both reactions, we delineate a unified mechanistic paradigm underpinned by an experimentally determined, thermodynamically consistent energy landscape wherein the observed kinetic constraints for RWGS and WGS depend solely on the direction through which this landscape is traversed. These insights elucidate surface reactions, structural dynamics, and O atom transport underlying water–gas shift reactions on ceria under working conditions, providing a robust foundation for understanding how variations in chemical potential drive changes in oxygen content to enable efficient CO and CO2 conversions.