Kinetics, Mechanism,\nand Thermodynamics of Ceria-Zirconia\nReduction

Ce<sub>0.5</sub>Zr<sub>0.5</sub>O<sub>2–<i>x</i></sub> (CZO) is widely used for the storage and reaction\nof O atoms\n(O*) in chemical looping and emissions control. Reductants react with\nO* to form vacancies (*) at rates limited by surface reactions with\nO*, replenished through fast diffusion through CZO crystals. The dynamics\nand mechanism of these surface reactions remain unresolved because\nO* stability and reactivity depend very strongly on the extent of\nCZO reduction during stoichiometric reactions. These thermodynamic\nnonidealities are evident from free energy penalties in removing O*\nthat increase sharply as intracrystalline O* concentrations decrease,\nleading to reduction rates that deviate from the expected linear dependence\nof rates on O* concentrations. Rates of CZO reduction by CO, at conditions\nresembling “cold start” of vehicle emissions systems,\ndecrease 10-fold when O* concentrations decrease by only a factor\nof 2; this nonlinearity reflects the strong effects of thermodynamic\nnonidealities on reaction dynamics. This study addresses and resolves\nthese mechanistic and practical matters using transition state theory,\na thermodynamic construct that rigorously accounts for the prevalent\nnonideal behavior. Such formalisms treat Ce<sub>0.5</sub>Zr<sub>0.5</sub>O<sub>2</sub> as an ideal solution and O*, *, surface-bound intermediates,\nand transition states as solutes within a well-mixed Ce<sub>0.5</sub>Zr<sub>0.5</sub>O<sub>2–<i>x</i></sub> solution\nwith excess free energies that depend strongly on extent of reduction.\nThe nonideal behavior of these solutes and the reactivity of O* in\nreactions with CO are related to the measured thermodynamics of O*\nthrough scaling relations, and the requisite kinetic parameters for\nthe ideal system are independently derived from a mechanism-based\ninterpretation of catalytic CO–O<sub>2</sub> reactions on stoichiometric\nCZO. These approaches and constructs lead to a kinetic model that\naccurately describes measured transient stoichiometric reduction rates,\nbut only when incorporated into reaction-convection equations that\nrigorously capture how the thermodynamic activities of kinetically\nrelevant reactants, transition states, and spectators evolve in time\nand space. These formalisms provide a general framework for the analysis\nof stoichiometric processes in strongly nonideal systems that are\nubiquitous in carbon capture, energy storage, and environmental remediation.