Kinetics, Mechanism, and Thermodynamics of Ceria-Zirconia Reduction
Andrew Hwang, Andrew “Bean” Getsoian, Enrique Iglesia
Abstract
Ce 0.5 Zr 0.5 O 2– x (CZO) is widely used for the storage and reaction of O atoms (O*) in chemical looping and emissions control. Reductants react with O* to form vacancies (*) at rates limited by surface reactions with O*, replenished through fast diffusion through CZO crystals. The dynamics and mechanism of these surface reactions remain unresolved because O* stability and reactivity depend very strongly on the extent of CZO reduction during stoichiometric reactions. These thermodynamic nonidealities are evident from free energy penalties in removing O* that increase sharply as intracrystalline O* concentrations decrease, leading to reduction rates that deviate from the expected linear dependence of rates on O* concentrations. Rates of CZO reduction by CO, at conditions resembling “cold start” of vehicle emissions systems, decrease 10-fold when O* concentrations decrease by only a factor of 2; this nonlinearity reflects the strong effects of thermodynamic nonidealities on reaction dynamics. This study addresses and resolves these mechanistic and practical matters using transition state theory, a thermodynamic construct that rigorously accounts for the prevalent nonideal behavior. Such formalisms treat Ce 0.5 Zr 0.5 O 2 as an ideal solution and O*, *, surface-bound intermediates, and transition states as solutes within a well-mixed Ce 0.5 Zr 0.5 O 2– x solution with excess free energies that depend strongly on extent of reduction. The nonideal behavior of these solutes and the reactivity of O* in reactions with CO are related to the measured thermodynamics of O* through scaling relations, and the requisite kinetic parameters for the ideal system are independently derived from a mechanism-based interpretation of catalytic CO–O 2 reactions on stoichiometric CZO. These approaches and constructs lead to a kinetic model that accurately describes measured transient stoichiometric reduction rates, but only when incorporated into reaction-convection equations that rigorously capture how the thermodynamic activities of kinetically relevant reactants, transition states, and spectators evolve in time and space. These formalisms provide a general framework for the analysis of stoichiometric processes in strongly nonideal systems that are ubiquitous in carbon capture, energy storage, and environmental remediation.