Andrew Hwang, Andrew “Bean” Getsoian, Enrique Iglesia
{"title":"氧化铈-氧化锆还原的动力学、机理和热力学","authors":"Andrew Hwang, Andrew “Bean” Getsoian, Enrique Iglesia","doi":"10.1021/acscatal.4c04771","DOIUrl":null,"url":null,"abstract":"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 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<sub>0.5</sub>Zr<sub>0.5</sub>O<sub>2</sub> as an ideal solution and O*, *, surface-bound intermediates, and 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 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<sub>2</sub> 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.","PeriodicalId":9,"journal":{"name":"ACS Catalysis ","volume":null,"pages":null},"PeriodicalIF":11.3000,"publicationDate":"2024-10-18","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":"0","resultStr":"{\"title\":\"Kinetics, Mechanism, and Thermodynamics of Ceria-Zirconia Reduction\",\"authors\":\"Andrew Hwang, Andrew “Bean” Getsoian, Enrique Iglesia\",\"doi\":\"10.1021/acscatal.4c04771\",\"DOIUrl\":null,\"url\":null,\"abstract\":\"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 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<sub>0.5</sub>Zr<sub>0.5</sub>O<sub>2</sub> as an ideal solution and O*, *, surface-bound intermediates, and 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 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<sub>2</sub> 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. 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Kinetics, Mechanism, and Thermodynamics of Ceria-Zirconia Reduction
Ce0.5Zr0.5O2–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 Ce0.5Zr0.5O2 as an ideal solution and O*, *, surface-bound intermediates, and transition states as solutes within a well-mixed Ce0.5Zr0.5O2–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–O2 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.
期刊介绍:
ACS Catalysis is an esteemed journal that publishes original research in the fields of heterogeneous catalysis, molecular catalysis, and biocatalysis. It offers broad coverage across diverse areas such as life sciences, organometallics and synthesis, photochemistry and electrochemistry, drug discovery and synthesis, materials science, environmental protection, polymer discovery and synthesis, and energy and fuels.
The scope of the journal is to showcase innovative work in various aspects of catalysis. This includes new reactions and novel synthetic approaches utilizing known catalysts, the discovery or modification of new catalysts, elucidation of catalytic mechanisms through cutting-edge investigations, practical enhancements of existing processes, as well as conceptual advances in the field. Contributions to ACS Catalysis can encompass both experimental and theoretical research focused on catalytic molecules, macromolecules, and materials that exhibit catalytic turnover.