The redox kinetics of ceria and its derivatives

Yuan, Weizi

doi:https://doi.org/10.21985/n2-93rf-cs65

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The redox kinetics of ceria and its derivatives

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The investigation of ceria-related materials has a long history since 1920s. The first publication on web of science traces back to 1928. Ever since that, more studies come out and surge starting from 1987. This material has attracted extensive attention due to its wide applications in electrochemical devices, catalysis, and thermochemical fuel production. The key to ceria’s applications is the capability to change the cerium oxidation state between 3+ and 4+, accompanied by generation or annihilation of oxygen vacancies upon change of temperature or oxygen activity. This capability is known as oxygen storage capacity (OSC). In catalysis and thermochemical fuel production, ceria-zirconia is one of the most popular derivatives as the incorporation of zirconium not only increases the OSC which corresponds to higher product conversion, but also enhances the thermal stability. The thermodynamic characteristics of ceria and ceria-zirconia is well established; however, the reaction kinetics remains unclear, which is very important in real applications as it relates to the production rate. This thesis explores the redox kinetics of ceria and ceria-zirconia materials. To begin with, in chapter 2, we examined the surface reduction which is correlated with the amount of active species in ceria and ceria-zirconia solid solutions with in-situ X-ray absorption near edge spectroscopy (XANES). The technique allows us to access the surface and the bulk by varying the incident angle under controlled temperature and atmosphere. For the first time, the surface Ce3+ concentration is experimentally quantified in ceria and ceria-zirconia solid solutions. In the meantime, the bulk Ce3+ determined by XANES was compared with literature thermogravimetric results to evaluate the effectiveness of the method. The conditions examined in XANES are relevant to the conditions used in thermochemical fuel production. In all circumstances, we observed substantial Ce3+ enrichment at the surface relative to the bulk. Surprisingly, the degree of enhancement is the highest in the absence of zirconium. This behavior stands in direct contrast to that of the bulk in which the Ce3+ concentration monotonically increases with increasing zirconium content. These results suggest that while zirconium enhances oxygen storage capacity in ceria, undoped ceria may have the higher surface activity. Overall, a complete profile of the surface Ce3+ concentration in ceria and ceria-zirconia was established under conditions relevant to thermochemical fuel production. At the end of the chapter, we discussed the surface termination effect on the Ce3+ concentration and the possibility of building up the in-situ depth profile of Ce3+ concentration in ceria and ceria-zirconia using different techniques. In chapter 3 and 4, the transport properties of undoped and 15% Zr doped ceria polycrystalline samples were investigated. The study includes two parts. The first part is measurement of the ambipolar bulk diffusion coefficient D ̃, followed by the measurement of surface reaction rate constant k_s. In both studies, we found that adding zirconium is detrimental to the kinetics, as 15% Zr doped ceria shows both lower k_s and D ̃ than undoped ceria. Measurements were performed under XANES-examined conditions. Complete examinations were performed only under oxidizing atmosphere. Challenges of performing measurements under reducing conditions mainly lie in two aspects: (i) difficulty in deconvoluting the bulk diffusion and the surface reaction brings challenge to extraction of convincing D ̃ and k_s; (ii) sluggish surface kinetics of Zr doped ceria renders challenges to maintain a stable atmosphere during the reaction which is required for the measurement. In the future, we will address these problems and complete the measurement of ceria-zirconia’s kinetics under reducing conditions. In the last part, chapter 5, we applied ceria as an oxide barrier layer between the electrode and electrolyte in the solid acid fuel cell (SAFC). The goal is to improve the stability of SAFCs by preventing reactions between cathode and electrolyte materials as well as increasing the catalytic activity towards oxygen reduction reaction (ORR) as platinum or palladium cathode materials supported by ceria and its derivatives are reported to show better activity towards ORR than the metals themselves.

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