Oxygen Storage Chemistry of Nanoceria

Sammanfattning: The versatile redox chemistry of ceria (CeO2) originates from its Ce4f electron, which plays the key role in changing the oxidation state of Ce between +IV and +III. Ceria is, among other things, a material that can act as a powerful oxygen buffer with a high oxygen storage capacity (OSC). This is used in many technical applications, such as the three-way catalyst, cleaning exhausts from gasoline vehicles. This thesis is concerned with the dramatic OSC effect observed experimentally in the literature for very small ceria nanoparticles (NPs) at lower temperatures, where the effect was found to be accompanied by the formation of superoxide ions (O2–).The main aim of the thesis work was to develop strategies to allow us to discover the origin of the OSC phenomenon, and to simulate temperature-programmed reduction (TPR) and temperature-programmed desorption (TPD) experiments and collect useful mechanistic insight about these processes. Quantum-mechanical (DFT) calculations, partly with modified DFT functionals, and later augmented by microkinetic (MK) modelling building on the DFT-results, made it possible to model the large and complex NP systems needed to make detailed comparisons between theory and experiment feasible.At first, a suitable DFT functional for nanoceria was needed. We turned to hybrid functionals, and more specifically, the non-local Fock exchange contribution within the hybrid functional HSE06 was explored. The amount that gave the best overall description was determined (15%, labeled HSE06' below) and was used in subsequent studies. Moreover, an accompanying HSE06'//PBE+U computational protocol was constructed (HSE06' energies calculated for pre-optimized structures at the PBE+U level); this made it possible to use the hybrid functional for large ceria systems.With the modified HSE functional, we scrutinized a previously proposed OSC model, namely the "supercharge" model for nanoparticles loaded on the outside with superoxide ions at low-coordinated ridge sites, enabled by the oxidation of Ce3+ to Ce4+. In the previous study, adsorption energies were calculated using the PBE+U density functional, which does not give adsorption energies in agreement with experiment. With the new HSE06' functional, together with the Redhead equation, we obtained an estimated oxygen desorption peak at ca. 415 K, in much better agreement with the experimental TPD peak at 440 K. However, this calculation could still not explain the large broadening of the experimental TPD spectrum. An oxygen adsorption energy model was then formulated which took Ce coordination and superoxide ion coverage into account. With microkinetic simulations based in this energy model, we achieved a broad simulated TPD signal, which was largely in agreement with the experimental spectrum.Finally, an improved “supercharge” model was assessed concerning its ability to mimic the temperature-programmed reduction (TPR) experiments reported in the literature for H2 interacting with ceria nanoparticles. We proposed that the reduction process follows a Langmuir-Hinshelwood reaction mechanism, which gave a simulated TPR spectrum in good agreement with the experimental results.In summary, the goals listed above were achieved: we managed to simulate TPD and TPR spectra, using a DFT-based MK approach; the results were in good agreement with experiment and useful mechanistic insight about these processes and the OSC mechanism was derived from the MK simulations and the DFT analyses.