Niobium Ceria-based Catalysis for Selected Chemical Applications

Kourosh Razmgar

The chapters in the thesis explore different aspects of ceria-based catalysis for variety of targets. Herein, we have investigated the efficiency of ceria catalyst for 1,3-butadiene and carbon monoxide low-temperature oxidation as well as thermochemical splitting of carbon dioxide and water. Ceria was chosen as the base catalyst for the experimentations due to its desirable redox properties; however, the oxide was modified with niobium (Nb) as trace metal element for improvement of catalytic characteristics. The review in the thesis initially covers the fundamentals, economic benefits, and application of ceria in the aforementioned reaction, highlighting the theoretical basis and importance of non-stoichiometric forms of ceria (CeO2-x). The effects of operational parameters and molecular alterations are contrasted as documented in the literature. The applied methodology for the experiments and computations are also briefly overviewed subsequently to give an insight into the research. Furthermore, the catalytic efficiency of ceria-supported niobium oxide catalysts are explored for the aforementioned reactions. The experiments are carried out in tubular reactor at defined temperature range and atmospheric pressure in conjunction with various characterization techniques. The results reveal the capability of the prepared catalysts for higher reactant conversion at relatively lower temperatures compared to the neat oxide surface. The mutual role of niobium in formation of active sites as well as its contribution to ceria non-stoichiometry can be regarded as the key factors for contributing to the desired catalytic activity. The density-functional-theory (DFT) calculations also reveals new mechanistic insights into the catalytic processes by portraying the reactions from a molecular point of view. Via DFT, it is shown that catalytic capacity of the Nb2O5/CeO2 catalysts stems from electronic attributes pertinent to a lower band gap and ionicity of Nb-O bonds in reference to pure ceria. The DFT calculations also demonstrates that Nb-decorated configuration possesses lower reaction barriers for converting the species to the corresponding products compared to the intact ceria structure. Besides, the study of Nb-doped ceria surface showed that niobium incorporation changes the surface charge behaviour of ceria and shifts the configuration towards metallic behaviour. Apart from the reactions over the ceria-based surface, the thesis also investigated the surface stability of the ceria either on its own or in adsorption system. It was observed that while the oxide is doped with niobium, the surface energy considerably reduces compared to the intact ceria. In case of adsorbed species over the surface, it was also found that unlike larger molecules like butadiene, hydrogen adsorption significantly stabilises the surface. In fact, the surface reduction via formation of hydroxyl (OH) groups was found to be energetically more favoured. To summarise, the thesis complements the scarce literature data on the oxidation and syngas production reactions over Nb2O5/CeO2 catalyst surface. It also provides kinetic models for evolution of product species over the catalysts as evidence for experiments. The results from the present work confirm that incorporation of niobium up to a certain content leads to significant improvements in catalytic activity; however, its further increment causes the reduced activity due to decrease in effective surface area.

Niobium Ceria-based Catalysis for Selected Chemical Applications

Files and links (1)

Abstract

Details

Metrics