Styrene production via ethylbenzene dehydrogenation (EBDH) is one of the ten most important petrochemical processes. Possessing highly reactive double bond which facilitates self-polymerization and polymerization with other monomers, styrene is the fourth utmost essential bulk monomer at present. Current commercial ethylbenzene dehydrogenation is highly endothermic reaction, thus highly energy demanding. Several alternatives to steam, conventionally used to supply heat have been widely investigated. In this thesis we report on the CO2 mediated EBDH in which CO2 has been used as a mild oxidant instead of stream. Reaction was solely employed as a model reaction to probe inherent surface reactivity of various low index crystal planes exposed at cerium oxide nanoparticles of distinct morphology. Results presented in this thesis revealed that addition of CO2 apparently has no effect on the catalytic performances of the investigated ceria nanoshapes (activity/ selectivity/ stability) in EBDH once the stable oxidation state and a semi-steady conversion have been attained. EBDH has been suggested in literature to proceed via a Mars Van Krevelen (MvK) mechanism over ceria nanoparticles. Reactions proceeding via a MvK apparently exhibit morphology dependent catalytic activity over ceria nanoparticles. This has further been related with the ease of an oxygen specie abstraction, i.e. the energy required for oxygen vacancy stabilization at low index ceria facets increasing in the order: (110) < (100) < (111) and their corresponding stability decreasing as follows: (111) > (110) > (100). Differential catalytic testing results, X-ray diffraction (XRD), high resolution scanning electron microscopy (HRSEM) and Raman spectroscopy results in this thesis suggested that CeZrOx cubes enclosed by highly inherently reactive (100) crystal planes exhibit greater intrinsic abundancy and higher inherent reactivity of oxygen vacancies in CO2 mediated EBDH as compared to CeZrOx spheres enclosed by mainly (111) crystal planes. Further, differential catalytic testing results clearly revealed that ceria cubes possess about two times higher catalytic activity per m2 for EBDH in presence and absence of CO2 as compared to ceria rods and particles. This has been ascribed to the higher abundancy of active lattice oxygen species at (100) crystal planes of ceria cubes as compared to (111) facets exposed at rods and particles. Removal of oxygen was qualitatively in agreement with the formation of oxygen vacancies and lattice distortion as observed by Raman spectroscopy. We suggested that these O species, consumed in nonselective EB conversion pathways generate the partly-reduced surface sites, which are active for the selective styrene formation. Similar trends: cubes >> rods ~ particles these nanoshapes exhibited in a reverse water gas shift (RWGS) reaction. Based on the results presented in this thesis cerium oxide cubes are highly promising catalysts for EBDH, CO2 mediated EBDH and RWGS reaction as compared to rods and particles. Results presented contribute to fundamental understanding of the structure-performance relationships in ceria catalysis which is a key in tailoring new and improving existent chemical processes.
|Award date||1 Jun 2016|
|Place of Publication||Enschede|
|Publication status||Published - 1 Jun 2016|