Fundamental aspects of ammonia oxidation on cobalt oxide catalysts

Abstract

The current thesis deals with the ammonia oxidation on cobalt oxide catalyst at the molecular level. The catalytic oxidation of ammonia to NO is crucial in the industrial process of nitric acid production. Cobalt oxide catalysts are being used together with platinum gauzes to reduce the production cost and emission of greenhouse gas N2O. However, the fundamentals of ammonia oxidation on cobalt oxides are not known. This thesis aims to provide insights into our fundamental understanding of ammonia oxidation on Co3O4 surfaces. The performance of cobalt oxide catalysts in the oxidation of NH3 strongly depends upon the exposed surface terminations. Results indicate that different surfaces of Co3O4 behave markedly differently in oxidative reactions due to the difference in binding energy and O recombination energies and oxygen vacancy formation. Overall, NH3 oxidation follows stepwise dehydrogenative route (NH3* → NH2* → NH* → N*) on Co3O4 surfaces. Desorption of lattice products results in the formation of O vacancy sites opening the way for a Mars-van Krevelen mechanism. The successive dehydrogenation of ammonia preferably occurs on the surfaces exposing active lattice O sites. Removal of active lattice O sites from the Co3O4 surfaces in the form of products results in the surface reduction. If the rate of reduction is faster than that of re-oxidation, a CoO-like phase might form. The formation of CoO in Co3O4 catalysts during NH3 oxidation not only reduces the NH3 conversion but also alters the selectivity towards N2 rather than NO due to weak ability of lattice O at the CoO surface to assist the hydrogen abstraction process. A surface with a lower oxygen vacancy formation energy and a higher binding energy of hydrogen exhibits a higher activity towards ammonia oxidation to NO.