Kinetics of Nitrous Oxide Decomposition over Heterogeneous Catalysts
This work studies the kinetics of nitrous oxide decomposition over alumina-based catalysts, specifically at the high temperatures and high nitrous oxide (N2O) concentrations that would be experienced in catalyst beds for monopropellant rocket thruster applications. High- and low- order models are developed to understand the interaction between reaction kinetics and mass transfer in monolith catalyst tubes. However, nitrous oxide decomposition is not observed on monolith catalyst tubes on account of their lower geometric surface area leading to a majority of the gas not coming into contact with the catalyst surface. Pellet-bed catalysts are studied for the remainder of this work, starting from experiments with a constant-volume batch reactor. The batch experiments demonstrate N2O decomposition over catalyst pellets, and a one-dimensional, time-varying model is developed to quantify the reaction rate based on measured temperature and pressure rise from experimental data. The reaction rates predicted by the model are significantly lower than predicted in the literature for the same catalysts. The inaccuracy is attributed to the fact that the model cannot capture N2O decomposition occurring during the first few seconds of filling the batch tube. Additionally, the simplified temperature distribution applied in the model may not be accurate, and obtaining a higher fidelity temperature distribution experimentally would require more advanced diagnostics.
The final experiment is a conventional flow-through pellet bed reactor which uses infrared spectroscopy to measure the concentration of nitrous oxide in the decomposed gas mixture. The analysis method incorporates uncertainties from infrared measurements and other sources, and initial activity results of a cobalt oxide-on-alumina catalyst are consistent with the literature. Results from additional testing indicate that manganese oxide catalysts are more active than nickel oxide or cobalt oxide catalysts. At weight loadings of ~10%, results indicate that the Arrhenius pre-exponential constant is roughly an order or magnitude greater for manganese oxide catalysts than cobalt or nickel oxide catalysts. The results also indicate hysteresis in catalytic activity of all oxides. Surface area and x-ray diffraction measurements do not reveal any permanent change in the surface area or crystal structure of these catalysts. The findings lead to the conclusion that the temperature and surrounding environment of the catalyst (either nitrous oxide or nitrogen during system purges) cause short-lived changes to the crystal structure of the active phase, leading to the observed hysteresis.
- Doctor of Philosophy
- Aeronautics and Astronautics
- West Lafayette