Catalysis of N2O decomposition [electronic resource] : efficient elimination of a greenhouse gas
Keywords: Aeronautics and Astronautics.

The overall goal of this research is to characterize the performance and learn the mechanics of catalysts that can help decompose N2O. The particular goal is to find an energy-efficient catalyst that is cost-effective, robust, and straightforward to implement. Two catalytic materials were studied, rhodium oxide (Rh2O3) and iron oxide (Fe2O3). Rh2O3 was expected to be efficient but expensive, whereas Fe2O3 was expected to be less efficient, but also less expensive and possibly less affected by contaminants and therefore more cost effective in the long term. A more efficient catalyst is defined here as that which requires less heat input for full decomposition when other factors are held constant. The primary application bering considered is decomposition of N2O used in operating theaters as an analgesic. The experimental side of this study shows that although Fe2O3 is a less energy- efficient catalyst than Rh2O3, it is effective enough to be worth consideration for use in a catalyst bed with a longer residence time or higher temperature than would be required for rhodium oxide. It could also be implemented as part of a multi-metal catalyst, mixed with Rh2O3 in order to reduce the amount of expensive rhodium ion solution that needs to be purchased, thereby providing an affordable catalyst for hospitals, water treatment plants, and other industries looking to implement this technology. Two catalyst beds were constructed to test the metal oxides. The first, fabricated from a refractory nickel alloy called Hastelloy-X, was supplied with N2O and a diulent gas directly from gas cylinders. The diluents were O2 and N2, with N2 serving as the control in an experiment to determine the effects of high concentra- tions of O2 on N2O decomposition. Since the active sites on the catalyst are oxygen vacancies, it was possible that O2 would interact with them, but no significant effect on the catalysis reaction was found for oxygen concentrations from 95% to 99% by moles. The second catalyst bed was fabricated from stainless steel. It was larger, and was designed to attach directly to the exhaust outlet of an anesthesia machine. This catalyst bed was designed to produce a minimal pressure difference appropriate to anesthesia. In both experiments, heat tape was wrapped around the catalyst bed and covered in insulation so that the percentage of N2O remaining in the exhaust could be measured over a range of bed temperatures. Tests using the stainless steel catalyst bed were run at typical anesthetic flow rates of 1 L/min N2O and 1 L/min O2. Along with the experimental studies, Rh2O3 and Fe2O3 crystals have been nu- merically simulated using Density Functional Theory (DFT). In the simulation, N2O molecules were placed at various points across the catalyst's potential energy surface in order to determine the minimum energy pathway that the reaction would naturally take. The simulation results were used to quantify the activation energy barrier along that path for the two metal oxides. It was determined that, according to DFT and under ideal circumstances and with one active site per unit cell, this activation barrier is negligible for the surfaces investigated.