THE DECOMPOSSITION OF NITROUS OXIDE : A CATALYTIC APPROACH

Abstract

In this thesis, the sol-gel derived zirconia and the sol-gel derived zirconia coated Al2O3 have been prepared, and both the sol-gel derived PbO-ZrO2 and the PbO-ZrO2/Al2O3 catalysts have been tested for high concentration N2O decomposition. In addition to the catalytic evaluation, material issues have also been addressed. The material system of PbO-ZrO2 shows high thermal and chemical stability over the reaction temperature range for complete gas-decomposition. With an atomic ratio of Pb:Zr = 1 :6, the PbO-ZrO2 gives the best catalytic performance; the catalyst has a metastable ZrO2 tetragonal phase. When the Pb to Zr atomic ratio is higher or lower than 1 :6, the activity drops significantly. The active sites for N2O decomposition can be ascribed to the presence of Pb cation pairs with mixed valence states. In the studies of doping gas (H2O, CO2, and O2) effects on N2O decomposition, it is found that O2 and CO2 have no obvious inhibition effect on the overall reaction, while H2O inhibits the catalysts for the decomposition reaction. To explore further on the above catalyst series, material system of PbO-ZrO2 with various Pb:Zr atomic ratios has been prepared by sol-gel method. The gel catalysts have been investigated spectroscopically by GC, XRD, and XPS with respect to their chemical reactivity, surface/bulk chemical composition, polymorphic stability, and phase transformation. As revealed in the current work, enrichment of lead at surface is found for low lead-content gels while surface lead-loss is observed in high lead containing gels. The surface Pb:Zr ratio for the most active catalyst is 1 :5, whose bulk phase corresponds to a metastable tetragonal structure of ZrO2 with Pb:Zr = 1 :6. Based on findings of this work, surface active sites responsible for N2O decomposition reaction can be described by a low oxygen-coordinated Pb-O-Zr surface complex with Pb:Zr = 1 :5. Investigations of general material issues also provide surface and bulk information on the perovskite compound PbZrO3 in the lead-poor region.