Abstract
Sol-gel production of catalyst supports has been investigated in order to produce homogeneous, high surface area alumina/zirconia materials. A novel microwave method of preparing colloidal Pd has been developed and a range of alumina/zirconia supported Pd catalysts has been produced. These have been tested for activity in terms of temperature programmed three way catalysis (simultaneous removal of NO, CO and C3H8 from a simulated car exhaust stream) and from the array of catalysts produced a suitable catalyst was chosen for further investigation. This catalyst had a 3% zirconia / 97% alumina support and contained ~1% Pd by weight and was examined for activity in the NO + CO + O2 system at temperatures below 500°C. Temperature programmed catalytic experiments revealed the reactions to be chemically controlled below ~400°C but diffusion controlled above this. CO temperature programmed reduction was used to examine the oxidation state of the Pd and revealed a complex Pd/PdO system to be present involving bulk PdO and surface oxide. Further kinetic studies showed that the reactions between NO, CO and O2 to have positive orders in all components. The reaction rates were stable over a wide range of conditions and the NO + CO reaction proved to have a low selectivity towards N2O. In-situ DBIFTS experiments alongside transient pulse work were used to illustrate low CO adsorption which indicated that these reactions proceeded via a redox mechanism in which Pd is oxidised by NO or O2 before being reduced by CO. The support material was examined by XRD following extended heating regimes and then compared to an identically treated sol-gel alumina. The zirconia doped alumina was found to possess a far greater thermal resistance to sintering than the pure alumina material and this was attributed to Zr4+ ions preventing the diffusion of Al3+ to form a-alumina.