Abstract
Using complexing-agent assisted sol-gel routes, mixed Pd/ceria-alumina and Pd/terbia-ceria-alumina catalysts were prepared. The materials have been characterised by means of ICP-MS, TEM, EDX, XPS, XRD, BET, TPR, TPO and TPD. Catalytic testing was carried out in a temperature programmed mode as well as isothermally, using synthetic exhaust gas mixtures with different air-to-fuel ratios. The obtained results were compared with those of traditionally impregnated Pd/ceria-alumina and PtRh/ceria-alumina. Evaluating the catalysts potential as three-way converters, it has been shown that as a result of the sol-gel preparation chosen, highly homogeneous materials were produced. These had (i) much higher oxygen storage potential (especially at low temperatures, T ≈ 400 - 500K), (ii) improved metal support interactions and (iii) lower CO and propane light-off temperatures (T50%(CO) ≈ 423K, T50%(C3H8) ≈ 593K for R ≥ 1). Under fuel-rich conditions an improved low temperature NO activity was shown for the ceria-containing materials. This was attributed to a ceria-mediated redox mechanism and an improved Pd-ceria interaction for these sol-gel-derived samples. The addition of terbia was found to promote the catalysts propane activity, especially under fuel-rich conditions. The application of the different catalysts as three-way converters has been discussed, with special emphasis on their potential during the cold-start period. In a further set of catalytic experiments, using less complex gas mixtures, a more comprehensive view of the detailed Pd chemistry involved in these new three-way catalysts was obtained.