Abstract
The HC and CO oxidation activity of Pt/Al2O3, Pd/Al2O3 and Pd-Pt/Al2O3 catalysts was compared under three exhaust-gas model conditions typical for stoichiometric gasoline as well as lean burn diesel by means of temperature programed conversion tests on laboratory test benches. In order to understand the differences in emission reduction performance, the impact of gas mixture composition was systematically studied with a particular focus on methane and formaldehyde conversion.
The catalysts used in this study were prepared by incipient wetness impregnation of Al2O3 followed by washcoating on ceramic honeycombs, and were pre-treated in an equal manner before each activity test.
Distinct differences in pollutant conversion were observed over the three catalysts under the investigated conditions. Pd-based monometallic and bimetallic catalysts revealed the lowest temperature for CH4 oxidation (<450 °C) under lean conditions, being lower under lean Diesel than Compressed Natural Gas (CNG) conditions. The reason for this behavior seems to be the higher water concentration in the CNG gas mixture. Furthermore, it was found that the presence of HCs shifts the conversion of methane to higher temperatures, most probably due to produced water during combustion of the HCs. Pt/Al2O3 on the other hand only converts methane at temperatures above 500 °C but no sensitivity to water or the amount of additional HCs was observed. The lowest light-out temperature for formaldehyde oxidation in excess of oxygen was measured on Pt/Al2O3 with full conversion below 125 °C. While this was the case for a model gas mixture of HCHO, H2O and O2, under simulated lean burn gas engine conditions, formaldehyde is oxidized at lower temperature on Pd-Pt/Al2O3 than on Pt/Al2O3 due to the strong inhibiting effect of CO on Pt-particles.
Under stoichiometric gasoline conditions, Pt had the highest activity for methane oxidation whereas Pd/Al2O3 and Pd-Pt/Al2O3 hardly convert methane below 500 °C. Unsaturated HCs and CO on the other hand were converted at lower temperature over Pd containing monometallic and bimetallic catalysts in comparison to Pt/Al2O3, showing once again the multiple and complex effects generated by variations in exhaust gas mixture and catalyst composition.
