Catalysis for Lean NOx Reduction - Aspects of Catalyst Synthesis and Surface Acidity

Doktorsavhandling, 2005

From both economic and environmental perspectives it is desirable to reduce the consumption of fossil fuels. One route to increase the fuel efficiency for vehicles is operation in oxygen excess. However, a major problem with this technique is the lean environment, which obstructs catalytic reduction of nitrogen oxides (NOx) to harmless nitrogen. This thesis focuses on key parameters affecting catalytic reduction of NOx under lean conditions. In specific surface acidity and nanoparticle synthesis have been investigated. The activity for lean NOx reduction by propene and propane, the selectivity for N2 formation, and the surface acidity were investigated with flow reactor experiments and Fourier Transform Infrared (FTIR) spectroscopy over model catalysts, with and without platinum. The NOx reduction activity is markedly dependent on the type of reducing agent. Propene and propane were used in the reaction over platinum supported catalysts and it was found that the samples that showed activity for NOx reduction with propane all contained Brønsted acid sites. Further, the N2 selectivity was enhanced (using propane) with increasing Brønsted-site density over these catalysts. For the platinum-free samples a clear correlation between the amount of Brønsted acid sites and the activity for NOx reduction, with propane, was observed. It is likely that the propane is activated over the Brønsted acid sites forming some kind of carbenium ions, which probably react with adsorbed NO+ species to form isocyanates that, in turn, may be hydrolysed to amine species. The NOx reduction by isopropylamine was followed in-situ by FTIR and the results indicate a rapid reaction, over Brønsted acid sites, compared to the corresponding reaction with propane. It is thus conceivable that amine species are possible reaction intermediates in the HC-SCR reaction under the present conditions. However, the hydrocarbon activation and the formation of NO+ species seem to be a prerequisite for the reaction to occur. Platinum nanoparticles were prepared in microemulsions and the resulting particles were used in model catalyst synthesis. The effect of surfactant type on the kinetics of platinum nanoparticle formation in water-in-oil microemulsions was studied with UV-visible spectroscopy. The rate of particle formation was found to be strongly dependent on the type of surfactant used in the microemulsion formulation. The difference in reactivity may be due to either droplet fusion and/or to the chemical microenvironment in the fused microemulsion droplets. Platinum nanoparticles were deposited on alumina support using two different methods. Both methods gave considerable particle agglomeration. However, the rate of addition of solvent, the choice of surfactant, and the pH value are of importance and a weak interaction between the platinum particles and the support seems favourable.