Quantitative studies of changes in microstructure and activity during ageing of supported catalysts

Licentiate thesis, 2015

Catalytic converters have substantially contributed to the improvement of air quality in populated areas in the last decades by abatement of the emission of hazardous components such as CO, unburned hydrocarbons and NOx. However, due to the exposure to high temperature and different components in the exhaust gas, the performance of the catalysts degrades with time. An understanding of the microstructural and chemical changes of the catalysts during different operating conditions is required in order to develop strategies to reduce the degradation. In the present work, a bimetallic Pt-Pd nanoparticle catalyst supported on gamma-alumina was investigated using high resolution electron microscopy and spectroscopy. The studies included both fresh and aged catalyst systems. In order to allow a quantitative evaluation of the particle size distributions and locations on the support, a new TEM specimen preparation method was developed. The new method allows for monitoring the evolution of the particle size distribution as well as the spatial difference in the porous oxide support. These microstructural observations were complemented by investigations of the chemical composition and alloying of the nanoparticles. Finally, CO oxidation experiments were performed to correlate the observed microstructural and compositional changes to the catalytic activity.