Studies of catalyst sintering during operating conditions

Licentiate thesis, 2014

The damaging effect of pollutants from exhaust gas of combustion engines on the human health and the environment is well recognized. Since the 70s, the three-way catalytic converter (TWC) has substantially improved the urban air quality by simultaneous conversion of CO, NOx and unburned hydrocarbons to O2 , N2 , CO2 and H2O. The active phase in the TWC is small precious metal particles (e.g. platinum, palladium, and rhodium) that are dispersed on a high surface area oxide support such as γ-alumina. One issue regarding the TWC is their limited durability due to the catalyst deactivation during operation. Sintering of nanometer sized metal particles at high temperature is one of the major reasons for the deactivation. Detailed understanding of sintering mechanisms and kinetics are crucial to design catalyst formulations with extended durability. However, the lack of suitable methods to follow particle sintering under technologically relevant conditions hinders the progress in this area. The present thesis addresses fundamental issues connected to platinum sintering on oxide supports by use of indirect nanoplasmonic sensing (INPS) and transmission electron microscopy (TEM). Moreover, indirect nanoplasmonic sensing is further developed as an operando tool for characterization of sintering processes on different supports and in different gas environments. In particular, the sintering kinetics and evolution of particle size distributions are studied during oxidizing conditions on alumina and silica. In the first part of this study, high time resolution sintering kinetic data are deduced by correlating the INPS data with ex situ TEM analysis. The obtained kinetic data are based on ensemble average of particles. Thus, to further study Pt sintering in more details, intermittent particle size distributions are analyzed as a function of time and temperature. Transitional bimodal size distributions are clearly observed during sintering of Pt on the two investigated supports.