Thermal Deactivation of a Three-Way Catalyst: Changes of Structural and Performance Properties

A standard commercial three-way catalyst was aged at 950°C for 24 hours in dry N₂ with 2% O₂. The performance of the aged and a fresh samples were characterized using a synthetic exhaust flow system.

The light-off temperature for all three pollutants on the aged sample was more than 70°C higher than for the fresh one. The effect of aging on steady-state performance at higher temperatures (>400°C) was more moderate.

In order to explain the decrease in activity the samples were analysed for their bulk and surface composition using electron microscopy (TEM/STEM/EDS), photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). In addition the precious metal dispersion were determined by CO chemisorption and the total area by standard BET measurement.

TEM micrographs showed that the metal particles containing platinum had an average diameter between 3-4 nm in the fresh sample but grow considerably in size upon aging. Very large sintered platinum particles (>1μm) were occasionally observed in the aged sample. The Pt/Rh ratio in these particles measured by STEM/EDS was twice as high on the aged catalyst compared to the fresh one, presumably due to a higher surface mobility for platinum. XPS measurements showed a higher degree of rhodium oxidation in the aged sample.