Microstructural Changes in Deactivated Noble Metal Catalysts
The structural deactivation of supported noble metal catalysts has been investigated for treatments at different temperatures and in different atmospheres.
The sintering rates of Pt/Al2O3, Rh/Al2O3, and Pt-Rh/Al2O3 catalysts were measured using temperature-programmed desorption of CO (CO-TPD) and transmission electron microscopy (TEM). Ageing was mainly performed in Ar, 4% H2, 2% O2, and 0.1% and 4% NO. The most rapid particle sintering occurred in NO atmospheres for catalysts containing Pt where sintering for the Pt/Al2O3 catalyst was observed at temperatures as low as 200 °C. The stability against sintering for the Pt/Al2O3 catalyst in different atmospheres was found to decrease in the order: Ar, H2
--> CO --> O2 --> NO
. In the case of the Rh/Al2O3 catalyst sintering was only a minor cause for deactivation. In oxidizing atmospheres at temperatures above 600 °C, Rh was found to interact strongly with the alumina support leading to a reduced accessibility to the Rh atoms. Bimetallic Pt- Rh/Al2O3 catalysts also sinter easily under oxidizing conditions but seem to have a higher stability against sintering in 0.1% NO at 500 °C compared to the Pt/Al2O3 catalysts.
The sintering of a bimetallic commercial three-way catalyst, with both rhodium and platinum supported on alumina/ceria was also investigated. Heat treatment in an oxidizing atmosphere of 2% O2/N2 at 850 °C for 24 hr resulted in very large plate like particles with sizes up to mm. The light-off temperature for all three pollutants (HC, CO, and NOx) of the aged sample was more than 70 °C higher than of the fresh one. The Pt/Rh concentration ratio in the sintered particles was higher than that in the small particles in the fresh catalyst as measured by energy dispersive X-ray analysis.
In vehicle aged commercial three-way catalysts poisoning compounds like calcium phosphates were frequently found. Large amounts of lead was also found associated with the noble metal particles in one of the samples. The presence of aluminum phosphates and cerium phosphates indicated that poisoning elements also had influenced the properties of the support.
Pure and noble metal impregnated alumina washcoats were, according to results from X-ray diffraction measurements, quite stable in dry air up to 1000 °C while a hydrothermal treatment in saturated steam at 814 °C for 2 hours resulted in the formation of larger alumina grains.