Ageing Mechanisms in Automotive Catalysts
Due to the slow and expensive nature of on-road and on-engine ageing, laboratory ageing and testing methods are becoming increasingly important, as well as mathematical catalyst simulation models. With the regulations concerning automotive emissions becoming more and more stringent it is important that these laboratory methods are developed, and catalyst ageing mechanisms understood, in order to continue to produce more efficient and durable catalytic control systems. This study uses a number of laboratory testing and ageing methods in order to investigate how effectively they correlate to, and describe, catalyst ageing on a vehicle. The activity testing of full size catalyst bricks was successfully conducted using the Catagen Labcat test system, and correlation with the light-off activity of cored catalyst samples in the Horiba SIGU 2000 examined using the QUB global catalyst model. Static ageing was also conducted in the laboratory, using the BAT equation to calculate ageing times for ageing temperatures relating to a RAT-A cycle. The thermal reactivity coefficients, R-factors, for static ageing in different ageing atmospheres were calculated and compared to those recommended by the EPA for dynamic ageing. Finally, using the results achieved from these laboratory ageing methods, recommendations were made as to how ageing models reviewed in the literature could be improved. From experimental results alone, it was seen that no correlation existed between the light-off performance of cored catalyst samples and the full size bricks from which they were taken. However, the QUB global catalyst model was able to consider variations in precious metal dispersion between samples, differences between inlet temperature and gas concentrations, inconsistencies in light-off ramp rates and heat transfer characteristics of the two reactors. The simulations performed were able to show good correlation between the test methods. However, the two test methods showed differently the activity rank of the samples, indicating the variation in precious metal loading and dispersion throughout each full size brick. In other results, static ageing for palladium loaded catalyst samples was found not to show ageing effects due to time for ageing temperatures below 1000 ᵒC, and does not correlate well to dynamic ageing. Static ageing methods were also shown to cause deactivation at a slower rate than dynamic methods. It was shown that the Toyota ageing expression would better predict catalyst deactivation if it incorporated an oxygen factor, to describe ageing when no oxygen is present. However, further testing is required if a more accurate ageing algorithm is to be developed.
Automotive catalysts, Catalyst ageing, Catalyst deactivation, Thermal ageing, Catalyst activity, Catalyst testing
Networked Digital Library of Theses and Dissertations