With emission legislations becoming ever more stringent there is an increased pressure on the after-treatment systems, and more specifically the three-way catalysts. With recent developments in emission legislations, there is requirement for more complex after-treatment systems and understanding of the aging process. With future legislation introducing independent inspection of emissions at any time under real world driving conditions throughout a vehicle life cycle this is going to increase the focus on understanding catalyst behavior during any likely conditions throughout its lifetime and not just at the beginning and end.
In recent years it has become a popular approach to use accelerated aging of the automotive catalysts for the development of new catalytic formulations and for homologation of new vehicle emissions. To accelerate the catalyst aging, the samples experience high temperatures of 800°C and higher on a recognized aging cycle for a specific time which can be related back to vehicle mileage. As opposed to using large gasoline engines, alternative bench-aging techniques are becoming more frequently used, including synthetic gas bench reactors. Bench reactors offer more flexibility, greater repeatability and opportunity for more precise control over variables providing greater development possibilities.
Whilst the body of understanding on catalyst deactivation and, in particular, catalyst aging is growing, there are still significant gaps in understanding, particularly how real world variations in temperature, flow rate and gas concentrations affect catalyst behavior.
Under normal driving conditions the catalyst can experience varying oxygen concentrations, such as under heavy acceleration or cruising down a hill will show a variation in oxygen from the engine emissions. The effect of varying oxygen concentrations has on the rate of aging is not fully understood and hence the total deactivation and efficiencies are not known throughout the catalyst lifetime. Current algorithms used in industry do not fully account for these variations in oxygen concentrations.
This paper presents a continuation of previous work into the investigation of the effect of varying oxygen concentration on the rate of catalyst aging. A number of commercially available palladium three-way catalysts were aged over a precise temperature cycle at varying oxygen concentrations for different aging times related back to a mileage. The results were analyzed in detail and compared with predictions based on the standard aging algorithm and with others proposed in literature.