Methodology for Estimating Ea for Catalyst Deactivation

Bukky  Oladipo, Umicore

Effectively assessing the performance deterioration of diesel emission control catalysts over their in-use lifetime is a very costly and time-consuming process. Typically, the aging process is accelerated using oven-, burner-, and engine bench-aging with appropriately select time @ temperature specification. For gasoline application, the Arrhenius expression Aexp(-Ed/RT) has been successfully applied where Ed, the activation energy of the deactivation mechanism, was suggested to be 150 kJ/mol and has been successfully applied to select the right aging time and temperature for many years.

This work describes a methodology for employing the Arrhenius expression to develop accelerated aging protocol for diesel catalysts. For a start, the assumption is made that the dependence of the rate of performance deterioration of the catalyst on aging duration is negligible relative to the dependence on aging temperature. Using SCR NOx control as an example, this allows representation of the NOx efficiency loss due to aging to be expressed as: dn/(nref*taging)=Aexp(-Ed/RT) where dn/taging is the rate of loss of NOx efficiency due to aging at temperature T for time duration taging. The loss of efficiency is normalized by an appropriate reference value nref, e.g. 100 for alpha = 1, 90 for alpha = 0.9 and so on. Performance data on SCR catalysts aged at various time @ temperature conditions was then analyzed to establish the pre-exponential factor A and the “activation energy” Ed for the SCR deactivation mechanism through aging.

A further potential application of this methodology relates to the understanding of changes in deactivation modes of diesel catalysts. For instance, the degreened mode of deactivation may be quite different from normal in-use aging. Also, catastrophic deactivation resulting from collapse of the internal structures may set in at elevated temperatures. With a well-designed aging time and temperature matrix, it may be possible to observe change of slope that will indicate differences in catalyst deactivation modes. Furthermore, the concept of Arrhenius representation of deactivation may lend itself to application for diesel catalyst modeling. Since the overall rates for the different catalytic reactions are usually expressed with the Arrhenius expression and the pre-exponential and activation energy factors parameterized with test data, it may be possible to include impact of aging by adjusting the model parameters using the Arrhenius factors determined from correlation of the aging data.

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