Identifying critical factors in the regeneration of NOx-trap materials under realistic conditions using fast transient techniques

Abstract

NOx storage and reduction (NSR) catalysts are considered to be one of the most promising solutions to meet the upcoming NOx exhaust emission regulations. However, despite the high level of activity in this area, it is surprising that most studies have been conducted under conditions that are far from realistic conditions. In the present work a fast transient apparatus has been developed to study NSR catalysts under realistic temporal conditions, i.e. where the storage time was 1–2min and the regenerative rich time was of the order of 1s. To exemplify the value of this methodology, the performance of a 1%Pt/17.5%Ba/γ-Al2O3 NSR catalyst was studied as a function of the temperature, rich phase duration, reductant concentration, the type of reductant and the effect of an inert gas purge. From these results it is clearly found that excess reductant to regenerate the trap at short regeneration times is advantageous and that hydrogen is a better reductant than CO. This is especially true at low temperatures where the rate of reduction of released NOx is not sufficiently fast to allow for complete regeneration of the trap. For a given quantity of reductant, a long, low concentration rich pulse regenerates the catalyst more effectively than a short, high concentration pulse. Readsorption of NOx released during the rich phase but not reduced quickly enough can be a significant cause of loss of trap performance. This study underlines the importance of the regeneration step in the NSR process and highlights the need to investigate these systems using fast transient techniques capable of reproducing the short regeneration times used in real systems.