From the reaction-kinetic modelling point of view, the NOx storage and reduction catalyst (NSRC) is currently one of the most complex automobile exhaust gas converters [1-7]. Primary application of NSR catalyst is elimination of the NOx emissions from diesel and lean-burn gasoline engines, where direct NOx reduction is difficult. In the course of a longer lean phase (economical engine operation with lean fuel mixture, excess of air and O2, typically lasting for few minutes) the NOx are adsorbed (stored) on the catalyst surface in the form of nitrites and nitrates. Then, the accumulated NOx are desorbed and reduced within a short rich phase (injection of rich fuel mixture, excess of reducing components – CO, H2 and HC, lasting several seconds).

It is known that several different nitrogen compounds may be produced depending on the gas composition (red/ox ratio) and length of the enrichment – the most frequent are NO, N2O, N2 and NH3 [6]. Molecular nitrogen is main desired product of the NOx reduction. NO peak at the beginning of the rich phase results from the sudden desorption of the NOx deposited on the surface. N2O is formed mostly around the light-off temperature of the used reducing agent as the product of incomplete NOx reduction. The main ammonia outlet peak appears several seconds later in the rich phase, when the catalyst surface along the monolith channel is reduced and excess of reducing agent is reached at the monolith outlet [6,7].

This contribution focuses on experimental evaluation of the NOx reduction dynamics and NH3/N2 product selectivity within the short rich phase, and consequent development of effective global reaction-kinetic model that is able to describe the observed phenomena. Lean/rich experiments with an industrial NOx storage catalyst monolith sample (PtRh/Ba/CeO2/γ-Al2O3 type) are performed in nearly isothermal laboratory micro-reactor [8] at different temperatures to evaluate dynamics of the NOx reduction. Lean/rich switches are performed first in the absence of NOx to evaluate the oxygen storage capacity, which influences the balance of the reducing agent along the monolith channel during the rich phase [3]. Then the lean/rich operation with the NOx present in the inlet gas is examined. The outlet ammonia peak is correlated with the breakthrough of the reducing agent. The breakthrough time increases with the effective oxygen and NOx storage capacity, i.e., increases with temperature. As a consequence, the highest NH3 selectivity (integrated over the lean+rich period) is observed at low temperatures, but above the light-off temperature for the used reducing agent. The observed order of light-off temperatures is H2 .lt. CO .lt. C3H6 [3]. At high temperatures, the NOx reduction activity and the NH3 selectivity are aproaching similar values for all three reducing agents studied. Water gas shift and steam reforming play an important role in the mutual transformation of reducing agents at higher temperatures.

The spatially distributed NSRC monolith model with global kinetics developed earlier [2-5] is extended by ammonia formation and decomposition reactions [7]. Model kinetic parameters are obtained by simplex optimisation method, using weighted sum of the differences between the simulation results and measurements as the objective function. The simulation results are in a good agreement with the observed dynamics of the NOx reduction, including the selectivity towards N2 and NH3.

Literature

[1] Epling, W.S., Campbell, L.E., Yezerets, A., Currier, N.W., Parks, J.E.II, Overview of the fundamental reactions and degradation mechanisms of NOx storage/reduction catalysts, Catal. Rev. 46 (2004) 163.

[2] Kočí P., Kubíček M., Marek M., Maunula T., Härkönen M., Modelling of catalytic monolith converters with low- and high-temperature NOx storage compounds and differentiated washcoat, Chem. Eng. J. 97 (2004) 131.

[3] Kočí, P., Schejbal, M., Trdlička, J., Gregor, T., Kubíček, M., Marek, M., Transient behaviour of catalytic monolith with NOx storage capacity, Catal. Today 119 (2007) 64.

[4] Güthenke, A., Chatterjee, D., Weibel, M., Waldbüßer, N., Kočí, P., Marek, M., Kubíček, M., Development and application of a model for a NOx storage and reduction catalyst, Chem. Eng. Sci. 62 (2007) 5357.

[5] Güthenke, A., Chatterjee, D., Weibel, M., Krutzsch, B., Kočí, P., Marek, M., Nova, I., Tronconi, E., Current status of modelling lean exhaust gas aftertreatment catalysts, in Adv. Chem. Eng. 33 (2007) in press.

[6] Epling, W.S., Yezerets, A., Currier, N.W., The effects of regeneration conditions on NOx and NH3 release from NOx storage/reduction catalysts. Appl. Catal. B: Environ. 74 (2007) 117.

[7] Kočí, P., Plát, F., Štěpánek, J., Kubíček, M., Marek, M., Dynamics and selectivity of NOx reduction in NOx storage catalytic monolith, Catal. Today (2008) submitted.

[8] http://www.vscht.cz/monolith