Background: In spite of the enormous achievements in the aftertreatment of exhaust gas emissions, the worldwide increasing number of vehicles represent a serious environmental problem due to vehicles' raw emissions, in particular, carbon dioxide, which has a strong impact on the greenhouse effect. A more efficient fuel consumption can be realized in Diesel and lean-operated engines, i.e., in excess of air (oxygen). Here, the problem is the formation of nitrogen oxides (NOx). Since improvements of the combustion process itself are not sufficient to meet future legislative limits, the development of a technique for the aftertreatment of NOx is urgently needed.
One of the most promising approaches is the NOx Storage and Reduction Catalyst (NSR) which utilizes the NOx storage on barium sites to form nitrates during the lean phase and their reduction to nitrogen in a rich atmosphere [1]. Detailed models, which are based on physical and chemical processes on the molecular level, are indispensable to exploit the full potential of this technique.

 

Experimental Work: The experimental work is based on well-defined model catalysts of monolithic structure and of varying complexity. Platinum and rhodium are chosen as noble metal components. The washcoat consists of γ-Al2O3 and for the more complex systems ceria and barium for the uptake of oxygen and nitrogen oxides were added. The investigations of the kinetics are carried out under isothermal conditions in a flat bed reactor [2] using a realistic model exhaust gas. Furthermore, lateral withdrawals allow the measurement of gas concentration profiles along the length of the catalyst. The experimental system is equipped with a fast responding mass spectrometer for the measurement of short lean/rich cycles.

Numerical Model: The numerical simulations are carried out using the software package DETCHEM [3, 4], which uses detailed reaction mechanisms for the conversion on the noble metals and the storage and reduction processes on the barium particles. DETCHEM is a FORTRAN based package that is designed to couple chemistry models with Computational Fluid Dynamics (CFD) programs. It applies hierarchically arranged detailed models from an atomic scale up to reactor scale. The core is a library for the description of species properties based on atomistic models and for reactions among gas-phase and surface species based on elementary step reaction mechanisms. Upon this, the two-dimensional flow field in a single channel is modeled using the boundary-layer assumption. Radial transport models include composition dependent diffusion coefficients in the gas phase and an effectiveness factor approach for the washcoat. Inlet conditions and NOx storage capacities of the single channel simulations vary in time.
The mechanism for the simulation of the oxidation and reduction processes is based on an elementary step reaction mechanism for platinum catalysts [4]. In this work, some reactions were modified and the mechanism was extended by reactions on support and storage media.

Barium Nitrate Coverage at 350°C

350°C

Simulated Ba(NO3)2 coverage of a Pt/Ba/Al2O3 catalyst (length: 0,2 m)
for a lean/rich cycle of 60/5 s at 350°C

Literature:

[1] W. Boegner, M. Kraemer, B. Krutzsch, S. Pischinger, D. Voigtlaender, G. Wenninger, F. Wirbeleit, M. S. Brogan, R. J. Brisley and et al., Applied Catalysis, B: Environmental, Vol. 7, No. 1-2 (1995), pp. 153-171.
[2] U. Tuttlies, V. Schmeisser and G. Eigenberger, Chemical Engineering Science, Vol. 59, No. 22-23 (2004), pp. 4731-4738.
[3] O. Deutschmann, S. Tischer, C. Correa, D. Chatterjee, S. Kleditzsch, V. M. Janardhanan, DETCHEM software package, 2.0 ed., http://detchem.de/, Karlsruhe, 2004.
[4] D. Chatterjee, O. Deutschmann and J. Warnatz, Faraday Discussions, Vol. 119, (2001), pp. 371-384.

Co-workers: Jan Koop , Marco Hartmann

Collaboration: V. Schmeißer, U. Tuttlies, G. Eigenberger, U. Nieken (University of Stuttgart), M. Tutuianu, W. Bessler (University of Heidelberg)

Publications:

Funding: Forschungsvereinigung Verbrennungskraftmaschinen (FVV) (DeNOx Modell III)

Further information: