The Kinetic Mechanisms of NH3-SCR Over V2O5-WO3/TiO2, Fe- and Cu-zeolite Catalysts

Enrico  Tronconi, Politecnico di Milano

The selective catalytic reduction of NOx with NH3/Urea (NH3-SCR) is nowadays recognized worldwide as the technology of choice to comply with the tightening NOx emissions standards for Diesel heavy-duty vehicles and passenger cars [1]. When first brought to the market over a decade ago, the mobile SCR technology was based on V2O5-WO3/TiO2 catalysts derived from those extensively used since the ‘80s to control NOx emissions from power stations and other stationary sources [2]. Such systems have been significantly improved and are still in use, but the latest generation of SCR converters mostly relies on zeolite catalysts promoted with transition metals (Cu, Fe) washcoated onto honeycomb monoliths [3,4]. In particular, the appearance of Cu-promoted small-pore zeolites has represented a significant step forward due to their enhanced low-T activity and hydrothermal stability.
In view of the model-based development of modern integrated aftertreatment systems, the elucidation of the SCR catalytic chemistry and kinetics is of key importance for the derivation of realistic and accurate simulation tools. In this contribution we will review comparatively the intrinsic kinetics of the NH3-NO/NO2 reactions over such three catalytic systems. Transient kinetic runs (e.g. isothermal concentration step changes, temperature programmed desorption and temperature programmed surface reaction experiments), designed to address the individual steps in the catalytic mechanisms, are primarily used to illustrate similarities and differences, in synergic combination with information from other techniques and from the literature [5-8].

[1] I. Nova and E. Tronconi (Eds.), “Urea-SCR Technology for deNOx After Treatment of Diesel Exhausts”, Fundamental and Applied Catalysis Series, Springer Science+Business Media, New York (2014).
[2] M. Koebel, M. Elsener, M. Kleemann, Urea-SCR: a promising technique to reduce NOx emissions from automotive diesel engines, Catal. Today 59 (2000) 335-345.
[3] A. Grossale, I. Nova, E. Tronconi, D. Chatterjee, M. Weibel,“NH3-NO/NO2 SCR for Diesel exhausts aftertreatment: reactivity, mechanism and kinetic modelling of commercial Fe- and Cu-promoted zeolite catalysts”, Topics in Catalysis, 52 (2009) 1837-1841
[4] M. Colombo, I. Nova, E. Tronconi, “A comparative study of the NH3-SCR reactions over a Cu-zeolite and a Fe-zeolite catalyst”, Catal. Today 151 (2010) 223-230.
[5] A. Grossale, I. Nova, E. Tronconi, D. Chatterjee, M. Weibel, ”The chemistry of the NO/NO2 – NH3 “Fast” SCR reaction over Fe-ZSM5 investigated by transient reaction analysis”, J. Catal. 256 (2008) 312-322.
[6] M. Colombo, I. Nova, E. Tronconi, “Detailed kinetic modelling of the NH3-NO/NO2 SCR reactions over a commercial Cu-zeolite catalyst for Diesel exhausts aftertreatment”, Catal. Today 197 (2012) 243-255.
[7] M.P. Ruggeri, I. Nova, E. Tronconi,“ Experimental Study of the NO Oxidation to NO2 Over Metal Promoted Zeolites Aimed at the Identification of the Standard SCR Rate Determining Step”, Topics in Catalysis 56 (2013) 109 – 113.
[8] M.P. Ruggeri, T. Selleri, M. Colombo, I. Nova, E. Tronconi, “Investigation of NO2 and NO interaction with a Fe-ZSM-5 Catalyst by Transient Response Methods and Chemical Trapping Techniques”, J. Catal. 328 (2015) 258–269.

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