Hydrothermally induced changes of lean NOx reduction over Cu-ZSM-5 and stability improvements by synthesis modification

Doctoral thesis, 2011

The growing concern about the global warming coupled with the increasing emissions of the greenhouse gas carbon dioxide, combined with the climbing oil price, drives the interest to more fuel efficient lean combustion engines for automotive applications. Such engines require new technical solutions for reduction of NOx in the oxygen rich exhaust gases, where a conventional three-way catalyst cannot be used. One of three main techniques to reduce NOx from lean-burn engines is hydrocarbon assisted selective catalytic reduction (HC-SCR) in which hydrocarbons from the fuel are used to selectively reduce NOx in the presence of excess oxygen. A copper ion-exchanged zeolite material, Cu-ZSM-5, is one of the catalysts with the ability to perform this NOx reduction reaction. However, this material successively loses its activity in hydrothermal environment. The objective of this thesis is to investigate the catalytic changes induced by the hydrothermal environment as well as the influence of the zeolite synthesis procedure on the hydrothermal stability and on the lean NOx reduction activity of Cu-ZSM-5. By using in-situ FTIR (Fourier transform infrared) spectroscopy, the species adsorbed onto the surface during the NOx reduction reaction was observed. It was found that the reactions between molecules, in which N to C bonds were developed and rearranged in such a way that N2 could be produced, were hindered upon hydrothermal treatment of the sample. On the other hand the oxidation reactions of NO or hydrocarbons, separately, were more or less unaffected.