Studies of the influence of ZSM-5 synthesis on activity and stability of Cu-ZSM-5 in leand NOx reduction

Licentiate thesis, 2008

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 exhaust gases, which are in excess oxygen, where a conventional three-way catalyst cannot be used to reduce NOx. One of the 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 oxygen. A copper ion-exchanged zeolite material, Cu-ZSM-5, is one of the better catalysts for this reaction. However, this material successively looses its activity in hydrothermal environment. The objective of this thesis project is to investigate the influence of the zeolite synthesis procedure on the hydrothermal stability and on the lean NOx reduction activity of Cu-ZSM-5. By varying the aluminium source and by introducing calcium hydroxide in the ZSM-5 synthesis, Cu-ZSM-5 samples were produced, for which different NOx reduction and hydrothermal stability was observed. One of the Cu-ZSM-5 samples lost about 55% of its catalytic NOx reduction activity after aging during 12 hours in a flow of 21% O2, 1% H2O in N2, while the others lost only about 10-30%. This investigation suggests that a paired aluminium distribution in ZSM-5, created in the synthesis by using AlCl3, stabilizes the corresponding Cu-ZSM-5 in HC-SCR reactions compared with the Cu-ZSM-5 where the parent ZSM-5 is synthesised using Al(NO3)3. Introduction of calcium hydroxide in the zeolite synthesis increases in some cases, and stabilizes, the NOx reduction performance of the corresponding Cu-ZSM-5 when Al(NO3)3 is used as aluminium source. This stabilizing effect of calcium hydroxide is not observed when AlCl3 is used as aluminium source. Keywords: HC-SCR, propene, Cu-ZSM-5, hydrothermal stability, lean NOx reduction