Abstract
Hydrothermal stability of mordenite type catalysts including synthetic (CuHM) and natural (CuNZA) zeolites has been examined under a simulated lean NO x wet condition. When the catalysts were hydrothermally aged at 800°C with 10% H 2O for 6 h, the NO removal activity at the reaction temperature of 450°C was higher for natural zeolite than for synthetic one. It was mainly due to the Si/Al ratio of the catalyst. CuHM and CuNZA catalysts were dealuminated to investigate the effect of Si content of the zeolites on the hydrothermal stability. After the aging at 800°C for 24 h in the presence of 10% H 2O, NO removal activity of the dealuminated CuHM catalysts at the reaction temperature of 500°C was in the order of the Si/Al ratio of the catalysts. Such a significant improvement of the deNO x performance was also observed for the dealuminated CuNZA catalysts. It reveals that the Si/Al ratio of zeolite catalysts is one of the crucial characteristics enhancing the water tolerance and hydrothermal stability of the catalysts for NO reduction under the lean NO x wet condition. In addition, the loss of NO removal activity of the catalysts upon the hydrothermal aging is mainly attributed to the chemical alteration of Cu 2+ ions on the catalyst surface through the structural collapse of the zeolite catalysts.
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