H-Mordenite Deactivation during the SCR of NOx. Adsorption and

H-Mordenite Deactivation during the SCR of NOx- Adsorption and Diffusion of Probe Molecules on Fresh and Deactivated Catalysts 

H-mordenites with various Si/Al ratios (5.9 - 16.9) have proved to be active for the SCR of NO with CH4 in the 400-600 C temperature range. However, they suffered an irreversible deactivation after an incursion at 650°C for 1 h under reaction stream, due to a dealumination process. While acid dealumination only affects the free exchange of gaseous molecules between the main channels and the side-pockets (as seen by t29Xe NMR), the aluminum extracted from the lattice of the mordenite during the SCR of NOx at 650°C (without water vapor in the feed) also hinders the diffusive transport along the main channels. 

Since Hamada et al. (1) reported that solids not containing transition metals catalyze the selective reduction of NO with hydrocarbons in the presence of O2, many research groups have studied this matter. Yogo et al. (2) reported that H-zeolites are able to reduce NO with CH4. They found that the selectivity for this reaction decreases in the order H-ferrierite H-mordenite H-ZSM5 H-Y. 

We have previously (3,4) characterized in detail a series of dealuminated H-mordenites with Si/Al ratios = 5.9, 7.3, 11.0, and 16.9 through FTIR spectroscopy, 27A1-MAS NMR, XRD, and nitrogen adsorption. We have also found a correlation between the aluminum content of the samples and the catalytic activity for the selective reduction of NO with CH4 in the presence of oxygen excess. Despite the aluminum content, catalysts are partially deactivated on dry stream at temperatures of 650 C or higher. 

In order to gain insight into these deactivation phenomena, we have investigated the adsorption and diffusion of probe molecules on fresh samples with different Si/Al ratios and deactivated on samples on stream during the SCR of NOx- While the i29Xe NMR of physisorbed Xenon was useful to study the exchange of gaseous molecules between the main 

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