NOx Adsorption and Transport in ZSM-5 Films

Doktorsavhandling, 2008

The present work dealt with a fundamental study of NOx adsorption-desorption and transport in ZSM-5 as films supported on cordierite monolith and as a part of composite membranes. The adsorption-desorption study was focused on developing a mechanistic understanding of NOx adsorption in ZSM-5 films including the effects of counter-ions, Si/Al ratios and film thicknesses on the adsorption. NOx adsorption-desorption measurements were carried out in a continuous flow reactor using binder-free ZSM-5 films grown on a cordierite monolith. Temperature programmed desorption (TPD) was used to determine thermal stability of the adsorbed species. To obtain detail information about the character of the adsorbed species, in-situ diffuse reflectance infrared spectroscopy measurements were carried out. It was found that various NOx adsorbed species with different thermal stabilities could be formed in the ZSM-5 depending on the counter-ions present and adsorption temperature. Si/Al and Na/Al ratios determined the adsorption capacity and mass transport resistance was observed to be of importance during transient adsorptions. The proposed adsorption mechanism combined with a mathematical transport model for NOx adsorption in a ZSM-5 film supported on a monolith support was able to reproduce the observed NOx adsorption-desorption phenomena which in turn offered a better insight into the NOx adsorption process in a thin ZSM-5 film. In addition, NOx transport in ZSM-5 films was further studied with the use of composite membranes in a Wicke-Kallenbach (WK) cell. It was found that the NOx transport in the ZSM-5 micropores was thermally activated and was influenced by surface adsorbed species. The presence of the adsorbed NO2 could hinder diffusion of non adsorbing components over a wide temperature range of 20C to 400C. The present work also showed that the measurement of component permeation through a membrane in the WK cell could be influenced by fluid dynamics and gas phase transport in the WK cell. However, the external mass transport correlation proposed through the present work can be useful to simplify the development of accurate transport models.