NO2 and N2 sorption in MFI films with varying Si/Al and Na/Al ratios

MFI crystals or films with controlled thicknesses and different Si/Al ratios were grown on seeded cordierite monoliths using a clear synthesis mixture with template or a template-free gel. The materials were analyzed by scanning electron microscopy, X-ray diffraction, inductively coupled plasma-atomic emission spectrometry, X-ray photoelectron spectroscopy, thermogravimetric analysis and sorption experiments using N₂ or NO₂ adsorbates. The films were uniformly distributed over the support surface. As expected, the specific monolayer N₂ adsorption capacity (mol/g_zeolite) was constant and independent of film thickness. The specific molar NO₂ adsorption capacity was significantly lower than the specific molar monolayer N₂ adsorption capacity, indicating that NO₂ is adsorbed at specific sites rather than evenly distributed in a monolayer. A number of NO₂ adsorption sites with varying strengths were observed by TPD experiments. At 30 °C, the amount of adsorbed NO₂ in the MFI films increased with increasing Al and Na content as opposed to the N₂ adsorption capacity, which was independent of these parameters. At 200 °C, the adsorbed amount of NO₂ was lower than at 30 °C and apparently independent on Al concentration in the Na-MFI films. These results indicate that different mechanisms are involved in NO₂ adsorption. NO₂ may adsorb weakly on Na⁺ cations and also react with silanol groups and residual water in the zeolite, the latter two results in more strongly bound species. Upon NO₂ adsorption, formation of NO was observed. This work represents the first systematic study of the effects of Al and Na content on NO₂ adsorption in MFI films.