Effect of H2O on the morphological changes of KNO3 formed on K2O/Al2O3 NOx storage materials: Fourier transform infrared and time-resolved X-ray diffraction studies

Abstract

Based on the combined FTIR and XRD studies, we report here that H 2O induces a morphological change of KNO3 species formed on model K2O/Al2O3 NOx storage-reduction catalysts. Specifically as evidenced by FTIR, the contact of H2O with NO2 preadsorbed on K2O/Al2O3 promotes the transformation from bidentate (surface-like) KNO3 species to ionic (bulk-like) ones irrespective of K loadings. Once H 2O is removed from the sample, a reversible transformation into bidentate KNO3 is observed, demonstrating a significant dependence of H2O on such morphological change. TR-XRD results show the formation of two different types of bulk KNO3 phases (orthorhomobic and rhombohedral) in an as-impregnated sample. Once H2O begins to desorb above 400 K, the former is transformed into the latter, resulting in the existence of rhombohedral KNO3 phase only. On the basis of consistent FTIR and TR-XRD results, we propose a model for the morphological changes of KNO3 species with respect to NO2 adsorption/desorption, H2O and/or heat treatments. Compared with the BaO/Al 2O3 system, K2O/Al2O3 shows some similarities with respect to the formation of bulk nitrates upon H2O contact. However, there are significant differences that originate from the lower melting temperature of KNO3 relative to Ba(NO3)2. © 2014 American Chemical Society.