Controlling the surface dispersion of BaO domains on NOx storage materials via TiO2 anchoring sites

Abstract

In an attempt to control the surface dispersion and the mobility of BaO domains on NOx storage materials, TiO2/TiOx anchoring sites were introduced on/inside the conventional γ-Al2O3 support matrix. BaO/TiO2/γ-Al2O3 ternary oxide materials were synthesized via two different sol-gel preparation techniques, with varying surface compositions and morphologies. The synthesized NOx storage materials were studied via XRD, Raman spectroscopy, BET surface area analysis, TPD, XPS, SEM, EDX-mapping and in situ FTIR spectroscopy of adsorbed NO2. NOx uptake properties of the BaO/TiO2/γ-Al2O3 materials were found to be strongly influenced by the morphology and the surface structure of the TiO2/TiOx domains. The presence of Ti4+ surface sites provide additional NOx adsorption sites which can store NOx predominantly in the form of bridged/bidentate nitrates. An improved Ba surface dispersion was observed for the BaO/TiO2/γ-Al2O3 materials synthesized via the co-precipitation of alkoxide precursors which was found to originate mostly from the increased fraction of accessible TiO2/TiOx sites on the surface. These TiO2/TiOx sites function as strong anchoring sites for surface BaO domains and can be tailored to enhance surface dispersion of BaO. TPD experiments suggested the presence of at least two different types of NOx species adsorbed on the TiO2/TiOx sites, with distinctively different thermal stabilities. The relative stability of the NOx species adsorbed on the BaO/TiO2/γ-Al2O3 system was found to increase in the following order: NO+/N2O3 on alumina < nitrates on alumina < surface nitrates on BaO < bridged/bidentate nitrates on large/isolated TiO2 clusters < bulk nitrates on BaO on alumina surface and bridged/bidentate nitrates on small TiO2 crystallites homogenously distributed on the surface < bulk nitrates on the BaO sites located on the TiO2 domains.