Nature of the Ti-Ba Interactions on the BaO/TiO2/Al2O3 NOx Storage System

A ternary oxide-based NOx storage material in the form of BaO/TiO2/gamma-Al2O3 was synthesized and characterized. Thermally induced structural changes occurring on the surfaces of the TiO2/gamma-Al2O3 and BaO/TiO2/gamma-Al2O3 systems were studied in a comparative manner within 300-1273 K via X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) spectroscopy, and BET surface area analysis. The surface acidity of the studied oxide systems was also investigated via pyridine adsorption monitored by in-situ Fourier transform infrared (FTIR) spectroscopy. BaO/TiO2/gamma-Al2O3 ternary oxide was synthesized by incorporating different loadings of (8-20 wt %) BaO onto the TiO2/gamma-Al2O3 support material, which was originally prepared using the sol-gel method. In the TiO2/gamma-Al2O3 binary oxide support material, anatase phase exhibited a relatively high thermal stability at T <= 1073 K. The presence of TiO2 domains on the surface of the alumina particles was found to alter the surface acidity of alumina by providing new medium-strength Lewis acid sites. SEM/EDX results indicate that in the BaO/TiO2/gamma-Al2O3 system, TiO2 domains present a significant affinity toward BaO and/or Ba(NO3)(2) resulting in a strong Ti-Ba interaction and the formation of overlapping domains on the surface. The presence of TiO2 also leads to a decrease in the decomposition temperature of the Ba(NO3)(2) phase with respect to the Ti-free Ba(NO3)(2)/gamma-Al2O3 system. Such a destabilization is likely to occur due to a weaker interaction between Ba(NO3)(2) and gamma-Al2O3 domains in the ternary oxide as well as due to the change in the surface acidity in the presence of TiO2. At relatively high temperatures (e.g., 873-1273 K) formation of complex structures in the form of BaTiO3, Ba1.23Al2.46Ti5.54O16, BaTiO5, and/or BaxAlyTizOn were also observed.