Infrared and TPD studies of nitrates adsorbed on Tb4O7, La2O3, BaO, and MgO/γ-Al2O3

NO and O-2 coadsorption on gamma-Al2O3-supported Tb4O3, La2O3. BaO, and MgO has been investigated by in situ infrared spectroscopy coupled with temperatnre-programmed decomposition and desorption. BaO/gamma Al2O3 and MgO/gamma-Al2O3 possess a higher NOx storage capability than Tb4O7/gamma-Al2O3 and La2O3/gamma-Al2O3. NO/O-2 coadsorbed on Tb4O7, La2O3, and BaO in the form of bridging bidentate, chelating bidentate, and monodentate nitrates, and on MgO in the form of bridging bidentate and monodentate nitrates via the reaction of adsorbed NO with adsorbed oxygen at 298 K. NO/O-2 coadsorbed as a chelating bidentate nitrate on TD4O7 and La2O3, and as a distinctive bridging bidentate nitrate on BaO and MgO via the reaction of adsorbed NO with surface lattice oxygen at 523 K. These various forms of adsorbed nitrate differ in structure and reactivity from Tb(NO3)(3), La(NO3)(3), Ba(NO3)(2), and Mg(NO3)(2), the precursor used to prepare metal oxides for NO/O-2 coadsorption. Temperature-programmed desorption (TPD) of chelating bidentate nitrate on Tb4O7, La2O3, and BaO produced primarily NO and O-2, With maxima at 640 and 670 K, respectively. TPD of bridging bidentate nitrate and monodentate nitrate on Tb4O7, La(2)o(3), and BaO produced NO and O-2 as major products and N-2 and N2O as minor products, at 320-500 K. Decomposition of bridging bidentate on MgO produced NO as a major product and N2O as a minor product at a peak temperature of 690 K, Peak temperatures for Tb(NO3)(3), La(NO3)(3), Ba(NO3)(2), and Mg(NO3)(2) decomposition occurred between those-for bridging and chelating nitrates. The difference in stability between chelating and bridging bidentate nitrates on various metal oxides/gamma-Al2O3 may provide a wide range of operating temperatures for NOx storage.