Infrared and TPD studies of nitrates adsorbed on Tb4O7, La2O3, BaO, and MgO/gamma-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 temper atnre-programmed decomposition and desorption. BaO/gamma Al2O3 and MgO/gamm a-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 o f bridging bidentate, chelating bidentate, and monodentate nitrates, and on MgO in the form of bridging bidentate and monodentate nitrates via the rea ction 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 bridgi ng bidentate nitrate on BaO and MgO via the reaction of adsorbed NO with su rface lattice oxygen at 523 K. These various forms of adsorbed nitrate diff er 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 coadsorp tion. Temperature-programmed desorption (TPD) of chelating bidentate nitrat e on Tb4O7, La2O3, and BaO produced primarily NO and O-2, With maxima at 64 0 and 670 K, respectively. TPD of bridging bidentate nitrate and monodentat e nitrate on Tb4O7, La(2)o(3), and BaO produced NO and O-2 as major product s and N-2 and N2O as minor products, at 320-500 K. Decomposition of bridgin g bidentate on MgO produced NO as a major product and N2O as a minor produc t 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 br idging and chelating nitrates. The difference in stability between chelatin g and bridging bidentate nitrates on various metal oxides/gamma-Al2O3 may p rovide a wide range of operating temperatures for NOx storage.