Adsorption and decomposition of NO on lanthanum oxide

The adsorption behavior of NO on La2O3, an effective catalyst for selective NO reduction with CH4 at temperatures above 800 K, depends upon the pretre atment as indicated by temperature-programmed desorption (TPD) and diffuse reflectance FTIR spectroscopy (DRIFTS). The use of isotopic O-18(2) exchang e and adsorption showed that oxygen adsorbed dissociatively by filling oxyg en vacancies and that both oxygen vacancies and lattice oxygen were mobile at high temperature. Oxygen pair vacancies were assumed to be created by de sorption of molecular oxygen and, upon cooling, a certain distribution of p air and single vacancies exists at the surface as the pair vacancies can re arrange due to oxygen ion migration. After La2O3 was pretreated at 973 K in He, exposure to NO at 300 K caused a brief reaction forming N2O, then gave three NO TPD peaks at 400, 700, and 800 K. The only O-2 desorption occurre d during the 800 K NO peak and gave an NO/O-2 ratio near unity. Oxygen chem isorption prior to NO admission eliminated the formation of N2O during NO a dsorption at 300 K, blocked the sites giving NO desorption at 700 K, but en hanced the NO and O-2 peaks at 800 K. TPD after (NO)-N-15-O-16 adsorption o n an La2O3 surface containing exchanged O-18 lattice anions, but no chemiso rbed O atoms, showed that both (NO)-N-15-O-16 and (NO)-N-15-O-18 desorbed a t 400 K, but only (NO)-N-15-O-16 was present in the 700 and 800 K desorptio n peaks, and O-16(2) again desorbed at 800 K. When both lattice exchange an d chemisorption of O-18(2) on the La2O3 surface were allowed before (NO)-N- 15-O-16 adsorption, (NO)-N-15-O-18 was desorbed at 400 and 800 K while O-16 (2), (OO)-O-16-O-18, and O-18(2) were also desorbed at 800 K; thus the NO p eak at 400 K involves exchange with surface lattice oxygen atoms, while the 800 K peak involves exchange with chemisorbed oxygen atoms. DRIFTS indicat ed the presence of anionic nitrosyl (NO-), hyponitrite (N2O2)(2-), chelated nitrite (NO2-), nitrito (ONO-), and bridging and monodenate nitrate (NO;) species. Consequently, the three NO TPD peaks were assigned as follows: 400 K, decomposition of nitrito, nitro, and bidentate nitrate species; 700 K, desorption from NO- and (N2O2)(2-) species; and 800 K, decomposition of mon odenate nitrate species into NO and O-2. A model of the La2O3 surface based on the (001) and (011) crystal planes is proposed to account for these dif ferent sites. Two types of oxygen pair vacancy sites with a different 0-0 s eparation appear to exist, with one forming (N2O2)(2-) species, and four ad ditional sites-(1) an oxygen single vacancy (2) a single vacancy and a latt ice oxygen atom, (3) a coordinative unsaturated lattice oxygen atom, and (4 ) adjacent lattice oxygen atoms-are proposed to explain the formation of NO -, nitrito (M-ONO-), chelated nitrites, and bridging nitrate species, respe ctively. Among these species, (N2O2)(2-) was detected by DRIFTS under react ion conditions at 800 K and is most likely to be an active intermediate dur ing NO decomposition. Monodentate nitrate species are also observed at 800 K, but are very stable and still present after purging at 800 K. (C) 2000 A cademic Press.