Adsorption and Decomposition of NO on Lanthanum Oxide

Abstract

The adsorption behavior of NO on La2O3, an effective catalyst for selective NO reduction with CH4 at temperatures above 800 K, depends upon the pretreatment as indicated by temperature-programmed desorption (TPD) and diffuse reflectance FTIR spectroscopy (DRIFTS). The use of isotopic 18O2 exchange and adsorption showed that oxygen adsorbed dissociatively by filling oxygen vacancies and that both oxygen vacancies and lattice oxygen were mobile at high temperature. Oxygen pair vacancies were assumed to be created by desorption of molecular oxygen and, upon cooling, a certain distribution of pair and single vacancies exists at the surface as the pair vacancies can rearrange 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 O2 desorption occurred during the 800 K NO peak and gave an NO/O2 ratio near unity. Oxygen chemisorption prior to NO admission eliminated the formation of N2O during NO adsorption at 300 K, blocked the sites giving NO desorption at 700 K, but enhanced the NO and O2 peaks at 800 K. TPD after 15N16O adsorption on an La2O3 surface containing exchanged 18O lattice anions, but no chemisorbed O atoms, showed that both 15N16O and 15N18O desorbed at 400 K, but only 15N16O was present in the 700 and 800 K desorption peaks, and 16O2 again desorbed at 800 K. When both lattice exchange and chemisorption of 18O2 on the La2O3 surface were allowed before 15N16O adsorption, 15N18O was desorbed at 400 and 800 K while 16O2, 16O18O, and 18O2 were also desorbed at 800 K; thus the NO peak at 400 K involves exchange with surface lattice oxygen atoms, while the 800 K peak involves exchange with chemisorbed oxygen atoms. DRIFTS indicated the presence of anionic nitrosyl (NO−), hyponitrite (N2O2)2−, chelated nitrite (NO2−), nitrito (ONO−), and bridging and monodenate nitrate (NO3−) 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 monodenate nitrate species into NO and O2. A model of the La2O3 surface based on the (001) and (011) crystal planes is proposed to account for these different sites. Two types of oxygen pair vacancy sites with a different O–O separation appear to exist, with one forming (N2O2)2− species, and four additional sites—(1) an oxygen single vacancy, (2) a single vacancy and a lattice 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, respectively. Among these species, (N2O2)2− was detected by DRIFTS under reaction conditions at 800 K and is most likely to be an active intermediate during NO decomposition. Monodentate nitrate species are also observed at 800 K, but are very stable and still present after purging at 800 K.