TPD, TPR and DRIFTS studies of adsorption and reduction of NO on La2O3 dispersed on Al2O3

Abstract

The surface properties of La2O3 supported on γ-Al2O3, as characterized by DRIFTS or temperature-programmed desorption (TPD), showed features more and more similar to unsupported La2O3 as the La2O3 loading increased. Optimum rates (mole/sg) were obtained when the bulk-like La2O3 surface area was maximized, and the 40% La2O3/Al2O3 catalyst had a rate high enough to possibly allow its use in combined-cycle power plants. TPD with La2O3/γ-Al2O3 and pure γ-Al2O3 after exposure to NO at 300K gave three NO desorption peaks and the only O2 desorbed coincided with the high-temperature NO peak near 800K. TPD after 15N16O adsorption on 18O-exchanged La2O3/γ-Al2O3 and γ-Al2O3 surfaces showed that a very small amount of 15N18O desorbed from the middle-temperature sites near 700K, while 15N18O and 16O18O desorbed from the high-temperature sites. Adsorption of NO on supported La2O3 resulted in the formation of nitrosyl, nitrite and nitrate species, and their thermal stabilities increased in the following order: NOδ+ (on Al2O3)≈bridged nitrate (on La2O3)<linear and chelated nitrito complexes (on Al2O3)≈NO− (on La2O3)<bidentate nitrate (on Al2O3)≈unidentate nitrate (on La2O3). The distribution of the adsorbed species between La2O3 and Al2O3 surfaces depends on the La2O3 loading. temperature-programmed reaction (TPR) experiments in which adsorbed NO reacted with either CH4 or CH4+O2 showed that the nitrate species which are stable at high temperatures are unlikely to be active intermediates in the catalytic reduction of NO with CH4. It is proposed that either O atoms or NO2 species adsorbed on oxygen vacancies are responsible for the activation of CH4.