The selective catalytic reduction of nitric oxide with methane over La2O3-CaO systems: Synergistic effects and surface reactivity studies of NO, CH4,O-2, and CO2 by transient techniques

Dispersing La2O3 crystallites in a 5 wt% La2O3-CaO mixed oxide system signi ficantly enhances the intrinsic rate of NO reduction by CH4 in the presence of 5% O-2 at 550 degrees C compared to pure La2O3 and CaO phases. A synerg istic effect between La2O3 and CaO crystallites due to doping of lanthana w ith Ca2+ ions at 800 degrees C is largely responsible for the observed cata lytic behavior. Support of this view was provided by photoluminescence stud ies and a large number of transient experiments for determining the surface reactivity of x wt% La2O3/CaO (x wt% = 0, 5, 80, 100) solids toward NO, CH 4, O-2 and CO2. The intrinsic site reactivity of the 5 wt% La2O3-CaO system at 550 degrees C (TOF = 6 x 10(-3) s(-1)) competes favorably with that of other similar oxides for the same reaction reported in the literature. X-ra y diffraction (XRD) and scanning electron microscopy (SEM) techniques have been used for crystal phases and particle morphology characterization of th e x wt% La2O3/CaO mixed oxide system. The information obtained from the XRD measurements was related to that obtained from the surface transient react ivity studies. By the addition of La2O3 crystallites to CaO crystallites in a wet mixing procedure followed by calcination in air at 800 degrees C, re sults in dramatic changes in the chemisorptive properties (amount and bond strength) of NO, O-2, and CO2 compared to the case of pure oxide phases. Pr etreatment of the catalyst surface with H-2 or CH4 was found to strongly af fect the amount of NO chemisorption and the kinetics of its desorption. The se alterations were found to strongly depend on catalyst composition. (C) 2 000 Academic Press.