Catalytic oxidation of toluene on Ce-Co and La-Co mixed oxides synthesized by exotemplating and evaporation methods

摘要

Ce-Co and La-Co mixed oxides were synthesized by two different methods: exotemplating and evaporation. The obtained catalysts were evaluated for volatile organic compounds (VOCs) abatement, using toluene as model molecule. The materials were characterized by N-2 adsorption at -196. C, X-ray diffraction (XRD), scanning electron microscopy (SEM), H-2 temperature-programmed reduction (H-2-TPR) and NH3 temperature-programmed desorption (NH3-TPD) in order to reveal the structure-activity relationship. The results obtained showed the superiority of mixed oxides compared to single oxides in toluene oxidation. Ce-Co mixed oxides were more active than La-Co samples. For Ce-Co materials, the exotemplating method produced catalysts which were more active than those prepared by the evaporation procedure. The former showed the best catalytic performances, with full conversion of toluene into CO2 at about 250 degrees C. Temperatures higher than 320 degrees C were required with single oxides. Characterization studies revealed strong interactions between Ce (or La) and Co, leading to a fine dispersion of oxide phases in binary systems. As a result, both the surface area and reducibility of the catalysts increase, which can be accounted for the higher performance of the mixed oxides. Furthermore, NH3-TPD studies showed a linear relationship between acidity and VOC oxidation activity. In fact, a high concentration of weak acid sites is required for high toluene oxidation activity. The results can be explained in terms of a Mars-van Krevelen type of mechanism, involving the adsorption of toluene and its subsequent oxidation by lattice and/or surface oxygen. (C) 2014 Elsevier B.V. All rights reserved.