The effects of excess manganese in nano-size lanthanum manganite perovskite on enhancement of trichloroethylene oxidation activity

Abstract

The effects of excess manganese in LaMn1+xO3+δ perovskite on enhancement of trichloroethylene (TCE) oxidation, as a chlorinated volatile organic compound (CVOC), in air are studied. Various LaMn1+xO3+δ (x=0.0–0.4) nanocatalysts were synthesized by a microwave-assisted “gel combustion” method and calcined at 600°C in air. XRD patterns of the samples confirm formation of perovskite structure and SEM micrographs exhibit spongy agglomerated nanoparticles with a wide size distribution and presence of large pores in their structure. TEM micrographs show the presence of 12–30nm nanoparticles of possibly highly dispersed amorphous and/or fine crystallites of MnOx, which are not detectable in XRD patterns. TPR results indicate that the oxygen overstoichiometry of LaMn1+xO3+δ catalysts increases with excess Mn. Enhancement of BET surface area of the catalyst samples as much as 2.5-fold and the maximum TCE oxidation activity are observed for the LaMn1.2O3+δ catalyst. As compared to LaMnO3+δ, T50 and T90 of TCE oxidation on LaMn1.2O3+δ are decreased by 150 and 174°C, respectively. Loss of BET surface areas of the catalysts after exposure to reaction conditions may be responsible for the observed initial deactivation. XPS spectra show the formation of LaOCl during the initial deactivation. Simple zero-order kinetics expression with respect to TCE, provides fairly good fits for the evolution of the TCE conversion on LaMn1+xO3+δ catalysts.