Influence of grinding on the catalytic properties of oxides

Abstract

The influence of grinding on the structure and catalytic performances of two families of oxides, namely the perovskite-type La 0.8Sr 0.2MnO 3± λ and the spinel-type Li–Mn–O, was investigated. Ball-milling of the well-known La 0.8Sr 0.2MnO 3± λ prepared either by a solid state reaction or by a sol-gel method led to (i) a decrease in carbon black combustion temperature of 100 °C ( T C=540 °C) and of 40 °C ( T C=505 °C) for ceramic and sol-gel ground 5-h samples, respectively, and (ii) to faster kinetics and higher rates of toluene conversion. A greater enhancement of the catalytic performances was obtained by using mechanical milled lithiated manganese oxides that are reported as promising catalyst candidates for the first time. Li–Mn–O catalysts were synthesized by room temperature mechanochemistry of a stoichiometric mixture of Li 2O and MnO 2 using various milling times (0< t milling<15 h). The nonstoichiometry, large surface area and disorder nature of the ground samples were of great benefit regarding catalytic applications. A remarkable decrease in the carbon black combustion temperature of 200 °C (from 650 to 450 °C) was obtained when using a mixture of Li 2O and MnO 2 ground for 3 or 4 h. This low T C value favorably compares with the T C of 500 °C of ceramic LiMn 2O 4, which shows, however, better catalytic performances than most of the perovskite-type oxides. The grinding proves to be efficient as well for volatile organic compounds (VOCs) combustion. The inactive ceramic LiMn 2O 4 exhibits a 100% toluene conversion rate for a temperature lower than 200 °C when ground 5 h.