Catalytic Activity for Methane Flameless Combustion and Thermal Stability of Nano-sized Lanthanum Cobaltites Doped with Ce, Pr and Tb

摘要

High surface area nano-sized catalysts with the general formula La_(0.9)M_(0.1)CoO_3 (M=Ce, Pr, Tb) were synthesised by a novel flame-spray-pyrolysis method. All the prepared samples possessed the LaCoO_3 perovskite-like structure and consisted of 30-60 nm particles, but existing as large (80-200 nm) agglomerates. Their surface area ranged from 45 to 60 m~2/g. All the catalysts showed a very high activity for methane flameless combustion, attaining 100% conversion at temperatures T_f, between 495 and 515 °C, depending on the nature of the M doping ion. The results of life-tests did not show any decreasing of catalytic activity after 50 h under the reaction conditions at Tf. The thermal stability under high temperature application was investigated by overheating the catalysts twice at 800 °C for 1 hr in a flowing reacting gas mixture, fast deactivation cycles, and then measuring methane conversion at T_f. All the catalysts showed a significant decrease of activity, depending on the chemical composition. Taking account of the results of SEM analysis, this was attributed to sintering, connected with the Gibbs instability of nano-sized materials.