On the effect of La–Cr–O– phase composition on diesel soot catalytic combustion
Résumé
A series of samples of La–Cr–O– perovskites were designed as catalysts for diesel soot combustion. They were prepared by using co-precipitation method, at ambient temperature using ammonia followed by a hydrothermal treatment (T = 200 °C, P = 20 atm, t = 24 h). All the chromium-containing precursors were then calcined at high temperature to develop the oxide catalyst. Two phase composition 86%LaCrO3–(14%) La2CrO6 or 94%LaCrO3–6%La2O3 were formed depending on the atmosphere of calcination (oxygen or hydrogen respectively) used. Their respective BET surface areas were 1.1 and 6.5 m2 g−1. Highly crystalline, pure phase of LaCrO3 and La2CrO6 powders were also prepared, with BET area of 4 and 3 m2 g−1, respectively. The crystalline structure and properties of all samples were characterised by X-ray diffraction (XRD), using Rietveld refinement, and temperature-programmed reduction (TPR) techniques. O2-TPD measurements performed on all samples showed the presence of suprafacial, weakly chemisorbed oxygen only for LaCrO3, which contributes actively to soot combustion. TPR study performed on all catalysts showed that while pure LaCrO3 and La2O3 samples did not reduce, the biphasic catalysts showed the presence of oxygen depletion peaks characteristic of lattice oxygen mobility in the samples at ca. 665 °C. Catalytic combustion of diesel soot was studied over all catalysts. The results showed that pure LaCrO3 exhibited significant catalytic activity which was sensitively affected by the modifier La2CrO6 or La2O3.