Key factors in Sr-doped LaBO3 (B = Co or Mn) perovskites for NO oxidation in efficient diesel exhaust purification

摘要

Perovskites have attracted attention in recent years as an economic alternative to noble metals in oxidation processes. Synthesis conditions of LaCoO3 and LaMnO3 perovskites have been studied varying citrate to nitrate molar ratio in the starting solution, pH and calcination protocol, with the aim of obtaining high purity perovskites, absence of impurities, and with enhanced textural properties. Once synthesis conditions were established, strontium was incorporated in the perovskite lattice as a textural and structural promoter, by substituting lanthanum with different doping levels, i.e. La0.9Sr0.1BO3, La0.8Sr0.2BO3, La0.7Sr0.3BO3, La0.6Sr0.4BO3 and La0.5Sr0.5BO3 with B = Co or Mn. The prepared solids were characterized in terms of crystalline phase identification (XRD), specific surface area (N-2 adsorption-desorption at -196 degrees C), reducibility and oxidation state of transition metal ions (H-2-TPR), quantification of adsorbed oxygen species (O-2-TPD) and surface elemental composition (XPS). Charge imbalance associated to strontium (Sr2+) incorporation in the perovskite lattice in substitution of lanthanum (La3+) was preferentially balanced by Mn4+ promotion in La1-xSrxMnO3 perovskites, whereas formation of oxygen vacancies seems to be the mechanism for charge compensation in La1-xSrxCoO3 perovskites, where Co ions remained as Co3+ ions. Strontium doped perovskites further improved NO conversion compared to the non-substituted formulations. The best NO oxidation performance was obtained with La0.7Sr0.3CoO3 and La0.9Sr0.1MnO3 samples, achieving maximum NO conversion of 83 and 65% at 300 and 325 degrees C, respectively. Higher oxidation capacity of La0.7Sr0.3CoO3 sample was associated to the higher oxygen mobility and exchange capacity between oxygen in the lattice and gas phase oxygen. It is worth noting that prepared perovskites presented far higher NO oxidation capacity than platinum-based NSR model catalysts, confirming perovskites as an economic alternative to catalyze NO oxidation reactions in automotive catalysis. (C) 2017 Elsevier B.V. All rights reserved.