Ba-doped vs. Sr-doped LaCoO3 perovskites as base catalyst in diesel exhaust purification

Abstract

Perovskites (ABO3) have emerged in recent years as an economic alternative to noble metals for oxidation-reduction reactions in diesel engines aftertreatment systems. Sr-doped La1-xSrxCoO3 perovskites have shown high efficiency in NO oxidation and limited NOx reduction efficiency. Alternatively, in this work, the effects of barium doping on NOx removal efficiency of LaCoO3 perovskite are explored with the aim of developing a more promising perovskite base catalyst. Bulk perovskites were prepared by substituting La3+ with increasing Ba2+ doping levels, i.e. La0.9Ba0.1CoO3, La0.8Ba0.2CoO3, La0.7Ba0.3CoO3, La0.6Ba0.4CoO3 and La0.5Ba0.5CoO3. The prepared catalysts were characterized in terms of crystalline structures identification (XRD), specific surface area (N2 adsorption-desorption at -196 °C), reducibility and oxidation state of Co ions (H2-TPR), concentration and strength of adsorbed oxygen species (O2-TPD) and surface basicity (CO2-TPD). The characterization results suggest that charge imbalance associated to Ba2+ accommodation in the perovskite lattice in substitution of La3+ leads to a preferential formation of oxygen vacancies. As a result, Ba-doped perovskites improve NO-to-NO2 conversion with respect to LaCoO3 perovskite. La0.7Ba0.3CoO3 perovskite shows the best NO oxidation efficiency (66 % at 350 °C). This fact is associated to the higher oxygen vacancies concentration, which favors the exchange capacity between oxygen in the lattice and in the gas phase. This sample also shows the best NOx storage and reduction efficiency (maximum NOx-to-N2 reduction of 40 % at 350 °C). On the one hand, NOx adsorption is promoted due to the best balance between NO oxidation capacity and NOx adsorption sites accessibility. On the other hand, the higher strength of NOx adsorption sites promotes a slower nitrates decomposition, which favors NOx reduction during rich period. The obtained results improve the NOx removal efficiency of reference La0.7Sr0.3CoO3 sample (maximum NOx-to-N2 reduction of 10 % at 350 °C). Thus, La0.7Ba0.3CoO3 catalyst is considered as a more promising base material for automotive applications.