Tailoring perovskite surface composition to design efficient lean NOx trap Pd–La1-xAxCoO3/Al2O3-type catalysts (with A = Sr or Ba)

Abstract

Here we report the influence of surface composition on the NOx removal efficiency of lean NOx trap Pd–La1-xAxCoO3/Al2O3-type catalysts. Three catalysts were prepared by the sequential impregnation of 30 wt.% of La1-xAxCoO3-type perovskites and 1.9 wt.% of Pd over alumina. The following perovskite compositions were used: La0.7Sr0.3CoO3, La0.7Ba0.3CoO3 or La0.5Ba0.5CoO3. The results of X-Ray diffraction, N2 adsorption-desorption at −196 °C, electron microscopy, temperature programmed techniques, and Raman and X-ray photoelectron spectroscopies demonstrated that lanthanum partial substitution by barium promoted the presence of Ba-based phases homogeneously distributed at the surface. This fact together with the higher basicity of Ba than Sr led to an increase of the surface basicity for Ba-doped samples. Likewise, Ba doping also favors the formation of small PdO particles homogenously distributed over the surface in close contact with the perovskite phase. Hence, the interactions between Pd and surface basic sites were also promoted. As a result, 1.9 wt.% Pd–30 wt.% La0.5Ba0.5CoO3/Al2O3 catalyst exhibited the highest NOx adsorption and reduction efficiency. Specifically, the NO global conversion and nitrogen production were as high as 90 % and 72 % at 350 °C, respectively. The enhancement of NOx storage capacity during the lean period is mainly assigned to the displacement of gas/solid equilibrium between NO2 and the available NOx adsorption sites due to the presence of higher concentration of basic sites at the surface. Meanwhile, the promotion of the NOx reduction capacity is due to the higher strength of NOx adsorbed species, which slows down the decomposition rate of NOx adsorbed species. Furthermore, the high proximity of Pd and perovskite phase favors the intermediate compounds diffusion. These results confirmed the excellent NOx removal efficiency of the 30 wt.% La0.5Ba0.5CoO3-based catalyst, even above than Pt-based model catalyst.