Palladium doped perovskite-based NO oxidation catalysts: the role of Pd and B-sites for NOx adsorption behavior via in-situ spectroscopy

Abstract

Perovskite-based materials (LaMnO3, Pd/LaMnO3, LaCoO3 and Pd/LaCoO3) were synthesized, characterized (via BET, XRD, Raman spectroscopy, XPS and TEM) and their NOx (x = 1,2) adsorption characteristics were investigated (via in-situ FTIR and TPD) as a function of the nature of the B-site cation (i.e. Mn vs Co), Pd/PdO incorporation and H2-pretreatment. NOx adsorption on of LaMnO3 was found to be significantly higher than LaCoO3, in line with the higher SSA of LaMnO3. Incorporation of PdO nanoparticles with an average diameter of ca. 4 nm did not have a significant effect on the amount of NO2 adsorbed on fresh LaMnO3 and LaCoO3. TPD experiments suggested that saturation of fresh LaMnO3, Pd/LaMnO3, LaCoO3 and Pd/LaCoO3 with NO2 at 323K resulted in the desorption of NO2, NO, N2O and N2 (without O2) below 700K, while above 700K, NOx desorption was predominantly in the form of NO + O2. Perovskite materials were found to be capable of activating N–O linkages typically at ca. 550K (even in the absence of an external reducing agent) forming N2 and N2O as direct NOx decomposition products. H2-pretreatment yielded a drastic boost in the NO oxidation and NOx adsorption of all samples, particularly for the Cobased systems. Presence of Pd further boosted the NOx uptake upon H2-pretreatment. Increase in the NOx adsorption of H2-pretreated LaCoO3 and Pd/LaCoO3 surfaces could be associated with the electronic changes (i.e. reduction of B-site cation), structural changes (surface reconstruction and SSA increase), reduction of the precious metal oxide (PdO) into metallic species (Pd), and the generation of oxygen defects on the perovskite. Mn-based systems were more resilient toward B-site reduction. Pd-addition suppressed the B-site reduction and preserved the ABO3 perovskite structure.