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

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2014-07-01
Say, Zafer
Dogac, Merve
Vovk, Evgeny I.
Kalay, Yunus Eren
KIM, Chang Hwan
LI, Wei
Ozensoy, Emrah
Perovskite-based materials (LaMnO3, Pd/LaMnO3, LaCoO3 and Pd/LaCoO3) were synthesized, characterized (via BET, XRD, Raman spectroscopy, XPS and TEM) and their NO (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 H-2-pretreatment. NO 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 LaMn03, Pd/LaMnO3, LaCoO3 and Pd/LaCoO3 with NO2 at 323 K resulted in the desorption of NO2, NO, N2O and N-2 (without 02) below 700 K, while above 700 K, NO desorption was predominantly in the form of NO + O-2. Perovskite materials were found to be capable of activating N-0 linkages typically at ca. 550 K (even in the absence of an external reducing agent) forming N-2 and N2O as direct NO decomposition products. H-2-pretreatment yielded a drastic boost in the NO oxidation and NO adsorption of all samples, particularly for the Cobased systems. Presence of Pd further boosted the NO uptake upon H-2-pretreatment. Increase in the NO adsorption of H-2-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 ABO(3) perovskite structure. (c) 2014 Elsevier B.V. All rights reserved.