engleski

Chemical synthesis of manganite catalysts for simultaneous oxidation of aromatic compounds

The development of efficient and affordable catalysts for the removal of volatile organic compounds is a pressing issue in environmental protection. Strontium-doped lanthanum manganites, La1-xSrxMnO3 (LSMO) have potential as catalysts, since the level of doping influences both the Mn oxidation state and oxygen non-stoichiometry and thus redox properties and possible catalytic activity of LSMO [1]. LSMO catalysts (x = 0, 0.1, 0.2 and 0.3) were prepared by the citrate-nitrate autocombustion (CNA) and coprecipitation synthesis. Pure manganite phases of desired composition were obtained in all cases, as confirmed by energy dispersive spectroscopy, X-ray diffraction and Rietveld refinement analysis. Oxygen content as calculated from occupancy factors matched the one determined by Mohr's salt permanganate titration. Morphology and porosity of prepared catalysts were investigated by scanning electron microscopy and N2 adsorption/desorption analysis. It was observed that both synthesis methods yield catalysts of similar average pore size diameter and specific surface area, but the pore size distribution differed: CNA-prepared catalysts had a multimodal pore size distribution, while the coprecipitated ones had a single maximum at 4 nm. Furthermore, the LSMO catalysts prepared by the CNA synthesis were voluminous with a porous ‘sponge-like’ morphology, which is expected to allow a better exchange of reactants and products. Catalytic activity was tested on oxidation of BTEX (benzene, toluene, ethylbenzene and o-xylene) gas mixture in the temperature range 373 – 723 K. Catalysts prepared by the CNA method have shown a higher catalytic activity, reaching 100 % conversion in catalytic oxidation of all components except benzene. Temperatures required to reach 50 % conversion were 10 – 50 K higher for catalysts prepared by the coprecipitation method. Catalytic activity was further increased by Sr-doping, as the presence of Mn3+/Mn4+ mixed valences increased their reducibility [2]. Samples prepared by coprecipitation had higher oxygen deficiency, which would decrease their reducibility and thus their activity as oxidative catalysts. The catalytic activity of these samples could be improved by controlling their morphology and crystallite size. References: 1. A. Žužić et al., Open Ceram. 5 (2021) 100063. 2. E.G. Vrieland, J. Catal. 32 (1974) 415-428.

autocombustion synthesis ; coprecipitation ; catalytic oxidation ; doped lanthanum manganites ; volatile organic compounds

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