Σύνθεση, χαρακτηρισμός και καταλυτική δραστικότητα μικτών πορώδων οξειδίων με βάση το μαγγάνιο

Abstract

The subject of investigation of the present study has been the synthesis and characterization of porous manganese based mixed oxides as well as their catalytic properties in heterogeneous reactions of environmental interest. A group of mesoporous materials was prepared by hydrolysis of the trinuclear complex [Mn3 0 (CH3C0 0 )6(pyr>3]C1 0 4, by adding drop wise deionized water oraqueous solutions of nitrate salts of the elements: Mg, Al, Fe, La, La/Sr, La/Ce and La/Sr/Ce. The precursors of the materials were studied by thermal analysis (Thermal gravimetry TG, Differential thermal analysis DTA, Differential thermal gravimetry DTG). After calcination at 300 400 and 500 °C the materials were tested by N2 adsorption at 77K and powder X-rays diffraction. The same materials after heating at 500 °C were examined in catalytic tests and their surface composition was also investigated by X-rays photoelectron spectroscopy (XPS). The following three catalytic reactions were studied: I) CH4 combustion in a mixture of 4.84% CH4 , 9.68% O2 in He with a GHSV=74000h‘\ Π) interconversion of 2% NO and 2% CO in a mixture in He with a GHSV=54000h'' and finally III) a lean de-NOx reaction in the gas mixture CH4/NO/O2 = 0.67%/0.2%/5% in He with a GHSV=20000h'1. The latter reaction was studied for four materials. In this reaction the influence of 4% H2O in the gas mixture was also investigated. Experiments of NO and O2 temperature programmed desorption were carried out in order to illuminate the factors controllingthe activity of these catalysts. The main remarks of the whole study are summarized as following:□ A new method involving the hydrolysis of the [Mn3 0 (CH3C0 0 )6 (pyr)3 ]C1 0 4 complex for preparing mesoporous and/or microporous mangenese based mixed oxides was applied successfully. The composition and the heating treatment influence the specific surface area and the mean pore size of the materials. Correspondingly, as determined by the vplots, the percentage of microporosity varies. □ The XRD studies showed that after heating treatment at 300 °C - 500 °C, the majority of materials are amorphous with high surface area. In some cases Mn0 2 was apparent after heating at 300°C while Mn20 3 is found after heating at 500 °C. The presence of Mn(III) at 500°C was proved of the XPS studies. α The gradual dropwise hydrolysis of the complex [MnjO(CHjCOO)6 (pyr)3 ]C1 0 4 by deionized water results in mesoporous mangenese oxides with MnC>2 crystal structure and low thermal stability. The presence of nitrates salts of Al, La, La/Sr,L$/Ce and/or La/Sr/Ce increases the specific surface area and improves the thermal stability of the materials. The material prepared in the presence of AIJ+ (Al-ΜηΟχ) possessed a very high surface area 711 m2/g. a The hydrolysis temperature "did'not influence significantly the surface features ofthe materials but the best results obtained at SO °C. Changes in the pH also did not influence significantly the surface features of the materials. As soon as the hydrolysis takes place the solution is buffered by the stoichiometric presence of pyridine and acetic acid that coexist as ligands in the complex coordination sphere. These compounds are readily formed with the addition of H2O. a The surface compositions of the materials, as investigated by XPS, proved the presence of Mn3* but also the presence of the rest of the added elements in lower surface concentration. In all cases except those of the hydrolysis in the presence of iron and cerium nitrate salts where the surface was enriched. Also in all materials the surface was enriched in oxygen. □ All the materials proved very reactive for the catalytic combustion of CH 4 and best case was the La-MnO* catalyst. This fact was attributed to the increased surface area and the surface presence of Mn(III) compared to the rest of the materials. A correlation of the surface presence of Μη(ΠΙ) and catalytic activitywas observed. For this same reaction a comparison between the present materials and catalysts reported in the literature showed that the materials are very active with CH« combustion in very low temperature (e.g. Temperature for the 50% of conversion is 366 °C). 0 For the reaction NO+CO, the conversion of the reactants appears even from room temperature. At low temperatures (<280 °C) the reaction proceeds through the route 2NO + CO -> N2O + CO2 . In higher temperatures the reaction proceeds through the route 2NO + 2CO -» N2 + 2 CO2 . The shift of thereaction path occurs within the same temperature region for all the materials. □ For the NO reduction by CH« in the presence of excess O2. the materials proved active in the temperature region of the diesel engines exhaust gases. The mixed oxide La-Sr-Ce-MnOx in particular found very reactive and efficient to convert NO to N2 . This behaviour was even better when H2O was added to the reaction mixture with selectivity towards N2 up to 96%. This stability was proved even after 20h test on stream. This excellent catalytic behavior was attributed to the interaction of surface with NO and the possible synergistic effect among the crystal phases of Μη2θ 3 and Ce0 2 .□ The catalytic superiority o f the La-Sr-Ce-MnOx, especially in cohiparison with 1% ' wt. RI1-A I2O 3 for the same reaction mixture using data from literature, provides further proof for the excellent catalytic activity of these solids.