Mixed oxides obtained from Co and Mn containing layered double hydroxides: Preparation, characterization, and catalytic properties |
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Authors: | František Kovanda Tomáš Rojka Vladimír Machovi? Lucie Obalová Tomáš Grygar |
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Institution: | a Department of Solid State Chemistry, Institute of Chemical Technology, Technická 5, 166 28 Prague, Czech Republic b Laboratory of Molecular Spectroscopy, Institute of Chemical Technology, Technická 5, 166 28 Prague, Czech Republic c Institute of Inorganic Chemistry, Academy of Sciences of Czech Republic, 250 68 ?e?, Czech Republic d Technical University of Ostrava, 17. listopadu 15, 708 33 Ostrava, Czech Republic e Institute of Chemical Process Fundamentals, Academy of Sciences of Czech Republic, Rozvojová 135, 165 02 Prague, Czech Republic |
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Abstract: | Co-Mn-Al layered double hydroxides (LDHs) with various Co:Mn:Al molar ratios (4:2:0, 4:1.5:0.5, 4:1:1, 4:0.5:1.5, and 4:0:2) were prepared and characterized. Magnesium containing LDHs Co-Mg-Mn (2:2:2), Co-Mg-Mn-Al (2:2:1:1), and Co-Mg-Al (2:2:2) were also studied. Thermal decomposition of prepared LDHs and formation of related mixed oxides were studied using high-temperature X-ray powder diffraction and thermal analysis. The thermal decomposition of Mg-free LDHs starts by their partial dehydration accompanied by shrinkage of the lattice parameter c from ca. 0.76 to 0.66 nm. The dehydration temperature of the Co-Mn-Al LDHs decreases with increasing Mn content from 180 °C in Co-Al sample to 120 °C in sample with Co:Mn:Al molar ratio of 4:1.5:0.5. A subsequent step is a complete decomposition of the layered structure to nanocrystalline spinel, the complete dehydration, and finally decarbonation of the mixed oxide phase. Spinel-type oxides were the primary crystallization products. Mg-containing primary spinels had practically empty tetrahedral cationic sites. A dramatic increase of the spinel cell size upon heating and analysis by Raman spectroscopy revealed a segregation of Co-rich spinel in Co-Mn and Co-Mn-Al specimens. In calcination products obtained at 500 °C, the spinel mean coherence length was 5-10 nm, and the total content of the X-ray diffraction crystalline portion was 50-90%. These calcination products were tested as catalysts in the total oxidation of ethanol and decomposition of N2O. The catalytic activity in ethanol combustion was enhanced by increasing (Co+Mn) content while an optimum content of reducible components was necessary for high activity in N2O decomposition, where the highest conversions were found for calcined Co-Mn-Al sample with Co:Mn:Al molar ratio of 4:1:1. |
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Keywords: | Layered double hydroxides Hydrotalcite-like compounds Thermal decomposition Cobalt-manganese mixed oxides VOC combustion N2O decomposition |
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