Mixed oxides obtained from Co and Mn containing layered double hydroxides: Preparation, characterization, and catalytic properties

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 Xray 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 1C in Co–Al sample to 120 1C 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 1C, 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.