Spinel ferrites as catalysts for light alkanes combustion

Abstract

The Spinel Ferrites of general formula MIIFe2 IIIO4 are advanced ceramics suitable for many technical applications. The preparation of such materials as nanoparticles is of high interest because these exhibits unique improved physicochemical properties suitable for catalytic combustion applications. In this chapter the synthesis, characterization and catalytic activity of the spinel Ferrites MIIFeIII 2O4, (M=Co, Ni and Cu), as catalysts for propane combustion is reported. The bulk catalysts were prepared by soft chemical procedure using the citrate route and characterized by XRD, TPR, FTIR, SEM-EDX, TEM and Mössbauer spectroscopy. The materials are active catalysts for the propane combustion. The temperature value to reach 90% propane conversion to CO2 and H2O is as low as 372ºC in the case of CoFe2O4 annealed at 400ºC, and a decline in activity is observed by increasing the temperature of annealing. The most relevant properties for the catalytic performance of hydrocarbon combustion seem to be: the presence of small particles of inverted spinel in the nanometric scale, with porous morphology, and convenient reducibility behavior. In addition to this, based on Mössbauer spectra, according to the distribution among the octahedral and tetrahedral sites, a partially inverted spinel structure, with higher occupation of the octahedral sites, was found. This feature would favor the availability of catalytic sites, mostly exposed at the surface of the spinel crystallites. The interaction between metallic cations results in the key factor for the catalytic process. The relationship between the magnetic properties of the materials and their catalytic behavior was demonstrated. On the other hand, in the case of the copper ferrite, supported over some ceramic materials (as Al2O3, ZrO2 and zeolites), an improvement in the catalytic activity with a shift of around 150C towards low temperature was observed in this material. The catalytic behavior of ferrospinels is comparable to those of the noble-metal- supported materials possessing the advantage of being much cheaper and more resistant to poisoning.