Catalytic combustion of methane over nickel cobalt based catalysts derived from hydrotalcite precursors.
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The bimetallic Ni-Co mixed oxide catalysts derived from hydrotalcite precursors were investigated for the catalytic combustion of methane in order to determine the kinetic of this oxidative reaction. The 5wt% metal loading catalysts were prepared by co-precipitation method and calcined at 600°C. Their textural properties were analysed by X-ray diffraction (XRD), X-ray fluorescence spectroscopy (XRF), Scanning electron microscopy coupled with X-ray energy dispersive spectroscopy (SEM-XEDS), N2 adsorption-desorption. The 12, 20, 30 and 40wt% metal loading catalysts were prepared and characterized by the former master student Bilal Yousaf. The characteristics of these catalysts agreed with the properties of mixed oxide derived from hydrotalcites described in the literature. Indeed, these mixed oxides display larger surface areas than their hydrotalcite precursors and contain spinel phases. However, their composition shows that a small amount of magnesium is lost during the synthesis and the samples contain a non negligible amount of zinc and copper. The methane oxidation over these catalysts was performed in a tubular reactor and followed by gas chromatography (GC). The methane conversion versus the furnace temperature was calculated based on the integrated peaks of the chromatograms. Evolution of the catalysts during the reduction and combustion steps was followed by Ultraviolet-Visible spectroscopy. This technique enabled to observe the two transitions in the cobalt reduction namely Co3+ to Co2+ and Co2+ to Co0. Moreover, it is assumed that cobalt is not totally oxidized during the combustion of methane, therefore the catalyst contains CoO. Moreover, it was observed a modification in the colour for some bimetallic catalysts which became blue/purple at the end of the tests. These blue/purple particles are assumed to be sintering CoO with Al2O3 forming CoAl2O4. Furthermore, the chromatograms and the methane conversion show an oscillation in the methane conversion over some low loading bimetallic Ni-Co catalysts. Finally, methane combustion over some catalysts investigated showed production of carbon monoxide at high temperature. Therefore, assumptions according to the literature to explain these observations are a cyclic oxidation/reduction and/or a deactivation (and recovery) of the catalyst by sintering and coke. Thus, this master thesis shows that these phenomena occurring during the methane combustion over these Ni-Co catalysts should understand before calculating the kinetic of this oxidative reaction with accuracy.