Preparation and Testing of Ab2o4 Spinels (a=Co, Cu, Mn; B=Fe, Cr) for Methane Combustion in Lean Mixtures

Salvador Ordóñez, Elisa F. Paz, Arcadio Fernández, and Fernando V. Díez. Chemical Engineering and Environmental Technology, University of Oviedo, Facultdad de Química, c) Julián Clavería s/n, Oviedo, 33006, Spain

Spinel-type oxides have been shown as promising catalysts for the total oxidation of methane, in terms of both catalytic activity and thermal stability. However, the role of the crystalline structure is not yet clear and relatively few combinations of the trivalent and bivalent cations have been studied. In this work, six different mixed oxides have been prepared and tested for the catalytic oxidation of methane. Fe and Cr were the trivalent metals and Mn, Co and Cu were the divalent ones. These metals have been selected because all of them are active for methane oxidation in the form of single oxides. The catalysts were prepared by co-precipitation from a solution of the nitrates of both metals (one trivalent and one divalent) in the stoichiometric ratio of the spinel. Resulting precipitates were calcined for 2 h at 400, 600 and 800 ºC in order to observe the influence of the calcining temperature in its crystalline structure. Prepared catalysts were characterised using nitrogen physisorption, X-ray diffraction and temperature-programmed reduction. As general trend, XRD analysis show that the crystallinity of the catalysts increases with calcining temperature, samples calcined at 600 and 800ºC show the presence of well-developed spinel structures, whereas samples calcined at 400ºC are the most amorphous. Important differences where found in surface area among the different spinels. Cr-Co and Cr-Mn are the spinels with highest surface area, ranging from 98 m2/g (Cr-Mn calcined at 400ºC) to 21 m2/g (Cr-Co calcined at 800ºC). By contrast, iron spinels present lower surfaces areas (1-10 m2/g), except in the case of the samples calcined at the lower temperature. TPR results show a strong interaction between both metals, leading to the absence of peaks below 900ºC, in most of the samples, even in the case of the poorly crystallised spinels. Combustion experiments were performed in a fixed bed reactor (600 mm long and with 9 mm internal diameter). Light-off curves were obtained by feeding the reactor with 1 L/min (s.t.p.) of 2000 ppmV methane in air (corresponding to typical values found in emissions from coke ovens and composting facilities), and working with a catalyst loading of 1 g. The activity of these catalysts for the combustion of methane was determined by recording their light-off curves from 200ºC to 700 ºC at the abovementioned conditions. Results show that Fe-based catalysts are less active than Cr-based catalyst, being observed a large influence of calcination temperature on catalysts activity. It was observed that Cr-Mn calcined at 800 ºC is the most active catalyst (in spite of its lower surface area, 24 m2/g), whereas Fe-Cu and Fe-Co are the less active catalysts, with conversions below 50 % at 650 ºC. Cr-Co catalysts and Cr-Mn calcined at 400 and 600 ºC have shown intermediate activity.