CuxCeMgAlO mixed oxide catalysts derived from multicationic LDH precursors for methane total oxidation

Abstract

A series of five Cu(x)CeMgAlO mixed oxides with different copper contents (x) ranging from 6 to 18 at. % with respect to cations, but with fixed 10 at. % Ce and Mg/Al atomic ratio of 3, were prepared by thermal decomposition of layered double hydroxide (LDH) precursors at 750 °C. The solid containing 15 at. % Cu, i.e. Cu(15)CeMgAlO, was also calcined at 550 and 650 °C. Powder XRD was used to characterize the crystalline structure and SEM-EDX was used to monitor the morphology and chemical composition of both as prepared and calcined materials. Additionally, the textural properties and the reducibility of the mixed oxide catalysts were studied by nitrogen adsorption/desorption and temperature programmed reduction with hydrogen (H2-TPR) techniques, respectively. X-ray photoelectron spectroscopy (XPS) was used to determine the chemical state of the elements on the catalyst surface and the diffuse reflectance UV–vis spectroscopy, to obtain information about the stereochemistry and aggregation of copper in the Cu-containing mixed oxides. Their catalytic properties in the total oxidation of methane, used as a volatile organic compound (VOC) model molecule, were evaluated and compared with those of an industrial Pd/Al2O3 catalyst. Their catalytic behavior was explained in correlation with their physicochemical properties. Cu(15)CeMgAlO mixed oxide was shown to be the most active catalyst in this series, with a T50 (temperature corresponding to 50% methane conversion) value of only ca. 45 °C higher than that of a commercial Pd/Al2O3 catalyst. This difference becomes as low as ca. 25 °C for the Cu(15)CeMgAlO system calcined at 550 °C. The influences of the contact time and of the methane concentration in the feed gas on the catalytic performances of the Cu(15)CeMgAlO catalyst have been investigated and its good stability on stream was evidenced.