Catalytic activity of Mn-substituted barium hexaaluminates for methane combustion

Abstract

The catalysts of hexaaluminate (BaMn _x Al_12−x O_19−δ , x = 1.0, 2.0, 3.0, 4.0) to be used in methane combustion have been successfully synthesized by co-precipitation method and supercritical drying. The crystalline structure and surface area of catalyst were characterized by X-ray diffraction (XRD) and nitrogen adsorption analysis of BET method. BET analysis revealed that the preparing and drying method proposed here provides stable materials with higher surface area of 51.4 m²/g in comparison to materials prepared using conventional ambient drying method for BaMn _x Al_12−x O_19−δ calcined at 1200° under oxygen. XRD analysis indicated that formation of a pure single phase BaMn _x Al_12−x O_19−δ occurred up to x = 3 in the case of Mn-substituted barium hexaaluminates. Incorporation of Mn in excess leads to BaAl₂O₄ phase formation. As far as the valence state of Manganese ions was concerned, the introduced Mn ions were either divalent or trivalent. The first Mn ions were introduced in the matrix essentially as Mn²⁺ and only for BaMn₃Al₉O_19−δ does manganese exist exclusively as Mn³⁺; the higher the Mn concentration, the higher the proportion of Mn³⁺. Catalytic activity for methane combustion has been measured for Mn-substituted barium hexaaluminates, light-off temperature was observed in the 512−624°C range. The highest activity was obtained for catalysts containing 3 Mn ions per unit cell, which reveals that the BaMn _x Al_12−x O_19−δ catalyst was a promising methane combustion catalyst with high activity and good thermal stability. Temperature programmed reduction (TPR) under hydrogen has been used to correlate the catalytic activity with the amount of easily reducible species.