Catalytic combustion of methane over copper- and manganese-substituted barium hexaaluminates

Copper- and manganese-substituted barium hexaaluminates were prepared by sol-gel method from metal alkoxides. The preparation conditions strongly influence the textural properties of the solids obtained. Manganese and copper occupy different crystallographic sites in the hexaaluminate structure: Mn3+ ions are located in octahedral sites, while Cu2+ enters only tetrahedral positions. The Cu sites are intrinsically more active than Mn sites for methane combustion, but the Cu-based catalysts are penalized by lower surface areas and by the lower limit of copper incorporation in the hexaaluminate matrix: manganese substitution for aluminium is possible up to approximate to 3 Mn per unit cell, while copper substitution is limited to about 1.3 Cu per unit cell. The catalytic activity of the Mn-substituted barium hexaaluminates increases with Mn substitution, the optimum composition being obtained when about 3 Mn ions per unit cell are incorporated, as regards the activity and the resistance to ageing at 1200 degrees C in the presence of steam. This is related not only to the Mn content, but also to the higher Mn3+/Mn2+ ratio when the amount of manganese introduced increases, as shown by TPR and Auger parameter measurements. The catalytic activity can be correlated with the fraction of reducible manganese species. (C) 1999 Elsevier Science B.V. All rights reserved.