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

Copper- and manganese-substituted barium hexaaluminates were prepared by so l-gel method from metal alkoxides. The preparation conditions strongly infl uence 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 met hane combustion, but the Cu-based catalysts are penalized by lower surface areas and by the lower limit of copper incorporation in the hexaaluminate m atrix: manganese substitution for aluminium is possible up to approximate t o 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 hexaalum inates increases with Mn substitution, the optimum composition being obtain ed when about 3 Mn ions per unit cell are incorporated, as regards the acti vity and the resistance to ageing at 1200 degrees C in the presence of stea m. 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 T PR and Auger parameter measurements. The catalytic activity can be correlat ed with the fraction of reducible manganese species. (C) 1999 Elsevier Scie nce B.V. All rights reserved.