XANES investigations and catalytic properties of coprecipitated AlF3/MgF2 phases

Abstract

Stepwise replacement of aluminium by magnesium in β-AlF₃ phases leads to considerable alterations in the structure and surface properties of the calcination products of co-precipitates. This is accompanied by significant changes in the catalytic activity for heterogeneous chlorine/fluorine exchange reactions. The maximum catalytic activity is reached between 10 and 20 atom% magnesium doping. Due to the decrease of the strength of Lewis acid sites with further increasing the Mg content, the catalytic activity is more and more diminished. The XRD patterns indicate a loss of crystallinity for β-AlF₃ with increasing Mg content. From Mg K-edge XANES and Al K-edge XANES spectra of Mg added β-AlF₃ phases one can conclude a partial incorporation of Mg into the β-AlF₃ lattice. Furthermore, a partial electron transfer from magnesium to the β-AlF₃ lattice resulting in an increased electron deficiency (increased Lewis acidity) at low Mg concentrations can be stated, a result which correlates well with the increased catalytic activity observed for co-precipitated phases with lower Mg contents. The insertion of Mg into the β-AlF₃ lattice is also supported by XPS studies on the same samples which show an increase of the modified Auger parameter for magnesium of about 0.4 eV, which indicates a change of the chemical environment of the ionized magnesium atom.