XAFS study of niobium species formed during the preparation of Nb/SiO 2 catalysts by the equilibrium adsorption method

Abstract

Nb K-edge XAFS has been investigated to clarify the most significant step controlling the dispersion of Nb species in Nb/SiO 2 catalysts prepared by the equilibrium adsorption method from impregnating solutions at various pH. In solution, the Nb ion is present as the [NbO(C 2O 4) 2H 2O] − complex regardless of pH. The Nb complex adsorbed on silica has the same structure as that in a solution of pH 4, while the complex is bonded to silica by an NbOSi bond in a solution of pH 0.8. After calcination at 773 K followed by evacuation, the Nb species on silica is stabilized as NbO 4 tetrahedra and/or its aggregates. The Nb species in a catalyst prepared in solution at pH 0.8 is highly dispersed, because of the formation of a tight bond between the Nb complex and the silica surface during equilibrium adsorption.