EXAFS spectroscopy as a tool to probe metal–support interaction and surface molecular structures in oxide-supported catalysts: application to Al2O3-supported Ni(ii) complexes and ZrO2-supported tungstates

Abstract

EXAFS spectroscopy is shown as a tool of prime importance to probe the formation of metal–oxygen–support bonds and unravel the surface molecular structure in oxide-supported systems through two examples: (i) a molecular metal complex (Ni(II) bisglycinate) characterized after impregnation and drying on Al₂O₃, and (ii) a tungsten oxide nanophase characterized after deposition on zirconia and high temperature thermal treatment (tungstated zirconia catalysts, i.e. WO_x/ZrO₂). Unlike other spectroscopic techniques, EXAFS at the Ni K-edge proves that a modest thermal activation during the impregnation step triggers the grafting of nickel(II) bisglycinate onto the support: Al next-nearest neighbours are detected when the impregnation is carried out at 60 °C instead of room temperature. Characterization of WO_x/ZrO₂ catalysts shows the presence of W next-nearest neighbours around tungsten, with W–W distances distinctive of edge-shared WO₆ octahedra only. The WO_x overlayer can thus be described as bidimensional, nanometric slabs of 4 to 5 WO₆ units on each side. In these slabs, W octahedra are interconnected to form a more condensed structure than the one present in bulk WO₃ (in which linkage through corners exists). Moreover, EXAFS results conclusively demonstrate that the WO_x overlayer is directly anchored to the ZrO₂ surface by means of W–O–Zr bonds with a W–Zr distance of 3.14 Å.