Niobia and silica niobia catalysts from sol gel synthesis: an X-ray photoelectron spectroscopic characterization
An X-ray photoelectron spectroscopy study of distinct series of niobia and silica-niobia mixed oxides, prepared via sol-gel at 1:2, 1:10 and 1:20 Nb/Si atomic ratios, is reported. The values of Nb 3d binding energy and the peak shapes for all the investigated samples exclude the presence of oxidation states other than Nb(V) at the surface. In the mixed oxides, the progressive Nb dilution in silica results in an increase in the values of the full width at half peak maximum of Nb 3d, which is particularly relevant on passing from the 1:10 to the 1:20 sample, while the symmetric shape of the 3d spin-orbit components is preserved even at the higher dilution. This is interpreted as due to a dramatic increase in Nb dispersion in the same sequence, which is further evidenced by a combination of XPS and X-ray-excited Auger results. While a progressive Nb dilution in silica would be expected to result in the narrowing of both the Si 2p and the Si KLL Auger peaks as the Si environment approaches that of pure silica, the effect experimentally observed by means of XPS is surprisingly the opposite. It is proposed that an unexpectedly growing fraction of Si atoms in a distinct environment from silica results at the higher Nb dilution. This fraction is assigned to the Si atoms engaged in Si-O-Nb bridging bonds, on the basis of XPS findings. The dramatic increase in the surface area values experimentally observed on passing from the 1:10 to the 1:20 silica-niobia sample, is therefore proposed to be due to a very high Nb dispersion.