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The sol-gel processing method was used as an alternative route to obtain Nb 2 O 5 phase homogenously dispersed in the SiO 2 matrix, improving the thermal stability of the Br^nsted acid sites, Nb-OH and Nb-OH-Si groups. The local niobium structure and the influence of the amount of niobia on the surface of the Nb 2 O 5 /SiO 2 system were studied by XAS and XPS, respectively. For the samples calcined at 423 and 873 K, the 3d 5/2 BE values are at ca. 208.2 eV, indicating an ionic character for Nb(V) species in the SiO 2 matrix, probably associated to Si-O-Nb linkages. The features of Nb K-edge XANES spectra of samples show the absence of Nb1⁄4O species. The Nb K-edge EXAFS oscillations exhibit a shoulder at ca. 5.6 A ̊À1 , which probably arises from Nb-O-Si. This fact corroborates the EXAFS simulation data of the second coordination shell, whose best fitting is achieved with three distances, two Nb-Nb lengths and one Nb-Si.

Author

Maria Suzana P Francisco; Richard Landers; Yoshitaka Gushikem

Publisher

Elsevier Science; Elsevier ; Elsevier Inc.; Elsevier BV (ISSN 0022-4596)

Published

2004

Language

EN