STUDY OF THE STRUCTURE OF NIOBIUM OXIDE BY X-RAY-ABSORPTION FINE-STRUCTURE AND SURFACE SCIENCE TECHNIQUES

The structure of niobium oxides was investigated by using XAFS and sur face science techniques. The origin of acidic property and catalysis o f niobium oxide is discussed by these results. From the EXAFS analysis , the structure of highly acidic hydrated niobium oxide is different f rom those of T-Nb2O5 and K8Nb6O19, which has been previously demonstra ted as the structure of hydrated niobium oxide, because Nb-Nb and Nb-O distances are significantly different from each other. There are at l east two types of Nb-O bonds, a tetrahedral form and an octahedral for m are existing for hydrated niobium oxide. The excess of negative char ge is present for Nb-O bond by applying Tanabe's hypothesis to single metal oxide consisting of at least two metal-O forms with different co ordination number. That seems to be the origin of the acidity of hydra ted niobium oxide. The ordered niobium oxides thin film on Pt(111) and their defect face were prepared by using UHV surface analysis apparat us, The oxygen vacancies of niobium oxide on Pt(111) were created by A r+ ion bombardment. H2O and CH3OH exhibit associated adsorption on the ordered NbOx and Nb2O5 surface and dissociative adsorption on the def ect Nb2O5 surface at room temperature. On the ordered NbOx and Nb2O5 s urface, no evidence of ethene adsorption at 200 K is found by HREELS, although ethene adsorbs molecularly on the defect Nb2O5 surface with a pi-complex formation. These results imply that oxygen vacancies of Nb Ox surface play an important role on the adsorption of the molecules.