Acidity of titania-supported tungsten or niobium oxide catalysts Correlation with catalytic activity

International audienceThe acidity of catalytic systems based on tungsten oxide or niobium oxide supported on titania was compared. Two series with metal contents up to 3.6 atom nm2 were prepared by incipient wetness impregnation of the titania support with ammonium metatungstate or niobium oxalate solutions. Characterization of both systems by X-ray diffraction and Raman spectroscopy studies did not show evidence of bulk metal oxide formation. The acidity was monitored by adsorption of 2,6-dimethylpyridine (2,6-lutidine) followed by infrared spectroscopy. The catalytic activity was tested for the reaction of isopropanol dehydration. At a reaction temperature of 403 K, WOx/TiO2 catalysts were inactive for a surface density ofW 1.2 Watom nm2. Above this loading, the activity increased progressively with increasing W content. Similar evolution was observed for the abundance of relatively strong Brønsted acid sites (i.e. able to retain lutidine at 573 K). In contrast, NbOx/TiO2 catalysts were essentially inactive at this reaction temperature and a higher reaction temperature (473 K) was required to reach a comparable catalytic activity. No threshold of Nb loading for the development of catalytic activity was observed. Similar behavior was evidenced for the abundance of medium strength Brønsted acid sites (able to retain lutidine at 523 K). For both systems, a direct correlation between the catalytic activity and the abundance of Brønsted acid sites was observed

Surface structure and catalytic performance of niobium oxides supported on titania

International audienceThe surface structure and activity of catalytic systems based on niobium oxide supported on titania were investigated. A series of catalysts with Nb content up to 3.6 atoms/nm2 was prepared by incipient wetness impregnation of the titania support with niobium oxalate solutions. Characterization of the system by X-ray diffraction (XRD) and Raman spectroscopy studies did not show evidence of niobium oxide formation. Surface analysis was performed by X-ray photoelectron spectroscopy (XPS), ion scattering spectroscopy (ISS), Raman and infrared spectroscopy. These results were correlated with Brønsted acidity and catalytic activity determined in our previous study. A direct correlation between the catalytic activity and the abundance of Brønsted acid sites had been observed. Analysis of the surface structure by infrared spectroscopy showed a similar evolution for the intensity of the band attributed to polymerized NbOx