Molecular Structure and Reactivity of the Group V Metal Oxides

ISRAEL E. WACHS; Y. CHEN; JIH-MIRN JEHNG; LAURA E. BRIAND; TSUNEHIRO TANAKA

CATALYSIS TODAY

Elsevier

Lugar: Amsterdam; Año: 2003 vol. 78 p. 13 - 24

0920-5861

The physical, electronic and reactivity properties of bulk and supported Group V metal oxides (V, Nb, Ta and Db) were compared at the molecular level. Dubnium is a very short-lived element, whose properties have not been extensively studied, but can be predicted from knowledge of the other members of the Group V metal oxides. Bulk V2O5 possesses platelet morphology with the active surface sites only located at the edges: primarily surface redox sites and some surface acidic sites. Bulk Nb2O5 and Ta2O5, as well as to be expected for bulk Db2O5, possess isotropic morphologies and the active surface sites relatively homogeneously dispersed over their surfaces: only surface acidic sites. However, the bifunctional bulk V2O5 was found to exhibit a much higher specific acidic catalytic activity than the acidic bulk Nb2O5 and Ta2O5, the latter being almost identical in their specific acidic catalytic activity. The bulk properties of the Group V metal oxides were essentially transferred to the analogous supported Group V metal oxides, where the active Group V metal oxides were present as a two-dimensional monolayer on various oxide supports. For supported vanadia catalysts, the active surface sites were essentially redox sites, with the exception of supported V2O5/Al2O3 that also contained strong acidic sites. For supported niobia and tantala catalysts, as well as to be expected for supported dubnia catalysts, the active surface sites were exclusively acidic sites. However, the TOFredox for the supported vanadia catalysts and the TOFacidic for the supported niobia and tantala catalysts varied over several orders of magnitude as a function of the specific oxide support with the electronegativity of the oxide support cation. However, the TOFredox varied inversely to that of the TOFacidic variation because of the opposite requirements of these active surface sites. Surface redox sites are enhanced by reduction and surface acidic sites are enhanced by stabilization (lack of reduction). The current fundamental understanding of the Group V metal oxides allows for the molecular engineering of their metal oxide applied catalytic materials.