Dynamic behavior of supported vanadia catalysts in the selective oxidation of ethane -: In situ Raman, UV-Vis DRS and reactivity studies

The coordination and oxidation states of surface vanadia species on different oxide supports were studied by in situ UV-Vis DRS and in situ Raman spectroscopy. Surface vanadia species remain essentially oxidized during the steady-state ethane oxidation reaction. Polymeric surface vanadia species are more reducible than isolated ones, but this has only a minor effect on the ethane oxidation reactions. It appears that only one surface V site is involved in the rate-determining step for ethane oxidation. The reducibility of supported vanadium oxide species corresponds with the TOF values, but not with the average oxidation state under steady-state reaction. Ceria- and niobia-supported vanadia catalysts do not follow this trend due to solid-state reaction between the surface vanadia species and the oxide support that decreases the number of exposed vanadia sites. This solid-state reaction does not appear to affect the nature of the active site, which is associated with the V-O-Support bond rather than with the terminal V=O bond. (C) 2000 Elsevier Science B.V. All rights reserved.