The structure of supported and unsupported vanadium oxide under calcination, reduction and oxidation determined with XAS

A vanadium oxide powder catalyst supported on titanium oxide (VOx/TiO2(anatase)) was investigated with in situ X-ray absorption spectroscopy (XAS). This system is applied in industry for partial oxidation processes. The structural changes of the supported vanadium oxide due to different treatments were determined: during a reduction-oxidation cycle at 623 K after calcination in air and after heating in inert atmosphere. Unsupported crystalline V2O5 powder was used as a reference system. After calcination in air of the VOx/TiO2(anatase) a V2O5 bulk crystal structure appears, while an octahedral co-ordination with a VO vanadyl bond is present after heating in inert atmosphere. The redox cycle performed on fully oxidized supported vanadium oxide induces similar structural changes as for unsupported V2O5: a reduction to V4+ with a slightly distorted VO octahedron and a re-oxidation to the initial V2O5 structure. However, when the supported vanadium oxide is reduced after heating in inert, a valence lower than V4+ is obtained and a different structure is found: a symmetrical VO octahedron is present with VO and VV distances identical to the TiO and TiTi distances in TiO2(anatase or rutile). After re-oxidation the V2O5 bulk structure is again obtained for the supported vanadium oxide.