Reactivity of Supported Vanadium Oxide Catalysts: The Partial Oxidation of Methanol

Abstract

The partial oxidation of methanol was used to probe the reactivity of the surface vanadium oxide redox sites present in supported vanadium oxide catalysts. Formaldehyde was the main oxidation product for all the supported vanadium oxide catalysts operating under differential reactor conditions. The methanol oxidation turnover frequency (TOF) of the surface vanadium oxide phase varies by three orders of magnitude when the support is changed from ZrO2/TiO2 (100 s−1) to SiO2 (10−3). The TOF of the surface vanadium oxide phase supported on Nb2O5 (10−1 s−1) and Al2O3 (10−2 s−1) have intermediate values with the surface vanadium oxide phase on Nb2O5 having a TOF very close to ZrO2 or TiO2. The TOF of the surface vanadium oxide phase on all the oxide supports is essentially independent of the vanadium oxide loading below monolayer coverage. The similar structures of the surface vanadium oxide phase on the different oxide supports as well as the independence of the TOF with respect to vanadium oxide surface coverage suggests that a structural difference is not responsible for the difference in reactivity of the various supported vanadium oxide catalysts. Similar activation energies are observed for all the supported vanadium oxide catalysts (19.6 ± 2.3 kcal/mol) which correspond to the CH bond breaking of the surface methoxy species to form formaldehyde. The similar activation energies and different TOFs of the supported vanadium oxide catalysts with respect to the oxide support imply that the specific oxide support influences the Arrhenius pre-exponential factor. In situ Raman studies during methanol oxidation suggest that the pre-exponential factor is determined by the number of participating surface vanadium oxide sites. The importance of the activity per surface vanadium oxide site in determining the pre-exponential factor was not investigated and may also be significant. The TOF for methanol oxidation is not related to the terminal VO bond strength, but appear to be related to the reducibility (Tmax) of the supported vanadium oxide catalysts. It is proposed that the number of participating surface vanadium oxide sites is most probably related to the reducibility of the V0Support bond since this bridging bond strength controls the reducibility of the supported vanadium oxide catalysts.