In situ IR, Raman, and UV-Vis DRS spectroscopy of supported vanadium oxidecatalysts during methanol oxidation

The application of in situ Raman, IR, and UV-Vis DRS spectroscopies during steady-state methanol oxidation has demonstrated that the molecular structu res of surface vanadium oxide species supported on metal oxides are very se nsitive to the coordination and H-bonding effects of adsorbed methoxy surfa ce species. Specifically, a decrease in the intensity of spectral bands ass ociated with the fully oxidized surface (V5+) vanadia active phase occurred in all three studied spectroscopies during methanol oxidation. The termina l V=O (similar to 1030 cm(-1)) and bridging V-O-V (similar to 900-940 cm(-1 )) vibrational bands also shifted toward lower frequency, while the in situ UV-Vis DRS spectra exhibited shifts in the surface V5+ LMCT band (>25,000 cm(-1)) to higher edge energies. The magnitude of these distortions correla tes with the concentration of adsorbed methoxy intermediates and is most se vere at lower temperatures and higher methanol partial pressures, where the surface methoxy concentrations are greatest. Conversely, spectral changes caused by actual reductions in surface vanadia (V5+) species to reduced pha ses (V3+/V4+) would have been more severe at higher temperatures. Moreover, the catalyst (vanadia/silica) exhibiting the greatest shift in UV-Vis DRS edge energy did not exhibit any bands from reduced V3+/V4+ phases in the d- d transition region (10,000-30,000 cm(-1)), even though d-d transitions wer e detected in vanadia/alumina and vanadia/zirconia catalysts. Therefore, V5 + spectral signals are generally not representative of the percent vanadia reduction during the methanol oxidation redox cycle, although estimates mad e from the high temperature, low methoxy surface coverage IR spectra sugges t that the catalyst surfaces remain mostly oxidized during steady-state met hanol oxidation (15-25% vanadia reduction). Finally, adsorbed surface metho xy intermediate species were easily detected with in situ IR spectroscopy d uring methanol oxidation in the C-H stretching region (2800-3000 cm(-1)) fo r all studied catalysts, the vibrations occurring at different frequencies depending on the specific metal oxide upon which they chemisorb. However, m ethoxy bands were only found in a few cases using in situ Raman spectroscop y due to the sensitivity of the Raman scattering cross-sections to the spec ific substrate onto which the surface methoxy species are adsorbed.