Surface Chemistry of Silica-Titania-supported Chromium Oxide Catalysts

The interaction of chromium oxide with different titania-silica supports has been examined. The oxide supports investigated consisted of SiO,, TiO, , physical mixtures of TiO, and SiO,, a surface titania overlayer on SiO, (10% of theoretical monolayer) and a surface silica overlayer on TiO, (80% of theoretical monolayer). The supported chromium oxide catalysts were structurally characterized with Raman spectroscopy (RS), diffuse reflectance spectroscopy (DRS) and electron paramagnetic resonance (EPR) and chemically probed by the methanol oxidation reaction. The RS and DRS characterization studies demonstrate that the supported chromium oxide phase is almost exclusively present as a surface Cr6+ overlayer for 1-3% CrO,, but traces of Cr3+ and Cr5+ are also detected. Under ambient conditions, the molecular structures of the surface chromates (monochromates vs. polychromates) are dependent on the net pH at the point of zero charge of the thin aqueous film at the surface of the supports. Under dehydrated conditions, the molecular structures are dependent on the specific support and its composition. Monochromates are dominant on SiO, and the surface silica overlayer on TiO, . Both polychromates and monochromates are present on the surface titania overlayer on SiO, , while surface polychromates are primarily present on TiO, as well as the TiO, and SiO, physical mixtures. These physical mixtures possess mainly polychromates because Cr6+ and Cr5+ preferentially migrate and coordinate to the titania phase. The surface chromate and Cr5+ species also appear to coordinate preferentially to the surface titania sites present in the surface titania overlayer on SiO,. The methanol oxidation probe reaction could not readily discriminate between the different surface chromate coordination sites because of the very similar reactivity and selectivity patterns of all the different surface sites present in the supported chromium oxide catalysts. The reactivity patterns, however, are consistent with the coordination of Cr oxide to the surface titania sites on SiO, .