Surface chemistry of silica–titania-supported chromium oxide catalysts

Abstract

The interaction of chromium oxide with different titania–silica supports has been examined. The oxide supports investigated consisted of SiO₂, TiO₂, physical mixtures of TiO₂ and SiO₂, a surface titania overlayer on SiO₂(10% of theoretical monolayer) and a surface silica overlayer on TiO₂(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% CrO₃, 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 SiO₂ and the surface silica overlayer on TiO₂. Both polychromates and monochromates are present on the surface titania overlayer on SiO₂, while surface polychromates are primarily present on TiO₂ as well as the TiO₂ and SiO₂ 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 SiO₂. 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 SiO₂.