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摘要
The monomeric, single-atom oxochromium species present on the surface of silica-supported Cr(VI) catalysts was characterized in detail using resonance Raman (RR) spectroscopy over a range of excitation wavelengths corresponding to the primary electronic transitions of Cr(VI)SiO2. The findings resolve a long-standing controversy regarding the possible contribution of mono-oxoCr(VI) sites, (SiO)(4)Cr=O, postulated to coexist with the well-established dioxoCr(VI) sites, (SiO)(2)Cr(=O)(2). Density functional theory (DFT) calculations and a normal coordinate analysis conducted using a chromasiloxane model cluster confirm prior assignments of bands in the nonresonant Raman spectrum at 986 and 1001 cm(-1) to the symmetric and antisymmetric stretching modes, respectively, of the dioxoCr(VI) sites. For all excitation energies, the symmetric stretch shows apparent resonant enhancement. Since all of the electronic transitions are strongly allowed, this finding is consistent with A-term enhancement. UV excitation at 257 nm (into the high energy electronic transition centered at 271 nm) also results in modest resonant enhancement of the antisymmetric stretch, due to the low average symmetry of the surface sites. Excitation at 351 nm (into the electronic transition centered at 343 nm) results in a strong increase in the relative intensity of the antisymmetric stretch, which is likely caused by B-term enhancement. Previously reported evidence for a mono-oxoCr(VI) site consists of a vibrational band observed at ca. 1011 cm(-1) and assigned to its Cr=O stretch. However, the band is observed only upon excitation into the lowest-energy electronic transition, at 439 nm. We show that excitation into this electronic transition causes photoinduced decomposition. The process depends on the laser power and duration of exposure, and it yields the band previously assigned to a mono-oxo species. The resonance Raman study reported here, in combination with our recent rigorous analysis of the corresponding electronic spectra, lead us to conclude that there is no credible spectroscopic evidence for the existence of mono-oxochromate species in highly dispersed Cr/silica materials.
- 出版日期2018-8-2
