Theoretical Analysis of d–d Transitions for the Reduced Cr/Silica System

Abstract

Cluster models are constructed for mono- and dinuclear Cr(II) sites and mononuclear Cr(III) sites on the Cr/SiO₂ Phillips catalyst and used to compute d–d transition energies and intensities. Mononuclear pseudo-tetrahedral Cr(II) gives rise to two bands of electric-dipole-allowed d–d transitions, at 8,400 and 12,300 cm⁻¹. This doublet is lowered in energy and intensity as the bond angle about chromium, ∠OCrO, opens up. The dinuclear site gives rise to bands at 5,200 and 10,300 cm⁻¹, consistent with calculations for a mononuclear cluster of comparable value for ∠OCrO. A tri-coordinated Cr(III) cluster shows bands of comparable oscillator strengths at energies of 11,000, 16,000, 18,000–20,000 and 33,000 cm⁻¹. The predicted bands correspond well with d–d bands in experimental diffuse reflectance spectra.