Raman spectroscopy of supported chromium oxide catalysts. Determination of chromium—oxygen bond distances and bond orders

An empirical correlation is described for relating Raman stretching frequencies of chromium—oxygen (Cr—O) bonds to their bond lengths in chromium oxide reference compounds. An exponential fit of crystallographically determined Cr—O bond lengths to Cr—O Raman symmetric stretching frequencies (800–1300 cm–1) is presented along with a relation between Cr—O bond strengths and Raman stretching frequencies. These empirical correlations have led to a systematic method for determining the coordination and bond lengths of chromates. The developed method is illustrated for chromates with unknown local structure in Bi2O3· K2CrO4 sillenite and for different supported chromium oxide catalysts. The obtained results are compared with those previously obtained by X-ray absorption spectroscopy.