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 fr equencies of chromium-oxygen (Cr-O) bonds to their bond lengths in chr omium oxide reference compounds. An exponential it of crystallographic ally determined Cr-O bond lengths to Cr-O Raman symmetric stretching f requencies (800-1300 cm(-1)) is presented along with a relation betwee n Cr-O bond strengths and Raman stretching frequencies. These empirica l correlations have led to a systematic method for determining the coo rdination and bond lengths of chromates. The developed method is illus trated for chromates with unknown local structure in Bi2O3 . K2CrO4 si llenite and for different supported chromium oxide catalysts. The obta ined results are compared with those previously obtained by X-ray abso rption spectroscopy.