In situ Raman spectroscopy of phase transformation in CrOx-Y2O3 system at elevated temperatures

A CrOx-Y2O3 sample was prepared by a deposition–precipitation method and phase transformation of the sample under N2 and air atmospheres was characterized by in situ Raman spectroscopy and X-ray diffraction (XRD) techniques. It was found that when the CrOx-Y2O3 sample was heated, CrO3 transformed to YCrO4 and then to YCrO3 and Cr2O3. Also, the transformation started from the surface region of the sample and then extended to the bulk, due to the fact that the phase transformation was detected by Raman spectroscopy at lower temperature compared to that by XRD. In addition, both atmosphere and temperature had influence on the phase transformation in the surface region, while the phase transformation in the bulk was merely dependent on the temperature. It was also found that low oxidation state Cr(III) species on the surface could be re-oxidized to high oxidation state Cr(V) or Cr(VI) species when the thermal treated sample was exposed to ambient air.