Vibrational spectroscopy of bulk and supported manganese oxides

Manganese oxide catalysts, both bulk and supported on gamma-Al2O3 and SiO2, have been studied by Raman and FTIR spectroscopies, their phase composition being determined by X-ray diffraction. The supported catalysts were prepared via pore volume impregnation from nitrate precursors, the atomic ratio of manganese to aluminium and silicon, respectively, being in the range from 0.5/100 to 18/100. The use of Raman analysis in the microscopic configuration allowed the spectra to be taken at different points of the surface and revealed the inhomogeneity of the catalyst. Besides the Raman features of the beta-MnO2 and alpha-Mn2O3 phases, other signals were assigned to isolated Mn2+ ions accommodated in tetrahedral vacancies on the support surface and to some epitaxial layers of gamma-Mn2O3 and manganese silicate, respectively. The FTIR spectra, though not as useful because of the strong bands of the support that overlap those of manganese oxides, support these findings.