Vibrational spectroscopy of bulk and supported manganese oxides

Abstract

Manganese oxide catalysts, both bulk and supported on γ-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 β-MnO₂ and α-Mn₂O₃ phases, other signals were assigned to isolated Mn²⁺ ions accommodated in tetrahedral vacancies on the support surface and to some epitaxial layers of γ-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.