Infrared and Raman study of interlayer anions CO₃^2–, NO₃^–, SO₄^2– and ClO₄^– in Mg/Al-hydrotalcite

Abstract

The difference in the local environment of CO₃^2-, NO₃^-, SO₄^2-, and ClO₄^- in Mg/Al-hydrotalcite compared to the free anions was studied by infrared and Raman spectroscopy. In comparison to free CO₃^2- a shift toward lower wavenumbers was observed. A band around 3000-3200 cm^-1 has been attributed to the bridging mode H₂O-CO₃^2-. The IR spectrum of CO₃^- hydrotalcite clearly shows the split ν₃ band around 1365 and 1400 cm^-1 together with weak ν₂ and ν₄ modes around 870 and 667 cm^-1. The ν₁ mode is activated and observed as a weak band around 1012 cm^-1. The Raman spectrum shows a strong ν₁ band at 1053 cm^-1 plus weak ν₃ and ν₄ modes around 1403 and 695 cm^-1. The symmetry of the carbonate anions is lowered from D_3h to C_2s resulting in activation of the IR inactive ν₁ mode around 1050-1060 cm^-1. In addition, the ν₃ shows a splitting of 30-60 cm^-1. Although NO₀ hydrotalcite has incorporated some CO₃^2- the IR shows a strong ν₃ mode at 1360 cm^-1 with a weak band at 827 cm^-1, and the ν₄ band is observed at 667 cm^-1, although it is largely obscured by the hydrotalcite lattice modes. The Raman spectrum shows a strong ν₁ mode at 1044 cm^-1 with a weaker ν₄ band at 712 cm^-1. The ν₃ mode at 1355 cm^-1 is obscured by a broad band due to the presence of CO₃^2-. The symmetry of NO₃^- did not change when incorporated in hydrotalcite. The IR spectrum of SO_4-hydrotalcite shows a strong ν₃ at 1126, ν₄ at 614 and a weak v₁ mode at 981 cm^-1. The Raman spectrum is characterized by a strong ν₁ mode at 982 cm^-1 plus medium ν₂ and ν₄ bands at 453 and 611 cm^-1; ν₃ cannot be identified as a separate band, although a broad band can be seen around 1134 cm^-1. The site symmetry of SO₄^2- is lowered from T_d to C_2ν. The distortion of ClO₄^- in the interlayer of hydrotalcite is reflected in the IR spectrum with both ν₃ and ν₄ bands split around 1096 and 1145 cm^-1 and 626 and 635 cm^-1, respectively. A weak ν₁ band is observed at 935 cm^-1. The Raman spectrum shows a strong ν₁ mode at 936 cm^-1 plus ν₂ and ν₄ bands at 461 and 626 cm^-1, respectively. A ν₃ mode cannot be clearly recognized, but a broad band is visible around 1110 cm^-1. These data indicative a lowering of symmetry from T_d to C_s.