Infrared and Raman study of interlayer anions CO32−, NO3−, SO42−and ClO4−in Mg/Al-hydrotalcite

The difference in the local environment of CO32-, NO3-, SO42-, and ClO4- in Mg/Al-hydrotalcite compared to the free anions was studied by infrared and Raman spectroscopy. In comparison to free CO32- a shift toward lower wavenumbers was observed. A band around 3000-3200 cm-1 has been attributed to the bridging mode H2O-CO32-. 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 D3h to C2s 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 CO32- 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 CO32-. The symmetry of NO3- did not change when incorporated in hydrotalcite. The IR spectrum of SO4-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 SO42- is lowered from Td to C2ν. 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 Td to Cs.