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

Abstract

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 D 3 h to C 2 s 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 NO3-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 ν1 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 T d to C 2 v . 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 .