The role of water in synthesised hydrotalcites of formula MgxZn6−xCr2(OH)16(CO3) · 4H2O and NixCo6−xCr2(OH)16(CO3) · 4H2O—an infrared spectroscopic study

摘要

Infrared spectroscopy has been used to characterise synthesised hydrotalcites of formula MgxZn6−xCr2(OH)16(CO3) · 4H2O and NixCo6−xCr2(OH)16(CO3) · 4H2O. The infrared spectra are conveniently subdivided into spectral features based (a) upon the carbonate anion (b) the hydroxyl units (c) water units. Three carbonate antisymmetric stretching vibrations are observed at around 1358, 1387 and 1482 cm−1. The 1482 cm−1 band is attributed to the CO stretching band of carbonate hydrogen bonded to water. Variation of the intensity ratio of the 1358 and 1387 cm−1 modes is linear and cation dependent. By using the water bending band profile at 1630 cm−1 four types of water are identified (a) water hydrogen bonded to the interlayer carbonate ion (b) water hydrogen bonded to the hydrotalcite hydroxyl surface (c) coordinated water and (d) interlamellar water. It is proposed that the water is highly structured in the hydrotalcite interlayer as it is hydrogen bonded to both the carbonate anion, adjacent water molecules and the hydroxyl surface.