FTIR study of low-temperature CO adsorption on MgAl-hydrotalcite and its calcined forms

The evolution of the Lewis acidity of a MgAl hydrotalcite sample (Mg/Al ratio of 4) during thermal treatment in vacuo at different temperatures was followed by means of FTIR spectroscopy of CO adsorbed at 85 K. The materials were also characterized by XRD, TG-DTA, and BET measurements. It was found that samples activated up to 623 K are characterized by weakly acidic protonic sites, the density of which sharply decreases at higher activation temperatures. This is associated with the water loss of the sample around this temperature. Lewis acid sites are detected on the surface after activation at 573 K. Their concentration increases with the activation temperature up to 873 K and then slightly decreases due to the sample sintering. A separate MgO phase was detected only after 923 K activation. (CO)-C-12-(CO)-C-13 coadsorption experiments evidence a small dynamic shift of v(CO) of 3.5 cm(-1), which indicates that the Lewis acid sites are cations located on regular crystal planes.