Adsorption of Formamide on Kaolinite Surfaces: A Combined Infrared Experimental and Theoretical Study

The interaction of formamide with kaolinite has been studied using transmission Fourier transform-infrared spectroscopy (FT-IR) and temperature programmed desorption (TPD) under ultra-high vacuum conditions. The theoretical harmonic infrared frequencies have been calculated using M05-2X/6-31G(d), 6-31G(d,p), and 6-31+G(d,p) levels of density functional theory, and general agreement with experiment has been found for condensed formamide and the formamide–kaolinite surface complexes. The kaolinite OH band positions change as a function of temperature, which have been previously correlated with surface reconstructions and changes in interlayer bond distances in the kaolinite. The temperature dependence of the kaolinite–OH and formamide vibrational frequencies can be sensitive probes of formamide interactions with the exposed and interlayer surfaces of kaolinite. Samples dosed directly with formamide show that the C═O stretching feature is broadened and shifts to lower frequency at low temperatures (67–90 K). The NH and CH stretches of formamide are also very weak at these temperatures. We interpret these features as manifestations of hydrogen bonding between the Al–OH octahedral kaolinite face (inner surface OH group) and formamide’s carbonyl group. Intercalation and subsequent bonding may be facilitated by the increased O–H···O bond lengths between kaolinite lattices and increased availability of Al–OH sites in the low-temperature regime. At 100–110 K, the C═O feature sharpens and shifts to a higher frequency, while the NH and CH stretches become more distinct. With increasing temperature, formamide diffusion and formamide–formamide interactions become more dominant. TPD analysis gives a mean binding energy of 11.7 ± 0.24 kcal/mol, which agrees well with the calculated values of adsorption energy and indicates strong physisorption.