First principles calculations of the adsorption of NH3 on a periodic model of the silica surface

For the first time, the interaction of a NH3 molecule with a periodic hydroxylated surface resulting from the (100) face of edingtonite has been studied ab initio by both the Hartree-Fock and B3-LYP levels using a localized Gaussian basis set and the fully periodic treatment as coded in the CRYSTAL98 program. For comparison, molecular clusters of different size and topology have also been adopted. The simulation mimics the physical adsorption of NH3 on a highly dehydroxylated amorphous silica surface, that being the OH density of the adopted (100) edingtonite face around 2 OH/nm(2). Geometries, binding energy, and the vibrational frequency of the surface OH groups perturbed by NH3 have been fully characterized. The basis-set dependence of the binding energy has also been addressed using basis sets for the periodic calculation that include both diffuse functions and multiple sets of polarization functions. BSSE and thermal corrections to the binding energy have also been included. Our best estimate of the DeltaH(0)(0) is -31.2 +/- 0.6 kJ/mol for the periodic model, to be compared with the experimental molar heat of adsorption of about -45 kJ/mol and with the isosteric heat of adsorption of about -37 kJ/mol. The anharmonic frequency shift Delta omega (01) caused by NH3 adsorption on the OH stretching mode has been computed to be -663 cm(-1), including estimated corrections for basis set and B3-LYP deficiencies, mechanical coupling, and BSSE. The comparison with the value of -950 cm(-1) measured at 4 K on amorphous silica shows a definite underestimation, in agreement with the too-high computed DeltaH(0)(0). It is concluded that the proposed periodic edingtonite slab behaves satisfactorily as a model for the dehydroxylated surface of siliceous materials, also requiring very few computer resources. Cagelike clusters, when properly designed, give results that are close to those obtained with the periodic model. The remaining differences between the computed properties are due to cooperative interactions of a long-range nature that are not accounted for by the finite cluster.