Probing the Basicity of Regular and Defect Sites of Alkaline Earth Metal Oxide Surfaces by BF3 Adsorption: A Theoretical Analysis

The basicity of regular and low‐coordinate (LC) sites (steps, edges and corners) at the surface of alkaline earths with NaCl structure (MgO, CaO, SrO, and BaO) has been investigated by using BF3 as a probe molecule. BO and BF distances; OBF bond angles; BF asymmetric stretching frequencies; O, B and F 1s core‐level binding energies; and the interaction energy of adsorbed BF3 were determined by means of DFT calculations on cluster models. These adsorption properties were compared with those of complexes of BF3 with molecules with various basicities (water, ammonia, phosphine, etc.). We show that many properties of adsorbed BF3, and in particular the experimentally accessible shifts in vibrational frequency, in B and F 1s core levels, and in BF3 desorption temperature, exhibit a linear correlation with the surface basicity as measured by the vertical ionization potential of the oxide anions. On the other hand, shifts of the O 1s core level binding energy do not provide a simple way to detect surface basicity. On a given oxide surface, the differeing basicities of various sites result in measurable differences in adsorption properties. This suggests the potential use of BF3 as a probe molecule for titrating LC sites on the surface of ionic oxides. The basicity of terrace, step, edge, and corner sites on the surface of MgO, CaO, SrO, and BaO was investigated theoretically by using BF3 as a probe molecule (the structure of a BF3 adduct adsorbed on a terrace oxide anion is shown in the picture). Many properties of adsorbed BF3, especially the experimentally accessible shifts in BF vibrational frequency, in B and F 1s core levels, and in BF3 desorption temperature, exhibit a linear correlation with the surface basicity as measured by the vertical ionization potential of the oxide anions. However, shifts in the O 1s core‐level binding energy are not quantitative indicators of surface basicity.