Chemistry with F Centers: Reduction of Organic Molecules on the Defective Potassium Chloride(100) Surface

Abstract

The local electronic structure and the resulting reactivity of crystalline surfaces are significantly influenced by the presence of defect centers. The most prominent type of point defects of alkaline-earth halides is the so-called F center, an isolated neutral halide vacancy where the remaining electron is localized at the defect site. In an earlier study [Chen, W.; Tegenkamp, C.; Pfnur, H.; Bredow, T., Phys. Rev. B, 2009, 79, p. 235419.], it has been demonstrated that the defect electron located in NaCl(100) surface F centers can be transferred to adsorbed salicylic acid. In this study, we extend this study to the KCl(100) surface and a wider variety of adsorbed organic molecules to reveal the general trends for this charge transfer. The adsorbed molecules comprise both aromatic annulenes and heteroaromatic systems as well as substituted hydrocarbons. As expected, the tendency toward charge transfer is in line with the molecular electron affinity (EA). However, even adsorbates with a negative EA up to −1.76 eV are reduced due to the strong electrostatic interaction with the defective surface. In particular, the lowest unoccupied orbitals of the adsorbed molecules are lowered near the F center so that electron transfer from the surface defect is facilitated. In some cases, partial electron transfer and mixing of molecular and defect orbitals are observed. In halogenated molecules like dichloromethane, 2,3,7,8tetrachlorodibenzodioxin, or chlorobenzene, halogen bond dissociation and a subsequent healing of the surface defect occur.

Mobility of F Centers in Alkali Halides

“…Final energy calculations were performed with a self-consistent dielectric-dependent global hybrid functional (sc-DDGH) based on PBE, denoted as sc-PBE0. In previous works, ,, we demonstrated that sc-PBE0 accurately describes the electronic properties of alkali halides. The exact Fock exchange fraction x is obtained by a self-consistent calculation based on the inverse of the dielectric constant ϵ ∞ , which is obtained by the response of the solid to an external electrical field.…”

“…The electronic properties of the F center are well studied with a plethora of theoretical methods. − Its reactivity was explored for NaCl and KCl both in experiments − and theoretically using density functional theory (DFT) studies. ,,− It acts as a strong, local reduction site, and readily transfers the defect electron to a wide range of organic molecules. This makes it an interesting candidate for catalytic reactions.…”

Nitrogen Activation on Defective Potassium Chloride and Sodium Chloride

“…Unless denoted otherwise, the GGA functional PBE with D3­(BJ) dispersion correction , was employed for geometry optimizations, as in our previous studies of defective alkali halides. ,, This method overestimates the experimental 0 K lattice parameter of KCl by about 1.5% and reproduces the low-temperature value for NaCl.…”

“…Its efficacy for alkali chlorides was shown in previous works. 17,42,52 The exchange functional contains a selfconsistently optimized Fock exchange fraction x, which is determined via the bulk dielectric constant ε ∞ . For KCl (NaCl), the optimized exchange fraction is x = 46.3% (42.8%).…”

“…This can be achieved in various ways, for example, by adding gaseous alkaline metal, , heating the solid, irradiation, , or through electrolysis. , The reactivity of the F center has been demonstrated experimentally. It is able to reduce different adsorbed molecules on the NaCl (100) surface , and split water. , Furthermore, several theoretical studies confirm the reductive power of the F center. − …”