Dipole effect on ethylene epoxidation: Influence of alkali metals and chlorine
Journal article, 2018
Ethylene epoxidation is one of the most important selective oxidation reactions in industry and is almost invariably carried out on silver catalysts. Empirically it is known that moderate selectivity of silver can be greatly enhanced with cesium and chlorine dopants, whereas a mechanistic understanding of the underlying reason remains elusive. In this work, we present a first-principles explanation of the dipole effect on reaction selectivity, which could, in principle, be effected by alkali metals. We show that, on a silver surface, alkali metals work in a similar fashion thanks to their low electronegative character. The selectivity is impacted by inducing electric dipoles on the catalytic surface, whereas the electronic properties of the catalyst remain largely unperturbed. Cesium, as the least electronegative metal, shows the most pronounced effect. This effect is shown to persist when alkali metals are treated as “naked” atoms or, more realistically, when oxidized. Additionally, alkali metals increase activity of the catalyst by favoring the rate-determining step of oxygen dissociation. Chlorine, which is strongly electronegative, acts differently. Subsurface chlorine would increase selectivity through the same mechanism, while surface chlorine stabilizes alkali metals and moderately increases selectivity. Interplay of both dopants is needed to precisely tailor the catalyst for the best performance.