Ordered phases in alkali redistribution during a catalytic surface reaction

Abstract

Reaction fronts in the O₂ + H₂ reaction on a Rh(110) surface predosed with potassium have been shown to be associated with a redistribution of the potassium from the oxygen freed to the still oxygen covered parts of the surface. As stable final state a stationary pattern results under reaction conditions formed by K + O coadsorption islands of macroscopic size. Here low energy electron microscopy (LEEM) in combination with mirror electron microscopy (MEM), photo electron emission microscopy (PEEM) and small area selected LEED (μ-LEED) were used to identify ordered phases in this process in situ and to resolve fine structures in the reduction fronts. In the O₂ + H₂ reaction without coadsorbed alkali metal a (2 × 2)p 2 mg and a c(2 × 6) were identified besides the c(2 × 8)–O and the (1 × 1) representing oxygen covered and oxygen freed surface, respectively. With coadsorbed potassium one finds in the front region a (2 × 2)p 2 mg and further inside the oxygen covered area a dominant (1 × 2) reconstruction with satellite spots reflecting a (n × 2) K + O coadsorption structure with n = 8–12. In the stationary pattern a (8 × 2) − K + O structure forms the core of the coadsorption islands while the boundary region exhibits a (2 × 2)p 2 mg − K + O overlayer as ordered phase.