Halogen Phases on Pd(110): Compression Structures, Domain Walls, and Corrosion

Abstract

Halogens are used as modifiers in catalytic processes, notably including photocatalytic watersplitting; as redox-active species in dye-sensitized solar cells; and as etchants in device fabrication. Atomically resolved studies of the adsorption and reactivity of halogens on Pt and Pd surfaces are scarce in view of the prominent role these metals play in the mentioned applications. The present study reports phases of halogens on Pd(110) in the submonolayer coverage range as monitored by temperature-programmed desorption, low-energy electron diffraction, scanning tunneling microscopy, and density functional calculations. Domain wall and compression structures are observed, as is typical for halogens (except fluorine) on transition and noble metals, but not on Pt, where the bonding is much more site-specific and a different succession of phases is found. The reactivity of Br toward Pd(110) is greater than that of Cl, as judged by the corrosive attack and by the bond strength derived from the thermal desorption temperature. The results confirm that an ionic-bonding picture based on a large metal-to-halogen charge transfer is not valid for Pd and Pt. Rather, the bonding is predominantly covalent, and the reactivity does not obey simple intuitive rules.