The Interaction of Bromide Ions with Pd(100) Single-Crystal Electrode Surfaces: Studies by UHV-EC

The influence of solution pH and electrode potential on the structure, composition, thermal stability and electrochemical reactivity of chemisorbed bromine on well-defined Pd(100) electrode surfaces was investigated by a combination of electrochemistry and ultrahigh vacuum surface spectroscopy. Br- ions chemisorbed oxidatively to form zerovalent Pd(100)- (2x2)-Br and Pd(100)-c(2x2)-Br adlattices. In acidic and neutral solutions, the surface coverages increased as the electrode potential was made more positive. The presence of Br- in solution induced anodic dissolution of Pd at pH < 7; however, at high overpotentials, the formation of a passivating film of PdBr2 precluded further surface dissolution. The surface coordination chemistry of Pd(100) exposed to Br- solutions at pH 10 resembled that of homogeneous halo-Pd complexes in which the only stable species at very alkaline solutions were metal hydroxides and/or metal-hydroxo complexes; that is, no Br chemisorption transpired on Pd(100) at pH 10.