ANODIC-DISSOLUTION AND REORDERING OF PD(110) INDUCED BY CHEMISORBED IODINE

Adsorbate-induced reordering and anodic dissolution, reported previous ly for Pd(111) and Pd(100) surfaces that contained an ordered iodine a dlattice, were examined at an I-pretreated Pd(110) electrode. Experime ntal measurements were based upon a combination of electrochemistry, e lectron spectroscopy, and scanning tunneling microscopy; remarkable co nsistency was observed between the ex situ and in situ results. Simila rities and differences exist between Pd(110) and the two other low-ind ex planes. The expected congruencies: (i) well-ordered iodine adlattic es (Pd(110)-c(2 x 2)-I and Pd(100)-pseudohexagonal-I) are formed spont aneously upon exposure of a Pd(110) surface to an aqueous solution of iodide, even when the surface was previously disordered by oxidation-r eduction cycles; (ii) anodic dissolution of the metal substrate occurs only in the presence of chemisorbed iodine; (iii) the I-catalyzed cor rosion does not alter the coverage of the iodine adlayer. The notable disparities: (a) reductive desorption of the chemisorbed iodine does n ot yield an ordered (1 x 1) surface; neither does the removal of iodin e by displacement with and subsequent oxidative desorption of CO; (b) anodic stripping of the metal surface disorders the structure of the i odine adlattice; no reordering takes place upon exposure of such disor dered surface to aqueous iodide; (c) the iodine-catalyzed corrosion oc curs selectively at step-edges along the {100} and {110} directions; d issolution at the {110}-directed step develops preferentially over tha t at the {100}-directed edge to form rectangular pits; (d) the propaga tion of new (smaller) pits at the bottom of the (enlarged) rectangular pits leads to progessive surface roughness. (C) 1997 Elsevier Science B.V.