Temperature-dependent of the electro-oxidation of the irreversibly chemisorbed as on Pt(111)

An overlayer of chemisorbed arsenic, As-chem, On Pt(111) having a surface c overage of theta (As) is prepared, and the impact of T variation on the sur face-oxidation behavior in 0.5 M aqueous H2SO4 is examined. Temperature var iation in the 273-313 Krange does not affect the AS(chem) surface coverage, but the cyclic-voltametry (CV) profiles undergo qualitative changes. The a nodic and cathodic peaks shift toward more-positive potentials, and a charg e-density redistribution between two anodic and cathodic features is observ ed, the latter effect being more pronounced in the anodic scans. At T = 313 K, the CV transients are symmetrical with respect to the potential axis. I n the case of AS(chem) overlayers having theta (As) well below 0.33, the CV profiles representative of the AS(chem) oxidation reveal only one peak. An analysis of the possible reaction pathways indicates that the surface oxid ation proceeds in one step involving transfer of three electrons and additi on of three OH groups, thus in formation of As(OH)(3chem). An analysis of t he As3+ and OH- radii indicates that the As(OH)(3chem) layer is densely pac ked. The existence of two anodic and two cathodic peaks in CV transients is explained in terms of formation two energetically slightly different struc tures of AS(OH)(3chem), their origin being two different ways of coordinati ng AS(OH)(3chem) to Pt(lll). The standard enthalpy of the surface process i s determined from the slope of the E/T versus 1/T plots, and the standard e nthalpy of formation of As(OH)(3chem), DeltaH(f)(o)(As(OH)(3chem)), is foun d to be -678 kJ mol(-1). The value of DeltaH(f)(o)(As(OH)(3chem)) is charac teristic of As being in the 3+ oxidation state and being attached to three OH groups. The conclusion that AS(OH)(3chem) is the species formed agrees w ith the CV, IR spectroscopy, and ex-situ scanning tunneling microscopy data which point to an oxygenated As3+ surface compound formed in the course of AS(chem) electro-oxidation.