New insight into the electro-oxidation of the irreversibly chemisorbed bismuth on Pt(111) through temperature-dependent research

The surface redox behavior of an overlayer of chemisorbed bismuth, Bichem (θBi=0.30), on Pt(111) in 0.5 M aqueous H2SO4 manifests itself through sharp CV features that are not affected by the scan rate (s) variation (for 1≤s≤100 mV s−1). The impact of T variation on the surface-oxidation behavior of Bichem was examined by T alteration in the 273–318 K range. The data show that the Bichem surface coverage is not affected upon T modification but the cyclic-voltammetry (CV) profiles undergo qualitative changes. The sharp anodic peak and the cathodic one shift towards less-positive potentials upon T increase. Despite some morphology changes in the CV profiles, the T variation does not affect the charge density distribution between the two anodic and two cathodic features. At θBi well below 0.30, say 0.20, the CV profiles representative of the Bichem oxidation reveal only one perfectly reversible peak. An analysis of the possible reaction pathways indicates that the Bichem surface oxidation proceeds in one step involving transfer of two electrons and addition of two OH groups. The standard enthalpy of the surface redox process is determined from the slope of the E/T versus 1/T plot, and then the standard enthalpy of formation of Bi(OH)2chem, ΔHf°(Bi(OH)2chem), is elucidated, its value being −441 kJ mol−1. It falls closely to 2/3 of the standard enthalpy of formation of the bulk-type Bi(OH)3, thus pointing to the nature of the surface compound. The different electro-oxidation behavior of Bichem with respect to Aschem (it undergoes electro-oxidation to As(OH)3chem) is explained in terms of surface electronic effects.