Electrochemical reactivity of 2,2',5,5'-tetrahydroxybiphenyl and related compounds adsorbed at pt (111) surfaces: studies by eels, leed, auger spectroscopy and cyclic voltammetry

Abstract

Adsorption and surface electrode reactions of a family of compounds containing the hydroquinone moiety, namely 2,2',5,5'-tetrahydroxybiphenyl (THBP), 2,5-dihydroxy-4-methylbenzylmercaptan (DMBM) and hydroquinone (HQ), have been studied at well-defined Pt (111) surfaces by means of cyclic voltammetry assisted by electron energy-loss spectroscopy (EELS), Auger electron spectroscopy and low-energy electron diffraction (LEED). Packing densities of THBP measured by Auger spectroscopy indicated adsorption predominantly with the rings parallel to the surface, although reversible electroactivity of the layer increased with increasing THBP concentration, indicating a small fraction of verticallyoriented molecules. In contrast, DMBM is adsorbed entirely through its sulfur atom with its ring perpendicular to the surface; the pendant 2,5-diphenyl moiety in adsorbed DMBM was reversibly electroactive. Adsorbed HQ was not reversibly electroactive at any adsorbate concentration. Vibrational spectra of the adsorbed species were obtained by means of EELS, and were compared with the infrared spectra of the parent compounds in KBr. EELS and IR spectra were closely similar except where adsorption changed the nature of the molecule: based upon virtual absence of the characteristic EELS O-H stretching band (near 3500 cm −1), the phenolic hydroxyl hydrogens of HQ and the horizontallyoriented form of THBP were removed upon adsorption, as was the mercaptan sulfhydryl hydrogen of DMBM. The EELS bands of polar groups such as OH are not broadened to the same extent as in the IR spectra of the solid compounds, evidently due to lesser intermolecular hydrogen bonding among such groups at the surface. LEED observations indicated that the THBP layer was structurally diffuse, while HQ formed a Pt (111) (3×3)-HQ adlattice and DMBM formed a Pt (111) (23 0.5× 2 0.5)R30 °-DMBM adlattice at packing densities slightly below saturation. Adsorbate orientation and mode of surface bonding exert a systematic influence on the product distribution of electrocatalytic oxidation of these well-characterized adsorbed organic intermediates, based upon cyclic voltammetry and potential-step chronocoulometry experiments. Also, these results have revealed that evacuation does not alter the composition or electrochemical properties of the chemisorbed layer formed from solution.