ELECTROCHEMICAL DEPOSITION OF HYDROGEN ON PLATINUM SINGLE-CRYSTALS STUDIED BY INFRARED-VISIBLE SUM-FREQUENCY GENERATION

Under- and overpotential deposition of hydrogen on polycrystalline platinum, Pt(100), Pt(110), and Pt(111) surfaces in sulfuric acidic medium is monitored in situ by infrared-visible sum-frequency generation (SFG). In the underpotential range, the vibrational signature between 1800 and 2020 cm(-1) reveals hydrogen bonding between the stable hydrogen adsorbed in terminal sites and the water molecules in the electrolyte. For Pt(100) and Pt(lll), the observed double SFG resonance is most consistent with a bonding configuration where water dimers are each bounded to three hydrogen atoms adsorbed on the relaxed substrate. In the overpotential range, an additional hydrogen adspecies appears with a similar vibrational fingerprint on all the investigated platinum surfaces and is tentatively assigned to the intermediate of the hydrogen evolution reaction. Its SFG resonance frequency near 1770 cm(-1) suggests a dihydride adsorption configuration. The detection at under- or overpotential of high frequency SFG resonances (approximate to 1050 cm(-1)) correlates with the presence of corrugations on the disordered Pt and reconstructed Pt(110) surfaces. (C) 1995 American Institute of Physics.