Adsorption of Phosphate Species on Pt(111) and Pt(100) As Studied by in Situ FTIR Spectroscopy

In situ FTIR results on the adsorption of phosphate species on Pt(111) and Pt(100) single crystal surfaces are presented and discussed. In weak acid solutions (fluoride electrolyte pH = 2.8), the adsorption of phosphate species on Pt(111) starts at about 0.4 V vs Pd/H₂ and the maximum adsorption occurs at about 0.6 V. At low potentials, two spectral features due to adsorbed H₂PO₄^- are observed at 1110 and 1000 cm^-1. With increasing potentials these two original bands are replaced by a new band located between 1150 and 1180 cm^-1, which is assigned to adsorbed HPO₄^2- species. In strongly acidic solution (pH = 0.23), undissociated H₃PO₄ molecules are adsorbed at low potentials. This species is characterized by a band between 1035 and 1050 cm^-1. With increasing adsorption potential adsorbed H₃PO₄ dissociates, generating H₂PO₄^-. For both solution pHs the deprotonation of adsorbed species is observed after the maximum of the anomalous voltammetric wave of the Pt(111) electrode. On Pt(100) and Pt(111) the adsorption of phosphate species shows almost identical behavior in mildly acidic solutions. Transformation from H₂PO₄^- to HPO₄^2- occurs on both single crystal surfaces at the same potentials.