Adsorption of phospbate 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/H-2 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 HPO42- 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 HPO42- occurs on both single crystal surfaces at the same potentials.