In-situ infrared spectroscopy of water and electrolytes adsorbed on a Pt(111) electrode surface in acid solution. Structural changes of adsorbed water molecules upon an electrode potential

At least two different solution species were absorbed on a Pt(111) electrode in acid solutions; one is neutral H2O, and the other the hydronium ion H3O+. The former neutral molecule chemisorbed through an oxygen lone pair to a hydrogen covered platinum surface. At potentials more positive than 0.4–0.6 V (NHE), the water molecule started to be replaced by specifically adsorbed anions (ClO4−, HSO4−). The latter hydronium ion could also be observed in an uppermost layer of a Pt(111) electrode at relatively wide potential ranges. The coverages of the surface water molecules were highest at the most negative potential. The remarkably higher νOH stretching frequencies of the water molecules indicate that the molecules are isolated from the outer bulk solution.