The reduction of hydrogen in the presence of bisulphate on platinum(111) electrodes

A new model for the electroreduction of hydrogen in acidic media is proposed. One of the still open problems is the structure of the intermediate compound of the hydrogen evolution reaction (HER) or alternatively the UPD hydrogen. We propose here a new form of this compound, namely (H₅O₂ ⁺)₃, a honeycomb, hydrogen-bonded lattice which is formed through a first-order phase transition by water and the hydronium H₃O⁺ ion under favourable conditions. The essential point of our theory is the use of a correct water model, which has to be tetrahedral and flexible. Then, as the polarity of the electrode is changed: (1) for positive electrodes the dipole of the water points down, a zig-zag chain of hydrogen-bonded water, is formed which is responsible for the 3^½ x 7^½ structure of the sulphate observed by STM. (2) For negative polarization the water dipole points up. The (bi)sulphate and the water form a periodic structure by incorporating 1/3 of hydronium, which then is adsorbed with a hydrogen down in the hollow sites of the Pt(111) electrode. This compound is neutral and forms a two-dimensional honeycomb arrangement of water with (or without) bisulphate ions. The model is consistent with all known experiments: it reproduces well the experimental voltammogram and the recent radiotracer measurements of bisulphate adsorption. Furthermore it gives a robust explanation of the 2/3 hydrogen yield for this surface.