  A new model for the intermediate compound of the hydrogen evolution reaction
(HER) is proposed, for the electrochemical reduction of hydrogen in the
presence of bisulfate on platinum(111). The formation of this compound, a
regular 2 dimensional honeycomb ice lattice, occurs by a first order phase
transitions that involves the reorientation of water molecules. The model is
analyzed using new and simple effective cluster approach which highlights the
relevant transitions in system. This method is based on the cluster variation
method used successfully in our previous work on the UPD of Cu onto Au(111),
and permits us to explore a large region of parameter space, an essential
feature to study this complex system. The theory makes full use of the
properties of the diffuse layer: The water molecule is reoriented as the
potential is changed. For positive potentials it forms linear chains which are
responsible for the $\sqrt 3 \times \sqrt 7$ structure of the sulfate observed
by STM. At negative potentials water turns so that its dipole points towards
the Pt. Then it will form a regular honeycomb network of hydrogen bonded
molecules, with the sulfate at the center of the hexagons. Then the bisulfate
is desorbed, leaving the honeycomb HER structure behind. Our model thus
provides an explanation of the well known fact that only 2/3 of the Pt atoms
participate in the electroreduction of the hydrogen. The theory implies
geometrical constraints to the water potential.
