Quantumchemical ab initio investigations of the second step of the electrochemical hydrogen reaction on GaAs(111)A: From a hydrogen chemisorption state to the H-2 molecule

We have performed ab initio HF- and MP2-calculations on a molecular surface cluster for the hydrogen evolution reaction on the GaAs(111)A-surface. Sta rting from chemisorbed H(ad)- or H3O(ad)-species on the surface, which were obtained in a previous work (Z. Phys. Chem. 203 (1998) 159) as the result of the first (Volmer) reaction step, we have examined different reaction pa ths for the second step via a Heyrovsky reaction. The essential feature for a successful attack of a second hydronium ion from the solution to an H(ad ) intermediate on the semiconductor surface is the negative charge on the H (ad)reaction centre. We find for the most probable path of the second Heyro vsky step an activation barrier of 1.01 eV (HF)/1.36 eV (MP2). Some first a ttempts to include the first solvation shell of the second hydronium ion in dicate to a preference of the same reaction path as for the attack of the n onsolvated hydronium ion but the activation barrier is enhanced.