Mechanism for oxidative dissolution of a Cr atom from a Pt surface: Molecular orbital theory

A molecular orbital study has been made of the oxidative abstraction of a substitutional surface Cr atom from the Pt(111) surface. The overall process studied is Cr(subst) + 4H(2)O(g) --> H2O . CrO3(g) + 6H(ads) (i). When solvated, the H2O . CrO3 molecule is expected to rearrange to H2CrO4(aq), and the adsorbed H atoms will be discharged to form H+(aq) plus electrons that enter the anode. The latter steps, which are not expected to be kinetically limiting, were not studied. The mechanism explored for forming H2O . CrO3 is one of sequential bonding of H2O to Cr and dehydrogenation, leading to strongly adsorbed Cr(OH)(3)(ads). A water molecule bonds, forming H2O . Cr(OH)(3)(ads) which then loses three hydrogen atoms to the surface. The product, H2O . CrO3(ads), is weakly held and desorbs easily. Each step leading to the product in eq i is calculated to have a low activation energy, However, HCrO4-(aq) remains stable only when the electrode is at a potential > 1.35 V (SHE), otherwise, it is expected to be reduced to Cr-III(aq) as it desorbs.