Mechanism for Oxidative Dissolution of a Cr Atom from a Pt Surface: Molecular Orbital Theory

Abstract

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) + 4H2O(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 CrIII(aq) as it desorbs.