ON THE LACK OF ACTIVITY OF SUBSTITUTIONAL SN ATOMS TOWARD THE ELECTROOXIDATION OF CO ON PT ANODES - MOLECULAR-ORBITAL THEORY

An atom superposition and electron delocalization molecular orbital (A SED-MO) study shows that tin atoms alloyed substitutionally into plati num electrode surfaces are inactive in generating OH(ads) for the elec tro-oxidation of the CO(ads) poison generated during the operation of methanol fuel cells. Such tin atoms, though they donate to Pt, are not good accepters for H2O lone-pair donation bond formation because of t he way in which their 5p orbitals mix with the Pt valence band. Thus s ubstitutional Sn atoms in the Pt surface do not attract or activate H2 O. OH is also found to adsorb weakly to substitutional Sn atoms compar ed to surface Pt atoms, the opposite of the diatomic SnO and PtO bond strengths. This is because the OH is essentially reduced by neighborin g Pt atoms and not the Sn atom to which it is bound. When bound to sub stitutional Sn atoms, OH is calculated to be relatively active in oxid izing CO(ads) on adjacent Pt atoms, but the inability of the surface t o generate such OH implies a different mechanism must be responsible f or the electrocatalysis, perhaps involving adsorbed Sn atoms or Sn com plexes.