Molecular orbital investigation of water reactions with tin hydroxide complexes in association with platinum electrodes

Calculations using the atom superposition and electron delocalization molecular orbital (ASED-MO) theory show that the redox reaction of Sn(OH)(2) to Sn(OH)(4) is likely to proceed by a mechanism of (i) collision of Sn(OH)(2) with H2O, which will promote electron loss to the electrode when the potential is the oxidation potential, (ii) deprotonation of the water molecule and electron loss to yield strongly bound OH in Sn(OH)(3)(+), (iii) immediate attack by H2O to form Sn(OH)(3)H2O+, and (iv) deprotonation to Sn(OH)(4). The possible transfer of OH to Pt electrode surfaces is of interest in fuel cell catalysis. OH(ads) is capable of oxidizing the anode poison CO(ads). Calculations indicate that Sn(OH)(3)(ads) is the best candidate thermodynamically for transferring OH(ads), but it is not kinetically active. Coordinating H2O to Sn in this adsorbed complex does not activate OH(ads) transfer. Sn(OH)(2) bound to a Pt surface step site does not activate H2O for OH(ads) formation. Neither does a Sn atom bound to a step site, or to a surface substitutional site, which was shown in a previous paper. It is concluded that tin does not promote the electrooxidation of CO adsorbed on Pt by a mechanism that involves OH(ads) as an oxidant.