Investigation of SO3 adsorption and dissociation on Pt electrode

In order to clarify the mechanism of SO3 electrolysis, we evaluated the configuration of SO3 adsorption on Pt (111) surface using the first-principles calculations by a slab model with a periodic boundary condition. In addition, we evaluated the chemical bonding states of SO3 in detail using the molecular orbital calculation on the basis of the calculations by a slab model. In the calculation by a slab model, we found two stable adsorbed SO3 configurations on Pt surface. In one configuration (A), the S and two O atoms were bound to Pt surface atoms, and in the other configuration (B), all the three O atoms were bound to Pt surface atoms, which was the most stable configuration. In the molecular orbital calculation, we found that S–O bond became weak by SO3 adsorption, and conjectured that isolated SO2 molecule could not be dissociated directly from SO3 adsorbed on Pt surface, and that SO3 dissociation proceeded through the intermediate state of the adsorbed SO2 and adsorbed O atom on Pt surface. We calculated SO3 dissociation path on the basis of this conjecture. In this calculation, we found that S–O bond was broken by crossing the atop site of Pt surface and that this energy barrier was 1.45 eV, and that SO3 dissociation was encouraged due to an anti-bonding nature between S and Pt surface atoms and the bond strengthening between dissociating O and Pt.