(544ac) Atomic-Level Insight into Oxygen Adsorption on (hkl) Platinum Surfaces and Implications for the Reactivity in the Oxygen Reduction Reaction

Rate predictions for the oxygen reduction reaction (ORR) in realistic nanoparticle and electrolyte environments and as a function of applied potential have remained a major challenge. We use new polarizable interatomic potentials for metals that are part of the Interface force field (IFF) to predict physisorption of oxygen molecules on Pt (110), (111), and (110) surfaces which precedes the chemisorbed state. The conditions correspond to dilute sulfuric acid solution with zero potential, potentials of ~1 V, and higher as tested in experiment. The free energy profiles as a function of distance from the metal surface show distinct facet-specific differences. Interactions of oxygen with (110) and (111) surfaces are favorable while a high barrier of ~6 kcal/mol prevents frequent direct contact of oxygen with the (100) surface. The energy barriers towards adsorption increase in the order Pt(110)≈Pt(111) < Pt(100), and correlates with trends in ORR activity in experiment from high to low. The differential adsorption is owed to the strong binding of a water adlayer on (100) surfaces with a high soft epitaxial fit and more vivid exchange between oxygen and water on (110) and (111) surfaces under constraints of hydrogen bonds. The adsorbed conformations of the oxygen molecules are also sensitive to surface structures and sites. Sulfate ion display site blocking effects in physisorption. The influence of electric potentials shows little to no effect on oxygen physisorption, even if increased by several orders of magnitude. The interatomic potentials are highly accurate to reproduce the metal lattice parameter, the surface energy, the water adsorption energy, and solvation energies of oxygen with deviations from experiment of 0.1%, 1%, 5%, and 1%, respectively, and fully compatible with IFF, CHARMM, AMBER as well as other common force fields. The next step are reactive simulations to obtain insight into the rate-determining step(s) and compute activation energies for different Pt and alloy surfaces, nanoparticle shapes, and electrolyte compositions in comparison with laboratory measurements.