DFT study of high performance Pt3Sn alloy catalyst in oxygen reduction reaction

The oxygen reduction reaction (ORR) is a multi-step catalytic process occurring at the cathode in fuel cells. As an alternative to a conventional Platinum catalyst, PtSn-based alloy catalyst experimentally presents an enhanced ORR activity compared with pure Pt catalysts. However, how the ORR reaction proceeds on PtSn is not yet well understood. On this context, a systematic study of O-2 reduction on the (1 1 1) facets of Pt3Sn based on periodic density functional theory (DFT) calculation is presented. With the charge transfer from Sn to Pt, d-band center shifts away from the Fermi level, the electronic structure thoroughly differs from that of pure Pt thus producing ligand (electronic) effect. The ORR intermediates (H, O, OH, O-2, OOH, H2O2, and H2O) species preferred site, adsorption configuration, binding energies, active barriers, rate constants, equilibrium constant are studied. Additionally, the corresponding transition states in seven elementary reactions are confirmed using the climbing image nudged elastic band (CI-NEB) method, and the thermodynamic and dynamic property in each reaction step are evaluated. Herein, the DFT results imply that on both the Pt3Sn(1 1 1) and Pt(1 1 1) surfaces, the ORR share the same mechanism following OOHad dissociation pathway (O-2ad -> OOHad -> O-ad -> OHad -> H2Oad). The rate-determining step of the ORR on the (1 1 1) surfaces is found to be the O-ad hydrogenation reaction which requires activation barrier of 0.66 eV on the Pt3Sn(1 1 1) surface and 0.77 eV on the pure Pt(1 1 1) surface, indicating the introduction of Sn significantly decreases the activation energy barrier. As the same temperature, the reaction rate of ORR on the Pt3Sn(1 1 1) surface is faster than that on the pure Pt(1 1 1) surface. Our thermodynamic and kinetic results verify the important role of tin in improving the catalytic activity of ORR.