Molecular modeling of electrocatalysis: The electroreduction of oxygen on platinum.

摘要

Density functional theory (DFT) and molecular dynamics (MD) techniques are used to study the most probable rate-determining step in electroreduction of oxygen on a platinum metal (M) surface: O2g+H +aq+ e-M→M-OOH ; Initially, chemisorption of the atomic and molecular oxygen on platinum is investigated using DFT. Clusters of size 2 to 6 are used to represent the metal surface. Electronic and thermodynamic interactions of the atomic and molecular oxygen with the metal clusters are thoroughly analyzed. Adsorbate/substrate binding energies, vibrational frequencies, and charge distributions are obtained. Two distinct states are found for molecular oxygen. The dissociation of adsorbed O₂ on the metal is investigated as a function of the cluster size.; Proton transfer processes in water and model electrolyte are studied. Hydrated proton clusters are found to be stable in both the gas phase and the solution. In the absence of an electric field, the barrier for proton transfer between H₃O+ and a water molecule depends on the hydrogen bond length O–H+…O. Under the effect of an electric field opposite to that of the system dipole, the activation barrier for proton transfer is significantly reduced. Regarding to the proton/electrolyte interactions, proton transfer from sulfonic acid group terminated Nafion side chain to the water molecules is characterized. Results from MD simulations indicate that the contact ion-pair formed between the sulfonic acid anion -SO₃− and the hydronium ion is very stable.; Furthermore, the potential energy surface (PES) for the proposed rds is analyzed on the basis of DFT calculations. We conclude that electron and proton transfers take place simultaneously, and the reaction activation barrier depends strongly on the degree of proton hydration, and on the metal surface charge.; To go beyond the cluster approximation, an effective Hamiltonian model proposed by Koper and Voth (J. Chem. Phys., 109, pp1991–2001, 1998) is applied to calculate the potential energy surface of oxygen dissociative adsorption/reduction reactions at the platinum/solution interface. The activation energy for the dissociative adsorption reaction is calculated as a function of the applied potential and the metal work function.