Design of Heterogeneous Catalysts and the Application to the Oxygen Reduction Reaction

Abstract

A method is developed to use ab initio calculations to predict what materials will have high catalytic activity for heterogeneous dissociative adsorption reactions. This method may be used to restrict the combinatorial possibilities of materials selection to a reasonable search space. The method is demonstrated for the test case of the oxygen reduction reaction, and it is shown that an Ag–Pt compound is superior to the standard Pt catalyst. Density functional theory was used to evaluate dissociative adsorption of oxygen on Ag _n (n = 4, 6, 8, 14), Pt _m (m = 2, 4, 8), and Ag _n Pt _m [(n, m) = (4, 2), (6, 2), (4, 4)] clusters. Stable adsorbed, dissociated, and activated states and energies were found. The AgPt compounds show enhanced performance over Ag and Pt clusters of comparable size. Calculated energy of associative and dissociative adsorption on Pt and Ag is in broad agreement with experiment. DFT models of oxygen adsorption and dissociation on slabs of Ag, Pt, and PtAg _x were found to agree with experiments and cluster models. A model is given to explain the reactivity of oxygen with Pt and Ag. A Pt/Ag bilayer and a random alloy are examined through experiment and simulation to show that it is possible to fine tune electronic properties, and therefore reactivity for oxygen dissociation. The reactivity of these compounds toward oxygen is generally intermediate to that of pure Ag and Pt; thus a AgPt alloy is expected to be a better low-temperature catalyst for oxygen dissociation than pure Ag or Pt under nearly reversible conditions. Indeed, experiments confirm an Ag₃Pt₂ alloy to show superior activity at lower than half the platinum loading. Stress analysis confirms that the altered electronic structure, and thus the enhanced catalytic activity, must be due to an alloying effect rather than a strain effect from lattice expansion.