Question

Dissociation of oxygen during oxygen reduction reaction

Answer 1

The oxygen reduction reaction (ORR) is a fundamental reaction related to various disciplines such as energy conversion, material dissolution or biology. Recently, particular interest focused on its essential role in fuel cells or lithium-air batteries. However, the mechanism of the ORR on metal surfaces remains unclear. The distinction between the ORR mechanisms is based on the number of proton-coupled electron transfer steps that precede the O–O bond breaking step (Fig. 1). Among these mechanisms, hydrogen peroxide can be formed as an intermediate of ORR only by the 2nd associative mechanism. Indeed, hydrogen peroxide has been detected under certain conditions during ORR, but it remains unclear whether it is a key intermediate of the dominant ORR mechanism or a side-product [1]. A detailed understanding of the interaction of H2O2 with metal surfaces is essential on the road to understanding the ORR mechanism.

In weakly adsorbing electrolytes such as HClO4, the total rate of H2O2 decomposition on polycrystalline Pt is controlled by mass transport, in the potential region between +0.2 VRHE and +1.5 VRHE. In this region, the currents in the cyclic voltammograms (CVs) scale with the thickness of the diffusion layer and with the bulk concentration of H2O2 [2,3]. In addition, electrolysis experiments performed under potentiostatic conditions indicate that the rate of decrease of H2O2 concentration with time is diffusion-limited, regardless of the applied potential. This means that during ORR in such a system, H2O2 cannot be detected in the electrolyte even if it is formed at the interface. In a peroxide-containing solution, the potential-dependent Pt surface state triggers the corresponding reaction: Upon interaction with reduced surface atoms at low potentials, H2O2 adsorbs dissociatively producing OHads, while upon interaction with an oxidized surface at high potentials, H2O2 gets oxidized to O2 by reducing the surface [3]. The measured current is the sum of the two partial currents restoring the thermodynamically preferred surface state at a given potential. Quantum chemical ab initio calculations (Fig. 2) showed that the activation barriers for either H2O2 dissociation or oxidation are easily overcome by the thermal energy and the reactions will proceed with a high rate at room temperature [2].

Answer 2

the recombination of oxygen atoms and isotopic exchange with molecular oxygen. The simplest heterogeneous catalytic process involving molecular oxygen is the dissociation of O2 into atoms together with the reverse reaction, the recombination of O-atoms. As the dissociation reaction is highly endothermic, temperatures greater than 1000° C are required before appreciable rates of reaction are observed and this restricts the range of catalysts that can be studied. At low pressures and at temperatures of 1000°-1500° C, the reaction proceeds on platinum via the dissociative oxygen chemisorption of oxygen and the desorption of O-atoms. The surface coverage with atomic oxygen is assumed to be low on account of the high temperature. The interaction of atoms or radicals is known to proceed rapidly even without a catalyst, but the large amount of heat evolved in reaction favors decomposition of the product. Solid catalysts, as third bodies, adsorb the energy released and dissipate it, stabilizing the O2 molecule formed. At the same time, a catalyst takes part in intermediate chemical reactions with the O-atoms and, as a result, the specificity of different surfaces is manifested.