An experimental and theoretical study of electroreduction of oxygen to hydrogen peroxide is presented. The experimental measurements of nitrided Ketjenblack indicated an onset potential for reduction of approximately 0.5 V (SHE) compared to the onset potential of 0.2 V observed for untreated carbon. Quantum calculations on cluster models of nitrided and un-nitrided graphite sheets show that carbon radical sites formed adjacent to substitutional N in graphite are active for O 2 electroreduction to H 2 O 2 via and adsorbed OOH intermediate. The weak catalytic effect of untreated carbon is attributed to weaker bonding of OOH to the H atom-terminated graphite edges. Substitutional N atoms that are far from graphite sheet edges will be active, and those that are close to the edges will be less active. Interference from electrochemical reduction of H atoms on the reactive sites is considered, and it is shown that in the potential range of H 2 O 2 formation the reactive sites are not blocked by adsorbed H atoms.
The slab band quantum computational approach in the Vienna ab initio simulation package (VASP) is used to calculate the adsorption energies of reactants, reaction intermediates, and products in O2 reduction and in water oxidation in acid on three crystallographic surfaces of pentlandite structure Co9S8. Reversible potentials for the reaction steps involving electron and proton transfer are determined by using the energies in a linear Gibbs free energy relationship. On the basis of these results, we find that the partially OH-covered (202) surface is active toward O2 reduction and should have overpotential behavior similar to that observed for platinum electrodes. One structure in the predicted four-electron reduction mechanism is novel: S2- provides an adsorption site for O following O-O bond scission, which, unlike the case of platinum electrodes, takes place prior to the first reduction step.
On the basis of spin-unrestricted hybrid gradient-corrected Becke, Lee, Yang, and Parr B3LYP density
functional calculations and the reaction site models Fe(NH2)2(NH3)2 for FeII, and Fe(NH2)2(NH3)2OH for
FeIII, FeII is predicted to be the active site for the four-electron reduction of oxygen by heat-treated iron
macrocycles. It is favored over FeIII in the first step of the mechanism because of a site blocking effect: H2O
bonds strongly to the FeIII site, blocking it against O2 adsorption, and it does not bond strongly to FeII. The
stronger bonding of the product of the first reduction step, OOH, to FeII compared to FeIII also helps by
contributing to a more positive reversible potential for its formation over FeII. Subsequent reduction steps
have high reversible potentials over both centers, paralleling an earlier study of oxygen reduction over a
single Pt site. However, the important difference compared to Pt is the hydrogen-bonding interaction between
(OHOH) bonded to FeII and a nitrogen lone-pair orbital in the N4 chelate. This is in addition to the O lone-pair donation bond to the FeII center and is proposed to prevent hydrogen peroxide from leaving as a two-electron reduction product, as it was predicted to do over a single Pt site, and provides a path for reduction
to water.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.