On the formation and diffusion of oxygen vacancies in non-stoichiometric mixed Co3-xMnxO4spinel structures from first-principles calculations

Using first-principles simulations, we focus on the study of Co₃O₄-Mn₃O₄mixed oxides, which have recently shown alluring features as thermochemical heat storage materials. We provide fundamental atomistic-level insight into the thermodynamics and kinetics of a series of non-stoichiometric Co_3-xMn_xO_4-y(0 ⩽x⩽ 3 andy= 0, 0.125, 0.250) bulk systems, by examining in detail the formation and diffusion processes of oxygen vacancies as a function of Mn content. We find a preference for the formation of vacancies atx= 1.5. And we predict a significant drop of diffusion barriers forx⩾ 1.5, when Mn atoms start to populate the spinel octahedral sites as Mn³⁺. Our results pave the way for better understanding the underlying mechanisms that govern oxygen vacancy dynamics in Co_3-xMn_xO₄in general, and, in particular, the reversible reduction and re-oxidation reactions of these promising mixed oxides for thermal energy storage. Nevertheless, some discrepancies are found between our calculations on bulk models and recent experimental insights from the literature, which suggests that surface and finite size effects might play an important role in controlling the observed macroscopic behavior of these materials during reversible reduction and re-oxidation cycles.

Rationalizing the promotional effect of Mn oxides in benzene combustion using an O 2p-band center descriptor

Our work sheds light on using the O 2p-band center as a useful electronic descriptor for understanding the variations in catalytic reducibility of transition metal oxides (TMOs) and the promotional effect of MnO2 during catalytic benzene combustion. The "volcano"-type activity plot, in conjunction with the reduction characteristic of the TMOs, ultimately reflects the Sabatier principle, which states that a good catalyst (i.e., MnO2) balances the capability of oxygen abstraction and uptake in the case of benzene combustion.

Tunable Redox Temperature of a Co3-xMnxO4 (0 ≤ x ≤ 3) Continuous Solid Solution for Thermochemical Energy Storage

Heat-storage technologies are well suited to improve the energy efficiency of power plants and the recovery of process heat. A good option for high storage capacities, especially at high temperatures, is storing thermal energy by reversible thermochemical reactions. In particular, the Co₃O₄/CoO and Mn₂O₃/Mn₃O₄ redox-active couples are known to be very promising systems. However, cost and toxicity issues for Co oxides and the sluggish oxidation rate (leading to poor reversibility) for Mn oxide hinder the applicability of these single oxides. Considering, instead, binary Co-Mn oxide mixtures could mitigate the above-mentioned shortcomings. To examine this in detail, here, we combine first-principles atomistic calculations and experiments to provide a structural characterization and observe the thermal behavior of novel mixed-metal oxides based on cobalt/manganese metals with the spinel structure Co_3-xMn_xO₄. We show that novel Co_3-xMn_xO₄ phases indeed enhance the enthalpy of the redox reactions, facilitate reversibility, and mitigate energy losses when compared to pure metal oxide systems. Our results expand therefore the limited list of currently available thermochemical heat-storage materials and pave the way toward the implementation of tunable redox temperature materials for practical applications.

One-step synthesis, electronic structure, and photocatalytic activity of earth-abundant visible-light-driven FeAl2O4

The development of inexpensive visible-light-driven photocatalysts is an important prerequisite for realizing the industrial application of photocatalysis technology. In this paper, an earth-abundant FeAl₂O₄ photocatalyst is prepared via facile solution combustion synthesis. Density functional theory and the scanning Kelvin probe technique are employed to ascertain the positions of the energy bands and the Fermi level. Phenol is taken as a model pollutant to evaluate the photocatalytic activity of FeAl₂O₄. The scavenger experiment results, ˙OH-trapping fluorescence technique, and electron spin resonance measurements confirm that the superoxide anion radical is the main active species generated in the photocatalytic process, which also further corroborates the proposed electronic structure of FeAl₂O₄. The degradation experiments and O₂ temperature programmed desorption results over various samples verify that the crystallinity degree is a more important factor than the oxygen adsorption ability in determining photocatalytic activity.