Reduction Of Transition Metal Oxides Research Articles

Facile synthesis of high entropy carbide-nickel based cermets by in-situ carbo-thermal reduction of transition metal oxides

Equimolar and non-equimolar high entropy carbide-nickel based cermets were successfully prepared by in-situ carbo-thermal reduction of constituent metal oxides in a single pressureless vacuum sintering cycle at 1420°C. The carbon content in quinary (Ti0.2V0.2Nb0.2Ta0.2W0.2)Cx-10wt.%Ni and senary (Ti0.2V0.2Nb0.2Ta0.2W0.1Mo0.1)Cx-10wt.% Ni high entropy carbide cermets was varied (x = 0.6–1.0). Two-phase HEC-Ni cermets were formed at x = 0.75–0.85, whereas M6C (η-phase) and graphite phase coexisted at lower and higher carbon contents (x = 0.6, 0.9 and 1.0) in both HEC 5 and HEC 6 systems. In HEC 5, submicrometric WC precipitates were observed, which were attributed to the limited solubility of hexagonal-WC within the in-situ formed HEC 5 phase, contrasting with HEC 6 where no such precipitations were formed across the entire carbon content range. The partial replacement of W with Mo in the (Ti0.2V0.2Nb0.2Ta0.2W0.1Mo0.1)Cx-10 wt% Ni cermets promoted solid solution formation at lower temperature, and the grain size and hardness decreased and the fracture toughness increased. Both HEC-Ni cermets showed a homogeneous distribution of elements within the carbide phase, resulting in (HV30) hardness around 16 GPa and a fracture toughness around 8 MPa.m1/2. The phase evolution was systematically investigated by interrupting the sintering cycle at 600–1400 °C, employing XRD analysis. Thermodynamic calculations were performed to predict the critical reaction temperatures for carbo-thermal reduction. To evaluate the intrinsic mechanical properties of each phase and establish the correlation with microstructural features, micromechanical mapping was performed utilizing the high-speed nano-indentation technique.

Recovery of LiNi0.5Mn0.3Co0.2O2 cathode material from spent lithium-ion batteries with oxygen evolution reduction in ammonium sulfate low-temperature molten salt

The advancement and extensive application of lithium-ion batteries (LIBs) as an energy storage device has led to an increase in the number of spent LIBs, and recycling spent LIBs is an urgent need to address their resource and environmental sustainability issues. Herein, an efficient and comprehensive recycling method for spent LiNi0.5Mn0.3Co0.2O2 cathode material (NMC 532), one of the most important materials for electric vehicles and grid storage applications, is developed based on ammonium sulfate low-temperature molten salt (LTMS) reduction with oxygen evolution, and the feasibility of the strategy is systematically investigated. The results demonstrate that (NH4)2SO4 LTMS can effectively realize the sulfate conversion of spent NMC 532, in which the sulfate conversion rates of Li, Ni, Mn, and Co all reach more than 90%. Through mechanism analysis, it is found that the induced superoxide ion (O2·-) and H+ are the keys to the reduction of transition metal oxides in LTMS. Meanwhile, O2·-, as a single electron reducing agent in the intermediate state, can be used to explain the reduction behavior of transition metal oxides in other ammonium salts, mineral acids, and H-containing salts in molten. Based on (NH4)2SO4 LTMS reduction, water leaching, thermodynamic analysis of coprecipitation, and a series of precipitation and separation experiments, this study provides an effective strategy for recycling spent cathode materials, which can realize the combination of spent material recycling and electrode material regeneration.

Mechanochemical upcycling of spent LiCoO2 to new LiNi0.80Co0.15Al0.05O2 battery: An atom economy strategy

The use of strong acids and low atom efficiency in conventional hydrometallurgical recycling of spent lithium-ion batteries (LIBs) results in significant secondary wastes and CO2 emissions. Herein, we utilize the waste metal current collectors in spent LIBs to promote atom economy and reduce chemicals consumption in a conversion process of spent Li1-xCoO2 (LCO) → new LiNi0.80Co0.15Al0.05O2 (NCA) cathode. Mechanochemical activation is employed to achieve moderate valence reduction of transition metal oxides (Co3+→Co2+,3+) and efficient oxidation of current collector fragments (Al0→Al3+, Cu0→Cu1+,2+), and then due to stored internal energy from ball-milling, the leaching rates of Li, Co, Al, and Cu in the ≤4 mm crushed products uniformly approach 100% with just weak acetic acid. Instead of corrosive precipitation reagents, larger Al fragments (≥4 mm) are used to control the oxidation/reduction potential (ORP) in the aqueous leachate and induce the targeted removal of impurity ions (Cu, Fe). After the upcycling of NCA precursor solution to NCA cathode powders, we demonstrate excellent electrochemical performance of the regenerated NCA cathode and improved environmental impact. Through life cycle assessments, the profit margin of this green upcycling path reaches about 18%, while reducing greenhouse gas emissions by 45%.

Reflection high-energy electron diffraction beam-induced structural and property changes on WO3 thin films

Reduction of transition metal oxides can greatly change their physical and chemical properties. Using deposition of WO3 as a case study, we demonstrate that reflection high-energy electron diffraction (RHEED), a surface-sensitive tool widely used to monitor thin-film deposition processes, can significantly affect the cation valence and physical properties of the films through electron-beam induced sample reduction. The RHEED beam is found to increase film smoothness during epitaxial growth of WO3, as well as change the electronic properties of the film through preferential removal of surface oxygen.

ChemInform Abstract: Reduction of Transition Metal Oxides and Related Systems with Hydrogen Atoms: Applications and Implications

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Autocatalytic effect in the processes of metal oxide reduction. I. Kinetic model of the reduction

Kinetic analysis of a model of the consecutive autocatalytic reaction has been carried out and the results of this analysis have been applied to the description of reduction of transition metal oxides. The proposed model is compared with the consecutive noncatalyzed reaction and the consecutive reaction in which the second step is limited by the nucleation process. It is shown that different mechanisms of the reduction can be distinguished on the kinetic bases.

Influence of the nature of the conduction band of transition metal oxides on catalytic activity for oxygen reduction

Catalytic activities for oxygen reduction of some transition metal oxides with metallic conductivity such as LaTiO 3, SrFeO 3, SrVO 3, SrRuO 3, V 0.2Ti 1.8O 3 and La 1-xSr xMnO 3 were investigated, and they as well as the activities of other oxides reported already were compared with the nature of their conduction bands. It was found that the catalytic activity of oxides having a σ * conduction band was high. The conclusion is drawn that in order for a transition metal oxide to have a high catalytic activity, (1) it must have a σ * band and (2) the band must contain electrons. This conclusion will be useful to predict the catalytic activity for oxygen reduction of transition metal oxides.