Metal Redox Research Articles

Streptomycetes as Microbial Cell Factories for the Biotechnological Production of Melanin.

Melanins are complex, polymeric pigments with interesting properties like UV-light absorbance ability, metal ion chelation capacity, antimicrobial action, redox behaviors, and scavenging properties. Based on these characteristics, melanins might be applied in different industrial fields like food packaging, environmental bioremediation, and bioelectronic fields. The actual melanin manufacturing process is not environmentally friendly as it is based on extraction and purification from cuttlefish. Synthetic melanin is available on the market, but it is more expensive than animal-sourced pigment and it requires long chemical procedures. The biotechnological production of microbial melanin, instead, might be a valid alternative. Streptomycetes synthesize melanins as pigments and as extracellular products. In this review, the melanin biotechnological production processes by different Streptomyces strains have been revised according to papers in the literature. The different fermentation strategies to increase melanin production such as the optimization of growth conditions and medium composition or the use of raw sources as growth substrates are here described. Diverse downstream purification processes are also reported as well as all the different analytical methods used to characterize the melanin produced by Streptomyces strains before its application in different fields.

High entropy alloy MoNbTaVW synthesized by metal-oxide reduction in a microwave plasma

A unique approach was used to synthesize the high entropy alloy MoNbTaVW via reduction of metal-oxide precursors in a microwave plasma. The metal-oxides underwent ball milling and consolidation before plasma annealing at 1800 °C for 1 h with hydrogen as feedgas. X-ray diffraction, scanning electron microscopy/energy dispersive x-ray analysis, and Vickers hardness testing reveal characteristics of the high-entropy alloy. This includes a predominantly single-phase body-centered cubic structure, homogeneous distribution of all five metals, and 6.8 ± 0.9 GPa hardness, comparable with other reports for the same five-metal high entropy alloy configuration. Localized microwave plasma particle sintering is evident from the microstructure. These results highlight the promising potential of microwave plasma as a fast, economical, and flexible processing tool for high entropy alloys.

Synthesis of nano- and ultra-fine refractory carbide powders by low-temperature calcium-carbothermic process and their grain growth during sintering

Metal carbides are widely used in various applications as catalysts, components of composite materials, cutting tools, and abrasive materials where the particle/grain size has a great impact on target properties. Here, we report a facile way to synthesize nano- and ultra-fine carbides of IVb-Vb groups, including high-entropy carbides, which are of interest nowadays due to their superior properties, by the combination of calcium-hydride reduction of metal oxides and the direct reaction of the reduced metals with carbon in one production step. By tailoring the synthesis parameters, it is possible to obtain the carbide powders of a size in the range of 50–1500 nm with high crystallinity. The average particle size is found to be dependent on the homologous synthesis temperature and follows a power law. We have also shown the possibility of obtaining ultrafine-grained bulk carbides (d = 0.24–0.9 μm) via spark-plasma sintering at a temperature of 1500–1650 °C with a relative density >95 % without using additives. The HEC (Ti,Zr,Nb,Hf,Ta)C has a higher activation energy of grain growth (collective recrystallization) compared to binary TaCx (x = 0.8 and 1), which confirms the effect of lattice distortion on diffusion coefficients. The suggested fabrication route can be widely used for nano and ultrafine carbide powder preparation on the industrial scale.

Trapped O2 and the origin of voltage fade in layered Li-rich cathodes

Oxygen redox cathodes, such as Li1.2Ni0.13Co0.13Mn0.54O2, deliver higher energy densities than those based on transition metal redox alone. However, they commonly exhibit voltage fade, a gradually diminishing discharge voltage on extended cycling. Recent research has shown that, on the first charge, oxidation of O2− ions forms O2 molecules trapped in nano-sized voids within the structure, which can be fully reduced to O2− on the subsequent discharge. Here we show that the loss of O-redox capacity on cycling and therefore voltage fade arises from a combination of a reduction in the reversibility of the O2−/O2 redox process and O2 loss. The closed voids that trap O2 grow on cycling, rendering more of the trapped O2 electrochemically inactive. The size and density of voids leads to cracking of the particles and open voids at the surfaces, releasing O2. Our findings implicate the thermodynamic driving force to form O2 as the root cause of transition metal migration, void formation and consequently voltage fade in Li-rich cathodes.

Homeostatic Solid Solution Reaction in Phosphate Cathode: Breaking High-Voltage Barrier to Achieve High Energy Density and Long Life of Sodium-Ion Batteries.

The stable phase transformation during electrochemical progress drives extensive research on vanadium-based polyanions in sodium-ion batteries (SIBs), especially Na3 V2 (PO4 )3 (NVP). And the electron transfer between V3+/4+ redox couple in NVP could be generally achieved, owing to the confined crystal variation during battery service. However, the more favorable V4+/5+ redox couple is still in hard-to-access situation due to the high barrier and further brings about the corresponding inefficiency in energy densities. In this work, the multilevel redox in NVP frame (MLNP) alters reaction pathway to undergo homeostatic solid solution process and breaks the high barrier of V4+/5+ at high voltage, taking by progressive transition metal (V, Fe, Ti, and Cr) redox couple. The diversified reaction paths across diffusion barriers could be realized by distinctive release/uptake of inactive Na1 site, confirmed by the calculations of density functional theory. Thereby its volume change is merely 1.73% during the multielectron-transfer process (≈2.77 electrons). MLNP cathode could achieve an impressive energy density of 440Wh kg-1 , driving the leading development of MLNP among other NASICON structure SIBs. The integration of multiple redox couples with low strain modulates the reaction pathway effectively and will open a new avenue for fabricating high-performance cathodes in SIBs.

Oxygen-vacancies rich CuFe2O4 catalyst as efficient peroxymonosulfate activator for enhanced oxytetracycline degradation: Performance and mechanism

Peroxymonosulfate (PMS) based advanced oxidation methods have gained attention for eliminating antibiotics like oxytetracycline (OTC) from wastewater. This study presents an innovative approach by incorporating oxygen vacancies (OVs) into CuFe2O4 to create OVs-CuFe2O4 materials. Compared to traditional CuFe2O4, OVs-CuFe2O4 offered superior catalytic performance due to its surface-rich OVs promoting rapid conversion of oxygen and efficient metal redox of Fe(Ⅱ)/Fe(Ⅲ) and Cu(Ⅰ)/Cu(Ⅱ). OVs-CuFe2O4/PMS achieved impressive 95.7 % OTC removal in 45 min with a rate constant of 0.107 min−1, which was 3.87 times of CuFe2O4/PMS. The OVs-CuFe2O4/PMS system exhibited a broad pH tolerance range of 3 to 11 with OTC removal efficiency above 89.3 %. The OVs-CuFe2O4/PMS system produced various free radicals and non-radical for OTC degradation. The OVs-CuFe2O4 also has good stability with removal efficiency of 81.9 % at fourth cycle. This research gives a strategy to construct oxygen vacancies enrich catalysts for efficient activation of PMS and treatment of antibiotics wastewater.

Separation of Li and Co From LiCoO2 Cathode Material Through Aluminothermic Reduction: Investigation of the Thermite Reaction

Aluminum can be used as a reductant for metal oxide reduction processes. This study investigates the reaction between Al with LiCoO2 in the context of recycling and separation of Li and Co from end-of-life battery cathode material. Specifically, this work attempts to investigate the initiation of the ignition of the thermite reaction. Both thermodynamic assessments and experimental work were carried out on the LiCoO2-Al system in the range of 750 °C to 1020 °C with three different amounts of Al additions in the sample, i.e., 11 wt pct, 20 wt pct, and 28 wt pct. It was found that the amount of Al (composition of the sample), the sample weight, and the initial heating temperature affect the occurrence of spontaneous ignition of the thermite reaction in the system leading to the partial/full melting of the sample. A function of Biot number and temperature was utilized to construct maps showing the onset of ignition where it was found that samples with large Biot numbers tend to ignite. In addition, higher Al addition, sample mass, and temperature were likely to generate ignition. The ignition was found to govern the type of end products of Li and Co; for example, the Li was distributed to gas as Li(g) and slag as LiAlO2 while Co could be extracted as Co metal or Co-Al alloy. The 11 wt pct and 20 wt pct Al addition to the samples resulted in a pure metallic cobalt product, whereas 28 wt pct Al addition resulted in CoAl alloy with a composition of 86.1 wt pct Co and 13.9 wt pct Al. The final product of the vaporized Li was in the form of Li(OH) due to the exposure to water vapor in the atmosphere upon collection. This aluminothermic approach is considered as a promising method to recover Li and Co from waste LiCoO2.

Unveiling charge compensation mechanisms in Na2/3MgxNi1/3-xMn2/3O2 cathode materials: insights into cationic and anionic redox

P2-type Na2/3Ni1/3Mn2/3O2 cathode material for sodium-ion batteries exhibits negligible voltage hysteresis during charge and discharge cycles, with oxygen redox mechanisms playing a pivotal, non-negligible role. Conversely, for the Na2/3Mg1/3Mn2/3O2 material featuring Mg substitution for Ni, O redox largely governs the entire charge and discharge process, accompanied by a pronounced voltage hysteresis. Upon combining these two materials – Na2/3MgxNi1/3-xMn2/3O2 – the transition metal and O redox processes, constituting the charge compensation mechanism, become paramount. These phenomena underscore the transition metal's influence on anionic redox, a topic with limited in-depth exploration in the existing literature. Within this study, we synthesized two materials, Na2/3Mg1/9Ni2/9Mn2/3O2 and Na2/3Mg2/9Ni1/9Mn2/3O2 (Mg-0.11, Mg-0.22), subsequently probing their transition metal and oxygen redox contributions to electrochemical capacity during the initial and 10th cycle charge and discharge processes by synchrotron-based mapping of resonant inelastic X-ray scattering and soft X-ray Absorption Spectroscopy. Our findings corroborate the presence of similar O redox contributions in the Mg-substituted materials. Intriguingly, the Mg-0.11 material exhibits a substantial enhancement in O redox following ten cycles, warranting further investigation due to its exceptional multiplicity, cycling performance, minimal voltage hysteresis, and potential impact on advancing sodium-ion battery commercialization.

Regulating Excess Electrons in Reducible Metal Oxides for Enhanced Oxygen Evolution Reaction Activity: A Mini-Review.

Identifying a universal activity descriptor for metal oxides, akin to the d-band center for transition metals, remains a significant challenge in catalyst design, largely due to the intricate electronic structures of metal oxides. This review highlights a major advancement in formulating the number of excess electrons (NEE) as an activity descriptor for oxygen evolution reaction (OER) on reducible metal oxide surfaces. We elaborate on the quantitative relationship between NEE and the adsorption properties of OER intermediates, and unveil the decisive role of the octet rule on the OER performance of these oxides. This insight provides a robust theoretical basis for designing effective OER catalysts. Moreover, we discuss critical experimental evidence supporting this theory and summarize recent advances in employing NEE as a guiding principle for developing highly efficient OER catalysts experimentally.

Photochemical tuning of dynamic defects for high-performance atomically dispersed catalysts.

Developing active and stable atomically dispersed catalysts is challenging because of weak non-specific interactions between catalytically active metal atoms and supports. Here we demonstrate a general method for synthesizing atomically dispersed catalysts via photochemical defect tuning for controlling oxygen-vacancy dynamics, which can induce specific metal-support interactions. The developed synthesis method offers metal-dynamically stabilized atomic catalysts, and it can be applied to reducible metal oxides, including TiO2, ZnO and CeO2, containing various catalytically active transition metals, including Pt, Ir and Cu. The optimized Pt-DSA/TiO2 shows unprecedentedly high photocatalytic hydrogen evolution activity, producing 164 mmol g-1 h-1 with a turnover frequency of 1.27 s-1. Furthermore, it generates 42.2 mmol gsub-1 of hydrogen via a non-recyclable-plastic-photoreforming process, achieving a total conversion of 98%; this offers a promising solution for mitigating plastic waste and simultaneously producing valuable energy sources.

Temperature-Controlled Hydrothermal Synthesis of α-MnO2 Nanorods for Catalytic Oxidation of Cyclohexanone.

This work describes the comparison of the catalytic performances of α-MnO2 nanorods synthesized by a facile hydrothermal approach at varying temperatures (140-200 °C). The structure and morphology of these nanorods were analyzed by XRD, N2-physisorption, NH3-TPD, Raman, SEM, HRTEM, and XPS. The prepared α-MnO2 nanorods also performed exceptionally well in the catalytic oxidation of cyclohexanone to dicarboxylic acids under mild reaction conditions. The characterization results conferred that there is a significant influence of hydrothermal temperatures on the textural properties, morphology, and catalytic activity. Notably, the α-MnO2 nanorods obtained from 180 °C hydrothermal conditions outperformed other catalysts with 77.3 % cyclohexanone conversion and 99 % selectivity towards acid products such as adipic acid (AA), glutaric acid (GA) and succinic acid (SA). The improved catalytic activity may be attributed to the interaction of the bifunctional Mn3+/4+ redox metal centres and surface acidic sites. The present oxidation reaction was found to be a promising eco-benign process with high selectivity for the production of commercially significant carboxylic acids from cyclohexanone.

Rational design of crystalline/amorphous nickel manganese phosphate octahydrate heterostructure for high-performance aqueous and all-solid-state asymmetric supercapacitors

Designing a rational architecture and combining diverse metal cations are of vital importance to obtain high-performance bimetallic compounds for energy storage purposes. Herein, a novel crystalline/amorphous nickel manganese phosphate octahydrate heterostructure grown in-situ on nickel foam was synthesized by a facile hydrothermal method followed by annealing in argon. The composite exhibits exceptional performance by adjusting the atomic ratio of redox metals (nickel and manganese) with multiple oxidation states. Specifically, the composite shows an excellent specific capacitance of 2351.6 F g−1 at a current density of 1 A g−1 and 67.3 % capacity retention after 3000 cycles at 10 A g−1. This remarkable performance can be attributed to the unique heterogeneous structure of the composite and the synergistic effect between two metals, improving the electronic conductivity. Density functional theory (DFT) calculations further confirm the reduction of band gap and conductivity enhancement. Furthermore, the crystalline portions offer not only active sites but also good mechanical support, while the amorphous components effectively promote ion diffusion and reaction. An aqueous asymmetric supercapacitor was assembled employing nickel manganese phosphate octahydrate as cathode and active carbon as anode, displaying a high energy density of 66.22 Wh kg−1 at a power density of 400 W kg−1 and outstanding cycling performance at 60 mA cm−2 (81.03 % capacitance retention after 15,000 cycles). Additionally, the fabricated all-solid-state asymmetric supercapacitor also displays a remarkable electrochemical performance.

Tuning Strong Metal‐Support Interactions for Enhancing Direct Deoxygenation of Biomass‐Lignin Derived Phenolics

AbstractCatalytic direct deoxygenation (DDO) of phenolic compounds to aromatics is an appealing approach for utilization of biomass‐lignin with minimal amount of H2 consumption. Metal/reducible metal oxide catalysts provide two functionalities, i. e., C−H bond formation (hydrogenation) and C−O bond breakage (deoxygenation), required for this reaction that takes place at the interfacial perimeter sites. Strong metal‐support interactions (SMSI) can profoundly alter the density and property of such sites. This short review summarized recent advances in tuning SMSI for enhancing DDO of phenolics. The approaches of varying reduction temperature, modulation of crystal facets and crystal phases of metal oxide, and reduction of mixed metal oxide were discussed for the origin for formation of SMSI, the degree of SMSI, and its consequence on the DDO performance. The analysis revealed that intermediate degree of SMSI with maximal density of metal/oxide interfacial perimeter site enhances DDO while minimizing unfavorable side reactions. This short review highlights the concept of tuning the interfacial perimeter sites via SMSI for reactions which require multiple functionalities and take place at the metal/oxide interface.

Production of synthesis gas from dry reforming of propane with carbon dioxide over ceria-promoted nickel foam catalysts

A series of nickel foam supported cerai catalysts (ceria/NiF) for dry reforming of propane with carbon dioxide were prepared and their performance were evaluated at temperatures in the range of 520-600°C. Among the catalysts examined the 4 wt.% CeO2/NiF exhibited better low-temperature activity for the DRP, which is attributed to the formation of well- isolated nanosize CeO2 particles and highly reducible metal oxide species over the NiF support. The XPS and temperature programmed reduction (TPR) data also supported the incorporation of ceria into the NiO surface layer formed over NiF support, which was substantially higher in 4% CeO2/NF catalyst. Due to these favorable properties, the 4 wt.% CeO2/NiF could be considered as an excellent catalyst for DRP.

Effect of the Interaction between Transition Metal Redox Center and Cyanide Ligand on Structural Evolution in Prussian White Cathodes.

Transition metal (TM) based Prussian whites, comprising a cyanide anion ((C≡N)-) and TM cations in an alternative manner, have been widely adopted as cathode materials for rechargeable batteries. Prussian whites are characterized by the TM electronic states that exclusively adopt low spin (LS) toward the C atom and high spin (HS) toward the N atom through the hybridized covalent bonding in the TM─C≡N─TM unit with the average oxidation states of the TM ions being 2+, considerably affecting the phase transition behavior upon the release and storage of carrier ions; however, there have been only a few studies on their associated features. Herein, Prussian whites with different HS TM ions were synthesized via coprecipitation and the phase transition behavior controlled by the π electron interaction between the cyanide anions and TM ions during battery operations was investigated. In situ X-ray characterizations reveal that the combined effect of π backdonation in the LS Fe-C unit and π donation in the HS TM-N unit effectively controls the bond length of the TM─C≡N─TM building unit, thus markedly influencing the lattice volume of a series of Prussian white cathodes during the charge/discharge process. This study presents a comprehensive understanding of the structure-property relationship of the Prussian white cathodes involving π electron interactions during battery operations.

Sulfidation of Ni-Fe-LDH for enhanced Fenton-like catalysis: Experimental validation and theoretical calculation

In this study, peroxymonosulfate (PMS) activation of Ni-Fe layered double hydroxide (Ni-Fe-LDH) toward to Tetracycline hydrochloride (TC) degradation was enhanced by sulfidation. When compared to the catalytic performance of Ni-Fe-LDH, the kinetic rate constant of the S-Ni-Fe-LDH/PMS system was 6.3 times higher than that of the Ni-Fe-LDH/PMS system. The degradation process in the S-Ni-Fe-LDH/PMS system was successfully achieved through the combined efforts of both radical and nonradical pathways, with singlet oxygen playing a key role in the process. The boosted performance brought by sulfidation was lie in: (i) the superior electrical conductivity; (ii) the low redox potential of S2- that facilitate metal redox in LDH; (iii) the production of 1O2 has been found to be increased in S-Ni-Fe-LDH compared to Ni-Fe-LDH, due to the higher energy desorption exhibited by S-Ni-Fe-LDH towards radical species; (iv) noteworthily, a new generation of high-valent metal-oxo species in S-Ni-Fe-LDH/PMS system. Consequently, the S-Ni-Fe-LDH/PMS system displayed satisfactory degradation efficiency in a broad pH range of 3–11 and even under the presence of inorganic ions and humic acid, as well as continuous-flow catalytic process. Based on the identification of transformation intermediates using HPLC-MS, three potential pathways for TC decomposition were deduced. The analysis conducted using ECOSAR demonstrated that the toxic effects decreased for the majority of transformation products during TC degradation. This work introduces new opportunities for designing highly efficient LDH-based catalysts for the decomposition of organic pollutants in water solutions.

Sunlight assisted highly efficient desorption of ammonia by redox graphene hybrid metal–organic framework photothermal conversion adsorbents

The capture, storage and release of NH3 play a pivotal role in sustainable energy applications. However, to simultaneously achieve high adsorption capacity and low energy desorption remains a challenge. In this work, a novel class of hybrid materials with photothermal conversion performance have been developed by doping redox graphene and metal–organic framework of MOF-303(Al). It is found that the optimized rGO@MOF-303(Al)-8% exhibits high NH3 uptake (17.0 mmol g−1) at 25.0 °C and 1.0 bar and exceptional photothermal conversion performance. Under the irradiation of simulated sunlight, its temperature can be raised to about 75 °C in 10 min. Even under direct natural sunlight irradiation, a stable equilibrium temperature of 53 °C may be achieved. Based on this excellent photothermal conversion performance, we investigated the desorption behavior of ammonia under visible light irradiation for the first time. We found that due to the breakdown of hydrogen bonds of NH3 and N-N, N–H sites of rGO@MOF-303(Al)-8% caused by the rapid rise of temperature, approximately 68% of NH3 could be effectively desorbed in 2-hour irradiation under ambient conditions. In addition, the optimized adsorbent demonstrated remarkable cycling stability. After undergoing 10 cycles, high adsorption capacity and efficient light-induced desorption of NH3 were still maintained Thus, the present study provides a novel concept for high-capacity adsorption and low-energy desorption of NH3 and other industrial gases.

Preparation of Li4SiO4 from lithium-ion battery cathode waste and diamond wire saw silicon powder using a two-step process

Li4SiO4 materials have excellent high-temperature CO2 adsorption properties. In this thesis, Li4SiO4 was produced by a two-step process by using Li+ from waste lithium-ion battery cathodes as a partial lithium source. The diamond wire saw silicon powder generated by the photovoltaic industry, was used as the silicon source. The reduction melting process of LiMeO2 (Me = Ni, Co, or Mn) in the battery cathode material was analyzed, and the effect of the Li: Si molar ratio in the melting system on the reduction of metal oxides was investigated. The effects of roasting temperature and roasting time on the adsorption properties of Mn-doped Li4SiO4 were also studied. The results showed that the maximum adsorption capacity of Mn-doped Li4SiO4 reached 29.44 wt% under a pure CO2 atmosphere, which was about 2.6 times higher than that of pure Li4SiO4. The saturated adsorption capacity reached 34.2 wt%. This study achieved the resource utilization of solid waste and may reduced the preparation costs of Li4SiO4 adsorbent, which may promote carbon capture technologies.

Proton-coupled electron transfer at a mis-metalated zinc site detected with protein charge ladders.

Distinguishing proton-coupled electron transfer (PCET) from uncoupled electron transfer (ET) in proteins can be challenging. A recent investigation [J. C. Koone, M. Simmang, D. L. Saenger, M. L. Hunsicker-Wang and B. F. Shaw, J. Am. Chem. Soc., 145, 16488-16497] reported that protein charge ladders and capillary electrophoresis can distinguish between single ET, PCET, and two-proton coupled ET (2PCET) by directly measuring the change in protein net charge upon reduction/oxidation (ΔZET). The current study used similar methods to assess PCET in zinc-free, "double copper" superoxide dismutase-1 (4Cu-SOD1), where one copper is bound at the copper site of each monomer and one copper is bound at the bridging zinc site, resulting in a quasi-type III Cu center. At pH 7.4, the net charge (Z) of the 4Cu-SOD1 dimer was unaffected by reduction of all four Cu2+ ions, i.e., ΔZ4ET = -0.09 ± 0.05 per dimer (-0.02 ± 0.01 per copper atom). These values suggest that PCET is taking place at all four Cu atoms of the homodimer. Molecular dynamics and Poisson-Boltzmann calculations suggest that a metal-coordinating histidine at the zinc site (His71) is the proton acceptor. These data show how ligands of a naturally occurring zinc site can help facilitate PCET when the right redox metal is bound.

Electrochemical benzylic deuteration of p-QMs enabling the synthesis of benzylic deuterated diarylmethanes.

Herein, electroreductive umpolung benzylic deuteration of p-QMs using cheap and easily accessible D2O as a deuterium source is reported. Various value-added benzylic deuterated diarylmethanes can be synthesized without the requirement of noble metal catalysts, redox reagents, and strong bases. The establishment of this protocol will provide an alternative strategy for acquiring benzylic deuterated diarylmethanes.