Mixed-valence Cobalt Research Articles

Boosting low-temperature NH3-SCR performance via mixed-valence cobalt dual active sites

Nitrogen oxides (NOx) have become a critical environmental concern, necessitating the development of highly efficient catalysts for temperature which is lower than 300 °C. Selective catalytic reduction with NH3 (NH3-SCR) is a process where ammonia is the reducing agent for selective catalytic reduction of nitrogen oxides in exhaust gases. The mechanism-property relationship between catalytic active sites and low-temperature denitrification efficiency, however, remains unclear. In this study, we synthesized a series of cobalt-modified nanoparticle catalysts using a scalable citric acid sol-gel method and conducted comprehensive characterizations. Our findings indicate that an optimal cobalt doping rate significantly enhances catalytic activity due to the synergistic effects of reduced nanoparticle size and the dual active sites of Co0/Co2+. Specifically, at a cobalt doping ratio of 1.0, the catalyst demonstrates peak SCR activity, achieving 85.5% NOₓ conversion at 120 °C and complete conversion at 150 °C. These cobalt-modified nanoparticle catalysts feature high deposition of doped Co ions on their surfaces, predominantly as metallic cobalt and CoOx nanoclusters. Compared to undoped samples, the cobalt-doped nanoparticle catalysts exhibit a greater specific surface area, increased acidity, enhanced chemisorbed oxygen, and improved redox properties, all contributing to superior denitrification efficacy. Additionally, Density Functional Theory (DFT) calculations provide assertive evidence into the reaction mechanism, showing that Co2+ sites effectively adsorb and activate NH₃, while Co0 sites are crucial for NO adsorption and activation. The synergistic interaction between Co2+ and Co0 facilitates NN coupling, enhancing overall reaction efficiency. These findings underscore the potential of cobalt-modified catalysts for practical industrial applications, offering robust performance over a wide temperature range and meeting stringent environmental regulations.

Enhanced Fenton-like process on Co9S8 catalyst with Mixed-Valence Cobalt(0)/Cobalt(Ⅱ)

Peroxymonosulfate (PMS)-based Fenton-like reactions have great potential to revolutionize wastewater treatment, but achieving highly efficient decontamination at low chemical input remains extremely challenging. In this study, a carbon carrier-supported Co9S8 catalyst with Mixed-Valence Cobalt(0)/Cobalt(Ⅱ) was prepared, which enables 70.72 % PMS decomposition-rate and complete, highly efficient tetracycline (TC) removal. Through the cyclic conversion pathways of polyvalent cobalt and the auto-decomposition of PMS, which produced SO4•-, 1O2, •OH, and high-valence metals, the material’s efficiency in degrading TC through PMS activation was effectively enhanced (99.92 %). S2-, as one of the active sites, not only promotes the participation of Co3+/Co2+/Co0 in the generation of reactive oxygen species but also significantly facilitates electron transfer to further increase the catalytic reaction rate. Through metal-chelating agents, it is illustrated that Co2+ directly activates PMS to produce reactive oxygen species (ROS), while Co3+ promotes cycling. The normalized kinetic constant of Co9S8/Co-C (105.6 min−1M−1) was even four times larger than that of Co9S8 (26.4 min−1M−1). Co9S8/Co-C exhibited rapid degradation over a wide pH range and extensive cycling experiments, demonstrating its versatility and stability in various environments. Possible degradation pathways were speculated using liquid chromatography-mass spectrometry (LC-MS). Toxicological evaluations and mung bean germination toxicity tests indicated that the Co9S8/Co-C+PMS system effectively reduced toxicity. It enables the complete degradation of TC while remarkably reducing the toxic intermediates, ensuring that the catalyst is suitable for wastewater treatment applications.

Synthesis, structure and catecholase activity of a trinuclear mixed valence cobalt complex

A trinuclear mixed valence cobalt complex, [L3CoIICoIII2(CH3COO)(OCH3)] (2), has been synthesized using a Schiff base ligand (H2L, 1). The ligand was prepared via 1:1 condensation of 3-Chloro salicylaldehyde and 3-Amino-1-propanol in methanol. The structure of 2 has been elucidated by single crystal X-ray crystallography. Comparing each set of the corresponding data of bond lengths and bond angles, presence of two different oxidation states (+2 and +3) of the cobalt metal ions was understood. Again the occurrence of mixed valence has been confirmed by X-ray photoelectron spectroscopy (XPS) study of 2. Interestingly, 2 acted as an efficient bio-mimic of catechol oxidase toward oxidation of 3,5-di‑tert-butyl catechol (3,5-DTBC) to 3,5-di‑tert-butyl quinone (3,5-DTBQ). Two different solvents, methanol (CH3OH) and acetonitrile (CH3CN), have been utilized as media for exploring catecholase activity of 2 and acetonitrile was observed to be a better medium since furnished greater turn-over number. 1HNMR spectroscopy suggests completion of the oxidation process within just ten minutes in acetonitrile. The ESI-MS spectrum of the substrate-catalyst mixtures in acetonitrile confirmed formation of Co(III)-3,5-DTSQ (3,5-DTSQ = 3,5-di‑tert butyl semiquinone) intermediates in the time scale of mass spectrum which could stand as a supporting evidence for the catalytic function of 2. However, all the substrate bound intermediates observed are mononuclear which indicates that the complex probably undergoes a dissociation in acetonitrile to generate mononuclear fragments for binding the substrate. Thus, on the basis of ESI-MS, 2 may be suggested as a pre-catalyst.

An insight into the non-covalent Cl•••Cl interaction in the solid state structure of carboxylate bridged trinuclear mixed valence cobalt(III/II/III) complexes with tetradentate N2O2 donor reduced Schiff base ligands

Two mixed-valence trinuclear cobalt(III)-cobalt(II)-cobalt(III) complexes, [CoII{(μ-L1)(μ-OOCC6H5NO2)CoIII(N3)}2] (1) and [CoII{(μ-L2)(μ-OOCC6H4NO2CH3)CoIII(N3}2] (2) have been synthesized using two tetradentate N2O2 donor 'reduced Schiff base' ligands, H2L1 {2,2-dimethyl-1,3-propanediyl)bis(iminomethylene)bis(4-chlorophenol}, and H2L2{2-hydroxy-1,3-propanediyl)bis(iminomethylene)bis(4-chlorophenol}, and 3-nitrobenzoic benzoic acid or 4-methyl 3-nitrobenzoic benzoic acid as anionic co-ligands. The complexes have been characterized by spectroscopic measurements and their solid state structures have been determined by single crystal X-ray diffraction analysis. The trinuclear cobalt(III)-cobalt(II)-cobalt(III) skeleton of both the complexes are linear and centrosymmetric. Both the central cobalt(II) and terminal cobalt(III) of both the complexes are hexa-coordinated. DFT calculations have been used to analysed some supramolecular assemblies of the complex that have been observed in the solid state, specifically the Cl···Cl interactions that result in the creation of a stable 2D supramolecular assembly in the solid state. QTAIM and NCI Plot computational tools have been used to characterize the interactions. Thermo-gravimetric (TG) analysis was used to examine the thermal behaviour of both complexes. The redox behaviors of both the complexes were studied by using Cyclic Voltammetry (CV).

Application of a distinctly bent, trinuclear, end-to-end azide bridged, mixed valence cobalt(iii/ii/iii) complex in the fabrication of photosensitive Schottky barrier diodes.

A mixed-valence trinuclear cobalt(iii)-cobalt(ii)-cobalt(iii) complex, [(μ-1,3-N3)Co3L(N3)3]·MeOH has been synthesized using a tetradentate N2O2 donor 'reduced Schiff base' ligand, H2L {1,3-bis(2-hydroxybenzylamino)2,2-dimethylpropane} and azide as anionic co-ligand. The complex has been characterised by elemental analysis, IR, UV-vis spectroscopy and single-crystal X-ray diffraction studies etc. The cobalt(iii)-cobalt(ii)-cobalt(iii) skeleton in the complex is non-linear and non-centrosymmetric. The redox behavior of the complex was studied by using Cyclic Voltammetry (CV). The complex is found to be a semiconductor material as confirmed by determining the band gap of this complex by experimental as well as theoretical studies. The band gap in the solid state has been determined experimentally. The conductivity of the synthesized complex based device improves considerably in illumination conditions from the non-illuminated conditions. The complex has also been used to fabricate Schottky barrier diodes.

Synthesis, Structure, and Magnetic Properties of Two {Co16IICo4III} Supertetrahedral T4 Clusters Formed from the In Situ Dissociation of a Tertiary N-Containing Ligand

Two mixed-valent cobalt clusters {[(CoII)16(CoIII)4(μ6-O)4(μ3-OH)12(OCH3)12(H2O)8(BMBC)4]·(NO3)8} (1) and {[(CoII)16(CoIII)4(μ3-OH)20(OCH2CH3)4(HOCH2CH3)8(BMBC)4]·(NO3)8} (2) were synthesized and constructed by a semiflexible dicarboxylic ligand 4′,4′-[ammoniobis(methylene)]-bis[(1,1′-biphenyl)-2-carboxylate] (BMBC–) with Co(NO3)2·6H2O under solvothermal systems. Interestingly, the ligand BMBC- is afforded from the in situ dissociation reaction of tertiary N-containing organic carboxylic acid ligands. The complexes present a beautiful supertetrahedral T4 cluster formed by an interior Co4IIIO4 cubane and four exterior Co4IIO4 cubanes bridged via μ6-O2 and μ3-OH– ions and stabilized by four BMBC– organic ligands. The complexes were confirmed by single-crystal X-ray diffraction (XRD), elemental analysis, infrared spectroscopy, and thermogravimetric analysis. Furthermore, the magnetic analyses reveal that the [Co20] cluster exhibits an antiferromagnetic behavior.

Magnetic Field-Enhanced Electrocatalytic Oxygen Evolution on a Mixed-Valent Cobalt-Modulated LaCoO3 Catalyst.

Extensive efforts to enhance the oxygen evolution reaction (OER) catalytic performance of transition metal oxides mainly concentrate on the extrinsic morphology tailoring, lattice doping, and electrode interface optimizing. Nevertheless, little room is left for performance improvement using these methods and an obvious gap still exists compared to the precious metal catalysts. In this work, a novel "mixed-valent cobalt modulation" strategy is presented to enhance the electrocatalytic OER of perovskite LaCoO3 (LCO) oxide. The valence transition of cobalt is realized by ethylenediamine post reduction procedure at room temperature, which further induces the variation of magnetic properties for LCO catalyst. The optimized LCO catalyst with Co2+ /Co3+ of 1.98 % exhibits the best OER activity, and the overpotential at 10 mA cm-2 current density is decreased by 170 mV compared pristine LCO. Impressively, the ferromagnetic LCO catalyst can perform magnetic OER enhancement. By application of an external magnetic field, the overpotential of LCO at 10 mA cm-2 can be further decreased by 20 mV compared to that of under zero magnetic field, which arises from the enhanced energy states of electrons and accelerated electron transfer process driven by magnetic field. Our findings may provide a promising strategy to break the bottleneck for further enhancement of OER performance.

Mixed-valence cobalt oxides bifunctional electrocatalyst with rich oxygen vacancies for aqueous metal-air batteries

As the bifunctional oxygen electrocatalysts, the mixed-valence metal oxide with multiple oxidation state of metal ions usually gives analogous alloy effect. However, the irreversible surface reconstruction of the electrocatalyst during the electrochemical catalysis usually changes the oxidation state of metal ions in the near-face to higher oxidation state especially in the mixed-valence metal oxide. For the modulation equilibrium, we adopt a new approach to prepare the mixed-valence cobalt oxide with the regulatable ratio of Co2+/Co3+ by inducing air at a certain temperature during the cooling procedure. The obtained Co/CoO/Co3O4/NCS with hollow nanosphere possesses multiple heterointerfaces accompanying rich oxygen vacancies. Together with the contribution of the mixed valence and oxygen vacancies, Co/CoO/Co3O4/NCS exhibits an exceptional ORR activity with a half-wave potential of 0.844 V and superb OER activity with a low overpotential of 285 mV at 10 mA cm−2. The as-fabricated Zn-air batteries with Co/CoO/Co3O4/NCS catalyst exhibit a remarkable open-circuit voltage (1.47 V), a competitive power density of 146.5 mW cm−2, and good durability without nearly no performance decay for over 400 h at 5.0 mA cm−2. More attractively, when constructed into Mg-air batteries, Co/CoO/Co3O4/NCS cathode realizes an excellent stable discharge voltage over 90 h at 20 mA cm−2.

Linear Magnetodielectric Effect and Huge Negative Magnetization in a Mixed-Valent Cobalt Manganite Spinel Oxide

Linear Magnetodielectric Effect and Huge Negative Magnetization in a Mixed-Valent Cobalt Manganite Spinel Oxide

Oxidative desulfurization and magnetic properties of a mixed-valence cobalt vanadate

Exploiting hydrothermal technique, a new cobalt-vanadate termed Co(en)2[VIV4VV2O14] was prepared, where en stands for ethylenediamine. For its elucidation, a combination of powder X-ray diffraction, FT-IR, X-ray single-crystal diffraction and elemental analysis was performed. The compound displays a 3D configuration consisting of mixed -valence vanadium (IV,V) oxide layers and metal–organic bridges. The catalytic potential of compound 1 for the heterogeneous oxidative sulfides is strong, and 1 is reusable without activity depletion. Furthermore, the magnetic behavior demonstrates the antiferromagnetic interactions in 1.

DFT study on CH⋯O, CH···SCN and S···π interaction energies in three dinuclear mixed valence cobalt(III/II) complexes with secondary diamine ligands having inner N2O2 and outer O4 compartments

Three dinuclear cobalt(III/II) mixed valence reduced Schiff base complexes, [(N3)CoIIIL(μ-O2CR1)CoII(N3)] (1), [(SCN)CoIIIL(μ-O2CR2)CoII(NCS)] (2) and [(SCN)CoIIIL(μ-O2CR3)CoII(NCS)] (3) {H2L= (2,2-dimethyl-1,3-propanediyl)bis(iminomethylene)bis(6-ethoxyphenol); R1CO2H = 4-methyl-3-nitrobenzoic acid; R2CO2H = benzoic acid; R3CO2H = 3-methyl-4-nitrobenzoic acid} have been synthesized and characterized by elemental analysis and spectral study. X-ray diffraction analysis was used to determine the structures of all three complexes. Spin density calculations have been used to determine the location of the paramagnetic cobalt(II) center in each of the dinuclear complexes. Moreover, several supramolecular assemblies (formed by CH⋯O, CH···SCN and S··· π and other interactions) observed in the solid state of the three complexes involving the pseudohalide as electron donors have been analysed using DFT calculations, molecular electrostatic potential (MEP) surfaces and characterized using the noncovalent interaction plot (NCI Plot) index.

Exploration of noncovalent interactions in the solid state structures of carboxylate bridged trinuclear mixed valence cobalt complexes using computational tools based on the topological analysis of the electron density

Three linear mixed valence trinuclear cobalt(III)-cobalt(II)-cobalt(III) complexes, [CoII{(μ-L1)(μ-OOCCH3)CoIII(OOCCH3)}2]⋅4H2O (1), [CoII{(μ-L2)(μ-OOCCH3)CoIII(OOCCH3)}2]⋅4H2O (2) and [CoII{(μ-L1)(μ-OOCPh)CoIII(OOCPh)}2] (3), have been synthesized using two N2O2 donor 'reduced Schiff base' ligands, H2L1 (2,2-dimethyl-1,3-propanediyl)bis(iminomethylene)bis(4-chlorophenol) and H2L2 (2,2-dimethyl-1,3-propanediyl)bis(iminomethylene)bis(4-bromophenol), and acetate or bezoate as anionic co-ligands. The complexes have been characterized by spectroscopic measurements and their solid state structures have been determined by single crystal X-ray diffraction analysis. DFT calculations have been performed to study the mixed valence nature of the complexes. Moreover, the structure directing role of several noncovalent interactions in the solid state of the complexes has been investigated. The interactions have been characterized by a combination of two computational tools based on the topological analysis of the electron density, which are the quantum theory of atoms-in-molecules (QTAIM) and the noncovalent interaction plot, since they are useful to reveal noncovalent interactions in the solid state.

A Highly Active Nitrogen‐Doped Mixed‐Phase Mixed‐Valence Cobalt Nanocatalyst for Olefins and Nitroarenes Hydrogenation

AbstractDevelopment of an inexpensive, environmentally friendly and robust recyclable heterogeneous catalyst for alkene and nitroarene hydrogenation is in high demand for the sustainable future of chemical industries. Herein, we have synthesize and characterized a trinuclear mixed valence cobalt complex 1 and nitrogen doped mixed phase cobalt (β‐Co(OH)2 and CoO) nanocatalyst (N@MPCoNC) as reported earlier. N@MPCoNC was synthesized via single source molecular precursor by reducing 1 by aq. N2H4 followed by calcinations. Both mixed‐valence cobalt metal complex (1) and nitrogen doped mixed phase cobalt nanocatalyst (N@MPCoNC) were utilized for alkenes and nitroarenes hydrogenation reaction, however excellent catalytic activity was noticed by N@MPCoNC with the 100 % conversion of styrene and 4‐nitrotoluene to the corresponding ethylbenzene and 4‐aminotoluene with 100 % selectivity under mild reaction condition. The scope of the heterogeneous N@MPCoNC was further evaluated for other alkenes and nitroarenes to obtain corresponding alkanes and aminoarenes with an excellent conversion and high selectivity at room temperature.

Estimation of the ability of the π-system of pseudohalides (azide and thiocyanate) to participate in CH···π interactions in cyclic hetero-tetranuclear cobalt(III)/sodium and linear trinuclear mixed valence cobalt(III/II/III) complexes

A cyclic tetranuclear cobalt(III)/sodium complex, [{(N3)CoIII(L1)Na(DMSO)}(μ1,3-N3)2{(N3)CoIII(L1)Na(OH2)}]2⋅0.5CH3OH (1) and a bent trinuclear mixed valence complex [CoII{(μ-L2)(μ-OAc)CoIII(NCS)}2]⋅CH3CN⋅0.5H2O(2) have been prepared and their molecular structures have been determined by X-ray crystallography [H2L1 = N,N'-bis(3-methoxysalicylidene)-2,2-dimethyl-1,3-diaminoproane, H2L2 = N,N'-bis(2-hydroxynaphthylmethylidene)propane-1,3-diamine]. Complex 1 contains two cobalt(III) and two sodium centers, while on the other hand, complex 2 contains a high-spin Co(II) and two terminal low-spin Co(III) centers in a CoIII(S = 0)-CoII(S = 3/2)-CoIII(S = 0) trimer, as confirmed by DFT calculations Theoretical calculations have been used to analyze the importance of CH···π interactions involving the π-systems of the pseudohalide ligands. Indeed apart from the formation of conventional π···π stacking and CH···π(arene) interactions both complexes exhibit CH···π(pseudohalide) interactions that have been evaluated energetically by using mutated dimers and characterized by means of QTAIM and NCI plot computational methods which can be used to clarify interactions in real space.

Oxidative Desulfurization and Magnetic Properties of a Mixed-Valence Cobalt Vanadate

Oxidative Desulfurization and Magnetic Properties of a Mixed-Valence Cobalt Vanadate

An unusual mixed-valence cobalt dimer as a catalyst for the anti-Markovnikov hydrophosphination of alkynes

A Co dimer with a 3e-reduced NNN pincer ligand [(PMe3)2CoII(L13−)CoI(PMe3)3] is an efficient catalyst for the hydrophosphination reaction of both internal and terminal alkynes including α-hydroxy-functionalized alkyne substrates with excellent stereo- and regioselectivity.

Cobalt Catalysts Enable Selective Hydrogenation of CO2 toward Diverse Products: Recent Progress and Perspective.

Selective hydrogenation of carbon dioxide (CO2) into value-added chemicals has aroused great interest. The chemical inertness of CO2 and diverse reaction pathways usually require the construction of enabled catalysts. To date, cobalt (Co) catalysts characteristic of metallic and/or divalent Co components show great potential for CO2 hydrogenation. To better regulate the CO2 hydrogenation, it is necessary to summarize the current progress of cobalt catalysts for selective hydrogenation of CO2. In this Perspective, first, hydrogenation of CO2 into methane over metallic Co sites is introduced. Second, hydrogenation of CO2 into methanol and C2+ alcohols is discussed by constructing mixed-valent cobalt sites. Third, hydrogenation of CO2 into light olefins and C5+ liquid fuels over cobalt-containing hybrid catalysts is introduced. Fourth, the reaction paths for selective hydrogenation of CO2 over cobalt catalysts are illustrated. Finally, the current challenges and prospects of cobalt-based nanocatalysts for hydrogenation of CO2 are proposed.

A supercapacitor electrode formed from amorphous Co3(PO4)2 and the normal spinel CoIICoIII2O4

Transition-metal oxides and phosphates can engage oxidation states that enhance reduction-oxidation (redox) reactions and can lead to excellent pseudo-capacitance behavior. In this study, amorphous Ar–Co3(PO4)2 (Ar ​= ​amorphous) was combined with the mixed valence cobalt oxide Co3O4 (or CoIICoIII2O4) to form a composite with a higher specific capacitance (531. 4 ​F/g) than pure Co3O4 (302.8 ​F/g) at 3 ​mV/s in 3 ​M KOH. The material was synthesized using an in-situ precipitation reaction of ammonium cobalt phosphate on Co3O4 followed by thermal treatment at 550 ​°C. Studies into the mechanism showed improved charge storage was linked to Co3O4 having rapid redox reactions on its surface. Cyclic stability studies revealed that the Ar–Co3(PO4)2/Co3O4 composite showed better stability with capacitance retention of 89.1% than when compared to pure Co3O4 at 76.5%. Both pure Co3O4 and Ar–Co3(PO4)2/Co3O4 contain mesopores with the former having a larger average pore volume. These findings demonstrate the potential of Ar–Co3(PO4)2 and Co3O4 as a promising electrode composite material in the fabrication of high-performance supercapacitor devices.

An insight to the spin density distribution and non-covalent interactions in a carboxylate bridged class-I mixed valence cobalt(II),cobalt(III) complex of quadruplet nature

A dinuclear mixed valence cobalt(II),cobalt(III) complex has been synthesized using a hexa-dentate (N2O4 donor) compartmental 'reduced Schiff base' ligand, thiocyanate and 4-methyl-3-nitrobenzonate as anionic co-ligands. The complex has been characterized by spectroscopic measurements and its solid state structure has been determined by single crystal X-ray diffraction analysis. The high spin configuration is 37 kcal/mol more stable than the low spin configuration at the PBE0-D3/def2-ZVP level of theory thus evidencing that the quadruplet nature of the complex, in good agreement with the experimental effective magnetic moment. The spin density distribution for the low spin shows that the unpaired electron is located at the dx2-y2 orbital. For the high spin configuration, the spin density distribution is more spherical due to the contribution of three d orbitals. There is some spin delocalization toward the oxygen and nitrogen atoms directly bonded to the cobalt(II) center; however, the spin density at the cobalt(III) is negligible. Non-covalent π-stacking interactions involving the 4-methyl-3-nitrobenzonate co-ligands have been studied, analysing the effect of the metal coordination on the interaction strength. The interaction has been also characterized by using the “non-covalent interaction plot” (NCI plot) method. The strength of the π–π stacking interaction is enhanced by the coordination to the cobalt centers, due to the amplification of the dipole–dipole force.

Nitrogen-Doped Mixed-Phase Cobalt Nanocatalyst Derived from a Trinuclear Mixed-Valence Cobalt(III)/Cobalt(II) Complex for High-Performance Oxygen Evolution Reaction.

Because of a continuous increase in energy demands and environmental concerns, a focus has been on the design and construction of a highly efficient, low-cost, environmentally friendly, and noble-metal free electrocatalyst for energy technology. Herein we report facile synthesis of the mixed-valence trinuclear cobalt complex 1 by the reaction of 2-amino-1-phenylethanol and CoCl2·6H2O in methanol as the solvent at room temperature. Further, 1 was reduced by using aqueous N2H4 as a simple reducing agent, followed by calcination at 300 °C for 3 h, yielding a nitrogen-doped mixed phase cobalt [β-Co(OH)2 and CoO] nanocatalyst (N@MPCoNC). Both 1 and N@MPCoNC were characterized by various physicochemical techniques. Moreover, 1 was authenticated by single-crystal X-ray diffraction studies. The hybrid N@MPCoNC reveals a unique electronic and morphological structure, offering a low overpotential of 390 mV for a stable current density of 10 mA cm-2 with high durability. This N@MPCoNC showed excellent electrocatalytic as well as photocatalytic activity for oxygen evolution reaction compared to 1.