Cobalt Species Research Articles

Hydrogen‐Induced Disproportionation of Samarium‐Cobalt Intermetallics Enabling Promoted Hydrogen Evolution Reaction Activity and Durability in Alkaline Media

AbstractTransition‐metal nanoparticles hold great promise as electrocatalysts for alkaline hydrogen evolution reaction (HER), however, addressing the simultaneous challenges of ensuring sufficient active sites, promoting favorable water dissociation, and optimizing binding energy toward hydrogen intermediates remains a formidable task. To overcome these hurdles, a novel gaseous hydrogen engineering strategy is proposed by in situ embedding cobalt nanoparticles within a samarium hydride matrix (Co/SmH2) via hydrogen‐induced disproportionation of SmCo5 particles for efficient alkaline HER. The as‐designed Co/SmH2 delivered an overpotential as low as 252 mV at 100 mA cm−2, surpassing the performance of pristine Co by 100 mV. Notably, this catalyst lasts remarkably long maintaining a durability at ≈500 mA cm−2 for 120 h. A combination of in situ Raman spectroscopy, in situ X‐ray absorption spectroscopy, density functional theory calculation and post‐HER characterizations unambiguously unveiled that the surface SmH2 transforms into samarium (hydr)oxide during electrocatalysis. This transformation not only inhibits the aggregation of the ultrafine cobalt nanoparticles but also significantly enhances the water dissociation and optimizes the binding energy of active cobalt species toward hydrogen intermediate, resulting in concurrent improvement of kinetics, thermodynamics, and stability of the HER process.

Electrochemical cobalt oxidation in chloride media

The green transition, despite recent advances in cobalt-free battery technologies, is still highly dependent on the availability of critical cobalt-based materials. Consequently, there has been increasing interest towards the development of new methods that maximize critical metals recovery from industrial hydrometallurgical solutions. In the current study, direct anodic oxidation of cobalt species from cobalt chloride solutions was studied as one alternative future strategy for cobalt recovery. Electrochemical methods were used (cyclic voltammetry, potentiostatic anodic deposition) and the effect of pH, temperature, and the concentration of cobalt and chloride ions on cobalt precipitation were investigated. The increase of pH and temperature was shown to stabilize the electrochemical oxidation of cobalt, while a decrease in cobalt concentration had a negative effect on precipitation. Scanning Electron Microscope, Atomic Force Microscopy and X-ray Photoelectron Spectroscopy were exploited to evaluate the morphology, structure, and composition of obtained anodic product. Calculated for potentiostatic anodic deposition (at highest studied potential of 1300 mV vs. Ag/AgCl) nucleation mechanism shows that the rate of nucleation for oxygen-cobalt species is faster than the subsequent growth rate of nuclei (instantaneous mechanism). XPS results confirmed that mixed Co3O4/Co(OH)2/CoOOH precipitate could be obtained by optimized anodic potentiostatic deposition in the range from 900 to 1150 mV and pH from 3 to 6 at 60 °C.

Preparation of Cobalt-Nitrogen Co-Doped Carbon Nanotubes for Activated Peroxymonosulfate Degradation of Carbamazepine.

Cobalt-nitrogen co-doped carbon nanotubes (Co3@NCNT-800) were synthesized via a facile and economical approach to investigate the efficient degradation of organic pollutants in aqueous environments. This material demonstrated high catalytic efficiency in the degradation of carbamazepine (CBZ) in the presence of peroxymonosulfate (PMS). The experimental data revealed that at a neutral pH of 7 and an initial CBZ concentration of 20 mg/L, the application of Co3@NCNT-800 at 0.2 g/L facilitated a degradation rate of 64.7% within 60 min. Mechanistic investigations indicated that the presence of pyridinic nitrogen and cobalt species enhanced the generation of reactive oxygen species. Radical scavenging assays and electron spin resonance spectroscopy confirmed that radical and nonradical pathways contributed to CBZ degradation, with the nonradical mechanism being predominant. This research presents the development of a novel PMS catalyst, synthesized through an efficient and stable method, which provides a cost-effective solution for the remediation of organic contaminants in water.

Facile construction of a cobalt-incorporated N-doped porous carbon from biomass for highly efficient removal of tetracycline

The performance of heterogeneous carbon-based catalysts in advanced oxidation processes (AOPs) significantly relies on their structure and surface active sites. Herein, the cobalt-incorporated N-doped porous carbon (Co-NC) was successfully prepared from tannin by utilizing its strong ability to coordinate with metal ions. The characterization results revealed that Co-NC-900 possessed a porous structure with abundant mesopores, a relatively large BET surface area (187.16 m2/g), multivalent cobalt species (Co0, Co2+, Co3+) and optimal surface nitrogen sites. As a result, Co-NC-900 demonstrated efficient PMS activation, resulting in a 97.4% removal of tetracycline (TC), along with a rate constant (kobs) of 0.131 min−1 and TOC removal efficiency of 55.3% within 30 min. The analysis of radical inhibition and EPR results revealed that TC degradation was attributed to both non-radical (1O2) and radical (•OH and SO4•-) pathways. In addition, the outperformance of Co-NC-900 compared to Co-C-900 and NC-900 highlights the essential contributions of both Co and N in PMS activation. Mechanism studies have revealed that the presence of multivalent Co species promotes the Co-based redox cycle. This, in association with the enhanced electron transfer efficiency facilitated by N doping sites, accelerates the rapid generation of reactive oxygen species (ROSs). This work provides a straightforward strategy to fabricate transition metal-incorporated N-doped porous carbon catalysts from eco-friendly biomass, contributing to the effective treatment of organic contaminant.

Synergistic cobalt‑copper metal-organic framework anchored amino-functionalized cellulose for antibiotic degradation: Interfacial engineering and mechanism insight.

Improving electron transfer rate of Co species and inhibiting aggregation of metal-organic frameworks (MOFs) particles are essential prerequisites for activating advanced oxidation process in wastewater treatment field. Here, we exploit Cu species with variable valence states to accelerate electron transfer of Co species and then to boost the unsatisfactory degradation efficiency for refractory pharmaceuticals via in-situ growth of copper and cobalt species on l-lysine functionalized carboxylated cellulose nanofibers. Utilizing the synergistic interplay of Co sites and deliberately exposed Cu0/Cu1+ atoms, the subtly designed catalyst exhibited a surprising degradation efficiency (~100 %) toward tetracycline hydrochloride within 10 min (corresponding to a catalytic capacity of 267.71 mg/g) without adjusting temperature and pH. Meanwhile, the catalyst displays good recyclability, well tolerance for coexisting ions and excellent antibacterial performance derived from the intrinsic antibacterial property of Cu-MOF. This research provided a novel strategy to construct MOFs-cellulose materials toward degrading various stubborn antibiotic pollutants.

Surface immobilization mechanisms of cobalt ions on hydroxyapatite catalyst supports

Cobalt deposition in an excess of solution was used to design Co-modified hydroxyapatite materials with various Co loadings and controlled dispersion. It is rationalized how the properties of hydroxyapatite supports (more or less stoichiometric compositions, nanorod or platelet morphologies and crystalline (100) zig-zag termination or non-apatitic hydrated external layer), influence the immobilization processes of Co by operating either with slightly acidic (natural) or basic pH of the suspension media. Four cobalt immobilization mechanisms impacting the final dispersion of Co on hydroxyapatites were identified by combining structural (XRD, 1H and 31P solid-state NMR, UV-Vis, Raman and X-ray fluorescence spectroscopies), surface (XPS) characterizations of Co-modified hydroxyapatites after drying and thermal treatment at 500 °C, and monitoring of the pH and the composition of the supernatant solutions during the Co deposition step. On dried Co-modified crystalline stoichiometric hydroxyapatite nanorods, cobalt is highly dispersed through cationic exchange or strong electrostatic adsorption (SEA) at slightly acidic or basic pH, respectively. After thermal treatment at 500 °C, only cation exchange preserved atomic dispersion of Co(II) ions since Co3O4 nanoparticles were observed on samples for which Co deposition occurred via SEA. On defective hydroxyapatite platelets, cobalt deposited at acidic natural pH could diffuse in an external non-apatitic layer, whereas under basic pH media, this surface layer was hydrolysed, resulting in the formation of a cobalt-substituted hydroxyapatite layer in which only a limited fraction of the surface cobalt species could be probed by XPS.

A Hydrogen Evolution Catalyst [Co2O2] Metallacycle Enables Regioselective Allene C(sp2)‐H Functionalization

AbstractSelective functionalization of allenic C(sp2)‐H is an ideal approach to upgrading simple allenes to synthetically useful allenes, albeit suffering from challenges associated with inert reactivity and inferior selectivity. Inspired by energy chemistry, a catalytic hydrogen evolution reaction (HER) strategy was leveraged to selectively activate weakly acidic allene C(sp2)‐H bonds in a reductive mode. An array of [Co2O2] metallacycle complexes were readily devised starting from amino acids, and they were demonstrated as robust HER catalysts, which would selectively break allenic C(sp2)‐H bonds to release hydrogen. With the newly developed HER catalyst, regioselective electrochemical functionalization of allenic C(sp2)‐H with alcoholic α C(sp3)‐H was unprecedentedly achieved. This strategy features excellent regioselectivity, unconventional chemoselectivity, good functional‐group tolerance (62 examples), and mild conditions. Mechanism experiments revealed a reactive hydroxy‐coordinated cobalt(II) species in the reaction. Density functional theory (DFT) calculations were also conducted to rationalize the regioselectivity observed in the reaction.

A Hydrogen Evolution Catalyst [Co2O2] Metallacycle Enables Regioselective Allene C(sp2)-H Functionalization.

Selective functionalization of allenic C(sp2)-H is an ideal approach to upgrading simple allenes to synthetically useful allenes, albeit suffering from challenges associated with inert reactivity and inferior selectivity. Inspired by energy chemistry, a catalytic hydrogen evolution reaction (HER) strategy was leveraged to selectively activate weakly acidic allene C(sp2)-H bonds in a reductive mode. An array of [Co2O2] metallacycle complexes were readily devised starting from amino acids, and they were demonstrated as robust HER catalysts, which would selectively break allenic C(sp2)-H bonds to release hydrogen. With the newly developed HER catalyst, regioselective electrochemical functionalization of allenic C(sp2)-H with alcoholic α C(sp3)-H was unprecedentedly achieved. This strategy features excellent regioselectivity, unconventional chemoselectivity, good functional-group tolerance (62 examples), and mild conditions. Mechanism experiments revealed a reactive hydroxy-coordinated cobalt(II) species in the reaction. Density functional theory (DFT) calculations were also conducted to rationalize the regioselectivity observed in the reaction.

Copper-cobalt dual-site on N-doped carbon nanotube with dual-promoted synergy for glucose electrochemical detection

Doping specific active sites and accelerating the decisive step of glucose catalysis to construct highly active glucose sensing electrochemical catalysts remains a major challenge for glucose sensing. Herein, we report the detailed design of Cu–Co dual active site N-doped carbon nanotube (CuCo-NCNTs) obtained by electrodeposition modification, programmed warming and calcination for electrochemical glucose detection. In the CuCo-NCNTs material system, Cu serves as the main active site for glucose sensing. Co with good adsorption of hydroxyl groups acts as the site providing hydroxyl groups to provide oxygen source for Cu oxidized glucose sensing. The synergistic effect between the two active sites in the Cu–Co system and the abundant micro-reactive sites exposed by carbon nanotubes greatly ensure the excellent electrocatalytic performance of glucose oxidation reaction. Therefore, CuCo-NCNTs have good electrocatalytic performance with a sensitivity of 0.84 mA mM−1 cm−2 and a detection limit of 1 μM, and also have excellent stability and specificity. DFT calculations elucidate the decisive steps of H-atom removal in the oxidation of glucose by Cu active site N-doped carbon nanotube (Cu-NCNTs) and Co active site N-doped carbon nanotube (CuCo-NCNTs) materials, illustrating the role of oxygen source provided by hydroxyl group adsorption in the electrochemical sensing process of glucose, thus demonstrating that the electrochemical sensing signal of glucose can be effectively enhanced when cobalt species that readily adsorb hydroxyl groups are introduced into the materials.

Eco-friendly spark-generated CoxOy nanoparticle-modified graphite screen-printed sensing surfaces for the determination of H2O2 in energy drinks

The modification of graphite screen-printed electrodes (SPEs) is reported using an eco-friendly and extremely fast method based on the direct cobalt pin electrode-to-SPE spark discharge at ambient conditions. This approach does not utilize any liquids or chemical templates, does not produce any waste, and allows the in-situ generation of CoxOy nanoparticles onto the electrode surface and the development of efficient electrocatalytic sensing surfaces for the determination of H2O2. Co-spark SPEs were characterized using scanning electron microscopy, energy-dispersive X-ray spectroscopy and x-ray photoelectron spectroscopy (XPS), revealing the formation of surface confined CoxOy nanoparticles and the diverse oxidation states of cobalt species. Co-spark SPEs were also characterized with cyclic voltammetry and electrochemical impedance spectroscopy. Redox transitions of the surface confined electrocatalysts are demonstrated by electrochemical polarization studies, showing the formation of different oxides (CoxOy), varying the XPS results. Amperometric measurements at 0.3 V vs. Ag/AgCl revealed a linear relationship between the current response and the concentration of H2O2 over the range 1 − 102 μM, achieving a limit of detection (3σ/m) of 0.6 μM. The interference effect of various electroactive species was effectively addressed by employing dual measurements in the absence and presence of the enzyme catalase. The analytical utility of the method was evaluated in antioxidant rich real-world samples, such as energy drinks, demonstrating sufficient recovery.Graphical

Materials Genes of CO2 Hydrogenation on Supported Cobalt Catalysts: An Artificial Intelligence Approach Integrating Theoretical and Experimental Data.

Designing materials for catalysis is challenging because the performance is governed by an intricate interplay of various multiscale phenomena, such as the chemical reactions on surfaces and the materials' restructuring during the catalytic process. In the case of supported catalysts, the role of the support material can be also crucial. Here, we address this intricacy challenge by a symbolic-regression artificial intelligence (AI) approach. We identify the key physicochemical parameters correlated with the measured performance, out of many offered candidate parameters characterizing the materials, reaction environment, and possibly relevant underlying phenomena. Importantly, these parameters are obtained by both experiments and ab initio simulations. The identified key parameters might be called "materials genes", in analogy to genes in biology: they correlate with the property or function of interest, but the explicit physical relationship is not (necessarily) known. To demonstrate the approach, we investigate the CO2 hydrogenation catalyzed by cobalt nanoparticles supported on silica. Crucially, the silica support is modified with the additive metals magnesium, calcium, titanium, aluminum, or zirconium, which results in six materials with significantly different performances. These systems mimic hydrothermal vents, which might have produced the first organic molecules on Earth. The key parameters correlated with the CH3OH selectivity reflect the reducibility of cobalt species, the adsorption strength of reaction intermediates, and the chemical nature of the additive metal. By using an AI model trained on basic elemental properties of the additive metals (e.g., ionization potential) as physicochemical parameters, new additives are suggested. The predicted CH3OH selectivity of cobalt catalysts supported on silica modified with vanadium and zinc is confirmed by new experiments.

Light-induced in-situ transformation from MOF to construct heterostructured Co3O4/Co catalyst for efficient photothermal catalytic oxidation

The construction of hybrid nanocatalysts with enhanced active surfaces/interfaces is crucial for the catalytic oxidative removal of volatile organic compounds (VOCs). In this study, we report an in-situ conversion approach to synthesize a heterostructured Co3O4/Co catalyst (Co@NC-0.5) for efficient photothermal catalytic oxidation of acetone. The catalysts were prepared by pyrolyzing a mixture of ZIF-67 and melamine. The exposed cobalt species underwent in-situ oxidation to form a Co3O4/Co heterostructure under photothermal conditions. This unique catalyst structure exhibited over 90 % conversion within 16 min under full solar spectrum irradiation and excellent reusability with continuous maintenance of 100 % acetone conversion rate after nine cycles performance tests. Notably, its specific mass activity under thermocatalytic conditions was 2.13 times higher than that of Co3O4. Our work provides a facile and effective strategy for developing highly active and durable non-precious metal catalysts suitable for various photothermal catalytic reactions.

Liquid-Phase Cyclic Chemiluminescence for the Identification of Cobalt Speciation.

Accurate discrimination of metal species is a significant analytical challenge. Herein, we propose a novel methodology based on liquid-phase cyclic chemiluminescence (CCL) for the identification of cobalt speciation. The CCL multistage signals (In) of the luminol-H2O2 reaction catalyzed by different cobalt species have different decay coefficients k. Thereby, we can facilely identify various cobalt species according to the distinguishable k values, including the complicated and structurally similar cobalt complexes, such as analogues of [Co(NH3)5X]n+ (X = Cl-, H2O, and NH3), Co(II) porphyrins, and bis(2,4-pentanedione) cobalt(II) derivatives. Especially, the number of substituent atoms also influences the k value greatly, which allows excellent discrimination between complexes that only have a subtle difference in the substituent group. In addition, linear discriminant analysis based on In provides a complementary solution to improve the differentiating ability. We performed density functional theory calculations to investigate the interaction mode of H2O2 over cobalt species. A close negative correlation between the adsorption energy and the k value is observed. Moreover, the calculation of energy evolutions of H2O2 decomposition into a double hydroxide radical shows that a high level of consistency exists between the activation energy barrier and the k value. The results further demonstrate that the decay coefficient of the CCL multistage signal is associated with the catalytic reactivity of the cobalt species. Our work not only broadens the application of chemiluminescence but also provides a complementary technology for speciation analysis.

Simultaneous spectrophotometric determination of Co (II) and Co (III) in acidic medium with partial least squares regression and artificial neural networks

This study aims at the application of two chemometric techniques to visible spectra of acetic acid solutions of Co (II) and Co (III) for simultaneous determination thereof. Spectral data of 145 samples in the range of 400–700 nm were used to build the models. Partial least squares regression models were developed for which latent variables were determined using internal cross-validation with a leave-one-out strategy and 3 and 2 latent variables were selected for Co(II) and Co(III) based on root mean square error of cross-validation. For these models, root mean square errors of prediction were 1.16 and 0.536 mM and coefficients of determination were 0.975 and 0.892 for Co (II) and Co (III). As an alternate method, artificial neural networks consisting of three layers, with 10 neurons in hidden layer, were trained to model spectra and concentrations of cobalt species. Levenberg-Marquardt algorithm with feed-forward back-propagation learning resulted root mean square errors of prediction of 0.316 and 0.346 mM for Co (II) and Co (III) respectively and coefficients of determination were 0.996 and 0.988.

Vitamin C-regulated CoAl- layered double hydroxide with oxygen vacancies to efficiently activate peroxydisulfate for sulfamethoxazole removal triggered via reactive oxygen and high-valent cobalt species

In this study, a vitamin C-regulated CoAl-layered double hydroxide with abundant oxygen vacancies was synthesized via a simple hydrothermal process. The resulting CoAl-layered double hydroxide was employed to activate peroxydisulfate for removal of sulfamethoxazole. The effect of the experimental parameters such as pH, catalyst dose and peroxydisulfate concentration on sulfamethoxazole removal was investigated. The current system exhibited excellent catalytic performance for sulfamethoxazole removal in a broad pH range (i.e., pH 3.0–11.0). Under the optimized condition, 94.2% of sulfamethoxazole was degraded within 15 min, accompanied by a 67.6% reduction in chemical oxygen demand. The effective sulfamethoxazole degradation could be attributed to four pathways. Firstly, the ≡ Co2+ in catalyst reacted with peroxydisulfate to generate reactive species, including SO4•−, •OH, O2•− and 1O2, which could degrade sulfamethoxazole. Secondly, the oxygen vacancies could modulate intrinsic electrons, resulted in the surface activation of catalyst and accelerated charge transfer, which was favorable for the degradation of sulfamethoxazole. Thirdly, the presence of vitamin C not only promoted the formation of oxygen vacancies but also expanded the interlayer spacing of layered double hydroxide. A large interlayer spacing facilitated the diffusion of peroxydisulfate and pollutants in the interlayer and improved the utilization efficiency of the active site. Lastly, the high-valent cobalt species exhibited excellent oxidation ability and enhanced the catalyst performance through continuously being employed as an electron acceptor. This study provided a valuable insight for the design and application of Co-based catalysts in peroxydisulfate-based advanced oxidation processes.

Cobalt catalyzed ethane dehydrogenation to ethylene with CO2: Relationships between cobalt species and reaction pathways

TiO2, ZrO2 and a series of TiO2-ZrO2 (TxZ1, x means the atomic ratio of Ti/Zr = 10, 5, 1, 0.2 and 0.1) composite oxide supports were prepared through co-precipitation, and then 3 wt% Co was loaded through wetness impregnation methods. The obtained 3 wt% Co/TiO2 (3CT), 3 wt% Co/ZrO2 (3CZ) and 3 wt% Co/TxZ1 (3CTxZ1) catalysts were evaluated for the oxidative ethane dehydrogenation reaction with CO2 (CO2-ODHE) as a soft oxidant. 3CT1Z1 catalyst exhibits excellent catalytic properties, with C2H4 yield, C2H6 conversion and CO2 conversion about 24.5 %, 33.8 % and 18.0 % at 650 °C, respectively. X-Ray Diffraction (XRD), in-situ Raman, UV–vis diffuse reflectance spectra (UV–vis DRS), H2 temperature-programmed reduction (H2-TPR), Electron paramagnetic resonance (EPR) and quasi in-situ X-ray Photoelectron Spectroscopy (XPS) have been utilized to thoroughly characterize the investigated catalysts. The results revealed that 3CT1Z1 produced TiZrO4 solid solution with more metal defect sites and oxygen vacancies (Ov), promoting the formation of Co2+-TiZrO4 structure. Furthermore, the presence of Ov and Ti3+can facilitate the high dispersion and stabilization of Co2+, as well as suppressing the severe reduction of Co2+, leading to superior ethane oxidative dehydrogenation activity. Besides, less Co0 is beneficial to ODHE reaction, because of its promotion effects for reverse water gas shift reaction; however, more Co0 results in dry reforming reaction (DRE). This work will shed new lights for the design and preparation of highly efficient catalysts for ethylene production.

Porous carbonaceous materials simultaneously dispersing N, Fe and Co as bifunctional catalysts for the ORR and OER: electrochemical performance in a prototype of a Zn-air battery.

Infiltration of the mesoporous structure of SBA-15 silica as a hard template with phenanthroline complexes of Fe3+ and Co2+ allowed the simultaneous dispersion of nitrogen, iron and cobalt species on the surface of the obtained carbonaceous CMK-3 silica replica, with potential as bifunctional heterogeneous catalysts for the cathodic oxygen reduction and evolution reactions (ORR and OER). The textural properties and mesopore structure depended on the composition of the material. The carbonaceous FeCoNCMK-3 (1/1), obtained with an Fe/Co molar ratio of 1/1, exhibited an ordered cylindrical mesoporous structure with a high mesopore volume, a rather homogeneous composition in terms of total and surface concentrations of iron and cobalt, and a balanced presence of pyridinic-, pyrrolic- and graphitic-N species. FeCoNCMK-3 (1/1) could improve the ORR kinetics by adsorption and reduction of O2 through the 4-electron mechanism with a current density of -17.37 mA cm-2, Eonset of 1.13 V vs. RHE and E1/2 of 0.75 V when compared to metal-free, monometallic or bimetallic electrocatalysts with a higher amount of cobalt than that of iron. In addition, FeCoNCMK-3 (1/1) exhibited activity for the OER, presenting lower values of Eonset (1.52 V), Ej10 (1.78 V) and the Tafel slope (76.3 mV dec-1) with respect to other catalysts. When evaluated as a cathode in a prototype of a Zn-air battery, FeCoNCMK-3 (1/1) exhibited a high open circuit voltage of 1.41 V, a peak power density of 66.84 mW cm-2, a large specific capacity of 818.88 mA h gZn-1, and cycling for 20 h but with deactivation upon cycling.

C-H bond activation by high-valent iron/cobalt-oxo complexes: a quantum chemical modeling approach.

High-valent metal-oxo species serve as key intermediates in the activation of inert C-H bonds. Here, we present a comprehensive DFT analysis of the parameters that have been proposed as influencing factors in modeled high-valent metal-oxo mediated C-H activation reactions. Our approach involves utilizing DFT calculations to explore the electronic structures of modeled FeIVO (species 1) and CoIVO ↔ CoIII-O˙ (species 2), scrutinizing their capacity to predict improved catalytic activity. DFT and DLPNO-CCSD(T) calculations predict that the iron-oxo species possesses a triplet as the ground state, while the cobalt-oxo has a doublet as the ground state. Furthermore, we have investigated the mechanistic pathways for the first C-H bond activation, as well as the desaturation of the alkanes. The mechanism was determined to be a two-step process, wherein the first hydrogen atom abstraction (HAA) represents the rate-limiting step, involving the proton-coupled electron transfer (PCET) process. However, we found that the second HAA step is highly exothermic for both species. Our calculations suggest that the iron-oxo species (Fe-O = 1.672 Å) exhibit relatively sluggish behavior compared to the cobalt-oxo species (Co-O = 1.854 Å) in C-H bond activation, attributed to a weak metal-oxygen bond. MO, NBO, and deformation energy analysis reveal the importance of weakening the M-O bond in the cobalt species, thereby reducing the overall barrier to the reaction. This catalyst was found to have a C-H activation barrier relatively smaller than that previously reported in the literature.

Enhanced Electrocatalytic Nitrate Reduction to Ammonia Using Functionalized Multi-Walled Carbon Nanotube-Supported Cobalt Catalyst.

Ammonia (NH3) is vital in modern agriculture and industry as a potential energy carrier. The electrocatalytic reduction of nitrate (NO3-) to ammonia under ambient conditions offers a sustainable alternative to the energy-intensive Haber-Bosch process. However, achieving high selectivity in this conversion poses significant challenges due to the multi-step electron and proton transfer processes and the low proton adsorption capacity of transition metal electrocatalysts. Herein, we introduce a novel approach by employing functionalized multi-walled carbon nanotubes (MWCNTs) as carriers for active cobalt catalysts. The exceptional conductivity of MWCNTs significantly reduces charge transfer resistance. Their unique hollow structure increases the electrochemical active surface area of the electrocatalyst. Additionally, the one-dimensional hollow tube structure and graphite-like layers within MWCNTs enhance adsorption properties, thus mitigating the diffusion of intermediate and stabilizing active cobalt species during nitrate reduction reaction (NitRR). Using the MWCNT-supported cobalt catalyst, we achieved a notable NH3 yield rate of 4.03 mg h-1 cm-2 and a high Faradaic efficiency of 84.72% in 0.1 M KOH with 0.1 M NO3-. This study demonstrates the potential of MWCNTs as advanced carriers in constructing electrocatalysts for efficient nitrate reduction.

Improved phenol degradation by catalytic wet air oxidation on the Co3O4 spinel catalysts supported on SBA-15 modified with TiO2

The removal of phenol from aqueous industrial effluents by catalytic wet-air oxidation (CWAO) is a challenging process that requires novel catalysts resistant to leaching of the active phases during residence time in a batch reactor. In this sense, this work demonstrated that this can be archived with the catalyst having the spinel phase Co3O4 supported on mesoporous silica SBA-15 doped with an optimized amount of TiO2 (3 wt%). In the phenol CWAO reaction carried out at 160 °C and 10 bar O2, this best Co3O4/TiO2/SBA-15 catalyst showed an increase of phenol conversion from 49.0 % to 68.3 % and of CO2 selectivity from 87 % to 98 %. The extensive catalyst characterization and overall activity results indicated that this was related to the strength and nature of the acid sites and the oxidation states of the cobalt oxide phases. Combined factors influenced the final behaviour of the catalyst, such as its intrinsic activity, dispersion of the active phase, inhibition of CoSiO4 phase formation, high Lewis acidity and the large amount of active Co3O4 spinel phase. The presence of TiO2 nanoparticles (NPs) decorating the support surface improved the stability of the catalyst by inhibiting the leaching of cobalt species during the reaction. After three reaction cycles, the most efficient Co/3Ti/S15 catalyst showed high CO2 selectivity (∼92.7 %) and low selectivity towards carboxylic acid (∼5.3 %) and polyphenolic compounds (∼2%). It is proposed that surface oxidation is the rate-determining step in the CWAO reaction over TiO2/SBA-15-supported Co3O4 spinel catalysts.