Cobalt Active Sites Research Articles

Defect-Driven Evolution of Oxo-Coordinated Cobalt Active Sites with Rapid Structural Transformation for Efficient Water Oxidation.

Reconstructing the surface nature of metal-organic frameworks (MOFs) as precatalytic structures is a promising methodology for improving electrocatalytic performance. However, regulating the structural evolution of MOFs during electrolysis remains highly uncontrollable and lacks an in-depth understanding of the role of in situ-derived active sites. Here, we suggest a simple approach to fine-tune the symmetry of Co-MOFs with an oxo-coordinated asymmetric coordination that acts as a prototypical structure motif for the oxygen evolution reaction (OER). Through a facile thermal treatment, the Co-N4 configuration of Co-MOFs transforms to the distorted Co-N3-oxo configuration of defective Co-ligand nanoclusters. By operando spectroscopic characterization, the reconstructed Co-N3-oxo structure enables a rapid structural transition toward homogeneous oxyhydroxides. Moreover, the defective nature of the precatalytic structure regulates the surface Co-O bonding environment with abundant μ2-O-Co3+ sites, thereby exhibiting highly enhanced OER activity with an overpotential of 256 mV at 10 mA cm-2 and excellent durability for 100 h, compared with the pristine Co-MOFs. Atomistic simulations reveal that the effect of OER intermediates on the oxyhydroxides gets distributed among neighboring Co ions, promoting balanced binding of the intermediates. This work highlights an effective strategy to design the MOF-based structure for optimizing the surface nature, thus enhancing the electrocatalytic activity.

Regulation of cobalt active sites by antimony to match adsorption configuration for propyne semihydrogenation

Regulation of cobalt active sites by antimony to match adsorption configuration for propyne semihydrogenation

Regulation of cobalt active sites by antimony to match adsorption configuration for propyne semihydrogenation

Regulation of cobalt active sites by antimony to match adsorption configuration for propyne semihydrogenation

Boosting HER performance of ZIF-67 via optimizing synthesis parameters in alkaline medium

Electrochemical water splitting offers a promising route to sustainable hydrogen production. Metal-Organic Frameworks (MOFs) have emerged as potential electrocatalysts due to their tunable structure and composition. We investigated the electrocatalytic activity of ZIF-67 for the hydrogen evolution reaction (HER) in alkaline media. Synthesis parameters, including temperature, reaction time, acidity, molar ratio of 2-methylimidazole (2-MIM) to cobalt source, cobalt source type, addition method of 2-MIM, and solvent type, were systematically varied to optimize the electrocatalytic properties of ZIF-67. The synthesized ZIF-67 electrocatalysts were characterized using BET, FESEM, EDS, TEM, XRD, FTIR, and UV–vis spectroscopy. The electrocatalytic properties were evaluated by cyclic voltammetry (CV), linear sweep voltammetry (LSV), electrochemical impedance spectroscopy (EIS), and chronoamperometry methods. The optimized ZIF-67 electrocatalyst exhibited remarkable performance, with a low overpotential of 171 mV, a small Tafel slope of 82 mV.dec−1, a low charge transfer resistance of 97 Ω, a high double-layer capacitance of 13.5 mF cm−2, and high stability for 15 h at the current density of 10 mA cm−2. The enhanced electrocatalytic activity of the optimized ZIF-67 was attributed to its high surface area, tunable pore structure, and the presence of active cobalt sites.

ZIF‐67 Derived Co‐LDH Nanocubes for Oxygen Evolution Reaction

AbstractTo address the substantial energy needs of the quickly evolving modern civilization, efforts are still needed to provide sustainable renewable energy sources. The electrocatalytic Oxygen evolution reaction (OER) is one of the essential technologies used in the various hydrogen production techniques. ZIF‐67 nanocubes (ZIF‐67 NCs) were synthesized in an aqueous solution and used as sacrificial templates. Cobalt nitrate hexahydrate was added under water bath conditions, resulting in the evolution of numerous small layered layers of cobalt hydroxide. This ultimately led to the formation of multilayered, three‐dimensionally crosslinked Co‐LDH with a porous networked cubic morphology. Hydrolysis of Co2+ at different concentrations produces different degrees of weak acidic environments, and the loose LDH on the cubic structure of the best catalyst, Co‐LDH‐1, exposes more active cobalt sites for the OER, which results in a high electrochemically active surface area, with an overpotential of 354 mV and a Tafel slope of 79.06 mV dec−1 at a current density of 10 mA cm−2, and good stability and low activation under alkaline condition also has good stability and low activation energy. It provides us an easy and practical plan for logically creating ZIF‐derived hydroxide materials, leading to the development of affordable and effective electrocatalysts.

Flower-like Z-scheme heterostructure of cobalt porphyrin/ZnIn2S4 for boosting photocatalytic solar fuel production

Molecular catalysts have attracted significant attention in photocatalytic CO2 reduction (PCR) due to well-defined structure and exceptional catalytic selectivity. However, they are still limited by recycling challenges and inherent instability. Forming an organic-inorganic composite can address these issues and enhance PCR performance simultaneously. Herein, we developed a flower-like Z-scheme heterojunction of cobalt porphyrin ([meso-tetra(4-sulfonatophenyl)porphyrin], CoTPPS) and ZnIn2S4 (ZIS). The optimized ZIS@CoTPPS exhibited a superior CO evolution rate of 388.26 μmol g−1 h−1, which is 8.96 times of pristine ZIS (43.32 μmol g−1 h−1). The superior PCR activity is contributed by the enhanced light absorption, and photoinduced charge separation facilitated by the Z-scheme structure and the cobalt active sites. Moreover, theoretical calculations confirm the heterojunction relies on a substantial interfacial coupling between the Zn atoms of ZIS and the sulfonic acid groups of CoTPPS through coordination assembly. This investigation affords a useful inspiration for consciously constructing novel ZIS-incorporated atomistic Z-scheme photocatalysts.

Radiation-induced preparation of nanoscale CoO@graphene oxide for activating peroxymonosulfate to degrade emerging organic pollutants

In this study, ionizing radiation was used to induce the in-situ formation of highly dispersed nanosized cobalt oxide on the surface of graphene oxide (R-Co-GO), which was highly effective for activating PMS to degrade sulfamethoxazole (SMX). R-Co-GO had the highest catalytic activity when 150 μL cobalt chloride hexahydrate solution was used in the precursor, and the pseudo first-order kinetic constant of SMX degradation was 0.07 min−1 with high mineralization efficiency (63.1 %) and high PMS utilization efficiency. The sulfate radicals and high-valent cobalt oxo were mainly responsible for SMX degradation. Mechanism analysis showed that cobalt active site dominated in PMS activation, which was responsible for the formation of sulfate radicals and high-valent cobalt oxo; while the carbon framework contributed to the formation of singlet oxygen. The R-Co-GO-150 had good catalytic activity and stability in five cycling experiments, in which SMX was completely degraded and the concentration of dissolved Co was below 0.1 mg/L. In addition, the R-Co-GO-150/PMS system could also degrade phenol, bisphenol A, atrazine and nitrobenzene effectively, confirming its wide applicability. This study provided a facile method to uniformly disperse the metal oxides on the surface of carbon materials, and an effective system for the removal of emerging organic pollutants from the actual wastewater.

Constructing Co@C nanoparticles with chainmail-structure for highly efficient hydroformylation of 1-hexene

Constructing highly active noble-metal-free heterogeneous catalysts is the crux for the hydroformylation of long-chain α-olefins. Herein, novel chainmail-structured cobalt oxide nanoparticles (CoO@C|SBA-15) are fabricated by the channel confinement strategy, which display excellent activity for the 1-hexene hydroformylation. Specifically, the obtained catalyst shows 100% 1-hexene conversion and 94% heptanal selectivity, as well as up to 2.0 linear to branch (l/b) ratio, outperforming most of heterogeneous noble metal catalysts. Remarkably, in-situ DRIFT spectra combined with density functional theory calculations disclosed the metallic Co derived from in-situ reconstruction of CoO is the key active site for 1-hexene hydroformylation. Specifically, the reconstructed Co site is responsible for the adsorption of 1-hexene, and the graphite carbon layer promotes the adsorption of H2 and CO by interacting with the reconstructed Co. Moreover, the graphite carbon layer provides a strong protective effect on the internal cobalt active sites and thus enhances the catalytic stability during the harsh condition. This work provides a new method for designing the low-cost heterogeneous catalyst and emphasizes the intrinsic active sites for 1-hexene hydroformylation reaction.

Enhancing hydrogen generation from sodium borohydride hydrolysis and the role of a Co/CuFe2O4 nanocatalyst in a continuous flow system

In this study, a series of cobalt-based spinel ferrites catalysts, including nickel, cobalt, zinc, and copper ferrites, were synthesized using the sol–gel auto-combustion method followed by a chemical reduction process. These catalysts were employed for accelerating hydrogen generation via the sodium borohydride hydrolysis process. A continuous stirred tank reactor was used to perform catalytic reactor tests. All samples were subjected to analysis using XRD, FESEM, EDX, FTIR, and nitrogen adsorption–desorption techniques. The results revealed that the cobalt-based copper ferrite sample, Co/Cu-Ferrite, exhibited superior particle distribution, and porosity characteristics, as it achieved a high hydrogen generation rate of 2937 mL/min.gcat. In addition, the higher electrical donating property of Cu-Ferrite which leads to the increase in the electron density of the cobalt active sites can account for its superior performance towards hydrolysis of NaBH4. Using the Arrhenius equation and the zero-order reaction calculation, activation energy for the sodium borohydride hydrolysis reaction on the Co/Cu-Ferrite catalyst was determined to be 18.12 kJ/mol. This low activation energy compared to other cobalt-based spinel ferrite catalysts confirms the catalyst's superior performance as well. Additionally, the outcomes from the recycling experiments revealed a gradual decline in the catalyst's performance after each cycle during 4 repetitive cycles. The aforementioned properties render the Co/Cu-Ferrite catalyst an efficient catalyst for hydrogen generation through NaBH4 hydrolysis.

Engineering d-band center of cobalt active sites via dual coordination with nitrogen-doped carbon nanotube and Ti3C2Tx MXene toward electrocatalytic oxygen reduction for H2O2 production

Manipulating the selective adsorption of oxygen-containing species is crucial for tailoring the electrocatalyst to select dual-electron pathway in the oxygen reduction reaction (2e- ORR). Herein, dual-coordination electrocatalyst structure (Co-NCNT/MXene) is proposed based on the modulation of oxygenated species adsorption by coordination microenvironment at the interface of metal nanocatalysts. The Co-NCNT/MXene achieves excellent H2O2 selectivity (95.25 %), yield (162 mg L-1h−1) and Faraday efficiency (94.81 %) for ORR, surpassing the most Co-based-electrocatalysts. In-situ EPR techniques reveal the selective *OOH desorption dynamic process on Co-NCNT/MXene. Density functional theory calculations confirm that asymmetric coordination of Co nanoparticles by oxygen-terminated Ti3C2-MXene and NCNT modulates the delocalized state of Co extranuclear electrons, resulting in a reduction of the d-band center of Co, thus turning the adsorption energy of *OOH within the Co-NCNT/MXene interface towards a path more conducive to H2O2 electrosynthesis. In-situ H2O2 generated in the ORR is applied to achieve excellent degradation of organic pollutants.

Le Chatelier’s principle to stabilize intrinsic surface structure of oxygen-evolving catalyst for enabling ultra-high catalytic stability of zinc-air battery and water splitting

Well-designed highly active intrinsic surface structure of a catalyst for oxygen evolution reaction (OER) often faces the problem of corrosion dissolution, which severely deteriorates its catalytic stability, and breaking this activity-stability trade-off has always been a huge and long-term challenge. Herein, we employ Le Chatelier’s principle to address this dissolution problem of the catalyst to maintain its intrinsic surface structure for highly efficient and ultra-stable OER. Taking highly active but susceptible oxygen vacancy-rich Co-W oxide as the model catalyst for alkaline OER, its dissolving kinetics is significantly passivated by introducing homo ions into the electrolyte to regulate the dissolution equilibrium, thus achieving ultralong operating stability without sacrificing its high activity. The theoretical calculations and in-situ experimental results unravel the underlying mechanism of this strategy, that is, the homo ions (tungstate ions) can be preferentially adsorbed on the catalyst surface to construct a local non-corrosive environment, thus effectively stabilizing the well-designed unsaturated active cobalt sites to maintain the intrinsic surface structure of the catalyst and preventing them from being reconstructed or transformed toward the unfavorable direction. Importantly, this strategy is also effective in zinc-air battery and electrolytic water splitting, achieving ultra-stable performance over 1200 h, which is of great importance for promoting the practical applications of low-cost non-noble metal-based catalysts in energy devices.

Molecular Co-Catalyst Confined within a Metallacage for Enhanced Photocatalytic CO2 Reduction.

The construction of structurally well-defined supramolecular hosts to accommodate catalytically active species within a cavity is a promising way to address catalyst deactivation. The resulting supramolecular catalysts can significantly improve the utilization of catalytic sites, thereby achieving a highly efficient chemical conversion. In this study, the Co-metalated phthalocyanine (Pc-Co) was successfully confined within a tetragonal prismatic metallacage, leading to the formation of a distinctive type of supramolecular photocatalyst (Pc-Co@Cage). The host-guest architecture of Pc-Co@Cage was unambiguously elucidated by single-crystal X-ray diffraction (SCXRD), NMR, and ESI-TOF-MS, revealing that the single cobalt active site can be thoroughly isolated within the space-restricted microenvironment. In addition, we found that Pc-Co@Cage can serve as a homogeneous supramolecular photocatalyst that displays high CO2 to CO conversion in aqueous media under visible light irradiation. This supramolecular photocatalyst exhibits an obvious improvement in activity (TONCO = 4175) and selectivity (SelCO = 92%) relative to the nonconfined Pc-Co catalyst (TONCO = 500, SelCO = 54%). The present strategy provided a rare example for the construction of a highly active, selective, and stable photocatalyst for CO2 reduction through a cavity-confined molecular catalyst within a discrete metallacage.

Efficient catalyst regeneration through solvent-based wax extraction for Co/Al2O3 Fischer-Tropsch catalysts

The regeneration of Fischer-Tropsch (FT) catalysts plays a crucial role in sustaining their catalytic activity and longevity in sustainable production processes. This research investigates the effectiveness of various solvents, including n-hexane, n-heptane, toluene, xylene, and the liquid fraction of FT products (LF-FT), for catalyst regeneration and extracting wax from the 15 wt%Co/Al2O3 catalyst. Toluene, with a unique combination of key parameters such as polarity, solubility, boiling point, aromatic structure, and chemical interactions, exhibited the highest wax removal efficiency of 90.6%. However, due to its 41 °C lower boiling point compared to toluene and very similar wax removal efficiency, n-hexane appears as a more energy-efficient and cost-effective option for this process. The regenerated catalysts showed comparable characteristics, including the textural properties (surface area, pore size, and pore volume), crystallinity, and reducibility, to fresh catalysts. The slightly lower reduction temperature in the regenerated catalyst indicates a higher possibility for the formation of cobalt active sites during the catalyst reduction process with hydrogen, leading to an increased potential for higher catalytic activity. The LF-FT, comprising paraffins, iso-paraffins, olefins, napthenes, and aromatics, provided a unique combination of aliphatic and aromatic moieties, enhancing the solubility of both polar and nonpolar components in FT wax and improving the wax removal efficiency to 91.5%. The higher boiling point and operating temperature of the LF-FT also contributed to its better performance. Efficient separation of hydrocarbons from solvents allowed for solvent reusability in subsequent FT wax removal cycles, making it advantageous for large-scale applications.

Ultrathin, Cationic Covalent Organic Nanosheets for Enhanced CO2 Electroreduction to Methanol.

Metalloporphyrins and metallophthalocyanines emerge as popular building blocks to develop covalent organic nanosheets (CONs) for CO2 reduction reaction (CO2RR). However, existing CONs predominantly yield CO, posing a challenge in achieving efficient methanol production through multielectron reduction. Here, ultrathin, cationic, and cobalt-phthalocyanine-based CONs (iminium-CONs) are reported for electrochemical CO2-to-CH3OH conversion. The integration of quaternary iminium groups enables the formation of ultrathin morphology with uniformly anchored cobalt active sites, which are pivotal for facilitating rapid multielectron transfer. Moreover, the cationic iminium-CONs exhibit a lower activity for hydrogen evolution side reaction. Consequently, iminium-CONs manifest significantly enhanced selectivity for methanol production, as evidenced by a remarkable 711% and 270% improvement in methanol partial current density (jCH3OH) compared to pristine CoTAPc and neutral imine-CONs, respectively. Under optimized conditions, iminium-CONs deliver a high jCH3OH of 91.7 mA cm-2 at -0.78 V in a flow cell. Further, iminium-CONs achieve a global methanol Faradaic efficiency (FECH3OH) of 54% in a tandem device. Thanks to the single-site feature, the methanol is produced without the concurrent generation of other liquid byproducts. This work underscores the potential of cationic covalent organic nanosheets as a compelling platform for electrochemical six-electron reduction of CO2 to methanol.

Fabrication of cobalt (II, III) oxide @nitrogen doped carbon/honeycomb ceramics with porous structure for room-temperature formaldehyde oxidation

The content of surface cobalt active sites and oxygen vacancy (VO) is vital to the catalytic performance of Co3O4 catalysts. Herein, a robust Co3O4@N doped C supported on honeycomb ceramics (Co3O4@NC-HC) catalyst with porous structure was prepared by an impregnation-calcination method using ZIF-67 as the precursor. The N2 calcining atmosphere and relatively closed environment are beneficial for the formation of more pyridine N, Co3+ surface active sites, and VO, which favors formaldehyde (HCHO) or O2 adsorption and activation. The higher content of NC in Co3O4@NC-HC than in Co3O4@NC-HC-open and Co3O4@NC-HC-air can disperse Co-containing catalysts better and promote electron transfer. The Co3O4@NC-HC catalysts exhibited excellent activity and stability for HCHO oxidation at room temperature. The 1.0 Co3O4@NC-HC sample with theoretical 7.7 wt% showed the maximum efficiency (96.5 %) for HCHO removal. The related catalytic mechanisms over Co3O4@NC/HC for HCHO oxidation were posed.

Langmuir-Blodgett Monolayer of Cobalt Phthalocyanine as Ultralow Loading Single-Atom Catalyst for Highly Efficient H2O2 Production.

The electrochemical production of H2O2 via the two-electron oxygen-reduction reaction (2e- ORR) has been actively studied using systems with atomically dispersed metal-nitrogen-carbon (M-N-C) structures. However, the development of well-defined M-N-C structures that restrict the migration and agglomeration of single-metal sites remains elusive. Herein, we demonstrate a Langmuir-Blodgett (LB) monolayer of cobalt phthalocyanine (CoPc) on monolayer graphene (LB CoPc/G) as a single-metal catalyst for the 2e- ORR. The as-prepared CoPc LB monolayer has a β-form crystalline structure with a lattice space for the facile adsorption of oxygen molecules on the cobalt active sites. The CoPc LB monolayer system provides highly exposed Co atoms in a well-defined structure without agglomeration, resulting in significantly improved catalytic activity, which is manifested by a very high H2O2 production rate per catalyst (31.04 mol gcat-1 h-1) and TOF (36.5 s-1) with constant production stability for 24 hours. To the best of our knowledge, the CoPc LB monolayer system exhibits the highest H2O2 production rate per active site. This fundamental study suggests that an LB monolayer of molecules with single-metal atoms as a well-defined structure works for single-atom catalysts.

Cobalt-based MOF-derived carbon electrocatalysts with tunable architecture for enhanced oxygen evolution reaction

Development of the hydrogen economy requires the design of catalysts that increase the rate of the accompanying sluggish kinetic oxygen evolution reaction (OER). This is a key process in electrochemical energy conversion and storage, such as water splitting and metal-air batteries. The OER needs high overpotential and typically expensive precious metal-based catalysts. Therefore, designing low-cost and efficient electrocatalysts for OER is of paramount importance. In addition to focusing on the number of active sites or high specific surface area, the correlation between catalyst particle shape and performance should be considered. This work presents an electrocatalytic activity comparison of cobalt-containing carbons with different morphologies in the OER process. Employing metal–organic frameworks as carbon and metal precursors, the materials in the shape of polyhedrons, needles, unique spherical hedgehogs, and sea urchins were obtained. The effect of MOF template infiltration with additional carbon source on the physicochemical properties of electrocatalysts was also examined. The furfuryl alcohol-impregnated needle-shaped particles were characterized by a high content of cobalt active sites, surrounded by nitrogen-containing graphite layers. Electrochemical tests confirmed their best activity (overpotential 317 mV@10 mA/cm2), long stability (up to 20 h), as well as low reagents diffusion limitations (Tafel slope 57 mV/dec up to 24 mA/cm2). The vertically aligned structure of the catalyst contributed to improved detachment of the oxygen bubbles produced.

High-valent cobalt active sites derived from electrochemical activation of metal-organic frameworks for efficient nitrate reduction to ammonia

Metal-organic frameworks (MOFs) exhibited great potential in the electrocatalytic reduction of nitrate to ammonia (eNO3RR), but the structural transformation and the identification of the active sites in the electrochemical process have rarely been reported. Herein, a series of MOFs have been fabricated for eNO3RR. The Co-based MOF (Co-TPA) is selected as a representative for exploration through in-situ and ex-situ techniques to reveal the structural evolution during eNO3RR. The structural transformation from the Co-TPA to electrochemically activated catalyst (Co-TPA-E) is verified, and it is found that the in-situ formed high-valent Co species (CoOOH) are the active sites for eNO3RR. Moreover, the formation of the high-valence CoOOH species can be further facilitated by introducing highly electronegative Cu ions into the Co-TPA-E, resulting in excellent catalytic activity with an NH3 yield rate of 1.12 mmol h−1 cm−2 at − 0.326 V vs. RHE and a maximum Faradaic efficiency of 99.62 %.

Effect of multilayered catalytic bed on temperature profiles and catalytic performance for Fischer–Tropsch synthesis over Co/Al2O3 catalyst in a multitubular fixed-bed reactor

Fischer-Tropsch synthesis (FTS) is a widely recognized process that catalytically converts syngas into higher hydrocarbons and oxygenates, which are ultimately upgraded into transportation fuels and chemicals. Effective temperature control and heat dissipation are crucial considerations due to the exothermic nature of the FT reaction. Underutilization of the catalytic bed is another challenge in fixed-bed reactors. The implementation of a multilayered catalytic bed, with the catalytic activity in an ascending order, has shown positive effects in addressing these issues. It mitigates adiabatic temperature rise, reduces the maximum reactor temperature, and minimizes catalyst deactivation at high temperatures. Moreover, this multilayered bed effectively addresses the underutilization issue by ensuring optimal utilization of the entire bed as the catalytic activity transitions from one layer to another during the reaction. Layers with a higher cobalt loading remain active until the completion of the reaction, resulting in minimal fluctuations in CO conversion. To achieve uniform temperature profiles, catalytic activities, and product distributions in all reactor tubes in a multitubular reactor, it is imperative to maintain consistent reaction conditions, utilize the same catalyst loading in all tubes, employ an appropriate heating system, and implement an efficient cooling system to dissipate the heat generated by the exothermic reaction. Furthermore, investigations into the properties of the used catalysts after the reaction indicate that catalyst deactivation primarily occurs due to the deposition of FT products on the catalyst surface and pores and blockage of active cobalt sites, and sintering of cobalt particles.

Efficient Selective Oxidation of Aromatic Alkanes by Double Cobalt Active Sites over Oxygen Vacancy-rich Mesoporous Co3O4.

The development of an efficient catalyst for selective oxidation of hydrocarbon to functional compounds remains a challenge. Herein, mesoporous Co3O4 (mCo3O4-350) showed excellent catalytic activity for the selective oxidation of aromatic-alkanes, especially for oxidation of ethylbenzene with a conversion of 42% and selectivity of 90% for acetophenone at 120 oC. Notably, mCo3O4 presented a unique catalytic path of direct oxidation of aromatic-alkanes to aromatic ketones rather than the conventional stepwise oxidation to alcohols and then to ketones. Density functional theory calculations revealed that oxygen vacancies in mCo3O4 activate around Co atoms, causing electronic state change from Co3+(Oh) → Co2+(Oh). Co2+(Oh) has great attraction to ethylbenzene, and weak interaction with O2, which provide insufficient O2 for gradual oxidation of phenylethanol to acetophenone. Combined with high energy barrier for forming phenylethanol, the direct oxidation path from ethylbenzene to acetophenone is kinetically favorable on mCo3O4, sharply contrasted to non-selective oxidation of ethylbenzene on commercial Co3O4.