Cobalt-carbon/silica nanocomposites prepared by pyrolysis of a cobalt 2,2'-bipyridine terephthalate complex for remediation of cationic dyes

Recent Advances in Carbon-Silica Composites: Preparation, Properties, and Applications

The thermal catalytic conversion of biomass is currently a prevalent method for producing activated carbon with superb textural properties and excellent adsorption performance. However, activated carbon suffers severely from its poor thermal stability, which can easily result in spontaneous burning. In contrast, silica material is famed for its easy accessibility, high specific surface area, and remarkable thermal stability; however, its broader applications are restricted by its strong hydrophilicity. Based on this, the present review summarizes the recent progress made in carbon-silica composite materials, including the various preparation methods using diverse carbon (including biomass resources) and silica precursors, their corresponding structure–function relationship, and their applications in adsorption, insulation, batteries, and sensors. Through their combination, the drawbacks of the individual materials are circumvented while their original advantages are maintained. Finally, several bottlenecks existing in the field of carbon-silica composites, from synthesis to applications, are discussed in this paper, and possible solutions are given accordingly.

Designing bimetallic zeolitic imidazolate frameworks (ZIFs) for aqueous catalysis: Co/Zn-ZIF-8 as a cyclic-durable catalyst for hydrogen peroxide oxidative decomposition of organic dyes in water

ZIF-8 is well known hybrid material that is self-assembled from inorganic and organic moieties. It has several potential applications due to its unique structure. One of these potential applications is in advanced oxidation processes (AOP) via a heterogeneous catalysis system. The use of modified ZIF-8/H2O2 for the destruction of the azo dye methyl orange (MO) is presented in this work to explore its efficacy. This work presents the bimetallic Co/Zn-ZIF-8 as an efficient catalyst to promote H2O2 oxidation of the MO dye. Co/Zn-ZIF-8 was synthesized through a hydrothermal process, and the pristine structure was confirmed using XRD, FTIR, and XPS. The Co/Zn-ZIF-8/H2O2 system successfully decolorized MO at the selected pH 6.5. It was found that more than 90% of MO (10 ppm) was degraded within only about 50 minutes. Proposed radical and redox mechanisms are presented for H2O2 decomposition where the redox mechanism is suggested to predominate via a Co(ii)/Co(iii) redox consecutive cyclic process.

Cerium d-Block Element (Co, Ni) Bimetallic Oxides as Catalysts for the Methanation of CO2: Effect of Pressure

Nickel– and cobalt–cerium bimetallic oxides were used as catalysts for the methanation of CO2 under pressure. The catalysts’ activity increases with pressure and an increase of just 10 bar is enough to double the yield of methane and to significantly improve the selectivity. The best results were those obtained over nickel–cerium bimetallic oxides, but the effect of pressure was particularly relevant over cobalt–cerium bimetallic oxides, which yield to methane increases from almost zero at atmospheric pressure to 50–60% at 30 bar. Both catalyst types are remarkably competitive, especially those containing nickel, which were always more active than a commercial rhodium catalyst used as a reference (5wt.% Rh/Al2O3) and tested under the same conditions. For the cobalt–cerium bimetallic oxides, the existence of a synergetic interaction between Co and CoO and the formation of cobalt carbides seems to play an important role in their catalytic behavior. Correlation between experimental reaction rates and simulated data confirms that the catalysts’ behavior follows the Langmuir–Hinshelwood–Hougen–Watson kinetic model, but Le Chatelier’s principle is also important to understand the catalysts’ behavior under pressure. A catalyst recycle study was also performed. The results obtained after five cycles using a nickel–cerium catalyst show insignificant variations in activity and selectivity, which are important for any type of practical application.

Hierarchical Graphitic Carbon-Encapsulating Cobalt Nanoparticles for Catalytic Hydrogenation of 2,4-Dinitrophenol

Cobalt hierarchical graphitic carbon nanoparticles (Co@HGC) (1), (2), and (3) were prepared by simple pyrolysis of a cobalt phenanthroline complex in the presence of anthracene at different temperatures and heating times, under a nitrogen atmosphere. The samples were used for the catalytic hydrogenation of 2,4-dinitrophenol. Samples (1) and (3) were prepared by heating at 600 °C and 800 °C respectively, while (2) was prepared by heating at 600 °C with an additional intermediate stage at 300 °C. This work revealed that graphitization was catalyzed by cobalt nanoparticles and occurred readily at temperatures of 600 °C and above. The nanocatalysts were characterized by Scanning Electron Microscopy SEM, energy dispersive X-ray analysis EDX, Raman, Xrd, and XPS. The analysis revealed the presence of cobalt and cobalt oxide species as well as graphitized carbon, while TEM analysis indicated that the nanocatalyst contains mainly cobalt nanoparticles of 3–20 nm in size embedded in a lighter graphitic web. Some bamboo-like multiwall carbon nanotubes and graphitic onion-like nanostructures were observed in (3). The structures and chemical properties of the three catalysts were correlated with their catalytic activities. The apparent rate constants kapp (min−1) of the 2,4-dinitrophenol reductions were 0.34 for (2), 0.17 for (3), 0.04 for (1), 0.005 (no catalyst). Among the three studied catalysts, the highest rate constant was obtained for (2), while the highest conversion yield was achieved by (3). Our data show that an increase in agglomeration of the cobalt species reduces the catalytic activity, while an increase in pyrolysis temperature improves the conversion yield. The nanocatalyst enhances hydrogen generation in the presence of sodium borohydride and reduces 2,4-dinitrophenol to p-diamino phenol. The best nanocatalyst (3) was prepared at 800 °C. It consisted of uniformly distributed cobalt nanoparticles sheltered by hierarchical graphitic carbon. The nanocatalyst is easily separated and recycled from the reaction system and proved to be degradation resistant, to have robust stability, and high activity towards the reduction reaction of nitrophenols.

Cobalt-Carbon Nanoparticles with Silica Support for Uptake of Cationic and Anionic Dyes from Polluted Water

Silica-supported hierarchical graphitic carbon sheltering cobalt nanoparticles Co-HGC@SiO2 (1) were prepared by pyrolysis at 850 °C of [Co(phen)(H2O)4]SO4·2H2O complex with silica in the presence of pyrene as a carbon source under nitrogen atmosphere. Nanocomposites (2) and (3) were obtained by acid treatment of (1) with HCl and HF acid, respectively. The nanocomposites showed rough hierarchical carbon microstructures over silica support decorated with irregular cobalt nanospheres and nanorods 50 to 200 nm in diameter. The nanoparticles consist of graphitic shells and cobalt cores. SEM, EDAX and TEM elemental mapping indicate a noticeable loss of cobalt in the case of (2) and loss of cobalt and silica in the case of (3) with an increase in porosity. Nanocomposite (3) showed the highest BET surface area 217.5 m2g-1. Raman spectrum shows defect D-band and graphitic G-band as expected in carbon nanostructures. PXRD reveals the presence of cobalt(0) nanoparticles. XPS indicates the presence of Co(II) oxides and the successful doping of nitrogen in the nanocomposites. Moreover, TEM elemental mapping provides information about the abundance of Si, Co, C, N and S elements in zones. Nanocomposite (1) showed maximum uptake capacity of 192.3 and 224.5 mg/g for crystal violet CV and methyl orange MO dyes, respectively. Nanocomposite (2) showed a capacity of 94.1 and 225.5 mg/g for CV and MO dyes, respectively. Nanocomposite (4) obtained after treatment of (1) with crystal violet proved successful adsorption of CV. Co-HGC (5) prepared without addition of silica has a capacity for CV equal to 192 mg/g, while it is 769.2 mg/g with MO. Electrostatics and π-π interactions of graphite and cobalt species in the nanocomposites with aromatic rings of cationic and anionic dyes are responsible for the adsorption. Yan et al. was the best model to describe column kinetics. The thomas column adsorption model showed that the maximum uptake capacity of (1) was 44.42 mg/g for CV and 32.62 mg/g for MO. for a column packed with 0.5 gm of (1) and dye concentration of 100 mg/L at a flow rate of 1 mL/min. The column was recycled three times with no noticeable clogging or degradation of nanocomposites. Thus, Co-HGC@SiO2 adsorbents can be used efficiently to treat water contaminated with cationic and anionic dyes.

Adsorption of Anionic Dyes from Wastewater onto Magnetic Nanocomposite Powders Synthesized by Combustion Method

In the present study, new magnetic nanocomposites were successfully prepared by combustion method, characterized by X-ray diffraction, Fourier transform infrared spectroscopy, magnetic measurements, N2 adsorption–desorption thermal analysis, and scanning electron microscopy, and tested as adsorbents for the removal of anionic dyes (Acid Yellow 42 and Acid Red 213) from aqueous solutions. The influence of process variables solution pH, adsorbent dose, initial dye concentration and temperature on the adsorption was evaluated. The best kinetic model that fitted with experimental data was a pseudo-second order model, and the equilibrium data were correlated by Langmuir isotherm model for the investigated dyes. Maximum removal efficiencies of 98.54% and 97.58% was obtained for Acid Yellow 42 and Acid Red 213, respectively, indicating the superior adsorption capacity of the new synthesized magnetic nanocomposites. The thermodynamic parameters indicated the spontaneous and endothermic nature of the adsorption process.

Hierarchical Porous Carbon Cobalt Nanocomposites-Based Sensor for Fructose

3D hierarchical graphitic carbon nanowalls encapsulating cobalt nanoparticles HPC-Co were prepared in high yield from solid-state pyrolysis of cobalt 2,2′-bipyridine chloride complex. Annealing of HPC-Co in air gave HPC-CoO, which consists of a mixture of crystallite Co3O4 nanospheres and nanorods bursting out of mesoporous carbon. Both nanocomposites were fully characterized using SEM, TEM, BET, and powder X-ray diffraction. The elemental composition of both nanocomposites examined using SEM elemental mapping and TEM elemental mapping supports the successful doping of nitrogen. The powder X-ray diffraction studies supported the formation of hexagonal cobalt in HPC-Co, and cubic crystalline Co3O4 with cubic cobalt in HPC-CoO. HPC-Co and HPC-CoO can be used as a modified carbon electrode in cyclic voltammetry experiments for the detection of fructose with limit of detection LOD 0.5 mM. However, the single-frequency impedimetric method has a wider dynamic range of 8.0–53.0 mM and a sensitivity of 24.87 Ω mM−1 for the electrode modified with HPC-Co and 8.0–87.6 mM and a sensitivity of 1.988 Ω mM−1 for the electrode modified with HPC-CoO. The LOD values are 3 and 4 mM, respectively. The effect of interference increases in the following order: ascorbic acid, ethanol, urea, and glucose. A simple method was used with negligible interference from glucose to measure the percentage of fructose in a corn syrup sample with an HPC-CoO electrode. A specific capacitance of 47.0 F/g with 76.6% retentivity was achieved for HPC-Co and 28.2 F/g with 87.9% for HPC-CoO for 3000 charge–discharge cycles. Thus, (1) has better sensitivity and specific capacitance than (2), because (1) has a higher surface area and less agglomerated cobalt nanoparticles than (2).

L-Ascorbic Acid-g-Polyaniline Mesoporous Silica Nanocomposite for Efficient Removal of Crystal Violet: A Batch and Fixed Bed Breakthrough Studies

In the present study, mesoporous silica nanoparticles (MSNs) synthesized through sol-gel process and calcined at 600 °C were further surface functionalized by a copolymer chain of L-ascorbic acid (AS) and polyaniline (PAni) by in situ free radical oxidative polymerization reaction. The surface modification of MSNs by AS-g-PAni was confirmed by using various analytical techniques, namely FTIR, XRD, SEM-EDX, TEM and AFM. The composition of AS-g-PAni@MS was found to be composed of C (52.53%), N (20.30%), O (25.69%) and Si (1.49%), with 26.42 nm as the particle size. Further, it was applied for the adsorption of crystal violet (CV) dye under batch, as well as fixed bed method. RSM-BBD was taken into consideration, to optimize the various operational parameters effecting the adsorption through batch method. To explore maximum efficiency of the material, it was further subjected to adsorption of CV under fixed bed method, using the variable bed heights of 3.7, 5.4 and 8.1 cm. Based on high value of regression coefficient (R2) and low value of RMSE given as (0.99, 0.02) for 3.7 cm, (0.99, 0.03), the breakthrough data were very well defined by the Thomas model, with optimum concurrence of stoichiometric adsorption capacity values. The external mass transfer equilibrium data were well fitted by the Langmuir model, with maximum monolayer adsorption capacity of 88.42 mg g-1 at 303K, 92.51 mg g-1 at 313 K, 107.41 mg g-1 at 313 K and 113.25 mg g-1 at 333 K. The uptake of CV by AS-g-PAni@MS was well defined by pseudo second order model with rate constant K2 = 0.003 L mg-1 min-1 for 50 and 0.003 L mg-1 min-1 for 60 mg L-1 CV. The adsorption reaction was endothermic with enthalpy (ΔH) value of 3.62 KJ mol-1 and highly efficient for treatment of CV-contaminated water for more the five consecutive cycles.

Novel cobalt-carbon@silica adsorbent

Recently, carbon nanostructures are of high importance due to their unique characteristics and interesting applications. Pyrolysis of anthracene with cobalt complex Co(2,2'-bipy)Cl2 (1), where (2,2'-bipy) is 2,2'-bipyridine, in the absence and presence of silica gave in high yield cobalt-carbon nanocomposite CoCNC (2) and CoCNC@SiO2 (3) at 600 °C and 850 °C, respectively. They were characterized using SEM, TEM, PXRD, Raman and XPS. (3) and (2) contain core-shell cobalt(0)/cobalt oxide-graphite with or without silica support. PXRD indicates that (2) contains crystalline hexagonal α-Co and cubic β-Co phases while (3) contains only cubic β-Co phase and silica. The structure of (2) is 3D hierarchical carbon architecture wrapping spherical and elliptical cobalt nanoparticles. (3) consists of graphitized structures around cobalt nanoparticles embedded in the silica matrix. XPS reveals that the nanocomposites contain oxygen functional groups that enhance uptake of cationic dyes. CoCNC@SiO2 (3) has higher capacity and thus is better adsorbent of Basic Violet 3 than CoCNC (2). The Langmuir adsorption capacity of (3) is 19.4 mg g-1 while column capacity is 12.55 mg g-1 at 25 °C. Freundlich isotherm and pseudo-second-order kinetic models fit well the adsorption data. Thermodynamics indicate that adsorption(3) is exothermic. Column regeneration was tested for three cycles and Yan et al. was found the best kinetic model.

Electrocatalytic Properties of 3D Hierarchical Graphitic Carbon-Cobalt Nanoparticles for Urea Oxidation

A 3D hierarchical graphitic carbon nanostructure encapsulating cobalt(0)/cobalt oxide nanoparticles (CoGC) has been prepared by solid-state pyrolysis of a mixture of anthracene and cobalt 2,2'-bipyridine terephthalate complex at 850 °C. Based on the Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) methods, the prepared material has high surface area (186.8 m² g^-1) with an average pore width of 205.5 Å. XPS reveals the functionalization of carbon with different oxygen-containing groups, such as carboxylic acid groups. The presence of metallic cobalt nanoparticles with cubic and hexagonal crystalline structures encapsulated in graphitized carbon is confirmed using XRD and TEM. Raman spectroscopy indicates a graphitization degree of I _D/I _G = 1.02. CoGC was cast onto a glassy carbon electrode and used for urea electrooxidation in an alkaline solution. The electrochemical investigation shows that the newly prepared CoGC has a promising electrocatalytic activity toward urea. The specific activity is 128 mA cm^-1 mg^-1 for the electrooxidation of 0.3 M urea in 1 M KOH at a relatively low onset potential (0.31 V vs Ag/AgCl). It can be mainly attributed to the morphological structure of carbon and the high reactivity of cobalt nanoparticles. The calculated charge-transfer resistance, R _ct, of the modified electrode in the presence of urea (10.95 Ω) is significantly lower than that in the absence of urea (113.5 Ω), which indicates electrocatalytic activity. The value of charge-transfer rate constant, k _s, for the anodic reaction is 0.0058 s^-1. Electrocatalytic durability in 1000 s chronoamperometry of the modified electrode suggests high structure stability. The modified electrode retained about 60% of its activity after 100 cycles as indicated by linear sweep voltammetry.