Browsing by Subject "Cobalt"

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Open Access
Electrocatalytic hydrogen evolution with cobalt–poly (4-vinylpyridine) metallopolymers
(Springer Netherlands, 2018) Kap, Zeynep; Ülker, Emine; Nune, Satya Vijaya Kuma; Karadaş, Ferdi
Abstract: A facile synthetic pathway using poly(4-vinylpyridine) as a polypyridyl platform is reported for the formation of a metallopolymer. Electrochemical studies indicate that the metallopolymer acts as an efficient H2 evolution catalyst similar to cobalt polypyridyl complexes. It is also observed that the metallopolymer is transformed to cobalt particles when a cathodic potential is applied in the presence of an acid. Electrochemical measurements indicate that an FTO electrode coated with these cobalt particles also acts as an efficient hydrogen evolution catalyst. Approximately 80 µmoles of H2 gas can be collected during 2 h of electrolysis at − 1.5 V (vs. Fc+/0) in the presence of 60 mM of acetic acid. A comprehensive study of the electrochemical and electrocatalytic behavior of cobalt-poly(4-vinylpyridine) is discussed in detail. Graphical Abstract: Poly(4-vinylpyridine) as a precursor for electrodeposited cobalt particles: a cobalt coat derived by a metallopolymer acts as an efficient H2 evolution catalyst. It can transform to a cobalt coat when a potential above − 1.1 V is applied in acid medium. Exchange current density of 10−2.67 mA cm−2 was observed from the Co-coat at − 1.5 V (vs. Fc+/0).
Open Access
Metal dicyanamides as efficient and robust water-oxidation catalysts
(Wiley Blackwell, 2017) Nune, S. V. K.; Basaran, A. T.; Ülker, E.; Mishra, R.; Karadas, F.
Non-oxide cobalt-based water-oxidation electrocatalysts have received attention recently for their relative ease of preparation, they are stable both in acidic and basic media, and they have higher turnover frequencies than cobalt oxides. Recent studies show that one of the main bottlenecks in the implementation of non-oxide systems to water splitting is the low number of active metal sites, which is in the order of nmol cm−2. Herein, a new series of non-oxide water-oxidation catalysts has been introduced to the field. Cobalt dicyanamides are observed to have around four times higher surface active sites and better catalytic performances than cyanide-based systems. Long-term catalytic studies (70 h) at an applied potential of 1.2 V and electrochemical studies performed in solutions in pH values of 3.0–12.0 indicate that the compounds are robust and retain their structures even under harsh conditions. Moreover, the addition of Ni impurities to cobalt dicyanamides is a feasible method to improve their catalytic activities.
Open Access
Oxygen partial pressure dependence of magnetic, optical and magneto-optical properties of epitaxial cobalt-substituted SrTiO3 films
(OSA - The Optical Society, 2015) Onbaşli, M.C.; Goto, T.; Tang, A.; Pan, A.; Battal, E.; Okyay, Ali Kemal; Dionne G.F.; Ross, C.A.
Cobalt-substituted SrTiO3 films (SrTi0.70Co0.30O3-δ) were grown on SrTiO3 substrates using pulsed laser deposition under oxygen pressures ranging from 1 μTorr to 20 mTorr. The effect of oxygen pressure on structural, magnetic, optical, and magneto-optical properties of the films was investigated. The film grown at 3 μTorr has the highest Faraday rotation (FR) and magnetic saturation moment (Ms). Increasing oxygen pressure during growth reduced Ms, FR and optical absorption in the nearinfrared. This trend is attributed to decreasing Co2+ ion concentration and oxygen vacancy concentration with higher oxygen partial pressure during growth. © 2015 Optical Society of America.
Open Access
Radiochemical and spectroscopic studies of cesium, barium, and cobalt sorption on some natural clays
(2000-08) Shahwan, Talal
The wide growth in the nuclear activities results in an increasing subsequent influx of radioactive wastes into the environment. This problem has manifested a great deal of interest aiming at finding out ways through which those wastes can be harmlessly isolated from the human environment. Geological disposal is considered as one of the most promising solutions that ensures a safe storage of radioactive wastes as long as their activities are above the accepted levels. Clay minerals are proposed as backfill buffering materials in the geological repositories that can delay the migration of the radionuclides through sorption and thus decrease the contamination of underground waters. The extent of retardation of the radionuclide migration is dependent on factors like time of contact, pH and Eh of groundwater, concentration, temperature and grain size of the mineral particles. In this study radiochemical, spectroscopic (ToF-SIMS, XPS), and X-ray diffraction techniques were applied to examine different aspects of the sorption behavior of cesium, barium and cobalt on three natural clay minerals containing primarily kaolinite, illite-chlorite, and bentonite. The elements cesium (Z=55), barium (Z=56), and cobalt (Z=27) have the radioactive isotopes superscript 137 Cs (half-life=30.17 years), superscript 140 Ba (half-life=12.79 days), and superscript 60 Co (half-life=5.3 y) which are important in radioactive waste management. The first two radionuclides are produced in high yields in nuclear fission, whereas the third is an activation product. The natural clay samples that were used in this study originated from natural mineralogical beds at Sindırgı, Afyon, and Giresun regions in Turkey. The characterization of these clay samples showed that the primary clay minerals were kaolinite in Sındırgı clay, chlorite and illite in Afyon clay, and montmorillonite in Giresun clay. Each of these clays possesses different structural properties that result in different sorption capabilities. Radiochemical batch experiments were carried out to examine the effects of time, concentration, and temperature on the sorption of cesium, barium and cobalt on clays. Solutions of these cations spiked with several microliters of the radionuclides 137 CS (half-life=30.1 y), 133 Ba (half-life=10.7 y), and 60 Co (half-life=5.3 y) were monitored using gamma-ray spectroscopy prior to and after each sorption experiment. These results showed that equilibrium is achieved within two days in all cases. The sorption data was adequately described by Freundlich and Dubinin-Radushkevich isotherm models. Based on the parameters of those isotherm models, it was found that sorption was nonlinear, and that bentonite showed the highest sorption affinity and sorption capacity towards the sorbed ions. The thermodynamic parameters indicated that while sorption of cesium and barium on the three clays is exothermic that of cobalt is endothermic. The obtained values of Gibbs free energy change, Delta G degrees, were generally in the 8-16 (kJ/mol) energy range that corresponds to ion exchange type sorption mechanism. Since sorption is mainly a surface phenomenon, part of our sorption studies were carried out using the surface sensitive techniques; Time of Flight- Secondary Ion Mass Spectroscopy (ToF-SIMS) and X-ray Photoelectron Spectroscopy (XPS). In addition, depth profiling up to 70 angstroms was performed using ToF-SIMS to investigate cesium, barium and cobalt concentrations through the clay surface. ToF-SIMS and XPS studies were helpful in figuring out the surface composition of different clays prior to and after sorption. Quantification of the depletion of different alkali and alkaline-earth metals initially contained within the analyzed clay surface showed that ion exchange plays a primary role in the sorption process. In addition, X-Ray Diffraction (XRD) technique was applied to figure out the mineralogical composition of the clay minerals used and examine any structural change a accompanying the sorption process. XRD spectra of the clay samples after sorption showed that -apart from some intensity reductions in some clay features-, no primary changes were detected in the sorption cases of cesium and cobalt. In barium sorption, however, features belonging to barium carbonate were present in the spectra corresponding to sorption on chlorite-illite and bentonite.
Open Access
Radiochemical study of Co2+ sorption on chlorite and kaolinite
(Akademiai Kiado Rt., 1999) Shahwan, T.; Erten, H. N.
In this work, the sorption behavior of Co(II) ions on natural chlorite and kaolinite as a function of time, concentration and temperature was studied. 60Co radiotracer method and the batch technique were used. The kinetic results indicated that about one day of contact time was enough to achieve equilibrium. The sorption process was described by Freundlich type isotherms. Sorption of Co(II) ions on both clays was found to be endothermic with ΔH(o) (kJ/mol) and ΔS(o) (kJ/mol·K) being 33 and 0.14 for kaolinite and 17 and 0.102 for chlorite, respectively. The magnitudes of the corresponding ΔG(o) values suggest that sorption occur mainly via an ion exchange mechanism on both clays.
Open Access
Sorption behavior of Co2+, Zn2+ and Ba2+ ions on alumina, kaolinite and magnesite
(Kluwer Academic Publishers, 1994) Erten, H. N.; Gokmenoglu, Z.
The sorption behavior of Ba2+, Co2+ and Zn2+ ions on alumina, kaolinite and magnesite have been investigated using the batch method.60Co,65Zn and133Ba were used as radiotracers. The mineral samples were separated into different particle size fractions using an Andreasen Pipette. The particle sizes used in the sorption experiments were all less than 38 μm. Synthetic groundwaters were used which had compositions similar to those from the regions where the minerals were recovered. The samples were shaken with a lateral shaker at 190 rpm, the phases were separated by centrifuging and adioactivity counted using a NaI(Tl) detector. Kinetic studies indicated that sorption onto the minerals took place in two stages with the slower process dominating. The highest sorption was observed on alumina. Both Freundlich and Dubinin-Radushkevich type isotherms were found to describe the sorption process well. The distribution ratio, Rd was found to be a function of the liquid volume to solid mass ratio. The Rd's for sorption on binary mixtures of minerals were experimentally determined and compared with those predicted from Rd values of each individual mineral. © 1994 Akadémiai Kiadó.
Open Access
Sorption studies of Cs+, Ba2+, and Co2+ ions on bentonite using radiotracer, ToF-SIMS, and XRD techniques
(De Gruyter Oldenbourg, 2001) Shahwan, T.; Erten H. N.
The sorption behaviour of Cs+, Ba2+, and Co2+ ions on bentonite were investigated using the radiotracer method, Time of Flight-Secondary Ion Mass Spectroscopy (ToF-SIMS), and X-Ray Diffraction (XRD). The sorption of Cs+ and Ba2+ were exothermic while sorption of Co2+ was endothermic. The sorption data were well described by Freundlich and Dubinin-Radushkevich isotherms. According to ToF-SIMS results Na+ and Mg2+ were the primary exchanging ions in bentonite. The XRD spectra showed that no structural changes were associated with the sorption of Cs+ and Co2+, and BaCO3 precipitate was formed upon the sorption of Ba2+ on bentonite.
Open Access
Synthesis and optical properties of Co and Zn-based metal oxide nanoparticle thin films
(Polish Academy of Sciences, 2017) Gungor, E.; Gungor, T.; Calıskan, D.; Özbay, Ekmel
ZnO, Co doped ZnO (ZnO:Co) and CoO thin films were deposited on glass substrates by using the spark discharge technique with Zn-Zn, Zn-Co and Co-Co metal electrodes (tips). The structural and optical properties of the films were characterized by X-ray diffraction, scanning electron microscopy measurements and UV-Vis spectrometry. Cubic phase reflection of CoO (200) was observed in the samples containing Co. The size of nanoparticles had varied between 38 nm and 200 nm in ZnO thin films. When Co electrode was used, spherical structure had deteriorated and clusters of particles, with smaller radii, were observed. In addition, when Co-Co electrode pairs were used, various cavities with different sizes were formed. Especially, it was observed that the optical transmittance had generally increased with the decreasing spark (charge) voltage, while increasing with the number of sparks. The Co-containing samples were green in color and it was observed that the loss of transmission appears in a specific region in the Co-doped ZnO thin films due to characteristic d-d transition of Co2+ ions. The thickness of the films had decreased with the increasing number of sparks. In addition, the band gap energy, Eg, evaluated by UV-Vis spectroscopy measurements has been shifted to higher wavelengths (red shift) for the ZnO:Co thin films.
Open Access
Synthesis and spectral studies of 5-[3-(1, 2, 4–triazolyl-azo]-2, 4-dihydroxybenzaldehyde (TA) and its Schiff bases with 1, 3-diaminopropane (TAAP) and 1, 6-diaminohexane (TAAH). Their analytical application for spectrophotometric microdet
(Pergamon Press, 2005) Khedr, A. M.; Gaber, M.; Issa, R. M.; Erten, H.
The new azo compound 5-[3-(1,2,4-triazolyl-azo]-2, 4-dihydroxy-benzaldehyde (TA) and its Schiff bases with 1,3-diaminopropane (TAAP) and 1,6-diaminohexane (TAAH) have been synthesized. The bands of diagnostic importance in the IR and the main signals in 1H NMR spectra are assigned. The electronic absorption spectra in pure organic solvents of different polarity and in buffer solutions of varying pH are investigated. The quantitative description of the solvent effect on the electronic absorption spectra is studied and their acid ionization constants are determined. Also, a new simple and sensitive method for the spectrophotometric microdetermination of Co(II) using these compounds (TA, TAAP and TAAH) as new chromogenic reagents is established. The developed method is successfully used for the determination of trace amounts of cobalt in authentic samples and calculation of the distribution ratio of cobalt adsorbed on bentonite and kaolinite clay minerals. © 2004 Elsevier Ltd. All rights reserved.
Open Access
Water oxidation electrocatalysis with a cobalt ‐ borate ‐ based hybrid system under neutral conditions
(Wiley-VCH Verlag, 2018) Turhan, Emine A.; Nune, Satya Vijaya Kumar; Ülker, E.; Şahin, U.; Dede, Y.; Karadaş, Ferdi
The development of new water oxidation electrocatalysts that are both stable and efficient, particularly in neutral conditions, holds great promise for overall water splitting. In this study, the electrocatalytic water oxidation performance of a new cobalt-based catalyst, Co3(BO3)2, with a Kotoite-type crystal structure is investigated under neutral conditions. The catalyst is also hybridized with CNTs to enhance its electrocatalytic properties. A remarkable increase in catalytic current along with a significant shift in the onset overpotential is observed in Co3(BO3)2@CNT. Additionally, CNT addition also greatly influences the surface concentration of the catalyst: 12.7 nmol cm−2 for Co3(BO3)2@CNT compared with 3.9 nmol cm−2 for Co3(BO3)2. Co3(BO3)2@CNT demands overpotentials of 303 and 487 mV to attain current densities of 1 and 10 mA cm−2, respectively, at pH 7. Electrochemical and characterization studies performed over varying pH conditions reveal that the catalyst retains its stability over a pH range of 3-14. Multi-reference quantum chemical calculations are performed to study the nature of the active cobalt sites and the effect of boron atoms on the activity of the cobalt ions.