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Öğe Ceria supported ruthenium(0) nanoparticles: Highly efficient catalysts in oxygen evolution reaction(Academic Press Inc Elsevier Science, 2019) Demir, Elif; Akbayrak, Serdar; Onal, Ahmet M.; Ozkar, SaimRuthenium(0) nanoparticles were successfully prepared on the surface of ceria (Ru-0/CeO2) and used as catalysts on glassy carbon electrode (GCE) in oxygen evolution reaction (OER) from water electrolysis at room temperature. Ru-0/CeO2 on GCE exhibits high catalytic activity for OER in alkaline solution. It provides a low onset potential of 1.57 V vs. RHE and low overpotential of 420 mV vs. RHE to reach a current density of 10 mA cm(-2). Ru-0/CeO2 on GCE shows no change in the onset potential value even after 6.1 h electrochemical measurement in OER. (C) 2018 Elsevier Inc. All rights reserved.Öğe Cobalt ferrite supported platinum nanoparticles: Superb catalytic activity and outstanding reusability in hydrogen generation from the hydrolysis of ammonia borane(Academic Press Inc Elsevier Science, 2021) Akbayrak, Serdar; Ozkar, SaimIn this work, platinum(0) nanoparticles are deposited on the surface of magnetic cobalt ferrite forming magnetically separable Pt-0/CoFe2O4 nanoparticles, which are efficient catalysts in H-2 generation from the hydrolysis of ammonia borane. Catalytic activity of Pt-0/CoFe2O4 nanoparticles decreases with the increasing platinum loading, parallel to the average particle size. Pt-0/CoFe2O4 (0.23% wt. Pt) nanoparticles have an average diameter of 2.30 +/- 0.47 nm and show an extraordinary turnover frequency of 3628 min(-1) in releasing 3.0 equivalent H-2 per mole of ammonia borane from the hydrolysis at 25.0 degrees C. Moreover, the magnetically separable Pt-0/CoFe2O4 nanoparticles possess high reusability retaining 100% of their initial catalytic activity even after ten runs of hydrolysis. The superb catalytic activity and outstanding reusability make the Pt-0/CoFe2O4 nanoparticles very attractive catalysts for the hydrogen generation systems in portable and stationary fuel cell applications. (C) 2021 Elsevier Inc. All rights reserved.Öğe Enhanced reactivity in a heterogeneous oxido-peroxido molybdenum(VI) complex of salicylidene 2-picoloyl hydrazone in catalytic epoxidation of olefins(Springer, 2017) Moradi-Shoeili, Zeinab; Zare, Maryam; Akbayrak, Serdar; Ozkar, SaimA molybdenum(VI) oxido-diperoxido complex of salicylidene 2-picoloyl hydrazine (sal-phz) was synthesized and successfully grafted onto chloro-functionalised Fe3O4 nanoparticles. The resulting heterogeneous and magnetically recoverable nanoscale catalyst MoO3(sal-phz)/Fe3O4 was characterized by physicochemical and spectroscopic techniques. The activity of this heterogeneous catalyst for the oxidation of olefins to corresponding epoxides was efficiently increased by increasing the reaction temperature up to 95 degrees C. The nanocatalyst proved to be efficient for the selective epoxidation of a variety of alkenes using t-BuOOH with high conversion and selectivity. Leaching and recycling tests showed that the nanocatalyst can be reused at least six times without significant decrease in efficiency.Öğe Group 4 oxides supported Rhodium(0) catalysts in hydrolytic dehydrogenation of ammonia borane(Pergamon-Elsevier Science Ltd, 2019) Tonbul, Yalcin; Akbayrak, Serdar; Ozkar, SaimRh3+ ions are first impregnated on Group 4 metal oxides (TiO2, ZrO2, HfO2) in aqueous solution and, then reduced with aqueous solution of NaBH4 to form rhodium(0) nanoparticles (NPs) on the oxide surface. The analyses reveal that Rh(0) NPs are highly dispersed on the surface of TiO2, ZrO2, HfO2. Rh-0/MO2 (M: Ti, Zr, Hf) NPs have high activity and reusability in releasing H-2 from the hydrolysis of ammonia borane with an initial turnover frequency of 643, 198, and 188 min(-1), respectively, at 25.0 +/- 0.1 degrees C. The reusability of Rh-0/ZrO2 and Rh-0/HfO2 catalysts is higher than that of the Rh-0/TiO2 catalyst. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.Öğe Immobilized Polyoxomolybdate Nanoclusters on Functionalized SBA-15: Green Access to Efficient and Recyclable Nanocatalyst for the Epoxidation of Alkenes(Wiley-V C H Verlag Gmbh, 2019) Bagherzadeh, Mojtaba; Hosseini, Hadighehsadat; Akbayrak, Serdar; Ozkar, SaimA heterogeneous nanocatalyst containing polyoxomolybdate immobilized on the internal surface of amino-functionalized SBA-15 pores was prepared and fully characterized by several analytical techniques. The textural property of supported material was found to retain important mesoporous features, compatible with its potential use as heterogeneous catalysts. The catalyst SBA-15-NH3+center dot{Mo-36}(-)shows excellent catalytic activity for the epoxidation of alkenes as compared to POM, which is attributed to the large specific surface area and highly ordered pore structure of SBA-15, allowing for easy access of reactants to the active sites and removal of products. The prepared catalyst has high recyclability performance without obvious loss of activity after six runs of epoxidation.Öğe Magnetically Isolable Pt0/Co3O4 Nanocatalysts: Outstanding Catalytic Activity and High Reusability in Hydrolytic Dehydrogenation of Ammonia Borane(Amer Chemical Soc, 2021) Akbayrak, Serdar; Ozkar, SaimThe development of a new platinum nanocatalyst to maximize the catalytic efficiency of the precious noble metal catalyst in releasing hydrogen from ammonia borane (AB) is reported. Platinum(0) nanoparticles are impregnated on a reducible cobalt(II,III) oxide surface, forming magnetically isolable Pt-0/Co3O4 nanocatalysts, which have (i) superb catalytic activity providing a record turnover frequency (TOF) of 4366 min(-1) for hydrogen evolution from the hydrolysis of AB at room temperature and (ii) excellent reusability, retaining the complete catalytic activity even after the 10th run of hydrolysis reaction. The outstanding activity and stability of the catalyst can be ascribed to the strong interaction between the platinum(0) nanoparticles and reducible cobalt oxide, which is supported by the results of XPS analysis. Pt-0/Co3O4 exhibits the highest TOF among the reported platinum-nanocatalysts developed for hydrogen generation from the hydrolysis of AB.Öğe Magnetically Separable Rh0/Co3O4 Nanocatalyst Provides over a Million Turnovers in Hydrogen Release from Ammonia Borane(Amer Chemical Soc, 2020) Akbayrak, Serdar; Tonbul, Yalcin; Ozkar, SaimCobalt(II,III) oxide nanopowders are used as supporting materials for rhodium(0) nanoparticles forming Rh-0/Co3O4 nanocatalysts, which can be prepared by impregnation and sodium borohydride reduction of Rh3+ ions on the surface of the oxide support. Magnetically separable Rh-0/Co3O4 nanoparticles are isolated from the reaction medium by an external magnet and characterized using various analytical techniques. Rh-0/Co3O4 nanoparticles are highly active and reusable catalysts with a long lifetime in hydrolytic dehydrogenation of ammonia borane (AB) at room temperature. Rh-0/Co3O4 nanoparticles with 0.5% wt Rh loading provide a turnover frequency of 1800 min(-1) and a total of 1.02 x 10(6) turnovers for H-2 evolution from the hydrolysis of AB at 25.0 +/- 0.1 degrees C. This turnover frequency is the second best value ever reported for the hydrolysis of AB, and the total turnover number of over a million is a record lifetime ever reported. Magnetically separable rhodium(0) nanoparticles are expectedly highly reusable catalysts and preserve their initial activity after the fifth run of hydrolysis. We also report the results of our study on the catalytic activity of Co3O4 nanopowders for the same dehydrogenation reaction.Öğe Magnetically separable rhodium nanoparticles as catalysts for releasing hydrogen from the hydrolysis of ammonia borane(Academic Press Inc Elsevier Science, 2019) Tonbul, Yalcin; Akbayrak, Serdar; Ozkar, SaimMagnetically separable catalysts attract considerable attention in catalysis due to their facile separation from the reaction medium. This propensity is crucial for efficient multiple use of precious noble metal nanoparticles in catalysis. In fact, the isolation of catalysts from the reaction medium by filtration and washing results usually in the loss of huge amount of activity in the subsequent run of catalysis. Although many transition metal nanoparticle catalysts have been reported for the H-2 generation from the hydrolysis of ammonia borane, there is no study reporting the magnetically separable rhodium based catalysts for the hydrolytic dehydrogenation of ammonia borane. Here, we report the preparation of rhodium(0) nanoparticles supported on the surface of Fe3O4 and CoFe2O4 magnetic nanopowders as the first example of magnetically separable rhodium nanocatalysts. The resulting magnetically separable Rh-0/Fe3O4 and Rh-0/CoFe2O4 nanoparticles are highly active, long-lived and reusable catalysts in H-2 generation from the hydrolysis of ammonia borane providing a turnover frequency value of 273 and 720 min(-1), respectively, at 25.0 +/- 0.1 degrees C. These magnetically separable catalysts show high reusability and long-term stability in the hydrolysis reaction. They retain their complete initial activity even after the 5th use releasing exactly 3.0 equivalent H-2 gas per mole of ammonia borane. The long-term stability tests show that Rh-0/Fe3O4 and Rh-0/CoFe2O4 nanoparticles provide a total turnover number of 125,000 and 245,000, respectively, in releasing H-2 from the hydrolysis of ammonia borane at room temperature. The long term stability and reusability of magnetically separable Rh-0/Fe3O4 and Rh-0/CoFe2O4 nanopartides make them attractive catalysts for hydrogen generation in fuel cell applications. (C) 2019 Elsevier Inc. All rights reserved.Öğe Nanoceria supported cobalt(0) nanoparticles: a magnetically separable and reusable catalyst in hydrogen generation from the hydrolysis of ammonia borane(Royal Soc Chemistry, 2017) Akbayrak, Serdar; Taneroglu, Oguzcan; Ozkar, SaimCobalt(0) nanoparticles supported on nanoceria (Co-0/CeO2) were prepared from the reduction of cobalt(II) ions impregnated on the surface of ceria nanopowders. The magnetic Co-0/CeO2 catalyst could be isolated from the reaction solution by using a magnet and characterized by using ICP-OES, XRD, TEM, EDX, BET and XPS techniques. The results reveal that cobalt nanoparticles with particle sizes in the range between 3.5 and 6.0 nm were successfully anchored on the surface of nanoceria. Co-0/CeO2 is a magnetically separable and reusable catalyst in hydrogen generation from the hydrolysis of ammonia borane retaining its initial catalytic activity (TOF = 7.0 min(-1)) even after the fifth use. For comparison, other non-noble metal nanoparticles were also supported on nanoceria. Ni-0/CeO2 and Cu-0/CeO2 provide TOF values of 1.7 min(-1) and 1.5 min(-1), respectively, while Fe-0/CeO2 is catalytically silent in the hydrolysis of ammonia borane at room temperature. The work reported here also includes the results of a kinetic study of the catalytic hydrolysis of ammonia borane depending on the temperature and catalyst concentration.Öğe Nanoceria-Supported Ruthenium(0) Nanoparticles: Highly Active and Stable Catalysts for Hydrogen Evolution from Water(Amer Chemical Soc, 2018) Demir, Elif; Akbayrak, Serdar; Onal, Ahmet M.; Ozkar, SaimRuthenium(0) nanoparticles supported on nanoceria (Ru-0/CeO2) were prepared by reduction of Ru3+ ions on the surface of ceria using aqueous solution of NaBH4. The Ru-0/CeO2 samples were characterized by advanced analytical tools and employed as electrocatalysts on the glassy carbon electrode (GCE) in hydrogen evolution from water. The GCE, modified by Ru-0/CeO2 (1.86 wt % Ru), provides an incredible electrocatalytic activity with a high exchange current density of 0.67 mA.cm(-2), low overpotential of 47 mV at j = 10 mA.cm(-2), and small Tafel slope of 41 mV.dec(-1). Moreover, this modified GCE provides an unprecedented long-term stability without changing the onset potential (33 mV) even after 10 000 scans in acidic water splitting at room temperature. The hydrogen gas, evolved during the water splitting using the Ru-0/CeO2 (1.86 wt % Ru) electrocatalyst, was also collected. The amount of the evolved H-2 gas matches well with the calculated value, which indicates the achievement of nearly 100% Faradaic efficiency.Öğe Nanozirconia supported ruthenium(0) nanoparticles: Highly active and reusable catalyst in hydrolytic dehydrogenation of ammonia borane(Academic Press Inc Elsevier Science, 2018) Tonbul, Yalcin; Akbayrak, Serdar; Ozkar, SaimNanozirconia supported ruthenium(0) nanoparticles (Ru-0/ZrO2) were prepared by impregnation of ruthenium(III) cations on the surface of zirconia followed by their reduction with sodium borohydride at room temperature. Ru-0/ZrO2 was isolated from the reaction solution by centrifugation and characterized by ICP-OES, XRD, TEM, SEM EDS and XPS techniques. All the results reveal that ruthenium(0) nanoparticles were successfully supported on zirconia and the resulting Ru-0/ZrO2 is a highly active and reusable catalyst for hydrogen generation from the hydrolysis of ammonia borane with a turnover frequency value of 173 min(-1) at 25 degrees C. The reusability and catalytic lifetime tests reveal that Ru-0/ZrO2 is still active in the subsequent runs of hydrolysis of ammonia borane preserving 67% of the initial catalytic activity even after the fifth run and Ru-0/ZrO2 provides 72,500 turnovers (mol H-2/mol Ru) before deactivation at 25 degrees C. Our report also includes the results of kinetic studies depending on the catalyst concentration and temperature to determine the activation energy (E-a = 58 2 kJ/mol) for hydrolytic dehydrogenation of AB. (C) 2017 Elsevier Inc. All rights reserved.Öğe Nickel(0) nanoparticles supported on bare or coated cobalt ferrite as highly active, magnetically isolable and reusable catalyst for hydrolytic dehydrogenation of ammonia borane(Academic Press Inc Elsevier Science, 2017) Manna, Joydev; Akbayrak, Serdar; Ozkar, SaimNickel(0) nanoparticles supported on cobalt ferrite (Ni-0/CoFe2O4), polydopamine coated cobalt ferrite (NP0/PDA-CoFe2O4) or silica coated cobalt ferrite (NP0/SiO2-CoFe2O4) are prepared and used as catalysts in hydrogen generation from the hydrolysis of ammonia borane at room temperature. Ni-0/CoFe2O4 (4.0% wt. Ni) shows the highest catalytic activity with a TOF value of 38.3 min(-1) in hydrogen generation from the hydrolysis of ammonia borane at 25.0 +/- 0.1 degrees C. However, the initial catalytic activity of Ni-0/NCoFe204 catalyst is not preserved in subsequent runs of hydrolysis. Coating the surface of cobalt ferrite support with polydopamine or silica leads to a significant improvement in the stability of catalysts. The TOF values of Ni-0/PDA-CoFe2O4 and Ni-0/(2)-CoFe2O4 are found to be 7.6 and 5.3 min(-1), respectively, at 25.0 +/- 0.1 degrees C. Ni-0/PDA-CoFe2O4 catalyst shows high reusability as compared to the Ni-0/CoFe2O4 and NP0/SiO2-CoFe2O4 catalysts in hydrolytic dehydrogenation of ammonia borane at room temperature. All the catalysts are characterized by using a combination of various advanced analytical techniques. The results reveal that nickel nanoparticles with an average size of 12.3 +/- 0.7 nm are well dispersed on the surface of PDA-CoFe2O4. (C) 2017 Elsevier Inc. All rights reserved.Öğe Oxazine containing molybdenum(VI)-oxodiperoxo complex immobilized on SBA-15 as highly active and selective catalyst in the oxidation of alkenes to epoxides under solvent-free conditions(Elsevier Science Bv, 2017) Zare, Maryam; Moradi-Shoeili, Zeinab; Esmailpour, Parvin; Akbayrak, Serdar; Ozkar, SaimMolybdenum(VI) oxodiperoxo complex containing an oxazine ligand, ([MoO(O-2)(2)(phox)], phox: 2-(2'-hydroxyphenyl)-5,6-dihydro-1,3-oxazine) was conveniently synthesized and immobilized onto chloro-functionalized mesoporous silica SBA-15 by covalent bonding between the chloropropyl group on the internal surface of the pores and the nitrogen atom of oxazine ligand yielding [MoO(O-2)(2)(phox)]/SBA-15. The resulting material was characterized by FT-IR, TGA, XRD, SEM, TEM, EDX, ICP-AES, BET and UV-vis spectroscopy. The heterogeneous catalyst [MoO(O-2)(2)(phox)]/SBA-15 was employed in the epoxidation of alkenes, exhibiting high catalytic performance and selectivity for epoxide similar to that of homogeneous catalyst, [MoO(O-2)(2)(Phox)] under solvent-free conditions. Finally, it was found that the heterogeneous catalyst had virtually no degree of leaching, which allowed its recyclability to nine cycles without loss of catalytic performance. (C) 2017 Elsevier Inc. All rights reserved.Öğe Preparation and characterization of a new CdS-NiFe2O4/reduced graphene oxide photocatalyst and its use for degradation of methylene blue under visible light irradiation(Springer, 2018) Bagherzadeh, Mojtaba; Kaveh, Reyhaneh; Ozkar, Saim; Akbayrak, SerdarIn this paper, CdS nanoparticles as a visible light active photocatalyst were coupled by NiFe2O4 and reduced graphene oxide (rGO) to form CdS-NiFe2O4/rGO nanocomposite by facile hydrothermal methods. The CdS-NiFe2O4/rGO nanocomposite shows enhanced photocatalytic activity for the degradation of methylene blue (MB) under visible light illumination. In addition to improved photocatalytic performance, this prepared nanocomposite shows increased photostability and is magnetically separable from the aqueous media. The degradation rate constant (k(app)) of the optimized photocatalyst, i.e. CdS-NiFe2O4 (0.05)/rGO 25wt% nanocomposite, was higher than the corresponding CdS and NiFe2O4 nanoparticles by factors of 11.1 and 8.9, respectively. The synergistic interactions between CdS, NiFe2O4 and rGO lead to enhanced surface area, reduced aggregation of the nanoparticles, decreased the recombination of photogenerated electron-hole pairs, and increased the charge separation efficiency and effective electron-hole generation transfer. According to the obtained results, a proposed mechanism of the photodegradation of MB under visible light irradiation is finally mentioned.Öğe Rhodium(0), Ruthenium(0) and Palladium(0) nanoparticles supported on carbon-coated iron: Magnetically isolable and reusable catalysts for hydrolytic dehydrogenation of ammonia borane(Pergamon-Elsevier Science Ltd, 2021) Akbayrak, Serdar; Cakmak, Gulhan; Ozturk, Tayfur; Ozkar, SaimWe report the synthesis of magnetically isolable ruthenium(0), rhodium(0), and palladium(0) nanoparticles, supported on carbon-coated magnetic iron particles, and their employment as catalysts in hydrolysis of ammonia borane. Carbon-coated iron (C-Fe) particles are obtained by co-processing of iron powders with methane in a radio frequency thermal plasma reactor. The impregnation of ruthenium(III), rhodium(III) and palladium(II) ions on the carbon-coated iron particles followed by aqueous solution of sodium borohydride leads to the formation of respective metal(0) nanoparticles supported on carbon coated iron, M-0/C-Fe NP (M 1/4 Ru, Rh, and Pd) at room temperature. M-0/C-Fe NPs are characterized using the ICP-OES, XPS, TEM, and EDX techniques and tested as catalysts for hydrolysis of ammonia borane at 298 K. The results reveal that Rh-0/C-Fe, Ru-0/C-Fe, Pd-0/C-Fe catalysts provide turnover frequency of 83, 93, and 29 min(-1), respectively, in this industrially important reaction. More importantly, these magnetically separable metal(0) nanoparticles show very high reusability with no noticeable activity loss in subsequent runs of hydrolysis evolving 3.0 equivalent H-2 per mole of ammonia borane. (C) 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.Öğe Ruthenium(0) nanoparticles supported on silica coated Fe3O4 as magnetically separable catalysts for hydrolytic dehydrogenation of ammonia borane(Pergamon-Elsevier Science Ltd, 2018) Sarica, Esra; Akbayrak, Serdar; Ozkar, SaimRuthenium(0) nanoparticles supported on bare or silica-coated magnetite are prepared by impregnation of ruthenium(III) ions followed by their reduction with aqueous solution of sodium borohydride on the surface of support. These magnetically isolable catalysts are used in hydrogen generation from the hydrolysis of ammonia borane at room temperature. They conserve their initial catalytic activity even after the fifth reuse in the hydrolysis reaction, Ruthenium(0) nanoparticles supported on bare magnetite and silica-coated magnetite provide turnover frequency values of 29 min(-1) and 127 min(-1) and in hydrolytic dehydrogenation of ammonia borane at 25.0 +/- 0.1 degrees C. Thus, coating of the surface of magnetite with silica results in a significant enhancement in catalytic activity of ruthenium(0) nanoparticles in hydrogen generation from the hydrolysis of ammonia borane. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.Öğe Titania, zirconia and hafnia supported ruthenium(0) nanoparticles: Highly active hydrogen evolution catalysts(Academic Press Inc Elsevier Science, 2018) Demir, Elif; Akbayrak, Serdar; Onal, Ahmet M.; Ozkar, SaimDesigning a cost-effective catalyst with high activity and stability for hydrogen evolution reaction (2H(+) + 2e(-) -> H-2) is a big challenge due to increasing demand for energy. Herein, we report the electrocatalytic activity of glassy carbon electrodes with group 4 metal oxides (TiO2, ZrO2, HfO2) supported ruthenium(0) nanoparticles in hydrogen evolution reaction. Electrochemical activity of modified electrodes is investigated by recording linear sweep voltammograms in 0.5 M H2SO4 solution. The results of electrochemical measurements reveal that among the three electrodes the glassy carbon electrode with Ruci/TiO2 (1.20% wt. Ru) exhibits the highest activity with a relatively small Tafel slope of 52 mV dec(-1), the highest exchange current density of 0.728 mA cm(-2), and the smallest overpotential of 41 mV at j = 10 mA cm(-2). Furthermore, it demonstrates superior stability in acidic solution with an unaltered onset potential for long term electrochemical measurement. (C) 2018 Elsevier Inc. All rights reserved.Öğe Tungsten(VI) oxide supported rhodium nanoparticles: Highly active catalysts in hydrogen generation from ammonia borane(Pergamon-Elsevier Science Ltd, 2021) Akbayrak, Serdar; Tonbul, Yalcin; Ozkar, SaimHerein, we report the use of tungsten(VI) oxide (WO3) as support for Rh0 nanoparticles. The resulting Rh0/WO3 nanoparticles are highly active and stable catalysts in H2 generation from the hydrolysis of ammonia borane (AB). We present the results of our investigation on the particle size distribution, catalytic activity and stability of Rh0/WO3 catalysts with 0.5%, 1.0%, 2.0% wt. Rh loadings in the hydrolysis reaction. The results reveal that Rh-0/WO3 (0.5% wt. Rh) is very promising catalyst providing a turnover frequency of 749 min-1 in releasing 3.0 equivalent H2 per mole of AB from the hydrolysis at 25.0 C-circle. The high catalytic activity of Rh-0/WO3 catalyst is attributed to the reducible nature of support. The report covers the results of kinetics study as well as comparative investigation of activity, recyclability, and reusability of colloidal(0) nanoparticles and Rh-0/WO3 (0.5 % wt. Rh) catalyst in the hydrolysis reaction. (c) 2021 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
