Browsing by Author "Ozkar, Saim"
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Article Citation Count: 98Copper(0) Nanoparticles Supported on Silica-Coated Cobalt Ferrite Magnetic Particles: Cost Effective Catalyst in the Hydrolysis of Ammonia-Borane with an Exceptional Reusability Performance(Amer Chemical Soc, 2012) Kaya, Murat; Zahmakiran, Mehmet; Ozkar, Saim; Volkan, Murvet; Chemical EngineeringHerein we report the development of a new and cost-effective nanocomposite catalyst for the hydrolysis of ammonia-borane (NH3BH3), which is considered to be one of the most promising solid hydrogen carriers because of its high gravimetric hydrogen storage capacity (19.6% wt) and low molecular weight. The new catalyst system consisting of copper nanoparticles supported on magnetic SiO2/CoFe2O4 particles was reproducibly prepared by wet-impregnation of Cu(II) ions on SiO2/CoFe2O4 followed by in situ reduction of the Cu(II) ions on the surface of magnetic support during the hydrolysis of NH3BH3 and characterized by ICP-MS, XRD, XPS, TEM, HR-TEM and N-2 adsorption-desorption technique. Copper nanoparticles supported on silica coated cobalt(II) ferrite SiO2/CoFe2O4 (CuNPs@SCF) act as highly active catalyst in the hydrolysis of ammonia-borane, providing an initial turnover frequency of TOF = 2400 h(-1) at room temperature, which is not only higher than all the non-noble metal catalysts but also higher than the majority of the noble metal based homogeneous and heterogeneous catalysts employed in the same reaction.. More importantly, they were easily recovered by using a permanent magnet in the reactor wall and reused for up, to 10 recycles without losing their inherent catalytic activity significantly, which demonstrates the exceptional reusability of the CuNPs@SCF catalyst.Article Citation Count: 32Hydroxyapatite-nanosphere supported ruthenium(0) nanoparticle catalyst for hydrogen generation from ammonia-borane solution: kinetic studies for nanoparticle formation and hydrogen evolution(Royal Soc Chemistry, 2014) Kaya, Murat; Gulcan, Mehmet; Zahmakiran, Mehmet; Ozkar, Saim; Kaya, Murat; Chemical EngineeringThe development of readily prepared effective heterogeneous catalysts for hydrogen generation from ammonia-borane (AB; NH3BH3) solution under mild conditions still remains a challenge in the field of "hydrogen economy". In this study, we report our finding of an in situ generated, highly active ruthenium nanocatalyst for the dehydrogenation of ammonia-borane in water at room temperature. The new catalyst system consists of ruthenium(0) nanoparticles supported on nanohydroxyapatite (RuNPs@nano-HAp), and can be reproducibly prepared under in situ conditions from the ammonia-borane reduction of Ru3+ ions exchanged into nanohydroxyapatite (Ru3+@nano-HAp) during the hydrolytic dehydrogenation of ammonia-borane at 25 +/- 0.1 degrees C. Nanohydroxyapatite-supported ruthenium(0) nanoparticles were characterized by a combination of advanced analytical techniques. The sum of their results shows the formation of well-dispersed ruthenium(0) nanoparticles with a mean diameter of 2.6 +/- 0.6 nm on the surface of the nanospheres of hydroxyapatite by keeping the host matrix intact. The resulting RuNPs@nano-HAp are highly active catalyst in the hydrolytic dehydrogenation of ammonia-borane with an initial TOF value of 205 min(-1) by generating 3.0 equiv. of H-2 per mole of ammonia-borane at 25 +/- 0.1 degrees C. Moreover, they are sufficiently stable to be isolated and bottled as solid materials, which can be reused as active catalyst under the identical conditions of first run. The work reported here also includes the following results: (i) monitoring the formation kinetics of the in situ generated RuNPs@nano-HAp by hydrogen generation from the hydrolytic dehydrogenation of ammonia-borane as the reporter reaction. The sigmoidal kinetics of catalyst formation and concomitant dehydrogenation fits well to the two-step, slow nucleation, followed by autocatalytic surface growth mechanism, P -> Q (rate constant k(1)) and P + Q -> 2Q (rate constant k(2)), in which P is Ru3+@nano-HAp and Q is the growing, catalytically active RuNPs@nano-HAp; (ii) the compilation of kinetic data for the RuNPs@nano-HAp catalyzed hydrolytic dehydrogenation of ammonia-borane depending on the temperature and catalyst concentration to determine the dependency of reaction rate on catalyst concentration and activation parameters (E-a, Delta H-#, and Delta S-#) of the reaction.Article Citation Count: 79Nanoceria-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, Saim; Chemical EngineeringRuthenium(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.Article Citation Count: 140Palladium(0) nanoparticles supported on silica-coated cobalt ferrite: A highly active, magnetically isolable and reusable catalyst for hydrolytic dehydrogenation of ammonia borane(Elsevier, 2014) Kaya, Murat; Kaya, Murat; Volkan, Murvet; Ozkar, Saim; Chemical EngineeringPalladium(0) nanoparticles supported on silica-coated cobalt ferrite (Pd(0)/SiO2-CoFe2O4) were in situ generated during the hydrolysis of ammonia borane, isolated from the reaction solution by using a permanent magnet and characterized by ICP-OES, XRD, TEM, TEM-EDX, XPS and the N-2 adsorption-desorption techniques. All the results reveal that well dispersed palladium(0) nanoparticles were successfully supported on silica coated cobalt ferrite and the resulting Pd(0)/SiO2-CoFe2O4 are highly active, magnetically isolable, and recyclable catalysts in hydrogen generation from the hydrolysis of ammonia borane with an unprecedented turnover frequency (TOF, calculated on the basis of the total amount of Pd) of 254 mol H-2 (mol Pd min)(-1) at 25 +/- 0.1 degrees C. The reusability tests reveal that Pd(0)/SiO2-CoFe2O4 are still active in the subsequent runs of hydrolysis of ammonia borane providing 100% conversion. Pd(0)/SiO2-CoFe2O4 provide the highest catalytic activity with a TOF value of 198 mol H-2 (mol Pd min)(-1) in the 10th use in hydrogen generation from the hydrolysis of ammonia borane as compared to the other palladium catalysts. The work reported here also includes the kinetic studies depending on the temperature to determine the activation energy of the reaction (E-a = 52 +/- 2 kJ/mol) and the effect of catalyst concentration on the rate of hydrolytic dehydrogenation of ammonia borane, respectively. (C) 2013 Elsevier B.V. All rights reserved.Article Citation Count: 44Ruthenium(0) nanoparticles supported on magnetic silica coated cobalt ferrite: Reusable catalyst in hydrogen generation from the hydrolysis of ammonia-borane(Elsevier Science Bv, 2014) Kaya, Murat; Kaya, Murat; Volkan, Murvet; Ozkar, Saim; Chemical EngineeringRuthenium(0) nanoparticles supported on magnetic silica-coated cobalt ferrite (Ru(0)/SiO2-CoFe2O4) were in situ generated from the reduction of Ru3+/SiO2-CoFe2O4 during the catalytic hydrolysis of ammonia-borane (AB). Ruthenium(III) ions were impregnated on SiO2-CoFe2O4 from the aqueous solution of ruthenium(III) chloride and then reduced by AB at room temperature yielding Ru(0)/SiO2-CoFe2O4 which were isolated from the reaction solution by using a permanent magnet and characterized by ICP-OES, XRD, TEM, TEM-EDX and XPS techniques. The resulting magnetically isolable Ru(0)/SiO2-CoFe2O4 were found to be highly reusable catalyst in hydrolysis of AB retaining 94% of their initial catalytic activity even after tenth run. Ru(0)/SiO2-CoFe2O4 provide the highest catalytic activity after the tenth use in hydrolysis of AB as compared to the other ruthenium catalysts. The work reported here also includes the formation kinetics of ruthenium(0) nanoparticles. The evaluation of rate constants for the nucleation and autocatalytic surface growth of ruthenium(0) nanoparticles at various temperatures provides the estimation of activation energy for both reactions; E-a = 116 +/- 7 kJ/mol for the nucleation and E-a = 51 +/- 2 kJ/mol for the autocatalytic surface growth of ruthenium(0) nanoparticles. The report also includes the activation energy of the catalytic hydrogen generation from the hydrolysis of AB (E-a = 45 +/- 2 kJ/mol) determined from the evaluation of temperature dependent kinetic data and the effect of catalyst concentration on the rate of hydrolysis of AB. (C) 2014 Elsevier B.V. All rights reserved.