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    Ceria supported rhodium nanoparticles: Superb catalytic activity in hydrogen generation from the hydrolysis of ammonia borane
    (Elsevier Science Bv, 2016) Akbayrak, Serdar; Tonbul, Yalcin; Ozkar, Saim
    We investigated the effect of various oxide supports on the catalytic activity of rhodium nanoparticles in hydrogen generation from the hydrolysis of ammonia borane. Among the oxide supports (CeO2, SiO2, Al2O3, TiO2, ZrO2, HfO2) ceria provides the highest catalytic activity for the rhodium(0) nanoparticles in the hydrolysis of ammonia borane. Rhodium(0) nanoparticles supported on nanoceria (Rh-0/CeO2) were prepared by the impregnation of rhodium(III) ions on the surface of ceria followed by their reduction with sodium borohydride in aqueous solution at room temperature. They were isolated from the reaction solution by centrifugation and characterized by a combination of advanced analytical techniques. The catalytic activity of Rh-0/CeO2 samples with various rhodium loading in the range of 0.1-4.0% wt. Rh was also tested in hydrogen generation from the hydrolysis of ammonia borane at room temperature. The highest catalytic activity was achieved by using 0.1% wt. rhodium loaded nanoceria. The resulting Rh-0/CeO2 with a metal loading of 0.1% wt. Rh show superb catalytic activity in hydrogen generation from the hydrolysis of ammonia borane with a record turnover frequency value (TOF) of 2010 min(-1) at 25.0 +/- 0.1 degrees C. The superb catalytic activity of Rh-0/CeO2 is ascribed to the reducible nature of ceria. The reduction of cerium(IV) to cerium(III) leads to a build-up of negative charge on the oxide surface which favors the bonding of rhodium(0) nanoparticles on the surface and, thus, their catalytic activity. Rh-0/CeO2 are also reusable catalysts preserving 67% of their initial catalytic activity even after the fifth use in hydrogen generation from the hydrolysis of ammonia borane at room temperature (TOF= 1350 min(-1). The work reported here also includes the kinetic studies depending on the temperature to determine the activation energy (E-a = 43 +/- 2 kJ/mol) and the effect of catalyst concentration on the rate of hydrolysis of ammonia borane. (C) 2016 Elsevier B.V. All rights reserved.
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    Ceria-supported ruthenium nanoparticles as highly active and long-lived catalysts in hydrogen generation from the hydrolysis of ammonia borane
    (Royal Soc Chemistry, 2016) Akbayrak, Serdar; Tonbul, Yalcin; Ozkar, Saim
    Ruthenium(0) nanoparticles supported on ceria (Ru-0/CeO2) were in situ generated from the reduction of ruthenium(III) ions impregnated on ceria during the hydrolysis of ammonia borane. Ru-0/CeO2 was isolated from the reaction solution by centrifugation and characterized by ICP-OES, BET, XRD, TEM, SEM-EDS and XPS techniques. All the results reveal that ruthenium(0) nanoparticles were successfully supported on ceria and the resulting Ru-0/CeO2 is a highly active, reusable and long-lived catalyst for hydrogen generation from the hydrolysis of ammonia borane with a turnover frequency value of 361 min(-1). The reusability tests reveal that Ru-0/CeO2 is still active in the subsequent runs of hydrolysis of ammonia borane preserving 60% of the initial catalytic activity even after the fifth run. Ru-0/CeO2 provides a superior catalytic lifetime (TTO = 135 100) in hydrogen generation from the hydrolysis of ammonia borane at 25.0 +/- 0.1 degrees C before deactivation. The work reported here includes the formation kinetics of ruthenium(0) nanoparticles. The rate constants for the slow nucleation and autocatalytic surface growth of ruthenium(0) nanoparticles were obtained using hydrogen evolution as a reporter reaction. An evaluation of rate constants at various temperatures enabled the estimation of activation energies for both the reactions, E-a = 60 +/- 7 kJ mol(-1) for the nucleation and E-a = 47 +/- 2 kJ mol(-1) for the autocatalytic surface growth of ruthenium(0) nanoparticles, as well as the activation energy of E-a = 51 +/- 2 kJ mol(-1) for the catalytic hydrolysis of ammonia borane.
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    Group 4 oxides supported Rhodium(0) catalysts in hydrolytic dehydrogenation of ammonia borane
    (Pergamon-Elsevier Science Ltd, 2019) Tonbul, Yalcin; Akbayrak, Serdar; Ozkar, Saim
    Rh3+ 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.
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    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, Saim
    Cobalt(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.
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    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, Saim
    Magnetically 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.
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    Nanoceria supported palladium(0) nanoparticles: Superb catalyst in dehydrogenation of formic acid at room temperature
    (Elsevier Science Bv, 2017) Akbayrak, Serdar; Tonbul, Yalcin; Ozkar, Saim
    Highly efficient dehydrogenation of formic acid (FA) at room temperature was achieved using palladium(0) nanoparticles supported on nanoceria (Pd-0/CeO2) as catalysts. Pd-0/CeO2 was prepared by impregnation of palladium(II) ions on the surface of ceria followed by their reduction with sodium borohydride in aqueous solution at room temperature. Pd((0)/CeO2 was isolated from the reaction solution by centrifugation and characterized by a combination of advanced analytical techniques. The catalytic activity of Pd-0/CeO2 samples with various Pd loading in the range 1.0-5.0% wt was tested in dehydrogenation of formic acid plus sodium formate with a molar ratio of FA/SF = 1/9. Pd-0/CeO2 with Pd loading of 2.27% wt shows superb catalytic activity in dehydrogenation of FA with a turnover frequency (TOF) value of 1400 h(-1) at 25.0 +/- 0.1 degrees C. The superb catalytic activity of Pd-0/CeO2 is ascribed to the reducible nature of ceria during the decomposition of FA (HCO2H -> CO2+H-2). The kinetic data, obtained by measuring the volume of pure H-2 gas, could be converted to the change in concentration of FA by considering the equilibrium between the formate ion and formic acid (HCO2H reversible arrow HCOO- + H3O+). The FA concentration versus time data fit to the first order kinetics with respect to the FA concentration. (C) 2017 Elsevier B.V. All rights reserved.
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    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, Saim
    Nanozirconia 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.
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    Palladium(0) nanoparticles supported on ceria: Highly active and reusable catalyst in hydrogen generation from the hydrolysis of ammonia borane
    (Pergamon-Elsevier Science Ltd, 2016) Tonbul, Yalcin; Akbayrak, Serdar; Ozkar, Saim
    Palladium(0) nanoparticles supported on nanoceria (Pd-0/CeO2) were prepared by the impregnation of palladium(II) ions on the surface of ceria followed by their reduction with sodium borohydride in aqueous solution at room temperature. Pd-0/CeO2 were isolated from the reaction solution by centrifugation and characterized by ICP-OES, XRD, TEM, SEM-EDS and XPS techniques. All the results reveal that palladium(0) nanoparticles were uniformly dispersed on ceria and the resulting Pd-0/CeO2 are highly active and reusable catalysts in hydrogen generation from the hydrolysis of ammonia borane with a turnover frequency value of 29 min(-1) at 25.0 +/- 0.1 degrees C. The catalytic activity of Pd-0/CeO2 in hydrogen generation from the hydrolysis of ammonia borane is higher compared to that of other palladium based catalysts such as Pd-0/ZrO2, Pd-0/SiO2, Pd-0/Al2O3, Pd-0/TiO2 under the same conditions. The catalytic activity of Pd-0/CeO2 samples with various palladium loading in the range of 1.0-5.0% wt Pd was tested in hydrogen generation from the hydrolysis of ammonia borane at 25.0 +/- 0.1 degrees C. The highest catalytic activity was achieved by using 1.18% wt palladium loaded ceria. The reusability tests reveal that Pd-0/CeO2 are still active in the subsequent runs of hydrolysis of ammonia borane preserving 47% of the initial catalytic activity even after the fifth run of hydrolysis. Our report also includes the results of kinetic study of the catalytic hydrolysis of ammonia borane depending on the temperature and catalyst concentration. (c) 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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    Reducible tungsten(VI) oxide-supported ruthenium(0) nanoparticles: highly active catalyst for hydrolytic dehydrogenation of ammonia borane
    (Tubitak Scientific & Technological Research Council Turkey, 2023) Akbayrak, Serdar; Tonbul, Yalcin; Ozkar, Saim
    Reducible WO3 powder with a mean diameter of 100 nm is used as support to stabilize ruthenium(0) nanoparticles. Ruthenium(0) nanoparticles are obtained by NaBH4 reduction of ruthenium(III) precursor on the surface of WO3 support at room temperature. Ruthenium(0) nanoparticles are uniformly dispersed on the surface of tungsten(VI) oxide. The obtained Ru0/WO3 nanoparticles are found to be active catalysts in hydrolytic dehydrogenation of ammonia borane. The turnover frequency (TOF) values of the Ru0/WO3 nanocatalysts with the metal loading of 1.0%, 2.0%, and 3.0% wt. Ru are 122, 106, and 83 min-1, respectively, in releasing hydrogen gas from the hydrolysis of ammonia borane at 25.0 degrees C. As the Ru0/WO3 (1.0% wt. Ru) nanocatalyst with an average particle size of 2.6 nm provides the highest activity among them, it is extensively investigated. Although the Ru0/WO3 (1.0% wt. Ru) nanocatalyst is not magnetically separable, it has extremely high reusability in the hydrolysis reaction as it preserves 100% of initial catalytic activity even after the 5th run of hydrolysis. The high activity and reusability of Ru0/WO3 (1.0% wt. Ru) nanocatalyst are attributed to the favorable metal-support interaction between the ruthenium(0) nanoparticles and the reducible tungsten(VI) oxide. The high catalytic activity and high stability of Ru0/WO3 nanoparticles increase the catalytic efficiency of precious ruthenium in hydrolytic dehydrogenation of ammonia borane.
  • Küçük Resim
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    Tungsten(VI) oxide supported rhodium nanoparticles: Highly active catalysts in hydrogen generation from ammonia borane
    (Pergamon-Elsevier Science LTD, 2021) Akbayrak, Serdar; Tonbul, Yalçın; Özkar, Saim
    Herein, 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.