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Hydrolysis of Sodium Borohydride and Ammonia Boranefor Hydrogen Generation Using Highly Efficient Poly(N-Vinyl-2-Pyrrolidone)-Stabilized Ru–Pd Nanoparticles asCatalystsMurat Rakapa

a Maritime Faculty, Yuzuncu Yil University, Van, TurkeyAccepted author version posted online: 03 Apr 2014.

To cite this article: Murat Rakap (2015) Hydrolysis of Sodium Borohydride and Ammonia Borane for Hydrogen GenerationUsing Highly Efficient Poly(N-Vinyl-2-Pyrrolidone)-Stabilized Ru–Pd Nanoparticles as Catalysts, International Journal of GreenEnergy, 12:12, 1288-1300, DOI: 10.1080/15435075.2014.895737

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International Journal of Green Energy (2015) 12, 1288–1300Copyright © Taylor & Francis Group, LLCISSN: 1543-5075 print / 1543-5083 onlineDOI: 10.1080/15435075.2014.895737

Hydrolysis of Sodium Borohydride and Ammonia Borane forHydrogen Generation Using Highly Efficient Poly(N-Vinyl-2-Pyrrolidone)-Stabilized Ru–Pd Nanoparticles as Catalysts

MURAT RAKAP

Maritime Faculty, Yuzuncu Yil University, Van, Turkey

The catalytic use of highly efficient poly(N-vinyl-2-pyrrolidone)-stabilized Ru–Pd nanoparticles (3.2 ± 1.0 nm) in the hydrolysis of sodiumborohydride and ammonia borane is reported. These were prepared by the co-reduction of two metal ions in ethanol/water mixture bythe alcohol reduction method and characterized by transmission electron microscopy, X-ray photoelectron spectroscopy, and UV-Vis spec-troscopy. These are recyclable and highly active for hydrogen generation from the hydrolysis of sodium borohydride and ammonia boraneeven at very low concentrations and temperature, providing a record number of average turnover frequency values (762 mol H2/molcat.min−1 and 308 mol H2/mol cat.min−1) and maximum hydrogen generation rates (22,889 L H2 min−1 (mol cat)−1 and 9364 L H2 min−1

(mol cat)−1) for sodium borohydride and ammonia borane, respectively. Poly(N-vinyl-2-pyrrolidone)-stabilized Ru–Pd nanoparticles provideactivation energies of 52.4 ± 2 and 54.5 ± 2 kJ/mol for the hydrolysis of sodium borohydride and ammonia borane, respectively.

Keywords: Ruthenium, Palladium, Nanoparticles, Sodium borohydride, Ammonia borane

Introduction

From the beginning of the last millennium, sodium borohydride(NaBH4, SBH) and more recently ammonia borane (H3N·BH3,AB) have been considered as potential solid hydrogen storagematerials. These appear to be suitable hydrogen sources becauseof a number of advantageous properties such as high hydrogenstorage capacity (10.8% wt for SBH and 19.6% wt for AB) whichmeets the US-DOE criteria for hydrogen storage materials (Leeet al. 2007), the optimal control on hydrogen generation rate bysupported catalysts, the acceptable hydrogen generation rate evenat low temperature, high solubility in water, and the availabilityand easy handling (Amendola et al. 2000). In addition, AB has anadvantage of the stability of aqueous solutions to self-hydrolysisover SBH, and therefore there is no need to add any base to stabi-lize the aqueous solutions of AB (Patel et al. 2008a). Both SBHand AB liberate hydrogen upon hydrolysis at room temperaturein the presence of suitable catalysts according to Equations (1)and (2) respectively.

NaBH4 (aq) + 2H2O (l)catalyst−→ NaBO2 (aq) + 4H2 (g) ,

(1)

Address correspondence to Murat Rakap, Department of Chemistry,Yuzuncu Yil University, 65080 Van, Turkey. E-mail: mrtrakap@gmail.comColor versions of one or more of the figures in the article can befound online at www.tandfonline.com/ljge.

H3NBH3(aq) + 2H2O(l)catalyst−→ NH+

4 (aq) + BO−2 (aq) + 3H2(g).

(2)

Various kinds of catalyst systems were tested for hydro-gen generation from the hydrolysis of SBH and AB (Table 1).Catalytic activities of some catalysts in terms of L H2 min−1

(g catalyst)−1 are as follows: Ni-C-B (2.608), Co-B/Ni foam(11), Co-W-B/Ni foam (15), Co-P (3.3), Co-B/MWCN (5.1),Ni-Ru nanocomposite (0.4), Co-Cr-B (3.4), CoB – attapulgite(1.175), Fe-Co-B/Ni foam (22), Co-B/clay (1.27), Co-Ni-B(4.23), Co-Cu-B (2.12), Ru-Pd-Pt (900), Co-Mo-B (210), CoO(8.333), Co-P/Ni foam (0.93), Co-C (10.4), and Co(0)-Hap(5) for the hydrolysis of SBH and Co(0) NPs (1116), Co-Bthin film (8.2), Co-Ni-P/Pd-TiO2 (0.06), Pd-PVB-TiO2 (0.642),Pd(0)-Hap (1.425), Co-Mo-B-P (2.75), and Co(0)-Hap (2.2) forthe hydrolysis of AB.

As can be seen from these values, nanoparticle-type cat-alysts provided good catalytic activities as expected due tosmall particle sizes. In addition, inclusion of second element tothe monometallic nanoparticles will definitely improve the cat-alytic properties. Therefore, the employment of highly activebimetallic-type nanoparticles as catalyst for hydrogen genera-tion from the hydrolysis of SBH and AB is recently focused.Although few articles including the use of some bimetallic-type catalyst for the hydrolysis of SBH and AB can be foundin the literature, there is no reported work on the catalyticactivity of (poly(N-vinyl-2-pyrrolidone), PVP)-stabilized Ru–Pdnanoparticles in hydrogen production from the hydrolysis ofSBH and AB up to now. Since both ruthenium and palladium

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Table 1. Some Catalyst Systems Tested for the Hydrolysis of SBH and AB

Sodium borohydride (SBH) Ammonia borane (AB)

Catalyst Reference Catalyst Reference

Ni-C-B Ingersoll et al. 2007 Ni1-xPtx Cheng et al. 2007Co-Mn-B Mitov et al. 2007 Cu@Cu2O Kalidindi et al. 2008Co-B/Ni foam Dai et al. 2008a Ni-SiO2 nanosphere Umegaki et al. 2009aCo-W-B/Ni foam Dai et al. 2008b Fe(0) NPs Yan et al. 2008Co-P Eom, Cho, and Kwon 2008 Ni(0)-PVP Umegaki et al 2009bNi(0)-PVP Metin and Özkar 2008 Co(0) NCs Fernandes, Patel, and Miotello 2009Co-B/MWCN Huang et al. 2008 Ru(0), Pd(0)/polym Metin, Sahin, and Özkar 2009Ni–Ru nanocomp Liu et al. 2009 Fe-Ni alloy Yan et al. 2009aAcetic acid Akdim, Demirci, and Miele 2009 Co(0) NPs Yan et al. 2010aCo(0)-zeolite-Y Rakap and Özkar 2009 Ni NPs Yan et al. 2009bCo(0)-polymer Metin and Özkar 2009 Co-Mo-B/Ni foam Dai et al. 2010Co-Cr-B Fernandes, Patel, and Miotello 2009 Co-B nanospindles Tong et al. 2010Fe-Co-B/Ni foam Liang, Wang, and Dai 2010 Co-B thin film Patel et al. 2010bCo-B/clay Tian, Guo, amd Xu 2010 Au-Ni/SiO2 Jiang et al. 2010Co-Ni-B Patel et al. 2010a Cu/Co3O4 NPs Yamada et al. 2010CoB -attapulgite Fernandes et al. 2009 Co-Ni-P/Pd-TiO2 Rakap, Kalu, and Özkar 2011bCo-Cu-B Ding et al. 2010 Pd-PVB-TiO2 Rakap, Kalu, and Özkar 2011cNixB Hua et al. 2003 Ni/SiO2 Metin, Özkar, and Sun 2010Ni-BMR07 Zhang et al. 2007 Co-SiO2 nanosphr Umegaki et al. 2010Ru–Pd–Pt Hu, Ceccato, and Raj 2011 Pd/GO Kılıç, Sencanlı, and Metin 2012Co-Mo-B Zhuang et al. 2013 Ru NPs Can and Metin 2012CoO Lu et al. 2012 Pt-CeO2 Wang et al. 2012Co-P/Ni foam Oh and Kwon 2012 Co-Mo-B-P Fernandes et al. 2012Co-C Zhu et al. 2012 Cu@FeNi Wang et al. 2012Fe(0)-polymer Dinç, Metin, and Özkar 2012 Ni-ZIF-8 Li, Aranishi, and Xu 2012Co(0)-Hap Rakap and Özkar 2012 Co(0)-Hap Rakap and Özkar 2012Co-Ni-P/Pd-TiO2 Rakap, Kalu, and Özkar 2011a Pd(0)-Hap Rakap and Özkar 2011

Fig. 1. UV-Vis absorption spectra of the aqueous solutions of RuCl3·3H2O, K2PdCl4, and PVP-stabilized Ru–Pd nanoparticles.

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Fig. 2. High resolution XPS spectra of Ru–Pd nanoparticles showing (a) Ru-3d, (b) Ru-3p, and (c) Pd-3d regions.

are known to have very good catalytic activities toward thosehydrolysis reactions, these were selected to prepare bimetallictype nanocatalysts. Herein, the employment of highly efficientPVP-stabilized Ru–Pd nanoparticles as catalyst for hydrogengeneration from the hydrolysis of both SBH and AB is reported.PVP-stabilized Ru–Pd nanoparticles were prepared by the alco-hol reduction method (Toshima and Hirakawa 1997), foundto be stable as colloidal dispersions for several months atroom temperature, and characterized by transmission electronmicroscopy (TEM), X-ray photoelectron spectroscopy (XPS),and UV-Vis spectroscopy. Moreover, the formation of alloy-typePVP-stabilized Ru–Pd nanoparticles was confirmed by compar-ing the catalytic activities of monometallic Ru–Pd nanoparticlesand their physical mixture with that of bimetallic nanoparticlesin the hydrolysis of AB. The kinetic studies were carried outdepending on the catalyst/substrate concentrations and the tem-perature. Although the cost of noble metal catalysts is assumedto be high, the high catalytic activity and recyclability ofthe PVP-stabilized Ru–Pd nanoparticles make them a very

promising candidate to be used as catalyst in developing efficientportable hydrogen generation systems using SBH or AB as solidhydrogen storage material since it would compensate the costconcerns.

Experimental

Materials

Ruthenium(III) chloride trihydrate (RuCl3·3H2O), potassiumtetrachloropalladate (K2PdCl4), poly(N-vinyl-2-pyrrolidone)(PVP-40), sodium borohydride (NaBH4), and ammonia borane(H3N·BH3) were purchased from Aldrich. Ethanol was pur-chased from Merck. Deionized water was distilled by a waterpurification system (Milli-Q system). All glassware and Teflon-coated magnetic stir bars were cleaned with acetone, followedby copius rinsing with distilled water before drying in an oven at150◦C.

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Fig. 3. TEM image of PVP-stabilized Ru–Pd nanoparticles taken at different magnifications (a – 50 nm, b –10 nm) and (b) correspondingEDX spectrum (c).

Fig. 4. Comparison of the catalytic activities of 0.3 mM of Ru and Pd monometallic nanoparticles, physical mixture of these, and PVP-stabilized Ru–Pd alloy nanoparticles in the hydrolysis of 0.100-M AB at 25.0 ± 0.1◦C.

Preparation of PVP-Stabilized Ru–Pd Nanoparticles

The PVP-stabilized Ru–Pd nanoparticles were prepared bythe alcohol reduction method developed by Toshima andHirakawa (1997) . First, solutions of ruthenium(III) chlo-ride trihydrate (0.25 mmol in 25-mL ethanol) and potassiumtetrachloropalladate (0.25 mmol in 25-mL water) were mixed and

poly(N-vinyl-2-pyrrolidone) (PVP-40, 2.5 mmol of monomericunits) was added to this solution as a protecting polymer. Thenthe mixed solution was refluxed at 90◦C for 2 h. The formedRu–Pd nanoparticles have a brownish black color, and are sta-ble for months at room temperature. The total concentration ofboth metals was kept at 5.0 mM in 50 mL of the water/ethanolsolution.

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Fig. 5. (a) Plot of the volume of the generated hydrogen gas versus time for the hydrolysis of 0.375-M SBH, and (b) plot of mol H2/molAB versus time for the hydrolysis of 0.100-M AB solutions in the presence of PVP-stabilized Ru–Pd nanoparticles at different catalystconcentrations at 25.0 ± 0.1◦C.

Method to Test the Catalytic Activity of PVP-Stabilized Ru–PdNanoparticles in the Hydrolysis of Sodium Borohydride andAmmonia Borane

The catalytic activity of the PVP-stabilized Ru–Pd nanoparticlesin the hydrolysis of SBH or AB in aqueous solution was deter-mined by measuring the rate of hydrogen generation. In all theexperiments, a jacketed reaction flask (50 mL) containing aTeflon-coated stir bar was placed on a magnetic stirrer (HeidolphMR-301) and thermostated to 25.0 ± 0.1◦C by circulating waterthrough its jacket from a constant temperature bath. Then agraduated glass tube (40 cm in height and 2.5 cm in diam-eter) filled with water was connected to the reaction flask tomeasure the volume of hydrogen gas to be evolved from the

reaction. In a typical experiment, 284 mg (7.47 mmol) of NaBH4

or 63.6 mg (2.00 mmol) of H3N·BH3 was dissolved in 20 mLof water. The solutions were transferred with a glass pipetteinto the reaction flask with the temperature set at 25.0 ± 0.1◦C.Then aliquots of PVP-stabilized Ru–Pd nanoparticles from thestock solution (5.0 mM) were added into the reaction flask. Theexperiment was started by closing the flask and the volume ofhydrogen gas evolved was measured by recording the displace-ment of water level at a stirring speed of 900 rpm. In additionto the volumetric measurement of hydrogen evolution, the con-version of SBH (δ = –42.1 ppm) (Guella et al. 2006) and AB(δ = –23.9 ppm) (Chandra and Xu 2006) to metaborate (δ =9.0 ppm) (Eom, Cho, and Kwon 2010) was also checked by 11BNMR spectroscopy.

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Fig. 6. Plots of the hydrogen generation rate versus catalyst concen-tration (both in logarithmic scale) for the hydrolysis of (a) 0.375-MSBH, and (b) 0.100-M AB in the presence of PVP-stabilized Ru–Pdnanoparticles (0.3 mM).

Kinetic Study of the Hydrolysis of Sodium Borohydride andAmmonia Borane Catalyzed by PVP-Stabilized Ru–PdNanoparticles

In order to establish the rate laws for the catalytic hydrolysisof SBH and AB using PVP-stabilized Ru–Pd nanoparticles, twodifferent sets of experiments were performed in the same wayas described in the previous section for each of these sub-strates. In the first set of experiments, the concentration ofthe substrate was kept constant (at 0.375 M for NaBH4 and0.100 M for H3N·BH3) and the concentration of catalyst wasvaried in the range of 0.1, 0.2, 0.3, 0.4, and 0.5 mM. In thesecond set of experiments, concentration of catalyst was keptconstant at 0.3 mM and concentrations of substrate were var-ied in the range of 0.375, 0.750, 1.125, 1.500, and 1.875 M

Fig. 7. Plots of the volume of the generated hydrogen gas versustime in the catalytic hydrolysis of (a) 0.375-M SBH, and (b) 0.100-MAB solutions in the presence of PVP-stabilized Ru–Pd nanoparticles(0.3 mM) at various temperatures (in the range of 15–35◦C for SBHand 10–30◦C for AB).

for NaBH4; and 0.100, 0.200, 0.300, 0.400, and 0.500 Mfor H3N·BH3.

Determination of Activation Energies for the PVP-StabilizedRu–Pd Nanoparticles Catalyzed by the Hydrolysis of SodiumBorohydride and Ammonia Borane

The hydrolysis of SBH (0.375 M) and AB (0.100 M) catalyzed byPVP-stabilized Ru–Pd nanoparticles (0.3 mM) was carried out byfollowing the same method described the above section at varioustemperatures (15, 20, 25, 30, and 35◦C for NaBH4 and 10, 15, 20,25, and 30◦C for H3N·BH3) to obtain the activation energies (Ea)for both hydrolysis reactions.

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Table 2. Activities in Terms of TOF Values of Various Catalyst Systems Tested in Hydrogen Generation from the Hydrolysis of AB (theTOF Values Were Either Directly Taken or Estimated from the Data Given in Respective References)

Catalyst TOF (mol H2.mol catalyst−1.min−1) Reference

PVP-stabilized Ru–Pd NPs 308.0 This studyLaurate-stabilized Rh(0) NCs 200.0 Durap, Zahmakıran, and Özkar 2009aZeolite-Rh(0) NCs 92.0 Zahmakıran and Özkar 2009Laurate-stabilized Ru(0) NCs 75.0 Durap, Zahmakıran, amd Özkar 2009bPt/C 55.4 Xu and Chandra 2007Co 44.2 Yan et al. 2010aIn-situ Co(0) NPs 39.8 Umegaki et al. 2009bRu@Al2O3 NPs 39.6 Can and Metin 2012Co35Pd65/C 35.7 Sun et al. 2011RGO-Pd NPs 26.3 Kılıç, Sencanlı, and Metin 2012PSSA-co-MA-stabilized Co(0) NCs 25.7 Metin and Özkar 2011PtO2 20.8 Xu and Chandra 2007Graphene-Pd(0) NPs 15.5 Metin et al. 2012Pt black 13.9 Xu and Chandra 2007Au@Co core-shell NPs 13.7 Yan et al. 2010bIn-situ Fe1-xNix NPs 10.9 Yan et al. 2009bPSSA-co-MA-stabilized Ni(0) NCs 10.1 Metin and Özkar 2011Electroplated Co-P 10.0 Eom, Cho, and Kwon 2010

Determination of Recyclability of PVP-Stabilized Ru–PdNanoparticles in the Hydrolysis of Sodium Borohydride andAmmonia Borane

The recyclability of PVP-stabilized Ru–Pd nanoparticles in thehydrolysis of SBH and AB was determined by a series ofexperiments started with a 20-mL solution containing 0.3-mMPVP-stabilized Ru–Pd bimetallic nanoparticles and 0.375-MSBH or 0.100-M AB at 25.0 ± 0.1◦C. When the completeconversion is achieved for each of these hydrolysis reactions,another equivalent of SBH or AB was added to reaction mix-ture immediately. The results were expressed as retained% initialcatalytic activity of PVP-stabilized Ru–Pd nanoparticles versusthe number of catalytic runs in the hydrolysis of SBH or ABsolution.

Characterization of PVP-Stabilized Ru–Pd Nanoparticles

Transmission Electron Microscopy analysis was carried outusing a JEOL-2010 microscope operating at 200 kV, fittedwith a LaB6 filament and has lattice and theoretical pointresolutions of 0.14 nm and 0.23 nm respectively. Sampleswere examined at a magnification between 100 and 1000 K.One drop of dilute suspension of the sample was depositedon the TEM grids and the solvent was then evaporated.The diameter of each particle was determined from enlargedphotographs. UV-Vis spectra were recorded on a Cary 5000(Varian) UV-Vis spectrophotometer. X-ray photoelectron spec-trum was taken by using SPECS spectrometer equipped witha hemispherical analyzer and using monochromatic Mg–Kα

radiation (1250 eV, the X-ray tube working at 15 kV and350 W). 11B NMR spectra were recorded on a BrukerAvance DPX 400 with an operating frequency of 128.15 MHzfor 11B.

Results and Discussion

Preparation and Characterization of PVP-Stabilized Ru–PdNanoparticles

The PVP-stabilized Ru–Pd nanoparticles were prepared fromthe co-reduction of the mixture of ruthenium(III) chloride trihy-drate and potassium tetrachloropalladate by the alcohol reductionmethod in the presence of PVP in ethanol–water mixture atrefluxing temperature. These were found to be highly stable formonths with any precipitation of metal particles at room tem-perature and characterized by UV-Vis spectroscopy and TEManalysis. The formation of PVP-stabilized Ru–Pd nanoparticlescould be followed by monitoring the UV-Vis electronic absorp-tion spectra as shown in Figure 1. The absorption peaks due toRu–Pd ions completely disappear after refluxing the solution,indicating the completion of the reduction of Ru–Pd ions inthe presence of PVP. Figure 2 shows the high resolution XPSspectra of PVP-stabilized Ru–Pd nanoparticles comprising 3dand 3p regions of ruthenium and 3d region of palladium. Twopeaks observed at 282.8 and 287.3 eV (Figure 2a) for Ru 3dand one peak observed at 464.8 eV (Figure 2b) for Ru 3p areindicative of Ru(0) (Wagner et al. 1979). In addition, two peaksobserved at 334.8 and 339.7 eV (Figure 2c) for Pd 3d are indica-tive of Pd(0) (Brun, Berthet, and Bertolini 1999). The slightshift (0.3 eV) to lower binding energies stems from increase inelectron densities around metal atoms in bimetallic nanoparticlesdue to interaction between Ru and Pd. There are no higher oxi-dation state peaks for both metals of the catalyst in the XPSspectra, indicating the protection of Ru(0) and Pd(0) species bythe attachment of PVP during catalyst preparation procedure.Figure 3 shows the TEM images taken at different magnifications((a) 50, (b) 10)) and the EDX spectrum (Figure 3c) of PVP-stabilized Ru–Pd nanoparticles, confirming alloy structure and

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Fig. 8. Arrhenius plots for the hydrolysis of (a) SBH (0.375 M)and (b) AB (0.100 M) catalyzed by 0.3 mM PVP-stabilized Ru–Pdnanoparticles.

1:1 ratio of Ru:Pd. These were found to have a mean particlesize of 3.2 ± 1.0 nm. Moreover, the formation of PVP-stabilizedRu–Pd nanoparticles rather than the physical mixtures of individ-ual monometallic nanoparticles was confirmed by comparing thecatalytic activities of monometallic Ru and Pd nanoparticles andtheir 1:1 physical mixture with the catalytic activity of 1:1 Ru–Pd bimetallic nanoparticles in the hydrolysis of AB. As shownin Figure 4, PVP-stabilized Ru–Pd nanoparticles provided amuch higher catalytic activity than the physical mixture of Ruand Pd monometallic nanoparticles, clearly indicating that thepresent catalyst comprises PVP-stabilized Ru–Pd bimetallic alloynanoparticles rather than a mixture of individual monometallicnanoparticles.

Kinetics of Hydrolysis of Sodium Borohydride and AmmoniaBorane Catalyzed by PVP-Stabilized Ru–Pd Nanoparticles

The PVP-stabilized Ru–Pd nanoparticles were found to be highlyefficient catalyst for the hydrolysis of SBH and AB solutions.Figure 5 shows the plot of the volume of generated hydrogengas versus time during the catalytic hydrolysis of 0.375-M SBHand plot of mol H2/mol AB versus time for the hydrolysisof 0.100-M AB solutions in the presence of PVP-stabilizedRu–Pd nanoparticles in different catalyst concentrations at 25.0± 0.1◦C. The linear hydrogen generation starts immediatelywithout an induction period and continues until the completehydrolysis of AB (hydrolysis of SBH was continued until 80%conversion was achieved, since it was enough to make nec-essary kinetic calculations). The quantity of NH3 liberatedduring the hydrolysis of AB has been found to be negligi-ble when the concentrations of catalyst and substrate are lowerthan 0.06 mol% and 6 wt%, respectively (Ramachandran andGagare 2007). As expected, the control tests using copper(II)sulfate trap with acid–base indicators resulted in no NH3 evo-lution in detectable amount in the experiments conducted inthis study.

Plotting the hydrogen generation rate, determined from thelinear portion of plots in Figure 5, versus catalyst concentration,both in logarithmic scales (Figure 6), gives a straight line with aslopes of 0.998 and 1.025, indicating that the hydrolytic dehydro-genation of SBH and AB is of first order with respect to catalystconcentration.

The effects of NaBH4 and H3N·BH3 substrate concentrationson the hydrolysis reactions were also studied by carrying outa series of experiments starting with various initial concentra-tions of SBH and AB while keeping the catalyst concentrationconstant at 0.3 mM. The hydrogen generation rates were foundto be practically independent of the NaBH4 and H3N·BH3 con-centrations, indicating that the hydrolysis reactions are of zeroorder with respect to the concentrations of NaBH4 and H3N·BH3.Consequently, the rate laws for the catalytic hydrolysis reac-tions of SBH and AB can be given as in Equations (3) and (4),respectively:

−4d [NaBH4]

dt= d [H2]

dt= k[catalyst], (3)

−3d [NH3BH3]

dt= d [H2]

dt= k[catalyst]. (4)

Determination of Activation Energies for the Hydrolysis ofSodium Borohydride and Ammonia Borane Catalyzed byPVP-Stabilized Ru–Pd Nanoparticles

Figure 7 shows the plots of the volume of the generated hydro-gen gas versus time in the hydrolysis of NaBH4 (0.375-M) andH3N·BH3 (0.100-M) solutions catalyzed by PVP-stabilized Ru–Pd nanoparticles (0.3 mM) at various temperatures (15, 20, 25,30, and 35◦C for NaBH4, and 10, 15, 20, 25, and 30◦C forH3N·BH3). It is worth to note that using PVP-stabilized Ru–Pd nanoparticles (0.3 mM) leads to 80% conversion for thehydrolysis of SBH within 5.33 min and complete hydrogen

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Table 3. The Apparent Arrhenius Activation Energy Values (Eaapp, kJ mol−1) for Various Catalyst Systems Tested in Hydrogen Generationfrom the Hydrolysis of SBH.

Catalyst Activation energy (kJ mol−1) Reference

Fe-Co-B/Ni foam 27 Liang, Wang, and Dai 2010Pd-C powder 28 Patel et al. 2008bCo-W-B/Ni 29 Dai et al. 2008bCo-B/Ni foam 33 Dai et al. 2008aCo/γ -Al2O3 33 Lee et al. 2007Intrazeolite Co(0) NCs 34 Rakap and Özkar 2009Co-Ni-B 34 Patel et al. 2010aNixB 38 Hua et al. 2003Co powder 42 Liu, Li, and Suda 2006Co/AC 44 Xu et al. 2008Co/C 46 Xu et al. 2008Ru/IRA-400 47 Xia and Chan 2005Ru/IR-120 50 Hsueh et al. 2008PVP-stabilized Ru–Pd NPs 52 This studyCo-Mn-B nanocomposites 55 Mitov et al. 2007Co-B/Attapulgite clay 56 Fernandes et al. 2009CoO nanocrystals 59 Lu et al. 2012o/SiO2 59 Shih et al. 2013Co-La-Zr-B 60 Loghmani and Shojaei 2013Co-P 60 Eom, Cho, and Kwon 2008Ni-Co-B 62 Ingersoll et al. 2007Ru-promoted sulphated Zr 76 Demirci and Garin 2008

Table 4. The Apparent Arrhenius Activation Energy Values (Eaapp, kJ mol−1) for Various Catalyst Systems Tested in Hydrogen Generationfrom the Hydrolysis of AB

Catalyst Activation energy (kJ mol−1) Reference

Pt/γ -Al2O3 21 Chandra and Xu 2007Rh/γ -Al2O3 21 Chandra and Xu 2007Fe-Co/C nano alloys 21 Qiu et al. 2013Ru/γ -Al2O3 23 Chandra and Xu 2007Pt0.65Ni0.35 NPs 39 Yang et al. 2009Pd/RGO NPs 40 Kılıç, Sencanlı, and Metin 2012Cu0.33Fe0.67 NPs 43 Lu et al. 2013PSSA-co-MA-stabilized Pd(0) NCs 44 Metin, Sahin, and Özkar 2009bNiCo-Pt nanoplates 46 Wen et al. 2013Laurate-stabilized Ru(0) NCs 47 Durap, Zahmakıran, and Özkar 2009bRu@Al2O3 NPs 48 Can and Metin 2012PVP-stabilized Ru–Pd NPs 54 This studyPSSA-co-MA-stabilized Ru(0) NCs 54 Metin, Sahin, and Özkar 2009bIntrazeolite Co(0) NCs 56 Rakap and Özkar 2009Ni0.97-Pt0.03 57 Cheng et al. 2007PVP-stabilized Co(0) NCs 63 Metin and Özkar 2009Zeolite-stabilized Rh(0) NCs 67 Zahmakıran and Özkar 2009Ru/C 76 Basu et al. 2009K2PtCl6 87 Mohajeri, T-Raissi, and Adebiyi 2007

release (3.0 mol H2/mol AB) for the hydrolysis of AB within3.25 min, corresponding to the record average turnover frequency(TOF) values of 762 mol H2/mol cat.min−1 and 308 mol H2/molcat.min−1 at 25.0 ± 0.1◦C, respectively. The TOF values ofdifferent catalyst systems for the hydrolysis of AB are givenin Table 2 for comparison. Moreover, PVP-stabilized Ru–Pd

nanoparticles provided record maximum hydrogen generationrates of 22889 L H2 min−1 (mol cat)−1 and 9364 L H2 min−1

(mol cat)−1 in the hydrolysis of SBH and AB, respectively.The apparent rate constants (kapp) for hydrogen generation

from the hydrolysis of SBH and AB were measured from thelinear portions of each plot given in Figure 7 at five different

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Hydrolysis of Sodium Borohydride and Ammonia Borane 1297

Fig. 9. Retained% catalytic activity of 0.3-mM PVP-stabilized Ru–Pd nanoparticles in successive catalytic runs for the hydrolysis of (a)0.375-M SBH and (b) 0.100-M AB at 25.0 ± 0.1◦C.

temperatures and used for the calculation of activation energiesfrom the Arrhenius plots (Figure 8). The apparent Arrhenius acti-vation energies (Eaapp) were found to be 52.4 ± 2 and 54.5± 2 kJ/mol for the hydrolysis SBH and AB, respectively. Forcomparison, the values of activation energy of various catalystsfor the hydrolysis of SBH and AB are given in Tables 3 and 4,respectively.

Recyclability of PVP-Stabilized Ru–Pd Nanoparticles in theHydrolysis of Sodium Borohydride and Ammonia Borane

The recyclability of PVP-stabilized Ru–Pd nanoparticles in thehydrolysis of SBH and AB was also investigated by successiveadditions of SBH or AB to the seventh cycle of both hydrolysisreactions. Figure 9 shows the results of recyclability tests for thehydrolysis of SBH and AB catalyzed by PVP-stabilized Ru–Pdnanoparticles. They retain 67% and 72% of their initial catalyticactivity in the hydrolysis of SBH and AB, respectively, even atthe seventh run% Retained catalytic activity was calculated on

the basis of assuming the first run 100% and then justifying sub-sequent runs with the first run, according to the duration in whichall SBH or AB were converted to the corresponding metaboratesreleasing hydrogen. Decrease in the catalytic activity of the cat-alyst in the hydrolysis of SBH and AB may be attributed to thedeactivation of nanoparticles’ surface by increasing the amountof metaborate, which decreases the accessibility of active sites(Clark, Whittell, and Manners 2007).

Conclusions

In summary, the study of the preparation and characterizationof PVP-stabilized Ru–Pd nanoparticles as a catalyst for thehydrolysis of SBH and AB has led to the following conclusionsand insights:

• PVP-stabilized Ru–Pd nanoparticles can be easily preparedfrom the co-reduction of corresponding Ru and Pd salts by thealcohol reduction method.

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• These are highly active catalysts for hydrogen generation fromthe hydrolysis of SBH and AB.

• These provide a record number of average TOF values(762 mol H2/mol cat.min−1 and 308 mol H2/mol cat.min−1)and maximum hydrogen generation rates (22,889 L H2 min−1

(mol cat)−1 and 9364 L H2 min−1 (mol cat)−1) for thehydrolysis of SBH and AB, respectively.

• Activation energies for the catalytic hydrolysis of SBH and ABin the presence of PVP-stabilized Ru–Pd nanoparticles werecalculated as 52.4 ± 2 and 54.5 ± 2 kJ/mol, respectively.

• The PVP-stabilized Ru–Pd nanoparticles can be regarded aspromising catalysts having high activity for practical applica-tions to supply hydrogen from the hydrolysis of SBH or AB forproton exchange membrane fuel cells.

Acknowledgment

The TEM and XPS analyses were carried out at the National HighMagnetic Field Laboratory (NHMFL) of Florida State Universityand Central Laboratory of Middle East Technical University,respectively.

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