Fuel Processing Technology 127 (2014) 88–96

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Fuel Processing Technology

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Superior reusability of metal catalysts prepared within poly(ethyleneimine) microgels for H2 production from NaBH4 hydrolysis

Sahin Demirci a, Nurettin Sahiner a,b,⁎a Faculty of Science & Arts, Chemistry Department, Canakkale Onsekiz Mart University, Terzioglu Campus, 17100 Canakkale, Turkeyb Nanoscience and Technology Research and Application Center (NANORAC), Canakkale Onsekiz Mart University, Terzioglu Campus, 17100 Canakkale, Turkey

⁎ Corresponding author at: Faculty of Science & Arts, ChOnsekiz Mart University, Terzioglu Campus, 17100 Can2180018x2041; fax: +90 2862181948.

E-mail address: sahiner71@gmail.com (N. Sahiner).

http://dx.doi.org/10.1016/j.fuproc.2014.06.0130378-3820/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 1 May 2014Received in revised form 6 June 2014Accepted 7 June 2014Available online xxxx

Keywords:HydrogelMicrogelPEI compositesNanogel compositeChemical hybridsChemical hydride catalysisH2 production

Polyethylene imine (PEI) particles were synthesized by a microemulsion polymerization technique, and wereused as template for metal nanoparticle preparation. The PEI particles were loaded with Co(II), Ni(II) andCu(II) ions from water and with CoCl2, NiCl2 and CuCl2 from ethanol and treated with NaBH4 to reduce the cor-responding metal nanoparticles in situ. The PEI-M (Co, Ni, and Cu) composites were used in hydrogen gen-eration from the hydrolysis of NaBH4 and NH3BH3 (AB). The PEI-Co composite catalyst system had bettercatalytic performances in the hydrolysis reactions of NaBH4 and AB than the other metal containingcomposites. The turnover frequency (TOF) value of PEI-Co in the hydrolysis of AB was found to be almost12.5 fold more than NaBH4 hydrolysis. It is reported here that the PEI template offered better characteristicsthan the other templates providing many advantages; for example, the metal loading capacity of PEIincreased by sequential loading and reduction cycles, and very high conversion (100%) and activity(100%) were obtained when PEI-Ni was used in fifteen consecutive runs for NaBH4 hydrolysis. The PEI-Mcomposites have mild activation energy in comparison to the literature but better, more flexible character-istics, and very fast kinetics in AB hydrolysis.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

Hydrogen is generally regarded as themost promising energy carrierfor future energy systems and can be obtained from various renewablesources such as water, plants, microorganisms and other biomass. Theuse of hydrogen as energy carrier can significantly affect greenhousegas emissions and reduce pollution [1,2]. Hydrogen is, in fact, a clean,carbon free and environmentally friendly, efficient energy carrier; andits combustion with oxygen emits only water vapor as by-product.The opportunity to use H2 as energy source for different devices suchas hydrogen fuel cell vehicles and portable electronics has urged scien-tists to seek better H2 carrier and storage systems. Chemical hydrides,such as NaBH4, MgH2, LiAlH4 and H3NBH3, have been widely studiedas storage materials for delivering H2 gas to fuel cells. Sodium borohy-dride (NaBH4), as the most convenient, economically suitable, andchemically stable material amongst the hydrides, has been intensivelyinvestigated as aH2 storagematerial. Thenon-flammability and stabilityin air, easily controlled hydrogen generation rate, side product recycla-bility, and highH2 storage efficiencies are other important advantages of

emistry Department, Canakkaleakkale, Turkey. Tel.: +90 286

NaBH4. The high theoretical gravimetric (10.9 wt.%) and volumetric H2

density of NaBH4 is another attractive feature [3–5]. The hydrolysisof NaBH4 is an exothermic reaction releasing approximately

˜ 300 kJ mol−1 [6], and this reaction can be carried out at ambient tem-peratures readily. Previous studies on hydrogen production fromNaBH4

are summarized in recent reviews [7–9]. Base-stabilized NaBH4 solutionhydrolyzes to hydrogen and sodiummetaborate (NaBO2) only when incontact with specific catalysts producing 2 mol extra H2 coming fromthe water in solution at the hydrolysis temperature.

NaBH4ðaqÞ þ 2H O→4H2 þ NaBO2 þ heat ð1Þ

Finally the by-product, namely sodiummetaborate (NaBO2), is envi-ronmentally friendly [10] and can be recycled for re-synthesis of NaBH4

[11,12].Many metal catalysts with different forms based on Ru and Pt have

been employed for the hydrolysis of NaBH4 [13–18]. However, non-noble catalysts such as Co or Ni-based catalysts have also been investi-gated [19–24] as potential replacement catalysts with comparable cata-lytic performances to noble metal nanoparticles.

Another hydrogen carrier, ammonia borane (AB), is also an attrac-tive candidate because of its high hydrogen capacity (19.6 wt.%) [25]and nontoxicity [26]. AB provides the release of 3 mol of H2 per mole

89S. Demirci, N. Sahiner / Fuel Processing Technology 127 (2014) 88–96

of AB in the presence of a suitable catalyst at ambient temperatures.

NH3BH3 þ ðaqÞ þ 2H O→ðNH4ÞBO2ðaqÞ þ 3H2ðgÞ ð2ÞIn recent decades, various catalyst systems including transition

metal salts, ions andmetal nanoclusters or their alloys have been testedin the hydrolysis of NaBH4 or AB [27–31].

Earlier we reported PEI particle synthesis and demonstrated its po-tential versatile applications [32]. Here, we report PEI-M (M: Co, Ni,Cu) composite preparation and their successful use in hydrolysis ofNaBH4 and AB. The effects of various parameters such as metal species,temperature, metal nanoparticle content, and reusability of PEI-M com-posites on the hydrolysis of NaBH4 and AB are reported. It was demon-strated that PEI-M nanoparticles do not need any co-catalyst, such asNaOH, for the hydrolysis reaction and can be repetitively used up toten times without any loss of conversion or catalytic activity. Amongstthe PEI-M (M:Co, Ni, Cu) catalyst composite systems, Co showed supe-rior catalytic performance.

2. Experimental

2.1. Materials

Polyethyleneimine (PEI, 50 wt.% soln. in water, Mn1:800 Mw:2000d:1.08) was purchased from Sigma-Aldrich and used as received forparticle preparation. Sodium bis(2-ethylhexyl) sulfosuccinate (AOT,98%, Aldrich) as surfactant, gasoline as solvent, and divinylsulfone(DVS, 98%) as crosslinker purchased fromMerck were used as received.Cobalt chloride hexahydrate (CoCl ·6H O, 98%, Acros), nickel chloridehexahydrate (NiCl2·6H2O, 98%, Acros) and copper chloride (CuCl2 an-hydrous, 98%, Acros) were employed as metal nanoparticle sources. So-dium borohydride (NaBH4, 98%,Merck)was used as reducing agent andfor preparation of metal nanoparticles. Sodium borohydride (NaBH4,98%, Merck) and ammonia-borane (NH3BH3, 97%, Aldrich) were usedfor production of hydrogen.

2.2. Synthesis of polyethyleneimine particles

Polyethyleneimine particles were synthesized according to the pre-viously reportedmethodwith somemodification by emulsion polymer-ization [32]. Initially, 750 μL of polyethyleneimine (PEI) solution wasdispersed in 30 mL of 0.1 M AOT solution in gasoline. After vortexmixing until a clear suspension was obtained, the crosslinker, DVS(50–100mol% relative to the PEI repeating unit), was added under con-tinuous mixing to disperse the DVS homogenously. The reactionproceeded for 1 h at ambient temperature under vigorous stirring at1200 rpm. Then, the obtained particles were precipitated in excess ofacetone and ethanol, and precipitated by centrifugation at 24,683 g for10min at 20 °C.Washing of particles was repeated five times to removesurfactant and finally the particles were dried by heat gun.

2.3. Metal nanoparticle preparation within polyethyleneimine (PEI)particles

To prepare metal nanoparticle ions Co(II), Ni(II) and Cu(II) in waterand metal salts CoCl2, NiCl2 and CuCl2 in ethanol were used. The metalions were loaded into PEI particles by placing certain amounts of PEIparticles (0.05–0.3 g) into 50 mL 1000 ppm Co(II), Ni(II), and Cu(II)ions in distilled water for 12 h at room temperature. Then the mixtureswere centrifuged and washed with distilled water and re-centrifuged(24,683 g) to remove the non-loaded metal ions. The metal ion-loadedPEI particles were transferred into 50 mL 0.1 M NaBH₄ solution andstirred at 1000 rpm for 4 h until gas evolution stopped. These PEI-Mcomposites (M: Co, Ni, Cu) were washed with distilled water by centri-fugation and used in the hydrolysis of NaBH₄ and NH3BH3. Also, thedried 0.1 g PEI particles were also placed in alcohol solutions containing

metal salts, i.e., in 0.1 M 50 mL CoCl2, NiCl2 or CuCl2 solution in ethanolfor 24 h. Then the metal salt-loaded PEI was placed in 50 mL 0.1 MNaBH₄ solution and stirred at 1000 rpm for 1 h until gas evolutionstopped. The metal nanoparticle-containing PEI was washed with etha-nol and dried with a heat gun.

The hydrolysis of NaBH4 orNH3BH3was carried out after placing cer-tain amounts (0.1 g) of PEI-M composites in a reaction flask containingeither 50 mM (0.0965 g) NaBH₄ in 50 mL distilled water or 50 mMNH3BH3 (0.0795 g) in 50 mL distilled water. The reaction conditionsfor hydrolysis reactions were the same: e.g., 50 mL 50 mM NaBH4 or50mMNH3BH3 and 1000 rpmmixing rate. The amount of produced hy-drogen was measured from a water filled inverted graded cylinder,based on the principle that the produced gas replaces the water. Theproduced gaswas passed through a concentratedH2SO4 solution to cap-ture the water vapor produced during hydrolysis reaction. The effect ofvarious parameters such as the type and amount ofM catalyst, aswell astemperature, on hydrolysis of NaBH4 and NH3BH3 was investigated.

2.4. Determination of the amount of metal nanoparticles within PEI-Mcomposites

The amounts of metal nanoparticles within PEI were determined byatomic absorption spectroscopy (Thermo, ICA 3500 AA SPECTRO) usingthe metal ion solution obtained by treating PEI-M composites with 5 M20 mL HCl three times for 8 h to dissolve the metal nanoparticles in thePEI-M composites.

3. Results and discussion

3.1. Synthesis and characterization of PEI particles

The branched PEI chains were connected via DVS to create PEI parti-cles as illustrated in Fig. 1(a). The synthesized PEI particles showedbroad particle size distribution ranging from tens of nanometers to sev-eral tens of μm as reported earlier [32]. The optical microscope imagesshown in Fig. 1(b), and (c) are PEI particles obtained by DVScrosslinking of Mn: 1800 g/mol of PEI taken directly from the reactionmixture after centrifugation at 24,000 g in dry and swollen in distilledwater. The scanning electron microscopy (SEM, Jeol JSM-5600) imagesof PEI particles are illustrated in Fig. 1(c) and (d). As can be clearlyseen PEI particles are spherical and size range is wide. The FT-IR spectraof PEI and PEI particles were obtained and the reaction of amine groupswith DVS was confirmed (data is not shown) [33]. The size of particleswas measured using dynamic light scattering (DLS) measurementsafter filtering with plankton fabric (pore size b 2 μm). DLS measure-ments were carried out using 1 mg/mL particle suspension in 10 mMKCl solutions. As illustrated in Fig. 2(a), thefiltered and non-filtered par-ticle sizesweremeasured as 514.4±6 and 1596.8±7nm, respectively.The surface charge of any particle is directly related to the existence offunctional groups and theirmeasurement is very important for catalyticand environmental applications. Therefore, zeta potential measure-ments of PEI particles were carried out by suspending the PEI particlesin pure water. The surface charge of the particles was +12.41 ±0.18mV, as illustrated in Fig. 2(b). It is reasonable that the amine groupsin PEI particles are both non-quaternized and quaternized, as they con-tain primary, secondary and tertiary amine groups providing positivecharges.

3.2. Metal nanoparticle preparation within PEI particles

Schematic presentation ofmetal nanoparticle preparation in PEI par-ticles is illustrated in Fig. 3(a). Different metal salts such as CoCl2, NiCl2and CuCl2 were dissolved inwater, and in ethanol, andwere loaded intothe PEI particles from their corresponding solutions. In aqueous envi-ronments, the metal ions such as Co(II), Ni(II), and Cu(II) are formedand loaded into PEI particles due to the interaction of the amine groups

(c)(b)

+

NH₂

NH₂

NH₂NH₂

NH₂

NH₂ NH₂NH₂

NH₂

NH₂

PEI DVS PEIparticles

Gasoline/AOT

(a)

(e)(d)

Fig. 1. (a) Schematic presentation of PEI particle synthesis, (b) and (c) optical microscope images of PEI particles.

90 S. Demirci, N. Sahiner / Fuel Processing Technology 127 (2014) 88–96

of PEI and metal ions via the ion–dipole route. In alcohol environments,the metal salts can dissolve in the molecular form of chloride salts, andare loaded into PEI particles due to the interaction of the positivelycharged amine groups on PEI particles, and the metal salts, again viaion–dipole interaction (ion from PEI and dipole from molecularly dis-solve metal salts) [32]. This is reasonable, as the zeta potential value ofPEI particles was +12.41 mV. Finally, after loading metals, whethermetals are in ion or salt form, they were reduced to their correspondingmetal nanoparticles by treatment with 50mL 0.1 M NaBH4. Uponmetalnanoparticle formation within PEI, the obtained PEI-Ms (0.1 g) weretreated with 20 mL 5 M HCl three times, and the amount of metal ionswas determined with atomic absorption spectroscopy (AAS) measure-ments. Table 1 summarizes the PEI-M composite metal contents. Inter-estingly, the amount of Co (18.1mg) is much less than the amount of Ni(103.2 mg) and Cu (108.2 mg) upon loading into the PEI particle fromaqueous environments. On the other hand, the amount of metal ionsin PEI is almost the same for Co (108.4 mg), Ni (107.2 mg), and Cu(112.5 mg) when loaded from ethanol solutions. So, the use of ethanolas solvent may be a better choice to load higher amounts of metal intoPEI particles for corresponding metal nanoparticle preparations. This ismore significant in the comparison of Co(II) and CoCl2 loading fromwater and ethanol, respectively, into PEI particles.

3.3. H2 production from the hydrolysis of NaBH4 and NH3BH3 catalyzed byPEI-M

In the generation of H2, chemical hydrides such as NaBH4 andNH3BH3 are widely used as H2 sources. Although H2 can be obtainedfrom these storage materials, generally metal nanocatalysts are re-quired for fast and efficient H2 production. As demonstrated inFig. 3(b), the hydrolysis of NaBH4 is much slower than the Conanoparticle-catalyzed reactions. The hydrolysis of NaBH4 is given inEq. (1) and the hydrolysis of AB is given in Eq. (2) and shows thatboth reactions produce 4 and 3 mol hydrogen, respectively. FromEq. (1), 2mol extra H2 is produced fromwater at the hydrolysis temper-ature which is a great advantage, however their use in real applicationsdepends on various factors e.g., theweight % of produced H2 in the totalweight fuel cells, or the reusability of the by-products and their interac-tionswithmetal catalysts orwith thewhole system. The reactionmech-anism for NaBH4 hydrolysis is given in Fig. 3(c). As can be clearly seenthe solvent,water, can contribute to theNaBH4 hydrolysis by generationof extra H2 on the metal catalyst surface.

As the template, PEI, is a versatile support material, metal ions fromcorresponding aqueous solutions or metal chloride salts from alcohol(ethanol) can be loaded into PEI networks. As shown in Fig. 4(a), PEI

0

20

40

60

80

100

120

filtered PEI particlesnot filtered PEI particles

Diameter (nm)

Inte

nsity

(a)

(b)

0

0.2

0.4

0.6

0.8

1

1.2

0 500 1000 1500 2000

0 5 10 15 20 25 30

PEI 1800

Zeta Potential (mV)

Pow

er

Fig. 2. (a) DLS results for PEI particles after and before filtration, 514.4 and 1596 nm, re-spectively, and (b) the surface charge of PEI particles, +12.41 mV.

91S. Demirci, N. Sahiner / Fuel Processing Technology 127 (2014) 88–96

particles loadedwith Co(II) and Ni(II) ions from aqueous environmentswere used in the hydrolysis of NaBH4 at 30 °C. Although the amounts ofmetal nanoparticles were the same (0.089 mmol), their hydrogen pro-duction rateswere different, and it was found that Co nanoparticles cat-alyzeNaBH4 hydrolysis faster than Ni nanoparticles as the same amountof H2 (252 mL) was produced in 114 and 180 min, respectively, underthe same reaction conditions. From now on, the hydrolysis conditionsare 50 mL 50mMNaBH4, or AB at 30 °C, and 1000 rpmmixing rate un-less otherwise stated. The hydrogen production rates versus timegraphs for PEI-M composites formed by loading from ethanol are illus-trated in Fig. 4(b). Again Co nanoparticles produced the same amountof hydrogen faster thanNi nanoparticles under the same reaction condi-tions with the same amount of catalyst used. To confirm that the PEIbinding ability is different for different metal ions from aqueous envi-ronments, we used 0.1 g PEI to load Co(II) and Ni(II) from their 50 mL1000 ppm solutions, and reduced to their corresponding metal nano-particles by the treatment of 50 mL 0.1 M NaBH4. Then the preparedPEI-M composites were used in the hydrolysis of NaBH4 as shown inFig. 4(c). As 0.1 g PEI contained 0.18 mmol Ni and 0.031 mmol Co, itseems PEI-Ni particle produced H2 faster (252 mL in 92 min) than PEI-Co (252 mL in 200 min) due to the higher amount of metal catalyst.To demonstrate the versatility of our PEI-M composite, we also testedtheir use in the hydrolysis of AB, and the corresponding graph is givenin Fig. 5. It is obvious from the figure that Co nanoparticles catalyzethe AB hydrolysis much faster than Ni and Cu when the amount of M(0.18 mmol) catalyst is used. About the same amount of H2 is producedfor each metal nanoparticle (~180 mL) in 3.2, 7 and 29.2 min in theorder of Co, Ni, and Cu.

The effect of temperature on hydrolysis of NaBH₄ and AB was inves-tigated to determine the activation parameters by conducting the hy-drolysis reactions at three different temperatures; 30, 50, and 70 °C.At these temperatures, same amounts of metal nanoparticle-containing PEI-M (M:0.18 mmol) composite particles were used for

thehydrolysis reactions. As shown in Fig. 6(a), it took PEI-Co composites55, 27 and 8 min at 30, 50 and 70 °C to produce the same amount of H2

(252mL) in the hydrolysis of NaBH4. Also, for the same reaction, it tookPEI-Ni composite 73, 32, and 10min at 30, 50 and 70 °C, respectively, asillustrated in Fig. 6(b). Additionally, to determine the effect of tempera-ture on AB hydrolysis, PEI-Co particles were used and the obtainedgraphs are shown in Fig. 6(c). As the Cometal nanoparticle catalyst pro-vided the fastest production rate in hydrolysis for H2, only PEI-Co com-posites were used in AB hydrolysis. As can be seen from Fig. 6(c), about180mLH2was produced in 180, 40, and 38 s for the reaction carried outat 30, 50 and 70 °C. Finally, the activation parameters for each of the PEI-M composite systems for NaBH4 and AB hydrolysis were calculated byusing the very well known Arrhenius and Eyring Equations whichwere given in Eqs. (3) and (4) respectively, and the obtained resultsare given in Table 2.

k ¼ Ae½−Ea =RT� ð3Þ

lnðk=TÞ ¼ −ðΔH=RÞð1=TÞ þ lnðkB=hÞ þ ΔS=R ð4Þ

where, k is the reaction rate constant and was calculated according to azero-order kinetic expression, Ea is the activation energy, T is the abso-lute temperature, kB is the Boltzmann constant (1.381 × 10−23 J K−1), his the Planck's constant (6.626 × 10−34 J·s),ΔH is the activation enthal-py, ΔS is the entropy and R is the gas constant (8.314 JK−1 mol−1).

As Table 2 reveals, the Ea is almost the same for Ni (Ea = 38.3 kJ/mol) as for Co (40.1 kJ/mol) catalyst systems, though the reaction rateof hydrolysis of NaBH4 is faster for Co. TheAB hydrolysis by PEI-Co is sig-nificantly faster than the hydrolysis of NaBH4 with lower Ea = 30.5 kJ/mol. The other parameters such as ΔS and ΔH values are also given inTable 2.

3.3.1. The effect of metal nanoparticle content on the hydrolysis of NaBH4

and ABIt is very well known that the amount of catalyst is very important

for fast catalytic reactions. Therefore, PEI metal nanoparticle contentswere increased by sequential metal loading and reduction cycles. Forexample, CoCl2 and NiCl2 were loaded and reduced from ethanol solu-tion three times to increase the metal nanoparticle content of PEI-Mcomposite catalyst systems, and they were tested for NaBH4 hydrolysisafter each loading and reduction cycle. As demonstrated in panel (a) ofFig. 7, the sizes of Cometal nanoparticles were between 20 and 100 nm,and the amounts Cometal nanoparticles within PEI microgels increasedwith the number of loading and reduction cycles. NaBH4 hydrolysis wascarried out after multiple loading and reduction cycles of Co and Ni par-ticles, and their H2 production versus time graphs were constructed asshown in Fig. 7(b) and (c), respectively, after each loading with time.It is clearly seen that with every loading and reduction cycle theamounts of Co and Ni nanoparticles increased, also resulting in an in-crease in hydrogen production rates. After the 3rd loading cycle theamount of Co was increased to 291.8 mg Co(II) whereas Ni(II) was in-creased to 378.6 mg per gram PEI particle, hence their H2 productionrates, and turn over frequency (TOF) valueswere also increased accord-ingly as given in Table 3. This increase is more pronounced for Ni thanCo, thus PEI-Ni particle hydrogen generation rate (HGR) increased to653 from 297 mL H2/(g of cat) (min). This increase was also observedfor PEI-Co composite systems to a lesser degree. Even though AB hydro-lysis was not carried out after multiple metal ion loading and reductioncycles (as AB hydrolysis was very fast), the TOF values were also calcu-lated and are shown in Table 3. The TOF value for AB was very high incomparison to NaBH4 hydrolysis. For example, the TOF value of PEI-Cowas calculated as 12.4 mol H /(mol cat) (min) for AB hydrolysis where-as the TOF value of PEI-Co was estimated at 0.995 mol H /(mol cat)(min) for NaBH4 hydrolysis after initial metal loading and reduction.

PEI particles

Co(II), Ni(II) or

Cu(II) or their chloride salts

Metal ions

NaBH4

Metal nanoparticles

PEI-M composite

0

50

100

150

200

250

300

0 50 100 150 200 250 300

NaBH₄NaBH4+PEINaBH₄+NaOHPEI-Co

Time (min)

Volu

me

of H

ydro

gen

(ml)

(a)

BH4-

H2O

BH

H

H

H

H

O

e-

H2O H2O

BH

HOH

H

H2

(c)

(b)NaBH4NaBH4+ PEINaBH4+ NaOHPEI-Co

Fig. 3. (a) Schematic presentation of the preparation of metal nanoparticles within PEI particles, and (b) NaBH4 hydrolysis in the presence of various conditions, and (c) the H2 productionmechanism from NaBH4 in the presence of metal nanocatalyst.

92 S. Demirci, N. Sahiner / Fuel Processing Technology 127 (2014) 88–96

These TOF valueswere 5.48 and 0.701mol H2/(mol cat) (min) for PEI-Niparticles for AB, and NaBH4 hydrolysis, respectively.

3.3.2. The reuse of PEI-M composite for hydrolysis of NaBH4

For catalytic systems to be used in industrial applications, somecriteria such as reusability, selectivity, shelf life, endurance, and eco-friendliness and so on, have paramount significance. Therefore, we test-ed 3rd time loaded and reduced PEI-Ni composite particles in thehydro-lysis of NaBH4 fifteen times in a row. We chose PEI-Ni as it containedmore Ni ions compared to Co ions after three loading and reduction cy-cles from ametal salt ethanol solution. The activitywas calculated basedon the initial hydrogen production rates e.g., taking the ratio of initialhydrogen production rate to the following hydrogen production ratesafter each use, and the percent conversion is calculated based on 100%conversion of NaBH4 that exists in the hydrolysis platform. The PEI-Nicomposite particles were used fifteen times consequently in hydrolysisreaction and 100% activity was obtained as illustrated in Fig. 8. This is a

Table 1Metal nanoparticle content of PEI-M composites from different loading environments.

Metal loading milieu Co(mg/g)

Ni(mg/g)

Cu(mg/g)

DI water 18.1 ± 2 103.2 ± 5 108.2 ± 6Ethanol 108.4 ± 5 107.2 ± 6 112.5 ± 6

very important result as up to now there has been no report in the liter-ature stating almost no decrease in the activity of a catalytic systemoverfifteen sequential uses in comparison to the similar research in litera-ture. In the literature, even with five or seven uses the activity goesdown to 85 or 90% at the best [18–23,33].

4. Conclusions

Here it was demonstrated that PEI polymeric particles with differentdimensions and positively charged zeta potential can provide very re-sourceful environments for the preparation of metal nanoparticlessuch as Co, Ni, and Cu within the PEI microgel network. The metalsthat can be used in particle formation can either be loaded into the PEInetwork from their aqueous environments or from their alcohol solu-tions and can then reduced to the corresponding metal nanoparticleby NaBH4 treatment. The prepared PEI-M composite systems can besuccessfully used in the production of H2 from the hydrolysis of NaBH4

and AB. The PEI-Co provided faster H2 generation in the hydrolysis ofboth chemical H2 storagematerials and had higher TOF values in the hy-drolysis of AB. This is very interesting, offering very important results insome real applications where fast hydrogen production rates are re-quired. It was also proven that PEI-Co composite can be used over andover, without any loss of % conversion and activity, and it was foundthat 100% was still achieved with no loss of activity at the end of fifteen

0

50

100

150

200

250

300

PEI-CoPEI-NiVo

lum

eof

Hyd

roge

n(m

l)

Time (min)

(a)

0

50

100

150

200

250

300

PEI-Et-OH NiPEI-Et-OH Co

Volu

me

of H

ydro

gen

(ml)

Time (min)

(b)

NiCo

(c)

0

50

100

150

200

250

300

0 50 100 150 200 250

0 20 40 60 80 100

0 50 100 150 200 250

PEI-NiPEI-Co

Volu

me

of H

ydro

gen

(ml)

Time (min)

Fig. 4. The effect of metal nanoparticle types on the hydrolysis of NaBH₄ (a) loaded fromaqueous environments (0.089 ± 0.002 mmol Co and Ni, metal nanoparticles), and (b)loaded from ethanol (0.183 ± 0.02 mmol Co, and Ni metal nanoparticles). (c) Hydrogenproduction rate vs. time for PEI-M (Co and Ni) composites prepared by using the sameamounts of PEI (0.1 g) from different metal ion aqueous solutions (Co: 0.031 and Ni:0.18 mmol [50 mM 50 mL aqueous NaBH4 solution, 30 °C, 1000 rpm]).

0

50

100

150

200

0 10 20 30 40

PEI-CoPEI-NiPEI-Cu

Time (min)

Vol

ume

of H

ydro

gen

(ml)

Fig. 5. The hydrogen production of PEI-M (M: Co, Ni, Cu) from the hydrolysis of NH3BH3

[reaction conditions: 0.183 ± 0.01 mmol M (M; Co, Ni and Cu), 50 mL 50 mM NaBH4,30 °C, 1000 rpm].

93S. Demirci, N. Sahiner / Fuel Processing Technology 127 (2014) 88–96

consecutive uses. Therefore, these types of PEI-M catalyst systems withvery commonmetal nanoparticles are economically feasible and can beused in advanced H2 powered devices for clean and environmentallyfriendly applications. In fact, our current research is directed to thesetopics.

Acknowledgments

This work was supported by the Scientific and Technological Re-search Council of Turkey (110T649).

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[12] H.K. Atiyeh, B.R. Davis, Separation of sodium metaborate from sodium borohydrideusing nanofiltration membranes for hydrogen storage application, InternationalJournal of Hydrogen Energy 32 (2007) 229–236.

[13] C. Crisafulli, S. Scirè, M. Salanitri, R. Zito, S. Calamia, Hydrogen production throughNaBH4 hydrolysis over supported Ru catalysts: an insight on the effect of the sup-port and the ruthenium precursor, International Journal of Hydrogen Energy 36(2011) 3817–3826.

[14] Z.T. Xia, S.H. Chan, Feasibility study of hydrogen generation from sodium borohy-dride solution for micro fuel cell applications, Journal of Power Sources 152(2005) 46–49.

[15] Y. Shang, R. Chen, G. Jiang, Kinetic study of NaBH4 hydrolysis over carbon-supportedruthenium, International Journal of Hydrogen Energy 33 (2008) 6719–6726.

[16] Y.H. Huang, C.C. Su, S.L. Wang, M.C. Lu, Development of Al2O3 carrier-Ru compositecatalyst for hydrogen generation from alkaline NaBH4 hydrolysis, Energy 46 (2012)242–247.

[17] J. Zhang, W.N. Delgass, T.S. Fisher, J.P. Gore, Kinetics of Ru-catalyzed sodium borohy-dride hydrolysis, Journal of Power Sources 164 (2007) 772–781.

[18] N. Sahiner, O. Ozay, E. Inger, N. Aktas, Controllable hydrogen generation by usesmart hydrogel reactor containing Ru nano catalyst and magnetic iron nanoparti-cles, Journal of Power Sources 196 (2011) 10105–10111.

[19] T. Turhan, Y. Avcıbası Güvenilir, N. Sahiner, Micro poly(3-sulfopropyl methacrylate)hydrogel synthesis for in situ metal nanoparticle preparation and hydrogen genera-tion from hydrolysis of NaBH4, Energy 55 (2013) 511–518.

[20] N. Sahiner, A.O. Yasar, Synthesis and modification of p(VI) microgels for in situmetal nanoparticle preparation and their use as catalyst for hydrogen generationfrom NaBH4 hydrolysis, Fuel Processing Technology 111 (2013) 14–21.

[21] N. Sahiner, A.O. Yasar, Metal nanoparticle preparation within modifiable p(4-VP)microgels and their use in hydrogen production from NaBH4 hydrolysis, Interna-tional Journal of Hydrogen Energy 38 (2013) 6736–6743.

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Fig. 6. The effect of temperature on the hydrolysis of NaBH4 catalyzed by (a) CoCl2, and (b)NiCl2 loaded from ethanol. (c) The effect of temperature on the hydrolysis of AB catalyzedby CoCl2 loaded from ethanol.

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[22] S. Sagbas, N. Sahiner, A novel p(AAm-co-VPA) hydrogel for the Co and Ni nanopar-ticle preparation and their use in hydrogel generation from NaBH4, Fuel ProcessingTechnology 104 (2012) 31–36.

Table 2The comparison of activation parameters; Ea, ΔH, and ΔS for the hydrolysis of NaBH4 andAB catalyzed by PEI-M composite catalysts.

Metal ions Loading medium: EtOH

NaBH4 AB

Ea(kJ/mol)

ΔH(kJ/mol)

ΔS(J/mol·K)

Ea(kJ/mol)

ΔH(kJ/mol)

ΔS(J/mol·K)

Co(II) 40.12 36.63 −179.81 30.54 27.26 −205.402Ni(II) 38.31 34.86 −186.88 – – –

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[25] J. Zhao, J. Shi, X. Zhang, F. Cheng, J. Liang, Z. Tao, et al., A soft hydrogen storage ma-terial: poly(methyl acrylate)-confined ammonia borane with controllable dehydro-genation, Advanced Materials 22 (2010) 394–397.

[26] Z.H. Lu, J. Li, A. Zhu, Q. Yao, W. Huang, R. Zhou, R. Zhou, X. Chen, Catalytic hydrolysisof ammonia borane via magnetically recyclable copper iron nanoparticles for chem-ical hydrogen storage, International Journal of Hydrogen Energy 38 (2013)5330–5337.

[27] O. Akdim, U.B. Demirci, P. Miele, A bottom-up approach to prepare cobalt-based bi-metallic supported catalysts for hydrolysis of ammonia borane, International Journalof Hydrogen Energy 38 (2013) 5627–5637.

[28] J.M. Yan, X.B. Zhang, S. Han, H. Shioyama, Q. Xu, Room temperature hydrolytic dehy-drogenation of ammonia borane catalyzed by Co nanoparticles, Journal of PowerSources 195 (2010) 1091–1094.

[29] H. Jiang, Q. Xu, Catalytic hydrolysis of ammonia borane for chemical hydrogen stor-age, Catalysis Today 170 (2011) 56–63.

[30] Q. Li, H. Kim, Hydrogen production fromNaBH4 hydrolysis via Co-ZIF-9 catalyst, FuelProcessing Technology 100 (2012) 43–48.

[31] Y. Chen, H. Kim, Use of a nickel-boride–silica nanocomposite catalyst prepared byin-situ reduction for hydrogen production from hydrolysis of sodium borohydride,Fuel Processing Technology 89 (2008) 966–972.

[32] N. Sahiner, Preparation of poly(ethylene imine) particles for versatile applications,Colloids and Surfaces A: Physicochemical and Engineering Aspects 433 (2013)212–218.

[33] N. Sahiner, Soft and flexible hydrogel templates of different sizes and various func-tionalities for metal nanoparticle preparation and their use in catalysis, Progress inPolymer Science 38 (2013) 1329–1356.

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Fig. 7. (a) TEM images of Co nanoparticles after the 1st, 2nd, and 3rd CoCl2 loading and reduction cycles within PEI microgels, and the effect of metal nanoparticle content on hydrolysis ofNaBH4 by three consecutive loading and reduction cycles of (b) CoCl2, and (c) NiCl2 loading from ethanol solution [reaction conditions: 50 mM 50 mL aqueous NaBH4, 30 °C, 1000 rpm].

95S. Demirci, N. Sahiner / Fuel Processing Technology 127 (2014) 88–96

Table 3Calculated turn over frequency (TOF) and hydrogen generation rates for hydrolysis of NaBH4.

Number of metal loading cycles Amounts of metal ions(mg/g)

Metal ion loading medium

EtOH

TOF mol H2/(mol cat) (min) Hydrogen generation rate mL H2/(g of cat) (min)

NaBH4

Cobalt1st

108.4 ± 0.5 0.994 407.73

Cobalt2nd

192.2 ± 0.6 0.805 354.72

Cobalt3rd

291.8 ± 0.7 1.349 568.15

Nickel1st

107.2 ± 0.6 0.701 297.17

Nickel2nd

219.7 ± 0.5 1.103 458.45

Nickel3rd

379.6 ± 0.8 1.523 653.44

ABCobalt 108.4 ± 0.5 12.35 5116Nickel 107.2 ± 0.6 5.48 2314Cupper 112.8 ± 0.3 1.64 657

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%

Fig. 8. The repetitive use of PEI-Ni composite particles in hydrolysis of NaBH4 [reactionconditions: 50 mM 50 mL aqueous NaBH4, 30 °C, 1000 rpm].

96 S. Demirci, N. Sahiner / Fuel Processing Technology 127 (2014) 88–96