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journal homepage: www.elsevier .com/locate/heMetal nanoparticle-embedded super porouspoly(3-sulfopropyl methacrylate) cryogel for H2production from chemical hydride hydrolysisSema Yildiz a, Nahit Aktas b, Nurettin Sahiner a,c,*

a Canakkale Onsekiz Mart University, Faculty of Sciences and Arts, Chemistry Department, Campus,

65080, Van, Turkeyb Yuzuncu Yil University, Engineering Faculty, Chemical Engineering Department, Campus, 65080, Van, Turkeyc Nanoscience and Technology Research and Application Center (NANORAC), Terzioglu Campus, 17100,

Canakkale, Turkeya r t i c l e i n f o

Article history:

Received 19 May 2014

Received in revised form

30 June 2014

Accepted 8 July 2014

Available online 4 August 2014

Keywords:

p(SPM) hydrogel/cryogel

Super macroporous p(SPM)-M com-

posite catalyst

Cryogenic hydrogel

H2 production

Chemical hydride hydrolysis* Corresponding author. Nanoscience and Takkale, Turkey. Tel.: 90 2862180018 2041; fa

E-mail address: sahiner71@gmail.com (Nhttp://dx.doi.org/10.1016/j.ijhydene.2014.07.00360-3199/Copyright 2014, Hydrogen Energa b s t r a c t

Poly(3-sulfopropyl methacrylate) (p(SPM)) cryogel was prepared under cryogenic conditions

(T 18 C) and used as template for in situ metal nanoparticle preparation of Co, Ni andCu. These metal nanoparticle-containing super macroporous cryogel composites were

tested for H2 production from hydrolysis of sodium borohydride (NaBH4) and ammonia

borane (AB). It was found that amongst p(SPM)-M (M: Co, Ni, and Cu) composite catalyst

systems, the catalytic performances of Co- and Ni-containing p(SPM) cryogel composite

catalyst systems were the same, however in hydrolysis of NH3BH3, the order of perfor-

mance of the catalysts was Co > Ni > Cu. Interestingly, p(SPM)-Co cryogel composite

demonstrated better catalytic performances in salt environments e.g., faster H2 production

rate in sea and tap water compared to DI water, and almost no effect of ionic strength of

the solution medium was observed, but the salt types were found to affect the H2 gener-

ation rate. Other parameters that affect H2 production rate such as metal type, tempera-

ture, water source, salt concentration, amount of metal nanocatalyst and reusability were

investigated. It was found that the hydrogen generation rate (HGR) was increased to

2836 90 from 1000 53 (ml H2)(g of Co min)1 by multiple loading and reduction cycles of

Co catalyst. Also, it was found that TOF values are highly temperature dependent, and

increased to 15.1 0.8 from 2.4 0.1 (mol H2)(mol catalyst min)1 by increasing the tem-

perature from 30 to 70 C. The activation energy, activation enthalpy and activation en-

tropy were determined as 40.8 kJ (mol)1, 37.23 kJ (mol K)1, and 170.87 J (mol K)1,respectively, for the hydrolysis reaction of NaBH4 with p(SPM)-Co catalyst system, and

25.03 kJ (mol)1, 22.41 kJ (mol K)1, and 182.8 J (mol K)1, respectively, for AB hydrolysiscatalyzed by p(SPM)-Co composite system.

Copyright 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.echnology Research andx: 90 2862181948.. Sahiner).35y Publications, LLC. PublApplication Center (NANORAC), Terzioglu Campus, 17100, Can-

ished by Elsevier Ltd. All rights reserved.

mailto:sahiner71@gmail.comhttp://crossmark.crossref.org/dialog/?doi=10.1016/j.ijhydene.2014.07.035&domain=pdfwww.sciencedirect.com/science/journal/03603199www.elsevier.com/locate/hehttp://dx.doi.org/10.1016/j.ijhydene.2014.07.035http://dx.doi.org/10.1016/j.ijhydene.2014.07.035http://dx.doi.org/10.1016/j.ijhydene.2014.07.035

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 1 4 6 9 0e1 4 7 0 0 14691Introduction

Hydrogels are diverse functional group-containing materials

with many applications such as template metal nanoparticle

preparation and catalysis [1]. As the size of the hydrogel can be

tuned from bulk to micro and/or nanometer, the pore size can

also be designed asmicroporous (2 nmand below),mesoporous

(between50and2nm)and super porousup to a fewmicrometer

[1]. Additionally, hydrogels can be chemicallymodified to allow

new functional groups for specific tasks e.g., metal ion absorp-

tion, etc. Interestingly hydrogels with binding ability to diverse

metal ions can also be utilized for the in situ synthesis of

differentmetalnanoparticles separatelyorsimultaneously [1,2].

Cryogels are, on the other hand, macroporous hydrophilic

materials prepared by cryo-polymerization of monomers in

the presence of ice crystals under frozen conditions [3,4].

Macroporous cryogels are prepared at temperatures below the

melting point of the solvent, which is generally water [5], and

upon thawing; the solid crystals of solvent generate large and

interconnected super macropores [6]. The crystalline struc-

ture of solvent molecules are responsible for the pores

occurring in a super-macroporous and sponge-like polymeric

interconnected network [7,8]. High porosity, super macropore

structures up to sizes of few hundreds of microns, high me-

chanical strength, and elasticity provide cryogels with great

advantages over hydrogels [9e11]. Cryogels can be used

instead of hydrogel in many places where these sponge-like

materials also have great potential applications as template

for inorganic material preparation, biotechnology, tissue en-

gineering [12], energy applications [13,14], and as drug delivery

devices in pharmaceuticals [15,16].

H2 is one of the most abundant elements on the earth, and

the use of hydrogen as a clean energy carrier is attracting great

attention recently as a promising alternative energy to con-

ventional fossil fuels [17,18]. The consideration of H2 as energy

material is unavoidable due to the increasing energy demands

of the world, as H2 can also be generated from sustainable and

renewable sources such as water, plants and biomass [19].

Recently, chemical H2 storage materials such as NaBH4,and AB, etc. with high hydrogen contents have been regarded

as promising hydrogen sources for fuel cells [20]. NaBH4 is the

most commonly studied borohydride compound because of

its high hydrogen storage capacity (as high as 10.8 wt%) and

the great stability of its solution even in high pH range [21,22].

NaBH4 generates hydrogen from a hydrolysis reaction occur-

ring according to the following reaction (Eq. (1)) [23e25].

NaBH4 2H2O Catalyst! NaBO2 4H2 (1)In the above reaction, various metals such as Co, Ni, Cu,

Ru, Pd, Pt and their alloys can be utilized as catalysts [26,27].

The reaction byproduct sodium metaborate (NaBO2) can be

recycled to produce NaBH4. Among the various reproduction

methods such as mechanical, electrical and thermochemical

processes, and H2 production even at low temperatures can be

possible with exothermic hydrolysis reactions [28e30]. The

thermochemical reaction is represented as follows (Eq. (2)):

NaBO2

s

2H2

g

xRe Catalyst! NaBH4 RexO2

l

(2)In this reaction, Re indicates the reducing agent such as

active metals (Mg, Al, Ca, etc.) and metal hydrides (MgH2, etc)

[28].

Anhydrous liquid NH3 and BH3 were reacted under pres-

sure to obtain NH3BH3 as resource for fuel cell applications

and it has high solubility in water [31]. NH3BH3 is stable in

aqueous solution at room temperature [32]. The hydrolysis

reaction of ammonia borane occurs according to Eq (3) [33]:

NH3BH3 2H2O Catalyst! NH4 BO2 3H2 (3)

This reaction is also catalyzed with many metal-based

catalysts, such as Co, Ni, Cu and their nanoparticles.

Currently the catalytic hydrolysis of AB with different for-

mulations of metal catalysts is being investigated [33e35].

Herein, p(SPM) cryogels with highly ionizable character

were prepared under cryogenic conditions, and used as tem-

plate in Co, Ni, and Cu nanoparticle preparation, then in the

catalysis of NaBH4, and AB for H2 generation. Various pa-

rameters affecting H2 production performance and different

reaction media were tested to improve H2 generation.Experimental

Materials

The monomer, 3-sulfopropyl methacrylate potassium salt,

(SPM, 98%, Aldrich), 1,4-dibromobutane (98%, Merck) and 1-

vinylimidazole (VI, 99%, Aldrich) as cross-linker, potassium

persulfate (KPS, 99%, SigmaeAldrich) as initiator; and

N,N,N0,N0-tetramethylethylenediamine (TEMED, 99%, Merck)as accelerator were used in p(SPM) cryogen preparation. Co-

balt (II) sulfate heptahydrate (CoSO4.7H2O, 99%, Merck), nickel

(II) sulfate heptahydrate (NiSO4.7H2O, 99%, SigmaeAldrich),

and copper (II) sulfate pentahydrate (CuSO4.5H2O, 99%, Sig-

maeAldrich) were used as metal sources, and sodium boro-

hydride (NaBH4, 98%, Merck) was used as reducing agent and

hydrogen (H2) source. Also, ammonia borane (NH3BH3, 97%,

Aldrich) was used as chemical hydride for hydrolysis re-

actions. Hydrogen chloride (HCl, 37%, Merck), methanol

(99.9%, SigmaeAldrich) and acetone (99.8%, SigmaeAldrich)

were used as received. NaOH (97%) was used to form the basic

reaction medium for reactions and 18.2 MU cm DI water was

used in all experiments.

Synthesis of p(SPM) cryogels

The synthesis of p(SPM) cryogels were carried out by a cry-

ogellation technique via free radical polymerization under

freezing conditions. In brief, 0.2 g (0.8119 mmol) SPM and

0.0443 g crosslinker (13.5 mol% with respect to monomer)

were dissolved in 3ml DIwater and 50 ml TEMED as accelerator

was added, vortex mixed then cooled in an ice bath for five

min. The initiator, 0.8 ml of aqueous KPS solution (1 mol%

with respect to monomer), was mixed with hydrogel pre-

cursors until a homogenous solution was obtained. Then, this

solution was placed into plastic straws of 8 mm in diameter,

and put in a freezer at 18 C. The cryogellation reactionproceeded for 24 h. The obtained cryogels were cut into 1 cm

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i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 1 4 6 9 0e1 4 7 0 014692lengths, and washed with DI water exhaustively for one day

changing the wash water every 8 h. The synthesized cryogels

were stored in DI water for further use.

Synthesis of Co, Ni and Cu nanoparticles within p(SPM)cryogel network

The metal nanoparticles within p(SPM) cryogel matrices were

synthesized by in situ reduction of metal ions within the

cryogels. Thewashed p(SPM) cryogels weighing about 2 g were

treated with 0.5 M 500 ml NaOH for two hours and washed

with DI water overnight, and then placed in 500 ml 1000 ppm

metal ion solutions (Co(II), Ni(II) and Cu(II)), by stirring at

1000 rpm at room temperature for 15 h to load the metal ions

into the cryogels. The metal ion-loaded p(SPM) cryogels were

washed with DI water for 2 h to remove unbound metal ions

and then treated with 50 ml 0.4 M NaBH4 for 1 h to obtain the

corresponding nanoparticles of the loaded metal ions in situ.

Again, after washing p(SPM)-M (M: Co, Ni and Cu) composite

cryogels in DI water a few times, they were dried at 50 C for15 h. The hydrogel composite systems containing in situ

synthesized Co, Ni and Cu were used for the hydrolysis of

NaBH4 and AB [2,36]. Multiple Co(II) loading and reduction

cycles of p(SPM) cryogel were carried out by using the same

metal ions and reduction concentration repetitively; up to five

times to increase the nanoparticle content in p(SPM) cryogels.Fig. 1 e (a) Representation of p(SPM) cryogel preparation with div

of metal ion-loaded p(SPM) cryogels, and their metal nanoparticl

with their TEM images.Characterization

The porosity of p(SPM) cryogels were visualized by optical

microscopy (Olympus bX53) and SEM (Jeol JSM-5600 LV) with

an operating voltage of 20 kV. The functional groups on the

prepared p(SPM) cryogels were analyzed with FT-IR spec-

troscopy (Thermo, Smart iTR) using ATR apparatus. To

determine the amounts of Co, Ni and Cu nanoparticles

within p(SPM)-M cryogels, the metal nanoparticle-

containing cryogel composites were treated with 50 ml 5 M

HCl solution for 15 h repetitively three times to dissolve

metal nanoparticles, and the eluted metal ion solution was

diluted at the ratio of 1/100 with DI water. Then the amounts

of metal ions were measured by atomic absorption spec-

troscopy (AAS, Thermo, ICA 3500 AA SPECTRO). The metal

nanoparticles within p(SPM) cryogels were imaged with TEM

(JEOL 2010, Japan) in a vacuum with an operating voltage of

200 kV.

Catalytic hydrolysis of NaBH4 and NH3BH3 catalyzed byp(SPM)-metal cryogel composites

The catalytic performances of metal nanoparticle-

containing p(SPM) cryogel composites were determined by

testing them in the hydrolysis of NaBH4 and NH3BH3 and by

measuring the generated H2. All the hydrolysis reactionsinylimidazole crosslinker, and (b) the digital camera images

e-containing forms as p(SPM)-M (M: Cu, Co, and Ni) cryogels

http://dx.doi.org/10.1016/j.ijhydene.2014.07.035http://dx.doi.org/10.1016/j.ijhydene.2014.07.035

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 1 4 6 9 0e1 4 7 0 0 14693were performed by using 50 mM 50 ml (0.0965 g) NaBH4solution containing 5 wt% NaOH, or 50 mM (0.0770 g) AB

with cryogel composites containing various amounts of

metal nanoparticles, in a 100 ml reaction flask. The hydro-

lysis reactions were carried out at 30 C, unless otherwisestated, and 1000 rpm mixing rate. All the experiments were

repeated three times, and the results are given as mean

values with standard deviations. The metal nanoparticle

content of p(SPM) cryogel was increased by multiple load-

ings and these cryogels were used in hydrolysis reactions.

To determine the activation parameters, the hydrolysis re-

action was completed at 30, 40, 50, and 70 C by using 0.15 gp(SPM)-Co catalyst in the hydrolysis of NaBH4 and AB under

the described reaction conditions. Different NaBH4 solutions

were prepared using different waters such as sea, tap and DI

water with different ionic strengths and used in NaBH4 hy-

drolysis using p(SPM)-Co cryogel composite catalyst

systems.Fig. 2 e (a) The optic microscope and SEM image of p(SPM) cryo

crosslinked p(SPM) cryogel.Results and discussion

Synthesis, and characterization of p(SPM)-M cryogelcomposites and their use in H2 production from chemicalhydride

It is known that hydrogels can be utilized as template for

metal nanoparticle preparation and in the catalysis of

different chemical reactions [1,2,21e26]. Super macroporous

cryogels can also be used as template for metal nanoparticle

preparation such as Co, Ni, and Cu within the super macro-

porous structure, and then as catalyst for H2 production from

hydrolysis of NaBH4 and AB [43]. Here, we report p(SPM) cry-

ogel preparation with a newly formed crosslinker prepared

from 1,4-dibromobutan and VI as crosslinker as illustrated in

Fig. 1(a). As shown in digital camera images in Fig. 1(b), the

p(SPM)-M cryogel assumes the color of the metal ions, andgel, and (b) FT-IR spectra of SPM monomer and di-VI

http://dx.doi.org/10.1016/j.ijhydene.2014.07.035http://dx.doi.org/10.1016/j.ijhydene.2014.07.035

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 1 4 6 9 0e1 4 7 0 014694turns black as an indication of the formation of metal nano-

particles. Metal nanoparticle-containing p(SPM) cryogels' TEMimages are also illustrated in Fig. 1(a). As can be seen themetal

nanoparticles within p(SPM) cryogels are evenly distributed

with a size of about few hundred nm in p(SPM)-M (M: Co, Ni,

and Cu) composite cryogels.

The optical microscope and SEM images of p(SPM) cryogels

are shown in Fig. 2(a). As can be seen from both images the

pore sizes of p(SPM) cryogel are a few tens of micrometers.

The FT-IR spectra of SPM monomer and di-VI crosslinked

p(SPM) cryogels were taken and the stretching frequencies of

functional groups were confirmed as shown in Fig. 2(b). The

specific vibrations for SPM monomer were determined at

2961 cm1 belonging to aliphatic eCH, 1716 cm1 for C]O,1178 cm1 for eSO3 asymmetric stretching vibrations, and1009e1036 cm1 for S]O stretching vibrations. The specificbands for the crosslinker were assigned as 3082 cm1 for eCHstretching in the ring, and 2979e2924 cm1 for aliphatic eCHvibrations. The band at about 1450 cm1 is assigned to eCNstretching in the ring, and 1157 cm1 for CeNeC asymmetricstretching vibrations [37]. p(SPM) cryogels crosslinked with di-

VI also produced these typical bands, 3082 cm1 for ringstretching of eCH, the 2979 cm1 vibrations for aliphatic eCH,

1716 cm1 for eC]O, 1458 cm1 for eCN in the VI ring,1178 cm1 for SO2 asymmetric stretching, and1036e1009 cm1 for S]O stretching frequencies [38,39].

To determine the exact amount of metal in p(SPM)-M

composite cryogels, the metal nanoparticles were dissolved

by three rounds of treatment with 50 ml 5 M HCl and the

amounts of metal ions in the total eluted solutions were

determined by AAS measurements. As given in Table 1, the

amounts of Co (II), Ni(II) and Cu(II) ions were 69.05, 54.58 and

82.05 mg per g dried p(SPM) cryogels, respectively. To increase

the amount of metal nanoparticles, as the catalytic reaction

rates are directly related to amount of metal nanoparticles,

Co(II) ions were loaded and reduced in p(SPM) cryogels up to 5

times. As shown in Table 1, the Co content of p(SPM)-Co

composite cryogel increased to 418.1 12.64 from69.05 3.06 mg g1 Co ions at end of the 5th cycle. Theamounts of Co ions after two, three, and four Co loading-

reduction cycles in p(SPM)-Co cryogels were found as

149.4 31.2, 253.2 7.4, and 328.8 9.2 mg g1 Co ions,respectively. It is obvious that the metal nanoparticle content

of p(SPM) cryogel can be increased by multiple loading and

reduction cycles of the desired metal ion, therefore, this cry-

ogel system is versatile and has great potential in different

advanced catalytic applications.Table 1 e The amount of metal nanoparticles withinp(SPM)-M (M:Co, Ni, and Cu) cryogel composite catalystmeasured with AAS.

Cryogel composite catalysts AAS (mg/g)

p(SPM)-Co 1st loading 69.1 3.1p(SPM)-Ni 54.2 3.6p(SPM)-Cu 82.1 1p(SPM)-Co 2nd loading 149.4 31.2p(SPM)-Co 3rd loading 253.2 7.4p(SPM)-Co 4th loading 328.8 9.2p(SPM)-Co 5th loading 418.1 12.6The effect of metal species on H2 generation from thehydrolysis of NaBH4 and NH3BH3

To determine the effect of the metal species on hydrogen

generation from the hydrolysis of NaBH4 and AB, equal

amounts of metal nanoparticles (0.175 mmol) within p(SPM)

cryogel (M: Co, Ni, Cu each 0.175 mmol) were utilized under

the same hydrolysis conditions. To make sure the number of

moles of metal nanoparticles are equal within p(SPM) cryogels

different amounts of p(SPM)-M were used, e.g., 0.15 g p(SPM)-

Co and 0.1905 g p(SPM)-Ni cryogel composites which both

contain 0.175 mmol Co and Ni were utilized in the hydrolysis

of 50 mM 50 ml aqueous NaBH4 solution with 5 wt% NaOH at

30 C and 1000 rpm mixing rate. As illustrated in Fig. 3(a),250 ml H2 gas was generated with p(SPM)-Co cryogels in about

30 min, whereas the equivalent amount of H2 gas was gener-

ated with p(SPM)-Ni cryogels at about the same time 30 min.

From the literature, it is well known that Co metal nano-

particles' catalytic performances are better thanNi particles inNaBH4 hydrolysis [1,2,17,22,25,26]. Therefore, p(SPM)-Co

composite performs better than p(SPM)-Ni cryogel composite

catalyst system for NaBH4 and NH3BH3. Furthermore, we also

tested p(SPM)-M cryogel composite system in NH3BH3 hydro-

lysis; 0.15 g p(SPM)-Co, 0.1905 g p(SPM)-Ni, and 0.136 g p(SPM)-

Cu, each containing 0.175 mmol metal nanoparticlesFig. 3 e (a) The effect of metal species on NaBH4 hydrolysis

by p(SPM)eM cryogel composite [M: Co and Ni each

0.175 mmol, 50 mM 50 ml aqueous NaBH4 solution

containing 5 wt % NaOH, at 30 C, 1000 rpm]. (b) The effectof metal species on ammonia borane hydrolysis by

p(SPM)eM cryogels composite [M: Co, Ni, and Cu each

0.175 mmol, 50 mM 50 ml aqueous AB solution at 30 C,1000 rpm].

http://dx.doi.org/10.1016/j.ijhydene.2014.07.035http://dx.doi.org/10.1016/j.ijhydene.2014.07.035

Table 2 e The comparison of HGR and TOF values for the hydrolysis of NaBH4 and AB by various cryogel composite catalystsystems.

p(SPM)-M cryogel(M:Co, Ni, Cu)composite

T (C) Metal content(mmol)

Hydrogen generation rates(mL H2)(g of Co min)

1Total turn over frequency

(mol H)(mol catalyst min)1

NaBH4 AB NaBH4 AB

p(SPM)-Ni 30 0.175 896 60 1591 105 2 0.1 3.8 0.2p(SPM)-Cu 30 0.175 e 700 8.2 e 1.8 0.1p(SPM)-Co 1st loading 30 0.175 1000 53 2469 130 2.4 0.1 5.8 0.3p(SPM)-Co 40 0.175 1555 82 4321 227 3.6 0.2 10 0.5p(SPM)-Co 50 0.175 2588 131 6402 337 5.7 0.3 14.2 0.7p(SPM)-Co 70 0.175 6859 361 8642 455 15.1 0.8 18 1p(SPM)-Co 2nd loading 30 0.4 1443 107 e 3.4 0.3 ep(SPM)-Co 3rd loading 30 0.7 1576 318 e 3.7 0.7 ep(SPM)-Co 4th loading 30 1.07 2222 126 e 5.3 0.3 ep(SPM)-Co 5th loading 30 1.4 2836 90 e 6.7 0.2 e

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 1 4 6 9 0e1 4 7 0 0 14695(0.175 mmol), were utilized in the hydrolysis of 50 ml 50 mM

AB at 30 C and 1000 rpm mixing rate as shown in Fig. 3(b).Although all of these composites generated 180 ml H2 (100%

conversion), the times to generate this amount were different

e.g., it took Co, Ni and Cu nanoparticle-containing p(SPM)

cryogels, 8, 12, and 24 min, respectively. Other characteristics

such as HGR (hydrogen generation rate) and TOF (turn over

frequency) were calculated for NaBH4 and AB hydrolysis by

p(SPM)-M (M: Co, Ni and Cu) cryogel composites, and are given

in Table 2. The TOF values are the numbers of moles of pro-

duced H2 per mole catalyst per unit time (min), and HGR is

defined as volume of the produced H2 per gram of catalyst per

unit time (min). The TOF values were calculated as

2.4 0.1 mol H2 (mol catalyst)1 for p(SPM)-Co, and

2.0 0.1 mol H2 (mol catalyst min)1 for p(SPM)-Ni for NaBH4

hydrolysis, and TOF values for AB hydrolysis reaction were

calculated as 5.8 0.3 mol H2 (mol metal min)1, 3.8 0.2 and

1.8 0.1 for Co, Ni and Cu nanoparticles within p(SPM) cryogelcomposite system. The HGR for Co, Ni, and Cu-loaded cryogels

for the AB hydrolysis reaction were 2469 130, 1591 105, and700 8.2 (ml H2) (g of metal min)

1, respectively.The HGR for the NaBH4 hydrolysis reaction were calculated

as 1000 53 (ml H2) (g of Co min)1 for p(SPM)-Co and

896.0 60 (ml H2) (g of Ni min)1 for p(SPM)-Ni; again con-

firming the better catalytic performance of Co composite

catalytic systems. It is apparent that the catalytic perfor-

mances of Co and Ni nanoparticle composite systems are

better than Cu nanoparticle composite systems for AB hy-

drolysis under the same conditions. In comparison to the

literature such as Ru nanoparticles that provide faster cata-

lytic performances [25], however, the metal catalysts reported

here such as Co, Ni, and Cu a are cheaper and readily available

[2,43].Fig. 4 e (a) The effect of multiple Co(II) ion loading and

reduction cycles on H2 production, (b) the change in

hydrogen generation rate (HGR) with number of Co(II) ion

loading and reduction cycles in hydrolysis of NaBH4 by

0.15 g p(SPM)-Co cryogel composite systems [50 mM 50 ml

aqueous NaBH4 solution containing 5 wt% NaOH, at 30 C,1000 rpm].The effect of metal loading on H2 generation rates

The effect of different metal amounts created by consecutive

loading-reduction of Co ions into p(SPM) cryogels on the hy-

drolysis of NaBH4 was determined by utilizing 0.15 g p(SPM)-

Co cryogel containing 0.175 mmol Co in the hydrolysis of

50 mM 50 ml aqueous NaBH4 solution containing 5 wt% NaOH

at 30 C. The p(SPM)-Co cryogel composites were used in thehydrolysis reaction after each Co(II) ion loading-reduction

cycle. As illustrated in Fig 4(a), the 1st time metal-loaded

cryogels generated 250 ml H2 in 25 min, whereas the 2nd,

3rd, 4th, and 5th time metal ion-loaded and reduced cryogel

composite generated the same amount of H2 in about 7.6, 5,

2.6, and 1.6 min, respectively. The total TOF values of 1st, 2nd,

3rd, 4th, and 5th time metal loadings were 2.4 0.1, 3.7 0.7,3.4 0.3, 5.3 0.3, and 6.7 0.2 mol H2 (mol metal min)

1,respectively. The HGR of 1st, 2nd, 3rd, 4th, and 5th time metal

ion loaded and reduced composites were calculated as

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Fig. 5 e The effect of temperature on (a) NaBH4, and (b) AB

hydrolysis catalyzed by p(SPM)-Co cryogels containing

0.175 mmol Co at 30, 40, 50, and 70 C [50 mM 50 ml NaBH4solution containing 5 wt% NaOH, and 50 ml 50 mM AB, at

1000 rpm].

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 1 4 6 9 0e1 4 7 0 0146961000 53, 1443 107, 1576 318, 2222 126, and 2836 90 (mlH2) (g of catalyst min)

1, respectively. As shown in Fig. 4(b), asthe number of the reloading and reduction cycles of the metal

ion is increased, the HGR is also increased, and in the inset the

HGR versus metal content is also presented confirming the

increase in HGR with amount of Co(II) almost linearly in-

creases in p(SPM) cryogels with multiple loading and reduc-

tion cycles. Therefore, it is obvious that the amounts of metal

nanoparticles can be increased by multiple loading and

reduction cycles to obtain the desired H2 rates by using the

same amount of p(SPM). This is a great advantage in real ap-

plications where a greater amount of catalyst in the same

volume of template is required to attain higher amounts of H2.

So, the presentedmethod here is versatile with great potential

in many applications.

The effects of temperature on the hydrolysis reaction

To examine the effect of temperature on NaBH4 and AB hy-

drolysis, the reactions were carried out at 30, 40, 50 and 70 C.The temperature was fixed by a thermocouple probe-

controlled oil bath. The hydrolysis reactions were performed

using 0.15 g p(SPM)-Co composite system containing

0.175 mmol Co under the standard reaction concentrations

(50mM50ml NaBH4with 5wt%NaOHor ABwith noNaOH). At

every temperature, 250 ml hydrogen gas was generated as

illustrated in Fig. 5(a). As the temperature was raised from 30

to 70 C the hydrolysis reaction time reduced from 25 to3.8 min for the NaBH4 hydrolysis reaction and from 6 to

2.2 min for the AB hydrolysis as demonstrated in Fig. 5(b).

Additionally, as illustrated in Table 2, the TOF and HGR values

increased for p(SPM)-Co catalyzed NaBH4 and AB hydrolysis at

30, 40, 50, and 70 C. The TOF values were calculated as2.4 0.1, 3.6 0.2, 5.7 0.3, and 15.1 0.8 mol H2 (mol metalmin)1 for NaBH4 hydrolysis, and 5.8 0.3, 10.0 0.5, 14.2 0.7,and 18.0 1.0 mol H2 (mol metal min)

1 for AB hydrolysis,respectively, at reaction temperatures of 30, 40, 50, and 70 C.The HGR at 30, 40, 50, and 70 C for the NaBH4 hydrolysis re-action were calculated as 1000 53, 1555 82, 2588 131, and6859 361 (ml H2) (g of metal min)

1, and for AB these valueswere 2469 130, 4321 227, 6402 337, and 8642 455 (ml H2)(g of metal min)1, respectively. According to the results, the

increase in the temperature increases the HGR and total TOF

values for both NaBH4 and AB hydrolysis reactions, and this

increase is more pronounced for AB hydrolysis than NaBH4hydrolysis. Comparing the TOF and HGR values of p(SPM)

cryogels with the literature, they provide some advantages

over the common hydrogels, for example, p(SPM)-Co com-

posite cryogels' TOF and HGR are 2.4 mol H2 (mol metal min)1

and 1000 (ml H2) (g of metal min)1, whereas p(AMPS)-Co

microgels' TOF and HGR values were reported as 0.34 mol H2(mol metal min)1 and 113.4 (ml H2) (g of metal min)

1. Simi-larly, macro p(AAGA)-Co composites' TOF and HGR valueswere reported as 1.88 mol H2 (mol metal min)

1 and 799 (ml

H2) (g of metal min)1 [22,40]. Interestingly, the TOF and HGR

values of p(SPM) cryogel, micro and macro hydrogels are not

very different e.g., p(SPM)-Co microgels TOF and HGR values

were calculated as 2.25 mol H2 (mol metal min)1 and 966 (ml

H2) (g of metal min)1, and these values are 2.25 mol H2 (mol

metal min)1 and 953 (ml H2) (g of metal min)1 for macrop(SPM)-Co hydrogels which are about the same [17,26]. Ac-

cording to results, the increase in the temperature increases

the TOF and HGR values for both NaBH4 and AB hydrolysis

reactions. From the Table 2, the catalyst performance of

p(SPM)-Co cryogels at 70 C shows higher activity than 3 timesmetal loaded-reduced cryogel p(4-VP)-Co catalyst in the liter-

ature [43].

To determine the activation parameters of the p(SPM)-Co

catalyzed hydrolysis reaction of NaBH4 and AB, lnk-(1/T) and

ln(k/T)-(1/T) graphs were constructed using the Arrhenius (Eq.

(4)) and Eyring (Eq. (5)) equations, respectively.

lnk lnA (Ea/RT) (4)

ln(k/T) ln(kB/h) (D#S/R) (D#H/R)(1/T) (5)

where, k is the rate constant, Ea is the activation energy, T is

the temperature, kB is the Boltzmann constant

(1.381 1023 J K1), h is the Planck constant (6.626 1034 J s),activation enthalpy, DH#, and entropy, DS#, and R is the gas

constant (8.314 J K1 mol1). The lnk-(1/T) graph, and ln(k/T)-(1/T) graphs were constructed and are illustrated in Fig. 6(a)

and (b), respectively. The activation energies for NaBH4 andAB

hydrolysis were calculated as 40.8, and 25.63 kJ (mol)1. TheseEa values are approximately the same value, 41.67 kJ mol

1 forbulk p(SPM)-Co [26], and also better than some of the studies

http://dx.doi.org/10.1016/j.ijhydene.2014.07.035http://dx.doi.org/10.1016/j.ijhydene.2014.07.035

Fig. 6 e The effect of temperature on the hydrolysis of

NaBH4, (a) lnk e 1/T(Arrhenius equation), and (b) ln k/T e 1/

T(Eyring equation). [Reaction conditions: p(SPM)-Co

containing 0.175 mmol, 50 mM 50 ml NaBH4 with 5 wt %

NaOH at 30, 40, 50, and 70 C, 1000 rpm].

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 1 4 6 9 0e1 4 7 0 0 14697with similar catalyst systems [2,22]. The activation enthalpy

and activation entropy for the hydrolysis reaction of NaBH4 by

p(SPM)-Co cryogel composite catalyst system were calculated

as 37.23 kJ (mol K)1 and 170.87 J (mol K)1, respectively, asgiven in Table 3. The activation enthalpy and activation en-

tropy of the AB hydrolysis reaction catalyzed by p(SPM)-Co

cryogel catalyst system were calculated as 22.41 kJ (mol K)1

and 182.8 J (mol K)1, respectively. Activation energy, acti-vation enthalpy, and activation entropy of p(SPM)-Co cryogels

composite are comparable to some other types of hydrogels,

microgel, and nanogel, etc. For example, the activation en-

ergies, enthalpy and entropy for NaBH4 hydrolysis catalyzedTable 3 e The activation parameters and rate constants of thecomposite systems at different temperatures.

Compound T (C) k (min)1 Ea kJ (mol)

NaBH4p(SPM)-Co 30 0.001 40.781

40 0.001

50 0.003

70 0.0071

AB

p(SPM)-Co 30 0.001 25.634

40 0.001

50 0.003

70 0.0071by p(VI)eCo microgel are 37.578 kJ (mol)1, 34.146 kJ (mol K)1,

and 191.22 J (mol K)1; for p(VI)eCo capsules are 24.6 kJ(mol)1, 21.45 kJ (mol K)1, and161.59 J (mol K)1; and for p(4-VP)-Co colloidal ionic liquid particles are 43.98 kJ (mol)1,40.38 kJ (mol K)1, and 178.22 J (mol K)1 [39e41]. The acti-vation parameters for AB hydrolysis catalyzed by p(VI)eCo

capsule particles were reported as 51.58 kJ (mol)1, 47.85 kJ(mol K)1, and 147.46 J (mol K)1 [42], and these values are25.6 kJ (mol)1, 22.4 kJ (mol K)1, and 182.8 J (mol K)1 forp(SPM)-Co cryogel composite systems. Therefore, it can be

assumed that p(SPM)-M cryogel systems have better or at least

comparable catalytic performances to similar studies in the

literature.

The effects of water on NaBH4 hydrolysis reactions

To determine the effect of the water that is used in the prep-

aration of NaBH4, different water sources such as sea, tap and

DI water were used in the preparation of NaBH4 and tested in

terms of H2 generation volume vs. time as illustrated in

Fig. 7(a). As can be seen interestingly H2 production under the

same conditions with sea water was much better than DI

water, and very close to tap water. This is an important

outcome of this investigation, as there is no need to use DI

water for potential real applications of NaBH4 hydrolysis,

seawater or tap water can be directly used in H2 production

with better performances. The increase in the catalytic per-

formance of p(SPM)-Co cryogel system could be due to the

presence of different salts that may prevent NaBO4 formation

on the metal catalyst surfaces. Therefore, to determine the

salt effect on NaBH4 hydrolysis, we also carried out the hy-

drolysis reaction of NaBH4 in the presence of different salts

such as 0.1 M CaCl2, NaCl, and KCl, and in NaCl solutions with

different ionic strength (0.01, 0.1 and 1 M) as given in Fig. 7(b)

and in the inset. All the hydrolysis reactions carried out in the

presence of NaCl produce about 250 ml H2 independent of

NaCl concentration, whereas the hydrolysis reaction carried

out in KCl and CaCl2 produces about 233 ml H2. It is apparent

that NaCl provides better catalytic performances than CaCl2and KCl, and the hydrolysis reaction is not affected by the

ionic strength of NaCl as given in the inset i.e., even 100 fold

more concentrated NaCl solution has no significant effect on

H2 generation rate. The reduction of H2 production rate with

KCl and CaCl2 can be attributed to the bigger size of K in

comparison to Na, and dual charge of Ca2 in comparison toNaBH4 and AB hydrolysis catalyzed by p(SPM)-Co cryogel

1 DH# kJ (mol)1 DS# J (mol k)1 R2

37.2261 170.87 0.9790.981

0.998

0.885

22.407 182.8 0.8740.946

0.959

0.950

http://dx.doi.org/10.1016/j.ijhydene.2014.07.035http://dx.doi.org/10.1016/j.ijhydene.2014.07.035

Fig. 7 e The effect of (a) different water (b) different salts

and the inset different concentration of NaCl solution on

the hydrolysis of NaBH4 by 0.15 g p(SPM)eCo cryogel

composite [50 mM 50 ml aqueous NaBH4 solution

containing 5 wt % NaOH, at 30 C, 1000 rpm].

Fig. 8 e The % conversion and activity of the p(SPM)-Co

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 1 4 6 9 0e1 4 7 0 014698Na ions. Calcium ions can be captured by sulfuric acid inp(SPM) cryogels and K may also be kept closer than Na to

cryogel-composite catalyst systems hindering the catalytic

activity of p(SPM)-Co cryogel composites.

cryogel composites in repetitive use (a) at 30 C (b) at 70 Cand (c) 3rd time Co(II) ion loaded and reduced p(SPM)-Co

composite catalyst system at 30 C [50 ml 50 mM NaBH4containing 5 wt % NaOH, 1000 rpm].The reusability of p(SPM)-Co cryogel in the hydrolysisreaction

To examine % conversion and the % activity of p(SPM)-Co

cryogel, they were utilized five times repeatedly in hydroly-

sis of NaBH4 reaction consequently in one day at different

temperatures. Conversion is calculated based on the theo-

retical value according to reaction (1) and activity is calculated

taking the initial H2 production rate to that of every use. As

illustrated in Fig. 8(a), 100% conversion and 100% activity was

obtained after each successive use for NaBH4 hydrolysis car-

ried out at 30 C catalyzed by p(SPM)-Co cryogel compositesystem. To determine the effect of reaction temperature on

the catalytic performance of p(SPM)-Co cryogel composite

system, the hydrolysis reactions were carried out at 70 C fivetimes consecutively, again as shown in Fig. 8(b); for each use

100% conversion and 100% activity were obtained. Moreover, 3

time Co(II) ion loaded and reduced p(SPM)-Co cryogel systems

that contain more Co metal nanoparticles (1st loading 69.1,

and 3rd loading 418.1 mg per gram cryogel) were also testedfor their reusability at 30 C as illustrated in Fig. 8(c). Again it isapparent that repetitive use, even with higher amounts of Co

in (SPM)-Co composite systems, does not alter the conversion

and catalytic activity of the catalyst systemas every time 100%

conversion and 100% activity is obtained. These results also

confirm that the p(SPM)-Co composite cryogel is better or su-

perior to most similar hydrogel metal composite systems.Conclusion

Supermacroporousp(SPM)cryogelswere synthesizedandused

as template in the preparation of Co, Ni, and Cu nanoparticles

and then inH2 generation from thehydrolysis ofNaBH4 andAB.

It was found that Co nanoparticle-containing p(SMP) cryogel

http://dx.doi.org/10.1016/j.ijhydene.2014.07.035http://dx.doi.org/10.1016/j.ijhydene.2014.07.035

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 1 4 6 9 0e1 4 7 0 0 14699composite catalyzes thehydrolysis reactions faster thanNi and

Cu. It was also demonstrated that the metal nanoparticle con-

tentofp(SPM)cryogels canbe increasedbymultiple loadingand

reduction cycles to obtain fast HGR e.g., 2836 (mL H2)(g of

Comin)1 for NaBH4 hydrolysis. Additionally, theHGR and TOFvalues for both hydrides were found to be highly temperature

dependent. Themild and comparable valueswith the literature

for activation energy, enthalpy, and entropy for the hydrolysis

reactions of NaBH4 and AB catalyzed by p(SPM)-Co catalyst

systemwere calculated as 40.8 kJ (mol)1, 37.2 kJ (mol K)1, and170.9 J (mol K)1, and 25.03 kJ (mol)1, 22.41 kJ (mol K)1, and182.8 J (molK)1, respectively.More importantly, itwas shownhere that p(SPM)-Co cryogel composite catalyst system per-

formed better in sea water and tap water than in DI water in

terms of fast H2 generation from NaBH4 hydrolysis, offering

great opportunity for direct use of these waters for NaBH4 so-

lution preparation in real applications. Also, the ionic strength

ofNaClwas foundnot to affect the hydrolysis ofNaBH4. Finally,

it was shown here that p(SPM)-Co cryogel composite system

can be used up to five times at 30 and 70 C without any loss ofcatalytic activity and conversion. To sum up, p(SMP)-M cryogel

composite system is versatile in the generation of H2 from

different aquatic environments and has great potential for

tunable advanced catalytic usage.r e f e r e n c e s

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Metal nanoparticle-embedded super porous poly(3-sulfopropyl methacrylate) cryogel for H2 production from chemical hydride h ...IntroductionExperimentalMaterialsSynthesis of p(SPM) cryogelsSynthesis of Co, Ni and Cu nanoparticles within p(SPM) cryogel networkCharacterizationCatalytic hydrolysis of NaBH4 and NH3BH3 catalyzed by p(SPM)-metal cryogel composites

Results and discussionSynthesis, and characterization of p(SPM)-M cryogel composites and their use in H2 production from chemical hydrideThe effect of metal species on H2 generation from the hydrolysis of NaBH4 and NH3BH3The effect of metal loading on H2 generation ratesThe effects of temperature on the hydrolysis reactionThe effects of water on NaBH4 hydrolysis reactionsThe reusability of p(SPM)-Co cryogel in the hydrolysis reaction

ConclusionReferences