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Metal ion-imprinted hydrogel with magneticproperties and enhanced catalytic performances inhydrolysis of NaBH4 and NH3BH3

Fahriye Seven a, Nurettin Sahiner a,b,*aCanakkale Onsekiz Mart University, Faculty of Sciences and Arts, Chemistry Department, Terzioglu Campus,

17100 Canakkale, TurkeybNanoscience and Technology Research and Application Center (NANORAC), Terzioglu Campus, 17100 Canakkale,

Turkey

a r t i c l e i n f o

Article history:

Received 2 June 2013

Received in revised form

26 August 2013

Accepted 14 September 2013

Available online 9 October 2013

Keywords:

Metal ion-imprinted hydrogels

Hydrogen production technologies

Controlled hydrogen production via

magnetic hydrogel-nanocatalyst

system

Chemical hydride hydrolysis

* Corresponding author. Canakkale Onsekiz17100 Canakkale, Turkey. Tel.: þ90 28621800

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

0360-3199/$ e see front matter Copyright ªhttp://dx.doi.org/10.1016/j.ijhydene.2013.09.0

a b s t r a c t

Metal ion-imprinted (IIH) poly(2-acrylamido-2-methyl-1-propansulfonic acid) p(AMPS)

hydrogels were prepared by using a free-radical polymerization technique in the presence

of metal ions (M ¼ Co (II) or Ni (II)). Using metal ion-imprinted hydrogels (IIHs), and non-

metal ion-imprinted (NIH) hydrogels as template for the preparation of Co and Ni cata-

lyst systems, the hydrolysis kinetics of NaBH4 and NH3BH3 were investigated. The catalytic

performances of IIHs and NIHs were compared in terms of effect on hydrolysis kinetics of

NaBH4 and NH3BH3. To increase the amounts of Co nanoparticles within p(AMPS) hydrogel

for better catalytic activity, several reloading and reduction cycles of Co (II) ions were

carried out, and the prepared p(AMPS)-Co composite catalyst systems were tested for

hydrogen generation from the hydrolysis of NaBH4. As the number of Co (II) loading and

reduction cycles increased, the amount of metal catalysts and the catalytic performance of

composites increased. Kinetics studies were carried out on three times Co (II) ion loaded

and reduced p(AMPS)-Co catalyst systems (containing 36.80 mg/g Co). Three time Co (II)-

loaded catalyst systems provided very fast hydrolysis kinetics for NaBH4, and provided

magnetic field responsive behavior. The hydrolysis reaction of NaBH4 was completed

within 50 s, under the described conditions at 60 �C. It was demonstrated that the syn-

thesized catalyst systems can be used ten times repetitively without significant loss of

catalytic activity (86.5%).

Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

1. Introduction useful techniques to generate specific binding sites within

Imprinting technologies such as molecular imprinting,

metal ion imprinting and macromolecular imprinting are

Mart University, Faculty18x2041; fax: þ90 286218. Sahiner).

2013, Hydrogen Energy P76

imprinted polymers. Metal ion-imprinted polymers (IIPs) are

composed of a polymer matrix template with target metal

ions through non-covalent interactions. Upon the removal

of Sciences and Arts, Chemistry Department, Terzioglu Campus,1948.

ublications, LLC. Published by Elsevier Ltd. All rights reserved.

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 8 ( 2 0 1 3 ) 1 5 2 7 5e1 5 2 8 415276

of materials from these matrices by using suitable tools,

selective binding sites or cavities are formed within the

polymer networks. IIPs have various applications and are

actively being investigated for environmental analysis [1],

food analysis [2], metal analysis [3], and sensors [4].

Hydrogen energy as a cleaner fuel is the focal point of much

research because of energy problems expected to occur in

the future [5e7]. Hydrogen has many attractive features as it

can be obtained from a variety of sources (plants, water,

coal, and biomass etc), and used as a renewable energy

carrier, combined with its high power density (142 MJ/kg)

and an energy density that is higher than other fossil fuels

(47 MJ/kg) [5,6,8]. However, the storage, transportation and

practical use in industry of the produced hydrogen obtained

from various resources such as hydrocarbons [9], metal al-

loys [10], activated carbon [11], and chemical hydrides [12]

for use by different process technologies such as stream

reforming [13], water splitting [14], gasification [15], and

hydrolysis reactions from hydrides [16] still have some

fundamental problems. Borohydrides are employed in

hydrogen storage and production due to easy handling

ability and the development of a practical use for hydrogen

energy. NaBH4 is the most commonly studied borohyride

compound owing to its safe properties, and economical cost

compared with other chemical hydrides with high hydrogen

storage capacity (theoretical value is 10.8 wt%). NaBH4 gen-

erates hydrogen from a hydrolysis reaction with a suitable

catalyst in alkaline medium at room temperature according

to (Eq. (1)) [6,17e19].

NaBH4 þ 2H2O!pðAMPSÞ�MðM:Co or NiÞNaBO2 þ 4H2 þ heat[ (1)

Even though NaBH4 has many advantages, the US Depart-

ment of Energy (DOE) recommended a no-go for NaBH4 for on-

board automotive hydrogen storage applications in 2007,

based on the same problems associated with practical appli-

cations, such as low catalytic activity, and hydrogen storage

capacity that could decrease down to 2.9 wt% with the for-

mation of sodiummetaborate (NaBO2), dependency on excess

amount of water in aqueous solution, and the solubility lim-

itation of NaBH4 [20]. A few different pathways could be used

to solve these problems. Reproduction of NaBH4 from

byproduct NaBO2 could be provided by use of variousmethods

such as mechanical, electrical and thermochemical processes

in reaction medium going back to hydrides from sodium

metaborate [21e23].

NaBO2ðsÞ þ 2H2ðgÞ þ xReðsÞ/NaBH4ðsÞ þ RexO2 (2)

In this equation, Re represents the reducing agents such as

active metals e.g., Mg, Ca, Na, Al, etc., or metal hydrides i.e.,

MgH2, CaH2, and so on. Another method reported by Shafir-

ovich [24] found that milled solid-state NaBH4/Ru-based

catalyst composite has a gravimetric hydrogen storage ca-

pacity as high as 7.3 wt% when a limited amount of water is

added.

Solubility of NH3BH3 (AB) in water is high (33.6 g AB/100 g

water at room temperature), and high stability in air with high

hydrogen capacity (19.6 wt%), different from other chemical

hydrides, could provide additional advantages. Hydrolysis

reaction of AB is described in Eq. (3) [25].

NH3BH3 þ 2H2O!pðAMPSÞ�MðM:Co or NiÞNH4

þ þ BO2� þ 3H2 heat[ (3)

Therefore, in this investigation we report both hydrolysis

reactions of sodium borohydride and ammonium borane by

using non-imprinted p(AMPS), and metal ion-imprinted

p(AMPS) providing magnetic properties. The use of IIHs for

hydrogen generation is a novel concept to increase the per-

formance of catalyst systems. Here, we demonstrate that the

capability of metal nanoparticles in the same hydrogel net-

works was increased by using different methods such as

metal ion imprinting and reloading cycles, as well as

providing additional benefits such as magnetic responsive-

ness and higher activity up to 86.5% over 10 consecutive uses.

2. Materials and methods

2.1. Materials

The monomer, 2-acrylamido-2-methyl-1-propansulfonic

acid (AMPS) (50 wt%. SigmaeAldrich), the crosslinker, N,N0-methylenebisacrylamide (MBA, 99%, Acros), the initiator

ammonium persulfate (APS, 99%, SigmaeAldrich), and the

accelerator N,N,N0,N0-tetraethylmethylenediamine (TEMED,

98% Acros) were used in hydrogel preparation. NiCl2$6H2O

(97%, SigmaeAldrich), CoCl2$6H2O (99%, SigmaeAldrich),

CuCl2 (99%, Acros), FeCl2$4H2O (SigmaeAldrich), and

FeCl3$6H2O (99%, Acros) were used as metal ion sources. So-

dium borohydride (NaBH4, 98%, Merck) and ammonium

borane (NH3BH3) were used as chemical hydrides for hydro-

lysis reactions. NaOH (97%) was used to form the basic reac-

tion medium for reactions and 18.2 M U cm DI water was used

in all experiments.

2.2. Preparation of the non-imprinted p(AMPS)hydrogels

Non-imprinted (NIH) p(AMPS) hydrogels were prepared by

free-radical polymerization of AMPS. The solution of 10 ml

(0.03 mol) AMPS was crosslinked with 0.0077 g MBA (0.25%

based on the monomer amount) in the presence of 0.0465 g

APS (1 mol% of total monomer), and 2 ml TEMED, and placed

in plastic straws at room temperature for 12 h. Upon solidi-

fication, hydrogels were cut into equal sizes, cleaned with

plenty of water by washing for 12 h, and dried in an oven at

45 �C.

2.3. Preparation of metal ion-imprinted p(AMPS)

IIHs were prepared by mixing various metal ions with AMPS

in aqueous environments, then carrying out free-radical

polymerization. For this purpose, 10 ml (0.03 mol) AMPS and

metal ions at 1:2 mole ratios (M:Co (II) or Ni (II)) were mixed

with 0.0167 g MBA (0.25% of the total monomer), 0.101 g APS

(1% of the total monomer) and 4 ml TEMED, at room temper-

ature. The mixture was placed in plastic straws of 0.25 cm

radius, and the reaction continued for 12 h. Then IIHs were

cut to equal cylindrical dimensions (w0.5 cm in length),

cleaned with plenty of distilled water for 12 h, and dried in an

oven at 45 �C.

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 8 ( 2 0 1 3 ) 1 5 2 7 5e1 5 2 8 4 15277

2.4. Preparation of magnetic p(AMPS) hydrogelsthrough metal ion-imprinted hydrogels

Metal ions were released from 100 mg IIHs by treatment with

100 ml 5 M HCl, and then washed with 100 ml 0.05 M NaOH to

remove excess protons coming fromHCl. Then as described in

a previous study [26], hydrogels were placed in an aqueous

metal ion solution containing 1:2 mole ratio of Fe (II)

(0.05 M):Fe (III) (0.1 M) ion mixture in 100 ml aqueous solution

for 12 h. Upon loading these ions into the network, these

hydrogels were washed with DI water to remove unbound

and/or physisorbed metal ions for 1 h. The iron ion laden

hydrogels were transferred into 100 ml 0.5 M sodium hy-

droxide solution to generate magnetic particles within

the hydrogel network, and again cleaned with distilled water

for 5 h.

Fig. 1 e (a) Digital camera images of 0.25% crosslinked bare, Co

removal of metal ions from 0.25% crosslinked IIH-p(AMPS) hyd

the p(AMPS) network.

2.5. Preparation of metal nanocomposites insidehydrogel matrices as catalysts

To prepare metal catalysts, p(AMPS) hydrogels were used as

template by two different routes. In the first one, 100 mg NIH

p(AMPS) hydrogels were placed in 100 ml 500 ppm metal ion

solution (M: Co (II), Ni (II) or Cu (II) ions) at 500 rpmmixing rate

at room temperature for 24 h, and the eluted metal ions were

removed from the polymeric networks by washing with

distilled water for 1 h. Thereafter, the reduction processes

were carried out with 100 ml 0.4 M NaBH4 solution for 4 h. In

the second method, the monomer, AMPS, was mixed with

metal ions in 1:2 mole ratios assuming the coordination

numbers with the metal ions (Co (II), Ni (II), and Cu (II)), and

the hydrogels formed in the presence of these metal ions.

Finally, these 100 mg IIHs (including M: Co (II) or Ni (II) ions)

(II), and Ni(II) ion-imprinted p(AMPS) hydrogels, and (b) the

rogels by HCl treatment and their schematic illustration in

(a)240

300

p(AMPS) impr. p(AMPS) discharged and reloaded impr. p(AMPS)-Co

ge

n (m

l)

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 8 ( 2 0 1 3 ) 1 5 2 7 5e1 5 2 8 415278

were reduced by treatment with 100 ml 0.4 M NaBH4 solution

for 4 h. As illustrated in Fig. 1(a), the bare p(AMPS) as (NIP-

p(AMPS)) is transparent whereas the metal ion-imprinted

p(AMPS) hydrogels assumed the color of the metal ions as

shown by Co (II)- and Ni (II)-imprinted p(AMPS) hydrogels (IIH

p(AMPS). Fig. 1(b) also depicts the removal ofmetal ions byHCl

treatment to obtain clear bare p(AMPS) hydrogels. These

hydrogels were washed with 0.05 M NaOH before metal ion

loading for in situ metal nanocatalyst preparation. For in situ

synthesis of metal nanoparticles; magnetic hydrogels were

loaded with Co (II) metal ions and were reduced with 100 ml

0.4 M NaBH4.

Cobalt (II) loaded 100 mg IIHs were also treated with 100ml

5 M HCl, and the obtained bare p(AMPS) were washed with

100 ml 0.05 M NaOH to remove protons coming from HCl for

activation. Then, these hydrogel matrices were used for Cu (II)

ion loading which differs from direct loading of Co (II) and Ni

(II) metal ions into the hydrogel networks. After Cu (II)

adsorption process, Cu metal nanoparticles were formed

again by reduction with NaBH4 in the same way.

To determine the amounts of Co, Ni, and Cu nanoparticles,

the metal nanoparticle-containing hydrogels were placed in

50 ml 5 M HCl solution for 12 h repetitively three times to

dissolvemetal nanoparticles from the polymeric network, and

the elution solution was diluted at the ratio of 1/150 with DI.

The amounts of metal ions released from the metal catalyst

were measured by Atomic Absorption Spectroscopy (AAS,

Thermo, ICA 3500 AA SPECTRO). Table 1 gives the amounts of

metal ions within the IIH p(AMPS) and NIH p(AMPS) prepared

in this investigations.

Time (min)

0

60

120

180

0 10 20 30

Vo

lu

me

o

f H

yd

ro

240

320discharged and reloded impr. p(AMPS)-Ni impr. p(AMPS)-Ni p(AMPS)-Ni

ge

n (m

l)

(b)

3. Results and discussion

3.1. Effect of various catalyst systems on hydrolysiskinetics of NaBH4 and NH3BH3

Hydrolysis of NaBH4 was performed by using 0.1 g catalyst

systems in the presence of 5 wt% NaOH within 50 ml 50 mM

NaBH4 (0.0965 g), at 30 �C and at 1000 rpmmixing rate. Fig. 2(a)

illustrates a comparison of Co nanoparticles catalytic perfor-

mances prepared in 0.1 g p(AMPS) hydrogel in three ways; 1)

Table 1 e The amounts of metal ion obtained by AASmeasurements per g dried hydrogels (mg/g) (Afternanoparticles were treated three times with 50 ml 5 MHCl).

Metal nanocomposites Amounts of metal nanoparticlesper g dried hydogels (mg/g)

IIH p(AMPS)-Co 88.5

Co-out-p(AMPS)-in-Co 131.3

NIH p(AMPS)-Co 110.9

IIH p(AMPS)-Ni 65.5

Ni-out-p(AMPS)-in-Ni 101.2

NIH p(AMPS)-Ni 91.6

Co-out-p(AMPS)-in-Cu 94.2

NIH p(AMPS)-Cu 90.2

Ni-out-p(AMPS)-in-Cu 91.9

MH p(AMPS)-Co 125.1

after hydrogel preparation and regular metal ion loading and

reduction, 2) Co (II) IIH p(AMPS) used directly in Co nano-

particle preparation, and 3) Co (II) IIH treated with HCl to

discharge Co (II), and reloaded with Co (II) and then reduced

with NaBH4 and finally used in H2 generation. Approximately

250 ml hydrogen gas was produced by all catalyst systems.

This amount of hydrogen production was completed in

different times: 30, 20 and 25 min for Co (II) IIH p(AMPS) (0.1 g

catalyst including 8.85 mg particles), after discharging and

reloading with Co (II) ions these p(AMPS) hydrogels (0.1 g

catalyst contains 13.17 mg particles), and non-imprinted

p(AMPS)-Co (0.1 g catalyst contains 11.09 mg particles) cata-

lyst systems, respectively. These times were 120, 67 and

83 min for Ni (II) ion-imprinted p(AMPS) (0.1 g catalyst con-

tains 6.55 mg particles), after discharging and reloading with

Ni (II) ions into p(AMPS) (0.1 g catalyst contains 10.12 mg

particles), and non-imprinted p(AMPS)-Ni (0.1 g catalyst con-

tains 9.16 mg particles), respectively, as depicted in Fig. 2(b).

As can be seen from Fig. 2, the absorption tendency of p(AMPS)

is higher for Co (II) then Ni (II) ions, and after removing metal

ions from imprinted p(AMPS) hydrogels, the amounts of the

same metal ions loaded increases resulting in more metal

nanoparticle formation for both Co(II) and Ni(II). Hence, the

hydrogen production of the discharged and reloaded p(AMPS)-

0

80

160

0 40 80 120

Vo

lu

me

o

f H

yd

ro

Time (min)

Fig. 2 e (a). Hydrogen production from hydrolysis of

sodium borohydride (NaBH4) by using 0.1 g IIH p(AMPS)-Co,

and discharging and reloading IIH p(AMPS)-Co, and non-

imprinted p(AMPS)-Co (containing 8.85, 13.17 and 10.09mg

particles). (b) Using 0.1 g IIH p(AMPS)-Ni, discharging and

reloading IIH p(AMPS)-Ni, and non-imprinted p(AMPS)-Ni

(containing 6.55, 10.12 and 9.16 mg particles). [Hydrolysis

reaction: 50 ml 50 mM NaBH4, 5 wt% NaOH, 30 �C,1000 rpm].

0

80

160

240

320impr. P(AMPS)-Co second loading third loading

Vo

lu

me o

f H

yd

ro

gen

(m

l)

(a)

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 8 ( 2 0 1 3 ) 1 5 2 7 5e1 5 2 8 4 15279

M (Co and Ni) catalyst systems is greater than the other metal

loading processes. Moreover, this effect is more pronounced

for Ni (II) as illustrated in Fig. 2(b). It is important to note that

the imprinting hydrogel network with metal ions during

hydrogel preparation provide pre-structuring of the hydrogel

networks generating suitable pores and/or voids. And upon

removing these metal ions from IIH-p(AMPS) hydrogels, and

reloading with the same metal ions and reducing to corre-

sponding metal nanoparticles produce the same amounts of

H2 faster e.g., compare 30 mine20 min for Co metal nano-

particle, and 120 mine67 min for Ni nanoparticles. This is a

significant outcome of this research that without changing

anything from catalyst systems just changing preparation

method improved the catalytic performances considerably.

This effect also shown for the preparation of different metal

nanoparticles in Co (II), and Ni (II) ions-imprinted-p(AMPS).

For example, we also carried out the hydrolysis of NH3BH3

with Cu metal nanoparticle-containing p(AMPS) hydrogels.

Furthermore, Co (II) and Ni (II) ion-imprinted p(AMPS) were

discharged from their matrices, and after activation with

NaOH (100 ml 0.05 M) these hydrogels were reloaded with Cu

(II) and used in the corresponding particle preparation and

hydrolysis of NH3BH3. As given in Fig. 3, after removal of Co

and Ni ions from imprinted p(AMPS) hydrogels, the Cu (II)

loading capacity of p(AMPS) increased about 4.5 and 2% for Co

(II)- and Ni (II)-imprinted p(AMPS) hydrogels.

The copper loaded catalyst system, p(AMPS)-Cu, contains

9.02 mg particles, whereas after the removal of cobalt ions

from imprinted p(AMPS)-Co and reloading with Cu (II) ions

p(AMPS) contained 9.42mg Cu particles, and after the removal

of nickel ions from imprinted p(AMPS)-Ni and reloading with

Cu (II) ions, p(AMPS) contained 9.19 mg Cu particles. It is

important to note that in all cases 0.1 g p(AMPS) were used in

hydrolysis reactions of NH3BH3 under the same reaction

conditions (0.1 g catalyst, 5 wt% NaOH, 50 ml 50 mM NH3BH3

(0.0795 g), at 30 �C, at 1000 rpm mixing rate). In all particle

formations from metal ions 100 ml 0.4 M NaBH4 was used.

Hydrolysis reactions of NH3BH3 were completed in

0

50

100

150

200

0 30 60 90

p(AMPS)-Cu

Cu in-p(AMPS)-Ni out

Cu in-p(AMPS)-Co outVo

lu

me o

f H

yd

ro

gen

(m

l)

Time (min)

Fig. 3 e Hydrogen production from hydrolysis of

ammonium borane (NH3BH3) by using 0.1 g p(AMPS)-Cu, Cu

(II) ions loaded after removal of Co(II) p(AMPS)-Co, and

again Cu (II) loaded after removal of Ni (II) ions from

p(AMPS)-Ni system (containing 9.02, 9.42 and 9.19 mg Cu

particles, respectively). [In all cases 0.1 g p(AMPS) was

used, and 50 ml 50 mM NH3BH3, 5 wt% NaOH, 30 �C,1000 rpm].

approximately 65 min producing 176 ml H2 for all catalyst

systems as shown in Fig. 3 with small differences in hydrogen

production rates attributed to small differences in the amount

of catalyst (Cu).

3.2. Effect of reloading studies on hydrolysis kinetics ofNaBH4 and NH3BH3

Hydrolysis reactions of NaBH4 andNH3BH3were carried out by

using 0.1 g imprinted p(AMPS)-Co composite catalyst systems

in the presence of 5 wt% NaOH with the aqueous solution of

50 ml 50 mM chemical hydrides (0.0965 g NaBH4 or 0.0795 g

NH3BH3), at 30 �C under 1000 rpmmixing rate. As illustrated in

Fig. 4(a) and (b), after reloading and reduction of 0.1 g

imprinted p(AMPS) hydrogels for the 2nd and 3rd times with

Co (II) ions, the hydrolysis of NaBH4 and NH3BH3 were carried

out. For the second loading, 0.1 g imprinted p(AMPS)-Co

nanoparticle-containing catalyst systems were placed in

100 ml 500 ppm cobalt solution for 12 h and then these ma-

terials were washed in 100ml distilled water for 1 h to remove

unbound metal ions from the hydrogels. Afterwards, the

composite materials were reduced with 100 ml 0.4 M NaBH4

solution, and again washed with distilled water for 30 min,

and finally, these nanocatalyst systems were used for the

hydrogenation reactions. The same process was applied one

0 10 20 30 40Time (min)

0

50

100

150

200

0 8 16 24 32

impr. P(AMPS)-Co second loading third loading

Vo

lu

me o

f H

yd

ro

gen

(m

l)

Time (min)

(b)

Fig. 4 e Hydrogen production from the hydrolysis of (a)

sodium borohydride (NaBH4), and (b) ammonium borane

(NH3BH3) by using 0.1 g imprinted p(AMPS)-Co, and after

second and third loading and reduction of p(AMPS)-Co

(containing 8.85, 23.92 and 36.80 mg particles) as a catalyst

[50 ml 50 mM NaBH4 and NH3BH3, 5 wt% NaOH, 30 �C,1000 rpm].

280magn. impr. P(AMPS)-Co third loading

l)

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 8 ( 2 0 1 3 ) 1 5 2 7 5e1 5 2 8 415280

more time for third loadings by using second time-loaded

p(AMPS)-Co nanoparticles. Due to the increase in the

amount of metal nanoparticle capacity of imprinted p(AMPS)-

Co composite systems, reloading and reducing of metal ions

increased the activity of catalyst systems tremendously for

both hydrolysis of NaBH4 and NH3BH3 reactions. Due to for-

mation of metal nanoparticles such as Co, Ni, Cu after first

reducing of loaded metal ions, the interaction between of

metal nanoparticles and SO3� groups are decreased in com-

parison to metal ions and SO3� groups. Therefore, it is possible

to load more metal ions into hydrogel network via ioneion

interactions between the metal ions e.g., Co(II) ions and SO3�

groups of the hydrogel network. In other words, the interac-

tion between M(II) and SO3� is stronger than the interaction

between Co0 and SO3�. Due to the differences in the in-

teractions, the total amounts of metal are increased with

increased number of reloading and reduction processes

within hydrogel networks. The metal nanoparticle loading of

imprinted p(AMPS) hydrogels increased to 23.92 mg/g from

8.85 mg/g at 2nd loading, and to 36.8 mg for 3rd loading. The

hydrolysis reaction of NaBH4 was completed in 30 min for

imprinted p(AMPS)-Co catalyst systems, 9 min for second

time-loaded p(AMPS) composite catalyst systems, and as low

as 4 min for third time-loaded systems as illustrated in

Fig. 4(a). The catalyst systems were also used for the hydro-

lysis reaction of NH3BH3. Although 176 ml H2 gas was pro-

duced by every catalyst system, the imprinted p(AMPS)-Co,

and the second, and third loading of p(AMPS)-Co provided

hydrolysis reaction times of 24, 6 and 3 min, respectively in

Fig. 5 e Digital camera images of 0.25% crosslinked

p(AMPS) hydrogels (a) third loading and reduced magnetic

p(AMPS)-Co hydrogels, and (b) ferrites containing magnetic

imprinted p(AMPS)-Co.

NH3BH3 hydrolysis. It is obvious that the multiple loading of

metal ions and reduction cycles have a great effect on the

hydrolysis of both hydrides, this is pertinent with the

increased amount of metal nanoparticles and faster hydrogen

generation. The most important outcome of the multiple

loadings of metal nanoparticles is the generation of magnetic

Co within the p(AMPS) hydrogels with the increase in the

amount of Co particles. As illustrated in Fig. 5, the Co

nanoparticle-containing p(AMPS) hydrogels becomemagnetic

field responsive under an externally applied magnetic field as

shown in Fig. 5(a), similar to the ferrite-containing p(AMPS) as

shown in Fig. 5(b), and can be both directed by an externally

applied magnetic fields. This property is very useful for the

control of H2 by an externally applied magnetic field. Previ-

ously we reported the use of magnetic ferrite particles for this

purpose [26,27], however, in this investigation the prepared Co

nanoparticles provided the magnetic behavior, which is very

advantageous requiring no magnetic ferrites, which are not a

catalysts for the hydrolysis of hydrides.

3.3. Comparison of ferrite-containing p(AMPS)-Co andmagnetic p(AMPS)-Co catalyst systems from IIH p(AMPS)for the hydrolysis of hydrides

Catalytic properties of ferrite-containing p(AMPS)-Co, and

magnetic p(AMPS)-Co catalyst systems obtained by three

0

70

140

210

0 10 20 30Time (min)

0

50

100

150

200

0 5 10 15 20 25 30

magn. impr. P(AMPS)-Co third loading

Vo

lu

me

o

f H

yd

ro

ge

n (m

l)

Time (min)

Vo

lu

me

o

f H

yd

ro

ge

n (m (a)

(b)

Fig. 6 e Hydrogen production from hydrolysis of (a) NaBH4,

and (b) NH3BH3 by using 0.1 g ferrite-containing magnetic

imprinted p(AMPS)-Co, and three times loaded and

reduced magnetic p(AMPS)-Co (containing 12.5 and

36.80 mg Co particles, respectively). [Reaction conditions:

50 ml 50 mM NaBH₄ and NH3BH3, 5 wt% NaOH, 30 �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 8 ( 2 0 1 3 ) 1 5 2 7 5e1 5 2 8 4 15281

consecutive loading and reduction cycles of Co (II) into

p(AMPS) hydrogels were used for hydrolysis reactions of

NaBH4 and NH3BH3 and compared. The Co (II) ion-imprinted

p(AMPS) were treated with HCl as mentioned above, and the

ferrites are formed in situ by the loading of Fe (II):Fe (III) mix

solution, and treated with 100 ml 0.5 M NaOH. Then this

ferrite-containing p(AMPS) was used for Co (II) loading

and then reduction to obtain ferrite containing-magnetic

p(AMPS)-Co catalyst systems. For this purpose, 0.1 g

hydrogel matrices used in the preparation of ferrites and Co

nanoparticles within p(AMPS), and three times Co (II) ion

loaded and reduced catalyst systems with inherently mag-

netic p(AMPS)-Co nanoparticles were used in hydrolysis

reactions under the same conditions: 5 wt% NaOH in

aqueous solutions of 50 ml 50 mM chemical hydrides

(0.0965 g NaBH4 and 0.0795 g NH3BH3), at 30 �C under

1000 rpm mixing rate. As illustrated in Fig. 6(a), the hydro-

lysis reaction of NaBH4 was completed in 35 min by ferrite-

containing magnetic p(AMPS)-Co systems (containing

(a)

0

70

140

210

280

0 1 2 3

Vo

lu

me o

f H

yd

ro

gen

(m

l)

Time

(c

lnln k

1/T

y

0

2500

5000

7500

10000

280 300

Hy

dro

ge

n G

en

era

tio

n

Ra

te

(m

l H

2)/(g

)(m

in

)

(b)

-6

-4.8

-3.6

-2.4

0.0028 0.0031 0.0034 0.0037

Fig. 7 e (a) The effects of temperature on the hydrolysis of NaB

temperature, (c) ln k versus 1/T (Arrhenius Eq.) and ln (k/T) versu

5 wt% NaOH, 1000 rpm mixing rate containing 36.8 mg Co nan

12.5 mg Co nanoparticles), whereas the same reaction was

completed in 4 min using three times Co (II) loaded and

reduced magnetic p(AMPS)-Co systems (containing 36.80 mg

nanoparticles). The TOF (Total Turnover Frequency) values

for magnetic p(AMPS)-Co and ferrite-containing p(AMPS)-Co

are 4.91 and 1.6 (mol H2) (mol catalyst)�1 (min)�1 respec-

tively. As can be seen the triple Co (II) ion loaded and

reduced catalyst system has larger TOF values. The hydro-

lysis of NH3BH3 with these ferrite-containing magnetic and

three times Co (II) ion loaded and reduced inherently mag-

netic p(AMPS)-Co catalyst took 27 and 3 min respectively as

shown in Fig. 6(b), and their TOF values are 1.47 and 3.97

(mol H2) (mol catalyst)�1 (min)�1 respectively. As can be

seen the inherently magnetic cobalt-containing p(AMPS)-Co

catalyst system has better TOF values. The inherently

magnetic catalytic Co nanoparticles provide additional ad-

vantages over magnetic ferrites as Co particles are

used instead of ferrites as both catalyst and magnetic

moieties.

4 5 6

20 °C

30 °C

40 °C

50 °C

60 °C

(min)

-11

-10

-9

-80.0028 0.00306 0.00332 0.00358

)

k/T

1/T

= 0.0017e0.0464x

R² = 0.9898

320 340T (K)

H4, and (b) the change in hydrogen production rate with

s 1/T (Eyring Eq.) [Reaction conditions 50 ml 50 m M NaBH₄,

oparticles].

Table 3 e Activation parameters of p(AMPS)-M compositecatalyst systems for the hydrolysis of NaBH4.

Composite catalystsystems

Ea(K J mol�1)

DH#

(K J mol�1)DS#

(J mol�1 K�1)

1st loaded p(AMPS)-Co 44.022 36.855 �157.857

2nd loaded p(AMPS)-Co 39.278 36.016 �159.131

3rd loaded p(AMPS)-Co 38.194 35.603 �161.750

1st loaded p(AMPS)-Ni 48.312 45.668 �177.101

75

100Conversion Activity

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 8 ( 2 0 1 3 ) 1 5 2 7 5e1 5 2 8 415282

3.4. The kinetics of NaBH4 hydrolysis via triple Co (II)ion loaded and reduced p(AMPS)-Co catalyst systems

The effect of the temperature on the hydrolysis reactions of

NaBH4 was determined by using inherently magnetic three

times Co (II) loaded and reduced IIH p(AMPS)-Co nano-

composite systems (containing 36.80 mg/g Co) at various

temperatures between 20 �C and 60 �C with 10 �C increments,

under certain reaction conditions of 50 ml 50 mM NaBH4

(containing 5 wt% NaOH), 1000 rpmmixing rate. As illustrated

in Fig. 7(a), the amounts of produced hydrogen gas for all

temperatures were approximately 250 ml, but the production

time for this amount of hydrogen was affected tremendously

by the change in temperature of the hydrolysis reaction. The

hydrolysis reaction carried out at 20 �Cwas completed in 300 s,

whereas the hydrolysis reaction carried out at 60 �C was

completed in 50 s, providing a 5 fold increase. As given in

Fig. 7(b), the hydrogen production rate assumes an exponen-

tial relationship with temperature and increases rapidly at

temperatures above room temperatures. The kinetics of the

hydrolysis reaction of NaBH4 were examined through graphs

of ln k versus 1/T and ln (k/T) versus 1/T as shown and by using

of Arrhenius (Eq. (4)) and Eyring (Eq. (5)) equations [29,30].

ln k ¼ ln A� ðEa=RTÞ (4)

lnðk=TÞ ¼ lnðkB=hÞ þ�DS#=R

�� �DH#=R

�ð1=TÞ (5)

Here, k is the reaction rate constant and was calculated

according to a zero-order kinetic expression that is indepen-

dent of reactant concentration. Ea is the activation energy, T is

the absolute temperature, kB is Boltzmann’s constant

(1.381 � 10�23 J K�1), h is Planck’s constant (6.626 � 10�34 J),

activation enthalpy is DH#, DS# is the entropy of activation and

R is the gas constant (8.314 J K�1 mol�1). The details regarding

reaction rate constants are given in Table 2. As can be seen

from Table 2, the hydrolysis reaction rate constants are

increasing with the increase in temperature as expected. To

determine the change in activation parameters after multiple

loading reduction of Co (II) in p(AMPS) hydrogels, the activa-

tion parameters including Ea after every metal loading and

reduction cycles for NaBH4 hydrolysis were calculated and

given in Table 3. As shown, using 0.1 g IIH p(AMPS) hydrogel,

Ea is decreasing slight after 1st, 2nd and 3rd time Co loaded

p(AMPS)-M catalyst system and determined as 44.022, 39.278,

and 38.194 k J mol�1, respectively. The other activation pa-

rameters such as, enthalpy and entropy for were also given in

Table 2 e The rate constant for NaBH4 hydrolysisreactions at different temperatures catalyzed by threetimes Co ion loaded-reduced p(AMPS)-Co catalystsystem.

Temperature (�C) Rate constant (mol/min)

20 0.01

30 0.014

40 0.028

50 0.041

60 0.063

Table 3 and do not change significantly that are about

DH# ¼ 35.6e36.86 k J mol�1, and DS# ¼ �157.857 to

�161.750 J mol�1 K�1. The IIH p(AMPS)-Co systems provided

similar Ea in comparison to our previously reported value that

is about 38.14 k J mol�1 [28]. The 3rd loaded p(AMPS)-Co

composite systems have lowest activation energy and high-

est catalytic performance among other catalyst systems using

p(AMPS) as template [28]. The 1st Ni loaded p(AMPS)-Ni were

also given Table 3 and calculated as 48.312 k J mol�1,

45.668 312 k J mol�1, and �177.101 J mol�1 K�1 for Ea, DH#, and

DS#, respectively which are higher in magnitude than Co

based catalyst systems.

3.5. The reusability of three times Co (II) loaded andreduced magnetic p(AMPS)-Co Nanocomposites

To determine the catalytic activity and reusability of three

times Co (II) loaded and reduced magnetic p(AMPS)-Co nano-

composites, the same catalyst system was used in 10

consecutive runs for the hydrolysis of NaBH4 (containing

36.80 mg Co/g hydrogel), each with 50 ml 50 mMNaBH4, 5 wt%

NaOH, at 30 �C and 1000 rpmmixing rate. The first use of 0.1 g

3rd loaded p(AMPS)-Co metal nanocatalyst was carried out at

described conditions, and then the catalyst systems was

filtered, and washed plenty of water at the end of NaBH4 hy-

drolysis reactions. After that, the hydrolysis reaction of NaBH4

again was done by the second time use of the same catalyst,

and same processes were carried out for other uses up to 10

times under the same reaction conditions. As shown in Fig. 8,

0

25

50

1 2 3 4 5 6 7 8 9 10

%

Run Number

Fig. 8 e The conversion and activity of the repetitive use of

triple Co (II) loaded and reduced p(AMPS)-Co

nanocomposite system in NaBH₄ hydrolysis [50 ml 50 mM

NaBH₄, 30 �C, 1000 rpm, 36.80 mg Co nanoparticle].

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 8 ( 2 0 1 3 ) 1 5 2 7 5e1 5 2 8 4 15283

each time 100% conversion was obtained, and the activity

reduced to 86.5% at the end of 10th use. This is better than our

previously reported values with additional magnetic advan-

tages [17,26,27]. So, these kinds of materials have better po-

tential in real energy processing technologies using an

externally controllable magnetic field.

4. Conclusion

Here, we revisit the use of p(AMPS) catalyst systems as metal

ion-imprinted (IIH) hydrogel template systems, preparing

hydrogels with the use of Co (II) and Ni (II) ions for the hy-

drolysis of NaBH4 and NH3BH3 as hydrogen generating sys-

tems. The multiple loading and reduction of Co (II) ions

provided inherently magnetic behavior.

In summary, the following results were obtained from this

investigation:

� Imprinting metal ions enhances catalytic activity of

p(AMPS)-Co and p(AMPS)-Ni catalyst systems, with the

second showing greater change.

� Kinetic studies of hydrolysis reactions of NaBH4 by using Co

(II) loadings and reductions for the preparation of p(AMPS)-

Co catalyst composites provided activation energies of

44.022, 39.278 and 38.194 k J mol�1, for the 1st, 2nd, and 3rd

loaded catalytic systems. The increase in the number of

metal ion loading and reduction cycles slightly decreasing

the activation energies.

� The triple Co (II) loaded and reduced p(AMPS)-Co showed

magnetic field responsive behavior and could be used for up

to ten repetitive uses without significant loss of its catalytic

activity.

Due to improved properties such as excellent chemical

stability, high catalytic activity, magnetic field responsive-

ness, reusability, and high capability to load metal ions, IIH

p(AMPS)-Co composites systems showed powerful perfor-

mances in hydrolysis of NaBH4 and NH3BH3 offering great

potential in real applications for H2 production in a control-

lable fashion.

Acknowledgments

This work is supported by the Scientific and Technological

Research Council of Turkey (110T649).

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