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Dehydrogenation characteristics of ammoniaboraneviaboron-based catalysts (CoeB,NieB,CueB)under different hydrolysis conditions

Aysel Kanturk Figen*

Department of Chemical Engineering, Yildiz Technical University, Istanbul 34210, Turkey

a r t i c l e i n f o

Article history:

Received 21 March 2013

Received in revised form

2 May 2013

Accepted 15 May 2013

Available online 15 June 2013

Keywords:

Ammonia borane

Hydrolysis

Boron catalyst

Non-stirring

Magnetic stirring

Ultrasonic irradiation

* Tel.: þ90 2123834774; fax: þ90 212383472E-mail addresses: ayselkanturk@gmail.co

0360-3199/$ e see front matter Copyright ªhttp://dx.doi.org/10.1016/j.ijhydene.2013.05.0

a b s t r a c t

In the present study, dehydrogenation characteristics of ammonia borane (NH3BH3) cata-

lyzed via boron-based catalysts under different hydrolysis conditions were investigated. A

series of boron-based catalysts (Co1�xeBx, Ni1�xeBx, and Cu1�xeBx, x: 0.25, 0.50, 0.75) were

prepared by solegel method. Gels were calcinated at different temperatures (250 �C, 350 �C,

and 450 �C) in order to obtain the boron-based catalysts. XRD characterizations revealed

that CoeB, NieB, and CueB crystalline structures were formed during calcination at 450 �C.

Hydrogen generation measurements were performed in order to determine the optimum

composition of the boron-based catalyst. The maximum hydrogen generation rates were

7607 ml min�1 gcat�1, 3869 ml min�1 gcat�1 and 1178 ml min�1 gcat�1 for Co0.75B0.25,

Ni0.75B0.25 and Cu0.75B0.25, respectively. Furthermore, the hydrolysis of NH3BH3 was per-

formed at 20 �C, 40 �C, 60 �C and 80 �C under magnetic stirring (750 rpm), ultrasonic irra-

diation and non-stirring in order to determine how these parameters effect hydrolysis.

Activation energies (Ea) were calculated by evaluation of the kinetic data. Under ultrasonic

irradiation, the Ea in the presence of Co0.75B0.25, Ni0.75B0.25 and Cu0.75B0.25 were

40.85 kJ mol�1, 43.19 kJ mol�1 and 48.74 kJ mol�1, respectively, which compares favorably

with results reported in the literature. Thus, the catalytic activities of the boron-based

catalysts were found to be Cu < Ni < Co and the best reaction condition for the catalytic

hydrolysis of NH3BH3 was determined to be non-stirring < magnetic stirring < ultrasonic

irradiation.

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

reserved.

1. Introduction dehydrocoupling: (1) Applying heat to NH3BH3 leads to

Ammonia borane (NH3BH3), which is also known as borazane,

is considered to be one of the leading candidates for a

hydrogen storage medium considering that its 19.6 wt% high

capacity is greater than the 2015 target of the U.S. Department

of Energy (9 wt% hydrogen for a material) [1,2].

Hydrogen generation from NH3BH3 can be performed by

three ways; thermolysis, hydrolysis and catalytic

5.m, akanturk@yildiz.edu.t2013, Hydrogen Energy P81

hydrogen release in three thermolysis steps at different tem-

peratures, each of which produces about 6.5 wt% hydrogen

[3e8]; (2) Hydrolysis provides 3 mol of hydrogen per

1 mol of NH3BH3 at ambient temperatures. However, the use

of a catalyst is required to generate hydrogen from the hy-

drolysis of NH3BH3 due to the strong BeN bond of NH3BH3.

Several types of catalysts such as transition-metals [9e11],

non-noble metals [12e15], solid acids [16], metal oxides [17],

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

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 ) 9 1 8 6e9 1 9 7 9187

and boron-based composites [18,19] are used in order to

accelerate the hydrolysis reaction. Current researches are

focused on heterogeneous catalysts which show both high

activity and cost-effectiveness; (3) Catalytic dehydrocoupling/

dehydrogenation of NH3BH3 adducts is of key importance in

this respect. In general, the catalyst-based systems are based

on 4d and 5d metals including Rh, Ir, Re, and Ru, for each of

which the cost is particularly high relative to their 3d coun-

terparts [20e26].

Among these three techniques, the hydrolysis of NH3BH3

seems to be most promising route when on-board applica-

tions in the last a few years are taken into consideration. The

development of low-cost and highly efficient catalysts is the

main challenge to be overcome in hydrolysis of NH3BH3. Non-

noblemetals such as Ni, Cu, and Co are the best candidates for

the preparation of metal oxide catalysts due to low prices and

high catalytic ability.

In the present study, detailed dehydrogenation character-

istics of NH3BH3 catalyzed via boron-based catalysts

(Co1�xeBx, Ni1�xeBx, and Cu1�xeBx, xB: 0.25, 0.50, 0.75) under

different hydrolysis conditions such as non-stirring, magnetic

stirring and ultrasonic irradiation are reported. There are no

literature reports focused on effects the different hydrolysis

conditions on dehydrogenation characteristics such as cata-

lytic activities and reaction parameters. Also, this is the first

example of using the H3BO3 as a boron source for synthesis of

CoeB, NieB and CueB catalysts with different compositions

via solegel techniques.

2. Experimental section

2.1. Materials and analytic techniques

All reagents used were of analytical grade. Nitrate salts of Co,

Ni and Cu [Co(NO3)2$6H2O, Ni(NO3)2$6H2O, Cu(NO3)2$3H2O]

from Merck, citric acid (C6H8O7, purity: 99.50%) from Carlo

Erba, boric acid (H3BO3, purity: 99.00%) from Eti Mine Works

General Management-Turkey and ammonia borane (NH3BH3,

purity: 97.00%) from Aldrich were used as received.

Boron-based catalysts were characterized by X-ray

diffraction (XRD, Philips Panalytical X’Pert-Pro, CuKa radia-

tion), BrunauereEmmetteTeller analyses (BET, Micromeritics

ASAP 2020) N2 adsorptive gas with multipoint modes, induc-

tively coupled plasma optical emission spectrometry (ICP-

OES, Perkin Elmer, Optima 2100 DV) and scanning electron

microscopy (SEM, CamScan/Apollo300).

2.2. Synthesis of boron-based catalysts via solegeltechnique

The series of boron-based catalysts (Co1�xeBx, Ni1�xeBx, and

Cu1�xeBx) were prepared by solegel method. Nitrate salts of

Co, Ni and Cu were used as a metal precursor, C6H8O7 was

used as an oligomer and H3BO3 was used as a boron source in

the present study. Products with different molar ratios (XB)

were obtained by using different concentrations (XB: 0.25,

0.50, 0.75) of Co, Ni and Cu salts in the initial solution. A

suitable amount of H3BO3 was dissolved in 200 ml distilled

water and then mixed with nitrate salts of Co, Ni and Cu with

varying proportions and C6H8O7 to make a uniformly mixed

solution. Then the aqueous solution was mixed with mag-

netic stirring for 2 h and gel structure was maintained. Sub-

sequently, the obtained gel was dried at approximately 100 �Cunder vacuum overnight to eliminate the remaining water

molecules. Furthermore, dried gel was calcined at 250, 350,

and 450 �C for 4 h to remove organic components from the

structure and in order to determine formation of the CoeB,

NieB and CueB crystalline structure. Boron-based catalysts

were structurally characterized using XRD, BET, ICP-OES and

SEM techniques.

2.3. Dehydrogenation characteristics of NH3BH3

In the present study, catalytic hydrolysis of NH3BH3 based on

a typical water-replacement procedure that includes 15 ml

hydrolysis e glass reactor with a temperature control system

was performed. The reactor was connected to a water-filled

inverted burette to measure the volume of hydrogen gas

that evolved from the 0.12 M NH3BH3 solution. The mea-

surement of hydrogen generation was performed at various

temperatures such as 20, 40, 60, and 80 �C. The amount of

boron-based catalyst was kept constant at 0.005 g in all ex-

periments. Constant reaction temperature was achieved

using a water cooling system combined with peristaltic

pump. Temperature was maintained in a �2 �C range during

all hydrolysis reactions. Hydrogen evaluation time was

measured by chronometer until no more hydrogen evolution

was observed. Reactions were performed under three

different conditions, non-stirring, magnetic stirring and ul-

trasonic irradiation, in order to investigate the characteristics

of NH3BH3 dehydrogenation catalyzed with boron-based

catalysts.

The procedure for hydrogen generation from NH3BH3

catalyzed by boron-based catalyst (Co1�xeBx, Ni1�xeBx, and

Cu1�xeBx) under different hydrolysis conditions (non-stirring,

magnetic stirring, ultrasonic irradiation) consisted of the

following three different sets of experiments: (1) Determina-

tion of optimum composition of boron-based catalysts, (2)

Catalytic hydrolysis of NH3BH3 under “non-stirring, magnetic

stirring, ultrasonic irradiation” conditions, (3) Kinetics of the

hydrolysis of NH3BH3 under “non-stirring, magnetic stirring,

ultrasonic irradiation” conditions.

2.3.1. Determination of optimum composition of boron-basedcatalystsIn the first set of experiments, hydrogen generation tests were

performed on 0.12 M NH3BH3 solutions at 60 �C under 750 rpm

magnetic stirring in order to determine the optimum com-

positions of boron-based catalysts. The amount of boron-

based catalysts was kept constant at 0.005 g while the

composition of the catalyst was varied (Co1�xeBx, Ni1�xeBx,

and Cu1�xeBx, x: 0.25, 0.50, 0.75). Optimum compositions of

boron-based catalysts were determined based on maximum

hydrogen generation rates.

2.3.2. Catalytic hydrolysis of NH3BH3 under “non-stirring,magnetic stirring, ultrasonic irradiation” conditionsIn the second set of experiments, hydrogen generation was

performed under non-stirring, magnetic stirring, and

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 ) 9 1 8 6e9 1 9 79188

ultrasonic irradiation conditions with boron-based catalysts

with the optimum composition that had been determined

from the first set of experiments. The same procedure was

applied in all experiments. Hydrogen generation tests were

performedwith 0.12MNH3BH3 solutions at 20, 40, 60 and 80 �Ctemperatures with 0.005 g catalyst. (1) Under the non-stirring

conditions, hydrogen evolution induced adequate mixing ef-

fect to allow the reaction to proceed; (2) Under magnetic stir-

ring conditions, reaction solutions were vigorously stirred at

750 rpmwith a Teflon-coated stir bar during the hydrolysis; (3)

Under ultrasonic irradiation, the hydrolysis glass reactor was

kept under ultrasonic waves (JPSELECTA, 3000839, 200 W) at a

frequency of 35 kHz.

2.3.3. Kinetics of the hydrolysis of NH3BH3 under “non-stirring, magnetic stirring, ultrasonic irradiation” conditionsIn the third set of experiments, the hydrogen generation rate

was converted into reactant, NH3BH3, concentration as

function of time for kinetic modeling. Zero, first and second

order reaction kinetic models were used to describe the ki-

netic behavior of the hydrolysis of NH3BH3. Zero-order anal-

ysis showed linear ðCNH3BH30 � CNH3BH3 Þ plots versus time for

the four different hydrolysis reaction temperatures. Zero-

order kinetics is independent of reactant concentration.

After determining the reaction order, a plot of Arrhenius

law’s slope gives activation energy of NH3BH3 catalytic hy-

drolysis where the intercept is the Arrhenius constant (Eqs.

(1), (2)). First-order analysis showed lnðCNH3BH30=CNH3BH3 Þplots versus time for three different hydrolysis reaction

temperatures (Eqs. (3), (4)). The second-order reaction model

showed linearity between ðð1=CNH3BH30 Þ � ð1=CNH3BH3 ÞÞ and

time (Eqs. (5), (6)).

Fig. 1 e XRD patterns of b

dCNH3BH3

dt¼ �rNH3BH3

¼ �kðTÞ (1)

�CNH3BH30

� CNH3BH3

� ¼ �kðTÞ$t (2)

dCNH3BH3

dt¼ �rNH3BH3

¼ �kðTÞ$CNH3BH3(3)

ln

�CNH3BH30

CNH3BH3

�¼ kðTÞ$t (4)

dCNH3BH3

dt¼ �rNH3BH3

¼ �kðTÞ$C2NH3BH3

(5)

�1

CNH3BH3

� 1CNH3BH30

�¼ �kðTÞ$t (6)

where CNH3BH3is the concentration, r is the rate of reaction, and

k is the reaction rate constant based on the solution volume.

2.3.4. Catalyst recyclabilityThe recyclability of the Co0.75B0.25, Ni0.75B0.25 and Cu0.75B0.25

catalysts under ultrasonic irradiations was investigated. Five

experiments were performed at 60 �C with 0.005 g catalyst.

The catalyst recyclability data were used to calculate the

turnover frequency (TOF, mole H2 mole�1 catalyst min�1) for

NH3BH3 hydrolysis reactions. In catalyst recyclability experi-

ments; after the first hydrogen generation reaction was

completed, the catalyst was kept in the reaction solution

under ambient conditions and another 1.2 mmol NH3BH3 was

added to the residual solution and the reaction was moni-

tored. This was repeated four times in order to determine the

recyclability.

oron based catalysts.

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 ) 9 1 8 6e9 1 9 7 9189

3. Results and discussion

3.1. Characterization of boron-based catalysts

Fig. 1 shows the XRD patterns of the boron-based cata-

lysts (Co1�xeBx, Ni1�xeBx, and Cu1�xeBx, xB: 0.25, 0.50,

0.75) which were synthesized by solegel method and

calcinated at 250, 350, and 450 �C. Amorphous structures

were formed after the 250 �C calcination as can be seen

in the XRD pattern of CoeB catalysts at different con-

centrations. As temperature increased, the crystal struc-

tures of Co0.25B0.75, Co0.5B0.5 and Co0.75B0.25 were changed.

Co3O4 was formed instead of CoeB at lower temperatures

for Co0.25B0.75. XRD results of Co0.5B0.5, calcinated at

450 �C, indicated that the structure included two crystals,

CoB2O4 and CoB2O5. However, Co0.75B0.25 calcinated at

450 �C only contained CoB2O4. All the boron-based cata-

lysts showed the same behavior as a function of tem-

perature. CoeB, NieB and CueB single structures occurred

after the 450 �C calcination for the Co0.75B0.25, Ni0.75B0.25

and Cu0.75B0.25 catalyst compositions. By considering the

phase characterization, it was concluded to start the

further characterization and hydrolysis experiments with

the metal (Co, Ni, Cu)/boron (B) ratio ¼ 0.75/0.25. Table 1

and Fig. 2 show texture and microstructure properties of

Co0.75B0.25, Ni0.75B0.25 and Cu0.75B0.25 catalysts. The

maximum specific surface area of 333.66 � 10.10 m2/g

was observed for the Ni0.75B0.25 catalyst with minimum

pore size (94.25 �A). Relatively porous structures due to

solegel synthesis technique were observed in the SEM

images at 5000� magnification and the average particular

sizes were close to each other. Catalytic activity of metals

is largely dependent upon the morphology and the sup-

port. In this work, the catalytic activity seems to be in-

dependent to the specific surface area and metal support

content of the catalyst is the most important effect on

the activity. The maximum specific surface area of

333.66 � 10.10 m2/g was observed for the Ni0.75B0.25

catalyst while the catalytic activities of the boron-based

catalysts were found to be Cu < Ni < Co. As can be

seen that metal support content of the boron based

catalyst defined the catalytic activity for hydrolysis of

NH3BH3.

Additionally, ICP-OES analysis was performed in order to

determine the chemical compositions of Co0.75B0.25, Ni0.75B0.25

and Cu0.75B0.25 catalysts. It was established that the ratio of

metal (Co, Ni, Cu)/Boron (B) was 3.

Table 1 e Texture properties of Co0.75B0.25, Ni0.75B0.25 andCu0.75B0.25 catalysts.

Catalyst Specificsurface area

(m2/g)

Pore size(�A)

Averageparticular size

(mm)

Co0.75B0.25 159.21 � 5.87 110.45 27.01 � 6.02

Ni0.75B0.25 333.66 � 10.10 94.25 37.26 � 7.09

Cu0.75B0.25 2.10 � 0.25 246.21 22.57 � 5.04

Fig. 2 e SEM images at 50003 magnification of Co0.75B0.25,

Ni0.75B0.25 and Cu0.75B0.25 catalysts.

3.2. Determination of optimum composition of boron-based catalysts

Table 2 shows the H2 generation rate for the hydrolysis of

NH3BH3 catalyzed by boron-based catalysts at 60 �C with

Table 2 e Hydrogen generation rate for the hydrolysis ofNH3BH3 catalyzed by boron based catalysts at 60 �C under750 rpm magnetic stirring.

Catalyst H2 generation rate(ml H2/gcat min)

Time span(min)

CoeB Co0.25B0.75 4813.00 1.08e4.28

Co0.50B0.50 5133.00 0.97e3.98

Co0.75B0.25 7607.00 0.63e2.87

NieB Ni0.25B0.75 519.00 17.50e46.70

Ni0.50B0.50 1334.00 7.39e18.70

Ni0.75B0.25 3869.00 1.43e5.48

CueB Cu0.25B0.75 428.00 3.42e39.20

Cu0.50B0.50 725.00 2.43e24.00

Cu0.75B0.25 1178.00 1.13e13.90

Fig. 3 e Hydrogen generation volume from the NH3BH3 hydrolys

different temperatures under non-stirring condition.

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 ) 9 1 8 6e9 1 9 79190

750 rpm magnetic stirring. For all the types of catalysts, H2

generation followed the same trend; increasing the metal

content in the catalyst structure (Co, Ni, Cu) led to faster H2

release. Time span values were taken into consideration in

calculating H2 generation rate. All of the catalysts exhibited

excellent catalytic activities, releasing all 3 mol of substrate

hydrogen.

Catalysts composed with a metal/boron ratio of 0.75/0.25

had better catalytic activity than the other series of metal/

boron ratios of 0.50/0.50 and 0.25/0.75. As a result, Co0.75B0.25

(7607 mlH2 min�1 gcat�1), Ni0.75B0.25 (3869 mlH2 min�1 gcat�1)

and Cu0.75B0.25 (1178mlH2/mlH2min�1 gcat�1) showed the best

catalytic activities of in their series. Meanwhile, the H2 gen-

eration rate of NH3BH3 via Co0.75B0.25 was the highest

is in the presence of Co0.75B0.25, Ni0.75B0.25 and Cu0.75B0.25 at

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 ) 9 1 8 6e9 1 9 7 9191

comparedwith theNi0.75B0.25andCu0.75B0.25catalysts,delivering

a maximum hydrogen release rate of 7607 mlH2 min�1 gcat�1

at 60 �C with a completion time of 3.11 min under 750 rpm

magnetic stirring.

3.3. Catalytic hydrolysis of NH3BH3 under “non-stirring,magnetic stirring, ultrasonic irradiation” conditions

The maximum hydrogen generation rates were observed in

the presence of Co0.75B0.25, Ni0.75B0.25 and Cu0.75B0.25 catalysts

in their series (Table 2). Therefore, process optimization of

NH3BH3 hydrolysis via these catalysts led to a series of ex-

periments performed under different conditions such as non-

stirring, magnetic stirring and ultrasonic irradiation, and at

four different temperatures in the range of 20e80 �C (Figs.

3e5). For three catalysts, reaction completion time decreased

with an increase of temperature from 20 �C to 80 �C under

Fig. 4 e Hydrogen generation volume from the NH3BH3 hydrolys

different temperatures under magnetic stirring at 750 rpm.

non-stirring, magnetic stirring and ultrasonic irradiation as

expected.

Fig. 3 shows the H2 generation volume under non-stirring

conditions. The reaction completion time of hydrolysis of

NH3BH3 in the presence of Co0.75B0.25 was decreased from

154.98min to 2.88min corresponding to the 60 �C temperature

ramp. In the case of Ni0.75B0.25, the decrease in the reaction

completion time was from 174.30 min to 7.82 min corre-

sponding to 60 �C temperature ramp. The reaction completion

time of hydrolysis of NH3BH3 via Cu0.75B0.25 was determined as

226.52 min at 20 �C and it was decreased to 8.90 min by

increasing temperature to 80 �C. Fig. 4 shows the H2 genera-

tion volume under magnetic stirring at 750 rpm. Reaction

completion times were determined as 20.92, 55.60 and

88.17 min for hydrolysis of NH3BH3 at 20 �C in the presence of

Co0.75B0.25, Ni0.75B0.25 and Cu0.75B0.25, respectively. When tem-

perature was increased to 80 �C, reaction completion times

is in the presence of Co0.75B0.25, Ni0.75B0.25 and Cu0.75B0.25 at

Fig. 5 e Hydrogen generation volume from the NH3BH3 hydrolysis in the presence of Co0.75B0.25, Ni0.75B0.25 and Cu0.75B0.25 at

different temperatures under ultrasonic irradiation.

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 ) 9 1 8 6e9 1 9 79192

dramatically decreased to 1.57, 2.92 and 5.92 min. Fig. 5 illus-

trates the hydrogen generation volume against to the reaction

completion times as a function of temperature under ultra-

sonic irradiation. The reaction completion time of hydrolysis

of NH3BH3 in the presence of Co0.75B0.25 was decreased from

12.87 min to 1.11 min when the temperature was increased

from 20 �C to 80 �C. In the case of Ni0.75B0.25, the reaction

completion time decreased from 34.97 min to 1.79 min cor-

responding to a 60 �C temperature ramp. The reaction

completion time of hydrolysis of ammonia via Cu0.75B0.25 was

51.75 min at 20 �C and it was decreased to 2.62 min by

increasing temperature to 80 �C. By considering non-stirring,

magnetic stirring and ultrasonic irradiation conditions in the

hydrolysis of NH3BH3, it was clearly seen that reaction

completion times were dependent on hydrolysis conditions.

Comparing hydrogen generation volumes against reaction

completion times allowed calculation of the H2 generation

rate at 20, 40, 60 and 80 �C under non-stirring, magnetic stir-

ring and ultrasonic irradiation (Fig. 6). The rate of H2 genera-

tion fromNH3BH3 depended on the hydrolysis conditions. The

insets in Fig. 5 show the increase of H2 generation rate values

(%) that were calculated based on non-stirring conditions. The

Fig. 6 e Hydrogen generation rate from the NH3BH3 hydrolysis in the presence of Co0.75B0.25, Ni0.75B0.25 and Cu0.75B0.25 at

different temperatures under non-stirring, under magnetic stirring at 750 rpm and ultrasonic irradiation. The inset shows

values the % hydrogen generation rate increase.

Table 3 e Activation energy (kJ/mol) for the hydrolysis ofNH3BH3 catalyzed by boron based catalysts at 20e80 �Cunder non-stirring, magnetic stirring and ultrasonicirradiation conditions comparedwith various catalysts inliterature.

Catalyst Activationenergy (kJ/mol)

Ref.

PSSA-co-MA stabilized

cobalt(0) nanoclusters

34 � 2 [29]

PVP stabilized Co 46 � 2 [35]

CoeB 44 [31]

Intrazeolite cobalt(0)

nanoclusters

56 � 2 [11]

Co/g-Al2O3 62 [28]

CoePeB on Ni foam 38.8 [30]

p(AMPS)-Co 52.8 [29]

K2PtCl6 87 [36]

2 wt% Ru/g-Al2O3a 23 [36]

2 wt% Rh/g-Al2O3a 21 [36]

2 wt% Pt/g-Al2O3a 21 [36]

Laurate stabilized Ru(0) 47 [9]

Co0.75B0.25

Non-stirring 51.84 At this work

Magnetic stirring 42.10

Ultrasonic irradiation 40.85

Ni0.75B0.25

Non-stirring 53.88 At this work

Magnetic stirring 44.16

Ultrasonic irradiation 43.19

Cu0.75B0.25

Non-stirring 56.84 At this work

Magnetic stirring 50.77

Ultrasonic irradiation 48.74

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 ) 9 1 8 6e9 1 9 79194

maximum rate of H2 generation of 24,156mlH2 gcat�1min�1 in

the presence Co0.75B0.25 was observed under ultrasonic irra-

diation at 80 �C. When the hydrolysis reaction was performed

at 40 �C under ultrasonic irradiation, H2 generation rate was

increased by 1566%. In order to increase the H2 generation

rate, the hydrolysis conditions should be changed instead of

the temperature. At the higher temperatures of 60 and 80 �C,magnetic stirring and ultrasonic irradiation resulted in

approximately the same % increase in H2 generation rate.

Minimum H2 generation rate via Ni0.75B0.25 was

126.96 mlH2 gcat�1 min�1 at 20 �C under non-stirring condi-

tions while themaximum value was 15,732mlH2 gcat�1 min�1

at 80 �C under ultrasonic irradiation. At lower temperatures,

magnetic stirring and ultrasonic irradiation showed the same

effect on hydrolysis rate and maximum % H2 generation rate

increase was obtained as 2752 at 40 �C under ultrasonic

irradiation.

Amongst these catalysts, Cu0.75B0.25 exhibited quite low

catalytic activity. Lower hydrogen generation rates were ob-

tained even at high temperature and under ultrasonic irradi-

ation. Nevertheless, under ultrasonic irradiation, hydrogen

production rate increased by 365%. A similar increase in the

rate of H2 generation was observed when using Cu0.75B0.25

catalyst.

As a result, for all the catalysts, the H2 generation rate

increased under ultrasonic magnetic stirring and ultrasonic

irradiation as opposed to non-stirring conditions.

3.4. Kinetics of the hydrolysis of NH3BH3 under “non-stirring, magnetic stirring, ultrasonic irradiation”conditions

The kinetics of the hydrolysis of NH3BH3 catalyzed by the

boron-based catalysts was studied. SUPFigs. 1e3 show the

zero, first and second-order kinetic plots created based on Eqs.

(1)e(6). The apparent Ea of NH3BH3 hydrolysis under different

hydrolysis conditions was calculated from the Arrhenius

plots. It is clear that the hydrolysis reaction obeyed the zero-

order kinetics for all the conditions. In other words;

hydrogen generation rate is practically independent from

NH3BH3 concentration. The apparent Ea values at different

hydrolysis conditions are illustrated in Table 3. Table 3 also

provides a comparison of the value of Ea in this work with

some other published values. It was determined that apparent

Ea was 40.85, 43.19 and 48.74 kJ mol�1 for Co0.75B0.25, Ni0.75B0.25

and Cu0.75B0.25, respectively under ultrasonic irradiation.

Activation energies of hydrolysis reactions changed by

applying the different conditions. It can be clearly seen that

magnetic stirring and ultrasonic irradiation improved the H2

generation characteristics as demonstrated by the decrease in

the apparent Ea and the increase the H2 generation rate. These

values compare favorablywith other reported results for CoeB

catalyst.While the value of the Ea for both CoeB catalystswere

relatively lower than the results for Co/g-Al2O3 (62 kJ mol�1)

[27], intra zeolite cobalt(0)nanoclusters (56 � 2 kJ mol�1) [11]

and p(AMPS)-Co (52.8 kJ mol�1) [28], the value of the Ea for

CoeB catalyst was relatively higher than the results for PSSA-

co-MA stabilized cobalt (0) nanoclusters (34 � 2 kJ mol�1) [29],

CoePeB on Ni foam (38.8 kJ mol�1) [30], and transition metal-

doped CoeB alloy catalysts (44 kJ mol�1) [31].

NH3BH3 hydrolysis is an equilibrium reaction and a stoi-

chiometricmole of water permole of NH3BH3 is between 2 and

4 and water in the hydrolysis reaction does not contribute to

generated H2 amount in contrast with NaBH4 hydrolysis. The

solubility of NH3BH3 in water at 23 �C is 26 wt% corresponding

to about 4.9 mol of H2O per mole of NH3BH3 (5.1 wt% H2). The

equation using the least amount of water is reported by

Brockman et al. [32]. In this study, in order to identify the

hydrogen storage characteristics of NH3BH3 in the presence of

boron based catalyst, H2 storage capacity (wt, % ¼ H2/NH3BH3)

and H2 weight density [wt, % ¼ H2/(NH3BH3 þ H2O þ catalyst)]

were calculated [33]. In the present hydrolysis system 0.193 g

of hydrogen was liberated per 1 g of the NH3BH3 and released

H2 to NH3BH3 ratio is 3.0, corresponding to 0.072 wt% of the

starting materials NH3BH3, H2O and catalyst. Also, hydrogen

storage capacity (wt%) was 19.3% (theoretical: 19.6%) calcu-

lated from ratio of H2 amount per mole of NH3BH3. It can be

concluded that boron based catalysts were the active for the

hydrogen generation for NH3BH3 hydrolysis system.

3.5. Catalyst recyclability

Fig. 7 shows the TOF (molH2 mol�1cat min�1) values for

recyclability under ultrasonic irradiation of 0.12 M NH3BH3

with 0.005 g Co0.75B0.25, Ni0.75B0.25 and Cu0.75B0.25 at 60 �C. ForCo0.75B0.25 catalyst, it was found that hydrolysis reaction was

completed within a range of 1.39e3.22 min with an initial

47.12 TOF value. The TOF value was decreased to 20.34 after

Fig. 7 e The plots of recyclability hydrolysis data of NH3BH3 at 60 �C under ultrasonic irradiation.

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 ) 9 1 8 6e9 1 9 7 9195

the 5th run. Ni0.75B0.25 and Cu0.75B0.25 catalysts showed the

same recyclability activities. Based on the TOF values of the

boron-based catalyst, at the end of the 5th run, Co-containing

catalyst preserved its activity more than the others. This

decrease in catalytic activity in subsequent runs may be

attributed to the passivation of catalyst surface by increasing

amount of byproduct (boron products, e.g. metaborate) after

the each run which decreases accessibility of active sites [34].

4. Conclusions

In the present study, characteristics of NH3BH3 dehydroge-

nation catalyzed via boron-based catalysts under different

hydrolysis conditions were investigated. The following points

result from this study:

1. CoeB, NieB and CueB single structures occurred after the

450 �C calcination of Co0.75B0.25, Ni0.75B0.25 and Cu0.75B0.25

catalysts.

2. For all the types of catalysts, H2 generation showed the

same trend; increasing the metal content in the catalyst

structure (Co, Ni, Cu) led to faster H2 release. Boron-based

catalysts composed of a metal/boron ratio of 0.75/0.25 had

best catalytic activities.

3. H2 generation rate using Co0.75B0.25 catalyst was the highest

compared with the Ni0.75B0.25 and Cu0.75B0.25 catalysts.

Among these catalysts, Cu0.75B0.25 exhibited quite low cat-

alytic activity in the hydrolysis of NH3BH3. Thus, the cata-

lytic activities of the boron-based catalystswere found to be

Cu < Ni < Co.

4. For the three catalysts, reaction completion times

decreased with the increase of temperature from 20 �C to

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 ) 9 1 8 6e9 1 9 79196

80 �C under non-stirring, magnetic stirring and ultrasonic

irradiation as expected.

5. In order to increase the H2 generation rate, the hydrolysis

conditions should be changed instead of the temperature.

6. At lower temperatures, magnetic stirring and ultrasonic

irradiation showed the same effect on hydrolysis rate.

7. Activation energies of hydrolysis reactions changed under

different conditions. It can be clearly seen that magnetic

stirring and ultrasonic irradiation led to a decrease of the

apparent Ea.

8. The best reaction conditions for the catalytic hydrolysis of

NH3BH3 were determined to be non-stirring < magnetic

stirring < ultrasonic irradiation.

Acknowledgments

The financial support of YTU 2012-07-01-GEP01 is

acknowledged.

Appendix A. Supplementary data

Supplementary data related to this article can be found at

http://dx.doi.org/10.1016/j.ijhydene.2013.05.081.

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