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 e n e r g y 3 4 ( 2 0 0 9 ) 1 1 8 5 – 1 1 9 9

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New insights into the mechanism of H2 generation throughNaBH4 hydrolysis on Co-based nanocatalysts studied bydifferential reaction calorimetry

Anthony Garron, Dariusz Swierczynski, Simona Bennici*, Aline Auroux*

Universite Lyon 1, CNRS, UMR 5256, IRCELYON, Institut de recherches sur la catalyse et l’environnement de Lyon, 2 avenue Albert Einstein,

F-69626 Villeurbanne, France

a r t i c l e i n f o

Article history:

Received 29 September 2008

Received in revised form

7 November 2008

Accepted 7 November 2008

Available online 23 December 2008

Keywords:

Cobalt nanoparticles

Calorimetry

NaBH4

Hydrolysis

Hydrogen storage

* Corresponding authors. Tel.: þ33 4 72 44 53E-mail addresses: simona.bennici@ircely

0360-3199/$ – see front matter ª 2008 Interndoi:10.1016/j.ijhydene.2008.11.027

a b s t r a c t

To our knowledge, the present study is the first investigation by liquid-phase calorimetry of

the mechanism of hydrogen generation by hydrolysis of sodium borohydride catalyzed by

Co2B nanoparticles generated in situ. The differential reaction calorimeter was coupled

with a volumetric hydrogen measurement, allowing a simultaneous thermodynamic and

kinetic study of the reaction. At the end of the reaction, the catalyst was characterized ex

situ by TEM, XRD, magnetism, N2 adsorption, TGA–DTA, and the liquid hydrolysis products

were analyzed by Wet-STEM and 11B-NMR. The in situ preparation method made it possible

to form nanoparticles (<12 nm) of Co2B which are the active phase for the hydrolysis

reaction. In semi-batch conditions, the Co2B catalyst formed in situ is subsequently reduced

by each borohydride addition and oxidized at the end of the hydrolysis reaction by OH� in

the presence of metaborate. A coating of the nanoparticles has been observed by calo-

rimetry and physico-chemical characterization, corresponding to the formation of a 2–

3 nm layer of cobalt oxide or hydroxide species.

ª 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights

reserved.

1. Introduction development of suitable and safe technologies for hydrogen

Hydrogen and fuel cell technologies are currently in an intense

development phase. If nowadays the H2 production methods

are well known and controlled, the storage and transportation

of the fuel remain major obstacles to its use [1]. As recom-

mended by the U.S. Department of Energy, a hydrogen storage

system should possess a minimum storage capacity of 6.5 wt%

and 62 kg m�3 in order to be used in a model fuel-cell vehicle

with a standard driving range of 560 km [2]. During the last

decade a lot of research effort has been put into the

98/79; fax: þ33 4 72 44 5on.univ-lyon1.fr (S. Benniational Association for H

storage, such as materials for high-pressure cylinders, lique-

faction processes, hydrogen adsorption materials, and metal

hydrides [3]. Although H2 adsorption capacities have recently

been brought up to values near 6–8 wt%, this storage method

requires a high pressure and low temperature [4]. On the other

hand, chemical hydrides have an excellent potential for high

energy density storage at room temperature and atmospheric

pressure [4]. In particular, NaBH4 based storage has been

intensively studied to evaluate its potential for portable,

automobile and stationary applications. It presents the

3 99.ci), aline.auroux@ircelyon.univ-lyon1.fr (A. Auroux).ydrogen Energy. 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 e n e r g y 3 4 ( 2 0 0 9 ) 1 1 8 5 – 1 1 9 91186

advantages of a high potential hydrogen density (max.

10.9 wt%)1 together with a safe and easy hydrogen release

through a hydrolysis reaction (Eq. (1)) that can be controlled

catalytically [5].

NaBH4ðsÞ þ 4H2OðlÞ/NaBðOHÞ4ðaqÞ þ 4H2ðgÞ (1)

DrH10 ¼ �250.5 kJ molNaBH4

�1 [6].

Sodium borohydride hydrolysis in aqueous solution can be

represented in terms of the overall stoichiometric equation

(Eq. (1)) where NaBH4 reacts with 4 molecules of water to

produce 4 molecules of H2 [5]. Numerous studies about the

catalytic generation of hydrogen from boron hydrides have

been published [7–29]. Although the reaction of NaBH4

hydrolysis has been studied since the discovery of sodium

borohydride by Stock [30], the theoretical, calculated energy of

the reaction is often cited in an inconsistent manner, and

actual experimental data are surprisingly rare [12,31,32].

The measurement of the heat evolved during a catalytic

reaction is important from both the practical and funda-

mental points of view. Firstly it is an essential tool for the

assessment of thermal risks related with the performance of

the reacting system at industrial scale (i.e. the capability of

a system to enter into a runaway reaction). This type of safety

data is particularly important for reactions like hydrogen

generation by hydrolysis of borohydrides, where the rapid

increase in temperature may result in a sharp pressure

increase. A precise determination of the heat of reaction is

needed for the design and evaluation of feasibility of an

industrially applicable system. Secondly, the thermodynamic

data are of primary interest for the determination of the

reaction mechanism.

Moreover, this reaction produces pure and slightly humid

hydrogen directly usable in a PEM fuel cell. The only byproduct

is sodium metaborate, which is soluble in water and envi-

ronmentally benign [33]. An appropriate catalyst is necessary

to carry out the reaction at a sufficiently high rate. While

expensive platinum and/or ruthenium based catalysts have

been developed and studied for this purpose [34,35], more

recently the attention has shifted towards cheaper catalytic

materials such as Ni–B or Co–B alloys [36,37]. In terms of their

activity/cost ratio, cobalt-based catalysts represent a very

interesting solution; however the morphology and stability of

these catalysts play a crucial role in their efficiency. The major

limiting step for the reaction is known to be the accessibility of

the active sites. That limitation can be overcome by the use of

nanoparticles, which are of great interest for catalytic reac-

tions because of their particular morphology, which leads to

important surface areas, large amounts of active sites, and

unique electronic properties [38,39]. In the 1990s, studies of

cobalt based nanocatalysts (Co2B) for sodium borohydride

hydrolysis by Glavee et al. demonstrated that Co2B nano-

particles can be generated by the rapid addition of a solution

1 Hydrogen storage capacity of sodium borohydride depends onthe quantity of water involved: for the theoretical reactionNaBH4 þ 2H2O/NaBO2 þ 4H2. The H2 generated amounts to 10.9 wt% of the whole system (NaBH4 þ H2O); for a solution con-taining 20 wt% of NaBH4 the storage capacity is 4.2 wt% H2; for30 wt% of NaBH4 the storage capacity is 6.4 wt% H2.

of CoCl2 salt on NaBH4 powder, whereas a slower addition

leads to Co(BO2)2 formation [40].

In spite of the abundance of kinetic data in the literature,

the thermodynamic features of the catalyzed hydrolysis are

not yet well understood, as the evolved energy depends on the

physical state and the hydration degree of borohydride and

metaborate [27,41] and on side reactions. The effect of acidity

[22] or surface reduction/oxidation phenomena should be also

taken into account, as they affect the mass and heat balance

of the system.

Calorimetric techniques, and liquid phase calorimetry in

particular, are promising methods to study catalytic reactions

[31,32,42–44]. Notably, the use of a differential reaction calo-

rimeter (DRC) makes it possible to determine the most

important thermodynamic data such as the heat of reaction

and heat capacity of the system [45,46].

In this work we show that the total enthalpy of the catalytic

process is strongly influenced by the evolution of the catalyst

during hydrolysis reaction and by water evaporation which is

related to NaBH4 concentration. The characterization of the

solid and liquid products formed during the reaction permits

a better understanding of the whole catalytic process. This

original approach combines the use of the calorimetric tech-

nique to study the catalytic reaction in aqueous phase with ex

situ characterization of the catalyst and the solution, thus

providing information about the reaction thermodynamics,

kinetics and mechanism.

2. Experimental

Prior to the experiments, all the chemicals (sodium borohy-

dride, cobalt chloride, commercial Co nanoparticles and

sodium hydroxide) were stored, handled and prepared in an

argon-purged glove box.

2.1. Calorimetric and catalytic test protocols in liquidphase

2.1.1. DRC measurementsThe experiments were performed using a SETARAM Differ-

ential Reaction Calorimeter (DRC) coupled with a drum-type

volumetric gas meter from Ritter.

In terms of the operating principle of its temperature

control, the DRC is classified as an isoperibolic calorimeter. It

is based on the differential measurement principle (similarly

to DSC or DTA) where two double-jacketed reaction vessels

run in parallel, one containing the sample and the other one

acting as a reference. The ‘‘sample’’ vessel is filled with the

reactants and the ‘‘reference’’ one with the solvent (NaOH

solution). The fluid circulating in the jackets maintains the

surroundings of the reactor at a constant temperature. It is

thermostated by a Julabo F32 system using oil (H2OS), making

it possible to work between�20 �C and 130 �C with a precision

of 0.1 �C. The calorimetric principle is based on the continuous

measurement of the temperature difference between the two

vessels during the experiment. In order to determine the

specific capacity (Cp) of the system and correlate the temper-

ature difference (DT ) to the heat flow, a Joule-effect calibra-

tion is performed before and after the reaction. The

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 e n e r g y 3 4 ( 2 0 0 9 ) 1 1 8 5 – 1 1 9 9 1187

corresponding energy is given by Q ¼ Cp � DT. The measured

heat corresponds to the global energy liberated by the reaction

for each addition of reactants. The endothermic effects

observed at the beginning of each peak correspond to the

addition of a reactive solution at a temperature slightly lower

than that of the thermostated vessel. The experimental error

has been estimated to be �1% of the measured heat, based on

three independent measurements in the same conditions. A

detailed description of the DRC, its principle and different

operating modes have been given by Andre et al. [45], and

examples of its potential applications are discussed by Nogent

et al. [44].

2.1.2. In situ catalyst preparation and catalytic testAll the solutions involved in the experiments were prepared

using distilled water saturated with N2. Each test was

repeated three times in order to verify the reproducibility. The

in situ preparation of the catalyst and subsequent catalytic

tests were performed in the ‘‘sample’’ vessel of the DRC

system.

The common experimental protocol was the following.

Firstly an amount of Co(H2O)6Cl2 (STREM Chemicals, 99%)

equivalent to 1 mmol of cobalt was dissolved in 20 ml of water

under nitrogen flow. The catalyst was then generated by

addition of 5 ml of an aqueous solution containing 5 mmol

(200 mg) of NaBH4 (STREM Chemicals, 98% purity). The

suspension of the in situ generated catalyst was then brought

to pH 14 by addition of 5 ml of an aqueous solution containing

30 mmol of NaOH (6 mol L�1) (Sigma–Aldrich, 98% purity).

The in situ generated catalyst (called ‘‘inCoB’’ henceforth)

was tested in a ‘‘semi-batch’’ regime by performing 4 subse-

quent additions of 10 ml of a NaBH4 solution stabilized by

10 mmol of NaOH (4 wt%). Two concentrations of sodium

borohydride were used: either 2 wt% (5.7 mmol in 10 ml) or

19 wt% (64.7 mmol in 10 ml); in the discussion below these will

be referred to as the low (LC) and high (HC) concentration

solutions, respectively. Both concentrations are representa-

tive of those commonly used in the literature [14,15,23–27,34].

It should be stressed that the real ‘‘effective’’ concentration of

NaBH4 in the reaction vessel diminishes with each subsequent

addition of NaBH4 solution, while the concentration of

metaborate product increases. However the comparison of

the catalyst performances between each addition is still

possible, since the overall kinetics of the reaction is of zero

order with respect to [BH4�] [26] and is not impacted by the

presence of metaborate at high borohydride/metaborate

ratios [12].

The reaction is performed at 30 �C and atmospheric pres-

sure, with stirring at 400 rpm in order to avoid diffusional

limitations. In addition to the measurement of the calori-

metric signal, the volume of hydrogen released was also

measured as a function of time (after drying in a liquid

nitrogen trap) using a RITTER TG 01 drum-type volumetric gas

meter with a precision of 2 ml.

Moreover, commercial cobalt nanoparticles provided by

STREM Chemicals (12 nm) were used as a reference catalyst

[46] and compared with our cobalt catalyst generated in situ.

This catalyst is hereafter referred to as ‘‘nCo’’. The specific

hydrogen generation rates (L min�1 gCo�1) for both catalysts

were computed relative to the amounts of Co.

2.2. Characterization of the solution after test

2.2.1. Scanning transmission electron microscopy (STEM)The observations were performed with a FEI XL 30 FEG ESEM

using ‘‘wet STEM’’ mode as described in [47,48]. Environ-

mental scanning electron microscopy (ESEM) was used to

enable the observation of wet samples and to avoid damaging

of the sample. The water layer was kept thin enough to ensure

the crossing of transmitted and scattered electrons by

adjusting the pressure and the temperature at a given value to

evaporate the necessary small amount of water from the

droplet.

2.2.2. 11B NMR11B liquid NMR measurements were carried out on a Bruker

Avance 250 spectrometer. Boric acid was used as reference.

2.3. Characterization of the catalyst after test

After the catalytic tests, the solid catalysts were filtered and

then dried under reduced pressure at room temperature. The

obtained solids were labeled ‘‘LC-inCoB’’ and ‘‘HC-inCoB’’ for

the systems issued from the reactions at low and high NaBH4

concentrations, respectively.

2.3.1. Chemical analysisThe metal content of the samples was determined by induc-

tively coupled plasma atomic emission spectroscopy (ICP-

AES) with a Flamme Perkin–Elmer M 1100 spectrometer after

dissolving the samples in H2SO4/HNO3, and then in a HCl

solution.

2.3.2. Specific surface and porosityN2-physisorption measurements were performed at �196 �C

(Micromeritics 2010 apparatus). The specific surface, pore

volume and pore size distribution were deduced from the

adsorption isotherms by using BET and BJH equations. Prior to

the adsorption measurements, the samples were outgassed in

vacuum at room temperature for 6 h.

2.3.3. Magnetism measurementsMagnetic measurements were performed by the Weiss

extraction method in an electromagnet providing fields up to

21 kOe (2.1 Tesla) at 25 �C. Magnetic measurements were

performed on the powders or on the suspension directly taken

from the reacting slurry. The amount of Co present in the

suspension after the magnetic measurement was determined

by chemical analysis (see Section 2.3.1).

2.3.4. TGA–DTAThermogravimetry/Differential thermal analysis experiments

were performed starting from room temperature up to 800 �C

with a heating rate of 5 �C min�1 under Ar flow on a TG-DTA

Setsys Evolution 12 apparatus from Setaram.

2.3.5. X-ray diffraction (XRD)The phase composition and crystalline state of the samples

were controlled by X-ray diffraction using a Bruker D5005

powder diffractometer where the sample is fixed while the X-

ray tube (Cu Ka1 þ a2;l ¼ 0.154184 nm) and the detector

0 600 1200 1800 2400 3000 3600-0.5

0.5

1.5

2.5

3.5

4.5

Time / s

Differen

tial T

em

peratu

re / °C

-0.5

0.5

1.5

2.5

3.5

4.5

Differen

tial T

em

peratu

re / °C

2357 2397 2437 2477 2517Time / s

0

100

200

300

400

H2 v

olu

me / m

l

a

b

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 e n e r g y 3 4 ( 2 0 0 9 ) 1 1 8 5 – 1 1 9 91188

rotate. X-ray diffraction patterns were recorded between 5

and 90� (2q) with a step size of 0.02� and an acquisition time of

8 s/step. In order to increase the signal/background ratio,

a zero background holder was used (made from a commercial

semiconductor grade silicon wafer grown and cut along the

[1.0.0]-axis, i.e, Si(100)).

Experiments at different temperatures (corresponding to

the phase changes determined by TG–DTA (Section 2.3.4))

were realized under N2 atmosphere, increasing the tempera-

ture in the XRD chamber starting from room temperature up

to 300, 550, and 650 �C respectively with a heating rate of

5 �C min�1.

2.3.6. Transmission electron microscopy (HRTEM andEFTEM)High resolution transmission electron microscopy images

were acquired by a 200 kV JEOL 2010 microscope, with a reso-

lution point of 0.195 nm, and equipped with a Link-Isis EDS X-

ray analyzer. All samples were ultrasonically dispersed in

ethanol at room temperature, and a drop of this suspension

was placed on a holey-carbon thin film supported on

a microscopy copper grid (3.05 mm, 200 mesh).

Analysis of the samples were also performed by EELS in

a LEO 912 energy filtered transmission electron microscope

(EFTEM) at 120 kV, to map the elemental composition of the

different phases.

Fig. 1 – Differential temperature signal (DRC calorimeter)

versus time: The first peak corresponds to a Joule effect and

the second peak to the in situ generation of cobalt boride

nanoparticles (inCoB) (a); Details of the calorimetric peak

and hydrogen generation during the inCoB formation (b).

3. Results and discussion

3.1. Measurement of the energy evolved during thehydrolysis of catalyzed borohydride solutions

3.1.1. In situ generation of the catalystIn Fig. 1a the plot of the differential temperature as a function

of time, measured in the DRC calorimeter, shows two peaks

corresponding to an initial Joule effect calibration and to the

catalyst generation, respectively. Fig. 1b reports a close-up of

the second peak and the corresponding generated volume of

hydrogen.

The initial Joule effect calibration represents an energy of

1200 J generated at constant power during 900 s. We can

notice that the differential temperature rises to a plateau

corresponding to the equilibrium between the energy

produced by the electrode and the energy dispersed in the

system. By contrast, for the generation of the active phase

corresponding to the second peak, the reaction is highly

energetic and much faster. The second peak is the result of

different thermal phenomena happening in a short time, and

at least two contributions to the measured enthalpy can be

deduced from the peak shape. Two different kinetic rates

could also be observed on the hydrogen generation curve

(Fig. 1b): in the first part the rate was about 3.75 mL s�1, while

for the final part it was about 0.8 mL s�1. To interpret these

results it is necessary to analyze the reaction mechanism. In

fact two parallel reactions occur when solutions of CoCl2 and

NaBH4 are put in contact at acidic or neutral pH. The first one

is the direct reaction of borohydride with the Hþ ions

(pH ¼ 6.3) provided by chloride present in the salt (Eq. (2)),

leading to the formation of boric acid:

HþðaqÞ þ 3H2OðlÞ þ BH�4ðaqÞ/4H2ðgÞ þH3BO3ðaqÞ (2)

DrH20 ¼ �285.1 kJ molNaBH4

�1 [6].

The other reaction is the formation of Co2B, the active

phase of the catalyst (Eq. (3)):

2Co2þðaqÞ þ 4BH�4ðaqÞ þ 3OH�ðaqÞ þ 9H2OðlÞ/Co2BðsÞ þ 12:5H2ðgÞ

þ3BðOHÞ�4ðaqÞ (3)

DrH30 ¼ �227.2 kJ molNaBH4

�1 [6,49].

These two reactions are very fast, corresponding to the first

part of the hydrogen generation at a rate of 3.75 mL s�1. This

high initial rate is accompanied by an important increase of the

differential temperature, as shown in Fig. 1b, and corresponds

to the in situ generation of Co2B nanoparticles. At this stage, in

the presence of the in situ generated catalyst, the remaining

amount of NaBH4 can react at basic pH to produce H2 and

B(OH)4� (Eq. (1)), which corresponds to the second and slower

part of the H2 evolution process (after 80 s). The reaction

proceeds at basic pH, the maximum concentration of metabo-

rate is 0.04 mol L�1, and the final pH of the solution is 9.85.

Considering the pH value of the buffer solution of boric acid and

borate (pKA ¼ 9.2), and the solubility domain of metaborates,

the main species present in the solution has to be BðOHÞ�4ðaqÞ.

The cobalt boride nanoparticles formed in situ have been

observed using the Wet-STEM technique; the pictures are

presented in Fig. 2. Large aggregates can also be observed,

Fig. 2 – Wet-STEM observation of the in situ generated

nanoparticles (inCoB).

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 e n e r g y 3 4 ( 2 0 0 9 ) 1 1 8 5 – 1 1 9 9 1189

corresponding to metaborate coalescing at low temperature.

The catalyst is present in the form of homogeneous nano-

metric particles well distributed in the liquid media. The heat

balance for the in situ generation of Co2B phase can be esti-

mated by a linear combination of the reaction energies for Eqs.

(1) and (3). In fact the pH of the solution allows us to neglect

the contribution of the acidic effect (2) to the global energy. By

applying a factor 2 for the generation of Co2B phase and

a factor 3 for the hydrolysis of the remaining NaBH4, the

theoretical value of the global energy corresponds to

0

100

200

300

400

500

0 200 400 600 800 1000 1200Time / s

H2 vo

lu

me / m

l

■ ad 1▲ ad 2+ ad 3● ad 4

■ ad 1▲ ad 2+ ad 3● ad 4

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

0 2000 4000 6000 8000Time / s

Differen

tial T

em

peratu

re / °C

a

b

Fig. 3 – Evolution versus time of the hydrogen generation (a, c)

successive additions of a NaOH-stabilized solution of sodium b

�241.2 kJ molNaBH4�1 . This value is in good agreement with the

measured total evolved heat of �242 kJ mol.NaBH4�1 (corre-

sponding to the second peak of Fig. 1a).

3.1.2. Comparison of the catalyst performances at low andhigh concentration of NaBH4

Fig. 3 shows the time evolution of the hydrogen volume and

differential temperature upon four successive additions of

NaBH4 solution on the catalyst generated in situ (inCoB), using

either a diluted solution LC (Fig. 3a and b) or a concentrated

solution HC (Fig. 3c and d). The hydrogen evolution curves for

each addition show similar profiles, with a kinetic response

that slightly decreases with each successive addition as the

concentration of metaborate and the total volume of the

reactive solution increase. In fact, the presence of residual

sodium metaborate during the reaction may lower the ability

of borohydride to reach the catalyst surface [24]. The decrease

in the hydrogen generation rate might be due to a competitive

adsorption of BH4� and BO2

�, or to an increasing coverage of the

active sites of the catalyst by BO2�, thus lowering the surface

accessibility of BH4� species [25]. The four differential

temperature peaks obtained after successive additions of LC

solution present very similar shapes, but with an increasing

width at half-height (thermokinetic parameter), confirming

that thermal transfer within the solution becomes increas-

ingly difficult as its viscosity increases (Fig 3b). Nonetheless,

the total area of the peak, and hence the total energy evolved

during the reaction, remain constant. On the other hand, the

peaks obtained using the high concentration solution (Fig. 3d)

present a different shape, which shows that several different

thermal phenomena occur simultaneously.

H2 vo

lu

me / m

l

■ ad 1▲ ad 2+ ad 3● ad 4

■ ad 1▲ ad 2+ ad 3● ad 4

Differen

tial T

em

peratu

re / °C

0

1000

2000

3000

4000

5000

6000

0 1000 2000 3000 4000 5000 6000 7000Time / s

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

18000 28000 38000 48000 58000Time / s

c

d

and of the differential temperature (b, d) for the four

orohydride: LC [ 2 wt% NaBH4; HC [ 19 wt% NaBH4.

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 e n e r g y 3 4 ( 2 0 0 9 ) 1 1 8 5 – 1 1 9 91190

The experimental data shown in Fig. 3 have been used to

determine the experimental reaction enthalpies and the

hydrogen generation rates. The reaction enthalpies have been

calculated by integration of the temperature difference (DT in

Fig. 3b, and d), taking into account the evolution of the specific

capacity (Cp) of the system as determined by the Joule-effect

calibrations performed before and after the reaction. The

hydrogen generation rates are determined from the first 50%

of conversion, i.e. the linear initial part of the curve of

hydrogen generation vs. time (Fig. 3a and c). At higher

conversion the kinetics of hydrogen generation is impacted by

the presence of metaborate [24].

Fig. 4 presents a comparison of the experimental reaction

enthalpies (Fig. 4a) and the hydrogen generation rates (Fig. 4b)

Fig. 4 – Comparison of the experimental reaction

enthalpy (a), and hydrogen generation rate (b) for the

hydrolysis of NaBH4 solutions at low and high

concentrations (LC: 2 wt%; HC: 19 wt%) on the in situ

generated catalyst (inCoB).

obtained with the catalyst generated in situ and the

commercial Co nanoparticles using either LC or HC solutions.

For the four additions of the diluted NaBH4 solution (LC), the

measured energies were in the range between �210 and

�222 kJ mol�1 on both the commercial catalyst (nCo) and that

prepared in situ (inCoB). The hydrogen generation rate for the

nCo catalyst was nearly constant at 1.6 L min�1 gCo�1, but for the

inCoB catalyst lower values were observed, decreasing from

1.2 to 0.9 L min�1 gCo�1. Since the global evolved energies

remained stable, the decrease in activity might be induced by

a modification of the liquid viscosity. Quite a different

behavior was observed in the case of the four additions of the

concentrated NaBH4 solution (HC). For the nCo catalyst the

measured reaction enthalpy was found to increase in absolute

value from 207 to 234 kJ mol�1, accompanied by a decrease in

the hydrogen generation rate from 1.8 to 1.2 L min�1 gCo�1. The

characterization of the LC-nCo catalyst after test was detailed

in [46]. For the HC-nCo catalyst the evolution of the catalytic

performances should correspond to a modification of the

carbonaceous residues present at the surface. Meanwhile,

for the inCoB catalyst the enthalpy was nearly constant

between �244 and �250 kJ molNaBH4�1 , with an almost constant

hydrogen generation rate of about 1.3 L min�1 gCo�1 for the first

three additions, changing to 1.2 L min�1 gCo�1 for the fourth

addition. Similar activities have been reported by Zhao et al.

and Jeong et al. for comparable Co-B catalysts [9,10].

A Wet-STEM experiment of the catalyst generated in situ,

taken after tests with the high concentration solution

confirmed that the morphology was preserved during the

reaction, as the nanoparticles remain well dispersed in the

liquid medium.

The enthalpy values measured for the inCoB catalyst

working with a high concentration of NaBH4 correspond well

to the theoretical enthalpy of �250.5 kJ molNaBH4�1 for sodium

borohydride hydrolysis in aqueous solution as expressed by

Eq. (1). This suggests that any difference from that theoretical

value, beyond the 7.4 kJ molNaBH4�1 that can be attributed to

water evaporation [32], should be ascribed to a contribution of

other side reactions. Taking into account that hydrolysis is the

main reaction, two hypotheses should be considered to

explain the enthalpy differences among the various catalysts:

(1) sodium metaborate is not the only hydrolysis product

(different selectivity of the catalyst);

(2) the catalyst is not stable in the reaction mediumd i.e.,

reactions related to catalyst modifications (reduction by

NaBH4 and oxidation by NaBO2) contribute significantly to

the measured enthalpy.

In order to test these two hypotheses we have character-

ized both the spent solution and the solid catalyst obtained

after test.

3.2. Characterization of the HC solution after test

Table 1 summarizes the compositions of the different solu-

tions before and after low or high concentration solution tests.

It can be observed that the compositions of the solutions after

test are very similar both at low and high concentrations. No

significant loss of boron was observed in the solutions, thus

Table 2 – Physicochemical properties of the solid reactionproducts, obtained by filtration of the solutions afterformation of the catalyst (inCoB) and after tests usingNaBH4 concentrations of 2 wt% (LC-inCoB) and 19 wt%(HC-inCoB)

Samplename

Bulkcomposition

wt%

Bulk Co/B(molar ratio)

Specificarea

(m2 g�1)

Averagepore

diameter(nm)

Co B

inCoB 59 6 1.80 50 10.0

LC-inCoB 70 7.5 1.68 43 8.4

HC-inCoB 55 5.8 1.72 57 15.5

Table 1 – Chemical analysis of the solutions after testswith low and high concentrations (2 and 19 wt% NaBH4),respectively

Catalyst Solution Concentration Na/B(molar ratio)

B(g L�1)

Na(g L�1)

– Low concentration

before catalytic test

6 36 2.82

nCo Low concentration

after test

4 31 3.64

inCoB Low concentration

after test

4 28 3.29

– High concentration

before catalytic test

70 171 1.15

nCo High concentration

after test

55 150 1.28

inCoB High concentration

after test

65 171 1.23

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 e n e r g y 3 4 ( 2 0 0 9 ) 1 1 8 5 – 1 1 9 9 1191

confirming that metaborate is the only boron product, and

excluding the formation of borane in the gas phase.

One interesting fact is that the reaction proceeds normally

for the fourth injection. In fact this injection is carried out in

a solution already containing 16.71 g of NaBO2 in 51.7 ml of

water, which corresponds to a boron concentration of

3.59 mol kg�1. This value is higher than that of a saturated

solution at 20 �C of Na2O,B2O3 8H2O and is corresponding to

2.49 mol kg�1. However, Tsuyumoto et al. [50] were able to

obtain a solution containing 5.24 mol kg�1 of B, and explained

its stability by the formation of polyborate ions such as

B9O10(OH)92�, B10O12(OH)8

2�, B11O14(OH)72�, and B12O16(OH)6

2�.

They also reported the presence of spherically shaped poly-

anions B18O282�, B20O32

4� and B20O324�; these heavy ions containing

important amounts of boron are considered to be the origin of

the super-solubility phenomenon.

In order to detect the formation of different boron-con-

taining species in our reaction experiments, we analyzed the

residual solution by 11B-NMR. The chemical shift observed for

all the spent solutions was w18 ppm, which corresponds to

the metaborate B(OH)4� species [51]. This result allows us to

exclude our first hypothesis postulating that the differences in

measured energies might be explained by the formation of

other boron-containing ions than metaborates. In order to

verify the second hypothesis concerning the instability and

evolution of the catalyst in the reacting medium, the solid

catalyst has been separated from the remaining solution at

the end of the test and characterized.

Fig. 5 – X-ray diffractograms of LC-inCoB and HC-inCoB

samples.

3.3. Characterization of the remaining catalyst after LCand HC tests

The specific surface areas, determined by the BET method,

were 43 and 57 m2 g�1 for LC-inCoB and HC-inCoB, respec-

tively. The LC-inCoB samples showed the presence of meso-

pores with maximum porosity at w20 nm, while for the

HC-inCoB this maximum is shifted to w3–4 nm; this is related

to the oxidation of the Co–B particles and closing of the pores.

Table 2 summarizes the characterization of the solids

obtained by filtration of the slurry mixture after the end of the

catalytic tests. The global composition of the LC-inCoB

powder is Co: 70.2, B: 7.6, O: 22.2 (in wt%), which corresponds

to an atomic Co/B ratio of 1.68, whereas the composition of

HC-inCoB was Co: 55, B: 5.8, O: 39.2, corresponding to a Co/B

ratio of 1.72. These values were lower than the stoichiometric

ratio of 2 for Co2B. Glavee et al. [40] have synthesized a sample

with the structure Co(BO2)2, in which the Co/B ratio is 0.50.

One might then assume that the global composition of the

catalysts corresponds to Co2B with part of the surface cobalt in

the form of oxidized cobalt phases due to the contact with

a highly oxidative alkaline solution at the end of the hydro-

lysis reaction.

The X-ray diffractograms of the catalysts after the LC and

HC reaction tests are presented in Fig. 5. The broad peaks in

the X-ray diffraction patterns indicate that the crystalline

phases had very low periodicity, typical of very small crys-

tallite sizes and approaching an amorphous material. The

XRD of LC-inCoB shows a broad peak at 2q ¼ 45�. As deter-

mined from the International Centre for Diffraction Data

(ICDD) and reported in [52,53], the main diffraction lines for

Co2B and for Co appear at 45.7� (d211 ¼ 1.983 A) and 44.3�

(d111 ¼ 2.0467 A), respectively. Due to the broad and amor-

phous-like XRD spectra (reported in Fig. 5) it was not possible

to clearly distinguish between these two cobalt species.

+

? CoOx(BO3)y(OH)z

?? ? ?

* CoO (00-043-1004)

? CoOx(BO3)y(OH)z

* CoO (00-043-1004)

+

*

*

*

* o

o Co hex (04-001-3273)+ Co cubic (04-001-2681)

o

++

+

550°C

650°C

300°C

25°C

a

In

ten

sity / a.u

.

5 25 45 65 85

?? ? ?▼

▼

▼

▼

▼ Co2B (00-025-0241)

o Co hex (04-001-3273)+ Co cubic (04-001-2681)

▼ Co2B (00-025-0241)

+ +

*

*

*

*

*

*o

o+

●

● Co2B2O5 (04-010-63017)

●● ●●

●●●+

+

550°C

650°C

300°C

25°C

2 θ / o

b

Fig. 7 – XRD spectra acquired at 25, 300, 550, and 650 8C for

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 e n e r g y 3 4 ( 2 0 0 9 ) 1 1 8 5 – 1 1 9 91192

Nevertheless, from the elemental analysis the main present

species was Co2B; consequently the peak centered at 45� can

be attributed to the Co2B phase.

Also the peaks centered around 35, 40, and 60� cannot be

univocally attributed. In fact, as reported in Refs 53 to 63, the

CoO, Co3O4, Co(BO2)2 and Co(OH)2 species display diffraction

patterns in the same zone. For the HC-inCoB sample the

contribution of oxidized cobalt phases was higher, as

demonstrated by the simultaneous decreasing of the peak at

45� and increasing of the other diffraction peaks.

In Fig. 6a and b, the TG-DTA curves for the LC-inCoB and

HC-inCoB are reported. Two endothermic peaks related to the

loss of physisorbed and structural water were observed

around 80 �C and 260 �C respectively for the HC-inCoB sample,

while only physisorbed water was detected for LC-inCoB. On

each sample exothermic peaks were detected at 471 and

463 �C respectively: at this temperature the formation of Co

can occur [54–56]. The integration of these peaks (referred to

the same sample weight) shows a relative I(LC-inCoB)/I(HC-

inCoB) ratio close to 2.2.

By comparing the XRD spectra of the same samples (Fig. 7a

and b for LC-inCoB and HC-inCoB respectively) collected at

various temperatures, and in particular at 300 and 550 �C (before

and after the first phase transformation), the disappearing of

the Co2B contribution and the apparition of Co in different

crystalline planes could be observed. This evidence supports

the transformation of Co2B to Co already observed by DTA.

A second exothermic peak was detected only for the HC-

inCoB sample at 606 �C and could be referred to the

-18

-14

-10

-6

-2

-8

-4

0

4

8471°C

80°C

exo

DTA

TG

a

20 220 420 620 820-18

-14

-10

-6

-2

Temperature / °C

-8

-4

0

4

8

606°C463°C

265°C82°C

exo

DTA

TG

Heat flo

w / µ

V

Weig

ht lo

ss / %

b

Fig. 6 – TG-DTA profiles versus temperature for the LC-

inCoB (a) and HC-inCoB (b) samples (heating rate of

5 8C minL1 under Ar flow (50 cm3 minL1)).

the LC-inCoB (a) and HC-inCoB (b) samples.

transformation: 2 CoO B2O3/Co2 B2O5 þ B2O3. This phenom-

enon can also be observed comparing the XRD spectra acquired

at 550 and 650 �C respectively on the HC-inCoB sample.

By coupling the DTA results (I(LC-inCoB)/I(HC-inCoB) ¼ 2.2)

with the analysis of the XRD results we can confirm that nearly

half of the Co2B present in the HC-inCoB sample has been

converted in CoO B2O3 by contact with the high concentration

solution of borate. Information about the ratio of ferromag-

netic cobalt (Co or Co2B) present in the sample were obtained

by comparing the measured saturation magnetization with

the specific saturation magnetization of bulk Co and cobalt

boride, 162 and 66 uemcgs g�1 respectively [40,57].

Fig. 8 visualizes the specific magnetization curves of the

catalysts in suspension in the reactive mixture just after the

hydrolysis reaction (Fig. 8a) and of the filtered and dried solids

(Fig 8b). The degree of oxidation was in the order inCoB < LC-

inCoB < HC-inCoB for the samples in suspension, and the

same order was maintained with the dried solids even if

the absolute value of saturation magnetization was lower. The

higher degree of oxidation of the HC-inCoB sample is also

confirmed by the values reported in Table 3 which for the

dried sample is 18 uemcgs g�1, corresponding to only w27% of

Co present in the form of Co2B, compared to the LC-inCoB

showing a magnetization of 32 uemcgs g�1, corresponding to

w45% of Co present in the form of Co2B.

The TEM micrographs of the dried catalysts LC-inCoB

(Fig. 9a, c and e) and HC-inCoB (Fig. 9b and d) show clusters

composed of Co2B particles oxidized on the surface. Larger

M / u

em

cg

s.g

Co

-1

H / kOe

Catalysts in suspension

Catalysts after filtration and drying

inCoB

LC-inCoB

HC-inCoB

LC-inCoB

HC-inCoB

00

10

20

30

40

50

5 10 15 20

00

10

20

30

40

50

5 10 15 20

a

b

Fig. 8 – Specific magnetization curves of the catalysts in

suspension in the reactive mixture after hydrolysis (a) and

of the solids obtained after filtration and drying (b)

(T [ 573 8C).

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 e n e r g y 3 4 ( 2 0 0 9 ) 1 1 8 5 – 1 1 9 9 1193

particles are observed for the HC-inCoB catalyst after test at

high concentration (Fig. 9b and d), where the conditions at the

end of the reaction are more oxidizing. The TEM patterns in

Fig. 9 reveal differences in the electronic contrasts of the

particles. A light gray coating surrounding higher-contrast

material is observed in Fig. 9c and d, which could be attributed

Table 3 – Magnetic properties of the solid reaction products in ssolution: after formation of the catalyst (inCoB) and after tests(HC-inCoB)

Samplename

Ms (uemcgs g Co�

Catalyst in suspension in

the spent reaction solution

inCoB 54

LC-inCoB 43

HC-inCoB 34

Catalyst after filtration and drying LC-inCoB 32

HC-inCoB 18

to the formation of an oxide layer surrounding the Co2B

particles in the spent hydrolysis solution or during the

washing and drying of the precipitate. A similar outer shell,

attributed to cobalt oxide, has also been observed by Petit et al.

[57] on Co2B nanoparticles obtained by reduction of cobalt

salts by sodium borohydride either in reverse micelles or in

a diphasic system. The same phenomenon was also observed

on iron- based systems by Zhang et al. [58] The micro- and

nanodiffraction patterns of the clusters of fresh precipitate

did not exhibit a sufficiently well- defined ring pattern to allow

crystal identification (Fig. 9e). It can be assumed that the

nanoparticles aggregated at the end of the reaction via the

formation of borate bridges between nanoparticles, yielding

the mesoporous system usually observed [10,18,23].

On the basis of the results of all these characterizations of

the catalysts after test, we will be able to propose a mecha-

nism for the operation of the catalyst, with successive steps of

reduction, catalytic hydrolysis, and oxidation at the end of the

reaction. The hydrogen generation rates of the catalysts

formed in situ are similar to those recently observed by Jeong

et al. [10], who have identified a poorly organized mesostruc-

tured solid; this feature can be related to the high value of the

magnetization, which may cause agglomeration during the

drying step and/or the formation of borate bridges between

the particles during the oxidation. However, during the reac-

tion, the formation of hydrogen on the surface of the catalyst

allows the segregation of the nanoparticles.

This behavior has been confirmed by performing elemental

mapping on inCoB-HC. The corresponding pictures are pre-

sented in Fig. 10.

The chosen zone for observation is constituted of hetero-

geneous agglomerates that provide a global view of all the

species present in the sample. In the upper part of the TEM

image (Fig. 10a) a dark agglomeration of nanoparticles created

during the drying step was observed.

A lighter veil was detected in the middle part of the picture,

while at its bottom two superposed and isolated nanoparticles

were clearly evidenced.

Cobalt was distributed all over the sample (Fig. 10b) with

higher density in the nanoparticles. On the contrary, oxygen

was completely absent in the nanoparticles while its presence

can be detected in the veil and between the nanoparticles,

demonstrating the presence of an oxidized layer around the

nanoparticles (Fig. 10d). Boron was present on and between

the nanoparticles, but only in very little amount in the veil

(Fig. 10c). These observations further demonstrate that the

uspension in the reactive mixture and after filtration of theusing NaBH4 concentration of 2 wt% (LC-inCoB) and 19 wt%

1) % Coas Co2B

Estimation ofparticles size (nm)

Particles >12 nm (%)

75 10 2

60 8 6

48 8 7

45 9 13

27 10 17

Fig. 9 – TEM micrographs of the dried catalysts LC-inCoB (a,

c, e), and HC-inCoB (b, d).

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 e n e r g y 3 4 ( 2 0 0 9 ) 1 1 8 5 – 1 1 9 91194

nanoparticles’ core is constituted of Co2B, surrounded by

a CoB–O phase, and that some cobalt is not incorporated in the

Co2B nanoparticles and remains free to form oxide or

hydroxide cobalt species.

3.4. Influence of the temperature: activation energy

The activation energy has been estimated for inCoB-HC by

studying the dependence of the rate of hydrogen production

on the temperature, in a temperature interval ranging from�5

to 70 �C. For this purpose, DRC experiments have been per-

formed while the catalyst was placed in a thermostated low

concentration solution, and then a second addition of highly

concentrated thermostated solution was performed. The

obtained results for the experiments performed at low

concentration and at high concentration are presented in

Fig. 11.

Except for the measurement performed at a temperature

of �5 �C, the variation of the reaction rate as a function of

temperature follows the Arrhenius law. The activation

energy determined in this manner is �42.7 kJ mol�1.

Compared to other results present in the literature, the in situ

generated catalyst is similar to bulk or supported Co2B

catalysts [7,36].

The amounts of heat evolved during these experiments at

various temperatures are presented in Table 4. At tempera-

tures lower than 0 �C the measured energies were similar for

the low and high concentration solutions. At slightly higher

temperatures, the less concentrated solution led to a lower

measured relative energy per hydrogen formed. On the other

hand, the measured energy decreased as a function of the test

temperature. This phenomenon is due to the formation of

gaseous water at higher temperatures. The temperature

measurement was carried out in the liquid media, so the

formation of water vapor induced an apparent loss of energy.

It can be deduced that the low concentration solution also led

to the formation of gaseous water. In fact, the presence of

large quantities of polar compounds such as B(OH)4� in

aqueous medium induces the formation of strong hydrogen

bonds, which leads to an increase in the vaporization

temperature. The influence of these interactions is less

pronounced in a diluted solution. Since the reaction products

are similar at low and high concentrations, the ability of the

less concentrated solution to form gaseous water is higher

than that of the more concentrated one thanks to the excess

of water. A similar decrease in the measured energy with

a low concentration solution has also been observed by Zhang

et al. [32].

The washed and dried catalyst inCoB-HC was reused in

a high concentration test. The resulting DRC profile and

specific hydrogen generation rate are presented in Fig. 12.

The specific hydrogen flow remained almost constant over

the entire duration of the experiment, with a value of

1.03 mL s�1 mmolCo�1. This value is close to the previously

obtained value over the catalyst generated in situ. The slight

decrease observed might be due to the presence of non-

reducible cobalt species.

A careful examination of the differential temperature on

Fig. 12 shows an exothermic effect at the beginning and at the

end of the reaction. Since the hydrogen generation rate

remains constant, the contribution of the hydrolysis reaction

to the energy evolved should also remain at a constant level

corresponding to the kinetic regime of the catalyst. Thus the

additional exothermic contributions correspond to modifica-

tions of the catalyst. This behavior can be explained by a three

steps mechanism.

According to the characterization results discussed above,

the surface of the catalyst is oxidized. At alkaline pH, in

presence of an oxidizing agent such as metaborate, the stable

species of cobalt might be Co(OH)2. So, in a first stage, we can

Fig. 10 – Elemental mapping of the HC-inCoB catalyst tested at high concentration (a), cobalt (b), boron (c), and oxygen (d).

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 e n e r g y 3 4 ( 2 0 0 9 ) 1 1 8 5 – 1 1 9 9 1195

expect the formation of Co2B by reduction and modification of

the oxidized cobalt surface layer by BH4�:

2CoðOHÞ2ðsÞ þ BH�4ðaqÞ/Co2BðsÞ þ 1=2H2ðgÞ þOH�ðaqÞ þ 3H2OðlÞ (4)

DrH ¼ DfH0(Co2B) þ DfH

0(OH�) þ 3DfH0(H2O) � 2DfH

0(Co(OH)2)

�DfH0(BH4

�).

-2

-1

0

1

2

3

4

5

3.4 3.6 3.8 4.0 4.2 4.4 4.610

4 1/RT

ln

(k)

Fig. 11 – Variation of the reaction rate as a function of

temperature (from -5 to 70 8C) at high (>) and low (,)

concentrations.

DrH ¼ �58.1 � 230 � 3 � 285.8 þ 2 � 539.7 � 48.2 ¼ �114.3

kJ molNaBH4�1 .

This reaction corresponds to the exothermic effect observed

at the beginning of the experiment, as observed by the DRC

(step I). It is also in agreement with the lower hydrogen gener-

ation rate generally observed at the beginning of the reaction.

Table 4 – Evolved energy as a function of temperature andconcentration of the solution

Reactiontemperature (�C)

Evolved energyat LC (kJ molH2

�1)Evolved energyat HC (kJ molH2

�1)

�5 �C �66.5 �63.2

0 �58.0 �62.0

10 �56.1 �61.2

20 �56.2 �63.8

30 �52.2 �60.9

40 �46.6 �54.0

50 �49.3 �53.6

60 �47.1 �48.6

70 �47.3 �46.2

Average values in the

range 0–30 �C

�55.6 �62.0

Tests performed with HC-inCoB catalyst.

Fig. 12 – Differential temperature and hydrogen production rate evolution versus time for the reaction performed at 30 8C

with the high concentrated solution in presence of 33 mg of HC-inCoB. (I) activation, (II) hydrolysis, (III) deactivation-

oxidation of the catalyst.

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 e n e r g y 3 4 ( 2 0 0 9 ) 1 1 8 5 – 1 1 9 91196

The second step (II) of the mechanism takes place when all

the reducible cobalt is in form of Co2B, and corresponds to the

catalyzed hydrolysis of borohydride (Eq. (1)).

The hypothesis that Co2B phase is the active phase for the

reaction is in agreement with the evidences provided from the

different characterizations and with the high hydrogen

storage capacity of this compound [59]. The exothermic effect

at the end of the experiment (step III) may correspond to the

surface reoxidation of cobalt boride to form nanocapsules,

a phenomenon that has been reported in the literature [58] for

Fe nanoparticles, according to the reaction:

Co2BðsÞþOH�ðaqÞþ7H2OðlÞ/2CoðOHÞ2ðsÞþBðOHÞ�4ðaqÞþ7=2H2ðgÞ (5)

Table 5 – Comparison of the experimental enthalpy data deter

Measured heat(kJ molNaBH4

�1 )Reacting system Catalyst

�287.8 12 wt% NaBH4 (s) þ H2O – M

P

�282.4 11.7 wt% NaBH4(s) þ 2.9 wt%

NaOH(s) þ H2O

– M

P

�311.7 11.4 wt% NaBH4(s) þ 2.8 wt%

NaOH(s) þ H2O

28 wt% iron

oxide þ NaBH4

M

P

�210 � 11 3 wt%NaBH4(s) þ H2O 5 wt%RuCl3þ NaBH4

�216 � 6 2 wt% NaBH4 (aq) þ 4 wt%

NaOH(aq) þ H2O

InCoB-LC L

re

�247 � 6 19 wt%NaBH4 (aq) þ 4 wt%

NaOH(aq) þ H2O

InCoB-HC H

re

DrH ¼ 2DfH0(Co(OH)2) þ DfH

0(B(OH)4�) � DfH

0(Co2B) � 7DfH0

(H2O) � DfH0(OH�).

DrH ¼ �2 � 539.7 � 1345.5 þ 58.1 þ 7 � 285.8 þ 230

¼ �136.2 kJ molNaBH4�1 .

This reaction explains the second exothermic effect, often

accompanied by an extra release of hydrogen, generally

observed with cobalt based catalysts.

3.5. Comparison of the measured enthalpies withexperimental data in the literature

Zhang et al. [32] have experimentally characterized the heat of

sodium borohydride hydrolysis using a microcalorimetric

mined for NaBH4 hydrolysis by different authors

Remarks Calorimeter type Ref.

ax. obs. Temp. 210 �C Pressure Tracking

Adiabatic Calorimeter (TIAX)

[31]

ressure 2025 psi

ax. obs. Temp.239 �C [31]

ressure 2112 psi

ax. obs. Temp.213 �C [31]

ressure 1998 psi

OmniCal CRC90 isothermal

micro-calorimeter

[32]

ow concentration

action

DRC This work

igh concentration

action

DRC This work

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 e n e r g y 3 4 ( 2 0 0 9 ) 1 1 8 5 – 1 1 9 9 1197

technique, in a system where solid NaBH4 reacts with water in

excess on a RuCl3 catalyst. Their measured average heat of

reaction was �210 � 11 kJ molNaBH4�1 which according to the

experimental conditions should correspond to the hydrolysis

reaction previously cited:

NaBH4ðsÞ þ 4H2OðlÞ/NaBðOHÞ4ðaqÞ þ 4H2ðgÞ (1)

The total heat evolved in this system should be the sum of the

energy of dissolution of NaBH4 (32 kJ mol�1) [60] and that of

the hydrolysis reaction (�250.5 kJ molNaBH4�1 , Eq.(1)), which

leads to DrH ¼ �218.5 kJ molNaBH4�1 . This energy corresponds to

the one calculated by Kojima et al. [35]. Thus it seems likely

that the measurement of the heat of reaction was affected by

the evaporation of water due to the use of a low concentration

solution of borohydride. For reference, a comparison of the

results published in the literature regarding the evolved

energies is given in Table 5.

4. Conclusions

Nanoparticles of cobalt boride generated in situ present totally

different shapes in the reaction media or after washing and

drying. This evolution of the catalyst is due to the formation of

borate and/or oxide species on the surface and to the

agglomeration of particles upon drying. In situ monitoring of

the reaction process is therefore very important, and

complementary to the recording of the rate of hydrogen

generation. In this respect, liquid-phase calorimetric methods

are very powerful.

By studying the in situ generation of cobalt nanoparticles

we can conclude that, starting from CoCl2, the first phenom-

enon that takes place is the direct reaction of borohydride

with acid in parallel with the formation of the active cobalt

boride phase. The second stage corresponds to the catalytic

reaction proper. Thus, the generation of the catalyst is easy

and leads to an active and stable catalyst.

A comparison between the performances obtained with

a low or high concentration NaBH4 solution indicates that the

actual reaction energy is around �247 kJ molNaBH4�1 , corre-

sponding to the formation of tetrahydrated sodium metabo-

rate species. At lower concentration the measured evolved

energy is lower, closer to that observed in the literature:

�216 kJ molNaBH4�1 . This lower energy results from an evapo-

ration of water during the generation of hydrogen.

The study of the catalytic reaction with the high concen-

tration solution on the cobalt nanoparticles generated in situ

leads to the conclusion that cobalt participates in the reaction

process in three successive steps:

(1) Reactivation of the catalyst upon a new addition of NaBH4

(Eq. (4)).

(2) Catalytic hydrolysis of borohydride by the cobalt boride

phase (Eq. (1)).

(3) Oxidation of cobalt boride (to cobalt borates, oxides and

hydroxides) (Eq. (5)).

Finally, the key parameter for the catalytic hydrolysis of

sodium borohydride over cobalt catalysts is the formation and

the stability of the cobalt boride phase, as shown by the

modification of the catalytic behavior of the commercial

nanoparticles during the high concentration test.

Acknowledgments

This work was supported by the ANR-PANH, CASTAFHYOR

project. The authors gratefully acknowledge the scientific

services of IRCELYON and in particular B. Jouguet for the TG-

DSC-MS analyses, S. Mangematin for the NMR analyses of

solutions, N. Cristin for performing the adsorption isotherms,

P. Mascunan for the ICP analyses, G. Bergeret for his help with

the powder XRD analyses, and F. Simonet, and B. Vacher for

the TEM and SEM analyses.

r e f e r e n c e s

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[3] Satyapal S, Read C, Adams J, Stetson N, Ordaz G, Gardiner M.DOE Hydrogen Program, Pre-solicitation meeting 2007, http://www.hydrogen.energy.gov/pdfs/precoe_presentation.pdf;October 2007.

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