The Molecular-Kinetic Approach to Hydrolysis of Boron Hydrides for Hydrogen Production

Article abstract of DOI:10.1134/S0023158419010075

Abstract: In this study, Langmuir–Hinshelwood and Michaelis–Menten kinetic models are applied to describe the kinetic behaviour of the Co–B catalyst in the hydrolysis of 0.12 M aqueous solutions of boron hydrides at temperatures from 22 to 60°C. Boron hyd

Full text of DOI:10.1134/S0023158419010075

ISSN 0023-1584, Kinetics and Catalysis, 2019, Vol. 60, No. 1, pp. 37–43. © Pleiades Publishing, Ltd., 2019.
The Molecular-Kinetic Approach to Hydrolysis of Boron Hydrides
for Hydrogen Production1
B. Coşkuner Filiza, * and A. Kantürk Figena
aChemical Engineering Department, Yildiz Technical University, Davutpasa Campus, Topkapi, Istanbul, 34210 Turkey
*e-mail: bilgecoskuner@gmail.com
Received May 11, 2018; revised June 12, 2018; accepted June 19, 2018
Abstract—In this study, Langmuir–Hinshelwood and Michaelis–Menten kinetic models are applied to
describe the kinetic behaviour of the Co–B catalyst in the hydrolysis of 0.12 M aqueous solutions of boron hydrides
at temperatures from 22 to 60°C. Boron hydrides are selected as sodium borohydride (NaBH4, 10 wt % NaOH)
and ammonia borane (NH3BH3). Based on the Langmuir–Hinshelwood kinetic approach, it is found that
under the same reaction conditions the NaBH4–Co–B catalyst interaction is more effective than that of the
NH3BH3–Co–B. According to the Langmuir–Hinshelwood model, apparent activation energies (Ea) for
the hydrolysis of NaBH4 and NH3BH3 over Co–B catalysts are calculated to be 45.38 and 57.37 kJ/mol,
respectively.
Keywords: boron hydrides, hydrolysis, Co–B catalyst, Langmuir–Hinshelwood, Michaelis–Menten
DOI: 10.1134/S0023158419010075
dissolving NaBH4 in the water BH4− anions are formed
INTRODUCTION
Catalytic hydrolysis of boron hydrides for hydrogen
production has been reported a long time ago. The
first study was published in 1953 by Schlesinger et al.
who described the sodium borohydride (NaBH4)
hydrolysis at ambient conditions and found that the
hydrogen generation rate is a function of the concen-
that react with water molecules to generate hydrogen
gas. The total hydrolysis of NaBH4 is given by
catalyst
(I)
.
NaBH4(I) + 4H2O(I) ⎯⎯⎯⎯→NaBO2(aq) + 4H2(g)
Ammonia borane (NH3BH3) is the latest popular
hydrogen storage material for hydrogen production
from boron hydrides. It is very competitive and alter-
tration of BH4− and pH value of BH4− solution [1].
NaBH4 belongs to the class of boron hydrides with native to NaBH4 with its higher hydrogen content
a hydrogen content of 10.8 wt %. It easily reacts with (19.5 wt %). NH3BH3 is stable in aqueous solutions
water and releases 4 moles of H2 for every one mole of
and this property is advantageous from the practical
the hydride. For this reason, its solutions are prepared point of view. NH3BH3 reacts with two moles of
at high pH by alkali agents such as NaOH, KOH and
catalysts are used to activate NaBH4 solutions [2]. By
water to generate three moles of hydrogen molecules
as seen below [3].
catalyst
(II)
NH3BH3(I) + 2H2O(I) ⎯⎯⎯⎯→(NH4)BO2(aq) + 3H2(g)
.
Kinetic modelling is a powerful tool used to inves-
tigate kinetics of heterogeneous reaction of metal cat-
alysed hydrolyses of boron hydrides. Various kinetic
models have been proposed as power law models.
Among these are zero-order, first-order second-order
and nth-order bimolecular kinetic models based on
Hydrolyses of these promising boron hydrides have
been investigated by many researchers using different
noble [4, 5] and base [6–8] metal catalysts. Among the
variety of catalysts, Co catalysts have a great potential due
to their good to excellent activities and cost-effectiveness
[9, 10]. Metals [11], nanoparticles [12], salts [13], oxides Langmuir–Hinshelwood, Michaelis–Menten and the
semi-empirical kinetics [17].
[14, 15], borides [9, 10] and Co based catalysts [16] were
repeatedly used to hydrolyse boron hydrides.
The Langmuir–Hinshelwood mechanism assumes
that the heterogeneous reaction takes place between
species adsorbed on the catalyst active sites and a sur-
1
The article is published in the original.
37

38
B. COŞKUNER FILIZ, A. KANTÜRK FIGEN
face reaction yields products and by-products, which [18]. In addition, this model at low temperatures and
are desorbed into the bulk medium. The reaction
mechanism includes three steps such as (i) equili-
brated adsorption of BH4 and H2O, (ii) the reaction
of the adsorbed BH4 and H2O and (iii) desorption of
concentrations obeys a zero order kinetic law, while
the first order kinetics is relevant to description at
higher temperatures [19]. Based on this knowledge,
we can depict the hydrolysis mechanism as shown in
products and by-products from the catalyst surface Scheme 1.
Langmuir–Hinshelwood mechanism
XBO2
H2
H2
H2
BH4–
BH4–
H2O
H2O
Catalyst active surface
Adsorption step
Reaction step
Desorption step
H2
Michaelis–Menten mechanism
H
2 H2
H2O
BH4–
XBO2
BH4–
Catalyst active surface
Adsorption step
Reaction step
Desorption step
Scheme 1. The Langmuir–Hishelwood and Michaelis–Menten mechanism of catalytic hydrolysis.
Originally, the Michaelis–Menten equation is pro-
It is well known that the behaviour of the boron
posed to outline the mechanism governing the enzy- hydrides hydrolysis depends on the nature and
matic sugar inversion, and it is also of use for describ- amount of catalysts and boron hydrides. Depending
ing heterogeneous catalysed reaction. The mechanism on reaction conditions, hydrolysis of boron hydrides
involves a three-step cycle.
in the presence of Co based catalysts shows zero- and
first-order reaction kinetics. Fernandes et al. tested
CoB catalysts prepared by the chemical reduction
method in an aqueous NH3BH3 solution and reported
Ea of the system as equal to 44.00 kJ/mol [3]. Qui et al.
prepared Co–B catalysts in situ via chemical reduc-
tion and carried out the catalytic hydrolysis of
NH3BH3 to generate hydrogen with activation energy
of 34.10 kJ/mol [13]. Metin et al. synthesized SiO2
embedded Co catalysts for hydrolysis of NH3BH3
solution and reported Ea of 42.00 kJ/mol [21]. Also
Luo et al. performed NH3BH3 hydrolysis in the pres-
ence of co-precipitation Co@M41S catalyst with Ea
found to be 55.00 kJ/mol [22]. Similarly, Ye et al. have
prepared Co–αAl2O3 catalysts and estimated Ea=
51.50 kJ/mol for NaBH4 hydrolysis [23]. Similarly, Xu
et al. prepared Co–αAl2O3 catalysts by the conven-
tional impregnation method and tested for it in the
hydrolysis of NH3BH3 with an activation energy of
62.00 kJ/mol of Ea [24]. Sahiner et al. prepared
(1) Reactant is adsorbed on the catalyst surface.
(2) Catalyst first reacts reversibly with the reactant,
forming a catalytic intermediate and a second reactant
interacts with catalytic intermediate.
(3) Intermediate decomposes to give the catalyst,
product (H2) and by-products (NaBO2/(NH4)BO2).
At this irreversible step the product and by-products
are desorbed from catalyst surface.
Based on this model (see Scheme 1), BH4− ions are
adsorbed on the active sites of catalytic surface and
MBH4 intermediate complex forms that reacts with
four water molecules from the aqueous phase to pro-
duce B(OH)4 species, desorption of which releases
4 moles of H2. Michaelis–Menten model satisfactorily
describes the NaBH4–Co–B interaction at high BH−
concentrations, but is hardly applicable at lower con4-
centrations of BH4− [20].
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THE MOLECULAR-KINETIC APPROACH
39
p(AMPS)–Co catalyst using adsorption and chemical responsible for hydrogen generation can be repre-
reduction methods and reported Ea = 42.00 kJ/mol for sented by the equation:
the NH3BH3 hydrolysis [12]. In our previous study, we
d[H2]
dt
vmax[C]
KM + [C]
(4)
=
,
synthesized Co–B amorphous catalysts via solid-state
reaction and Ea was determined to be 48.07 kJ/mol [25].
where
is the maximum H2 generation rate in log-
vmax
The present study is concerned with kinetic inves-
tigations of the hydrolysis of boron hydrides with the
Co–B catalyst. Two different molecular kinetic mod-
els were applied to shed some light into the kinetics of
hydrolysis of two different boron hydrides (NaBH4
and NH3BH3). The results were compared to charac-
terize and describe the Co–B catalysts behaviour in
the presence of 0.12 M aqueous solutions of NaBH4
and NH3BH3. In addition, by using Langmuir–Hin-
shelwood and Michaelis–Menten kinetic models, the
kinetic mechanism of catalytic reaction between cata-
lyst and boron hydrides was analysed in detail. It was
found that under the same conditions the NaBH4–
Co–B catalyst interaction was more effective than that
of the NH3BH3–Co–B system.
arithmic scale of the hydrolysis reaction and KM is
Michaelis–Menten constant. In this method, the use
of the Lineweaver–Burk plot makes it easier to deter-
mine these values and the Eq. (4) becomes:
KM
−r vmax[C] vmax
1
1
(5)
=
+
.
EXPERIMENTAL
In the present study, Co–B catalyst was prepared
using a sol–gel method according to the procedure
described in our previous study. Amorphous to X-ray
Co–B catalyst (Co : B molar ratio ≈0.29) had a surface
area of 122.70 m2/g with particle size <500 nm [28].
All reagents used in this research were of analytical
reagent grade. NH3BH3 (97% purity) from Aldrich
was used as received. NaBH4 with a purity level ≥96%
was supplied by Fluka. Sodium hydroxide NaOH was
purchased from Labor Technic, and used as a stabi-
lizer for NaBH4 solutions. To gain insight into the
mechanistic aspects of the Co–B catalysed hydrolysis
of 0.12 M NaBH4 in an alkaline medium and 0.12 M
NH3BH3 the kinetics of the reaction was investigated
by hydrogen generation measurements. Two 0.12 M
boron hydride solutions were prepared by dissolving
boron hydrides in deionized water. In addition, to sta-
bilize NaBH4, basic aqueous solution was prepared
with 10 wt % NaOH. A constant amount of the cata-
lyst (19 wt %) was maintained in all hydrolysis runs.
The Co–B catalysed hydrolysis tests of boron
hydrides were performed at temperatures from 22 to
60°C in a 15-mL hydrolysis-glass reactor, which was
immersed in a water bath with a temperature con-
trolled system ( 2°C). The generated hydrogen was
collected by the water displacement procedure. Out-
put of reactor was connected with a piece of rubber
hose to transfer the evolved hydrogen to the water-
filled inverted burette in order to follow with the
burette the increase in volume of H2 generated during
the hydrolysis. Hydrogen evaluation time was mea-
sured simultaneously with the chronometer. The
hydrolysis-glass reactor was kept under magnetic stir-
ring at 500 rpm during the hydrolysis.
THEORETICAL
Now we can consider how Langmuir–Hinshel-
wood and Michaelis–Menten models can be used to
explain the mechanism governing the hydrolysis of
boron hydrides in the presence of the Co–B catalyst.
In accord with the kinetic model of Langmuir–Hin-
shelwood, reactants are initially adsorbed on the sur-
face of the catalyst and then the reaction between
adsorbed species occurs as a second step to generate
hydrogen and at the last step hydrogen is evolved and
by-products are desorbed from the catalyst surface.
Since water was in excess in the hydrolysis reaction
medium, its concentration was assumed to be con-
stant. Based on these assumptions, the kinetic models
were applied with simplified representation of the
adsorbed species [2, 26].
Hydrogen generation rate can be expressed as:
KaC
1 + KaC
dC
dt
(1)
= −rLWCH = −k
,
where
,
,
and
are the adsorption constant,
rLWCH
Ka C k
concentration of hydride, Arrhenius rate constant and
reaction rate, respectively. Calculation of
(Eq. (2))
Ka
is fulfilled by minimizing the correlation coefficients
derived from the data recorded at 40 and 60°C.
min Ka f (Ka) = (1 − R420°C) + (1 − R620°C).
Integrating Eq. (1) gives the reaction rate:
(2)
C
1




0
RESULTS AND DISCUSSION
The change in molarity of boron hydride solutions
(3)
ln
+ (C − C) = kt.
0
Ka  C 
Michaelis–Menten kinetic model envisages a as a function of the hydrolysis time over the Co–B cat-
reversible adsorption on the active sites. The main step alyst is shown in Fig. 1. Both boron hydrides showed a
in this model is kinetically controlled adsorption of the similar pattern of behaviour illustrating the tempera-
reactant on the catalyst surface [20, 27]. The reaction ture dependence of the hydrolysis depth, although
KINETICS AND CATALYSIS Vol. 60 No. 1 2019

40
B. COŞKUNER FILIZ, A. KANTÜRK FIGEN
(a)
0.12
0.08
0.04
22°C
HGR: 260.36 mLH2 min–1 g–1
IT: 150 min
40°C
HGR: 857.27 mLH2 min–1 g–1
IT: <1 min
2
60°C
1
HGR: 2938.4 mLH2 min–1 g–1
IT: <1 min
3
0
100
200
Time, min
300
400
(b)
0.12
0.08
0.04
22°C
HGR: 174.64 mLH2 min–1 g–1
IT: 100 min
40°C
HGR: 357.00 mLH2 min–1 g–1
IT: 6 min
2
60°C
HGR: 1081.06 mLH2 min–1 g–1
IT: <1 min
1
3
0
100
200
300
Time, min
Fig. 1. Hydrolysis of hydride solutions containing light weight complexes in 10 wt % NaOH + 0.12 M NaBH solution (a) and
4
0.12 M NH BH solution (b) at different temperatures: (1) 22°C, (2) 40°C, (3) 60°C. The ranges of experimental errors are indi-
3
3
cated on the curves.
their H2 generation rates (HGR) were different. As the
temperature rise, the number of boron hydride parti-
cles colliding with the Co–B catalyst increases on a
pro rata basis. The H2 generation rates given in Fig. 1
are nearly three times higher for NaBH4 than for
NH3BH3. At room temperature, induction time (IT),
the time needed to develop strong interaction between
active sites and boron hydride particles, was longer for
NaBH4 (∼150 min) than for NH3BH3 (∼100 min).
While lower temperatures (22°C) required more time
for the catalyst to react with boron hydrides actively,
hydrolysis reaction almost immediately started at
higher temperatures (≥40°C).
Langmuir–Hinshelwood kinetic model was
applied to our results in an attempt to determine the
adsorption constant
(Eq. (2)). Although both sub-
Ka
strates were hydrolysed in the presence of the same
catalyst, the values were determined to be 611.00
Ka
and 525.50 L/mol for NaBH4 and NH3BH3, respec-
tively (Table 1). The divergence can be ascribed to dif-
ferent molecular interactions. This means that, under
certain conditions, NH3BH3 molecules are stronger
adsorbed on the Co–B catalyst surface than NaBH4
molecules. While NH3BH3 is stable in water, stability
of NaBH4 is associated with higher pH values and such
stabilizers as NaOH or KOH need to be used in aque-
ous solutions.
According to information published in literature
and derived from our earlier studies, the best results for
H2 generation were obtained when alkaline NaBH4
solutions containing 10 wt % NaOH were used [29].
Experiments with NaBH4 solutions with a higher or
lower NaOH concentration showed a decreased
hydrogen generation rate [23]. The hydrolysis of
NaBH4 over Co–B catalysts surface occurred in three
Table 1. Langmuir–Hinshelwood kinetic constants for the
hydrolysis of boron hydrides
Ka,
Ea,
Boron
k,
hydrides
mol L/min
L/mol
kJ/mol
NaBH4
NH3BH3
611.00
525.50
45.38
57.37
11.18
16.15
main steps: (i) adsorption of BH4−, (ii) hydrolysis reac-
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(a)
41
(a)
0.15
0.10
0.05
0.15
3
3
2
1
2
1
0.10
0.05
0
200
Time, min
200
0
100
Time, min
200
300
(c)
1/T, 1/K
0.0029
–4
0.0031
0.0033
0.0035
y = –6900x + 16.15
R2 = 0.999
–6
–8
y = –5458x + 11.17
R2 = 1
Fig. 2. Langmuir–Hinshelwood kinetics for hydrolysis of light weight complexes in hydride solutions of 10 wt % NaOH + 0.12 M
NaBH (a) and 0.12 M NH BH (b) for different temperatures: (1) 22°C, (2) 40°C, (3) 60°C. (c) Arrhenius plots for Langmuir–
4
3
3
Hinshelwood kinetics calculated for different boron hydrides (r—NaBH , j—NH BH ).
4
3
3
tion of BH4− and (iii) desorption of H2 and by-products
Table 2 shows the
and
values registered for
KM
vmax
from the catalyst surface. Adsorption of BH4− on the
catalyst surface can be considered as the rate-deter-
mining step [30]. In NaBH4 solutions stabilized by
both boron hydride solutions at different tempera-
tures. The Michaelis constant
between the reactant and the active phase of catalyst.
explains the affinity
KM
The value of
depends on the nature of the catalyst
KM
NaOH BH4− ions compete with NaOH for adsorption
sites on the catalyst surface [2].
and the substrate and on operating conditions such as
temperature and pH. The influence of temperature on
values indicates that this parameter improves cat-
KM
Figure 2 shows application of the Langmuir–Hin-
shelwood kinetic model to the process of H2 genera-
tion by hydrolysing 0.12 M solution of boron hydrides
alytic efficiency under the conditions chosen for our
experiments. As the temperature was increased from
at different temperatures. Arrhenius equation was also 22 to 40°C and then to 60°C
values found for
KM
used to compare Ea values calculated for different NH3BH3 solutions decreased by ∼26 and ∼32%,
boron hydrides (Fig. 2c). In contrast to values, gen- respectively. Lower values of KM characterise more
Ka
efficient catalysts. The nature of reactants and reac-
tion conditions can also affect the hydrolysis kinetics
as evidenced by the results shown in Table 2. Lower
eration of H2 from NaBH4 proceeds with a lower acti-
vation energy (45.38 kJ/mol) than that from NH3BH3
(57.37 kJ/mol).
values obtained for NaBH4 compared to those
KM
Michaelis–Menten kinetic model was performed
by the determination of the logarithmic phase of H2
generation data of 0.12 M boron hydride solutions
(Fig. 3). Three different reaction temperatures showed
a logarithmic phase in different time and/or concen-
tration range. As it could be seen from Lineweaver–
found for NH3BH3 apparently suggest that the catalyst
affinity of the NaBH4–Co–B system is superior to
that of the NH3BH3–Co–B catalyst.
According to the Langmuir–Hinshelwood model,
the value corresponds to the total number of adsorp-
k
Burk plots, NaBH4 and NH3BH3 also have different tion sites on the catalyst surface. However in the light
values. of our findings the value can be defined as the reac-
k
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42
B. COŞKUNER FILIZ, A. KANTÜRK FIGEN
(a)
(b)
8000
6000
4000
2000
0
1500
1000
500
22°C
40°C
–25
25
75
125
175
0
100
200
1/C, L/mol
300
1/C, L/mol
(d)
1/T, 1/K
(c)
0.0029
0.0031
0.0033
0.0035
300
60°C
–4.5
–5.5
–6.5
–7.5
y = –4523x + 8.640
R2 = 0.995
200
100
y = –6254x + 14.000
R2 = 0.990
0
50
100
150
1/C, L/mol
Fig. 3. Application of Michaelis–Menten kinetic model for hydrolysis of light weight complex in hydride solutions (10 wt %
NaOH + 0.12 M NaBH (r) and 0.12 M NH BH (j)) for different temperatures: 22°C (a), 40°C (b) and 60°C (c). (d) Arrhenius
4
3
3
plots for Michaelis–Menten kinetics calculated for different boron hydrides.
tion rate for the elementary step of adsorption (Table 2).
Moreover, based on the Michaelis–Menten kinetic
model, the hydrolysis of NaBH4 solutions demon-
CONCLUSIONS
In this study, the heterogeneous reaction of hydro-
lysis of boron hydrides in the presence of the Co–B
catalyst was investigated. Langmuir–Hinshelwood
and Michaelis–Menten kinetic models provide two
molecular approaches. While Langmuir–Hinshel-
wood kinetic model suggests a surface reaction
between reactants initially adsorbed on active sites,
Michaelis–Menten kinetic model envisages reversible
adsorption on active sites. In our experiments water
was taken in excess and its concentration may be taken
strates higher
and lower Ea values than that of
vmax
NH3BH3 solutions (Table 2). Comparing the applica-
bility of kinetic models, a conclusion can be made that
the hydrolysis data of boron hydrides obeyed Lang-
muir–Hinshelwood kinetic model with 1 and 0.9996
correlation co-factors. From the kinetic results, it can
be inferred that hydride complexes initially absorbed
on the Co–B catalysts react to generate hydrogen.
Table 2. Michaelis–Menten kinetic constants of boron hydrides hydrolysis reaction
vmax, mol L–1 min–1
KM, mol/L
Ea, kJ/mol
k, mol L/min
Tempera-
ture, °C
NaBH4
NH3BH3
NaBH4
NH3BH3
NaBH4
37.61
NH3BH3
NaBH4
NH3BH3
22
40
60
0.0012
0.0032
0.0069
0.0008
0.0022
0.0090
0.033
0.028
0.011
0.0051
0.0038
0.0035
52.00
8.64
14.00
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THE MOLECULAR-KINETIC APPROACH
43
11. Cho, K.W. and Kwon, H.S., Catal. Today, 2007,
constant. Correspondingly, simplifications in our cal-
culations were warranted. According to Langmuir–
Hinshelwood and Michaelis–Menten kinetic models,
Ea values for NaBH4 and NH3BH3 were found to be
45.38, 57.37 kJ/mol and 37.61, 52.00 kJ/mol, respec-
tively. Comparing results of kinetic investigation car-
ried out at the same temperature and concentration of
boron hydrides with allowance for correlation co-fac-
tor values, it can be postulated that the Langmuir–
Hinshelwood kinetic model provided a more relevant
explanation for the behavior of our system.
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no. 2012-07-01-GEP01) for its financial support.
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