Reaction intermediate/product-induced segregation in cobalt-copper as the catalyst for hydrogen generation from the hydrolysis of sodium borohydride

Article abstract of DOI:10.1039/c6ra22998k

Cobalt is the most attractive catalyst for hydrogen generation from the hydrolysis of sodium borohydride, NaBH₄, but its potential is further improved when it is combined with an inactive element like copper. Accordingly, several cobalt-copper catalysts (Co_xCu_1-x, with x as a mole ratio equal to 0, 0.1, 0.25, 0.5, 0.75, 0.9 or 1) were prepared. Under our conditions, Co_0.9Cu_0.1 shows the best performance, being able to complete H₂ evolution in <4 min (vs. <7 min for Co). However, Co_0.9Cu_0.1 is not as stable as expected; after the first cycle, the catalytic activity in terms of the H₂ generation rate halves, and then remains quite constant for additional cycles (up to five under our conditions). XPS measurements show that the surface composition of Co_0.9Cu_0.1 is subject to changes during hydrolysis; the anti-segregation of copper concomitantly takes place with the segregation of cobalt. This is explained through the occurrence of borate-induced segregation, favored due to the well-known strong affinity of cobalt for borate species. In other words, the catalytic activity of cobalt can be improved through combination with copper but, under our conditions, it cannot be stabilized. This is evidenced and discussed in detail herein.

Full text of DOI:10.1039/c6ra22998k

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Reaction intermediate/product-induced segregation for cobalt-
copper as catalyst in hydrogen generation by hydrolysis of sodium
borohydride
Received 00th January 20xx,
Accepted 00th January 20xx
H. Kahri,a,b V. Flaud,c R. Touati,b P. Miele,a U. B. Demircia
DOI: 10.1039/x0xx00000x
Cobalt is the most attractive catalyst for hydrogen generation by hydrolysis of sodium borohydride NaBH4, but its potential
is further improved when it is combined with an inactive element like copper. Accordingly, several cobalt-copper catalysts
(CoxCu1-x with x as a mole ratio equal to 0, 0.1, 0.25, 0.5, 0.75, 0.9 or 1) were prepared. In our conditions, Co0.9Cu0.1 shows
the best performance, being able to complete the H2 evolution in <4 min (vs. <7 min for Co). However, Co0.9Cu0.1 is not as
stable as expected: after a first cycle, the catalytic activity in terms of H2 generation rate halves, and then remains quite
constant for additional cycles (up to five in our conditions). XPS measurements show that the surface composition of
Co0.9Cu0.1 is subject to changes during hydrolysis: anti-segregation of copper concomitantly with segregation of cobalt
takes place. This is explained by occurrence of borate-induced segregation favored by the well-known strong affinity of
cobalt with borate species. In other words, the catalytic activity of cobalt can be improved by combination with copper
but, in our conditions, it cannot be stabilized. This is evidenced and discussed in details herein.
www.rsc.org/
the catalytic reaction and the nature of the catalytically active
site remain unclear; for more details, the reader is invited to
go through two reviews dedicated to cobalt in hydrolysis of
Introduction
Sodium borohydride NaBH4 in (alkaline) aqueous solution is a
liquid-state hydrogen carrier. Since the early 2000s, it has
shown to be attractive in the field, mainly by the fact that it
generates four molecules of hydrogen in ambient conditions
by hydrolysis:1,2
NaBH4.5,6
One of the weaknesses of cobalt is that it deactivates
because of surface passivation due to strong adsorption of
borates.7 Less adsorption may be expected by modifying the
electronic structure of cobalt through alloying with a transition
metal,8 especially an element with low catalytic activity in
hydrolysis of NaBH4 like copper. To our knowledge, the effect
of copper on the stability (or durability) of cobalt over cycles
has not been demonstrated yet, even though there are few
reports that mainly show improved catalytic activity; this
beneficial effect is explained by prevention of the catalytic
particles agglomeration or increase of the electron density of
cobalt.9-11 With another liquid-state hydrogen carrier, i.e.
aqueous ammonia borane NH3BH3, cobalt-copper was found
to have better stability than pure cobalt. No significant activity
loss was observed for cobalt-copper films in five consecutive
cycles and this was explained in terms of synergetic effect of
the binary components.12 Similar results were reported for
cobalt-copper nanoparticles encapsulated in the pores of a
metal-organic framework and the catalyst stability over five
hydrolysis cycles was attributed to geometric effects.13 With
cobalt-copper nanoparticles supported onto carbon or silica
after ten catalytic reuses, a 5-7% activity decrease was found
without agglomeration and leaching throughout the runs.14,15
However, in one report, cobalt-copper was found to be
unstable over cycles. For binary nanoparticles made of 80
mol% of cobalt and supported on hierarchically porous carbon,
the hydrogen generation rate gradually decreased until being
NaBH4 + 4H2O
The as-generated hydrogen is generally assumed to be pure
and the by-product is borate, precisely sodium
→
NaB(OH)4 + 4H2
(2)
a
tetrahydroxyborate NaB(OH)4. Conversions of 100% and
tunable hydrogen generation rates can be easily achieved in
the presence of an accelerator, generally a metal-based
catalyst, sometimes an acid.3 Actually, catalysis of hydrolysis of
NaBH4 has been much investigated within the past decade.1-3
Cobalt is the most attractive metal to be used as main
element of catalysts for hydrolysis of NaBH4. It is cheaper and
more abundant than noble metals like platinum and
ruthenium, and more interestingly, it is able to be as active as
these noble metals. Catalytic activity of cobalt can be tuned by
nano-/micro-sizing, anchoring onto supports like alumina or
carbon, adding dopants like boron or phosphorus, and alloying
with another transition metal.4 However, the role of cobalt in
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divided by a factor of about 2 after four cycles. According to was 17.5 and the weight ratio NaBH4/CoxCu1-x was 8. At the
the authors, this was likely caused by the oxidation of the beginning of the work, the temperature for the screening tests
catalytically active sites during hydrolysis of NH3BH3.16 In our was set at 40°C to allow fast successive experiments. For the
opinion, this is unlikely since aqueous solution of NH3BH3 is a stability experiments (with 5 successive cycles; details are
reducing medium; oxidation might take place post-hydrolysis.
given in sub-section 3.2), similar experimental conditions were
In the present article, cobalt-copper binary particles were used but, on the basis of the results of the kinetic study, the
targeted to be used in hydrolysis of NaBH4. Cobalt was doped temperature was 30°C. By decreasing the hydrolysis
with copper and the content of the latter was tuned to get the temperatures, it was targeted to have slower kinetics because
best catalytic activity (i.e. total conversion of 100% and highest deactivation (poisoning), if any, is more pronounced at lower
hydrogen generation rate) over one hydrolysis cycle. In our temperatures (slower adsorption/desorption rates onto the
conditions, cobalt-copper with a theoretical content of 10% of catalyst surface).
copper showed the best performance and consequently was
After the screening, two samples of the best CoxCu1-x
tested over five cycles. Loss of catalytic activity occurred, catalyst, respectively in fresh and used states, were
especially in terms of hydrogen generation rate. The selected characterized by X-ray diffraction (XRD; X’Pert Pro
catalyst was then characterized in order to better understand diffractometer, using copper Kα radiation with λ = 1.5406 Å;
the reason(s) of deactivation. This is reported and discussed in the patterns were analyzed with the help of the software
details hereafter.
X’Pert HighScore), scanning electron microscopy (SEM; Hitachi
S4800 microscope), and X-ray photoelectron spectroscopy
(XPS; ESCALAB 250 from Thermo Electron with
a
Experimental
monochromatic source, Al Kα ray 1486.6 eV; analyzed surface
of 400 µm diameter; binding energies (BE) of all core levels
Cobalt nitrate hexahydrate Co(NO3)2⋅6H2O (Sigma-Aldrich),
copper chloride dihydrate CuCl2⋅2H2O (Sigma-Aldrich), sodium
referred to the C−C of C1s carbon at 284.8 eV).
borohydride NaBH4 (Acros Organics), L-ascorbic acid C6H8O6
(Sigma-Aldrich) and sodium hydroxide NaOH (Sigma-Aldrich)
were used as received. Sodium borohydride was stored and
handled in an argon-filled glove box (MBraun M200B, O2 < 0.1
ppm, H2O < 0.1 ppm). Ultrapure water (Millipore milli-Q,
resistivity > 18.2 MΩ cm) was used.
The cobalt-copper CoxCu1-x (x = 0, 0.1, 0.25, 0.5, 0.75, 0.9,
1, and x as a mole ratio) catalysts were prepared according to
a procedure inspired from references.17,18 For every synthesis,
15 mg of CoxCu1-x were targeted. Typically, in a two-neck
round-bottom flask of 100 mL, the required masses of one or
two of the salts Co(NO3)2⋅2H2O and/or CuCl2⋅6H2O were
solubilized in 10 mL of water containing C6H8O6 (mole ratio
C6H8O6/(Co+Cu) of 4). The solution was kept under stirring, and
ultrasonicated 5 min. Then, 5 mL of an aqueous solution of
NaBH4 (mole ratio NaBH4/(Co+Cu) of 2) was added dropwise
and under stirring. The solution was kept under stirring for 10
min. A black suspension of particles formed. The catalyst was
finally recovered after centrifugation (3500 rpm, 15 min),
washing (3 times with water and ethanol) and drying (80°C, 2
h).
Figure 1. Screening of Co, Co0.9Cu0.1, Co0.75Cu0.25, Co0.5Cu0.5, Co0.25Cu0.75
,
Co0.1Cu0.9 and Cu: hydrogen evolution curves by hydrolysis of NaBH4
performed at 40°C (15 mg of catalyst, 120 mg of NaBH4, 1 mL of alkaline
water, [NaOH] = 0.5 M). For clarity, two graphs with different time scales are
proposed.
The CoxCu1-x catalysts were screened in hydrolysis of
NaBH4. The hydrogen evolution experiments were performed
as follows. In a glassy tube-like reactor, 15 mg of CoxCu1-x and 1
mL of water were introduced to be ultrasonicated for 10 min.
The reactor was immersed in an oil bath kept at constant
temperature (e.g. 40°C for the screening tests, and 30-60°C for
the kinetic study) and connected to a colored-water filled
inverted burette. To start the hydrogen generation by
hydrolysis of NaBH4, i.e. the catalytic reaction, 1 mL of alkaline
solution (NaOH, 0.5 M) of NaBH4 (120 mg) was injected in the
reactor. The hydrogen evolution was followed for 60 min and
was stopped even though the hydrolysis was not completed
(this was the case for two catalysts, i.e. Cu and Co0.1Cu0.9; cf.
sub-section 3.1). In our conditions, the mole ratio H2O/NaBH4
Results and discussion
Screening of the CoxCu1-x catalysts
The hydrogen evolution curves obtained with the CoxCu1-x
catalysts are reported in Figure 1. For clarity, the curves of Co,
Co0.9Cu0.1
, Co0.75Cu0.25 and Co0.5Cu0.5 and those of Co,
Co0.25Cu0.75, Co0.1Cu0.9 and Cu are plotted in separate graphs. In
our experimental conditions, the catalyst made of pure Cu is
not active in hydrolysis of NaBH4 whereas pure Co is active
with a total conversion of 100% reached in 7 min. The
hydrogen evolution curve of Co is typical with an induction
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period (slow hydrogen generation rate) at the beginning of the respective apparent activation energy values. The results are
hydrolysis due to the surface activation (by reduction of shown in Figures 2 and . They were found to be 16.5 and 43
3
1
oxidized cobalt species), followed by fast hydrolysis (ca. 250 kJ mol− for Co0.9Cu0.1 and Co respectively, indicating the
mL H2 evolved in 4 min). These observations are consistent beneficial effect of copper when combined to cobalt. A direct
with the catalytic features of these pure metals as reported in comparison with data available in the open literature is
previous reports.1-6
generally irrelevant because of discrepancies in the
The addition of cobalt to copper leads to more active experimental conditions, but it may be indicative to report few
1
catalytic particles. At 30°C, Co0.1Cu0.9 is negligibly active with examples. With respect to Co, the energy of 43 kJ mol−
ca. 15 mL H2 generated in 60 min, but Co0.25Cu0.75 is much favorably compares with the values generally reported for
more active with a reaction completed in 30 min. With a cobalt-based catalysts (40-60 kJ mol−1).4 However, the energy
further increase of the Co content, better catalytic results found for Co0.9Cu0.1 is low in comparison to the data available
were obtained. With Co0.5Cu0.5 and Co0.75Cu0.25, the hydrolysis in the open literature;4 similar values were otherwise reported
reactions are completed within 9 and 7 min respectively. The in hydrolysis of NH3BH3 for bimetallic FeCo nano-alloys.19
best result can be observed with Co0.9Cu0.1, 4 min being
enough to generate the 4 moles of H2 per mole of NaBH4. It is
even better than pure Co. Unlike for Co, there is no induction
period with these bimetallic catalysts.5,6
The previous results confirm that combining cobalt with
copper leads to synergetic effect via interactions between
both metals and thus to better reactivity and catalytic
activity.9-16 Electronic and/or geometric effects could
rationalize such reactivity changes. For example, theoretical
calculations have predicted up-shift of the d band center of
cobalt in the presence of copper (i.e. change in transition state
energy) as well as strong segregation of the latter (i.e. surface
enrichment by copper).8 It seems then likely that: copper
dilutes cobalt with formation of more small cobalt-based
active sites; and copper allows a better reducibility of oxidized
surface cobalt. The consequences are a very short/negligible
induction period and faster hydrogen generation kinetics.
Figure 3. Co: (left) hydrogen evolution curves by hydrolysis of NaBH4
performed at 30, 40, 50 and 60°C (15 mg of catalyst, 120 mg of NaBH4, 1 mL
of alkaline water, [NaOH] = 0.5 M); (right) determination of the apparent
activation energy by plotting ln(r) as a function of 1/T (Arrhenius equation).
Figure 2. Co0.9Cu0.1: (left) hydrogen evolution curves by hydrolysis of NaBH4
performed at 30, 40, 50 and 60°C (15 mg of catalyst, 120 mg of NaBH4, 1 mL
of alkaline water, [NaOH] = 0.5 M); (right) determination of the apparent
activation energy by plotting ln(r) as a function of 1/T (Arrhenius equation).
Figure 4. Stability of Co0.9Cu0.1 over 5 hydrolysis cycles at 30°C: after each
cycle the catalyst was kept in the spent fuel (10 min) and then a new batch
of NaBH4 (120 mg in 1 mL of alkaline solution) was added. For clarity, the
successive curves are plotted every 30 min (x axis).
From the screening, Co0.9Cu0.1 and Co were selected and
additional hydrogen evolution experiments were performed
for both catalysts at 30, 50 and 60°C while keeping the other
experimental conditions identical. The curves allowed
determining the hydrogen generation rates (denoted r) that
were used with the Arrhenius equation to calculate the
Stability of Co0.9Cu0.1 over 5 cycles
In a first approach, the stability of Co0.9Cu0.1 over 5 cycles at
30°C was studied so that after each cycle the catalyst was kept
in the spent fuel (10 min) and then a new batch of NaBH4 (120
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mg in 1 mL of alkaline solution) was added. The results are features are unlikely. There may be also other reason(s). This is
reported in Figure 4. The catalytic activity of Co0.9Cu0.1 is discussed hereafter.
gradually degraded. The total conversion reaches 89% after
Characterization of Co0.9Cu0.1
the 5th cycle and the hydrogen generation rate decreases from
1
78.5 to 18 mL min− . Likely explanations may be catalyst
To better understand the reasons of the aforementioned
catalyst deactivation, two samples of Co0.9Cu0.1 were prepared
in high amounts to be characterized in fresh and used states
(denoted F-Co0.9Cu0.1 and U-Co0.9Cu0.1). The former state
corresponds to the freshly prepared catalyst. The latter state
corresponds to the catalyst used in hydrolysis for 5 cycles; it
was washed and dried before the characterizations. For both,
SEM observations (Figure 6) showed agglomerated particles
that could consist of metallic particles embedded in a matric
made of borates as reported in reference.21
deactivation by surface poisoning (adsorption of borate by-
products),5-7 or agglomeration of the metallic particles.9-11
Another explanation may be the changes of the solution
features (higher volume, more and more borate by-products,
increased viscosity, etc.); it is known that such variations may
negatively impact the hydrolysis reaction.1-3,20
In a second approach, the stability of Co0.9Cu0.1 over 5
cycles at 30°C was studied so that after each cycle the catalyst
was extracted from the spent fuel (by centrifugation), washed
with water and ethanol (to remove most of the surface-
The XRD patterns are reported in Figure 7. As reported for
adsorbed by-products)7, and dried (80°C,
1 h). Such
experimental approach is preferable to avoid the catalytic
activity degradation due to changes of the features of the
hydrolysis medium. The results are reported in Figure 5. The
catalyst is clearly stable in terms of total conversion with 100%
after each cycle. As the solution features were identical for
each cycle in this second approach, it can reasonably
concluded that the decrease of the total conversion in the
series of tests of the first approach were due to changes of the
solution features like the increasing amount of borates in the
hydrolysis medium.
Figure 6. SEM images of F-Co0.9Cu0.1 (left) and U-Co0.9Cu0.1 (right), the scale
being 300 nm.
many of the cobalt-based catalysts reduced by a boron hydride
(i.e. NaBH4, NH3BH3),5,6,21 Co0.9Cu0.1, which is predominantly
made of cobalt, is amorphous. With particular attention, two
diffraction peaks of low intensity can be seen at 2θ values of
43.4° and 50.5°; they may be ascribed to the planes (111) and
(200) of metallic copper (Ref. 01-085-1326).
Figure 5. Stability of Co0.9Cu0.1 over 5 hydrolysis cycles at 30°C: after each
cycle the catalyst was extracted from the spent fuel (centrifugation),
washed with water and ethanol, and dried at 80°C for 1 h. For clarity, the
successive curves are plotted every 15 min (x axis).
In Figure 5, the activity of the catalyst and the hydrogen
generation rates are affected: after the first cycle, it decreases
1
from 78.5 to 38 mL min− , and then it is almost constant for
Figure 7. XRD patterns of F-Co0.9Cu0.1 (fresh state) and U-Co0.9Cu0.1 (used
state). The broad peak at 2θ 12-14° is due to the Kapton film we generally
use to prevent samples from air and moisture.
the other cycles (31-36 mL min−1). In other words, the addition
of copper does not prevent cobalt from loss of catalytic activity
over very few hydrolysis cycles in our experimental conditions.
According to the existing literature,5-7,9-11 this may be
explained by surface-adsorbed borate by-products (inducing
poisoning), or agglomeration of the metallic particles. As
discussed in the previous paragraph, changes of the solution
The fresh and used Co0.9Cu0.1 samples were analyzed by
XPS (Figure 8). For attribution, the binding energy (BE) values
were compared to databases,22 and selected articles of the
open literature.5,6,21 The presence of both metals was
confirmed. The Co 2p binding energies (BEs) are typical of Co
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(II). The Co 2p3/2 BE of both F-Co0.9Cu0.1 and U-Co0.9Cu0.1 is the catalytic activity of cobalt can be improved by combination
781.4 eV and can be attributed to Co(OH)2. The Cu 2p peaks with copper but in our conditions the catalytic activity of
can be ascribed to Cu or Cu2O which generally display similar cobalt cannot be stabilized.
spectra in the range of BEs from 930 to 960 eV.22,23 Boron was
found on the surface of both samples and the B 1s BE at 191.9
eV is attributed to boron oxides/borates (formed by hydrolysis
of NaBH4 during the reduction and hydrolysis steps). Note that,
with such a cobalt-based catalyst, the borates are the matrix
that embedded and implies agglomeration of the metallic
5,6,21
particles; this is illustrated in Figure 9 for U-Co0.9Cu0.1
.
The
O 1s BE is 531.7 eV for both materials, consistently with B−O
and Co O environments. To sum up these XPS results, one can
−
state that F-Co0.9Cu0.1 and U-Co0.9Cu0.1 mainly consist of
Co(OH)2, Cu and borates.
Figure 9. SEM image of U-Co0.9Cu0.1, the scale being 167 nm. The metallic
particles (black) are embedded in a matrix made of borates.
Conclusions
Cobalt is a good candidate as main element of catalyst for
hydrolysis of NaBH4, but its potential is further improved when
it is combined with copper. In our conditions, we
demonstrated that Co0.9Cu0.1 is more active than Co, the
hydrogen evolution being completed in less than 4 and 7 min
respectively. Furthermore, significantly different apparent
1
activation energies were found (16.5 and 43 kJ mol−
,
respectively). These results confirm the beneficial effect of
copper on the catalytic activity of cobalt. The addition of
copper is also expected to improve the stability of cobalt by
reducing or avoiding deactivation, generally due to strongly
adsorbed borate by-products. This is in fact the principal
expectation from copper. However, in our conditions,
Co0.9Cu0.1 does not show stable activity over several hydrolysis
cycles. The hydrogen generation rate decreases, being divided
by a factor of 2 after the first hydrolysis. Like pure Co,
Co0.9Cu0.1 deactivates when reused in hydrolysis of NaBH4. To
explain the reason of such deactivation, Co0.9Cu0.1 was
characterized in fresh and used states. XPS measurements
showed that the surface composition of Co0.9Cu0.1 is subject to
changes. Quantification of the surface elements showed the
stoichiometry Co0.92Cu0.08 for the fresh sample, but Co0.98Cu0.02
for the used one. It is shown, for the first time, that anti-
segregation of copper concomitantly with segregation of
cobalt takes place upon hydrolysis. These surface phenomena
then modify the catalytic surface. The surface enrichment of
cobalt is much likely due to borate-induced segregation, which
is clearly detrimental to the synergetic effect achieved for the
freshly-prepared catalyst. In conclusion, the catalytic activity of
cobalt can be improved by combination with copper but in our
conditions it cannot be stabilized. Further efforts are required
to stabilize cobalt-based multimetallic catalysts in hydrolysis of
NaBH4. Alternatives to copper, addition of a third element to
e.g. Co0.9Cu0.1 and/or heat treatments may be few of the first
paths to be explored.
Figure 8. XPS spectra of F-Co0.9Cu0.1 (blue lines) and U-Co0.9Cu0.1 (red lines):
focus on the regions of Co 2p, Cu 2p, B 1s and O 1s.
The spectra focusing on the Cu 2p region also shows a peak
at ca. 927 eV belonging to Co 2s (Figure 8). The relative
intensity between the peak of Cu 2p3/2 and Co 2s decreases
after hydrolysis, suggesting more cobalt after hydrolysis. The
surface atomic concentrations of the elements were
determined from the photoelectron peaks areas using the
atomic sensitivity factors.24 For the fresh sample, it was found
the formulae Co0.92Cu0.08, in good agreement with the target
Co0.9Cu0.1. However, for the used sample, it was found a
decrease of the copper content, with Co0.98Cu0.02
. This
indicates a compositional change after reaction: the catalyst
surface has been enriched with cobalt. Given that, unlike
copper, cobalt is known to have strong affinity with the
hydrolysis intermediates (hydroxyborate) and products
(borates), the surface enrichment may be explained by borate-
induced segregation.
The loss of catalytic activity of Co0.9Cu0.1 can thus be
explained as follows. Because of the strong affinity of cobalt
with hydroxyborates and borates, cobalt segregates and
enriches the particles surface when put into contact with
aqueous
NaBH4
whereas
copper
anti-segregates.
Consequently, Co0.9Cu0.1 acts like pure Co, that is, the
beneficial effect of copper is lost, at least partially, leading to
deactivation over cycles by borates adsorption. In other words,
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Acknowledgements
The authors acknowledge the University of Monastir for H.K.’s
scholarship. H.K. acknowledges Mr. Salem Ould-Amara
(University of Montpellier) for his time assistance.
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RSC Advances
View Article Online
DOI: 10.1039/C6RA22998K
Manuscript ID
Title
RA-ART-09-2016-022998
Reaction intermediate/product-induced segregation for cobalt-copper as
catalyst in hydrogen generation by hydrolysis of sodium borohydride
Umit B. DEMIRCI
Corr. Author
TOC
Bimetallic cobalt-copper catalysts in hydrolysis of NaBH4 are instable over cycles because of surface
borate-induced cobalt segregation