Nanocatalysts for hydrogen production from borohydride hydrolysis: graphene-derived thin films with Ag- and Ni-based nanoparticles

Article abstract of DOI:10.1039/C8TA05834B

Generation of hydrogen is one of the greatest challenges nowadays, especially chemically obtaining from the simple, straightforward borohydride hydrolysis reaction. Herein we are interested in this reaction for producing hydrogen using various nanocatalys

Full text of DOI:10.1039/C8TA05834B

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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS2 nanocages as a lithium ion battery anode material
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Nanocatalysts for hydrogen production from borohydride
hydrolysis: graphene-derived thin films with Ag and Ni-based
nanoparticles
Received 00th January 20xx,
Accepted 00th January 20xx
Leandro Hostert, Eduardo G. C. Neiva,§ Aldo J. G. Zarbin and Elisa S. Orth*
DOI: 10.1039/x0xx00000x
The generation of hydrogen is one of the greatest challenges nowadays, especially when chemically obtained from the
simple, straightforward borohydride hydrolysis reaction. Herein we are interested in this reaction for producing hydrogen
using various nanocatalysts which are derived from graphene (reduced graphene oxide, rGO) and combined with metal-
based nanoparticles (Ag , Ni and Ni(OH)2). Overall, four promising catalysts were obtained using the liquid-liquid interfacial
route in order to obtain highly homogeneous thin films. We highlight the novelty in employing thin films for the generation
of hydrogen gas, in which the reaction kinetics was thoroughly elucidated, obeying first-order profiles. In fact, very high
catalytic activity were observed using low amounts of nanocatalysts (~ 0.1-0.8 mg), with the maximum rate of hydrogen
production reaching nearly 33x103 mL-1 min-1 g-1. The nanocomposite rGO/Ni is reported here for the first time, which also
showed the most prominent activity. The isolated components alone were not efficient, evidencing the synergism in the
nanocomposites produced. Finally, the nanocatalysts were consecutively recycled up to 10 cycles, without losing activity.
Various characterizations taken during the reactions and after the reuse confirmed that the typical nature of the
composites is maintained intact.
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limiting the use and application on a large scale. A promising
alternative comes with the use of cheaper materials, such as
Co9 and Ni,10 supported on organic matrices, increasing surface
Introduction
Since the early 19th century, scientists have identified
hydrogen (H2) gas as a potential energy vector. Its combustion
has the highest heating value if compared to others fuels and
when employed in fuel cell leads to water as by-product, which
is highly environmentally friendly.1, 2 Most of the H2 produced
at the industrial scale is described by the steam reform process
or as a by-product of petroleum refining. However, both
methods have limitations or high environmental drawbacks.
Recently, various methods have been applied for the
production of H2 such as fuel cells, electrolysis, solar
generator, biomass conversion, photocatalysis, electrolysis
coupled with photovoltaics and thermochemical water
splitting, hydrolysis reactions.3-5 The latter is still poorly
exploited despite its simple, straightforward and highly
sustainable approach, i.e. a chemical reaction not involving
very hazardous substances, under mild conditions (ambient)
and without significant energy input (in contrast to electrolysis
for example). Moreover, most of the methods for H2
generation rely on catalysts based on nobles metals such as:
Au,6 Pt7 and Ru8. Conversely, such metals are expensive, thus
area and consequently increasing the catalytic performance,
but these methods can still present high cost or handling
difficultly.
In this context, a new class of catalysts that benefit from
synergism using graphene derivatives combined with various
metal-based nanoparticles (NPs) emerges, presenting a high
catalytic increase for several reactions11-14 due to exceptional
optical,15 mechanical16 and electronic properties.14 Graphene
stands out as a promising scaffold for nanocatalysts because it
has, among several interesting properties, high surface area,17
adsorption capacity18 and electronic properties.14 In fact, when
combined with NPs, unique improved characteristic can be
obtained, in contrast to the isolated precursors. This has
furnished many studies towards novel nanocomposites, for
example of graphene with NPs for H2 generation, using TiO2,19
CdS,20, 21 ZnS,22 Cu2O,23 Ni24, among others.
Seeking applications, various concerns arise. For example,
controlling and modulating the H2 rate production is
technologically important for safety issues. Related to this is
the need to be able to project any interruption during the
generation, if desired. Surely a thorough elucidation of the
reaction kinetics of these processes can be greatly insightful.
Another important issue to address is how to handle these
nanocomposites (e.g., powder), which can be limiting. In the
past years, some of us has demonstrated that thin films
obtained from the liquid-liquid (L/L) interfacial method
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emerges as
a
promising alternative.25-27 By this route method was employed which was fully described previously
nanocomposites (derived from graphene and polymers/NPs) for rGO/Ni(OH)2,28 although is reported herein for the first
can be easily self-assembled at water/toluene interface and time for rGO/Ni. Briefly, two approaches were adopted
further deposited as highly homogeneous thin films with great starting from GO and Ni acetate as precursors using two
o
adherence in any substrate of interest, allowing direct temperatures: 190 and 198 C, which lead to rGO/Ni(OH)2 and
applications for example as energy devices,25 sensors,28 rGO/Ni, respectively. The proportion in mass used was 1:1 and
catalysts for organophosphates cleavage26 and SERS 1:0.5 (GO:Ni) for rGO/Ni(OH)2 and rGO/Ni, respectively. As
substrates.26 Most recently, two novel routes have been mentioned, for rGO/Ni(OH)2, the synthesis has already been
reported for obtaining thin film nanocomposites between described28 so herein we specify in more detail the novel
28
reduced graphene oxide (rGO) and Ag29 or Ni(OH)2 NPs. The synthesis of rGO/Ni. For this, 5 mg of GO were dispersed in 20
first one consists in the concomitant formation of Ag NPs and mL of ethylene glycol using ultrasonic bath for 60 min. Upon
rGO using sodium borohydride as the reducing agent at addition of Ni acetate, the system was refluxed for 2 h at 198
water/toluene interface system.29 For the second one, the °C. At the end of this period, the material was filtered, washed
polyol method was proposed, in which the material is obtained with 200 mL of deionized water and dried at 70 °C for 1 h. For
in a single step and where ethylene glycol acts as solvent, obtaining the thin films, 1 mg of rGO/Ni(OH)2 or rGO/Ni was
reducing agent and stabilizer for the NPs. 28
added to a round bottom flask containing 40 mL of deionized
Among the several possible applications with these thin Milli-Q water and dispersed assisted with an ultrasonic bath
film materials described above, the production of H2 has not during 1 h. In the following, 40.0 mL of toluene was added to
been explored. Thus, Ag nanocomposites have shown to be the flask and the system was stirred (2500 rpm) for 12 hours,
excellent SERS substrates29, while with Ni(OH)2 multiple in which upon stirring interruption, the film spontaneously
applications as batteries, electrochromic device and glycerol migrated to the L/L interface. The film was transferred with
electrochemical sensors have been reported.28 Herein, we are the aid of a micropipette to a 100 mL beaker containing water,
interested in broadening the application of Ag and Ni(OH)2 toluene and the substrate of interest. As the thin film is self-
nanocomposites for H2 production. Further,
a
novel assembled, it reorganizes itself at the L/L interface, which is
graphene/Ni nanocomposite (rGO/Ni) is described, which is removed by pulling the substrate toward the interface using a
also applied and evidenced the most promising results. homemade dip-coating equipment,31 aiming a steady and
Overall, four thin film nanocomposites were obtained derived homogeneous deposition. Subsequently, the films were oven-
from graphene and Ag, Ni or Ni(OH)2 NPs that promote the dried at 70 °C for 1 h.
borohydride hydrolysis reaction. The nanocatalysts evaluated
The nanocomposites with Ag were synthetized as
show among the highest catalytic activity ever reported, with described previously29 and the main difference from the Ni-
remarkable recycling features, up to 10 cycles. A thorough based materials given above is that with Ag, the thin films
kinetic analysis is also carried out in order to guide the were obtained directly from the L/L interfacial reaction
understanding of the catalytic activity and also comparison medium. Hence, the synthesis of the nanocomposite as well as
with the literature.
the deposition of the films are obtained in a one-pot system. In
summary, AgNO3 was reacted in a water/toluene system with
two different carbon precursors: GO and rGO, which led to the
nanocomposites rGO1/Ag and rGO2/Ag, respectively. The
proportion in mass adopted was 1:5 of GO:AgNO3 and 1:2 of
rGO:AgNO3 for obtaining rGO1/Ag and rGO2/Ag, respectively.
For the reduction process NaBH4 was used. After the reaction
occurs, a film is assembled at the interface. Both the aqueous
and organic phases were repeatedly washed (10 times) by
Experimental section
All reagents were used without pre-treatment: 99.8%
AgNO3 (Sigma-Aldrich), NaBH4 98% (Acros), toluene 99%
(Neon), H2SO4 95-98% (Carlo Erba), graphite (Graflake 99580,
99%, Nacional de Grafite SA), NaNO3 99% (Vetec), KMnO4 99%
(Synth), HCl 37% (Impex), H2O2 aqueous solution 30% (Vetec),
ethylene glycol 99,5% (Carlo Erba), glycerol (CAAL) and NaOH
98% (Vetec). Ni(CH3COO)2·4H2O (Vetec) was previously dried
under low pressure at 100 oC for 5ꢀhours before used. All
solutions were prepared with deionized Milli-Q water.
replacing with fresh water/toluene with the aid of
a
micropipette. Finally, for obtaining the film, the same
procedure described for the Ni-derived composites was
carried, i.e., the film is transferred to a beaker and further
deposited. An overall summary of the samples obtained are
shown in Figure 1.
Synthesis and characterization of the nanocomposites
GO was synthesized according to the modified Hummers
method described before.30 For the synthesis of the
nanocomposites between Ni/Ni(OH)2 and rGO, the polyol
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Fig. 1 Schematic illustration for synthesis of the nanocomposites and their application for hydrogen production. Photographs are also shown for the thin films deposited over glass
and during the hydrogen production stage.
Likewise, thin films of the nanocomposites of solely Ag, Ni was related to the amount of gas produced. The ambient
or Ni(OH)2 NPs were also obtained for comparison purposes, temperature was maintained constant at 21°C. Figure 1
following analogous procedures given above, although their illustrates the H2 production. The system was completely
films did not present themselves homogeneously. A film of isolated in order to guarantee that all the gas produced is
rGO was also obtained for comparison, as described in the collected. The amount of H2 produced (volume) was measured
literature.29
as a function of time and further used for the kinetic studies.
For the characterization, the thermal analysis (TG) The kinetic profiles were fitted using the program Origin, by
measurements were performed on a TGA/SDT Q 600, under non-linear regressions using the Levenberg-Marquardt
synthetic air atmosphere, with a heating ramp of 5
ray diffractometry (XRD) analysis was performed on
Shimadzu XRD-6000 equipment, with CuKα radiation (λ = section.
°
C min-1. X- algorithm to find the least squares sum. A detailed description
of the equations fitted is further discussed in the results
a
1.5418 Å), operating at 40 kV voltage and 30 mA current,
For the recycling study, the film was removed from the
directly on the films deposited over the glass substrate. A low- vessel, followed by washing and drying. Then it was reused
angle accessory was used in order to minimize the consecutively as described above in a reaction medium
contribution of the substrate. Raman spectroscopy was containing fresh NaBH4. The characterization (SEM, Raman,
collected using a Renishaw spectrophotometer coupled to an XRD) carried out at varying reaction times (1h, 3h, 6h and 12h)
optical microscope with a spatial resolution of 1 μm (50x lens). required an individual synthesis for each measurement. For
The excitation lines used were He-Ne (632.8 nm) and Ar+ this, the thin film was removed from the solution and dried at
(514.5 nm) with an incidence power of less than 1 mW, in a 60 oC for 1h.
total of 15 accumulations (10 s per accumulation). All spectra
were recorded with the films deposited on Si/SiO2 substrates.
Results and discussion
Scanning electron microscopy (SEM) images were collected in
a FEG-SEM Tescan, using a voltage of 10 kV and secondary
Characterization of the nanocomposites
electron detector. For these analyses the films were deposited
The nanocomposites were thoroughly characterized and
on Si/SiO2.
fully agree with previous reports for those composites already
29
described.28,
Figure 2 highlights most of the relevant
H2 production
The catalytic hydrolysis of NaBH4 was carried out by
characterization for the Ni-based nancomposites. XRD shows
peaks at 42.5o and 51.6o for rGO/Ni, correlated to the sets of
planes (111) and (200) of the face-centred cubic structure of Ni
(Figure 2A). For rGO/Ni(OH)2, the peaks at 7.7°, 14.5°, 34.4°
and 60.5° were depicted, relative to planes (001), (002), (100)
and (110), respectively, of the α-Ni(OH)2 structure (Figure
2C).28
immersing in the reaction medium the thin films: rGO1/Ag,
rGO2/Ag, rGO/Ni(OH)2 and rGO/Ni deposited over glass (both
sides) with a total area of 15 cm2. Firstly, NaBH4 (227.2 mg)
was dissolved in 40.0 mL deionized water in a closed vessel
and the system was kept under stirring (1000 rpm). Then the
film was placed in the solution and the generated gas (H2) was
collected in an inverted measuring cylinder, containing water,
immersed in a 2.0 L beaker and the volume of water displaced
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Figure
3
presents some of the most important
characterizations for the Ag-derived nanocomposites. XRD
confirms the formation of the Ag NPs in rGO1/Ag and rGO2/Ag
due to the presence of 38.0° and 44.2° peaks, which are
related to the crystalline planes (111) and (200), respectively,
of the face-centred cubic Ag (Figure 3A).29 UV-Vis spectra
(Figure 3B) taken using quartz substrate shows for rGO a band
at 270 nm due to the π-π* transition of aromatic C=C.33 In the
nanocomposites, an additional band is observed which is
related to the Ag plasmon band, commonly found at
approximately 400 nm, but for rGO2/Ag is centred at 340 nm.
This displacement may be related to the shape of the NPs, size
or even the interaction with rGO.29 SEM images (Figure 3C-D)
show rGO sheets with homogeneously distributed Ag NPs. In
some cases, it is possible to verify Ag agglomerates.
In summary, all characterizations indicate the successful
synthesis of the four nanocomposites derived from rGO and
Ag/Ni/Ni(OH)2 NPs, which displayed different morphologies. It
is important to note that these nanocomposites rely on the
passivating effect of rGO on the NPs formed. Indeed, results
have shown that the carbon precursors (GO, rGO) can act as
nucleating sites for the growth of various NPs, thus controlling
their size, distribution and shape. In many cases, this approach
Fig. 2
and (
(
A
) SEM for rGO/Ni(OH)2
) XRD for rGO and rGO/Ni; (
B) SEM for rGO/Ni; (C) XRD for rGO/Ni(OH)2
D
The rGO peak is present at 23.1° relative to the plane (002).32
In fact, the difference in the reaction temperature is
responsible for the structural modification observed within the
Ni-derived materials. SEM images evidence the presence of
the typical wrinkled sheets of rGO. For rGO/Ni (Figure 2B), it is
possible to depict a surface coated with Ni microparticles with
a size of 0.5 to 40 μm, which are actually composed of
aggregated smaller particles (nanometric scale). In the case of
rGO/Ni(OH)2, the NPs are not distinguished, although this is
expected.28 Further TGA, SEM/EDS and Raman analysis for
rGO/Ni is given in the Supplementary Information (SI).
has shown optimized properties due to synergistic effects and
broaden applications.26,
28, 29
Henceforth, we applied the
nanocomposites for an imperative application: H2 production.
H2 production
The production of H2 was carried out by promoting the
borohydride hydrolysis, shown in reaction 1. The volume
produced of H2 was monitored as a function of time and the
kinetic profiles obtained are shown in Figure 4, along with rGO
for comparison. Another important comparison is with the
bare NPs (without rGO), which didn´t show any significant
activity.
-
-
BH4 (aq) + 2H2O(l)
ꢀ
BO2 (aq)
+
4H2(g)
(Reaction 1)
It is possible to evidence from Figure 4, that the volume of
H2 produced was higher for all nanocomposites compared to
rGO, despite it still presenting some activity. In all cases, the
amount of nanocatalyst employed is very low (~ 0.12-0.77 mg).
Actually, rGO/Ni showed the greatest behaviour producing
nearly 400 mL of H2 in just 5 h. Similar H2 volume was also
obtained with rGO1/Ag but with higher reaction time. The
catalytic behaviour among the Ag-materials can be related to
the amount of Ag used in the synthesis which is higher for
rGO1/Ag, which in fact produced more H2. For the Ni-
materials, rGO/Ni(OH)2showed the lower activity probably
related to the stability of Ni(OH)2, that can be reduced to
metallic Ni under the reaction conditions.
Fig. 3 (A) XRD for rGO1/Ag and rGO2/Ag; (B) UV-Vis spectra rGO1/Ag, rGO2/Ag and
rGO; (C) SEM for rGO1/Ag and (D) SEM for rGO2/Ag.
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deduced,37 authors have shown that Equation 1 can be
successfully fitted in volume vs time profiles, where VH2(t) is the
Max
volume of H2 at a certain time t, VH2 is the maximum of H2
produced and k is the rate constant. The fit curves (Figure 4)
show good correlations giving the k values of 0.010, 0.0029,
0.0044 and 0.016 min-1 for rGO/Ni, rGO1/Ag, rGO/NiOH2 and
rGO2/Ag, respectively. Further, for employing the comparison
method 1, the maximum rate of H2 production (MRP, unity of
mL-1 min-1 g-1) defined by a mathematical treatment37 can be
t
calculated using Equation 2, where VH2 is the total volume of
H2 produced in the reaction, k is the rate constant obtained
from the kinetic profile and mcat is the total mass of catalyst
used.
VH2ꢀtꢁꢂVHm2axꢀ1-ꢃꢄꢅꢆ
ꢁ
( Eq. 1)
Fig. 4 Volume of H2 produced vs time, from the borohydride reaction promoted
by the nanocatalysts (~0.12-0.77 mg). The solid lines corresponds to the fit using
Eq.1
t
ꢇꢅ
cat
MRP ꢂ V
(Eq. 2)
H2
Method 1
m
Interpreting the kinetics for H2 production has shown to be
The value of MRP was calculated for the nanocomposites
evaluated herein as well as for other studies reported in the
literature that obey first order kinetics. Figure 5 shows the
results obtained here (highlighted with asterisk) along with
data from other studies, for example, Co-B nanoparticles,37 2D
nitrogenated polymer encapsulated Co3O4 (Co3O4@C2N),39
ionic liquid functionalized graphene oxide with CuO
(rO/IL/Cu)40 and graphene Co-Zn-B nanocomposite (Co-Zn-
B/G)41. In fact, Co-based materials seem to be very promising
for H2 generation, evidenced by the many reports involving
them. Nonetheless, the nanocomposites evaluated herein
present very high activity with MRP values among the highest,
especially for rGO/Ni. This impressive activity can be
accounted to several effects such as the low mass of catalysts
used which is homogeneously distributed over the substrate
due to the L/L interfacial method and synergistic effect
between rGO and the NPs that favour rapid kinetics (high k
and volume) for the reaction of interest.
a
great challenge nowadays. Different mechanisms for
hydrolysis of borohydride catalysed by metallic NPs have been
widely discussed with several kinetic models proposed: zero
order, first order, second-order, power law, Langmuir-
Hinshelwood, Michaelis-Menten and semi-empirical kinetics.34
According to Rangel et al.34 the order of the reaction can
change according to the molar ratio borohydride/catalysts:
36
high ratios lead to zero-order kinetics35, and low ratios to
first-order kinetics.37 All the previously highlighted
mechanisms present two important steps: i) adsorption of
borohydride on the surface of the catalyst and ii) reaction of
the adsorbed species on the catalyst surface. The first step is
characterized by the BH4- ions chemisorbed on the Ni/Ag metal
-
atoms, then the H- is transferred from BH4 to the metal atom
and forming H- hydridic. This species reacts with water
molecule to produce H2 and OH- and the later reacts with B to
produce BH3(OH)-.38 The higher catalytic activity for the
materials with NPs is usually correlated to the higher
superficial area and the increase of electron density. In
addition, the effect of graphene-based supports in these
reactions are usually attributed to improving charge transfer
and increasing the specific surface area.9
Although data in Figure 4 consistently evidence the high
efficiency of the nanocatalysts, we were further interested in
elucidating the kinetic profiles and also comparing with other
data in the literature. Indeed, these comparisons can be tricky
because it should consider the volume of H2 produced, the
time/rate required and the mass of catalyst used. Due to that,
herein we present different approaches for analysing the data
which are thoroughly described as follows. Based on the
literature, two different methods were used to evaluate and
compare the H2 production with other studies: i) method 1
considers the rate constant from first order kinetics, combined
with the volume produced and mass of catalysts;37 ii) method
2 considers only the time where the greatest volume of H2 is
produced and is also related to the mass of catalyst. More
specifically, for method 1, the kinetic profiles given in Figure 4
were fitted according to first order equations. Previously
Fig. 5 Values of MRP (Eq 2) for the borohydride hydrolysis promoted by the
nanocatalysts (*) and compared with data form the literature Co-B,37
Co3O4@C2N,39 GO/IL/Cu40 and Co-Zn-B/G41
.
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Calculation of MRP is an important tool for understanding
Journal Name
Table 1. Values of OP using Eq. 3 for the nanocatalysts evaluated and other studies
reported, for borohydride hydrolysis. a
the H2 production kinetics, although its comparison to other
data in the literature is limited since solely studies obeying first
order kinetics can be used. Due to this, another widely used
method was also applied,42 namely method 2. For this, the
optimal production (OP) of H2 was estimated considering
Nanocatalysts
OP (mL min-1 g-1)
3.0 x104
1.4 x104
1.0 x104
1.3 x104
3.0 x105
2.3 x104
4.4 x103
3.0 x103
2.2 x103
1.3 x103
1.2 x103
3.0 x102
17.4
b
rGO/Ni
b
rGO/Ni(OH)2
b
rGO1/Ag
op
Equation 3, where VH2 is the maximum volume of H2
b
rGO2/Ag
produced during the reaction at a given optimal time top. For
example, in the case of rGO/Ni, this optimal condition is ~ 400
mL with 5h. Although this method doesn´t consider the kinetic
profile fitting, it has the advantage of broadening comparison
with other studies that present vary kinetic behaviours (order)
or when the kinetics are not followed. Thus, this is an attempt
to compare the “best” result of H2 production (volume
generated) of a given study, normalized by the time required
to overcome this and also the mass of catalyst. Table 1
presents an overall comparison of various OP values calculated
using method 2 for the nanocatalysts herein and other
reported in the literature. Analysis of Table 1 shows that the 4
nanocatalysts evaluated present among the greatest value of
OP. When comparing with similar components of our
nanocatalysts (carbon nanomaterial or Ag/Ni NPs), the results
stand out, with exception of the catalyst Pt/Pd atoms on CNT
paper.41 Although this material shows high activity, its
preparation is not so feasible, i.e., requires a monolayer
Pt/Pd atoms on CNT paper 43
Ru NPs with PPy46
Co-B-O NPs42
Co–B37
NaOH-treated CoZnB/GP41
PAN/CoCl2/CNT47
Cryogel p(AAm)Co48
49
NiB/NiFe2O4
Ag/MWCNT50
a Considers the total mass of catalyst employed except for “Pt/Pd atoms on CNT
b
paper” and “Ru NPs with PPY”, which consider solely the mass of metal. This
work
A recycling study was also carried for all nanocomposites,
in which the same thin film was reused for up to 10 cycles in
different reactions. For all nanocatalysts, the value of OP
observed in the first cycle was overall maintained in the
consecutive cycles (see SI) and we highlight the most
representative results for rGO/Ni, which was reused 10 times
(Figure 6B). For example, for rGO/Ni, 365 and 335 mL mL of H2
were produced in the first and fifth cycle, respectively.
Analysing the OP values, we observe 3.0, 4.5 and 6.0 x104 mL
min-1 g-1 for the first, fifth and tenth cycle, respectively,
evidencing no significant loss of activity. Figure 6C-E indicates
that the overall characteristics of the film/material are
maintained comparing analysis before and after the reaction.
It should be noted that in the case of rGO/Ni(OH)2, its activity
varied (~30%) within the cycles probably due to the reduction
of the NPs during the reaction. Surely, recycling is one of the
great advantages of the thin film nanocatalysts, along with its
easily handling. Hence, after the completeness of the reaction,
the thin film can be removed from the reaction medium and
reused consecutively simply by washing, drying and re-
immersing in new reaction medium. Indeed, the film is
strongly adhered to the substrate, thus is not lixiviated to the
solution, as proven by characterizations taken after the
reaction (film and solution). Finally, the thin film addresses
several stability and safety issues, since it can be easily
handled and removed from the medium. In addition, the
production of H2 can be strategically controlled. Thereby, a
more controlled production can be obtained by using for
example rGO/Ni(OH)2 or the Ag-derived nanocatalysts. Albeit,
higher volumes in shorter periods could be projected using
rGO/Ni. In addition, any necessary interruption in the
generation of H2 can be easily done in the proposed system,
which is highly important for safety issues, simply by raising
the thin film from the borohydride solution.
elemental dispersion of Pt and Pd atoms on
a
coated/functionalized carbon nanotube paper.43 In addition,
this study considers for mcat, the mass of solely metal, which is
very low, compared to the total mass of catalysts, as
considered herein. Notwithstanding, both methods 1 (MRP)
and 2 (OP) strongly support that the nanocatalysts proposed
herein present very high catalytic activity towards H2
production (Figure 5 and Table 1). The distinguished catalytic
activity between Ag and Ni-based materials could be explained
by the different interaction forces of Ag and Ni with H2, that
although is not focus of the present study, has been
extensively discussed elsewhere.44, 45 Another important factor
could be associated to the morphological difference of the
NPs.
op
H2
V
Method 2
OP ꢂ m
(Eq. 3)
ꢈꢉ
ꢇꢆ
cat
Figure 6 summarizes some results regarding the reuse of
rGO/Ni nanocatalyst, presenting OP values over the cycles,
photographs and SEM images of the films before and after the
reaction, as well as XRD taken during the reaction. XRD
obtained at different reaction times during the borohydride
hydrolysis don´t evidence any significant change in the
material, which is desired (Figure 6A). Similar conclusions were
obtained from SEM and Raman analyses obtained analogously
for rGO/Ni and the other nanocatalysts (shown in SI).
6 | J. Name., 2012, 00, 1-3
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and could help guide towards a more concise understanding of
the catalytic activity. In addition, the catalyst composition can
greatly tune the kinetics as observed by varying the NPs nature
(Ag, Ni and Ni(OH)2). The amount of H2 produced and the time
required can be modulated according to the needs. Moreover,
the feasibility of handling the catalysts and of interrupting the
production is also important safety issues, which we also
observed in our systems.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
Fig. 6 Analysis for rGO/Ni (A) XRD taken at different borohydride hydrolysis
reaction time; (
B
) OP values for H2 production for consecutive cycles using the
) SEM
) SEM image for the same film in D, obtained after
Authors acknowledge the financial support from CNPq,
CAPES, Fundação Araucária, National Institute of Science and
Technology of Carbon Nanomaterials (INCT-Nanocarbono) and
L’Oréal-UNESCO-ABC.
same film; (C) Photograph of the films before and after the reaction; (D
image before reaction and (
the reaction.
E
Conclusions
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Page 9 of 9
Journal of Materials Chemistry A
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DOI: 10.1039/C8TA05834B
Nanocatalysts for hydrogen production from borohydride hydrolysis:
graphene-derived thin films with Ag and Ni-based nanoparticles
Leandro Hostert, Eduardo G. C. Neiva, Aldo J. G. Zarbin and Elisa S. Orth*
High values of maximum rates of hydrogen production are obtained using four thin film
nanocomposites, that showed recycling features.