Inverse relation between the catalytic activity and catalyst concentration for the ruthenium(0) nanoparticles supported on xonotlite nanowire in hydrogen generation from the hydrolysis of sodium borohydride

Article abstract of DOI:10.1016/j.molcata.2016.09.001

Ruthenium(0) nanoparticles supported on xonotlite nanowire (Ru(0)/X-NW) were prepared by the ion exchange of Ru³⁺ ions with Ca²⁺ ions in the lattice of xonotlite nanowire followed by their reduction with sodium borohydride in aqueous solution at room temperature. Ru(0)/X-NW show high catalytic activity and long life time in hydrogen generation from the hydrolysis of sodium borohydride with a turnover frequency value up to 305?min^?1 and a total turnover number of 63,100?mol H₂/mol Ru in hydrogen generation from the hydrolysis of sodium borohydride at 25.0?±?0.1?°C. Moreover, the catalytic activity of Ru(0)/X-NW shows an inverse dependence on the catalyst concentration. Ru(0)/X-NW at different ruthenium concentrations were characterized by high resolution transmission electron microscopy. The results reveal that Ru(0)/X-NW have tunable particle size depending on the metal concentration. An increase in the mean particle size of ruthenium(0) nanoparticles is observed when the metal concentration increases. The smaller particle size distribution at low ruthenium concentrations exhibits higher catalytic activity in hydrogen generation from the hydrolysis of sodium borohydride. The reaction kinetics of the catalytic hydrolysis of sodium borohydride was also studied.

Full text of DOI:10.1016/j.molcata.2016.09.001

Accepted Manuscript
Title: Inverse relation between the catalytic activity and
catalyst concentration for the ruthenium(0) nanoparticles
supported on xonotlite nanowire in hydrogen generation from
the hydrolysis of sodium borohydride
¨
Author: Serdar Akbayrak Saim Ozkar
PII:
S1381-1169(16)30373-9
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2016.09.001
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Journal of Molecular Catalysis A: Chemical
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Revised date:
Accepted date:
16-7-2016
21-8-2016
1-9-2016
¨
Please cite this article as: Serdar Akbayrak, Saim Ozkar, Inverse relation
between the catalytic activity and catalyst concentration for the ruthenium(0)
nanoparticles supported on xonotlite nanowire in hydrogen generation from the
hydrolysis of sodium borohydride, Journal of Molecular Catalysis A: Chemical
http://dx.doi.org/10.1016/j.molcata.2016.09.001
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Inverse relation between the catalytic activity and catalyst concentration for the
ruthenium(0) nanoparticles supported on xonotlite nanowire in hydrogen generation from
the hydrolysis of sodium borohydride
Serdar Akbayrak, Saim Özkar*
Department of Chemistry, Middle East Technical University, 06800, Ankara, Turkey
*To whom correspondence should be addressed: Department of Chemistry, Middle
East Technical University, 06800 Ankara, Turkey. Tel: +90 312 210 3212, Fax:
+90 312 210 3200, e-mail: sozkar@metu.edu.tr
Keywords: Sodium borohydride; Hydrogen; Hydrolysis; Ruthenium nanoparticles; Xonotlite
nanowire; Inverse TOF dependence
1

Graphical Abstract
251658240
2

Research highlights
 Formation of ruthenium(0) nanoparticles supported on xonotlite nanowire during the
hydrolysis of SB.
 Turnover frequency decreases with the catalyst concentration and particle size.
 High catalytic activity with an initial TOF of 305 min-1 in hydrogen generation from the
hydrolysis of SB at 25 °C.
 A reusable and long-lived catalyst providing 63,100 turnovers in hydrogen generation from the
hydrolysis of SB.
3

ABSTRACT
Ruthenium(0) nanoparticles supported on xonotlite nanowire (Ru(0)/X-NW) were prepared by
the ion exchange of Ru3+ ions with Ca2+ ions in the lattice of xonotlite nanowire followed by their
reduction with sodium borohydride in aqueous solution at room temperature. Ru(0)/X-NW show
high catalytic activity and long life time in hydrogen generation from the hydrolysis of sodium
borohydride with a turnover frequency value up to 305 min−1 and a total turnover number of
63,100 mol H2 / mol Ru in hydrogen generation from the hydrolysis of sodium borohydride at
25.0 ± 0.1 °C. Moreover, the catalytic activity of Ru(0)/X-NW shows an inverse dependence on
the catalyst concentration. Ru(0)/X-NW at different ruthenium concentrations were characterized
by high resolution transmission electron microscopy. The results reveal that Ru(0)/X-NW have
tunable particle size depending on the metal concentration. An increase in the mean particle size
of ruthenium(0) nanoparticles is observed when the metal concentration increases. The smaller
particle size distribution at low ruthenium concentrations exhibits higher catalytic activity in
hydrogen generation from the hydrolysis of sodium borohydride. The reaction kinetics of the
catalytic hydrolysis of sodium borohydride was also studied.
4

Introduction
Hydrogen has attracted great interest as a globally accepted clean energy carrier. However, secure
storage and effective release of hydrogen are very important in the application of hydrogen
energy [1]. Among the chemical hydrides tested as hydrogen storage materials for onboard
applications [2], sodium borohydride (NaBH4, SB) has been one of the most promising
candidates due to its high stability in alkaline solution, non-flammability, non-toxicity in nature,
and high hydrogen storage capacity of 10.8 wt% [3]. The hydrolysis of sodium borohydride
releases 4 equivalents of H2 per mole of sodium borohydride according to the equation 1 [4].
catalyst
NaBH4(aq) + 2H2O(l)
NaBO2(aq) + 4H2(g)
(1)
Since the hydrolysis occurs only in the presence of a suitable catalyst [5] many transition metal
nanoparticles including cobalt, nickel, ruthenium, palladium and platinum have been tested as
catalyst for this industrially important reaction [6]. Among them, ruthenium is one of the most
efficient catalysts in hydrogen generation from the hydrolysis of sodium borohydride [7,8]. Since
ruthenium(0) nanoparticles encapsulated in cavities of the zeolite Y support with 3-dimensional
confinement have been reported to show the highest catalytic activity in hydrogen generation
from the hydrolysis of sodium borohydride [9,10], metal nanoparticles supported on the surface
of nanowires are also expected to provide high catalytic activity in the same reaction. However,
there exist no report on the use of one-dimensional support for the metal nanoparticles catalyst in
the hydrolysis of sodium borohydride. Our recent report [11] has shown that ruthenium(0)
nanoparticles supported on xonotlite nanowire (Ca6(Si6O17)(OH)2, X-NW) [12,13] provide high
catalytic activity (a turnover frequency value up to 135 min-1) and unprecedented catalytic
5

lifetime (TTO = 134,100) in hydrogen generation from the hydrolysis of ammonia borane at 25.0
± 0.1 ºC. These results prompted us to test these ruthenium(0) nanoparticles supported on
xonotlite nanowire, Ru(0)/X-NW as catalyst in hydrogen generation from the hydrolysis of
sodium borohydride. Indeed, we observed that Ru(0)/X-NW are also highly active and long lived
catalyst in this reaction. They provide a turnover frequency value up to 305 min−1 and 63,100
turnovers in hydrogen generation from the hydrolysis of sodium borohydride at 25.0 ± 0.1 °C
before deactivation. Furthermore, we observed an inverse relation between the catalytic activity
(turnover frequency, TOF) and the concentration of ruthenium catalyst. Since the true TOF is
expected to be constant for a given metal containing catalyst in a given reaction under specified
conditions [14], understanding this inverse relation between the TOF and precatalyst
concentration is important. The use of Ru(0)/X-NW as catalyst in hydrogen generation from the
hydrolysis of sodium borohydride provides a unique opportunity to investigate the inverse
relation between the TOF and ruthenium concentration in terms of increasing particle size. Thus,
we were able to report the results of the first investigation on the decrease in catalytic activity
with the increasing catalyst concentration in hydrogen generation from the hydrolysis of sodium
borohydride.
Experimental section
Materials
RuCl3ｷ 3H2O, Ca(NO3)2ｷ 4H2O, Na2SiO3ｷ 9H2O, and NaBH4 were purchased from Aldrich.
Milli-Q System was used to obtain deionized water. All glassware and Teflon-coated magnetic
stir bars were cleaned with acetone, followed by copious rinsing with water before drying at 150
ｰ C.
Characterization
6

Transmission electron microscopy (TEM) was performed on a JEM-2100F (JEOL) microscope
operating at 200 kV. Samples were examined at magnification between 100 and 400 K. The
details for the characterization of Ru(0)/X-NW can be seen in our recent study [11].
Preparation of xonotlite nanowire [Ca6(Si6O17)(OH)2]
Xonotlite nanowire (X-NW) was prepared by following the procedure described elsewhere [15].
Ca(NO3)2ｷ 4H2O and Na2SiO3ｷ 9H2O were dissolved in water in two separate flasks to obtain
0.5 M solution for each. The calcium nitrate solution was added dropwise into the sodium silicate
solution at room temperature under stirring to obtain a white suspension. The Ca/Si molar ratio
was kept at 1.0. The suspension was transferred into the Teflon-lined stainless-steel autoclave and
heated at 200 ｰ C for 24 h. After cooling down to room temperature, the suspension was filtered
and washed with water and anhydrous ethanol for three times, respectively. The liquid residue
was removed by vacuum filtration, and the remaining xonotlite nanowires were dried at 120 ｰ C
for 24 h.
Preparation of ruthenium(III) ion exchanged xonotlite nanowire (Ru3+/X-NW)
Ruthenium(III) cations were introduced onto the xonotlite nanowires by ion exchange of 400 mg
xonotlite nanowires in 100 mL aqueous solution of RuCl3·3H2O (21.83 mg). The slurry was
stirred at room temperature for 3 days. The solid Ru3+/X-NW was isolated by centrifugation and
washed with 100 mL of water. The solid remnant was dried under vacuum (10−3 Torr) at 80 °C.
In situ formation of ruthenium(0) nanoparticles supported on xonotlite nanowire (Ru(0)/X-
NW) and concomitant catalytic hydrolysis of SB
Ruthenium(0) nanoparticles supported on xonotlite nanowire were in situ generated from the
reduction of Ru3+/X-NW during the catalytic hydrolysis of sodium borohydride by following the
procedure as described elsewhere [11]. Before starting the catalyst formation and concomitant
catalytic hydrolysis of sodium borohydride, a 20 mL jacketed reaction flask with a Teflon-coated
7

stir bar was placed on a magnetic stirrer (Heidolph MR-301) and thermostated to 25.0 ± 0.1 °C by
circulating water through its jacket from a constant temperature bath. Then, a graduated glass
tube (60 cm in height and 3.0 cm in diameter) filled with water was connected to the reaction
flask to measure the volume of the hydrogen gas generated from the hydrolysis of sodium
borohydride. Next, desired amount of Ru3+/X-NW (1.37 wt.% Ru) powder was dispersed in 10
mL water in the reaction flask thermostated at 25.0 ± 0.1 °C. Then, sodium borohydride (38.6 mg,
1.0 mmol) was added into the flask and the reaction mixture was stirred at 1000 rpm. After a
short induction period, ruthenium(0) nanoparticles were formed and the concomitant hydrolysis
of sodium borohydride started. The volume of hydrogen gas evolved was measured by recording
the displacement of water level every 1 min at constant atmospheric pressure of 693 Torr. The
reaction was stopped when no more hydrogen evolution was observed. All the hydrogen volume
data were corrected by subtracting the self hydrolysis data of sodium borohydride to obtain the
volume of hydrogen generated only from the catalytic hydrolysis.
Determination of apparent activation energy for the hydrolysis of sodium borohydride
catalyzed by Ru(0)/X-NW
In a typical experiment, the hydrolysis reaction was performed starting with 10 mL of 100 mM
sodium borohydride solution and 10 mg Ru3+/X-NW (1.37 wt.% ruthenium, [Ru] = 0.135 mM) at
various temperatures of 25, 30, 35, 40, 45 °C to determine the apparent activation energy.
Determination of the catalytic lifetime of Ru(0)/X-NW in the hydrolysis of sodium
borohydride
The catalytic lifetime of Ru(0)/X-NW in the hydrolysis of sodium borohydride was determined
by measuring the total turnover number (TTO). Such a lifetime experiment was started with a
100 mL solution containing 0.135 mM Ru(0)/X-NW and 30 mM sodium borohydride at 25.0 ±
0.1 °C. When all the sodium borohydride present in the solution was completely hydrolyzed,
8

more sodium borohydride was added and the reaction was continued in this way until no more
hydrogen evolution was observed.
Results and Discussion
Xonotlite nanowire (X-NW) was prepared by following the procedure described elsewhere [15].
Fig. 1 exhibits the SEM image, SEM-EDX spectrum and TEM images of xonotlite nanowire
indicating that (i) xonotlite nanowire has an average diameter of 36 nm and length of 1.4 m, and
(ii) there is no impurity detected in addition to the framework elements of xonotlite nanowire
(Ca, Si, O). Ruthenium(0) nanoparticles supported on xonotlite nanowire (Ru(0)/X-NW) were in
situ generated from the reduction of ruthenium(III) ions impregnated on X-NW during the
hydrolysis of sodium borohydride at room temperature. When sodium borohydride is added to
the suspension of xonotlite nanowire containing ruthenium(III) ions, both reduction of
ruthenium(III) to ruthenium(0) and hydrogen release from the hydrolysis of sodium borohydride
occur concomitantly. The progress of ruthenium(0) nanoparticles formation and concomitant
hydrolysis of sodium borohydride was followed by monitoring the change in H2 pressure which
was then converted into the equivalent H2 per mole of sodium borohydride, using the known 4:1
H2/SB stoichiometry (Eq. (1)). Fig. 2, shows the plot of equivalent H2 generated versus time for
the hydrolysis of NaBH4 starting with Ru3+/X-NW precatalyst (0.406 mM Ru) and 100 mM
NaBH4 in 10 mL aqueous solution at 25.0 ± 0.1 °C. After a short induction period of 2 min, the
hydrogen generation starts and continues almost linearly until the release of 4 equiv. H2 per mole
of NaBH4. The observation of an induction period and a sigmoidal shape of dehydrogenation
curve indicates the formation of ruthenium(0) nanoparticles with a two-step, nucleation plus
autocatalytic growth mechanism [16,17]. Formation kinetics of the Ru(0)n nanoparticle catalyst
9

can be obtained using the hydrogen release from the hydrolysis of NaBH4 as reporter reaction
[18] given in Scheme 1, where P is the added precursor Ru3+/X-NW and Q is the growing Ru(0)n
nanoparticles on the surface of xonotlite nanowire. The hydrogen generation from the hydrolysis
of NaBH4 will accurately report on and amplifies the amount of Ru(0)n nanoparticle catalyst Q
present if the dehydrogenation rate is fast in comparison to the rate of nanoparticles formation.
Sigmoidal kinetics can be seen in Fig. 2 and fit well by the Finke–Watzky two-step, nucleation,
and autocatalytic growth mechanism of nanoparticle formation [16]. The observation of a
sigmoidal dehydrogenation curve and its curve-fit to the slow, continuous nucleation, P → Q
(rate constant k1) followed by autocatalytic surface growth, P + Q → 2Q (rate constant k2)
kinetics are very strong evidence for the formation of metal(0) nanoparticles catalyst from a
soluble transition-metal complex in the presence of reducing agent [16]. The rate constants
2
determined from the nonlinear least-squares curve-fit in Fig. 2, given below, are k1 = 1.82 × 10−
1
1
1
min− and k2 = 0.089 ×102 M− min− . The mathematically required correction has been made to k2
for the stoichiometry factor of 246 as described elsewhere [17] but not for the “scaling factor”;
that is no correction has been made for the changing the number of Ru atoms on the growing
1
metal surface [17]. The large value of k2/ k1 ratio (4.92 × 102 M− ) is indicative of the high level
kinetic control in the formation of ruthenium(0) nanoparticles from the reduction of the precursor
ruthenium(III) ions on the surface of xonotlite nanowire (rate constants k2 and k1 were calculated
according to the kinetic model equation described elsewhere [11]).
Ru(0)/X-NW was isolated from the reaction solution by centrifugation, copious washing with
water, and drying under vacuum (10−3 Torr) at 80 °C and characterized by ICP-OES, XRD, SEM,
SEM-EDX, TEM, XPS and the N2 adsorption–desorption technique (See our recent study [11]
10

for the details of the catalyst characterization). It is noteworthy that XPS analysis shows the
existence of ruthenium(0) in the samples studied.
Hydrogen generation from the hydrolysis of sodium borohydride catalyzed by Ru(0)/X-NW
A control experiment was performed to check whether xonotlite nanowire show any catalytic
activity in the hydrolysis of sodium borohydride before starting the investigation on the catalytic
activity of Ru(0)/X-NW. In a control experiment starting with 1.0 mmol SB and 10 mg powder of
xonotlite nanowire in 10 mL water at 25.0 ± 0.1 °C or 45.0 ± 0.1 °C, no hydrogen generation was
observed in 1 h at both temperatures. This observation indicates that the hydrolysis of sodium
borohydride does not occur at an appreciable rate in the presence of xonotlite nanowire in the
temperature range used in this study. Ruthenium(0) nanoparticles supported on xonotlite
nanowire, Ru(0)/X-NW, are found to be highly active catalyst in hydrogen generation from the
hydrolysis of sodium borohydride yielding 4.0 equivalent H2 gas per mol of SB in the same
temperature range.
Fig. 3a shows the plots of equivalent H2 gas generated per mole of sodium borohydride versus
time during the catalytic hydrolysis of 100 mM SB solution using Ru(0)/X-NW with a loading of
1.37% wt Ru in different ruthenium concentration at 25.0 ± 0.1 °C. In each of the experiments,
hydrogen evolution starts immediately without induction period and continues almost linearly
until the consumption of sodium borohydride present in the solution. The hydrogen generation
rate was determined from the linear portion of each plot in Fig. 3a and plotted versus the initial
concentration of ruthenium, both in logarithmic scale, in Fig. 3c. This plot gives straight line with
a slope of 0.40 indicating that the order of the catalytic hydrolysis of sodium borohydride is 0.40
with respect to the ruthenium concentration. Since all the hydrogen volume versus time plots are
11

linear, it is conceivable that the hydrogen evolution is zero order with respect to sodium
borohydride concentration.
The turnover frequency (TOF) value for hydrogen generation from the hydrolysis of sodium
borohydride (100 mM) at 25.0 ± 0.1 °C was determined from the hydrogen generation rate in the
linear portion of each plot given in Fig. 3a. The TOF value of Ru(0)/X-NW catalyst (Table 1) is
as high as 305 min-1 (mol H2/(mol Ru×min)) which corresponds to 1110 mL H2/(min×gcat) in the
hydrolysis of sodium borohydride at 25.0 ± 0.1 °C. Turnover frequency decreases with the
increasing initial concentration of ruthenium in the Ru(0)/X-NW catalyst, down to a value of 35
min-1 for the 0.542 mM Ru. An inverse relation between the TOF and ruthenium concentration
was observed (Fig.3b) and ascribed to the increasing size of ruthenium(0) nanoparticles. In fact,
inverse relationship between the apparent turnover frequency and the concentration of precatalyst
or catalyst has been reported for different reactions with different catalysts such as hydrogenation
by cobalt(III) acetylacetonate [19], alcohol oxidation by palladium(II) acetate [20],
hydrogenation of styrene by osmium [21], Heck reaction by palladium [22,23], heterolysis by
cobalt [24], cyclohexene hydrogenation by iridium [25], and methanolysis by rhodium [26]. With
one exception of iridium hydrogenation catalyst [25], in all the reports the inverse relationship
has been ascribed to the increasing size of nanoparticles. However, only in the case of
methanolysis of ammonia borane catalyzed by rhodium nanoparticles, experimental evidence has
been provided showing that the decrease in catalytic activity with the increasing catalyst
concentration is due to the increasing particles size [26]. Herein we also report that in hydrogen
generation from the hydrolysis of sodium borohydride catalyzed by ruthenium(0) nanoparticles,
the inverse relation between the TOF and catalyst concentration is in fact due to increasing
particle size of Ru(0)/X-NW. Fig. 4 shows the TEM images of Ru(0)/X-NW at different
ruthenium concentration along with the associated particle size histogram. The particle size for
12

Ru(0)/X-NW at three different ruthenium concentrations were determined and given in Table 1.
Expectedly, size of ruthenium(0) nanoparticles increases as ruthenium concentration increases
while the TOF value decreases in the same direction (Fig. 5). The TOF value of Ru(0)/X-NW in
hydrogen generation from the hydrolysis of sodium borohydride decreases as the particle size of
the ruthenium(0) nanoparticles increases with the increasing initial concentration of ruthenium
(Fig. 5a and Fig. 5b). However, when the TOF values are corrected for the number of ruthenium
atoms on the surface of nanoparticle, calculated as described elsewhere [25,27], the inverse
relation between the catalytic activity and the size of nanoparticles becomes less perceptible
(Table 1). This also indicates that the inverse relation between the catalytic activity and the
concentration of ruthenium is indeed a size issue.
Catalytic activity of Ru(0)/X-NW in hydrolysis of sodium borohdyride at room temperature can
be compared to that of other reported catalysts such as Co-NP-B-MA (TOF= 48 min-1) [28],
PVP/Ru-Pd NPs (TOF= 762 min-1) [29], Ni(0) nanoclusters stabilized on PVP (4250 mL H2/
(min×gcat)) [30], Co-B embedded on B thin film (3375 mL H2/ (min×gcat) [28], acetate stabilized
Ru(0) nanoclusters (96,800 mL H2/ (min×gcat)) [31].
Finally, we performed the catalytic hydrolysis of sodium borohydride in the presence of Ru(0)/X-
NW at constant NaBH4 (100 mM in 10 mL) and ruthenium (0.135 mM Ru) concentrations at
various temperatures in the range of 25-45 °C in order to obtain the apparent activation energy
(Ea). The rate constant for the hydrogen generation reactions was calculated from the slope of the
linear part of each hydrogen evolution per mole of sodium borohydride versus time plot prepared
at various temperatures (Fig. 6a). The apparent activation energy calculated from the slope of the
regression line (Fig. 6b) is Ea = 75 ± 2 kJ/mol. The apparent activation energy for the hydrolysis
of sodium borohydride catalyzed by Ru(0)/X-NW is comparable with the literature values
reported for various catalysts such as Ni(0) nanoclusters stabilized on PVP (Ea = 48 kJ/mol) [30],
13

hydrogen phosphate stabilized Ni(0) NPs (Ea = 54 kJ/mol) [32], acetate stabilized Ru(0)
nanoclusters (Ea = 41 kJ/mol) [31], Fe-Co NPs (Ea = 35.6 kJ/mol) [33], 5 wt % Pt/carbon black
(Ea = 44 kJ/mol) [28], 10 wt% Pd/carbon black (Ea = 52 kJ/mol) [28], Co-NP-B-MA (Ea = 30
kJ/mol) [28], Co–La–Zr–B nano allloy (Ea = 60 kJ/mol) [34], PVP/Ru-Pd NPs (Ea = 52 kJ/mol)
[29], Ni(0)/ Zeolite (Ea = 64 kJ/mol) [35]. It is noteworthy that there is no obvious relation
between apparent activation energy value and catalytic activity of a catalyst.
Catalytic lifetime of Ru(0)@X-NW was determined by measuring the total turnover number
(TTO) in hydrogen generation from the hydrolysis of sodium borohydride. A catalyst lifetime
experiment starting with 10 mg Ru(0)@X-NW in 50 mL of solution of sodium borohydride (100
mM) at 25.0 ± 0.1 °C reveals a minimum of 63,100 turnovers over 101 h in hydrogen generation
from the hydrolysis of sodium borohydride before deactivation of the catalyst (Fig. 7). The
deactivation of Ru(0)/X-NW catalyst can be attributed to a decrease in the accessibility of active
sites of ruthenium nanoparticles due to the passivation of metal surface by metaborate ions which
accumulate in solution as the reaction proceeds. In addition, the increase in viscosity of the
solution as the reaction proceeds might hinder the diffusion of sodium borohydride, which results
in decreasing the catalytic activity [36]. In fact, Ru(0)/X-NW could not be separated from the
reaction medium after the lifetime experiment because of the difficulties arising from the high
viscosity of reaction medium due to the accumulating metaborate ion. It was impossible to
separate small amount of the catalyst (10 mg) from the highly viscous reaction medium
-
containing more than 1 gram BO2 ion.
Although it is difficult to propose a specific reason to explain the high activity and long lifetime
of Ru(0)/X-NW catalyst, the factors making contribution to the stability include the nature of
supporting material (such as surface area, porosity, structure), the identity of metal (generally
14

ruthenium provides high stability), the preparation method of catalysts (such as impregnation, ion
exchange method) and the metal-support interactions. In our case, the main contribution to the
high stability of Ru(0)/X-NW catalyst is likely from the preparation technique used for the
ruthenium(0) nanoparticles: The ruthenium(III) ions are ion-exchanged with calcium(II) ions of
the X-NW and then reduced by sodium borohydride whereby the cation-vacancies are re-
occupied by sodium ions. In this way, the integrity of support is maintained after nanoparticle
formation on the surface of X-NW. The favorable interaction of ruthenium(0) nanoparticles with
the surface of support might also provide significant contribution to the high activity and long life
time of Ru(0)/X-NW catalyst.
Hydrolysis of sodium borohydride reaction was continued until hydrogen gas evolution was
slowed down to the self-hydrolysis level. Self-hydrolysis of sodium borohydride occurs even at
ambient temperature at much slower rates compared to the hydrolysis catalyzed by Ru(0)/X-NW.
Under our experimental conditions, the self-hydrolysis of sodium borohydride liberates nearly 12
mL of H2 gas in 1 h while the catalytic hydrolysis of sodium borohydride in the presence of
Ru(0)/X-NW can generate 182 mL H2 gas in 1 h at 25.0 ± 0.1 °C. The self-hydrolysis of sodium
borohydride is slow but not negligible, and therefore, we have taken it into account during the
catalytic life time test. Note that all the hydrogen volume data were corrected by subtracting the
self hydrolysis data of NaBH4 in order to obtain the volume of hydrogen generated only from the
catalytic reaction.
As shown in Fig. 7, Ru(0)/X-NW provide an unprecedented TTO value for hydrogen generation
from the hydrolysis of sodium borohydride as compared to other catalysts such as hydrogen
phosphate stabilized Ni(0) NPs (TTO= 1450), acetate stabilized Ru(0) nanoclusters (TTO= 5170)
[31], Ni(0) nanoclusters stabilized on PVP (TTO = 8700) [30], Ni(0)/ Zeolite (TTO=19,400)
[35].
15

Conclusion
In summary, ruthenium(0) nanoparticles supported on xonotlite nanowire (Ru(0)/X-NW) show
high catalytic activity and long life time providing an initial turnover frequency of 305 min−1 and
63,100 turnovers over 101 h in hydrogen generation from the hydrolysis of sodium borohydride
at 25.0 ± 0.1 °C. Most interestingly, an inverse relation between the catalytic activity of Ru(0)/X-
NW and the catalyst concentration is observed for this hydrolysis reaction. The decrease in
turnover frequency with the increasing concentration of ruthenium was shown to due to the
increase in particle size with the increasing catalyst concentration. The mean particle size of
Ru(0)/X-NW, formed from the reduction of Ru3+/X-NW during the hydrolysis of sodium
borohydride at room temperature, increases with the increasing initial ruthenium concentration.
Thus, the size of ruthenium(0) nanoparticles supported on xonotlite nanowire can be tuned by
varying the initial concentration of precatalyst. As the mean particle size increases, the fraction
of catalytically active ruthenium atoms on the surface over the total ruthenium atoms decreases,
so does the catalytic activity.
The facile preparation, high catalytic activity, high stability, and long life time of ruthenium(0)
nanoparticles supported on the surface of xonotlite nanowires make them promising candidate to
be exploited as catalyst in releasing hydrogen from the hydrolysis of sodium borohydride which
has been considered as hydrogen storage materials for portable application systems.
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18

Figure Captions:
Figure 1 (a) SEM image, (b) SEM-EDX spectrum, (c) and (d) TEM images of xonotlite nanowire with two different
magnifications.
Figure 2 Plot of equivalent H2 generated versus time for the hydrogen generation from the hydrolysis of sodium
borohydride starting with 30 mg Ru3+/ X-NW (with a loading of 1.37 wt% Ru) as catalyst and 1.0 mmol NaBH4 in
10 mL of aqueous solution at 25.0 ± 0.1 °C. The sigmoidal curve fits well to the two-step mechanism for the
ruthenium(0) nanoparticle formation.
Figure 3 (a) mol H2/mol NaBH4 versus time graph depending on the ruthenium concentration in Ru(0)/X-NW for
the hydrolysis of SB (100 mM) at 25.0 ± 0.1 °C. (b) The TOF values of the catalyst at different Ru concentrations.
(c) The logarithmic plot of hydrogen generation rate versus the Ru concentration; ln(rate) = 0.40 ln[Ru] + 1.87.
Figure 4 TEM images of Ru(0)/X-NW for (a) 0.0135 mM Ru, (b) 0.0338 mM Ru, (c) 0.203 mM Ru (the TEM
images were taken from the samples harvested after the hydrolysis of sodium borohydride) and Histograms showing
particle size distribution of (d) 0.0135 mM Ru (mean diameter: 2.64 ± 0.09 nm), (e) 0.0338 mM Ru (mean diameter:
3.27 ± 0.15 nm), (f) 0.203 mM Ru (mean diameter: 4.40 ± 0.40 nm)
Figure 5 (a) The plots of TOF and particle size of Ru(0)/X-NW versus the ruthenium concentration, (b) The plot of
TOF versus the particle size of Ru(0)/X-NW for hydrogen generation from the hydrolysis of sodium borohydride at
25.0 ± 0.1 °C.
Figure 6 (a) The evolution of equivalent hydrogen per mole of SB versus time plot for the hydrolysis of SB starting
with Ru(0)/X-NW (0.135 mM Ru) and 100 mM SB at various temperatures. (b) The Arrhenius plot for the Ru(0)/X-
NW catalyzed hydrolysis of SB. Ink= -8959.66(1/T) + 31.06.
Figure 7 The variation in turnover number (TON) and turnover frequency (TOF) during the catalytic lifetime
experiment performed starting with 10 mg Ru(0)/X-NW (ruthenium loading = 1.37% wt Ru, and [Ru] = 0.0271 mM)
in 50 mL solution of sodium borohydride at 25.0 ± 0.1 °C.
19

Scheme 1 Illustration of the hydrogen release from the catalytic hydrolysis of sodium borohydride as reporter
reaction: P is the precursor Ru3+/X-NW and Q is the growing Ru(0)n nanoparticles on the surface of xonotlite
nanowire.
20

Table 1 The apparent turnover frequency (TOFapp=mol H2×mol Ru-1×min-1), the corrected turnover frequency
(TOFcorrected, the value corrected for the surface atoms), and the particle size of Ru(0)/X-NW with standard deviation
in parentheses depending on the ruthenium concentration ([Ru]) of the catalyst in hydrogen generation from the
hydrolysis of sodium borohydride at 25.0 ± 0.1 °C. a TOFapp values were calculated from the total ruthenium content
without correction by the fraction of active sites.b TOFcorrected values were determined by estimating the fraction of
surface atoms which were calculated from the ratio of number of surface Ru atoms to the total number of Ru atoms.
The total number of Ru atoms in a nanoparticle was estimated by using the equation given elsewhere [24], n = (N0
ρ(4/3)π(D/2)3)/W, where n = number of Ru atoms, N0 = 6.022 × 1023 mol−1, ρ = room-temperature density of
ruthenium (12.45 g/cm3), D = diameter of ruthenium nanoparticles, W= atomic weight of ruthenium (101.07 g/mol).
The number of surface Ru atoms was estimated from full-shell “magic number” approach described elsewhere [24].
From these calculations, the percentage surface atoms were found as 42%, 36% and 27% for the Ru nanoparticles
with average diameters of 2.64 nm, 3.24 nm and 4.4 nm, respectively. The TOFapp is divided by the surface fraction
to obtain the TOFcorrected value.
[Ru] mM
TOFapp (min-1)a
Particle size (nm)
TOFcorrected (min-1)b
(standard deviation)
0.0135
0.034
0.068
0.135
0.203
0.271
0.406
0.542
305
209
105
79
2.64 (0.96)
726
3.24 (0.95)
581
-
-
-
-
70
4.40 (1.40)
259
48
-
-
-
-
-
-
41
35
21

Fig.1
22

Fig.2
23

Fig. 3
24

Fig.4
25

Fig.5
26

Fig.6
27

Fig.7
28