Ceria-supported ruthenium nanoparticles as highly active and long-lived catalysts in hydrogen generation from the hydrolysis of ammonia borane

Article abstract of DOI:10.1039/c6dt01117a

Ruthenium(0) nanoparticles supported on ceria (Ru⁰/CeO₂) were in situ generated from the reduction of ruthenium(iii) ions impregnated on ceria during the hydrolysis of ammonia borane. Ru⁰/CeO₂ was isolated from the reaction solution by centrifugation and characterized by ICP-OES, BET, XRD, TEM, SEM-EDS and XPS techniques. All the results reveal that ruthenium(0) nanoparticles were successfully supported on ceria and the resulting Ru⁰/CeO₂ is a highly active, reusable and long-lived catalyst for hydrogen generation from the hydrolysis of ammonia borane with a turnover frequency value of 361 min^-1. The reusability tests reveal that Ru⁰/CeO₂ is still active in the subsequent runs of hydrolysis of ammonia borane preserving 60% of the initial catalytic activity even after the fifth run. Ru⁰/CeO₂ provides a superior catalytic lifetime (TTO = 135:100) in hydrogen generation from the hydrolysis of ammonia borane at 25.0 ± 0.1 °C before deactivation. The work reported here includes the formation kinetics of ruthenium(0) nanoparticles. The rate constants for the slow nucleation and autocatalytic surface growth of ruthenium(0) nanoparticles were obtained using hydrogen evolution as a reporter reaction. An evaluation of rate constants at various temperatures enabled the estimation of activation energies for both the reactions, E_a = 60 ± 7 kJ mol^-1 for the nucleation and E_a = 47 ± 2 kJ mol^-1 for the autocatalytic surface growth of ruthenium(0) nanoparticles, as well as the activation energy of E_a = 51 ± 2 kJ mol^-1 for the catalytic hydrolysis of ammonia borane.

Full text of DOI:10.1039/c6dt01117a

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DOI: 10.1039/C6DT01117A
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PAPER
Ceria-Supported Ruthenium Nanoparticles as Highly Active and
Long-Lived Catalyst in Hydrogen Generation from the Hydrolysis
of Ammonia Borane
Received 00th January 20xx,
Accepted 00th January 20xx
a
a,b
a
DOI: 10.1039/x0xx00000x
Serdar Akbayrak, Yalçın Tonbul and Saim Özkar*
0
www.rsc.org/
Ruthenium(0) nanoparticles supported on ceria (Ru /CeO
2
) were in situ generated from the reduction of ruthenium(III)
0
ions impregnated on ceria during the hydrolysis of ammonia borane. Ru /CeO
2
were isolated from the reaction solution by
centrifugation and characterized by ICP-OES, BET, XRD, TEM, SEM-EDS and XPS techniques. All the results reveal that
0
ruthenium(0) nanoparticles were successfully supported on ceria and the resulting Ru /CeO
2
is highly active, reusable and
long-lived catalyst for hydrogen generation from the hydrolysis of ammonia borane with a turnover frequency value of 361
−
1
0
min . The reusability tests reveal that Ru /CeO
2
are still active in the subsequent runs of hydrolysis of ammonia borane
0
preserving 60% of the initial catalytic activity even after the fifth run. Ru /CeO
2
provides superior catalytic life time (TTO =
1
35,100) in hydrogen generation from the hydrolysis of ammonia borane at 25.0 ± 0.1 °C before deactivation. The work
reported here also includes the formation kinetics of ruthenium(0) nanoparticles. The rate constants for the slow
nucleation and autocatalytic surface growth of ruthenium(0) nanoparticles were obtained by using the hydrogen evolution
as reporter reaction. The evaluation of rate constants at various temperatures provides the estimation of activation
energies for both reactions, E
a
= 60 ± 7 kJ/mol for the nucleation and E
a
= 47 ± 2 kJ/mol for the autocatalytic surface
a
= 51 ± 2 kJ/mol for the catalytic hydrolysis of
growth of ruthenium(0) nanoparticles, as well as the activation energy of E
ammonia borane.
achieved only in the presence of a suitable catalyst. Therefore,
the development of efficient and stable catalysts for hydrogen
release from AB is a challenge in using ammonia borane as
hydrogen storage material for fuel cell applications.
Ruthenium is one of the most active catalysts in hydrogen
generation from the hydrolysis of AB and there exists a wealth
of reports on hydrolysis of AB using ruthenium based catalysts
Introduction
Hydrogen has been considered as a clean, nontoxic, and
abundant energy carrier in transition from fossil fuels to the
renewable energy sources on the way towards a sustainable
1
, 2,3
energy future.
However, safe and efficient storage of
hydrogen is still the main obstacle in the widespread
application of hydrogen energy.
14
15
16
4
, 5
such as Ru(0)/MIL-96, Ru(0)@SiO , Ru/Carbon, Ru/γ-
2
Among the chemical
17 , 18
19
20
Al O ,
Ru/SiO -CoFe O
2 4
and Ru/graphene. However,
2
3
2
hydrides, tested as hydrogen storage materials for onboard
6,7
there are few reports indicating the catalytic life time of
ruthenium catalysts in the hydrolysis of AB. This is due to the
short life time of them. The main reason for the short life time
of metal nanoparticles catalysts is their instability against
agglomeration. Using suitable supports to prevent
agglomeration of metal nanoparticles is the most common
applications, ammonia borane (NH -BH , AB) appears to be
3
3
the most promising candidate for chemical hydrogen storage
due to its 19.6% wt hydrogen content, high stability, and
environmental benignity.
the widely accepted and well-studied way of hydrogen release
from ammonia borane because of favourable kinetics and mild
8,9
10,11
Hydrolysis in aqueous medium is
21
12,13
way to improve the activity and stability of catalysts. Metal
reaction conditions (Eq. 1).
2
2,23
24
25,26
oxides,
polymers, carbonaceous materials
or metal-
2
7
organic frameworks have widely been used to stabilize metal
nanoparticles. There has been growing interest on ceria as
supporting material for noble metals due to its high chemical
28
However, the hydrogen release from ammonia borane can be
29
stability.
Ceria is considered potent to improve the
performance of late transition metals through strong
30
metal−support interaction. However, the mechanism of the
promoting effect for cerium oxides has not been well
established. Ceria supported metal nanoparticles have been
This journal is © The Royal Society of Chemistry 2016
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used as catalysts in many chemical reactions such as oxidation were examined at magnification between 300 VaienwdArt5ic0le0Onklinxe.
3
1
32
of organic compounds, hydrogenation of aromatics,
Scanning electron microscope (SEM) ima
and soot a JEOL JSM-5310LVat 15 kV and 33 Pa in a low-vacuum mode
So far, a variety of noble and non-noble metal without metal coating on aluminum support. The X-ray
based catalysts using different supporting materials have been photoelectron spectroscopy (XPS) analysis was performed on a
D
ge
O
s
I:
w
10
e
.1
r
0
e
39
t
/
a
C
k
6
e
D
n
T0
u
1
s
1
i
1
n
7A
g
33 , 34
decomposition of N O and acetic acid,
2
35,36
combustion.
21
tested in the hydrolysis of AB. There exists only one report Physical Electronics 5800 spectrometer equipped with a
on the use of ceria containing metal nanoparticles catalyst in hemispherical analyzer and using monochromatic Al Kα
37
the hydrolysis of ammonia borane. However, that study radiation of 1486.6 eV, the X-ray tube working at 15 kV, 350 W
11
deals with the formation of Pt-CeO hybrid on the surface of and pass energy of 23.5 keV. B NMR spectra were recorded
2
graphene oxide.
on a Bruker Avance DPX 400 with an operating frequency of
1
1
Herein we report the in situ generation, characterization, 128.15 MHz for B.
catalytic use and formation kinetics of ruthenium(0) 2.3. Preparation of Ru(0) nanoparticles supported on ceria
0
nanoparticles supported on ceria, Ru /CeO . Since reducing Ceria (500 mg) was added to a solution of RuCl ·xH O (57.7
2
3
2
the particle size of support from the microcrystalline to the mg) in 100 mL H O in a 250 mL round bottom flask. This slurry
2
3+
nanocrystalline regime (from > 1 μm to < 100 nm) can enhance was stirred at room temperature for 4 h. The Ru /CeO
2
the catalytic activity of the supported materials due to sample was isolated by centrifugation and washed with 50 mL
38
increase in the external surface area of support, nanosize of distilled water and the remnant was dried under vacuum
−
3
ceria with an average size of 25 nm was used as support for (10 Torr) at 60 °C for 12 h.
the ruthenium(0) nanoparticles. Ruthenium(III) ions 2.4. In situ formation of ruthenium(0) nanoparticles
0
impregnated on the surface of ceria were reduced by supported on ceria Ru /CeO and concomitant catalytic
2
0
ammonia borane forming the Ru /CeO during the hydrolysis hydrolysis of AB
2
0
of AB. Ru /CeO could be isolated from the reaction solution by Ruthenium(0) nanoparticles supported on ceria were in situ
centrifugation and characterized by ICP-OES, BET, XRD, TEM, generated from the reduction of Ru /CeO during the catalytic
2
3+
2
SEM-EDS and XPS techniques. All the results reveal the hydrolysis of AB. Before starting the catalyst formation and
formation of well dispersed ruthenium nanoparticles concomitant catalytic hydrolysis of AB, a jacketed reaction
0
supported on the surface of ceria particles. Ru /CeO has flask (20 mL) containing a Teflon-coated stir bar was placed on
2
remarkable catalytic activity in hydrogen generation from the a magnetic stirrer (Heidolph MR-301) and thermostated to
-1
hydrolysis of AB with a turnover frequency of 361 min at 25.0 25.0 ± 0.1 °C by circulating water through its jacket from a
0
±
0.1°C. We also report that Ru /CeO is a reusable and long- constant temperature bath. Then, a graduated glass tube (60
2
lived catalyst with a total turnover number of 135,100 in cm in height and 3.0 cm in diameter) filled with water was
hydrogen generation from the hydrolysis of AB at room connected to the reaction flask to measure the volume of the
0
temperature. Ru /CeO has the longest catalytic life time as hydrogen gas to be evolved from the reaction. Next, 20 mg
2
3+
compared to the other ruthenium catalyst used in hydrolysis of powder of Ru /CeO (1.90 wt. % Ru) was suspended in 10 mL
2
ammonia borane. Our report also provides valuable insight on distilled water in the reaction flask thermostated at 25.0 ± 0.1
the kinetics of nanoparticle formation.
°C. Then, 31.8 mg AB (1.0 mmol H N·BH ) was added to the
3 3
suspension in the flask and the reaction medium was stirred at
000 rpm. After a short induction period of less than 2.0 min,
ruthenium(0) nanoparticles were formed and the catalytic
hydrolysis of AB 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.
2
. Experimental
1
2
.1 Materials
Ruthenium(III) chloride trihydrate (RuCl ·3H O), ammonia
borane (AB, 97%) and ceria (CeO , 25 nm, BET surface area
3
2
2
2
4
8.1 m /g) were purchased from Aldrich. Deionized water was
distilled by water purification system (Milli-QSystem). All
glassware and Teflon-coated magnetic stir bars were cleaned
with acetone, followed by copious rinsing with distilled water
before drying in an oven at 150 ᵒC.
2
.5. Determination of activation energy for the hydrolysis of
0
AB catalyzed by Ru /CeO
2
In a typical experiment, the hydrolysis reaction was performed
starting with 10 mL of 100 mM (31.8 mg) AB solution and 20
2
.2. Characterization
3+
0
mg Ru /CeO (1.90 wt. % Ru), [Ru] = 0.38 mM) at various
2
The ruthenium contents of the Ru /CeO samples were
2
temperatures (25, 30, 35, 40 °C) in order to obtain the
determined by Inductively Coupled Plasma Optical Emission
Spectroscopy (ICP-OES, Leeman-Direct Reading Echelle) after
each sample was completely dissolved in the mixture of
activation energy.
0
2
.6. Reusability of Ru /CeO in the hydrolysis of AB
2
After the complete hydrolysis of AB started with 10 mL of 100
HNO /HCl (1/3 ratio). FEI Tecnai G2 F30 transmission electron
3
3+
mM AB (31.8 mg H NBH ), and 40 mg Ru /CeO (1.90 wt. %
3
3
2
microscope (TEM) was used to image ruthenium nanoparticles
on ceria. A small amount of powder sample was dispersed in
ethanol and one drop of this solution was placed on the holey
carbon grid of the transmission electron microscope. Samples
Ru, [Ru] = 0.76 mM) at 25.0 ± 0.1 °C, the catalyst was isolated
as dark grey powder by centrifugation, washed with 50 mL of
−
3
water, and dried under vacuum (10 Torr) at 60 °C. The
2
| Dalton Trans., 2016, 00, 1-3
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0
yielding the rate constants k
autocatalytic surface growth steps.
1 2
for the slow, continuous nucleation and k for the
isolated samples of Ru /CeO were weighed and redispersed in
1
2
40
View Article Online
0 mL solution of 100 mM AB for a subsequent run of
DOI: 10.1039/C6DT01117A
hydrolysis at 25.0 ± 0.1 °C. Material loss has been taken into
account for the calculation of initial TOF values for each run.
0
2
.7. Determination of the catalytic lifetime of Ru /CeO in
2
hydrolysis of AB
The catalytic lifetime of Ru /CeO in the hydrolysis of AB was
0
2
determined by measuring the total turnover number (TTO).
Such a lifetime experiment was started with a 100 mL solution
Scheme 1: Illustration of the hydrolysis of ammonia borane as reporter reaction
0
containing 0.19 mM Ru /CeO and 50 mM AB at 25.0 ± 0.1 °C.
39
2
for the formation of ruthenium(0) nanoparticles
:
P
is the precursor
When all the AB present in the solution was completely ruthenium(III) ion impregnated on CeO
hydrolyzed, more AB was added and the reaction was
2
and
Q
is the growing Ru(0)
n
nanoparticles.
continued in this way until no H gas evolution was observed.
2
The rate constants obtained from the nonlinear least squares
3
. Results and Discussion
-
4
-1
curve-fit using the function in Eq. 2 are k =(6.92±0.67)×10 s
1
Ruthenium(0) nanoparticles formation and concomitant
-1 -1
and k =1.75±0.13 M .s . Note that the mathematically
2
hydrogen generation
required correction has been made to k for the stoichiometry
2
Ruthenium(0) nanoparticles supported on ceria were in situ
generated from the reduction of ruthenium(III) ions
42
factor of 1053 as described elsewhere, but not for the
“scaling factor”; that is, no correction has been made for
3+
impregnated on nanoceria (Ru /CeO ) during the catalytic
2
changing the number of ruthenium atoms on the growing
metal surface.
hydrolysis of ammonia borane following the procedure
39
described elsewhere. Both the reduction of ruthenium(III) to
ruthenium(0) and H evolution from the hydrolysis of ammonia
2
for ammonia borane,
borane occur concomitantly when AB solution is added to a
ꢃꢄ
3+
ꢆꢇꢆ[ ꢀ]ꢈ
suspension of Ru /CeO . The progress of ruthenium(0)
ꢃꢅ
2
[
ꢀ] ꢂ
ꢆꢆ
(2)
ꢁ
ꢃꢄ
nanoparticle formation and concomitant hydrolysis of
ammonia borane could be followed by monitoring the change
ꢉꢇ
ꢊꢋꢌꢍ(ꢃꢄꢇꢃꢅꢆ[ ꢀ] )ꢁ
ꢈ
ꢃꢅꢆ[ ꢀ]ꢈ
for equiv. H per mole of AB,
2
in H volume, which was then converted into the equivalent H2
ꢉ
ꢔ
2
ꢃ
ꢃ
ꢆꢇꢆꢕꢒꢖ
ꢉ
per mole of AB, using the known 3:1 H /AB stoichiometry (Eq.
2
[
]
ꢋꢎꢏꢐꢑꢒꢓ ꢂꢕꢒꢖꢗ
ꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆ
ꢔ ꢁ
ꢉ
ꢃ
ꢔ
ꢔ
1
). Fig. 1 shows the plot of equivalent H per mole of AB versus
ꢉꢇ
ꢊꢋꢌꢍꢘꢃ ꢇꢃ ꢕꢒꢖꢙꢁ
2
ꢃ
ꢕꢒꢖ
3+
time for the hydrolysis of AB starting with Ru /CeO
2
precatalyst (0.095 mM Ru) and 100 mM AB at 25.0 ± 0.1 °C. The kinetic parameters for the formation of ruthenium(0)
After a short induction period of less than 2 min, the hydrogen nanoparticles on different supports during the hydrolysis of
generation starts and continues almost linearly until the ammonia borane are listed in Table 1 for comparison. It is
release of 3 equivalents of H per mole of AB.
noteworthy that ceria can provide higher kinetic control than
The kinetic data in Fig. 1 fit well by the Finke-Watzky 2-step that of the similar support titania. This emphasizes the special
2
40,41,42
mechanism
consisting of slow, continuous nucleation (P nature of ceria in stabilizing the metal nanoparticles.
→
→
Q, rate constant k ) and autocatalytic surface growth (P + Q
1
2Q, rate constant k ) given in Scheme 1 whereby P is the Table 1: Comparison of the rate constants for the continuous slow nucleation (k
1
) and
2
2 n
autocatalytic growth (k ) of Ru(0) nanoparticles on different supporting materials in
precursor ruthenium(III) ion impregnated on CeO and Q is the
growing Ru(0) nanoparticles.
2
situ formed during the hydrolysis of ammonia borane at 25.0 ± 0.1 °C.
4
0
n
induction time
k
2
-
1
Precatalyst
[Ru] mM
k
1
(s )
-1 -1
Ref.
(
min)
(M s )
k
2
/k
1
3
+
-3
-4
3
Ru /TiO
2
0.480
0.378
less than 1
less than 1
1.78×10
7.66×10
1.33 0.75×10
[43]
[44]
3
+
4
3
Ru @MWCNT
8.33
1.95
1.1×10
2.5×10
3
+
−4
This
study
Ru /CeO
2
0.380
less than 2
7.91×10
1.09×10
3
+
−5
4
4
Ru @X-NW
0.271
0.196
14
6
0.351
1.05
3.2×10
2.2×10
[45]
[39]
3
+
-5
Ru @HAp
4.67×10
Characterization of Ru(0) nanoparticles supported on ceria
0
Ruthenium(0) nanoparticles supported on ceria (Ru /CeO )
2
were isolated from the reaction solution by centrifugation,
−
3
copious washing with water, and drying under vacuum (10
Torr) at 60 °C and characterized by ICP-OES, BET, XRD, SEM,
2
Fig. 1 The evolution of equivalent H per mole of AB versus time plot for the
3+
hydrolysis of AB starting with Ru /CeO
2
precatalyst (0.095 mM Ru) and 100 mM SEM-EDS, TEM and XPS techniques. Ruthenium content of
0
AB at 25.0 ± 0.1 °C. The kinetic data curve-fit well by the 2-step mechanism Ru /CeO was found to be 1.90 wt % by ICP-OES.
2
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Fig. 3 exhibits the SEM image and SEM-EDS spectrum of
View Article Online
0
Ru /CeO with a ruthenium loading of 1.9
D
0
O
%
I:
w
10
t
.1
i
0
n
3
d
9
i
/
c
C
a
6
t
D
in
T
g
01
that
1
17
A
2
ruthenium is the only element detected in the sample in
addition to the framework elements of ceria (Ce, O).
0
Fig. 2 Powder XRD patterns of (a) CeO
2
and (b) Ru /CeO
2
with a 1.90% wt Ru loading.
Fig. 2 shows the XRD pattern of ceria powders and
0
ruthenium(0) nanoparticles supported on ceria (Ru /CeO )
2
with a ruthenium loading of 1.90% wt Ru. The comparison of
the XRD patterns clearly shows that there is no change in the
characteristic diffraction peaks of ceria. This observation
indicates that ceria remains intact after reduction of
ruthenium(III) ions; in other words, there is no noticeable
alteration in the framework lattice or loss in the crystallinity of
the host material. Note that there is no observable peak
attributable to ruthenium nanoparticles in Fig. 2b, most likely
as a result of low ruthenium loading on ceria.
Fig. 4 (a) STEM, (b) STEM-EDX spectrum taken from the area shown by the red
0
0
rectangle in (a), (c-e) high resolution TEM images of Ru /CeO
2
with a ruthenium loading
Fig. 3 (a) SEM image and (b) SEM-EDS spectrum of Ru /CeO
loading.
2
with a 1.90% wt Ru
of 1.90% wt, Ru(0) nanoparticles were indicated by white arrows, (f) high resolution
TEM image of CeO
2
.
4
| Dalton Trans., 2016, 00, 1-3
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PAPER
experiment starting with 1.0 mmol of AB and 20 mVigewoAfrtpicolewOndliener
Fig. 4 shows the STEM and high resolution TEM images of of CeO (the same amount as the one use vity
D
d
OI
i
:
n
10
c
.
a
10ta39ly/Cti
6
c
D
a
T
c
0
t
1
i
1
1
7
A
2
0
Ru /CeO with a ruthenium loading of 1.90% wt. STEM-EDX tests) in 10 mL of water at 25.0 or 40.0 ± 0.1 °C, no hydrogen
2
(Fig. 4b), taken from the red area in Fig 4a, depicts highly evolution was observed in 1 h at both temperatures. This
dispersed ruthenium(0) nanoparticles on the surface of ceria. observation indicates that the hydrolysis of AB does not occur
The size of Ru(0) nanoparticles on the surface of ceria were in the presence of CeO in the temperature range used in this
2
0
investigated by high resolution TEM in different magnifications study. However, Ru /CeO is found to be a highly active
2
(Fig. 4c-e). Ruthenium(0) nanoparticles of 2.0-5.0 nm size are catalyst in the hydrolysis of AB, generating 3.0 equivalent H2
well dispersed on ceria nanopowder. High resolution TEM gas per mole of AB in the same temperature range.
image of pure ceria in Fig. 4f exhibits the well ordered
crystalline nature of ceria powders. The lattice fringes of
ruthenium(0) nanoparticles have a separation of 0.211 nm
which corresponds to the spacing of the (111) planes of
18
hexagonal Ru while the larger lattice spacing of 0.33 nm
matches that of the (111) crystal planes of cubic fluorite-
46
structured CeO2.
0
The surface composition of Ru /CeO and the oxidation state
2
of ruthenium were also studied by XPS technique. The survey-
0
scan XPS spectrum of Ru /CeO with a ruthenium loading of
2
1
.90% wt. in Fig. 5 shows that ruthenium is the only element
detected in addition to the ceria framework elements (Ce, O)
in agreement with the STEM-EDX result. High resolution X-ray
0
photoelectron spectrum of a Ru /CeO sample given in the
2
inset of Fig. 5 shows a prominent band at 279.8 eV which can
readily be assigned to Ru(0) 3d5/2 in the nanoparticles by
47
comparing with the value of ruthenium metal 280 eV. It is
noteworthy that the Ru(0) 3d3/2 peak overlaps with the C 1s
peak at 283.1 eV coming from air with a percent atomic ratio
of 7.09 (C 1 s/Ru 3d). Because of this overlap, only the peak at
2
79.8 eV can with certainty be assigned to Ru(0) 3d5/2. The
48
higher energy peaks might be of carbon or ruthenium.
Fig.6 (a) mol
2 3 3
H /mol H N·BH versus time graph depending on the ruthenium
concentration for hydrogen generation from the hydrolysis of AB (100 mM) at 25.0 ±
◦
0
.1 C. (b) The plot of hydrogen generation rate versus the concentration of Ru, both in
logarithmic scale; ln(rate) = 0.83 ln[Ru] + 4.13.
Fig. 6a shows the evolution of equivalent hydrogen per mole
of AB versus time plot for the hydrolysis of AB (100 mM) using
0
Ru /CeO catalyst (1.90% wt Ru) in different ruthenium
2
concentrations at 25.0 ± 0.1 °C. In each experiment, hydrogen
evolution starts after a short induction period of less than 2
minutes and continues almost linearly until complete
conversion of the substrate, thus generating 3.0 equivalents of
H2 per mole of AB. All the experimental data fit well to the 2-
step mechanism yielding the rate constants k of the slow,
1
continuous nucleation (P → Q) and k of the autocatalytic
2
0
surface growth (P + Q → 2Q) for the formation of ruthenium(0)
Fig. 5 X-ray photoelectron (XPS) spectrum of Ru /CeO
2
sample with ruthenium loading
of 1.90% wt. Ru. The inset gives the high resolution scan of Ru 3d and deconvolution.
nanoparticles catalyst from the reduction of ruthenium(III)
40
ions during the hydrolysis of AB (Table 2). Although it is not
possible to get a direct correlation between the rate constant
0
Catalytic activity of Ru /CeO in hydrolysis of AB
2
Before starting with the investigation on the catalytic activity k1 for nucleation and the catalyst concentration, the rate
0
of Ru /CeO in hydrogen generation from the hydrolysis of constant k for autocatalytic surface growth slightly increases
2
2
ammonia borane, a control experiment was performed to with the increasing concentration of ruthenium.
check whether ceria itself shows any catalytic activity in the
same reaction in the same temperature range. In a control
This journal is © The Royal Society of Chemistry 2016
Dalton Trans., 2016, 00, 1-3 | 5
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Ple aDs ea l dt oo nn To rt a and sj ua sc tt imo na sr gins
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Paper
Dalton Transactions
2
0
1 2
Table 2. The rate constants k of the slow, continuous nucleation, P → Q, and k of the
Ru/graphene (1.9 nm). Although average particle size of
View Article Online
auto-catalytic surface growth, P + Q → 2Q for the formation of ruthenium(0)
nanoparticles catalyst from the ruthenium(III) ions during the hydrolysis of AB at
various concentration of ruthenium in the reaction mixture.
Ru@MIL-96 (2.0 nm) and Ru@MIL-101 (2.8 nDmO)I:a1r0e.1l0o3w9/eCr6tDhTa0n11th17aAt
of Ru/CeO2 they have low TOF value. The lower catalytic effect
of metal organic framework (MOF) supported ruthenium
nanoparticles may be attributed to diffusion controlled kinetics
of MOFs.
-
1
4
-1 -1
-1
a
[
Ru] (mM)
.095
k
1
(s )×10
k
2
(M s )
TOF (min )
0
6.92 ± 0.66
8.70 ± 1.15
7.91 ± 1.39
6.04 ± 1.26
1.75 ± 0.13
1.78 ± 0.14
1.95 ± 0.14
2.09 ± 0.13
361
299
274
249
0
.19
.38
.76
0
0
a
TOF values were not corrected by the fraction of active sites.
The turnover frequency (TOF) for hydrogen generation from
the hydrolysis of AB (100 mM) at 25.0 ± 0.1 °C was determined
from the hydrogen generation rate in the linear portion of
plots given in Fig. 6a for experiments starting with 100 mM AB
0
plus Ru /CeO with a loading of 1.90% wt. Ru. The TOF value of
2
0
-1
-1
Ru /CeO catalyst is as high as 361 min (mol H · mol Ru
·
2
2
-
1
min ) in hydrolysis of AB at 25.0 ± 0.1 °C. Turnover frequency
decreases with the increasing initial concentration of
ruthenium in the reaction mixture as shown in Table 2. The
inverse dependence of the TOF on the precatalyst
concentration is a known phenomenon in literature which
indicates that the lower concentration of precatalyst forms
smaller particles which exhibit higher catalytic activity for
49
50,51
some reactions.
Table 3. The turnover frequency and activation energy values of ruthenium based
catalysts used in hydrolysis of ammonia borane.
Catalyst
TOF
min )
600
429.5
Metal/AB
molar ratio
0.002
0.00425
0.00095
E
a
Ref
-
1
(
(kj/mol)
12.7
34.81
51
Ru/graphene
Ru/C
20
16
This
Study
44
52
53
43
14
15
54
Ru/CeO
2
361
Ru(0)@MWCNT
Ru@SBA-15 NCs
329
316
313
241
231
200
180
178
172.5
137
135
83.3
0.00094
0.002
0.0017
0.0006
0.0039
0.0025
0.005
33
34.8
37.4
70
48
38
54
51
45.6
58
Ru/g-C
Ru@TiO
Ru@MIL-96
Ru@SiO
3
N
4
2
2
Ru(0)/PSSA-co-MA
Ru@MIL-101
Ru(0)/SiO
Ru/HAp
Ru/X-NW
0.008
55
19
39
45
2
-CoFe
O
2 4
0.00097
0.00392
0.00271
0.00375
77
46
2
Ru@Al O
3
18
TOF values of the reported ruthenium catalysts used in the
hydrolysis of AB at room temperature are given in Table 3 for
comparison. As clearly seen from the comparison of values
0
listed in Table 3, Ru /CeO provides catalytic activity greater
2
than most of the ruthenium based catalysts such as Ru@Al O ,
2
3
Ru@X-NW, Ru(0)/Hap, Ru/SiO -CoFe O , Ru@MIL-101,
2
2 4
Ru(0)/PSSA-co-MA, Ru@SiO , Ru@MIL-96, Ru@TiO , Ru/g-
2
2
C N , Ru(0)@MWCNT, in hydrogen generation from the
3
4
hydrolysis of ammonia borane. However, the catalytic activity
Fig. 7 (a) The evolution of equivalent hydrogen per mole of AB versus time plot for the
0
3+
hydrolysis of AB starting with Ru /CeO
2
precatalyst (0.380 mM Ru) and 100 mM AB at
of Ru /CeO is lower than that of Ru/C and Ru/graphene. The
2
0
various temperatures. (b) The Arrhenius plot for nucleation of ruthenium(0)
lower catalytic activity of Ru /CeO compared to Ru/C and
2
nanoparticles, ln(k ) = −7229/T + 17.14. (c) The Arrhenius plot for the autocatalytic
1
Ru/graphene may be attributed to larger particle size of surface growth of ruthenium(0) nanoparticles, ln(k
2
) = −5680/T + 19.68. (d) The
Arrhenius plot for hydrogen generation from the catalytic hydrolysis of ammonia
1
6
ruthenium(0) nanoparticles than those of Ru/C (1.7 nm) and
borane, ln(kobs) = −6129/T + 23.82
6
| Dalton Trans., 2016, 00, 1-3
This journal is © The Royal Society of Chemistry 2016
Please do not adjust margins

Page 7 of 9
Ple aDs ea l dt oo nn To rt a and sj ua sc tt imo na sr gins
Dalton Transactions
PAPER
The kinetics of ruthenium(0) nanoparticle formation and powder material was weighed and then redisperVsieewdAirnticl1e0OnmlinLe
concomitant hydrogen generation from the hydrolysis of of solution containing 100 mM AB. A sec
D
o
O
ndI: 1
r
0
u
.1
n
03
o
9
f
/C
h
6
y
D
d
T
r
0
o
1
l
1
ysis
1
7
A
ammonia borane were also followed at various temperature in was started immediately and continued until the completion
order to get an idea on the energy barrier for the slow of hydrogen evolution. This was repeated five times. Figure 8
nucleation and autocatalytic surface growth of metal(0) shows the catalytic activity, TOF relative to TOF value in the
0
nanoparticles as well as for the hydrolysis reaction. Fig. 7a first run, of Ru /CeO in the subsequent catalytic hydrolysis of
2
shows the evolution of equivalent H per mole of AB versus 100 mM AB performed by using the catalyst isolated after the
2
3+
time for the hydrolysis of AB starting with Ru /CeO
previous run at 25.0 ± 0.1 °C. These tests reveal that the
2
0
precatalyst (0.380 mM Ru) and 100 mM AB at four different Ru /CeO catalyst is still active in the subsequent runs of
2
temperatures. For each temperature, experimental data curve- hydrolysis of ammonia borane providing a release of 3.0
40
0
fits well to the 2-step mechanism yielding the rate constants equivalent H per mole of AB. After the fifth run, Ru /CeO
2
2
k of the slow, continuous nucleation (P → Q) and k of the preserve 60% of their initial catalytic activity. The decrease in
1
2
autocatalytic surface growth (P + Q → 2Q) for the formation of catalytic activity in successive runs can be attributed to the
ruthenium(0) nanoparticles catalyst from the ruthenium(III) agglomeration of nanoparticles on the surface of ceria during
ions during the hydrolysis of ammonia borane (Table 4). From the isolation and redispersion processes. Indeed, the TEM
the Arrhenius plots constructed by using the values of rate image in Fig. 9 of the sample harvested after the fifth run of
constants k and k at various temperatures in Fig. 7b and c, hydrolysis indicates this agglomeration.
1
2
one can obtain the activation energy, E = 60 ± 7 kJ/mol for the
a
nucleation and E = 47 ± 2 kJ/mol for the autocatalytic surface
a
growth of ruthenium(0) nanoparticles, respectively. The large
value of k /k ratio also given in Table 4 is indicative of the high
2
1
level kinetic control in the formation of ruthenium(0)
nanoparticles from the reduction of the precursor
4
0
ruthenium(III) ions on the surface of ceria.
1 2
Table 4. The rate constants k of the slow, continuous nucleation, P → Q, and k of the
autocatalytic surface growth, P + Q → 2Q for the formation of ruthenium(0)
nanoparticles catalyst from the reduction of ruthenium(III) ions during the hydrolysis of
AB, and the observed rate constant kobs for hydrogen generation from the catalytic
hydrolysis of AB at various temperatures.
-
1
4
-1 -1
-1
Temp °C
k
1
(s )×10
k
2
(M s )
k
2
/k
1
k
obs (min )
0
Fig. 8 Percentage of initial catalytic activity (TOF value in the first run) of Ru /Ceria ([Ru]
3
3
3
3
25
30
35
40
7.91 ± 1.39
12.60 ± 1.38
18.30 ± 3.00
25.40 ± 4.15
1.95 ± 0.14
2.37 ± 0.12
3.55 ± 0.26
4.72 ± 0.35
2.47×10
1.87×10
1.94×10
1.86×10
27.0
35.0
49.0
71.0
=
0.760 mM) retained in successive runs after the reuse for the hydrolysis of AB (100
mM) at 25.0 ± 0.1 °C.
Activation energy for hydrogen generation from the hydrolysis
0
of AB catalyzed by Ru /CeO2 could be determined by
evaluating the temperature dependent kinetic data presented
in Fig. 7a. The rate constants for the hydrogen generation at
different temperatures were calculated from the slope of
linear portion of each plot given in Fig. 7a and used for the
calculation of activation energy (E = 51 ± 2 kJ/mol) from the
a
Arrhenius plot in Fig. 7d. The activation energy for the
0
hydrolysis of ammonia borane catalyzed by Ru /CeO2 is
comparable to the literature values reported for the same
reaction using other ruthenium catalysts listed in Table 3.
0
Reusability test of Ru /CeO was performed using the catalyst
2
0
Fig. 9 TEM image of Ru /Ceria sample harvested after the fifth run of hydrolysis of AB
at 25.0 ± 0.1 °C. White arrows indicate the spots of ruthenium.
isolated from the reaction solution following previous run.
After the completion of hydrogen generation from the
hydrolysis of ammonia borane starting with 0.760 mM
0
Catalytic lifetime of Ru /CeO was determined by measuring
the total turnover number (TTO) achieved in hydrogen
2
0
Ru /CeO plus 100 mM AB in 10 mL aqueous solution at 25.0 ±
2
0
.1 °C, the catalyst was isolated by centrifugation and washing
generation from the hydrolysis of ammonia borane (Fig. 10). A
0
with water and dried under vacuum at 60 °C for 12 h. All of the
catalyst lifetime experiment starting with 10 mg Ru /CeO in
2
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Dalton Trans., 2016, 00, 1-3 | 7
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Paper
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-
5
0 mL of solution of AB at 25.0 ± 0.1 °C reveals a minimum of the passivation of metal surface by metaborate ions (BO )
2
View Article Online
1
35,100 total turnovers over 250 h in hydrogen generation which accumulate in solution as reaction p
D
r
O
o
I:
c
1
e
0
e
.1
d
039/C6DT01117A
s
.
from the hydrolysis of AB before deactivation of the catalyst.
CONCLUSION
Ruthenium(0) nanoparticles supported on ceria were
reproducibly prepared from the reduction of ruthenium(III)
3+
ions impregnated on ceria (Ru /CeO ) during the catalytic
2
hydrolysis of ammonia borane at room temperature. The
formation kinetics of ruthenium(0) nanoparticles supported on
ceria and the concomitant hydrogen generation from the
hydrolysis of ammonia borane could be followed by
monitoring the amount of hydrogen gas evolved. Using the
hydrogen evolution from the hydrolysis of ammonia borane as
reporter reaction enables one to study the formation kinetics
of ruthenium(0) nanoparticles from the reduction of
Fig. 10 The variation in turnover number (TON) and turnover frequency (TOF) during ruthenium(III) ions impregnated on ceria. Sigmoidal data of
0
the catalytic lifetime experiment performed starting with 10 mg Ru /Ceria (ruthenium
loading = 1.90% wt Ru, and ꢀRuꢁ = 0.190 mM) in 50 mL solution of AB at 25.0 ± 0.1 °C.
equivalent H per mole of AB versus time curve-fit well to the
2
4
2
2
-step mechanism yielding the rate constants for continuous,
0
slow nucleation and autocatalytic surface growth of
As shown in Fig. 11, Ru /CeO provide the highest TTO value
2
0
ruthenium(0) nanoparticles. Ru /CeO were isolated from the
ever reported for the hydrolysis of AB using ruthenium
2
4
5
reaction solution by centrifugation and characterized by a
catalysts such as Ru@X-NW (TTO = 134,100), Ru(0)@HAp
3
9
43
combination of advanced analytical techniques. Ruthenium(0)
(
TTO = 87,000), Ru(0)/TiO (TTO = 71,500), Ru(0)NP/PSSA-
2
0
54
44
nanoparticles of 2-5 nm size supported on ceria (Ru /CeO )
co-MA (TTO = 51,720), Ru(0)@MWCNT (TTO = 26,400),
RuNPs@ZK-4 (TTO = 36,700), Ru(0)NP/laurate (TTO =
900). The high activity and long lifetime of Ru /CeO can be
2
56
show high catalytic activity in hydrogen generation from the
57
0
hydrolysis of ammonia borane yielding 3.0 equivalent H per
5
2
2
0
mole of ammonia borane. Ru /CeO provide a turnover
attributed to the strong adsorption capacity, high surface area
of nanoceria and nonporous structure of ceria which
minimizes the diffusion problem. This finding indicates that the
role of ceria as support is crucial. After the lifetime
experiment, upon filtration of the resulting solution no
leaching of the ruthenium into the solution was detected by
ICP-OES, indicating that ruthenium(0) nanoparticles still
remain on the surface of ceria.
2
-1
frequency value up to 361 min and unprecedented catalytic
life time with a total turnover number of 135,100 in hydrogen
generation from the hydrolysis of ammonia borane at 25.0 ±
0
.1 °C. Temperature dependent kinetics data on ruthenium(0)
nanoparticles formation and concomitant hydrogen
generation from the hydrolysis of ammonia borane were used
to obtain the activation energy values for the slow nucleation
and autocatalytic surface growth of ruthenium(0)
nanoparticles as well as for the catalytic hydrolysis of ammonia
borane. Activation energy for hydrogen generation from the
0
hydrolysis of ammonia borane catalyzed by Ru /CeO (Ea = 51
2
±
2 kJ/mol) is comparable to the literature values reported for
the same reactions by using ruthenium(0) nanoparticles
0
supported on different materials. Ru /CeO is a reusable
2
catalyst as it provides the complete hydrolysis of AB even in
the fifth use. High catalytic activity, long catalytic life time and
0
simple preparation procedures make Ru /CeO very attractive
2
catalyst in hydrogen generation from the hydrolysis of
ammonia borane for the fuel cell applications.
Acknowledgements
2
Fig. 11 Catalytic lifetime of reported ruthenium catalysts (TTO = mol H /mol Ru) in the
Partial support by Turkish Academy of Sciences (TUBA) is
gratefully acknowledged. We would like to thank İlker Yıldız for
XPS analyses.
hydrolysis of AB at room temperature.
As shown in Fig. 10, the observation that the TOF value
decreases as the reaction proceeds indicates the deactivation
of ruthenium(0) nanoparticles catalyst. The deactivation of
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8
| Dalton Trans., 2016, 00, 1-3
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