Highly active metastable ruthenium nanoparticles for hydrogen production through the catalytic hydrolysis of ammonia borane

Article abstract of DOI:10.1002/smll.201303507

Late transition metal nanoparticles (NPs) with a favorably high surface area to volume ratio have garnered much interest for catalytic applications. Yet, these NPs are prone to aggregation in solution, which has been mitigated through attachment of surface ligands, additives or supports; unfortunately, protective ligands can severely reduce the effective surface area on the NPs available for catalyzing chemical transformations. The preparation of 'metastable' NPs can readily address these challenges. We report herein the first synthesis of monodisperse metastable ruthenium nanoparticles (RuNPs), having sub 5 nm size and an fcc structure, in aqueous media at room temperature, which can be stored for a period of at least 8 months. The RuNPs can subsequently be used for the catalytic, quantitative hydrolysis of ammonia-borane (AB) yielding hydrogen gas with 21.8 turnovers per min at 25 °C. The high surface area available for hydrolysis of AB on the metastable RuNPs translated to an E_a of 27.5 kJ mol^-1, which is notably lower than previously reported values for RuNP based catalysts. Metal nanoparticles are becoming increasingly important in a variety of practical applications on account of their unique properties, especially for catalysis. Herein, we report the straightforward preparation of metastable ligand-free ruthenium nanoparticles (RuNPs) in aqueous solution that exhibit an fcc structure and which were found to be particularly active in the hydrolysis of ammonia borane (AB) at room temperature.

Full text of DOI:10.1002/smll.201303507

Hydrogen Production
Highly Active Metastable Ruthenium Nanoparticles for
Hydrogen Production through the Catalytic Hydrolysis of
Ammonia Borane
Enass K. Abo-Hamed, Timothy Pennycook, Yana Vaynzof, Chris Toprakcioglu,
Alexandros Koutsioubas, and Oren A. Scherman*
L ate transition metal nanoparticles (NPs) with a favorably high surface area to volume
ratio have garnered much interest for catalytic applications. Yet, these NPs are prone
to aggregation in solution, which has been mitigated through attachment of surface
ligands, additives or supports; unfortunately, protective ligands can severely reduce the
effective surface area on the NPs available for catalyzing chemical transformations.The
preparation of ‘metastable’ NPs can readily address these challenges. We report herein
the first synthesis of monodisperse metastable ruthenium nanoparticles (RuNPs),
having sub 5 nm size and an fcc structure, in aqueous media at room temperature,
which can be stored for a period of at least 8 months. The RuNPs can subsequently
be used for the catalytic, quantitative hydrolysis of ammonia-borane (AB) yielding
hydrogen gas with 21.8 turnovers per min at 25 °C. The high surface area available for
hydrolysis of AB on the metastable RuNPs translated to an Ea of 27.5 kJ mol−1, which
is notably lower than previously reported values for RuNP based catalysts.
1. Introduction
E. K. Abo-Hamed, Dr. O. A. Scherman
Melville Laboratory for Polymer Synthesis
Department of Chemistry
University of Cambridge
Lensfield Road, Cambridge CB2 1EW, UK
Tel: +44 1223 334370
Hydrogen is among the leading candidates as an alternative
energy source for the future. Whilst hydrogen gas is used in a
variety of ways to generate energy, the most efficient process
entails its conversion to electrical energy via fuel cell technol-
E-mail: oas23@cam.ac.uk
ogies.[1] Fundamental technology exists for both the storage
Dr. T. Pennycook
Department of Materials
of hydrogen as well as its on-board production;[2] how-
ever, improvement of current methods is crucial to achieve
more efficient practical applications. On account of its high
hydrogen content (19.6 wt%), high stability under fuel cell
reaction conditions, and non toxicity,[3] ammonia-borane
(AB) meets the 2015 US Department of Energy (DOE)
targets for a hydrogen resevoir of 9 wt% and 81 g L−1.[4]
As the catalytic hydrolysis of AB can in principle provide
stoichiometric conversion to hydrogen gas at room tem-
perature,[5,6] the development of an economic and efficient
catalyst that operates under mild conditions is of paramount
importance.[7]
Catalytic nanoparticles (NPs) have attracted a great
deal of interest among both scientific and industrial com-
munities owing to the unique properties derived from their
characteristic large surface area-to-volume ratios.[8] In order
to achieve their small size during preparation, and to pre-
vent subsequent particle coalescence due to the enhanced
surface tension associated with small particle sizes,[9,10] NPs
University of Oxford
Parks Road, Oxford OX1 3PH, UK
(b) SuperSTEM Laboratory
STFC Daresbury Campus
Keckwick Lane, Daresbury WA4 4AD, UK
Dr. T. Pennycook
SuperSTEM Laboratory, STFC Daresbury Campus
Keckwick Lane, Daresbury, WA4 4AD, UK
Prof. Y. Vaynzof
Centre for Advanced Materials (CAM)
Im Neuenheimer- Feld 227, 69120 Heidelberg, Germany
Prof. C. Toprakcioglu
Department of Physics
University of Patras
Patras 26500, Greece
Dr. A. Koutsioubas
Synchrotron Soleil, Beamline SWING
Sain Aubin BP48, F-91192 Gif Sur Yvette Cedex, France
DOI: 10.1002/smll.201303507
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E. K. Abo-Hamed et al.
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Figure 1. Formation of the metastable RuNP. a) Ruthenium ions in solution; b) nucleation, forming initial seeds upon introducing NaBH4; c) growth
of seed dimensions, which occur when further ruthenium ions are introduced to the formed nuclei, eventually leading to the final stable state;
d) metastable cluster, when remaining ruthenium ions are no longer reduced.
2. Results and Discussion
are generally stabilized through the introduction of sur-
factants or surface-bound ligands and stabilizing agents.[11]
While the NPs must be covered with protective ligands for The growth of the NPs to their final (stable) size is gener-
the purpose of stability and to allow them to be used in ally determined by the amount of stabilizing ligands present
catalytic applications, the presence of the same protective during reduction.[22–24] It has also been reported, that cluster
ligands can also reduce substrate accessibility on the cata- growth stops only when all of the metal ion precursor has been
lytic NP surface. These competing factors have presented the consumed, with further growth only occurring by aggregation
synthetic chemist with a dilemma, as a compromise between of small into large clusters; importantly, the relative rates for
these competing issues must be sought. We report herein the all of these steps are responsible for the resultant size of the
preparation of metastable ruthenium nanoparticles (RuNPs) NPs.[25] Watzky et al. suggest that an intermediate autocatal-
through the reduction of RuCl3 precursor with sodium boro- ysis step occurs between steps (i) and (ii), where the surface of
hydride. Our method not only offers versatility and simplicity the nuclei acts to reduce proximate metal ions in solution.[26]
in the preparation of organic ligand-free nanoparticles, but
Metastable RuNPs were prepared in 1:1 (v/v) aqueous
also allows for enhanced exposure of the catalytic surface of ethanol solution, where NaBH4 was applied to reduce the
⋅
3
the NPs.
RuCl H2O precursor to the NPs, without further stabilizing
Ruthenium complexes are among the most widely additives and agents being employed. We hypothesized that
studied materials due to their catalytic performance. Some instead of adding external stabilizing ligands, the aggregation
ruthenium complexes have recently been shown to be process could be arrested in a quasi-reduced state, as meta-
effective in the hydrolysis of AB.[12–15] A number of dif- stable RuNPs, whereby a collection of both Ru3+ and Cl− ions
ferent shapes, structures and compositions of RuNPs have prohibit further agglomeration as illustrated in Figure 1. To
also been investigated for their catalytic activity in a wide examine our hypothesis, X-ray photoelectron spectroscopy
range of chemical transformations.[16–18] As the size of NPs (XPS) measurements were carried out on the metastable
has a marked effect on the overall catalytic activity,[19] their RuNPs 48 hours after reduction. Figure 2 shows the C1s-Ru3d
formation is of great consequence and is controlled by an XPS spectrum of a 0.5 mM sample. The spectrum consists of
aggregation process during reduction of the metal ions. In two C1s peaks and Ru3d doublets; the low energy Ru3d dou-
principle the particle size can be determined by influencing blet (Ru3d5 peak at binding energy of 280.4 eV) is assigned
the relative rates of nucleation and cluster growth.[20] Tra- to Ru0. XPS data therefore confirms that the RuNPs observed
ditionally, in order to achieve such stable aggregates on the by TEM (Figure 4) consist of Ru0. It was also found that
nanometer scale, the formation process, has been carried out only 19.04% of the Ru signal originates from Ru0, while the
in the presence of stabilizing ligands. Aggregation kinetics remaining Ru originates from a high-energy Ru3d doublet
have been evaluated using a classical Derjaguin, Landau, (Ru3d5 peak at binding energy of 281.9 eV), which is assigned
Verwey and Overbeek (DLVO) model, whereby the inter- to RuCl3. Additionally, C1s peaks at 285.6 eV (C-C) and
action between two particles is the sum of the electrostatic 288.0 eV (C=O) are observed in the XPS. The initial concen-
⋅
3
repulsions controlled by the NP surface potentials, and all tration of RuCl H2O played a decisive role in determining
of the interparticle van der Waals attractions are controlled the final size of the RuNPs, (vide infra). In order to minimize
by the Hamaker coefficient for the bulk metal. Particle col- possible side effects stemming from variations in temperature
lisions by Brownian motion give rise to substantial agglom- or differences in ionic strength on NP growth, varying con-
eration, if repulsive interactions or stabilizing ligands are not centrations of the metal precursor were used for the forma-
present. However, according to Turkevich and co-workers, tion of RuNPs whilst keeping the reaction temperature and
there are three reaction stages in forming metal NPs: (i) ionic strength, governed by the sodium borohydride reducing
an initial nucleation step, followed by (ii) aggregation, and agent, constant. Borohydrides have a tendency to be adsorbed
finally (iii) growth.[21]
onto the surface of the NPs acting as ion sources during the
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Highly Active Metastable Ruthenium Nanoparticles for Hydrogen Production through the Catalytic Hydrolysis
Therefore, the mean distance between the NPs arising from
the SAXS data is 79 nm. However, a rough estimate for the
same distance can be calculated from the concentration of
the NPs in solution.TEM results indicate that the mean diam-
⋅
3
eter of RuNPs prepared from a 1 mM RuCl H2O solution is
approximately 1.2 nm (see Figure 3c). Assuming a spherical
shape, it is possible to estimate the average distance between
NPs in solution. Thus, for a 1 mM sample (which has a total
concentration of dispersed metallic ruthenium of 0.2 mM
and NP volume fraction of ca. 1.6 × 10−6) the mean distance
between NPs is calculated to be roughly 83 nm, which is
commensurate with the spacing (79 nm) associated with the
peak position. The inset of Figure 3b shows SAXS patterns
that correspond to a series of samples obtained by consecu-
tive dilutions of the original 1 mM sample composition. As
can be seen, the correlation peak shifts slightly to lower
q-values but persists at least up to a dilution by a factor of
16, while obscuring the underlying form factor of the NPs
and preventing reliable extraction of the particle radius from
the scattering data. The presence of a prominent correlation
Figure 2. XPS spectra collected on metastable RuNPs in solution
(0.5 mM).
growth step.[27] As the covalent ruthenium-chloride bonds peak at such a low volume fraction of the dispersed phase is
break in the RuCl3 precursor, electrostatic repulsions arise indicative of the strong electrostatic repulsion experienced by
and a competition develops between all free ions in solution the charged NPs. The long range of the inter-particle repul-
for their adsorption onto the NP surface. After all the boro- sion implied by the data can be attributed to the fact that
hydride ions are consumed and oxidized to the corresponding the NPs must be highly charged. This observation would also
borates, the remaining ruthenium ions likely serve as the main account for the remarkable stability of the RuNPs despite
stabilizing agent giving rise to the metastable NPs.
the absence of any added protective ligand on the NP surface
To further understand the stability of the RuNPs in solu- in this system. It is indeed noteworthy that the SAXS spec-
tion, small angle X-ray scattering (SAXS) measurements trum of a 1 mM solution, when measured again after 30 days,
were carried out. Figure 3 a shows the SAXS pattern of a was found to be identical to the original spectrum, within
1 mM metastable RuNP solution.The scattering curve is dom- experimental error, demonstrating the lack of any significant
inated by a strong correlation peak that exhibits a maximum structural changes in the system over this timescale.
max
at q
= 0.00796 Å−1. Although correlation peaks can appear
One of the key problems that could emerge from the
in interacting systems without necessarily implying the exist- absence of stabilizing ligands over time is the loss of con-
ence of a special correlation distance, it is nevertheless of sistent NP shape and structure on account of agglomeration.
interest to compare the spacing (2π/q max ) suggested by the To examine this, transmission electron microscopy (TEM) and
position of the peak, to the mean distance between the NPs. aberration corrected high angle annular dark field scanning
transmission electron microscopy (HAADF STEM) were con-
ducted to characterize the particle size distribution and struc-
ture.A number of solutions of RuNPs varying in concentration
and aged for 24 hours were deposited on holey carbon grids
for the purpose of this analysis. Representative TEM micro-
graphs are presented in Figure 4 and the analysis of their cor-
responding size distribution is shown in Figure 4f with all of
the corresponding raw data in Figure S1 (Supporting Infor-
mation). One trend is readily apparent: as the precursor con-
centration increases, there is a concomitant decrease in the
average particle size of the RuNPs from 2.76 nm (for 0.3 mM)
to 0.97 nm (1.1 mM). Additionally, as the precursor concen-
tration is increased (see TEM images in Figure 4a–e) there
is a progressive increase in the appearance of sub-nm ‘seeds’.
Moreover, there is a marked difference in appearance from
Figure 4c, where the majority of the NPs are larger, to Figure 4d
where there appears to be a co-existence of both large and
much smaller ‘seed’ particles. Thus, when the precursor con-
centration reaches 0.9 mM and above, the NP growth process
appears to be shortened at the expense of the seeding step.
This implies that a competition between the over abundance
Figure 3. a) Small angle scattering profile of a 1 mM metastable RuNP
solution. b) SAXS patterns associated with a series of samples obtained
by consecutive dilutions of the original composition. c) TEM image of a
1 mM metastable RuNPs solution.
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E. K. Abo-Hamed et al.
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structure as a nanoparticle phase observed
so far was reported recently by Kitagawa
et al.[28] Interestingly, it was observed that
the fcc structure was obtained only when a
precursor of Ru(acac)3 was applied in the
presence of a poly(N-vinyl-2-pyrrolidone)
stabilizing agent. In comparison, our
method realizes RuNPs with an fcc struc-
⋅
ture for the first time using RuCl 3 H2O as
a precursor in aqueous media, where there
are no added stabilizing agents or supports.
In the absence of stabilizing agents, the
weakly-coordinating chloride anions do not
provide enough stabilization for the metal
NPs;[29] therefore, the Ru3+ ions remaining
in solution must play an important role in
both the overall NP growth process and in
the surface stabilization of the final clus-
ters, forming part of the ‘protecting’ layer.
As more ‘protecting’ ions are available at
higher initial concentration of the precursor,
Figure 4. TEM images of metastable RuNPs where initial concentration of precursor used is:
(a) 0.3 mM, (b) 0.5 mM, (c) 0.7 mM, (d) 0.9 mM and (e) 1.1 mM. Scale bars = 5 nm. (f) Plot
of initial precursor concentration vs. RuNP average size distribution obtained by counting at
least 200 NPs.
of seed particles develops during the growth step if the parti- the observation that smaller particles exist at higher concen-
cles are partially stabilized by borohydride ions on the surface. trations corresponds well with this explanation, as a constant
To gain further insight into the atomic structure of the amount of reducing agent was applied across this series. When
metastable RuNPs, samples were subjected to HAADF STEM the concentration of borohydride ions decreases over the course
analysis. The images obtained show that despite the lack of of metal reduction, it was expected that the stability of the the
stabilizing ligands, freestanding RuNPs are indeed formed formed particles would also decrease with time as a partial sta-
(see Figure S2d, Supporting Information) and possess a face bilizing agent was consumed consistent with previous sugges-
centred cubic (fcc) structure viewed down the <110> direc- tions.[30] However, in our case, the notable long-term stability
tion as shown in Figure 5 with a horizontal lattice spacing of observed for the metastable RuNPs after all borohydride is con-
2.75 Å and a lattice constant of approximately 3.9 Å. Agglom- sumed, might be attributed to the large amount of Ru3+ ions still
erations of particles were also observed; however, these often present in solution. The presence of these ions was confirmed
took the form of several nanocrystals (NCs) with small areas by XPS, and correlates well with the SAXS data reflected by
of their surfaces in contact with one another, nevertheless the maintained interparticle distance enhanced by electrostatic
maintaining a relatively high area of exposed nanocrystal- repulsion.
line surface (see Figure S2a–c, Supporting Information). Fur-
In addition to testing the effect of precursor concentra-
thermore, these agglomerated particles appeared to possess tion on NP size and stability, several other variables were
the same fcc crystal structure when aligned with the electron tested for their contribution to particle stability in solu-
beam. To the best of our knowledge, the first and only fcc tion, including agitation and the type of alcohol used in
the aqueous solution. Varying the alcohol from ethanol
to either methanol or propanol was found to be ineffec-
tive and reduced the overall NP stability, while a 1:1 (v/v)
water:ethanol mixture was found to be the best composition.
When a magnetic stir bar was used during reduction, the sta-
bility of the produced NPs was remarkably decreased; the stir
bar serves as a nucleation surface, which accelerated aggrega-
tion to larger particles and eventually led to precipitation. In
stark contrast, when gentle swirling was applied during metal
reduction with a bench-top incubator, RuNPs of a control-
lable size were formed and exhibited enhanced stability for
more than 8 months. In synthetic methods reported in the
literature, the ligands employed limit the catalytic surface
available for chemical transformation, thereby necessitating
ligand removal and subsequent growth in particle size or
surface oxidation.[31,32] In contrast, our method provides a
metastable approach towards the formation of high surface
area RuNPs, thus potentially offering a general route for the
Figure 5. HAADF STEM image of a fcc structured (red circles) RuNP
synthesis of long-term ion-stabilized NPs in the absence of
typical organic or inorganic ligands with control over size.
obtained from 0.3 mM initial concentration of precursor, scale bar =
2 nm.
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Highly Active Metastable Ruthenium Nanoparticles for Hydrogen Production through the Catalytic Hydrolysis
Figure 6b. Thus, a higher rate of reaction was achieved for
140
120
100
80
(a)
smaller, more active NPs on account of the overall surface
area to volume ratio for the NP solution. Moreover, a double
logarithmic plot of hydrogen generation rate as a function
of catalyst concentration (Figure 6b) yields a slope of 1.01,
strongly indicative of a first-order hydrolysis reaction.
0.3 mM
0.5 mM
0.7 mM
0.9 mM
1.1 mM
The effect of the AB substrate concentration on hydrolysis
was also investigated by carrying out the reaction with different
AB concentrations of 50, 100, 150 and 200 mM, while keeping
the the RuNP concentration constant at 0.5 mM. Here, H2 gas
generation was found to be independent of the AB concen-
tration (see Supporting Information, Figure S3a). Figure S3b
(Supporting Information) shows the double logarithmic plot
of H2 as a function of AB concentration with a slope of 0.06,
confirming that the 2.2 nm RuNPs (produced at 0.5 mM), is
roughly zero-order with respect to the AB concentration. The
zero-order rate dependency highly suggests that AB adsorption
onto the surface of the RuNP catalysts is a rate-limiting step.[34]
Finally, the catalytic hydrolysis of AB by the metastable
RuNPs was carried out at different temperatures. In order to
60
40
20
0
0
1
2
3
4
5
6
7
8
9
Time [min]
(b) 4.4
4.2
4.0
3.8
3.6
3.4
3.2
3.0
a
determine the activation energy, E , for the hydrolysis reac-
tion, a 200 mM solution of AB was subjected to the 0.5 mM
RuNPs over a temperature range of 25–45 °C. Figure 7a shows
the plot of H2 gas produced as a function of time monitored
for five different temperatures (25, 30, 35, 40 and 45 °C). The
ln rate = 1.01ln[Ru]+4.09
2.8
(a)
140
-1.3 -1.1 -0.9 -0.7 -0.5 -0.3 -0.1 0.1
120
100
80
ln[Ru]
Figure 6. a) Plot of time vs. volume of hydrogen generated from AB
hydrolysis by 0.3–1.1 mM metastable RuNPs at room temperature ([AB]
= 200 mM). b) Plot of hydrogen generation rate vs. the concentration of
Ru, both in logarithmic scale.
25 oC
60
30 oC
35 oC
As the high surface area RuNPs displayed long-term
stability in aqueous media, they were particularly attractive
for the catalytic hydrolysis of ammonia borane in water.[33]
In order to establish the rate law for the hydrolysis of AB
by our metastable RuNPs, the as-synthesized RuNPs were
introduced to aqueous solutions of AB at 25 °C with a very
low stirring rate. For every 10 mL of AB solution (2 mmol,
200 mM) added to a 10 mL dispersion of RuNPs, the H2 gas (b)
evolved from the hydrolysis reaction was collected in a buret
and measured every minute. To ensure that the gas forma-
tion consisted solely of hydrogen and did not contain any
ammonia via the direct decomposition of AB, the gas pro-
duced was tested by an acid-base titration. Variation of RuNP
concentration, reaction temperature and AB concentration
were all tested to elucidate an AB hydrolysis kinetic profile.
To examine the effect of RuNP concentration on the
hydrolysis reaction, five different concentrations (0.3, 0.5, 0.7,
0.9 and 1.1 mM) were chosen such that they were identical
to those previously investigated in the RuCl3 reduction (see
Figure 4f). Figure 6a shows a plot of the volume of H2 gas
produced as a function of time. The rate of hydrogen gen-
eration was directly determined from a linear fit of the data.
For all five concentrations, a stoichiometric amount of H2
40
40 oC
20
45 oC
0
0
1
2
3
4
5
6
Time [min]
3.00
2.54
2.08
1.62
1.16
0.70
ln K = -8108.6(1/T)+28.2
0.0031 0.0032 0.0033 0.0033 0.0034
1/T [k-1]
Figure 7. a) Plot of time vs. volume of hydrogen gas generated from AB
hydrolysis by 0.5 mM metastable RuNPs at room temperature ([AB] =
rate increased with catalyst concentration as clearly seen in 200 mM). b) Arrhenius plot.
(6 mmol) was generated in a rapid manner. The H2 evolution
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E. K. Abo-Hamed et al.
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Table 1. H 2 generation from aqueous AB catalysed by Ru-based
catalysts.
entry
catalyst
particle size
(nm)
Ea (kJ mol−1
)
ref
1
2
Ni-Ru alloy NPs
9
37
54
[12]
[36]
C1s (C-C)
Ru3d5 Ru(0)
PSSA-co-MAa)
1.9
(a)
Ru(0) nanoclusters
3
4
5
6
7
8
laurate-Ru(0) nanoclusters
Ru nanolusters/C
Ru(0)NPs-H2PO4
Ru/C
2.6
1.7
2.9
–
47
34.81
92.4
76
[37]
[38]
[39]
[16]
[17]
[40]
75.47%
(b)
24.53%
RuNPs/MCNTb)
2.5
2.5
33
Ru3d5 RuOx
Ru3d5 RuOx
Ru3d5 Ru(0)
280 282
Binding Energy [eV]
48
Ru Al O
@
2
2
284
286
288
9
metastable RuNPs
2.2
27.5
this study
a)PSSA-co-MA=poly(4-styrenesulfonic acid-co-maleic acid) stabilized; b)Ruthenium(0) nano-
particles supported on multiwalled carbon nanotubes.
Figure 8. XPS spectra of the same 0.5 mM RuNPs sample (a) before
(- - dashed line), and (b) after completion of a complete hydrolysis of
2 mmol AB ( — solid line).
metastable RuNPs in a 1:1 (v/v) water:ethanol mixture. The
⋅
rate constants were derived from the linear part of the plot preparation process was concentration dependent on the RuCl 3
a
for each temperature, and were used to calculate the E of H2O precursor with respect to control over the NP size. Despite
27.5 kJc·mol−1 from the Arrhenius plot shown in Figure 7 b. the lack of a conventional organic or inorganic stabilizing ligands
The catalytic performance of the RuNPs could be further present on the NP surface, metastable RuNPs were shown to be
characterized by the turnover frequency (TOF), which was stable for many months likely on account of surrounding Ru3+
−1
⋅
calculated to be 21.8 mol H 2 (mol catalyst c· min) .
ions, the presence of which was confirmed by XPS. Moreover, a
Interestingly, the metastable RuNPs remained in solution consistent interparticle distance between the NPs in solution was
during the entire hydrolysis of the AB (2 mmol) substrate. To readily observed by SAXS measurements; we believe, that this is
further investigate the oxidation state of the NPs after a full attributed to the repulsion forces arising from these surrounding
catalytic cycle, a dispersion of RuNPs (0.5 mM) was isolated ions. Thus, the highly charged NP surfaces appear to gain long-
and subjected to XPS. Figure 8 displays the XPS spectra both term stability and control over size through charge-charge repul-
prior to (dashed line) and after (solid line) the hydrolysis reac- sion as opposed to direct ligand attachment. Additionally, the
tion. The spectrum after a full catalytic cycle consists of two metastable RuNPs exhibit an fcc structure and were shown to
Ru3d doublets and a single C1s peak (285.5 eV). The doublet be a promising catalytically active material for production of
at 280.4 eV (as before catalysis Figure 8 a), is assigned to Ru0; H2 through the hydrolysis of ammonia-borane in water at room
however, the Ru0 peak contributes to over 75% of the Ru temperature. The activation energy of 27.5 kJ mol−1 for the cata-
signal (Figure 8 b) after a complete cycle. Moreover, we do lytic hydrolysis was found to be remarkably low and resulted in
not observe any evidence for Ru3+ or Cl− in the sample. As no a turnover number of 21.8 per minute, rendering the metastable
protective ligands are attached to the NPs and the protective RuNPs as an extremely promising candidate for the production
Ru3+ ions no longer surround the NPs, a contribution from an of hydrogen gas under mild conditions for practical applications.
oxidized Ru species (Ru3d5 peak at 281.4 eV) was observed,
which has likely occurred during sample preparation (in air).[35]
Table 1 lists the particles sizes and activation energies for
the previous reports on AB hydrolysis catalyzed by ruthenium
species in comparison to our method reported here. While we
4. Experimental Section
⋅
3
Chemicals: RuCl H2O, NaBH4 (99.99%), ammonia-borane
have not quite achieved the smallest RuNP species reported (97%) and all solvents used were purchased from Sigma-Aldrich
a
(see Table 1 entries 4 vs. 9), the E for our metastable NPs and used as received.
is substantially less than any previously reported Ru-based
system. Moreover, our metastable RuNPs are simple to pre- 30 mM RuCl 3 H2O in distilled water was first prepared and soni-
General Metastable RuNP Preparation: Stock solution of
⋅
pare and remain stable in solution without any support.
cated for 20 min. To a conical flask, 40 mL of 1:1 (v/v) aqueous
ethanol was added and different amounts of 0.012, 0.02, 0.028,
0.036 and 0.044 mmol of a stock solution was added to obtain
specific concentrations (0.3, 0.5, 0.7, 0.9 and 1.1 mM, respec-
tively). The solution was sonicated for 1 min and 1 eq of (0.1 M)
3. Conclusion
Our report represents the first example whereby RuNPs are sta- NaBH4 of 1:1 (v/v) aqueous ethanol was immediately added with
bilized without protective organic ligands or additional supports swirling for 15 min using a bench-top incubator at 300 rpm at room
while simultaneously exhibiting a high catalytic activity. We temperature. The colour of the solution was immediately changed
have demonstrated a simple yet efficient method to prepare from light to dark brown. The solution was left to age for 24 h.
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Highly Active Metastable Ruthenium Nanoparticles for Hydrogen Production through the Catalytic Hydrolysis
Hydrolysis of AB by Metastable RuNPs: The catalytic activity of supported by an ERC Starting Investigator Grant (ASPiRe, 240629)
metastable RuNPs for the hydrolysis of AB was evaluated by meas-
uring the rate of hydrogen generation produced in a conventional
water-filled gas burette set-up. Prior to starting the catalytic activity
test, a jacketed reaction flask (25 mL) containing a Teflon-coated stir
bar was placed on a magnetic stirrer and thermostated to 25.0 ( )
1 °C by circulating water through its jacket from a constant temper-
ature bath. Then, a burette filled with water was connected to the
reaction flask to measure the volume of the hydrogen gas released
from the reaction mixture. A 10 mL dispersion of RuNPs was loaded
into the reaction flask and 64 mg (2 mmol) NH3BH3 (corresponding
to generation of a maximum 6 mmol equivalet to134 mL H2 gas at
25.0 ( ) 1 °C and 0.91 atm pressure) was added into the catalyst
solution under 800 rpm stirring rate. The volume of hydrogen gas
evolved was measured by recording the displacement of water
level every 30 s. A 11B NMR spectrum of the solution after one day
showed only the signals attributed to metaborate at 9 ppm.
Characterization: Transmission electron microscopy (TEM)
characterization was carried out by a FEI Philips Tecnai 20 TEM
under an accelerating voltage of 200 kV. Samples were pre-
pared by applying one drop of the as-prepared dispersion onto a
Holey®carbon coated copper TEM grid (300 mesh).
High angle annular dark field (HAADF) images were acquired
on a Nion UltraSTEM 100, operated at 100 kV, equipped with a
cold field emission electron source and a corrector capable of neu-
tralizing up to fifth order aberrations.[41]
11B NMR spectra were recorded on a BRUKER Avance 500MHz
spectrometer operating at 160.48 MHz.
Photoemission spectroscopy: The PES samples were trans-
ferred to the ultrahigh vacuum (UHV) chamber (ESCALAB 250Xi)
for XPS measurements. XPS measurements were carried out using
a XR6 monochromated Alkα X-ray source (hv = 1486.6 eV) with a
650 µm spot size and a pass energy of 20 eV.
and an ESPRC Programme grant (sNaP, EP/G060649/1). STEM
experiments were performed at SuperSTEM, the EPSRC UK national
facility for aberration-corrected STEM.
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Received: November 12, 2013
Published online: April 28, 2014
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