In Situ Formed Ruthenium(0) Nanoparticles Supported on TiO2 Catalyzed Hydrogen Generation from Aqueous Ammonia-Borane Solution at Room Temperature under Air

Article abstract of DOI:10.1080/15533174.2014.988808

Herein, the authors report that TiO₂ supported ruthenium nanoparticles (Ru(0)/TiO₂) during hydrolysis starting with RuCl3/TiO2 precatalyst act as highly active, long-lived, and reusable nanocatalyst in the hydrogen generation from th

Full text of DOI:10.1080/15533174.2014.988808

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Synthesis and Reactivity in Inorganic, Metal-Organic,
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In-Situ Formed Ruthenium(0) Nanoparticles Supported
on TiO2 Catalyzed Hydrogen Generation from Aqueous
Ammonia-Borane Solution at Room Temperature under
Air
Nihayet Konuşa, Yaşar Karataşa & Mehmet Gulcana
a Department of Chemistry, Science Faculty, Yüzüncü Yıl University, Van, 65080, Turkey
Accepted author version posted online: 31 Jul 2015.
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To cite this article: Nihayet Konuş, Yaşar Karataş & Mehmet Gulcan (2015): In-Situ Formed Ruthenium(0) Nanoparticles
Supported on TiO2 Catalyzed Hydrogen Generation from Aqueous Ammonia-Borane Solution at Room Temperature under Air,
Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry, DOI: 10.1080/15533174.2014.988808
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In-Situ Formed Ruthenium(0) Nanoparticles Supported on TiO2 Catalyzed Hydrogen
Generation from Aqueous Ammonia-Borane Solution at Room Temperature under Air
Nihayet Konuş, Yaşar Karataş, Mehmet Gulcan*
Department of Chemistry, Science Faculty, Yüzüncü Yıl University, Van, 65080, Turkey
* Corresponding Author: mehmetgulcan@yyu.edu.tr;
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ABSTRACT
Herein, we report that TiO2 supported ruthenium nanoparticles (Ru(0)/TiO2) during the
hydrolysis of starting with RuCl3/TiO2 precatalyst, which act as highly active, long-lived and
reusable nanocatalyst in the hydrogen generation from the hydrolysis of ammonia-borane
(NH3BH3) at room temperature. The resulting Ru(0)/TiO2 catalyze hydrogen generation from the
hydrolysis of ammonia-borane with an initial turnover frequency (TOF) value of 200 min−1 at 25
± 0.1 °C. More importantly, Ru(0)/TiO2 are stable enough to be isolated and bottled as solid
material, which can be reused as active catalyst under the identical conditions of the first run.
Keywords: Ruthenium; Nanoparticles; Titanium(IV) Oxide; Ammonia-Borane; Hydrogen
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INTRODUCTION
Hydrogen is a globally accepted clean energy carrier, which could overcome the world energy
problem and slim down the environmental pollution caused by using fossil fuels as energy
source.[1] However, the safe and efficient storage of hydrogen is still one of the trickiest
problems in the “Hydrogen Economy”.[2] Because of this concern, there has been a rapidly
growing interest for the development of solid hydrogen storage materials that provide the
combination of high volumetric/gravimetric storage capacity, adequate kinetics, reversibility,
low cost and low toxicity, as the majority of them are impractical for the current high-pressure
and cryogenic hydrogen storage systems.[3, 4] In this context, boron based chemical hydrides[56-7]
and B-N adducts[891011-12] have been tested as solid hydrogen storage materials. Among these
solid materials, ammonia-borane (AB; NH3BH3) seems to be much better suited for this purpose
due to its high gravimetric hydrogen storage capacity (19.6 % wt), non-toxicity, low molecular
weight (30.7 g.mol-1) and inclination for bearing protic (N–H) and hydridic (B–H) hydrogens,
which can be „discharged‟ and „recharged‟ in different chemical transformations.[13] The
hydrolytic dehydrogenation of AB generates 3 equiv. of H2 (per mole of AB) at room
temperature (RT) under air only in the presence of suitable catalyst (1).[8,13,14,-15]
catalyst
NH3BH3 + 2H2O
NH4BO2 + 3H2
at RT under air
Regardless of the difficulty in recycling of the (meta)borate anion, the hydrogen evolution
from the catalytic hydrolysis of ammonia-borane has some features, which make it capable for
prospective applications; (i) AB has high solubility in water (33.6 g NH3BH3/100 g H2O),[16] (ii)
it occurs at appreciable rate only in the presence of a suitable catalyst at room temperature, (iii)
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the catalytic hydrolysis of AB is exothermic (ΔH = -155.97 kJ/mol).[17] Since the hydrolytic
dehydrogenation occurs only in the presence of a suitable catalyst, many transition metals or
their compounds have been tested as catalyst for this important reaction.[18] Although,
homogeneous catalysts have already showed notable activities in the hydrolysis of AB,[1920-21] the
current effort has been focused on the employment of heterogeneous catalysts because of their
considerable advantages in the catalytic reactions including simple product separation and
[25, 26]
catalyst recovery.[22] Among the heterogeneous catalysts Rh,[23, 24] Pt[23, 24] and Ru
provide significant catalytic activities in the hydrolysis of AB.
metals
Of particular importance, ruthenium based heterogeneous catalysts[26-Error! Bookmark not defined.]
can catalyze the hydrolytic dehydrogenation of AB under mild conditions with acceptable rates
at 3 equiv. of H2 (per mole of AB) generation. Unfortunately, most of them used in these
schemes suffer from difficult isolation,[26, 27, 28] low activity,[29, 30, 31] low stability,[27,28] and time
consuming synthesis procedures.[26, 28, 32, 33] Therefore, the development of a simply prepared,
highly active and reusable catalyst that operates under mild conditions remains a difficult
challenge in the catalytic hydrolysis of AB.
In this study, we report our results on the in-situ generation of ruthenium(0) nanoparticles
supported on TiO2, hereafter referred to as Ru(0)/TiO2, and their notable catalytic performance in
terms of the activity, lifetime and reusability in the hydrolytic dehydrogenation of AB at room
temperature under air. Ru(0)/TiO2 can simply and reproducibly be prepared by the liquid phase
impregnation of RuCl3 on TiO2 matrix, followed by their in-situ AB reduction during the
hydrolysis of AB. After a short induction time period, the resulting Ru(0)/TiO2 catalyzes the
dehydrogenation of aqueous AB solution at room temperature with an initial turnover frequency
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(TOF) of 200 min-1. Moreover, Ru(0)/TiO2 were also found to be long-lived catalyst, which
provide 60000 total turnovers in hydrogen generation from the hydrolysis of ammonia-borane
over 17 h before deactivation. More importantly, Ru(0)/TiO2 feature notable resistance against
agglomeration and leaching throughout the catalytic runs. In fact, when the isolated Ru(0)/TiO2
are reused, they retain 65 % of their initial activity even at 5th reuse in the hydrolytic
dehydrogenation of AB.
EXPERIMENTAL
Materials
Titanium(IV) oxide (TiO2, ≥99 %), ruthenium(III) chloride trihydrate (RuCl3.3H2O),
ammonia-borane (NH3BH3, 97 %), D2O and BF3·(C2H5)2O were purchased from Sigma-Aldrich.
Deionized water was distilled with a water purification system (Thermo Scientific Barnsted
Nanopure System). All glassware and Teflon-coated magnetic stir bars were cleaned with
acetone, followed by copious rinsing with doubly deionized water under ultrasonication, and
finally dried in an oven at 423 K.
Characterization
TEM samples were prepared by dropping one drop of dilute suspension on copper coated
carbon TEM grid and the solvent was then evaporated at room temperature under reduced
pressure (10-3 torr). The conventional TEM was carried out on a JEOL JEM-200CX transmission
electron microscopes operating at 120 kV. The ruthenium content of Ru(0)/TiO2 was determined
by inductively coupled plasma atomic emission spectroscopy (ICP-OES; ULTIMA 2-HORIBA
Jobin-Yvon) after the powdered sample was completely dissolved in the mixture of HNO3:HCl
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with a 1:3 ratio. XPS analysis was performed on a Kratos AXIS ultra imaging X-ray
photoelectron spectrometer using monochromatic Al Kα radiation (1486.6 eV, the X-ray tube
working at 15 kV, 350 W and pass energy of 23.5 keV). At the end of the hydrolysis reaction,
the resulting solutions were filtered and the filtrates were collected for 11B NMR analysis. 11B
NMR spectra were recorded on a Bruker Avance DPX 400 with an operating frequency of
128.15 MHz. D2O and BF3·(C2H5)2O were used as a lock and an external reference, respectively.
Preparation of ruthenium(III) supported on titanium(IV) oxide (Ru3+/TiO2) precatalyst
and the general procedure for the in-situ generation of Ru(0)/TiO2 during the hydrolytic
dehydrogenation of ammonia-borane
Ruthenium(III) cations were introduced onto TiO2 support by the liquid phase impregnation in
a 10 ml aqueous solution that contains 100 mg of TiO2 and 2.6 mg RuCl3.3H2O for 12 h at room
temperature. The sample was then filtered by suction filtration using a Whatman-1 filter (Ø = 9
cm), washed three times with 100 mL of deionized water to remove chloride ions, and dried at
353 K in the oven. The in-situ formation of Ru(0)/TiO2 and the concomitant hydrolytic
dehydrogenation of AB were performed in a typical jacketed, three-necked reaction flask
thermostatted at 25 ± 0.1 °C and connected to the water-filled cylinder glass tube. In a typical
experiment, 123 mg of Ru3+/TiO2 (with 0.66 % wt Ru loading corresponding to 8.0 μmol Ru)
was weighted and placed into a reaction flask containing a new 5/16 in. × 5/8 in. stir bar. Next,
10.0 mL of 100 mM AB solution was added to the reaction flask and the reaction timer was
started (t = 0 min). Hydrogen gas generation from the catalytic reaction solution was followed
using a typical water-filled gas buret system and recording the displacement of the water level in
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the gas buret until no more hydrogen evolution was observed. When no more hydrogen
generation was observed, the experiment was stopped, the reactor was disconnected from the
water-filled tube, and the hydrogen pressure was released and a small aliquot from the reaction
solution in the culture tube was withdrawn for 11B NMR analysis.
Isolability and reusability of Ru(0)/TiO2 in the hydrolytic dehydrogenation of ammonia-
borane
After the first run of the catalytic hydrolysis of 100 mM AB in 10 mL H2O starting with 123
mg of Ru3+/TiO2 (8.0 μmol Ru) at 25 ± 0.1 °C, the catalyst was isolated by suction filtration,
washed three times with 20 mL of deionized water, and dried under N2 gas purging at room
temperature then transferred into the glove box. The dried samples of Ru(0)/TiO2 were weighted
and used again in the hydrolytic dehydrogenation of 100 mM AB in 10 mL H2O and the same
procedure was repeated up to 5th catalytic cycle. The results were expressed as percentage of the
retained initial catalytic activity of Ru(0)/TiO2 in the hydrolysis of AB versus number of
catalytic runs.
Catalytic lifetime of Ru(0)/TiO2 in the hydrolytic dehydrogenation of ammonia-borane
The catalytic lifetime of Ru(0)/TiO2 formed in-situ during the hydrolytic dehydrogenation of
AB was determined by measuring total turnover number (TTO). This experiment was started
with 123 mg of Ru3+/TiO2 (8.0 μmol Ru) precatalyst in a 20 mL solution of 100 mM AB. When
all the AB in the solution was completely hydrolyzed, more AB was added and the reaction was
continued in this way until no hydrogen gas evolution was observed.
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RESULTS AND DISCUSSION
In-situ generation of Ru(0)/TiO2 during the hydrolysis of AB starting with Ru3+/TiO2
precatalyst at room temperature under air
Ruthenium nanoparticles supported on titanium(IV) oxide (Ru(0)/TiO2) were prepared by the
liquid phase impregnation of Ru3+ ions onto TiO2, followed by the in-situ reduction of
ruthenium(III) cations in the Ru3+/TiO2 precatalyst during the hydrolytic dehydrogenation of AB.
The evolution of Ru(0)/TiO2 and associated hydrolysis of AB can be followed by monitoring the
changes in hydrogen pressure, which was then converted into the equivalent H2 per mole of AB,
using the known 3:1 H2/AB stoichiometry (1). Fig. 1 shows the equivalent hydrogen generated
Figure 1
per mole of AB versus time plot for the hydrolytic dehydrogenation of AB starting with
Ru3+/TiO2 precatalyst at room temperature under air. The formation kinetics of Ru(0)/TiO2
catalyst can be obtained by using the catalytic hydrolysis of AB as the reporter reaction,[34,35]
(Scheme 1), in which P is the added precatalyst (Ru3+/TiO2) and Q is the growing supported
ruthenium nanoparticles (Ru(0)/TiO2). The catalytic hydrolysis of AB will straightforwardly
report and intensify the amount of Ru(0)/TiO2 catalyst, Q, present if the rate of AB hydrolysis is
fast in comparison to the ruthenium(0) nanoparticles formation rate. Sigmoidal kinetic can be
seen in Fig. 1 fits well by the Finke-Watzky two-step nucleation and autocatalytic growth
mechanism of nanoparticles formation.[36,37] The surveillance of a sigmoidal hydrogen generation
curve and its stimulated fit to the slow, continuous nucleation P→Q (rate constant k1) followed
by autocatalytic surface growth P+Q→2Q (rate constant k2) kinetics is strongly declarative of the
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formation of a metal(0) nanoparticles catalyst from a precatalyst in the presence of a reducing
agent.[34-37] The rate constants determined from the nonlinear least-squares curve fit from Fig. 1
are k1 = 9.68×10-2 min-1 and k2 = 90.11 M-1. min-1 (the mathematically required correction has
been made to k2 for the stoichiometry factor of 83.3, as described elsewhere,[34] but not for the
“scaling factor”; that is, no correction has been made for the changing number of Ru atoms on
the growing metal surface).
After the generation of 3 equiv. of H2, the resulting Ru(0)/TiO2 were simply isolated by
11
filtration and B NMR spectrum of the aliquot taken from the reaction solution showed that the
formation metaborate (δ = 8.7 ppm, s).[38] Fig. 2 show TEM images of Ru(0)/TiO2, which reveals
that the formation of ruthenium(0) nanoparticles on TiO2 with an average diameter of 5.51 ±
0.91 nm (Fig. 2(c)). The oxidation state of ruthenium in the Ru(0)/TiO2 sample was also
investigated by X-ray photoelectron spectroscopy (XPS). The high resolution XPS spectrum
(Fig. 3) of Ru(0)/TiO2 shows a prominent peak at 462 eV, which can readily be assigned to
Ru(0) 3p3/2,[39] thus revealing that the reduction of Ru3+ species during the hydrolysis of AB.
Figure 2
Figure 3
The effect of ruthenium loading on the catalytic activity of Ru(0)/TiO2 in the hydrolytic
dehydrogenation of AB
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The effect of ruthenium loading (wt %) on the catalytic activity of in-situ generated
Ru(0)/TiO2 in the hydrolytic dehydrogenation of AB was investigated by performing a series of
experiments for the catalytic AB hydrolysis starting with Ru3+/TiO2 precatalysts with different
ruthenium loadings in the range of 0.25 – 3.60 wt %. Fig. 4(a) shows the variation in the
observed initial TOF values with respect to the ruthenium loadings (% wt). The trend in the
catalytic activities of Ru(0)/TiO2 with different Ru loadings mirrors the accessibility of
ruthenium(0) nanoparticles supported on TiO2 by the substrate.
Figure 4
The best catalytic activity (TOF = 200 min-1) is obtained by using Ru3+/TiO2 precatalyst
containing 0.66 wt % Ru, in which the in-situ generated ruthenium(0) nanoparticles exist mostly
on the surface and readily accessible. The small size of Ru(0)/TiO2 containing 0.66 wt% Ru may
be another reason for its high activity. In fact, TEM image taken from Ru(0)/TiO2 sample with
3.60 wt % Ru loading (Fig. 4(b)) indicates the existence of larger ruthenium(0) nanoparticles and
agglomerates on the surface of TiO2 support. It should also be noted that, TOF value provided by
our Ru(0)/TiO2 catalyst is the apparent lower limit as it is not corrected for the amount of
ruthenium that is on the surface of the Ru(0)/TiO2[40,41] but it is still comparable with the majortiy
of the previous ruthenium based catalysts tested in the same reaction (see Table 1).
Table 1
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Initial kinetic studies and the determination of activation energy (Ea) for the in-situ
generated Ru(0)/TiO2 catalyzed hydrolytic dehydrogenation of AB
Fig. 5(a) presents the plot of mol of H2 generated per mole of AB versus time for the catalytic
◦
dehydrogenation of AB starting with different concentrations of Ru in water at 25 ± 0.1 C. As
expected from an in-situ formed catalyst, a linear hydrogen generation starts with induction
period and continues until the generation of 3 equiv. of H2. The dehydrogenation rate,
determined from the nearly linear portion of the plots, increases with the catalyst concentration.
Plotting the dehydrogenation rate versus ruthenium concentration (both on logarithmic scales)
gives a straight line with a slope of 0.72 (Fig. 5(b)).
Figure 5
That is, an apparent first-order dependence on the catalyst concentration is observed. The
deviation from the first-order kinetics with respect to catalyst concentration might be due to the
initial stage deactivation of some ruthenium nanoparticles by the borate anion [BO2]−, which acts
as a catalyst poison by significantly decreasing the accessibility of active sites.[42]
The effect of the substrate concentration on the hydrolytic dehydrogenation rate was also
studied by performing a series of experiments starting with variation of the initial concentration
◦
of AB while keeping the catalyst concentration constant at 25 ± 0.1 C. Fig. 6(a) shows the
volume of H2 generated versus time for the hydrolytic dehydrogenation of AB starting with
different concentrations of AB at constant Ru concentration. In a substrate concentration higher
than 90 mM, the catalytic dehydrogenation of AB appears to be zero-order in the substrate
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concentration, while at lower substrate concentration; one observes first-order dependence (Fig.
6(b)).
Figure 6
The activation energy for the in-situ generated Ru(0)/TiO2 catalyzed hydrolytic
dehydrogenation of AB was also determined by using the temperature dependent kinetic data
shown in Fig. 7(a). The observed rate constants were calculated from the nearly linear portions
of the curves at different temperatures and used for the construction of Arrhenius (Fig. 7(b)) plot,
which gives us the activation energy of Ea = 87.4 kJ/mol for the in-situ generated Ru(0)/TiO2
catalyzed hydrolytic dehydrogenation of AB.
Figure 7
Reusability of the in-situ generated Ru(0)/TiO2 in the hydrolytic dehydrogenation of AB
The isolability and reusability of Ru(0)/TiO2, as two crucial measures in heterogeneous
catalysis, were also tested in the hydrolytic dehydrogenation of AB. After the generation of 3
equiv. of H2 per AB and formation of Ru(0)/TiO2 starting with Ru3+/TiO2 precatalyst in the first
run, the catalyst was simply isolated by suction filtration then dried in vacuo and bottled under
inert atmosphere. When reused the isolated Ru(0)/TiO2 were found to be still highly active in the
hydrolytic dehydrogenation of AB. They retain > 65 % of their initial catalytic activity even at
the 5th catalytic run, with the generation of 3 equiv. H2 per mole AB (Fig. 8(a)).
The reusability performance of Ru(0)/TiO2 is also better than some of the previously reported
Ru catalysts; laurate[27] and PSSA-co-MA[28] stabilized ruthenium(0) nanoparticles and
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ruthenium(0) nanoparticles supported on carbon[32] and carbon nanotube[33] tested in this
reaction (see Table 1). The decrease in the activity of Ru(0)/TiO2 can be explained by the
agglomeration of surfactant-free ruthenium(0) nanoparticles on the surface of TiO2 into some
extent as evidenced by the representative TEM image of Ru(0)/TiO2 sample harvested from the
5th catalytic run (Fig. 8(b)).
Figure 8
More importanly, the leaching tests performed by ICP-OES analyses of Ru in the filtrate
collected from each cycles confirm that ruthenium(0) nanoparticles are not leaching into the
reaction solution from TiO2 support. A control experiment was also performed to show that the
hydrolytic dehydrogenation of AB is completely stopped by the removal of Ru(0)/TiO2 from the
reaction solution.
Catalytic lifetime of the in-situ generated Ru(0)/TiO2 in the hydrolytic dehydrogenation of
AB
In a catalyst lifetime experiment started with Ru3+/TiO2 precatalyst, the in-situ generated
Ru(0)/TiO2 were found to provide total turnover number (TTON) 60000 in the hydrolytic
dehydrogenation of AB over 15 h before deactivation. An initial TOF value of 200 min-1 was
obtained; however, the average TOF value was calculated to be 100 min-1. The observation that
the TOF value decreases as the reaction proceeds indicates the deactivation of the ruthenium(0)
nanoparticles catalyst. Moreover, it should be noted that the in-situ formed Ru(0)/TiO2 provide
approximately 45-fold higher TTON than the bulk ruthenium(0) metal formed in-situ during the
hydrolysis of AB started with support-free RuCl3 (TTON = 2000)[27] under the same conditions.
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CONCLUSIONS
In summary, our study of the in-situ generated Ru(0)/TiO2 catalyzed hydrolytic
dehydrogenation of AB has led to the following conclusions and insights: (i) TiO2 supported
ruthenium(0) nanoparticles, Ru(0)/TiO2, were reproducibly prepared from the in-situ reduction
Ru3+/TiO2 during the hydrolytic dehydrogenation of ammonia- borane at room temperature; (ii)
The preliminary characterization of the resulting catalytic material by using ICP-OES, XPS, and
TEM techniques reveal that the formation of well-dispersed ruthenium(0) nanoparticles with an
average diameter of 5.51 ± 0.91 nm within TiO2; (iii) Ru(0)/TiO2 provide notable catalytic
activity (lower-bound TOFinital = 200 min-1) among all the heterogeneous ruthenium catalysts
tested in the hydrolytic dehydrogenation of AB at room temperature; (iv) Ru(0)/TiO2 have great
stability against to sintering and leaching, that make them highly reusable catalyst, when
redispersed they retain 65 % their initial activity even at the 5th catalytic run in the hydrolytic
dehydrogenation of AB with the generation of 3 equiv. of H2 per mole of AB; (v) Morover,
Ru(0)/TiO2 were also found to be long-lived catalyst in the hydrolytic dehydrogenation of AB so
that they provide total turnover number (TTON) 60000, which is 30-fold higher TTON than the
bulk ruthenium(0) metal formed in-situ during the hydrolysis of AB started with support-free
RuCl3; (vi) The quantitative kinetic studies depending on the catalyst/substrate concentrations
and temperature indicates that Ru(0)/TiO2 catalyzed hydrolytic dehydrogenation of AB is first-
order in catalyst concentration and zero order in AB concentration (>90 mM). The activation
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energy was also estimated for the in-situ generated Ru(0)/TiO2 catalyzed hydrolytic
dehydrogenation of AB (Ea = 87.4 kJ/mol); (vii) The kinetic data, collected for the Ru(0)/TiO2
formation and concomitant hydrolytic dehydrogenation of AB fit well to the two-step mechanism
for the nanoparticles formation [36, 37]: continuous nucleation P→Q (rate constant k1) followed
by autocatalytic surface growth P+Q→2Q (rate constant k2). At this concern, after the known
catalytic dehydrocoupling of DMAB in water,[43] the hydrolytic dehydrogenation of AB is the
second reporter reaction takes place in water for the nanoparticles formation kinetics.
ACKNOWLEDGMENT
Authors thanks to Presidency of Scientific Research Projects of University of Yüzüncü Yıl for
the financial support.
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20. Graham, T.W.; Tsang, C.W.; Chen, X.; Guo, R.; Jia, W.; Lu, S.M.; Sui-Seng, C.; Ewart,
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21. Fortman, G.C.; Slawin, A.M.Z.; Nolan, S.P. Highly active iridium(III)–NHC system for
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26. Can, H.; Metin, Ö. A facile synthesis of nearly monodisperse ruthenium nanoparticles
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31. Basu, S.; Brockman, A.; Gagare, P.; Zheng, Y.; Ramachandran, P.V.; Delgass, W.N.;
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36. Watzky, M.A.; Finke, R.G. Transition metal nanocluster formation kinetic and
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40. The TOF and TTO values reported herein are those typically reported. TOF = (mol of H2
generated/total mol of catalyst time; TTON = (TOF) (time). That is, the reported
 
values of TOF and TTON are not corrected for the amount of metal that is on the surface
of the catalyst and/or the actual number of active sites. For the calculation of initial TOF
values the initial rates41 were considered.
41. Wilkins, R.G. Kinetics and mechanism of reactions of transition metal complexes, 2nd
Ed.; VCH: New York, 1991.
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42. Clark, T.J.; Whittell, G.R.; Manners, I. Highly efficient colloidal cobalt- and rhodium-
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Table Captions
Table 1. Catalyst systems (in chronological order) and their performances in terms of activation
energy, activity, and reusability in the hydrolytic dehydrogenation of AB.
entry
Pre(catalyst)
ex or in-situ
Ea
TOF
Reusability
Ref.
Formation
(kJ/mol) (min-1)
(% of retaining
initial activity)
Laurate Stabilized
Ru NPs
1
2
ex-situ
in-situ
47
54
75
18
53 % at 5th reuse
27
28
PSSA-co-MA
Stabilized Ru NPs
Not Demonstrated
3
4
5
6
7
8
Rubulk@C
ex-situ
in-situ
in-situ
ex-situ
ex-situ
ex-situ
76
34
33
28
48
58
113
430
329
91
Not Demonstrated
25 % at 5th reuse
41 % at 4th reuse
85 % at 5th reuse
90 % at 10th reuse
92 % at 5th reuse
31
32
33
29
26
25
Ru NPs@C
Ru NPs@CNT
Ru NPs@ZK-4
Ru NPs@Al2O3
83
RuNPs@micro-
137
HAp
9
RuNPs@TiO2
in-situ
87
200
65 % at 5th reuse
this
study
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Figure Captions
Figure 1. Plot of [H2]/[NH3BH3] versus time (min.) for the in-situ generated Ru(0)/TiO2
catalyzed hydrolysis of AB starting with Ru3+/TiO2 precatalyst ([AB] = 100 mM; [Ru] = 0.8 mM
in 10.0 mL H2O) at 25 ± 0.1 ºC and its curve fit (red) to F–W two-step nucleation and
autocatalytic growth mechanism (k1 = (9.68 ± 0.76) ×10-2 min-1, k2 = (90.11 ± 3.07) M-1. min-1
and R2 = 0.9979) for TiO2 supported ruthenium(0) nanoparticles formation.
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Figure 2. (a-b) TEM images (in different magnifications) of Ru(0)/TiO2 formed in-situ during
the hydrolytic dehydrogenation of AB starting with Ru3+/TiO2 precatalyst ([AB] = 100 mM, [Ru]
= 0.8 mM in 10.0 mL H2O) at room temperature under air (c) corresponding size histogram of
Ru(0)/TiO2.
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Figure 3. High resolution X-ray photoelectron spectrum of Ru(0)/TiO2 formed in-situ during the
hydrolytic dehydrogenation of AB.
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Figure 4 (a) Plot of initial TOF values versus Ru loadings (wt %) for the catalytic hydrolysis of
AB, (b) TEM image of Ru(0)/TiO2 formed in-situ during the hydrolytic dehydrogenation of AB
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