Dramatic Synergy in CoPt Nanocatalysts Stabilized by "click" Dendrimers for Evolution of Hydrogen from Hydrolysis of Ammonia Borane

Article abstract of DOI:10.1021/acscatal.8b04498

Hydrolysis of ammonia borane (AB) is a very convenient source of H₂, but this reaction needs catalytic activation to become practical under ambient conditions. Here this reaction is catalyzed by bimetallic late transition-metal nanoparticles (N

Full text of DOI:10.1021/acscatal.8b04498

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Article
Dramatic Synergy in CoPt Nanocatalysts Stabilized by “Click”
Dendrimers for Hydrogen Evolution from Hydrolysis of Ammonia Borane
Qi Wang, Fangyu Fu, Sha Yang, Marta Martinez Moro, Maria de los Angeles
Ramirez, Sergio E. Moya, Lionel Salmon, Jaime Ruiz, and Didier Astruc
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b04498 • Publication Date (Web): 24 Dec 2018
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Dramatic Synergy in CoPt Nanocatalysts Stabilized
by “Click” Dendrimers for Hydrogen Evolution from
Hydrolysis of Ammonia Borane
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[a]
[a]
[b]
[c]
[c]
Qi Wang, Fangyu Fu, Sha Yang, Marta Martinez Moro, Maria de los Angeles Ramirez,
Sergio Moya, [c] Lionel Salmon, Jaime Ruiz, and Didier Astruc*
[d]
[a]
[a]
a
b
º
ISM, UMR CNRS N 5255, Univ. Bordeaux, 33405 Talence Cedex, France.
Department of Chemistry and Centre for Atomic Engineering of Advanced Materials, Anhui
Province Key Laboratory of Chemistry for Inorganic/Organic Hybrid Functionalized Materials,
Anhui University, Hefei, Anhui 230601, China
c
Soft Matter Nanotechnology Lab, CIC biomaGUNE, Paseo Miramón 182. 20014. Donostia-San
Sebastián, Gipuzkoa, Spain.
d
Laboratoire de Chimie de Coordination, UPR CNRS 8241, 31077 Toulouse Cedex, France
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ABSTRACT
Hydrolysis of ammonia borane (AB) is a very convenient source of H , but this reaction needs
2
catalytic activation to become practical under ambient conditions. Here this reaction is catalyzed
by bimetallic late transition-metal nanoparticles (NPs) that are stabilized and activated by “click”
dendrimers. These dendrimers 1 and 2 contain respectively 27 or 81 triethylene glycol terminal
groups and 9 or 27 1,2,3-triazole ligands located on the dendritic tethers. A remarkable synergy
between Pt and Co in the Pt-Co/“click” dendrimer nanocatalysts is disclosed. These Pt-Co/“click”
dendrimer catalysts are much more efficient for hydrolysis of AB than either “click” dendrimer-
stabilized Co or Pt analogues alone. The best catalyst Pt Co /1 stabilized by the nona-triazole
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“
click” dendrimer 1 achieves a TOF of 303 mol ·mol
·min (606 mol ·mol ·min ) at 20
H2
catal.
H2
Pt
±
1°C. The AB hydrolysis reaction catalyzed by Pt Co /1 is boosted by NaOH, the TOF value
1
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-1
-1
-1
-1
reaching 476.2 mol ·mol
·min (952.4 mol ·mol ·min ), one of the very best results ever
H2
catal.
H2
Pt
obtained for this reaction. The presence of a percentage of Pt as low as 25% in the CoPt nanoalloy
provides a reaction rate higher than that obtained with the pure Pt NP catalyst alone. The kinetics
involves in particular a kinetic isotope effect k /k = 2.46 obtained for the hydrolysis reaction
D
H
with D O, suggesting that a O-H bond of water is cleaved in the rate-determining step. Tandem
2
reactions were carried out for the hydrogenation of styrene with hydrogen generated from the
hydrolysis of AB. Performing this tandem reaction with D O shows deuteration of the
2
ethylbenzene products confirming O-D cleavage and H/D scrambling on the bimetallic NP
surface. Finally a full reaction mechanism is proposed. This dramatic synergy type should also
prove useful in a number of other catalytic systems.
KEYWORDS: Synergy; nanocatalyst; cobalt; dendrimer; hydrolysis; ammonia borane
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INTRODUCTION
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st
Hydrogen energy is considered as the most promising clean energy source in the 21 century.
H production upon hydrolysis of ammonia borane (AB) has attracted much attention owing to its
2
–
1
low molecular weight (30.87 g∙mol ), high hydrogen content (19.6 wt%), and high stability in
1
solution. Duan and Chen recently reported mechanistic studies for the hydrolysis of AB, showing
+
- 2
that this reaction finally produces NH B(OH) :
4
4
+
-
H N∙BH + 4 H O  NH B(OH) + 3 H
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2
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Intensive efforts have recently been devoted to the investigation of potentially efficient and
3
,4
economical catalysts for AB hydrolysis. Noble-metal nanoparticles (NPs), in particular Pt-based
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catalysts, have been shown to undergo high hydrogen evolution rates for this reaction. Non-
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noble-metal NP catalysts are by far less efficient than noble metal NPs, however. For example,
under the same conditions PtNPs and RhNPs showed much higher efficiency than CoNPs and
7
NiNPs for the hydrolysis of AB, but the practical use of precious metals in catalysis is limited
owing to their high costs and global scarcities.
Dendrimers have been extensively used as supports and shown to be highly efficient in catalysis,8
9
including for the encapsulation or stabilization of transition-metal NP catalysts. Their use to
1
0
generate hydrogen from hydrogen-rich precursors has been pioneered by Xu’s group, but they
have very rarely been used for this purpose. Only two reports are known, and they deal with
monometallic NPs.7,10 There is so far no report of dendrimer-stabilized bimetallic nanocatalyst for
hydrogen generation. Here we report a series of bimetallic NPs with narrow size distributions
1
1
stabilized by “click” dendrimers for optimized catalysis of AB hydrolysis using the possible
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synergy between the two metals. The two dendrimers 1 and 2 that are terminated by triethylene
glycol (TEG) with an arene core and containing 27 and 81 TEG tethers are of zeroth and first
generation respectively (Figure 1). These dendrimers serve as NP stabilizers and activators. The
homogenerous bimetallic catalysts are prepared by mixing the two precursor metal salts with one
of the dendrimers in water, followed by reduction of the mixture of the transition-metal salts by
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These bimetallic nanocatalysts are characterized by transmission electron microscopy (TEM),
high-resolution transmission electron microscopy (HRTEM), high-angle annular dark-field
scanning transmission electron microscopy (HADDF-STEM), energy dispersive X-ray
spectroscopy (EDS) and X-ray photoelectron spectra (XPS). Among these click-dendrimer
templated bimetallic NPs, the nanoalloy Pt Co /1 is shown to be the most active and efficient
1
1
-1
-1
nanocatalyst for AB hydrolysis, achieving a TOF value of 952.4 mol ·mol ·min . The
H2
Pt
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influence of NaOH on the reaction efficiency for various bimetallic NPs is investigated. Kinetic
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isotope effects (KIEs) using D O instead of H O and tandem reaction for hydrogenation of
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styrene are also examined in the study of the mechanism of this reaction.
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Figure 1. Dendrimer structures 1 and 2.
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RESULTS AND DISCUSSION
Synthesis and Characterizations of the Pt-Co/1 NPs.
The synthesis of the “click” dendrimers 1 and 2 was conducted following previous reports.11
The Pt-Co/1 NPs were prepared by mixing dendrimer 1 with two metal salts in water and stirring
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was continued for 1h (Scheme 1 and SI).
Scheme 1. Synthesis of the Pt-Co/1 NPs.
TEM images were recorded for the Pt-Co/1 NPs series, and the average particle sizes were
between 1.5 nm and 2.0 nm (Figure S3-5). Very fine and Pt Co /1 NPs with narrow size
1
1
distribution were indicated by TEM with an average size around 2 nm (Figure 2).
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Figure 2. (a) TEM image (insert is the HRTEM image) and (b) size distribution of Pt Co /1.
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A good crystallinity of Pt Co /1 NPs was confirmed as shown by the clear lattice fringes in
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HRTEM (Figure 2a insert). The spacing in the lattice is approximately 2.2 Å, if it is assumed that
the crystallographic plane of the Pt Co nanoalloy is {111} (JCPDS 43-1358). The analysis by
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Energy Dispersive X-Ray Spectroscopy (EDS) of the Pt Co /1 NPs showed that the Pt and Co
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elements were present in the 011 area (Figure S6), and the alloy structure of Pt Co /1 NPs was
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verified by the HAADF-elemental mapping spectra (Figure S7). The EDS measurements clearly
showed that the alloys did not contain any monometallic NPs.
The X-ray photoelectron spectra (XPS) of the Pt Co /1 NPs showed binding energies of 71.1 and
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4.4 eV for the 4f7/2 and 4f , respectively, indicating the presence of metallic Pt(0) (Figure 3a).
5/2
In addition a 778.6-eV binding energy was found for Co 2p , showing the existence of Co(0).
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/2
Another peak around 782.0 eV demonstrated that there is some Co(II) species in the Pt Co /1 NP
1
1
sample, probably due to partial oxidation of Co(0) just before or during the XPS experiment
Figure 3b). Indeed, catalytic experiments were conducted just after the syntheses and were not
(
subjected to such sample handling. Therefore it is believed that the Co(II) species was not present
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during catalysis. Since the intradendritic triazole ligands are neutral, surface atoms are indeed
expected to be Co(0) and Pt(0).
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Figure 3. XPS spectrum of (a) Pt 4f and (b) Co 2p in Pt Co /1 nanocatalyst. The presence of some
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Co(II) most probably results from partial oxidation of the sample just before or during the XPS
experiment.
Synergistic catalysis of ammonia borane hydrolysis by various Pt-Co NPs/1.
The hydrolysis of AB catalyzed by various Pt-Co/1 NPs (1 mol%) stabilized by dendrimer 1
was carried out at 20 ± 1°C (Table 1, see the kinetics in Figure 4). The compositions of the Pt-
Co/1 NPs were adjusted by variation of the precursor molar ratios. The naming Pt-Co/1 NPs will
be used in the following text for alloys of NPs containing both PtNPs and CoNPs in any
proportions in dendrimer 1. The proportions of the metals in the alloy will be indicated by the
numbers following the Pt and Co symbols (for instance Pt Co for equal amounts of Pt and Co in
1
1
the alloy, etc.).
Pt is known as a very efficient catalyst for the hydrolysis of AB. For example, the PtNPs/1 led to a
−
1
−1
TOF value of 120 mol ·mol catal ·min . The performance of CoNPs/1, however, was not very
H2
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−1
−1 7,15
good for this reaction, its TOF value being only 8.8 mol ·mol
·min . Interestingly, when
H2
catal
Co was introduced to form a Pt-Co alloy NP, the catalyst Pt Co /1 led to a TOF value of 163.6
1
1
−
1
−1
mol ·mol
·min for the hydrolysis of AB. Pt Co /1 was the best catalyst among the whole
1 1
H2
catal
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Pt-Co NP series, and the TOF value decreased from its maximum value when the Co proportion in
the Pt-Co alloy was higher than 1/2. For instance the TOF value of Pt Co /1 for this reaction was
1
2
−
1
−1
1
50 mol ·mol
·min . All the alloyed Pt-Co NPs gave better results than the catalyst PtNPs/1
H2
catal
or CoNPs/1. In particuler it is remarkable that the introduction of only 25% of Pt to Pt-Co alloy
led to a better nanocatalyst than PtNPs/1 alone (Figure 4). In addition, with a mixture of PtNPs/1
and CoNPs/1 (ratio of Pt:Co of 1:1) and a total catalyst amount fixed at 1%, AB hydrolysis was
conducted, and 6 min was needed for complete H generation. This is slower than with 1%
2
Pt Co /1 NPs or even than 1% PtNPs/1 NPs (Figure S14), demonstrating that there is no synergy
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with the physical mixture of these CoNPs/1 and PdNPs/1.
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Table 1. Sizes of the cores and efficiencies of various Pt-Co/1 NPs catalysts at 20 ± 1°C in the
presence or absence of 0.3 M NaOH.
[
b]
TOF (mol ∙
TOF in the presence of
H2
Catalyst[a]
Average size (nm)
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molcat−1∙ min−1)
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163.6
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Pt Co /1
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Pt Co /1
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Pt Co /1
138.5
8.8
200
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CoNPs/1[c]
18.8
[
a] 1.0 mol % Pt-Co/1 NPs have been used in the hydrolysis of AB catalyzed at 20 ± 1°C with an excess (10/1)
of NaBH ; [b] TOF = mol released / (molcatalyst × reaction time(min)) for all atoms (see also Table S1 for TOF
4
H2
by surface atoms); [c] experiment results from a previous study.7
Figure 4. (a) Hydrogen evolution and (b) TOF comparison for NH BH hydrolysis catalyzed by 1
3
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mol% of various Pt-Co/1 nanocatalyst.
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hydrolysis with NaOH have been tested earlier. It was found that NaOH had a positive influence
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on the AB hydrolysis catalyzed by CoNPs/1, but a negative influence on PtNPs/1. So AB
hydrolysis experiments catalyzed by the series of Pt-Co/1 NPs with NaOH were conducted
(Figure 5), and the influence of various NaOH quantities (0.1 M to 0.4 M) was further
investigated. The results show that the reaction rates were boosted, higher TOF values being
obtained for Pt-Co/1 NPs in the presence of NaOH. The best TOF value of 257.1 mol ·molcatal−1·
H2
−
1
min was reached by Pt Co /1 with 0.3 M NaOH (Table S2 and Figure S15), and the TOF value
1
1
decreased with higher NaOH concentration such as 0.4 M.
Figure 5. (a) Hydrogen evolution and (b) TOF comparison for NH BH hydrolysis catalyzed by 1
3
3
mol% of various Pt-Co/1 nanocatalyst in the presence of 0.3 M NaOH.
−
1
−1
The TOF value of PtNPs/1 decreased from 120 to 78.3 mol ·mol
·min after addition of 0.3
H2
catal
M NaOH, but the presence of NaOH had a positive influence on the Pt-Co/1 NPs series.
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Remarkably all the Pt-Co/1 NPs in the various Co/Pt proportions tested present very high
synergistic effects between these two metals with 0.3 M NaOH. The synergy culminates for equal
amounts of Co and Pt in the alloy, as in the absence of NaOH, but it is much higher with NaOH
than in its absence in all proportions of Co/Pt( Figure 6).
0
1
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9
0
1
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0
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0
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9
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1
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6
7
8
9
0
Figure 6. TOF comparison of NH BH hydrolysis catalyzed by 1 mol% of various Pt-Co/1
3
3
nanocatalyst with and without 0.3 M NaOH.
High selectivity of Pt Co /1 compared to other bi-metallic/1 NPs.
1
1
A series of “click” dendrimer-supported bimetallic NPs of the late transition metals, Rh Co /1,
1
1
Ru Co /1, Pt Ni /1 and Pt Cu /1 were synthesized using the same method as that used for the
1
1
1
1
1
1
syntheses of the nanocatalysts Pt Co /1. TEM images have been recorded for these bimetallic NPs
1
1
(Figure S8-11). The reactions of AB hydrolysis catalyzed by these bimetallic NPs/1 (1 mol%)
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with and without 0.3 M NaOH have been conducted at 20 ± 1°C (Table 2, see kinetics on Figure
7
).
Furthermore, the NPs Pt Co stabilized by the first generation dendrimer 2 was synthesized
1
1
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
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0
1
2
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9
0
1
2
3
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9
0
1
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9
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1
2
3
4
5
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7
8
9
0
1
2
3
4
5
6
7
8
9
0
(
Figure S12), and the catalytic performances of the Pt Co NPs stabilized by two dendrimers
1 1
were compared for AB hydrolysis. The Pt Co NPs stabilized by dendrimer 1 of generation 0
1
1
showed slightly better results than those obtained using dendrimer 2 of first generation. The latter
−
1
−1
8b
led to a TOF value of 100 mol ·mol
·min . The dendrimer generation effect was found to
H2
catal
be weak, probably due to the interdendritic and/or NP stabilization.
Table 2. Sizes of the cores and efficiencies of the bimetallic NPs/1 catalysts with and without 0.3
M NaOH at 20 ± 1°C.
[b]
TOF (mol ∙
H2
TOF with 0.3M
Catalyst[a]
Average size (nm)
molcat−1∙min−1)
NaOH
Pt Co /1
2.0
1.8
2.1
1.8
1.8
2.0
163.6
94.7
33.3
94.7
12
257.1
200
1
1
Rh Co /1
1
1
Ru Co /1
64.3
1
1
Pt Ni /1
133.3
17.6
1
1
Pt Cu /1
1
1
Pt Co /2
100
163.6
1
1
[
a] 1.0 mol % bimetallic NPs were utilized in the catalysis of AB hydrolysis with excess of NaBH (10/1) at 20
4
± 1°C; [b] TOF = molH2 released / (molcatalyst × reaction time(min)) for all atoms (see also Table S3 for TOF
related to surface atoms).
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The comparison showed that Pt Co /1 exhibited the best efficiency among these bimetallic NP
1
1
catalysts, followed by the nanocatalysts Rh Co /1 and Pt Ni /1. The same TOF value of 94.7
1
1
1
1
0
1
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9
0
−1
−1
mol ·mol
·min has been obtained for the latter two catalysts. The TOF value of PtNPs/1
H2
catal
−
1
−1
was 120 mol ·mol
·min , which indicated that there was no synergy between Pt and Ni. A
H2
catal
similar conclusion was reached for Rh and Co, because the TOF value of RhNPs/1 was 120
−1
−1 7
mol ·mol
·min . All the reactions of hydrolysis of AB catalyzed by these bimetallic NPs
H2
catal
were accelerated with 0.3 M NaOH, as shown by comparison with the results obtained without
NaOH (Figure S16-17).
Figure 7. Comparison of the evolution of H upon reaction between NH BH and H O catalyzed
2
3
3
2
by 1 mol% bimetallic NPs/1 (a) without NaOH and (b) in the presence of 0.3 M NaOH.
The durability of the best catalyst Pt Co /1 has been checked because of its importance for
1
1
practical applications (Figure 8). Another AB equivalent (1 mmol) was injected into the flask after
the end of a reaction, and the second round was started. The AB hydrolysis reactions have been
th
repeated in this way several times. The results showed that up to the 5 cycle the Pt Co /1
1
1
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th
nanocatalyst still presented a good catalytic property for AB hydrolysis. After the 5 catalyst
recycling the size of the catalyst has now been re-measured by TEM indicating size increase
(Figure S13). It is believed that NP size increase and agglomeration and concomitant activity
1
1
1
1
1
1
1
1
1
1
2
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0
1
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1
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4
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8
9
0
th
decrease in the 5 recycling shown in Figure 8 are due to the accumulation of the sodium and
ammonium products of the reaction adsorbed on the NP surface.
Figure 8. Plots of the volume of hydrogen vs. time for AB hydrolysis catalyzed by 1 mol%
Pt Co /1 during the tests of reusability.
1
1
Mechanistic studies for the AB hydrolysis reaction catalyzed by Pt Co /1
1
1
The determination of the second Damköhler number (Da = 0.02), much lower than unity,
II
allowed ruling out mass-transfer-induced diffusion control (see S.I.). The slope of the logarithmic
plot of hydrogen generation vs. Pt Co /1 concentration is 1.16 (Figure 9), which shows that the
1
1
reaction is first order in concentration of catalyst.
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Figure 9. Plots of the times of the catalyzed AB hydrolytic dehydrogenation catalyzed by
Pt Co /1 nanocatalyst with various catalyst amounts (a), and plots of the rates of H generation vs.
1
1
2
concentration of the nanocatalyst Pt Co /1 both in natural logarithmic scale (b).
1
1
The AB hydrolysis reaction is zero order in concentration of AB (the slope of the logarithmic plot
of hydrogen generation vs. AB concentration is a nearly horizontal line; slope: 0.09 in Figure 10),
which rules out activation of AB alone in the rate-determining step.
Figure 10. Plots of the volume of hydrogen generated vs. time for the hydrolysis of AB catalyzed
by 1 mol% of Pt Co /1 nanocatalyst (a), and plots of the H generation rate vs. AB concentration
1
1
2
both in natural logarithmic scale (b).
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The time dependence measurement of hydrogen generation at various temperatures has led to the
-1
determination of a low activation energy (E = 28.8 kJ∙mol for Pt Co /1; Figure 11 and
a
1
1
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8
9
0
calculation in the SI).
Figure 11. Plots of the hydrogen volume vs. time for AB hydrolysis catalyzed at various
temperatures by 1 mol% of Pt Co /1 catalyst (a), and kinetic data obtained from the Arrhenius
1
1
plots (b).
The isotopic experiment using D O instead of H O was conducted for the Pt Co /1-catalyzed AB
2
2
1
1
hydrolysis, and the kinetic results are presented in Figure 12, showing a kinetic isotope effect
KIE) of 2.46. The kinetic studies, and in particular the latter experiment, suggests that water O-H
bond is cleaved in the rate-determining step.
(
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Figure 12. H evolution upon NH BH hydrolysis with H O and D O catalyzed by 1 mol% of the
2
3
3
2
2
nanocatalyst Pt Co /1 (KIE = 2.46).
1
1
In conclusion the results obtained here show a strong and remarkable bimetallic synergy between
cobalt and platinum optimized in the Co Pt alloy, in particular with 0.3 M NaOH. This synergy
1
1
also involves the “click” dendrimer supports, in particular the intradendritic 1,2,3-triazole ligands
that have proven to be essential.9g, 11c, 16 Previously the coordination of various transition-metal
NPs to the triazole ligands of the same dendrimer 1 near the dendrimer periphery has been shown
to be highly beneficial for NP catalysis. This has been demonstrated in very efficient catalysis by
PdNP/1 with ppm Pd amounts for various carbon-carbon cross coupling reactions (Miyaura-
Suzuki, Sonogashira, and Heck),11c, 16b-d by CuNP/1 for “click” reactions,9g, 16e and by a variety of
transition metal NPs/1 for nitrophenol reduction.16f-g Altogether these “click” dendrimer 1 and 2,
i.e. containing the intradendritic 1,2,3-triazole ligands, have been shown to boost catalysis of a
various reactions by molecular, ionic and NP catalysts. The electronic donicity of the triazole
ligand onto the NP surfaces is responsible for the superior catalytic activity of these NP
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surfaces,16e in particular here for the difficult oxidative addition in the rate-determining step of a
O-H bond of water. Note that the fully stable “click” dendrimers 1 and 2 are also used in catalytic
amounts and recovered unchanged in the aqueous phase at the end of the reactions. The present
results are among the very best obtained compared to results on related bimetallic nanocatalysts in
the literature as shown in the comparative Table S4.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
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3
3
3
3
3
3
4
4
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4
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0
The practical usages of hydrogen produced in the AB hydrolysis reaction are not only for
hydrogen storage, but also for its direct use in situ, for tandem reactions, and for mechanistic
studies of the AB hydrolysis reaction. Examples of the latter applications are shown here. Tandem
reactions were carried out for the hydrogenation of styrene with hydrogen generated from AB
hydrolysis with D O. In a sealed system consisting in two communicating chambers (Figure S18),
2
the hydrogen was produced from AB hydrolysis with D O, the reaction being catalyzed by
2
1
7
Pt Co /1 in the left tube. This hydrogen is used for the hydrogenation of styrene catalyzed by
1
1
1
8
Ni Pt@ZIF-8 in the right tube (Figure 13, see details in the Supporting Information). The
2
1
reaction mixture of styrene hydrogenation was stirred at 50 °C for 12 h, then the H NMR and GC
1
mass spectra of the hydrogenation product were checked. The integration of the H NMR peak
1
around 1.2 and 2.6 ppm showed a reduced intensity compared to that of ethylbenzene in its H
NMR spectrum (Figure S19). Three peaks around 106, 107 and 108 were found the in mass
spectrum (Figure S20), showing that there were three hydrogenation products of styrene, which
contained respectively 0, 1 and 2 D atoms in the ethyl substituent.
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Figure 13. Tandem reaction for hydrogenation with “HD” generated from AB hydrolysis
catalyzed by Pt Co /1. On the arrow, “HD” represents a mixture of H , HD and D (see text).
1
1
2
2
Several mechanisms have been proposed for the late transition metal NP-catalyzed AB
hydrolysis. Xu et al proposed an attack of water onto an activated complex provoking cleavage of
3
the B-N bond and hydrolysis of NH generating hydrogen. Jagirdar’s group proposed attack of
3
-
19
water on a transient M-H bond by analogy with the hydrolysis of BH . Na and Ma proposed a
4
- 20
-
+
similar mechanism related to the hydrolysis of BH . Fu suggested of BH OH NH formation
4
3
4
2
1
2
18
and attack of water to generate hydrogen. Chen and Fu suggested O-H bond cleavage in the
water molecule in the rate-determining step.
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A mechanistic proposal based on our experimental results follows. Hydrogen bonding
[
H NBH H]…H-OH resulting from the hydridic property of the B-H bond is suggested here to
3 2
play a key role. A hydridic BH is proposed to transfer its H atom to the NP surface, which
formally corresponds to an oxidative addition of this B-H bond onto the surface. Oxidative
1
1
1
1
1
1
1
1
1
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2
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0
addition of the water O-H bond proceeds in the rate-determining step, as essentially indicated
-
from the large KIE (k /k = 2.46) with D O. The favorable OH effect for bimetallic NPs/1 is in
H
D
2
-
accord with coordination of OH onto the NP surface, which makes the NP more electron-rich and
-
beneficial to the oxidative addition of H O. On the contrary, OH has a negative effect on AB
2
hydrolysis catalyzed by PtNPs/1. The reason probably is that H-bonding between H O and AB is
2
sufficient to enhance O-H oxidative addition by the most electron-rich PtNP surface. Accordingly
there is no need at this point to further enrich the electron density of the monometallic PtNP
-
-
surface by OH coordination. Furthermore, the added OH would partly occupy the surface active
sites of PtNPs, inhibiting the oxidative addition of the O-H bond of water, thereby decelerating the
reaction. In this step, synergy between a Pt atom and a Co is likely to operate. The more electron-
rich Pt NP atoms would most probably form both Pt-H bonds due to the high strength of the Pt-H
bonds, while the less electron-rich Co atom would bind the electron-rich OH group to form a
Co-OH bond. The di-hydride-NP species would then easily form H by reductive elimination as is
2
2
2
well known in transition-metal organometallic chemistry and catalysis. The tandem reaction with
D O showing deuteration of the ethylbenzene products confirms H/D exchange subsequently to
2
O-D cleavage and H/D scrambling on the NP surface before reductive elimination. Given the KIE
-
value and the favorable effect of OH on the reaction rate, it is not possible that protonation of AB
by water (heterolytic H O or D O cleavage) proceeds before the rate-limiting step. A tentative
2
2
reaction mechanism is represented in Scheme 2. Reductive elimination of H is accompanied by
2
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reductive elimination of the two fragments NH BH - and OH- to form the hydroxy intermediate
3
2
NH BH OH. This first overall step (a) thus corresponds to the substitution in ammonia borane of
3
2
an H atom by OH on boron. Each of the two subsequent H evolution steps (b) and (c) are
2
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proposed to proceed by the same mechanism leading to adsorbed B(OH) NH that upon
3
3
+
-
hydrolysis produces NH B(OH) (d).
4
4
Scheme 2. Suggested mechanism for the AB hydrolysis reaction catalyzed by Pt Co /1.
1
1
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CONCLUDING REMARKS
In conclusion, two generations of “click” dendrimers stabilized various bimetallic NPs that
catalyze AB hydrolysis producing 3 mol hydrogen. The reaction order in AB is zero, and it is one
in NP catalyst. A remarkably highly positive synergistic effect is discovered for the nanocatalysts
Pt-Co/1 containing various proportions of Co and Pt NPs in the nanoalloys, so that the presence of
even down to 25% Pt in the nanocatalyst Pt-Co/1 produces a catalyst that is more efficient than
PtNP/1. NaOH increases the reaction rate, except for PtNPs. The maximum synergistic effect is
disclosed both with and without 0.3 M NaOH with nanoalloys containing equal amounts of Co
and Pt. This dramatic synergy between Pt and Co is highly selective compared to other bimetallic
NPs stabilized by the dendrimer 1. With other supports these PtCo nanoalloys are on the contrary
less efficient. Thus the synergy also involves the dendrimer, in particular its intradendritic 1,2,3-
triazole ligands that activate the NP surface. The TOF value reached with Pt Co /1 is 476.2
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-1
-1
-1
-1
mol ·mol
·min (952.4 mol ·mol ·min ) for the production of 3 mol hydrogen completed
H2 Pt
H2
catal.
in 70 s at 20°C (Table S1), one of the best results ever recorded. The kinetic isotope effect (k /k
H
D
= 2.46) with D O indicates that the cleavage of the water O-H bond proceeds in the rate-
2
determining step facilitated by hydrogen bonding between a B-H bond of AB and a H atom of
water. This mechanism is suggested to also involve another oxidative addition, that of a hydridic
B-H bond of AB. This is followed by reductive eliminations of both H and NH BH OH and two
2
3
2
+
-
analogous overall steps before hydrolysis to NH B(OH) . The tandem reaction with D O that
4
4
2
involves AB hydrolysis-styrene hydrogenation producing deuterated ethylbenzenes confirms O-D
cleavage in the rate-limiting step and H/D scrambling on the NP surface before reductive
elimination of HD, H , and D .
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ASSOCIATED CONTENT
Supporting Information
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The Supporting Information is available free of charge on the ACS Publications website at DOI:
XXX. General data, experimental details, characterizations and kinetics of AB hydrolysis.
AUTHOR INFORMATION
Corresponding Author
didier.astruc@u-bordeaux.fr
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
Financial support from the China Scholarship Council (CSC) of the People’s Republic of China
(grants to Q.W. and F.F., the Centre National de la Recherche Scientifique (CNRS), the
University of Bordeaux and CIC biomaGUNE at San Sebastian are gratefully acknowledged.
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