Ultrahigh Catalytic Activity of l-Proline-Functionalized Rh Nanoparticles for Methanolysis of Ammonia Borane

Article abstract of DOI:10.1002/cssc.201802157

The synthesis of ultrafine and well-distributed rhodium nanoparticles (NPs) with high efficiency toward methanolysis of ammonia borane (AB) is crucially important but challenging. A facile approach has been developed for synthesizing ultrafine and uniform Rh NPs deposited on carbon by using the small soluble organic molecule (SOM) of l-proline (PRO) as capping agent (Rh-PRO/C). The enrichment of N,O-coordination sites for the metal precursor by using PRO was found to be the key to the synthesis Rh-PRO/C. The as-prepared Rh-PRO/C showed high catalytic activity for ammonia borane methanolysis with the highest total turnover frequency (TOF) of 1035 mol_H2 (mol_Rh min)^?1 under basic conditions, which was three times higher than that of the state-of-the-art Rh-based catalysts. The excellent catalytic performance of Rh-PRO/C was ascribed to the well-dispersed Rh NPs and the PRO-functionalized metal surface, which can provide more active sites for the reaction. The merit of size-controlled synthesis combined with metal NP surface modification by SOMs is likely to be beneficial in various catalytic fields.

Full text of DOI:10.1002/cssc.201802157

Accepted Article
Title: Ultrahigh Catalytic Activity of L-Proline-Functionalized Rh
Nanoparticles for Methanolysis of Ammonia Borane
Authors: Yi Wang, Wenxiu Luo, Wei Cheng, Min Hu, Qi Wang, Xia
Cheng, Yun Zhang, Daojiang Gao, Jian Bi, and Guangyin Fan
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ChemSusChem
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Ultrahigh Catalytic Activity of L-Proline-Functionalized Rh
Nanoparticles for Methanolysis of Ammonia Borane
Wenxiu Luo,[a] Wei Cheng,[a] Min Hu, Qi Wang,[a] Xia Cheng,[a] Yun Zhang,[a] Yi Wang*,[a] Daojiang
Gao,[a] Jian Bi,[a] and Guangyin Fan*[a]
Abstract: The synthesis of ultrafine and well-distributed rhodium
nanoparticles (NPs) with high efficiency toward methanolysis of
ammonia borane (AB) is crucially important but challenging.
Herein, we developed a facile approach for synthesizing ultrafine
and uniform Rh NPs deposited on carbon by using the small
soluble organic molecules (SOM) of L-proline (PRO) as capping
agent (Rh-PRO/C). The enrichment of N, O-coordination sites
for metal precursor by PRO was found to be the key to the
synthesis of such Rh-PRO/C material. The as-prepared Rh-
PRO/C showed ultrahigh catalytic activity for AB methanolysis
with the highest total turnover frequency (TOF) of 1035 mol H2
(molRh min)-1 under base conditions, which was three times
higher than that of the state-of-the-art Rh-based catalysts. The
excellent catalytic performance of Rh-PRO/C was ascribed to
the well-dispersed Rh NPs and the PRO-functionalized metal
surface, which can provide more active sites for the reaction.
The merit of size-controlled synthesis combined with metal NP
surface modification by SOM is likely to be beneficial in various
catalytic fields.
temperature.[4]
Various catalytic systems consisting of homogeneous and
heterogeneous metal catalysts, such as metal cations,
monometallic nanoparticles (NPs), alloys, and core-shell
catalysts have been studied for catalytic AB methanolysis.[5]
Among them, supported Rh and Ru-based catalysts have been
regarded as the more active ones compared with other metal-
based catalysts because of their well-dispersed catalytic active
sites on the support (Table S1).[6] For instance, Özkar et al.
reported the synthesis of nanosilica-stabilized Rh NPs as an
efficient catalyst for AB methanolysis with a tumover frequency
(TOF) of 168 mol H2 (molRh min)-1.[7] Recently, organic porous
materials including covalent organic frameworks,[8] metal-
organic frameworks,[9] porous organic polymers,[10] and ionic
liquids,[11] were all customarily employed to encapsulate metal
NPs. For example, Xu et al. reported a “soluble” Rh/CC3-R-
homo nanocatalyst, which combined the merits of both homo-
and heterogeneous catalysts, to achieve a high catalytic activity
for AB methanolysis with TOF of 215 mol H2 (molRh min)-1.[12]
Zhou et al. used a homogenous Ru@PCC-2 catalyst to obtain a
high TOF of 304 mol H2 (molRu min)-1 for AB methanolysis.[13] As
described above, the soluble organic molecules (SOM)-
homogenized heterogeneous catalysts exhibited higher
performance than their heterogeneous counterparts, because
SOMs play a vital role to control the size and dispersion of metal
NPs, resulting in the exposure more catalytic active sites in the
reaction solution.[14] Despite the great progresses that have been
achieved, the catalysts stillsuffer from relatively low
dehydrogenation rates for AB methanolysis compared with the
AB hydrolysis (the highest TOF of 2010 mol H2 (molRh min)-1)[15]
and complicated synthetic procedures. Thus, the development of
simple and accessible catalysts with excellent catalytic
performance for AB methanolysis is still a pending issue.
Introduction
The development of secure hydrogen-storage materials and
efficient ways for H2 evolution have attracted considerable
attention due to their wide applications in the hydrogen economy,
power density and limited environmental impact.[1] Ammonia
borane (NH3BH3, AB) is considered as a leading candidate for
chemical hydrogen storage materials because of its high
gravimetric hydrogen capacity of 19.6 wt%, high solubility, and
long-term stability in solution at room temperature.[2] When
proper catalysts are used, three equivalent H2 per mole of AB
can be evolved through hydrolysis or methanolysis under
ambient conditions (Eq. 1).[3] Notably, the catalytic AB
methanolysis is of profound interest compared with AB
hydrolysis because of the following advantages: 1) higher
solubility and stability of AB in methanol under ambient
conditions; 2) release of high purity of hydrogen without
poisonous NH3 gas; 3) possible applications at low temperatures
(≤0 °C) by suitable catalysts; and 4) efficient regeneration of by-
product with LiAlH4 in the presence of NH4Cl at room
Herein, we demonstrated an efficient strategy of L-
proline(PRO)-functionalized Rh NPs with highly catalytic activity
toward AB methanolysis. PRO was employed as size controller
for synthesis of ultrasmall and uniform Rh NPs for the first time.
The presence of strong anchoring groups of PRO with
intrinsically enriching N, O-coordination sites would facilitate the
binding of metal ions. The formation of nucleation sites around
PRO can confine the overgrowth of Rh NPs and consequently
control their particle size.[16] To the best of our knowledge, no
studies have reported on the use of PRO coordination effect to
tailor the size and dispersion of metal NPs. Furthermore, the
highly soluble PRO can not only homogenize the catalyst in
solution, but also functionalize metal NPs as a ligand (we
[a]
[*]
W. Luo, W. Cheng, M. Hu, Q. Wang, Dr. Y. Zhang, Prof. Y. Wang,
Prof. D. Gao, Prof. J. Bi and Prof. G. Fan
College of Chemistry and Materials Science, Sichuan Normal
University
Chengdu 610068, China
Corresponding author: wangyi@sicnu.edu.cn;
Fanguangyin@sicnu.edu.cn
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10.1002/cssc.201802157
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named this as bound ligand). The as-synthesized ultrafine Rh
NPs were immobilized and stabilized by carbon subsequently.
Combined with ultrasmall size and bound-ligand effect, the
resulting Rh-PRO/C catalyst dramatically facilitated the catalytic
performance for AB methanolysis.
replacement with N-M bond formation.[17] At the same time, the
distinct signals of 1626 cm-1 in IR spectroscopy were present in
Rh3+-PRO, typical for O-C=O bond of the PRO. For Rh-PRO/C,
the observation of characteristic peaks confirmed the presence
of PRO in Rh-PRO/C. The interaction between PRO and Rh3+
suppressed the motion of reduced Rh atoms when Rh3+-PRO
was exposed to NaBH4; hence, Rh NPs were restricted within
the PRO layer, and part of the coordinated PRO ligands was
preserved on the surface of NPs. Furthermore, because N in
PRO contains lone pair electrons, it can still interact with the NP
surface atoms, thereby modifying he catalytic active sites of the
metal particles.[12] The bonding mode of Rh NPs functionalized
with PRO condition was well consistent with the results of
Kunz.[18] The UV-vis spectrum of Rh3+ exhibited an absorption
band at 518 nm. When NaBH4 was added to the solution, the
Rh3+ absorption band disappeared, indicating the successful
reduction of Rh3+ to Rh0 (Figure 1b).[19]
Result and Discussion
Scheme 1. The schematic illustration for preparing of Rh-PRO/C catalyst.
The synthesis of Rh-PRO/C catalyst is illustrated in Scheme 1.
Typically, PRO was dissolved into methanol to form a colorless
solution. When rhodium chloride was introduced, the color of the
solution turned yellow, which was obviously different from the
color of rhodium chloride methanol solution (Figure S1a-c) and
indicated the coordination between PRO and Rh3+ ions. After the
mixture was reduced by sodium borohydride (NaBH4), the color
of the solution immediately changed into dark brown without any
precipitates, indicating the efficient Rh3+ reduction and well
dispersion of the resulting PRO-Rh (Figure S2b). Subsequently,
carbon acting as support was added into the mixture to
immobilize the Rh NPs, and the Rh-PRO/C catalyst with 2.5 wt%
Rh loading was obtained finally.
The PRO molecule enriching N, O-coordination sites can
coordinate with Rh3+ precursors, which was confirmed by
Pneumatikakis.[17] We further investigated the coordination
between PRO and Rh3+ using 1H NMR and IR spectroscopic
studies. The strength of 1H signals of free PRO decreased
obviously upon the addition of Rh3+, indicating the successful
coordination of PRO and Rh3+ (Figures S3 and S4). The good
solubility and suitable coordination capacity of PRO with Rh3+
ensured the promising template for synthesis of well-dispersed
Rh NPs with ultrasmall sizes. The IR spectra were investigated
to further verify the successful coordination of PRO and Rh3+
Figure 1. (a) IR spectra of (I) PRO, (II) Rh3+-PRO and (III) Rh-PRO/C,
respectively. (b) The UV-vis absorption spectra of (I) Rh3+-PRO and (II) Rh-
PRO. (c) XPS survey data of Rh-PRO/C and Rh/C. (d-f) XPS spectra of N1s
,
O1s and Rh3d in Rh-PRO/C, respectively.
The X-ray photoelectron spectroscopy (XPS) survey scans of
the Rh-PRO/C and Rh/Care shown in Figure 1c. Both the
samples showed the presence of Rh and C elements on the
carbon surface, indicating the successful anchoring of Rh.
Compared with Rh/C, additional XPS signal of N element was
observed in Rh-PRO/C, verifying the presence of PRO on the
carbon surface. The high-resolution XPS spectra of N, O,and Rh
in Rh-PRO/C were recorded (Figure 1d-f). The binding energies
at 399.80 and 400.10 eV corresponded to the NH and zwitterion
(Figure 1a). The IR spectrum of PRO showed
a broad
absorption peak in the range of 3761-3245 cm-1 and
characteristic peaks at 2974 cm-1, which were assigned to the
stretching vibrations of O-H and N-H in the amide. When Rh3+
ion was chelated by the amino groups of PRO,
a new
characteristic peak at 1044 cm-1 appeared, which corresponded
to the formation of Rh3+-PRO complex. The lack of NH vibrations
in the IR spectrum was potentially assigned to proton
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+
structures of the NH2 species in PRO, respectively, which is
consistent with the results of the XPS spectra of PRO.[17] The
O1s was fitted by two states at bonding energies of 532.3 and
534 eV, which can be ascribed to the C-O and O=C-O of PRO,
respectively. The clear existence of Rh3+ (309.5 eV) in Rh-
PRO/C, likely in the form of oxide, can be attributed to the
ultrasmall size of Rh NPs, which were easily oxidized upon
exposure to air at ambient temperature. In addition, RhOx might
originate from the surface oxidation of Rh (0) NPs during XPS
sampling.[20] Given the presence of oxygen-containing groups in
PRO, the presence of Rh-O component can be inevitable on the
Rh NP surfaces, which further confirmed the Rh NPs surface-
functionalized by PRO. Powder X-ray diffraction (XRD) was
carried out to confirm that the PRO effectively controlled the Rh
NPs into small sizes and immobilized by carbon (Figure S5). All
the XRD patterns of the samples showed two broad peaks
located at 2θ = 25° and 43°, which are almost consistent with
the peak shape of the carbon support.[21] No visible diffraction
peaks of Rh were observed, which was probably attributed to
the ultrasmall size of Rh NPs and the low loading of Rh in PRO-
Rh/C. The XRD results were well consistent with the
transmission electron microscopy (TEM) results.
The morphology and particle size of Rh NPs for Rh-PRO/C,
Rh-PRO, and Rh/C samples were characterized by TEM. As
shown in Figure 2a and 2b, ultrasmall and well-dispersed Rh
NPs with a mean diameter of 1.78 nm were achieved in Rh-PRO.
For Rh-PRO/C, the Rh NPs were homogeneously distributed on
carbon matrix without the changes of morphology and particle
size (Figure 2c, d). The Rh NPs were single crystals and faceted,
as confirmed by the well-defined lattice fringes with a spacing of
0.25 nm. A slight aggregation and larger size of Rh NPs (2.35
nm) were observed without PRO for the Rh/C sample (Figure 2e,
f). TEM results provided compelling evidence that the PRO
played a vital role in controlling the particle size and dispersion
of Rh NPs, serving as an efficient size controller. The ultrafine
Rh NPs combined with nearly homogeneous dispersion exposed
abundant active surface atoms for catalytic applications.
Figure 3. H2 generation rate of AB methanolysis catalyzed by: (a) Rh-
PRO/Cand Rh/C, respectively. (b) Rh-PRO/C and Rh-PRO/C with 0.3
M
NaOH, respectively. (c) Stability test of the Rh-PRO/C catalyst for AB
methanolysis. (d) TOF values and stability tests for AB methanolysis catalyzed
by Rh-PRO/C, Rh-PRO and Rh-PVP/C, respectively.
The catalytic performance of the prepared catalysts for AB
methanolysis was evaluated at 25 °C and atmospheric pressure.
The AB methanolysis reaction was started when AB solution
(1.0 M, 1.0 mL) was added into the methanol solution (5.0 mL) in
the presence of a catalyst. The hydrogen generated was
measured by recording the drained water. Before starting the
activity test of Rh-PRO/C, the catalytic activity of pure PRO,
carbon, and Rh NPs was tested for AB methanolysis to generate
hydrogen. Pure PRO and carbon as catalysts were incapable of
catalyzing the AB methanolysis, and no hydrogen generation
was observed (Figure S7a, b). Pure Rh NPs can completely
catalyze AB methanolysis to generate hydrogen with a poor TOF
of 64.9 mol H2 (molRh min)-1 because of the serious Rh
aggregations without PRO and carbon support (see Figures S2a
and S7c). The Rh/C catalyst with Rh loading of 2.5 wt% showed
catalytic activity with a TOF of 273 mol H2 (molRh min)-1.
Surprisingly, in the presence of PRO, the Rh-PRO/C with the
same Rh loading (2.5 wt%) provided an efficient performance for
hydrogen release, giving the initial turnover frequency of 410.8
mol H2 (molRh min)-1 (Figure 3a). The catalytic activity was
Figure 2. (a and b) TEM images and corresponding size distribution of Rh
NPs in Rh-PRO. (c and d) HRTEM images and corresponding size distribution
of Rh NPs in Rh-PRO/C. (e and f) TEM images and corresponding size
distribution of Rh NPs in Rh/C.
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considerably higher than the previously reported performance
achieved.[12],[13a] The presence of OH- has been reported to
promote the rate-determining step of O-H bond cleavage in H2O
and activate AB molecule in the hydrolysis of AB to H2.[22]
Therefore, the promoted effect of NaOH on the catalytic activity
of Rh-PRO/C for methanolysis of AB was investigated in detail.
A certain amount of sodium hydroxide (NaOH) was added to the
activity (see Figure 3d and Table S2, entries 8-11). Leaching
tests showed that no noticeable catalyst loss was observed
during the stability process. Thus, after the completion of the
methanolysis reaction, the Rh-PRO/C was isolated for further
characterizations. Notably, the XRD pattern of the recovered Rh-
PRO/C displayed no obvious peaks of Rh or RhOx species due
to the small sizes of Rh NPs, and no change was observed as
reactor stirred for 20 min and then subjected to AB methanolysis. compared with the fresh one (FigureS5). The binding energies at
307.8, 312.5, 309.0 and 313.7 eV were ascribed to Rh0 3d5/2
The H2 generation rates increased with the increase of NaOH
concentrations in the range of 0-0.3 M but decreased with the
further increase of NaOH concentration (0.4 M). Specifically, the
Rh-PRO/C catalytic system with 0.3 M NaOH displayed an
ultrahigh activity toward AB methanolysis, which was completed
within 25 s, giving the TOF of 1035 mol H2 (molRh min)-1, which
was three-fold higher that of AB methanolysis. These results
demonstrated that the catalytic performance of Rh-PRO/C
toward methanolysis of AB in the presence of NaOH (0.3 M)
would be improved significantly (Figure 3b). Control experiment
indicated that no H2 release was observed for AB methanolysis
without Rh-PRO/C in NaOH methanol solution, suggesting that
NaOH acted as a promoter rather than a catalyst toward AB
methanolysis (Figure S7d). These results indicated that optimum
level of NaOH in the reaction would increase the electron
density of the metal NP surfaces and benefit the interaction of
AB molecule.[23] Nevertheless, too much OH- adsorbed on the
metal NP surface can reduce the promotion effect, leading to the
decrease of catalytic activity. In addition, the basic additive can
,
Rh0 3d3/2, Rh3+ 3d5/2, and Rh3+ 3d3/2, respectively. However, the
binding energy of Rh(0) in the Rh-PRO/C shifted negatively by
0.6 eV because of the interaction between metal and PRO
(Figure 4a). The TEM images showed that Rh NPs were
uniformly dispersed on carbon, which is well consistent with the
XRD results. Note that the diameter of Rh NPs exhibited no
obvious increase after the fourth stability test (Figure 4b,c).
These results confirmed that the Rh-PRO/C was not influenced
during the catalysis of AB to produce hydrogen. Thus, the
decrease in the catalytic activity might be attributed to the
increasing viscosity of the solution and deactivation effect of the
-
increasing by-product B(OCH3)4 concentration during the
methanolysis process. The by-products of AB methanolysis can
be adsorbed on the surface of metal NPs and consequently
cover the metal active sites.[26]
decrease the concentration of NH4 (NH4 + OH- → NH3·H2O) in
reaction solution and accelerate the shift of the equilibrium to the
product side.[24] The NH3·H2O is easily soluble in methanol or
+
+
+
water solution and can slightly dissociate into NH4 and OH-. In
our catalytic system, the hydrogen generated was measured by
recording the drained water, and no ammonia was found in the
collected gas. The above results indicated that the negative
+
effect of NH4 can be inhibited by the addition of NaOH, which
neutralized the NH3·H2O in methanol solution. On the other hand,
the negative effect of Cl- can be inhibited by the addition of OH-,
25]
which neutralized the HCl solution.[22,
Consequently, the
successful promotion of the dehydrogenation rate of AB
methanolysis was proved through the observation of NaOH,
which is well with the literature.[23c]
The stability and durability of the catalyst are greatly
significant in practical application. For example, after the first AB
methanolysis catalyzed by Rh-PRO/C in the presence of 0.3 M
NaOH, which gave a TOF value of 1035 mol H2 (molRh min)-1, 1
mmol of AB was then added to the remaining methanol solution
that contained the catalyst for the next reaction. The Rh-PRO/C
catalyst can be reused for four times with 100% conversion of
AB, and three equivalents of hydrogen per mole of AB were
released. The activities of the catalyst after the first run
decreased to 70%, and after the fourth run, it retained 52 % of
the initial catalytic activity (Figure 3c). In parallel, although the
Rh-PRO homogenous catalyst showed efficient catalytic activity
for AB methanolysis with a TOF value as high as 1125 mol H2
(molRh min)-1, the solution of Rh-PRO without carbon became
clear and metal particles were reunited after the first cycle. The
catalytic activity of the second cycle was only 1.2% of the first
catalytic activity. In addition, the durability of the Rh-PVP/C
catalyst was also very poor. For Rh-PVP/C catalyst, the catalytic
activity of the second cycle was only 5.4% of the first catalytic
Figure 4. (a) Stoichiometric hydrogen evolution containing 1 mmol AB at
various Rh concentrations at 298 K. (b) The plot of H2 generation rate versus
the catalyst amount for AB methanolysis. (c) Dehydrogenation rate as
a
function of temperature in AB methanolysis. (d) The plot of H2 generation rate
versus1/T for AB methanolysis.
The plot of hydrogen generated versus time for AB
methanolysis over Rh-PRO/C with different Rh concentrations at
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25°C is shown in Figure 4d. With the increase of initial
concentration of Rh, the hydrogen generation rate of AB
methanolysis was raised. The logarithmic plot of the hydrogen-
evolution rate versus Rh concentration gave a straight line with a
slope of 0.77, indicating the first order with respect to the Rh
concentration for AB methanolysis. The activation energy for the
AB methanolysis calculated by the slope of the fitting line based
on Arrhenius plot was 38.2 kJ/mol, which was notably lower than
that of Rh-based catalysts,[20, 27] indicating the kinetic superiority
of Rh-PRO/C toward AB methanolysis. Interestingly, the catalyst
Rh-PRO/C still had high activity for AB methanolysis even at
0 °C with TOF of 289 mol H2 (molRh min)-1 and excellent
performance for AB ethanolysis in ethanol at 25 °C with TOF of
628 mol H2 (molRh min)-1 (Table S2, entries 12 and 13). Notably,
ethanol is a commonly-used solvent in many fields and less
poisonous than methanol. The prominent catalytic activity of Rh-
PRO/C system at low temperature and in low poisonous solvent
is critically important for practical applications. Carbon disulfide
(CS2) poisoning test revealed that the PRO-Rh/C catalytic AB
methanolysis was heterogeneous, as confirmed by the complete
cessation on addition of CS2.
of their characterizations, the presence of PRO in PRO-Rh/C
catalyst highly promoted the efficiency in AB methanolysis.
Some capping agents have been reported to exhibit strong
affiliation to the metal NP surface, which might block the
exposure of the active atoms, leading to the deactivation of
metal NP catalysts.[28] Very interestingly, in the Rh-PRO/C
catalytic system, PRO as a capping agents, through suitable
coordination with metal precursor,not only restricted the
overgrowth of metal particles, but also modified the metal NP
surface as a bound ligand. Consequently, the ultrasmall and
uniform dispersion of functionalized-Rh NPs were obtained in
methanol solution, leading to the excellent performance in AB
methanolysis.
Figure 6. A plausible mechanism for Rh-PRO/C catalyzed AB methanolysis.
A mechanism was proposed to explain the catalytic activity of
Rh-PRO/C catalysts observed in AB methanolysis (Figure 6).
Under the base conditions, the bonding model of PRO-
functionalized Rh NPs was formed (I). The first step of the
reaction was the formation of activated complex species on the
metal particle surface through the diffusion of AB.[29] The
carboxylic ion of PRO would interact with the hydrogen of the
“BH3” part of AB to form O-H bonds, while the “NH3” part of AB
combines with the Rh surface (II).[3d] Once the intermediate is
formed, the concerted dissociation of the B-N bonding effectively
weakened. Subsequently, the attack of CH3OH molecule to the
Figure 5. (a) Catalytic activities of Rh nanoparticles catalysts modified by
different ligands for the AB methanolysis. (b) The corresponding TOF values
of AB methanolysis over the Rh-CA/C, Rh-PVP/C, Rh-PRO/C, Rh-EG/C, Rh-
TEA/C and Rh-EA/C catalysts, respectively.
To illustrate the important role of PRO ligand in the
methanolysis of AB, a series of experiments was designed
under the same alkali conditions mentioned above. We chose
some representative capping agents, including ethylenediamine
+
weakened B-N bond would easily generate and dissociate NH4
cation (III), which was neutralized with OH- in the reaction
solution (IV). Finally, a H2 molecule couldbe extracted from the
system by further attack of another adjacent CH3OH on the
H3B(OCH3) unit (V). Further systematic studies of the proposed
mechanism are being pursued by our group and will be
discussed in separate reports.
(EA),
triethylamine
(TEA),
ethylene
glycol
(EG),
polyvinylpyrrolidone (PVP), and citric acid (CA), to prepare Rh
NPs with the same Rh loading (2.5 wt%) under identical
conditions (Figure 5). A negative effect was observedwith the
capping agents in dehydrogenation efficiency. Although all the
SOMs have good solubility in methanol solution, Rh-EA/C, Rh-
TEA/C, and Rh-CA/C showed very low catalytic activities toward
AB methanolysis. Rh-PVP/C and Rh-EG/C required a longer
time than Rh-PRO/Cto complete the methanolysis reaction.[28] In
addition, Rh-PRO/C catalyst was isolated from the reaction
solution by centrifugation and reused for AB methanolysis,
Conclusions
We report a facile in situ approach for the synthesis of PRO-
functionalized-Rh NPs for remarkably improving the catalytic
dehydrogenation rate of AB methanolysis. The soluble small
organic molecular (SOM) PRO was simultaneously employed as
size controller and bound ligand for Rh catalyst, which could
supply abundant active surfaces for AB methanolysis. The
substantial alkali condition and OH- groups were beneficial for
the activation of AB molecule in the methanolysis process of AB
to H2. The obtained Rh-PRO/C system has the following merits:
1) PRO enriching N,O-coordination sites was employed for the
synthesis of ultrasmall and uniform Rh NPs. To the best of our
yielding
a
TOF of 512 mol H2 (molRh min)-1, which was
considerably lower than that of in situ Rh-PRO/C catalyst. The
activity degradation over isolated Rh-PRO/C for AB
methanolysis was possibly assigned to the leaching of PRO by
centrifugation and washing because PRO played a vital role in
the catalytic performance as a surface ligand. In addition, the
decrease of catalytic activity can be attributed to the inevitable
loss of catalyst during isolation. By comparing the catalytic
performance of controlled experiments and analyzing the results
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knowledge, no studies have used PRO coordination to tailor the
size of metal NPs. 2) The obtained Rh-PRO/C catalyst showed
ultrahigh performance in catalytic dehydrogenation of AB
methanolysis with TOF of 1035 mol H2 (molRh min)-1 under base
condition, which was three times higher than that of the state-of-
the-art reported. 3) The catalyst Rh-PRO/C still showed
excellent performance for AB ethanolysis in ethanol and high
activity for AB methanolysis even at 0°C, which was also
reported for the first time. The strategy of using SOM size-
control synthesis and functionalized metal NPs will likely open a
new frontier on the design and preparation of metal NP materials
for diverse potential applications.
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This work was financially supported by the Natural Science
Foundation of Sichuan Provincial Department of Education
(17ZA0330), the Opening Project of Key Laboratory of Green
Chemistry of Sichuan Institutes of Higher Education (LZJ1603),
and the National Natural Science Foundation of China
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Keywords: Ammonia borane • L-Proline • Methanolysis • Rh
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10.1002/cssc.201802157
ChemSusChem
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Entry for the Table of Contents
FULL PAPER
W. Luo, Wei Cheng, M. Hu, Q. Wang,
Dr. Y. Zhang, Prof. Y. Wang, Prof. D.
Gao, Prof. J. Bi and Prof. G. Fan
Page No. – Page No.
Ultrahigh Catalytic Activity of L-
Proline-Functionalized
Rh
The L-proline with intrinsically enriching N, O-coordination sites was employed as
size controller and bound ligand for synthesis of ultrasmall and uniform
functionalized Rh NPs for the first time. The ultrafine functionalized Rh NPs were
supported on carbon and showed ultrahigh catalytic activity for the methanolysis of
ammonia borane with a TOF value of 1035 mol H2 (molRh min)-1 under base
conditions, which was three times higher than that of the state-of-the-art reported.
Nanoparticles for Methanolysis of
Ammonia Borane
This article is protected by copyright. All rights reserved.