Highly dispersed surfactant-free nickel nanoparticles and their remarkable catalytic activity in the hydrolysis of ammonia borane for hydrogen generation

Article abstract of DOI:10.1002/anie.201202055

Highly active: Surfactant-free Ni nanocatalysts were highly dispersed into the nanoporous carbon support MSC-30 through a dry process, and the obtained Ni@MSC-30 catalyzed the generation of H₂ from the hydrolysis of ammonia borane at room temperature (see picture). The improvement of the performance of surfactant-free non-noble metal nanocatalysts is a promising step toward the application of NH₃BH₃ as an applicable chemical hydrogen storage material. Copyright

Full text of DOI:10.1002/anie.201202055

Angewandte
Chemie
DOI: 10.1002/anie.201202055
Heterogeneous Catalysis
Highly Dispersed Surfactant-Free Nickel Nanoparticles and Their
Remarkable Catalytic Activity in the Hydrolysis of Ammonia Borane
for Hydrogen Generation**
Pei-Zhou Li, Arshad Aijaz, and Qiang Xu*
Hydrogen has been considered as one of the best alternative
energy carriers to satisfy the increasing demand for a sustain-
synthesis of non-noble surfactant-free Ni NPs through a dry
process and their excellent catalytic activity in hydrolysis of
[
1,2]
able and clean energy supply.
Controlled storage and
NH BH3 for hydrogen generation with a total turnover
3
release of hydrogen are the widely known challenging
technologies on the way towards a fuel-cell-based hydrogen
economy. Owing to its high hydrogen content, high stability at
room temperature, and nontoxicity, the ammonia borane
frequency (TOF, calculated on the basis of the total amount
of Ni) value as high as 30.7 mol of H per mol of Ni per min at
2
room temperature, which is the highest one among all of the
Ni nanocatalysts ever reported for this reaction (Table S1 in
[
5]
(
NH BH ) complex has been identified as one of the most
the Supporting Information). The remarkable improvement
of catalytic performance of Ni NPs on this hydrogen
generation reaction indicated that the strategy of using
a surfactant-free dry process could be an effective approach
for the synthesis of highly active metal nanocatalysts for
heterogeneous catalytic reactions.
3
3
[3–5]
attractive candidates for chemical hydrogen storage.
Catalytic hydrolysis can generate 3 mol of hydrogen per mol
of NH BH3 at room temperature, which presents a high
3
hydrogen capacity up to 9.0 wt% of the starting materials
(
NH BH and H O), thus making itself an effective approach
3 3 2
[
4,5]
for the release of hydrogen stored in NH BH . So far many
Owing to its accessible porosity, large surface area, high
chemical; thermal; and mechanical stability nanoporous
3
3
catalyst systems have been tested for hydrogen generation
[
4,5]
[7]
from hydrolysis of NH BH ,
the highest activity.
among which platinum shows
carbon, Maxsorb MSC-30, was selected as catalyst support.
3
4b]
3
[
However, concerning the element
For the synthesis of Ni NPs through a surfactant-free dry
preparation process, the small volatile molecule nickelocene
abundance and related economic issues, it is clearly a desired
goal to prepare low-cost catalysts with high catalytic activity
for the terminal practical application of this reaction system in
the fuel cell.
([NiCp ], Cp = cyclopentadienyl) was used as the precursor of
2
Ni, and
a
series of MSC-30-supported [NiCp2]
([NiCp ]@MSC-30) samples were synthesized using chemical
2
Ni catalysts, as versatile non-noble metal catalysts, have
attracted much attention owing to their activity in various
catalytic reactions as well as in catalyzing hydrogen gener-
vapor deposition (CVD) through heating the separated
[NiCp ] and MSC-30 in a Schlenk tube in different [NiCp ]/
2
2
[
8,9]
MSC-30 ratios of starting materials.
The samples of
[
5,6]
ation from hydrolysis of NH BH .
Various Ni nanoparti-
Ni@MSC-30 (1x, x = a–d) with different Ni loadings were
obtained by reducing the related above intermediate samples,
[NiCp ]@MSC-30 (2x, the [NiCp ]/MSC-30 weight ratio was
3
3
cles (NPs) have been developed by decomposition of Ni
complexes in the presence of surfactants, which are nearly
unavoidable for preventing the NPs from aggregation in the
2
2
2:1 for 2a, 1.5:1 for 2b, 1:1 for 2c, 0.5:1 for 2d), in a H /Ar
2
[5,6]
À1
solution-phase synthetic methods.
As the catalytic process
flow (50 mol% H , 50 mLmin ) at 3008C for three hours.
2
takes place on the metal surfaces, the presence of a protective
surfactant shell around the metal NPs is unfavorable for
catalytic applications. Therefore the preparation of NPs
without the existence of any protective surfactant signifi-
cantly benefits both academic researches and practical
applications of NP catalysts. Herein, we report the successful
When comparing them with the pristine nanoporous
MSC-30, an appreciable decrease in the amount of N2
adsorption was observed for both the [NiCp ] dispersed
2
MSC-30 and the Ni NPs dispersed into MSC-30. Based on the
N adsorption the saturation deposition of [NiCp ] was found
2
2
to be achieved at the [NiCp ]/MSC-30 ratio of 2:1 (w/w), and
2
a relative increase in the amount of N adsorption happened
2
after reducing [NiCp ] molecules to Ni NPs by hydrogen
2
[
*] P.-Z. Li, Dr. A. Aijaz, Prof. Dr. Q. Xu
National Institute of Advanced Industrial Science and Technology
(Figure 1), thus indicating that [NiCp ] molecules and Ni NPs
2
were successfully dispersed into the host framework of MSC-
(
AIST)
3
0 as in the case of metal NPs loaded to ZIF-8, MIL-101, and
Ikeda, Osaka 563-8577 (Japan)
E-mail: q.xu@aist.go.jp
[10]
other porous materials.
The powder X-ray diffraction
(
PXRD) patterns of the as-prepared samples exhibited that
P.-Z. Li, Prof. Dr. Q. Xu
Graduate School of Engineering, Kobe University
Nada Ku, Kobe, Hyogo 657-8501 (Japan)
0
two small broad peaks around 44.38 and 51.68 owing to Ni
were detected for all of the prepared samples of 1,
[5a,8]
which
further proved the Ni loading in MSC-30. The X-ray photo-
electron spectroscopy (XPS) investigation of 1a show that
well-defined peaks with binding energies of 852.7 and
870.2 eV were detected for the 2p3/2 and 2p1/2 levels, respec-
[
**] We thank JSPS, AIST, and Kobe University for financial support. P.-Z.
Li thanks JSPS for a fellowship (DC).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201202055.
Angew. Chem. Int. Ed. 2012, 51, 6753 –6756
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
6753

.
Angewandte
Communications
presence of surfactants indicates that the surfactant-free dry
process is an effective approach for the synthesis of highly
active metal catalysts.
Then we extended this work to other porous materials.
MCM-41, a commonly used mesoporous silica support, also
with accessible porosity, large surface area, and high stabil-
[
12]
ity,
was selected to compare the influence of different
porous supports on the catalytic activity of the supported
catalysts. The preparation of Ni@MCM-41 (3x) via
[
NiCp ]@MCM-41 (4x) was carried out through the same
2
process as that for Ni@MCS-30, and the same phenomena
were observed based on N adsorption, PXRD, and XPS
2
Figure 1. N adsorption isotherms (77 K) of 1a, 2a, and pristine
nanoporous MSC-30. Curves with filled and empty circles denote the
[8]
2
measurements. The saturation deposition of [NiCp ] was
2
found to be achieved at the [NiCp ]/MCM-41 ratio of starting
2
N adsorption and desorption, respectively. STP=standard temper-
2
materials of 1.5:1 (w/w), and the corresponding Ni@MCM-41
ature and pressure.
(3a) was found to have a Ni loading of 19.5 wt%. The
complete hydrogen release using 3a as catalyst in the same
reaction system as had been used for 1a needed about
0
[8,11]
0
tively, of metallic Ni of the loaded Ni NPs.
Even after Ar
etching for 240 min, the intensity of the Ni species remains
the same, thereby further indicating the highly uniform
dispersion of Ni NPs in the host framework of MSC-30.
The catalytic activities on hydrolysis of aqueous NH BH3
were tested for the prepared samples. Among all the samples
of 1 with different Ni loadings (1a, 18.2 wt%; 1b, 14.9 wt%;
c, 11.7 wt%; 1d, 7.1 wt%, determined by inductively
coupled plasma (ICP) analyses, see Table S2 in the Support-
ing Information), 1a has the highest activity, with which the
reaction can be completed (H /NH BH = 3.0) within 6.5 min
19.5 min, giving a TOF value of 9.3 mol of H per mol of Ni
2
[
8]
per min. Compared with 1a, 3a gives a lower TOF value,
while it is also among the highest values ever reported
(Table S1 in the Supporting Information). Taking into
account that the same conditions for preparation and catalytic
reaction were used, the only explanation should be that the
structures of the selected supports, MSC-30 and MCM-41,
have high influences on the catalyst preparation and therefore
the catalytic results.
3
1
[
8]
The morphologies of MSC-30- and MCM-41-supported Ni
NPs were further characterized by transmission electron
microscopy (TEM). Both bright-field TEM and high-angle
annular dark-field scanning TEM (HAADF-STEM) images
showed that the Ni NPs were highly dispersed into the
frameworks of MSC-30 in 1a; these results were in good
2
3
3
(
Ni/NH BH = 0.016) at room temperature (Figure 2), giving
3 3
a TOF value as high as 30.7 mol of H per mol of Ni per min,
2
agreement with the observations of N adsorption, PXRD,
2
and XPS measurements (Figure 3). The mean diameter of the
dispersed Ni NPs in 1a was (6.3 Æ 1.7) nm. Although the sizes
are a little larger, the catalytic activity is much higher than
that of surfactant-protected Ni NPs, such as the reported
[5e,g]
3
.2 nm Ni NPs;
this difference in catalytic activity should
be due to the much more effective exposed surface area of the
surfactant-free Ni NPs compared to that of surfactant-
[
5]
protected Ni NPs. The TEM and HAADF-STEM images
also showed that the Ni NPs were deposited in MCM-41.
However, different from Ni@MSC-30, large aggregated
nanoparticles were observed in Ni@MCM-41 3a, and the
mean diameters of the dispersed Ni NPs in 3a were even
higher than 9.5 nm, which is much larger than the diameters in
Figure 2. Hydrogen generation by hydrolysis of aqueous NH BH
3
3
(
2 mmol in 2 mL water) in the presence of 1a (
&
, Ni/
NH BH =0.0155), 1b (
*
, Ni/NH BH =0.0127), 1c (~, Ni/
3
3
3
3
NH BH =0.0100), and 1d (!, Ni/NH BH =0.0060) at room temper-
3
3
3
3
ature.
1
a; this difference in diameters corresponds to the observa-
which, notably, is the highest TOF value among all the nickel
tion that the catalytic activity of 3a was much lower than that
of 1a. The TEM results provide the direct evidence that the
Ni NPs have been highly dispersed into the nanoporous
materials, MSC-30 and MCM-41, and the morphologies and
catalytic activities of the dispersed Ni NPs highly depend on
the structures of selected supports.
[5]
catalysts ever reported for this reaction. Moreover, when
comparing it with the reported poly(vinylpyrrolidone) (PVP)-
[5d]
stabilized Ni NPs, the surfactant-free Ni@MCS-30 sample
shows a catalytic activity that is more than seven times higher.
The durability/stability was also tested for 1a by adding
additional equivalents (2 mmol) of NH BH into the reacted
As suggested previously, the catalytic process takes place
on the metal surfaces where activated complex species might
be formed through interactions between the NH BH mole-
3
3
aqueous solution under ambient atmosphere. There was no
significant decrease in catalytic activity even after five runs of
3
3
[8]
[5a]
the hydrolysis reaction for 1a. The remarkable improve-
cule and the metal particle surface, and therefore finding
a method to increase the effective surface area of non-noble
ment compared with the Ni NPs that were prepared in the
6
754
www.angewandte.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2012, 51, 6753 –6756

Angewandte
Chemie
the Ni loadings based on ICP results were 19.5, 14.0, and 7.2 wt%,
respectively.
Hydrolytic dehydrogenation of NH BH :
[
4a,d,8]
The catalytic
3
3
reactions were carried out by mixing the aqueous suspension of
Ni@MSC-30 or Ni@MCM-41 (10 mg) and aqueous NH BH (63.5 mg
3
3
in 2 mL water) in a round bottom flask at room temperature, and
a gas burette was used for monitoring the gas evolution.
Received: March 15, 2012
Published online: May 24, 2012
Keywords: heterogeneous catalysis · hydrogen · nanoparticles ·
.
nickel · supported catalysts
[
1] a) L. Schlapbach, A. Zꢀttel, Nature 2001, 414, 353 – 358; b) Z.
Xiong, C. K. Yong, G. Wu, P. Chen, W. Shaw, A. Karkamkar, T.
Autrey, M. O. Jones, S. R. Johnson, P. P. Edwards, W. I. F. David,
Nat. Mater. 2008, 7, 138 – 141; c) K.-J. Jeon, H. R. Moon, A. M.
Ruminski, B. Jiang, C. Kisielowski, R. Bardhan, J. J. Urban, Nat.
Mater. 2011, 10, 286 – 290.
Figure 3. TEM images of a) 1a and b) 3a, and HAADF-STEM images
of c) 1a and d) 3a reveal that Ni NPs are dispersed into the supports,
MSC-30 and MCM-41, respectively.
[
2] a) W. Grochala, P. P. Edwards, Chem. Rev. 2004, 104, 1283 –
1315; b) J. Graetz, Chem. Soc. Rev. 2009, 38, 73 – 82; c) T. C.
Johnson, D. J. Morris, M. Wills, Chem. Soc. Rev. 2010, 39, 81 – 88;
d) H.-L. Jiang, S. K. Singh, J.-M. Yan, X.-B. Zhang, Q. Xu,
ChemSusChem 2010, 3, 541 – 549; e) U. Sanyal, U. B. Demirci,
B. R. Jagirdar, P. Miele, ChemSusChem 2011, 4, 1731 – 1739.
metal nanocatalysts will be a powerful strategy for developing
efficient and economical catalysts in this catalytic hydrogen
generation system. This work demonstrated that, based on
careful selection of catalyst supports, the strategy of using
a surfactant-free dry process could be an effective approach
for the synthesis of highly active metal nanocatalysts.
[3] a) A. Gutowska, L. Li, Y. Shin, C. M. Wang, X. S. Li, J. C.
Linehan, R. S. Smith, B. D. Kay, B. Schmid, W. Shaw, M.
Gutowski, T. Autrey, Angew. Chem. 2005, 117, 3644 – 3648;
Angew. Chem. Int. Ed. 2005, 44, 3578 – 3582; b) T. He, Z. Xiong,
G. Wu, H. Chu, C. Wu, T. Zhang, P. Chen, Chem. Mater. 2009, 21,
2315 – 2318; c) H. Kim, A. Karkamkar, T. Autrey, P. Chupas, T.
In summary, highly dispersed surfactant-free Ni NPs have
been successfully deposited into porous carbon, Maxsorb
MSC-30, through a dry process, which gives NPs that exhibit
remarkably high catalytic activity for hydrogen generation
from hydrolysis of ammonia borane. Detailed investigations
showed that the structures of the selected supports have high
influences on the morphologies and catalytic results of
deposited surfactant-free metal NPs. This remarkable
improvement of the catalytic performance of the non-noble
metal catalysts demonstrates a promising step towards the
application of ammonia borane as a feasible chemical hydro-
gen storage material in a fuel-cell-based hydrogen economy.
Proffen, J. Am. Chem. Soc. 2009, 131, 13749 – 13755.
[4] a) M. Chandra, Q. Xu, J. Power Sources 2006, 156, 190 – 194;
b) M. Chandra, Q. Xu, J. Power Sources 2007, 168, 135 – 142;
c) J.-M. Yan, X.-B. Zhang, S. Han, H. Shioyama, Q. Xu, Angew.
Chem. 2008, 120, 2319 – 2321; Angew. Chem. Int. Ed. 2008, 47,
2287 – 2289; d) J.-M. Yan, X.-B. Zhang, T. Akita, M. Haruta, Q.
Xu, J. Am. Chem. Soc. 2010, 132, 5326 – 5327.
[
5] a) Q. Xu, M. Chandra, J. Power Sources 2006, 163, 364 – 370;
b) T. Umegaki, J.-M. Yan, X.-B. Zhang, H. Shioyama, N.
Kuriyama, Q. Xu, J. Power Sources 2009, 191, 209 – 216; c) T.
Umegaki, J.-M. Yan, X.-B. Zhang, H. Shioyama, N. Kuriyama, Q.
Xu, Int. J. Hydrogen Energy 2009, 34, 3816 – 3822; d) J.-M. Yan,
X.-B. Zhang, S. Han, H. Shioyama, Q. Xu, Inorg. Chem. 2009, 48,
7389 – 7393; e) ꢁ. Metin, S. ꢁzkar, S. Sun, Nano Res. 2010, 3,
676 – 684; f) C. Y. Cao, C. Q. Chen, W. Li, W. G. Song, W. Cai,
ChemSusChem 2010, 3, 1241 – 1244; g) ꢁ. Metin, V. Mazumder,
S. ꢁzkar, S. Sun, J. Am. Chem. Soc. 2010, 132, 1468 – 1469; h) P.-
Z. Li, K. Aranishi, Q. Xu, Chem. Commun. 2012, 48, 3173 – 3175.
Experimental Section
[
8,9]
Preparation of Ni@MSC-30:
For 1a, the samples of [NiCp2]
[6] a) D. J. C. Yates, W. F. Taylor, J. H. Sinfelt, J. Am. Chem. Soc.
1964, 86, 2996 – 3001; b) J. Park, E. Kang, S. U. Son, H. M. Park,
M. K. Lee, J. Kim, K. W. Kim, H.-J. Noh, J.-H. Park, C. J. Bae, J.-
G. Park, T. Hyeon, Adv. Mater. 2005, 17, 429 – 434; c) N. Mahata,
A. F. Cunha, J. J. M. Orfao, J. L. Figueiredo, Catal. Commun.
2009, 10, 1203 – 1206; d) S. K. Singh, A. K. Singh, K. Aranishi, Q.
Xu, J. Am. Chem. Soc. 2011, 133, 19638 – 19641.
(120 mg) and MSC-30 (60 mg) were placed separately in a Schlenk
tube. After evacuating for 30 min at room temperature, the reaction
tube was closed under static vacuum and heated to 1108C for 24 h,
and then the [NiCp ]@MSC-30 (2a) was obtained. Ni@MSC-30 (1a,
2
1
8.2 wt% Ni loading based on ICP result) was obtained by reducing
À1
2
a in a H /Ar flow (50 mol% H , 50 mLmin ) at 3008C for three
2
2
hours. For 1b, 1c, and 1d the weight ratios of starting materials
[NiCp ]/MSC-30) were 90 mg/60 mg, 60 mg/60 mg, and 30 mg/60 mg,
[7] Maxsorb MSC-30, also known as AX21, is a high-surface-area
super-activated nanoporous carbon in powder shape, now
commercialized by Kansai Coke and Chemical Co. Ltd.
[8] See the Supporting Information.
[9] a) S. Hermes, M. K. Schrçter, R. Schmid, L. Khodeir, M. Muhler,
A. Tissler, R. W. Fischer, R. A. Fischer, Angew. Chem. 2005, 117,
6394 – 6397; Angew. Chem. Int. Ed. 2005, 44, 6237 – 6241;
b) Y. K. Park, S. B. Choi, H. J. Nam, D.-Y. Jung, H. C. Ahn, K.
(
2
respectively, and the Ni loadings based on ICP results were 14.9, 11.7,
and 7.1 wt%, respectively.
[
8,9]
Preparation of Ni@MCM-41:
were prepared in a similar procedure as those of Ni@MSC-30. For 3a,
b, and 3c the weight ratios of starting materials ([NiCp ]/MCM-41)
The samples of Ni@MCM-41
3
2
were 90 mg/60 mg, 60 mg/60 mg, and 30 mg/60 mg, respectively, and
Angew. Chem. Int. Ed. 2012, 51, 6753 –6756
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
6755

.
Angewandte
Communications
Choi, H. Furukawa, J. Kim, Chem. Commun. 2010, 46, 3086 –
088.
10] a) H.-L. Jiang, B. Liu, T. Akita, M. Haruta, H. Sakurai, Q. Xu, J.
Am. Chem. Soc. 2009, 131, 11302 – 11303; b) H.-L. Jiang, T.
Akita, T. Ishida, M. Haruta, Q. Xu, J. Am. Chem. Soc. 2011, 133,
[11] C. D. Wagner, W. N. Riggs, L. E. Davis, J. F. Moulder, Handbook
of X-ray Photoelectron Spectroscopy: A reference Book of
Standard Spectra for Use In X-Ray Photoelectron Spectroscopy
(Eds: G. E. Muilenberg), Physical Electronics Division Perkin
Elmer Corp., Minnesota, 1978, pp. 80 – 81.
3
[
1
304 – 1306; c) X. Gu, Z.-H. Lu, H.-L. Jiang, Q. Xu, J. Am.
[12] C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli, J. S.
Beck, Nature 1992, 359, 710 – 712.
Chem. Soc. 2011, 133, 11822 – 11825.
6
756
www.angewandte.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2012, 51, 6753 –6756