Unique reactivity in Pt/CNT catalyzed hydrolytic dehydrogenation of ammonia borane

Article abstract of DOI:10.1039/c3cc48027e

We demonstrate an unprecedented H₂ generation activity in the hydrolytic dehydrogenation of ammonia borane over acid oxidation- and subsequent high temperature-treated CNT immobilized Pt nanocatalysts to combine the merits of defect-rich and ox

Full text of DOI:10.1039/c3cc48027e

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Unique reactivity in Pt/CNT catalyzed hydrolytic
dehydrogenation of ammonia borane†
Cite this: Chem. Commun., 2014,
50, 2142
Wenyao Chen,a Jian Ji,a Xuezhi Duan,*a Gang Qian,a Ping Li,a Xinggui Zhou,a
De Chenb and Weikang Yuana
Received 18th October 2013,
Accepted 28th December 2013
DOI: 10.1039/c3cc48027e
www.rsc.org/chemcomm
We demonstrate an unprecedented H2 generation activity in the metal NPs.12–14 An attempt is therefore necessary to clarify the nature
hydrolytic dehydrogenation of ammonia borane over acid oxida- of the relationship between the surface chemistry and textural proper-
tion- and subsequent high temperature-treated CNT immobilized ties of carbon supports, Pt particle size and H2 generation activities in
Pt nanocatalysts to combine the merits of defect-rich and oxygen AB hydrolysis over supported Pt nanocatalysts.
group-deficient surfaces and unique textural properties of supports
as well as optimum particle size of Pt.
In this work, the H2 generation activities (TOFH , molH
2
2
molPt minÀ1) in Pt-catalyzed hydrolytic dehydrogenation of
À1
AB will be correlated with the types of carbon supports, the
Hydrogen is an ideal energy carrier for a long-term solution to current defects and the surface concentration of oxygen groups on the
energy problems owing to its high energy-efficiency and non-polluting supports, and the Pt particle size, aiming at a rational design of
nature, and its safe and efficient storage is of great significance to the high-performance Pt nanocatalysts.
fulfill the goal of hydrogen-powered transport systems.1 With a high
Two types of carbon supports, i.e., activated carbon (AC) and
hydrogen capacity of 19.6 wt%, good stability and being non-toxic, close carbon nanotubes (CNTs), were used to immobilize Pt NPs by
ammonia borane (AB, NH3BH3) has recently attracted considerable incipient wetness impregnation for AB hydrolysis, in which the Pt
attention as a hydrogen reservoir.2 Typically, the hydrogen stored in particle size was tuned by changing Pt loading (Table S1 and Fig. S1,
AB can be released by thermal dehydrogenation and hydrolysis. ESI†). Both support effects and size effects of Pt catalysts in principle
Compared to the thermal dehydrogenation, the hydrolysis, have a significant influence on the catalytic activity, and thus
+
À
proceeding by means of NH3BH3 + 2H2O - NH4 + BO2
+
excluding the size effects will make a fair comparison for the support
3H2m, is more attractive because of the mild reaction conditions effects. It can be clearly seen that at the identical Pt particle size,
and high H2 selectivity.3–5 Various catalysts have been tested for CNT supported Pt nanocatalysts exhibit much higher TOFH than
2
AB hydrolysis, among which Pt exhibits the highest activity.2,3
AC supported Pt nanocatalysts (Fig. 1a), indicating a profound
Taking into account the practical applications of this system, support effect. The reasons for this particular behavior could be
many efforts have been made to reduce the cost of supported Pt related to the unique surface chemistry and textural properties
catalysts and achieve high Pt utilization by preparing Pt-based alloy (e.g., the absence of microporosity with elimination of mass-transfer
and core–shell nanoparticles (NPs) as well as highly dispersed Pt limitation and the introduction of curvatures of graphene sheets with
NPs.3,6–11 Compared with oxide supports, carbon supports have the a strong interaction with Pt NPs) of close CNTs with respect to AC.14
controllable arrangement of the carbon atoms in terms of primary Unexpectedly, during the batch AB hydrolysis reaction, both Pt/CNT
and secondary structures, and exhibit unique and tunable surface and Pt/AC nanocatalysts with a low Pt loading of 0.5 wt% show much
chemistry (e.g., defects and oxygen groups) and textural properties lower volume of H2 generation than those with high Pt loadings
(e.g., pore structures and curvature of graphene sheets), which (Fig. 1b and Fig. S2, ESI†). This strongly implies that the difference in
opens up an unprecedented opportunity to tailor the properties the textural properties of the two carbon supports is not the dominant
(e.g., electronic properties and metal particle size) of supported reason, while the difference in the surface chemistry is mainly
responsible for the above support effect.
a State Key Laboratory of Chemical Engineering, East China University of Science
and Technology, 130 Meilong Road, Shanghai 200237, China.
E-mail: xzduan@ecust.edu.cn; Fax: +86-21-64253528
To explore the difference between the surface chemistry of
CNTs and that of AC, Raman, TG and XPS techniques are
employed, and the results are provided in Fig. 2 and Table S2
(ESI†). Obviously, CNTs have lower ID/IG and thus less disorders
(defects) as well as lower surface concentration of oxygen
groups than AC, in which the oxygen groups on the surfaces
b Department of Chemical Engineering, Norwegian University of Science and
Technology, Trondheim 7491, Norway
† Electronic supplementary information (ESI) available: Experimental procedures
and characterization/catalytic test results. See DOI: 10.1039/c3cc48027e
2142 | Chem. Commun., 2014, 50, 2142--2144
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to form the activated complex species, which has been assigned as
the rate-determining step.9,15 As a result, it enhances the H2
generation from AB hydrolysis. This is one reason for the higher
activity of Pt/CNT catalysts. Moreover, these Pt catalysts especially
Pt/AC catalysts exist clear shoulders in the XPS Pt 4f spectra at the
side of high binding energies (ca. 78 eV), which could be linked to
the number of Pt–O bonds (i.e., ionic Pt) from the interaction of Pt
NPs with oxygen groups. Performing the deconvolution analysis of
Pt 4f spectra of 1.5 wt% Pt catalysts (Fig. S4, ESI†) for example, Pt/
CNT catalysts are found to have lower content of ionic Pt and
higher content of metallic Pt than Pt/AC catalysts. This is another
reason for the higher activity of Pt/CNT catalysts.
However, notably, for the low Pt loading of 0.5 wt%, Pt nano-
catalysts show lower binding energy of Pt 4f7/2 (Fig. 2d), which may
be due to Pt NPs preferentially anchored on the electron-rich oxygen
groups not the electron-deficient defects and thus the electron
transfer from surface oxygen groups to Pt.14,16,17 It is expected that
Pt NPs immobilized on CNTs with more defects and the lower
surface concentration of oxygen groups in principle show the
improved Pt 4f7/2 binding energies as well as H2 generation activity.
To test this idea, pristine CNTs treated at a high temperature of
800 1C under an Ar atmosphere (i.e., CNTs-HT) are used to
immobilize Pt NPs for AB hydrolysis. As expected, CNTs-HT exhibits
higher ID/IG and thus more defects (Fig. S5a, ESI†) as well as lower
surface concentration of oxygen groups (i.e., nO/nC = 0.0059 from
XPS) than CNTs. The 0.5 wt% Pt/CNTs-HT catalyst shows much
higher Pt 4f7/2 binding energy and content of metallic Pt (Fig. S5b,
ESI†), which results in much higher volume and the rate of
hydrogen generation (Fig. 1b) than the 0.5 wt% Pt/CNT catalyst.
The preliminary results indicate that manipulating the surface
chemistry of CNTs could be an effective method to remarkably
increase the activity of Pt catalysts.
Fig. 1 (a) TOFH as a function of Pt particle size over Pt nanocatalysts, and
(b) volume of H22generation as a function of time over Pt/AC, Pt/CNTs and
Pt/CNTs-HT catalysts with 0.5 wt% of Pt loading. Reaction conditions:
30 1C, nPt/nAB = 4.7 Â 10À3 and CAB = 0.01 g mLÀ1
.
To this end, an acid oxidation and subsequent high tempera-
ture method is further employed to treat pristine close CNTs for
introducing more defects on CNTs, and the as-obtained carbon
supports are used to immobilize Pt NPs for optimizing their
activities, in which only acid oxidation treated CNTs and acid
oxidation followed by high temperature-treated CNTs in an Ar
atmosphere at 800 1C are labelled as CNTs-O and CNTs-O-HT,
respectively. As shown in Fig. 2 and Table S2 (ESI†), the acid
oxidation followed by high temperature treatment significantly
increases the number of defects, while the acid oxidation treat-
ment remarkably increases the surface concentration of oxygen
groups on CNTs. Correspondingly, the positive and negative shifts
of Pt 4f7/2 binding energies are observed over Pt/CNTs-O-HT and
Pt/CNTs-O nanocatalysts, respectively. These results indicate that
the defects on CNTs promote the electron transfer from Pt to
carbon supports, while the oxygen groups reverse the net electron
transfer towards the opposite direction. This is in good agreement
Fig. 2 (a) Raman spectra of AC, CNTs, CNTs-O and CNTs-O-HT sup-
ports. (b) TG patterns of AC, CNTs, CNTs-O and CNTs-O-HT supports
under an Ar atmosphere. (c) Molar ratio of oxygen to carbon from XPS for
AC, CNTs, CNTs-O and CNTs-O-HT supported 1.5 wt% Pt catalysts. (d)
Binding energy (B.E.) of Pt 4f7/2 over different Pt nanocatalysts as a function
of Pt particle size (Note: for the same carbon support, the smaller sized Pt
nanocatalyst corresponds to the lower Pt loading (Table S1, ESI†)).
of both carbon supports mainly consist of the electron donating with previous results.16,17 In addition, CNTs-O and CNTs-O-HT
groups (e.g., phenolic hydroxyl) (Fig. S3, ESI†). Correspondingly, Pt/ show slightly higher surface area and pore volume than pristine
CNT catalysts show the positive shifts of the Pt 4f7/2 binding CNTs from N2-BET measurements (Table S1, ESI†), indicating that
energies compared to Pt/AC catalysts, in which the signals are the ends of the as-treated CNTs are still close in most cases.16 This
mainly assigned to Pt0 species (i.e., metallic Pt). In other words, the will help ensure that the supported Pt NPs mainly locate on the
metallic Pt NPs supported on CNTs have higher binding energies. external surface of close CNT supports, and thus the confinement
These surfaces seem to have stronger interaction with AB molecules effect of Pt catalysts is almost negligible in this work.
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Fig. 1a shows TOFH as a function of Pt particle size over Pt@MIL-101 (i.e., Pt NPs immobilized inside the pores of MIL-101)
2
Pt/CNTs-O-HT catalysts as well as TOFH of Pt/CNTs-O at 30 1C. It is catalyst. The higher activity of the Pt/CNTs-O-HT catalyst could be
2
expected that at the identical Pt particle size, the defect-rich CNTs- related to not only the unique textural properties of the support,
O-HT immobilized Pt nanocatalysts exhibit much higher TOFH
which have no mass-transfer limitations and good capacity
2
compared to Pt/CNT nanocatalysts, while the oxygen group-rich as electron reservoirs, but also the distinct surface chemistry.
CNTs-O immobilized Pt nanocatalyst shows much lower TOFH . As illustrated in Scheme 1, the electron-deficient defects on CNTs
2
This again verifies the fact that the surface oxygen groups and effectively transfer the electrons from Pt to carbon supports and thus
defects on CNTs are detrimental and beneficial to the Pt catalysis, promote AB hydrolysis reaction to generate more H2, while the
respectively. Moreover, TOFH of Pt/CNTs-O-HT increases with Pt electron-rich oxygen groups reversely transfer electrons from carbon
2
particle size to a maximum, i.e., B567 molH molPtÀ1 minÀ1, at the supports to Pt and thus inhibit the reaction.
2
mean size of B1.3 nm followed by a decline with a further increase
In summary, we have demonstrated the defects not the oxygen
in the size, indicating a significant structure sensitivity for AB groups on CNTs responsible for the improved Pt catalytic activity in AB
hydrolysis over Pt/CNTs-O-HT catalysts. To our knowledge, this is hydrolysis. It is a facile and effective method to introduce a number of
the first study by manipulating the surface chemistry of CNTs and defects on CNTs by acid oxidation and subsequent high temperature
Pt particle size to remarkably enhance the activity in AB hydrolysis, treatments. Such defect-rich CNTs-O-HT support immobilized Pt nano-
which will help future work on the catalyst design of Pt/C and the catalysts are highly active, and the optimum sized Pt catalyst shows an
reaction mechanism.
unprecedented H2 generation activity up to B567 molH molPtÀ1 minÀ1
2
Table 1 displays a comparison of the activities of Pt/CNTs-O- at 30 1C. The methodology reported here could be applicable for bi- or
HT with various Pt-based catalysts taken from the literature. multi-metallic and other metallic catalysts.
Clearly, the as-prepared Pt/CNTs-O-HT catalyst shows superior
This work was financially supported by the Natural Science
activity, and in particular B2.1 times higher TOFH at 25 1C Foundation of China (21306046 and 21276077) and the 111
2
than the most active supported Pt catalyst (i.e., Pt/g-Al2O3) Project of Ministry of Education of China (B08021).
reported so far as well as slightly higher than the 2.0 wt%
Notes and references
Table 1 Activities of various Pt-based catalysts for AB hydrolysis
1 (a) L. Schlapbach and A. Zuttel, Nature, 2001, 414, 353;
(b) W. Grochala and P. P. Edwards, Chem. Rev., 2004, 104, 1283;
(c) S. Satyapal, J. Petrovic, C. Read, G. Thomas and G. Ordaz, Catal.
Today, 2007, 120, 246; (d) A. W. C. van den Berg and C. O. Arean,
Chem. Commun., 2008, 668; (e) U. Eberle, M. Felderhoff and
F. Schuth, Angew. Chem., Int. Ed., 2009, 48, 6608.
2 (a) T. B. Marder, Angew. Chem., Int. Ed., 2007, 46, 8116; (b) B. Peng
and J. Chen, Energy Environ. Sci., 2008, 1, 479; (c) C. W. Hamilton,
R. T. Baker, A. Staubitz and I. Manners, Chem. Soc. Rev., 2009,
38, 279; (d) U. B. Demirci and P. Miele, Energy Environ. Sci., 2009,
2, 627; (e) H. L. Jiang, S. K. Singh, J. M. Yan, X. B. Zhang and Q. Xu,
ChemSusChem, 2010, 3, 541; ( f ) M. Yadav and Q. Xu, Energy Environ.
Sci., 2012, 5, 9698.
3 M. Chandra and Q. Xu, J. Power Sources, 2007, 168, 135.
4 O. Metin, V. Mazumder, S. Ozkar and S. H. Sun, J. Am. Chem. Soc.,
2010, 132, 1468.
5 S. F. Li, Y. H. Guo, W. W. Sun, D. L. Sun and X. B. Yu, J. Phys. Chem.
C, 2010, 114, 21885.
6 F. Y. Cheng, H. Ma, Y. M. Li and J. Chen, Inorg. Chem., 2007, 46, 788.
7 C. F. Yao, L. Zhuang, Y. L. Cao, X. P. Ai and H. X. Yang, Int.
J. Hydrogen Energy, 2008, 33, 2462.
Catalyst
nmetal/nAB
T (1C)
TOF
567
Ref.
H2
Pt/CNTs-O-HT
Pt/CNTs-O-HT
Pt/g-Al2O3
Pt/g-Al2O3
Pt/CeO2
Pt/C
Pt black
PtO2
K2PtCl4
PtRu/C
0.0047
0.0047
0.018
0.018
0.018
0.018
0.018
0.018
0.018
0.03
30
25
30
25
25
25
25
25
25
25
25
25
25
This work
This work
3
3
18
19
19
19
19
7
468a
261
222
182
111
14
21
9
8b
Ni0.33@Pt0.67/C
Co0.32Pt0.68/C
Pt@MIL-101
0.018
0.038
0.0029
81b
67b
414
8
9
20
a
b
Obtained from the data in Fig. S6 (ESI). Estimated from the slope of
the fitting line.
8 X. J. Yang, F. Y. Cheng, J. Liang, Z. L. Tao and J. Chen, Int.
J. Hydrogen Energy, 2011, 36, 1984.
9 X. J. Yang, F. Y. Cheng, Z. L. Tao and J. Chen, J. Power Sources, 2011,
196, 2785.
10 M. Wen, S. Q. Zhou, Q. S. Wu, J. Y. Zhang, Q. N. Wu, C. X. Wang and
Y. Z. Sun, J. Power Sources, 2013, 232, 86.
11 M. Chandra and Q. Xu, J. Power Sources, 2006, 156, 190.
12 Y. H. Zhang, M. L. Toebes, A. van der Eerden, W. E. O’Grady, K. P. de
Jong and D. C. Koningsberger, J. Phys. Chem. B, 2004, 108, 18509.
13 D. S. Su, S. Perathoner and G. Centi, Chem. Rev., 2013, 113, 5782.
14 J. Zhu, A. Holmen and D. Chen, ChemCatChem, 2013, 5, 378.
15 Q. Xu and M. Chandra, J. Power Sources, 2006, 163, 364.
16 X. M. Wang, N. Li, J. A. Webb, L. D. Pfefferle and G. L. Haller, Appl.
Catal., B, 2010, 101, 21.
17 I. Fampiou and A. Ramasubramaniam, J. Phys. Chem. C, 2012, 116, 6543.
18 X. Wang, D. P. Liu, S. Y. Song and H. J. Zhang, Chem. Commun.,
2012, 48, 10207.
19 Q. Xu and M. Chandra, J. Alloys Compd., 2007, 446–447, 729.
20 A. Aijaz, A. Karkamkar, Y. J. Choi, N. Tsumori, E. Ronnebro,
T. Autrey, H. Shioyama and Q. Xu, J. Am. Chem. Soc., 2012,
134, 13926.
Scheme 1 Possible pathway for the hydrolytic dehydrogenation of AB
over two kinds of Pt particles immobilized on close CNT supports. Note:
higher (yellow) and lower (brown) binding energy of Pt 4f.
2144 | Chem. Commun., 2014, 50, 2142--2144
This journal is ©The Royal Society of Chemistry 2014