Anchoring ultrafine RhNi nanoparticles on titanium carbides/manganese oxide as an efficient catalyst for hydrogen generation from hydrous hydrazine

Article abstract of DOI:10.1039/c8nj04766a

The development of a cost-effective catalyst with high activity and 100% selectivity for hydrogen production from hydrous hydrazine under mild conditions is desirable for fuel cell applications. In this work, a series of RhNi/MnO_x-MXene NPs are prepared using a simple one-step wet-chemical method. The synthesized RhNi/MXene catalysts are characterized by XPS, TEM, SEM and ICP-AES. The ultrafine RhNi nanoparticles of 2.8 nm size are well dispersed on the bi-support (MnO_x-MXene) surface and avoid the aggregation of RhNi nanoparticles. This unique nanocatalytic system shows an active performance toward N₂H₄·H₂O decomposition under mild conditions. The optimized Rh_0.7Ni_0.3/MnO_x-MXene NPs exhibit excellent catalytic properties, and the corresponding TOF value can reach 1101.9 h^?1 with 100% H₂ selectivity under mild conditions. The remarkable catalytic performance is attributed to the mild interactions of the bi-support to the NPs, which not only stabilizes the NPs to maintain good dispersion but also leaves sufficient surface active sites to facilitate the catalytic reaction.

Full text of DOI:10.1039/c8nj04766a

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Anchoring ultrafine RhNi nanoparticles on
titanium carbides/manganese oxide as an efficient
catalyst for hydrogen generation from hydrous
hydrazine†
Cite this: New J. Chem., 2018,
42, 20001
Bing Yin,a Qingtao Wang,b Tong Liu *b and Guanhui Gao*c
The development of a cost-effective catalyst with high activity and 100% selectivity for hydrogen
production from hydrous hydrazine under mild conditions is desirable for fuel cell applications. In this
work, a series of RhNi/MnOx-MXene NPs are prepared using a simple one-step wet-chemical method.
The synthesized RhNi/MXene catalysts are characterized by XPS, TEM, SEM and ICP-AES. The ultrafine
RhNi nanoparticles of 2.8 nm size are well dispersed on the bi-support (MnOx-MXene) surface and avoid
the aggregation of RhNi nanoparticles. This unique nanocatalytic system shows an active performance
toward N2H4ÁH2O decomposition under mild conditions. The optimized Rh0.7Ni0.3/MnOx-MXene NPs exhibit
excellent catalytic properties, and the corresponding TOF value can reach 1101.9 hÀ1 with 100% H2 selectivity
under mild conditions. The remarkable catalytic performance is attributed to the mild interactions of the
bi-support to the NPs, which not only stabilizes the NPs to maintain good dispersion but also leaves
sufficient surface active sites to facilitate the catalytic reaction.
Received 19th September 2018,
Accepted 5th November 2018
DOI: 10.1039/c8nj04766a
rsc.li/njc
N2H4ÁH2O decomposition, which not only promote the com-
plete decomposition of N2H4ÁH2O but also simultaneously
Introduction
The development of a hydrogen (H2) economy requires a simple suppress NH3 production, is important. Despite recent progress
and low-cost technology for H2 manufacture and storage.1–12 shows that Ni-based nanocatalysts possess high efficiency and
However, the discovery of safe and efficient H2 storage media is selectivity toward N2H4ÁH2O decomposition, their poor kinetics
still one of the biggest challenging obstacles for on-board are below acceptable levels.31
hydrogen fuel applications.13–19 Recently, hydrous hydrazine
H2NNH2 - N2 + 2H2
(1)
(2)
such as hydrazine monohydrate (N2H4ÁH2O) has been proposed
as a promising candidate due to its high H2 content of 8%,
low material cost and stable liquid state at a wide range of
temperature.20–26 Moreover, its production of nitrogen (N2) is
3H2NNH2 - N2 + 4NH3
Recently, it has been reported that amorphous oxide possesses
the only byproduct besides H2 through the reaction as repre- high concentrations of unsaturated coordination sites, that is
sented in eqn (1). Ideally, two moles of H2 in the absence of CO to say, several dangling bonds on the amorphous phase help
can be efficiently produced from one mole of N2H4ÁH2O, in providing high surface area and strong metal–support inter-
whereas the undesired reaction pathway as in eqn (2) should actions superior to the crystalline one.32–34 In addition, in order
be avoided, because the generated NH3 will poison the H2 fuel to further avoid agglomeration of metallic nanocatalysts and
cell.27–30 Therefore, from the viewpoint of practical applica- maximize their stability, a suitable solid support is required to
tions, the development of highly efficient catalysts toward anchor well and disperse the active metallic nanoparticles (NPs).
Graphene-like transition metal carbide (MXene, Ti3C2(OHxF1Àx)2),
as a promising solid support for a new nanocatalyst, has been
widely investigated.35–40 The studies show that the abundant
functional groups on MXene surfaces, such as Ti–OH and Ti–F
bonds, not only immobilize the nanoparticles during reduction
but also improve the hydrophilicity of the resultant catalysts.
Therefore, the utilization of amorphous oxide/MXenes to
immobilize to Ni-based nanocatalysts might boost the kinetic
rate as well as selectivity, and stability during the N2H4ÁH2O
a School of civil engineering, Qingdao University of Technology, Qingdao 266033,
China
b College of Materials Science and Engineering, Qingdao University of Science and
Technology, Zhengzhou Rd 53, 266000 Qingdao, China.
E-mail: liutong@qust.edu.cn
c
¨
¨
Paul-Drude-Institut fur Festkorperelektronik, Hausvogteiplatz 5–7, 10117 Berlin,
Germany. E-mail: gao@pdi-berlin.de
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c8nj04766a
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decomposition process. However, this has been rarely reported X-ray photoelectron spectroscopy (XPS) measurement was performed
in the past decades. using an ESCALAB 250Xi spectrophotometer. The metal contents
Here, we creatively synthesize RhNi NPs supported on the of the catalyst were analyzed using inductively coupled plasma
bi-support MnOx-MXene surface via a one-step wet-chemical atomic emission spectroscopy (ICP-AES) on a Leeman PROFILE
approach. Interestingly, the prepared RhNi/MnOx-MXene nano- SPEC. Scanning electron microscopy (SEM) and energy dispersive
catalysts show a high kinetic rate as well as 100% H2 selectivity spectroscopy (EDS) were performed and elemental mapping
and excellent stability, which indicate that the bi-support can images were obtained using an FEI Nova NanoSEM 450.
efficiently promote the catalytic kinetics toward the dehydro-
genation of N2H4ÁH2O in this nanocatalytic system.
Results and discussion
The morphologies of pristine MXenes were characterized for
the first time by scanning electron microscopy (SEM). As shown
in Fig. S1 (ESI†), it can be found that pristine MXene exhibits an
Experiment
Synthesis of RhNi/MnOx-MXene NPs
obvious lamellar structure with a highly rough surface. Fig. 1a–c
show the typical transmission electron microscopy (TEM) and
high-resolution TEM (HRTEM) images of Rh0.7Ni0.3/MnOx-
MXene hybrid. As shown in Fig. 1a and b, all the resultant
RhNi NPs are highly dispersed on the surface of MnOx-MXene
with an average particle size of around 2.8 nm (Fig. S2, ESI†).
The narrow size and shape distributions are mainly attributed
to the ‘confine effect’ from the MnOx-MXene surface, which can
efficiently anchor the RhNi NPs and control their distribution
during the synthesis process. The HRTEM analysis of Rh0.7Ni0.3
NPs suggests that the metallic NPs are in the crystalline state
with a lattice fringe space of 0.213 nm, which differs from
the fcc (111) plane of Rh (0.219 nm) and the (011) plane of
Ni (0.203 nm), and thus further indicates the formation of the
RhNi/MnOx-MXene NPs were facilely synthesized by the
co-reduction of the metal precursors and MXene. Scheme 1
shows the synthesis route to the Rh0.7Ni0.3/MnOx-MXene nano-
catalyst. In a typical experimental procedure, 100 mg MXene
was dissolved in 2 mL water in a two-neck round-bottom flask
(30 mL). Then, the aqueous solution containing rhodium chlor-
ide (0.042 mmol), nickel chloride (0.018 mmol) and manganese
chloride (0.06 mmol) was added into the MXene solution,
and the resultant mixture was kept under sonication for 3 h.
Subsequently, a 0.5 mL aqueous solution of sodium boro-
hydride (NaBH4) (1.3 mol LÀ1) was added into the resultant mixture
under vigorous stirring for 2 h. Finally, the Rh0.7Ni0.3/MnOx-MXene
nanocatalyst was obtained by washing with deionized water
and centrifugation. The synthesis of Rh0.7Ni0.3/MnOx-MXene
NPs with different Rh/Ni molar ratios was carried out following
the above-mentioned process through adjusting the Rh/Ni
molar ratio of initial metal precursors. Furthermore, to discuss
the effect of MnOx on catalytic properties, the Rh0.7Ni0.3/MnOx-
MXene NPs with different molar contents of Mn were prepared
by the same method by altering the initial amount of Mn
RhNi alloy. The real molar ratio of Rh : Ni : Mn in Rh0.7Ni0.3
/
MnOx-MXene is determined to be 0.349 : 0.149 : 0.502 by induc-
tively coupled plasma-atomic emission spectrometry (ICP-AES),
which agrees well with the energy-dispersive X-ray (EDX) results
(0.352 : 0.145 : 0.503).
precursor. For comparison, Rh0.7Ni0.3/MnOx-rGO, Rh0.7Ni0.3
/
MnOx-XC 72R, Rh0.7Ni0.3/MnOx-multiwalled carbon nanotubes
(MNCTs, Rh0.7Ni0.3/MnOx-MNCTs) and Rh0.7Ni0.3/MnOx sam-
ples were also synthesized by a similar procedure using NaBH4
as the reducing agent.
Morphological, structural and compositional characterizations
Transmission electron microscopy (TEM) and high-resolution
TEM (HRTEM) images were obtained using a Tecnai G2 F30
S-Twin instrument with a field emission gun operating at 200 kV.
Scheme 1 Schematic representation of synthesis of the RhNi/MnOx-
Fig. 1 TEM images (a and b), HRTEM images (c) and EDX (d) of Rh0.7Ni0.3/
MXene catalyst.
MnOx-MXene.
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Fig. 2 The elemental mapping images of Rh0.7Ni0.3/MnOx-MXene (a) and the corresponding elemental mapping images for C (b), Ti (c), Mn (d), O (e),
F (f), Rh (g) and Ni (h).
Fig. 2 shows the elemental mapping images of the Rh0.7Ni0.3
/
introduced on the Ti3C2X2 surface, which can be used as an
MnOx-MXene hybrid and the corresponding elemental mapping ‘‘anchor’’ to enhance the hydrophilicity of the support to
analysis. It can be seen that the elements of Rh and Ni are simultaneously facilitate the stabilization of the resultant
co-distributed in these NPs and homogenously dispersed on the RhNi NPs, resulting in a fine dispersion and good stability of
position of C, Ti, O, F and Mn elements, further indicating the the synthesized nanocatalyst in this heterogeneous system.
formation of RhNi and their homogeneous distribution on the The binding energies of Ni0 2p3/2 and Ni0 2p1/2 are located at
surface of the MnOx-MXene hybrid.
856.8 eV and 873.9 eV, respectively, which are attributed to
In order to investigate the chemical states and composition metallic Ni0, whereas the binding energies of Rh0 3d5/2 and
of as-prepared Rh0.7Ni0.3/MnOx-MXene, X-ray photoelectron Rh0 3d3/2 observed at 307.5 eV and 311.6 eV, respectively,
spectroscopy (XPS) analysis after Ar sputtering has been carried correspond to metallic Rh0. Moreover, the valence states of Mn in
out. Fig. 3 shows the XPS spectra of RhNi/MnOx-MXene in the the three specimens are mainly Mn2+, Mn3+ and Mn4+. Based the
survey, and individual C 1s, Ti 2p, Mn 2p, O 1s, F 1s, Rh 3d and above-mentioned XPS results, the synthesized RhNi/MnOx-MXene
Ni 2p regions. It can be seen from the XPS results that the C 1s NPs are regarded as the composition of RhNi particles supported
spectrum can be divided into 4 peaks, which is attributed on MnOx-MXene. From the above-mentioned TEM and XPS
to different kinds of bonds such as C–Ti, C–C, C–OH, and results, it can be suggested that the as-synthesized RhNi
HO–CQO. Additionally, the presence of F and O suggest that NPs have been formed and well dispersed on the bi-support
the functional groups such as OH and F are successfully MnOx-MXene surface, which will result in improving the
Fig. 3 Spectroscopic characteristics of Rh0.7Ni0.3/MnOx-MXene. Survey spectrum (a), and XPS spectra of C 1s, Rh 3d, Ni 2p, Mn 2p, Ti 2p, O 1s and
F 1s (b–h).
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can release 2.6 equiv. N2 + H2 within 25 min. The superior
catalytic performance is mainly attributed to the strong inter-
face interaction between metal and carrier, especially that the
absence of MnOx would facilitate the electron transfer between
the metallic catalyst and the bi-support. The effect of MnOx
amount on the catalytic properties is investigated, and the
results show that the optimum mole fraction of MnOx is 0.5
(Fig. S3, ESI†). Increasing or decreasing this optimum value will
lead to the suppression of catalytic activity.
In addition, RhNi/MnOx-MXene nanocatalytic systems with
different metal compositions are also tested toward N2H4ÁH2O
decomposition. The catalytic performance is strongly depen-
dent on the molar ratio of Rh/Ni. Both Rh/MnOx-MXene and
Ni/MnOx-MXene monometallic systems show certain activity
toward N2H4ÁH2O decomposition, in which 1.9 and 0.86 equiv.
(H2 + N2) per N2H4ÁH2O was released in 25 min, respectively.
While for the RhNi bimetallic system, although the catalytic
performance is improved with the addition of Ni, adjustment of
the molar ratio of Rh : Ni to some other values will result in a
change in the catalytic properties and H2 selectivity (Fig. 4b),
and thus the optimum molar ratio of Rh : Ni is determined
to be 7 : 3 and the TOF value will reach 1101.9 hÀ1 with 100% H2
selectivity. The reason for the enhanced performance of Rh0.7Ni0.3
Fig. 4 Time course plots for N2 + H2 production from N2H4ÁH2O decom-
position over Rh0.7Ni0.3/MnOx-MXene, Rh0.7Ni0.3/MnOx-rGO, Rh0.7Ni0.3
/
MnOx-XC 72R, Rh0.7Ni0.3/MnOx-MNCTs, Rh0.7Ni0.3/MnOx, and support-
free Rh0.7Ni0.3 (a); with different Rh/Ni molar ratios (b); catalyzed by
Rh0.7Ni0.3/MnOx-MXene from 1st to 6th cycles (c); TEM image of the
Rh0.7Ni0.3/MnOx-MXene NPs after the sixth cycle (d). The molar ratio of
catalyst/N2H4ÁH2O = 0.03.
catalytic properties toward N2H4ÁH2O decomposition in this may be mainly attributed to the synergetic effect between Rh and
bimetallic heterogeneous system. Ni, which can efficiently adjust the surface electronic states of
The as-synthesized Rh0.7Ni0.3/MnOx-MXene NPs have been bimetallic nanocatalysts, particularly related to the local strain
used as catalysts for the N2H4ÁH2O decomposition at 50 1C with and effective atomic coordination number at the surface, resulting
a constant molar ratio of catalyst/N2H4ÁH2O = 0.03 (Fig. 4a). in an obvious improvement for N2H4ÁH2O decomposition.41
For comparison, Rh0.7Ni0.3/MnOx-rGO, Rh0.7Ni0.3/MnOx-XC
To study the effect of the OHÀ group on determining the
72R, Rh0.7Ni0.3/MnOx-MNCTs and Rh0.7Ni0.3/MnOx samples catalytic properties toward N2H4ÁH2O decomposition, the per-
were also synthesized and characterized under the same con- formance of hydrogen generation from an aqueous hydrazine
ditions. Interestingly, among the above catalysts, Rh0.7Ni0.3
/
system with/without NaOH catalyzed by Rh0.7Ni0.3/MnOx-
MnOx-MXene exhibits the best catalytic performance. It is clear MXene was further investigated. Although both samples with/
that, over the Rh0.7Ni0.3/MnOx-MXene catalyst, 3.0 equiv. of gas without NaOH reach 100% H2 selectivity, it should be noted
(N2 + H2) can be produced in only 3.63 min with the corres- that the kinetic rate of N2H4ÁH2O decomposition could be
ponding turnover frequency (TOF) of 1101.9 hÀ1; while over the efficiently facilitated by the addition of NaOH amount com-
Rh0.7Ni0.3/MnOx-GO, Rh0.7Ni0.3/MnOx-XC 72R, and Rh0.7Ni0.3
/
pared to that without NaOH (Fig. S4, ESI†). KOH is also used to
MnOx samples, the reaction can be completed between 5.2 replace NaOH, and no obvious difference is observed in the
and 19.8 min, corresponding to the TOF value of 202.1 and catalytic properties (Fig. S5, ESI†), which suggests that the
769.2 hÀ1, respectively. Even worse, Rh0.7Ni0.3/MnOx-MNCTs OHÀ group indeed determines the N2H4ÁH2O decomposition.
Table 1 Catalytic activities of different catalysts for N2H4ÁH2O decomposition
Catalyst
Solvent/medium
Temp. (1C)
Selectivity for H2 (100%)
TOF (hÀ1
)
Ea (kJ molÀ1
)
Ref.
Rh0.7Ni0.3/MnOx-MXene
Rh55Ni45/Ce(OH)
Rh34Ni66@ZIF-8
Pt0.6Ni0.4/PDA-rGO
(Ni3Pt7)0.5–(MnOx)0.5/NPC-900
PtNi/CeO2
Ni3Pt7/BNG-1000
Ni0.9Pt0.05Rh0.05/La2O3
Ni0.9Pt0.1/Ce2O3
Ni0.58Pt0.42/graphene
Ni0.9Pt0.1/MIL-101
Rh0.8Ni0.2@CeOx/rGO
(Ni3Pt7)0.5–(MnOx)0.5
Ni/CeO2
Aqueous NaOH
Aqueous NaOH
Aqueous NaOH
Aqueous NaOH
Aqueous NaOH
Aqueous NaOH
Aqueous NaOH
Aqueous NaOH
Aqueous NaOH
Aqueous NaOH
Aqueous NaOH
Aqueous NaOH
Aqueous NaOH
Aqueous NaOH
50
50
50
50
50
50
30
25
25
30
30
30
25
50
100
100
100
100
100
100
100
100
100
100
100
100
100
100
1101.9
395
140
2056
706
286
199.4
45.9
28.1
434
140
36.4
120
36.9
38.8
58.1
33.39
50.15
38.7
28.4
This work
47
48
49
50
51
52
53
24
54
55
33
56
57
42.3
23.9
48.4
58
34
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Thanks to the optimization of NaOH promotion, the TOF Acknowledgements
increases by 3-fold (Fig. S6, ESI†), which indicates that the
This work was financially supported by the National Natural
Science Foundation of China (Project No. 51303188).
addition of 3 M NaOH will decrease the concentration of
undesired N2H5 in the aqueous catalytic system. Additionally,
+
in view of the chemical equilibrium, a strong alkaline environ-
ment will inhibit the generation of the basic NH3 byproduct,
which increases the selectivity to H2.42–46
Notes and references
To obtain the activation energy (Ea) toward N2H4ÁH2O decom-
position catalyzed by Rh0.7Ni0.3/MnOx-MXene catalysts, the catalytic
properties at different measured temperatures (25–60 1C) were
tested, and the results are presented in Fig. S6 (ESI†). The gas
(N2 + H2) production from N2H4ÁH2O decomposition can be
finished in 2.37, 3.63, 5.33, and 14 min at 333, 323, 313, and
298 K and shows the corresponding TOF value of 1690, 1101.9, 750,
and 285 hÀ1, respectively, which is higher that the reported values
in Table 1. The Arrhenius plot of ln TOF vs. 1/T for the catalyst is
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