The “electric-dipole” effect of Pt–Ni for enhanced catalytic dehydrogenation of ammonia borane

Article abstract of DOI:10.1016/j.jallcom.2020.156253

In this work, we prepared the PtNi alloy nanoparticles on Al₂O₃ (Al₂O₃–PtNi) via a simple wet-grinding method followed by a thermal reduction process. The as-prepared Al₂O₃–PtNi displayed a higher TOF of 426.84 mol_H2 (mol_Pt·min)^?1 toward the dehydrogenation of AB (ammonia borane) than Al₂O₃–Pt and Al₂O₃–Ni. Based on the theoretical calculations, an “electric-dipole” effect generated by the neighboring Pt–Ni atoms was found to facilitate the catalytic dehydrogenation process of ammonia borane (AB). The external electric field produced by the Pt–Ni dipole can elongate the B–H bond and increase the negative charges of H atoms on the BH₃ group, which helps to activate the B–H bond in the AB molecule, and thus enhances the kinetic process of catalysis.

Full text of DOI:10.1016/j.jallcom.2020.156253

Journal Pre-proof
The “electric-dipole” effect of Pt–Ni for enhanced catalytic dehydrogenation of
ammonia borane
Bin Wang, Laifei Xiong, Hanjing Hao, Hairui Cai, Pengfei Gao, Fuzhu Liu, Xiaojing
Yu, Chao Wu, Shengchun Yang
PII:
S0925-8388(20)32617-7
DOI:
https://doi.org/10.1016/j.jallcom.2020.156253
JALCOM 156253
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 2 April 2020
Revised Date: 24 June 2020
Accepted Date: 29 June 2020
Please cite this article as: B. Wang, L. Xiong, H. Hao, H. Cai, P. Gao, F. Liu, X. Yu, C. Wu, S. Yang, The
“electric-dipole” effect of Pt–Ni for enhanced catalytic dehydrogenation of ammonia borane, Journal of
Alloys and Compounds (2020), doi: https://doi.org/10.1016/j.jallcom.2020.156253.
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CRediT authorship contribution statement:
Bin Wang: Conceptualization, Investigation, Writing - original draft, Funding
acquisition. Laifei Xiong: Investigation. Hanjing Hao: Investigation. Hairui Cai:
Formal analysis, Resources. Pengfei Gao: Formal analysis. Fuzhu Liu: Formal
analysis. Xiaojing Yu: Validation, Resources. Chao Wu: Supervision, Writing - review
& editing. Shengchun Yang: Supervision, Writing - review & editing.

Graphic Abstract

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Title* The “electric-dipole” effect of Pt-Ni for enhanced catalytic
dehydrogenation of ammonia borane
Bin Wang,1,§,* Laifei Xiong,1,§ Hanjing Hao,1,§Hairui Cai,1 Pengfei Gao,2 Fuzhu Liu,1
Xiaojing Yu,3 Chao Wu4,*, Shengchun Yang1,*
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1. School of Science, MOE Key Laboratory for Non-equilibrium Synthesis and
Modulation of Condensed Matter, State Key Laboratory for Mechanical Behavior
of Materials, Xi'an Jiaotong University, Xi’an 710049, People's Republic of
China
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2. Northwest Institute of Nuclear Technology, Xi'an 710024, People's Republic of
China
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3. School of Materials Science and Engineering, Xi'an University of Technology,
Xi'an 710048, People's Republic of China
4. Frontier Institute of Science and Technology, Xi’an Jiaotong University, Xi’an
710054, People's Republic of China
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* Corresponding authors: bin_wang@xjtu.edu.cn; chaowu@mail.xjtu.edu.cn;
ysch1209@mail.xjtu.edu.cn
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§ These authors contributed equally.
Abstract
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In this work, we prepared the PtNi alloy nanoparticles on Al2O3 (Al2O3-PtNi) via a
simple wet-grinding method followed by a thermal reduction process. The
as-prepared Al2O3-PtNi displayed a higher TOF of 426.84 molH2 (molPt·min)-1 toward
the dehydrogenation of AB (ammonia borane) than Al2O3-Pt and Al2O3-Ni. Based on
the theoretical calculations, an “electric-dipole” effect generated by the neighboring
Pt-Ni atoms was found to facilitate the catalytic dehydrogenation process of ammonia
borane (AB). The external electric field produced by the Pt-Ni dipole can elongate the
B-H bond and increase the negative charges of H atoms on the BH3 group, which
helps to activate the B-H bond in the AB molecule, and thus enhances the kinetic
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process of catalysis.
Keywords:
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The “Electric-dipole” effect; Pt-Ni alloy nanoparticles; Electron transfer; Ammonia
borane; Hydrogen evolution
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1. Introduction
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As a clean and source-independent energy carrier, hydrogen possesses a higher
gravimetric energy density (120 MJ/kg) than gasoline (44 MJ/kg), and can be used to
operate environment-friendly fuel cells with H2O as the only byproduct[1]. However,
it is still a challenge to controllably store and release 5-13 kg of H2 to enable a
300-mile driving range with no excessive weight added to the fuel-cell-powered
vehicle[2]. For this onboard H2 storage in vehicular applications, the U.S. Department
of Energy (DOE) set a gravimetric hydrogen capacity (GHC) of 5.5 wt% or
volumetric hydrogen capacity (VHC) of 40 g/L as the target for the year 2025 and 6.5
wt% (50 g/L) as the ultimate target[3].
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Among various approaches, such as compressed gas[4], liquid H2[5], material
absorption[6], etc., chemical hydrogen storage in materials composed of light
elements has drawn lots of attention. Ammonia borane (NH3BH3, AB), a stable,
non-toxic, non-flammable B-N compound, is considered as one of the leading
candidates for hydrogen storage due to its high GHC of 19.6 wt% and VHC of 140
g/L, which are greater than the U.S. DOE’s target. It has been reported that both
hydrolysis[7] and thermolysis[8] can be performed to produce hydrogen from AB and
the obtained final byproduct can also be utilized to regenerate AB[9]. Compared with
thermolysis for 100% decomposition of AB at high temperature, the catalytic
hydrolysis provides a convenient approach to release three equivalents of pure
hydrogen from AB in the presence of an appropriate catalyst (NH3BH3 + 2H2O →
NH4BO2 + 3H2) at room temperature. Thus, it is highly desirable to develop excellent
catalysts with high activity, reusability and large-scale facile preparation to meet the
terminal practical requirements of this system in fuel cells.
Various noble metal catalysts like Rh, Pd, Ru nanoclusters, as well as the
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supported noble metals (Ru, Rh, Pd, Pt, and Au) over Al2O3, C and SiO2, have been
widely studied to produce hydrogen by the hydrolysis of AB[10]. Among them,
platinum-based catalysts have exhibited remarkable performance in this catalytic
reaction. Chen et al. fabricated Pt-WO3 with dual-active-sites to boost hydrogen
evolution from ammonia borane for the first time[11]. In such unique catalyst, Pt and
WO3 acted as the active for ammonia borane and H2O activation, respectively.
However, the high cost and limited reserve of platinum greatly hindered their practical
applications. Therefore, it is desirable to develop a low-cost catalyst with high
hydrolysis activity. Recently, many non-noble metals, such as Ni, Co, Fe, etc., have
been studied for the catalytic dehydrogenation of AB[5]. For example, Mahyari et al.
prepared Ni nanoparticles immobilized on three-dimensional nitrogen-doped
graphene-based frameworks with a TOF of 41.7 molH2 molNi-1 min-1 for the hydrolytic
dehydrogenation of ammonia borane[9]. Hu et al. reported that Co nanoparticles
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supported by amine-rich polyethyleneimine and graphene oxide (PEI-GO/Co
)
-1
showed an enhanced catalytic performance with a TOF of 39.9 molH2 molCo min-1
for hydrogen evolution from AB solution [12]. Although many attempts have been
made to develop an efficient non-noble catalyst for catalytic hydrolysis AB, there still
exists a catalytic activity gap between the noble metals and non-noble metals.
Another efficient strategy to develop low-cost catalysts is to minimize the usage
of noble metal via introducing transition metals to form bimetallic nanoparticles (NPs)
catalysts or core-shell structured catalyst[13-17]. Ge et al. developed a new strategy to
synthesize PtNi/NiO clusters coated by small-sized hollow silica for highly efficient
hydrogen evolution from the hydrolysis of NH3BH3[17]. Li et al. reported a galvanic
replacement method to synthesize atomically dispersed Pt on the surface of Ni
particles supported by CNT and found the dissociation of an O-H bond in H2O is
rate-determining step (RDS)[18]. Furthermore, based on the results of kinetic isotope
effect (KIE) measurements, Li et al. confirmed that RDS for AB hydrolysis was the
breaking of an O-H bond in H2O[19]. Zhu et al. immobilized ultrafine AuNi alloy
nanoparticles into MIL-101. The authors found that the AuNi/MIL-101 displayed a
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high activity for hydrogen evolution from the catalytic hydrolysis of AB[20].
Core-shell structured Pt@Ni NPs were also synthesized by Qi et al. in hollow SiO2
nanocapsules for the catalytic hydrolysis of AB[21]. It is well known that there is a
strong electronic interaction between different metal atoms when introducing a second
metal into the host metal due to the difference of their electronegativity[22, 23].
However, up to now, it is still unclear about the effect of electronic interaction
between different metal atoms on the kinetics of AB hydrolysis.
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In this work, by taking Al2O3 as catalyst support due to its low cost, high surface
area, and high abundance[24], we synthesized Al2O3-PtNi with high catalytic activity
and long-term durability for hydrogen generation from AB via a simple wet-grinding
method followed by a thermal reduction process. Various characterizations, such as
XRD, TEM, XPS, etc., were conducted to analyze the catalysts in-depth, and the
kinetics of catalytic dehydrogenation of AB were studied systemically. An
“electric-dipole” effect of neighboring Pt-Ni atoms was found to facilitate the
catalytic dehydrogenation process of ammonia borane (AB). The as-prepared
Al2O3-PtNi catalyst displayed a higher TOF of 426.84 molH2 (molPt·min)-1 toward the
dehydrogenation of AB in contrast to Al2O3-Pt and Al2O3-Ni.
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2. Experimental section
2.1 Materials:
Nickel chloride (NiCl2·6H2O) was purchased from Aladdin Ltd. (Shanghai,
China). Ammonia borane (AB) and Aluminium oxide (Al2O3) were purchased
from Tianjin Kemiou Chemical Reagent Co., Ltd. Nitric acid (HNO3) was
purchased from Tianjin Fuyu Fine Chemical Co., Ltd. Chloroplatinic acid was
purchased from Kunming Institute of Precious Metals. The deionized water
used in all of the experiments was 18.25 MΩ cm. All the chemical reagents
used in the preparation were analytical grade and without any further
purification.
2.2 Synthesis of Al2O3-PtNi, Al2O3-Pt and Al2O3-Ni
The Al2O3-PtNi were synthesized by a wet-gridding method followed by a thermal
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reduction process. Typically, 500 mg of Al2O3 and 30.5 mg of NiCl2·6H2O were
added into an agate mortar. Then, 0.658 mL 195 mM H2PtCl6 solution and 2 mL H2O
were added into the mixture. Subsequently, vigorous grind the mixture until the
sample became a dry powder. The powder sample was placed in a tube furnace at
400°C under the H2/Ar atmosphere for 2 h. After cooling, the sample was washed in
deionized water several times and dried under 60°C in a vacuum oven for 5h. The
final obtained powder was designated as Al2O3-PtNi. The Al2O3-Ni and Al2O3-Pt were
prepared by the same process except for no addition of H2PtCl6 or NiCl2·6H2O,
respectively. The loadings of Pt and Ni in Al2O3-Pt and Al2O3-Ni were 5wt%. The Pt
loading in Al2O3-PtNi was 5 wt%, consistent with that of Al2O3-Pt, and the molar ratio
of Pt:Ni was 1:1.
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2.3 Catalytic hydrolysis of AB
The procedure for catalytic hydrolysis of AB was conducted as previously reported[13,
25]. Typically, 50 mg catalyst was dispersed in 6 mL H2O in a two-neck round-bottom
flask, which was placed in a water bath at 25 oC, under continuous magnetic stir. The
flash was purged with N2 for 20 min and connected to an inverted measuring cylinder
filled with water via a rubber tube. Then, 4 mL solution contained 2 mmol AB
(corresponding to the generation of a maximum 6 mmol = 134 mL H2 gas) was
injected into the flask using a syringe. The volume of generated H2 was measured by
monitoring the displacement of water in the cylinder periodically. The reaction was
stopped when no hydrogen generation was observed.
2.4 Calculation method
The turnover frequency (TOF) value was calculated by the following equation:
ꢀ
ꢆ
ꢁꢂ
ꢁꢂ
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TOF = ꢀ
=
Eq(1)
×ꢅ
ꢇꢇ.ꢈ×ꢀ ×ꢅ
ꢃꢄ
ꢃꢄ
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where the nH2 is the mol of the generated hydrogen, nPt is the mol of Pt in the reaction
system, VH2 is the volume of the generated hydrogen, and t is the reaction time. The
unit of TOF in this work was molH2 molPt-1 min-1.
The activation energy (Ea) for the catalytic hydrolysis of AB was evaluated based on
the Arrhenius equation:
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ln ꢉ = ln ꢊ − ꢋꢌ/ꢍꢎ
Eq(2)
The rate constant K (mL/min) of H2 evolution was determined by the volume of
generated H2 in the first 1 min for all catalysts, where the volume of evolved H2
linearly increased with increasing reaction time. Ea is the activation energy. R is the
gas constant. T is the reaction temperature.
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2.5 Durability test of the catalyst
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The process of durability test of the as-prepared catalyst was similar to the procedure
of catalytic hydrolysis of AB. Typically, 50 mg catalyst was dispersed in 6 mL H2O in
a two-neck round-bottom flask under continuous magnetic stir. The first run was
started when a 4 mL solution contained 2 mmol AB was injected into the flask using a
syringe. The volume of generated H2 was measured by monitoring the displacement
of water in the cylinder periodically. After the completion of the 1st-run hydrolysis
reaction, the catalyst was collected by centrifugation, washed with water. The
obtained catalyst was reused for the next run of hydrolysis of AB. Such cycle tests
were repeated 6 times.
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2.6 Characterizations
The X-ray diffraction (XRD) patterns were obtained from a PANalytical X’pert MPD
Pro diffractometer operated at 40 kV and 40 mA using Ni-filtered Cu Kα irradiation
(Wavelength 1.5406 Å). The morphology was characterized by scanning electron
microscope (SEM, JEOL JSM-7000F) at an acceleration voltage of 15 kV. The
transmission electron microscopy (TEM) images, high-resolution TEM (HRTEM)
images, and the high angle annular dark field-scanning transmission electron
microscopy (HAADF-STEM) images were obtained from a JEOL JEM-F200
transmission electron microscope at an accelerating voltage of 200 kV. An
OXFORDMAX-80 energy-dispersive X-ray detector (EDX) which was mounted in
the above TEM was used to conduct elemental analysis. X-ray photoelectron
spectroscopy (XPS) measurements were conducted on a Thermo Fisher ESCALAB
Xi+ with monochromatic Al Kα radiation (hν = 1486.69 eV) and with the pressure of
sample analysis chamber under high vacuum <5×10-10 mbar. All binding energies
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were referenced to the C 1s peak at 284.8 eV. The total Pt and Ni contents in the
as-prepared sample was measured by Inductively Coupled Plasma Mass Spectrometer
(ICP-MS) (NexIONTM 350D, PerkinElmer, USA).
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2.7 Theoretical methods
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Spin-polarized DFT calculations were implemented by using the Vienna ab initial
simulation package (VASP) with the projected augmented wave (PAW) potentials[26].
The Perfew-Burke_Ernzerhof (PBE) functional was used to describe the
exchange-correlation potential[27]. The energy cutoff was set to 400 eV. A
Monhorst-Pack k-point mesh of 3×3×1 was used for sampling the Brillouin zone for a
(2×2) PtNi (1 1 1) supercell. Four atomic layers were included and the bottom two
layers were fixed during the optimizations. A 15 Å vacuum layer was added to
minimize the layer interactions. The converge criteria for energy and force are 10-4 eV
and 0.05 eV/Å, respectively. The supercell models of PtNi and the AB molecule are
shown in Figure S1-2, respectively.
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The adsorption energy of molecule X (X = AB, H2O) was calculated as:
Eadsorption = E(X+surf.) - Esurf. – EX
Eq(3)
where E(X+surf.), Esurf. and EX were the total energy of X adsorbed surface, clean surface
and X molecule, respectively.
The MP2 method was applied by using Gaussian software package to explore the
structure and properties of BH3NH3, the geometry of BH3NH3 were optimized in the
external electric fields of 0 and 0.0320 a.u. (1 a.u. = 5.142 × 1011 V m-1) at the
MP2/6-311+G(2df,p) level[28]. The external electric field was applied along the
direction from the N atom to the B atom. The natural bond orbital (NBO) analyses
were conducted at the QCISD/aug-cc-pVTZ level.
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3. Results and discussion
The Al2O3-PtNi catalysts were prepared via the wet-grinding method followed by a
thermal reduction process. A simplified illustration of the synthesis process for
Al2O3-PtNi is depicted in Figure 1a. Briefly, the precursor was obtained by grinding
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the mixture of H2PtCl6, NiCl2, Al2O3, and H2O until the mixture became dry powder.
Then, the as-prepared precursor was calcined under the H2/Ar atmosphere at 400°C.
The finally obtained powder was the Al2O3-PtNi catalyst. The morphology of the
as-prepared Al2O3-PtNi was characterized by transmission electron microscopy
(TEM). Figure 1a shows a representative TEM image of Al2O3-PtNi. It can be seen in
Figure 1b that PtNi NPs with high contrast, which are indicated by white arrows, are
well monodispersed on the surface of Al2O3. The TEM image of Al2O3-Pt in Figure
S3a shows that the Pt nanoparticles were distributed on the surface of Al2O3, but some
Pt nanoparticles were slightly aggregated on Al2O3 (Figure S3b). The average size of
PtNi nanoparticles is measured to be ca. 2.4 ± 0.5 nm (Figure S4), which was smaller
than that of Pt nanoparticle (2.8 ± 0.6 nm, Figure S3c). Compared with Pt
nanoparticles in Al2O3-Pt, the smaller size and well-distribution of PtNi in Al2O3-PtNi
should also be favorable to the hydrolysis of AB molecule. Figure 1c shows the
enlarged TEM image of the dotted box in Figure 1b. The high-resolution TEM image
(HRTEM) of PtNi (inset in Figure 1c), which is recorded from the rectangular area in
Figure 1c, reveals a well-resolved lattice spacing of 0.21 nm, consistent with the (111)
lattice spacing of the PtNi alloy.[29] The corresponding EDX spectrum (Figure S5)
further confirmed the existence of Al, O, Pt and Ni elements in Al2O3-PtNi. Moreover,
the HAADF-STEM image of Al2O3-PtNi and elemental mapping distribution were
recorded in Figure 1d. It can be confirmed that the Al and O were uniformly
distributed through the as-prepared Al2O3-PtNi. While Pt and Ni elements show the
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same distribution without the segregation of each other throughout the image area,
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revealing the successful synthesis of homogeneous PtNi alloy nanoparticles that are
deposited on the surface of Al2O3. Furthermore, inductively coupled plasma mass
spectrometry (ICP-MS) characterization was conducted to analyze the Pt and Ni
content in Al2O3-PtNi. The mass fractions of Pt and Ni elements were 5 % and 1.5%,
respectively, suggesting that the atomic ratio of Pt/Ni was approximately 1:1.
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Figure 1. a) Schematic illustration of the synthesis process for Al2O3-PtNi. b) The TEM image of
Al2O3-PtNi, the PtNi nanoparticles are indicated by white arrows; c) the enlarged TEM image of
the dotted box area in a), the inset is the HRTEM image of the rectangular area in b); d) the
HAADF-STEM image of Al2O3-PtNi and the corresponding EDX elemental mapping of Al, O, Pt
and Ni, respectively.
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The chemical state and surface characterization of Pt and Ni atoms in the
as-prepared samples were investigated through X-ray photoelectron spectroscopy
(XPS). All of the XPS signals of Pt, Ni, Al, and O were detected in the Al2O3-PtNi
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sample (Figure S6). The high-resolution XPS spectra of Al 2p, Pt 4f regions for
Al2O3-Pt and Al2O3-PtNi samples are shown in Figure 2a. The peaks located at 74.5
eV corresponds to the Al 2p of Al2O3[30-32], and no shift of the peak position for Al
2p is observed. The other peaks at around 71.3 eV can be assigned to Pt 4f7/2 of
metallic Pt[33, 34], and the Pt 4f7/2 peak of Al2O3-PtNi shows a negative shift by 0.2
eV in contrast to that of Al2O3-Pt, suggesting a strong electron interaction between the
Pt and Ni atoms in Al2O3-PtNi. However, the corresponding Pt 4f5/2 peaks of metallic
Pt at around 74.6 eV were not observed due to the weak peaks of Pt 4f5/2 overlapped
by the strong peaks of Al 2p at 74.5 eV. The high-resolution XPS spectra of Ni 2p for
Al2O3-Ni and Al2O3-PtNi (Figure 2b) is well fitted with two spin-orbit doublets of Ni
2p3/2 and Ni 2p1/2 at around 855.8 and 874.8 eV, respectively. The peaks at ca. 880.1
and 862.5 eV with intense signals are the shake-up satellites arising from the
multi-electron excitation[35]. Further deconvolution of Ni 2p3/2 and Ni 2p1/2 spectra
for Al2O3-Ni reveal two different chemical environments for Ni. The peaks at 852.2
and 869.5 eV can be assigned to the metallic Ni(0), while the peaks located at 855.9
and 873.5 eV are ascribed to the Ni(II) oxidation state, which may be ascribed to the
interaction between Ni and lattice O atom in Al2O3. Compared with Al2O3-Ni, the
peak positions of Ni 2p3/2 and Ni 2p1/2 spectra for Al2O3-PtNi were positively shifted
by 0.2 eV. When Pt was alloyed with Ni metal, an electron-withdrawing effect would
be occurred from Ni atoms to Pt atoms due to the larger electronegativity of Pt (2.28)
than that of Ni (1.91), thus resulting in the lower Pt binding energy and higher Ni
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binding energy of Al2O3-PtNi, respectively. Figure S7 shows the X-ray diffraction
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patterns of the as-prepared samples. The XRD peak for PtNi(111) was detected, and
the relatively low intensity of PtNi(111) than that of Pt(111) suggested the smaller size
of PtNi in contrast to those of Pt, which was consistent with the result of TEM images
for the two sample (Figure S3).
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Figure 2. a) The high-resolution Al 2p & Pt 4f XPS profiles of Al2O3-PtNi and Al2O3-Pt,
respectively; b) The high-resolution Ni 2p XPS profiles of Al2O3-PtNi and Al2O3-Ni, respectively.
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It is well known that the dative bond of B-N in AB is formed by the donation of the N
lone-pair electrons to the empty p-orbital of B[36]. The difference in electronegativity (χ)
of the component hydrogen (χH=2.20), nitrogen (χN=3.04) and boron atoms (χB=2.04)
result in the heterogeneous charge distribution in the NH3BH3 molecule[37, 38]. Thus, the
H atoms attached to B are electronegative and those attached to N are
electropositive[39-41]. As reported by Chandra et al.[42], during the catalytic
dehydrogenation of AB in aqueous solution over metal catalyst, an activated complex
species between AB and the metal surface was formed first and then it was
immediately attacked by H2O molecules, leading to the dissociation of the B-N bond.
The resulted BH3 intermediate was hydrolyzed quickly to release H2 together with the
formation of borate ion. It has been reported that the metal catalyst surface can react
with the H atom within AB molecules to form the activated complex species, which is
the prerequisite to generate hydrogen[13, 43, 44]. Based on the above
dehydrogenation process of AB, a plausible mechanism of catalytic hydrogen
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evolution from AB over Al2O3-PtNi is proposed, as shown in Figure 3. As evidenced
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by the XPS result in Figure 2, electron naturally transfers from a less electronegative
Ni atom to a more electronegative Pt atom, forming an electric dipole between the
neighboring Pt-Ni atom pair (Figure 3, left). The newly generated local electric field
from the Pt-Ni dipole could affect the electron distribution in the AB molecule
(Figure 3, middle), and the electron-redistributed AB molecule may be more easily
interacted with the PtNi catalyst to form the activated complex(Figure 3, right).
Consequently, under the effect of Pt-Ni electric dipole, Al2O3-PtNi is believed to display
higher catalytic activity toward the dehydrogenation of AB in contrast to Al2O3-Pt and
Al2O3-Ni.
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Figure 3. The plausible process of catalytic dehydrogenation of AB over Al2O3-PtNi with the help
of the Pt-Ni dipole.
To verify the assumption mentioned above experimentally, the catalytic activity
of the as-prepared Al2O3-Pt, Al2O3-Ni, and Al2O3-PtNi were evaluated by the catalytic
dehydrogenation of AB tests. Briefly, 4 mL of AB solution (2 mmol, 500 mM) was
added into a 6 mL dispersion of as-prepared catalyst. The H2 gas generated from the
catalytic hydrolysis of AB reaction was collected in a water-filled inverted measuring
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cylinder and measured every ten seconds. Figure 4a shows the time course for
hydrogen production from the hydrolysis of AB. No hydrogen was detected in the
hydrolysis experiment over Al2O3-Ni even after 290 s, suggesting that Al2O3-Ni was
inactive for the catalytic hydrolysis of AB in our case. For both Al2O3-Pt and
Al2O3-PtNi, it was clear that no introduction periods were observed for the catalytic
hydrolysis of AB over these two samples, and the molar ratio of generated H2/AB was
3/1 during the catalytic course, revealing the complete dehydrogenation of AB. When
Al2O3-Pt was used as catalyst, it took 270 s to 100% decompose AB with a TOF value
of 95.07 molH2·(molPt·min)-1 (Table S1). Compared with Al2O3-Pt, Al2O3-PtNi
provided a TOF of 315.17 molH2·(molPt·min)-1 (Table S1), which was ca. 3.32 times
that of Al2O3-Pt, for the complete dehydrogenation of AB in only 90 s, providing solid
evidence on the superior ability of Al2O3-PtNi toward catalytic hydrolysis of AB. This
enhancement reasonably attributed to the effect of Pt-Ni electric dipole, which would
be discussed later in this article in-depth. Since the durability of the catalyst is also a
key parameter for its practical application, we also investigate the stability of
Al2O3-PtNi by recycling the catalyst for six runs. As shown in Figure 4b, although the
activity of Al2O3-PtNi showed a slight decrease, 100% AB was completely
decomposed in 4 min even after 6 runs, demonstrating that Al2O3-PtNi possessed
excellent durability during the recycling experiment. The slightly decreased activity
of Al2O3-PtNi may be due to the loss of catalytic materials during the separation
process of the catalyst after each run (the amount of catalyst was decreased from the
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initial 50 mg to final 43 mg after the sixth run).
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Figure 4. a) Time course for hydrogen production from AB over Al2O3-Pt, Al2O3-Ni, and
Al2O3-PtNi, respectively; b) Durability test for the hydrogen generation from aqueous AB in the
presence of Al2O3-PtNi catalyst; c) Plot of time vs. hydrogen evolution from AB hydrolysis over
10-100 mg Al2O3-PtNi at room temperature ([AB] = 200 mM); d) Plot of hydrogen generation rate
vs. the concentration of Pt, both in logarithmic scale; e) Plot of time vs. the volume of hydrogen
gas generated from AB hydrolysis by 50 mg Al2O3-PtNi at different temperatures; f) Arrhenius
plot of ln[K] vs. the reciprocal reaction temperature 1/T in the temperature range of 20-47 oC.
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To study the reaction kinetics of AB hydrolysis, the time courses of hydrogen
evolution in the presence of Al2O3-PtNi with different mass were tested. When the
mass usage of Al2O3-PtNi was 10 mg, 20 mg, 50 mg, and 100 mg, the concentration
of Pt in the reaction solution was 0.24, 0.48, 1.20 and 2.40 mM, respectively. As
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shown in Figure 4c, the time for completion of hydrolysis AB was decreased with the
increase of catalyst mass from 10 to 100 mg, revealing that the increased
concentration of Pt in the reaction solution can accelerate the hydrolysis rate of AB.
Based on the results in Figure 4c, the reaction rates (K) can be determined from the
linear portion of each plot. Figure 4d shows the relation between the reaction rates
and the Pt concentration in logarithmic scale. It can be seen that the line slope is 1.04,
clearly demonstrating that the hydrolysis of AB was first-order with respect to the Pt
concentrations. Furthermore, we investigated the effect of temperature on the
hydrogen evolution rate from the hydrolysis of AB. As shown in Figure 4e, the tests
of catalytic hydrolysis of AB over Al2O3-PtNi were conducted at different
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o
temperatures in the range of 20 to 47 C. It can be seen that the time for the
completion of hydrolysis of AB was decreased from 160 to 30 s when the reaction
o
temperature was increased from 20 to 47 C, with the TOF increased from 199.19 to
1106.61 molH2·(molPt·min)-1 (Table S2), clearly confirming that temperature has a
great influence on the hydrolysis kinetics of AB. Moreover, another parameter to
assess the effectiveness of the catalyst is to evaluate its ability in reducing the
activation barrier related to the hydrolysis of AB. Figure 4f shows the Arrhenius plots
of ln[K] vs. the reciprocal reaction temperature 1/T in the temperature range of 20-47
oC. From the corresponding Arrhenius plots, the activation energy of Al2O3-PtNi was
calculated to be 45.679 kJ/mol. When comparing with most reported Pt-based
catalysts, our samples displayed a comparable catalytic performance in hydrolysis of
AB, as shown in Table S3.
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To further explore the role of the local electric field generated by the Pt-Ni dipole
in the process of AB hydrolysis, a series of Density Functional Theory (DFT)
calculations were conducted. Bader charge analysis revealed that ca. 0.3 e was
transferred from Ni atoms to the neighboring Pt atoms in the (111) facet of PtNi, as
shown in Figure S8, which led to the formation of Pt-Ni dipole, consistent with the
XPS result (Figure 2). The absorption energy of AB and H2O molecule on the (111)
facet of PtNi was estimated to be ca. -0.919 eV and -0.195 eV, respectively (Table S4),
revealing that AB molecule was preferentially absorbed on the PtNi (111) surface
instead of H2O molecule. It can be seen that one of the H atom on BH3 group was
bonded to a surface Pt atom (Figure 5a). In a free NH3BH3 molecule, the lengths of
B-H bond and B-N bond are 1.209 Å and 1.649 Å, respectively (Table S7). Once the
AB molecule absorbed on PtNi(111) surface, DFT calculations revealed that the B-H
and B-N bond length are 1.361 Å and 1.602 Å, which are increased by 0.152 Å and
decreased by 0.047 Å relative to the free molecule, respectively. This suggested that
the B-H bond in the AB molecule was activated by priority. Further, the influence of
the local electric field by the Pt-Ni dipole on the AB molecule was simulated by
calculating the molecule in the external electric field of 0.032 a.u.. With the external
electric field applied in the direction of the local field, more negative charges
accumulated to the BH3 group (-0.334 |e| to -0.422|e|, Table S5), which was
transferred from NH3 group via the “B-N bond” channel (Figure 5b). The increased
negative charges in the H atom attached on B atom (-0.10690 |e| to -0.13776 |e|, Table
S6) could enhance the interaction between AB molecules and the vacant 5d band of Pt
in Pt-Ni dipole (Figure 5b), thus promoting the formation of the activated complex,
which is the prerequisite to generate hydrogen[13, 43, 44]. The structure analysis
indicated that the electric field (0.032 a.u.) from Ni atom to Pt atom made the B-N
bond length shorter (1.649 Å to 1.613 Å, Table S7), but the B-H bond length became
longer (1.209 Å to 1.224 Å, Table S7), suggesting that the local electric field can also
activate the B-H bond in AB molecule. These analyses suggested that the activation of
AB molecule on the PtNi catalyst can be attributed to the local electric field assisted
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electronic structure of PtNi, thus enhancing the kinetic process of catalysis.
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Figure 5. a) The adsorption configuration of AB molecule on PtNi (111) surface and b)
the proposed mechanism for the effect of Pt-Ni dipole on the hydrolysis of AB.
4. Conclusion
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In summary, compared with Al2O3-Pt, the TOF value toward the dehydrogenation of
AB was greatly enhanced when introducing the Ni into Pt atoms to form PtNi alloy on
Al2O3. Due to the larger electronegativity of Pt than that of Ni atoms, the electron
donation from Ni atoms to Pt atoms led to the formation of electric dipole between a
neighboring Pt-Ni atom pair. In the external local field generated by the Pt-Ni dipole,
the redistribution of electrons in AB molecule resulted in the elongated B-H bond
length and the increased negative charges in the H atom on BH3 group, contributing to
the activation of the B-H bond and the enhanced interaction between the AB molecule
and the Pt atom in Pt-Ni dipole. Therefore, this unique “electric-dipole” effect was the
reason for the higher catalytic activity of Al2O3-PtNi toward the dehydrogenation of
AB in contrast to Al2O3-Pt and Al2O3-Ni. This work shed a light on the catalytic
process of the molecule with a permanent dipole under the local field generated from
the electric dipole on the surface of alloy catalyst.
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Acknowledgments
This work is supported by National Natural Science Foundation of China (No.
51802255), the National Key Research and Development Program of China
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(2017YFE0193900), Key Scientific and Technological Innovation Team of
Shaanxi province(2020TD-001), China Postdoctoral Science Foundation (No.
2017M623161), the China Fundamental Research Funds for the Central
Universities, and the Fundamental Research Funds and the World-Class
Universities (Disciplines) and the Characteristic Development Guidance Funds
for the Central Universities. The theoretical calculations were performed using
the HPC Platform of Xi’an Jiaotong University. We thank Liqun Wang,
Xiaojing Zhang and Chao Li for the help of data analyses, and we also thank
the characterization support, such as TEM, XPS, ICP-MS, from the Instrument
Analysis Center of Xi’an Jiaotong University.
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Highlights
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ò
ò
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An electric dipole is formed between the neighboring Pt and Ni atom.
The Pt-Ni dipole generates a local external electric field.
The electric field increases negative charges in H atoms on BH3 group and B-H length.
The electric field helps to improve the interaction between AB molecule and Pt atom.
The electric dipole effect leads to the higher catalytic activity of Al2O3@PtNi.

Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships
that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered
as potential competing interests: