Synthesis of nickel phosphide nanorods for hydrotreating reactions

Article abstract of DOI:10.1016/j.catcom.2019.03.006

Nickel phosphate (NiPO) nanofibers with high surface area (288 m²/g) were obtained from phosphoric acid and nickel nitrate in an ionic liquid ([Bmim]Br) via an ionothermal synthesis process at 150 °C. Heating the nanofibers under a H₂ atmosphere at 700 °C led to the formation of Ni₂P nanorods, ~50 nm in diameter and 100–200 nm in length. The Ni₂P nanorod catalyst exhibited a much higher turnover frequency in the hydrodesulfurization of dibenzothiophene than the previously reported Ni₂P catalysts with nanoparticle, nanosheet or irregular block morphologies, which may be attributable to the morphology effect of the nanorods.

Full text of DOI:10.1016/j.catcom.2019.03.006

Accepted Manuscript
Synthesis of nickel phosphide nanorods for hydrotreating
reactions
Yu Zhao, Shidong Wang, Ning Li, Qi Zhang, Guixian Li, Jianyi
Shen
PII:
S1566-7367(19)30073-1
DOI:
https://doi.org/10.1016/j.catcom.2019.03.006
CATCOM 5644
Reference:
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
24 October 2018
25 February 2019
5 March 2019
Please cite this article as: Y. Zhao, S. Wang, N. Li, et al., Synthesis of nickel phosphide
nanorods for hydrotreating reactions, Catalysis Communications, https://doi.org/10.1016/
j.catcom.2019.03.006
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ACCEPTED MANUSCRIPT
Revised manuscript for Catalysis Communications
Synthesis of nickel phosphide nanorods for hydrotreating reactions
1
1
1
1
1
2
Yu Zhao* , Shidong Wang , Ning Li , Qi Zhang , Guixian Li and Jianyi Shen
1
School of Petrochemical Engineering, Lanzhou University of Technology, Lanzhou
2
7
30050, Gansu, China; School of Chemistry and Chemical Engineering, Nanjing
University, Nanjing 210093, Jiangsu, China
*
Corresponding author.
Permanent address: School of Petrochemical Engineering, Lanzhou University of
Technology, Lanzhou 730050, Gansu, China
Tel. and Fax: +86-931-7823125
E–mail: yzhao@lut.edu.cn
Abstract:
2
Nickel phosphate (NiPO) nanofibers with high surface area (288 m /g) were
obtained from phosphoric acid and nickel nitrate in an ionic liquid ([Bmim]Br) via an
ionothermal synthesis process at 150 ºC. Heating the nanofibers under a H2
atmosphere at 700 ºC led to the formation of Ni P nanorods, ~50 nm in diameter and
2
1
00–200 nm in length. The Ni P nanorod catalyst exhibited a much higher turnover
2
frequency in the hydrodesulfurization of dibenzothiophene than the previously
reported Ni P catalysts with nanoparticle, nanosheet or irregular block morphologies,
2
which may be attributable to the morphology effect of the nanorods.
Key words: Nickel phosphate nanofibers, Ionothermal synthesis, Ni P nanorods,
2
Hydrotreating reactions
1

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1
. Introduction
Metal phosphide catalysts (e.g., Ni P) have received a lot of attention due to their
2
high activity in hydrodesulfurization (HDS) and hydrodenitrogenation (HDN)
reactions when compared to traditional Co(Ni)-Mo-S catalysts [1], and their increased
thermal stability in a variety of sulfur-containing environments [2]. The latest report
shows that Ni P is not only used as an excellent HDS catalyst, but also has extended
2
application in other important reactions, such as the dehydrogenation [3] and
hydrodeoxygenation reactions [4-6]. In addition, Ni P has been extensively studied as
2
an electrode material in recent years [7,8], which shows its wide application
prospects.
There have been many studies reporting various methods used to prepare Ni P,
2
such as the temperature programmed reduction (TPR) of a range of nickel precursors
and phosphorous sources in flowing H at high temperature [9], phosphorization of
2
2

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nickel or nickel oxides using PH [10], treatment of nickel in trioctylphosphine (TOP)
3
[
11], the self-oxidation-reduction reaction between nickel cations and hypophosphite
or phosphite at low temperature [12,13], etc. Among these methods, the TPR method
is the easiest and most stable, and thus, the most widely used. However, the high
temperatures used in the TPR process may lead to the agglomeration and sintering of
the Ni P particles, which results in bulk crystals. Ni P crystals with specific
2
2
morphology are difficult to obtain using this method and has been rarely reported to
date.
The synthesis of inorganic nanomaterials in ionic liquids has attracted a lot of
attention since Morris et al. defined the term “ionothermal synthesis” in 2004 and
prepared aluminophosphate zeolite analogues in ionic liquids or eutectic mixtures [14].
In this method, the ionic liquid acts as both the solvent and template, which makes the
resulting mixture simpler. Ionic liquids have negligible vapor pressure, so the
synthesis process can be carried out using an open system, which eliminates the
danger of high pressures and makes the process easier. Many metal phosphates
nanomaterials have been prepared using ionic liquids as solvent, such as
aluminophosphate zeolite [15], zirconium phosphate [16], germanium phosphate [17]
and vanadium phosphate catalysts [18]. However, the synthesis of nickel phosphate
with special morphology using an ionothermal method has rarely been reported.
In this study, nickel phosphate nanofibers with high surface area were prepared
using an ionothermal process. Hydrotreatment of this nanofiber precursor using the
TPR method led to the formation of a Ni P catalyst with nanorod morphology. The
2
Ni P nanorod catalyst was used in the HDS of dibenzothiophene (DBT) and
2
hydrogenation of tetralin in a model diesel sample, which displayed higher catalytic
activity than that of the corresponding Ni P nanosheet and nanoparticle catalysts,
2
3

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which was attributed to the morphology effect.
2
. Experimental
2
.1 Catalyst preparation
The [Bmim]Br ionic liquid was prepared according to a previous process [15].
The nickel phosphate nanofibers were prepared using an ionothermal method. A total
of 2.91 g (0.01 mol) of Ni(NO ) •6H O and 1.73g (0.015 mol) of 85% phosphoric
3
2
2
acid were added to 87.7 g (0.4 mol) of the as-prepared [Bmim]Br with constant
stirring. The mixture was stirred at 50 ºC for a period of time to ensure sufficient
dissolution had occurred. The resulting solution was then heated to 150 ºC and heated
at reflux for 72 h and then allowed to cool to room temperature. The resulting
precipitate was filtered, washed three times with deionized water and two times with
n-butyl alcohol, and dried at 120 ºC for 12 h to give the nickel phosphate nanofibers
(NiPO-IL) as a green solid.
To obtain the Ni P nanorods, a TPR step was carried out. The NiPO-IL precursor
2
was heated under an atmosphere of flowing H (40 mL/min) to 400 ºC at a heating
2
rate of 5 ºC/min and then to 700 ºC at a heating rate of 1 ºC/min and finally kept at
7
00 ºC for 2 h. After being cooled down to room temperature under an H atmosphere,
2
the material was passivated in 0.5% O /N for 12 h. The resulting sample was named
2
2
Ni P-IL.
2
For comparison, a nickel phosphate sample was also prepared via a traditional
co-precipitation method using water as the reaction solvent, which was named
NiPO-H O. The NiPO-H O precursor was then subjected to the TPR process under
2
2
the same reaction conditions described above to prepare the Ni P sample, which was
2
signed as Ni P-H O.
2
2
2
.2 Characterization
4

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The samples were characterized by N adsorption-desorption measurements,
2
X-ray diffraction (XRD), the microcalorimetric adsorption of CO, scanning electron
microscopy (SEM). Experimental details can be seen in the supplementary
information.
2
.3 Catalytic tests
The catalytic tests were carried out for the HDS of DBT and hydrogenation of
tetralin in a model diesel sample containing 1.72% DBT (3000 ppm S), 5% tetralin
and 0.5% n-octane (as an internal standard) in n-nonane (solvent). A fixed-bed reactor
was used to perform the reactions. The reaction conditions can be seen in the
supplementary information.
3
. Results and discussion
3
.1 Textural and structural properties
Nickel phosphate nanofibers can be obtained via an ionothermal synthesis
process using Ni(NO ) •6H O and phosphoric acid in [Bmim]Br. Fig. 1(a) shows the
3
2
2
SEM images of the final NiPO-IL material. It can be seen that the NiPO-IL sample
exhibited a uniform nanofiber morphology with diameters in the range of 10–20 nm
and several hundred nm in length. The reduction of this nanofiber precursor under a
H atmosphere at 700 ºC led to the formation of the Ni P nanorod catalyst, 100–200
2
2
nm in length and 30–50 nm in diameter, as shown in Fig. 1(b). For comparison, the
nickel phosphate sample prepared using a traditional co-precipitation method in water
possessed an irregular block morphology (not shown). Thus, the ionic liquid played
an important role in the synthesis of nickel phosphate with nanofiber morphology due
to both its solvent and template effect. Nickel phosphates with nanofiber or nanotube
morphologies have also been reported in past reports. However, almost all these
materials are prepared using hydrothermal methods under a high temperature and high
5

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pressure environment and require the addition of a template agent, which is obviously
more complex and dangerous than the ionothermal method described in this paper.
During the preparation of Ni P via the TPR method, the reported Ni P products
2
2
showed an irregular block morphology due to the significant aggregation of the
particles at high temperature (650–750 ºC). However, a Ni P catalyst with a uniform
2
nanorod morphology could be still obtained after the TPR treatment at 700 ºC in this
work due to the high stability of the NiPO nanofiber precursor.
Fig. 2 shows the XRD patterns of the as-prepared NiPO and those obtained after
reduction in flowing H at 400 ºC and 700 ºC, respectively. The nickel phosphate
2
nanofibers prepared via an ionothermal process exhibited a series of broad peaks in
the range of 25–35°, indicating that it was prepared in a mainly amorphous phase.
After treatment of this nanofiber precursor in H at 400 ºC, the XRD patterns of the
2
product still showed a broad peak in the range of 25–35°, which showed the high
stability of the NiPO-IL precursor. When the reduction temperature was increased to
7
00 ºC, sharp diffraction peaks were detected, which were all consistent with
crystalline Ni P (JCPDS No. 74-1385) indicating that a high-purity Ni P catalyst was
2
2
prepared using the TPR process. According to Scherrer equation, the average sizes of
Ni P crystallites were estimated to be about 33.7 nm which was confirmed by SEM.
2
Table 1 summarizes the specific surface area, pore volume and pore diameter of
the as-prepared samples. The nickel phosphate nanofibers (NiPO-IL) prepared via an
2
ionothermal synthesis method possessed a high surface area of 288 m /g, while the
2
surface area of the NiPO-H O sample prepared in water was only 44 m /g. The pore
2
3
volume of NiPO-IL was 1.69 cm /g, while that of NiPO-H O was small. Fig. 3 shows
2
the N adsorption-desorption isotherms and pore size distribution curves obtained for
2
the NiPO-IL and NiPO-H O samples. NiPO-H O only possessed a very low
2
2
6

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adsorption capacity and showed an extremely weak pore size distribution peak at ~6
nm. However, NiPO-IL exhibited a high adsorption capacity and the N2
adsorption-desorption isotherms were typical of type IV with H3 model hysteresis
loops, which indicated a mesoporous structure. The corresponding pore size
distribution of NiPO-IL showed a relatively narrow peak at ~25 nm, which was
probably from the fiber packing gaps.
After heat treatment of NiPO-IL under an H atmosphere at 700 ºC, the
2
2
as-obtained Ni P-IL catalyst possessed a surface area of 14 m /g, which was
2
significantly decreased when compared with the NiPO-IL precursor due to its
deoxidation process and structural modification at high temperature. However, the
surface area of the Ni P-IL catalyst was still much higher than that of Ni P-H O (2
2
2
2
2
m /g ) prepared via the TPR process of NiPO-H O.
2
3
.2 The surface properties as-probed by the microcalorimetric adsorption of CO
The adsorption of CO can be used to titrate the number of surface active sites in
a Ni P catalyst [19]. Thus, the microcalorimetric adsorption of CO on the Ni P
2
2
catalysts prepared in this work was carried out and the results were shown in Fig. 4.
The Ni P/NiPO nanosheet catalyst (Eut-NP3-H) reported in our previous work [20]
2
was prepared from the self oxidation-reduction reaction of nickel hypophosphite in
the eutectic mixture and possessed an initial heat of 79 kJ/mol. The initial heats for
the Ni P-IL sample prepared in this work was 74 kJ/mol, which was similar with that
2
of Eut-NP3-H indicating a similar number of active sites are present on the surface of
these Ni P catalysts. The saturation coverage of CO was measured to be about 38 and
2
1
9 mol/g for the Eut-NP3-H nanosheet and Ni P-IL nanorod catalysts, respectively.
2
Although the Ni P-IL catalyst possessed a lower number of surface active sites on
2
Ni P than the Eut-NP3-H catalyst, considering the surface area of the Eut-NP3-H
2
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2
sample was much higher than that of the Ni P-IL nanorod catalyst (133 m /g vs. 14
2
2
m /g), the adsorption quantity of CO per surface area observed for the Ni P-IL
2
nanorod sample was much higher than that of the Eut-NP3-H sample, which was
attributed to the morphology effect. For comparison, the Ni P-H O sample prepared
2
2
in water possessed an extremely low adsorption quantity of CO (3 mol/g) and small
initial heat, indicating the low number of active sites in this sample due to its low
surface area.
3
.3 Catalytic behavior
The as-synthesized Ni P-IL nanorod catalyst and two other catalysts (Eut-NP3-H
2
and Ni P-H O) were used for the HDS of DBT and the hydrogenation of tetralin in a
2
2
-1
model diesel sample. At 3.1 MPa, a weight hourly space velocity (WHSV) of 4 h
and 360 ºC, the conversion of DBT was observed to be 60.2% and 75.8% on Ni P-IL
2
and Eut-NP3-H, respectively, which was much higher than that observed on the
Ni P-H O catalyst (44.8%). Two products (biphenyl and cyclohexylbenzene) were
2
2
detected for the HDS of DBT over the catalysts in this work. The selectivity to
cyclohexylbenzene and biphenyl was found to be about 15.2 and 84.8%, respectively,
over the Ni P-IL at 360 ºC, which was similar to that for the Eut-NP3-H (13.7 and
2
8
6.3%, respectively) [20], indicating that the reactions were all mainly through the
direct desulfurization (DDS) mechanism over the two catalysts. Although the
performance of Ni P-IL was lower than that of the Eut-NP3-H catalyst, a full
2
comparison of the intrinsic activity over the two catalysts could not obtained from the
numerical value of the conversions due to the great differences between the surface
area of the two catalysts. Thus, the inherent HDS activity of the Ni P catalysts were
2
calculated in the form of the turnover frequency (TOF) based on two standards, which
was the number of active sites and surface area, respectively. The HDS TOF based on
8

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the number of active sites was calculated according to the equation TOFactivity sites
=
QX/S, where Q is the molar rate of DBT, X is the conversion and S is the number of
moles of sites loaded (the mass of catalysts loaded multiplied by the adsorption
quantity of CO). The value of TOFactivity sites obtained for the Eut-NP3-H nanosheet
-1
catalyst was obtained to be 0.0021 s , while that for Ni P-IL nanorod catalyst was
2
-1
0
.0033 s . Obviously, the inherent HDS activity of the Ni P nanorod catalyst was
2
higher than that of the nanosheet catalyst, which was also much higher than the other
nanoparticle metal phosphide catalysts reported in the literature (such as Ni P/SiO
2
2
-1
-1
(
0.0015 s ) [21] and CoP/γ-Al O (0.00177 s ) [22]) as seen in Fig. 5. For the HDS
2 3
TOF based on the surface area of the catalysts, it was also calculated according to the
equation TOFsurface areas = QX/S, where Q is the molar rate of DBT, X is the conversion,
while the substitutional S is the total surface area loaded (the mass of catalysts loaded
by the BET specific surface area). The calculated value of TOFsurface areas obtained for
-1
-2
the Ni P-IL nanorod catalyst was 0.0045 mol•s •m , while that for the Eut-NP3-H
2
-1
-2
nanosheet catalyst was only 0.00059 mol•s •m . The HDS TOF
for the Ni2P
surface areas
nanorod catalyst was an order of magnitude higher than that observed for the
nanosheet catalyst. Upon combining the two series of results mentioned above, the
inherent HDS activity of the Ni P nanorod catalyst was higher than that of the
2
nanosheet catalyst and the other irregular morphology catalysts probably due to the
morphology effect [23].
The number of multi-ring hydrocarbons in diesel fuel is also an important
parameter for fuel oil quality because they have low cetane numbers. In this work,
tetralin was chosen as a model compound to study the catalytic hydrogenation of
aromatic rings. At 360 ºC, the conversion of tetralin over the Ni P-IL nanorod catalyst
2
was about 8.2%, which was similar with that of the Eut-NP3-H nanosheet catalyst
9

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(
8.9%), while that over the Ni P-H O catalyst was only 3.1% (see Fig. S1 in SI). In
2
2
consideration of the great differences observed between the Ni P-IL and Eut-NP3-H
2
catalyst, the Ni P-IL nanorod catalyst was superior over the Eut-NP3-H catalyst.
2
Under the current reaction conditions, the hydrogenation of tetralin led to the
formation of decalin while the dehydrogenation of tetralin produced naphthalene. Due
to the cetane number of naphthalene being zero, the dehydrogenation of tetralin
should be restrained. At 360 ºC, the selectivity to decalin (total of trans- and
cis-decalin) over the Ni P-IL nanorod catalyst reached 42.2%, which was much higher
2
than that observed over the Eut-NP3-H (28.2%) and Ni P-H O (only 6.7%) catalysts,
2
2
indicating the high hydrogenation activity of the Ni P-IL nanorod catalyst due to the
2
morphology effect.
4
. Conclusions
Phosphoric acid and nickel nitrate were heated at 150 ºC in an ionic liquid
([Bmim]Br) resulting in the formation of nickel phosphate (NiPO) nanofibers with a
2
high surface area of 288 m /g. Heating this material under an atmosphere of H at 700
2
ºC using a TPR process led to the formation of Ni P nanorods, ~50 nm diameter and
2
1
00–200 nm length. Ni P catalysts prepared with specific morphologies using a TPR
2
method have been rarely reported because the high temperature used in the process
often leads to agglomeration and sintering of the Ni P particles, which commonly
2
results in an irregular block morphology. The hydrogenation activity over the
as-synthesized Ni P nanorod catalyst was investigated using the HDS of DBT and
2
hydrogenation of tetralin in a model diesel sample. Due to the morphology effect, the
Ni P nanorods catalyst exhibited a much higher turnover frequency (TOF) in the HDS
2
of DBT and hydrogenation of tetralin than the previous reported Ni P catalysts with
2
nanoparticle, nanosheet or irregular block morphologies.
10

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Acknowledgements:
This work was supported by the NSFC (21203081 and 21763016) and Gansu
Province of China (18YF1GA062).
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Tables
Table 1: Surface properties of the catalysts.
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Table 1: Surface properties of the catalysts
Samples
Surface area
Pore volume
Average Pore
diameter (nm)
2
3
(
m /g)
(cm /g)
NiPO-IL
288
44
14
2
1.69
0.06
--
25.0
5.7
--
NiPO-H O
2
Ni P-IL
2
Ni P-H O
--
--
2
2
15

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Figure captions
Figure 1: SEM images of NiPO-IL (a) and Ni P-IL (b).
2
Figure 2: XRD patterns of as-prepared NiPO and after reduction in flowing H at 400
2
ºC and 700 ºC, respectively.
Figure 3: N adsorption-desorption isotherms of NiPO-IL and NiPO-H O. Inset shows
2
2
the corresponding pore size distributions.
Figure 4: Microcalorimetric adsorption of CO over the samples.
16

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Figure 5: The HDS TOF based on active sites over nanorod Ni P-IL catalyst and
2
previous reported metal phosphide catalysts.
Figure 1: SEM images of NiPO-IL (a) and Ni P-IL (b).
2
17

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

Ni P
2





NiPO-700H
NiPO-400H
NiPO





1
0
20
30
40
50
60
70
80
2
Theta (degree)
Figure 2: XRD patterns of as-prepared NiPO and after reduction in flowing H at 400
2
ºC and 700 ºC, respectively.
18

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1200
4

NiPO-IL
3
2
1
0
900
600
300
0
 NiPO-H O
2
0
10
20
30
40
50
Pore diameter (nm)
0.0
0.2
0.4
0.6
0.8
1.0
P/P0
Figure 3: N adsorption-desorption isotherms of NiPO-IL and NiPO-H O. Inset shows
2
2
the corresponding pore size distributions.
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100

Eut-NP3-H
80
60
40
20
0
 Ni P-IL
2

Ni P-H O
2
2
0
10
20
30
40
CO coverage (mol/g)
Figure 4: Microcalorimetric adsorption of CO over the samples.
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3
.5
.0
.5
.0
.5
.0
.5
.0
3
2
2
1
1
0
0
Ni P-IL Eut-NP3-H Ni P/SiO
CoP/Al O3
2
2
2
2
nanorod nanosheet nanoparticle nanoparticle
Figure 5: The HDS TOF based on active sites over nanorod Ni P-IL catalyst and
2
previous reported metal phosphide catalysts.
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Highlights



Nickel phosphate nanofibers were obtained via an ionothermal synthesis process.
Heating the nanofibers in H at 700 ºC led to the formation of Ni P nanorods.
2
2
The Ni P nanorod catalyst exhibited high activity for the hydrotreating reactions.
2
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