Facile Preparation of Ni2P with a Sulfur-Containing Surface Layer by Low-Temperature Reduction of Ni2P2S6

Article abstract of DOI:10.1002/anie.201510599

Preparation of Ni₂P by temperature-programmed reduction (TPR) of a phosphate precursor is challenging because the P-O bond is strong. An alternative approach to synthesizing Ni₂P, by reduction of nickel hexathiodiphosphate (Ni₂P₂S₆), is presented. Conversion of Ni₂P₂S₆ into Ni₂P occurs at 200-220 °C, a temperature much lower than that required by the conventional TPR method (typically 500 °C). A sulfur-containing layer with a thickness of about 4.7 nm, composed of tiny crystallites, was observed at the surface of the obtained Ni₂P catalyst (Ni₂P-S). This is a direct observation of the sulfur-containing layer of Ni₂P, or the so-called nickel phosphosulfide phase. Both the hydrodesulfurization activity and the selective hydrogenation performance of Ni₂P-S were superior to that of the catalyst prepared by the TPR method, suggesting a positive role of sulfur on the surface of Ni₂P-S. These features render Ni₂P-S a legitimate alternative non-precious metal catalyst for hydrogenation reactions.

Full text of DOI:10.1002/anie.201510599

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201510599
German Edition: DOI: 10.1002/ange.201510599
Hydrocarbon Processing
Facile Preparation of Ni2P with a Sulfur-Containing Surface Layer by
Low-Temperature Reduction of Ni2P2S6
Song Tian, Xiang Li,* Anjie Wang, Roel Prins, Yongying Chen, and Yongkang Hu
Abstract: Preparation of Ni2P by temperature-programmed
reduction (TPR) of a phosphate precursor is challenging
counterparts.[5] It was not until 1996, when Robinson et al.[6]
reported that Ni2P had higher activity in quinoline hydro-
À
because the P O bond is strong. An alternative approach to
denitrogenation (HDN) than
a
commercial sulfided
synthesizing Ni2P, by reduction of nickel hexathiodiphosphate
(Ni2P2S6), is presented. Conversion of Ni2P2S6 into Ni2P occurs
at 200–2208C, a temperature much lower than that required by
the conventional TPR method (typically 5008C). A sulfur-
containing layer with a thickness of about 4.7 nm, composed of
tiny crystallites, was observed at the surface of the obtained
Ni-Mo/Al2O3 catalyst, that they again caught the attention
of the scientific community as a new family of hydrotreating
catalysts. Among the investigated TMPs (Fe2P, CoP, MoP, WP,
and Ni2P for example), Ni2P is the most active catalyst in the
simultaneous hydrodesulfurization (HDS) of dibenzothio-
phene (DBT) and HDN of quinoline.[7] In recent years, TMPs
have been reported as promising for many other reactions;
such as hydrodeoxygenation, hydrogen evolution, and selec-
tive hydrogenation.[8]
À
Ni2P catalyst (Ni2P S). This is a direct observation of the
sulfur-containing layer of Ni2P, or the so-called nickel phos-
phosulfide phase. Both the hydrodesulfurization activity and
the selective hydrogenation performance of Ni2P-S were
superior to that of the catalyst prepared by the TPR method,
suggesting a positive role of sulfur on the surface of Ni2P-S.
These features render Ni2P-S a legitimate alternative non-
precious metal catalyst for hydrogenation reactions.
There are various methods for synthesizing TMPs, of
which the most commonly used is the temperature-pro-
grammed reduction of metal phosphate precursors in flowing
H2 at elevated temperature (TPR).[9] The P O bond in
À
phosphate is strong, therefore its reduction requires high
temperatures (generally > 5008C).[9] One approach to achieve
lowering of the reduction temperature is to use phosphorus
sources other than phosphate (for example, dihydrogenphos-
phite,[10] hypophosphite,[11] PH3,[12] tris(trimethylsilyl)phos-
phine, or trioctylphosphine[13]); or to perform the solvother-
mal reduction with Na3P or phosphorus (yellow or red).[9]
However, according to Da Silva et al.[14] and Wang et al.,[15]
these methods present disadvantages that may limit their
P
hosphorus reacts with most elements of the periodic table
to form a diverse class of compounds known as phosphides.[1]
Many metal phosphides accept several stoichiometries, pro-
viding a large number of structures.[2] According to the metal/
phosphorus ratio (M:P), the transition-metal phosphides
(TMPs) can be classified as metal-rich phosphides
(M:P > l), monophosphides (M:P = l), and phosphorus-rich
phosphides (M:P < 1). Phosphorus-rich TMPs are semicon-
ducting and are considerably less stable than metal-rich
compounds.[3] Metal-rich TMPs are covalent compounds and
usually possess metallic character.[3] They are hard, electrical
conductors, and have high thermal stabilities and resistance to
chemical attack, and thus attract much attention as catalytic
materials for hydrogenation reactions.[3]
application. Another alternative is to use precursors contain-
[16]
À
À
ing P S bonds, which are easier to break than P O bonds.
In 1996, two years before the TPR method was employed by
Li et al. for the synthesis of TMPs,[17] Robinson et al. obtained
a sulfur-free Ni2P catalyst by decomposing a nickel thiophos-
phate precursor (denoted by the authors as NiPS3) under
a 10 vol% H2S/H2 atmosphere.[6] Nevertheless, the NiPS3
precursor was synthesized in this study by reacting stoichio-
metric quantities of elementary nickel, red phosphorus, and
sulfur at high temperature (7008C) and over a long reaction
period (3.5 days). By adopting a soft-chemistry route, NiPS3
can be synthesized by reaction of NiCl2 or Ni(NO3)2 with
Li2PS3 at room temperature.[16]
As early as the 1950s, the metal-rich dinickel phosphide,
NiP0.584, was reported to be active in vapor-phase reduction of
nitrobenzene with hydrogen into aniline and water.[4] There-
after, the hydrogenation, dimerization, polymerization, and
hydroformylation–carbonylation performance of TMPs were
examined in the 1970s and 1980s.[5] They were found to
possess lower hydrogenation activity than their metallic
One of the unique properties of TMPs relative to their
metallic counterparts is that sulfur plays a positive role in
some reactions performed over TMPs. For both Ni2P and
MoP, the most active HDS site has been identified as a surface
phosphosulfide that is generated during reaction.[18] Kibs-
gaard and Jaramillo found that introduction of sulfur onto the
surface of MoP produced a molybdenum phosphosulfide
catalyst with superb activity and stability for hydrogen
evolution in acidic environments.[19] The surface sulfidation
of Ni2P is more difficult than that for MoP. Sun et al.
demonstrated that MoP/SiO2 can be sulfided with a mixture
of thiophene/H2, whereas more severe sulfiding conditions
[*] S. Tian, Dr. X. Li, Prof. Dr. A. Wang, Y. Chen, Prof. Y. Hu
State Key Laboratory of Fine Chemicals, School of Chemical
Engineering, Dalian University of Technology
No. 2 Linggong Road, Dalian 116024 (P.R. China)
E-mail: lixiang@dlut.edu.cn
Prof. Dr. R. Prins
Institut für Chemie- und Bio-Ingenieurwissenschaften, ETH Zürich
Vladimir-Prelog-Weg 1-5/10, 8093 Zürich (Switzerland)
Supporting information for this article can be found under http://dx.
doi.org/10.1002/anie.201510599.
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and an H2S/H2 mixture were required to transform Ni2P/SiO2
the peak at 2q = 14.08 became the most intense diffraction
into its phosphosulfide counterpart.[20]
line (Supporting Information, Figure S2), suggesting that
some of the Ni2P2S6 species was converted to NiPS3.
Apparently, conversion of Ni2P2S6 into Ni2P is a result of
reduction of Ni2P2S6 in H2 rather than thermal decomposition.
Reduction of Ni2P2S6 starts at a temperature between 200 and
2208C, much lower than that required by the conventional
TPR method (typically 5008C), but also lower than that
reported for the conversion of NiPS3 into Ni2P. Because of the
slow kinetics of reduction, Ni2P2S6 was reduced to Ni2P after
8 h at 2208C (Supporting Information, Figure S3). By means
of in situ XRD and EXAFS, LobouØ et al. observed that Ni2P
can be obtained from a NiPS3 sample prepared by a soft-
chemistry approach at a reduction temperature of 3008C,
whereas a well-crystallized NiPS3 precursor (PDF card
78-0499) was first reduced into a Ni5P4 intermediate at
3508C, with Ni2P formation at 5008C.[16] Besides temperature,
H2 pressure is another determining factor in the reduction of
Ni2P2S6 into Ni2P. At 0.1 MPa H2 pressure, no Ni2P was
formed at temperatures up to 3008C after an 8 h reduction of
Ni2P2S6 (Supporting Information, Figure S4). H2 flow rate is
not as important for reduction as temperature and H2
pressure. At 1.0 MPa H2 pressure and 3008C, a decrease of
H2 flow rate from 100–25 mLminÀ1 did not affect the
formation of Ni2P (Supporting Information, Figure S5).
The XPS spectra of Ni2P samples prepared by the
conventional TPR method (Ni2P-TPR), and the reduction
of Ni2P2S6 at 3008C with 1.0 MPa H2 at a flow rate of
100 mLminÀ1 for 2 h (Ni2P-S), are illustrated in Figure 2. Two
broad peaks were observed in the Ni 2p spectrum of Ni2P-
TPR. The major peak at approximately 852.1 eV is related to
reduced Nid+ (0 < d < 2) in Ni2P, while the other peak at about
855.5 eV corresponds to Ni2+.[22] In the P 2p spectrum of Ni2P-
TPR, the doublet at 129.4 and 130.2 eV can be assigned to P
bonded to Ni in the form of a phosphide, whereas the most
intense peak at 133.8 eV indicates a surface PO43À species.[19]
Herein, we report the preparation of Ni2P by reduction of
nickel hexathiodiphosphate (Ni2P2S6) at a temperature as low
as 2208C. Ni2P2S6 was obtained at room temperature from the
solid reaction of NiCl2 and Na4P2S6. A distinct sulfur-
containing layer was observed at the surface of the derived
Ni2P catalyst, which might explain its high HDS and selective
hydrogenation performances in comparison with those of the
Ni2P catalyst prepared by the conventional TPR method.
Ni2P2S6 was first synthesized by reacting NiCl2 with
Na4P2S6, a product of the reaction of Na2S and PCl3.[21] In
the XRD pattern of the sample (Supporting Information,
Figure S1), the ratio of the intensity of the peak at 2q = 36.28
relative to the peak at 2q = 14.08, and particularly the
presence of a peak at 2q = 45.78, indicate that the major
phase in the sample was Ni2P2S6 (PDF card 33-0952) rather
than the NiPS3 precursor (PDF card 78-0499) described in
a previous study.[16] A small amount of impurity, probably NiS
(PDF card 65-3419), was also detected.
Ni2P2S6 was treated in a tubular reactor under a H2 or Ar
flow for 2 h, at temperatures ranging from 200–3008C, and
a pressure of 1.0 MPa. Figure 1 shows the development of the
3À
The presence of Ni2+ and PO4 species must result from air
exposure of the sample before XPS measurement. In the case
of Ni2P-S, Ni2+ became the predominant Ni species. Shifting
of the major P 2p peak to 135.1 eV arises from P2O5.[23]
Apparently, the surface oxidation behavior of Ni2P-S is
Figure 1. Evolution of the XRD patterns of Ni2P2S6 during reduction
into Ni2P in a H2 flow (1.0 MPa and a H2 flow rate of 100 mLminÀ1).
!
=impurity.
XRD pattern of the Ni2P2S6 sample after H2 treatment
(1.0 MPa H2 at a 100 mLminÀ1 flow rate) over an increasing
temperature range. At 2008C new peaks with very low
intensities were detected at 40.78 and 55.08, respectively,
corresponding to the (111) and (211) reflections of the Ni2P
phase (PDF card 74-1385). At 2208C a mixture of Ni2P2S6 and
Ni2P was obtained. Above 2608C Ni2P became the dominant
phase. Some weak impurity peaks were also present in the
XRD pattern above 2408C. The nature of the impurity cannot
be determined on the basis of the present data, and might be
phosphate, phosphorus sulfide, sulfur, or a mixture of these
compounds. Further research is needed to determine the
nature of the unknown impurity.
In Ar atmosphere (1.0 MPa and 100 mLminÀ1), no Ni2P
was formed under the temperatures investigated. Moreover,
Figure 2. XPS spectra of Ni2P-TPR and Ni2P-S in the A) Ni 2p and P 2p
regions, and B) S 2p region.
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different from that of Ni2P-TPR upon exposure to air.
Ni2P-S is easier to oxidize than Ni2P-TPR, as discussed
below. Sulfur was only detected in Ni2P-S. The surface S:Ni
ratio determined by XPS was 0.41, which was 5.6 times larger
than that measured by XRF (0.073), suggesting that sulfur is
enriched in the surface of Ni2P-S. The S 2p peak (Figure 2B)
can be deconvoluted into two Gaussian peaks, located at 161.4
and 162.9 eV, assigned to sulfide species (S2À) and thiolate-
type sulfur, respectively.[18b] These sulfur species are similar to
those observed in the so-called “phosphosulfide” phase.[18b]
particles, which is in agreement with the MoP passivation
layer reported in our previous study.[24] On the other hand,
Ni2P-S particles were surrounded by a thick layer (about
4.7 nm) composed of tiny crystallites. The lattice spacing in
the (111) plane of some surface crystallites was measured as
0.24 nm. This value is slightly larger than that for the bulk
(0.22 nm), which suggests incorporation of sulfur originating
from H2S released during Ni2P2S6 reduction. This theory is
supported by an absence of sulfur in the bulk phase. Assuming
that all the sulfur species are located in the thick surface layer
with a S:Ni ratio of 0.41 (determined by XPS), and that Ni
species are uniformly distributed in Ni2P-S, and considering
the thickness of the layer (4.7 nm), as well as the average
crystallite size of Ni2P-S (100 nm), the S:Ni ratio in Ni2P-S is
estimated to be 0.10. This value is close to that measured by
XRF (0.073). In fact, the surface S:Ni ratio (0.41) is only
slightly higher than that of a stable nickel phosphosulfide
phase (Ni3PS, 0.33) predicted by density functional theory
calculations.[25] The combined results suggest that reduction of
Ni2P2S6 creates a phosphosulfide phase at the surface, leaving
the bulk of the catalyst almost intact as Ni2P.
Ni2P-TPR demonstrated
a
CO uptake of 4.7 mmolgÀ1,
whereas that of Ni2P-S was almost zero. It seems that the
presence of sulfur on the surface of Ni2P-S blocks the
adsorption of CO.
TEM images demonstrate that particles of Ni2P-TPR
were heterogeneously distributed with an average particle
size of 304 nm, which was much larger than that for Ni2P-S
(100 nm; Supporting Information, Figure S6). This might be
a result of the low temperature (3008C) employed during
reduction of Ni2P2S6 into Ni2P. At higher resolution
(Figure 3), lattice fringes were clearly seen in TEM images
of both samples with an interplane distance of 0.22 nm
corresponding to the (111) plane of Ni2P, confirming the
formation of the Ni2P phase. However, the surface of Ni2P-S
was quite different from that of Ni2P-TPR. An amorphous
layer with a thickness of approximately 1.5 nm, resulting from
O2 passivation, was observed at the surface of the Ni2P-TPR
Different surface structures are likely to be responsible
for the differing surface oxidation behaviors of Ni2P-TPR and
Ni2P-S, revealed by XPS (Figure 2). Based on the study of
Pakes et al. regarding anodic film growth on InP,[23a] we
propose that the surface phosphorus species of Ni2P-TPR are
immobile. Outward migration of Ni species leads to formation
3À
of Ni oxide, while inward migration of oxygen yields PO4
species at the interphase (Supporting Information,
Scheme S1). For Ni2P-S, tiny crystallites at the surface may
react with oxygen much more vigorously than the surface of
bulk Ni2P-TPR particles, leading to the formation of Ni oxide
and P2O5. However, it is still not possible to separate these
surface phases. Further experimental and theoretical work is
needed to fully understand their nature.
The HDS performance of Ni2P-S and Ni2P-TPR were
evaluated using DBT as a sulfur-containing molecule. DBT
undergoes HDS by two parallel pathways (Supporting
Information, Scheme S2): direct desulfurization (DDS), and
hydrogenation (HYD). DDS leads to formation of biphenyl
(BP), while HYD yields mainly cyclohexylbenzene (CHB)
and dicyclohexane. Because the hydrogenation of BP to CHB
is negligible in the presence of DBT, BP selectivity (SBP) is
used as a measure of the DDS pathway, while (1-SBP
)
represents the HYD pathway. The conversion of DBT over
Ni2P-S was two times greater than that over Ni2P-TPR
(Table 1). Such a significant increase in the HDS activity of
Ni2P-S cannot only be attributed to a decrease in particle size,
because Ni2P-S and Ni2P-TPR demonstrated different reac-
tion pathway selectivities for the HDS of DBT. Moreover, it
has been determined by Oyama et al.[26] that the HDS of DBT
over Ni2P is a structure insensitive reaction. The HYD
pathway selectivity of Ni2P-S was 8% higher than that of
Ni2P-TPR (Table 1), indicating that the HYD pathway is
enhanced relative to DDS after formation of the sulfur-
containing layer.
The low HDS reactivity of DBT and its alkylated
derivatives is usually attributed to a steric hindrance effect
that prevents adsorption of sulfur atoms on the catalytic site.
Figure 3. TEM images of Ni2P-TPR and Ni2P-S.
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5008C for 24 h. The solid product was thoroughly washed with water
Table 1: The catalytic performances of Ni2P-TPR and Ni2P-S: HDS of
DBT, and selective hydrogenation of phenylacetylene, naphthalene,
nitrobenzene, and p-chloronitrobenzene.
and ethanol to remove NaCl. After drying at 1208C for 6 h a black
powder of Ni2P2S6 was obtained. The phosphate precursor of Ni2P-
TPR was prepared by a co-precipitation method. An aqueous
solution of (NH4)2HPO4 (1.77 g) in deionized water (10 mL) was
added dropwise to a solution of Ni(NO3)2·6H2O (3.90 g) in deionized
water (15 mL) with stirring. The resulting precipitate was stirred
while the water was evaporated to obtain a solid product, which was
dried at 1208C for 12 h and calcined at 5008C for 6 h to obtain the
final oxidic precursor with a Ni:P molar ratio of 1. This value is
exactly the same as that of Ni2P2S6. Further details regarding
preparation of Ni2P, characterization techniques, as well as hydro-
desulfurization and hydrogenation reactions are summarized in the
Supporting Information.
Reaction
Conversion [%]
Selectivity [%]
Ni2P-S Ni2P-TPR Ni2P-S Ni2P-TPR
HDS of DBT
50.2
90.2
24.8
10.1
33.5[a] 26.1[a]
95.6
99.7
99.2
95.7
89.2
95.4
95.3
94.4
85.4
84.2
43.3
51.2
65.4
18.7
[a] HYD pathway selectivity, (1ÀSBP)”100%.
Acknowledgements
This work was financially supported by the Natural Science
Foundation of China (21173033, U1162203, and 21473017),
the Fundamental Research Funds for the Central Universities
(DUT13LK18), the PetroChina Innovation Foundation
(2014D-5006-0402), the State Key Laboratory of Fine Chem-
icals (KF1306), and the Scientific Research Fund of Liaoning
Provincial Education Department (L2013023).
Hydrogenation of the aromatic ring, which removes steric
hindrance, remarkably enhances the HDS reactivity of these
refractory polyaromatic sulfur-containing compounds. Our
previous work,[8c] and that of Carenco et al.,[8d] has demon-
strated that Ni2P is promising for the chemoselective hydro-
genation of alkynes. Therefore, we further studied the
selective hydrogenation of phenylacetylene, naphthalene,
nitrobenzene, and p-chloronitrobenzene over Ni2P-S and
Ni2P-TPR. The results are listed in Table 1. The two catalysts
exhibited high selectivities (ꢀ 89%) toward partially hydro-
genated products. For all reactions a significant increase was
observed in the activity of Ni2P-S relative to Ni2P-TPR, as
well as a minor increase in product selectivities. For instance,
conversion of styrene over Ni2P-S was 90%, 9 times higher
than that over Ni2P-TPR. The conversion of p-chloronitro-
benzene increased from 18% over Ni2P-TPR to 43% over
Ni2P-S without facilitating the dechlorination reaction,
because selectivity for p-chloroaniline was equally high over
both catalysts (about 95%). The above features and a facile
preparative route suggest that Ni2P-S is a legitimate alter-
native to non-precious metal catalysts, such as Ni2P-TPR, in
selective hydrogenation reactions.
In summary, reduction of Ni2P2S6 is a facile and promising
method for preparation of Ni2P. Conversion of Ni2P2S6 into
Ni2P occurs at temperatures as low as 200–2208C and at
1.0 MPa H2 pressure. A distinct sulfur-containing layer with
a thickness of about 4.7 nm, composed of tiny crystallites, was
observed at the surface of the obtained Ni2P catalyst. This is
a direct observation of the sulfur-containing layer of Ni2P, or
the so-called nickel phosphosulfide phase. Sulfur occurred as
a mixture of S2À and thiolate species. Both the HDS activity
and selective hydrogenation performance of Ni2P-S were
superior to that of Ni2P-TPR, suggesting a positive role of the
surface sulfur species in catalytic processes.
Keywords: hydrodesulfurization · nickel phosphides ·
selective hydrogenation · sulfur-containing layer
How to cite: Angew. Chem. Int. Ed. 2016, 55, 4030–4034
Angew. Chem. 2016, 128, 4098–4102
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sealed in a quartz tube under vacuum (1.3 ” 10À2 Pa N2) and heated at
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Received: November 16, 2015
Revised: January 20, 2016
Published online: February 17, 2016
4034 www.angewandte.org
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 4030 –4034