A novel synthesis of Ni2P/MCM-41 catalysts by reducing a precursor of ammonium hypophosphite and nickel chloride at low temperature

Article abstract of DOI:10.1016/j.apcata.2013.05.015

A novel method to prepare Ni₂P/MCM-41 catalysts at low reduction temperature based on ammonium hypophosphite and nickel chloride by temperature programmed reduction is described. The catalysts were prepared using incipient wetness impregnation of the siliceous MCM-41 support with aqueous solutions of ammonium hypophosphite and nickel chloride, followed by reducing the obtained precursor at 483-663 K for 2 h in flowing H₂, to form Ni₂P catalysts. The catalysts were characterized by H₂ temperature-programmed reduction (H₂-TPR), X-ray diffraction (XRD), N₂-adsorption specific surface area measurements (BET), CO uptake, transmission electron microscope (TEM), and X-ray photoelectron spectroscopy (XPS). With sample of initial P/Ni molar ratio >0.5, the Ni₂P was successfully obtained at lower reduction temperature, and a high initial P/Ni molar ratio favors the formation of Ni₂P at lower temperature. Using less oxidic phosphorus precursor of hypophosphite enabled the Ni₂P to be formed at low reduction temperature. Evaluation of the activity for DBT HDS of the catalysts shows that the catalyst prepared with initial P/Ni ratios of 2 exhibited the highest activity. At a reaction temperature of 613 K, a pressure of 3.0 MPa, a H₂/oil ratio of 500 (V/V), and a weight hourly space velocity (WHSV) of 2.0 h^-1, the HDS conversion reached 99%, and no catalyst deactivation was observed within 120 h.

Full text of DOI:10.1016/j.apcata.2013.05.015

Applied Catalysis A: General 462–463 (2013) 247–255
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Applied Catalysis A: General
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A novel synthesis of Ni2P/MCM-41 catalysts by reducing a precursor
of ammonium hypophosphite and nickel chloride at low temperature
Hua Songa,b,∗, Min Daia,d, Hualin Songc,∗∗, Xia Wana,d, Xiaowei Xua
a College of Chemistry & Chemical Engineering, Northeast Petroleum University, Daqing 163318, China
b Provincial Key Laboratory of Oil & Gas Chemical Technology, College of Chemistry & Chemical Engineering, Northeast Petroleum University, Daqing
163318, Heilongjing, China
c Department of Image College Mudanjiang Medical University, Mudanjiang 157011, Heilongjing, China
d CPE Survey and Design Institute of Xinjiang Oil, Karamay 834000, Xinjiang, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel method to prepare Ni2P/MCM-41 catalysts at low reduction temperature based on ammonium
hypophosphite and nickel chloride by temperature programmed reduction is described. The catalysts
were prepared using incipient wetness impregnation of the siliceous MCM-41 support with aqueous
solutions of ammonium hypophosphite and nickel chloride, followed by reducing the obtained precur-
sor at 483–663 K for 2 h in flowing H2, to form Ni2P catalysts. The catalysts were characterized by H2
temperature-programmed reduction (H2-TPR), X-ray diffraction (XRD), N2-adsorption specific surface
area measurements (BET), CO uptake, transmission electron microscope (TEM), and X-ray photoelectron
spectroscopy (XPS). With sample of initial P/Ni molar ratio >0.5, the Ni2P was successfully obtained at
lower reduction temperature, and a high initial P/Ni molar ratio favors the formation of Ni2P at lower
temperature. Using less oxidic phosphorus precursor of hypophosphite enabled the Ni2P to be formed at
low reduction temperature. Evaluation of the activity for DBT HDS of the catalysts shows that the catalyst
prepared with initial P/Ni ratios of 2 exhibited the highest activity. At a reaction temperature of 613 K, a
pressure of 3.0 MPa, a H2/oil ratio of 500 (V/V), and a weight hourly space velocity (WHSV) of 2.0 h−1, the
HDS conversion reached 99%, and no catalyst deactivation was observed within 120 h.
Received 8 January 2013
Received in revised form 23 April 2013
Accepted 12 May 2013
Available online 20 May 2013
Keywords:
Nickel phosphide
Ammonium hypophosphite
Nickel chloride
MCM-41
Hydrodesulfurization
Dibenzothiophene
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
alternative hydrodesulfurization catalysts [5,6]. Among the phos-
phides, nickel phosphide (Ni2P) shows excellent activity for the
Driven by the growing demand for fuel oil and the increasing use
of heavy crude oil, sulfur removal from fuels has received and will
continue to attract much attention in the coming years. Moreover,
more and more rigorous environmental regulations limiting the
emissions of sulfur dioxide and the continuing decline in the quality
of petroleum feedstocks have made sulfur removal becomes one of
the paramount problems in the refining industry. To solve these low
levels of sulfur, much of the research over the past decade has aimed
at improving the classic catalysts, which are based on molybdenum
sulfide and promoted with cobalt or nickel, as well as finding new
catalyst materials [1–4].
hydrotreatment of fuels [7,8]. All recent reports of Ni2P catalysts
were mostly produced by temperature programmed reduction
(TPR) of metal nickel salt together with ammonium phosphate
in hydrogen [5,9,10]. The nickel phosphate precursor is prepared
mainly by impregnation of the support with (NH4)2HPO4 (or
NH4H2PO4) and Ni(NO3)2 solutions, then dried and calcined. After
a subsequent temperature programmed reduction in hydrogen,
the desired Ni2P catalyst is formed. The method of temperature
programmed reduction is convenient and simple, but has the dis-
advantage of requiring a high reduction temperature. Berhault
et al. [11] had observed that the phosphate reduction to phos-
phide did not start until the reduction temperature is raised to
823 K and the selective formation of Ni2P phase begins at reduction
temperatrue of 923 K. The P O bond is strong, and its reduction
requires high temperature. Moreover, hydrogen atoms are avail-
able only after metal nickel particles have formed, because the
nickel particles are necessary for hydrogen molecules dissociat-
ing to hydrogen atoms [1]. These active hydrogen atoms can spill
over to the phosphate and reduce it to phosphorus or phosphine.
The phosphorus or phosphine can then react with the metal par-
ticles to form Ni2P. The strong P O bond and the surface diffusion
In recent years, transition metal phosphides, which have high
activity and stability, have been found to be quite promising as
∗
Corresponding author at: College of Chemistry & Chemical Engineering, North-
east Petroleum University, Daqing 163318, China. Tel.: +86 0459 6503167;
fax: +86 0459 6506498.
∗∗
Corresponding author. Tel.: +86 0453 6984321; fax: +86 0453 6586656.
E-mail addresses: songhua2004@sina.com (H. Song), songhualin@126.com
(H. Song).
0926-860X/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2013.05.015

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H. Song et al. / Applied Catalysis A: General 462–463 (2013) 247–255
of the H atoms are responsible for the high reduction tempera-
ture which leads to larger catalyst particles, relatively low catalytic
activity and the almost exclusive formation of phosphorus [1].
There have been other approaches to preparing Ni2P catalysts, such
as solvothermal reactions [12–16], thermal decomposition of nickel
thiophosphate (NiPS3) [17], mixing of trioctylphosphine (TOP) with
metal salts [18–23], and co-reaction of metal or metal oxide with
phosphines [1]. However, the need for extreme conditions has
limited all these approaches. Recently, it has been reported that the
Ni2P were prepared by thermal decomposition of hypophosphites
in a static protecting gas atmosphere [24–28]. This route is mild
and does not require a high temperature. Moreover, Cecilia and
Infantes-Molina found the formation of Ni2P also can be achieved
by temperature-programmed reduction of phosphite-based pre-
cursors (Ni(HPO3H)2) at a relatively low temperature[29,30]. In
general, using the hypophosphites as phosphorus sources provides
a new method to prepare Ni2P catalyst under mild conditions.
under the setting conditions of 40 kV, 30 mA, scan range from 10
to 80◦ at a rate of 10◦/min.
The typical physico-chemical properties of supports and cata-
lysts were analyzed by BET method using Micromeritics adsorption
equipment of NOVA2000e. All the samples were outgassed at 473 K
until the vacuum pressure was 6 mm Hg. The adsorption isotherms
for nitrogen were measured at 77 K.
Transmission electron microscope (TEM) examinations were
performed using the JEM-1010 instrument supplied by JEOL. The
samples were dispersed in ethanol and placed on a carbon grid
before TEM examinations.
The CO uptake was measured using pulsed chemisorption.
About 1.0 g of catalyst was pretreated in a quartz reactor to remove
the passivation layer by heating up to 613 K at a rate of 2 K/min in
H2 with flowrate of 20 mL/min for 2 h, and then naturally cooled
to room temperature in a continuous H2 flow and an He flow at
30 mL/min was used to flush the catalyst for 30 min to achieve an
adsorbate-free. After pretreatment, 1 mL pulses of CO were injected
into a flow of He (30 mL/min), and the CO uptake was measured
using a TCD. CO pulses were repeatedly injected until the response
from the detector showed no further CO uptake after consecutive
injections. Assuming a 1:1 adsorption stoichiometry between CO
and metal atoms, this value corresponds to the metal site density
on the catalyst surface.
The X-ray photoelectron spectroscopy (XPS) spectra were
acquired using ESCALAB MKII spectrometer under vacuum. XPS
measurements have been performed using monochromatic Mg
K␣ radiation (E = 1253.6 eV) and equipped with a hemi-spherical
analyzer operating at fixed pass energy of 40 eV. The recorded
photoelectron binding energies were referenced against the C 1s
contamination line at 284.8 eV.
In this paper, we demonstrate
a method for preparing
Ni2P/MCM-41 catalysts at lower reduction temperature. The cat-
alyst precursors were prepared by impregnation of an ammonium
hypophosphite and nickel chloride solution with MCM-41 support,
followed by reduction of the precursors in a flow of H2 at 483–663 K
for 2 h, to obtain the Ni2P catalysts. Compared with preparation of
the catalyst using the conventional TPR method, these conditions
are mild because the reduction temperature has decreased by about
200 K.
2. Experimental
2.1. Catalyst synthesis
Siliceous MCM-41 was synthesized using tetraethyl orthosili-
cate (TEOS) as the silica source and cetyltrimethylammonium
bromide (CTAB) as the template, following the procedure as
described in the literature [31].
2.3. Catalytic activity test
The HDS of DBT was carried out in a flowing high-pressure
fixed-bed reactor using a feed consisting of a decalin solution of
DBT (1 wt%). The conditions of the HDS reaction were 553–613 K,
3.0 MPa, WHSV = 2 h−1, and hydrogen/oil ratio of 500 (V/V). The cat-
alyst was pressed in discs, crushed and sieved with 30–60 mesh.
Prior to reaction, 0.5 g of the catalysts were pretreated in situ with
flowing H2 (40 mL/min) at 613 K for 2 h. Sampling of liquid prod-
ucts was started 6 h after the steady reaction conditions had been
achieved. Liquid samples were collected every hour and analyzed
by FID gas chromatography with a GC-14C–60column. The total
conversion was calculated from the ratio of converted dibenzothio-
phene/initial dibenzothiophene. Turnover frequency (TOF) values
of the samples containing nickel phosphide were calculated using
Eq. (1) [32]:
The supported Ni2P catalyst precursors were prepared by
impregnating an ammonium hypophosphite (NH4H2PO2) and
nickel chloride (NiCl2·6H2O) solution with the mesoporous MCM-
41. The precursors, with different initial P/Ni molar ratios (0.5, 1,
2, and 3), were prepared with Ni loading of 12 wt%. In a typical
experiment, 4.0 g NH4H2PO2 and 5.9 g NiCl2·6H2O were dissolved
in 20 mL of deionized water at room temperature to form a uni-
form solution (the initial molar ratio of P/Ni is 2). The MCM-41 was
wet-impregnated with the above solution for 8 h. After evapora-
tion of water, the impregnated solid was dried at 333 K and then
directly reduced in a fixed-bed reactor by heating to 483–663 K at
a rate of 2 K/min in a flow of H2 (200 mL/min), held for 2 h, then
naturally cooled to room temperature in a continuous H2 flow. The
obtained catalyst was passivated in O2/N2 mixture (0.5 vol.% of O2)
with flowrate of 20 mL/min for 2 h. The precursors obtained before
reducing and catalysts were named NiCl2-NH4H2PO2/MCM(X) and
Ni-P/MCM(X-Y), respectively, where X and Y are the initial P/Ni
molar ratio and reduction temperature, respectively.
F
X
TOF =
(1)
W M
where F is the molar rate of DBT fed into the reactor (␮mol s−1), W
is the weight of catalyst (g), X is the conversion of DBT (%), and M
is the mole of sites loaded which is decided by the CO uptake.
2.2. Catalyst characterization
3. Results and discussion
The reducibility of precursors was characterized by the H2
temperature-programmed reduction (H2-TPR) using a quartz U-
tube reactor (inner diameter of 6 mm), in which 0.05 g of catalyst
was loaded in the thermostatic zone. Reduction was conducted at
a heating rate of 10 K/min in a 10 vol.% H2/Ar flow (30 ml/min).
The TPR spectrum was determined using a thermal conductivity
detector (TCD) to monitor hydrogen consumption.
3.1. H2-TPR analysis
H2-TPR profiles of MCM-41-supported NiCl2, NH4H2PO2 and
catalyst precursors with different initial P/Ni molar ratios are
shown in Fig. 1. With sample (a) (NiCl2/MCM), the hydrogen con-
sumption peak attributed to the reduction of Ni2+ to Ni is observed
around 640 K, while with sample (b) the peaks were weaker, and
then which is hardly observed with increasing P content (sam-
ples (c)–(d)). This indicates that the P content in the precursor
X-ray diffraction (XRD) analysis of the samples were carried out
on a D/max-2200PC-X-ray diffractometer using CuK␣ radiation

H. Song et al. / Applied Catalysis A: General 462–463 (2013) 247–255
249
Fig. 1. H2-TPR profiles of MCM-41 supported NiCl2, NH4H2PO2, and NiCl2-
NH4H2PO2/MCM(X). (a) NiCl2, (b) P/Ni = 0.5, (c) P/Ni = 1, (d) P/Ni = 2, (e) P/Ni = 3, (f)
NH4H2PO2.
Fig. 2. XRD patterns of the catalysts prepared with different initial Ni/P molar ratios
and reduced at different temperature. (a) P/Ni = 0.5, (b) P/Ni = 1, (c) P/Ni = 2, (d)
P/Ni = 3.
has a great effect on the reduction of Ni2+ to Ni by H2. For sam-
ples (b)–(e) (NiCl2-NH4H2PO2/MCM(X)), some significant negative
peaks are found at about 483 K, this maybe due to some gas (such
as HCl and NH3) has been produced during the heating process,
and therefore the TCD appears a negative signal opposite of H2
consumption. These negative peaks correspond to the decomposi-
tion of the precursors. However, the sample (f) (NH4H2PO2/MCM)
shows a hydrogen consumption peak around 483 K, which indi-
cates that phosphorus species come from the decomposition of
NH4H2PO2 can be reduced at low temperature. Hydrogen con-
sumption peaks with samples (b)–(e) attributed to the reduction of
phosphorus species is observed around 593 K, which is higher than
that observed in the sample (f). This indicates that in the catalyst
precursors, the nickel is likely to be in the form of hypophosphite
and reaction occurs at the intrinsic reduction temperature of this
main hypophosphite phase. The sample (b)–(e) also show the H2
consumption decrease with the increase in phosphorous loadings.
Moreover, some weak peaks of H2 consumption appeared at about
663 K, which indicates that a small amount of phosphorous species
has been reduced.
It has been reported that the reduction of Ni2P from a pre-
cursor prepared by (NH4)2HPO4 (or NH4H2PO4) and Ni(NO3)2 do
not occur until the temperature reaches 823 K [11]. Such a high
reduction temperature is attributed to the highly thermodynamic
stability of P O bond and the surface diffusion of the H atoms [1].
But our H2-TPR analysis shows that compare with the precursor
prepared by (NH4)2HPO4 (or NH4H2PO4) and Ni(NO3)2, the reduc-
tion of Ni2P from a precursor prepared by NiCl2 and NH4H2PO2
could be performed at a lower temperature. The reduction tem-
perature has decreased by about 200 K. This may be due to the P
species with less valence electron can be easily reduced, because P
in NH4H2PO2 has one valence electron, which is four less than that
of P in (NH4)2HPO4 (or NH4H2PO4). Our method which use less oxi-
dic phosphorus precursor of hypophosphite enables the Ni2P to be
formed at even lower reduction temperature. These observations
are similar to the results reported by Cho et al. [16].
3 reduced at 483, 593 and 663 K are presented in Fig. 2. All the
patterns show a broad feature at 2ꢀ = 25◦ due to the amorphous
nature of mesoporous MCM-41 [29]. As shown in Fig. 2(a) (ini-
tial P/Ni molar of 0.5), Ni phase is detected at 44.4, 51.8, and 76.7◦
(PDF: 65–2865) with sample reduced at 483 K. When the temper-
ature rises above 593 K, the intensity of Ni phase increased and an
amorphous Ni12P5 phase formed. When the temperature is up to
663 K, the Ni phase disappeared, and the Ni12P5 phase is detected
at 32.6, 35.8, 38.4, 41.7, 44.4, 46.9 and 48.9◦ (PDF: 22–1190). The
situation is somewhat different when the initial P/Ni molar ratio
increased. With initial P/Ni molar of 1 (Shown in Fig. 2(b)), Ni phase
and amorphous Ni12P5 phase are detected at 483 K, and both Ni
phase and Ni12P5 phase are observed at 593 K. With the temper-
ature rises to 593 K, the Ni2P phase is detected at 40.6, 44.5, 47.1,
54.1, 54.8, 66.1, 72.5 and 74.5◦ (PDF: 03–0953) at 663 K. With ini-
tial P/Ni molar of 2 and 3 (Fig. 2(c) and (d)), the pure Ni2P phase
is detected, and the peak intensity increases with the reduction
temperature increased from 483 to 593 K and then maintains the
intensity with the reduction temperature up to 663 K. The XRD
analysis results show that formation of active phases follows the
order Ni, Ni12P5 and Ni2P, which coincides with the previously
published studies [33]. Ni2P was not obtained for sample prepared
with initial P/Ni molar of 0.5, probably because of lack of P and
partial loss of P due to the formation of PH3 or P during reducing
process. Prins et al. [34] have pointed out the possibility for for-
mation of volatile species such as elemental P or phosphine (PH3)
during the reduction processes, and then the formed PH3 reacts
with nickel to form Ni, Ni12P5 or Ni2P phase [35]. At temperature of
483 K, greater proportions of PH3 is formed by the decomposition
of phosphorous rich precursors and react with nickel particles to
form phosphorous rich phosphides such as Ni12P5 and Ni2P. With
temperature increases to 593 K, the Ni12P5 phase transforms to
Ni2P phase for the sample with initial P/Ni molar ratio of 1 and
the peaks of Ni2P phase become more intense for the samples with
initial P/Ni molar ratio of 2 and 3, indicating PH3 was formed at
this temperature again, and which promoted the formation of Ni2P
phase. At temperature of 663 K, Ni2+ can be reduced by H2, and
then which is reduced to Ni12P5 or Ni2P. Therefore, the advantage
of using NiCl2 and NH4H2PO2 as materials to prepare Ni2P is that the
synthesis process can be carried out under a relative low reduction
temperature.
3.2. XRD
In order to optimize the experimental conditions to obtain the
desired Ni2P phase, the influence of the initial P/Ni molar ratio and
reduction temperature on the formation of Ni2P were studied. The
XRD patterns of the samples with initial P/Ni ratios of 0.5, 1, 2, and

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H. Song et al. / Applied Catalysis A: General 462–463 (2013) 247–255
Table 1
Textrual characterization of support and catalysts.
Sample
BET area
Pore volume
Average pore
diameter (nm)
(m2 g−1
)
(cm3 g−1
)
MCM-41
1012
507
815
308
616
214
462
83
0.816
0.363
0.584
0.211
0.425
0.138
0.301
0.042
0.113
3.2
2.8
2.9
2.7
2.7
2.6
2.5
2.0
2.3
Ni-P/MCM(0.5–483)
Ni-P/MCM(0.5–663)
Ni-P/MCM(1–483)
Ni-P/MCM(1–663)
Ni-P/MCM(2–483)
Ni-P/MCM(2–663)
Ni-P/MCM(3–483)
Ni-P/MCM(3–663)
196
3.3. Textural properties
Table 1 summarizes the textural properties of support and cat-
alysts reduced at 493 and 583 K. The surface area and pore volume
of the MCM-41 support are 1012 m2 g−1 and 0.816 cm3 g−1, respec-
tively. It can be seen that these catalysts suffer a loss of surface area
and pore volume, especially the catalysts rich in phosphorus. This
may be due to the formation of a phosphosilicate, which causes a
shrinkage in the MCM-41 structure [36]. Abu et al. [37] have also
found that metal phosphides supported on MCM-41 have much
lower surface areas than pristine MCM-41, indicating that some of
the MCM-41structure is lost during the preparation of metal phos-
phide. Furthermore, Table 1 shows the P content had significant
effect on the surface area and pore volume. The surface area and
pore volume decease with decreasing of P contents, which can be
contributed to the excess P evolved during the reduction covers on
the outer surface of catalysts. For the catalysts with same initial
P/Ni molar ratios, the catalysts reduced at 663 K have higher sur-
face area than that of the catalysts reduced at 483 K. The low surface
area of catalysts reduced at 483 K is attributed to the presence of
un-reduced hypophosphites.
Fig. 3. XPS spectra of the Ni-P/MCM(X-663) with different initial P/Ni molar ratios.
bulk Ni2P and 133.5 for surface P5+ (phosphate) due to the superfi-
cial oxidation of Ni2P particles [33]. As shown in the Fig. 3(b), only
the Ni-P/MCM(2–663) shows the P␦− band at 129.4 eV obviously,
and the binding energy shifts lightly to a lower value, which indi-
cates that there is a interaction between the Ni2P particles and the
MCM-41 support. The Ni2P phase in sample of Ni-P/MCM(1–663)
is too little that the P␦− is hardly observed. Because of the cov-
erage of excess P on the surface, the sample of Ni-P/MCM(3–663)
hardly exhibit the P␦− band. All the samples show the broad bands
at 134–135 eV, which are assigned to P5+ (phosphate) species [42],
and which implies that there is a large mount of phosphate over
the catalysts.
3.4. XPS
In order to gain further insight into the surface composition and
the valence states of Ni2P, the XPS spectrum of MCM-41 supported
Ni2P samples with initial P/Ni molar ratio of 1, 2 and 3 reduced
at 483 and 663 K was obtained. Table 2 shows the binding energy
values and Ni/P and Ni/Si atomic ratios for all the samples.
For the Ni-P/MCM(X-663) catalysts, Ni 2p core level spec-
trum involves two contributions (Fig. 3(a)). The first is assigned
to Ni␦+ in Ni2P phase and centered at 852.6–853.5 eV [25], and
the second at 856.4–857.3 eV, corresponding to the possible inter-
action of Ni2+ ions with phosphate ions, as a consequence of a
superficial passivation, along with the broad shake-up peak at
approximately 6.0 eV higher than that of the Ni2+ species [38].
This satellite peak is due to divalent species [39,40], although
in the literature has also been assigned to trivalent and oxy-
sulfided nickel species [29,41]. Other broad peaks, centered
at high binding energy side, are assigned to the Ni 2p sig-
nal from oxidized Ni species [25]. Comparing contributions at
852.6–852.7 eV, we can see that the intensity of the Ni␦+ is in the
order Ni-P/MCM(2–663) > Ni-P/MCM(3–663) > Ni-P/MCM(1–663).
The weakest contribution observed with Ni-P/MCM(1–663) is
understandable, the lower P content in precursor leads to less Ni2P
phase formed (Fig. 2 XRD patterns, sample (b)). With P/Ni initial
P/Ni molar ratio of and 3, P content in precursor is enough to forma-
tion of Ni2P, but the excess P covers on the outer surface of catalyst,
and this will leads to influence the detection of Ni␦+. The binding
energy for Ni 2p in the Ni2P phase is very close to that reported
for Ni0 species (852.0–853.0 eV) [39]. Meanwhile, with regards to
P 2p binding energy, it has been reported to be 129.5 eV for P␦− on
In order to investigate the Ni 2p and P 2p core level of the cat-
alysts obtained at 483 K, the sample with initial P/Ni molar of 2
and 3 were considered. As shown in Fig. 4(a), both the samples
exhibit the Ni␦+ and Ni2+ species centered at 852.4–852.7 eV and
856.5–856.6 eV, respectively. However, the intensity of Ni␦+ species
bands of the catalysts reduced at 483 K is weaker than that of the
catalysts reduced at 663 K (Fig. 3(a)), indicating a small amount of
Ni2P is formed at 483 K. This conclusion could be confirmed by the
results obtained by XRD, where the weak diffraction lines of Ni2P
were observed. Fig. 4(b) shows P␦−is not observed in the two sam-
ples as was found in the catalyst Ni-P/MCM(2–663), while the P5+
band at 134.6 eV remains, indicating phosphite ions are not totally
reduced under the experimental conditions. Moreover, the P 2p sig-
nal centered at 133.0–133.7 eV, which is assigned to phosphorous
in the form of HPO3H−, as previously reported by Infantes-Molina
and Cecilia et al. [29,30]. This H−PO3H−species is due to the dispro-
portionation reaction of H2PO2
.
XPS analyses were also used to calculate the surface P/Ni
and Ni/Si atomic ratios (Table 2). If we consider the superficial

H. Song et al. / Applied Catalysis A: General 462–463 (2013) 247–255
251
Table 2
Spectral parameters obtained by XPS analysis.
Sample
Binding energy (eV)
[0,8-9]Superficial
atomic ratio
Ni 2p3/2
Ni2+
P 2p
Ni2P
PO4
HPO3H−
Ni2P
P/Ni
Ni/Si
3−
Ni-P/MCM(1–663)
Ni-P/MCM(2–483)
Ni-P/MCM(2–663)
Ni-P/MCM(3–483)
Ni-P/MCM(3–663)
857.3
856.6
857.3
856.5
857.3
852.6
852.4
852.7
852.7
852.6
134.2
134.7
134.5
134.6
134.8
–
133.0
–
133.6
–
129.3
129.3
129.4
129.2
129.6
1.543
5.618
2.994
6.098
4.132
0.803
0.057
0.097
0.053
0.081
P/Ni atomic ratio, the theoretical ratio corresponding to precur-
sor materials would be 1, 2, and 3, respectively. However, after
reduction, these ratios increase considerably. This confirms that
indicating phosphorous species in catalysts increases the disper-
sion of nickel on support [38,43].
there is an enrichment of phosphorous on the surface of catalysts,
3.5. CO uptake
3−
although it is essentially present as PO4
forming a passivation
layer in the surface of Ni2P [29]. Moreover, Table 2 also shows
the P/Ni atomic ratios of Ni-P/MCM(2–483) and Ni-P/MCM(3–483)
are higher than that of Ni-P/MCM(2–663) and Ni-P/MCM(3–663),
respectively. This indicates that some phosphorous can be reduced
at 663 K and partial loss of phosphorous due to the formation
of PH3 or P during reducing process [38]. As for the superficial
Ni/Si atomic ratio, the Ni-P/MCM(2–483) and Ni-P/MCM(3–483)
show the lowest Ni/Si atomic ratio, which may be due to the
surface of catalysts are covered by excess phosphorous. The super-
ficial Ni/Si atomic ratio of the catalysts obtained at 663 K is in the
order Ni-P/MCM(2–663) > Ni-P/MCM(1–663) > Ni-P/MCM(3–663),
CO uptake data measured at room temperature for the catalysts
which obtained Ni2P phase was used to estimate the number of
active sites, and the data is shown in Table 3. The CO uptake over the
catalysts reduced at 663 K are in the order Ni-P/MCM(2–663) > Ni-
P/MCM(3–663) > Ni-P/MCM(1–663). The lowest CO uptake over
Ni-P/MCM(1–663) is easily accountable, because less active Ni2P
phase was formed with the catalyst lower initial P/Ni molar ratio.
The considerable lower CO uptake of Ni-P/MCM(3–663) catalyst
indicate that excess P covers on the outer surface of the catalyst
when the P/Ni molar ratio is higher, which led to the decrease
in the amount of exposed nickel atoms used for adsorption of CO
[38]. Although CO molecules may adsorb on P sites, their amount
may be very small [44]. The possibility of the catalyst surface
blocked by phosphate species for the samples rich in phosphorus
is in accordance with BET and XPS results. The high CO uptake of
Ni-P/MCM(2–663) also indicates that there are highly dispersed
Ni2P particles in Ni-P/PMCM(2–663). Moreover, compared with the
Ni-P/MCM(2–663) and Ni-P/MCM(3–663) reduced at higher tem-
perature, Ni-P/MCM(2–483) and Ni-P/MCM(3–483) show lower
values of CO uptake, which may be due to the less Ni2P phase was
reduced and the blocking by phosphate species at lower reduction
temperature.
3.6. TEM
TEM images of the Ni-P/MCM(2–663) catalyst is shown in Fig. 5.
The morphologies and particle sizes of the sample can be seen in the
TEM images of Fig. 5(a). The Ni2P particle size ranged from approx-
imately 6 to 18 nm, and which are homogenously dispersed on the
MCM-41 support. Unlike the typical stacked morphologies of Mo
and W sulfides, Ni2P are not layered and form spherical particles
that can be well dispersed on supports [45]. After reducing, the cat-
alysts were subjected to a flow of a 0.5 vol.% O2/N2 mixture at room
temperature, so that a thin oxide layer formed on the outer surfaces
of the particles, to prevent deep oxidation of the catalysts upon air
exposure. Fig. 5(b) shows evidence of the passivation layer on a
Ni2P particle in the Ni-P/MCM(2–663) catalyst. A light gray band
with a thickness of approximately 1.5 nm can be seen to extend
around the external edge of the Ni2P particle. This observation is
consistent with the results reported by Sawhill et al. [46]. Fig. 5(c)
shows the visible lattice spacing is about 0.22 nm, consistent with
the d-spacing value for {1 1 1} crystallographic planes of the Ni2P
phase.
3.7. The formation mechanism of Ni2P
To investigate the formation mechanism of the Ni2P prepared
by this method, the analysis of the catalysts (initial P/Ni molar ratio
Fig. 4. XPS spectra of the Ni-P/MCM(X-483) with different initial P/Ni molar ratios.

252
H. Song et al. / Applied Catalysis A: General 462–463 (2013) 247–255
Table 3
CO uptake and DBT HDS performance over Ni2P catalysts.
Sample
Reaction temperature (K)
Conversion (%)
Selectivity (%)
BP
CO uptake (␮mol g−1
)
TOF (10−3 s−1
)
CHB
553
573
593
613
553
573
593
663
553
573
593
613
37.5
42.8
46.1
50.0
77.4
83.2
90.4
99.9
60.1
67.5
74.5
83.1
80.5
83.8
87.3
90.1
60.2
65.5
69.8
72.3
72.3
78.8
81.6
85.6
19.5
16.2
12.7
9.9
39.8
34.5
30.2
27.7
27.7
22.2
18.4
14.4
0.665
0.759
0.818
0.887
0.778
0.836
0.909
1.004
0.725
0.814
0.898
1.002
Ni-P/MCM(1–663)
17
30
25
Ni-P/MCM(2–663)
Ni-P/MCM(3–663)
of 2) obtained at different temperature and without passivating,
as well as the precursor, are considered as representative (Fig. 6).
Fig. 6(a) shows the corresponding Ni 2p core level spectrum of pre-
cursor and reduced sample, respectively. The precursor spectrum
shows two Ni 2p3/2 signals, one with a binding energy value of
856.3 eV which can be attributed to Ni2+, and another one of the
broad shake-up satellite. This observation is in agreement with the
results reported by Cecilia et al. [29]. The Ni 2p3/2 signal for cat-
alysts possess a small band located at about 852.7 eV, assigned to
Ni␦+ forming Ni2P. The Ni2+ species can also be seen in the catalysts
reduced at 483 and 593 K, but it can be hardly seen in the catalysts
reduced at 663 K. Similarly, the P 2p signal for the precursor and
the catalysts reduced at different temperature is shown in Fig. 6(b).
If we consider the precursor spectra, the band centered at 132.7 eV
can be assigned to phosphorous in the form of HPO2−, and no high
valence of phosphorous is observed, indicating the valence of phos-
phorous do not been modified during the preparation procedure.
After reducing at 483 K, a small band at about 129.1 eV is observed,
and which can be attributed to the P␦− forming Ni2P. Moreover,
the bands at 133.6 and 134.8 eV appears in the catalyst reduced at
483 K, and which can be attributed to HPO3H− and PO43−, respec-
tively [29]. When the reduction temperature rises up to 593 K, the
band of PO43− is invisible, and the band of HPO3H− becomes weak,
but the band of P␦– grows strong. This indicates that the HPO3H2−
3−
and PO4 species have been reduced to low valence of phospho-
rous (such as P␦− or other P species). With rising the reduction
temperature up to 663 K, only the band of P␦− can be observed,
indicating the HPO3H2− has been reduced completely.
In order to study the volatile species produced during the reduc-
tion process, we choose the activated carbon as the adsorbent to
collect the effluent during reduction. The experimental configu-
ration is shown in Fig. 7(a). After absorbing the effluent during
reduction of catalysts reduced at 483, 593 and 663 K, the adsorbents
were analyzed by the XPS to identify the elemental composition of
the volatile products, and the result is shown in Fig. 7(b). It can be
seen that oxygen (the binding energy of 533 eV) is observed in acti-
vated carbon for all the samples reduced at different temperature.
It comes from the adsorbed O2 or the adsorbed H2O. For catalyst
reduced at 483 K, the phosphorous, chlorine and nitrogen elements
can be observed at about 130, 200 and 400 eV, respectively, which
indicates that the PH3 (or P), HCl and NH3 may be produced during
the reduction process. It is worthwhile to note that the band of oxy-
gen grows stronger after adsorption, implying the H2O is produced.
With rising the reduction temperature up to 593 and 663 K, only
the phosphorous and oxygen elements are still detected, indicating
there is just reduction of phosphate and hypophosphite.
Guan et al. [24] have put forward the mechanism of the prepara-
tion of Ni2P catalysts by the heat treatment of a mixed salt precursor
(NiCl2 and NaH2PO2) in a static Ar protecting gas atmosphere. Our
Fig. 5. Low and high resolution TEM micrographs of the Ni-P/MCM(2-663) catalyst.
(a) Low resolution TEM micrograph, (b) and (c) high resolution TEM micrographs.

H. Song et al. / Applied Catalysis A: General 462–463 (2013) 247–255
253
Fig. 6. XPS spectra of the Ni-P/MCM(2-Y) obtained at different temperature and
without passivating.
Fig. 7. The experimental configuration for gathering the volatile products (a) and
the XPS results of the absorbed activated carbon (b).
Ni(HPO3H)2 and H3PO4 were formed during reduction at 483 K. The
XPS (Fig. 7(b)) of absorbed activated carbon shows phosphorous,
oxygen, chlorine and nitrogen elements confirms the formation of
PH3, H2O, HCl and NH3 at reduction temperature of 483 K. And the
H2-TPR of NH4H2PO2 shows a H2 consumption peak at about 483 K
indicates the H2 (Fig. 1(f)) has also been involved at this temper-
ature. In a word, the following reaction may describe the process
occurred at reduction temperature of 483 K:
research describes catalysts prepared in a flowing H2 atmosphere.
Thus, the formation process of Ni2P may be somewhat different
from that reported by Guan et al. We propose that several possible
reactions occur during the synthesis of Ni2P catalyst. The equations
for the complete reaction would consist of three steps.
At the temperature of 483 K, the presence of some weak diffrac-
tion peaks of Ni2P (XRD analysis, Fig. 2(c)) and presence of the PH3,
HCl and NH3 (XPS analysis, Fig. 7(b)) indicate that the dispropor-
tionation reaction of NH4H2PO2 can produce PH3, then the PH3
reduces the NiCl2 to Ni2P. However, this process is not the dom-
inating reduction process. This is because the week peak of the
P␦− forming Ni2P and large amounts of unreduced P species, such
as HPO3H− and PO43−, can be observed from the XPS analysis of
the catalyst reduced at 483 K and without passivation (Fig. 6(b)).
Cecilia et al. [29] has found that the Ni(H2PO3)2 was amorphous,
but the Ni3(PO4)2 can be detected by XRD. The XRD shows no
other phase except Ni2P phase (Fig. 2(c)), confirming there is no
Ni3(PO4)2 formed. Therefore, it is reasonable to speculate that the
6NiCl2 + 12NH4H2PO2 + 5H2 = 2Ni2P + 2Ni(HPO3H)2
+ 2H3PO4 + 12HCl + 12NH3
+ 4H2O + 4PH3
(2)
At the temperature of 593 K, the XRD analysis shows the crystal-
lization of Ni2P grows stronger (Fig. 2(c)), and the XPS analysis of the
catalyst reduced at 593 K and without passivation also shows a peak
of the P␦− grows stronger (Fig. 6(b)), indicating more Ni2P were

254
H. Song et al. / Applied Catalysis A: General 462–463 (2013) 247–255
Fig. 8. The HDS activity of the catalysts with different initial Ni/P molar ratios.
Fig. 9. Stability of HDS activity over the Ni-P/MCM(2-663) catalyst. Temperature,
Pressure, 3.0 MPa; WHSV, 2 h−1; H2/oil ratio = 500 (V/V).
613 K; pressure, 3.0 MPa; WHSV, 2 h−1; H2/oil ratio = 500 (V/V).
3−
formed at this tempeature than at 483 K. And the peak of PO4
disappeared and the band of HPO3H− became weak (Fig. 6(b)).
The H2-TPR shows a marked H2 consumption peak at this tem-
perature indicates a reduction process occured (Fig. 1(d)). The
XPS of absorbed activated carbon (Fig. 7(b)) shows phosphorous
and oxygen elements, this confirms the formation of PH3 and
H2O at this temperature. Infantes-Molina and Cecilia et al. [29,30]
have also successfully prepared the Ni2P catalyst by H2 reduction
of a Ni(H2PO3)2 precursor at 373–773 K. The reduction process
occurred for 593 K through the following reaction, which can well
coincide with the results showed by XRD and XPS that more Ni2P
was formed at this temperature than 483 K:
with temperature (Table 3), suggesting that the DBT HDS over Ni2P
is a thermodynamically favorable reaction. Ni-P/MCM(2–663) pos-
sesses the highest TOF values what indicates a higher effectiveness
of Ni2P phase on this sample. However, within the reaction temper-
ature range of 593–613 K, the Ni-P/MCM(3–663) shows TOF values
close to the Ni-P/MCM(2–663). It is possible that the sample lost
some phosphorus during reaction, and the active sits covered by
excess P were exposed again. Oyama et al. [38] have reported that
the prolonged exposure of catalyst at the reaction temperature
caused the Ni2P phase transform Ni12P5 phase in the phosphorus
deficient catalysts, which indicated that there was loss of phospho-
rus during the HDS reaction.
DBT undergoes HDS by two parallel pathways: the direct desul-
furization (DDS) and the hydrogenation (HYD). DDS leads to the
formation of biphenyl (BP), while HYD yields mainly cyclohexyl-
benzene (CHB) and small amount TH-DBT and HH-DBT. Because
the transformation of BP to CHB is negligible in the presence of
DBT, BP selectivity (SBP) is used as a measure of DDS pathway, and
CHB represents the HYD pathway [47]. For Ni2P catalysts, the main
products of HDS reaction were BP and CHB, hence the selectivity
of reaction products can be calculated considering BP and CHB as
the only products obtained. Table 3 illustrates the variations of BP
and CHB yields with temperature over MCM-41 supported Ni2P
obtained at 483 K and 663 K. It can be seen that BP is formed in
greater proportions in all cases, with DDS being the favored reaction
route. The yield of BP increased with temperature for each catalyst.
The selectivity to CHB of Ni-P/MCM(2–663) is highest and that of
other samples are lower. As reported by the literature [48], there
are two types of sites in Ni2P, Ni(1) sites with tetrahedral coordina-
tion and Ni(2) sites with square pyramidal coordination, both Ni(1)
and Ni(2) sites are present on large crystallites but that the Ni(2)
sites are more numerous on the more highly dispersed samples. The
lower coordination Ni(1) sites are responsible for desulfurization by
the DDS pathway by taking on a sulfur atom while the Ni(2) sites
are the high-activity sites that carry out HDS by the hydrogenation
(HYD) route. Hence, we conclude that the acceleration of the HYD
pathway over the catalysts must be associated with the dispersion
of active phase. The Ni-P/MCM(2–663) catalyst has good dispersion
(as the CO uptake analysis), this may lead to more Ni(2) sites and
less steric hindrance during sufur adsorption of reactants, resulting
in a higher CHB selectivity.
4Ni(HPO3H)2 + 25H2 = 2Ni2P + 6PH3 + 24H2O
(3)
(4)
H3PO4 + H2 = H3PO3 + H2O
Compared with the catalyst reduced at 593 K, the XRD shows
the peaks intensities of Ni2P of catalyst reduced at 663 K remain
unchanged (Fig. 2(c)), and the XPS also shows the P␦− peak intensity
did not changed (Fig. 6(b)) demonstrating no new Ni2P was formed.
Moreover, only the P␦− can be observed from the XPS analysis
indicating HPO3H− was completely reduced. The XPS of absorbed
activated carbon (Fig. 7(b)) shows phosphorous and oxygen ele-
ments, indicates the formation of PH3 and H2O during reduction.
The H2-TPR shows a weak H2 consumption peak confirms a reduc-
tion process occurred at 663 K (Fig. 1(d)). In conclusion, a reduction
of H3PO3 has occurred.
H3PO3 + H2 = PH3 + H2O
(5)
3.8. HDS performance
The above analysis shows the sample with initial P/Ni molar
ratio of 1, 2 and 3 reduced at 663 K can be assigned to the Ni2P phase.
To investigate their catalytic properties, the catalysts are evaluated
by the HDS of DBT, and the result is shown in Fig. 8. In all cases, the
conversion values increase gradually with the temperature. Com-
paring the three catalysts we can conclude that Ni-P/MCM(2–663)
is more active than two other catalysts for HDS reaction, because
Ni-P/MCM(2–663) shows higher conversion values in the temper-
ature range of 280–340 K. The likely reason is that, with lower P
content, such as the samples with P/Ni of 1, less Ni2P was formed
(such as the XRD and XPS analysis). Nevertheless, at higher P con-
tent level, such as the samples with P/Ni of 3, excess P was formed
on the catalyst surface, which resulted in the Ni2P active phase was
blocked, and thus the catalyst shows low desulfurization activity.
The DBT conversion and CO uptake were used to calculate the
HDS TOF for the catalysts. The HDS TOF of each catalyst increased
Fig. 9 shows the results of Ni-P/MCM(2–663) catalyst stability
for HDS of DBT within 120 h. It can be known from Fig. 9 that
the activity of catalyst increases gradually in the initial 6 h, and
the conversion of DBT close to 99% after 6 h and kept for 114 h
without activity loss under the given reaction condition. The Ni-
P/MCM(2–663) catalyst improves its catalytic capacity with initial

H. Song et al. / Applied Catalysis A: General 462–463 (2013) 247–255
255
6 h, which suggests that there is a formation of an intermediate
phase which is more active than Ni2P. The surface modification of
Ni2P phase during the HDS reaction and the explanation of its excel-
lent behavior with initial time on stream have been considered in
the literature as consequence of the presence of a superficial phos-
phosulfide as active phase with a stoichiometry represented by
NiPxSy [49,50]. In a recent study of Ni2P/SBA catalysts, Korányi et al.
[42] reported that the presence of a superficial phosphosulfide with
a composition Ni2.4P1.0S0.24 is the real active phase of HDS reaction.
Moreover, Ni2P has better sulfur resistance than metal carbides and
nitrides which transform into metal sulfides with normal activity
under HDS conditions [1], the conversion of DBT maintains at 99%
in the case of DBT content in feed is as high as 1 wt% as shown in
Fig. 9. Although the surface of Ni2P takes up sulfur atoms under HDS
conditions, the kernel of the Ni2P particles remains intac [44,51,52].
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4. Conclusions
Ni2P catalysts were successfully prepared by H2 reduction of a
solid mixture of NH4H2PO2 and NiCl2·6H2O precursor, and by using
a silica mesoporous material such as MCM-41 as material support.
This method is energy saving because of no need of calcination step
and a low reduction temperature is required. Both the XRD and XPS
analysis showed that the phosphorus content in precursor has an
effect on the formation of the active phase. With sample of initial
P/Ni molar ratio >0.5, the Ni2P was successfully obtained at lower
reduction temperature, and a high initial P/Ni molar ratio favors
the formation of Ni2P at lower temperature (483 K). Using of less
oxidic phosphorus precursor of hypophosphite enabled the Ni2P
to be formed low reduction temperature. Ni2P catalysts prepared
by this method proved to be highly active for DBT HDS. The stabil-
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120 h time on stream whilst displaying an improved conversion of
close to 99% with high selectivity of BP. The results suggest that
this synthesis provides a potential new and general route for the
preparation of supported Ni2P catalyst.
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