The synthesis and mechanistic studies of a highly active nickel phosphide catalyst for naphthalene hydrodearomatization

Article abstract of DOI:10.1039/C7RA00250E

Although HDS and HDN reactions over transition metal phosphides have been widely studied, few publications about aromatic hydrodearomatization (HDA) over transition metal phosphides are found. Using ordered mesoporous Al-MCM-41 (Al-M) as the support, a series of supported nickel phosphide catalysts with different Ni/P molar ratios and loadings have been prepared by a temperature-programmed reduction and characterized. The HDA activity of naphthalene was measured in a fixed bed reactor at 250-330 °C and 3 MPa. The results showed that the catalyst with initial Ni/P molar ratio of 1.25 and 15 wt% loading displayed the highest HDA activity as well as 99.0% of selectivity of decalin at 270 °C, which is even higher than that of 0.5 wt% Pt-Al-M catalyst. As a comparison, the HDA performance of various catalysts was also examined. The results revealed that the presence of framework aluminum favors HDA processes and a synergistic effect between nickel phosphide and the suitable acidity resulted in an improvement of the catalytic activity. Finally, the possible reaction pathway of naphthalene hydrogenation (HYD) on nickel phosphide catalysts was proposed. Taking into consideration the high catalytic activity, Al-M supported nickel phosphide can be considered as a very efficient HDA catalyst to decrease the contents of aromatics in the fuels.

Full text of DOI:10.1039/C7RA00250E

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The synthesis and mechanistic studies of a highly
active nickel phosphide catalyst for naphthalene
hydrodearomatization†
Cite this: RSC Adv., 2017, 7, 8677
a
a
ab
*
*
Guoxia Yun, Qingxin Guan and Wei Li
Although HDS and HDN reactions over transition metal phosphides have been widely studied, few
publications about aromatic hydrodearomatization (HDA) over transition metal phosphides are found.
Using ordered mesoporous Al-MCM-41 (Al-M) as the support, a series of supported nickel phosphide
catalysts with different Ni/P molar ratios and loadings have been prepared by
a temperature-
programmed reduction and characterized. The HDA activity of naphthalene was measured in a fixed bed
reactor at 250–330 ꢀC and 3 MPa. The results showed that the catalyst with initial Ni/P molar ratio of
1.25 and 15 wt% loading displayed the highest HDA activity as well as 99.0% of selectivity of decalin at
270 ꢀC, which is even higher than that of 0.5 wt% Pt–Al-M catalyst. As a comparison, the HDA
performance of various catalysts was also examined. The results revealed that the presence of
framework aluminum favors HDA processes and a synergistic effect between nickel phosphide and the
suitable acidity resulted in an improvement of the catalytic activity. Finally, the possible reaction pathway
of naphthalene hydrogenation (HYD) on nickel phosphide catalysts was proposed. Taking into
consideration the high catalytic activity, Al-M supported nickel phosphide can be considered as a very
efficient HDA catalyst to decrease the contents of aromatics in the fuels.
Received 7th January 2017
Accepted 23rd January 2017
DOI: 10.1039/c7ra00250e
rsc.li/rsc-advances
thus signicantly improve the cetane number of diesel streams
and improve the combustion characteristics. The products of
1. Introduction
aromatics HYD are considered as an ideal component of jet
fuel, as they have concurrently appropriate quality and volume
caloric value. In this regard, this opinion is consistent with that
of Yan et al. They highlight a detailed overview of the develop-
ment related to energy production and environmental
remediation.3
Population increase, urbanization, and rising living standards
have rapidly increased global energy consumption and envi-
ronmental burdens in the last half century. Energy consump-
tion is expected to continue to increase dramatically in the
coming years along with its associated environmental issues.
Thus more stringent environmental regulations and fuel spec-
ications on diesel are being tightened in both the United
States and Europe, which not only put impelling emphasis on
the contents of aromatics in the fuels, but also lay special focus
on the density and cetane number.1 Because aromatics in fuels,
i.e., diesel and jet fuels, not only lower the quality and produce
undesired exhaust emissions but also have potentially
hazardous and carcinogenic effects.2 Therefore, HYD of
aromatics becomes one of the most important unit operations
in modern oil-renery and conventional hydroprocessing tech-
nology is used to reduce aromatics in fuels through the HDA
reaction. And HYD can produce cycloalkane from aromatics and
Generally, HYD is a highly exothermic reaction and thus
thermodynamically favored at lower temperature. A lot of
information about naphthalene HYD and formation of decalin
have been reported in literature.4 Therefore, searching out
excellent HYD catalysts will provide potentiality for decreasing
the aromatic content in the fuels and regarding them as high
thermal stable jet fuel. Conventional catalysts in aromatic HYD
are suldes based on Mo or W and promoted with Ni or Co
supported on alumina.5 The drawbacks of these catalysts
include severe operating conditions such as high temperature
and high pressure. Furthermore, only moderate levels of satu-
ration of aromatic content under typical hydrotreating condi-
tions can be obtained due to equilibrium limitations.
Meanwhile, as proposed by Wang and Zhou et al.,6 many
researchs have shown that the noble metal-based catalysts have
attracted extensive attention for a wide range of applications in
many reactions, as well as the HDA reaction. Noble metal-based
catalysts, including Pt,7 Pd,8 Rh,9 or bimetallic systems of these
noble metals, show distinctly high catalytic activities towards
aCollege of Chemistry, Key Laboratory of Advanced Energy Materials Chemistry
(Ministry of Education), Nankai University, Tianjin, 300071, China. E-mail: weili@
nankai.edu.cn; qingxinguan@nankai.edu.cn
bCollaborative Innovation Center of Chemical Science and Engineering, Tianjin,
300071, China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c7ra00250e
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naphthalene HYD, due to the efficient ability for dissociating generally demands a catalyst with high product diffusivity and
hydrogen. However, noble metal has high cost and the presence selectivity for aromatics HYD, molecular sieves with high
of sulfur strongly inuences their catalytic activity due to the surface area as well as the pore size and acidity are potential
poisoning of active sites. Recently, transition metal phosphides catalyst supports. Furthermore, proper pore structure and
have attracted attention not only because they are very active in acidity favored the dispersion of the active phases.
the hydrotreating reactions but also because they are much
Although HDS and HDN reactions over transition metal
more stable than noble metals and metal carbides and nitrides phosphides have been widely studied, few publications about
in the atmosphere of H2S.10 A number of results have shown aromatic HDA over transition metal phosphides are found.
that Ni2P was the most active catalyst for both hydro- Here in this work, we prepared the Al-M supported nickel
denitrogenation (HDN) and hydrodesulfurization (HDS) reac- phosphide HYD catalysts with different Ni/P molar ratios and
tions over the past several years.11–13 Oyama et al. compared HDS loadings in the precursors. The catalytic performance of the
and HDN activity over different phosphides and found that nickel phosphide catalysts and the reaction pathway of naph-
their activity order follows Fe2P < CoP < MoP < WP < Ni2P.
thalene HYD were studied in detail. Compared to Pt–Al-M, the
As common supports, silica, alumina, resin, active carbon, NixP–Al-M catalyst exhibited much better HYD activity and
clay, and microporous zeolite have been widely used, while decalin selectivity.
diffusion limitation limit their potential applications for cata-
lysing bulky molecules.14 Diffusion and catalytic tests have also
proved that mesoporous zeolite, especially MCM-41, can over-
2. Experimental
2.1 Materials
come the mass transfer limitation and exhibit excellent catalytic
properties for the conversion of bulky molecules.15,16 However,
purely siliceous mesoporous materials are electrically neutral
frameworks, no acidity was formed on their surface. Recent
studies have been proved that the supports with suitable acidity,
such as SiO2–Al2O3,17 USY,18 SAPO-11,19 H-ZSM-5,20 morden-
ites,21 H-beta zeolite,22 etc., have positive effects on the HDA
activity. These effects have been attributed to the synergistic
effects between acid sites and transition metal phosphides
particles. Firstly, it was claimed that electron-decient metal
particles (Md+) were formed in a transition metal phosphide/
zeolite bifunctional catalyst due to the partial electron trans-
fer from the active particles to acidic sites of the zeolite
Nickel nitrate (Ni(NO3)2$6H2O) and ammonium dihydrogen
phosphate (NH4H2PO4, 99%) were purchased from Tianjin
Guangfu Technology Development Co., Ltd. Chloro platinic
acid (H2PtCl6$6H2O, 99%) was obtained from Aladdin Indus-
trial Corporation. The supports, mesostructured hexagonal
frame-work, MCM-41 and Al-M were obtained from Tianjin
Chem. Sci. Ltd., China and the content of Al in Al-M accounts
for about 5 wt%. All of the materials were analytical grade and
utilized without further purication.
2.2 Catalyst preparation
supports. Secondly, Ni2P behaves somewhat like hydrogenase. A A series of high-performance catalysts have been synthesized by
spillover based HYD pathway in which aromatic hydrocarbons temperature-programed reduction of nickel phosphate precur-
were adsorbed on acidic sites and hydrogenated by the sors according to the following steps: [1] rstly, the support was
hydrogen from active particles would occur in zeolite supported dried at 100 ꢀC overnight prior to use. The oxide precursor was
catalyst. Namely, the acid sites of the support provide additional prepared by incipient wetness impregnating Al-M with
active sites on which aromatic are hydrogenated by the spillover a mixture aqueous solution of Ni(NO3)2$6H2O and NH4H2PO4,
hydrogen.23 Therefore, much effort has been devoted to the followed by drying at 80 ꢀC for 12 h, and calcination at 500 ꢀC for
enhancement of the surface acidity with the aim of using them 2 h in air; [2] the oxide precursor was reduced at 650 ꢀC for 2 h
as catalyst support or catalyst for acid-catalyzed reactions. under owing H2 (60 mL minꢁ1) by temperature-programmed
Aluminum may be incorporated into the mesoporous structure reduction (TPR) to obtain the active nickel phosphide catalyst.
by adding an aluminum source to the synthesis mixture. The Finally, the product was cooled to ambient temperature under
substitution of silicon by aluminum usually unbalances the owing H2 and was passivated for 1 h under owing 1% O2/N2.
neutral structure of silica, which may generate acid sites. The In one series, the loadings on the Al-M was chosen to be 10, 15,
nature and strength of such acid sites, as well as the amount of 20, 25, and 30 wt% with Ni/P molar ratio of 1.25. In another
trivalent cation added, may have an impact on the catalytic and series, the Ni/P molar ratios of the precursors were 0.5, 1, 1.25,
adsorption properties of these materials and suit them for 1.5, 2, and 3 with a 15 wt% loading.
applications in several processes.24 Introduction of foreign ions,
In addition, xing the initial Ni/P atomic molar to 1.25,
for instance, aluminum ions, into the silicate framework (that various supports supported nickel phosphide catalysts with
means, a decrease in the Si/Al ratio) has been proved to be an nickel loading of 15 wt% was also prepared following the same
efficient route for modifying the surface acidity of Si-MCM-41 procedure. For comparison purposes, Ni–Al-M catalyst with Ni
and the Lewis acidity of Si-MCM-41 could be enhanced signi- loading of 15 wt% was prepared by a procedure similar to that of
¨
cantly aer modication with aluminum ions. The Bronsted the supported nickel phosphides except for the addition of the
acidity of aluminosilicates generally arises from the presence of ammonium dihydrogen phosphate. Catalysts are designated
accessible hydroxyl groups or in the form of ‘‘bridging’’ Si– according to the loading (M) of the active phase and the Ni/P ratio
(OH)–Al hydroxyl groups (structural hydroxyls) associated with (Y) as M wt% NixP–Al-M-(Y), in which x represents the actual Ni/P
4-coordination framework aluminium.25 As such a process values aer reaction. The Pt loading in catalysts were 0.5 wt%.
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The supported Pt catalysts were obtained through the wet co-
The reaction procedure was stated in detail here. The cata-
impregnation with an appropriate amount of aqueous H2PtCl6- lysts were pelleted, crushed, and a particle size fraction of 20–40
$6H2O. The impregnation lasted for 24 h. Aer that, the sample mesh was used for the test. And catalysts were diluted with
was dried in air at 80 ꢀC for 12 h, and nally calcined at 350 ꢀC for quartz sand before loaded into the reaction tube. Before reac-
2 h in air. At last the catalyst was cooled to ambient temperature tion, the catalysts were reduced at the same temperature as
and washed in deionized water for twice followed by drying at preparation for 1 h in owing H2. Aer prereduction, the system
ꢀ
ꢀ
80 C for 12 h. In this way, Pt supported on Al-M was prepared was adjusted to the desired reaction temperature (250 C) and
and denoted as 0.5 wt% Pt–Al-M, which was used as a reference. pressure (3.0 MPa). The feedstock containing naphthalene in
cyclohexane (5 wt%) was continuously pumped into the reactor
at the rate of 0.17 mL minꢁ1. The weight hourly space velocity
2.3 Catalyst characterizations
(WHSV) was equal to 1 hꢁ1, and the H2/oil ratio was kept at 500
The catalysts were studied by a series of characterizations as
(v/v).
follows:
Liquid products were collected every hour in all assays. The
Powder X-ray diffraction patterns (XRD) were recorded by
reactant and products were analysed using a gas chromato-
a Bruker D8 focus diffractometer using a Cu-Ka radiation. The
graph (Agilent 7890A), equipped with a HP-5 capillary column
measurement was made in the 2q range of 10–80ꢀ at 40 kV and 40
and a ame ionization detector (FID). Products compositions
mA. The size and morphology of as-obtained samples were
were identied by GC-MS (Agilent 7890A-5977C). The HYD
observed by eld emission scanning electron microscope
performance of catalysts was determined by the naphthalene
(FESEM, JEOL, JSM-7500F) linked with energy-dispersive X-ray
conversion and product selectivity.
spectrometer (EDS). The samples were plated with gold before
For TOF calculation, we used the same denition of TOF like
measurement to avoid the electric charge accumulation. Trans-
other works do, which is moles of reactant converted divided by
mission electron microscopy (TEM) images and the HAADF-
moles of active site. As follows, the TOF was calculated using
STEM mapping analysis were obtained on electron microscope
eqn (1):
(FEI Tecnai, G2 F20) with a link energy-dispersive X-ray (EDX,
EDAX, TEAM) system for elemental mapping operated at an
FA0 XA
TOF ¼
(1)
W COuptake
accelerating voltage of 200 kV equipped with a eld emission
source. The elemental composition of the catalysts was deter-
mined by ICP-OES using an SPECTRO ARCOS ICP-OES Thermo
Optical Emission Spectrometer. The specic surface area of
materials was analysed using nitrogen adsorption–desorption
isotherms measured at 77 K on a BELSORP Mini instrument with
preprocessing the samples in vacuum at 150 ꢀC for 12 h. The
specic surface area data and the pore size distribution analysis
were calculated by Brunauer–Emmett–Teller (BET) and nonlocal
density functional theory (NLDFT) method from adsorption
isotherm, respectively. Temperature-programmed desorption
(TPD) experiments were performed with a Micromeritics Chem-
isorb 2750 instrument. All samples were pretreated in N2 at
where FA0 is the molar rate of reactant fed into the reactor (mmol
sꢁ1), W is the catalyst weight (g), COuptake is the uptake of
chemisorbed CO (mmol gꢁ1), and XA is the reactant conversion
(%). FA0$XA represents the moles of reactant converted per
second (mmol sꢁ1), and W$COuptake represents the moles of
active site (mmol).
3. Results and discussion
3.1 X-ray diffraction
XRD patterns were used to identify the crystalline phases
formed aer reduction. In order to study the effect of surface
modication on the crystal structure of supported nickel
phosphide catalysts, the structures of NixP–Al-M obtained from
various initial Ni/P molar ratios at a loading of 15 wt% NixP were
determined in Fig. 1(a), as well as a Ni2P and Ni12P5 reference.
From the Fig. 1(a), it can be easily found that a broad diffraction
line located at 2q ¼ 23ꢀ is due to the amorphous nature of
mesoporous structure for all of the catalysts.26 Interestingly, the
sample prepared with initial Ni/P ratio of 1/2 shows a pure Ni2P
phase and the main diffraction peaks positions clearly appear
around 40.7ꢀ, 44.6ꢀ, 47.4ꢀ, 54.2ꢀ, and 54.8ꢀ, which correspond to
the (111), (201), (210), (300), and (211) crystal faces of Ni2P
(PDF# 65-1989), respectively. In particular, when initial Ni/P
ratio exceeds 1/1 and further increase, the peak of the Ni2P
phase declined and the Ni12P5 phase emerged. Diffraction
peaks associated with Ni12P5 at 2q ¼ 38.3ꢀ, 41.7ꢀ, 46.7ꢀ, and
48.9ꢀ (PDF# 22-1190) were observed in addition to those of Ni2P.
300 C for 2 h with a ow rate of 25 mL minꢁ1. The desorption
ꢀ
step was performed from 50 ꢀC to 800 ꢀC with a heating rate of
10 ꢀC minꢁ1. X-ray photoelectron spectroscopy (XPS) was carried
out using a Kratos Axis Ultra DLD spectrometer employing
a monochromatic Al Ka X-ray source (hn ¼ 1486.6 eV), hybrid
(magnetic-electrostatic) optics, and a multichannel plate and
delay line detector with the C 1s peak at 284.6 eV as the internal
standard. All XPS spectra were recorded using an aperture slot of
300 ꢂ 700 mm, survey spectra were recorded with a pass energy of
160 eV, and high resolution spectra with a pass energy of 40 eV.
CO uptake was obtained on Micromeritics Chemisorb 2750
instrument using 100 mL loop ring. CO chemisorption was taken
at 30 ꢀC in the owing He (100 mL minꢁ1). The samples were pre-
reduced in owing H2 at corresponding temperature for 1 h.
2.4 Catalytic activity evaluation
Naphthalene was used to investigate the catalysts HYD perfor- Namely, with increasing Ni/P molar ratio, the excess of P
mance. HDA reaction was carried out in a continuous ow xed- decreases, the conditions for formation of the Ni2P phase
bed reactor equipped with a high-pressure pump.
become less favorable, so the intensities of the Ni2P diffraction
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excessive P content, the active nickel phosphide surface covered
by the excess P. This is discussed in below. In a word, only pure
Ni2P phase is observed with samples with higher P content,
while with the sample with lower P content both Ni2P and
Ni12P5 phases were observed. Furthermore, with P content fell
to the lowest level, crystalline Ni formed in addition to little
Ni12P5.
Fig. 1(b) shows the XRD patterns of NixP–Al-M with various
NixP loadings from 10 wt% to 30 wt% at an initial Ni/P molar
ratio of 1.25. As shown in Fig. 1(b), X-ray line-broadening
analysis of the samples indicated a weak trend toward larger
crystallite sizes with increasing sample loading. As will be seen,
when the NixP loading reached 10 wt%, no prominent NixP
diffraction peak was found compared with the other samples
due to the low loading and high distribution of NixP on the
support. So it was found that a low loading sample was partic-
ularly difficult to prepare with the right stoichiometry. This may
represent a limitation for the preparation of samples consisting
of compounds from two elements like nickel phosphide. As the
NixP loading increased from 15 wt% to 20 wt%, a Ni12P5 peak
emerged and the intensity of the Ni12P5 diffraction peaks
strengthened, nally reached the highest at 20 wt%. With
further increase in the NixP loading, the diffraction peak of
Ni12P5 obviously decreased. Interestingly, when the NixP
loading reached 30 wt%, mainly Ni2P diffraction peaks were
observed and the Ni12P5 diffraction peaks nearly disappeared.
Moreover, excess loading leads to larger particle sizes of NixP
and correspondingly decreases the activity. The HDA activities
of the NixP–Al-M catalysts were strongly dependent on the types
and dispersion of the nickel phosphide. In this study, the
formation of NixP (Ni2P and/or Ni12P5) on the surface of Al-M
could be decided by controlling the loading. And it was found
that catalysts of intermediate loading had high activity, as
Fig. 1 XRD patterns of (a) 15 wt% NixP–Al-M obtained from various
initial Ni/P molar ratios and (b) NixP–Al-M-(1.25) with various NixP
loadings.
peaks decrease. As previously proposed by Oyama et al., the shown in below. Besides, low-angle powder X-ray diffraction
lower P content led to formation of Ni12P5, which is considered patterns of the samples are depicted in Fig. S1.† Only the most
as the intermediate in Ni to Ni2P. And more phosphorus was intense characteristic peak of MCM-41 structure can be
needed to form Ni2P.27 The formation of Ni12P5 and Ni at the low observed for 0.5 wt% Pt–Al-M apart from supports. This fact
P content is understandable, as Ni12P5 contain a relatively lower indicates that the structural order of MCM-41 decreases for the
proportion of P than Ni2P. Ni2P is formed by the further reaction supported catalysts, very likely due to the uneven distribution of
between Ni12P5 and phosphorus. This is the result of the partial active phase particles inside the pores. Meanwhile, the wide-
loss of phosphorous due to the formation of volatile P species, angle XRD patterns of Al-M support along with the corre-
such as PH3, during reducing process, leading to a decrease in sponding Pt-loaded catalyst are displayed in Fig. S2.† For 0.5
the amount of P on the surface. Oyama et al. also found that if wt% Pt–Al-M, diffraction peaks of platinum are hardly visible
the Ni/P ratio of precursor is equal to the stoichiometric ratio of probably owing to the low content. Crystallite size of various
Ni2P, not only Ni2P phase is formed, but also a P-decient phase catalysts also have evaluated based on Scherrer equation. The
Ni12P5 was formed. Therefore, it should be noted that an corresponding results are listed in last column of Table S1.† In
amount of phosphorus above stoichiometric proportions was practical terms, the crystallite sizes of samples are in close
necessary in order to obtain the desired stoichiometry. As the agreement with the results of particle size distribution accord-
Ni/P ratio further increased, with regard to 15 wt% Ni–Al-M, the ing to TEM images, as described in Fig. 3 below.
new diffraction peaks at 44.4ꢀ, 51.7ꢀ, and 76.3ꢀ assigned to
(111), (200), and (220) planes of crystalline metal Ni (PDF# 1-
120) were clearly observed besides those of supports. These
3.2 SEM
results demonstrate that the initial Ni/P molar ratio in the To further conrm the composition of the catalysts and the
precursor has profound impact on the structure of NixP catalyst. effective loading of the NixP in the materials. Fig. 2 shows the
Namely, different Ni/P ratios can result in different crystal SEM and EDS mapping images of different catalysts (red dot is
phases of the catalyst and we also found that the catalyst with nickel atom and yellow dot is phosphorus). Fig. 2(a), (b) and (d)
initial Ni/P ratio of about 1/2 had not an excellent activity. At show that the Ni–Al-M has better metal dispersion than the
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Fig. 2 The SEM and EDS mapping images of different catalysts (red Fig. 3 The TEM images of NixP–Al-M catalysts synthesized at 650 ꢀ
C
dot refer to nickel atom, and yellow dot refer to phosphorus atom), (a) from Ni(NO3)2$6H2O and NH4H2PO4 precursors with a mole ratio of
Ni/P ¼ 1.25, and with different loadings: (a) 10 wt%, (b) 15 wt%, (c) 20
15 wt% Ni–Al-M, (b) 15 wt% NixP–Al-M-(1.25), (c) 15 wt% NixP–Al-M-
(1.25), and (d) 30 wt% NixP–Al-M-(1.25).
wt%, (d) 30 wt%. Inset is the corresponding particle size distribution
histogram.
NixP–Al-M. Moreover, the distribution of NixP particle is rela-
tively uniform, no obvious sintering and aggregation was
observed. Compared to Fig. 2(b) and (d) show that Ni and P
dispersion decreased signicantly with the increase in NixP
loading of the same Ni loading. Fig. 2(c) gives the mole ratio of
Ni/P ¼ 1.71 measured by EDS. Considering that EDS is a semi-
quantitative analysis method, and there may be due to a partial
loss of the phosphorus during preparation, the mole ratio of Ni/
P ¼ 1.71 is a reasonable value. In addition, previous work had
shown that phosphorus is removed as PH3 during the reduction
procedure and that some P was le on the better dispersed
supported Ni2P catalysts, even though higher temperatures
were applied for reducing these well dispersed samples.28 This
again is consistent with stronger interactions between Ni and P
in the smaller crystallites. In addition, Fig. S3† shows the SEM,
mapping image (cyan dot refer to platinum atom), and EDS of
0.5 wt% Pt–Al-M catalyst. This study reveals that small Pt
particles with a rather homogeneous size distribution are well-
dispersed on the Al-M, as shown in Fig. S3.† Besides, the
amount of the platinum is 0.46 wt% measured by EDS.
XRD result using the Scherrer formula. A magnied high resolu-
tion TEM image of 15 wt% NixP–Al-M approximately yields d-
spacing values of 0.221 nm, which was consistent with the {111}
crystallographic plane of Ni2P, in good agreement with the calcu-
lated values (PDF# 65-1989), as shown in Fig. 3(d). In contrast, the
mapping analysis of 15 wt% NixP–Al-M also further conrmed the
presence and good distributions of nickel and phosphorus
elements in Fig. S4.† Also of note that the highest nanodispersity
of the NixP nanoparticles was observed on the 15 wt% NixP–Al-M
for all catalysts. It shows a spherical shape with good dispersion
of nanocrystalline NixP on the Al-M support. Inset show the
consequent size distribution histograms of NixP-based catalysts.
The micrographs obtained for each catalyst were analyzed to
determine the average particle diameter and size distribution of
the NixP nanoparticles. A measurement of at least 200 nano-
particles, assuming virtually spherical and randomly orientated
particles was taken for the construction of the size distribution
histograms. The NixP nanoparticles are homogeneously distrib-
uted over the supports. The sizes of the NixP nanoparticles for all
the catalysts are about in the range of 5–80 nm.
3.4 ICP-OES
3.3 TEM
The composition of the catalysts is further measured by ICP-
OES and the results were summarized in Table S2.† Obvi-
ously, as seen in Table S2,† it is clearly indicated that the
experimental values were much larger than theoretical values of
Ni/P molar ratio. It might be due to the extra phosphorous
except volatilized as PH3 and residual POxnꢁ species during the
reduction process, partly situated in the surface of the catalyst,
which is further explained in detail below.
Fig. 3 shows the representative TEM images of NixP–Al-M catalysts
synthesized at 650 ꢀC from Ni(NO3)2$6H2O and NH4H2PO4
precursors with a mole ratio of Ni/P ¼ 1.25, and with different
loadings: (a) 10 wt%, (b) 15 wt%, (c) 20 wt%, (d) 30 wt%, and inset
is the corresponding particle size distribution histogram. The TEM
images exhibited a large number of NixP, fairly uniform globular
nanoparticles with a mean particle size of about 5–10 nm, were
observed to be supported on Al-M, indicating NixP has good
dispersion on the Al-M support. Furthermore, the results clearly
show that the distributions of the nanoparticles exhibited poor
3.5 N2 sorption–desorption experiment
dispersion and signicantly aggregation, along with the increase Fig. S5† shows the N2 adsorption–desorption isotherms of (a) 15
in the NixP loading from 10 to 30 wt%. Correspondingly, the wt% NixP–Al-M with various Ni/P molar ratios, (b) NixP–Al-M-
particle size of nickel phosphides increased, which is similar to the (1.25) with different NixP loadings, (c) MCM-41, Al-M,
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physically mixed MCM-41 + g-Al2O3, denoted as (M + Al)
supports, and 0.5 wt% Pt–Al-M (d) the corresponding BJH pore
size distributions of supports and 0.5 wt% Pt–Al-M catalyst. As
can be seen from Fig. S5(a)–(c),† all the samples of N2 isotherms
are basically identical to pure supports. The samples present
a reversible adsorption–desorption type IV isotherm character-
istic with a hysteresis loop according to the IUPAC classica-
tion, showing the presence of some mesopores.29 This type of
hysteresis corresponds to aggregates or agglomerates of parti-
cles which form pores in the form of cle such as plates or sharp
particles such as buckets, with uniform distribution. As for Al-M
and (M + Al) supports, it was very similar to that of the pure
MCM-41, the adsorption and desorption lines were almost
overlapped, nearly leading to no hysteresis loop formation. The
coincidence of the adsorption and desorption lines could be
ascribed to a similar mechanism for the two processes, which
reects the absence of adsorbate pore blocking. The absence of
a hysteresis loop and the sharp curvature along the second
stage, suggest the existence of uniform and cylindrical channels
throughout the materials. The pore size distributions obtained
by BJH analysis of these curves show no marked differences
among the samples as we can see from Fig. S5(d).† Pore
distribution for all of the samples was mono-modal showing
that they belong to the mesoporous materials. The pore diam-
eters for all samples were almost same to original MCM-41,
suggesting that NixP particles were basically limited on the
external surface of the mesoporous support and only some of
the MCM-41 was destroyed aer NixP loading. In addition, the
corresponding textural characterization of the supports and
catalysts are presented in Table S1.† The BET specic surface
area and pore volume of the MCM-41 were 893 m2 gꢁ1 and 0.96
cm3 gꢁ1, respectively. However, the incorporation of alumina in
the MCM-41 matrix resulted in a slight decrease on the surface
area and the pore volume, which can be attributed to the
stability of the siliceous material and also a decrease in the pore
volume due to the blockage of some from pores of the MCM-
41.30 The date also indicated that the deposition of NixP and Pt
on the support surface causes a greater drop in the surface area
and pore volume. Meanwhile, both the BET surface areas and
pore volumes of the samples varied from high to low, with
corresponding NixP loadings increasing from 10 to 20 and 30
wt%. One reason is that active component loadings increased
the catalyst density so that the proportion of Al-M in the total
quality of catalysts correspondingly decreased. The other is that
a number of pores of the supports were blocked/lled by NixP
during the catalyst preparation. However, no signicantly
change in the pore diameter is observed from catalysts
compared to their corresponding samples, indicating that no
additional P is deposited on the walls of the pores.
Fig. 4 NH3-TPD profiles of the MCM-41, Al-M supports, and the
supported NixP catalysts with different Ni/P molar ratios.
observed in three temperature regions. The regions of the Tmax
ꢀ
(temperature maximum) from 20 to 200 C, Tmax from 200 to
500 ꢀC, and Tmax higher than 500 ꢀC are referred to as low-
temperature, moderate-temperature and high-temperature
regions, corresponding to weak acidic sites, intermediate
acidic sites, and strong acidic sites, respectively.31 The principal
objective of this work is to increase the stability as well as the
acidity of the support by adding alumina, which play a vital role
in the HDA and product selectivity during hydrotreating.
Generally, MCM41 support presented the lowest amount of acid
sites. As expected, the addition alumina to the MCM-41
supports, the surface acidity increases with decreasing the Si/
Al molar ratio. Desorption peak around low-temperature was
¨
assigned to Lewis and Bronsted acid sites of weak acidic
strength, while desorption peak about high-temperature was
¨
assigned to intermediate Bronsted acid sites (usually bridging
Si–(OH)–Al groups).32–34 It is demonstrated that there are the
¨
weak and medium strength Bronsted acid sites on Al-M, which
correspond to the NH3 desorptions with the peaks at about 120
and 400 ꢀC, respectively, as shown in Fig. 4. However, when Ni
or NixP was supported on Al-M, it is composed of two types of
¨
weak acid sites, Lewis and Bronsted, which are ascribed to the
electron-decient Ni site and to the P–OH, Si(OH)Al group,
respectively. Since the electron-deciency of these nickel ions is
very high and nickel in nickel phosphide exists in positive
charges,35 they may play a role of Lewis sites. Also an additional
amount of weak acidic sites was found with a shi of the peak to
lower temperature shown in Fig. 4. The acid sites on Al-M still
maintained, but some of them could be covered by Ni or NixP
particles. Besides, as indicated in Fig. 4, Ni–Al-M had more
¨
medium strength Bronsted acid sites but less weak strength
acid sites than NixP–Al-M. In other words, there is less medium
¨
strength Bronsted acid sites on NixP–Al-M, which is due to the
3.6 NH3-TPD
coverage of acid sites with P. The amount and strength of acid
sites are related not only to the activity of naphthalene HYD but
also to the extent of concomitant further cracking and coke
disposition. Meanwhile, the total acidity amounts are listed in
Table S4.† Moreover, NH3-TPD proles for supports MCM-41,
NH3-TPD was performed to elucidate the relationship between
the catalysts activity and types of acidic sites on the catalysts.
Fig. 4 shows the NH3-TPD proles of the MCM-41, Al-M
supports, and the supported NixP catalysts with different Ni/P
molar ratios. In a typical NH3-TPD prole, peaks are generally
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physically mixed M + Al, Al-M supports, and 0.5 wt% Pt–Al-M
catalyst are depicted in Fig. S6.† In general, these proles for
Pt-based catalyst consist of two peaks. One appears at a low
temperature range of around 150–200 ꢀC while the other
appears at a high temperature range of around 350–450 ꢀC. The
low and high temperature peaks correspond to the weak and
strong acid sites, respectively.36 In this study, as shown in
Fig. S6,† aer the addition of Pt, two patterns for NH3 desorp-
tion appeared. The rst curve was due to the NH3 adsorption on
pure mesoporous surface. The second curves were due to the
NH3 adsorption on the impregnated Pt species on Al-M.36
Obviously, the NH3 desorption peak temperatures and areas
increase, which illustrates that the addition of Pt can effectively
enlarge the number and strength of acid sites over 0.5 wt% Pt–
Al-M catalyst. In particular, the NH3 amounts desorbed at low
temperature reached about 3 times in 0.5 wt% Pt–Al-M
compared with pure Al-M.
3.7 XPS
In order to gain further insight into the surface composition of
surface oxidation layer and the valence states of active compo-
nents, the XPS of catalysts was performed. The XPS Ni 2p and P
2p spectra of 15 wt% NixP–Al-M-(1.25) catalyst are presented in
Fig. 5(a) and (b). All the spectra were decomposed by tting
using Origin Pro. 8.5, in which the black lines represent the
experimental data, the red lines are the envelopes of decom-
posed features and the other lines are the Gaussian components
peaks, as shown in Fig. 5. According to the result in Fig. 5(a), we
can see that the Ni 2p core level spectrum involves two contri-
butions, which correspond to the spin-orbital splitting of the Ni
2p3/2 and Ni 2p1/2 lines (about 17 eV), respectively. Ni 2p3/2 core-
level spectrum consists of three components, the rst of which
centered at 851.6–853.5 eV can be assigned to reduced Nid+
species in the NixP phase, and the second is at 856.3–857.6 eV,
corresponding to Ni2+ ions interacting possibly with phosphate
ions as a consequence of a supercial passivation,37 along with
the broad satellite peak at approximately 6.0 eV higher than that
of the Ni2+ species, and this shake-up peak is ascribed to diva-
Fig. 5 XPS spectra in the Ni 2p and P 2p regions for 15 wt% NixP–Al-
M-(1.25) catalyst: (a) Ni 2p core level spectra and (b) P 2p core level
spectra.
1.40 Moreover, an active site with a high electron density favors
the formation of a p back bond between the aromatic ring and
active sites, which promotes the HYD of the aromatic ring.
Accordingly, XPS analyses are also used to calculate the surface
Ni/P atomic ratios as well as surface compositions of the cata-
lysts before their use. The XPS analysis results for 15 wt% NixP–
MCM-41-(1.25) and 15 wt% NixP–Al-M-(1.25) catalysts, given in
Table S3.† It can be seen that as prepared catalysts exhibited
lower contents values of Ni, P and the surface Ni/P ratios
determined using XPS are far below the theoretical Ni/P ratio
corresponding to the precursors, which shows that surplus
phosphorous tend to distribute on the surface of the catalysts.
´
lent species. This observation is consistent with that of Koranyi
et al.38 Other broad peaks on the high binding energy side can
be assigned to the Ni 2p1/2 components. The P 2p binding
energy spectrum features a low intensity peak at 129.5 eV due to
P
dꢁ of the reduced metal phosphides and a peak at 134.5 eV due
3ꢁ
to surface metal phosphate species in PO4 arising from the
supercial oxidation of the nickel phosphide particles. The
result is similar to other report.39 Generally, the higher binding
energy indicates a lower electron density. The Ni 2p binding
energies in NixP phase are much higher than reported values for
3.8 Hydrogenation activity
Ni metal (852.5–852.9 eV) and lower than those reported for Ni Previous studies have shown that the activity of aromatic HYD
in NiO (854.1 eV),40 which is inuenced by the formation of the depended not only on the amounts of accessible active sites, but
greater interaction with the support, indicating that the Ni in also on the surface acid properties.41 Typically, the naphthalene
NixP carries a partial positive charge (d+), where 0 < d < 2, HYD, which was a rst-order reaction. There are three major
whereas the P 2p binding energies are below the value reported products formed from naphthalene on these catalysts: trans-
for elemental phosphorus (130.2 eV), revealing the small decalin, cis-decalin and tetralin. Meanwhile, trans- and cis-dec-
transfer of electron from Ni to P in nickel phosphide and alin were integrated as decalin. The NixP loadings and Ni/P ratio
a partial negative charge (dꢁ) carried on P surface, where 0 < d < had been reported to have an important role in catalytic activity.
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To investigate the catalytic activity of NixP–Al-M, the HYD of naphthalene conversion and selectivity to decalin, which were
ꢀ
naphthalene was conducted in a xed bed reactor at tꢁh1e 100% and 99.9% at 270 C. Fig. S7† present HDA activity per-
ꢀ
conditions of P ¼ 3 MPa, T ¼ 250–330 C and WHSV ¼ 1 h
.
formed over the temperature range of 250–330 ꢀC. It shows the
Fig. 6(a) shows the inuence of NixP loadings on HDA activity effect of temperature on the selectivity. Wherein, the decrease in
and selectivity for naphthalene over NixP–Al-M catalysts at a Ni/ selectivity to decalin with increase in reaction temperature and
P ratio of 1.25 aer 6 h on-stream. As depicted in Fig. 6(a), it this appears to be common for all the catalysts, which is prob-
could be observed that all catalysts showed high naphthalene ably due to thermodynamic equilibriums limitations and coke
conversion. With the increasing of the loading, the catalytic formation that can hinder the access to the active sites, as it has
activity and selectivity rst increases and then decreases. been previously found.42 Correspondingly, the conversion
Accordingly, samples with low loadings showed low selectivity toward naphthalene and the selectivity to decalin went through
ꢀ
to decalin. When the NixP loading was 10 wt%, the selectivity to a maximum at about 270 C. As mentioned earlier, the sample
decalin was only 44.7%, and when the loadings reached 20 wt% with NixP loading of 15 wt% gave highest activity for decalin
and 25 wt%, the selectivity to decalin were 98.0% and 95.9%, formation. Therefore, we studied the effect of initial Ni/P ratio
respectively. But further increasing of loading led to decline of with the NixP loading of 15 wt% on the HYD of naphthalene.
activity and selectivity. When the NixP loading rises to 30 wt%, The results of HYD of naphthalene over those samples with
the selectivity to decalin rapidly decreased to 36.3%. The HDA different Ni/P molar ratios at the level of 15 wt% loading and
properties decrease as a result of the lack of catalytic activity a reaction temperature of 270 ꢀC were exhibited in Fig. 6(b). As
centers, the poor NixP dispersion and the smaller surface area a general trend, it was obvious that the HDA activity altered with
led by excessive load. Combined with the XRD, SEM, and TEM the change of the Ni/P molar ratios and the corresponding
analysis, these results suggest that the loading content and naphthalene conversion and selectivity towards decalin rst
dispersion both have an effect on the HDA properties. More- increased and then decreased with the increase of the Ni/P
over, the one with 15 wt% loading had went through the highest molar ratio, as shown in Fig. 6(b). Much the same as above
analysis, two main products are detected: decalin and tetralin.
No rupture products were detected except tetralin and decalin,
indicating the acidity on the NixP–Al-M sample could not cata-
lyze the open-ring reaction of naphthalene. The product selec-
tivity with Ni/P ratio of 1/2 mainly contained tetralin, and the
decalin content is lowest, only about 2.0%, and that of other
samples are higher. Along with the increase of initial Ni/P ratio,
and the selectivity of decalin increases dramatically. The sample
with an initial Ni/P ratio of 1.25 has the highest HYD conversion
and the selectivity to decalin, about 100% and 99.9%, which is
higher than that of the other four samples, with Ni/P ratios of
1.0, 1.5, 2.0, and 3.0. The likely reason is that, on the one hand,
with lower P content, some P-decient Ni12P5 or Ni metal phase
are formed, which have been found to be less active in HYD
reaction. Nevertheless, on the other hand, at higher P content
level, the extra phosphorous partially located at the outer
surface of the catalyst,43 further covering the active sites and
leading to blocking some apertures of the NixP–Al-M catalysts,
as it has been previously mentioned. Moreover, it is possible
affected by the relatively large crystallite size of nickel phos-
phide, which should reduce the number of available centers for
HYD activity. Similaring to the optimal Ni/P ratio of 1.25 for the
SiO2 supported Ni–P catalyst reported in the published litera-
tures,44 our work also shows the optimal Ni/P ratio is 1.25.
Besides, the detailed product distributions obtained with
different temperature are also shown in Fig. S8.† It can be seen
that product distributions over the six catalysts showed
a similar current: the selectivity of decalin increased with
reaction temperature until a plateau was noticed at around
270 ꢀC. When temperature was approximately above 300 ꢀC,
concentrations of both trans-decalin and cis-decalin declined,
and concentration of tetralin slightly increased. This indicates
a high temperature does not promote further HYD. Fig. S9†
Fig. 6 Naphthalene conversion and selectivity to decalin (a) over
NixP–Al-M with different NixP loadings and (b) over NixP–Al-M with
compares the conversions and selectivities obtained from
naphthalene HYD over Ni-, NixP-, and Pt-supported catalysts. As
different Ni/P molar ratio in precursor as 0.5, 1.0, 1.25, 1.5, 2.0, and 3.0
at 270 ꢀC and 3 MPa.
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shown in Fig. S9,† the result showed that 15 wt% NixP–Al-M has replace of noble metals in the future. Thus it is easy to under-
the highest naphthalene conversion and decalin selectivity than stand and conclude that the naphthalene HYD activity is rst
15 wt% Ni–Al-M, 15 wt% NixP–MCM-41, 0.5 wt% Pt–Al-M and related to the NixP dispersion. The second factor that affects the
physical mixed 15 wt% NixP–(M + Al) samples, which indicates catalytic activity is the electron density of the NixP particles.
that the support plays important role and this implies again the Thirdly, the acid sites and hydroxyls provide additionally active
occurrence of a synergistic effect between the support and NixP. sites and favour spillover hydrogen, which also contributes to
Combined with characterization results mentioned above, it HYD activity. Furthermore, long-term stability test for the 15
seems that the electronic state of NixP surface, acid site number wt% NixP–Al-M-(1.25) catalyst at 270 ꢀC is displayed in Fig. S11.†
as well as strength, and particle size provide signicant effects As shown in Fig. S11,† during the activity testing, catalysts have
on the catalytic performance. The MCM-41 surface is homoge- not shown deactivation trend on the whole, which indicated
neous and had hardly any acid sites.45 The interaction between that 15 wt% NixP–Al-M-(1.25) catalyst showed good activity and
the supported NixP and the MCM-41 support is very weak. Some stability. In order to get rid of the inuence of different exposed
NixP sintering and agglomeration to large particles occurs active sites on different catalysts, we chose TOF as a signicant
during calcination and reduction. However, the aluminum the activity of per active site in unit time. It is reported that the
promoters affect the acid property and the electron property as numbers of active sites on the NixP surfaces can be titrated by
well as improving the dispersion of the NixP particles via the adsorption of CO since CO species could effectively titrate
isolation and anchor effects. The improvement in NixP disper- active centers of NixP via the formation of p back-bonding of
sion, which implies more active sites, is sure to exert an effect higher-electron density NixP species, according to which the
on the catalytic activity. Combining the SEM and TEM results turnover frequency (TOF) values for the hydrotreating reactions
with the naphthalene conversions of the catalysts, it is obvious can be calculated to gain an insight on the number of naph-
that the higher the NixP dispersion, the better the naphthalene thalene molecules converted to decalin on a catalytic site per
HYD activity. Based on above results, these facts clearly indi- second.46 The corresponding CO chemisorption uptakes and
cated that the surface acidity of supported nickel phosphide turnover frequency (TOF) results based on CO adsorption are
catalysts played an important role on the adsorption of naph- presented in the last two columns of Table S4.† Seen from the
thalene for the HYD. And a more detailed comparison of the Table S4,† the TOFs of the all catalysts tested were nearly the
differences in the product distribution over these catalysts with same, as anticipated.
various temperature is displayed in Fig. S10(a)–(e).† As we can
see from Fig. S10,† an almost complete conversion of naph-
3.9 Possible mechanism
thalene into decalin was observed over the reaction temperature
It has been widely reported that the naphthalene HYD occurred
range of 250–330 ꢀC. 15 wt% NixP–Al-M catalyst mostly con-
in two consecutive steps and a reaction pathway could be
verted naphthalene into decalin at lower temperature, in
gured out as shown in Scheme 1: conversion to tetralin in the
contrast with the 15 wt% Ni–Al-M catalyst. It also can be obvi-
rst step through sequential cis-addition steps of two dis-
ously seen that the tetralin selectivity steadily increased with the
sociatively adsorbed hydrogen atoms, followed by further
further raise of reaction temperature along with the decrease of
formation of decalin in the second step according to the
naphthalene conversion due to the equilibrium limitation of 15
following formula:
wt% NixP–MCM-41 and 15 wt% NixP–(M + Al) catalysts. At
ꢀ
270 C, the 15 wt% NixP–MCM-41 catalyst showed a naphtha-
C10H8 + 2H2 / C10H12 + 3H2 / C10H18 (ref. 44)
(2)
lene conversion of 99.0% with the tetralin and decalin selec-
tivities of 39.3 and 60.8%, respectively. And at 330 ꢀC, the
tetralin and decalins selectivities were almost 2 : 1 at the
naphthalene conversion of 98.7%. The 15 wt% NixP–(M + Al)
catalyst showed a moderate level of naphthalene conversion
and tetralin selectivity, compared with 15 wt% NixP–MCM-41
and 15 wt% NixP–Al-M. Above results validate the fact that the
acidity of the support plays a signicant role in the HYD of
aromatic molecules and the catalyst prepared by depositing
NixP on acidic supports shows higher activity than the catalyst
prepared by depositing NixP on non-acidic supports as reported
elsewhere. So the higher activity of NixP–Al-M, compared with
the 15 wt% NixP–MCM-41 and 15 wt% NixP–(M + Al) catalysts,
can be attributed to the stronger surface acidity of the Al-M
support which coupled with hydrogen spillover effect favoring
the HYD of aromatics. The 15 wt% NixP–Al-M catalysts exhibited
high decalin selectivity at high naphthalene conversion level
while the reference 0.5 wt% Pt–Al-M catalyst showed a relatively
low HYD activity towards naphthalene. This implied the high
Decalin has two isomers, which are trans- and cis-decalin,
and can be used as thermal stable jet fuel in extreme condition.
Hence, the selectivity to decalin is used in our study as the main
criterion to evaluate the catalytic performance. Pang et al.
HYD abilities of the NixP–Al-M catalysts and may take the Scheme 1 Reaction pathway of naphthalene hydrogenation.
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studied naphthalene HYD and found that the HYD rate of
naphthalene to tetralin was an order of magnitude faster than
that of tetralin to decalin.47 Some evidence showed that olens
played an important role in the reaction scheme. Naphthalene
react to D9,10-octalin, which is either directly hydrogenated to
cis-decalin at high pressure or isomerized to intermediate D1,9
-
octalin. D1,9-octalin is, in turn, hydrogenated to cis- and trans-
decalin. Tetralin reacted directly with hydrogen to form cis-
decalin, or through the intermediate D1,9-octalin. But tetralin
could not convert directly into trans-decalin. It needed to
convert to D1,9-octalin and then to trans-decalin. As aforemen-
tioned, D1,9-octalin is widely reported to be the most reactive
intermediate product in the reaction tetralin / decalin,
therefore, the formations of trans- or cis-decalin are closely
related to this intermediate. Based on the conformation
between the intermediate D1,9-octalin and the catalyst surface,
the possible formation of trans- or cis-decalin could be inferred
considering the syn nature of hydrogen addition, which means
that when the H atoms are added to a double bond, both H
atoms are always on the same side.48 Dokjampa et al. proposed
that the formation mechanisms of trans- or cis-decalin depen-
ded on the hydrogen atom in position 10 facing towards or away
from the catalyst surface.48 When the hydrogen atom in position
10 of this intermediate is oriented facing towards the catalyst
surface, the HYD of the double bond results in the formation of
cis-decalin, because hydrogen addition to a double bond is
always syn (both H atoms are added on the same side). By
contrast, when the hydrogen atom in position 10 is oriented
facing away from the catalyst surface, the syn HYD addition
results in the formation of trans-decalin. According to our
results, the very good catalytic performance is due to the
interaction between the support acid sites and the HYD sites on
bifunctional catalysts and a possible reaction mechanism for
Fig. 7 Proposed reaction schematic of naphthalene HDA over highly
active NixP–Al-M catalyst.
due to the enriched electron density on the aromatic ring with
p-bonds.24 Secondly, the NixP supported on the acidic Al-M
surface might be electron-decient due to the electron charge
transfer from NixP to acidic sites, leading to changing the
surface charge density of nickel and formation of electron
decient metal sites in close vicinity of the support acid sites,
increasing the activity. Thus the adsorption of naphthalene on
the electron-decient nickel might be enhanced due to the
enhanced electron donor from the naphthalene to nickel atoms.
Namely, the electron-decient surface nickel atoms on the
acidic, which favored the donation of electron charges from
naphthalene to the electron-decient d orbitals of surface
nickel. Apparently, the surface acidity also favored the adsorp-
tion and activation of aromatic rings.50
4. Conclusions
the HYD of naphthalene on nickel phosphide catalyst can be In summary, using Al-M as the support, NixP–Al-M, a series of
proposed as depicted in Fig. 7. One side, NixP can be ascribed to supported nickel phosphide catalysts with different Ni/P molar
the effects and behavior of the P sites and the formation of Ni–P ratios and loadings have been prepared by a temperature-
bonds. First, in NixP, the concentration of Ni is diluted by the programmed reduction method. The naphthalene HDA result
presence of P, the formation of Ni–P bonds induces relatively shows that both the Ni/P ratio and loading of the catalysts have
minor perturbations in the electronic properties of nickel.49 profound effect on the naphthalene HYD performance. The
Namely, a weak ligand effect (minor stabilization of the Ni 3d results showed that the catalyst with initial Ni/P molar ratios of
levels and a small Ni / P light charge transfer) that allows and 15 wt% loading displayed the highest HDA activity as well
a reasonably high activity for the dissociation of naphthalene as 99.0% of selectivity of decalin at 270 ꢀC, which is even higher
and molecular hydrogen,49 forming spillover of activated than that of 0.5 wt% Pt–Al-M catalyst. As a comparison, the HDA
hydrogen from HYD sites to the acidic support. Second, the performance of various catalysts were also examined. The
number of exposed active Ni sites present in the surface results revealed that the presence of framework aluminum
decreases due to an ensemble effect of P, which prevents the favors HDA processes and a synergistic effect between nickel
system from the deactivation.49 Third, the P sites are not simple phosphide and the suitable acidity resulted in an improvement
spectators and provide moderate bonding to the products of the of the catalytic activity. These results indicated that the catalyst
decomposition of naphthalene and the H adatoms necessary for performance is related to its acid property, pore structure, high
HYD.49 NixP–Al-M is a highly active HDA catalyst by obeying dispersion, the number of active sites, and the intimate contact
Sabatier's principle: good bonding with the reactants, and between NixP and the support. Finally, the possible reaction
moderate bonding with the products. On the other side, pathway of naphthalene HYD on nickel phosphide catalysts was
a possible explanation for the inuence of the acidic sites in the proposed. Considering the high HDS, HDN, and HDA activities
HYD activity of the NixP–Al-M catalyst may be mainly related to of nickel phosphide catalysts, NixP–Al-M can be considered as
the migration of electron charge. Detailed reasons are as a very efficient HYD catalyst to produce clean fuels. In our future
follows: rstly, the surface acidity favored the adsorption of work, the catalytic activities of nickel phosphide catalysts for
naphthalene since naphthalene can be taken as a Lewis base HDS and HDA of jet fuel will be studied.
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24 R. S. Araujo, D. C. S. Azevedo, C. L. Cavalcante Jr, A. Jimenez-
Lopez and E. Rodrıguez-Castellon, Microporous Mesoporous
Mater., 2008, 108, 213–222.
25 B. J. Liu, Q. L. Meng, H. Wang and Q. M. Meng, Acta Phys.-
Chim. Sin., 2010, 26, 3257–3262.
Acknowledgements
This work was nancially supported by the NSFC (21376123,
U1403293, 21603107), MOE (IRT-13R30 and 113016A), NSFT
(15JCQNJC05500), and the Research Fund for 111 Project
(B12015).
´
´
26 J. A. Cecilia, A. Infantes-Molina, E. Rodrıguez-Castellon and
´
´
A. Jimenez-Lopez, J. Catal., 2009, 263, 4–15.
27 Y. Y. Shu and S. T. Oyama, Carbon, 2005, 43, 1517–1532.
28 X. Q. Wang, P. Clark and S. T. Oyama, J. Catal., 2002, 208,
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