Probing the synergistic effect of Mo on Ni-based catalyst in the hydrogenation of dicyclopentadiene

Article abstract of DOI:10.1016/j.apcata.2019.01.026

Mo promoted Ni/γ-Al₂O₃ catalysts were synthesized by an incipient wetness co-impregnation method. The micro-structure, surface composition and adsorption characteristics of these catalysts were investigated by N₂ adsorption-desorption isotherms, XRD, HRTEM, XPS, TPR and dicyclopentadiene-TPD. The hydrogenation of dicyclopentadiene (DCPD) to endo-tetrahydrodicyclopentadiene (endo-THDCPD) was selected to evaluate the catalytic performance. The results showed Mo species improved dispersity of nickel oxide on the support surface and inhibit formation of spinel NiAl₂O₄. The nickel oxide could be reduced to Ni nanoparticles at relatively lower temperature because of its excellent dispersity and weakened interaction with the support. Meanwhile, the aggregation of metallic Ni on catalysts were markedly inhibited with the increasing of Mo content. Mo species also changed the adsorption mode of DCPD on Ni-based catalysts, and hence improved DCPD adsorption strength and capacity on catalysts and further changed hydrogenation mechanism of DCPD. The catalytic properties of NiMo_X/γ-Al₂O₃ catalysts showed that the hydrogenation activity was increased by adding Mo to Ni-based catalyst within limits. When the ratio of Mo to Ni was 0.2, the NiMo_0.2/γ-Al₂O₃ catalyst displayed the highest activity (TOF = 134.2 h^?1) and the best selectivity (99.7%). Compared with Ni/γ-Al₂O₃ catalyst, the hydrogenation time reduced from 6 h to 3 h and the amount of by-product C₅ fraction significantly decreased from 2.4% to 0.3%.

Full text of DOI:10.1016/j.apcata.2019.01.026

Accepted Manuscript
Title: Probing the synergistic effect of Mo on Ni-based
catalyst in the hydrogenation of dicyclopentadiene
Authors: Zhuqing Fang, Daxin Shi, Na Lin, Airu Li, Qin Wu,
Qiqi Wang, Yun Zhao, Caihong Feng, Qingze Jiao, Hansheng
Li
PII:
S0926-860X(19)30042-0
DOI:
Reference:
https://doi.org/10.1016/j.apcata.2019.01.026
APCATA 16966
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
6 November 2018
9 January 2019
29 January 2019
Please cite this article as: Fang Z, Shi D, Lin N, Li A, Wu Q, Wang Q, Zhao Y,
Feng C, Jiao Q, Li H, Probing the synergistic effect of Mo on Ni-based catalyst
in the hydrogenation of dicyclopentadiene, Applied Catalysis A, General (2019),
https://doi.org/10.1016/j.apcata.2019.01.026
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Probing the synergistic effect of Mo on Ni-
based catalyst in the hydrogenation of
dicyclopentadiene
Zhuqing Fanga, Daxin Shia*, Na Linb, Airu Lia, Qin Wua, Qiqi Wanga, Yun Zhaoa, Caihong
Fenga, Qingze Jiaoa, c, Hansheng Lia*
a School of Chemistry and Chemical Engineering, Beijing Institute of Technology,
Beijing 100081, China.
b School of Materials Science and Engineering, Beijing Institute of Technology,
Beijing 100081, China.
c School of Materials and Environment, Beijing Institute of Technology, Zhuhai,
Zhuhai 519085, China.
* Corresponding authors.
E-mail addresses: shidaxin@bit.edu.cn (D. Shi), hanshengli@bit.edu.cn (H. Li)
Graphical Abstract

Highlights
 Mo species significantly promoted the hydrogenation of DCPD and inhibited the
decomposition of DCPD.
 The aggregation of metallic Ni was restrained by Mo species.
 The adsorption modes of DCPD on catalyst surface changed with the introduction of
Mo
Abstract
Mo promoted Ni/γ-Al2O3 catalysts were synthesized by an incipient wetness co-
impregnation method. The micro-structure, surface composition and adsorption
characteristics of these catalysts were investigated by N2 adsorption-desorption isotherms,
XRD, HRTEM, XPS, TPR and dicyclopentadiene-TPD. The hydrogenation of
dicyclopentadiene (DCPD) to endo-tetrahydrodicyclopentadiene (endo-THDCPD) was
selected to evaluate the catalytic performance. The results showed Mo species improved
dispersity of nickel oxide on the support surface and inhibit formation of spinel NiAl2O4.
The nickel oxide could be reduced to Ni nanoparticles at relatively lower temperature
because of its excellent dispersity and weakened interaction with the support. Meanwhile,
the aggregation of metallic Ni on catalysts were markedly inhibited with the increasing of
Mo content. Mo species also changed the adsorption mode of DCPD on Ni-based catalysts,
and hence improved DCPD adsorption strength and capacity on catalysts and further changed
hydrogenation mechanism of DCPD. The catalytic properties of NiMoX/γ-Al2O3 catalysts
showed that the hydrogenation activity was increased by adding Mo to Ni-based catalyst
within limits. When the ratio of Mo to Ni was 0.2, the NiMo0.2/γ-Al2O3 catalyst displayed
the highest activity (TOF = 134.2 h−1) and the best selectivity (99.7%). Compared with Ni/γ-
Al2O3 catalyst, the hydrogenation time reduced from 6 h to 3 h and the amount of by-product
C5 fraction significantly decreased from 2.4% to 0.3%.
Keywords: Dicyclopentadiene; Hydrogenation; Ni-Mo bimetallic catalyst; Adsorption
mode

1. Introduction
Endo-tetrahydrodicyclopentadiene (endo-THDCPD) is not only a high-energy-density
solid fuel [1] but also an important intermediate for the synthesis of adamantine [2-4] and
exo-tetrahydrodicyclopentadiene (exo-THDCPD) [5-9]. The synthesis of endo-THDCPD
has been attracting much attention in recent years. Generally, endo-THDCPD was
synthesized by the hydrogenation of dicyclopentadiene (DCPD) [10,11] which was separated
from the C5 by-product of naphtha pyrolysis process [12,13]. There are two unsaturated
double bonds in the chemical structure of DCPD, one is the double bond in norbornene ring
(NB-bond) and the other is the double bond in cyclopentene ring (CP-bond). DFT simulation
demonstrate the NB-bond strength is weaker than that of the CP-bond. Hence, in the
hydrogenation process, NB-bond is saturated more easily than CP-bond. As a result, the
majority of the hydrogenation intermediate is 8,9-dihydrodicyclopentadiene (8,9-DHDCPD)
[14] (Scheme 1A). Another aspect has to be mentioned that DCPD can be decomposed to
CPD in higher temperature [15,16]. The decomposition reaction (Scheme 1B) is the main
reason for the reduction of the selectivity of endo-THDCPD in the hydrogenation of DCPD
[16,17].
In previous works, Pd-based and Ni-based catalysts have been investigated for the
hydrogenation of DCPD. Pd-based catalysts always show a high catalytic activity and
selectivity with low reaction temperature. Pd/Al2O3 catalyst shows high activity for the
hydrogenation of DCPD at low temperatures (90 °C) [10,11]. Pd/C catalyst also shows a
good performance in the DCPD hydrogenation reaction, with the yield of endo-THDCPD
reaching 97% in 30 min at 60 °C [18]. Nevertheless, due to the weak interaction between the
metallic Pd and the support, the active centers of the Pd-based catalysts are easily sintered or
lost during hydrogenation process. Furthermore, the Pd-based catalysts are susceptible to be
poisoned by sulfide groups [19,20]. Besides, the high cost and rarity of Pd also limit its
application in industry. Therefore, Ni-based catalysts as a substitute have been attracting lots

of researcher's interests in DCPD hydrogenation field.
Raney Ni is the only commercial hydrogenation catalyst in endo-THDCPD industry
because of its low cost and high catalytic performance [21]. However, it suffers from inferior
mechanical strength, poor reusability and complicated post-treatment procedure [22].
Therefore, it is necessary to find a suitable catalyst to replace Raney Ni. Zou et al. [14]
investigated the hydrogenation of DCPD over SRNA-4 catalyst which was a kind of
amorphous alloy Ni. The results showed that SRNA-4 exhibited significantly higher activity
than Raney Ni under relatively mild conditions and the two-stage heating operation was more
suitable to avoid the decomposition of DCPD. Wang et al. [22] prepared a kind of supported
skeletal Ni catalyst that displayed better compressive strength than Raney Ni and good
stability in the fixed-bed reactor. The above two catalysts were a kind of bulk Ni-based
catalyst, which all had the problems of low activity and selectivity. The supported Ni-based
catalyst can solve these problems because of great stability, low cost and high dispersion. Mi
et al. [23] and Du et al. [24] prepared two kinds of Ni/γ-Al2O3 catalysts with 15% and 30%
Ni content and obtained the yield of endo-THDCPD reaching 95% and 93%, respectively.
Sun et al. [21] prepared Ni-based catalysts using HY and α-Al2O3 as supports and the results
showed that the mesoporous structure of α-Al2O3 could promote catalytic hydrogenation
reaction because of sufficient contact of Ni atoms and DCPD molecules. Although supported
Ni-based catalyst had unique advantages, the process of the DCPD hydrogenation should be
carried out at high temperature in order to improve catalytic activity, which leaded to the
tendency of DCPD decomposition and a bad endo-THDCPD selectivity. So, it is necessary
to develop a highly active Ni-based catalyst to speed up the hydrogenation process and inhibit
the decomposition of DCPD.
Ni-Mo bimetallic catalysts as a kind of hydrogenation catalysts are often used in the
hydrogenation of unsaturated bonds [25-29]. In most of the bimetallic catalysts, Mo always
acts as a promoter to modify the catalytic activity. The bimetallic Ni-Mo catalysts always
show a higher hydrogenation activity than single metal Ni catalysts because the addition of
Mo can improve the dispersion of Ni and effectively inhibit the strong interaction between

Ni and support, increase the hydrogen storage active site of Ni and change the outer layer
valence electron structure of Ni, modify the activity and selectivity of the catalyst [30-32].
To the extent of our good knowledge, using NiMo/γ-Al2O3 as a catalyst for the hydrogenation
of DCPD and inhibiting the thermal decomposition of DCPD have not yet been reported.
In this work, a series of the NiMo/γ-Al2O3 bimetallic catalysts were synthesized by
using an incipient wetness co-impregnation method. The structure, physicochemical
properties, and catalytic performance of these catalysts were investigated. Probing the
synergistic Mo/Ni molar ratio of NiMoX/γ-Al2O3 bimetallic catalysts for the hydrogenation
of DCPD and investigating the influence of Mo on the chemistry, structure and catalytic
activity of NiMoX/γ-Al2O3 are the main purpose of the present work.
2. Experimental section
2.1. Materials
Ni(NO3)2·6H2O (Sinopharm Chemical Reagent Co., Ltd), (NH4)6Mo7O24·4H2O (Alfa
Aesar) and γ-Al2O3 (Shandong Ruiding Technology Co., Ltd.) were used as purchased. γ-
Al2O3 was dried for 6 h at 120 °C before use.
2.2. Preparation and activation of catalysts
The preparation of NiMo/γ-Al2O3 catalysts was achieved by incipient wetness co-
impregnation of γ-Al2O3 with mixed aqueous solution of Ni(NO3)2·6H2O and
(NH4)6Mo7O24·4H2O. The impregnated sample was dried at 120 °C overnight, followed by
calcination in air at 350 °C for 3 h. Before the hydrogenation, the precursor was activated by
reduction in a horizontal quartz tubular reaction chamber under atmospheric pressure at
500 °C for 3 h. The Ni mass content was fixed as 10wt.%, and Mo mass content was varied
with the change of molar ratios Mo/Ni at 0, 0.02, 0.05, 0.1, 0.2 and 0.5. The prepared catalysts

were labeled as Ni/γ-Al2O3, NiMo0.02/γ-Al2O3, NiMo0.05/γ-Al2O3, NiMo0.1/γ-Al2O3,
NiMo0.2/γ-Al2O3 and NiMo0.5/γ-Al2O3, respectively. And their precursors were denoted as
NiO-MoO3 X/γ-Al2O3 (X = 0.02, 0.05, 0.1, 0.2 and 0.5) and NiO/γ-Al2O3. Besides, single
metal Mo sample (10wt.%, Mo/γ-Al2O3) was prepared as control, and its precursor was
assigned as MoO3/γ-Al2O3.
2.3. Characterization of catalyst and support
N2 adsorption-desorption isotherms of catalysts were measured by using BELSORP-
max to estimate the specific surface area and pore size distribution. The specific surface areas
were estimated by using the Brunner-Emmet-Teller (BET) method, and the pore size
distributions were calculated by the Barrett-Joyner-Halenda (BJH) method from the
desorption isotherms. X-ray powder diffraction (XRD) was analyzed by a Philips X’Pert Pro
Multipurpose X-ray diffractometer with Cu Kα radiation. Temperature-programmed
reduction (TPR) and DCPD temperature-programmed desorption (DCPD-TPD) were
performed on a dynamic adsorption instrument (TP-5076) equipped with a thermal
conductivity detector (TCD). In the TPR experiment, the sample (100 mg) was pretreated at
300 °C in Ar for 1 h and then cooled to room temperature. A gas mixture of H2 (5%)/Ar
(95%) was used as the reducing agent with a total flow rate of 50 mL/min. The samples were
heated to 980 °C (10 °C/min) and the TPR profile was recorded simultaneously. For DCPD-
TPD, firstly, precursors had been reduced in-situ at 500 °C (10 °C/min) in H2 (5%)/He (95%)
for 1 h, and the adsorption of DCPD over the catalyst was controlled at 30 °C for 30 min.
After that the sample was purged with He for 1 h and then DCPD-TPD was performed in the
range of 30-650 °C (10 °C/min). The desorbed DCPD was detected by TCD. The
transmission electron microscopy (TEM), high-resolution TEM (HRTEM), high angle
annular dark field-scanning transmission electron microscopy (HAADF-STEM) and EDS-
elemental mapping tests were performed on a JEOL JEM-2100F system operated at 300 kV.
X-ray photoelectron spectroscopy (XPS) spectra were recorded using a Thermo ESCALAB

250XI. Binding energy was calibrated using adventitious carbon at 284.8eV.
2.4. Catalytic testing
The hydrogenation of DCPD was carried out in a 100 ml stainless steel autoclave
(Shanghai laibei Scientific Instruments Co., Ltd.) equipped with a mechanical agitator, a
pressure controller and a temperature controller. Cyclohexane was selected as the reaction
solvent. Prior to the reaction, 0.5 g pre-reduced catalyst, 5 g DCPD and 50 g cyclohexane
were introduced in the reactor vessel. The reaction conditions were set at 150 °C, 3.5 MPa
and a stirring rate of 600 rpm. A back-pressure valve was used to guarantee constant pressure
in the autoclave. Liquid samples of 1 ml were taken from the reactor every hour and analyzed.
The quantitative analysis of liquid mixtures was analyzed by a gas chromatograph (GC-2014,
SHIMADZU, Japan) equipped with a HP-5MS column (30 m × 0.25 mm × 0.25 μm) and a
flame ionization detector (FID). The area normalization method was used to quantitatively
analyze the conversion of DCPD and the yield of each product. The turnover frequency (TOF)
of the hydrogenation of DCPD on the catalytic sites (Ni) of the catalysts were calculated by
dividing the number of DCPD converted per hour by the number of exposed Ni atoms on the
surface of the catalysts determined by XPS.
3. Results and discussion
3.1. Structures of NiMoX/γ-Al2O3 catalysts
N2 adsorption-desorption isotherms and the pore size distributions of the oxide
precursors were shown in Fig. 1. From Fig. 1a, all precursors and γ-Al2O3 support exhibited
similar type-IV N2 adsorption-desorption isotherms, which had typical H2 shape hysteresis
loops at P/P0 = 0.6-1.0 [33,34]. This denoted that Mo and Ni oxides did not change the
mesoporous structure of γ-Al2O3 support [35]. A single peak distribution centered at about
7-8 nm was observed in all cases (Fig. 1b). This was in line with the relatively high specific

surface areas and the small mean pore diameters determined for all samples (Table 1).
As shown in Table 1, All oxide precursors showed the same structural characteristics as
γ-Al2O3, such as high surface area (198-236 m2/g), relative large pore volume (0.46-0.64
cm3/g) and mean pore diameter (9.2-10.8 nm). With the increase of the Mo loading, the
surface area and the pore size presented a subtle decrease. This indicated that the pores were
not obviously blocked by the active components and the supported species were well
dispersed on the γ-Al2O3 support [36]. And this result was also proved by XRD patterns.
Fig. 2a illustrated the XRD patterns of catalyst precursors and the support γ-Al2O3.
Three main peaks at 2θ equaled to 37.60°, 45.79° and 66.76° revealed the presence of γ-
Al2O3 phase (JCPDS 29-0063) in all samples. No diffraction peaks derived from NiO, MoO3
or other phases such as NiMoO4 and NiAl2O4 detected in NiO-MoO3 X/γ-Al2O3 (X = 0.02,
0.05, 0.1, 0.2 and 0.5) and NiO/γ-Al2O3 precursors, which meant that the precursors maintain
the base structure of γ-Al2O3 and the oxides of Ni and Mo uniformly dispersed on the surface
of support without obvious agglomeration [35,37-39] and the same conclusion was also
drawn from the characterization of N2 adsorption-desorption isotherms.
Fig. 2b illustrated the XRD patterns of NiMoX/γ-Al2O3 (X = 0.02, 0.05, 0.1, 0.2 and 0.5)
and Ni/γ-Al2O3 catalysts. Besides the broad peak of γ-Al2O3, a relatively narrow diffraction
peak at 51.85° (JCPDS 04-0850) corresponding to the metallic Ni was shown in the XRD
pattern of the Ni/γ-Al2O3 catalysts. It meant that the aggregation of metallic Ni occurred
during the high temperature reduction process which led to the larger Ni particles appeared
on the surface of the γ-Al2O3 support. Comparing with diffraction peak of Ni/γ-Al2O3, the
peak of NiMoX/γ-Al2O3 (X = 0.02, 0.05, 0.1, 0.2 and 0.5) at 51.85° became broader and its
intensity became lower regularly with the increase of Mo loading until no obvious Ni
diffraction peaks appeared in the XRD patterns of NiMo0.2/γ-Al2O3. This led to the
conclusion that the average particle size of Ni was decreasing [26], which indicated that the
aggregation of metallic Ni could be restrained by adding Mo species [40,41] and this effect
gradually strengthened with the rise of Mo loading. However, when the content of Mo was
further increased, the patterns were almost the same in both NiMo0.5/γ-Al2O3 and NiMo0.2/γ-

Al2O3. It illustrated that the inhibitory effect of Mo could be optimal when the ratio of Mo/Ni
reached 0.2.
The TEM images visualized much better that NiMo0.2/γ-Al2O3 (Fig. 3 B1) showed a
better dispersion of metal particles compared with Ni/γ-Al2O3 (Fig. 3 A1). The HRTEM
images of Ni/γ-Al2O3 and NiMo0.2/γ-Al2O3 catalysts were given in Fig. 3 A2 and B2. As can
be seen, the dark black near-spherical shaped Ni particles were located on the surface of γ-
Al2O3 support (gray color, no obvious lattice fringe) and the crystal planes of the (111) of
metallic nickel (about 0.203ꢀnm) were clearly observed. Compared to Ni/γ-Al2O3, it could be
clearly observed from NiMo0.2/γ-Al2O3 that the size of Ni particles significantly decreased.
It meant that Mo species were able to inhibit the sintering process of Ni particles and
improved the dispersity of metallic Ni [42,43]. More HRTEM micrographs of NiMoX/γ-
Al2O3 (X = 0.02, 0.05, 0.1, and 0.5) catalysts were given in the supporting information (Fig.
S1). Seen from these pictures, the size of Ni particles decreased gradually with increasing of
Mo content which was well consistent with the XRD results of the reduced catalysts (Fig.
2b). HAADF-STEM and EDS-elemental mapping tests for the all catalysts were performed
to further investigate the elemental distributions. As can be seen in Fig. S2, all the supported
elements (Ni and Mo) were distributed homogeneously on the γ-Al2O3 which was also
confirmed the conclusion drawn from TEM.
3.2. Reducibility of NiMoX/γ-Al2O3 catalysts
The reducibility of the supported oxide species and their interaction with γ-Al2O3
support were studied by TPR. Fig. 4a illustrated the TPR profiles of MoO3/γ-Al2O3, NiO/γ-
Al2O3 and NiO-MoO3 X/γ-Al2O3 (X = 0.02, 0.05, 0.1, 0.2 and 0.5). The TPR curve of MoO3/γ-
Al2O3 had two hydrogen consumption peaks centered at 475 °C and 878 °C [37,44]. The
former could be assigned to the reduction of Mo6+ → Mo4+, and the latter was attributed to
the further reduction of Mo4+ → Mo0. The TPR profile of NiO/γ-Al2O3 catalyst presented a
distinct H2 consumption peak centered at 524 °C and a broad peak at about 754 °C. As shown

in Fig. 4b, the first, strong, peak at the range of 350-550 °C could be deconvoluted into two
NiO species [26]. The shoulder peak at low temperature was attributed to the reduction of
loosely attached NiO crystals with rather low dispersion. The main peak arose from well
dispersed NiO exhibiting moderate interaction with support surface. The second, minor,
broad band was assigned to the reduction of NiAl2O4, which was strongly bonded to the
support and hard to be reduced [40]. The peak area of 524 °C was several times larger than
that of 754 °C that meant the content of NiO was much more than NiAl2O4. The TPR profiles
of NiO-MoO3 X/γ-Al2O3 (X = 0.02, 0.05, 0.1, 0.2 and 0.5) were more similar to NiO/γ-Al2O3
rather than MoO3/γ-Al2O3 for the reason that the content of Mo species was low. Compared
NiO-MoO3 X/γ-Al2O3 (X = 0.02, 0.05, 0.1, 0.2 and 0.5) with NiO/γ-Al2O3, there were two
interesting phenomena. Firstly, with the increasing of Mo content, the relative content of well
dispersed NiO increased significantly. More details were given in the supporting information
(Fig. S3 & Table S1). Secondly, the centered temperatures of the first H2 consumption peaks
gradually shifted to the low temperature range (508°C → 480°C) (Fig. 4a). An explanation
was proposed for these phenomena that the incorporation of Mo species promoted the
dispersion of NiO and decreased their particle size [41], and hence increased the reducibility
of NiO over these five catalysts. As mentioned earlier, the reduction of Mo-oxo species had
two main H2 consumption peaks at around 475 °C and 878 °C, and the latter peak area was
larger than that of the former peak. So the reduction of NiAl2O4 and the further reduction of
Mo-oxo species from Mo4+ to Mo0 were the two main effects on the second peak (TR2) of
NiO-MoO3 X/γ-Al2O3 (X = 0.02, 0.05, 0.1, 0.2 and 0.5) TPR profiles. Owing to the existence
of Mo species, the TR2 peak area of NiO-MoO3 X/γ-Al2O3 (X = 0.02, 0.05, 0.1, 0.2 and 0.5)
increased with the rise of Mo content. Moreover, an obvious down-shift of TR2 peaks,
compared with 878 °C, occurred due to the catalysis of metallic Ni. The variation content of
NiAl2O4 on the surface of NiO-MoO3 X/γ-Al2O3 (X = 0.02, 0.05, 0.1, 0.2 and 0.5) could not
be obtained from this pattern, and this was going to be investigated by the XPS.
To attain further insight into the atomic ratios and chemical state of surface metals in all
catalysts, XPS measurements were carried out for the NiMoX/γ-Al2O3 (X = 0.02, 0.05, 0.1,

0.2 and 0.5) and Ni/γ-Al2O3 catalysts. The average atomic composition on the surface of the
samples were obtained from the total peak area and relative correction factors of Ni2p3/2 and
Mo3d photoelectrons. As shown in Table 2, the percentage composition of Ni was basically
stable at 2.2-2.4%, and the calculated values of Ni/Mo atomic ratio were in line with the
actual values in catalysts preparation. Due to the low content of Mo, Mo was not detected on
the surface of NiMo0.02/γ-Al2O3 catalyst.
The experimental and fitted Ni2p3/2 spectra peaks of the six Mo-Ni bimetallic catalysts
were shown in Fig. 5. The XPS pattern of Ni2p3/2 could be deconvoluted into three chemical
species (Ni0, NiO and NiAl2O4). Three peaks centered at 853.1 ± 0.1 eV, 855.9 ± 0.1 eV and
856.9 ± 0.1 eV were assigned to Ni0, NiO and NiAl2O4, respectively [36]. Moreover, from
the total peak area of Ni and three kind of Ni species, the molar ratios of Ni0/Ni, NiO/Ni and
NiAl2O4/Ni had been obtained. The above parameters were present in Table 2. It could be
seen that most of Ni element existed in the form of nickel oxide and only a portion of nickel
oxide was reduced to zero-valent state. This corresponded to the pattern of TPR that only a
part of NiO could be reduced to metallic Ni at 500°C. Moreover, the molar ratio of Ni0/Ni
varied from 0.16 (Ni/γ-Al2O3) to 0.28 (NiMo0.2/γ-Al2O3), with the increasing of Mo content.
This demonstrated that the reducibility of NiO was improved by the co-existence of Mo
species. However, NiMo0.5/γ-Al2O3 and NiMo0.2/γ-Al2O3 exhibited almost the same
proportion of Ni species. This conclusion conformed with the result that drawn from XRD
that the optimal additive proportion of Mo was 0.2. Furthermore, it could also be seen that
the molar ratio of NiAl2O4/Ni decreased with increasing of Mo loading. The possible reasons
were that the well dispersed Mo species weakened the nickel species-support interactions,
and thus inhibited the formation of spinel NiAl2O4 during the catalyst preparation. Thence,
a conclusion could be drawn that the presence of Mo improved the dispersion and reduction
of NiO and decreased the formation of NiAl2O4.
Fig. 6 showed the Mo3d XPS patterns of NiMoX/γ-Al2O3 (X = 0.1, 0.2 and 0.5)
catalysts. Three doublets Mo3d5/2-Mo3d3/2 corresponded, respectively, to the three valence
states of Mo. The binding energies at 229.3 ± 0.1 eV and 231.6 ± 0.1 eV were ascribed to the

3d5/2 and 3d3/2 levels of Mo0 [31], while 232.0 ± 0.1 eV and 234.3 ± 0.1 eV associated with
Mo4+ species of the MoO2 phase [46], and finally 232.9 ± 0.1 eV and 235.4 ± 0.1 eV related
to Mo6+ in unreduced MoO
[46,47]. From the Table 2, it should be noted that the catalysis
3
of metallic nickel promoted the reduction of Mo4+ to Mo0 at a relatively low temperature, so
there was a portion of Mo phase on the catalyst surfaces. Along with the Mo content
increasing, the ratio of Mo0/Mo increased. Moreover, the proportion of Mo4+ showed a
significant difference between these three catalysts, that it reached the maximal value in
NiMo0.2/γ-Al2O3 catalyst. This was in line with the result of DCPD-TPD. Finally, it could be
seen that most of Mo element existed in the form of MoO3 phase without being reduced.
Hence, a conclusion could be drawn that the relative contribution of these three valence states
could be adjusted by altering the molar ratio Mo/Ni in catalyst. The relevance between the
DCPD-TPD characterization results and the ratio of Mo+4/Mo allowed us to make a
conjecture about the role of Mo in the hydrogenation process.
3.3. Adsorption-desorption properties of NiMoX/γ-Al2O3 catalysts
A large number of literatures have shown that the unsaturated carbon-carbon double
bond has two adsorption states on the metal catalyst surface [48-50]. One is weakly
adsorption of double bond with π-bond site, the other is strongly adsorption with di-σ-bond
site. Therefore, there were eight possible adsorption states of DCPD on the surface of Ni-Mo
bimetallic catalysts (Scheme 2). Scheme 2 (C-3) adsorption mode was hard to be formed due
to the non-parallel structure of two unsaturated bond. Scheme 2 (A-2) and Scheme 2 (C-2)
were also rare on the surface of catalyst due to the NB-bond strength was weaker than the
CP-bond [14].
To investigate the adsorption-desorption properties of DCPD on samples, DCPD-TPD
measurements were carried out for the NiMoX/γ-Al2O3 (X = 0.02, 0.05, 0.1, 0.2 and 0.5),
Ni/γ-Al2O3, Mo/γ-Al2O3 catalysts and γ-Al2O3 support (Fig. 7). As far as DCPD-TPD results
were concerned, γ-Al2O3 and Mo/γ-Al2O3 both only had one distinct low temperature

desorption peak illustrating that the adsorption strength of DCPD on γ-Al2O3 or Mo/γ-Al2O3
was weak. For γ-Al2O3, the peak derived from the desorption of DCPD on the Lewis acid
sites of γ-Al2O3. For Mo/γ-Al2O3, the peak area was larger than that from γ-Al2O3, indicating
the adsorption of DCPD occurred on Mo species (Mo4+). And this adsorption strength was
stronger than that of γ-Al2O3 due to the up-shift of the peak. However, two peaks (Td1 and
Td2) were observed from Ni/γ-Al2O3 catalyst implying that the DCPD had two adsorption
states on the Ni/γ-Al2O3 catalyst with different strength. The aforementioned statement that
Td1 at relatively low temperature (113 °C) was ascribed to weakly π-bonded DCPD desorbed
from metallic Ni (Scheme 2B), and Td2 at relatively high temperature (309 °C) corresponded
to the desorption of di-σ-bonded DCPD species (Scheme 2A). By comparing Ni/γ-Al2O3
with Mo/γ-Al2O3, it could be concluded that Ni was the active site to form strongly di-σ-
bonded adsorption, which was the main active adsorption of DCPD.
More interestingly, in the profiles of the four Mo-Ni bimetallic catalysts NiMoX/γ-Al2O3
(X = 0.02, 0.05, 0.1 and 0.2), with the increasing of Mo/Ni ratio to 0.2, firstly, the peak area
for both low and high temperature desorption peak area increased simultaneously, indicating
that the addition of Mo element raised the DCPD adsorption capacity of catalysts. And the
improving dispersion of metallic Ni was the main reason for the increase of the high
temperature peak area. Secondly, the second desorption peak of DCPD gradually shifted to
the higher temperature, demonstrating that Mo promoter enhanced the adsorption strength of
DCPD over bimetallic catalyst. The possible reason was that the addition of Mo species
increased the amount of mixed-pin adsorption mode (Scheme 2 (C-1)), in which NB-band
adsorbed on metallic Ni with di-σ-bond form and CP-band adsorbed on Mo species (Mo4+)
with π-bond form. However, things were different when the ratio of Mo/Ni increased to 0.5.
there was no significant increase or shifting in the peak area and temperature of NiMo0.5/γ-
Al2O3, compared with NiMo0.2/γ-Al2O3. This was in line with the result of XPS (Table 2,
column 9) that the value of Mo4+/ Mo reached the summit at NiMo0.2/γ-Al2O3. It further
illustrated that the optimal ratio of Mo/Ni was located at 0.2. As testified by the catalysts
evaluation, the increasing of DCPD adsorption capacity and strength was beneficial to the

hydrogenation reaction of DCPD.
3.4. Catalysts evaluation
Fig. 8a showed the DCPD conversion rate using any one of Ni/γ-Al2O3, NiMoX/γ-Al2O3
(X = 0.02, 0.05, 0.1, 0.2 and 0.5) and Mo/γ-Al2O3 catalysts. The DCPD could achieve
complete conversion within 6 h over all catalysts, except for Mo/γ-Al2O3. It took 4 h to reach
97% conversion of DCPD on Ni/γ-Al2O3 and NiMo0.02/γ-Al2O3 catalysts. With the further
increasing of Mo content, the conversion rate of DCPC was gradually accelerating.
Conversion speed of DCPD on NiMo0.2/γ-Al2O3 was two times faster than Ni/γ-Al2O3 or
NiMo0.02/γ-Al2O3 catalysts. Only in 2 h the conversion of DCPD reached 97% under
NiMo0.2/γ-Al2O3. This clearly indicated that Mo was promoting the hydrogenation of DCPD
on Ni-based catalysts. However, things were different as more Mo added to 0.5. The
conversion rate had a slight decline compared with that over NiMo0.2/γ-Al2O3.
Fig. 8b showed the yield of intermediate 8,9-DHDCPD under different catalysts. Apart
from Mo/γ-Al2O3, the content of 8,9-DHDCPD firstly increased and then decreased, but the
generation and conversion rate of 8,9-DHDCPD were markedly different. For Ni/γ-Al2O3
and NiMo0.02/γ-Al2O3, it took approximate 2 h, the contents of 8,9-DHDCPD reached the
maximum value and then gradually decreased. However, for NiMoX/γ-Al2O3 (X = 0.05, 0.1,
0.2 and 0.5), it took only 1 h to reach the peak. This illustrated the generation and conversion
rate of 8,9-DHDCPD could be enhanced by adding Mo to Ni-based catalyst within limits.
Another interesting phenomenon was that the maximum content of intermediate 8,9-
DHDCPD in the reaction system decreased with the ratio of Mo increasing from 0.05 to 0.2.
The decreased of the maximum intermediate content meant that the reaction rate ratio of the
second hydrogenation step to the first hydrogenation step raised.
Fig. 8c showed the curves of the yield of product endo-THDCPD. Adding Mo
accelerated the producing of endo-THDCPD. When the amount of Mo (Mo/Ni=0.02) was
low, there was no acceleration effect. And when the amounts of Mo (Mo/Ni=0.05, 0.1 and

0.2) were larger, the acceleration effect was obviously improved. Nevertheless, As the
continuation of Mo content increased after loading over 0.2, the evaluation result of
NiMo0.5/γ-Al2O3 deviated from the above conclusion. Under the Mo/γ-Al2O3 catalyst, the
product endo-THDCPD was almost undetected, which further illustrated that Ni was the
main active component of this system.
Turnover frequency (TOF) reflected the intrinsic activity of the active sites (exposed
surface Ni metal) in the catalyst, and the changes of TOF could be observed by adjusting
Mo/Ni value (Fig. 9). The mass content of Ni was fixed at 10wt.% for all catalysts. However,
with the increasing of Mo content, the TOF on Ni-based catalysts increased, and the optimum
molar ratio of Mo/Ni was located at 0.2, with the TOF value of 134.2 h−1, which was twice
as that of Ni/γ-Al2O3 (TOF = 67.2 h−1). When the content of Mo increased to 0.5, the catalytic
activity of NiMo0.5/γ-Al2O3 declined (TOF = 106.8 h−1).
Three possible reasons could be applied to explain why the catalytic performance of
NiMoX/γ-Al2O3 (X = 0.02, 0.05, 0.1 and 0.2) were gradually enhanced. The first was that
both the dispersity and the active surface area of Ni were increased with the addition of Mo
content. Another reason was the increasing reducibility of NiO and the decrease of inactive
nickel aluminate on the surface of the catalyst. Third, and perhaps most significant, was that
both the adsorption capacity and the adsorption strength of DCPD on NiMoX/γ-Al2O3 had
increased (Fig. 7). It was generally known that hydrogenation of unsaturated double bond on
the surface of heterogeneous catalyst needed at least three steps: adsorption, surface reaction
and desorption. DCPD had two unsaturated double bonds (NB-bond and CP-bond). For Ni/γ-
Al2O3 catalyst, most of adsorption DCPD species were in the form of Scheme 2 (A-1) that
only NB-bond adsorbed on the surface of catalyst. There would be a process in which the
8,9-DHDCPD was desorbed from catalyst after the NB-bond was hydrogenated and then the
8,9-DHDCPD was adsorbed to catalyst again and hydrogenated the CP-bond (Scheme 3 A).
This might be the reason why the hydrogenation rate was slow and the concentration of 8,9-
DHDCPD was high for Ni/γ-Al2O3. Nevertheless, with the addition of Mo, the mixed-pin
adsorption mode (Scheme 2 (C-1)) appeared. For this adsorption mode, a surface migration

of intermediate product happened when the NB-bond hydrogenation was finished. The
adsorption site of 8,9-DHDCPD moved from Mo species (π-bond site) to adjacent metallic
Ni (di-σ-bond site). This led to a possible hydrogenation pathway that a portion of the
intermediate product continued to hydrogenate on the catalyst surface without undergoing a
desorption process (Scheme 3B). With the increasing of Mo content from 0.02 to 0.2, the
proportion of mixed-pin adsorption mode on the surface of catalyst was increased
simultaneously, but a small part of mono-pin adsorption mode (Scheme 2 (A-1)) still existed
in the surface of catalysts. This could explain why the hydrogenation rate of NiMoX/γ-Al2O3
(X = 0.02, 0.05, 0.1 and 0.2) were sped up in turn and the concentration of 8,9-DHDCPD
showed a decrease tendency. For NiMo0.5/γ-Al2O3, the decreasing of specific surface area,
pore size and the value of Mo4+/Mo might be the reason for the decrease of catalyst activity.
It was also interesting that, with the increment of Mo content on Ni-based catalysts, the
yield of by-product C5 fraction (cyclopentadiene & cyclopentene & cyclopentane) showed a
significantly decline trend (Fig. 8d). For Ni/γ-Al2O3, the final yield of C5 fraction was about
2.4%. For the NiMo0.2/γ-Al2O3, that figure was only 0.3%. This could be attributed to the
enhanced DCPD adsorption strength on the NiMo0.2/γ-Al2O3 catalyst surface, thus promoting
the DCPD hydrogenation pathway (Scheme 1A). Therefore, one conclusion could be fairly
drawn that the addition of Mo could effectively inhibit the decomposition of DCPD during
the hydrogenation process. And this had a great significance for industrial production.
In summary, there were significant differences among the seven catalysts in the
conversion rate of DCPD, the formation and consumption rate of 8,9-DHDCPD and its
maximum yield, the production rate of endo-THDCPD and the yield of by-product. All these
phenomena pointed to the conclusion that the addition of Mo species could not only increase
the hydrogenation rate of Ni-based catalysts, inhibit the decomposition of DCPD, but also
change the relative hydrogenation rate of CP-bond and NB-bond. However, the change in
content of Ni and nickel aluminate could not affect or change the relative hydrogenation rate
of CP-bond and NB-bond. So it might be that the change of the catalytic mechanism caused
this phenomenon.

4. Conclusion
Aseries of Mo-promoted Ni/γ-Al2O3 catalysts were synthesized by an incipient wetness
co-impregnation method, and the supported species did not influence the structure of the
support. The dispersion of NiO on precursors were promoted and the aggregation of metallic
Ni on catalysts were restrained with increasing of Mo content. More catalytic active centers
(Ni0) formed on the support due to the weakened strength of the nickel species-support
interactions. Furthermore, the addition of Mo changed the adsorption mode of DCPD on
catalyst surface, which had a significant effect on the performance of the catalysts. The
catalytic properties of NiMoX/γ-Al2O3 (X = 0.02, 0.05, 0.1, 0.2 and 0.5) catalysts showed that
in a certain range, the hydrogenation activity increased with the rise of Mo amount, and the
optimal ratio of Mo/Ni was located at 0.2. The NiMo0.2/γ-Al2O3 catalyst displayed the highest
activity (TOF=134.2 h−1) and the best selectivity (99.7%). Compared with Ni/γ-Al2O3
catalyst, the hydrogenation time reduced from 6 h to 3 h and the amount of by-product C5
fraction significantly decreased from 2.4% to 0.3%.
Acknowledgements
We are very grateful to the National Natural Science Foundation of China (NNSFC) for
the support (No.21576025).

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Table Captions
Table 1
Symbols and several characteristics of the precursors (specific pore
volume (Vp), specific surface area (SBET) and mean pore diameter (MPD)).
Table 2
Quantitative surface analysis of NiMoX/γ-Al2O3 (X = 0.02, 0.05, 0.1, 0.2
and 0.5) and Ni/γ-Al2O3.
Scheme Captions
Scheme 1
Scheme 2
Scheme 3
Two reaction pathway during the hydrogenation of DCPD.
Adsorption modes of DCPD on catalyst surface.
Proposed hydrogenation mechanism of DCPD on the catalyst.
Figure Captions
Figure 1
N2 adsorption-desorption isotherms (a) and pore size distributions (b) of
the catalyst precursors NiO-MoO3 X/γ-Al2O3 (X =0.02, 0.05, 0.1, 0.2 and
0.5), NiO/γ-Al2O3 and γ-Al2O3.
Figure 2
XRD patterns of catalyst precursors NiO-MoO3 X/γ-Al2O3 (X = 0.02, 0.05,
0.1, 0.2 and 0.5), NiO/γ-Al2O3 and γ-Al2O3 (a), fresh catalysts NiMoX/γ-
Al2O3 (X = 0.02, 0.05, 0.1, 0.2 and 0.5) and Ni/γ-Al2O3 (b).
Figure 3
Figure 4
HRTEM micrographs of Ni/γ-Al2O3 (A1-A2) and NiMo0.2/γ-Al2O3 (B1-
B2) catalysts.
TPR profiles of catalyst precursors MoO3/γ-Al2O3 NiO/γ-Al2O3 and NiO-
MoO3 X/γ-Al2O3 (X = 0.02, 0.05, 0.1, 0.2 and 0.5) (a), deconvolution of the
reduction curve of the NiO/γ-Al2O3 (b).
Figure 5
Figure 6
Figure 7
Ni2p3/2 XPS patterns of NiMoX/γ-Al2O3 (X = 0.02, 0.05, 0.1, 0.2 and 0.5)
and Ni/γ-Al2O3.
Mo3d XPS patterns of NiMo0.1/γ-Al2O3 and NiMo0.2/γ-Al2O3 and
NiMo0.5/γ-Al2O3.
DCPD-TPD profiles of NiMoX/γ-Al2O3 (X = 0.02, 0.05, 0.1, 0.2 and 0.5),

Ni/γ-Al2O3, Mo/γ-Al2O3 catalysts and γ-Al2O3 support.
Figure 8
Figure 9
Conversion of DCPD (a), yield of 8,9-DHDCPD (b), yield of endo-
THDCPD (c) and yield of C5 fraction (d) varying with reaction time (mass
ratio of catalyst to DCPD to cyclohexane is 1:10:100; temperature 150 °C;
hydrogen pressure 3.5 MPa; stirring rate 600 rpm)
Turnover frequency (TOF, h-1) of various Ni-based catalysts for
hydrogenation of DCPD

Table 1
Symbols and several characteristics of the precursors (specific pore volume (Vp), specific
surface area (SBET) and mean pore diameter (MPD)).
Sample
Vp (cm3/g)
0.64
SBET (m2/g)
236
MPD (nm)
10.8
10.5
10.2
9.8
γ-Al2O3
NiO/γ-Al2O3
0.56
213
NiO-MoO3 0.02/γ-Al2O3
NiO-MoO3 0.05/γ-Al2O3
NiO-MoO3 0.1/γ-Al2O3
NiO-MoO3 0.2/γ-Al2O3
NiO-MoO3 0.5/γ-Al2O3
0.54
213
0.53
218
0.49
201
9.7
0.49
209
9.4
0.46
198
9.2
Table 2
Quantitative surface analysis of NiMoX/γ-Al2O3 (X = 0.02, 0.05, 0.1, 0.2 and 0.5) and Ni/γ-
Al2O3.
SAC a (atomic %)
Sample
Ni/Mo Ni0/Ni
NiO/Ni
NiAl2O4/Ni
Mo0/Mo
Mo4+/Mo
Mo6+/Mo
Ni
Mo
Ni/γ-Al2O3
2.4
2.3
2.2
2.2
2.2
2.2
0.16
0.17
0.21
0.24
0.28
0.27
0.68
0.68
0.66
0.64
0.63
0.64
0.16
0.15
0.13
0.12
0.09
0.09
_
_
_
_
_
_
NiMo0.02/γ-Al2O3
NiMo0.05/γ-Al2O3
NiMo0.1/γ-Al2O3
NiMo0.2/γ-Al2O3
NiMo0.5/γ-Al2O3
_
_
_
_
0.1
0.3
0.5
1.2
0.04
0.14
0.23
0.54
_
_
_
0.15
0.18
0.25
0.18
0.28
0.21
0.67
0.54
0.54
a Surface atomic composition

Scheme 1. Two reaction pathway during the hydrogenation of DCPD.

Scheme 2. Adsorption modes of DCPD on catalyst surface.

Scheme 3. Proposed hydrogenation mechanism of DCPD on the catalyst.

Fig. 1. N2 adsorption-desorption isotherms (a) and pore size distributions (b) of the catalyst
precursors NiO-MoO3 X/γ-Al2O3 (X =0.02, 0.05, 0.1, 0.2 and 0.5), NiO/γ-Al2O3 and γ-Al2O3.

Fig. 2. XRD patterns of catalyst precursors NiO-MoO3 X/γ-Al2O3 (X = 0.02, 0.05, 0.1, 0.2
and 0.5), NiO/γ-Al2O3 and γ-Al2O3 (a), fresh catalysts NiMoX/γ-Al2O3 (X = 0.02, 0.05, 0.1,
0.2 and 0.5) and Ni/γ-Al2O3 (b).