Preparation, characterization and catalytic performance of rod-like Ni-Nb-O catalysts for the oxidative dehydrogenation of ethane at low temperature

Article abstract of DOI:10.1039/c9cy00519f

Ni-Nb mixed oxides are a system of high-efficiency catalysts for the oxidative dehydrogenation of ethane to ethylene (ODHE) in the presence of O₂. In this work, a series of Ni-Nb-O catalysts with various Nb contents (0, 5, 10, 15, 20 and 30 at%) were prepared by a PEG-4000-assisted one-pot hydrothermal method and evaluated as catalysts for the ODHE at low temperature. The Ni-Nb-O catalysts present excellent catalytic activity and selectivity to ethylene with promising stability. The prepared catalysts were characterized by means of XRD, N₂ adsorption-desorption, SEM, TEM, H₂-TPR, O₂-TPR and Raman spectroscopy. XRD and TEM results explicitly manifest that a Ni-Nb solid solution formed, stemming from the Nb doped into the NiO crystal lattice. The reducibility is improved and unselective oxygen species are eliminated, as evidenced by H₂-TPR and O₂-TPD results. A comparison of the catalytic properties between bulk NiO and Ni-Nb-O catalysts was made, and Ni-Nb-O catalysts present improved efficiency for the production of ethylene. The significantly enhanced selectivities of ethylene are obtained over Ni-Nb-O catalysts, in which ethane conversion is also improved slightly. The elevated catalytic performance could be associated with smaller NiO crystal size, regular structure, reduction of electrophilic O^- species and formation of a Ni-Nb solution. The catalytic performance of the catalysts changes with the variation of Nb content, which indicates that the incorporation of niobium into the NiO phase plays a critical role in the adjustment of catalytic performance. The Ni-Nb-O catalysts containing an optimum niobium content of 15 and 20 at% display higher catalytic performance with 47.5% and 47% ethylene yield at 400 °C, respectively. The Ni₈₅Nb₁₅ composite is stable and an ethylene selectivity of ～84% could be sustained after 56 h on-stream operation at 350 °C.

Full text of DOI:10.1039/c9cy00519f

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DOI: 10.1039/C9CY00519F
ARTICLE
Preparation, Characterization and Catalytic Performance of Rod-
like Ni-Nb-O Catalysts for the Oxidative Dehydrogenation of
Ethane at Low Temperature
Received 00th January 20xx,
Accepted 00th January 20xx
Lian Kong, a Dong Li, a Zhen Zhao,*ab Jianmei Li, b Linlin Zhao, a Xiaoqiang Fan, a Xia Xiao a and Zean
DOI: 10.1039/x0xx00000x
Xie b
Ni-Nb mixed oxides are the system of high-efficiency catalysts for the oxidative dehydrogenation of ethane to ethylene
(ODHE) in the presence of O2. In this work, a series of Ni-Nb-O catalysts with various Nb contents (0, 5, 10, 15, 20 and 30
at.%) were prepared by the PEG-4000-assisted one-pot hydrothermal method and evaluated as catalysts for the ODHE at
low temperature. The Ni-Nb-O catalysts present excellent catalytic activity and selectivity to ethylene with promising
stability. The prepared catalysts were characterized by means of XRD, N2 adsorption-desorption, SEM, TEM, H2-TPR, O2-
TPR and Raman spectroscopy. XRD and TEM results explicitly manifest that Ni-Nb solid solution formed, stemming from
the Nb doped into NiO crystal lattice. The reducibility is improved and the unselective oxygen species are eliminated,
evidenced by H2-TPR and O2-TPD results. A comparison in catalytic properties between bulk NiO and Ni-Nb-O catalysts was
made, and Ni-Nb-O catalysts present improved efficiency for the production of ethylene. The significantly enhanced
selectivities of ethylene are obtained over Ni-Nb-O catalysts, in which ethane conversion is also improved slightly. The
elevated catalytic performance could be associated with smaller NiO crystal size, regular structure, reduction of
electrophilic O- species and the formation of Ni-Nb solution. The catalytic performance of the catalysts changes with the
variation of Nb content, which indicates that the incorporation of niobium into NiO phase plays a critical role in the
adjustment of catalytic performance. The Ni-Nb-O catalysts containing optimum niobium content of 15 and 20 at.% display
o
higher catalytic performance with 47.5% and 47% ethylene yield at 400 C, respectively. The Ni85Nb15 composite is stable
and the ethylene selectivity of ~84% could be sustained after 56 h on-stream operating at 350 oC.
Various kinds of catalytic materials have been investigated for
the ODHE reaction.5-11 In reported studies, the polynary mixed
oxide catalysts have attracted much attention worldwide served as
Introduction
Ethylene as an intermediate for producing polyethylene, ethylene
oxide and ethylene dichloride is one of the most important
chemical material in petrochemical industry. Although over 110
million tons of ethylene are produced every year,1,2 it is still not
enough to meet ever-increasing demand. Steam cracking of long-
chain hydrocarbons (T > 800 oC) is used to manufacture ethylene in
the industry,3 which is an energy intensive process. In recent year,
the oxidative dehydrogenation of ethane to ethylene (ODHE) with
O2 molecular as oxidant has attracted much attention due to the
lower energy consumption and reaction temperature compared
with the conventional steam cracking. However, the ethylene
derived from the ODHE is easily oxidized to carbon oxides (CO and
CO2) due to the presence of oxygen (O2). Thus, a key factor for the
industrial implementation of ODHE is not only to improve the C2H6
conversion but also to enhance the C2H4 selectivity.4
an effective catalytic system for the ODHE reaction at low
temperature,12-16 which will be the most possibly in place of steam
cracking of hydrocarbons. These catalysts could be divided into two
categories based on the elemental components of catalysts: Mo-V-
Nb oxides and Ni-based mixed oxides. So far, MoVTeNbO mixed
metal oxide is one of the most preferable catalysts for the ODHE
reaction (~75% ethylene yield at 400 oC).17 However, the
complicated preparation of MoVTeNbO catalyst limits its practical
application.
Recent studies have shown that bulk NiO exhibits high activity
and low selectivity in the ODHE reaction at low temperature. In
order to promote the selectivity of ethylene, various transition
metals (e.g. Nb, Ta, W and Ti, etc.) are incorporated into the lattice
of nickel oxide to modify the physicochemical property of bulk NiO,
especially electronic changes via forming the Ni-metal solid
solution.16,18-20 As expected, the significantly higher ethylene yield
could be achieved over those catalysts than bulk NiO. Therefore,
the doping of a second metal into NiO could effectively improve
catalytic behavior. The literature believe that the increase of
selectivity to ethylene is attributed to the reduction of
nonstoichiometric oxygen species (O-) in bulk NiO, which catalyzes
a. Institute of Catalysis for Energy and Environment, College of Chemistry and
Chemical Engineering, Shenyang Normal University, Shenyang 110034, China.
E-mail: zhaozhen163@163.com; zhenzhao@cup.edu.cn
b. State Key Laboratory of Heavy Oil Processing, China University of Petroleum, 18#
Fuxue Road, Chang Ping, Beijing 102249, China.
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the ethane to COx by adding the heteroatom. Nonetheless, Nb5+ stirring at room temperature until PEG-4000 completely dissolved.
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cation is still considered to be the most effective promoter among Afterwards, 5.0 g Ni(NO)3·6H2O was added iDnOtoI: t1h0e.10a3b9o/Cve9CmY0ix0t5u1r9eF.
these metals and is extensively investigated. Lemonidou and Finally, 10 mL NH3·H2O (25%) was added dropwise to the Ni-Nb
Heracleous systematically studied the effect of different transition mixture. After stirring 6 h at room temperature, the slurry was
metals (Li, Mg, Al, Ga, Ti, Ta and Nb) on the catalytic performance transferred into a Teflon autoclave and aged at 100 oC for 24 h. The
for the ODHE.21 The results showed that the catalytic performances obtained powder was centrifuged (7000 r min-1, 10 min), washed
were closely related to the nature of the dopant in Ni-based mixed with deionized for several times and dried at 80 oC for another 24 h
oxides. The higher valence state of Nb was prone to bind oxygen under static conditions. All as-prepared samples were calcined in a
o
species, resulting in the higher ethylene selectivity. They inferred muffle oven at 450 C in air for 4 h. These catalysts are denoted as
that the amount of unselective electrophilic oxygen species NixNby, where x and y represent the atomic percentage of Ni and
depending on the nature of the dopant could increase or reduce Nb, respectively.
catalytic activity.
For comparison, pure NiO and the Ni-Nb-O catalyst (Ni:Nb atomic
It is generally believed that the catalytic performance may ratio = 85:15, denoted as Ni85Nb15-nt) were also prepared by the
change along with the preparation method of the catalysts owing to same process except for the addition of Nb precursor and PEG-
the alteration of physicochemical properties, such as specific area, 4000, respectively.
morphology, structures of active phases and sites, etc. Research
Catalysts characterization
results also supported the idea that the catalytic activity of Nb-Ni-O
X-ray diffraction patterns (XRD) of all the catalysts were analyzed
catalysts was intrinsically dependent on the preparation method.
using a powder X-ray diffractometer (Bruker D8 Advance and
RIGAKU U IV) with Cu Kα radiation (λ = 0.15406 nm, 40 kV and 40
For instance, Ni-Nb-O catalysts, reported by Heraclous et al.,16 were
synthesized via the evaporation method from nickel nitrate and
mA). And the NiO average crystal size was calculated derived from
ammonium niobium oxalate. The highest ethylene yield of 46% over
(200) diffraction peak via the Scherrer formula (D = Kλ/Bcosθ, K-
Scherrer constant, θ-Bragg angle and B-half peak width). 23
Ni85Nb15Ox catalyst was achieved. Subsequently, Zhu and co-
workers synthesized the Nb-doped NiO catalysts adopting the sol-
The Brunauer-Emmet-Teller (BET) surface areas were obtained
gel method and different experimental results were gained at W/F
o
using Micromeritics TriStar II 3020 porosimetry analyzer at -196 C.
Before the test, all samples were pretreated at 300 oC for 3 h.
The morphologies of all the samples were investigated on
of 0.6 g s cm-3 (the ethane conversion and ethylene selectivity were
o
19% and 76% over Nb0.15-Ni0.85O catalyst at 350 C, respectively),22
attributed to the formation of amorphous Nb-rich oxide layers.
SU8010 field emission scanning electron microscope (SEM). The
Besides, a green and effective method of solid-state grinding was
catalysts were treated by spray-gold under an Ar atmosphere in
developed for preparing the Nb-doped NiO catalyst.14 The crystallite
order to make a better research about the structure. High-
size is smaller than those prepared by the evaporation method at
resolution transmission electron microscopy (HRTEM) images were
high Nb contents. Unfortunately, the ethylene productivity over the
taken using a JEM-2100 electron microscope operating at 200 kV,
optimized Ni-Nb-O catalyst was inferior to that of Nb-doped NiO
coupled with the selected area electron diffraction (SAED) and
catalyst synthesized from the evaporation method. Moreover,
energy dispersive X-ray spectroscopy (EDX). Samples for the TEM
combustion synthesis is also an alternative method for synthesizing
observations were dispersed in absolute alcohol and dropped on
Ni-Nb composites, but a high chemical energy would be needed for
micro grids. And the ethanol was evaporated at 50 oC.
the spontaneous combustion of a mixture. In brief, the preparation
Raman spectroscopy measurements were carried out by a
method plays a crucial role in the catalytic performance of Ni-based
Renishaw inVia Reflex Raman spectrometer with 532 nm laser
mixed oxide catalyst.
excitations. The laser power was operated at 10 mW and 3
Encouraged by the aforementioned methods, the aim of this
acquisitions were recorded for each catalyst.
work is to prepare the Nb-doped NiO catalysts by the hydrothermal
H2 temperature-programmed reduction (H2-TPR) measurements
method for modulating the physicochemical properties. Not only
were performed on a Quantachrome Autosorb-iQ instrument (USA)
the transformation of material morphology occurs with the addition
with a thermal conductivity detector (TCD). The operation was
of Nb, but also the high-efficiency catalytic performance over Ni-
described as follows: 0.1 g catalyst placed in a U-quartz tube reactor
Nb-O catalysts are obtained for the ODHE at low temperature
.
were pretreated with Ar flow at 450 oC for 30 min, followed by
cooling to room temperature and purging in a H2/Ar gas mixture for
60 min. Then the temperature was raised from room temperature
to 800 oC with a heating rate of 10 oC min-1 in a 10 % H2/Ar flow (30
mL min-1).
Experimental
Catalysts preparation
Ni(NO)3·6H2O and polyethylene glycol 4000 (HO(CH2CH2O)nH, Mw
=
The temperature-programmed oxygen desorption (O2-TPD)
experiments were carried out in a Micromeritics AutoChem II 2920
Chemisorption Analyzer equipped with a TCD detector. Prior to the
measurements, 0.2 g catalyst was pretreated with O2 at 450 oC for 1
h. Cooling to room temperature and purging in He for 1 h (50 mL
min-1), and then the oxygen was desorbed from room temperature
to 850 oC at a heating rate of 15 oC min-1.
3600-4400, PEG-4000) (Sinopharm Chemical Reagent Co., Ltd),
ammonia solution (NH3·H2O, 25%, Tianjin Guangfu Chemical
Research Institute), niobium (V) oxalated hydrate (C10H5Nb20·xH2O,
Alfa Aesar) were used without further purification.
A series of Ni-Nb mixed oxides were synthesized by the PEG-
4000-assisted one-pot hydrothermal method. Briefly, appropriate
amount of niobium (V) oxalated hydrate was dissolved into 100 mL
deionized water. And 8.0 g PGE-4000 was added under vigorous
Catalytic activity testing
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The catalytic reaction was carried out in a fix-bed glass reactor “NiO-like” phase, indicating that niobium oxide with VvieewryArttiinclye Osnizlienes
(internal diameter: 6 mm; external diameter: 8 mm). The typical is well dispersed on the surface of NiO DoOrI: n10io.1b0i3u9m/C9uCnYi0fo0r5m19lFy
test conditions were as followed: 0.3 g catalyst with 40-60 mesh disperses into NiO lattice. However, the crystallinity of pure NiO
was loaded in the center of glass reactor with silica-sand to gradually weakens with the increase of Nb content. Moreover, an
o
maintain stable volume of the catalytic bed. The gas compositions extremely weak and broad peak is detected at 2θ = 27 at high Nb
were 10 vol.% C2H6 and 10 vol.% O2 in N2 (the total flow rate: 30 mL content (> 15 at.%) indicating the formation of amorphous niobium
min-1) and the flow rates were controlled with mass flow phase. But no crystalline Nb2O5 diffraction peak is observed
controllers. The ODHE reaction was performed in the reaction manifesting a well dispersion of Nb in the lattice of NiO. The lattice
o
temperature range of 250-400 C at ambient pressure. Prior to the constant and particle size of NiO phase were calculated by the
evaluation of catalytic performance, all catalysts were pre-heated in Bragg’s formula and the Scherrer equation, respectively. The
O2 and N2 stream for 30 min. The products were analyzed using an physico-chemical parameters of all the samples are presented in
on-line gas chromatography (SP3420, Beijing, China) with two FID Table 1. The lattice constant of NiO decreases gradually with
detectors and two columns (Porapak Q and TDX-01). Ethane and increasing the Nb content up to 20 at.%. Since the ionic radius of
ethylene were detected using a Porapak Q column and CO and CO2 Ni2+ (0.069 nm) is greater than that of Nb5+ (0.064 nm), the lattice
were analyzed on a TDX-01 column, respectively. All gases (C2H6, constant decreases by the incorporation of Nb5+ ions into NiO
Air, N2, H2 and O2) (Dalian Date Standard Gas Company, China) were lattice for the substitution of some Ni2+ cations.24 These results
used without further purification.
along with similar ones from reference 16 make clear to us that Ni-
Under our reaction conditions, C2H6 and CO2 were primary Nb solid solution formed in the Ni-Nb-O samples prepared via
products and a trace amount of CO was also detected. Ethane hydrothermal method. In addition, the lattice constant increases
conversion and selectivity of ethylene were estimated on the with further increasing Nb content to 30 at.%, which may indicate
carbon balance. In order to obtain the correlation of conversion and the quantity of niobium into NiO lattice achieves a maximum.
o
selectivity at constant temperature (350 C), W/F ratio was varied Moreover, the particle sizes of NiO in Ni-Nb-O catalysts are
by changing the mass of catalysts, where W was the mass of significantly less than pure NiO sample. The minimal crystallite size
catalyst and F was the total flow. The stability of catalyst (Ni85Nb15 (13.8 nm) is gained in Ni85Nb15 sample. The decrease of crystallite
as the representative) was also evaluated at constant reaction size is attributed to the incorporation of Nb5+ into the lattice of NiO
conditions (0.3 g catalyst, C2H6:O2:N2 = 1:1:8, F = 30 mL min-1, T = for perturbing crystallization of NiO. Smaller size of NiO particles
o
350 C). Moreover, the “blank experiment” was checked under the increases the quantity of accessible surface active sites, thereby
reaction conditions, where the ethane conversion was less than enhances the catalytic activity.
0.5%.
Results
Characterization of NiO and Ni-Nb-O catalysts
Fig. 1. XRD patterns of Ni-Nb mixed oxide catalysts with different
Nb contents prepared by hydrothermal method
The XRD patterns of NiO and Ni-Nb-O samples with different Nb
contents are shown in Fig. 1. For bulk NiO sample, the characteristic
diffraction peaks at 2θ = 37.1o, 43.2o, 62.7o, 74.9o and 79.0o could
be assigned to the (111), (200), (220), (311) and (222)
crystallographic planes of the cubic structure of NiO, respectively.
For Ni-Nb mixed oxide catalysts, no characteristic diffraction peaks
of niobium oxide are observed except for the diffraction lines of
Fig. 2 Scanning electron micrograph images of NiO and Ni-Nb-O
catalysts: (A) NiO; (B) Ni95Nb5; (C) Ni90Nb10; (D) Ni85Nb15; (E)
Ni80Nb20 and (F) Ni70Nb30
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NiO sample, indicating the introduction of Nb into NiO lattice to
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DOI: 10.1039/C9CY00519F
Table 1 The physico-chemical parameters of NiO and Ni-Nb-O form the Ni-Nb composite. The corresponding SAED pattern is
catalysts
shown in Fig. 3F. The diffraction patterns match pretty well with the
NiO which is shown in Fig. 3C, manifesting that the NiO crystal
structure is also maintained after the addition of niobium.
Meanwhile, the result also shows that no crystalline Nb2O5 is
observed, in agreement with the XRD results. All these results
demonstrate that the NiO structure is not influenced by the
morphology of catalyst. Fig. 3G presents the Ni and Nb content in
Ni85Nb15 mixed oxide from the EDX analysis result. The obtained
result verifies that the catalyst is constitutive of Ni, Nb and O
elements. However, from the inset figure in Fig. 3G, the Nb/Ni atom
percentage is 13.6%, which is less than the nominal value of 17.6%.
It is possible that a part of Nb species with the weak interaction
between niobium and NiO were washed away in the synthesis
process.
Nb/(Ni + Nb)
atomic ratio
(nominal)
Nb/(Ni + Nb)
atomic ratio
(by ICP)
NiO average
crystal size
(nm)
NiO lattice
constant
(Å)
SBET
(m2 g-1)
Catalyst
NiO
0
0
45
101
115
79
18.3
15.6
14.3
13.8
15.5
16.8
4.1786
4.1786
4.1752
4.1750
4.1718
4.1724
Ni95Nb5
Ni90Nb10
Ni85Nb15
Ni80Nb20
Ni70Nb30
0.05
0.1
0.05
0.08
0.13
0.21
0.25
0.15
0.2
56
0.3
41
SEM is an important means for the observation and study of the
morphology of NiO and Ni-Nb-O catalysts. The SEM images of NiO
and Ni-Nb mixed oxide catalysts are presented in Fig. 2A-F. As
shown in Fig. 2A, bulk NiO is composed of nano-sheets with
different sizes. The surface morphology changes obviously due to
the addition of 5 at.% Nb (Fig. 2B). A large number of nano-flakes
have self-assembled the flower-like Ni95Nb5 catalyst. The ultrathin
“nano-petals” are constructed of numerous nanoparticles (inserted
figure in Fig. 2B). One possibility is that the complexation between
Ni2+ and NH3·H2O is destroyed partly by the incorporation of Nb5+
cations, and the self-assembly process of Ni(OH)2 nanosheets for
the oriented attachment occurs due to the existence of PEG-4000.
Further increasing Nb content, the morphology changes from the
flower-like architecture to rod-like structure, shown in Fig. 2C-2F.
The inserted figures in Fig. 2E and 2F present that the rod-like Ni-Nb
mixed oxides are also formed with the accumulation of abundant
nanoparticles. A possible reason about the change of morphology
is that the complexation between Ni2+ and NH3·H2O is completely
destroyed and the disordered granular Ni(OH)2 crystal nucleus form
owing to the addition of abundant Nb5+ ions. And the Ni(OH)2
particles spontaneously aggregate together into the nano-rods
under the reaction conditions.
Fig. 3 TEM images, HRTEM images and SAED pattern of NiO sheet
(A, B and C) and Ni85Nb15 mixed oxide (D, E and F) and the EDX
elemental analysis of Ni85Nb15 mixed oxide (G)
The microstructure of NiO and Ni85Nb15 mixed oxide were further
investigated by transmission electron microscopy (TEM). The TEM
images, HRTEM images, SAED results and EDX elemental analysis of
NiO sheet and Ni85Nb15 mixed oxide are shown in Fig. 3. The TEM
image clearly sheds light on that bulk NiO is composed of nano-
sheet with many porosities as presented in Fig. 3A. And the HRTEM
image of NiO sample exhibited in Fig. 3B indicates that the
interlayer distance of lattice fringe is 0.210 nm, corresponding to
the (200) crystal plane of NiO. Moreover, SAED pattern was further
investigated, as displayed in Fig. 3C. The typical polycrystalline rings
could be assigned to the (111), (200), (220) and (311) crystal plane
of cubic NiO, respectively,25 comfirming the formation of a well
crystalline characteristic of NiO structure. The TEM image of
Ni85Nb15 mixed oxide in Fig. 3D confirms that the rod-like product
consists of a large number of nano-particles with various sizes. The
observed results are consistent with SEM images. The HRTEM
image in Fig. 3E was analyzed in order to have a profound
understanding for the microstructure of Ni85Nb15 mixed oxide. The
estimated fringe spacing of 0.238 nm is greater than the value of
Fig. 4 N2 adsorption-desorption isotherms of NiO and Ni-Nb-O
catalysts
Fig. 4 displays the N2 adsorption-desorption isotherms of NiO and
Ni-Nb-O catalysts. All measured samples present the III-type
isotherms with H3 hysteresis loops, which can be attributed to the
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formation of intergranular pore accumulated with nano-particles. pronounced reduction peak around 334 C and a shoulder peak is
View Article Online
DOI: 10.1039/C9CY00519F
The BET specific surface areas of NiO and Ni-Nb-O catalysts depend observed at 256 C, which have been assigned to the Ni2+ cation to
on Nb content, as shown in Table 1. NiO exhibits the specific surface metallic nickel transformation and the reduction of Ni3+→Ni2+ cation,
area of 45 m2 g-1. After the introduction of Nb species into NiO, an respectively.27,30 The appearance of shoulder peak indicates that
increase in specific surface area is observed (Table 1). The specific pure NiO synthesized by hydrothermal method is non-
surface area of Ni95Nb5 and Ni90Nb10 composite are 101 m2 g-1 and stoichiometric material.22 The shoulder peak of 256 oC gradually
115 m2 g-1, respectively. The further increasing of Nb content, i.e., disappears after adding niobium into NiO. The phenomenon may
when Nb content is greater than 15 at.%, the specific surface area indicate that Ni3+ cations react with niobium species and change
of Ni-Nb-O catalyst presents an unexpected decrease from 79 m2 g-1 into Ni2+ sites, which enables the reduction of Ni3+ species. The
o
to 41 m2 g-1. Furthermore, the Table
1
includes the ICP reduction peak at 334 oC becomes wider and shifts to the lower
characterization results for all the Ni-Nb mixed oxide catalysts. It is temperature attributed to the incorporation of Nb species into NiO
observed that the Nb contents are slightly lower than the nominal lattice, in line with the reported results of Nb-doped NiO catalysts.16
value, which is in agreement with EDX analysis.
These results suggest the presence of more reducible species in Ni-
The Raman spectra of NiO and Ni-Nb mixed oxide catalysts are Nb composites and the reducibility of Ni-Nb-O catalysts closely
depicted in Fig. 5. The pure NiO catalyst mainly presents three depends on the Nb content. With increasing Nb content from 5 to
bands at around 500, 691 and 1065 cm-1. The first band can be 20 at.%, the corresponding reduction temperature shifts ranging
assigned to Ni-O stretching mode,16 and the second two bands are from 330 to 300 oC. The main peaks are ascribed to the reduction of
related to the v1 vibration mode of carbonate groups or NiO Ni2+ to Ni0 in Ni-Nb solid solution. However, as the further increase
phase.26,27 In addition, a weak shoulder near 382 cm-1 is also of Nb content, two principal reduction peaks around 335 oC and 450
observed, which could be ascribed to the transverse vibration mode oC are observed in Ni70Nb30 composite, likely indicating the
involving the rhombohedra deformation or to the stretching presence of two reducible species. The first one should be
vibrations of non-stoichiometric Ni-O.24,28 A new Raman peak is attributed to the reduction of Ni2+ species and the appearance of
detected centered at 790 cm-1 after the introduction of niobium second peak is correlation with the formation of amorphous
into NiO system and, as expected, the peak intensity increases with niobium phase, which is consistent with the XRD result. The
the increase of niobium content. The band is attributed to the reduction temperature of crystalline NiNb2O6 phase in literature is
vibration modes of bridging Ni-O-Nb bonds,27 in accordance with nearly 480 oC and is significantly higher than the value in our
XRD data (shown in Fig. 1), possibly indicating the interaction experiment.31 Moreover, the characteristic diffraction peaks of
between Ni and Nb ions and the formation of Ni-Nb solid solution. crystalline NiNb2O6 phase are not observed form XRD patterns.
Moreover, the blue shift of Ni-O vibration for the Ni-Nb mixed oxide Thus, the crystalline NiNb2O6 phase might not exit in our samples.
takes place compared with the bulk NiO, from 500 to 522 cm-1. The The lower reduction temperature of Nb-doped NiO catalysts
shift of this band further confirms that there is a strong interaction (except for Ni70Nb30 composite) may indicate that the addition of
between Ni2+ and Nb5+ cations. Furthermore, a new band near 250 proper content of Nb is beneficial to the enhancement of
cm-1 is detected for the Ni-Nb-O catalysts with Ni/Nb atomic ratio reducibility, thereby improving the catalytic activity of Ni-Nb-O
above 9. This Raman band is attributed to the bending vibration of catalysts for the ODHE.
the Nb-O-Nb linkage,29 demonstrating the existence of Nb2O5 nano-
particles. But unfortunately, Nb2O5 nano-particles in the Ni-Nb
mixed oxide catalysts are not detected by XRD analysis, which could
indicate that the crystalline size is too small to generate a
diffraction pattern.27
Fig. 6 Hydrogen temperature-programmed reduction profiles of
NiO and the Ni-Nb mixed oxide catalysts
The O2-TPD profiles of NiO and Ni85Nb15 mixed oxide are
Fig. 5 Ambient Raman spectra of NiO and Ni-Nb-O catalysts presented in Fig. 7. As we known, pure NiO is non-stoichiometric
synthesized by the hydrothermal method
oxide with a certain amount of cationic deficiency.32 But the oxygen
excess is suppressed by doping higher valence cation into NiO
The reducibilities of NiO and Ni-Nb-O catalysts were measured by lattice according to the principle of controlled valence.33
temperature-programmed reduction in the H2 stream (H2-TPR) and desorption peak centered at around 688 oC is observed for NiO
the results are shown in Fig. 6. The profile of pure NiO shows a catalyst, which is attributed to the strong oxygen release of non-
A
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stoichiometric oxygen (O- species), in accordance with O2-TPD composite synthesized by hydrothermal method arVeiewcoAmrticpleaOranblinlee
results of pure NiO, as reported previously.34 The desorption peak with the those of Ni-Nb mixed oxides prepared by evaporation
DOI: 10.1039/C9CY00519F
shifts to lower temperature (644 oC) and the peak intensity method,16 but they are greater than those over Ni-Nb composite
22
weakens after the introduction of niobium, indicating the reduction catalysts via solid grinding,14 citrate method
or one-pot
of non-stoichiometry oxygen species.
combustion method.35 However, the different trend is observed
that a high ethane conversion (67%) is also obtained even when Nb
contents are greater than 15 at.%, which are much higher than
those reported in the literature. 14, 16, 22
It could be noted that the selectivity to ethylene reaches a
o
maximum at around 300 C, then decreases with further increasing
the reaction temperature due to the over-oxidation process of
ethylene at higher temperature (Fig. 8B). A positive effect of
selectivity to ethylene is observed by the addition of Nb into NiO.
The selectivities to ethylene over Ni-Nb mixed oxide catalysts are
higher than that of bulk NiO catalyst. However, unlike the
monotonous improvement of ethane conversion noticed with an
increase of Nb content, the selectivity of ethylene is limited by the
Nb content. And the selectivities of catalysts are in the order of
Ni85Nb15 ˃ Ni80Nb20 ˃ Ni90Nb10 ˃ Ni70Nb30 ˃ Ni95Nb5 ˃ NiO. The Nb
Fig. 7 Temperature-programmed oxygen desorption (O2-TPD)
profiles of NiO and Ni85Nb15 mixed oxide catalysts
Catalytic performance of Ni-Nb mixed oxide catalysts in content of 15 at.% is definitely better for the promotion of
ODHE
selectivity to ethylene. It is illustrated that the introduction of
moderate niobium species could restrain the deep oxidation of
ethane. Moreover, the ethylene selectivities over Ni85Nb15 mixed
oxide catalyst are approximately two times higher than those over
NiO catalyst within the full operating temperature range. Indeed,
the moderate Nb content has
a positive influence on the
improvement of C2H4 selectivity. The selectivity to ethylene
increases firstly and then decreases as the increase of Nb content.
Therefore, the higher ethylene selectivity could be achieved when
the optimal Nb content is 15 at.% or 20 at.%.
Table 2 The catalytic performances for ODHE over NiO and Ni-Nb
mixed oxide catalysts. Reaction conditions: C2H6:O2 = 1:1 and W/F =
0.6 g s mL-1
Fig. 8 Ethane conversion (A) and ethylene selectivity (B) as a
function of reaction temperature over NiO and Ni-Nb-O catalysts.
Reaction condition: C2H6:O2 = 1:1 and W/F = 0.6 g s mL-1
Selectivity (%)
Sample
NiO
T (oC)
Conv.C2H6 (%)
YC2H4 (%)
The effect of reaction temperature on the catalytic performance of
as-prepared NiO and Ni-Nb-O catalysts were carried out in the
temperature range of 250-400 oC under steady state conditions
(C2H6:O2:N2 = 1:1:8 and Ftotal = 30 mL min-1). The only products are
ethylene and COx. The curves of ethane conversion and ethylene
selectivity versus reaction temperature over NiO and Ni-Nb-O
catalysts are presented in Fig. 8 A and B, respectively. Clearly, the
ethane conversion and selectivity to ethylene depend strongly on
the reaction temperature and the doping metal content. As shown
C2H4
CO
CO2
300
350
400
300
350
400
300
350
400
300
350
400
300
350
400
300
350
400
7.8
43.5
39.1
36.9
55.7
55.3
52.3
70.4
66.7
59.9
87.9
84.0
71.7
85.5
81.6
70.0
66.6
60.5
53.8
3.3
0.05
0.03
0.7
0.1
0.06
0.4
0.1
0.03
0.9
0.2
0
52.4
60.8
63.1
43.6
44.6
47.7
29.2
33.2
40.0
11.2
15.8
28.3
13.9
18.3
30.0
33.1
39.5
46.2
3.4
26.9
54.7
8.6
10.5
20.2
4.8
Ni95Nb5
Ni90Nb10
Ni85Nb15
Ni80Nb20
Ni70Nb30
29.8
60.4
9.9
16.5
31.6
7.0
31.4
62.8
11.9
35.4
66.2
14.7
38.0
67.2
16.5
44.2
67.5
20.9
37.6
10.5
29.7
47.5
12.6
31.0
47.0
11.0
26.7
36.3
o
in Fig. 8A, the ODHE reaction could be triggered below 250 C. The
ethane conversion gradually increases with temperature rising, but
o
those are very low when the reaction temperature is below 300 C
for all the catalysts. It increases dramatically with further increasing
the reaction temperature out of the range. The higher catalytic
activity is associated with the high reaction temperature which is
favorable for the C-H bond cleavage of ethane molecular. The
highest ethane conversion is about 67% over Ni80Nb20 and Ni70Nb30
catalyst at 400 oC (as shown in Table 2). Moreover, the ethane
conversion over Ni85Nb15Ox catalyst is 66% at the same temperature,
slightly less than those over Ni80Nb20 and Ni70Nb30 mixed oxide
catalysts. And the values of ethane conversion over Ni85Nb15
0.7
0.1
0
0.3
0.04
0.03
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The relationship of ethylene yield and space time yield (in 10-2
catalyst shows an initial C2H4 selectivity of 86.3% and displays a
slight drop from 86.3% to 82.0% with extending the contact time.
View Article Online
DOI: 10.1039/C9CY00519F
h -1
-1
gC H
g
) against the Nb/(Ni + Nb) atomic ratio at 350 and 400
cat
2
4
oC are presented in Fig. 9. Obviously, the volcano type curves are
obtained for both of them with the increasing of Nb/(Ni + Nb)
atomic ratio. The yield of ethylene firstly increases from 10.5% to
Whereas the selectivity of COx slightly increases with increasing the
contact time, which may indicate that COx produces by the direct
oxidation of C2H6. In addition, the selectivity to ethylene over Ni-
Nb-O catalyst synthesized by hydrothermal method is quite stable
in the range of 0.2-0.8 g s mL-1, which is superior to those of
published Ni-Nb-O catalysts for ODHE by solid grinding or
o
o
31.0% and 20.2% to 47.5% at 350 C and 400 C with increasing Nb
content to 20 at.%, respectively. And then they decline with a
further increase of Nb content up to 30 at.%. Moreover, the C2H4
yield also increases with rising the reaction temperature, which is
due to the fact that the high reaction is beneficial to the
transformation of ethane under the experimental conditions. In
evaporation.14,22 Meanwhile,
a drop of C2H4 selectivity with
increasing of C2H6 conversion is observed, in accordance with the
results in literature.37 However, a result is presented in Fig. 11 that
the selectivity to ethylene is not equal to 100% when ethane
conversion is zero. The trend indicates that COx is mainly originated
from ethane and ethylene direct oxidation (so-called consecutive
and parallel reaction).
g
g -1
addition, the maximum ethylene space time yield of 0.36
h -1
C2H4 cat
o
is obtained over Ni85Nb15 mixed oxide catalyst at 400 C. The
value is higher than those over Ni-Nb-O catalysts reported in
literature under the same GHSV (6000 cm3 g-1 h-1).22,36 The results
could indicate more reactive lattice oxygen existed in rod-like Ni-
Nb-O catalysts in comparison with particulate catalysts.
Fig. 11 Ethylene selectivity as a function of ethane conversion over
o
Ni85Nb15 mixed oxide catalyst in the range of 250-400 C. Reaction
condition: C2H6:O2 = 1:1 and W/F = 0.6 g s mL-1
STY
Fig. 9 The dependence of ethylene yield and
in 10-2
gC H
C2H4
2
4
h -1
-1
g
cat
on the Nb loading in the ODHE at different temperature.
Reaction condition: C2H6:O2 = 1:1 and W/F = 0.6 g s mL-1
Fig. 12 Time-on-stream catalytic performance of Ni85Nb15 catalyst
for ODHE. Reactive conditions: C2H6:O2 = 1:1 and W/F = 0.6 g s mL-1,
reaction temperature of 350 oC
Fig. 10 Effect of contact time (W/F, g s mL-1) on the catalytic
performance of the Ni85Nb15 mixed oxide catalyst for ODHE.
Reactive conditions: m = 0.1-0.4 g, C2H6:O2 = 1:1, total flow = 30 mL
min-1, T = 350 oC
For the industrial application, the stability of catalyst is of greater
importance. Based on the above experimental results, the
Ni85Nb15Ox as a representative catalyst due to its superior catalytic
performance was studied for a continuous evaluation of catalytic
performance at 350 oC for 56 h. Ethane conversion, selectivity to
ethylene and yield of ethylene versus time-on-stream are plotted in
Fig. 12. It is clearly observed that the ethane conversion of
Ni85Nb15Ox catalyst shows a decrease from 35.4% to 30.4%. In
addition, the selectivity to ethylene is maintained until 56 h and
The effect of contact time (W/F) on catalytic performance over
o
Ni85Nb15Ox catalyst at 350 C was investigated in order to provide a
better understanding of the reaction pathway for the formation of
ethylene. The corresponding results are shown in Fig. 10. The
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decreases slightly from ~84.0% to ~83.0%. Therefore, the yield of catalysts. The previously published Ni-Nb-O catalyst VbieywtAhrteiclseoOln-glineel
22
ethylene also shows a decreased trend (from 29.8% to 25.3%) with method presented high catalytic performanDceO.I: 10H.1o0w3e9/vCe9r,CtYh0e05s1o9lF-
the time-on-stream. These results indicate that Ni-Nb-O catalysts gel procedure was relatively complicated due to involving several
prepared by hydrothermal method have high stability, and will steps. In the present study, rod-like Ni-Nb-O catalysts prepared by
possibly be potential materials for the industrial application. hydrothermal method are very active for ethane ODH and the
Moreover, the good stability of Ni-Nb-O catalysts suggests that the preparation method is very simple, which could show extensive
structures of active phases are invariable under given reaction prospects for the practical application. All the Ni-Nb-O catalysts
conditions.
present higher catalytic performance than bare NiO catalyst. Such
positive effect of the mixed oxides could be attributed to their
preferable redox properties, the formation of Ni-Nb solid solution
and small crystal size. The phenomenon is consistent with the
previous results for Ni-Nb-O catalysts prepared by the other
methods, such as evaporation and combustion synthesis.16,35 The
highest ethylene yield of ~47.5% (66.2% ethane conversion with
71.7% ethylene selectivity) over rod-like Ni85Nb15Ox catalysts is
Effect of PEG-4000 on the catalytic performances of Ni85Nb15
mixed oxide
The catalytic activities over Ni85Nb15 mixed oxides with and
without the addition of PEG-4000 from 250-400 oC are
depicted in Fig. 13. It could be noted that the addition of PEG-
4000 changes the catalytic performance of Ni85Nb15 mixed
oxides. In comparison with Ni85Nb15 mixed oxide, the Ni85Nb15-
nt oxide shows both the lower ethane conversion and
selectivity to ethylene. The suppressive activity of Ni85Nb15-nt
oxide may be attributed to the high concentration of
amorphous NiNb2O6 and low specific surface area (26 m2 g-1).
o
gained at 400 C. Such an ethylene yield is even higher than those
over Ni-Nb-O catalysts synthesized by solid-state grinding14 and
one-pot combustion synthesis.35
As mentioned above, the single factor could not explain the
difference of catalytic performance for the Ni-Nb-O catalysts.
Combined the results of characterization and activity evaluation, it
is found that catalytic performance are dependent on their
structure, reducibility and composition. The results of XRD, HRTEM
and SAD patterns manifest that Ni-Nb solid solution formed by
incorporating Nb species into NiO lattice. The formation of Ni-Nb
solution seems to be a key component for improving the catalytic
performance of Ni-Nb composite catalysts. The SEM and TEM
results indicate that the introduction of Nb element changes the
morphology of catalysts, and the TEM images also indicate that NiO
structure is retained and Ni-Nb solid solution forms. Moreover, it is
sure that surface area definitely affects the catalytic performance of
catalysts. However, the catalytic performance is not proportional to
the surface area in this work. In fact, the rod-like Ni85Nb15Ox and
Ni80Nb20Ox catalysts with lower surface area achieve high catalytic
properties. These results show that there are other factors, besides
the surface area, to affect the catalytic performance of the studied
catalysts. To be best of our knowledge, the lower NiO particle size
(displayed in Table 1) is considered a possible factor for the
improvement of catalytic performance. In addition, it is believed
that the catalytic performances are associated with the reducibility
of Ni-Nb-O catalysts demonstrated by the H2-TPR results. The
improved activity with decreasing reduction temperature (334 oC →
300 oC) for Ni-Nb-O catalysts could be well understood which
combined with the results of H2-TPR and activity evaluation. With
gradually increasing Nb content, in parallel with the configurational
evolution (from flake to rod structure), the corresponding electronic
structure of these catalysts also varies, evidenced by Raman spectra
and H2-TPR patterns. It is observed that the well reducibility of Ni-
Nb mixed oxide seems to be beneficial for the elimination of β-
hydrogen,39 therefore, improving the catalytic activity.
Fig. 13 Ethane conversion and selectivity of ethylene as a function
of reaction temperature over Ni85Nb15 mixed oxides with or
without using PEG-4000. Reaction conditions: C2H6:O2 = 1:1 and
W/F = 0.6 g s mL-1
The XRD patterns (measured using a RIGAKU U IV) of
Ni85Nb15 and Ni85Nb15-nt mixed oxides displayed in Fig. S1 in
the Supporting Information show the characteristic peaks of
NiO phase. However, the sharper peaks with strong intensity
imply a better crystallinity of NiO in Ni85Nb15-nt composite and
a broad peak at 2θ = 28o assigned to the amorphous NiNb2O6
phase is also detected. These results demonstrate that PEG-
4000 plays an important role in the structure of Ni-Nb-O
catalyst.
Discussion
In summary, the catalytic performance of NiO-based catalysts could
be enhanced by the incorporation of Nb species into NiO lattice for
the ODHE reaction studied in the literature,16,38 owing to the
reduction of electrophilic oxygen species and the formation of Ni-
Nb solid solution. And the high ethylene yield of 46% was achieved
It is generally considered that the catalytic properties over oxide
catalysts for ODHE are certainly related to the type of active oxygen
species. The results of O2-TPD indicate that the relative content of
non-stoichiometric oxygen species, which is responsible for the
deep oxidation of C2H6 to CO2, decreases due to the incorporation
of niobium into NiO lattice to form Ni-Nb solid solution.
Consequently, the selectivity to ethylene is improved. Moreover,
o
over Ni-Nb-O catalyst at 400 C.16 In addition, the catalytic activity
also depends on the preparation method and morphology of
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1
2
3
4
T. Ren, M. Patel and K. Blok, Energy, 2006, 31, 425V-4ie5w1A.rticle Online
the XRD, Raman and TEM do not determine the existence of inert
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0.2), the high selectivity of ethylene is still obtained, probably due
to the good homogeneity. Conclusively, the high ethylene
selectivity seems to depending on the decrease of electrophilic O-
species caused by the doping of Nb into NiO lattice.
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Conclusions
In this work, the Nb-doped NiO catalysts were synthesized,
characterized and evaluated for the ODHE at low temperature.
From the foregoing results, some conclusions can be drawn as
follows:
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(1) The Nb-Ni-O catalysts were synthesized by the one-pot
hydrothermal method using PEG-4000 as a template. This method is
easy-operating and low-cost.
(2) The morphology (flaky nano-material → rod-like material) of
mixed oxide is essentially affected via changing the Nb/Ni atom
ratio. Compared with bulk NiO catalyst, the rod-like Ni-Nb-O
catalysts have smaller crystal size of NiO and lower reduction
temperature. Those are favorable for improving the catalytic
performance. The results of XRD and TEM explicitly unveil the
formation of Ni-Nb solid solution by virtue of the incorporation of
niobium into NiO lattice. Moreover, the reduction of non-
stoichiometric oxygen species after doping Nb species into NiO
lattice is also in favor of the improvement of ethylene selectivity.
(3) The experimental results demonstrate that rod-like Ni-Nb-O
catalysts synthesized in a facile method are also very efficient for
the ODHE at low temperature. The promising catalytic results of
47.5% ethylene yield at 66.2% ethane conversion with 71.7%
ethylene selectivity were achieved at 400 oC with W/F = 0.6 g s mL-1.
The high ethane conversion (67%) over Ni-Nb mixed oxides with
high Nb content (> 15 at.%) is also obtained, which is greater than
those obtained over the catalysts prepared by evaporation method
and solid grinding. In addition, the Ni85Nb15 mixed oxide prepared
by one-pot hydrothermal method is very stable on stream for 56 h
at 350 oC .
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Conflicts of interest
There are no conflicts of interest to declare.
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Acknowledgements
The authors are grateful for financial support from Natural Science
Foundation of China (91845201; 91545117; 21761162016), the
Engineering Technology Research Center of Catalysis for Energy and
Environment, Major Platform for Science and Technology of the
Universities in Liaoning Province, Liaoning Province Key Laboratory
for Highly Efficient Conversion and Clean Utilization of Oil and Gas
Resources, the Engineering Research Center for Highly Efficient
Conversion and Clean Use of Oil and Gas Resources of Liaoning
Province
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Notes and references
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