Anti-sintering ZrO2-modified Ni/α-Al2O3 catalyst for CO methanation

Article abstract of DOI:10.1039/c5ra28057e

To obviously reduce the sintering behavior of Ni particles in Ni/α-Al₂O₃ catalysts during CO methanation reaction, ZrO₂ was deposited onto the Ni/α-Al₂O₃ catalyst surface by a modified impregnation method. It was observed that the ZrO₂-modified catalyst with an appropriate amount of ZrO₂ showed enhanced catalytic activity due to the increased H₂ uptake and Ni dispersion. During the 103h-lifetime test, the ZrO₂-modified catalyst exhibited higher stability and anti-sintering performance than the unmodified one, because the partial coverage of ZrO₂ particles could effectively prevent Ni particles from sintering during the reaction at high temperatures. The findings obtained in this study should be conducive to the design and development of supported metal catalysts for high temperature reactions with enhanced stability.

Full text of DOI:10.1039/c5ra28057e

View Article Online
View Journal
RSC Advances
This article can be cited before page numbers have been issued, to do this please use: Q. Liu, F. Gu, Z.
Zhong, G. Xu and F. Su, RSC Adv., 2016, DOI: 10.1039/C5RA28057E.
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
Accepted Manuscripts are published online shortly after
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. This Accepted Manuscript will be replaced by the edited,
formatted and paginated article as soon as this is available.
You can find more information about Accepted Manuscripts in the
Information for Authors.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the Ethical guidelines still
apply. In no event shall the Royal Society of Chemistry be held
responsible for any errors or omissions in this Accepted Manuscript
or any consequences arising from the use of any information it
contains.
www.rsc.org/advances

Page 1 of 32
RSC Advances
View Article Online
DOI: 10.1039/C5RA28057E
Anti-sintering ZrO2-modified Ni/α-Al2O3 catalyst for CO methanation
Qing Liua, Fangna Gua,*, Ziyi Zhongb, Guangwen Xua, Fabing Sua,*
a State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese
Academy of Sciences, Beijing 100190, China
b Institute of Chemical Engineering and Sciences, A*star, 1 Pesek Road, Jurong Island 627833,
Singapore
*Corresponding author. Tel.: +86ꢀ10ꢀ82544850; Fax: +86ꢀ10ꢀ82544851. Eꢀmail address:
fngu@ipe.ac.cn (F. Gu); fbsu@ipe.ac.cn (F. Su).
1

RSC Advances
Page 2 of 32
View Article Online
DOI: 10.1039/C5RA28057E
Abstract
To obviously reduce sintering behavior of Ni particles in Ni/αꢀAl2O3 catalysts during CO
methanation reaction, ZrO2 was deposited onto the Ni/αꢀAl2O3 catalyst surface by a modified
impregnation method. It was observed that the ZrO2ꢀmodified catalyst with proper amount of ZrO2
showed enhanced catalytic activity due to the increased H2 uptake and Ni dispersion. During the
103hꢀlifetime test, the ZrO2ꢀmodified catalyst exhibited higher stability and antiꢀsintering
performance than the unmodified one, because the partial coverage of ZrO2 particles could
effectively prevent Ni particles from sintering during the reaction at high temperatures. The findings
obtained in this study should be conducive to the design and development of supported metal
catalysts for high temperature reactions with enhanced the stability.
2

Page 3 of 32
RSC Advances
View Article Online
DOI: 10.1039/C5RA28057E
1. Introduction
In the last several decades, CO methanation reaction from syngas (CO + 3H2 → CH4 + H2O) has
attracted intensive attention from both academia and industry.1,2 In this reaction, the syngas can be
obtained via gasification reaction of coal or biomass, and Ni/γꢀAl2O3 catalysts are often employed
due to their relatively high activity and low cost.3,4 However, these catalysts often suffer from Ni
sintering during the methanation process because of its strongly exothermic nature5,6 as well as the
presence of water vapor, which is one of the byproducts of methanation and can accelerate the Ni
particle sintering.7,8 In addition, the surface acidity of γꢀAl2O3 support easily causes coking
formation on the catalyst Ni/γꢀAl2O3 catalysts, and the phase transformation of γꢀAl2O3 during the
high temperature reaction process can lead to the fast deactivation due to collapse of the pore
structure and burying of the Ni particles. Therefore, considering the above drawbacks of γꢀAl2O3
support, some researchers paid their attentions to more stable, acidꢀfree and inert αꢀAl2O3
support.9,10 Although, partial success has been achieved, the Ni sintering of Ni/αꢀAl2O3 catalysts
seems to be more serious due to the low surface area of αꢀAl2O3 support and the weak interaction
between NiO species and the support.9,10 Hence, the antiꢀsintering property of Ni/αꢀAl2O3 catalyst
should be further improved.
Various approaches have been employed to stabilize metal particles, e.g., by adding inorganic
oxide via chemical vapor deposition, atomic layer deposition, encapsulating with dendrimer,
grafting, or formation of the wellꢀdefined cavities or channels, such as core–shell and ordered
mesoporous structures.11ꢀ14 However, there are still some drawbacks for these methods, such as the
poor control on the shell thickness, the need of special equipment as well as the associated high cost,
limiting their wide and largeꢀscale application. ZrO2 is an inorganic oxide having high thermal
stability as well as chemical inertness, and having been reported as the efficient promoter15ꢀ17 or
3

RSC Advances
Page 4 of 32
View Article Online
DOI: 10.1039/C5RA28057E
support18,19 in the catalysts for methanation reaction. However, the sinteringꢀresistance of these
catalysts is usually far from ideal except for the ones with ordered mesoporous structure.17,18
Therefore, improving the antiꢀsintering property of the catalysts prepared by the conventional
methods is in demand.
Recently, several reports showed that the construction of an oxideꢀonꢀmetal surface structure can
enhance both the catalytic activity and stability of metal catalysts dramatically, because they not
only combine the function of the metal nanoparticles, but also bring unique collective and
synergetic catalytic properties compared to the uncovered metal catalysts.20ꢀ22 Following the
previous work on the catalysts with specific oxideꢀonꢀmetal structures in our group for CO
methanation,14,15 the ZrO2ꢀmodified Ni/αꢀAl2O3 catalysts were prepared by a modified impregnation
method in this work. To the best of our knowledge, there is no report in the literature for such route
for CO methanation reaction. In addition, in order to obtain a deep insight into the interrelationship
among the structure, activity and stability, we investigated the catalytic performances of the
catalysts and characterized their structures. It is found that the ZrO2 nanoparticles can distribute
over the surface of the catalyst and even partially cover some Ni particles, which can restrain the
growth of the Ni particles and result in enhanced stability in the lifetime test.
2. Experimental
2.1. Catalyst preparation
Nickel (II) nitrate hexahydrate (Ni(NO3)2·6H2O) and ethanol were analytical grade (Sinopharm
Chemical Reagent Co. Ltd., China). Zirconium nꢀbutoxide (Zr(OBu)4) (purity 80 wt% in nꢀbutanol)
was used without further treatment (SigmaꢀAldrich). The αꢀAl2O3 was obtained by calcination of
the commercial γꢀAl2O3 (purity >95% GongYiHuaYu Alumina Co. Ltd., China, 300 m2 g–1) at
4

Page 5 of 32
RSC Advances
View Article Online
DOI: 10.1039/C5RA28057E
1200 °C in air for 5 h.
2.1.1. Preparation of NiO/αꢀAl2O3 catalyst
The NiO/αꢀAl2O3 catalyst was prepared by the wet impregnation method according to our
previous report.14 Typically, 0.39 g Ni(NO3)2·6H2O was dissolved in 20 mL deionized water,
followed with addition of 1.00 g of the αꢀAl2O3 powder, then the slurry was kept under vigorous
o
stirring at room temperature overnight. After evaporation of the liquid at 80 C, the sample was
o
calcined at 400 C for 2 h in air. The obtained NiO/αꢀAl2O3 catalyst was denoted as 10NA with a
NiO loading of 10 wt%.
2.1.2. Preparation of ZrO2ꢀmodified NiO/αꢀAl2O3 catalyst
ZrO2ꢀmodified NiO/αꢀAl2O3 catalyst was prepared by the modified impregnation method after
the above procedure. Typically, for each cycle, 1.00 g 10NA was immersed in a Zr(OBu)4 ethanol
solution (5 mL 0.05 M) for 1 min in a sand core funnel to allow adsorption of Zr(OBu)4 onto the
10NA. After filtrated and washed twice with anhydrous ethanol, 5 mL 0.1 M ethanol aqueous
solution was added sequentially within 1 min to initiate the reaction of the preꢀadsorbed Zr(OBu)4
with H2O to form the desired ZrO2 precursor. The substrates were then dried at 100 oC in air. Such a
modification cycle (Scheme 1) could be repeated for several times to achieve the desired amount of
o
ZrO2. The obtained samples were finally calcined at 400 C for 2 h and denoted as 10NA@ZrO2ꢀx
(x = 5, 10 and 15), where x represents the cycle times. In addition, the bulk ZrO2 was obtained
through the calcination of the hydrolyzed Zr(OBu)4 under the same calcination condition as 10NA.
5

RSC Advances
Page 6 of 32
View Article Online
DOI: 10.1039/C5RA28057E
Scheme 1. Illustration of the one impregnation cycle of the ZrO2ꢀmodified Ni/Al2O3 catalyst.
2.2. Catalysts characterization
o
N2 adsorption was measured at −196 C (Quantachrome surface area & pore size analyzer NOVA
3200e) and degassed at 200 ºC for 4 h under vacuum. The specific surface area was determined
according to the BrunauerꢀEmmettꢀTeller (BET) method. PANalytical X'Pert PRO MPD was used
to record the Xꢀray diffraction (XRD) patterns with the Cu Kα radiation (λ = 1.5418 Å) at 40 kV
and 40 mA. Quantachrome chemBET pulsar TPR/TPD was used to record both H2
temperatureꢀprogrammed reduction (H2ꢀTPR) and H2 temperatureꢀprogrammed desorption
(H2ꢀTPD) profiles. Similar with our previous report,13 0.05 g sample was used in the H2ꢀTPR
o
o
measurement. After being heated from room temperature to 200 C at 10 C min–1 and maintained
for 1 h in a He flow, the sample was cooled to room temperature and followed by heating to 1000
o
oC at 10 C min–1 in the 10 vol % H2/Ar flow (30 mL min–1). In the H2ꢀTPD measurement, 0.2 g
o
catalyst was used and reduced in situ in the H2/Ar flow at 650 C for 2 h, then the sample was
cooled to room temperature and saturated with H2 for 1 h. After being flushing with Ar for 1 h, the
o
sample was heated to 600 C at 10 oC min–1 in the Ar flow (30 mL min–1). The signal was detected
using a thermal conductivity detector (TCD). The dispersion of Ni was calculated using the method
described in our previous work.13 The morphology of the samples was observed by scanning
electron microscopy (SEM) equipped with energyꢀdispersive Xꢀray spectroscopy (EDS)
(JSMꢀ6700F, JEOL, Japan) and transmission electron microscopy (TEM) (JEMꢀ2010F, JEOL,
6

Page 7 of 32
RSC Advances
View Article Online
DOI: 10.1039/C5RA28057E
Japan) under a working voltage of 200 kV. The surface chemical composition of the reduced
catalysts was measured using Xꢀray photoelectron spectroscopy (XPS) test conducted on a VG
ESCALAB 250 spectrometer (Thermo Electron, U.K.) with a nonꢀmonochromatized Al Kα XꢀRay
source (1486 eV). In addition, the exact composition of the catalysts was determined by using a
Thermo Scientific iCAP 6300 inductively coupled plasma atomic emission spectrometry (ICPꢀAES).
Moreover, thermogravimetric (TG) analysis was conducted on a Seiko Instruments EXSTAR
TG/DTA 6300 in the temperature range of room temperature to 1000 oC (10 oC min–1) in air.
2.3. Catalytic measurement
o
The catalysts (20–40 mesh) were reduced at 650 C in pure H2 (100 mL min–1) for 2 h and then
cooled to the starting reaction temperature in H2 in a fixedꢀbed quartz tubular reactor (10 mm I.D.)
at 0.1 MPa. Then the mixed H2 and CO as well as N2 (as an internal standard) (H2/CO/N2 = 3/1/1)
were introduced into the reactor at a weight hourly space velocity (WHSV) of 30000 mL g‒1 h‒1.
The inlet and outlet gases were analyzed on line by Micro GC 3000A (Agilent Technologies) with a
TCD detector. The lifetime test of CO methanation was performed at 500 oC, 0.1 MPa, 60000 mL g‒
1
h‒1. Hydrothermal treatment for catalysts aging was also carried out in a fixed bed quartz tube
reactor under 0.1 MPa.17 Prior to the treatment, the catalyst (10NA and 10NA@ZrO2ꢀ5) was
reduced at 650 oC in pure H2 for 1 h and then subjected to 90 vol% H2O/H2 at 650 oC for 8 h. After
being cooled to room temperature in pure H2, the obtained samples were denoted as 10NAꢀHT and
10NA@ZrO2ꢀ5ꢀHT, respectively. The CO conversion, CH4 selectivity and yield are defined as
follows:13
FCO,in − FCO,out
XCO (%) =
×100
CO conversion:
FCO,in
7

RSC Advances
Page 8 of 32
View Article Online
DOI: 10.1039/C5RA28057E
F
CH4 ,out
SCH (%) =
×100
CH4 selectivity:
4
FCO,in − FCO,out
XCO × S
F
CH4 ,out
CH4
Y CH (%) =
=
×100
CH4 yield:
4
100
FCO,in
Here, X is the conversion of CO, S is the selectivity of CH4, Y is the yield of CH4, Fi, in and Fi, out
are the volume flow rates of species i (i=CO or CH4) at the inlet and outlet.
3. Results and discussion
3.1. Characterization of the catalysts
All the calcined samples show the similar N2 adsorptionꢀdesorption isotherms with Type IV
isotherms and H4 hysteresis loops (Fig. 1A), suggesting the textural structure still remains after the
o
incorporation of ZrO2 into 10NA. After reduction at 650 C for 1 h, there is no obvious change in
their isotherms (Fig. 1B), and the hysteresis loops of the samples locate at P/P0 = 0.75–1.0,
indicating the pores are from the aggradation of nanoparticles.9 Table 1 lists the surface areas of the
calcined and reduced samples. Overall, the reduced samples have the lower surface areas than the
calcined ones, and the surface areas of 10NA@ZrO2ꢀx samples are slightly increased with the
increase of the ZrO2 amount, because the small ZrO2 nanoparticles can make the surface of the
catalyst much coarser. In addition, the exact compositions of the above catalysts are determined by
ICPꢀAES and listed in Table 1. The ZrO2 loading varies from 3.10 to 12.05 wt%, and the real Ni
loading of 10NA@ZrO2 is slightly lower than 10 wt% due to the addition of ZrO2 to 10NA.
Fig. 2 shows the morphology and XRD patterns of the 650oCꢀreduced catalysts. In 10NA, the Ni
nanoparticles are highly dispersed over the support with the average Ni size of 10.3 nm (Fig. 2a),
which is consistent with the result of XRD in Table 1. ZrO2 nanocrystallites can be easily identified
in the TEM images due to the obvious contrast difference between αꢀAl2O3 and ZrO2. It is seen that
8

Page 9 of 32
RSC Advances
View Article Online
DOI: 10.1039/C5RA28057E
some ZrO2 particles are distributed on the surface of the catalysts (Fig. 2b‒c), and even cover the
dominating surface at higher ZrO2 loadings (Fig. 2d). The coverage of ZrO2 species are expected to
improve the interaction between NiO and the support, and restrain the growth of the Ni particles
during the reduction and catalytic reaction at high temperatures. Actually, the average Ni particle
sizes are about 7.2, 7.5 and 7.0 nm in 10NA@ZrO2ꢀ5 (Fig. 2b), 10NA@ZrO2ꢀ10 (Fig. 2c) and
10NA@ZrO2ꢀ15 (Fig. 2d), respectively, which is smaller in size compared to that of 10NA. Fig. 2e
shows the HRTEM image of the reduced 10NA@ZrO2ꢀ15, which reveals more information on the
distribution of ZrO2 particles. The observed lattice spacing of ca. 0.20 nm corresponds to the Ni
(111) plane,13 and the ones at ca. 0.26 and 0.30 nm to the αꢀAl2O3 (104) plane and the tetragonal
ZrO2 (tꢀZrO2) (011) plane, respectively. Clearly, the ZrO2 species covers part of the catalyst surface,
which should include some Ni particles, while the rest Ni particles are distributed among the ZrO2
particles. Furthermore, the results of the elemental mappings and EDS of the reduced
10NA@ZrO2ꢀ15 confirm that both Zr and Ni elements are well dispersed across the whole Al2O3
support (Fig. 3). In Fig. 2f, the observed peaks at 25.6, 35.2, 43.4 and 57.9o correspond to (012),
(104), (113), and (116) planes of αꢀAl2O3 (JCPDS 01ꢀ082ꢀ1468), while those at 44.5, 51.8 and 76.6o
to (111), (200) and (220) planes of metallic Ni (JCPDS 01ꢀ070ꢀ1849). After the ZrO2 addition, new
diffraction peaks at 30.3, 50.4 and 60.2o are observed in the XRD patterns, which correspond to
(011), (112) and (121) planes of tꢀZrO2 (JCPDS 00ꢀ050ꢀ1089), while the ones at 28.2 and 31.5o
attributing to (‒111) and (111) planes of monoclinic ZrO2 (mꢀZrO2). Clearly, the mixed crystal
phases of ZrO2 exist in the reduced catalysts. Furthermore, the Ni particle sizes are calculated using
the DebyeꢀScherrer equation and listed in Table 1. Overall, the Ni particle sizes of the
ZrO2ꢀmodified catalysts are smaller than that of 10NA, and the Ni particle size calculated through
XRD is consistent with that estimated from TEM, which confirms that the steric hindrance of ZrO2
9

RSC Advances
Page 10 of 32
View Article Online
DOI: 10.1039/C5RA28057E
can restrict the sintering of the Ni particles during the reduction process at high temperature.
Fig. 4A presents the H2ꢀTPR profiles of the catalysts as well as the bulk ZrO2. It should be
pointed out that there is no any reduction peak for ZrO2, and hence all the hydrogen consuming
peaks of the catalysts are attributed to the NiO reduction. There are three reduction peaks at about
305, 362 and 524 oC in the profile of 10NA, which corresponds to NiO with weak (305 and 362 oC)
o
or middle (524 C) interactions with the support.3 The reduction peaks at low temperature are
o
disappeared after the ZrO2 addition, and shifted to high temperature range (475 and 616 C),
because the NiO species is covered partially by ZrO2 particles, which significantly increases the
difficulty of the reduction of NiO species.3
Fig. 4B shows the H2ꢀTPD profiles of all the catalysts and the bulk ZrO2. There is a negligible H2
adsorption peak for ZrO2, which is quite similar to the results in the literature,15 and thus all the
hydrogen desorption peaks in the H2ꢀTPD profiles of the catalysts can be contributed to the Ni
species. The H2ꢀTPD profile of 10NA shows two main H2 desorption peaks at around 153 and 296
oC. After the ZrO2 addition, there is an obvious increment in the integrated area of the
o
o
lowꢀtemperature peak (153 C) as compared with that of 10NA; Meanwhile, the peak at 296 C is
disappeared, indicating the presence of more highlyꢀdispersed Ni atoms.3 However, the intensity of
the H2 desorption peak decreases gradually with the increase of the ZrO2 addition due to the
coverage of some exposed Ni particles by excess ZrO2. The hydrogen uptakes and Ni dispersion of
the catalysts are listed in Table 1. The modification of ZrO2 is beneficial for enhancing the hydrogen
uptakes and Ni dispersion. Among these catalysts, 10NA@ZrO2ꢀ5 has the highest total H2 uptake of
44.0 ꢁmol g−1 and the Ni dispersion of 6.5%. In addition, we notice that the Ni dispersion of
10NA@ZrO2ꢀ5 is almost 2 times higher than that of 10NA, which is contradictory to the fact that
their Ni particle size difference is considerable small (1.5 nm). This may be understood from the
10

Page 11 of 32
RSC Advances
View Article Online
DOI: 10.1039/C5RA28057E
results of H2ꢀTPR, in detail, there is a stronger interaction between Ni species and support in
10NA@ZrO2ꢀ5 than that in 10NA (Fig. 4A). Thus the sintering degree of Ni particle in the former
should be lower during the 650oCꢀreduction process, resulting in the formation of more exposed
surface active Ni atoms with high density of surface defects,13 and much improved Ni dispersion.
However, for 10NA@ZrO2ꢀ10 and 10NA@ZrO2ꢀ15, the excess ZrO2 has the negative effect on H2
uptake because of the severe coverage of Ni species. In all, only the proper amount of ZrO2 can lead
to the improved dispersion of Ni particle in this work.
The XPS spectra of the reduced 10NA and 10NA@ZrO2ꢀ5 catalysts are shown in Fig. 5. The
reduced 10NA catalyst shows the metallic Ni (Ni0) peak at around 852.3 eV, the peaks at 855.8 and
861.5 eV are assigned to the Ni oxide states (Ni2+) because catalysts were oxidized in air during the
sample transfer and preparation (Fig. 5a).16 After addition of ZrO2, there is no obvious change of the
Ni0 binding energy, indicating ZrO2 species is a structure promoter rather than the electron promoter
in the 10NA@ZrO2ꢀx catalysts for CO methanation. The similar Ni0/(Ni0+Ni2+) of the two catalysts
indicates the similar reducibility of Ni spices in the different catalysts. The O 1s spectra are
displayed in Fig. 5b. There is only one peak in the O 1s spectra of 10NA, while two kinds of
oxygen species can be observed in 10NA@ZrO2ꢀ5. The low binding energy peak was assigned to
the surface lattice oxygen (OI), and the other is the chemisorbed oxygen (OII)18 which is the typical
characteristic of ZrO2. The adsorbed oxygen of 10NA@ZrO2ꢀ5 can supply more oxygen vacancies
which are favorable for eliminating deposited carbon and improving the stability of the catalyst.
3.2. Catalytic performances of the catalysts
o
The CO methanation reaction was carried out in the temperature range of 260−440 C at 0.1 MPa,
30000 mL g−1 h−1, and the results are shown in Fig. 6a–c. For 10NA, the CO conversion increases
11

RSC Advances
Page 12 of 32
View Article Online
DOI: 10.1039/C5RA28057E
o
with the increase of the reaction temperature and reaches the maximum of 87% at 440 C; at the
same time, the CH4 selectivity drops with the rise of the temperature due to the side reactions such
as waterꢀgas shift and inversed methane CO2 reforming reaction.23 It can be seen that the addition of
ZrO2 considerably improves the catalytic activities, and 10NA@ZrO2ꢀ5 shows the best catalytic
o
performance, whose CO conversion and CH4 yield can reach 94% and 74% respectively at 400 C.
However, excess ZrO2 is adverse for the catalytic performance, and the activities of 10NA@ZrO2ꢀx
(x = 10 and 15) decrease with the further increase of the ZrO2 loading. The order of the activity of
the different catalysts is consistent with that of the H2 uptake, which is similar to the literature
results.3,24 It should be pointed out that the order of CH4 selectivity of the different catalysts is
opposite with their order of CO conversion, indicating the side reactions can also be significantly
enhanced in the presence of the more active catalysts. In all, only the proper amount of ZrO2 can
remarkably enhance the CO methanation, and the excess ZrO2 has the adverse effect on it due to the
coverage of the active sites.
In order to evaluate the lifetime of the catalyst in a short time in laboratory, a 103hꢀlifetime test
of 10NA@ZrO2ꢀ5 was carried out at high temperature and WHSV (500 °C, 60000 mL g–1 h–1, 0.1
MPa), and 10NA was also evaluated as a comparison (Fig. 6d‒f). During the test, the CO
conversion and CH4 yield over 10NA are decreased by 17.8% and 13.1%, respectively, although the
CH4 selectivity (Fig. 6e) is still comparable. On the contrary, the CO conversion and CH4 yield over
10NA@ZrO2ꢀ5 show the decrement of 6.7% and 5.5%, respectively. Clearly, 10NA@ZrO2ꢀ5
exhibits a better stability especially at high temperature and WHSV. In Fig. 7, it can be seen that in
the spent 10NA (Fig. 7A) and 10NA@ZrO2ꢀ5 (Fig. 7B), the Ni nanoparticle size is 12.2−25.1 and
7.0−12.5 nm, respectively. Compared with the aforementioned TEM images of the reduced catalysts
(Fig. 2), the antiꢀsintering of Ni particle on 10NA@ZrO2ꢀ5 is significant. At the same time, the
12

Page 13 of 32
RSC Advances
View Article Online
DOI: 10.1039/C5RA28057E
average Ni particle size of the spent 10NA calculated from XRD pattern is 17.2 nm, while that of
the spent 10NA@ZrO2ꢀ5 is 9.5 nm, only slightly larger than that of the reduced catalysts (Table 1),
further confirming the antiꢀsintering property of 10NA@ZrO2ꢀ5. We believe that the antiꢀsintering
performance can be further improved through optimization of the amount of ZrO2. Moreover, the
amount of carbon deposited on the spent catalysts can be measured by TG analysis, and the result is
presented in Fig. 7D. The TG curves of the samples exhibit a rise in the temperature range of 200 to
o
430 C, which results from the oxidation of the metallic Ni.25 Furthermore, the decrement of mass
loss percentage of the spent 10NA catalyst is larger than that of 10NA@ZrO2ꢀ5, indicating a higher
resistance of 10NA@ZrO2ꢀ5 to coke formation. The TEM image of the spent 10NA catalyst further
confirms the existence of the filamentous carbon. However, the amount of the deposited carbon of
the two catalysts is close, suggesting that coke may be not the main factor of deactivation. In short,
the sintering of Ni particles leads to the decrease of the activity of 10NA in the lifetime test, and
10NA@ZrO2ꢀ5 exhibits high catalytic activity and stability as well as much improved resistance to
Ni sintering.
3.3. Hydrothermal stability of the catalysts
Recently, researchers have reported that H2O26 and OH anions (NaOH in water)27 could
significantly enhanced the activity and selectivity of Ni based catalysts for CꢀO bond
hydrogenolysis of lignin model compounds and organosolv lignin. Considering water is one of the
byproducts in CO methanation and additional steam in the feed gas is an often used strategy to
avoid the generation of hot spots in the catalyst bed and to reduce carbon deposition in
industry,9,17,28 the hydrothermal stability of 10NA@ZrO2ꢀ5 was examined, and that of 10NA was
also carried out as the reference. Fig. 8 shows the catalytic properties of 10NA@ZrO2ꢀ5ꢀHT and
13

RSC Advances
Page 14 of 32
View Article Online
DOI: 10.1039/C5RA28057E
10NAꢀHT after the hydrothermal treatment. Compared with the fresh catalysts in Fig. 6,
10NA@ZrO2ꢀ5ꢀHT shows comparable catalytic activity and selectivity; while the activity of
10NAꢀHT decreases drastically, suggesting that the introduction of proper amount of ZrO2 is useful
to stabilize Ni particles on αꢀAl2O3 support. In addition, the Ni particle sizes of the two
hydrothermally treated catalysts were also estimated using XRD analysis (Fig. 9). The Ni particle
size grows from 10.3 (Table 1) to 25.3 nm in 10NAꢀHT, which is even larger than that of the spent
10NA catalyst (17.2 nm); while the change in 10NA@ZrO2ꢀ5ꢀHT is from 8.8 (Table 1) to 12.8 nm,
which further indicates the superior antiꢀsintering property of 10NA@ZrO2ꢀ5. In addition, there is
no obvious change of crystallization of ZrO2 during the hydrothermal treatment.
3.4. Schematic diagram of the catalysts
Combing and considering all the above results, a schematic diagram illustrating the preparation
process and the structureꢀproperty relationship of ZrO2ꢀmodified Ni/Al2O3 catalyst is depicted and
shown in Fig. 10. The calcined 10NA successively adsorbs Zr(OBu)4 and H2O during the
preparation process, and amorphous ZrO2 (aꢀZrO2, XRD pattern not shown here) is obtained after
the hydrolysis of Zr(OBu)4 and the calcination (Fig. 10a‒b). After reduction, tetragonal and
monoclinic phases of ZrO2 (tꢀZrO2 and mꢀZrO2) will be formed, and the ZrO2 particles that are
interspersed among the nickel species can act as a physical barrier to restrict the growth of the Ni
particles and enhance the antiꢀsintering of the Ni species at high temperatures. However, the excess
ZrO2 may be beneficial to the improved stability, but it has adverse effect on the catalytic activity
due to the coverage of some exposed Ni atoms.
4. Conclusions
14

Page 15 of 32
RSC Advances
View Article Online
DOI: 10.1039/C5RA28057E
ZrO2ꢀmodified Ni/αꢀAl2O3 catalysts were prepared by a modified impregnation method and used
for CO methanation to produce synthetic natural gas. Compared to the unmodified catalyst, the
10NA@ZrO2ꢀ5 catalyst shows superior catalytic activity, coinciding with its large H2 uptake and
o
high Ni dispersion. In a 103hꢀlifetime test conducted under the conditions at 500 C, 0.1 MPa and
60000 mL g‒1 h‒1, 10NA@ZrO2ꢀ5 displays a high stability and resistance to Ni sintering. Further
characterizations indicate that in the preparation process, ZrO2 particles are formed and interspersed
among the nickel particles to restrain the growth of the Ni particles during the reduction and
catalytic reaction at high temperatures, resulting in the high stability in the lifetime test. It is
expected that this approach for catalyst preparation can be widely applied in the metalꢀbased
catalysts to enhance their antiꢀsintering performance in some highꢀtemperature reactions.
Acknowledgements
The authors gratefully acknowledge the supports from the National High Technology Research and
Development Program 863 (No. SS2015AA050502), the Fund of State Key Laboratory of
Multiphase complex systems (No. MPCSꢀ2015ꢀAꢀ06), the National Natural Science Foundation of
China (No. 21476238), the National Basic Research Program (No. 2014CB744306), the Open
Research Fund of State Key Laboratory of Multiphase Complex Systems (No. MPCSꢀ2014ꢀDꢀ03)
and “Strategic Priority Research Program” of the Chinese Academy of Sciences (Nos.
XDA07010100 and XDA07010200). Z. Zhong (Zhong_ziyi@ices.aꢀstar.edu.sg) works in Institute
of Chemical Engineering in Singapore, and also holds an adjunct associate professor position in the
School of Chemical & Biomedical Engineering, Nanyang Technological University (NTU) in
Singapore.
15

RSC Advances
Page 16 of 32
View Article Online
DOI: 10.1039/C5RA28057E
References
1.
2.
3.
C. H. Bartholomew, Appl. Catal. A Gen., 2001, 212, 17ꢀ60.
A. M. Cao, R. W. Lu and G. Veser, Phys. Chem. Chem. Phys., 2010, 12, 13499ꢀ13510.
D. C. Hu, J. J. Gao, Y. Ping, L. H. Jia, P. Gunawan, Z. Y. Zhong, G. W. Xu, F. N. Gu and F.
B. Su, Ind. Eng. Chem. Res., 2012, 51, 4875ꢀ4886.
4.
5.
J. J. Gao, Q. Liu, F. N. Gu, B. Liu, Z. Y. Zhong and F. B. Su, RSC Adv., 2015, 5,
22759ꢀ22776.
J. J. Gao, Y. L. Wang, Y. Ping, D. C. Hu, G. W. Xu, F. N. Gu and F. B. Su, RSC Adv., 2012, 2,
2358ꢀ2368.
6.
7.
W. R. Kang and K. B. Lee, Korean J. Chem. Eng., 2013, 30, 1386ꢀ1394.
S. Kawi, Y. Kathiraser, J. Ni, U. Oemar, Z. Li and E. T. Saw, ChemSusChem 2015, 8,
3556ꢀ3575.
8.
9.
J. Kopyscinski, T. J. Schildhauer and S. M. A. Biollaz, Fuel, 2010, 89, 1763ꢀ1783.
Y. J. Liu, J. J. Gao, Q. Liu, F. N. Gu, X. P. Lu, L. H. Jia, G. W. Xu, Z. Y. Zhong and F. B. Su,
RSC Adv., 2015, 5, 7539ꢀ7546.
10. J. J. Gao, C. M. Jia, M. J. Zhang, F. N. Gu, G. W. Xu and F. B. Su, Catal. Sci. Technol., 2013,
3, 2009ꢀ2015.
11.
12.
S. R. Li and J. L. Gong, Chem. Soc. Rev., 2014, 43, 7245ꢀ7256.
H. Liu, C. Y. Guan, X. Li, L. Y. Cheng, J. B. Zhao, N. H. Xue and W. P. Ding,
ChemCatChem 2013, 5, 3904ꢀ3909.
13.
14.
Q. Liu, J. J. Gao, F. N. Gu, X. P. Lu, Y. J. Liu, H. F. Li, Z. Y. Zhong, B. Liu, G. W. Xu and F.
B. Su, J. Catal., 2015, 326, 127ꢀ138.
Q. Liu, J. J. Gao, M. J. Zhang, H. F. Li, F. N. Gu, G. W. Xu, Z. Y. Zhong and F. B. Su, RSC
16

Page 17 of 32
RSC Advances
View Article Online
DOI: 10.1039/C5RA28057E
Adv., 2014, 4, 16094ꢀ16103.
15.
Q. Liu, F. N. Gu, J. J. Gao, H. F. Li, G. W. Xu and F. B. Su, J. Energy Chem., 2014, 23,
761ꢀ770.
16.
17.
F. H. Meng, Z. Li, F. K. Ji and M. H. Li, Int. J. Hydrogen Energy, 2015, 40, 8833ꢀ8843.
Q. Liu, F. N. Gu, X. Y. Wang, G. J. Jin, H. F. Li, F. Gao, Z. Y. Zhong, G. W. Xu and F. B. Su,
RSC Adv., 2015, 5, 84186ꢀ84194.
18.
19.
20.
21.
X. Y. Wang, Q. Liu, J. X. Jiang, G. J. Jin, H. F. Li, F. N. Gu, G. W. Xu, Z. Y. Zhong and F. B.
Su, Catal. Sci. Technol., 2015, DOI: 10.1039/C5CY01482D.
W. Y. Teoh, D. E. Doronkin, G. K. Beh, J. A. H. Dreyer and J. D. Grunwaldt, J. Catal., 2015,
326, 182ꢀ193.
J. Zhang, M. Ibrahim, V. Collière, H. Asakura, T. Tanaka, K. Teramura, K. Philippot and N.
Yan, J. Mol. Catal. A Chem., 2016, DOI: http://dx.doi.org/10.1016/j.molcata.2016.01.014.
N. A. Mashayekhi, Y. Y. Wu, M. C. Kung and H. H. Kung, Chem. Commun., 2012, 48,
10096ꢀ10098.
22.
23.
G. D. Li and Z. Y. Tang, Nanoscale, 2014, 6, 3995ꢀ4011.
J. R. RostrupꢀNielsen, K. Pedersen and J. Sehested, Appl. Catal. A Gen., 2007, 330,
134ꢀ138.
24.
25.
26.
A. M. Zhao, W. Y. Ying, H. T. Zhang, H. F. Ma and D. Y. Fang, J. Nat. Gas Chem., 2012, 21,
170ꢀ177.
H. Y. Ma, L. Zeng, H. Tian, D. Li, X. Wang, X. Y. Li and J. L. Gong, Appl. Catal., B
Environ., 2016, 181, 321ꢀ331.
J. Zhang, J. Teo, X. Chen, H. Asakura, T. Tanaka, K. Teramura and N. Yan, ACS Catal.,
2014, 4, 1574ꢀ1583.
17

RSC Advances
Page 18 of 32
View Article Online
DOI: 10.1039/C5RA28057E
27.
28.
H. Konnerth, J. Zhang, D. Ma, M. H. G. Prechtl and N. Yan, Chem. Eng. Sci., 2015, 123,
155ꢀ163.
G. J. Jin, F. N. Gu, Q. Liu, X. Y. Wang, L. H. Jia, G. W. Xu, Z. Y. Zhong and F. B. Su, RSC
Adv., 2016, 6, 9631ꢀ9639.
18

Page 19 of 32
RSC Advances
View Article Online
DOI: 10.1039/C5RA28057E
Table and Figure Captions
Table 1 Physical and chemical properties of the samples.
Fig. 1. N2 adsorption isotherms of the calcined (A) and reduced (B) samples: (a) 10NA, (b)
10NA@ZrO2ꢀ5, (c) 10NA@ZrO2ꢀ10, and (d) 10NA@ZrO2ꢀ15 (For clarity, the isotherm of
10NA, 10NA@ZrO2ꢀ5, 10NA@ZrO2ꢀ10 and 10NA@ZrO2ꢀ15 was vertically shifted for 80,
55, 35 and 0 cm3 g–1, respectively).
Fig. 2. TEM images of the reduced samples: (a) 10NA, (b) 10NA@ZrO2ꢀ5, (c) 10NA@ZrO2ꢀ10,
and (d and e) 10NA@ZrO2ꢀ15; and XRD patterns of the reduced catalysts (f).
Fig. 3. SEM image of the reduced 10NA@ZrO2ꢀ15 (a), elemental mapping images of O (b), Al (c),
Ni (d), and Zr (e); Energyꢀdispersive Xꢀray spectroscopy (EDS) analysis of this sample (f).
Fig. 4. H2ꢀTPR profiles (A) and H2ꢀTPD profiles (B) of the catalysts: (a) 10NA, (b) 10NA@ZrO2ꢀ5,
(c) 10NA@ZrO2ꢀ10, (d) 10NA@ZrO2ꢀ15 and (e) ZrO2.
Fig. 5. XPS spectra of the reduced catalysts: (a) Ni 2p and (b) O 1s.
Fig. 6. Catalytic properties of the catalysts: (a) CO conversion, (b) CH4 selectivity, and (c) CH4
yield; and lifetime test of 10NA and 10NA@ZrO2ꢀ5 catalysts: (a) CO conversion, (b) CH4
selectivity, and (c) CH4 yield.
Fig. 7. TEM images of the spent samples: (A) 10NA, and (B) 10NA@ZrO2ꢀ5; XRD patterns of the
spent catalysts (C): (a) 10NA, and (b) 10NA@ZrO2ꢀ5; and TG curves of the spent catalysts
in air (D) (Inset: TEM image confirming the carbon filament).
Fig. 8. Catalytic properties of the catalysts after hydrothermal treatment: (a) CO conversion, (b)
CH4 selectivity, and (c) CH4 yield.
Fig. 9. XRD patterns of the catalysts after hydrothermal treatment.
Fig. 10. Schematic diagram illustrating the preparation of the ZrO2ꢀmodified Ni/αꢀAl2O3 catalyst: (a)
19

RSC Advances
Page 20 of 32
View Article Online
DOI: 10.1039/C5RA28057E
the calcined 10NA, (b) the calcined 10NA@ZrO2ꢀ15, and (c) the reduced 10NA@ZrO2ꢀ15
(Note: a, t and m is the abbreviation of amorphous, tetragonal and monoclinic, respectively.)
20

Page 21 of 32
RSC Advances
View Article Online
DOI: 10.1039/C5RA28057E
Table 1
Ni particle size
(nm)
XRDc TEMd
SBETa (m2 g–1)
NiO
ZrO2
H2 uptake
De
Catalysts
content content
(wt%)b (wt%)b
(ꢁmol g–1) (%)
Calcined Reduced
10NA
25
27
29
30
12
17
19
21
–
–
10.3
8.8
9.3
9.2
10.3
7.2
23.9
44.0
33.1
23.0
3.6
6.5
4.9
3.4
10NA@ZrO2ꢀ5
10NA@ZrO2ꢀ10
10NA@ZrO2ꢀ15
9.53
9.10
8.35
3.10
7.21
12.05
7.5
7.0
a Surface area of the sample derived from BET equation.
b The exact composition of the calcined catalysts was determined by ICPꢀAES.
c
Particle size estimated from the XRD diffraction peak (2θ = 44.6) using the DebyeꢀScherrer
equation.
d Average particle size estimated from the TEM images.
e Ni dispersion calculated from the H2ꢀTPR and H2ꢀTPD results.
21

RSC Advances
Page 22 of 32
View Article Online
DOI: 10.1039/C5RA28057E
(A)
(a)
(B)
(a)
100
80
60
40
20
0
125
100
75
50
25
0
(b)
(c)
(b)
(c)
(d)
(d)
0.0
0.3
0.6
0.9
P/P0
0.0
0.3
0.6
0.9
P/P0)
Relative Pressure
(
)
Relative Pressure
(
Fig. 1
22

Page 23 of 32
RSC Advances
View Article Online
DOI: 10.1039/C5RA28057E
(a)
(b)
(c)
2
4
6
8
10 12 14
4
6
8
10 12 14
3
6
9
12 15 18 21
Particle Size (nm)
Particle Size (nm)
Particle Size (nm)
ꢃ
ZrO2 (Tetragonal)
ꢁ
ZrO2 (Monoclinic)
ꢂ
(f)
ꢀ
Ni
α−Al2O3
(e)
(d)
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂꢂ
ꢂ
ꢀ
ꢂ
ꢂ
ꢀ
10NA
ꢁ
ꢃ
ꢃ
ꢁ
ꢃ
10NA@ZrO -5
2
10NA@ZrO -10
2
10NA@ZrO -15
2
30
45
60
75
4
6
8
10
12
2 Theta (degree)
Particle Size (nm)
Fig. 2
23

RSC Advances
Page 24 of 32
View Article Online
DOI: 10.1039/C5RA28057E
(b)
(e)
(c)
(f)
(a)
(d)
O
Al
Ni
Zr
Fig. 3
24

Page 25 of 32
RSC Advances
View Article Online
DOI: 10.1039/C5RA28057E
362 oC
(A)
(B)
(a)
153 oC
524 oC
616 oC
296 oC
305 oC
(a)
475 oC
(b)
(c)
(b)
(c)
(d)
(e)
(d)
(e)
300
600
900
100 200 300 400 500 600
Temperature (oC)
Temperature (oC)
Fig. 4
25

RSC Advances
Page 26 of 32
View Article Online
DOI: 10.1039/C5RA28057E
(b)
O 1s
Ni 2p3/2
(a)
Ni2+
OI
OII
Ni2+
Ni0
10NA@ZrO2-5
10NA@ZrO2-5
10NA
10NA
534
864
860
856
852
532
530
528
Bing Energy (eV)
Binding Energy (eV)
Fig. 5
26

Page 27 of 32
RSC Advances
View Article Online
DOI: 10.1039/C5RA28057E
100
80
60
40
20
0
(a)
(c)
(b)
10NA@ZrO2-15
10NA@ZrO2-10
10NA@ZrO2-5
10NA
70
50
30
10
100
80
10NA@ZrO2-15
10NA@ZrO2-10
10NA@ZrO2-5
10NA
10NA@ZrO2-15
60
10NA@ZrO2-10
10NA@ZrO2-5
10NA
0.1 MPa, 30000 mL g-1 h-1
0.1 MPa, 30000 mL g-1 h-1
0.1 MPa, 30000 mL g-1 h-1
40
250 300 350 400 450
Temperature (oC)
250 300 350 400 450
Temperature (oC)
250 300 350 400 450
Temperature (oC)
(e)
(f)
(d)
80
60
40
20
0
60
40
20
0
75
60
45
30
10NA
10NA@ZrO2-5
10NA
10NA@ZrO2-5
10NA
10NA@ZrO2-5
500 oC, 0.1 MPa, 60000 mL g-1 h-1
500 oC, 0.1 MPa, 60000 mL g-1 h-1
500 oC, 0.1 MPa, 60000 mL g-1 h-1
0
20
40
Time (h)
60
80 100
0
20
40
Time (h)
60
80 100
0
20
40
60
80 100
Time (h)
Fig. 6
27

RSC Advances
Page 28 of 32
View Article Online
DOI: 10.1039/C5RA28057E
(A)
(B)
2
1
10NA@ZrO2-5-spent
(D)
ꢀ
Ni
101
(C)
ꢀ
100
ꢀ
(a)
99
10NA-spent
(b)
98
75
35
45
55
65
300
600
900
Temperature (oC)
2 Theta (degree)
Fig. 7
28

Page 29 of 32
RSC Advances
View Article Online
DOI: 10.1039/C5RA28057E
100
80
60
40
20
0
90
70
50
30
10
(a)
(b)
100
80
(c)
10NA@ZrO -5-HT
10NA@ZrO -5-HT
2
10NA-HT
2
10NA-HT
10NA@ZrO -5-HT
2
10NA-HT
60
0.1 MPa, 30000 mL g-1 h-1
0.1 MPa, 30000 mL g-1 h-1
0.1 MPa, 30000 mL g-1 h-1
40
250 300 350 400 450
Temperature (oC)
250 300 350 400 450
Temperature (oC)
250 300 350 400 450
Temperature (oC)
Fig. 8
29