Development of Highly Stable Ni-Al2O3 Catalysts for CO Methanation

Article abstract of DOI:10.1007/s10562-020-03486-4

Abstract: A series of Ni-Al₂O₃ catalysts with attractive catalytic activity and thermal stability have been developed via a facile hydrolysis-precipitation technique (xNiAl-HP, x representing the weight percentage of NiO). Based on the high nickel active surface area, good dispersion of NiO and high content of α- and β₁-type NiO, outstanding CO conversion (99%) and CH₄ yield (96%) can be achieved over 40NiAl-HP under mild conditions [T = 220?°C, P = 1.0?MPa, and WHSV = 20,000?mL/(g·h)]. The reaction-sintering at high temperature (800?°C) evolves through: (a) the migration of nickel crystals, (b) the phase transformation from γ- to θ- and then to α-Al₂O₃, and (c) the agglomeration of catalyst particles. The 40NiAl-HP catalyst could still maintain CO conversion over 99% at 300?°C after being used at 800?°C for 100?h. Herein, the high catalytic activity, good sintering resistance and noble-metal-/promoter-free nature might make 40NiAl-HP a competitive candidate for industrial methanation. Graphic Abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-020-03486-4

Catalysis Letters
https://doi.org/10.1007/s10562-020-03486-4
Development of Highly Stable Ni‑Al2O3 Catalysts for CO Methanation
Zhifeng Qin1,4,5 · Hongyan Ban1 · Xiaoyue Wang1 · Zhibin Wang1 · Yanxia Niu2 · Ying Yao3 · Jun Ren1 · Liping Chang1 ·
Maoqian Miao1 · Kechang Xie1 · Congming Li1
Received: 12 August 2020 / Accepted: 1 December 2020
© The Author(s), under exclusive licence to Springer Science+Business Media, LLC part of Springer Nature 2021
Abstract
A series of Ni-Al2O3 catalysts with attractive catalytic activity and thermal stability have been developed via a facile
hydrolysis-precipitation technique (xNiAl-HP, x representing the weight percentage of NiO). Based on the high nickel active
surface area, good dispersion of NiO and high content of α- and β1-type NiO, outstanding CO conversion (99%) and CH4
yield (96%) can be achieved over 40NiAl-HP under mild conditions [T=220 °C, P=1.0 MPa, and WHSV=20,000 mL/
(g·h)]. The reaction-sintering at high temperature (800 °C) evolves through: (a) the migration of nickel crystals, (b) the phase
transformation from γ- to θ- and then to α-Al2O3, and (c) the agglomeration of catalyst particles. The 40NiAl-HP catalyst
could still maintain CO conversion over 99% at 300 °C after being used at 800 °C for 100 h. Herein, the high catalytic activ-
ity, good sintering resistance and noble-metal-/promoter-free nature might make 40NiAl-HP a competitive candidate for
industrial methanation.
Graphic Abstract
Keywords CO methanation · Ni-Al2O3 · Hydrolysis-precipitation · Low temperature · Sintering resistance
1 Introduction
Supplementary Information The online version of this article
(https://doi.org/10.1007/s10562-020-03486-4) contains
supplementary material, which is available to authorized users.
Natural gas (NG), as a kind of clean energy with a high
heating value (37.26−28.10 MJ/m3) and a high energy ef-
ciency, has been widely applied in many felds such as chem-
ical and power industries, transportation fuel and residential
use etc. [1]. In view of the increased demand of natural gas,
the development of synthetic natural gas (SNG) is of vital
* Zhifeng Qin
qinzhifeng@tyut.edu.cn
* Congming Li
licongming0523@163.com
Extended author information available on the last page of the article
Vol.:(0123456789)
1 3

Z. Qin et al.
importance in some regions of the world [2]. Especially in
China, the strategy of “Coal to Natural Gas” has been highly
developed in recent years due to the primary energy struc-
ture of China (70% coal, 20% oil and 4% natural gas) [3]. To
reduce the dependence on coal and oil, the government aims
at increasing the proportion of natural gas in the total energy
system [4, 5]. The production of SNG from coal/biomass/
coke oven gas is in line not only with the tendency towards
a clean utilization of coal, but with the goal of changing the
primary energy mix [6–8].
2 Experimental
2.1 Catalyst Preparation
2.1.1 Preparation of Ni‑Al2O3 Catalysts
by Hydrolysis‑Precipitation
Typically, known quality of nickel nitrate hexahydrate
[Ni(NO3)2, 0.21 mol/L] and sodium aluminate [NaAlO2,
0.90 mol/L] were dissolved into 100 mL of deionized water
separately under vigorous stirring. The Ni(NO3)2 solution
was added into the aluminum precursor solution at a rate of
2 mL/min. The suspension was stirred by a magnetic stirrer
and its pH was maintained at about 10. The mixture was
aged for 12 h at 70 °C, and the resultant white precipitate
was collected via fltration. After being washed with water
(30 mL×3), the obtained light green powder was dried at
100 °C for 12 h and then calcined at 450 °C for 2 h under
N2 atmosphere in a tubular furnace. The Ni-Al2O3 catalysts
were labelled as xNiAl-HP (x=10, 20, 30, 40 and 50, rep-
resenting the weight percentage of NiO). The catalysts were
reduced at 500 °C for 2 h before catalytic reaction (named
as R-xNiAl-HP). The 40NiAl-HP catalysts employed in the
reaction at 800 °C were named as U-40NiAl-HP-Z (Z=2,
24 and 100, representing the reaction time).
In recent years, carbon monoxide (CO) methanation, has
been regarded as a competitive technology to aford SNG
from syngas. The catalysts based on group VIII metals (e.
g. Ni, Rh, Ru, Pd and Pt) have been widely developed for
CO methanation [6–17]; among them, Ni-Al2O3 catalysts
have attracted much attention due to the superior activity in
methanation and relatively low cost [8–11, 18–26]. However,
active Ni component is easily to be sintering following quick
deactivation, and Al2O3 commonly sufer from unsatisfac-
tory thermal stability [20–24]. Considering the strongly exo-
thermic nature of CO methanation, it is critical to develop
catalysts with high activity and good stability under harsh
conditions. Very recently, MCR-2X catalyst was reported
to maintained the relative high activity for CO methanation
beyond two years at an exit temperature of 675 °C, it seems
important to improve the sintering resistance of catalysts
under series conditions [11, 26].
2.1.2 Preparation of Ni‑Al2O3 by Co‑Precipitation
So far, numerous researches have focused on the enhance-
ment of the catalytic activity and thermal stability of Ni-
Al2O3 catalysts via introducing promoters (e.g. La, Na, Mg,
W) [16, 17, 24–26] and varying preparation methods (e.g.
impregnation, solution combustion, co-precipitation, homo-
geneous precipitation) [16–26]. Considering the low cost of
common Al2O3 support, it is promising to develop a facile
preparation technique for Ni-Al2O3 catalysts that exhibits
superior catalytic activity and thermal stability rather than
introducing special promoters.
The aqueous solutions of [Ni(NO3)2, 0.21 mol/L] and alu-
minum nitrate [Al(NO3)3, 0.47 mol/L] were obtained by
dissolving Ni(NO3)2·6H2O and aluminum nitrate nonahy-
drate [Al(NO3)3·9H2O] in 100 mL of deionized water,
respectively. An aqueous solution of NaOH (2 mol/L) was
selected as precipitant. The nitrate precursor solution was
added to the precipitant solution at a rate of 2 mL/min. The
suspension was stirred by a magnetic stirrer and its pH was
maintained at about 10. After aging at 70 °C for 12 h, the
precipitate was fltered, washed, dried at 100 °C for 12 h and
calcined at 450 °C for 2 h under N2 atmosphere in a tubular
furnace (denoted as 40NiAl-CP). The 40NiAl-CP catalyst
reduced at 500 °C for 2 h was denoted as R-40NiAl-CP, and
the used 40NiAl-CP catalysts at 800 °C were denoted as
U-40NiAl-CP-Z (Z=2, 24 and 100, representing the reac-
tion time).
Herein, a series of Ni-Al2O3 catalysts with high cata-
lytic activity and good sintering resistance in CO methana-
tion were synthesized by hydrolysis-precipitation method.
In view of the fact that few reports have focused on the
implications of a reaction temperature that is considerably
higher than that commonly adopted in industrial applica-
tions (650 °C), the catalytic performance at low temperature
(220 °C) and lifetime test at harsh temperature (800 °C) of
the prepared Ni-Al2O3 catalysts have been investigated. The
correlation of the catalytic activity and thermal stability with
the textural structures was also concerned and discussed in
detail.
2.2 Catalyst Characterization
The specifc surface areas and pore volumes were measured
by nitrogen adsorption/desorption at −196 °C on a BeiShiDe
3H-2000PS2 instrument. Before test, the samples were
treated under dynamic vacuum at 250 °C for 4 h to remove
1 3

Development of Highly Stable Ni-Al2O3 Catalysts for CO Methanation
the adsorbed impurities. XRD spectra were recorded on a
Rigaku D/Max 2500 with a CuKα X-ray source in a range
of 2θ from 10° to 85°. The crystal size of Ni was calculated
using the Scherrer–Warren equation [10]. H2 chemisorp-
tion characterization was carried out on a Micrometrics
AutoChem II 2920 instrument. Prior to test, 100 mg cata-
lyst was reduced in a H2 fow at 500 °C for 1 h. Then, the
catalysts were purged in Ar at 500 °C for 1 h and cooled
down to 30 °C. Finally, pure H2 was pulsed for 10 min to
achieve saturated adsorption. The H2 capacity and nickel
surface area were calculated based on the assumption that
one H atom combined with one surface Ni atom and the
cross-sectional area of atomic nickel was 6.49× 10–20 m2/
Ni-atom [10]. Temperature-programmed reduction of H2
(H2-TPR) was carried out on Micrometrics AutoChem II
2920. About 100 mg catalyst loaded in a quartz tube was
purged in an Ar fow of 25 mL/min at 300 °C for 30 min
and then cooled down to room temperature. The catalyst
was heated to 910 °C at a rate of 10 °C·min−1 with a fow of
10vol% H2/Ar (20 mL·min−1). X-ray photoelectron spectros-
copy (XPS) was conducted on an ESCALab 250Xi electron
spectrometer with a non-monochromatized Mg Kα radia-
tion (1253.6 eV). Scanning electron microscopy (SEM) and
X-ray energy dispersive spectrometry (EDS) were performed
on a JSM-6700F electron microscope. Transmission electron
microscopy (TEM) was taken on JEM-2100F at an accelera-
tion voltage of 200 keV.
Nout
y
CH4,out
YCH
=
× 100%
(2)
4
Ninyco,in − Noutyco,out
where Nin or Nout denotes as the fow rate (mL·h−1) at the
inlet or outlet and yin or yout represents the molar fraction of
CO2 or CH4 at the inlet or outlet.
3 Results and Discussion
3.1 Catalyst Characterization
3.1.1 Textural Structures of the xNiAl‑HP Catalysts
with Diferent Ni Contents
Both the Ni species and crystal structure of the Al2O3 sup-
port are essential to the catalytic activity of the Ni-Al2O3
catalysts [10]. XRD patterns of the as-synthesized and as-
reduced catalysts were conducted to determine their crystal
structures (Fig. 1). The peaks representing NiO [JCPDS
47–1049, solid line in Fig. 1a], Ni [JCPDS 87–0712, solid
line in Fig. 1b], and γ-Al2O3 [JCPDS 10–0425, dashed line
in Fig. 1a, b] can be observed for the catalysts prepared by
hydrolysis-precipitation (xNiAl-HP) [7, 8, 21, 24], suggest-
ing that the γ-Al2O3 phase was formed in the xNiAl-HP cata-
lysts during calcination at 450 °C. With increasing nickel
content from 10 to 50 wt.%, the difraction peaks of NiO and
Ni become stronger and narrower, while those of γ-Al2O3 are
weakened gradually, indicating that the crystal sizes of NiO
and Ni gradually increase with increasing nickel content.
On the other hand, the crystal size of Ni on the xNiAl-HP
catalysts, calculated from the width of the Ni (111) difrac-
tion lines using the Scherrer equation, shows an increase
from 6.7 to 18.4 nm with increasing nickel content (Table 1).
The Ni2p3/2 XPS spectra of the catalysts were detected
to verify the nickel species and crystal structure of Al2O3
(Fig. 2). The signals at BEs above 855 eV in the xNiAl-HP
and 40NiAl-CP catalysts are located between the BEs of
NiO (854.1 eV) and NiAl2O4 (856.1 eV) with the corre-
sponding shake-up satellite peak (861 eV), further indicat-
ing the presence of NiO and NiAl2O4 [27, 28]. The binding
energy of Ni2p3/2 on the xNiAl-HP catalysts is gradually
decreased with increasing nickel loading, suggesting that
the interaction between nickel and the Al2O3 support tends
to be weakened. The satellite peak of Ni2+ ions can still be
observed in all cases, the intensity of which is gradually
enhanced with increasing nickel loading, indicating that the
xNiAl-HP catalysts with higher nickel content contain more
nickel species in the oxidation state [29, 30].
2.3 Catalyst Evaluation
The CO methanation reaction was performed in a stainless
steel fxed bed reactor (Φ20×3×550 mm) equipped with a
quartz tube (Φ12×2×500 mm, as shown in Fig. S1). Typi-
cally, 500 mg catalyst was placed in the constant temperature
zone of the reactor, and reduced at 500 °C for 2 h with a gas
fow of 50 vol.% H2/N2 (30 mL·min−1). The gas mixture
of 25 vol.% CO and 75 vol.% H2 was then introduced to
study the catalytic activity at 200–300 °C and catalytic sta-
bility at 800 °C. The catalytic experiments were performed
at 1.0 MPa with a weight hourly space velocity (WHSV)
of 20,000 mL·g−1·h−1. H2O was removed from the exit gas
through a cold trap after reaction for 1 h, and the dry gas
entered into an on-line GC-900-SD gas chromatograph
equipped with a TCD detector and a 2 m carbon molecular
sieve (TDX-1) column. The CO conversion and CH4 yield
were calculated using Eqs. (1) and (2):
CO conversion (Xco,%):
Ninyco,in − Noutyco,out
H2-TPR measurements were carried out to characterize
the interaction between the nickel species and the Al2O3
support. Based on the peaks in the H2-TPR spectra (see
Fig. 3), the reduced NiO species can be divided into four
Xco =
× 100%
(1)
Ninyco,in
CH4 yield (YCH ,%):
4
1 3

Z. Qin et al.
Fig. 1 XRD patterns of the
xNiAl-HP and 40NiAl-CP
catalysts before (a) and after (b)
reduction at 500 °C
Table 1 Textural properties of
the xNiAl-HP and 40NiAl-CP
catalysts
Catalyst
Specifc surface Pore volume Pore diameter Ni crystal- H2 uptake
Ni sur-
areas (m2·g−1 a
)
(cm3·g−1 b
)
(nm)b
lite size
(μmol·g−1 d
)
face area
(nm)c
(m2·g−1
)
d
cat
10NiAl-HP 260
20NiAl-HP 264
30NiAl-HP 277
40NiAl-HP 270
50NiAl-HP 202
40NiAl-CP 257
0.41
0.42
0.44
0.45
0.60
0.43
4.4
4.4
4.8
5.2
9.0
4.5
6.7
8.4
10.2
11.4
18.4
11.8
36.2
86.6
95.1
225.5
93.8
147.3
2.8
6.8
7.4
17.6
7.3
11.5
aThe BET specifc surface areas, derived from the BET equation
bAverage pore volume, obtained from BJH desorption
cNiO and Ni crystallite size, calculated by the Debye–Scherrer equation
dDetermined by hydrogen chemisorption
types: α-type in the temperature region lower than 400 °C,
β1- and β2-type in the temperature region of 400–700 °C and
γ-type in the temperature region higher than 700 °C [28,
31, 32]. The α-type NiO can be identifed as the surface
amorphous one that has no and/or weak interaction with the
support [21, 31]. The β-type NiO can be classifed into Ni-
rich mixed oxide phase (β1) and Al-rich mixed oxide phase
(β2) that possesses a stronger interaction with the support
than the former one [21–23, 29, 33]. The γ-type NiO pos-
sesses spinel structure (such as NiAl2O4) which can strongly
interact with the support and cannot be reduced until tem-
perature reaches 700 °C [21–23, 29, 33]. The gaussian ftting
analyses of the H2-TPR patterns of the catalysts are shown
in Table S1. Generally, the peak areas of the α- and β-type
NiO that represent the content of α- and β-type NiO are
increased gradually with increasing nickel content for the
NiAl-HP catalysts. Compared with the 40NiAl-CP sample,
the 40NiAl-HP catalyst exibited the larger amount of α- and
β1-type NiO species, which is benefcial to the low temera-
ture reactivity [10, 31].
The nickel active surface area is correlated to the number
of surface active sites, which can directly afect the catalytic
activity [10, 26, 33–35]. The active surface area of Ni spe-
cies were analyzed via H2 chemisorption technique [27].
As shown in Table 1, the calculated active surface areas
of the xNiAl-HP catalysts follow the order of 40NiAl-
HP>30NiAl-HP>50NiAl-HP>20NiAl-HP>10NiAl-HP,
which may induce a higher catalytic activity of 40NiAl-HP.
The physical properties of the Ni-Al2O3 catalysts were
examined via N2 adsorption–desorption (see Fig. S2). As
shown in Table 1, the BET specifc surface areas and pore
volumes of the prepared catalysts are in the range of 202–277
m2/g and 0.41–0.60 cm3/g, respectively. The specifc sur-
face area of the xNiAl-HP catalysts increased initially with
1 3

Development of Highly Stable Ni-Al2O3 Catalysts for CO Methanation
Fig. 2 Ni2p3/2 XPS spectra of the xNiAl-HP and 40NiAl-CP catalysts
Fig. 3 H2-TPR profles of the xNiAl-HP and 40NiAl-CP catalysts
increasing nickel content and then declined as the nickel
loading reaches 50 wt.%, while both the pore volume and
average pore size of the catalysts increased throughout. The
decreased specifc surface area and increased pore size of
50NiAl-HP were probably due to the growth of the catalyst
particles.
those on 40NiAl-CP, indicating a higher content of α- and
β1-type NiO on 40NiAl-HP. In addition, as listed in Table 1,
the specifc surface area, pore volume and pore diameter of
40NiAl-HP are larger than that of 40NiAlP-CP, indicating
the easy difusion of reactants and products on 40NiAl-HP
in the CO methanation reaction.
TEM and EDS patterns of 40NiAl-CP and 40NiAl-HP
were detected to determine the NiO distribution on the cata-
lysts. As shown in Fig. 4, the crystal size of NiO on 40NiAl-
CP is ca. 4–12 nm with an average size of 6.9 nm, slight
larger than that on 40NiAl-HP which is about 3–10 nm with
an average size of 5.9 nm. The Ni crystal size, calculated
using the Scherrer equation, showed the same trend. The
EDS patterns indicated that there was a higher nickel content
on the surface of 40NiAl-HP than that of 40NiAl-CP, which
would lead to more surface active sites.
3.1.2 Structural Comparison of the Catalysts Prepared
by Hydrolysis‑Precipitation and Co‑Precipitation
To gain insight into the efect of preparation method on the
textural properties of the Ni-Al2O3 catalysts, the textural
structures of the catalysts prepared by hydrolysis-precipi-
tation (40NiAl-HP) and co-precipitation (40NiAl-CP) were
compared. As shown in Fig. 1, the peaks representing NiO,
Ni and γ-Al2O3 can also be observed in the XRD pattern of
40NiAl-CP, suggesting a similar texture of 40NiAl-CP with
40NiAl-HP. However, the NiO crystal size on 40NiAl-HP is
slightly smaller than that on 40NiAl-CP (Table 1), indicating
a better dispersion of NiO on 40NiAl-HP. In the Ni2p3/2 XPS
spectrum of 40NiAl-CP, the binding energy of NiAl2O4 is
higher than that of 40NiAl-HP, verifying a larger amount of
NiAl2O4 spinel on the surface of 40NiAl-CP (see Fig. 2).
In the H2-TPR patterns as shown in Fig. 3, the reduction
peaks of α- and β1-type NiO on 40NiAl-HP are stronger than
It could be concluded that the catalyst prepared by
hydrolysis-precipitation (40NiAl-HP) possesses higher
specifc surface area, better dispersion of NiO, and larger
nickel active surface area than that prepared by co-precip-
itation (40NiAl-CP). To further understand the textural
diference of the two catalysts, the mechanisms of hydrol-
ysis-precipitation and co-precipitation were analyzed
(see Scheme 1). It was known that pure sodium alumi-
nate was alternatively expressed as NaAlO2 or NaAl(OH)4
1 3

Z. Qin et al.
Fig. 4 TEM, size distribution and EDS images of fresh 40NiAl-CP (a–c) and 40NiAl-HP (d–f)
Scheme 1 Schematic models of the hydrolysis-precipitation process (a) and co-precipitation process (b)
(hydrated) [36], and gibbsite and Al(OH)4– are present in
Ni + Al(OH)−4(aq) + OH(−aq) = Ni(OH)2 ↓ + Al(OH)3
2+
(aq)
the aqueous solution of sodium aluminate [37, 38]. Dur-
ing the hydrolysis-precipitation process, the Al(OH)3 and
Ni(OH)2 precipitates are simultaneously generated via the
(3)
However, the co-precipitation process exhibits diferent
mechanisms [Scheme 1b]. Upon the synchronous addition
of the solutions of Ni(NO3)2 and Al(NO3)3 to the NaOH
solution, Al3+ and Ni2+ react quickly with OH– to produce
–
reaction between Ni2+ and Al(OH)4 [Scheme 1a]. With
the continuous addition of Ni(NO3)2, the crystal nuclei of
Al(OH)3 and Ni(OH)2 gradually grows up. The reaction
equation is as follows:
1 3

Development of Highly Stable Ni-Al2O3 Catalysts for CO Methanation
Al(OH)3 and Ni(OH)2 precipitates. However, Al3+ pre-
cipitates into Al(OH)3 in the pH range of 4–10 and exists
–
as D4h-sym-metry Al(OH)4 in the pH range of 8–12.5
–
[36–41]. Thus, Al(OH)3 is rapidly dissolved into Al(OH)4 in
the alkali solution in the early stage of the co-precipitation
reaction. With the continuous addition of Ni(NO3)2 and
Al(NO3)3, the crystal nuclei of Ni(OH)2 continues to grow
up and the concentration of Al(OH)4– keeps rising, followed
by decreased pH of the mixed solution. With the rapid con-
–
suming of OH–, Al(OH)4 reacts with Al3+ to form Al(OH)3
after the secondary sedimentation. As a result, Al(OH)3
would be deposited on the surface of Ni(OH)2 and reduce
the nickel active surface area of the catalyst. The reaction
equation is as follows:
Ni + Al + Al(OH)−4(aq) + 4OH(−aq) = 2Al(OH)3 ↓ + Ni(OH)2 ↓
2+
(aq)
3+
(aq)
(4)
3.2 Catalytic Performance and Structure‑Sensitivity
at Low Temperatures
The catalytic performance of the Ni-Al2O3 catalysts in CO
methanation was assessed at relatively low temperatures
(200–300 °C) at 1.0 MPa and WHSV of 20,000 mL/(g·h),
and the results are shown in Fig. 5. The catalysts with dif-
ferent nickel loadings were studied; among them, 40NiAl-
HP performs optimum activity, which can aford high CO
conversion (99%) and CH4 yield (96%) at a low temperature
of 220 °C. The CH4 yield over the other xNiAl-HP catalysts
can reach up to 95% when the reaction temperature is raised
to 300 °C.
Fig. 5 Efect of reaction temperature on CO conversion (a) and CH4
yield (b) over diferent catalysts
It is noted that the sequence of the catalytic activity
of the xNiAl-HP catalysts follows the order of 40NiAl-
HP>30NiAl-HP>50NiAl-HP>20NiAl-HP>10NiAl-HP,
which correlated well with the nickel active surface area
(see Table 1). The decrease of CO conversion and CH4 yield
as the nickel loading is increased to 50 wt% should result
from the blocking of the external surface of the catalyst by
large metal aggregates. It can be concluded that the excel-
lent catalytic activity should be mainly attributed to the high
nickel active surface area, which can transform to abundant
“exposed” active sites available for the reactants. And just
for this reason, the catalytic activity of 40NiAl-HP is higher
than that of 40NiAl-CP. Moreover, the activity of Ni-Al2O3
catalysts is also consistent with the content of α- and β1-type
NiO species, further indicating that α- and β1-type NiO play
an important role in CO methanation [10, 31].
which no doubt increase the cost of CO methanation. The
40NiAl-HP catalyst developed in this work exhibits facile
preparation method, noble-metal/promoter-free nature, and
outstanding catalytic performance at extremely low tem-
perature (220 °C), which would highly reduce the energy
consumption and/or operation cost of CO mathanation.
3.3 Long‑Term Catalytic Stability
and Reaction‑Sintering at High Temperature
Considering the fact that CO methanation is highly exother-
mic and the released heat can lead to catalyst deactivation,
it is of urgent to detect the catalytic stability of the catalysts
under harsh conditions. However, the reaction temperatures
and times employed in the literatures (<600 ℃,<320 h, see
Table S3) are generally lower than that commonly adopted
in industrial application (650 °C, 1 year) [11]. To reliably
refect the guaranteed lifetime for industrial methanation, a
100 h stability of 40NiAl-HP was tested under the follow-
ing conditions: H2/CO=3, T=800 °C, WHSV=20,000 mL/
(g·h), P=1.0 MPa. As shown in Fig. 6a, the CO conversion
To evaluate the potential of the Ni-Al2O3 catalysts for
industrial application, the catalytic performance of 40NiAl-
HP was compared with typical reported catalysts (see
Table S2). To aford a high catalytic activity, noble metal
(Ru), additional promoters (La, B, Zr, CNT, CeO2) and/or
special supports (SiC, Mo2C, TiO2) are generally required,
1 3

Z. Qin et al.
observed on the catalysts as-reduced at 500 °C for 2 h, as
shown in Fig. 7a. With increasing reaction time, the difrac-
tion peaks of Ni become stronger, and meanwhile, γ-Al2O3
is gradually transformed to θ-type after being used at 800 °C
for 24 h [43–46]. After 100 h, a further transformation from
θ- to α-Al2O3 occured on the catalysts [Fig. 7d, [43–45]].
The porous properties of the Ni-Al2O3 catalysts treated at
800 °C for diferent times were examined (Fig. S3). The spe-
cifc surface area and pore volume of the catalysts declined
obviously after being used at 800 °C (see Table 2), which
might be explained by the crystal transformation of Al2O3 as
well as the shrinkage of the catalyst skeleton [47]. Moreover,
the specifc surface area and pore volume of 40NiAl-HP
decline by 82.2% and 36.8% respectively, while those of
40NiAl-CP decline by 86.5% and 41.2% respectively, indi-
cating a rapid sintering speed of 40NiAl-CP than that of
40NiAl-HP.
H2 chemisorption can further evidence the catalyst sin-
tering. The nickel active surface areas of fresh and spent
catalysts as-reduced at 500 °C and post-reacted at 800 °C
were determined (see Table 2). For both the 40NiAl-HP and
40NiAl-CP catalysts, a gradual decrease in the nickel active
surface area with reaction time can be observed, confrming
that sintering could reduce the active sites on catalysts. The
active surface areas of 40NiAl-HP and 40NiAl-CP decreased
by 94.8% and 97.0% respectively, further indicating the sin-
tering speed of 40NiAl-CP is higher than that of 40NiAl-HP.
SEM images of the 40NiAl-CP and 40NiAl-HP cata-
lysts post-reacted at 800 °C for diferent times are shown
in Fig. 8. One can see that the irregular catalysts gradually
migrate, gather and fnally evolve to regular spheres with
increasing reaction time. For 40NiAl-CP, the average par-
ticle size increases from 70.2 to 126.1 nm as the reaction
time increased from 2 to 100 h. Under the same condition,
the average particle size of 40NiAl-HP increased from 32.8
to 79.2 nm. Meanwhile, the Ni crystal size in the 40NiAl-
CP catalyst gradually increases from 11.8 to 68.6 nm at the
reaction time of 100 h (see Table 2), which is larger than that
in the 40NiAl-HP catalyst (54.5 nm). Based on the analysis
of structure characterization, it is revealed that the prepara-
tion method had a great efect on the the textural structures
of the NiAl-based catalyst which is derectly related to the
catalytic activity.
Fig. 6 a Catalytic stability of 40NiAl-HP and 40NiAl-
CP at high temperature; Reaction conditions: H2/CO=3:1,
WHSV=20,000 mL·g−1·h−1, T=800 °C, P=1.0 MPa; b Efect of
reaction temperature on CO conversion over 40NiAl-HP and 40NiAl-
CP after high temperature tests
and CH4 yield gradually decrease during the entire 100 h on
stream, which is probably attributed to the catalyst sintering.
Figure 6b showed that the 40NiAl-HP catalyst, previously
tested at 800 °C for 100 h, can still aford a CO conversion
over 99% at 300 °C, which is satisfactory with the demand
of industrial production [42]. However, CO conversion over
40NiAl-CP only reaches 60% at 380 °C and therefore it is
inadequate for industrial demand. In conclusion, the 40NiAl-
HP catalyst not only showed superior catalytic activity at
low temperature but exhibited good stability at high tem-
perature, thereby demonstrating that the catalyst prepared by
hydrolysis-precipitation was more suitable for high tempera-
ture methanation instead of the conventioanl one prepared
by co-precipitation method.
The sintering mechanism of the Ni-Al2O3 catalysts can
be deduced (see Scheme 2). The catalyst particles gradu-
ally migrate and agglomerate into larger particles with
increasing reaction time at 800 °C, leading to the growth
of the nickel and alumina crystals. Meanwhile, γ-Al2O3 is
transformed into θ-type after being used for 24 h. When the
reaction time is increased to 100 h, the crystal particles of
NiO and Al2O3 grow continuously, and θ-Al2O3 is further
transformed to α-type. Both the crystal growth of the cata-
lysts and γ-to-α phase transformation of Al2O3 lead to the
To gain insight into the sintering process of the 40NiAl-
HP and 40NiAl-CP catalysts, their textural properties after
being treated at harsh temperatures were analyzed. Figure 7
shows the XRD patterns of the 40NiAl-HP and 40NiAl-CP
catalysts post-reacted at 800 °C for diferent times. The char-
acteristic difraction peaks of Ni and γ-Al2O3 can be clearly
1 3

Development of Highly Stable Ni-Al2O3 Catalysts for CO Methanation
Fig. 7 XRD patterns of 40NiAl-
HP and 40NiAl-CP as-reduced
at 500 °C for 2 h (a) and post-
reacted at 800 °C for 2 h (b),
24 h (c) and 100 h (d)
Table 2 Textural properties of 40NiAl-HP and 40NiAl-CP post-treated at high temperatures
Catalyst
Treatment conditions
Specifc surface
areas (m2·g−1 a
Pore volume
(cm3·g−1 b
Pore diameter
(nm)b
Ni crystal
size (nm)c
H2 uptake
Ni sur-
)
)
(μmol·g−1 d
)
face area
(m2·g−1
)
d
cat
40NiAl-HP
500 °C, 2 he
800 °C, 2 h
800 °C, 24 h
800 °C, 100 h
500 °C, 2 he
800 °C, 2 h
800 °C, 24 h
800 °C, 100 h
258
112
78
46
245
87
0.38
0.36
0.32
0.24
0.34
0.33
0.26
0.20
5.0
8.4
11.2
13.0
4.5
10.9
11.4
16.9
11.4
17.4
25.3
54.5
11.8
22.4
31.8
68.6
225.5
55.4
31.3
11.7
147.3
33.5
13.4
4.4
17.6
4.3
2.4
0.9
11.5
2.6
1.1
0.4
40NiAl-CP
62
33
b
c
aSpecifc surface areas are derived from the BET equation; Obtained from BJH desorption average pore volume; NiO and Ni crystallite size,
calculated by the Debye–Scherrer equation; dDetermined by hydrogen chemisorption. eReduced at 500 °C for 2 h
1 3

Z. Qin et al.
Fig. 8 SEM and particle distribution images of a U-40NiAl-CP-2, b U-40NiAl-CP-24, c U-40NiAl-CP-100, d U-40NiAl-HP-2, e U-40NiAl-
HP-24, and f U-40NiAl-HP-100. Scale bars in SEM images: 200 nm
signifcant decrease of the specifc surface area and active
surface area, by which the catalytic activity at low tempera-
ture declines.
the migration and growth of NiO crystallites, the γ-to-θ-
to-α phase transformation of Al2O3, and the difusion and
agglomeration of the catalysts. A 99% of CO conversion can
still be achieved over 40NiAl-HP at 300 °C after being used
at 800 °C for 100 h, making it a competitive candidate for
industrial methanation.
4 Conclusions
Acknowledgements This work was supported by the Technology
Innovation Project for Excellent Talents of Shanxi Province (No.
201805D211037), Coal Based Key Scientifc and Technological Pro-
ject of Shanxi Province (No. MJH2016-03).
A series of Ni-Al2O3 catalysts have been prepared via
hydrolysis-precipitation for CO methanation, and the efect
of the textural structures on catalytic activity at low tem-
perature and long-term catalytic stability at high temperature
has been established. Herein, 40NiAl-HP catalyst exhibits
optimal activity at low temperature (220 °C), which can
catalyze CO into CH4 with excellent CO conversion (99%)
and CH4 yield (96%). The high content of α- and β1-type
NiO, good dispersion of NiO, and high nickel active sur-
face area are responsible for the superior catalytic activity of
40NiAl-HP. The sintering of the catalysts at 800 °C induces
Compliance with Ethical Standards
Conflict of interest The authors declare that they have no confict of
interest.
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Afliations
Zhifeng Qin1,4,5 · Hongyan Ban1 · Xiaoyue Wang1 · Zhibin Wang1 · Yanxia Niu2 · Ying Yao3 · Jun Ren1 · Liping Chang1 ·
Maoqian Miao1 · Kechang Xie1 · Congming Li1
1
4
Key Laboratory of Coal Science and Technology, Ministry
of Education and Shanxi Province, Taiyuan University
of Technology, Taiyuan 030024, China
Post-Doctoral Research Station of Shanxi Institute
of Environmental Planning, Taiyuan 030002, China
5
Postdoctoral Mobile Station of Environmental Science
and Engineering, Tsinghua University, Beijing 100084,
China
2
3
School of Chemistry and Chemical Engineering, Taiyuan
University of Technology, Taiyuan 030024, China
Department of Chemistry and Chemical Engineering,
Taiyuan Institute of Technology, Taiyuan 030008, China
1 3