The effect of synthesis parameters on ordered mesoporous nickel alumina catalyst for CO2 methanation

Article abstract of DOI:10.1016/j.apcata.2017.10.012

A series of ordered mesoporous nickel alumina catalysts were synthesized via the evaporation induced self-assembly technique (EISA). Varying synthesis parameters such as the type of acid, nickel loading, calcination temperature as well as synthesis method

Full text of DOI:10.1016/j.apcata.2017.10.012

Accepted Manuscript
Title: The effect of synthesis parameters on ordered
mesoporous nickel alumina catalyst for CO methanation
2
Authors: Ali Aljishi, Gabriel Veilleux, Jose Augusto
Hernandez Lalinde, Jan Kopyscinski
PII:
S0926-860X(17)30497-0
DOI:
Reference:
https://doi.org/10.1016/j.apcata.2017.10.012
APCATA 16443
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
10-7-2017
11-10-2017
13-10-2017
Please cite this article as: Ali Aljishi, Gabriel Veilleux, Jose Augusto Hernandez
Lalinde, Jan Kopyscinski, The effect of synthesis parameters on ordered
mesoporous nickel alumina catalyst for CO2 methanation, Applied Catalysis A,
General https://doi.org/10.1016/j.apcata.2017.10.012
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Revised Manuscript submitted to Applied Catalysis A
The effect of synthesis parameters on ordered mesoporous nickel
alumina catalyst for CO methanation
2
Ali Aljishi, Gabriel Veilleux, Jose Augusto Hernandez Lalinde, Jan Kopyscinski*
Department of Chemical Engineering, McGill University,
3
610 University Street, Montreal, Quebec H3A 0C5, Canada
*
To whom all correspondence should be addressed:
Tel.: +1 514 398 4276
jan.kopyscinski@mcgill.ca
Graphical Abstract

2

Highlights




Compared to hydrochloric and citric acid, using nitric acid during the synthesis led to ordered
mesoporous catalysts with higher surface area and superior catalytic activity.
Evaporation induced self-assembly (also known as "one pot") synthesis technique produced
catalysts with highly dispersed nickel clusters with very small diameter of around 2.8 nm.
Ordered mesoporous nickel alumina catalyst calcined at 700°C exhibited the best results
towards CO
At lower reaction temperatures (<400°C) the CO
direct dissociation of CO
2
methanation.
2
methanation proceeds most likely via the
2
.
Abstract
A series of ordered mesoporous nickel alumina catalysts were synthesized via the evaporation induced
self-assembly technique (EISA). Varying synthesis parameters such as the type of acid, nickel loading,
calcination temperature as well as synthesis method influenced the catalyst morphology and its activity
towards CO
2
methanation. Catalyst prepared without acid formed macroporous structures with a very
2
-1
low surface area (47 m g ), whereas using a mixture of hydrochloric and citric acid resulted in
2
-1
incomplete formation of mesoporous micelles with surface area of 173 m g . On the other hand, using
nitric acid lead to complete formation of long cylindrical micelles with a combined surface areas up to
2
-1
2
60 m g and highly dispersed nickel clusters with a size of 3-5 nm. An optimum calcination
temperature of 700°C was determined yielding the highest CO conversion and CH selectivity. This
2
4
catalyst displayed a stable performance and did not exhibit any sign of deactivation during a 150 h test.
Catalysts calcined at lower and higher temperatures had smaller surface areas as well as lower catalytic
activity.
3

Keywords: CO
2
methanation • Ordered mesoporous nickel alumina catalyst • Catalyst synthesis and
characterization
4

1
Introduction
Energy storage is a pressing topic in the development of renewable energies such as wind and solar,
mainly due to the intermittencies of electricity production. Growing interest has been observed towards
the Power-to-Gas process (P2G) in which excess electrical energy is used to produce hydrogen via
water-electrolysis, which is subsequently converted with captured CO
2
to grid compatible gas
(
methane). The P2G process combines electricity storage and CO utilization leading potentially to a
2
closed carbon cycle (Circular Economy)[1]. Beside water-electrolysis, the methanation is the most
important and technically challenging step in the P2G process. Current areas of research focus on (1)
evaluating suitable carbon oxides sources (i.e., biogas, power plants, extraction from air), (2) catalyst
design, and (3) reactor and process design including heat integration.
The main overall reactions are the CO
2
methanation (eq. 1), the competing reverse water-gas-shift
reaction (eq. 2), which leads to undesired CO, and the subsequent CO methanation (eq.3).
표
ꢃꢄ
ꢃꢄ
퐶푂 + 4퐻 → 퐶퐻 + ꢁ퐻 푂
∆퐻 = −165 푘퐽 푚ꢂ푙
(1)
(2)
(3)
2
2
ꢀ
2
푅
표
ꢃꢄ
퐶푂 + 퐻 ↔ 퐶푂 + 퐻 푂
∆퐻 = 41 푘퐽 푚ꢂ푙
2
2
2
푅
표
퐶푂 + 3퐻 ↔ 퐶퐻 + 퐻 푂
∆퐻 = −ꢁ06 푘퐽 푚ꢂ푙
2
ꢀ
2
푅
The reactions involved are catalyzed by supported Ni, Ru, and Rh systems of which the latter two noble
metals are more active than Ni-based catalysts [2–4]. Nickel might still be the first-choice due to lower
costs and its high catalyst stability shown for syngas/CO methanation (eq.3) [5]. For the CO
2
methanation, however, the stability of Ni-catalysts is not proven on a commercial scale.
New catalysts have been developed of which the ordered mesoporous alumina catalysts (OMA) show
great promise due to larger surface areas, larger pore volumes and uniform pore size distribution
compared to traditional catalyst systems allowing for a better interaction between the metal-oxide and
the support resulting a high metal dispersion [6,7]. In addition, the metallic nanoparticles are stabilized
5

due to the confinement effect of the mesoporous framework that suppresses sintering and
agglomeration of the metal atoms (e.g., Ni) even at high reaction temperatures. Therefore, ordered
mesoporous alumina materials are considered an ideal catalyst support for reforming [7–9], oxidation
[
10,11] and hydrogenation [12] reactions.
Ordered mesoporous alumina catalysts can be synthesized by various methods as outlined in [13–15].
The “one-step” facile route with evaporation induced self-assembly (EISA) has become one of the most
adopted techniques due to its simple and flexible procedure [14,15]. In EISA, the precursors are mixed
together at the desired concentrations until a homogeneous solution is obtained, which is then dried and
calcined. A polymeric surfactant/copolymer is often used as soft template and mixed with a polar
organic solvent and acid. Using a non-polar solvent like toluene would result in a water-in-oil emulsion
and would lead to rod formation rather than a mesoporous structure. Providing an acidic environment is
key in inducing the mesoporous structure since the polymerization and cross-linking rate are too fast at
a pH of 6-8.5 [14,15]. Nitric acid as well as mixtures of hydrochloric and citric acid have been used in
the synthesis of alumina and silica based catalysts. Besides the acidic environment, the calcination
temperature influences the catalyst structure and morphology hence its activity as observed for the
partial oxidation of methane [11].
Recently, Ni-OMA catalysts were tested for CO
2
methanation and showed higher CO
2
conversion and
CH selectivity compared to non-mesoporous nickel alumina catalysts [12]. Considering the promising
4
results, this work deals with the development of alumina based ordered mesoporous catalysts using
EISA synthesis method with various synthesis parameters. The objective was to study the effect of
different Ni loadings, calcination temperatures, and the acid used on the catalyst morphology and
activity towards the CO methanation at different reaction temperatures and gas hourly space velocities.
2
6

2
2
Experimental
.1 Catalyst Preparation
EISA technique was chosen as it was reported to be a robust mesoporous catalyst synthesis pathway.
®
Approximately 1 g of triblock copolymer (Pluronic P-123, Sigma Aldrich) was dissolved in 20 ml of
anhydrous ethanol until a homogeneous solution was obtained. Subsequently 2-4 g of aluminum-
isopropoxide (≥ 98%, Sigma Aldrich) and up to 1.5 g of nickel (II) nitrate hexahydrate (≥ 98.5%,
Sigma Aldrich) were added to achieve the desired catalyst mass and nickel target loadings (Table 1).
Nitric acid (68 wt%, Fisher Scientific) or a mixture of hydrochloric (37 wt%, Fisher Scientific) and
citric acid (Sigma Aldrich) was then added to provide the acidic environment to enhance mesopore
formation. The solution was stirred for 5 h at 600 rpm at room temperature. The homogenized solution
obtained, was dried at 60°C for 48 h and subsequently calcined in a muffle furnace with heating rate of
-1
1
°C min for 5 h at the targeted calcination temperature (i.e., 400, 500, 700 or 900°C).
Samples with three different nickel loadings (5 wt%, 15 wt% and 30 wt%) and four different
calcination temperatures (400-900°C) were prepared. The catalysts were named OMA-15Ni-500,
where OMA refers to ordered mesoporous alumina, 15Ni refers to the nickel loading in wt% and 500 to
the calcination temperature in degree Celsius (°C). OMA-500 denotes to the support only, calcined at
5
00°C; whereas catalysts prepared without no acid or with hydrochloric + citric acid, were labeled
OMA-500-No Acid or OMA-500-HCl and OMA-15Ni-500-HCl, respectively (Table 1).
In addition, a 15 wt% Ni/OMA catalyst was synthesized via wet impregnation for comparison. First,
the OMA support was prepared via the aforementioned technique and calcined at 500°C. Secondly, the
OMA support was added slowly to the nickel (II) nitrate hexahydrate solution and stirred vigorously
(~600 rpm) for 6 h. The mixture was dried for 24 h at 100°C and then calcined at 500°C with heating
-
1
rate of 1°C min for 5 h. The catalyst was named 15Ni/OMA-500.
7

All of the produced catalyst samples were sieved into three different particle size categories (45-90 µm,
9
0-125 µm, and 125-150 µm) using a DUAL D-4326 motorized sieve.
2
.2 Catalyst Characterization
Various characterization techniques were used to study catalyst morphology, crystallinity and
composition. Sieved particles with 45-90 µm were used in all characterization techniques, whereas
particles of 125-150 µm were used in H
measurements.
2
temperature programmed reduction and activity
Inductively Coupled Plasma - Optical Emission Spectroscopy (ICP-OES, iCAP 6500 dual view
Thermo Scientific) was used to assess the actual nickel concentration in each sample. Prior to the
analysis, 100 mg of the catalyst sample was digested in a mixture of 2 ml of nitric acid (67 wt%, Fisher
Scientific) and 3 ml of hydrochloric acid (37 wt%, Fisher Scientific) at 95°C for 3 h.
2
N adsorption/desorption measurements (-196°C) were conducted using Micromeritics Tristar 3000
BET analyzer to determine total surface area, pore size distribution and pore volume. Before the
analysis, the samples were degassed under vacuum for 12 h at 200°C.
X-Ray Diffraction (XRD) was used to identify the crystallinity of the catalyst. Analyses were
conducted on a Bruker D8 Discovery X-Ray Diffractometer with two-dimensional VANTEC-500
detector and CuK
α
(λ = 1.54056 Å) radiation source. The measurements were done with a scan rate of
° min at a tube voltage of 40 kV and a tube current of 20 mA.
Temperature program reduction (H -TPR) experiments were carried out in fixed bed reactor setup
coupled with a calibrated mass spectrometer (Pfeiffer Omnistar GSD 301). Approximately, 100 mg of
-1
5
2
-1
fresh calcined catalyst was heated under Ar atmosphere (40 ml
N
min , 99.999%, Megs) with a rate of
-1
1
0°C min to 310°C for 3 h to remove moisture. After cooling the sample to room temperature, H
2
-1
(
99.999%, Megs) was introduced and the sample was heated at a rate of 8.5°C min to 950°C while the
8

corresponding mass to charge ratio for H
2
O (m/z = 18) was recorded. Flow rates of H
min (subscript N denotes normal condition with T = 0°C and 1 bar).
Temperature programmed oxidation was carried out for both fresh and spent OMA-15Ni-700 (used at
00 °C for 150 h) catalysts using a thermogravimetric analyzer (TGA, Q500 TA instrument). Note, the
fresh catalyst was calcinated, while the spent catalyst was passivated after the long-term experiment.
Approximately, 9 mg of sample was placed in the TGA and heated in presence of N to 130°C at the
rate of 5°C min , while the weight change was recorded. After a holding time of 30 minutes to remove
2
and Ar were
-1
respectively set to 10 and 40 ml
N
4
2
-1
-1
moisture, the sample was then heated in Air to 900°C at 5°C min to combust possible carbon
deposition.
Volumetric hydrogen uptake (chemisorption) was conducted in an Autosorb iQ (Quantachrome) gas
sorption instrument. Approximately 100 mg of fresh calcined sample was placed in a U-shaped quartz
-1
tube, preheated in He at 120°C with a heating rate of 20°C min for 0.5 h, then reduced with H
vol% H in He or 100 vol% H ) at its main peak reduction temperature obtained from H -TPR (Fig. 3)
for another 3 h, then evacuated for 1 h and subsequently cooled to 40°C, where all adsorption
2
(5
2
2
2
2
-1
measurements were taken at pressures ranging from 0.05 bar to 0.8 bar. Specific surface area (m g ),
metal dispersion (D in %) and average crystallite size (d in nm) were calculated based on the amount of
2
H adsorbed assuming an atomic stoichiometric ratio of H/Ni = 1 and a hemispherical cluster with a
-3
nickel density of 8.9 g cm .
Transmission electron microscopy (TEM) of fresh and calcined catalyst was carried out on FEI Tecnai
G2 F20 with 200 kV. Prior to the analysis, a thin (100 nm) section of the catalyst was prepared using a
dual-beam focused ion beam (FEI Helios 600 NanoLab, Hillsboro) equipped with a gallium ion source.
9

The ICP-OES, H
2
-TPR, N
2
-physisorption and H -chemisorption analyses were repeated at least twice
2
for each batch to assure that catalysts prepared with the same recipes at different days had the same
properties.
2
.3 Activity Measurements
The catalyst performance was tested in a fixed bed reactor (FBR) system at atmospheric pressure. The
setup included mass flow controllers, gas mixing station, tube furnace, condenser and gas analyzer
(
mass spectrometer). The reactant gases, Ar (99.999%, Megs), H
2
(99.999%, Megs) and CO (99.99%,
2
Praxair), were mass flow controlled using calibrated Vögtlin red-y smart controller GSC (Switzerland).
The reactor was stainless steel (SS316) with a double tube counter-current flow configuration. The
inner tube (ID = 4.57 mm) was closed with a 5 μm thick stainless steel frit on which the catalyst was
placed. The exit gas line was electrically heated between 150-180°C to avoid condensation of water. A
split stream of the exit gas was analyzed using a calibrated mass spectrometer (Pfeiffer Omnistar GSD
3
01), while the rest of the exit gas was exhausted after condensation. The experiments were performed
in the 300 to 500°C temperature range with gas hourly space velocities (GHSV) ranging from 45-115
cat h-1. The reaction temperature was controlled by a K-type thermocouple inserted into the
catalyst bed. Prior to the experiments, the catalysts were reduced with 20 vol% H in Ar for 3 h at their
respective main peak reduction temperature determined via H -TPR (Fig 3). The reactions were
performed at 1 bar with H /CO ratio of 5:1. Argon gas was used as an internal standard. The catalyst
-1
N
L g
2
2
2
2
was subjected to each experimental condition for at least 3 h. The OMA-15Ni-700 catalyst was used in
a 150 h long-term run to study the catalyst stability. The test was performed at 400°C with a GHSV of
9
1 L
N
g
cat h-1. After the reaction, all catalysts were passivated and then removed, stored and analyzed.
-1
Conversion of carbon dioxide (XCO2), and product selectivity (S
i
) were defined per eqs. 4 and 5.
1
0

푛
̇ꢉꢊꢇ,푖ꢋꢃ 푛
̇ꢉꢊꢇ,ꢌ푢푡
푋
(4)
(5)
ꢅꢆ ꢈ
ꢇ
푛
̇ꢉꢊꢇ,푖ꢋ
푛
̇
푖
푆ꢍ = 푛
̇ꢉꢊꢇ,푖ꢋꢃ 푛
̇ꢉꢊꢇ,ꢌ푢푡
Where ꢎꢍ
hydrocarbons were detected during all activity experiments.
To ensure the reproducibility of the results, three different batches of the catalysts (e.g., OMA-15Ni-
00) were synthesized using the exact procedures and tested in the reactor under the same conditions
and reduced at the same temperature. The results (CO conversion and CH selectivity) varied within 
% and were very small compared to the change in conversion and selectivity obtained for different
catalysts.
̇
is the molar flow rate of species i (i.e., CH
4
, CO). Only CH
4
and CO, but no C
2
+
5
2
4
3
3
3
Results and Discussion
.1 Characterization of fresh catalyst
.1.1 N Adsorption/desorption analysis
adsorption/desorption isotherms and the pore size distributions of the synthesized supports and
3
2
N
2
nickel catalysts are depicted in Fig. 1. All the samples prepared with nitric acid showed a type-IV
isotherm with H1 hysteresis loop, indicating the formation of uniform cylindrical mesoporous structure
[
11]. Acidic media were important in creating mesoporous structure; samples prepared without acid did
not exhibit a type-IV isotherm (Figure 1A and B labeled OMA-500-No Acid) and had a very small total
2
-1
2 -1
surface of 47 m g , of which 43 m g was macroporous (Table 1). Lack of acidity in the synthesis
media resulted in low tendency towards mesopore formation as the copolymer became less hydrophilic
[
16].
The type of acid used during the synthesis played an important role in the formation of organized
mesoporous structures. The catalyst prepared with hydrochloric and citric acid had a smaller
1
1

2
-1
mesoporous surface area (134 m g for OMA-500-HCl) compared to the one prepared with nitric acid
2
-1
3-
-
(
258 m g for OMA-500), see Table 1. This might be related to the role of anion (e.g., NO , Cl ) and
their binding strengths when adsorbed on the positively charged surfactant during the synthesis [17].
3
-
The binding strength of NO ions was sufficient to promote long micelles, hence, a more ordered and
organized structure.
Table 1 Sample overview: Synthesis parameter (target Ni loading, calcination temperature Tcal, acid used) and N
2
adsorption/desorption results (total and mesoporous surface area, average pore size, pore volume).
a
b
c
d
Ni
wt%]
T
[°C]
cal
Acid
used
S
BET
2
S
Meso
2
D
Pore
V
Pore
Sample
-1
-1
3
-1
[
[m g ] [m g ] [nm] [cm g ]
OMA-500-No Acid
OMA-500-HCl
0
500
none
47
43*
11.6
0.14
0
500
400
500
700
400
500
700
900
500
500
500
500
HCl+CA 173
134
7.2
0.31
OMA-400
0
HNO
HNO
HNO
HNO
HNO
HNO
HNO
HNO
HNO
HNO
3
3
3
3
3
3
3
3
3
3
175
262
202
140
242
206
123
217
174
135
153
258
190
124
234
198
115
206
168
122
72
14.0
8.0
0.60
0.52
0.42
0.41
0.60
0.42
0.28
0.52
0.58
0.29
0.25
OMA-500
0
OMA-700
0
8.3
OMA-15Ni-400
OMA-15Ni-500
OMA-15Ni-700
OMA-15Ni-900
OMA-05Ni-500
OMA-30Ni-500
15
15
15
15
5
11.7
10.0
8.2
8.9
9.4
30
15
15
12.7
8.5
15Ni/OMA-500
OMA-15Ni-500-HCl
HCl+CA 83
12.7
a
0
S
BET = BET total specific surface area obtained from adsorption data in the p/p range from 0.05-0.2; all
2
-1
b
reported data are within  4 m g based on repeated analysis. SMeso = mesoporous surface area
determined via subtracting the microporous surface area. DPore = average pore diameters calculated using
Barrett-Joyner-Halenda (BJH) method; VPore = pore volume was obtained at p/p = 0.97; * macropores.
c
d
0
Surface area and pore volume were influenced by the calcination temperature and catalyst loading as
2
-1
depicted in Figure 1C to H. The maximum mesoporous surface area for support (258 m g for OMA-
2
-1
5
5
00) and nickel catalyst (234 m g for OMA-15Ni-500) was found for a calcination temperature of
00°C (Table 1). Calcination temperatures of 400 or 900°C yielded a much smaller surface area of 153
1
2

2
-1
to 115 m g for samples with and without nickel (Table 1). Interestingly, the surface area for the
samples calcined at 400°C exhibited a bimodal pore size distribution (Figure 1D and F), probably due
to the incomplete combustion of the soft template (P123 copolymer). The reduction in surface area and
pore volume for the samples calcined at 900°C might be associated with the reconstruction of the
surface and/or thermal collapsing of the mesoporous structure.
A temperature of 500°C for the calcination has been used as basis to investigate the influence of the
2
-1
nickel loading. Adding nickel reduced the mesoporous surface area slightly (e.g., 258 m g and 234
2
-1
m g for OMA-500 and OMA-15Ni-500, respectively). However, calcined at 700°C the sample with
2
-1
and without nickel had about the same mesoporous surface area of 194  4 m g . In contrast, adding
nickel to the solution containing hydrochloric and citric acid during the synthesis reduced the total
2
-1
surface area significantly from 173 to 83 m g (OMA-500-HCl and OMA-15Ni-500-HCl), implying
that nitric acid is a better reagent. Similar results have been reported for mesoporous silica materials
[
17]. Using hydrochloric and citric acid might require a much longer induction time.
Increase in nickel loading to 30 wt% led to a decrease in the mesoporous surface area, larger pores and
less uniform pore size distribution (Table1 and Figure 1H).
2
-1
The catalyst prepared via impregnation (15Ni/OMA-500) had a mesoporous surface area of 122 m g ,
2
-1
which is significantly smaller than 234 m g for OMA-15Ni-500. A considerable number of pores
might be blocked by adding nickel clusters to mesoporous framework. Thus, following the EISA
technique yielded in highly organized samples with larger surface areas compared to the sample
prepared via impregnation.
1
3

3
.1.2 ICP analysis
ICP-OES was used to confirm the presence of nickel in the catalysts. Results showed that the actual
nickel loadings were close to the targeted theoretical values indicating an effective synthesis of the
ordered mesoporous nickel alumina catalyst (Table 2).
Table 1 Target and actual nickel loadings for ordered mesoporous catalysts determined by ICP-OES.
Catalyst Code
OMA-05Ni-500
OMA-15Ni-500
OMA-30Ni-500
Target [wt%]
Actual [wt%]
4.7 ± 0.1
5.0
15.0
30.0
13.5 ± 1.3
28.6 ± 3.2
3
.1.3 TEM analysis
The fresh and calcined OMA-15Ni-500 catalyst was analyzed with TEM. During the preparation, the
catalyst was cut with a focused ion beam along the axis of the pores and not perpendicular. Thus, the
typical hexagonally shape of the pores with p6mm symmetry could not be confirmed. However,
aligned cylindrical pores were clearly visible (Fig. 2 A and B). The average pore diameter determined
with nitrogen physisorption measurements was 9-11 nm and smaller than observed via TEM. Here, the
cylindrical micelles had a diameter between 50-100 nm containing multiple holes in the wall along the
axis with approximately 10-20 nm in diameter. Highly dispersed nickel clusters with a crystallite sizes
between 2 to 5 nm were observed as well (Fig. 2C), which were much smaller compared to values
reported in the literature [18], but similar to the values determined via hydrogen chemisorption (see
section 3.1.5). Analysis with an image processing software (ImageJ) showed that the average nickel
crystallite size was around 2.8 nm.
1
4

3
.1.4 H
2
-TPR analysis
Temperature programmed reduction (H
2
-TPR) was used to investigate the interactions between nickel
and the alumina support. The H
spectrometry.
2
O signal (m/z = 18) was monitored and recorded by mass
The synthesis method affects the TPR results as illustrated in Fig. 3 A. The catalysts prepared via the
one-pot ESIA technique OMA-15Ni-500 (with nitric acid) and OMA-15Ni-500-HCl (with citric and
hydrochloric acids) have similar catalyst TPR characteristics. From 300°C to 400°C the first H O peaks
2
were observed corresponding to weak NiO interactions, whereas the reduction peaks for strong the NiO
interactions were detected between 575°C and 590°C for OMA-15Ni-500 and OMA-15Ni-500-
HCl, respectively. The impregnated sample (15Ni/OMA-500) exhibited a different TPR behavior with
a much broader H
2
O peak consisting of three peaks; the first at around 400°C, the second at around
6
00°C and the third at 710°C. The impregnated sample, even though calcined at the same temperature
of 500°C produced NiO clusters with a much stronger interaction than other two sample. This might
indicate a higher degree of crystallinity of the NiO clusters from the 15Ni/OMA-500 catalyst (Fig 4A).
With increasing nickel loading the reduction peak did only vary slightly from 575°C to 620°C (Fig.
3
B). The influence of calcination temperature was studied for the OMA-15Ni catalyst. A broader H
2
O
peak at around 550°C was observed for the OMA-15Ni-400 compared to the OMA-15Ni-500 (Fig.
3
7
C). At calcination temperatures of 700°C and 900°C, the TPR peaks shifted to higher temperatures of
80°C and 870°C, respectively, indicating a stronger interaction of the nickel with the alumina support,
which might be related to the formation of nickel aluminates. Small peaks appeared between 400-
00°C for both catalysts calcined above 700°C. This small reduction peak might correspond to weak
NiO interaction with the OMA support. TPR measurements were also conducted on the support (OMA-
00), but did not result in any H O formation (Fig. 3B), which verifies that the observed peaks are
related to the reduction of NiO only.
5
5
2
1
5

3
.1.5 H -uptake
2
The mesoporous surface area of the OMA support impregnated with nickel (15Ni/OMA-500) was
about half of that for the sample prepared with the one-pot EISA method (OMA-15Ni-500, Table 1),
-1
2
but had a 25% higher H -uptake and specific surface area (Table 3, 179 vs. 142 µmol g , 14 vs. 11 m
2
-1
g ) for the same nickel loading. This might suggest that some nickel atoms were encapsulated by the
alumina framework during the one-pot synthesis and not accessible. The sample synthesized with
hydrochloric and citric acid had a much smaller H -uptake and specific surface area (Table 3, 30 µmol
2
-1
2
-1
g and 2.3 m g ), demonstrating that the type of acid used is important for both the formation of the
mesoporous structure and distribution of the active metal.
The effect of the calcination temperature on the H
2
-uptake shows that with higher temperature the H -
2
-1
uptake decreased from 166 to 96 µmol g , while the average crystallite size increased from 3.5 to 6.1
nm for OMA-15Ni-400 to OMA-15Ni-900 (Table 3). The crystallite size determined with TEM were in
the same range and matched nicely with the H -chemisoption measurements.
2
-
1
2
-1
Table 3 H
2
-uptake [µmol g ], specific surface area [m g ], average nickel crystallite size [nm] and
metal dispersion [%].
H
2
-uptake
Specific surface Crystallite size, Metal dispersion,
area [m g ]
Catalyst Code
-
1 a
2
-1 a
d [nm]b
D [%]b
[
µmol g ]
OMA-15Ni-500
142
179
30
11.1
4.1
11.1
1
5Ni/OMA-500
14.0
2.3
3.2
19.4
3.5
4.0
6.1
6.2
4.2
3.8
14.3
2.6
OMA-15Ni-500-HCl
OMA-15Ni-400
OMA-15Ni-700
OMA-15Ni-900
OMA-05Ni-500
OMA-30Ni-500
166
144
96
13.0
11.3
7.5
13.3
11.5
7.7
32
2.6
8.2
295
152
23.1
11.9
12.1
12.2
OMA-15Ni-500*
a
-1
based on replicated analyses all values for H
2
-uptake and specific surface area are within  8 µmol g and
2
-1
b

0.6 m g , respectively. based on ICP measurements (total nickel content) the values reported for the
1
6

average nickel crystallite size and metal dispersion are within d  0.4 nm and D  1.1 % (absolute),
respectively. * reduced with 100% H
2
;
-1
The H
2
-uptake increased as expected with the nickel loading from 32 to 295 µmol g for OMA-05Ni-
5
00 to OMA-30Ni-500. However, the average crystallite size decreased slightly from 6.2 to 4.2 nm. In
most cases, an increase in the metal loading results in an increase of the crystallite size, but for the
ordered mesoporous catalyst prepared via EISA technique the opposite was observed. Again, this
suggest that some nickel atoms were encapsulated by the alumina framework during the one-pot
synthesis, especially for the OMA-05Ni-500 sample with a high alumina/nickel ratio. A minimum
amount of nickel might be needed to achieve a good distribution and dispersion.
Prior to the chemisorption, the samples were reduced with 5 vol% H
temperature (see section 3.1.4 and Fig. 3) for 3 h. For comparison, the OMA-15Ni-500* sample was
reduced with 100 vol% H prior to the chemisorption measurement to assure a complete removal of the
oxygen and formation of the active zero-valent nickel (NiO + H  Ni + H O). Both catalysts reduced
with 5 vol% and 100 vol% H had a comparable H -uptake and average crystallite size (i.e., 142 vs.
2
in He at its TPR-peak
2
2
2
2
2
-1
1
52 µmol g and 4.1 vs. 3.8 nm), indicating a similar degree of catalyst reduction.
3
.1.6 XRD analysis
Fig. 4 shows the XRD patterns of the catalysts synthesized using different methods, calcination
temperatures and nickel loadings. Reference patterns for NiAl , NiO and γ-Al are also shown for
2
O
4
2
O
3
comparison and taken from the International Centre for Diffraction Data (ICDD) with the powder
diffraction file (PDF) #010-0339, #044-1159 and #050-074, respectively. Samples prepared via wet
impregnation (15Ni/OMA-500) and with hydrochloric acid (OMA-15Ni-500-HCl) exhibited distinct
and sharp NiO diffraction peaks (Fig. 4A), representing a higher degree of crystallinity compared to the
OMA-15Ni-500 sample. For the latter, the absence of specific NiAl
2
O
4
, NiO and γ-Al
2
O
3
peaks
indicate that alumina and nickel were homogenously mixed and amorphous in nature. The same was
1
7

observed for the sample calcined at 400°C (Fig. 4B). When the calcination temperature was increased
to 700°C, crystallinity of the support started to increase and small diffraction peaks were observed on
the (3 1 1), (4 0 0) and (4 4 0) lattice planes of γ-alumina. The sample calcined at 900°C had a highly
crystalline γ-alumina structure. However, the sample was not thermally stable as the surface area and
pore volume dropped by 50% compared to sample calcined at 500°C (Table 1).
Diffraction peaks associated with NiO (i.e., 2 = 37.2°, 43.2° and 62.8°; Fig. 4B) were very broad for
the nickel containing catalysts, indicating amorphous structure. The diffraction peaks became more
pronounced with higher nickel loadings and crystalline structure. Again, no peaks associated with
NiAl
2
4
O were detected, even for the OMA-30Ni-500 sample.
To study the structural change during the synthesis, OMA-30Ni-500 was analyzed by XRD before and
after calcination as well as in its reduced form. Prior to the calcination the dried catalyst showed only a
small diffraction peaks for NiO (i.e., 2 = 43.2°) and a broader peak at around 20-26° corresponding to
carbon of the soft template (Fig. 5). The transition to wider and broader peaks at 2 = 43.2° upon
analysis of the calcined catalyst revealed the formation NiO, which was mostly reduced to Ni after 3 h
under reduction conditions (T = 580°C). Only a small peak for NiO was observed at 37.2° as the
catalyst was passivated. The peak at 2 = 66° is associated with the crystal structure of Al
2
3
O that
started to appear at temperatures higher than 500°C.
3
.2 Activity measurements
3
.2.1 Effect of synthesis parameter
As discussed in section 3.1, the synthesis method significantly influenced the catalyst properties and
thus the catalytic activity. The catalyst prepared with nitric acid (OMA-15Ni-500) exhibited a higher
catalytic activity towards CO
2
methanation over the temperature range of 300-500°C compared to the
catalysts prepared via impregnation and with hydrochloric + citric acid, 15Ni/OMA-500 and OMA-
1
8

2
-1
1
5Ni-500-HCl, respectively (Fig. 6). Larger mesoporous surface area (234 vs. 122 vs 72 m g ) and
3
-1
larger pore volume (0.6 vs. 0.29 vs. 0.25 cm g ) of the OMA-15Ni-500 sample made the highly
dispersed active nickel clusters easily accessible, which resulted in higher CO conversions and CH
selectivities at every reaction temperatures (300-500°C) tested (Fig. 6A and B; Table 1). Even though
the H -uptake of the impregnated catalyst (15Ni/OMA-500) was 25% higher than for the OMA-15Ni-
00 sample (Table 3), the mesoporous surface area was 50% smaller leading to a lower CO
2
4
2
5
2
conversion. This indicates that both active and mesoporous surface area are important catalyst
parameters.
The CO
theoretical equilibrium conversion at a temperature of 450°C. Thereafter, the CO
the CH selectivity declined following the equilibrium. At 450°C the OMA-15Ni-500 achieved its
maximum CH selectivity of 0.982  0.01, whereas only a value 0.778 and 0.959 was attained at 300°C
and 500°C, respectively. In all the experiments, no C + and higher hydrocarbons were detected as
illustrated with the MS signal at mass-charge ratio m/z = 0 to 45 in Fig. 7. Thermodynamically,
temperatures lower than 400°C are favorable for a high CO conversion, but due to kinetic limitations
2
conversion for the OMA-15Ni-500 increased until it reached a maximum close to the
2
conversion as well as
4
4
2
2
temperatures of 400 to 450°C were needed. In contrast, CO formation is favoured at higher
temperatures; however, in the current study CO was produced at lower temperatures only. For example,
at 300°C selectivity values of SCO = 0.21, SCO = 0.35 and SCO = 0.56 were determined for OMA-15Ni-
5
00, 15Ni/OMA-500 and OMA-15Ni-500-HCl, respectively. Again, this shows clearly that the EISA
synthesis with the use of nitric acid is the best method for this application.
The reaction mechanisms for the CO and CO methanation differ slightly and depend not only on the
2
catalyst (e.g., active metal, promoter, support) but also the operating conditions. No unified mechanism
exists, rather, multiple reaction pathways have been proposed assuming different elementary steps and
1
9

adsorbed surface intermediates [19]. For example, it has been hypothesized that the C-O bond cleavage
occurs on the catalyst surface via direct dissociation (i.e., from CO2,ads  COads + O  Cads + O) or via
hydrogen-assisted dissociation (e.g., COHads, HCOOads surface intermediate) [20–22]. For the latter, it
is assumed that hydrogen lowers the energy of the C-O bond activation leading eventually to a Cads or
CHx,ads carbon surface intermediate, which is then stepwise hydrogenated to methane. The results of the
current study might indicate, that at lower temperatures the CO
dissociation to form a COads intermediate. COads formation and desorption to CO seems to be faster
than the stepwise hydrogenation to CH . However, without detailed analysis of the surface
2
methanation proceeds via direct
4
intermediates using for example diffuse reflectance FTIR (DRTIFS) experiments, no concrete
conclusions about the reaction mechanism can be drawn.
3
.2.2 Effect of calcination temperature
Catalytic activity also depends on the calcination temperature. With increasing calcination temperature
from 400 to 700°C, the conversion of CO as well as the selectivity of CH for all reaction temperatures
2
4
increased (Fig. 8). A further increase of the calcination temperature to 900°C, however, showed a huge
decline of the catalytic activity. For example, at a reaction temperature of 350°C, the conversion of
CO
2
increased from 0.374 to 0.618 for calcination temperatures of 400 to 700°C and then decreased to
0
.140 for a calcination temperature of 900°C. The results are in good agreement with the surface area
analysis in which the samples calcined at 400 and 900°C (OMA-15Ni-400 and OMA-15Ni-900) had
the smallest mesoporous surface areas and pore volumes. Furthermore, the mesoporous structure of the
OMA-15Ni-900 catalyst became more crystalline as illustrated in Fig. 4B and the specific surface area
was 30% smaller than for the OMA-15Ni-500 and OMA-15Ni-700 sample (Table 3).
3
.2.3 Effect of nickel loading
As the nickel loading increased from 5 wt% to 30 wt% the CO
selectivity increased. For example, at 350°C CO conversion values of 0.23, 0.45 and 0.69, and CH
2
conversion as well as the CH
4
2
4
2
0

selectivity values of 0.783, 0.911 and 0.966 were determined for OMA-05Ni-500, OMA-15Ni-500 and
OMA-30Ni-500, respectively (Fig. 9). It is very likely, that the 30 wt% nickel catalyst provided more
active sites (confirmed with H -chemisorption measurements) for the competitive adsorption of
2
hydrogen and thus stepwise hydrogenation of the assumed COads surface intermediate compared to the
catalyst with a lower nickel loadings. The current results do not allow the determination of the relative
surface coverage of each catalyst. For that steady-state isotopic transient kinetic analysis combined
with diffuse reflectance FTIR and mass spectrometry (SSITKA-DRIFTS-MS) experiments need to be
conducted.
3
.2.4 Effect of space velocity
The influence of the gas hourly space velocity (GHSV) was investigated for the OMA-15Ni-700
catalyst since it exhibited the best overall performance towards CO methanation. The experiments
cat h-1. At temperatures, lower than 400°C the
selectivity declined with increasing GHSV. Whereas at 450 and
00°C the opposite trend was observed. The highest CO conversion and CH selectivity was achieved
cat h-1 at 400°C. The GHSV values used in this study were considerably higher
2
-
1
were performed at GHSV values from 45 to 115 L
N
g
CO conversion as well as the CH
2
4
5
2
4
-1
for GHSV of 45 L g
N
cat h-1 in [12,23], but achieving similar high CH
selectivity
-
1
than those in other reports such as 15 L
N
g
4
values.
3
.2.5 Long-term experiment
The OMA-15Ni-700 catalyst was subjected to a long-term run at 400°C with H
2
/CO
2
= 5 at a GHSV of
9
1 L g
N
cat h-1. After continues operation of more than 150 h on stream, the catalyst did not exhibit any
-1
sign of deactivation, the CO
respectively (Fig. 12 A).
2
conversion and CH selectivity were stable with ~83% and 97%,
4
2
1

3
.3 Characterization of used catalyst
.3.1 N Adsorption/desorption analysis
3
2
BET surface area analysis was carried out for the spent catalysts with different nickel loadings. Surface
2
-1
3
-1
area and pore volume of the spent catalysts were reduced by 40-50 m g and 0.02-0.06 cm g (Table
4
), respectively. This might be associated with thermal shrinkage of the mesoporous framework and/or
sintering of weakly interacted nickel clusters due to the high reduction temperatures of 575 to 620°C,
which were considerably higher than the calcination temperature of 500°C and the reaction
temperatures.
Table 4 BET surface area analysis for fresh and spent catalysts.
2
-1
3
-1
Sample
S
BET (m g )
Vpore (cm g )
fresh
spent
fresh
spent
OMA-05Ni-500
OMA-15Ni-500
OMA-30Ni-500
OMA-15Ni-700*
217
177
0.52
0.50
242
174
206
191
136
176
0.60
0.58
0.42
0.54
0.52
0.38
*
used in 150 h long term experiment
However, this shrinkage did not lead to a collapse of the mesoporous structure as they showed type IV
isotherms with H1-hysteresis and narrow pore size distributions (not shown), demonstrating good
thermal stability of the mesoporous catalyst. The catalyst used for the 150 h test (OMA-15Ni-700),
2
-1
exhibited a similar reduction of surface area (by 30 m g ), confirming the excellent stability.
3
.3.2 XRD of used catalysts
XRD patterns of the spent catalysts of various nickel loadings are displayed in Fig.11. All the catalysts
were subjected to reduction and reaction conditions as each catalyst was used for at least 3 consecutive
days. The transformation of NiO to Ni can be observed clearly via XRD where the diffraction peaks of
Ni became obvious at higher Ni loadings (15-30 wt%) at 2 = 44.4°, 51.8° and 76.3°. The diffraction
2
2

peak at 66° for OMA-30Ni-500 corresponds to γ-Alumina which was most likely formed during the
reduction.
3
.3.3 Temperature programmed oxidation
To further investigate the catalyst stability, temperature programmed oxidation (TPO) of the fresh and
used catalyst (150 h) were conducted. The fresh catalyst exhibited a weight reduction by 4 wt% on dry
basis upon heating in Air to 500°C, which can be associated with the devolatilization of adsorbed
species. A further increase in the temperature resulted in small weight increase for yet unknown
reasons. The spent catalyst showed a completely different behavior; no weight decrease was measured.
Instead, the weight of the used catalyst increased by approximately 3 wt% on dry basis in the range of
1
50 to 300°C with a total increase of 8.4 wt% at 900°C. This, result clearly demonstrates the absence of
any carbon deposition and the oxidation of the passivated catalyst (i.e., 2 Ni + O  2 NiO).
Conclusions
2
4
A systematic approach was followed to investigate the relationship between the synthesis of nickel
containing ordered mesoporous alumina catalysts and its structure and catalytic activity toward CO
2
methanation. Type of acid used, calcination temperature, nickel loading and synthesis method affected
the catalyst morphology and activity significantly. Obtaining a complete mesoporous structure was the
key for a good catalytic performance. Using nitric acid during the “one-step evaporation induced self-
assembly - EISA” synthesis allowed the formation of long and cylindrical micelles with small and
highly dispersed nickel clusters. Catalysts prepared via wet impregnation had smaller surface area and
lower CO methanation activity. Besides the acid used, the calcination temperature was one of the most
2
important synthesis parameters. 700°C was found to be the optimum calcination temperature in the
present study leading to large mesoporous and specific surface areas, and achieving the highest CO
2
2
3

conversion and CH selectivity over the whole temperature range studied. In addition, the catalyst
4
proved to be stable under reaction conditions for more than 150 h without any sign of deactivation.
Understanding how the synthesis parameters of ordered mesoporous alumina catalysts affect the CO
methanation activity is key towards better, more robust and thermally stable catalysts.
2
Acknowledgements
The authors thank Monique Riendeau for conducting the N adsorption/desorption and Ranjan Roy and
2
Andrew Golsztajn for the ICP-OES measurements. The authors acknowledge the financial support
from the Natural Sciences and Engineering Research Council of Canada (NSERC, RGPIN/04685-
2
014).
2
4

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2
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2
6

List of Tables
Table 1 Sample overview: Synthesis parameter (target Ni loading, calcination temperature Tcal, acid
used) and N adsorption/desorption results (total and mesoporous surface area, average pore
2
size, pore volume).
Table 2 Target and actual nickel loadings for ordered mesoporous catalysts determined by ICP-OES.
-1
2
-1
Table 3 H -uptake [µmol g ], specific surface area [m g ], average nickel crystallite size [nm] and
2
metal dispersion [%].
Table 4 BET surface area analysis for fresh and spent catalysts.
2
7

List of Figures
Fig. 1 Top row: N adsorption/desorption isotherms and bottom row: pore size distribution for catalysts
2
synthesized with (A, B) various acids, (C-F) different calcination temperatures and (G, H)
different nickel loadings.
Fig. 2 TEM of fresh OMA-15Ni-500; (A) scale 200 nm, (B) scale 50 nm and (C) scale 5 nm including
crystallite size distribution.
Fig. 3 H
2
-TPR results for OMA catalysts synthesized with different (A) different methods (i.e., with
, HCl, and wet impregnation; OMA-15Ni-500, OMA-15Ni-500 HCl, and 15Ni/OMA-500,
HNO
3
respectively), (B) different nickel loadings and (C) different calcination temperatures.
Fig. 4 XRD patterns of the nickel containing OMA catalysts synthesized with (A) different methods
(
i.e., with HNO
5Ni/OMA-500, respectively), (B) different calcination temperatures and (C) different nickel
loadings. Reference patters of NiAl , NiO and γ-Alumina are PDF #010-0339, PDF #044-
159, and PDF #050-074, respectively.
3
, HCl, and wet impregnation; OMA-15Ni-500, OMA-15Ni-500 HCl, and
1
2
O
4
1
Fig. 5 XRD patterns of OMA-30Ni-500 after drying, calcination and reduction. Reference patters of
NiO and Ni are PDF #044-1159 and PDF #04-010-6148, respectively.
Fig. 6 (A) Conversion of CO
catalysts synthesized with HCl, HNO
00 and 15Ni/OMA-500, respectively (all catalyst calcined at 500°C). Symbols denote
2
and (B) selectivity of CH
4
as a function of reaction temperature for
3
, and wet impregnation; OMA-15Ni-500 HCl, OMA-15Ni-
5
experimental data; solid orange line denotes chemical equilibrium; dotted lines are for guidance
only.
Fig. 7 Mass spectrum of the reaction products for experiments conducted at 350°C and 450°C over
OMA-15Ni-500 catalyst.
Fig. 8 (A) Conversion of CO
2
and (B) selectivity of CH as a function of reaction temperature for
4
catalysts synthesized at different calcination temperatures (i.e., from 400 to 900°C). Symbols
denote experimental data; solid orange line denotes chemical equilibrium; dotted lines are for
guidance only.
Fig. 9 (A) Conversion of CO
2
and (B) selectivity of CH as a function of reaction temperature for
4
catalysts synthesized at different nickel loading (i.e., from 5 to 30 wt%). Symbols denote
experimental data; solid orange line denotes chemical equilibrium; dotted lines are for guidance
only.
2
8

Fig. 10 (A) Conversion of CO
2
and (B) selectivity of CH as a function of GHSV and reaction
4
temperature for the OMA-15Ni-700 catalyst.
Fig. 11 Comparison of XRD patterns of fresh and used catalyst. Reference patters of NiO and Ni are
PDF #044-1159 and PDF #04-010-6148, respectively.
Fig. 12 (A) Conversion of CO
catalyst with H /CO = 5/1, GHSV = 91 L
OMA-15Ni-700 catalyst before and after the long-term test.
2
and selectivity of CH as a function of time for the OMA-15Ni-700
4
g
cat h-1 and T = 400°C. (B) TGA-TPO results for the
-
1
2
2
N
2
9