Highly durable Ru catalysts supported on CeO2 nanocomposites for CO2 methanation

Article abstract of DOI:10.1016/j.apcata.2019.03.011

Taking advantages of the high heat tolerance, large specific surface area, and rough surface morphology created by agglomeration of fine primary particles, a solvothermally prepared CeO₂ aggregate, SiO₂–CeO₂ nanocomposite, and TiO₂–CeO₂ nanocomposite are proposed to be used as sintering-resistant catalyst supports in highly exothermic reactions. Well-dispersed Ru metal catalysts were deposited on support surfaces by the precipitation–deposition method. The methanation of CO₂ by H₂, which is a highly exothermic reaction, was selected as a probe reaction in order to evaluate the catalytic activity and sintering-resistant capability of Ru catalysts deposited on those prepared supports. As expected, the low temperature (150–200 °C) activity and CH₄ production of the Ru catalysts on the prepared CeO₂ aggregate and TiO₂–CeO₂ nanocomposites were better than those on commercial CeO₂ aggregates. Moreover, long-term stability (400 °C, 24 h; 50–300 °C, 10 cycles) was also achieved in the catalysts on these prepared supports.

Full text of DOI:10.1016/j.apcata.2019.03.011

Accepted Manuscript
Title: Highly Durable Ru Catalysts Supported on CeO2
Nanocomposites for CO2 Methanation
Authors: Hien Thi Thu Nguyen, Yoshitaka Kumabe, Shigenori
Ueda, Kai Kan, Masataka Ohtani, Kazuya Kobiro
PII:
S0926-860X(19)30119-X
DOI:
Reference:
https://doi.org/10.1016/j.apcata.2019.03.011
APCATA 17016
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
14 January 2019
11 March 2019
18 March 2019
Please cite this article as: Nguyen HTT, Kumabe Y, Ueda S, Kan K,
Ohtani M, Kobiro K, Highly Durable Ru Catalysts Supported on CeO2
Nanocomposites for CO2 Methanation, Applied Catalysis A, General (2019),
https://doi.org/10.1016/j.apcata.2019.03.011
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Highly Durable
Ru
Catalysts
Supported
on
CeO2
Nanocomposites for CO2 Methanation
Hien Thi Thu Nguyen,a Yoshitaka Kumabe,a Shigenori Ueda,b Kai Kan,a,c,d
Masataka Ohtani,a,c,d* Kazuya Kobiroa,c,d*
a School of Environmental Science and Engineering, Kochi University of Technology.
185 Miyanokuchi, Tosayamada, Kochi 782-8502, Japan.
b Synchrotron X-ray Station at SPring-8, and Synchrotron X-ray Group, National Institute
for Materials Science (NIMS).
1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5148, Japan.
c Laboratory for Structural Nanochemistry, Kochi University of Technology.
185 Miyanokuchi, Tosayamada, Kochi 782-8502, Japan.
d Research Center for Material Science and Engineering, Kochi University of Technology.
185 Miyanokuchi, Tosayamada, Kochi 782-8502, Japan.
E-mail: ohtani.masataka@kochi-tech.ac.jp;
kobiro.kazuya@kochi-tech.ac.jp
URL: http://kobiken.jindo.com/
Graphical abstract
1

Highlights
[1] Solvothermally prepared CeO2-based materials including CeO2 aggregates, SiO2–
CeO2 nanocomposites, and TiO2–CeO2 nanocomposites were examined as
sintering-resistant supports for Ru catalyst in highly exothermic CO2 methanation.
[2] High catalytic activity and durability of these prepared Ru catalysts with low-
temperature catalytic activities and constant catalytic performances were
demonstrated by heat-cycle test and long-term stability tests.
[3] Remained good dispersion of small Ru nanoparticles and no enlargement of CeO2
crystallites of the supports proved the sintering suppression effect of solvothermally
prepared CeO2-based nanocomposites.
2

Abstract
Taking advantages of the high heat tolerance, large specific surface area, and rough
surface morphology created by agglomeration of fine primary particles, a solvothermally
prepared CeO2 aggregate, SiO2–CeO2 nanocomposite, and TiO2–CeO2 nanocomposite
are proposed to be used as sintering-resistant catalyst supports in highly exothermic
reactions. Well-dispersed Ru metal catalysts were deposited on support surfaces by the
precipitation–deposition method. The methanation of CO2 by H2, which is a highly
exothermic reaction, was selected as a probe reaction in order to evaluate the catalytic
activity and sintering-resistant capability of Ru catalysts deposited on those prepared
supports. As expected, the low temperature (150–200 °C) activity and CH4 production of
the Ru catalysts on the prepared CeO2 aggregate and TiO2–CeO2 nanocomposites were
better than those on commercial CeO2 aggregates. Moreover, long-term stability (400 °C,
24ꢀh; 50–300 °C, 10 cycles) was also achieved in the catalysts on these prepared
supports.
Keywords: Ru/CeO2; Ru/SiO2CeO2; Ru/TiO2CeO2; sinter-stable supports; CeO2
composites; CO2 methanation.
1. Introduction
The suppression of thermal sintering of metal nanoparticles and the thermal
deformation of catalyst supports in supported catalysts are critical issues in practice,
especially when catalysts are applied to high temperature catalytic processes [1–3]. In
general, nanosized metal particles and their supports tend to migrate under severe
reaction conditions, such as high temperature, generating enlarged agglomerates with a
morphological change as well as losing their surface area and catalytic activity [4–7].
3

Therefore, it is crucial to retain the original size and morphology of both metal
nanoparticles and catalyst supports. Several anti-sintering strategies in both chemical
and physical approaches, for example, alloying, ligand-assisted pinning, encapsulation
(e.g., core shell and core sheath), and fixing metal nanoparticles on defects of catalyst
supports, have been investigated [8–14]. These strategies, however, are still in progress.
Recently, we developed a versatile and facile one-pot/single-step solvothermal
approach to fabricate metal oxide nanoparticle assemblies such as SiO2, TiO2, ZrO2, and
CeO2 with submicron-sized special morphologies, named micro/mesoporously
architected roundly integrated metal oxide (MARIMO) [15–17]. MARIMO
nanocomposites consisting of several metal oxides are also easily obtained by the
solvothermal approach. The obtained MARIMO assemblies consist of an ultrafine nano-
concave–convex surface with a huge surface area. For example, the TiO2 MARIMO
assembly consists of numerous primary nanoparticles with ca. 5ꢀnm diameter, and the
specific surface area exceeds 200ꢀm2/g and reaches 400ꢀm2/g. Therefore, the TiO2
MARIMO assembly with its large specific surface area and nano-concave–convex
surface structure is expected to be an excellent support for catalyst metal nanoparticles,
because the nano-concave–convex surface structure can disperse metal nanoparticles
well and prevent metal nanoparticles from migrating to aggregates. Indeed, the use of
the TiO2 MARIMO assembly as a catalyst support successfully enhanced the dispersion
of Au nanoparticles on the surface and suppressed the sintering of Au nanoparticles
during highly exothermic CO oxidation [18]. However, the heat tolerance of the TiO2
support turned out to be insufficient in this reaction, and the support gradually started to
be sintered, which impelled us to study better supports with higher heat tolerance.
Among metal oxides utilized for catalytic applications, CeO2 is an attractive material
because of its high and unique catalytic activity derived from the Ce3+/Ce4+ redox change
[19–26]. Additionally, a possible strong interaction between CeO2 and metal
nanoparticles such as Ru, Pd, and Pt would enhance metal dispersion, catalytic
4

efficiency, and suppression of migration of active metal nanoparticles [27–33]. Moreover,
the properties of CeO2 can be tuned or enhanced by mixing with other metal oxides at a
nano-level to yield CeO2 nanocomposites. Actually, we reported that the SiO2–CeO2
nanocomposites with a 1ꢀ:ꢀ1 Si/Ce mole ratio exhibited a large specific surface area
exceeding 300ꢀm2/g, as well as excellent heat tolerance with almost no serious damage
under heating at either 700 °C for 24ꢀh or 850 °C for 3ꢀh, compared to monocomponent
CeO2 [34]. With these advantages, CeO2 assemblies and CeO2 nanocomposites with
ultrafine surface roughness, which are synthesized by our original solvothermal
synthesis method, are highly expected to be effective and sintering-resistant catalyst
supports, especially when the reaction is performed at an elevated temperature.
Herein, we report the heat tolerance of newly prepared CeO2-based materials, as
sintering-resistant supports, including CeO2 assemblies, SiO2–CeO2 nanocomposites,
and TiO2–CeO2 nanocomposites. Methanation of CO2 by H2 catalyzed by Ru, which is
one of the most important reactions for CO2 recycling [35–37], was selected as a probe
reaction to estimate sintering resistance of the catalysts, since the reaction is known as
a highly exothermic reaction (∆H298K = -165 kJ/mol). When the methanation is performed
in a small scale, temperature control of the catalyst bed is relatively easy because of
small amount of generated reaction heat from the exothermic reaction. However, in the
case of large scale reaction, such as bench-top scale or more, total amount of the
reaction heat is quite much. In this case, temperature control of the catalyst bed becomes
seriously important. If the methanation reaction meet “runaway (out of control)”
accidentally, the catalysts will be exposed to high temperature, which surely leads to
sintering of the catalyst Ru nanoparticles. Once sintering of the Ru nanoparticles occurs,
catalytic activity will be lost and no recovery can be expected even after the catalyst bed
temperature is lowered. In order to avoid such catastrophic situation, the reactant gases
are sometimes diluted by inert gas or the reactor is cooled from the outside not to reach
such high temperature, which need more cost practically. Then, if the catalysts for the
5

exothermic reactions could tolerate such accidental and/or unexpected temperature
jump, it would be very meaningful to realize sintering-resistant catalytic systems. Thus,
for exothermic CO2 methanation, sintering suppression of the supported metal
nanoparticle catalyst under unexpected high temperature conditions is extremely
important in order to keep the catalysts stable.
2. Experimental Section
2.1. Materials
Methanol, cerium nitrate hexahydrate (Ce(NO3)3·6H2O), tetraethyl orthosilicate
(Si(OEt)4), titanium tetraisopropoxide (Ti(OiPr)4), sodium hydroxide (NaOH), ruthenium
chloride
trihydrate
(RuCl3·3H2O),
N,N,N′,N′-tetramethylethylenediamine
((CH3)2NCH2CH2N(CH3)2, TMEDA), commercial cerium oxide (commercial CeO2), and
washed sea sand (425~850ꢀμm) were purchased from FUJIFILM Wako Pure Chemical
Corporation. All chemicals were used as received without further purification.
2.2. Preparation of the CeO2 Assembly, SiO2–CeO2 Nanocomposite, and TiO2–CeO2
Nanocomposite
Synthesis of the CeO2 assembly and SiO2−CeO2 nanocomposite was carried out by the
solvothermal reaction of a precursor solution including Ce(NO3)3·6H2O (0.1 mol/L), Si(OEt)4
(0.1 mol/L), and TMEDA (0.2 mol/L) in methanol (3.5 mL) at 300 °C according to a similar
procedure described previously [34]. The TiO2–CeO2 nanocomposite with a 1:1 molar ratio
of Ti/Ce was prepared by a similar procedure using Ti(OiPr)4 instead of Si(OEt)4.
2.3. Preparation of Ru Catalysts
Supported Ru catalysts were prepared by the precipitation–deposition method with
an intended Ru amount of 3ꢀwt%. The powdery support (1ꢀg, commercially available and
6

solvothermally prepared) was dispersed in 27ꢀmL of reverse osmosis water prior to the
addition of 80ꢀmg of RuCl3·3H2O. NaOH solution (0.1ꢀmol/L) was added slowly to the
suspension with vigorous stirring to adjust the pH to 8.0~8.5. The mixture was vigorously
stirred for another 3ꢀh, and then the mixture was centrifuged. The obtained precipitate
was collected and washed three times with water and dried at 60 °C for 12ꢀh in an oven.
The obtained powdery product was reduced in a mixed gas stream of H2 and N2 (40%
H2 and 60% N2) at 200 °C for 3ꢀh. The obtained 3ꢀwt% Ru catalysts supported on the
commercial CeO2 assembly, solvothermally prepared CeO2 assembly, solvothermally
prepared SiO2–CeO2 nanocomposite, and solvothermally prepared TiO2–CeO2
nanocomposite are denoted as Ru/commercial CeO2, Ru/CeO2, Ru/SiO2–CeO2, and
Ru/TiO2–CeO2, respectively.
2.4. Characterization
Transmission electron microscopy (TEM) images were taken using a JEOL JEM-
2100F microscope. High-angle annular dark-field scanning TEM (HAADF-STEM) and
energy-dispersive X-ray spectroscopy (EDX) were performed on an Oxford INCA X-Max
80 EDX spectrometer with the above TEM instrument. Inductively coupled plasma optical
emission spectroscopy (ICP-OES) was measured on a Hitachi High-Tech Science
PS3520UV-DD spectrometer. Nitrogen (N2) adsorption/desorption experiments were
conducted on a MicrotracBEL BELSORP-mini II. The specific surface area was
calculated using the Brunauer–Emmett–Teller (BET) method through obtained N2
adsorption–desorption isotherms. The crystalline phases of the resultant nanoparticle
nanocomposites were identified by X-ray diffraction (XRD) on a Rigaku SmartLab
diffractometer using graphite-monochromated Cu Kα radiation. Hard X-ray photoelectron
spectroscopy (HAXPES) measurements were performed at BL15XU of SPring-8. The
excitation photon energy was fixed to 5.95ꢀkeV, and the total energy resolution was set
to 240ꢀmeV.
7

The reducibility of these prepared catalysts was studied by temperature-programmed
reduction of hydrogen (H2-TPR) with a flow-type reactor (BELCAT II; MicrotracBEL). The
catalysts (50 mg) were pretreated in a mixed gas flow of O2/Ar (20% O2 and 80% Ar) at
300 °C for 30ꢀmin prior to H2-TPR. The H2-TPR experiment was executed in a
temperature range of 40–900 °C, with a ramping rate of 10 °C/min, in a mixed gas of 6%
H2 in Ar.
Pulsed chemisorption using CO gas (CO chemisorption) was carried out to
characterize the Ru active sites of the catalysts. 100ꢀmg of the catalysts was pretreated
by H2 at 120 °C for 15ꢀmin prior to CO adsorption with a mixed stream of 5% CO in He
at room temperature.
2.5. Evaluation of Catalytic Activity
The catalytic activity of the Ru catalysts for CO2 methanation was tested using the
flow-type reactor (BELCAT II; MicrotracBEL). Before performing the reaction, the catalyst
sample (100ꢀmg) diluted by the washed sea sand (100ꢀmg) was packed into the reactor
and then pretreated under a H2 stream at 120 °C for 15ꢀmin. Then, CO2 methanation was
conducted by feeding an inlet gas stream of CO2/H2/Ar (5% CO2, 20% H2, and 75% Ar)
at a total flow rate of 20ꢀmL/min. The outlet gases were analyzed by a gas chromatograph
(GC3200; GL Sciences) using a thermal conductivity detector.
The stability and durability of the catalysts against sintering were evaluated through
three different experiments: three-run test, 10-cycle test, and long-term stability test. In
these catalytic tests, the reaction processes were sequentially repeated several times or
constantly kept under high temperature conditions. For the three-run test, the whole CO2
methanation process in a temperature range of 150–600 °C was sequentially repeated
three times. The temperature step and the reaction time at each temperature were kept
at 50 °C and for 30ꢀmin, respectively. The 10-cycle test was carried out by repeating CO2
methanation 10 times at a low temperature of 50 °C and a high temperature of 300 °C
8

alternately. In each cycle of the 10-cycle test, the reaction time was kept at 30 min at
each temperature. The long-term stability test was performed over a period of 24ꢀh at a
constant temperature of 400 °C. In the cases of the three-run test and the 10 cycle test,
100 mg of catalysts were used, while 25 mg of catalysts were used in the long-term
stability test in expectation of earlier activity loss of the catalysts.
3. Results and Discussion
3.1. Properties of Ru Catalysts Supported on CeO2-Based Materials
Due to the fact that the large specific surface area and excellent heat tolerance of
catalyst supports are quite suitable for good dispersion and stabilization of catalyst
metals, our reported SiO2–CeO2 nanocomposites are potential supports to suppress the
sintering of catalyst metals. We also selected the TiO2–CeO2 nanocomposite as a
different type of support here, because some interactions between catalyst metals and
TiO2 supports can be expected [38,39].
Dispersion of metal nanoparticles on supports is an essential factor that directly
influences the catalytic activity in supported catalysts. The properties of the as-prepared
catalysts were investigated using TEM, HAADF-STEM, EDX, CO chemisorption, XRD,
and HAXPES measurements (Table 1, Figure 1, and Figures S1–S5). The TEM
observations and CO chemisorption of the as-prepared catalysts reveal that the Ru
particle size on commercial CeO2 (Ru/commercial CeO2) was quite large (7.7ꢀnm by
TEM) as compared to those of Ru on the prepared CeO2 (Ru/CeO2), the SiO2–CeO2
nanocomposite (Ru/SiO2–CeO2), and the TiO2–CeO2 nanocomposite (Ru/TiO2–CeO2)
(Figure 1). In the case of Ru/CeO2, the Ru nanoparticles were too small to be recognized
in the TEM image. In the cases of Ru/SiO2–CeO2 and Ru/TiO2–CeO2, the average Ru
nanoparticle sizes were measured to be so small, 0.6 and 1.0ꢀnm, respectively.
The average crystallite sizes of CeO2 in Ru/CeO2 (6.1ꢀnm, Scherrer equation),
Ru/SiO2–CeO2 (1.7ꢀnm), and Ru/TiO2–CeO2 (2.2ꢀnm) are much smaller than that in
9

Ru/commercial CeO2 (49ꢀnm), resulting in a much larger specific surface area of
Ru/CeO2 (93ꢀm2/g), Ru/SiO2–CeO2 (180ꢀm2/g), and Ru/TiO2–CeO2 (177ꢀm2/g) than that
of Ru/commercial CeO2 (4.0ꢀm2/g). In addition, the Ru dispersivity of Ru/CeO2, Ru/SiO2–
CeO2, and Ru/TiO2–CeO2 determined by CO pulse experiment is much better than that
of Ru/commercial CeO2. Homogeneous distribution of Ru nanoparticles was also
confirmed by STEM/EDX measurement of the supported Ru catalysts. The Ru contents
quantified by ICP-OES analysis were 3.7, 3.4, 3.0, and 2.9ꢀwt% for Ru/commercial CeO2,
Ru/CeO2, Ru/SiO2–CeO2, and Ru/TiO2–CeO2, respectively. These results are consistent
with Ru contents from STEM-EDX analysis (Table 1). Thus, the Ru nanoparticle and
CeO2 crystallite sizes of the prepared Ru/CeO2, Ru/SiO2–CeO2, and Ru/TiO2–CeO2 are
much smaller than those of Ru/commercial CeO2.
Next, the XRD patterns of the prepared catalysts Ru/CeO2, Ru/SiO2–CeO2, and
Ru/TiO2–CeO2 showed only reflection peaks ascribed to cubic CeO2, whereas no peaks
derived from Ru, SiO2, and/or TiO2 were detected (Figure S4), indicating that SiO2 and
TiO2 in the nanocomposites existed in amorphous phases. The existence of Ru on the
CeO2 and SiO2–CeO2 supports was also confirmed by HAXPES measurements (Figure
S5).
The H2-TPR experiments were performed in order to clarify the surface properties of
the catalyst supports and Ru nanoparticles on the supports (Figure 2). As a result, three
types of responses were mainly observed for the prepared Ru/CeO2, Ru/SiO2–CeO2,
and Ru/TiO2–CeO2 (Figures 2b–2d), whereas two responses were found for
Ru/commercial CeO2 (Figure 2a). The peak that appeared at 100–200 °C can be
attributed to the reduction of ruthenium oxides [40,41]. The peaks at 250–350 °C and
around 800 °C can be ascribed to the reduction of the surface and the bulk of CeO2,
respectively [27,42–44]. In addition, a peak was observed at 550–600 °C only in the case
of Ru/TiO2–CeO2, which can be assigned to the reduction of TiO2 [45]. Therefore, the
surface oxidation states of the prepared Ru/CeO2, Ru/SiO2–CeO2, and Ru/TiO2–CeO2
10

are different from that of Ru/commercial CeO2. Moreover, in the case of the prepared
Ru/CeO2, Ru/SiO2–CeO2, and Ru/TiO2–CeO2, the reduction of Ru species around 100–
200 °C initiated at a lower temperature range, suggesting the existence of some
interactions between Ru and the supports favorable to H2 reduction. Notably, H2 uptakes
of 2.90 mmol/g for Ru/SiO2‒CeO2 and 1.99 mmol/g for Ru/TiO2‒CeO2 at below 250°C
were considerably higher than those of the Ru catalysts on the monocomponent CeO2,
which would indicate their greater reduction of Ru species (Table S1). Moreover, those
values remarkably exceeded the theoretically required H2 amount of 0.59 mmol/g to
convert RuO2 to metallic Ru, suggesting that a partial reduction of the composite
supports are included.
3.2. Catalytic activity and durability of Ru catalysts supported on CeO2-Based
Materials
The prevention of sintering is an important requirement for catalysts to achieve a long
lifetime. Here highly exothermic CO2 methanation was selected as a probe reaction in
order to check the sintering resistance of the catalysts. The catalytic activity and
durability of the prepared catalysts were studied by a three-run experiment in a
temperature range of 150–600 °C. The obtained CH4 yields in the range of 100–400 °C
are given in Figure 3. The detailed CO2 consumption, CH4 yield, and CO formation over
the catalysts are shown in Figure S6.
When Ru/commercial CeO2 was used as a catalyst, the maximum CH4 yield was
achieved at 350 °C to reach 55% (Figure 3a). However, when the prepared Ru/CeO2,
Ru/SiO2–CeO2, and Ru/TiO2–CeO2 catalysts were used, higher yields of ca. 80% were
obtained (Figures 3b–3d). The biggest advantage of our composite catalysts was seen
in repeated reactions. When the reaction was repeated twice in the case of
Ru/commercial CeO2, the CH4 yield profile drastically shifted to a higher temperature;
namely, the catalytic activity clearly decreased (Figure 3a). On the contrary, almost no
11

deactivation was observed in the prepared Ru/CeO2 even after the third run of the
reaction (Figure 3b). Interestingly, the low temperature activity at 150–200 °C was
obviously improved in the cases of Ru/SiO2–CeO2 and Ru/TiO2–CeO2 nanocomposites
when the reactions were repeated (Figures 3c and 3d). It is difficult to put forward a
conclusive discussion. However, this could be ascribed to the enhanced interactions
between the support nanoparticles and the Ru metal particles under high temperature
conditions. A similar positive effect of heating on the catalytic activity of supports was
reported for calcined Ru/CeO2 and Ru/TiO2 catalysts with strengthened interactions
between Ru and ceria supports as well as between Ru and titania supports [32,46,47].
In order to clarify the sintered structure of both metal particles and supports, the
morphological changes of the catalysts after the three-run test were directly investigated
by TEM and HAADF-STEM observations (Figure 4), and the estimated Ru mean
diameters are listed in Table 1.
Figure 4a shows the existence of large-sized Ru nanoparticles deposited on the
commercial CeO2 support, whose particle size changed from 7.7 to 9.6ꢀnm (Table 1).
However, the existence of a small amount of much larger Ru nanoparticles (20–30ꢀnm)
was confirmed (Figure 5a), indicating that the conditions of the three-run test were so
severe for the Ru/commercial CeO2 catalyst to cause sintering. In contrast, uniform Ru
dispersions on CeO2, SiO2–CeO2, and TiO2–CeO2 were retained after the three-run test,
as demonstrated by HAADF-STEM images and EDX mappings, where no Ru
agglomerations were observed (Figures 4b–4d). The growth of only small Ru
nanoparticles was observed in the cases of Ru/SiO2–CeO2 and Ru/TiO2–CeO2, and the
particle size still remained 1–2ꢀnm (Table 1, Figures 5b and 5c). Interestingly, no
enlargement of the CeO2 crystallites was observed even after the three-run test (Table
1), revealing the effective sintering suppression of Ru nanoparticles as well as CeO2
crystallites on/in both SiO2–CeO2 and TiO2–CeO2 supports, where the introduced
amorphous SiO2 and TiO2 in the supports could suppress the crystal growth of CeO2
12

nanoparticles. These discussed results indicate that remained good dispersion of Ru
nanoparticles on the prepared CeO2-based supports during CO2 methanation could be
a reason for maintained activity of the catalysts.
As a result, the Ru/TiO2–CeO2 nanocomposite exhibited the best performance of not
only the low temperature activity and the maximum yield, but also the sintering resistance
of Ru nanoparticles and CeO2 crystallite in CO2 methanation. As presented in Figure 6,
the Ce3d HAXPES spectra of the supports showed Ce3+/Ce4+ coexistence in the
prepared CeO2 composite supports, whereas only the Ce4+ oxidation state was formed
in the case of the commercial CeO2. The peak positions of Ce4+ and Ce3+ marked by
yellow and green in Figure 6, respectively, were obtained from the literatures [38,48].
Moreover, it is noted that the TiO2–CeO2 composite exhibited a favored Ce3+ formation
that could be responsible for its higher catalytic activity as compared to the prepared
monocomponent CeO2. These results agree well with the previous literature where the
Ce–Ti interface was reported to strongly promote the Ce3+ existence because of the
electron contact [38]. It is reported that Ce3+ sites created on the CeO2 surface are
involved in activation of CO2, contributing to a better activity of catalysts [19,27,49,50].
Indeed, Ce3+ sites act as active sites for adsorption and dissociation of CO2 to form
carbonaceous intermediates which are rapidly hydrogenated into CH4 molecules.
Therefore, the abundant Ce3+ formation on the surface of the CeO2-based catalysts is
advantage in promoting CO2 methanation. Thus, the prepared CeO2, SiO2–CeO2, and
TiO2–CeO2 nanocomposites turned out to be promising sintering-resistant catalyst
supports for the elevated temperature process of CO2 methanation.
Calcined Catalysts
According to the above-mentioned activity enhancement of Ru catalysts supported
on CeO2-based materials after the three-run test, CeO2, SiO2–CeO2, and TiO2–CeO2
were calcined at 500 °C for 2ꢀh prior to Ru deposition in the expectation of catalytic
13

activity enhancement, yielding Ru/calcined CeO2, Ru/calcined SiO2–CeO2, and
Ru/calcined TiO2–CeO2, respectively. The crystallite sizes of CeO2 in Ru/CeO2,
Ru/calcined SiO2–CeO2, and Ru/calcined TiO2–CeO2 (Table 2) are quite comparable to
those after the three-run test (Table 1). The specific surface areas of the calcined
catalysts are a little bit larger than those after the three-run test. When the calcined
catalysts were used for CO2 methanation, Ru/calcined CeO2 and Ru/calcined TiO2–CeO2
exhibited higher activities than that of Ru/calcined SiO2–CeO2 (Figure 7). The low-
temperature catalytic activity of Ru/calcined CeO2 (150‒250 °C) towards CO2
methanation in the present study is comparable with those in the literatures [19,27].
Notably, an obvious enhancement of low temperature activity at 150–200 °C was
observed in the case of Ru/calcined TiO2–CeO2.
3.3. Long-Term Stability Test of the Ru Catalysts
As mentioned in the former section, the prepared CeO2, SiO2–CeO2, and TiO2–CeO2
supports led to a high activity and high sintering resistance for Ru catalysts as compared
to the commercial CeO2 support. In order to further support the aforementioned
discussions, the stability and durability of the catalysts were investigated through two
different experiments: a 10-cycle test and a long-term experiment. In the 10-cycle test,
severe heat stress was placed on the catalysts, where CO2 methanation was repeated
10 times at a low temperature (50 °C) and a high temperature (300 °C) alternatively. In
each cycle, the reaction time was 30 min at each reaction temperature. Figure 8a clearly
shows stable CH4 production over Ru/CeO2 during the 10-cycle experiment. Likewise, a
stable catalytic performance with almost no change in the CH4 yield between the 1st and
the 10th cycles was observed when the Ru/TiO2–CeO2 nanocomposite was used (Figure
8c). However, under similar conditions, the catalytic performance over Ru/commercial
CeO2 decreased gradually by each cycle, clearly indicating catalytic activity loss by
thermal sintering (Figure S7). In addition, the long-term stability of Ru/CeO2 was
14

confirmed by a constant CH4 yield as the time-on-stream increased to 24ꢀh (Figures 8b
and 8d) at 400 °C. In contrast, the long-term stability test for 24 h at 400 °C with the use
of Ru/commercial CeO2 catalyst resulted in 2% initial yield of CH4, but the yield reduced
gradually to 0% within 10 h. Thus, the three prepared CeO2, SiO2–CeO2, and TiO2–CeO2
nanocomposites were effective supports in enhancing the catalyst activity and durability
of Ru catalysts for CO2 methanation. Their further applications in other high temperature
systems are highly expected.
4. Conclusions
A
porous CeO2 aggregate, SiO2–CeO2 nanocomposite, and TiO2–CeO2
nanocomposite, all of which consisted of numerous amounts of small primary particles
(<5ꢀnm) with a huge surface area, were solvothermally prepared as catalyst supports for
highly exothermic reactions. Ru nanoparticles were deposited on the support surface by
the precipitation–deposition method. The prepared catalyst supports of CeO2 aggregate,
SiO2–CeO2 nanocomposite, and TiO2–CeO2 nanocomposite dispersed Ru nanoparticles
very well on the surface. When they were used as catalysts for CO2 methanation by H2,
the Ru catalysts on the prepared CeO2 aggregate and TiO2–CeO2 exhibited a better low
temperature (150–200 °C) activity and long-term stability (400 °C, 24ꢀh) than those on
the commercial CeO2 aggregate. Thus, we succeeded in preparing sintering-resistant
catalyst supports for high temperature reactions.
Acknowledgments
We acknowledge the financial support from the Creation of New Business and
Industry Program through the Kochi Prefectural Industry-Academia-Government
Collaboration Research Promotion Operation and by the Japan Society for the Promotion
of Science Grants-in-Aid for Scientific Research (JSPS KAKENHI, grant number
15

15K06560). We also acknowledge for TEM measurements from Nanotechnology and
Research Center for Material Science and Engineering, Kochi University of Technology.
The HAXPES measurements were performed under the approval of the NIMS
Synchrotron X-ray Station (proposal numbers 2014A4900, 2014B1051, 2015A4900,
2016A4905, 2016A4905, and 2018A4910). In addition, the authors would like to thank
the staffs of BL15XU, NIMS, and SPring-8 for their help at the beamline. The authors are
also grateful to HiSOR, Hiroshima Univ. and JAEA/SPring-8 for the development of
HAXPES at BL15XU of SPring-8.
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Table 1. Properties of CeO2 assemblies and CeO2 nanocomposites supported Ru catalysts.
Ru particle size (nm)
Ru content (wt%)
CeO2
Specific
Ru dispersivityc
Samplea
crystallite
sizef (nm)
surface
(%)
From TEMb
7.7 ± 3.2
From CO chemisorptionc
From ICPd From EDXe
areag (m2/g)
Ru/commercial CeO2
Ru/CeO2
15.0
1.6
2.6
1.6
—
9.0
83.4
52.3
81.2
—
3.7
3.4
3.0
2.9
4.0
3.3
3.1
2.4
1.2
4.2
4.0
3.1
1.9
4.2
4.5
3.2
49
4.0
h
—
6.1
1.7
2.2
48.7
10.3
1.7
2.1
93
As-prepared
Ru/SiO2–CeO2
Ru/TiO2–CeO2
Ru/commercial CeO2
Ru/CeO2
0.6 ± 0.3
1.0 ± 0.2
9.6 ± 4.8
180
177
3.9
g
—
—
—
39.6
61.1
68.3
After a three-
run test
Ru/SiO2–CeO2
Ru/TiO2–CeO2
1.2 ± 0.4
1.4 ± 0.3
—
—
—
—
a The Ru catalysts (3 wt%) deposited on commercial CeO2, prepared CeO2, SiO2–CeO2, and TiO2–CeO2 supports.
b Estimated by TEM images from at least 50 Ru particles.
c Calculated from CO adsorption measurements.
d Quantified by ICP-OES measurement.
e Quantified by STEM/EDX analysis on TEM.
f The Scherrer equation was used.
g The BET method was used. h Too small to be estimated by TEM.
20

Table 2. CeO2 crystallite size and specific surface area of the Ru catalysts
prepared on the different calcined supports.
Specific surface areac
Samplea
CeO2 crystallite sizeb (nm)
(m2/g)
Ru/calcined CeO2
8.9
2.6
2.3
63.5
120
119
Ru/calcined SiO2–CeO2
Ru/calcined TiO2–CeO2
a The Ru catalysts (3 wt%) deposited on the supports calcined at 500 °C for 2ꢀh.
b Estimated by Scherrer equation.
c The BET method was used.
21

Figure captions
Figure 1. TEM and/or STEM/EDX images and Ru size distribution on the prepared
catalysts: (a) Ru/commercial CeO2, (b) Ru/CeO2, (c) Ru/SiO2–CeO2, and (d) Ru/TiO2–
CeO2.
Figure 2. H2-TPR profiles of the prepared catalysts: (a) Ru/commercial CeO2, (b)
Ru/CeO2, (c) Ru/SiO2–CeO2, and (d) Ru/TiO2–CeO2.
Figure 3. CH4 production of a three-run test over the catalysts: (a) Ru/commercial CeO2,
(b) Ru/CeO2, (c) Ru/SiO2–CeO2, and (d) Ru/TiO2–CeO2. The reaction process for each
run was carried out in a temperature range of 150600 °C at a gas flow rate of 20ꢀmL/min
(5% CO2, 20% H2, and 75% Ar). The described process of each run was then
sequentially repeated three times. For ease of recognition of the efficiency difference in
catalysis, a small amount of catalyst (100ꢀmg) was used for this experiment. When a
larger amount of catalyst (1ꢀg) was used, 100% conversion of CO2 and 100% yield of
CH4 were easily achieved at 250 °C.
Figure 4. TEM images, HAADF-STEM images, and EDX mappings of Ce, Si, and Ru
elements of the catalysts: (a) Ru/commercial CeO2, (b) Ru/CeO2, (c) Ru/SiO2–CeO2, and
(d) Ru/TiO2–CeO2 after the three-run test.
Figure 5. Ru particle size distributions in the as-prepared state and after the three-run
test of the catalysts: (a) Ru/commercial CeO2, (b) Ru/SiO2–CeO2, and (c) Ru/TiO2–CeO2.
Figure 6. HAXPES Ce3d spectra of (a) commercial CeO2, (b) prepared CeO2, and (c)
prepared TiO2–CeO2. The peak positions of Ce4+ and Ce3+ marked by yellow and green,
respectively, were obtained from the literature [38,48].
Figure 7. CH4 production of CO2 methanation over (a) Ru/calcined CeO2, (b) Ru/calcined
SiO2–CeO2, and (c) Ru/calcined TiO2–CeO2. The CO2 methanation test was carried out
in the temperature range of 150–600 °C at a gas flow rate of 20ꢀmL/min (5% CO2, 20%
H2, and 75% Ar). The reaction time for each temperature step was kept at 30ꢀmin.
22

Figure 8. CH4 yield of CO2 methanation using (a, b) Ru/CeO2 and (c, d) Ru/TiO2–CeO2
in the 10-cycle test at 50 °C and 300 °C (the graph represents only the results at 300 °C)
and for 24ꢀh at 400 °C, respectively. In each cycle of the 10-cycle test, the reaction time
was 30 min at each reaction temperature of 50 °C and 300 °C. In the cases of (a) and
(c), 100 mg of catalysts were used, while 25 mg of catalysts were used in the cases of
(b) and (d) in expectation of earlier activity loss of the catalysts in the long-term stability
tests.
23

Figure 1.
Figure 2.
24

Figure 3.
25

Figure 4.
26

Figure 5.
Figure 6.
27

Figure 7.
Figure 8.
28