CO2 methanation over Ni supported on Carbon–ZrO2: An optimization of the composite composition

Abstract

Currently, the most common catalysts for CO2 methanation reaction are based on Ni supported on metal oxides. However, such catalysts require high operation temperatures and present stability issues, which have been tackled by the use of expensive metal oxides such as CeO2 or ZrO2.

In this study, the decrease in the amount of ZrO2 in ZrO2-based catalysts was addressed through the preparation of composites of ZrO2 and carbon materials (activated carbon (AC) and carbon nanotubes (CNTs)). The optimization of the carbon:ZrO2 ratio demonstrated an optimal value of 50:50 in the case of AC:ZrO2, and 70:30 for CNT:ZrO2. With this composite composition, the possibility of enhancing the performance of the catalyst by functionalizing the carbon material was evaluated, and it was demonstrated that reduced AC (AC-R) with increased Lewis basic sites showed the best performance for AC-based composites, achieving a CO2 conversion of 79 % and CH4 selectivity of 98.9 % at 400 °C, whereas the catalysts supported on the pristine CNT:ZrO2 composite presented the highest CO2 conversion of 82.1 % and CH4 selectivity of 99.3 % at 400 °C.

Notably, promotion with Fe was studied in the best performing support (CNT:ZrO2 (70:30) and it was shown that it enabled an improvement in terms of CO2 conversion and optimal temperature, achieving a CO2 conversion of 85 % and CH4 selectivity of 99.5 % at a lower temperature of 370 °C. This catalyst demonstrated to be highly stable for 70 h of time on stream with no apparent modifications on its chemical composition or microstructure.

This work demonstrates that combining the properties of carbon materials and ZrO2 can be an interesting approach to obtain high performing catalysts for CO2 methanation.

Full text

Catalysis Today 422 (2023) 114215
Available online 20 May 2023
0920-5861/© 2023 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-
nc-nd/4.0/).
CO
2
methanation over Ni supported on Carbon–ZrO
2
: An optimization of
the composite composition
Inˆ
es F. Quatorze
a
,
b
,
1
, Liliana P.L. Gonçalves
a
,
b
,
1
, Yury V. Kolen’ko
c
, O. Salom´
e G.P. Soares
a
,
b
,
*
,
M. Fernando R. Pereira
a
,
b
a
LSRE-LCM - Laboratory of Separation and Reaction Engineering, Laboratory of Catalysis and Materials, Faculty of Engineering, University of Porto, Rua Dr. Roberto
Frias, 4200-465 Porto, Portugal
b
ALiCE - Associate Laboratory in Chemical Engineering, Faculty of Engineering, University of Porto, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal
c
Nanochemistry Research Group, International Iberian Nanotechnology Laboratory, Avenida Mestre Jos´
e Veiga, 4715-330 Braga, Portugal
ARTICLE INFO
Keywords:
CO
2
Methanation
Ni-based catalysts
ZrO
2
Carbon nanotubes
Activated carbon
ABSTRACT
Currently, the most common catalysts for CO
2
methanation reaction are based on Ni supported on metal oxides.
However, such catalysts require high operation temperatures and present stability issues, which have been
tackled by the use of expensive metal oxides such as CeO
2
or ZrO
2
.
In this study, the decrease in the amount of ZrO
2
in ZrO
2
-based catalysts was addressed through the prepa-
ration of composites of ZrO
2
and carbon materials (activated carbon (AC) and carbon nanotubes (CNTs)). The
optimization of the carbon:ZrO
2
ratio demonstrated an optimal value of 50:50 in the case of AC:ZrO
2
, and 70:30
for CNT:ZrO
2
. With this composite composition, the possibility of enhancing the performance of the catalyst by
functionalizing the carbon material was evaluated, and it was demonstrated that reduced AC (AC-R) with
increased Lewis basic sites showed the best performance for AC-based composites, achieving a CO
2
conversion of
79 % and CH
4
selectivity of 98.9 % at 400 ◦C, whereas the catalysts supported on the pristine CNT:ZrO
2
com-
posite presented the highest CO
2
conversion of 82.1 % and CH
4
selectivity of 99.3 % at 400 ◦C.
Notably, promotion with Fe was studied in the best performing support (CNT:ZrO
2
(70:30) and it was shown
that it enabled an improvement in terms of CO
2
conversion and optimal temperature, achieving a CO
2
conversion
of 85 % and CH
4
selectivity of 99.5 % at a lower temperature of 370 ◦C. This catalyst demonstrated to be highly
stable for 70 h of time on stream with no apparent modications on its chemical composition or microstructure.
This work demonstrates that combining the properties of carbon materials and ZrO
2
can be an interesting
approach to obtain high performing catalysts for CO
2
methanation.
1. Introduction
The conversion of CO
2
into value-added products (methane, meth-
anol, ethanol, etc.) has become one of the solutions proposed to manage
its presence in our atmosphere [1]. From the possible products, methane
has appeared as an interesting option since the CO
2
methanation
(Sabatier reaction – Eq. (1)) is the most thermodynamically favorable
reaction of CO
2
hydrogenation [2–4].
CO2(g) + 4H2(g)→CH4(g) + 2H2O(g)ΔH298k= − 165kJ mol−1(1)
Since CO
2
is a very stable molecule, its catalytic activation at
favoured thermodynamical conditions remains a challenge, and, as
such, a proper catalyst capable of high CO
2
conversion and CH
4
selec-
tivity is of utmost importance [5,6].
The most common catalysts for this reaction consist of metallic
nanoparticles, such as Ni, Ru, and Fe, dispersed in a porous support, for
example, metal oxides (Al
2
O
3
, CeO
2
, MgO, TiO
2
, SiO
2,
ZrO
2
), zeolites, or
carbon materials [2,4,7–16]. Among these, Ni has become the preferred
choice and focus of several studies due to its lower cost, high activity,
and CH
4
selectivity. This metal, however, presents low activity at tem-
peratures below 300 ◦C and is susceptible to thermal sintering aggra-
vated by the formation of hot spots during the reaction, leading to
* Corresponding author at: LSRE-LCM - Laboratory of Separation and Reaction Engineering, Laboratory of Catalysis and Materials, Faculty of Engineering, Uni-
versity of Porto, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal.
E-mail address: salome.soares@fe.up.pt (O.S.G.P. Soares).
1
Both authors contributed equally
Contents lists available at ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
https://doi.org/10.1016/j.cattod.2023.114215
Received 30 November 2022; Received in revised form 25 March 2023; Accepted 17 May 2023

Catalysis Today 422 (2023) 114215
2
deactivation [5,17]. Therefore, different approaches have been used to
improve the performance of the methanation catalysts, being one of
them the use of improved Supporting materials [18]. Interestingly, the
properties of the Supporting material, such as surface area, pore size,
surface chemistry, and metal–support interaction (MSI) can be tailored
in order to have a positive inuence on the performance of the catalyst
[19–21]. The selection of the support can, for example, inuence the size
of the Ni nanoparticles, due to MSI, which can prevent sintering and
consequently reduce catalyst deactivation [22,23].
It is well known that the most used catalyst supports for the CO
2
methanation reaction are metal oxides, and of these, rare earth metal
oxides, such as CeO
2
, are particularly good candidates to achieve
optimal performance for this reaction. As a rare earth metal, CeO
2
is a
scarce and expensive material, and thus its use should be reduced;
consequently, other metal oxides with similar properties, such as ZrO
2
,
have been explored as an option. ZrO
2
has presented good results as
support for this reaction [24–27] due to the basic nature of its surface
and the presence of oxygen vacancies. Additionally, it has been shown
that the addition of others materials to zirconia, such as Ce and Pr, may
lead to a further increase in the catalytic performance [27]. However,
the high price of this material is still a limitation.
Carbon nanomaterials have been drawing attention, in recent years,
due to their unique characteristics like thermal conductivity and sta-
bility as well as tunable surface chemistry and texture, allowing the
catalysts supported on these materials to surpass the conventional metal
oxide-supported ones [15,28–30]. In particular, carbon nanotubes
(CNTs) have been shown to prevent the previously mentioned hot spots,
extending the catalysts’ lifetime [15].
In a previous study [31], we have demonstrated that forming a
composite of activated carbon and CeO
2
could lead to a support with
enhanced properties to be applied in CO
2
methanation. The prepared
catalyst demonstrated enhanced activity compared to our previously
reported Ni on reduced activated carbon [5], achieving a similar activity
to a Ni on CeO
2
catalyst with signicant reduction in the usage of CeO
2
.
Herein, we present the possibility of associating ZrO
2
with carbon
materials and take advantage of the properties of both supporting ma-
terials in order to obtain active and stable catalysts for CO
2
methanation.
Carbon materials and ZrO
2
composites were prepared and an optimi-
zation study was conducted over these composites as supports for CO
2
methanation Ni-based catalysts. Firstly, the carbon:ZrO
2
ratio in the
supporting material was varied, which was followed by the study on the
inuence of the addition of Fe to the system.
2. Materials and methods
2.1. Materials
Activated Carbon (AC) NORIT GAC 1240 PLUS, acquired from
CABOT; Carbon Nanotubes (CNTs) NC3100, bought from Nanocyl,
Melamine M2659 (99 %) was purchased from Sigma-Aldrich, ZrO
2
(SZ
31164) from Saint-Gobain NorPro, HNO
3
(65 %, AnalaR NORMAPU®)
was purchased from VWR.
Nickel Nitrate (Ni(NO
3
)
2
•6H
2
O (>97 %)) was purchased from
Sigma-Aldrich, Fe(NO
3
)
3
•9H
2
O (98–101 %) was acquired from Thermo
Fisher Scientic, Ultrapure Water (18.2 MΩcm
−2
) was produced by the
Milli-Q Advantage A10 system (Millipore).
2.2. Carbon materials functionalization
Reduced activated carbon (AC-R) was prepared by submitting 3.5 g
of the original AC (NORIT GAC 1240 PLUS) to a thermal treatment at
900 ◦C, for 1 h, with a H
2
ow rate of 100 cm
3
min
−1
. The samples were
heated under N
2
at 10 ◦C min
−1
until the desired temperature, and then
the ow was changed to H
2
.
N-doped carbon nanotubes (CNT-N) were prepared by ball milling
0.6 g of CNTs and 0.39 g of melamine, the N-precursor, in a Retsch
MM200 equipment for 4 h, at 15 vibrations s
−1
. Finally, the samples
were annealed at 600 ◦C (at a heating rate of 10 ◦C min
−1
) under a
100 cm
3
min
−1
ow rate of N
2
.
2.3. Composites preparation
Three different AC-ZrO
2
composites were prepared by ball milling a
mixture of activated carbon (AC) and zirconia (ZrO
2
) in the Retsch
MM200 with proportions of 50:50, 70:30 and 90:10 for a total mass of
2 g, for 30 min at 10 vibrations s
−1
. The same is true for the CNT-ZrO
2
composites except only 1 g of the 50:50 and 70:30 proportions and 0.7 g
of the 90:10 composite was ball milled at a time due to space constric-
tions of the milling vessel.
2.4. Catalysts preparation
All catalysts were prepared using the incipient wetness impregnation
(IWI) method. 0.5 g of each support was placed in an Erlenmeyer ask
under ultrasonic vibrations, to which an aqueous solution of Ni(NO
3
)
2
•
6 H
2
O was added (in the case of monometallic catalysts). After 90 min,
the ask was removed, covered and dried overnight at 100 ◦C.
In the case of bimetallic catalysts, a sequential impregnation was
applied, where the dried, nickel-containing catalyst was again placed
under ultrasonic vibration, adding an aqueous solution of Fe(NO
3
)
3
•
9H
2
O and, nally, removing and drying the catalyst overnight at 100 ◦C.
The nal step was to reduce all samples at an optimal temperature
determined by temperature programmed reduction (H
2
-TPR). The cat-
alysts were then placed in a quartz reactor and subjected to a 100 cm
3
min
−1
ow rate of N
2
, after which it was reduced by an H
2
ow rate of
100 cm
3
min
−1
for 3 h.
2.5. Characterization of the prepared materials
The resultant catalysts and their supports were characterized by H
2
-
temperature programmed reduction (H
2
-TPR), N
2
physisorption, H
2
-
pulse chemisorption, temperature programmed desorption (TPD), X-ray
photoelectron spectroscopy (XPS), powder X-ray diffraction analysis
(XRD), elemental analysis (EA) and transmission electron microscopy
(TEM). Additional information can be found in the Supplementary ma-
terial (SM).
2.6. Catalytic experiments
All catalytic experiments were performed on a microactivity XS15
equipment by PID Eng & Tech, in a quartz reactor (d
int
=1 cm). The
analysis of the products was carried out on a gas chromatograph (GC
1000 from DANI) equipped with a thermal conductivity detector (TCD)
and GS-CarbonPLOT capillary column, using He as a carrier gas and N
2
as the internal standard.
The catalysts were pre-treated for 30 min and 1 bar, in situ, with a
40 cm
3
min
−1
ow rate of H
2
at the reduction temperature. Afterwards,
under a 50 cm
3
min
−1
ow rate of He, the temperature was reduced to
100 ◦C. Finally, maintaining the previous ow rates, the temperature
was increased from 100 ◦C to 500 ◦C to evaluate catalytic performance
at different temperatures.
In order to study the stability of the best performing catalyst, it was
subjected to reaction conditions for an extended time on stream (70 h) at
250 ◦C (to obtain a CO
2
conversion below thermodynamic equilibrium).
The CO
2
conversion (X
CO2
) and CH
4
selectivity (S
CH4
) were deter-
mined as shown in Eqs. (2) and (3), assuming CO as the only by-product.
XCO2(%) = FCO2,in −FCO2,out
FCO2,in
(2)
SCH4(%) = FCH4,out
FCH4,out +FCO,out
×100 (3)
I.F. Quatorze et al.

Catalysis Today 422 (2023) 114215
3
Where F
CO2,in
and F
CO2,out
correspond to the CO
2
molar ow rate (mol
min
−1
) in the inlet and outlet gas ow, respectively, and F
CH4,out
and F
CO,
out
are the molar ow rate (mol min
−1
) of CH
4
and CO in the outlet gas
ow, respectively.
3. Results
3.1. Optimization of carbon:ZrO
2
composite proportion
Considering the high catalytic performance obtained previously
using carbon materials and ZrO
2
as support for Ni-based catalysts for
CO
2
methanation [5,15], in this work, the optimization of a composite of
ZrO
2
and carbon materials was performed to obtain a high performing
material for this application.
Catalytic testing began by determining the optimal carbon:ZrO
2
composite proportion, among 90:10, 70:30 and 50:50, for a xed nickel
(Ni) loading of 15 % (wt. %) (Fig. 1). The optimal Ni loading was
determined based on preliminary experiments.
Regarding the optimal composition of the AC:ZrO
2
composite
(Fig. 1), a clear trend was observed, both in terms of X
CO2
and S
CH4
, in
which a higher amount of ZrO
2
led to a better performance of the Ni
catalyst. The best-performing composite, from this set, was the one with
a 50:50 proportion of AC and ZrO
2
– Ni/AC-ZrO
2
(50:50), with X
CO2
=76.4 % and S
CH4
=98.5 % at 430 ◦C. On the other hand, for CNT, an
optimal CNT:ZrO
2
proportion was found at 70:30, in which decreasing
(90:10) or increasing (50:50) the ZrO
2
content led to a decrease in the
catalyst performance. The topmost catalyst from this set, Ni/CNT-ZrO
2
(70:30), achieved a X
CO2
=82.1 % and S
CH4
=99.3 % at 400 ◦C.
Importantly, it is clear that comparing the AC:ZrO
2
−supported set
with the CNT:ZrO
2
−supported set of catalysts, the results obtained by
the CNT-based composites, in terms of X
CO2
, were higher than those
obtained with the AC-based composites.
The possibility of increase in the performance of the catalysts
through functionalization of the carbon materials was studyed through
N-doping and increase of the number of Lewis basic sites by thermal
reduction (see the SM, Fig. S1). However, in the case of the CNT:ZrO
2
composites, the functionalization was not found to lead to an increase in
the performance.
3.2. Evaluation of the effect of the addition of Fe
After optimising the composite Supporting material, the possibility
of adding Fe as a second metal on the topmost performing CNT:ZrO
2
support, CNT:ZrO
2
(70:30), was evaluated. A bimetallic catalysts (with
different Ni:Fe loadings) on this support was prepared to study the in-
uence of the introduction of iron (Fe) on the catalytic performance
(Fig. S2).
Interestingly, at temperatures below 370 ◦C, all the bimetallic cata-
lysts, except the one with a Ni:Fe ratio of 10:10, presented higher X
CO2
,
when compared to the monometallic Ni/CNT-ZrO
2
(70:30) (Fig. S2).
Above this temperature, only the topmost performing catalyst 18:2Ni:
Fe/CNT-ZrO
2
(70:30) outperforms the monometallic one, with a
maximum X
CO2
=85.1 % and S
CH4
=99.5 % at 370 ◦C.
Comparing to Ni on pristine CNT catalysts (Ni/CNT), both of the
highest performing catalysts of this work, Ni/CNT-ZrO
2
(70:30) and
18:2Ni:Fe/CNT-ZrO
2
(70:30), presented higher X
CO2
in the range of
temperatures tested (Fig. 2). Interestingly, when compared to Ni/ZrO
2
,
our monometallic Ni/CNT-ZrO
2
(70:30) presented slightly lower X
CO2
,
while the bimetallic 18:2Ni:Fe/CNT-ZrO
2
(70:30) outperformed this
commonly used catalyst (Fig. 2).
Finally, the stability over long time on stream (TOS) of the best
performing catalyst, 18:2Ni:Fe/CNT-ZrO
2
(70:30), was evaluated
(Fig. 3). This sample remained stable for at least 70 h under the studied
reaction conditions, both in terms of X
CO2
and S
CH4
.
3.3. Characterization
The catalytic results obtained with the prepared materials allowed
for a systematic optimization of the preparation of carbon:ZrO
2
sup-
ported catalysts for CO
2
methanation. However, understanding the
properties of the materials is fundamental for the rational development
of new catalysts. Therefore, an extensive characterization of the pre-
pared materials was performed in order to correlate them with the
catalytic performance.
Regarding the textural properties of the materials, the specic
Fig. 1. Comparison of the catalytic performance, in terms of X
CO2
and S
CH4
, at different temperatures, for Ni supported on AC:ZrO
2
(a,b) and CNT:ZrO
2
(c,d)
composites with different carbon:ZrO
2
proportions. The thermodynamic equilibrium curves were calculated using a model based on the minimization of Gibbs free-
energy of the existing species. Methanation conditions: P=1 bar; GHSV =60 000 cm
3
g
−1
h
−1
; CO
2
:H
2
(V:V) =1:4.
I.F. Quatorze et al.

Catalysis Today 422 (2023) 114215
4
surface area (S
BET
), total pore volume (V
p
) and micropore volume (V
mi-
cro
) of the catalysts and the respective Supporting materials were
determined through their N
2
adsorption-desorption isotherms at
−196 ◦C (Fig. S3) and the obtained results can be consulted in Tables S1
and S2. Interestingly, the functionalized materials presented a slighlty
lower S
BET
, both for AC and CNT, whereas V
p
does not seem to suffer
signicant changes.
Additionally, the composites of carbon:ZrO
2
are shown to have lower
S
BET
values than their non-ZrO
2
-containing counterparts. This was ex-
pected since ZrO
2
has a lower specic surface area (20 m
2
g
−1
) than AC
(810 m
2
g
−1
) and CNT (209 m
2
g
−1
). Interestingly, there was an
exception for CNT-ZrO
2
(90:10), which presented a slight increase in
S
BET
. This might be explained by a possible opening of the ends of the
CNTs and decreasing of their entanglement caused by the ball milling
procedure; since this sample presents only 10 % of ZrO
2,
the S
BET
is
mainly related to the CNTs.
Regarding the Ni catalysts, any given catalyst presents lower S
BET
, V
p
and V
Micro
values than their respective support. The introduction of
metallic nanoparticles (NPs) in the Supporting materials leads to an
increase in mass without a signicant increase in the surface area and
total pore volume, leading to a smaller S
BET
and V
p
.
The elemental analysis (Table S3) conrmed the success of the N-
doping procedure on CNTs, with an increase of N of 7.5 % in CNT-N,
compared with the pristine CNT. Additionally, temperature pro-
grammed desorption (TPD) conrmed the success of the removal of
oxygen groups during the reduction treatment in AC-R, with a decrease
in the amount of CO and CO
2
released (Table S4).
The XRD patterns of the most promising samples (Fig. S4) demon-
strated the presence of graphitic carbon, together with ZrO
2
, in a
mixture of monoclinic and tetragonal forms. Additionally, Ni peaks were
found for all samples, and in the case of the bimetallic one, 18:2NiFe/
CNT-ZrO
2
(70:30), Ni and Fe peaks appear overlapped. Moreover, with
the overlapping of NiO and ZrO
2
peaks, it is not possible to distinguish
the NiO peaks.
Interestingly, the Ni crystallite size (d
Ni
), determined by the Scherrer
equation from the XRD patterns (Table S5), was found to be higher in the
case of Ni/AC R-ZrO
2
(50:50) (d
Ni
=19 nm) than Ni/CNT-ZrO
2
(70:30)
(d
Ni
=14 nm), which could be the reason for the better catalytic per-
formance of the latter sample. Additionally, after alloying with Fe, the
Ni crystallite size is even lower (d
Ni
=11 nm in 18:2NiFe/CNT-ZrO
2
(70:30)), possibly explaining the even better catalytic performance of
this sample. It is important to note, however, that Ni and Fe peaks
appear overlapped, and thus, the crystallite size in the bimetallic sample
is merely an estimate.
The specic metallic surface area (S
M
) was determined for the
topmost performing catalysts from the AC-ZrO
2
and CNT-ZrO
2
-sup-
ported set of catalysts, by H
2
pulse chemisorption, and is presented in
Table 1. Remarkably, the catalysts supported on CNT-ZrO
2
composite,
Ni/CNT-ZrO
2
(70:30) and 18:2NiFe/CNT-ZrO
2
(70:30), present the
highest S
M
. The H
2
TPR prole of the best performing Ni/AC-R-ZrO
2
(50:50) (Fig. S5) demonstrated the presence of two well established
peaks: one at 312 ◦C, corresponding to Ni species interacting weakly
with the support and another at 345 ◦C, corresponding to Ni species that
interact more strongly with the support. Regarding Ni/CNT-ZrO
2
(70:30), the rst and second peaks occur at slightly higher temperatures
322 ◦C and 352 ◦C, respectively, and additionally, it presents a peak at a
higher temperature of 403 ◦C. The stronger metal support interaction on
Ni/CNT-ZrO
2
(70:30), in comparison with Ni/AC-R-ZrO
2
(50:50) is
indicated by the higher reduction temperatures of the different Ni spe-
cies in this sample, and also by the higher S
M
.
STEM analysis coupled with STEM-EDX mapping (Fig. 4) demon-
strated that, on 18:2NiFe/CNT-ZrO
2
(70:30), ZrO
2
agglomerates appear
mixed with CNTs, while the Ni and Fe nanoparticles are dispersed in
both in the CNTs and the ZrO
2
.
3.4. Characterization after the stability test
Even though the topmost performing sample, 18:2NiFe/CNT-ZrO
2
(70:30), demonstrated to be stable under reaction conditions after 70 h,
an additional characterization study was performed to understand if the
Fig. 2. Comparison of the catalytic performance, in terms of X
CO2
(a) and S
CH4
(b), at different temperatures, for the high performing catalysts prepared in this work
(Ni/CNT-ZrO
2
(70:30) and 18:2NiFe/CNT- ZrO
2
(70:30)), and Ni/CNT and Ni/ZrO
2
. The thermodynamic equilibrium curves were calculated using a model based on
the minimization of Gibbs free-energy of the existing species. Methanation conditions: P=1 bar; GHSV =60 000 cm
3
g
−1
h
−1
; CO
2
:H
2
(V:V) =1:4.
Fig. 3. Stability testing results in terms of X
CO2
and S
CH4
over 70 h TOS of the
best performing catalyst 18:2Ni:Fe/CNT-ZrO
2
(70:30). Methanation conditions:
P=1 bar; GHSV =60 000 cm
3
g
−1
h
−1
; CO
2
:H
2
(V:V) =1:4, T=250 ◦C.
Table 1
H
2
consumption, and specic metallic area (S
M
) for the topmost performing
catalysts.
Sample H
2
Consumption [
μ
mol g
−1
] S
M
[m
2
g
cat
−1
]
Ni/AC-R-ZrO
2
(50:50) 9.83 0.77
Ni/CNT-ZrO
2
(70:30) 34.8 2.72
18:2NiFe/CNT-ZrO
2
(70:30) 33.0 –
I.F. Quatorze et al.

Catalysis Today 422 (2023) 114215
5
sample underwent signicant changes. Importantly, it is worth to note
that the limitations of the experimental protocols and sample handling
should be taken into account. Namely, although characterization was
performed on the reduced catalysts, exposure to ambient air can
partially oxidize the surface of transition-metal nanoparticles. However,
before the catalytic testing, the fresh catalysts undergo an in situ
reduction pre-treatment, meaning that metallic nanoparticles are
considered when interpreting their catalytic activity.
XPS analysis of the as-synthesised sample (Fig. 5) demonstrated that
the surface of the catalyst is composed of a mixture of Ni and NiO. From
the deconvolution of the Ni 2p
3/2
spectra, it was possible to observe that
in the samples before and after reaction, three main peaks appear, one at
853.2 ±0.1 eV, corresponding to Ni
0
, and the other two at 855.0
±0.3 eV and 856.5 ±0.1 eV, corresponding to Ni
2+
. The Ni 2p
3/2
spectrum is consistent with a thin, predominantly Ni
2+
, nickel oxide
layer on metallic Ni, typical for nanoscale oxidation layers on Ni and Ni
compounds [31–33]. Importantly, after reaction the position of the
deconvoluted peaks remains similar (Fig. 5); with only a very slight
decrease in the relative atomic percentage of metallic Ni in comparison
with that of the as-synthesised sample (Table S6).
The C 1 s spectra (Fig. S6) demonstrated that the CNTs were not
hydrogenated, since the oxidized C components in the C 1 s remained
unchanged after the reaction. Furthermore, the C 1 s shake-up compo-
nent, which was the component that would suffer a reduction resulting
from the physical damage of the CNTs, was not reduced after reaction
[15,34–36].
Fe 2p spectrum (Fig. S6) overlaps strongly with Ni LMM Auger peaks;
therefore, it is difcult to be deconvoluted into multiple peaks, so its
deconvolution was not done. The XPS spectra of the remaining elements
and a brief discussion can be found in the SM (Fig. S6) [24,35,37–39].
TEM analysis of the 18:2NiFe/CNT-ZrO
2
(70:30) sample before and
after reaction (Fig. S7) indicates that there is no signicant change in the
microstructure of the catalyst. It is important to note that the imaging
contrast between the Ni and ZrO
2
nanoparticles is very low, thus it is
difcult to differentiate the two types of nanoparticles in the electron
microscopy images. Therefore, it was not possible to obtain a clear
particle size distribution.
4. Discussion
In this work, we present a systematic study of the use of carbon:ZrO
2
composites as support for Ni-based catalysts for CO
2
methanation.
The optimization of the ZrO
2
loading in the composite supports of the
Ni-based catalysts demonstrated that the catalysts with the optimal ZrO
2
in each set of tested materials were Ni/AC-ZrO
2
(50:50) and Ni/CNT-
ZrO
2
(70:30) (Fig. 1).
Notably, in the range of reaction temperatures evaluated in the
study, all of the Ni nanocatalysts supported on CNT:ZrO
2
composites
presented a higher X
CO2
than those supported on the AC:ZrO
2
counter-
parts. In previous works that studied the use of carbon materials as
catalyst support, the X
CO2
values reported for CNT-supported catalysts
are commonly higher than those reported for AC-supported catalysts [5,
15], evaluated in similar conditions. The better performance of the
CNT-based materials in comparison with AC-based materials can be
explained by the higher metallic Ni surface area (Table 1) of the
CNT-based catalysts, resulting from the smaller Ni nanoparticles
(Table S5), but also by the high thermal conductivity of the CNTs that
should prevent the possible hotspots caused by CO
2
methanation reac-
tion – an exothermic reaction [40]. The improved metallic Ni surface
area can be a consequence of the absence of micropores in the CNTs, a
Fig. 4. Low-magnication STEM image of the topmost performing catalyst 18:2Ni:Fe/CNT-ZrO
2
(70:30), together with the respective STEM–EDX maps for C, Zr, Ni,
and Fe.
Fig. 5. High-resolution XPS data for the Ni 2p
3/2
region collected from as-
synthesized and TOS-tested 18:2NiFe/CNT-ZrO
2
(70:30). Symbols: raw data;
black lines: overall ts; coloured lines: ts of individual components; dashed
lines: background.
I.F. Quatorze et al.

Catalysis Today 422 (2023) 114215
6
mesoporous material (Fig. S3, Tables S1 and S2). In the case of the
AC-based catalysts, the presence of micropores in the Supporting ma-
terial leads to a lower accessible surface area for the metal precursor
during the preparation of the material; leading, in this case, to a lower
metallic Ni surface area. Another possible explanation for the different
metallic surface area would be the different MSI with the different
supports; however, this is not the case as the TPR proles present similar
peak temperatures indicating that the MSI should be similar in both
CNT-ZrO
2
and AC-R-ZrO
2
composite supports.
As explained above, due to the oxygen vacancies and basic nature of
the surface, ZrO
2
is an excellent support for Ni-based catalysts in CO
2
methanation. However, being an expensive metal oxide, reducing the
quantities used is attractive. In this work, we demonstrated that by
forming a CNT composite we could obtain activity and selectivity results
nearing those with pure ZrO
2
as support.
Finally, the study of the addition of Fe to the best performing catalyst
– Ni/CNT-ZrO
2
(70:30) – headed to the nding that adding Fe to this
catalyst led to the development of a high-performing catalyst. The
18:2NiFe/CNT-ZrO
2
(70:30) with 18 wt. % of Ni and 2 wt. % of Fe
achieved a X
CO2
=85.1 % and S
CH4
=99.5 % at 370 ◦C, values that
compare favourably to the results found in the literature on Ni based
catalysts supported either on ZrO
2
and carbon materials. The higher
performance of this material compared to our monometallic Ni/CNT-
ZrO
2
(70:30) does not seem to stem from its higher metal content (20 wt.
% cf. 15 wt. %), since as can be observed in Fig. S2, the catalyst
15:5NiFe/CNT-ZrO
2
(70:30) presents the same and 18:6 NiFe/CNT-ZrO
2
(70:30) an even higher metal content than the best performing 18:2
NiFe/CNT-ZrO
2
(70:30), and both have lower catalytic activity. Addi-
tionally, these two catalysts, 15:5 NiFe/CNT-ZrO
2
(70:30) and 18:6
NiFe/CNT-ZrO
2
(70:30) present a higher metal loading than the
monometallic Ni/CNT-ZrO
2
(70:30) and a similar catalytic performance.
Therefore, it is clear that the synergy between NiFe and an adequate
proportion of each metal is fundamental for a better performance of the
catalyst.
Importantly, this high performing catalyst did not suffer micro-
structural changes after 70 h of TOS, and no carbon gasication was
observed after the stability experiment, demonstrating the high stability
of the material.
Comparatively, previous studies on carbon materials as supports led
to lower X
CO2
at higher optimal temperatures than our 18:2NiFe/CNT-
ZrO
2
(70:30). Namely, Hu et al. [41] studied Ni catalysts supported on
reduced graphene oxide (RGO), achieving X
CO2
=78.4 % at 350 ◦C,
although it was more active and selective than Ni/AC and Ni/γ-Al
2
O
3
.
Additionally, in our previous work [5], 15 wt. %Ni/ACR achieved X
CO2
=76.0 % and S
CH4
=97 % at a higher optimal T=450 ◦C, while 15 wt.
%Ni 5 wt. %Fe/ACR achieved X
CO2
=77.0 % and S
CH4
=98 % at a
higher optimal T=430 ◦C. In the case of CNTs, Wang et al. [42]
demonstrated the good performance of Ni on N-doped CNT of X
CO2
=80
% and S
CH4
=99 % at optimal T=360 ◦C. Additionally, in our previous
work [15] Ni on N-doped CNT achieved X
CO2
=81.2 % and S
CH4
=99.2
% at optimal T=400 ◦C.
Regarding catalysts with ZrO
2
as support, for example, Jia et al. [24]
demonstrated that a Ni/ZrO
2
catalysts could achieve X
CO2
=79.1 % and
S
CH4
=96.7 % at T=350 ◦C. They attributed the good performance of
this plasma-prepared catalyst to the existence of Ni-ZrO
2
interfacial sites
with more oxygen vacancies that facilitated CO
2
activation and metha-
nation. Ashok et al. [25] studied a series of Ni/CeO
2
-ZrO
2
catalysts and
their topmost performing catalyst obtained X
CO2
=55 % and S
CH4
>99.6 % at T=275 ◦C. They attributed the best performance of this
catalyst to the enhanced metal-support interactions between Ni and Ce.
Romero-S´
aez et al. [26] investigated a Ni/Zr/CNT catalyst prepared by
co-impregnation and obtained X
CO2
≈55 % and S
CH4
>95 % at
T≈400 ◦C.
A more comprehensive list of the catalytic activity reported in the
literature for several catalysts can be consulted in Table S7 [5,15,24–27,
31,41–48].
Both carbon materials (specically CNTs) and ZrO
2
have demon-
strated to be good candidates as support for Ni-based methanation cat-
alysts. In this work, we showed that by associating both materials, a high
performing catalyst could be obtained. Likely, as a result of joining the
high basicity of ZrO
2
and the high surface area and high thermal con-
ductivity of CNT. Furthermore, the addition of Fe led to an improvement
in X
CO2
and a decrease in the optimal temperature, with a catalyst with
excellent stability. The excellent stability of the catalyst allied to the
minimal chemical and structural changes after the long TOS experiment
indicate that there is no poisoning of the surface.
5. Conclusions
Herein we performed a systematic study on the optimization of a
carbon:ZrO
2
Supporting material for Ni-based catalysts for CO
2
metha-
nation, in order to try to obtain a catalyst with improved performance
compared to the commonly used ones. This study demonstrated that, in
general, CNT-based composites outperformed AC ones on this applica-
tion. Most likely, due to the absence of micropores in CNT, which leads
to a larger surface area accessible for the metal nanoparticles (higher
metallic dispersion), and also due to the higher thermal conductivity,
which prevents the formation of hotspots. Importantly, a ratio of CNT:
ZrO
2
of 70:30 demonstrated to be optimal in order to achieve a Ni
catalyst with higher X
CO2
and S
CH4
.
Finally, the addition of Fe was studied in this material, which was
demonstrated to be benecial. The 18:2NiFe/CNT-ZrO
2
(70:30) catalyst
obtained X
CO2
=85.6 % and S
CH4
=99.9 % at T=370 ◦C, a result that
compares favourably with those reported in the literature in systems
composed either by carbon materials or ZrO
2
as support for Ni catalysts.
Notably, this catalyst presented an excellent stability over 70 h of TOS.
The systematic study presented herein provides evidence that the
preparation of carbon:metal-oxides composites could be an interesting
approach to achieving high performing catalysts for CO
2
methanation
reaction.
Declaration of Competing Interest
The authors declare that they have no known competing nancial
interests or personal relationships that could have appeared to inuence
the work reported in this paper.
Data Availability
Data will be made available on request.
Acknowledgements
This work was nancially supported by LA/P/0045/2020 (ALiCE),
UIDB/50020/2020 and UIDP/50020/2020 (LSRE-LCM), funded by na-
tional funds through FCT/MCTES (PIDDAC), by Move2LowC project,
reference POCI-01–0247-FEDER-046117, conanced by Programa
Operacional Competitividade e Internacionalizaç˜
ao (POCI); Programa
Operacional Regional de Lisboa, Portugal 2020 and the European Union,
through the European Regional Development Fund (ERDF), and by
Project HyGreen&LowEmissions, reference NORTE-01–0145-FEDER-
000077, Co-nanced by the European Regional Development Fund
(FEDER), through the North Portugal Regional Operational Programme
(NORTE2020). O.S.G.P.S. acknowledges FCT funding under the Scien-
tic Employment Stimulus - Institutional Call CEECINST/00049/2018.
Appendix A. Supporting information
Supplementary data associated with this article can be found in the
online version at doi:10.1016/j.cattod.2023.114215.
I.F. Quatorze et al.

Catalysis Today 422 (2023) 114215
7
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