Catalytic hydrogenation of CO2 to methane over supported Pd, Rh and Ni catalysts

Article abstract of DOI:10.1039/c6cy02536f

As a step in production of so-called electrofuels, ambient pressure CO₂ hydrogenation has been investigated over different catalytic model systems based on metal particles (Pd, Rh and Ni) supported on various metal oxides (SiO₂, Al

Full text of DOI:10.1039/c6cy02536f

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process may require a different catalyst9. An important challenge
for atmospheric pressure CO2 hydrogenation is to design catalysts
that efficiently can suppress the rWGS reaction as well as forma-
tion of other by-products in favour of methanol and/or methane
formation.
(NH3)4Pd(NO3)2 (Alfa Aesar). The specific amount of precursor
solution to obtain 3 wt.% of the metal was added to 3 g of each
support. The impregnated silica, alumina and ceria samples were
instantly frozen with liquid nitrogen for 24 h and finally calcined
◦
in air at 550 C for 1 h. After freeze-drying, the impregnated
◦
MCM-41 sample was calcined in air at 450 C for 1 h.
Hydrogenation of CO2 to methane (eq. (2)) is the most ad-
vantageous reaction with respect to thermodynamics (∆G298= -
130.8 kJmol−1) among the CO2 conversion reactions. Previous
work has reported different transition metals such as Ru, Rh, Ni
and Pd to be highly active and selective for the methane forma-
For the Ni/ZSM-5 catalyst, 3 g ZSM-5 (SAR 27, Akzo Nobel)
was added to a beaker together with 1.5 liter 0.025 molar nickel
nitrate solution. After 24 h of stirring at room temperature the
solution was filtered and washed with Milli-Q water. The aqueous
◦
tion by CO2 hydrogenation, especially at low temperatures10–12
Among these, the supported Ni catalysts were reported to have
the highest selectivity to form methane13
.
ion-exchanged zeolite was then dried at 110 C for 24 h before
◦
calcination in air at 450 C for 1 h starting from room temperature
◦
with a heating rate of 5 C/min. Table 1 gives an overview of the
.
prepared catalysts.
In the present work we have synthesised and studied industri-
ally relevant polycrystalline catalysts that can bind CO2 and facil-
itate H2 dissociation and further reaction with activated CO2 to
form methane at atmospheric pressure conditions. We also con-
sider possible methanol formation, even though no methanol has
been observed to form in the present study, under the investi-
gated conditions. A series of metal oxide supported catalysts con-
taining Pd, Ni and Rh have been prepared with impregnation and
ion-exchange methods, characterised and evaluated in terms of
activity and selectivity for CO2 hydrogenation. The kinetic study
showed an increased conversion of CO2 to methane and CO at
elevated temperature, with varying selectivity depending on the
active phase. The Rh/Al2O3 and Rh/CeO2 catalysts were singled
out as the ones giving the highest CO2 conversion and lowest
selectivity for the rWGS reaction in the methanation reaction, fol-
lowed by the Ni/CeO2 catalyst. Furthermore, ceria is concluded
to be the most active support for hydrogenation of CO2, prob-
ably due to a participating role in CO2 adsorption and a highly
dispersed active phase. In addition, the CO2 methanation reac-
tion mechanism was investigated by in situ diffusive reflectance
Fourier transform infrared spectroscopy and the results revel clear
differences between Rh/Al2O3 and Rh/CeO2 catalysts.
Monolith samples were prepared by coating corderite mono-
liths with approximately 200 mg of the powder samples through
dip coating. Colloidal alumina sol (Disperal, Sasol) was used as
binder for all samples. The monolith samples were thereafter
◦
dried at 90 C in air until all water evaporated and finally cal-
◦
cined at 600 C for 5 min.
The crystal structure of the as-prepared samples was studied by
X-ray diffraction (XRD) under ambient atmosphere using a Bruker
AXS D8 ADVANCE diffractometer with Cu-Kα radiation equiva-
lent to 0.15418 nm. The diffraction data was recorded in the 2θ
◦
◦
range of 20-66 with incremental steps of 0.03 for 28 minutes.
The specific surface area of the catalysts was determined by
nitrogen sorption at 77 K (Micrometrics Tristar 3000) using the
Brunauer Emmet and Teller (BET) method14. The samples were
◦
dried in vacuum at 230 C for 3 h before the measurement.
2.2 Flow-reactor experiments
The catalytic performance of the catalysts was analysed in a flow
reactor. An FTIR instrument (MultiGas 2030, MKS Instruments)
was used to measure the composition of the effluent gas. The
flow reactor, described elsewhere15, consists of a quartz tube
wherein the sample was positioned. Heating of the inlet gas and
the sample occurred via resistive heating of a metal coil surround-
ing the reactor tube. Both the inlet gas and the catalyst tempera-
ture were measured by separate thermocouples. Feed gases were
mixed from H2, CO2 and Ar and introduced into the reactor via
individual mass flow controllers, providing a total flow of 2000
ml/min, corresponding to a gas hourly space velocity (GHSV) of
about 60000 h−1. Prior to each experiment the samples were pre-
2 Experimental section
2.1 Catalyst preparation and ex situ characterization
In this study, 13 different catalysts were prepared with Pd, Ni and
Rh supported on silica, alumina, ceria, MCM-41 and ZSM-5. The
samples with silica, alumina, ceria and MCM-41 were prepared
by incipient wetness impregnation, while the Ni/ZSM-5 sample
was prepared by ion-exchange as described below. Powders of sil-
ica (Kromasil Silica KR-300-10, Akzo Nobel Eka Chemicals), alu-
mina (Puralox SBa200, Sasol) and ceria (99.5 H.S.A. 514, Rhône-
◦
treated at 450 C under a flow of 5% oxygen followed by a flow
of 5% hydrogen, each for 10 minutes. The catalytic performance
of the catalysts was measured under a flow of 1% CO2 and 5 %
H2, corresponding to a molar ratio of 1:5, for 10 minutes at four
◦
Poulenc) were calcined in air at 600 C for 2 h starting from room
◦
temperature with a heating rate of 5 C/min to remove carbona-
◦
different temperatures (450, 350, 250, and 150 C).
ceous impurities and stabilise the structure of the support materi-
als. Since MCM-41 (Sigma-Aldrich) is more fragile, a lower calci-
◦
2.3 In situ FTIR spectroscopy measuremets
nation temperature (450 C) was used for this material. Precur-
sor solutions of nickel and rhodium were prepared by dissolving
Ni(NO3)2×6H2O (Alfa Aesar) and Rh(NO3)3×2H2O (Alfa Aesar)
salts in Milli-Q water (18 MΩ/cm). To increase the solubility of
the rhodium salt, 25 droplets of 70% HNO3 were added. The
precursor for palladium was an aqueous solution of the complex
The in situ FTIR spectroscopy experiments were performed in
diffusive reflectance (DRIFT) mode with a BRUKER Vertex 70
spectrometer equipped with a nitrogen cooled MCT detector and
a high-temperature stainless steel reaction cell (Harrick Praying
MantisTM High Temperature Reaction Chamber) with KBr win-
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Table 1 Metal loading, calcination temperature and specific surface area for the Pd-, Ni-, and Rh-based catalyst samples used in this study.
◦
Sample
Metal loading ( wt.%) Calcination temperature ( C) Specific surface area (m2/g)
Pd
3.0
3.0
3.0
Ni
-
-
-
-
3.0
3.0
3.0
3.0
1.5
-
Rh
-
-
-
-
-
-
-
Pd/SiO2
Pd/Al2O3
Pd/CeO2
550
550
550
450
550
550
550
450
450
550
550
550
450
315
187
131
-
311
182
128
-
Pd/MCM-41 3.0
Ni/SiO2
Ni/Al2O3
Ni/CeO2
Ni/MCM-41
Ni/ZSM-5
Rh/SiO2
Rh/Al2O3
Rh/CeO2
Rh/MCM-41
-
-
-
-
-
-
-
-
-
-
-
-
3.0
3.0
3.0
3.0
310
180
125
-
-
-
-
dows. The temperature of the sample holder was measured by
a thermocouple (type K) and controlled by a PID regulator (Eu-
rotherm). Feed gases were introduced into the reaction cell via
individual mass flow controllers, providing a total flow of 100
ml/min in all experiments. Moreover, the H2 feed was introduced
via a high-speed gas valve (Valco, VICI) in order to provide precise
transients. Prior to each experiment the samples were pre-treated
the samples, which were found to correspond to the diffraction
patterns of PdO, NiO and Rh2O3, seen as coloured marks above
the diffraction patterns (Figure 1 a), c) and d)). The XRD results
indicate that the metal particle sizes are different for the met-
als supported on each oxide. The metals supported by cerium
oxide nearly have diffraction peaks, which might indicate that
Pd, Ni, and Rh are in amorphous phase or in extreme small crys-
talline size (below 3 nm), while the metals supported on silica
show diffraction peaks for PdO, NiO and Rh2O3. In addition, the
broadening of the diffraction peak gives insight into the size of the
crystallites: sharp peaks indicate larger crystallites, while broader
peaks indicate smaller crystallites. The Ni-silica sample has a very
sharp XRD peak for NiO, suggesting a much larger particle size of
Ni, compared to the Pd-silica or Rh-silica samples which show
broader diffraction peaks. For the alumina supported samples,
only Pd-alumina has some reflections originating form PdO, sug-
gesting that this sample has larger particles compared to the other
alumina supported samples investigated. For the ion-exchanged
sample Ni/ZSM-5, no new reflections are observed compared to
the pattern for ZSM-5 suggesting that Ni is highly dispersed in
this sample.
◦
at 350 C with 5% O2 in Ar for 10 min and 0.8% H2 in Ar for 10
min and then cooled in Ar to the desired temperature where a
background spectrum was collected. The experiment started by
introducing a flow of 0.2% CO2 and 0.8% H2 to the reaction cell.
After 10 min the H2 flow was turned off for 10 min and rein-
troduced again for 10 minutes. The measurements were repeated
five times giving a total length of the experiments of 100 minutes.
The hydrogen pulse-response measurements were performed at
◦
different temperatures (400, 350, 300, and 250 C). The region
−1
between 790-3800 cm was investigated with a spectral resolu-
tion of 8 cm−1. The product stream was continuously analysed
with mass spectrometry (Hiden Analytical, HPR-20 QIC) follow-
ing the m/e ratios 2 (H2), 15 and 16 (CH4), 18 (H2O), 28 (CO),
40 (Ar), and 44 (CO2).
3 Results and discussion
3.2 Methanation reaction over Pd-, Ni-, and Rh-based cata-
lysts
3.1 Ex situ characterization of as-prepared samples
The specific surface areas and the X-ray diffraction patterns of
the as-prepared powder catalyst samples are presented in Table
1 and Figure 1, respectively. The surface area measurements re-
veal that all samples initially have high specific surface areas. The
silica supported catalysts had the highest surface area (310-315
m2/g), followed by the alumina (180-187 m2/g) and the ceria
(125-131 m2/g) supported catalysts. The surface area measure-
ments of MCM-41 and ZSM-5 were cancelled due to the long
measurement times, which indicates that the high surface areas
are preserved after metal impregnation and calcination. Figure 1
shows the XRD patterns for the as-prepared Pd-, Ni- and Rh-based
catalysts. It can be observed that the pure metal oxide patterns
Figure 2 shows the catalytic performance of the metal-promoted
silica, alumina and ceria samples for CO2 hydrogenation. The
inlet gas concentrations to the flow reactor are 1% CO2 and 5%
H2 in Ar and each marker in the figure represents the measured
average outlet concentration over a 10 min period. The main re-
action products formed during CO2 hydrogenation are CH4, CO
and H2O. No hydrocarbon compounds except methane are ob-
served. Due to high background, the water signal is not included
in Figure 2.
The temperature dependence of CO2 hydrogenation can clearly
be seen for all samples. A clear trend is the increased formation
of CO at high temperature, due to the reverse water gas shift
reaction16. The ceria supported catalysts show the highest CO2
conversion and CH4 production over the temperature range in-
◦
are unaffected by calcination at 600 C and impregnation with
Pd, Ni and Rh, suggesting that the metals are well dispersed over
the oxide supports. Additional peaks are observed for some of
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(b)
(b)
(a)
Silica
Alumina
(c)
Ceria
*
Rh
*
*
*
*
*
*
Ni
*
*
*
*
*
*
*
Pd
*
*
*
Support
30
20
40
50
60 20
30
40
2Theta (O)
50
60 20
30
40
2Theta (O)
50
60
2Theta (O)
(d)
(e)
MCM-41
*
ZSM-5
Ni
*
*
*
*
*
*
*
20
30
40
2Theta (O)
50
60 20
30
40
2Theta (O)
50
60
Fig. 1 X-ray diffraction patterns of the support materials (blue) and as-prepared Pd (orange), Ni (green) and Rh (black) catalysts (3 wt.%) supported
on a) silica b) alumina c) ceria and d) MCM-41. e) shows the XRD pattern for 1.5 wt.% Ni (dark green) ion-exchanged into ZSM-5.
◦
no reaction was observed at 150 C during the previous test, the
vestigated, followed by the alumina supported catalysts. This can
be attributed to metal particle size effects. As discussed above,
the catalysts supported on ceria have no metal XRD peaks sug-
gesting amorphous or very small metal particles, followed by the
alumina supported catalysts. Among the studied catalysts, the
Rh/Al2O3 catalyst shows the lowest CO production and highest
investigation focused on measuring the activity at 250 (blue), 350
◦
◦
(orange) and 450 C (grey). At 250 C, the outlet concentration
of CO2 is the same as the inlet concentration for both experi-
◦
ments, which means that no reaction occurs. At 350 and 450 C
the outlet concentrations of CO2 and the formation of methane
are similar, whereas the formation of CO is considerably higher
at the highest temperature. Above an inlet CO2 concentration of
1%, the increased CO formation gives rise to a slightly higher CO2
conversion. Initially, for low inlet concentrations of CO2, the left
column shows a higher CH4 formation at 350 compared to 450
◦
methane selectivity, up to 350 C. Over this temperature the CH4
production declines and the selectivity towards CO increases. The
Ni/CeO2 sample shows the highest CO2 conversion compared to
the other Ni catalysts investigated, in agreement with previous
studies13. The silica supported catalysts show the overall low-
est methane formation also suggested by the larger metal particle
sizes as obtained from the XRD measurements. Further, a signifi-
cant difference in product selectivity between the active metals is
observed.
◦
C. The formation of CH4 then decreases slightly with increased
◦
inlet concentration of CO2 and becomes lower than at 450 C for
an inlet CO2 concentration of 1.5%. In the right column one can
see a linear increase in CH4 formation with the same gradient
◦
at both 350 and 450 C when the inlet H2 concentration is in-
Using MCM-41 as a support gives similar results as the silica
supported catalysts described above, with a slightly higher selec-
tivity towards CH4. However, for Ni/ZSM-5, almost no conversion
of CO2 is observed. The results over the metal-promoted MCM-41
and ZSM-5 samples are shown in Figure S1 (see Electronic Sup-
plementary Information (ESI)†). Due to the low catalytic activity
for CO2 hydrogenation (at least below 400 C), the results of sil-
ica, MCM-41 and ZSM-5 catalysts will not be further reported in
this study.
creased. At the same time, the formation of CO decreases linearly
with a less steep slope. To obtain higher CO2 conversion and CH4
formation it is necessary to increase the H2:CO2 ratio.
From all studied catalysts in this work, the ones supported on
ceria are found to have the highest activity for CO2 hydrogena-
tion, followed by the alumina supported catalysts. No consider-
able methanol production was observed over the studied catalysts
under ambient pressure conditions and temperatures up to 450
◦
◦
The results of a more extended investigation of CO2 hydrogena-
tion over the Rh/CeO2 catalyst are shown in Figure 3. The ex-
periment is carried out by measuring the steady-state outlet gas
concentrations for inlet concentrations between 0.3 and 1.5% of
CO2 (left panel), and between 1 and 5% H2 (right panel). Since
C. In addition, the results reveal that the catalysts preparation
method and structure play an important role on the catalytic ac-
tivity and/or selectivity. The catalysts prepared by impregnation
show the highest CO2 conversion activity and the comparison be-
tween the different catalysts prepared by impregnation reveals
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Silica
Alumina
Ceria
1.0
0.9
0.8
0.7
0.6
0.5
0.4
Pd
CO
CO
CO
2
2
2
Rh
Ni
CH
CH
CH
4
4
4
0.4
0.3
0.2
0.1
0.0
CO
CO
CO
0.4
0.3
0.2
0.1
0.0
150 200 250 300 350 400
Temperature (C)
150 200 250 300 350 400
Temperature (C)
150 200 250 300 350 400
Temperature (C)
Fig. 2 Steady-state outlet concentration of CO2 (top), CH4 (middle) and CO (bottom) as a function of temperature during the reaction between 1%
◦
CO2 and 5% H2 in Ar between 150 and 450 C at a total flow rate of 2000 ml/min at atmospheric pressure over the silica, alumina and ceria supported
Pd (orange), Ni (green) and Rh (black) catalysts.
CO
CO
250 oC
350 oC
450 oC
2
2
1.2
0.8
0.4
0.0
CH
CO
0.4
CH
4
4
0.4
0.2
0.0
CO
0.2
0.1
0.0
0.6
0.8
1.0
1.2
1.4
1
2
3
4
5
CO concentration (%)
H concentration (%)
2
2
Fig. 3 Steady-state outlet concentration of CO2 (top), CH4 (middle) and CO (bottom) over Rh/CeO2 as a function of inlet concentration of CO2 at
constant H2 concentration of 5% (left column) and as a function of inlet concentration of H2 at constant CO2 concentration of 1% (right column) in Ar
gas with a total flow rate of 2000 ml/min at atmospheric pressure.
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Table 2 Assignment of IR absorption bands within the wavenumber
−1
region 1100-3700 cm observed in this study.
sorbed CO species disappear about 2 min after the H2 flow has
been turned off. In addition, some weak components at lower
wavenumbers are observed and will be discussed below (see Fig-
ure 5 description).
−1
Wavenumber (cm
1200-1600
1720
1805
2020
)
Species
Reference
18,23
Formate and carbonate-like species
Bridge-bonded CO on Rh/Ce interface
Bridge-bonded CO on Rh
21
19,20
18,19
23
The color map for the Rh/CeO2 sample in Figure 4 (mid-
dle panels) shows a slightly different behaviour, formate and
carbonate-like species18,23 form during the exposure to CO2 and
CO species linearly bonded on Rh
2800-3000
3000-3700
CH vibrations
x
◦
H2 at 350 C, which can be observed in the IR region 1200-1600
24
Hydroxyl region
−1
cm (Figure 4, middle panels). In addition, a smaller peak cen-
−1
tred at 2020 cm appears corresponding to CO species linearly
adsorbed on Rh (carbonyl), forming at the same time with the
formate and carbonate species. When the H2 flow is turned off
the intensity of the formed surface species decreases. The bands
plotted in the bottom panel (formates and carbonates/carbonyl)
show a sharp increase at the switches from CO2 to CO2+H2 flow
followed by a sharp decrease about four minutes after the H2 flow
is turned off.
that the catalysts with the smallest metal particles (i.e. the cata-
lysts supported on ceria) show the highest activity and selectivity
for CO2 methanation.
The most prominent result in this study is observed for the
Rh/CeO2 catalyst that managed to convert 44% CO2 almost ex-
◦
clusively to CH4 at 350 C. An even higher CO2 conversion is
◦
observed at 450 C but, due to CO formation through the reverse
The results for the Ni/CeO2 sample are similar as for the
Rh/CeO2 sample with the strong peaks appearing between 1200-
water-gas shift reaction, the CH4 selectivity decreases. Further
evaluation of the Rh/CeO2 catalyst revealed evidence of a max-
−1
1600 cm corresponding to formate and carbonate-like species
◦
imum CH4 formation around 380 C. A linear relationship be-
(Figure 4, right panels). No carbonyl species are observed for
the Ni/CeO2 sample during CO2 hydrogenation. A slower forma-
tion rate of formates and carbonate species is observed for the Ni
sample compared to the Rh sample, even though they are much
stronger for the Ni/CeO2 sample. When the H2 flow is turned off,
a continuous decline of the formate and carbonate band is ob-
served for about five min. It is interesting to note the hysteresis
behaviour for the formate and carbonate species observed for the
CeO2 supported catalysts during the CO2 hydrogenation, which is
likely due to the active ceria support.
tween CH4 formation and H2:CO2 ratio was observed at both 350
and 450 C (Figure 3). The experiments for inlet CO2 concen-
◦
trations below 0.5% confirm that a high H2:CO2 ratio leads to
total conversion towards CH4 formation. Ceria-based materials
have aroused increasing interest as supports or catalysts toward
CO2 conversion due to the unique structural properties resulting
from oxygen vacancies and reversible valence change (Ce4+ and
17
Ce3+
)
.
3.3 Spectroscopic surface speciation during CO2 methana-
tion
In order to investigate possible effects of the support, the exper-
iments were repeated for pure alumina and ceria samples at 350
◦
In situ FTIR spectroscopy in diffuse reflectance mode has been
employed to study the surface interaction of CO2 and H2 for se-
lected Rh- and Ni-based catalysts at temperatures between 250-
C and the results are shown in Figure S2 (see Electronic Sup-
plementary Information (ESI)†). Some weak carbonate/formate
species are observed to form on the ceria sample, while alumina
shows no clear adsorption bands, suggesting that ceria can inter-
act with CO2. Additionally, no clear changes are observed for the
two samples during the CO2+H2 versus the CO2 periods.
◦
400 C, in order to provide insights to the CO2 methanation reac-
tion mechanism. The DRIFTS results for the hydrogen pulse ex-
periments under a continuous flow of CO2 for Rh/Al2O3 (left pan-
els), Rh/CeO2 (middle panels) and Ni/CeO2 (right panels) at 350
To better understand the changes that the formed surface
species undergo during the transient experiment, a number of
IR spectra are shown in Figure 5 when the flow of CO2+H2 (48-
50 min) is changed to only CO2 flow (51-57 min) for Rh/Al2O3
(left panel), Rh/CeO2 (middle panel), and Ni/CeO2 (right panel)
◦
C are shown in Figure 4. The top panels show the color coded
intensities (blue corresponds to low intensity, red to high inten-
−1
sity) of the IR spectra in the interval 790-3800 cm versus time.
The bottom panels show the time evolution of the integrated IR
−1
◦
peak areas between 1200-1600 cm corresponding to carbon-
at 350 C. The spectra reveal noticeable changes of the posi-
tion/intensity of the IR bands during the cycle. All significant
−1
ates and formate species, and around 2020 cm corresponding
to carbonyls. By presenting the data in color maps the dynamics
of the evolution of the IR adsorption bands during the different
phases of the experiment is clearly displayed. A peak centred
around 2350 cm−1, corresponding to gaseous carbon dioxide, is
visible in all spectra.
changes observed in the spectra are contained in the region be-
−1
tween 1200 and 2100 cm
. The insert numbers indicate the
time (min) in the dynamic experiment where the spectra were
recorded. The major changes in the IR spectra take place after
the switch from CO2+H2 flow (48-50 min) to CO2 flow only (51-
57 min).
The interaction of CO2 and H2 (in the feed) with the sur-
face of the catalysts generates different surface species, which
are assigned as illustrated in Table 2. For the Rh/Al2O3 sample,
For the Rh/Al2O3 sample (Figure 5, left panel), a strong peak
corresponding to linearly adsorbed CO species on Rh (IR band
a strong peak around 2020 cm corresponding to linearly ad-
around 2020 cm−1), appears during CO2 hydrogenation in agree-
−1
11
sorbed CO species on Rh (Rh0-CO, carbonyl)18,19, appears dur-
ing CO2 hydrogenation, suggesting CO2 dissociation. The ad-
ment to previous reports on CO2 methanation over Rh/Al2O3
A broad band around 1805 cm
.
−1
is also detected, which pre-
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Rh/CeO2
Ni/CeO2
Rh/Al2O3
3500
3000
2500
2000
1500
1000
x 0.1
formates+carbonates
carbonyls
0
100
20
40 60
Time (min)
80
100
0
20
40 60
Time (min)
80
100 0
20
40 60
Time (min)
80
Fig. 4 Transient hydrogenation of 0.2% CO2 over Rh/Al2O3 (left panels), Rh/CeO2 (middle panels) and Ni/CeO2 (right panels) during periodic
◦
variation of the feed gas composition between 0.8% H2+0.2% CO2 and 0.2% CO2 at 350 C for 10 min. The top panels show the color coded
−1
intensities (blue corresponds to low intensity, red to high intensity) of the IR bands in the region 790-3800 cm versus time. The bottom panels show
−1
−1
the IR peak areas between 1200 and 1600 cm representing formate and carbonate species (blue lines) and around 2020 cm representing
carbonyls (red lines).
Rh/CeO2
Rh/Al2O3
Ni/CeO2
57
56
57
56
55
57
56
55
54
55
54
54
53
53
52
51
53
52
51
52
51
50
50
49
48
50
49
49
48
48
3500 3000 2500 2000 1500
Wavenumber (cm-1)
3500 3000 2500 2000 1500
Wavenumber (cm-1)
3500 3000 2500 2000 1500
Wavenumber (cm-1)
−1
Fig. 5 Evolution of IR absorption bands in the wavenumber region 1100-3800 cm for the Rh/Al2O3 (left panel), Rh/CeO2 (middle panel) and
Ni/CeO2 (right panel) catalysts exposed to CO2 and H2 while changing the feed gas composition from 0.8% H2+0.2% CO2 (48-50, red spectra) and
◦
0.2% CO2 (51-57, blue spectra) at an inlet gas temperature of 350 C. The insert numbers indicate the time (min) in the dynamic experiment where
the spectra were recorded.
1–10 | 7

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DOI: 10.1039/C6CY02536F
viously has been attributed to bridge-bonded CO on Rh (Rh0
-
,
port and its reaction with adsorbed H species formed on the metal
which leads to a formate intermediate (COOH) at the metal-
support interface. The formates can give rise to CO species ad-
sorbed on the metal by the rWGS reaction, which subsequently
. The second mechanism (eq.
(5) below) involves the direct dissociation of CO2 to CO and O
2
CO)19,20
wavenumbers by the end of the CO2+H2 cycle ( 1720 cm
.
This band broadens and seems to appear at lower
−1
denoted by the dotted lines in Figure 5) suggesting the forma-
tion of bridged bonded CO species positioned between Rh and
Al atoms from the support, similar to previous reports on CO ad-
sorption between Rh and Ce atoms21. Some additional weaker
are hydrogenated to methane25
adsorbed on the metal surface, with adsorbed CO being subse-
−1
10,12,19,26
peaks in the region 1700-1500 cm appear towards the end of
quently hydrogenated to CH4
.
rWGS
the CO2+H2 cycle (49-50 min) corresponding to carbonate or
formate-like species. Two minutes after the H2 flow is turned off,
the band related to carbonyl species adsorbed on Rh disappears
CO2(support)+ H(metal)
CO(metal)+OH(support)
COOH(interface)
(4)
(5)
CO2
CO(metal)+O(metal)
−1
In contrast to several previous studies on the methanation
mechanism, Upham et al.27 suggest another mechanism for CO2
methanation over Ru-doped ceria, where CH4 is not formed
through intermediates of CO or a formate. IR spectroscopy mea-
surements instead corresponds to a variety of surface carbonate
spectra. This strenghtened the idea of direct CO2 adsorption and
formation of carbonates on ceria supported catalysts, without ad-
sorption and dissociation on the metal phase. The formed car-
bonates react with hydrogen to produce methane that directly
desorbs from the surface of the catalysts.
and the band around 1700 cm shifts towards lower wavenum-
bers (1650 cm−1) indicating the formation of different surface
−1
carbonates. A band centred around 1520 cm is clearly visi-
ble through the entire time the sample is exposed to CO2 (51-57
min).
For the Rh/CeO2 sample (Figure 5, middle panel), strong peaks
are observed in the IR region between 1590-1330 cm−1, together
with a weaker absorption band around 2020 cm−1. The intensity
of the formed surface species during CO2 hydrogenation (formate
and carbonate, and carbonyl species) shows a very slow diminu-
tion directly after the H2 flow is turned off and only a broad peak
The DRIFTS results presented in this work give insights on the
methanation reaction mechanism and differences are observed
between the studied catalysts. In Figures 4 and 5 the surface
species formed when a mixture of CO2+H2 followed by a flow
of CO2 interacts with the surface of Rh/CeO2, Rh/Al2O3 and
Ni/CeO2 are shown. As discussed above, for the Rh based cat-
alysts, CO species adsorbed on metal are formed, suggesting that
CO2 dissociates on these catalysts. Since no carbonyl species are
formed under the flow of CO2 only, it is clear that CO2 dissocia-
tion is favored by the presence of hydrogen which is likely to be
dissociated on the metallic Rh surface. In contrast, no adsorbed
CO species are observed on the Ni catalyst, suggesting a differ-
ent reaction mechanism over this sample. The ability of CO2 to
dissociate over Rh/γ-Al2O3 has been evidenced by in situ DRIFTS
experiments, which show the presence of bands that correspond
to Rh-CO (carbonyls)12. Similar results have been observed over
−1
centered around 1460 cm remains when the sample is exposed
to CO2 only, corresponding mostly to carbonate species22
.
For the Ni/CeO2 sample (Figure 5 right panel), during the tran-
sition between CO2+H2 to only CO2 flow mostly the intensity of
−1
the formates and cabonates peaks in the region 1300-1600 cm
decreases. The spectra during the CO2 hydrogenation phase (48-
50 min) resemble well the spectra recorded for the Rh/CeO2 sam-
ple without the formation of carbonyl species. There are differ-
ences between the two samples during the CO2 flow only (51-
57 min). Stronger peaks are observed for the Ni/CeO2 sample
during the entire experiment. In addition to the carbonate and
−1
formate species, new peaks appear around 2800 cm and 3500-
23
3700 cm−1, corresponding to C-H and O-H vibrations from CH
x
and hydroxyl groups24, respectively. The presence of CH species
x
on the Ni containing species suggest the formation of different
hydrocarbonated compounds on the surface of the catalysts, in
contrast to the Rh containing samples, where only methane in
the gas phase is detected suggesting the direct desorption of CH4
as soon as it forms on the surface. The behaviour of the Ni/CeO2
sample during the CO2 flow is slightly different compared to the
Rh/CeO2 and Rh/Al2O3 catalysts discussed above and is likely
that more formate species form on this sample.
Rh/TiO2 catalysts28
.
Due to the limited wavelength resolution we are unable to sep-
arate between the different formate and carbonate species in the
IR region between 1200-1700 cm−1. However, the observed peak
−1
for C-H stretching at around 2800-2900 cm for the Ni-ceria
sample indicates that more formates are formed on this sample,
while for the Rh containing samples the absence of this peak
may suggest the predominance of carbonate species, which is
also supported by the presence of a smaller peak around 840
3.4 Reaction mechanism during CO2 methanation
−1
cm for these samples (IR region not shown), characteristic of
Recently, it has been demonstrated that CO2 can react with H2
over Rh/γ-Al2O3 to produce methane at room temperature and
atmospheric pressure12. However, in the present study, we see
that higher temperatures are needed for methane to be produced
from CO2 and H2 over Rh/Al2O3. No significant methane produc-
carbonate species. We observe the formation of CO in the gas
phase from the rWGS reaction over the Rh/CeO2 catalyst (Figure
2). Additionally, the formation of formate and carbonate species
is accompanied by the concomitant apparition of adsorbed CO
species, suggesting the first reaction mechanism (through for-
mate/carbonate species) over this sample. In contrast, no CO
in the gas phase is observed for the Rh/Al2O3 catalyst up to 350
◦
tion was observed below 250 C (see Figure 2).
Different mechanisms have been proposed for CO2 methana-
tion over a variety of noble metal-based catalysts. The first mech-
anism (eq. (4) below) involves the adsorption of CO2 on the sup-
◦
C, suggesting the direct dissociation of CO2, in agreement with
previous reports10–12, where formates were proposed to be spec-
8 |
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DOI: 10.1039/C6CY02536F
tator species and to not contribute significantly to methane for-
Abild-Pedersen, S. Zander, F. Girgsdies, P. Kurr, B.-L. Kniep, M.
Tovar, R. W. Fischer, J.K. Nørskov, R. Schlögl, Science, 2012,
336, 893-897.
mation for the methanation reaction11. No carbonyl bands are
observed for the Ni/CeO2 catalyst, only formate and carbonate
bands suggesting a formate/carbonate pathway reaction mecha-
nism. The DRIFTS results suggest that the metal-support interface
for the Rh/CeO2 and Ni/CeO2 catalysts may play an important
role in the CO2 hydrogenation reaction, similar to the reports on
9 F. Studt,I. Sharafutdinov, F. Abild-Pedersen, C. F. Elkjær, J.
S. Hummelshøj, S. Dahl, I. Chorkendorff, J. K. Nørskov, Nat.
Chem., 2014, 6, 320-324.
10 A. Beuls, C. Swalus, M. Jacquemin, G. Heyen, A. Karelovic, P.
Ruiz, Appl. Catal. B, 2012, 113-114, 2-10.
Cu/CeO /TiO (111)29
.
x
2
11 A. Karelovic, P. Ruiz, Appl. Catal. B: Env., 2012, 113-114, 237-
249.
4 Conclusions
Hydrogenation of CO2 at ambient pressure conditions has been
studied over a series of supported Pd-, Rh- and Ni-based cat-
alysts prepared by impregnation. High selectivity to methane
and high CO2 conversion are obtained over the Rh-based cata-
lysts. Our results show that it is possible to produce methane
from CO2 and H2 at ambient pressure and relatively low temper-
ature over Rh/CeO2, Rh/Al2O3 and NiCeO2 catalysts. Methane
is the only hydrocarbon product observed in our analysis. The
results of in situ DRIFTS experiments show that CO2 adsorption
and dissociation occurs over the Rh/Al2O3 catalyst in the pres-
ence of H2, resulting in the formation of linear Rh-CO species,
which accounts for the majority of the adsorbed species formed
12 M. Jacquemin, A. Beuls, P. Ruiz, Cat. Today, 2010, 157, 462-
466.
13 S. Tada, T. Shimizu, H. Kameyama, T. Haneda, R. Kikuchi,
Int. J. Hydrog. Energy, 2012, 37, 5527-5531.
14 S. Brunauer, P. H. Emmett and E. Teller, Journal of the Ameri-
can Chemical Society, 1938, vol. 60, 309-319.
15 P.-A. Carlsson, S. Mollner, K. Arnby, M. Skoglundh, Chem Eng.
Sci., 2004, 59, 4313-4323.
16 D. Pakhare, J. Spivey, Chem. Soc. Rev., 2014, 43, 7813-7837.
17 F. Wang, M. Wei, D.G. Evans, X. Duan, J. Mater. Chem. A,
2016, 4, 5773-5783.
18 F. Solymosi, A. Erdohelyi, T. Bansagi, J. Chem. Soc. Faraday
Trans. 1, 1981, 77, 2645-2657.
◦
during CO2 hydrogenation at 350 C. In contrast the results show
that the methanation reaction mechanism is different over the
Rh/CeO2 catalyst where CO is formed through formate and car-
bonate intermediate species.
19 I.A. Fisher, A.T.Bell, J. Catal., 1996, 162, 54-65.
20 H.Y. Luo, H.W. Zhou, L.W. Lin, D.B. Liang, C. Li, D. Fu, Q. Xin,
J. Catal., 1994, 145, 232-234.
The results, strenghtened by previous studies indicate that a
metal oxide like ceria can interact with CO2 and thus, be a part
of and promote the hydrogenation reaction by direct formation of
surface carbonates and formates.
21 A. Kiennemann, R. Breault, J.-P. Hindermann, M. Laurin, J.
Chem. Soc. Faraday Trans. 1, 1987, 83, 2119-2128.
22 M. Marwood, R. Doepper, A. Renken, Appl. Catal. A., 1997,
151, 223-246.
5 Acknowledgement
23 R.J. Behm, S. Eckle, Y. Denkwitz, J. Catal., 2010, 269, 255-
268.
This work was financially supported by the Swedish Research
Council through the Röntgen-Ångström collaborations "Catalysis
on the atomic scale" (No. 349-2011-6491) and "Time-resolved in
situ methods for design of catalytic sites within sustainable chem-
istry" (No. 349-2013-567) and the Competence Centre for Cataly-
sis, which is financially supported by Chalmers University of Tech-
nology, the Swedish Energy Agency and the member companies:
AB Volvo, ECAPS AB, Haldor Topsøe A/S, Volvo Car Corporation,
Scania CV AB, and Wärtsilä Finland Oy.
24 G.Busca, Phys. Chem. Chem. Phys., 1999, 1, 723-736.
25 D.I. Kondarides, P. Panagiotopoulou, X.E. Verykios, J. Phys.
Chem. C, 2011, 115, 1220-1230.
26 L.F.Liotta, G.A. Martin, G. Deganello, J. Catal., 1996, 164,
322-333.
27 D.C. Upham, A.R. Derk, S. Sharma, H. Metiu, E.W. McFarland,
Catal. Sci. Technol., 2015, 5, 1783-1791
28 A. Karelovic, P. Ruiz, J. Catal., 2013, 301, 141-153.
29 J. Graciani, Science, 2014, 345, 546-550.
References
1 G.A. Olah, Catal. Lett., 2004, 93, 1-2.
2 I. Ganesh, Renewable and Sustainable Energy Reviews, 2014,
31, 221-257.
3 W. Wang, S. Wang, X. Ma, J. Gong, Chem. Soc. Rev., 2011, 40,
3703-3727.
4 G.W. Busser et al., ACS Catal., 2015, 5(9), 5530-5539.
5 D.T. Whipple, P.J.A, Kenis, J. Phys. Chem. Lett., 2010, 1(24),
3451-3458.
6 J.N. Park, E.W. McFarland, J. Catal., 2009, 266, 92-97.
7 G.C. Chinchen, P.J. Denny, J.R. Jennings, M.S. Spencer, K.C.
Waugh, Appl. Catal., 1988,36, 1-65.
8 M. Behrens, F. Studt, I. Kasatkin, S. Kühl, M. Hävecker, F.
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Rh/Al O
Rh/CeO
Ni/CeO
2
2
3
2
3000
2000
1000
CO
CH
2
20
10
4
CO
20
30 40
Time (min)
50
60 20
30 40
Time (min)
50
60 20
30 40
Time (min)
50
60
◦
TOC: CO2 methanation over Rh/Al2O3, Rh/CeO2 and Ni/CeO2 at 350 C highlighting the different surface speciation during reaction.
10 |
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