Hydrogenation of CO2 over supported noble metal catalysts

Article abstract of DOI:10.1016/j.apcata.2017.05.026

The catalytic activity of supported noble metal catalysts for the CO₂ methanation reaction has been investigated with respect to the nature (Rh, Ru, Pt, Pd), loading (0.1–5.0?wt.%) and mean crystallite size (1.3–13.6?nm) of the dispersed metall

Full text of DOI:10.1016/j.apcata.2017.05.026

Accepted Manuscript
Title: Hydrogenation of CO over supported noble metal
2
catalysts
Author: Paraskevi Panagiotopoulou
PII:
S0926-860X(17)30234-X
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2017.05.026
APCATA 16248
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
24-3-2017
16-5-2017
24-5-2017
Please cite this article as: Paraskevi Panagiotopoulou, Hydrogenation
of CO2 over supported noble metal catalysts, Applied Catalysis A,
Generalhttp://dx.doi.org/10.1016/j.apcata.2017.05.026
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Hydrogenation of CO over supported noble metal catalysts
2
Paraskevi Panagiotopoulou*
School of Environmental Engineering
Technical University of Crete, GR-73100 Chania, Greece
Tel. No: +30 28210 37770
e-mail: ppanagiotopoulou@isc.tuc.gr
*
To whom correspondence should be addressed.
Graphical abstract
1E-11
1E-12
1E-13
Ru -CO
x% Ru/TiO2
x% Ru/TiO2
x
dRu =4.2 nm
dRu =2.1 nm
(TiO )Ru-CO
2
Ru -CO
x
n+
Ru -(CO)x
2
200 2100 2000 1900 1800
1.0 1.5 2.0 2.5 3.0 3.5 4.0
-
1
Wavenumber (cm )
dRu (nm)
1

Research Highlights:




Rh and Ru catalysts are more active for the CO
catalysts, which promote the RWGS.
2
methanation compared to Pt and Pd
CO
2
methanation reaction is structure sensitive and is enhanced with increasing mean
Ru crystallite size.
The reactive surface intermediates could potentially be CO species linearly bonded on
reduced Ru crystallites (Ru -CO).
x
The relative intensity of the band due to Ru
particle size.
x
-CO species increases with increasing Ru
ABSTRACT
The catalytic activity of supported noble metal catalysts for the CO
2
methanation reaction has
been investigated with respect to the nature (Rh, Ru, Pt, Pd), loading (0.1-5.0 wt.%) and
mean crystallite size (1.3-13.6 nm) of the dispersed metallic phase. It has been found that the
turnover frequency (TOF) of CO
2
conversion for TiO -supported catalysts increases
2
following the order of Pd < Pt < Ru < Rh, with Rh being about 3 times more active than Pd.
Selectivity toward methane depends strongly on the noble metal catalyst employed and is
significantly higher for Rh and Ru catalysts compared to Pt and Pd, which mainly promote
production of CO via the RWGS reaction. Conversion of CO
increases significantly with increasing Ru loading in the range of 0.1-5.0 wt.%. Results of
kinetic measurements show that the CO hydrogenation reaction is structure sensitive, i.e.,
catalytic activity is strongly influenced by metal crystallite size. In particular, for Ru/TiO
and Ru/Al catalysts, the normalized turnover frequency (TOF divided by the length of the
perimeter of the metal-support interface) increases by two orders of magnitude with
increasing ruthenium crystallite size in the range of 0.9-4.2 nm and 1.3-13.6 nm, respectively.
2
at a given temperature
2
2
O
2 3
FTIR experiments provide evidences that CO
2
hydrogenation reaction occurs via intermediate
n+
formation of adsorbed CO species (Ru
x
-CO, Ru (CO)
x
, (TiO )Ru-CO) produced via the
2
RWGS reaction. Part of this species interacts with adsorbed hydrogen atoms producing
2

methane, whereas the remaining species desorbs to yield CO in the gas phase. The CO
2
hydrogenation pathway does not change with variation of Ru crystallite size. However, the
relative intensity of the band due to CO species linearly bonded on reduced Ru crystallites
(
Ru
x
-CO) increases significantly with increasing Ru particle size, indicating that Ru -CO
x
species could potentially being the reactive surface intermediates.
Keywords: CO
2
methanation; Ru, Rh, Pt, Pd; TiO
2
; Al
2
3
O ; structure sensitivity; DRIFTS
1
. INTRODUCTION
Fossil fuels consumption is responsible for the significant emissions of CO
2
in the
atmosphere, which, in nowadays, is considered as one of the main greenhouse gases resulting
in global warming and therefore, major climate changes [1-3]. Thus, efforts have been made
during the last decades in order to eliminate and utilize CO
2
. Among the proposed utilization
processes, CO hydrogenation is of special interest, since it is not only able to reduce
2
effectively carbon dioxide, but also opens up novel pathways for producing high added value
chemicals and fuels [4-6]. Depending on the operating conditions and catalyst employed, the
interaction of CO
2
with hydrogen results in the formation of a variety of products, including
hydrocarbons [2, 6].
carbon monoxide, methane, methanol, formic acid and C
2
Methanation of CO
2
(Eq. 1) takes place efficiently at atmospheric pressure and
o
moderate temperatures (300-400 C) and offers certain advantages provided that hydrogen is
generated from renewable energy sources (e.g. water electrolysis, biomass etc.) [1, 2, 4-7].
o
CO 4H  CH 2H O
ΔH = -165 kJ/mol
(Eq. 1)
2
2
4
2
In particular, methane produced can be used as synthetic substitute of natural gas in chemical
and petrochemical industry and/or as energy carrier in power plants following well
established methods for its storage and transfer [3-5].
3

It is generally accepted that the CO hydrogenation proceeds through intermediate
2
formation of CO via the reverse water-gas shift (RWGS) reaction (Eq. 2), followed by CO
hydrogenation to methane [8-10].
o
CO H  CO H O
ΔH = 41.1 kJ/mol
(Eq. 2)
2
2
2
However, depending on the reaction conditions and catalyst employed, part of the produced
CO cannot be further hydrogenated toward CH and is desorbed in the gas phase, resulting in
low methane selectivities. Thus, it is important to develop active and stable CO
catalysts, able to convert selectively CO to CH
4
2
methanation
o
2
4
at relative low temperatures (< 400 C).
Hydrogenation of carbon dioxide toward methane can be catalyzed by several metal
catalysts, including Ni [2, 3, 11-14], Ru [7, 15-21], Rh [1, 4, 5, 15, 22], Pt [15], Pd [6, 15, 23]
Ir [24] and Co [25] supported on various metal oxides (e.g. TiO
2
, SiO
2
, Al
2
O
3
, CeO
2
, ZrO ).
2
Generally, Ru, Rh and Ni catalysts exhibit high activity for the reaction, which is also
affected by the nature of the support [13, 15, 26-28]. Alumina and titania supported catalysts
present high activity and selectivity toward methane and thus, they are widely studied. It has
been found that metal-support interactions can modify the catalytic properties of the active
metallic phase (i.e. morphology, dispersion etc.) and thus, affect the CO hydrogenation
2
activity [26, 27]. Metal loading and crystallite size affect catalytic performance of supported
metal catalysts in a manner which depends on operating conditions and metal-support
combination employed [4, 6, 18, 26, 29-31]. For example, it is generally accepted that a high
Ru or Pd loading is necessary to produce selectively methane in CO
6]. On the other hand, although catalysts with high Ni loadings exhibit high CO
conversions they tend to enhance the RWGS reaction, resulting in lower methane selectivities
26]. In the case of Rh/Al catalysts, CO methanation activity is favored over large Rh
2
hydrogenation [6, 18,
2
2
[
2
O
3
2
o
particles at low reaction temperatures (<185 C), whereas at high reaction temperatures no
effect has been observed [4].
4

In the present study, the catalytic performance of TiO
metals for the title reaction is investigated with respect to the nature, loading and mean
crystallite size of the metallic phase. The relation between CO hydrogenation activity and
2
- and Al
2
3
O -supported noble
2
physicochemical properties of the dispersed metallic phase is discussed. Τhe effect of metal
crystallite size on the nature and relative population of adsorbed surface species produced
during CO
2
methanation is investigated over Ru/TiO catalysts employing in situ FTIR
2
spectroscopy technique.
2
2
. EXPERIMENTAL
.1 Catalyst preparation and characterization
Noble metal catalysts supported on TiO
2
(Degussa P25) and γ-Al
2
3
O (Alfa products)
were synthesized employing the wet impregnation method [32]. The metal precursor salts
used were Rh(NO
3
)
3
, Ru(NO)(NO
3
)
3
, (NH
3
)
2
Pt(NO
2
)
2
and (NH
3
)
2
Pd(NO
2
)
2
. Impregnation of
o
o
o
catalysts was followed by drying at 110 C for 24 h and reduction at 300 C (400 C for Ru
catalysts) in flowing hydrogen for 2 h. The nominal metal loading of the catalysts thus
prepared varied between 0.1 and 5 wt.%. In the case of Ru/Al
2
3
O catalysts, three additional
samples with 5 wt.% Ru content were prepared in an attempt to increase Ru crystallite size.
o
This was achieved by calcination of the dried samples in air at 600, 650 or 700 C for 2 or 4 h,
followed by reduction with H as described above.
2
Carriers and dispersed noble metal catalysts were characterized with respect to their
specific surface area employing nitrogen physisorption at 77 K (B.E.T. technique) with the
use of a Micromeritics (Gemini III 2375) instrument [32]. The exposed metallic surface area,
metal dispersion and mean crystallite size were estimated employing selective chemisorption
of H
2
with the use of a modified Fisons Instruments (Sorptomatic 1900) apparatus [32, 33].
o
o
o
Measurements have been conducted at 25 C for Rh and Pt, 60 C for Pd and 100 C for Ru
5

samples. Calculations of the exposed metallic surface area, metal dispersion and mean
crystallite size have been performed using standard equations described in detail elsewhere
[32, 33]. The structural characteristics of selected catalysts were also investigated employing
Transmission Electron Microscopy (TEM) following standard procedures [10, 33].
2
.2 Catalytic performance tests
The catalytic performance for the CO hydrogenation reaction was evaluated using a
2
fixed bed reactor, operating at atmospheric pressure. The reactor consisted of two 6 mm O.D.
sections (22-cm long) of quartz tube, which served as inlet and outlet to and from a quartz cell
of 8 mm O.D. (1-cm long), in which the catalyst sample was placed by means of quartz wool.
The temperature of the catalyst was measured by a K-type thermocouple, which ran through
the cell. Heating of the reactor was provided by an electric furnace, the temperature of which
was controlled using a second K-type thermocouple placed between the reactor and the walls
of the furnace. The pressure drop in the catalyst bed was measured by means of a pressure
indicator. The flow of the gases (CO
2
, H , He) was measured and controlled with the use of
2
three mass-flow controllers (M+W Instruments). The gas mixture was fed to the reactor
through stainless steel tubing, whereas, when desired, the gas stream was introduced to a by-
pass loop by means of a common set of switch valves. The exit of the reactor was connected
to a three-way valve, which allows selection of introduction of the gas stream to the GC or to
a bubble flow meter, which was used for measuring the total flow rate at the exit of the
reactor. The reactor effluent was analyzed using a gas chromatograph (Shimadzu) equipped
with two packed columns (Porapak-Q, Carboxen) and two detectors (TCD, FID), operating
with He as the carrier gas. Determination of the response factors for the TCD and FID
detectors was achieved using gas streams of known compositions.
o
Catalytic performance tests were carried out in the temperature range of 170-450 C
using 100 mg (particle size: 0.18< d < 0.25 mm) of catalyst and a total flow rate of 150 cm3
6

-1
o
min . Prior to each experiment the catalyst was reduced in situ at 300 C for 1 h under
3
-1
5
1
5
0%H /He flow (60 cm min ), followed by purging with He and decrease of temperature to
2
o
3
-1
70 C. The sample was then exposed to the reaction mixture (150 cm min ) consisting of
%CO
2
+20%H (in He), and the conversions of reactants as well as the product distribution
2
were determined using the gas chromatograph described above. The temperature was then
o
stepwise increased up to 450 C and similar measurements were obtained. It should be noted
that the reaction was carried out for 1 hour in each temperature, so as steady state conditions
to be achieved. Carbon dioxide conversion (XCO2) and selectivities tοward CH
CO) were calculated following the expressions described in detail in the SI.
The intrinsic reaction rates were measured under differential reaction conditions, i.e. for
4
(SCH4) and CO
(
S
conversions of reactants lower than 10%. These results, along with the measurements of
metal dispersion, were used to calculate the turnover frequencies (TOFs) of carbon dioxide
conversion, defined as moles of CO
2
converted per moles of surface noble metal atom per
second (s ). The equations used are described in the SI.
.3 In situ FTIR spectroscopy
The CO hydrogenation reaction was investigated by performing in situ FTIR
-1
2
2
spectroscopy experiments under continuous flow conditions with the use of a Nicolet 6700
FTIR spectrometer. The spectrometer was equipped with a diffuse reflectance (DRIFT) cell
(Spectra Tech), an MCT detector and a KBr beam splitter [8]. Briefly, the catalyst powder
o
3
-1
was placed into the sample holder and heated at 450 C under He flow (30 cm min ) for 10
3
-1
o
min followed by reduction with 20%H in He (30 cm min ) at 300 C for 60 min. The
2
3
-1
o
reduced sample was then heated in flowing helium (30 cm min ) at 450 C for 10 min and
subsequently cooled down to room temperature (RT). In the cooling stage, background
spectra were recorded at selected temperatures. Finally, the flow was switched to the reaction
3
-1
mixture, which consisted of 1%CO
2
+4%H (in He) (30 cm min ) and the first spectrum was
2
7

recorded at RT after 15 minutes-on-stream. The temperature was then increased in a stepwise
o
mode up to 450 C and spectra were collected at selected temperatures. In each temperature an
equilibration for 15 minutes-on-stream was carried out prior to spectrum recording.
3
3
. RESULTS AND DISCUSSION
.1 Catalyst characterization
The specific surface areas of commercial TiO
2
and Al
2
3
O used as supports in the
2
2
present study were measured with the BET method and found to be 42 m /g and 83 m /g,
respectively. Similar measurements have been conducted over the dispersed noble metal
catalysts and results showed that the specific surface areas were not altered upon addition of
metal. The physicochemical characteristics of synthesized catalysts are summarized in Table
1
, where metal loading (XMe), dispersion (D) and mean crystallite size (dMe) are listed for all
samples investigated. It is observed that metal dispersion and crystallite size vary
significantly in the range of 7-99% and 0.9-13.6 nm, respectively, depending on the nature
and loading of the metallic phase, as well as on the nature of the support employed.
The structural characteristics of 0.5% and 5%Ru/TiO
by HRTEM and the images obtained are shown in Fig. S1. It is observed that both 0.5% and
%Ru/TiO catalysts comprise fairly homogeneously dispersed spherical Ru particles of
approximately 1.8 and 4.0 nm diameter, respectively. Ruthenium crystallite sizes obtained by
TEM analysis agree well with those estimated based on H chemisorption measurements
Table 1).
.2 Effect of the nature of the metallic phase on CO
The effect of the nature of the dispersed metal on catalytic performance for the CO
hydrogenation reaction was investigated over Rh, Ru, Pd and Pt catalysts of the same metal
loading (0.5 wt.%) supported on TiO . Results obtained are summarized in Fig. 1A, where
the conversion of CO obtained over noble metal catalysts is plotted as a function of reaction
2
catalysts were also investigated
5
2
2
(
3
2
methanation activity
2
2
2
8

temperature. Under the present experimental conditions both CO methanation and RWGS
2
reactions are equilibrium limited. Thus, the equilibrium conversion, predicted by
thermodynamics, is also shown for comparison.
It is observed that, under the experimental conditions employed, Rh catalyst exhibits
significantly higher CO conversions, in the entire temperature range, compared to Ru and Pt,
2
whereas Pd is significantly less active than the other samples examined. In particular, Rh-
o
loaded sample is activated at temperatures higher than 150 C and XCO2 reaches equilibrium at
o
ca 380 C. The conversions curves of Ru and Pt catalysts are shifted toward higher
o
temperatures, compared to that of Rh, while temperatures higher than 450 C are required to
achieve equilibrium CO
2
conversions. For the least active Pd catalyst, XCO2 is lower than 10%
o
at temperatures as high as 440 C.
In addition to methane, hydrogenation of CO also results in the production of
2
appreciable amounts CO, via the RWGS reaction, which is strongly affected by the nature of
the dispersed metallic phase. Representative results can be seen in Fig. 1B where selectivities
o
toward CO and CH
4
at 350 C are plotted for the investigated catalysts. Interestingly,
o
methane selectivity at 350 C is 100% for Rh, 86% for Ru, 1% for Pt and practically zero for
Pd catalyst. It should be noted that SCH4 decreases from 90 to 79% with increasing
temperature for Ru/TiO
unaffected by temperature for Rh, Pt and Pd/TiO
is strongly favored over Pt and Pd catalysts, which are not able to methanate CO
under the present experimental conditions employed. Results are in agreement with previous
studies over noble metal catalysts (Rh, Ru, Pt, Pd) supported on Al [15] and CeO [34],
and can be explained considering the mechanistic pathway proposed for the title reaction. It
has been suggested that CO hydrogenation proceeds through intermediate formation of CO
at the metal-support interface via the RWGS reaction. Part of CO interacts with hydrogen
2
catalysts (not shown for clarity), whereas it remains practically
catalysts. Results indicate that the RWGS
at least
2
2
2
O
3
2
2
9

leading to methane production, whereas the rest is desorbed to yield CO in the gas phase. The
enhancement of each pathway depends on the catalyst and experimental conditions
employed, and determines selectivity toward CH
of Fig.1B suggest that the adsorbed CO species produced via the RWGS reaction at the
interface between Pt (or Pd) and TiO support prefer to desorb yielding CO in the gas phase
rather than to interact with hydrogen producing CH
Turnover frequencies of CO conversion (TOFCO2) over the four catalysts are
4
(methanation) or CO (RWGS) [8]. Results
2
4
.
2
summarized in the Arrhenius-type diagram of Fig. 1C. It is observed that the intrinsic activity
o
per exposed metal atom varies by a factor of 2.9 at 250 C in the order of Rh > Ru ~ Pt > Pd.
However, the specific activity takes completely different values taking into account the
selectivity toward methane formation. In particular, the specific reaction rate of methane
formation (not shown for clarity) is 1.5 times higher for Rh catalyst compared to Ru and
practically zero for Pt and Pd catalysts. The high activity of Rh and Ru catalysts for CO
2
methanation has been reported by several authors and has been attributed to the rapid
hydrogenation of the intermediate CO, resulting in higher methane production [15, 26, 28,
4].
3
The apparent activation energies (E
a
) of the CO methanation reaction over the four
2
catalysts examined were calculated from the slopes of the fitted lines shown in Fig. 1C and
results are summarized in Table 1. It is observed that the apparent activation energy depends
on the nature of the metallic phase, taking values between 55.6 kJ/mol for Ru and 71.1
kJ/mol for Rh. Similar values have been previously reported over Al
metal catalysts [15, 24].
2
3
O -supported noble
3
3
.3 Effect of metal loading and crystallite size
.3.1 CO methanation activity
The effect of metal loading on catalytic performance was investigated over ruthenium
2
1
0

catalysts (0.1-5.0 wt.%) supported on TiO . The mean crystallite size of Ru (dRu) in this series
2
of catalysts increases from 0.9 to 4.2 nm with increasing ruthenium content from 0.1 to 5.0
wt.%, respectively (Table 1). Results obtained are shown in Fig. 2A, where it is observed that
increasing Ru loading from 0.1 to 5.0 wt.% results in a progressive and significant shift of the
CO
2
conversion curve toward lower temperatures. In particular, the catalyst with the lowest
o
Ru loading (0.1 wt.%) is activated at temperatures higher than 250 C, whereas temperatures
o
higher than 450 C are required in order XCO2 to reach equilibrium conversions. On the other
o
hand, the 5 wt.% Ru-loaded sample is already active at 165 C and XCO2 reaches equilibrium
o
conversions at 375 C (Fig. 2A).
The specific reaction rate of CO
2
conversion over the Ru/TiO samples of variable
2
metal loading and crystallite size are shown in the Arrhenius-type diagram of Fig. 2B. It is
observed that the rate per surface Ru atom does not follow a monotonic trend with respect to
the size of Ru crystallites. Turnover frequency takes similar values for the 0.1 and 0.5 wt.%
loaded catalysts, which characterized by Ru crystallite size of 0.9 and 2.2 nm, respectively.
TOF increases with increasing metal crystallite size up to 3.2 nm and decreases for the
sample with larger Ru particles of 4.2 nm (5.0 wt.%). As discussed above, CO
2
hydrogenation proceeds through the formation of CO, as an intermediate, via the RWGS,
followed by CO hydrogenation to methane. The RWGS reaction has been found to take place
at the metal carrier interface [35]. This implies that the interface between the Ru and TiO is
2
promoting this catalytic step. Therefore, the interfacial area between the metal particles and
the support material participates directly in the reaction pathway, and influences the observed
catalytic activity.
The length of the metal/support interface decreases with increasing metal particle size,
and therefore, the specific rate would be expected to decrease with increasing metal particle
size. However, if specific activity is enhanced with the size of the metal crystallites, the
1
1

opposite tendency is expected. In order to eliminate the factor attributed to the length of the
interface, TOF was normalized with respect to the interfacial length. The total perimeter of
the metal/support interface per Ru surface area, I , was calculated following Eq. 3, which is
0
based on our previous study [33] and developments reported by Duprez et al. [36], assuming
hemispherical particles.
2
m
A      AW
(Eq. 3)
Ru
2
Ru
I0 

minterface / m

N  S
AV
Ru
2
Ru
Where: A is the metallic surface area

m
,
/ gρ is

the specific weight of the metal
m
Ru
3
(
12.3×106
gRu / m
Ru ), AWRu is the atomic weight of the metal (101.07g / molRu, NAV is the
)
Ru
2
3
Avogadro number (6.023×10 atomRu / molRu), SRu is the surface area occupied by a surface
metal atom (8.6∙10-20 Ru2/Ru atom) and β is the coefficient depending on the particle shape
m
(
β=33.3 for a hemispherical particle). As it was expected the length of the metal/support
interface (I
0
) decreases progressively with increasing ruthenium crystallite size (Fig.S2A).
and results
, are presented in the Arrhenius-type diagram of Fig. 3A. It is observed that
normalized specific activity for the CO hydrogenation reaction at a given temperature
Turnover frequencies presented in Fig. 2B were normalized with respect to I
obtained, TOF/I
0
0
2
increases progressively (by two orders of magnitude) with increasing ruthenium crystallite
size, from 0.9 to 3.2 nm, whereas it does not change appreciably for particle sizes up to 4.2
o
nm. This can be clearly seen in Fig. 3B, where the TOF/I
mean Ru crystallite size (dRu) for all TiO
0
at 250 C is plotted as a function of
2
-supported catalysts investigated. The increase of
the normalized TOF which is observed in Fig. 3A and B can be attributed to the influence of
Ru crystallite size alone on specific catalytic activity. Based on previous studies, the WGS
reaction over supported Ru catalysts is structure insensitive with respect to the metal, i.e. the
TOF does not depend on metal loading or crystallite size [32, 35]. Thus, although the RWGS
and CO hydrogenation reactions run in parallel under the present experimental conditions,
2
1
2

only the rate of CO
argued that CO hydrogenation reaction is structure sensitive and is enhanced with increasing
mean Ru crystallite size.
The effect of Ru crystallite size on catalytic activity has been also investigated over
catalysts of variable ruthenium content (Fig.4). Results of H chemisorption
2
hydrogenation is affected by the size of Ru particles. It may then be
2
Ru/Al
2
O
3
2
measurements showed that increasing Ru loading from 0.5 to 5 wt.% results in an increase of
Ru particles size in the narrow range of 1.3-1.6 nm (Table 1). In order to increase metal
o
crystallite size, 5%Ru/Al
investigated for the CO
2
O
3
catalysts were calcined at 600-700 C for 2-4 hours and
2
hydrogenation reaction. An increase of Ru particles up to 13.6 nm
was achieved (Table 1), which was accompanied by a decrease of the length of the
metal/support interface (Fig. S2B). Results of kinetic measurements are shown in Figs.4A
and B, where it is observed that the normalized specific activity, TOF/I
improved with increasing Ru crystallite size from 1.3 to 13.6 nm. Obviously, the beneficial
effect of increasing the size of Ru particles on catalytic activity is also operable for Al
support. Comparison of Figs.4 and 5 shows that the effect is higher in the case of Ru/TiO
catalyst, taking into account that the size of Ru crystallites varies in the narrow range of 0.9-
.2 nm compared to the wider range of 1.3-13.6 nm for Ru/Al catalysts.
It is of interest to know that, for both TiO - and Al -supported catalysts, the
0
, is significantly
O
2 3
2
4
2
O
3
2
2
O
3
increase of intrinsic activity with increasing ruthenium crystallite size, at a given temperature,
is accompanied by an increase of methane selectivity from 90 to 100% and from 80 to 100%,
respectively (Fig 5). Catalysts with low ruthenium content (or small Ru particles) favor the
formation of CO, which is diminished substantially for higher ruthenium loadings (or large
Ru particles). Results agree well with the above discussion indicating that the hydrogenation
of the intermediate produced carbon monoxide takes place faster with increasing Ru
crystallite size (decreasing the length of the metal/support interface). Results of Fig.5 are in
1
3

agreement with previous studies over Pd/Al
was found to be higher (by a factor of 2-3) over 5%Pd/Al
.5%Pd/Al [6]. Similarly, SCH4 has been found to be improved over large Ru particles
dispersed on TiO [37] or Al [37, 38].
It can be concluded that CO hydrogenation on Ru is structure sensitive and increases
with increasing mean Ru crystallite size or that larger Ru particles are significantly more
efficient in catalyzing CO hydrogenation. The structure sensitivity of dispersed noble metals
for the CO methanation reaction has been investigated by several authors and results seems
2
O
3
catalysts, where selectivity toward methane
2
O
3
compared to that measured over
0
O
2 3
2
2
O
3
2
2
2
to depend on the metal-support combination employed, the experimental conditions and the
range of metal particles size investigated [4, 6, 18, 31, 38, 39]. For example, Karelovic et al.
[
4] found that large Rh particles supported on Al
2
O
3
are more active than smaller particles at
o
o
temperatures lower than 150 C, whereas above 200 C the intrinsic activity does not depend
on metal particle size. Kowalczyk et al. [39] reported that TOF for CO
2
methanation reaction
catalysts.
increases with decreasing Ru dispersion over Ru/Al , Ru/MgO and Ru/MgAl
2
O
3
2
O
4
Moreover, it has been found that the rate of methane production per surface Rh atoms
increases with increasing metal particle size up to 7 nm, whereas it does not vary significantly
for larger Rh particles [31]. As it will be discussed below, most of the studies agree that the
beneficial effect of large metal particles on catalytic activity is attributed to the higher
population of active sites which is enhanced over catalysts of high metal content.
Regarding the apparent activation energy of CO
show that it varies in the range of 51.5-71.1 kJ/mol for Ru/TiO
Ru/Al without presenting any trend with respect to metal particle size. This can be
attributed to the fact that two reactions, RWGS and CO hydrogenation, run simultaneously
2
hydrogenation, results (Table 1)
2
and 61.9-93.7 kJ/mol for
2
O
3
2
under the present conditions, each one of which depends in a different manner on
temperature, nature of the support and metal crystallite size. This is in accordance to the
1
4

results of Aksoylu et al.[40], who found that E
Ni/Al catalysts without depending on Ni loading (5-25 wt.%). Similar values of the
apparent activation energy have been previously reported for Rh/Al (60.7-95.0 kJ/mol) [4]
and Rh/TiO (71.1-120.1 kJ/mol) [31] catalysts. However, the authors found that E depends
a
varies in the range of 59.4-72.8 kJ/mol over
2
O
3
2
O
3
2
a
on Rh crystallite size and that smaller Rh particles exhibit higher activation energies.
Interestingly, Kwak et al. [38] found that the apparent activation energy of CO formation was
lower (by ~20 kJ/mol) than that of CH
4
formation, regardless of Ru loading. The authors
follows different pathways or takes place on
suggested that the formation of CO and CH
4
different active sites, indicating that CO cannot be considered as an intermediate of methane
formation. This should not be the case for the results of the present study. Although the
apparent activation energy of CO formation was not possible to be estimated for all catalysts
examined, in most cases it takes values lower or equal to that of methane formation implying
that methanation of CO
.3.2 CO methanation studied by DRIFTS
Results of Figs.2-5 clearly show that ruthenium crystallite size affects strongly CO
methanation activity. In an attempt to identify the nature of active sites and adsorbed species,
which drives selectivity for CO hydrogenation reaction with respect to Ru crystallite size,
the interaction of CO /H mixture with the catalyst surface was investigated over 0.5% and
%Ru/TiO catalysts employing in situ FTIR spectroscopy. It should be noted that, with the
exception of the CO /H ratio (1/4), the experimental conditions (e.g. mass of catalyst, total
2
proceeds through intermediate CO formation.
3
2
2
2
2
2
5
2
2
2
flow rate, feed composition etc.) used in DRIFT experiments are not identical with those used
during catalytic performance tests under steady state conditions. Moreover, previous studies
have shown that DRIFT cells may present some bypass of the catalyst bed [41]. Therefore,
FTIR results of the present study cannot be used to extract kinetic information, but only
qualitative information related to the nature and variation of the relative population of
1
5

adsorbed surface species with reaction temperature and catalyst employed.
In Figure 6Α are shown DRIFT spectra obtained over the 0.5%Ru/TiO catalyst under
2
o
CO
that interaction of the catalyst with 1%CO
results only in the development of bands below 1700 cm (trace a) which, are due to formate
2
hydrogenation reaction conditions in the temperature range of 25-450 C. It is observed
o
2
+4%H mixture at temperatures lower than 100 C
2
-1
-1
(
bands at ca. 1553, 1382 and 1318 cm ) and bicarbonate (bands at ca. 1660, 1646 and 1426
-1
cm ) species associated with the support [8, 31, 42-44]. However, increase of temperature at
o
-1
1
50 C (trace c) results in the development of a band at 2060 cm , two overlapping peaks
-1
-1
located at 2020 and 1967 cm , and a broad weak band at 1837 cm , indicating the formation
-1
of Ru-bonded carbonyl species. The band at 2060 cm is due to multicarbonyl species
n+
n+
adsorbed on partially oxidized Ru sites (Ru (CO)
x
) [8-10, 45], which disappears from the
o
catalyst surface upon heating at temperatures higher than 300 C. The latter sites can be
formed by the oxidative disruption of small Ru clusters with the participation of hydroxyl
groups of the support, which is enhanced over well-dispersed Ru catalysts [10, 46, 47]. The
-1
broad band at 1837 cm can be attributed to bridge-bonded CO on reduced Ru crystallites [8,
-1
1
9]. The peak at 2020 cm appears at wavenumbers corresponding to CO species linearly-
-1
bonded on reduced Ru sites (Ru -CO) [8-10, 19, 45, 46], whereas the peak at 1967 cm can
x
be attributed to terminal CO species adsorbed on Ru sites located at the metal-support
interface ((TiO
2
)Ru-CO) [8-10, 48]. The relative intensity of the (TiO )Ru-CO band
2
o
decreases with increasing temperature up to 250 C, whereas the opposite is observed for the
o
band assigned to Ru
of the Ru
x
-CO, which dominates in the spectra obtained up to 450 C. The red shift
-1
o
-1
o
x
-CO band from 2020 cm at 150 C (trace c) to 1991 cm at 450 C (trace i) is
consistent with the decrease of the dipole-dipole coupling with decreasing coverage [9, 49].
n+
In our previous study, under CO/H
2
conditions, it has been found that Ru (CO)
x
species are interconverted to Ru -CO species most probably due to H
x
2
-induced reductive
1
6

n+
agglomeration of Ru sites to Ru
x
[8, 10]. This may also be the case for the results of the
-1
present study since the decrease of the relative intensity of the band at 2060 cm is
-1
accompanied by an increase of the band at 2020 cm . The intensity of the latter band starts
o
-1
decreasing above 300 C, where a new weak band at 3015 cm assigned to gas-phase CH
not shown) [9] appears, indicating the onset of CO methanation reaction.
Based on our previous studies, in the absence of H in the feed, the interaction of CO
with Ru/TiO catalyst results only in the development of bands due to carbonate species
associated with TiO support, whereas no bands are detected in the ν(CO) frequency region
8, 9]. This indicates that CO is mainly adsorbed on the support surface [50], and that
does not take place, in agreement with results obtained over Pd/Al
[51] catalysts. Results of Fig.6A show that Ru-bonded
carbonyl species can be detected in the presence of H in the gas stream, indicating that
hydrogen facilitates the conversion of CO to CO. It has been previously suggested that
hydrogen adsorbed on Ru surface is diffused at the metal-support interface where it reacts
with CO , which is mainly adsorbed on TiO , yielding formate and, eventually, adsorbed CO
species according to [6, 9, 10, 18, 42, 51]:
CO2(TiO2 )  H(Ru) (COOH)(Ru-TiO2) CO(Ru) OH(TiO2 )
4
(
2
2
2
2
2
[
2
dissociation of CO
2
2
O
3
[6], Ru/Al
2
O
3
[18] and Rh/Al
2
O
3
2
2
2
2
(Eq. 4)
This scheme is consistent with the results of Fig. 6A, where the relative population of formate
o
species associated with the support starts decreasing at temperature higher than 150 C where
the adsorbed CO species start to be developed. Part of this species may interact with adsorbed
hydrogen atoms to form methane in the gas phase, whereas the rest is possibly desorbed
yielding CO in the gas phase, in a manner which depends on the catalyst and experimental
conditions employed [10]. The elimination of (TiO )Ru-CO species from the catalyst surface
2
o
already at 250 C, i.e. prior to initiation of CO
2
methanation reaction provides evidence that
1
7

this species cannot be further hydrogenated toward methane and most probably is desorbed
producing gas phase CO.
In Figure 6B are shown the in situ DRIFT spectra obtained from 5%Ru/TiO
under CO hydrogenation reaction conditions. Qualitative comparison with results obtained
from 0.5%Ru/TiO (Fig. 6A) shows the following differences: (a) The formation of Ru-
2
catalyst
2
2
bonded carbonyls species takes place at lower temperatures for the 5%Ru/TiO
2
catalyst; (b)
n+
-1
The relative intensity of the band due to Ru (CO)
x
species (band at ca. 2060 cm ) is
significantly lower over the catalyst of higher Ru content; (c) The relative intensity of the
-
1
Ru
the band attributable to (TiO
by that of Ru -CO species or it does not appear at all; (d) The relative population of
formate/carbonate species (<1700 cm ) is higher over the sample with smaller Ru crystallites
due to the higher exposed surface of TiO support.
The formation of adsorbed CO species at lower temperatures for the catalyst
containing 5% Ru implies that CO conversion initiates at lower temperatures with increasing
x
-CO band (ca. 2015 cm ) is significantly higher over the 5%Ru/TiO
2
catalyst, whereas
2
)Ru-CO species is either significantly lower and superimposed
x
-1
2
2
Ru crystallite size in agreement with results of Figs. 2 and 3. The higher relative intensity of
n+
the band due to multicarbonyl species on partially oxidized Ru sites over the 0.5%Ru/TiO
catalyst can be explained taking into account that the catalyst with smaller Ru particles is
characterized by higher metal/support interface. As a result, the interaction of ruthenium
2
atoms with the hydroxyl groups of TiO
partially oxidized Ru sites. The higher metal/support interface is also responsible for the
detection of (TiO )Ru-CO species on the surface of 0.5%Ru/TiO catalyst. On the other hand,
the observed increase of the relative intensity of the band assigned to Ru -CO species on the
surface of the most active 5%Ru/TiO catalyst implies that CO species linearly bonded on
reduced Ru crystallites could potentially being precursors of methane. This agrees well with
2
support is enhanced, leading to the creation of
2
2
x
2
1
8

results reported in our recent publication, using a different CO
2
/H reaction mixture, where it
2
has been found that the mechanistic pathway for the CO methanation reaction does not
2
change with increasing Ru crystallite size, and that the key factor which determines catalytic
activity is the relative population of the active intermediate carbonyl species [10]. As
suggested above, in the case of 0.5%Ru/TiO
support interface ((TiO )Ru-CO) most possibly prefer to desorb producing gas phase CO
rather than to interact with adsorbed hydrogen atoms producing CH , indicating that they are
inactive for the title reaction [10]. However, the formation of (TiO )Ru-CO species is
eliminated with increasing Ru loading or crystallite size. This is accompanied by an increase
of the relative intensity of the band due to the potentially active Ru -CO species, which
dominates on the surface of 5%Ru/TiO catalyst, and may be related to the higher methane
selectivity observed in Fig.5. Results are in agreement with previous studies over Rh/TiO
31], Ru/Al [18] and Pd/Al [6] catalysts, which have demonstrated that the different
relative population of reactive sites/intermediates is responsible for the structure sensitivity of
the CO methanation reaction.
Moreover, since the formation of Ru-bonded carbonyl species takes place via
interaction of CO adsorbed on TiO with hydrogen atoms adsorbed on Ru surface (Eq. 4),
2
catalyst, the adsorbed CO species at the metal-
2
4
2
x
2
2
[
2
O
3
2
O
3
2
2
2
the higher methanation activity observed with increasing Ru crystallite size (Fig.3) can be at
least in part attributed to the higher population of adsorbed hydrogen species on the surface
of the catalyst with higher Ru loading. This is in agreement with results reported by Wang et
al. [6], who suggested that formate reduction to CO is facilitated on the surface of
5
%Pd/Al
2
O
3
catalyst due to the catalyst ability to provide higher concentration of activated
catalyst.
hydrogen species compared to that of 0.5%Pd/Al
2
O
3
1
9

4
. CONCLUSIONS
Results of the present study show that catalytic performance and selectivity to reaction
products for the CO methanation reaction depend strongly on the nature of the metallic
2
phase. Methanation activity is higher for Rh catalyst compared to Ru, whereas Pd and Pt are
practically inactive for the title reaction, since they tend to enhance the RWGS reaction.
Catalytic performance and selectivity toward methane are greatly enhanced with increasing
Ru loading. Turnover frequency of CO
metal/support interface, is significantly improved with increasing ruthenium crystallite size
from 0.9 to 4.2 nm for Ru/TiO catalysts, and from 1.3-13.6 nm for Ru/Al catalysts.
DRIFTS experiments carried out over 0.5% and 5%Ru/TiO catalysts showed that the
interaction of CO /H mixture with the catalyst surface results in the development of Ru-
2
conversion, normalized with the perimeter of the
2
2
O
3
2
2
2
bonded carbonyl species, the nature of which does not depend on metal loading or crystallite
size, implying that the reaction pathway is the same for both catalysts examined. Methanation
of CO
reaction, followed by their hydrogenation to methane. Τhe reactive surface intermediates
could potentially be CO species linearly bonded on reduced Ru crystallites (Ru -CO), as
indicated by the increase of the relative intensity of the band assigned to Ru -CO species with
2
proceeds via intermediate formation of adsorbed CO species produced via the RWGS
x
x
increasing Ru crystallite size. It can be proposed that the structure sensitivity of the reaction
may be related to variation of the relative population of reactive surface intermediates.
2
0

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Figure captions
Figure 1. (A) Conversions of CO
2
as a function of reaction temperature, (B) selectivity
o
toward CH
CO
on TiO
.18<d
rate: 150 cm min .
Figure 2. Effect of metal loading on the (A) catalytic performance and (B) turnover
frequency of CO conversion obtained over Ru catalysts supported on TiO
4
and CO at 350 C and (C) Arrhenius plots of turnover frequencies of
2
conversion obtained over Rh, Ru, Pt and Pd catalysts (0.5 wt.%) supported
. Experimental conditions: Mass of catalyst: 100 mg; particle diameter:
<0.25 mm; Feed composition: 5%CO, 20% H (balance He); Total flow
2
0
p
2
3
-1
2
2
.
Experimental conditions: same as in Fig. 1.
Figure 3. (A) Arrhenius plots of normalized reaction rates obtained over Ru/TiO catalysts
2
with various crystallite sizes. (B) Effect of Ru crystallite size on the normalized
o
reaction rates obtained over Ru/TiO
2
catalysts at 250 C. Experimental conditions:
same as in Fig. 1.
Figure 4. (A) Arrhenius plots of normalized reaction rates obtained over Ru/Al
2
3
O catalysts
with various crystallite sizes. (B) Effect of Ru crystallite size on the normalized
o
reaction rates obtained over Ru/Al
conditions: same as in Fig. 1.
2
O
3
catalysts at 250 C. Experimental
Figure 5. Effect of mean Ru crystallite size (dRu) of (A) TiO
2
- and (B) Al
2
3
O -supported
o
catalysts on the selectivity toward CH
conditions: same as in Fig. 1.
4
and CO at 250 C. Experimental
Figure 6. DRIFT spectra obtained over the (A) 0.5% Ru and (B) 5% Ru/TiO catalysts
2
o
following interaction with 1%CO
2
+4%H
2
(in He) at 25 C for 15 minutes and
o
subsequent stepwise heating at 450 C under the same flow.
2
3

100
(A)
80
60
40
20
0
Rh
Ru
Pt
Pd
2
00 250 300 350 400 450
o
Temperature ( C)
1
00
(
B)
CH4
CO
8
6
4
2
0
0
0
0
0
Pd
Pt
Ru
Rh
1
Pt
(C)
Ru
Pd
Rh
0.1
0.01
1.6
1.8
2.0
2.2
-
1
1
000/T (K )
Figure 1
2
4

100
(A)
x% Ru/TiO2
80
60
40
20
0
0.1
0
1
.5
.0
2.0
5.0
1
50 200 250 300 350 400 450
o
Temperature ( C)
(B)
0.1
x% Ru/TiO2
0.1
0.5
1.0
2.0
5.0
0.01
1.6
1.8
2.0
2.2
2.4
-
1
1
000/T (K )
Figure 2
2
5

dRu (nm)
(
A) x% Ru/TiO2
0
2
2
3
4
.9
.1
.4
.2
.2
1
1
1
E-11
E-12
E-13
1.6
1.8
2.0
2.2
2.4
-1
1
000/T (K )
1E-11
1E-12
1E-13
(
B) x% Ru/TiO2
o
T=250 C
1.0 1.5 2.0 2.5 3.0 3.5 4.0
dRu (nm)
Figure 3
2
6

(
A)x% Ru/Al O3
2
1E-10
1E-12
1E-14
dRu (nm)
1
1
1
7
9
1
.3
.5
.6
.4
.4
3.6
1.6
1.8
2.0
2.2
-
1
1000/T (K )
(
B) x% Ru/Al O3
2
1E-10
1E-11
1E-12
o
T=250 C
2
4
6
8
10
12
14
dRu (nm)
Figure 4
2
7

100
(A)
80
60
40
20
0
CH4
x% Ru/TiO2
o
T=250 C
CO
1.0 1.5 2.0 2.5 3.0 3.5 4.0
dRu (nm)
100
(
B)
CH4
80
60
40
20
0
x% Ru/Al O
2
3
o
T=250 C
CO
2
4
6
8
10
12
14
dRu (nm)
Figure 5
2
8

1
922
(A)
(B)
0.05
0.05
1
991
o
4
50 C
i
o
i
450 C
o
4
00 C
h
h
o
4
00 C
o
3
50 C
g
f
o
g
f
3
50 C
o
3
00 C
2
004
2
068
o
300 C
o
2
50 C
2
000
e
d
1981
967
o
2
50 C
o
e
2
00 C
1
538
1622
2061
1
438
1
378
o
1
2015
2
020
200 C
1
321
1
837
2
060
1558
d
c
1367
1
624
o
1
660
1
50 C
c
o
1
50 C
1
646
b
o
1
00 C
1
553
o
1
00 C
1
426
1576
1
365
1
1
630
1
382
1410
b
a
327
1
661
1
318
2
018
1248
a
o
o
2
5 C
2
5 C
2
200 2000 1800 1600 1400 1200
2200 2000 1800 1600 1400 1200
-1
-1
Wavenumber (cm )
Wavenumber (cm )
Figure 6
2
9