Evidence of anti-coking behavior of La0.8Sr0.2Cr 0.98Ru0.02O3 as potential anode material for Solid Oxide Fuel Cells directly fed under methane

Article abstract of DOI:10.1016/j.jcat.2012.03.012

La_0.8Sr_0.2CrO₃ (LSC) based Ru catalysts are very active in methane steam reforming. Nevertheless, they can be easily poisoned under water-deficient conditions. Ru can be deposited as metallic ruthenium particles decorating the LSC grains or be inserted as Ru ions in the perovskite structure. Both Ru-promoted LSC catalysts were studied in methane steam reforming under water-deficient conditions and characterized after testing. Catalytic activity tests showed that ruthenium metal species are deactivated under water-deficient atmosphere, while ruthenium species inserted in LSC presented a remarkable stability and catalytic activity where residual steam plays a key role. Very unreactive carbon species responsible for deactivation were detected by temperature-programmed oxidation and transmission electron microscopy over metallic ruthenium species. Such species were not observed when ruthenium species are inserted and stabilized into the LSC structure. La_0.8Sr_0.2Cr_0.98Ru _0.02O₃ appears therefore as a highly promising anti-coking anode material for Solid Oxide Fuel Cells directly fed with methane or natural gas and operating under water-deficient conditions.

Full text of DOI:10.1016/j.jcat.2012.03.012

Journal of Catalysis 290 (2012) 158–164
Contents lists available at SciVerse ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Evidence of anti-coking behavior of La Sr Cr0.98Ru0.02O as potential anode
0
.8 0.2
3
material for Solid Oxide Fuel Cells directly fed under methane
T. Caillot , G. Gauthier b,1, P. Delichère , C. Cayron , F.J. Cadete Santos Aires
a,
⇑
a
b
a
a
IRCELYON, Institut de Recherches sur la Catalyse et l’Environnement de LYON, UMR 52562 CNRS/Université Lyon1, Avenue Albert Einstein, 69626 Villeurbanne, France
Commissariat à l’Energie Atomique – DTEN/STN/LT2N, 17 rue des Martyrs, Grenoble, F-38054 cedex 9, France
b
a r t i c l e i n f o
a b s t r a c t
Article history:
La0.8Sr0.2CrO3 (LSC) based Ru catalysts are very active in methane steam reforming. Nevertheless, they
can be easily poisoned under water-deficient conditions. Ru can be deposited as metallic ruthenium par-
ticles decorating the LSC grains or be inserted as Ru ions in the perovskite structure. Both Ru-promoted
LSC catalysts were studied in methane steam reforming under water-deficient conditions and character-
ized after testing. Catalytic activity tests showed that ruthenium metal species are deactivated under
water-deficient atmosphere, while ruthenium species inserted in LSC presented a remarkable stability
and catalytic activity where residual steam plays a key role. Very unreactive carbon species responsible
for deactivation were detected by temperature-programmed oxidation and transmission electron micros-
copy over metallic ruthenium species. Such species were not observed when ruthenium species are
Received 1 February 2012
Revised 15 March 2012
Accepted 15 March 2012
Available online 21 April 2012
Keywords:
Internal steam reforming of methane
Solid Oxide Fuel Cell
Ru-promoted Sr-substituted lanthanum
chromite
3
inserted and stabilized into the LSC structure. La0.8Sr0.2Cr0.98Ru0.02O appears therefore as a highly prom-
Internal reforming
Carbon deposition
ising anti-coking anode material for Solid Oxide Fuel Cells directly fed with methane or natural gas and
operating under water-deficient conditions.
Ó 2012 Elsevier Inc. All rights reserved.
1
. Introduction
One great advantage of the Solid Oxide Fuel Cell (SOFC), com-
tion. This allows the reaction to be delocalized over the entire
electrode surface and the steam content to be reduced since water
produced by hydrogen oxidation is used to convert methane. Con-
version of CO can proceed either via electrochemical oxidation or
by the water–gas shift reaction.
pared to other fuel cells, is that hydrocarbon fuels like natural
gas or its major constituent, i.e., methane, can be converted
directly. Due to their high operating temperature of the SOFC
However, low water concentrations can produce important car-
bon deposition. For example, dehydrogenation of hydrocarbons oc-
curs with the most widely used anode, the cermet Ni/YSZ. This
leads to the deterioration of anode performance. Some authors
have proposed to decrease the conversion rate of nickel in order
to avoid carbon deposition. Morimoto et al. have added iron to
the Ni/YSZ cermet [5]. Iron addition decreases conversion rate of
nickel and adjusts the thermal expansion coefficient toward that
of YSZ but important carbon deposition was observed for low
water concentrations. Insertion of nickel in strontium-doped
lanthanum chromite has also been tested [6]. Nevertheless, the
conversion rate decreased and some nickel/chromium agglomera-
tions were observed with nickel content higher than 5 mol%. Nickel
(
800–1000 °C), several reactions with methane may be considered,
such as methane combustion, partial oxidation, or methane
reforming. In many respects, internal steam reforming is the most
promising concept since the reaction takes place directly in the an-
ode compartment; this improves heat exchange between the endo-
thermic reforming reaction and the exothermic electrochemical
hydrogen oxidation within the stack [1]. It can also significantly re-
duce the requirement for cell cooling which is usually done by
flowing excess air through the cathode [2]. However, its develop-
ment is hindered by the endothermicity of the reaction which pro-
duces local cooling in the cell and by carbon deposition due to
methane cracking or by the Boudouard reaction [3].
Another model refers to the concept of gradual internal steam
reforming [4]. This theory is based on a coupling between the
steam reforming reaction and the electrochemical hydrogen oxida-
2
addition to Ce–ZrO and/or to Ce–YSZ has also been studied. These
supports have been reported as promising but a deterioration of
conversion rate was observed at low hydrogen pressure [7–10].
Gadolinium doped ceria catalysts have also been tested but
changes in both surface area and redox properties were observed
and related to catalytic deactivation [11]. Other metals than nickel
can also promote the reforming reaction, such as cobalt and some
noble metals such as Pt, Rh, Ir, and especially Ru [12–15].
⇑
Corresponding author.
E-mail address: thierry.caillot@ircelyon.univ-lyon1.fr (T. Caillot).
Present address: Escuela de Ingeniería Química, Universidad Industrial de
1
Santander, Bucaramanga, Colombia.
0
021-9517/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved.
http://dx.doi.org/10.1016/j.jcat.2012.03.012

T. Caillot et al. / Journal of Catalysis 290 (2012) 158–164
159
Previous work clearly established that Ru-promoted La0.8Sr0.2-
CrO (LSC) catalysts are promising anode materials for SOFCs di-
rectly fed with methane and integrating the concept of gradual
internal steam reforming [16]. Depending on catalyst structure,
very different catalytic behaviors with time on stream were ob-
served. Ru can be inserted as Ru ions in perovskite structure
Transmission Electron Microscopy characterizations were carried
out with a JEOL 2000FX microscope before catalytic runs and with
a JEOL JEM 2010 microscope after catalytic runs, both operated at
200 kV and equipped with EDX spectrometers. The latter micro-
3
6
scope is equipped with a LaB thermoionic gun and an UHR pole-
piece which has a point-to-point resolution of 0.196 nm; it also
allows the analysis by EDX of small individual nanoparticles down
to 3 nm [17–21] with probes down to 5 nm providing sufficient
current to make EDX analysis within acceptable times (less than
60 s). Temperature-programmed oxidation (TPO) experiments
were carried out after 1 and 24 h of steam reforming reaction. After
catalytic testing, the catalysts were cooled down to room temper-
(
LSCRu). In this case, the conversion rate in steam reforming
strongly oscillated with time on stream. Ru could alternatively be
deposited as metallic Ru particles decorating the LSC grains (Ru/
LSC). In the latter case, the conversion rate in steam reforming
was maximized and fairly stable with time on stream. Neverthe-
less, carbon deposition was more important. However, in both
cases, conversion rates were so intense (close to 100% with the
2
ature in reaction mixture and purged 30 min in N . The quartz tube
2
00 mg of catalyst used) that investigations after reaction were
limited.
The aim of this paper is to characterize both types of Ru-pro-
moted La0.8Sr0.2CrO anode materials in relation with the catalytic
reactor was then transferred on TPO apparatus. The samples were
À1
then exposed to a 0.98% O
2
in He (1.8 L h ) with a constant heat-
À1
ing rate of 20 °C min from room temperature to 950 °C. The anal-
ysis of gases was carried out continuously with an on-line mass
spectrometer (Pfeiffer Vacuum Omnistar Quadrupole). The species
3
activity in methane steam reforming under water-deficient condi-
tions. Samples loaded with 1% Ru were prepared and characterized
before and after catalytic testing by XRD, TEM, XPS, and Chemical
Analysis. Catalysts quantities used for catalytic tests were opti-
mized and temperature-programmed oxidation was used to mea-
sure carbon formation during the reaction and to follow the
oxidation state of catalysts.
followed were O
CO (m/e = 28). Calibration of CO
surements of CO formation and O
were performed in a KRATOS Axis Ultra DLD with a monochro-
matic Al K
2
(m/e = 32), CO
and O
consumption. XPS experiments
2
(m/e = 44), H
2
O (m/e = 18), and
2
2
allowed quantitative mea-
2
2
a
X-ray source, and a spot size aperture of 300 Â
700
5
l
m. The base pressure in the analysis chamber was better than
À8
 10 Pa. XPS spectra of Ru3p, Sr3d, La3d, Cr2p, O1s, and C1s
were measured at a pass energy of 20 eV. A charge neutralizer is
used to control charges effects on powder samples.
2
. Experimental procedure
2.1. Catalyst preparation
3
. Results
Ru/LSC and LSCRu were synthesized according to methods pre-
3.1. Catalysts characterization
viously described [16]. Table 1 gives the composition of the two
samples. The specific area was measured using the Brunauer–
Emmet–Teller (BET) method with nitrogen at 77 K and the compo-
sition was determined by ICP chemical analysis at the CNRS Central
Analysis Service (Vernaison – France). Both samples were sieved
The results obtained by X-ray diffraction are similar to those ob-
tained previously [16]. The diffractogram for Ru/LSC clearly shows
LSC and metallic Ru peaks whereas the diffractogram for LSCRu
only shows the peaks corresponding to a trigonal perovskite phase.
Fig. 1 presents a typical TEM Ru/LSC image. In this micrograph,
small (10–40 nm) ruthenium (identified by EDX analysis) nanopar-
ticles inhomogeneously dispersed on the surface of LSC grains are
clearly observed. Such small nanoparticles were never observed on
LSCRu where rather large grains (500 nm in average) without
specific shape were detected (Fig. 2). EDX analysis reveals the
presence of La, Sr, Cr, and Ru within those grains. The surface com-
position of the two samples obtained by XPS is presented in
Table 2. The presence of carbon (respectively 7.8% and 5.7% for
Ru/LSC and LSCRu) due to the preparation method (carbonate pre-
cipitation) can be noted for both samples. For Ru/LSC, the surface
ruthenium content is overestimated due to the presence of metal-
lic Ru nanoparticles. However, for this sample, it can be noted that
the surface of LSC particles is La-rich and (Cr and Sr)-poor when
compared to the bulk composition. The binding energy measured
for Ru-3p3/2 peak (461.9 eV) corresponds to values obtained for
metallic Ru [22]. The same difference between surface and bulk
compositions is observed for LSCRu and the surface Ru content is
higher than the bulk Ru content. The binding energy measured
for Ru-3p3/2 (464.2 eV) corresponds to values obtained for Ru(IV)
ions [23]. This indicates as it was also indirectly suggested by the
EDX results that Ru is well-inserted in the perovskite structure as
Ru(IV) ions in substitution of chromium ions as it was intended [16].
between 40 and 80
lm before characterization and catalytic tests
in order to have uniform samples.
2.2. Activity tests
Before activity measurements, catalysts were pre-treated in situ
in N
2
at 800 °C for 2 h. Catalytic runs were performed at atmo-
spheric pressure in a conventional flow system at 800 °C with a
CH
4
/H
2
O ratio of 10 (50% CH
4
, 5% H
2
O, N
2
balance, total flow rate
À1
of 6.5 L h ). Samples (from 10 to 200 mg) were held on quartz
wool in a 2 mm inside diameter U-shaped quartz reactor. Steam
was added to the feed by flowing the CH
saturator maintained at 33 °C. H O concentration was measured
with an EdgeTech Dew Prime I hygrometer. The reactor effluents
CH , CO, H , CO , and O ) were periodically (every 5 min) analyzed
4 2
/N mixture through a
2
(
4
2
2
2
by a gas microchromatograph (Varian CP 2003 QUAD) equipped
with high sensitivity (5 ppm) thermal conductivity detectors
(
TCD).
2.3. Characterization methods
XRD measurements were performed at room temperature using
a Brücker D8ADVANCE diffractometer using Cu-K-L2,3 radiation.
Table 1
Results of chemical analysis and BET measurements.
a
Specific area (m2
g )
À1
Symbols
wt% La
wt% Sr
wt% Cr
wt% Ru
Calculated formula by assuming O
0.92 wt%Ru/La0.804Sr0.196Cr1.007
La0.809Sr0.191Cr0.979Ru0.021
3
Ru/LSC
LSCRu
47.59
48.72
7.3
7.27
22.31
22.11
0.93
0.91
O
3
1.9
1.0
O
3
a
O was not analyzed.

1
60
T. Caillot et al. / Journal of Catalysis 290 (2012) 158–164
paths and consequent diffusion limitations. The mass applied for
catalytic testing was thus an intermediate value of 20 mg. Fig. 3
shows the evolution of the H , CO, and CO concentrations ob-
2
2
tained in the flowing reaction products during the catalytic steam
reforming test at 800 °C using 20 mg of Ru/LSC. The initial behavior
of this catalyst confirms that it is both very active (93 ± 2% of water
conversion) and very selective (very low CO
water-gas shift reaction) [24–27]. However, the catalyst slowly and
gradually deactivates with reaction time. H and CO concentration
2
concentration due to
2
are respectively 10% and 3% by the end of the catalytic test. The
water conversion decreased to 66 ± 2% after 24 h reaction. Further-
more, a small amount of CO is detected. Therefore, for a ratio
2
methane/water of 10, Ru/LSC deactivates and becomes less selec-
tive during methane steam reforming reaction.
In the case of LSCRu, the oscillating behavior of the catalytic
activity evidenced previously [16] is observed for masses ranging
between 50 and 200 mg. For masses of 40 mg and lower, this
behavior disappears. The mass retained for the catalytic test is thus
4
0 mg; results obtained for 20 mg were similar; however, sensitiv-
ity is higher with a larger amount of catalyst namely for the TPO
measurements that we will present in Section 3.3.1. Fig. 4 shows
the evolution of the H , CO, and CO concentrations obtained in
2 2
1
00 nm
the flowing reaction products during the catalytic steam reforming
test at 800 °C using 40 mg of LSC Ru. This catalyst is less active
Fig. 1. TEM micrograph of Ru/LSC powder.
(
2
42 ± 2%) and less selective (presence of CO ) than Ru/LSC. After
an activation period of 40 minutes, the conversion rate is gradually
recovered, stabilizes after 14 h reaction, and then remains constant
for the rest of the catalytic test. Between 14 and 24 h reaction, H
CO, and CO concentrations measured in the flowing reaction prod-
ucts are 8.3%, 2.4%, and 0.3%, respectively. The water conversion is
3 ± 2%. Several catalytic tests were performed that showed the
2
,
2
5
reproducibility of these results. During one of these catalytic tests,
the catalyst did not show any deactivation over the 72 h reaction.
Both samples were characterized after reaction by elemental
chemical analysis. The composition of both samples did not change
after the catalytic tests within the experimental uncertainty of the
technique. In particular, the Ru concentrations are identical before
and after reaction indicating that no sublimation of ruthenium oxi-
x
des RuO occurred during the catalytic tests.
3
3
.3. Catalysts characterizations after catalytic testing
.3.1. TPO
In order to determine the origin of the deactivation of Ru/LSC,
we have performed two temperature-programmed oxidations of
this sample: one after 1 h reaction (before deactivation) and the
other after 24 h reaction.
5
00 nm
Fig. 5 shows the profiles of O
of Ru/LSC (20 mg) after 1 h of catalytic test at 800 °C. The CO
2
and CO
2
obtained during the TPO
pro-
Fig. 2. TEM micrograph of LSCRu powder.
2
file shows two maxima at 430 and 580 °C. These two types of
carbon are generally identified in the literature [28–31] as poly-
3.2. Catalytic tests
morphic carbon C
b
and graphitic carbon Cgraph. The amounts of
À1
Different tests were performed to determine the catalyst mass
deposited carbon are presented in Table 3: 54
l
mol g
for Cb
À1
to be ultimately used in the catalytic test. For masses of 40 mg
and higher, the conversion rate of Ru/LSC is close to 100%. For
masses of 10 mg and lower, the catalyst amount is not sufficient
to completely cover the quartz wool leading to preferential flow
and 29
l
mol g for Cgraph. The total amount of carbon is rather
low (0.1 wt%) after 1 h reaction.
The profile of the consumed oxygen represents the oxygen con-
sumed by the oxidation of the deposited carbon and consequent
Table 2
Surface composition and binding energy of ruthenium obtained by XPS.
wt% Sr
wt% Cr
wt% La
wt% Ru
wt% O
wt% C
Binding energy Ru 3p3/2 (eV)
Ru/LSC
LSCRu
3.3
5.5
7.0
14.8
16.2
15.8
45.2
51.1
45.1
7
2.5
3
21.9
19.0
24.2
7.8
5.7
4.9
461.9
464.2
462.8
LSCRu after 24 h steam reforming

T. Caillot et al. / Journal of Catalysis 290 (2012) 158–164
161
1
1
1
4
2
0
8
6
4
2
0
Table 3
Temperatures of peaks maxima and amounts of
oxidation and CO
reaction at 800 °C with CH
O
2
consumed for catalyst re-
2
released during TPO experiments of samples after 1 h and 24 h
4
/H
2
O = 10.
H
2
T (°C)
CO
2
released
T
(°C)
O
2
consumed
À1
À1
(l
mol g
)
(l
mol g
)
Ru/LSC after
1
430
580
54 ± 3
29 ± 2
290 30 ± 5
700 96 ± 8
h
Ru/LSC after
24 h
430
53 ± 3
280 35 ± 5
690 93 ± 8
CO
CO
570, 650, 700, 54 ± 4
00
8
LSCRu after
1
430
25 ± 2
260 34 ± 5
2
h
0
200
400
600
800
1000
1200
1400
Time / min
0.07
1
Fig. 3. [H
reforming of CH
rate = 6.5 L h , feed composition = 50% CH
2
] (
D
), [CO] (+), and [CO
2
] (Â) concentrations obtained upon catalytic
over Ru/LSC at 800 °C: sample weight = 20 mg, total flow
, 5% H O, N balance.
0.06
0.99
0.98
0.97
0.96
0.95
0.94
0.93
4
À1
4
2
2
O
2
: m/e = 32
0
0
0
0
.05
.04
.03
.02
Re-oxidation
1
1
1
4
2
0
8
6
4
2
0
H
2
0.01
0
CO
2
: m/e = 44
250
5
0
450
650
850
Temperature / °C
CO
CO
Fig. 6. CO
2
(m/e = 44) and O
2
(m/e = 32) profiles obtained during TPO of Ru/LSC after
/H O = 10. TPO conditions: sample weight = 20 mg,
.98% O in He, flow rate = 1.8 L h . ‘‘Re-oxidation’’ corresponds to O consumption
2 2
2
0
4 h reaction at 800 °C with CH
4
2
À1
2
for catalyst re-oxidation.
0
200
400
600
800
1000
1200
1400
Time / min
of the support LSC while the second peak is attributed to the re-
oxidation of ruthenium.
Fig. 4. [H
reforming of CH
2
] (
D
), [CO] (+), and [CO
2
] (Â) concentrations obtained upon catalytic
over LSCRu at 800 °C: sample weight = 40 mg, total flow
, 5% H O, N balance.
3
Unlike LaCrO , the oxidation state of LSC may vary. This is ex-
4
3+
2+
À1
plained by the partial substitution of La ions by Sr ions that
yields a charge defect which is compensated by the oxidation of
rate = 6.5 L h , feed composition = 50% CH
4
2
2
3
+
4+
a part of the Cr ions into Cr ions and/or by the formation of oxy-
gen defects [32–34]. For 20% substitution, the formula of LSC can
0
0
0
0
0
0
0
.07
.06
.05
.04
.03
.02
.01
0
1
evolve from La0.8Sr0.2CrO2.9 (reduced form) to La0.8Sr0.2CrO
3
(oxi-
0.99
0.98
0.97
0.96
0.95
0.94
0.93
dized form).
The amounts of consumed oxygen are shown in Table 3. For LSC,
2
O : m/e = 32
À1
the consumption of oxygen (30
l
mol g ) indicated that, after 1 h
Re-oxidation
of catalytic test, the formula of the support was La0.8Sr0.2CrO2.985
which is very close to the oxidized form.
The amount of consumed oxygen for the oxidation of ruthenium
À1
(96
l
mol g ) corresponds, within the experimental uncertainty,
to the amount of dioxygen necessary to oxidize the amount of
supported metallic ruthenium, present in the sample, to RuO
2
CO
2
: m/e = 44
À1
(
92
lmol g ). All ruthenium nanoparticles are thus in the metallic
50
250
450
650
850
form after 1 h of catalytic test.
Température / °C
Fig. 6 shows the profiles of O
2
, CO
2
, and ‘‘re-oxidation’’ obtained
during the TPO of Ru/LSC (20 mg) after 24 h of catalytic test at
00 °C.
The profile ‘‘re-oxidation’’ presents exactly the same character-
Fig. 5. CO
2
(m/e = 44) and O
h reaction at 800 °C with CH
2
.98% O in He, flow rate = 1.8 L h . ‘‘Re-oxidation’’ corresponds to O consumption
2
(m/e = 32) profiles obtained during TPO of Ru/LSC after
8
1
0
4
/H O = 10. TPO conditions: sample weight = 20 mg,
2
À1
2
istics after 24 h of catalytic test and after 1 h of catalytic test and
the amounts of dioxygen detected are also similar (Table 3). Thus,
no change in the oxidation state of both phases was evidenced dur-
ing the catalytic test.
for catalyst re-oxidation.
formation of CO
the catalyst. In order to quantify the latter, we added in Fig. 5 the
‘re-oxidation’’ profile that corresponds to the difference between
the total amount of consumed oxygen and the amount of con-
sumed oxygen for the oxidation of deposited carbon (and CO for-
mation). The profile obtained in this way presents two maxima
290 and 700 °C). The first peak is attributed to the re-oxidation
2
and the oxygen consumed during the oxidation of
‘
The CO
800 °C) corresponding to five types of carbon with different reac-
tivity behavior toward oxygen. C and Cgraph, already detected after
1 h of catalytic test, are always present. The amount of C
tical to the one determined after 1 h of catalytic test (53
2
profile shows five maxima (430, 570, 650, 700, and
2
b
b
is iden-
mol g ).
À1
(
l

1
62
T. Caillot et al. / Journal of Catalysis 290 (2012) 158–164
0
0
0
0
0
0
0
.07
.06
.05
.04
.03
.02
.01
0
1
0.99
0.98
0.97
0.96
0.95
0.94
0.93
Re-oxidation
2
O :m/e = 32
CO
2
: m/e = 44
50
250
450
650
850
Fig. 8. High resolution TEM micrograph of Ru/LSC powder after 24 h reaction at
800 °C with CH /H O = 10. A layer of carbonaceous species is clearly observed
covering the Ru nanoparticles (black double-arrow) and the support (white double-
Temperature / °C
4
2
arrow) surface.
Fig. 7. CO
2
(m/e = 44) and O
h reaction at 800 °C with CH
2
.98% O in He, flow rate = 1.8 L h . ‘‘Re-oxidation’’ corresponds to O consumption
2
(m/e = 32) profiles obtained during TPO of LSCRu after
1
0
4
/H O = 10. TPO conditions: sample weight = 40 mg,
2
À1
2
for catalyst re-oxidation.
It is, however, difficult to estimate the amounts corresponding to
the other forms of deposited carbon. The total amount of deposited
À1
carbon (107
l
mol g : 0.13 wt%) has increased by comparison
with the one measured after 1 h of catalytic test. The carbon depo-
sition is thus a slow but continuous process during the whole cat-
alytic test.
As for Ru/LSC, we have performed two TPOs of the LSCRu sam-
ple after 1 h of catalytic test (when activity reaches its lowest va-
[
4 -4 1]
lue) and after 24 h of catalytic test. Fig. 7 shows the O
‘re-oxidation’’ obtained during the TPO of LSCRu (40 mg) after
h of catalytic test at 800 °C. The CO profile presents one maxi-
mum at 430 °C. This oxidation temperature corresponds to that ob-
tained for C with Ru/LSC. The total amount of deposited carbon
2 2
, CO , and
‘
1
2
b
À1
(
25 lmol g ) corresponds to 0.03 wt% after 1 h of catalytic test; it
is much lower than for Ru/LSC and no trace of Cgraph was evidenced.
The ‘‘re-oxidation’’ profile presents only one broad peak with
the maximum at 260 °C similar to the one corresponding to the
Fig. 9. High resolution TEM micrograph of LSCRu powder after 24 h reaction at
00 °C with CH /H O = 10. No carbon is observed at the surface. Crystallographic
orientation of the presented region as well as the resolved interplanar distances and
8
4
2
À1
À1
oxidation of LSC in Ru/LSC (34
tively). The formula of the catalyst is calculated to be La0.809Sr0.191
2.986 after 1 h of catalytic test. The oxidation state
lmol g and 30 lmol g , respec-
angles between planes are indicated.
Cr0.979Ru0.021
O
3.3.3. XPS
corresponds, within the experimental uncertainty, to the obtained
for LSC in Ru/LSC. The TPO of LSCRu performed after 24 h of cata-
lytic test neither shows the formation of CO nor the consumption
2
of dioxygen (results not shown). Thus, there is no building-up of
carbon on LSCRu after stabilization of the conversion rate during
catalytic test. Furthermore, the catalyst is in its oxidized form after
Only LSCRu was analyzed by XPS after the catalytic tests since
the carbon layer on Ru/LSC hinders its analysis. The surface of
LSCRu contains more strontium after testing than before test; there
is thus segregation of strontium in preference to lanthanum during
the catalytic test. It is also worth noting that the amount of carbon
detected on the surface of this catalyst after reaction is lower than
the amount detected before reaction. This catalyst not only resists
to carbon deposition during methane steam reforming in water-
deficient conditions but it is also capable of self cleaning (at least
partially) the carbon already present on its surface.
2
4 h of catalytic test.
3.3.2. Electron microscopy
Fig. 8 shows a high resolution TEM image of Ru/LSC after 24 h of
methane steam reforming in water-deficient conditions. The cata-
lyst surface is totally coated with a rather uniform carbon layer
4. Discussion
(
ꢀ5 nm). The carbon building-up occurs not only over the ruthe-
nium nanoparticles but also on the LSC support. Conversely, for
LSCRu (Fig. 9) no trace of carbon is observed on the surface after
2
lographic planes are clearly resolved at the surface. The region pre-
sented in Fig. 9 is in the [4–41] orientation with resolved (1 1 0),
Our results show that the steam reforming reaction is the main
reaction in the reactor together with a small contribution from
water-gas shift reaction for both catalysts. The contribution of
methane cracking reaction yields only several tens of micrograms
over 24 h for Ru/LSC (from TPO). Although this is significant on
the surface of the catalyst (TEM), it is almost impossible to detect
it on each measure point of the catalytic test.
4 h of catalytic test. The surface is perfectly clean and the crystal-
(
0 1 4), and (À1 0 4) interplanar distances. Measured distances
(d
(1 1 0) ꢀ 0.273 nm; d(1 0 4) = d(0 À1 4) ꢀ 0.271 nm) and angles (an-
gle between (1 1 0) and (0 1 4) ꢀ 60.3°; angle between (0 1 4) and
À1 0 4) ꢀ 59.2°) are very close to the values expected (d(1 1 0)
.27464 nm; (1 0 4) = d(0 À1 4) = 0.27253 nm and angle between
1 1 0) and (1 0 4) = 60.2553°; angle between (1 0 4) and (0 À1 4) ꢀ
9.4895°, respectively) and correspond to the trigonal phase identi-
fied by XRD.
(
0
(
5
=
4.1. Ru/LSC
d
The amount of oxygen consumed for the oxidation of ruthe-
nium shows that the ruthenium is in metallic form during the cat-
alytic test. Matsui et al. studied the mechanisms of dry reforming

T. Caillot et al. / Journal of Catalysis 290 (2012) 158–164
163
of methane over supported Ru catalysts on different supports. Dur-
ing reaction, ruthenium reacts with CH to form Ru–CH and with
CO to form Ru–O and CO. Oxygen is then transferred from Ru–O
toward Ru–CH yielding CO and metallic ruthenium. The XRD anal-
ysis of the phases after the catalytic test shows the presence of
both RuO and metallic ruthenium [35]. Ferreira-Aparício et al. ob-
tained similar results with Ru/SiO [36]. The authors proposed a
deactivation since the reduced form of the catalyst is less active
than the oxidized form; the decrease in conversion rate modifies
the reaction mixture composition leading to a re-oxidation of the
catalyst and a stabilization of the conversion rate.
The TPO performed after 1 h of catalytic test (when activity is at
its lowest value) shows only the presence of a very reactive carbon
4
x
2
x
x
x
2
2
b
form C (Table 2). Moreover, no carbon deposit was detected up to
Langmuir–Hinshelwood type mechanism in which the two reac-
tants dissociate on the metallic phase. In our case, ruthenium only
exists in the metallic form after testing which suggests that only
methane dissociates on the metal. The mechanism is thus certainly
different and the LSC support, used in the present work, must play
an important role in the dissociation of steam as it has already
been shown for other supports in the case of methane steam
reforming [37,38] as well as in the case of the dry reforming of
methane [39,40].
The ‘‘re-oxidation’’ profiles, obtained by TPO, after 1 h and 24 h
of catalytic test show two peaks (Figs. 5 and 6) attributed to each of
the two phases (support and catalyst). The second peak, corre-
sponding to the oxidation of ruthenium, is very broad (from 450
to 950 °C). This very slow ruthenium oxidation can be explained
the end of the catalytic test. The other stable forms of carbon ob-
served for Ru/LSC do not appear for LSCRu. Thus, unlike metallic
ruthenium, Ru(IV) ions do not lead to the formation of stable car-
bon species during the catalytic test.
Furthermore, the catalytic activity behavior of LSCRu in meth-
ane steam reforming is very different from the one observed for
Ru/LSC which confirms that Ru configuration within the material
catalyst is different between the two samples. In particular, LSCRu
is both less active and less selective than Ru/LSC. This can be ex-
plained by the fact that ruthenium ions do not present the same
activity as metallic ruthenium and also by the fact that those ions
are less accessible since they are inserted in the perovskite struc-
ture. However, unlike Ru/LSC, LSCRu does not deactivate with reac-
tion time; it is thus much more stable than Ru/LSC even under a
water-deficient reaction mixture at 800 °C. This study confirms
LSCRu thermal stability [44] as well as its catalytic stability during
methane steam reforming in water-deficient conditions [25].
The ‘‘re-oxidation’’ profile after 1 h of catalytic test shows very
low dioxygen consumption all over the TPO temperature range.
However, the amount of consumed oxygen is similar to the one de-
tected for the oxidation of LSC for Ru/LSC. This result shows that
the insertion of ruthenium in the perovskite structure modifies
its oxidation properties.
The XPS study after reaction showed a segregation of strontium
to the surface of the catalyst during methane steam reforming. This
segregation of strontium decreases the number of oxygen surface
defects and modifies the redox properties of the catalyst; numer-
ous studies show a modification of redox properties of perov-
skite-containing ruthenium under reducing conditions [25] or
under inert gas after oxidation [45].
After stabilization of the activity, LSCRu is in the oxidized form
despite a reaction mixture very deficient in water. Unlike our pre-
vious study, with a higher catalyst mass (200 mg), in the same con-
ditions [16], no oscillation was observed even during the 72 h
catalytic test. It is worth noting that the catalytic conversion never
attains 100% with 40 mg of catalyst whereas total conversion was
achieved with 200 mg of catalyst. Residual steam is thus in contact
with the catalyst during the whole test (water conversion lower
than 100%). These results indicate that this residual steam certainly
plays a very important role in methane steam reforming reaction
mechanism over LSCRu. The presence of steam is directly con-
nected to the extinction of the oscillating behavior of the catalyst.
Thus, the conversion rate of the catalyst depends on the reaction
mixture and, in particular, on the presence or absence of residual
steam. In other words, for LSCRu, the catalytic activity behavior
clearly depends on the conversion rate.
2
by the formation of a surface RuO layer on the Ru nanoparticles
that acts as a barrier for the diffusion of oxygen as it has been ob-
served by Balek et al. for the oxidation of metallic ruthenium (30–
5
2
0 nm) to RuO [41]. It is important to note that the two profiles
obtained after 1 h and 24 h of catalytic test are similar. These re-
sults show clearly that the observed deactivation is neither due
to a change in the oxidation state of the phases present nor to a
phase modification during the catalytic test.
Ru/LSC is composed by two phases: metallic ruthenium and the
LSC support. The catalytic test of this sample shows a slow and
continuous deactivation of the catalyst during the whole test
(Fig. 3). The catalytic test conditions (water-deficient reaction mix-
ture) lead thus to a deterioration of the catalyst. This deactivation
could be explained by the partial sublimation of ruthenium in the
form of RuO oxides during the catalytic test [42]. However, the
x
amount of ruthenium in the sample, measured by chemical analy-
sis, is the same before and after reaction. Furthermore, no deposit
was observed on the walls of the reactor. Thus, the decrease in con-
version rate cannot be explained by the sublimation of ruthenium
x
in the form of RuO oxides.
The TPO performed after 24 h of catalytic test (after deactiva-
tion) reveals the presence of five forms of carbon. By comparison
with the TPO performed after 1 h of catalytic test (before deactiva-
tion), three other (very stable) carbon forms were detected. These
rather unreactive carbonaceous species are certainly responsible
for the observed deactivation of Ru/LSC. In other words, the exper-
imental conditions that were used in this work (water-deficient
reaction mixture) significantly modify the capability of metallic
ruthenium to resist to the deposition of carbon and this study
clearly establishes that the sample Ru/LSC does not meet the crite-
ria required for its application as an anode material in SOFCs oper-
ating with natural gas and using the gradual internal methane
reforming concept.
The present study clearly establishes that, unlike Ru/LSC, LSCRu
meets the criteria required for its application as an anode material
in SOFCs operating with natural gas and using the gradual internal
methane reforming concept.
4.2. LSCRu
The different characterizations performed on this catalyst
clearly showed that LSCRu is composed by a single perovskite
phase where ruthenium is inserted in the structure as Ru(IV) ions
in substitution of chromium ions in perovskite B sites [43].
At the beginning of the catalytic test (Fig. 4), a quick deactiva-
tion followed by a slower reactivation is observed prior to the sta-
bilization of the activity. This apparent induction effect can be
explained in following way: at the beginning of the catalytic test,
the methane-rich mixture reduces the catalyst which provokes a
5. Conclusion
The results obtained in this study show that ruthenium in
metallic form deactivates during methane steam reforming in
water-deficient conditions (CH /H O = 10). This deactivation is ex-
4 2
plained by the formation of very stable carbonaceous species at the
surface of the catalysts during reaction. Conversely, when ruthe-
nium is inserted in the perovskite structure as Ru(IV) ions, no car-

1
64
T. Caillot et al. / Journal of Catalysis 290 (2012) 158–164
bon build-up and no deterioration of the catalyst were evidenced.
Furthermore, the conversion rate of the latter catalyst remains con-
stant over 72 h after an activation period. The LSCRu catalyst ap-
pears as a promising anode material for SOFCs operating with
natural gas and using the gradual internal methane reforming
concept.
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