Nanostructured La0.8Sr0.2Fe0.8Cr0.2O3 Perovskite for the Steam Methane Reforming

Article abstract of DOI:10.1007/s10562-016-1885-4

Abstract: This work describes the synthesis of La_0.8Sr_0.2Fe_0.8Cr_0.2O₃ perovskite using a polymerization chemical route, for application on steam methane reforming reaction, compared to the LaFeO₃ oxide. Results derived from X-ray diffraction (XRD), X-ray photoelectron spectroscopy and transmission electron microscopy (TEM) confirmed the presence of a prevalent orthorhombic structure related with a nanostructured solid (15–33?nm). The methane conversion of La_0.8Sr_0.2Fe_0.8Cr_0.2O₃ oxide is very high compared to the LaFeO₃ oxide at 700 °C. The stability test was performed during 240?h, using the catalyst under similar reaction conditions and the deactivation was low. Post reaction analyses by TEM, XRD and TPO showed growth of crystallite sizes, absence of carbon deposition, but no structural modifications, which suggest that the promoters prevent carbon deposition and resist structure changes during the steam methane reforming reaction. Graphical Abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-016-1885-4

Catal Lett (2016) 146:2504–2515
DOI 10.1007/s10562-016-1885-4
Nanostructured La Sr Fe Cr O Perovskite for the Steam
0
.8 0.2
0.8
0.2
3
Methane Reforming
1
2
2
Jairo A. Gómez-Cuaspud · Carlos A. Perez · Martin Schmal
Received: 1 September 2016 / Accepted: 5 October 2016 / Published online: 22 October 2016
Springer Science+Business Media New York 2016
©
Abstract This work describes the synthesis of
Graphical Abstract
La Sr Fe Cr O perovskite using a polymerization
0.8 0.2 0.8 0.2
3
chemical route, for application on steam methane reforming
reaction, compared to the LaFeO 3 oxide. Results derived
from X-ray diffraction (XRD), X-ray photoelectron spec -
troscopy and transmission electron microscopy (TEM) con-
firmed the presence of a prevalent orthorhombic structure
related with a nanostructured solid (15–33 nm). The meth-
ane conversion of La 0.8Sr Fe Cr O oxide is very high
0.2
0.8 0.2
3
compared to the LaFeO oxide at 700 °C. The stability test
3
was performed during 240 h, using the catalyst under simi-
lar reaction conditions and the deactivation was low. Post
reaction analyses by TEM, XRD and TPO showed growth
of crystallite sizes, absence of carbon deposition, but no
structural modifications, which suggest that the promoters
prevent carbon deposition and resist structure changes dur -
ing the steam methane reforming reaction.
Keywords Chromium-iron perovskite ·
Methane reforming · Post reaction analyses
1
Introduction
The continuous interest for production of energy from the
use of hydrogen as fuel in new devices has emerged, as one
of the most highlighted alternatives for energy production
in several and critical sectors of an economy based on the
consumption of fossil fuels [1, 2]. The advantages of hydro-
gen as fuel are high efficiency, low emissions, and relatively
low cost, but such process depends strongly on the catalytic
promoters of the catalyst, enabling higher activity and sta -
bility on the steam methane reforming reaction [ 3, 4]. As
known, the methane steam reforming is composed by three
reversible reactions, with the formation of syngas and CO2,
ꢀ
Martin Schmal
schmal@peq.coppe.ufrj.br
1
Grupo de Integridad y Evaluación de Materiales (GIEM),
Instituto para la Investigación e Innovación en Ciencia
y Tecnología de Materiales (INCITEMA), Universidad
Pedagógica y Tecnológica de Colombia,
Av. Central del Norte 39-115, Tunja, Colombia
2
Centro de Tecnologia, Federal University of Rio de Janeiro-
NUCAT/COPPE/UFRJ, Bloco G, sala 228,
CEP 21945-909 Rio de Janeiro, RJ, Brazil
1
23

Nanostructured La0.8Sr0.2Fe0.8Cr0.2O Perovskite for the Steam Methane Reforming
2505
3
besides the exothermic water–gas shift reaction, favoring
of La1 Ce CoO and claimed that the maximum solubility
−x x 3
the H production, as follows [5–7]:
of cerium in the perovskite lattice was x≤0.05, Oliva et al.
2
[21] investigated this structure for different x, and showed
°
CH + H O ⇔ CO+3H ⇒ ∆H = +206 kJ/mol
(1)
that for high content there is segregation of CeO . The iso-
2
4
2
2
298
structural substitution favors the formation of vacancies.
Therefore, in this work aims to the synthesis of a
°
CO + H O ⇔ CO +H ⇒ ∆H = −41 kJ/mol
(2)
(3)
2
2
2
298
La Sr Fe Cr O pure perovskite structure, using the
0
.8 0.2 0.8 0.2
3
°
CH +2H O ⇔ CO +4H ⇒ ∆H = +165 kJ/mol
combustion method to obtain nanoparticles, testing with
steam methane reforming reaction and comparing with the
4
2
2
2
298
Diverse synthesis of active materials for promoting the steam pure LaFeO oxide. Characterizations before and post reac-
3
methane reforming reaction have been suggested, mainly
based on the perovskite structure, since these oxides have
the possibility to combine different oxidation states and ele-
ments in its structure [8]. According to Mihai et al. [9] some
of the most interesting perovskite compositions are the
tion allowed explaining possible deactivation causes and
structure modifications.
2 Experimental
substituted systems based on ferrite LaFeO composition,
3
presenting high activity and stability; however, according
to Zhao et al. [ 10], one of the most challenging problem is
the carbon deposition on the solid surface and consequently
the deactivation of the catalyst. Transition metals based on
perovskite-type oxides have variety of properties attributed
to the ability of replacing the cations, creating iso-structural
2.1 Preparation of Perovskite
The La Sr Fe Cr O oxide was prepared by the com -
x
1−x
x
1−x
3
bustion technique, using nitrates of La(NO
) ·6H O
3 2
3
(99.9%), Sr(NO ) (99.9 %), Fe(NO ) ·9H O (98.9%),
3
2
3 3
2
Cr(NO ) ·9H O 99.9 % and citric acid monohydrated all
3
3
2
solids of A1 A B B O type [ 11]. The substitution can
have led to the stabilization of unusual oxidation state of the were dissolved in 20 mL of absolute ethanol until com
from Merck. Stoichiometric quantities of each solid nitrate
−x
x
1−y
y
3
-
-
cations, besides the creation of anionic and cationic vacan -
cies. The valence, stoichiometry and presence of vacancies
in the oxide structure can interfere on the catalytic activity
plete dissolution and magnetic stirring, as reported else
where [22]. The citric acid was added at 0.5:1 molar ratio
with respect to the total concentration of metal salts in dis -
[12].
solution. The solution was kept under reflux at 120°C for
Therefore, the partial substitution of A-site cations by
12 h until formation of a viscous liquid, which was sub
-
Sr and of B-site cations by Cr modifies the properties of
ferrite compositions, increasing the dispersion of metallic
particles, and thus the catalytic activity and stability. In this
regard, Zhang et al. [ 13] reported that A and B-site metals
sequently heated at 150°C under air flux in an oven, until
complete solvent evaporation. The obtained gel was heated
at 300°C for 30 min until the initiation of a combustion pro-
cess, which consumes most of the organic phase. The car -
bonaceous remnant was removed by a thermal treatment at
800°C under oxygen flow for 3 h. The sample was kept in a
chamber under controlled humidity (20 %), before physico-
chemical characterization. The stoichiometric composition
was calculated based on the total oxidizing and reducing
valences (N/C), assuming total combustion of citrate spe -
cies as shown in Eq. 4 [18].
have proportional effect on the amount of oxygen mobil
-
ity under reaction conditions. Li et al. [ 14–16] found that
the oxide La 0.8Sr FeO is highly effective for the partial
0.2
3
oxidation of methane, because it facilitates the ion oxygen
mobility. On other hand, similar compositions based in
LaBO perovskites, where B =V, Cr, Mn, Fe, Co, Ni were
3
studied and showed that the stability of the LaCoO
was
3
lower than of the LaFeO 3 oxide [ 10, 15]. However, struc -
tures based on La Sr Fe M O oxide, modified by
−
NO3
)
+ C H O → 6CO + N + 9H O + oxides
(4)
0
.8 0.2 1−x
x
3
(
6
18
7
2
2
2
transition cations, such as Cr, Rh and Co, depend on synthe-
sis methods to obtain nanoparticles, being one of the most
imperative requirements in the field of active materials for
catalytic uses. Arai et al. [ 17], Goldwasser et al. [ 18] and
Zhang et al. [ 19] studied the effect of the synthesis meth -
ods for obtaining perovskites modified with chromium,
where the cation provoked surface defects and vacancies,
increasing the activity of the catalyst and the selectivity of
hydrogen formation. However, the maximum solubility of
metal in the lattice of the perovskite-type structure is con -
troversial. Białobok et al. [20] prepared different samples
2.2 Characterization of Catalysts
The chemical composition of the oxide was obtained using
X-ray fluorescence (XRF) technique, using a Philips MagiX
Pro apparatus. Sample was pressed as pellet and analyzed
quantitatively. The specific area BET was evaluated by
nitrogen adsorption isotherms at −196°C, using the ASAP-
2020 apparatus (Micromeritics), degassing at 350 °C over-
night to remove residual humidity.
123

2
506
J. A. Gómez-Cuaspud et al.
The crystalline structure, was determined by X-ray dif -
fraction, in a PANalytical X’Pert PRO MPD equipment,
cleaning and cooling, the gas valve was switched to the gas
mixture and adjusted until stabilization, GSHV=18000 h
−1
using Cu K radiation (λ=1.54186 Å) between 10° and 90°,
and a contact time of 0.20s. The reaction was studied under
isothermal condition at 700 °C and exhaust gases were ana-
lyzed by gas chromatography (HP 5890 series II) equipped
with a packed column HAYESEP Q of 5.5 m long, ¼″ pore
diameter and an inner diameter of 80/100 and a TCD detec-
tor. Finally, the flame detector allowed evaluating the for-
mation of minor species.
α
with steps of 0.05°. Refinement, indexing and the simu-
lation of the diffraction patterns were done with Fullprof
software [23], which allowed us to establish the crystallo -
graphic structure of the oxides and to perform quantitative
phase analysis. The crystallite size was calculated using the
highest diffraction peaks, using the Scherrer equation, tak -
ing the value of half peak width and using a constant of
Main products from methane reaction were hydrogen,
methane, carbon dioxide, carbon monoxide, ethylene, eth -
ane, acetaldehyde and diethyl ether. The conversion of
methane was calculated from carbon products in molar bal-
0
.89, as reference.
X-ray photoelectron spectroscopy (XPS), were done on
a Thermo Scientific Escalab 250 XI spectrometer, using Al
−
9
K radiation at a pressure of 10 mbar. First, a survey spec-
ance (on dry basis), as shown in Eq. 5, where γ is the ratio
α
Ci
trum was collected using pass energy of 100eV, in the bind-
ing energy interval 0–1300 eV, with energy steps of 1 eV.
of carbon content of product i to carbon content of methane
molecule, and y is the molar fraction of carbon content
Ci
Then, characteristic photoemission regions for each element products in the effluent flow [24]. The molar selectivity was
present were acquired using energy of 20 eV, with steps of
.05 eV. Atomic concentrations were calculated from the
defined as the ratio of moles of one product and the total
moles of products, based on experimental values as follows:
0
peak areas, assuming a Shirley-type background of inelastic
electrons, using a proper sensitivity factor for each photo -
emission line measured.
Σ γCi yCi
XMethane (%) =
×100
(5)
(6)
YMethane + Σ γC i yC i
The transmission electron microscopy analysis (TEM),
γCi yCi
were performed on a JEOL 2100 equipment using a LaB
6
Si (%) =
×100
thermionic gun operated with an acceleration voltage of
Σ γC i yC i
2
00 kV, equipped with a CCD imaging system. For analysis,
the sample was ground to obtain fine powders. Which were
sieve to 400 U.S. standard mesh and dispersed in a test tube
with 5.0 mL of water, each tube was place in an ultrasonic
equipment for a period of 30 min, after which a drop of the
top of each tube was take and dried at 45 °C for respective
analysis.
2.4 Post Reaction Analyses
After 10 days on stream, under steam methane reforming
reaction conditions, the catalyst was removed and analyzed
by X-ray diffraction, to evaluate the oxide state. Eventual
carbon deposition and other surface modifications were
Temperature programmed reduction (TPR-H ) was per -
2
formed in a Micromeritics AutoChemII instrument equipped tested with transmission electron microscopy. Finally, the
with a TCD detector. The sample was heated at 200 °C for
temperature programmed oxidation tests (TPO) were per -
formed to investigate their oxidation behavior. TPO/TPR
cycles were carried out in a Micromeritics AutoChemII
instrument equipped with a TCD detector. Upstream of the
2
5
h, flowing pure helium and then reduced with a mixture of
−1
% H /He (30 mL min ), rising up to the maximum tem -
2
−1
perature reduction at 10°C min . The H consumption was
2
measured using a thermal conductivity detector.
TCD, a CO trap was installed to adsorb water and humidity.
In each TPO tests, the sample was exposed to a 20 mL min
2
−1
2
.3 Catalytic Test
flow of 2% O in Helium balance. The temperature was first
2
ramped from 25 to 1000 °C, then maintained at 1000 °C for
The catalysts were tested in the steam methane reform
ing. The reaction was performed running for 10 days in an
-
1 h, and finally ramped down to 30°C, at constant heating
and cooling rate (5 °C min ). Prior to the admission of the
−
1
experimental unit consisting of a set of mass flow-meters
and a reactor coupled to a resistive furnace. The reaction
feed rate mixture (CH /H O/Ar in gas phase), was 8/24/68
mL min , respectively, pre-heated at 200°C before catalyst
contact in a U shape quartz micro-reactor with 12 mm ID
and 250 mm long at 700 °C. The sample was supported on
quartz wool.
O flow, the sample was evacuated at 120°C in Helium
2
−1
(20 mL min ) and cooled down to 30 °C. In the TPO/TPR
−1
cycles, the TPR tests were performed in 20 mL min of
4
2
−1
5% H in Argon balance. The sample was heated from 25
2
−
1
to 1000°C at 5°C min , maintained at 1000°C for 1 h, and
finally cooled to room temperature under inert atmosphere
−
1
(20 mL min He). After each TPR, a TPO test was per
-
The sample 200 mg was dried at 300 °C for 30 min prior
catalytic tests under helium flow (30 mL min ). After
formed and three cycles were repeated to verify the stability
of the material.
−1
1
23

Nanostructured La0.8Sr0.2Fe0.8Cr0.2O Perovskite for the Steam Methane Reforming
2507
3
3
Results and Discussion
quantitative Rietveld phase analysis gives the mass fractions
La Sr Fe Cr O and cell parameters, in accordance
0
.8 0.2 0.8 0.2
3
Chemical composition was calculated by X-ray fluores-
cence, as shown in Table 1. These final composition was
close to the proposed nominal values, which allowed
evaluate the stoichiometry of proposed composition of
La Sr Fe Cr O oxide. The specific surface area
with Gabroska et al. [ 27] and Thirumalairajan et al. [ 28].
Crystallite sizes for the perovskite phases were estimated
using the highest diffraction peak and Scherrer equation,
taking the value of half peak width (β), adjusted to a Lorent-
zian function and using a constant of 0.89 as reference, ind-i
cating the presence of nanometric crystallites distributed
around 25 nm. It can clearly be seen that the crystallite size
depends strongly of time and temperature of the combus -
tion process, enabling sinter of the powder due to the large
surface area, and more important, obtaining a single crystal-
line phase, avoiding the loss of surface properties during
the synthesis. These results were compared with the ICDD
databases, which permits to identifying the space group, the
0
.8 0.2 0.8 0.2
3
2
−1
was 60 m g , which can be attributed to the lower auto-
combustion temperature in the synthesis process, in accor -
dance with Phokha et al. [24]. Such results, confirm that the
synthesis method affects significantly obtaining a homog-
enous material with high surface area when compared to
2
−1
the LaCoO perovskite (<10 m g ) [25], using Pechini´s
3
method of preparation, which can be attributed to the elimi-
nation of volatile components in the combustion process.
Therefore, an effective way to produce very active materials crystal system and cell parameters, as shown in Table 2.
with higher porous structure [ 26]. Previously, we observed
Transmission electron microscopy analyses (TEM) of
La Sr Fe Cr O sample, showed irregular aggregates
composed by small crystallites around 25 nm (Fig. 2). In
principle, these pictures are related with the texture and the
relief formed by elimination of solvent and volatile species
during the combustion of organic compounds in the ignition
for perovskite-type oxides La 1 M CoO (M =Ce, Sr) that
−x
x
3
0.8 0.2 0.8 0.2
3
the insertion of lower x=0.2, generated solid solution with
a single phase of perovskite, however, higher contents
favored phase segregation [25].
The XRD pattern of La
Sr Fe Cr O catalyst
3
0
.8 0.2 0.8 0.2
after polymerization-combustion is shown in Fig. 1. The
Table 2
Comparison between crystalline properties of
La0.8Sr0.2Fe0.8Cr0.2O oxide obtained by means XRD and TEM analy-
sis
3
Table 1 Elemental composition of La
derived from X-ray fluorescence
0.8Sr0.2Fe0.8Cr0.2O oxide
3
Method
Average
crystallite
size (nm)
Space
group
Crystal system
Cell
param-
eters (Å)
Oxides
Composition (%)
LaFeO3
La0.8Sr0.2Fe0.8Cr0.2O3
XRD
TEM
41.1
Pbn (62) Orthorhombic
a=5.532
b=5.553
c=7.835
a=5.425
b=5.453
c=7.812
La O3
67.10
−
59.30
4.71
2
SrO
Fe O
32.89
29.06
6.91
2
3
41.0
Pbn
(62–63)
Orthorhombic
Cr O
−
99.99
2
3
Total
99.98
Fig. 1 XRD pattern of La0.8
S
r0.2Fe0.8Cr0.2O oxide obtained
3
by citrate method and compared
with corresponding Rietveld
refinement
123

2
508
J. A. Gómez-Cuaspud et al.
Fig. 2 Transmission electron microscopy images at different magnifications of La0.8Sr0.2Fe0.8Cr0.2O oxide
3
[
29]. With increasing calcination temperature, the spheri-
The data were collected and normalized for obtaining
regular distributions of crystallite sizes with regular mor
cal morphology and crystallinity of the particles increased,
which implies on growth of small particles occurring dur -
ing the posterior calcination process. In fact, the micro
structure and uniformity of the synthesized powder can
affect significantly the density of the sintered ceramic and
the catalytic properties [ 30, 31]. The transmission electron
microscopy results in Fig. 2 allowed a statistical counting
in all micrographs, obtained by reference in a population of
-
phology. These results confirm previous X-ray diffraction
analysis indicating nanometric crystallites. Figure 3 dis -
plays the histogram of particle size distribution, showing
unimodal distribution with a marked Gaussian behavior,
revealing the presence of nanometric particles varying from
15 to 33 nm. These results can be attributed to the ultrafine
structure of the solid and the dispersive effect of the water
vapor evolved during the combustion process, in accordance
-
2
50 particles.
Fig. 3 Histogram distribution
of particle size for a population
of 250 particles of La0.8Sr0.2Fe0.
8Cr O oxide
100
80
0.2
3
60
40
20
0
15-18
18.1-21.1
21.2-24.2
24.3-27.3
27.4-30.4
30.5-33.5
Crystallite diameter (nm)
1
23

Nanostructured La0.8Sr0.2Fe0.8Cr0.2O Perovskite for the Steam Methane Reforming
2509
3
with Huang et al. [ 32]. While the particles sizes are in the
desired range of the nanoscale level, the different size distr-i
bution can probably affect the sintering of the final product
of the solid-state.
around 600 °C. According to Jeong et al. [ 34], the iron-rich
solid material can be reduced in four single steps at differ -
ent temperatures, while the structure collapses, according
to Eqs. 6–9.
From transmission electron microscopy data and X-ray
diffraction analysis, it was possible to calculate the unit
cell of the La 0.8Sr Fe Cr O oxide, using the ELMIX
0
La O + 3H → 2La + 3H O
(7)
(8)
0.2
0.8 0.2
3
2
3
2
2
software. The simulation of crystalline parameters shown
in Fig. 4 and Table 2, are consistent with the experimental
values obtained from X-ray diffraction and Rietveld calcu -
lations [19]. The structural refinements were performed by
means of Fullprof software packages in the orthorhombic
space group Pbn. A polynomial function, and pseudo-Voigt
function was employed to model the peak shapes while a
sixth order polynomial for the background, zero, LP factor,
scale, pseudo-Voigt profile function (U, V, W and X), lattice
parameters, atomic coordinates and isothermal temperature
factors, in accordance with procedure established by Singh
et al. [33].
0
SrO + H → Sr + H O
2
2
0
Fe O + H → 2FeO + H → Fe + H O
(9)
2
3
2
2
2
Cr O + H → 2CrO + H O
(10)
2
3
2
2
The profile shows that the reduction of the oxide is facili-
tated by the reduction of Fe O and Cr O at lower tem -
2
3
2
3
peratures. Additionally, the substitution of La provoked
vacancies, and thus the formation of Fe and Cr ions in coor-
−
dination with the O2 species.
The TPR-H 2 profile of La Sr Fe Cr O oxide is
shown in Fig. 5. The maximum reduction signal occurred
at 423°C and was almost complete at 450 °C, besides a tail
The XPS analyses and surface composition were deter -
mined for the perovskite-type La 0.8Sr Fe Cr O oxide.
Figure 6, shows the spectrum referred to the internal levels of
0.8
0.2 0.8 0.2
3
0.2
0.8 0.2
3
La3d, Sr3d, Fe2p, Cr2p and Sr3p for La0.8Sr Fe Cr O
3
0.2
0.8 0.2
oxide. The sample displays one peak for Fe 2 p3/2 centered
3
+
at 710.4eV, which is assigned to Fe and a peak of Cr 2p 3/2
centered at 576.0 eV, for Sr 3d5/2 centered at 131.9 eV and
for lanthanum for 3 d5/2 is centered at 837.5 eV, which
according to Hueso et al. [ 35] belongs to the lattice of the
perovskite structure. Data relative to oxygen was analyzed
and found an O 1 s signal at 528.9 and 531.2 eV, which is
assigned to the oxygen of hydroxide or carbonate species
[25, 36].
Fig. 4 Unit cell of La 0.8Sr0.2Fe0.8Cr0.2O oxide obtained by ELMIX
3
software
Fig. 5 TPR-H profile for La0.8Sr0.2Fe0.8Cr0.2O oxide (10 % H -He
Fig. 6
X-ray photoelectron spectroscopy images for
2
3
2
−
1
rate 10°C min
)
La0.8Sr0.2Fe0.8Cr0.2O oxide
3
123

2
510
J. A. Gómez-Cuaspud et al.
Detailed spectrum of lanthanum La 3 d, as shown in
Fig. 7a exhibits binding energies (BE) at 833.7 and a shake-
up satellite peak at 837.5 eV, which are assigned to La
reported by Li et al. [40]. The migration and segregation of
SrO in the perovskite structure from the bulk to the surface
and probably reaction of SrO with Cr species may be occur
[41].
3
+
in the lattice [37]. When modified with strontium, the XPS
spectrum presents a BE of La 4d at 101.15 eV (not shown),
3
+
corresponding to La in the lattice and traces of segregated
La O at the surface, linked to carbonate ions. Literature
Cr −Sr − O + CrO + SrO → SrCrO
(11)
3
4
2
3
showed that the shift to higher binding energies 105.01 and
08.38 eV are assigned to segregated lanthanum, like LaO
Besides, the segregated species may also exist with the
SrCrO , as shown in the spectrum of Sr 3 d5/2 in Fig. 7b
for BE of Sr at 132.2 eV, in agreement with Chen et al.
[41]. The excess of segregation is probably attributed to the
decomposition of the perovskite structure.
1
2
3
3
2
+
(
103.0 eV), La(OH) (106.1 eV) and carbonate species [25,
3
3
7].
As shown in Fig. 6 the BE positions of O1s spectra yield
2
−
two peaks. The stronger peak is associated with O
at 528.87 (529.6) eV, which is related to oxygen in the
ions
The XPS spectra Fe 2 p3/2 and Fe 2 p1/2 of the Fe O
2
3
sample are shown in Fig. 7c. The Fe 2 p spectrum shows
3
+
perovskite structure, indicating that the oxygen ions remain
coordinated in structure [38]. The peak at 531.16 eV is due
that 29 % of Fe ions are related to the doublet of BE at
3
+
710.46–723.81 eV. These Fe species are located at the
−x
3+
to an intermediate oxidation state for oxygen [O ] and may
be related to carbonate compounds as well as to defects,
such as oxygen vacancies [39].
surface suggesting that part of Fe
ions were inserted in
the lattice. The Fe 2p3/2 peak is narrower and stronger than
Fe 2p1/2 and the area of the Fe 2 p3/2 peak is greater than
of the Fe 2p1/2, because its spin–orbit coupling, according
to Yamashita et al. [ 42]. According the literature, the peak
position of Fe 2p3/2 is around 710.6 and 711.2 eV [43–45].
The Fe 2 p3/2 peak has associated a satellite peak that is
shifted approximately by 8 eV relative to the main Fe 2p3/2
Separated spectrum of Sr 3 d5/2 in Fig. 7b showed BE
of Sr2 at 131.92 and 133.77 eV. The BE at 131.92 (0.61)
+
2
+
eV evidences the presence of Sr
at the surface and in
the SrO2 sub-layer of the perovskite structure, while the
−x
band at 133.77 eV suggests the presence of carbonates, as
Fig. 7 Slow scan X-ray photoelectron spectroscopy images fora La 3d, b Sr 3d5/2c Fe 2p3/2 and Fe 2p1/2 and d Cr 2p3/2 oxide components, with
corresponding binding energies
1
23

Nanostructured La0.8Sr0.2Fe0.8Cr0.2O Perovskite for the Steam Methane Reforming
2511
3
peak. Yamashita et al. [42] obtained binding energies of Fe
p3/2 and Fe 2p1/2 of 711.0 (standard deviation SD=0.01)
La Sr Fe Cr O oxide and compared to sample
0.8 0.2 0.8 0.2 3
2
(LaFeO ) in powder form. Blank results did not show activ-
3
and 724.6 eV (SD =0.17), respectively, in good agreement
with our work. The satellite peak at 717.92 eV is close to
that reported in the literature [718.8 (SD =0.13) eV] and
does not overlap either the Fe 2 p3/2 or Fe 2 p1/2 peaks. In
addition, another satellite peak appears at 731.5 eV, which
may be associated to a satellite peak of Fe 2p1/2. According
to the literature, the Fe 2 p3/2 spectrum of Fe O does not
ity. The reaction was performed after elimination of diffu -
sion or mass transport effects. The conversion, H /CO ratio
2
and selectivity for H and CO, for three experiments showed
2
good reproducibility.
The initial methane conversion for the
La Sr Fe Cr O oxide was 97 ± 2%, while the H /CO
0
.8 0.2 0.8 0.2
3
2
ratio was close to 3.0 at 700 °C, as expected for a methane
steam reforming reaction:
3
4
exhibit a satellite [43]. The absence of the satellite peak was
confirmed in present study.
Figure 7d shows the XPS spectra of Cr 2 p3/2. It dis -
plays three distinct BE of Cr 2 p3/2 at 576.06, 577.7 and
CH + H O → CO + 3H (∆Ho
= 206 kJ / mol)(12)
298 K
4
2
2
5
79.34 eV, which are assigned to Cr O Cr O and SrCrO
4
The average methane conversion, using the reference sam -
2
3,
2
5
respectively, in good agreement with that reported by Chen
et al. [41]. These results agreed with XRD diffraction pat -
tern after Rietveld refinement, as presented in Table 3.
Zhang et al. [ 43] investigated by XPS the electronic
structure of Fe ions in the SrFeO 3 perovskite oxide and
concluded that there is a large increase of charge on Fe sites
that led to a chemical shift to a higher BE.
ple (LaFeO ) was 66.3 ± 1.08%. The H/CO ratio was of
3
2
the order of 3.0, suggesting high formation of H . Tests dur-
2
ing 240 h on stream under isothermal conditions at 700 °C,
showed a methane conversion of 95 % and excellent selec-
tivity to carbon monoxide and hydrogen of 93 and 94 %,
respectively.
In fact, XPS results showed the presence of chromium
and iron at the surface with the formation of ion species at
4
+
Indeed, XPS results showed the presence of an interme -
−x
diate oxidation state for oxygen [O ] and may be related
to carbonate compounds as well as to defect, such as oxy -
gen vacancies. Results showed also the presence of iron at
the interface, which are active centers for CH reforming.
4
In fact, Ghaffari et al. [ 44] observed that for LaFeO , the
3
3
+
4+
iron in the perovskite structure contains Fe and Fe ions,
3
+
2−
the surface with the formation of Fe
/O species at the
and that with increasing Fe content it promotes a signifi-
2
+
3+
2+
interface, besides, the presence of Sr at the surface, which
resulted in the formation of vacancies.
cant growth Fe ions and besides, the presence of Sr
at
the surface resulted in the formation of vacancies, in accor-
dance with our results.
3
.1 Activity and Selectivity
The methane conversion at 700
°C for the
La Sr Fe Cr O oxide is very high, compared to the
0
.8 0.2 0.8 0.2
3
The steam methane reaction was performed at 700
°C,
reference sample, which can be attributed to the strong
dependence vacancies, in accordance with the literature
[45–48]. The steam reforming reaction on multicomponent
oxides suggests a redox mechanism, where CH 4 may also
combine with oxygen of the lattice. The reduced oxide may
be oxidized by water, increasing hydrogen concentration
and allowing the formation of intermediate species by an
associative mechanism. The main reaction occurs between
the bidentate intermediate and the terminal hydroxyl groups
−
1
at space velocity (GSHV) 18.000
h , with a contact
time of 0.20 s. Catalytic tests were carried out on the
Table 3 Binding energy, bulk (XRF) and surface (XPS) atomic con -
centrations of La0.8Sr0.2Fe0.8Cr0.2O sample
3
Element
Photo- Binding energy Bulk atomic
Surface
emis- (eV)
sion
line
concentration atomic con-
on the oxide, which is then decomposed to form H and car-
by XRF (%)
centration by
XPS (%)
2
bonate species according to Sun et al. [49].
The stability test was performed during 240
similar reaction conditions. Results are displayed in Fig. 8.
The deactivation of the La Sr Fe Cr O oxide was low
h under
La
Sr
3d
3d
3d5/2
3d5/2
837.5 16.67
6.2
5.7
131.9 4.16
133.8
3
d3/2
2p3/2
p1/2
2p3/2
p1/2
1s
0.8 0.2 0.8 0.2
3
compared to the reference sample (LaFeO ). In fact, XPS
Fe
Cr
O
2p
2p
1s
1s
710.5 16.67
723.8
5.6
2.8
3
2
results evidenced clearly the presence of segregated lantha-
num, like La O , La(OH) and carbonate species. Besides,
576.1 4.16
585.5
2
3
3
2
Sr showed also the formation of SrO
sub-layer and car -
2−x
528.9 58.33
531.2
54.2
25.6
bonates, such as SrCrO [50]. Therefore, the post reaction
4
1
s*
analyses are important to provide a clue on the main factors
that may affect the stability of the promoted catalyst.
C
1s
284.6
288.3
–
1s*
123

2
512
J. A. Gómez-Cuaspud et al.
0
50
100
150
200
250
300
100
100
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
o
90
80
70
60
50
40
30
20
10
0
T=700 C
(
La Sr Fe Cr O )
0
.8 0.2 0.8 0.2
3
(
LaFeO3
)
0
50
100
150
200
250
300
Fig. 10 X-ray diffraction pattern of La 0.8Sr0.2Fe0.8Cr0.2O oxide after
3
catalytic test during 240 h
Time (h)
Fig. 8 Stability test during 240 h for steam methane reaction on
La0.8Sr0.2Fe0.8Cr0.2O and LaFeO as reference catalysts
deactivation of the catalyst is usually attributed to sintering,
which depends largely on the temperature, time exposure
and the nature of the reducing environment, among other
factors. However, the oxide resists structural changes during
the steam methane reforming reaction.
3
3
3
.2 Post-reaction Analysis
The La 0.8Sr Fe Cr O oxide was analyzed by TEM,
0.2
0.8 0.2
3
XRD and TPO after the stability test. TEM analyses of the
sample does not evidence carbon deposition, as shown in
Fig. 9, suggesting that the catalyst presents resistance to ca-r
bon deposition.
Temperature programmed oxidation (TPO) of the spent
sample, using 2% O (v/v) to simulate a condition of stability
2
at high oxygen conversion is displayed in Fig. 11. The TPO
profile shows the re-oxidation peak centered around 350°C
and no carbon. The peak around 210°C may be attributed to
the reduction of binary phases of Sr-Cr, in agreement with
Zheng et al. [ 51]. Repeated TPR/TPO cycles confirm the
results.
XRD analysis does not present any secondary struc
ture or composition alteration, discarding the presence
of contaminant phases or phase change during the reac
-
-
tion, as shown in Fig. 10. Calculations, showed that the of
space group, crystal and cell parameters of material, using
ICDD databases, retain the same structural characteristics.
Noteworthy are the XPS results, showing the reduction
3
+
2+
4+
3+
of the redox couples Fe /Fe and Cr /Cr . In analogy
with the mechanism proposed by Tanaka et al. [ 52] for the
LaFeO structure, we propose for this catalyst LaFeCrO
The crystallite size, calculated using the highest diffrac
tion signals D (0 1 1) , D(0 0 2) and D (2 1 1) by means Debye–
Scherrer equation, exhibited growth of the crystallite sizes,
about 220–310 nm, using the principal diffraction signals
of La0.8Sr Fe Cr O sample after catalytic test, ratify -
-
3
3
6
+
(Cr ) structure the following reaction mechanism:
CH4(g) + CrO + Vacancy → CrO + CO
3
2
(13)
0.2
0.8 0.2
3
+
2H + 2Vacancy
2
ing that crystallite size was almost 81 % larger than before
the test. This fact justifies the deactivation process. The
CrO + H O − 2 Vacancy → CrO + H + Vacancy (14)
2 2 v 3 2
Fig. 9 TEM micrographs at different amplifications for La0.8Sr0.2Fe0.8Cr0.2O oxide after 240 h on stream
3
1
23

Nanostructured La0.8Sr0.2Fe0.8Cr0.2O Perovskite for the Steam Methane Reforming
2513
3
La Sr Fe Cr O phase. The physicochemical char
-
0
.8 0.2 0.8 0.2
3
acterization showed the presence of an active nanometric
catalyst of homogeneous appearance and an excellent stoi -
chiometric composition. XPS results showed the presence
3
+
2+
4+
3+
2+
Fe /Fe and Cr /Cr , besides, the presence of Sr
the surface, which resulted in the formation of vacancies.
Tests during 240 h on stream under isothermal conditions at
00°C showed high methane conversion of 95% and excel-
at
7
lent selectivity to hydrogen and lower deactivation when
compared to the LaFeO as reference.
3
Post reaction analyses by TEM, XRD and TPO showed
absence of carbon deposition, growth of crystallite sizes.
However, the structure of the La Sr Fe Cr O was not
0
.8 0.2 0.8 0.2
3
affected and no phase changes were observed during the
reaction, which suggest that the structure resists structure
modification during the steam methane reforming reaction
and that the promoters prevent carbon deposition.
Fig. 11
Temperature programmed oxidation profiles of
−1
La0.8Sr0.2Fe0.8Cr0.2O oxide using 2 % O in He at 20 mL min as
3
2
function of temperature
Acknowledgments CNPQ, CAPES, FAPERJ, FINEP and NUCAT
and funds for supporting this research.
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