Elucidation of the role of support in Rh/perovskite catalysts used in ethanol steam reforming reaction

Article abstract of DOI:10.1016/j.cattod.2020.12.013

Rh/perovskite catalysts were prepared and evaluated in ethanol steam reforming reaction. The samples were characterized by SBET, XRD, XPS, TPR, TG and SEM techniques. It could be possible to obtain perovskites supports with high purity, stability and suitable specific surface areas. The partial substitution of La with Ca and Ce in a LaAlO₃ based perovskite structure was reached and changes in its porosity and surface features were detected. The inclusion of Ca into the structure increased specific surface area of support and Ce inclusion activated oxygen mobility on catalyst surface. The supports influenced the degree of reduction of Rh species at the surface of catalysts. All catalyst showed complete ethanol conversion on 6 h on stream. The best catalytic performance was achieved with Rh/LaAlO3 0.3 % catalyst which presented the highest mean value of H₂ product distribution, low production of byproducts (CH₄ and CO) and presented an excellent long term experience performance during 24 h on stream. Rh/LaCeAlO3 0.3 % presented the lowest carbon accumulation during 6 h on stream but it suffered deactivation after 14 h on stream. The deactivation was attributed to the partial oxidation of Rh° species to Rh³⁺ which are less active favoring a reaction pathway that produce carbonaceous deposits.

Full text of DOI:10.1016/j.cattod.2020.12.013

Catalysis Today xxx (xxxx) xxx
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Catalysis Today
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Elucidation of the role of support in Rh/perovskite catalysts used in ethanol
steam reforming reaction
a
b
a
a,
´
Asiel Hernandez Martínez , Eduardo Lopez , Luis E. Cadús , Fabiola N. Agüero *
a Instituto de Investigaciones en Tecnología Química (INTEQUI), CONICET-UNSL, Facultad de Química Bioquímica y Farmacia, Almirante Brown 1455, 5700, San Luis,
Argentina
b Planta Piloto de Ingeniería Química (PLAPIQUI), UNS – CONICET, Bahía Blanca, Argentina
A R T I C L E I N F O
A B S T R A C T
Keywords:
Rh/perovskite catalysts were prepared and evaluated in ethanol steam reforming reaction. The samples were
characterized by SBET, XRD, XPS, TPR, TG and SEM techniques. It could be possible to obtain perovskites
supports with high purity, stability and suitable specific surface areas. The partial substitution of La with Ca and
Ce in a LaAlO3 based perovskite structure was reached and changes in its porosity and surface features were
detected. The inclusion of Ca into the structure increased specific surface area of support and Ce inclusion
activated oxygen mobility on catalyst surface. The supports influenced the degree of reduction of Rh species at
the surface of catalysts. All catalyst showed complete ethanol conversion on 6 h on stream. The best catalytic
performance was achieved with Rh/LaAlO3 0.3 % catalyst which presented the highest mean value of H2 product
distribution, low production of byproducts (CH4 and CO) and presented an excellent long term experience
performance during 24 h on stream. Rh/LaCeAlO3 0.3 % presented the lowest carbon accumulation during 6 h
on stream but it suffered deactivation after 14 h on stream. The deactivation was attributed to the partial
oxidation of Rh◦ species to Rh3+ which are less active favoring a reaction pathway that produce carbonaceous
deposits.
Steam reforming
Ethanol
Supported catalyst
Perovskites
Rhodium
C2H5OH + 3H2O→2CO2 + 6H2
There are additional side reactions in which, byproducts are
obtained:
2CO ↔ CO2 + C
(1)
1. Introduction
Hydrogen interest lies in its several applications such as esterifica-
tion and transesterification of polyunsaturated compounds to obtain
final products of industrial interest like biodiesel; large scale ammonia
production; combined heat and power (CHP) plants or fuel cell electric
generators; and also the direct use as fuel in hydrogen powered cars
[1–7]. The feasibility of using hydrogen as direct fuel has been studied
[8–10]. The comparative study of technical and environmental issues
between fossils fuels and hydrogen energy reveals hydrogen economy as
an environmental solution [11]. There are several process for hydrogen
production such as electrolysis, photolysis and thermolysis of water,
biological reactions, gasification and pyrolysis of biomass, steam
reforming and partial oxidation of hydrocarbons [12,13]. One of the
main feedstock of hydrogen production via steam reforming is natural
gas which is a not a renewable source. A good alternative can be ethanol
steam reforming (ESR) since it can be obtained from biomass as
renewable source [14,15], as represented by the following overall
stoichiometric equation:
(2)
(3)
(4)
(5)
(6)
(7)
(8)
(9)
CO2 + H2 ↔ CO + H2O
C2H4O + H2O →3H2 + 2CO
CH4→2H2 + C
CO + H2 ↔ H2O + C
CO + 3H2 → CH4 + H2O
C2H5OH→C2H4 + H2O
n C H → [C H →coke
]
4
n
2
4
2
* Corresponding author.
E-mail address: fnaguero@unsl.edu.ar (F.N. Agüero).
https://doi.org/10.1016/j.cattod.2020.12.013
Received 25 May 2020; Received in revised form 30 October 2020; Accepted 14 December 2020
Available online 5 January 2021
0920-5861/© 2020 Elsevier B.V. All rights reserved.
Please cite this article as: Asiel Hernández Martínez, Catalysis Today, https://doi.org/10.1016/j.cattod.2020.12.013

A.H. Martínez et al.
Catalysis Today xxx (xxxx) xxx
Rh on ethanol steam reforming has not been reported. The main
research in this line have been related to Rh supported over single or
doped oxides.
C2H5OH→CH4 + CO + H2
CH4 + 2H2O→4H2 + CO2
(10)
(11)
The purpose of this work is to elucidate the role of the support in the
textural, structural and chemical properties of Rh/ perovskite based
catalysts and thus its behavior in ethanol steam reforming reaction. For
this reason, LaAlO3 perovskite partially substituted with Ca or Ce in the
A site was studied.
Although reforming, is not a new method to obtain hydrogen, new
research in this field have been reported [16–25]. Novelty in this area
lies in the design and development of functional materials, based on
particles dispersed on external surfaces, allowing high hydrogen yield a
low distribution to byproducts. In the last two decades a lot of work has
been done on the modification strategies to minimize carbon formation
during ESR [26].
2. Experimental
Noble metals (Pt, Pd, Rh, Au and Ru), and transition metals (Cu, Co
and Ni) as well as the
2.1. Catalyst preparation
combinations of both [27–30] have been widely studied as active
metal for ESR reactions. Rh based catalysts are considered as the most
The support perovskites were prepared by the citrate method [49].
La(NO3)3 x 6H2O (Fluka Analytical), Ca(NO3)2 x 4H2O (Sigma Aldrich),
Al(NO3)3 x 6H2O (Fluka Analytical) and Ce(NO3)3x 6H2O (Sigma
Aldrich) were dissolved in water and were added to an excess citric acid
solution (Sigma Aldrich). The resulting solution was slowly evaporated
under vacuum in a rotavapor at 65 ◦C until a gel was obtained. This gel
was dried in a vacuum oven, slowly increasing the temperature to
100 ◦C and maintaining this temperature overnight, to produce a solid
amorphous citrate precursor. The resulting precursor was milled and
then calcined in air at 800 ◦C for 2 h with a 10 ◦C heating ramp
obtaining three supports designated as La1-x-yCexCayAlO3 (x = 0 when
y = 0.1 and x = 0.05 when y = 0). Rh based catalysts were obtained by
wet impregnation method. Rh(NO3)3 x H2O (Sigma Aldrich) aqueous
solution was used to impregnate the previously prepared supports. Then
samples were dried in a vacuum oven at 100 ◦C and then calcined to
800 ◦C in order to obtain catalysts with three levels of rhodium charge
(0.1; 0.3 and 0.5 %). The samples were named as: Rh/LaMAlO3 (0.1, 0.3
or 0.5 %) (M = 0, Ca or Ce)
–
active metal due to their excellent C C and C–H bond rupture affinity,
high water gas shift (WGS) activity and high resistance towards carbon
deposition [17–19,22,25,31–35]. The nature of the support is also
important in terms of accomplishing most adequate acid-base charac-
teristics for the process in each case. Additives or promoters are also
employed in this sense like alkaline metals. Reactive supports are also
used in many cases in order to modify the redox properties of the active
metal as well as to directly intervene in relevant steps of the reaction as
bifunctional promoter. It is important to know the interface between
metal and support, rather than the individual components themselves,
which features the active region and presents new specific characteris-
tics itself which are different from those of the metal and the support [7].
The selection of support plays an important role in long-term catalyst
operation. Acidic supports, induce ethanol dehydration to produce
ethylene, which is a source of coke formation (Eqs. 8 and 9). Dehydra-
tion can be depressed by adding K to neutralize the acidic support, or by
using basic supports, i.e. and MgO or CaO. Frusteri et al. [36] evaluated
catalytic performance of MgO-supported Pd, Rh, Ni, and Co for
hydrogen production by ethanol steam reforming. Rh/MgO showed the
best performance in terms of ethanol conversion and long term experi-
ence. Osorio-Vargas investigated Rh catalysts supported over bare
γ-Al2O3 and γ-Al2O3 modified with different promoters (15 wt% La2O3
and/or 2.5 wt% CeO2) and conclude that pathways depend on the
characteristics of the support. They found that the extent and type of
carbon deposited on catalyst is affected by the support. They also re-
ported that resistance toward carbon formation of Rh/2.5Ce-A and
Rh/2.5Ce-LA could be mainly due to the redox property of ceria.
CeO2-based materials have high oxygen storage capacity and oxygen
mobility. These characteristics are related to their rapid reduction/ox-
idation capability [37]. Sharma et al. [31] found that H2 distribution
was higher for Rh/CeZrO2 catalyst (62.9 %) as compared to Rh/Al2O3
(59.3 %) under optimized reaction conditions. Campos et al. [38] re-
ported bimetallic Rh (0.25, 0.5, 0.75, 1.0 wt.%)-Ni 10 wt%) promoted
with La2O3(15 wt%) and CeO2(10 wt%)/alumina supported catalysts
for ethanol steam reforming and a comparison to their monometallic
counterparts was reported. Ce containing catalysts were reported as the
most active due to high oxygen mobility that enhanced oxidation re-
actions, particularly of CO towards CO2, effectively promoting the water
gas shift reaction [39].
2.2. Catalysts characterization
2.2.1. Thermogravimetric analysis
Thermogravimetric analysis was performed using a Shimadzu DTG-
60 with 10 mg of the sample at a heating rate of 10 ◦C /min from
ambient to 900 ◦C in 50 mL/min air atmosphere.
2.2.2. X-ray photoelectron spectroscopy
XPS data were obtained with a Multitechnic UniSpecs equipment
with a dual X-ray source of Mg/Al and a hemispheric analyzer PHOIBOS
150. A pass energy of 30 eV and an Al anode operated at 100 W was
used. The pressure was kept under 29 × 10ꢀ 8 mbar. Samples were
previously reduced at 400 ◦C for 1 h in 50 mL/min 5% of H2/N2 stream.
2.2.3. X-ray diffraction (XRD)
XRD patterns were obtained by using a Rigaku Ultima IV diffrac-
tometer operated at 30 kV and 25 mA with Cu
Kα radiation
(λ = 0.15418 nm) and a Nickel filter.
2.2.4. Temperature programmed reduction (TPR)
The TPR was performed in a quartz tubular reactor using a mass
spectrometer as detector. Samples of 100 mg were used. The reducing
gas was a mixture of 5 % vol. H2/N2, at a total flow rate of 30 mL/min.
The temperature was increased at a rate of 10 ◦C /min from room
temperature to 900 ◦C. The degree of reduction of each catalyst was
calculated as follows:
Several structures such as multiple oxide solid solution, zeolites,
perovskites and spinel have also been used as supporting frameworks
[39–42]. Perovskite type support with general formula ABO3 is a
promising material as catalyst supports for ethanol steam reforming. Its
A-site ion is generally occupied by an alkaline earth or lanthanide metal
with a large ionic radius to stabilize the whole structure, and B-site ion is
generally a transition metal element. Perovskite possess a special ability
in oxygen mobility and storage, which is specifically suitable to be used
as the support of catalysts. This can be beneficial for deposited carbon
removal [43]. Only a few reported works have studied perovskites type
support (ABO3) as catalysts precursors for rhodium dry reforming and
selective hydrogenation [44–48]. Nevertheless, perovskite support for
H2 consumption from TPR experiment
H2 required for complete reduction
Degree of reduction (f) =
(12)
The amount of H2 consumed was calculated using the area obtained
from the TPR profile of each catalyst, while the H2 required for complete
reduction was calculated from the total number of moles of Rh present in
the catalyst. The following reduction reactions were considered for
calculating the amount of H2 required for complete reduction of Rh
2

A.H. Martínez et al.
Catalysis Today xxx (xxxx) xxx
oxides:
Rh3+ + 1.5H2 → Rh0 + 3H+
thermocouple was placed directly inside the catalytic bed. The con-
densable compounds were collected by means of a condenser at 10 ◦C
using a Peltier-based device. The noncondensable fraction was quanti-
fied by gas chromatography (HP 4890D) using Porapak QS-AW and
Molecular Sieve 0.5 nm columns and a TCD. Liquids were analyzed, only
for the long term experience, by GC (HP5890) with a Porapak N column
and TCD. Moreover, the total volumetric flow rate of the gaseous stream
and the liquid condensation rate were quantified in order to close
element balances and ensure the reliability of the experiments.
(13)
2.2.5. Specific surface area measurements (SBET)
The specific surface area of samples was calculated by the BET
method from the nitrogen adsorption isotherms obtained at 77 K. A
Gemini V from Micromeritics apparatus was used. Samples were pre-
viously degassed at 200 ◦C overnight.
2.3. Catalytic test
3. Results
The catalytic evaluation of the prepared catalysts was performed in a
lab set-up which includes dosing and evaporation of the liquid feed
mixture (water + ethanol), electrical heating, and quantitative analysis
of the exit gaseous stream. The liquid feed was dosed by an Eldex Piston
Metering Pump, 1/4" stainless head directly from a storage tank. The
subsequent vaporization and overheating of this mixture was performed
by using heating tapes. A furnace with temperature control was
employed to hold a quartz reactor in which the catalyst was disposed. A
K-type thermocouple was used to register the reactor temperature. The
product gas composition was analyzed by gas chromatograph with a The
gas stream exiting the reactor was conducted through a thermostatized
line to the detection system composed by GC 9A Shimadzu equipped
with a Carbosphere packed column and Buck Scientific chromatograph
with a Porapak Q packed column both provided with a TCD. Catalysts
3.1. Supports and catalysts characterization
3.1.1. X-ray diffraction (XRD)
XRD patterns of all samples are shown in Fig. 1. As it can be seen the
support perovskites presented a good crystallinity and the diffraction
lines were identified as those corresponding to LaAlO3 phase (Crystal-
lography Open Database ID: 2206576). Rh impregnated catalysts pre-
sented the same patterns. Diffraction lines corresponding to segregated
phases of lanthanum, calcium, aluminum, cerium or rhodium oxides
were not observed with this technique. XRD patterns measured in step
mode for the perovskite supports were refined using the Rietveld
method with the Fullprof Suite Toolbar [51,52]. An excellent fit was
obtained between observed and calculated XRD profiles for this model,
as illustrated in Fig. 2 for LaAlO3, LaCaAlO3 and LaCeAlO3 perovskites
respectively. The main obtained parameters are summarized in Table 1.
The paramount parameters to analyze is the effective doping of Ca and
Ce in the base perovskite structure, however, the last value is impossible
to refine from X-ray pattern because La and Ce cations are virtually
indistinguishable in terms of scattering factors. In spite of this, the Ce
content in the perovskite structure also generates changes in the lattice
(unit-cell volume and atomic distances) which can be used to estimate
the Ce substitution. Regarding to the changes in the unit-cell volume, it
would be considered that; rLa3+ = 0.136 nm, rCe3+ = 0.134 nm,
rCe4+ = 0.114 nm, rCa2+ = 0.134 nm. The substitution of Ce3+ (or
Ce4+) instead La3+ cations induces a reduction in the unit-cell. The in-
clusion of Ca2+ would have produce a decrease of cell dimensions,
however an increase of cell parameter and volume were observed. An
were grounded up to a particle diameter between 297 and 500 μm. In
order to moderate thermal effects, dilution by mixing with quartz
(similar particle diameter) was adopted. 50 mg of catalyst with a ratio
catalyst:inert 1:20 wt/wt were used. The samples were previously
reduced at 400 ◦C with 50 mL/min of 5% H2/N2 for 1 h, followed by
purging at the same temperature with 50 mL/ min of He during 30 min.
Catalytic runs were conducted at 500 ◦C and 1.2 bar. A water:ethanol
feed molar ratio of 5:1 was selected (S/C = 2.5) with a Weight Hourly
Space Velocity (WHSV) = 10ꢀ 3mLliq/(mgcat min)-1 and using 50 mL/
min of helium as carrier. Ethanol conversion (XEtOH), products distri-
bution (Si) as inert- and water-free molar fractions of the main outlet
compounds (i.e., H2, CH4, CO2 and CO) and yield to hydrogen (YH2, only
for long term experience) were estimated as:
(C2H5OHin ꢀ C2H5OHout
)
%XEtOH =
× 100
(14)
(15)
C2H5OHin
n
nj
Si = i × 100
H2
out
YH2
=
× 100
(16)
6C2H5OHin
Where ni is moles of each gaseous compound and nj is the sum of all
compound moles as products. H2out is moles of H2 produced and is moles
of ethanol fed to the system and C2H5OHout is moles of ethanol in the
outlet current.
The overall carbon balance for catalytic tests was calculated using
the following equation [50]:
(
)
Cin
Cout
Carbon balance = 100 ꢀ
× 100
(17)
Where Cin is moles of carbon from ethanol feed and Cout represents the
sum of moles of carbon outlet from CH4, CO2, CO, C deposited, and
eventually, ethanol, acetaldehyde and ethylene. The amount of carbon
deposited on the spent catalytic bed was calculated by TG.
Long term experiences were conducted in a fully automated kinetic
unit (Microactivity Effi, Micromeritics). The liquid feed mixture was
injected with a high-performance liquid chromatography pump (Gilson
307). A stainless-steel reactor (inner diameter 9 mm) containing the
powder catalyst was disposed inside an electrical oven. A K-type
Fig. 1. XRD of Rh catalyst supported over LaAlO3, LaCaAlO3 and LaCeAlO3.
3

A.H. Martínez et al.
Catalysis Today xxx (xxxx) xxx
Fig. 3. TPR profiles of Rh supported catalysts.
Table 2
Specific surface area, H2 uptakes from TPR, degree of reduction, Carbon percent
lost from TG results.
Sample
Area
Uptakes (mol (H2)gꢀ 1
(Rh))
f (%)
% C
(m2 gꢀ 1
)
(TG)
LaAlO3
10.11
7.42
–
–
–
Rh/LaAlO3 0.1 %
Rh/LaAlO3 0.3 %
Rh/LaAlO3 0.5 %
LaCaAlO3
Rh/LaCaAlO3 0.1
%
0.55
0.26
0.21
–
37.76
17.96
14.08
–
1.85
2.01
2.15
–
7.16
7.39
Fig. 2. Rietveld plots from XRD data for LaAlO3, LaCeAlO3 and LaCaAlO3.
21.82
17.05
0.52
35.97
5.06
Table 1
Rh/LaCaAlO3 0.3
%
21.78
21.62
0.25
0.20
17.15
13.74
4.69
6.18
Tabulated ionic radii [53] and parameters obtained after Rietveld refinement.
Rh/LaCaAlO3 0.5
%
Support
Radii
Cell parameter
(nm)
Cell volume
(nm)
Bond distance
(nm)
(nm)
LaCeAlO3
Rh/LaCeAlO3 0.1
%
15.07
–
0.20
–
–
2.34
La3+
:
La-O: 0.26782
Al-O: 0.18938
La-O: 0.26785
Ca-O:
10.83
13.50
LaAlO3
0.37875
5.4330
0.1360
Rh/LaCeAlO3 0.3
%
11.14
9.99
0.17
0.14
11.85
9.38
1.31
2.17
Ca2+
:
La0.9Ca0.1AlO3
0.37880
5.4350
0.1340
0.26785
Rh/LaCeAlO3 0.5
%
Al-O: 0.18940
Ce3+
:
La-O: 0.26745
0.1340
La0.95Ce0.05AlO3
0.37822
5.4110
Ce-O:
Ce4+
:
content. Catalysts prepared with LaCeAlO3 support presented the lowest
values.
0.26745
0.1140
Al-O: 0.18912
3.1.3. Specific surface area (BET)
increase in the density of oxygen vacancies, which are common in these
materials, can increase the unit-cell volume. Thus, the different
(La/Ce/CaAl)-O distances between support perovskites are also sensitive
of cations doping in the structure.
Table 2 shows specific surface area data of all synthetized samples.
SBET of LaAlO3, was around 10 m2/g and it increases with the partial
substitution of La with Ce (15 m2/g) and in a higher extend with Ca
substitution obtaining almost the double of its value (22 m2/g).
The impregnation of these supports with Rh slightly decreased sur-
face areas of each support but there were no differences with Rh content.
3.1.2. Temperature programmed reduction (TPR)
Reduction profiles of supported catalysts is presented in Fig. 3. At
low temperatures, Rh/LaAlO3 0.1 % catalyst presents a single reduction
signal at 300 ◦C which intensity increases with the increase in Rh con-
tent. Rh/LaAlO3 0.5 % presents another signal at around 160 ◦C, and
these catalysts also present a reduction peak at high temperature around
750 ◦C. Rh/LaCaAlO3 (0.1 and 0.3 %) catalysts present only one
reduction signal at around 270 ◦C, while Rh/LaCaAlO3 0.5 % shows also
a small H2 uptake at even lower reduction temperature. TPR profiles of
Rh/LaCeAlO3 series were quite different. They present a main reduction
signal at around 300 ◦C which intensity increases with the increase in Rh
content, and additional broad signals at lower temperatures. They also
present a broad reduction signal between 400 and 600 ◦C. The degree of
reduction was calculated from H2 uptake results (see Table 2). As it can
be seen the degree of reduction decreases with the increase in Rh
3.1.4. X-ray photoelectron spectroscopy (XPS)
Fig. 4 shows the deconvolution of O1s peak of the three supports.
These spectra show a main signal between 529.2–533.6 eV. The
deconvolution of this signal gives two components: a component at low
binding energy (529.2–531.3 eV) attributed to lattice oxygen (O2ꢀ ) and
another component (over 531.3 eV) assigned to surface oxygen (Os)
[54]. As it was expected, the most abundant component in all sample is
lattice oxygen. Os/O2ꢀ ratio is summarized in Table 3. This value is not
considerably affected by calcium inclusion in site A (Os/ O2- = 0.07),
however, cerium inclusion produces a considerable increase in oxygen
vacancy (Os/ O2- = 0.357). Rhodium impregnation results in small
variations, but keeping the tendency of cerium substituted support. The
deconvolution of Ce 3d spectrum for La0.95Ce0.05AlO3 is shown in Fig. 5.
4

A.H. Martínez et al.
Catalysis Today xxx (xxxx) xxx
Ce 3d XPS spectrum is dominated by two features. Four peaks corre-
sponding to the pairs of spin-orbit doublets can be identified in the Ce 3d
spectrum from Ce(III) oxides, in good agreement with other authors
[55–57]. The highest binding energy peaks, u’ and v’ respectively
located at about 903.8 ± 0.1 eV and 885.5 ± 0.1 eV are the result of a
Ce 3d9 4f1 O 2p6 final state. The lowest binding energy states u◦ and v◦
respectively located at 899.1 ± 0.1 eV and 881.9 ± 0.1 eV are the result
of Ce 3d9 4f2 O 2p5 [58]. It may be notice a peak at 916.5 eV, however,
the signal is near to background level due to low concentrations of Ce in
the perovskite. According to other authors [55,59,60], the component
around 916.5 eV, commonly labeled u’’, is a fingerprint of Ce(IV) cation.
Nevertheless, this peak could not be deconvoluted, so there is no pres-
ence of Ce(IV) in the sample or at least is present in a very low con-
centration at the catalyst surface.
La/Al Atomic ratio is 1.09 for base perovskite, is superior to nominal
value (1) that indicate lanthanum excess over surface. In the case of
calcium and cerium partial substituted supports this relation decrease to
0.57 and 0.63 respectively, which could indicate inclusion of Ca and Ce
cations in La position as it was verified previously by Rietveld refine-
ment [61].
Rh/La + Al atomic ratio, increases with the increase in Rh content
for Rh/LaAlO3 catalysts. Rh/LaCaAlO3 catalysts presented higher values
of Rh/La + Al + Ca ratio than Rh/LaAlO3 catalysts but the highest
values were observed on Rh/LaCeAlO3 catalysts.
Fig. 4. Deconvolution of O1s peak from XPS for LaAlO3, LaCaAlO3
and LaCeAlO3.
Fig. 6 displays the XPS spectra collected in the region of Rh 3d core
levels characterized by the two spin orbit components doublet, Rh 3d5/
2 and Rh 3d3/2. Rh 3d5/2 peak can be decomposed in two components,
the first one in the range 306.5–307 eV and the second one in the range
308.5–309 eV. The first component is attributed to metallic rhodium
(Rh◦), while the second one is attributed to positively charged rhodium
species; Rh3+ (Rh2O3) [62,63]. As it can be seen, Rh◦/Rh3+ atomic ratio
value (Table 3) increases with the increase in Rh content for each sup-
port. The higher values of these ratios were observed on Rh/LaAlO3
catalysts. Rh/LaCeAlO3 catalysts presented the lowest values.
Table 3
Atomic relationships obtained by XPS.
Catalyst/
Os/
O2-
La/Al
Rh/La + Al
Rh0/
Rh3+
Relationship
LaAlO3
0.080
0.115
0.046
0.032
1.09
Rh/LaAlO3 0.1%
Rh/LaAlO3 0.3%
Rh/LaAlO3 0.5%
0.74
1.42E-02
3.07E-02
3.45E-02
Rh/
2.24
4.41
7.75
0.73
0.77
La þ Ca/
Al
3.2. Catalytic activity results
La þ Al þ Ca
LaCaAlO3
0.072
0.073
0.185
0.124
0.59
Rh/LaCaAlO3 0.1%
Rh/LaCaAlO3 0.3%
Rh/LaCaAlO3 0.5%
0.61
2.08E-02
3.60
4.27
5.11
3.2.1. Catalytic tests
0.57
3.10E-02
Fig. 7 shows the catalytic performance of catalysts as mean values
from 6 h on stream catalytic tests under ethanol steam reforming. The
performance parameters selected were ethanol conversion and, H2, CH4,
CO, and CO2 products distribution (as mentioned in section 2.3, reported
as inert- and water-free molar fractions). Total ethanol conversion was
reached in all cases with exceptionally high fractions of H2 amounting
ca. 80 %. Additionally, reduced fractions of CO and CH4 were attained in
general. The presence of acetaldehyde, ethane and ethylene, the most
common precursors of carbon, were not detected in any case.
Rh/ LaAlO3 0.3 % catalyst presented the highest H2 molar fractions
and low mean values of CO (around 3.5 %) and CH4 (around 4%). It is
interesting to note that Rh/LaCeAlO3 catalysts presented lower H2 outler
molar fractions than Rh/LaAlO3 catalysts, but these catalysts presented
the lowest CH4 values. Rh/LaCaAlO3 catalysts showed the lowest H2
outlet contents with the consequent highest CH4 ammounts.
0.62
5.30E-02
La þ Ce/Al
0.70
Rh/La þ Al þ Ce
LaCeAlO3
0.357
0.264
0.149
0.334
Rh/LaCeAlO3 0.1%
Rh/LaCeAlO3 0.3%
Rh/LaCeAlO3 0.5%
0.65
5.52E-01
5.95E-01
5.03E-01
0.69
1.21
4.06
0.72
0.65
3.2.2. Long-term experiences
Long term experiences were performed with the two catalysts with
the best performance (Rh/LaAlO3 0.3 %) and the one with lower
byproducts distributions (Rh/LaCeAlO3 0.3 %). Catalyst were evaluated
during 24 h on-stream (Figs. 8 and 9 respectively). The Rh/LaAlO3 0.3 %
catalyst presented high long term experience performance with total
ethanol conversion along almost the whole test. H2 yield remained about
60 %, while low mean values of CH4 and CO distributions of 12 and 6 %,
respectively, were measured.
Fig. 5. Deconvolution of Ce peak from XPS for LaCeAlO3.
The Ce 3d 3/2, 5/2 spectra are composed of two multiplets (v and u)
corresponding to the spin-orbit split 3d 5/2 and 3d 3/2 core holes. The
spin-orbit splitting is about 18.1 eV. Each spin-orbit component of the
Rh/LaCeAlO3 0.3 % also presented a good performance during 8
reaction hours with similar values of H2 yield and CO and CH4 distri-
butions as the Rh/LaAlO3 0.3 % sample. However, it started to
5

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3.3.2. SEM
SEM Micrographs of spent catalysts after 6 h time on stream are
shown in Fig. 11. It can be noticed that Rh/LaAlO3 0.3 % catalyst has
fine isolated carbon fibers. Catalysts prepared on Ca substituted perov-
skite presented a higher accumulation of agglomerated fibers. It is
interesting to note that carbon fibers were not observed on catalysts
supported on LaCeAlO3.
Fig. 12 shows SEM micrographs of Rh/LaAlO3 0.3 % and Rh/
LaCeAlO3 0.3 % after the long term experience of 24 h on stream. Car-
bon filaments were observed on both catalysts. Visually Rh/LaAlO3 0.3
% showed more conglomerations of filaments of carbon than Rh/
LaCeAlO3 0.3 %.
4. Discussion
4.1. Support characterization
The support plays an important role in the final features of the cat-
alysts and thus in the catalytic activity. In this work, perovskite-type
supports of high purity, thermal stability and non-reducible properties
were selected. LaAlO3 perovskite was used as a base perovskite support
and the partial substitution of La with Ce or Ca, in order to induce
morphological, structural and surface modifications was studied. The
citrate method [49] was chosen as synthesis method since it is known to
obtain solids with high purity and suitable specific surface areas in
comparison to ceramic methods. As it can be seen in Table 2, the specific
surface area of LaAlO3 was increased with the partial substitution of A
site with Ce or Ca cations. In a previous article, similar results were
obtained with a La1-xCaxAl1-yNiyO3 perovskites [61]. Probably, the
presence of Ce or Ca modified the carboxylate precursor decomposition
during the synthesis procedure, and thus, different textures were ob-
tained. Merino et al., [64]also observed an increase in surface area when
Ca was introduce into LaCoO3 perovskite structure. They attribute this
to structural disorder that leads to a delay in the crystallite growth, due
to the incorporation of Ca into the perovskite structure. The inclusion of
Ca or Ce is viable due to similarity of ionic radii between La3+ and Ca2+
or Ce3+ (r La3+=0.136 nm; r Ca2+ = 0.134 nm; Ce 3+) and this was
corroborated by the XRD analysis and the Rietveld refinement (Figs. 1, 2
and Table 1). The three supports showed high crystallinity and purity,
since the only phase detected was LaAlO3, with the absence of lines
corresponding to segregated phases. Ce inclusion into the perovskite
structure evidenced a decrease of the unit cell volume and the modifi-
cation of bond distances. However, the inclusion of Ca increased the unit
cell volume in spite of its lower ionic radio. An increase in the density of
oxygen vacancies, which are common in these materials, can increase
the unit-cell volume. Oxygen vacancies were also detected at the surface
of supports by an XPS analysis of O1s. As it is shown in Table 3, Os/O2ꢀ
ratio increased with Ca and Ce inclusion. The presence of oxygen va-
cancies is an important feature of catalysts supports used in steam
reforming reactions since it could be beneficial for the oxidation process
of carbon deposition. Supports as oxygen vacancy levels can be sort as
below: LaCeAlO3>LaCaAlO3>LaAlO3 (Fig. 5). The highest oxygen va-
cancies content was found at the surface of LaCeAlO3 support which
could be due to the high oxygen storage capacity of Ce. Additionally, the
inclusion of cations could be deducted from the differences in La/Al and
La(Ca or Ce)/Al atomic ratio values.
Fig. 6. Deconvolution of Rh 3d peak from XPS for (a) LaAlO3, (b) LaCaAlO3
and (c) LaCeAlO3.
deactivate slowly after that 8 h, decreasing the ethanol conversion and
the H2 yield to final values of around 60 and 47 %, respectively, at 24
reaction hours. Ethylene, acetaldehyde and unconverted ethanol were
detected, during this deactivation period, in liquid phase.
3.3. Characterization of spent catalysts
3.3.1. Thermogravimetric analysis of spent catalysts
4.2. Rh supported catalyst characterization
Fig. 10 shows thermogravimetric analysis of catalysts after ethanol
steam reforming and data are listed in Table 2. The weight loss is
attributed to the combustion of the carbon deposited. It can be notice
that the highest weight loss was observed for LaCaAlO3 support. The
lowest weight lost was observed with catalysts supported on LaCeAlO3.
For all catalytic test, 6 h on stream, the carbon balance is satisfactory
with a deviation less than 10 % with values between 0.55 and 9.94 %.
The nature of Rh particles generated by the impregnation method
could not be inferred from XRD results since Rh content in too small and
it is under the detection limits of the technique. Therefore, TPR results
were used to obtain information about the nature of the Rh phases and
its interaction with the supports. All catalyst presented signals at low
reduction temperatures, that have been associated to both RhOx nano-
crystallites with different sizes or RhOx species with low interaction with
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Fig. 7. Products distribution of catalytic test during 6 h on stream.
Fig. 8. Products distribution of catalytic test and ethanol conversion during 24 h on stream for Rh/LaAlO3 0.3 % catalyst.
the support [65]. Catalysts prepared with LaAlO3 presented also addi-
tional reduction signals at high temperatures indicating the presence of
Rh species with stronger interaction with the support. This was also
found on different catalytic systems [66–68]. Ruckenstein et al. [66]
observed in the reduction profiles of Rh/La2O3, Rh/MgO, Rh/Y2O3 and
Rh/Ta2O5 a high temperature peak associated with the reduction of
LaRhO3, MgRh2O4, YRhO3 and RhTaO4 species, respectively. Boukha
et al. [69]attributed the higher reduction signal to the incorporation of
Rh in the hydroxyapatite structure of the support. In this work the peak
at high temperature would indicate a significant interaction between
rhodium oxide and lanthanum [70], which is in excess at the surface in
the case of LaAlO3 as it was detected by XPS results. Catalysts supported
on LaCeAlO3 presented additionally a broad reduction signal between
400 and 600 ◦C. This signal would indicate the reduction of Ce species. It
has been published that the surface Ce species reduction are facilitated,
via hydrogen spillover, by the presence of Rh. The hydrogen spillover is
evidenced by the migration of hydrogen atoms from a metal, which is
active in the dissociative adsorption of hydrogen, to an oxide, which by
itself had no activity for dissociative hydrogen adsorption and then
reduced oxide at lower temperatures [71]. Hou et al. found that for
Rh/CeO2 catalyst, hydrogen, once dissociated on metallic Rh, was able
to spill over to CeO2 and reduced it at lower temperatures [72]. The
degree of reduction calculated from H2 uptake presented in Table 3 was
lower in LaCeAlO3 support. This was also in accordance with XPS re-
sults. As it was mentioned, the Rh◦/Rh3+ ratio values for Rh/LaCeAlO3
were lower than those for Rh/LaAlO3 which could be due to the high
oxygen mobility that is able to produce Ce support considering its higher
oxygen vacancies, also observed by XPS results, producing the oxidation
of Rh species. Considering the same support, it can be observed that the
degree of reduction decreases with the increased in Rh content. XPS
results also corroborate this at the surface of catalysts where a decrease
in Rh◦/Rh3+ was detected. Evidently, the final disposition of Rh particles
influenced also the reduction behavior. Ligthart et al. [73] studied the
influence of Rh particles using ZrO2, CeO2, CeZrO2 and SiO2 as supports
on the catalytic performance in methane steam reforming (E11). They
pointed out that the nature of the support influenced the dispersion and
the reduction degree of the metal phase. They observed that more
disperse particles were more easily oxidized. The same can be observed
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Fig. 9. Products distribution of catalytic test and ethanol conversion during 24 h on stream for Rh/LaCeAlO3 0.3 % catalyst.
Fig. 10. Thermogravimetric analysis of catalysts after ESR.
in this work, catalysts supported on LaCeAlO3 presented also the highest
Rh/La + Al + Ce atomic ratio which could give an idea of the Rh
dispersion at the surface of support. Rh/La + Al + Ce atomic ratio in the
case of catalysts supported on LaCaAlO3 perovskite were higher than
Rh/La + Al for LaAlO3. This could be explained considering the higher
specific surface area generated with Ca inclusion into the perovskite
structure.
monoatomic carbon [66]. The weight lost over 450 ◦C could be assigned
to the combustion of more organized carbon filaments which are
observed in all catalysts. The weight lost observed over 600 ◦C mainly
with Rh/LaCaAlO3 catalysts could be due to the combustion of a
graphitic type carbon. These catalysts presented the lowest H2 amount
and the highest CH4 production. It has been reported that formed carbon
can encapsulate the metal particles or cover the support and both of
̋
these lead to losses in activity [67]. Erdohelyi et al. proposed that the
deactivation of supported noble metal catalysts was caused by the
accumulation of acetate-like species over the support. They suggested
that these species inhibited the migration of ethoxy species from the
support to the metal particles and, thus, delayed their decomposition
[68]. It is interesting to note Rh/LaCaAlO3 (0.1, 0.3 and 0.5 %) catalysts
showed the highest levels of CO2 distribution (Fig. 7). Thus, a pathway
through Boudard (E2) and Reverse Carbon Gasification reaction (E6),
consuming almost all produced CO could be proposed. These are the
main ways of carbon deposition, additionally, reverse carbon gasifica-
tion would contribute to reduce H2 production. The low CH4 distribution
values for these catalysts would indicate methane production by a
methanation process (E7).
4.3. Catalytic activity in ESR and characterization of spent catalysts
The catalysts activity during ethanol steam reforming was evaluated
in terms of ethanol conversion and products distribution as a function of
time on stream for 6 h-length experiences (Fig. 7). All catalysts attained
complete conversion and displayed the same products but with different
distribution. The main observed products were H2, CH4, CO, CO2. The
presence of acetaldehyde, ethane and ethylene, the most common pre-
cursors of carbon, were not detected in any case. However, SEM analysis
(Fig. 11) and TG results (Fig. 10, Table 2) indicated the presence of
carbon filaments all over the prepared catalysts. The higher amount of
carbon was detected with catalysts supported on LaCaAlO3. TG results
revealed the kind of carbon produced. It has been published that the
weight lost observed over 300 ◦C could be associated to the presence of
Catalysts prepared with LaAlO3 support presented the best catalytic
performance in general, indeed, Rh/LaAlO3 0.3 % presented the highest
8

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Fig. 11. SEM micrograph of catalyst used in 6 h ESR reaction for (a, b) Rh/LaAlO3 0.3 % (c, d) Rh/LaCaAlO3 0.3 % and (e, f) Rh/LaCeAlO3 0.3 %.
mean value of H2 distribution and lowest amount of CO. Intermediate
amounts of Rh appears as a compromise balance between sufficient
active sites and prevention of agglomerates formation by surface
migration. These results could indicate a high component of Water Gas
Shift reaction on catalytic pathway. High moderate CO2 production in
contraposition with low CO sustain a Water Gas Shift preference. A high
production of H2 and a moderate carbon accumulation (Table 2) would
discard Boudard reaction as a possible pathway for Carbon formation.
Participation of lanthanum phase through formation of La2O2CO3 could
explain the differences in carbon formation on these catalysts. As it was
mentioned in the results section, these catalysts presented La species in
excess at the surface of catalysts determined by XPS analysis (Table 3). It
has been reported that the La2O2CO3 specie can react with carbon–metal
formed in its vicinity, to produce two molecules of CO [74]. Thus, La
species would act as an intermediate, promoting the reaction between
the carbon species and CO2.
support would oxidize the solid carbon at the surface of catalyst. Simi-
larly, in presence of lattice oxygen, Rh oxidation may also take place.
The presence of Rh3+ species in higher content in Rh/LaCeAlO3 catalysts
was confirmed by XPS and TPR results. This could be the reason of the
lower activity of these catalysts than those prepared on LaAlO3 support,
since Rh3+ has less activity than Rh◦ species. Additionally, the low
distribution of CH4 would indicate that the methane decomposition re-
action is favored, which would be the responsible of carbon deposition
in these catalysts. Rh/LaCeAlO3 0.3 % catalyst presented the best per-
formance of this catalysts series.
The catalysts long term experience performance during ethanol
steam reforming was evaluated in terms of ethanol conversion and
products distribution as a function of time on stream for 24 h. Catalyst
with higher H2 production, Rh/LaAlO3 0.3 %, and Rh/LaCeAlO3 0.3 %
which produced the lowest amount of carbon were selected for the long
term experiences (Figs. 8 and 9). Rh/LaAlO3 0.3 % catalyst presented an
excellent long term experience performance during the test, with total
conversion of ethanol during the 24 h. The same products distribution as
in the test for 6 h was observed. H2 yield remained about 60 %, while
low mean values of CH4 and CO distributions of 12 and 6 %, respec-
tively, were obtained. This sample also produced filament carbon spe-
cies as it was showed in Fig. 12. However, deactivation of catalyst could
not be observed during 24 on stream. Evidently, the higher amount of
As it was observed by SEM and TG results, (Figs. 10, 11 and Table 2)
the lowest amount of carbon filaments were obtained with catalysts
supported on LaCeAlO3 perovskite. These results could be due to the
redox property of Ce-based materials, its high oxygen storage capacity
and oxygen mobility as it was determined by XPS analysis. Carbon
monoxide can adsorb and react with the lattice oxygen on the surface of
Ce to form carbon dioxide. As a lattice oxygen provider, Ce based
9

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Fig. 12. SEM micrograph of catalyst used in 24 h ESR reaction for (a, b) Rh/LaAlO3 0.3 % and (c, d) Rh/LaCeAlO3.0.3 %.
reduced Rh species and a La excess at the surface of catalysts would be
the responsible of the excellent catalytic long term experience perfor-
mance of this catalyst. This result was similar to that obtained by
Campos et al. using similar conditions for Rh catalysts in long term ex-
periences. They, also, suggested that Rh catalyst favors the WGS reaction
decreasing the rate of carbon deposition [38].
catalytic activity in ethanol steam reforming reaction. It could be
possible to obtain perovskites supports with high purity, long term
experience performance and suitable specific surface areas. The partial
substitution of La with Ca and Ce in a LaAlO3 based perovskite structure
was reached and changes in its porosity and surface features were
detected. The inclusion of Ca into the structure increased specific surface
area of support and Ce inclusion activated oxygen mobility on catalyst
surface. The supports influenced the degree of reduction of Rh species at
the surface of catalysts. All catalyst showed complete ethanol conversion
over 6 h on stream. The best catalytic performance was achieved with
Rh/LaAlO3 0.3 % catalyst which presented the highest mean value of H2
distribution and low production of byproducts (CH4 and CO) and pre-
sented an excellent long term experience performance during 24 h on
stream. Rh/LaCeAlO3 0.3 % presented the lowest carbon accumulation
during 6 h on stream but it suffered deactivation after 14 h on stream.
The deactivation was attributed to the partial oxidation of Rh◦ species to
Rh3+ which are less active favoring a reaction pathway that produce
carbonaceous deposits.
Rh/LaCeAlO3 0.3 % presented a good performance during the first
14 h with similar values of H2 yield and CO and CH4 distributions as the
Rh/LaAlO3 0.3 % sample. However, it started to deactivate slowly after
that 14 h, decreasing the ethanol conversion and the H2 yield to final
values of around 60 and 47 %, respectively. Deactivation could be
related also to the oxidation-reduction ability of Ce. This phenomenon
would be favorable at the beginning of the reaction in order to oxidize
carbon and/or CO minimizing carbon deposition. But redox properties
of Ce species present at the support would oxidize Rh◦ to Rh3+ in a long-
term experiment decreasing the activity of Rh sites. The products dis-
tribution obtained with this catalyst also suggests a reverse WGS reac-
tion when deactivation was evident. The presence of ethylene,
acetaldehyde and unconverted ethanol during this long term experience
would indicate the reason of catalyst deactivation. Considering the
observed products, one of the main causes of deactivation in this catalyst
would be due to the deposition of carbonaceous species by ethanol
dehydration to ethylene, followed by polymerization to coke, as it was
observed by SEM [75] that would block the catalyst metal sites. Moraes
et al. evaluated the performance of a Rh/SiCeO2 catalyst deposited on
monolith in a long duration test of ESR. In contrast to Rh/LaAlO3 0.3 %
long term experience, the monolith supported catalyst retained com-
plete stability and no coke was observed [76].
CRediT authorship contribution statement
´
Asiel Hernandez Martínez: Investigation. Eduardo Lopez: Vali-
dation, Investigation, Resources, Writing - review & editing. Luis E.
Cadús: Conceptualization, Supervision, Resources. Fabiola N. Agüero:
Resources, Writing - review & editing, Visualization, Project adminis-
tration, Funding acquisition.
Declaration of Competing Interest
The authors report no declarations of interest.
5. Conclusions
In this work a series of Rh/perovskites catalyst were synthetized in
order to elucidate the role of support on the Rh particles features and its
10

A.H. Martínez et al.
Catalysis Today xxx (xxxx) xxx
[37] P. Osorio-Vargas, C.H. Campos, R.M. Navarro, J.L.G. Fierro, P. Reyes, Appl. Catal.
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The financial support from Universidad Nacional de San Luis,
CONICET, ANPCyT is gratefully acknowledged. The authors are grateful
to E. Ilincheta for her valuable contribution during the long term
experience measurements.
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