Hydrogen production by steam reforming of ethanol on a RhPt/CeO2/SiO2 catalyst: Synergistic effect of the Si:Ce ratio on the catalyst performance

Article abstract of DOI:10.1016/j.apcata.2016.06.014

The effect of the Si content (0–100%) on a RhPt/CeO₂-SiO₂ catalyst was studied with respect to the steam reforming of ethanol (SRE) for H₂ production. Activity tests were performed between 400 and 700?°C at atmospheric pressure; the stability was tested for 72?h in time-on-stream (TOS) and under several startup/shutdown perturbations. The maximum benefits were achieved with 33?mol% Si content in the support. The new RhPt/CeO₂-SiO₂ (RhPt/CeSi-33) catalyst delivered a H₂ yield of 5.2?±?0.2?mol H₂/mol EtOH inlet, higher than the 4.0?±?0.1?mol H₂/mol EtOH inlet over the reference RhPt/CeO₂ catalyst. The extra H₂ production on RhPt/CeSi-33 was ascribed to the additional water activation on the SiO₂ surface. A Si content of 33% in the support (RhPt/CeSi-33, Si:Ce ratio of 1:2) was found to provide the best catalyst. This Si content in the RhPt/CeSi-33 reduced catalyst sample promoted the following changes compared to the RhPt/CeO₂ reference catalyst: (i) decreased the basicity of the support (from 341 to 259?μL CO₂/g catalyst), (ii) reduced the crystalline size of CeO₂ (from 46.0 to 22.6?nm), (iii) increased the catalyst surface area (from 36 to 129?m²/g catalyst), and (iv) decreased the active particle size (from 5.6 to 5.1?nm). The RhPt/CeSi-33 catalyst maintained its H₂ yield by 72?h TOS and during the on/off test, showing promising features for future use in fuel cells.

Full text of DOI:10.1016/j.apcata.2016.06.014

Applied Catalysis A: General 523 (2016) 283–293
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Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Feature article
Hydrogen production by steam reforming of ethanol on a
RhPt/CeO2/SiO2 catalyst: Synergistic effect of the Si:Ce ratio on the
catalyst performance
Bernay Cifuentes, María Hernández, Sonia Monsalve, Martha Cobo∗
Energy, Materials, and Environment Laboratory, Department of Chemical Engineering, Universidad de La Sabana, Campus Universitario Puente del Común,
Km. 7 Autopista Norte, Bogotá, Colombia
a r t i c l e i n f o
a b s t r a c t
Article history:
The effect of the Si content (0–100%) on a RhPt/CeO2-SiO2 catalyst was studied with respect to the steam
reforming of ethanol (SRE) for H2 production. Activity tests were performed between 400 and 700 ◦C
at atmospheric pressure; the stability was tested for 72 h in time-on-stream (TOS) and under several
startup/shutdown perturbations. The maximum benefits were achieved with 33 mol% Si content in the
support. The new RhPt/CeO2-SiO2 (RhPt/CeSi-33) catalyst delivered a H2 yield of 5.2 0.2 mol H2/mol
EtOH inlet, higher than the 4.0 0.1 mol H2/mol EtOH inlet over the reference RhPt/CeO2 catalyst. The
extra H2 production on RhPt/CeSi-33 was ascribed to the additional water activation on the SiO2 surface.
A Si content of 33% in the support (RhPt/CeSi-33, Si:Ce ratio of 1:2) was found to provide the best catalyst.
This Si content in the RhPt/CeSi-33 reduced catalyst sample promoted the following changes compared
to the RhPt/CeO2 reference catalyst: (i) decreased the basicity of the support (from 341 to 259 ␮L CO2/g
catalyst), (ii) reduced the crystalline size of CeO2 (from 46.0 to 22.6 nm), (iii) increased the catalyst surface
area (from 36 to 129 m2/g catalyst), and (iv) decreased the active particle size (from 5.6 to 5.1 nm). The
RhPt/CeSi-33 catalyst maintained its H2 yield by 72 h TOS and during the on/off test, showing promising
Received 30 March 2016
Received in revised form 8 June 2016
Accepted 10 June 2016
Available online 11 June 2016
Keywords:
CeO2-SiO2 supports
Ethanol
Hydrogen
Rh-Pt catalyst
Steam reforming
features for future use in fuel cells.
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
internal combustion engine [4]; additionally, bioethanol burning
generates pollution [5]. In contrast, proton exchange membrane
The current reliance on fossil fuels has caused pollution, deple-
tion of natural resources, and socioeconomic instability in several
regions of the world [1]. Therefore, the development of sustain-
able, alternative energy sources is desired. H2-based energy models
have attracted interest due to their high energy density (141.6 kJ/g).
In addition, H2 can be obtained from renewable sources, does not
generate pollutants [2], and is thermodynamically favorable for
producing electricity in fuel cells (FCs) [2]. Bioethanol produced
from renewable sources could be a sustainable H2 source. Partic-
ularly in Colombia, bioethanol has emerged as a promising fuel
because the Colombian government has established a series of
incentives to promote its production via sugar cane fermentation
[3]. The objective is to blend the bioethanol with gasoline (10%
v/v) for transport applications. However, only ∼20% of the chemi-
cal energy stored in ethanol is available for mechanical work in an
FCs can achieve an electrical efficiency of ∼50% [6]. Thus, the con-
version of ethanol to H2 could benefit bioethanol production and
promote new businesses in Colombia.
H2 can be produced by the steam reforming of ethanol (SRE).
This method produces a greater amount of H2 per mole of ethanol
[7] and is more economical compared to other methods, such as
auto-thermal reforming or ethanol oxidation [8]. Different reac-
tion routes (that depend on the reaction conditions, nature of the
support, and the active phase) will produce a complex set of prod-
ucts, including hydrogen (H2), carbon monoxide (CO), methane
(CH4), carbon dioxide (CO2), ethane (C2H6), ethylene (C2H4), and
acetaldehyde (CH3CHO) (Table 1) [9–12].
The nature of the catalyst and the operational conditions directly
affects the SRE. Noble metals are promising candidates for the SRE
[13]. Specifically, Rh-Pt bimetallic catalysts appear to have a high
activity in the SRE and the water-gas shift reaction (WGSR) and a
low susceptibility to coke formation [8,14]. In addition, the support
plays a critical role in the SRE [15]. Basic supports have significant
advantages in terms of selectivity and resistance to deactivation
∗
Corresponding author.
E-mail address: martha.cobo@unisabana.edu.co (M. Cobo).
http://dx.doi.org/10.1016/j.apcata.2016.06.014
0926-860X/© 2016 Elsevier B.V. All rights reserved.

284
B. Cifuentes et al. / Applied Catalysis A: General 523 (2016) 283–293
Table 1
Network of chemical reactions that can occur during the steam reforming of ethanol [9–11].
Reaction
Description
CH3CH2OH+3H2O → 2CO2+6H2 (1)
(Steam reforming – ethanol) Eq. (1)
(Water-gas shift reaction) Eq. (2)
(Dehydrogenation – ethanol) Eq. (3)
(Dehydration – ethanol) Eq. (4)
(Decomposition – ethanol) Eq. (5)
(Decomposition – acetaldehyde) Eq. (6)
(Steam reforming – acetaldehyde) Eq. (7)
(Steam reforming – methane) Eq. (8)
(Dehydrogenation – methane) Eq. (9)
(Steam reforming – ethylene) Eq. (10)
CO+H2O ꢀ CO2 + H2
(2)
(3)
(4)
(5)
(6)
CH3CH2OH ꢀ CH3CHO + H2
CH3CH2OH ꢀ C2H4 + H2O
CH3CH2OH ꢀ CH4 + CO + H2
CH3CHO ꢀ CH4 + CO
CH3CHO + H2O ꢀ CH4 + CO2 + H2(7)
CH4 + H2O ꢀ CO + 3H2
2CH4 ꢀ C2H4 + 2H2
(8)
(9)
(10)
C2H4 + 4H2O ꢀ 2CO2 + 6H2
Table 2
[1]. They promote H2 formation during the SRE and contribute to
the conversion of CO in the WGSR [13]. CeO2 has been proposed
as a promising basic support for Rh [16], Pt [17], and Rh-Pt [18]
catalysts because it promotes a high dispersion of the active sites.
Likewise, the Cen+ species of CeO2 interact strongly with the active
sites and facilitate the decomposition of the alcohols. However,
the main disadvantage of CeO2 is its low surface area compared to
other supports, which affects the number of active sites available
for the reaction. Changing the catalyst structure using other matri-
ces can positively influence the performance of the catalyst. Mixed
supports, such as CeO2-YSZ [19], CeO2-ZrO2 [20], and SiO2-ZrO2
[21] showed a greater stability, selectivity, and affinity between
the metal and support than their individual use. SiO2 has special
features despite its lower selectivity compared to CeO2 [22]. Specif-
ically, SiO2 has a higher surface area and larger average pore size,
which allows for a better dispersion and stabilization of the metals
[23]. These features allow for the integration of SiO2 into the matri-
ces mixed with other carriers to promote higher activity, selectivity,
and stability [24], and allow the exploitation of the advantages of
both materials.
This study aims to identify an active, selective, and stable cat-
alyst for H2 production from SRE for its further use with real
bioethanol samples and under FC operation conditions. The effect
of the Si:Ce ratios in a RhPt/CeO2–SiO2 catalyst on the catalytic
activity, H2 yield, and product distribution was evaluated by the
response surface methodology (RSM) [25]. The use of response sur-
faces allowed for the selection of a Si:Ce ratio suitable to obtain an
active and selective material for the SRE. Stability tests in contin-
uous operation and under several startup/shutdown cycles (on/off
tests) were conducted to identify the performance of the catalysts
under actual operation conditions. A stoichiometric steam/ethanol
(S/E) molar ratio of 3 was used in all experiments to simulate the
bioethanol concentration after flash separation in the commercial
samples procured from Colombia. Characterization tests, such as
X-ray diffraction (XRD), CO2-temperature programmed desorption
(CO2-TPD), Brunauer–Emmett–Teller (BET) surface area analysis,
thermogravimetric analysis (TGA), energy-dispersive X-ray (EDX),
and high-resolution transmission electron microscopy (HR-TEM)
were also included to determine the changes to the internal struc-
ture of the support caused by SiO2.
RhPt/CeO2-SiO2 catalysts evaluated in the steam reforming of ethanol.
Catalysta
Si/(Ce+Si) (mol%)
Si:Ce (molar ratio)
RhPt/Ce
0
0:1
1:5
1:2
1:1
1.4:1
1:0
RhPt/CeSi-17
RhPt/CeSi-33
RhPt/CeSi-50
RhPt/CeSi-60
RhPt/Si
17
33
50
60
100
a
The numerical affixes for the RhPt/CeSi catalysts correspond to the theoretical
Si mol%.
added to the support. Previous RSM analysis in our group deter-
mined that these metal loadings were the most appropriate to
increase the H2 yield (data not shown). The required amounts of
active metals were obtained from rhodium (III) chloride hydrate
(RhCl3·H2O) (Aldrich Chem. Co., USA) and chloroplatinic acid hex-
ahydrate (H2PtCl6·6H2O) (Aldrich Chem. Co., USA) solutions in
water and were slowly added onto the support by the incipient
wetness co-impregnation method. To achieve co-impregnation, the
metal-support mixture was continuously stirred, dried at 105 ◦C for
24 h, and calcined at 700 ◦C in a muffle for 2 h. All the calcined solids
were sieved using an 80-mesh sieve to ensure particle sizes of less
than 177 ␮m. These samples were the “fresh” catalysts (F).
2.2. Catalytic tests
RhPt/CeSi catalysts were evaluated in a fixed-bed reactor (inter-
nal diameter (ID) = 12 mm) at atmospheric pressure under kinetic
control (avoiding external and internal mass transfer limitations).
The reactor was coupled to an electric furnace (Applied Test
Systems, USA) with temperature control. Before entering the
reactor, the liquid ethanol:water mixture (stoichiometric molar
ratio S/E = 3, 0.03 mL/min) was pumped to an evaporator (200 ◦C,
atmospheric pressure) using a metering pump (Simdos 02, KNF
Neuberger, USA) and mixed with N2 carrier gas (300 mL/min). The
RhPt/CeSi (50 mg) catalyst was diluted by mixing with inert quartz
particles (80-mesh) to complete a 300 mg total catalytic bed. The
plug flow reactor condition was achieved by eliminating the back
mixing and channeling by maintaining a ratio of 50 between the
catalyst bed height and catalyst particle size (L/Dp) and a ratio
of 60 between the reactor internal diameter and catalyst particle
size (D/Dp) [26]. Before the reaction, the catalysts were reduced in
10% H2/N2 (300 mL/min) at 700 ◦C for 1 h. This temperature was
selected based on the maximum work temperature of the catalyst
(700 ◦C) and the temperature-programmed reduction of the fresh
catalysts (data shown in Supplementary material, A). These sam-
ples were the “reduced” catalysts (R). An elevated gaseous hourly
space velocity (GHSV) was used to avoid the mass transfer lim-
itations in the system. Catalytic activity tests on RhPt/CeSi were
conducted at a GHSV of 65,200 h−1, at an atmospheric pressure
between 400 and 700 ◦C in 20 ◦C increments (continuous sequence,
25 min at each temperature). The samples used in the SRE were the
2. Experimental methodology
2.1. Catalyst synthesis
RhPt/CeO2-SiO2 catalysts with different Si:Ce molar ratios
(Table 2) were prepared by mixing Ce(NO3)3·6H2O and SiO2 in dif-
ferent proportions. For this, the required amount of Ce(NO3)3·6H2O
(99.5%, Alfa Aesar, USA) was dissolved in water and slowly added
to SiO2 (Merck, Germany). The support mixture was continuously
stirred, dried at 105 ◦C for 24 h, and calcined at 500 ◦C in a muf-
fle for 4 h. Rh (0.4 wt%) and Pt (0.4 wt%) metal total loadings were

B. Cifuentes et al. / Applied Catalysis A: General 523 (2016) 283–293
285
“used” catalysts (U). The mole fractions of ethanol, water, and N2
2.4. Catalytic characterization
in the vapor phase were 0.018, 0.054, and 0.928, respectively.
The SRE catalytic stability tests were conducted at 680 ◦C for
72 h time-on-stream (TOS) for each reduced catalyst using the con-
ditions and parameters described above. The samples used in these
stability tests were the “time-on-stream” catalysts (TOS). Simi-
larly, samples of these catalysts were tested in startup/shutdown
cycles, which were called on/off tests. Initially, the catalyst sam-
ples were evaluated by 6 h TOS at 680 ◦C, then the system was
turned off and allowed to cool to 200 ◦C for 1 h. The system was
then heated again (10 ◦C/min) to 680 ◦C and maintained for 16 h
TOS. This procedure was repeated to complete 72 h. These sam-
ples were the “on/off” catalysts (O–O). In addition, thermodynamic
equilibrium calculations (Supplementary material, B) were simu-
lated using Aspen-Hysys 7.3 software (Aspen Tech Inc., USA) by
considering all of the possible reactions shown in Table 1.
RhPt/CeSi catalysts were analyzed by XRD, CO2-chemisorption,
BET surface area analysis, and TGA to determine their character-
istics and stability in the SRE. The crystal structure of the samples
was analyzed by ex-situ XRD using an XPert Empyrean Series II unit
(PANalytical, UK) with an X’Celerator Detector and Cu-K␣ radiation
(␭ = 1.541874 Å) by scanning in the 2␪ = 5–90◦ range. The minimum
step size of Phi was 0.1, the detection limit in 2␪ was 150◦, and the
peak position deviation was ≤ 0.002◦. The fresh, reduced, and spent
samples were located in the XRD chamber without any pretreat-
ment. For the RhPt/CeSi catalysts, the crystallite size was calculated
from the XRD peak widths using the Scherrer equation (Eq. (14))
at 28.6◦, where ␤ is the half-height width of the XRD peak in radi-
ans for the plane peak and ␪ is the angle between the incident and
diffracted beams in radians [30].
The compounds in the product stream, including N2, H2, CH4,
CO, CO2, ethanol, and ethylene, were quantified online using a
Clarus 580 gas chromatograph (GC, Perkin Elmer, USA). The GC was
equipped with a Carboxen 1010 plot column (30 m, 0.53 mm ID,
Restek, USA) connected to a thermal conductivity detector (TCD)
and an Innowax column (30 m, 0.53 mm ID, Perkin Elmer, USA)
connected to a flame ionization detector (FID). Elemental balances
between the inlet ethanol-water feed and the outlet products were
measured in all tests. A test was declared effective when the ele-
mental carbon balance was ∼100%. The ethanol conversion (XEtOH),
yield, and mole distribution for each detected product were cal-
culated according to Eqs. (11)–(13), where FEtOH,in is the initial
theoretical mole flow (mol/min) of ethanol at ambient tempera-
ture, FEtOH,out is the mole flow (mol/min) of unreacted ethanol in
the product stream detected by GC at time t, and Fi is the mole flow
(mol/min) of product i (H2, CO, CH4, CO2, C2H4, or C2H6).
0.89ꢀ
␤cos(ꢁ)
dhkl
=
(14)
The catalyst surface area was determined by the single-point
BET surface method, measured with 30% N2/He (28 mL/min) as the
adsorption gas, and at liquid nitrogen temperature on a ChemBET
Pulsar TPR/TPD unit (Quantachrome instruments, USA). Samples
were previously degassed in He (40 mL/min) at 150 ◦C for 2 h. The
relative basicity of the RhPt/Ce, RhPt/CeSi-33, and RhPt/Si reduced
catalysts was analyzed by CO2 chemisorption [31] in the same unit.
Samples were previously degassed in He (40 mL/min) at 150 ◦C for
1 h and reduced at a rate of 10 ◦C/min with 5%H2/N2 flow at 700 ◦C
for 1 h. Then, pulses of pure CO2 (65 ␮L pulse every 5 min) were
injected into the sample until saturation.
The TGA analysis was performed in a thermogravimetric ana-
lyzer (Mettler Toledo, USA). Each sample (30 mg) was pretreated
in pure N2 at 100 ◦C and subsequently heated to 1000 ◦C at a rate
of 5 ◦C/min with air flow of 100 mL/min to burn the deposits. The
inclusion of SiO2 in the support may alter the oxygen storage capac-
ity of the material [32]. This could favor the adsorption of O2 onto
the support with increasing temperature and cause the catalyst to
gain weight, which would limit the direct calculation of carbon
deposition on the catalyst. Hence, the weight loss was expressed
as mg carbon/gcat h deposited on the spent catalysts. The back-
ground of the reduced catalyst samples was subtracted from the
spent samples.
Finally, HR-TEM analysis was performed for the reduced, TOS,
and O–O catalyst samples using a Tecnai F20 Super Twin TMP
microscope (FEI, USA; at 200 kV with a GATAN US 1000XP-P cam-
era, 0.1 nm resolution) coupled to an EDX X-MAX detector (Oxford
Instruments, UK). The samples were dispersed in ethanol by ultra-
sonic vibration and dropped on a carbon film-coated copper grid.
The active metal particles were measured with the ImageJ soft-
ware, counting at least 150 particles/image. The average surface
area weighted diameter of the active sites was calculated according
to Eq. (15), where ni is the number of particles and di is the parti-
cle diameter [33]. In addition, particle size distribution histograms
were constructed [34].
FEtOH,in − FEtOH,out
XEtOH
=
(11)
(12)
(13)
FEtOH,in
Fi
FEtOH,in
Yieldi =
Fi
ꢀ
Molefractioni =
× 100%
Fi
2.3. Statistical analysis
The RSM is a powerful tool to improve processes because it can
determine the effect of multiple variables on a response of inter-
est [25]. This technique is widely used in industrial assessments
and research projects. Nevertheless, the RSM is poorly understood
in some areas of chemistry and chemical engineering. Recently,
the use of the RSM in new areas, such as analytical chemistry
[27], chromatography [28], and even catalysis [29], has allowed for
process optimization and successful material evaluations. Experi-
mental results were evaluated with the Design Expert 8 software
(Stat-Ease, Inc., USA) to generate response surfaces that allowed for
the analysis of the effect of the Si:Ce ratio on the catalytic perfor-
mance. The adjustment of the response surfaces was validated by
the probability (Prop. F), lack-of-fit test, and variability with respect
to the experimental data (R2). The Prop. F value (also known as p-
value or probability) and lack-of-fit are related to the significance
of the model and its terms. In a significant model, a Prop. F value of
less than 0.1 is expected. Similarly, the lack-of-fit is a statistical test
to indicate if the model fits well. This value must be higher than 3
(non-significant) [27]. Favorable operation zones were identified in
the SRE for a high production of H2 and a low amount of byproducts.
ꢀ
ꢀ
nid3
ds =
⁄
inidi2
(15)
¯
i
i
3. Results and discussion
3.1. Effect of the inclusion of SiO2 into the RhPt/CeO2 catalysts on
the steam reforming of ethanol
RhPt/CeSi catalysts with different Si contents (Table 2) were
tested during the SRE. Fig. 1 shows the ethanol conversion and
product yields over the RhPt/CeO2-SiO2 catalysts between 400
and 700 ◦C as response surfaces. The analysis of variance of these

286
B. Cifuentes et al. / Applied Catalysis A: General 523 (2016) 283–293
Fig. 1. (a) Ethanol conversion (XEtOH) and yields of (b) H2, (c) CH4, (d) CO, (e) CO2, and (f) ethylene (C2H4) as a function of temperature over the RhPt/CeSi catalysts with different
Si:Ce ratios. Metal loading of Rh = 0.4 and Pt = 0.4 wt% in all catalysts. Response surface: Quartic model, R2 > 0.95, Prop. F « 0.1 (significant) and lack-of-fit » 3 (non-significant).
Reaction conditions: S/E = 3, 50 mg catalyst, and GHSV = 65,200 h−1
.
response surfaces showed a Prop. F « 0.1 (significant), lack-of-fit
» 3 (non-significant), and R2 > 0.9, indicating that these models fit
well, could be used to make predictions about the response, and the
experimental data can be trusted [27]. Values slightly above 100%
in the ethanol conversion (Fig. 1a) were due to the statistical error
and the adjustment of the mathematical model (R2 < 1). Detailed
statistical model results are included in Supplementary material,
C.
The catalysts with Si in their structure showed a higher activity
(Fig. 1a), though the most noticeable effect of Si was detected on
the H2 yield (Fig. 1b). In a stoichiometric feed of S/E = 3, the maxi-
mum value of mol H2/mol ethanol was 6.0 (Eq. (1)). In this work, a
maximum of 5.3 mol H2/mol EtOH at 700 ◦C with the RhPt/SiCe-33
catalyst was obtained, compared to 4.0 and 3.1 when only CeO2 or
SiO2 were used as supports, respectively. Revesz [35] proposed that
SiO2 and its vacancies (V) could interact with H+ and OH− species
from H2O to form H2 and SiOH species at high temperatures (Eq.
(16)–(19)), increasing the H2 yield. However, the positive effect
of SiO2 in RhPt/Ce has a limit. When Si exceeds 33 mol%, a dra-
matic decrease of the H2 yield was observed (Fig. 1b). Hu et al. [36]
reported that the H2 production and CH4 oxidation decreased when
the Si:Ce molar ratio in the Ni/CeO2-SiO2 catalysts was greater
than 0.75, which could be related to the acidity of the support
by the presence of SiO2. In this work, a decrease in the basicity
of the support was observed when SiO2 was present (Section 3.3.2
CO2 chemisorption). Moreover, Fig. 1c shows that the CH4 yield
increased when the Si content was beyond 40 mol%, indicating that
the catalyst lost its effectiveness to reform CH4, which reduced the
H2 production (Fig. 1b). Hence, the excess acid in the RhPt/CeSi cat-
alysts with Si > 40% could disfavor the CH4 conversion and increase

B. Cifuentes et al. / Applied Catalysis A: General 523 (2016) 283–293
287
Table 3
Comparison of various catalytic systems for the steam reforming of ethanol (SRE).
Description
This work
Other authors
Calles et al. [40]
Mondal et al. [26]
Barroso et al. [42]
Hou et al. [16]
Arslan et al. [21]
Active metal(s)
Support
Total metal loading (wt%)
Rh-Pt
CeO2-SiO2
0.8
Ni
Rh-Ni
CeO2-ZrO2
31
9
0.5
600
99
3.6
–
Co
Rh
CeO2
1
Ni
CexZr1-x O2/SiO2
7
CeO2/Al2O4
8
ZrO2-Silicate
6
4
–
600
100
5.8
74
5
S/E
3
3.7
7.0
600
100
5.7
–
4.8
60.3
650
100
5.2
–
3
a
WHSV (h−1
)
15.7
680
100
5.2
71
72
10.9
650
100
–
72
70
–
T (◦C)
Ethanol conversion (%)
H2 yield (mol H2/mol EtOH reacted)
H2 (mol%)
TOS (h)
On/off test (h)
8
–
36
–
7
–
72
–
a
Weight hourly space velocity (WHSV) was calculated as ethanol mass flow rate/mass of catalyst.
the ethylene formation (Fig. 1f), in turn decreasing the H2 yield
from the SRE.
Si − O − Si + H2O ꢀ 2SiOH
Si − V − Si + H2O ꢀ SiOH + SiH
2SiOH ꢀ 2Si − O − Si + H2
2SiH ꢀ Si − V − Si + H2
(16)
(17)
(18)
(19)
The CO yield (Fig. 1d) was closely related to the H2 production
(Fig. 1b). Hou et al. [16] proposed that higher yields of CO com-
pared to CO2 could be related to the CH4 reforming (Eq. (8)) and
reverse WGSR (Eq. (2)), which are favored above 600 ◦C with basic
catalysts. Hence, the CO2 yield increased in the presence of Si in
the support because SiO2 favored the catalyst acidity and the fur-
ther water dissociation [35], which could promote the WGSR (Eq.
(2)) on the RhPt/CeSi catalysts (and promote CO2 formation). In
addition, small traces of ethylene were detected especially in zones
where CO2 decreased, which could indicate that the reversed steam
reforming of ethylene (Eq. (10)) could be occurring.
To identify the operating conditions that favored the SRE, the
ethanol conversion and product yields were analyzed for the
response surfaces. A higher H2 production and lower CH4 and ethy-
lene formations were obtained with the complete conversion of
ethanol (>600 ◦C). Between 680 and 700 ◦C on the RhPt/CeSi cata-
lysts, there were no significant changes in the ethanol conversion
(100 and 99.9%, respectively) or for the H2 yield (5.2 and 5.3 mol
H2/mol EtOH inlet, respectively). RhPt/CeSi-33 (Si:Ce = 1:2 molar
ratio) promoted the highest H2 yield and CO production, and dimin-
ished the amount of CH4 compared to the RhPt/Ce (Si = 0%) catalyst.
Hence, the performance of the RhPt/CeSi-33 catalyst was compared
to the RhPt/Ce reference catalyst in stability tests at 680 ◦C.
3.2. Catalyst stability
Fig. 2. Product distribution of H2, CO, CO2, and CH4 as a function of (a) time-
on-stream (TOS) and (b) on/off cycles (O–O) on RhPt/Ce (open dashed lines) and
RhPt/CeSi-33 (closed dashed lines). The thermodynamic equilibrium was predicted
by Aspen-Hysys (solid lines). Reaction conditions: S/E = 3, GHSV = 70,600 (RhPt/Ce)
and 65,200 h−1 (RhPt/CeSi-33) at 680 ◦C.
The integration of the catalysts in extensive applications is one
of the major challenges in the development of a sustainable energy
model based on H2 [2]. The stabilities of RhPt/Ce and RhPt/CeSi-
33 catalysts were evaluated in both continuous and intermittent
operations to determinate their performance under operational
conditions similar to those required for extensive applications.
obtained throughout the tests on both catalysts, confirming their
high catalytic activity. In the case of RhPt/Ce, the H2 presence
slightly increased during the first 10 h with a decrease in CO2 and
CH4. Between 10 and 50 h TOS, the product distribution remained
approximately constant, without any CH4 presence, and close to
the thermodynamic equilibrium (solid lines) predicted by Aspen-
Hysys. This indicates that reactions 1, 7, and 8 (Table 1) dominated
the process. At approximately 50 h TOS, small amounts of CH4 were
detected, and the H2 concentration decreased progressively, show-
ing a possible decrease in the ability of RhPt/Ce to reform CH4 with
3.2.1. Continuous stability test
FCs are typically used in stationary applications (e.g., power
supplies for buildings) [37]. In this type of application, FCs are
used for long periods without interference, requiring a continu-
ous H2 supply. Thus, RhPt/Ce and RhPt/CeSi–33 were subjected to
a 72 h TOS stability test at 680 ◦C to identify their behavior under
continuous operation conditions. Fig. 2a shows the product distri-
bution and ethanol conversion. The total ethanol conversion was

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B. Cifuentes et al. / Applied Catalysis A: General 523 (2016) 283–293
Table 4
time [38]. During the test, the H2 yield was 4.0 0.1 mol H2/mol
EtOH on the RhPt/Ce catalyst.
Crystal sizes and weight loss of RhPt/SiCe catalysts.
In contrast, RhPt/CeSi–33 provided an increase in the H2 con-
centration accompanied by a progressive decrease in CO2 and CH4
during the first 18 h TOS. With time, the H2 concentration was
higher than the thermodynamic prediction. This could be related
to the presence of chemical reactions other than those shown in
Table 1. Previous reports indicate that CeO2 [39], SiO2 [35], and
O-vacancies can favor the water dissociation altering product dis-
tribution. These chemical mechanisms were not considered in the
thermodynamic analysis made in Aspen-Hysys, but support the
presence of the reactions depicted in Eqs. (16)–(19), as previously
analyzed. The CH4 reforming (Eq. (8)) and WGSR (Eq. (2)) pro-
gressively improved with TOS in this catalyst. Calles et al. [40]
studied the effect of Ce addition to Ni/SiO2 catalysts during 8 h
TOS, reporting that the CeO2-SiO2 support could favor the amount
of surface oxygen, improving the H2 mole fraction due to carbon
gasification with TOS. The H2 yield over the RhPt/CeSi-33 cata-
lyst was 5.2 0.3 mol H2/mol EtOH during the whole test, which is
1.2 units higher that the RhPt/Ce catalyst. This constant, higher H2
yield could make RhPt/CeSi-33 interesting for stationary processing
using FCs for electricity generation.
Catalysts
CeO2 average crystallite
size (Dhkl, nm)a
Weight loss of spent
(U, TOS and O–O)
catalyst samplesb (mg
carbon/gcat h)
Fresh
Reduced
Used
RhPt/Ce
14.8
9.8
8.5
8.1
5.9
–
46.0
28.0
22.6
20.9
41.8
–
43.7
35.6
18.4
32.2
49.7
–
–
–
27.4
0.2
0.4
0.7
1.0
5.3
5.5
0.6
0.4
4.4
RhPt/CeSi-17
RhPt/CeSi-33
RhPt/CeSi-50
RhPt/CeSi-60
RhPt/Si
RhPt/Ce-TOS
RhPt/Ce-O-O
RhPt/CeSi-33- 8.5
14.8
14.8
46.0
46.0
26.9
TOS
RhPt/CeSi-33- 8.5
O-O
26.9
44.5
3.6
Measured by aXRD and bTGA.
ture reports. Although the reaction conditions and catalytic regimes
were different, all catalytic systems had a high activity in the SRE,
achieving a total ethanol conversion above 600 ◦C, H2 yields higher
than 5 mol H2/mol EtOH (except for Mondal et al. [26]), and H2
concentrations above 70%. We reported the lowest metal loading
(0.8 wt%) and a high weight hourly space velocity (WHSV) because
of working with active noble metals. However, stability tests are
important to determine the catalyst performance over time. In this
work, a continuous stability test of 72 h TOS was performed (the
longest of all studies included in Table 3). In addition, our catalysts
were evaluated in an on/off test to determinate their possible use
in mobile applications with FCs. Such tests are novel in the SRE.
We are currently testing this catalyst with real bioethanol samples
under longer stability tests for continuing H2 supply to FCs.
In this way, XRD, CO2-TPD, BET surface area, TGA, and HR-TEM
analyses were employed to identify the possible effect of SiO2 inclu-
sion on the RhPt/Ce catalyst and the changes after the TOS and
on/off stability tests to investigate the characteristics that caused
this catalyst to produce more H2.
3.2.2. On/off stability tests
Electricity generation for mobile applications, such as vehicles
and drones, is one of the more widespread uses of FCs. In these
applications, the H2 supply is intermittent (i.e., while a car is in
motion, it should ensure a continuous flow of H2 into the FC; when
the car is parked, H2 is not required). On/off tests were performed
(Fig. 2b) to evaluate the catalyst potentials for future use in mobile
applications with FCs. Both catalysts, RhPt/Ce and RhPt/CeSi-33,
provided a total conversion of ethanol. RhPt/Ce showed CH4 for-
mation at the beginning of every start-up cycle; however, this was
progressively reduced with time. Recently, Gholami et al. [41] stud-
ied Pd catalysts for vehicle converts, reporting that CH4 conversion
decreased at low temperatures (450 ◦C) due to a reversible inhi-
bition by H2O. According to these authors, water can be adsorbed
on active sites at low temperatures, limiting the CH4 conversion.
However, when the temperature increased, water was desorbed
and released from the active sites, but this process required time to
stabilize. The other products (H2, CO, and CO2) remained close to
the thermodynamic limit predicted by Aspen-Hysys. Accordingly,
the on/off cycles temporarily altered the capacity of the catalysts
to reform CH4, which was progressively mitigated in each cycle.
In contrast, RhPt/CeSi-33 presented changes in product dis-
tribution (closed dashed lines in Fig. 2b). Initially, RhPt/CeSi-33
required 3 cycles to stabilize the H2 concentration. Similarly, CH4
was detected in cycle (1) and was progressively reduced until cycle
(5). In addition, CO progressively increased and CO2 decreased. In
cycles (4) and (5), the product distribution was stable and similar. In
cycle (6), CO content started to decrease (∼33%) and CO2 increased
to reach concentrations close to the thermodynamic limit predicted
by Aspen-Hysys, a behavior similar to that observed in cycle (1)
and over RhPt/Ce. Several authors [22,41,32] have reported that
the on/off and oxidation/reduction cycles favor structural changes
in the catalysts promoted by the temperature and atmosphere. In
general, the performance of the catalysts tends to stabilize after
several cycles.
3.3. Catalyst characterization
3.3.1. X-ray diffraction
Previous studies determined that mixed supports could pro-
mote changes in the crystalline structure of the metal oxides
[43,44]. Therefore, XRD was developed to identify the possible
alteration in the crystal size of CeO2 with respect to the presence of
SiO2 in the catalyst. Fig. 3 shows the XRD diffraction patterns of all
analyzed catalyst samples. Diffraction patterns for fresh RhPt/CeSi
catalysts (Fig. 3a) showed a characteristic peak around 2␪ = 21.9◦ in
RhPt/Si, corresponding to the amorphous silica peak [45]. RhPt/CeSi
(Si = 17, 50, and 60%) and RhPt/Ce (Si = 0%) peaks were observed at
2␪ = 28.6◦, 33◦, 47.4◦, 56.6◦, 76.3◦, and 79.1◦, which corresponded
to a representative cubic fluorite structure in CeO2 for (111), (200),
(220), and (311) reflections [45]. The presence of these peaks indi-
cates that CeO2 was present in very finely divided nano-sized
crystals [44], which were formed at calcination temperatures lower
than 900 ◦C [46]. However, the peak intensities decreased con-
sistently with respect to SiO2. This behavior can be ascribed to a
reduction in the crystalline size of CeO2 by the presence of Si [45],
which can form amorphous structures. The crystalline size of CeO2
in the samples is shown in Table 4. In fresh catalysts, the CeO2 crys-
tal size decreased gradually from 14.8 nm on RhPt/Ce (Si = 0%) to
5.9 nm on RhPt/CeSi-60. Hu et al. [36] reported that the reduction
of the crystalline size of CeO2 was by the lattice constriction of CeO2
when Si4+ entered the CeO2 lattice due to the difference between
Similar to the TOS tests, the H2 yields during the on/off test
were 4.1 0.2 and 5.1 0.3 mol H2/mol EtOH over RhPt/Ce and
RhPt/CeSi-33, respectively. The higher H2 yield on the RhPt/CeSi-33
catalyst was interesting for further process developments (espe-
cially in continuous operation, where RhPt/CeSi-33 showed an
increase of the H2 mole fraction with the progressive elimination of
CH4). Table 3 compares the catalytic results obtained in this work
with those of other catalytic systems for the SRE from recent litera-

B. Cifuentes et al. / Applied Catalysis A: General 523 (2016) 283–293
289
Fig. 3. X-ray diffraction patterns of the RhPt/CeO2-SiO2 catalysts: (a) fresh, (b) reduced, and (c) used after the conversion-temperature cycle (U), and (d) used after the
stability tests (TOS) and startup/shutdown cycles (O–O).
the atomic radii of the two ions. No other crystalline phases were
observed in the fresh samples of RhPt/CeSi by XRD.
No obvious changes were identified between the reduced
(Fig. 3b) and used samples (Fig. 3c), indicating that the SRE
conversion-temperature cycle did not significantly affect the sup-
port phases. Fig. 3d shows the XRD patterns for fresh, reduced,
used, TOS, and O–O samples of RhPt/CeSi-33 catalysts, and Table 4
shows the average CeO2 crystallite size of these samples. The used
and TOS RhPt/CeSi-33 samples presented a critical decrease of the
peak intensities, which was related to an increase of the average
CeO2 crystalline size (Table 4) and a possible carbon deposition
(see Section 3.3.4 TGA). Hu et al. [36] reported that the Ce/Si molar
ratio in Ni/CeO2-SiO2 catalysts altered the surface area, acidity, and
the average crystalline size of CeO2, which could promote changes
of the CH4 conversion and H2 selectivity. Thus, the Si:Ce ratio of
RhPt/CeSi-33 possibly contributed to the altered crystalline struc-
ture, which could be linked to the highest H2 yield on this catalyst
in both the activity (Fig. 1b) and continuous stability (Fig. 2a) tests.
Moreover, Rocchini et al. [32] suggested that interactions between
Ce and Si could alter the oxygen storage capacity (OSC) of the
support; in an oscillating environment, such as the auto exhaust
catalysis, the OSC can change. The average CeO2 crystallite size in
RhPt/CeSi-33-(O–O) increased close to the size in RhPt/Ce, which
indicates a possible decrease in the Ce-Si interaction during the
on/off cycles. This decrease in Ce-Si synergy could be related to
the similar behavior of both catalysts to produce CO, CO2, and H2
in cycle (6) (Fig. 2b). Likewise, the possible changes in the struc-
ture of the support and carbon deposition could affect the ability
of the catalyst to desorb the CHx species observed in the on/off test
(Fig. 2b).
Similar to the fresh samples, a fluorite CeO2 structure was
identified in the reduced RhPt/CeSi catalysts (Fig. 3b). However,
the crystal size of CeO2 listed in Table 4 shows increases by fac-
tors of 3.1, 2.9, 2.7, 2.6, and 7.1 between the fresh and reduced
samples of RhPt/Ce, RhPt/CeSi-17, RhPt/CeSi-33, RhPt/CeSi-50, and
RhPt/CeSi-60, respectively. Thus, thinner peaks show an increase
of the average crystallites size of CeO2. Nevertheless, the inclusion
of SiO2 in the support mitigated the increase of the crystal size
in the RhPt/CeSi catalysts after reduction until a Si content of 50
mol%. When the Si content was above 60 mol%, the crystal size suf-
fered a larger increase (7.1 times). Revesz [35] suggested that the
H+ species could cause a structural modification of SiO2 at temper-
atures above 300 ◦C under a H2 atmosphere, which would affect
the average CeO2 crystal size. In addition, the RhPt/CeSi-33 and
RhPt/CeSi-50 reduced samples showed a peak at about 2␪ = 31◦,
indicating that a portion of CeO2 could form a new crystalline
structure. Rocchini et al. [32] reported a similar peak in samples of
CeO2-SiO2 reduced between 500 and 1000 ◦C. These authors pro-
posed the formation of a new Ce9.33(SiO4)6O2 phase favored by the
H2 atmosphere and stoichiometric amounts of CeO2 and SiO2.
Regarding to active metals, only the reduced sample of RhPt/Si
showed a small peak at 2␪ = 39.9◦, which was assigned to Pt (111)
[47]. In the other catalysts, the diffraction peaks of either Pt or Rh
were not detected because of the low metallic loadings (0.4 wt% for
each metal) [48].

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B. Cifuentes et al. / Applied Catalysis A: General 523 (2016) 283–293
fresh catalysts. Thus, the inclusion of more than 50% Si did not rep-
F
resent an increase in the catalyst surface area, probably due to the
interaction of the H+ species with the SiO2 surface, which changed
the Si structure [36]. Thus, when these catalysts were tested in the
SRE, the elevated Si content did not improve the H2 yield.
200
150
100
50
R
U
TOS
O-O
In the RhPt/CeSi-33 (TOS) and RhPt/CeSi-33 (O–O) samples, the
crystallite size of the CeO2 increased (Table 4), while the BET surface
area of these samples presented an additional decrease compared
to the reduced and used samples (Fig. 4). RhPt/Ce reduced its BET
surface area by 8% and 6% after the TOS and O–O tests, respectively,
compared to the reduced RhPt/Ce sample. In contrast, RhPt/CeSi-
33-TOS and RhPt/CeSi-33-O–O presented a reduction of 35% and
46%, respectively, of the BET surface area compared to the reduced
RhPt/CeSi-33 sample (129 m2/g). This additional decrease of the
BET surface area could be related to the extra interaction between
the SiO2 and H+ species from steam water [35], favoring a further
increase of the CeO2 crystalline size and coke formation over the
catalytic surface (TGA results, Section 3.3.4). According to these
results, the presence of SiO2 in the support promoted an increase
of the RhPt/CeSi surface area in the fresh catalysts due to a decrease
of the CeO2 crystal size. However, when RhPt/CeSi was subjected
to a reductive environment and steam, SiO2 could interact with the
0
0
10
20
30
40
50
60
Si/(Ce+Si)
Fig. 4. BET surface area of the RhPt/CeSi catalysts. F: fresh, R: reduced, U: used during
a conversion-temperature cycle, TOS: used during the stability tests, and O–O: used
during the on/off cycles.
H
+ species, favoring a growth of the CeO2 crystals and promoting a
decrease of the BET surface area.
3.3.2. CO2-chemisorption
CeO2 is a well-known basic catalyst and the addition of SiO2 can
change this property. Hence, CO2 chemisorption was conducted
to analyze the changes of the reduced catalyst’s relative basicity.
RhPt/Ce (341 ␮L CO2/g), RhPt/CeSi-33 (259 ␮L CO2/g), and RhPt/Si
(89 ␮L CO2/g) showed that SiO2 indeed reduced the relative basic-
ity of the catalyst, which is congruent with previous reports [36,45].
Thus, the lower basicity of RhPt/CeSi compared to the RhPt/Ce cat-
alyst could be responsible for the increase in CH4 formation when
the Si content was >40 mol% (Fig. 1c); as well as the slight increase of
the ethylene yield on the RhPt/CeSi catalysts compared to RhPt/Ce
(Fig. 1f) and the coke formation (Section 3.3.4). Some catalysts
used in the SRE have a deficiency to reform CH4 and a favorabil-
ity to produce ethylene, which was linked to excessive acid sites
in the catalysts [22,24,40]. Therefore, RhPt/CeSi-33 could provide
an interesting equilibrium between basicity (to maximize H2 and
avoid excess of ethylene and coke formation) and acidity (to pro-
mote CH4 formation and H2O activation), which would provide a
very active and selective catalyst for the SRE.
3.3.4. Thermogravimetric analysis
Coke formation during the SRE was evaluated by TGA in air.
Table 4 shows the TGA results for the used and aged catalysts. Car-
bon deposition on used samples (U) increased proportionally with
the SiO2 content in the support; 50 mol% Si increased the amount
of carbon by a factor of 26 compared to the RhPt/Ce catalyst after
one reaction cycle. When RhPt/Ce was used in the TOS and O–O
tests, carbon deposition was of only 0.6 and 0.4 mg carbon/gcat h,
respectively. On the contrary, the weight loss was of 4.4 and 3.6 mg
carbon/gcat h on RhPt/CeSi-33 after TOS and O–O tests respectively.
Therefore, the increase of the support acidity promoted ethylene
formation and polymerization to produce carbonaceous species
[36,40]. Differences between basic and acidic supports on carbon
deposits can be extracted from the literature. Sanchez et al. [50]
tested a RhPt/CeO2 catalyst for the SRE during 20 h TOS, reporting
a carbon deposition of 0.52 mg carbon/gcat h. On the other hand,
Barroso et al. [42] reported higher carbon deposition (7–28 mg
carbon/gcat h) after 7 h TOS in the SRE on a Co/CeO2/MgAl2O4 cata-
lyst. The authors ascribed the carbon deposition to the low S/E ratio
of 4 and the acidity of the support. In this way, the high weight loss
observed by TGA in the used RhPt/CeSi samples can be promoted
by the higher acidity of the supports containing Si.
3.3.3. BET surface area
The increase of the surface area of the catalyst is closely related
to its catalytic performance, because more space would be avail-
able for active site allocation and interaction between the species
present in the reaction [49]. SiO2 has a higher surface area and
average pore size, allowing for a better dispersion and stabiliza-
tion of the metals [23]. For this reason, SiO2 is commonly included
in matrices with other carriers to promote a higher surface area and
to improve the catalytic performance. Fig. 4 shows the surface area
of the RhPt/CeSi catalysts.
The surface area of the RhPt/CeSi catalyst was directly pro-
portional to the Si:Ce ratio in the fresh samples. Their surface
area increased from 44 m2/g in RhPt/Ce (Si = 0%) to 193 m2/g in
RhPt/CeSi-60 (Fig. 4). The fresh RhPt/Si (Si = 100%) showed the high-
est surface area of 480 m2/g (data not shown). However, the catalyst
reduction with 5% H2/N2 flow that was applied to fresh samples
decreased their BET surface area (R catalysts), and an inflexion point
was observed at RhPt/CeSi-50. After this point, the BET surface area
decreased with respect to the Si content. Structural modifications
of SiO2 caused by H2 reduction with fresh catalysts with Si contents
above 60 mol% increased the crystalline size of CeO2 (XRD results,
Section 3.3.2). This pretreatment reduced the surface area of the
3.3.5. High-resolution transmission electron microscopy
The size of the active particles consistent of Rh and Pt of the
reduced, TOS, and O–O catalyst samples of RhPt/Ce and RhPt/CeSi-
33 were studied by HR-TEM to evaluate the sintering of the active
phase, metal dispersion, and interplanar sizes of the catalysts. Fig. 5
shows the HR-TEM micrographs of these materials and particle
size distributions of Rh-Pt on the catalysts. The mean particle sizes
obtained from the HR-TEM analysis are listed in Table 5.
The crystalline structure and particle size distribution of RhPt/Ce
can be observed in Fig. 5a, c, e, and g. We previously reported the
ability of CeO2 to disperse Rh-Pt in a 0.6 wt% Rh-0.2 wt% Pt/CeO2
catalyst, probably forming Rh-Pt alloys with Pt enrichment [52],
which was also confirmed here by EDX semi-quantitative analysis
(data not shown). However, in that report [52], we did not observe
particle sintering after 27 h TOS. Here, both the Rh(0.4 wt%)-
Pt(0.4 wt%)/Ce (TOS) and (O–O) samples aged by 72 h showed an
increase of the active particle sizes (Table 5), indicating sintering
during the SRE. This was more significant in RhPt/Ce-TOS, which

B. Cifuentes et al. / Applied Catalysis A: General 523 (2016) 283–293
291
Fig. 5. HR-TEM images and particle size distribution histograms of the reduced, TOS, and O–O samples of the catalysts: (a, c, e, g) RhPt/Ce and (b, d, f, h) RhPt/SiCe-33.
showed a decrease of the H2 mole fraction after about 50 h of con-
tinuous stability tests (Fig. 2a). Kaila et al. [53] reported sintering
on a RhPt/ZrO2 catalyst during the auto-thermal reforming of simu-
lated gasoline due to high concentrations of Pt (∼0.5 wt%) that could
promote sintering. In addition, Tanaka et al. [54] proposed that Pt
was more sensitive to sintering than Rh owing to its lower melt-

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B. Cifuentes et al. / Applied Catalysis A: General 523 (2016) 283–293
Table 5
Interplanar sizes of the RhPt/Ce (Si = 0%) and RhPt/SiCe-33 catalysts.
Catalysts
CeO2 interplanar sizes
(nm)a
Particle interplanar
sizes (nm)a
Metal average
Metal dispersion (%)b
surface-area-weighted
diameter (ds, nm)a
RhPt/Ce (R)
RhPt/Ce (TOS)
RhPt/Ce (O-O)
RhPt/CeSi-33 (R)
RhPt/CeSi-33 (TOS)
RhPt/CeSi-33 (O-O)
0.28, 0.31
0.27, 0.31
0.27, 0.31
0.28, 0.31, 0.44
0.27, 0.32, 0.42
0.27, 0.32
0.19, 0.24
0.19, 0.24
0.19, 0.24
0.19, 0.26
0.18, 0.24
0.18, 0.21
5.6
9.7
7.8
5.1
7.8
9.7
19
12
14
22
14
11
Measured by aHR-TEM. b Dispersion defined as 1.1/ds [32,51].
ing point and migration sensitivity. Thus, the higher content of Pt
(0.4 wt%) in the current RhPt/Ce catalyst could favor the enrichment
of Pt on the CeO2 surface and favor active site agglomeration.
The reduced RhPt/CeSi-33 sample presented a similar mean
particle size (5.1 nm, Table 5) as RhPt/Ce (5.6 nm), indicating the
formation of similar Rh-Pt alloys on these catalysts. Thus, the pres-
ence of SiO2 in the support seems not to affect the formation of
the Rh-Pt alloys. The active metal dispersion (Table 5) was slightly
higher in the reduced RhPt/CeSi-33 (22%) compared to the RhPt/Ce
(19%) catalyst, which could be linked to the decrease of the CeO2
average crystallite size (Table 4) and the increase of the BET surface
area by the presence of SiO2 in the support. The highest surface
area and small crystalline size of RhPt/CeSi compared to RhPt/Ce
(Sections 3.3.1 and 3.3.3) could allow for a better distribution of
the active sites [36,45], thus improving the catalytic performance
in the SRE (Fig. 1). In contrast, the samples used in the stability
tests, RhPt/CeSi-33 (TOS) and RhPt/CeSi-33 (O–O), presented sin-
tering that was similar to RhPt/Ce. Nevertheless, the sintering in
RhPt/CeSi-33 (TOS, 54%) was lower than in RhPt/Ce (TOS, 71%) when
compared to their respective reduced catalysts. This showed that
the small CeO2 average crystallite size (Section 3.3.1) and higher
BET surface area (Section 3.3.3) of this catalyst contributed to mit-
igate the sintering during continuous operation.
The interplanar distance (d-spacing) of CeO2 was measured by
HR-TEM (Table 5). The typical distance of 0.31 nm related to CeO2
(100) and 0.27 nm to CeO2 (111) [55,56] were identified in all of the
RhPt/Ce samples. Mullins et al. [57] proposed that both CeO2 (100)
and CeO2 (111) formed during the water dissociation and regener-
ation of oxygen vacancies on CeO2. In addition, the d-spacing in the
RhPt/CeSi-33 catalyst samples indicated the presence of CeO2 (100)
and CeO2 (111). According to Rocchini et al. [32], the presence of
the Ce9.33(SiO4)6O2 phase favored an increase of the CeO2 interpla-
nar distance. An additional distance of ∼0.43 nm was observed on
the RhPt/CeSi-33 (R) and RhPt/CeSi-33 (TOS) samples, which could
be linked to a reduction of the average crystallite size of CeO2 and
the Ce9.33(SiO4)6O2 phase formation observed by XRD.
ratio of 1:2) was found to provide the best catalyst. This Si con-
tent in the RhPt/CeSi-33 reduced catalyst changed the following
properties compared to the RhPt/CeO2 reduced reference sam-
ple: (i) decreased the basicity of the support (from 341 to 259 ␮L
CO2/g catalyst), (ii) reduced the crystalline size of CeO2 (from 46.0
to 22.6 nm), (iii) increased the catalyst surface area (from 36 to
129 m2/g catalyst), and (iv) decreased the active particle size (from
5.6 to 5.1 nm). These characteristics allowed for a more active mate-
rial when RhPt/CeSi-33 was compared to a RhPt/CeO2 reference
sample. A maximum H2 yield of 5.2 0.2 mol H2/mol EtOH inlet
was obtained over this catalyst, 30% more than for RhPt/CeO2 under
the same reaction conditions. This extra H2 was related to the
possible ability of SiO2 to interact with OH− and H+ species, favor-
ing the decomposition of water. Further, the RhPt/CeSi-33 catalyst
maintained its H2 yield during 72 h TOS and on/off cycles, showing
promising features for its forthcoming use in stationary and mobile
FC applications. This catalyst is going to be tested for the SR of real
bioethanol samples under FCs operation conditions.
Conflicts of interest
This manuscript has not been published or presented elsewhere
in part or in its entirety, and is not under consideration by another
journal. All authors approve of the manuscript and agree with sub-
mission to your esteemed journal. There are no conflicts of interest
to declare.
Author contributions
Bernay Cifuentes and Martha Cobo contributed equally to this
work. María Hernández and Sonia Monsalve participated in the
evaluation and characterization of the RhPt/SiCeO2 catalysts. All
authors contributed to the writing and review of this document.
Acknowledgments
The interplanar distances of the Rh and Pt active metals mea-
sured by HR-TEM are shown in Table 5. The distances of 0.19
and 0.23 nm correspond to cubic Pt (200) [58] and Rh (111) [59],
respectively. The presence of other ions in the molecular arrange-
ment could slightly alter the interplanar distance of Pt [58], which
could explain the other distances listed in Table 5 (0.18, 0.24, and
0.26 nm). Thus, the difference of the interplanar distance could indi-
cate the inclusion of Rh in the molecular arrangement of Pt [58].
According to the EDX and interplanar distances, both Rh and Pt are
present in the active sites of the catalyst.
The authors are grateful to the Universidad de La Sabana for the
financial support of this work through project ING-135.
Appendix A. Supplementary data
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.apcata.2016.06.
014.
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