Oxidative steam reforming of ethanol over M: XLa2- xCe1.8Ru0.2O7- δ (M = Mg, Ca) catalysts: Effect of alkaline earth metal substitution and support on stability and activity

Article abstract of DOI:10.1039/c9ra08385e

Alkaline earth metal substitutions on the A-site of pyrochlore oxide M_xLa_2-xCe_1.8Ru_0.2O_7-δ (M = Mg, Ca) were studied as catalyst materials for oxidative/autothermal steam reforming of ethanol (OSRE/ATR). The as-prepared oxides were synthesized by a combustion method and characterized using powder X-ray diffraction (PXRD), and X-ray photoelectron and absorption spectroscopy (XPS and XAS). PXRD Rietveld analysis and elemental analysis (ICP-AES) support the formation of a pyrochlore-type structure (space group Fd3m) with a distorted coordination environment. The substitution of Mg²⁺ and Ca²⁺ ions affects the oxidation states of Ce^4+/3+ and Ruⁿ⁺ ions and creates oxygen vacancies, which leads to enhanced catalytic activity and reduced ethylene selectivity. A long-term stability test showed optimized catalysts Mg_0.3La_1.7Ce_1.8Ru_0.2O_7-δ and Ca_0.2La_1.8Ce_1.8Ru_0.2O_7-δ with S_H2 = 101(1)% and S_H2 = 91(2)% under OSRE conditions. The initial operation temperatures were lower than that of the unsubstituted catalyst La₂Ce_1.8Ru_0.2O_7-δ. Catalysts supported on La₂Zr₂O₇ showed stable OSRE/ATR performance and low carbon deposition compared to catalysts supported on Al₂O₃. We ascribe the enhanced activity to well-dispersed alkaline earth metal and Ru ions in a solid solution structure, synergistic effects of (Mg, Ca)²⁺/Ce^3+/4+/Ruⁿ⁺ ions, and a strong catalyst-support interaction that optimized the ethanol conversion and hydrogen production.

Full text of DOI:10.1039/c9ra08385e

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Oxidative steam reforming of ethanol over
MxLa2ꢀxCe1.8Ru0.2O7ꢀd (M ¼ Mg, Ca) catalysts:
effect of alkaline earth metal substitution and
support on stability and activity†
Cite this: RSC Adv., 2019, 9, 39932
Ho-Chen Hsieh,ac Ping-Wen Tsai,a Yuan-Chia Chang,a Sheng-Feng Weng,a
ab
Hwo-Shuenn Sheu,cd Yu-Chun Chuangd and Chi-Shen Lee
*
Alkaline earth metal substitutions on the A-site of pyrochlore oxide MxLa2ꢀxCe1.8Ru0.2O7ꢀd (M ¼ Mg, Ca)
were studied as catalyst materials for oxidative/autothermal steam reforming of ethanol (OSRE/ATR). The
as-prepared oxides were synthesized by a combustion method and characterized using powder X-ray
diffraction (PXRD), and X-ray photoelectron and absorption spectroscopy (XPS and XAS). PXRD Rietveld
analysis and elemental analysis (ICP-AES) support the formation of a pyrochlore-type structure (space
2+
group Fd3m) with a distorted coordination environment. The substitution of Mg and Ca2+ ions affects
ꢀ
the oxidation states of Ce4+/3+ and Run+ ions and creates oxygen vacancies, which leads to enhanced
catalytic activity and reduced ethylene selectivity. A long-term stability test showed optimized catalysts
Mg0.3La1.7Ce1.8Ru0.2O7ꢀd and Ca0.2La1.8Ce1.8Ru0.2O7ꢀd with SH ¼ 101(1)% and SH ¼ 91(2)% under OSRE
2
2
conditions. The initial operation temperatures were lower than that of the unsubstituted catalyst
La2Ce1.8Ru0.2O7ꢀd. Catalysts supported on La2Zr2O7 showed stable OSRE/ATR performance and low
carbon deposition compared to catalysts supported on Al2O3. We ascribe the enhanced activity to well-
Received 14th October 2019
Accepted 20th November 2019
dispersed alkaline earth metal and Ru ions in a solid solution structure, synergistic effects of (Mg, Ca)2+
/
DOI: 10.1039/c9ra08385e
Ce3+/4+/Run+ ions, and a strong catalyst–support interaction that optimized the ethanol conversion and
rsc.li/rsc-advances
hydrogen production.
Hydrogen production from the reforming of ethanol is one
of the potential fuel processes that could help alleviate our
dependence on fossil fuels.10–13 As a result, it has been widely
1. Introduction
Energy is indispensable in our daily lives and about 80% of
energy relies on the combustion of fossil fuels. The production
of pollutants such as COx, NOx, CHx, and SOx generated during
energy generation from fossil fuels severely damages our envi-
ronment.1–3 To resolve the prospective crisis, many researchers
have devoted their attention and efforts to renewable energy
resources.4–6 Among these available sources, hydrogen is
a prospective energy carrier because it burns cleanly without
emitting pollutants, and it has a high energy density (120–142
MJ kgꢀ1) compared to other hydrocarbon fuels.7–9
investigated. In recent years, many researchers have investi-
gated ethanol conversion that combines oxygen, water, and
ethanol. The addition of oxygen facilitates exothermic reactions
from total or partial oxidation to provide energy for endo-
thermic steam reforming reactions. Depending on the condi-
tions, the processes are named oxidative (OSRE) or autothermal
(ATR) steam reforming procedures. The OSR/ATR process is
considered to be an energy efficient and attractive ethanol
conversion process.14
To date, the most studied catalysts for OSRE processes are
transition metal-based catalysts (e.g. Co, Ni, Ru, Rh and Ir) with
supporting materials such as Al2O3, ZrO2 and SiO2.15–17 The
catalysts employed for the OSRE process may be degraded due
to a sintered catalyst and carbon deposition. Supporting mate-
rials also play an important role in maintaining an even
distribution of the catalyst and suppressing the carbon depo-
sition rate.18 Some studies show the promotion effect over
supporting materials by adding either alkali or alkaline metal
oxides. For example, alkaline earth or rare earth oxides (MgO,
CaO, La2O3, CeO2, and Y2O3) were used as promoter materials to
aDepartment of Applied Chemistry, National Chiao Tung University, 1001 University
Rd., Hsinchu 30010, Taiwan. E-mail: chishen@mail.nctu.edu.tw
bCenter for Emergent Functional Matter Science, National Chiao Tung University,
Hsinchu 30010, Taiwan
cGraduate Degree Program of Science and Technology of Accelerator Light Source,
National Chiao Tung University, Hsinchu 30010, Taiwan
dNational Synchrotron Radiation Research Center, Hsinchu, 30010, Taiwan
† Electronic supplementary information (ESI) available: Detailed information can
be found in the ESI, including PXRD, XAS, XPS, SEM, TGA, and so on. See DOI:
10.1039/c9ra08385e
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enhance the stability of catalysts on OSRE because they provide catalyst and degraded performance. To prevent the solid state
more basic sites on the oxide surface to facilitate the reaction between the catalyst and support, we utilized La2Zr2O7
adsorption/activation of CO2, reduce carbon deposition, and (LZO) as the supporting material. It signicantly improved the
prevent the aggregation of active catalysts.19–23 Graschinsky et al. stability of the catalyst in the OSRE process.46
studied Rh(1%)/MgAl2O4/Al2O3 for OSRE and found enhanced
In this work, alkaline earth metal substituted on an A-site
performance due to the effect of MgAl2O4 on the surface of pyrochlore structure was studied in order to understand the
Al2O3.24 Another way to promote catalytic activity is to include effect of the A-site substitution on the activity of the catalyst in
the uorite-type structure CeO2 as the promoter material with the OSRE. The mix-occupied A-sites may modify the oxidation
an active metal catalyst. Deluga et al. demonstrated that using states of active metal ions in the B-sites (Ce4+/3+ and Run+) and
5 wt% Rh/CeO2/Al2O3 and Pt/CeO2 as catalysts in a double-bed create oxygen vacancies. The aim of this work was to study the
reactor could promote hydrogen selectivity reaching 130% at C/ effect of substituted alkaline earth metals on the A-site of
O ꢁ 0.7 in OSRE.25 In our previous work, we studied the effect of a pyrochlore structure on the catalytic performance of the OSRE
controlled morphologies of ceria nanocrystals, Ce1ꢀxMxO2 (M ¼ process. We systematically investigated the substitution of La3+
Ti, Zr, and Hf), on the catalytic activity of 5 wt% (Ru, Rh)/ with alkaline earth cations Mg2+/Ca2+ on the A-site to Mx-
Ce1ꢀxMxO2 catalysts, which revealed high performance due to La2ꢀxCe1.8Ru0.2O7ꢀd (M ¼ Mg, Ca), and its OSRE performances
the specially exposed facets on the catalyst; however, the facets were compared to La2Ce1.8Ru0.2O7ꢀd. The effect of the sup-
were unstable aer long-term use.26
porting material on the OSRE performance was determined
Recently, mixed-metal oxides with active metal ions have under the OSR/ATR processes using catalysts supported by
also been found to exhibit good activity as catalysts in relation Al2O3 or La2Zr2O7.
to the ethanol reforming process.27 Various metal oxides, such
as perovskite, spinel, hydrotalcites, nano-oxyhydride, and
2. Experiment
2.1 Synthesis and characterization
pyrochlore, have been reported for OSRE process.28–36 Chen et al.
investigated the effect of a LaBO3 (B ¼ Ni, Co, Fe, Mn) perovskite
structure on OSRE. The thermal stability and oxygen vacancies Alkaline earth metal substituted pyrochlore oxides MxLa2ꢀx
-
affected the catalytic activity and stability for OSRE.37 Sania M. Ce1.8Ru0.2O7ꢀd (M ¼ Mg, Ca) with controlled stoichiometries of
de Lima et al. studied the perovskite structure of La1ꢀxCexNiO3 M/La (M ¼ Mg, Ca) were prepared by the sol–gel method. All
on OSRE. Good performance was found with a high oxygen methods use an approach described elsewhere in detail.46 The
storage capacity and oxygen mobility on OSRE.38 Wang et al. materials of the two groups with Ca- and Mg-doped samples
studied La1ꢀxCaxFe1ꢀxCoxO3 as a catalyst for SRE/OSRE and were made at 600 C and 900 ꢂC for 5 h, respectively.
ꢂ
found that the A-site substituted with Ca2+ and cobalt ions
Detailed structural properties for the materials were char-
leaving/entering the lattice of the perovskite structure enhanced acterized by powder X-ray diffraction (PXRD) (Bruker D8
activity and suppressed the sintering of the catalyst.39 M. Advance Bragg-Brentano-type, l ¼ 1.5418 A). These samples
˚
˚
Morales et al. investigated the catalytic activity and stability of were measured using a wavelength of 0.688 A at Beam Line
La0.6Sr0.3CoO3ꢀd on SRE and OSRE. This showed higher selec- 01C2, NSRRC, Hsinchu, Taiwan. The standard sample LaB6 was
tivity of hydrogen with a good stability for OSRE.40 Huang et al. used to correct the experimental environment at the beamline
investigated the NiAl2O4–FeAl2O4 spinel oxide structure as station. These data were measured at room temperature and
a catalyst for the ATR of ethanol; it showed an optimized used the Rietveld renement method using GSAS
hydrogen selectivity of ꢁ109% at 600 C.41 Espitia-Sibaja et al. programming.47–49
ꢂ
investigated layered oxide hydrotalcites containing CoMgAl
For synchrotron-based X-ray absorption experiments, XANES
mixed ions for OSRE, which showed that the amount of Co3+ and EXAFS analyses were carried out at Beam Line 01C1.
species catalysts affected the content of the products.42 Pirez Powder samples were uniformly spread onto adhesive tape to
and Fang et al. used the catalysts CeNiHzOy and Mg2AlNixHzOy measure data using a Lytle detector in the uorescence mode.
nano-oxyhydride materials in OSRE that produced about a 45% The XAS spectra of Ru K-edge were concomitantly recorded in
yield of hydrogen with low activation energy.43,44
the energy range of 21.867–22.86 keV. The normalization of the
Our group reported on the metal substituted pyrochlore- XANES data was accomplished according to previous proce-
based catalysts that display enhanced catalytic performance dures using the AUTOBK program algorithm. EXAFS data
and stability for OSRE. The sintering effect on the catalyst was analysis was performed with the program FEFF 702.50–52 We
suppressed, and the amount of deposited carbon was reduced. recorded the XPS spectra of Mg2p, Ca2p, C1s, Ru3d, O1s, La3d
The catalyst, La2Ce1.8Ru0.2O7 (LCRO), showed a high perfor- and Ce3d core levels (ESCA PHI 1600 instrument, Al anode ¼
mance of OSRE, and the synergetic effect between the Run+ and 1486.6 eV). The spectra energy was calibrated using the C1s
Ce3+/4+ ions enabled the catalyst to remain active during the peak (binding energy 284.8 eV). The pressure in the chamber
OSRE process.45 Lithium ion substituted metal oxides, Lix- was less than 6.7 ꢃ 10ꢀ7 Pa for measurements. All spectra were
La2ꢀxCe1.8Ru0.2O7ꢀd (LLCRO), created oxygen vacancies that analyzed with soware package XPSPEAK4.1.53
correlated with the formation of high oxidation states cations of
Run+/Ce4+. However, the initial study showed that the as- by using Temperature-Programmed Reduction (China Chro-
prepared pyrochlore catalysts interacted with the conventional matography 660 instrument). The gas owed (50 mL minꢀ1
Al2O3 support during the stream test and led to a decomposed through the silica tube, comprising 10% H2 and 90% Ar.
The reduction temperature of the materials can be employed
)
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Approximately 60–100 mg of samples were loaded into a silica for H2, CO, CO2, and a ame-ionization detector (FID) for CH4,
tube. The ꢀfu1rnace temperature was raised to 900 ꢂC at a rate of C2H4, C2H5OH, and CH3CHO. The carbon balance was calcu-
10 ꢂC min , and then, cooled with a furnace from 900 ꢂC down lated ꢄ5%. The general expression to calculate the selectivity of
to room temperature. The quantitative analysis of hydrogen product species is dened below:
consumption was integrated by PeakFitv4.11 soware, using
CuO as the calibration standard.
Ethanol conversionð%Þ
The surface area of the materials was usually determined by
the Brunauer–Emmett–Teller method, which was analyzed by
Micrometrics sorptometer Tri Star 3000. The particle size and
morphology of the materials were acquired with scanning
electron microscopy (JEOL JSM-7401F FE-SEM), and the images
were examined at accelerating voltages with several
magnications.
½moles CðCO2 þ CO þ CH4 þ C2H4 þ CH3COHÞin reformateꢅ
½moles CðC2H5OHÞin feedꢅ
¼
mole of product species
Selectivityð%Þ ¼
n ꢃ mole of fuel ꢃ XEtOH
n ¼ 1 for C2-species; 2 for C1-species; n ¼ 3 for hydrogen
Hydrogen yieldð%Þ
2.2 Catalyst preparation and performance
ðmoles of H2 in reformateÞ
¼
ꢃ 100
All catalysts were prepared by means of the impregnation
method. Commercial g-Al2O3 and lab-made La2Zr2O7 were used
as the supporting materials, sieved in 16 to 18 mesh and
immersed in that catalyst-ethanol mixed solution. The sup-
porting material of La2Zr2O7 was prepared according to the
previous procedure.46 The mixed solution was dried at 90 ꢂC,
and then, the entire procedure was repeated at least ve times to
prepare metal oxide well dispersed on the supporting materials.
ðmaximum thertical moles of H2Þ
3. Results
3.1 Physicochemical properties of MxLa2ꢀxCe1.8Ru0.2O7ꢀd
(M ¼ Mg, Ca) solid solution
The as-prepared catalysts, MgxLa2ꢀxCe1.8Ru0.2O7ꢀd (MLCRO), Fig. 1A and B show the X-ray diffraction pattern of Mg- and Ca-
CaxLa2ꢀxCe1.8Ru0.2O7ꢀd (CLCRO), and La2Ce1.8Ru0.2O7ꢀd substituted MxLa2ꢀxCe1.8Ru0.2O7ꢀd (M ¼ Mg, Ca). The major
(LCRO), were used on Al2O3 and La2Zr2O7 to test the catalytic diffraction peaks correspond to the pyrochlore phase with
2+
ꢀ
reaction. The weight ratio of the catalyst to the supporting a cubic lattice of high-symmetry Fd3m (no. 227). As the Mg
material g-Al2O3 was 1 : 10, and the supporting material, content increased, diffraction peaks of MgxLa2ꢀxCe1.8Ru0.2O7ꢀd
La2Zr2O7, at 1 : 20 were prepared for xing the weight patterns shied to a high 2q range, indicating a trend to
percentage of the catalyst. The catalysts used were analyzed with a reduction in the unit cell dimension. Mg2+ (C.N. ¼ 8, 0.89 A)
˚
Raman spectroscopy, using a 5 mW He–Ne laser system working ions are smaller than La3+ (C.N. ¼ 8, 1.16 A) ions, and it was
˚
at 633 nm (THORLABS, HRR170 instrument). The powders were expected that the unit cell parameters would be reduced upon
dispersed on the microslide, and each datum was acquired for Mg2+ replacement with La3+ ions. The results shown in Fig. 1(C)
a counting time of 60 s. Measurements of TGA used a thermal are consistent with Vegard's law for the La3+ substitution for the
analyzer (NETZSCH STA 409PC) and the long-term used smaller Mg2+ cations. A single phase of MgxLa2ꢀxCe1.8Ru0.2O7ꢀd
ꢀ1
ꢂ
ꢂ
samples were heated to 800 C at a rate of 10 C min in an (MLCRO) was observed for x ¼ 0.0–0.3 (denoted as LCRO,
oxygen ow of 60 mL minꢀ1
.
MLCRO01-03), and a La2O3 (P3m1, ICSD 96196) impurity phase
ꢀ
Experimental OSRE was carried out in a xed-bed quartz started to appear when x ¼ 0.4. On the other hand, when the
reactor (4 mm inner diameter and 12 cm length). The mass ow size of Ca2+ (C.N. ¼ 8, 1.12 A) is similar to that of La3+, the
˚
of 0.5 L minꢀ1 (STP) used with 150–320 mg of the catalysts and substitution of Ca2+ into the A sites results in the lattice
support would equal a GHSV of 1.6 ꢃ 105 hꢀ1. The carbon-to- remaining close to the unsubstituted La2Ce1.8Ru0.2O7ꢀd (LCRO).
oxygen ratio was determined by the concentration of the A single phase was observed for x ¼ 0.0–0.2, and diffraction
ethanol and air, a factor that would affect the total and partial peaks from La2O3 impurities was observed when x was higher
oxidation processes. A heating furnace was used to control the than 0.2. The results indicate that calcium ions were substituted
vaporization temperatures of the fuels and catalytic bed in the into the CaxLa2ꢀxCe1.8Ru0.2O7ꢀd (CLCRO) lattice in a range of
OSRE. Under OSR and ATR of ethanol, the fuels were pumped 0.0 < x < 0.2 (denoted as CLCRO01, 02).
into the reactor which contained the three zone red furnace.
The synchrotron X-ray diffraction of the MxLa2ꢀxCe1.8Ru0.2-
The rst and second zone was maintained at 220–240 ꢂC to mix O7ꢀd (M ¼ Mg, Ca) was characterized. The powder data on the
the evaporating gas. A catalytic bed was loaded into the third highest substituted samples, MLCRO03 and CLCRO02, were
heating zone, and the furnace temperature was preheated to rened by the Rietveld method, and these results are presented
initially activate the fuels. The third heating zone could be in Fig. S1 and Table S1.† The unsubstituted LCRO (P-type
ꢀ
maintained at activation temperature in OSRE; in contrast, the structure, Fd3m, no. 227) phase was used as an initial model.
furnace temperature of the third heating zone was turned off The occupation of the Ru/Ce was xed with the as-prepared
aer the catalytic reaction proceeded to an autothermal reac- samples for MxLa2ꢀxCe1.8Ru0.2O7ꢀd (M ¼ Mg, Ca). The rene-
tion. These products were analyzed by a gas chromatograph ment of occupation and thermal parameters was constrained
(Agilent GC 7890A) with a thermal-conductivity detector (TCD) due to the correlation of both. There were two cation sites and
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Scheme 1 Crystal structures of 1/8 unit cell of pyrochlore structure.
surface areas of all the samples exhibited a lower surface area
within a small range of 5 to 16 m2 gꢀ1. The morphology and
particle size were investigated with SEM (Fig. S2†), showed
uneven shapes and particle sizes aggregated in a range of 10–
25 nm due to a higher annealing temperature.
3.2 X-ray photoelectron spectra (XPS)
The surface properties, as well as the oxidation states of the
constituent elements of MxLa2ꢀxCe1.8Ru0.2O7ꢀd (M ¼ Mg, Ca),
were investigated by XPS. The XPS results of MLCRO were taken
as a representative example. For MLCRO catalysts, Fig. 2(A)
shows the La3d spectra of the substituted pyrochlore type
structures. Four pairs of signals were observed from the La3d
that correspond to the 3d5/2 and 3d3/2 levels for MLCRO. The
binding energies of La3+ were 838 eV (La3d5/2) and 855 eV
(La3d3/2) with their satellites at 834 eV and 851 eV, respectively.
Other weak signals centered at 836 eV (La3d5/2) and 853 eV
(La3d3/2) eV with their corresponding satellite peaks at 832 eV
and 849 eV, which are attributed to the effects of structural
distortion due to the Mg2+ and Run+ ions substitution on A and
B sites, respectively.54
Fig. 1 Phase identification of MxLa2ꢀxCe1.8Ru0.2O7ꢀd (M ¼ Mg, Ca): (A)
powder X-ray diffraction of MLCRO0-05; (B) CLCRO0-04, and (C) cell
constant refinement.
The XPS spectra of Mg2p and Ca2p are shown in Fig. S3†
with peaks centered at 50 eV (Mg2p), 347 eV (Ca2p1/2) and
350 eV (Ca2p3/2), respectively. The peaks correspond to the
three anion sites in the environment. The special crystallo-
graphic site 16d coordination was a distorted cube for a triva-
lent rare-earth cation; the special site 16c was a distorted
octahedral for a tetravalent metal cation. There were three
anion sites: the 48f and 8b sites were for O2ꢀ ions; and the 8a
site was a vacant oxygen site, as shown in Scheme 1. The O (48f)
atoms were connected to two (Mg, Ca)/La atoms and two Ru/Ce
atoms. The O (48f) (x, 1/8, 1/8) positions for the parameter, x,
increased from 0.367(2) to 0.392(2) for Mg- and 0.384(3) for the
Ca-substituted structure due to partially substituted alkaline
earth metal ions on the A-site, respectively. This affected the
interatomic distances and bond angles, as shown in Table S1.†
The results indicated that the A-site alkaline metal substitution,
the structural environment was distorted and the oxidation
state of Ce/Ru and oxygen vacancy needed to maintain the
charge balance.
Table 1 Physicochemical properties of the solid solution MxLa2ꢀx
-
Ce1.8Ru0.2O7ꢀd (M ¼ Mg, Ca)
x
0.0
0.1
0.2
0.3
˚
Cell parameter/A
Crystal size/nma
Mg contentb
11.128(3)
9.9(1)
N/A
11.130(6)
14.86(9)
0.10
11.120(4)
13.5(1)
0.21
11.104(7)
12.32(9)
0.32
Surface area/m2 gꢀ1
14.17
6.47
9.89
9.07
x
0.0
11.128(3)
9.9(1)
N/A
14.17
0.1
0.2
˚
Cell parameter/A
11.137(6)
17.3(1)
0.13
11.145(5)
24.8(2)
0.20
Particle size/nma
Ca contentb
Surface area/m2 gꢀ1
16.11
10.89
a
The crystal size was measured using Scherrer equation in XRD.
Calculated from ICP-AES results.
b
The specic surface areas, ICP-AES, and crystal size data for
the materials as prepared are summarized in Table 1. The
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be deconvoluted to several peaks in a range of 280–292 eV,
indicative of multiple oxidation states of Run+ ions.56,57
The O1s spectra for MLCRO materials are shown in Fig. 2(C).
It has a broad asymmetry peak which can be deconvoluted into
two peaks. It presents the two binding energies, the O48f and
O8b, due to the electronegativity effect. The electronegativity of
the A-site (Mg/La) ions was smaller than for the B-site (Ce/Ru).
The higher binding energy of O48f surrounded by a 2 A (Mg/
La) site and 2 B(Ru/Ce) site, was comparable to the lower
binding energy of O8b (4A (Mg/La)). Thus, the peak could be
assigned to O48f with the binding energy of 531 eV and the
binding energy of 528 eV could be assigned to O8b in the O1s
spectrum.58 The results indicate that the Mg-substituted
samples on the A-site not only affected the La–O distance, but
also created mixed oxidation states of Ce3+/4+ and Run+ that led
to the distribution of oxygen vacancies. The XPS analyses from
Ca-substituted samples are similar to the Mg-substituted
samples, and their results are summarized in Fig. S4, Tables
S2 and S3.†
3.3 X-ray absorption spectroscopy (XAS)
The oxidation state and local atomic environment in MxLa2ꢀx
-
Ce1.8Ru0.2O7ꢀd (M ¼ Mg, Ca) were investigated by an X-ray
absorption near-edge structure (XANES) spectrum (Fig. 3). For
comparison of the XANES of the Ru K-edge region of series
materials, Ru-foil, RuCl3, and RuO2 powders were used as
references and measured using the same procedure. The pre-
edge region of LCRO, MLCRO03, and CLCRO02 are shown in
Fig. 3. According to the rst derivatives of the Ru XANES spectra,
the apparent pre-edges of MLCRO03 and CLCRO02 are higher
than that of RuO2, indicative of an oxidation state higher than
4+ for all Ru ions in Mg- and Ca-substituted pyrochlores in
Fig. S5 and Table S4.† In order to evaluate the oxidation state of
Ru ions in the pyrochlore structure, we analyzed the EXAFS
tting results obtained from the Fourier transform into the R-
space as FT [k3c(k)], shown in Fig. S6 and S7.† The EXAFS tting
˚
results are shown in Table 2. The peak at around 1.93 A is
Fig. 2 Peak deconvolution of MLCRO0-03: (A) La3d; (B) Ce3d, and (C)
O1s in X-ray photoelectron spectra.
associated with electronic backscattering between Ru and O
atoms in the rst shell. The Ru–O bond length was affected by
the electronic structure due to the mixed oxidation state of Run+
.
MLCRO of the Mg2+ ion and the CLCRO of the Ca2+ ion in the
structure. The Ce3d spectra can be assigned as the result of the
mixed oxidation states of Cen+ cations; the results are shown in
Fig. 2(B). The relative compositions of Ce3+ and Ce4+ ions were
evaluated with peak integrations; the results are shown in Table
S2.† Peaks U0 and V0 centered at 884.0 and 903.0 eV resulted
from the contribution of Ce3+ ions. The peaks (V, V00, V000) and (U,
U0, U000) corresponded to 3d5/2 and 3d3/2 and were assigned for
the Ce4+ states, showing that the increased Mg2+ substitution
affected a relative amount of Ce4+ to Ce3+.55 The Ru3d spectra
show binding energies in a range of 281.7–283.7 eV for Ru3d5/2
(Fig. S3(C), Table S3†). The reported data on the Ru3d5/2 signal
of Ru4+ were in the range of 280.7–281.0 eV, and the binding
energies high oxidation states Run+ (n > +4) were located at
282.5–282.6 eV and 283.3 eV, respectively. The broad signal can
Fig. 3 The Ru K-edge XANES spectra of MLCRO (M ¼ Mg, Ca) with
references.
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be between 150 ꢂC and 350 ꢂC. The reduction proles could be
convoluted into two temperature regions. It could be assigned
to the reduction temperature region at 200 ꢂC and 300 ꢂC by Ru
ions in the structure. A reduction in Run+ (n S 4) to Ru3+ was
Table 2 EXAFS fitting results for MxLa2ꢀxCe1.8Ru0.2O7ꢀd (M ¼ Mg, Ca)
MgxLa2ꢀxCe1.8Ru0.2O7ꢀd x ¼ 0
x ¼ 0.1
1.979(4)
6
x ¼ 0.2
1.991(6)
6
x ¼ 0.3
1.983(5)
6
˚
Ru–O (A)
1.969(7)
N/nuꢀm2bbera
R-Factorc (%)
6
ꢂ
ꢂ
2
˚
observed at around 210 C and 270 C. The second reduction
peak was attributed to the reduction of Ru3+ and Ru2+/0 centered
at around 270 ꢂC and 380 ꢂC. The reduction of Ce4+ to Ce3+
specꢂies was not clearly observed at a temperature higher than
400 C. The initial reduction temperature gradually increased
and reached 200 ꢂC, which correlated with the modied
oxidation state of Run+ ions close to Ru4+ (Table S3†). There was
a minor change in the initial reduction temperature in MLCRO
(x ¼ 0.1) which rose to ꢁ250 ꢂC when x ¼ 0.2 and 0.3. As for the
Ca-substituted samples, the temperature shied to high
temperature ꢁ260 ꢂC. Both Mg- and Ca-substituted samples
show higher initial reduction temperatures than that of non-
doped LCRO, indicative of a reduced oxidation state of Ru
ions. When the active metal composition of Ru/Ce ions was
xed, the highest H2 consumption was 0.91 mmol gꢀ1 for
MLCRO0-03 and 1.44 mmol gꢀ1 for CLCRO0-02, as shown in
Fig. S8(A).† The average H2 molecule (two electron) consumed
0.57 and 0.89 mmol mmolꢀ1 Ru for MLCRO01-03 and
CLCRO01-02, as shown in Fig. S8(B),† respectively. The results
indicate that the Mg- and Ca-substituted MxLa2ꢀxCe1.8Ru0.2O7ꢀd
affected the oxidation states of Run+ and Ce3+/4+ on the oxide
surface.
s (A
)
0.0067(3) 0.0057(2) 0.0046(2) 0.0065(2)
0.1
0.05
0.1
0.09
E0 (eV)
ꢀ1.0
ꢀ0.7
ꢀ0.2
ꢀ1.1
CaxLa2ꢀxCe1.8Ru0.2O7ꢀd
x ¼ 0.1
1.991(9)
6
0.0075(3)
0.12
x ¼ 0.2
1.99(1)
6
0.0082(4)
0.1
˚
Ru–O (A)
N/nuꢀm2bbera
2
˚
s (A
)
R-Factorc (%)
E0 (eV)
ꢀ0.34
ꢀ0.17
N/number: coordination number. s2: Debye–Waller factor. R-
a
b
c
Factor: the goodness of the t.
In comparison with RuO2, CLCRO02, MLCRO03, and LCRO, the
averaged bond lengths of Ru–O were 2.02(1), 1.99(1), 1.983(5),
˚
and 1.969(7) A, respectively. We found that the bond distance of
Ru–O was different due to the effect of the electronic structure
of Ru. The rened Ru–O lengths for MLCRO03 and CLCRO02
were close to RuO2 and longer than that of LCRO, suggesting
the similar bond strength to RuO2.
3.4 TPR process analysis and H2 reduction ability
Fig. 4 depicts the TPR proles of MxLa2ꢀxCe1.8Ru0.2O7ꢀd (M ¼
3.5 Catalytic performance
Mg, Ca), showing the contribution of Ru ions in the reduction to
Catalytic activity as a function of temperature was studied, and
the results are shown in Fig. 5 and S9.† The initial activation
temperatures of MLCRO03 and CLCRO02 decreased to 350 and
280 ꢂC, respectively, which were lower than that of LCRO/Al2O3
(400 ꢂC), indicating that alkaline-substitution had markedly
increased the activity of the catalyst. Once the process was
initiated, the ethanol conversions remained ꢁ100%.
3.5.1 Effect of carbon-to-oxygen ratio. The ethanol conver-
sion under the OSRE process was tested for a pure phase of
MLCRO01-03 and CLCRO01-02 supported on Al2O3. The lowest
operating temperatures for Mg- and Ca-substituted pyrochlore
catalysts were 350 ꢂC to 280 ꢂC. Once the process had been
initiated, the furnace temperature was xed at the corre-
sponding temperature for MLCRO (350 ꢂC) and CLCRO (280 ꢂC).
The effect of oxygen concentration was evaluated from carbon-
to-oxygen ratio (C/O) ¼ 0.4 to 0.7 in order to evaluate the cata-
lytic activity and the results are shown in Fig. 6 and S10.† The
oxygen concentration was controlled by adjusting the C/O
ratios. The results indicated that the MLCRO03 and CLCRO02
catalysts exhibit the highest H2 selectivity with a full ethanol
conversion at C/O ¼ 0.6. As the C/O ratio increased to 0.7, the
ethanol conversion was incomplete under a low O2 partial
pressure. The selectivity of ethylene (C2H4) was increased and
formation of coke was observed at C/O ¼ 0.7 for both catalysts,
leading to catalytic degradation.
3.5.2 Time-on-stream test. The long-term stability on OSRE
performances of LCRO, MLCRO03, and CLCRO02 supported on
Al2O3 was carried out for 48 h. The initial activating
Fig. 4 Temperature programmed reduction profile of: (A) MLCRO0-
03 and (B) CLCRO0-02.
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Fig. 5 The activation temperature of MLCRO (M ¼ Mg, Ca) catalysts
for: (A) selectivity of H2 and (B) selectivity of C2H4 (H2O/ethanol ¼ 3
Fig. 6 Performance in OSRE with: (A) MLCRO01-03/Al2O3 (H2O/
ethanol ¼ 3, GHSV ¼ 1.6 ꢃ 105 hꢀ1, T ¼ 350 ꢂC) and (B) CLCRO01-02/
Al2O3 with H2 selectivity (H2O/ethanol ¼ 3, GHSV ¼ 1.6 ꢃ 105 hꢀ1, T ¼
280 ꢂC).
and GHSV ¼ 1.6 ꢃ 105
h
ꢀ1).
temperatures of this series were 400, 350, and 280 ꢂC for LCRO,
MLCRO03, and CLCRO02, respectively. The main products were
H2, CO and CO2; CH4 and C2H4 were in trace proportions. The
performances are shown in Fig. 8. The average ethanol
conversions for OSRE were 100(3)%, 100.0(8)%, and 100(2)%,
and it remained stable during the 105 h catalytic reaction. The
major product was H2, and the average selectivities were
90(1)%, 90.1(3)%, and 90.1(8)% for LCRO, MLCRO03, and
CLCRO02, respectively. The initial selectivities of CO for these
catalysts were 42.0(1)%, 39.5(7)%, and 39.9(1)%, and the initial
selectivities for CO2 were 54.64(3)%, 57.1(5)%, and 56.4(3)%,
respectively. During the OSRE process, CO was gradually
increased to 48.9(4)%, 49.2(1)%, and 46(1)%, and CO2 was
decreased to 49.0(4)%, 47.7(1)%, and 52(1)% for LCRO,
MLCRO03, and CLCRO02, respectively. Small amounts of CH4,
C2H4, and CH3CHO (<5%) were detected. Compared to the
catalysts supported on Al2O3, the catalysts supported on
corresponding selectivity, including XEtOH and SH , are shown in
2
Fig. 7. The hydrogen selectivity gradually decreased for LCRO/
Al2O3 operating at 400 ꢂC. MLCRO03/Al2O3 and CLCRO02/Al2O3
exhibited stable hydrogen production under the condition of C/
O ¼ 0.6. Low activities were observed for CLCRO02 due to the
reaction being maintained at a lower temperature than
MLCRO03.
We tested the enduring catalytic stability for a MLCRO03
catalyst supported on Al2O3 for 350 h, as shown in Fig. S11.† The
initial ethanol conversion for OSRE was almost 100%. The
major products were H2, CO, and CO2, and small amounts of
CH4 and C2H4 (selectivity less than 5%) were formed. The
activity remained stable during the 48 h experiment without
degradation. However, the selectivity of hydrogen initially
reached over 100% and gradually degraded to ꢁ90% aer
a 100 h test; it then decreased to 80% aer the 320 h test.
Overall, the average hydrogen selectivity was approximately
80%, and the ethanol conversion was around 90% for 350 h.
The enduring catalytic activity of LCRO, MLCRO03, and
CLCRO02 catalysts was supported on lab-made La2Zr2O7 for
La2Zr2O7 remained active and stable under
exothermic reaction.
a vigorous
3.5.3 Characterizations of catalysts used. The catalysts
used were characterized by powder X-ray diffraction. For cata-
lysts supported by Al2O3, the results are shown in Fig. S12(A);†
they indicate that the structure of the LCRO catalyst collapsed
due to the interaction between the catalyst and Al2O3 support-
ing material for the impurity phase of LaAlO3. Under the OSRE
105 h (T ¼ 400 C, GHSV of 160 000 hꢀ1, and C/O of 0.6). The
ꢂ
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Fig. 7 The long-term use of LCRO/Al2O3, MLCRO03/Al2O3, and
CLCRO02/Al2O3: (A) ethanol conversion and (B) H2 selectivity for
400 ꢂC, 350 ꢂC, and 280 ꢂC (OSR, H2O/ethanol ¼ 3 and GHSV ¼ 1.6 ꢃ
105 ꢀ1).
h
reaction, the particles were gradually sintered and carbon
deposition was built up, reducing the catalytic activity. The
powder diffraction patterns of the MLCRO03 catalyst used were
Fig. 8 The long-term use of: (A) LCRO/LZO, (B) MLCRO03/LZO, and
individually collected on the fresh catalyst for 48 h and 350 h, as (C) CLCRO02/LZO for 400 ꢂC (OSR, H2O/ethanol ¼ 3 and GHSV ¼ 1.6
ꢃ 105
h
ꢀ1).
shown in Fig. S12(B).† In general, the PXRD prole of the as-
prepared catalyst exhibited broadened diffraction peaks that
can be indexed with the phase of MLCRO03 and Al2O3. During
the time on stream in OSRE, the structure of the catalyst was
stable for 48 h during the on stream test without exhibiting any
sign of impurities. However, the pattern of the used catalyst for
350 h showed diffraction peaks of LaAlO3. The diffraction peaks
of MLCRO03 were barely observable, indicative of structural
decomposition due to the interaction between the catalyst and
the support. For the LCRO02 catalyst used, the powder pattern
contained the major phase LCRO02 and minor phase of LaAlO3,
indicative of solid–state interaction also having occurred,
causing catalytic deactivation, as shown in Fig. S12(C).† The
structural decomposition could be promoted under high
temperatures. SEM images of fresh and used catalysts for 48 h
and 350 h are shown in Fig. S13.† This shows that as the OSRE
reaction withstands the destruction of the pore structure in
Fig. S13(B),† an increased catalyst size and production of
a carbon ber were observed, as shown in Fig. S13(C).†
Raman prole of the MLCRO03/Al2O3 (350 h) catalyst exhibits
a strong signal between 1200 and 1400 cmꢀ1 contributed by
a carbonaceous species. On the other hand, the Raman prole
of the CLCRO02/Al2O3 (45 h) catalyst showed two broad bands
centered at ꢁ1300 cmꢀ1 and ꢁ1600 cmꢀ1 corresponding to the
disordered carbonaceous and ordered defective graphitic
species. The intensities of the two signals were about the same,
indicating that similar amounts of disordered/ordered carbo-
naceous species were used on the catalyst. The results suggest
that the ethanol dehydration reaction caused the formation of
polymeric carbon via the ethylene precursor, which was due to
the acidic property of the Al2O3 support. It is possible that the
carbon deposition of both catalysts was attributed to the
decomposition and aggregation of the catalysts.
For catalysts supported by LZO in long-term OSRE, as shown
in Fig. 10(B), the weight losses shown by TGA measurements of
used catalysts were 0.6% for non-doped, 0.1% for Mg-, and 0.5%
for Ca-substituted catalysts, respectively. The Raman spectrum
obtained for the used catalysts supported by a LZO catalyst were
almost free of signals from carbonaceous species for Mg- and
Ca-substituted catalysts, as seen in Fig. 11(B) and S17,†
respectively. The results obtained for the TGA and Raman
In Fig. 10(A), TGA measurements on used Al2O3-supported
catalysts showed weight losses of used MLCRO03 and CLCRO02
catalysts of 3.8% and 7.7%, respectively. Raman spectra of fresh
and used catalysts supported by Al2O3 are shown in Fig. 11(A)
and S17(A),† respectively. The MLCRO03/Al2O3 (48 h) showed
a weak and broad signal between 1000 and 2000 cmꢀ1. The
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The element distribution of catalysts aer the OSR of ethanol
was measured by element-mapping analysis using EDX on SEM,
as shown in Fig. S18–S20.† From the images, it can be seen that
the active metals were still evenly distributed on the used
catalysts.
4. Discussion
Based on the EXAFS analyses, the order on the Ru–O length is
LCRO < MLCRO03 < CLCRO02 (Table 2), which reects the
relative strength of the Ru–O bond and the oxidation state of
Run+ ions.59,60 The results of Rietveld renements indicate that
the shi in the O (48f) position affected the coordination envi-
ronments, which caused structural distortion during the metal
ions substitutions on the A-site. The order of bond distances
B–O (B ¼ Ru/Ce) is LCRO < CLCRO02 < MLCRO03, which were
contributed to by the combination of mixed occupied Run+ and
Ce3+/4+ cations. In general, both the EXAFS and Rietveld anal-
yses suggest that the average oxidation for the Run+ in the B-site
of MxLa2ꢀxCe1.8Ru0.2O7ꢀd is less than that of the unsubstituted
LCRO but higher than for RuO2. Since the substitution of an
alkaline earth metal into LCRO also reduced the oxidation state
Fig. 9 The long-term use of: (A) MLCRO03/LZO and (B) CLCRO02/
of the A-site, this would create oxygen vacancies in the lattice
LZO (ATR, H2O/ethanol ¼ 3 and GHSV ¼ 1.6 ꢃ 105
h
ꢀ1).
61
maintaining the charge balance of MxLa2ꢀxCe1.8Ru0.2O7ꢀd
.
Overall, alkaline earth metal substitution affects the relative
composition of Run+ and Ce4+/3+ ions and oxygen vacancies. The
proposed oxidation state for ruthenium ion is consistent with
the TPR experiment for the reduction of temperature on
spectra indicate that carbon deposition on catalyst-supported
LZO was signicantly reduced compared to that of catalysts
supported by Al2O3. It is clear that the supporting material
played an important role regarding these catalysts; it not only
suppressed carbon deposition but also maintained the cata-
lysts' stability during the OSRE process.
MxLa2ꢀxCe1.8Ru0.2O7ꢀd
.
The concentration of Run+ ions with a high oxidation state
plays an important role in the initial oxidation of ethanol during
the OSRE process. In OSRE, the functional Ce3+ and Ce4+
showed that the Ce3+ could modify the oxygen vacancies and
Fig. 10 TGA and DSC profiles of: (A) OSR, MxLa2ꢀxCe1.8Ru0.2O7ꢀd (M ¼ Fig. 11 The Raman spectrum of fresh and used catalysts: (A)
Mg, Ca) on Al2O3 and (B) OSR, on LZO.
MLCRO03/Al2O3 and (B) MLCRO03/LZO.
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react with a methyl group to produce CO2 and suppress the support that would lead to reduced ethylene selectivity. The
formation of CH4. The reduced Ce3+ creates an oxygen vacancy proposed effect on the substituted Mg/Ca was checked with the
and increases oxygen mobility, which promotes the cleavage of relative selectivity of ethylene on the temperature dependent
the O–H bonds (ethanol and water) on catalysts.62,63
OSRE experiments. As shown in Fig. 5(B), the selectivity of C2H4
The activation temperature of these catalysts decreased in for LCRO is the highest compared to MLCRO03 and CLCRO02
the order of MLCRO03/Al2O3 (350 ꢂC) > CLCRO02/Al2O3 (280 in the temperature range of 280–600 ꢂC. The differences on
ꢂC), which is lower than that of LCRO/Al2O3 (400 ꢂC), indicating ethylene selectivity were reduced as the temperature increased,
that the A-site substitution increased the activity of the catalyst. favoring the ethanol decomposition reaction. It is clear that the
Since the MxLa2ꢀxCe1.8Ru0.2O7ꢀd (M ¼ Mg, Ca) catalysts contain catalysts with alkaline metal reduced the yield of ethylene
the same amount of active metal and similar particle sizes, the signicantly; this is an important factor in reducing carbon
effect of a substituted alkaline earth metal is signicant. The deposition in the OSRE process. The surface property of LZO
oxidation states of Run+/Ce4+/3+ ions and oxygen vacancies might exhibit a basic/neutral property that prevents the dehy-
enhance activity and stability during the OSRE process. The dration reaction of ethanol and leads to a reduced carbon
interaction of Run+–O–Ce4+/3+ attributed to ethanol oxidation. deposition rate, as shown in Fig. S22.†
The Run+ and oxygen vacancies may adsorb ethanol and induce
The catalyst support on LZO was tested under the auto
oxidization to form ethyl aldehyde, accompanied by reduced thermal process. The ratios between H2O and C2H5OH, carbon-
Ru/Ce ions. Furthermore, the adsorbed oxygen molecule or to-oxygen, and GHSV were xed, and the temperature control
atom could induce decomposition of the ethyl aldehyde to was stopped when the conversion process was activated. The
produce H2, CO, and CO2.12
results are shown in Fig. 9 and summarized in Table 3. The
The interaction between the catalyst and support was average ethanol conversions were 94.3(6)% and 100(2)% for
investigated by TPR, as shown in Fig. S21.† The TPR experiment MLCRO03 and CLCRO02 catalysts, respectively. Both MLCRO03
showed that the catalysts were active in the temperature range and CLCRO02 catalysts exhibited nearly 87(1)% and 85.4(5)%
of 150–700 ꢂC. The TPR proles of MLCRO03/LZO catalysts initial H2 selectivity which gradually decreased to about
reveal two broad signals in the temperature region at 200– 68.5(2)% and 78(2)% in the 105 h test. The initial selectivity
350 ꢂC and 350–700 ꢂC. Similar results were observed for values of CO were 47.3(9)% and 44.7(3)% which increased to
ꢂ
CLCRO02/LZO for temperature ranges of 150–350 C and 350– 62.4(2)% and 52(3)% during the stream test. The average mole
650 ꢂC. These reduction signals for both catalysts shied to ratios of H2/CO were 1.96 and 2.38 for MLCRO03 and CLCRO02,
a higher temperature compared to the TPR plots for unsup- respectively. For the minor products, the average selectivities of
ported MLCRO03 and CLCRO02, indicative of strong interac- CH4 were 3.44(4)% and 4.0(2)%, and the selectivities of C2H4
tions between the catalyst and support. The proles also show were lower than 1%, respectively. The CH3CHO was observed
that the rst reduction peak for CLCRO02/LZO is lower than for both catalysts, indicating that the catalytic reactions for
that for MLCRO03/LZO. The results suggest that the relative these catalysts favored the dehydrogenation reaction. The
amount of high oxidation state for Run+ ions (n > 4) for selectivity of CH3CHO for MLCRO03 was higher than that of
CLCRO02/LZO is higher than for MLCRO03/LZO. The high CLCRO02, indicative of incomplete ethanol conversion. The
oxidation state of Run+ ions is considered an important factor in sintering of both catalysts was not obvious from the powder X-
initializing the ethanol oxidation reaction to acetaldehyde. The ray diffraction analyses (Fig. S16†). Overall, the MLCRO03 and
results from TPR measurements on catalysts supported by LZO CLCRO02 remained stable for 105 h proceeding with the ATR of
support the low activation temperature on the CLCRO02/LZO the ethanol reaction.
catalyst.
The temperatures of the catalyst during the OSRE and ATR
Studies show that the ethanol dehydrogenation and dehy- experiments were monitored by a K-type thermocouple on the
dration routes on OSRE were dependent on the catalyst and reaction tube close to the reaction spot. According to the results
acidic/basic of the support. It has been reported that catalysts on XPS, EXAFS, and Rietveld analyses, the oxidation states of
using an Al2O3 support favored the dehydration pathway to
produce ethylene and induce carbon deposition. Alkaline metal
oxides like MgO and CaO are known to be basic promoters used
Table 3 Comparison of long-term use between MLCRO03/LZO and
to modify a Al2O3 support.64 The metal/oxide catalyst modied
CLCRO02/LZO in OSR and ATR of ethanol
by MgO/CaO is favored for the dehydrogenation reaction on the
active sites to produce acetaldehyde and hydrogen. The pres-
ence of Mg and Ca also enhances water adsorption on the
catalyst that facilitates the adsorption of oxygen or hydroxyl
radicals from the catalyst. The combined effects lead to sup-
pressed ethylene formation, promoting water adsorption for
WGS reaction, suppressing the formation of coke, and main-
taining the catalyst's stability. The effect of the MgO/CaO-
treated catalyst on the reaction route of SRE suggested that
the alkaline earth metal substituted pyrochlore may exhibit
a similar inuence on mitigating the acidity of the catalyst and
MLCRO03/
LZO
S
SCO
S
S
S
S XEtOH
CH3CHO
H2
CO2
CH4
C2H4
OSRE (%)
ATRE (%)
90.1(3) 45.6(3) 51.8(3) 3.28(3) 0.045(7) 0
100.0(8)
74.4(2) 57.9(3) 36.7(3) 3.44(4) 0.293(5) 1.58(3) 94.3(6)
SCO SCO SCH SC H SCH CHO XEtOH
90.1(7) 44.2(6) 53.0(6) 3.44(8) 0.036(1) 0
CLCRO02/LZO SH
2
2
4
2
4
3
OSRE (%)
ATRE (%)
100(1)
81(1) 51.4(7) 44.5(9) 4.0(2) 0.037(3) 0.11(5) 100(2)
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both catalysts were changed, and oxygen vacancies were created the OSR/ATR of ethanol. The insertion of Mg2+ and Ca2+ into the
in the lattice, which affected the activation temperature on the LCRO structure changed the composition of Ce3+/Ce4+, the
ethanol conversion. For the OSRE process, the temperatures oxidation state of Run+, and oxygen vacancies to maintain
were xed in accordance to their activation temperatures of charge balance. The content of alkaline earth metal cations in
350 ꢂC for MLCRO03 and 280 ꢂC for CLCRO02, respectively. The the pyrochlore structure not only modied the geometric and
selectivity of the major products (H2, CO, CO2) remained close electronic properties, but also affected the active temperature of
for both catalysts during the stream test. The average tꢂemper- the OSRE reaction and suppressed the formation of ethylene.
atures for MLCRO03 and CLCRO02 catalysts were 625 C and The even distribution of Run+ and Ce4+/3+ ions in the structure
605 ꢂC, respectively. The effect of the temperature control is provided stable activity for the OSRE process. The supporting
unfavorable for the WGS reaction on the performance, but material (La2Zr2O7) exhibits strong catalyst support interaction,
rather for methane steam reforming. The observed selectivity of which effectively suppress the sintering of catalyst and deposi-
CO and CO2 was close for both catalysts. However, there was tion of carbonaceous species under oxidative/autothermal
a signicant difference in the ATR reaction in terms of their steam reforming of ethanol.
product distributions. During the autothermal reaction, the
selectivity of CO/CO2 for MLCRO03 was higher/lower than that
of CLCRO02 catalysts, which indicates a low hydrogen produc-
Conflicts of interest
tion on the MLCRO03 catalyst. The activation temperature for The authors declare no competing nancial interest.
CLCRO02 was lower than that of MLCRO03, and a high Ce3+
content on the surface CLCRO02 was benecial to the WGS
reaction to promote CO conversion.65 The average temperatures
Acknowledgements
on the reaction zone under the ATR process for MLCRO03 and We are grateful to Prof. Hiro-o Hamaguchi, Prof. Hirotsugu
ꢂ
ꢂ
CLCRO02 were 425 C and 420 C, respectively. The low oper- Hiramatsu, Miss Chen-Hua Wang, and Miss Szu-Hua Chen
ational temperature for CLCRO02 may be due to its low acti- regarding the Raman experiments provided by the Ultimate
vation temperature and the high catalytic activity of the catalyst. Spectroscopy and Imaging Laboratory (USIL). We thank Dr Jyh-
Although the temperature difference was minor, the combined Fu Lee, Dr Ting-Shan Chan, and Mr Chung-Kai Chang at the
effect of the reduced temperature and high activity of CLCRO02 National Synchrotron Radiation Research Center for the XRD
facilitated more CO conversion that resulted in increased and XAS experiments, and Prof. Hsin-Tien Chiu for the BET
hydrogen selectivity.66,67 Characterizations on the used catalyst measurements. This work is supported by the Ministry of
aer the ATR process also showed minor carbon deposition Science and Technology, Taiwan (Grant No. MOST 107-2113-M-
without signs of structural decomposition.
009-006) and the Center for Emergent Functional Matter
A comparison of the ethanol conversion and hydrogen Science of National Chiao Tung University from The Featured
production observed over the OSRE process is summarized in Areas Research Center Program within the framework of the
Table S5.† Some metal/metal oxide catalysts with non-noble and Higher Education Sprout Project by the Ministry of Education
noble metal have been reported as possessing high activity in (MOE) in Taiwan.
OSRE processes. The yields of H2 production were around 41–
69%.25 Metal oxides as catalysts, like perovskite and spinel
phases, were also reported with hydrogen yields of 45–68%.38,41
References
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CLCRO02/LZO under OSRE were 54.0%, 53.9%, and 54.3%,
respectively, which are comparable to the catalyst with Rh metal
in the OSRE process. Under the auto-thermal condition,
CLCRO02/LZO exhibits a high activity with a 100% ethanol
conversion and 49% yield of hydrogen. The H2 yield of the
CLCRO02 catalyst was more than that of Li- and Mg substituted
catalysts due to its low temperature operation that promoted
the WGS reaction to produce additional hydrogen. This study
and our previous studies clearly demonstrate that the metal-
substituted pyrochlore catalysts with mixed-valence states of
active metals facilitated the C–C and C–H bond due to the
dynamic redox reactions. The LZO supported catalyst not only
induced metal-support interaction but also maintained the
catalyst's stability in the OSRE process.
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