Enhanced oxygen exchange capacity in nano-structured vanadia-ceria multi-phase oxygen carriers for solar thermal fuel production

Article abstract of DOI:10.1039/c9ta06471k

Developing an efficient redox material is a fundamental and crucial step in sustainable hydrocarbon fuel production via solar energy-driven thermochemical redox cycles. Vanadium being the 20th most abundant element in the earth's crust, with excellent cat

Full text of DOI:10.1039/c9ta06471k

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Enhanced oxygen exchange capacity in nano-
structured vanadia–ceria multi-phase oxygen
carriers for solar thermal fuel production†
Cite this: J. Mater. Chem. A, 2019, 7,
27347
b
a
a
Asim Riaz,a Muhammad Umair Ali,
Wojciech Lipinski
*
and Adrian Lowe
*
´
Developing an efficient redox material is a fundamental and crucial step in sustainable hydrocarbon fuel
production via solar energy-driven thermochemical redox cycles. Vanadium being the 20th most
abundant element in the earth's crust, with excellent catalytic properties combined with other oxide
materials can potentially be a suitable candidate for efficient solar fuel production. Here, we report a first
demonstration of a facile strategy to effectively utilize vanadium oxide with ceria for thermochemical
H2O and CO2 splitting with superior oxygen exchange capacity and efficient H2 and CO yields of 68%
more than the pure nano-structured ceria. We observe a synergic effect in oxygen exchange capacity
and ion mobility in the vanadia–ceria binary phase system, where vanadia provides more reducing states
for the hydrocarbon oxidation, while ceria acts as an oxygen buffer for the re-oxidation of reduced
vanadia. We demonstrate that the addition of a carbon dioxide step between the methane partial
oxidation and H2O splitting step activates the oxygen carrier by creating sufficient oxygen vacancies for
the controlled oxidation of methane and avoiding methane cracking for over 100 cycles. The
extraordinarily high oxygen exchange capacity observed in 25% V–CeO2 resulted in superior and pure H2
yields of 220 mL gꢀ1 during the water splitting step of sequential water and carbon dioxide splitting,
while maintaining a H2/CO ratio close to 2. Finally, these findings suggest that the facile combination of
the extraordinary catalytic properties of vanadia and superior oxygen ion mobility of ceria can be
a powerful approach for an efficient and effective solar thermochemical fuel production.
Received 17th June 2019
Accepted 1st November 2019
DOI: 10.1039/c9ta06471k
rsc.li/materials-a
carriers is a key factor in determining the fuel yield, which
directly depends upon the operating temperature and atmo-
sphere. Inert gas sweeping (IGS)2,5–10 and methane partial
1. Introduction
Direct production of synthesis gas (CO/H2) via solar energy-
driven thermochemical splitting of carbon dioxide and water
is a promising renewable energy source for fuel consuming
sectors. However, extensive research is required to develop
methods to dissociate CO2 and H2O into CO and H2 at
temperatures less than 2000 K for sustainable and economical
renewable fuels. In addition, gas separation problems at such
high temperatures are an ever existing challenge.1,2 Thermo-
chemical splitting of CO2 (CDS) and H2O (WS) via two step
reduction–oxidation (redox) cycles mediated by active metal
oxides as oxygen carriers has been extensively studied and
proven to be highly effective in mitigating the above mentioned
challenges whilst achieving large scale production of synthesis
gas.3,4 The quantity of cyclic reduction–oxidation of oxygen
oxidation (MPO)1,11–17 are the most commonly studied metal
oxide reduction methods, where MPO promotes the reduction
of metal oxide at temperatures as low as 1073 K when compared
to the operating temperature of IGS, i.e., T > 1673 K.9,12,18,19 This
MPO-driven metal oxide reduction ensures high solar-to-fuel
efficiencies of up to 54%, compared to 19% in IGS
cycles.11,19,20 Consequently, synthesis gas can be produced at an
industrial scale with adjustable H2-to-CO ratios required for
gas-to-liquid fuel conversion by
process.21
a Fischer–Tropsch (FT)
High oxygen exchange capacity with fast fuel production
rates and high cyclability under harsh operation conditions are
challenging to achieve. Ceria has emerged as a highly attractive
redox material choice for thermochemical syngas production
due to its fast kinetics and fuel selectivity.1,3,22–24 However, ceria
possesses low oxygen exchange capacities (below 0.25 molO/
molCe) and is highly dependent on the geometrical structure
including surface area, pore size and particle size for efficient
oxygen ion mobility.1,3,25–30 Ceria supported on Al2O3, SiO2, MgO
etc. has reportedly improved its initial oxygen exchange capacity
with structural and morphological stability over several cycles at
aResearch School of Electrical, Energy and Materials Engineering, The Australian
National University, Canberra, ACT 2601, Australia. E-mail: wojciech.lipinski@anu.
edu.au; adrian.lowe@anu.edu.au; Tel: +61 2 612 57896; +61 2 612 54881
bDepartment of Materials Science and Engineering, College of Engineering, Peking
University, Beijing 100871, China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c9ta06471k
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temperatures as high as 1273 K.31 On the other hand, doping prepared by dissolving cerium(III) nitrate hexahydrate (CeN3O9-
ceria with transition metals (Mn, Cu, Zr, Fe etc.) results in stable $6H2O, Aldrich) in ethanol and stirred until complete dissolu-
reaction rates due to enhanced surface and bulk oxygen tion. For the precursor solution of xVCeO2, separate solutions of
mobility and reactivity.14,15,32–36 However, doped ceria has lower stoichiometric concentration of vanadium(V) oxytripropoxide
reaction kinetics than pure ceria, which results in overall low (C9H21O4V, Aldrich) and CeN3O9$6H2O in ethanol were mixed
fuel production performance.33
and stirred at room temperature until complete homogeneous
The catalytic nature of vanadia has been utilized for depol- solutions were achieved. The precursor solutions were dried at
lution reactions via selective oxidation of hydrocarbons.37,38 353 K and then red at 1173 K for 2 h. The ramp rate for drying
Kodama et al. reported V2O5 as an oxygen carrier, yielding and ring was kept constant at 3 K minꢀ1. The synthesis sche-
25.9% CO, 5.1% CO2 and 83.5% and 16.5% selectivity for CO matic for these oxygen carriers is shown in Fig. 1a.
and H2 respectively, during MPO at 1173 K.39 Vanadia-
supported structures on SiO2, TiO2, Al2O3, MgO and CeO2
have extensively been studied for hydrocarbon oxida-
2.2. Thermochemical cycling
tion.37,38,40–42 A combination of the high oxygen ion mobility of The cycling thermochemical redox performance of pure nano-
CeO2 and the high reduction extent of surface vanadia species structured ceria and CV25 was evaluated in a vertical-tube
near the ceria makes the Ce–V–O system an attractive candidate reactor placed inside an infrared gold image furnace (P4C-
for hydrocarbon oxidation. In addition, vanadia–ceria stabilizes VHT, Advance Riko). 250 ꢁ 5 mg powder samples were placed
the surface Ce3+ sites by moderating the valence change ability between 2 ꢁ 0.3 mm thick layers of highly porous and
of the Ce3+/Ce4+ pair with cyclic formation of CeVO4/CeVO3 in temperature resistant alumina-based brous wool (AlBF-1, 97%
the redox activity.41 Thus, the study of Ce–V–O systems is vital Al2O3 and 3% SiO2, ZIRCAR). The samples and wool layers were
for high temperature reforming of methane for solar fuel centrally located in the alumina tube to ensure uniform heat-
production with high and stable reaction rates.
ing, as shown in Fig. 1b. Powder mass was recorded before and
Here, we report a method for stabilizing the reaction rates aer cycling to monitor the weight loss at the appropriate part
and structural parameters of ceria by utilizing the vanadia–ceria of the process. A weight loss of 15 ꢁ 2 mg was observed aer
binary phase system, synthesized by a facile liquid phase cycling which may be due to cycle chemistry and possible le-
precursor combustion method. Compared to the conventional over powder on the inner walls of the alumina tube. Lower
impregnation method, liquid phase precursor combustion material activity was observed for sample masses greater than
methods deliver better control of particle size, morphology and 300 mg because the small diameter of the alumina tube limits
dispersion of vanadia particles on the ceria. Structural studies the mass transfer in solid–gas reactions during redox cycling.
of Ce–V–O systems revealed that surface vanadia species on Gas ow rates are regulated with ow rate controllers (F201CV,
ceria play an important role in achieving stable reaction rates Bronkhorst), operated using an in-house developed LabVIEW
over 100 cycles during H2O and CO2 splitting at 1173 K. (National Instrumental™) program code which is integrated
Oxidation of Ce–V–O during CDS, WS, WS–CDS and WS–CDS with mass ow controllers and pneumatically actuated valves
aer MPO also reveals interesting facts about metal–CO2 and (1315R, Swagelok). The sample temperature was monitored
metal–H2O interactions. We observed that sequential WS and using a B-type thermocouple sealed in an alumina sheath,
CDS cycles enhanced the hydrogen purity and production yield located directly under the packed samples. The composition of
during the WS step. In addition, higher and more stable MPO the product gases was recorded using a quadrupole mass
rates were achieved when compared to pure nano-structured spectrometer (OmniStar™ GSD 320, Pfeiffer Vacuum).
ceria. We also observed that the combination of vanadia and
The tubular reactor was purged with argon (Ar grade 5.0)
ceria greatly stabilized the microstructure and specic surface with a ow rate of 500 mL minꢀ1 before every reduction and
area of pure nano-structured ceria. We believe that this study oxidation reaction, to remove any surface gas species (H2, CO
may be useful in the development of new, efficient oxygen and CO2) from the sample/tube/tting surfaces, which may
carriers for upscale production of solar fuel via thermochemical potentially affect the product gas composition. The reactor was
redox reactions. Specically, the sequential WS and CDS cycling heated in an inert atmosphere (Ar, 500 mL minꢀ1) from
can be utilized on the wide range of oxygen carriers reported ambient temperature to an optimized isothermal operating
earlier, e.g., recently developed Ce-doped Mn3O4 (ref. 43) and temperature of 1173 K at a ramp of 100 K minꢀ1. The powder
La-doped strontium manganates.10
samples were reduced in a gas ow of 20 mL minꢀ1 CH4 (grade
4.5), diluted with 230 mL minꢀ1 Ar ow. CO2 splitting by
reduced metal oxide samples was carried out in the presence of
4% vol of CO2 (grade 4.5) with a ow of 10 mL minꢀ1 diluted
with a 240 mL minꢀ1 Ar ow. For water splitting, steam vapor
2. Experimental
2.1. Materials preparation
Ultra-ne, nano-structured CeO2 and xVCeO2 (x ¼ 0%: CeO2, was generated at 368 K in a bubbler lled with DI water. An Ar
25%: CV25, 50%: CV50, 75%: CV75, 100%: V2O5) nanoparticles ow of 30 mL minꢀ1 passed through the bubbler to carry the
were synthesized by a facile liquid phase precursor decomposi- steam vapors and was further diluted by an Ar ow of 220
tion method.44 The synthesis method offered upscale production mL minꢀ1 before delivering the vapors into the reactor tube.
of nanoparticles with good control on the particle size and The total gas ow during the reduction and oxidation step was
morphology. Briey, the precursor solution for CeO2 was kept constant at 250 mL minꢀ1
.
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Fig. 1 Schematic illustration of the synthesis procedure: (a) flow diagram of the synthesis of vanadia–ceria mixed-metal oxide nano-powders. (b)
Schematic diagram indicating the tubular reactor set-up utilized for performing redox cycle experiments, mass-spectrometer used to analyze
the reactant and product gases and the temperature program control.
Inert gas sweeping was done with argon at a ow rate of 500 K-Alpha source with a spot size of 200–900 mm was operated at
mL minꢀ1 before and aer each reduction and oxidation step a power of 180 W (15 kV ꢃ 12 mA). The total pressure in the
and repeated for each cycle. The sequence and duration of the chamber was maintained at around 10ꢀ8 mbar. Two different
gas ow during CO2 and H2O cycles was Ar (5 min)/CH4 (20 spots on a 5 ꢃ 5 mm area of sample with an estimated depth of
min)/Ar (5 min)/CO2 or H2O (10 min). In the case of coupled 4 mm were scanned with an energy of 160 eV. The pass energy
H2O and CO2 splitting cycles the sequence for MPO–WS–CDS was reduced to 40 eV for high resolution spectra. The data
was Ar (5 min)/CH4 (20 min)/Ar (5 min)/H2O (7 min)/Ar (5 min)/ processing was performed using CasaXPS processing soware
CO2 (7 min) and for MPO–CDS–WS the gas ow sequence was Ar version 2.3.18 (Casa Soware Ltd, Teignmouth, UK). The
(5 min)/CH4 (20 min)/Ar (5 min)/CO2 (7 min)/Ar (5 min)/H2O (7 aliphatic carbon peak C 1s at 284.8 eV was used as a reference
min). The redox performance of nano-powders was tested over for the binding energies of elements acquired in the survey
10 preliminary cycles, followed by a stability test over 100 cycles. spectra.
Changes in the structure and chemical nature of the oxide
Structural analysis of the redox materials was carried out by
samples were studied aer the 1st reduction, 10 cycles and 100 utilizing a Raman imaging microscope (Renishaw Plc, model
cycles during the CO2 and H2O splitting phase.
2000) coupled with an Olympus BH2 microscope. The micro-
scope is equipped with a motorized XYZ stage, air-cooled CCD
detector and a CCD camera. The samples were excited by using
a 785 nm NIR laser source and spectra were recorded in
a Raman shi range of 100–1200 cmꢀ1. The samples were
exposed for 20 s with an accumulation of 3 and the power was
adjusted from 0.01% to 0.5% (<6 MW) according to the sample
response.
2.3. Materials characterization
Powder samples were characterized before and aer the ther-
mochemical cycles. X-ray diffraction (XRD) analysis was carried
out using a D2 phaser diffractometer (Bruker). The samples
˚
were scanned using a Cu Ka (1.54 A) radiation source with an
operating voltage of 30 kV and a current of 10 mA. XRD patterns
were recorded with a scanning rate of 0.75ꢂ minꢀ1 in a 2q range
of 10–80ꢂ at an increment of 0.02ꢂ. Crystalline domain size was
determined by applying the Scherrer equation on the most
intense peaks obtained in the XRD patterns. Phase quantica-
tion was carried out by the Rietveld renement technique
utilizing Match 3 V. 3.8.1.143 (Crystal Impact) soware.
X-ray photoelectron spectroscopy (XPS) analysis was per-
formed using a Thermo Fisher ESCALAB 250Xi X-ray photo-
electron spectrometer (XPS) microprobe equipped with a 180ꢂ
double focusing hemispherical analyzer. A monochromatic Al
A eld emission scanning electron microscope (FESEM, Zeiss
Ultraplus) was utilized to observe the morphology of nano-
structured powders before and aer thermochemical redox
cycles. Particle size distribution and lattice plane spacing were
determined using
a high-resolution transmission electron
microscope (HR-TEM, JEOL 2100F) with an operating voltage of
200 kV. The samples were deposited onto lacey carbon-coated
200 mesh copper grids. Primary particle size and lattice plane
spacing were acquired via the ImageJ image processing soware.
Energy dispersive X-ray spectroscopy mapping (EDX) was
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performed via the scanning transmission electron microscopy Micromeritics. Powders were degassed at 523 K under vacuum
(STEM) mode on a JEOL 2100F, to acquire information about the (0.1 mBar) for 3 h prior to the measurements.
chemical composition of 25% VCeO2 (CV25) before and aer
MPO–WS–CDS cycles. The area of interest was scanned using the
STEM probe to obtain elemental mapping and EDX spectra.
3. Results
Information about the specic surface area and porosity of
nano-structured powder samples was acquired via N2 adsorp-
tion–desorption isotherms at 7 K by utilizing a TriStar II,
A structural comparison of as-prepared pure nano-structured
ceria and CV25 nanoparticles is shown in Fig. 2a and b, repre-
senting a similar nanoscale morphology consisting of agglom-
erated semi-spherical particles with an average diameter of 15 ꢁ
Fig. 2 Chemical and morphological characterization of pure nano-ceria and CV25: HRTEM images representing the size and morphology of as-
prepared (a) pure ceria and (b) CV25, nanoparticles. Inset in (b) shows d-spacings of vanadium and cerium present in CV25. EDS spectra of (c) as-
prepared and (d) reduced CV25 nanoparticles. STEM EDS (e) grey scan and (f) overlay mapping of reduced CV25 showing the distribution of Ce
(green), V (blue) and C (red) in samples after methane partial oxidation.
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nm. High-resolution transmission electron microscopy
Journal of Materials Chemistry A
3
Evolution of the CeVO4 phase can be observed from the
(HRTEM) analysis of samples reveals similar semi-spherical growth of shoulder peaks at 2q ¼ 48.2ꢂ and doublets at 2q ¼
nanoparticles exposing two distinct lattice spacings of 32.87ꢂ with the increase in V greater than 25%. Controlled
0.437 nm and 0.307 nm corresponding to the (010) and (111) conversion of CeO2 and V2O5 into CeVO4 was observed up to
planes of vanadia and ceria, respectively (Fig. 2b). These nd- 25% V loading, where a mixture of CeO2 and CeVO4 with a ratio
ings conrm the two distinct phases of ceria and cerium of CeO2 : CeVO4 (ꢄ3) is achieved. However, a further increase in
vanadate, observed in the XRD patterns with crystallite sizes of vanadium concentration results in a drastic decline in the CeO2
12 nm and 13 nm for pure nano-structured ceria and CV25 concentration by 96% with 75% V addition. This can be visu-
respectively.
alized by the ratio of CeO2 to CeVO4 intensities (ICeO /ICeVO )
2 4
Energy dispersive X-ray spectroscopy (EDX) analysis reveals evaluated from XRD patterns of as-prepared vanadia–ceria
distinct emission energy peaks of Ce, V and carbon content systems, presented in Fig. S2.† Large quantities of V2O3 up to
before and aer methane partial oxidation of CV25, as shown in 46.1% are also observed along with only 17% CeVO4 with 75% V
Fig. 2b and c. The SRTEM EDS mapping of the reduced CV25 addition. Thus, a V–Ce–O system with 25% vanadia loading is
sample represents the elemental distribution of vanadium, investigated as a suitable candidate material to study the
cerium and carbon aer the methane partial oxidation reaction, combined effect of CeO2 and CeVO4 phases for solar fuel
as shown in Fig. 2e and f. The intensity of the carbon peak is production performance evaluation.
higher in the reduced CV25 sample than in the as-prepared
CV25 sample, which indicates possible carbon deposition on V3+ at high temperatures due to the leaching of oxygen from
the metal oxide. CeVO4, resulting in additional peaks of the CeVO3 phase in
Methane partial oxidation promotes the conversion of V5+ to
XRD patterns of as-prepared nanoparticles are presented in CV25, as shown in Fig. 3b. Lower CeO2 content is also observed
Fig. 3a. Pure ceria consists of a dominant peak of the triclinic in reduced CV25, which is evident from the decline in the CeO2
crystal structure with a low-angle shoulder of cubic CeO2. As- peak intensity near 33ꢂ. Aer 10 consecutive MPO–CDS and
prepared nano-structured ceria is calcined at 600 ꢂC and MPO–WS–CDS cycles, oxygen recovery is highest resulting in up
900 ꢂC to further investigate this behavior. Low temperature to 65% of CeO2 and 25% CeVO4. However, CV25 aer MPO–WS
calcination resulted in amorphous-like triclinic structures (i.e. and MPO–CDS–WS redox cycles contains a signicant amount
very small grain sizes) while at the higher calcination temper- of CeVO3 phase up to 23.7%, which indicates an incomplete
ature, a more dened crystalline triclinic ceria is seen with oxidation of CeVO3 to CeVO4. This phenomenon can be related
shoulder peaks of cubic ceria. A comparison of XRD patterns to the carbon deposition on oxygen carriers which hinders the
obtained from calcined, commercial, reference triclinic and re-oxidation of reduced species, resulting in few oxygen species
cubic ceria structures is presented in Fig. S1 in the ESI.† Addi- in the material being available for an efficient methane
tion of vanadium results in an increase in the CeVO4 phase reforming reaction. Consequently, it results in further methane
with V concentrations greater than 25%, where high vanadium cracking with high H2 products and low CO emission, resulting
contents promote the conversion of CeO2 and V2O5 into CeVO4 in a high H2/CO ratio. This is evident from the syngas produc-
via the following solid state reaction.40
tion rates and yields of MPO–WS cycles. However, addition of
a CDS step aer WS oxidizes carbon from the surface of the
oxygen carrier which reduces methane cracking during the MPO
step. This can easily be veried by comparing the H2/CO ratios
1
V2O5 þ 2CeO2/2CeVO4 þ O2
(1)
2
Fig. 3 Structural analysis of vanadia–ceria multi-phase systems before and after redox cycles: (a) XRD patterns representing the structural
evolution in pure nano-ceria with the addition of vanadium xVCeO2 (x ¼ 0%, 25%, 50%, 75% and 100%) with discrete phases of CeO2, CeVO4,
V2O5 and CeVO3. (b) Phase analysis of CV25 before and after thermochemical redox cycling highlighting the reduction and oxidation behaviour
of CeO2 and CeVO4 during fuel production.
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of CV25 during MPO–WS, MPO–WS–CDS and MPO–CDS–WS. WS cycles, indicating high CeVO3 contents, which supports the
High fuel production yields during MPO–CDS and MPO–WS– XRD ndings. The intense O 1s peak at 529.7 eV is observed due
CDS cycles with high fuel selectivity are achieved when to the presence of surface oxygen species (Fig. 4b). Two peaks
compared to that of the MPO–WS and MPO–CDS–WS cycles, associated with V 2p3/2 and V 2p1/2 in pure V2O5 and CV25
which will be explained later.
samples were observed at ꢄ517.2 eV and ꢄ524.1 eV, respec-
The surface of the CV25 and ceria samples was further tively. These peak positions correspond to the V5+ oxidation
investigated by X-ray photoelectron spectroscopy (XPS), as pre- state of vanadium.46 A shi in the V 2P3/2 peak towards low
sented in Fig. 4. The oxidation states of cerium and vanadium binding energy is observed in CV25 samples aer reduction and
ions were determined from the XPS spectra. The presence of all other redox cycles (Fig. 4d). This shi corresponds to the
tetravalent cerium ions can be conrmed from the Ce 3d peaks presence of V3+ species along with V5+ due to the co-existence of
observed in the pure nano-structured ceria and CV25 samples, CeVO4 and CeVO3 phases, as discussed in the XRD section.
as shown in Fig. 4a. The Ce 3d 3/2, 5/2 peaks are shied towards
XPS data are utilized to quantify the atomic% of Ce, V and
higher binding energies with addition of V, i.e. CV25, which oxygen in terms of Ce/V and V/O ratios, as shown in Table 1. The
corresponds to the presence of a 3+ oxidation state in the Ce/V ratio showed a decline of 3–5% in the CV25 sample aer
cerium.45 The intensity of the dominant peak at ꢄ917.0 eV the redox cycles, indicating the loss of vanadium due to possible
associated with the Ce4+ ions increases aer reduction of CV25 volatilization of vanadium at high temperatures.47 Segregation
indicating the formation of CeVO3, (Fig. 4c). The Ce4+ peak at of CeO2 and CeVO4 phases may also be a reason for a non-
ꢄ917 eV in the CV25 sample aer MPO–WS cycling is more homogeneous vanadium distribution, resulting in less vana-
intense as compared to that of the MPO–CDS and MPO–CDS– dium in the area scanned using the XPS probes. No signicant
Fig. 4 Surface analysis of vanadia–ceria mixed-metal oxides: (a) XPS spectra representing the Ce4+/Ce3+ oxidation state in pure nano-ceria and
CV25; (b) XPS spectra representing the V5+ oxidation state and surface oxygen in pure vanadia and CV25. XPS spectra of the CV25 sample after
MPO, MPO–CDS, MPO–WS, MPO–WS–CDS and MPO–CDS–WS redox cycles representing change in (c) Ce 3d and (d) V 2P signature peaks.
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Table 1 Elemental quantification (atomic %) of surface vanadium,
cerium and oxygen species in CV25 samples before and after redox
cycles. The elemental data are obtained from XPS analysis and quan-
tified including Ce, V, O and C
bands related to the V–O–Ce and V–O–V bridging modes of
CeVO4 at 787.84 cmꢀ1 and 799.21 cmꢀ1, along with the bands at
861.25, 379.54 and 259.19 cmꢀ1 conrm the formation of
CeVO4. The disappearance of the V]O band at 970–1030 cmꢀ1
and a decline in the intensity of the CeO2 band at 461.87 cmꢀ1
are also related to the conversion of CeO2 and V2O5 into CeVO4
by the solid–state reaction (eqn (1)).
A decrease in the ratios of the signature peaks of CeVO4 at
861 cmꢀ1 to CeO2 at 461 cmꢀ1 is observed aer the redox cycles,
which indicates a decrease in CeVO4 concentration and
formation of CeVO3. This ratio in CV25 is lowest aer MPO–WS
and highest aer MPO–CDS cycling. The XPS ndings on
possible vanadium loss and oxygen depletion in CV25 samples
aer MPO–WS and MPO–CDS–WS redox samples can be veri-
ed by the decline in the signature peak of the V]O species at
122.25 cmꢀ1. In contrast, this peak is most intense aer
reduction and MPO–CDS, followed by MPO–WS–CDS redox
cycling, indicating high concentrations of available surface
vanadia species for methane partial oxidation reaction. This
nding supports the better reforming performance during
MPO–CDS and MPO–WS–CDS redox cycling as compared to that
of MPO–WS and MPO–CDS–WS redox cycling.
Oxygen exchange capacity and redox rates of pure nano-
structured ceria and CV25 were evaluated via 10 consecutive
two-step thermochemical redox cycles. The reduction and
oxidation behaviors and performance in terms of fuel produc-
tion rates and yields were evaluated during MPO–CDS, MPO–
WS, MPO–WS–CDS and MPO–CDS–WS redox cycling. Methane
conversion was calculated using eqn (E1) (ESI†), to investigate
the material's behavior during the methane partial oxidation
reaction. Table T1 (ESI†) shows the methane conversion by pure
ceria and CV25 samples during the MPO–CDS, MPO–WS, MPO–
WS–CDS and MPO–CDS–WS redox cycle tests performed in this
study. CV25 clearly shows a higher methane conversion when
compared to that of pure ceria in all four-cycle tests. Comparing
the methane conversion seen in the four types of cycle tests, it is
highest during MPO–CDS followed by the MPO–WS–CDS redox
Elemental
concentration (%)
Elemental
ratios
Sample ID
V
Ce
30
O
V/O
Ce/V
CeO2
—
55
—
—
V2O5
CV25
35.24
20.02
19.11
18.3
16.66
16.07
13.57
47.26
46.67
44.22
41.13
34.98
41.81
35.76
0.75
0.62
0.65
0.4
0.55
0.57
0.68
12.5
12.42
7.28
9.22
9.23
9.29
0.43
0.43
0.44
0.48
0.38
0.38
Reduced CV25
CV25 aer MPO–CDS
CV25 aer MPO–WS
CV25 aer MPO–WS–CDS
CV25 aer MPO–CDS–WS
change in the overall values of V/O ratios is observed because
both vanadium and oxygen species show a decline on the CV25
surface aer cycling. However, the depletion of surface oxygen
species is higher in CV25 aer MPO–WS and MPO–CDS–WS
when compared to that of the MPO–CDS and MPO–WS–CSD
redox cycles. The Ce/V ratio is also subjected to a decrease in Ce
and V species aer redox cycles. The lower content of cerium
suggests that vanadium-containing phases dominate the CV25
surface reactions during redox cycles when compared to ceria.
The nature of the interactions between ceria and vanadia is
further investigated from the information obtained by Raman
spectroscopy, as shown in Fig. 5. Pure nano-structured ceria and
CV25 were analyzed before and aer redox cycles, as shown in
Fig. 5a and b, respectively. A Raman spectrum of as-prepared
pure vanadia nanoparticles was used as a reference. The pres-
ence of ceria phase in pure ceria and CV25 is conrmed by the
band at 464.88 cmꢀ1, corresponding to the F2g symmetry of the
cubic structure. The Raman bands at 279.14 and 711.14 cmꢀ1
validate the formation of crystalline V2O5 nanoparticles along
with the surface vanadia species (V]O) at 122.25 and 970–
1030 cmꢀ1, in pure nano-vanadia samples.48,49 Two Raman
Fig. 5 Structural characterization of oxygen carriers: (a) Raman spectra of as-prepared pure nano-ceria, pure vanadia and CV25 nanoparticles,
representing the presence of fingerprint peaks of CeO2, V2O5 and CeVO4; (b) Raman spectra of CV25 nanoparticles as-prepared and after MPO,
MPO–CDS, MPO–WS, MPO–WS–CDS and MPO–CDS–WS redox cycles.
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cycles and lowest in the MPO–WS cycles, indicating inferior result in lower CO selectivity. Carbon deposition on the oxygen
methane reforming during MPO–WS cycling. carriers can be associated with this, which may possibly promote
High rates of syngas production during the reduction step of methane cracking instead of reforming of methane, resulting in
MPO–CDS by CV25 are observed with an average rate of 12.96 high H2/CO ratios being obtained during MPO–WS cycling.
mL minꢀ1 gꢀ1 and 4.88 mL minꢀ1 gꢀ1 for H2 and CO respectively,
Reaction kinetics play a critical role in the H2 and CO redox
Fig. 6a and c. The calculated H2/CO ratio was close to 2 with an rate proles, which are greatly affected by the amount of reac-
average value of around 2.15, indicating controlled methane tant gasses reaching the oxygen carrier. In a xed bed reactor
partial oxidation (Table T2†). In contrast, the average production design, the homogeneous concentration of reactant gases
rates for H2 and CO produced by pure nano-structured ceria were reaching the oxygen carriers is limited by the gas ow through
only 7.12 mL minꢀ1 gꢀ1 and 2.47 mL minꢀ1 gꢀ1, respectively. solid materials, which greatly inuences the CH4-to-O2 ratios. It
However, methane cracking is observed aer the rst cycle of the is observed that in the beginning of the MPO reaction, small
MPO–WS process, resulting in signicant increases in the H2/CO amounts of CO2 are produced due to low CH4 concentrations, as
ratio reaching up to a value of around 4.59 and 3.38 in pure nano- shown in real time gas proles obtained from the mass spec-
structured ceria and CV25 respectively. Moreover, the water trometer (Fig. S4†). As the required concentration of CH4 is
splitting rate was close to 10 mL minꢀ1 gꢀ1 – more than double achieved and stabilized, no CO2 is observed, and reaction rates
that of the pure nano-ceria ca. 4.25 mL minꢀ1 gꢀ1, as shown in of H2 and CO are increased.
Fig. S3a and S3c.† The CO selectivity S(CO) was determined from
Sequential WS and CDS reaction tests aer the MPO step
the cycle data, by utilizing eqn (E2) in the ESI.† A higher value of were performed to investigate the oxidation behavior of these
S(CO) is obtained for CV25 during the MPO–WS–CDS cycles when nano-structured oxygen carriers in CO2 and H2O as oxidizing
compared to that of pure nano-structured ceria. Elevated media (Fig. 6b and d). In the rst attempt, WS–CDS cycles were
production rates of CO2 in the MPO step of MPO–WS cycling performed aer the MPO reduction of oxygen carriers, while
Fig. 6 Thermochemical redox cycle performance evolution in terms of production rates of H2, CO and CO2 during MPO–CDS and MPO–WS–
CDS cycles on: (a) and (b) pure nano-structured ceria; (c) and (d) CV25.
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ushing with Ar for 5 min at 500 mL minꢀ1 before each whole duration of each reduction and oxidation step of every
oxidation step. The reduction rate prole for both the oxygen cycle is presented in Tables T2 and T3.† Evidently, CV25
carriers changed aer the rst cycle, where an increase in the H2 produced nearly double the yield of H2 and CO when compared
production rate was observed aer 4 min of reduction and to pure nano-structured ceria, with a near ideal H2/CO ratio of
reached its maximum value aer the maximum rate of CO was 2.12 and CO selectivity up to 90.14%, during MPO–CDS redox
achieved. A signicant improvement in the H2/CO ratio is cycles. Despite the high H2 (78.2 mL gꢀ1) and CO (19.9 mL gꢀ1
)
achieved in the MPO step of the MPO–WS–CDS redox cycle yields produced by pure ceria, the H2 yield during oxidation is
when compared to simple MPO–WS cycles. The highest calcu- still lower than that produced by CV25. This is possible because
lated H2/CO ratio from CV25 was 3.1 at the 5th cycle, subse- of less oxygen vacancies produced during the methane partial
quently decreasing and becoming stable at around 2.21. oxidation step of the MPO–WS redox cycles, which results in low
However, in the case of pure nano-structured ceria, the ratio H2 yields in the H2O splitting reaction.
appeared to be unstable and uctuated from 3.4 to 2.8. The H2
Introducing a CDS step aer the WS reaction improves the
production rates of pure nano-structured ceria were observed to overall methane reforming process and lowers the H2/CO ratio.
be unstable during the WS step of the MPO–WS–CDS cycles, In addition, the H2 yield during the WS step is higher and more
despite achieving rates as high as 10.21 mL minꢀ1 gꢀ1 for a few stable in MPO–WS–CDS cycling than in the simple MPO–WS
cycles. In contrast, CV25 exhibited high and stable rates of water redox cycles. However, utilizing WS as a complementary step for
splitting reactions up to 15.12 mL minꢀ1 gꢀ1, with considerable CDS in MPO–CDS–WS cycling exhibits inefficient reduction of
stability throughout the 10 cycles. This nding indicates oxygen carriers in the MPO step, as observed in the MPO–WS
possible structural stability in pure nano-structured ceria cycles. A considerable amount of H2 up to 3.4 mL gꢀ1 is produced
induced by the surface vanadia particles, which additionally by CV25 in the WS step along with a small amount of CO2
improve the ion mobility in the structure.
(around 1.1 mL gꢀ1). However, a drop in the CO yield during the
The counter effect of steam on methane reforming is studied MPO and CDS step of the MPO–CDS–WS cycle is observed in pure
by performing MPO–CDS–WS cycling (Fig. S3b and S3d†). Aer nano-structured ceria when compared to the simple MPO–CDS.
the rst cycle, the H2 production rate increased signicantly
The average total CO yield calculated for pure nano-structured
with a H2/CO ratio reaching 6.3 and 4.8 for pure nano- ceria and CV25 during the reduction and oxidation reactions is
structured ceria and CV25, respectively. The CO production shown in Fig. 7. Methane partial oxidation by CV25 resulted in H2
rate during the MPO step did not undergo a substantial increase and CO yields of up to 130.12 mL gꢀ1 and 87.34 mL gꢀ1
,
for CV25 and a considerable amount of CO2 was observed. respectively, which is more than double of that produced by pure
However, deterioration in the CO production rates was observed ceria during MPO–CDS redox cycles. A decline in CO yields by
in pure nano-structured ceria. The presence of H2 in the CDS 300% in CV25 and 170% in pure ceria was observed aer the
oxidation step indicates the presence of residual methane in the addition of the WS step aer CDS during MPO–CDS–WS cycling.
system. It is worth noting that the H2 during the second The H2 products showed a considerable increase, suggesting an
oxidation (i.e. WS), clearly reveals the presence of oxygen inefficient reforming of methane. Similar to rate-based infor-
vacancies in the structure of the oxygen carriers due to an mation, yields of H2 produced during the methane partial
incomplete re-oxidation during the CDS cycle. Moreover, the oxidation showed a drastic increase during MPO–WS redox
presence of a small amount of CO2 also indicates the possible cycles. Addition of the CDS step aer the WS step, i.e. MPO–WS–
carbon deposition on the surface of the oxygen carriers during CDS, not only improved the H2/CO ratio but also increased the H2
MPO and CDS reactions.
yields resulting from splitting of H2O. The reasons for this
The total fuel production data along with information about phenomenon have been discussed in previous sections.
H2/CO ratios and CO selectivity S(CO), recorded during the
Fig. 7 Thermochemical redox cycle performance evaluation in terms of average of total H2 and CO yield during the period of: (a) reduction and
(b) oxidation steps of 10 MPO–CDS, MPO–WS, MPO–WS–CDS and MPO–CDS–WS cycles on pure nano-ceria and CV25. Error bars represent
the deviation of maximum and minimum yield from the average yield value.
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The stability test for pure nano-structured ceria and CV25 5 cycles post 100 cycles are presented in Fig. 8 and S5.† A typical
was performed through 100 cycles of the MPO–CDS and MPO– decline in the production rates and yields is observed in both
WS–CDS reactions. The fuel production rate yields per cycle and oxygen carriers indicating possible sintering and incomplete
Fig. 8 Performance evaluation of CV25 during MPO–CDS and MPO–WS–CDS cycles: (a) and (b) H2 and CO production rates during 100 cycles;
(c) and (d) average yield calculated after each step of 5 cycles; (e) and (f) syngas production rates during 5 redox cycles performed after the “100
cycle stability test”.
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oxygen recovery (Fig. 8a, b, S5a and S5b†). The fuel yield is
Overall, the vanadia–ceria binary phase redox oxide per-
calculated by taking the average yield over 5 cycles during the formed better than pure nano-structured ceria by achieving
run of 100 cycles. A 47% and 63% increase in the fuel yield is 53% higher H2 and 51% higher CO yields during the methane
observed in CV25 over pure nano-structured ceria during MPO– partial oxidation and a 47% higher CO yield in the CO2 splitting
CDS and MPO–WS–CDS redox cycles, respectively, as shown in step in MPO–CDS reactions over 100 cycles. High yields of H2
Fig. 8c, d, S5c and S5d.† The long-term stability test of the pure and CO are achieved in CV25 during sequential WS and CDS
nano-structured ceria and CV25 was further investigated by reactions (MPO–WS–CDS), with a 48% and 45% increase
measuring the oxygen exchange capacity by performing 5 cycles respectively, when compared to pure nano-structured ceria. The
of MPO–CDS and MPO–WS–CDS reactions aer the 100 redox H2 and CO yields during the reduction and oxidation reactions
cycles. Notably, the H2 and CO production rates of pure nano- of MPO–WS–CDS and IGS-WS–CDS cycles is tabulated in Table
structured ceria and CV25 were largely unchanged and main- 2 and compared to the current benchmark material (ceria) and
tained a high performance, similar to the as-prepared material, other redox oxides reported recently, where MPO-driven
as shown in Fig. 8e, f, S5e and S5f.†
reduction of oxygen carriers results in higher reduction
The structural reorganization of the materials under inves- extents with efficient syngas yields, a controlled H2/CO ratio
tigation was evaluated by the determination of specic surface (ꢄ2) is another important aspect of an effective syngas
area and pore volume via BET analysis, before and aer 100 production process. Notably, a mass-specic CO yield of 91 mL
cycles. The as-prepared nano-structured ceria possessed gꢀ1 is achieved with CV25 samples during CO2 splitting (e.g.
a higher specic surface area of 81.77 m2 gꢀ1 compared to that 70% higher than the recently reported CO yield on La0.6Ca0.4
-
of CV25 with a surface area of 65.25 m2 gꢀ1. Despite a 79% MnO3 during reverse water gas shi reactions62), which is
decline in surface area, CV25 exhibited a high and consistent attributed to the extraordinary oxygen exchange capacity of
oxygen exchange capacity during and aer the 100 cycles. On CV25. Gao et al. reported a high CO yield of 397 mL gꢀ1 during
the other hand, a 92% decrease in the surface area in pure nano- the CO2 splitting step carried out for over 70 min.1
structured ceria is observed aer the 100-cycle test. This shows
However, the long reduction durations resulted in a high H2/
that vanadia–ceria multi-phase systems not only support the CO ratio of 4.47, which deviates signicantly from the H2/CO
redox capability for fuel production, but also avoid drastic ratio (i.e. 2) required for an ideal methane partial oxidation
deterioration through structural re-organization.
reaction. Pure nano-structured ceria in this study depicted
Table 2 Performance comparisons of solar fuel production by oxygen carriers evaluated in this study and other benchmark materials reported
recently. tc: operating time; Nc: number of cycles; Tred: reduction temperature; Tox: oxidation temperature; YH : hydrogen yield; YCO: CO yield;
2
WS: water splitting; CDS : CO2 splitting; MPO: methane partial oxidation; IGS: inert gas sweeping; RWGS: reverse water gas shift; FBAR: fixed bed
alumina reactor; PCTSR: porous ceria tube solar reactor; CRSR: cavity receiver solar reactor; FBQR: fixed bed quartz reactor; FBR: fixed bed
reactor; SD-TGA: solar driven thermo-gravimetric analysis; MRS: micro-reactor system; FBRa: fluidized bed reactor; SFR: stagnation flow reactor;
QUTR: quartz U-tube reactor
Reduction
Oxidation
Tred
YH
YCO
YH
YCO
Reactor
2
2
Material
Process
tc (h) Nc (#) (K)
mL gꢀ1 mL gꢀ1 TOX (K) mL gꢀ1 mL gꢀ1 H2 : CO type
Ref.
CeO2
CeO2
25% V$CeO2
25% V$CeO2
25% V$CeO2
CeO2
CeO2
CeO2
CeO2
3% Ce$Mn3O4
3% Ce$Mn3O4
3% Ce$Mn3O4
Ce0.8Zr0.2O2
Ce0.7Fe0.3O3
CeO2/SiC
MPO–CDS
MPO–WS–CDS 66.7
MPO–CDS
MPO–WS
MPO–WS–CDS 66.7
66.7
100
100
100
10
100
500
10
10
100
100
4
2
1
10
3
6
1173 122
1173 168
1173 261
1173 665
1173 320
58
79
118
230
145
—
1173
1173
1173
1173
1173
1073
1100
1173
1173
1173
1173
1173
773
—
71
—
160
220
14
2
—
—
—
—
254
19
23
27
117
5
49
—
91
5
2.09
2.14
2.18
2.89
2.21
—
FBAR
FBAR
FBAR
FBAR
FBAR
PCTSR
CRSR
FBAR
FBAR
FBAR
FBAR
FBAR
FBQR
FBR
This work
This work
This work
This work
This work
3
56
25
1
1
1
1
57
28
66.7
6.7
2
IGS-WS
167
>8
12
46.7
46.7
11.7
2
>2.7
>6.3
0.7
1773
1700
—
—
—
0.9
55
35
29
397
81
—
—
—
—
—
—
—
3
IGS-WS–CDS
MPO–CDS
MPO–CDS
MPO–CDS
MPO–CDS
MPO–WS
MPO–WS
MPO–WS
MPO–WS
MPO–WS
MPO–WS
MPO–WS
IGS-WS
—
—
1173 81
1173 44
1173 77
1173 640
1173 846
39
16
20
143
253
14
14
10
79
6
2.09
2.74
3.47
4.47
3.34
2.08
1.89
1.72
2.00
7.40
—
973
29
1123 27
1273 17
1223 159
1073 45
973
1273
1123
1073
1123
1623
1273
823
SD-TGA 58
FBR
MRS
FBRa
SFR
Fe3O4
15.5
> 27
35
16
59
60
61
62
LaFeO3/CeO2
LaFe0.9Ni0.1O3
CoFe2O4/Al2O3
Sr0.6La0.4Mn0.6Al0.4O3 IGS-CDS
La0.6Ca0.4MnO3 RWGS
30
10
3
80
5
1123
1623
1623
823
—
—
—
—
—
—
—
—
150
2
—
4.6
40
3.3
—
—
—
SFR
QUTR
—
28
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Fig. 9 Proposed mechanism for the contribution of CeO2 and CeVO4 during reduction and oxidation reaction for high syngas production than
pure ceria.
a higher H2 and CO yield of 122 mL gꢀ1 and 58 mL gꢀ1, while by modulating the valence change in C4+/Ce3+ due to the cyclic
retaining the H2/CO ratio close to 2, as compared to other re- formation of CeVO4 to CeVO3, which potentially can suppress
ported ceria structures.1,25 However, the multi-phase vanadia– the combustion of hydrocarbons during the redox cycles.41
ceria system outperformed the pure nano-structured ceria by Hence, the co-existence of CeO2 and CeVO4 phases demon-
producing H2 (261 mL gꢀ1) and CO (118 mL gꢀ1) with a H2/CO strates a synergic effect in the redox reactions resulting in better
ratio of 2.18 during the MPO reaction and 91 mL gꢀ1 of CO in syngas yields as compared to pure ceria, as shown in Fig. 9.
the oxidation step of 100 cycles of MPO–CDS reactions.
Improved water splitting yields are observed with the combi-
5. Conclusions
nation of CDS reaction on CV25 over the 100 cycles with
a maximum H2/CO ratio of 2.21 when compared to MPO–WS
Facile and low-cost synthesis of oxygen carriers, possessing
with a H2/CO ratio of 2.89 in pure nano-structured ceria.
extraordinary oxygen ion mobility coupled with superior cata-
lytic properties may lead to broad acceptance for use in ther-
mochemical fuel production with high redox rates. This study is
the rst demonstration of the vanadia–ceria two phase system
4. Discussion
for thermochemical fuel production via methane partial
oxidation followed by CO2, H2O, CO2–H2O and H2O–CO2 split-
ting. Suitable amounts of vanadium in CeO2 results in a multi-
phase system consisting of CeO2 and CeVO4, which increases
syngas production rates by 68% during H2O splitting and 47%
CO2 splitting reactions, when compared to pure nano-
structured ceria. Efficient re-oxidation of CeVO3 to CeVO4
plays a critical role in order to obtain high syngas yields, which
is achieved by coupling H2O and CO2 splitting reactions. The
long-term cycling performance of these redox oxide pairs
demonstrates that the oxygen exchange in the vanadia–ceria
system during the redox reactions is highly reversible and can
be performed up to hundred cycles, while maintaining high
redox rates. In conclusion, these ndings provide evidence that
these binary phase systems can be used for the large-scale fuel
production via improved thermochemical redox cycles.
Ceria possesses high bulk oxygen mobility in its non-
stoichiometric cubic structure and is efficient in creating
oxygen vacancies upon reduction. Matta et al.37 have reported
poor ethane oxidation on ceria-supported vanadia at 550 ꢂC.
Addition of vanadia up to 5 wt% loading in ceria results in
deterioration in the catalytic activity of ceria. The high density
of surface vanadia readily reacts with ceria and forms CeVO4,
which may not be a fully reversible reaction due to oxygen
leaching. Ceria exists in both 3+ (CeVO4) and 4+ (CeO2) oxida-
tion states in ceria–vanadia multi-phase systems due to segre-
gated phases of CeVO4 and CeO2. The formation reaction of
1
CeVO4 also gives a oxygen molecule towards the methane
2
oxidation and results in the CeVO3 phase consisting of Ce4+ and
V3+, where re-oxidation of CeVO3 consumes oxygen from the
oxidation medium and forms CeVO4 and re-oxidized ceria.
Consequently, co-existence of CeO2 and CeVO4 results in more
oxygen species for reduction and oxidation reactions compared
to pure ceria. The high density of surface vanadia species
contributes the most in the formation of CeVO4 because of its
high extent of reduction,40 whilst the V5+ species interact with
the ceria and promote its reduction from Ce4+ to Ce3+,
depending upon the reducing power of hydrocarbon and V5+
loading. The re-oxidation of cerium vanadate occurs via oxygen
buffering by cerium, where cerium assists the re-oxidation of
the reduced metal oxide system.40,49–55,63–65 Moreover, the inter-
Funding
This project was supported by the Australian Research Council
´
(ARC Future Fellowship FT140101213 by W. Lipinski).
Conflicts of interest
action of surface vanadia with ceria stabilizes the surface Ce3+ There are no conicts to declare.
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Journal of Materials Chemistry A
24 S. M. Haile and W. C. Chueh, US Pat. 8480923B2, 2009, vol. 1,
pp. 1–17.
Acknowledgements
´
25 X. Gao, A. Vidal, A. Bayon, R. Bader, J. Hinkley, W. Lipinski
This study used the facilities and the scientic and technical
assistance at the Centre of Advanced Microscopy at the
Australian National University. We are grateful to Dr Michael
Gao, Colin Carvolth and Kevin Carvolth for their assistance with
setting up the IR furnace experiments.
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