Solar Thermochemical Conversion of CO2 into Synthetic Fuels via Ferrite Based Redox Reactions.

Abstract

In this contribution, we report the synthesis NiFe2O4 and CoFe2O4 redox materials via sol-gel method. For the synthesis of these materials via sol-gel approach, the Ni, Co, and Fe precursors were first dissolved in ethanol with the help of sonic energy. Once the metal precursors were dissolved, propylene oxide (PO) was added dropwise to the well mixed solution as a gelation agent to achieve gel formation. As-prepared gels were aged, dried and subsequently calcined in presence air. The calcined powder obtained was characterized towards its phase/chemical composition, particle morphology, and specific surface area (SSA). Derived NiFe2O4 and CoFe2O4 redox materials were further investigated towards thermochemical splitting of CO2 into solar fuels by performing several reduction/re-oxidation cycles using a thermogravimetric analyzer (TGA).

Full text

1
SOLAR THERMOCHEMICAL CONVERSION OF CO2 INTO SYNTHETIC FUELS VIA
FERRITE BASED REDOX REACTIONS
Rahul Bhosale1,*, Ivo Alxneit2, Anand Kumar1, LJP van den Broeke1, Jamila Folady1, Dareen
Dardor1, Shahd Gharbia1, Mehak Jilani1, Mashail Shaif Alfakih1, Noura Nabil Elshrbajy1,
Mahsa Tarsad1, Fatima Abdulaqder Almkhyari1
1Department of Chemical Engineering, College of Engineering, Qatar University, PO Box
2713, Doha, Qatar.
2Solar Technology Laboratory, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland.
*Corresponding Author: rahul.bhosale@qu.edu.qa
Abstract
In this contribution, we report the synthesis NiFe2O4 and CoFe2O4 redox materials via
sol-gel method. For the synthesis of these materials via sol-gel approach, the Ni, Co, and Fe
precursors were first dissolved in ethanol with the help of sonic energy. Once the metal
precursors were dissolved, propylene oxide (PO) was added dropwise to the well mixed
solution as a gelation agent to achieve gel formation. As-prepared gels were aged, dried and
subsequently calcined in presence air. The calcined powder obtained was characterized
towards its phase/chemical composition, particle morphology, and specific surface area (SSA).
Derived NiFe2O4 and CoFe2O4 redox materials were further investigated towards
thermochemical splitting of CO2 into solar fuels by performing several reduction/re-oxidation
cycles using a thermogravimetric analyzer (TGA).
Introduction
Solar radiation is an essentially inexhaustible energy source that delivers about 100,000
TW to the earth. To harvest the solar radiation and to convert it effectively into renewable fuels
from H2O and captured CO2 provides a promising path for a future sustainable energy
economy [1]. Solar fuel production via metal oxide (MO) based thermochemical H2O and/or
CO2 splitting reactions are considered as one of the promising new technologies for fulfillment
of future energy requirement and steadying the greenhouse gas concentration in the earth’s
atmosphere. In comparison to the high temperature direct thermolysis of H2O and/or CO2
(needs more than 2500oC), the MO based thermochemical cycles are advantageous as: a)
these cycles requires lower temperatures as compared to thermolysis, b) no explosive mixture
formation as the production of H2/CO and O2 can be carried out in two different steps, and c)
environmentally and thermodynamically more feasible in comparison with thermolysis [3-4].
Production of solar fuels via MO based thermochemical reactions is a two-step process.
In the first step, the MO is reduced into a lower valence MO or metal with the help of solar
energy. The reduced MO is further re-oxidized in the second step via H2O and/or CO2 splitting
reactions. Among the many MOs investigated so far for solar fuel production, in recent years,
research has been focused towards mixed ferrite materials [1-11]. These redox materials are
appealing as their reduced form is a solid and hence the separation of a gaseous metal (or
MO) and O2, a challenging and compulsory step in cycles based on ZnO/Zn or SnO2/SnO/Sn

2
[12-16] can be eliminated. A typical ferrite based solar thermochemical H2O/CO2 splitting
process is presented in Fig. 1.
Fig.1. Typical ferrite based two-step solar thermochemical H2O/CO2 splitting process.
According to the results reported in previous investigations, it was observed that Ni-
doped (NiFe2O4) and Co-doped ferrite materials (CoFe2O4) showed promising towards solar
H2 production via thermochemical water splitting process. Based on this background, this
investigation reports the utilization of NiFe2O4 and CoFe2O4 redox materials towards solar
thermochemical CO production via CO2 splitting reaction. Respective ferrite materials were
synthesized using sol-gel approach and the derived materials were characterized using
various analytical methods such as powder x-ray diffractometer (XRD), BET surface area
analyzer, scanning (SEM) and transmission electron microscopy (TEM), and an inductively
coupled plasma spectrometer (ICP). Sol-gel derived NiFe2O4 and CoFe2O4 were examined
towards their thermal reduction and CO2 splitting ability by performing successive
thermochemical cycles using a thermogravimetric analyzer (TGA).
Experimental
Synthesis of ferrite materials
For the synthesis of NiFe2O4 and CoFe2O4 redox materials via sol-gel method, the
nitrate salts of Ni or Co, and Fe were added in ethanol and this mixture was sonicated to
dissolve the metal salts in the solvent. Once the metal precursors were dissolved in ethanol,
predetermined amount of propylene oxide (PO) was added dropwise as a gelation agent and
the gel formation was achieved. As-synthesized gel was aged by keeping undisturbed for 24 h.

3
After aging, the gel was broken and dried using a vacuum oven operated at 100oC. The dried
ferrite powder obtained was further calcined rapidly upto 600oC in air using a muffle furnace.
Fig. 2 represents a typical synthesis route followed in this investigation for the production of
CoFe2O4 and similar approach was used for the synthesis of NiFe2O4.
Fig.2. Sol-gel route used for the synthesis of ferrite materials.
Characterization of ferrite materials
Phase purity, crystallite size, morphology, specific surface area, and elemental
composition of the sol-gel derived ferrite powders were analyzed using a Panalytical XPert
MPD/DY636 powder X-ray diffractometer, a Zeiss Supra 55VP field-emission scanning
electron microscope (SEM), a FEI – Tecnai G2 200kV transmission electron microscope
(TEM), a BET surface area analyzer (Micromeritics, ASAP 2420), and an ICP (Thermo, iCAP
6500).
TGA set-up and procedure
The thermal reduction and the subsequent CO2-splitting cycles were performed by
using the NiFe2O4 and CoFe2O4 redox materials derived via sol-gel method using a
thermogravimetric analyzer (TGA, Netzsch STA 409). Approximately 50 mg of ferrite powder
was placed in an alumina crucible. Thermal reduction of ferrites was carried out at 1400oC for
60 minutes in presence of 100 ml/min of Ar as an inert gas. After completing the thermal
reduction step, the temperature was lowered to 1000oC to perform the CO2 splitting reaction.
The CO2 was admitted (50 ml of CO2/min + 50 ml of Ar/min ) for 30 min to induce the re-

4
oxidation of the ferrites. The mass loss/increase observed is correlated with the O2
release/uptake of the sample during the reduction/re-oxidation step. O2 and CO evolution was
determined by on-line gas chromatography (GC, VARIAN, CP-4900, Micro GC 2 channel
system)) of the off gas flow.
Results and Discussion
Powder XRD analysis was performed to identify the phase composition of the sol-gel
derived NiFe2O4 and CoFe2O4 redox materials. The XRD patterns for both materials calcined
at 600oC in air are presented in Fig. 3. As per the results obtained, both ferrite materials
showed strong XRD peaks/reflections and a well-defined spinel cubic structure with high
degree of crystallinity. XRD reflections related to Impurities such as NiO, CoO, pure metals i.e.
Ni, Co, or Fe were not observed. When compared with the ICDD patterns of pure iron oxide
(Fe3O4), it was understood that due to the inclusion of Ni or Co metal into spinal crystal
structure of iron oxide, the diffraction patterns of derived NiFe2O4 and CoFe2O4 redox
materials were observed to be shifted as compared to the pure iron oxide. For further
confirmation of compositional purity, ICP analysis was also performed and the results obtained
indicate that phase pure NiFe2O4 and CoFe2O4 redox materials were produced via sol-gel
approach utilized in this investigation (Table 1).
Fig. 3. Powder XRD patterns for NiFe2O4 and CoFe2O4 redox materials.
Crystallite size of single crystalline domains of sol-gel derived NiFe2O4 and CoFe2O4
redox materials were calculated using Scherrer equation. The crystallite size for NiFe2O4 was
observed to be 29 nm whereas for CoFe2O4 it was equal to 31 nm. Further information about
the particle size and morphology of the derived ferrite materials was gathered by performing
SEM and TEM analysis experiments. The results obtained via SEM and TEM analysis indicate
Arbitrary counts
2θ
NiFe2O4
CoFe2O4

5
that the synthesis of ferrites via sol-gel method yields into nano-crystals of NiFe2O4 and
CoFe2O4 redox materials. The specific surface area (SSA) and porosity of the ferrite materials
was determined using a BET surface area analyzer. In case of NiFe2O4, the SSA was equal to
36.52 m2/g and pore volume was observed to be 0.0312 cm3/g. Likewise, in case of CoFe2O4,
the SSA and pore volume were observed to be equal to 37.43 m2/g and 0.0722 cm3/g.
Table 1: ICP analysis results for sol-gel derived ferrite materials.
The redox relativity and thermal stability of the sol-gel derived NiFe2O4 and CoFe2O4
redox materials during thermochemical CO2 splitting cycles was examined by performing
multiple thermal reduction and re-oxidation experiments using a thermogravimetric analyzer
(TGA). Prior conducting actual thermochemical CO2 splitting experiments using NiFe2O4 and
CoFe2O4 redox materials, a baseline run was performed under identical experimental
conditions in absence of the ferrite powder (using empty alumina crucible). This allows
subtracting artifacts due to buoyancy effects or caused by changing the gas composition.
Four consecutive thermochemical cycles were performed to investigate the thermal
reduction and CO2 splitting ability of the sol-gel derived NiFe2O4 and CoFe2O4 redox materials.
The results obtained during the TGA analysis are reported in Fig. 4. Thermal reduction step
was carried out at 1400oC for 60 min in presence of pure inert Ar. On the other hand, CO2
splitting experiment was performed at 1000oC for 30 min in presence of a gas mixture consists
of CO2 and Ar (50:50 molar ratio). According to the results reported in Fig. 4, it can be seen
that the redox capacity of both sol-gel derived NiFe2O4 and CoFe2O4 redox materials does not
deteriorate i.e. approximately the same mass loss and mass gain is observed in all
thermochemical cycles.
Amounts of evolution of O2 and CO during in consecutive thermochemical cycles
performed using sol-gel derived NiFe2O4 and CoFe2O4 redox materials were determined with
the help of % weight changes (TGA analysis) and the data obtained via gas chromatography.
During the 1st thermal reduction step, both ferrites produced a very high amount of O2 (1.37%
mass loss in case of NiFe2O4 and 1.37% mass loss in case of CoFe2O4). This disproportionally
large weight loss occurs due to the desorption of physisorbed water from the ferrite samples.
In contrast to this, for the 2nd, 3rd, and 4th thermochemical CO2 splitting steps, the % mass loss
and mass gain by both ferrite materials were observed to be constant. In case of NiFe2O4, an
average of 269 μmol/g·cycle O2 and 360 μmol/g·cycle of CO were produced with CO to O2
ratio equal to 1.34. Likewise, sol-gel derived CoFe2O4 produce 273 μmol/g·cycle O2 and 424
μmol/g·cycle of CO with CO to O2 ratio equal to 1.55. Obtained results indicate that the
thermal reduction ability of both ferrites is identical; however, CoFe2O4 is capable of producing
higher amount of CO as compared to NiFe2O4. Furthermore, these results also shows that sol-
gel derived NiFe2O4 and CoFe2O4 redox materials are capable of producing higher amounts of
solar fuels as compared to that of the previously investigated redox materials such as doped
ceria redox materials [12-21].
Material
Ni:Fe ratio
(prepared)
Ni:Fe ratio
(calcined)
Final
Composition
Ni-ferrite
1:2
1:2
NiFe2O4
Co-ferrite
1:2
1:2
CoFe2O4

6
Fig. 5. TGA of CO2-splitting experiments performed using sol-gel derived NiFe2O4 and
CoFe2O4 redox materials.
Conclusions
In this study, NiFe2O4 and CoFe2O4 redox materials were effectively derived via sol-gel
method by using nitrate salts of Ni, Co, and Fe, ethanol as solvent, and PO as gelation agent.
Characterization performed with the help of powder XRD indicate that the sol-gel derived
NiFe2O4 and CoFe2O4 redox materials are phase pure in composition with absence of any
oxide or metal impurities. The crystallite/particle size and material morphology analyzed via
quantitative XRD and SEM/TEM reveal formation of NiFe2O4 and CoFe2O4 nanocrystals. The
solar fuel production capacity of both ferrite materials were further examined in the
temperature range of 1000oC to 1400oC using a thermogravimetric analyzer. TGA and GC
results showed that thermal reduction capacity of both ferrites is identical, however CO2
splitting ability of CoFe2O4 was observed to be reasonably higher as compared to the NiFe2O4
redox material. Nevertheless, both materials produced highest amounts of O2 and CO as
compared to previously investigated redox materials (at similar operating conditions).
Acknowledgments
The authors gratefully acknowledge the financial support provided by the Qatar University
(QUUG-CENG-CHE-13/14-4), Indo-Swiss Joint Research Program (ISJRP, grant #138852),
and Swiss Federal Office of Energy.
Time (min)
Temperature (oC)
Weight Change (%)
CoFe2O4
NiFe2O4

7
References
1. Bhosale, R., Alxneit, I, van den Broeke, Leo L. P., Kumar, A., Jilani, M., Gharbia, S.,
Folady, J., and Dardor, D. (2014), “Sol-gel synthesis of nanocrystalline Ni-Ferrite and
Co-Ferrite redox materials for thermochemical production of solar fuels”, Proceedings
of Materials Research Society Symposium (Accepted – In Press).
2. Bhosale, R., Shende, R., and Puszynski, J. (2010), “H2 generation from thermochemical
water-splitting using sol-gel synthesized ferrites (MxFeyOz, M= Ni, Zn, Sn, Co, Mn, Ce)”,
Proceedings of AIChE Annual Meeting.
3. Shende, R., Puszynski, J., Opoku, M., Bhosale, R. (2009), “Synthesis of novel ferrite
foam material for water-splitting application”, Proceedings of NSTI-Nanotech
Conference & Expo. ISBN 978-1-4398-1782-7, 201, 1.
4. Kodama, T., Gokon, N., and Yamamoto, R. (2010), “Thermochemical two-step water
splitting by ZrO2-supported NixFe3−xO4 for solar hydrogen production”, Solar Energy, 82,
73.
5. Scheffe, J., Li, J., and Weimer, A. (2010), “A spinel ferrite/hercynite water-splitting
redox cycle”, International Journal of Hydrogen Energy, 35, 3333.
6. Bhosale, R., Shende, R., and Puszynski, J. (2010), “H2 generation from thermochemical
water splitting using sol-gel derived Ni-ferrite”, Journal of Energy and Power
Engineering, 4, 27.
7. Bhosale, R., Shende, R., and Puszynski, J. (2010), “H2 generation from thermochemical
water-splitting using sol-gel synthesized Zn/Sn/Mn-doped Ni-ferrite”, International
Review of Chemical Engineering, 2, 852.
8. Bhosale, R., Shende, R., and Puszynski, J. (2012), “Thermochemical water-splitting for
H2 generation using sol-gel derived Mn-ferrite in a packed bed reactor”, International
Journal of Hydrogen Energy, 37, 8223.
9. Bhosale, R., Khadka, R., Shende, R., and Puszynski, J. (2011), “H2 generation from
two-step thermochemical water-splitting reaction using sol-gel derived SnxFeyOz”,
Journal of Renewable and Sustainable Energy, 3, 063104-1.
10. Bhosale, R., Shende, R., and Puszynski, J. (2012), “Sol-gel derived NiFe2O4 modified
with ZrO2 for hydrogen generation from solar thermochemical water-splitting reaction”,
Proceedings of Materials Research Society Symposium, 1387.
11. Fresno, F., Yoshida, T., Gokon, N., Fernandez-Saavedra, R., and Kodama, T. (2010),
“Comparative study of the activity of nickel ferrites for solar hydrogen production by two-
step thermochemical cycles”, International Journal of Hydrogen Energy, 35, 8503.
12. Loutzenhiser, P., and Steinfeld, A. (2011), “Solar syngas production from CO2 and H2O
in a two-step thermochemical cycle via Zn/ZnO redox reactions: Thermodynamic cycle
analysis”, International Journal of Hydrogen Energy, 36, 12141.
13. Stamatiou, A., Loutzenhiser, P., and Steinfeld, A. (2010), “Solar syngas production from
H2O and CO2 via two-step thermochemical cycles based on Zn/ZnO and FeO/Fe3O4
redox reactions: Kinetic analysis”, Energy & Fuels, 24, 2716.
14. Loutzenhiser, P., Meier, A., and Steinfeld, A. (2010), “Review of the two-step H2O/CO2-
splitting solar thermochemical cycle based on Zn/ZnO redox reactions”, Materials, 3,
4922.

8
15. Charvin, P., Abanades, F., Lemont, F., and Flamant, G. (2008), “Experimental study of
SnO2/SnO/Sn thermochemical systems for solar production of hydrogen”, AIChE
Journal, 54, 2759.
16. Abanades, F. (2012), “CO2 and H2O reduction by solar thermochemical looping using
SnO2/SnO redox reactions: Thermogravimetric analysis”, International Journal of
Hydrogen Energy, 37, 8223.
17. Scheffe, J., Jacot, R., Patzke, G., and Steinfeld, A. (2013), “Synthesis, characterization
and thermochemical redox performance of Hf, Zr and Sc doped ceria for splitting CO2”,
Journal of Physical Chemistry C, 177, 24104.
18. Chueh, W. C., Falter, C., Abbott, M., Scipio, D., Furler, P., Haile, S., and Steinfeld, A.
(2010), “High-flux solar-driven thermochemical dissociation of CO2 and H2O using
nonstoichiometric ceria”, Science, 330, 1797.
19. Le Gal, A., and Abanades, S. (2011), “Catalytic investigation of ceria-zirconia solid
solutions for solar hydrogen production”, International Journal of Hydrogen Energy, 36,
4739.
20. Abanades, S., and Gal, A. L. (2012), “CO2 splitting by thermo-chemical looping based
on ZrxCe1−xO2 oxygen carriers for synthetic fuel generation”, Fuel, 102, 180.
21. Le Gal, A., Abanades, S., and Flamant, G. (2011), “CO2 and H2O splitting for
thermochemical production of solar fuels using nonstoichiometric ceria and
ceria/zirconia solid solutions”, Energy & Fuels, 25, 4836.