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Thermochemical splitting of CO2 using Co-precipitation synthesized Ce0.75Zr0.2M0.05O2-δ (M = Cr, Mn, Fe, CO, Ni, Zn) materials
Fuel 256 (2019) 115834
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Thermochemical splitting of CO2 using Co-precipitation synthesized Ce0.75Zr0.2M0.05O2-δ (M = Cr, Mn, Fe, CO, Ni, Zn) materials
T
Gorakshnath Takalkar, Rahul R. Bhosale , Fares AlMomani ⁎
Department of Chemical Engineering, College of Engineering, Qatar University, P. O. Box – 2713, Doha, Qatar
ARTICLE INFO
ABSTRACT
Keywords: CO2 splitting Thermochemical cycles Ceria-zirconia solid solution Co-precipitation method Redox reactivity Solar fuels
This work reports the investigation of the redox reactivity of Ce0.75Zr0.2M0.05O2-δ (M = Cr, Mn, Fe, Ni, Co, Zn) materials towards thermochemical CO2 splitting (CS) cycle. The Ce0.75Zr0.2M0.05O2-δ materials were prepared via co-precipitation method and the derived materials were characterized to determine the phase/elemental composition and microstructural morphology. The powder X-ray diffraction (PXRD) analysis indicate formation of phase pure Ce0.75Zr0.2M0.05O2-δ materials with no metal or metal oxide impurities. The analysis performed using scanning electron microscopy confirms production of agglomerated roundish particles of Ce0.75Zr0.2M0.05O2-δ materials. Synthesized Ce0.75Zr0.2M0.05O2-δ materials were further tested, using a thermogravimetric analyzer (TGA), to determine their redox reactivity towards CS reactions. The obtained results indicate that all the Ce0.75Zr0.2M0.05O2-δ materials possess better thermal reduction (TR) and CS aptitude as compared to previously studied phase pure ceria and transition metal doped ceria oxides. The obtained results further indicate that, except for Ce0.75Zr0.2Mn0.05O2-δ material, all the other Ce0.75Zr0.2M0.05O2-δ materials were capable of releasing higher amounts of O2 during TR performed at 1400 °C as compared to Ce0.75Zr0.25O2-δ. Overall in ten thermochemical cycles, the Ce0.75Zr0.2Zn0.05O2-δ showed the highest O2 releasing capacity (105.1 μmol/g·cycle) and the Ce0.75Zr0.2Ni0.05O2-δ indicated the maximum CO production aptitude (170.5 μmol/g·cycle).
1. Introduction Gas to liquid (GTL) technology, based on the Fischer–Tropsch (FT) process, is a well-known option to convert natural gas i.e., CH4 into synthetic liquid fuels [1,2]. Pearl GTL (from Shell) located in Qatar is one of the largest capacity GTL plant. This plant operates in the following three steps: 1) conversion of CH4 into synthesis gas (mixture of H2 and CO), 2) conversion of synthesis gas in to liquid hydrocarbons, 3) cracking and isomerization to produce the synthetic fuels such as diesel, kerosene, etc. This process has the potential to produce synthetic fuels from CH4, however emission of CO2 is considered as one of the major concerns. As an alternative to the GTL process, a metal oxide (MO) based solar-assisted two-step thermochemical H2O splitting (WS) and CO2 splitting (CS) redox cycle can be considered for the production of syngas. As shown in Fig. 1, the combination of solar thermochemical syngas production cycle and the catalytic FT process provides a sustainable and environment friendly route for the generation of synthetic fuels from H2O and CO2. The MO-based WS/CS cycle is advantageous as it a) decreases the operating temperatures and b) resolves the issues associated with the ⁎
gas separation allied with the direct WS/CS splitting reactions. The reactions allied to the solar thermochemical WS/CS cycle are presented below. Thermal reduction (TR) step:
MO(x )
MO(x
)
+
2
O2
(1)
WS/CS step:
MO(x
)
+ H2 O/CO2
MO(x ) + H2 / CO
(2)
A variety of MOs such as zinc oxide [3–5], tin oxide [6–7], iron oxide [8–9], ceria and doped ceria [10–13], perovskites [14–17], ferrites [18–20], other MOs [21–23] were utilized until now as the redox active materials for the splitting of H2O/CO2. Among the listed MOs, because of their a) higher oxygen storage capacity (OSC), b) faster kinetics associated with the TR and WS/CS reactions, and c) excellent thermal stability, the ceria based redox materials were considered as the state of the art redox materials for solar thermochemical WS/CS cycles. Chueh et al. [24] have demonstrated the feasibility of CeO2 towards stable fuel generation via WS and CS reactions over 500
Corresponding author. E-mail address: [email protected] (R.R. Bhosale).
https://doi.org/10.1016/j.fuel.2019.115834 Received 27 January 2019; Received in revised form 13 June 2019; Accepted 17 July 2019 Available online 05 August 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.
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Fig. 1. Schematic of the combined solar thermochemical H2O/CO2 splitting cycle and catalytic Fischer Tropsch Process for the production of synthetic fuels.
thermochemical cycles. To further improve the fuel production capacity of CeO2, several researchers have explored Zr-doped ceria materials (ceria zirconia solid solutions). Miller et al. [25] prepared and utilized the CeO2-ZrO2 monolithic structures for thermochemically splitting the H2O/CO2. Thermochemical WS ability of the CeO2-ZrO2 solid solution was compared with the other CeO2-MOX solid solutions (where, M = Mn, Fe, Co, Cu, and Zn) by Abanades et al. [26] and reported that the CeO2-ZrO2 solid solution is the foremost choice in terms of the redox reactivity towards TR and WS steps. Kaneko et al. [27] tested the O2 releasing capacity of the Ce1-xZrxO2 (x = 0.1, 0.2, 0.3) solid solutions by performing the TR in presence of high O2 partial pressure and reported that the amount of O2 released (nO2 ) by Ce0.8Zr0.2O2-δ was greater than the CeO2 and other ceria-zirconia solid solutions. Le Gal and Abanades [28] reported that the Ce0.75Zr0.25O2-δ showed better TR and WS aptitude as compared to Ce0.9Zr0.1O2-δ and Ce0.5Zr0.5O2-δ for multiple TR (at 1400 °C) and WS steps (at 1050 °C). The thermodynamic and experimental exploration of Ce1-xZrxO2 (x = 0.05, 0.1, 0.15, 0.2) towards WS cycles was reported by Hao et al. [29] and the obtained results shows that the H2 production was enhanced with the increase in the Zr concentration (optimum range 15–20%). A study performed by Scheffe et al. [30] shows that the inclusion of Zr and Hf in the CeO2 fluorite structure was responsible for the improvement in the CO production via CS reaction. The Ce1-xZrxO2 redox materials were tested by Abanades and Le Gal [31] and reported that the 10% incorporation of Zr in the ceria crystal structure was advantageous to achieve CO production up to 130 μmol/g·cycle. According to Call et al. [32], the Ce0.85Zr0.15O2 material was capable of attaining the maximum nO2 = 155 μmol/g·cycle and nCO = 305 μmol/ g·cycle as compared to the other Ce1-xZrxO2 materials (where, x = 0–0.375) in four thermochemical CS cycles. Zhao et al. [33] and Jiang et al. [34] reported that the reduction in the activation energy associated with the CS reaction was responsible for the production of higher amount of CO by ceria-zirconia solid solutions in comparison to the phase pure ceria material. Recently, Munich et al. [35] reported that the ceria-zirconia solid solutions can be considered as promising redox materials based on their high solar-to-fuel energy conversion efficiency. These authors further added that the inclusion of the alkaline metals and transition metals as the dopants in the ceria-zirconia solid solutions resulted into improvement in the H2 production via WS reaction. To achieve production of high quality syngas, in addition to the H2, CO generation via CO2 splitting cycle is also an important step. Therefore, in this investigation, the influence of incorporation of transition metals such as Cr, Mn, Fe, Co, Ni, and Zn in the ceria-zirconia crystal structure on the
thermochemical CS ability was examined. The Ce0.75Zr0.2M0.05O2-δ (where, M = Cr, Mn, Fe, Co, Ni, Zn) were synthesized using co-precipitation method. As-prepared Ce0.75Zr0.2M0.05O2-δ materials were analyzed using multiple characterization techniques. A high temperature thermogravimetric analyzer (TGA) was utilized to determine the redox reactivity of each Ce0.75Zr0.2M0.05O2-δ material by performing multiple CS cycles. 2. Experimental sections 2.1. Synthesis and characterization of Ce0.75Zr0.2M0.05O2-δ materials Synthesis of Ce0.75Zr0.2M0.05O2-δ materials using co-precipitation method was conducted by using following precursors (purchased from Sigma Aldrich, USA): Ce(III)(NO3)2·6H2O, Ni(II)(NO3)2·6H2O, Co(II) (NO3)2·6H2O, Zn(NO3)2·6H2O, Cr(III)(NO3)3·9H2O, Mn(II)(NO3)2·4H2O, Zr(IV)N2O7·H2O, Cr(III) (NO3)3·9H2O, Fe(III)(NO3)2·3H2O and 28% ammonium hydroxide. Deionized (DI) water was utilized for the dissolution of metal precursors. Gas cylinders containing a) Ar with a stated purity of 99.999% and b) feedstock gas mixture containing 50% CO2 in Ar were acquired from a local supplier. The amounts of metal precursors needed for the synthesis of 1 g of Ce0.75Zr0.2M0.05O2-δ material were dissolved in excess DI water. To this solution, an aqueous ammonium hydroxide was added (as a precipitating agent) until the pH of the solution reaches ~10. This aqueous solution was further stirred for 24–48 h (with a maintained pH ~10) to complete the precipitation reaction. The obtained precipitate of Ce0.75Zr0.2M0.05O2-δ was washed multiple times with DI water using a vacuum filtration unit (to remove the unreacted chemicals). The washed solid particles were dried at 120–130 °C for multiple hours to remove the moisture. Finally, the dried particles were crushed into a fine powder and this powder was further calcined up to 1000 °C in air for 3–4 h using a muffle furnace. For simplicity, the Ce0.75Zr0.2M0.05O2-δ materials were abbreviated as: Ce0.75Zr0.2Cr0.05O2-δ (CZCr), Ce0.75Zr0.2Mn0.05O2-δ (CZMn), Ce0.75Zr0.2Fe0.05O2-δ (CZFe), Ce0.75Zr0.2Co0.05O2-δ (CZCo), Ce0.75Zr0.2Ni0.05O2-δ (CZNi), and Ce0.75Zr0.2Zn0.05O2-δ (CZZn). Crystallographic phase composition of the Ce0.75Zr0.2M0.05O2-δ materials was determined by using a Panalytical XPert MPD/DY636 powder X-ray diffractometer with CuKα radiation. The scanning electron microscope equipped with the energy dispersive spectroscopy (SEM/EDS, Nova Nano 450, FEI) was applied to identify and distinguish the overall particle morphology and chemical composition of the synthesized Ce0.75Zr0.2M0.05O2-δ materials. 2
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Fig. 2. Thermochemical experimental set-up involving a Setaram SETSYS Evolution, TGA-DSC instrument.
2.2. Thermochemical redox cycles
CS steps. To eliminate the influence of buoyancy on the final results, a baseline TGA profile obtained by conducting a blank run (by using an empty alumina crucible only) at operating conditions identical to the CS experiments was subtracted from the TGA profile obtained during the actual experiment. The mass loss ( mloss ) reported during the TR step and the mass gain ( mgain) recorded during the CS step were directly attributed to the nO2 and nCO by Ce0.75Zr0.2M0.05O2-δ materials as per Eqs. (3) and (4).
The thermochemical CS reactivity of the Ce0.75Zr0.2M0.05O2-δ materials was estimated by using a TGA setup (Setaram SETSYS Evolution, TGA-DSC) depicted in Fig. 2. The heating furnace installed in the TGA set-up was consists of a graphite-heating element, which was protected by an alumina tube. A temperature controller was utilized to maintain the temperature of the TGA set-up at the desired value. During the TGA experiments, a continuous flow of chilled water was supplied to the setup to maintain the temperature of exhaust stream (leaving the TGA setup) at ambient conditions. Ultrapure inert Ar was circulated through a) the heating furnace to avoid the oxidation of the graphite-element and b) the TGA balance to dodge the prospect of pollution of the balance and the sample chamber from the vapors generated during the TGA experiments. Mass flow controllers and pressure transducers were installed in the TGA set-up to maintain the carrier and protective inert Ar gas flowrates. Other details associated with the Setaram SETSYS Evolution TGA-DSC set-up were already reported in our previous studies [36]. The TR and CS ability of each Ce0.75Zr0.2M0.05O2-δ material was estimated by performing multiple thermochemical CS cycles using roughly 50 mg of the reactive sample. This Ce0.75Zr0.2M0.05O2-δ powder was placed on the hanging balance with the help of an alumina crucible and the TR and CS runs were carried out at 1400 °C for 60 min and 1000 °C for 30 min, respectively. The TR step was performed in the presence of inert Ar whereas the CS step was conducted by using a feed gas mixture containing 50% CO2 in Ar. The mass flowrates of both Ar and the feed gas mixture were maintained at 100 ml/min during TR and
no2 =
nco =
mloss (Mo2 × mce0.75 Zr 0.2 M0.05 o2 )
(3)
mgain (Mo × mce0.75 Zr 0.2 M0.05 o2 )
(4)
where, MO2 Molecular weight of O2 MO Molecular weight of O mce0.75 Zr 0.2 M0.05 o2 Mass of Ce0.75Zr0.2M0.05O2-δ materials 3. Results and discussion 3.1. Characterization of Ce0.75Zr0.2M0.05O2-δ All the Ce0.75Zr0.2M0.05O2-δ materials synthesized via co-precipitation method were first characterized via PXRD analysis. The PXRD patterns associated with each Ce0.75Zr0.2M0.05O2-δ material are presented in Fig. 3a. Each PXRD pattern shows formation of a single-phase cubic fluorite Fm-3m structure which is identical to that of the phase 3
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Fig. 3. PXRD patterns of the co-precipitation synthesized Ce0.75Zr0.2M0.05O2-δ materials, a) 2θ = 20° to 80°, and b) 2θ = 28° to 29.5°.
pure ceria material. The absence of any metal or metal oxide impurities further provided a confirmation that the transition metal dopants were successfully incorporated in the ceria-zirconia fluorite structure. As the crystal ionic radii of Cr, Mn, Fe, Co, Ni, and Zn are lower as compared to the Ce and Zr, the peaks associated with the Ce0.75Zr0.2M0.05O2-δ materials were shifted towards the lower 2θ angle as compared to the Ce0.75Zr0.25O2-δ solid solution (Fig. 3b). The results obtained after performing the EDS analysis further validated the formation of Ce0.75Zr0.2M0.05O2-δ materials. The EDS patterns associated with each Ce0.75Zr0.2M0.05O2-δ material is presented in Fig. 4 and the chemical compositions are reported in Table 1. In addition to the phase composition, the PXRD analysis was also utilized to estimate the average crystallite size of each Ce0.75Zr0.2M0.05O2-δ material by applying the Scherrer equation [37]. The results obtained indicate that there is no fixed trend allied with the variation in the average crystallite size of Ce0.75Zr0.2M0.05O2-δ materials as a function of the change in the dopant metal. The average crystallite size of all the Ce0.75Zr0.2M0.05O2-δ materials was observed to be in the range of 30–50 nm. The maximum and minimum average crystallite size was noticed in case of the CZFe (49.5 nm) and CZCr (30.5 nm), respectively. The microstructural morphology of the co-precipitation synthesized Ce0.75Zr0.2M0.05O2-δ materials was analyzed by performing SEM analysis. The images obtained for CZCr, CZMn, CZFe, CZCo, CZNi, and CZZn are presented in Fig. 5 which shows agglomerated roundish particle morphology (for all the derived materials). Furthermore, it was understood that the material morphology was not affected due to the inclusion of Cr, Mn, Fe, Co, Ni, and Zn in the Ce0.75Zr0.2M0.05O2-δ crystal structure.
structure. The plots reported further indicate that the kinetics associated with the CS step was much faster as compared to the TR step. At the beginning, the redox reactivity of the Ce0.75Zr0.2M0.05O2-δ materials was determined by performing one thermochemical cycle. The TGA profiles allied with the TR and CS steps of the first thermochemical cycle are reported in Figs. 7 and 8. As expected, the mass of each Ce0.75Zr0.2M0.05O2-δ material was reduced during the TR step. The TGA profiles reported in Fig. 7 shows that all the Ce0.75Zr0.2M0.05O2-δ materials continue to lose weight for the entire TR duration. The kinetics associated with the TR of CZZn was the fastest and for the TR of CZNi was the slowest. The mloss allied with the first TR step was converted into nO2 by each Ce0.75Zr0.2M0.05O2-δ material by using Eq. (3) and the numbers obtained are reported in Table 2. The data presented in Table 2 shows that the CZZn was capable of releasing highest nO2 (707.6 µmol/g) and the CZMn liberated the lowest nO2 (130.5 µmol/ g) as compared to the remaining Ce0.75Zr0.2M0.05O2-δ materials. The obtained results also indicate that the CZCo and CZNi possess the capacity to release identical nO2 during TR at 1400 °C. As presented in Fig. 8, the CZCo represents the quickest and CZMn shows the lethargic re-oxidation kinetics during the CS step performed at 1000 °C in comparison to the remaining Ce0.75Zr0.2M0.05O2-δ materials. Similar to the TR step, all the Ce0.75Zr0.2M0.05O2-δ materials were continue to gain weight during the entire duration assigned for the CS step (30 min). This observation indicate that the time assigned for the CS step was not enough to completely re-oxidize the Ce0.75Zr0.2M0.05O2δ materials. The nCO produced by each Ce0.75Zr0.2M0.05O2-δ material (Table 2), during 30 min of CS step, backs the above-mentioned statement. The numbers listed in Table 1 imply that all the Ce0.75Zr0.2M0.05O2-δ materials were partially re-oxidized as the nCO /nO2 ratio was significantly less than 2.0. Based on the time allocated for the CS step, the CZNi produced the uppermost (227.3 µmol/g) and CZCr generated the lowermost nCO (167.0 µmol/g) as compared to other Ce0.75Zr0.2M0.05O2-δ materials. As the nCO /nO2 for the first thermochemical cycle was very low, it was essential to determine if this ratio persists during multiple thermochemical cycles. In this viewpoint, three consecutive thermochemical cycles were performed by conducting the TR and CS steps at conditions identical to the first cycle. The variation in the gain and loss of the Ce0.75Zr0.2M0.05O2-δ mass over three cycles is displayed in Fig. 9. Furthermore, the mloss and mgain recorded for the three cycles were converted into nO2 and nCO by each Ce0.75Zr0.2M0.05O2-δ material
3.2. Redox reactivity of Ce0.75Zr0.2M0.05O2-δ After characterization, the evaluation of the redox reactivity of the calcined Ce0.75Zr0.2M0.05O2-δ powders was conducted by using a TGA set-up. For all the TGA experiments, the TR and CS steps were performed at 1400 °C (for 60 min) and 1000 °C (for 30 min). A typical TGA profile obtained in case of CZNi material (attained after subtracting the blank run) is presented as an example in Fig. 6. The plot associated with the TR step indicate mass loss in two sections: 1) below 1000 °C due to the dissipation of moisture and other chemical impurities and b) above 1000 °C due to the release of O2 from the Ce0.75Zr0.2M0.05O2-δ fluorite 4
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Fig. 4. EDS patterns of the co-precipitation synthesized a) CZCr, b) CZMn, c) CZFe, d) CZCo, e) CZNi, and f) CZZn. Table 1 Chemical composition, Ce/Zr/M (where M = transition metal cation) molar ratio (as-prepared and from EDS), and the abbreviations assigned to each Ce0.75Zr0.2M0.05O2-δ material. Abbreviation
Ce/Zr/M (as-prepared)
Ce/Zr/M (from EDS)
Composition
CZCr CZMn CZFe CZCo CZNi CZZn
0.750/0.200/0.050 0.750/0.200/0.050 0.750/0.200/0.050 0.750/0.200/0.050 0.750/0.200/0.050 0.750/0.200/0.050
0.743/0.216/0.048 0.752/0.211/0.049 0.744/0.199/0.050 0.741/0.205/0.049 0.739/0.208/0.051 0.751/0.198/0.050
Ce0.743Zr0.216Cr0.048O2-δ Ce0.752Zr0.211Cr0.049O2-δ Ce0.744Zr0.199Cr0.050O2-δ Ce0.741Zr0.205Cr0.049O2-δ Ce0.739Zr0.208Cr0.051O2-δ Ce0.751Zr0.198Cr0.050O2-δ
(Table 3). The nO2 by each Ce0.75Zr0.2M0.05O2-δ material in the second cycle was observed to be considerably inferior than the nO2 liberated in the first cycle. For instance, the nO2 by CZCr, CZFe, and CZNi was decreased by 116.7 µmol/g, 138.1 µmol/g, and 45.7 µmol/g, respectively in cycle 2. Similarly, the nCO by each Ce0.75Zr0.2M0.05O2-δ material was also reduced in the second cycle. The magnitude of the drop in the nCO in the
second cycles by CZMn (30.3 µmol/g) was the highest and by CZCr (2.5 µmol/g) was the lowest as compared to the remaining Ce0.75Zr0.2M0.05O2-δ materials. In case of the third cycle, the nO2 by each Ce0.75Zr0.2M0.05O2-δ material continue to drop considerably. For instance, the percentage of fall in the nO2 by Ce0.75Zr0.2M0.05O2-δ materials during the third cycle as compared to the second one can be arranged as follows: CZZn (36.8%) > CZMn 5
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Fig. 5. SEM images of the co-precipitation synthesized Ce0.75Zr0.2M0.05O2-δ materials.
(23.7%) > CZFe (21.8%) > CZCr (15.7%) > CZCo (13.7%) > CZNi (6.0%). Contrast to the decline in the nO2 , the nCO by most of the Ce0.75Zr0.2M0.05O2-δ materials in the third cycle was roughly identical to the nCO in the second cycle, except for the CZMn and CZNi materials. The results described in the previous paragraphs indicate that, although the CS ability of the Ce0.75Zr0.2M0.05O2-δ materials attained stability, the TR aptitude of each material was not steady during first three cycles. Hence, as it is crucial to identify the long-term redox reactivity of the derived Ce0.75Zr0.2M0.05O2-δ materials, ten consecutive thermochemical cycles were performed and the TGA profiles obtained are presented in Fig. 10. Similar to the first three cycles, the nO2 and nCO by each Ce0.75Zr0.2M0.05O2-δ material during ten cycles was estimated by using the mass variation recorded by the TGA and the equations listed in the experimental section. During the determination of the average nO2 and nCO , to avoid inaccuracy, the data acquired by the TGA during the first
cycle was not considered. The nO2 and nCO by all the Ce0.75Zr0.2M0.05O2δ materials during each cycle (cycle 2 to cycle 10) is presented in Fig. 11 and Fig. 12, respectively. To compare with the Ce0.75Zr0.2M0.05O2-δ materials, the co-precipitation synthesized Ce0.75Zr0.25O2 was also tested for ten consecutive thermochemical cycles. The comparison between the Ce0.75Zr0.2M0.05O2-δ materials and other previously investigated ceria based materials, based on the average nO2 , nCO , and nCO /nO2 (from cycle 2 to cycle 10), is presented in Table 4. The data presented in Fig. 11 shows that most of the Ce0.75Zr0.2M0.05O2-δ materials attained stability in terms of nO2 at around cycle 4 or cycle 5. When compared with each other, the highest nO2 = 105.1 μmol/g·cycle was recorded in case of CZZn and the lowest nO2 = 56.6 μmol/g·cycle was observed for CZMn (from cycle 2 to cycle 10). Based on the average nO2 from cycle 2 to cycle 10, the Ce0.75Zr0.2M0.05O2-δ materials can be arranged as: CZZn > CZNi > CZFe > CZCr > CZCo > CZMn. 6
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Fig. 6. A typical TGA profile obtained in case of CZNi material.
Fig. 8. TGA profiles obtained in case of CS of Ce0.75Zr0.2M0.05O2-δ at 1000 °C (30 min). Table 2 nO2 , nCO , and CO/O2 molar ratio related to the Ce0.75Zr0.2M0.05O2-δ materials during the first thermochemical cycle. Ce0.75Zr0.2M0.05O2-δ materials
nO2 (µmol/g)
nCO (µmol/g)
nCO /nO2
CZCr CZMn CZFe CZCo CZNi CZZn
218.6 130.5 285.6 152.6 152.6 707.6
167.0 177.1 168.9 176.8 227.3 188.7
0.76 1.36 0.59 1.16 1.49 0.27
Fig. 7. TGA profiles obtained in case of TR of Ce0.75Zr0.2M0.05O2-δ at 1400 °C (60 min).
When compared with the previously investigated phase pure ceria and transition metal doped ceria oxides, the average nO2 by each Ce0.75Zr0.2M0.05O2-δ material was considerably higher (Table 4). The average nO2 by CZZn, CZNi, CZFe, CZCr, CZCo, and CZMn was greater than Ce0.9Zn0.1O2, Ce0.9Ni0.1O2, Ce0.9Fe0.1O2, Ce0.9Cr0.1O2, Ce0.9Co0.1O2, and Ce0.9Mn0.1O2 by 54.6 μmol/g·cycle, 42.9 μmol/ g·cycle, 38.0 μmol/g·cycle, 30.4 μmol/g·cycle, 31.9 μmol/g·cycle, and 15.4 μmol/g·cycle, respectively. When compared with the Ce0.75Zr0.25O2, except for CZMn, all the Ce0.75Zr0.2M0.05O2-δ materials showed higher TR aptitude. For instance, the average nO2 by CZZn, CZNi, CZFe, CZCr, and CZCo was higher than Ce0.75Zr0.25O2 by 32.9%, 20.4%, 19.8%, 11.7%, and 8.3%, individually. Fig. 12 indicate that the CZCr, CZMn, CZNi, and CZCo attained the CS stability (in terms of stable nCO ) by third cycle. In case of CZFe and CZCo, the stability in terms of constant nCO was achieved at around
Fig. 9. TGA profiles obtained for three cycles in case of Ce0.75Zr0.2M0.05O2-δ materials.
cycle 5. The computations further shows that the CZNi was capable of producing the highest nCO = 170.5 μmol/g·cycle and the aptitude of the CZMn was the lowest towards CS reaction (nCO = 108.2 μmol/g·cycle). The average nCO by the Ce0.75Zr0.2M0.05O2-δ materials follows the following order: CZNi > CZZn > CZCr > CZCo > CZFe > CZMn. Similar to the average nO2 , the average nCO by each Ce0.75Zr0.2M0.05O2-δ material was observed to be greater than the previously studied phase pure ceria and transition metal doped ceria oxides. In terms of numbers, the CZZn, CZNi, CZFe, CZCr, CZCo, and 7
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Table 3 nO2 and nCO by Ce0.75Zr0.2M0.05O2-δ materials during three cycles. Ce0.75Zr0.2M0.05O2-δ Materials
CZCr CZMn CZFe CZCo CZNi CZZn
Cycle 1
Cycle 2
Cycle 3
nO2 (μmol/g)
nCO (μmol/g)
nO2 (μmol/g)
nCO (μmol/g)
nO2 (μmol/g)
nCO (μmol/g)
218.6 130.5 285.6 152.6 152.6 707.6
167.0 177.1 168.9 176.8 227.3 188.7
101.9 88.6 147.5 115.1 107.0 201.7
164.6 146.8 155.5 167.5 202.8 175.8
86.0 67.6 115.3 99.3 100.6 127.4
167.0 134.7 153.6 162.3 192.7 171.7
Table 4 Comparison between Ce0.75Zr0.2M0.05O2-δ and other previously investigation ceria based redox materials based on the average nO2 and nCO from cycle 2 to cycle 10.
Fig. 10. TGA profiles obtained for ten cycles in case of Ce0.75Zr0.2M0.05O2-δ materials.
Materials
Average nO2 (μmol/g·cycle)
Average nCO (μmol/g·cycle)
CeO2 [36] Ce0.9Cr0.1O2 [36] CZCr Ce0.9Mn0.1O2 [36] CZMn Ce0.9Fe0.1O2 [36] CZFe Ce0.9Co0.1O2 [36] CZCo Ce0.9Ni0.1O2 [36] CZNi Ce0.9Zn0.1O2 [36] CZZn Ce0.9Zr0.1O2 [36] Ce0.75Zr0.25O2
47.6 49.4 79.8 41.2 56.6 50.0 88.0 45.0 76.9 45.7 88.6 50.5 105.1 41.0 70.5
95.6 88.8 150.7 82.2 108.2 96.3 126.3 91.6 128.0 92.1 170.5 103.3 154.2 79.7 135.2
CZMn produced uppermost average nCO as compared to Ce0.9Zn0.1O2, Ce0.9Fe0.1O2, Ce0.9Cr0.1O2, Ce0.9Co0.1O2, and Ce0.9Ni0.1O2, Ce0.9Mn0.1O2 by a factor of 1.49, 1.85, 1.31, 1.69, 1.39, and 1.31, respectively. The comparison further shows that the CZNi, CZZn, and CZCr were capable of producing higher, CZCo and CZFe were capable of generating comparable, and CZMn produced lower nCO in comparison with the Ce0.75Zr0.25O2 material. By considering the average nO2 and nCO from cycle 2 to cycle 10, the average nCO / nO2 allied with each Ce0.75Zr0.2M0.05O2-δ material was computed and presented in Fig. 13. The plot indicate that the average nCO / nO2 was highest in case of CZNi (1.92) and lowest for the CZFe material (1.44). The Ce0.75Zr0.2M0.05O2-δ materials can be categorized as CZNi ~ CZMn > CZCr > CZCo > CZZn > CZFe in terms of average nCO / nO2 . These results further indicate that the re-oxidation aptitude of CZNi, CZMn, and CZCr was superior as compared to CZCo, CZZn, and CZFe.
Fig. 11. nO2 by each Ce0.75Zr0.2M0.05O2-δ material from cycle 2 to cycle 10.
Fig. 13. Average nCO / nO2 related to each Ce0.75Zr0.2M0.05O2-δ material (from cycle 2 to cycle 10).
Fig. 12. nCO by each Ce0.75Zr0.2M0.05O2-δ material from cycle 2 to cycle 10.
8
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4. Summary and conclusions
2018. https://doi.org/10.1016/j.ceramint.2018.06.096. [12] Bhosale RR, Kumar A, AlMomani F, Alxneit I. Sol-gel derived CeO2-Fe2O3 nanoparticles: Synthesis, characterization and solar thermochemical application. Ceram Int 2015. https://doi.org/10.1016/j.ceramint.2016.01.042. [13] Bhosale RR, Takalkar G, Sutar P, Kumar A, AlMomani F, Khraisheh M. A decade of ceria based solar thermochemical H2O/CO2splitting cycle. Int J Hydrogen Energy 2018. https://doi.org/10.1016/j.ijhydene.2018.04.080. [14] Scheffe JR, Weibel D, Steinfeld A. Lanthanum-strontium-manganese perovskites as redox materials for solar thermochemical splitting of H2O and CO2. Energy Fuels 2013;27:4250–7. https://doi.org/10.1021/ef301923h. [15] Demont A, Abanades S. Solar thermochemical conversion of CO2 into fuel via twostep redox cycling of non-stoichiometric Mn-containing perovskite oxides. J Mater Chem A 2015. https://doi.org/10.1039/c4ta06655c. [16] Takacs M, Hoes M, Caduff M, Cooper T, Scheffe JR, Steinfeld A. Oxygen nonstoichiometry, defect equilibria, and thermodynamic characterization of LaMnO3perovskites with Ca/Sr A-site and Al B-site doping. Acta Mater 2016. https://doi.org/10.1016/j.actamat.2015.10.026. [17] Gálvez ME, Jacot R, Scheffe J, Cooper T, Patzke G, Steinfeld A. Physico-chemical changes in Ca, Sr and Al-doped La-Mn-O perovskites upon thermochemical splitting of CO2 via redox cycling. Phys Chem Chem Phys 2015. https://doi.org/10.1039/ c4cp05898d. [18] Muhich CL, Aston VJ, Trottier RM, Weimer AW, Musgrave CB. First-principles analysis of cation diffusion in mixed metal ferrite spinels. Chem Mater 2016;28:214–26. https://doi.org/10.1021/acs.chemmater.5b03911. [19] Scheffe JR, McDaniel AH, Allendorf MD, Weimer AW. Kinetics and mechanism of solar-thermochemical H2 production by oxidation of a cobalt ferrite–zirconia composite. Energy Environ Sci 2013;6:963. https://doi.org/10.1039/c3ee23568h. [20] Bhosale RR, Pasala X, Yelakanti S, Puszynski JA, Shende RV. Solar thermochemical water-splitting for H2 generation using sol-gel derived ferrite nanomaterials. Tech. Proc. 2012 NSTI Nanotechnol. Conf. Expo. NSTI-Nanotech; 2012. p. 2012. [21] Bhosale RR. Thermochemical H2production via solar driven hybrid SrO/ SrSO4water splitting cycle. Int J Hydrogen Energy 2018. https://doi.org/10.1016/ j.ijhydene.2018.02.053. [22] Bhosale RR, Kumar A, AlMomani F, Ghosh U, Khraisheh M. A comparative thermodynamic analysis of samarium and erbium oxide based solar thermochemical water splitting cycles. Int J Hydrogen Energy 2017. https://doi.org/10.1016/j. ijhydene.2017.03.172. [23] Bhosale RR, Sutar P, Kumar A, Almomani F, Ali MH, Ghosh U, et al. Solar hydrogen production via erbium oxide based thermochemical water splitting cycle. J Renew Sustain Energy 2016. https://doi.org/10.1063/1.4953166. [24] Chueh WC, Abbott M, Scipio D, Haile SM. High-flux solar-driven thermochemical dissociation of CO 2 and H2O using ceria redox reactions. Science (80-) 2010;63:2010. https://doi.org/10.1123/science.1197834. [25] Miller JE, Allendorf MD, Diver RB, Evans LR, Siegel NP, Stuecker JN. Metal oxide composites and structures for ultra-high temperature solar thermochemical cycles. J Mater Sci 2008. https://doi.org/10.1007/s10853-007-2354-7. [26] Abanades S, Legal A, Cordier A, Peraudeau G, Flamant G, Julbe A. Investigation of reactive cerium-based oxides for H2 production by thermochemical two-step watersplitting. J Mater Sci 2010. https://doi.org/10.1007/s10853-010-4506-4. [27] Kaneko H, Taku S, Tamaura Y. Reduction reactivity of CeO2-ZrO2oxide under high O2partial pressure in two-step water splitting process. Sol Energy 2011. https://doi. org/10.1016/j.solener.2011.06.019. [28] Le Gal A, Abanades S. Catalytic investigation of ceria-zirconia solid solutions for solar hydrogen production. Int J Hydrogen Energy 2011. https://doi.org/10.1016/ j.ijhydene.2011.01.078. [29] Hao Y, Yang CK, Haile SM. Ceria-zirconia solid solutions (Ce1–xZrxO2−δ, x ≤ 0.2) for solar thermochemical water splitting: a thermodynamic study. Chem Mater 2014;26:6073–82. https://doi.org/10.1021/cm503131p. [30] Scheffe JR, Jacot R, Patzke GR, Steinfeld A. Synthesis, characterization, and thermochemical redox performance of Hf4+, Zr4+, and Sc3+ doped ceria for splitting CO2. J Phys Chem C 2013. https://doi.org/10.1021/jp4050572. [31] Abanades S, Le Gal A. CO 2 splitting by thermo-chemical looping based on Zr xCe 1xO 2 oxygen carriers for synthetic fuel generation. Fuel 2012. https://doi.org/10. 1016/j.fuel.2012.06.068. [32] Call F, Roeb M, Schmücker M, Bru H, Curulla-Ferre D, Sattler C, et al. Thermogravimetric analysis of zirconia-doped ceria for thermochemical production of solar fuel. Am J Anal Chem 2013. https://doi.org/10.4236/ajac.2013.410A1005. [33] Zhao Z, Uddi M, Tsvetkov N, Yildiz B, Ghoniem AF. Enhanced intermediate-temperature CO 2 splitting using nonstoichiometric ceria and ceria-zirconia. Phys Chem Chem Phys 2017;19:25774–85. https://doi.org/10.1039/C7CP04789D. [34] Jiang Q, Zhou G, Jiang Z, Li C. Thermochemical CO2 splitting reaction with CexM1xO2-δ (M=Ti4+, Sn4+, Hf4+, Zr4+, La3+, Y3+ and Sm3+) solid solutions. Sol Energy 2014;99:55–66. https://doi.org/10.1016/j.solener.2013.10.021. [35] Muhich CL, Blaser S, Hoes MC, Steinfeld A. Comparing the solar-to-fuel energy conversion efficiency of ceria and perovskite based thermochemical redox cycles for splitting H2O and CO2. Int J Hydrogen Energy 2018. https://doi.org/10.1016/j. ijhydene.2018.08.137. [36] Takalkar GD, Bhosale RR, Kumar A, AlMomani F, Khraisheh M, Shakoor RA, et al. Transition metal doped ceria for solar thermochemical fuel production. Sol Energy 2018. https://doi.org/10.1016/j.solener.2018.03.022. [37] Patterson AL. The scherrer formula for X-ray particle size determination. Phys Rev 1939;56:978–82. https://doi.org/10.1103/PhysRev. 56.978.
By performing multiple thermochemical CS cycles, the redox reactivity of Ce0.75Zr0.2M0.05O2-δ materials (where, M = Cr, Mn, Fe, Co, Ni and Zn) was estimated. The Ce0.75Zr0.2M0.05O2-δ synthesis was achieved by using co-precipitation method in which ammonium hydroxide was utilized as the precipitating agent. In addition, a TGAbased high temperature reactor set-up was developed and utilized to perform multiple TR and CS steps in the temperature range of 1000 to 1400 °C by using the derived Ce0.75Zr0.2M0.05O2-δ materials. The PXRD analysis of the synthesized materials confirms formation of a singlephase Ce0.75Zr0.2M0.05O2-δ cubic fluorite Fm-3m structure. The average crystallite size of all the Ce0.75Zr0.2M0.05O2-δ materials was observed to be in the range of 30–50 nm. In addition, the SEM analysis indicate agglomerated roundish particle morphology of all the Ce0.75Zr0.2M0.05O2-δ materials synthesized via co-precipitation method. The results obtained in the TGA analysis notify that most of the Ce0.75Zr0.2M0.05O2-δ materials attained stability in terms of nO2 at around cycle 4 or cycle 5. Furthremore, the steady nCO was noticed in case of CZCr, CZMn, CZNi, and CZCo by the third cycle and for CZFe and CZCo by the fifth cycle. The TR and CS aptitude of all the Ce0.75Zr0.2M0.05O2-δ materials was observed to be higher as compared to the previously studied phase pure ceria and transition metal doped ceria oxides. In terms of numbers, among the all Ce0.75Zr0.2M0.05O2-δ materials explored in this study, the CZNi appears to be the best choice in terms of higher average nCO = 170.5 (μmol/g·cycle) and maximum average nCO / nO2 = 1.92 (in ten consecutive thermochemical cycles). Acknowledgments This publication was made possible by the NPRP grant (NPRP8-3702-154) from the Qatar National Research Fund (a member of Qatar Foundation). The statements made herein are solely the responsibility of author(s). References [1] Wood DA, Nwaoha C, Towler BF. Gas-to-liquids (GTL): A review of an industry offering several routes for monetizing natural gas. J Nat Gas Sci Eng 2012. https:// doi.org/10.1016/j.jngse.2012.07.001. [2] Velasco JA, Lopez L, Velásquez M, Boutonnet M, Cabrera S, Järås S. Gas to liquids: A technology for natural gas industrialization in Bolivia. J Nat Gas Sci Eng 2010. https://doi.org/10.1016/j.jngse.2010.10.001. [3] Loutzenhiser PG, Meier A, Steinfeld A. Review of the Two-Step H2O/CO2-Splitting solar thermochemical cycle based on Zn/ZnO redox reactions. Materials (Basel) 2010;3:4922–38. https://doi.org/10.3390/ma3114922. [4] Bhosale R, Kumar A, AlMomani F, Gupta RB. Solar thermochemical ZnO/ZnSO4 water splitting cycle for hydrogen production. Int J Hydrogen Energy 2016. https:// doi.org/10.1016/j.ijhydene.2017.02.190. [5] Bhosale RR. Thermodynamic efficiency analysis of zinc oxide based solar driven thermochemical H2O splitting cycle: Effect of partial pressure of O2, thermal reduction and H2O splitting temperatures. Int J Hydrogen Energy 2018. [6] Abanades S, Charvin P, Lemont F, Flamant G. Novel two-step SnO2/SnO watersplitting cycle for solar thermochemical production of hydrogen. Int J Hydrogen Energy 2008;33:6021–30. https://doi.org/10.1016/j.ijhydene.2008.05.042. [7] Bhosale RR, Kumar A, Sutar P. Thermodynamic analysis of solar driven SnO2/SnO based thermochemical water splitting cycle. Energy Convers Manag 2017;135:226–35. https://doi.org/10.1016/j.enconman.2016.12.067. [8] Bhosale RR, Kumar A, Van Den Broeke LJP, Gharbia S, Dardor D, Jilani M, et al. Solar hydrogen production via thermochemical iron oxide-iron sulfate water splitting cycle. Int J Hydrogen Energy 2015;40:1639–50. https://doi.org/10.1016/j. ijhydene.2014.11.118. [9] Scheffe JR, Allendorf MD, Coker EN, Jacobs BW, McDaniel AH, Weimer AW. Hydrogen production via chemical looping redox cycles using atomic layer deposition-synthesized iron oxide and cobalt ferrites. Chem Mater 2011;23:2030–8. https://doi.org/10.1021/cm103622e. [10] Scheffe JR, Steinfeld A. Thermodynamic analysis of cerium-based oxides for solar thermochemical fuel production. Energy Fuels 2012;26:1928–36. https://doi.org/ 10.1021/ef201875v. [11] Bhosale RR, Takalkar GD. Nanostructured co-precipitated Ce0.9Ln0.1O2(Ln = La, Pr, Sm, Nd, Gd, Tb, Dy, or Er) for thermochemical conversion of CO2. Ceram Int
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Full Length Article
Thermochemical splitting of CO2 using Co-precipitation synthesized Ce0.75Zr0.2M0.05O2-δ (M = Cr, Mn, Fe, CO, Ni, Zn) materials
T
Gorakshnath Takalkar, Rahul R. Bhosale , Fares AlMomani ⁎
Department of Chemical Engineering, College of Engineering, Qatar University, P. O. Box – 2713, Doha, Qatar
ARTICLE INFO
ABSTRACT
Keywords: CO2 splitting Thermochemical cycles Ceria-zirconia solid solution Co-precipitation method Redox reactivity Solar fuels
This work reports the investigation of the redox reactivity of Ce0.75Zr0.2M0.05O2-δ (M = Cr, Mn, Fe, Ni, Co, Zn) materials towards thermochemical CO2 splitting (CS) cycle. The Ce0.75Zr0.2M0.05O2-δ materials were prepared via co-precipitation method and the derived materials were characterized to determine the phase/elemental composition and microstructural morphology. The powder X-ray diffraction (PXRD) analysis indicate formation of phase pure Ce0.75Zr0.2M0.05O2-δ materials with no metal or metal oxide impurities. The analysis performed using scanning electron microscopy confirms production of agglomerated roundish particles of Ce0.75Zr0.2M0.05O2-δ materials. Synthesized Ce0.75Zr0.2M0.05O2-δ materials were further tested, using a thermogravimetric analyzer (TGA), to determine their redox reactivity towards CS reactions. The obtained results indicate that all the Ce0.75Zr0.2M0.05O2-δ materials possess better thermal reduction (TR) and CS aptitude as compared to previously studied phase pure ceria and transition metal doped ceria oxides. The obtained results further indicate that, except for Ce0.75Zr0.2Mn0.05O2-δ material, all the other Ce0.75Zr0.2M0.05O2-δ materials were capable of releasing higher amounts of O2 during TR performed at 1400 °C as compared to Ce0.75Zr0.25O2-δ. Overall in ten thermochemical cycles, the Ce0.75Zr0.2Zn0.05O2-δ showed the highest O2 releasing capacity (105.1 μmol/g·cycle) and the Ce0.75Zr0.2Ni0.05O2-δ indicated the maximum CO production aptitude (170.5 μmol/g·cycle).
1. Introduction Gas to liquid (GTL) technology, based on the Fischer–Tropsch (FT) process, is a well-known option to convert natural gas i.e., CH4 into synthetic liquid fuels [1,2]. Pearl GTL (from Shell) located in Qatar is one of the largest capacity GTL plant. This plant operates in the following three steps: 1) conversion of CH4 into synthesis gas (mixture of H2 and CO), 2) conversion of synthesis gas in to liquid hydrocarbons, 3) cracking and isomerization to produce the synthetic fuels such as diesel, kerosene, etc. This process has the potential to produce synthetic fuels from CH4, however emission of CO2 is considered as one of the major concerns. As an alternative to the GTL process, a metal oxide (MO) based solar-assisted two-step thermochemical H2O splitting (WS) and CO2 splitting (CS) redox cycle can be considered for the production of syngas. As shown in Fig. 1, the combination of solar thermochemical syngas production cycle and the catalytic FT process provides a sustainable and environment friendly route for the generation of synthetic fuels from H2O and CO2. The MO-based WS/CS cycle is advantageous as it a) decreases the operating temperatures and b) resolves the issues associated with the ⁎
gas separation allied with the direct WS/CS splitting reactions. The reactions allied to the solar thermochemical WS/CS cycle are presented below. Thermal reduction (TR) step:
MO(x )
MO(x
)
+
2
O2
(1)
WS/CS step:
MO(x
)
+ H2 O/CO2
MO(x ) + H2 / CO
(2)
A variety of MOs such as zinc oxide [3–5], tin oxide [6–7], iron oxide [8–9], ceria and doped ceria [10–13], perovskites [14–17], ferrites [18–20], other MOs [21–23] were utilized until now as the redox active materials for the splitting of H2O/CO2. Among the listed MOs, because of their a) higher oxygen storage capacity (OSC), b) faster kinetics associated with the TR and WS/CS reactions, and c) excellent thermal stability, the ceria based redox materials were considered as the state of the art redox materials for solar thermochemical WS/CS cycles. Chueh et al. [24] have demonstrated the feasibility of CeO2 towards stable fuel generation via WS and CS reactions over 500
Corresponding author. E-mail address: [email protected] (R.R. Bhosale).
https://doi.org/10.1016/j.fuel.2019.115834 Received 27 January 2019; Received in revised form 13 June 2019; Accepted 17 July 2019 Available online 05 August 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.
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Fig. 1. Schematic of the combined solar thermochemical H2O/CO2 splitting cycle and catalytic Fischer Tropsch Process for the production of synthetic fuels.
thermochemical cycles. To further improve the fuel production capacity of CeO2, several researchers have explored Zr-doped ceria materials (ceria zirconia solid solutions). Miller et al. [25] prepared and utilized the CeO2-ZrO2 monolithic structures for thermochemically splitting the H2O/CO2. Thermochemical WS ability of the CeO2-ZrO2 solid solution was compared with the other CeO2-MOX solid solutions (where, M = Mn, Fe, Co, Cu, and Zn) by Abanades et al. [26] and reported that the CeO2-ZrO2 solid solution is the foremost choice in terms of the redox reactivity towards TR and WS steps. Kaneko et al. [27] tested the O2 releasing capacity of the Ce1-xZrxO2 (x = 0.1, 0.2, 0.3) solid solutions by performing the TR in presence of high O2 partial pressure and reported that the amount of O2 released (nO2 ) by Ce0.8Zr0.2O2-δ was greater than the CeO2 and other ceria-zirconia solid solutions. Le Gal and Abanades [28] reported that the Ce0.75Zr0.25O2-δ showed better TR and WS aptitude as compared to Ce0.9Zr0.1O2-δ and Ce0.5Zr0.5O2-δ for multiple TR (at 1400 °C) and WS steps (at 1050 °C). The thermodynamic and experimental exploration of Ce1-xZrxO2 (x = 0.05, 0.1, 0.15, 0.2) towards WS cycles was reported by Hao et al. [29] and the obtained results shows that the H2 production was enhanced with the increase in the Zr concentration (optimum range 15–20%). A study performed by Scheffe et al. [30] shows that the inclusion of Zr and Hf in the CeO2 fluorite structure was responsible for the improvement in the CO production via CS reaction. The Ce1-xZrxO2 redox materials were tested by Abanades and Le Gal [31] and reported that the 10% incorporation of Zr in the ceria crystal structure was advantageous to achieve CO production up to 130 μmol/g·cycle. According to Call et al. [32], the Ce0.85Zr0.15O2 material was capable of attaining the maximum nO2 = 155 μmol/g·cycle and nCO = 305 μmol/ g·cycle as compared to the other Ce1-xZrxO2 materials (where, x = 0–0.375) in four thermochemical CS cycles. Zhao et al. [33] and Jiang et al. [34] reported that the reduction in the activation energy associated with the CS reaction was responsible for the production of higher amount of CO by ceria-zirconia solid solutions in comparison to the phase pure ceria material. Recently, Munich et al. [35] reported that the ceria-zirconia solid solutions can be considered as promising redox materials based on their high solar-to-fuel energy conversion efficiency. These authors further added that the inclusion of the alkaline metals and transition metals as the dopants in the ceria-zirconia solid solutions resulted into improvement in the H2 production via WS reaction. To achieve production of high quality syngas, in addition to the H2, CO generation via CO2 splitting cycle is also an important step. Therefore, in this investigation, the influence of incorporation of transition metals such as Cr, Mn, Fe, Co, Ni, and Zn in the ceria-zirconia crystal structure on the
thermochemical CS ability was examined. The Ce0.75Zr0.2M0.05O2-δ (where, M = Cr, Mn, Fe, Co, Ni, Zn) were synthesized using co-precipitation method. As-prepared Ce0.75Zr0.2M0.05O2-δ materials were analyzed using multiple characterization techniques. A high temperature thermogravimetric analyzer (TGA) was utilized to determine the redox reactivity of each Ce0.75Zr0.2M0.05O2-δ material by performing multiple CS cycles. 2. Experimental sections 2.1. Synthesis and characterization of Ce0.75Zr0.2M0.05O2-δ materials Synthesis of Ce0.75Zr0.2M0.05O2-δ materials using co-precipitation method was conducted by using following precursors (purchased from Sigma Aldrich, USA): Ce(III)(NO3)2·6H2O, Ni(II)(NO3)2·6H2O, Co(II) (NO3)2·6H2O, Zn(NO3)2·6H2O, Cr(III)(NO3)3·9H2O, Mn(II)(NO3)2·4H2O, Zr(IV)N2O7·H2O, Cr(III) (NO3)3·9H2O, Fe(III)(NO3)2·3H2O and 28% ammonium hydroxide. Deionized (DI) water was utilized for the dissolution of metal precursors. Gas cylinders containing a) Ar with a stated purity of 99.999% and b) feedstock gas mixture containing 50% CO2 in Ar were acquired from a local supplier. The amounts of metal precursors needed for the synthesis of 1 g of Ce0.75Zr0.2M0.05O2-δ material were dissolved in excess DI water. To this solution, an aqueous ammonium hydroxide was added (as a precipitating agent) until the pH of the solution reaches ~10. This aqueous solution was further stirred for 24–48 h (with a maintained pH ~10) to complete the precipitation reaction. The obtained precipitate of Ce0.75Zr0.2M0.05O2-δ was washed multiple times with DI water using a vacuum filtration unit (to remove the unreacted chemicals). The washed solid particles were dried at 120–130 °C for multiple hours to remove the moisture. Finally, the dried particles were crushed into a fine powder and this powder was further calcined up to 1000 °C in air for 3–4 h using a muffle furnace. For simplicity, the Ce0.75Zr0.2M0.05O2-δ materials were abbreviated as: Ce0.75Zr0.2Cr0.05O2-δ (CZCr), Ce0.75Zr0.2Mn0.05O2-δ (CZMn), Ce0.75Zr0.2Fe0.05O2-δ (CZFe), Ce0.75Zr0.2Co0.05O2-δ (CZCo), Ce0.75Zr0.2Ni0.05O2-δ (CZNi), and Ce0.75Zr0.2Zn0.05O2-δ (CZZn). Crystallographic phase composition of the Ce0.75Zr0.2M0.05O2-δ materials was determined by using a Panalytical XPert MPD/DY636 powder X-ray diffractometer with CuKα radiation. The scanning electron microscope equipped with the energy dispersive spectroscopy (SEM/EDS, Nova Nano 450, FEI) was applied to identify and distinguish the overall particle morphology and chemical composition of the synthesized Ce0.75Zr0.2M0.05O2-δ materials. 2
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Fig. 2. Thermochemical experimental set-up involving a Setaram SETSYS Evolution, TGA-DSC instrument.
2.2. Thermochemical redox cycles
CS steps. To eliminate the influence of buoyancy on the final results, a baseline TGA profile obtained by conducting a blank run (by using an empty alumina crucible only) at operating conditions identical to the CS experiments was subtracted from the TGA profile obtained during the actual experiment. The mass loss ( mloss ) reported during the TR step and the mass gain ( mgain) recorded during the CS step were directly attributed to the nO2 and nCO by Ce0.75Zr0.2M0.05O2-δ materials as per Eqs. (3) and (4).
The thermochemical CS reactivity of the Ce0.75Zr0.2M0.05O2-δ materials was estimated by using a TGA setup (Setaram SETSYS Evolution, TGA-DSC) depicted in Fig. 2. The heating furnace installed in the TGA set-up was consists of a graphite-heating element, which was protected by an alumina tube. A temperature controller was utilized to maintain the temperature of the TGA set-up at the desired value. During the TGA experiments, a continuous flow of chilled water was supplied to the setup to maintain the temperature of exhaust stream (leaving the TGA setup) at ambient conditions. Ultrapure inert Ar was circulated through a) the heating furnace to avoid the oxidation of the graphite-element and b) the TGA balance to dodge the prospect of pollution of the balance and the sample chamber from the vapors generated during the TGA experiments. Mass flow controllers and pressure transducers were installed in the TGA set-up to maintain the carrier and protective inert Ar gas flowrates. Other details associated with the Setaram SETSYS Evolution TGA-DSC set-up were already reported in our previous studies [36]. The TR and CS ability of each Ce0.75Zr0.2M0.05O2-δ material was estimated by performing multiple thermochemical CS cycles using roughly 50 mg of the reactive sample. This Ce0.75Zr0.2M0.05O2-δ powder was placed on the hanging balance with the help of an alumina crucible and the TR and CS runs were carried out at 1400 °C for 60 min and 1000 °C for 30 min, respectively. The TR step was performed in the presence of inert Ar whereas the CS step was conducted by using a feed gas mixture containing 50% CO2 in Ar. The mass flowrates of both Ar and the feed gas mixture were maintained at 100 ml/min during TR and
no2 =
nco =
mloss (Mo2 × mce0.75 Zr 0.2 M0.05 o2 )
(3)
mgain (Mo × mce0.75 Zr 0.2 M0.05 o2 )
(4)
where, MO2 Molecular weight of O2 MO Molecular weight of O mce0.75 Zr 0.2 M0.05 o2 Mass of Ce0.75Zr0.2M0.05O2-δ materials 3. Results and discussion 3.1. Characterization of Ce0.75Zr0.2M0.05O2-δ All the Ce0.75Zr0.2M0.05O2-δ materials synthesized via co-precipitation method were first characterized via PXRD analysis. The PXRD patterns associated with each Ce0.75Zr0.2M0.05O2-δ material are presented in Fig. 3a. Each PXRD pattern shows formation of a single-phase cubic fluorite Fm-3m structure which is identical to that of the phase 3
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Fig. 3. PXRD patterns of the co-precipitation synthesized Ce0.75Zr0.2M0.05O2-δ materials, a) 2θ = 20° to 80°, and b) 2θ = 28° to 29.5°.
pure ceria material. The absence of any metal or metal oxide impurities further provided a confirmation that the transition metal dopants were successfully incorporated in the ceria-zirconia fluorite structure. As the crystal ionic radii of Cr, Mn, Fe, Co, Ni, and Zn are lower as compared to the Ce and Zr, the peaks associated with the Ce0.75Zr0.2M0.05O2-δ materials were shifted towards the lower 2θ angle as compared to the Ce0.75Zr0.25O2-δ solid solution (Fig. 3b). The results obtained after performing the EDS analysis further validated the formation of Ce0.75Zr0.2M0.05O2-δ materials. The EDS patterns associated with each Ce0.75Zr0.2M0.05O2-δ material is presented in Fig. 4 and the chemical compositions are reported in Table 1. In addition to the phase composition, the PXRD analysis was also utilized to estimate the average crystallite size of each Ce0.75Zr0.2M0.05O2-δ material by applying the Scherrer equation [37]. The results obtained indicate that there is no fixed trend allied with the variation in the average crystallite size of Ce0.75Zr0.2M0.05O2-δ materials as a function of the change in the dopant metal. The average crystallite size of all the Ce0.75Zr0.2M0.05O2-δ materials was observed to be in the range of 30–50 nm. The maximum and minimum average crystallite size was noticed in case of the CZFe (49.5 nm) and CZCr (30.5 nm), respectively. The microstructural morphology of the co-precipitation synthesized Ce0.75Zr0.2M0.05O2-δ materials was analyzed by performing SEM analysis. The images obtained for CZCr, CZMn, CZFe, CZCo, CZNi, and CZZn are presented in Fig. 5 which shows agglomerated roundish particle morphology (for all the derived materials). Furthermore, it was understood that the material morphology was not affected due to the inclusion of Cr, Mn, Fe, Co, Ni, and Zn in the Ce0.75Zr0.2M0.05O2-δ crystal structure.
structure. The plots reported further indicate that the kinetics associated with the CS step was much faster as compared to the TR step. At the beginning, the redox reactivity of the Ce0.75Zr0.2M0.05O2-δ materials was determined by performing one thermochemical cycle. The TGA profiles allied with the TR and CS steps of the first thermochemical cycle are reported in Figs. 7 and 8. As expected, the mass of each Ce0.75Zr0.2M0.05O2-δ material was reduced during the TR step. The TGA profiles reported in Fig. 7 shows that all the Ce0.75Zr0.2M0.05O2-δ materials continue to lose weight for the entire TR duration. The kinetics associated with the TR of CZZn was the fastest and for the TR of CZNi was the slowest. The mloss allied with the first TR step was converted into nO2 by each Ce0.75Zr0.2M0.05O2-δ material by using Eq. (3) and the numbers obtained are reported in Table 2. The data presented in Table 2 shows that the CZZn was capable of releasing highest nO2 (707.6 µmol/g) and the CZMn liberated the lowest nO2 (130.5 µmol/ g) as compared to the remaining Ce0.75Zr0.2M0.05O2-δ materials. The obtained results also indicate that the CZCo and CZNi possess the capacity to release identical nO2 during TR at 1400 °C. As presented in Fig. 8, the CZCo represents the quickest and CZMn shows the lethargic re-oxidation kinetics during the CS step performed at 1000 °C in comparison to the remaining Ce0.75Zr0.2M0.05O2-δ materials. Similar to the TR step, all the Ce0.75Zr0.2M0.05O2-δ materials were continue to gain weight during the entire duration assigned for the CS step (30 min). This observation indicate that the time assigned for the CS step was not enough to completely re-oxidize the Ce0.75Zr0.2M0.05O2δ materials. The nCO produced by each Ce0.75Zr0.2M0.05O2-δ material (Table 2), during 30 min of CS step, backs the above-mentioned statement. The numbers listed in Table 1 imply that all the Ce0.75Zr0.2M0.05O2-δ materials were partially re-oxidized as the nCO /nO2 ratio was significantly less than 2.0. Based on the time allocated for the CS step, the CZNi produced the uppermost (227.3 µmol/g) and CZCr generated the lowermost nCO (167.0 µmol/g) as compared to other Ce0.75Zr0.2M0.05O2-δ materials. As the nCO /nO2 for the first thermochemical cycle was very low, it was essential to determine if this ratio persists during multiple thermochemical cycles. In this viewpoint, three consecutive thermochemical cycles were performed by conducting the TR and CS steps at conditions identical to the first cycle. The variation in the gain and loss of the Ce0.75Zr0.2M0.05O2-δ mass over three cycles is displayed in Fig. 9. Furthermore, the mloss and mgain recorded for the three cycles were converted into nO2 and nCO by each Ce0.75Zr0.2M0.05O2-δ material
3.2. Redox reactivity of Ce0.75Zr0.2M0.05O2-δ After characterization, the evaluation of the redox reactivity of the calcined Ce0.75Zr0.2M0.05O2-δ powders was conducted by using a TGA set-up. For all the TGA experiments, the TR and CS steps were performed at 1400 °C (for 60 min) and 1000 °C (for 30 min). A typical TGA profile obtained in case of CZNi material (attained after subtracting the blank run) is presented as an example in Fig. 6. The plot associated with the TR step indicate mass loss in two sections: 1) below 1000 °C due to the dissipation of moisture and other chemical impurities and b) above 1000 °C due to the release of O2 from the Ce0.75Zr0.2M0.05O2-δ fluorite 4
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Fig. 4. EDS patterns of the co-precipitation synthesized a) CZCr, b) CZMn, c) CZFe, d) CZCo, e) CZNi, and f) CZZn. Table 1 Chemical composition, Ce/Zr/M (where M = transition metal cation) molar ratio (as-prepared and from EDS), and the abbreviations assigned to each Ce0.75Zr0.2M0.05O2-δ material. Abbreviation
Ce/Zr/M (as-prepared)
Ce/Zr/M (from EDS)
Composition
CZCr CZMn CZFe CZCo CZNi CZZn
0.750/0.200/0.050 0.750/0.200/0.050 0.750/0.200/0.050 0.750/0.200/0.050 0.750/0.200/0.050 0.750/0.200/0.050
0.743/0.216/0.048 0.752/0.211/0.049 0.744/0.199/0.050 0.741/0.205/0.049 0.739/0.208/0.051 0.751/0.198/0.050
Ce0.743Zr0.216Cr0.048O2-δ Ce0.752Zr0.211Cr0.049O2-δ Ce0.744Zr0.199Cr0.050O2-δ Ce0.741Zr0.205Cr0.049O2-δ Ce0.739Zr0.208Cr0.051O2-δ Ce0.751Zr0.198Cr0.050O2-δ
(Table 3). The nO2 by each Ce0.75Zr0.2M0.05O2-δ material in the second cycle was observed to be considerably inferior than the nO2 liberated in the first cycle. For instance, the nO2 by CZCr, CZFe, and CZNi was decreased by 116.7 µmol/g, 138.1 µmol/g, and 45.7 µmol/g, respectively in cycle 2. Similarly, the nCO by each Ce0.75Zr0.2M0.05O2-δ material was also reduced in the second cycle. The magnitude of the drop in the nCO in the
second cycles by CZMn (30.3 µmol/g) was the highest and by CZCr (2.5 µmol/g) was the lowest as compared to the remaining Ce0.75Zr0.2M0.05O2-δ materials. In case of the third cycle, the nO2 by each Ce0.75Zr0.2M0.05O2-δ material continue to drop considerably. For instance, the percentage of fall in the nO2 by Ce0.75Zr0.2M0.05O2-δ materials during the third cycle as compared to the second one can be arranged as follows: CZZn (36.8%) > CZMn 5
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Fig. 5. SEM images of the co-precipitation synthesized Ce0.75Zr0.2M0.05O2-δ materials.
(23.7%) > CZFe (21.8%) > CZCr (15.7%) > CZCo (13.7%) > CZNi (6.0%). Contrast to the decline in the nO2 , the nCO by most of the Ce0.75Zr0.2M0.05O2-δ materials in the third cycle was roughly identical to the nCO in the second cycle, except for the CZMn and CZNi materials. The results described in the previous paragraphs indicate that, although the CS ability of the Ce0.75Zr0.2M0.05O2-δ materials attained stability, the TR aptitude of each material was not steady during first three cycles. Hence, as it is crucial to identify the long-term redox reactivity of the derived Ce0.75Zr0.2M0.05O2-δ materials, ten consecutive thermochemical cycles were performed and the TGA profiles obtained are presented in Fig. 10. Similar to the first three cycles, the nO2 and nCO by each Ce0.75Zr0.2M0.05O2-δ material during ten cycles was estimated by using the mass variation recorded by the TGA and the equations listed in the experimental section. During the determination of the average nO2 and nCO , to avoid inaccuracy, the data acquired by the TGA during the first
cycle was not considered. The nO2 and nCO by all the Ce0.75Zr0.2M0.05O2δ materials during each cycle (cycle 2 to cycle 10) is presented in Fig. 11 and Fig. 12, respectively. To compare with the Ce0.75Zr0.2M0.05O2-δ materials, the co-precipitation synthesized Ce0.75Zr0.25O2 was also tested for ten consecutive thermochemical cycles. The comparison between the Ce0.75Zr0.2M0.05O2-δ materials and other previously investigated ceria based materials, based on the average nO2 , nCO , and nCO /nO2 (from cycle 2 to cycle 10), is presented in Table 4. The data presented in Fig. 11 shows that most of the Ce0.75Zr0.2M0.05O2-δ materials attained stability in terms of nO2 at around cycle 4 or cycle 5. When compared with each other, the highest nO2 = 105.1 μmol/g·cycle was recorded in case of CZZn and the lowest nO2 = 56.6 μmol/g·cycle was observed for CZMn (from cycle 2 to cycle 10). Based on the average nO2 from cycle 2 to cycle 10, the Ce0.75Zr0.2M0.05O2-δ materials can be arranged as: CZZn > CZNi > CZFe > CZCr > CZCo > CZMn. 6
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Fig. 6. A typical TGA profile obtained in case of CZNi material.
Fig. 8. TGA profiles obtained in case of CS of Ce0.75Zr0.2M0.05O2-δ at 1000 °C (30 min). Table 2 nO2 , nCO , and CO/O2 molar ratio related to the Ce0.75Zr0.2M0.05O2-δ materials during the first thermochemical cycle. Ce0.75Zr0.2M0.05O2-δ materials
nO2 (µmol/g)
nCO (µmol/g)
nCO /nO2
CZCr CZMn CZFe CZCo CZNi CZZn
218.6 130.5 285.6 152.6 152.6 707.6
167.0 177.1 168.9 176.8 227.3 188.7
0.76 1.36 0.59 1.16 1.49 0.27
Fig. 7. TGA profiles obtained in case of TR of Ce0.75Zr0.2M0.05O2-δ at 1400 °C (60 min).
When compared with the previously investigated phase pure ceria and transition metal doped ceria oxides, the average nO2 by each Ce0.75Zr0.2M0.05O2-δ material was considerably higher (Table 4). The average nO2 by CZZn, CZNi, CZFe, CZCr, CZCo, and CZMn was greater than Ce0.9Zn0.1O2, Ce0.9Ni0.1O2, Ce0.9Fe0.1O2, Ce0.9Cr0.1O2, Ce0.9Co0.1O2, and Ce0.9Mn0.1O2 by 54.6 μmol/g·cycle, 42.9 μmol/ g·cycle, 38.0 μmol/g·cycle, 30.4 μmol/g·cycle, 31.9 μmol/g·cycle, and 15.4 μmol/g·cycle, respectively. When compared with the Ce0.75Zr0.25O2, except for CZMn, all the Ce0.75Zr0.2M0.05O2-δ materials showed higher TR aptitude. For instance, the average nO2 by CZZn, CZNi, CZFe, CZCr, and CZCo was higher than Ce0.75Zr0.25O2 by 32.9%, 20.4%, 19.8%, 11.7%, and 8.3%, individually. Fig. 12 indicate that the CZCr, CZMn, CZNi, and CZCo attained the CS stability (in terms of stable nCO ) by third cycle. In case of CZFe and CZCo, the stability in terms of constant nCO was achieved at around
Fig. 9. TGA profiles obtained for three cycles in case of Ce0.75Zr0.2M0.05O2-δ materials.
cycle 5. The computations further shows that the CZNi was capable of producing the highest nCO = 170.5 μmol/g·cycle and the aptitude of the CZMn was the lowest towards CS reaction (nCO = 108.2 μmol/g·cycle). The average nCO by the Ce0.75Zr0.2M0.05O2-δ materials follows the following order: CZNi > CZZn > CZCr > CZCo > CZFe > CZMn. Similar to the average nO2 , the average nCO by each Ce0.75Zr0.2M0.05O2-δ material was observed to be greater than the previously studied phase pure ceria and transition metal doped ceria oxides. In terms of numbers, the CZZn, CZNi, CZFe, CZCr, CZCo, and 7
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Table 3 nO2 and nCO by Ce0.75Zr0.2M0.05O2-δ materials during three cycles. Ce0.75Zr0.2M0.05O2-δ Materials
CZCr CZMn CZFe CZCo CZNi CZZn
Cycle 1
Cycle 2
Cycle 3
nO2 (μmol/g)
nCO (μmol/g)
nO2 (μmol/g)
nCO (μmol/g)
nO2 (μmol/g)
nCO (μmol/g)
218.6 130.5 285.6 152.6 152.6 707.6
167.0 177.1 168.9 176.8 227.3 188.7
101.9 88.6 147.5 115.1 107.0 201.7
164.6 146.8 155.5 167.5 202.8 175.8
86.0 67.6 115.3 99.3 100.6 127.4
167.0 134.7 153.6 162.3 192.7 171.7
Table 4 Comparison between Ce0.75Zr0.2M0.05O2-δ and other previously investigation ceria based redox materials based on the average nO2 and nCO from cycle 2 to cycle 10.
Fig. 10. TGA profiles obtained for ten cycles in case of Ce0.75Zr0.2M0.05O2-δ materials.
Materials
Average nO2 (μmol/g·cycle)
Average nCO (μmol/g·cycle)
CeO2 [36] Ce0.9Cr0.1O2 [36] CZCr Ce0.9Mn0.1O2 [36] CZMn Ce0.9Fe0.1O2 [36] CZFe Ce0.9Co0.1O2 [36] CZCo Ce0.9Ni0.1O2 [36] CZNi Ce0.9Zn0.1O2 [36] CZZn Ce0.9Zr0.1O2 [36] Ce0.75Zr0.25O2
47.6 49.4 79.8 41.2 56.6 50.0 88.0 45.0 76.9 45.7 88.6 50.5 105.1 41.0 70.5
95.6 88.8 150.7 82.2 108.2 96.3 126.3 91.6 128.0 92.1 170.5 103.3 154.2 79.7 135.2
CZMn produced uppermost average nCO as compared to Ce0.9Zn0.1O2, Ce0.9Fe0.1O2, Ce0.9Cr0.1O2, Ce0.9Co0.1O2, and Ce0.9Ni0.1O2, Ce0.9Mn0.1O2 by a factor of 1.49, 1.85, 1.31, 1.69, 1.39, and 1.31, respectively. The comparison further shows that the CZNi, CZZn, and CZCr were capable of producing higher, CZCo and CZFe were capable of generating comparable, and CZMn produced lower nCO in comparison with the Ce0.75Zr0.25O2 material. By considering the average nO2 and nCO from cycle 2 to cycle 10, the average nCO / nO2 allied with each Ce0.75Zr0.2M0.05O2-δ material was computed and presented in Fig. 13. The plot indicate that the average nCO / nO2 was highest in case of CZNi (1.92) and lowest for the CZFe material (1.44). The Ce0.75Zr0.2M0.05O2-δ materials can be categorized as CZNi ~ CZMn > CZCr > CZCo > CZZn > CZFe in terms of average nCO / nO2 . These results further indicate that the re-oxidation aptitude of CZNi, CZMn, and CZCr was superior as compared to CZCo, CZZn, and CZFe.
Fig. 11. nO2 by each Ce0.75Zr0.2M0.05O2-δ material from cycle 2 to cycle 10.
Fig. 13. Average nCO / nO2 related to each Ce0.75Zr0.2M0.05O2-δ material (from cycle 2 to cycle 10).
Fig. 12. nCO by each Ce0.75Zr0.2M0.05O2-δ material from cycle 2 to cycle 10.
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4. Summary and conclusions
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By performing multiple thermochemical CS cycles, the redox reactivity of Ce0.75Zr0.2M0.05O2-δ materials (where, M = Cr, Mn, Fe, Co, Ni and Zn) was estimated. The Ce0.75Zr0.2M0.05O2-δ synthesis was achieved by using co-precipitation method in which ammonium hydroxide was utilized as the precipitating agent. In addition, a TGAbased high temperature reactor set-up was developed and utilized to perform multiple TR and CS steps in the temperature range of 1000 to 1400 °C by using the derived Ce0.75Zr0.2M0.05O2-δ materials. The PXRD analysis of the synthesized materials confirms formation of a singlephase Ce0.75Zr0.2M0.05O2-δ cubic fluorite Fm-3m structure. The average crystallite size of all the Ce0.75Zr0.2M0.05O2-δ materials was observed to be in the range of 30–50 nm. In addition, the SEM analysis indicate agglomerated roundish particle morphology of all the Ce0.75Zr0.2M0.05O2-δ materials synthesized via co-precipitation method. The results obtained in the TGA analysis notify that most of the Ce0.75Zr0.2M0.05O2-δ materials attained stability in terms of nO2 at around cycle 4 or cycle 5. Furthremore, the steady nCO was noticed in case of CZCr, CZMn, CZNi, and CZCo by the third cycle and for CZFe and CZCo by the fifth cycle. The TR and CS aptitude of all the Ce0.75Zr0.2M0.05O2-δ materials was observed to be higher as compared to the previously studied phase pure ceria and transition metal doped ceria oxides. In terms of numbers, among the all Ce0.75Zr0.2M0.05O2-δ materials explored in this study, the CZNi appears to be the best choice in terms of higher average nCO = 170.5 (μmol/g·cycle) and maximum average nCO / nO2 = 1.92 (in ten consecutive thermochemical cycles). Acknowledgments This publication was made possible by the NPRP grant (NPRP8-3702-154) from the Qatar National Research Fund (a member of Qatar Foundation). The statements made herein are solely the responsibility of author(s). References [1] Wood DA, Nwaoha C, Towler BF. Gas-to-liquids (GTL): A review of an industry offering several routes for monetizing natural gas. J Nat Gas Sci Eng 2012. https:// doi.org/10.1016/j.jngse.2012.07.001. [2] Velasco JA, Lopez L, Velásquez M, Boutonnet M, Cabrera S, Järås S. Gas to liquids: A technology for natural gas industrialization in Bolivia. J Nat Gas Sci Eng 2010. https://doi.org/10.1016/j.jngse.2010.10.001. [3] Loutzenhiser PG, Meier A, Steinfeld A. Review of the Two-Step H2O/CO2-Splitting solar thermochemical cycle based on Zn/ZnO redox reactions. Materials (Basel) 2010;3:4922–38. https://doi.org/10.3390/ma3114922. [4] Bhosale R, Kumar A, AlMomani F, Gupta RB. Solar thermochemical ZnO/ZnSO4 water splitting cycle for hydrogen production. Int J Hydrogen Energy 2016. https:// doi.org/10.1016/j.ijhydene.2017.02.190. [5] Bhosale RR. Thermodynamic efficiency analysis of zinc oxide based solar driven thermochemical H2O splitting cycle: Effect of partial pressure of O2, thermal reduction and H2O splitting temperatures. Int J Hydrogen Energy 2018. [6] Abanades S, Charvin P, Lemont F, Flamant G. Novel two-step SnO2/SnO watersplitting cycle for solar thermochemical production of hydrogen. Int J Hydrogen Energy 2008;33:6021–30. https://doi.org/10.1016/j.ijhydene.2008.05.042. [7] Bhosale RR, Kumar A, Sutar P. Thermodynamic analysis of solar driven SnO2/SnO based thermochemical water splitting cycle. Energy Convers Manag 2017;135:226–35. https://doi.org/10.1016/j.enconman.2016.12.067. [8] Bhosale RR, Kumar A, Van Den Broeke LJP, Gharbia S, Dardor D, Jilani M, et al. Solar hydrogen production via thermochemical iron oxide-iron sulfate water splitting cycle. Int J Hydrogen Energy 2015;40:1639–50. https://doi.org/10.1016/j. ijhydene.2014.11.118. [9] Scheffe JR, Allendorf MD, Coker EN, Jacobs BW, McDaniel AH, Weimer AW. Hydrogen production via chemical looping redox cycles using atomic layer deposition-synthesized iron oxide and cobalt ferrites. Chem Mater 2011;23:2030–8. https://doi.org/10.1021/cm103622e. [10] Scheffe JR, Steinfeld A. Thermodynamic analysis of cerium-based oxides for solar thermochemical fuel production. Energy Fuels 2012;26:1928–36. https://doi.org/ 10.1021/ef201875v. [11] Bhosale RR, Takalkar GD. Nanostructured co-precipitated Ce0.9Ln0.1O2(Ln = La, Pr, Sm, Nd, Gd, Tb, Dy, or Er) for thermochemical conversion of CO2. Ceram Int
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