Co-precipitation synthesized nanostructured Ce0.9Ln0.05Ag0.05O2−δ materials for solar thermochemical conversion of CO2 into fuels

Synthesis, characterization, and application of Ce0.9Ln0.05Ag0.05O2−δ materials (where, Ln = La, Pr, Nd, Sm, Gd, Tb, Dy, Er) for the thermochemical conversion of CO2 reported in this paper. The Ce0.9Ln0.05Ag0.05O2−δ materials were synthesized by using an ammonium hydroxide-driven co-precipitation method. The derived Ce0.9Ln0.05Ag0.05O2−δ materials were characterized via powder X-ray diffraction, scanning electron microscope, and electron diffraction spectroscopy. The characterization results indicate the formation of spherically shaped Ce0.9Ln0.05Ag0.05O2−δ nanostructured particles. As-prepared Ce0.9Ln0.05Ag0.05O2−δ materials were further tested toward multiple CO2 splitting cycles by utilizing a thermogravimetric analyzer. The results obtained indicate that all the Ce0.9Ln0.05Ag0.05O2−δ materials produced higher quantities of O2 and CO than the previously studied pure CeO2 and lanthanide-doped ceria materials. Overall, the Ce0.911La0.053Ag0.047O1.925 showed the maximum redox reactivity in terms of O2 release (72.2 μmol/g cycle) and CO production (136.6 μmol/g cycle).


Introduction
The worldwide human population is rising at a quick pace, which inherently demands a large amount of energy for consumption [1]. Current energy production largely depends on the utilization of fossil fuels. The excessive use of petroleum-based resources results in a continuous CO 2 discharge [2,3]. This constant release of CO 2 is considered as one of the primary reasons for the variation in the climate parameters [4]. The environmental distress occurring due to increases in CO 2 emission generated more interest in the conversion of CO 2 into value-added products.
One of the possible options for CO 2 utilization is to convert the captured CO 2 into fuels. Solar thermochemical cycles driven based on the metal oxide (MO)-based redox reactions can split CO 2 into CO ( Fig. 1) [5,6]. The solar CO produced can be combined with the solar H 2 (produced via solar thermochemical splitting of water) for the manufacturing of the solar syngas, which can be further utilized in the catalytic Fischer-Tropsch process [7].
The MO-based solar-driven thermochemical conversion of CO 2 is a two-phase process. In the first phase, the MO is reduced with the help of concentrated solar power, and in phase 2, the reduced MO is reoxidized again via a CO 2 splitting reaction. Zinc oxide [8,9], tin oxide [10,11], iron oxide [12,13], CeO 2 [14][15][16], doped ceria [17][18][19][20], ferrites [21][22][23][24], perovskites [25][26][27][28], and others [29][30][31][32] have been utilized for the solar thermochemical conversion of H 2 O and CO 2 . Among all these, the phase pure CeO 2 is considered as one of the promising options due to its faster reaction kinetics and better thermal stability. Although the CeO 2 is beneficial for the thermochemical conversion of H 2 O and CO 2 , a lower fuel production capacity is one of the major limitations associated with this MO.
Recently, Bhosale and Takalkar [33] reported that the doping of lanthanides such as La, Pr, Nd, Sm, Gd, Tb, Dy, and Er into CeO 2 fluorite cubic crystal structure improved the thermal reduction (TR) capacity of the CeO 2 . It was also reported that although the TR capability of CeO 2 was improved, only Ce 0.9 La 0.1 O 2 was capable of producing higher quantities of CO than CeO 2 via CO 2 splitting reaction. In another investigation, Takalkar et al. [19] reported that the inclusion of Ag in the transition metal-doped ceria considerably improved the TR as well as CO 2 splitting (CDS) capacity.
Based on the results reported in our previous investigations, in this study, the synthesis, characterization, and application of Ce 0.9 Ln 0.05 Ag 0.05 O 2-d materials (where, Ln = La, Pr, Nd, Sm, Gd, Tb, Dy, Er) for the thermochemical conversion of CO 2 is reported. Synthesis of the Ce 0.9 Ln 0.05 Ag 0.05 O 2-d materials is achieved via a co-precipitation method.
The derived materials are further tested for multiple thermochemical CDS cycles by utilizing a thermogravimetric analyzer (TGA) setup. The TR and CDS capacity of the Ce 0.9 Ln 0.05 Ag 0.05 O 2-d materials was estimated and compared with the previously studied phase pure CeO 2 and lanthanide-doped ceria materials.

Experimental
Preparation and characterization of Ce 0.9 Ln 0.05 Ag 0.05 O 22d Nitrate-based precursors of ceria, silver, and all lanthanides were acquired from Sigma-Aldrich, USA. Figure 1 Overall process of production of liquid fuels by using solar syngas generated via MO-based solar thermochemical H 2 O/CO 2 splitting cycle.
Aqueous 28% NH 3 OH as a precipitating agent was procured from the same supplier. Ultrapure deionized (DI) water (produced through Direct-Q system, Millipore, France) was utilized for the preparation of nitrate solution. An ultrapure grade Ar gas (purity 99.999%) and 50% CO 2 ? 50% Ar gas mixture are ordered from the Buzwair Scientific and Technical Gases, Doha, Qatar.
The synthesis of redox Ce 0.9 Ln 0.05 Ag 0.05 O 2-d materials was achieved via co-precipitation of the hydroxide method. As shown in Fig. 2, an aqueous mixture of selected metal precursors was prepared by dissolving them in 300 ml of deionized water. To this mixture, aqueous ammonium hydroxide (28%) was added to attain a pH approximately equal to 10. The mixture further stirred for 24 h with a maintained pH * 10. Once the stirring was stopped, the precipitate of Ce 0.9 Ln 0.05 Ag 0.05 O 2-d material was allowed to settle via gravity (mixture kept undisturbed overnight). The next morning, the supernatant liquid was decanted, and the obtained precipitate was washed several times by deionized water with the help of a vacuum filtration unit. The obtained filtered cake of Ce 0.9 Ln 0.05 Ag 0.05 O 2-d was dried (120°C, 5 h), crushed, and annealed (Nabertherm Furnace) up to 1000°C for 4 h in the presence of air. The obtained annealed powder was analyzed for the determination of the phase/elemental composition and morphology by using Panalytical XPert powder X-ray diffractometer (PXRD) and scanning electron microscope (SEM, Nova Nano 450, FEI) equipped with the electron diffraction spectroscopy (EDS).

CO 2 splitting experiments
The Ce 0.9 Ln 0.05 Ag 0.05 O 2-d materials were experimentally tested in a TGA setup (Setaram Instrumentation, France), which is shown in Fig. 3. The experimental parameters utilized to perform the thermochemical cycles are given in Table 1. Approximately, 50 mg of the calcined Ce 0.9 Ln 0.05-Ag 0.05 O 2-d powder was charged in an alumina (100 ll) crucible, and then placed inside the heating furnace of the TGA. Before performing the TGA experiments, the residual air filling the hollow space near to the furnace was evacuated by applying a vacuum followed by sweeping by the inert Ar. Chilled water (generated by Julabo FC 1600T) was utilized to decrease the exiting gas stream temperature. Additional details related to the TGA setup and the experimental procedure are already described in our previous studies [23,29]. Multiple TR and CDS steps were performed by considering the operating parameters given in Table 1.
The mass variations allied with the Ce 0.9 Ln 0.05-Ag 0.05 O 2-d materials during the TR and CDS steps were documented after subtracting the blank TGA experimental data from the actual TGA experimental data. The amount of O 2 liberated (lmol/g) during the TR step and the quantity of CO produced during the CDS step are calculated by utilizing Eqs. (1) and (2).

Results and discussion
After synthesizing the Ce 0.9 Ln 0.05 Ag 0.05 O 2-d materials, the next important step was to determine the phase composition of the derived materials. PXRD profiles of the calcined Ce 0.9 Ln 0.05 Ag 0.05 O 2-d materials are shown in Fig. 4a, b. The presented patterns indicate a cubic fluorite crystal structure of the Ce 0.9 Ln 0.05 Ag 0.05 O 2-d materials, similar to the one reported in the case of CeO 2 [23]. The PXRD profiles shown in Fig. 4a further indicates the absence of the formation of any metal or metal oxide impurities. As shown in Fig. 4b, the Ce 0.9 Ln 0.05 Ag 0.05 O 2-d material peaks shift toward either lower or higher 2h angle (based on the crystal ionic radii of the dopants). This shift in the peaks further confirmed the successful incorporation of the dopants inside the ceria fluorite crystal structure. The formation of nominally phase pure Ce 0.9 Ln 0.05 Ag 0.05 O 2-d materials was also assured via EDS analysis (results given in Table 2). In order to determine the crystallite size, a widely used Scherrer equation and the PXRD data associated with the Ce 0.9 Ln 0.05 Ag 0.05 O 2-d materials were utilized.   After estimating the composition and crystallite size of each Ce 0.9 Ln 0.05 Ag 0.05 O 2-d material, the morphology was scrutinized by performing the SEM analysis. The SEM images obtained looks very similar to each other and indicate the formation of roughly spherical particles of Ce 0.9 Ln 0.05 Ag 0.05 O 2-d . The microscopic SEM analysis further confirmed that the particles were agglomerated. It was also observed that the average particle size was very close to the crystallite sizes given in Table 2. The representative SEM images of Pr 5 Ag 5 Ce, Tb 5 Ag 5 Ce, Gd 5 Ag 5 Ce, and Er 5 Ag 5 Ce are shown in Fig. 5.
The redox performance of the Ce 0.9 Ln 0.05 Ag 0.05 O 2-d materials is estimated by performing the thermochemical CDS experiments using the TGA setup. Ce 0.9 Ln 0.05 Ag 0.05 O 2-d materials thermally reduced at 1400°C (10 K/min) for 60 min in the presence of the inert Ar (100 ml/min) and then re-oxidized at 1000°C by using a feed gas mixture containing 50% CO 2 ? 50% Ar (100 ml/min). As an example, Fig. 6 represents a TGA profile of La 5 Ag 5 Ce material obtained during the first CDS cycle. As shown in Fig. 6, during the TR step, the mass of the La 5 Ag 5 Ce material reduced by 0.511 mg, and during the CDS step, the weight of the La 5 Ag 5 Ce material increased by 0.126 mg. These mass variations further converted into respective redox performances in terms of n O 2 released (320.2 lmol/g) and n CO produced (157.8 lmol/g) by using Eqs. (1) and (2).
The mass variations associated with the Ce 0.9-Ln 0.05 Ag 0.05 O 2-d materials recorded during the first cycle are shown in Fig. 7a, b. The n O 2 released and n CO produced by each Ce 0.9 Ln 0.05 Ag 0.05 O 2-d material was computed based on the obtained TGA profiles and given in Table 3. The data given in Table 3 show that the Pr 5 Ag 5 Ce was capable of releasing a higher amount of O 2 at 1400°C than the other Ce 0.9 Ln 0.05-Ag 0.05 O 2-d materials. Likewise, the CO production aptitude of La 5 Ag 5 Ce was the uppermost when compared with the remaining Ce 0.9 Ln 0.05 Ag 0.05 O 2-d materials. The numbers listed in the n CO =n O 2 ratio column shows that the re-oxidation ability of the  Interesting to note that the n CO produced by each Ce 0.9 Ln 0.05 Ag 0.05 O 2-d material was considerably lower than the n O 2 released during the first cycle. The two probable reasons for these results are (a) poor reoxidation ability of the Ce 0.9 Ln 0.05 Ag 0.05 O 2-d materials or (b) additional mass loss during the first TR reduction due to the release of volatile chemicals from the Ce 0.9 Ln 0.05 Ag 0.05 O 2-d materials (which remained unburnt during the calcination step). For further investigation of this matter, the Ce 0.9 Ln 0.05-Ag 0.05 O 2-d materials were tested for three cycles (by maintaining the same operating conditions utilized in  the case of the first cycle). Figure 8 shows the variations in the mass of the Ce 0.9 Ln 0.05 Ag 0.05 O 2-d materials during the successive three thermochemical cycles. Besides, Fig. 9a and b compares the n O 2 released and n CO produced by each Ce 0.9 Ln 0.05-Ag 0.05 O 2-d material from cycle 1 to cycle 3. Figures 8 and 9 show that the n O 2 released by all the Ce 0.9 Ln 0.05 Ag 0.05 O 2-d materials during cycle 1 was considerably higher than cycle 2. For example, the n O 2 released by La 5 Ag 5 Ce, Nd 5 Ag 5 Ce, Gd 5 Ag 5 Ce, and Dy 5 Ag 5 Ce in cycle 2 was lower by 77.9%, 64.6%, 76.6%, and 80.0% as compared to cycle 1. The comparison between cycle 2 and cycle 3 shows a different story. The n O 2 released by all the Ce 0.9 Ln 0.05 Ag 0.05-O 2-d materials in cycle 3 was slightly less when compared to cycle 2. For instance, the n O 2 released by La 5 Ag 5 Ce, Nd 5 Ag 5 Ce, Gd 5 Ag 5 Ce, and Dy 5 Ag 5 Ce in cycle 3 was lower by 2.0%, 0.0%, 14.8%, and 1.8% than cycle 2. Based on the results given in Figs. 8 and   9, it can be concluded that the prime reason for the more substantial O 2 evolution in cycle 1 was the additional loss in the mass of the Ce 0.9 Ln 0.05 Ag 0.05-O 2-d materials due to the release of volatile chemicals.
In the case of the CDS step, the n CO produced by each Ce 0.9 Ln 0.05 Ag 0.05 O 2-d material first decreased in cycle 2 (compared to cycle 1) and remained approximately stable in cycle 3 (compared to cycle 2). For example, the n CO produced by La 5 Ag 5 Ce, Nd 5 Ag 5 Ce, Gd 5 Ag 5 Ce, and Dy 5 Ag 5 Ce in cycle 2 was 11.1%, 20.0%, 7.4%, and 24.5% lower than cycle 1. In contrast, the %decrease in the n CO produced by La 5 Ag 5-Ce, Nd 5 Ag 5 Ce, Gd 5 Ag 5 Ce, and Dy 5 Ag 5 Ce in cycle 3 was dropped to 1.1%, 0.5%, 0.9%, and 1.2% when compared with the cycle 2 data. The n CO =n O 2 ratio of all the Ce 0.9 Ln 0.05 Ag 0.05 O 2-d materials increased significantly in cycle 2 when compared with cycle 1. For instance, the n CO =n O 2 ratio of La 5 Ag 5 Ce, Nd 5 Ag 5 Ce, The results obtained during cycle 3 indicate that most of the Ce 0.9 Ln 0.05 Ag 0.05 O 2-d materials are moving toward achieving a stable redox reactivity. For attaining further confirmation about the stable redox reactivity, the Ce 0.9 Ln 0.05 Ag 0.05 O 2-d materials were tested for ten successive cycles. The TGA profiles associated with the ten cycles are shown in Fig. 10. It was already confirmed that the data obtained in cycle 1 is misleading, and hence the TGA analysis was more focused on the remaining nine cycles. The n O 2  As per the data given in Fig. 11, the La 5 Ag 5 Ce, Pr 5 Ag 5 Ce, and Nd 5 Ag 5 Ce shows a stable release of O 2 from cycle 2 to cycle 10. The Gd 5 Ag 5 Ce indicates redox stability in terms of constant O 2 release from cycle 3 to cycle 10. For the rest of the Ce 0.9 Ln 0.05-Ag 0.05 O 2-d materials, i.e., Sm 5 Ag 5 Ce, Tb 5 Ag 5 Ce, Dy 5 Ag 5 Ce, and Er 5 Ag 5 Ce, a steady n O 2 evolution was realized after cycle 5 or cycle 6. In terms of average n O 2 released from cycle 2 to cycle 10, La 5 Ag 5 Ce (72.2 lmol of O 2 /g cycle) and Tb 5 Ag 5 Ce (60.2 lmol of O 2 /g cycle) were observed to be the best and worst among all the Ce 0.9 Ln 0.05 Ag 0.05 O 2-d materials.
As shown in Fig. 12, the La 5 Ag 5 Ce, Pr 5 Ag 5 Ce, Nd 5 Ag 5 Ce, Gd 5 Ag 5 Ce, and Dy 5 Ag 5 Ce showed a stable production of CO from cycle 2 to cycle 10. In contrast, a constant n CO production in the case of Sm 5 Ag 5 Ce, Tb 5 Ag 5 Ce, and Er 5 Ag 5 Ce was noticed from cycle 6 to cycle 10. In terms of the average n CO produced from cycle 2 to cycle 10, the Ce 0. According to the data given in Fig. 13, the re-oxidation ability (average n CO / n O 2 ratio) of the La 5 Ag 5 Ce and Tb 5 Ag 5 Ce was the highest as compared to the rest of the Ce 0.9 Ln 0.05 Ag 0.05 O 2-d materials. Based on n O 2 released and n CO produced from cycle 2 to cycle 10, the La 5 Ag 5 Ce appears to be the most excellent candidate among all the Ce 0.9 Ln 0.05 Ag 0.05 O 2-d materials investigated in this study. Table 4 reports the comparison of Ce 0.9 Ln 0.05-Ag 0.05 O 2-d materials with the CeO 2 and Ce 0.9 Ln 0.1 O 2 materials. The results given in Table 4 shows that all the Ce 0.9 Ln 0.05 Ag 0.05 O 2-d materials were capable of higher n O 2 release (except for Sm 5 Ag 5 Ce) and n CO production than CeO 2 and their corresponding

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