Co-precipitation synthesized nanostructured Ce_0.9Ln_0.05Ag_0.05O_2−δ materials for solar thermochemical conversion of CO₂ into fuels

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 CO₂ discharge [2, 3]. This constant release of CO₂ is considered as one of the primary reasons for the variation in the climate parameters [4]. The environmental distress occurring due to increases in CO₂ emission generated more interest in the conversion of CO₂ into value-added products.

One of the possible options for CO₂ utilization is to convert the captured CO₂ into fuels. Solar thermochemical cycles driven based on the metal oxide (MO)-based redox reactions can split CO₂ into CO (Fig. 1) [5, 6]. The solar CO produced can be combined with the solar H₂ (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 CO₂ 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 re-oxidized again via a CO₂ splitting reaction. Zinc oxide [8, 9], tin oxide [10, 11], iron oxide [12, 13], CeO₂ [14‐16], doped ceria [17‐20], ferrites [21‐24], perovskites [25‐28], and others [29‐32] have been utilized for the solar thermochemical conversion of H₂O and CO₂. Among all these, the phase pure CeO₂ is considered as one of the promising options due to its faster reaction kinetics and better thermal stability. Although the CeO₂ is beneficial for the thermochemical conversion of H₂O and CO₂, 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 CeO₂ fluorite cubic crystal structure improved the thermal reduction (TR) capacity of the CeO₂. It was also reported that although the TR capability of CeO₂ was improved, only Ce_0.9La_0.1O₂ was capable of producing higher quantities of CO than CeO₂ via CO₂ 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 CO₂ splitting (CDS) capacity.

Based on the results reported in our previous investigations, in this study, the synthesis, characterization, and application of Ce_0.9Ln_0.05Ag_0.05O_2−δ materials (where, Ln = La, Pr, Nd, Sm, Gd, Tb, Dy, Er) for the thermochemical conversion of CO₂ is reported. Synthesis of the Ce_0.9Ln_0.05Ag_0.05O_2−δ 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_0.9Ln_0.05Ag_0.05O_2−δ materials was estimated and compared with the previously studied phase pure CeO₂ and lanthanide-doped ceria materials.

After synthesizing the Ce_0.9Ln_0.05Ag_0.05O_2−δ materials, the next important step was to determine the phase composition of the derived materials. PXRD profiles of the calcined Ce_0.9Ln_0.05Ag_0.05O_2−δ materials are shown in Fig. 4a, b. The presented patterns indicate a cubic fluorite crystal structure of the Ce_0.9Ln_0.05Ag_0.05O_2−δ materials, similar to the one reported in the case of CeO₂ [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_0.9Ln_0.05Ag_0.05O_2−δ material peaks shift toward either lower or higher 2θ 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_0.9Ln_0.05Ag_0.05O_2−δ 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_0.9Ln_0.05Ag_0.05O_2−δ materials were utilized. Table 2 indicates that the variation in the crystallite size of the Ce_0.9Ln_0.05Ag_0.05O_2−δ materials does not follow any specific trend. As per the obtained results, the Ce_0.9Ln_0.05Ag_0.05O_2−δ materials can be arranged as follows based on their average crystallite size: La₅Ag₅Ce > Dy₅Ag₅Ce > Sm₅Ag₅Ce > Pr₅Ag₅Ce > Tb₅Ag₅Ce > Gd₅Ag₅Ce > Er₅Ag₅Ce > Nd₅Ag₅Ce.

After estimating the composition and crystallite size of each Ce_0.9Ln_0.05Ag_0.05O_2−δ 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_0.9Ln_0.05Ag_0.05O_2−δ. 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 Pr₅Ag₅Ce, Tb₅Ag₅Ce, Gd₅Ag₅Ce, and Er₅Ag₅Ce are shown in Fig. 5.

The redox performance of the Ce_0.9Ln_0.05Ag_0.05O_2−δ materials is estimated by performing the thermochemical CDS experiments using the TGA setup. Ce_0.9Ln_0.05Ag_0.05O_2−δ 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% CO₂ + 50% Ar (100 ml/min). As an example, Fig. 6 represents a TGA profile of La₅Ag₅Ce material obtained during the first CDS cycle. As shown in Fig. 6, during the TR step, the mass of the La₅Ag₅Ce material reduced by 0.511 mg, and during the CDS step, the weight of the La₅Ag₅Ce material increased by 0.126 mg. These mass variations further converted into respective redox performances in terms of \( n_{{{\text{O}}_{2} }} \) released (320.2 µmol/g) and \( n_{\text{CO}} \) produced (157.8 µmol/g) by using Eqs. (1) and (2).

The mass variations associated with the Ce_0.9Ln_0.05Ag_0.05O_2−δ materials recorded during the first cycle are shown in Fig. 7a, b. The \( n_{{{\text{O}}_{2} }} \) released and \( n_{\text{CO}} \) produced by each Ce_0.9Ln_0.05Ag_0.05O_2−δ material was computed based on the obtained TGA profiles and given in Table 3. The data given in Table 3 show that the Pr₅Ag₅Ce was capable of releasing a higher amount of O₂ at 1400 °C than the other Ce_0.9Ln_0.05Ag_0.05O_2−δ materials. Likewise, the CO production aptitude of La₅Ag₅Ce was the uppermost when compared with the remaining Ce_0.9Ln_0.05Ag_0.05O_2−δ materials. The numbers listed in the \( n_{\text{CO}} /n_{{{\text{O}}_{2} }} \) ratio column shows that the re-oxidation ability of the Nd₅Ag₅Ce was better than the other Ce_0.9Ln_0.05Ag_0.05O_2−δ materials.

Interesting to note that the \( n_{\text{CO}} \) produced by each Ce_0.9Ln_0.05Ag_0.05O_2−δ material was considerably lower than the \( n_{{{\text{O}}_{2} }} \) released during the first cycle. The two probable reasons for these results are (a) poor re-oxidation ability of the Ce_0.9Ln_0.05Ag_0.05O_2−δ materials or (b) additional mass loss during the first TR reduction due to the release of volatile chemicals from the Ce_0.9Ln_0.05Ag_0.05O_2−δ materials (which remained unburnt during the calcination step). For further investigation of this matter, the Ce_0.9Ln_0.05Ag_0.05O_2−δ 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_0.9Ln_0.05Ag_0.05O_2−δ materials during the successive three thermochemical cycles. Besides, Fig. 9a and b compares the \( n_{{{\text{O}}_{2} }} \) released and \( n_{\text{CO}} \) produced by each Ce_0.9Ln_0.05Ag_0.05O_2−δ material from cycle 1 to cycle 3.

Figures 8 and 9 show that the \( n_{{{\text{O}}_{2} }} \) released by all the Ce_0.9Ln_0.05Ag_0.05O_2−δ materials during cycle 1 was considerably higher than cycle 2. For example, the \( n_{{{\text{O}}_{2} }} \) released by La₅Ag₅Ce, Nd₅Ag₅Ce, Gd₅Ag₅Ce, and Dy₅Ag₅Ce 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 \( n_{{{\text{O}}_{2} }} \) released by all the Ce_0.9Ln_0.05Ag_0.05O_2−δ materials in cycle 3 was slightly less when compared to cycle 2. For instance, the \( n_{{{\text{O}}_{2} }} \) released by La₅Ag₅Ce, Nd₅Ag₅Ce, Gd₅Ag₅Ce, and Dy₅Ag₅Ce 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 O₂ evolution in cycle 1 was the additional loss in the mass of the Ce_0.9Ln_0.05Ag_0.05O_2−δ materials due to the release of volatile chemicals.

In the case of the CDS step, the \( n_{\text{CO}} \) produced by each Ce_0.9Ln_0.05Ag_0.05O_2−δ material first decreased in cycle 2 (compared to cycle 1) and remained approximately stable in cycle 3 (compared to cycle 2). For example, the \( n_{\text{CO}} \) produced by La₅Ag₅Ce, Nd₅Ag₅Ce, Gd₅Ag₅Ce, and Dy₅Ag₅Ce in cycle 2 was 11.1%, 20.0%, 7.4%, and 24.5% lower than cycle 1. In contrast, the %decrease in the \( n_{\text{CO}} \) produced by La₅Ag₅Ce, Nd₅Ag₅Ce, Gd₅Ag₅Ce, and Dy₅Ag₅Ce in cycle 3 was dropped to 1.1%, 0.5%, 0.9%, and 1.2% when compared with the cycle 2 data. The \( n_{\text{CO}} /n_{{{\text{O}}_{2} }} \) ratio of all the Ce_0.9Ln_0.05Ag_0.05O_2−δ materials increased significantly in cycle 2 when compared with cycle 1. For instance, the \( n_{\text{CO}} /n_{{{\text{O}}_{2} }} \) ratio of La₅Ag₅Ce, Nd₅Ag₅Ce, Gd₅Ag₅Ce, and Dy₅Ag₅Ce increased by 1.49, 1.01, 1.13, and 1.09 in cycle 2 when compared with cycle 1. The \( n_{\text{CO}} /n_{{{\text{O}}_{2} }} \) ratio of all the Ce_0.9Ln_0.05Ag_0.05O_2−δ materials in cycle 2 and cycle 3 were almost identical.

The results obtained during cycle 3 indicate that most of the Ce_0.9Ln_0.05Ag_0.05O_2−δ materials are moving toward achieving a stable redox reactivity. For attaining further confirmation about the stable redox reactivity, the Ce_0.9Ln_0.05Ag_0.05O_2−δ 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 \( n_{{{\text{O}}_{2} }} \) released and \( n_{\text{CO}} \) produced by each Ce_0.9Ln_0.05Ag_0.05O_2−δ material from cycle 2 to cycle 10 are shown in Figs. 11 and 12.

As per the data given in Fig. 11, the La₅Ag₅Ce, Pr₅Ag₅Ce, and Nd₅Ag₅Ce shows a stable release of O₂ from cycle 2 to cycle 10. The Gd₅Ag₅Ce indicates redox stability in terms of constant O₂ release from cycle 3 to cycle 10. For the rest of the Ce_0.9Ln_0.05Ag_0.05O_2−δ materials, i.e., Sm₅Ag₅Ce, Tb₅Ag₅Ce, Dy₅Ag₅Ce, and Er₅Ag₅Ce, a steady \( n_{{{\text{O}}_{2} }} \) evolution was realized after cycle 5 or cycle 6. In terms of average \( n_{{{\text{O}}_{2} }} \) released from cycle 2 to cycle 10, La₅Ag₅Ce (72.2 μmol of O₂/g cycle) and Tb₅Ag₅Ce (60.2 μmol of O₂/g cycle) were observed to be the best and worst among all the Ce_0.9Ln_0.05Ag_0.05O_2−δ materials.

As shown in Fig. 12, the La₅Ag₅Ce, Pr₅Ag₅Ce, Nd₅Ag₅Ce, Gd₅Ag₅Ce, and Dy₅Ag₅Ce showed a stable production of CO from cycle 2 to cycle 10. In contrast, a constant \( n_{\text{CO}} \) production in the case of Sm₅Ag₅Ce, Tb₅Ag₅Ce, and Er₅Ag₅Ce was noticed from cycle 6 to cycle 10. In terms of the average \( n_{\text{CO}} \) produced from cycle 2 to cycle 10, the Ce_0.9Ln_0.05Ag_0.05O_2−δ materials can be arranged as: La₅Ag₅Ce > Nd₅Ag₅Ce ~ Pr₅Ag₅Ce > Gd₅Ag₅Ce > Tb₅Ag₅Ce > Sm₅Ag₅Ce ~ Er₅Ag₅Ce > Dy₅Ag₅Ce. According to the data given in Fig. 13, the re-oxidation ability (average \( n_{\text{CO}} \) / \( n_{{{\text{O}}_{2} }} \) ratio) of the La₅Ag₅Ce and Tb₅Ag₅Ce was the highest as compared to the rest of the Ce_0.9Ln_0.05Ag_0.05O_2−δ materials. Based on \( n_{{{\text{O}}_{2} }} \) released and \( n_{\text{CO}} \) produced from cycle 2 to cycle 10, the La₅Ag₅Ce appears to be the most excellent candidate among all the Ce_0.9Ln_0.05Ag_0.05O_2−δ materials investigated in this study.

Table 4 reports the comparison of Ce_0.9Ln_0.05Ag_0.05O_2−δ materials with the CeO₂ and Ce_0.9Ln_0.1O₂ materials. The results given in Table 4 shows that all the Ce_0.9Ln_0.05Ag_0.05O_2−δ materials were capable of higher \( n_{{{\text{O}}_{2} }} \) release (except for Sm₅Ag₅Ce) and \( n_{\text{CO}} \) production than CeO₂ and their corresponding Ce_0.9Ln_0.1O₂ materials. For instance, the \( n_{{{\text{O}}_{2} }} \) released by La₅Ag₅Ce, Pr₅Ag₅Ce, Nd₅Ag₅Ce, Gd₅Ag₅Ce, Tb₅Ag₅Ce, Dy₅Ag₅Ce, and Er₅Ag₅Ce was higher by 21.8 μmol of O₂/g cycle, 23.1 μmol of O₂/g cycle, 23.0 μmol of O₂/g cycle, 18.6 μmol of O₂/g cycle, 0.20 μmol of O₂/g cycle, 12.1 μmol of O₂/g cycle, and 8.50 μmol of O₂/g cycle as compared to Ce_0.9La_0.1O₂, Ce_0.9Pr_0.1O₂, Ce_0.9Nd_0.1O₂, Ce_0.9Gd_0.1O₂, Ce_0.9Tb_0.1O₂, Ce_0.9Dy_0.1O₂, and Ce_0.9Er_0.1O₂, respectively. Similarly, the La₅Ag₅Ce, Pr₅Ag₅Ce, Nd₅Ag₅Ce, Sm₅Ag₅Ce, Gd₅Ag₅Ce, Tb₅Ag₅Ce, Dy₅Ag₅Ce, and Er₅Ag₅Ce produced 1.39, 1.44, 1.52, 1.32, 1.49, 1.43, 1.29, and 1.37 times higher CO when compared with the Ce_0.9La_0.1O₂, Ce_0.9Pr_0.1O₂, Ce_0.9Nd_0.1O₂, Ce_0.9Sm_0.1O₂, Ce_0.9Gd_0.1O₂, Ce_0.9Tb_0.1O₂, Ce_0.9Dy_0.1O₂, and Ce_0.9Er_0.1O₂, respectively. The overall results of this investigation indicate that the incorporation of Ag in the Ln-doped ceria was beneficial to improve the redox performance of Ce_0.9Ln_0.05Ag_0.05O_2−δ materials.

The PXRD and EDS analysis have confirmed the formation of nominally phase pure Ce_0.9Ln_0.05Ag_0.05O_2−δ materials via co-precipitation of the hydroxide method. The average crystallite size of the derived Ce_0.9Ln_0.05Ag_0.05O_2−δ materials was in the range of 32–64 nm. The SEM analysis further established a spherical nanostructured particle morphology of the Ce_0.9Ln_0.05Ag_0.05O_2−δ materials. The SEM analysis also indicates that the doping of the lanthanides and silver does not have any significant effect on the morphology of Ce_0.9Ln_0.05Ag_0.05O_2−δ materials. Based on the results associated with the TGA analysis, the redox reactivity of the Ce_0.9Ln_0.05Ag_0.05O_2−δ materials can be ranked as: Ce_0.911La_0.053Ag_0.047O_1.925 > Ce_0.921Nd_0.049Ag_0.050O_1.940 ~ Ce_0.892Pr_0.051Ag_0.051O_1.886 > Ce_0.889Gd_0.054Ag_0.050O_1.884 > Ce_0.905Tb_0.051Ag_0.046O_1.909 > Ce_0.896Sm_0.048Ag_0.053O_1.890 ~ Ce_0.899Er_0.050Ag_0.055O_1.900 > Ce_0.910Dy_0.053Ag_0.048O_1.923. The TGA results also indicate that the Ce_0.911La_0.053Ag_0.047O_1.925 and Ce_0.905Tb_0.051Ag_0.046O_1.909 possess the highest re-oxidation ability as compared to the rest of the Ce_0.9Ln_0.05Ag_0.05O_2−δ materials. The overall results of this investigation indicate that the inclusion of Ag into the Ln-doped ceria considerably improved the O₂ releasing and CO production capability of the Ce_0.9Ln_0.05Ag_0.05O_2−δ materials as compared to the phase pure CeO₂ and Ce_0.9Ln_0.1O₂ materials. After estimating the CO production capacity, our research team is currently focused on the production of H₂ by using the Ce_0.9Ln_0.05Ag_0.05O_2−δ materials.