Co-Precipitation Synthesized Ag-Doped Ceria Redox Material (ACRM) for the Thermochemical Conversion of CO₂ into Solar Fuels

Abstract

In this investigation, an effort was made to introduce Ag into the CeO₂ fluorite crystal lattice to form Ce_0.99Ag_0.01O_2-δ (ACRM) using an ammonium hydroxide-assisted co-precipitation method. The resulting powder obtained after the co-precipitation reaction, filtration, and drying was annealed at 800 °C in a muffle furnace to obtain crystalline ACRM. The phase composition and microstructure of the synthesized ACRM were analyzed using a powder X-ray diffractometer (PXRD) and a scanning electron microscope (SEM). The characterized ACRM powder was then subjected to multiple thermochemical thermal reduction (TR) and CO₂ splitting (CDS) cycles using a high-temperature thermogravimetric analyzer (TGA). The TR step was conducted using Ar gas as an inert atmosphere, maintaining the temperature at 1400 °C for 60 min. Subsequently, the same powder was subjected to the CDS step by treating it with a gaseous mixture of 50% CO₂ and Ar gas at 1000 °C for 30 min. ACRM displayed stable redox reactivity towards thermochemical CDS cycles by generating an average of 50.9 μmol of O₂/g·cycle and 101.6 μmol of CO/g·cycle, respectively, over 10 thermochemical cycles.

1. Introduction

The escalating world population has resulted in an accompanying uptick in the demand for energy supply. Presently, this energy requirement stands at 18 TW, a figure that is expected to soar to 30 TW by 2050 [1]. The predominant fulfillment of the current energy demand is achieved using non-renewable fossil fuels. The limited reserves of fossil fuels, coupled with their excessive usage, lead to the release of a significant amount of CO₂, one of the harmful greenhouse gases [2,3]. The concentration of CO₂ in the earth’s atmosphere has risen from 325 ppm in 1970 to 420 ppm in 2022, and estimates indicate that it will surge to 525 ppm by 2050. Given that emitted CO₂ is a significant contributor to global warming, there is a pressing need for carbon-neutral and renewable alternative energy sources to replace fossil fuels [4].

The efficient conversion of water (H₂O) and carbon dioxide (CO₂) into fuels using solar radiation presents a sustainable, long-term alternative for reducing fossil fuel consumption and exploiting solar energy [5,6]. Among the different methods for inducing solar thermochemical cycles, metal oxide (MO)-based redox reactions have shown efficiency in splitting H₂O and CO₂ into H₂ and CO [7,8]. The MO-based thermochemical cycle operates at a relatively lower temperature compared with direct thermolysis [9].

Numerous nonvolatile metal oxides, including but not limited to ferrites [10,11], ceria [12,13], and perovskites [14,15], have been extensively researched for their capacity to facilitate isothermal and non-isothermal thermochemical splitting of H₂O (WS) and CO₂ (CDS), leading to the production of solar fuel [16,17]. In the realm of metal oxides, ceria (CeO₂) has emerged as a promising redox material thanks to its enduring stability, rapid reaction kinetics, high vacancy concentration at elevated temperatures, and ease of accessibility [18,19]. The thermal reduction (TR) of ceria is achieved by releasing O₂ at high temperatures during the first step, followed by re-oxidation with H₂O or CO₂ into H₂ and CO in the second step. The resulting syngas (a mixture of CO and H₂) can be converted into storable and transportable liquid hydrocarbons via the Fischer–Tropsch (FT) process [20]. Ultimately, the two-step thermochemical cycle using ceria offers a renewable, environmentally friendly, and long-term solution to meet future energy demand while mitigating CO₂ emissions.

In 2006, Abanades et al. [21] explored ceria for WS in H₂ via thermochemical cycles. They found that CeO₂ can be thermally reduced to Ce₂O₃ at 2223 K under reduced pressure due to its favorable redox properties. Rhodes et al. [19] conducted a thorough examination of the degree of the redox reaction by using ceria in conjunction with CO generation via CDS for over 2000 thermochemical cycles. The results of their investigation revealed a significant elongation in thermal stability with an average non-stoichiometry of 0.0197 and retention of reactivity of 86.4%. Chueh et al. [22] demonstrated that non-stoichiometric ceria and a solar cavity receiver reactor can be used for thermochemical WS splitting for up to 500 redox cycles, achieving a solar-to-fuel efficiency of 0.7 to 0.8%. A study used an isothermal pressure swing reactor bed with ceria and CO₂ as feedstock. The study demonstrated a total of 102 CDS cycles throughout 7.5 h, with an average CO production yield of 0.079 µmol/g·s [23].

Scheffe and Steinfeld [24] analyzed the impact of dopants such as Y₂O₃, Sm₂O₃, Gd₂O₃, CaO, and SrO on oxygen nonstoichiometry in ceria for splitting CO₂ and H₂O at different temperatures and oxygen partial pressures. Their research reveals that higher dopant concentration increases oxygen vacancies in ceria at lower temperatures. Kaneko et al. [25] have used a combustion synthesis method to produce ceria doped with transition metal ions such as Mn, Fe, Ni, and Cu. They studied the process of TR and re-oxidation for WS to generate H₂. Their research showed that the doping of transition metal cations resulted in higher production of H₂ compared with pure CeO₂. Bhosale et al. [26] conducted a study on incorporating Hf⁺⁴ and Zr⁺⁴ dopants into CeO₂ for redox thermochemical cycling. The study findings revealed that a 5% dopant concentration of both cations (individually) resulted in high CO generation, with thermal stability lasting over 20 multiple cycles as determined by thermogravimetry setup.

Meng et al. [27] studied the effect of metal cation doping on CeO₂ for solar thermochemical WS cycles. Ce_0.9Hf_0.1O₂ produced the highest amount of H₂ at 500 °C and exhibited the least deactivation during the TR step at 1500 °C. It outperformed CeO₂-MOx (M = Mn, Fe, Ni, and Pr). A recent study conducted by Takalkar et al. [28] investigated the impact of eight transition metal cations (M = Fe, Co, Ni, Cu, Zn, Mn, Cr, Zr) as dopants into a CeO₂ for multiple cycles towards CO generation by CO₂. The study found that in most cases, the transition-doped CeO₂ produced higher amounts of CO in ten cycles than pure CeO₂. Rhodium-doped ceria [29] has been successfully employed for long-term methane generation, simultaneously splitting CO₂ and H₂O. The findings suggest that Ce_0.99Rh_0.01O_2-δ materials possess an enhanced oxygen storage capacity and exhibit activity towards methane formation.

The existing body of research suggests that including various metal cations (such as transition, rare earth, and noble) into the structure of ceria leads to alterations in the thermal, chemical, kinetic, and physical properties. Most research efforts have been dedicated to doping transition metal cations in the ceria crystal structure. The outcomes have been less than promising, except for cases involving Zr and Hf. Consequently, the decision was made to redirect our focus toward utilizing noble metals as dopants for ceria to facilitate the solar fuel production cycle. To date, only Rh noble metal has been doped in CeO₂ for the simultaneous splitting of CO₂ and H₂O into hydrocarbon through solar power [29]. In this study, we have used a more abundant and low-cost precious metal, silver (Ag¹⁺), as a dopant in the ceria structure [1% Ag-doped ceria redox materials (ACRM): Ce_0.99Ag_0.01O_2-δ] for the thermochemical redox cycle, which splits CO₂ into CO and O₂. We prepared the ACRM using a co-precipitation method and tested it for the CDS by performing multiple thermochemical cycles using a TGA set-up. The typical schematic of the ACRM redox CDS cycle is presented in Figure 1.

2. Experimental

2.1. Materials

To synthesize the ACRM (1% Ag-doped in CeO₂: Ce_0.99Ag_0.01O_2-δ) via the co-precipitation method, nitrate precursors of cerium and silver were procured from Sigma Aldrich (St. Louis, MO, USA). In addition to the metal precursors, Sigma Aldrich was also contacted for the purchase of a precipitating agent, an aqueous ammonia hydroxide (28%). Ultra-high pure Ar (99.999% purity, 2 ppm O₂), and a 50/50 CO₂/Ar gas mixture were ordered from Buzwair Scientific and Technical Gases supplier (Doha, Qatar). A de-ionized water production unit installed in the laboratory (Millipore, ultrapure Type 1) was utilized to supply de-ionized water for the co-precipitation synthesis of SCRM. All chemicals were used as is without any pre-treatment step.

2.2. Synthesis of ACRM

The co-precipitation method [30] employed for synthesizing ACRM is described in detail in Figure 2. The process involved weighing the cerium and silver nitrates and adding them to 300 mL of deionized water in a beaker. Subsequently, the metal precursor nitrates were allowed to dissolve in the deionized water with a continuous stirring mechanism. Aqueous ammonium hydroxide (28%) was used as a precipitating agent, adding dropwise to the aqueous mixture of metal precursors. The precipitation was achieved by stirring the aqueous mixture and adding the aqueous ammonium hydroxide (25%) dropwise while maintaining the solution pH at 10. The co-precipitation experiment was allowed to continue for 24 h while maintaining the pH at 10 to achieve the maximum possible precipitation. The beaker was then set aside overnight to allow the precipitate to settle. The following day, the supernatant liquid was decanted, and the wet ACRM precipitate was transferred to a vacuum filtration unit. The obtained precipitate was washed several times with deionized water to remove impurities and chemicals, and the pH of the water collected after washing was monitored. The washing process was stopped when the pH reached neutral. The filtered wet solids collected on the filtration paper were then scraped and transferred to a drying chamber operated at 100 °C. The ACRM powder obtained after drying was crushed using a ceramic mortar and pestle, and the fine powder sample was annealed at 800 °C for 4 h in a muffle furnace. The annealed ACRM powder sample was stored in a dry atmosphere for characterization.

2.3. Characterization of ACMR

The ACRM powder that was obtained after annealing underwent a comprehensive characterization process. The first step involved using a Panalytical XPert MPD/DY636 powder X-ray diffractometer (PXRD), which was operated using CuKα radiation (λ = 0.15418 nm), with a voltage of 45 kV and a current of 20 mA. The pre- and post-reaction ACRM powder underwent scanning for 2θ in the range of 20° to 90°, with a recording time of 5 s. Additionally, to analyze the particle morphology of the ACRM particles, Scanning (SEM, Nova Nano 450, FEI) and Transmission Electron Microscopes (TEM, Tecnai G2 S Twin FEG, FEI) were utilized.

2.4. Thermochemical Redox Cycles

The functionality of the catalytic ACRM synthesized through co-precipitation was confirmed using a thermogravimetric analyzer (TGA, SETSYS Evo.,) sourced from SETARAM, France. The TGA setup (Figure 3), comprising a heating furnace with an enclosed alumina tube containing a graphite heating element, a temperature-regulating system, and a reaction chamber, was utilized for this purpose. A continuous flow of protective gas (Ar) was passed through the furnace to prevent damage to the graphite heating element from oxidizing gases. A chiller that provided ice-cold water to the furnace maintained the setup’s temperature. A balance was positioned in the top section of the TGA setup, and the carrier gas (Ar) flowed from top to bottom to prevent oxidation or contamination caused by sample vapors. Two mass flow controllers were employed to measure and monitor the flow rates of the gases used in the TGA experiments- one for Ar and the other for the CO₂/Ar mixture. A Pt-Rh thermocouple (type B) was placed near the alumina crucible to measure and regulate the temperature of the powder sample. Prior to conducting any TGA experiments, the air or O₂ trapped inside the hollow of the furnace was removed. This evacuation procedure was performed using a two-step mechanism: first, by applying a vacuum, and then by purging the hollow with an inert carrier gas (Ar).

To perform the TR and CDS experiments, a precise amount of approximately 100 mg of SCRM powder was loaded into a 100 µL platinum crucible and positioned accurately on a hanging balance. Several thermochemical cycles were conducted at temperatures ranging from 1000 to 1400 °C. The TR phase was carried out using inert Ar gas at 1400 °C for 60 min. Afterward, the CDS phase was performed by exposing the ACRM powder to a 50% CO₂-Ar gaseous mixture at 1000 °C for 30 min. During the thermal reduction phase, the overall mass of the sample decreased due to the release of O₂ from ACRM. On the other hand, during the CDS phase, the overall mass of the sample increased due to the re-oxidation of SCRM. The amounts of O₂ released and CO produced during multiple thermochemical cycles were calculated using mass loss and gain values per the given equations.

$$n_{O_2}={\displaystyle \frac{∆m_{loss}}{\left(M_{O_2}\times m_{ACRM}\right)}}$$

(1)

$$n_{CO}={\displaystyle \frac{∆m_{gain}}{\left(M_O\times m_{ACRM}\right)}}$$

(2)

3. Results and Discussions

As explained in Section 2, ACRM was synthesized using a co-precipitation method. After synthesis, the calcined powder of ACRM was characterized using PXRD and SEM, and the characterization results were compared with pure CeO₂ material prepared using the co-precipitation method. Figure 4 compares the PXRD patterns of CeO₂ and ACRM. ACRM exhibits sharp crystalline peaks associated with a cubic fluorite crystal, similar to CeO₂, as seen in PXRD peaks. According to the PXRD patterns, there is no evidence of metallic Ag or Ag-oxide impurities, as no peaks are associated with them. Upon analyzing the zoomed section of the PXRD (2θ = 28° to 30°), it has been observed that there is a noticeable shift in the peaks for the ACRM compared with the CeO₂ due to the structural strain [31]. It is a well-known fact that such peak shifts can generally be attributed to the replacement of the Ce⁺⁴ cation by another metal cation. Therefore, based on the shift in the peaks recorded in Figure 4b, it can be inferred that the incorporation of Ag in the CeO₂ crystal structure has been successfully achieved, and the formation of ACRM has been accomplished via the co-precipitation method.

The microstructural analysis of the ACRM synthesized through co-precipitation was examined using SEM. The SEM image in Figure 5a reveals that the particles exhibit a rough, spherical shape and appear agglomerated. The average particle size of the co-precipitation-synthesized ACRM was determined using ImageJ software (Version 1.54j). Figure 5b–e presents the particle size analysis across four distinct sections of the SEM image. The numbers on each image in Figure 5b–e indicate that the average particle size of the co-precipitation-synthesized ACRM falls within the 80 to 90 nm range.

The accurate particle size range for co-precipitation synthesized ACRM was determined while avoiding particle agglomeration through TEM analysis. The TEM image in Figure 6 provided further insight and confirmed that the average particle size for co-precipitation synthesized ACRM falls within 25 to 30 nm. Discrepancies between the SEM and TEM analysis results arose from the sample preparation method employed for TEM analysis, which prevented particle agglomeration and allowed for observing individual particles rather than agglomerated ones.

The ACRM powder, which was characterized, underwent a thermochemical CDS reaction test using a TGA setup. The Calisto software (Version 2.10), embedded in the TGA setup, was used to track the sample’s mass variations during the TR and CDS stages. The procedure for conducting thermochemical redox reactions is explained in Section 2. Consecutively, several thermochemical redox cycles were carried out, employing thermal reduction at 1400 °C (60 min) and re-oxidation with CO₂ conversion into CO at 1000 °C (30 min). Blank TGA data were gathered under similar experimental conditions and utilized to subtract artifacts to eliminate the effect of buoyancy variation of gaseous components.

One thermochemical cycle was performed to estimate the redox reactivity of co-precipitation synthesized ACRM in terms of mass loss or gain percentage during the TR and CDS steps. A TGA profile obtained for the cycle is presented in Figure 7. In this cycle, 101.95 mg of ACRM was loaded in the TGA. During the TR and CDS steps, 0.652 mg of mass loss and 0.201 mg of mass gain were recorded. The TGA plot confirms that the TR rate was slower than the CDS rate. The mass loss and gain recorded during the TR and CDS steps were converted into the amounts of O₂ released and CO produced using Equations (1) and (2). The amount of O₂ released from ACRM during the TR step (199.8 μmol/g) was observed to be 152.3 μmol/g higher than that of the pure CeO₂ material. Similarly, the amount of CO produced by ACRM at 1000 °C (123.2 μmol/g) was also higher than the CeO₂ material (95 μmol/g). The results indicate that incorporating Ag into the CeO₂ structures is advantageous in improving the O₂-releasing and CO-producing ability of pure CeO₂.

The findings illustrated in Figure 7 demonstrate that adding Ag into CeO₂ structures is beneficial in enhancing the O₂-releasing and CO-producing efficacy of pure CeO₂. However, the amount of CO produced by ACRM via CDS was notably low compared with the O₂ released. Moreover, drawing conclusions based on a single thermochemical cycle is not ideal, and therefore, we decided to evaluate the ACRM towards CDS in three consecutive thermochemical cycles. Figure 8 depicts the TGA plots obtained for the three consecutive thermochemical redox cycles carried out using ACRM. The amount of O₂ released and CO produced during each CDS cycle was determined based on mass loss and gain data obtained from TGA, and the results are presented in

Table 1. Amounts of O₂ released and CO produced by ACRM and pure CeO₂ during three consecutive thermochemical CDS cycles.

Materials	Cycle 1		Cycle 2		Cycle 3
	${\mathbf{n}}_{{\mathbf{O}}_2}$ Released (μmol/g)	${\mathbf{n}}_{\mathbf{C}\mathbf{O}}$ Produced (μmol/g)	${\mathbf{n}}_{{\mathbf{O}}_2}$ Released (μmol/g)	${\mathbf{n}}_{\mathbf{C}\mathbf{O}}$ Produced (μmol/g)	${\mathbf{n}}_{{\mathbf{O}}_2}$ Released (μmol/g)	${\mathbf{n}}_{\mathbf{C}\mathbf{O}}$ Produced (μmol/g)
CeO2	47.1	94.2	47.8	95.6	46.9	93.8
ACRM	199.8	123.2	51.2	102.4	50.9	101.8

.

Based on data reported in Table 1, it has been observed that the TR ability of the ACRM has significantly decreased during the second cycle. As per the published literature, the mass loss of the reactive sample during the first TR step can be attributed to the release of O₂ and volatile components. Furthermore, during the calcination step at 800 °C, some of the chemicals used during the synthesis process were not wholly burned and remained unburnt. These chemicals were subsequently released during the TR step at 1400 °C. However, it was found that the amounts of O₂ released and CO produced by ACRM materials, calculated based on the mass loss recorded during the first TR step, were misleading. Therefore, data obtained during the first thermochemical cycle were excluded from further analysis of the ACRM materials to avoid any misinterpretation. According to experimental data obtained from the second and third thermochemical cycles, it is evident that ACRM has shown an increase of 6.64% and 7.86% in the release of O₂ and production of CO, respectively, as compared with CeO₂. These findings confirm that incorporating Ag within the crystal structure of CeO₂ positively affects both its TR and CDS abilities.

Ten consecutive thermochemical cycles were conducted to assess the ACRM’s redox reactivity, phase, and microstructural stability over the long term. The TGA profiles obtained during these ten cycles are presented in Figure 9, while the quantities of O₂ released and CO produced for each cycle, excluding cycle 1, are illustrated in Figure 10. Based on the reported trends, it is evident that the redox reactivity of ACRM remained stable for ten consecutive thermochemical cycles. The average quantities of O₂ released and CO produced from cycle 2 to cycle 10 for ACRM are 50.9 μmol of O₂/g·cycle and 101.6 μmol of CO/g·cycle, respectively. The co-precipitation synthesized ACRM exhibited higher O₂-releasing and CO-producing abilities, by 7.07% and 6.99%, respectively, across the entire ten thermochemical cycles compared with pure CeO₂.

This study aimed to investigate the stability of ACRM in terms of its phase composition and microstructure, in addition to the redox reactivity. The post-reaction ACRM obtained after ten consecutive thermochemical CDS cycles was characterized using PXRD and SEM. The PXRD patterns of the pre- and post-reaction ACRM are depicted in Figure 11. As per the PXRD analysis, the phase composition of the ACRM remained unchanged even after undergoing ten thermochemical CDS cycles.

SEM analysis examined the ACRM samples obtained after 1 and 10 thermochemical cycles. Sintering of ACRM after the first cycle was observed in the SEM image presented in Figure 12a, indicating a significant increase in particle size from nanometers to microns. However, the SEM image in Figure 12b showed no further increase in particle size after performing 10 thermochemical cycles. This finding confirms that sintering of ACRM occurs upon exposure to high operating temperatures during the first cycle. No further sintering is observed once the material is stabilized after the initial thermochemical cycle. This is consistent with the levels of O₂ released and CO produced by ACRM, as higher levels were recorded during the first cycle, while significantly lower amounts were observed during the second cycle, which could be attributed to ACRM sintering. Conversely, from the second to the tenth cycle, the redox reactivity of ACRM remained consistent, as no further sintering was observed.

The investigation conducted on the redox reactivity of CeO₂ with Ag doping has shown promising results. Our research indicates that the amounts of O₂ released and CO produced by ACRM are not extremely high compared with those produced by pure CeO₂. However, it is important to note that we have only tested one composition of ACRM (1% Ag-doped in CeO₂: Ce_0.99Ag_0.01O_2-δ). Besides, the findings of this study provide a pathway to improve the redox reactivity of CeO₂ by incorporating Ag in its crystal structure.

Our future plans entail investigating the impact of varying doping levels of Ag in the ceria crystal structure (ranging from 1 to 10%) on the thermochemical conversion of H₂O and CO₂ into solar fuels. Additionally, we will conduct advanced material characterizations, such as TEM mapping and XPS analysis [32], to analyze the elemental distribution and electronic structural characteristics of the Ce, Ag, and O elements.

4. Summary and Conclusions

A co-precipitation synthesis approach was employed to successfully dope Ag within the CeO₂ crystal, forming ACRM, to improve the redox reactivity of pure CeO₂. The PXRD peaks analysis revealed that ACRM manifests distinctive crystalline peaks consistent with a cubic fluorite crystal, similar to CeO₂. Notably, no metallic Ag or Ag-oxide impurities were detected in the PXRD patterns, indicating their absence. Additionally, SEM analysis confirmed the formation of ACRM nanoparticles. In TGA-based thermochemical CDS experiments, the co-precipitation synthesized ACRM exhibited higher O₂-releasing and CO-producing abilities, 7.07% and 6.99%, respectively, over ten thermochemical cycles, compared with pure CeO₂. The PXRD analysis conducted on the reacted ACRM indicated that the chemical structure remained unaltered. However, SEM analysis confirmed that the sintering of ACRM occurred due to the high-temperature operation. The results of our research on CeO₂ with Ag doping are promising, indicating that ACRM can enhance the redox reactivity of CeO₂.