Utilizing Limestone Alone for Integrated CO₂ Capture and Reverse Water-Gas Reaction in a Fixed Bed Reactor: Employing Mass and Gas Signal Analysis

Abstract

The integrated CO₂ capture and utilization coupled with the reverse water-gas shift reaction (ICCU-RWGS) presents an alternative pathway for converting captured CO₂ into CO in situ. This study investigates the effectiveness of three calcium-based materials (natural limestone, sol-gel CaCO₃, and commercial CaCO₃) as dual-functional materials (DFMs) for the ICCU-RWGS process at intermediate temperatures (650–750 °C). Our approach involves a fixed-bed reactor coupled with mass spectrometry and in situ Fourier transform infrared (FTIR) measurements to examine cyclic CO₂ capture behavior, detailed physical and chemical properties, and morphology. The in situ FTIR results revealed the dominance of the RWGS route and exhibited self-catalytic activity across all calcium-based materials. Particularly, the natural limestone demonstrated a CO yield of 12.7 mmol g⁻¹ with 100% CO selectivity and 81% CO₂ conversion. Over the 20th cycle, a decrease in CO₂ capture capacity was observed: sol-gel CaCO₃, natural limestone, and commercial CaCO₃ showed reductions of 44%, 61%, and 59%, respectively. This suggests inevitable deactivation during cyclic reactions in the ICCU-RWGS process, while the skeleton structure effectively prevents agglomeration in Ca-based materials, particularly in sol-gel CaCO₃. These insights, coupled with the cost-effectiveness of CaO-alone DFMs, offer promising avenues for efficient and economically viable ICCU-RWGS processes.

1. Introduction

A series of environmental issues in terms of the excessive emission of CO₂ has sparked widespread concerns [1]. The control of greenhouse gas emissions through Carbon Capture, Utilization, and Storage (CCUS) is acknowledged as a pivotal method in mitigating CO₂ emissions, garnering significant attention within the global research community [2]. Nevertheless, the CCUS process typically includes the capture, pressurized liquefaction, transportation, and conversion of CO₂, rendering the entire CO₂ reduction procedure intricate and costly. To simultaneously alleviate CO₂ emissions and energy consumption, a novel concept known as integrated carbon capture and utilization (ICCU) has been proposed [3,4,5]. In contrast to conventional CCUS methods, which treat CO₂ capture from flue gas [6] and CO₂ hydrogenation into liquid fuels as separate processes [7,8], ICCU eliminates the need for compression, liquefaction, and transportation of CO₂. This integration reduces energy losses associated with multiple temperature fluctuations and has the potential to enhance overall energy efficiency [9]. Noteworthy ICCU technologies currently under extensive study encompass CO₂ capture coupled with in situ methanation [10,11], dry reforming of methane (DRM) [12,13], and the reverse water gas shift reaction (RWGS) [14,15]. In the ICCU-RWGS process, CO₂ and H₂ are introduced separately for CO₂ capture in the flue gas and hydrogenation via water electrolysis, as shown in Figure 1. With the introduction of H₂ during the utilization phase, the captured CO₂ liberated from sorbents is directly reduced and converted in situ to yield CO in a single reactor. Additionally, research has demonstrated that the hydrogen-to-carbon ratio in ICCU-RWGS can surpass the thermodynamic constraints of traditional RWGS, yielding a more favorable ratio for utilization as a feedstock in the Fischer–Tropsch Synthesis (FTS) process [16,17]. Consequently, ICCU-RWGS emerges as a preferable option for reducing the expenses of the CO₂ compression and transportation mid-process while concurrently enhancing the environmental and economic advantages of producing high-value-added chemicals.

One major challenge of ICCU-RWGS technology revolves around the advancement of dual-function materials (DFMs) that encompass both CO₂ adsorbent and catalyst functionalities. Specifically, alkali and alkaline metal oxides or carbonates, such as CaO [18], SrO [19], MgO [20], Na₂CO₃, Na₂O, K₂O, and K₂CO₃ [21], operating within a temperature range from ambient to 900 °C have been utilized as CO₂ adsorbents within DFMs. The catalytic elements embedded in these DFMs predominantly include noble metals (Ru, Rh) [22,23] and transition metals (Ni, Fe, Co) [15,24]. To align with the reaction temperatures (600–800 °C) required for the RWGS process, high-temperature adsorbents like CaO, BaO, and SrO are commonly employed [19,25,26]. When evaluating DFMs, several essential criteria come into play, including high CO₂ sorption capacity, elevated CO₂ conversion rates, superior product selectivity, and robust operational stability. In particular, CaO stands out as a widely employed high-temperature CO₂ adsorbent, showcasing self-catalytic activities during the carbonate hydrogenation process [27,28]. This inherent functionality not only enhances the process’s viability for commercialization but is also cost effective, abundant, and environmentally sustainable. Furthermore, utilizing CaO alone for high-purity syngas production has demonstrated noteworthy results, with a CO yield of 4.15 mmol/g-CaO, up to 75% CO₂ conversion, and 100% CO selectivity achievable at 700 °C within a fixed bed system [29]. Research on limestone hydrogenation under fluidized and isothermal conditions via a micro-fluidized bed thermogravimetric analysis coupled with mass spectrometry (MFB-TGA-MS) system has yielded promising outcomes, achieving 79.1% CO₂ conversion and approximately 100% CO selectivity at 710 °C [30]. These pioneering studies suggest the feasibility of utilizing the ICCU-RWGS process in conjunction with CaO-alone materials without necessitating additional catalytic components. However, despite the success of CaCO₃/CaO material systems as DFMs within ICCU technology, further detailed exploration into their catalytic efficacy and stability is warranted.

From a reactor standpoint, traditional techniques such as thermogravimetric analysis (TGA) [31,32], packed-bed reactors [33,34,35,36,37], and MFB-TGA-MS [30,38] under fluidized conditions are the primary methods for analyzing integrated CO₂ capture and utilization-reverse water gas shift (ICCU-RWGS) processes. In fixed-bed reactors, high reactivity and stability are observed under isothermal conditions at 650 °C using a Ca₁Ni_0.1Ce_0.033 catalyst, which demonstrates a notable CO₂ capture capacity of 14.10 mmol CO₂ g⁻¹ and a CO production rate of 7.3 mmol CO g⁻¹ without deactivation over 20 cycles [39]. A nickel-cerium-based CaO material synthesized via the sol-gel method exhibits a CO₂ capture capacity of 15.34 mmol CO₂ g⁻¹ and a CO production rate of 5.97 mmol CO g⁻¹, although it experiences an 11.6% loss in capacity over 10 cycles in a fixed-bed reactor [40]. Under fluidized and isothermal conditions, non-catalytic gas–solid reactions (CaO + CO₂ → CaCO₃) coexist with catalytic gas–solid reactions (CO₂ + H₂ → CO + H₂O), resulting in the gradual coating of Cu particles by CaCO₃ during carbonation. This phenomenon prevents Cu sintering but slightly reduces catalytic activity [41]. For material screening, fixed-bed reactors are primarily used in the ICCU field due to their ease of operation and convenience.

Therefore, understanding the reactive pathway, performance, and cyclic stability of calcium-based materials without additional catalyst components in ICCU-RWGS is urgent. To gain a deeper understanding of DFMs, particularly those derived from CaO-alone material, this study presents novel investigations through a comparative analysis conducted in a fixed-bed reactor coupled with mass spectrometry, employing mass and gas signal analysis. The study focuses on three calcium-based sorbents: natural limestone, commercially available carbonate, and synthetic materials (sol-gel CaCO₃). Furthermore, the research points out the reaction pathway through temperature-programmed surface reactions, employing in situ Fourier transform infrared (FTIR) spectroscopy to uncover the impact of ICCU temperature variations. The carbon capture and utilization process involving the three materials was evaluated across a temperature range spanning 650 to 750 °C, with increments of 25 °C. By examining these materials at different temperatures, the study aims to provide fresh insights into the strategic design of robust solid DFMs tailored for ICCU applications. Overall, the cost-effectiveness of CaO-alone DFMs offers promising pathways for efficient and economically viable ICCU-RWGS processes.

2. Experimental

2.1. Material Preparation

Natural limestone (Hubei Jingmen Shunzhan Cement Industrial Corp., Hubei, China), commercial CaCO₃ (98%, Shanghai Titan Scientific Corp., Shanghai, China), and sol-gel CaCO₃ were used for Ca-Based DFMs in the ICCU-RWGS process. Specifically, the sol-gel CaCO₃ was synthesized through a simple one-pot sol-gel method [41]. Calcium nitrate tetrahydrate (Ca(NO₃)₂·4H₂O) with citric acid at a molar ratio of 1:2.5 (sum of metal cations/citric acid) was dissolved in 80 mL deionized water at room temperature. This was followed by mixing the dissolved cations with ethylene glycol at a molar ratio of 1.5:1 (ethylene glycol/citric acid) at 40 °C. The mixture was then heated at 80 °C to form a gel. The precursor gel was dried at 110 °C overnight and then calcined in a box furnace at 800 °C with airflow. The precursor particle was finally carbonated in the tube furnace with 15% CO₂ at 550 °C for 1 h to obtain the sol-gel CaCO₃. After crushing and grinding, the three nominated DFMs (natural limestone, commercial CaCO₃, and sol-gel CaCO₃) were carefully sieved to particle size ranges of 250–450 μm.

2.2. Material Characterization

X-ray diffraction (XRD) patterns (2 theta: 20°–70°) were obtained using a D8 Advance diffractometer with Cu Kα radiation with a continuous scanning rate of 2 degree min⁻¹. Scanning electron microscope (SEM) images and energy-dispersive spectroscopy (EDX) mappings were recorded on a scanning transmission electron microscope (Zeiss Merlin, Oberkochen, Germany). Raman spectra was carried out using a Raman spectrometer (LabRAM HR Evolution, Horiba, Japan) with a He-Ne green laser (532 nm). The elemental compositions of natural limestone were quantified on inductively coupled plasma-optical emission spectroscopy (ICP-OES, ICAP 7400). The carbon and sulfur content of natural limestone was detected by the Carbon/Sulfur Analyzer (ELTRA CS-800).

2.3. Temperature Programmed Procedure

The H₂ temperature-programmed surface reaction (H₂-TPSR) was performed on the DFMs after the removal of surface species at 150 °C in a 30 mL min⁻¹ N₂ (100 vol%) flow for 30 min. All of the DFMs were then heated from 150 °C to 850 °C in a H₂/N₂ (10 vol%) flow. The gaseous products were collected online using a Fourier transform infrared (FTIR) spectrometer (Thermo Nicolet iS20) equipped with a gas cell to track the gas signals, including CO, CO₂, CH₄, H₂O.

2.4. Integrated CO₂ Capture and Utilization System

The performance evaluation for CO₂ capture and in situ hydrogenation to CO was performed in a fixed-bed system coupled mass spectrometry. As shown in Figure 2a, the system consists of a quartz reactor, an electric furnace, a gas supply system, and a measuring system. The quartz reactor (od: 23 mm, id: 20 mm, L: 1000 mm) is designed to be a fixed bed, and a porous quartz plate with a thickness of 5 mm is used as the gas distributor. The reactor was heated by an external electric furnace with the temperature monitored by a K-type thermocouple. For the bed temperature, one K-type thermocouple was measured in the material layer. The gas flow rate was controlled with a mass flow control (MFC, Sevenstar, Beijing, China). The composition of the effluent stream was monitored by an on-line gas mass spectrometry (Tilon LC-D200M). The entire quartz reactor without contacting any part of furnace rests entirely on the weighing transducer (XPE 1203S, Mettler Toledo Corp., Zürich, Switzerland) to measure the real-time mass signals with a readability of 1 mg. Based on the gas profile and mass signals, the precise conversion information of ICCU would be effectively obtained. As shown in Figure 2b, to ensure the consistency of gas signal and mass signal on samples, the obtained data would be verified whether in the ±10% error margin. This indicates that the gas signals and mass signals as a function of time were also confirmed during CO₂ capture and in situ hydrogenation steps.

Typically, DFMs were heated to specified temperature with a rate of 30 °C/min in Ar (200 mL/min). After the temperature is stabilized, switch Ar to 15 vol% CO₂/Ar (200 mL/min) for 30 min CO₂ capture. The DFMs were then purged in Ar (200 mL/min) for 5 min, following with in situ hydrogenation in pure H₂ (200 mL/min) for 30 min.

Furthermore, the CO formation (${\mathrm{q}}_{\mathrm{C}\mathrm{O}}$), CH₄ formation (${\mathrm{q}}_{\mathrm{C}{\mathrm{H}}_4}$), and CO₂ formation (${\mathrm{q}}_{\mathrm{C}{\mathrm{O}}_2}$) were calculated from the gas profile via Equations (1)–(3).

$${\mathrm{q}}_{\mathrm{C}\mathrm{O}}[\mathrm{m}\mathrm{m}\mathrm{o}\mathrm{l}{\mathrm{g}}^{-1}]={\displaystyle \frac{1}{\mathrm{W}}}{\displaystyle \underset{{\mathrm{t}}_{{\mathrm{H}}_2,\mathrm{i}\mathrm{n}}}{\overset{{\mathrm{t}}_{{\mathrm{H}}_2,\mathrm{o}\mathrm{u}\mathrm{t}}}{\int }}{\mathrm{F}}_{\mathrm{C}\mathrm{O},\mathrm{o}\mathrm{u}\mathrm{t}}(\mathrm{t})\mathrm{d}\mathrm{t}}$$

(1)

$${\mathrm{q}}_{\mathrm{C}{\mathrm{H}}_4}[\mathrm{m}\mathrm{m}\mathrm{o}\mathrm{l}{\mathrm{g}}^{-1}]={\displaystyle \frac{1}{\mathrm{W}}}{\displaystyle \underset{{\mathrm{t}}_{{\mathrm{H}}_2,\mathrm{i}\mathrm{n}}}{\overset{{\mathrm{t}}_{{\mathrm{H}}_2,\mathrm{o}\mathrm{u}\mathrm{t}}}{\int }}{\mathrm{F}}_{\mathrm{C}{\mathrm{H}}_4,\mathrm{o}\mathrm{u}\mathrm{t}}(\mathrm{t})\mathrm{d}\mathrm{t}}$$

(2)

$${\mathrm{q}}_{\mathrm{C}{\mathrm{O}}_2}[\mathrm{m}\mathrm{m}\mathrm{o}\mathrm{l}{\mathrm{g}}^{-1}]={\displaystyle \frac{1}{\mathrm{W}}}{\displaystyle \underset{{\mathrm{t}}_{{\mathrm{H}}_2,\mathrm{i}\mathrm{n}}}{\overset{{\mathrm{t}}_{{\mathrm{H}}_2,\mathrm{o}\mathrm{u}\mathrm{t}}}{\int }}{\mathrm{F}}_{\mathrm{C}{\mathrm{O}}_2,\mathrm{o}\mathrm{u}\mathrm{t}}(\mathrm{t})\mathrm{d}\mathrm{t}}$$

(3)

where the ${\mathrm{F}}_{\mathrm{C}\mathrm{O},\mathrm{o}\mathrm{u}\mathrm{t}}$, ${\mathrm{F}}_{\mathrm{C}{\mathrm{H}}_4,\mathrm{o}\mathrm{u}\mathrm{t}}$, and ${\mathrm{F}}_{\mathrm{C}{\mathrm{O}}_2,\mathrm{o}\mathrm{u}\mathrm{t}}$ are the molar flow rates of CO, CH₄, and CO₂ (mmol min⁻¹) in the outlet gas, respectively. W (g) is the sample mass.

Finally, the CO selectivity (${\mathrm{S}}_{\mathrm{C}\mathrm{O}}$), the CO₂ conversion (${\mathrm{X}}_{\mathrm{C}{\mathrm{O}}_2}$), and CO yield (${\mathrm{Y}}_{\mathrm{C}\mathrm{O}}$) were determined via Equations (4) and (5).

$${\mathrm{S}}_{\mathrm{C}\mathrm{O}}={\displaystyle \frac{{\mathrm{q}}_{\mathrm{C}\mathrm{O}}}{{\mathrm{q}}_{\mathrm{C}\mathrm{O}}+{\mathrm{q}}_{\mathrm{C}{\mathrm{H}}_4}}}\times 100\%$$

(4)

$${\mathrm{X}}_{\mathrm{C}{\mathrm{O}}_2}={\displaystyle \frac{{\mathrm{q}}_{\mathrm{C}\mathrm{O}}+{\mathrm{q}}_{\mathrm{C}{\mathrm{H}}_4}}{{\mathrm{q}}_{\mathrm{C}\mathrm{O}}+{\mathrm{q}}_{\mathrm{C}{\mathrm{H}}_4}+{\mathrm{q}}_{\mathrm{C}{\mathrm{O}}_2}}}\times 100\%$$

(5)

3. Results and Discussion

3.1. Phase and Structural Characterization of the Ca-Based DFMs

Figure 3a presents the XRD patterns of the fresh Ca-based dual functional materials (DFMs). The characteristic peaks of the CaCO₃ phase were detected in all the samples, and no impurity phases were observed, indicating the high purity of the three fresh DFMs. This purity is further confirmed by the detailed ICP results, as shown in Table S1. To further investigate the surface properties, Raman spectra were obtained on fresh DFMs, as shown in Figure 3b. The Raman spectra of commercial CaCO₃ and sol-gel CaCO₃ reflected strong CaCO₃ peaks (151, 278, 709, 1083 cm⁻¹) and no carbon species on the sample surface. In contrast, the Raman spectra of natural limestone reflected carbon species, indicated by the D and G peaks at 1380 and 1586 cm⁻¹, respectively.

Table 1. Physical properties of the fresh Ca-based DFMs.

Sample	Carbon Composition (%)	Main Element (>0.01%)	BET Surface Area (m2 g−1)	Crystallite Size (nm)
Natural limestone	0.03	Ca, Mg, Al, Fe, K, Sr	1.77	78.9
Commercial DFMO3	-	Ca, Mg, Al	1.21	58.9
Sol-gel DFMO3	-	Ca	5.61	42.1

summarizes the carbon composition, main elements, surface area, and crystallite size. The carbon content on the natural limestone is consistent with carbon/sulfur analysis, indicating that the surface is slightly covered (0.03%) (Table S2). Figure 3c–e show SEM images and EDX mappings of Ca and O on the fresh DFMs. The EDX mappings reveal that the surface of all samples is evenly covered with elements Ca and O. This consistency aligns with the XRD and Raman results, where strong CaCO₃ diffraction peaks and Raman intensities were obtained, respectively. However, the SEM images indicate different morphologies among the three samples. Natural limestone and commercial CaCO₃ reveal smooth and dense surfaces, whereas sol-gel CaCO₃ shows a porous particle surface. This trend is similar to the BET results, with natural limestone and commercial CaCO₃ exhibiting relatively low surface areas (1.77 and 1.21 m² g⁻¹, respectively), while sol-gel CaCO₃ demonstrates a higher surface area (5.61 m² g⁻¹). The crystallite sizes of CaCO₃ were determined using the CaCO₃ (104) peak at 2 theta = 29.4° for all samples. The order of crystallite size increase was natural limestone (78.9 nm) > commercial CaCO₃ (58.9 nm) > sol-gel CaCO₃ (42.1 nm). The highest crystallite size of natural limestone may be attributed to the incorporation of trace metals, as indicated by the ICP results. Overall, the high purity of the DFMs, confirmed by these characterizations, makes them suitable candidates for the ICCU process.

3.2. Integrated CO₂ Capture and Utilization Performance of the Ca-Based DFMs

To determine the reaction region and pathway of carbonate hydrogenation, in situ FTIR and Temperature Programmed Surface Reaction (TPSR) analyses were conducted to elucidate the hydrogenation process on various DFMs (natural limestone, sol-gel CaCO₃, and commercial CaCO₃). The species were identified based on the absorbance intensities of their characteristic vibrational modes: ν(C=O) at 2337 cm⁻¹ and 2360 cm⁻¹ for CO₂, ν(C≡O) at 2171 cm⁻¹ and 2113 cm⁻¹ for CO, δ(H−O−H) at 1559 cm⁻¹ for H₂O, and ν(C−H) at 3016 cm⁻¹ for CH₄. Figure 4 illustrates a uniform single-step hydrogenation process across all samples, indicative of CaCO₃(s) → CaO(s) conversion. From the gas composition standpoint, the peak temperatures of CO₂ for sol-gel CaCO₃, commercial CaCO₃, and natural limestone were 706 °C, 726 °C, and 726 °C, respectively, indicating the unconverted CO₂ released from the sorbents of DFMs. Similarly, the peak temperatures of CO for sol-gel CaCO₃, commercial CaCO₃, and natural limestone were 706 °C, 736 °C, and 736 °C, respectively. The lowest peak temperature of CO for sol-gel CaCO₃ is likely due to its porous structure, which provides a better reaction interface during the hydrogenation process. Conversely, the presence of trace elements in natural limestone negatively impacted its reaction performance compared to sol-gel CaCO₃ and commercial CaCO₃. The inset in Figure 4 shows that in situ FTIR spectra analysis revealed the products were composed of CO₂ and CO, with no CH₄ detected. The absence of CH₄ suggests a high selectivity toward CO, especially at temperatures above 550 °C, primarily via the RWGS pathway. This observation aligns with the thermodynamic preference for CO production at higher temperatures and corroborates previous studies using CaO-alone material [30]. Therefore, the reaction temperature for this study was set between 650 °C and 750 °C.

Figure 5a illustrates the CO₂ capture and utilization performance using natural limestone in a fixed-bed system coupled with mass spectrometry at 700 °C. The reaction curves can be generally divided into three primary regions: CO₂ capture, inert gas purge, and hydrogenation processes. Initially, as CaCO₃ is the primary reactant, a portion of CO₂ is released during the heating stage, defining the first hydrogenation phase as the pre-hydrogenation stage. Following pre-hydrogenation, the system is purged with Ar to eliminate residual gases and subsequently exposed to simulated flue gas (15 vol% CO₂ balanced with Ar) for CO₂ capture. After Ar purging, a continuous flow of pure H₂ initiates the hydrogenation step within the fixed bed. The investigation focused on the three DFMs in terms of CO₂/CO/CH₄ yield, CO₂ conversion, and CO selectivity at 700 °C, as shown in Figure 5b. Notably, all DFMs demonstrated exceptional CO selectivity (~100%) with CO₂ conversions ranging between 75 and 81% without the addition of any catalyst components, leading to a lower cost compared to conventional DFMs. Among them, sol-gel CaCO₃ exhibited the highest CO yield (11.34 mmol g⁻¹), attributed to its higher CO₂ capture capacity (15.10 mmol g⁻¹) compared to natural limestone (13.96 mmol g⁻¹) and commercial CaCO₃ (12.49 mmol g⁻¹). This indicates that CO yield is strongly influenced by the CO₂ capture capacity of the DFMs. Figure 5c illustrates the impact of temperature on the reactivity of DFMs. Higher temperatures (e.g., 725 °C and 750 °C) resulted in lower CO₂ conversion (65–75%) across all DFMs. This suggests that the rapid decomposition rate of CaCO₃ at higher temperatures significantly influences the overall CO₂ conversion, leading to more unconverted CO₂ being released into the atmosphere. The increased reaction temperature correlated with an increased amount of unconverted CO₂ (blue bar), which presented a similar trend across the performance of all three DFMs. Regarding CO selectivity, all DFMs demonstrated 100% selectivity during the hydrogenation process. Notably, sol-gel CaCO₃ maintained good reaction performance between 650 °C and 750 °C, indicating that the particle structure plays a key role during the reaction of CaO-alone material. Considering the reactive performance and efficiencies of DFMs in both CO₂ capture and utilization, the temperature of 700 °C was deemed optimal. Despite sol-gel CaCO₃ exhibiting the highest CO yield in CO₂ capture and conversion, the cost-effective and abundantly available natural limestone emerged as more competitive than synthesized and state-of-the-art dual functional materials.

3.3. Cyclability of CO₂ Capture and Utilization on Ca-Based DFMs

To comprehensively understand the cyclability of the integrated CO₂ capture and utilization processes of the DFMs, a series of experiments were conducted using a fixed-bed system coupled with mass spectrometry at 700 °C over 20 cycles (Figure 6). The carbonation stage involved 15 vol% CO₂, and the hydrogenation stage involved pure H₂. Throughout the 20-cycle ICCU-RWGS process, all DFMs experienced a decline in CO₂ capture and carbonation conversion as the number of cycles increased. However, at the relatively mild reaction temperature of 700 °C (compared to the conventional calcination temperature of 850–900 °C in the calcium-looping process), the decrease in CO₂ capture capacity was slower, particularly for sol-gel CaCO₃. Specifically, in the initial cycle, natural limestone, commercial CaCO₃, and sol-gel CaCO₃ exhibited CO₂ capture capacities of 15.78 mmol g⁻¹, 16.23 mmol g⁻¹, and 15.10 mmol g⁻¹, respectively. By the 10th cycle, these capacities declined to 9.09 mmol g⁻¹, 8.76 mmol g⁻¹, and 10.88 mmol g⁻¹, respectively, corresponding to decreases of 42%, 46%, and 28%. By the 20th cycle, the capacities further reduced to 6.17 mmol g⁻¹, 6.65 mmol g⁻¹, and 8.44 mmol g⁻¹, representing decreases of 61%, 59%, and 44%, respectively. Notably, sol-gel CaCO₃ retained 58% of its CO₂ capture capacity after the 20-cycle carbonation and hydrogenation process, significantly higher than the 20–30% retention observed in conventional calcination processes [42]. Despite these declining trends in CO₂ capture capacity, the CO₂ conversion of the DFMs remained relatively stable, consistently in the 70–80% range. The average CO₂ conversion percentages for natural limestone, commercial CaCO₃, and sol-gel CaCO₃ were 71.76%, 76.05%, and 73.83%, respectively, indicating stability in the CO₂ conversion released from sorbents. Throughout the ICCU-RWGS process, all DFMs exhibited nearly 100% CO selectivity.

To date, there have been several studies on the ICCU-RWGS process using calcium-based DFMs.

Table 2. A comparison of ICCU-RWGS performance of Ca-based DFMs to those reported in literatures.

Sample	Conditions	CO2 Capture Capacity (mmol g−1)	CO2 Conversion (%)	CO Yield (mmol g−1)	Loss-in-Capacity (%)	Ref.
Mable dust	650 °C, 10% CO2, 5% H2	9.4	85.2	8.0	47 cycles, 30	[27]
Fe5Co5Mg10CaO	650 °C, 10% CO2, pure H2	9.2	88.5	8.2	10 cycles, 4	[15]
CaO-CaGa0.5Zr0.5(O)	650 °C, 15% CO2, pure H2	14.55	87.8	12.29	20 cycles, 13	[33]
Ca1Ni0.1	650 °C, 15% CO2, 5% H2	15.0	46.0	6.9	20 cycles, 33	[39]
Ca1Ni0.1Ce0.033		14.1	51.1	7.3	20 cycles, stable
Ni/CS-P30-C	650 °C, 10% CO2, 5% H2	13.86	39.8	5.52	17 cycles, 49	[18]
stableNi/CS-P30-C-P		8.95	59.6	5.33	17 cycles, 26
NiCaCe-SG	650 °C, 10% CO2, 10% H2	15.34	92.4	5.97	10 cycles, 12	[40]
Natural limestone	700 °C, 15% CO2, pure H2	15.8	81.1	12.7	20 cycles, 61	This study
Commercial CaCO3		16.2	79.8	13.6	20 cycles, 59
Sol-gel CaCO3		15.1	75.1	11.7	20 cycles, 44

presents a comparison of the ICCU-RWGS performance of Ca-based DFMs. The CO₂ capture capacity ranges from 8.95 to 16.2 mmol g⁻¹ for fresh DFMs on both synthetic and natural materials, indicating high CO₂ capture performance in calcium-based materials. Regarding CO yield, most DFMs exhibit the capability of CO formation with high CO selectivity, including CaO-alone materials, with yields ranging from 6.9 to 13.6 mmol g⁻¹ for fresh samples. This trend is also reflected in CO₂ conversion rates, which range from 46.0% to 96.4%. However, from a cyclic stability standpoint, natural and non-modified materials experience a rapid decline during cyclic reactions compared to synthetic materials. In terms of loss in capacity, marble dust exhibited a 30% decrease, followed by sol-gel CaCO₃ (44%), commercial CaCO₃ (59%), and natural limestone (61%). Synthetic materials demonstrated more stable cyclic performance during the ICCU-RWGS process. Notably, Ca₁Ni_0.1Ce_0.033 stands out as the best-performing DFM over 20 cycles, due to the well-dispersed CeO₂, which acts as a physical barrier that effectively prevents the growth and agglomeration of CaO crystallites and NiO species [39]. Therefore, given the outstanding performance in CO₂ capture capacity, CO₂ conversion, CO yield, and cyclic stability, appropriate modification of calcium-based materials for cyclic reactions is crucial.

To provide substantial evidence for elucidating the cyclic morphology of the spent DFM particles, SEM images in Figure 7 complement these findings by offering visual evidence of changes in the physical properties of the spent Ca-based DFMs after 20 cycles. After 20 cycles, some micro-grade pores were still present on the surface of sol-gel CaCO₃, although some areas showed signs of sintering. When zoomed in, cavities were visible inside the sol-gel CaCO₃. In contrast, natural limestone and commercial CaCO₃ appeared coarser and rougher, with few micro-grade pores remaining. According to Bhatia, a porous surface is beneficial for the carbonation reaction [43], indicating that sol-gel CaCO₃ absorbed CO₂ with high efficiency in the present study due to its abundant pores, which facilitate the diffusion of CO₂ into the surface of the DFM. This suggests better diffusion of CO₂ towards the CaO and more stable micro-structures of the sorbents, especially on sol-gel CaCO₃. This is in good agreement with the observation and previous study that the finer pores and skeleton structure of calcium-based material can mitigate agglomeration and sintering during the cyclic reactions [28,44]. Overall, a certain degree of sintering was observed in all DFMs. Among them, sol-gel CaCO₃ maintained a skeleton-like structure with micro-grade pores as the number of cycles increased, unlike the non-skeleton-like structure of natural limestone and commercial CaCO₃. This highlights the importance of modifying Ca-based materials to enhance their performance.

4. Conclusions

This study investigated the CO₂ capture performance of CaO-alone dual functional materials (DFMs) prepared from natural limestone, commercially available carbonate, and synthetic materials (sol-gel CaCO₃). A comparative thermodynamic analysis of the three CaO-alone DFMs was performed over 20 cycles of the ICCU-RWGS process. Firstly, the in situ FTIR results revealed the dominance of the reverse water-gas shift reaction (RWGS) route instead of methanation and exhibited self-catalytic activity across all CaO-alone DFMs. Additionally, the influence of temperature on CO₂ conversion, CO yield, and cyclability was examined within the range of 650 to 750 °C. The results indicated that the loss in CO₂ capture capacity for the sol-gel CaCO₃ decreased from 15.1 to 8.44 mmol g^-¹ over 20 cycles. A certain level of deactivation occurred in the ICCU-RWGS process due to CaO agglomeration. The abundant pores and skeleton structure inside the sol-gel CaCO₃ particles facilitated the diffusion of CO₂ into the surface of the sorbent, resulting in enhanced CO₂ capture performance compared to natural limestone and commercial CaCO₃. This study offers a promising pathway for CaO-alone DFMs to achieve carbon neutrality using ICCU technology.