Highly selective and stable Cu-based dual-function materials for integrated CO2 capture and in-situ conversion to CO

Highly selective and stable Cu-based dual-function materials for integrated CO₂ capture and in-situ conversion to CO

3.1. Characterization of DFMs

To understand the relationship between Cu content and the textural properties, the textural properties of DFMs with varying Cu loadings and 30 % NaCa sorbent, including surface area, total pore volume, and average pore size, are presented in Table 1. Additionally, their N₂ adsorption–desorption isotherms are illustrated in Fig. S1. All the investigated DFMs illustrated isotherms with mixed characteristics of types II and IV according to the IUPAC classification of adsorption isotherms with hysteresis. Therefore, it could be inferred that mesoporous and macroporous structures coexist in these materials [41]. At relative pressure near unity, there is a lack of adsorption plateau for all samples, suggesting a macroporous structure. Furthermore, the narrow hysteresis area indicates a minority of mesopores [28].

Table 1. DFMs textural properties.

Materials	BET surface area (m²/g)	pore volume (cm³/g)	average pore diameter (nm)	CuO crystallite size (nm)*
NaCa₃₀Al₇₀	51	0.28	22	−-
Cu₅NaCa₃₀Al₆₅	36	0.21	24	6.3
Cu₁₀NaCa₃₀Al₆₀	33	0.27	33	13.1
Cu₁₅NaCa₃₀Al₅₅	33	0.28	34	25.1

*Calculated by Scherrer’s equation using the full width at half maximum of the most intense diffraction line in XRD patterns.

According to the data presented in Table 1, the BET surface area decreased from 51 in the case of the sample with zero Cu content (NaCa₃₀Al₇₀) to 33–36 m²/g by adding different Cu loadings, likely due to the reduced γ-Al₂O₃ content. Conversely, the pore volume initially decreased with the addition of 5 % Cu (Cu₅NaCa₃₀Al₆₅) but further increases in Cu led to higher pore volumes comparable to NaCa₃₀Al₇₀. Additionally, the pore diameter of DFMs increased by Cu, with higher Cu content resulting in larger pore openings. Similarly, it is reported that incorporating Cu into Ni-CaZr DFM resulted in a bigger pore opening by generating larger piled mesopores [42].

XRD analysis was performed to reveal the effect of Cu content on the crystalline phases of DFMs. The XRD patterns of as-prepared DFMs with varying Cu loading are depicted in Fig. 2. The diffraction peaks corresponding to lime CaO (ref. code: 96–900-6696), tenorite CuO (ref. code: 96–901-5925), calcite CaCO₃ (ref. code: 96–901-5482), Na₂CO₃ (ref. code: 96–152-7607), mayenite Ca₁₂Al₁₄O₃₃ (ref. code: 96–901-1738), and γ-Al₂O₃ (ref. code: 96–101-0462) are detected in the XRD patterns of the calcined DFMs. Notably, with an increase in Cu content, there is a notable growth in the intensity of peaks associated with tenorite, especially at 2θ = 35.550°, 38.667°, and 48.841°. The crystallite size of CuO was calculated through Scherrer’s equation employing the full width at half maximum (FWHM) of the most intense diffraction line at 2θ = 38.667°. The results indicated a significant increase in CuO crystalline size with the rise in Cu content, measuring 6.3, 13.1, and 25.1 nm for DFMs containing 5 %, 10 %, and 15 % Cu, respectively. The variation in CuO particle size can affect its interaction with the γ-Al₂O₃ support and reducibility of DFM, which will be further elucidated through H₂-TPR experiments.

The emergence of the inert mayenite phase (Ca₁₂Al₁₄O₃₃) suggests the successful diffusion of Ca²⁺ into the alumina oxide framework [43] during calcination at 700 °C. This inert phase could serve as a physical barrier against sintering, thereby enhancing the stability of the DFMs. Further diffusion of Ca²⁺ into the Ca₁₂Al₁₄O₃₃ could potentially result in a more stable calcium aluminate phase, Ca₉Al₆O₁₈ [44]. However, such a transition was not observed in this case, likely due to the relatively low calcination temperature.

CO₂-TPD measurements were conducted to investigate the basic sites and basicity of the DFMs. The resulting profiles are depicted in Fig. 3. According to the literature, the basic sites are divided into three distinguished regions: i) weak basic sites where CO₂ desorption occurs up to 250 °C, ii) medium basic sites characterized by CO₂ desorption taking place between 250 °C and 700 °C, and iii) strong basic sites with CO₂ desorption at temperatures surpassing 700 °C [45,46]. The basicity of DFMs is an essential factor affecting their performance in the cyclic ICCC-RWGS process since the temperature at which CO₂ is desorbed from the DFMs should be aligned with the reaction temperature [45]. In the case of the RWGS reaction, medium basic sites are the most favorable.

As seen in Fig. 3, none of the prepared samples illustrate weak basic sites originating from the surface hydroxyl groups. These sites effectively bond with CO₂ and promote the formation of bicarbonates [[46], [47], [48]]. In the case of NaCa₃₀Al₇₀, a distinct peak with high density appears at around 666 °C, indicating the presence of medium basic sites, which can be attributed to the formation of carbonates by chemical bonding between ionic metal–oxygen Lewis pairs and CO₂ [49]. In contrast, in profiles of samples containing Cu, two distinct peaks with lower densities are observed, indicating the interference of Cu into the basic sites. The first peak is in the range of medium sites, and the second is in the range of strong sites. Compared to the NaCa₃₀Al₇₀, the peak attributed to medium basic sites (first one) appears at lower temperatures for all DFMs, and an increase in Cu content further shifted the center of this peak to lower temperatures. Specifically, the DFMs containing 5, 10, and 15 % Cu exhibited peaks centered at approximately 630 °C, 608 °C, and 606 °C, respectively. This observation underscores that the presence of Cu facilitates the decomposition of carbonates and the subsequent release of CO₂, highlighting the synergistic interplay between Cu and the sorbent (NaCa). The shift is significant when 5 % Cu is added to NaCa₃₀Al₇₀, moving the center of the peak from 666 °C to 630 °C. By increasing the Cu content to 10 %, the interaction between Cu and NaCa sorbent intensifies, resulting in another notable shift from 630 °C to 608 °C. However, such a shift is not observed when Cu content increases to 15 %. This could be explained by the fact that after reaching 10 %, further increases in Cu content do not enhance the interaction between Cu and NaCa sorbent. Instead, Cu particles agglomerate and increase in size, as indicated by XRD measurements (Table 1).

In contrast, the second peak occurs at higher temperatures, indicating the presence of strong basic sites that can be attributed to the presence of oxygen ions (O^2–) [49]. These oxygen vacancies are likely introduced by Cu species on the surface of DFMs [[50], [51], [52]]. These vacancies are recognized as active sites for CO₂ sorption, as they promote electron transfer between oxygen-containing adsorbates and the catalyst surface [53]. Additionally, oxygen vacancies contribute to the generation of strong basic sites [49,53], which enhance the chemisorption of acidic molecules such as CO₂ and enhance its reduction [54]. Notably, the profiles demonstrate that the intensity of this peak increased with an increase in Cu content, confirming the Cu role in forming the strong basic sites. Table 2 presents the quantification of the density of basic sites. Moreover, the center of this peak shifted to even higher temperatures by an increase in Cu content, measuring approximately 829 °C, 853 °C, and 868 °C, respectively, for the DFMs containing 5, 10, and 15 % Cu. By comparing the CO₂-TPD profiles of NaCa₃₀Al₇₀ and DFMs containing Cu, it could be inferred that Cu plays a significant role in the basicity of the DFMs through its interaction with the NaCa sorbent.

Table 2. Basic sites quantification from CO₂-TPD profiles.

Materials	Density of basic sites, µmol CO₂/m²	Distribution of basic sites, %
Materials	Density of basic sites, µmol CO₂/m²	Medium sites, 250–700 °C	Strong sites, > 700 °C
NaCa₃₀Al₇₀	22.55	100	0
Cu₅NaCa₃₀Al₆₅	11.91	79	21
Cu₁₀NaCa₃₀Al₆₀	18.54	57	43
Cu₁₅NaCa₃₀Al₅₅	19.04	38	62

The reducibility of the prepared DFMs was examined through H₂-TPR analysis. In Fig. 4, a distinct peak is observed in the profile of all DFMs compared to the NaCa₃₀Al₇₀ profile, indicating the reduction of copper oxides. The reduction of CuO can occur through two mechanisms: i) a two-step reduction process, where Cu²⁺ first reduces to Cu¹⁺ at around 200 °C, followed by the reduction of Cu⁺¹ to Cu⁰ at temperatures above 500 °C, and ii) a one-step reduction process, where Cu²⁺ directly reduces to Cu⁰ in the approximate temperature range of 200 to 300 °C [55]. Thereby, based on the presence of a single peak below 300 °C in the profiles of the DFMs, this peak can be attributed to the one-step reduction mechanism [56]. Furthermore, increased Cu content resulted in a higher peak intensity, indicating greater H₂ consumption. Based on the TPR profiles, it can be observed that Cu₅NaCa₃₀Al₆₅ was reduced at higher temperatures (peak center around 245 °C) compared to two other DFMs with more Cu content (230 and 207 °C, for 10 and 15 % of Cu, respectively). This can be explained by the smaller CuO particles, measured from XRD patterns, within the porous structure of alumina, which enhances the interaction between the metal oxide and γ-Al₂O₃ support. As a result, the copper species become less accessible for H₂ activation and more challenging to reduce [56]. This increase in Cu content also resulted in a noticeable shift towards lower temperatures for the peak. As the Cu content increases, multiple layers of CuO species may form on the surface of the γ-Al₂O₃ support [57]. The interaction between the outer layer of CuO and the support is not strong enough, facilitating the formation of amorphous CuO species that can be reduced at lower temperatures [57].

NaCa₃₀Al₇₀ exhibited two minor peaks at high temperatures, approximately at 705 and 858 °C, indicating the reduction of stable carbonates. However, the presence of 5 % Cu resulted in smaller peaks occurring at lower temperatures, around 690 and 843 °C. Upon exposure of the DFM to H₂, CuO undergoes reduction, transforming into active metallic Cu. The resulting metallic Cu facilitates the activation of H₂, leading to the presence of H species on the catalyst surface (H_(ads)) [58]. This, in turn, facilitates the reduction of CaCO₃, causing the peaks to occur at lower temperatures than NaCa₃₀Al₇₀. Interestingly, in DFM containing 10 % Cu (Cu₁₀NaCa₃₀Al₆₀), the peaks related to CaCO₃ decomposition were shifted significantly to around 380 °C, indicating the profound synergistic interplay between Cu and the NaCa sorbent. A higher amount of metallic Cu resulted in more available H_(ads) on the DFM surface, intensifying this interplay. Those peaks are no longer present in the case of 15 % Cu (Cu₁₅NaCa₃₀Al₅₅), which might be engaged in the broad reduction shoulder of the CuO peak.

3.2. DFM performance in CO₂ sorption-regeneration

The CO₂ sorption-regeneration performance of the prepared DFMs was evaluated through Intelligent Gravimetric Analysis experiments. Additionally, their stability was investigated through 12 cycles of consecutive sorption and regeneration. Fig. 5 illustrates the behavior of DFMs under cyclic CO₂ capture and regeneration at 650 °C.

In the first cycle, NaCa₃₀Al₇₀ exhibits a CO₂ capture of 2.34 mmol/g. However, the regeneration rate (decarbonation) at 650 °C is significantly lower, with only 50 % of the captured CO₂ released (1.15 mmol/g) during the defined regeneration period. Consequently, only half of the capture sites were available for the subsequent sorption cycle, significantly reducing CO₂ capture. Upon adding 5 % Cu, the CO₂ uptake in the first cycle decreased from 2.34 to 1.73 mmol/g. Interestingly, with 10 % Cu, the CO₂ capture increased to 2.37 mmol/g, comparable to NaCa₃₀Al₇₀. However, further increasing the Cu content to 15 % resulted in a subsequent decrease in CO₂ capture (1.93 mmol/g), though it remained higher than the Cu₅NaCa₃₀Al₆₅. This suggests that a 10 % Cu content represents the optimum amount for maximizing the CO₂ capture efficiency of DFMs.

Remarkably, all DFMs exhibited an enhanced CO₂ release capability compared to NaCa₃₀Al₇₀. In the first regeneration cycle, DFMs containing 5 %, 10 %, and 15 % of Cu released 1.31, 1.50, and 1.41 mmol/g of CO₂, respectively, surpassing the 1.15 mmol/g released by NaCa₃₀Al₇₀. This indicates that the presence of Cu facilitates the decarbonation process, underscoring the positive impact of the synergistic interplay between Cu and NaCa sorbent on the behavior of DFMs. A transitional metal can increase oxygen vacancies and mobility, enhancing the carbonation/decarbonation kinetics of a Ca-based sorbent [[59], [60], [61]]. According to the proposed CO₂ sorption mechanism outlined in Equations (8), (9), oxygen vacancies introduced by Cu are advantageous for both CO₂ diffusion and the mobility of O^2– ions [14,62].

(8)(9)

In a study conducted by Al Mamoori et al. [31], the XRD patterns of Na-promoted CaO were compared before and after exposure to CO₂. The analysis revealed the emergence of peaks corresponding to sodium calcium carbonate (Na₂Ca(CO₃)₂) and CaCO₃ following CO₂ introduction. These findings suggest the presence of two primary pathways for CO₂ capture by the prepared DFMs (Eqs. (10), (11)).

(10)(11)

Notably, all DFMs demonstrated remarkable stability throughout the 12 cycles without any sign of deactivation. In each cycle, the sorption curve consistently reached the maximum point of the first cycle, represented by the red line.

3.4. Integrated CO₂ sorption and in-situ conversion to CO (ICCC-RWGS) isothermally

The performance of DFMs in the integrated CO₂ capture and in-situ conversion by RWGS was evaluated isothermally at 650 °C using a fixed-bed reactor, as described in Section 2.5. 100 mg of DFMs with varying Cu content were loaded in the reactor and subjected to the ICCC-RWGS process. The dynamic evolution of CO₂ and CO concentrations is depicted in Fig. 6, including a blank test (empty reactor) as a benchmark.

The CO₂ sorption period can be delineated into two distinct phases. The fast and kinetically-controlled carbonation phase represents the major part of CO₂ capture, where the CO₂ profiles significantly deviate from the blank test. Following that is the diffusion-controlled phase in which the CO₂ profiles of DFMs approach and converge with the blank test profile. In this region, carbonation persists but at a slower rate, with diffusion resistance influencing the reaction rate. However, the specific behavior of DFMs exhibits slight variations based on Cu loading, leading to different amounts of CO₂ capture. CO is promptly detected upon introducing H₂ to the reactor, signifying the high activity of DFMs in CO₂ hydrogenation, including CO₂ release from carbonates and its subsequent reaction with activated H₂. Nonetheless, the observation of unreacted CO₂ exiting the reactor during the hydrogenation step, as indicated by the continuation of the blue line (CO₂ concentration) into the hydrogenation phase in Fig. 6, suggests that the rate of CO₂ release from the DFMs slightly surpasses the rate of RWGS. Remarkably, throughout the entire hydrogenation step, no CH₄ was detected, which underlines the exceptional selectivity of DFMs towards CO.

A notable observation in Fig. 6 is the absence of CO generation during the CO₂ capture step. A significant production of CO in this step, arising from the direct CO₂ reduction by active Cu sites within the reduced DFMs (Eq. (12)), was reported previously [37,38]. This phenomenon is not unique to copper, as Fe-based DFMs have also exhibited similar behavior [12]. The CO produced in this stage exits the reactor mixed with the CO₂-free gas, which will be vented to the atmosphere, posing an environmental concern. However, the sorbent-catalyst interplay influences the direct reduction of CO₂. Previous studies highlighted a synergistic interaction between potassium and copper, where K partially covered the Cu species, reducing direct contact with CO₂ and lowering CO production [37,38]. The advantage of having no CO generation during CO₂ capture in the case of prepared DFMs in this study can be attributed to the interplay between the NaCa sorbent and Cu species, wherein Cu is shielded from direct oxidation with CO₂.

(12)

It is worth noting that the complete regeneration of the NaCa sorbent of the DFMs by the hydrogenation process takes less than 20 min (Fig. 6). In contrast, experiments conducted using the IGA setup showed that complete regeneration under an N₂ atmosphere at the same temperature requires 120 min (Fig. S3). This behavior implies the beneficial effect of H₂, where the Cu catalyst activates the hydrogen molecule, and the generated H_(ads) atoms at the DFM surface (Eq. (13) [58]) play an essential role in facilitating carbonate decomposition, as confirmed by H₂-TPR experiments.

(13)

3.4.1. Effect of Cu loading

Fig. 7 and Table 3 quantify the DFM behavior with different Cu loadings across a complete isothermal ICCC-RWGS cycle at 650 °C, illustrated in Fig. 6. As shown, except for the remarkable 100 % CO selectivity, which remains constant across all three DFMs, their performance significantly depends on the extent of Cu loading. Cu₁₀NaCa₃₀Al₆₀ achieved the highest CO₂ capture (2.52 mmol/g), CO production (2.10 mmol/g), and CO₂ conversion (89.3 %). The CO₂ capture and conversion trend in these experiments agree with the results of the sorption-regeneration and activity in RWGS reaction tests presented in Fig. 5 and Fig. S2, respectively.

Table 3. Details of DFMs performance during a complete ICCC-RWGS cycle at 650 °C over the DFMs with different Cu loadings.

DFM	CO₂ capture (mmol/g)	CO₂ loss during purge (mmol/g)	CO production (mmol/g)	CO₂ Conversion* (%)	CO selectivity (%)
Cu₅NaCa₃₀Al₆₅	1.73	0.07	1.40	84.6	100
Cu₁₀NaCa₃₀Al₆₀	2.52	0.17	2.10	89.3	100
Cu₁₅NaCa₃₀Al₅₅	2.16	0.21	1.67	85.6	100

*The reported CO₂ conversions are based on the remaining CO₂ on DFMs after the N₂ purge.

Cu loading affects the physicochemical properties of the DFMs, leading to different behaviors under an identical ICCC-RWGS process. The enhanced CO₂ capture capacity exhibited by Cu₁₀NaCa₃₀Al₆₀ compared to Cu₅NaCa₃₀Al₆₅ and Cu₁₅NaCa₃₀Al₅₅ can be attributed to physicochemical properties that are optimum for 10 wt% Cu loading, and not due to the amount of Ca loading, which varies from around 23 to 26 % for all three DFMs (determined by ICP analysis). Firstly, although all three DFMs present similar BET surface areas (refer to Table 2), the pore volume and pore size of Cu₁₀NaCa₃₀Al₆₀ and Cu₁₅NaCa₃₀Al₅₅ are notably higher than those of Cu₅NaCa₃₀Al₆₅. This enhancement in porosity likely facilitates more efficient CO₂ diffusion into the DFM structure during the diffusion-controlled carbonation stage. Therefore, the relatively lower CO₂ uptake observed for Cu₅NaCa₃₀Al₆₅ is attributed to its more restricted pore network, which limits CO₂ mobility and increases resistance to its diffusion into the porous network of the DFM. Secondly, surface basicity, which directly influences CO₂ chemisorption, is also Cu-content dependent. Insightful quantitative data from CO₂-TPD analysis (Table 2) reveals that both Cu₁₀NaCa₃₀Al₆₀ and Cu₁₅NaCa₃₀Al₅₅ exhibit a higher density of basic sites compared to Cu₅NaCa₃₀Al₆₅, an additional reason for the comparatively lower CO₂ capture capacity of Cu₅NaCa₃₀Al₆₅. However, a crucial distinction in the distribution of the basic sites becomes apparent in the case of DFMs containing 10 % and 15 % Cu with similar surface area, pore size, and total basicity. Cu₁₀NaCa₃₀Al₆₀ possesses a significantly higher proportion (57 %) of medium-strength basic sites (250–700 °C), which aligns closely with the temperature regime of the ICCC-RWGS process (650 °C). In contrast, Cu₁₅NaCa₃₀Al₅₅, while having comparable total basicity, features a larger fraction of strong basic sites (>700 °C), which are less accessible or active under the operating temperature of the ICCC-RWGS process. This optimal alignment of basic site distribution in Cu₁₀NaCa₃₀Al₆₀ results in more effective CO₂ sorption performance under working temperature.

Regarding CO₂ conversion, the higher activity of Cu₁₀NaCa₃₀Al₆₀ over Cu₅NaCa₃₀Al₆₅ is ascribed to the presence of more catalytically active Cu sites, enhancing the RWGS activity. However, increasing the Cu content beyond 10 wt% to 15 wt% (Cu₁₅NaCa₃₀Al₅₅) leads to catalyst agglomeration and an increase in the average particle sizes of the Cu species, as determined by XRD measurements (from 13.1 nm for 10 % Cu to 25.1 nm for 15 % Cu). This agglomeration indicates a 92 % increase in average Cu particle size despite only a 50 % increase in Cu content. Therefore, it could be interpreted that the active surface area of Cu is reduced. This diminishes the catalytic effectiveness of Cu₁₅NaCa₃₀Al₅₅ and leads to a drop in CO₂ conversion despite the higher Cu loading.

In summary, Cu₁₀NaCa₃₀Al₆₀ achieves a collaborative combination of optimized pore structure for diffusion, suitable basic site distribution for CO₂ chemisorption, and well-dispersed Cu active sites for catalytic conversion. None of these are individually sufficient, but collectively, they result in the highest performance among the studied formulations.

The N₂ purge, executed between the CO₂ capture and hydrogenation steps, serves to remove residual free carbon dioxides that are not captured by the DFM from the reactor. This step ensures a more precise evaluation of DFM performance. Moreover, for the commercialization of ICCC-RWGS, the N₂ purge is essential to prevent direct contact of hydrogen with the oxygen usually present in industrial flue gases. When comparing the CO₂ profiles of the blank test with those of DFMs during the N₂ purge, a notable observation emerged: a greater quantity of CO₂ exits the reactor in the case of DFMs. This phenomenon suggests that some of the captured CO₂ on the DFMs was released due to the decomposition of carbonates during the N₂ purge at 650 °C.

The extent of CO₂ released during the purge was found to be dependent on the Cu loading, as illustrated in Fig. 7a (purple bars). As discussed in Section 3.2, the presence of Cu facilitates the carbonate decomposition and release of CO₂ from the DFMs under an inert atmosphere. Consequently, a higher Cu content results in a greater release of CO₂ during the N₂ purge. The amount of CO₂ released during the purge is considered a loss and does not contribute to the RWGS reaction. Consequently, the amount of CO₂ available for hydrogenation on the DFM after the N₂ purge is less than the initially captured quantity.

3.4.2. Effect of temperature

To evaluate the effect of temperature on DFM behavior, Cu₁₀NaCa₃₀Al₆₀, which exhibited the best performance, was selected for further investigation. Its behavior was evaluated at four different temperatures: 550 °C, 600 °C, 650 °C, and 700 °C. Fig. 8 illustrates the DFM behavior during a cycle of ICCC-RWGS, while Table 4 and Fig. 9 quantify its performance at the tested temperatures. As shown in Fig. 8, the rate of CO₂ hydrogenation and the resulting CO peak concentration significantly depend on temperature. Increasing the temperature from 550 °C to 700 °C increases the hydrogenation rate (CO formation rate) from 0.024 mmol/min.g to 0.140 mmol/min.g, while the CO peak increases from 0.14 % at 550 °C to 3.1 % at 700 °C. This can be attributed to the fact that carbonate decomposition is an endothermic process, leading to an increased rate of CO₂ release with rising temperatures. Consequently, this results in a higher hydrogenation rate and CO peak concentration. Notably, the CO selectivity of Cu₁₀NaCa₃₀Al₆₀ remains consistently at 100 %, exhibiting no variation with temperature changes.

Table 4. The performance of Cu₁₀NaCa₃₀Al₆₀ over the ICCC-RWGS process at different temperatures.

Temperature (°C)	CO₂ capture (mmol/g)	CO₂ loss in Purge (mmol/g)	Avail. CO₂ after purge (mmol/g)	CO prod. (mmol/g)	CO₂ Conv. (%)	Hydrogenation Time (min)
550	1.88	0.00	1.88	1.85	98.5	∼77
600	1.98	0.00	1.98	1.92	97.1	∼24
630	2.41	0.07	2.34	2.19	93.5	∼20
650	2.52	0.17	2.35	2.10	89.3	∼15
700	2.17	0.49	1.68	1.26	74.9	∼9

According to the data presented in Table 4 and Fig. 9, the CO₂ capture capacity of DFM also depends on the operating temperature increasing from 1.88 mmol/g at 550 °C to 2.52 mmol/g at 650 °C. At 550 °C and 600 °C, no CO₂ was released from the DFM during the N₂ purge, indicating that the decomposition of carbonates under an inert atmosphere is unattainable at these temperatures. In contrast, approximately 6.8 % of initial captured CO₂ is lost during the N₂ purge at 650 °C. Nevertheless, the remaining CO₂ on the DFM after the N₂ purge (2.35 mmol/g) surpasses the CO₂ capture capacity of DFM at 550 °C and 600 °C. At 700 °C, the CO₂ capture of the DFM is significantly reduced compared to 650 °C, dropping from 2.52 to 2.17 mmol/g. This disparity is further emphasized during the N₂ purge, where 0.49 mmol/g of CO₂ (≈22.5 % of initial captured CO₂) was released and exited the reactor at 700 °C, compared to 0.17 mmol/g at 650 °C. Consequently, only 1.68 mmol/g of CO₂ remains on the DFM for the hydrogenation step, even less than the amount at 550 °C (1.88 mmol/g).

Despite the slow hydrogenation rates at 550 °C and 600 °C, the DFM showcased a high catalytic activity. This is evident in the remarkable CO₂ conversion of 98.5 % and 97.1 %, respectively, with negligible unreacted CO₂. It is well established that the decarbonation process of Ca-based sorbents is temperature-dependent, with increasing rate by temperature [65]. Consequently, it could be said that the limiting factor for the hydrogenation rate at 550 °C and 600 °C lies not in the catalytic activity of DFM but in the decomposition rate of carbonates at these temperatures. Raising the temperature to 650 °C accelerates the decarbonation rate, surpassing the RWGS reaction rate. This imbalance results in a notable release of unreacted CO₂ during the hydrogenation step and a reduction in the CO₂ conversion to 89.3 %. At 700 °C, the intensified decarbonation rate further reduces the CO₂ conversion to 74.9 %.

The influence of temperature on DFM performance is crucial, particularly in determining CO₂ capture capacity and dictating the hydrogenation rate by balancing the decarbonation and RWGS rates. Increasing the temperature from 600 °C to 650 °C increased CO₂ capture by 0.53 mmol/g (∼27 %), although 0.17 mmol/g of that was lost during the N₂ purge due to the increased decarbonation rate. Additionally, CO₂ conversion significantly decreased from 97.1 to 89.3 %. To achieve a better balance between decarbonation and RWGS rates, enhance CO₂ conversion and minimize CO₂ loss during the N₂ purge, an additional ICCC-RWGS experiment was conducted at 630 °C, a midpoint between 600 °C and 650 °C. Fig. 9 presents a comprehensive comparison, encompassing the effect of temperature on CO₂ capture, CO₂ loss during the purge, CO production, and CO₂ conversion across all investigated temperatures.

At 630° C, the initial CO₂ capture is 2.41 mmol/g, slightly lower than the 2.52 mmol/g at 650 °C. However, the CO₂ loss during the N₂ purge is less, measuring 0.07 mmol/g (2.9 % of the total CO₂ capture) compared to 0.17 mmol/g at 650 °C (6.8 % of the total CO₂ capture). This resulted in a comparable availability of remaining CO₂ on DFM for the subsequent hydrogenation step. Moreover, the slower decarbonation rate at 630 °C reduced the escape of unconverted CO₂ from the reactor during the hydrogenation step. This slower decarbonation rate led to a noteworthy increase in CO₂ conversion, from 89.3 % to 93.5 %. This indicates a more favorable balance between the decarbonation and RWGS reaction rates. Consequently, the production of CO at 630 °C exceeded that at 650 °C, with values of 2.19 and 2.10 mmol/g, respectively. Considering factors such as CO production, CO₂ conversion, and hydrogenation time, it can be concluded that 630 °C represents the optimum temperature among the tested temperatures for the cyclic ICCC-RWGS process at isothermal conditions.

Within the literature, two mechanisms are proposed for the reduction of CO₂ by DFMs. The first mechanism suggests that CO₂ is released through a decarbonation process and then spills over to the catalyst sites adjacent to the sorbent, where it undergoes hydrogenation in the presence of activated H₂. [6,[66], [67], [68]]. The second mechanism suggests that H₂ is first activated on the catalyst sites, and then the activated hydrogen spills over to the nearby sorbent sites, where the reduction of the captured CO₂ occurs [38,69]. In the present study, the analysis of the performance of DFM across various temperatures revealed that the decomposition of carbonates is a critical factor influencing the overall rate of CO production. Notably, as the temperature increased from 550 °C to 600 °C, prompting a more rapid decarbonation process, the rate of CO production experienced a corresponding boost. However, further temperature increases resulted in a decarbonation rate surpassing the RWGS reaction rate, diminishing the CO₂ conversion. Hence, it can be inferred that, in this case, the hydrogenation mechanism involves the initial carbonate decomposition and release of CO₂ from the sorbent, followed by its spillover onto the Cu catalyst sites, where the reduction with dissociated hydrogens occurs.

3.4.3. Effect of oxygen presence during CO₂ capture

The impact of O₂ presence during the CO₂ capture phase on the performance and stability of Cu₁₀NaCa₃₀Al₆₀ was examined at 630 °C over 12 consecutive cycles of the ICCC-RWGS process. A flow of 5.5 % CO₂ and 4 % O₂, balanced with He, was employed to emulate realistic conditions. A benchmark test was concurrently conducted under ideal conditions (O₂ exempt) using a flow of 5.5 % CO₂ balanced with He for comparative analysis. Throughout these experiments, the duration of each step (CO₂ capture and hydrogenation) was set at 15 min, with a 5-minute N₂ purge between steps. Fig. 10 depicts the evolution of CO₂ and CO concentrations at the reactor outlet during the first cycles. The CO₂ profiles during the early capture stage are nearly identical for both conditions. However, as CO₂ capture progresses and carbonates are formed on the DFM surface, a discernible deviation between the two profiles emerges, persisting until the end of the capture step. The CO₂ profile in the presence of O₂ exhibits a notably higher concentration than the ideal condition, showcasing a diminished CO₂ capture, dropping from 2.05 mmol/g under ideal conditions to 1.82 mmol/g when O₂ is present.

O₂ presence during the CO₂ capture phase led to the oxidation of Cu. One potential consequence of Cu oxidation in the presence of O₂ is a reduction in the catalyst’s basicity. This change can influence the CO₂ sorption behavior of the DFMs. To assess the impact of this oxidation on Cu basicity, CO₂-TPD experiments were conducted on the Cu₁₀Al₉₀ catalyst in both its reduced and oxidized states. The NaCa sorbent was excluded from the catalyst to compare the basicity of metallic Cu and CuO more accurately. The results are illustrated in Fig. S4, along with the TPD profile of Cu₁₀NaCa₃₀Al₆₀ for comparative purposes. Fig. S4a shows that both the oxidized and reduced catalysts exhibited a small peak around 780 °C, indicative of the basic site attributed to the oxygen vacancies introduced by Cu. Fig. S4b provides a closer view of these peaks, showing overlapping profiles. The oxidation process led to a reduction in the basicity of the catalyst. Upon oxidation, oxygen atoms are incorporated into the catalyst lattice, leading to the formation of CuO and the healing or filling of these vacancies [53], thereby decreasing the overall basicity of the catalyst in its oxidized form compared to the reduced state. However, the decline in the density of basic sites was marginal compared to the basicity of the DFM containing 30 % NaCa sorbent.

Additionally, the oxidation of Cu could cover the active sites of the sorbent in close proximity due to the increased volume of CuO species compared to metallic Cu, thereby diminishing their direct interaction with passing CO₂. Furthermore, the greater volume of CuO, compared to Cu, may intensify the resistance against CO₂ diffusion alongside the carbonate layer during the diffusion-controlled carbonation phase, resulting in a reduced capture of CO₂ within the same time frame.

Another interesting observation in Fig. 10 is the sharper CO peak in hydrogenation phase when O₂ was present during the capture phase, in contrast to the ideal condition. This can be attributed to the increased hydrogenation rate, resulting in a higher CO concentration. During the hydrogenation step, CuO is reduced back to its metallic state. Given the exothermic nature of CuO reduction [70,71], the released heat could intensify the decomposition of neighboring carbonates. The heightened decarbonation rate subsequently increased the hydrogenation rate, resulting in a CO peak with a higher concentration. On the other hand, the enhanced decarbonation rate also led to a greater release of unconverted CO₂ during the hydrogenation step, resulting in a lower CO₂ conversion compared to the ideal condition, dropping from 95.0 % to 88.1 % when O₂ is present. The reduced amount of captured CO₂, coupled with the lower conversion, contributed to a lesser formation of CO in the presence of O₂ compared to the ideal condition.

A critical consideration for the practical application of DFM in flue gases containing O₂ is the rapid reduction of the oxidized catalyst following the capture step. In the case of Cu₁₀NaCa₃₀Al₆₀, CuO reduction occurs rapidly, enabling the detection of CO simultaneously with the introduction of H₂ without any discernible delay, as demonstrated in Fig. 10.

Bobadilla et al. [72] highlighted the negative impact of O₂ on both the CO₂ capture and conversion over FeCrCu-K DFM. The DFM experienced a significant reduction in CO₂ conversion, falling from approximately 87 % under ideal conditions to around 60 % in the presence of O₂. The authors speculated that the competitive sorption of O₂ on the DFM led to a diminished CO₂ capture, consequently altering the state of the active catalyst and resulting in a lower conversion. In contrast, the DFM proposed here exhibited only a marginal decrease in CO₂ conversion, declining from 95.0 % to 88.1 %, underscoring their high activity under realistic conditions.

Fig. 11 illustrates the cyclic behavior of Cu₁₀NaCa₃₀Al₆₀ over 12 cycles of ICCC-RWGS under both ideal and realistic conditions. After 12 cycles under ideal conditions, the DFM exhibited a reduction in CO₂ capture of only 7.9 %, underscoring the high stability of the sorbent. The catalytic activity of the DFM, linked to the presence of Cu species, also exhibited impressive stability, with a marginal drop from 95.0 % to 90.9 % after 12 cycles, suggesting a high resistance of the Cu particles against sintering. The presence of mayenite (Ca₁₂Al₁₄O₃₂), identified in the XRD pattern of the DFM (Fig. 2), can serve as a physical barrier, preventing sintering and agglomeration of both sorbent and Cu particles. Interestingly, when O₂ was present during CO₂ capture, the CO₂ conversion exhibited remarkable stability, showing no signs of deactivation over 12 cycles (88.1 % and 89.1 % at the 1st and the 12th cycles, respectively (Fig. 11)). Similarly, CO₂ capture displays enhanced stability in this scenario, experiencing only a 3.8 % decrease after 12 cycles. This suggests that the oxidation of Cu, which typically has a detrimental effect on the CO₂ capture and conversion, surprisingly contributes positively to the stability of the DFM. It can be inferred that CuO serves as an additional physical barrier against the agglomeration of sorbent particles while simultaneously inhibiting Cu sintering, thereby intensifying the overall stability of the DFM.

It is essential to recognize that, under realistic conditions, in addition to CO₂ and O₂, flue gases can contain water vapor, sulfur oxides (SO_X), and nitrogen oxides (NO_X), depending on the fuel source and burning conditions. Therefore, future studies investigating the effect of SO_X and NO_X could be interesting. It has been addressed that while the carbonation process is reversible, the sulfation process is not and may significantly damage the performance of DFMs.

3.5. Non-isothermal ICCC-RWGS process

As observed in section 3.4.2 (refer to Table 4), at 650 °C, the initial CO₂ capture was the highest among all tested temperatures. However, due to the relatively high rate of decarbonation at this temperature, the CO₂ loss during the purge increased, and the conversion decreased compared to lower temperatures. From these observations, it can be concluded that while CO₂ capture at 650 °C improves capture capacity, conducting the purge and hydrogenation steps at lower temperatures could minimize CO₂ loss at the purge step and enhance the conversion at the hydrogenation step, improving overall performance. Therefore, we conducted a new set of non-isothermal experiments where CO₂ capture was performed at 650 °C, followed by purge and hydrogenation at a lower temperature (such as 630 °C, 620 °C, and 610 °C) to obtain a better balance between the rate of decarbonation and RWGS reaction, leading to a higher CO₂ conversion. In this non-isothermal ICCC-RWGS process, a new DFM composition featuring 10 % Cu, 80 % NaCa, and 10 % γ-Al₂O₃ denoted as Cu₁₀NaCa₈₀Al₁₀ was utilized.

Throughout this experiment, the CO₂ capture was conducted at 650 °C using a flow of 10 % CO₂/He for 30 min, followed by a controlled ramp-down to the target temperature (1st cycle: 630, 2nd cycle: 620, and 3rd cycle: 610 °C) at a rate of 10 °C/min under the same gas flow conditions. Subsequently, the reactor underwent a 5-minute purging step with N₂, followed by a 30-minute hydrogenation step with pure H₂. To prepare for the subsequent capture step, the reactor then experienced a ramp-up with a rate of 10 °C/min to 650 °C, maintaining the H₂ flow, and finally, it underwent a 5-minute N₂ purge before the following CO₂ capture.

Fig. 12 illustrates the CO₂ capture, conversion, and CO production of Cu₁₀NaCa₈₀Al₁₀ across three consecutive non-isothermal ICCC-RWGS cycles. To compare the performance of the DFM under non-isothermal conditions with that under isothermal conditions, a separate isothermal ICCC-RWGS experiment was conducted at 630 °C, and the results are also shown in Fig. 12. In the case of the non-isothermal condition, during the N₂ purge, CO₂ losses were 0.12, 0.07, and 0.02 mmol/g at 630 °C, 620°C, and 610 °C, respectively. These values correspond to 2.3 %, 1.2 %, and 0.4 % of the total CO₂ captured, which is negligible compared to the initial capture capacity, especially at 610 °C. Reducing the hydrogenation temperature from 630 °C to 610 °C increases the CO₂ conversion from 86.5 % to 91.0 %. The reduced CO₂ loss during the purge step and a higher conversion at lower temperatures resulted in the highest CO production at 610 °C. The higher conversion at this lower temperature is attributed to the slower decarbonation rate being more closely aligned with the RWGS rate. However, this slower decarbonation rate also extends the hydrogenation time, requiring 30 min for complete hydrogenation at 610°C, matching the CO₂ capture duration. Consequently, further reducing the hydrogenation temperature is impractical, making 610 °C the optimal temperature among the three tested for conducting the hydrogenation step within a timeframe aligning with the CO₂ capture duration.

By comparing the performance of the DFM under isothermal conditions at 630 °C and during the first cycle of non-isothermal operation (with CO₂ capture at 650 °C and hydrogenation at 630 °C), higher CO₂ capture and CO production were observed when the capture step was performed at 650 °C, as expected. However, the CO₂ loss during the purging step and the CO₂ conversion remained nearly constant, as the N₂ purge and hydrogenation steps were conducted at the same temperature of 630 °C under both conditions.

To gain deeper insight into the cyclic performance and stability of Cu₁₀NaCa₈₀Al₁₀, a test comprising 18 non-isothermal cycles of ICCC-RWGS was conducted, with CO₂ capture at 650 °C and hydrogenation at 610 °C as depicted in Fig. 13. The DFM exhibited remarkable stability with an increasing CO₂ capture and CO formation trend throughout the cyclic operation. CO₂ capture capacity increased from 5.19 mmol/g in the first cycle to a maximum of 6.10 mmol/g by the 14th cycle. The increase in CO₂ capture capacity is due to the self-reactivation of CaO sorbent. This process is driven by forming a soft-porous framework due to the cyclic carbonation-decarbonation process. In the synthesis process, the high-temperature calcination (in the case of this study, 700 °C) yields a sintered CaO structure, constituting a rigid skeleton. However, through the initial cycles, particle expansion (carbonation step) and shrinkage (decarbonation step) lead to the rearrangement of the porous structure, transforming the rigid skeleton into a soft one. With an increased surface area, the obtained porous structure enhances the carbonation stage in subsequent cycles [73,74]. CO production also demonstrated an ascending trend, with a maximum of 5.51 mmol/g at the 14th cycle. Moreover, throughout all 18 cycles, the CO₂ conversion remained consistent, hovering around 91 %. Moreover, across all 18 cycles, CO selectivity was remarkably constant at 100 %. A comparison summarizing recent DFM studies is presented in Table S1. However, it is important to mention that a thorough comparison may not be feasible because the experimental conditions and DFM compositions concerning both sorbent and catalyst content vary a lot across all studies. Nevertheless, we consider that the developed DFMs in this study are among the best-performing DFMs in terms of CO₂ conversion and cyclic stability, which are essential in this process.

To evaluate the effect of the cyclic ICCC-RWGS process on the porous structure of Cu₁₀NaCa₈₀Al₁₀, the BET tests were conducted on fresh and used DFM after 18 cycles (Table 5). The surface area, pore volume, and pore size showed clear improvements after the 18 cycles, indicating enhanced textural properties through self-reactivation.

Table 5. Textural properties of the fresh and used DFM.

Materials	BET surface area (m²/g)	pore volume (cm³/g)	average pore diameter (nm)
Cu₁₀NaCa₈₀Al₁₀-fresh	10	0.07	28
Cu₁₀NaCa₈₀Al₁₀-used	13	0.11	34

The EDS mapping displayed in Fig. 15a illustrates the uniform distribution of Ca and Cu throughout the fresh DFM. Remarkably, this consistent distribution of both elements persisted after 18 cycles (Fig. 15b), with no evidence of sintering or agglomeration. Although some agglomerated alumina particles were observed within both the fresh and used DFMs, there was no indication of an increase in alumina particle size or sintering after being used for 18 cycles.

3.6. H₂ temperature-programmed surface reaction (H₂-TPSR)

All the DFMs prepared in this study exhibited a remarkable 100 % CO selectivity under the ICCC-RWGS process, a crucial factor in practical applications, across all the tested temperatures (from 550 °C to 700 °C). Nevertheless, to identify the selectivity of DFMs in CO₂ hydrogenation towards producing CO by RWGS reaction as a function of temperature, an H₂ temperature-programmed surface reaction (H₂-TPSR) on Cu₁₀NaCa₈₀Al₁₀ was performed as described in section 2.2, and the results are presented in Fig. 16. The ability of the DFMs to inhibit the simultaneous occurrence of alternative hydrogenation pathways, particularly methanation, in conjunction with the RWGS reaction is crucial. This is essential because the competing reactions could significantly reduce the CO selectivity of the DFMs, undermining their effectiveness in practical applications. Therefore, understanding and managing the reaction pathways is one of the keys to optimizing DFM performance.

Up to approximately 483 °C, neither CO, CH₄, nor CO₂ is detected at the reactor outlet, indicating that no CO₂ hydrogenation or carbonate decomposition occurs below this temperature. The first detection of CO happens around 483 °C, whereas literature indicated that H₂ activation can take place around 350 °C on Cu sites [37,38]. This implies that carbonate decomposition is necessary for the hydrogenation reaction, even in the presence of activated H₂ starting around 350 °C. At 483 °C, decarbonation initiates with slow kinetics, slower than that of the RWGS reaction, leading to the conversion of all released CO₂ to CO. As the temperature increases, the rate of decarbonation accelerates, evident from the increase in CO concentration, peaking around 668 °C. On the other hand, at approximately 600 °C, CO₂ begins to be detected, indicating that at this temperature, the rate of decarbonation exceeds that of RWGS, allowing some released CO₂ from carbonate decomposition to escape the reactor unconverted.

Throughout the entire temperature range investigated, methane was only detected between 544 °C and 604 °C, peaking at a mere 2 ppm, which is negligible compared to the peak of CO at 3.21 %, rendering it invisible in the figure. This observation suggests that the only hydrogenation pathway occurring at the Cu₁₀NaCa₈₀Al₁₀ surface is the RWGS reaction, and the DFM does not activate the methanation reaction. The DFM performance, including CO selectivity, is highly contingent upon the interaction and synergy between Cu and the sorbent. For instance, Hyakutake et al. [37] reported a CO selectivity of 93.4 % at 350 °C for Cu-K/Al₂O₃ DFM, whereas Cu-Ba/Al₂O₃ exhibited 74.0 % CO selectivity under identical conditions. Moreover, the presence of Na can act as a promoter, enhancing CO selectivity by hindering methane formation. It is reported that during CO₂ hydrogenation, Na impedes the adsorption of generated CO on catalyst sites and further reduction to methane [75]. Consequently, the remarkable CO selectivity of our DFMs can be attributed to the collaborative interplay between Cu and NaCa sorbent.

This observation further supports our earlier discussion of the CO₂ reduction mechanism to CO on our DFMs, as discussed in section 3.4.2. While H₂ dissociation can occur on Cu at temperatures as low as 350 °C [38], Fig. 16 shows that the initial detection of CO occurs around 483 °C, which is significantly higher than the temperature required for H₂ activation by Cu. Consequently, it can be inferred that dissociated hydrogen is insufficient for carbonate hydrogenation. Instead, the decomposition of carbonates must happen, initiated at 483 °C. Therefore, we consider that the mechanism of CO formation involves the release of CO₂ from carbonates followed by its spillover to Cu sites, where dissociated hydrogen is already present for the hydrogenation of CO₂ to CO and water. However, to obtain a deeper understanding of the underlying mechanism of the ICCC-RWGS process, additional investigations employing advanced spectroscopic techniques such as DRIFTS are necessary. These methods can provide valuable information on the nature and evolution of surface intermediates and adsorbed species during the reaction, which is essential for unraveling the reaction pathway.

May 12, 2025 at 05:39PM
https://www.sciencedirect.com/science/article/pii/S1385894725042007?dgcid=rss_sd_all

3.1. Characterization of DFMs

3.2. DFM performance in CO2 sorption-regeneration

3.4. Integrated CO2 sorption and in-situ conversion to CO (ICCC-RWGS) isothermally