Effect of the Ni-to-CaO Ratio on Integrated CO₂ Capture and Direct Methanation

Abstract

Direct methanation in an integrated CO₂ capture and utilization system has recently gained considerable attention as a promising approach owing to its simplified process and lower requirement of total thermal energy as compared to conventional CO₂ capture and utilization techniques. This study formulated macroporous structured Ni/CaO catal-sorbents by controlling the Ni-to-CaO ratio. The influence of this ratio on the CO₂ capture (capacity and kinetics) and direct methanation performances (productivity and kinetics) was evaluated at 500 °C. CO₂ capture combined with direct methanation experiments revealed that 10Ni/CaO exhibited the best CO₂ capture capacity, kinetics, and CH₄ productivity with the thermal stability of Ni and CaO species.

1. Introduction

Carbon dioxide (CO₂) emissions are an important cause of global climate change and a major challenge our planet is facing today. The burning of fossil fuels, such as coal, oil, and gas, for energy and transportation is the primary source of CO₂ emissions. These emissions trap heat in the atmosphere and lead to global warming, which has various adverse effects on the environment, including sea-level rise, extreme weather events, and the loss of biodiversity [1,2,3,4,5]. CO₂ capture and utilization (CCU) technology captures CO₂ emissions from various sources and converts them into value-added products. This technology has gained considerable attention as a means to mitigate climate change and greenhouse gas emissions. The captured CO₂ can be used to produce fuels, chemicals, and other products through various conversion methods, such as biological, chemical, and electrochemical processes [3,6]. By utilizing the captured CO₂, CCU technology not only reduces emissions but also contributes to the circular economy by producing sustainable products. In addition, CO₂ utilization may decrease the dependency on fossil fuels, thereby promoting a sustainable future. With continued research and development, CCU technology may become a feasible solution to the challenges of climate change while supporting economic growth [3,7].

Recently, there has been a growing interest in the integrated CO₂ capture and utilization (ICCU) technology that uses dual functional materials to combine CO₂ capture and utilization in a single reaction system. ICCU technology is promising for mitigating climate change by reducing CO₂ emissions while producing valuable products. This technology involves capturing CO₂ from industrial processes, transport, and power plants, followed by its utilization in various green technologies [8,9,10,11,12,13,14,15,16,17,18,19,20,21]. Calcium looping technology is one of the effective methods for CO₂ capture, which employs a calcium oxide (CaO)-based solid sorbent to chemically absorb CO₂ [12,22,23,24]. However, the sintering-induced decrease in CO₂ capture capacity during carbonation and regeneration cycles limits the efficiency of these sorbents [23]. To improve their multicycle stability, metal oxide stabilizers are added to the intraparticle region of CaO, which increases the length of diffusional paths, decreases the sintering rate, and enhances the textural properties of these sorbents [12,22,23,24].

Owing to the characteristics of ICCU technology, where CO₂ capture and conversion must occur continuously in the same temperature range, only CO₂ methanation through the Sabatier reaction is suitable for intermediate temperature conversion. The methanation reaction is the simplest and the most efficient catalytic CO₂ utilization process, which converts CO₂ and H₂ to CH₄, which is an energy carrier with the least CO₂ emission per unit of energy among fossil fuels [17,18,19,20,25,26,27]. Direct methanation after CO₂ capture in flue gas conditions through Ni-CaO occurs as follows [28,29]:

CO₂ capture: NiO/CaO(s) + CO₂(g) ↔ NiO/CaCO₃(s),

Direct methanation: NiO/CaCO₃(s) + 4H₂(g) ↔ Ni⁰/CaO(s) + CH₄(g) + 2H₂O(g).

In our previous paper, an ICCU-direct methanation process using a 10Ni/CaO catal-sorbent was proposed, and the optimum temperature was determined to be 500 °C, considering the high CO₂ capture capacity, CH₄ selectivity, and thermal stability of the catal-sorbent [28,29]. Herein, Ni/CaO catal-sorbents with different Ni-to-CaO ratios were synthesized by the sol–gel method to determine the effect of these ratios on the macroporous of CaO and Ni dispersion. Textural structure of the Ni/CaO catal-sorbents is critically important since the pore structure enables rapid CO₂ diffusion for CO₂ capture capacity and the Ni dispersion promotes catalytic activity. Their CO₂ capture performance (capacity and kinetics) and direct methanation performance (productivity and kinetics) according to the textural structure were studied. Moreover, the cycle stability of the catal-sorbents in CO₂ capture and direct methanation was revealed. The optimal Ni/CaO ratio was proposed through CO₂ capture and direct methanation performance results and analysis.

2. Results and Discussion

2.1. Characterization

The ICP-OES metal content, BET surface area, nanoparticle size, and porosity of 2Ni/CaO, 10Ni/CaO, and 20Ni/CaO catal-sorbents are summarized in

Table 1. Metal contents, BET surface area, nanoparticle size, and porosity of the 2Ni/CaO, 10Ni/CaO, and 20Ni/CaO catal-sorbents in their fresh states.

	Metal Contents	BET Surface	Nanoparticle Size (nm)	Porosity
	(wt.%)	Area (m2/g)		(%)
2Ni/CaO	Ni: 1.6	4.59	1307	72.9
	CaO: 98.4
10Ni/CaO	Ni: 9.3	9.9	605.8	74.68
	CaO: 90.7
20Ni/CaO	Ni: 18.0	7.77	771.6	74.39
	CaO: 82.0

. The Ni metal contents of the 2Ni/CaO, 10Ni/CaO, and 20Ni/CaO catal-sorbents prepared by the citrate sol–gel method are 1.6, 9.3, and 18.0 wt.%, respectively, which are consistent with the desired metal loading. The BET surface area of 2Ni/CaO, 10Ni/CaO, and 20Ni/CaO catal-sorbents are 4.59, 9.90, and 7.77 m²/g, respectively, and 10Ni/CaO has the smallest nanoparticle size of 605.8 nm. As can be seen in Figure 1, 2Ni/CaO has a dense morphology, with interconnected nanoparticle structures and poor porosity. Notably, as the Ni-to-CaO ratio increases the porous structures expand, and the nanoparticle size decreases dramatically; 2Ni/CaO, 10Ni/CaO, and 20Ni/CaO exhibited porosities 72.9, 74.7, and 74.4%, respectively. In addition, the Hg porosimeter revealed that macroporous structures > 100 nm increase as the Ni-to-CaO ratio increases (Figure S1). 10Ni/CaO exhibited exceptional BET surface area, nanoparticle size, and porosity among xNi/CaO catal-sorbents. Figure 1d–f present the TEM images and the Ni crystallite size distribution of the catal-sorbents. The energy dispersive spectroscopy (EDS) mapping on the catal-sorbents revealed that Ni nanoparticles of uniform size are dispersed uniformly throughout CaO (Figures S2–S4). Individual Ni particles in 2Ni/CaO, 10Ni/CaO, and 20Ni/CaO had an average crystallite size of 20.2, 27.0, and 45.4 nm, respectively (

Table 2. Ni crystallite size, Ni dispersion, and Ni surface area of the 2Ni/CaO, 10Ni/CaO, and 20Ni/CaO catal-sorbents.

	Ni Crystallite Size a (nm)	Dispersion b (%)	Ni surface Area b (m2/gNi)
2Ni/CaO	20.2	2.43	16.19
10Ni/CaO	27.0	0.11	0.72
20Ni/CaO	45.4	0.09	0.58

^a Measured by TEM. ^b Calculated from CO chemisorption.

). 2Ni/CaO exhibited the highest Ni dispersion (2.43%) and Ni surface area (16.19 m²/gNi), and Ni dispersion and Ni surface area decreased with the increase in the Ni-to-CaO ratio, leading to poor catalytic activity.

In Figure 2a, the XRD patterns of 2Ni/CaO, 10Ni/CaO, and 20Ni/CaO in the fresh state display NiO (JCPDS No. 89-7131) and CaO phase (JCPDS No. 48-1467). In addition to the NiO phase, a very small and broad peak of metallic Ni is observed for 20Ni/CaO. 2Ni/CaO, 10Ni/CaO, and 20Ni/CaO have CaO crystallite sizes of 49.7, 50.1, and 32.2 nm, respectively. The crystallite size of CaO decreased with increasing Ni-to-CaO ratio, and the smallest CaO in 20Ni/CaO is expected to be the fastest kinetics in CO₂ capture. In Figure 2b, the XPS spectra of Ni 2p_3/2 revealed that Ni²⁺ exists in two states within the catal-sorbents. The peak with the lowest binding energy (853.5–854.5 eV) corresponds to the bulk NiO (Ni²⁺ (I)), whereas the peak with a higher binding energy (855.5–856 eV) is attributed to the NiO interacting with CaO (Ni²⁺ (II)) [29,30,31]. The intensity ratio of Ni²⁺ (I) and Ni²⁺ (II) increased as the Ni concentration increased from 2 to 20 wt.%. This is because for catal-sorbents with larger Ni crystallite sizes (e.g., 20Ni/CaO), there may be a greater increase in the amount of bulk NiO relative to the amount of NiO interacting with CaO. In the XRD results, very small peaks of Ni⁰ were observed for 20Ni/CaO. The H₂-TPR profiles of Ni/CaO catal-sorbents (Figure S5) revealed that NiO phases can be converted to metallic Ni under direct methanation conditions. The H₂-TPR curves for 2Ni/CaO, 10Ni/CaO, and 20Ni/CaO exhibit two groups of peaks at maximum temperatures of ~380 °C and 450–500 °C. The lower temperature event is attributed to the bulk reduction of NiO, indicating minimal or no interaction with the CaO phase, whereas the higher temperature event corresponds to NiO interacting strongly with CaO [31].

2.2. Evaluation of Catal-Sorbent Performance

CO₂ capture and direct methanation in the ICCU process during five cycles under 10 vol.% CO₂ and 10 vol.% H₂O at 500 °C using the catal-sorbents and the corresponding profiles are depicted in Figure 3. The breakthrough curves of 2Ni/CaO, 10Ni/CaO, and 20Ni/CaO are shown in Figure 3a. The breakthrough time slightly increased with increasing Ni-to-CaO ratio because of the enhanced macroporous structures and the smaller size of CaO, despite the decrease in CaO amount. In the initial cycle, the total CO₂ capture capacities of 2Ni/CaO, 10Ni/CaO, and 20Ni/CaO, which were calculated from the breakthrough curves, were 5.89, 8.96, and 9.87 mmol CO₂/g catal-sorbent, respectively (

Table 3. CO₂ capture and direct methanation performances of the 2Ni/CaO, 10Ni/CaO, and 20Ni/CaO catal-sorbents over five consecutive cycles at 500 °C.

		CO2 Capture	Direct Methanation
	Cycle	CO2 Captured (mmol/g)	CH4 Produced (mmol/g)	CH4 Selectivity (%)	Carbon Balance (%)
2Ni/CaO	1	5.89	5.88	100	99.8
	2	5.75	5.75	100	100
	3	5.33	5.33	100	100
	4	4.84	4.83	100	99.8
	5	4.45	4.44	100	100
10Ni/CaO	1	8.96	8.95	100	99.9
	2	8.94	8.93	100	99.9
	3	9.24	9.24	100	100
	4	8.70	8.70	100	100
	5	8.75	8.75	100	100
20Ni/CaO	1	9.87	9.72	100	98.5
	2	8.70	8.62	100	99.1
	3	8.16	7.98	100	97.8
	4	7.39	7.21	100	97.6
	5	7.15	6.97	100	97.5

). 20Ni/CaO showed the highest CO₂ capture capacity owing to the smallest crystallite size of CaO and improved macroporous structure. Figure 3b shows the CH₄ concentration for 2Ni/CaO, 10Ni/CaO, and 20Ni/CaO during direct methanation. Further, it was observed that the maximum CH₄ concentration in the direct methanation step decreased with increasing Ni-to-CaO ratio. However, 20Ni/CaO exhibited the highest CH₄ productivity at the initial cycle as it has the highest CO₂ capture capacity, and all CO₂ was converted to CH₄ (i.e., 100% CH₄ selectivity) during direct methanation (Table 3).

The catalytic activity of the catal-sorbents in the direct methanation step was quantified by calculating the apparent turnover frequency (TOF), as shown below [32]:

$$\mathrm{TOF}({\mathrm{s}}^{-1})=\frac{r_{{CH}_4}·M}{W·D·x}$$

(1)

where $r_{{CH}_4}$ denotes the CH₄ production rate during direct methanation (mol/s), M is the atomic mass of Ni (g/mol), W indicates the weight of the catalyst packed into the reactor (g), D denotes the dispersion of Ni particles on the surface of the catalyst, and x denotes the loading weight ratio of Ni in the catalyst. Figure 3c shows the apparent TOF of the catal-sorbents obtained at the highest CH₄ concentration during direct methanation in the initial cycle. The TOF increased from 0.1707 s⁻¹ for 2Ni/CaO to 0.4621 s⁻¹ for 10Ni/CaO and then decreased to 0.2232 s⁻¹ for 20Ni/CaO. Figure 3d shows the CH₄ productivity over five cycles of ICCU methanation steps for the catal-sorbents. The CO₂ capture capacities of 2Ni/CaO and 20Ni/CaO catal-sorbents decreased gradually over multiple cycles, resulting in a gradual decline in CH₄ productivity at the subsequent direct methanation step (Figure S6). Further, it was observed that the breakthrough time of 2Ni/CaO slightly decreased as the cycle test progressed. Nevertheless, the kinetics of direct methanation in 2Ni/CaO were stable throughout the cycles. In contrast to 2Ni/CaO, the breakthrough time of 20Ni/CaO was stable. However, the CO₂ capture capacity decreased in subsequent reaction cycles because of the incomplete regeneration of CaCO₃ by slow direct methanation. 20Ni/CaO exhibited a relatively sluggish direct methanation rate, and the rate was decreasing as the number of reaction cycles increased, leading to a decrease in CH₄ productivity from 9.72 to 6.97 mmol CH₄/g catal-sorbent. 10Ni/CaO exhibited excellent thermal stability with high CH₄ productivity (8.95–8.75 mmol CH₄/g catal-sorbent) over five cycles of ICCU methanation because of its high CO₂ capture capacity, fast carbonation, and direct methanation by the improved macroporous CaO structure and Ni dispersion. The CaO phase in the catal-sorbents is converted to CaCO₃ by CO₂ absorption after the first carbonation at 500 °C for 2 h, and the CaO conversion ratio increases with increasing Ni-to-CaO ratio (Figure 4a), corresponding to the CO₂ capture performance presented in Figure 3a. As shown in Figure 1b, NiO is reduced to Ni, and CaCO₃ is regenerated to CaO/Ca(OH)₂ phases during direct methanation. However, for 20Ni/CaO, not all CaCO₃ is regenerated after the fifth direct methanation because of the slow methanation rate, as mentioned above. After the sixth CO₂ capture, the Ni phase is converted back to the NiO phase by the oxidation of metallic Ni by CO₂ or H₂O in the CO₂ capture step [28,29]. It is observed that the CaO conversion ratio in 2Ni/CaO decreased, compared to the first CO₂ capture. This might be because 2Ni/CaO has the largest CaO crystallite size and the poorest macroporous structures among the catal-sorbents, and CO₂ cannot diffuse through the thick layer of CaCO₃ to come in contact with the inner CaO phase. For 10Ni/CaO, the conversion of CaO to CaCO₃ increased, which may be due to the redispersion of the CaO phase. However, for 20Ni/CaO, the increase in CaO conversion after the sixth CO₂ capture is caused by incomplete decarbonation during direct methanation.

After five consecutive cycles of CO₂ capture and direct methanation, the crystallite size of Ni species in 2Ni/CaO and 10Ni/CaO increased slightly from 20.2 and 26.9 to 32.3 and 37.8 nm, respectively (Figure 5). Despite the increase in the crystallite size of Ni in 2Ni/CaO and 10Ni/CaO catal-sorbents, the kinetics of direct methanation maintained stability because of the relatively high Ni dispersion. 20Ni/CaO shows high bulk NiO (Ni²⁺ (I)) in the XPS results, which explains the large Ni particle size. The large size of the Ni particles means that the dispersion of Ni is low. This exhibited a sluggish direct methanation rate (Figure S6). 20Ni/CaO has a higher amount of bulk NiO compared to the amount of NiO interacting with CaO (Ni²⁺ (II)) due to the large particle size of Ni. Many bulk NiO produced rapid sintering of Ni species after five consecutive cycles in CO₂ capture and direct methanation (Figure 5f). The increase in the Ni crystallite size correlates with the gradual deactivation of CH₄ productivity as the cycle progresses.

3. Materials and Methods

3.1. Catal-Sorbent Synthesis

Nickel (II) nitrate hexahydrate (Ni(NO₃)₂·6H₂O) and calcium nitrate tetrahydrate (Ca(NO₃)₂·4H₂O) were used as Ni and Ca precursors, respectively, and citric acid was used as a chelating agent. The 2Ni/CaO, 10Ni/CaO, and 20Ni/CaO catal-sorbents were prepared by citrate sol–gel techniques [28,29]. Metal precursors and citric acid were dissolved in deionized water, and the solution was vigorously stirred at 50 °C for 1 h. Then, the solution was dried overnight in an oven at 100 °C to obtain a gel. The resulting dry gel was finely ground and calcined in a furnace at 800 °C for 5 h (temperature ramp rate of 10 °C/min). The resulting catal-sorbents were designated as xNi/CaO, where x represents the weight percentage of the metallic Ni based on the catal-sorbents.

3.2. Catal-Sorbent Characterization

The metal contents (Ni and Ca) of the catal-sorbents were determined using inductively coupled plasma–optical emission spectrometry (ICP-OES, OPTIMA 4300 DV, PerkinElmer, Waltham, MA, USA). Textural properties such as the Brunauer, Emmett, and Teller (BET) surface area, pore volume, average pore size, and nanoparticle size were estimated from the N₂ adsorption–desorption isotherms at −196 °C using Micromeritics ASAP 2020 (Norcross, GA, USA) analyzer. The Ni dispersion and surface area were determined by a CO chemisorption experiment using a Micromeritics ASAP 2020 analyzer. The average pore size was calculated by the Barrett–Joyner–Halenda method, and macroporosity and pore size distribution were measured by a Hg porosimeter (AutoPore V 9620, Micromeritics, Norcross, GA, USA). The morphology of the catal-sorbents was determined through field emission scanning electron microscopy (FE-SEM, S-4800, Hitachi, Tokyo, Japan), and the crystalline phase was determined through field emission transmission electron microscopy (FE-TEM, JEM-2100F, JEOL, Tokyo, Japan). The chemical composition of the surface of the catal-sorbents was obtained using an X-ray photoelectron spectrometer (XPS) using a NEXSA (ThermoFisher, Waltham, MA, USA) equipped with an Al Kα source. Differential thermal gravity curves on an SDT Q600 thermogravimetric analyzer were used to evaluate the reduction properties. The crystallographic structure of the catal-sorbents was determined by X-ray diffraction (XRD) analysis (Phillips X’PERT X-ray diffractometer, PANalytical, Amsterdam, Netherlands) using a Cu Kα radiation source. The crystallite size of CaO in the catal-sorbents was calculated by the Scherrer equation. In addition, thermogravimetric analysis (TGA, N-1000, Scinco, Seoul, Korea) was utilized to assess the cyclic regeneration properties of the catal-sorbents during CO₂ capture and direct methanation (CO₂ capture: pure CO₂ and direct methanation: pure H₂ at 500 °C).

3.3. Reaction Process

The CO₂ capture and direct methanation in the ICCU process was carried out using 0.5 g of catal-sorbents in a packed-bed reactor (inner diameter of ½ inch). The reactor was placed in an electric furnace at atmospheric pressure. The reaction temperatures during CO₂ capture and direct methanation were maintained at 500 °C. At the CO₂ capture step, 10 vol.% CO₂, 10 vol.% H₂O, and balance N₂ were made to flow through the packed bed with 40 mL/min of total flow rate. At the direct methanation step, 90 vol.% H₂ and balance N₂ were passed after purging the reactor. N₂ was used as the balancing and standard gas. The outlet gases were analyzed through gas chromatography (GC, Agilent 6890, Agilent Technologies, Santa Clara, CA, USA) equipped with two thermal conductivity detectors (TCD). A Porapak Q packed column was used to separate N₂, CO₂, and H₂O during CO₂ capture, whereas a Carboxen-1000 packed column was used to separate H₂, N₂, CH₄, and CO₂ during direct methanation step.

4. Conclusions

Herein, Ni/CaO catal-sorbents were prepared by the sol–gel method to determine the effect of the Ni-to-CaO ratio on CO₂ capture and direct methanation performances. With increasing Ni-to-CaO ratio, the macroporous structures and crystallite size of Ni species increased, whereas the extent of nanoparticle agglomeration and the crystallite size of CaO decreased. The CO₂ capture breakthrough time slightly increased with increasing Ni-to-CaO ratio because of the enhanced macroporous structures and the smaller size of CaO. In the direct methanation step, 20Ni/CaO showed the lowest methane production due to the large Ni particle size. 10Ni/CaO showed the best TOF results (0.4621 s⁻¹) in the initial cycle with appropriate Ni dispersion and macroporous structure. The CO₂ capture and CH₄ production performances (capacity and kinetics) in 2Ni/CaO and 20Ni/CaO catal-sorbents decreased gradually over five consecutive cycles. After five consecutive cycles of CO₂ capture and direct methanation, despite the increase in the crystallite size of Ni species in 2Ni/CaO and 10Ni/CaO catal-sorbents, the kinetics of direct methanation maintained stability because of the relatively high Ni dispersion. 10Ni/CaO exhibited excellent thermal stability with high CH₄ productivity over five cycles in ICCU methanation because of its high CO₂ capture capacity, fast carbonation, and direct methanation by the improved macroporous CaO structure and Ni dispersion. Overall, the 10Ni/CaO catal-sorbent showed promise for cost-effective CO₂ capture and conversion value-added products. In the future study, long-term stability tests will be conducted to study deactivation and its effect on catalyst performance.