Coke-promoted Ni/CaO catal-sorbents in the production of cyclic CO and syngas

Abstract

Compared with conventional CO₂ capture and utilization (CCU), the integrated CCU (ICCU) process has attracted attention in reducing total thermal energy and ensuring a simplified process. CO₂ capture and the subsequent dry reforming of methane (DRM) during the ICCU employing Ni/CaO catal-sorbents has been proposed for the conversion of waste CO₂ with CH₄ into syngas. Here, coke-promoted Ni/CaO catal-sorbents were fabricated via CH₄ pretreatment, and a highly efficient ICCU process for producing CO and syngas was proposed. High CO₂ conversion and high CO production were achieved in the CO₂ conversion step employing the reverse Boudouard reaction in tandem with CO₂ capture. In the following CH₄ conversion step, the spent catal-sorbents were regenerated into syngas via a reaction with CH₄, and the carbon sources for the reverse Boudouard reaction were supplied by CH₄ decomposition. The C-Ni/CaO catal-sorbent exhibited excellent performances of CO₂ capture capacity, and CO and H₂ productivities in consecutive CO₂ and CH₄ conversions. However, the coke-promoted Ni/CaO catal-sorbents were not completely regenerated in the CH₄ conversion step after the 5^th cycle, which might be due to the sintering of Ni⁰/NiO and CaO/CaCO₃ materials and the excess amount of coke deposited on the surface of C-Ni/CaO catal-sorbents.

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Introduction

An increase in CO₂ emissions from the excessive combustion of fossil fuels has rapidly increased its atmospheric concentration, and this is the main cause of the climate emergency.^1–3 To achieve a low-carbon or carbon-neutral society, many researchers have extensively investigated innovative CO₂ mitigation technologies, such as efficient electrical energy production and consumption; coal gasification; and CO₂ capture, utilization, and storage (CCUS), in the past few decades.⁴ CO₂ can be utilized as a carbon source to produce fuels and value-added chemicals in several CO₂ green technologies.^4–6 CO₂ capture and utilization (CCU) is among the most effective climate mitigation technologies for separating CO₂ from large-scale industrial, as well as other emission sources; transporting the separated CO₂ to specific sites; and utilizing it for different chemical processes.^7,8

Calcium looping (CaL) technology is considered a potential inexpensive technology for separating CO₂; it utilizes regenerable calcium oxide (CaO)-based solid sorbents with excellent CO₂ sorption capacity (786 mg_CO2 g_sorbent⁻¹ or 17.8 mmol_CO2 g_sorbent⁻¹): CaO_(s) + CO_2(g) ↔ CaCO_3(s).^9–12 However, the application of CaL technology in the conventional CCU process requires high total energies for carbonation and subsequent decarbonation in a temperature swing system, followed by the transportation and conversion of CO₂via several catalytic reactions.^11,13–15 Additionally, the CaO particles undergo severe thermal sintering, which causes a gradual decrease in their CO₂ capture performance (capacity and kinetics) under conventional CaL conditions (carbonation: 10–15% CO₂ at 650 °C and decarbonation: concentrated CO₂ at 950 °C).^9–11

Dry reforming of methane (DRM), which converts two greenhouse gases (CO₂ and CH₄) into industrial feedstock syngas (H₂ and CO), is widely investigated using various techniques such as thermal catalysis,^16–21 electrocatalysis,^22–24 and photocatalysis.^25–29 Among these techniques, thermal catalysis is a way to convert CO₂ with a fast reaction rate, high selectivity, and large throughput. Considering that H₂ is industrially produced from methane, DRM is considered more economical for CO₂ utilization.^30–32 Ni-based catalysts are the most investigated for DRM because of their low cost and comparable activities to noble metal catalysts.^{17,19,21,24,33,34} Apart from the hydrogenation of CO₂ (methanation and rWGS), the most significant factor that affected the stability of the severe process during DRM was carbon deposition.³⁵ The formation of carbon during DRM was mainly ascribed to the decomposition of CH₄ (CH_4(s) ↔ C_(s) + 2H_2(g)) and the Boudouard reaction (2CO_(g) ↔ C_(s) + CO_2(g)).³⁵ To achieve high conversion and avoid catalytic coking, DRM should be conducted at a high temperature (870–1040 °C) with a CH₄/CO₂ ratio of 1 because carbon deposition proceeds from the decomposition of CH₄ and the Boudouard reaction in the temperature range of 557–700 °C.³⁵

Some researchers have recently attempted to combine effectively CaL technologies with utilization of CO₂ using catal-sorbents or dual-functional materials (DFMs) to develop a novel integrated CCU (ICCU) process for reducing the total thermal energy, as well as achieving a simplified process compared with that of conventional CCU.^15,36–45 The catal-sorbents or DFMs consist of CaO as a CO₂ sorbent and catalysts such as Ru, Rh, or Ni metals.^{15,36,38,40–42,46–50} First, CO₂ is captured by CaO to form CaCO₃ in the CO₂ capture step, then a reactive gas such as H₂, CH₄ or C₂H₆ is flowed to regenerate CaCO₃ to CaO, activate catalysts, and convert captured CO₂ to value added chemicals via methanation,^{15,36,38,40,41,49} reverse water gas shift (rWGS),^42,50 or dry reforming of hydrocarbons.^46–48

Presently, only a few studies have attempted to develop catal-sorbents for CaL combined with DRM in the ICCU process using Ni/CaO catal-sorbents.^15,48 In the CO₂ capture step, CaO in the Ni/CaO catal-sorbents reacts with CO₂, forming CaCO₃: NiO/CaO_(s) + CO_2(g) ↔ NiO/CaCO_3(s). During the subsequent DRM step, the NiO phase was reduced to metallic Ni by CH₄, and the captured CO₂ in the spent catal-sorbents was completely desorbed into the CH₄ stream, after which the released CO₂ reacted with CH₄ to produce syngas: NiO/CaCO_3(s) + CH_4(g) ↔ Ni⁰/CaO_(s) + 2CO_(g) + 2H_2(g).⁵¹ Moreover, a higher thermodynamic equilibrium was observed in the conversion of CaCO₃ by CH₄ compared with the conversion with H₂ at a high temperature (>600 °C). Kim et al. demonstrated the feasibility of coupled CO₂ capture and conversion reactions performed at 720 °C using a mixture of CaO (CO₂ sorbent) and Ni/MgO–Al₂O₃ (DRM catalyst).⁵² The released CO₂ (regeneration of the CO₂ sorbent) is converted into syngas with an approximately equimolar ratio of H₂ to CO with minimal CO₂ slip; however, the CO₂ uptake decreases during 10 cycles from thermal sintering of CaO at 720 °C. Tian et al. coupled the CaL-based CO₂ capture and DRM in ICCU employing a CaO/Ni bi-functional sorbent catalyst to exploit the possibility of producing syngas from waste CO₂ in an energy-efficient manner.¹⁵ Compared with conventional CaL processes, the captured CO₂ was desorbed with superior reaction kinetics, and the desorbed CO₂ reacted with CH₄ at 800 °C when Ni-catalyzed CH₄ reforming was integrated with CaCO₃ regeneration, following Le Chatelier's principle. However, the CO₂ capture capacity of the material gradually decreased with the cycle number because of the thermal sintering of the CaO phase. This low amount of CO₂ in the following DRM step marginally decreased the syngas yield and coke formation. Although the deposited carbon could be gasified via the reverse Boudouard reaction in the following CO₂ capture step, the deposited carbon is accumulated in the DRM process.

The DRM process was conducted at a high temperature (>800 °C) to avoid carbon deposition and achieve high conversions of CO₂ and CH₄. Nevertheless, carbon was still deposited on the surface of the Ni phase of the Ni/CaO catal-sorbents during DRM. Moreover, the CO₂ capture capacity decreased gradually because of the thermal sintering of CaO at a high temperature, and this decreased the production of syngas in the subsequent DRM process. Here, coke-promoted Ni/CaO (C-Ni/CaO) catal-sorbents were developed through CH₄ pretreatment, after which a highly efficient process of producing CO and syngas at a relatively low temperature was proposed. The effect of the CH₄ treatment temperature on coke, which was deposited on the coke-promoted Ni/CaO catal-sorbents, was investigated in detail. Moreover, the C-Ni/CaO catal-sorbents were applied to hybrid CO and syngas production via CO₂ and CH₄ conversions during cyclic ICCU.

Experimental

Materials

Nickel(II) nitrate hexahydrate (Ni(NO₃)₂·6H₂O), calcium nitrate tetrahydrate (Ca(NO₃)₂·4H₂O), and citric acid were purchased from Sigma-Aldrich. Ca(NO₃)₂·4H₂O and Ni(NO₃)₂·6H₂O were utilized as the precursors of CaO and Ni, respectively. Citric acid was employed as a gelling agent.

Synthesis of the catal-sorbent

Desired amounts of Ca(NO₃)₂·4H₂O, Ni(NO₃)₂·6H₂O, and citric acid were dispersed in deionized water, and the mixture was continuously stirred for 1 h. The mixed solution was evaporated for 12 h in a drying oven at 100 °C until a dried gel was formed. The dried gel was calcined at 800 °C in a muffle furnace for 5 h in air with the temperature ramp rate of 10°C min⁻¹. The resulting sample was denoted as a NiO/CaO catal-sorbent. Based on the ICP analysis, the Ni metal content in the NiO/CaO catal-sorbents was found to be 9.3 wt% (Table S1†).

Pretreatment of the NiO/CaO catal-sorbent

Before the tests, the as-prepared NiO/CaO catal-sorbents were pretreated in H₂ or CH₄ to reduce NiO to Ni⁰. In the presence of 10% H₂, the NiO/CaO catal-sorbents were treated at 500 °C for 2 h, which were denoted as Ni/CaO catal-sorbents. In the presence of 10% CH₄, the NiO/CaO catal-sorbents were treated at 600, 650, and 700 °C for 2 h to deposit carbon on the surface of the Ni, which were denoted as C-Ni/CaO catal-sorbents.

Characterization

The temperature-programmed reduction (TPR) profiles of the NiO/CaO catal-sorbents were obtained by 10% H₂ and 10% CH₄ treatments from 100 °C to 800 °C, respectively, at a temperature ramping rate of 10 °C min⁻¹. The outlet gases were recorded via a gas chromatograph (GC) equipped with thermal conductivity detectors (TCD). The crystal structure of the catal-sorbents was analyzed with a Cu-Kα radiation source on a Phillips X'PERT X-ray diffractometer at the Korea Basic Science Institute, Daegu. The crystallite size of the metal phase was calculated by using the Scherrer equation. Raman spectroscopy was performed to determine the degree of graphitization in the C-Ni/CaO catal-sorbents. Derivative thermogravimetry-temperature-programmed oxidation (DTG-TPO) was conducted from 100 °C to 800 °C, with a temperature ramping rate of 10 °C min⁻¹ to determine the type of coke species that was deposited in the C-Ni/CaO catal-sorbents. The crystalline phase was determined by field-emission transmission electron microscopy (FE-TEM, JEOL, JEM-2100F), which was performed at the Korea Basic Science Institute, Daegu.

CO production and syngas production tests

The production of CO, and the subsequent production of syngas in the ICCU process, were determined by monitoring their concentrations via GC. The prepared catal-sorbent (0.5 g) was packed in a fixed-bed reactor with a diameter of ½-inch, and the reactor was placed in an electric furnace at atmospheric pressure. Prior to the tests, the Ni/CaO catal-sorbents were pretreated with CH₄ as mentioned above. During each CO₂ conversion and subsequent CH₄ conversion process, the reaction temperature was maintained at 600, 650, and 700 °C. In the CO₂ conversion step, 10 vol% CO₂ and balance N₂ were passed through the bed for 2 h at 40 mL min⁻¹. When the CO₂ concentration of the outlet gas matched that of the inlet CO₂ feed gas, the gas composition was changed to pure N₂ to purge CO₂ from the reactor for 2 min. Afterwards, the gas composition was changed to 10 vol% CH₄. During the subsequent conversion of CH₄, the reaction temperature was maintained at 600 °C, 650 °C, and 700 °C. Moreover, N₂ gas was employed as the standard and balance gas. To prevent the condensation of water vapor, the inlet and outlet lines of the reactor were maintained at temperatures greater than 100 °C employing a heating tape before the injection of gas into the reactor and GC column. The outlet gases were analyzed on a gas chromatograph (Agilent 6890), which was equipped with two TCDs. A Carboxen 1000 packed column was connected to one TCD to analyze the N₂, CO₂, and H₂O gases; a Porapak Q packed column was connected to the other TCD to analyze the H₂, N₂, CO, CH₄, and CO₂ gases. The amount of coke deposited at each cycle was calculated using eqn (1)–(4)

Coke deposited in the CH₄ treatment step (mmol g⁻¹) = CH₄ consumed − CO produced (1)

Coke accumulated (mmol g⁻¹) = eqn (1) − eqn (2) + eqn (3) (4)

Results and discussion

Process description

We propose a cyclic production of CO and syngas in the ICCU process using the coke-promoted Ni/CaO (C-Ni/CaO) catal-sorbent as illustrated in Fig. 1; the main reactions that occurred in this process are summarized in Table 1. Regarding the CO₂ conversion step, the reverse Boudouard reaction, CO₂ splitting, and CO₂ capture processes proceeded simultaneously with C-Ni/CaO to produce CO gas in a high yield while also capturing CO₂: C-Ni/CaO_(s) + CO_2(g) ↔ NiO/CaCO_3(s) + CO_(g). Regarding the CH₄ conversion step, the decomposition of CH₄, partial oxidation of CH₄, and DRM proceeded simultaneously in the presence of NiO/CaCO₃ to produce syngas in a high yield while achieving the complete regeneration of CaCO₃: NiO/CaCO_3(s) + CH_4(g) ↔ C-Ni/CaO_(s) + CO_(g) + H_2(g). Given that the process proposed in this study outweighs a high CO and syngas production from CO₂ and CH₄ conversion as well as multicycle stability by suppressing the thermal sintering of Ni or CaO, the optimum CH₄ pretreatment and reaction temperatures are further examined in this paper.

Fig. 1 Schematics of CO and syngas production in the ICCU process utilizing C-Ni/CaO.

Table 1 Major reactions in the CO₂ and CH₄ conversion steps in the ICCU process

CO2 conversion	CH4 conversion
C(s) + CO2(g) ↔ 2CO(g)	NiO(s) + CH4(g) ↔ Ni^0(s) + CO(g) + 2H2(g)
Ni^0(s) + CO2(g) ↔ NiO(s) + CO(g)	CaCO3(s) + CH4(g) ↔ CaO(s) + 2CO(g) + 2H2(g)
CaO(s) + CO2(g) ↔ CaCO3(s)	CH4(g) ↔ C(s) + 2H2(g)

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Fig. 2 shows the TPR profiles of the NiO/CaO catal-sorbents under (a) 10% H₂ and (b) 10% CH₄ treatment conditions from 100 °C to 800 °C, with a temperature ramping rate of 10 °C min⁻¹ after the pretreatment with N₂ for 1 h at 200 °C to desorb the adsorbed gases. The H₂ consumption curves of the NiO/CaO catal-sorbents exhibited two reduction peaks, which corresponded to the reducible NiO species in the NiO/CaO catal-sorbents: NiO_(s) + H_2(g) ↔ Ni⁰_(s) + H₂O_(g).^15,45 The lower and higher temperature peaks at 500 °C and 600 °C were attributed to the reductions of the free NiO and NiO interacting with the CaO support, respectively. During the reduction by CH₄, CO was detected between 500 °C and 600 °C, which corresponded to the same reduction peaks observed under the H₂ condition. CO and H₂ were produced via the partial oxidation of CH₄ employing NiO as an oxygen source: NiO_(s) + CH_4(g) ↔ Ni⁰_(s) + CO_(g) + 2H_2(g).⁵³ Notably, the H₂/CO ratio was higher than the value (∼2) obtained from the partial oxidation of CH₄, and the CH₄ consumption and H₂/CO ratio increased as the reaction temperature increased. The improved production of H₂ during the reduction of CH₄ could be attributed to the decomposition of CH₄ at temperatures above 450 °C: CH_4(g) ↔ C_(s) + 2H_2(g).^51,54

Fig. 2 TPR profiles of the NiO/CaO catal-sorbents under the (a) 10% H₂ and (b) 10% CH₄ conditions from 100 to 800 °C at a temperature ramping rate of 10 °C min⁻¹.

Fig. 3 shows the XRD patterns of the catal-sorbents in the calcined (NiO/CaO) and reduced states employing H₂ at 800 °C (Ni/CaO) or CH₄ at 650 °C (C-Ni/CaO). The catal-sorbents exhibited sharp peaks corresponding to the CaO phase (JCPDS no. 48-1467) in all the states. In addition to the CaO phase, the XRD patterns of the catal-sorbents in the calcined state (NiO/CaO) contained the NiO phase (JCPDS no. 43-1003) with small peaks of the Ca(OH)₂ phase (JCPDS no. 84-1263). The XRD patterns of the catal-sorbents after they were reduced by H₂ at 800 °C (Ni/CaO) exhibited the metallic Ni phase (JCPDS no. 70-1849) without the NiO phase, indicating the complete reduction of NiO to Ni⁰. After they were reduced by CH₄, deposited carbon phases and the Ni⁰ phase are observed. This result corresponds to the TPR results in Fig. 1. Almost no phase change was observed in the XRD patterns of the C-Ni/CaO samples that were reduced by CH₄ at 600, 650, and 700 °C (Fig. S1†).

Fig. 3 XRD patterns of the Ni/CaO catal-sorbents in the calcined (NiO/CaO) and reduced states employing H₂ at 800 °C (Ni/CaO) or CH₄ at 650 °C (C-Ni/CaO): () NiO, () Ni⁰, () CaO, () Ca(OH)₂ and (▲) coke.

Raman spectroscopy, DTG-TPO, and TEM analyses afforded additional information on the deposition of coke during the reduction by CH₄. The phase chemistry of the coke, which was deposited on the C-Ni/CaO, as revealed by Raman spectroscopy (Fig. 4a), exhibited two sharp characteristic bands in all the samples. The D band at 1340 cm⁻¹ was ascribed to the presence of disordered or amorphous carbons, and the G band at 1580 cm⁻¹ corresponded to the well-crystallized graphite phase.^55–57 The ratios of the intensities of the D to G bands (I_D/I_G) of coke, which was deposited on the CH₄-reduced Ni/CaO catal-sorbents at 600, 650, and 700 °C, were 1.01, 0.79, and 0.77, respectively. This indicated that the graphitization degrees of the carbon increased with increasing CH₄ reduction temperatures. The DTG-TPO analyses of C-Ni/CaO at different CH₄ reduction temperatures were performed to determine the type of coke that was deposited under air conditions (Fig. 4b). Two types of oxidation peaks were observed: a small one at ∼380 °C and a large one at 600–700 °C. Following the literature, the small peaks at a lower combustion temperature were attributable to the combustion of encapsulating coke with an amorphous nature, whereas the latter peaks at a higher temperature corresponded to the filamentous carbon.^33,58 The DTG-TPO results indicated that the amorphous coke deposit decreased while the filamentous carbon increased with increasing CH₄ reduction temperature; this is consistent with the Raman spectra. Fig. S2a† shows that the Ni particles with a crystallite size of 15–20 nm in the Ni/CaO sample were well dispersed in the NiO/CaO sample (Fig. S3†), and this corresponds to the crystallite size obtained by XRD (Fig. 3). After the CH₄ treatment at 650 °C (Fig. S2b†), no change was observed in the crystallite size of the Ni particles, whereas the amorphous and filamentous coke deposits exhibited changes (Fig. S4†); these results correspond to the Raman and TPO results.

Fig. 4 Coke characterization: (a) Raman spectra, (b) DTG-TPO of the C-Ni/CaO catal-sorbents that were reduced at different temperatures (600 °C, 650 °C, and 700 °C).

Fig. 5 shows the CO₂ consumption and CO production performances of the Ni/CaO and C-Ni/CaO catal-sorbents in the CO₂ conversion step under the 10 vol% CO₂ condition at 650 °C. The Ni/CaO and C-Ni/CaO catal-sorbents were prepared by pretreating the as-prepared NiO/CaO samples with H₂ and CH₄ at 800 °C and 650 °C, respectively. The total CO₂ consumption capacity and CO productivity of the Ni/CaO catal-sorbent were 12.2 mmol_CO2/g⁻¹ and 1.05 mmol_CO g⁻¹, respectively (Fig. 5a). The metallic Ni in Ni/CaO reacted with CO₂ to yield CO via CO₂ splitting, as follows: Ni⁰_(s) + CO_2(g) ↔ NiO_(s) + CO_(g) (Fig. S5a†). However, most CO₂ reacted with CaO and was stored as the CaCO₃ phase (11.6 mmol_CO2 g⁻¹): CaO_(s) + CO_2(g) ↔ CaCO_3(s). The conversion of CO₂ decreased rapidly to zero after achieving complete CO₂ capture and splitting (Fig. 5b and c). Conversely, the total CO₂ consumption capacity and CO productivity of C-Ni/CaO were 30.2 mmol_CO2 g⁻¹ and 36.9 mmol_CO g⁻¹, respectively. Notably, the total CO₂ consumption capacity and CO productivity of C-Ni/CaO were significantly improved via the reverse Boudouard reaction: C_(s) + CO_2(g) ↔ 2CO_(g). Nevertheless, the carbon deposited in C-Ni/CaO are observed in XRD analysis after the CO₂ conversion step (Fig. S5b†). The amount of CO produced from CO₂ splitting employing Ni⁰ as a reductant was assumed to be insignificant. Additionally, the CO₂ gas was also stored as CaCO₃ (11.7 mmol_CO2 g⁻¹), which is the same as the amount of CO₂ that was captured by the Ni/CaO catal-sorbent. Therefore, the simultaneous CO₂ capture and reverse Boudouard reaction ensured that the C-Ni/CaO catal-sorbent maintained a high CO₂ conversion, as well as CO mole fraction, at the early stage of the reaction (∼40 min). After saturation of CO₂ capture, the amount of CO that was produced via the reverse Boudouard reaction was maintained, thus decreasing the CO₂ conversion and mole fraction of CO (Fig. 5b and c). Although the reverse Boudouard reaction with increasing CH₄ reduction temperatures increased the production of CO, C-Ni/CaO catal-sorbents exhibited the highest CO₂ conversion and CO mole fraction at the early stage of the reaction at 650 °C (Fig. S6†). This is because CaO sorbents exhibited the highest CO₂ capture performance at 650 °C. At 700 °C, however, the catal-sorbents did not show enhancement of CO₂ conversion at the early stage of the reaction because the excess carbon deposition in the CH₄ treatment step blocks the access of the CO₂ to CaO phase. Owing to the unfavorable conditions, the thermodynamic equilibrium has revealed that CO₂ splitting at a low temperature (<900 °C) could not achieve high CO production and complete CO₂ conversion.⁵³ However, C-Ni/CaO could achieve high CO₂ conversion and the production of a large amount of CO at relatively low temperature via the reverse Boudouard reaction with CO₂ capture: C-Ni⁰/CaO_(s) + CO_2(g) ↔ NiO/CaCO_3(s) + CO_(g).

Fig. 5 CO₂ consumption and CO production performances in the CO₂ conversion step: (a) CO₂ consumption and CO production capacity, (b) CO₂ conversion and (c) CO mole fraction obtained with Ni/CaO and C-Ni/CaO under the 10 vol% CO₂ condition at 650 °C.

Fig. 6 exhibits syngas productivity of the CH₄ conversion step employing the as-prepared NiO/CaO and NiO/CaCO₃ catal-sorbents under the 10 vol% CH₄ condition at 650 °C. NiO/CaCO₃ was obtained from C-Ni/CaO after the conversion of CO₂ at 650 °C. The syngas productivities of NiO/CaO were 48.4 mmol_H2 g⁻¹ and 0.94 mmol_CO g⁻¹, respectively. A low amount of CO is produced via the partial oxidation of CH₄ utilizing NiO as the oxygen source: NiO_(s) + CH_4(g) ↔ Ni_(s) + CO_(g) + 2H_2(g) (Fig. S7a†). Contrarily, H₂ was produced via the partial oxidation, as well as the decomposition, of CH₄. The CH₄ conversion performance decreased dramatically at the early stage of the reaction (∼40 min) after the partial oxidation of CH₄ (reduction of nickel). Coke, which was deposited on the surface of nickel might contribute significantly to the gradual deactivation of CH₄ conversion as the reaction proceeded, by blocking the access of the reactant gases to the catalytic sites.^52,57 Conversely, the NiO/CaCO₃ catal-sorbent shows higher syngas production (110.6 mmol_H2 g⁻¹ and 18.5 mmol_CO g⁻¹) and initial CH₄ conversion compared with NiO/CaO. The improved CH₄ conversion by NiO/CaCO₃ at the early stage of the reaction (∼120 min) was due to DRM with CO₂ that was stored as CaCO₃, which produced syngas (CO and H₂), as follows: CaCO_3(s) + CH_4(g) ↔ CaO_(s) + 2CO_(g) + 2H_2(g) (Fig. S7b†). However, the NiO/CaCO₃ catal-sorbent shows rapid deactivation at the early stage compared to NiO/CaO catal-sorbents because the high CO concentration from enhanced CH₄ conversion promoted the Boudouard reaction: 2CO_(g) ↔ C_(s) + CO_2(g). The amount of CO, which was produced by the partial oxidation of CH₄ employing metallic NiO as an oxidant, was assumed to be negligible. Moreover, the conversion of CH₄ decreased dramatically, and there was no observed production of CO after DRM; the production of H₂via the decomposition of CH₄ proceeded. As the reaction temperature increased, the conversion of CH₄, as well as the production of H₂, increased because of its favorable thermodynamics regarding the decomposition of CH₄ (Fig. S8†). At 700 °C, the NiO/CaCO₃ catal-sorbent shows a relatively stable CH₄ conversion because the Boudouard reaction is unfavorable at temperatures >700 °C.¹⁵ However, the amount of CO produced from DRM at 700 °C was low because the amount of CO₂ that was captured in the CO₂ conversion step was the lowest. Therefore, improved CH₄ conversion and syngas production were achieved via DRM and CH₄ decomposition at 600–700 °C: C-Ni⁰/CaO_(s) + CO_2(g) ↔ NiO/CaCO_3(s) + CO_(g). Additionally, almost all the captured CO₂ was utilized in the production of syngas, and the carbon source was supplied for the production of CO in the CO₂ conversion step from the decomposition of CH₄ at relatively low temperatures.

Fig. 6 Syngas production performances in the CH₄ conversion step: (a) syngas (H₂ and CO) productivity and capacity for desorbing CO₂, (b) CH₄ conversion, and (c) syngas mole fraction obtained by utilizing NiO/CaO and NiO/CaCO₃ under the 10 vol% CH₄ condition at 650 °C.

The long-term stability of C-Ni/CaO was also investigated in ten consecutive cycles of CO₂ and CH₄ conversions (CO₂ conversion: 10 vol% CO₂ for 2 h, and CH₄ conversion: 10 vol% CH₄ for 4 h at 650 °C) and these results are illustrated in Fig. 7a and summarized in Table S2.† As shown in Fig. 7a, CO production via coke gasification in the CO₂ conversion step remained unchanged, whereas that via DRM in the CH₄ conversion step decreased slightly. This is because the captured CO₂ was not desorbed completely in the subsequent CH₄ conversion step after the 5^th cycle (Fig. 7b), leading to a decrease in CO₂ capture capacity in the following cycles of CO₂ conversion. This phenomenon might be due to the sintering of Ni⁰ and CaO/CaCO₃ materials (Table S3†). Moreover, H₂ productivity decreased (∼48.3 mmol_H2 g⁻¹) gradually and the amount of coke accumulated reached a maximum value (∼85.8 mmol_C g⁻¹) after 7 cycles of CH₄ conversions and remained unchanged. During the multicycle stability tests, the C-Ni/CaO catal-sorbents exhibited excellent amount of CO and H₂ productivity in cyclic CO₂ and CH₄ conversions after saturation of coke deposition. However, the Ni and CaO/CaO materials are sintered during the cycles, as observed by XRD (Fig. 7b and Table S3†), leading to decrease in performances of CO₂ capture capacity and CO and H₂ productivity. The combined H₂/CO molar ratio that was obtained from the CO₂ and CH₄ conversion steps was constant (∼1.0). Additionally, the desired H₂/CO ratios for producing value-added chemicals, e.g., liquid fuels, light olefins, and higher alcohol, could be controlled by changing the reaction time in the CH₄ conversion step.

Fig. 7 Long-term stabilities of C-Ni/CaO in ten consecutive cycles of CO₂ and CH₄ conversions: (a) CO and syngas (H₂ and CO) productivities in the CO₂ and CH₄ conversion steps, as well as the combined values of the H₂/CO ratios of CO₂ and CH₄ conversions, respectively; (b) XRD patterns of C-Ni/CaO in ten consecutive cycles of CO₂ and CH₄ conversions (() NiO, () Ni⁰, () CaO, () CaCO₃, () Ca(OH)₂ and (▲) coke).

Conclusions

Here, we proposed a highly efficient strategy for producing CO and syngas in a cyclic ICCU process employing C-Ni/CaO to reduce the total thermal energy and achieve a simplified process compared with that of conventional CCU. The C-Ni/CaO catal-sorbents were fabricated by the CH₄ pretreatment of NiO/CaO. Further, they were applied to the production of CO and syngas in a cyclic ICCU process. In the CO₂ conversion step, coke, which was deposited on the surface of the Ni catalysts, was gasified by CO₂via the reverse Boudouard reaction, and the residual CO₂ was simultaneously captured via CaO, thus enabling the high consumption of CO₂, as well as the high production of CO. In the following CH₄ conversion step, CaCO₃ was regenerated, and the desorbed CO₂ was converted into syngas by CH₄. Moreover, the carbon source for the production of CO in the CO₂ conversion step was the decomposition of CH₄. The C-Ni/CaO catal-sorbent exhibited excellent performances of CO₂ capture capacity, and CO and H₂ productivities in consecutive CO₂ and CH₄ conversions. However, while CO productivity in the CO₂ conversion step was constant, the coke-promoted Ni/CaO catal-sorbents were not completely regenerated in the CH₄ conversion step after the 5^th cycle. This might be due to the sintering of Ni⁰/NiO and CaO/CaCO₃ materials and the excess amount of coke deposited on the surface of C-Ni/CaO catal-sorbents.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea (No. 20182010600530).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d1se01136g

‡ Seongbin Jo and Jong Heon Lee contributed equally to this work.

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