Bifunctional catalysts for the coupling processes of CO₂ capture and conversion: a minireview

Abstract

The integrated CO₂ capture and utilization (ICCU) process can simultaneously achieve the reduction of CO₂ emissions and the production of high-value products. Bifunctional catalysts with coupled catalytic and adsorption functions show great potential in the realization of ICCU. The aim of this study is to comprehensively review the recent research progress in the development of bifunctional catalysts. We first review the bifunctional catalysts used in the integrated CO₂ capture and hydrogenation reaction including the RWGS and methanation reaction (ICCU-RWGS and ICCU-methanation). The effects of the adsorbent and catalyst components of bifunctional catalysts on the performance of ICCU-RWGS and ICCU-methanation including adsorption efficiency and catalytic activity are summarized. Then, we present the bifunctional catalysts applied in the integrated CO₂ capture and dry reforming reaction (ICCU-DR). The optimization of bifunctional catalysts to improve their cyclic stability is reviewed. Finally, the applications of bifunctional catalysts that were developed for the integrated ICCU and sorption-enhanced steam reforming (SESR) reaction are demonstrated.

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1. Introduction

As one of the most serious problems facing modern society today, global warming has been caused by the continuous increase of CO₂ content in the atmosphere over the past century and a half.¹ The excessive CO₂ contributes to serious environmental problems such as rising sea levels, extreme weather, and ocean acidification.² Meanwhile, recent studies have confirmed that the rising CO₂ concentration threatens seafood production and degrades the nutritional value of rice.^3,4 In this context, many researchers have been working on CO₂ conversion and utilization processes for the past few decades to achieve the anthropogenic carbon cycle.^5,6 Among them, the catalytic conversion of CO₂ in the thermochemical process has received extensive attention as it not only can reduce CO₂ emission but also directly produces a variety of valuable chemicals and fuels.⁷ Unfortunately, these processes often require purified CO₂ as a reactant, where multiple steps including CO₂ capture, sequestration and transportation are needed,⁸ which inevitably increases the costs and complicates system configuration.

In comparison, the integration of CO₂ capture and utilization (ICCU) into a single process offers an alternative more promising strategy for the direct conversion of CO₂ into valuable products, which saves the significant energy, facilities, and expense of the conventional catalytic conversion with separated process units.⁸ Therefore, there is increasing interest in the ICCU technology. To date, various ICCU processes have been developed through a combination of different CO₂ conversion and CO₂ capture processes, like the integrated hydrogenation reaction and CO₂ capture^9,10 and the integrated dry reforming reaction and CO₂ capture.¹¹ Different sources of CO₂, like simulated flue gas conditions,¹² air,¹³ and the sorption-enhanced steam reforming reaction (SESR),¹⁴ have also been studied in ICCU processes. On the other hand, ICCU processes can be coupled with chemical looping reforming reactions to improve energy efficiency.^15,16

In general, ICCU processes need a sorbent to capture CO₂ and a catalyst to subsequently convert the captured CO₂. The sorbent and catalyst particles can be physically mixed in a single bed or packed separately in distinct beds.¹⁷ This strategy of physical mixing has the advantages of easy realization and low complexity,¹⁸ as the optimization of the sorbent and catalyst particles can be carried out independently. In addition, ICCU can also be performed using bifunctional catalysts that incorporate both catalytic and adsorption capabilities into one material.^19,20 The intimate contact between different functional sites in a bifunctional catalyst can enhance mass and heat transfer of the conversion process, so it shows unique performance advantages in terms of ICCU technology. Moreover, the bifunctional catalyst can also reduce the volume of the reactor required, which is more attractive for ICCU process intensification.

Bifunctional catalysts typically consist of alkaline earth or alkali metals (K₂O, Na₂O, Li₂O, MgO, and CaO) for capturing CO₂ and transition metal catalysts (Ni, Ru, Pt, and Rh) for CO₂ conversion. This minireview focuses on an overview of bifunctional catalysts applied to various ICCU processes, including the integrated CO₂ capture and methanation reaction (ICCU-methanation), reverse water–gas shift reaction (ICCU-RWGS), and dry reforming of methane (ICCU-DRM) and other alkanes. On the other hand, this review also discusses the application of bifunctional catalysts for the integrated process of CO₂ captured from the SESR reaction for ICCU.

2. Bifunctional catalysts for the integrated CO₂ capture and hydrogenation reaction

Reducing gases (H₂, CH₄, or other alkanes) are usually required to convert captured CO₂ and simultaneously regenerate the bifunctional catalyst in the ICCU process. Among them, hydrogen is the most commonly used reducing agent,^21,22 especially green hydrogen obtained from water electrolysis technology using renewable energy sources such as solar, biomass, and wind. The integrated CO₂ capture and hydrogenation reaction (ICCH, Fig. 1) can produce CO and CH₄ through the RWGS reaction and methanation reaction, respectively.

4H₂ + CO₂ → CH₄ + 2H₂O ΔH_298K = −165 kJ mol⁻¹ (1)

CO₂ + H₂ → CO + H₂O ΔH_298K = 41.2 kJ mol⁻¹ (2)

Fig. 1 Conceptual diagram of ICCH processes.

2.1. Bifunctional catalysts for ICCU-RWGS

The ICCU-RWGS process is usually more feasible to achieve at high temperatures (>500 °C) because of the endothermic nature of the RWGS reaction. To meet the requirements of high temperature conversion, the adsorptive component of bifunctional catalysts is usually CaO, which is a high temperature CO₂ adsorbent.^9,23,24 A few studies have also reported Na salts and K salts as adsorptive components for bifunctional catalysts.^25,26 On the other hand, although the ICCU-RWGS process often operates with bifunctional catalysts, it can be also performed using transition-metal-free bifunctional catalysts. Removing the active metal component from the bifunctional catalyst can reduce the material costs. On the other hand, it also avoids the deactivation problem of sintering or coking faced by transition metal-based bifunctional catalysts.²⁷

2.1.1. Transition-metal-free bifunctional catalysts for ICCU-RWGS. Kosaka et al. have reported that Na/γ-Al₂O₃ can be used as a bifunctional catalyst for the ICCU-RWGS process at 450–500 °C, and 90% conversion of CO₂ with 95% selectivity to CO was obtained from 400 ppm CO₂.²⁸ However, Na/γ-Al₂O₃ has low capture capacity of CO₂, which needs to be further improved. Compared with alkali metals, CaO may be a better choice because of its high CO₂ adsorption capacity. Wu et al. reported that CaO-alone as both an effective adsorbent and catalyst can efficiently achieve CO₂ capture and in situ conversion in the ICCU-RWGS process.²⁷ 75% conversion of the captured CO₂ with a 4.15 mmol g_CaO⁻¹ yield of CO was achieved at 700 °C. Meanwhile, no CH₄ was detected during the entire conversion process, and the captured CO₂ could be completely converted to CO. Recently, they have successfully applied natural marble in the ICCU-RWGS process, and a high CO₂ capture capacity of 9.4 mmol g⁻¹ with 85% conversion of the captured CO₂ and almost 100% CO selectivity was reported at 650 °C,²⁹ which improves the cost-effectiveness of ICCU technology by using cheap and robust materials. Li et al. have further studied the ICCU-RWGS performance of limestone under iso-thermal conditions using micro-fluidized bed thermogravimetric analysis coupled with mass spectrometry (MFB-TGA-MS, Fig. 2).³⁰ They showed that the CO₂ conversion and CO selectivity can reach 79.1% and ∼100% at 710 °C, respectively. Moreover, the generated CaO has self-catalytic activity, resulting in a 5 times higher decomposition rate of limestone in H₂ than in Ar.

Fig. 2 Schematic diagram of MFB-TGA-MS.³⁰

Although high reaction temperature is beneficial for CO generation, the CO₂ conversion will decrease with the increase of reaction temperature due to the enhanced CO₂ desorption kinetics.²⁷ Typically, the inevitable decomposition of CaCO₃ limits the CO₂ conversion to no more than 80% during the utilization stage.³¹ Recently, Wu et al. proposed a novel strategy to improve the CO₂ conversion of the ICCU-RWGS process by promoting direct hydrogenation of carbonates and hindering the decomposition process of CaCO₃.³¹ They reported potassium-promoted CaO (K-CaO, Fig. 3) as a bifunctional catalyst with a CO₂ conversion of >95% and a CO selectivity of nearly 100% during 50 hydrogenation cycles at 650 °C. The result showed that K species participated in the carbonation process of CaO to form stable K₂Ca(CO₃)₂ with septal K₂CO₃ and CaCO₃ layers. And the formation of K₂Ca(CO₃)₂ facilitated the direct gas–solid hydrogenation of carbonates and highly suppressed the CaCO₃ decomposition reaction. This work provides a simple and efficient design strategy for bifunctional catalysts used in ICCU-RWGS.

Fig. 3 (a) Schematic diagram and key reactions of ICCU. (b) The real time and (c) cyclic ICCU performance with 20 mol% K–CaO as a bifunctional catalyst at 650 °C.³¹

2.1.2. Transition metal-containing bifunctional catalysts for ICCU-RWGS. The transition-metal-free bifunctional catalysts have a cost advantage in the ICCU-RWGS process, but the limited kinetics of CO formation should not be underestimated due to the absence of catalytic sites. On the other hand, bifunctional catalysts containing active catalytic sites also provide more possibilities for optimizing ICCU-RWGS processes by lowering the reaction temperature. Urakawa et al. developed an FeCrCu/K/MgO–Al₂O₃ bifunctional catalyst and showed that the ICCU-RWGS reaction could be conducted at 450–550 °C.³² Recently, they found that a temperature of 350 °C is required on a K-promoted Cu/γ-Al₂O₃ catalyst to activate the dissociation of H₂ and generate the active site to capture CO₂.²⁵ However, CH₄ is often detected in these studies because of the exothermic nature of CO₂ methanation. Shimizu et al. reported that Na-promoted Pt nanoparticles supported by Al₂O₃ (Pt–Na/Al₂O₃) could act as an effective bifunctional catalyst and found that Na species could inhibit the adsorption of the formed CO on the surface of Pt nanoparticles, suppressing the successive reduction of CO to CH₄, thus leading to high selectivity to CO at a mild reaction temperature of 350 °C.²⁶ In general, bifunctional catalysts with alkali metals as sorbent components used in the ICCU-RWGS process can operate at relatively low temperatures (350–550 °C), but face the problem of CH₄ generation and low capacity of CO₂ capture.

CaO-based sorbents are the more commonly used adsorption components of bifunctional catalysts in ICCU-RWGS reactions due to their low price, high theoretical CO₂ uptake capacity, and wide availability.³³ Additionally, the high decomposition temperature of CaCO₃ matches the endothermic nature of the RWGS reaction.⁹ Jo et al. showed that a Ni–CaO bifunctional catalyst had a CO₂ adsorption capacity of up to 14.8 mmol CO₂/g and showed very good ICCU-RWGS performance with a CO yield of 11.0 mmol CO/g at 700 °C.³⁴ Zhao et al. prepared a Ni–CaO bifunctional catalyst derived from carbide slag, and it exhibited a CO₂ capture capacity of 13.28 mmol g⁻¹ and a CO productivity of 5.12 mmol g⁻¹ at 650 °C.²¹ Currently, the main challenge faced by Ni–CaO bifunctional catalysts in ICCU-RWGS is that the CO₂ adsorption capacity and CO yield decrease rapidly with the number of cycles at high temperatures (>650 °C). This is because the operating temperature of the conversion stage (>650 °C) is higher than the Tammann temperature of CaCO₃ (530 °C), which easily leads to the significant sintering-induced deactivation of CaO and thus the decease of CO₂ adsorption capacity.³⁵ On the other hand, the Ni species also suffer from obvious particle agglomeration, leading to the loss of active sites.³³

2.1.3. Highly stable bifunctional catalysts for ICCU-RWGS. Doping inactive stabilizers is deemed as the effective strategy to inhibit the deactivation of Ni–CaO bifunctional catalysts.^23,24,33 Wu et al. investigated the influence of different stabilizers (Al₂O₃, CeO₂, TiO₂, and ZrO₂) on Ni–CaO.²³ The ICCU-RWGS performance of these samples showed the following order: Ni/Al₂O₃–CaO < Ni/ZrO₂–CaO < Ni/TiO₂–CaO < Ni/CeO₂–CaO. 56.1% conversion of CO₂ and nearly 100% CO selectivity with a 2.68 mmol g⁻¹ yield of CO were achieved on Ni/CeO₂–CaO at 650 °C. Furthermore, after 20 cycles, the decrease in CO and C1 yields for Ni/CeO₂–CaO was less than 5%, with a CO selectivity greater than 99%. The superior ICCU-RWGS activity of Ni/CeO₂–CaO can be attributed to its stronger basicity, optimal Ni dispersion and improved reducibility of NiO. It was reported that the oxygen vacancies generated on the surface of CeO₂ could directly dissociate CO₂, thereby improving CO₂ conversion and CO selectivity.³⁶ Furthermore, well-dispersed CeO₂ can also act as a physical barrier to effectively inhibit the agglomeration of CaO and NiO species, ensuring the stability of the Ni/CeO₂–CaO catalyst. Zhao et al. showed that the concentration of oxygen vacancies in the bifunctional catalyst could be further boosted by doping ZrO₂–CeO₂ binary metal oxides.³³ The desired Ni/CaO–6ZrO₂–6CeO₂ promoted the activation and conversion of CO₂ to produce CO. Wang et al. also showed that metal oxide (MgO, ZrO₂, La₂O₃, and CeO₂) modified Ni/CaO bifunctional catalysts significantly improved the cyclic stability compared with Ni/CaO.²⁴ Ni/La₂O₃–CaO exhibited the best cyclic stability with a slight decrease of adsorption capacity from 13.4 mmol g⁻¹ in the 1st cycle to 12.9 mmol g⁻¹ in the 10th cycle, suggesting that La₂O₃ can also serve as an effective physical barrier to suppress the growth of CaO and Ni particles. Recently, Xu et al. showed that Na incorporation had a positive effect on the stability of Ni–CaO bifunctional catalysts.³⁷

The preparation method for Ni–CaO bifunctional catalysts can also improve their stability. Zhao et al. compared three methods for the synthesis of Ni–CaO: acidification/impregnation, acidification/impregnation combined with citric acid complexation, and wet-mixing. The results showed that the Ni–CaO synthesized by the second method had stable CO₂ adsorption capacity and CO yields for 17 cycles.²¹ They ascribed this to a more uniform dispersion of components accompanied by smaller particles and better textural properties. Wang et al. synthesized a three-dimensional (3D) Ni/CaO network composed of mesopores and macropores as a bifunctional catalyst,³⁸ which could preserve the stabilization of the structure and inhibit the growth of CaO particles. A CO₂ adsorption capacity of 9.7 mmol g⁻¹ with 57.5% conversion of CO₂ was achieved and maintained for 20 cycles on the 3D Ni/CaO.

Ca–Fe bifunctional catalysts have also been used in the ICCU-RWGS reaction by combining the Fe/Fe₃O₄ redox cycle and calcium looping. Unlike Ni–CaO bifunctional catalysts, Ca–Fe can produce CO during both capture and conversion stages (Fig. 4), which is ascribed to the re-oxidation of Fe by CO₂ and the RWGS reaction, respectively.³⁹ The CO yield in the conversion stage was significantly smaller than that in the capture stage (1.80 molCO kg_CaO⁻¹vs. 142.95 molCO kg_Fe2O3⁻¹) at 650 °C,³⁹ due to the high re-oxidation rate of Fe and the poor catalytic activity of Fe₃O₄ or Fe for the RWGS reaction. By doping Ni into the Ca–Fe material, Ni₁Fe₉–CaO showed a superior performance with a CO yield of 11.3 mol kg⁻¹, a CO₂ conversion of 82.5% and a CO selectivity of 99.9% at 650 °C.⁴⁰ Hu et al. reported Fe_xCo_yMg₁₀CaO as a heterojunction-redox catalyst for the ICCU-RWGS reaction, and the best sample exhibited a stable CO₂ adsorption capacity of 9.0–9.2 mmol g⁻¹, nearly 90% conversion of CO₂ and almost 100% selectivity to CO.⁹ The sintering of CaO was suppressed by the hybrid doping of highly dispersed Co and Fe oxides and MgO. Furthermore, they demonstrated the stable scalability of the ICCU-RWGS process through scale-up studies with experiments and computer simulations.

Fig. 4 Process diagrams of (a) Ca–Fe and (b) Ca–Ni bifunctional catalysts in the ICCU-RWGS reaction.³⁹

2.2. Bifunctional catalysts for ICCU-methanation

The CO₂ methanation reaction is thermodynamically favorable at low temperatures due to its exothermic nature.^41,42 However, low reaction temperatures will lead to severe kinetic limitations.⁸ On the other hand, if the temperature of the methanation reaction is high (>400 °C), the RWGS will occur, so the selectivity to CH₄ at high temperatures may face challenges. Therefore, the ICCU-methanation reaction is often operated at intermediate temperatures (200–400 °C). Noble metals (Rh and Ru) and Ni have been extensively employed as catalytic components in the ICCU-methanation due to their high CO₂ methanation activity.^43,44 As for adsorbents, alkaline earth (Mg, Ba, and Ca) or alkali (K, Na, and Li) metals are usually used due to their capture temperature of 300 to 400 °C.^45–47 Additionally, the high specific surface area of the support (γ-Al₂O₃) is needed to maximize adsorption sites and disperse catalyst particles.⁴⁸

2.2.1. Alkali metal-based bifunctional catalysts for ICCU-methanation. Ru–Na₂CO₃/γ-Al₂O₃ is one of the most widely studied bifunctional catalysts for ICCU-methanation. Duyar and co-workers investigated the isothermal ICCU-methanation process by using a 5%Ru–10%Na₂CO₃/γ-Al₂O₃ bifunctional catalyst at 320 °C, and a methanation capacity of 1.05 mol kg⁻¹ was achieved.⁴⁹ Later on, Farrauto et al. studied the effect of cyclic aging on the ICCU-methanation with 5%Ru–6.1%Na₂O/γ-Al₂O₃ (“Na₂O” derived from hydrogenated Na₂CO₃) at 320 °C.⁵⁰ The result showed the stable performance of ICCU-methanation for over 50 cycles (equivalent to 80 h of operation). Cobo et al. conducted a mechanism study of ICCU-methanation on 5%Ru–6.1%Na₂O/γ-Al₂O₃ by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS).⁵¹ CO₂ was first adsorbed on the Al–O⁻–Na⁺ species to form bidentate carbonates, which then overflowed to the Ru–support interface for methanation with formates as intermediaries. In the work of González-Velasco,⁵² Na₂O and NaOH were identified as storage sites for CO₂, with Na₂O being more active in CO₂ adsorption. To achieve high methanation activity, the content of Ru in bifunctional catalysts is usually high (>5%). A promising way to reduce the Ru content while maintaining the high catalytic activity of bifunctional catalysts is to replace some of the Ru with Ni.^53–55 González-Velasco et al. showed that 1%Ru/10Ni%–16%Na₂CO₃/γ-Al₂O₃ presented a selectivity of more than 95% for CH₄ in the temperature range of 320 °C to 400 °C, and a CH₄ yield of 137 μmol g⁻¹ with a fast formation rate was achieved at 360 °C.⁵³ Jeong-Potter et al. further explored the effect of Ni loading (1%, 5%, 10%, 20%) on Ru–Na₂O/γ-Al₂O₃.⁵⁴ Among them, 0.5%Ru/5Ni%–6.1%Na₂O/γ-Al₂O₃ had the highest CO₂ adsorption capacity while 0.5%Ru/20Ni%–6.1%Na₂O/γ-Al₂O₃ was the most stable. Ni–Na₂O/γ-Al₂O₃ has also been used in the ICCU-methanation reaction.^55,56 González-Velasco et al. showed that the highest CH₄ productivity of 177 μmol g⁻¹ and 90% selectivity to CH₄ were achieved on 10%Ni–10%Na₂CO₃/γ-Al₂O₃.⁵⁶ The Ni-alone bifunctional catalyst is often used in an O₂-free environment, and is not suitable for O₂-containing applications (flue gas or air) as it oxidizes and cannot be reactivated under methanation conditions (300–400 °C).^54,55,57 In contrast, Ru-based bifunctional catalysts exhibit good stability in an O₂-containing environment because of the high reducibility of RuO_x species (Fig. 5).

Fig. 5 Cyclic aging study on 5%Ru–6.1%Na₂O/γ-Al₂O₃.⁵⁰

Other alkali metals (K and Li) are also used in ICCU-methanation as sorbent components of bifunctional catalysts. Farrauto et al. demonstrated that a CO₂ sorbent capacity of 496.4 mmol kg⁻¹ and a CH₄ yield of 466.9 mmol kg⁻¹ were achieved at 320 °C on 5%Ru–7.01%K₂O/γ-Al₂O₃.⁵⁸ Cimino et al. reported a 0.50 mmol g⁻¹ yield of CH₄ with a CH₄ selectivity of 99.9% on 0.89%Ru–5%Li/γ-Al₂O₃ at 280 °C.⁵⁹ The formed Li-aluminate captures and stores CO₂ at moderate temperatures (230–350 °C). They further studied the effect of alkali metals (K, Na, Li) on the ICCU-methanation performance of 1%Ru/γ-Al₂O₃.⁴⁶ The CO₂ capture capacity was in the order Ru–Li/γ-Al₂O₃ > Ru–Na/γ-Al₂O₃ ≥ Ru–K/γ-Al₂O₃. Meanwhile, Li–Ru/γ-Al₂O₃ displayed the highest methanation activity. However, in another study, it was found that the Ru–Li/γ-Al₂O₃ bifunctional catalyst just showed a similar performance to Ru–Li/γ-Al₂O₃, which was worse than Ru–K/γ-Al₂O₃ and Ru–Na/γ-Al₂O₃.⁴⁷

2.2.2. MgO-based bifunctional catalysts for ICCU-methanation. MgO or MgO-based materials are good CO₂ adsorbent components of bifunctional catalysts in ICCU-methanation processes since the reversible transformation of MgO–MgCO₃ occurs at 300 °C–400 °C.⁶⁰ Sun et al. reported that nearly 100% capture of CO₂ with the successful production of CH₄ was achieved in prolonged cycles at low temperatures (≤250 °C) using 2D-layered Ni–MgO–Al₂O₃ nanosheets as a bifunctional catalyst.⁶¹ However, the CO₂ adsorption capacity and CH₄ yield of MgO-based bifunctional catalysts are usually limited due to the high lattice enthalpy of MgO.⁶² The introduction of alkaline metal salts (AMS) has been conducted to appreciably improve the slow adsorption rate and cycle stability of MgO-based bifunctional catalysts.^62–64 Zhao et al. showed that NaNO₃-promoted Ni/MgO exhibited good ICCU-methanation performance with a high CO₂ adsorption capacity (6.46 mmol g⁻¹) and CH₄ yield of 0.85 mmol g⁻¹ at 300 °C.⁶² Gilliard-AbdulAziz et al. constructed a Ru/K₂CO₃–MgO bifunctional catalyst, and it achieved a stable CH₄ yield of 1.07–1.19 mmol g⁻¹ and 100% selectivity to CH₄ at the carbonation temperature of 150 °C and methanation temperature of 320 °C.⁶³ In addition, they found that the Ru crystallite size in Ru/K₂CO₃–MgO was smaller than that in Ru/MgO due to the presence of the K₂RuO₃ phase. Hu et al. developed a Ni_xCo_y-AMS/CaMgO bifunctional catalyst by loading alkaline metal salts (AMS) on porous CaMgO composites, and the AMS were composed of KNO₃ (0.28 mmol), LiNO₃ (0.22 mmol), Na₂CO₃ (0.125 mmol), and K₂CO₃ (0.125 mmol).⁶⁴ Ni_xCo_y-AMS/CaMgO had excellent ICCU-methanation performance with 93.4% CO₂ conversion and 88% CH₄ selectivity at 350 °C.

2.2.3. CaO-based bifunctional catalysts for ICCU-methanation. As the pioneer of ICCU-methanation, Farrauto's group reported one of the earliest CaO-based bifunctional catalysts.⁶⁵ They constructed Ru–CaO/γ-Al₂O₃ bifunctional catalysts by dispersing CaO and Ru on porous γ-Al₂O₃ and studied the effect of different weight loadings of the CaO adsorbent (1–10 wt%) and Ru catalyst (1.1–10.6 wt%) on the ICCU-methanation performance. The results indicated that 5%Ru–10%CaO/γ-Al₂O₃ as the optimum composition realized a CH₄ yield of 0.50 mol kg⁻¹ at 320 °C. They further showed that a higher CH₄ yield of 0.70 mol kg⁻¹ was achieved on 5Rh–10%CaO/γ-Al₂O₃, suggesting that Rh–CaO-based bifunctional catalysts were more active than those containing Ru and CaO.⁴⁹ However, the high cost of Rh limited their use. González-Velasco et al. thought that high reaction temperatures were required for CaO-based bifunctional catalysts because of the formation of highly stable carbonates on CaO.⁵² They conducted the ICCU-methanation reaction at a temperature of 400 °C on 4%Ru–15%CaO/γ-Al₂O₃ and achieved a maximum production of 414 μmol g⁻¹ for CH₄. At high operating temperatures, the support in bifunctional catalysts can be eliminated, thus increasing the CaO content and making full use of the CaO sorbent capacity. Kim et al. reported an excellent CO₂ uptake (8.96 mmol g⁻¹) and CH₄ yield (8.34 mmol g⁻¹) with 96% CH₄ selectivity at 500 °C using 10%Ni–CaO.⁶⁶ Later on, they studied the effect of H₂ concentration and reaction temperature on ICCU-methanation over 10Ni–CaO.³⁴ The intermediate temperature (500–600 °C) and high H₂ concentration were conducive to the formation of CH₄. Wu et al. studied the effect of Ni loading on the ICCU-methanation performance of Ni–CaO.²² They found that 1%Ni–CaO at 550 °C exhibited a low CO₂ conversion of 38% and 58% selectivity for CH₄. However, when the Ni loading was increased to 10%, the CO₂ conversion and CH₄ selectivity improved to 45% and 69%, respectively. The lower methanation activity in the former case was attributed to volume expansion caused by the carbonation process, which led to Ni active sites being covered by formed CaCO₃. Zhou et al. investigated the effect of doping a series of metal oxides (MgO, MnO₂, La₂O₃, Y₂O₃, Al₂O₃, ZrO₂, and CeO₂) on 10Ni–CaO.¹⁸ Thanks to the good thermal stability at 600 °C, 10Ni–ZrO₂/CaO showed the best ICCU-methanation performance in 20 cycles. They proposed a two-step mechanism (Fig. 6) in which CO₂ was first released from CaCO₃ decomposition, followed by its conversion to CO at CaO sites and to CH₄ at Ni sites.

Fig. 6 Illustration of the ICCU-methanation over Ni–CaO bifunctional catalysts.¹⁸

3. Bifunctional catalysts for integrated CO₂ capture and dry reforming reactions

Dry reforming of CH₄ simultaneously utilizes two major greenhouse gases to produce syngas, thus receiving wide attention. The reaction can combine with CO₂ capture to realize the ICCU-DRM. Other light alkanes (C₂H₆ and C₃H₈) have also been used as reduction gases in the ICCU process to achieve the integrated CO₂ capture and dry reforming of alkane (ICCU-DRA). In addition to the production of syngas, the ICCU-DRA process can also produce light olefins (C₂H₄ and C₃H₆),^67,68 which is appealing to chemical industry. The ICCU-DR reaction process requires high temperatures (>600 °C) due to the highly endothermic nature of DR reactions.^14,69 CaO or CaO-based materials as high-temperature adsorbents have been used in bifunctional catalysts for the ICCU-DR reaction. On the other hand, other high-temperature adsorbents (sodium zirconate or lithium silicate) have also been studied.⁷⁰

CH₄ + CO₂ → 2H₂ + 2CO ΔH_298K = 247 kJ mol⁻¹ (3)

2CO₂ + C₂H₆ → 4CO + 2H₂ ΔH_298K = 429 kJ mol⁻¹ (4)

CO₂ + C₂H₆ → CO + C₂H₄ + H₂O ΔH_298K = 178 kJ mol⁻¹ (5)

3CO₂ + C₃H₈ → 6CO + 4H₂ ΔH_298K = 620 kJ mol⁻¹ (6)

CO₂ + C₃H₈ → CO + C₃H₆ + H₂O ΔH_298K = 164 kJ mol⁻¹ (7)

3.1. Bifunctional catalysts for ICCU-DRM

Heriberto Pfeiffer et al. studied the decomposition of carbonated samples in the CH₄ atmosphere on a NiO–CaO bifunctional catalyst and reported that syngas was produced by the CH₄ reforming process between 400 and 600 °C.⁷¹ Unfortunately, they only showed the H₂ production. Tian et al. constructed Ni–CaO bifunctional catalysts by the Pechini sol–gel method and conducted the ICCU-DRM reaction at a CO₂ capture temperature of 600 °C and a dry reforming temperature of 800 °C.¹⁶ An average cyclic utilization efficiency of 65% for CO₂ was obtained on the optimized Ni–CaO bifunctional catalyst with 86% average cyclic conversion for CH₄ (Fig. 7). Meanwhile, a maximum syngas yield of 10.4 mmol g⁻¹ for a H₂/CO molar ratio of 1.1 was reported. More importantly, they indicated that the ICCU-DRM process consumed 22% less energy than the conventional DRM reaction. Huang et al. investigated the synergistic effect of CaO and Ni at the interface (Fig. 8) and proposed that the Ni–CaO interface provided another pathway for CO₂ activation and reduced the energy barrier for CHO formation, thus ensuring long-term stability and high efficiency of the catalyst.¹¹ Hu et al. suggested that controlling the adsorptive/catalytic interface by balancing the size and loading of Ni nanoparticles on porous CaO was key to achieving ultra-high CH₄ and CO₂ conversions of 96.0% and 96.5%, respectively, at 650 °C.⁷²

Fig. 7 The performance of the ICCU-DRM reaction for the Ni–CaO bifunctional catalyst. (A) Syngas yield and the molar ratio of H₂ to CO and (B) CO₂ and CH₄ conversion with the molar ratio of their consumption.¹⁶

Fig. 8 Schematic diagram of the reaction mechanism of ICCU-DRM on the Ni–CaO catalyst.¹¹

Sintering of active metals and carbon deposition at high temperature often lead to the deactivation of bifunctional catalysts, which limits the application of the ICCU-DRM process. Kawi et al. synthesized a porous CeO₂-modified CaO microsphere loaded with Ni as a bifunctional catalyst.⁷³ The optimized 5Ni–CeO₂/CaO with an 85 : 15 molar ratio of Ca/Ce showed constant CO₂ uptake while retaining ∼80–90% of its initial syngas yield over 9 cycles at 650 °C. The introduction of CeO₂ as a promoter enhanced the CaO affinity to CO₂ and the reducibility of Ni, thereby significantly improving the low-temperature activity for DRM at 650 °C. Additionally, CeO₂ can also be used as a structure stabilizer to improve the sintering resistance of Ni and CaO particles, thus maintaining the stability of the ICCU-DRM reaction. Later on, they prepared a novel Ni–CaO-based bifunctional catalyst by co-loading CeO₂ and Ni particles onto ZrO₂-coated CaCO₃.⁷⁴ The ZrO₂ layer could stabilize Ni and CaO particles to maintain catalytic activity and CO₂ capture capacity. The addition of CeO₂ can eliminate carbon deposition and activate CH₄ and CO₂. Recently, they synthesized a mesoporous Ni–CaO bifunctional catalyst by pre-doping trace amounts of Ni into the CaO matrix, which realized the tandem distribution of Ni in the CaO framework.⁷⁵ The ICCU-DRM performance indicated that the obtained Ni/NiCa had 38% higher CO₂ adsorption capacity and 35% higher CO₂ and CH₄ conversions than Ni/Ca. Meanwhile, higher stability was also observed on the Ni/NiCa bifunctional catalyst.

Mendoza-Nieto et al. applied alkaline zirconates (Li₂ZrO₃ and Na₂ZrO₃) as bifunctional catalysts to the ICCU-DRM process.⁷⁶ They found that Na₂ZrO₃ had the higher CO₂ capture. H₂ was produced via the DRM reaction at 450–750 °C and the partial oxidation of methane (POM) above 750 °C. In another report, they further studied the NiO doping on Na₂ZrO₃ for the ICCU-DRM reaction.⁷⁰ It was shown that the addition of NiO increased the H₂ production. Moreover, a sharp decrease in reaction temperatures was observed for the NiO-doped Na₂ZrO₃. Although sodium zirconate or lithium silicate materials can be used as bifunctional catalysts in ICCU-DRM, their poor adsorption capacity will limit their application compared to high capacity CaO-based materials.

3.2. Bifunctional catalysts for ICCU-DRA

Ethane (C₂H₆) and propane (C₃H₈) are also used as reactants in the ICCU-DR process. Rezaei et al. prepared γ-Al₂O₃ supported Ni-impregnated CaO and MgO as bifunctional catalysts and successfully applied them in integrated CO₂ capture and DR of ethane (ICCU-DRE) to produce syngas.⁷⁷ The performance results indicated that the adsorbed CO₂ could be released completely. And the best performing sample exhibited 100% conversion of C₂H₆ and 65% conversion of CO₂ at 650 °C. Meanwhile, the H₂/CO ratio of the obtained syngas was 1.30, meeting the requirement for major chemical production processes. They further synthesized K–Ca and Cr-impregnated H-ZSM-5 bifunctional catalysts for the production of C₂H₄ by the ICCU-DRE process.⁷⁸ The performance experiments were conducted under adsorption conditions of 600 °C and 10% CO₂/Ar and reaction conditions of 700 °C and 5% C₂H₆/Ar. On the optimal bifunctional catalyst, the CO₂ adsorption capacity was 5.2 mmol g⁻¹, the conversion of C₂H₆ was 25%, and the selectivity to C₂H₄ was 88%, respectively. Recently, they proposed 3D-printed metal-oxide-CaO/ZSM-5 (V, Ga, Ni, and Ti) structured monoliths as bifunctional catalysts.⁶⁷ V–CaO/ZSM-5 as the best sample achieved a high CO₂ capture of 5.4 mmol g⁻¹. Meanwhile, 36.5% C₂H₆ conversion, 65.2% CO₂ conversion, and 98% C₂H₄ selectivity with a 35.8% yield of C₂H₄ were also shown.

The CO₂ capture process can also be coupled with the dry reforming of propane reaction (ICCU-DRP).^68,79 Hu et al. conducted the study of ICCU-DRP on Pt/ZrO₂–5CaO to produce syngas.⁶⁸ The result showed that the CO₂ adsorption capacity of Pt/ZrO₂–5CaO was 10.3 mmol g⁻¹, which could be completely converted to CO at 600 °C. Meanwhile, they found that 77% of C₃H₆ was reformed with CO₂, and 23% of C₃H₈ underwent dehydrogenation. The high dissociation activity of Pt to C–H and C–C bonds might be the reason responsible for the low productivity of C₃H₆. Rezaei et al. reported that C₃H₆ could be produced from the ICCU-DRP process with 3D-printed CaO/M@ZSM-5 (M = Ni-, Ti-, Ga-, or V-oxide) monoliths as bifunctional catalysts.⁷⁹ Among them, 3D-printed CaO/Ti@ZSM-5 achieved 76% conversion of CO₂ and 39% selectivity to C₃H₆. Besides alkanes, Yu et al. demonstrated that an ethanol and glycerol mixture could be used as a hydrogen donor in the ICCU-DR reaction.⁸⁰ Nearly 100% conversion of the ethanol and glycerol mixture and 92% syngas selectivity with a 1.2 ratio of H₂/CO were obtained on the 10Ni/CaCO₃ bifunctional catalyst.

4. Bifunctional catalysts for integrated SESR and CO₂ conversion

Although the mixture of CO₂ and N₂ is often used for CO₂ adsorption in ICCU processes, there is also ongoing research on the effects of O₂ and H₂O by studying simulated flue gas and air. The ICCU can be further combined with other CO₂-producing processes, such as the sorption-enhanced steam reforming (SESR) reaction.^15,81 The SESR reaction can produce high-purity hydrogen (>96%) through a single-step integration of SR, WGS and in situ CO₂ capture.^82,83 To continuously produce hydrogen by SESR, a decarbonation step must be performed to regenerate the adsorbent (usually CaO) or bifunctional catalyst. The decarbonation reaction typically occurs at high temperatures (>700 °C) in an inert atmosphere to decompose carbonates and release CO₂, resulting in the dilution of captured CO₂ by carrier gas, which poses challenges for further CO₂ utilization. The integrated SESR reaction and CO₂ utilization (ICCU-SESR) offers a promising solution. Yu et al. demonstrated the technical feasibility of the coupled process between SESR of glycerol (SESRG) and dry reforming of methane (ICCU-SESRG, Fig. 9) with Ni–CaO–Ca₁₂Al₁₄O₃₃ as a bifunctional catalyst.¹⁴ 98.53% high-purity H₂ was produced during SESRG at 550 °C, and 95.3% conversion of CH₄ and syngas with a H₂/CO molar ratio of less than 2 were achieved during DRM at 700 °C. Meanwhile, the ICCU-SESRG process was stable over ten cycles. Later on, they reported that the temperature of DRM could be lowered to 650 °C by increasing the decomposition activity of CH₄ over a Pd-promoted Ni–Ca–Al bifunctional catalyst in the ICCU-SESRG process,⁴ resulting in co-production of a hydrogen-enriched stream (98.3%) and syngas with a H₂/CO molar ratio less than <3. Meanwhile, the sorption-enhanced effect only decreased by 25% after ten cycles due to the lower regeneration temperature. Wu et al. reported that CO₂ could be simultaneously captured by CaO in the production of syngas from lignin gasification.⁵ And these captured CO₂ can be effectively converted into CO to achieve zero CO₂ emissions.

Fig. 9 Schematic diagram of the ICCU-SESRG process.¹⁴

5. Outlook

Bifunctional catalysts play a crucial role in ICCU processes, determining the unique performance advantages and system efficiency. Therefore, great efforts have been made to develop bifunctional catalysts. The ICCU-methanation reaction is often operated at intermediate temperatures (200–400 °C) to balance the kinetics and endothermic nature of the methanation reaction. Alkali earth metals (Na, K and Li) and MgO, as adsorbent components of bifunctional catalysts, are more suitable for the ICCU-methanation process because of the CO₂ adsorption properties at low and medium temperatures. The main problems faced by these bifunctional catalysts are their low adsorption rate and cycling stability. Dispersing them on supports with high specific surface areas and porous structures is currently the main optimization method, but it is necessary to pay attention to the compromise between the carrier and the amount of adsorbent. In addition, it may be more potential to realize thin-layer bifunctional catalyst by optimizing the preparation method. CaO-based bifunctional catalysts can also be used in ICCU-methanation due to their excellent CO₂ adsorption capacity. However, the high stability of CaCO₃ requires that the ICCU-methanation reaction is operated at high temperatures (>550 °C) to achieve appropriate kinetics, which leads to the occurrence of RWGS, so the selectivity to CH₄ is challenged. The key to the optimization of CaO-based bifunctional catalysts is to achieve fast kinetics and high CH₄ selectivity of the ICCU-methanation reaction by improving the methanation performance of bifunctional catalysts at low temperatures.

The ICCU-RWGS process is usually operated at high temperatures due to the endothermic nature of the RWGS reaction, and CaO-based bifunctional catalysts are most commonly used in this process. Although the ICCU-RWGS reaction can be realized over transition-metal-free bifunctional catalysts with high selectivity to CO, the high conversion of captured CO₂ is difficult to achieve because of the rapid decomposition of CaCO₃ and the limited kinetics of CO formation at high temperatures (>650 °C). Transition-metal-containing bifunctional catalysts, especially Ni–CaO, provide more choices to realize high CO selectivity and CO₂ conversion by lowering the reaction temperature (500–600 °C). On the other hand, the low temperature can also inhibit the deactivation caused by sintering at high temperatures, which is another key problem of CaO-based bifunctional catalysts for ICCU-RWGS. Therefore, the ICCU-RWGS process focuses on developing bifunctional catalysts that inhibit CH₄ by-products at low temperatures to simultaneously achieve high CO₂ conversion, CO selectivity and stability.

ICCU-DRA is a more attractive and potential process because it can produce high value-added products, especially C₂H₄ and C₃H₆. CaO-based materials are the most commonly used bifunctional catalysts in ICCU-DRA processes due to the highly endothermic nature of dry reforming reactions requiring high operating temperatures. Compared with ICCU-methanation and ICCU-RWGS, ICCU-DRA is a more complex process with side reactions, so the selectivity to the product needs to be regulated. Additionally, carbon deposits also need to be considered. The work of bifunctional catalysts for ICCU-DRA is mainly focused on the optimization of catalytic performance by adjusting the active metal components. To enable the ICCU-DRA process, three major issues of bifunctional catalysts remain to be solved: (1) high selectivity to products, especially the co-production of lower olefins; (2) high stability, including the resistance to sintering and carbon deposition; (3) isothermal operation from CO₂ capture to conversion. Meanwhile, further studies can be conducted to couple different sources of CO₂ such as flue gas, air and the sorption-enhanced steam reforming (SESR) reaction in the ICCU process.

Data availability

No primary research results, software or code have been included and no new data were generated or analysed as part of this review.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 22078106), the Natural Science Foundation of Guangdong Province (2024B1515040016, 2024A1515030255) and the Guizhou Provincial Science and Technology Plan Project Natural Science Key Project (ZK[2022] 028).

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