Integrated Technology of CO₂ Adsorption and Catalysis

Abstract

This paper discusses the integrated technology of CO₂ adsorption and catalysis, which combines adsorption and catalytic conversion, simplifies the traditional process, reduces energy consumption, and improves efficiency. The traditional carbon capture technology has the problems of high energy consumption, equipment corrosion, and absorbent loss, while the integrated technology realizes the adsorption, conversion, and catalyst regeneration of CO₂ in a single reaction system, avoiding complex desorption steps. Through micropore confinement and surface electron transfer mechanism, the technology improves the reactant concentration and mass transfer efficiency, reduces the activation energy, and realizes the low-temperature and high-efficiency conversion of CO₂. In terms of materials, MOF-based composites, alkali metal modified oxides, and carbon-based hybrid materials show excellent performance, helping to efficiently adsorb and transform CO₂. However, the design and engineering of reactors still face challenges, such as the development of new moving bed reactors. This technology provides a new idea for CO₂ capture and resource utilization and has important environmental significance and broad application prospects.

1. Introduction

With the rapid development of society, various industries are booming. In recent years, the global GDP has increased rapidly. From 2014 to 2024, the global GDP has increased by 100% [1]. But, at the same time, for the rapid development of economy and the happy life of mankind, environmental problems have also suffered serious damage, including water pollution [2], soil pollution [3] and air pollution [4]. Among them, air pollution is particularly serious, such as acid rain [5], haze [6], secondary pollution [7], and global warming [8]. In recent years, climate warming has seriously affected human normal life. Since the middle of the 20th century, the annual emission of burning fossil fuels has increased every decade [9], from nearly 11 billion tons per year in the 1960s to an estimated 36.6 billion tons in 2023 [10]. In 2024, the global climate was significantly affected by the El Nino phenomenon, which was manifested in the intensification of global warming caused by the abnormal rise in sea water temperature [11,12]. This year not only broke the temperature control target of the Paris Agreement but also set a record daily maximum temperature of 17.16 °C [13]. The Scripps Institution of Oceanography (SIO) of the United States released data showing that, in April 2022, the concentration of carbon dioxide in the atmosphere reached the highest level since human history [14]. The observation of the SIO is based on the Keeling curve, which is a curve drawn by scientists measuring the concentration of carbon dioxide in the atmosphere in Hawaii. It has been observed since 1958 and has become the first evidence to confirm the rapid increase of carbon dioxide concentration [14]. According to the observation of the SIO, by April 2022, the average monthly carbon dioxide concentration in the atmosphere had reached 420.23 ppm, which was much higher than the 315.00 ppm recorded by the Keeling curve in 1958 [15]. After the end of 2024, the carbon dioxide concentration monitored by the Mauna Loa Global Atmospheric Background Observatory in Hawaii will increase by about 3.6 ppm to 427 ppm, much higher than the 280 ppm monitored before the climate crisis caused by the large-scale combustion of fossil fuels. In order to reduce the global greenhouse effect, many researchers have begun to develop a variety of carbon removal technologies. At present, the most promising technology is carbon capture, utilization, and storage technology (CCUs) [16].

Carbon capture technology (CC) mainly includes post-combustion capture, pre-combustion capture, and oxygen-enriched combustion capture [17]. Post-combustion capture technology mainly uses chemical absorbents (such as amine absorbents) to absorb carbon dioxide in flue gas [18]. In the absorption process, a large amount of flue gas needs to be fully contacted with the absorbent, which requires a lot of energy to drive the gas-liquid mass transfer process in the absorption tower [19]. Amine absorbents must be heated to release CO₂. This regeneration step is highly energy-intensive. Generally, each ton of carbon dioxide captured needs to consume about 3.0–3.5 gigajoules (GJ), which significantly increases the energy consumption cost of the whole carbon capture process [20]. Pre-combustion capture is the process of first gasifying hydrocarbon fuels such as coal, natural gas, or biomass with oxygen (or pure oxygen obtained from air separation) at high temperature and high pressure to obtain “syngas” mainly composed of CO and H₂. Next, let the synthesis gas undergo a water gas shift reaction (CO+H₂O → CO₂+H₂) to convert the majority of the carbon elements into high concentrations of CO₂. Subsequently, CO₂ is separated, compressed, stored, or utilized using physical or chemical absorbents such as Selexol, Rectisol, MDEA, etc. The remaining “clean fuel” containing almost only H₂ is then used to generate electricity in gas turbines or boilers. In order to improve the absorption efficiency, it is often necessary to pretreat the flue gas, using methods such as cooling, dust removal, etc. These pretreatment processes also need to consume additional energy. Amine absorbents are corrosive to a certain extent and will cause corrosion to equipment such as absorption towers and pipelines. Long-term operation may lead to equipment damage and increase equipment maintenance and replacement costs [21]. For example, some amine absorbents will accelerate the corrosion rate of metal equipment in environments of high temperature and high humidity, which will shorten the service life of the equipment. In addition, the volatility of amine absorbents also brings certain potential safety hazards [22,23]. If the absorbent leaks, it will cause harm to the environment and the health of operators, such as respiratory tract and skin irritation [23]. Absorbent loss via flue gas or thermal degradation raises costs and lowers capture efficiency. The regeneration efficiency of the absorbent is also a key issue. If the regeneration is not complete, the absorption capacity of the absorbent will be reduced, and the absorbent needs to be replaced or supplemented frequently, further increasing the operation cost and complexity [24,25,26].

Pre-combustion capture technology usually requires gasification or partial oxidation of fossil fuels to first convert them into syngas (the main components are carbon monoxide and hydrogen), then convert carbon monoxide into carbon dioxide and more hydrogen through a conversion reaction, and finally separate carbon dioxide through physical or chemical methods [27,28,29]. This series of reaction processes requires complex process equipment, such as a gasifier, shift reactor, separation device, etc., and the equipment investment cost is high. For example, the construction of a large-scale capture device before coal gasification and combustion may cost hundreds of millions of dollars, which is a huge economic burden for many enterprises and limits the wide application of this technology [30]. Pre-combustion capture technology has strict requirements on fuel quality. Impurities in fuel (such as sulfur and nitrogen) will affect the efficiency and product quality of gasification and shift reaction. For example, sulfide may cause catalyst poisoning, reduce the activity of the shift reaction, and then affect the generation and separation of carbon dioxide. In order to meet the requirements of pre-combustion capture technology, the fuel needs to be pretreated to remove impurities, which adds additional processing costs and process complexity [31].

Oxygen-enriched combustion capture technology needs to provide high-purity oxygen to replace air for combustion, so as to produce a high concentration of carbon dioxide flue gas for subsequent capture [32]. At present, air separation technology is mainly used to produce oxygen in industry, which requires a lot of energy [33]. Oxygen-enriched combustion requires high-purity oxygen, which is typically produced through air separation techniques such as low-temperature distillation, membrane separation, or adsorption separation. Low-temperature distillation technology is mature but consumes high energy, requiring air to be cooled to extremely low temperatures (about −196 °C) for separation. Membrane separation technology is relatively energy saving, but it requires high-temperature operation, and the performance and durability of membrane materials still need to be improved. Although adsorption separation technologies such as pressure swing adsorption and temperature swing adsorption operate at lower temperatures, they require frequent pressure or temperature changes to regenerate the adsorbent, and their energy consumption is also not low. In the process of oxygen-enriched combustion, due to the high concentration of oxygen, the combustion temperature is usually higher than that of traditional air combustion. This may require additional equipment and technology to handle high-temperature combustion products, such as high-temperature heat exchangers and refractory materials. Although high-temperature combustion can improve thermal efficiency, an effective heat recovery system is needed to utilize this heat; otherwise, it will lead to energy waste [34]. Oxygen-enriched combustion will change the combustion characteristics, with faster combustion speed and higher temperature, which may increase the thermal stress of combustion equipment and affect the stability and service life of combustion equipment. For example, in combustion equipment such as boilers, oxygen-enriched combustion may raise the furnace wall temperature and increase the thermal fatigue risk of furnace wall materials. The high concentration of oxygen also brings certain potential safety hazards, such as increasing the risk of fire and explosion [34]. In the oxygen-enriched combustion system, it is necessary to strictly control the oxygen concentration and mixing ratio to prevent danger caused by excessive local oxygen concentration. In addition to the high concentration of carbon dioxide, the flue gas generated by oxygen-enriched combustion may also contain other pollutants, such as nitrogen oxides (NOx), water vapor, etc. The presence of these pollutants will increase the difficulty and cost of flue gas treatment. For example, the emission of nitrogen oxides needs to be treated by a special denitration device, and the condensation and separation of water vapor also needs to consume additional energy and equipment [35,36]. These bottlenecks of traditional carbon capture technology limit its large-scale and low-cost popularization and application. Therefore, it is necessary to continuously research and develop new technologies or optimize existing technologies to improve the efficiency and economy of carbon capture.

Stepwise adsorption-catalysis is a multi-stage method for gas purification and pollutant removal. [37,38]. However, the energy consumption pain point of this process is mainly concentrated in the thermal desorption link, especially when the thermal desorption energy consumption accounts for more than 60% of the total system cost. Under the programmed heating conditions of 150–250 °C, Hopcalite requires approximately 0.35–0.40 kJ/g of adsorbent and 0.9–1.1 kJ/μmol of toluene oxidation. CeO₂ nanorods have low catalytic activity and slightly lower energy consumption per unit mass (≈0.30 kJ g⁻¹), but the amount of toluene oxidation increases to 5–6 kJ μmol⁻¹. UiO-66-SO₃H can be regenerated at 150–200 °C, with the lowest energy consumption per unit mass (≈0.25 kJ g⁻¹), and the energy consumption per μmol of toluene is comparable to Hopcalite (0.8–1.0 kJ). Converted into energy consumption per ton of CO₂, all three fall within the range of 0.6–1.0 GJt⁻¹ CO₂, significantly better than MEA’s 3.0–3.5 GJt⁻¹ CO₂. But overall, the energy consumption for regeneration is relatively high [38]. In the step-by-step adsorption catalysis process, after adsorbing a certain amount of pollutants or target substances, the adsorbent needs to restore its adsorption capacity by thermal desorption. The desorption process of many adsorbents (such as activated carbon, molecular sieve, etc.) needs to be carried out at a higher temperature, usually 150–300 °C or even higher [39]. For example, some high-capacity activated carbon adsorbents may need to be heated to more than 250 °C in order to completely desorb the adsorbed organic matter. This high-temperature condition requires a large amount of heat input, resulting in a significant increase in energy consumption. High desorption temperature not only increases the direct heating energy consumption but may also cause a series of thermal management problems [40].

The high energy consumption of the thermal desorption process directly leads to a significant increase in energy costs. If the thermal desorption energy consumption accounts for more than 60% of the total system cost, it means that most of the operating costs are used to provide the heat required for desorption [41,42,43]. Huang et al. [40] found that the Pd/AC system requires only 0.10 kWh of thermal energy to oxidize 1 ton of CO₂. Calculated at 0.05 USD·kWh⁻¹, the regeneration cost is approximately USD 0.005·t⁻¹ CO₂, which is only one thousandth of that of traditional amine absorbents (USD 5–6·t⁻¹ CO₂). For example, when natural gas or electric energy is used as heat source, the fluctuation of the energy price will directly affect the operating cost of the system. Taking electric heating as an example, assuming that the desorption process needs to consume 100 kwh of electric energy per hour and the electricity price is 0.1 US dollars/kWh, the hourly cost of thermal desorption alone is as high as USD 10. If the system runs 24 h a day, the energy consumption cost of thermal desorption alone is as high as USD 240, accounting for the vast majority of the total cost of the system. In order to meet the demand of high energy consumption for thermal desorption, the system needs to be equipped with efficient heating equipment and a thermal management system. The purchase cost of this equipment is high; for example, high-efficiency electric heaters, heat exchangers, and thermal insulation materials require a large investment. In addition, the frequent heating and cooling process will lead to thermal fatigue and wear of the equipment, increasing the maintenance and replacement costs of the equipment. For example, the heating elements may be damaged due to long-term high-temperature operation and need to be replaced regularly, which further increases the operating cost of the system.

The research and development of adsorbents that can efficiently desorb at low temperatures is one of the key directions to reduce the energy consumption of thermal desorption. For example, by modifying activated carbon [43] and developing new metal-organic framework (MOF) adsorbents [44], the desorption temperature of adsorbents can be reduced, thus reducing the energy input required for thermal desorption. Some studies have shown that, by introducing specific functional groups or composite materials [45,46] on the surface of activated carbon, the desorption temperature can be significantly reduced while maintaining a high adsorption capacity. This provides a new way to reduce the energy consumption of thermal desorption.

The integrated technology of CO₂ adsorption and catalysis is an innovative technology that organically combines the adsorption and catalytic conversion process of CO₂ [47,48]. It effectively simplifies the traditional multi-step CO₂ treatment process by realizing the adsorption, conversion, and catalyst regeneration of CO₂ in the same reaction system [49]. There are two core ideas for achieving CO₂ adsorption and conversion on the same solid: first, saturate with pure CO₂ or CO₂-containing gas flow adsorption and then switch to gases such as H₂, CH₄ and heat up to trigger the reaction; alternatively, a mixture of CO₂ and reactants can be introduced from the beginning to achieve simultaneous adsorption and reaction. In the traditional CO₂ treatment process, after the adsorbent adsorbs CO₂, a desorption operation is often required to realize the regeneration of the adsorbent. This not only increases the complexity of the process but also decreases the performance of the adsorbent due to changes in temperature, pressure, and other conditions during desorption. However, the in situ conversion method of the integrated technology of adsorption and catalysis of carbon monoxide directly converts the adsorbed carbon monoxide on the adsorbent without separate desorption steps [50,51]. For example, in some adsorption catalytic integrated systems based on metal-organic frameworks (MOFs), the adsorbed CO₂ can directly participate in the catalytic reaction in the pores of MOFs, omitting the desorption link and greatly simplifying the whole process [52].

The desorption process usually requires a lot of energy to change the adsorption equilibrium between the adsorbent and CO₂. For example, thermal desorption requires heating the adsorbent to release the adsorbed CO₂. However, the high energy consumption desorption process is avoided by the integrated technology of CO₂ adsorption and catalysis [53]. Taking the chemical adsorbent as an example, the adsorbed CO₂ can be converted into more valuable chemicals (such as methanol, carbonate, etc.) through in situ catalytic conversion after it adsorbs CO₂. During the conversion process, no additional high-temperature or high-pressure conditions are required to desorb CO₂, thus significantly reducing the energy consumption of the whole process [54,55]. The desorption process may lead to a large amount of CO₂ release. If it is not properly controlled, it may cause safety problems, such as gas leakage and other dangerous situations in the process of high-pressure desorption. The in situ conversion method avoids such a large number of CO₂ release links, making the whole process safer and more controllable. For example, in some industrial tail gas treatment scenarios, the integration of adsorption and catalysis technology can complete the treatment of CO₂ in a closed reaction system, reducing the risk of CO₂ leakage.

Three configurations can be used to complete the adsorption reaction on the same solid: Fixed bed alternating circulation—by switching valves, the same particle is first enriched with CO₂ and then reacted with reducing gas. The equipment is simple but requires a high-pressure valve group. Solid-state circulation—separating the adsorber from the reactor, allowing particles to flow continuously between the two vessels with a low pressure drop and continuous operation, suitable for large-scale facilities. Single reactor coupling—simultaneous adsorption and reaction in a fluidized bed, with the simplest structure but high difficulty in rate matching.

In the process of closed-loop circulation, the adsorbent can be used repeatedly. Each adsorbed CO₂ can be effectively transformed, and the transformed adsorbent can re-adsorb CO₂. For example, when zeolite molecular sieve is used as an integrated material of adsorption and catalysis, the pore structure of zeolite molecular sieve can not only adsorb CO₂ but also serve as a place for catalytic reaction. After catalytic conversion, the structure of zeolite molecular sieve remains basically unchanged and can re-adsorb new CO₂, realizing the efficient recycling of adsorbent and improving the utilization rate of resources [56,57]. In addition to the solid circulation scheme, a fixed bed can also be used, and the same adsorbent particles can undergo adsorption and desorption/reaction steps in sequence through valve switching. Some wastes may be generated by traditional CO₂ treatment methods, such as waste adsorbents. The closed-loop circulation mode of the integrated technology of CO₂ adsorption and catalysis makes the adsorbent continuously recycled in the whole process, reducing the generation of waste adsorbents. Moreover, through catalytic conversion, CO₂ is converted into useful chemicals, which avoids the emission of CO₂ as a greenhouse gas and is more friendly to the environment. For example, in the process of converting CO₂ into methanol, methanol is an important chemical raw material, realizing the resource utilization of waste [58]. The closed-loop design makes the whole system more stable. The performance of the adsorbent can remain relatively stable during the cycle, because each adsorption conversion regeneration process is carried out under similar conditions. Compared with the traditional step-by-step treatment process, the integrated technology reduces the system fluctuation caused by improper connections between different steps. For example, in the continuous industrial production process, the integrated technology of adsorption and catalysis can stably process the gas containing CO₂, ensuring the continuity of production and the stability of product quality [59].

2. Technical Principle and Coordination Mechanism

Microporous confinement refers to that in the micropores of microporous materials such as metal-organic frameworks (MOFs) [60], molecular sieves [61], and porous carbon materials [62]; the reactant molecules are confined in a limited space. The size of these micropores is usually at the nanometer level, which can significantly restrict and regulate the reactant molecules.

In the micropore-confined environment, CO₂ molecules are adsorbed to the inner surface of micropores. Due to the limited volume of micropores, the local concentration of CO₂ molecules in micropores will be significantly higher than that in the external environment. For example, in some MOFs with a high specific surface area and rich microporous structure, CO₂ molecules can be densely adsorbed on the inner wall of micropores. This high local concentration makes it easier for the reactant molecules to contact with other reactants or catalysts in the micropores, thus accelerating the reaction rate. Taking MOFs as an example, their microporous structure can be like a “molecular cage” to confine CO₂ molecules in a specific space, making the local concentration of CO₂ molecules in the micropores several times or even dozens of times higher than in the free space. This high local concentration provides more favorable conditions for the catalytic reaction [60]. Micropore confinement not only improves the local concentration of reactants [63] but also enhances the interfacial mass transfer efficiency. In the micropore, the interaction between the CO₂ molecule and the inner wall of the micropore (such as van der Waals force, hydrogen bond, etc.) shortens the diffusion path of the molecule and reduces the mass transfer resistance. This enhancement of interfacial mass transfer enables reactants to reach the active site of the catalyst faster, thus improving the overall efficiency of the reaction. For example, in porous carbon materials, the microporous structure can provide a large number of active sites, and, at the same time, it is easier for CO₂ molecules to contact these sites through the confinement effect. This enhancement of interfacial mass transfer enables the reaction to be completed in a shorter time, improving the selectivity and yield of the reaction [62].

Microporous confinement can significantly improve the reaction rate by increasing the local concentration of reactants and enhancing the interfacial mass transfer efficiency [64]. In the integrated technology of adsorption and catalysis, this means that the conversion of CO₂ can be completed in a shorter time, which improves the processing efficiency of the whole system [65]. For example, in the synthesis of methanol from CO₂ hydrogenation, the reaction rate can be increased several times or even higher by using microporous confined catalysts. This is because the high local concentration in the micropores makes it easier for CO₂ molecules to react with hydrogen molecules. At the same time, the enhancement of interfacial mass transfer also reduces the residence time of reactants on the catalyst surface and improves the reaction efficiency. Microporous confinement can also improve the selectivity of the reaction [66]. Because the size and shape of micropores can screen reactant molecules, only molecules with an appropriate size can enter the micropores to participate in the reaction. This size selectivity can reduce the occurrence of side reactions and improve the selectivity of the target product [67]. For example, in the cycloaddition reaction of CO₂ with epoxy compounds, microporous confined catalysts can selectively promote the reaction of CO₂ with specific epoxy compounds and inhibit the occurrence of other side reactions. This is because the space limitation in the micropores allows only suitable reactant molecules to bind to the active sites of the catalyst, which improves the selectivity of the reaction. Microporous confinement can reduce the activation energy by increasing the local concentration of reactants and enhancing the interfacial mass transfer efficiency. This means that the reaction can be carried out at a lower temperature, thus reducing energy consumption [68].

CO₂ molecules first bind to Lewis acid-base sites on the catalyst surface by physical or chemical adsorption. Physical adsorption mainly depends on van der Waals force, while chemical adsorption involves electron transfer. For example, on the surface of metal oxide catalysts, CO₂ molecules can be adsorbed by forming chemical bonds with surface oxygen atoms (Lewis base sites) or metal cations (Lewis acid sites). The adsorbed CO₂ molecule is activated by electron transfer under the action of Lewis acid-base sites. Specifically, the Lewis acid site can accept the electron pair of the CO₂ molecule, which reduces the density of the electron cloud of the C=O bond and weakens the strength of the C=O bond; Lewis base sites can provide electron pairs to further promote the activation of CO₂ molecules. For example, by adjusting the type and density of Lewis acid-base sites, the activation mode of the CO₂ molecule can be controlled to achieve different catalytic conversion products. For example, in the cycloaddition reaction between CO₂ and epoxy compounds, the Lewis acid site on the catalyst surface can promote the activation of CO₂ molecules, making it easier to react with epoxy compounds to produce products such as cyclic carbonates [69].

3. Progress of Key Material System

3.1. Metal–Organic Framework (MOF)-Based Composites

Metal–organic framework (MOF)-based composites have become ideal materials for the adsorption and conversion of carbon dioxide (CO₂) gas due to their high specific surface area, rich porous structure, adjustable chemical active sites, and excellent thermal and chemical stability [70,71]. These characteristics not only improve the adsorption capacity and selectivity of CO₂ but also enhance the long-term stability of the material under harsh conditions. Li et al. [72] prepared two nickel-based metal-organic framework (MOF) materials, Ni-MOF-74 and Ir-MOF-74, and carried out a detailed study on the adsorption performance of carbon dioxide (CO₂), as well as its kinetics and thermodynamics. It was found that the two materials exhibited high CO₂ adsorption capacity at different optimal activation temperatures, which were 1.9401 mmol/g and 1.9475 mmol/g (at 30 °C and 0.16 bar), respectively. The pseudo-first-order kinetic model and the Langmuir isothermal model were used to fit the experimental data. It was revealed that the CO₂ adsorption process of the two materials followed the pseudo-first-order kinetic model, and there was entropy enthalpy complementarity in the CO₂ capture process.

In addition, MOF-based composites can achieve multi-functional synergy by compounding with other materials, such as promoting their catalytic conversion while adsorbing CO₂, improving conversion efficiency, and reducing energy consumption. Li et al. [69] studied a porous metal-organic framework (MOF) with Lewis acid-base bifunctional sites for the efficient adsorption of carbon dioxide (CO₂) and catalytic conversion to cyclic carbonate. The research team successfully constructed a three-dimensional (3D) MOF [Ni₃(BTC)₂(MA)(H₂O)](DMF)₇ (denoted as Ni (II)-MOF-1) composed of 1,3,5-benzoic acid (H₃BTC), melamine (MA), and Ni (II) ions by the solvothermal method. The material has a rich microporous structure and high CO₂ adsorption capacity. Its Lewis base property and a large number of micropores give it excellent affinity for CO₂. At room temperature, Ni (II)-MOF-1 exhibited high catalytic efficiency for the cycloaddition reaction of CO₂ with small epoxy compounds. In addition, Ni (II)-MOF-1 also showed good stability and recyclability, and its catalytic activity did not decrease significantly after repeated recycling. Based on the experimental results, the research team proposed the synergistic mechanism of a Ni(II)-MOF-1/Bu₄NBR catalyst system in the conversion of carbon monoxide; that is, the Ni (II) site acts as Lewis acid to activate epoxy compounds, while the nitrogen atom in melamine acts as Lewis base to activate carbon monoxide, ultimately promoting the formation of cyclic carbonate. This study provides a new idea for the development of efficient and stable CO₂ adsorption and conversion materials, and it is expected to realize the efficient utilization of CO₂ in industry.

Liu et al. [73] found that, under the treatment of non-thermal plasma (NTP), the structural stability of MOFs (Cu BTC, Zr BPDC, Zn MEIM, and Zr BDC) and their adsorption and conversion performance of CO₂ were greatly affected. Among them, the structure of Cu BTC was damaged under the treatment of NTP, the adsorption capacity of CO₂ decreased by 46.0%, and the catalytic performance was unstable under high input power. While Zr BDC showed excellent structural stability, its crystal and pore structure were almost unaffected by NTP treatment, and the adsorption capacity of CO₂ remained stable. As a catalyst, NTP-assisted CO₂ conversion efficiency was significantly improved, with good stability and selectivity. Jin et al. [74] constructed this ternary composite (PUC) by encapsulating Pt nanoparticles in NH₂-UIO-66 and combining it with Cu TCPP nanosheets, as shown in the Figure 1. This structure changes from the traditional type ii heterojunction to the cascade Z heterojunction, which significantly enhances the separation efficiency and lifetime of photogenerated carriers, thus greatly improving the activity and stability of photocatalytic CO₂ reduction. The experimental results show that the co-generation rate of the PUC under visible light is 40.2 μmol·g⁻¹ h⁻¹, which is nearly five times higher than that of the original type ii heterojunction, and maintains stable performance in three consecutive reaction cycles.

Figure 1. (a) Preparation of ternary PUC nano-heterostructure. (b) The 3D distribution profile of contact potential on PUC particles analyzed by KPFM. The scanning area is represented in (i), while the contours measured before and after the introduction of light are represented in (ii,iii), respectively. (c,d) Schematic diagrams of built-in IEF and heterojunction structures of UC (c) and PUC (d), respectively, which illustrate the transfer path of photogenerated charge carriers. (e) Schematic diagram of cascade electron transfer in the PUC driven by cooperative heterojunction. (f) The relative IEF strength of the sample shows that the significantly enhanced internal driving force promotes the spatial separation of charge carriers in the PUC [74].

Figure 1. (a) Preparation of ternary PUC nano-heterostructure. (b) The 3D distribution profile of contact potential on PUC particles analyzed by KPFM. The scanning area is represented in (i), while the contours measured before and after the introduction of light are represented in (ii,iii), respectively. (c,d) Schematic diagrams of built-in IEF and heterojunction structures of UC (c) and PUC (d), respectively, which illustrate the transfer path of photogenerated charge carriers. (e) Schematic diagram of cascade electron transfer in the PUC driven by cooperative heterojunction. (f) The relative IEF strength of the sample shows that the significantly enhanced internal driving force promotes the spatial separation of charge carriers in the PUC [74].

Liang et al. [52] prepared a new two-dimensional metal-organic framework (MOF) material, especially the monolayer and bilayer porphyrin-based MOF (called MOF1 and MOF2, respectively). As shown in Table 1, the bilayer MOF2 shows 100% selectivity to reduce CO₂ to CO under simulated sunlight, without any cocatalyst or photosensitizer, and can be recycled for at least three times. In addition to CO, a small amount of CH₄ and H₂ were also detected. The photogenerated charge transfer kinetics of MOFs were studied by photoluminescence (PL), time-resolved PL, and electrochemical impedance spectroscopy (EIS). The results show that the photo-generated charge recombination of mof1 is significantly inhibited, and the charge separation efficiency is higher. DFT calculation showed that the Mn metal atom in the porphyrin center was the active site for CO₂ adsorption and catalysis, which effectively improved the CO₂ reduction activity of MOF materials. Porphyrin ligands act as visible light collection units, and photogenerated electrons are transferred to porphyrin metal centers. CO₂ adsorbed on the active sites is reduced to CO. The adsorption energy of the H₂O molecule is weak, which helps to reduce the generation of H₂ and improve the selectivity of CO.

Table 1. Adsorption and catalytic performance of MOFs.

Material	Temperature (°C)	Adsorption Capacity (mmol/g)	Conversion Rate	Ref.
Ni-MOF-74	30	1.9401	95%	[72]
Ir-MOF-74	30	1.9475	95%	[72]
MOF2	30	1.9475	100%	[52]
La-5-SIP-MOF	25	8.47	98%	[71]
Cu-BDC	25	4.72	18%	[73]
Zr-BTC	25	1.64	26%	[73]

Table 1. Adsorption and catalytic performance of MOFs.

Material	Temperature (°C)	Adsorption Capacity (mmol/g)	Conversion Rate	Ref.
Ni-MOF-74	30	1.9401	95%	[72]
Ir-MOF-74	30	1.9475	95%	[72]
MOF2	30	1.9475	100%	[52]
La-5-SIP-MOF	25	8.47	98%	[71]
Cu-BDC	25	4.72	18%	[73]
Zr-BTC	25	1.64	26%	[73]

3.2. Alkali Metal-Modified Oxides

The preparation methods of alkali metal-modified oxides mainly include the coprecipitation method, impregnation method, and physical mixing method. The coprecipitation method involves the following steps: mix alkali metal salt solution with a metal oxide precursor solution, obtain uniform composite materials by controlling precipitation conditions, and then calcine to obtain the catalyst. The impregnation method involves immersing the oxide carrier in an alkali metal salt solution, followed by drying and calcination. These methods can effectively control the distribution and content of alkali metals in oxides, thus affecting the performance of catalysts [75]. Yong et al. [76] synthesized M-Al-PMOF (M=Na, K) composites by adding alkali metals (Na, K) into Al PMOF and systematically studied their effects on the adsorption and photocatalytic reduction of CO₂. It was found that the alkali metal doping significantly increased the specific surface area and the number of alkaline sites of the materials, thus enhancing the adsorption performance of CO₂. In particular, the adsorption capacity of 1-K-Al-PMOF was 45% higher than that of the original Al PMOF. In addition, the doped material shows improvement in light absorption, band gap reduction, and photogenerated electron hole pair recombination inhibition, which significantly improve the photocatalytic reduction efficiency of CO₂ to CO, and the CO yield of 1-K-Al-PMOF is increased by 6.6 times. Li et al. [77] found that ROCS combines the performance of solid oxide fuel cells (SOFCs) and solid oxide electrolysis cells (soecs) to achieve efficient conversion between electrical energy and chemical energy. Using CO/CO₂ as an energy storage medium helps to achieve “carbon neutrality” and renewable energy storage. The working principle is shown in Figure 2. In the study, La_0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF) was modified by doping with low electronegativity alkali metals (Li or Na). The results show that alkali metal doping can effectively improve the oxygen vacancy concentration and oxygen migration ability in LSCF and then enhance its CO₂ reduction kinetics. At 800 °C, the chemical surface exchange coefficients (Kchem) of Li-doped and Na-doped LSCF are 9.06 × 10⁻⁴ and 11.93 × 10⁻⁴ cm s⁻¹, respectively, which are 13.1% and 48.9% higher than those of undoped LSCF. The diffusion coefficient (Dchem) of the chemical body reached 8.86 × 10⁻⁵ and 10.73 × 10⁻⁵ cm s⁻¹, which were 39.7% and 69.2% higher than LSCF. The single cell using Na-doped LSCF as a fuel electrode has an electrolytic current density of 1.07 A cm⁻² at 800 °C/1.5 V and a maximum power density (Pmax) of 207 mW cm⁻² (using 50% Co–50% CO₂ as fuel). In the process of pure CO₂ electrolysis, the single cell showed the best performance of CO₂ electrolysis, the current density was 1.81 A cm⁻² (800 °C/1.5 V), and it still maintained good stability after 20 h durability test under the condition of pure CO₂ as fuel.

Figure 2. (a) Schematic diagram of charge compensation mechanism, (b) carbon dioxide electrolysis reaction mechanism, and (c) schematic diagram of working principle of RSOC energy conversion [72].

Figure 2. (a) Schematic diagram of charge compensation mechanism, (b) carbon dioxide electrolysis reaction mechanism, and (c) schematic diagram of working principle of RSOC energy conversion [72].

Alkali metal-modified oxides can significantly improve the adsorption capacity of CO₂. Cui et al. [78] also synthesized a new type of M−Mg/dobdc (M = Li, Na, K) composite by adding alkali metals (Li, Na, K) into the mg/dobdc metal-organic framework (MOF) and carried out experimental research on its dynamic adsorption performance of CO₂. It was also found that the introduction of alkali metals can coordinate with the active sites of metal centers and show high affinity and activation ability for CO₂. After alkali metal modification, 0.5 K-Mg/dobdc had the best dynamic CO₂ adsorption performance, and the adsorption capacity reached 14.93 mmol/g (0.1 MPa, 25 °C), which is 3.44 times that of unmodified mg/dobdc, which is significantly better than the previously reported CO₂ adsorption performance. In addition, the Yoon Nelson model was used to fit the experimental data to verify the experimental results, and the influencing factors of adsorption were further discussed through the kinetic model and internal particle diffusion model. Bian et al. [79] prepared a composite material based on MgAl layered double hydroxides (LDHs) and improved the adsorption capacity of CO₂ at medium temperature (200–400 °C) by adding carbon molecular sieves (CMSs) and alkali metal nitrates ((Li_0.3Na_0.18K_0.52) No₃, referred to as LINAK). The results showed that, after adding 15% CMS and 30 mol% LINAK, the adsorption capacity of LDH/CMS composites reached 1.32 mmol/g at 300 °C, which was nearly seven times higher than that of unmodified LDH. This significant improvement is attributed to the synergy between CMS and nitrate. The addition of CMS improved the specific surface area and pore structure of LDHs, while the introduction of nitrate significantly enhanced the number of alkaline sites of the material, thereby improving the adsorption capacity of CO₂. In addition, the study also found that the composite exhibited the best adsorption performance after calcination at 550 °C for 3 h, and the adsorption capacity reached the maximum at 300 °C. With the increase in the mg/Al molar ratio, the adsorption capacity increased gradually and reached 1.50 mmol/g when the Mg/Al ratio was 20:1. In particular, in the process of air roasting at 550 °C, the combustion of the CMS and the decomposition of CO₃²⁻ produce CO₂, which reacts with molten nitrate on the surface to generate carbonate ions, further enhancing the adsorption capacity of the material.

In addition, the catalytic regulation of alkali metals (especially potassium and cesium) occurs at the interface between metals and metal oxides, especially in the application of CO₂ hydrogenation. As an effective catalyst promoter, alkali metals can significantly enhance the catalytic activity and adjust the product distribution [70]. The results show that alkali metals can stabilize the key intermediates (such as *HCOO, *COOH, etc.) in the hydrogenation of CO₂ by reducing the work function of the metal surface, enhancing the electronic polarization and electro-static interaction with the reaction intermediates, so as to improve the activation and conversion efficiency of CO₂. For example, at the K/Cu_xO/Cu (111) interface, the introduction of K significantly enhanced the stability of *HCOO and promoted the conversion of CO₂ to formic acid and methanol. In addition, the morphology and distribution of alkali metals will change with the change in reaction conditions. From dispersed clusters to aggregated islands, this structural diversity directly affects its catalytic performance. However, the mechanism of action of alkali metals in actual powder catalysts is still unclear. Future research needs to establish a connection between the model catalyst and the actual catalyst. Through the combination of theoretical calculation and experiment, we can deeply understand the structural changes in alkali metals under reaction conditions and their influence on catalytic performance, so as to provide theoretical guidance for the design of efficient CO₂ hydrogenation catalyst.

Alkali metal modified oxides have shown good application prospects in a variety of CO₂ catalytic conversion reactions, such as the hydrogenation of CO₂ to methanol, the conjugate addition reaction between CO₂ and epoxy compounds, etc. In the hydrogenation of CO₂ to methanol, alkali metal-modified oxides can significantly improve the yield and selectivity of methanol. In addition, alkali metal-modified oxides can also be used for the photocatalytic oxidation of volatile organic pollutants to produce harmless products such as CO₂ [80].

3.3. Carbon-Based Hybrid Materials

As one of the main greenhouse gases, carbon dioxide (CO₂) emission reduction and resource utilization have become the focus of global attention. The catalytic conversion of CO₂ into high-value-added chemicals or fuels not only helps to alleviate the greenhouse effect but also realizes the recycling of carbon resources. Carbon-based hybrid materials show great potential in the field of CO₂ catalysis due to their unique physical and chemical properties and controllable structure [81]. Metal-free carbon-based materials have rich natural resources, high temperature stability, a high specific surface area, excellent conductivity, environmental friendliness, and chemical inertia to acid and alkali. By introducing defects or heteroatoms (such as nitrogen, sulfur, phosphorus, etc.), the electronic structure can be adjusted, and the catalytic performance can be optimized. Zhang et al. [62] prepared a carbon-based catalytic material (BPO₄@BCN-800) designed and synthesized by doping B, N, P, and other main group elements, which has Lewis acid-base sites and Bronsted acid sites and can efficiently catalyze the fixation of CO₂ under mild conditions. The material is prepared by the sol-gel method with porous carbon as the carrier, showing excellent catalytic performance. In particular, the BPO₄@BCN-800 material carbonized at 800 °C shows 99% yield and selectivity for the cycloaddition reaction of CO₂ and epoxides under the condition of KI as the cocatalyst and can also achieve 99% yield at room temperature. Additionally, the catalyst retains its high level of activity even after five cycles, and it has good recyclability and reusability. The study also proposed a possible reaction mechanism, as shown in Figure 3; that is, Lewis acid-base sites and Bronsted acid sites synergize to promote the enrichment and transformation of CO₂.

Figure 3. BPO4@BCN-800 possible mechanism of catalytic epoxide–CO₂ cycloaddition reaction [62].

Figure 3. BPO4@BCN-800 possible mechanism of catalytic epoxide–CO₂ cycloaddition reaction [62].

Metal/carbon hybrid materials are metal nanoparticles or single atoms loaded on carbon substrates, which combine the high activity of metals with the excellent stability and conductivity of carbon materials. For example, Ni/N-doped carbon materials exhibit excellent performance in the reduction of CO₂. David M. Koshy et al. [82] directly proved the monatomic dispersion of nickel (Ni) atoms in nitrogen-doped carbon materials and their coordination structure with nitrogen. It is also found that, in the Ni/N N-doped carbon material (NiPACN), Ni atoms are dispersed in the carbon matrix as a single atom, and Ni and N on carbon based materials, indicating the existence of Ni–N coordination. NiNxCy fragments detected by TOF SIMS further confirmed the existence of the Ni–N bonding environment. The distribution of these fragments is similar to that of the molecular catalyst Ni phthalocyanine (NIPC), indicating that there may be a NiN₄ active site similar to the molecular catalyst in NiPACN. The results provide direct evidence for understanding the active site structure of Ni/N-doped carbon materials in the electrochemical CO₂ reduction reaction (CO₂RR), and they provide important structural information for the design of next-generation heterogeneous catalysts. Hong et al. [83] studied the synergistic effect of carbon nitride (CN) and carbon black (CB) mixed support on the silver (Ag) catalyst in the electrochemical carbon dioxide reduction reaction (CO₂RR). It was found that the mixed carrier not only improved the electrochemical activity of the catalyst but also enhanced the selectivity for carbon monoxide (CO). By loading Ag on a mixed carrier composed of CN and CB, the prepared AgCN₇CB₃ catalyst achieved a Faraday efficiency (FE) of 96% and a partial current density of −287 mA/cm² for CO at −1.6 V, and it maintained stability for 160 h at a current density of −100 mA/cm². Compared with the single support catalyst, the mixed support catalyst showed significant advantages in CO productivity and stability. The results show that CN provides the affinity of CO₂ and improves the dispersion of active metals, while CB provides high conductivity and increased hydrophobicity. The synergistic effect of the mixed carrier not only improves the mass transfer efficiency but also maintains the hydrophobicity of the catalyst, thus promoting the reduction of CO₂ to CO.

Carbon-based materials derived from the metal-organic framework (MOF) are prepared by pyrolysis of MOF precursors, which have a highly ordered pore structure and rich active sites, and can effectively promote the adsorption and activation of CO₂. Ma et al. [84] found that Sb doping mainly exists in the catalytic matrix in the form of an alloy. Trace Sb doping (≤3.1%) can effectively inhibit the formation of *H species and promote the formation of *OCHO species, thus significantly improving the selectivity of Bi-based materials to formic acid. However, it has been demonstrated that excessive Sb doping (≥5%) can exacerbate the formation of *H and enhance the hydrogen evolution reaction (HER), which is deleterious to the formation of formic acid and Bi@C. In comparison with other materials, Sb doped with trace Sb/Bi@C has been shown to exhibit enhanced electrode stability and increased catalytic longevity under electrochemical operation conditions. This phenomenon has been attributed to the substantial enhancement of electrode stability that is observed upon the introduction of Sb Bi@C. In addition, a spherical physical structure of carbon-based nanomaterials is present on the material surface.

In the above study, although the catalyst showed good catalytic performance and cycling stability under laboratory conditions, impurities in flue gas (such as SO₂, NOₓ, particulate matter, etc.) can significantly affect the performance and lifespan of the catalyst in practical applications. These impurities may compete with CO₂ for adsorption sites, reducing the selectivity of the catalyst. The deposition of matter on the surface of the catalyst has been shown to result in the blockage of pores, thereby affecting the performance of the catalyst. Furthermore, the process has been observed to induce undesirable side reactions, leading to the generation of unanticipated by-products. This, in turn, has the potential to further diminish the efficiency and selectivity of the catalyst. For example, SO₂ may undergo irreversible reactions with active sites on the catalyst surface, forming deposits and leading to catalyst deactivation [83]. Therefore, in order to improve the stability and efficiency of catalysts in practical applications, further research is needed on how to effectively remove or reduce the influence of impurities by methods such as pre-treating flue gas, designing more stable catalyst structures, or developing new catalyst materials.

4. Reactor Design and Engineering Challenges

With the global emphasis on carbon dioxide (CO₂) emission reduction and resource utilization, the catalytic conversion technology of CO₂ has become a research hotspot. However, in the process from laboratory to industrialization, the design and engineering of the reactor face many challenges. Yuya Ono et al. [85] developed a moving bed reactor for the hydrogenation of carbon dioxide (CO₂), as shown in Figure 4. The aim is to achieve efficient CO₂ capture and conversion through dual functional materials (DFMs) to improve the concentration of gaseous products such as methane. DFM composed of sodium, nickel, and aluminum oxide was used to conduct a CO₂ hydrogenation experiment in a moving bed reactor. In the experiment, DFM particles pre-adsorbed with CO₂ were continuously fed into the reactor at a rate of 2.5 g/min, and the continuous recovery rate of methane was as high as 57% at 350 °C with a hydrogen flow rate of 65 mL/min. Through the temperature programmed reaction test, it was found that the maximum methane production appeared at 350 °C, which was consistent with the peak methane concentration observed in the moving bed experiment. The results show that the moving bed reactor can effectively overcome the limitations of the fixed bed reactor, which cannot continuously recover products, and the low concentration of products in the circulating fluidized bed reactor. By optimizing the reactor design and operating conditions, it is expected to further improve the methane production, which provides important technical progress for the industrial application of CO₂ capture and conversion.

Figure 4. Schematic diagram of carbon dioxide capture and conversion integrated process using dynamic adsorption materials (DFMs) in a particle circulation system: (a) Circulating fluidized bed system, (b) a mixed system consisting of a fluidized bed (carbon dioxide capture) and a moving bed (carbon dioxide hydrogenation). (c) Schematic diagram of the moving bed reactor for the experiment [85].

Figure 4. Schematic diagram of carbon dioxide capture and conversion integrated process using dynamic adsorption materials (DFMs) in a particle circulation system: (a) Circulating fluidized bed system, (b) a mixed system consisting of a fluidized bed (carbon dioxide capture) and a moving bed (carbon dioxide hydrogenation). (c) Schematic diagram of the moving bed reactor for the experiment [85].

Yolanda a. Criado et al. [86] developed a new type of countercurrent moving bed carbonator, as shown in Figure 5, for capturing carbon dioxide (CO₂) through calcium oxide (CAO) and calcium hydroxide (Ca(OH)₂), so as to achieve the goal of capturing CO₂ from scattered small emission sources and even the atmosphere. The research team conducted experiments in a 3 m-long moving bed reactor with an inner diameter of 0.15 m, using calcium-based adsorbent particles with different particle sizes (mm and cm). Different concentrations of CO₂ (up to 8.5% volume fraction), different gas flow rates (0.5–2 m/s), and inlet gas temperatures (250–630 °C) in the simulated flue gas were tested. The results showed that the carbonation zone was formed at the optimal temperature (550–700 °C), and the capture rate of CO₂ exceeded 99% in some experiments. When Ca(OH)₂ is used as the adsorbent of CO₂, the molar conversion rate of calcium carbonate (CaCO₃) is about 0.7. A reactor model was proposed, and the experimental data were compared with the model prediction. The model can better predict the conversion efficiency of solid and gas. This study provides a new technical scheme for the capture of dispersed small-scale CO₂ emission sources, which is expected to realize the purification of CO₂ and the regeneration of adsorbent by transporting CO₂ in the form of CaCO₃ to the centralized calcination plant.

Figure 5. (a) Process flow chart of decoupled calcium cycle (d-Cal) of distributed carbon dioxide emission source. (b) Pipeline and instrument flow diagram of TRL4-5 moving bed [86].

Figure 5. (a) Process flow chart of decoupled calcium cycle (d-Cal) of distributed carbon dioxide emission source. (b) Pipeline and instrument flow diagram of TRL4-5 moving bed [86].

5. Conclusions

As an innovative carbon dioxide treatment method, the integrated technology of adsorption and catalysis effectively simplifies the traditional multi-step carbon dioxide treatment process by organically combining the adsorption and catalytic conversion process, significantly reducing energy consumption and improving the safety and economy of the system. This technology uses microporous confinement and a surface electron transfer mechanism to improve the local concentration of reactants and the efficiency of interfacial mass transfer, reduce the activation energy of reaction, thus realizing the efficient conversion of CO₂ at a low temperature, and generate high-value-added chemicals, such as methanol and carbonate. In terms of key material systems, metal-organic framework (MOF)-based composites, alkali metal modified oxides, and carbon-based hybrid materials all exhibit excellent CO₂ adsorption and catalytic conversion properties. MOF-based composites with a high specific surface area and rich porous structure can efficiently adsorb and convert CO₂. Alkali metal-modified oxides can further improve the conversion efficiency of CO₂ by enhancing the alkalinity and adsorption capacity of the materials. Carbon-based hybrid materials have broad application prospects in the field of catalytic conversion of carbon monoxide due to their unique physical and chemical properties and controllable structure. However, from laboratory to industrial application, the reactor design and engineering still face many challenges. The development of new reactors, such as the moving bed reactor and countercurrent moving bed carbonator, provides important technical support for improving the conversion efficiency of CO₂ and product recovery. However, alkali metal modification may lead to the decline of the thermal stability of the catalyst, and the poisoning and deactivation of the catalyst still need to be further solved. In general, the integrated technology of CO₂ adsorption and catalysis provides new ideas and solutions for the efficient capture and resource utilization of CO₂, which has important environmental significance and broad application prospects. Future research needs to further optimize the material properties and reactor design, improve the stability and economy of the system, and promote the industrial application of this technology.