Research Advances on Nitrogen-Doped Carbon Materials in CO_x Hydrogenation

Abstract

The excessive consumption of fossil fuels has resulted in massive carbon emissions and serious ecological and environmental crises. Therefore, achieving the efficient utilization of waste carbon sources is considered as an important pathway to addressing the aforementioned issues in the context of carbon neutrality. Developing and designing suitable catalyst materials has become the key to converting CO_x into valuable platform chemicals and value-added liquid fuels (e.g., CO, CH₄, CH₃OH, and C₂₊ hydrocarbons). A moderate interaction between nitrogen-doped carbon materials and active metals is more favorable for the progress of the CO_x hydrogenation reaction compared to traditional metal oxide carriers. In this work, we comprehensively summarize the synthesis methods of N-doped carbon materials and the relevant research progress in the field of CO_x hydrogenation. In addition, a general assessment of carbon-based catalysts for CO_x hydrogenation reactions, concerning the support and metal properties, the activity and product selectivity, and their interactions is systematically discussed. Finally, this review discusses the roles of N-doped carbon materials, the current challenges, and future development directions.

1. Introduction

With the acceleration of industrialization, the demands for energy-related industrial chemicals and fuel is rapidly increasing. However, the production of the aforementioned chemicals and fuels is heavily reliant on traditional fossil energy (e.g., coal, oil, and natural gas), causing considerable CO₂ emissions and a serious energy crisis. To mitigate the greenhouse effect and address the energy crisis, C1 chemistry has become an important and attractive research field in chemistry and catalysis [1,2,3]. Therefore, the conversion of C1 molecules (e.g., CO and CO₂) into high-value chemicals has attracted widespread attention. The hydrogenation of the C1 molecule via the Fischer–Tropsch synthesis (FTS) pathway is considered as one of the promising directions for developing sustainable and clean alternative energy. Therefore, it is necessary to vigorously promote in-depth research on CO₂ conversion and Fischer–Tropsch synthesis.

FTS reactions are regarded as the key to producing clean fuels and high-value-added chemicals via the syngas-mediated pathway [4], which can be produced from coal, biomass, and natural gas in the existing industry [1,5]. The essential steps in the FTS process are the adsorption and activation of CO and H₂, as well as C-C coupling. Generally, the obtained products via FTS reactions follow the Anderson–Schulz–Flory (ASF) distribution [6]. The derived gasoline, diesel, and aviation kerosene can effectively reduce reliance on fossil fuels and provide possibilities for developing alternative energy sources. In addition, the generated low-carbon olefins and high-carbon alcohols have tremendous application prospects. Currently, the combustion of fossil fuels and human activities have led to a rapid increase in CO₂ concentrations in the atmosphere. Hence, the raw materials of the FTS reactions have extended to the atmospheric CO₂ to achieve their efficient utilization and mitigate climate change. Therefore, researchers have also conducted extensive studies in related fields. In this work, CO_x is used to replace CO₂ and CO, and we review the state-of-the-art research progress in the field of CO_x hydrogenation, which poses a major challenge for carbon emissions reduction. Therefore, researchers have extensively explored CO₂ capture, storage, and utilization technologies [7,8,9]. Currently, CO₂ has become a strategic chemical raw material for transport fuels and various chemicals [10,11], and the diversification of its products (CH₄, C₂–C₄, C₅₊, etc.) has led to extensive research on this reaction. CO₂ hydrogenation is a concept for comprehensive carbon resource utilization with broad prospects. It can not only reduce a series of environmental issues caused by the greenhouse effect but also reduce excessive reliance on fossil fuels in social development [12,13].

The catalyst performances (e.g., catalytic activity, targeted product selectivity, and stability) of CO_x hydrogenation can be regulated by adjusting the catalyst composition and structure [14,15], support materials [16], and additives. Among the aforementioned factors, the specific surface area, pore structure, and interaction with the active components of the support materials significantly affect the dispersion of active components, the adsorption/desorption properties of reactants and products, the catalytic activity, product selectivity, and catalyst stability [17]. Particularly, the strong interaction between traditional metal oxide supports (SiO₂, Al₂O₃, TiO₂, MgO) and active components hinders their reduction, resulting in poor catalytic performance. For instance, the strong metal-support interaction between cobalt and Al₂O₃, SiO₂, and TiO₂ leads to the formation of Co₂SiO₂, Co₂AlO₄, or CoTiO₄, respectively, which are reducible only at high temperatures [18]. Compared to traditional supports, carbon materials (activated carbon (AC), carbon nanotubes (CNTs), carbon nanofibers (CNFs), carbon spheres (CSs), etc.) have weak metal-support interactions, which might overcome these drawbacks [19,20]. And the advantages of carbon materials, such as their diverse morphological structures, high specific surface area, easily controllable surface properties through doping, and good stability, make them widely used as catalyst supports [21].

Inert carbon carriers can be modified through subtle structural modifications, thereby affecting catalytic performance. In this case, doping the carbon matrix with heteroatoms has become one of the ways to regulate catalytic systems [22]. In recent years, metals supported on nitrogen-doped carbon materials have shown excellent performance as heterogeneous catalysts. Nitrogen-doped carbon materials have origins dating back to the 1960s, but their use as catalyst supports has only emerged recently, leading to significant advancements in heterogeneous catalysis. The major advantage of nitrogen-doped carbon materials lies in the ability to modulate the interactions between the support and the metals through electron interactions induced by introducing heteroatoms on the carbon material surface [23,24]. Additionally, doping nitrogen atoms introduces vacancies and defects on the carbon material surface. These defects can serve as anchoring sites for active metals, enhancing the stability of the catalyst. N is the most explored element among various dopants (B, P, F, S), and nitrogen-doped carbon materials also exhibit a wide range of potential applications [25,26]. The introduction of N atoms can effectively anchor the metal components, increase the dispersion of active components, and reduce their agglomeration. Moreover, with the incorporation of the N element, alkaline sites are provided, which is favorable for the adsorption/activation of weakly acidic CO₂. The N element also plays a role in weakening the C-O bond and enhancing the C-C bond, resulting in excellent catalytic performance in CO_x hydrogenation reactions [27,28,29].

Many precious metals and non-precious metals are currently loaded onto nitrogen-doped carbon materials and widely applied in the catalysis field (photocatalysis, electrocatalysis, thermocatalysis) [30,31]. Substantial efforts and progress have been made for FTS or CO₂ hydrogenation over the past few decades [32,33,34,35,36,37]. They have systematically summarized the application of efficient catalysts in CO_x hydrogenation reactions. However, none of them have summarized the application of nitrogen-doped carbon materials as carriers in CO_x hydrogenation reactions, implying that a comprehensive overview on this emerging topic would have great merit. The surface physical and chemical properties of carbon materials can be changed after the introduction of nitrogen atoms, which is beneficial to make the desired products more dominant. Therefore, it is necessary to summarize the recent research progress in this field to provide feasible suggestions for the design of efficient catalysts. In this review, we first introduce the preparation methods of nitrogen-doped carbon materials and the key factors influencing the N species structure. Second, the application of nitrogen-doped carbon materials as catalyst supports in CO_x hydrogenation is discussed, summarizing the role of N species in the reaction (including strategies for regulating the proportion of nitrogen species, the impact of material surface properties, and a preliminary exploration of the reaction mechanism). Finally, it discusses the potential challenges in this field to provide some guidance for the development of advanced N-doped carbon materials.

2. N-doped Carbon Material and Preparation Methods

2.1. N Type of Doping Species

The nitrogen species in nitrogen-doped carbon materials can be mainly divided into four types: pyridinic nitrogen, pyrrolic nitrogen, graphitic nitrogen, and pyridinic nitrogen oxide. Characterization techniques are crucial for elucidating the physicochemical properties of materials and understanding their catalytic mechanisms. The characterization techniques for elucidating the role of nitrogen-doped carbon materials in CO_x hydrogenation reactions can be broadly categorized as follows [22,38]: The type of nitrogen species can be distinguished using XPS spectra. The pyridinic N peak appears around 398.6 eV, the pyrrolic N peak appears around 400.5 eV, and the graphitic N peak appears around 401.3 eV. Additionally, pyridinic N, pyrrolic N, and graphitic N species can also be distinguished using XANES and EELS spectra.

Based on the current experimental and theoretical research results on CO_x hydrogenation, the first three nitrogen species have been widely applied in the relevant fields due to their superior catalytic abilities.

In general, pyridinic nitrogen is located in the edge or defective hexagonal carbon layers. Furthermore, some studies have proposed that a pyridinic nitrogen atom is bonded to two sp2-hybridized carbon atoms. Plus, the pyridinic nitrogen atom provides a p for the π system. The increasing electron density promotes the nucleation and growth of nanoparticles on nitrogen-doped carbon supports, and then enhance the interaction between the metal and the support [39]. As for pyrrolic nitrogen, the atom is incorporated into a pentagonal ring, which can provide two p electrons to the π electron system. Due to the unstable pentagonal structure, pyrrolic nitrogen species can gradually transform into graphitic nitrogen species when the temperature is higher than 800 °C [40]. However, considering the geometric effects, the anchoring abilities of pyridinic nitrogen and pyrrolic nitrogen are greater than that of graphitic nitrogen [41].

The physicochemical properties (e.g., acidity/basicity, wettability, charge transfer rate, and electronic structure) of carbon materials would change after the introduction of nitrogen species. More specifically, pyridinic nitrogen species have unhybridized lone pair electrons, which provide a strong electron-donating ability and effective Lewis base sites. Nitrogen-doped carbon catalysts that are rich in pyridinic nitrogen species can enhance the adsorption of CO₂ (Lewis acid) in CO₂ hydrogenation reactions, improving catalytic performances. However, the alkalinity of the carbon material doped that it is doped with relatively weak due to the weak electron donation capability of graphite nitride species [42]. For example, Feng et al. [43] revealed that the introduction of nitrogen species into carbon materials can provide metal anchoring sites on material surfaces and improve the electronic properties of transition metal species using theoretical calculations. In terms of wettability, carbon materials with sp2 hybridized structures have strong hydrophilicity, and the introduction of nitrogen species can increase the proportion of sp2 hybridized structures in the material, thereby enhancing its hydrophilicity [44]. Additionally, the introduction of nitrogen species can generate functional groups on the surface of carbon materials, improving the interaction between the carbon material surface and reactant molecules or active species through dipole-dipole interactions, hydrogen bonding, or covalent bonding [45]. Moreover, the introduction of nitrogen species can significantly influence the charge distribution in carbon materials by utilizing the difference in electronegativity. Generally, untreated carbon materials have weak charges on their surfaces. However, the introduction of nitrogen species can greatly affect the charge distribution in carbon materials, resulting in the generation of vacancies or defect sites in the carbon structure. According to the work of Su et al. [46], it was found that the pyrrolic nitrogen species formed during the preparation of Fe-based nitrogen-doped carbon catalysts can introduce certain defect sites in the material, effectively improving the coordination environment of the Fe species and enhancing its ORR reaction properties. Furthermore, due to the delocalization effect, doped nitrogen atoms can alter the electronic structure of carbon materials at nitrogen doping sites, which leads to local charge accumulation and changes in the local electronic state density. This effectively promotes electron transfer reactions and an acceleration of the adsorption and/or activation of reactant molecules [47].

2.2. Preparation Methods of Nitrogen-Doped Carbon Materials

Nitrogen-doped carbon materials (i.e., nitrogen-doped graphene, nitrogen-doped porous carbon, and nitrogen-doped carbon nanofibers) chemically bond or incorporate nitrogen atoms into the carbon material framework. Given the similar size of nitrogen and carbon atoms, the substitution of nitrogen atoms for carbon atoms causes less damage to the carbon material framework, which maintains the stability of the carbon materials [48,49]. Therefore, nitrogen-doped carbon materials have been widely applied in the field of catalysis. Typically, nitrogen-doped carbon materials can be prepared through post-doping and in situ doping methods. Meanwhile, their different types and morphologies can be achieved by leveraging disparities in the precursor types, preparation processes, and the introduction of nitrogen components.

2.2.1. Post-Doping Method

The post-doping method, as a classical method for preparing nitrogen-doped carbon materials, has captured considerable interest among researchers owing to its straightforward operational nature. It can be used to yield the nitrogen contents and proportions of nitrogen species by varying the reaction, reaction time, nitrogen precursor species, and types of carbon materials. To be specific, it involves a reaction between pre-synthesized carbon materials and nitrogen precursor species (such as ammonia, urea, dicyandiamide, melamine, etc.) to prepare nitrogen-doped carbon materials under high temperatures. Typically, this method requires a long-term treatment under high temperatures to ensure the successful introduction of nitrogen species. Herein, the nitrogen contents and proportions of nitrogen species can be changed by varying the reaction temperature, reaction time, nitrogen precursor species, and types of carbon materials.

The treatment temperature is considered as a major factor influencing the nitrogen content of nitrogen-doped carbon materials. For example, when treating graphene oxide and activated carbon in an NH₃ atmosphere between 500 and 600 °C, the nitrogen content is highest, and is more than twice the nitrogen content of samples calcined at temperatures above 800 °C [50,51]. Wang et al. also found that as the pyrolysis temperature increased from 500 °C to 800 °C [52], the nitrogen content remarkably decreased from 19.8 to 2.08%. It can be seen that the pyrolysis temperature in an ammonia atmosphere has a significant impact on the nitrogen content of the material. In addition to the nitrogen content, the treatment temperature also has a considerable influence on the types of nitrogen species. Wang’s research revealed that as the treatment temperature increased, the unstable structure of pyrrolic nitrogen resulted in a reduction in its content, and there was also a decreasing trend in the content of pyridinic nitrogen.

Compared to conventional high-temperature pyrolysis methods, the microwave-assisted method can reduce the preparation time for nitrogen-doped carbon materials to even be as short as a few seconds [29]. It also conforms to the current trend of environmental friendliness and has gradually gained attention in recent years [53]. In the microwave method, the appropriate nitrogen precursors (e.g., urea, ammonium hydroxide, and ammonium carbonate) generally have advantages such as low cost, being environmentally friendly, and low pyrolysis temperatures. For example, Ali et al. [54] used urea as the nitrogen precursor and synthesized a mixture of carbon nanotubes and urea under microwave conditions. The nitrogen-doped carbon material was obtained in about 10 min at a power of 700 W, where the nitrogen content was close to 7%, and the nitrogen species included pyridine, pyrrole, and various other groups.

2.2.2. In Situ Doping Method

The in situ doping method can be regarded as a one-step synthesis approach for nitrogen-doped carbon materials. In this method, the nitrogen source and carbon source precursor materials are added simultaneously during the preparation process, and then they are treated under a high-temperature condition in an inert atmosphere or via hydrothermal carbonization. Notably, the specific morphologies or structures can be flexibly adjusted by selecting different precursors and preparation conditions in this method [55,56,57]. Common precursors include polyaniline (PANI), polypyrrole (PPy), urea, biomass-based materials [58], and nitrogen-containing metal–organic framework materials (such as a zeolitic imidazolate framework) [59]. In the in situ doping method, preparation parameters such as the carbonization temperature and the type of nitrogen source precursor play crucial roles in determining the types of nitrogen species. For instance, the pyrolysis temperature significantly affects the nitrogen content, the ratio of the different nitrogen species, and the graphitization degree of the carbon material [60]. Therefore, synthesizing nitrogen-doped carbon materials with specific structures poses significant challenges in in situ synthesis.

N-doped carbon materials derived from biomass have attracted significant attention due to the unique pore structure generated during their preparation process. Geng et al. [61] utilized spirulina as a precursor for nanoporous biochar preparation via in situ nitrogen–oxygen heteroatom co-doping through a pre-carbonization coupling pyrolytic activation process. The effects of the activation temperature and the KOH mass ratio on the carbon properties were investigated. The results indicated that both the temperature and the amount of activator significantly affect the physical properties of the material, such as the pore distribution and the specific surface area. Su et al. [62] used birch bark as the carbon source for biochar preparation for the first time. N-doped porous biochar was synthesized with one-step co-activation pyrolysis by adding an activator (NaOH) and a nitrogen source (urea) during the preparation process. The results indicated that nitrogen doping not only provides more reactive binding sites on the surface of carbon materials, but also enhances the reactivity of biochar by adjusting the electronic structure of the material framework. Liu et al. [63] prepared novel N-doped biochar via the one-step pyrolysis of algal sludge without external nitrogen sources. High-temperature pyrolysis leads to a more enriched pore structure on the material surface, which is more favorable for the adsorption and decomposition of sulfadiazine. Additionally, high-temperature pyrolysis also increases the defects in the material. Glaydson et al.’s research also indicates that N doping significantly alters the pore structure of the material (primarily mesopores) and increases the surface defects of the material [64]. The enriched pore structure enables the loading of a large number of active components, and the increased defects also play a role in anchoring the active species. Therefore, nitrogen-doped carbon materials hold great potential in the field of catalyst supports. Among the various carbon sources, Pinus sylvestris is advantageous for producing biochar due to its high lignin content (25.5%). Wang et al. [65] used Pinus sylvestris as a carbon source and mixed it with urea (nitrogen source), then calcinated the mixture in a tube furnace under a N₂ atmosphere. The obtained materials exhibited a pore structure with both micropores and mesopores at different pyrolysis temperatures. However, as the pyrolysis temperature increased from 500 °C to 700 °C, the pore size, pore volume, and specific surface area of the materials all showed an increasing trend, indicating the significant impact of the pyrolysis temperature on the final material structure. Lignin, as the second most abundant biomass resource in the plant kingdom (after cellulose), is often used as a carbon source for synthesizing nitrogen-doped carbon materials. Zhong et al. [66] mixed a solution containing lignin and melamine, and then pyrolyzed them in a tube furnace, generating materials with a thin carbon-sheet-like, three-dimensional structure. In addition to the aforementioned nitrogen sources, organic compounds such as polypyrrole and polyaniline contain nitrogen elements themselves, making direct pyrolysis easier compared to adding external nitrogen sources. For instance, Sudhakaran et al. [67] used polypyrrole to synthesize nitrogen-doped carbon nanotubes (NCNTs) via pyrolysis in a N₂ atmosphere. It can be observed that these carbon nanotubes were composed of amorphous carbon, and nitrogen elements were uniformly distributed on the carbon nanotubes. Li et al. [68] used self-synthesized nanofiber polyaniline as a precursor, and mixed it with KOH (an activating agent). The nitrogen-doped porous carbon fibers were generated by pyrolyzing the sample in a N₂ atmosphere. It was found that the large pores between the nanofibers served as efficient gas transport channels.

Zeolitic imidazolate framework materials (ZIFs) possess a high specific surface area, a large pore volume, and a rich microstructure. Therefore, they have been important precursors for the preparation of nitrogen-doped carbon materials, and have been a research hotspot in recent years. Based on the advantages of the ZIF precursors, nitrogen-doped carbon materials with different structures, such as nanosheets [55], nanoparticles [56], and core-shell structures [69], have been designed. For example, Zhao et al. [55] synthesized nanosheet-like ZIF-8 using water as a solvent, and the material still maintained its sheet-like structure after pyrolysis. Zhang et al. [70] synthesized ZnCo-ZIF materials with particle sizes of around 10 nm by adjusting the preparation parameters, and found that nitrogen-doped carbon nanosheets could be prepared during further carbonization. On the basis of the encapsulation effect of MOF materials on metal components, Shi et al. [69] successfully prepared nitrogen-doped carbon nanotubes coated with ZnO in a core–shell structure (ZnO@NCNT), and a further etching of ZnO with an acid solution formed hollow nitrogen-doped carbon nanofibers with diameters ranging from 70 to 190 nm. In addition, considering MOFs as precursors, as well as the unique interaction between metal species and organic species, the preparation of supported single-atom catalysts can be realized, and the structural properties of metal sites can be controlled. Zheng et al. [71] used a two-dimensional layered bimetallic Zn/Fe-MOF as a precursor and prepared Fe-supported nitrogen-doped carbon catalysts with a single-atom/single-atom pair Fe-N4 structure. The loading amount of the Fe species was nearly 4%, and the nitrogen content reached 10%.

In summary, the current methods for preparing nitrogen-doped carbon materials have their respective advantages, but there are still challenges that are difficult to control. These issues include a low sample yield, the inability to precisely construct nitrogen species, and the stringent preparation conditions that may damage the carbon material or the support material’s structure. Therefore, developing milder synthesis conditions while achieving an efficient and controllable preparation of catalytically active components is currently a key focus in the research on nitrogen-doped carbon materials.

3. Catalytic Application of Nitrogen-Doped Carbon Materials in CO Hydrogenation

With the advancement of industrialization, fossil energy is becoming increasingly depleted, and developing alternative clean energy is becoming urgent. Fischer–Tropsch synthesis (FTS) is the essential process of converting syngas into high-value-added hydrocarbons. Studies have found that the interaction between different supports (such as alumina, silica, and carbon material) and metals is crucial for FTS reactions. Carbon supports, with their high specific surface area, excellent thermal stability, and weak metal–support interactions [5], have been widely used. Introducing nitrogen atoms into carbon materials can further regulate the metal–support interaction, promoting the dispersion of the active components [72]. In recent years, the nitrogen doping of carbon materials has become a promising strategy to improve catalytic performance. Through a review of the current research literature, we find that nitrogen-doped carbon materials are mainly applied in the production of long-chain hydrocarbons in FTS reactions. The following content is a brief description of the aforementioned application, which is classified according to the main active components, Co and Fe.

3.1. Cobalt-Based Catalysts

Co-based catalysts have been widely used in FTS reactions due to their high C₅₊ selectivity, high single-pass conversion rate, and low CO₂ selectivity. However, the strong interaction between the traditional metal oxide supports and the active components leads to a decrease in the utilization of the active phases [73]. Recently, nitrogen-doped carbon materials have shown excellent catalytic performance in FTS due to the formation of suitable and favorable metal–support interactions [74].

Kafrani et al. [75] impregnated Co and Ru onto a nitrogen-doped graphene aerogel (NGA) and carbon nanotubes (CNTs). By comparing the selectivity of the two catalysts, it was found that when an NGA was used as the support, the product tended to generate methane due to enhanced H₂ adsorption on the material surface, resulting in an increased H/C ratio. However, CNTs without nitrogen doping showed higher selectivity towards C₅₊ hydrocarbons. Although the C₅₊ selectivity of the NGA was not as high as that of the CNTs, the high dispersion and reduction rate of the active components in the N-doped materials significantly improved their catalytic performance compared to the CNTs. Wang et al. [76] prepared three-dimensional nitrogen-doped graphene aerogel catalysts with different Co loadings using a hydrothermal method, and investigated the effects of the Co content and Co phase on the catalytic performances. The results showed that when Co participated in the gelation of the aerogel, an excessive Co loading resulted in encapsulation within the material, reducing the reaction activity. Moreover, the phase change in active Co species was found to affect the activity of the catalyst. In more detail, as the reaction progressed, the Co phase in the in situ synthesized catalyst transformed from the fcc phase to the hcp phase, resulting in an increase in the catalytic activity with the extension of the reaction time (as shown in Figure 1). By changing the Co loading method (in situ/post-treatment), the authors found that no phase transition from the fcc to the hcp phase was observed in the post-treated catalyst, indicating no increase in catalytic activity. As mentioned earlier, different treatment conditions can have impacts on the structure of the materials. For example, Zhan et al. [77] used a pyrolysis-oxidation–reduction approach to treat the materials, resulting in the formation of a polyhedron structure consisting of nitrogen-doped nanotubes embedded with hollow CoNPs (as shown in Figure 2). In addition, they also studied the effects of different treatment methods on the catalyst structure and the relevant catalytic performances. The H-Co@NCNHP catalyst exhibited a CO conversion of 16.4% and a C₅₊ selectivity of 90.4% at 250 °C, 2 MPa, and 28,400 mLg_cat⁻¹h⁻¹. The catalyst was not deactivated after 120 h of operation. H-Co NPs that underwent the oxidation–reduction process exhibited a stronger CO/CH_x adsorption ability compared to S-Co NPs that did not undergo this process. Additionally, DFT calculations showed that the electronic structure of the H-Co NPs enhanced the CO dissociation adsorption and reduced the energy barrier for C-C coupling. This undoubtedly enhanced the reactivity and the possibility of long-chain hydrocarbon formation.

In addition to conventional carbon nanotubes as supports, carbon nanosheets and carbon nanospheres are also commonly used for N-modification. Taghavi et al. [78] investigated the effect of N-functionalization on FTS by using pure carbon nanosheets and nitrogen-doped carbon nanosheets as supports and loading Co on their surfaces. The characterization results showed that N doping increased the surface defects of the material, providing more anchor points for a better dispersion of Co and smaller active phase particles. Chernyak et al. [79] conducted experiments using pristine/oxidized/N-doped carbon nanosheets as supports and loading Co. By treating the carbon nanosheets using different pathways, the author found that Co/GNFox (oxidized nanosheets) exhibited the highest anti-sintering capability. N doping and oxidation treatment significantly reduced the particle size of the active component. Meanwhile, they reduced the catalyst’s specific surface area and pore size, which hindered the transfer and diffusion of products and impeded the formation of C₅₊. Although the C₅₊ selectivity decreased, the CO conversion was greatly improved. Dlamini et al. [80] utilized N-doped hollow carbon spheres as supports, which promoted both the dispersion and prevention of Co sintering. Cheng et al. [81] constructed N sites on carbon nanospheres and adjusted the N content by changing the pyrolysis temperature. Anchoring the N enhanced the dispersion of Co, and increased the exposure of active sites on the catalyst surface. The electron-rich Co facilitated the adsorption and dissociation of CO, which led to the generation of more C* on the catalyst surface. Then, CO-TPD showed that the catalyst with the highest pyrrolic N content had the highest CO₂ desorption temperature (as shown in Figure 3), indicating stronger Co-C bonding, which increased the abundance of C* species and was beneficial for C-C coupling. The author considered the N-anchoring ability in the following order: pyrrolic N > pyridinic N > graphitic N.

To address the issue of the strong interaction between the traditional support, Al₂O₃, and the metal, Co, Guo et al. [82] coated the surface of Al₂O₃ with a layer of N-doped carbon shell. The nitrogen-doped carbon (NC) layer on the outer surface of the Al₂O₃ weakened the interaction between the metal and the support, making it more difficult for the Co to enter the lattice of the Al₂O₃ and facilitating its reduction. Additionally, the incorporation of N enhanced the reducibility of the catalyst. Particularly, the pyrrolic N, which increased the electron density on the surface of Co, led to strengthened C-Co bonds and weakened C=O bonds.

The summary of previous experimental research for cobalt-based catalysts is presented in

Table 1. Performance of various cobalt-based catalysts in CO hydrogenation.

Catalyst	Pressure (bar)	Temperature (°C)	GHSV	H2:CO Ratio	CO Conversion (%)	C5+ Selectivity	Reference
Co-Ru/NGA	25	280	10,000 b	1	75.25	17.2	[75]
2.7%Co/NGA	10	250	2000 b	2	9.0	88.2	[76]
H-Co@NCNHP	20	250	28,400 b	2	16.4	90.4	[77]
Co/GHFox	20	240	6000 *	2	31.9	30.0	[79]
10Co/N-HCSs900	10	220	N.A.	2	34.0	75.8	[80]
Co/NCS-500	20	220	N.A.	2	N.A.	74.4	[81]
Co/Al2O3-15CN	20	250	N.A.	2	71.3	77.8	[82]
Co/SiO2-CN	20	250	27,000 b	2	34.4	76.3	[83]
10Co/N-MC	20	230	1000 a	2	N.A.	73.0	[84]
Co/NCNTs-2	20	230	6750 b	2	51.2	83.4	[85]
Co/A-NCNTs	20	230	6750 b	2	74.3	80.0	[86]
15Co/NMC-2	20	240	1000 a	2	N.A.	61.2	[87]

^a GHSV, h⁻¹; ^b WHSV, mL/g/h; * NmL/g/h.

.

3.2. Iron-Based Catalysts

Fe-based catalysts have been applied for CO hydrogenation due to their low cost and excellent FTS reaction activity. Carbon materials have been found to facilitate the carburization of Fe, promoting the formation of more active iron carbide phases [88]. Introducing nitrogen atoms into carbon materials can achieve similar effects to that of K promoters, namely increasing the surface basicity of the material and modifying the electron density of the active components. Therefore, the catalytic performance of the Fischer–Tropsch synthesis (FTS) reaction is further enhanced [89].

Tian et al. [90] utilized ammonium citrate ferrite to simultaneously load Fe and dope N into activated carbon. Raman spectroscopy revealed that the graphitization degree of the activated carbon remained unchanged before and after N doping, indicating that N entered the Fe lattice rather than the carbon material itself, thereby enhancing the electron transfer between the N and the Fe. To investigate the influences of different active phases encapsulated in a carbon shell on catalytic performances, Lee et al. [91] prepared three different catalysts (i.e., Fe@C/NPC, Fe₃C@C/NPC, Fe₅C₂@C/NPC), as shown in Figure 4. The results showed that Fe₅C₂@C/NPC has high loading and dispersion, achieving a CO conversion of 96.4%, an FTS activity of 4.3 × 10⁻⁴ mol_COg_Fe⁻¹s⁻¹, and a hydrocarbon yield of 4.71 g_HCg_cat⁻¹h⁻¹. The encapsulation of the carbon shell reduced the catalyst sintering and significantly enhanced the stability. N atoms adjusted the interface interaction, and increased the catalyst’s surface alkalinity. Then, the CO adsorption and dissociation were enhanced. The advantages of carbon materials were also demonstrated in Tang’s study [92] on inhibiting the migration of Fe species and maintaining their high dispersion during the reaction. Furthermore, the increase in the graphitization degree of the carbon material enhanced the electron transfer from N atoms to Fe. It was also shown that N species strengthened the Fe-C bond and weakened the C-O bond. The stable N-doped carbon shell structure enabled the catalyst to achieve a gasoline selectivity of nearly 50%. This is mainly because the thickness of the carbon shell hindered the outward diffusion of olefins, increasing the possibility of further hydrogenation and enhancing the selectivity towards long-chain hydrocarbons. In addition, N can act as an anchoring site for active metals, facilitating dispersion. Liu et al. [89] used an impregnation method to load 40 wt% Fe onto nitrogen-rich mesoporous carbon as an efficient catalyst for FTS. The results showed that the high pore volume of mesoporous carbon and the anchoring effect of N achieved the dispersion of high loading Fe (as shown in Figure 5).

MOFs have abundant porosity and an adjustable functionality, which are widely used as precursors for nitrogen doping [93]. ZIF-8 is a typical example, where, upon thermal decomposition at temperatures above 900 °C, all the Zn can be evaporated to generate pure N-doped carbon material [94]. Furthermore, ZIF-8 can be divided into two categories, cubic and nanosheet forms, providing more choices for catalyst preparation [95]. Zhao et al. [55] prepared N-doped nanosheets through the thermal decomposition of ZIF-8 nanosheets and used them as catalyst supports (as shown in Figure 6). Compared with traditional activated carbon and cubic ZIF-8, the advantage of nanosheets lies in their higher degree of graphitization, which is beneficial for electron transfer. In more detail, the CO conversion significantly increased from 7.7% (AC) and 22.1% (cubic) to 45.9%. XPS characterization analysis indicated that pyrrole N has a stronger electron-donating capacity than pyridine N and graphite N. TEM images (as shown in Figure 7) of the catalyst after use revealed that N anchoring reduced the sintering of the active phases and slowed down the deactivation, highlighting the indispensable role of N doping in catalyst stability. Furthermore, according to the work of Zhao et al. [96], it was found that the thermal decomposition temperature significantly affected the total N content and the proportion of N species in the materials. Increasing the thermal decomposition temperature within a reasonable range can improve the proportion of pyrrolic N, providing a stronger electron-donating ability to Fe.

The microwave-assisted preparation of N-doped carbon materials has attracted extensive attention from researchers due to its extremely short preparation time. Guo et al. [29] used microwave treatment instead of traditional calcination, greatly reducing the preparation time of NC materials (within seconds). The authors found that by adjusting the microwave treatment time, the particle size of the catalytic active component could be changed. In more detail, increasing the treatment time resulted in a decrease in the particle size from 120 nm to 10 nm. The ammonia treatment time can modify the total amount and configuration of N. With a 12 h ammonia treatment and a 10 s microwave treatment, the catalyst exhibited a CO conversion of 97.2% and a C₅₊ selectivity of 40% at 320 °C and 2 MPa. Park et al. [97] prepared a catalyst consisting of carbon nanotubes, N-doped reduced graphene oxide, and nano-sized Fe particles through microwave-assisted and CO activation. Under a space velocity of 210 NL g_cat⁻¹h⁻¹, the STY value reached 4.4 × 10⁻³ mol_COg_Fe⁻¹s⁻¹, and the CO conversion reached 80.5%. The high activity is attributed to the unique structure of the catalyst, which possesses a high surface area and a three-dimensional pore system that enhances mass transfer.

The uncontrollable nature of N species in N-doped carbon materials makes it difficult for researchers to elucidate the specific role of individual N species. In response to the complexity of N species in current N-doped carbon materials, Liu et al. [98] prepared carbon nanotubes containing only pyrrolic N through a simple hydrothermal method (as shown in Figure 8). It was found that the content of pyrrolic N was positively correlated with C₅₊ selectivity, which was mainly because pyrrolic N enhanced the alkalinity of the catalyst surface and the electron density around the active components. More specifically, the addition of N can enhance the interaction between the active metal and the support, and it also increases the reduction temperature of Fe. The formation of iron carbide is closely related to the degree of reduction, and a high degree of reduction favors the formation of iron carbide. Therefore, N doping increases the selectivity of C₅₊ hydrocarbons. A similar conclusion can be proven in the work of Lama et al. [99]. They modified CNTs with N atoms, and found that N doping stabilized smaller Fe particles (nanoscale) and could even stabilize atomic-level Fe species. The formation of iron carbide was accelerated (rapid activation capability), which could be attributed to the formation of an electron junction between the Fe species and the NC layer, enhancing the binding between CO and Fe.

In summary, although the active components (Fe/Co) may differ, N-doped carbon materials share many similarities in the mechanism of the CO hydrogenation reaction. N species have a significant impact on the surface acidity/basicity and the electronic structure around the active components of the catalyst. Furthermore, N-doped carbon materials can provide abundant anchoring sites, reducing the sintering of active components during the reaction process.

The summary of previous experimental research for iron-based catalysts is presented in

Table 2. Performance of various iron-based catalysts in CO hydrogenation.

Catalyst	Pressure (bar)	Temperature (°C)	GHSV	H2:Co Ratio	CO Conversion (%)	C5+ Selectivity	Reference
Fe/CNS(1000)	10	340	9000 b	1	45.9	39.2	[55]
15%Fe/AG(12h)-W(10)	20	320	5 #	2	97.2	40.0	[29]
40Fe/N1	10	260	5 #	1	74.7	67.0	[89]
FeN-AC	20	320	15,000 a	1	59.7	18.1	[90]
Fe5C2@C/NPC	15	340	42 *	1	96.4	33.5	[91]
FeC-800	20	300	N.A.	2	47.0	52.0	[92]
Fe/CNS-KOH(700, 0.6)	10	340	9000 b	1	87.0	55.6	[96]
Fe5C2@Ns-rGO/CNT	15	340	210 *	1	80.5	30.0	[97]
Fe/CNTs-NaU	20	270	4500 b	1	60.0	90.8	[98]
FeBN2	20	270	1500 a	2	47.0	48.0	[100]
Fe-MIL-88B-NH2/C	20	300	36,000 a	1	81.8	50.7	[101]
Fe/NCNTs-10	20	270	4500 b	1	45.0	76.0	[102]
Fe/NG−16.4	5	340	600 a	1	21.1	19.8	[103]
Fe/NCSver	8	275	2700 a	N.A.	50.0	51.6	[104]
Fe/N-CNT-h	8	275	2400 a	2	70.1	60.9	[105]
2.3 wt% Co/N-CSs	8	230	1200 a	2	N.A.	47.1	[106]

^a GHSV, h⁻¹; ^b WHSV, mL/g/h; * NmL/g/h; ^# g·h/mol.

.

4. Catalytic Application of Nitrogen-Doped Carbon Materials in CO₂ Hydrogenation

In order to achieve global carbon neutrality, the efficient utilization of waste CO₂ has been a concern in both industry and academia. Generally, CO₂ can be treated as an important carbon source, and can be converted into valuable long-chain hydrocarbons with the aid of green H₂ [107,108]. The obtained long-chain hydrocarbons can be used as an alternative to traditional fossil fuels owing to their high volumetric energy density. The products from CO₂ hydrogenation exist in the form of C-H and C-C chemical bonds, known for their high volumetric energy density. These products are not only core components in the industry but also the foundation of modern society. Consequently, research in this field has garnered significant attention in recent years. Below, we will analyze and summarize the different performances of CO₂ hydrogenation catalysts, as well as reasons for using them, from the perspective of different products.

4.1. CO₂ Hydrogenation to CO over Nitrogen-Doped Carbon Materials

The reverse water–gas shift (RWGS) reaction involves the conversion of CO₂ and H₂ into CO and H₂O [109]. The produced CO can be used as a feedstock for FTS to synthesize liquid fuels and high-value-added chemicals. Li et al. [110] loaded different metals (Fe, Co, Ni, and Cu) onto N-doped carbon materials derived from ZIF-8, and found that the reactivity order was Ni = Fe > Co > Cu (as shown in Figure 9). The experiment result shows that pyridinic N and metal carbides can capture and enrich CO₂, and the number of pyridinic N sites correlates positively with the alkaline sites on the catalyst surface. Moreover, DFT calculations indicate that CO* tends to desorb rather than further hydrogenate, resulting in a high CO selectivity (as shown in Figure 10). Bruno et al. [111] covered SiO₂ with a layer of N-doped carbon, which led to a higher CO selectivity compared with catalysts without the N-doped carbon layer. This can be attributed to a weakened CO adsorption on the Ni-NC surface. However, catalysts without the N-doped carbon layer exhibited better CH₄ selectivity, indicating that the presence of the NC carbon layer can significantly alter product selectivity. In other words, the multilinear binding of CO with Ni on the surface of pure Ni catalysts is stronger than the linear binding of CO with Ni on the NC-Ni surface, making pure Ni more prone to promoting the formation of methane, while NC-Ni is more prone to resulting in CO generation. Li et al. [112] prepared catalysts with single atomic Co-N4 and Co nanoparticle structures. It was found that the atomic-level Co had a weaker CO adsorption, resulting in a preference for CO as the product. The nanoparticles had a stronger H₂ activation ability than the single atom, leading to a higher tendency for CH₄ production (selectivity of 99%). Considering that the anchoring effect of N promotes the formation of atomic-level Co (as shown in Figure 11), the quantity of N atoms plays a crucial role in the formation of single atoms. An insufficient N content leads to the formation of nanoparticles. Additionally, the electron structure of Co is adjusted to form Coⁿ⁺ species, optimizing the adsorption and desorption of intermediate products. Besides the commonly used Ni- and Fe-based catalysts, Mo-based catalysts have also been developed for the RWGS reaction. Zhang et al. [113] used N-doped graphene to support various metal single atoms and performed systematic DFT calculations, revealing that the VIB group metals exhibited significant advantages in the RGWS reaction. Particularly, Mo can provide electrons to CO₂ molecules, facilitating the spontaneous generation of CO and O*, and decreasing the hydrogenation barrier of O* to 1.10 eV. Jiang et al. [114] used Mo as the active component in the RWGS reaction. The prepared catalyst had a MoN₃ structure, which could effectively adsorb CO₂ and convert it to CO. Under a low H₂ partial pressure, the catalyst showed a conversion of around 30% at 500 °C and a low H₂ partial pressure, with close to a 100% CO selectivity. DFT calculations indicated that the transition state energy during CO generation was at most 0.82 eV, much lower than the 2.16 eV required for CH₄ formation, making it more favorable for CO selectivity. Compared to Mo/C, Mo/NC has fewer stable CO adsorption sites, making CO easier to desorb. The performance data of catalysts in the published literature are summarized in

Table 3. Performance of various catalysts in CO₂ hydrogenation to CO.

Catalyst	Pressure (bar)	Temperature (°C)	GHSV	H2:CO Ratio	CO2 Conversion (%)	CO Selectivity	Reference
Ni/ZIF-8-C	1	420	15,000 b	4	43.8	42.7	[82]
Fe/ZIF-8-C	1	420	15,000 b	4	43.8	42.6	[82]
5%Co-N-C	1	500	6000 b	4	52.4	98.3	[84]
Mo/NC	1	500	8000 b	3	46.3	100	[86]

^b WHSV, mL/g/h.

.

4.2. CO₂ Hydrogenation to CH₄ over Nitrogen-Doped Carbon Materials

Converting CO₂ to CH₄ through hydrogenation is also known as the Sabatier reaction. Methane is characterized by its easy storage and transportation, and it can be used as a fuel and a precursor for the production of other chemicals [115,116]. Stasi et al. [117] proved the potential of N doping to enhance the performance of catalysts for CO₂ hydrogenation to methane. The authors introduced Ce and N separately into Ni-based catalysts and compared their catalytic effects. Both additives increased the catalyst’s surface alkalinity. While the effect of N was not as good as that of Ce, an increase in the N content showed an upward trend in the conversion rate and selectivity. In order to investigate the influence of N doping on the reaction, David et al. [27] directly introduced Fe and N into the carrier through CVD to maximize the interaction between the particles and the carrier. The catalyst doped with N showed an increased CO₂ conversion rate and CH₄ selectivity, but a significant decrease in CO selectivity. The TPD characterization data showed that N-doped carbon materials exhibited an increased adsorption strength for CO₂ and CO, possibly due to the dipole moment similarity between C-N and CO₂/CO. However, the absence of a dipole moment in hydrocarbon products made them more prone to desorption, leading to a decrease in the selectivity of long-chain hydrocarbons. To verify the effect of N, Na was introduced as an additive to mask the effect of N. The product distribution exhibited catalytic effects opposite to the N-doped carbon materials, validating the authors’ assumption. To elucidate the advantages of carbon materials over traditional metal oxide supports, Roldan et al. [118] compared Ru catalysts supported on different carriers (CNT, CNF, NCNF, Al₂O₃). Among all the carbon supports, CNF performed the worst. The carbon materials exhibited stronger CO₂ adsorption compared to Al₂O₃. And with the incorporation of N into carbon materials, the interaction between the CO₂ chemical adsorption intermediates and the nitrogen atoms was enhanced.

Wu et al. [119] prepared non-metal-loaded nitrogen-doped carbon materials for CO₂ hydrogenation and compared the reaction results before and after N doping. The results showed that the introduction of N increased the surface Lewis basic sites and enhanced the CO₂ adsorption. The catalyst activity (an increased CO₂ conversion rate, increased CH₄ selectivity, and a lowered reaction starting temperature) and the number of pyridinic N sites showed a significant positive linear relationship, while pyrrolic N and graphitic N did not exhibit this relationship. Wang et al. [120] also demonstrated that the addition of N would result in CO₂ concentration gradients around the active components. They prepared a Ni-based catalyst supported on N-doped nanotubes, which efficiently converted CO₂ selectively into methane. Under a high gas hourly space velocity of 120,000 mLg⁻¹ h⁻¹, the STY of methanol in the product was 5 mol_CH4g_Ni⁻¹h⁻¹. Moreover, the catalyst exhibited a significantly enhanced resistance to coke deposition. Wang et al. [65] used urea as the N source, and red pine as the C source to prepare materials through in situ thermolysis, with the metal Ru loaded as the methanation catalyst. The catalyst with the highest pyridinic N content achieved a CO conversion rate of 92% and a CH₄ selectivity of 99.7% under the reaction conditions of 380 °C, 1 MPa, and H₂/CO₂ = 4. An XPS analysis revealed a higher content of zero-valent metal in the N-doped material, indicating that N altered the electronic state of the active metal, thereby affecting its reducibility. The increased zero-valent Ru improved the H₂ dissociation, leading to an increased CH₄ selectivity. Anchoring sites on the carrier reduced the aggregation and leaching of active sites during the reaction. In contrast to other researchers, the total N content in the material increased with the increasing pyrolysis temperature, possibly due to the favorable combination of N atoms and carbon materials at high temperatures. Gholampour et al. [56] investigated N-doped carbon materials derived from two different structures (nanosheets/cube) of ZIF-67 for methanation. It was found that ZIF-67-L (nanosheets) had smaller Co particles and a higher number of N species (especially pyridinic N, which was believed to exhibit the strongest Lewis basicity and have a unique effect on CO₂ adsorption). The catalyst showed good performance in a 60 h stability test. Liliana et al. [121] compared the catalytic performance of Ni/CNT and Ni/CNT-N. The N-doped CNT exhibited a stronger interaction with the metal, Ni, resulting in an atomic-level dispersion of Ni in the catalyst. The specific metal surface area of CNT-N was 3.5 times higher than that of CNT, indicating better dispersion of the active metal, Ni. Gödde et al. [122] achieved a CO₂ conversion rate of 51.4% and a CH₄ selectivity of 95.8% (as shown in

Table 4. Performance of various catalysts in CO₂ hydrogenation to CH_4.

Catalyst	Pressure (bar)	Temperature (°C)	GHSV	H2:Co Ratio	CO2 Conversion (%)	CH4 Selectivity	Reference
BCCe30Ni20	1	400	13,200 a	4	65.0	95.0	[27]
NGQDs/Al2O3	1	400	18,000 b	4	61.8	55.2	[117]
Ni/N-CNTs	1	360	120,000 b	4	75.0	94.0	[118]
			60,000 b		81.0	98.0	[118]
Ru/N-ABC-600	10	380	6000 b	4	93.8	99.7	[65]
Co/C(L)	1	300	12,000 b	4	44.0	84.0	[56]
Co/C(67)	1	300	12,000 b	4	33.0	56.0	[56]
Ni/CNT-N	1	400	60,000 b	4	81.2	99.2	[119]
40Ni/NCNT	1	340	50,000 b	4	51.4	95.8	[120]

^a GHSV, h⁻¹; ^b WHSV, mL/g/h.

) by adjusting the loaded Ni content on NCNT. Despite the high loading of Ni, the anchoring effect of N kept the overall Ni particle size below 10 nm, providing a large accessible Ni metal surface area, even at high loading.

4.3. CO₂ Hydrogenation to C₂₊ over Nitrogen-Doped Carbon Materials

With the in-depth study of CO₂ hydrogenation, single-carbon atom compounds (e.g., CO and CH₄) have been widely reported. In recent years, C₂₊ compounds have gained attention from researchers due to their potential conversion into high-value compounds such as olefins and aromatics [123]. Liu et al. [124] used nitrogen-doping materials derived from the pyrolysis of ZIF-8 as the carrier of the Fe-based catalyst, and the experimental results showed that the carbon carrier containing N species exhibited a better C₂₊ olefin selectivity and C₅₊ selectivity compared to traditional carbon carriers. Dong et al. [57] prepared a Fe-Co bimetallic N-doped catalyst using different pyrolysis temperatures. With the increase in the pyrolysis temperature, the total N content decreased, and the proportion of pyridinic N decreased (as shown in Figure 12). Generally, the pyrolysis temperature also has a significant influence on the carbonization degree of N-doped carbon carriers, the particle size, and the carburization degree of active components. Zhang et al. [125] addressed the issue of the poor activity of catalysts for CO₂ hydrogenation to olefins by confining Fe₃O₄-FeC_x heterojunction active sites within N-doped graphene shells (as shown in Figure 13). Under reaction conditions of 3.0 MPa, 320 °C, and GHSV = 4800 mLg_cat⁻¹h⁻¹, the catalyst achieved a CO₂ conversion of 54.5% (as shown in

Table 5. Performance of various catalysts in CO₂ hydrogenation to C_2+.

Catalyst	Pressure (bar)	Temperature (°C)	GHSV	H2:CO Ratio	CO2 Conversion (%)	C2+ Selectivity	Reference
FeZn-NC	30	320	7200 b	3	29.3	63.5	[124]
FeCo/NC-600	20	320	6240 b	3	37.0	49.4	[58]
0.8Fe@N-OMC	30	320	4800 b	3	54.5	82.9	[127]
Co-Fe@(N)	40	300	6000 a	4	58.0	44.0	[128]

^a GHSV, h⁻¹; ^b WHSV, mL/g/h.

) and a C₂⁼-C₄⁼ selectivity of 65.63%. The role of N-MOC is to enrich alkaline sites and enhance CO₂ adsorption while inhibiting H₂ adsorption and dissociation. FeN_x formed after the interaction between the carrier and Fe can be converted into active sites of FeC_x, enhancing the activity of the FTS reaction. Moreover, the FeC_x particles in the N-doped catalyst were smaller, which is beneficial for structure-sensitive FTS reactions. At the same time, the shell formed by N-MOC prevented the contact between the active sites and water, avoiding the sintering of the heterojunction, and increasing the stability of the catalyst (with a good stability over 100 h of operation). Peng et al. [126] prepared a catalyst with a nitrogen-doping layer coated on a Co-Fe alloy, which achieved over a 50% C₂₊ selectivity and more than a 30% selectivity of ethylene and propylene under reaction conditions of 300 °C and 5 bar. Theoretical calculations showed that the NC carrier significantly altered the electronic density of the Co-Fe alloy, favoring the formation of C-C bonds.

4.4. CO₂ Hydrogenation to CH₃OH over Nitrogen-Doped Carbon Materials

Methanol, as one of the potential products of CO₂ hydrogenation, is widely used for solvents, fuels, and platform chemicals. Limleamthong et al. [127] used nitrogen-doped porous carbon as a support, as well as loading Cu-Zn/Ce. The catalyst showed a space–time yield of 512 mg_MeOH g_cat⁻¹ h⁻¹ at a reaction temperature of 250 °C and pressure of 1.5 MPa. Sun et al. [128] synthesized catalysts dominated by different N species through different treatment methods and discussed the role of different N species in CO₂ hydrogenation to methanol. The results showed that pyridinic N could lower the reduction temperature of CuO, and inhibit the growth of Cu particles. Moreover, pyridinic N could promote the dispersion of Cu and enhance the dissociation adsorption of H₂. Similar conclusions were obtained in the study of Deerattrakul et al. [129]. Ma et al. [130] prepared CuZnAl catalysts with different contents of N-doped graphene via co-precipitation for methanol synthesis. The results indicated that there was an optimal value for the content of N-doped graphene (10 wt%). Initially, as the nitrogen-doping carbon material’s content increased, it led to the generation of oxygen vacancies, promoting the reduction of Cu species. As the NC content continued to increase, the H₂ adsorption was enhanced and a reduction was facilitated. However, with the further increase in the nitrogen-doping carbon materials, the Cu sites could be covered, making the Cu reduction more difficult. Donphai et al. [131] used PBZ-derived C materials and doped them with different N sources (i.e., ammonia, urea, and arginine). Among the three N sources, arginine led to a decrease in the pore structure and specific surface area of the catalyst, which was not favorable for the loading of active components. Additionally, the low N content resulted in a decrease in CO₂ adsorption. The other two N sources also slightly altered the physical structure. Thermogravimetric analysis showed an enhanced thermal stability in the N-doped materials. However, the three different N sources did not have a significant impact on the reaction results. The performance data of catalysts in the published literature are summarized in

Table 6. Performance of various catalysts in CO₂ hydrogenation to CH₃OH.

Catalyst	Pressure (bar)	Temperature (°C)	GHSV	H2:CO Ratio	CO2 Conversion (%)	STY of Methanol (mg g−1cat h−1)	Reference
CZ/NCMK-3U	15	250	2444 a	3	43.0	512.0	[127]
CC/NCMK-3U	15	250	2444 a	3	30.0	367.0	[127]
CZ/CNTs-N	30	260	3600 b	3	11.5	102.2	[128]
15%CuZn/NrGOae-H	15	250	2444 a	3	20.5	329.4	[129]
15%CuZn/NrGOae-A	15	250	2444 a	3	16.4	264.4	[129]
15%CuZn/NrGOae-U	15	250	2444 a	3	24.2	405.5	[129]
10NG-CZA	30	200	10 #	3	8.2	N.A.	[130]
Cu-Ru-Ce/Am-P123	15	210	2400 a	3	27.0	530	[131]
Cu-Ru-Zr/Am-P123	15	210	2400 a	3	37.0	642	[131]
Cu-Ru-Ce/U-P123	15	210	2400 a	3	26.0	504	[131]
Cu-Ru-Zr/U-P123	15	210	2400 a	3	36.0	639	[131]

^a GHSV, h⁻¹; ^b WHSV, mL/g/h; ^# g·h/mol.

.

4.5. CO₂ Hydrogenation to CHOOH over Nitrogen-Doped Carbon Materials

Formic acid is an important chemical which possesses a good hydrogen storage capacity (by weight and volume). Therefore, it is considered to be an ideal hydrogen storage material [132], and a feasible CO₂ conversion and utilization solution. Yang et al. [133] prepared a nitrogen-doped, hollow, carbon-supported PdAg alloy catalyst for formic acid production. The changes in the thermal decomposition temperature affected the material’s specific surface area (which decreased as the temperature increased and the carbon shell became thinner), the dispersion of active components (an increased particle size with higher temperature), N content (which decreased with higher temperature), and the formation of Pd-N bonds. The introduction of N increased CO₂ adsorption and H₂ dissociation, while reducing the reaction activation energy. The decrease in carbon material defects upon the addition of the active phase indicated their combination, highlighting the important role of the N anchoring effect. Ahn et al. [134] loaded Ru onto N-doped mesoporous hollow carbon spheres. Through cyclic tests, it was found that Ru would leach out, but the amount of Ru leaching was reduced after N doping, benefiting from the strong binding energy between the Ru species and the pyridinic N and pyrrolic N sites, thereby enhancing the catalyst’s stability. Catalysts with a higher content of graphitic N exhibited serious leaching in cyclic tests, suggesting that the interactions between pyridinic N, pyrrolic N, and Ru were stronger than those with graphitic N. DFT calculations showed that the adsorption energy of Ru and pyridinic N was lower than that of graphitic N, indicating greater stability of Ru-pyridinic N. However, experimental results showed an improved catalytic performance with an increasing proportion of graphitic N species, implying that Ru-graphitic N might be the active site for the reaction. Jaleel et al. [135] found through calculations that among the various modes of N and Ru interaction, the bipyridine N site had the most stable binding with Ru (as shown in Figure 14), which could reduce the loss of Ru³⁺ during the reaction. Additionally, nitrogen doping increased the electron density of Ru. Zhang et al. [136] incorporated main group metals (B, Al, Ga) into N-doped materials and designed catalysts with a frustrated Lewis acid/base pair structure. Simulation calculations revealed that all three catalysts were favorable for the activation and dissociation of H₂. Particularly, the catalysts designed with Al and Ga displayed a lower activation energy for the rate-determining step, indicating their potential as promising catalysts.

Compared to CO hydrogenation, the study of N-doped carbon materials in CO₂ hydrogenation started later. The surface alkalinity of nitrogen-doped carbon materials is stronger compared to traditional supports, making it easier for CO₂ (Lewis acid) to adsorb. Moreover, the anchoring effect of N species themselves allows researchers to load a larger amount of active components on the carbon support, with high dispersion. N-doped carbon materials have unique advantages compared to traditional supports. The performance data of catalysts in the published literature are summarized in

Table 7. Performance of various catalysts in CO₂ hydrogenation to CHOOH.

Catalyst	Pressure (bar)	Temperature (°C)	Reaction Time(h)	H2:CO Ratio	Formate Conc (M)	TON	Reference
[email protected]	20	100	2	1	N.A.	640	[135]
Ru/N-MCHS-900	80	120	2	1	0.950	7550	[136]
2Ru/NC750	80	120	2	1	1.374	18,212	[136]

.

5. Concluding Remarks

In the face of the dual crisis of energy and environment, researchers have conducted extensive research on CO_x hydrogenation conversion and utilization. The advantage of carbon materials over traditional carriers lies in the weaker metal–carrier interaction. Furthermore, the confinement effect of carbon materials allows for the easy separation of active components, effectively preventing their aggregation and leaching during the reaction process. The role of N doping in carbon materials is as follows: nitrogen serves as an anchoring site for the better dispersion of active components, leading to a higher utilization of metals. The anchoring effect can also prevent the sintering of active components during catalytic reactions. The electron-donating effect of N atoms increases the surface alkalinity of the catalyst, facilitating the adsorption of CO₂. It can be observed that N-doped carbon materials not only increase the adsorption of the reactants (CO₂) but also promote their dissociation. Compared to traditional metal oxide carriers, nitrogen-doped carbon materials have unique advantages.

According to current research findings, there are several challenges in the application of nitrogen-doped carbon materials in CO_x hydrogenation reactions. Firstly, the main challenge in this field is the high cost of nitrogen-doped carbon material synthesis, which hinders its large-scale application in CO_x hydrogenation. Developing simple and cost-effective synthesis methods is crucial. Secondly, the diversity of nitrogen species in nitrogen-doped carbon materials hinders the understanding of the specific roles of certain nitrogen species. Thirdly, carbon materials have poor stability in oxidative atmospheres, posing difficulties in catalyst preparation and regeneration. Fourth, among the numerous synthesis methods, there is a question of how to select the most favorable method for the reaction. It is particularly important to compare the effects of different doping methods on the reaction while controlling the physical and chemical properties of the materials to be similar. Lastly, nitrogen atoms on the surface of nitrogen-doped carbon materials may be lost under harsh reaction conditions, leading to the sintering of the active components.

Currently, many investigations have been performed in this regard, but many issues of CO_x hydrogenation to value-added products have not been solved yet.

• Methods to improve the stability of nitrogen atoms on carbon materials and increase the nitrogen content need to be explored in relation to the material itself. On the one hand, nitrogen atoms serve as effective anchoring sites for active components, but the quantity of nitrogen atoms tends to decrease with the increasing pyrolysis temperature of the material. On the other hand, high pyrolysis temperatures often enhance the graphitization of carbon materials, facilitating electron transfer. These two processes seem contradictory and it appears difficult to achieve both simultaneously. If the interaction between nitrogen atoms and carbon materials can be strengthened (adding functional groups to the surface of the material), allowing nitrogen atoms to remain well-preserved on the surface during high-temperature pyrolysis, it would contribute to further enhancing the catalytic activity of the catalyst. Currently, there are limitations in our understanding of the role of nitrogen species. Nitrogen-doped carbon materials prepared using the current doping methods have complex nitrogen species, and it is difficult to precisely control the content of the corresponding components. This greatly restricts our understanding of the specific roles of nitrogen species (such as which of pyridine N and pyrrole N has the stronger electron-giving ability and the stronger anchoring ability), and causes significant confusion in explorations of the synthesis and mechanisms of subsequent catalytic materials. It is challenging to introduce a specific nitrogen species onto the surface of traditional carbon materials. Therefore, it is crucial to modify the surface of carbon materials or synthesize them through the pyrolysis of raw materials containing specific nitrogen species. Carbon materials with specific nitrogen species can help us to understand the active sites of reactions and provide important support for speculating reaction pathways.
• With the rapid development of porous carbon technology, nitrogen-doped carbon materials with a tunable pore size and hierarchical pore structure can be developed. N-doped carbon materials with adjustable microporous and mesoporous structures can be prepared, featuring a layered structure that connects mesopores and macropores. This facilitates the investigation of the pore structure’s effect on the reaction. Micro-porous structures can spatially restrict the distribution of active components and promote the formation of nanoscale particles. At the same time, the layered structure is beneficial for improving the mass and heat transfer of the catalyst, which has a positive effect on enhancing the reaction activity and improving the product distribution.
• A comprehensive insight into active species and their interactions, along with the interactions among the active centers with support, are of paramount significance for catalyst design and tailoring. Therefore, in situ characterization techniques (such as XRD, TEM, and IR spectroscopy) should be employed to distinguish active sites. Additionally, analysis methods such as XPS and DFT calculations are needed to identify the major surface species and explain the elusive reaction mechanisms of CO_x hydrogenation to value-added products. Moreover, in situ structural and surface studies can be conducted under working conditions to validate the proposed mechanisms.

The purpose of this review is to summarize the recent research on the application of N-doped carbon materials in CO_x hydrogenation. It aims to provide insights for the future development of N-doped carbon materials.