Highly efficient CO₂ hydrogenation to long-chain linear α-olefins via CO intermediate enrichment over Na/FeMn/ZrO₂ catalysts

Abstract

Although significant progress has been made in the oriented conversion of CO₂ to long-chain linear α-olefins (LAOs), cooperatively regulating C–O bond activation and C–C coupling via tailored catalyst microstructures remains a persistent challenge. Herein, a highly efficient Na/FeMn/ZrO₂ catalyst has been fabricated through a covalent anchoring strategy, which achieves a LAOs/C₄₊ selectivity of 68% and an O/P ratio of 5.1 in CO₂ hydrogenation to LAOs. There is a pronounced interaction between Fe species and MnCO₃ in Na/FeMn/ZrO₂ catalysts, which promotes the formation and stabilization of iron carbides. Meanwhile, Fe₅C₂–ZrO₂ interfaces possess strong adsorption capacity for CO intermediates, resulting in the accumulation of generated CO on the Fe₅C₂ active sites. The higher CO concentration on the Fe₅C₂–ZrO₂ interface is beneficial to the C–C coupling reaction, thereby significantly improving the production of high-value olefins. These results will provide a theoretical basis and guidance for developing efficient catalysts for the oriented conversion of CO₂ to LAOs.

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Introduction

With rapid economic development, fossil energy consumption continues to rise, emitting substantial anthropogenic carbon dioxide (CO₂) into the environment and causing serious environmental consequences.^1–5 Directly converting CO₂ into chemicals via integration with green hydrogen technology not only reduces overdependence on fossil fuels and alleviates CO₂-induced environmental problems, but also provides an effective strategy for CO₂ resource utilization and high value-added chemical production.^6–9 CO₂ hydrogenation can produce CO,¹⁰ light olefins,^11,12 gasoline,¹³ methanol,¹⁴ jet fuels,¹⁵ and long-chain linear α-olefins.¹⁶ Among these products, long chain linear α-olefins (LAOs, C⁼₄₊ a terminal carbon–carbon double bond) are important chemical intermediates that are used in synthetic lubricants, new polymers, high-octane gasoline, corrosion inhibitors, and agricultural chemicals.^17–19 Currently, LAOs are mostly generated by thermal cracking of petroleum resources. Therefore, the oriented conversion of CO₂ coupled with green hydrogen to LAOs is important for the sustainable development of feedstocks.

CO₂ hydrogenation to LAOs mainly involves the reverse water–gas shift (RWGS) reaction and C–C coupling reaction. In this process, CO₂ is first activated to CO via RWGS, and then LAOs are obtained through C–C coupling.^20,21 Fe-based catalysts are widely used for CO₂ hydrogenation to LAOs due to their combination of Fe-oxides for the RWGS reaction and Fe-carbides for the C–C coupling reaction, as well as low CH₄ selectivity.^22,23 Meanwhile, the modified Fe-based catalyst exhibited excellent catalytic activity and olefin selectivity in CO₂ hydrogenation reactions.^24,25 This demonstrated the excellent potential of modified Fe-based catalysts in CO₂ hydrogenation to LAOs. However, the generated H₂O and CO₂ can oxidize the Fe carbides, thereby weakening the C–C bond coupling ability and resulting in insufficient LAOs selectivity. Therefore, the rational construction of Fe-based catalysts for the CO₂ hydrogenation to LAOs is essential to improve LAOs selectivity.

Currently, research on Fe-based catalysts for CO₂ hydrogenation to LAOs mainly focuses on promoter modification, catalyst structure optimization, and active-site environment modulation.^26–31 The Zn promoter reduced the particle size of Fe-based catalysts and increased the adsorption of H₂, thus improving CO₂ hydrogenation activity.³² The Na promoter efficiently enhanced CO* and H* activation, facilitated C–C coupling, and suppressed the secondary hydrogenation of olefins, thereby improving both catalytic activity and LAOs selectivity.³³ Meanwhile, the Na promoter could be enriched on the catalyst surface, which can improve olefin selectivity by inhibiting the secondary hydrogenation of primary olefins.³⁴ Sr and Na co-modified Fe-based catalysts exhibited stable catalytic performance in CO₂ hydrogenation to LAOs. SrCO₃ promoted the formation and stabilization of the FeC_x phase. Meanwhile, the Sr promoter strengthened electronic interactions between Na and Fe, thereby improving C–O bond dissociation and C–C coupling.³⁵ A Cu–Fe catalyst consisting of Cu, Fe-oxides and Fe carbides achieved a C⁼₄₊ selectivity of 66.9% under atmospheric pressure. The synergistic interaction between Fe₅C₂ and the Cu–Fe₅C₂ interface promoted the generation of long-chain olefins.³⁶

Although significant progress has been made in CO₂ hydrogenation to LAOs, effective control of C–O bond activation and C–C bond coupling through interface structures remains challenging. Developing highly efficient catalysts to enhance C–C coupling is crucial for improving LAOs selectivity. By modulating promoter content, the active-phase composition (Fe₃O₄ and Fe₅C₂) can be effectively regulated to optimize RWGS/FTS reaction matching. Fe–Mn–K catalysts exhibited excellent catalytic performance with a CO₂ conversion of 32.9% and LAOs selectivity of 45.7%.³⁷ Significant progress has been made in understanding the stabilizing and promoting effects of Mn on iron carbides in CO₂ hydrogenation reactions. The Mn promoter significantly reduced the rates of reduction and carbonization reactions, inducing pronounced reaction-induced evolution of the Fe₅C₂ phase. A surface layer of MnO converting Fe₅C₂ effectively inhibited CH₄ formation, while promoting the production of light olefins and C₅₊ hydrocarbons.³⁸ Guo et al.³⁰ employed multiple in situ techniques to reveal the structural evolution of Fe-based catalysts during CO₂ hydrogenation to hydrocarbons. In the initial reduction phase, Fe species undergo carbonization to form Fe₃C, which subsequently transforms into Fe₅C₂. The generated H₂O during CO₂ hydrogenation to LAOs could oxidize Fe₅C₂ to the Fe₃O₄ phase. The Fe₅C₂–ZrO₂ interfacial structure enhanced the adsorption of key CO intermediates, forming a CO-enriched microenvironment on the Fe₅C₂ active phase surface. This significantly increases the partial pressure of CO on the Fe₅C₂ phase, thereby reducing the adsorption energy of H₂O on the Fe₅C₂ surface.^39,40 Consequently, it effectively improved catalyst stability and C–C coupling capability. The introduction of Mn promoted the dispersion of Fe species and enhanced the formation of Fe oxides, while the addition of the K promoter favored the generation of Fe carbides, thus improving carbon-chain growth capability. The incorporation of Mn and K effectively modulates the balance between the RWGS and FTS reactions by promoting the formation and conversion of CO intermediates.^37,41 In FeZr catalysts, oxygen vacancies in ZrO₂ promote CO₂ adsorption and activation.⁴² Although significant progress has been achieved with K/FeMn catalysts in CO₂ hydrogenation to LAOs, research has mainly focused on regulating the RWGS and FTS active sites through the Mn promoter to achieve high olefin selectivity. In contrast, K–FeZr catalysts primarily modulate CO₂ adsorption and H₂ dissociation through oxygen vacancies, exhibiting excellent selectivity toward light olefins in CO₂ hydrogenation. Therefore, the Fe₅C₂–ZrO₂ interfacial structure can create a CO-enriched microenvironment at the Fe₅C₂ active sites by utilizing the strong CO adsorption capacity of oxygen vacancies on the ZrO₂ surface. This effectively increases the partial pressure of CO on the Fe₅C₂ surface and reduces the adsorption energy of H₂O, thereby suppressing the oxidation of the Fe₅C₂ active phase by H₂O and significantly enhancing the C–C coupling capability.⁴³ Meanwhile, the interfacial structure not only facilitates the transport of key intermediate species, but also lowers the activation energy barrier for C–C coupling.³⁶ Therefore, constructing Fe₅C₂–ZrO₂ interfacial catalysts should effectively promote LAOs synthesis in CO₂ hydrogenation.

In this work, we report a Na/FeMn/ZrO₂ catalyst that demonstrated high catalytic performance for CO₂ hydrogenation to LAOs. Under reaction conditions of 320 °C, 1.5 MPa, and 7500 mL g_cat⁻¹ h⁻¹, it achieved a LAOs selectivity in C₄₊ hydrocarbons of 68.0% and an O/P ratio of 5.1. The structure–performance relationships were revealed by various characterization techniques. The strong CO adsorption capacity at the Fe₅C₂–ZrO₂ interface enriched CO intermediates on Fe₅C₂ active sites, promoting C–C coupling reactions. Meanwhile, strong interfacial interactions enhanced synergy among various active sites, resulting in superior selectivity for high-value olefins.

Results and discussion

Active phase characterization

The active phase composition of the catalysts was analyzed by XRD. The calcined samples of Fe, Na/Fe, and Na/FeMn exhibited characteristic diffraction peaks of the Fe₂O₃ phase (JCPDS, 33-0664) (Fig. S1). However, the diffraction peak intensities of the Fe₂O₃ phase in the Na/FeMn/SiO₂ and Na/FeMn/ZrO₂ catalysts were much weaker (Fig. S1). XRD patterns of the reduced catalysts are shown in Fig. 1. The characteristic diffraction peaks of the Fe phase (JCPDS, 06-0696) appeared in the reduced Fe and Na/Fe catalysts, while a mixed phase of Fe, FeMnO_x, and Mn₂O (JCPDS, 07-0230) appeared in the reduced Na/FeMn catalyst. However, no significant Fe phases were detected in the reduced Na/FeMn/SiO₂ and Na/FeMn/ZrO₂ catalysts, indicating that the reduced Fe species were uniformly dispersed on SiO₂ and ZrO₂. Only characteristic diffraction peaks of the Fe₃O₄ phase (JCPDS, 75-1609) were detected in the spent Fe catalysts. Compared with the pure Fe catalyst, the Na/Fe catalyst exhibited a weak Fe₅C₂ phase (JCPDS, 89-2544), indicating that the Na promoter was beneficial to the formation of the Fe₅C₂ phase.⁴⁴ The MnCO₃ phase (JCPDS, 86-0173) appeared in the spent Na/FeMn catalyst, which stabilized the active phase and improved the catalytic activity owing to the thermal stability and electronic interactions of alkaline earth metal carbonates.⁴⁵ No significant Fe carbide phase was detected in the spent Na/FeMn/SiO₂ catalysts, because the interaction between Fe species and SiO₂ hydroxyl groups from Fe–O–Si bonds was unfavorable for the formation of iron carbides. However, both Fe₃O₄ and Fe₅C₂ phases (JCPDS, 89-2544) appeared in the spent Na/FeMn/ZrO₂ catalyst, indicating that Fe species can be further carbonized to form Fe₅C₂. The strong interaction between Fe and ZrO₂ could promote iron carbide formation.⁴⁰

Fig. 1 XRD patterns of the reduced (a) and spent (b) catalysts.

In situ XRD analysis was performed to reveal the phase evolution during H₂ reduction and CO₂ hydrogenation reaction processes over the Na/FeMn/ZrO₂ catalyst. The characteristic diffraction peaks of Fe₂O₃ and ZrO₂ were mainly detected on the Na/FeMn/ZrO₂ catalyst (Fig. S2). The characteristic diffraction peaks of Fe₂O₃ were weak, indicating that the Fe species were uniformly distributed on ZrO₂. The interaction between the complex (formed by the ethylene diamine tetraacetic acid and Fe species) and hydroxyl groups at oxygen vacancies of ZrO₂ was reported to form small sized Fe₂O₃.⁴⁶ Increasing the reduction temperature to 350 °C caused disappearance of Fe₂O₃ diffraction peaks with no detectable Fe₃O₄ diffraction peaks (Fig. 2a). That is, Fe₂O₃ could be completely reduced at 400 °C and the reduced Fe species were uniformly dispersed on the ZrO₂ surface. After reduction, the system was cooled to 320 °C and then the feedstock (H₂/CO₂ = 3) was introduced. As the reaction progressed, the characteristic peaks of Fe₃O₄, Fe₅C₂, and MnCO₃ appeared. The reappearance of Fe₃O₄ was attributed to the oxidation of FeO or Fe during the reaction (Fig. 2b). The resultant Fe₃O₄ and Fe₅C₂ were the active sites for the RWGS and FTS reactions, respectively. Importantly, ZrO₂ remained unchanged in its monoclinic phase, indicating its stable presence during the CO₂ hydrogenation reaction (Fig. S3).

Fig. 2 In situ XRD patterns of the Na/FeMn/ZrO₂ catalyst during H₂ reduction (a) and the CO₂ hydrogenation reaction (b). Reduction process: pure H₂, 30 mL min⁻¹. Reaction process: H₂/CO₂ = 3, 320 °C, 30 mL min⁻¹, atmospheric pressure.

Morphology and textural characteristics

SEM images clearly exhibited the microscopic morphology of the catalysts. The fresh Na/FeMn/ZrO₂ catalysts exhibited a stack of spherical nanoparticles around 50 nm (Fig. S4). Compared with the fresh Na/FeMn/ZrO₂ catalyst, the spent catalyst displayed larger particles and elongated crystals, due to the formation of some MnCO₃ and FeMnO_x species during the reaction. The microstructure of fresh and spent Na/FeMn/ZrO₂ catalysts was further characterized using the HR-TEM technique, and the results are shown in Fig. 3 and S5. The fresh Na/FeMn/ZrO₂ catalysts possessed an irregular crystal structure with rough edges, and the particle size of the Na/FeMn/ZrO₂ catalysts increased after the reaction (Fig. 3a and c). The intimacy of catalyst components had a great influence on the catalyst performance. The HAADF-STEM images and the corresponding EDS elemental maps of the fresh and spent Na/FeMn/ZrO₂ catalysts were analyzed in detail. The elements Fe, Mn, and Na in the Na/FeMn/ZrO₂ catalysts were almost uniformly distributed on the ZrO₂ support (Fig. 3e and S5).

Fig. 3 TEM images of the fresh (a and b) and spent (c and d) Na/FeMn/ZrO₂ catalysts. STEM image of the spent Na/FeMn/ZrO₂ catalyst (e) and the corresponding EDS elemental mapping images.

Meanwhile, Na, Fe, Mn, and Zr species exhibited relatively close contact (Fig. S6). The closely packed MnCO₃ effectively stabilized the Fe₅C₂ active phase, inhibiting the oxidation that generated H₂O molecules and thereby promoting C–C coupling reactions.⁴⁷ To further characterize the Fe₅C₂–ZrO₂ interface structure, EDS mapping of Fe and Zr elements was performed (Fig. S7). The mapping of Fe and Zr in the Na/FeMn/ZrO₂ catalyst clearly revealed an interfacial structure.

To further elucidate the elemental composition of the catalyst, ICP analysis was performed, and the results are summarized in Table S1. The elemental compositions of the Na/FeMn, Na/FeMn/SiO₂, and Na/FeMn/ZrO₂ catalysts were found to be consistent with theoretical values. The surface area and pore size distribution of the synthesized catalysts were analyzed using N₂ adsorption–desorption measurements, and the results are summarized in Table S2. The addition of the Na promoter to the Fe catalyst increased its specific surface area (from 23.7 m² g⁻¹ to 25.0 m² g⁻¹) and enlarged the average pore size from 11.5 nm to 13.8 nm. Compared with Na/Fe, Na/FeMn exhibited smaller specific surface area, pore size, and pore volume due to nanoparticle aggregation caused by the promoter. Na/FeMn/SiO₂ catalysts had the smallest specific surface area and pore size. Relative to Na/FeMn, Na/FeMn/ZrO₂ showed significantly higher specific surface area and pore volume alongside reduced pore size. These changes indicate that ZrO₂ improved the dispersion of active sites by increasing the specific surface area, which was beneficial for the reaction and diffusion of feedstocks/intermediates.

Chemical state, reduction and chemisorption behavior

To determine the chemical state of the catalyst, we carried out XPS analysis of the spent catalysts with the results shown in Fig. 4. The binding energies of the Fe 2p spectra of all spent catalysts exhibited two characteristic peaks at 710.8 eV and 723.9 eV, which were assigned to Fe 2p_3/2 and Fe 2p_1/2 of iron oxide species, respectively (Fig. 4a).⁴⁸ The Na/Fe, Na/FeMn, Na/FeMn/SiO₂, and Na/FeMn/ZrO₂ catalysts displayed characteristic peaks at 706.8 eV, which were attributed to Fe carbide species.⁴⁸ No characteristic peaks of Fe carbide were detected in the fresh catalyst and the reduced catalyst. Meanwhile, no such signal was detected in the Fe catalyst, indicating that Na and Mn promoters play a crucial role in the formation of Fe carbide species. Mn 2p XPS spectra exhibited two characteristic peaks at 641.7 eV and 653.5 eV, which corresponded to Mn 2p_3/2 and Mn 2p_1/2 of manganese oxide species, respectively (Fig. S8). However, the spent catalysts displayed two shoulder peaks, which were attributed to the characteristic features of Mn²⁺ in MnCO₃ species (Fig. S8). Notably, the C 1s binding energy of Na/FeMn, Na/FeMn/SiO₂, and Na/FeMn/ZrO₂ catalysts shifted to 289.2 eV, which was higher than that of Fe and NaFe catalysts (at 288.7 eV) (Fig. 4b). The apparent change in binding energy further revealed strong electron transfer and interfacial interaction between Fe species and Mn species.³⁰ The O 1s spectra displayed two characteristic peaks corresponding to oxygen vacancy and lattice oxygen, appearing at 532.1 and 529.6 eV, respectively (Fig. 4c and S11).⁴⁹ Fresh Na/FeMn samples show higher oxygen vacancies and lattice oxygen, while fresh Na/FeMn/SiO₂ samples predominantly contain lattice oxygen. After reduction, the oxygen vacancies in the Na/FeMn catalyst decreased significantly, while those in Na/FeMn/SiO₂ and Na/FeMn/ZrO₂ samples increased. The oxygen vacancy was beneficial to the CO₂ activation.⁵⁰ Although the spent Na/FeMn/SiO₂ sample possessed abundant oxygen vacancies, its catalytic activity and olefin selectivity were relatively low. This was primarily attributed to Fe species interacting with silicon hydroxyl groups to form Fe₂SiO₄, which was difficult to further reduce into Fe₅C₂, resulting in insufficient C–C coupling active sites in the Na/FeMn/SiO₂ catalyst.^51,52 The Zr 3d binding energy of Na/FeMn/ZrO₂ exhibited two peaks at 183.8 eV and 181.4 eV, which were attributed to ZrO₂ species (Fig. S12). The binding energy of the Si 2p spectra of Na/FeMn/SiO₂ appeared at 101.8 eV, which was associated with SiO₂ species (Fig. S13). These results indicated that Mn and ZrO₂ promoted the dissociation of lattice oxygen to generate oxygen vacancies during the reduction process, thereby improving CO intermediate adsorption and C–C coupling. The surface elemental compositions of the spent catalyst are listed in Table S3. Compared with Fe, the Na/Fe catalyst exhibited lower surface Fe content and higher C content, indicating that the Na promoter enhanced the carbonization capability of Fe species. With the introduction of the Mn promoter, the surface Fe content decreased, which was attributed to the Mn species migrating to the surface of Fe species.³⁸ Compared with the Na/FeMn/SiO₂ catalyst, the Na/FeMn/ZrO₂ catalyst exhibited higher carbon content, indicating greater susceptibility to carbonization and formation of Fe₅C₂ active sites.

Fig. 4 Fe 2p (a), C 1s (b), and O 1s (c) XPS spectra of the spent catalysts. (d) H₂-TPR profiles of the synthesized catalysts.

H₂-TPR experiments were performed to evaluate the interactions between the species. In Fig. 4d, H₂ consumption peaks at 365 °C and 527 °C for the Fe catalyst represented the gradual reduction of Fe₂O₃ to Fe. Specifically, the H₂ consumption peak at 365 °C was attributed to the reduction of Fe₂O₃ to Fe₃O₄, while the broad peak near 527 °C was attributed to the reduction of Fe₃O₄ to FeO and the further reduction of FeO to Fe. Compared with the Fe catalyst, Na/Fe exhibited a H₂ consumption peak with higher temperature, indicating that the Na promoter inhibited the reduction of Fe₂O₃. MnO₂ samples exhibited three reduction peaks at 308 °C, 422 °C, and 510 °C (Fig. S14). The reduction peak at 361 °C for Na/FeMn was weakened, while the high-temperature peaks increased, indicating that the transition metal enhanced the Fe–O bond energy.⁵³ For the Na/FeMn catalyst, H₂ consumption peaks near 361 °C and 462 °C were attributed to the reduction of Mn species. Relative to the Na/Fe catalyst, the Na/FeMn catalyst exhibited significantly lower H₂ consumption, with all Fe species reduction peaks shifting to higher temperatures. This indicates that Mn species strengthened the Fe–O bond energy.⁵⁴ In comparison, the reduction peaks of the Na/FeMn/SiO₂ catalysts all shifted to high temperature, which was attributed to the formation of the Fe–O–Si bond.⁵⁵ Compared with Na/FeMn catalysts, the reduction peak of the Na/FeMn/ZrO₂ catalyst shifted to a lower temperature, which was beneficial to the reduction of Fe species.⁵³ This shift was attributed to ZrO₂ enhancing the migration and reactivity of hydrogen species.⁵⁶ These results further indicated strong interactions among Fe, Mn, and Zr species, which had a positive effect on the stability of iron carbide species. The H₂ consumption of as-prepared catalysts is summarized in Table S4. Compared with the Fe catalyst, the Na/Fe catalyst exhibited lower low-temperature H₂ consumption and higher high-temperature H₂ consumption. The introduction of the Mn promoter enhanced the formation of FeMnO_x spinel, which hindered the reduction of both Fe and Mn species, leading to decreased H₂ consumption. The H₂ consumption of Na/FeMn/SiO₂ and Na/FeMn/ZrO₂ decreased due to reduced Fe content from the introduced supports. Compared with Na/FeMn/SiO₂, the Na/FeMn/ZrO₂ catalyst exhibited higher low-temperature H₂ consumption, while lower high-temperature consumption, indicating greater susceptibility to H₂ reduction.

To elucidate the CO adsorption of the Na/FeMn/ZrO₂ catalyst, CO-TPD analysis was performed on reference ZrO₂ and Na/FeMn/ZrO₂ catalysts (Fig. S15). The ZrO₂ sample exhibited a weak CO desorption peak at 291 °C, indicating its relatively low CO adsorption. Compared with the ZrO₂ sample, the Na/FeMn/ZrO₂ catalyst exhibited a pronounced CO desorption peak at 313 °C, indicating that the Na/FeMn/ZrO₂ catalyst with an Fe₅C₂–ZrO₂ interfacial structure possesses strong CO adsorption. The strong adsorption capacity for CO enabled the formation of a high partial pressure of CO on the surface of the Na/FeMn/ZrO₂ catalyst, thereby inhibiting the adsorption of H₂O molecules on the Fe₅C₂ active sites and effectively promoting the C–C coupling reaction.⁴³ CO-DRIFTS was employed to further investigate the adsorption configuration and strength of CO on the surface of the Na/FeMn/ZrO₂ catalyst (Fig. S16). The adsorption and desorption spectra of the Na/FeMn/ZrO₂ catalyst exhibited peaks at 2170 cm⁻¹ and 2118 cm⁻¹, respectively, which was attributed to the adsorption of gaseous CO. Peaks observed at 2068 cm⁻¹, 2054 cm⁻¹, and 2031 cm⁻¹ correspond to linear CO adsorption. These linear CO adsorption peaks markedly intensify with extended adsorption time. The adsorbed CO could enhance the partial pressure of CO at the Fe₅C₂ active sites, thereby promoting C–C coupling reactions.^43,57

The effective adsorption of CO₂ on the active sites of the catalysts could promote the activation of C–O bonds and the generation of CO intermediates. CO₂-TPD analysis was employed to elucidate CO₂ adsorption behavior of the catalysts (Fig. S17). The Na/Fe catalyst displayed two desorption peaks, where the low- and high-temperature peaks were attributed to the interaction of CO₂ molecules with weak and strong base sites, respectively. Compared with the Na/Fe catalyst, Na/FeMn catalysts exhibited stronger CO₂ desorption peaks at 200 °C, suggesting that Na and Mn promoters enhanced CO₂ adsorption. The desorption peak of CO₂ for the Na/FeMn/SiO₂ catalyst was significantly weakened, indicating that the Na/FeMn/SiO₂ catalyst had a weaker adsorption capacity for CO₂.⁵⁸ The Na/FeMn/ZrO₂ catalyst exhibited a strong CO₂ desorption peak near 100 °C, indicating that the introduction of ZrO₂ increased CO₂ adsorption capacity despite weak binding strength. These results indicated that the introduction of Mn species and ZrO₂ modulated the amount and strength of CO₂ adsorption.⁵⁹ The suitable CO₂ adsorption on catalyst active sites promoted the generation of Fe-carbide active phases, which improved the yield of high-value olefins.

Mössbauer spectroscopy analysis

⁵⁷Fe Mössbauer spectroscopy of the spent Na/FeMn/ZrO₂ catalysts further identified the Fe species in the catalysts, and the results are shown in Fig. 5. The detailed composition and comparison of the different phases are listed in Table S5. Small amounts of Fe₃O₄ species were contained in the spent Na/FeMn/ZrO₂ catalysts, which were mainly detected as Fe₅C₂(II), Fe₅C₂(I), and Fe₅C₂(III), accounting for 31.7%, 26.2%, and 29.9%, respectively.^60–62 Therefore, the synergistic effect of Fe₅C₂ and Fe₃O₄ active sites promoted the highly selective synthesis of high value olefins from CO₂ hydrogenation.

Fig. 5 ⁵⁷Fe Mössbauer results of the spent Na/FeMn/ZrO₂ catalyst (a); CO signal (b) and H₂ signal (c) under CO pre-adsorption and H₂-TPSR measurements.

To further investigate the adsorption, activation, and dissociation strength of CO intermediates, H₂-TPSR measurements were performed on the Na/FeMn and Na/FeMn/ZrO₂ catalysts. In Fig. 5b, the Na/FeMn catalyst exhibited two CO desorption peaks, and the desorption peaks were primarily concentrated in the temperature range of 450 °C to 750 °C. Relative to Na/FeMn, Na/FeMn/ZrO₂ exhibited attenuated CO desorption intensity with peaks shifting to higher temperatures, providing evidence of strengthened CO adsorption. Consequently, substantial CO accumulated at the Fe₅C₂–ZrO₂ interface, where it rapidly migrated to Fe₅C₂ sites, facilitating C–C coupling reactions. Meanwhile, two H₂ consumption peaks appeared on the surface of the Na/FeMn catalyst. The H₂ consumption of the Na/FeMn/ZrO₂ catalyst significantly increased, indicating that the enriched CO underwent dissociation. These results indicated that the Na/FeMn/ZrO₂ catalyst with an Fe₅C₂–ZrO₂ interfacial structure could enrich CO at the interface. The enriched CO underwent dissociation and C–C coupling at Fe₅C₂ active sites, promoting CO₂ hydrogenation toward high-value olefins.

Catalytic performance

The catalytic performance of the prepared catalysts for CO₂ hydrogenation to LAOs is shown in Fig. 6. Detailed results on product distribution, O/P ratio, and LAOs yield are summarized in Table S6. The Fe catalyst exhibited a CO₂ conversion of 29.3%, a LAOs/C₄₊ ratio of 4.1%, and a LAOs yield of 0.2%. Compared with the Fe catalyst, Na/Fe displayed significantly higher CO₂ conversion and C₅₊ selectivity, alongside markedly lower CH₄ selectivity and LAOs selectivity (Fig. 6a). Na content impacts CO₂ hydrogenation by reducing the hydrogen adsorption and activation capacity, thereby reducing the coverage of hydrogen species.⁶³ The primary reaction of CO₂ hydrogenation is suppressed. Compared with the unpromoted Fe catalyst, Na/Fe catalysts exhibited lower selectivity toward CH₄ and C₂–C₄ hydrocarbons. The inhibitory effect of Na on CH₄ and light alkane formation could be attributed to its regulation of the electronic properties of iron carbides, which enhanced the generation of surface carbon-containing species and hydrogen-containing species.⁶⁴ Furthermore, the Na promoter was beneficial to the production of C₅₊ hydrocarbons by modulating the surface carbon-to-hydrogen ratio, primarily following a catalytic surface polymerization mechanism.⁶⁵ The promotion of CO disproportionation (Boudouard reaction) also increased carbonization in Na-promoted catalysts, facilitating the formation of Fe₅C₂ and accelerating C–C coupling reactions.⁶⁶ The Na/FeMn catalyst exhibited higher catalytic activity and LAOs selectivity with a CO₂ conversion of 30.2%, a LAOs/C₄₊ selectivity of 54.8%, a light olefin selectivity of 35.9%, and a LAOs yield of 6.3%. The introduction of the Na promoter decreased CH₄ selectivity from 38.2% to 22.0%, which was attributed to the electron transfer from Na to the Fe₅C₂ phase. The resulting increase in electron density at Fe sites facilitated the CO dissociation and destabilized adsorbed H atoms on the Fe surface, thereby promoting the hydrogenation of carbon species.⁶⁷ The introduction of Mn further enhanced the electron-donating ability of Na and promoted the formation of MnCO₃, leading to Na enrichment on the Fe₅C₂ surface and providing abundant basic sites. The steric hindrance effect induced by high-density sodium enrichment on the Fe₅C₂ surface impairs chain growth capability.⁴⁷ Studies indicated that Na and Mn synergistic effects could enhance RWGS and FTS. Nevertheless, as the Mn/Fe molar ratio increased from 1 : 3 to 1 : 1, CO selectivity reverses, suggesting that higher Mn content weakens FTS activity. Compared with the Na/Fe catalyst, the Na/FeMn catalyst exhibited lower CO₂ conversion and higher CO selectivity. This decline in activity was likely due to coverage of active sites by the Mn promoter. When Mn was added to the Na/Fe catalyst, Mn species dispersed uniformly on the Fe₅C₂ surface. The steric hindrance caused by these Mn species reduced the ability of intermediates to undergo C–C coupling on the Fe₅C₂ phase, thereby inhibiting the formation of C₅₊ hydrocarbons.⁶⁸ The suppressed C–C coupling allowed CO generated via RWGS to accumulate, increasing CO selectivity. These results are consistent with those reported in the literature.⁶⁸ Compared with the Na/FeMn catalyst, Na/FeMn/SiO₂ showed significantly lower CO₂ conversion and LAOs selectivity but higher CO selectivity. This resulted from different reduction/activation of Fe species, which limited chain growth active sites and prevented the generated CO from being activated to undergo C–C coupling for the formation of LAOs.⁶⁹ The Na/FeMn/ZrO₂ catalyst possessed higher CO₂ conversion and LAOs selectivity, but lower CO and CH₄ selectivity than Na/FeMn/SiO₂ (Fig. S18). Among these catalysts, the Na/FeMn/ZrO₂ catalyst possessed a higher LAOs yield of 6.7%. Importantly, the LAOs/C₄₊ selectivity and O/P ratio of the Na/FeMn/ZrO₂ catalyst reached 68.0% and 5.1, respectively (Fig. 6b). For the C₂₊ product distribution, the Fe catalyst had higher selectivity for light alkanes, while Na/Fe exhibited higher light olefin selectivity and C₅₊ alkane selectivity. With Mn promotion, the Na/FeMn catalyst showed significantly higher light olefin and C₅₊ olefin yields than Na/Fe. This indicated that Na and Mn promoters suppressed secondary hydrogenation of primary olefins, thereby boosting selectivity toward high-value olefins. Among these catalysts, the Na/FeMn/ZrO₂ catalyst possessed the highest high-value olefin selectivity (61.3%) and O/P ratio (5.1). The Fe₅C₂–ZrO₂ interfacial structure effectively enhanced the synergism among various active sites and CO intermediate adsorption. Moreover, the CO generated by the RWGS reaction on FeMn sites could be effectively adsorbed at this interface. The significantly increased CO partial pressure on Fe₅C₂ enhanced C–C coupling capability while suppressing olefin secondary hydrogenation, thereby promoting high-value olefin formation and specifically improving LAOs selectivity.

Fig. 6 Catalytic performance toward CO₂ hydrogenation. (a) The hydrocarbon distribution, CO₂ conversion, and CO selectivity. (b) LAOs selectivity, LAOs/C₄₊ ratio, and O/P ratio of catalysts. Reaction conditions: H₂/CO₂ = 3, 1.5 MPa, 320 °C, 7500 mL g_cat⁻¹ h⁻¹, TOS = 10 h. The LAOs/C₄₊ ratio represents the LAO fraction within the C₄₊ hydrocarbons.

Reaction mechanism for CO₂ hydrogenation to LAOs

Compared with other investigated catalysts, Na/FeMn/ZrO₂ exhibited the best high-value olefin selectivity in CO₂ hydrogenation. To reveal the key intermediate species during the reaction, in situ DRIFTS analysis was performed over Na/FeMn/ZrO₂. The peaks in the wavenumber range of 2400 cm⁻¹ to 2300 cm⁻¹ were attributed to the adsorption of gaseous CO₂ (Fig. S19), indicating that the catalyst possessed a strong adsorption capacity for CO₂ molecules.²⁵ As shown in Fig. 7, the vibrational peaks in the range of 1710–1200 cm⁻¹ were assigned to the adsorbed carbonate and formate species ( and HCOO*). These intermediates were formed through the interaction of oxygen vacancies on the catalyst surface with CO₂.⁷⁰ The vibration peak at 1672 cm⁻¹ was attributed to the symmetric and asymmetric stretching vibrations of the bidentate carbonate species. As the reaction proceeded, vibration peaks corresponding to carbonate and bicarbonate species appeared at 1370 cm⁻¹ and 1308 cm⁻¹.⁷¹ At a reaction temperature of 200 °C (Fig. 7a), the vibration peaks at 1370 cm⁻¹ and 1297 cm⁻¹ did not change significantly as the reaction proceeded. This result indicated that the generated species did not fully participate in the subsequent reaction. At a higher reaction temperature of 320 °C (Fig. 7b), the vibrational peaks of species gradually increased as the reaction proceeded, indicating that the initially generated species were rapidly involved in the subsequent reaction. CO intermediates enriched at the Fe₅C₂ active site were beneficial to the C–C coupling.

Fig. 7 In situ DRIFTS of CO₂ hydrogenation to LAOs over the Na/FeMn/ZrO₂ catalyst. (a) H₂/CO₂ = 3, 200 °C, 30 mL min⁻¹ (b) H₂/CO₂ = 3, 320 °C, 30 mL min⁻¹ (reduced conditions: 400 °C, H₂, 30 mL min⁻¹). (c) CO adsorption energy at the Fe₅C₂ and Fe₅C₂–ZrO₂ interfaces. (d) Comparison of energy barriers of CH₂ + CH₂ over Fe₅C₂ and the Fe₅C₂–ZrO₂ interface. (e) Reaction scheme for high-value olefin synthesis from CO₂ hydrogenation over the Na/FeMn/ZrO₂ catalyst.

To further demonstrate CO enrichment on Fe₅C₂ active sites enabled by the Fe₅C₂–ZrO₂ interface, we compared CO adsorption energies on Fe₅C₂ and Fe₅C₂–ZrO₂ using density functional theory (DFT) calculations. To simulate the Fe₅C₂–ZrO₂ interface, we constructed Fe₅C₂(11−2) on the ZrO₂(−111) surface (Fig. S20). As depicted in Fig. 7c, the CO adsorption energy on Fe₅C₂ and the Fe₅C₂–ZrO₂ interface was 1.97 eV and 2.12 eV, respectively. The higher CO adsorption energy on the Fe₅C₂–ZrO₂ interface suggested stronger CO adsorption capacity. The Fe₅C₂–ZrO₂ interface promoted CO adsorption, thereby enriching CO at the Fe₅C₂ active sites. The enriched CO could accelerate the occurrence of C–C coupling reactions. To gain a deeper understanding of the reaction mechanism, we calculated the energy barrier for C–C coupling. CO₂ hydrogenation to high value olefins is a complex process involving multiple possible steps. For simplicity, we employed CH₂ + CH₂ to represent the C–C coupling reaction between CH₂ intermediates. The energy barrier for CH₂ insertion on the Fe₅C₂ surface reached up to 0.81 eV. For the Fe₅C₂–ZrO₂ interface, the energy barrier for CH₂ insertion decreased to 0.64 eV. These results indicated that the Fe₅C₂–ZrO₂ interface not only enabled CO enrichment at Fe₅C₂ active sites, but also lowered the C–C coupling energy barrier, thereby promoting C–C coupling reactions toward high-value olefin production. Therefore, the possible reaction pathways for CO₂ hydrogenation to high-value olefins on Na/FeMn/ZrO₂ catalysts with Fe₅C₂–ZrO₂ interface structures are proposed in Fig. 7e. Notably, the synergistic action of each active sites promoted the RWGS and FTS reactions of CO₂ hydrogenation. CO₂ was adsorbed on the active sites as carbonate species, which underwent hydrogenation to bicarbonate and subsequently to formate species, followed by further hydrogenation yielding dissociated CO intermediates. The CO generated via RWGS was further hydrogenated into species on iron carbide active sites, and then proceeded to C–C coupling for the formation of high-value olefins. Importantly, Fe₅C₂–ZrO₂ significantly enhanced CO adsorption capacity, enriching CO intermediates on Fe₅C₂ active sites and selectively promoting C–C coupling reactions, thereby producing high-value olefins.

Conclusions

In this work, a Na/FeMn/ZrO₂ catalyst featuring an Fe₅C₂–ZrO₂ interfacial structure was prepared via a covalent anchoring strategy. This catalyst exhibited excellent LAOs selectivity and O/P ratio in the directed hydrogenation of CO₂ to LAOs, achieving a LAOs/C₄₊ selectivity of 68% and an O/P ratio of 5.1. During reduction, Mn and ZrO₂ promoted oxygen vacancy formation through lattice oxygen dissociation, thereby enhancing CO₂ adsorption and C–O bond activation. The strong interaction between Fe species and MnCO₃, driven by the electronic effect of alkaline earth metal carbonates, effectively improved the catalytic activity. Concurrently, the Fe₅C₂–ZrO₂ interfaces demonstrated stronger CO adsorption capacity. Elevated CO partial pressure on the Fe₅C₂ surface facilitated the C–C coupling reaction. The synergistic interplay of multiple active sites suppressed the secondary hydrogenation of primary olefins while effectively promoting the generation of LAOs.

Author contributions

Kangzhou Wang: investigation, formal analysis, writing – original draft. Tong Liu: investigation. Pengqi Hai: software. Shunnosuke Fujii: investigation. Chufeng Liu: formal analysis. Hanyao Song: methodology. Caixia Zhu: investigation. Guangbo Liu: supervision. Jianli Zhang: funding acquisition. Zhou-jun Wang: writing – review & editing. Noritatsu Tsubaki: writing – review & editing.

Conflicts of interest

There are no conflicts to declare.

Data availability

The authors confirm that the data supporting the findings of this study are available within the article and its supplementary information (SI). Supplementary information is available. See DOI: https://doi.org/10.1039/d5sc08926c.

Acknowledgements

This work was financially supported in part by the National Natural Science Foundation of China (22368041 and 22378017), the Key Research and Development Program of Ningxia (Talent-Introduction Program, 2023BSB03061), and the Ningxia Natural Science Foundation (2024AAC03049). P. Hai acknowledges the Hefei Advanced Computing Center.

Notes and references

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