Iron–Potassium on Single-Walled Carbon Nanotubes as Efficient Catalyst for CO2 Hydrogenation to Heavy Olefins

Novel heterogeneous Fe-based catalysts for carbon dioxide hydrogenation to long chain α-olefins-A review

The thermocatalytic conversion of carbon dioxide (CO2) into high value-added chemicals provides a strategy to address the environmental problems caused by excessive carbon emissions and the sustainable production of chemicals. Significant progress has been made in the CO2 hydrogenation to long chain α-olefins, but controlling C-O activation and C-C coupling remains a great challenge. This review focuses on the recent advances in catalyst design concepts for the synthesis of long chain α-olefins from CO2 hydrogenation. We have systematically summarized and analyzed the ingenious design of catalysts, reaction mechanisms, the interaction between active sites and supports, structure-activity relationship, influence of reaction process parameters on catalyst performance, and catalyst stability, as well as the regeneration methods. Meanwhile, the challenges in the development of the long chain α-olefins synthesis from CO2 hydrogenation are proposed, and the future development opportunities are prospected. The aim of this review is to provide a comprehensive perspective on long chain α-olefins synthesis from CO2 hydrogenation to inspire the invention of novel catalysts and accelerate the development of this process.

Hollow carbon-based materials for electrocatalytic and thermocatalytic CO2 conversion

Electrocatalytic and thermocatalytic CO2 conversions provide promising routes to realize global carbon neutrality, and the development of corresponding advanced catalysts is important but challenging. Hollow-structured carbon (HSC) materials with striking features, including unique cavity structure, good permeability, large surface area, and readily functionalizable surface, are flexible platforms for designing high-performance catalysts. In this review, the topics range from the accurate design of HSC materials to specific electrocatalytic and thermocatalytic CO2 conversion applications, aiming to address the drawbacks of conventional catalysts, such as sluggish reaction kinetics, inadequate selectivity, and poor stability. Firstly, the synthetic methods of HSC, including the hard template route, soft template approach, and self-template strategy are summarized, with an evaluation of their characteristics and applicability. Subsequently, the functionalization strategies (nonmetal doping, metal single-atom anchoring, and metal nanoparticle modification) for HSC are comprehensively discussed. Lastly, the recent achievements of intriguing HSC-based materials in electrocatalytic and thermocatalytic CO2 conversion applications are presented, with a particular focus on revealing the relationship between catalyst structure and activity. We anticipate that the review can provide some ideas for designing highly active and durable catalytic systems for CO2 valorization and beyond.

Tailoring the CO2 Hydrogenation Performance of Fe-Based Catalyst via Unique Confinement Effect of the Carbon Shell

Even though Fe-based catalysts have been widely employed for CO2 hydrogenation into hydrocarbons, oxygenates, liquid fuels, etc., the precise regulation of their physicochemical properties is needed to enhance the catalytic performance. Herein, under the guidance of the traditional concept in heterogeneous catalysis-confinement effect, a core-shell structured catalyst Na-Fe3 O4 @C is constructed to boost the CO2 hydrogenation performance. Benefiting from the carbon-chain growth limitation, tailorable H2 /CO2 ratio on the catalytic interface, and unique electronic property that all endowed by the confinement effect, the selectivity and space-time yield of light olefins (C2 = -C4 = ) are as high as 47.4 % and 15.9 g molFe -1  h-1 , respectively, which are all notably higher than that from the shell-less counterpart. The function mechanism of the confinement effect in Fe-based catalysts are clarified in detail by multiple characterization and density functional theory (DFT). This work may offer a new prospect for the rational design of CO2 hydrogenation catalyst.

Selective conversion of carbon dioxide into heavy olefins over Ga modified delafossite-CuFeO2

Ga-modified CuFeO2 used as an efficient catalyst for CO2 hydrogenation to heavy olefins (C=5+) can achieve a high heavy olefin selectivity of 53.5%, which lies at a high level among reported catalysts, at a single pass CO2 conversion of 41.5%. It also displays an excellent long-term stability over 100 h, exhibiting its promising potential for industrial applications.

The Effect of Mn, Al Doping on the CO2 Hydrogenation Performance of CaCO3 -Supported Fe-Based Catalysts

With increasingly serious environmental problems caused by the greenhouse effect, it has also become essential to reduce the concentration of CO2 in the atmosphere. In this paper, CaCO3 -supported Fe-based catalysts doped with Mn, Al, and K are prepared by a straightforward method and used for CO2 hydrogenation. The fresh and spent catalysts were characterized by SEM-EDS, BET, TG, CO2 -TPD, XRD, and XPS. The experimental results show that the highest CO2 conversion rate of Fe10Mn2Al10Ca is 35.99 %, the maximum FTY value is 293.98 μmolCO2  ⋅ g Fe - 1 ${{\rm{g}}_{{\rm{Fe}}}^{ - 1} }$  ⋅ s-1 , the maximum O/P value is 6.61, and the lowest CO selectivity is 32.21 %. At the same time, according to the characterization results, the doping of Mn and Al increased the Fe3 O4 /FeCx ratio. As the Fe3 O4 /FeCx ratio increases, the proportion of short-chain hydrocarbons (CH4 , C2-4 ) in the products increases, and the proportion of long-chain hydrocarbons (C5+ ) decrease. Therefore, the co-doping of Mn and Al promotes the conversion of CO and reduces its selectivity, and promotes the formation of light olefins. Finally, it is hoped that this study can provide a reference for further research on CaCO3 -supported Fe catalysts.

In situ catalytic cells for x-ray absorption spectroscopy measurement

In catalysis, determining the relationship between the dynamic electronic and atomic structure of the catalysts and the catalytic performance under actual reaction conditions is essential to gain a deeper understanding of the reaction mechanism since the structure evolution induced by the absorption of reactants and intermediates affects the reaction activity. Hard x-ray spectroscopy methods are considered powerful and indispensable tools for the accurate identification of local structural changes, for which the development of suitable in situ reaction cells is required. However, the rational design and development of spectroscopic cells is challenging because a balance between real rigorous reaction conditions and a good signal-to-noise ratio must be reached. Here, we summarize the in situ cells currently used in the monitoring of thermocatalysis, photocatalysis, and electrocatalysis processes, focusing especially on the cells utilized in the BL14W1-x-ray absorption fine structure beamline at the Shanghai Synchrotron Radiation Facility, and highlight recent endeavors on the acquisition of improved spectra under real reaction conditions. This review provides a full overview of the design of in situ cells, aiming to guide the further development of portable and promising cells. Finally, perspectives and crucial factors regarding in situ cells under industrial operating conditions are proposed.

Effect of Halide Anions on the Electroreduction of CO2 to C2 H4 : A Density Functional Theory Study

The halide anions present in the electrolyte improve the Faradaic efficiencies (FEs) of the multi-hydrocarbon (C₂₊ ) products for the electrochemical reduction of CO₂ over copper (Cu) catalysts. However, the mechanism behind the increased yield of C₂₊ products with the addition of halide anions remains indistinct. In this study, we analysed the mechanism by investigating the electronic structures and computing the relative free energies of intermediates formed from CO₂ to C₂ H₄ on the Cu (100) facet based on density functional theory (DFT) calculations. The results show that formyl *CHO from the hydrogenation reaction of the adsorbed *CO acts as the key intermediate, and the C-C coupling reaction occurs preferentially between *CHO and *CO with the formation of a *CHO-CO intermediate. We then propose a free-energy pathway of C₂ H₄ formation. We find that the presence of halide anions significantly decreases the free energy of the *CHOCH intermediate, and enhances desorption of C₂ H₄ in the order of I^- >Cl^- >Br^- >F^- . Lastly, the obtained results are rationalized through Bader charge analysis.

High-yield production of liquid fuels in CO2 hydrogenation on a zeolite-free Fe-based catalyst

Catalytic conversion of CO₂ to long-chain hydrocarbons with high activity and selectivity is appealing but hugely challenging. For conventional bifunctional catalysts with zeolite, poor coordination among catalytic activity, CO selectivity and target product selectivity often limit the long-chain hydrocarbon yield. Herein, we constructed a singly cobalt-modified iron-based catalyst achieving 57.8% C₅₊ selectivity at a CO₂ conversion of 50.2%. The C₅₊ yield reaches 26.7%, which is a record-breaking value. Co promotes the reduction and strengthens the interaction between raw CO₂ molecules and iron species. In addition to the carbide mechanism path, the existence of Co₃Fe₇ sites can also provide sufficient O-containing intermediate species (CO*, HCOO*, CO₃ ²*, and ) for subsequent chain propagation reaction via the oxygenate mechanism path. Reinforced cascade reactions between the reverse water gas shift (RWGS) reaction and chain propagation are achieved. The improved catalytic performance indicates that the KZFe-5.0Co catalyst could be an ideal candidate for industrial CO₂ hydrogenation catalysts in the future.

Recent Insight in Transition Metal Anchored on Nitrogen-Doped Carbon Catalysts: Preparation and Catalysis Application

The design and preparation of novel, high-efficiency, and low-cost heterogeneous catalysts are important topics in academic and industry research. In the past, inorganic materials, metal oxide, and carbon materials were used as supports for the development of heterogeneous catalysts due to their excellent properties, such as high specific surface areas and tunable porous structures. However, the properties of traditional pristine carbon materials cannot keep up with the sustained growth and requirements of industry and scientific research, since the introduction of nitrogen atoms into carbon materials may significantly enhance a variety of their physicochemical characteristics, which gradually become appropriate support for synthesizing supported transition metal catalysts. In the past several decades, the transition metal anchored on nitrogen-doped carbon catalysts has attracted a tremendous amount of interest as potentially useful catalysts for diverse chemical reactions. Compared with original carbon support, the doping of nitrogen atoms can significantly regulate the physicochemical properties of carbon materials and allow active metal species uniformly dispersed on the support. The various N species in support also play a critical role in accelerating the catalytic performance in some reactions. Besides, the interaction between support and transition metal active sites can offer an anchor site to stabilize metal species during the preparation process and then improve reaction performance, atomic utilization, and stability. In this review, we highlight the recent advances and the remaining challenges in the preparation and application of transition metal anchored on nitrogen-doped carbon catalysts.

Modified fischer-tropsch synthesis: A review of highly selective catalysts for yielding olefins and higher hydrocarbons

Global warming, fossil fuel depletion, climate change, as well as a sudden increase in fuel price have motivated scientists to search for methods of storage and reduction of greenhouse gases, especially CO2. Therefore, the conversion of CO2 by hydrogenation into higher hydrocarbons through the modified Fischer–Tropsch Synthesis (FTS) has become an important topic of current research and will be discussed in this review. In this process, CO2 is converted into carbon monoxide by the reverse water-gas-shift reaction, which subsequently follows the regular FTS pathway for hydrocarbon formation. Generally, the nature of the catalyst is the main factor significantly influencing product selectivity and activity. Thus, a detailed discussion will focus on recent developments in Fe-based, Co-based, and bimetallic catalysts in this review. Moreover, the effects of adding promoters such as K, Na, or Mn on the performance of catalysts concerning the selectivity of olefins and higher hydrocarbons are assessed.

Hydrogenation of Carbon Dioxide to Value-Added Liquid Fuels and Aromatics over Fe-Based Catalysts Based on the Fischer–Tropsch Synthesis Route

Hydrogenation of CO2 to value-added chemicals and fuels not only effectively alleviates climate change but also reduces over-dependence on fossil fuels. Therefore, much attention has been paid to the chemical conversion of CO2 to value-added products, such as liquid fuels and aromatics. Recently, efficient catalysts have been developed to face the challenge of the chemical inertness of CO2 and the difficulty of C–C coupling. Considering the lack of a detailed summary on hydrogenation of CO2 to liquid fuels and aromatics via the Fischer–Tropsch synthesis (FTS) route, we conducted a comprehensive and systematic review of the research progress on the development of efficient catalysts for hydrogenation of CO2 to liquid fuels and aromatics. In this work, we summarized the factors influencing the catalytic activity and stability of various catalysts, the strategies for optimizing catalytic performance and product distribution, the effects of reaction conditions on catalytic performance, and possible reaction mechanisms for CO2 hydrogenation via the FTS route. Furthermore, we also provided an overview of the challenges and opportunities for future research associated with hydrogenation of CO2 to liquid fuels and aromatics.

Vacancy engineering of two-dimensional W2N3 nanosheets for efficient CO2 hydrogenation

Peaking carbon emissions and achieving carbon neutrality have become the consensus goal of the international community to solve the environmental problems threatening mankind caused by accumulative greenhouse gases like CO₂. Herein we proposed vacancy engineering of two-dimensional (2D) topological W₂N₃ for efficient CO₂ hydrogenation into high value-added chemicals and fuels. Spherical aberration corrected scanning transmission electron microscopy (Cs-corrected STEM) confirmed a large amount of N vacancies on the catalyst surface, which significantly reduced the energy barrier for the formation of the essential intermediates of *CO and *CHO as revealed by density functional theory (DFT) calculations. Consequently, the highly stable catalyst exhibited efficient CO₂ hydrogenation superior to many previous reports with a maximum CO₂ conversion rate of 24% and a high selectivity of 23% for C₂₊ hydrocarbons. This work provided not only insight into the vacancy-controlled CO₂ hydrogenation mechanism, but also fresh ammunition to bring the remaining potential of 2D topological transition metal nitrides in the field of catalysis.

Ambient-pressure hydrogenation of CO2 into long-chain olefins

The conversion of CO₂ by renewable power-generated hydrogen is a promising approach to a sustainable production of long-chain olefins (C₄₊⁼) which are currently produced from petroleum resources. The decentralized small-scale electrolysis for hydrogen generation requires the operation of CO₂ hydrogenation in ambient-pressure units to match the manufacturing scales and flexible on-demand production. Herein, we report a Cu-Fe catalyst which is operated under ambient pressure with comparable C₄₊⁼ selectivity (66.9%) to that of the state-of-the-art catalysts (66.8%) optimized under high pressure (35 bar). The catalyst is composed of copper, iron oxides, and iron carbides. Iron oxides enable reverse-water-gas-shift to produce CO. The synergy of carbide path over iron carbides and CO insertion path over interfacial sites between copper and iron carbides leads to efficient C-C coupling into C₄₊⁼. This work contributes to the development of small-scale low-pressure devices for CO₂ hydrogenation compatible with sustainable hydrogen production.

The conversion of CO2 by renewable power-generated hydrogen is a promising approach to a sustainable production of long-chain olefins. Here the authors report a Cu-Fe catalyst which achieves the hydrogenation of CO2 into long-chain olefins under ambient pressure via the synergy of carbide mechanism and CO insertion mechanism.

Towards the development of the emerging process of CO2 heterogenous hydrogenation into high-value unsaturated heavy hydrocarbons

The emerging process of CO₂ hydrogenation through heterogenous catalysis into important bulk chemicals provides an alternative strategy for sustainable and low-cost production of valuable chemicals, and brings an important chance for mitigating CO₂ emissions. Direct synthesis of the family of unsaturated heavy hydrocarbons such as α-olefins and aromatics via CO₂ hydrogenation is more attractive and challenging than the production of short-chain products to modern society, suffering from the difficult control between C-O activation and C-C coupling towards long-chain hydrocarbons. In the past several years, rapid progress has been achieved in the development of efficient catalysts for the process and understanding of their catalytic mechanisms. In this review, we provide a comprehensive, authoritative and critical overview of the substantial progress in the synthesis of α-olefins and aromatics from CO₂ hydrogenation via direct and indirect routes. The rational fabrication and design of catalysts, proximity effects of multi-active sites, stability and deactivation of catalysts, reaction mechanisms and reactor design are systematically discussed. Finally, current challenges and potential applications in the development of advanced catalysts, as well as opportunities of next-generation CO₂ hydrogenation techniques for carbon neutrality in future are proposed.

Progress on the natural asphalt applications as a new class of carbonious heterogeneous support; synthesis of Na[Pd-NAS] and study of its catalytic activity in the formation of carbon-carbon bonds

In continuation of our recent research on introducing natural asphalt as a new carbonious, eco-friendly, highly economical support, and also in addition to our plan to develop its application in heterogeneous catalyst chemistry, palladium grafted on natural asphalt sulfonate (Na [Pd-NAS]), was prepared and characterized using usual spectroscopy techniques. This new carbon-based heterogeneous nanocatalyst was successfully applied as an efficient catalyst for the Suzuki, Stille and Heck reactions under mild and sustainable conditions. The reaction of various aryl halides with triphenyltin chloride, phenylboronic acid or n-butyl acrylate provided the corresponding products with moderate to good yields. Na [Pd-NAS] was characterized by FT-IR spectroscopy, scanning electron microscopy, energy-dispersive spectroscopy, X-ray diffraction, inductively coupled plasma, thermogravimetric analysis techniques and N₂ adsorption-desorption measurement. SEM image illustrated that the Na [Pd-NAS] has vermicular and flaky shapes. According to the IUPAC classiication, the sample exhibited IV type curves. More importantly, this ligand-free catalyst is stable under the reaction conditions. Besides, the catalyst was separated by simple filtration and reused for the several times without any deterioration in its activity. In this research we report Na[Pd-NAS] as a versatile and reusable nanocatalyst for the C-C coupling reactions.

Adjusting the CO2 hydrogenation pathway via the synergic effects of iron carbides and iron oxides

The synergic effects of iron carbides and iron oxides were used to adjust the reaction pathway to form alkenes or ethanol.

Tobacco stem-derived nitrogen-containing porous carbon with highly dispersed Ni–N sites as an efficient electrocatalyst for CO2 reduction to CO

A cost-effective electrocatalyst with highly dispersed Ni–N sites was prepared by tobacco stem-derived nitrogen-containing porous carbon.

Controlled Nanostructure of Zeolite Crystal Encapsulating FeMnK Catalysts Targeting Light Olefins from Syngas

Light olefins (C₂⁼-C₄⁼) are important basic raw materials in chemical industries. Direct production of light olefins from syngas using zeolite encapsulation catalysts shows great potential due to their regulation of product distribution in the Fischer-Tropsch process. Herein, we report a series of silicalite-1 zeolite-encapsulated FeMnK catalysts with distinct nanostructures, including FeMnK@S-1, FeMnK@Hol-S-1, and FeMnK@HM-S-1. It was found that the FeMnK@HM-S-1 catalyst (encapsulation of FeMnK oxide in hollow mesoporous silicalite-1 crystal) had an enhanced C₂⁼-C₄⁼ selectivity of 49% at a CO conversion of 12%. Our results revealed that superior light olefins selectivity of the FeMnK@HM-S-1 catalyst was achieved by the synergic effect between the inherent silicalite-1 micropores and the hollow mesoporous structure, which is responsible for restricting heavy hydrocarbon (C₅⁺) formation, maximizing C₂-C₄ hydrocarbons selectivity, quickly removing the primary light olefin products, and increasing the O/P ratio. We demonstrated that the enhanced CO adsorption and the declined H₂ adsorption (lower [H*]/[C*] ratio) over the FeMnK@HM-S-1 catalyst could also facilitate the olefin synthesis. This work provides guidance for reasonable designing of F-T catalysts to tailor product selectivity.

Novel Heterogeneous Catalysts for CO2 Hydrogenation to Liquid Fuels

Carbon dioxide (CO₂) hydrogenation to liquid fuels including gasoline, jet fuel, diesel, methanol, ethanol, and other higher alcohols via heterogeneous catalysis, using renewable energy, not only effectively alleviates environmental problems caused by massive CO₂ emissions, but also reduces our excessive dependence on fossil fuels. In this Outlook, we review the latest development in the design of novel and very promising heterogeneous catalysts for direct CO₂ hydrogenation to methanol, liquid hydrocarbons, and higher alcohols. Compared with methanol production, the synthesis of products with two or more carbons (C₂₊) faces greater challenges. Highly efficient synthesis of C₂₊ products from CO₂ hydrogenation can be achieved by a reaction coupling strategy that first converts CO₂ to carbon monoxide or methanol and then conducts a C-C coupling reaction over a bifunctional/multifunctional catalyst. Apart from the catalytic performance, unique catalyst design ideas, and structure-performance relationship, we also discuss current challenges in catalyst development and perspectives for industrial applications.