The conversion of CO2 into CO is an important step in CO2 utilization to achieve clean fuels and value‐added chemicals. Herein, we explored the pyrolysis of zeolitic imidazolate framework‐8 (ZIF‐8) ...loaded with different amounts of Ni2+ to obtain Ni−Zn carbide (Ni3ZnC) embedded in N‐doped carbon. Ni is present in the intermetallic compound, while Zn excess remains on the N‐doped carbon. The Ni3ZnC phase catalyzes the selective hydrogenation of CO2 into CO via the reverse water gas shift reaction, reaching 100 % CO selectivity at ∼30 % CO2 conversion at 450 °C and atmosphere pressure (CO2 : H2=1 : 4, GHSV=30000 mL gcat−1 h−1). The methanation reaction of CO2/CO, which is usually favored over Ni catalysts, is suppressed. The selectivity to CO at the expense of CH4 is related to the stability of chemisorbed CO in the Ni3ZnC surface, which is lower compared to Ni surfaces. The Ni3ZnC@NC catalyst is selective towards CO over a wide range of conditions, including high pressure, that is usually required for the conversion of CO to hydrocarbons and alcohols via the Fisher‐Tropsch synthesis (FTS) process. Contrarily, a classical Ni/SiO2 catalyst prepared by impregnation produces CH4 under high pressure.
The Ni3ZnC phase catalyzes the selective hydrogenation of CO2 into CO via the reverse water gas shift (RWGS) reaction, while the methanation reaction of CO2/CO, which is usually favored over Ni catalysts, was completely suppressed, even at high pressures. This finding opens the door for the integration of RWGS/Fisher‐Tropsch synthesis (FTS) tandem process to achieve the conversion of CO2‐to‐liquids.
Reverse water gas shift（RWGS） reaction can be served as a pivotal stage of transitioning the abundant CO2 resource into chemicals or hydrocarbon fuels, which is attractive for the CO2 utilization and ...of eventually significance in enabling a rebuilt ecological system for unconventional fuels. This concept is appealing when the process is considered as a solution for the storage of renewable energy, which may also find a variety of potential end uses for the outer space exploration. However, a big challenge to this issue is the rational design of high temperature endurable RWGS catalysts with desirable CO product selectivity. In this work, we present a comprehensive overview of recent publications on this research topic,mainly focusing on the catalytic performance of RWGS reaction over three major kinds of heterogeneous catalysts, including supported metal catalysts, mixed oxide catalysts and transition metal carbides. The reaction thermodynamic analysis, kinetics and mechanisms are also described in detail. The present review attempts to provide a general guideline about the construction of well-performed heterogeneous catalysts for the RWGS reaction, as well as discussing the challenges and further prospects of this process.
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•Na/ZnFe2O4 catalyst produced long-chain olefinic hydrocarbons from CO2 hydrogenation.•Various zeolites were combined with Na/ZnFe2O4 to form a versatile hybrid catalyst ...platform.•Hybrid catalysts demonstrated successful hydrocarbon selectivity control.•Silica coating on zeolites enhanced the effects of shape selectivity of the zeolites.
Catalytic CO2 hydrogenation faces great challenges in both reaction rates and selectivity to desired high-value products. Herein, we present a one-pot reaction platform that converts CO2 into various long-chain hydrocarbons selectively by combining a Na/ZnFe2O4-based catalyst for CO2 activation and carbon–carbon coupling, and a zeolite for fine-tuning the selectivity of desired products by exploiting its shape selectivity. Thus, the Na/ZnFe2O4 catalyst without zeolite produces highly olefinic diesel range hydrocarbons, and a hybrid catalyst with ZSM-5 produces highly aromatic gasoline range hydrocarbons, that with ZSM-11 produces branched kerosene range hydrocarbons, and that with SSZ-13 produces hydrocarbons rich in C2-C4 olefins. In all cases, high CO2 conversions of over 35% and low CO selectivity of near 10% are maintained. Therefore, the hybrid catalyst platform proposed here demonstrates that an elaborate catalyst design enables fine-tuning of the reaction pathway of CO2 hydrogenation to produce selectively versatile value-added hydrocarbons.
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•The size of Fe3O4 nanoparticles on the h-BN surfaces can be controlled by small Pt additions into synthesis media.•FePt/BN is high-productive catalyst in CO2 hydrogenation ...reaction.•In situ TEM annealing revealed a unique mechanism of FePt/BN core–shell structure formation.•According to MD bimetallic FePt nanoparticles formation can be induced by Pt atoms diffusion toward the surface of Fe@Pt core–shell system.
Hexagonal boron nitride (h-BN) nanosheets are a promising material for various applications including catalysis. Herein, h-BN-supported Fe-based catalysts are characterised with respect to CO2 hydrogenation reaction. Heterogeneous Fe3O4/BN, Fe3O4(Pt)/BN, and FePt/BN nanostructures are obtained via polyol synthesis in ethylene glycol. The sizes of Fe3O4 nanoparticles and their distributions over h-BN surfaces depend on the amount of H2PtCl6 added to the synthesis media. Bimetallic FePt nanoparticles are formed when Pt content is high enough. In situ TEM analysis shows the formation of core–shell h-BN@FePt nanoparticles during heating that prevents FePt NPs from further sintering during the catalytic process. The mechanism of Fe and Pt interaction is elucidated based on the molecular dynamic simulations. The FePt/BN nanomaterials show significantly higher CO2 conversion rate compared to the Fe3O4/BN and Fe3O4(Pt)/BN heterogeneous nanomaterials and exhibit almost 100% selectivity to carbon monoxide. The Fe3O4/BN and Fe3O4(Pt)/BN nanomaterials show better selectivity to hydrocarbons. The possible reaction pathways are discussed based on the calculated sorption energies of all reactants, intermediate compounds, and reaction products. The study highlights pronounced catalytic properties of the developed system and reveals a unique interaction mechanism between its components increasing their stability.
Hydrogen fuel production from methane cracking is a sustainable process compared to the ones currently in practice due to zero greenhouse gas emissions. Also, carbon black that is co-produced is ...valuable and can be marketed to other industries. As this is a high-temperature process, using solar energy can further improve its sustainability. An integrated solar methane cracking system is proposed where hydrogen and carbon products are sent to fuel cells to generate electricity. The CO2 exhaust stream from the carbon fuel cell is captured and reacted with hydrogen in the CO2 hydrogenation unit to produce liquid fuels – Methanol and dimethyl ether. The process is simulated in Aspen Plus®, and its energy and exergy efficiencies are evaluated by carrying out a detailed thermodynamic analysis. In addition, a sensitivity analysis is performed on various input parameters of the system. The overall energy efficiency of 41.9% and exergy efficiency of 52.3% were found.
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•Energy and exergy analysis of an integrated methane cracking system.•Clean electricity generation using hydrogen and carbon black.•One-step DME synthesis using captured CO2 for hydrogen carrier production.•Aspen Plus simulation of the overall process.•Overall energy and exergy efficiencies of 41.9% and 52.3%, respectively.
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•Potassium strongly increases the catalytic activity of the Fe-Al-O spinel in RWGS.•Potassium added to Fe5C2 suppresses the methanation increasing the FTS activity.•Potassium ...stabilizes surface iron atoms in Fe-Al-O and Fe5C2 phase in reduced form.•Oxygen vacancies at the Fe-Al-O spinel surface are potential active sites in RWGS.•Carbon vacancies at the Fe5C2 surface are potential active sites in methanation.
The effect of potassium was tested with unpromoted and K-promoted Fe-Al-O oxide and Fe5C2 carbide materials, formed during CO2 hydrogenation from Fe-Al-O spinel. Each one of the tested catalysts contained a single phase (oxide or carbide). 2wt% potassium enhanced the RWGS rate of reaction on the oxide phase tenfold compared with the unpromoted oxide. This correlated with increase of Fe2+/Fe3+ ratio (XPS) determining oxygen vacancies as active sites for RWGS. Potassium suppressed the methanation rate on the carbide catalyst by a factor of five and increased the CO FTS rate to C2+ hydrocarbons by a factor of 1.4. EFTEM images and elemental profiles of unpromoted carbide nanocrystals measured after testing in CO hydrogenation displayed an amorphous surface layer enriched with oxygen. Potassium stabilized the surface iron atoms in reduced form. The observed effects of potassium on carbide phase were explained by a model implementing carbon vacancies for methanation and near-metallic iron atoms for FTS.
A reaction-coupling strategy is often employed for CO2 hydrogenation to produce fuels and chemicals using oxide/zeolite bifunctional catalysts. Because the oxide components are responsible for CO2 ...activation, understanding the structural effects of these oxides is crucial, however, these effects still remain unclear. In this study, we combined In2O3, with varying particle sizes, and SAPO-34 as bifunctional catalysts for CO2 hydrogenation. The CO2 conversion and selectivity of the lower olefins increased as the average In2O3 crystallite size decreased from 29 to 19 nm; this trend mainly due to the increasing number of oxygen vacancies responsible for CO2 and H2 activation. However, In2O3 particles smaller than 19 nm are more prone to sintering than those with other sizes. The results suggest that 19 nm is the optimal size of In2O3 for CO2 hydrogenation to lower olefins and that the oxide particle size is crucial for designing catalysts with high activity, high selectivity, and high stability.
The effect of In2O3 particle size on CO2 hydrogenation over In2O3/SAPO-34 bifunctional catalysts was studied. In2O3 particles with a size of 19 nm are the most beneficial for CO2 hydrogenation to lower olefins.
•CO2 hydrogenation to methanol, ethanol and hydrocarbons via SACs is discussed.•Biomass conversion through thermocatalysis, electrocatalysis and photocatalysis methods via SACs is ...summarized.•Challenges and future development prospects of SACs are highlighted.
The utilization of fossil fuels has brought unprecedented prosperity and development to human society, but also caused environmental pollution and global warming triggered by excess greenhouse gases emission. For one thing, the excess emission of carbon dioxide (CO2), which has a negative impact on global temperature and ocean acidity, needs to be controlled. For another, the depletion of fossil fuels will eventually force people to seek alternative carbon sources to maintain a sustainable economy. Thus, using renewable energy to convert CO2 and biomass into value-added chemicals and fuels is a promising method to overcome urgent problems. The hydrogenation of CO2 is very important to mitigate the greenhouse effect caused by CO2, while biomass conversion can produce alternative renewable biofuels and green chemicals. As a kind of promising catalyst, heterogeneous single-atom catalyst (SAC) has received extensive attention in the past decades. SACs combine the advantages of homogeneous catalysts with uniform active sites and heterogeneous catalysts that are easily separable. In this review, we will give a comprehensive overview of the latest progress in CO2 selective hydrogenation and biomass conversion via SACs.
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