Fundamentals and Recent Progress in Magnetic Field Assisted CO2 Capture and Conversion

CO2 capture and conversion technology are highly promising technologies that definitely play a part in the journey towards carbon neutrality. Releasing CO2 by mild stimulation and the development of high efficiency catalytic processes are urgently needed. The magnetic field, as a thermodynamic parameter independent of temperature and pressure, is vital in the enhancement of CO2 capture and conversion process. In this review, the recent progress of magnetic field-enhanced CO2 capture and conversion is comprehensively summarized. The theoretical fundamentals of magnetic field on CO2 adsorption, release and catalytic reduction process are discussed, including the magnetothermal, magnetohydrodynamic, spin selection, Lorentz forces, magnetoresistance and spin relaxation effects. Additionally, a thorough review of the current progress of the enhancement strategies of magnetic field coupled with a variety of fields (including thermal, electricity, and light) is summarized in the aspect of CO2 related process. Finally, the challenges and prospects associated with the utilization of magnetic field-assisted techniques in the construction of CO2 capture and conversion systems are proposed. This review offers a reference value for the future design of catalysts, mechanistic investigations, and practical implementation for magnetic field enhanced CO2 capture and conversion.

Catalytic Sabatier Process under Thermally and Magnetically Induced Heating: A Comparative Case Study for Titania-Supported Nickel Catalyst

In the present paper, we compare the activity, selectivity, and stability of a supported nickel catalyst in classical heating conditions and in magnetically activated catalysis by using iron wool as a heating agent. The catalyst, 5 wt% Ni supported on titania (Degussa P25), was prepared via an organometallic decomposition method and was thoroughly characterized by using elemental, microscopic, and diffraction techniques. In the event of magnetic induction heating, the % CO₂ conversion reached a maximum of ~85% compared to ~78% for thermal conditions at a slightly lower temperature (~335 °C) than the thermal heating (380 °C). More importantly, both processes were found to be stable for 45 h on stream. Moreover, the effects of magnetic induction and classical heating over the catalyst evolution were discussed. This study demonstrated the potential of magnetic heating-mediated methanation, which is currently under investigation for the development of pilot-scale reactors.

l-Lysine Stabilized FeNi Nanoparticles for the Catalytic Reduction of Biomass-Derived Substrates in Water Using Magnetic Induction

The reduction of biomass-derived compounds gives access to valuable chemicals from renewable sources, circumventing the use of fossil feedstocks. Herein, we describe the use of iron-nickel magnetic nanoparticles for the reduction of biomass model compounds in aqueous media under magnetic induction. Nanoparticles with a hydrophobic ligand (FeNi₃ -PA, PA=palmitic acid) have been employed successfully, and their catalytic performance is intended to improve by ligand exchange with lysine (FeNi₃ -Lys and FeNi₃ @Ni-Lys NPs) to enhance water dispersibility. All three catalysts have been used to hydrogenate 5-hydroxymethylfurfural into 2,5-bis(hydroxymethyl)furan with complete selectivity and almost quantitative yields, using 3 bar of H₂ and a magnetic field of 65 mT in water. These catalysts have been recycled up to 10 times maintaining high conversions. Under the same conditions, levulinic acid has been hydrogenated to γ-valerolactone, and 4'-hydroxyacetophenone hydrodeoxygenated to 4-ethylphenol, with conversions up to 70 % using FeNi₃ -Lys, and selectivities above 85 % in both cases. This promising catalytic system improves biomass reduction sustainability by avoiding noble metals and expensive ligands, increasing energy efficiency via magnetic induction heating, using low H₂ pressure, and proving good reusability while working in an aqueous medium.

Magnetically Induced Catalytic Reduction of Biomass-Derived Oxygenated Compounds in Water

The development of energetically efficient processes for the aqueous reduction of biomass-derived compounds into chemicals is key for the optimal transformation of biomass. Herein we report an early example of the reduction of biomass-derived oxygenated compounds in water by magnetically induced catalysis. Non-coated and carbon-coated core-shell FeCo@Ni magnetic nanoparticles were used as the heating agent and the catalyst simultaneously. In this way it was possible to control the product distribution by adjusting the field amplitude applied during the magnetic catalysis, opening a precedent for this type of catalysis. Finally, the encapsulation of the magnetic nanoparticles in carbon (FeCo@Ni@C) strongly improved the stability of the magnetic catalyst in solution, making its reuse possible up to at least eight times in dioxane and four times in water.

Induction Heating of Magnetically Susceptible Nanoparticles for Enhanced Hydrogenation of Oleic Acid

Radio frequency (RF) induction heating was compared to conventional thermal heating for the hydrogenation of oleic acid to stearic acid. The RF reaction demonstrated decreased coke accumulation and increased product selectivity at comparable temperatures over mesoporous Fe₃O₄ catalysts composed of 28-32 nm diameter nanoparticles. The Fe₃O₄ supports were decorated with Pd and Pt active sites and served as the local heat generators when subjected to an alternating magnetic field. For hydrogenation over Pd/Fe₃O₄, both heating methods gave similar liquid product selectivities, but thermogravimetric analysis-differential scanning calorimetry measurements showed no coke accumulation for the RF-heated catalyst versus 6.5 wt % for the conventionally heated catalyst. A different trend emerged when hydrogenation over Pt/Fe₃O₄ was performed. Compared to conventional heating, the RF increased the selectivity to stearic acid by an additional 15%. Based on these results, RF heating acting upon a magnetically susceptible nanoparticle catalyst would also be expected to positively impact systems with high coking rates, for example, nonoxidative dehydrogenations.

Commercial Cu2 Cr2 O5 Decorated with Iron Carbide Nanoparticles as a Multifunctional Catalyst for Magnetically Induced Continuous-Flow Hydrogenation of Aromatic Ketones

Copper chromite is decorated with iron carbide nanoparticles, producing a magnetically activatable multifunctional catalytic system. This system (ICNPs@Cu₂Cr₂O₅) can reduce aromatic ketones to aromatic alcohols when exposed to magnetic induction. Under magnetic excitation, the ICNPs generate locally confined hot spots, selectively activating the Cu₂Cr₂O₅ surface while the global temperature remains low (≈80 °C). The catalyst selectively hydrogenates a scope of benzylic and non‐benzylic ketones under mild conditions (3 bar H₂, heptane), while ICNPs@Cu₂Cr₂O₅ or Cu₂Cr₂O₅ are inactive when the same global temperature is adjusted by conventional heating. A flow reactor is presented that allows the use of magnetic induction for continuous‐flow hydrogenation at elevated pressure. The excellent catalytic properties of ICNPs@Cu₂Cr₂O₅ for the hydrogenation of biomass‐derived furfuralacetone are conserved for at least 17 h on stream, demonstrating for the first time the application of a magnetically heated catalyst to a continuously operated hydrogenation reaction in the liquid phase.

Determination of the surface temperature of magnetically heated nanoparticles using a catalytic approach

Herein we describe a new method for the determination of the surface temperature of magnetically heated nanoparticles in solution using the temperature dependency of the catalytic performances of iron carbide nanoparticles coated with ruthenium (Fe_2.2C@Ru) for acetophenone hydrodeoxygenation. A correlation between nanoparticle surface temperature and magnetic field could be established. Very high surface temperatures could be estimated in different solvents, which were also found similar at a given magnetic field and well above some solvent boiling points.

A setup to measure the temperature-dependent heating power of magnetically heated nanoparticles up to high temperature

Magnetic heating, namely, the use of heat released by magnetic nanoparticles (MNPs) excited with a high-frequency magnetic field, has so far been mainly used for biological applications. More recently, it has been shown that this heat can be used to catalyze chemical reactions, some of them occurring at temperatures up to 700 °C. The full exploitation of MNP heating properties requires the knowledge of the temperature dependence of their heating power up to high temperatures. Here, a setup to perform such measurements is described based on the use of a pyrometer for high-temperature measurements and on a protocol based on the acquisition of cooling curves, which allows us to take into account calorimeter losses. We demonstrate that the setup permits to perform measurements under a controlled atmosphere on solid state samples up to 550 °C. It should in principle be able to perform measurements up to 900 °C. The method, uncertainties, and possible artifacts are described and analyzed in detail. The influence on losses of putting under vacuum different parts of the calorimeter is measured. To illustrate the setup possibilities, the temperature dependence of heating power is measured on four samples displaying very different behaviors. Their heating power increases or decreases with temperature, displaying temperature sensibilities ranging from -2.5 to +4.4% K^-1. This setup is useful to characterize the MNPs for magnetically heated catalysis applications and to produce data that will be used to test models permitting to predict the temperature dependence of MNP heating power.

Novel Magnetic Nanohybrids: From Iron Oxide to Iron Carbide Nanoparticles Grown on Nanodiamonds

The synthesis and characterization of a new line of magnetic hybrid nanostructured materials composed of spinel-type iron oxide to iron carbide nanoparticles grown on nanodiamond nanotemplates is reported in this study. The realization of these nanohybrid structures is achieved through thermal processing under vacuum at different annealing temperatures of a chemical precursor, in which very fine maghemite (γ-Fe2O3) nanoparticles seeds were developed on the surface of the nanodiamond nanotemplates. It is seen that low annealing temperatures induce the growth of the maghemite nanoparticle seeds to fine dispersed spinel-type non-stoichiometric ~5 nm magnetite (Fe3−xO4) nanoparticles, while intermediate annealing temperatures lead to the formation of single phase ~10 nm cementite (Fe3C) iron carbide nanoparticles. Higher annealing temperatures produce a mixture of larger Fe3C and Fe5C2 iron carbides, triggering simultaneously the growth of large-sized carbon nanotubes partially filled with these carbides. The magnetic features of the synthesized hybrid nanomaterials reveal the properties of their bearing magnetic phases, which span from superparamagnetic to soft and hard ferromagnetic and reflect the intrinsic magnetic properties of the containing phases, as well as their size and interconnection, dictated by the morphology and nature of the nanodiamond nanotemplates. These nanohybrids are proposed as potential candidates for important technological applications in nano-biomedicine and catalysis, while their synthetic route could be further tuned for development of new magnetic nanohybrid materials.

Magnetically Induced CO2 Methanation Using Exchange-Coupled Spinel Ferrites in Cuboctahedron-Shaped Nanocrystals

Magnetically induced catalysis can be promoted taking advantage of optimal heating properties from the magnetic nanoparticles to be employed. However, when unprotected, these heating agents that are usually air-sensitive, get sintered under the harsh catalytic conditions. In this context, we present, to the best of our knowledge, the first example of air-stable magnetic nanoparticles that: 1) show excellent performance as heating agents in the CO₂ methanation catalyzed by Ni/SiRAlOx, with CH₄ yields above 95 %, and 2) do not sinter under reaction conditions. To attain both characteristics we demonstrate, first the exchange-coupled magnetic approach as an alternative and effective way to tune the magnetic response and heating efficiency, and second, the chemical stability of cuboctahedron-shaped core-shell hard CoFe₂ O₄ -soft Fe₃ O₄ nanoparticles.

Transition Metal Carbides (TMCs) Catalysts for Gas Phase CO2 Upgrading Reactions: A Comprehensive Overview

Increasing demand for CO2 utilization reactions and the stable character of CO2 have motivated interest in developing highly active, selective and stable catalysts. Precious metal catalysts have been studied extensively due to their high activities, but their implementation for industrial applications is hindered due to their elevated cost. Among the materials which have comparatively low prices, transition metal carbides (TMCs) are deemed to display catalytic properties similar to Pt-group metals (Ru, Rh, Pd, Ir, Pt) in several reactions such as hydrogenation and dehydrogenation processes. In addition, they are excellent substrates to disperse metallic particles. Hence, the unique properties of TMCs make them ideal substitutes for precious metals resulting in promising catalysts for CO2 utilization reactions. This work aims to provide a comprehensive overview of recent advances on TMCs catalysts towards gas phase CO2 utilization processes, such as CO2 methanation, reverse water gas shift (rWGS) and dry reforming of methane (DRM). We have carefully analyzed synthesis procedures, performances and limitations of different TMCs catalysts. Insights on material characteristics such as crystal structure and surface chemistry and their connection with the catalytic activity are also critically reviewed.

Ultrastable Magnetic Nanoparticles Encapsulated in Carbon for Magnetically Induced Catalysis

Magnetically induced catalysis using magnetic nanoparticles (MagNPs) as heating agents is a new efficient method to perform reactions at high temperatures. However, the main limitation is the lack of stability of the catalysts operating in such harsh conditions. Normally, above 500 °C, significant sintering of MagNPs takes place. Here we present encapsulated magnetic FeCo and Co NPs in carbon (Co@C and FeCo@C) as an ultrastable heating material suitable for high-temperature magnetic catalysis. Indeed, FeCo@C or a mixture of FeCo@C:Co@C (2:1) decorated with Ni or Pt-Sn showed good stability in terms of temperature and catalytic performances. In addition, consistent conversions and selectivities regarding conventional heating were observed for CO₂ methanation (Sabatier reaction), propane dehydrogenation (PDH), and propane dry reforming (PDR). Thus, the encapsulation of MagNPs in carbon constitutes a major advance in the development of stable catalysts for high-temperature magnetically induced catalysis.