Research Progress on Magnetic Catalysts and Its Application in Hydrogen Production Area

Abstract

The noncontact heating technology of IH targets heat directly where it is needed through the electromagnetic energy adsorption and conversion of magnetic materials. Unlike conventional heating methods, the heat generated by electromagnetic induction of magnetic materials can be applied directly into the reactor without heating the entire device; this new heating method is not only more energy efficient but also safer, cleaner and more sustainable if renewable electricity is adopted; moreover, magnetic catalysts can be recovered and reused by separating chemical reactants and products from the catalyst by the application of a magnetic field, and it can provide the required heat source for the reaction without altering its catalytic properties. Magnetic catalysts with an electric field have been applied to some industrial areas, such as the preparation of new materials, catalytic oxidation reactions, and high-temperature heat absorption reactions. It is a trend that is used in the hydrogen production process, especially the endothermic steam reforming process. Therefore, in this paper, the heat release mechanism, properties, preparation methods and the application of magnetic catalysts were presented. Highlights of the application and performance of magnetic catalysts in the hydrogen production area were also discussed.

1. Introduction

Traditional fossil energy resources are limited and over-dependence on it produces greenhouse gas emissions to the environment. The utilization of renewable energy is an important way to transform energy and reduce CO₂ emissions [1,2]. The sources of renewable energy such as wind and solar energy, have an instability and intermittently in the process of energy utilization. The use of energy storage technology provides renewable energy storage for power generation to maintain the stability of industrial production. For example, hydrogen reforming can significantly reduce CO₂ emissions by using renewable energy power generation. Wismann’s research team at the Technical University of Denmark used an electrically heated reformer to demonstrate attractive perspectives for the application of renewable energy-based surplus power chemical production [3].

Electromagnetic induction heating technology has been exploited for a wide range of applications in the past, involved in steel metallurgy [4,5], biomedical technologies for drug release [6,7,8,9,10], and disease treatment by magnetic hyperthermia [11,12,13] and thermo-magnetism [14]; moreover, nowadays, due to the successful application of catalytic oxidation reaction [15] and strong endothermic reaction [16] by magnetic nanoparticle heating, it has been widely concerned by researchers. The heating can be generated directly on the catalyst active site by using induction heating technology, which can substantially exceed the volume temperature of the surrounding reaction medium and lead to significant accelerations of chemical reactions [17]; it is also important that the reactor wall is not exposed to the high temperatures in this process. IH is an alternative heating method, which can overcome the heat transfer limitations encountered in the conventional “contact” heating devices. In addition, it has the advantages of fast start-up response [18,19], low thermal inertia and high heating efficiency [20,21]. It is possible to heat quickly and turns off the reactor quickly [16]. Therefore, electromagnetic induction heating technology can solve the problems involved in the catalytic reaction process, such as reactor simplification, process enhancement, and cost reduction [22]. Based on these advantages, magnetic catalysts are beginning to be used in strongly endothermic methane steam reforming reactions for hydrogen production by using renewable energy power generation, as shown in Figure 1.

Steam methane reforming is still the dominant large-scale industrial process for hydrogen production. The catalysts currently used in the industrial sector consist of alumina particles, which have a low thermal conductivity and a high resistance to heat transfer [23]. To enable heat to be generated directly from within the catalyst, the peer researchers speculated whether induction heating technology could be used to solve these problems and improve energy use efficiency; moreover, many researchers have done a lot of work on induction heating for hydrogen reforming. In 1983, Ovenston et al. [24] first applied electromagnetic induction heating to single magnetic catalysts for endothermic reforming of hydrocarbons for hydrogen production. Pérez Camacho et al. [25] also used Calcium titanium ore magnetic catalysts to drive methane reforming for hydrogen production by using induction heating technology. Vinum et al. [26] used CoNi and Cu@CoNi as induction heating catalysts for hydrogen production in steam reforming of methane. Almind [27] et al. optimized the induction heating steam methane reforming process by controlling the frequency and geometry of the alternating magnetic field and coils; this evidence shows that peer researchers have experimented with techniques that differ from traditional heating methods and have had successful results in these areas. The application of this technology not only reduces CO₂ emissions, but also reduces the dependence of the steam reforming process on non-renewable energy sources for heating.

Electrolysis is the most important method to obtain hydrogen from water [28]. It is a well-established technology based on the production of hydrogen and oxygen by applying a direct electric current to water to dissociate it. The hydrogen obtained with this technique has a high purity (99.999 vol%) [29,30]. Although electrolyzers have been in use for a long time, formidable challenges still exist and hinder the practical application of large-scale, energy-efficient, and economically viable water electrolysis [31,32]. Therefore, a new strategy is presented here to address these challenges by increasing the local temperature of the electrolysis modules through induction heating thereby reducing their overpotential. Future applications may often require coupling them with renewable energy sources to produce clean hydrogen and contribute to grid load peaking. Both researchers aim to improve the energy efficiency of these processes in order to promote their sustainability; moreover, considering ferrite-based solar thermochemical water splitting for hydrogen production also requires a high reaction temperature [33,34,35]. Therefore, magnetic catalyst induction heating has prospective applications in the fields of electrolytic water, solar thermochemical water splitting and ammonia decomposition for hydrogen production with high flexibility in energy storage and conversion in the future [36]. Despite the positive performance of induction heating in these fields, it still faces some challenges, such as magnetic catalysts are currently reported to have complicated preparation processes, low catalytic activity, and difficulties in their temperature measurement during heating.

Given the prospect of magnetic catalysts in future green energy applications, this paper discussed the effects of catalyst particle size, shape, and components on the heating mechanism and thermal efficiency of magnetic catalysts from exothermic mechanism; further, it summarized the preparation methods of magnetic catalysts; finally discussed the application progress of magnetic catalysts including in the field of hydrogen production.

2. Magnetic Catalyst Induction Heating Theory and Its Influencing Factors

The characteristic of electromagnetic induction heating is to directly supply radio frequency energy to the magnetic material for heating, which can shorten the heating cycle and reduce the energy loss; moreover, the size, shape, and fraction of magnetic catalytic particle all affect the heating process [37]. Since magnetic eddy current heating is mainly produced in materials of above cm scale [38], whereas the magnetic catalysts have smaller core sizes and the magnetic catalyst composites are not good conductors. Therefore, the effect of the thermal effect of the current action is very small and will not be discussed in depth here. Under the action of the external alternating magnetic field, the rotation of magnetic particles drives the direction of magnetic moment to change to cause resistance, and this heat generation from resistance to heat energy is called relaxation heating. For magnetic nanoparticles (MNP), the magnetic particle surface appears anisotropic due to the disruption of the symmetry of the local environment and the variation of the crystal field [39].

In general, the Néel relaxation heating efficiency can be expressed as [22].

$$SAR\left(w/g\right)=\frac{P}{\rho }$$

where P is the volumetric power dissipation (kw/m³) and ρ is the material density (kg/m³). The volumetric power P can be expressed as: $P=\pi f{\mu }_0{\chi }_0{H}_0^2\frac{2\pi f\tau }{1+{\left(2\pi f\tau \right)}^2}$, where f is the frequency of the applied AC magnetic field (HZ), μ₀ is the magnetic permeability in vacuum (H/m), χ₀ is the magnetization, H₀ is the magnetic field amplitude (KA/m), and τ is the relaxation time (s). Normally, in the presence of an external magnetic field, the magnetic moment of the particle is rotated to reach the direction corresponding to the minimum energy, denoted by ΔE~KV_V required to overcome the energy potential difference. To achieve high heating rates, avoid allowing Néel relaxation to dominate [40]. Therefore, the Neel relaxation time τ is expressed as [41].

$$\tau ={\tau }_0{e}^{\left(\frac{\mathsf{\Delta }E}{K_{B}T}\right)}$$

where τ₀ is the time-referred front factor. It depends on numerous parameters such as temperature, spin-magnetization ratio, saturation magnetization strength, anisotropy constant, and energy barrier height. K_V is the magnetic anisotropy constant (J/m³), V is the average particle volume (m³), K_B is the Boltzmann constant (J/K), and T is the temperature (K). For magnetic particles of spherical shape, the heating efficiency is expressed as [22]:

$$SAR\left(w/g\right)=\frac{\sigma {\left(\pi \mu rfH\right)}^2}{5\rho }$$

where σ is the material conductivity (S/m), μ is the magnetic permeability in vacuum (H/m), r is the average particle radius (m), f is the frequency of the applied AC magnetic field (Hz), H is the magnetic field amplitude (KA/m), and ρ is the material density (kg/m³).

Figure 2 shows the dependence of the magnetic energy of the nanomagnet on the direction of its magnetization vector. The direction that minimizes this magnetic energy is called the anisotropic direction or easy axis. The magnetic energy increases with the tilt angle between the magnetization vector and the easy axis directions. The magnitude of the variation of this curve is called anisotropic energy [42]. Néel relaxation defines fluctuations caused by jumps in magnetic moments between different easy-axis directions. The Brownian relaxation characterizes the viscous rotation of the entire particle [16]. Figure 3 shows the two components of the magnetic relaxation of the magnetic fluid [42].

In addition, the heat generated by the resistance of the magnetic material to the magnetization process by being magnetized in an external alternating magnetic field is called hysteresis heating [43]. The hysteresis line (magnetization vs. field) of a magnetic medium is shown in Figure 4. Hysteresis heating is used to be expressed in terms of the area. When the external magnetic field is sufficiently powerful it causes the rotation of the internal magnetic moment of the material to align with the external field, and the maximum magnetization strength reached in this state is called the saturation magnetization strength Ms [44]. As the magnetic field strength decreases, the spin is no longer aligned with the magnetic field and the overall magnetization strength decreases. In a ferromagnetic material, the remanent magnetic moment is maintained at zero fields. The magnetization strength at zero fields is called the remanent magnetization Mr.

Hysteresis loss, Néel or Brownian relaxation may occur simultaneously during the remagnetization of magnetic nanoparticles in an alternating magnetic field, but much depends on the average particle size of the magnetic nanoparticles [40]. The efficiency of magnetic catalyst hysteresis heating is expressed as [22]:

$$SAR\left(w/g\right)=f{\mu }_0\oint MdH$$

where 𝑓 is the frequency of the applied AC magnetic field (HZ), μ₀ is the magnetic permeability in vacuum (H/M), M is the magnetization strength (Am² k^–1), and H is the magnetic field amplitude.

In the application of magnetic catalysts, it is needed to analyze the effects of sample particle size, material composition, activity loading, sample Curie temperature and spatial arrangement on the heating efficiency (SAR) and temperature rise characteristics of magnetic catalysts [27]. The heating efficiency is influenced by the coil geometry, the number of turns, height, input power, and electromagnetic field parameters outside the reactor tube. The basic principle of eddy current heating is the formation of a circle of induction current in the radial direction of the material surface in an alternating magnetic field. Since the material has a tiny resistance, the intensity of the eddy current is extremely powerful, which is generates a relatively high temperature on the material surface. Eddy currents are commonly used in industry to melt metals. Since the skin effect increases the current density closer to the surface of the material, the phenomenon of the skin effect results in the effective area decreasing and the resistance increasing. Most importantly, it also produces thermal effects of thecatalytic material [45]. Due to the skin effect and the proximity effect caused by the Joule heat of the pipe, which will result in the power loss in the coil [45]. The skinning depth δ is defined as the effective penetration depth of electromagnetic waves in conductive materials and can be defined as [22]:

$$\delta =\frac{1}{\sqrt{\sigma \pi f\mu }}$$

where σ is the material conductivity (s/m), μ is the magnetic permeability (H/m), and f is the frequency of the applied AC magnetic field (Hz).

Due to the presence of electromagnetic field-induced eddy currents, it is not feasible to measure temperature using conventional thermocouples; moreover, when the temperature of catalytic material is above 500 °C, fiber optic probes can be used to monitor the temperature [46]; however, complex heat transfer problems are involved when measuring the temperature of porous composites, so it is impossible to accurately describe the conductivity as a function of temperature, field strength, time, and frequency because at this time temperature becomes the main influencing factor. The other two characteristic parameters of the induction heating catalytic process are the Curie temperature (TC) and the blocking temperature (TB). The T values here denote the phase change of the material magnetic moment from ferromagnetic (FM) to paramagnetic (PM) and from ferromagnetic (FM) to superparamagnetic (SPM), respectively. That is, the Curie temperature indicates the temperature limit at which a given FM sample loses its permanent magnetism, and therefore the temperature limit at which the material is heated by hysteresis losses [22]. For materials less than 20 nm in diameter, heat is generated linearly and proportional to the applied magnetic field. For diameters above 20 nm, the Stoner–Wohlfarth model (ferromagnetic and subferromagnetic states) can be used to describe the material heat production process, and the response is nonlinearly varying [41]. In this case, Fourier’s law is no longer suitable for heat calculation, and the sample exhibits ballistic heat transfer internally. These above factors create barriers to the simulation of heat transfer on the nanoscale for induction heating of magnetic catalysts, so experimental probe methods can provide better temperature estimates [46].

3. Synthesis of Magnetic Catalysts

According to magnetic catalysts different structures, it can be classified into three types: magnetic cores alone, encapsulated or modified on the surface of magnetic cores, and multi-component active nanomaterials magnetic core-loaded catalysts, respectively [7]. In the synthesis of magnetic catalysts, it needs to be considered that the material properties of magnetic nuclei, such as the coupling effect between magnetic moments within the material that generates spontaneous magnetization intensity; the shape size of magnetic nuclei particles, components, magnetic properties, catalytic properties, curie temperature, linear response, etc. Both of them have had an effect on the heating efficiency and may appear to the agglomeration phenomenon of magnetic nuclei during the reaction process, etc.

3.1. Magnetic Catalyst Cores

Magnetic catalysts are mainly used in high temperature gas/liquid phase reactions, catalytic hydrogen production and magnetic thermotherapy. There are different selection and preparation methods for magnetic catalyst magnetic core materials in various fields. The key parameters of high temperature gas/liquid phase reactions and catalytic hydrogen production are involved in material heating efficiency, catalytic performance and Curie temperature, etc. Commonly used preparation methods include co-precipitation, microemulsion, sol-gel, hydrothermal reaction, ultrasonic decomposition, microwave radiation, biosynthesis, etc. [47]. Metals, alloys, oxides and compounds can be used as materials for the preparation of magnetic cores.

Zhao et al. [48] prepared nano magnetite core catalyst material modified by methoxy polyethylene glycol (MPEG) technology, and the MPEG acted as a high boiling point solvent, reducing agent and modifier at the same time. It is no need to further surfactants. Zhang et al. [49] synthesized hydrophilic Fe₃O₄ nanoparticle catalysts with an average diameter of 15 nm by direct reduction of FeCl₃ and FeCl₂ in an aqueous solution. The heating characteristics of Fe₃O₄ nanoparticles under a 50–500 kHz alternating magnetic field were studied. The results showed that the Néel relaxation heating efficiency increased linearly with the increase of the external alternating magnetic field. Fang et al. [50] prepared a magnetic bimetallic NiCo₂O₄ magnetic core by coprecipitation method. In the condensation reaction, the reactant conversion was 99%, and the catalytic activity of the catalyst will not be significantly reduced after 20 times cycles. Malik et al. [51] prepared fexco100-x magnetic nanoparticles with different stoichiometry (the range of x is from 0 to 100). The characterization results showed that the catalyst particles had small aggregation and surface oxidation, and the magnetization of iron increased after adding cobalt alloy. Rafienia et al. [52] synthesized spinel ferrite nanoparticles (SFNs) by solvothermal technology, and summarized the effects of key parameters such as temperature, reaction time, solvent, capping agent and reducing agent. Among the methods for the preparation of magnetic nuclei, the solvothermal preparation technology has the advantages of controlled size, precise shape distribution, high crystallinity, and no post-annealing treatment, so it has been widely used in the synthesis of SFNs.

3.2. Surface Modified and Coated Magnetic Catalysts

The high specific surface area and surface holes of the magnetic nuclei nanoparticles determine their high catalytic activity in the reaction; however, “bare” magnetic nuclei often exhibit instability in reactions, and agglomeration is inevitable during the reaction [53]. In order to avoid the reaction and agglomeration of the magnetic core with the active component of the load, and to maintain the stability of the magnetic core, the magnetic core is often surface modified to different catalytic processes. Silicon dioxide, polymers, carbon, etc. can all be used to modify the surface of the magnetic core [47]. Silicon dioxide is highly stable and can be used for surface modification of magnetic nuclei to avoid charge transfer between magnetic nuclei and catalytic sites, ensuring catalytic activity and stability. Therefore, many researchers have done a lot of work on this. As shown in Figure 5. Ceylan et al. [54] made palladium nanoparticles by ammonium combined with tetrachloropalladate reduction precipitation using the property that silica coating can be functionalized on the surface of magnetic nuclei, and the experiments showed that the surface modified catalyst had good catalytic activity in a microfluidic fixed bed reactor.

Morales et al. [55] synthesized two silica-coated magnetite nanoparticle composites using co-precipitation by the Stöber method. Chaudhuri et al. [56] used a co-precipitation method to attach gold nanoparticles to the surface of magnetic nanostructured core-shell particles composed of Fe₃O₄ magnetic cores and silica shells, and the synthesized nanostructured particles exhibited superparamagnetic properties. Deng et al. [57] prepared core-shell structured magnetite particles with an average diameter of 40 nm and silica coating at room temperature using methanol, ethanol, isopropanol and n-propanol solvents with relatively uniform dispersion. Fajaroh et al. [56] prepared silica-coated magnetite nanoparticles in sodium silicate solution by an electrochemical technology, and their stability studies showed that the silica layer could effectively protect magnetite from being converted to other oxides. In addition, to use silica to coat the magnetic core, Houlding et al. [58] investigated the mechanochemical synthesis of TiO₂/NiFe₂O₄ magnetic catalysts with the advantage that the catalyst’s magnetic and catalytic properties can be optimized independently. Encapsulated magnetic nucleation nanoparticles can also be used in medical applications. Cervantes et al. [59] prepared iron oxide nanoparticles with an average diameter of 13.5 nm, magnetic saturation of 34 (emu/g), and a blocking temperature of 285 K with 1, 2-phenylene glycol coating for magnetothermal therapy by a co-precipitation method. Yin et al. [60] showed good 4-NP catalytic hydrogenation performance and efficient magnetic separation of NiCo/BCNTs, an encapsulated NiCo alloy/carbon nanotube nanocomposite prepared by a one-step calcination method using nickel acetate and cobalt acetate as the source of magnetic particles and dicyandiamide as the carbon source. It can be seen that the modification of the surface of the magnetic core using the coating material SiO₂ can avoid the reaction of the magnetic core with the active component of the load and the agglomeration phenomenon between the magnetic core.

3.3. Active Component Loaded Magnetic Catalyst

Magnetic nuclei with catalytic function and cladding magnetic catalysts have the advantage of simple preparation, but there are disadvantages such as cumbersome posterior, low yield, low separation, recovery or reuse efficiency, and less catalyst surface activity site, and low reactivity and selectivity. Therefore, researchers have proposed new techniques to improve them. Liu et al. [61] synthesized Ni/Fe nanoparticles by sol-gel method and prepared composite catalysts with TiO₂ loadings of 8.9, 16.7, 26.0 and 32.0 wt%, respectively, and investigated the effect of titanium dioxide content on their amide synthesis reaction activity and stability. Marbaix et al. [62] successfully synthesized a series of stoichiometry-controlled Fe1- xCox alloy nanoparticles. As shown in Figure 6, the bimetallic NPs shows high magnetization strength and low coercivity compared to Fe (0) NPs. Their SAR values increase with increasing Co content and their Higher Curie temperature makes them suitable for induction heating catalytic reactions.

Xu et al. [63] prepared magnetic Fe₃O₄NPs by a solvothermal synthesis method using three noble metal nanoparticles (Au, Pt and Pd) immobilized in the shell layer of Fe₃O₄@COF by NaBH₄ reduction. They prepared Fe₃O₄@COF-Au catalysts were used for the reduction reactions of 4-NP and methylene blue (MB) with NaBH₄, and this multifunctional composite showed excellent catalytic activity and stability with easy recovery. Li et al. [64] synthesized ferromagnetic nanocatalysts by selectively depositing noble metal nanoparticles on Fe₃O₄/graphene composites, which has the advantage of good dispersion and high catalytic performance. Since the surface of Fe₃O₄NPs has more NH2 groups than graphene, it is favorable for the selective deposition of precious metals. Liu et al. [65] synthesized Fe₃O₄@C@β-CD by covalent bonding with epichlorohydrin under strongly alkaline conditions for the first time, then, the β-CD on the surface of MFC reacted with p-aminothiophenol (pATP) to form MFC@β-CD-pATP composites. Metal nanoparticle catalysts of ternary nanocomposite MFC@β-CD-pATP were prepared by adsorbing pre-prepared gold, silver and platinum nanoparticles (AuNPs, AgNPs, PtNPs) on the surface of MFC@β-CD-pATP. Wang et al. [66] used silver nanoparticles modified with magnetic composites to prepare magnetic catalysts for the degradation of organic dyes. As shown in Figure 7, the carboxyl modified magnetic composite (Fe₃O₄@PS-COOH) was synthesized for the first time, and then the Ag ions were adsorbed on the Fe surface and the Fe₃O₄@PS microspheres were reduced in situ on its surface. Finally, the catalytic performance of Fe₃O₄@PS@Ag composites was evaluated in the degradation of organic dyes (4-NP, RhB and MB). The results showed that the Fe₃O₄@PS@Ag catalyst could be recovered from the solution and reused more than seven times by magnets.

4. Application of Magnetic Catalyst

4.1. Catalysis Materials Synthesis

Magnetic nanoparticles (MNPs) are suitable for multiphase catalysis because of their easy handling, recovery, and recycling properties. Its use as a “magnetic catalyst” has been expanded by modifying or heterogenizing the surface of the catalytically active metal [22]. Piner et al. [67] investigated a new route for the synthesis of graphene, where induction heating of metal foils under an alternating magnetic field was able to produce high quality graphene compared to chemical vapor deposition techniques. Ceylan et al. [54] used silica coated magnetic catalysts for microfluidic fixed bed reactor catalytic synthesis applications. The study shows that magnetic catalytic materials are used for continuous flow reactions and that the experimental setup is simpler than heating a flow-through reactor by microwave radiation, and that the effective heating in the electromagnetic field is mainly based on the magnetic heating of Fe₂O₃, Co and Ni and other materials. Liu et al. [61] prepared composite catalysts with a mass of 0.2 kg that exhibited magnetic coercivity, and since the magnetic coercivity depends on the magnetic particle size, the same coercivity for different catalysts indicates that the NiFe₂O₄ particle size is not affected by the introduction of TiO₂ shells. Houlding [58] et al. carried out RF heating using TiO₂/NiFe₂O₄ magnetic catalyst in a miniature packed-bed flow reactor, as shown in Figure 8, and the initial amide yield of T₁-SP_25-0.5 catalyst was increased by 12% compared to T₁-P_25-0.5 catalyst. Irreversible deactivation was observed on both catalysts. Compared with the T₁-SP_25-0.5 catalyst, T₁-P_25-0.5 not only deactivates more rapidly but also has lower steady-state activity.

García-Aguilar et al. [68] proposed a new method for the preparation of magnetic zeolite-based catalysts that allows more efficient RF heating. Zadražil et al. [69] studied remotely controlled gas-phase heat absorption catalytic reactions. The remote control of the reaction process was achieved by introducing iron particles into the alumina catalyst and placing the composite catalyst in an RF magnetic field. As shown in Figure 9, there is almost no delay in the ethylene yield response and the amount of ethylene produced is proportional to the duration of the state.

Liu et al. [61] used a magnetic composite catalyst with TiO₂@NiFe₂O₄ core-shell structure for the direct synthesis of amides in a continuous reactor heated by radio frequency and optimized its heating and catalytic performance. The initial reaction rate was increased by 60% under RF heating. The catalyst deactivation rate was higher in the conventionally heated reactor due to the presence of a radial temperature gradient. Chaudhuri et al. [70] investigated a study using oxygen or air to oxidize allyl alcohol and benzyl alcohol to aldehyde and ketone, respectively. As shown in Figure 10, gold nanoparticles are attached to the surface of Fe₃O₄ and silica nanostructured core-shell particles. Since the newly synthesized nanostructured particles are superparamagnetic, they can be inductively heated in an external alternating electromagnetic field. The heating characteristic curve of the external oscillating electromagnetic field frequency of 25 kHz is basically consistent with those of the non-functionalized nanostructured particles1.

The Knoevenagel condensation reaction has a widely used application. For instance, Fang et al. [50] developed an efficient magnetic bimetallic NiCo₂O₄ nanocatalyst by co-precipitation for the Knoevenagel condensation reaction of various benzaldehyde with malononitrile to be investigated. Marin et al. [71] used an easily available FeNi₃@NiNPs catalyst for the synthesis of biomass-derived molecules. Their results showed that the catalyst has better magnetic properties and catalytic performance. The activity and reactant conversion of FeNi₃@Ni under oil bath heating and magnetic heating were also comparatively studied, and the reactant conversion was higher under electromagnetic induction heating. Yan et al. [72] synthesized a new low-cost MCN bifunctional catalyst with excellent ORR and OER performance. The catalyst used a simple electrostatic spinning method to dope metallic cobalt nanoparticles into macroporous carbon nanofibers, directly enhanced the oxygen catalytic activity by applying a moderate (350 mT) magnetic field intensity, and finely controlled the MCN film thickness by adjusting the electrostatic spinning time, etc.

Highly fluorescent products can also be synthesized by using heat generated by magnetic nanoparticles in an external alternating magnetic field to drive catalytic chemical reactions. Yassine et al. [73] used positively charged polyelectrolytes to assemble catalytic microstructures in the presence of negatively charged magnetic and gold nanoparticles, and they used heat generated by microscale electromagnetic induction to drive the reduction of edged asphaltenes to the highly fluorescent product tryhalocin. Magnetic heating is considered to be an effective way to perform catalytic reactions with heating sources and catalytically active components deposited on the carrier. Magnetic heating can be used for hydrodeoxygenation reactions with lower loading of catalytically active components and low H₂ pressure. Asensio et al. [15] synthesized FeC@RuNPs magnetic nanoparticles and explored the feasibility of hydrodeoxygenation of aromatic and aliphatic ketones by magnetic heating in solution by combining the high thermal power of Fe_2.2CNPs and the high catalytic activity of RuNPs. After optimizing the reaction conditions for the hydrodeoxygenation of acetophenone and its derivatives, the reaction was extended to furfural and hydroxymethylfurfural at low H₂ pressure (3 bar). The close contact between the heater and the catalyst promotes the reaction and increases the activity of the catalyst.

4.2. Endothermic Steam Reforming for Hydrogen Production

Steam methane reforming to hydrogen (SMR) remains the proven technology for the current large scale hydrogen production. Steam reforming is a strong endothermic reaction, and it always reacts at high temperatures (700–900 °C). The common reaction mechanisms of SMR are as follows [74].

$${\mathrm{CH}}_4+{\mathrm{H}}_2\mathrm{O}\leftrightarrow \mathrm{CO}+3{\mathrm{H}}_2$$

$${\mathrm{CH}}_4+2{\mathrm{H}}_2\mathrm{O}\leftrightarrow {\mathrm{CO}}_2+4{\mathrm{H}}_2$$

$$\mathrm{CO}+{\mathrm{H}}_2\mathrm{O}\leftrightarrow {\mathrm{CO}}_2+{\mathrm{H}}_2$$

SMR is a complex reaction. Previous studies have focused on improving it by designing an appropriate catalyst and operating procedures [75]. One problem with fixed bed reactors is the occurrence of low and high temperature zones caused by poor heat transfer which can lead to sintering in the hot areas and low conversion in the cold areas [76]. In a word, conventional hydrogen production device usually has a large temperature gradients and high temperature carbon buildup on the catalyst, and more importantly, which will even cause the catalyst to deactivate; however, microreactors exhibit much higher heat and mass transport rates than conventional large-scale reactors [77]. By designing miniature reactors, it is possible to reduce catalyst carbon deactivation and obtain a uniform temperature distribution [78]. But this strategy reduces the ability to produce hydrogen on a large scale because of the reactor size limitation. In contrast, electromagnetic induction heating has high energy utilization efficiency, because it can directly heat transfer to the catalytic active site, and which is not limited by heat transfer. As shown in Figure 11. In addition, induction heating SMR technology can also reduce CO₂ emissions, it can be used as a transition technology to meet the global demand for H₂ and reduce dependence on non-renewable energy use.

In the study of methane reforming to hydrogen driven by electromagnetic induction heat, many researchers have done a lot of work on induction heating for hydrogen reforming. For instance, Ovenston et al. [24] used electromagnetic induction heating of a single magnetic catalyst for its heat-absorbing reforming to hydrogen for the first time in 1983. Pérez-Camacho et al. [25] carried out a study of methane reforming to hydrogen by induction heating using a chalcogenide catalyst. The results showed that the conversion rate varies with the current. When input the maximum current is 110.5 A, the methane conversion reaches 90%, while the methane conversion reaches a maximum of 95% at an input current of 98.1 A, which is close to thermodynamic equilibrium; moreover, the catalyst bed reaches 853 °C at this stage. Despite it having a high conversion rate, however, the SEM showed that sintering of the chalcogenide catalyst occurred. Mortensen et al. [79] proposed a novel flow reactor system with magnetic Ni-CoNP alloy on a magnesium aluminate (MgAl₂O₄) spinel carrier to provide hysteresis heat for the reforming hydrogen production reaction, as shown in Figure 12, where the hydrogen production efficiency increases with the increase in induction heater power; moreover, the highest methanol conversion was achieved at a reactant flow rate of 20 NL/h.

Vinum et al. [26] proposed bottom-up NPs engineering instead of the common metal impregnation method to prepare CoNi and Cu loaded on CoNi (Cu⊂CoNi) magnetic materials as catalysts for inductively heated methane steam reforming. At a reactant flow rate of 152 NL/h, the Cudoped catalyst has achieved 95% CH₄ conversion. For the same results, the newly prepared catalyst requires about 15% lower magnetic field strength than that applied with the common CoNi catalyst. Nguyen et al. [80] used binary Cu-Ni, Cu−Co, and ternary Cu−Ni−Co catalysts for methane double reforming process to produce hydrogen by using induction heating technology. As shown in Figure 13. The catalyst can catalyze the reaction with the expected temperature range of the reforming reaction or even lower. They also prepared monometallic Cu−based catalysts in order to better evaluate the activity of binary and ternary catalysts. In addition, the literature56 also explored the activation mechanism of this catalyst in the presence of oscillating magnetic fields. Their findings shown that the presence of both magnetic and conductive components in the catalyst system facilitates the hysteresis loss and joule effect mechanisms. Due to Joule heating, the Cu−Co catalyst temperature is higher than its Curie temperature value.

Varsano et al. [81] used Ni₆₀Co₄₀ alloy as a magnetic induction heating for methane dry reforming for hydrogen production. At a reaction temperature of 950 °C, compared to the conventional heating case, although the methane conversion and hydrogen yield advantages of this experimental setup are not significant, but it has precise temperature regulation and energy-saving features. Scarfiello et al. [16] used Ni₆₀Co₄₀ nanoparticles loaded on γ−Al₂O₃ to prepare catalyst samples with two different metal loadings and particle size distributions; it can simultaneously catalyze methane steam reforming reactions and supply the heat required for the reactions in situ. As shown in Figure 14, the Ni-Co-based catalyst surface temperature is sufficient to achieve a methane conversion of 80%.

Fache et al. [82] developed a numerical simulation theory of induction heating of magnetic particles in a fixed bed and investigated the effect of thermal power on temperature. The results showed that the parameter sensitivity of induction heating is higher than that of the conventional device and its yield is higher. Almind et al. [27] optimized the performance of induction heating for methane steam reforming by controlling the frequency of the alternating magnetic field and the coil geometry; their findings showed that the system efficiency can approach 90% when induction heating is used for large-scale H₂ production. Similarly, Almind et al. [83] obtained Co-Ni nanoparticle catalysts by varying the cobalt to nickel ratio and investigated its magnetic tunability experimentally. The results showed that high Ni content leads to a large initial hysteresis area and lower Curie temperature, while high Co content produces smaller hysteresis heating and lower intrinsic activity. But this allows for a higher operating temperature. In the further study, Almind et al. [84] modulated the heating and catalytic capacity of the material by changing the composition of the nanoparticles. They prepared magnetic material catalysts with different components to optimize the device performance, and finally they obtained that the maximum variation of hysteresis loss with temperature and applied magnetic field was obtained in the particle size range of 24–31 nm. Further, the literature 60 also investigated the power loss in an induction-heated benchtop reformer and obtained energy efficienies greater than 80% when scaled up to industrial conditions. Scale-up analysis shown that the system is competitive with commercially available electrolytic hydrogen production methods when only energy requirements are considered.

In a word, the direct heating of the catalytic system provides a fast heat transfer and thereby overcomes the heat-transfer limitation of the industrial-scale steam reformer; moreover, according to

Table 1. Methane reforming to hydrogen catalyst current status of induction heating.

Catalyst	Reaction Temperature	Methane Conversion Rate	Advantages	Literature Sources
Ni60Co40	720 °C	80%	Easily scalable to mass production	Scarfiello et al. [16]
CoNi Cu⊂CoNi	715 °C	90.95%	Cu-doped catalyst has achieved 95% methane conversion	Pérez- Vinum et al. [26]
CoNi	800 °C	90%	Easy to prepare and High activity	Almind et al. [27]
NiCo/MgAl2O4	780 °C	98%	Direct heating, High catalytic activity and High methane conversion rate	Mortensen et al. [79]
Cu-Co	500 °C	62%	The Cu-Co catalyst preserved its catalytic stability for at least 50 h at 500 °C.	Nguyen et al. [80]
Ni60Co40	850 °C	70%	Minimize catalyst deactivation	Varsano et al. [81]
Co50Ni50	767 °C	98%	High methane conversion rate and No support material required	Almind et al. [83]

, direct heating of the catalyst by induction heating technology not only has no thermal gradient (thermal inertia) and enables a rapid start of the reaction, but also maintains a high level of methane conversion in comparison with other literature [85,86,87]. Chemically, magnetic induction catalysts have a general catalytic mechanism with general catalysts. It rather focuses on highlighting the key advantages of IH over conventional heating methods coming from the targeted and rapid control of the heating rate of hydrogen production. IH reaction scheme has shown how energy saving and process intensification can be achieved by transferring electromagnetic energy generated by an external alternating field directly to the magnetic core of its catalytic system and in turn to the catalyst particles decorating active site.

4.3. Hydrogen Production from Electrolytic Water

Electrolysis of water is considered an alternative technology to alleviate the crisis of non-renewable energy sources, and the process of water decomposition requires a thermodynamic potential over electric potential greater than 1.23 V [88]. During the process of water electrolysis, hydrogen ions move towards the cathode, while hydroxide ions move towards the anode. The half reactions occurring on the cathode and anode, respectively, can be written as [89].

$$\mathrm{Cathode}: 2{\mathrm{H}}^{+}+2\mathrm{e} \to {\mathrm{H}}_2$$

$$\mathrm{Anode}: 2{\mathrm{OH}}^{-} \to \frac{1}{2}{\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}+2\mathrm{e}$$

The overall chemical reaction of the water electrolysis can be written as

$${\mathrm{H}}_2\mathrm{O} \to \frac{1}{2}{\mathrm{O}}_2+{\mathrm{H}}_2$$

The commercial electrolyzers have two options operating environments, either they work at neutral pH or in alkaline environments. Both electrodes and proton exchange membranes use precious metals, which raises the cost, but of course, this is mainly used for small-scale applications. In alkaline media, the electrodes are made of mixed metal oxides based on Fe, Ni, and Co cations [90]. This catalyst can be used to reduce the hydrogen precipitation reaction (HER) and oxygen precipitation reaction (OER) overpotential [88]; moreover, different thickness of the electrode catalyst also has a significant effect on HER. [91] Proton exchange membrane electrolyzers are hampered by the high cost and low availability of their precious metal catalysts, while alkaline electrolyzers suffer from low power density and lack of durability. Therefore, Niether et al. [92] prepared 15 nm FeC-Ni nanocatalytic particles by using Ni active ingredient coated with iron carbide. As shown in Figure 15, hydrogen production by catalytic electrolysis of water was heated under an external alternating magnetic field at 300 kHZ. The authors investigated the effect of FeC-Ni magnetic nanoparticles on the electrocatalytic performance of hydrogen precipitation reaction and oxygen precipitation reaction in alkaline aqueous solution. The overpotential values required for oxygen and hydrogen precipitation were found to be reduced by 200 mV and 100 mV, respectively. Noticeably, FeC-Ni is located on the carbon fibre, Ni is uniformly distributed on the surface of FeC core-shell, and electrons are transferred to FeC-Ni through the carbon fibre, which makes the hydrogen ions of water molecules get electrons to convert to hydrogen on the Ni catalyst. produce local heat to raise the temperature of the FeC–Ni NPs and therefore accelerate the OER, One of the OER kinetic boosts is equivalent to raising the temperature of the cell to 200 °C, while in reality the cell temperature only increases by 5 °C.

Xiong et al. [93] used hydrothermal and furnace calcination methods to synthesize 16 electrocatalysts with different morphologies and structures, which include monometallic hydroxides, bimetallic hydroxides, and metal nitrides. Among them, Ni-MoO₂/NF-IH and NiFe LDH/NF-IH exhibited excellent HER and OER performance in alkaline solutions; Ni-MoO₂/NF-IH as cathode and NiFe LDH/NF-IH as anode could achieve a current density of 10 mA cm^–2 with an electrolytic cell voltage of only 1.50 V. The catalytic performance is better than that of Pt/C/NF and RuO₂/NFNiMoO₄ catalysts heated in a conventional way. The reason is that the NF-IH structure is beneficial to increase the specific surface area, which can provide more active sites for electrocatalytic reactions. Su et al. [92] designed and synthesized 3D helical cone catalysts with edge active sites MoS₂ for hydrogen production from electrolytic water by a modified chemical vapor deposition (CVD) method and tilted substrates. The formation of a micro-vortex inside the helical pyramidal MoS₂ nanosheets under AMF makes full use of magnetic heating, and its special structure allows electron transport directly along the helical orbits to improve electron transfer efficiency. The NiCr alloy investigated by Li et al. [94] has fast heating rates and better electrocatalytic properties under the action of an external magnetic field. Liu et al. [95] rapidly synthesized catalysts loaded with Ru nanoparticles on carbon paper in a short time based on a new method of magnetic induction heating (MIH), which was used to catalyze the hydrogen precipitation reaction (HER) of electrolytic water production. The overpotential is reduced and the current density is increased. They provided a new idea for the preparation of electrodes for hydrogen production from electrolytic water.

4.4. Other Hydrogen Production Technologies

Badakhsh et al. [96] used NiCrAl foam as a catalyst for ammonia decomposition to produce hydrogen, and the method of Joule heat generated by NiCrAl under alternating magnetic field was used to improve the performance of ammonia decomposition for hydrogen production. Wu et al. [97] first added metal particles to biomass in an induction heating reactor for biomass pyrolysis. The effect of temperature on the product properties during pyrolysis was also investigated, as shown in Figure 16. The yield of hydrogen at 600 °C was about 25 mL/g, which showed that induction heating has a promising application in hydrogen production from biomass pyrolysis despite the low hydrogen yield.

4.5. Catalytic Wet Peroxide Oxidation

Catalytic wet peroxide oxidation (CWPO) is a favorable technological means for treating organic pollutants in wastewater under mild reaction conditions [98]. The reaction temperature is the main influencing factor for the non-homogeneous CWPO reaction. The current conventional heating methods involve in steam heating, microwave heating, and resistance heating [99]. This type of heating is often characterized by the transfer of heat from the reactor to the liquid phase zone and finally to the catalyst surface, which increases the irreversible loss of energy utilize and reduces the its efficiency [100]; however, electromagnetic induction heating technology can generate heat directly through the magnetic heat catalyst, which is low energy loss and a fast response to reaction initiation. Munoz et al. [101] used a modified magnetite mineral (Fe₃O₄-R₄₀₀) as a catalyst for the treatment of antibiotic sulfamethoxazole (SMX) contaminants in wastewater. The results showed a significant increase in the apparent kinetic constant values, and the CWPO reaction rate constant values at the highest electromagnetic field strength reached three times that of the conventional heating method. Chen et al. [98] used a fixed-bed reactor combined with a high-frequency induction heating reactor system for the treatment of dye substrates in an aqueous solution. The results showed that electromagnetic induction heating allowed the catalyst particles to reach higher reaction temperatures. The energy is transferred directly to the dispersed particle interface without acting on the aqueous phase, which greatly improves the reaction efficiency. As shown in Figure 17. Jing et al. [102] prepared a core-coated catalyst (mNiO/C) from sponge iron by nickel impregnation and calcination. mNiO/C had the highest catalytic activity with 80% degradation of wastewater dyes under optimal conditions.

5. Conclusions

Electromagnetic induction heating technology is relatively new when used in the catalytic field. It is expected to solve the general problems of conventional endothermic reaction where heat is not uniformly transferred to the reaction zone of the catalyst bed of the reactor system, heat transfer from the outer surface of the reactor to the catalyst bed resulting in large temperature gradients and uneven temperature distribution, and slow heating/cooling rate. Therefore, this work first summarized the heat release mechanism and performance of magnetic catalysts, then, we introduced the preparation methods of magnetic catalysts, finally, we reviewed the applications of magnetic catalysts. It is worthwhile to pay attention to the application and performance of magnetic catalysts in the field of hydrogen production. Overall, the advantages of a transition from a contact to contactless heat management in catalysis based on IH technology are manifold: more favorable energy balance, process intensification, reactor setup simplification, reduced safety issues, minor operational costs, and increased process productivity.

Heat transfer from magnetic nanoparticles and mass transfer from catalytically active surfaces belong to the heat and mass transfer problems at the micro and nano-scale, in this case, Fourier’s law is no longer suitable for heat calculation. Considering the complexity of heat dissipation at the nanoscale, the theoretical model still cannot predict the temperature accurately, limiting the simulation of their heat transfer processes at the micro and nano-scale. The relationship between heat transfer and surface temperature of magnetic nanoparticles at the micro/nanoscale needs to be further revealed to optimize the heating process of magnetic nanoparticles. There are less studies on the combination of electromagnetic induction heating with high thermal efficiency (SAR) with metal nanoparticles or metal coatings with catalytic properties. The current stage of magnetic catalyst preparation commonly uses SiO₂-coated magnetic nuclei or nanometallic particles with high catalytic activity (e.g., Ag) modified on the surface of magnetic particles, the preparation process of which is often complicated and costly.

6. Perspectives

The development of induction heating catalysis scheme in the future has the following views. Firstly, materials with high superthermal efficiency (SAR) need to be combined with highly active catalytic performance components to analyze the effect of temperature on the internal lattice of magnetic particles. Secondly, further research is needed to reduce the preparation cost of magnetic nanocatalytic particles and to develop nanoparticles with high active catalytic components, high stability, and high magnetic response properties. Thirdly, together with the possibility of fast plant startup, holds promise for these materials as competitors for classical hydrogen plants or as part of ammonia plants in a future hydrogen economy and especially for ad hoc small-scale demands. The temporal and spatial resolution of temperature measurements near magnetic NPs during the application of Hydrogen production will be an obstacle for advancements in induction heating applications. Indeed, the technology of renewable energy generation and hydrogen production by electrolysis of water can significantly reduce CO₂ emission. Because of the high energy utilization rate of induction heating hydrogen production technology. In the future, it can be used as a powerful technical means to solve the contradiction between energy utilization and environmental conservation faced by people.