Microwave-Assisted Catalytic Deconstruction of Plastics Waste into Nanostructured Carbon and Hydrogen Fuel Using Composite Magnetic Ferrite Catalysts

Finding new catalysts and pyrolysis technologies for efficiently recycling wasted plastics into fuels and structured solid materials of high selectivity is the need of time. Catalytic pyrolysis is a thermochemical process that cracks the feedstock in an inert gas environment into gaseous and liquid fuels and a residue. This study is conducted on microwave-assisted catalytic recycling of wasted plastics into nanostructured carbon and hydrogen fuel using composite magnetic ferrite catalysts. The composite ferrite catalysts, namely, NiZnFe2O4, NiMgFe2O4, and MgZnFe2O4 were produced through the coprecipitation method and characterized for onward use in the microwave-assisted valorization of wasted plastics. The ferrite nanoparticles worked as a catalyst and heat susceptor for uniformly distributed energy transfer from microwaves to the feedstock at a moderate temperature of 450°C. The type of catalyst and the working parameters significantly impacted the process efficiency, gas yield, and structural properties of the carbonaceous residue. The tested process took 2–8 minutes to pulverize feedstock into gas and carbon nanotubes (CNTs), depending on the catalyst type. The NiZnFe2O4-catalyzed process produced CNTs with good structural properties and fewer impurities compared to other catalysts. The NiMgFe2O4 catalyst performed better in terms of hydrogen evolution by showing 87.5% hydrogen (H2) composition in the evolved gases. Almost 90% of extractable hydrogen from the feedstock evolved during the first 2 minutes of the reaction.


Introduction
Plastic manufacturing has increased twentyfold over the past ffty years, which is predicted to continue in the coming years.By 2050, the yearly output of plastics will approach 500 million tons, using up as much as 40% of the world's crude oil [1].More than 80% of plastic ever manufactured is fnished in the environment due to disposal of plastic materials in landflls and marine and coastal environments.Currently, at most, 2% of plastic production comes from forming renewable resources.Furthermore, only 14% of plastic waste is recycled and only 2% of that material is recovered for use in applications that require the same or equivalent quality materials.Currently, mechanical recycling is the primary process used to recycle plastics, and this method usually entails collecting, sorting, and cleaning the material before processing it to create new products [2].As the need for plastic in the modern world grows, so does the amount of waste plastic.Te two most popular methods for treating waste plastics produced by the municipal solid waste stream are landflling and incineration [3].Te energy content of used plastics is lost when disposed of in landflls.Although energy is recovered during incineration, the carbon content of the plastics is primarily converted to carbon dioxide (CO 2 ) and released into the atmosphere.Te thermal cracking of plastic waste via pyrolysis is among the top favorable chemical recycling methods.
Plastic is thermally decomposed during pyrolysis into a liquid like oil or wax and small quantities of gaseous products or solid residue leftovers [4].Te pyrolysis of waste plastic for H 2 production has been abundantly researched.Various catalysts have also been researched to make the procedure efcient and increase hydrogen production [5].More work on catalyst development is needed to improve the yield and structural properties of the solid residue for its practical use as a nanomaterial.In the present research, the role of ferrite catalysts was determined in producing CNTs as a solid residue and hydrogen gas from the wasted plastics [6].Ferrite nanoparticles play a dual role in microwaves energized catalytic pyrolysis of plastic waste.Almost all plastics are thermal insulators and do not absorb heat energy directly from the incident microwaves.Ferrite nanoparticles work as heat susceptors for absorbing energy from microwaves and transferring it to plastic molecules [7].Concurrently, they provide abundant active sites for redox reactions and nucleation of CNTs.Ferrite nanoparticles have redox capabilities, which means they can easily switch between diferent oxidation states.Tis redox behavior benefts CNTs growth because it allows for the activation and regeneration of catalytic sites during the synthesis process.Te plastic molecules decompose into carbon, hydrogen, and other gases and byproducts on receiving energy from ferrite nanoparticles.Te carbon atoms get adsorbed onto the catalyst surface and then difuse and dissolve into the catalyst nanoparticles [8].Te high thermal stability, surface area, and chemical reactivity of the magnetic catalyst facilitate the localized heating in this process.Te dissolved carbon atoms begin to form nanotube structures.High thermal stability is important for maintaining catalytic activity throughout the growth of CNTs.Conventional pyrolysis often produces tar and char residue, which can interfere with the efciency of the process.Ferrite nanoparticles can aid in the decomposition and cracking of tar and char, preventing their accumulation and improving the conversion of plastic into valuable products.Te catalytic properties of ferrite nanoparticles can enhance the cracking of long-chain hydrocarbons, reducing the formation of undesired byproducts.Magnetic ferrite catalysts are more efective in producing hydrogen due to their high ability to break C-C bonding in the plastic chain and low cost.
Microwaves can permeate solid materials to commence volumetric heating, as opposed to traditional heating, which can only achieve surface heating.In microwave pyrolysis of plastics, an absorbent dielectric substance is often used to induce heat transfer to the plastic feedstock.Magnetic catalysts like NiZnFe 2 O 4 , NiMgFe 2 O 4 , and MgZnFe 2 O 4 absorb more microwave energy than other materials because of their high dielectric loss factor [9]. Tese catalysts enhance the quality of the process leading to better selectivity of the products even at moderate temperatures compared to conventional pyrolysis.Tis catalyst efectively minimizes the formation of tar and char by breaking organic molecules.Microwave interactions with these catalysts result in a high Snoek's limit, complex permeability, and saturation magnetization.Many researchers have indicated that microwave-assisted heating is a viable method for the pyrolysis of various feedstocks, particularly polymers [10,11].Microwave-catalytic pyrolysis is a complicated thermochemical process that is regulated by various factors such as process temperature, microwave absorbents, and catalyst types [12].Microwave pyrolysis is gaining popularity due to signifcant advantages over traditional heating processes, such as contactless volumetric heating, simple heat distribution, fast processing, better energy efciency, and high pyrolytic oil or combustible gas yield.Zhou et al. [13] assumed that temperature is an important parameter in defning product distribution and energy utilization by the pyrolysis of plastic waste.Microwave heating does not heat the entire sample due to a small temperature diferential from the outside surface, which could result in multiple side reactions and the formation of unnecessary byproducts.However, waste plastics absorb low microwave heat energy because they are microwave transparent.Combined with microwave absorbers, such as carbon compounds and inorganic oxide, plastic can easily be degraded.It is also been observed that microwaves strongly interact with magneticbased catalysts [14].More microwave energy can be absorbed by coupling ferromagnetic species with the magnetic components of the microwave feld.At lower temperatures, the reaction may occur by multiple mechanisms because of continuous heating mechanisms from conventional heating techniques [15].CNTs and hydrogen are described as the major carbonaceous products other than CO 2 , methane (CH 4 ), ethylene (C 2 H 4 ), carbon monoxide (CO), etc., during the pyrolysis of plastics.H 2 can be utilized as an efcient energy source due to high energy density (120.7 kJ/g) and ecofriendly chemistry [16].
Te exact mechanism of CNT nucleation during such recycling processes is not fully understood; however, it is widely assumed to involve the formation of atomic carbon, its absorption onto the catalyst surface, and then difusion and precipitation onto nanotube structures.One notable advantage of ferrite nanoparticles is their magnetic nature.Te magnetic properties of ferrite nanoparticles can be exploited to manipulate and control the alignment and orientation of CNTs during growth.Externally applied felds can also infuence the growth direction and alignment of CNTs on the catalyst surface.For microwave pyrolysis applications, catalysts with a high dielectric loss factor are the best choice [17].When exposed to microwave radiation, ferrite nanoparticles experience a phenomenon known as dielectric heating.Te electromagnetic feld of microwaves interacts with the electric dipoles in the ferrite nanoparticles to cause dielectric heating [18].Tis interaction causes the ferrite to rapidly alternate its magnetic polarity, resulting in energy absorption and heat generation.Tis work reports a microwave-assisted catalytic pyrolysis approach for achieving high hydrogen yield and structured CNTs by   [20].Te surface topology and size of the catalyst samples were analyzed by scanning electron microscopy (SEM).Te crystal structure of the catalyst samples was probed by using the X-ray difraction (XRD) spectroscopy method.Te optical traits of the synthesized samples were checked by producing UV-vis spectra.Fourier transform infrared (FTIR) spectra of the catalysts were taken at room temperature between 500 and 4500 cm −1 for the characterization of functional groups and bond formation.

Pyrolysis Experiments.
Te reported catalytic pyrolysis involved the catalytic decomposition of plastic at 450 °C in an oxygen-free chamber.Pyrolysis is the most reliable and straightforward process for recycling plastic waste into oil, H 2 gas, and valuable solid products.But the end products from this process need to be chemically improved and further purifed.Te pyrolysis process incorporates an appropriate catalyst to overcome these challenges.Figure 1 illustrates the experimental setup used in the present study.Te catalytic microwave pyrolysis of plastic was conducted in a multimode microwave reactor operated with a 2450 MHz source [21].Te experimental setup contains a gas supply system, a microwave source, a temperature sensors, connecting tubes, condensers, an oil container, cold traps, gas analyzer, and a cold-water supply.A roundbottomed fask was used as a feedstock container, which is transparent to the incident microwaves.To remove any residual air, the pyrolysis chamber was purged with nitrogen gas at a fowrate of 200 ml min −1 for 20 minutes just before starting the microwave pyrolysis process [22].Te microwave reactor then operated for 8 minutes at 1000 W power.
A mass fow controller was used to control the gas fowrate.
Te fowrate and composition of the produced gases was measured with a gas analyzer.Te gas product was estimated using the obtained mass relative to the total weight of feedstock (equation ( 1)).Te yield of H 2 is calculated by dividing the moles of H 2 by the total mass of feedstock (equation ( 2)).Te efciency of hydrogen production is calculated by dividing the theoretical hydrogen mass in plastic by the total hydrogen mass in all gas products (equation ( 3)).
Gas Yield Hydrogen Yield mmol/g P  � m H 2 m P . (2) where m g is the mass of gas, m p is the plastic waste,m H 2 is the moles of H 2 , m th is the theoretical H 2 mass in plastic waste, and m gH is the total H 2 mass present in all gaseous products [23].Te yield of the produced gas can be determined using equations ( 1)-( 3).
For pyrolysis experiments, 40 g of feedstock and 10 g of the catalyst were mixed uniformly in a weight ratio of 4 : 1. Te feedstock was placed in the microwave oven in a roundbottomed quartz fask [24].Tree sets of experiments were conducted using NiZnFe 2 O 4 , NiMgFe 2 O 4 , and MgZnFe 2 O 4 at 450 °C [25].Te catalyst plays an essential role in the reaction because it produces the desired products at a low temperature.Magnetic ferrite catalysts catalyze the reaction and work as microwave heat susceptors.Temperature sensors were added to the setup to monitor the feedstock and gas outlet temperature.Te feedstock temperature was maintained at 450 °C throughout the pyrolysis process.Liquid oil was condensed and collected using a one-necked fask and a cooling column was cooled using chilled water.Te noncondensable gas was analyzed using a gas analyzer [26].Scientifca 2.4.Characterization of the Product.SEM and XRD techniques were employed to characterize the fresh catalysts and the solid products of the pyrolysis of plastic.Te parameters of the products, such as structure, grain size, lattice parameter, and unit cell volume, were studied by producing XRD patterns.Te crystallinity was evaluated from the ratio of the integrated peak area for the crystalline phase and amorphous fraction area.Te average crystallite size was determined using the Scherrer equation.SEM was used to analyze morphology and to identify the clusters and pores on the product surface.Termogravimetric analysis (TGA) was performed to check thermal stability of the samples.FTIR spectra of the pyrolysis products and catalysts were produced at room temperature between 500 cm −1 and 4500 cm −1 to characterize functional groups and the bond formation of the samples.Te gas analyzer was utilized to assess the composition of the evolved gas.

Structural Characteristics and Optical Response of
Catalysts.Te crystalline solid structure of the prepared catalysts was analyzed using the most prominent difraction peaks in the XRD patterns.Peak variations have been used to determine and explain multiple phases and specifcations of the produced ferrite catalysts [27] [28].
A prominent peak for (311) phase was selected to calculate the grain size of the catalyst nanoparticles [29].Te prepared samples showed the cubical spinel phase structure and Fd-3m space group.Debye-Scherrer's equation was considered to estimate the crystallite size as where λ is wavelength of X-ray (1.5406 Å), θ is the difraction angle, β represents the FWHM of the XRD peak, and 0.9 is the Scherrer's constant [30] 3(a).Maximum absorption was observed between the wavelength range of 570-680 nm, indicating a violet shift in the absorption spectrum [31].Te band gap of these catalyst nanoparticles was calculated using Tauc's relation.
In equation ( 5), constant "A" depends on the probability of the transition, α is the absorption coefcient, "E g " is band gap, "hv" is the energy of the incident photon.We calculated the [band gap] of the synthesized nanoparticles as a function of the curve between the optical band gap energy at X-axis and the curve between (αhv) 2 and (αhv) 1/2 at y-axis using an extrapolation of the curve between the optical band gap energy at x-axis.All these graphs are shown in Figure 3(b).Nanoparticles of NiZnFe 2 O 4 , NiMgFe 2 O 4 , and MgZnFe 2 O 4 have a band gap of 2.1 eV, 2.5 eV, and 1.9 eV, respectively [32].
Figure 4 shows the morphology of NiZnFe 2 O 4, NiMg-Fe 2 O 4, and MgZnFe 2 O 4 catalysts.Te SEM images revealed the rough morphology of all catalysts having irregular particle shapes and sizes.Some degree of agglomeration was also observed among the catalyst particles.MgZnFe 2 O 4 showed fne particle size, followed by NiMgFe 2 O 4 and NiZnFe 2 O 4 catalysts [33].Magnetic interaction could be a possible reason for agglomeration among the catalyst particles.NiZnFe 2 O 4 particles were relatively more dispersed and larger in size with a reduced degree of agglomeration.Te high aggregation of nanoparticles may reduce the surface area and thereby density of active sites to perform the catalytic reaction [34].
Te active sites also reduce with an increase in the particle size.NiZnFe 2 O 4 catalyst showed the least aggregation of particles but a large particle size.At the same time, the particle shape and boundaries of this catalyst were more defned than the other catalyst.Tese characteristics could signifcantly boost the number of active sites and hereinafter the catalytic activity of the catalysts.Terefore, NiZnFe 2 O 4 was regarded as a good candidate to demonstrate good catalytic activity to pyrolyze HDPE [35].Te mean particle size of NiZnFe 2 O 4 , NiMgFe 2 O 4 , and MgZnFe 2 O 4 catalysts was measured to be 64 nm, 51 nm, and 48 nm, respectively [36].
FTIR spectrum represented diferent transmittance bands in catalysts because of the vibration of functional groups and the vibration of ions at the lattice sites.Due to the vibrations of functional groups, the bands usually appear in 500-4500 cm −1 range.Te FTIR spectra of NiZnFe 2 O 4, NiMgFe 2 O 4 , and MgZnFe 2 O 4 catalysts are represented in Figure 5. Te vibrating modes at ∼3452, 2095, 1658, 1348, 1104, 853, and 654 cm −1 demonstrated the functional groups of catalyst samples and the pure phasic spinel formation [37].Te bands at 3452 and 1658 cm −1 represent O-H stretching and bending vibrations of water molecules.Te bands between 1513 and 1663 cm −1 range are caused by symmetric and asymmetric C-O group vibrations [38].Te band around 1348 cm −1 represents the existence of moisture.Te stretching of C-O bonds is comparable to the vibrating band at 1104 cm −1 .Te bands below 1000 cm −1 are attributed to octahedral M-O and tetrahedral stretching vibration modes [39].

Role of Catalyst Properties in Pyrolysis.
Te structural and morphological traits of a catalyst play a crucial role in infuencing the activation energy of the reactions involved in pyrolysis and nucleation of CNTs.Te studied magnetic catalysts would increase the reaction rate and make the pyrolysis process more efcient by giving an alternate reaction pathway with lower activation energy.Te size and shape of catalyst particles can infuence the accessibility of reactant molecules to active sites.Smaller particles may have more surface area and exposed active areas, resulting in lower activation energy.Furthermore, due to diferences in electrical and geometric attributes, various surface facets or crystal structures of catalysts can have diferent catalytic activities.Te tested catalysts showed the same electronic and geometric properties but slightly diferent particle sizes [36].Although NiZnFe 2 O 4 catalyst had the least particle aggregation, the largest particle size, shape, and boundaries of the particles were more defned than the other catalysts.Tese properties could signifcantly increase the number of active sites and, as a result, the catalytic activity of the catalysts.NiZnFe 2 O 4 was recognized as a promising candidate for pyrolyzing HDPE with a strong catalytic activity [35].Te active sites on the magnetic catalyst surface aided in the adsorption and activation of reactant molecules, allowing the reaction to proceed more quickly.As a result, the reactions involved had lower activation energy.
Te precise process of CNT nucleation during catalytic pyrolysis is unknown.It is widely thought, however, that the process involves the synthesis of atomic carbon, its absorption into the catalyst surface, and subsequently, difusion and precipitation onto nanotube structures [17].Reactant adsorption can stabilize the transition state, lowering the energy required to achieve the activated complex and the activation energy.Since all the tested catalysts were redox active, they transfer electrons between the catalyst surface and reactant molecules, facilitating the reaction and reducing the activation energy for the formation of   6 Scientifca nanotubes.Since magnetic catalysts were composite materials, the iron oxide works in conjunction with nickel (Ni), magnesium (Mg), and zinc (Zn), leading to synergistic effects.Te combined action of diferent catalytic materials can further decrease the activation energy and improve the overall catalytic performance.

Characteristics of the Solid Residue.
Te surface morphology of CNTs is displayed in Figure 6.It is observed that CNTs produced using NiZnFe 2 O 4 as a catalyst revealed threadlike structures with nonuniform diameters and lengths.Most CNTs exhibited clear boundaries and shapes, while some joined to form clustered structures, consistent with earlier documented studies [8,40].Such structure formations are assigned to van der Waals forces, entropydriven agglomeration, and electrostatic interactions between CNTs. Figure 6(a) shows relatively longer CNTs, while Figure 6(b) displays the formation of CNTs with complex structures in the presence of NiMgFe 2 O 4 catalyst [41].Some particles of the ferrites remained rounded, identifying the presence of unutilized catalyst even after the pyrolysis process was complete.Te presence of catalyst particles in CNTs can be attributed to strong chemical bonding between the catalyst and CNTs, which reduces the difusion of the catalyst into the feedstock [42].It was also witnessed that the CNTs produced in the presence of NiMgFe 2 O 4 displayed large diameters as compared to other catalysts.Figure 6(c [43].Tese arguments agree with the diameter of the produced CNTs in the presence of diferent catalysts with diferent particle sizes [44].
Te functional groups in the compounds were identifed based on their absorption frequencies.According to Figure 7, bands were observed at 1089 cm −1 corresponding to carboxyl bond oscillations and at 1453 cm −1 , indicating aromatic ring bonds [45].FTIR analysis showed the formation of the aromatic benzene group at 1182 cm −1 and a signifcant carbon absorption bond peak observed for �C-H in the range of 2105−2095 cm −1 [46].Other characteristic peaks of CNTs include CH at 1425-1434 cm −1 and C-N at 1136 cm −1 due to stretching vibration and 1040 cm −1 representing the functional groups of CoC of HA.Te vibration of C�C can be seen in the range of 748-768 cm −1 [47].Furthermore, C�O bonding vibration in CNTs is recorded between the range of 1575 and 1595 cm −1 .Te peak detected at around 756 cm −1 is attributed to tetrahedral site Zn-O stretching, while the band detected at 689 cm −1 is attributed to Fe-O vibration at the octahedral sites [48].Te results from this study demonstrated the presence of functional groups as well as spectrum vibrations of the carboxyl group and amide bonds in the product.
Te TGA is depicted in Figure 8, indicating the thermal behavior of the residue of catalytic pyrolysis of plastic.Te weight of the samples is plotted at diferent temperatures to demonstrate thermal shifts in the solid material on the TGA curve.Since the pyrolysis was conducted at 450 °C, the residual mass may contain char, unspent catalyst, and other contaminants present in the feedstock composition [49].TGA analysis demonstrated small quantities of amorphous carbon in the product obtained with NiZnFe 2 O 4 catalyst.On the other hand, NiMgFe 2 O 4 and MgZnFe 2 O 4 showed slightly higher amorphous content in the solid product [50].Te TGA analysis showed 7.8%, 8.6%, and 12.2% amorphous contents in the carbon product obtained with NiZnFe 2 O 4 , NiMgFe 2 O 4 , and MgZnFe 2 O 4 , respectively.Tese fndings suggest that the amorphous content can be reduced signifcantly by deliberately choosing the composite magnetic catalyst.One reason for having high purity in the solid product obtained with NiZnFe 2 O 4 is the relatively defned particle shape and low agglomeration of the particles compared to the other catalysts.Tese results are consistent with the fndings of SEM analysis.Te amorphous content may include the unspent catalyst, amorphous carbon, and other impurities in the form of chemicals used during the fabrication of the plastic.Some char may also be present in the solid residue, which may convert to other forms during heating above 450 °C.
Te thermographs indicate that the onset temperature of product degradation is 380 °C, with a peak temperature of 608 °C.Table 2 shows the initial and oxidation temperatures of all products.A similar behavior was observed in TGA curves in the case of all catalysts with slightly difering thermal decomposition temperatures.Te graph shows that the samples exhibit good stability at temperatures below 350 °C.No substantial weight loss was detected when the temperature was less than 380 °C, as shown in Figure 8.At a temperature of ∼350 °C, there was no sign of oxidation in any of the carbon samples [51].A two-step weight loss mechanism can be seen in the composites.Initially, degradation occurs at a temperature between 382 and 410 °C.Te second degradation occurs between 620 and 632 °C; thereafter, no change in weight loss is observed over temperature.Tis indicates that the temperature selected for pyrolysis of HDPE was appropriate for achieving maximum efciency from the process [52].
3.4.Gas Analysis.Te pyrolytic gas composition difers from natural gas, as it contains alkenes, dienes, alkanes, and alkynes.Removing unsaturated hydrocarbons is essential for properly functioning gas turbines, gas engines, and fuel cells.Te degradation of plastic waste produces noncondensable low-weight hydrocarbons (C 1 -C 5 ).Te gas formation depends on the process temperature and the type of catalyst used.Te oxygenated compounds in the waste show that the obtained gases are primarily hydrocarbons ranging from C 1 -C 5 , H 2 , CO, to CO 2. H 2 is widely used as a feedstock in producing plastics, steel, pharmaceuticals, and chemicals [53].Using H 2 as green energy for vehicles will help reducing environmental pollution and climate change.Waste plastic can be used to produce hydrogen.Acomb et al. [54] used a fxed-bed reactor to obtain hydrogen gas from plastic pyrolysis.Figures 9(a Te NiMgFe 2 O 4 catalyst produced more hydrogen compared to other catalysts.Tis catalyst had a smaller particle size and relatively more agglomeration among the particles.Te gas was composed of H 2 , CO, CH 4 , C 2 H 4 , and CO 2 [55].Te hydrogen concentration of 84 vol%, 87 vol%, and 85 vol% was recorded with NiZnFe 2 O 4 , NiMgFe 2 O 4 , and MgZnFe 2 O 4 catalysts, respectively.During the pyrolysis process, the maximal hydrogen evolved from HDPE was 127.9 mmol/g, 137.1 mmol/g, and 131.5 mmol/g, respectively [56].Te efciency of H 2 production was calculated by dividing the total mass of hydrogen present in all gas products by the theoretical mass of hydrogen in plastic [57].Table 3 illustrates the gas composition (vol%) produced from microwave pyrolysis of HDPE with feedstock to catalyst ratio of 4 : 1 at 450 °C.Te produced gas mainly composed of a high value of H 2 , a small amount of CH 4 , CO, CO 2, C 2 H 4 , and other impurities.

FTIR Analysis of the Liquid Product.
FTIR analysis was used to analyze the functional groups of HDPE pyrolytic oil.Te oils were examined in the range of 500-4500 cm −1 .Te FTIR spectra of liquid oil produced from plastic waste are shown in Figure 10.Te chemical compounds in the oil product are provided in Table 4. Te spectrum indicates average hydrocarbon vibrations in the oil.No signifcant diference was observed in FTIR spectra of oils produced with NiZnFe 2 O 4 , NiMgFe 2 O 4 , and MgZnFe 2 O 4 catalysts.Based on the observed absorption bands, the FTIR spectrum can be classifed into 3100−2800 cm −1 range, 1700−1300 cm −1 range, and 1000−600 cm −1 range [58].Alkane groups were observed between 2840 and 2640 cm −1 , indicating the functional groups of alkenes CH 3 stretching bands [59].Te highest absorption peak intensity was observed at 1687 cm −1 , corresponding to the C�O stretching band of conjugated aldehydes.Te aromatic amine's C-N stretching bending vibration was demonstrated by an intense peak at a frequency range of 1342−1260 cm −1 [60].Te band at 1090 cm −1    10 Scientifca feedstock needs to be selected [62].Te used magnetic nanoparticles possessed both microwave absorption and catalytic characteristics [63].A signifcant amount of hydrogen gas and carbon residue was obtained by using magnetic catalysts.Tese catalysts improved the capability of microwave energy transfer into heat energy.Tey enabled the pyrolysis temperature to rise more quickly, increasing the steam-reforming pyrolysis that produced H 2 and CNTs.During pyrolysis, the organic materials rapidly crack at 450 °C to produce mainly liquid oil [64].Te catalyst increases the speed of the reaction, converting waste plastic into useful products.Te yields of end products are afected by the sample's residence time.Prolonged residence time increases the decomposition of the primary products, which causes the creation of thermally stable products, such as lightweight hydrocarbons or noncondensable gas [66].Te product distribution was split into 3 steps during pyrolysis.Te yield of gas produced by the process increased from 80% to 90% in the frst phase, solid residue increased from 15% to 22%, and just a small quantity of oil was found.Te gas production reduced from 80% to 75% in the second step, but char residue increased from 35% to 40% and oil slightly increased.In comparison to prior stages of product yield evaluation, the oil and gas yields approached 13% and 14.5%, respectively, in the third step, while CNTs were boosted to 76.5%.Table 5 depicts how various pyrolytic factors infuenced the yield of liquid oil, gas, and solid.
A comparison of this study with the published literature is provided in Table 5. Tis comparison only included the studies conducted on catalytic pyrolysis of plastics at 450 °C.Tis temperature is taken as an optimum condition in most of the published literature [67][68][69].Rahman et al. [67] utilized Zeolite catalyst to produce 80.01% gas yield with a minimum solid yield of 3.2%.Our study was focused on the high yield of structured carbon residue.Babatabar et al. [69] used Ni-Cu/AC-Cao catalyst to produce 49.62% gas and 44%

Conclusions
We have ofered a straightforward one-step microwaveinitiated catalytic approach for rapidly transforming pulverized HDPE plastic waste into combustible gases and valuable carbon-based substances.NiZnFe 2 O 4 catalyst had the least particle aggregation and large particle size.However, the shape and boundaries of NiZnFe

Figure 10 :
Figure 10: Functional groups of compounds observed in FTIR of liquid oil.

Figure 11 :
Figure 11: Time on stream evolution of products obtained with (a) NiZnFe 2 O 4 , (b) NiMgFe 2 O 4 , (c) MgZnFe 2 O 4, and (d) composition of product at the end of the process.

Table 1 :
XRD analysis data of ferrite catalysts based on Figure3.

Table 2 :
Summary of TGA analysis of CNTs.

Table 3 :
Composition of gas evolved during the pyrolysis of HDPE using diferent magnetic catalysts.
Tese catalysts could rapidly raise the pyrolysis temperature and increase CNTs and H 2 production.Compared with NiMgFe 2 O 4 , the solid yield and hydrogen gas with the NiZnFe 2 O 4 and MgZnFe 2 O 4 catalysts increased, while the liquid yield decreased to 9.1 wt% with NiMgFe 2 O 4 .Te concentration of H 2 produced by the pyrolysis process increased to 84% when NiMgFe 2 O 4 served as the catalyst.Figures 11(a)-11(c) show the product yield, distribution, and composition over time.

Table 4 :
Summary of FTIR analysis of oil product.

Table 5 :
A comparison of catalyst efect on the product yield from pyrolysis of plastics at 450 °C.Uthpalani et al. [11]used SiO 2 /Al 2 O 3 catalyst to produce a high oil yield (74.05%) and a very low gas yield (5.08%).In our case, we produced a high yield of structured carbon residue (76.5%) using NiMgFe 2 O 4 catalyst, a maximum oil yield of 11% using NiZnFe 2 O 4 catalyst and a gas yield of 13% using MgZnFe 2 O 4 catalyst.Tese three magnetic catalysts showed slight diferences in oil, gas, and solid residue production.
2 O 4 catalyst particles were more defned than the other catalysts.Tis catalyst was recognized as a promising candidate for pyrolyzing into structure carbon.TGA analysis of the solid carbon residue showed 7.8%, 8.6%, and 12.2% amorphous contents in the carbon product produced using NiZnFe 2 O 4 , NiMgFe 2 O 4 , and MgZnFe 2 O 4 catalysts, respectively.Tese fndings suggest that the amorphous content can be reduced signifcantly by deliberately choosing the composite magnetic catalyst.Almost 87% of the H 2 from HDPE was extracted in about 2-8 minutes.Te H 2 production was 137.1 mmol/g, and the H 2 concentration in the produced gases reached 87.5 vol%.A high yield of structured carbon residue (76.5%) was produced using NiMgFe 2 O 4 catalyst.Te MgZnFe 2 O 4 yielded the highest gas amount of 13%.