Role of MgO in Al2O3‐supported Fe catalysts for hydrogen and carbon nanotubes formation during catalytic methane decomposition

Abstract

Catalytic methane decomposition is a promising technology for reducing the reliance on fossil fuels and mitigating the effects of climate change by producing clean hydrogen and value‐added carbon without the emission of greenhouse gases. The aim of the study was to investigate the use of Al₂O₃‐modified MgO doped iron‐based catalysts for the catalytic decomposition of methane. The catalysts were synthesized using the impregnation method and characterized using various analysis techniques, including Brunauer, Emmett, and Teller, temperature programmed reduction, temperature programmed oxidation, X‐ray diffraction, thermal gravimetric analysis, Raman, scanning electron microscopy, and transmission electron microscopy. The activity of the synthesized catalysts was tested in a packed‐bed reactor with a gas flow rate of 20 mL/min at a temperature of 800°C. The investigation focuses on the influence of incorporating magnesium into alumina catalysts with MgO concentration ranging from (20%–70%), where higher magnesium levels improve catalytic activity by creating more active sites, positively impacting methane decomposition. Enhanced catalyst reducibility and increased particle dispersion lead to improved catalytic properties despite the reduced surface area. The FA70M and FA63M catalysts exhibited almost the same catalytic characteristics and the highest stability and methane conversion among the catalysts investigated, reaching 87% and 85% at 800°C for 120 min. Moreover, both catalysts showed hydrogen yields of 86% and 85%, respectively. The introduction of MgO further increased the total carbon yield from 103% with FA and 39% for FM to 114% and 120% for the respective catalysts (FA70M and FA63M). During the methane decomposition reaction, carbon nanotubes of varying diameters were produced. Higher iron loading resulted in a positive trend.

INTRODUCTION

Global initiatives on sustainable and green energy resources and large methane reserves have stimulated increased research on methane-to-hydrogen conversion. Methane is a potent greenhouse gas that has major effects on the environment, and reducing methane emissions can decrease the rate of climate change. The catalytic methane decomposition (CDM) process is a new method for producing clean hydrogen from abundant natural gas resources. Unlike traditional methods such as steam reforming, CMD does not release CO₂ into the atmosphere, offering a promising technology for clean energy production. This can help diversify our energy sources and potentially reduce our reliance on fossil fuels. CDM minimizes waste generation like carbon byproducts and creates new opportunities for material development. In other words, it provides a near-zero carbon footprint pathway to hydrogen generation, contributing to reducing greenhouse gas emissions and mitigating climate change.¹ Hydrogen is an environmentally benign fuel with high heating value and CO_x-free emission, whereas carbon has many industrial applications such as metal extraction, water purification, and pharmaceuticals.^2,3

Methane molecule (CH₄) consists of four identical sp³-hybridized C–H bonds and thus is a highly stable molecule at room temperature and pressure, having a bond energy of 435 kJ/mol. The non catalytic thermal decomposition of methane is highly endothermic (ΔH_298K = 74.8 kJ/mol) and occurs at temperatures of or above 1200°C, where energy is required for C–H bond breaking^4–6: 1$<math altimg="urn:x-wiley:20500505:media:ese31867:ese31867-math-0001" display="block" wiley:location="equation/ese31867-math-0001.png" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mrow><msub><mtext>CH</mtext><mrow><mn>4</mn><mo>(</mo><mi mathvariant="normal">g</mi><mo>)</mo></mrow></msub><mrow><mo>\unicode{x02192}</mo><msub><mi mathvariant="normal">C</mi><mrow><mo>(</mo><mi mathvariant="normal">s</mi><mo>)</mo></mrow></msub></mrow><mo>\unicode{x0002B}</mo><msub><mrow><mn>2</mn><mi mathvariant="normal">H</mi></mrow><mrow><mn>2</mn><mrow><mo>(</mo><mi mathvariant="normal">g</mi><mo>)</mo></mrow></mrow></msub><msub><mrow><mo>\unicode{x02206}</mo><mi mathvariant="normal">H</mi></mrow><mrow><msup><mn>298</mn><mn>0</mn></msup><mi mathvariant="normal">k</mi></mrow></msub><mo>\unicode{x0003D}</mo><mn>74.8</mn><mo>\unicode{x0200A}</mo><mtext>kJ</mtext><mo>/</mo><mtext>mol</mtext><mo>.</mo></mrow></mrow></math>$

As a result, using heterogeneous catalysts is essential for lowering the activation energy. Common catalysts for methane decomposition include Ni, Fe, and Co.⁷ Iron-based catalysts are a viable choice for the catalytic decomposition of methane (CDM) due to their efficient performance and eco-friendly qualities. Additionally, the partially filled 3d orbitals of iron assist in hydrocarbon dissociation by accepting electrons.^1,8 The iron-based catalyst for the CH₄ dissociation reaction is found to support selective. Silica supported Iron is completely inactive for this reaction whereas Fe dispersed over silica-alumina is quite active.^9,10 Fe/Al₂O₃ catalysts show great potential for clean methane decomposition (CDM) due to their affordability, acceptable activity, and potential for further optimization. The CH₄ decomposition reaction over Fe-doped alumina has drawn much attention from the catalytic community because of its high activity.⁹ Ibrahim et al.¹¹ reported a study of iron catalysts with different Fe loadings supported on alumina and they found that increasing the amount of Fe in Fe/Al₂O₃ catalysts led to a higher H₂ yield in CDM reactions at 700°C. However, when the Fe content exceeded 42%, the H₂ yield decreased due to a reduction in the catalyst's surface area. The authors attribute the high catalytic activity and carbon formation to adequate metal-support interaction. The addition of MgO to Al₂O₃ to form composite support for metal catalysts is an effective approach due to its unique properties of low acidity, resistance to high temperatures, and good interaction with metals.¹² Also, the resulting MgAl₂O₄ spinel is renowned for its distinctive attributes, including a high melting point, chemical resistivity, minimal thermal expansion, and impressive mechanical durability.¹³ Moreover, the mesoporous nanocrystalline structure and extensive surface area of magnesium aluminate composite make it a promising candidate as a support material for a wide range of catalytic reactions.¹⁴ Gac et al.¹⁵ observed that adding MgO to the Ni/Al₂O₃ catalyst increased the active surface area. The addition of MgO to the Ni/Al₂O₃ caused smaller Ni crystallites to form, resulting in more effective adsorption sites due to strong metal-support interaction and changes in the catalyst's microstructure. Moreover, the increase in MgO content boosted the rate of methane decomposition during the initial stages of the reaction. The impact of doping MgO over ZrO₂ over iron-based catalyst was investigated by Bayazed et al.¹⁶; The result of catalytic activity showed that the addition of MgO enhanced the methane decomposition. This study explores the performance of a Fe-based catalyst in the catalytic decomposition of methane for hydrogen and nanostructured carbon production. The study investigates the influence of MgO content (ranging from 20% to 70%) on alumina-supported iron catalysts for methane decomposition. Prior research explored MgO-doped catalysts, but this study systematically investigates a wider range of MgO concentrations (20%–70%) to identify the optimal level for maximizing hydrogen yield and carbon production. This work goes beyond activity by analyzing how MgO content affects catalyst reducibility, which is crucial for methane decomposition. At 800°C, the effect of different MgO loadings on MgO-modified Al₂O₃ support was used for Fe-based catalysts. This study aims to determine the ideal amount of MgO loading on Fe/Al₂O₃ that generates the greatest hydrogen output and nanostructured carbon. The objective is to examine the influence of different levels of MgO on the properties of the catalyst, such as surface area and reducibility, and their association with catalytic performance. The approach taken involves: creating Fe-based catalysts with varying MgO content supported on alumina; employing various techniques (Brunauer, Emmett, and Teller [BET], temperature programmed reduction [TPR], temperature programmed oxidation [TPO], X-ray diffraction [XRD], thermal gravimetric analysis [TGA], Raman, scanning electron microscopy [SEM], transmission electron microscopy [TEM]) to analyze the catalysts’ structural and textural properties; performing methane decomposition reactions at 800°C to assess the catalysts’ performance in terms of methane conversion, hydrogen yield, and carbon production, and Correlating the characterization results with the catalytic activity to understand the impact of MgO content on the overall process.

EXPERMENTAL

Materials

The ferric nitrate nonahydrate (Fe (NO₃)₃·9H₂O, 99%) served as the active metal for the supported catalysts, while the supports of alumina, magnesia, and magnesia-modified alumina, were obtained as a gift from SASOL.

Catalyst preparation

The wet impregnation technique was employed to synthesize the catalysts. Stoichiometric amounts of the iron precursor and the oxide supports were dissolved in 20 mL of ultrapure water. This step led to the formation of a solution containing the precursor and support materials. The solution underwent magnetic stirring for 2 h at 80°C to ensure thorough mixing and even dispersion of the precursor on the support. The resulting mixture was left to dry overnight at 120°C, eliminating any residual water or solvents. The dried mixture was then subjected to calcination at 600°C for 3 h. The high-temperature treatment transformed the precursor-support combination into a stable solid form, promoting the creation of active catalyst sites. Finally, the catalyst material was crushed into a powder form, which would be employed in chemical reactions. The catalysts prepared were labeled as FA, FM, and FAxM (Table 1), where F stands for iron, A for alumina, and M for magnesia, and x denotes the weight percentage of magnesium oxide in the alumina support (20%, 30%, 63%, or 70%). Additional samples with 10% and 30% Fe (10 FA70M and 30 FA70) were prepared to study the effect of Fe loading. Figure 1 depicts the catalyst preparation scheme.

Table 1 Textural properties and description of the fresh catalysts.

Catalyst	Catalyst abbreviation	SA (m2/g)	PV (cm3/g)	PS (nm)
20Fe/Al2O3	FA	157	0.48	10.97
20Fe/MgO	FM	48	0.25	20.39
20Fe/Al2O3 + 20 mg	FA20M	145	0.21	5.04
20Fe/Al2O3 + 30 mg	FA30M	113	0.15	5.28
20Fe/Al2O3 + 63 mg	FA63M	89	0.12	5.59
20Fe/Al2O3 + 70 mg	FA70M	64	0.10	6.11

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Catalytic activity test

The CMD reaction experiments were conducted under atmospheric pressure in a fixed-bed stainless steel tubular reactor with a 9.1 mm inner diameter and a length of 30 cm.¹⁷ In a typical activity test, quartz wool is placed in the tubular reactor, and on top of this, a fixed amount (0.15 g) of catalyst is deposited. A K-type thermocouple, positioned at the center of the catalyst bed, monitors the reactor's temperature. Before the CMD reaction, the catalysts are subjected to a reduction in a hydrogen flow (40 mL/min) at 800°C for 60 min. Subsequently, the reactor temperature is adjusted to the desired value (e.g., 800°C) under a nitrogen flow (15 mL/min) to eliminate any remaining H₂. Once the desired reaction temperature is achieved, methane and nitrogen, with a CH₄-to-N₂ ratio of 1.5, are introduced into the reactor at a total flow rate of 20 mL/min, resulting in a gas hourly space velocity of 8000 mL/h g_cat. The composition of the reactor outlet is analyzed using online gas chromatography equipped with a thermal conductivity detector (TCD). CH₄ conversion, hydrogen, and carbon yield calculations are as follows: 2$<math altimg="urn:x-wiley:20500505:media:ese31867:ese31867-math-0002" display="block" wiley:location="equation/ese31867-math-0002.png" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mrow><msub><mi mathvariant="normal">X</mi><mrow><mtext>CH</mtext><mn>4</mn></mrow></msub><mo>\unicode{x02007}</mo><mo>(</mo><mo>$ 3$<math altimg="urn:x-wiley:20500505:media:ese31867:ese31867-math-0003" display="block" wiley:location="equation/ese31867-math-0003.png" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mrow><msub><mi mathvariant="normal">Y</mi><mrow><mi mathvariant="normal">H</mi><mi mathvariant="normal">\unicode{x02082}</mi></mrow></msub><mo stretchy="false">(</mo><mo>$ 4$<math altimg="urn:x-wiley:20500505:media:ese31867:ese31867-math-0004" display="block" wiley:location="equation/ese31867-math-0004.png" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mrow><mtext>Carbon yield</mtext><mo stretchy="false">(</mo><mo>$where [CH₄, in] represents the initial methane in the feed, [CH₄, out] and [H₂, out] are the methane and hydrogen output, respectively. W_P is the weight of the products after the reaction, and W_cat is the weight of the fresh catalyst.

Catalyst characterization

The study involved analyzing Fe-based catalysts through various techniques. X-ray diffraction patterns were obtained using a Miniflex Rigaku diffractometer, utilizing Cu-K X-ray radiation with an operating voltage of 40 kV and a current of 40 mA. Nitrogen sorption isotherms were determined at −196°C using a Micromeritics Tristar II 3020 surface area and porosity analyzer after outgassing the samples at 250°C for 3 h to eliminate any adsorbed gases or vapors. The pore size distributions of the samples were assessed based on the adsorption profiles of isotherms, employing the Barrett–Joyner–Halenda (BJH) model. Micromeritics AutoChem II 2920 was used to perform temperature-programmed reduction and oxidation on both Fresh and used catalysts. Hydrogen temperature-programmed reduction (H₂-TPR) was conducted on the freshly synthesized catalysts. This analysis covered a temperature range of 50–800°C and utilized a mixture of 10% H₂/Ar with a flow rate of 40 mL/min in the TPR analysis. TPO measured carbon deposits on the spent catalysts. It was performed using a 10% O₂/He mixture flowing at 40 ml/min. The TPO analysis was conducted over a temperature range of 50°C to 900°C. The formation of coke and the amount of carbon deposits on the catalyst surfaces were quantified using a thermogravimetric analyzer (Shimadzu-TGA). In the TGA experiment, about 10–15 mg of the used catalyst was heated in an inert atmosphere from room temperature to 1000°C at a rate of 20°C/min. The change in weight was observed to measure the carbon content as it burned off during the heating process. To assess the extent of graphitization of the carbon deposits on the used catalysts, laser Raman spectroscopy with an NMR-4500 instrument was employed. This measurement utilized a 5x magnification objective lens and a beam excitation wavelength of 532 nm, with spectra recorded within the 1350–1600 cm⁻¹ range. Morphological changes in calcined samples were investigated through SEM utilizing a JEOL JSM-7100F. An in-depth examination of nanostructured carbon deposits on spent catalysts was conducted using a JEOL JEM-1400 TEM operating at an accelerating voltage of 120 kV.

RESULTS AND DISCUSSION

Fresh catalyst characterization

N₂ adsorption/desorption measurements are crucial for characterizing the textural properties of fresh catalysts in methane decomposition, as shown in Figure 2.¹⁶ The isotherms conform to Type IV, indicating mesoporous structures with diverse pore sizes and shapes. The FM and FA catalysts exhibit distinct hysteresis loop types (H3-type and H1-type, respectively), loop respectively. It indicates the presence of cylindrical mesopores of average diameter 10.97 over FA catalyst and the presence of uniformly distributed slit-shaped pores with average diameters of 20.39 nm over FM catalyst. Additionally, for the FAxM catalysts, the hysteresis loop pattern shifts to H2, signaling a transition in the pore structure from cylindrical -shaped to Ink bottle-shaped mesopores.¹⁸ The incorporation of MgO into the mesoporous Al₂O₃ support led to a notable reduction in surface area, pore volume, and average pore size due to nanoparticle nucleation within the pores.¹⁹ The Mg-doped samples showed surface areas and pore volumes ranging from 145.45 to 64.63 m²/g and 0.21 to 0.10 cm³/g, respectively. FA20M had the highest surface area and pore volume, while FA70M had the lowest as indicated in Table 1. The FA catalyst had a surface area of 157 m²/g, while the FM sample had the lowest surface area at 48 m²/g but the highest pore size at 20.39 nm. As the MgO content increased in the FAxM samples, pore size also increased, indicating that adding MgO to the support material affects the catalyst's texture, which can impact its catalytic performance.

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The crystalline structure and phase identification of the fresh Fe-based catalysts were studied by using the powder XRD technique. For different oxide supports (i.e., MgO, Al₂O₃), the XRD patterns of Fe-based catalysts are presented in Figure 3. FA catalyst as shown in Figure 3A has individual Rhombohedral Fe₂O₃ phase at 2θ = 24.2°, 33.2°, 35.6°, 40.9°, 49.5°, 54.1°, 62.4°, and 64.1°; (JCPDS 01-084-0306) and cubic Al₂O₃ phase at 2θ = 39.5, 45.8°, and 66.8°; (JCPDS 00-001-1303). FM catalyst has cubic Magnetite Fe₃O₄ phase at 2θ = 30.1°, 35.4°, 37.1°, 43.1°, 56.9°, 62.5°, and 73.9°; (JCPDS 00-019-0629), cubic MgO phase at 2θ = 36.8°, 42.9°, 62.3°, 74.6, and 78.6; (JCPDS 00-004-0829). The incorporation of magnesia alongside alumina results in a notable reduction in the intensity of all peaks. In contrast to the FA and FM catalysts, the FAxM catalysts consistently as shown in Figure 3 display lower intensity, indicating a more efficient dispersion of iron throughout the catalyst. In the case of FA20M and FA30M, the presence of the Fe₂O₃ phase is in contrast to the Fe₃O₄ phase. Furthermore, for FA30M, the appearance of cubic magnesium ferrite Fe₂MgO₄ phases is observed at 2θ = 30.1°, 35.4°, 43.1°, and 56.9°; (JCPDS 00-036-0398). In FA20M, peaks at 2θ = 45.1°, 58.9°, and 65.5°, are associated with MgAl₂O₄ species corresponding to JCPDS reference number 01-084-0377. Conversely, for FA63M and FA70M, diffraction peaks are identified at 2θ = 36.8°, 42.9°, 62.3°, and 78.6°, indicating the presence of the cubic MgO phase. Interestingly, only Fe₂O₃ phase is observed at 2θ = 35.6°and 62.4° whereas the Fe₃O₄ and the Fe₂MgO₄ and MgAl₂O₄ species are not detected.

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H₂-TPR is a significant analytical tool for assessing both the catalyst's metal-support interactions and its reducibility. It also serves to confirm the suitable reduction temperatures for the catalyst.¹⁶ Figure 4 displays the H₂-TPR curves for the freshly prepared catalysts. The initial peak in the 300°C to 500°C temperature range is attributed to the phase transformation of Fe₂O₃ (hematite) to Fe₃O₄ (magnetite). Simultaneously, the broader and more intense reduction peaks observed between 500°C and 700°C are assigned to the reduction of the magnetite (Fe₃O₄) phase to the FeO (Wüstite) phase. The reduction peak profile from 700°C to 900°C is attributed to the reduction of FeO into metallic Fe° species.²⁰ XRD study over the current catalyst system shows the absence of FeO phase whereas H₂-TPR study of magnesia incorporated catalyst system shows the absence of reduction peak for Fe₃O₄ to FeO. It indicates that the H₂-TPR peak >700°C should be attributed to the reduction of Fe₃O₄ to metallic Fe directly. Meanwhile, Reduction peaks with maxima at 725°C, 737°C, 773°C, and 749°C for FAxM (x = 20, 30, 63, 70), respectively, are likely associated with the reduction of remaining Fe₃O₄ species to zero-valent metallic Fe species.^21,22 Increasing MgO content further enhances the reducibility of the catalysts. Notably, all FAxM catalysts exhibit a shift to higher temperatures and display a broad reduction peak in the 600–900°C region, indicating a strong interaction of Fe with the support and improved dispersion on the surface. There is a noticeable rise in the Intensity in high-temperature regions (>700°C) with increasing Mg/Al ratio. Among the examined catalysts, FA63M and FA70M show the highest levels of hydrogen consumption, approximately 141 cm³/g STP and 150 cm³/g STP, respectively, while FA and FM catalysts exhibit the lowest hydrogen consumption, of about 45 cm³/g STP and 88 cm³/g STP, respectively according to Table 2. It is interesting to note that a high-temperature reduction peak (>700°C) is absent/feebly present over FM. It indicates the absence of reduction of iron oxide into metallic Iron over FM. The MgO-support inhibits the reduction of FeO to Fe or Fe₃O₄ to Fe. Metallic iron is an active site for CH₄ decomposition reaction and the absence of metallic iron over FM may affect the activity seriously. FA catalyst encompasses sharp reduction peaks in the range of 300-500°C range (attributed to the reduction of Fe₂O₃ to Fe₃O₄) and another overlapping reduction peak in the range of 500–750°C (attributed to the reduction of Fe₃O₄ → FeO → Fe).

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Table 2 Hydrogen consumption for the fresh catalysts.

Catalyst	Temperature at maximum (°C)	Quantity (cm³/g STP)a	(% reduction)b
FA	428–670	45	37.5
FM	430–529	88	73.3
FA20M	461–725	83	69.2
FA30M	493–737	108	90
FA63M	477–773	141	117.5
FA70M	476–749	150	125

Catalytic activity

Figure 5 displays the result of the catalytic decomposition of methane using 20 wt.% Fe-supported catalysts over 120 min. The reaction was conducted at atmospheric pressure and 800°C while the feed was kept at a space velocity of 8000 mL/(hg_cat). The results illustrate the impact of the composite supports on Fe catalysts. CH₄ conversion of 77% and 30% were achieved for FA and FM respectively. However, over time, the activities of both catalysts declined. The low distribution of active metallic Fe sites across the catalyst surface is the likely cause of the low activity over the Fe/MgO catalyst (FM). This implies that a considerable amount of Fe may exist in bigger aggregates, which would limit the amount of reactant molecules’ available active sites. The FA catalyst showed a CH₄ conversion reduction of 63%, while the FM catalyst experienced an 8% decrease. The primary cause of the reduction in CH₄ conversion is the carbon that has been deposited on the active Fe surfaces. The modification of the alumina support with MgO led to an enhancement in activity and stability. The FAxM catalysts outperformed both FM. The initial methane conversions after 4 min on-stream were 80.1%, 79.1%, 83.4%, and 85.4% for FA20M, FA30M, FA63M, and FA70M, respectively. The rise of concentration of metallic Fe upon increasing the content of MgO is evident in H₂-TPR. So, the rise in activity upon Mg addition is attributed to rise in the concentration of metallic Fe over the catalyst surface. After 120 min, the methane conversion rates were 77.7%, 67.5%, 84.5%, and 85.2%. Notably, FA63M and FA70M catalysts exhibit superior catalytic methane dissociation activity and stability, primarily attributed to the abundant active sites (metallic Fe) on their surfaces, as indicated by XRD and TPR data. This suggests a more effective dispersion of iron throughout the catalyst. This results in prolonged exposure of active Fe metal to the reactant gas, leading to increased H₂ production over an extended reaction time. In contrast, FA20M and FA30M catalysts experience a non-stable decline in catalytic decomposition activity. This decline is primarily linked to the consumption of iron oxide in the formation of the Fe₂MgO₄ phase. In Figure 5B, the H₂ yield of FAxM catalysts increases with higher loading of magnesia. This is due to active site concentration rise on the catalyst surface, which improved reducibility with higher magnesia loadings. The FA70M catalyst demonstrates the highest H₂ yield, reaching around 85%.¹⁴ The catalytic activity test had to be stopped after 120 min due to the rise in reactor pressure caused by substantial carbon accumulation on these catalysts. Figure 5C shows the final carbon yield achieved with different catalysts: 103 wt.% for FA, 39 wt.% for FM, 91 wt.% for FA20M, 110 wt.% for FA30M, 114 wt.% for FA63M, and 120 wt.% for FA70M. Supporting Information S1: Figure S1 illustrates the impact of reaction temperature on methane conversion and hydrogen yield using the FA70M catalyst. The FA70M catalyst works better at higher temperatures to turn methane into useful products. In just 1 h, methane conversion jumps from 38% at 600°C to a peak of 87% at 800°C. The comparison of catalytic activity of the present work and that in literature was performed in terms of CH₄ conversion at different reaction/catalytic conditions such as reaction temperature and time on stream (TOS). The present catalysts displayed comparable results as shown in Table 3.

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Table 3 Comparison of the results of this work with those reported earlier for supported catalysts.

Catalyst	Rx. temp. (°C)	TOS (h)	Conversion (%)	Ref.
CH4, Initial	CH4, Final
20Fe/Al2O3	700	3	47	65	[23]
60Fe/Al2O3	700	4	77	74	[11]
50Fe/MgO	700	7	10	45	[24]
30Fe/MgO+ZrO2	800	3	74	29	[16]
FA63M	800	2	83.4	84.5	[a]
FA70M	800	2	85.4	85.3

Spent catalyst characterization

The type and amount of deposited carbon on the spent catalyst after the stability test at 800°C were evaluated using TPO analysis and the results are represented in Figure 6A. The peaks at 500–700°C represent highly structured carbon nanoforms characterized by crystalline graphitic arrangements.¹⁶ The FAxM catalysts presented around 700°C prominent and intense oxidation peaks in the TPO patterns. This signifies the presence of a highly organized crystalline structure within the carbon material. The presence of carbon does not hinder the catalytic activity, but it can potentially block the feed gas passing through the reactor by increasing the pressure drop.^25–27 The spent catalysts were also analyzed using TGA to identify the extent of weight loss as a function of temperature (Figure 6B). The percentage of weight loss reflects the removal of volatile components and the oxidation of carbon from the catalyst's surface.²⁸ Additionally, TGA helps assess the type of carbon-containing substances present in an oxidizing environment.²⁹ The TGA curves showed minimal weight loss below 450°C, suggesting that only a small amount of amorphous carbon had been deposited on the catalysts. The significant decline in TGA curves between 500°C and 700°C resulted from the combustion of carbon nanotubes (CNTs).³⁰ Compared to other catalysts, FA70M showed the highest weight loss resulting from the carbon deposition, consistent with its superior activity and stability. Figure 6B illustrates that the FM catalyst exhibited the lowest weight loss of 39%. In contrast, the FA catalyst had a weight loss of 66%, which increased to 75%, 72%, and 80% on doping different amounts of MgO. This implies that alumina support modification with Mg led to increased carbon formation.³¹ Analyzing the spent catalysts through spectroscopy proves to be a valuable method for assessing the quality of the carbon nanomaterials that have formed on the catalyst surface.²¹ Supporting Information S1: Figure S2 exhibits the DTGA profiles for the catalysts spent at 800°C. The DTGA showed that the rate of weight loss is maximum at 600°C, where the combustion of amorphous and graphitic carbon took place. This conforms with the drop profile of the TGA. Figure 6C provides a visual representation of the typical Raman spectra obtained from these spent catalysts. All catalysts exhibit two well-defined peaks, prominently situated at around 1340 and 1580 cm⁻¹, which correspond to the D and G bands, respectively. In general, the D band is indicative of the existence of disordered carbon structures, including defects on carbon nanotube walls and the presence of amorphous carbon. On the other hand, the G band is attributed to the presence of highly organized, graphitic crystalline carbon structures.³² Hence, by examining the intensity ratio (I_D/I_G) of the D band (I_D) with the intensity (I_G) of the G band, valuable insights can be extracted. The analysis results reveal that the D band's intensity is relatively low for the FM, FA20M, FA63M, and FA70M samples, while it's notably high for the FA and FA30M samples. Specifically, the calculated I_D/I_G ratio for the FA, FM, FMG20, FA30M, FA63M, and FA70M samples stands at 1.21, 0.61, 0.78, 1.04, 0.57, and 0.55, respectively. This illustrates that, as a general trend, samples with a lower I_D/I_G ratio tend to contain carbon species that are highly crystalline, well-ordered, and predominantly graphitized. Of all the catalysts, FA70M demonstrated the lowest I_D/I_G ratio, approximately 0.55, signifying a high degree of crystallinity and graphitization in the resulting carbon nanotubes. In contrast, the FA catalyst displayed the highest I_D/I_G ratios, suggesting decreased crystallinity due to a lack of graphitized carbon content. This indicates that altering the support by incorporating 70 wt%MgO resulted in an enhanced degree of graphitization during the carbon formation process. A two-phonon process gives rise to the weaker peak known as the G' band in Raman spectroscopy of carbon nanomaterials, which is indicative of the presence of sp2-bonded carbon structures. These structures are a distinguishing characteristic of materials such as few-layer graphene, single-walled nanotubes, and multiwalled nanotubes. The G′ band provides additional information about sp2-bonded carbon and can provide more precise details regarding the sort of nanomaterial-based on its properties, even if it is occasionally associated with the creation of single-layer graphene. Figure 6C in the revised manuscript shows the G′ band around 2700 cm⁻¹.

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The SEM technique was utilized to investigate the changes in the morphology of freshly calcined FA70M catalysts and to identify both surface characteristics and carbon accumulation on the spent catalyst, as illustrated in Figure 7.¹⁶ In Figure 7A, the FMG70 catalysts display analogous rough surfaces and small grain particles, forming sizable clusters with indistinct crystal interfaces and irregular sizes. The smaller particle sizes observed in Figure 7A for the FA70M catalysts suggest that they can alleviate agglomeration and improve the dispersion of iron species on the support surface.³³ SEM analysis of spent catalysts, as depicted in Figure 7B, reveals a significant presence of carbon nanotubes covering the catalyst surface. The real length of nanotubes is difficult to measure due to their complex structure; lengths can reach several micrometers.³⁴

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The internal structure of the FA70M catalyst was investigated by TEM analysis and the images are shown in Figure 8. From Figure 8A, it is clear that the metal oxide particles were randomly inter-aggregated. The dark spots shown in the TEM images represent the Fe species which are found to be highly dispersed on the surface of the agglomerated metal oxide species. The catalyst's metal particle size distribution contributes to the generation of carbon nanotubes (CNTs) with a diameter distribution that is not uniform, spanning the range of 10 to 30 nanometers as shown in Figure 8B.²⁴ Further, a TEM study was performed to investigate spent catalysts and filamentous carbon in-depth morphological investigation. As regards the coke formation, Figure 8C shows the different typologies formed during the reaction. Mainly, irregular filaments form with different sizes, which often depend on Fe particle size. As regards the length of filaments, it is not easy to calculate; however, the length could be close to 1 micron. Furthermore, it can be seen that Fe is also encapsulated by coke. In this case, the size of Fe ranges from 28 to 72 nm. The dark spots shown in Figure 8C represent the presence of Fe metal species.^28,35

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Effect of iron loadings

Figure 9A explores how different iron loadings affect methane decomposition rates, providing crucial insights into catalyst stability and deactivation from carbon deposition. The best-obtained catalyst FA70M will be tested for different Fe loadings. At lower iron content (10% Fe), referred to here as 10FA70M, exhibited an initial CH₄ conversion of 73% and deactivated sharply to reach 7% after 2 h time-on-stream, leading to a deactivation factor of 91% and carbon yield of 44% (Table 4), possibly due to sintering during activation, limiting access to the active phase. Higher iron loadings (e.g., 30% Fe) referred to here as 30FA70M, show a milder conversion drop (85% to 78%), a deactivation factor of 9%, and a higher carbon yield of 180%, emphasizing the importance of maintaining a balanced metal-to-support ratio. The most stable performance is seen with a FA70M catalyst (D.F. = 0.2%), consistent over 2 h. Previous research by Ibrahim et al.¹¹ noted increased H₂ yield with Fe loading up to 42%, but exceeding this reduced yield due to decreased surface area. Mohamed et al.²¹ found higher Fe loading improved catalytic performance in methane conversion and carbon yields. Table 4 shows that, despite the 10FA70M catalyst having the highest surface area and pores, its lower iron content led to lower methane decomposition rates and stability compared to the FA70M and 30FA70M catalysts ^21,36

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Table 4 Surface area of fresh and used Fe loading catalysts.

Catalyst	Surface area (m²/g)	Carbon yield (%)	Deactivation factor (D.F., %)a
Fresh	Used
10FA70M	105.4	73.6	44	91.03
FA70M	64.62	59.3	120	0.22
30FA70M	33.1	61.7	180	9.07

The TPO for the different Fe-loading spent catalysts at 800°C is shown in Figure 9B. The oxidation peak temperature for the filamentous carbon broadened as the Fe loading on the catalyst increased. It indicates an enhanced deposit of carbon with a higher loading of Fe which also needs a larger range of temperature to oxidize under air. Kazemi et al.²⁵ indicated that the higher loading of nickel oxide led to an increase in the oxidation peak area, confirming a raised quantity of carbon formation in the samples. From TPO, it seems that having more iron on the catalyst's surface encourages CNT growth, while less iron leads to soft carbon.³⁷ Supporting Information S1: Figure S3 depicts TPR profiles for catalysts with varying iron (Fe) content. The peaks around 450–800°C correspond to the reduction of iron oxides to metallic iron. These peaks intensify and broaden with increasing Fe loading, indicating a greater amount of iron oxide to reduce. Interestingly, the peaks also shift to higher temperatures with higher Fe content, suggesting a stronger interaction between Fe and the catalyst support, making reduction slightly more difficult. Supporting Information S1: Figure S4 presents TGA profiles for catalysts with varying iron (Fe) content. It is apparent the highest weight loss occurs at around 600°C and the amount of the deposited carbon is in line with the respective activity of the catalysts.

CONCLUSIONS

In this study, Fe-supported catalysts were synthesized using the wet impregnation method and employed in CMD. The MgO-modified Al₂O₃ catalyst with varying MgO loadings at 800°C was investigated. The addition of magnesium reduced the BET surface area from 157 to 64 m² g⁻¹ Fe/MgO has dispersed distribution of metallic Fe active sites across the catalyst surface, so it showed the least activity. In the case of Fe supported over MgO-modified Al₂O₃, the concentration active sites “Fe” is increased with increasing proportion of MgO. Over FA63M and FA70M catalysts; CH₄ conversions are 85.5%, and 87.1%, H₂ yields are 85.1% and 86.5 and carbon yields are 114 and 120 wt.% after 120-min TOS, respectively. The analysis of deposited nanocarbon revealed the formation of highly graphitized and crystalline multiwalled carbon nanotubes. Furthermore, increasing iron loading enhanced the formation of carbon nanotubes during the reaction. Spent-FA contained the lowest degree of graphitization in their nanotube whereas FA70M had a carbon nanotube with the highest degree of graphitization. Upon higher Fe loading, the quantity of carbon deposit is increased and it also needs a larger range of temperature to oxidize under air.

AUTHOR CONTRIBUTIONS

Mohammed O. Bayazed: methodology; formal analysis; data curation; validation; software; writing—original draft. Ahmed S. Al-Fatesh: funding acquisition; project administration; data curation; conceptualization; investigation; writing—review and editing; methodology. Ahmed A. Ibrahim: methodology; conceptualization; data curation; writing—original draft. Ahmed E. Abasaeed: conceptualization; investigation; software; methodology; formal analysis. Anis H. Fakeeha: review, data curation; validation; software; methodology; formal analysis. Abdulaziz I. Alromaeh: data curation; validation; formal analysis. Frusteri Francesco: data curation; validation; software; methodology; formal analysis. Jehad K. Abu-Dahrieh: funding acquisition; writing—review and editing; methodology.

ACKNOWLEDGMENTS

The authors would like to extend their sincere appreciation to Researchers Supporting Project number (RSP2024R368), King Saud University, Riyadh, Saudi Arabia. J. A. D. acknowledges Queen's University Belfast for supporting research.

CONFLICT OF INTEREST STATEMENT

The authors declare no conflict of interest.

DATA AVAILABILITY STATEMENT

The data sets used or analyzed during the current study are available from the corresponding author on reasonable request.

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