Alumina Coated with Titanium Dioxide Supported Iron for Hydrogen Production and Carbon Nanotubes via Methane Decomposition

Abstract

Research on converting methane to hydrogen has gained more attention due to the availability of methane reserves and the global focus on sustainable and environmentally friendly energy sources. The decomposition of methane through catalysis (CDM) has excellent potential to produce clean hydrogen and valuable carbon products. However, developing catalysts that are both active and stable is a highly challenging area of research. Using titanium isopropoxide as a precursor and different loadings of TiO₂ (10 wt.%, 20 wt.%, and 30 wt.%), alumina has been coated with TiO₂ in a single-step hydrothermal synthesis procedure. These synthesized materials are examined as possible support materials for CDM; different wt.% of iron is loaded onto the synthesized support material using a co-precipitation method to enhance the methane conversion via a decomposition reaction. The result shows that the 20 wt.% Fe/20 wt.% Ti-Al (20Fe/20Ti-Al) catalyst demonstrates remarkable stability and exhibits superior performance, reaching a conversion rate of methane of 94% with hydrogen production of 84% after 4 h. The outstanding performance is primarily due to the moderate interaction between the support and the active metal, as well as the presence of the rutile phase. The 20Fe/30Ti-Al catalyst exhibited lower activity than the other catalysts, achieving a methane conversion of 85% and hydrogen production of 79% during the reaction. Raman and XRD analysis revealed that all the catalysts generated graphitic carbon, with the 20Fe/20Ti-Al catalyst specifically producing single-walled carbon nanotubes.

1. Introduction

Hydrogen stands out as one of the most promising candidates among the potential solutions being considered. It offers a viable option for addressing the environmental challenges. This is because industrial growth has resulted in a significant release of CO₂ into the air. The world’s energy supply relies heavily on fossil fuels like oil, coal, and natural gas [1]. Hydrogen gas has been regarded as a significant energy carrier and fuel for the 21st century due to its potential benefits in various industrial applications and its ability to minimize environmental impact; in addition, H₂ has already been extensively used in the chemical processing industry, electronics, and food processing [2,3,4]. The current method for large-scale hydrogen production is the widely adopted and fully developed steam methane reforming (SMR) process. This process involves the production of both carbon monoxide (CO) and carbon dioxide (CO₂) as byproducts [5]. Over the past few decades, hydrogen production has been constrained by its sources, prompting researchers to explore various alternatives. One such alternative that has garnered significant attention is the decomposition of methane into molecular hydrogen and elemental carbon. This approach holds great potential as a viable solution for hydrogen production [6].

The process of methane decomposition Equation (1) is moderately endothermic, and it takes 37.8 kJ to produce 1 mole of H₂ instead of the 63 kJ required by the SMR process [1,2].

(1)$\mathrm{C}{\mathrm{H}}_4\to \mathrm{C}+2{\mathrm{H}}_2\Delta {\mathrm{H}}_0=+75 \mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{e}$

Typically, this reaction occurs at temperatures higher than 1300 °C for non-catalytic reactions. However, by employing a suitable catalyst, it is possible to lower the operating temperature and still achieve efficient methane decomposition rates. In the catalytic methane decomposition (CMD), commonly used catalysts include transition metals such as nickel (Ni) [3], iron (Fe) [4], and cobalt (Co) [5], supported by various metal oxides like alumina (Al₂O₃) and magnesium oxide (MgO) [6]. These materials play a crucial role in the economic feasibility of the process. Their high electrical and thermal conductivity and microporosity provide added value, making them suitable for various applications. For instance, they can be utilized in double-layer capacitors, electrocatalysts, carbon nanofiber-based composites, or as precursors for graphitic materials.

Ni-based catalysts have been extensively researched and sought after due to their impressive activity levels [7]. However, despite being used less as an active metal in CMD reactions, iron-based catalysts have garnered attention in the field of CMD, primarily because Fe-based catalysts offer the advantages of being more cost-effective and, importantly, more environmentally friendly [8,9,10]. So, to ensure that the CMD process is both environmentally friendly and cost-effective, it would be a great idea to utilize an affordable catalyst that can efficiently convert methane into hydrogen without the need for catalyst regeneration or mixing with carbon materials [11]. Iron (Fe) is an up-and-coming option because its partially filled 3d orbitals enable it to facilitate the dissociation of hydrocarbons by accepting electrons [12]; this positions iron as a highly effective candidate for this purpose. Furthermore, Fe catalysts offer the advantage of operating at higher temperatures (700–950 °C) due to their higher melting point than Ni catalysts (500–700 °C). This capability is vital for achieving more efficient thermodynamic conversion, particularly in CMD reactions [13].

Numerous techniques have been investigated to enhance the efficiency and stability of Fe-based catalysts. These efforts include incorporating various promoters, employing different reactor types and supports, utilizing diverse synthesis methods, and optimizing operational environments. The goal was to increase the catalyst’s surface area, enhance the dispersion of the active metal on the support, and refine the electronic states of the metal. At the same time, efforts are made to reduce catalyst sintering [14,15]. Certain metal oxide materials such as CeO₂ [16], MgO [17], ZrO₂ [18], La₂O₃ [19], TiO₂ [20], and Al₂O₃ [21] are adequate supports in thermal catalytic methane decomposition processes. Alumina is a popular choice for catalyst support in methane decomposition reactions due to its large surface area, excellent thermal conductivity, and metal-support solid interaction [22,23]. However, employing alumina as a supporting material for a single oxide has drawbacks, such as the formation of spinel structures and difficulty reduction [24]. There are three naturally occurring crystal phases of titanium: rutile, anatase, and brookite [25]. The most stable of these is rutile. Supporting catalysts made of rutile and anatase-phase TiO₂ are frequently employed [20]. The combination of rutile-anatase-phase TiO₂ is effective as a location where non-noble metal species deposit like iron. However, pure titanium has a relatively low surface area of 70 m²/g and a low pore volume of 0.3 cm³/g [26]. A growing trend involves the creation of alumina–titanium composites by integrating titanium into alumina to enhance its properties. This synthesis method has demonstrated significant improvements in the mechanical strength, activity, and thermal stability of alumina, while also offering a larger surface area compared to titanium alone. Hydrothermal synthesis is a widely used technique for preparing catalysts, particularly for mixed metal oxide materials. This method is known to have several advantages over conventional production methods. For instance, it enables the synthesis of high-quality thermodynamically stable crystals at lower temperatures, and the process can be completed in a single step [27]. The main factors that can influence the textural properties of hydrothermally synthesized powders are the type of solvent, autoclave temperature, aging time, and calcination temperature [28]. Coating TiO₂ with alumina through hydrothermal synthesis enhances the photocatalytic activity of alumina, resulting in a higher density of hydroxyl groups and increased strength [29].

This work investigates a new method of coated alumina with titanium dioxide as the catalyst support for methane decomposition using the hydrothermal synthesis protocol while varying the loadings of titanium dioxide. Furthermore, the study is extended to evaluate the effect of different iron loadings on the as-synthesized support material to enhance methane conversion through a decomposition reaction. The BET, X-ray diffraction (XRD), temperature-programmed reduction (TPR), transmission electron microscopy (TEM), temperature-programmed oxidation (TPO), and Raman spectroscopy techniques are utilized for the characterization of the catalysts. The result indicates that loading titanium dioxide plays a crucial role in CMD performance.

2. Results and Discussion

2.1. Structure Analysis

N₂-physisorption analysis is employed to determine the effect of adding iron to the 20Ti-Al support on its textural properties. The results, presented in Table 1, show a notable decrease in surface area following the incorporation of iron into the 20Ti-Al support. The support 20Ti-Al had a surface area of 229.86 m²/g. This surface area decreased remarkably to 175.51 m²/g after loading 10% iron for the 10Fe/20Ti-Al catalyst; the same behavior occurred for another catalyst, noting that increasing loading of iron decreased the surface area of the catalyst. For instance, 20Fe/20Ti-Al and 30Fe/20Ti-Al have surface area 155.07 m²/g and 133.29 m²/g, respectively. Moreover, the pore volumes of the 20Fe/20Ti-Al and 30Fe/20Ti-Al catalysts were both measured at 0.49 cm³/g, which is lower than the pore volume of the 10Fe/20Ti-Al catalyst, which was 0.54 cm³/g. The results are shown in Table 1. Decreasing surface area and pore volume up to 20 wt% Fe over 20TiAl confirms the deposition of iron-oxide phases into the pores [30]. Upon increasing Fe loading further (beyond 20 wt.%), the particle size of iron increases and larger Fe-based particles do not enter in the pore and remain outside the pore. So, concerning 20Fe/20Ti-Al, the surface area of 30Fe/20Ti-Al decreases but pore volume remains preserved. Figure 1A shows the N₂ adsorption-desorption isotherms of the Fe-containing catalysts. As seen in Figure 1A, the adsorption-desorption isotherms of all the catalysts fall under the type IV category and exhibit H3 hysteresis loops. This classification is consistent with mesoporous materials slit shape/plate-like pores according to the IUPAC classification [31]. Figure 1B confirms this observation through the pore size distribution analysis. It reveals that the pore diameters were significantly increased with decreased iron loading in the catalyst composition. However, it is essential to note that despite these changes, most of the pores in all synthesized catalysts still fell within the mesoporous range.

2.2. Reducibility

H₂-TPR is used to analyze the impact of different iron loadings on the reducibility and the degree of interaction between metal and support in the freshly calcined catalysts (i.e., X wt.% Fe/20Ti-Al).

The H₂-TPR profiles in Figure 2A for three catalysts show three similar and distinct drop peaks across various temperatures. These peaks are often mainly attributed to the progressive decline of Fe₂O₃ to zero valence Fe (Fe₂O₃–Fe₃O₄–FeO–Fe). The first peak of the reduction within 350–450 °C can be attributed to the reduction of bulk hematite species into magnetite (α-Fe₂O₃–Fe₃O₄). Conversely, the second peak of the reduction located within the temperature range of 520–600 °C can be assigned to the further reduction of magnetite into metallic iron (Fe₃O₄–Fe). In addition, the third reduction peak that appears in 20Fe/20Ti-Al and 30Fe/20Ti-Al at temperatures above 700 °C belongs to the reducing of FeO to metallic Fe [32,33]. The observation of the reduction peaks of 30Fe/20Ti-Al shifts towards higher temperatures than other catalysts; this is explained by a stronger interaction between the (Ti-Al) support and Fe. Furthermore, it can be noted that the peak intensity of the 30Fe/20Ti-Al catalyst is higher than that of other catalysts, as can be confirmed by the consumption of hydrogen in Table 1, which indicates the presence of the highest number of reducible species than the rest catalyst.

In contrast, the 20Fe/20Ti-Al catalyst shows moderate interaction between iron and support according to hydrogen consumption, as shown in Table 1. Based on current research, it has been observed that the development of graphitic carbon nanofibers resulted from a weak interaction between the metal and the support [34]. Figure 3A displays the temperature-programmed reduction profiles of several catalysts with different loadings of TiO₂ calcined at 500 °C. The impact of various loadings of TiO₂ does not change the reduction behavior of all the catalysts significantly since the calcination temperature is constant and three reduction peaks are visible for three catalysts, as seen in the first peak at 400 °C, the second at 550 °C, and the third at 700 °C. In addition, Table 2 shows that hydrogen consumption of the 20Fe/10Ti-Al and 20Fe/30Ti-Al catalysts were higher than 20Fe/20Ti-Al, indicating that metal interacts strongly with the bulk of the support and forms such species that are difficult to reduce ultimately.

2.3. X-Ray Diffraction

The X-ray diffraction (XRD) patterns of composite titania-alumina-supported iron catalysts were analyzed after calcination at 500 °C, as presented in Figure 2B. The peak trends of all three catalysts demonstrated remarkable similarities. The 10Fe/20Ti-Al, 20Fe/20Ti-Al, and 30Fe/20Ti-Al catalysts have diffraction peaks at 2θ = 30°, 2θ = 35.2°, 2θ = 37.6°, 2θ = 53.9°, 2θ = 57.2°, 2θ = 62.9°, 2θ = 66.7°, and 2θ = 75.3°. The diffraction peaks at 2θ = 30.2° and 2θ = 57.2° are found in all catalysts related to hematite (JCPDS: 00-025-1402). The diffraction peaks at 2θ = 37.6° and 2θ = 66.7° correspond to Al₂O₃ (JCPDS: 00-029-0063) [35]. Meanwhile, the peaks identified at 2θ = 35.2° and 2θ = 53.9° indicate TiO₂ in the rutile phase (JCPDS: 021-1276); in addition, the peaks identified at 2θ = 62.9° and 2θ = 75.3° indicate TiO₂ in the anatase phase (JCPDS: 021-1272) [29]. Furthermore, it is noted that no discernible diffraction peaks were observed for Fe-aluminates. This may be attributed to the moderate calcination temperature of 500 °C applied to all prepared catalysts or other factors, such as the highly dispersed nature of Fe₂O₃ particles and their moderate interaction with the (20Ti-Al) support, as shown in the TPR analysis. Moreover, the XRD result indicates no differences in intensity from the result in Figure 2B when loading various TiO₂ on the catalysts, as shown in Figure 3B.

Table 2 provides the textural properties of the 20Fe/10Ti-Al and 20Fe/30Ti-Al catalysts at different loading of TiO₂. Upon increasing the loading of TiO₂ from 10% to 30%, the specific surface area underwent a slight reduction, resulting in a decrease to 166.98 m²/g (20Fe/10Ti-Al) and 152.78 m²/g (20Fe/30Ti-Al) from their initial values of the support 239.69 m²/g (10Ti-Al), 233.03 m²/g (30Ti-Al). Agglomeration of metal particles is responsible for the decrease in textural aspects.

2.4. Catalytic Activity

2.4.1. Effect of the Fe Loading in the Catalyst (i.e., Fe X wt.%/20 Ti-Al) on the Performance

The performance of 20Ti-Al-supported iron catalysts in methane decomposition reaction is studied to determine the effect of different loading of iron (10 wt.%, 20 wt.%, and 30 wt.%) on the catalytic performance. Figure 4 illustrates the catalytic decomposition activities of methane conversion and hydrogen yield over these catalysts as a function of time on stream (TOS). Before starting the reaction, the H₂ atmosphere is brought down to 800 °C for 1 h to activate all the catalysts. The activity is then carried out at an operating temperature of 800 °C at a gas hourly space velocity (GHSV) of 6 L gcat⁻¹ h⁻¹.

As shown in Figure 4A, the 10Fe/20Ti-Al catalyst started with initial methane conversion 83% lower than the 20Fe/20Ti-Al and 30Fe/20Ti-Al catalysts. As the reaction duration increases, the 10Fe/20Ti-Al catalyst shows a modest rise in activity after about 120 min, reaching 86%. The reaction is then continually completed with 86% methane conversion in 240 min. As shown in Figure 4A, an increase in iron loading to 20 wt.% and 30 wt.% boosted the methane conversion, most likely due to an increase in the amount of iron’s active sites on the support. The highest methane conversion activity is achieved over the 20Fe/20Ti-Al and 30Fe/20Ti-Al catalysts, which starts with an initial CH₄ conversion of around 89%, and with time on stream, an increase in the methane conversion is also observed at about 96 min with 93%; methane conversion stabilized and remained unchanged during the rest of the test time on stream. Although the 10Fe/20Ti-Al catalyst had the largest surface area and pore volume, it had the lowest iron content. As a result, its methane conversion rate is lower than the 20Fe/20Ti-Al and 30Fe/20Ti-Al catalysts. Moreover, the pore size of the 10Fe/20Ti-Al catalyst is more significant than that of the other catalysts, providing direct evidence of the catalysts’ insensitivity toward methane decomposition reaction in this study. The opposite trend is noticed in contrast to previous research, which claimed that textural characteristics had a more significant impact on activity than the metal content in the catalysts [26,36]. It can be concluded that the presence of the anatase and rutile phases affected all catalyst performance in terms of activity and stability; thus, because the anatase phase can transform irreversibly into a more thermal stable rutile phase at higher temperatures while rutile phase maintains its structure without change phase transformation at higher temperatures [37,38]. In addition, the formation of graphitic carbon on the 20Fe/20Ti-Al catalyst surface appeared to play a positive role, as seen in the TEM analysis. This graphitic layer might have acted as a shield, hindering further carbon encapsulation and preserving catalyst activity. Thus, the impact of carbon formation is complex and depends on the type and morphology of the carbon species formed with its ordered structure, graphitic carbon can offer improved stability compared to amorphous carbon. Additionally, the used XRD examination indicates the presence of iron carbide species, which may boost C-H bond activation during methane breakdown and contribute to catalytic activity. As seen in the carbon accumulation, the three catalysts’ carbon generation compares favorably to their methane conversion efficiency. Additionally, a comparison is made between the catalysts used in this work and different iron catalyst systems documented in the literature, as shown in Table 3.

2.4.2. Effect of the TiO₂ Loading in the Support (i.e., 20 Fe/X wt.% Ti-Al) on the Performance

Given that the 20% iron catalyst outperformed the other catalysts tested in this regard, the impact of various TiO₂ loadings on catalyst efficiency was also investigated, with a GHSV of 6 L gcat⁻¹ h⁻¹. The outcomes are displayed in Figure 5A. As shown, an increase in the loading of TiO₂ by up to 30% reduces the methane conversion, as seen in Figure 5A. With a titania-based system, there are two major limitations (1). The formation of stable FeTiO₃ by reaction of FeO and TiO₂ (2) phase transformation between stable rutile phase to unstable anatase phase [42,43]. In XRD, the crystalline peak of FeTiO₃ is also not observed. Over 20Fe/30TiAl, the peak intensity of the unstable phase (anatase) is increased whereas the intensity of the stable phase (Rutile) phase is decreased (Figure S1). The growth of the unstable phase induces more diffusion of TiO_x [44] and partial coverage of active sites by TiO_x, which may result in inferior catalytic activity. In the case of the 20Fe/10Ti-Al catalyst, the initial methane conversion of 89% increased slightly to 90%, and the conversion was stable and remained unchanged in the rest of the tested time on stream. Methane conversion increased to 90% at the beginning and then somewhat to 93% during the experiment when TiO₂ loading was increased to 20%. Nevertheless, the initial activity decreased with additional TiO₂ loading increases up to 30%. The catalyst initially converted methane at an 83% rate, as seen in Figure 5A, but as time passed on stream, the catalyst’s conversion rate gradually decreased. Nevertheless, a steady state-like behavior with approximately 85% methane conversion was seen after on-stream for about 100 min.

2.5. Characterization of Used Catalyst

2.5.1. TGA-Used Catalyst

TGA is an analytical technique that proves helpful in assessing several aspects related to the carbon left over from the CMD process on the catalyst surface. These characteristics include its yield, thermal stability, and nature. TGA thermo-graphs of the used catalysts are represented in Figure 6B for three catalysts: 10Fe/20Ti-Al, 20Fe/20Ti-Al, and 30Fe/20Ti-Al, which are subjected to this analysis up to 1000 °C.

Based on the findings, weight loss initiates at temperatures as low as 580 °C, as indicated in all TGA profiles. Figure 6B illustrates the percentage weight loss of various used catalysts, showing that the 20Fe/20Ti-Al catalyst experienced the most significant weight loss percentage, surpassing about 90%. In contrast, the 10Fe/20Ti-Al and 30Fe/20Ti-Al catalysts show the second-highest weight loss percentage, at around 80%. When analyzing the weight loss curve of three catalysts, it is essential to mention that a considerable reduction in carbon weight is observed beyond 600 °C. This trend indicates that the catalysts could generate carbon in the form of carbon nanotubes (CNTs) or graphitic carbon, which can be validated by conducting a thorough Raman spectroscopic study of the catalyst.

2.5.2. Raman Spectroscopy-Used Catalyst

The spent catalysts are analyzed using Raman spectroscopy to verify the resultant carbon nanomaterials on the surface. The Raman spectra of the employed catalysts are shown in Figure 7, where two distinct peaks are visible at approximately 1342 cm⁻¹ and 1581 cm⁻¹, respectively. These peaks are associated with the D and G bands. The D band is typically attributed to disordered carbon species, such as amorphous carbon, and defects in carbon nanotubes (CNTs) walls. In contrast, the G band is believed to be due to graphitic crystalline carbon [45]. Analyzing the intensity ratio (I_D/I_G) of the D band (I_D) about the intensity (I_G) of the G band can yield important information [46].

Based on the analysis results, the D band’s intensity was relatively weak in all samples. The calculated I_D/I_G ratios for 10Fe/20Ti-Al, 20Fe/20Ti-Al, and 30Fe/20Ti-Al are 0.66, 0.88, and 0.87, respectively. These ratios imply that crystalline, highly graphitized, and well-ordered carbon species are present in all samples. With the lowest I_D/I_G ratio of roughly 0.66 among the catalysts, 10Fe/20Ti-Al generated CNTs with strong crystallinity and graphitization.

2.5.3. TPO-Used Catalyst

Temperature-programmed oxidation (TPO) is a powerful method for identifying the type of carbon present on spent catalyst surfaces. Carbon deposits formed during methane decomposition can be gasified at specific temperature intervals, such as below 250 °C for atomic carbon, between 250 °C and 600 °C for amorphous carbon, and above 600 °C for graphitic carbon [41]. The TPO profiles of catalysts 10Fe/20Ti-Al, 20Fe/20Ti-Al, and 30Fe/20Ti-Al are shown in Figure 8A. These profiles peak in the graphitic carbon temperature range, suggesting that the carbon deposited on the surface is graphitic. Unlike amorphous carbon, which tends to impede the active sites and cause deactivation, this type of carbon deposition permits the active sites to remain accessible to the feed [42]. This is probably the reason that, despite having the largest amount of carbon deposits, all three catalysts remained stable and active throughout the test.

2.5.4. XRD for Used Catalyst

Figure 8B illustrates the carbon deposition from methane decomposition by three catalysts through their respective XRD patterns. It is strongly suggested by the observed patterns that the diffraction peaks at 2θ values of 26° and 43° are indicative of the production of crystalline carbon on the surface of the catalyst. Moreover, the presence of iron carbide is implied by the detection of additional diffraction peaks at 2θ values of 39°, 54°, and 67° [47]. It is important to highlight that the 20Fe/20Ti-Al catalyst displayed a significantly higher carbon peak intensity when compared to the 10Fe/20Ti-Al and 30Fe/20Ti-Al catalysts. O₂-TPO and Raman’s results also validate the presence of maximum graphitization over 20Fe/20Ti-Al. This intensity variation can be ascribed to the catalyst’s enhanced crystallinity and higher hydrogen generation on the 20Fe/20Ti-Al [48].

2.5.5. TEM-Fresh and Used Catalyst

Figure 9A show the TEM images of the fresh catalyst and the 20Fe/20Ti-Al catalyst’s particle size distributions after it was calcined at 500 °C. The iron catalyst supported on the coated alumina revealed that iron particles dispersed over the support seem to be well dispersed. The lighter spots in the background structure indicated the presence of the coated alumina with TiO₂. On the other hand, The TEM image of the used catalyst displays the carbon configuration of the 20Fe/20Ti-Al catalyst, showcasing the presence of single-walled carbon nanotubes, as shown in Figure 9B. The tip-growth mechanism is responsible for the formation of carbon on the 20Fe/20Ti-Al catalysts, with the active iron remaining on top of the single-walled carbon nanotubes due to weak interaction, as discussed in the reducibility section. This phenomenon occurs as metal particles detach from the support and form carbides. As a result, the catalysts demonstrate exceptional stability and activity over the course of the response.

2.5.6. Characterization of the Used 20Fe/X wt.%Ti-Al Catalyst

Thermal gravimetric analysis was used to determine how much carbon compounds were produced on the wasted catalyst during the methane breakdown reaction (TGA). This analysis was performed on catalysts synthesized at different loadings of titanium dioxide (10 wt.%, 20 wt.%, and 30 wt.%), as shown in Figure 10A. According to the TGA analysis Figure 10A, the relative weight loss of 20Fe/10Ti-Al, 20Fe/20Ti-Al, and 20Fe/30Ti-Al catalysts varied depending on the loading of titanium dioxide. The weight loss percentage for the respective catalysts was 89%, 90%, and 80%. The characteristics of every catalyst exhibiting varying TiO₂ loadings suggested that every carbon deposited was graphitic. The TGA analysis’s conclusions will be supported by the Raman analysis’s results. Furthermore, the Raman spectroscopic study of the carbon product crystallinities created on the catalysts during the reaction of methane decomposition is shown in Figure 10B. The findings show that the I_D/I_G ratios of every catalyst were smaller than unity. According to Figure 10B, the I_D/I_G ratio dropped for catalysts with 20Fe/10Ti-Al, 20Fe/20Ti-Al, and 20Fe/30Ti-Al when the loading TiO₂ amount decreased. Additionally, all of the catalysts used underwent temperature-programmed oxidation (TPO) examination to identify the different forms of carbon deposited. The corresponding results have been depicted in Figure 10C. It is evident from the graph that the spent catalysts for 20Fe/10Ti-Al and 20Fe/20Ti-Al exhibit comparable intensities in their graphic regions, implying a higher concentration of graphic carbon. Conversely, the Fe/30Ti-Al catalyst displays a lower concentration of graphic carbon, as confirmed by the Raman spectroscopy Figure 10B. Moreover, the result of the used XRD confirmed that the 20Fe/20Ti-Al catalyst had more activity than other catalysts with graphic carbon types, as shown in Figure 10D.

2.6. Limitation and Future Work

The developed catalysts are viable to demonstrate significant potential across various industries, including natural gas, power generation, biomass conversion, and ammonia production. The active site of catalyst is iron, which is cost-effective as well as widely available. They facilitate the conversion of methane to hydrogen, help reduce NO_x emissions through selective catalytic reduction in power plants, enhance the economic viability of biomass conversion, and contribute to making the Haber–Bosch process more sustainable for ammonia production. However, there are challenges to overcome. For instance, to scale up the process, efficient heat-management strategies are essential for controlling exothermic reactions and maintaining optimal operating temperatures. This could entail advanced reactor designs and cooling systems. It is crucial to thoroughly investigate long-term catalyst stability and the mechanisms of deactivation under industrial operating conditions. Additionally, developing effective regeneration strategies for deactivated catalysts is vital for ensuring economic viability. Integrating the methane decomposition process into existing oil refining and petrochemical infrastructure will require careful consideration of feedstock availability, product handling, and potential safety hazards. A comprehensive techno-economic analysis is necessary to evaluate the economic feasibility of the scaled-up process, considering factors such as capital investment, operating costs, and product value. Future work should include conducting pilot-scale experiments under conditions that closely resemble industrial environments. This is essential for validating laboratory-scale findings and addressing potential challenges associated with scaling up. Ongoing research and development efforts should prioritize improving catalyst stability, durability, and resistance to deactivation. Additionally, optimizing reaction conditions such as temperature, pressure, and space velocity is crucial for maximizing process efficiency and reducing operating costs. Finally, conducting a comprehensive life-cycle assessment is necessary to evaluate the environmental and economic impacts of the scaled-up process, ensuring sustainable development.

3. Materials and Methods

3.1. Materials

The experiment’s chemicals were bought from Alfa Aesar in Karlsruhe, Germany. Among the substances are Fe (NO₃)₃·9H₂O (purity 98%), titanium (IV) isopropoxide Ti [OCH(CH₃)₂]₄ (purity 95%), and the alumina support (Al₂O₃) (purity 99.97%) was supplied by Daiichi Kigenso Kagaku Kogyo Co., Ltd., Osaka, Japan, and Ethanol (purity 96%), Hydrochloric Acid (concentration 37%), Sodium hydroxide pellets (purity 97%) were supplied by Sigma Aldrich in Eschenstr, Germany, without any additional treatment. The experiment also utilized a crucible, a crusher, deionized water, and a stainless steelautoclave.

3.2. Catalyst Synthesis

TiO₂ and alumina support are prepared by hydrothermal method; typically, the desired amount of titanium isopropoxide (TTIP) and 15 mL ethanol are stirred at room temperature for 1 h, and HCl solution (0.5 M concentration) is added dropwise and continuously stirred for 1 h. First, the specified amount of alumina is added to the mixture and stirred continuously for 2 h. Then, the mixture is transferred to a 100 mL Teflon-lined stainless-steel autoclave, heated in an oven at 180 °C for 12 h, and allowed to cool to room temperature. After that, the solid is filtered and dried at 100 °C for 12 h, followed by heat treatment in the air in a muffle furnace at a rate of 7 °C/min up to 350 °C for 2 h. The obtained supports are 10 wt.% Ti-Al, 20 wt.% Ti-Al, and 30 wt.% Ti-Al and termed as 10Ti-Al, 20Ti-Al, and 30Ti-Al, respectively. Next, the iron precursor, i.e., Fe (NO₃)₂·9H₂O, is dissolved in distilled water, and the solution is gradually added to the prepared support while stirring continuously at 60 °C for 3 h and using 0.5 M of NaOH to control the pH at 9. The resulting catalysts are then dried at 100 °C for a whole night before being calcined for 5 h at 500 °C, as depicted in the schematic diagram of preparation in Figure 11. The obtained catalysts are 10 wt.% Fe/10Ti-Al, 20 wt.% Fe/10Ti-Al, and 30 wt.% Fe/10Ti-Al and termed as 10Fe/10Ti-Al, 20Fe/10Ti-Al, and 30Fe/10Ti-Al, respectively and so on. The catalyst characterizations are discussed in the Supplementary File as S2.

3.3. Catalyst Testing

Methane decomposition catalytic reactions are performed at 800 °C and atmospheric pressure in a fixed-bed tubular reactor with a diameter of 0.9 cm (i.d.) and a 30-cm-long stainless-steel tube. The reactor tube is placed inside a temperature-controlled vertical oven. Initially, 0.15 g of the catalyst is subjected to reduction for 1 h using 30 cm³ min⁻¹ H₂ at 800 °C. Then, to get rid of any remaining H₂, the system is purged with N₂ for 15 min. After that, the mixture CH₄:N₂ (1:2) is passed at a rate of 15 mL min⁻¹ for 4 h to begin the methane conversion process. Gas chromatography (Shimadzu GC 2014, Tokyo, Japan) is utilized to analyze the product gas composition, which is connected online to the reactor and equipped with packed columns and a thermal conductivity detector (TCD).

The detail of the catalytic reactor setup is given in the Supplementary File as S1. The Schematic diagram of the experimental setup for catalytic hydrogen production is shown in Figure S2.

The methane conversion and hydrogen yield were calculated using the following equation.

(2)$\mathrm{M}\mathrm{e}\mathrm{t}\mathrm{h}\mathrm{a}\mathrm{n}\mathrm{e} \mathrm{C}\mathrm{o}\mathrm{n}\mathrm{v}\mathrm{e}\mathrm{r}\mathrm{s}\mathrm{i}\mathrm{o}\mathrm{n}\left(\%\right)={\displaystyle \frac{{\mathrm{C}\mathrm{H}}_{4\left(\mathrm{i}\mathrm{n}\right)}-{\mathrm{C}\mathrm{H}}_{4\left(\mathrm{o}\mathrm{u}\mathrm{t}\right)}}{{\mathrm{C}\mathrm{H}}_{4\left(\mathrm{i}\mathrm{n}\right)}}}\times 100$

(3)${\mathrm{H}}_2 \mathrm{Y}\mathrm{i}\mathrm{e}\mathrm{l}\mathrm{d}\left(\%\right)={\displaystyle \frac{[\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{e}\mathrm{s} \mathrm{o}\mathrm{f} {\mathrm{H}}_2 \mathrm{P}\mathrm{r}\mathrm{o}\mathrm{d}\mathrm{u}\mathrm{c}\mathrm{e}\mathrm{d}]}{[2\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{e}\mathrm{s} \mathrm{o}\mathrm{f} {\mathrm{C}\mathrm{H}}_4 \mathrm{i}\mathrm{n} \mathrm{t}\mathrm{h}\mathrm{e} \mathrm{f}\mathrm{e}\mathrm{e}\mathrm{d}]}}\times 100$

(4)$\mathrm{C}\mathrm{a}\mathrm{r}\mathrm{b}\mathrm{o}\mathrm{n} \mathrm{y}\mathrm{i}\mathrm{e}\mathrm{l}\mathrm{d}\left(\%\right)={\displaystyle \frac{\mathrm{W}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t} \mathrm{o}\mathrm{f} \mathrm{d}\mathrm{e}\mathrm{p}\mathrm{o}\mathrm{s}\mathrm{i}\mathrm{t}\mathrm{e}\mathrm{d} \mathrm{c}\mathrm{a}\mathrm{r}\mathrm{b}\mathrm{o}\mathrm{n} \mathrm{o}\mathrm{n} \mathrm{t}\mathrm{h}\mathrm{e} \mathrm{c}\mathrm{a}\mathrm{t}\mathrm{a}\mathrm{l}\mathrm{y}\mathrm{s}\mathrm{t}}{\mathrm{W}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t} \mathrm{o}\mathrm{f} \mathrm{m}\mathrm{e}\mathrm{t}\mathrm{a}\mathrm{l} \mathrm{c}\mathrm{a}\mathrm{t}\mathrm{a}\mathrm{l}\mathrm{y}\mathrm{s}\mathrm{t}}}\times 100$

4. Conclusions

The hydrothermal synthesis method was used to coat alumina with TiO₂, using titanium isopropoxide as a precursor with various TiO₂ loading and iron as the active metals with different ratios. The research investigated how adding TiO₂ to alumina supports affects the performance and durability of iron-based catalysts used in methane decomposition reactions. The study found that adding TiO₂ improved the dispersion of active iron and promoted the interaction between iron and TiO₂-Al support, significantly enhancing methane conversion and catalyst stability. The impact of using different loading of iron and TiO₂ (10 wt.%, 20 wt.%, and 30 wt.%) was investigated; increasing the iron ratio exhibited the supremacy of activity and stability performance; the 20Fe/20Ti-Al and 30Fe/20Ti-Al catalysts presented a CH₄ conversion of 94% and an H₂ yield of 84%. Alternatively, the 10Fe/20Ti-Al showed slightly weak activity in methane conversion of 86% with a hydrogen yield of 79%. On the other hand, adding TiO₂ of more than 20% to the alumina support decreased the activity of the catalyst. The catalysts 20Fe/10Ti-Al and 20Fe/20Ti-Al achieved a methane conversion rate of 92% and 94%, respectively, while producing a hydrogen yield of 81% and 84%. On the other hand, the catalyst 20Fe/30Ti-Al showed slightly lower activity with a CH4 conversion rate of 86% and an H₂ yield of 78%. Additionally, according to the TEM image, the type of carbon present in catalysts is CNTs.

Author Contributions

Conceptualization, H.A., A.H.F. and S.F.A.; Methodology, H.A. and A.S.A.-F.; Software, H.A., S.F.A. and A.E.A. Validation, A.E.A., A.S.A.-F., A.E.A. and A.H.F.; Formal analysis, H.A., S.F.A. and A.E.A.; Investigation, H.A. and S.F.A.; Resources, A.S.A.-F., F.M.A.-A. and S.B.A.; Data curation, H.A., A.I.O. and A.H.F.; Writing—original draft preparation, H.A.; Writing—review and editing, A.E.A., S.F.A. and A.S.A.-F.; Visualization, H.A., A.E.A., A.A.I. and A.S.A.-F.; Supervision, A.H.F. and A.S.A.-F.; Project administration, A.S.A.-F., A.E.A., A.I.O., F.M.A.-A. and A.H.F.; Funding acquisition, A.H.F., A.I.O. and S.B.A. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

The original contributions presented in the study are included in the article and Supplementary Materials.

Acknowledgments

The authors would like to express their sincere appreciation to the Researchers Supporting Project number (RSP2025R368) at King Saud University in Riyadh, Saudi Arabia.

Conflicts of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Footnotes

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Figures and Tables

Figure 1. N2 adsorption-desorption isotherms (A) and pore size distribution (B) of fresh catalyst (Fe X wt.%/20Ti-Al).

Figure 2. H2-TPR spectra (A) and XRD patterns (B) for freshly loaded iron catalysts on 20Ti-Al calcined at 500 °C.

Figure 3. H2-TPR spectra (A) and XRD patterns (B) of fresh catalysts with various ratios of 20Ti-Al-supported iron catalysts calcined at 500 °C.

Figure 4. Methane conversion time on stream (A) and hydrogen yield time on stream (B) for Fe-based catalysts loaded on 20Ti-Al support calcined at 500 °C, CH4/N2= 1:2, T = 800 °C, P = 1 atm, and GHSV = 6 L gcat−1 h−1.

Figure 5. Time on Stream (TOS) of methane conversion (A) and hydrogen yield (B) against time on stream for iron-based catalysts with various ratios of Ti-Al support calcined at 500 °C, CH4 /N2 = 1:2, T = 800 °C, P = 1 atm, GHSV = 6 L gcat−1 h−1.

Figure 6. Carbon Yield of the catalysts (A) and TGA curves (B) for spent Fe-based catalysts loaded on 20Ti + Al support calcined at 500 °C for a reaction time of 240 min at 800 °C.

Figure 7. Raman spectra of spent Fe-based catalysts (i.e., Fe X wt.%/20Ti-Al) after a reaction time of 240 min at 800 °C.

Figure 8. TPO profiles (A) and XRD patterns (B) used iron-based catalysts (i.e., Fe X wt.%/20Ti-Al) after a reaction time of 240 min at 800 °C.

Figure 9. TEM images of the (A) unused Fe-based catalysts (i.e., Fe X wt.%/20Ti-Al) and particle distribution curve and (B) used iron-based catalysts (i.e., Fe X wt.%/20Ti-Al) after a reaction time of 240 min at 800 °C.

Figure 10. Thermal gravimetric analysis (A), Raman spectra (B), TPO (C), and X-ray diffraction (D) curves for the used iron-based catalysts with various ratios of Ti-Al support calcined at 500 °C for a reaction time of 240 min at 800 °C.

Figure 11. A schematic diagram of the preparation procedure.

Table 1

Characterization of the various percentages of iron supported on 20Ti-Al catalysts (i.e., Fe X wt.%/20Ti-Al).

Catalyst’s Name	Surface Area BET (m2/g)	Pore Volume(cm3/g)	Pore Diameter(nm)	Total Hydrogen Consumption (cm3/g STP)	(DR) a (%)
20Ti-Al	229	0.62	9.42		------
10Fe/20Ti-Al	175	0.54	13.50	34.49	40.15
20Fe/20Ti-Al	155	0.49	12.29	30.96	34.57
30Fe/20Ti-Al	133	0.49	10.39	80.84	93.63

(Reduction Degree) ^a (%) = (Hydrogen consumed during H₂-TPR/theoretical H₂ needed to finish the reduction).

Table 2

Textural properties of fresh with various loading of TiO₂ on the alumina support in the Fe/X wt.%Ti-Al catalyst and hydrogen consumption of catalyst during TPR.

Catalyst	Surface Area BET (m2/g)	Pore Volume(cm3/g)	Pore Size(nm)	Total Hydrogen Consumption (cm3/g STP)	(DR) a (%)
10Ti-Al	239	0.68	10.04		------
20Fe/10Ti-Al	166	0.54	11.69	67.87	74.91
20Ti-Al	229	0.62	9.42		------
20Fe/20Ti-Al	155	0.49	10.59	30.96	34.57
30Ti-Al	233	0.54	7.98		-----
20Fe/30Ti-Al	152	0.52	11.86	65.41	75.23

(Reduction Degree) ^a (%) = (Hydrogen consumed during H₂-TPR/theoretical H₂ needed to finish the reduction).

Table 3

A comparison between the outcomes reported in earlier literature and the current work activity.

Catalyst’s Name	Iron Content (%)	Reaction Temperature (°C)	Methane Conversion (%)	Hydrogen Yield (%)	Ref.
Fe/CeO2-ZrO2	15	700	-----	83	[39]
Fe2O3/Y2O3-Al2O3	3	750	29	-----	[40]
Fe/La2O3 + ZrO2	40	800	79	84	[41]
Fe/20Ti-Al	10	800	91	82	This work
Fe/20Ti-Al	20	800	94	85	This work
Fe/20Ti-Al	30	800	94	84	This work

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal15020122/s1, Catalytic Reactor Setup S1; Catalyst Characterization S2; Figure S1. The X-ray diffraction of the catalyst in the selected Bragg’s angle. Note: Here, R and A signify the Rutile and Anatase phases; Figure S2. Schematic diagram of the experimental setup for catalytic hydrogen production.

References

1. Fan, Z.; Weng, W.; Zhou, J.; Gu, D.; Xiao, W. Catalytic Decomposition of Methane to Produce Hydrogen: A Review. J. Energy Chem.; 2021; 58, pp. 415-430. [DOI: https://dx.doi.org/10.1016/j.jechem.2020.10.049]

2. Catalan, L.J.J.; Rezaei, E. Coupled Hydrodynamic and Kinetic Model of Liquid Metal Bubble Reactor for Hydrogen Production by Noncatalytic Thermal Decomposition of Methane. Int. J. Hydrogen Energy; 2020; 45, pp. 2486-2503. [DOI: https://dx.doi.org/10.1016/j.ijhydene.2019.11.143]

3. Rastegarpanah, A.; Rezaei, M.; Meshkani, F.; Dai, H.; Arandiyan, H. Thermocatalytic Decomposition of Methane over Mesoporous Ni/XMgO·Al₂O₃ Nanocatalysts. Int. J. Hydrogen Energy; 2018; 43, pp. 15112-15123. [DOI: https://dx.doi.org/10.1016/j.ijhydene.2018.06.057]

4. Sun, H.; Ren, S.; Ji, X.; Song, W.; Guo, Q.; Shen, B. Doping Fe and Zn to Modulate Ni Nanoparticles on IM-5 for Methane Decomposition to Form Hydrogen and CNTs. Int. J. Hydrogen Energy; 2023; 48, pp. 13081-13096. [DOI: https://dx.doi.org/10.1016/j.ijhydene.2022.12.230]

5. Silva, R.R.C.M.; Oliveira, H.A.; Guarino, A.C.P.F.; Toledo, B.B.; Moura, M.B.T.; Oliveira, B.T.M.; Passos, F.B. Effect of Support on Methane Decomposition for Hydrogen Production over Cobalt Catalysts. Int. J. Hydrogen Energy; 2016; 41, pp. 6763-6772. [DOI: https://dx.doi.org/10.1016/j.ijhydene.2016.02.101]

6. Zhao, G.; Pan, X.; Zhang, Z.; Liu, Y.; Lu, Y. A Thin-Felt Pd-MgO-Al₂O₃/Al-Fiber Catalyst for Catalytic Combustion of Methane with Resistance to Water-Vapor Poisoning. J. Catal.; 2020; 384, pp. 122-135. [DOI: https://dx.doi.org/10.1016/j.jcat.2020.01.013]

7. Sotnikova, A.; Ivantsov, M.; Kulikov, A.; Kulikova, M. Influence of MgO Promoution to Ni-Based Composites in Hydrogen Production Methane Decomposition Process. Int. J. Hydrogen Energy; 2024; 57, pp. 1208-1220. [DOI: https://dx.doi.org/10.1016/j.ijhydene.2024.01.123]

8. Yang, M.; Li, S.; Deng, Y.; Baeyens, J.; Zhang, H. Effect of Fe-Loading in Iron-Based Catalysts for the CH₄ Decomposition to H₂ and Nanocarbons. J. Environ. Manag.; 2023; 346, 118999. [DOI: https://dx.doi.org/10.1016/j.jenvman.2023.118999] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/37751646]

9. Ahmad, A.; Hamdani, I.R.; Srinivasakannan, C.; Al Shoaibi, A.; Hossain, M.M. Catalytic Cracking of Methane to Hydrogen and Carbon: Scale-up Perspective. Int. J. Hydrogen Energy; 2024; 54, pp. 1212-1230. [DOI: https://dx.doi.org/10.1016/j.ijhydene.2023.12.042]

10. Dawkins, M.; Saal, D.; Marco, J.F.; Reynolds, J.; Dann, S. An Iron Ore-Based Catalyst for Producing Hydrogen and Metallurgical Carbon via Catalytic Methane Pyrolysis for Decarbonisation of the Steel Industry. Int. J. Hydrogen Energy; 2023; 48, pp. 21765-21777. [DOI: https://dx.doi.org/10.1016/j.ijhydene.2023.03.022]

11. Kazemi, S.; Alavi, S.M.; Rezaei, M. Influence of Various Spinel Materials Supported Ni Catalysts on Thermocatalytic Decomposition of Methane for the Production of CO_x-Free Hydrogen. Fuel Process Technol.; 2023; 240, 107575. [DOI: https://dx.doi.org/10.1016/j.fuproc.2022.107575]

12. Zhao, X.; Teng, Q.; Tao, H.; Tang, W.; Chen, Y.; Zhou, B.; Sang, J.; Huang, S.; Guan, W.; Li, H. et al. FeCo Alloy-Decorated Proton-Conducting Perovskite Oxide as an Efficient and Low-Cost Ammonia Decomposition Catalyst. Catalysts; 2024; 14, 850. [DOI: https://dx.doi.org/10.3390/catal14120850]

13. Liang, W.; Yan, H.; Chen, C.; Lin, D.; Tan, K.; Feng, X.; Liu, Y.; Chen, X.; Yang, C.; Shan, H. Revealing the Effect of Nickel Particle Size on Carbon Formation Type in the Methane Decomposition Reaction. Catalysts; 2020; 10, 890. [DOI: https://dx.doi.org/10.3390/catal10080890]

14. Soliman, A.M.S.; Tschentscher, R.; Akporiaye, D.; Al-Rawashdeh, M. Effect of Ni/SiO₂ Catalyst Preparation Method on Methane Decomposition and CO₂ Gasification Cycles. Fuel; 2024; 368, 131585. [DOI: https://dx.doi.org/10.1016/j.fuel.2024.131585]

15. Biondo, L.D.; Manera, C.; Aguzzoli, C.; Godinho, M. DoE-Driven Thermodynamic Assessment of COX-Free Hydrogen Production from Methane Decomposition. Catal. Commun.; 2024; 187, 106874. [DOI: https://dx.doi.org/10.1016/j.catcom.2024.106874]

16. Tang, L.; Yamaguchi, D.; Burke, N.; Trimm, D.; Chiang, K. Methane Decomposition over Ceria Modified Iron Catalysts. Catal. Commun.; 2010; 11, pp. 1215-1219. [DOI: https://dx.doi.org/10.1016/j.catcom.2010.07.004]

17. Ko, D.H.; Kang, S.C.; Lee, C.W.; Im, J.S. Effects of Support Porosity of Co-Mo/MgO Catalyst on Methane Catalytic Decomposition for Carbon and Hydrogen Production. J. Ind. Eng. Chem.; 2022; 112, pp. 162-170. [DOI: https://dx.doi.org/10.1016/j.jiec.2022.05.008]

18. Al -Fatesh, A.S.; Kasim, S.O.; Ibrahim, A.A.; Al-Awadi, A.S.; Abasaeed, A.E.; Fakeeha, A.H.; Awadallah, A.E. Catalytic Methane Decomposition over ZrO₂ Supported Iron Catalysts: Effect of WO₃ and La₂O₃ Addition on Catalytic Activity and Stability. Renew. Energy; 2020; 155, pp. 969-978. [DOI: https://dx.doi.org/10.1016/j.renene.2020.04.038]

19. Pudukudy, M.; Yaakob, Z.; Jia, Q.; Takriff, M.S. Catalytic Decomposition of Methane over Rare Earth Metal (Ce and La) Oxides Supported Iron Catalysts. Appl. Surf. Sci.; 2019; 467–468, pp. 236-248. [DOI: https://dx.doi.org/10.1016/j.apsusc.2018.10.122]

20. Shen, Y.; Lua, A.C. Sol–Gel Synthesis of Titanium Oxide Supported Nickel Catalysts for Hydrogen and Carbon Production by Methane Decomposition. J. Power Sources; 2015; 280, pp. 467-475. [DOI: https://dx.doi.org/10.1016/j.jpowsour.2015.01.057]

21. Caballero, M.; Del Angel, G.; Rangel-Vázquez, I.; Huerta, L. Hydrogen Production by Methane Decomposition on Pt/γ-Alumina Doped with Neodymium Catalysts and Its Kinetic Study. Catal. Today; 2020; 349, pp. 106-116. [DOI: https://dx.doi.org/10.1016/j.cattod.2018.05.024]

22. Pinilla, J.L.; Suelves, I.; Lázaro, M.J.; Moliner, R.; Palacios, J.M. Parametric Study of the Decomposition of Methane Using a NiCu/Al₂O₃ Catalyst in a Fluidized Bed Reactor. Int. J. Hydrogen Energy; 2010; 35, pp. 9801-9809. [DOI: https://dx.doi.org/10.1016/j.ijhydene.2009.10.008]

23. Al-Fatesh, A.S.; Fakeeha, A.H.; Khan, W.U.; Ibrahim, A.A.; He, S.; Seshan, K. Production of Hydrogen by Catalytic Methane Decomposition over Alumina Supported Mono-, Bi- and Tri-Metallic Catalysts. Int. J. Hydrogen Energy; 2016; 41, pp. 22932-22940. [DOI: https://dx.doi.org/10.1016/j.ijhydene.2016.09.027]

24. Fakeeha, A.H.; Khan, W.U.; Al-Fatesh, A.S.; Abasaeed, A.E.; Naeem, M.A. Production of Hydrogen and Carbon Nanofibers from Methane over Ni–Co–Al Catalysts. Int. J. Hydrogen Energy; 2015; 40, pp. 1774-1781. [DOI: https://dx.doi.org/10.1016/j.ijhydene.2014.12.011]

25. Hu, C.; Qiu, C.; Zhang, W.; Song, J.; Meng, Q.; Yuan, Q.; Wang, T. Insights into the Role of Pt Promoter in Co/TiO₂ Catalysts for CO Hydrogenation. Catalysts; 2024; 14, 922. [DOI: https://dx.doi.org/10.3390/catal14120922]

26. Ahmed, H.; Alotibi, M.F.; Fakeeha, A.H.; Ibrahim, A.A.; Abasaeed, A.E.; Osman, A.I.; Al-Awadi, A.S.; Alarifi, N.; Al-Fatesh, A.S. Hydrogen Production via Methane Decomposition over Alumina Doped with Titanium Oxide-Supported Iron Catalyst for Various Calcination Temperatures. ChemistryOpen; 2023; 13, e202300173. [DOI: https://dx.doi.org/10.1002/open.202300173] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/38085118]

27. Modeshia, D.R.; Darton, R.J.; Ashbrook, S.E.; Walton, R.I. Control of Polymorphism in NaNbO₃ by Hydrothermal Synthesis. Chem. Commun.; 2008; pp. 68-70. [DOI: https://dx.doi.org/10.1039/B814601B]

28. Mobini, S.; Meshkani, F.; Rezaei, M. Surfactant-Assisted Hydrothermal Synthesis of CuCr2O₄ Spinel Catalyst and Its Application in CO Oxidation Process. J. Environ. Chem. Eng.; 2017; 5, pp. 4906-4916. [DOI: https://dx.doi.org/10.1016/j.jece.2017.09.027]

29. Cano-Casanova, L.; Amorós-Pérez, A.; Ouzzine, M.; Lillo-Ródenas, M.A.; Román-Martínez, M.C. One-Step Hydrothermal Synthesis of TiO₂ with Variable HCl Concentration: Detailed Characterization and Photocatalytic Activity in Propene Oxidation. Appl. Catal. B Environ.; 2018; 220, pp. 645-653. [DOI: https://dx.doi.org/10.1016/j.apcatb.2017.08.060]

30. Ren, X.Y.; Cao, J.P.; Zhao, S.X.; Zhao, X.Y.; Liu, T.L.; Feng, X.B.; Li, Y.; Zhang, J.; Bai, H.C. Encapsulation Ni in HZSM-5 for Catalytic Hydropyrolysis of Biomass to Light Aromatics. Fuel Process Technol.; 2021; 218, 106854. [DOI: https://dx.doi.org/10.1016/j.fuproc.2021.106854]

31. Kumar, R. Surface Characterization Techniques: From Theory to Research; Walter de Gruyter: Berlin, Germany, 2022; pp. 1-208. [DOI: https://dx.doi.org/10.1515/9783110656480]

32. Liu, Q.; Qiao, Y.; Tian, Y.; Gu, F.; Zhong, Z.; Su, F. Ordered Mesoporous Ni-Fe-Al Catalysts for CO Methanation with Enhanced Activity and Resistance to Deactivation. Ind. Eng. Chem. Res.; 2017; 56, pp. 9809-9820. [DOI: https://dx.doi.org/10.1021/acs.iecr.7b02174]

33. Karimi, S.; Meshkani, F.; Rezaei, M.; Rastegarpanah, A. The Influence of Promoters on the Catalytic Behavior of the Ni/SiO₂.MgO Catalysts for Thermocatalytic Decomposition of Methane Reaction. Fuel; 2024; 372, 131945. [DOI: https://dx.doi.org/10.1016/j.fuel.2024.131945]

34. Lamouroux, E.; Serp, P.; Kalck, P. Catalytic Routes towards Single Wall Carbon Nanotubes. Catal. Rev.-Sci. Eng.; 2007; 49, pp. 341-405. [DOI: https://dx.doi.org/10.1080/01614940701313200]

35. Zhang, J.; Jin, L.; Hu, H.; Xun, Y. Effect of Composition in Coal Liquefaction Residue on Catalytic Activity of the Resultant Carbon for Methane Decomposition. Fuel; 2012; 96, pp. 462-468. [DOI: https://dx.doi.org/10.1016/j.fuel.2011.12.075]

36. Barati Dalenjan, M.; Rashidi, A.; Khorasheh, F.; Ardjmand, M. Effect of Ni Ratio on Mesoporous Ni/MgO Nanocatalyst Synthesized by One-Step Hydrothermal Method for Thermal Catalytic Decomposition of CH4 to H2. Int. J. Hydrogen Energy; 2022; 47, pp. 11539-11551. [DOI: https://dx.doi.org/10.1016/j.ijhydene.2022.01.185]

37. Zhu, X.; Han, S.; Feng, W.; Kong, Q.; Dong, Z.; Wang, C.; Lei, J.; Yi, Q. The Effect of Heat Treatment on the Anatase–Rutile Phase Transformation and Photocatalytic Activity of Sn-Doped TiO₂ Nanomaterials. RSC Adv.; 2018; 8, pp. 14249-14257. [DOI: https://dx.doi.org/10.1039/C8RA00766G]

38. Boytsova, O.; Zhukova, I.; Tatarenko, A.; Shatalova, T.; Beiltiukov, A.; Eliseev, A.; Sadovnikov, A. The Anatase-to-Rutile Phase Transition in Highly Oriented Nanoparticles Array of Titania with Photocatalytic Response Changes. Nanomaterials; 2022; 12, 4418. [DOI: https://dx.doi.org/10.3390/nano12244418]

39. Ramasubramanian, V.; Ramsurn, H.; Price, G.L. Hydrogen Production by Catalytic Decomposition of Methane over Fe Based Bi-Metallic Catalysts Supported on CeO₂–ZrO₂. Int. J. Hydrogen Energy; 2020; 45, pp. 12026-12036. [DOI: https://dx.doi.org/10.1016/j.ijhydene.2020.02.170]

40. Al Mesfer, M.K.; Danish, M.; Shah, M. Synthesis and Optimization of Hydrotalcite Derived Ni-Fe-Cu Based Catalysts for Catalytic Methane Decomposition Process Using the Design of Experiment Approach. J. Taiwan Inst. Chem. Eng.; 2021; 128, pp. 370-379. [DOI: https://dx.doi.org/10.1016/j.jtice.2021.08.045]

41. Fakeeha, A.H.; Kasim, S.O.; Ibrahim, A.A.; Al-Awadi, A.S.; Alzahrani, E.; Abasaeed, A.E.; Awadallah, A.E.; Al-Fatesh, A.S. Methane Decomposition Over ZrO₂-Supported Fe and Fe–Ni Catalysts—Effects of Doping La₂O₃ and WO₃. Front. Chem.; 2020; 8, 317. [DOI: https://dx.doi.org/10.3389/fchem.2020.00317] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/32411666]

42. Jeong, M.H.; Sun, J.; Young Han, G.; Lee, D.H.; Bae, J.W. Successive Reduction-Oxidation Activity of FeO_x/TiO₂ for Dehydrogenation of Ethane and Subsequent CO₂ Activation. Appl. Catal. B Environ.; 2020; 270, 118887. [DOI: https://dx.doi.org/10.1016/j.apcatb.2020.118887]

43. Hanaor, D.A.H.; Sorrell, C.C. Review of the Anatase to Rutile Phase Transformation. J. Mater. Sci.; 2011; 46, pp. 855-874. [DOI: https://dx.doi.org/10.1007/s10853-010-5113-0]

44. Shah, M.; Bordoloi, A.; Nayak, A.K.; Mondal, P. Effect of Ti/Al Ratio on the Performance of Ni/TiO₂-Al₂O₃ Catalyst for Methane Reforming with CO₂. Fuel Process Technol.; 2019; 192, pp. 21-35. [DOI: https://dx.doi.org/10.1016/j.fuproc.2019.04.010]

45. Mohamed, A.T.; Ali, S.; Kumar, A.; Mondal, K.C.; El-Naas, M.H. Evaluation of Highly Active and Stable SiO₂ Supported Fe-Based Catalysts for the Catalytic Methane Decomposition into COx Free Hydrogen and CNTs. Catal. Commun.; 2023; 180, 106703. [DOI: https://dx.doi.org/10.1016/j.catcom.2023.106703]

46. Chen, L.; Noreña, L.E.; Wang, J.A.; Limas, R.; Arellano, U.; Vargas, O.A.G. Promoting Role of Amorphous Carbon and Carbon Nanotubes Growth Modes of Methane Decomposition in One-Pot Catalytic Approach. Catalysts; 2021; 11, 1217. [DOI: https://dx.doi.org/10.3390/catal11101217]

47. Zhou, L.; Enakonda, L.R.; Harb, M.; Saih, Y.; Aguilar-Tapia, A.; Ould-Chikh, S.; Hazemann, J.L.; Li, J.; Wei, N.; Gary, D. et al. Fe Catalysts for Methane Decomposition to Produce Hydrogen and Carbon Nano Materials. Appl. Catal. B Environ.; 2017; 208, pp. 44-59. [DOI: https://dx.doi.org/10.1016/j.apcatb.2017.02.052]

48. Echegoyen, Y.; Suelves, I.; Lázaro, M.J.; Sanjuán, M.L.; Moliner, R. Thermo Catalytic Decomposition of Methane over Ni–Mg and Ni–Cu–Mg Catalysts: Effect of Catalyst Preparation Method. Appl. Catal. A Gen.; 2007; 333, pp. 229-237. [DOI: https://dx.doi.org/10.1016/j.apcata.2007.09.012]