Because of the dominance of fossil fuels, hydrogen is derived mostly from them (Kayfeci et al., 2019). The main processes involved in hydrogen production include catalytic steam reforming of light hydrocarbons, partial oxidation of heavy hydrocarbons, coal gasification, and methane decarburization. The most common method used worldwide is the steam reforming of natural gas (Kim et al., 2018). As shown in the Figure 4, the set up required during steam reforming reaction is quite simple and contains a furnace, a temperature modulator, membrane module, flow and gas meters (Kim et al., 2018). This method is far more environment-friendly and highly efficient (∼70–80%) (Grigoriev et al., 2006; Shiva Kumar and Himabindu, 2019) than other methods that produce hydrogen through fossil fuels (Abe et al., 2019).

Despite the rampant production of hydrogen from fossil fuels, those methodologies pose serious concerns, especially environment-related issues and higher production costs. Being a greenhouse gas, it poses a lot of concern towards environmental sustainability. Hence, hydrogen production’s better alternative is from renewable resources like geothermal sources, biomass, wind energy, etc., (Avcıoğlu et al., 2019). There are methods available that permit the chemical conversion of renewable resources to give rise to hydrogen. Such strategies include biomass gasification, steam reforming, among others. The hydrogen produced is used in metallurgical purposes, electronics development, and various other chemical applications (Srilatha et al., 2017).

Metal catalysts like Ni/Al₂O₃ are better preferred during SMR reactions because of their inexpensive nature (Siew et al., 2014). The most extensively studied metal catalysts for this process are from the VIII group of the periodic table. Their properties, like the size effect (Vogt et al., 2020), support effect (Wang et al., 2018), promoter effects (Cao et al., 2020), and coordination states (Rogers et al., 2016), have also been well researched. Even the external conditions of methane reforming like the temperature, pressure, and kind of reactor used, are considered while using the metal catalysts.

Transitions metals like Fe and Ni that have semi-filled d-orbitals are also used in SMR reactions because of their enhanced stability and activity (Syed Muhammad et al., 2018; Inaba et al., 2019). Ni catalysts are far more convenient for TMD via SMR reactions when carried out at 500–600°C. However, this temperature range is lower than the conditions needed to achieve an equilibrium state because Ni deactivates at higher temperature conditions. On the contrary, Fe functions efficiently at increased temperatures and is comparatively less expensive (Frontera et al., 2017). The most commonly used metal catalysts are the transition metal catalysts like Ni, Co, and Fe. Because of the enhanced turnover rates and being cheaply available, Ni catalysts are better preferred. Studies have also shown that, in the presence of elements like rhodium, Ni gets easily reduced, which increases the methane conversion, thereby increasing the total hydrogen production (Kim et al., 2015). Table 1 shows the data related to a few recent reports about hydrogen production via POM method using Ni-based catalysts supported on various catalysts.

Like the other processes, the most widely used metal catalyst is based on the transition element, Ni (Farsi and Mansouri, 2016), because of its cheap availability and reaction efficiency. The Ni available in the catalyst system usually remains below 20% due to its highest activity observed at that concentration (Haynes and Shekhawat, 2011; Scapinello et al., 2017). This could occur because of a decrease in Ni dispersion with increasing Ni amount. Studies have reported that catalysts prepared via microwave assistance impregnation methods show enhanced activity (Hosseini et al., 2017). Moreover, microwave radiations enhance the rate at which Ni gets deposited on the catalyst’s surface during their preparation. The efficiency of such a catalyst and their influence in methane reforming reactions was observed in the studies carried out by Roh and colleagues (HS et al., 2002). The authors explained this behavior by the catalysts due to the high interaction with the catalysts’ active phase. Table 2 shows the data related to a few recent reports about hydrogen production via autothermal reforming reactions.

Many catalysts like the ones based on nickel, zeolites, alumina, platinum, ruthenium, and others based on alkali metals have been studied (Okolie et al., 2019). Ni-based catalysts have shown high-end results during the gasification process because of their ability to allow reforming reactions (Nikolaidis and Poullikkas, 2017). Studies have revealed that alumina silicate-based catalysts are far more effective than Ni-based catalysts during the gasification process (Arregi et al., 2018). Nickel-based catalysts are often used when hydrogen is the only product needed. Its activity is dependent on the presence or absence of the support material and other additives (Kannaiyan et al., 2016). Table 3 shows the data related to a few recent reports about hydrogen production via direct biomass gasification method using Ni-based catalysts supported on various catalysts.

Metal catalysts show enhanced activity and high chemical and mechanical efficiency and stability during the water-splitting reaction. Metals like Ni, Co, Pt, and others are very commonly used during the reaction (Konsolakis et al., 2016). Among these, Ni catalysts have shown the highest effectiveness for hydrogen production because of the resultant large yield during the reaction (Saikia et al., 2020; Wang et al., 2021b). However, metal poisoning by sulfur and carbon deposition on their actives sites results in metal deactivation. Hence, even carbon-based catalysts are used to overcome the shortcomings of metal-based catalysts (Dufour et al., 2010; Harun et al., 2020).

Most metal catalysts that are used in this reaction get deactivated on carbon deactivation on their surface. The separation of the coke deposited on the surface can be a cumbersome process. We have discussed catalyst reactivation in the following sections. Because of the additional reactivation process for catalysts, sometimes no catalysts are preferred. Moreover, metal-based catalysts are also promising entities for the process because they still show better stability and efficiency than carbon-based catalysts.

Among the commonly used metal catalysts, nickel has shown high conversion rates even at 500°C temperatures. While, without a catalyst, the reaction proceeds at a higher temperature of 700°C (Brandon and Kurban, 2017). Apart from Ni, even Fe-based catalysts have shown to be inexpensive and widely used for the industrial production of hydrogen via methane pyrolysis (Qian et al., 2020). APR reactions that produce hydrogen have been studied for various catalysts, including iron, nickel, palladium, rhodium, iridium, etc. These elements have been noted to show high activity as catalysts during aqueous phase reforming for hydrogen production. Among these, platinum, palladium, and nickel-tin alloys have displayed enhanced activity from hydrogen production. It has been noted that when and if the supports demonstrate basic or neutral nature, then the selectivity during hydrogen production and its selectivity increases. Table 4 shows the data related to a few recent reports about hydrogen production via APR method using Pt-based catalysts supported on various catalysts.

As in the previously discussed methodologies for hydrogen production, Ni-based catalysts have the maximum catalytic activity and efficiency. It helps in adjusting the ratio of CO_x and hydrogen during the conversion of methane. The highest activity of this catalyst is observed at 780°C (Widjaya et al., 2018). However, as discussed previously, the Ni catalysts get deactivated due to carbon deposits. On the contrary, Ni is relatively cheap and readily available; hence, it is an ideal catalyst and shows optimal activity.

Metal supported catalysts like catalysts supported by Ni are also preferred in SMR reactions. They are relatively cheaper and available with ease. Moreover, they show enhanced activity. This was observed in the study carried out by Karaismailoglu and colleagues (Karaismailoglu et al., 2019). They used Ni-based catalysts attached to yttria. The authors developed a catalyst via the sol-gel method. They noticed its activity between 390 and 845°C and observed that the maximum amount of coke was formed when the temperature was elevated. Few studies indicate that methane decomposition can occur without the reduction of catalysts (Musamali and Isa, 2018). Such methods are relatively more inexpensive. Moreover, it has been noted that hydrogen production reduces this way. Table 5 shows the data related to a few recent researches about hydrogen production via SMR method using Ni-based catalysts supported on various catalysts.

The most commonly used metal-supported catalysts during the direct biomass gasification process include Ni-based catalysts having K₂CO₃ support. Even fundamental catalysts like MgO and CaO are also used during the reaction (Anis and Zainal, 2011). However, these metal-supported catalysts make use of sorbents that are eco-friendly to enhance the catalyst’s overall sensitivity. Limitations also persist in the noble metal-supported catalysts that the ones supported by rhodium (Asadullah et al., 2004). Among the metal catalysts like nickel, cobalt, palladium, iron, and others, none of these shows a very high efficiency on their own during the thermo catalytic water splitting reactions. Hence elements like silica, alumina, magnesia, carbon are used simultaneously along with the core catalysts. These metal support catalysts enhance the total available surface area on the catalyst so that more active sites can be made available to enhance the reaction process (Srilatha et al., 2017).

The second metal support is introduced in a catalyst to increase the life of the catalyst. For instance, when the second metal support is introduced in Ni-based catalysts, the activity and its stability have shown to be exponentially increased (Sánchez‐Bastardo et al., 2020). The most popularly used metal-supported catalysts include the ones supported by palladium and copper. Because of the larger surface area, the catalyst deactivation does not occur immediately, and hence the catalysts remain stable for longer durations. The support available for the catalysts allows for equilibrium between the methane dissociation and carbon diffusion rates. Certain metals like palladium and copper enhance the reduction process. Moreover, weak bonds between metals and their supports also improve metals’ reducing capacity; when there are strong bonds between metal and the support in a catalyst, the catalyst’s dispersion and stability increases.

Tao and colleagues studied the effect of nickel-iron-cobalt alloy in APR reaction (Tao et al., 2020). The authors observed that the nickel-iron allowed for better catalytic activity during the reaction than the lone nickel and iron catalysts. Moreover, when cobalt is added to the catalyst, the catalyst’s overall activity was shown to increase. A higher conversion rate of the reactants was observed in aqueous phase reforming, and more hydrogen production was recorded. Moreover, the selectivity of hydrogen was also shown to increase tremendously. This catalyst was shown to undergo reduction at even low temperatures of 300°C. However, the activity was recorded to increase with increasing temperature. Maximum activity was observed at ∼660°C.

Earth metal catalysts have been shown to play a useful role during hydrogen production during coal gasification. Dolomite (CaCO₃.MgCO₃) has very high efficiency and does not need to undergo any regeneration after the catalytic process (Widjaya et al., 2018). Other than those, nickel-aluminum catalysts and nickel-olivine catalysts have also shown practical usage during coal gasification of methane for hydrogen production (Mei Wu et al., 2014). Ni-based catalysts, in general, are involved in removing tars and excess methane.

Studies also indicate that the performance of a given catalyst for TMD depends on the material of carbon used as the support (Yang et al., 2015). Among these, graphene has better activity when compared to unsupported catalysts (Hasnan et al., 2020a). This was seen in the work of Lua and Wang (2013). They studied non supported Ni-based catalysts in TMD and discovered that CH₄ must be inserted in the reaction mixture at a reduced temperature for an efficient reaction process. Unsupported catalysts like NiCuAl (Suelves et al., 2006) and NiCuMg (Moliner et al., 2008) exhibit high activity and stability. They provide an opportunity to develop metal crystals that allow for the reaction to proceed for prolonged durations. Hence, non-supported catalysts also play a vital role in hydrogen production. Despite being cheaply available, transition metal catalysts like the ones based on Ni are easily deactivated when on stream. This may occur because of frittage, deposition of carbon on the surface, or further reactions with the substrates (Pantaleo et al., 2016). The activity of these catalysts depends on their active forms and the support used during the reaction. The size of the metal particles is an essential feature that determines their intrinsic activity and their deactivation rate. These features are found to be inversely proportional to the size of the metal particles (Kosinov et al., 2019).

When another metal is attached to these metal-based catalysts like Ni-based catalysts, the resultant catalyst’s stability dramatically increases. Metals like rhodium, platinum, palladium, iridium have been used in metal catalysts to improve their overall strength. This has been seen in many studies (Alvarez-Galvan et al., 2019). Of all the metals used to support other metal catalysts, rhodium is the most promising of all (Tanaka et al., 2010). This is because it aids in the non-noble metal to remain a metal by disallowing the excess transfer of hydrogen atoms from noble metal to non-noble metal catalysts (Arandiyan and Li, 2012). As discussed above, Ni catalysts serve as the most popular option for catalysts during hydrogen production. Apart from this, palladium, copper, and other metals are also used as catalysts. However, these metals alone sometimes lead to the deactivation of the catalysts. This happens because of the carbon deposition at the catalysts’ surface.

This deactivation can be prevented by adding metals like iron, platinum, gold, etc., along with the primary metal catalyst to improve the catalysts’ overall activity and efficiency. This was shown in a study done by Dantas and colleagues (Dantas et al., 2010). The scientists reported that the catalyst supported on silver allowed for the increased conversion of methane and accounted for the stability of the catalyst function and activity. In another study, Kaori Yoshida and coworkers discovered that Ni catalysts supported on palladium give the most promising results during hydrogen production (Yoshida et al., 2009). Such metal-supported catalysts have shown to provide more hydrogen/CO ratio when compared to unsupported catalysts. The supported catalysts support enhanced catalytic activity, enhanced stability, and larger active surface area for the catalyst to function. Any support gives porosity that gives increased contact with reactants. Moreover, the nature of contact between the support element and the catalyst, and the bond between them determines the reaction rate during hydrogen production. Studies have shown that the addition of Mg, Co, Zn elements increases Ni-based catalysts (Zhu et al., 2011). Also, ZrO₂ is an unbound or non-supported catalyst that leads to increased efficiency for hydrogen production via an autothermal oxidation reaction (Jeong et al., 2006). In addition to these catalysts, bifunctional catalysts can also be used to help improve the reaction rate of hydrogen production to increase the hydrogen yield. The proposed structure of such bifunctional catalysts has been shown in Figure 10.

Of all the catalysts studied for the gasification process, alkali metals were the first to be researched (Francke et al., 2018). Unsupported catalysts based on natural minerals, noble gas, and synthetically available triggers have also been studied and developed for hydrogen production via direct biomass gasification (Al-Rahbi and Williams, 2017; Yang et al., 2019). These catalysts can undergo rapid action, decrease the amount of tar production, and increase total hydrogen gas. Sometimes even microorganisms are used that are chemically active. Such microbes tend to generate electricity by producing electrons. When these organisms are used as “catalysts,” the process becomes very economical and highly efficient. Electrochemically active microbes are used in a cell with protons, electrons, and other chemical substances to ease the reaction. Because of the coupling of such components, direct production of hydrogen occurs (Kim et al., 2017; Ma et al., 2021). The dissolved organic components get oxidized, and the protons are reduced to give hydrogen.

Metals like nickel, iron, and cobalt have also been used in methane pyrolysis reactions without the need for a supporting structure (Srilatha et al., 2017). This is because of their enhanced activity, temperature conditions, and ability to produce solid carbon as the byproduct. These metal catalysts also allow solubility and diffusion of carbons through catalysts’ surface. Of all the catalysts mentioned above, Ni has the highest activity and least toxicity for this reaction (Ouyang et al., 2019). However, these elements undergo deactivation at very high temperatures (>600°C). In the case of unsupported catalysts, the catalytic activity during aqueous phase reforming was noted to be (He et al., 2013): Platinum  =  nickel > ruthenium > rhodium  =  palladium > iridium

Of all the catalysts, Ni has shown to display the lowest reformation of the reactants and allows for efficient hydrogen production. Alkali metals derived from acids have also shown efficiency during the catalytic process – like the alkali metal salts derived from weak acids. These include potassium carbonate, potassium sulfide, sodium carbonate, and sodium sulfide. During these reactions, lower temperatures are better preferred, that is, between 700 and 925°C (Mei Wu et al., 2014). These catalysts are added to the gasifier present on coal or char. These catalysts, however, are not quickly recovered after the gasification process. Moreover, elements present in the catalyst, like sulfur, can sometimes poison the catalysts during the reaction.

Various transition metals supported on oxides are also used as catalysts in TMD reactions (Shen and Lua, 2015a). These include metals like Ni, Fe, and Co, and others. Various studies (Khan et al., 2016; Abdullah et al., 2017; Chen et al., 2020b; Baig et al., 2021) have shown the effects of different metal oxide supported catalysts like La₂O₃, ZiO₂, Al₂O₃, SiO₂, and TiO₂ involved in producing hydrogen (Kim et al., 2011; El Hakim et al., 2021). In a study by Ibrahim et al., it was found that Fe over La₂O₃ showed promising results for TMD at temperatures ranged between 500 and 700°C. Another study by Shen and colleagues on Ni supported on TiO₂ revealed that this catalyst showed higher stability and expressed a high hydrogen production rate as compared to a catalyst unsupported by Ni (Shen and Lua, 2015b). Of all studies on metal oxides supported on carbon substances, silica-based materials serve as better catalysts because of their increased surface area. They also possess enhanced thermal stability, changeable pore size, and increased diffusion ability (Di Michele et al., 2019). This makes them an ideal candidate to be used in metal oxide supported catalysts for hydrogen production.

The behavior of metal oxide supported catalysts has shown to be far more convenient and useful in POM reactions. Ni-based oxide supported catalysts like the ones kept on Al₂O₃, La₂O₃, ZrO₂, CeO_2, etc. are developed through solid-state reaction and wet impregnation. These catalysts, however, are easily deactivated because of the rapid deposition of carbon. Moreover, contamination because of Ni nanoparticles may also result in their deactivation. However, the metal catalysts that are supported on ceria and alumina have shown to be very promising. This is because of the large surface area of the Ni nanoparticles on the alumina surface and the available spots in ceria structures that allows for the direct adsorption on those sites (Singha et al., 2017).

On the contrary, catalysts attached to Mg-based structures that rapidly undergo deactivation due to the creation of NiO/MgO irreducible complexes in the solution. Metal oxides like La₂O₃ allow for decreased carbon formation because of the high scattering of Ni nanoparticles (Kumar et al., 2019). Moreover, these structures also cause lanthanum oxycarbonate development that causes further gasification of carbon structures. ZrO₂ also plays an essential role in serving as the support structure for metal catalysts. However, these oxides reduce the oxygen presence that is essential in POM reaction (Hongloi et al., 2019).

Among the metal oxide supported catalysts that are well studied, Ni/Al₂O₃ has gained wide popularity because of its cheap availability and high stability (Alvarez-Galvan et al., 2019). Other metal oxide supports include MgO, SiO₂, TiO₂, CeO₂ etc. Furthermore, Al₂O₃ disallows catalyst frittage and increases the overall mechanical stability of the catalysts. Beyond 973K, the sensitivity of the Al₂O₃ based metal oxide catalysts also increases. Hence, any changes in Al₂O₃ support create catalysts that directly affect the catalysts (Koh et al., 2007). In some instances, transition metal oxides like CeO₂, ZrO₂, and Cr₂O₃ are used along with alumina components (Silva et al., 2012). However, oxygenates rapidly undergo dehydration and give ethylene, resulting in carbon deposition and catalyst deactivation (Ali et al., 2016; Rogers et al., 2016). Studies have shown that CeO₂ support increases the stability and activity of Ni-based catalysts (Arora and Prasad, 2016). Moreover, the addition of Ce to ZrO₂ results in strong interaction between the individual substances present on the catalyst. Also, Ce causes enhanced storage capacity of oxygen and allows for the synthesis of transferrable oxygen during the reaction process. Hyun Seog Toh and colleagues studied multiple metal oxide supported catalysts (like MgO, Ce-ZrO₂, MgAl₂O₄) however, Ni catalysts supported on MgAl₂O₃ showed the maximum efficiency and stability (Roh et al., 2001; Shang et al., 2017).

Metal oxides of alkaline earth metals are favorable for gasification reaction because they permit reformation reactions (Nikolaidis and Poullikkas, 2017). When metal catalysts like Cu catalysts are introduced into the reaction, additional elements like promoters are also used. This is to decrease the amount of non-required products (like the soot and tar) and increase the total hydrogen produced. The metal oxides, including the oxides of transition metals, are alkali earth metals are often introduced as the promoters during the reaction (Gupta et al., 2011; Qu et al., 2020).

The ZnO/Zn redox pairs have shown to have better superiority among other metal oxide supported catalysts during hydrogen production via this reaction (Steinfeld, 2002). However, this metal oxide pair is not feasible for the large-scale production of hydrogen. Moreover, efficient separation of the products poses a challenge (Koepf et al., 2016). Hence, because of the shortcomings, CeO₂ is a better candidate for water splitting reaction and hydrogen production (Gokon et al., 2013). However, these catalysts function at high temperatures of 1700–3000 K. Other metal oxide supported catalysts include the redox pairs of Mn₂O₃/MnO, SnO₂/SnO, Fe₂O₄/Fe, etc. The enhanced surface area of platinum supported on Al₂O₃ and ZrO₂ also shows very high activity. However, these catalysts exhibit rapid deactivation too. Platinum catalyst supported on TiO₂ also show very high stability. However, they too get deactivated when used for prolonged durations.

Fe catalysts supported on other metal oxides like Al₂O₃ based catalysts have been studied for this reaction and have shown promising results (Zhou et al., 2016). When such catalysts are used, they offer in situ catalysis during methane pyrolysis. This was seen in the work put forward by Yeheskel and coworkers (Yeheskel and Epstein, 2011). The authors discussed when Fe(CO)₅ and Fe(C₂H₅)₂ are decomposed. Fe clusters are produced during the pyrolysis reaction. Besides, various other gases are also produced as by-products. These gases produced, however, undergo further decontamination steps.

Metal oxide supports like Al₂O₃ and Fe₃O₄ have shown to be very efficient in hydrogen synthesis via aqueous steam reforming (Gumina et al., 2019). Valenzuela and colleagues were the first teams who developed this process for hydrogen production (Valenzuela et al., 2006). They employed Pt catalysts supported on Al₂O₃. High activity of the catalyst structure and large hydrogen production was noted during the reaction. In another study, the catalytic activity of CeO₂-ZrO₂ and CeO₂-TiO₂ was studied (Chen et al., 2017a). It was noted that both sets of catalysts showed high activity and efficiency. Moreover, these catalysts, including Pt-Al₂O_3, were observed to show high catalytic activity and were recyclable.

Among the metal oxide supported catalysts, K₂CO₃ has shown high catalytic activity during the gasification reaction compared to Ni catalysts supported on metal oxides. It has been observed that hydrogen production is increased two times when K₂CO₃ is used as a catalyst (Li et al., 2010). This could be because of the high and uniform solubility of the catalyst in the reaction mixture. Other catalysts like SiO₂, KAlSiO₄, NiO, and KAlSi₃O₈ have been shown to undergo rapid deactivation owing to large carbon deposits on their surface. Other metal oxide-supported catalysts like ZnO were shown to exhibit quick activity in contrast to SnO₂ catalysts; the hydrogen yield was also enhanced.

Noble metal catalysts like ruthenium and rhodium used in the SMR process exhibit increased activity and high stability (Chen et al., 2017b). Other noble metals like platinum and iridium show enhanced electrocatalytic activity and can be attached to other elements to give better catalytic activity (Swain et al., 2018). However, there are a few drawbacks to these catalysts including their high cost of production and instability (Digraskar et al., 2019). These can be overcome by reducing the size of the noble metals attached to porous carbon structure. This increases the overall efficiency of catalysts. Furthermore, they can be attached to other transition metals to enhance the catalyst’s activity (Li et al., 2019). These steps can be incorporated at an industrial level to reduce their overall cost of production.

Noble metals like palladium, rhodium, platinum, iridium also serve as noble metal-based catalysts in POM reaction (Figen and Baykara, 2018; Ma et al., 2019). Studies have shown the reducing nature of noble metals like ruthenium that results in lanthanide oxide-supported ruthenium structures that are used catalysts in POM (Lian et al., 2018). Owing to the enhanced performance of noble metal catalysts, these catalysts have gained a great deal of attention from the scientific community. In a study by Alvarez-Galvan and colleagues, it was discovered that rhodium, when used as the supporting material of metals like Ni, promotes the further reduction of the catalyst, thereby improving the overall catalytic activity during POM (Alvarez-Galvan et al., 2019). It has been noted that the catalytic activity of ruthenium-based catalysts is significantly greater at greater flow rates than Ni-based catalysts (Wysocka et al., 2019). However, the method used to prepare the catalyst and the supporting structure also determines their activity (Horn and Schlögl, 2015).

Noble metal-based catalysts show a better and bigger ratio of hydrogen and CO than metal oxide supported catalysts. For instance, when present in the Ni-based catalyst, palladium disallows the oxidation of catalyst and maintains its reduced state. The bigger the noble metal clusters of the catalyst, the stronger the interaction and, hence, the more stable the catalyst’s overall structure (Mosinska et al., 2020). This was seen in the study by Keiichi Tomishige and coworkers. The authors reported that Ni and Pt catalysts restrict the formation of carbon structures on the catalysts. In another study, Antonio Vita and coworkers studied CeO₂ based catalysts attached to metals like rhodium, platinum, nickel used in autothermal reforming reactions (Vita et al., 2015). Of all the catalysts studied by the authors, catalysts based on CeO₂ showed the least stability, while catalysts based on Ru showed very high methane conversion rates and high hydrogen yields.

Rhodium based catalysts are the most popularly used noble metal-based catalysts for the gasification process. This is because of the enhanced selectivity and cost ineffectiveness. However, these are relatively quite expensive. Regardless, noble metal-based catalysts, including rhodium, palladium, and platinum, among others, have been found to have enhanced activity, stability, and increased resistance to deposition of carbon on the catalysts’ surface (Hakouk et al., 2018). Among the noble metal-based catalysts, platinum has shown the maximum efficiency for the water-splitting reaction. These elements act as electron magnets and decrease the potential for hydrogen production (Wang et al., 2015). When supported on TiO₂, electron transfers occur from TiO₂ to platinum. On the contrary, noble metals like rhodium do not show very efficiency during this reaction, and hence the amount of hydrogen production is lowered (Jana et al., 2010).

Like the metal-based catalysts, the noble metal-based catalysts show better activity and stability than metal catalysts without supporting materials. This was seen in the work put forward by Pudukudy and colleagues (Pudukudy et al., 2018). They developed techniques that could synthesize hydrogen devoid of carbon-based co-products. They used ceria along with platinum-based nickel catalysts. The authors noted that there were no separate peaks in regards to platinum observed. This meant that the addition of the noble metal platinum permitted good dispersion on the catalyst surface. Moreover, the overall activity was noted to be highly efficient in the addition of platinum. Hence, such noble metal-based catalysts are also considered to increase the catalyst’s overall activity. Various kinds of catalysts that are used in hydrogen production by methane pyrolysis are shown in Table 6.

A few years after the first APR activity for hydrogen production was recorded, Erbatur and colleagues then investigated APR using various catalysts. They noted that platinum displayed the best catalytic efficiency and activity during the reaction (Meryemoglu et al., 2010). After platinum, ruthenium, and palladium recorded the highest activity and catalytic efficiency. This could be because of the availability of strong minerals that are essential for hydrolytic activity. Moreover, the mineral acids result in the drainage of the noble metals into the solution. This thus limits the recycling of the catalysts. However, their high expense is the sole disadvantage of using noble metal-based catalysts.

Ruthenium based catalysts have shown five to ten times better activity than other noble metal-based catalysts (Molino et al., 2018). The only drawback is the high rate of deactivation of ruthenium-based catalysts because of the carbon supporting material often used along with this catalyst. During the reaction, carbon is also shown to be utilized, which reduces ruthenium activity. Other elements, like platinum and palladium, have also shown promising catalytic activity. They can overcome the drawbacks posed by nickel-based catalysts. Their only drawback is their expensive availability. Tomishige and coworkers discovered that the order of catalyst activity to be (Tomishige et al., 2004): rhodium > palladium > platinum > nickel  =  ruthenium

Therefore, among most catalysts, noble metal-based catalysts show the maximum activity during coal gasification reaction. Many studies have carried out to understand the catalytic process of the various techniques. Various kinds of catalysts have been used for hydrogen production via the TMD process. Some of these are comprehensively discussed below in Table 7 .