Heterogeneous catalysts applied in the SCWG process can be broadly divided into two categories, namely metal oxides and transition metals. The recovery and recycling of heterogeneous catalysts are relatively easier compared to those of homogeneous catalysts [47]. Heterogeneous catalysts are more active, resulting in efficient and improved gasification efficiency [48]. They are also more selective for specific products by promoting desired reactions.

Nickel-based catalysts are the most widely used heterogeneous catalysts in SCWG because of their high activity compared to other expensive transition metal catalysts. Ni-based catalysts require comparatively lower temperatures and promote biomass gasification with higher efficiency. However, Ni-based catalysts can also consume the produced H₂, CO, and CO₂ due to their high methanation activity, producing CH₄ [56]. Furusawa et al. [57] used the Ni/MgO catalyst in the SCWG of lignin. They studied its regenerative capabilities by recovering and reusing the catalyst thrice. The catalyst showed satisfactory regenerative capability before suffering from deactivation due to the formation of carbon and Mg(OH)₂.

Zhang et al. [58] studied the SCWG of glucose and compared the activities and H₂ selectivities of Ni, Co, Ru, and Cu transition metals on γ-Al₂O₃, AC, and ZrO₂ supports. Both 10%Ni/γ-Al₂O₃ and 10%Ru/Al₂O₃ demonstrated the highest catalytic activities and H₂ selectivities. The order of activity of the supports for the Ni catalyst was: γ-Al₂O₃ > ZrO₂ > AC. Due to satisfactory results with 10%Ni/γ-Al₂O₃, further enhancement with Na, K, Mg, and Ru promotors was also studied. The addition of the 0.5%K promoter on 10%Ni/γ-Al₂O₃ significantly increased the H₂ yield by favoring the water–gas shift reaction.

Azadi et al. [28] studied the SCWG of various lignocellulosic feedstocks (e.g., glucose, fructose, cellulose, pulp, xylan, bark, and lignin) using five transition metals catalysts (e.g., Ni/Al₂O₃, Ru/C, Raney nickel, Ni/hydrotalcite, and Ru/Al₂O₃). The activities of Ni/Al₂O₃ and Ni/hydrotalcite catalysts for SCWG demonstrated the highest H₂ selectivities. In contrast, Raney nickel showed the lowest H₂ selectivity. Ni/α-Al₂O₃ and Ni/hydrotalcite also demonstrated low CH₄ yields at high temperatures and longer reaction times. The high H₂ selectivities of Ni/α-Al₂O₃ and Ni/hydrotalcite were attributed to the lower nickel dispersion and large crystallite sizes of Ni/α-Al₂O₃ and Ni/hydrotalcite catalysts compared to Raney nickel. The high nickel dispersion of Raney nickel strongly favored C–O bond cleavage compared to Ni/Al₂O₃ and Ni/hydrotalcite catalysts, thus explaining the low H₂ selectivity of Raney nickel. The authors also reported that among all feedstocks, lignin was the most resistant to SCWG because of its branched polymeric structure. The lowest gas yield obtained from lignin was attributed to potential deactivation of the catalysts due to its sulfur content.

Azadi et al. [27] compared Ni catalysts on different support materials, including γ-Al₂O₃, α-Al₂O₃, activated carbon, carbon nanotubes (CNT), hydrotalcite, MgO, SiO₂, silica gel, TiO₂, ZrO₂, and various zeolites in the SCWG of glucose. The 20%Ni/α-Al₂O₃ catalyst showed the highest H₂ selectivity, and Ni/CNT demonstrated high H₂ yields (17–24 mmol/g) and high stability with maximum carbon gasification efficiency. On the other hand, Ni/MgO demonstrated a better H₂ yield (26 mmol/g) and satisfactory carbon gasification efficiency. Due to its low cost and high stability, the authors further investigated the Ni/α-Al₂O₃ catalyst by varying Ni loading and using promoters. Tin increased the H₂ selectivity but decreased the catalytic activity, whereas alkali promoters increased the carbon gasification efficiency but decreased the H₂ selectivity. Lu et al. [50] also studied Ni-based catalysts with various promoted Al₂O₃ supports (e.g., CeO₂/Al₂O₃, MgO/Al₂O₃, La₂O₃/Al₂O₃, and ZrO₂/Al₂O₃) in the SCWG of glucose. CeO₂/Al₂O₃ showed the highest H₂ yield, followed by La₂O₃/Al₂O₃, ZrO₂/Al₂O₃, Al₂O₃, and MgO/Al₂O₃.

Onwudili and Williams [53] investigated the catalytic SCWG of various plastic wastes with Ru and Ni catalysts. By increasing RuO₂ loading up to 5 wt% in the SCWG of low-density polyethylene, the H₂ yield rose from 1 to 9.9 mol/kg at 450 °C in 1 h. However, the subsequent increase in RuO₂ loading from 5 wt% to 20 wt% decreased the H₂ yield to 4.9 mol/kg while increasing the hydrogen gasification and carbon gasification efficiency. By using a 20 wt% RuO₂-γ-Al₂O₃ catalyst, polypropylene produced a high H₂ yield and the highest carbon gasification efficiency of 99%. High- and low-density polyethylenes also showed similar gas yields, whereas polystyrene produced the lowest yields of C₂-C₄ gases. Low-density polyethylene demonstrated the highest H₂ yield, followed by polystyrene, polypropylene, and high-density polyethylene.

Adamu et al. [59] studied Ce-mesoAl₂O₃ support impregnated with Ni in the SCWG of glucose. Ce-mesoAl₂O₃ had superior support properties compared to γ-Al₂O₃, such as moderate acidity, which helped to reduce coke formation and enabled high metal loading with low agglomeration. The Ni(20)/Ce-Al₂O₃ catalyst exhibited a very high H₂ yield of 10.2 mol/mol of glucose. The meso-form led to the cracking of large intermediates such as tar compounds. Furthermore, Ce helped to improve the thermal stability of the alumina support.

Lu et al. [51] compared Ni, Cu, and Fe transition metals supported on MgO in the SCWG of wheat straw. The H₂ yields varied with the application of different catalysts in the following order: Ni/MgO > Fe/MgO > Cu/MgO. Due to excellent H₂ selectivity with Ni, the authors explored various supports, such as basic oxides (MgO and ZnO), acidic oxide (Al₂O₃), and amphoteric oxide (ZrO₂). The H₂ selectivities of Ni-supported catalysts varied in the order of Ni/MgO > Ni/ZnO > Ni/ Al₂O₃ > Ni/ZrO. Although the type of support had a minimal effect on H₂ yield, a significant effect was observed on the decrease in CO yield. Basic oxide supports such as MgO and ZnO favored water–gas shift reactions, thus increasing H₂ yields. The acidic support such as Al₂O₃ did not enhance the water–gas shift reaction. Hence, Ni/Al₂O₃ showed nearly double the CO yield as compared to the Ni/ZnO and Ni/MgO catalysts.

Okolie et al. [54] performed the SCWG of soybean straw using different Ni-based catalysts, catalyst supports, and promoters. ZrO₂ and Al₂O₃ proved to be the most effective supports for Ni-based catalysts. Both 10%Ni-ZrO₂ and 10%Ni-Al₂O₃ demonstrated higher H₂ yields than other catalyst supports (e.g., CNT, SiO₂/Al₂O₃, SiO₂, and AC). Therefore, the authors further studied the effects of K, Na, and Ce promotors on Ni-based catalysts supported by ZrO₂ and Al₂O₃. The 10%Ni-1%Ce/ZrO₂ catalyst demonstrated the highest H₂ yield of 10.9 mmol/g, followed by 10%Ni-1%K/ZrO₂ and 10%Ni-1%Na/ZrO₂. The relative increment in H₂ yield and total gas yield without using any promoters was more substantial with the Ce and K promotors than with the Na promotor. However, the Na promotor showed the highest H₂ yield with the Al₂O₃ support compared to the K and Ce promotors. The 10%Ni-1%Na/Al₂O₃ catalyst demonstrated the highest H₂ yield (10.8 mmol/g) compared to 10%Ni-Ce/Al₂O₃ and 10%Ni-1%K/Al₂O₃. The 10%Ni-1%Ce/ZrO₂ catalyst demonstrated an improved H₂ yield and excellent catalytic performance. Further analysis revealed that the Ce promotor could store oxygen species and eliminate coke formation and sintering of the catalysts, resulting in its high performance.

Su et al. [60] investigated the effects of La₂O₃ in promoting the Ni-La₂O₃/θ-Al₂O₃ catalyst in the SCWG of food waste. La enhanced the water–gas shift reaction, resulting in a high H₂ yield. La also inhibited the methanation reaction, which is a major limitation of Ni-based catalysts. La also improved the metal dispersion, which increased the catalytic activity. Chowdhury et al. [61] also reported that Ni/Al₂O₃ with an La promoter can lead to excellent catalytic activity in the SCWG of food waste. Ni/9%La-Al₂O₃ showed high H₂ and gas yields. La improved the mesoporous structure and increased the dispersion of Ni, which enhanced the water–gas shift reaction and increased the H₂ yield. Ni/9%La-Al₂O₃ also demonstrated high stability, which could be attributed to its better anti-carbon deposition property.

Mastuli et al. [62] compared doped and supported Zn and Ni catalysts on MgO support in the SCWG of oil palm frond. The doped catalysts had high surface areas, high stability, and high-activity basic sites, resulting in high H₂ yields compared to supported catalysts. Zn-based catalysts showed higher H₂ yields than Ni-based catalysts for both supported and doped catalysts. Mastuli et al. [63] further investigated the structural and catalytic effects of Mg_1−xNi_xO nanomaterial as a catalyst. They synthesized Mg_1−xNi_xO nanomaterial via a self-propagating combustion method in the SCWG of oil palm frond. As the Ni content increased, the cell volume decreased linearly. This increased the specific surface area and improved the basic properties of the catalyst. The Mg_0.8Ni_0.2O catalyst with the highest Ni content demonstrated the highest gas and H₂ yields.

Li et al. [64] demonstrated that the formation of the char layer could be minimized using co-precipitated Ni-Mg-Al catalysts. They varied the Mg-Al molar ratio in the catalyst and investigated its effects in the SCWG of glucose. The catalysts favored H₂ production, resulting in high H₂ selectivity. Furthermore, Mg inhibited graphitic carbon formation because of its neutralizing action on alumina acidic sites, thus increasing the lifespan of the catalysts. However, the subsequent increase in Mg loading formed the MgNiO₂ complex, which limited the activity of Ni metal.

Li et al. [65] also studied the stability and activities of various wet-impregnated Mg-promoted Ni catalysts on Al₂O₃ and CNT supports in the SCWG of glycerol. The stability studies showed the loss of Al, which resulted in deactivation of the Mg-promoted Ni-Al₂O₃ catalysts. Both the Ni/α-Al₂O₃ and Ni/γ-Al₂O₃ catalysts showed poorer stability and regenerability over repeated use than the Ni/CNT catalyst.

Li and Guo [66] compared the catalytic action of Mg-promoted Ni/Al₂O₃ catalysts synthesized via the co-precipitation and wet impregnation methods for a variety of feedstocks, such as glycerol, cellulose, glucose, poplar leaf, corncob, phenol, and sawdust. The results showed that the co-precipitated Ni-Mg-Al catalysts were more stable than the wet-impregnated Ni-Mg-Al catalysts. This was due to the growth of the crystal size of the wet-impregnated Ni-Mg-Al catalysts in SCW. Among different feedstocks, the co-precipitated Ni-Mg-Al catalysts were more active for the gasification of water-soluble organics as compared to real lignocellulosic biomasses.

Kang et al. [67] explored and proposed a detailed catalytic mechanism of Ni-Co supported on Mg-Al in the SCWG of lignin. The 2.6%Ni-5.2%Co/2.6%Mg-Al catalyst prepared via the co-precipitation method demonstrated high total gas and H₂ yields due to significant improvement in its coke resistance ability. They also concluded that the co-precipitation method was more efficient than the wet-impregnated method. Norouzi et al. [68] showed that the addition of Ru on Fe-Ni/γ-Al₂O₃ could enhance gas yields while minimizing char formation. Another study by Lu et al. [50] showed that the addition of the Ce promoter on Ni/γ-Al₂O₃ was also capable of reducing coke and carbon deposition.

Catalysts synthesized in SCW have demonstrated high stability through their ability to reduce sintering. The supercritical water synthesis (SCWS) method for catalyst design provides better control over the size and shape of the nanoparticle without any requirement for organic solvents or precipitants. A few studies on SCWS synthesis of Ni-based catalysts on various supports (e.g., ZrO₂, Ce-ZrO₂, Al₂O₃, Mg-Al₂O₃, CNT and AC) have been reported for the SCWG of biomass [69,70]. SCWS-synthesized Ni/MgO-Al₂O₃ catalysts demonstrated the highest activities and stability. Despite their increased specific surface areas and pore volumes, SCWS-synthesized Ni/CeO₂-ZrO₂ catalysts showed no promotional effects when Ce was used. This was because of the low Ni particle dispersion in the Ni/CeO₂-ZrO₂ catalysts. However, as compared to sol-gel prepared catalysts, which have bigger bulk NiO particles, the SCWS-synthesized catalysts showed high dispersion and stable crystalline structures. After multiple use cycles, the SCWS-synthesized catalysts retained their high dispersion, whereas sol-gel-prepared catalysts experienced growth in size. This allowed the SCWS-prepared catalysts to maintain their high activities over repeated use, as opposed to catalysts prepared using conventional methods that may lose their activity over repeated use. Additionally, SCWS-synthesized catalysts are also synthesized in an environmentally friendly way as they do not require any organic solvents or robust chemical compounds.

Li et al. [71] studied and proposed a catalytic mechanism in the SCWG of dewatered sewage sludge and various model compounds using AlCl₃ combined with Ni, KOH, or K₂CO₃ catalysts and oxidants (e.g., H₂O₂, K₂S₂O₈, and CaO₂). AlCl₃-H₂O₂ demonstrated the highest gas yields, followed by AlCl₃-K₂S₂O₈. AlCl₃ combined with Ni, KOH, CaO, or K₂CO₃ catalysts resulted in low H₂ yields as compared to AlCl₃ alone. However, using K₂S₂O₈ or H₂O₂ alone decreased the H₂ yield. The H₂ yield decreased, and gasification efficiency increased with a rise in the addition of oxidants. Interestingly, AlCl₃-H₂O₂ (8:2) showed the highest gas yield, followed by AlCl₃-K₂S₂O₈ (8:2) and AlCl₃. For the AlCl₃-catalyzed SCWG of the model compound, glycerol resulted in the highest H₂ yield, followed by guaiacol, glucose, alanine, and humic acid. Al₂Cl₃-H₂O₂ increased the H₂ yield of humic acid by 17% but decreased the H₂ yields of glucose and glycerol by 20% and 12%, respectively, compared to the AlCl₃ catalyst. The authors also proposed a catalytic mechanism in the SCWG of dewatered sewage sludge with an AlCl₃-H₂O₂ catalyst. They proposed that AlCl₃ promoted the cleavage of the C–C bond with Al₃⁺ ions. The Al₃⁺ ions increased the acidity of SCW by reacting with water and forming Al(OH)₃ and H⁺ ions. Al(OH)₃ further underwent dehydration to form AlO(OH), which formed precipitates in water. The H⁺ and Cl⁻ ions enhanced the gasification of intermediate compounds to produce H₂, thus increasing the H₂ yield. H₂O₂ further enhanced the gasification of benzene-containing monomers by favoring the steam reforming reaction. In the case of sewage sludge, H⁺ generated via Al₃⁺ deposition further enhanced the ring-opening activity of H₂O₂ to promote the decomposition of benzene-containing monomers into small molecules. These small organic molecules were further gasified by the combined catalytic effects of Cl⁻ and H⁺ ions to increase H₂ yields.

Although Ni-based catalysts demonstrate improvement in gasification efficiency, they suffer from deactivation mainly because of tar formation and coke deposition [72]. Despite the high activity of Ni/γ-Al₂O₃-based catalysts, they still suffer from various issues, such as sintering, formation of Ni/Al₂O₄ complexes, and transformation of the γ-Al₂O₃ phase to the α-Al₂O₃ phase. These issues significantly hamper the catalysts’ stability. This is a severe issue for alumina-supported catalysts due to the ready conversion of intermediate products adsorbed on the acidic site into carbon, which deactivates Ni-based catalysts. The addition of alkali promoters can suppress cracking and polymerization reactions. Alkali promoters can also neutralize the acidic sites of alumina supports. Thus, alkali promotors can significantly reduce carbon formation.

Ru-based catalysts with promising metal dispersion are more reactive at low temperatures than Ni-based catalysts [73]. Ru-based catalysts have higher surface areas and distribution than Ni-based catalysts. Therefore, high surface area and more metal distribution can be achieved with relatively low Ru metal loading on the support material. Nguyen et al. [74] also confirmed that Ru-based catalysts show higher catalytic activities per metallic mass than Ni-based catalysts. Additionally, Ru-based catalysts are highly resistant to oxidation and hydrothermal conditions compared to Ni-based catalysts. Ru-based catalysts have higher activities toward hydrogenation and C–C bond cleavage [75]. When compared to other expensive transition metals, Ru-based catalysts exhibit the highest activity and H₂ selectivity.

As opposed to Ni-based catalysts, Ru-based catalysts are more susceptible to deactivation by sulfur poisoning [76]. To overcome sulfur sintering, a sacrificial agent with a relatively high affinity towards sulfur can be used to protect Ru from sulfur sintering. Peng et al. [77] used ZnO as a sacrificial agent with Ru/C catalysts to study the SCWG of microalgae (Chlorella vulgaris). ZnO showed high mechanical stability and sulfur adoption performance, which minimized Ru metal sintering. Despite Ru-based catalysts having high surface areas, high dispersion, and high catalytic performance, the relatively low cost of Ni-based catalysts makes them preferable for large-scale industrial applications over Ru-based catalysts.

Kang et al. [29] also observed that Ru/Al₂O₃ showed the highest metal dispersion compared to Ni-based catalysts. They concluded that 5%Ru/Al₂O₃ demonstrated a higher H₂ yield than the 5%Ni/Al₂O₃ catalyst in the SCWG of cellulose and lignin. Therefore, for the same metal loading, Ru-based catalysts had higher H₂ yields than Ni-based catalysts. Nanda et al. [55] compared Ru/Al₂O₃ with Ni/Si-Al₂O₃, K₂CO₃, and Na₂CO₃ catalysts in the SCWG of waste cooking oil. The order of H₂ yield was Ru/Al₂O₃ > Ni/Si-Al₂O₃ > K₂CO₃ > Na₂CO₃. The effects of metal loading showed that 5 wt% Ru/Al₂O₃ resulted in the maximum H₂ yield.

The superior catalytic performance of Ru/Al₂O₃ catalysts has also been reported in the SCWG of glucose and guaiacol [75,78]. In the SCWG of glucose, the Ru/Al₂O₃ catalyst inhibited the production of furfural and 5-hydroxymethylfurfural while favoring the degradation of intermediates such as phenols, ketones, acids, and arenes [75]. Enhanced gasification of intermediates improved process efficiency and increased total gas and H₂ yields while preventing the formation of char. During the SCWG of guaiacol, Ru/Al₂O₃ catalysts enhanced the conversion of phenol to cyclohexanol by favoring the hydrogenation reaction and the conversion of cyclohexanol to hexanone or hexenol by favoring ring-opening reactions [78]. Hexanone and hexenol can further decompose into small gaseous molecules, including H₂. Thus, Ru/Al₂O₃ improved H₂ and total gas yields while minimizing char and tar formation.

Zhang et al. [58] observed the effects of Ni and Ru bimetallic catalysts supported on γ-Al₂O₃. They recommended the use of Ni and Ru bimetallic catalysts supported on γ-Al₂O₃ in the SCWG of glucose to achieve high activity and H₂ selectivity. Hossain et al. [52] further investigated various bimetallic Ni-Ru/Al₂O₃-supported aerogel catalysts. Ni-Ru/Al₂O₃ aerogel catalysts demonstrated 1.3- and 1.6-times higher H₂ yields than mesoporous and wet-impregnated synthesized Ni-Ru/Al₂O₃ catalysts for the same amount of metal loading. The aerogel catalysts showed high and uniform metal particle dispersion with strong interaction between the support and active metal. The high catalytic performance of the aerogel catalysts was due to the supercritical CO₂ drying step during aerogel catalyst synthesis, which improved the surface area and reactant diffusivity. A significant decrease in coke formation was also observed with the aerogel catalysts due to their low acidity. This resulted in high stability and activities of the aerogel catalysts.

Tushar et al. [79] confirmed the catalytic effects of Ni and Ru catalysts. They investigated ten different combinations of Ni and Ru catalysts on various supports, such as γ-Al₂O₃ and ZrO₂. Overall, Ni-Ru/γ-Al₂O₃-ZrO₂ demonstrated the maximum H₂ yields and high carbon gasification efficiency. Ni-Ru/γ-Al₂O₃-ZrO₂ also demonstrated high stability and activities over repeated use. In another study, dual-component catalysts having equal amounts of Ru/C-Ru/C demonstrated better catalytic activities than single-component catalysts [80].

Yang et al. [81] investigated the kinetics and intermediate products of Ni-Ru/Al₂O₃ bimetallic catalysts for the SCWG of phenol. They proposed that phenol converted into an enol intermediate via a partial hydrogenation reaction. Furthermore, enol rapidly formed cyclohexanone. This observation was different from the mechanism proposed by Zhu et al. [78] where cyclohexanone was considered as an intermediate product for the formation of cyclohexanol. The kinetic study revealed that phenol was more difficult to gasify than the intermediate compounds. Interestingly, steam reforming of cyclohexanone was not the main contributor to H₂ production due to its lower concentration than phenol.

Apart from Ni and Ru, other transition metals such as Pt, Co, and Rh (supported or unsupported) are also used as heterogeneous catalysts in the SCWG process. Karakuş et al. [49] investigated Pt/Al₂O₃ and Ru/Al₂O₃ catalysts in the SCWG of 2-propanol. Their results showed that the H₂ selectivity of Pt/Al₂O₃ was relatively higher than that of Ru/Al₂O₃ due to enhancement of the methanation reaction, which produced CH₄ at the expense of H₂. Pairojpiriyakul et al. [82] used Co-based catalysts on a variety of supports, such as α-Al₂O₃, ZrO₂, γ-Al₂O₃, La₂O₃, and yttria-stabilized zirconia (YSZ), in the SCWG of glycerol. The highest H₂ yield was obtained with Co/YSZ. In addition, increasing the Co loading up to 10% improved the gasification efficiency of glycerol and H₂ production. However, a further increase in the Co loading decreased both H₂ yield and glycerol conversion.

Deactivation, sintering, and poisoning of heterogeneous catalysts by sulfur or coke is still a major challenge. Additionally, heterogeneous catalysts oxidize the elemental sulfur and chlorine in biomass to acids. Retention of these acids in the liquid products of SCWG poses a serious challenge for its disposal and/or recycling. The non-polar nature of SCW dissolves the organic compounds during hydrothermal gasification but the inorganic components, including the active metal (catalyst) and mineral matter (catalyst support), can precipitate and form agglomerates in the reactor if not removed properly. The gradual deposition of these precipitates and agglomerates can corrode the reactor during high-temperature and high-pressure operations [83]. Nevertheless, more advancements are needed to address these challenges to synthesize suitable heterogeneous catalysts with high activity, regenerability, and stability, with resistance to sintering and deactivation.

Metal oxide catalysts are rarely used in the SCWG process and very little literature is available on their catalytic performance in SCWG processes. They are generally used as supports to improve the stability and activities of metal-supported catalysts. The most common metal oxides used in SCWG processes are RuO₂, ZrO₂, and CeO₂. Cao et al. [84] compared different metal oxides catalysts such as V₂O₅, MnO₂, Cr₂O₃, Fe₂O₃, CuO, Co₂O₃, ZnO, MoO₃, ZrO₂, SnO₂, CeO₂, and WO₃ in SCWG of glucose. Among all metal oxide catalysts, Cr₂O₃, CuO, and WO₃ showed high gasification efficiencies compared to Fe₂O₃, ZnO, and ZrO₂. The H₂ yields decreased with almost all metal oxide catalysts, except Cr₂O₃, which improved the H₂ yield.

Various co-precipitated binary metal oxide catalysts, such as CeO₂-ZrO₂, CuO-ZnO, and Fe₂O₃-Cr₂O₃, have demonstrated high catalytic performance in SCWG [85,86]. Cao et al. [85] showed that in the SCWG of lignin, the CuO-ZnO catalyst demonstrated high catalytic performance with a high H₂ yield and better gasification efficiency, followed by Fe₂O₃-Cr₂O₃ and CeO₂-ZrO₂. However, in the SCWG of cellulose, Fe₂O₃-Cr₂O₃ showed a greater H₂ yield and high carbon gasification efficiency, followed by CuO-ZnO and CeO₂-ZrO₂. This was due to the higher oxygen content of cellulose compared to lignin. Thus, oxygen released by metal oxide catalysts had less pronounced effects in the SCWG of cellulose. Additionally, the H₂ yield from cellulose was less than that from lignin, which also decreased the reducibility of the reaction medium. The catalytic mechanism of binary metal oxide catalysts showed that CeO₂ was the main active component in the CeO₂-ZrO₂ catalyst [86]. CeO₂ distributed on ZrO₂ released active oxygen via redox reactions to enhance the SCWG process. ZrO₂ also absorbed active H₂ and small intermediates to increase contact between the intermediates and CeO₂ for improved catalytic performance. In CuO-ZnO, Cu was the main active component, which released oxygen species. ZnO acted as a structural stabilizer, promotor and absorbent for sulfur in the CuO-ZnO supported catalyst.

Onwudili [87] studied the detailed catalytic mechanism of RuO₂/γ-Al₂O₃ in the SCWG of municipal solid waste. RuO₂/γ-Al₂O₃ drastically increased H₂, CH₄, and CO₂ yields while significantly improving gasification efficiency. The high yield of H₂ was due to enhancement of the water–gas shift reaction by the catalytic action of RuO₂/γ-Al₂O₃. In addition, the enhancement of methanation of CO or CO₂ and hydrogenolysis of C–C hydrocarbons resulted in a high CH₄ yield. Improvement in the yields of the reduction product (CH₄) and oxidation product (CO₂) indicated the involvement of the RuO₂/γ-Al₂O₃ catalyst in Ru(IV) and Ru(0) cyclic redox reactions. Reduction of Ru(IV) into Ru(0) was essential for the SCWG process, whereas oxidation of Ru(0) into Ru(IV) was necessary for the catalytic process. The primary synergetic effects were due to the improvement of the dispersion of RuO₂ on γ-Al₂O₃, which resulted in enhanced carbon gasification efficiency.

Samiee-Zafarghandi et al. [88] compared MnO₂/SiO₂ and NiO/SiO₂ catalysts in the SCWG of microalgae Chlorella. MnO₂/SiO₂ demonstrated the highest H₂ yield (1.1 mmol/g) compared to NiO/SiO₂ (0.6 mmol/g) and non-catalytic SCWG (0.2 mmol/g). Therefore, NiO/SiO₂ was less active than the supported MnO₂/SiO₂. Borges et al. [89] investigated the Ni/Fe₂O₄ catalyst in the SCWG of Eucalyptus wood chips. Ni/Fe₂O₄ enhanced the H₂ yield and decreased the char yield. Further investigation showed that Ni/Fe₂O₄ favored the water–gas shift and steam reforming reactions, thus increasing H₂ yield and decreasing CH₄ yield. It also demonstrated good stability and recyclability despite the coke deposit [90].