A Review of the Design and Performance of Catalysts for Hydrothermal Gasification of Biomass to Produce Hydrogen-Rich Gas Fuel

Supercritical water gasification has emerged as a promising technology to sustainably convert waste residues into clean gaseous fuels rich in combustible gases such as hydrogen and methane. The composition and yield of gases from hydrothermal gasification depend on process conditions such as temperature, pressure, reaction time, feedstock concentration, and reactor geometry. However, catalysts also play a vital role in enhancing the gasification reactions and selectively altering the composition of gas products. Catalysts can also enhance hydrothermal reforming and cracking of biomass to achieve desired gas yields at moderate temperatures, thereby reducing the energy input of the hydrothermal gasification process. However, due to the complex hydrodynamics of supercritical water, the literature is limited regarding the synthesis, application, and performance of catalysts used in hydrothermal gasification. Hence, this review provides a detailed discussion of different heterogeneous catalysts (e.g., metal oxides and transition metals), homogeneous catalysts (e.g., hydroxides and carbonates), and novel carbonaceous catalysts deployed in hydrothermal gasification. The article also summarizes the advantages, disadvantages, and performance of these catalysts in accelerating specific reactions during hydrothermal gasification of biomass, such as water–gas shift, methanation, hydrogenation, reforming, hydrolysis, cracking, bond cleavage, and depolymerization. Different reaction mechanisms involving a variety of catalysts during the hydrothermal gasification of biomass are outlined. The article also highlights recent advancements with recommendations for catalytic supercritical water gasification of biomass and its model compounds, and it evaluates process viability and feasibility for commercialization.


Introduction
Owing to increased growth in the population as well as urban and industrial development, global energy consumption has witnessed a dramatic rise over the years. Currently, 80% of the global energy demand is met by fossil fuels such as coal, natural gas, gasoline, and diesel. It cannot be denied that fossil fuels have long-term adverse effects on the environment and ecosystems, including global warming, an increase in greenhouse gas emissions, acid rain, and changes in weather patterns, to name a few [1]. On a global scale, CO 2 emissions from the usage of fossil fuels such as coal, crude oil, and natural gas amount to 15, 12, and 8 billion tons, respectively [2]. Gradually phasing away from fossil fuels and seeking alternative and renewable sources of energy are urgently required.
Biofuels produced from renewable sources such as lignocellulosic biomass, livestock manure, microalgae, municipal solid waste, and sewage sludge are desirable alternatives to fossil fuels for meeting future energy demands and reducing carbon emissions [3,4].
to fossil fuels for meeting future energy demands and reducing carbon emissions [3,4]. Hydrogen (H2) has proven to be a clean alternative source to fossil fuels for meeting energy demands because of its zero carbon emissions, higher heating value of 140 MJ/kg, and adiabatic flame temperature of approximately 2100 °C. The combustion products of H2 are water and heat energy, compared to the combustion of fossil fuels which emits greenhouse gases such as CO2, CO, CH4, SOx, and NOx. In addition to being considered the fuel of the future, hydrogen is also utilized in a wide variety of commercial applications such as fuel cells, upgrading crude oil, synthesis of fine chemicals, metallurgy, pharmaceuticals, and the aerospace industry [5]. A main advantage of H2 is its ability to produce clean electricity through fuel cells [6]. H2 is also a valuable precursor in the production of various commodity and specialty chemicals, such as methanol, ammonia, alcohol, and aldehydes, through various catalytic and non-catalytic thermochemical conversion processes [7,8]. H2 is also extensively used by refineries in hydrotreating processes such as hydrodeoxygenation [9], hydrodenitrogenation [10], hydrodesulfurization [10], and hydrodemetallization [11] to upgrade crude oil and bio-oil to transportation-grade fuels. The sustainable nature of H2 and its increasing demand in many industrial and commercial sectors has entrenched it as an integral component of the circular economy.
Although hydrogen gas has no color, its production routes have designated it different colors categorization. Hydrogen can be categorized as brown, grey, blue, green, pink, yellow, turquoise, and white based on its production from a wide variety of sources and technologies ( Figure 1). Based on the source and production technology employed, hydrogen can be classified into different colors such as brown H2 (gasification of coal), grey H2 (steam reforming of methane), blue H2 (steam reforming of methane with carbon capture), green H2 (electrolysis using electricity from renewables), pink H2 (electrolysis using electricity from nuclear energy), turquoise H2 (methane pyrolysis), yellow H2 (electrolysis using electricity from solar power), and white H2 (geological H2 in underground deposits) [12]. Currently, the major route for the synthesis of H2 is the steam reforming of methane, which contributes to approximately 95% of global H2 production [13]. Nearly 250,000 standard cubic feet of CO2 is emitted per 1 million standard cubic feet of H2 produced from the steam reforming of CH4 [14]. Despite the significantly larger carbon footprint of the steam reforming of methane process, it is still commercially applied today. Although not widely commercialized, several sustainable pathways for hydrogen production from Currently, the major route for the synthesis of H 2 is the steam reforming of methane, which contributes to approximately 95% of global H 2 production [13]. Nearly 250,000 standard cubic feet of CO 2 is emitted per 1 million standard cubic feet of H 2 produced from the steam reforming of CH 4 [14]. Despite the significantly larger carbon footprint of the steam reforming of methane process, it is still commercially applied today. Although not widely commercialized, several sustainable pathways for hydrogen production from alternative sources are also available, such as electrolysis, photocatalysis, hydrothermal gasification, dark fermentation, and photo-fermentation [15].
The hydrothermal gasification conversion route is capable of sustainably producing H 2 via renewable lignocellulosic biomass sources. This process utilizes water at either subcritical or supercritical conditions as a green solvent and reaction medium to disintegrate complex organic substrates to gases such as H 2 , CH 4 , CO, and CO 2 [16]. When the temperature and pressure of water exceed its critical points of 374 • C and 22.1 MPa, respectively, supercritical water (SCW) is generated [17]. On the other hand, water is transformed into subcritical water when the temperature and pressure of water are slightly below or near its critical points.
SCW experiences a significant change in its properties compared to liquid water at room conditions, imparting unique properties such as faster kinetics, a non-polar nature, and excellent solubility of gaseous molecules with the absence of interphase transfer boundaries [18,19]. Due to these versatile properties, supercritical water gasification (SCWG) can convert recalcitrant feedstocks with high moisture content into gaseous fuels enriched with combustible gases such as H 2 and CH 4 . SCWG also does not require biomass drying because of its aqueous reaction medium, making the process energy efficient [20]. Due to these advantages, SCWG has recently gained popularity as an environmentally friendly process to produce H 2 from waste feedstocks.
The main products of SCWG are gases (e.g., H 2 , CO, CO 2 , CH 4 , and C 2+ ), hydrochar, and liquid effluents. Hydrochar is a carbon-rich solid product resulting from depolymerization, dehydrogenation, decarboxylation, deamination, and aromatization of the organic feedstock used in SCWG [21]. Further activation and functionalization of hydrochar can enhance its surface area and properties for a wide variety of applications, such as solid fuel, adsorbent, catalyst support, activated carbon, carbon sequestration product, reinforcing material for composites, and soil amendment agent [22][23][24]. The liquid effluents resulting from the hydrothermal decomposition of biomass contain alcohols, furfurals, carboxylic acids, esters, ethers, aliphatics, aldehydes, ketones, and phenolics [25]. Some of these degradation compounds may further polymerize to form tar, which is a challenging component that causes plugging as well as heat and mass transfer limitations in the processors [26]. It should be noted that process conditions such as temperature, reaction time, pressure, and feedstock concentration largely impact the yields and composition of gases, liquids, and hydrochar from the SCWG of biomass [20].
Catalysts also play an important role in improving the process efficiencies of SCWG, especially carbon gasification efficiency and selectivities of gases, by regulating specific reaction mechanisms [27,28]. Several homogeneous and heterogeneous catalysts have been designed and investigated for the SCWG of different biomasses. However, the literature on the application of different catalysts in SCWG appears to be scattered. In addition, indepth knowledge is scarce on understanding the different reaction pathways, mechanisms, and product properties impacted by homogeneous and heterogeneous catalysts in SCWG. Hence, this review paper attempts to categorically summarize the recent advancements in different homogeneous and heterogeneous catalysts used in SCWG. Furthermore, the challenges and shortcomings of different catalysts are also identified, followed by a discussion and recommendations for the effective design of catalysts, catalytic supports, and promoters used in the SCWG of biomass to produce high-value gaseous fuels.

Homogeneous Catalysts Used in Hydrothermal Gasification
Homogeneous catalysts used in SCWG generally consist of alkali metal and hydroxide catalysts. Table 1 summarizes some notable studies on the application of homogeneous catalysts in the SCWG process [29][30][31][32][33][34][35]. Homogeneous catalysts promote water-gas shift reactions by favoring C-C bond breakup, thus improving H 2 yields [36]. The water-gas shift reaction results in the formation of H 2 and CO 2 because of the reaction of CO and H 2 O. The produced H 2 can further react with the reactive intermediates generated by the catalytic action of homogeneous catalysts to increase overall gas yields [31]. Homogeneous catalysts usually have rapid conversion rates and can be used in both batch and continuous reactors. Homogeneous catalysts are also cost-effective with negligible sintering [26]. Nanda et al. [35] Su et al. [37] reported a base-catalyzed mechanism of alkali metals that enabled the water-gas shift reaction. The degradation intermediates were anions comprising hydroxides, carbonates and formates. Mixing the carbonates in water produced CO 2 and hydroxides. Hydroxides can further combine with CO to produce formate. Further decomposition of formaldehyde can generate H 2 . Watanabe et al. [38] reported an ioniccatalyzed mechanism of alkali metals in the SCWG of methanol. They proposed that the ionic species stabilized the methanol by protonation or disassociation. Methanol then oxidized into CO, and protons stabilized the produced CO. CO 2 was formed via the Molecules 2023, 28, 5137 5 of 23 oxidation of CO, and hydroxide ions favored the water-gas shift reaction to convert CO into CO 2 . Thus, the oxidization of CO to CO 2 was enhanced by the hydroxyl ions. Figure 2 represents a simplified catalytic mechanism of potassium metal in the SCWG of biomass [39]. Sınaǧ et al. [32] compared K 2 CO 3 (a homogeneous catalyst) with Raney nickel (a heterogeneous catalyst) in the SCWG of glucose. Their results showed that the catalytic action of K 2 CO 3 enhanced H 2 production while suppressing the formation of phenols for improved gasification efficiency. K 2 CO 3 demonstrated superior catalytic activity as compared to Raney nickel. K 2 CO 3 showed higher yields of H 2 and CO 2 than Raney nickel, which confirmed its catalytic action to promote the water-gas shift reaction. The catalytic mechanism of K 2 CO 3 in enhancing water-gas shift via formate (HCOO − K + ) formation is presented in the following equations. The produced potassium formate further reacts with excess water to generate H 2 with KHCO 3 , which decomposes into CO 2 and K 2 CO 3 .
water-gas shift reaction. The degradation intermediates were anions comprising hydroxides, carbonates and formates. Mixing the carbonates in water produced CO2 and hydroxides. Hydroxides can further combine with CO to produce formate. Further decomposition of formaldehyde can generate H2. Watanabe et al. [38] reported an ionic-catalyzed mechanism of alkali metals in the SCWG of methanol. They proposed that the ionic species stabilized the methanol by protonation or disassociation. Methanol then oxidized into CO, and protons stabilized the produced CO. CO2 was formed via the oxidation of CO, and hydroxide ions favored the water-gas shift reaction to convert CO into CO2. Thus, the oxidization of CO to CO2 was enhanced by the hydroxyl ions. Figure 2 represents a simplified catalytic mechanism of potassium metal in the SCWG of biomass [39]. Sınaǧ et al. [32] compared K2CO3 (a homogeneous catalyst) with Raney nickel (a heterogeneous catalyst) in the SCWG of glucose. Their results showed that the catalytic action of K2CO3 enhanced H2 production while suppressing the formation of phenols for improved gasification efficiency. K2CO3 demonstrated superior catalytic activity as compared to Raney nickel. K2CO3 showed higher yields of H2 and CO2 than Raney nickel, which confirmed its catalytic action to promote the water-gas shift reaction. The catalytic mechanism of K2CO3 in enhancing water-gas shift via formate (HCOO − K + ) formation is presented in the following equations. The produced potassium formate further reacts with excess water to generate H2 with KHCO3, which decomposes into CO2 and K2CO3.
Madenoğlu et al. [30] studied the kinetics effects of K2CO3 in the SCWG of cellulose, lignin, and their mixtures. Their results showed that K2CO3 promoted the rates of gasification reactions and prevented the formation of char. Both gas and aqueous phase yields increased at the expense of char yield due to the catalytic effects of K2CO3. K2CO3 also favored the water-gas shift reaction, thus increasing the H2 yield.
Sınaǧ et al. [40] studied the catalytic effect of K2CO3 on glucose, phyto-mass (plant residues without proteins), and zoo-mass (meat residues containing proteins). The addition of K2CO3 had a significant influence in promoting the water-gas shift reaction during Madenoglu et al. [30] studied the kinetics effects of K 2 CO 3 in the SCWG of cellulose, lignin, and their mixtures. Their results showed that K 2 CO 3 promoted the rates of gasification reactions and prevented the formation of char. Both gas and aqueous phase yields increased at the expense of char yield due to the catalytic effects of K 2 CO 3 . K 2 CO 3 also favored the water-gas shift reaction, thus increasing the H 2 yield.
Sınaǧ et al. [40] studied the catalytic effect of K 2 CO 3 on glucose, phyto-mass (plant residues without proteins), and zoo-mass (meat residues containing proteins). The addition of K 2 CO 3 had a significant influence in promoting the water-gas shift reaction during the SCWG of glucose and enhanced H 2 production. However, its catalytic effects in promoting water-gas shift during the SCWG of phyto-mass and zoo-mass were minimal.
Nanda et al. [35] compared four different homogeneous catalysts, Na 2 CO 3 , K 2 CO 3 , NaOH, and KOH, in the SCWG of Timothy grass. An increase in catalyst loading from 1% to 3% increased the total gas yield, as well as H 2 , CH 4, and CO 2 yields, but decreased the CO yield for all catalysts. This indicated the catalytic action of alkali catalysts promoted gasification efficiency and the water-gas shift reaction. KOH showed the highest H 2 yield of 9 mol/kg, followed by K 2 CO 3 , NaOH, and Na 2 CO 3 . A similar trend was observed for total gas yields. The highest total gas and H 2 yields with KOH were explained by its catalytic action to promote the water-gas shift reaction. On the other hand, NaOH enhanced the methanation reaction with the consumption of H 2, increasing CH 4 yields. Nanda et al. [41] also confirmed the superior catalytic effects of KOH in the SCWG of fructose where KOH showed a higher H 2 yield than NaOH with nearly three times more H 2 yield than non-catalytic reactions.
Yanik et al. [42] compared the activities and selectivities of K 2 CO 3 , Trona, red mud, and Raney nickel catalysts in the SCWG of cotton stalk, corncob, and tannery wastes. Their results showed that all four catalysts significantly enhanced H 2 yields by favoring watergas shift and reforming reactions. K 2 CO 3 demonstrated the highest H 2 yield with no CO detected in the gas products. However, the catalytic activity of Trona was analogous to that of K 2 CO 3 . Ferreira-Pinto et al. [43] investigated the effects of NaOH, KOH, and Na 2 CO 3 catalysts in the SCWG of lactose. The increase in H 2 yield was highest with NaOH, followed by Na 2 CO 3 and KOH. All catalysts inhibited char formation and significantly reduced the total organic carbon content in the reactants, indicating high gasification efficiencies.
Alkali catalysts can also significantly reduce the sulfur content in gas products. High sulfur content in gas products is a serious issue as its combustion can release SO x . Sulfur can also deactivate and poison the catalysts as well as corrode pipelines. Feng et al. [34] used different homogeneous catalysts (e.g., KOH, K 2 CO 3 , NaOH, Na 2 CO 3 , and activated carbon or AC) in the SCWG of sewage sludge. K 2 CO 3 showed the best desulfurization effect and limited the H 2 S and SO 2 contents to around 140 ppm and 200 ppm, respectively. The order of desulfurization effects of catalysts was found to be: K 2 CO 3 > Na 2 CO 3 > NaOH > KOH > AC. KOH demonstrated the highest H 2 yield and selectivity. Alkali catalysts converted the unstable sulfur compounds into stable sulfur compounds by promoting cyclization and oxidation reactions, thus preventing the migration of sulfur into gas and liquid products.
Zhong et al. [44] investigated the catalytic performance of KOH, K 2 CO 3 , KMnO 4 , and H 2 O 2 on polycyclic aromatic hydrocarbons (PAHs) and gas formation during the SCWG of coking sludge. Their results showed that the PAH content decreased in the catalytic SCWG experiments. The catalytic action of KOH was attributed to its ability to promote free radical reactions during SCWG. These free radicals promote ring-opening reactions of PAHs, leading to their decomposition. KOH led to a higher H 2 yield than K 2 CO 3 because of an improved water-gas shift reaction through the formation of a formate intermediate and hydroxyl ions. These hydroxyl ions efficiently capture CO 2 produced from the watergas shift reaction. This shifted the equilibrium of the water-gas shift reaction towards the products side, thus producing more H 2 . Despite the numerous advantages of homogeneous catalysts, they can easily cause reactor plugging and corrosion in the reactor [45]. The recovery of homogeneous catalysts is also difficult compared to that of heterogeneous catalysts, which adds to overall process expenditures [46].

Heterogeneous Catalysts Used in Hydrothermal Gasification
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. A summary of promising studies on the use of heterogeneous catalysts in SCWG is presented in Table 2 [27,[49][50][51][52][53][54][55].

•
The highest carbon gasification efficiency of 99% was achieved with polypropylene followed by high-density polyethylene, low-density polyethylene, and polystyrene.

•
The highest H 2 yield in the non-catalytic run was achieved with low-density polyethylene followed by polystyrene, polypropylene, and high-density polyethylene. • Compared to only using NiO, the bimetallic catalyst with RuO 2 increased the H 2 yield and reduced C 2 -C 4 gas yields.
Onwudili and Williams [53] Molecules 2023, 28, 5137 8 of 23 Nickel-based catalysts are the most widely used heterogeneous catalysts in SCWG because of their high activity compared to other expensive transition metal catalysts. Nibased catalysts require comparatively lower temperatures and promote biomass gasification with higher efficiency. However, Ni-based catalysts can also consume the produced H 2 , CO, and CO 2 due to their high methanation activity, producing CH 4 [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 H 2 selectivities of Ni, Co, Ru, and Cu transition metals on γ-Al 2 O 3 , AC, and ZrO 2 supports. Both 10%Ni/γ-Al 2 O 3 and 10%Ru/Al 2 O 3 demonstrated the highest catalytic activities and H 2 selectivities. The order of activity of the supports for the Ni catalyst was: γ-Al 2 O 3 > ZrO 2 > AC. Due to satisfactory results with 10%Ni/γ-Al 2 O 3 , further enhancement with Na, K, Mg, and Ru promotors was also studied. The addition of the 0.5%K promoter on 10%Ni/γ-Al 2 O 3 significantly increased the H 2 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/Al 2 O 3 , Ru/C, Raney nickel, Ni/hydrotalcite, and Ru/Al 2 O 3 ). The activities of Ni/Al 2 O 3 and Ni/hydrotalcite catalysts for SCWG demonstrated the highest H 2 selectivities. In contrast, Raney nickel showed the lowest H 2 selectivity. Ni/α-Al 2 O 3 and Ni/hydrotalcite also demonstrated low CH 4 yields at high temperatures and longer reaction times. The high H 2 selectivities of Ni/α-Al 2 O 3 and Ni/hydrotalcite were attributed to the lower nickel dispersion and large crystallite sizes of Ni/α-Al 2 O 3 and Ni/hydrotalcite catalysts compared to Raney nickel. The high nickel dispersion of Raney nickel strongly favored C-O bond cleavage compared to Ni/Al 2 O 3 and Ni/hydrotalcite catalysts, thus explaining the low H 2 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-Al 2 O 3 , α-Al 2 O 3 , activated carbon, carbon nanotubes (CNT), hydrotalcite, MgO, SiO 2 , silica gel, TiO 2 , ZrO 2 , and various zeolites in the SCWG of glucose. The 20%Ni/α-Al 2 O 3 catalyst showed the highest H 2 selectivity, and Ni/CNT demonstrated high H 2 yields (17-24 mmol/g) and high stability with maximum carbon gasification efficiency. On the other hand, Ni/MgO demonstrated a better H 2 yield (26 mmol/g) and satisfactory carbon gasification efficiency. Due to its low cost and high stability, the authors further investigated the Ni/α-Al 2 O 3 catalyst by varying Ni loading and using promoters. Tin increased the H 2 selectivity but decreased the catalytic activity, whereas alkali promoters increased the carbon gasification efficiency but decreased the H 2 selectivity. Lu  Onwudili and Williams [53] investigated the catalytic SCWG of various plastic wastes with Ru and Ni catalysts. By increasing RuO 2 loading up to 5 wt% in the SCWG of lowdensity polyethylene, the H 2 yield rose from 1 to 9.9 mol/kg at 450 • C in 1 h. However, the subsequent increase in RuO 2 loading from 5 wt% to 20 wt% decreased the H 2 yield to 4.9 mol/kg while increasing the hydrogen gasification and carbon gasification efficiency. By using a 20 wt% RuO 2 -γ-Al 2 O 3 catalyst, polypropylene produced a high H 2 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 C 2 -C 4 gases. Low-density polyethylene demonstrated the highest H 2 yield, followed by polystyrene, polypropylene, and high-density polyethylene.
Adamu et al. [59] studied Ce-mesoAl 2 O 3 support impregnated with Ni in the SCWG of glucose (Figure 3). Ce-mesoAl 2 O 3 had superior support properties compared to γ-Al 2 O 3 , such as moderate acidity, which helped to reduce coke formation and enabled high metal loading with low agglomeration. The Ni(20)/Ce-Al 2 O 3 catalyst exhibited a very high H 2 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  Okolie et al. [54] performed the SCWG of soybean straw using different Ni-based catalysts, catalyst supports, and promoters. ZrO 2 and Al 2 O 3 proved to be the most effective supports for Ni-based catalysts. Both 10%Ni-ZrO 2 and 10%Ni-Al 2 O 3 demonstrated higher H 2 yields than other catalyst supports (e.g., CNT, SiO 2 /Al 2 O 3 , SiO 2 , and AC). Therefore, the authors further studied the effects of K, Na, and Ce promotors on Ni-based catalysts supported by ZrO 2 and Al 2 O 3 . The 10%Ni-1%Ce/ZrO 2 catalyst demonstrated the highest H 2 yield of 10.9 mmol/g, followed by 10%Ni-1%K/ZrO 2 and 10%Ni-1%Na/ZrO 2 . The relative increment in H 2 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 H 2 yield with the Al 2 O 3 support compared to the K and Ce promotors. The 10%Ni-1%Na/Al 2 O 3 catalyst demonstrated the highest H 2 yield (10.8 mmol/g) compared to 10%Ni-Ce/Al 2 O 3 and 10%Ni-1%K/Al 2 O 3 . The 10%Ni-1%Ce/ZrO 2 catalyst demonstrated an improved H 2 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 La 2 O 3 in promoting the Ni-La 2 O 3 /θ-Al 2 O 3 catalyst in the SCWG of food waste. La enhanced the water-gas shift reaction, resulting in a high H 2 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/Al 2 O 3 with an La promoter can lead to excellent catalytic activity in the SCWG of food waste. Ni/9%La-Al 2 O 3 showed high H 2 and gas yields. La improved the mesoporous structure and increased the dispersion of Ni, which enhanced the water-gas shift reaction and increased the H 2 yield. Ni/9%La-Al 2 O 3 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 H 2 yields compared to supported catalysts. Zn-based catalysts showed higher H 2 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−x Ni x O nanomaterial as a catalyst. They synthesized Mg 1−x Ni x O 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 0.8 Ni 0.2 O catalyst with the highest Ni content demonstrated the highest gas and H 2 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 H 2 production, resulting in high H 2 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 MgNiO 2 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 Al 2 O 3 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-Al 2 O 3 catalysts. Both the Ni/α-Al 2 O 3 and Ni/γ-Al 2 O 3 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/Al 2 O 3 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 coprecipitated 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 (Figure 4). The 2.6%Ni-5.2%Co/2.6%Mg-Al catalyst prepared via the co-precipitation method demonstrated high total gas and H 2 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-Al 2 O 3 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-Al 2 O 3 was also capable of reducing coke and carbon deposition.
lyst 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 Mgpromoted 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 wetimpregnated Ni-Mg-Al catalysts. This was due to the growth of the crystal size of the wetimpregnated 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 (Figure 4). 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., ZrO 2 , Ce-ZrO 2 , Al 2 O 3 , Mg-Al 2 O 3 , CNT and AC) have been reported for the SCWG of biomass [69,70]. SCWS-synthesized Ni/MgO-Al 2 O 3 catalysts demonstrated the highest activities and stability. Despite their increased specific surface areas and pore volumes, SCWS-synthesized Ni/CeO 2 -ZrO 2 catalysts showed no promotional effects when Ce was used. This was because of the low Ni particle dispersion in the Ni/CeO 2 -ZrO 2 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 AlCl 3 combined with Ni, KOH, or K 2 CO 3 catalysts and oxidants (e.g., H 2 O 2 , K 2 S 2 O 8 , and CaO 2 ). AlCl 3 -H 2 O 2 demonstrated the highest gas yields, followed by AlCl 3 -K 2 S 2 O 8 . AlCl 3 combined with Ni, KOH, CaO, or K 2 CO 3 catalysts resulted in low H 2 yields as compared to AlCl 3 alone. However, using K 2 S 2 O 8 or H 2 O 2 alone decreased the H 2 yield. The H 2 yield decreased, and gasification efficiency increased with a rise in the addition of oxidants. Interestingly, AlCl 3 -H 2 O 2 (8:2) showed the highest gas yield, followed by AlCl 3 -K 2 S 2 O 8 (8:2) and AlCl 3 . For the AlCl 3 -catalyzed SCWG of the model compound, glycerol resulted in the highest H 2 yield, followed by guaiacol, glucose, alanine, and humic acid. Al 2 Cl 3 -H 2 O 2 increased the H 2 yield of humic acid by 17% but decreased the H 2 yields of glucose and glycerol by 20% and 12%, respectively, compared to the AlCl 3 catalyst. The authors also proposed a catalytic mechanism in the SCWG of dewatered sewage sludge with an AlCl 3 -H 2 O 2 catalyst. They proposed that AlCl 3 promoted the cleavage of the C-C bond with Al 3 + ions. The Al 3 + 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 H 2 , thus increasing the H 2 yield. H 2 O 2 further enhanced the gasification of benzene-containing monomers by favoring the steam reforming reaction. In the case of sewage sludge, H + generated via Al 3 + deposition further enhanced the ring-opening activity of H 2 O 2 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 H 2 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-Al 2 O 3 -based catalysts, they still suffer from various issues, such as sintering, formation of Ni/Al 2 O 4 complexes, and transformation of the γ-Al 2 O 3 phase to the α-Al 2 O 3 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.

Ruthenium-Based Catalysts
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. Rubased 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 H 2 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/Al 2 O 3 showed the highest metal dispersion compared to Ni-based catalysts. They concluded that 5%Ru/Al 2 O 3 demonstrated a higher H 2 yield than the 5%Ni/Al 2 O 3 catalyst in the SCWG of cellulose and lignin. Therefore, for the same metal loading, Ru-based catalysts had higher H 2 yields than Ni-based catalysts. Nanda et al. [55] compared Ru/Al 2 O 3 with Ni/Si-Al 2 O 3 , K 2 CO 3 , and Na 2 CO 3 catalysts in the SCWG of waste cooking oil. The order of H 2 yield was Ru/Al 2 O 3 > Ni/Si-Al 2 O 3 > K 2 CO 3 > Na 2 CO 3 . The effects of metal loading showed that 5 wt% Ru/Al 2 O 3 resulted in the maximum H 2 yield.
The superior catalytic performance of Ru/Al 2 O 3 catalysts has also been reported in the SCWG of glucose and guaiacol [75,78]. In the SCWG of glucose, the Ru/Al 2 O 3 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 H 2 yields while preventing the formation of char. During the SCWG of guaiacol, Ru/Al 2 O 3 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 H 2 . Thus, Ru/Al 2 O 3 improved H 2 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-Al 2 O 3 . They recommended the use of Ni and Ru bimetallic catalysts supported on γ-Al 2 O 3 in the SCWG of glucose to achieve high activity and H 2 selectivity. Hossain et al. [52] further investigated various bimetallic Ni-Ru/Al 2 O 3 -supported aerogel catalysts. Ni-Ru/Al 2 O 3 aerogel catalysts demonstrated 1.3-and 1.6-times higher H 2 yields than mesoporous and wet-impregnated synthesized Ni-Ru/Al 2 O 3 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 CO 2 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-Al 2 O 3 and ZrO 2 . Overall, Ni-Ru/γ-Al 2 O 3 -ZrO 2 demonstrated the maximum H 2 yields and high carbon gasification efficiency. Ni-Ru/γ-Al 2 O 3 -ZrO 2 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/Al 2 O 3 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 H 2 production due to its lower concentration than phenol.

Other Heterogeneous Catalysts
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/Al 2 O 3 and Ru/Al 2 O 3 catalysts in the SCWG of 2-propanol. Their results showed that the H 2 selectivity of Pt/Al 2 O 3 was relatively higher than that of Ru/Al 2 O 3 due to enhancement of the methanation reaction, which produced CH 4 at the expense of H 2 . Pairojpiriyakul et al. [82] used Co-based catalysts on a variety of supports, such as α-Al 2 O 3 , ZrO 2 , γ-Al 2 O 3 , La 2 O 3 , and yttria-stabilized zirconia (YSZ), in the SCWG of glycerol. The highest H 2 yield was obtained with Co/YSZ. In addition, increasing the Co loading up to 10% improved the gasification efficiency of glycerol and H 2 production. However, a further increase in the Co loading decreased both H 2 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 hightemperature 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
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 RuO 2 , ZrO 2 , and CeO 2 . Cao et al. [ [85,86]. Cao et al. [85] showed that in the SCWG of lignin, the CuO-ZnO catalyst demonstrated high catalytic performance with a high H 2 yield and better gasification efficiency, followed by Fe 2 O 3 -Cr 2 O 3 and CeO 2 -ZrO 2 . However, in the SCWG of cellulose, Fe 2 O 3 -Cr 2 O 3 showed a greater H 2 yield and high carbon gasification efficiency, followed by CuO-ZnO and CeO 2 -ZrO 2 . 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 H 2 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 CeO 2 was the main active component in the CeO 2 -ZrO 2 catalyst [86]. CeO 2 distributed on ZrO 2 released active oxygen via redox reactions to enhance the SCWG process. ZrO 2 also absorbed active H 2 and small intermediates to increase contact between the intermediates and CeO 2 for improved catalytic performance. In CuO-ZnO, Cu was the main active component, which released oxygen species ( Figure 5). 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 RuO 2 /γ-Al 2 O 3 in the SCWG of municipal solid waste. RuO 2 /γ-Al 2 O 3 drastically increased H 2 , CH 4 , and CO 2 yields while significantly improving gasification efficiency. The high yield of H 2 was due to enhancement of the water-gas shift reaction by the catalytic action of RuO 2 /γ-Al 2 O 3 . In addition, the enhancement of methanation of CO or CO 2 and hydrogenolysis of C-C hydrocarbons resulted in a high CH 4 yield. Improvement in the yields of the reduction product (CH 4 ) and oxidation product (CO 2 ) indicated the involvement of the RuO 2 / γ-Al 2 O 3 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 RuO 2 on γ-Al 2 O 3 , which resulted in enhanced carbon gasification efficiency.
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 ( Figure  5). 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.

Novel Carbon-Based Catalysts Used in Hydrothermal Gasification
Carbon-based supports can also be used with transition metals in the SCWG of biomass. Their high surface areas along with the renewable and biodegradable nature of activated carbon and other carbon-based supports make them sustainable catalytic Samiee-Zafarghandi et al. [88] compared MnO 2 /SiO 2 and NiO/SiO 2 catalysts in the SCWG of microalgae Chlorella. MnO 2 /SiO 2 demonstrated the highest H 2 yield (1.1 mmol/g) compared to NiO/SiO 2 (0.6 mmol/g) and non-catalytic SCWG (0.2 mmol/g). Therefore, NiO/SiO 2 was less active than the supported MnO 2 /SiO 2 . Borges et al. [89] investigated the Ni/Fe 2 O 4 catalyst in the SCWG of Eucalyptus wood chips. Ni/Fe 2 O 4 enhanced the H 2 yield and decreased the char yield. Further investigation showed that Ni/Fe 2 O 4 favored the water-gas shift and steam reforming reactions, thus increasing H 2 yield and decreasing CH 4 yield. It also demonstrated good stability and recyclability despite the coke deposit [90].

Novel Carbon-Based Catalysts Used in Hydrothermal Gasification
Carbon-based supports can also be used with transition metals in the SCWG of biomass. Their high surface areas along with the renewable and biodegradable nature of activated carbon and other carbon-based supports make them sustainable catalytic materials. Table 3 summarizes some notable studies on the use of carbon-based catalysts for SCWG processes [65,71,[91][92][93][94]. Taylor et al. [95] compared Ni/AC and Ru/AC with other catalysts such as KOH, Trona, dolomite, and Borax in the SCWG of wood chips. Both Ni/AC and Ru/AC demonstrated higher H 2 yields because of improved water-gas shift compared to other non-carbonaceous catalysts.  Osada et al. [94] Yamaguchi et al. [96] investigated various metals (e.g., Ru, Ni, Pt, Rh and Pd) supported on activated carbon in the SCWG of woody biomass. The Ru/AC catalysts demonstrated the highest gas yields, followed by Rh/AC, Pt/AC, Pd/AC, and Ni/AC. Ru/AC showed the highest activity for lignin gasification. However, it showed an inferior H 2 yield, which was due to enhancement of the methanation reaction, which consumed H 2 . Interestingly, the Pd/AC catalyst demonstrated the highest H 2 yield, followed by Ru/AC, Pt/AC, Rh/AC, and Ni/AC. Thus, Pd/AC showed the best H 2 yield but poor gas yields, whereas Ni/AC showed the lowest gas and H 2 yields. Activated carbon also improved the H 2 yield over a wide range of reaction temperatures.
Osada et al. [94] investigated TiO 2 and activated carbon as supports for Ru catalysts in the SCWG of lignin, cellulose, and sugarcane bagasse. Ru/AC demonstrated the highest gasification efficiency with near-complete gasification of sugarcane bagasse in 15 min. For the same amount of Ru metal, Ru/AC showed slightly higher activity as compared to Ru/TiO 2 catalysts. This was due to the high Ru metal dispersion of 51% in the Ru/AC catalyst as compared to 27% metal dispersion in Ru/TiO 2 . However, the gas yield and composition of both catalysts were the same when 100% carbon conversion was achieved. This indicated that the equilibrium gas yield and composition did not have any correlation with metal dispersion. For the Ru/AC catalysts, repetitive use increased H 2 selectivity but decreased CH 4 selectivity due to disintegration of the active sites for the methanation reaction. However, Ru/AC suffered from deactivation since its activity decreased significantly after repetitive use. Therefore, more active and durable AC-based catalysts need to be developed to overcome these challenges. Yamaguchi et al. [96] reported that Ru/γ-Al 2 O 3 demonstrated high gasification activity but low stability as the crystallographic phase of γ-Al 2 O 3 transformed into α-Al 2 O 3 .
CNT is another carbon-based support that has a large surface area, high heat conductivity, excellent chemical and physical stability, and a tunable porous structure. Among SCWS-prepared metal-impregnated carbon catalysts, CNT-based catalysts showed higher activities and stability than active carbon and Al 2 O 3 supported catalysts [97]. At reaction conditions of 480 • C, 25 MPa, and 10-50 h, Ni/CNT resulted in the highest H 2 , CO, CH 4 , and total gas yields, followed by Ni/AC, Ni/Al 2 O 3 , and Ni catalysts. Ni/CNT maintained its high activity even at a longer reaction time of 50 h, whereas Ni/AC and Ni/Al 2 O 3 significantly dropped their activities after 30 h of use. This was primarily due to the leaching of active Ni metal in the Ni/AC and Ni/Al 2 O 3 catalysts.
Rashidi and Tavasoli [98] evaluated the effects of a copper promoter on Ni/CNT catalysts in the SCWG of sugarcane bagasse. Cu-promoted Ni/CNT was found to increase the H 2 and total gas yields but decreased the CH 4 yield. Thus, Cu-promoted Ni/CNT catalysts overcome the methanation tendency of Ni, which is a major limiting factor of Ni-based catalysts. Azadi et al. [28] reported that Ni-Cu/CNT showed a nearly ten-fold increase in H 2 yield and 40 times less CH 4 yield with a significant reduction in CO 2 yield. Thus, Cu-promoted Ni/CNT catalysts have high H 2 selectivity and low CH 4 and CO 2 selectivities. Li et al. [65] confirmed the high catalytic stability over repeated use of Ni/CNT catalysts in the SCWG of glycerol. de Vlieger et al. [99] also showed the high stability of Pd/CNT catalysts in the SCWG of ethylene glycerol. Pt/CNT exhibited no mass loss with no change in the size and distribution of Pt particles on CNT during SCWG.
Carbonaceous materials such as hydrochar and biochar are also potential materials for the development of catalysts. Safari et al. [91] investigated the performance of catalysts developed from the hydochars of green algae (Cladophora glomerata) and wheat straw in the SCWG of almond shells. The high amounts of alkali and alkaline earth metals in the hydrochar samples enhanced the cracking of biopolymers and favored the water-gas shift reaction, thus increasing the H 2 yield. The total gas yield and H 2 fraction were selectively improved from 26.7 mmol/g and 41% in the non-catalytic run to 29.2 mmol/g and 58%, respectively, when wheat straw hydrochar was used as the catalyst. The total gas yield and H 2 concentration also increased to 31.1 mmol/g and 60%, respectively, when green algae hydrochar was used as the catalyst in the SCWG of almond shells.
Another novel method for catalytic SCWG is the in-situ impregnation of metal nanoparticles in biomass feedstock. This approach can overcome the issue of deactivation encountered by conventional catalysts and help to reduce the cost of catalyst preparation. Nanda et al. [93] carried out the SCWG of pinewood and wheat straw impregnated with Ni-nanoparticles. Ni-impregnated biomasses demonstrated high H 2 , CO 2 , and CH 4 yields compared to the raw feedstocks. Huang et al. [100] also used in-situ-generated Ni particles using nickel acetate as a precursor for the gasification of glucose in SCW. In-situ-generated Ni catalysts demonstrated superior catalytic performance compared to nickel wire catalysts. They also proved the role of in-situ generated Ni particles from nickel acetate in enabling the catalytic production of H 2 during SCWG of glucose.
Kumar and Reddy [101] investigated the SCWG of in-situ Ni-impregnated sugarcane bagasse and lemon peels and compared the results with the Raney nickel catalyst. They used nickel nitrate hexahydrate salt as a precursor for the in-situ generation of nickel nanoparticles. Both Ni-impregnated biomasses demonstrated significantly higher gas yields, H 2 yields, and carbon gasification efficiencies than Raney nickel. Ni-impregnated sugarcane bagasse achieved higher carbon gasification efficiency, gas yield, and H 2 yield than Ni-lemon peel. Kumar and Reddy [92] also performed the SCWG of banana pseudostem using impregnation of Ni, Ru, and Fe metals onto the biomass as the support material. The H 2 yields and gasification efficiencies of the metals were in order of Ni > Ru > Fe. The superior performance of Ni to act as an in-situ nanocatalyst is due to its ability to effectively cleave C-H and C-C bonds for improved reforming reactions [102]. However, very little literature is available on the development of in-situ nanocatalysts impregnated onto biomass for proper assessment of their robustness, stability, regeneration, and postgasification compared to commercially available homogeneous and heterogeneous catalysts. One of the limitations in the design of such novel catalysts can be the presence of lignin and other mineral matter in the biomass [103,104], which can hinder the penetration of catalytic nanoparticles within the cell wall. Therefore, more research is needed for a better understanding of such catalysts and to address these limitations.

Conclusions and Perspectives
SCWG is a promising technology for the sustainable production of H 2 due to its various advantages over other thermochemical processes. SCWG has shown its potential for converting a wide variety of low-value biomasses into high-value H 2 -rich gas products. This can serve as a green alternative to the steam methane reforming process due to the renewable and clean nature of biomass sources compared to fossil fuels. However, SCWG requires high energy input to achieve supercritical conditions. Nonetheless, catalysts are used to achieve high gas yields and process efficiencies even at near-critical conditions.
Various homogeneous and heterogeneous catalysts have been studied to achieve high H 2 yields at low temperatures in SCWG processes. Although homogeneous catalysts are suitable compared to heterogeneous catalysts, they suffer from recovery issues. This also increases the cost of the process and hinders its use in large-scale industrial applications. On the other hand, heterogeneous catalysts are relatively easier to recover, but they can suffer from deactivation. Deactivation of heterogeneous catalysts can occur for various reasons, such as fouling, poising, sintering, and char formation. Transition metal catalysts (e.g., Ni, Cu, Co, and Ru) have demonstrated excellent performance in enhancing SCWG reactions. Ru-and Ni-based catalysts are the most widely used catalysts owing to their superior performance in SCWG processes, especially in water-gas shift, hydrogenation, and methanation reactions. Novel catalysts such as activated carbon, char, CNT, and lignocellulosic biomass impregnated with catalytic nanoparticles have demonstrated promising potential to achieve comparable catalytic performance and renewability in SCWG reactions.
It cannot be denied that SCWG is an innovative and viable technology for producing combustible gases with higher selectivity to individual gas components using catalysts. However, a detailed study of the economic viability and technical feasibility of these catalysts is needed. New developments in the field of catalysts can facilitate the commer-cialization of SCWG technology. Extensive research strategies are required to tackle the unique challenges faced by SCWG technology that prevents its scalability and commercialization. Some of the common challenges are associated with reactor corrosion, plugging due to salt and mineral precipitation, the requirement of special reactor set-up resistant to high temperatures, high pressures, and corrosion, coking of catalyst supports, as well as catalyst poisoning, sintering, and deactivation. The techno-economic, environmental, and lifecycle viability of SCWG technology on a commercial scale is also contingent on the efficient conversion of feedstocks, catalyst recovery, regeneration and reuse, effective separation of gas, liquid, and solid products, as well as upgrading and applications of main products and co-products. Nonetheless, SCWG remains an appealing technology with many benefits in the use of water as a source of aqueous reaction media to valorize complex feedstocks and pollutants under environmentally benign conditions while addressing the issues of waste management and clean energy recovery.