A Review on Catalysts Development for Steam Reforming of Biodiesel Derived Glycerol; Promoters and Supports

: In the last decades, environmental crises and increasing energy demand have motivated researchers to investigate the practical techniques for the production of clean fuels through renewable energy resources. It is essential to develop technologies to utilize glycerol as a byproduct derived from biodiesel. Glycerol is known as a sustainable and clean source of energy, which can be an alternative resource for the production of value-added chemicals and hydrogen. The hydrogen production via steam reforming (SR) of glycerol using Ni-based catalysts is one of the promising approaches for the entry of the hydrogen economy. The purpose of this review paper is to highlight the recent trends in hydrogen production over Ni-based catalysts using the SR of glycerol. The intrinsic ability of Ni to disperse easily over variable supports makes it a more viable active phase for the SR catalysts. The optimal reaction conditions have been indicated as 650–900 ◦ C, 1 bar, and 15 wt% Ni in catalysts for high glycerol conversion. In this review paper, the e ﬀ ects of various supports, di ﬀ erent promoters (K, Ca, Sr, Ce, La, Cr, Fe), and process conditions on the catalytic performance have been summarized and discussed to provide a better comparison for the future works. It was found that Ce, Mg, and La have a signiﬁcant e ﬀ ect on catalytic performance as promoters. Moreover, SR of glycerol over hydrotalcite and perovskite-based catalysts have been reviewed as they suggest high catalytic performance in SR of glycerol with improved thermal stability and coke resistance. More speciﬁcally, the Ni / LaNi 0.9 Cu 0.1 O 3 synthesized using perovskite-type supports has shown high glycerol conversion and su ﬃ cient hydrogen selectivity at low temperatures. On the other hand, hydrotalcite-like catalysts have shown higher catalytic stability due to high thermal stability and low coke formation. It is vital to notice that the primary concern is developing a high-performance catalyst to utilize crude glycerol e ﬃ ciently.


Introduction
Energy demand is inevitably increasing referring the population growth and considering environmental issues such as climate change and air pollution, development of novel technologies for the production of clean fuels is inventible and rapidly increasing. In this regard, hydrogen is a potential clean Energy demand is inevitably increasing referring the population growth and considering environmental issues such as climate change and air pollution, development of novel technologies for the production of clean fuels is inventible and rapidly increasing. In this regard, hydrogen is a potential clean fuel with high energy density and no CO2 emission when burning [1][2][3]. Furthermore, hydrogen can be used for the production of value-added chemicals such as ammonia, methanol, polyurethane, formic acid, hydrochloric acid, formaldehyde, urea, etc.
This wide demand for hydrogen prompts the development of facile technologies for the production of hydrogen as a key clean component used in many industries [4][5][6]. Extensive studies are undergoing to implement the hydrogen economy with commercialization vision to achieve a sustainable process. Hydrogen is conventionally produced via the steam reforming (SR) of methane [7]. The SR of methane generally is carried out at a temperature around 800 to 1000 °C, and the main process additionally includes water gas shift reactors and pressure swing adsorption (PSA) for hydrogen purification. The major disadvantages of this process would be CO2 emission, PSA costs, consumption of fossil fuels, and high capital costs [8][9][10][11]. Even though hydrogen is a clean fuel, many researchers have still endeavored to find a clean feedstock to substitute fossil fuels for the production of hydrogen. Biodiesel as a fuel owns excellent features such as renewability, lower sulfur content and non-toxicity, biodegradability, and high cetane number, which these properties can serve as a petroleum substitute with increasing market demands in recent years [12]. Crude glycerol is the major by-product (by 10 wt%) in biodiesel production, whereas the transesterification of triglycerides, including animal fats, vegetables, and seed oils, is carried out [13][14][15]. Therefore, the hydrogen production using glycerol is suggested as a sustainable process by which it can utilize the crude glycerol through various pathways such as partial oxidation, steam reforming, and autothermal reforming [10]. Figure 1 illustrates the catalytic reaction of triglycerides with ethanol, whereas glycerol and ethyl esters of fatty acids are produced. Glycerol is mainly used in the pharmaceutical, food industries, and production of various chemicals such as lactic acid, acrolein, citric acid, 1,3-propanediol, antifreeze, and hydrogen [16,17]. Furthermore, the application of such sustainable and cheap resources is more important when we notice that based on the environmental policy in the European Union, biodiesel's use has been considered to grow drastically in the early future [18]. As shown in Figure 2, by implementing different processes, a variety of products can be obtained through the conversion of glycerol. Glycerol is also known as a key chemical building block, and it can be used as a feed for the production of hydrogen via SR of glycerol which is biochemical conversion [19,20].   The hydrogen produced by SR of glycerol is highly dependent on the catalyst, and process conditions. An appropriate catalyst must cleave the C-H, C-C and C-O bonds. Numerous catalysts, including metals: Pt, Rh, Re, Pd, Ru, Ir, Co, Cu, and Ni have been studied [21][22][23]. However, bearing in mind the great costs and short availability of noble metals, practically in the point of commercialization of industrial processes, the development of Ni-based catalysts is more advantageous [24,25]. The main purpose of this paper is to review the evaluation of Ni-based alumina catalysts in the SR of glycerol for the efficient production of hydrogen with an eye on the perovskitetype and hydrotalcite based catalysts.

Perspective of Catalysts
Generally, the catalyst composition regarding the SR of glycerol typically consists of transition metals such as nickel (Ni) or noble metals such as platinum (Pt), ruthenium (Ru), and palladium (Pd), supported on alumina or perovskite-type catalysts that are doped with promoters to prevent the coke formation. The high costs of the noble metals shifted researchers to substitute them with the low-cost and available metals such as Ni [26]. Considering the SR of glycerol, C-C, C-H and O-H bond cleavages with conserving the C-O bonds are essentially important [27]. The hydrocarbon (C-C, O-H, and C-H) bonds can easily break down in the presence of Ni, with the latter also capable of enhancing the water-gas shift reaction (WGS). Using supports such as aluminum oxide (Al2O3) can lead to improving the metal diffusion, obtaining appropriate acid-base sites, and consequently decreasing the coke deposition on the surface of the catalyst. Al2O3 is a metal oxide with proper thermal stability and specific surface area [28]. In recent decades, perovskite-type catalysts because of their special crystal structure are more attractive to researchers who focused on the hydrogen economy [29].
The catalytic application of hydrotalcite-like compounds and their derivatives have received extensive attention in the academic and industrial researches. Hydrotalcite-like (HTL) materials are double layered anionic clays with a 2D nanostructure considering the packed arrangement of OH groups where weak bonding between interlayer anions and structural sheets initiates the ion exchange feature and its physicochemical properties influenced by these anions. Its chemical formula is Mg6Al2(OH)16CO34H2O, and the double hydroxides are layered. Hydrotalcites usually exist in nature with different forms, such as foliated, contorted plates or fibrous masses [30][31][32].

Steam Reforming of Glycerol
In recent years, the SR of glycerol as a process to utilize the crude glycerol obtained from biodiesel production plants has attracted many researchers. The main objective is to produce hydrogen from a renewable biomass resource and furthermore making biodiesel with more economical benefits [29,33]. In this review paper, we focused on the SR of glycerol for the efficient production of hydrogen. The SR of glycerol Equation (1), includes glycerol decomposition Equation (2) and water-gas shift reaction Equation (3): The hydrogen produced by SR of glycerol is highly dependent on the catalyst, and process conditions. An appropriate catalyst must cleave the C-H, C-C and C-O bonds. Numerous catalysts, including metals: Pt, Rh, Re, Pd, Ru, Ir, Co, Cu, and Ni have been studied [21][22][23]. However, bearing in mind the great costs and short availability of noble metals, practically in the point of commercialization of industrial processes, the development of Ni-based catalysts is more advantageous [24,25]. The main purpose of this paper is to review the evaluation of Ni-based alumina catalysts in the SR of glycerol for the efficient production of hydrogen with an eye on the perovskite-type and hydrotalcite based catalysts.

Perspective of Catalysts
Generally, the catalyst composition regarding the SR of glycerol typically consists of transition metals such as nickel (Ni) or noble metals such as platinum (Pt), ruthenium (Ru), and palladium (Pd), supported on alumina or perovskite-type catalysts that are doped with promoters to prevent the coke formation. The high costs of the noble metals shifted researchers to substitute them with the low-cost and available metals such as Ni [26]. Considering the SR of glycerol, C-C, C-H and O-H bond cleavages with conserving the C-O bonds are essentially important [27]. The hydrocarbon (C-C, O-H, and C-H) bonds can easily break down in the presence of Ni, with the latter also capable of enhancing the water-gas shift reaction (WGS). Using supports such as aluminum oxide (Al 2 O 3 ) can lead to improving the metal diffusion, obtaining appropriate acid-base sites, and consequently decreasing the coke deposition on the surface of the catalyst. Al 2 O 3 is a metal oxide with proper thermal stability and specific surface area [28]. In recent decades, perovskite-type catalysts because of their special crystal structure are more attractive to researchers who focused on the hydrogen economy [29].
The catalytic application of hydrotalcite-like compounds and their derivatives have received extensive attention in the academic and industrial researches. Hydrotalcite-like (HTL) materials are double layered anionic clays with a 2D nanostructure considering the packed arrangement of OH groups where weak bonding between interlayer anions and structural sheets initiates the ion exchange feature and its physicochemical properties influenced by these anions. Its chemical formula is Mg 6 A l2 (OH) 16 CO 3 4H 2 O, and the double hydroxides are layered. Hydrotalcites usually exist in nature with different forms, such as foliated, contorted plates or fibrous masses [30][31][32].

Steam Reforming of Glycerol
In recent years, the SR of glycerol as a process to utilize the crude glycerol obtained from biodiesel production plants has attracted many researchers. The main objective is to produce hydrogen from a renewable biomass resource and furthermore making biodiesel with more economical benefits [29,33]. In this review paper, we focused on the SR of glycerol for the efficient production of hydrogen. The SR of glycerol Equation (1), includes glycerol decomposition Equation (2) and water-gas shift reaction Equation (3): Catalysts 2020, 10, 910 4 of 22 The glycerol decomposition: Water-gas shift reaction (WGS): The SR of glycerol may include a couple of secondary reactions, for instance, the methanation, Equations (4) and (5), methane dry reforming, Equation (6), and coke formation, Equations (7)-(10) [34,35]: It is crucial to notice that in the decomposition of glycerol, methane can be formed as an intermediate. Therefore, a high-performance catalyst which can perform both SR of methane and WGS reaction is needed to produce syngas and convert CO to CO 2 , respectively [36].
Coke deposition leads to the catalyst deactivation and is considered as one of the main issues in the SR of glycerol. Therefore, many researchers have investigated developing a durable catalyst and enhanced reaction conditions. In this regard, it was found that the reaction conditions for SR of glycerol generally would be as follows: 700 • C, 1 bar, steam to carbon molar ratio of H 2 O/C 3 H 8 O 3 = 9 to 12. It must be noted that the operation at such a high temperature would be tremendously difficult because the glycerol oxygen content is mildly high, and it causes lower thermal stability [37,38]. This fact implies the importance of implementing an efficient catalyst bearing the sintering and avoiding coke formation in the process. In other words, in the catalytic SR of glycerol, the development of a high-performance catalyst is an important factor for the commercialization of this process [39].
Many researchers have studied the effect of synthesis methods of the catalyst with modified supports (MgO, CeO 2 , Al 2 O 3 , and TiO 2 ) and promoters (using various transition metals including Co, Cu, Zr, Ce, Rh, Ru, Pt and Pd, and Fe) to obtain high yields [40][41][42]. Ming et al. [43] reported that when using bare alumina as a catalyst, the hydrogen yield was 39%. However, when the catalyst was modified as Ni/Al 2 O 3 , Co/Al 2 O 3 , and La/Al 2 O 3 , these yields reached 47.7, 43.8, and 54.5%, respectively. The Ni/La/Co/Al 2 O 3 catalyst showed the highest hydrogen yield, 77.7%. Kousi et al. [44] stated that the enhanced activity and high hydrogen yield in the SR of glycerol could be achieved by introducing La 2 O 3 as a promoter to the Ni/Al 2 O 3 catalyst. Table 1 listed a summary of process conditions for the SR of glycerol over Ni-based catalysts. * Attapulgite.

The Effect of Support in SR of Glycerol over Ni-Based Catalysts
The effect of support was discussed concerning the glycerol conversion and H 2 selectivity in the SR of glycerol using various Ni-based catalysts. The interaction of the metal with support is a crucial parameter that influences the catalytic activity and the coke formation.
In order to decrease the coke deposition and improve catalytic activity, MgO, CeO 2 , and TiO 2 have been used as supports that provide sufficient interaction with metals [60]. Buffoni et al. [61] investigated the effect of ZrO 2 and CeO 2 addition to the commercial α-Al 2 O 3 . According to Figure 3, the Ni/CeO 2 /Al 2 O 3 catalyst exhibited better catalytic stability with glycerol conversion of 90% and an H 2 yield of 86%. The role of Ce as an inhibitor to prevent the secondary dehydration reactions was proved.  Moon et al. [62] investigated Ni catalysts supported on Al2O3-SiO2 in terms of Ni content and the role of the support, SiO2 (0, 5, 70 wt%) The result has shown that 15% of Ni is the optimum amount of nickel for high glycerol conversion ( Table 2). It was found that Ni/SiO2(70)-Al2O3 catalyst with a high surface area enhanced the catalytic activity. This could be due to the ability of this catalyst to reduce NiO to Ni easily and prevent the formation of NiAl2O4.  Moon et al. [62] investigated Ni catalysts supported on Al 2 O 3 -SiO 2 in terms of Ni content and the role of the support, SiO 2 (0, 5, 70 wt%) The result has shown that 15% of Ni is the optimum amount of nickel for high glycerol conversion (Table 2). It was found that Ni/SiO 2 (70)-Al 2 O 3 catalyst with a high surface area enhanced the catalytic activity. This could be due to the ability of this catalyst to reduce NiO to Ni easily and prevent the formation of NiAl 2 O 4 . In another research, Menezes et al. [63] studied the Ni catalysts supported on alumina, niobia, and the combination of these two supports as 10 wt% Ni supported on niobia/alumina. Niobia plays a crucial role in improving the Ni dispersion and consequently decreasres the coke deposition. Figure 4 shows the catalytic performance results in the SR of glycerol over the synthesized catalysts at 500 • C for 30 h. It was found that the Ni/Nb 2 O 5 -Al 2 O 3 catalyst exhibited higher glycerol conversion and hydrogen yield about 80 and 50%, respectively. Bastan et al. [64] prepared a series of nanostructured Ni/X catalysts (X = Al 2 O 3 , MgO, and MgO-Al 2 O 3 ) via the co-precipitation method. It was declared that the Ni MgO-Al 2 O 3 had shown better catalytic performance in SR of glycerol.  The carbon nanotubes (CNTs) exhibited several outstanding physicochemical properties such as significant electron conductivity, proper mechanical strength, and a porous structure with a high surface area. Therefore, these materials have been considered with numerous applications specifically as support in heterogeneous catalysis. The important feature of using CNTs in catalytic reactions is the possibility to modify the chemical composition and functionalization of the surface, which these improvements lead to sufficient interactions between the metal and support [65][66][67][68]. Shuzhuang et al. [65] suggested that Ni (15%) supported on the CNT could demonstrate high catalytic activity owning 86.4% of glycerol conversion for gaseous products with 72.9% for the hydrogen selectivity at low temperature (375 °C). This observation could be due to improved reducibility, enhanced thermal conductivity, and better active phase dispersion. Santiago et al. [28] studied the SR of glycerol over activated carbon and used various metal oxides such as MgO, La2O3, and Y2O3 as the promoter. It was found that the catalyst promoted with Mg as NiMg/AC-OX showed higher Ni dispersion, which led to a better catalytic performance ( Figure 5). Furthermore, a lower amount of the methane in the product distribution reveals that as expected, the SR mechanism performed well in this catalyst [28]. The carbon nanotubes (CNTs) exhibited several outstanding physicochemical properties such as significant electron conductivity, proper mechanical strength, and a porous structure with a high surface area. Therefore, these materials have been considered with numerous applications specifically as support in heterogeneous catalysis. The important feature of using CNTs in catalytic reactions is the possibility to modify the chemical composition and functionalization of the surface, which these improvements lead to sufficient interactions between the metal and support [65][66][67][68]. Shuzhuang et al. [65] suggested that Ni (15%) supported on the CNT could demonstrate high catalytic activity owning 86.4% of glycerol conversion for gaseous products with 72.9% for the hydrogen selectivity at low temperature (375 • C). This observation could be due to improved reducibility, enhanced thermal conductivity, and better active phase dispersion. Santiago et al. [28] studied the SR of glycerol over activated carbon and used various metal oxides such as MgO, La 2 O 3 , and Y 2 O 3 as the promoter. It was found that the catalyst promoted with Mg as NiMg/AC-OX showed higher Ni dispersion, which led to a better catalytic performance ( Figure 5). Furthermore, a lower amount of the methane in the product distribution reveals that as expected, the SR mechanism performed well in this catalyst [28].

The Effect of Promoters in SR of Glycerol over Ni-Based Catalyst
The major drawback of the Ni-based catalysts is catalyst deactivation through sintering and coke formation. This issue causes side reactions as well. Using various promoters (e.g., noble or transition metals) has been suggested to overcome this issue through enhancement of catalytic stability [69]. Araque et al. [70] reported that the condensable molecules such as acrolein, acetaldehyde, and hydroxyacetone can be obtained through the SR of glycerol, which can mainly initiate the coke formation. It is mentioned that ethylene is also known as a coke precursor. Frusteri et al. [71] showed that ethanol hydration was suppressed by using basic promoters, and consequently, the ethylene formation was decreased. It was concluded that the alkaline promoters would have a similar effect as the hydration process in the SR of glycerol. Iriondo et al. [72] investigated the addition of Ce, Zr, and La as promoters to Ni-based alumina catalysts. The addition of promoters showed enhancement in the hydrogen selectivity; however, each promoter would have a different effect in the reaction mechanism. For instance, it was found that strong Ni-ZrO2 interactions would occur, but on the other hand, introducing Zr to alumina support lowers Ni dispersion as well.
Furthermore, Ce and La could boost the catalytic stability in terms of resistance to the coke deposition. Gallegos and et al. [73] investigated the Ni-Ce-based catalysts for the SR of glycerol (10 wt% C3H8O3 in water). As a result, the catalyst with 20 mol% of nickel content among other Ni-Ce catalysts showed the optimum ratio that could minimize CO and CH4 as two major byproducts.
In other research, Demsash et al. [74] studied Ni-based catalysts promoted with Ce in different loadings of Ni (5, 10, 15 wt%), and Ce (5, 10 wt%). Ce:Ni:Al2O3 = 5:10:100 was selected as the optimum loading with sufficient catalytic activity and stability over 16 h. The glycerol conversion and hydrogen selectivity were reported to 86 and 96% at 650 °C, respectively. In another work, Shao et al. [38] studied the Ni/CeZrO catalysts with different Ce content. They declared that at higher contents of Ce up to 77 wt%, the BET surface area was maximized and showed high enhanced catalytic activity

The Effect of Promoters in SR of Glycerol over Ni-Based Catalyst
The major drawback of the Ni-based catalysts is catalyst deactivation through sintering and coke formation. This issue causes side reactions as well. Using various promoters (e.g., noble or transition metals) has been suggested to overcome this issue through enhancement of catalytic stability [69]. Araque et al. [70] reported that the condensable molecules such as acrolein, acetaldehyde, and hydroxyacetone can be obtained through the SR of glycerol, which can mainly initiate the coke formation. It is mentioned that ethylene is also known as a coke precursor. Frusteri et al. [71] showed that ethanol hydration was suppressed by using basic promoters, and consequently, the ethylene formation was decreased. It was concluded that the alkaline promoters would have a similar effect as the hydration process in the SR of glycerol. Iriondo et al. [72] investigated the addition of Ce, Zr, and La as promoters to Ni-based alumina catalysts. The addition of promoters showed enhancement in the hydrogen selectivity; however, each promoter would have a different effect in the reaction mechanism. For instance, it was found that strong Ni-ZrO 2 interactions would occur, but on the other hand, introducing Zr to alumina support lowers Ni dispersion as well.
Furthermore, Ce and La could boost the catalytic stability in terms of resistance to the coke deposition. Gallegos and et al. [73] investigated the Ni-Ce-based catalysts for the SR of glycerol (10 wt% C 3 H 8 O 3 in water). As a result, the catalyst with 20 mol% of nickel content among other Ni-Ce catalysts showed the optimum ratio that could minimize CO and CH 4 as two major byproducts.
In other research, Demsash et al. [74] studied Ni-based catalysts promoted with Ce in different loadings of Ni (5, 10, 15 wt%), and Ce (5, 10 wt%). Ce:Ni:Al 2 O 3 = 5:10:100 was selected as the optimum loading with sufficient catalytic activity and stability over 16 h. The glycerol conversion and hydrogen selectivity were reported to 86 and 96% at 650 • C, respectively. In another work, Shao et al. [38] studied the Ni/CeZrO catalysts with different Ce content. They declared that at higher contents of Ce up to 77 wt%, the BET surface area was maximized and showed high enhanced catalytic activity in the SR of glycerol. Moon et al. [75], also investigated the effect of cerium on the Ni-Fe/Al 2 O 3 catalyst, and the obtained results were listed in Table 3.
Cerium and iron as two efficient prompters increased the reducibility of the Ni-based catalysts. Therefore, the active sites of the NiFeCeAl catalyst were reduced easily than those of the other catalysts. Moreover, cerium decreased sintering and coke deposition [75].
Lukman et al. [76], studied the production of hydrogen through the SR of glycerol over Ni-Ce-Cu/hydroxyapatite catalysts. The catalyst with the composition of Ni:Ce:Cu = 3:7.5:7.5 wt% showed a high glycerol conversion and hydrogen yield of 97 and 57%, respectively.
Sanchez et al. [77] studied the addition of La on the Ni/Al 2 O 3 catalyst for SR of glycerol. The results showed that the addition of La led to an increase in the surface area and decreased the coke formation. The Ni/La 2 O 3 /Al 2 O 3 catalyst exhibited enhanced catalytic stability during the SR of glycerol. Furthermore, it was expected that La could also influence the Ni particle distribution and, consequently, decrease the sintering of active sites through stronger metal support interactions. Chao Wang et al. [78] suggested the addition of CaO to the Ni-based alumina catalyst as a multifunctional catalyst for SR of glycerol and the subsequently CO 2 sorbent unit. The catalytic performance tests for SR of glycerol were performed at 550 • C and H 2 O/C 3 H 8 O 3 molar ratio of 9 as multi cycles with regeneration processes. It was found that the catalyst with the composition of NiO:CaO:Al 2 O 3 = 41.21:30.77:28.02 wt% showed high-purity hydrogen.
Bobadilla et al. [79] studied the effect of Ni-Sn alloying in the SR of glycerol over the promoted Ni-Sn/CeMgAl catalyst. Based on the SEM images of the fresh and used catalysts (Figure 6), it was observed that the growth of filamentous carbons on the surface of the Sn promoted catalyst was decreased. This phenomenon can be due to the core-shell system, whereas a layer of Sn clusters surrounds Ni. The adsorption capacity of the glycerol on the active sites was also enhanced, and the bimetallic configuration of Ni and Sn in the Ni-Sn/CeMgAl catalyst exhibited a better catalytic activity and durability.
Oemar et al. [80] studied the addition of Sr to Ni/La 2 O 3 catalyst via co-impregnation and sequential impregnation methods for the SR of toluene. They showed that besides the positive effect of Sr as a promoter on the enhancement of catalytic activity, the preparation method and sequences of the Sr addition are also important. Ni-Sr/La 2 O 3 (NSL) and Sr/Ni/La 2 O 3 (SNL) catalysts showed different performances, whereas a higher catalytic activity was attributed to the SNL. It is important to notice that Sr in the SNL catalyst has a weaker interaction with Ni particles; however in the case of NSL, the Sr seems to be placed between La 2 O 3 and Ni, and somehow influenced their electronic states. A similar approach can be investigated for the SR of glycerol.
Calles et al. [81] investigated the promoter effect of Mg and Ca on Ni-based SBA-15 catalysts in SR of glycerol at 600 • C. The glycerol conversion and hydrogen yield of 98% were observed for the Ni/Ca/SBA-15 catalyst. Figure 7 shows that the Ca has decreased the Ni particle size over the SBA-15 support and enhanced the catalytic performance compared to the bare sample of Ni/Ca/SBA-15. Narrow sized Ni particles could provide stronger interactions with the support, which can improve the coke resistance feature of the catalyst. It is believed that the presence of structural defective carbon deposits facilitates the gasification process and avoids the formation of encapsulated filamentous carbon. Choong et al. [82] also confirmed that the addition of Mg and Ca can improve the catalytic performance through the activation of water adsorption which eventually facilitates the steam gasification of deposited coke on the catalyst surface.
Moon et al. [83] studied the alkali metals (Ca, Sr, and K) promoted Ni-based catalysts in SR of glycerol. The catalysts were prepared via the dry impregnation method at various calcination temperatures (750, 850, and 950 • C corresponding to A, B, and C, respectively), and Ni loading was kept at 15 wt%, and Ca, Sr, and K loadings were 1 wt%, respectively. Table 4 shows the physicochemical properties of the synthesized promoted Ni-based catalysts. Oemar et al. [80] studied the addition of Sr to Ni/La2O3 catalyst via co-impregnation and sequential impregnation methods for the SR of toluene. They showed that besides the positive effect of Sr as a promoter on the enhancement of catalytic activity, the preparation method and sequences of the Sr addition are also important. Ni-Sr/La2O3 (NSL) and Sr/Ni/La2O3 (SNL) catalysts showed different performances, whereas a higher catalytic activity was attributed to the SNL. It is important to notice that Sr in the SNL catalyst has a weaker interaction with Ni particles; however in the case of NSL, the Sr seems to be placed between La2O3 and Ni, and somehow influenced their electronic states. A similar approach can be investigated for the SR of glycerol.
Calles et al. [81] investigated the promoter effect of Mg and Ca on Ni-based SBA-15 catalysts in SR of glycerol at 600 °C. The glycerol conversion and hydrogen yield of 98% were observed for the Ni/Ca/SBA-15 catalyst. Figure 7 shows that the Ca has decreased the Ni particle size over the SBA-15 support and enhanced the catalytic performance compared to the bare sample of Ni/Ca/SBA-15. Narrow sized Ni particles could provide stronger interactions with the support, which can improve the coke resistance feature of the catalyst. It is believed that the presence of structural defective carbon It was observed that the BET surface area and pore volume of the catalysts were reduced at higher calcination temperature, while the particle growth is also indicated. At high calcination temperatures, the phase transition of Al 2 O 3 support from γ to δ or the θ phase is more probable which can be the main reason for the morphology change of the catalyst as well. Moreover, this change in the morphology can be interpreted to the addition of alkali promoters as it was expected to observe bigger catalyst particles.
The addition of promoters can also lead to the plugging of the pore cavities of the support. The results of catalytic performance test for the SR of glycerol over the prepared catalysts are listed in Table 5. In the case of the catalysts that were calcinated at higher temperatures, the methane selectivity and coke formation were reduced, but the conversion and hydrogen selectivity were almost constant. deposits facilitates the gasification process and avoids the formation of encapsulated filamentous carbon. Choong et al. [82] also confirmed that the addition of Mg and Ca can improve the catalytic performance through the activation of water adsorption which eventually facilitates the steam gasification of deposited coke on the catalyst surface. Moon et al. [83] studied the alkali metals (Ca, Sr, and K) promoted Ni-based catalysts in SR of glycerol. The catalysts were prepared via the dry impregnation method at various calcination temperatures (750, 850, and 950 °C corresponding to A, B, and C, respectively), and Ni loading was kept at 15 wt%, and Ca, Sr, and K loadings were 1 wt%, respectively. Table 4 shows the physicochemical properties of the synthesized promoted Ni-based catalysts. Araque et al. [70] reported that the condensable molecules such as acrolein, acetaldehyde, and hydroxyacetone could be obtained through the SR of glycerol, which can mainly initiate the coke formation. It is mentioned that ethylene is also known as a coke precursor. Frusteri et al. [71] showed that by using basic promoters, ethanol hydration is suppressed, and ethylene formation was decreased. Hence, there is a possibility that the alkaline promoters will have the same impact as the hydration process in the SR of glycerol.    Figure 8 shows the resulsust of thermogravimetric analysis (TGA) for the used catalysts in the SR of glycerol, it was concluded that high calcination temperatures could improve the coke resistance and led to more durable catalysts. Among synthesized catalysts, the Sr-Ni/γ-Al 2 O 3 catalyst showed the best stability over coke deposition in the SR of glycerol [83]. Based on the research conducted by Oemar et al. [80] it has also been confirmed that the addition of Sr could drastically improve the surface catalytic activity in the SR of toluene. Bobadilla et al. [79] reported that Ni-based catalysts doped with Sn can increase the stability of the catalyst and decrease the coke formation. In another research, Kitamura et al. [84], investigated the SR of glycerin Ni/ZrO 2 catalysts. The addition of CaO to the support ZrO 2 enhanced the catalytic performance. In this regard, the synthesized NiO/CaO-ZrO 2 catalyst exhibited glycerin conversion and hydrogen yield of 89 and 74% at 600 • C, respectively. In addition, the coke deposition was also reduced from 4.2 to 2%.
Catalysts 2020, 10, x FOR PEER REVIEW 13 of 23 decreased. Hence, there is a possibility that the alkaline promoters will have the same impact as the hydration process in the SR of glycerol. Figure 8 shows the resulsust of thermogravimetric analysis (TGA) for the used catalysts in the SR of glycerol, it was concluded that high calcination temperatures could improve the coke resistance and led to more durable catalysts. Among synthesized catalysts, the Sr-Ni/γ-Al2O3 catalyst showed the best stability over coke deposition in the SR of glycerol [83]. Based on the research conducted by Oemar et al. [80] it has also been confirmed that the addition of Sr could drastically improve the surface catalytic activity in the SR of toluene. Bobadilla et al. [79] reported that Ni-based catalysts doped with Sn can increase the stability of the catalyst and decrease the coke formation. In another research, Kitamura et al. [84], investigated the SR of glycerin Ni/ZrO2 catalysts. The addition of CaO to the support ZrO2 enhanced the catalytic performance. In this regard, the synthesized NiO/CaO-ZrO2 catalyst exhibited glycerin conversion and hydrogen yield of 89 and 74% at 600 °C, respectively. In addition, the coke deposition was also reduced from 4.2 to 2%. In another research work, Moon et al. [85] studied the addition of Fe, Ce, La, and Cr to Ni-based catalysts for the SR of glycerol in various reaction conditions (350~650 °C and 1~8 bar). The contents of Ni, Fe, and X were maintained at 15, 3, and 1% respectively, whereas X stands for Ce, La, and Cr. Table 6 summarized the physicochemical properties of synthesized catalysts. The Ni-Fe-Ce/Al2O3 catalyst showed a higher surface area and Ni dispersion. Table 6. Physicochemical properties of the prepared Ni-based Fe promoted catalysts [85] (used with copyright permission).  In another research work, Moon et al. [85] studied the addition of Fe, Ce, La, and Cr to Ni-based catalysts for the SR of glycerol in various reaction conditions (350~650 • C and 1~8 bar). The contents of Ni, Fe, and X were maintained at 15, 3, and 1% respectively, whereas X stands for Ce, La, and Cr. Table 6 summarized the physicochemical properties of synthesized catalysts. The Ni-Fe-Ce/Al 2 O 3 catalyst showed a higher surface area and Ni dispersion.  Figure 9a shows the TGA profiles of the deposited coke on the catalysts after the SR of glycerol. It was found that the first weight loss at a temperature range of 50~200 • C was related to physisorbed H 2 O and the second weight loss at 400~500 • C was caused by carbonaceous components adsorbed on the surface of the catalyst. The ceria promoted Ni-Fe-Ce/Al 2 O 3 catalyst showed lower carbonaceous deposition than the other catalysts. Go et al. [75] proposed that CeO 2 is capable of storing oxygen molecules at the catalyst surface, which hinders the coke formation.

Catalyst Composition (wt%) BET Surface Area (m 2 /g) * Metal Dispersion (%) ** Ni Fe Ce La Cr
Catalysts 2020, 10, x FOR PEER REVIEW 14 of 23 Figure 9a shows the TGA profiles of the deposited coke on the catalysts after the SR of glycerol. It was found that the first weight loss at a temperature range of 50~200 °C was related to physisorbed H2O and the second weight loss at 400~500 °C was caused by carbonaceous components adsorbed on the surface of the catalyst. The ceria promoted Ni-Fe-Ce/Al2O3 catalyst showed lower carbonaceous deposition than the other catalysts. Go et al. [75] proposed that CeO2 is capable of storing oxygen molecules at the catalyst surface, which hinders the coke formation.  Table 7 presents the summary of glycerol conversion and hydrogen selectivity results in the SR of glycerol using promoted Ni-based catalysts. The Ni-Fe-Ce/Al2O3 catalyst has also exhibited significant catalytic performance with glycerol conversion and H2 selectivity of 94.1 and 64.0%, respectively. Table 7. The catalytic performance in the SR of glycerol over Ni-based Fe modified catalysts [85] (used with copyright permission). Reaction condition: 450 °C and GHSV = 7300 mL/g-cat·h.

Catalyst
It was found that the introduction of Ce avoids the formation of unsaturated hydrocarbons via the secondary dehydration reactions and consequently decreases the coke deposition. Figure 9b illustrates TGA results for the synthesized catalysts in the SR of glycerol at 450 °C and various pressures (1~8 bar). It was observed that the coke deposition had been increased when the pressure in the SR of glycerol was increased, which reveals the positive impact of atmospheric pressure for avoiding coke formation.
As already discussed, the Ca and Mg had increased the catalytic performance of the Ni-based catalysts through reducing the Ni particle size with a better dispersion and stronger interaction on the support. Huang et al. [86] studied the addition of transition metals (La, Ce, Y, Zr, or Mo) as the second promoter to the modified Ni/Al2O3 catalyst with Ca and Mg (nominated as CMA). This approach assists in decreasing the acidic sites in order to increase the catalytic stability in the SR of glycerol. Table 8 summarized the reaction results for SR of glycerol over synthesized catalysts. It was found that the NiMoLa/CMA significantly increased the catalytic performance, including glycerol conversion (99%), hydrogen selectivity (63.2%), and catalyst stability in 42 h.  Table 7 presents the summary of glycerol conversion and hydrogen selectivity results in the SR of glycerol using promoted Ni-based catalysts. The Ni-Fe-Ce/Al 2 O 3 catalyst has also exhibited significant catalytic performance with glycerol conversion and H 2 selectivity of 94.1 and 64.0%, respectively. Reaction condition: 450 • C and GHSV = 7300 mL/g-cat·h.
It was found that the introduction of Ce avoids the formation of unsaturated hydrocarbons via the secondary dehydration reactions and consequently decreases the coke deposition. Figure 9b illustrates TGA results for the synthesized catalysts in the SR of glycerol at 450 • C and various pressures (1~8 bar). It was observed that the coke deposition had been increased when the pressure in the SR of glycerol was increased, which reveals the positive impact of atmospheric pressure for avoiding coke formation.
As already discussed, the Ca and Mg had increased the catalytic performance of the Ni-based catalysts through reducing the Ni particle size with a better dispersion and stronger interaction on the support. Huang et al. [86] studied the addition of transition metals (La, Ce, Y, Zr, or Mo) as the second promoter to the modified Ni/Al 2 O 3 catalyst with Ca and Mg (nominated as CMA). This approach assists in decreasing the acidic sites in order to increase the catalytic stability in the SR of glycerol. Table 8 summarized the reaction results for SR of glycerol over synthesized catalysts. It was found that the NiMoLa/CMA significantly increased the catalytic performance, including glycerol conversion (99%), hydrogen selectivity (63.2%), and catalyst stability in 42 h.

SR of Glycerol over the Perovskite-Based Catalyst
Perovskite is defined in the form of ABO 3 , where A and B present the rare-earth cation and transition metal, respectively. The corners of the unit cell are generally concurred by the A sites while B sites are placed in the center with a coordination number of 12 and 6, respectively [87]. Perovskite type oxides have received significant attention because of their unique crystal structures in chemical processes such as steam reforming of various hydrocarbons such as methane, ethanol, and glycerol [88][89][90][91]. Metallic particles with high dispersion could be achieved through the reduction when using perovskite as the support. This feature known as high oxygen mobility can drastically decrease the coke deposition on the catalyst surface in steam reforming reactions.
Furthermore, in terms of hydrogen production, the catalytic performance was improved when using perovskite supported catalysts. In this regard, Kamonsuangkasem et al. [29] found that in the reduced CeAlO 3 perovskite, the interaction of Ni and alumina was decreased which consequently influenced the amount of Brønsted acid sites. Therefore, dehydration of condensable intermediates was fascinated, and more hydrogen was produced. In this regard, Franchini et al. [92] studied the SR of glycerol using La 1−x Ce x NiO 3 catalysts, and they also confirmed that the mentioned behavior could be attributed to the presence of both Ce and perovskite. Through their work, it was found that to prepare a catalyst which includes CeO 2 -La 2 O 3 with improved coke resistance, considering higher contents of Ce could be obliging. In another research, Xie et al. [93] studied the SR of glycerol using La 0.7 Ce 0.3 NiO 3 perovskite-type mixed oxides synthesized by the co-precipitation method in various calcination temperatures and concentrations. The prepared samples were nominated as LT-X-Y whereas T stands for the calcination temperature (700, 900), X shows the molar concentration of nitrate solution (0.03~3), and Y is the molar concentration of the precipitant (0.1~1.9). Figure 10 illustrates the glycerol conversion and yield for the gaseous products in the SR of the glycerol over synthesized catalysts in this work. It was found that the L700-0.3-0.6 catalyst showed lower coke deposition and catalytic stability with proper activity. Furthermore, for all synthesized La 0.7 Ce 0.3 NiO 3 perovskite-type mixed oxide catalysts the low yield of CH 4 represents that the hydrogenation between produced hydrogen and the CO or CO 2 was almost omitted.
Moon et al. [94] studied the SR of glycerol for the production of hydrogen at low temperatures using Cu decorated perovskite catalysts through the vapor phase reaction conditions. Table 9 shows the catalytic performance for SR of glycerol. Figure 10. The glycerol conversion and yield for the gaseous products in SR of the glycerol over LT-X-Y perovskite-type mixed oxides (575 °C, H2O/ C3H8O3 molar ratio of 15, and 49.7 g h/mol) [93] (redrawn with copyright permission).
Moon et al. [94] studied the SR of glycerol for the production of hydrogen at low temperatures using Cu decorated perovskite catalysts through the vapor phase reaction conditions. Table 9 shows the catalytic performance for SR of glycerol. The (LaNi0.9Cu0.1O3) catalysts were synthesized by a perovskite precursor, exhibiting high catalytic performance in SR of glycerol at 650 °C, which could be a result of uniform dispersion of Cu particles. Moreover, the addition of Cu assisted in operating the reaction at a lower temperature as the resistance towards the coke deposition was improved. This approach also led to a significant decrease in the Ni sintering and consequently retaining the catalytic activity. It was understood that configuration of Cu-Ni nanoparticles on the perovskite-type oxides had enhanced the glycerol decomposition and dehydration process as well [94].
Aman et al. [88] studied the synthesized LaNiO3 and LaCoO3 via reverse microemulsion in SR of glycerol for the production of hydrogen. They found that at a low temperature of 400 °C and H2O/ C3H8O3 molar ratio of 15, the synthesized LaNiO3 has approximately three times more catalytic activity compared to the LaCoO3 catalyst. It is important to notice that the SR of glycerol using Nibased catalysts owns a lower activation energy compared to the Co-based catalysts. Therefore, it is expected that Ni-based catalysts would have better catalytic performance in terms of glycerol conversion and hydrogen selectivity at low temperature while the Co-based catalyst is less capable for the coke deposition at higher temperatures. Moreover, the methane yield could be considered as a parameter for evaluating how C-C bond cleavages occur over the synthesized catalysts. For  The (LaNi 0.9 Cu 0.1 O 3 ) catalysts were synthesized by a perovskite precursor, exhibiting high catalytic performance in SR of glycerol at 650 • C, which could be a result of uniform dispersion of Cu particles. Moreover, the addition of Cu assisted in operating the reaction at a lower temperature as the resistance towards the coke deposition was improved. This approach also led to a significant decrease in the Ni sintering and consequently retaining the catalytic activity. It was understood that configuration of Cu-Ni nanoparticles on the perovskite-type oxides had enhanced the glycerol decomposition and dehydration process as well [94].
Aman et al. [88] studied the synthesized LaNiO 3 and LaCoO 3 via reverse microemulsion in SR of glycerol for the production of hydrogen. They found that at a low temperature of 400 • C and H 2 O/ C 3 H 8 O 3 molar ratio of 15, the synthesized LaNiO 3 has approximately three times more catalytic activity compared to the LaCoO 3 catalyst. It is important to notice that the SR of glycerol using Ni-based catalysts owns a lower activation energy compared to the Co-based catalysts. Therefore, it is expected that Ni-based catalysts would have better catalytic performance in terms of glycerol conversion and hydrogen selectivity at low temperature while the Co-based catalyst is less capable for the coke deposition at higher temperatures. Moreover, the methane yield could be considered as a parameter for evaluating how C-C bond cleavages occur over the synthesized catalysts. For instance, in the case of Ni-based catalysts (Ni ex LaNiO 3 ), more methane was produced compared to the Co ex LaNiO 3 which resembles that the rate of C-C bond cleavage is higher. Ramesh et al. [95] investigated the SR of the glycerol over synthesized LaNiO 3 with and without templates. The core idea was to study the effect of various particle sizes of NiO in SR of glycerol (Table 10). It was found that even though the LaNiO 3 synthesized with the template showed a lower BET surface area, but a better catalytic performance compared to NiAl 2 O 3 was obtained over this catalyst at 550 and 650 • C. Wu et al. [96] studied the SR of glycerol over the La 1-x Ca x NiO 3 perovskite-type oxides. They found that the partial substitution of lanthanum with calcium could drastically influence the physicochemical properties of the catalyst. In fact, the perovskite-type oxides are capable of providing the uniform distribution of the nickel, lanthanum, and/or calcium. Furthermore, after the characterization of the used catalysts, it was found that the coke formation over active sites could be the major reason for the catalyst deactivation. Narrow nickel particles that have a stronger interaction with the support can show higher catalytic stability during the SR of glycerol.

The SR of Glycerol over Hydrotalcite-Based Catalyst
Hydrotalcites are double-layered hydroxides known as the layered synthetic minerals anionic clays. These brucite-like layers have the general formula of [M II 1−x M III x (OH) 2 ] x+ [A n− x/n ·yH 2 O] x− , whereas the M(II) and M(III) stand for the divalent and trivalent cations. The n-valent anion number of interlayer molecules of H 2 O was shown by A n− and y, respectively. Calcination of these types of double-layered hydroxides ends with a uniform dispersion of MII and MIII oxides. In this regard, it was believed that the Ni-Mg-Al mixed oxides derivate from hydrotalcite had presented a proper catalytic performance in various catalytic processes such as dry and steam reforming of methane [97][98][99][100] and ethanol [101][102][103][104]. Furthermore, the Cu-Mg-Al hydrotalcite type catalysts also showed superior catalytic activity in the water-gas shift (WGS) reaction [105]. The bimetallic of Ni and Cu catalyst has been initially studied for the SR of ethanol with decreased coke deposition [106][107][108], however even though it was expected that this configuration could be efficiently applied for the SR of glycerol, but Manfro et al. [109] investigated the Ni-Cu hydrotalcite based catalysts with 20 wt% of nickel oxide and 0, 5, and 10 wt% of copper oxide. It was recognized that the addition of the copper had a negative impact on the conversion of glycerol, and the maximum conversion was obtained for the bare Ni-based catalyst. Furthermore, the results demonstrated that the Ni10Cu exhibited higher glycerol conversion in comparison to the Ni5Cu catalyst.
Dahdah et al. [110] investigated the SR of glycerol over Ru-Mg-Al hydrotalcite-derived mixed oxides synthesized through co-precipitation (COP) and impregnation (IMP). The reaction conditions for the SR of glycerol were as following: 400~700 • C, H 2 O/C 3 H 8 O 3 (molar ratio) = 9, and the stability test was carried out at 600 • C. The Ru-Mg-Al (IMP) exhibited sufficiently better catalytic performance, which could be due to the ensemble effect and the facilitated accessibility to the active sites. In another research work, Simone et al. [111] suggested that Pt-based catalysts supported on hydrotalcite consist of Mg and Al in the SR of glycerol. It was reported that the hydrotalcite catalyst with Mg/Al = 3 showed higher catalytic activity than the hydrotalcite catalyst with Mg/Al = 5. In this regard, Moon et al. [112] studied nanosized Ni-based hydrotalcite-like catalysts for application in the SR of glycerol. The Ni/Al 2 O 3 catalyst was synthesized by the impregnation method as a reference for the comparative analysis, and the Ni-MgAl (Ni-based hydrotalcite) catalyst was synthesized through the solid phase crystallization method. The reaction condition in a fixed bed reactor was as follows: 700~800 • C, 1 bar, H 2 O/C 3 H 8 O 3 (molar ratio) = 9, 10 wt% of glycerol, GHSV = 10,000 h −1 .
It was found that the particle size of Ni in the Ni/MgAl catalyst is smaller than that in the Ni/Al 2 O 3 catalyst. The catalyst deactivation in the SR of glycerol was a consequence of sintered Ni particles and filamentous carbon deposited on the surface of the catalyst. A negligible amount of coke was formed on the surface of the Ni/MgAl catalyst, with a coke formation rate per weight of the catalyst as 1.7 × 10 −2 g carbon/(h. gcat) which is much smaller than that over the Ni/Al 2 O 3 catalyst, 4.18 × 10 −2 g carbon/(h. gcat).

Conclusions
This paper reviewed and categorized the current state of knowledge on bio-hydrogen production through the SR of glycerol obtained from transesterification. Nickel-based catalysts are conventionally used for various SR of hydrocarbons, including glycerol. Carrying out the process at lower temperatures can lead to reducing the cost of energy for the commercialization of the process. It was found that the optimal temperature could be around 650 • C, while the pressure is always maintained at atmospheric conditions. For enhancement of the efficiency in the SR of glycerol, various promoters were implicated, whereas Ce, Mg, and La were found to have a greater influence on lowering the coke deposition and sintering. Perovskite-type supports have exhibited stability as a result of the incorporation of other metal ions into the perovskite structure. In this regard, Ni-based catalysts supported on the ZrO 2 and promoted with La showed high conversion at low temperatures. It was found that by adjusting the Ni content supported on the SiO 2 -Al 2 O 3 catalyst, the catalytic performance can be sufficiently enhanced at low temperatures (350~550 • C). Using perovskite-type catalysts can lead to superior hydrogen selectivity and glycerol conversion. The LaNi 0.9 Cu 0.1 O 3 catalysts synthesized by a perovskite precursor has shown high catalytic performance in SR of glycerol. It was suggested that nanosized Ni-based hydrotalcite-like catalysts for application in the SR of glycerol could be considered as potential candidates. The hydrotalcite catalysts can offer higher thermal stability and coke resistance. As for future work, it is suggested to simulate the feed composition for the development of the core technology for handling crude glycerol that contains various impurities based on the biomass source.