Current advances in syngas (CO + H2) production through bi-reforming of methane using various catalysts: A review

Today, bi reforming of methane is considered as an emerging replacement for the generation of high-grade synthesis gas (H2:CO 1⁄4 2.0), and also as an encouraging renewable energy substitute for fossil fuel resources. For achieving high conversion levels of CH4, H2O, and CO2 in this process, appropriate operation variables such as pressure, temperature and molar feed constitution are prerequisites for the high yield of synthesis gas. One of the biggest stumbling blocks for the methane reforming reaction is the sudden deactivation of catalysts, which is attributed to the sintering and coke formation on active sites. Consequently, it is worthwhile to choose promising catalysts that demonstrate excellent stability, high activity and selectivity during the production of syngas. This review describes the characterisation and synthesis of various catalysts used in the bi-reforming process, such as Ni-based catalysts with MgO, MgOeAl2O3, ZrO2, CeO2, SiO2 as catalytic supports. In summary, the addition of a Ni/SBA-15 catalyst showed greater catalytic reactivity than nickel celites; however, both samples deactivated strongly on stream. Ce-promoted catalysts were more found to more favourable than Ni/MgAl2O4 catalyst alone in the bireforming reaction due to their inherent capability of removing amorphous coke from the catalyst surface. Also, Lanthanum promoted catalysts exhibited greater nickel dispersion than Ni/MgAl2O4 catalyst due to enhanced interaction between the metal and * Corresponding author. School of Engineering, Edith Cowan University, 270 Joondalup Dr, Joondalup, 6027, WA, Australia ** Corresponding author. School of Engineering, Edith Cowan University, 270 Joondalup Dr, Joondalup, 6027, WA, Australia E-mail addresses: U.mohanty@ecu.edu.au (U.S. Mohanty), Muhammad.ali.2@kaust.edu.sa (M. Ali), M.azhar@ecu.edu.au (M.R. Azhar), A.keshavarz@ecu.edu.au (A. Keshavarz), S.iglauer@ecu.edu.au (S. Iglauer). Available online at www.sciencedirect.com


Introduction
Due to increasing evidence of global warming in the present century, scientists at the UN Intergovernmental Panel on Climate Change have reached a consensus for reduction of greenhouse gas emissions, especially carbon dioxide, to the atmosphere [1e8]. This has also prompted steering committees of industrialised countries to assess their energy strategies based on mitigation of greenhouse gas emissions [9e14] (see Table 1).
Extensive literature has covered on the various alternatives for cleaner energy sources [ 233], their economic aspects and advantages to mitigate CO 2 emissions and reduce environmental pollution [234e241]. Among the list of proposed alternative energy sources, hydrogen appears to be the most promising large-scale fuel due to its efficient storage over time and clean combustion [2,244]. Recent, there has been enormous interest in hydrogen and it's been increasing rapidly due to its potential applications in fuel cells. They also serve as an excellent replacement to batteries in the field of portable electronics, internal combustion engines as well as power plants. The demand for hydrogen in the i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 6 ( 2 0 2 1 ) 3 2 8 0 9 e3 2 8 4 5 'CaO Ni/Ca 3% 98.5 2.0 Drastic reduction in the carbon deposition 850 [108] (continued on next page) i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 6 ( 2 0 2 1 ) 3 2 8 0 9 e3 2 8 4 5 most important sector of road transport is depicted in Fig. 1, which illustrates that the annual hydrogen demand is projected to surge from 25 tons in 2020 to 945.5 thousand tons in 2045. Furthermore, the soaring demand for hydrogen in Japan [33] can be illustrated in Fig. 2. This shows a gradual increase in hydrogen demand from 2015, which will increase rapidly to 21 million tons in 2035.
Additionally, electric engines in any vehicle are energised by electricity from fuel cells, which is generated by conversion of clean and environmentally friendly hydrogen (and oxygen from the air) in fuel cells. A schematic diagram representing hydrogen supply from various sources, and its applications, are illustrated in Fig. 3.
It has been established that CO 2 emission levels [35] to the atmosphere can be significantly decreased by substitution of traditional fuels such as diesel, gasoline and carbon with higher (H/C) ratio fuels such as natural gas or biomass, as shown in Fig. 3. Therefore, production of hydrogen from hydrocarbons is regarded as the most economic and efficient way of achieving a significant degree of reduction in the emissions of greenhouse gases. Natural gas is a nonrenewable energy source; essentially a blend of lighter hydrocarbons existing in the basement of gas accumulations present in porous rock which might or might not be associated with oil. It is mainly constituted of saturated hydrocarbons, mainly methane, with butane and propane in insignificant quantities, and other compounds composed of inorganic gases. Production of synthesis gas comprising of a mixture of CO along with purified H 2 being obtained from natural gas by using various catalyst and is currently the most preferred choice (Table 1). With the advent of the hydrogen economy, there has been an increased focus on the transformation of petroleum gas into more ecologically friendly hydrogen fuel.
The Global carbon dioxide emissions from various industrial processes and fossil fuel combustion have been estimated to be around 35.7 billion tons [36] annually, which has contributed to increased global warming [242,243]. Therefore, it is imperative to develop clean technologies for the utilisation of fossil fuels [245e247] and to introduce alternative greener fuels for inhibiting the adverse effects of greenhouse gas emissions and subsequent climatic changes. Among various alternative fuels, hydrogen [37] is considered to be a sustainable energy carrier and offers near zero end-use emissions of greenhouse gases and pollutants [38].
For the creation of clean fuel, like hydrogen, natural gas needs to undergo a catalytic process described as natural gas reforming. Reforming is the most common technique used in industries for production of synthesis gas via through one of three reforming processes i.e., partial oxidation of methane (POM) [39], steam reforming of methane (SRM) [40e48] and CO 2 reforming of methane (DRM) [49e55]. SRM is a fully developed generation technique which utilises steam at high temperature (700e1000 C) for the production of H 2 from natural gas. During SRM, CH 4 interacts with steam with pressures ranging from 3 to 25 bar using catalyst to produce H 2 , CO and a moderate quantity of CO 2 . Eventually during the water-gas shifting reaction, steam and CO 2 interact to generate CO and more H 2 using an efficient catalyst. Steam reforming of methane requires rigorous energy input because of its endo-thermicity i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 6 ( 2 0 2 1 ) 3 2 8 0 9 e3 2 8 DRM: POM can deliver syn gas with a H 2 /CO proportion of 2.0. Nonetheless, controlling this process is an arduous task due to the danger associated with explosions [70] and the presence of hot spots Also, Partial oxidation of methane needs an air separation unit (ASU), which markedly impacts the expenses associated with the reforming plant. Because of these disadvantages associated with POM, combined steam and dryreforming of methane (CSDRM), where H 2 O is used in conjunction with CO 2 , has been considered as a worthwhile strategy for the mass production of syn gas with a H 2 /CO proportion of 2.0 [71,72]. The CSDRM can generate syn gas with flexible H 2 /CO proportions, which can be effectively controlled by modifying the feed gas (H 2 O, CO 2 and CH 4 ) composition. The utilized procedure is alternatively known as bi-reforming (BRM) where a 3/2/1 proportion of CH 4 along with CO 2 and steam produces a gas blend with basically a 2/1 proportion of H 2 to CO. This formed gas is also called 'met gas' to underline its distinction from broadly utilized syngas blends of different H 2 /CO proportions. The formation of syngas with this ratio has potential applications in Fischer-Tropsch operations for the preparation of long hydrocarbon chains [73e75]. as well as in the production of methanol [76e78].
Furthermore, bi-reforming [79e94] of CH 4 has captivated massive interest from both environmental and industrial perspectives. CO 2 and CH 4 are the most abundant carboncontaining, ozone-depleting substances from an environmental perspective, which can be used successfully in this reaction and can undergo conversion to useful chemical products. In reality, the combination of both steam and dry reforming provides a more pragmatic route for enhancing the H 2 /CO ratio compared to the introduction of CH 4 [95e97]. Additionally, this method possesses the merit of producing synthesis gas by using methane and carbon dioxide which are coined as greenhouse gases.   i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 6 ( 2 0 2 1 ) 3 2 8 0 9 e3 2 8 4 5 It has been reported [98] that at lower temperatures higher conversion of methane can be achieved in the bi-reforming process. In addition to the operating conditions, catalysts also play a crucial role in bi-reforming reactions. One of the most important advantages of bi-reforming [85,98e104] is that the consumption of major greenhouse gases occurs, thereby creating a significant environmental impact. These gases are water vapour, which accounts for 36e70% of the feed gas, CO 2 at 9e26%, CH 4 at 4e9% and ozone (O 3 ) for the rest (3e7%) [105]. Hence, there has been a renewed interest in the application of these gases via bi-reforming of methane towards the production of value-added chemicals that are useful for both scientific and industrial communities.
Additionally, bi-reforming technology can be regarded as a method for enhancing the caloric value of biogas, which is composed of CO 2 , H 2 and CH 4 through the solar reforming process [106,107]. One of the biggest stumbling blocks for the methane reforming process is related to the sudden catalyst deactivation, which might be due to sintering and coke formation on the active sites [108,109]. CH 4 decomposition (Eq. (3)), CO disproportionation (Eq. (4)) and CO reduction (Eq. (6)) are the primary processes that lead to coke formation. The reaction in Equation (3) shows an endothermic reaction that is highly favourable at higher temperatures and lower pressures, whereas Equations (4) and (5) are exothermic in nature and favoured at lower temperatures [110] and higher pressures through the reverse water gas shift reaction (Eq. (6)).
Since catalysts deactivation is caused by formation of coke from the above reactions (4), (5) and 6, hence, it is desirable to establish promising catalysts that demonstrate greater selectivity, excellent stability and activity during the production of syngas. Several investigations [49,50,111], have been reported for assessing the most suitable catalyst for syngas production employing different technologies. Common catalysts that have been used in reforming reactions include catalysts such as copper, nickel supported by transition metals and other supported noble metal catalysts such as ruthenium, platinum, rhenium. Several noteworthy reviews reporting on various innovations recorded in catalyst development for DRM reactions have mainly focussed on catalysts configurations [112], the influence of process parameters [50], noble metal catalysts [49], coke deposition and management [111], development of oxygen carriers in chemical looping [113], Ni and Ni-based catalysts [51], low temperature dry reforming [114], and advances in synthesis of catalysts with mesoporous SBA-15 support [115]. Fidalgo et al. [36], conducted a review on carbon black catalysts and activated carbon which have the unique characteristic of operating without being deactivated by carbon deposition. The catalysts role on methane decomposition and carbon dioxide reforming of CH 4 was assessed, and the characteristics of carbon deposits during CO 2 reforming of CH 4 were listed. The influence of nanocatalysts [38] on the oxidative coupling, steam reforming and CO 2 reforming of CH 4 has been previously reported, which suggested that methane conversion over a nanocatalyst occurred significantly than the ordinary catalyst and there existed no interdependence between the average particle size of nanoparticles and the conversion of methane.
Pakhare et al. [49], reviewed DRM for catalysts based on metals such as palladium, platinum, rhenium and ruthenium, which involved the role of these elements on the mechanism, deactivation, kinetic behaviour of these catalysts. Abdullah et al. [51], conducted a comprehensive review on the potential of nickel based catalysts employed during syngas production using dry reforming process. Their result suggested that strong metal support interactions were dependent on the catalyst supports and these factors were responsible for highest coke resistance, high thermal resistance and greater stability. The authors also examined the synthesis of catalyst supports from cellulosic materials and stressed the enhanced catalytic activity of the cellulose in the DRM reaction due to its superior mechanical strength and distinct structure.
However, to the author's knowledge there is a lack of comprehensive literature on the synthesis, characterisation and the role of catalysts and their promoters in the generation of synthesis gas during bi -reforming of methane (BRM). Therefore, the present review encompasses in details the role of various catalysts: Ni-based, Co-based, Ru-based, mesoporous and La-based, on BRM process. Additionally, the review describes the recent progress relating to the most relevant topics on catalysts used in bi-reforming technology.

Nickel-based catalysts
Though Ni-based catalysts [116] are inexpensive, they display exhibit superior performance in comparison to precious metal catalysts. Nevertheless, sintering and formation of carbon affects the sudden deactivation of catalysts. Since bireforming employs low S/C ratios for adjusting the H 2 /CO ratio, hence catalysts involving Ni undergo deactivation by carbon deposition [76]. Two main methods have been documented to diminish the deactivation of catalyst due to formation of coke. One method described the effect of promoters such as lanthanum [117], cerium, magnesium and calcium [118,119], on the characteristics of the catalysts during the reforming process and other was aimed at controlling the particle size at the nano-level in the active metals.
It has been established that during bi-reforming [120e122] and DRM, deactivation of Ni supported catalysts occur due to coke formation. Hence, it is highly imperative for the development of most active and stable catalysts in bi-reforming. Several authors [120,121,123], have developed nickel catalyst of high activity and stability supported by CeeZrO 2 , ZrO 2 and MgO during the DRM process. Several literature studies on the effect of nickel catalysts and their promoters during bireforming of methane have been documented in the present review. For example, Roh et al. [121] employed numerous supported Ni catalysts during bi-reforming reactions for production of syngas having H 2 /CO ¼ 2. The supported Ni catalysts were prepared by incipient wetness method with Ni(NO 3 ) 2 . The Ni catalysts were supported by small nanoparticles of ZrO 2 or MgO which were highly active and stable for BRM. Fig. 4 illustrates scanning electron microscopic (SEM) images of catalysts used in the reaction and the coke formation observed when subjected to 800 C. The authors observed that the degree of carbon formation and shape varied with different catalysts. Ni catalyst with MgOeAl 2 O 3 as support generated filamentous coke but of insignificant intensity (Fig. 4a). However, Ni catalyst with MgO as support generated a lot of coke from the filaments (Fig. 4b) during occurrence of BRM reaction while Ni/ZrO 2 exhibited a worm-like coke feature (Fig. 4c). Similar shape of coke ( Fig. 4d) was observed for Ni/CeO 2 catalyst. Nevertheless, Ni/aeAl 2 O 3 generated rod shaped-like coke (Fig. 4e).
The authors [121] also made a comparative study between the Ni/MgOeAl 2 O 3 catalyst and the commercial Ni catalyst supported with MgAl 2 O 4 (Fig. 5)   catalyst. Also, coke formation was more severe with the commercial Ni/MgAl 2 O 4 catalyst than with Ni/MgOeAl 2 O 3 , which was attributed to the efficient dispersion of Ni [124] supported MgOeAl 2 O 3 . The high activity and stability of Ni/ MgO Al 2 O 3 catalyst was attributed to the beneficial role of MgO which resulted from basic property, fine dispersion of nanosized Ni and strong interaction of Ni to the support. From Fig. 6 it was clearly observed that the CH 4 conversion was highest for Ni catalyst supported by MgO Al 2 O 3 and approximately 90% methane underwent conversion which continued for 2 h. However, rapid catalyst deactivation occurred in the case of Ni a-alumina catalyst with changes in time attributed to carbon formation. Moreover, Ni/MgO catalyst exhibited around 60% CH 4 conversion and Ni/ZrO 2 demonstrated around 70% CH 4 conversion. Both the catalysts were found to be highly stable during the reforming process. Nevertheless, the conversion of methane for Ni catalyst supported by CeO 2 was initially 57% which decreased to 50% followed by its saturation. Results revealed that Ni/MgOeAl 2 O 3 possessing lowest nickel oxide crystallite size exhibited highest stability along with higher CH 4 conversion with time on stream.

Catalytic nickel membrane
Ryi et al. [125] conducted tests over a catalytic nickel membrane during bi-reforming of methane for a shorter residence time of 120 ms for various CO 2 /H 2 O ratios in the range of 0e1.0, along with (H 2 O þ CO 2 )/CH 4 ratio of around 3.0 in the reactant feed for temperatures ranging from 923 to 1023 K.
The purpose of this study was to examine the performance of bi-reforming of methane over a catalytic nickel membrane for the GTL (gas to liquid) process. GTL process possess two advantages. One is that carbon formation is reduced due to the oxidation of carbon precursor species and a desirable H 2 /CO can be achieved by adjusting CH 4 /H 2 O/CO 2 ratio in the feed stream. Porous wall of catalytic nickel membrane was chosen for reforming studies since hydrogen passed through the catalytic nickel membrane was faster than the other gases because of viscous and Knudsen flow. Generally, the catalyst that contained relatively small size pore was more affected by internal diffusion than the one which has large sized pores.
The results revealed that the change in the feed ratio of CO 2 /H 2 O strongly affected the conversion of methane and furthermore an increase in the feed ratio of CO 2 /H 2 O at a temperature of 923 K (Fig. 7) decreased the methane conversion. The authors noted a very high conversion of methane in the range of 92.7e96% above 973 K, when the CO 2 /H 2 O feed ratios were in the range of 0e1.0 during bi-reforming of methane. The authors ascertained that with change in the CO 2 /H 2 O ratio during the reforming reaction, a change in H 2 / CO also occurred. Hence H 2 /CO molar ratio obtained at 973 k were 8.1, 5.7, 3.7 & 2 when the molar ratio of CO 2 /H 2 O were 0, 0.11, 0.33 and 1.0. However, increase in temperature to 1023 K changed the H 2 /CO molar ratio to 7.5, 5.3, 3.4 and 1.8 respectively for similar values of H 2 O/CO 2. Additionally, the CO 2 registered an increase with increasing temperature attributed to CO 2 reforming of methane occurring at higher temperatures, which remained almost constant at ! 973 K attributed to the limitations of CH 4 as the reacting species [125].  i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 6 ( 2 0 2 1 ) 3 2 8 0 9 e3 2 8 4 5

Ni/Al 2 O 3 based catalyst
Al-Nakoua and El-Naas [126] experimented with different molar proportions of H 2 O/CH 4 and CO 2 /CH 4 in a detachable reactor covered by catalyst B Nickel (33%)ÀChromium (5.6%)À Barium (11%)/La 2 O 3 (19%) and catalyst A which represents Ni (49%)/Al 2 O 3 (51%). The author observed that rapid carbon deposition was observed at 700 C, 1 atm during dry reforming of methane. However, when CO 2 reforming was performed in conjunction with steam reforming reaction in thinner channels deposited with a thinner layer of catalyst, a reduction in carbon deposition was noticed on the surface. The authors determined the equilibrium compositions of the CH 4 and CO 2 reactants, which are shown in Figs. 9 and 10 at pressures of 1, 2, 3, 4, 5, 10, and 20 bar respectively. Fig. 8 shows that CH 4 conversion was highest (85e90%) at pressures ranging from 1 to 3 bar and at temperatures in the range of 810À900 C.
Similarly, CO 2 conversion ( Fig. 9) was found to be above 80% in the pressure range of 1e3 bar and temperatures varying from 840 to 900 C. SEM studies (Fig. 10) revealed that    i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 6 ( 2 0 2 1 ) 3 2 8 0 9 e3 2 8 4 5 cracks formed in catalyst A possessed a length of 250 mm and width of about 10 mm, whereas for catalyst B the cracks were formed with a width of 20 mm and was spread up-to a certain length where the cracks were interconnected. Furthermore, the results [126] established that there was a significant improvement in the catalyst stability when the H 2 /CO was around 2.2 during bi-reforming of methane. These conditions were appropriate for Fischer-Tropsch applications and synthesis of methanol. Additionally, the authors observed a fivefold increase in the resistance of coke formation displayed by Ni/Al 2 O 3 catalyst An upon addition of Cr, Ba, and La 2 O 3 during a continuous reaction time of 140 h. The SEM results were also supported by EDX analysis.
The results of catalyst activity test on Ni/Al 2 O 3 in the ratio of 1:1 for Catalyst A is represented in Fig. 11. The catalyst film exhibited 50% conversion of CH 4 and 15e10% conversion of CO 2 when the reaction was continuously operated for 24 h at 630 C. Furthermore, with increase in inlet pressure from 1 to 23 psig, carbon deposition was noticed. The flow rate of CO 2 was 0.2 mol/h and the flow rate of CH 4 was 0.8 mol/h and the steam: carbon ratio was 0.51. However, NieCreBa/La 2 O 3- 4 conversion in the range of 50e75% and conversion of CO 2 increased from 20% to 60%.
The authors found a reduction in the conversion percentage ( Fig. 12) of methane and carbon dioxide when the reactor pressure of steam was increased up to 42 psig during continuous operation from 25 to 90 h. However, the conversion percentage of methane and carbon dioxide underwent an increase with further increase in temperature. Since catalyst deactivation has been caused by carbon deposition, hence suppression of coke formation was important. This was only achieved by optimization of the H 2 O/CH 4 and CO 2 /CH 4 and feed ratios.
Formation of coke is usually attributed to the following reactions 9 and 10: Major amount of coke formed in the temperature range of 850 to 900 C [127] resulted from disproportionation reaction involving carbon monoxide (reaction 9) and pyrolysis of methane (reaction 10).
Son et al. [128], observed that Ni/g-Al 2 O 3 catalyst was rendered stable by pre-treatment with steam at a temperature operated at 850 C. Ni/Al 2 O 3 based catalysts are relatively cheap because precious metals are not used and these catalysts can operate stably with high activity under excess steam. Ni/g-Al 2 O 3 catalyst used in this study was prepared by incipient wetness method. Thermodynamically, the catalyst promoted very high conversion of CH 4 (98.3%) and CO 2 (82.4%) when subjected to bi-reforming of methane for 200 h and resulting in H 2 /CO ratio of 2.01. Furthermore, the results revealed that the conventional catalyst system produced 15.4% coke after 200 h while the mass of carbon deposited was around 3.6% for catalysts exposed to steam. This novel steam pre-treatment technique significantly increased the resistance towards carbon formation in the presence of catalysts, thereby improving both long-term stability and activity.
Transmission electron images of fresh untreated Nickel Aluminium and fresh steam-treated NiAl (WNiAl) catalysts shown in Fig. 13 (a) and (c) revealed that it was difficult for  i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 6 ( 2 0 2 1 ) 3 2 8 0 9 e3 2 8 4 5 distinguishing nickel nanoparticles dispersed in the Nickel Aluminium catalyst, owing to their small size. However, the WNickel Aluminium catalyst showed the presence of distinct Ni nanoparticles in the range greater than 10 nm. Severe carbon deposition (Fig. 13b) was noticed for Ni/Al catalysts treated with steam for 200 h. The shape resembled to wiretype resembled carbon and the size of the nanoparticles were enhanced from 4.2 nm to 23.5 nm from Hydrogen chemisorption measurements. Nevertheless, carbon coke with wire typed shape did not appear in the WNickel Aluminium catalyst (Fig. 13d).
Two alumina supported Ni catalysts with pore sizes of 5.4 nm and 9 nm were synthesized and tested in the bireforming process [129] for the production of hydrogen rich gases. Structural and functional characterisation of catalysts showed that Ni/Al 2 O 3 with the largest pore size exhibited better characteristics i.e. higher capacity to adsorb CO 2 , higher surface area, higher proportion of stronger catalytic sites for hydrogen adsorption and lower Ni crystallite sizes. At all the investigated temperatures, for a CH 4 : CO 2 : H 2 O molar ratio of 1:0.48:1.2, a (H 2 þCO) mixture with H 2 :CO ratio around 2.5 was obtained. The optimum conditions for the production of hydrogen rich gases, were CH 4 : CO 2 : H 2 O ¼ 1:0.48:6.1 and 600 C.
Dan et al. [130] have investigated the role of Ni/Al 2 O 3 , Ni/ MgOeAl 2 O 3 and Ni/La 2 O 3 eAl 2 O 3 with bimodal pore structure in the bi-reforming process. The authors observed that La 2 O 3 and MgO promoted catalysts presented better functional and structural properties. Among all the catalysts, Ni/La 2 O 3 eAl 2 O 3 was found to be the catalyst with best stability and activity. The presence of both lanthanum and magnesium oxides contributed to excellent dispersion and stabilization of Ni nanoparticles on the catalyst surface. The catalytic activity for the bi-reforming process increased in the order Ni/Al 2 O 3 (-

NieCe based catalysts
Lanthanide group metals (La, Ce) have been reported [131e133], to be efficient promoters for Ni-based catalysts. Recently, literature reports have suggested [120,121], that during the bi-reforming of methane smaller nanoparticles of CeeZrO 2 ,ZrO 2 and MgO supported by Ni catalysts were found to be highly stable and active. Koo et al. [134], used a stable and extremely active magnesium oxide promoted Nickel/Al 2 O 3 catalyst to investigate catalytic activity and coke formation during bi-reforming for potential applications in gas to liquid (GTL) processes. In their study, the incipient wetness technique was employed to synthesise Ni/Al 2 O 3 catalysts with different concentrations of MgO. The authors used H 2 -chemisorption, CO 2 -temperature programmed desorption (TPD), BET analysis, and X-ray diffraction (XRD) to examine the characteristics of the prepared catalysts. Furthermore, the authors established that by changing the feed ratio of H 2 O/ CO 2 , a H 2 /CO ratio of 2 was obtained during the bi-reforming reaction. Additionally, catalysts containing 20 wt % magnesium oxide (MgO) showed high coke resistance and excellent catalytic performance during the bi-reforming reaction. MgO addition to the catalyst formed a stable MgAl 2 O 4 spinel phase at high temperatures and was quite effective in eliminating formation of coke by enhancing the adsorption of CO 2 because of higher base strength on the surface of the catalyst. SEM images of reduced Ni/MgO/Al 2 O 3 catalysts with changing concentrations of MgO content are illustrated in Fig. 14.
In particular, Cerium oxide has been widely recognized as an efficient promoter for Ni-based catalysts. This is because the redox properties of Ce 4þ /Ce 3þ results in easier gasification of the settled coke on the surface of the catalyst and also helps in storage and delivering of active oxygen thereby enhancing the dispersion of Ni. In another study [132], Ce-promoted Ni/ MgAl 2 O 4 catalysts synthesized by co-impregnation showed higher metal dispersion than Ni/MgAl 2 O 4 catalyst alone and demonstrated outstanding reducibility properties at lower temperatures of around 550 C, as established by XPS. The authors found that the catalytic activities of NieCe/MgAl 2 O 4 catalyst were the highest and it generated enormous coke resistance during the bi-reforming reaction performed at lower temperatures with Ce/Ni ratio of 0.25. These were due to stronger metal-support interactions and powerful dynamic oxygen movement through close contact with NieCe. Furthermore, when no Ce was present, the NiO crystallite size in the Ni/MgAl 2 O 4 catalyst was observed to be enormous at a value of 11.0 nm and indicated a lower metal scattering of 3.49%. The authors used BrunauereEmmetteTeller, (BET) adsorption H 2 -chemisroption, CO 2 -TPD and TPR to ascertain the crystallite size of NiO, basicity and reduction temperature of the catalysts. Results revealed that the nickel oxide (NiO) crystallite size, reduction degree and dispersion of the metal were significantly affected by cerium addition to the Ni/ MgAl 2 O 4 catalyst.
The authors [132] also employed Raman spectroscopy in the range of 1200e1800 cm -1 to investigate coke formation in the presence of Nickel Cerium/MgAl2O4 catalysts with varying Cerium/Nickel ratios. The spectra in Fig. 15 revealed two peaks in the vicinity of 1600 cm À 1 and 1350 cm À 1 which corresponds to G band and D band. The role of the G band is to provide useful information related to the electronic characteristics of filamentous carbon [135] while the D band arose from imperfect and polycrystalline graphite. Additionally, Cepromoted Ni/MgAl2O4 showed a decrease in peak intensity with a Cerium/Nickel ratio of 0.25 due to minimal coke formation on the surface of the catalyst in comparison to Ni/ MgAl2O4 catalyst without addition of cerium. These results were also in accordance with results obtained from TGA studies: quantification of coke deposition by TGA established a rise in graphitic and amorphous carbon with an increase in Ce/Ni ratio to 1, which was further confirmed by the increase in D band peak intensity (Fig. 15) for amorphous carbon. The advantages of using a Ce-promoted Ni/MgAl2O4 catalyst in  i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 6 ( 2 0 2 1 ) 3 2 8 0 9 e3 2 8 4 5 the bi-reforming reaction relate to its inherent ability to eliminate formation of amorphous coke in comparison to the Ni/MgAl2O4 catalyst.
Recently, there has been renewed interest in developing Ce1-xÀZrxO 2 catalytic systems [136]. It has been established that addition of zirconium oxide (ZrO2) to cerium oxide results in significant improvement in the oxygen storage capacity of cerium oxide, its thermal stability, metal dispersion and its redox properties. These improvements were attributed to the preferential replacement of Ce4þ with Zr4þ ion existing in the structure of the lattice surrounding cerium oxide (CeO 2 ).
[136e138], The Ce1-xeZrxO2 catalytic unit has also been regarded as an outstanding material for support in Ni-based catalyst systems [139e141]. CeO 2 eZrO 2 has been reported to be an effective promoter for the Ni/q-Al 2 O 3 catalytic system and helps in significant suppression of coke formation with a high catalytic stability [142] Bae et al. [143], investigated the catalytic activity of Ni/MgAl 2 O 4 catalyst in presence of cerium oxide-zirconium oxide (CeO 2 eZrO 2 ) during combined steam and CO 2 reforming of methane. The synthesis of the catalysts were performed by employing an impregnation technique followed by co-precipitation process of CeO 2 eZrO 2 components.
Furthermore, the basic supports such as MgO or exhydrotalcite MgAl 2 O 4 employed in this study possessed beneficial effects such as minimising coke formation due to the reduced acidic site density [144,145]. The Cerium oxidezirconium oxide (CeO 2 eZrO 2 ) component demonstrated a key role in the conversion of CO 2 by increasing CO 2 activation when contacted with crystallites of nickel. The catalysts synthesized by co-precipitation technique showed higher catalytic characteristics in comparison to catalysts synthesized by successive impregnation of Ni on support of MgAl 2 O 4 with cerium zirconium oxide (CeO 2 eZrO 2 ).
Addition of lanthanum to Ni/Al 2 O 3 catalysts inhibited the agglomeration of Ni particles due to the enhancement of strong metal to support interaction (SMSI). SMSI of catalysts was reported to enhance thermal stability [145e147]. Park et al. [148], synthesized 10 wt % NickelexLanthanum/MgAl 2 O 4 catalysts where x ranges from 0 to 5% by the co-impregnation technique during the bi-reforming of coke oven gas (COG). They conducted aging treatment with a H 2 : H 2 O: N 2 ratio of 1:10:1.25 with temperature around 900 C run for 50 h. The results revealed an increase in the Ni crystallite size for all the investigated catalysts subjected to ageing. Furthermore, the lanthanum promoted catalysts exhibited greater nickel dispersion than Ni/MgAl 2 O 4 catalyst due to their enhanced interactions between the metal and support. Results from catalytic tests performed at 900 C and at a pressure of 5 atmospheric pressure for 40 h with a CH 4 : H 2 O: CO 2 :H 2 :CO: N 2 ratio of 1:1.2:0.4:2:0.3:0.3 also revealed that aged Nie2.5La/ MgAl 2 O 4 catalyst showed maximum sinter stability and activity due to its enhanced nickel dispersion and surface area.
Furthermore, the role of Ce/Zr ratio on the catalytic activity of NieCe x Zr 1Àx O 2 catalyst and coke formation was demonstrated by Roh et al. [131], during the bi-reforming reaction. The authors used co-precipitation method to synthesise NieCeeZrO 2 catalysts having different ratios of CeO 2 /ZrO 2 for syngas production having potential applications in gas to liquid (GTL) processes. 15% NieCe 0.8 Zr 0.2 O 2 demonstrated excellent stability and highest activity during BRM which was attributed to the dispersion of nickel oxide having higher oxygen storage capacity and intimate contact with the support. Recently, it has been reported [131] that Cerium content along with nickel-cerium loading technique has an significant effect on the transfer of O 2 occurring between nickel (Ni) and cerium (Ce). Studies have also shown an improvement in coke resistance of 12 wt % Nickel/a-Al 2 O 3 catalyst in the bi-reforming reaction. [133,149], However, the characteristics of supports play a marked effect on coke formation [132]. For example, a-Al 2 O 3 support caused carbon deposition due to its acidity [150]; active nickel metal and Al 2 O 3 supports underwent interaction to form inactive NiAl 2 O 4 , resulting in deactivation of the catalyst [134]. Additionally, Baek et al. [151], observed higher coke resistance and enhanced catalytic stability of NieCe/ MgAl 2 O 4 (MgO/Al 2 O 3 ¼ 3/7) in comparison to NieCe/q-Al 2 O 3 in bi-reforming process.
Gao et al. [152] develop an ideal NieCe/ZSM-5 catalyst by the impregnation method for the bi-reforming process. The authors noted that by adjusting the parameters properly, highest conversion i.e. 99% and 94% of CH 4 and CO 2 to syngas was achieved in presence of NieCe/ZSM-5 catalyst. Furthermore, the catalyst did not show any deactivation and maintained high activity for 40 h. SEM, XRD and H 2 -TPR analysis further established the structure as well as composition of the catalysts and provided better understanding of the catalytic performance.
Chen et al. [153] synthesized highly dispersed mNi/xL/Si catalysts by one-pot sol-gel process and applied to the bireforming process for syngas production. Results revealed that the addition of lanthanum improved the stability, catalytic activity as well as the coke resistance of these catalysts. The17.5Ni/3.0LaeSi catalyst prepared using ethylene glycol and poly (ethylene glycol) displayed the best catalytic activity, coke resistance and stability. Additionally, the H 2 /CO ratios in the product gas were tuned by varying the C/S ratios in the feed.

NieMgO based catalysts
Jabbour et al. [154], employed a one-pot method followed by evaporation-induced self-assembly (EISA) to synthesise two types of catalyst namely Ni 5% M 5% where M represents Ca or Mg and Ni x% where X corresponds to 5e10 wt % along with packing of mesoporous Al 2 O 3 .
Low cost and widely available Mg 2þ and Ca 2þ containing salts were used as the additives based on their potential to yield basic properties (in their oxide form) and their positive impact on bi-reforming process [155]. Temperature programmed reduction (TPR) of calcined Ni-loaded samples displayed a strong reduction peak at higher temperatures ranging from 550 to 800 C (Fig. 16bef), which was ascribed to the reduction of oxidised nickel (Ni) undergoing stronger interaction with the support present in the mixed spinel phase [156]. The author observed that there was no peak signifying weakly-bounded Ni species below a reduction temperature of around 500 C, which was similar to the finding for non-porous impregnated alumina samples [82,157]. The authors in their studies noted that after the reduction process, the catalysts demonstrated higher dispersion of Ni within the arranged oxide cavity, possessed elevated activities and also i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 6 ( 2 0 2 1 ) 3 2 8 0 9 e3 2 8 4 5 showed long-lasting stability in bi-reforming of CH 4 performed at 800 C. They also observed that a relationship existed between carbon deposition and reactivity level in the presence of Mg free catalysts. SEM (Fig. 17A, B) and TEM (Fig. 17C, D) images for spent Ni 10% Al 2 O 3 clearly identified long carbon filaments on the exterior area of the alumina containing grains, with some grains which were found to more protected than others. These images also showed some Ni nanoparticles containing coke were situated either at the boundary between the support and the filament embedded into it. These images also resembled carbon nanotubes nucleation with a 'closed end' consisting a nanoparticle at either their closure or located inside the tip [158e160]. For both Ni 5% Mg 5% Al 2 O 3 and Ni 5% Ca 5% Al 2 O 3 , a peak was observed at very high temperatures of above 800 C, as shown in Fig. 18A (e and f); this was a possible indication of free metallic Ni existing under stronger interactions or may have been related to the reduction of Mg-or Ca-derived species. Furthermore, a linear correlation was established between H 2 uptake and Ni content, which confirmed that all Ni used in the synthesis was completely reclaimed in the solid after preparation. The authors also reported that, due to the  endothermic nature of the bi-reforming reaction, the conversion of both CH 4 and CO 2 decreased at lower temperatures, and CO 2 conversion was more significant below 700 C.
Results also revealed that there was a beneficial effect when 5 wt% magnesium or calcium was used for the conversion of both CH 4 and CO 2 represented by d and e (Fig. 18A,  B), compared to catalyst containing 5 wt % Nickel and without additive. The enhancement in the reactivity in presence of 5 wt (wt%) magnesium or calcium was higher than with Ni 7.5% Al 2 O 3 despite the lower Ni content reported in previous literature [161e163]. Jabbour et al. [154], established that in addition to high activity levels, doping of the Ni catalysts with Mg or Ca additive resulted in excellent catalytic stability for high temperature bi-reforming operations. The authors found that mesoporous catalysts synthesized by one-pot method served as an ideal candidate for catalysing met gas production from biomass-related natural resources. The beneficial effect of nickel confinement in the pores was twofold, one in protecting the metal nanoparticles against sintering phenomenon and second against coking due to steric constraints.
Kang et al. [164], synthesized core shell structured Ni catalysts Ni/MgOeAl 2 O 3 and Ni/Al 2 O 3 and via technique coined multi-bubble sono-luminescence and conducted tests using these catalysts for the bi-reforming process. The authors observed that Ni catalysts constituting of 10% Ni loaded on Aluminium oxide or MgOeAl 2 O 3 exhibited exemplary performance during the steam reforming of methane, achieving 97% conversion of CH 4 at a temperature of 750 C. Additionally, methane conversion was 96% at 850 C during dry reforming of CH 4 and demonstrated greater thermal stability for the initial duration of 50e150 h. The results also established that supported Ni catalysts demonstrate excellent performance in both mixed and auto-thermal reforming of CH 4 , where satisfactory thermal stability was noted for the first 50 h. An interesting observation was that no significant carbon formation was obtained on surface of the investigated catalysts after the reforming reaction. Very recently, Koo et al. [118], synthesized nickel catalysts in the nanoscale by employing a mixture of magnesium oxideÀaluminium oxides (MgOeAl 2 O 3 ) obtained from a structure resembling hydrotalcite. Their results revealed an enhancement in the coke resistance with various mixed ratios of Mg/Al for the generation of syngas during bi-reforming for applications in GTL processes.
Mesoporous SBA-15 has aroused enormous interest among researchers in steam reforming [165,166], and CH4 dry reforming [68,167], process due to its high surface area, high silanol group density, uniformity of pores and enhancement in active metal dispersion with smaller crystallite size [168]. A group of Nickel/SBA-15 catalysts with Ni content ranging from 5 to 15 wt % were synthesized by Huang et al. [169], along with 10% Ni/MgO/SBA-15 catalysts with MgO content ranging from 1 to 7 wt (wt %) during combined steam and dry reforming reaction in a continuous micro-reactor. XRD, H 2 -TPR and CO 2 -TPD techniques were used to investigate the structure of catalysts. The authors observed that selectivity of carbon monoxide (CO) for these reactions was almost 100% and they also noticed that with the change in the molar ratio of H 2 O/ CO 2 , there can be effective control of the H 2 /CO ratio. After reaction at 850 C for more than 120 h with 10 wt % of Ni/SBA-15 catalyst, the conversion of methane underwent a decrease from 98% to 85% while the conversion of CO 2 reduced from 86% to 53%, respectively. Additionally, the catalyst containing 3% MgO/SBA-15 loaded with Ni demonstrated excellent catalytic activity after a reaction for 620 h and the CO 2 conversion over this catalyst underwent a decrease from 92% to 77%, while no change in CH 4 conversion was observed. Furthermore, certain changes in the MgO promoter enhanced the Ni 0 species dispersion and resulted in an increase in the adsorption affinity of CO 2 , thereby inhibiting coke deposition and retarding the deactivation phenomenon.
MgeAl mixed oxides derived from hydrotalcite-like materials are reported [118,154], to exhibit higher activity and stability in bi-reforming process due to its basic property, enhanced steam and CO 2 adsorption, strong Ni to support interaction and fine dispersion. These catalysts were synthesized using various preparation methods such as impregnation of pre-calcined carriers or simultaneous co-precipitation of the mostly nitrate-based solution of all the constituents. Roohollahi et al. [170], synthesized numerous Ni-based catalysts supported on mesoporous MgOeAl 2 O 3 resembling a   [171]. The synthesis of hydrotalcite-like components was performed at an optimized pH of 10, which were then calcined at various calcination temperatures from 500 to 800 C to obtain a homogenous texture. Results from the bi-reforming reaction conducted on the catalysts at 800 C for 36 h with feed stock constitution of CH 4 : CO 2 : H 2 O ¼ 1.0:0.4:0.8 at GHSV ¼ 150,000 mL$gcat À1 h À1 revealed that the sample derived from the carrier calcined at 700 C exhibited the lowest nickel crystallite size (2.68 nm) and largest nickel surface area (25.01 m 2 /g). The results also established excellent conversion efficiencies for CH 4 (93.7%) and CO 2 (75.2%) and higher resistance to coke formation. The high resistance to carbon formation was due to the enhanced strength of basic sites formed in the catalyst carrier during the calcination of MgeAl hydrotalcite-like components at 700 C.
He et al. [172] investigated the role of nickel nanoparticles supported on the binary MgeAl metal oxide catalysts during bi-reforming of CH 4 . The successful synthesis of Ni/MgO, Ni/ Mg x Al y O, and Ni/Al 2 O 3 catalysts were also supported by XRD, TEM, and FT-IR results. The TPR profile revealed that the reduction temperature of Ni species underwent a slight decrease upon addition of Al due to the formation of the NiAl 2 O 4 phase. Furthermore, the XPS spectra demonstrated that Ni/MgO and Ni/Al 2 O 3 produced higher amounts of Ni 0 after H 2 reduction.

NiOeCaO based catalysts
NiOeCaO catalyst has been reported [173] to exhibit high selectivity, activity and productivity in the oxidative conversion of methane to synthesis gas. Choudhary et al. [174], have reported the role of NiOeCaO catalysts during SRM, DRM and combined steam and CO 2 reforming of methane to produce CO and H 2 at varying temperatures ranging from 700 to 850 C and gas hour space velocities (5000 to 70,000 cm 3 g À1$ h À1 ) They characterised the catalysts using various techniques including XRD, XPS and TPR. Their results revealed that the catalysts demonstrated high activity/selectivity during all of the reforming processes tested. When CO 2 reforming was performed in conjunction with steam reforming process a drastic reduction in carbon deposition from 25.96% to 1.08% was observed for a feed composition of CH 4 :CO 2 :H 2-O ¼ 1.0:0.55:0.55 [174]. Furthermore, the authors noted that when the feed composition was maintained for CH 4 : H 2 O ¼ 1:1 during the steam reforming reaction, the reaction characteristics were outside the coke formation control. Nevertheless, for the dry reforming reaction with a reactant feed composition of CH 4 : CO 2 in the ratio of 1:1, the coke formation was obtained from a gas mixture formed at equilibrium.
The authors [174] also noticed that complete conversion of methane to syngas with 100% selectivity consisting of both CO 2 and H 2 and during bi-reforming reaction at 800 C with GHSV ranging from 20,000 to 30,000 cm 3 g À1 h À1 . The authors observed that by changing the carbon dioxide/steam (CO 2 / H 2 O) ratio in the reactant feed, a significant improvement in the bi-reforming process occurred and also a desirable H 2 /CO ranging from 1.5 to 2.5 was seen. TPR studies were performed to measure changes in the concentration of H 2 owing to reduction of nickel oxide in the catalyst. The TPR curves (Fig. 19) revealed maximum value in the range 400 and 450 C, in accordance with the maximum peak temperature observed around 418 C attributed to reduction of bulk nickel oxide.

NieSiO 2 based catalysts
Chen et al. [153] synthesized mNi/xLa/Si catalysts with efficient dispersion characteristics and comprising of various weight contents of nickel and lanthanum by using sol-gel method, and tested these catalysts for bi-reforming of CH 4 to generate syngas. The authors noticed an increase in the stability, catalytic characteristics and an enhancement in the Fig. 19 e TPR studies using Ni/CAO catalyst of various proportions, adapted from Ref. [174]. resistance of carbon deposited during bi-reforming in presence of mNi/xLa/Si catalysts upon addition of lanthanum. The 17.5Ni/3.0La/Si catalyst prepared using ethylene glycol and poly (ethylene glycol) demonstrated excellent coke resistance and catalytic activity. Additionally, modification of the carbon/sulphur (C/S ratios) in the reactant caused tuning of the H 2 /CO ratios in the gas generated as products. Furthermore, when the bi-reforming reaction was performed in presence of 17.5Ni/3.0La/Si catalyst produced a H 2 /CO ratio of about 2 for the C/S ratio of 0.5.
Ni-phyllosilicate (PS) intermediates were used to synthesise [175] NickeleSiO 2 eMgO materials for its application in bireforming of methane., and the role of reaction temperature as well as steam on the reforming process were also investigated. The results revealed that catalytic performance was excellent and resulted in 80% conversion of CH 4 and 60% CO 2 conversion respectively, at 750 C for 140 h in presence of a Nie30 wt % SiO 2 e55 wt % MgO catalyst. Furthermore, carbon deposition was found to be stable when the H 2 /CO ratio was maintained at 2. The catalytic behaviour of the investigated catalyst was ascribed to its structural stability, acidic strength and enhanced basicity for the reforming reaction conducted at high temperatures. The presence of nickel-magnesium comprising phyllosilicates in the reduced catalysts were established by TEM and XRD technique. Furthermore, a TPR profile of around 750 C substantiated the presence of strong interlinkage between nickel and Silicon dioxideeMagnesium support species. A representative schematic diagram of this is illustrated in Fig. 20.
Jabbour et al. [82], used an one pot method (Fig. 21) for synthesis of mesoporous nickelealumina catalyst containing 5 wt % Nickel and possessing an ordered structure. From their observations, the ordered Niealumina sample exhibited excellent stability in comparison to non-porous and impregnated catalyst during the bi-reforming process at 800 C over 40 h. The conversion percentage of methane was consistent with the thermodynamically expected variants. The authors also noted that nickel catalyst loaded with SBA-15 demonstrated enhanced catalytic activity than Ni/celites, however both these catalysts underwent rapid deactivation on stream which was attributed to the partial re-oxidation of the Ni active phase under the investigated conditions (see Fig. 22).
SBA-15 support has been employed for suppressing carbon formation in steam reforming reactions [166,167], and has aroused significant interest due to its high surface area, high silanol group density, pore uniformity. An incipient wetness method was employed by Singh et al. [176], to synthesise SBA-15Àpacked Ni catalyst by impregnating nickel nitrate onto the SBA-15 support. They found that the surface area decreased from 669.5 m 2 g À1 to 538.6 m 2 g À1 with the change in catalyst support from SBA-15 to 10 wt % Nickel/SBA-15 catalyst was confirmed by BET surface area analysis. Analysis by H 2 -TPR demonstrated the complete reduction of NiO nanoparticles beyond 576.85 C where the temperature of reduction from nickel oxide to metallic nickel was completely dependent on metal-support interactions which was correlated to the location, confinement effect and crystallite size of nickel oxide. CO 2 and H 2 O had a significant role in controlling formation of carbon during bi-reforming of methane due to their unique capability in converting the partially dehydrogenated C x H 1-x to a mixture of CO and H 2 . The authors observed that carbon dioxide conversion and methane conversion was 58.9%, and 61.6% respectively. Furthermore, the resulting H 2 /CO ratio was found to be 2.14 during the combined CO 2 and steam reforming of CH 4 under stoichiometric conditions. A steep increase in the H 2 and CO yield was noticed while increasing  the CO 2 /(CH 4 þ H 2 O) ratio, and a considerable decrease in the ratio of both hydrogen and carbon monoxide ratio ranging from 2.14 to 1.83 was observed with a decrease in the H 2 O/ (CH 4 þ CO 2 ) ratio. Furthermore, Ni/SBA-15 exhibited higher resistance towards both coking and sintering which was related to the efficient distribution of nickel particles and steric effects caused by SBA-15.
The synthesis, catalytic activity and characterisation studies on Ni/SBA-15 catalysts during BRM has been reported [177]. The authors observed that 25 wt % Nickel/SBA-15 SGM catalyst showed the maximum conversion of CH 4 (23%, 548 C), which was followed Ni/SBA-15 HTM (CH 4 z 20% at 548 C) and 10% CH 4 conversion was achieved in presence of 25 wt % Ni/SBA-15 CG catalyst. CO 2 and CH 4 conversion were found to be 82% and 23% at 548 C, respectively. These differences in the catalytic activity were related to the degree of availability of active metal for the reaction. Due to excellent catalyst activity of these catalysts, these catalysts were employed for the formation of membrane reactor with hollow fibres and catalytic hollow fibres.
The authors employed commercially available SBA-15 for comparison. SEM micrographs (Fig. 23) revealed a needle shaped particle having a grain size of around 0.6 mm (A1 and A2). The SBA-15 particles synthesized by the sol-gel method did not display a homogeneous shape and consisted of a hard shell covering smaller particles whose grain size was approximately around 0.1 mm.
Mesoporous siliceous SBA-15 material has been used as support for preparing active metal catalysts in several reforming processes [66,178]. The mesoporous SBA-15 support possessed uniform mesopores with thick framework walls, high thermal stability and wide specific surface area [179], [. Additionally, the ordered hexagonal mesostructure of SBA-15 support provided a confinement effect to anchor the nanoparticles inside its channels and also prevented deposition of carbonaceous species metal sintering [115]. Siang et al. [180], used the incipient wetness impregnation technique to synthesise stable and active boron (B) aided catalyst for bireforming of methane. Results revealed that B 2 O 3 and nickel oxide particles were scattered on the outer area of SBA-15 support possessing higher surface area. Additionally, the authors observed an enhancement in catalytic activity that underwent a linear increase with temperature due to the endothermic behaviour of the catalysed process. They obtained H 2 /CO molar ratio of 2.7 and 67.3% of CH 4 conversion at 799.8 C which was highly significant for downstream Fischer-Tropsch (FT) applications. Furthermore, XPS measurements revealed that B facilitated the adsorption of CO 2 through the electron transfer to the Ni cluster at the neighbourhood, thereby improving its catalytic activity. More importantly, analysis by XRD and Raman showed that boron doped catalyst was completely free from graphitic and amorphous carbon deposition. This was due to the incorporation of B into the octahedral sites occupied by NiO, resulting in inhibition of carbonaceous deposits.
Encapsulation of Ni particles in a suitable support material has been reported [181] to enhance the sintering resistance and coke resistance of Ni catalysts. The introduction of promoters namely rare-earth metals; metal oxides, alkaline earth and alkali metals is also one of the effective strategy to prevent the sintering of active sites/supports and enhance the coke-resistant ability of catalysts [182]. Chen et al. [153], synthesized highly dispersed mNi/xLa-Si catalysts by employing one pot sol-gel process by varying the weight percentages of nickel and lanthanum. These catalysts were subsequently applied to generate syngas during bi-reforming of CH 4 . The authors observed that La addition enhanced the stability, coke resistance and catalytic activity of mNi/x LanthanumeSilicon catalysts. The 17.5Ni/3.0LaSi catalyst prepared by employing poly (ethylene glycol) and ethylene glycol displayed the coke resistance, maximum selectivity and catalytic activity. One notable observation was that a H 2 / CO ratio of about 2 was obtained when the carbon to sulphur ratio was maintained at 0.5, for the 17.5Ni/3.0LaSi catalyst, suitable for potential applications in Fischer-Tropsch synthesis.

NieSn catalysts
Literature reports [183] have established that CeO 2 eAl 2 O 3 combinations are potential supports for reforming reactions. Furthermore, the redox properties of CeO 2 resulted in a significant improvement in the oxidation of deposits thereby enhancing the lifetime of the catalysts [184,185]. Furthermore, second metal addition promoted the formation of an active phase along with modification of the support. Bimetallic systems [186] have been known to display superior catalytic activity and increased the resistance of carbon formation in comparison to their own counterparts. The bimetallic combination of NieSn has proved to be of considerable interest in reforming reactions. Additionally, the dispersion of nickel over the catalyst surface has been shown to be enhanced in the presence of Sn [187].
Straud et al. [188], synthesized a set of multicomponent advanced catalysts composed of Sn, CeO 2 and Ni/Al 2 O 3 . A schematic diagram representing the production of syngas in the presence of the investigated catalysts are shown in Fig. 23. The authors observed that addition of minute amounts of the investigated dopants improved the performance of methane reforming using CO 2 . From their results it was noticed that a multicomponent Sn 0.02 Nickel/Cerium-Al catalyst showed excellent catalytic characteristic and remained active over a long period of 92 h. The catalyst also demonstrated an exceptional level of stability and conversion during BRM. Comparison of dry reforming and BRM reactions over the Sn0.02Ni/CeeAl catalyst at 700 C revealed that H 2 /CO ratio remained above 1.6 for 24 h. This suggested that the catalyst could generate high quality synthesis gas by introduction of water into the reforming mixture.
Therefore, the addition of water established the suitability of the Sn0.02Ni/CeeAl catalyst for bi-reforming of CH 4 . Furthermore, the results also revealed that presence of ceria created high storage capacities for oxygen and changed both the acidic and basic characteristics of support thereby enhancing the catalyst performance. The multicomponent i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 6 ( 2 0 2 1 ) 3 2 8 0 9 e3 2 8 4 5 catalyst Sn0.02Ni/CeeAl proved to be active over period of 92 h and fared well over a range of space velocities and temperatures.
The remarkable level of stability and excellent conversions seen in the bi-reforming process has proved the versatility of Sn0.02Ni/CeeAl catalyst which can be upgraded to variety of CO 2 containing feed stocks.

Effect of mineralisers on ZrO 2 Àsupported Ni catalysts
Literature reports [189,190]have established the role of ZrO 2 as an excellent support for reforming reactions because of its higher oxygen mobilisation, excellent thermal stability as well as its unique basic and acidic properties. The reinforced interaction between nickel and zirconium oxide makes zirconium oxide an effective support for a nickel based catalyst.
Agli et al. [191], reported that basic mineralisers affect the nucleation, rearrangement and crystallisation of gel made of zirconia during the synthesis of zirconium oxide. Hence, Zhao et al. [190], used the hydrothermal method [192,193] with various mineralisers along with L-arginine ligand-using wetness impregnation technique [194] to synthesise Ni/ZrO 2 supports for bi-reforming of methane. Results from their studies revealed that the catalysts performance depended on texture of the zirconium oxide support and its morphology was also highly affected by the mineraliser amount. In this study, the authors synthesized Zirconium oxide support with a mole ratio sodium acetate/Zr 4þ as 0.5 denoted by (SAc 0.5 ). ZrO 2 Àsupported Ni catalyst was synthesized by employing sodium acetate where the mole ratio was N SAc/Zr ¼ 0 and also showed increased catalytic activity in comparison to catalyst i.e zirconium oxide synthesized using (SC) where the ratio was N sc /Z r ¼ 0.5. The authors established from the results that sodium acetate would serve as a suitable mineraliser for making an excellent ZrO 2 support and also in terms of its stability and activity. Furthermore, the authors [190] observed that, in general, the addition of different amounts of mineralisers to ZrO 2 supports had a significant effect on textural properties, which in turn affected the behaviour of the Nisupported catalysts on zirconium oxide and also influenced the catalytic activity of the Nickel/Zirconium oxide catalysts during bi-reforming of methane.
The TEM micrographs in Fig. 24 show that all the investigated zirconium oxide supports resembled cobblestone like structure, with dimension of mesopores. Interestingly, reduction in pore volume and pore diameter along with expansion in the surface area was observed when the S Ac /Zr molar ratio rose from 0.5 to 2.0. This provided the Ni/ZrO 2 catalyst with a bigger crystallite size but also caused lower dispersion compared to Ni/ZrO 2 (SA c0.5 ). From these studies, the authors noted lowering in the sintering resistance of nickel in Ni/ZrO 2 (SAc 2.0 ) catalyst than Ni/ZrO 2 (SAc 0.5 ), which was attributed to its imperfect interaction between nickel and zirconium oxide as established by H 2 -Temperature programmed reduction. Fig. 25 displays the catalytic characteristics of the prepared Ni/ZrO 2 catalysts with varying amount of mineralizers present in the support.  [190]. i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 6 ( 2 0 2 1 ) 3 2 8 0 9 e3 2 8 4 5 The figure above revealed the initial activity in the order of Ni/ZrO 2 (SAc 0.5 ) z Ni/ZrO 2 (Non) > Ni/ZrO 2 (SC 0.5 ) > Ni/ZrO 2 (SAc2.0) respectively. Nevertheless, the Ni/ZrO 2 catalyst without any acetate in the figure showed least stability among all the catalysts.

Co-based catalysts
Itkulova et al. [195], used Group VIII metals(0.25e1 wt %) along with alumina as a support to synthesise 5% bimetallic Cobased catalysts. The bimetallic Co constituted catalysts were   [195]. i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 6 ( 2 0 2 1 ) 3 2 8 0 9 e3 2 8 4 5 synthesized by impregnation of Al 2 O 3 with solutions comprising of both cobalt and platinum compounds followed by a thermal treatment. The authors investigated the stability of these catalysts by varying the temperature (300e800 C), composition of feed mixture and space velocity (SV) (500À3000 h À1 ) during bi-reforming as well as DRM. The authors observed that methane conversion was almost 100% at 750 C and 770 C for 5 wt % CoePt (9:1)/Al 2 O 3 catalyst during both DRM and BRM, However, the results in Fig. 26 a reveal a decrease in CO 2 conversion during the reforming process (Fig. 26 b) performed over the entire temperature range compared with DRM due to the suppression of CO 2 eCH 4 reaction by the competing CH 4 eH 2 O interaction. The authors also observed a surge in the H 2 /CO ratio from 0.84 to 1.0 when 20 vol % steam was added to the feed with equal amounts of CH 4 and CO 2 .
The effect of Pt on BRM for various feed compositions was also investigated [195]. Results in Fig. 27 revealed an enhancement in the catalytic activity with increased platinum loading varying from 0.25 to 1 wt %. It was also noted that higher temperatures were necessary for the total conversion of CH 4 when there was a decrease in platinum content in the catalyst. It was established that addition of 10e30% steam had a marked effect on the conversion of CH 4 , which further decreased the temperature required for conversion of methane and an increase in the ratio of H 2 /CO. Syngas produced during bi-reforming of methane over 5% CoePt/Al 2 O 3 catalyst showed a desirable H 2 /CO ratio >1. Pt  i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 6 ( 2 0 2 1 ) 3 2 8 0 9 e3 2 8 4 5 was responsible for the formation and stabilization of highly dispersed and reduced bimetallic nanoparticles. Itkulova et al. [196] have investigated the role of 5% CoePt catalysts modified with 0.25e0.5 mass% Pt supported on alumina and modified with zirconia (ZrO 2 ) with amounts ranging from 5 to 10 mass% of Zr in the bi reforming process in the temperature range of 300e755 C, and CO 2 /CH 4 in 1:1 ratio. The results revealed that introduction of 20 vol% of steam into the CO 2 eCH 4 feed was highly beneficial to the performance of the bi-reforming process. The improved performance of the 5%CoePt/Al 2 O 3 eZrO 2 catalysts was attributed to the synergistic effect caused by the combination of two reactions i.e. dry and steam reforming of methane.

PtÀbased catalysts
The major issue affecting commercialisation of the reforming process is coke formation, which causes deactivation of catalysts. The most effective way for decreasing coke formation is by coupling CO 2 with steam. It has been established that the support plays a significant role in suppressing formation of coke on Group VIII metals during the CO 2 reforming of CH 4 [197e200]. Several researchers [201,202], have demonstrated that the addition of promoter such as cerium led to a marked improvement in the activity of catalyst, stability and also decreased the sintering of ZrO 2 during calcination performed at high temperature. Literature reports [203e207], have also established that PteZrO 2 catalysts demonstrate high stability and activity under extreme deactivating environment.
The activity for CO 2 reforming of methane has been investigated by Noronha et al. [208], on PteZrO 2 (Fig. 28 a) and PteCeeZrO 2 (Fig. 28b) catalysts under CH 4 :CO 2 molar proportion of 2:1. The authors noticed that the conversions of CH 4 and CO 2 decreased slightly in the presence of PteZrO 2 catalysts (see Fig. 29).
Interestingly, after the removal of water the conversion of carbon dioxide and methane remained roughly constant, and at the same level as after interaction with water (Fig. 28a). A more drastic reduction in H 2 /CO from 0.82 to 0.45 was also noticed after 22 h. However, the authors observed that DRM of CH 4 in presence of H 2 O occurred differently with PteCeeZrO 2 PteZrO 2 catalysts. Furthermore, CH 4 and CO 2 conversion underwent a decrease upon addition of water during the ongoing reaction on the PteZrO 2 (Fig. 28 a) catalysts, with the decrease in conversion of CO 2 being significant. This was attributed to the reaction between excess H 2 and higher amount of CO 2 through water-gas shift reaction [209] in the reverse mode. The reduced stability observed in the PteZrO 2 catalyst was related to the diminishing of oxygen vacancies on the support and oxidation behaviour. Furthermore, Temperature Programed Oxidation analysis established that water addition enhanced the amount of mass of carbon deposited on the surface. Nevertheless, PteCeeZrO 2 exhibited excellent stability in presence of H 2 O and its stability was due to higher number of vacancies caused by O 2 on the support. Both the Pt catalysts with and without Ce were relatively stable during CO 2 reforming of methane performed at 105 psig.

Porous NiÀbased catalysts
BRM is considered to be an endothermic process which involves optimization of the temperature within the bed containing the catalyst and also a large amount of heat transfer into the reaction system occurs with the aid of external source. This suggests that catalysts used in these processes should have greater thermal conductivity, which can be attained by employing metallic supports [210]. Several authors have reported the role of numerous catalysts that operate on these supports [211,212].
A promising under layer for Ni catalysts is MgO due to its high thermal stability, ability to decrease carbonisation and ability to easily form solid solutions with NiO, also aids in promoting the dispersion of reduced nickel crystallites [213]. There have been numerous studies on supported Ni catalyst with a MgO under layer: supports on metal foams [213], porous Ni plate [212] and Al 2 O 3 eSiO 2 [214].
Danilova et al. [215], reported the synthesis of thick porous Ni ribbon (pNirb) with a MgO under layer supported by Ni catalyst on the top. The under layer constituting magnesium oxide was synthesized by packing of the support with MgNO 3 solution, then subjected to drying followed by calcination performed at 550 C in presence of air designated as Support 1 and the calcination performed in flowing H 2 was known as Support 2. The catalyst was reduced under the atmosphere of flowing H 2 at 750 C was termed catalyst I and the catalyst reduced at 900 C was termed catalyst II. Use of these supported catalysts resulted in 49% and 56% conversion of CH 4 with support 1 and support 2, respectively. The greater activity of support 2 in comparison to support 1 was attributed to efficient dispersion of Ni crystallites that was produced from solid solution reduced in presence of reaction medium. The authors further remarked that catalyst II (2.7% Ni/(p Nirb þ8.6% MgO) and 4.0% Ni/(pNirb þ10.4% MgO) exhibited excellent stability for CH 4 conversion over a period of 18 h compared with catalyst I (4.6% Ni/(pNirb þ6.0% MgO) and 4.6% Ni/ (pNirb þ 6.0% MgO) under the following conditions:

Nanocatalysts
Recently, nanocatalysts have attracted much attention [38]. Nanocatalysts show better selectivity, outstanding stability and higher activity, due to their special crystal structure, higher amount of surface atoms in comparison to their microsized counterparts and larger specific surface area [216]. Many works [217] have shown that catalyst preparation with larger surface area affected significantly the physical and chemical properties, which can only be achieved by a nanocatalyst.
Khani et al. [218], synthesized novel M/ZnLaAlO 4 nanocatalysts where M consists of 3%Ru, 10% Ni and 3% Pt using wet impregnation technique and characterised these by using TPR, FT-IR, TEM, XRD, FE-SEM, Thermogravimetric analysis and Differential Thermal Analysis. The authors evaluated the catalytic characteristics of the these catalysts in the SRM, DRM and BRM of methane at temperatures varying from 600 to 800 C at different gas hourly space velocities values of 10,500, 7000, 3500 h À1 . TGA revealed that the nanocatalysts namely 3% Pt/ZnLaAlO 4 and 3% Ru/ZnLaAlO 4 did not exhibit any coke formation during SRM, which was also supported by FE-SEM (Fig. 30).
The authors noted that an increase in temperature during the bi-reforming (BRM) of methane increased the CH 4 conversion, however decreased conversion of CO 2 (Fig. 31). Furthermore, Fig. 30 shows that 3% Ruthenium/ZincLaAlO 4 demonstrated the lowest activity while 10%Ni/g-Al 2 O 3 showed marked activity for CO 2 conversion. Additionally, among the four tested catalysts (Figs. 30), 3% Ru/ZnLaAlO 4 displayed the highest catalytic CH 4 conversion. The authors observed a reduction in H 2 /CO ratio with the rise in temperature from 600 to 700 C for the investigated catalysts used in the bi-reforming process. The authors observed 3% Ru/ZnLaAlO 4 a H 2 /CO of   2.1 at a temperature of 800 C, while for 10% Ni/g-Al 2 O 3 catalyst a lowest value of 1.6 was obtained., 3% Ru/ZnLaAlO 4 was considered as the potential catalyst for potential applications based on its resistance to formation of carbon and catalytic efficiency in BRM, dry reforming and SRM. TPR profiles of the nano-catalysts showed lowest reduction temperatures at the onset for 3%Pt/ZnLaAlO 4 , 10% Ni/ZnLaAlO 4 , 3%Ru/ZnLaAlO 4 at 264 C, 333 C and 230 C respectively.
Potdar et al. [219], noted that the nanocatalyst NieCeeZrO 2 synthesized by employing co-precipitation technique exhibited excellent coke resistance and highly stable catalytic activity attributed to the greater mobility of oxygen in the carbon dioxide reforming of methane and higher surface area. Roh et al. [121], demonstrated that higher stability and activity of Ni/MgOeAl 2 O 3 catalyst with nano dimensions was due to the beneficial effects of magnesium oxide (MgO) namely stronger interaction between nickel and support, basicity, enhanced steam adsorption and also the crystallite size of nanosized NiO. Sadykov et al. [220], investigated the role of catalysts made of nanocomposites in the bi-reforming reaction. Nanocomposite catalysts consisting of nickel particles implanted into an oxide matrix of Yttrium or Scandinavium-stabilised Zr (YSZ, ScSZ) mixed with doped CeeZr oxides or LanthanumePraseody-miumeManganeseeChromiumeOxygen (La-Pr-Mn-Cr-O) perovskite along with promoters namely Pd, Ru and Pt were synthesized via different routes [101].

Ruthenium based catalysts
Soria et al. [221], investigated the role of H 2 O along with Ru/ ZrO 2 eLa 2 O 3 catalyst placed in a fixed-bed Palladium reactor with membrane during bi-reforming of methane. The authors observed that addition of H 2 O along with CO 2 during the reforming reaction significantly affected the catalyst activity. Fig. 32 shows that the presence of small concentrations of H 2 O (1e2 vol %) did not affect the conversion of CH 4 appreciably, but an increase in steam to 5 vol % did result in increased CH 4 conversion.
Furthermore, the CO 2 conversion gradually decreased with increasing concentration of H 2 O from 1 to 5 vol %, and the CO 2 conversion exhibited lower values below 330, 375 and 450 C for water content ranging from 1 to 5 vol %. Furthermore, at a designated temperature, the composition of syngas (H 2 /CO ratio) was altered with the change in the concentration of H 2 O feed.
The authors [221] also investigated the stability of the Ru/ ZrO 2 eLa 2 O 3 catalyst during both bi-reforming and carbon dioxide reforming of methane at 500 C. It was observed that without steam presence in the reaction feed, the Ru/ZrO 2-eLa 2 O 3 catalyst was very stable and 15% of deactivation was noticed. Fig. 33 shows that the addition of water had a marked effect on the stability, which increased in a significant manner with the increase in steam amount. The deactivation values were 5%, 11%, 8% for addition of 5, 1, 2 vol % H 2 O, respectively.
Research has indicated that steam addition to the CO 2 during bi-reforming of CH 4 affects the reaction parameters in a temperature-dependent manner, which is noteworthy for the generation of high purity H 2 using Pd-based membrane technology.

Perovskite-type oxides
Chaudhary and Mandal [101] demonstrated the CH 4 conversion of synthesis gas in presence of NdCOO 3 perovskite-type oxides used as a catalyst during BRM of methane. Results from their studies revealed that H 2 O and CH 4 conversion along with the H 2 /CO ratio, were greatly affected by the feed ratio of CO 2 /H 2 O during the reforming process. Furthermore, the heat of reaction was strongly affected by relative concentration of oxygen in the reactant feed, space velocity and temperature. NdCOO 3 perovskite-type catalyst proved to be highly efficient for carbon-free bi-reforming process.
The Sr-doped NieLa 2 O 3 catalyst has been reported [222] to generate the highest CH 4 and CO 2 activity along with the highest resistance to carbon deposition over the catalyst surface which was attributed to considerable involvement of a large amount of mobile lattice oxygen species as a result of CeH activation in dry reforming. Yang et al. [223], have reported the role of Sr addition to LaNiO 3 perovskite catalysts during the bireforming process. Mineralogical characterisation by XRD revealed a distortion in the perovskite lattice and generation of alien phases such as La 2 ÀxSr x NiO 3 ± d and Sr 0.5 La 1.5 NiO 4 . The authors [223] observed that the reduction behaviour was affected by the presence of these phases. The results also revealed that strontium oxide adsorbed CO 2 during the bi-reforming reaction and formed strontium carbonate (SrCO 3 ), which possessed the unique ability of inhibiting carbon sources by producing La 2 O 2 CO 3 . The addition of Sr particles covered the support sites thereby resulting in large-sized Ni particles by decreasing the interlinkage between the support and active metals. The authors [223] recommended using a small amount of Sr in the perovskite-based catalyst for obtaining greater resistance to carbon deposition. Furthermore, larger Ni particles were formed, with diameters of 30.8, 29.9, 27.6 nm for concentrations of 10%, 50%, 30% SrO containing La 2 O 3 e NiO 3 catalysts in comparison to a La 2 O 3 eNiO 3 particle size of 13.5 nm.
Kim et al. [224], investigated the bi-reforming reaction of methane employing mixed oxides of La, Sr and Ni packed on b-SiC catalysts loaded with Al 2 O 3 for assessing the conversion of carbon dioxide at a certain concentration of Al 2 O 3 used as a modifier. The authors found that though all the investigated tested catalysts provided close activation energies values, the increase in dispersion of aluminium oxide on silicon carbide with 10 wt % Al 2 O 3 as modifier was in agreement with the higher distribution of perovskite containing La 2 NiO 4 crystallites. Additionally, larger amounts of adsorption of carbon dioxide on the efficiently distributed basic Sr and La oxides were also responsible for enhanced carbon dioxide conversion. CO 2 and CH 4 conversion also correlated well to the IA1/ INi ratios obtained from XPS analyses.

Ni foam as the catalytic support
Park et al. [225], have described the use of various foam catalyst embedded with metals to enhance the heat transfer of reaction during BRM process. The authors characterised heat transfers based on the Nusselt number and also used a pellet shaped catalyst to improve the heat transfer of the foam catalyst. The results revealed that the Nusselt number of foam catalyst packed with metals was larger than the pellet catalyst used conventionally. Additionally, uniform temperature distribution was noticed in the reformer throughout the catalyst bed along with the foam catalyst.  i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 6 ( 2 0 2 1 ) 3 2 8 0 9 e3 2 8 4 5 Images of the various metallic foam catalysts are shown in Fig. 34. Fig. 35 shows that the uncoated Al 2 O 3 and bare Ni foam were reactive without Ni catalyst loading. Additionally, the bare Ni foam displayed greater methane conversion than the uncoated aluminium oxide. The results established that the Ni foam exhibited a significant role in improving heating characteristics of the catalyst bed and mass transfer inside the reactor. However, after wash coating of a layer of Al 2 O 3 on the nickel foam, the conversion of carbon dioxide and methane increased to 34% and 69.1% and, respectively.
The syngas flow rate along with molar ratio of hydrogen/ carbon monoxide in presence of Nickel/Al 2 O 3 /Nickel foam, uncoated Nickel foam, uncoated Al 2 O 3 bead and Al 2 O 3 /Ni foam are shown in Fig. 36. The syngas flow rate exhibited a similar behaviour both for carbon dioxide and methane conversions. Nevertheless, the H 2 /CO molar ratio was different and furthermore molar ratio of H 2 /CO of uncoated Ni and uncoated Al 2 O 3 bead was less than 2.0. The metallic foam catalyst is potentially useful for GTL-FPSO applications based on the enhanced mechanical properties of the catalyst and compactness of the reformer. Additionally, higher selectivity levels and activity are associated with nickel inside the coating layer, which serves as active sites for methane, water and carbon dioxide.

Carbide-based catalysts
Brush et al. [181] reported the ability of Ni/Mo 2 C to catalyse the bi-reforming of methane. The authors noted that by altering the ratio of carbon dioxide: water, the resulting Hydrogen: Carbon monoxide (H 2 :CO) ratio could be changed from 0.91 to 3.0, which covers a wide range of H 2 :CO ratios common to various hydrocarbon syntheses. Most importantly, the catalytic activity changed from very high (50% conversion) to very low (10% conversion) within a time interval of 10 min. Additionally, for various inlet feed compositions similar performance was exhibited by the catalyst. However, enhanced activity was followed by greater deactivation shortly after the exposure to stream.
Claridge et al. [226], synthesized Mb and W carbide materials of larger surface area to assess the performance of carbide catalysts formed from non-metals for various CH 4 reforming reactions including bi-reforming. Their study revealed that carbides activity was similar to those of iridium and ruthenium catalysts used in reforming of methane. i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 6 ( 2 0 2 1 ) 3 2 8 0 9 e3 2 8 4 5 Nevertheless, conversion values were in agreement with the values obtained during thermodynamic equilibrium. HRTEM images revealed the absence of deposited macroscopic on the catalysts during the reforming process.

Conclusions and future outlook
The present review article demonstrates a comprehensive review of various catalysts, including Ni, Co, Rh and Pt-based catalysts, used for BRM. It also describes the role of various promoters and supports on the conversion efficiencies of CH 4 , CO 2 and H 2 /CO ratio. Ni catalysts supported by smaller nanoparticles of CeeZrO 2 ZrO 2 , MgO were observed to be extremely stable and active for BRM processes. The degree of coke formation was dependent on each investigated catalyst. Coke formation was found to be more severe in commercial Ni/MgAl 2 O 4 catalyst than Ni/MgOeAl 2 O 3 catalyst, which was attributed to higher dispersion of Nickel over MgOeAl 2 O 3 .
Bi-reforming of methane over a catalytic nickel membrane for the GTL (gas to liquid) process produced a very high conversion of methane in the range of 92.7e96% above 973 K, when the CO 2 /H 2 O feed ratios were in the range of 0e1.0. GTL process possessed two advantages. One was that carbon formation was reduced due to the oxidation of carbon precursor species and a desirable H 2 /CO was achieved by adjusting CH 4 /H 2 O/CO 2 ratio in the feed stream. Cerium containing Ni/MgAl 2 O 4 catalysts synthesized by both co -precipitation and impregnation technique exhibited higher metal dispersion than Ni/MgAl 2 O 4 alone and showed marked reducibility at lower temperatures around temperature 550 C as confirmed by XPS since the redox properties of Ce 4þ / Ce 3þ resulted in easier gasification of the settled coke on the surface of the catalyst and also helped in storage and delivering of active oxygen thereby enhancing the dispersion of Ni. The catalyst Ni/g-Al 2 O 3 promoted very higher conversion of carbon dioxide (82.4%) and methane (98.3%) when subjected to bi-reforming of methane for 200 h since the Ni/ Al 2 O 3 catalyst exhibited characteristics such as high metal dispersion, high catalytic activity large specific surface area, and stronger metal support interaction respectively. Presence of MgO in 20 wt % MgO/Ni catalyst was quite effective in preventing coke formation due to the formation stable MgAl 2 O 4 spinel phase at higher temperatures. The presence of CeO 2 in Ce 1-x eZr x O 2 catalytic systems caused a marked improvement in redox properties, thermal stability, and promotion of metal dispersion and also enhanced the oxygen storage ability of CeO 2 . A drastic reduction in carbon deposition from 25.96% to 1.08% was observed for a feed composition of CH 4 : CO 2 : H 2 O ¼ 1.0:0.55:0.55 when CO 2 reforming was performed in conjunction with steam reforming process in presence of NiOeCaO catalyst due to its high selectivity, activity and productivity in the oxidative conversion of methane to synthesis gas. Nickel/Santa Barabara Amorphous À15 (SBA-15) exhibited enormous resistance both towards sintering and coking. 10% Nickel/3% MgO/Santa Barabara Amorphous 15 catalyst demonstrated higher catalytic performance after reaction for 620 h since the mesoporous SBA-15 support possessed uniform mesopores with thick framework walls, high thermal stability and wide specific surface area. Bimetallic systems such as Sn 0.02 Nickel/Cerium-Al catalyst displayed superior catalytic activity, increased the resistance of carbon formation and remained active over a long period of 92 h in comparison to their own counterparts. The remarkable level of stability and excellent conversions seen in the bi-reforming process has proved the versatility of Sn0.02Ni/CeeAl catalyst which could be upgraded to variety of CO 2 containing feed stocks. Ni-based catalysts supported on mesoporous MgOeAl 2 O 3 resembling a MgeAl hydrotalcite structure with Mg/Al ratio of 1 demonstrated excellent conversion efficiencies for CH 4 (93.7%) and CO 2 (75.2%) and higher resistance to coke formation due to its basic property, enhanced steam as well as CO 2 adsorption, strong Ni to support interaction of these catalysts. The excellent catalytic performance of Ni 30 wt % SiO 2 55 wt % MgO catalyst was ascribed to its acidic strength, enhanced basicity and structural stability under high temperatures. NdCOO 3 perovskite-type mixed-oxide catalysts proved to be highly efficient for carbon-free bi-reforming process. PteCeeZrO 2 catalyst exhibited excellent stability in presence of H 2 O and its stability was attributed to the greater number of O 2 vacancies present in the support. Lanthanum promoted catalysts exhibited greater nickel dispersion than Ni/MgAl 2 O 4 catalyst due to their enhanced interactions between the metal and support. Nie2.5La/MgAl 2 O 4 catalyst showed maximum sinter stability and activity due to its enhanced nickel dispersion and surface area.
It is highly essential to develop Ni-based catalysts containing bi-metallic and tri-metallic configuration since Ni displayed stronger stability and enhanced activity. However, the biggest constraint in this approach is the coke formation. Bi-reforming process is endothermic and would require higher activation energy to achieve the synthesis. Future studies should be undertaken to design an appropriate bimetallic and tri-metallic catalyst that can be suitable at lower temperatures. Furthermore, the approach towards catalyst formation has a significant influence on the catalyst's capability. Therefore, selecting a suitable catalyst and its synthesis technique can provide improved SMSI, superior activity, enhanced Ni dispersion on the catalytic support and stability against coke formation.
Among the evaluated inlet feed compositions, conducting bi-reforming process under a stoichiometric feed composition (CH 4: CO 2: H 2 O ¼ 3:1:2) is considered the ideal one for selective production of syngas within the operating temperature of 750e800 C range. One of the major drawback associated with nano-based catalysts is that the up scaling of catalyst preparation from laboratory batches to continuous industrial production. Henceforth, development of reproducible and economical synthetic strategies is imperative for linking all advantages of nano-based catalysts to large-scale metgas generation facilities.

XPS
X-ray photo electron Spectroscopy XRD X-ray diffraction FT Fischer-Tropsch Synthesis i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 6 ( 2 0 2 1 ) 3 2 8 0 9 e3 2 8 4 5

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