Steam reforming or steam methane reforming (SMR) is the reaction where steam and hydrocarbons, such as natural gas or refinery feed stock, react in a reformer at temperature of 800˚C - 900˚C and moderate pressure (around 30 bar) in the presence of metal based catalyst for the production of syngas [5] . Syngas reacts further to give more hydrogen and carbon dioxide via the water gas shift (WGS) reaction, which is a side reaction in steam reforming. Steam reforming of natural gas produces syngas with a H₂:CO molar ratio close to 3.

Figure 3 illustrates the tubular steam reformer of Linde Company [6] .

Most SMR units include two sections, namely a radiant and a convective section. Reforming reactions take place inside the radiant section. In the convective section, heat is recovered from the hot product gases for preheating the reactants feeds and for generating superheated steam.

Because the process of steam reforming of methane is the reforming process that leads to obtaining syngas with the highest H₂/CO ratio, this type of reforming process is considered ideal to obtain a hydrogen gas flow of

high purity from syngas. It is the most widely applied method of producing syngas from natural gas and represents 50% of the global processes of conversion of natural gas for hydrogen production. This percentage reaches 90% in the U.S. Steam reforming of methane is an endothermic process and, therefore, requires very high temperatures, which makes this process very expensive.

Heat Exchange Reformers

Basically, a heat exchange reformer is a steam reformer where the heat required for the reaction is supplied predominantly by convective heat exchange. The heat can be supplied from flue gas or process gas or in principle by any other available hot gas. When the heat and mass balance on the process (catalyst) side only is considered, there is no difference between heat exchange reforming and fired tubular reforming, where the heat transfer is predominantly by radiation. This means that all process schemes using heat exchange reforming will have alternatives where the function of the heat exchange reformer is performed in a fired reformer. The process schemes differ “only”, in the amount of heat in flue gas and/or process gas and in the way this heat is utilized.

Types of heat exchange reformers

Three different concepts for heat exchange reformer design have been commercialized by various companies. The three concepts are illustrated in Figure 4.

Types A and B in Figure 4 can be used with all types of heating gas, whereas type C can only be used when the desired product gas is a mixture of the heating gas and the reformed product gas.

It occurs when a sub-stoichiometric fuel-air mixture is partially combusted in a high temperature reformer [8] , and it produces hydrogen rich syngas. Partial oxidation is an exothermic reaction and, thus, considered more economic than the processes of steam reforming or dry reforming, because it requires a smaller amount of thermal energy. On the other hand, the partial oxidation is considered an expensive process because it requires a flow of pure oxygen. Thus, there is a warning of danger inherent in the process of partial oxidation of methane, since the two reagents (CH₄ and O₂) can cause an explosion if the reaction is not conducted with the necessary care [9] .

The reactor design of POX and CPOX is presented as a scheme in Figure 5.

POX reactor simply comprises two zones, first the flame part where the hydrocarbons, oxygen, and possibly low amounts of steam react together and second a heat exchanger that recovers the excess heat after the reaction.

In non-catalytic partial oxidation, the production of syngas depends on the air-fuel ratio at operating temperature of 1200˚C - 1500˚C without a catalyst [10] . A non-catalytic partial oxidation process was developed by Texaco and Shell which results in high syngas yields at high temperature and pressures [11] .

The use of catalyst in the production of syngas lowers the required reaction temperature to around 800˚C - 900˚C [10] . In the CPOX reaction, methane is converted with oxygen (or air) over noble metal (Pt, Rh, Ir, Pd) and non-noble metal (Ni, Co) catalysts to syngas in a single step process. CPOX has been studied extensively during the past decade. Many studies have focused on the reaction mechanism [13] ; reactor configurations [14] - [16] ; reactor simulations [17] [18] as well as novel catalyst synthesis [19] to improve the process efficiency.

Catalytic partial oxidation can be used only if the sulfur content of natural gas is below 50 ppm. Higher sulfur content would poison the catalyst, so non-catalytic partial oxidation should be used for such fuels.

Two reaction mechanisms have been proposed: one is the “direct mechanism” in which CH₄ and O₂ react on the adsorbed state on the catalyst surface to yield CO and H₂ (Equation 3); the second one is the so-called “combustion-reforming mechanism”. In this latter mechanism, CH₄ and O₂ first form H₂O and CO₂ (Equation 4), and then dry (Equation 5) and steam reforming (Equation 1) reactions producing CO and H₂.

In addition to these reactions, other side reactions eventually occur. These include

and the formation of solid carbon by the Boudouard reaction

Chemical-Looping Reforming and Combustion

Chemical-looping reforming is a novel process for partial oxidation of hydrocarbon fuel where oxygen is

brought to the fuel by a solid oxygen carrier [20] . Chemical-looping reforming has been examined in a laboratory reactor consisting of two interconnected fluidized beds (Figure 6). Particles of NiO and MgAl₂O₄ were used as bed material, oxygen carrier and reformer catalyst. Natural gas was used as fuel. The reactor temperature was 820˚C - 930˚C. There was a continuous circulation of oxygen carrier particles between the fluidized-beds. In the fuel reactor the oxygen carrier was reduced by the fuel, which in turn was partially oxidized to H₂, CO, CO₂ and H₂O. In the air reactor the oxygen carrier was re-oxidized with air. Formation of solid carbon was noticed for some cases. Addition of 25 vol% steam to the natural gas reduced the carbon formation substantially. H₂ production by chemical-looping reforming with CO₂ capture has also been examined in a process study, where it was found that an overall efficiency of 81% including CO₂ sequestration is possible.

Figure 6 illustrates both chemical-loop reforming and combustion

MeO is the oxygen carrier in its oxidized form while Me is the reduced form.

Suitable oxygen carriers include metal oxides such as Fe₂O₃, NiO, CuO and Mn₃O₄.

If the fuel is CH₄, the oxygen carrier is NiO and the reactor temperature is 1200 K, reaction (8) occurs in the air reactor.

Regeneration: (8)

In the fuel reactor, reactions (equations 9, 10, 1 and 5) may occur, depending on the air ratio. Steam or CO₂ could be added to the fuel to enhance the relative importance of reaction (1) or reaction (5) respectively. This could be used to adjust the H₂/CO ratio in produced synthesis gas or to suppress formation of solid carbon in the fuel reactor. For chemical-looping combustion as much fuel as possible should be completely oxidized according to reaction (9).

Oxidation: (9)

Partial oxidation: (10)

Chemical-looping reforming is similar to chemical-looping combustion, but complete oxidation of the fuel is prevented by using low air to fuel ratio. Hence chemical-looping reforming can be described as a method for partial oxidation of hydrocarbon fuels that is utilizing chemical-looping as a source of oxygen. This is a considerable advantage compared to conventional technology since the need for expensive and power consuming air separation is eliminated.

The Short Contact Time-Catalytic Partial Oxidation (SCT-CPO) Technology

Precise knowledge of the mechanism of CPOX reaction is of vital importance because of the different thermal effects, which indeed affect both the design and heat management of industrial units [21] .

Initial observation on the occurrence of short contact time hydrocarbon oxidation processes were reported in the years 1992-1993 [22] . These processes have been deeply studied since then, and the number of scientific articles published every year on this topic, is still high. Hickman and Schmidt [23] [24] demonstrated that near complete conversion of methane to mostly hydrogen and carbon monoxide could be achieved at reaction times as short as 1 ms, promising dramatic reduction in a reactor size and complexity, as compared to existing syngas production technologies.

The fast and selective chemistry that is originated is confined inside a thin (<1 mm) solid−gas inter-phase zone surrounding the catalyst particles. Here, the molecules spend 10 - 6 s at temperatures variable between 600˚C - 1200˚C. A key issue for the technological exploitation is in the possibility of avoiding the propagation of reactions into the gas phase that has to remain at a “relatively low” temperature. This condition favors the formation of primary reaction products (namely CO and H₂) inhibiting chain reactions. Indeed some experimental studies whose results have been partially described in literature [25] - [29] indicate that partial and total oxidation products are directly produced through parallel and competing surface reactions and that the formation of partial oxidation products is favored under SCT conditions due to the very high surface temperatures.

By proper choice of the operating conditions, surface temperatures are locally much higher than those predicted by thermodynamic equilibrium calculations assuming adiabatic reactors. The occurrence of the reactions in these local environments determines in some cases conversion and selectivity values higher than those predicted by the thermodynamic equilibrium at the reactor exit temperatures [30] . Moreover, the very high surface temperatures inhibit catalyst deactivation phenomena related to chemical poison effects [31] [32] . For these and other related reasons, this chemical process is carried out in very small reactors having a very high flexibility towards reactant flow variations. It has also been found that several hydrocarbon feed stocks, even containing sulfur and aromatic compounds can be fed to a SCT-CPO reactor for producing synthesis gas. Now a long term R&D effort is approaching the industrialization phase of a technology whose main advantages concern are:

i) Small dimensions technical and operational simplicity.

ii) Possibility of modular construction of pre-fabricated and skid mounted units.

iii) Flexibility towards feedstock composition & production capacity.

iv) Reduction of investment costs and energy consumption.

v) Reduction of CO₂ production and possibility of an almost complete CO₂ capture in case of H₂ production plants.

Figure 7 illustrates the main characteristics of SCT-CPO technology application devoted to H₂ production and CO₂ removal [33] .

Like other reforming processes of methane, the purpose of the auto-thermal reforming is the production of syngas. Although auto-thermal reforming is an old idea, to date there are only a few commercial sites. The H₂/CO

ratio of the syngas obtained in the auto-thermal reforming is a function of the gaseous reactant fractions introduced in the process input. Thus, the H₂/CO ratio can be 1 or 2 [34] . Natural gas is mixed at high temperature with a mixture of oxygen and steam and ignited in a combustion chamber (see Figure 8) originating sub-stoi- chiometric flames that can be represented with both equations:

Reactions carried out in the Combustion zone ≈2200 ˚K are given by Equations (4) and (6), and those carried out in the reforming zone 1200 - 1400 ˚K are given by Equations (1) and (5).

By proper adjustment of oxygen to carbon and steam to carbon ratios, the partial combustion in the thermal zone supplies the heat for completing the subsequent endothermic steam and CO₂ reforming reactions [36] . The product gas composition at the exit of the reactor results very close to the thermodynamic equilibrium of an adiabatic reactor, especially in large scale processes [37] . ATR is also utilized as a “secondary reformer” (for lowering the CH₄ residue) and it is placed after a primary SMR in syngas plants integrated with Ammonia synthesis reactors. In this case the “secondary” ATR is fed with the syngas produced from SMR and Air.

New Auto-Thermal Reactor (KBR)

KBRATR reactor contains a combustion zone at the top and a catalyst filled bed at the bottom. The feedstock is mixed with a sub-stoichiometric amount of oxidant and burned in the combustion zone. There is an intermediate conical recirculation section (see Figure 9), where the hot gases continue to react, but are far from equilibrium.

The resultant gases are passed over the catalyst in the bottom section to achieve as close to an equilibrium mixture as possible.

ATRs are attractive when used in combination with a reforming exchanger. They are also suited for making large volumes of synthesis gas, especially with hydrogen/carbon monoxide ratios such as 1.5/1 - 3/1. These ratios are desirable for synthesis of higher molecular weight hydrocarbons. ATRs have limited commercial experience. One belongs to SASOL in South Africa, which uses ATRs licensed by Lurgi out of Germany. KBR-de- signed ATRs have been installed in ammonia plants in Kitimat, Canada and Liaohe, China [38] . There are a handful of other ATRs installed in commercial operation.

Since CO₂ is available in large quantities and at low costs, CO₂ can be used in place of steam for reforming.

Therefore, the dry reforming which is reforming of methane with CO₂ seems to be a promising technology for the production of syngas. Dry reforming of methane (DMR) is a process that uses waste carbon dioxide to produce syngas from natural gas. The synthesis gas produced by steam reforming has high H₂/CO ratio which is not suitable for Fischer-Tropsch synthesis in the production of long chain higher hydrocarbons due to the excess hydrogen which suppresses chain growth and decreases the selectivity of higher hydrocarbons [39] . Conversely, methane reforming with CO₂ plays an important role in the industries due to the production of syngas with a low H₂/CO ratio (≈1.0) which can be preferentially used for production of liquid hydrocarbons in Fischer-Tropsch synthesis network specifically those based on iron catalyst [40] .

Dry reforming reaction (Equation (5)) is slightly more endothermic than steam reforming. It is favored by low pressure and high temperature [41] . The presence of CO₂ gives rise to more chances of carbon formation on catalyst surface due to production of CO and consumption of H₂ via RWGS reaction Equation 11.

The dry reforming of methane with CO₂ has received special attention in recent years due to two main reasons:

i) It produces syngas with a H₂: CO molar ratio that is suitable for products including F-T fuels and DME.

ii) The reaction consumes two types of greenhouse gases, CO₂ and CH₄ [42] [43] .

The main disadvantage of dry reforming of methane is the significant deposition of carbon on the surface of the catalyst, which contributes to the reduction of its useful life. The main challenge for the industrial application of the reforming of methane with CO₂ is related to the development of active catalytic materials, but with a very low coke formation rate, either on the catalysts or in the cold zones of the reactor. The carbon formation in this process can be controlled by using a support that favors the dissociation reaction of CO₂ into CO and O, the last species being the responsible for the cleaning of the metallic surface [44] .

It is a new process designed for the direct production of synthesis gas with desirable H₂/CO ratios by reforming methane or natural gas using flue gas from fossil fuel based electric power plants without pre-separation of CO₂. These flue gases are regarded as major source of CO₂ emission in the U.S. Generally the compositions of flue gases depend on the types of fossil fuels used in power plants. Flue gases from natural gas-fired power plants typically contain:

8% - 10% CO₂, 18% - 20% H₂O, 2% - 3% O₂, and 67% - 72% N₂;

Flue gases from coal-fired boilers primarily contain:

12% - 14% CO₂, 8% - 10% H₂O, 3% - 5% O₂, 72% - 77% N₂, and trace amount of NO_x, SO_x, and particulates [62] .

It is hypothesized that tri-reforming be a synergetic combination of CO₂ reforming (Equation 5), steam reforming (Equation 1) and methane oxidation reactions (Equation 3 and Equation 4). Therefore, tri-reforming is expected to encompass a number of unique features. One major feature is its ability to convert CO₂ in flue gas without CO₂ separation while avoiding the use of pure CO₂ and the severe problem of carbon deposition encountered in CO₂ reforming system [63] - [67] . Currently most of pure CO₂ is obtained from CO₂ separation processes (e.g. absorption, adsorption, and membrane separation) that are often energy-intensive and costly. Some separation processes could lower the power plant energy output as much as 20% [68] .

Other features of tri-reforming include that there is no need to handle pure oxygen and it directly produces synthesis gas with a desirable H₂/CO ratio (e.g. H₂/CO = 1.5 - 2). Furthermore, oxygen in flue gas may help to ease the reaction energy requirement as encountered in CO₂ reforming alone or steam reforming alone. In general, the new tri-reforming process concept is consistent with the goals of DOE Vision 21 for power plants with respect to decreasing green house gas emission, improving power generation efficiency and co-producing fuels and chemicals [69] .

It should be pointed out that the H₂/CO ratio in synthesis gas is important since synthesis gas with different H₂/CO ratios has different applications in industry. The current major application of synthesis gas (not hydrogen) includes methanol synthesis and Fischer-Tropsch (F-T) synthesis that require synthesis gas with a H₂/CO ratio close to 2. However, synthesis gas directly produced from CO₂ reforming of methane has H₂/CO ratio close to 1. Hence, this kind of synthesis gas (H₂/CO ratio ≤1) requires further treatment in order to be applied in methanol and F-T synthesis.

Similarly synthesis gas produced from steam reforming cannot be directly applied in methanol or F-T synthesis either since the H₂/CO ratio of synthesis gas produced from steam reforming is usually larger than 3. Although methane partial oxidation produces synthesis gas with a H₂/CO ratio of 2, methane partial oxidation is difficult to control due to its exothermic feature and is dangerous and expensive due to the handling of pure oxygen. Tri-reforming, however, is expected to readily produce synthesis gas with the desired H₂/CO ratios of 1.5 ~ 2 by manipulating tri-reforming reactant compositions under relatively mild reaction conditions.

The concept of tri-reforming using power plant flue gas was first proposed by [62] [70] . Before 1999, several papers were published on the study of combined CO₂ reforming and partial oxidation reaction [71] - [73] and simultaneous steam and CO₂ reforming of methane in the presence of oxygen [74] . The results in these papers have indicated that combined reforming is feasible. However, the new tri-reforming process still faces a number of challenges. The future challenges include, for example, effective conversion of CO₂ in the presence of O₂ and H₂O; the heat management; the minimization of the effect of SOx and NOx in flue gas on tri-reforming process; the management of inert gas N₂ in flue gas; and the integration of new process into power plants.

Tri reforming can also be used for converting and utilizing CO₂-rich natural gas [75] as some natural gas resources contain up to 50 vol% CO₂ which are not yet utilized commercially due to the high CO₂ concentration. Tri-reforming process concept was recently proposed and developed at the Pennsylvania State University [62] [75] , and by independent studies on tri-reforming catalysts [76] - [79] .

Figure 13 depicts the process concept.

Steam reforming is the main reforming process of methane that is predominantly utilized because it has the greatest value for H₂/CO ratio, i.e., the product of the reforming process is a gas flow considered ideal for the development of the catalytic process of obtaining a gas hydrogen flow of high purity. However, as the process of steam reforming is considered too expensive, the other types of catalytic chemical processes are considered as alternative processes for carrying out the reforming of methane and they were developed with the aim of making savings in thermal energy consumption required for the catalytic process to occur. The choice of process type to reforming of methane must take into consideration the economic viability of the process related to the destination to be given to the syngas produced.

Partial oxidation and auto-thermal reforming are good choices to produce syngas when the value of H₂/CO ratio is adequate and especially when it comes to reduce the consumption of thermal energy, a most important factor. In short, it can be said that the selection of the type of catalytic chemical process of reforming of methane depends on the type of application of the syngas produced. A comparison of syngas generation technologies using natural gas as feed is shown in Table 1.

Zirconia [98] , lanthana [99] [100] , ceria [101] and ceria-zirconia [102] [103] oxides have been recently reported as promoters of methane reforming reactions. Incorporation of a 5 wt% ZrO₂ to a base Ni/SiO₂ catalyst resulted in excellent performance for the CPOX reaction O₂ mixture in a fluidized-bed reactor. Chawla et al. [104] prepared Nickel catalyst by impregnation method using support-Al₂O₃ and different types of promoters to improve activity, stability and selectivity in order to reduce coke formation and to achieve long-term operation. Nickel catalysts promoted by the ZrO₂ shows higher dispersion of the metal particle on the surface of the support than the un-promoted catalysts [105] . It has been found that the ZrO₂, CeO₂, K₂O and MgO promoted 10% Ni/-Al₂O₃ catalysts exhibited good activity, stability and long-term operation as compared to the un-promoted catalysts.

Iron has also been used as a promoter. Park et al. [106] prepared a set of mesoporous nickel-iron-alumina xerogel catalysts with different iron loadings. The catalyst formula “20Ni₄FeAl” reviled the finest nickel dispersion, the highest nickel surface area and the best catalytic performance in the steam reforming of LNG.

Perovskite oxides have also been extensively used as precursors of supported metal catalysts. Perovskites are mixed oxides with a general stoichiometry of ABO₃, where A and B can be partially substituted by other metals.

Most of the perovskites studied have a lanthanide and/or alkaline earth metal in the A site, and the active metal in the B site. After reduction, a highly dispersed metal supported in the lanthanide or alkaline earth oxide is obtained [107]

Perovskite structures of the type CaTiO₃, SrTiO₃, BaTiO₃ and LaAlO₃ have been used as supports by [108] to prepare supported nickel catalysts. All these supported Ni catalysts showed better performance than Ni/Al₂O₃ reference catalyst. Specifically, the Ni/ LaAlO₃ was the most active catalyst which suppressed the hot spot formation at the catalyst inlet. It has been shown that the LaNiO₃ perovskite renders small Ni particles deposited on a La₂O₃ substrate upon reduction. The resulting catalyst exhibited over 90% CH₄ conversion at 800˚C with H₂ and CO production at a ratio close to 2:1 [109] . Traditional catalyst preparation methods involve the precipitation and/or impregnation techniques; the latter has broadly been used for the preparation of Ni-supported catalysts for different areas of catalysts preparation methods [110] . However, the conventional impregnation method does not provide adequate control over the final size, morphology and dispersion of active metal particles. In the literature several other preparation methods such as surfactant assisted route to reduce the particle size of the support material, sol-gel, mixed oxides solid solution micro emulation and combustion synthesis were investigated as alternatives to traditional methods [111] .

On conventional reforming catalysts, discrete metal nano crystals (typically 1.15 nm) are dispersed on support particles that are one to several orders of magnitude larger than the supported metal nano particles. However, when the particle sizes of an oxide support are reduced to such an extent that they become comparable to that of the active metal particles, the oxide may deviate dramatically from its function as a conventional catalyst support. Such metal/oxide catalyst with size-comparable metal and oxide nano crystals may be better called a metal/oxide nano composite rather than an “oxide-supported” metal catalyst [112] .

When the sizes of zirconia particles become smaller than 25 nm, the oxide forms nano composite catalysts with size-comparable Ni-metal nano crystals (10 - 15 nm). The nano composite catalysts show extremely stable catalysis, which is in strong contrast with the deactivating Ni catalyst supported on bigger zirconia particles (>25 nm). Energy dispersive analysis of X-rays focused on individual particles showed little contamination between Ni-metal and zirconia nano crystals. This raises the possibility of tailoring the catalytic behavior of oxide-supp- orted metal catalysts by reducing the particle size of oxide to make high performance nano composite catalysts [112] .

The reason for high stability of nanoco mposite Ni/ZrO₂ remains unclear. It could be due to the enhancement of oxygen transfer ability of zirconia particles smaller than 25 nm or by formation of nano composite with high percentage of metal/oxide boundary or perimeter CO₂, which in turn increase oxidative removal of carbon atoms to produce CO [112] .

Application of the MgO nanocrystals for support of nickel catalyst was also successful and gave promising results for highly active as well as very stable Ni/MgO catalysts for the dry reforming of methane [113] . The work of Ruckenstein and Hu, [96] showed that NiO/MgO catalysts prepared by impregnation of nickel nitrate onto MgO powders containing 7 - 10 nm nano crystals developed stable activities for the reforming reaction after reduction at 1063 K.

Mesoporous materials when used as the support could control the size of nano particles by the diameter of their pores [111] [114] . It has been found that appropriately prepared mesoporous, nano crystalline pure tetragonal zirconia could result in an active and stable nickel based catalyst for dry reforming reaction [115] . Under relatively low temperatures and low carbon dioxide to methane ratios which thermodynamically favors coke formation, long-term stable performance was observed over 5% Ni catalyst. The addition of CeO₂ to the support was found to increase the surface area of the resulting zirconia powder [116] . The use of alkaline promoter [117] as well as CeO₂ and La₂O₃ [118] further improved the stability and activity of the resulting catalysts under conditions otherwise coke formation is extensive.

Most recently, an innovative steam- and/or CO₂-reforming designated as Thermo-Neutral Reforming (TNR) has been introduced by [119] after several years studies on ultra-rapid catalytic reactions. The reformer can be reduced two-order magnitude compared with traditional steam reformers, because the large endothermic heat of reforming is compensated by the large exothermic heat of complete combustion on the same catalyst surface without supply of heat from outside of the reactor.

By applying the extremely compact size of the TNR system to the successive syngas converters packed newly developed catalysts, highly effective ultra clean fuels such as MeOH, DME, and sulfur-free & non-aromatic high octane number gasoline can be produced effectively with non-expensive costs.

Recent advances in the steam reforming catalyst have been done through the CO₂ reforming associated with the CO₂ mitigation against the global warming crisis [119] . The common sense of the steam reforming has been that the excess steam is necessary to prevent coke formation by the reaction between deposited carbon and steam to convert to carbon mono oxide and hydrogen, while in case of dry reforming with CO₂ there is no opportunity to avoid coke formation. Inui [120] investigated a novel catalyst to avoid coke formation even under the reaction condition of CO₂ reforming. As criteria to develop this ideal catalyst; the following performances have been required:

・ No coke formation,

・ High sulfur tolerance,

・ Ultra-rapid reaction rate,

・ High-temperature resistance,

・ Low temperature start-up in a very short time,

・ Non toxic, and

・ Low production cost.

This novel catalyst has both catalytic functions of combustion and steam reforming for hydrocarbons, the thermo-neutral reactions (TNR) on the same catalyst surface could be realized [120] . As the result, the reactor size could be reduced to two-orders of magnitude that of traditional hydrocarbon steams reforming (HSR) method [121] . Catalyst composition of the four-component catalyst (wt%): 10Ni-6.0Ce₂O₃-1.0Pt-0.2Rh. The synergistic effect of the four-component catalyst on CO₂ reforming of methane is shown in Figure 14 [122] .