Production of hydrogen from fossil fuel: A review

Abstract

Production of hydrogen, one of the most promising alternative clean fuels, through catalytic conversion from fossil fuel is the most technically and economically feasible technology. Catalytic conversion of natural gas into hydrogen and carbon is thermodynamically favorable under atmospheric conditions. However, using noble metals as a catalyst is costly for hydrogen production, thus mandating non-noble metal-based catalysts such as Ni, Co, and Cu-based alloys. This paper reviews the various hydrogen production methods from fossil fuels through pyrolysis, partial oxidation, autothermal, and steam reforming, emphasizing the catalytic production of hydrogen via steam reforming of methane. The multicomponent catalysts composed of several non-noble materials have been summarized. Of the Ni, Co, and Cu-based catalysts investigated in the literature, Ni/Al₂O₃ catalyst is the most economical and performs best because it suppresses the coke formation on the catalyst. To avoid carbon emission, this method of hydrogen production from methane should be integrated with carbon capture, utilization, and storage (CCUS). Carbon capture can be accomplished by absorption, adsorption, and membrane separation processes. The remaining challenges, prospects, and future research and development directions are described.

Graphical abstract

Keywords

methane / catalytic conversion / natural gas / hydrogen production / CCUS

1 Introduction

Hydrogen is considered to be the key component in the decarbonization of industrial processes and energy sectors for climate change mitigation [1]. In transport and other hard-to-decarbonize sectors, hydrogen is being developed rigorously to replace fossil fuels [2]. However, pure hydrogen is not available on earth in any large quantity; it needs to be produced from other available natural resources that are also economic on earth. Consequently, any large-scale commercial use of hydrogen requires energy-efficient production with economic and environmentally friendly methods, i.e., a low-cost hydrogen, such as 1 $/kg targeted by the US Department of Energy [3], that is produced in a mass production, would be desirable.

Many methods are available for hydrogen production from various natural resources, as illustrated in Fig.1 [4]. Hydrogen can be produced in large quantities from fossil fuels, using the methods of hydrocarbon pyrolysis, partial oxidation, autothermal and steam reforming [5]. It can also be produced from biomass through biological processes, such as bio-photolysis, dark fermentation, photo fermentation, and through thermochemical processes including gasification, pyrolysis, combustion, and liquefaction [6]. However biological processes are typically slow with a small scale and low rate of yield for a gas mixture that often has a low concentration of hydrogen [7]. Thermochemical processes have a potential for scaled up production of hydrogen, but they are limited by the low energy density of biomass per unit of earth surface area; and the plantation, harvest, collection and transport of biomass to a central thermochemical processing plant limits the plant size and the cost of hydrogen produced [8].

Hydrogen production from water, such as via thermolysis, photolysis, and electrolysis, has been considered as the cleanest and most convenient method as water is clean and abundantly available on Earth [9]. Thermolysis of water for hydrogen production requires heating to an excessively high temperature, and the efficiency and durability of a robust reactor is also a challenge [10]. Photolysis of water requires the light penetration into the water body, thus limiting the size of the reactor, hence the rate of hydrogen production as well [11]. One of the ideal methods for hydrogen production that is being vigorously pursued is the electrolysis of water with electricity generated from renewable energy resources such as solar, wind, hydro, and nuclear, although nuclear energy may not be the generally agreed upon or chosen form of primary energy [12]. Water electrolysis includes alkaline, ceramic, and proton exchange membrane (PEM) as the electrolyte. Alkaline electrolyzer is a mature technology, normally available in large size and operates with a steady electricity input, hence not compatible with renewable intermittent electricity; it also has a low energy efficiency [13]. Solid ceramic electrolyzer is still in the stage of development, and it might become mature in the future [14]. PEM water electrolyzer has a high energy efficiency, can operate with an unsteady electricity input, and it can be designed in various sizes ranging from small modular reactors to large ones. Hence PEM electrolyzer is the favored method for hydrogen production. However, it requires the use of a large quantity of noble metals such as iridium and platinum as the catalysts for its anode and cathode reaction, and the annual world production of the iridium and platinum is limited, several times less than what might be needed in PEM electrolyzer for the production of hydrogen [15].

At present, about 75 Mt of hydrogen is produced annually worldwide with an additional 45 Mt per year as part of a mix of gases [16]. An overwhelming portion of 96% of global hydrogen is produced from fossil fuels [17–22], while only 4% from renewable resources [23]. The majority of the hydrogen is used as chemical compound, rather than as fuel. The amount of hydrogen used as fuel currently is equivalent to only 3% of global final energy demand [24]. However, to achieve net zero emissions by the middle of this century, hydrogen as fuel is expected to increase significantly, contributing up to 18%–24% of final energy demand [25–28]. This dictates that a significant increase in hydrogen production is required immediately and compounded over the years in order to meet the expected requirement.

Currently, hydrogen is most economically produced from fossil fuels in large quantities. In this regard, fossil fuels are expected to remain a major source of hydrogen in the future [29]. Most of the hydrogen produced in the United States is derived from fossil fuels that can be converted catalytically via steam reforming to release hydrogen from their hydrocarbon molecules. In the United States, about 95% of hydrogen is produced by reforming natural gas in large central plants [23]. It is an important pathway to produce hydrogen in the near future.

To reduce the cost of hydrogen production, noble metals such as platinum, gold, ruthenium, and silver are currently used as catalysts. The development of catalysts based on non-noble metals, including Ni, Co, and Cu, has been vigorous, which is the main focus of this review.

Production of hydrogen from fossil fuels causes inevitable CO₂ emissions. Further even by 2050, the transport sector is still expected to emit 0.7 Gt of CO₂, and globally a total amount of 7.6 Gt of CO₂ would have to be captured from the use of fossil fuels for utilization and storage [30]. Tab.1 illustrates the numerous hydrogen production methods and sources that are commercially available currently at a large scale [4,17,21,22,31–37]. The majority of hydrogen (96%) is produced by using a non-renewable process, specifically steam reforming of methane (SRM) [38]. Therefore, it is essential for hydrogen production from fossil fuels to couple with carbon capture, utilization, and storage (CCUS) to avoid greenhouse gas emissions into the atmosphere. This production route is quite effective from a greenhouse gas perspective since CCUS on hydrogen assets can capture as much as 90% [39] of greenhouse gas emissions. The production of hydrogen from fossil fuels with CCUS also provides a temporal pathway until renewable hydrogen production technology is maturing.

Therefore, the objective of this paper is to review the various methods of hydrogen production from fossil fuels, emphasizing the catalytic process of hydrogen production as the most cost-effective and efficient method. Various non-precious catalysts being developed for the catalytic conversion of natural gas (methane) to hydrogen via steam reforming are examined, and different methods for the separation and capture of carbon dioxide gas are also evaluated. This paper ends with a section on the current trends and future direction of research and development for the production of hydrogen from natural gas.

2 Methods of hydrogen production from fossil fuels

Fossil fuel processing offers an efficient and low-cost alternative to non-conventional hydrogen extraction with higher hydrogen yields. A comparison of various fossil fuel processing methods for the production of hydrogen is presented in Tab.2. Hydrogen can be extracted through the breakdown of hydrocarbons as a primary reaction product and as by-products using these methods which rely on the contents of fossil fuels [4]. The extraction process involves high-temperature thermal decomposition of hydrocarbons, or modification of the molecule structure. There are multiple ways to initiate the thermal reformation of hydrocarbons, including partial oxidation, autothermal reformation, and steam reformation [7]. In contrast, fossil fuels undergo thermal decomposition during the pyrolysis process. The hydrogen selectivity of these processes is typically assured by catalyzing these processes. In addition, optimal conditions are necessary for the highest yield and the highest efficiency of the process. The following subsections address the major mechanisms related to these processes.

2.1 Hydrocarbon pyrolysis

Hydrocarbon pyrolysis refers to the process of producing hydrogen through the thermal decomposition of hydrocarbons. A thermo-catalytic decomposition is used to produce hydrogen and carbon from hydrocarbons (e.g., methane, ethane, propane) by an endothermic reaction known as pyrolysis. In general, the reaction is

(1){\mathrm{C}}_n{\mathrm{H}}_m\to n\mathrm{C}+0.5m{\mathrm{H}}_2

Natural gas (methane) can be converted into CO_x-free hydrogen through thermo-catalytic decomposition for use in PEM fuel cells, oil refineries, ammonia, and methanol production [29]. The reaction temperature is high, typically at or above 700 °C, which can be lowered by using suitable catalysts [52]. Methane reacts as

(2){\mathrm{C}\mathrm{H}}_4\to \mathrm{C}+2{\mathrm{H}}_2({\mathrm{\Delta }H}_{\mathrm{R}}^0=74.9\mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l})

where {\mathrm{\Delta }H}_{\mathrm{R}}^0 is the enthalpy of reaction under the standard condition of 25 °C and 1 atm producing solid carbon and gaseous hydrogen. Solid carbon is a valuable chemical/material and has many industrial uses [53]; it has a price of 0.4–2.0 $/kg for additional economic gain, and excess unsold carbon can be stored in geological storage or unused coal mines for filling. It is much more economical to handle, transport, and store than gaseous carbon dioxide [54]. The solid carbon contains energy (heating value), which can be released through the conventional combustion for practical use, for example; if the solid carbon is used as chemical material or for storage, this amount of energy contained within the solid carbon is not utilized.

Pyrolysis does not emit greenhouse gases, but it is extremely inefficient, although it is widely known. Its efficiency is considerably lower using fossil fuels than using renewable resources [52]. Further, pyrolysis process can cause other environmental problems, although not directly. The pyrolysis process is often modified to minimize the above-mentioned limitations, and one of the modifications involves using molten metal bubble columns in natural gas pyrolysis [40]. The energy barrier to pyrolysis initiation is reduced by interactions between the bubbles and feedstocks. It was concluded [55] that the process was economically impractical unless its by-products were sold, or carbon tax was deducted. As a result, this process might be more practical for some developing countries that have not yet introduced a large-scale carbon tax.

In spite of molten metal-based catalysts ensuring impurity-free hydrogen production, industrial-scale implementation is not feasible due to their high costs. To reduce the high conversion temperature, metal oxides were used as catalysts in another study where hydrocarbons were diluted with argon [41]. As a result, the optimum temperature was halved compared to that of Ref. [55]. The cost was relatively low, yet extra investments are often required to avoid soot formation. Another cost-effective use of fossil fuel pyrolysis was the assessment and comparison of the H₂ production potential from multiple feedstocks using molten mixtures of Ni and Bi. Propane has a higher conversion efficiency than methane and ethane because its chemical bonds are weaker [43]. As a result, propane conversion can be considered as a techno-economically feasible process, since catalysts and reactors are more readily available and cheaper than in other processes.

2.2 Partial oxidation

The partial oxidation process involves providing oxygen and water (or steam) to mix with the hydrocarbons, and the heat released from the hydrocarbon oxidation with oxygen is used to convert the hydrogen in the hydrocarbon and steam into hydrogen gas. This process can be accomplished with or without catalytic effect and accompanied with the production of carbon dioxide. Sulfur needs to be removed from the hydrocarbon feedstock before the pyrolysis process. Notice that the amount of oxygen provided is not sufficient for the complete oxidation of hydrocarbons, hence the name of the partial oxidation process. As a result, syngas (a mixture of CO and H₂) is formed, and water-gas shift (WGS) reaction follows converting CO into CO₂ while producing more hydrogen gas from steam. The partial oxidation can be catalytic or non-catalytic, expressed as [7]

(a) Reforming process:

Catalytic reaction:

(3){\mathrm{C}}_n{\mathrm{H}}_m+0.5n{\mathrm{O}}_2\to n\mathrm{C}\mathrm{O}+0.5m{\mathrm{H}}_2({\mathrm{\Delta }H}_{\mathrm{R}}^0=-36\mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l})

Non-catalytic reaction:

(4)\begin{array}{rl}& {\mathrm{C}}_n{\mathrm{H}}_m+n{\mathrm{H}}_2\mathrm{O}\to n\mathrm{C}\mathrm{O}+\left(n+0.5m\right){\mathrm{H}}_2\\ & ({\mathrm{e}.\mathrm{g}.,\mathrm{\Delta }H}_{\mathrm{R}}^0=206\mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{f}\mathrm{o}\mathrm{r}{\mathrm{C}\mathrm{H}}_4)\end{array}

(b) WGS reaction:

(5)\mathrm{C}\mathrm{O}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{C}\mathrm{O}}_2+{\mathrm{H}}_2({\mathrm{\Delta }H}_{\mathrm{R}}^0=-41\mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l})

(c) Methanation process:

(6)\mathrm{C}\mathrm{O}+3{\mathrm{H}}_2\to {\mathrm{C}\mathrm{H}}_4+{\mathrm{H}}_2\mathrm{O}({\mathrm{\Delta }H}_{\mathrm{R}}^0=-206\mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l})

Since heavy oil residues and coal substances have low hydrogen and carbon ratios, the process is highly applicable to these feedstocks [43]. As a result, coal gasification is sometimes called coal combustion. The steam reforming process combines with the partial oxidation process to produce a considerable amount of H₂. The process can also be combined with CCUS to limit carbon emissions. It is standard for developed countries to use this partial oxidation process for industrial purposes. However, partial oxidation-mediated hydrogen production is often not as popular as it should be because of its high capital and operational costs. There are also challenges associated with this process, such as frequent cleaning, preventing coke formation, and handling reaction conditions.

In different studies, catalytic partial oxidation was found to be the most prominent fossil fuel-mediated hydrogen production method [56]. In contrast, noncatalytic partial oxidation was the most popular method for biomass processing [57]. For maximum catalyst stability during the partial oxidation, nickel-catalyzed partial oxidation was used in conjunction with alumina and zirconia [43]. The effectiveness of the combined catalysis is higher than that of common metallic catalysis mechanisms based on X-ray diffraction measurements. A modified catalyst containing aluminum showed a maximum conversion efficiency in this study. As a result of temperature oscillations, the surface area of the catalyst fluctuates, increasing the reaction rate and efficiency when the surfaces of the catalyst have more miniature and high-affinity potentials at higher temperatures. In addition to the temperature oscillations observed during the partial oxidation of methanol, flow rate variances were also observed during the process [58]. Therefore, temperature oscillations are a suitable mechanism for determining the optimal conditions for hydrogen production through partial oxidation in industrial applications.

There are other operating and design parameters, other than catalysts used and operating temperature, which can be used to increase the hydrogen yield and catalyst recovery efficiency. Fluidized bed design and operation can have a significant impact on the hydrogen yield, for example, and they are being investigated to determine the best combinations of the design and operation condition for specific contexts. This technology relies on oxidation as its primary mechanism of the reaction. This implies the need for appropriate oxygen donors to start the process. Metallic oxides are oxidizing agents in most cases, as mentioned in the previous examples. The use of nitrous oxide (N₂O) as an oxygen donor was found to be a viable alternative to traditional oxidizing agents in a fluidized bed study [45].

There are three modes of operation for fluidized bed reactors, including bubbling, circulating, and dual beds [59]. A bubbling fluidized bed (BFB) introduces fuel from the bottom or side of the bed. If the velocity of the gasification agent exceeds the minimum fluidization velocity, the bed will begin to bubble. A cyclone is used to clean the syngas produced at the top of the reactor [60]. A circulating fluidized bed (CFB) is composed of two operating units, namely a fast velocity riser reactor and a circulating loop cyclone [61]. As a result, it is treated at a higher gas velocity (superficial flow velocity) than BFB in a more drastic fluidization process. As product syngas and bed particles rise to the surface, they are separated in cyclones and recirculated into risers. As a result, greater efficiency could be achieved overall. The dual fluidized bed (DFB) system or two-stage fluidized bed system is composed of two fluidized bed reactors interconnected by a solid looping design. There are three different types of BFBs: two BFBs, two CFBs, or a BFB and a CFB. Both the pyrolysis and combustion reactions are controlled independently by the two fluidized beds that are connected [62].

2.3 Autothermal reforming

A steam reforming process and an exothermic partial oxidation process combine to produce hydrogen through autothermal reforming. The reactor is filled with oxygen and steam simultaneously in contrast with partial oxidation reaction where only oxygen is present, and the reforming and oxidation reactions occur simultaneously. Combining the two processes results in this reaction [7]

(7)\begin{array}{rl}& {\mathrm{C}}_n{\mathrm{H}}_m+0.5n{\mathrm{H}}_2\mathrm{O}+0.25n{\mathrm{O}}_2\to \\ & n\mathrm{C}\mathrm{O}+\left(0.5n+0.5m\right){\mathrm{H}}_2({\mathrm{\Delta }H}_{\mathrm{R}}^0=84\mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l})\end{array}

Most of these reactions occur simultaneously, similar to steam reforming and partial oxidation, but they are separated in the steam reforming process. It is recommended that the sulfur in hydrocarbons be removed at the beginning of the process, followed by the extraction of hydrogen and other gaseous by-products using optimal pressures and temperatures. A subsequent step is to perform selective hydrogen extraction from the gaseous mixture, called membrane-based adsorption [63]. This method can produce pure elemental hydrogen.

An autothermal reforming process most commonly uses nickel catalysis to increase reaction rates. The hydrogen yield was significantly increased when nickel was combined with other catalytic metals in multiple percentage ratios [46]. Similarly, a study of methane processing to produce hydrogen provided similar trends in process efficiency enhancement where Ni²⁺ reducibility contributed to reaction rates [47]. Elemental ratios can also be considered as determinants of process efficiency and hydrogen yield, depending on the type of feedstock used. When certain conditions are met, this ratio regulates the carbon-carbon cleavage rate, which alters the strength of the bond [46]. Furthermore, modified catalysts calcined under certain conditions showed an increase of 4.8% in conversion efficiency. To determine appropriate reforming conditions, it is important to review the process operating costs carefully.

2.4 Steam reforming

Hydrocarbons and steam are converted into hydrogen gas and carbon dioxide in the steam reforming process through a number of reforming steps. In addition to producing synthesized gas, the reforming process involves WGS, gas purification, and other steps [7]. Hydrogen is currently most commonly produced via steam reformation of natural gas [2]. The reason for this is that the hydrogen in the steam (water) is also converted into hydrogen gas, substantially increasing the hydrogen yield in the process. This process consists of three stages. In the first stage, methane is reformatted at high temperatures (800–900 °C) via a highly endothermic catalytic process ({\mathrm{\Delta }H}_{\mathrm{R}}^0=206\mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l} for CH₄) . In the second stage, there is the catalytic WGS reaction. The first part occurs at 400–500 °C to reduce carbon monoxide to 5%, and the second occurs at 177–250 °C to reduce CO to 1%; in the third stage, the H₂-CO₂ mixture is separated using pressure-swing adsorption (PSA) [64,65]. Besides the steam generation section and the desulphurization unit, other auxiliary steps are needed for this process. However, this method of generating hydrogen mixes it with CO and CO₂, and separating the two requires a complicated process.

The chemical reaction equations illustrating the steam reforming process are as follows [66].

(a) Steam reforming reaction:

(8){\mathrm{C}\mathrm{H}}_4+{\mathrm{H}}_2\mathrm{O}\to \mathrm{C}\mathrm{O}+{3\mathrm{H}}_2({\mathrm{\Delta }H}_{\mathrm{R}}^0=206\mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l})

Further transformations can be made for the CO produced as a by-product [66]. Typically, the water gas shift reaction results in CO₂ and hydrogen,

(b) WGS reaction:

(9)\mathrm{C}\mathrm{O}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{C}\mathrm{O}}_2+{\mathrm{H}}_2({\mathrm{\Delta }H}_{\mathrm{R}}^0=-41.16\mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l})

(c) Direct steam reforming reaction:

(10){\mathrm{C}\mathrm{H}}_4+{2\mathrm{H}}_2\mathrm{O}\to {\mathrm{C}\mathrm{O}}_2+{4\mathrm{H}}_2({\mathrm{\Delta }H}_{\mathrm{R}}^0=165.12\mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l})

From the above description, it is clearly observed that SRM provides the highest yield for hydrogen gas production. Further, SRM is the least expensive and most widely accepted method for hydrogen production. Therefore, the following section focuses on the natural gas (methane) for hydrogen gas production, including the most suitable catalyst for hydrogen production in the SRM process, since it is a catalytic conversion process involving methane reaction with steam. In SRM processes, Ni-based catalysts are commonly used due to their low cost and high activity [67].

3 Catalytic hydrogen production from methane

Currently, natural gas accounts for 48% of hydrogen production, oil for 30%, and coal for 18%, while only 4% of global hydrogen is produced from renewable resources [23]. A significant decline in the share of oil and coal within total energy consumption is expected in the next few years due to the rising importance of natural gas, as shown in Fig.2 [68]. Methane is the primary component of natural gas, a feasible raw material due to the abundance of natural gas reserves in the world, which accounts for an annual production of 70 Mt of hydrogen [69,70]. In terms of fossil resources, methane has the largest share, producing the highest amount of energy per unit mass of carbon dioxide emitted [71]. The substitution of coal with methane would result in significant reductions in emissions.

As the most mature industrial process for hydrogen production, steam reforming of fossil fuels, particularly natural gas, is the primary method for producing hydrogen. The US has produced more than 10 Mt of hydrogen annually through the SRM process which is an industrial process that produces hydrogen from methane sources (natural gas) at high pressures (300–2500 kPa) with steam at temperatures of 700–1000 °C. Apart from hydrogen, CO and relatively small amounts of CO₂ are also obtained from SRM, which enables the conventional production of syngas (H₂ and CO) [72,73]. In combination with the WGS reaction, the co-product CO from SRM may act as a catalyst to accelerate the decomposition of methane into hydrogen and carbon species [74]. Carbon species were then further reacted with oxygen in a classification reaction to yield CO/CO₂ and recover active sites.

In the field of methane conversion, SRM is the leading industrial process for the production of hydrogen and syngas [67,75]. The process flow diagram for SRM for hydrogen production is demonstrated in Fig.3 [76]. SRM was implemented in the industrial sector soon after 1930 [77]. However, research and industrial efforts to improve the catalytic performance and the material properties of the reactor tubes have continued. As of now, SRM remains an important scientific topic. SRM technology development is a long-term project to improve efficiency and cost-effectiveness [78,79].

This approach mainly has three drawbacks that need to be resolved. The first issue is that methane is highly stable and difficult to activate. To proceed with the reaction at high temperatures and pressures, the SRM process requires additional energy and specially designed instruments, which also introduces mass transfer and heat transfer issues. Secondly, the design of catalysts needs to be improved. Meanwhile, noble metal catalysts such as Pt, Au, Ru, and Ag show superior stability and high SRM activities [80,81]. However, the high cost and limited availability of noble metals limit their application. Finally, one of the by-products for the SRM process is CO₂, especially when coupled with WGS that yields approximately 9–14 kg(CO₂)/kg(H₂). In addition to the strong greenhouse effect of CO₂, its handling increases the cost of the SMR process [82].

The most widely utilized catalysts for SRM are Ni, Co, Cu, and Mo-based catalysts due to their low cost and high activity [83]. Nevertheless, Ni-based catalysts have been the commonly used catalysts in the SRM process due to their low cost and high activity. Commercial Ni-catalysts face two significant challenges: coking and sintering. It is preferred to use Ni catalysts modified with promoters, solid solution catalysts, novel supported Ni catalysts, and self-supported Ni catalysts to avoid coking and sintering [84–86].

The following section explores non-noble-based alloy catalysts for hydrogen production. In atmospheric conditions, the catalytic conversion of natural gas into hydrogen and carbon is thermodynamically advantageous. A limitation of their practical application is the availability and cost of noble metals as catalysts, namely Pt, Au, Ru, and Ag. However, non-noble transition metal-based catalysts tend to be less expensive and more efficient. Therefore, non-noble materials such as Ni, Co, and Cu-based alloys are needed to catalyze hydrogen production. In recent years, the catalysis of natural gas for hydrogen production has undergone significant development, which will be reviewed in Section 4. An overview of the properties and cost of the non-noble metal catalyst is provided in Tab.3 [87].

4 Non-noble-based alloy catalysts for hydrogen production

In general, transition metals of group VIII are suitable for use as catalysts in the SRM process. The sulfides of Mo and W as well as the carbides of Mn and their oxides have all been demonstrated for their excellent catalytic activity [88–90]. It is known that noble metals, such as Au, Pt, Pd, Rh, Ru, and others, have the highest catalytic activity and stability. However, they are limited in known reserves and very costly as well for commercial SRM [91,92]. On the other hand, significant progress has been made for Ni and other transition metal-based catalysts including Co and Cu-based alloys. As a non-noble metal, Ni is available in abundance and is very affordable, making it an excellent active metal for large-scale applications. A further advantage of Ni is that it can break C−H and O−H bonds easily [93,94]. However, Ni-based catalysts typically deactivate rapidly during the reforming process as a result of sintering and carbon buildup on their surfaces [95,96]. Because of complexities and cost in catalyst regeneration and replacement, developing better SRM catalysts requires inhibiting or mitigating Ni-based catalyst deactivation.

Fujimoto & Ohba [97] reported the methane decomposition and aromatization could be achieved efficiently using metal catalysts based on various substrates, including zeolites and silica. They examined methane decomposition in bulk, on a SiO₂ substrate, and in mesoporous SiO₂ using Co-, Ni-, Cu-, Mo- and Ru-based nano-catalysts. Specifically, the bulk nano-catalyst crystallite size was controlled to 80, 30, and 3 nm, respectively, for both nonporous and mesoporous SiO₂. As a result of the nano-catalyst size effect and the adsorption potentials of the SiO₂ mesopores, the nano-catalysts on mesoporous SiO₂ produced hydrogen from methane decomposition in high yields. When compared with previous hydrogen production tests, the Ni nano-catalysts on mesoporous SiO₂ produced hydrogen from 650 K, which was the lowest temperature. Furthermore, the catalytic activity was maintained at 650 and 800 K for over 15 h.

Fig.4 displays the temperature at which hydrogen is produced and aromatized based on crystallite size. In the bulk nano-catalysts, the activity was independent of crystallite size. However, even though its crystallite size was similar to that of the other bulk nano-catalysts, the bulk Mo nano-catalyst demonstrated a significant hydrogen production activity at 850 K. Similar hydrogen production was observed with non-porous SiO₂ nano-catalysts between 30 and 70 nm in size [98]. There was no expectation that the bulk nano-catalysts would have a similar or lower activity. The porous SiO₂ nano-catalysts, however, showed a higher activity for hydrogen production with considerably smaller nano-catalysts. There was no de-hydro-aromatization of methane observed for Cu, Mo, or Ru. The high activity of nano-catalysts on porous SiO₂ was caused by the size effect of the nano-catalysts and the confinement effect of the SiO₂ pores [97].

Traditional SRM processes have used Ni-based catalysts due to their low cost and high activity. Coking and sintering remain significant issues for Ni-based SRM catalysts, leading to recent SRM research to improve their coking and sintering resistance. Aside from Ni-catalysts, Co, Cu, and Mo-based catalysts have also been demonstrated for their superior catalytic performance. It was found that some of these catalysts had higher catalytic activities than Ni-based catalysts, even at a lower reaction temperature of 700 °C. In the next few subsections, Ni, Co, and Cu based catalysts will be examined in detail.

4.1 Ni-based catalysts

In the SRM process, nickel-based catalysts are most often used due to their low price and high activity. Ni-based SRM catalysts have been targeted for improvement in coke and sintering resistance due to the presence of significant coking and sintering problems (Tab.2). New promoters have been developed, solid-state catalysts based on Ni solids have been explored, supported Ni catalysts have been tuned and self-supported Ni catalysts have been constructed.

Nb improved the SRM performance of Ni/Al₂O₃ catalyst by suppressing the coke formation on the catalyst because of the strong interactions between Ni and Al₂O₃ [99]. A niobium/alumina catalyst was added to methane steam reforming to examine the correlation among the structural, textural, and acid/base properties. The catalysts containing 5 wt.%–10 wt.% Nb₂O₅ had a higher methane conversion at 800 °C and were not deactivated for 24 h, but 20 wt.% Nb₂O₅ had an inhibited coke formation. Fig.5 manifests the schematic of Ni/Al₂O₃ promoted by Nb and the percentage of CH₄ conversion for different compositions of Nb [99]. The CH₄ conversion rate for Ni/Al₂O₃ reached 98% with a 5 wt.% Nb loading, which is 10.4% higher than that of the pristine sample. At 20 wt.% Nb, aggregation of Ni became extreme, and the catalytic activity was reduced. A perovskite oxide suitable as a catalyst promotion agent affected not only the interaction of Ni and Al₂O₃, but also the number and size of oxygen vacancies [100]. As a result of 15 wt.% CaZrO₃, Ni/\text{α}-Al₂O₃ was able to convert 67% of CH₄ at 700 °C with a molar ratio of steam to carbon (S/C ratio) of 1.0.

The only non-metal promoter studied so far is boron [101]. Ni-based catalysts could be made more stable and active by preventing the formation of coke. At first, the pristine catalyst could only convert 55% of CH₄ and lost 21% of its initial activity after 10 h, but 1 wt.% B doped catalyst enabled the conversion of 61% at first and maintained 56% after 10 h. However, significant additions of B may reduce the activity of the catalyst by blocking all the step sites of Ni catalyst first and occupying octahedral sites just below the surface.

Improvements have been made in the catalytic activity, coking resistance, structural modification of SRM catalysts for Ni-based catalysts, and other non-noble metal catalysts (Co, Cu, Mo, etc.) as well as expensive noble metal catalysts (Au, Pt, Pd, Rh, Ru, etc.) [102]. A variety of modified SRM processes have been developed, including the thermos-photo hybrid, the sorbent enhanced SRM, the oxidative SRM, the chemical looping SRM, the plasma enhanced SRM, and the electrical field enhanced SRM, and their advantages and limitations are compared along with critical perspectives to help illuminate further research and development required.

A type of solid solution catalyst based on Ni and magnesium oxide is Ni/MgO, which forms fine Ni particles with an exceptionally high stability. The active Ni species is reduced from the parent solid solution [103]. A solid solution catalyst consisting of Ni_0.03Mg_0.97O with well-dispersed Ni metallic particles can convert 90% CH₄ with a low S/C ratio of 1.0 and maintain stability for 70 h with a high coke resistance. The Ni particles which were homogenously distributed on the surfaces of Ni_0.4Mg_0.6O solid solution supports totally converted CH₄ for 1000 h without deactivation [104]. The tri-compound Ni-Mg-Al catalysts have been engineered to drive efficient SRM in a wide range of S/C ratios from 1.0 to 4.0. In addition, they can drive SRM at a ratio of 4.0 and temperatures as low as 550 °C. On Ni-Mg-Al catalysts, full conversion of CH₄ is still possible [105]. Further, Ni_0.5Mg_2.5AlO₉ catalyzed reactions better than Ni/ZrO₂/Al₂O₃ and Ni/La-Ca/Al₂O₃ with a residence time of 20 ms [106].

Barati Dalenjan reported a study on the synthesis of Ni/MgO catalysts by using the hydrothermal method and obtained physicochemical and catalytic data for the thermos-catalytic decomposition of CH₄ into H₂. It was shown that the methane conversion and hydrogen yield increased from 28.3% to 48.6% and 33.2% to 53.2%, respectively, when the nickel ratio was increased from 10 wt.% to 40 wt.%; and the size and amount of nickel particles influenced the catalyst stability. The highest initial activity (49.6% methane conversion) and the highest stability (above 45% hydrogen yield for 180 min) were achieved for catalysts with 40 wt.% and 30 wt.%.

To better understand the morphology of synthesized catalysts, transmission electron microscopy (TEM) analysis is used for catalysts with a nickel content of 40 wt.%. Fig.6(a) and Fig.6(b) show TEM images and particle sizes of nickel at 40 wt.% Ni/MgO. Analyses of TEM confirm the synthesis of mesoporous nanostructured catalysts [107]. Furthermore, Fig.6(a) indicates that the black points are NiO particles distributed uniformly in the image.

Ni catalysts experience a stability problem and react under challenging conditions such as high temperatures and pressures. The SRM process has been continually improved over the past few years, including fabricating cost-effective and efficient catalysts, and designing advanced SRM processes. Various methods have been employed to enhance Ni-based catalyst activity and stability, including adding promoters, modifying the structure of the catalyst, and constructing unsupported Ni-catalysts. For example, high CH₄ conversion rates of 95% and 98% have been achieved for Ce-doped Ni-Al₂O₃ catalyst [108], 98% for Nb-doped Ni-Al₂O₃ catalyst [99], 97.2% for K₂Ti_xO_y-doped Ni-Al₂O₃ catalyst [109], and 97% for Ni/Ce-ZrO₂ catalyst [110]. The performance of these catalysts indicates that Ce, Nb, and K₂Ti_xO_y are excellent promoters for Ni-based catalysts. In the low temperature SRM process, NiO-MgO catalysts such as Ni_0.4Mg_0.6O provide 100% CH₄ conversion; when tested in partial oxidation of CH₄, spc-Ni_0.5/Mg_2.5Al provided high CH₄ conversion despite its high space velocity (9×105 mL·h⁻¹·g·cat⁻¹)[111]. Tab.4 provides more studies on the non-noble metal-based catalysts, most of which are Ni-based.

4.2 Co-based catalysts

Promising results have been achieved for Co-based catalysts in the SRM process [67]. A major problem stemmed from the oxidation of metallic Co by H₂O leading to the deactivation of the Co catalysts. In this case, noble metals could be added to the Co/Al₂O₃ catalyst to provide a more stable metallic state and to decrease the susceptibility of the catalyst to degradation during SRM. As a result of the hydrogen spillover effect, Pd, Pt, Ru, and Ir significantly reduced the reduction temperatures of both Co₃O₄ and Co surface species [130]. Although Pd, Ir, and 0.3 wt.% Pt produced the higher conversion of CH₄ on Co/Al₂O₃ at 700 °C with an S/C ratio of 4.0, Pt had the highest CH₄ conversion, H₂ production, and stability. Adding transition metals other than noble ones to Co catalysts could improve their activity to a certain extent. The Co/Mg/Al catalyst promoted by La and Ce achieved a CH₄ conversion of 85% at 700 °C and an S/C ratio of 2.0 [131,132]. The unpromoted catalyst had considerable coke formation at an S/C ratio of 0.5, whereas the La and Ce promoted catalysts had a high resistance to carbonation. In addition, Co oxidation still shows some deactivation. It has been demonstrated that Co-Pt-Zr-La/Al₂O₃ catalysts can accomplish a nearly complete CH₄ conversion (99.3%) at 750 °C with an S/C ratio of 1.25 without any carbon deposition or sintering [129]. At high temperatures, Co-based catalysts become inactivated for its tendency to sintering and its lack of metal carbide cracking [133,134].

In a study by Avdeeva et al. [135], cobalt catalysts synthesized by different methods were investigated under varying operating conditions. The highest conversion was found in co-precipitated 60 wt.%–75 wt.% Co/Al₂O₃ at the temperature of 475–500 °C. Abdelbaki et al. [136] reported a study on the Nickel oxides catalyst supported on \text{γ}-alumina (Ni-loading ranging from 5 wt.% to 30 wt.% NiO) were synthesized and tested in the oxidative dehydrogenation of ethane to determine the importance of the Ni–O support interaction. There is a progressive increase in ethylene selectivity with 15 wt.% Ni–O leading to 88%–95%. However, the selectivity for ethylene in oxidative dehydrogenation (ODH) of ethane is further reduced to 72% when Ni–O is added to Al₂O₃ at a concentration of 30 wt.%. In addition, with the exception of catalysts made from Co/Al₂O₃-SiO₂ and Co/MgO, the conversion rates were 77% and 86%, respectively [137,138].

Awadallah et al. [138] reported a study on the effectiveness of combining one metal from group VI (25 wt.% of Cr, Mo, or W) with 25 wt.% of cobalt metal supported with MgO for the catalytic decomposition of methane to hydrogen free of CO_x and carbon nanotubes (CNTs). A TEM image of the deposited carbon over the Co catalysts, which was collected after running the methane catalytic decomposition reaction at 700 °C for seven hours, is shown in Fig.7. It is apparent that all the different composed Co catalyst produced dark robe-like tubular multi-walled carbon nanotube (MWCNT) texture with hollow core. The fabricated CNTs are composed with curled and twisted to form sinuous material with length beyond several microns. Additionally, no imperfections were detected at the external walls, signifying that the produced MWCNTs on all cocatalyst retain a superior quality and graphitization. It is shown in Fig.7(a) that the external diameter of MWCNTs attained over the 50 wt.% Co/MgO catalysts is in the range of 6–14 nm. It is apparent that the incorporation of 25 wt.% Mo into 25 wt.% Co/MgO catalyst yields CNTs with the tiniest and thinnest diameters distribution of 3–11 nm compared to using other catalysts (Fig.7(c)). Specific defects were detected on the external walls of MWCNTs formed on the Co-Cr/MgO catalyst (Fig.7(b)). The MWCNTs produced from the Co-W/MgO catalyst acquire a larger diameter in the range of 8–24 nm, as shown in Fig.7(d).

4.3 Cu-based catalyst

Pure Cu metal has been found less active for SRM than metallic Pt, Pd, Rh, and Ni. The reaction steps are more energetically difficult in pure Cu metal [132]. Recently, Cu has been shown to act as a catalyst promoter for Ni catalysts [139] and Co₆Al₂ supported 5 wt.% Cu catalysts exhibits a promising SRM activity [128]. At 700 °C (5 wt.% Cu/Co₆Al₂), an S/C ratio of 3.0 could nearly be achieved without coke formation. As Cu content was increased, copper oxide agglomerated and deactivated the reaction. As a result of the enhanced reverse water gas shift reaction (RWGS), higher Cu contents resulted in a higher selectivity to CO [67].

Sajjadi et al. [140] synthesized a Cu and Co doped Ni/Al₂O₃ nano-catalyst via impregnation and sol-gel methods. At different mixtures of CH₄/CO₂ and different gas hourly space velocities, the samples were used to examine the CO₂-reforming of methane at atmospheric pressure and temperature. In addition to displaying excellent surface areas, fewer particle sizes, a more stable supporting structure, and improved morphology, Ni-Co/Al₂O₃ exhibits excellent properties due to the synergistic effect of sol-gel synthesis and cobalt additions. It produces the highest yield of products (98.21% for H₂ and 95.64% for CO), the closest molar ratio of H₂ to CO (0.92–1.05), and the most stable conversion during the 1440-min stability test. Of the prepared nano-catalysts, Ni-Co/Al₂O₃ demonstrated the most effective catalytic performance and was shown to be an efficient catalyst for the dry reforming of methane. Although Ni-Cu/Al₂O₃ had a stable yield, it exhibited a lower catalytic activity and CO/H₂/CO ratio than the unprompted nano-catalysts.

CO_x-free hydrogen and carbon nanofibers can be produced with 50 wt.% Ni–10 wt.% Fe–n wt.% Cu/Al₂O₃ (n = 0, 5, 10, 15) catalysts [141]. Carbon buildup on the nickel surface is inhibited by the high affinity of copper for graphite structures that prevent the catalyst from deactivation. At the temperatures higher than 700 °C, the catalytic effect is enhanced considerably.

Fig.8 shows the surface morphology and nature of carbon deposited on (a) 50 wt.% Ni/SiO₂ and (b) 50 wt.% Ni–10 wt.% Cu/SiO₂ catalysts [142]. The majority of carbon is found to be composed of carbon filaments, which are potentially valuable. These catalysts have a diameter of a few nanometers and micrometers. Due to the interwoven nature of the carbon filaments, it was not possible to determine the exact length of these nanofibers. In some cases, nanofibers may be longer than one millimeter. The morphology of the carbon nanofiber was significantly influenced by the reaction temperature. According to Refs. [143–145], CNTs form perfectly (solid) at lower temperatures, while they do not at higher temperatures. It has been suggested that the change in the diameter of CNTs caused by changes in reaction temperature is due to different rates of nanocarbon nucleation [146].

Fig.9 shows the TEM analysis of as-grown CNTs on (a) 50 wt.% Ni/SiO₂ and (b) 50 wt.% Ni–10 wt.% Cu/SiO₂ catalysts at 750 °C [142]. The entanglement of amorphous carbon and nanoparticles forms carbon filaments. The TEM micrographs of the 50 wt.%–10 wt.% Cu/SiO₂ catalysts showed that the CNTs were filled with the Ni-catalyst. There was a sharp contrast between the Ni-filled catalyst and the CNTs around it. In contrast, catalysts deposited nanotubes of different sizes and lengths. Fig.9(a) also depicts that the type of CNTs has a similar size to the metallic Ni-particles. Hence, a Ni metal particle produces a carbon nanofiber of the same size.

The use of non-noble transition metal- (i.e., Co, Cu, and Mo) based catalysts for SRM has been found promising. Some of the catalysts showed a higher catalytic activity than Ni-based catalysts at higher temperatures. As the reaction temperature of MoC₂/Al₂O₃ was lower (700 °C versus 750 °C), a higher CH₄ conversion rate was achieved (95%) [90]. In spite of being inexpensive, the catalysts were poorly reducible [130], easily deactivated (because of oxidation of the metallic species), and easily aggregated at high temperatures [120, 127–130]. These issues must be addressed in order for transition metal catalysts to be widely used in practice.

Today, ceramic-supported nickel is the most widely used SRM catalyst since it is relatively superior in terms of performance and price but sintering and coke formation remain its main drawbacks [147]. The performance of Ni-based catalysts can be improved by various promoters, and the best promoters are noble metals in terms of reactivity, coke suppression, and reducibility, but they are expensive for general commercial applications. Consequently, non-noble metals and metalloids have received more attention from researchers. The activity of WGS reactions can be increased by using Co or Cu, which can produce more hydrogen from the reaction. Zirconium, yttrium, and lanthanum are suitable textural promoters because they increase surface area and metal dispersion. Ceria or magnesia are effective at tuning the surface acidity of silica for long-term stability. Silicon is commonly used as catalyst support in its oxide and carbide forms. In addition to Ni-based catalysts, Co, Cu, and Mo-based catalysts have also been demonstrated for their superior catalytic performance for SRM. Some even showed a higher catalytic activity than Ni-based catalysts, i.e., MoC₂/Al₂O₃ exhibited significantly a good performance of around 95% CH₄ conversion, even at a lower reaction temperature of 700 °C [90]. In contrast, most catalysts exhibit poor reducibility, easy deactivation (due to oxidation of the metal species), carbon deposition, and high temperature-induced aggregation. These issues must be addressed for these transition metal catalysts to be widely used in the practical hydrogen production process [67].

5 Hydrogen production integrated with carbon capture, utilization, and storage

The production of hydrogen from fossil fuels results in the formation of a substantial amount of CO₂ as a by-product, i.e., the production of approximately 9 kg(CO₂)/kg(H₂) during the SRM process [148]. About 830 Mt CO₂ per year is emitted globally due to H₂ production from fossil fuels, an amount that is equivalent to the combined total CO₂ emissions of the UK and Indonesia [149]. Therefore, it is essential to capture CO₂ from SRM processes for storage and/or utilization to curb CO₂ emissions for climate change mitigation and for clean production of hydrogen [150,151].

Commercial production of hydrogen from fossil fuels involves a number of steps. The first is to form syngas, commonly through gasification or reforming, including partial oxidation, autothermal reforming and SRM. The second is to convert the syngas to hydrogen and CO₂ through WGS in the high- and low-temperature shift reactions. In the shifted syngas, there exist some unconverted and/or partially converted combustibles, including CH₄, CO, possibly H₂S, N₂, and/or argon, in addition to hydrogen and CO₂. CO₂ and other impurities are separated from hydrogen by the hydrogen purification system.

To produce hydrogen with CO₂ capture, the part of the process that separates the syngas must be increased following syngas production. This can be accomplished most easily by adding a second separation unit. Currently, the focus is on obtaining sufficiently pure hydrogen and CO₂ for transport and storage. The process would result in three streams: one stream of purified hydrogen, one stream of purified CO₂, and an off-gas stream that contains a mixture of trace hydrogen, trace CO₂, and other compounds such as CO, CH₄, and N₂.

Fig.10 illustrates a general route for hydrogen production with CO₂ capture [152]. As part of the gas separation system, it is critical to select the right sequence of CO₂ capture and hydrogen purification. SRM utilizes fuel burning in a furnace to provide heating while gasification uses fuel oxidation inside the reactor (the reactor is internally heated). As a result of gasification, fuel oxidation delivers heat (the reactor being externally heated). In the latter case, fossil fuel is directly fed into the reformer, which produces flue gas. The CO₂ captured in this case would represent around 60% of the CO₂ from the overall process if it was only captured from the syngas. The flue gas must be captured separately (post-combustion capture) for a higher CO₂ capture ratio.

Currently, PSA is the most commonly used method for hydrogen purification, with solvent absorption being the most mature technique for CO₂ capture. Additionally, a lower temperature/cryogenic separation can be achieved with novel separation methods, such as membranes and sorbents. The development of advanced methods to produce hydrogen with CO₂ capture is also underway, including membrane reactors, sorption-enhanced hydrogen production [152]. To produce hydrogen with CO₂ capture, relevant separation methods include adsorption, absorption, membranes, and cryogenic/low temperature processes.

5.1 Adsorption process

An adsorbent (usually formed out of solid material) is used to fix molecules on its surface through the physical process of adsorption. This phenomenon allows gases to be purified by adsorbing them with different affinities to various types of adsorbents. When purifying hydrogen, it is possible to select different types of adsorbents so that all types of impurities will be adsorbed, and hydrogen will pass through with very limited adsorption. It is possible to use adsorbents which selectively bind CO₂ in the gas mixtures to capture CO₂.

To achieve an excellent separation performance, it is essential to select the right adsorbents. There have been many studies exploring CO₂ adsorbents. Several recent studies have focused on developing zeolite for CO₂ adsorption [153]. A variety of zeolite classes have been reported such as ZSM-20 [154], 5A [155], natural zeolites [156], ZSM-5 [154,156], 13X, H-ZSM-5-30, H-ZSM-5-80 [157]. In addition to these studies, several other supports have been used for zeolite, such as silica [158], NaX, NaY, NaY-10 [157], Na, and B on ZSM-5 [159]. Moreover, activated carbon is also a useful adsorbent material [160].

PSA processes in a post-combustion thermal power plant environment use zeolites as natural adsorbents. It has been demonstrated that zeolites are more effective in adsorbing CO₂ than activated carbons under post-combustion conditions due to their superior capacity and selectivity [161]. Zeolite 13X and NaY proved the most effective at CO₂ partial pressures found in flue gases [162]. Several studies suggest that zeolite 5A is a promising adsorbent because of its higher volumetric capacity and less severe heat effect [163]. However, these features may be relevant only to thermal swing processes. The hydrophilic nature of zeolites is their main drawback [164]. As a result of water vapor present in the flue gas, these adsorbents are adversely affected in terms of their capacity and surface area available for sorption. A potential alternative is metal-organic frameworks (MOFs) [165]. The highest surface area MOFs can adsorb CO₂ even in water vapor, which is extremely promising. A high degree of adsorptive selectivity has also been observed in materials MOFs with functionalized surfaces [166]. Moreover, amine-functionalized adsorbents have an excellent CO₂ selectivity (particularly over N₂) and a high capacity for CO₂ adsorption at low-pressure levels. Their applicability to standard PSA-based systems is doubted by sharp adsorption isotherms, high energy requirements for regeneration, and possible degradation of amines at high temperatures [167].

Adsorbent materials and separation processes were found to need improvement after pre-combustion analysis. The use of activated carbons as adsorbents in cold PSA processes is feasible. Regarding their potential, MOFs are extremely interesting for cold PSA processes. To improve the performance of the process, it might be necessary to adjust their adsorbent properties to specific operating conditions. It is suggested that potassium-promoted hydrotalcite be used in hot PSA processes [161].

There are a number of types of adsorption processes, such as PSA, adsorption by temperature swing (TSA) or adsorption by electric swing (ESA) [152]. Since PSA can allow for pressure swings with minimal energy penalty, it is most relevant for hydrogen and CO₂ separation from syngas.

The PSA process is the most commonly used method for hydrogen purification, as it can obtain very pure hydrogen (sometimes exceeding 99.99 mol.% [168]). Hydrogen plants have used this process since the early 1980s [169]. Sircar & Golden [168] and Ritter & Ebner [169] had provided a comprehensive overview of the process.

A high-pressure adsorbent column is used in PSA to purify hydrogen from syngas. Impurities are removed through adsorption by the absorbent, and purified hydrogen passes through with little adsorption. When adsorbents are saturated with the impurities, they are regenerated by lowering the pressure to desorb impurities, which are purged out of the column by hydrogen flow. Fig.11 shows the adsorption and desorption steps [152]. At ambient temperatures, PSA units are typically operated with feed pressures of 20–60 atm (low temperatures promote adsorption). The pressure drop between the feed gas stream and the hydrogen product stream is small, typically less than 100 kPa, and the PSA off-gas stream containing the impurities exits the column at near atmospheric pressure, usually 1.1–1.7 atm [168]. PSA can also be designed to operate at a pressure less than the atmospheric pressure, the so-called vacuum swing adsorption (VSA). In the PSA process, hydrogen product gas can be purified to 98 mol.%–99 mol.% H₂ with 60%–95% hydrogen recovery rate, depending on the composition of the feedstock [168,169].

5.2 Absorption process

The use of CO₂ absorption in natural gas sweetening, first introduced in the 1930s with tri-ethanolamine (TEA) [170], is a commercially mature technology. In hydrogen plants, it was commonly used until PSA became commercially available in 1980, which allowed for production of high-purity hydrogen at the same cost [169,171]. Hydrogen is still purified in ammonia plants using this technology with a subsequent methanator [169].

Separation by absorption of gases is conducted using a scrubber column (absorber) in which a liquid solvent is brought in contact with gas and certain contaminants are absorbed. Afterwards, the rich solvent is heated and/or depressurized, then passed through a regeneration column (stripper) which creates one stream of components absorbed, and one stream of lean solvent that is fed back to the scrubber column. Fig.12 illustrates the principle behind the CO₂ absorption from syngas [152].

5.3 Membrane separation process

A membrane is a selective barrier that can be used to separate certain components more easily than others, because they allow selected components to pass through more easily. Those portions of the feed that pass through the membrane is referred to as permeates, while those parts that will not pass through is called retentates. For membranes, it is essential that their selectivity be high, their flux be high, they be low cost, they be mechanically stable, and they be chemically stable. Molecules are transported through membranes by partial pressure differences, and depending on the process, there may also be a need for compression power [152].

Hydrogen production with CO₂ capture can also use both hydrogen and CO₂ selective membranes. Hydrogen permeates through hydrogen selective membranes to become a purified hydrogen gas but at a pressure lower than the feed gas, whereas impure CO₂ retains at high pressures in the feed gas side. When CO₂ selective membranes are used, CO₂ enriched permeate is produced at a lower pressure, while CO₂ depleted retentate is produced at high pressures. A polymeric membrane is the most mature type of hydrogen selective membrane. For many years, low-temperature process streams have been recovered with these systems, because they can operate at temperatures below 100 °C [169,172,173]. In addition to being able to withstand high pressure drops, they are relatively inexpensive as well [174].

5.4 Carbon capture, utilization, and storage

Hydrogen can be produced through several methods, including “green hydrogen” from water electrolysis with renewable electricity, “gray/black hydrogen” from fossil fuels that release CO₂, and “blue hydrogen,” which mitigates CO₂ emissions associated with hydrogen production processes from fossil fuels [175]. Another complexity related to blue H₂ plants is storing or utilizing the CO₂ captured from these SRM processes. It is essential and has been demonstrated for CO₂ capture and subsequent storage in underground reservoirs [176].

It was reported by Dou et al. [177] that hydrogen production from chemical looping steam reforming (CLSR) utilizing an oxygen transport catalyst was significantly enhanced by CO₂in situ capture, which shifted the chemical equilibrium and reduced energy consumption. This study described an integrated chemical looping hydrogen production process that combines sorption-enhanced chemical looping steam reforming (SE-CLSR) of glycerol with CO₂ capture and conversion in a single integrated process. As a separation method, NiO/NiAl₂O₄ was oxidized and used as a steam reforming catalyst, NiO/NiAl₂O₄ was reduced and used as a CO₂-CH₄ dry reforming catalyst, as well as inexpensive dolomite as a CO₂ sorbent. The results indicated that hydrogen concentrations of above 92.5% were generated in one step and that CO₂ levels were reduced to zero during the pre-CO₂ breakthrough period. A reduced NiO/NiAl₂O₄ catalyst was preferred for CO₂-CH₄ dry reforming at 850 °C. The production of syngas with a molar ratio of H₂ to CO between 0.8 and 1.1 was achieved following the in-situ extraction of the CO₂ dolomite absorbed in the process. NiO/Al₂O₄ with a content of 15 wt.% NiO had the highest hydrogen and syngas yields and maintained a high performance throughout the experimental period. A schematic illustration of the proposed system for SE-CLSR with in situ capture and utilization of CO₂ is shown in Fig.13.

A study by Ren et al. [178] demonstrated the use of colloidal solution combustion to produce a mesoporous Ni/CeO₂ catalyst with a high specific surface area for the production of syngas associated with the dry reforming of glycerol. Among the catalytic performance experiments, 5 wt.% Ni/CeO₂ exhibited the highest conversion rates of glycerol and CO₂, respectively, at 84.1% and 38.4%. As a result, the output molar ratio of H₂ to CO ranges from 1.47 to 1.75. Among the catalysts, 5 wt.% Ni/CeO₂ has the lowest apparent activation energy (52.83 kJ/mol), hence it has the best catalytic performance.

Dou et al. [179] used a SE-CLSR of ethanol for in situ CO₂ removal, and evaluated the effects of sorbent addition on hydrogen production in an alternating fixed-bed reactor utilizing a mixture of NiO/Al₂O₃ oxygen carrier catalysts (OCs) and CaO-based sorbents under moderate operating conditions (600 °C, 1.0 atm, 3 °C). It was found that the NiO component in the OC was first reduced by ethanol, and the reduced OC was responsible for the catalytic steam reforming and WGS required to produce hydrogen. As a result of the efficient removal of CO₂ by CaO based sorbent, the process was also intensified substantially. Generally, the best molar ratio of sorbent to OC (Ca/Ni ratio) is 2.0 to 3.0, and the highest hydrogen selectivity and feed conversion were obtained at a Ca/Ni ratio of 3.0. The production of hydrogen was inhibited by a higher Ca/Ni ratio due to the fact that the OC particles were rounded and diluted by the sorbent. The removal of CO₂ by solid sorbents induces ethanol dehydration and C−C bond cleavage, changing the hydrogen production route of conventional CLSR. An alternating fixed-bed reactor integrated oxidization, steam reforming, WGS, and CO₂ capture in situ to produce continuous, high-purity hydrogen.

Gonzalez-Diaz et al. [180] analyzed the energy, carbon emission, and economy of hydrogen production from methane transported as ammonia. In the assessment, enhanced oil recovery is adopted as an alternative for utilizing CO₂, which involves extracting crude oil using captured CO₂. It has been determined that hydrogen production with carbon capture and captured CO₂ can be used for enhanced oil recovery compared to conventional oil production and that CO₂ emissions could be reduced by 54.8% from 97.4 to 44 t/h. A significant reduction in hydrogen transportation costs can also be achieved by using liquid ammonia as a carrier. The cost for carbon emissions can be reduced through the revenue generated by selling CO₂, facilitating hydrogen production if incorporating carbon capture and utilization.

Katebah et al. [181] examined two strategies for reducing specific CO₂ emissions to capture and compress CO₂ for use in subsequent processes and to utilize electrolysis to produce higher levels of hydrogen. They analyzed CO₂ mitigation to examine its effect on gray hydrogen production without emission mitigation. It is shown that electrolysis integration is less attractive than heat and power integration schemes for the SRM process, which can reduce emissions by 90% with only a moderate increase in hydrogen production costs of 13%, with CO₂ capture and compression for later sequestration or utilization. They compared this blue hydrogen production with an alternative electrolysis production in the context of renewable and fossil energy generation and electricity mix considering the life-cycle emissions involved.

According to Ali Khan et al. [148], spatial techno economic frameworks can be used to assess blue hydrogen SRM projects with carbon captures and utilization. In an Australian case study, zoning filters were used to identify four potential SRM hubs and eight carbon dioxide injection sites for hydrogen generation. Their analysis determined that the levelized cost of hydrogen (LCH₂) varied across Australian states based on natural gas pricing and storage costs. The techno-economic framework was modeled so that it could be applied worldwide, particularly to blue hydrogen production integrated with CO₂ utilization opportunities. They considered using an SRM unit for CO₂ capture and subsequent storage. It is of interest from the perspective of the environmental footprint and certification that the highest levels of CO₂ emissions are captured from the SRM unit [182,183]. As a result, they considered CO₂ capture technologies that have the highest capture rate after combustion. This is the reason for the authors to choose a mono-ethanolamine (MEA) solvent unit to capture CO₂ due to its high levels of commercial maturity and technology readiness level of 8 [184,185], its widespread deployment worldwide, its superior CO₂ capture rate of more than 90%, and its suitability for post-combustion capture.

Another study considered the case that the CO₂ captured from the CCUS unit is diverted to a CO₂ electrolyser (with capacities ranging from 1 to 50 MW) for the production of valuable products (electrochemical CO₂ reduction to formate/formic acid and methanol) [186]. Formic acid was previously established as a highly probable commercial CO₂RR (reduction reaction of CO₂) pathway due to its high conversion performance and competitive retail pricing [187,188]. The electrolyser was modeled based on the performance data of a state-of-the-art bench-scale alkaline electrolyser that had a Faradaic efficiency of up to 80% at 3.5 V with a current density of 140 mA/cm² [184]. In addition, a 50% CO₂ conversion per pass was considered for the electrolysis and the system included a PSA system that recycled untreated CO₂ (to improve conversion) from by-products such as O₂/H₂ production. The by-products, primary hydrogen, were retailed as an additional product. Fig.13 illustrates a proposed layout for hydrogen production integrated with CCUS [148].

A workflow is shown in Fig.14 [148] as a sequence of four distinct blocks: SRM plant, carbon capture system, carbon storage system, and carbon utilization system. Block 1 contains the SRM unit with a H₂ plant, while Block 2 contains the carbon capture unit that separates CO₂ from flue gases. Block 3 involves transporting and injecting captured CO₂ underground, while Block 4 utilizes a CO₂ electrolyser to convert it into formic acid. Depending on the operating scenario of the SRM, these blocks are combined differently.

6 Current trends and future direction of research

Hydrogen extraction from fossil fuels is an efficient, low-cost alternative to non-conventional sources. Hydrogen can be extracted from fossil fuels by breaking down hydrocarbons as a primary reaction product and by-products. It is essential that hydrogen can be produced in an energy-efficient manner using eco-friendly and economical methods to be used on a large scale for commercial purposes. These processes are typically catalyzed to ensure hydrogen selectivity. Additionally, optimal conditions are essential for achieving the highest yield and efficiency. Several methods are used to produce hydrogen from fossil fuels, including hydrocarbon pyrolysis, partial oxidation, autothermal reforming, and steam reforming, with SRM being the most economical and preferred method for hydrogen production commercially.

An endothermic reaction called pyrolysis triggers natural gas conversion into hydrogen and carbon (solid). To develop an accurate process model, it is necessary to accurately determine the kinetics of methane pyrolysis, including deactivation. The use of modeling can help narrow the scope of the research and development, in relation to significant design and operational parameters. Methane pyrolysis produces solid carbon as a by-product with many industrial uses. It is possible to convert solid carbon into thermodynamically stable and practically useful materials, which can be used in the construction of fuel cells, batteries, and other applications. The extra carbon can be stored in geological storage or unused coal mines for filling. However, solid carbon contains a significant amount of energy that could be utilized otherwise. To improve the economics of methane pyrolysis, it is also important to develop methods to separate carbon and improve the quality of the carbon deposited.

It is standard for developed countries to use the partial oxidation process for industrial purposes. However, partial oxidation-mediated hydrogen production may not be as popular as it should be because of its high capital and operational costs. There are also challenges associated with this process, such as frequent cleaning, preventing coke formation, and handling reaction conditions. To increase reaction rates, nickel catalysis is commonly used in auto-thermal reforming. Adding Ni to other catalytic metals in multiple percentage ratios increased hydrogen yields significantly. Similar trends in process efficiency enhancement were found in hydrogen production, where it was demonstrated that Ni²⁺ reducibility contributed to reaction rate enhancement. Among the various means of generating hydrogen, SRM provides the highest yield because the hydrogen in the steam (water molecule) is also converted into hydrogen gas.

At present, natural gas produces 48% of hydrogen, oil 30%, and coal 18%, while renewable sources produce only 4% of hydrogen. In the next few years, oil and coal will significantly decline the share of total energy consumption due to the increasing importance of natural gas. In methane conversion, SRM is the most efficient means of producing hydrogen and syngas. However, research and development efforts have continued to improve the catalytic performance and material properties of reactor tubes.

Previously, the catalyst used in the SRM process was catalytic active noble metals such as Au, Pt, Pd, Rh, Ru, and other noble metals. However, these noble metals are relatively limited in natural reserves and too expensive for SRM. Recently significant advancements have been made for Ni-based catalysts, and other transition metal-based catalysts, such as Ni, Co, and Cu-based alloys, for the SRM process. It is also necessary to develop a catalyst support which is an essential component of catalyst stability and reactivity. Among the non-noble metals, Ni is the most valuable active metal for large-scale applications because of its abundant natural resources and low price. Ni-based SRM catalysts have been targeted to improve coking and sintering resistance due to significant coking and sintering problems. In contrast, most catalysts exhibit poor reducibility, easy deactivation (due to oxidation of the metal species), carbon deposition, and high temperature-induced aggregation. These issues must be addressed for these transition metal non-noble catalysts to be widely used in the practical hydrogen production process.

The production of hydrogen from fossil fuels has to be thoroughly investigated in order to mitigate CO₂ emissions. During the production of gray hydrogen, significant amounts of CO₂ are released into the atmosphere (9 kg of CO₂ is produced for every kilogram of hydrogen). There are also advanced methods being developed to produce hydrogen with CO₂ captures, such as membrane reactors and sorption-enhanced hydrogen production. The most common method for hydrogen purification is PSA, while the most mature technique for capturing CO₂ is solvent absorption. Adsorbent selection is crucial for achieving excellent separation performance. A number of studies have been focused on the development of zeolites and MOFs for adsorbing CO₂. Membrane separation requires high selectivity, high flux, low cost, and chemical and mechanical stability. Finally, a spatial techno-economic framework is the most appropriate method for assessing the viability of hydrogen production with SRM and CCUS. Further, significant research and development is required on the utilization of carbon dioxide for value-added products instead of expensive storage.

7 Summary and concluding remarks

This paper provides a comprehensive overview of various methods for the production of hydrogen from fossil fuels, including hydrocarbon pyrolysis, partial oxidation, autothermal reforming, and steam reforming, with a focus on the SRM, because SRM is the most economical and preferred method for the commercial production of hydrogen. The current SRM process involves the use of catalysts made of noble metals, such as Pt, Au, Ru, and Ag. To reduce the cost of hydrogen production, the development of non-noble metals as catalyst, such as Ni, Co, and Cu alloys, is essential and is examined. Ni-based catalysts are highly efficient, and their synthesis can be scaled up easily. They are the most common commercial non-noble metal-based catalysts for SRM. However, these non-noble metal-based catalysts need improvement for stability and broader reaction conditions such as at lower operating temperatures and pressures. Various methods have been investigated for improved activity and stability, such as introducing promoters, novel catalyst structure and supports, and unsupported Ni catalysts. Some of the high-performance Ni-based catalysts include Ce- and Nb-doped Ni-Al₂O₃, as well as NiO-MgO catalysts such as Ni_0.4Mg_0.6O for the low-temperature SRM process. In addition to Ni-based catalysts, Co, Cu, and Mo-based catalysts have also been demonstrated for their superior catalytic performance, such as MoC₂/Al₂O₃. On the other hand, most of these non-noble metal-based catalysts exhibit poor reducibility, easy deactivation due to oxidation of the metal species, carbon deposition, and high temperature-induced aggregation. These technical issues must be resolved before their widespread commercial applications. Hydrogen production from fossil fuels accompanies a significant amount of CO₂ generation, requiring the capture, utilization, and/or storage of CO₂. CO₂ capture requires its separation from the SRM product gas mixture via adsorption, absorption, or the membrane separation process. A spatial techno-economic framework is the most appropriate method for assessing the viability of hydrogen production with SRM and CCUS. Carbon utilization for value-added chemicals is desired as opposed to carbon storage which is expensive.

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