Catalytic Dry Reforming of Methane

Summary

Natural gas is among the conventional fossil fuels which remain the global primary energy source. Moreover, the demand for natural gas is anticipated to increase in the coming decades. However, due to low energy density and high storage costs, natural gas consumption has not met its full potential. In some commercial practices, natural gas is flared due to the absence of economic viability around its usage. The combustion of all fossil fuels, including natural gas, produce large amounts of greenhouse gases that mainly consist of carbon dioxide. In addition to carbon dioxide, methane is also a significant pollutant in greenhouse gases. Although, methane is relatively lesser in quantity, it is more detrimental to the ozone layer than carbon dioxide. The dry reforming process was introduced to capture carbon dioxide and methane to produce synthesis gas. The industrialization of the process has not been achieved yet despite its positive environmental aspects.  The major hurdle in the commercialization of this process is catalyst deactivation due to coke deposition and sintering. A vast majority of researchers are working on designing and developing such catalysts, which do not deactivate at the operation conditions of dry reforming of methane and are resistant to coke deposition/sintering. The most commonly employed catalyst for this process is nickel. Moreover, various novel catalysts utilizing noble metals, silica foams, and zeolites have recently been studied for dry reforming. This article gives an overview of the dry reforming process, the types of reactors used, and the performance of the catalysis used in dry reforming.

Introduction

Fossil fuels are the reason for a strong connection between energy and the environment. The need for energy-efficient clean systems is growing daily due to swiftly depleting natural energy reserves and stringent environmental laws. Fossil fuels have been regarded as the primary energy source for the last few decades [1, 2]. However, the growing consumption and rapid depletion calls for alternate energy sources/technological innovations to consume oil & gas in an environmentally friendly manner [3].

The estimates show a huge (around 40%) surge in the annual global demand for natural gas. The global natural gas demand is anticipated to be 25% by 2040 [3]. The current forecast also shows that natural gas reserves are anticipated to fill the demand for another 200 years.

Aside from the natural gas reserves, anaerobic digestion of organic matter generates methane and carbon dioxide in almost equal proportions that can be tapped for functional purposes [4]. So far, natural gas consumption has not reached its full potential due to low energy density, low critical temperature, and high transportation costs.

Numerous studies carried out in the 1990s showed that anthropogenic actions were the leading cause of the generation of greenhouse gases (i.e., methane, carbon dioxide, nitrogen oxides) and global warming [5-7]. The statistics publicized by Exxon Mobil show that carbon dioxide emissions are anticipated to reach 43 gigatons under new policies by the year 2035. Thus, the emission and generation of greenhouse gases due to anthropogenic activities remain the predominant cause of global warming.

Even though the quantity of methane is lesser than carbon dioxide, methane’s global warming potential is 28-36 times higher than carbon dioxide. Eventually, this takes part in radiative imbalance at the global level [8]. Therefore, there was a great need for a process to eliminate methane and carbon dioxide produced globally effectively.

Dry Reforming

The dry reforming of methane (DRM) was invented almost two centuries ago, in 1888 [9]. In 1928, many years after introducing dry reforming of methane, Fisher and Tropsch experimented with the reforming method. They ran trials with various elements as catalysts for this process. The elements included iron, nickel, copper, cobalt, molybdenum, and tungsten.

They also tried various support materials for these catalysts, like clay support, silica, magnesium oxide, and magnesium oxide mixed with aluminum oxide. Nickle and cobalt showed promising results among all these elements. The catalyst performance was also increased as the concentration of aluminum oxide was increased. However, the catalyst had a significant drawback, i.e., it deactivated with coke formation.

Interest in dry methane reforming was rekindled back in 2010 with the findings that this process could be used to reduce greenhouse gases. Moreover, it can also be used to produce long-chain hydrocarbons or in the oxygenation of dimethyl ether, acetic acid, and oxo-alcohols. Various metallic elements, either noble or non-noble, have been utilized so far as a catalyst for the dry reforming of methane [10].

Compared to non-noble metals, noble metals such as iridium, rhodium, platinum, and palladium have shown a much higher tendency to stop coke deposition. Although the price of noble metals is way more than its competitors, i.e., non-noble metals. Therefore, the most cost-effective balance is a combination of noble and non-noble metals. This combination facilitates dispersion of active metals and offers more reactive sites [11].

The scientists extensively investigated the usage of supports for the catalyst due to their huge potential of restricting coke formation and enhancing catalyst performance by increased dispersion. In addition to the role of catalysts enhancers, the promoters were also investigated for the synthesis and performance during the dry reforming process [12].

Nowadays, methane conversion to more valuable products has become vital because it is a significant constituent of the natural gas extracted from oil and gas reservoirs and landfills. Therefore, methane consumption to generate higher-valued products is actively pursued, usually accomplished by syngas production.

Various techniques have been introduced to transform methane into syngas. These technologies include dry reforming, steam reforming, auto-thermal reforming, and partial oxidation. These techniques are differentiated by the type of oxidation agent utilized, the syngas composition (i.e., H₂/CO ratio), the types of catalysts used, and the reaction mechanism [13, 14].

Reactors

Continuous fixed bed reactors (FBRs) are extensively utilized in the dry reforming processes due to their comparatively less complex design and economic viability (Figure 1). Some researchers have also attempted the application of fluidized bed reactors (FIBRs) for dry reforming. In this regard, Tomishige et al. [15] compared FBRs with FIBRs and concluded that Ni_0.15Mg_0.85O catalyst demonstrated improved catalytic activity in FIBRs due to lesser carbon accumulation.

Figure 1. Schematic illustration of a continuous fixed bed reactor

Agrawal and Srivastava [16] also reported better performance of the catalyst when they performed a simulation of dry reforming and found a methane conversion of around 98% at 800 °C. Although these reactors showed poor thermomechanical and thermochemical durability, excess quantity of catalyst and higher reaction temperatures were required. Therefore, most research works have focused on generating new designs of reactors.

Many studies have discussed membrane reactors that combine hydrogen generation and separation in one step. Sumrunronnasak et al. [17] studied the performance of a 5% Ni/Ce_0.6Zr_0.4O₂ catalyst in a membrane reactor made of Pd₇₆Ag₁₉Cu₅ alloy. The results showed significant improvement in the conversion of reactants when compared to the conventional reactor. The improved conversion was due to the separation of hydrogen by a membrane, which shifted the reaction equilibrium forward and prevented the parallel water-gas shift reaction.

In a recent study, Garcia-Garcia et al. [18] carried out a comparison of the catalytic performance of a conventional FBR, a tubular membrane reactor, and a newly developed hollow fiber membrane reactor were compared. The new reactor is consisted of a packed catalyst bed surrounded by a hollow membrane of palladium coated alumina, as shown in Figure 1. The amount of methane converted in the hollow fiber membrane reactor was 72%. Whereas, in the fixed bed reactor, it was only around 34% with respect to the thermodynamic equilibrium. The conversion of reactants in hollow fiber membrane was comparable to the tubular membrane reactor.

Figure 2. The catalyst bed scheme of the hollow fiber membrane reactor [19]

In another study, Liang et al. [20] studied a new BaFe_0.9Zr_0.05Al_0.05O₃ membrane reactor to observe simultaneous N₂O (a greenhouse gas) dissociation and syngas generation. A significantly high conversion of methane (97 %) and almost complete N₂O transformation were attained at 900 °C. The arrangement demonstrated better stability with no apparent catalytic activity or selectivity loss after 100 hours of reaction.

Micro-reactors are commonly utilized in multiphase reactions due to their excessively large surface-to-volume ratio and efficient heat and mass transport capabilities [21]. These reactors exhibit better conversion rates as comparted to the conventional tube-type fixed bed reactors. However, micro-reactors also face a few challenges, like blockage and choking [22].

Another variant of reactors of solar thermal types have also been investigated recently for dry reforming. As the name suggests, solar radiations are utilized in these reactors to carry out reactions at elevated temperatures. In a study, the conversion of carbon dioxide and methane was analyzed in a photocatalytic reactor by using ZnO semiconductor immobilized on a stainless-steel mesh. Ultraviolet illumination was used as the energy source in this reactor. The conversion of methane was around 16%. Whereas, for carbon dioxide it was around 12 % at room temperature [23].

Catalyst

The design of catalysts for dry reforming of methane is based on applying the intrinsic properties of the catalyst components by developing an understanding of the detailed interaction of these components for maximum performance and increased service life of these catalysts.

A variety of configurations, morphologies, and topologies have been investigated from time to time to assess the combined effect of the interactions of catalyst components. These interactions affect various factors like active metal dispersion, basicity, redox property, oxygen mobility, particle size, size distribution, reducibility, and mass transfer limitations of the catalyst [24, 25].

The understanding of these interactions has played a vital role in anticipating the reaction energetics, thermodynamics, mechanisms, and reactor design of dry reforming of methane. The main hurdle in generating syngas via dry reforming of methane is to strike a balance between the price of the catalyst, its performance, and selectivity.

In dry reforming, base metals have been used as an economical alternative to noble metals as catalysts. However, the fast deactivation of base metal catalysts due to sintering, generation, and accumulation of coke at reaction conditions is a significant problem. On the other hand, noble metal catalysts like iridium, palladium, platinum, and rhodium have shown better performance in terms of activity, selectivity, and stability during the dry reforming of methane. However, the feasibility of their commercial scale utilization has still not been possible due to their high price, resulting in their limited availability [26, 27].

The suitability of various catalysts for the dry reforming of methane were studied comprehensively. Nevertheless, there has been only minor success in finding their commercial viability, mainly because they tended to deactivate due to coke generation and sintering [28, 29]. The focus of researchers has shifted from high-performance noble metals to base metals, primarily due to their high price and scarcity. Nickel-based catalysts have shown excellent results comparable to noble metals regarding catalytic performance and cost- effectiveness [24].

Numerous approaches have been adopted to mitigate the issue of coke deposition and sintering of nickel catalysts. The approaches include [30-32],

• Changing the nature of active metal
• Introduction of promoters having essential characteristics
• Improvement of interaction between active metals and support materials
• Reducing the particle size of the catalyst
• Use of various pre-treatment and synthesis methods
• Investigation of various combinations of metals and support materials

Several research studies have demonstrated that attaining an ideal coking-resistant and sintering-free catalyst cannot be achieved by merely tuning a single parameter. Therefore, it is crucial to study the synergistic interaction of numerous parameters in designing an effective and efficient catalyst for the dry reforming of methane. In addition to adjusting various parameters, the improvement in catalyst performance has been attempted through the arrangement of different active metals, support materials, and promoters.

Catalyst Synthesis

The characteristics and performance of the catalysts are significantly affected by the synthesis method of the catalyst itself. The synthesis method mainly affects the size of the metal particles, and it also changes the metal-support interactions [33]. The components of given catalysts and the quality of materials affect the decision to adopt a particular synthesis method. Sol-gel method, co-precipitation, and impregnation are the frequently employed synthesis methods.

Incipient & wet impregnation

Both methods are commonly used for the development of catalysts. They are based on the capillary pressure developed due to the interaction of the solid and liquid phases. It also depends on the adsorption of the active species on the support material. This method offers many advantages over other methods, such as simplicity and reproducibility. Another advantage is the possibility of managing the metal distribution over the support to some extent [34]. On the other hand, this method only allows fewer weight loadings, and the interaction of the metal with support materials is weaker [35].

Co-precipitation

In co-precipitation, a precipitating agent is used to precipitate the active metals present in the solution. In an investigation, Wang et al. [36] showed that in contrast to the impregnated or sol-gel methods, interaction between the metal oxides in a co- precipitation method leads to a less active and stable Ni/MgO catalyst. This method is not typically preferred because the precipitating agents may decrease the catalysis activity.

Sol-gel method:

In these methods, the precursors are mixed in a solution to form a colloidal system with the help of hydrolysis and condensation. This method can easily manipulate catalyst parameters like surface area, particle size, and distribution of pore sizes. Moreover, it is also helpful in enhancing the thermal stability and deactivation resistance of the catalysts [37].

Comparing sol-gel methods and impregnation for synthesis of cobalt and copper- promoted Ni/Al₂O₃-ZrO₂ catalyst shows that the former method imparts better catalytic activity. The increased activity is due to the consistent particle size, better surface area, and significant scattering of active metals. The catalysts prepared by the sol-gel method show an increase in vibrations of the phenol function group on the surface compared to those prepared by the impregnation method. This increased vibration of the phenol function group enhances the basic characteristics [19].

Besides conventional catalyst preparation methods, novel methods like combustion, hydrothermal, and micro-emulsion have also been investigated. M. Awais et al. [38] developed two types of nickel-based nano-catalysts for dry reforming with the help of polyol and surfactant-assisted procedures. The catalyst prepared from the polyol-assisted method was prepared in ethylene glycol (EG) medium. On the other hand, the catalysts that were developed through the surfactant-assisted method used trimethylammonium bromide as a surfactant.

The comparison of the two types of catalysts showed that polyol catalysts demonstrated better catalytic activity and selectivity. On the contrary, the surfactant- assisted catalysts showed improved coke deposition resistance, attributed to improved basicity. Therefore, the authors argued that both catalyst synthesis methods carried great potential.

In recent years, non-thermal plasma has attracted significant attention as the latest technology. In this regard, Rahemi et al. developed numerous catalysts for dry reforming by using non-thermal plasma treatment. They developed Ni/AI₂O₃ and Ni/Al₂O₃—ZrO2 catalysts through impregnation followed by plasma treatment methods. The catalysts were labeled NA-I, NA- P, NAZ-I, and NAZ- P, respectively [39].

The NAZ-P catalyst demonstrated better activity (methane and carbon dioxide conversion around 98 %) than the NAZ-I and NA-P catalysts. This improved performance was attributed to the smaller particle size and better nickel oxide dispersion in the NAZ-P in comparison to the NAZ-I. This is mainly due to the presence of visible agglomerates in the NAZ-I (see Figure 3). However, the plasma treatment method has the downside of being an energy-intensive process and the utilization of costly apparatus [22]. These factors become a severe challenge to the commercial application of this process.

Figure 3. TEM images showing active phases in (a) NAZ-I, (b) NA-P, and (c) NAZ-P [19]

Catalyst Deactivation

The most significant milestone not yet achieved is to develop a stable catalyst that resists swift deactivation. Numerous efforts have been made to use a combination of various factors like the promoting agent, support material, and active metals to obtain a catalyst that is immune to deactivation. However, these efforts have not proven fruitful. Most commonly employed nickel -based catalysts still have the drawback of coke formation, poor heat resistance, and oxidation of active metals.

So far, the deactivation of catalysts is deemed an unavoidable outcome of dry reforming. Efforts are underway to reverse, delay or completely inhibit the catalyst deactivation. Dry reforming is a heat-intensive operation, and the catalysts quickly lose their activity by carbon deposition, increased size of active crystallites, structural damage of support materials, etc. [40].

Coke Formation

Coke formation occurs due to condensation or disintegration of hydrocarbons on the catalyst surface. Coke primarily consists of lengthy polymerized chains of hydrocarbons. The feed ratio can somewhat help anticipate the tendency of carbon formation [41]. It was shown that smaller feed ratios, in terms of hydrogen-carbon and oxygen-carbon, generally facilitate coke formation. The hydrogen-carbon ratio of 2 and oxygen-carbon ratio of 1 has the highest possibility of carbon formation in the dry reforming of methane. Three mechanisms have been proposed to explain coke deposition on the metal surface.

1. Complete inactivation of active metals by encapsulation
2. Obstruction of active metal sites by the formation of a single layer through chemisorption or by the formation of multiple layers through physical adsorption
3. Destruction of the support structure by deposition of active filamentous carbon into the porous structure till fractured

Coke is formed when carbon monoxide reacts with the active metal species and disintegrates to generate adsorbed carbon particles (i.e., C_ɑ). When these particles interact with each other, polymeric carbon filaments are produced (Cᵦ). The C_ɑ and Cᵦ phases are highly reactive at elevated temperatures, and their reaction produces graphitic carbon (C_ɣ). The loss of catalytic activity by carbon deposition is a strong function of the deactivation rate. The deactivation rate is often referred to as the rate of coke generation minus the gasification rate of the same. This can be represented by r_d = r_f – r_g, where r_d is the deactivation rate, r_f is the carbon or coke generation rate, and r_g is the gasification rate. Thus, carbon formation and deposition happen when the gasification rate exceeds the carbon formation rate [42].

Metal Sintering

In metal sintering, the active metals in a catalyst undergo sintering at high temperatures. The activity of the catalyst is lost due to reduced surface area. The reduction in surface area results from the enlargement of metal particles or breakdown of pore structure in the support. Metal sintering takes place because of the presence of moisture from the reverse water gas shift reaction. It usually takes place at elevated temperatures, commonly above 700 °C.

The growth of active metal crystallites can take place by either of the two pathways,

1. Crystallite migration in aggregated form: Here the complete crystallite is in motion across the surface of the support material by collision and accumulation
2. Atomic movement: In this pathway, the metal atoms released from the crystallite are captured by the bigger crystallites after moving across the support surface

It has been shown that both of these mechanisms are followed simultaneously during dry reforming. The process typically occurs slowly because the reaction temperature is not high enough. However, if the process begins, it is not possible to reverse the variations that have already occurred [43, 44].

Recent Trends And Outlook

The design of efficient anti-sinter and anti-coking dry reforming catalysts is expected to be the key challenge in the future. The type of bimetallic system in the reforming catalyst dictates its performance and long-term stability. There is a need to develop advanced synthesis methods that can significantly assist in the design of such catalysts. Likewise, considering the process economics, the bimetallic approaches independent of noble metals are of major interest.

In a bimetallic catalyst, the difference of surface energy among the metals may lead to an undesirable surface aggregation. Such aggregation is often dependent on the reaction conditions. The bimetallic catalysts approach offers a number of merits concerning the monometallic counterparts. The cost of conducting dry reforming at high pressures has been estimated to be less than the ambient pressure conditions. However, dry reforming at elevated pressures is under developed primarily due to the following reasons,

• Excessive formation of coke at higher pressures
• A tradeoff between the process economics and reactants conversions

The high temperature kinetically favors the dissociation of methane into lighter alkanes/alkenes. These components are mainly responsible for the deposition of excessive carbon-based components over the catalyst surface. Optimizing the reaction conditions and the physicochemical properties of bimetallic catalysts needs extensive investigation under high operating pressures. The findings might enable the application of the approach to practical reforming applications. Some of the other advancements include,

• Researchers have found that metallic oxides (like copper, iron, and nickel) show good activity for dry reforming of methane under reducing conditions.
• The application of microwaves during reaction helps overcome thermal mass transfer limitations by enhancing heat transport within the reactant mixture and improving reaction kinetics.
• Highly permeable, thin-film composite polymeric membranes exhibit excellent separation properties for CO₂/H₂ mixtures generated during reforming. Integration of these membranes within continuous fixed bed reactors enhances process efficiency due to their ability to simultaneously remove reaction products (CO₂ and H₂).
• Adding nonthermal plasmas to the reaction environment increases the partial pressure of active radicals and oxygen species. This provides additional kinetic assistance for methane activation and subsequent reactions.

Advanced techniques such as XPS, SFG, or EXAFS spectroscopy could greatly assist in handling the challenges of bimetallic and support-influenced catalysis. In this regard, several mechanistic studies have been conducted, and various reaction pathways have been suggested. These combined investigations are expected to highlight the possible reaction mechanisms and kinetics using bimetallic catalysis for dry reforming [45].

Conclusions

Over the years, the dry reforming of methane into syngas has encountered several challenges in achieving commercial-scale implementation. The primary one includes the deactivation of the catalyst through coke deposition and sintering. The most commonly used Nickel-based catalysts are susceptible to these deactivation mechanisms. This leads to a considerable reduction in the catalytic performance over time.

As a result, extensive research has been conducted to develop effective strategies to mitigate catalyst deactivation in dry reforming. One approach to overcoming catalyst deactivation is through catalyst modification by changing the catalyst compositions and adding promoters. For example, adding cerium or lanthanum as promoters to nickel catalysts has been shown to improve their resistance to coke deposition and sintering. The choice of catalyst support also plays a critical role in mitigating catalyst deactivation.

Various supports, such as alumina, silica, and zeolites, have been investigated for their impact on catalyst performance. Supports with a large surface area and strong interaction with active metal can reduce coke deposition and sintering. Despite significant progress, an optimal solution to the problem of catalyst deactivation in dry reforming has not been achieved yet. Further research is needed to fully understand the fundamental mechanisms of catalyst deactivation and develop innovative strategies to enhance catalyst stability, activity, and selectivity.

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