Thermocatalytic Decomposition of Methane: A Review on Carbon-Based Catalysts

Abstract

The global initiatives on sustainable and green energy resources as well as large methane reserves have encouraged more research to convert methane to hydrogen. Catalytic decomposition of methane (CDM) is one optimistic route to generate clean hydrogen and value-added carbon without the emission of harmful greenhouse gases, typically known as blue hydrogen. This Review begins with an attempt to understand fundamentals of a CDM process in terms of thermodynamics and the prerequisite characteristics of the catalyst materials. In-depth understanding of rate-determining steps of the heterogeneous catalytic reaction taking place over the catalyst surfaces is crucial for the development of novel catalysts and process conditions for a successful CDM process. The design of state-of-the-art catalysts through both computational and experimental optimizations is the need of hour, as it largely governs the economy of the process. Recent mono- and bimetallic supported and unsupported materials used in CDM process have been highlighted and classified based on their performances under specific reaction conditions, with an understanding of their advantages and limitations. Metal oxides and zeolites have shown interesting performance as support materials for Fe- and Ni-based catalysts, especially in the presence of promoters, by developing strong metal-support interactions or by enhancing the carbon diffusion rates. Carbonaceous catalysts exhibit lower conversions without metal active species and largely result in the formation of amorphous carbon. However, the stability of carbon catalysts is better than that of metal oxides at higher temperatures, and the overall performance depends on the operating conditions, catalyst properties, and reactor configurations. Although efforts to summarize the state-of-art have been reported in literature, they lack systematic analysis on the development of stable and commercially appealing CDM technology. In this work, carbon catalysts are seen as promising futuristic pathways for sustained H₂ production and high yields of value-added carbon nanomaterials. The influence of the carbon source, particle size, surface area, and active sites on the activity of carbon materials as catalysts and support templates has been demonstrated. Additionally, the catalyst deactivation process has been discussed, and different regeneration techniques have been evaluated. Recent studies on theoretical models towards better performance have been summarized, and future prospects for novel CDM catalyst development have been recommended.

Figure 1

(a) Influence of the reaction parameters on the equilibrium conversion of methane (temperature and pressure). Reprinted with copyright permission from ref (171). Copyright Elsevier 2021. (b) Effect of using a catalyst in the CDM reaction. Reprinted with copyright permission from ref (172). Copyright Elsevier 2017. (c) Illustration of a gas–solid heterogeneous reaction mechanism for the formation of carbon nanomaterials (CNMs). Reprinted with copyright permission from ref (30). Copyright Elsevier 2021. Copyright 2020 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences.

Figure 2

(a) Schematic of the CDM reaction via dissociative methane adsorption. (b) Relative energies of CH₄ decomposition on the Ni–Al₂O₃ (001) catalyst surface. Reprinted with copyright permission from ref (31). Copyright Elsevier 2017.

Figure 3

Schematic diagram of a nondissociative model of methane adsorption. Reprinted with copyright permission from ref (31). Copyright Elsevier 2017.

Figure 4

Methane conversion over (a) 30% Ni on various zeolites at 550 °C and (b) 20% Co on various supports at 500 °C. Panel a was reprinted with copyright permissions from ref (45). Copyright Elsevier 2007. Panel b was reprinted with copyright permissions from (46). Copyright American Chemical Society 2004.

Figure 5

Classification of carbon-based catalysts based on degrees of order.

Figure 6

(a and b) Methane decomposition over AC from two sources: ACPS:AC from palm shells and NORIT:commercial AC from hardwood at different temperatures. Reprinted with copyright permission from ref (116). Copyright Elsevier 2010. (c) Methane decomposition over activated carbon from olive stones and (d) N₂ sorption isotherms at −196 °C. Reprinted with copyright permission from ref (117). Copyright Elsevier 2017.

Figure 7

Initial CDM rates of conversion on ACs (a) with respect to the carbon surface area and (b) versus the BET surface area of fresh ACs. Figures have been reprinted with copyright permissions from refs (32) and (119). Copyright Elsevier 2005 and 2004.

Figure 8

Dependence of the CH₄ conversion of ACs on (a) particle size and (b) different temperatures. (c) Influence of catalyst particle size on turnover costs. Figures have been reprinted with the copyright permissions from refs (119) and (120). Copyright Elsevier 2004 and 2022.

Figure 9

Illustration of functionalized active centers on the surface of a carbon catalyst due to the introduction of acidic groups. Figure is reprinted with copyright permission from ref (30). Copyright Elsevier 2021.

Figure 10

Schematic of filamentous carbon formation during a CDM reaction over a Ni catalyst: (a) tip growth and (b) base growth. The figure has been reprinted with copyright permission from ref (145). Copyright Elsevier 1997.

Figure 11

A schematic of the CDM flow diagram for developing models for the CDM reactions.