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.

Catalytic Dehydrogenation of Methane to Hydrogen and Carbon Nanostructures with Fe–Mo Bimetallic Catalysts

“…Fe, on the other hand, is quite stable at higher temperatures and reportedly has the highest rate of carbon diffusion, making it a suitable candidate for the CDM reaction, as the reaction needs to be conducted at high temperatures due to thermodynamic limitations. 24 Additionally, Fe-based catalysts also facilitate easy separation of the metal catalyst from the product carbon through exposure to a magnetic field, as the economics of the CDM process is governed by the stability and regeneration/recycling of the catalyst. In the literature, methane conversion for Fe-based catalysts has been reported to improve with promotional metals, such as Cu, Mo, Mn, W, and Pd.…”

“…However, Ni is unstable at temperatures higher than 600 °C, with a rapid decline in activity at higher temperatures due to the sintering effect. Fe, on the other hand, is quite stable at higher temperatures and reportedly has the highest rate of carbon diffusion, making it a suitable candidate for the CDM reaction, as the reaction needs to be conducted at high temperatures due to thermodynamic limitations . Additionally, Fe-based catalysts also facilitate easy separation of the metal catalyst from the product carbon through exposure to a magnetic field, as the economics of the CDM process is governed by the stability and regeneration/recycling of the catalyst.…”

Thermo-Catalytic Decomposition Comparisons: Carbon Catalyst Structure, Hydrocarbon Feed and Regeneration

“…Yet carbon catalysts are more stable at high temperatures, i.e., offering a longer lifetime with feed flexibility while being relatively immune to poisoning [13,18]. Moreover, facilitated by material similarity and low cost, they can become part of the TCD carbon, avoiding any subsequent recovery or removal processes [13,19]. In carbon materials, there is no singular definition of active sites; they are often described as including lattice dislocations, low-coordination sites, vacancies, discontinuities, edges, defects, and other energetic abnormalities [20,21].…”

“…Literally dozens of carbons have been tested, including graphite, diamond powder, carbon nanotubes, glassy carbon, fullerene soot, fullerenes C60/70, acetylene black, coal char, and ordered mesoporous carbons (CMK materials) [13,19,24]. A less ordered structure is generally recognized as advantageous because it hosts more high-energy sites (i.e., active sites).…”

“…Long-term, carbon materials deactivate, though most tend to settle to some steady state, albeit with low activity-carbon blacks in particular [13,19,25,26]. Oppositely porous carbons exhibit a greater level of deactivation, reflecting their pore-based capacity for carbon deposition [13,19]. As a means of retaining activity, various hydrocarbons have been co-fed with methane to retain activity [13,19,27].…”