**4. Dry reforming of methane**

DRM approach is the most promising of all techniques, as it utilizes two greenhouse gases (CO2 and CH4) to generate industry-significant syngas while at the same moment lowering excessive greenhouse gas emissions. The DRM method is also cheaper than other techniques, as it eliminates the complicated gas separation of finished products. It generates the ratio H2:CO that can be used to synthesize oxidized chemicals and F-T synthesis long-chain hydrocarbons. DRM can also be extended to biogas (CO2, CO, and CH4) as a raw material for cleaner and ecofriendly fuels. DRM syngas is also a solar or nuclear energy storage facility [84]. Since reaction is endothermic, the process is generally carried out at temperatures between 450 and 900°C. In addition, the utilization of a catalyst is required in order to obtain acceptable CH4 conversions. The practical application of the DRM faces many significant obstacles and one of the most significant is the deactivation of the catalysts due to carbon formation during CH4 decomposition and CO2 disproportionate responses. Working at elevated temperatures and low CH4:CO2 ratios (<1) is useful from a thermodynamic point of perspective to prevent carbon formation. From an industrial point of perspective, however, work at mild temperatures and CH4 would be much more desirable: CO2 ratios close to one. Nevertheless, circumstances under which thermodynamic carbon formation is favored [85].

In this context, the DRM's attempts focus on developing a catalyst that demonstrates elevated activity and stability and low carbon formation and price at the same moment. In one of the first works related to the DRM, Fischer and Tropsch studied different metals belonging to groups 8, 9 and 10 (Ni, Co, Fe, Mo, W, Y, Cu). Among them, only Ni and Co showed a good activity (XCH4 ≈ 90%). Years later,

*Catalysts for the Simultaneous Production of Syngas and Carbon Nanofilaments… DOI: http://dx.doi.org/10.5772/intechopen.101320*

Gadalla et al. [86] tested different commercial Ni-based catalysts, obtaining CH4 conversions near 100% during 70 h of operation. Nonetheless, in order to avoid carbon deposition and catalyst deactivation they used CH4:CO2 ratios below 0.5 and temperatures above 900°C. Due to their high activity and lower carbon formation as compared to Ni, noble metals have been extensively studied as catalysts for the DRM [87]. However, their high cost and low availability make other metals more attractive from an industrial point of view. Due to their reduced cost compared to noble metals, Ni, Co and Fe were also widely researched and in the last years, bimetallic catalysts have stood out.

In order to synthesize an enhanced catalyst, these catalysts aim to potentiate the features of both metals. Ni-Co bimetallic catalysts showed a very healthy conduct among them. In any event, carbon deposition issues are even more important when using biogas. Biogas usually has higher CH4:CO2 ratios than one that ultimately leads to bigger quantities of carbon depositions that quickly deactivate the catalysts. However, distinct types of carbon are created during the decomposition of hydrocarbons and luckily not all of them are directly liable for the deactivation of catalysts. The sort and location of carbon atoms is more important than the amount generated when considering catalytic activity, according to Pinilla et al. [88]. Generally, only carbon encapsulation is directly liable for deactivation of the catalyst owing to active center coverage, while other carbon structures, such as carbon nanofilaments, can only cause operational issues when manufactured in big amounts as reactor blockage.
