# Hydrogen Production in Methane Decomposition Reactor Using Solar Thermal Energy
## 1 Introduction  
The increasing reality of global warming effects owing to the burning of fossil fuels has prompted the need to invest in renewable energy sources in order to reduce greenhouse gas emissions. Of all the renewable energy sources available, hydrogen is gaining global attention as the preferred eco-friendly energy source due to its pollution-free combustion process. In Korea, for example, steps are being taken to ensure that the country becomes the leading global hydrogen economy by 2050. As a result, policies have been initiated, and studies are being conducted to guide the establishment of an industrial ecosystem based on hydrogen vehicles and fuel cells, and the creation of a hydrogen production and supply system without carbon emissions. Until recently, hydrogen was produced via steam methane reforming (SMR), as in Equation (1) [1]. However, dry reforming of methane (DRM), as shown in Equation (2), and partial oxidation of methane (POM) method, as shown in Equation (3), are currently being developed. The above methods generate CO\({}_{\text{x}}\) as a by-product, which requires a separate costly process for removal. Therefore, catalytic methane decomposition (CMD), as shown in Equation (4), is a preferred alternative. To decompose methane through direct pyrolysis, a temperature of 1200 \({}^{\circ}\)C or higher is required; however, if the CMD method is used, a reasonable yield can be obtained while reducing the temperature considerably. In addition, carbon, a by-product generated with hydrogen, is cheaper than CO\({}_{\text{x}}\), and the separated carbon can be used by tire and paint manufacturing industries [2,3]. High temperature heat is required for the preceding four processes, and when fossil fuels are used, air pollutants, including carbon, are released. However, hydrogen can be produced without carbon emission, by supplying heat through concentrated solar energy and using the CMD process [4; 5]. \[CH_{4}+H_{2}O\to CO+3H_{2}\quad\Delta H_{298}^{o}=206\;\mathrm{kJ/mol} \tag{1}\]  
\[CH_{4}+CO_{2}\to 2 CO+2H_{2}\quad\Delta H_{298}^{o}=247\;\mathrm{kJ/mol} \tag{2}\]  
\[CH_{4}+0.5O_{2}\to CO+2H_{2}\quad\Delta H_{298}^{o}=-23\;\mathrm{kJ/mol} \tag{3}\]  
\[CH_{4}\to C+2H_{2}\quad\Delta H_{298}^{o}=75\;\mathrm{kJ/mol} \tag{4}\]  
The catalysts used in the CMD process are largely divided into metal and carbon catalysts. Metal catalysts mainly consist of nickel, iron, and cobalt. Noble metals, owing to their excellent catalytic properties can also be used; however, this is impractical due to the high cost. Compared to the carbon catalyst, the activation temperature of the metal catalyst was lower (450-750 \({}^{\circ}\)C). The initial activity was high, but the stability at high temperatures was low. Although the risk of damage is lower than that of the carbon catalyst during the regeneration of the catalyst, both catalysts require separate heat supply in the process of removing deposited carbon; this process is costly and harmful to the environment, since CO\({}_{\mathrm{x}}\) is generated. Carbon catalysts have been used in CMD owing to their availability, durability, low cost, abundant porosity, and molecular activation. Due to its good stability at high temperatures, it is suitable for use in an environment where high temperature solar heat is concentrated during the day. In addition, there are several uses of carbon commercially [6; 7; 8]. Studies have shown that carbon black (CB) or activated carbon (AC) catalysts are mainly used as carbon catalyst materials. The initial reaction rate of the AC catalyst was high, and the amount of carbon generated before deactivation was relatively small, whereas for the CB catalyst, the initial reaction rate was low, and the amount of carbon generated before deactivation was relatively large [8; 9; 10; 11; 12; 13]. The need for an efficient reactor to supply heat required for the CMD process through solar heat has encouraged several studies to be conducted on the subject matter. [2; 4; 7; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24]. Abanades et al.