# Prospects and Technical Challenges in Hydrogen Production through Dry Reforming of Methane
## 1 Introduction  
The energy transition is currently one of the major global challenges and will require the development of various new technologies allowing the economical production of low-carbon energy vectors. Hydrogen (H\({}_{2}\)) is an energy vector that can be used as a fuel or as raw material in the chemical industry and in the production of synthetic fuels (ammonia, synthetic CH\({}_{4}\), synthetic liquid fuels and methanol) [1]. Hence, due to its large use in high-emitting sectors such as transport and industry, H\({}_{2}\) (when produced in a sustainable way) can be a key enabler of the energy transition. H\({}_{2}\) is currently produced from fossil fuels, biomass or water. Currently, steam reforming of methane (SRM, Equation (1)) is the main process used for the dedicated H\({}_{2}\) production [1]. However, the production of H\({}_{2}\) from fossil resources, such as natural gas or coal, is responsible for significant CO\({}_{2}\) emissions. According to the International Energy Agency (IEA), 10 tons of CO\({}_{2}\) are generated per ton of H\({}_{2}\) when it is produced from natural gas. In order for H\({}_{2}\) to have a significant impact on the energy transition, it has to be produced in a sustainable way. Water electrolysis using renewable electricity is the main technology currently investigated since it produces high-purity H\({}_{2}\) from renewable energy and water. However, there are still some drawbacks related to high costs, technologyreadiness level, renewable electricity supply and the coupling of electrolyzers with variable renewable energy depending on the type of electrolyzer used [2]. There are already a large number of reviews available in the open literature about H\({}_{2}\) production through water electrolysis [3; 4]. Hence, this technology will not be discussed in the present paper. Another alternative technology that has been considered in the literature for H\({}_{2}\) production is dry reforming of methane (DRM, Equation (2)). This process allows the conversion of two major greenhouse gases (GHG) into a mixture of H\({}_{2}\) and carbon monoxide (CO) with a H\({}_{2}\)/CO ratio of 1. \[\mathrm{SRM:}\ \mathrm{CH_{4}+H_{2}O\rightleftharpoons 3H_{2}+CO\ \ \mathrm{\Delta H _{298K}=206\ kJ\ mol^{-1}}} \tag{1}\]  
\[\mathrm{DRM:}\ \mathrm{CH_{4}+CO_{2}\rightleftharpoons 2H_{2}+2 CO\ \ \mathrm{\Delta H _{298K}=247\ kJ\ mol^{-1}}} \tag{2}\]  
However, there are issues preventing the use of this process at larger scale, especially for H\({}_{2}\) production. DRM is a highly endothermic reaction and both CH\({}_{4}\) and CO\({}_{2}\) conversion are favored at high temperatures, especially above 600 \({}^{\circ}\)C. Figure 1 presents the equilibrium concentrations as a function of temperature for all the species when DRM is operated at atmospheric pressure [5]. Several side reactions (Equations (3)-(6)) producing solid carbon and water are also possible and they not only lead to catalyst deactivation, but they also decrease the final H\({}_{2}\)/CO of the syngas produced, which is a major issue when DRM is considered for H\({}_{2}\) production. For temperatures above 800 \({}^{\circ}\)C, reverse water gas shift (RWGS) reaction (Equation (6)) is especially favored, which reduces the H\({}_{2}\) production [6]. \[\mathrm{Methane\ cracking:}\ \mathrm{CH_{4}\rightleftharpoons C_{(s)}+2H_{2}\ \ \mathrm{\Delta H _{298}=75\ kJ/mol}} \tag{3}\]  
\[\mathrm{Boudouard\ reaction:}\ \mathrm{2 CO\equiv CO_{2}+C_{(s)}\ \ \mathrm{\Delta H _{298}=-172\ kJ/mol}} \tag{4}\]  
\[\mathrm{Steam\ gasification\ of\ carbon:}\ \mathrm{C_{(s)}+H_{2}O\rightleftharpoons CO+H_{2}\ \mathrm{\Delta H _{298}=131\ kJ/mol}} \tag{5}\]  
\[\mathrm{RWGS:}\ \ \ \mathrm{CO_{2}+H_{2}\rightleftharpoons H_{2}O+CO\ \mathrm{\Delta H _{298}=41\ kJ/mol}} \tag{6}\]  
Figure 1: Thermodynamic equilibrium plot for DRM as a function of temperature (T = 0–1000 \({}^{\circ}\)C) at 1 bar and CH\({}_{4}\)/CO\({}_{2}\) = 1. Reprinted from [5] in the terms of CC-BY license. Copyright 2022 [5].