# Trapping and Methanation of CO\({}_{2}\) in a Domestic Microwave
## 1 Introduction  
CO\({}_{2}\) emissions represent a major fraction of greenhouse gas emissions induced by human activities and should be reduced to limit climate change [1]. CO\({}_{2}\) capture and utilization (CCU) will play a prominent role in restructuring the energy sector and the fight against global warming. CO\({}_{2}\) could be trapped within large industrial units (e.g., power plants, boilers, and cement manufacture) and reprocessed on-site or remotely. The hydrogenation of CO\({}_{2}\) into convenient fuels such as CH\({}_{4}\) through the Power-to-Gas (PG) technology is of strong interest [2]. CH\({}_{4}\) is produced through Sabatier reaction (Equation (1)), which is catalyzed by Ru- or Ni-based catalysts [3]. \[\text{CO}_{2}+4\text{ H}_{2}\rightarrow\text{CH}_{4}+2\text{ H}_{2}\text{O} \tag{1}\]  
\[\text{CO}_{2}+\text{H}_{2}\rightarrow\text{CO}+\text{H}_{2}\text{O} \tag{2}\]  
Note that the reverse water-gas shift (RWGS) reaction (Equation (2)) is often proposed as an intermediate step during methanation [4]. The H\({}_{2}\) needed for these reactions should preferably be obtained from water electrolysis using renewable or nuclear energy. CH\({}_{4}\) could be then directly injected into the already existent pipeline networks or storage infrastructures. Developing an effective system that could combine CO\({}_{2}\) capture and conversion to CH\({}_{4}\) in a single unit is of particular interest for the efficiency of PtG plants. Amine solutions are used industrially for CO\({}_{2}\) trapping [5], and a recent process was proposed that could produce CH\({}_{4}\) at 170 \({}^{\circ}\)C in a multiphasic batch reactor by mixing CO\({}_{2}\)-saturated amine solutions with a Ru-based heterogeneous catalyst [6]. Yet, processing high-pressure multiphasic batch reactors is costly and complex. Simpler isothermal sequentialprocesses solely based on solid traps and catalysts, so-called \"dual function materials\" (DFMs), have been proposed [7; 8; 9; 10]. DFMs contain a CO\({}_{2}\)-storing component, typically an alkali or Earth-alkali oxide, which exhibit a strong basic character [11; 12; 13], and a catalytic metal able to hydrogenate CO\({}_{2}\) to methane, usually Ru or Ni or alloys of those [7; 8; 9; 10]. Na-exchanged zeolites exhibit some of the highest CO\({}_{2}\) adsorption capacity under direct air capture conditions [14]. The release of the trapped CO\({}_{2}\) implies an energy cost, which can be minimized by using oxides of weaker basicity such as zirconia [15]. Energy savings can also be achieved by using microwave (MW) heating, which does not require heating up the whole reactor assembly [16]. [PERSON] et al. recently reported that the use of microwave heating for CO\({}_{2}\) desorption from zeolite 13X increased the adsorption/desorption cycling productivity and potentially reduced the energy penalty of sorbent regeneration [17]. In addition, these authors determined an apparent activation energy of the MW-assisted desorption which is significantly lower than that obtained via conventional heating. [PERSON] et al. also showed that desorption of a solid guanidine carbonate under MW heating is up to 17 times faster than conventional conductive heating at 160 \({}^{\circ}\)C, resulting in a 40% electrical energy reduction [18]. Moreover, energy savings for desorption can rise up to 94% when the sorbent is microwave-transparent (i.e., when only the adsorbed molecules are heated up) [19]. Tubes, fitting and reactors can be made of microwave-transparent materials such as polytetrafluoroethylene PTFE (=Feflon(r)) or quartz [20; 21], meaning that those also would not consume electromagnetic energy and remain at low temperatures. Such a design should allow for further energy saving and facilitate process design. We recently reported on the use of a simple domestic MW oven to carry the combined trapping and combustion of volatile organic compounds (i.e., toluene, n-decane and formaldehyde) [22].