# Trapping and Methanation of CO\({}_{2}\) in a Domestic Microwave
## 3 Results and Discussion
### CO\({}_{2}\) Trapping Followed by Desorption and Methanation under MW
The sample was first exposed to 2% CO\({}_{2}\) + 20% O\({}_{2}\) in N\({}_{2}\) for 5 min, then purged under N\({}_{2}\) for 5 min, then left for 5 min under 20% H\({}_{2}\) in N\({}_{2}\) with a total flow of 50 mL min\({}^{-1}\). The MW were then turned on for 5 min still under 20% H\({}_{2}\) in N\({}_{2}\) with a total flow of 50 mL min\({}^{-1}\). The significant proportion of CO (ca. 20%) in the reaction products is not desirable, and a different catalyst bed configuration was attempted by mixing directly the Ru catalyst with the iron oxide (which can heat up under MW in the absence of SiC) and zirconia to favor fast reduction and higher activity of Ru (Figure 8A). SiC was left out to limit excess temperature, and the mixture thus used became hot under MW irradiation but never glowed red. Figure 6: Amount of carbon released (i.e., sum of CO, CO\({}_{2}\) and CH\({}_{4}\)) by the various elements of the sorbent-catalyst system described in Figure 5A. Figure 7: CO\({}_{2}\) conversion and selectivity to CO and methane obtained over a configuration (**Left**) including and (**Right**) lacking a sorption bed. The reaction sequence and catalyst bed are the same as those detailed in Figure 5.  
The desorption of compounds was fast and essentially completed in 100 s (Figure 8A), but unreacted CO2 was the main species released. Nonetheless, no CO was observed over five consecutive runs, CH4 being the only product (Figure 8B). The quantity of methane produced per total bed mass somewhat declined before stabilizing to about 35 umol g-1 over the five runs. This value is about 10-fold lower than those reported by Farrauto over some of the best dual function materials (DFM) operated in conventional ovens [8]. Yet, we believe that this is promising for a system that has not yet been optimized and offers a lot of parameters to adjust (e.g., sorbent, MW-absorber, catalyst, bed configuration, and MW power). The nature of the iron phases present after use were determined with XRD analyses (Figure 9). The used Fe-based material, collected after the experiment reported in Figure 7, contained metallic iron [37; 38] and FeO [39] as the main phases, with only traces of Fe3O4 [40] left. These two phases (metallic iron and FeO) are known to absorb better MW than FeIII compounds, as discussed elsewhere [41; 42; 43; 44]. It is thus important to avoid full oxidation of the iron phases. The abilities of the FeO phase to store CO2 as a carbonate and to absorb MW are much greater than that of metallic iron, thus stabilizing FeO appears important for future investigations. Figure 8: Experiment using a catalyst bed made of a mixture of 111 mg of Fe3O4 + 146 mg of ZrO2 and 28 mg of Ru/SiO2. The sorbent bed was empty. (**A**) Products released as a function of time under MW. (**B**) CO2 conversion and selectivity to CO and methane over five consecutive runs. (**C**) Corresponding production of methane per total mass of the bed. The reaction sequence is the same as that detailed in Figure 5.  
Figure 9: XRD pattern of the (**A**) fresh and (**B**) used Fe3O4 material. The present study reports for the first time, to the best of our knowledge, that CO2 desorption and methanation can be readily achieved in a domestic microwave oven. The economics of the process would need to be estimated in detail, but probably once the sorbent, MW absorber, methanation catalyst and the process conditions would have been optimized further from the present proof of concept. Since an earlier study reported up to 94% energy savings for an MW-based desorption processes [19], the present method can be regarded as highly promising that could lead to an inexpensive trapping-methanation technology.