# Trapping and Methanation of CO\({}_{2}\) in a Domestic Microwave
## 3 Results and Discussion
### CO\({}_{2}\) Trapping Followed by Desorption and Methanation under MW  
In the following pulsed experiments, CO\({}_{2}\) adsorption was carried out at room temperature using a 2% CO\({}_{2}\) + 20% O\({}_{2}\) in N\({}_{2}\) for 5 min, which was always sufficient to saturate the sorbent used. The system was 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 power was then turned on for 5 min still under 20% H\({}_{2}\) in N\({}_{2}\) with a total flow of 50 mL min\({}^{-1}\), while monitoring the gases evolved from the system. ZrO\({}_{2}\) poorly absorbed MW and did not release CO\({}_{2}\) if not mixed with SiC. SiC did not adsorb and release any CO\({}_{2}\) when used alone. In the first pulse experiment shown, the sorption bed was filled with a mixture of ZrO\({}_{2}\) + SiC, while the catalyst bed contained both a mixture of SiC + Fe\({}_{3}\)O\({}_{4}\) as MW absorbers, followed by Ru/SiO\({}_{2}\) acting as a methanation  
Figure 4: (**A**) _Operando_ DRIFTS spectra collected over the used Ru/SiO\({}_{2}\) sample under 2% CO\({}_{2}\) + 20% H\({}_{2}\) in N\({}_{2}\) at various temperatures. A KBr spectrum was used as background. (**B**) Comparison of the _operando_ DRIFTS spectra of the fresh and used Ru/SiO\({}_{2}\) samples under 2% CO\({}_{2}\) + 20% H\({}_{2}\) in N\({}_{2}\) at 180 \({}^{\circ}\)C. The spectra were normalized to silica overtones to ensure identical optical pathlengths. (**C**) Corresponding Arrhenius-like plots of the natural logarithm of the rate of methane formation against the reciprocal temperature for both the fresh and used Ru/SiO\({}_{2}\) catalysts. catalyst (Figure 5A). Figure 5B shows the compounds desorbed as a function of time during MW irradiation. Only a small concentration of CO\({}_{2}\) was released, followed by CO and then a larger peak of methane. These data are consistent with the steady flow experiment reported in Figure 3, in which CO appeared as a primary reaction product, followed by methane. The heating rate of the Ru catalyst seemed insufficient to convert the CO formed initially via RWGS (Equation (2)) over the Fe\({}_{3}\)O\({}_{4}\). Experiments were carried out to determine the amount of equivalent C (i.e., sum of CO, CO\({}_{2}\) and CH\({}_{4}\)) released by the main components of the system described in Figure 5A. As mentioned earlier, SiC did not release any carbon-containing compound (Figure 6) and the same was observed with the Ru/SiO\({}_{2}\) catalyst. Interestingly, Fe\({}_{3}\)O\({}_{4}\) led to a large amount of products, about 3-fold higher than the ZrO\({}_{2}\). It must be concluded that the Fe constituent formed some carbonate species when exposed to the CO\({}_{2}\) + O\({}_{2}\) stream. The CO\({}_{2}\) released by zirconia under our conditions was about 150 \(\upmu\)mol g\({}^{-1}\), while the use of amine-based methods can lead to reversible sorption capacity as high as 3500 \(\upmu\)mol g\({}^{-1}\)[36]. It is therefore clear that the adsorption on solids is unlikely to match that obtained in amine solutions. Some of the interests of solids lie in easier handling, lower toxicity and higher thermal stability. The gain in terms of released species obtained using a sorbent bed with ZrO\({}_{2}\) appeared thus limited. A limited interest was also observed in terms of product selectivity by comparing a series of experiment with and without the sorption bed (Figure 7). Similar high CO\({}_{2}\) conversion and selectivity to methane were obtained in both cases. Figure 5: (**A**) Configuration used: 350 mg of SiC + 32 mg of ZrO\({}_{2}\) were placed in the sorption bed and a mixture made of 354 mg of SiC + 91 mg of Fe\({}_{3}\)O\({}_{4}\) followed by 25 mg of Ru/SiO\({}_{2}\) were placed in the catalyst bed. (**B**) Products released as a function of time under MW.