The distance between catalytic sites and sorbent is important for DFMs performance. The two functional sites need sufficient distance to work properly without negative effects on each other. However, a close distance between catalytic and adsorption sites could also be beneficial for synergetic reactions. This is also affected by the ICCU system (e.g. product, reduction agent, temperature).
The following sentences are copied from this reference (https://doi.org/10.1016/j.ccst.2022.100052) regarding the influence of methods for combining catalytic and adsorption sites.
“The synthesis approach and application method of the Ru-based DFMs also play a critical role in their performance in the ICCC-Met process. For example, when impregnating the 10.6%Ru over the 10%CaO/γ-Al2O3, it showed a higher CH4 yield than the DFM impregnating 10%CaO over 10%Ru/γ-Al2O3. This was caused by the loss of Ru sites after doping with CaO, and/or the increased dispersion of Ru on the CaO/γ-Al2O3 surface (Duyar et al., 2015). When physically mixing the 10%CaO/γ-Al2O3 and 10%Ru/γ-Al2O3 on the other hand, the materials showed inferior performance to the sample prepared with the impregnation method, indicating the proximity of CaO and Ru should benefit the methanation process (Duyar et al., 2015). In another study, physically mixed DFM of 17%NaNO3/MgO and Ru/Al2O3 at a mass ratio of 2 (sample labeled as MP) was compared with the configuration of putting them in two separate beds (sample labeled as 2B) at the same mass ratio by Park et al. (Park et al., 2021) (figure results shown below). The MP configuration showed much lower CO2 uptake capacity (0.4 vs. 2.62 mol/kg), as well as methanation capacity (0.38 vs. 1.51 mol/kg) than 2B regardless of Ru loading at 300°C. It was hypothesized that in the tightly bound MP sample, molten NaNO3 may transfer to the Al2O3 support causing the loss of promotion functionality of the NaNO3 in CO2 sorbent (Park et al., 2021). The catalytic performance of an intimate mixture of Ru-BaO/Al2O3 with that of the mechanical mixed BaO/Al2O3+Ru/Al2O3 was compared by Porta et al. (Porta et al., 2021b). The mechanical mixture showed a much lower CH4 yield (0.097 mol/kg) than the intimate mixture (153 mol/kg) at a similar Ru and Ba loading amount, even though the CO2 sorption capacities were comparable. This indicated that the synergistic effect of Ru and Ba likely facilitated the fast spillover of CO2 to the Ru sites for methanation. For the intimate mixture, a small amount of CO was detected with CH4 selectivity higher than 98%, while the CH4 selectivity was 100% for the mechanical mixture.”
ICCU-methanation performance using two types of DFMs configurations (2B- two separate beds; MP- mixed-then-pelletized)