https://doi.org/10.1016/j.fuel.2022.123420
“The ICCM process is a combination of several steps, including CO2 capture over the physical mixed Li, Na, K-doped MgO (Eq.5), the purge of residual CO2, and CO2 methanation accompanied with the regeneration of adsorbents (Eqs. (1) and (6)).(5)CO2+MgO↔MgCO3(6)4H2+MgCO3=MgO+CH4+2H2O
The performances of ICCM over Ru/CeO2-MgO with different CeO2 morphologies are shown in Fig. 6. Owing to the partial desorption of CO2 from the adsorbent, the following analysis focuses on the desorbed one-carbon (C1) species (CO2 and CH4) in the 2nd stage. The desorption capacity of C1 species over combined materials, representing the CO2 throughput, was evaluated using the sum of CO2 and CH4 yield. All the detected C1 species come from the adsorbed CO2 in the 1st stage of ICCM. The order of C1 species desorption capacity is Ru/particle-CeO2-MgO (0.49 mmol/gDFM) < Ru/rod-CeO2-MgO (0.60 mmol/gDFM) < Ru/cube-CeO2-MgO (1.29 mmol/gDFM), as shown in Fig. 6. Furthermore, there are significant carbonate peaks on the spent combined materials of Ru/cube-CeO2-MgO (Fig. 7) after releasing the most abundant C1 species, indicating its enhanced CO2 adsorption compared to the other two materials.”
“Fig. 6. ICCM performance over Ru/CeO2-MgO with different CeO2 morphologies. (Adsorbing in 100 ml min−1 35% CO2/N2 at 300 °C for 1 h; purging in 50 ml min−1 N2 for 10 min, reacting in 50 ml min−1 5% H2/N2 at 300 °C.)”
“Fig. 7. XRD patterns of fresh and spent Ru/CeO2-MgO catalysts.”
“On the contrary, there is no distinct carbonates peak of spent Ru/rod-CeO2-MgO and Ru/particle-CeO2-MgO, representing the sufficient regeneration of MgO after the hydrogenation step. It is speculated that the differences in CO2 adsorption over the three Ru/CeO2-MgO combined materials are related to the different amounts of oxygen vacancies on the surface of CeO2 with different morphologies. Compared to the rod and particle-CeO2, the higher oxygen vacancies on the surface of cube-CeO2 might benefit the formation of bicarbonates [53] to enhance the CO2 adsorption capacity of combined materials.
However, oxygen vacancies cannot significantly improve the catalytic methanation in ICCM. For example, higher CO2 conversion was achieved using the combined materials of Ru/rod-CeO2-MgO (55.7%) and Ru/particle-CeO2-MgO (59.8%) compared to Ru/cube-CeO2-MgO (2.7%). It is concluded that better Ru dispersion and SMI promote the catalytic performance of CO2 utilization. Furthermore, there is no CO generation in ICCM over Ru/CeO2-MgO owing to the excellent CH4 selectivity of Ru. The limited hydrogen spillover from Ru metals to supports might also contribute to high CH4 selectivity [32].
Several factors contribute to the significantly improved CO2 conversion and CH4 yield of Ru/rod-CeO2-MgO and Ru/particle-CeO2-MgO. (1) The better dispersion of Ru (indicated from TEM analysis, Fig. 3) on rod-CeO2 and particle-CeO2 can increase the reactant contact with the Ru active sites. (2) The higher BET surface area could facilitate the accessibility of active sites. (3) The abundant interaction between Ru active metals and CeO2 supports [17], [55] can act as the center of catalytic activities. (4) The abundant oxygen vacancies would lead to the enhanced CO2 absorption performance. Therefore, It is suggested that the optimal composition of the combined materials with CeO2 as support is to have abundant SMIs with a well-dispersed active metal site, while the adjustment of oxygen vacancies is more related to the CO2 capacity in ICCM.”