| Reducing the emission of CO2 has become a worldwide concerned problem. However, CO2 capture is the essential prerequisite for CO2 storage or utilization. Layered double hydroxides (LDHs) derived mixed metal oxides (LDO) are considered to be one of suitable CO2 adsorbents for sorption enhanced water gas shift (SEWGS) technology with high surface area and abundant surface basicity, which has been a hot topic. Yet LDO still has many problem as CO2 adsorbents due to its low capture capacity, low mechanism property etc. Based on this, this paper extends the method to synthesize the LDH and carbon materials (graphene oxide/carbonnanotube) composites as CO2 sorbents and perform a comprehensive and systematic study on the influence of synthesis condition, CO2 capture capacity, synthesis method, thermal stability, continuous adsorption/desorption cycling stabilities.To obtain the LDH composites, we first systematically studied the synthetic method of LDH single nanosheets, and synthesized four kinds of LDH single nanosheets. During the characterization, we explored a simple and reliable method for determining the delamination degree of LDHs in formamide by analyzing the formed LDH dispersion gels using XRD. In addition, we also demonstrated that the solution with turbidity does not mean it has not been completely exfoliated.The LDH and graphene oxide (GO) composites were synthesized based on the "electrostatic self-assembly" between the delaminated positively charged Mg3Al-NO3 LDH single sheets and the negatively charged GO monolayer. By rigorous measurements of CO2 absorption performance of the four composites, tests of applying the synthetics reaction conditions are determined. In particular, Mg3-Al-NO3 LDH-NS/GO composite with 6.54 wt% GO showed the maximum adsorption capacity (0.47 mmol g-1), which is more than twice larger than the pure Mg3Al-NO3 LDH. The characteristics of the novel adsorbents were investigated using XRD, FE-SEM, HR-TEM, FT-IR, BET, and TGA to confirm the structure, morphology, composites forms etc. It also provided the explanation of the optimization of CO2 capture capacity.The influence of adsorption conditions was also investigated. The results showed when the calcination temperature was 400℃, the composite got the maximum CO2 capture capacity. It is also proved that LDH/GO composite shows good CO2 capture capacity in a wide temperature range. The maximum CO2 capture takes place at 60℃ (1.0 mmol g-1). At the intermediate temperature range, the maximum CO2 capture took place at 200℃. The Mg3Al-NO3 LDH-NS/GO (VR=1:0.7) composite also showed good CO2 adsorption/desorption cycling tests during 22 cycles. The amount of adsorption was 0.35 mmol g-1 much higher than the pure Mg3Al-NO3 LDH which indicated that the compounds shows an excellent recycling performance. The adsorption capacity of CO2 was significantly enhanced by loading with K2CO3, and therefore, the best load volume was determined. Moreover, the CO2 capture capacity of the adsorbent could be further greatly increased to 0.6 mmol g-1 by doping with 15 wt% K2CO3 which was 2.7 times of pure Mg3Al-NO3 LDH.A comprehensive and comparative study on the influence of synthesis method of LDH/oxidized carbon nanotube (LDH/OCNT) composites on their CO2 capture performance was conducted. Three types of LDH/OCNT composites including Mg3Al-NO3 LDH-NS/OCNT, Mg3Al-NO3 LDH/OCNT, and Mg3Al-CO3 LDH/OCNT were prepared using "electrostatic self-assembly" and "direct co- precipitation" methods. For Mg3Al-NO3 LDH-NS/OCNT, the optimal OCNT weight loading was also 9.1 wt% and the CO2 capture capacities were significantly improved to 0.43 mmol g-1, which is nearly twice larger than the pure Mg3Al-NO3 LDH. It suggested that the synthetic method of the composites had little effect on the CO2 capture capacity.The influence of adsorption conditions was also investigated. And the Mg3Al-NO3 LDH-NS/OCNT composite also showed good CO2 adsorption/desorption cycling stability. Moreover, the CO2 capture capacity of the Mg3Al-NO3 LDH-NS/OCNT (VR=1:1) composite could be further greatly increased to 0.69 mmolg-1 by doping with 15 wt% K2CO3 which was three times of pure Mg-Al-NO3 LDH which indicated that the LDH/OCNT composites had the potential for practical applications.Finally, the CO2 adsorption sites and mechanisms of active site formation in LDH-derived mixed oxides were further studied. It was proposed that the active Mg-0 species might be induced either by the substitution of Mg2+ by Al3+ in periclase, the MgO lattice, or by the diffusion of Al3+ out of the octahedral brucite layers by carefully examining the structural changes during thermal treatment using XRD and solid state NMR. |
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