Influence of Alkali Metals on Layered Double Hydroxides Supported on Graphene Oxide for CO2 Adsorption
The effect of alkali metals on the CO2 adsorption performance of unsupported and GO supported layered double hydroxides is investigated. A comparative study of promoting the effect of sodium, potassium and caesium on the adsorbents is presented. In additi
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Influence of Alkali Metals on Layered Double Hydroxides Supported on Graphene Oxide for CO2 Adsorption
6.1
Introduction
The use of supporting materials for layered double oxides (LDOs) has been demonstrated to be an effective approach to improve their stability in temperature-swing CO2 adsorption cycles. Simultaneously, the intrinsic adsorption capacity of LDOs has been observed to increase although this requires high support contents that lead to very diluted hybrids with low CO2 capacities per volume of adsorbent [1–3]. Graphene oxide appears as an outstanding support for LDOs since very modest amounts need to be added to observe a marked improvement in their multicycle stability (Chap. 5). Due to this high mass efficiency, the density of the adsorbents is not significantly reduced and adequate volumetric capacities are obtained. However, the capacities of LDO-GO hybrids are still relatively low and therefore it is necessary to explore strategies to increase the amount of CO2 adsorbed without compromising the enhancement in the stability gained by the use of GO. The presence of alkali ions on metal oxide surfaces is known to increase their basicity and therefore many researchers have observed a promoting effect of sodium, potassium and caesium on the CO2 capacity of LDOs [4–7]. Bulky monovalent cations are weaker Lewis acids than divalent (Mg2+) and trivalent (Al3+) cations and this leads to the higher basicity of alkali-doped hydrotalcites. Some studies suggest that the interaction between the alkali salts and the aluminium oxide centres in the LDO plays a key role in the formation of strong basic sites which are more active for CO2 adsorption [5]. Alternatively, the gain in capacity observed has been attributed to a higher concentration of surface defects created by the presence of the univalent alkali ions [7]. In the literature, most reports on alkali promotion of LDOs have focused on the assessment of unsupported materials [5, 6, 8, 9]. Recently Meis et al. [7] studied the effect that Na and K (either residues left in the washing step or deliberately impregnated) have on the adsorption capacity of LDOs supported on carbon © Springer International Publishing Switzerland 2016 D. Iruretagoyena Ferrer, Supported Layered Double Hydroxides as CO2 Adsorbents for Sorption-enhanced H2 Production, Springer Theses, DOI 10.1007/978-3-319-41276-4_6
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nanofibers. A significant increase in the CO2 uptake was observed in all cases although the capacities were slightly higher for the samples with alkali residues. The alkali metals seemed to have no effect on the stability of the hybrids. However, the authors only reported 10 adsorption-desorption cycles and the adsorption step was carried out in the presence of water which is known to slow down the rate of deactivation of LDOs [8]. Furthermore, very large carbon nanofibers contents (90 wt%) were used in the study and therefore it is not justifiable to extrapolate this observation to lower support loadings wh
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