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Title: Supported layered double hydroxides as CO2 adsorbents for sorption-enhanced H2 production
Author: Iruretagoyena Ferrer, Diana
ISNI:       0000 0004 5918 3956
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2014
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Sorption enhanced hydrogen production is considered an attractive technology to improve the efficiency of the water gas shift reaction (WGS). It requires the development of selective catalysts and CO2 adsorbents that are sufficiently stable to tolerate cyclic regeneration. The present work focuses on the assessment of the adsorption performance of novel layered double hydroxides (LDHs) supported on multi-walled carbon nanotubes (MWCNTs) and graphene oxide (GO) for application in sorption-enhanced processes. Emphasis is placed on the stability and capacity of hybrids prepared with relatively low carbon support loadings as the CO2 uptake per total volume of composite dictates the size of the industrial units. The coprecipitation of Mg(NO3)2 and Al(NO3)3 onto well-dispersed MWCNTs or GO is shown to be an effective preparation method that ensures an adequate interaction between the LDH and the support, leading to an increase in the CO2 uptake per mass of LDH and improving significantly the stability of the pure LDH. Compared to other supports such as alumina or carbon nanofibers, LDH platelets can be stabilised with lower loadings of nanotubes or graphene oxide. The mass efficiency of the stabiliser is found to be particularly high in the case of GO, possibly due to a better charge and geometric compatibility. Graphene oxide does not to modify significantly the number or chemistry of the CO2 adsorption sites of hydrotalcites but helps to maintain the surface heterogeneity which is otherwise lost during temperature-activated regeneration. The changes in heterogeneity and capacity can be described by the three-parameter Toth isotherm. Alkali ions are shown to increase the capacity of unsupported and supported hydrotalcites without influencing the multicycle stability. Co-adsorption of water enhances the stability and increases the uptake of CO2 without any evidence of carbon gasification in the case of hybrids. Under dry and wet conditions, all materials show fast adsorption kinetics which can be approximated by the linear driving force model. A thorough thermodynamic analysis demonstrates that besides sorption enhanced water gas shift, the in situ removal of CO2 when water gas shift and methanol decomposition (methanol to shift) are conducted simultaneously is an attractive alternative to lift the H2 yield while autothermal conditions are procured. The temperature range of both enhanced processes coincides with the operation window of the adsorbents developed in this research (573-773 K). A preliminary screening shows that Cu/ZnO/Al2O3 and Pt/CeO2 are promising candidates to catalyse water gas shift and methanol decomposition.
Supervisor: Chadwick, David ; Hellgardt, Klaus Sponsor: Consejo Nacional de Ciencia y Tecnología ; SEP ; Imperial College London
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral