Abstract
Graphene oxide (GO) and multi-walled carbon nanotubes (MWCNT) have been previously used independently as active supports for layered double hydroxides (LDH), and found to enhance the intrinsic CO2 sorption capacity. However, the long-term stability of the materials subjected to temperature-swing adsorption (TSA) cycles still requires improvement. In this contribution, GO and MWCNT are hybridized to produce mixed substrates with improved surface area, and compatibility for the subsequent deposition of LDH platelets, compared to either phase alone. The incorporation of a robust and thoroughly hybridized carbon network considerably enhances the thermal stability of activated, promoted LDH over twenty cycles of gas adsorption-desorption (96% of retention of the initial sorption capacity at the 20th cycle), dramatically reducing the sintering previously observed when either GO or MWCNT were added separately. Detailed characterization of the morphology of the supported LDH, at several stages of the multicycle adsorption process, shows that the initial morphology of the adsorbents is more strongly retained when supported on the robust hybrid GO/MWCNT network; the CO2 adsorption performance correlates closely with the specific surface area of the adsorbents, with both maximized at small loadings of a 1:1 ratio GO:MWCNT substrate.
Highlights
Layered double hydroxides (LDH) are lamellar hydroxides of the family of hydrotalcites (HT), with general formula (M2+1-x M3+x (OH)2)x+(Am-x/m nH2O)x, where M2+, M3+ and Am- most commonly represent Mg2+, Al3+ and CO32- respectively (Brucite), many other compositions occur.[1]
Amongst the materials tested as supports in these applications, carbon nanostructures (CN), including carbon nanofibers (CNF),[12] multi walled-carbon nanotubes (MWCNT)[13, 14] and graphene oxide (GO),[15, 16] have shown the most promise, compared to more conventional systems based on alumina or zeolites.[17, 18]
Initially, the hybrid substrates were analyzed without depositing Layered Double Hydroxides (LDH) platelets; mixed substrates were prepared at different Graphene oxide (GO)/multi-walled carbon nanotubes (MWCNT) ratios of 10:1, 3:1, 1:1, 1:3, 1:5, and compared to the pure GO (1:0) and MWCNT (0:1) forms
Summary
Layered double hydroxides (LDH) are lamellar hydroxides of the family of hydrotalcites (HT), with general formula (M2+1-x M3+x (OH)2)x+(Am-x/m nH2O)x-, where M2+, M3+ and Am- most commonly represent Mg2+, Al3+ and CO32- respectively (Brucite), many other compositions occur.[1]. The multicycle stability of CN-LDH still has considerable scope for improvement; for instance, for the best materials reported so far (GO-LDH), the loss of CO2 adsorption performance over 20 cycles 78 of gas adsorption-desorption under dry conditions remained a significant 15 - 40% (though improved 79 compared to the 50 - 60% loss for pure LDH).[16, 31] Despite their promise, neither MWCNT nor GO are ideal supports: MWCNT, though offering a mechanically strong network for LDH deposition, require higher loadings to match the performance improvement provided by GO, as the surface area is not as intrinsically high, and the geometry less well matched. The pore-size distribution of the samples was calculated from desorption branch using the Barrett−Joyner−Halenda (BJH) method
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