Investigating CO2 Capture in Ionic Liquids Using in Situ Synchrotron Techniques

Abstract

This thesis details three in situ synchrotron studies probing the interactions of ionic liquid (IL) thin films with CO2. ILs are liquid salts with bulky ions showing a wide range of interesting properties. An emerging class called superbasic ILs (SBILs) show great promise as alternatives to current carbon capture solvents. However, more fundamental studies delving into the molecular interactions of SBILs with CO2 are required before they see widescale industrial use. In this work, interfacial interactions were probed using near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) and near-edge X-ray absorption fine structure (NEXAFS) at synchrotron facilities. Experimental results were complemented by computational simulations carried out using density functional theory (DFT). The studies presented here aim to explore chemical and physical interactions at the IL/gas interface, examine the reordering of ions upon exposure to gas, and discover how CO2 competes with other gases at different depths into IL thin films.
Thin films of the SBIL [P66614][benzim] were prepared on a rutile TiO2 (110) surface and exposed to CO2 at near-ambient pressures. SBILs show highly structured behaviour when deposited as thin films, and results indicate that reaction with CO2 causes a reordering of IL molecules. [benzim]– anions orient 27° ± 5° from the surface normal before exposure to CO2 and approximately 54° ± 4° from the surface normal when they react with CO2. NAP-XPS shows evidence of irreversible CO2 absorption and a greater concentration of CO2-reacted anions in the deeper layers.
Thin films of the SBIL [P66614][124Triz] were deposited onto rutile TiO2 (110) using in situ electrospray deposition (ESD). To the best of our knowledge, this is the largest IL to be successfully deposited via ESD. Depth profiling NAP-XPS reveals that competitive absorption between CO2 and H2O in [P66614][124Triz] varies with depth into the thin film. A greater concentration of CO2 absorbs in the bulk layers, and does so reversibly, while more H2O ad/absorbs at the surface reversibly. CO2 that is ad/absorbed at the surface does so irreversibly.
Depth profiling NAP-XPS and NEXAFS measurements were carried out on an electrosprayed thin film of the SBIL [P66614][Tetz] upon exposure to CO2, NO, and a CO2 + NO mixture. The plane of the ring of the [Tetz]– anion orients 10° ± 3° from the surface normal prior to exposure to gas, before reorienting to 37° ± 2° and 64° ± 2° from the surface normal upon exposure to NO and CO2, respectively. This corroborates with the orientational behaviour of [P66614][benzim] upon exposure to CO2. Irreversibly absorbed NO in the bulk layers of the [P66614][Tetz] thin film inhibits CO2 absorption in these deeper layers. CO2 was therefore only found to ad/absorb at the surface layers, but this reaction is reversible.
These results provide new insights into CO2 absorption in SBILs by studying fundamental interactions at the liquid/gas interface, the reversibility of chemical reactions, and the reordering of ions. Understanding interfacial behaviour is an important step towards implementing ILs into gas capture applications. Results may also inform designs of IL-based technologies such as SCILL/SILP catalysts.