In situ XPS of competitive CO2/H2O absorption in an ionic liquid

Abstract

Superbasic ionic liquids (SBILs) are being investigated as potential carbon dioxide (CO₂) gas capture agents, however, the presence of H₂O in the flue stream can inhibit the uptake of CO₂. In this study a thin film of the SBIL trihexyltetradecylphosphonium 1,2,4-triazolide ([P₆₆₆₁₄][124Triz]) was deposited onto rutile TiO₂ (110) using in situ electrospray deposition and studied upon exposure to CO₂ and H₂O using in situ near-ambient pressure x-ray photoelectron spectroscopy (NAP-XPS). The molar uptake ratio of gas in the electrosprayed SBIL (n_gas:n_IL) was calculated to be 0.3:1 for CO₂, 0.7:1 for H₂O, and 0.9:1 for a CO₂/H₂O mixture. NAP-XPS taken at two different depths reveals that the competitive absorption of CO₂ and H₂O in [P₆₆₆₁₄][124Triz] varies with sampling depth. A greater concentration of CO₂ absorbs in the bulk layers, while more H₂O adsorbs/absorbs at the surface. The presence of H₂O in the gas mixture does not inhibit the absorption of CO₂. Measurements taken during exposure and after the removal of gas indicate that CO₂ absorbed in the bulk does so reversibly, whilst CO₂ adsorbed/absorbed at the surface does so irreversibly. This is contrary to the fully reversible CO₂ reaction shown for bulk ionic liquids (ILs) in literature and suggests that irreversible absorption of CO₂ in our highly-structured thin films is largely attributed to reactions at the surface. This has potential implications on IL gas capture and thin film IL catalysis applications.