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.