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

Abstract

Superbasic ionic liquids (SBILs) are being investigated as potential carbon dioxide (CO _2 ) gas capture agents, however, the presence of H _2 O in the flue stream can inhibit the uptake of CO _2 . In this study a thin film of the SBIL trihexyltetradecylphosphonium 1,2,4-triazolide ([P _66614 ][124Triz]) was deposited onto rutile TiO _2 (110) using in situ electrospray deposition and studied upon exposure to CO _2 and H _2 O 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 CO _2 , 0.7:1 for H _2 O, and 0.9:1 for a CO _2 /H _2 O mixture. NAP-XPS taken at two different depths reveals that the competitive absorption of CO _2 and H _2 O in [P _66614 ][124Triz] varies with sampling depth. A greater concentration of CO _2 absorbs in the bulk layers, while more H _2 O adsorbs/absorbs at the surface. The presence of H _2 O in the gas mixture does not inhibit the absorption of CO _2 . Measurements taken during exposure and after the removal of gas indicate that CO _2 absorbed in the bulk does so reversibly, whilst CO _2 adsorbed/absorbed at the surface does so irreversibly. This is contrary to the fully reversible CO _2 reaction shown for bulk ionic liquids (ILs) in literature and suggests that irreversible absorption of CO _2 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.

Keywords

• ionic liquids
• carbon capture
• x-ray photoelectron spectroscopy
• thin films
• electrospray