In-situ XPS studies of CO₂ capture by aqueous monoethanolamine (MEA) solutions

XPS on a micro liquid jet has been used to study solutions of monoethanolamine (MEA), which is commonly used in gas stream scrubbing for carbon dioxide capture. It is likely that interactions between CO2 and the aqueous MEA solution at the gas/liquid interface are important to this process, yet there is little information available concerning the spatial distribution of species at the interface of such solutions. In the present work, aqueous solutions of MEA with a range of pH values as well as solutions of MEA reacted with CO2 have been measured using tunable synchrotron radiation from the BESSY facility in Berlin, where the photoelectron kinetic energy can be varied to obtain depth dependent composition information. N1s photoemission spectra allow for the identification of protonated verses unprotonated MEA by the different binding energies of the two species, and likewise, C1s spectra allow for the determination of CO2-reacted verses unreacted MEA. Depth profiling shows that deprotonated MEA is more surface active than the protonated species and reveals the spatial distribution of CO2-reacted products at the liquid/vapor interface of solution. The mechanism of the reaction of CO2 with aqueous solutions of monoethanolamine will be discussed in light of our results.