Operando Surface-Enhanced Raman-Scattering (SERS) for Probing CO2 Facilitated Transport Mechanisms of Amine-Functionalized Polymeric Membranes.

Abstract

This work describes a new operando surface enhanced Raman spectroscopy (SERS) platform that we developed for use with polymeric membranes that includes (1) a method for preparing SERS-active polymer membranes and (2) a permeation cell with optical access for SERS characterization of membranes under realistic operating conditions. This technique enables the direct correlation of membrane structure to its performance under realistic operating conditions by combining in situ SERS characterization of the molecular structure of polymer membranes and simultaneous measurement of solute permeation rates on the same sample. Using the new operando SERS technique, this work aims to clarify the unknown mechanisms by which reactive amines facilitate CO2 transport across polyvinylamine (PVAm), a prototypical facilitated transport membrane for CO2 separations. We show that a small amount of plasmonic silver particles added to the PVAm solution prior to knife-casting selectively enhances the sensitivity to detection of chemical intermediates (e.g., carbamate) formed in the PVAm film due to the surface-enhanced Raman scattering effect with only minimal effect on the CO2 permeance and selectivity of the membrane. Operando SERS characterization of PVAm during exposure to humidified CO2/CH4 biogas mixtures at room temperature shows that CO2 permeates across PVAm primarily as carbamate species. This work clarifies the previously unknown mechanism of CO2 facilitated transport across PVAm and establishes a new operando SERS platform that can be used with a wide range of polymer membrane systems. This technique can be used to elucidate fundamental transport mechanisms in polymer membranes, to establish reliable structure-performance relationships, and for real-time diagnostics of membrane fouling, among other applications.