Enhancing the surface sensitivity of in-situ/operando characterization of palladium membranes through polarization modulation and synthesis of optically smooth palladium thin films

Abstract

Operando infrared reflection absorption spectroscopy (IRAS) is a powerful technique for elucidating the fundamental mechanisms by which reactive gases such as CO, C3H6 and H2S poison (inhibit) H2 permeation across palladium (Pd) and Pd-alloy H2 separation membranes, by integrating in-situ IRAS detection of chemical species adsorbed on the membrane surface with simultaneous measurement of H2 permeation rates on the same sample. Achieving high sensitivity to detection of surface-adsorbed species under realistic operating conditions is a significant challenge, however, because infrared absorption from gas-phase species can obscure the relatively weak absorption bands associated with surface-adsorbed species, and IRAS requires optically smooth Pd membrane surfaces to specularly reflect the infrared light at grazing angle incidence. This work addresses both of these challenges by (1) installing a photoelastic modulator in the infrared beam path to modulate the polarization of the infrared light between s- and p-polarization (PM-IRAS), and (2) by developing a new electroless plating method for synthesis of optically smooth Pd thin film membranes. The net result is a ~10× enhancement in the sensitivity to detection of surface-adsorbed CO in the presence of gas-phase CO, relative to IRAS characterization with commercial Pd foil. This work represents a significant advancement in the development and application of surface-sensitive operando membrane characterization, which is a relatively new research methodology that is expected to have a significant impact on membrane science in the coming years because of its unique ability to directly correlate membrane atomic/molecular structure to its performance under realistic operating conditions.