(622c) Operando PM-IRAS+Raman Spectroscopy for Elucidating Poisoning Mechanisms of Pd-Based Hydrogen Separation Membranes in Complex Reaction Mixtures

Pd-based membranes can separate hydrogen from mixed gases with high selectivity due to the unique hydrogen transport mechanism. Molecular H₂ dissociatively adsorbs on the catalytically active surface of the membrane, producing H atoms which diffuse through the interstices of the bulk metal lattice, and recombine on the downstream surface. Only H atoms can diffuse through the membrane so selectivity over other gases is essentially infinite. However, other gases that are common in hydrogen-containing gas streams, such as propylene and H₂S, can severely poison Pd-based membranes and inhibit hydrogen transport. Development of membranes that resist poisoning is hindered by a lack of a fundamental understanding of how these reactive gases inhibit hydrogen transport. More specifically, it is not clear: (1) what compounds are formed when these gases react with Pd-based membranes, (2) where these compounds are located (surface versus bulk), (3) how these compounds inhibit hydrogen transport (catalytic surface poisoning versus slowing bulk H atom diffusion), or (4) how the Pd-alloy composition influences (1) through (3). This lack of fundamental understanding is due in part to the lack of commercially-available tools that are capable of monitoring the evolution of the membrane structure in-situ under realistic permeation conditions. To address this problem, our group is developing a multi-mode operando spectroscopy instrument to provide new scientific insights into membrane poisoning mechanisms. This instrument uses both polarization-modulation infrared-reflection absorption spectroscopy (PM-IRAS) and Raman spectroscopy to interrogate the membrane composition in-situ under realistic permeation conditions while trans-membrane hydrogen permeation rates are measured simultaneously. Thus, this technique can correlate microscopic surface processes to macroscopic permeation rates and elucidate membrane poisoning mechanisms in more detail than is possible with ex-situ techniques. In this talk, I will discuss the design and performance of this spectroscopic tool using hydrogen permeation across Pd and Pd-Ag alloys in the presence of CO as a model reaction. I will show that CO poisoning of hydrogen permeation across the membrane can be correlated to CO adsorbed on specific sites on the membrane surface.