Palladium-based membranes are attractive for hydrogen separation applications, such as membrane reactors, due to their rare combination of high permeability and high selectivity for hydrogen separation from mixed gas streams. Their near-infinite selectivity for hydrogen separation results from the unique hydrogen transport mechanism. Molecular H2 dissociatively adsorbs on the catalytically active surface of the membrane, producing H atoms which diffuse through 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 CO, 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 demonstrate how this tool can elucidate poisoning mechanisms of Pd, Pd-Cu, and Pd-Ag alloy membranes in great detail, by correlating inhibition of hydrogen transport to CO adsorbed on specific sites on the membrane surface.

Operando PM-IRAS+Raman spectroscopy for elucidating surface poisoning mechanisms of Pd-based hydrogen separation membranes in complex reaction mixtures

2019 Spring National Meeting

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