(754g) Operando PM-IRAS for Elucidating Poisoning Mechanisms of Catalytic Membrane Surfaces

Catalytic palladium (Pd)-based membranes could enable new energy technologies, such as membrane reactors for modular chemical process intensification, due to their unique ability to separate H₂ from mixed gas streams with infinite selectivity at reaction temperatures. However, other reactive gases that are common to H₂-containing gas streams, such as CO, C₃H₆, and H₂S, can severely inhibit (poison) H₂ permeation across the membrane. While the stability of Pd-based membranes has improved incrementally over the years by alloying Pd with various elements, a membrane that resists poisoning over a wide range of operating conditions has not been discovered to date, and it is not well-understood why some alloy compositions resist poisoning more than others. Limited fundamental understanding of the poisoning mechanisms hinders development of membranes that resist poisoning. To provide new scientific insights into membrane poisoning mechanisms, we have developed a new operando spectroscopy research tool that uses polarization-modulation infrared-reflection absorption spectroscopy (PM-IRAS) to interrogate the interactions between reactive gases and the membrane surface in-situ under realistic permeation conditions while trans-membrane H₂ permeation rates are measured simultaneously. Thus, this technique can correlate microscopic surface poisoning processes to macroscopic H₂ permeation rates and elucidate membrane poisoning mechanisms in more detail than is currently possible with ex-situ characterization techniques. In this talk, I will demonstrate how this tool can elucidate the mechanisms by which CO inhibits H₂ permeation across Pd, PdCu, and PdAg membranes in greater detail than ever before. While CO inhibits H₂ permeation across Pd, PdCu, and PdAg membranes by adsorbing only on Pd sites on the membrane surface and blocking H₂ dissociative adsorption on these sites, the adsorption geometry of CO on Pd (bridging), PdCu (linear), and PdAg (three-fold hollow) is significantly different, which results in dramatic differences in the H₂ permeation behavior. This work demonstrates that operando spectroscopic observation of membrane surfaces is critical for elucidating fundamental poisoning mechanisms, and for rational design of membranes that resist poisoning.