Monday, March 31, 2008 - 12:10 PM

H2S Interactions with Pd Alloy Membrane Surfaces

Bret H. Howard1, Bryan D. Morreale1, James B. Miller1, Dominic R. Alfonso1, and Casey P. O'Brien2. (1) National Energy Technology Laboratory, (2) Carnegie Mellon University

Coal gasification coupled with water-gas shift and CO2 sequestration is generally recognized as a viable approach to supply the near- to mid-term hydrogen demand for the United States while simultaneously addressing the global warming concerns linked to the release of CO2 to the atmosphere. The gasifier effluent stream contains primarily H2 and CO2 with the balance being CO and H2O as well as minor constituents such as H2S and NH3. Dense metal membranes based on Pd alloys or utilizing a Pd alloy component such as a surface coating, for example, remain a promising technology for the separation and purification of hydrogen from the gasifier effluent stream and, concurrently, the concentration of CO2. However, the impurities present, especially H2S, can have adverse effects on the mechanical and chemical performance and stability of membranes containing a Pd alloy component. These effects include corrosion, possibly leading to failure and catalytic poisoning. Therefore, developing an understanding of the interactions of H2S with these membrane materials, both at the micro- and macro-scales, is critical to developing practical dense metal membranes for gasification applications.

 In this work, membrane performance measurements over a range of temperatures and Pd alloy compositions, coupled with materials characterization by both bulk and surface methods and computational modeling, are being utilized to provide a foundation for understanding Pd alloy corrosion in the presence of H2S-containing gas mixtures and the effect of sulfur on the dissociative adsorption of molecular hydrogen on Pd and Pd-alloy surfaces. Two modes of sulfur-induced deactivation are observed during membrane flux testing depending on the specific set of conditions employed. A corrosive decay mechanism is associated with formation of a thick, low diffusivity, Pd4S scale layer on the membrane surface. For corrosive decay, both the surface region and top layer of the scale display Pd4S stoichiometry. In addition, Pd-terminated Pd4S low-index faces appear to provide the catalytic activity for hydrogen dissociation. The catalytic poisoning mechanism does not appear to be associated with the formation of a bulk sulfide. Results suggest that the formation of a thin, sulfur-rich (relative to Pd4S) overlayer effectively inhibits the dissociation reaction severely degrading performance. The results of these studies will be utilized to guide future membrane design improvements.


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