XPS Study of the Influence of CO2 on the H2 Flux through a Composite Membrane Made of Palladium and Porous Stainless Steel

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Plating a Pd membrane on the outer surface of a porous stainless steel (PSS) tube facilitates rapid H2 permeation with an absolute selectivity; the permeation is based on the solution-diffusion transport mechanism. Reforming of carbon hydride is a well-known method of manufacturing H2. CH4 dry reforming is performed in a membrane reactor using a thin Pd membrane. In this reaction, chemisorbed CO2 is known to affect H2 permeation behavior. The effect on H2 permeation and membrane stability were investigated by performing a permeation test and by XPS. After H2 was permeated through the Pd membrane, CO2 was flowed over the membrane for 1 h, and H2 was once again permeated through the membrane. In the experiment, the temperatures were 623, 723, and 823 K, the differential pressure was 0.1 MPa, and the feed rate was 10 mL/min. CO2 was replaced by He and the experiment was performed under the same conditions. After the second H2 permeation, the H2 flux increased with the time and attained its original value. The time required to attain the original value was dependent on the gas and the temperature. In the case of the He flow, the time required was only 5 min and was independent of the temperature. On the other hands, in the case of the CO2 flow, the required times were 10 min and 25 min at 623 K and 723 K or 823 K, respectively. After the CO2 flow, the XPS spectra of the surface of the Pd membrane were measured. The spectra showed that PdO was formed on the Pd membrane by oxidation of the Pd surface. The PdO was formed by chemisorbed CO2, and the H2 permeability was degraded by PdO; however, PdO was easily reduced to Pd by the H2 flow. When the experiment involving CH4 dry reforming was performed in the Pd membrane reactor with conventional Pt-based catalysts and a sweep gas for 12 h at 873 K, the H2 flux, H2 selectivity, and H2 recovery were ca. 6 mL/min, 98% (calculated by excluding the sweep gas), and 80%, respectively.

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