XPS Study of the Influence of CO_2 on the H_2 Flux through a Composite Membrane Made of Palladium and Porous Stainless Steel
Plating a Pd membrane on the outer surface of a porous stainless steel (PSS) tube facilitates rapid H<sub>2</sub> 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 H<sub>2</sub>. CH<sub>4</sub> dry reforming is performed in a membrane reactor using a thin Pd membrane. In this reaction, chemisorbed CO<sub>2</sub> is known to affect H<sub>2</sub> permeation behavior. The effect on H<sub>2</sub> permeation and membrane stability were investigated by performing a permeation test and by XPS. After H<sub>2</sub> was permeated through the Pd membrane, CO<sub>2</sub> was flowed over the membrane for 1 h, and H<sub>2</sub> 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. CO<sub>2</sub> was replaced by He and the experiment was performed under the same conditions. After the second H<sub>2</sub> permeation, the H<sub>2</sub> 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 CO<sub>2</sub> flow, the required times were 10 min and 25 min at 623 K and 723 K or 823 K, respectively. After the CO<sub>2</sub> 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 CO<sub>2</sub>, and the H<sub>2</sub> permeability was degraded by PdO; however, PdO was easily reduced to Pd by the H<sub>2</sub> flow. When the experiment involving CH<sub>4</sub> 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 H<sub>2</sub> flux, H<sub>2</sub> selectivity, and H<sub>2</sub> recovery were ca. 6 mL/min, 98% (calculated by excluding the sweep gas), and 80%, respectively.
- Journal of chemical engineering of Japan
Journal of chemical engineering of Japan 43(8), 745-750, 2010-12-01