Hydrogen-induced high-temperature segregation in palladium silver membranes

Area of Science:

• Materials Science
• Chemical Engineering
• Physical Chemistry

Background:

• Metal membranes offer high H2 permeability and selectivity at elevated temperatures.
• Applications include water gas shift and steam reforming in membrane reactors.
• Understanding high-temperature behavior of PdAg-ceramic membranes is crucial.

Purpose of the Study:

• Investigate the behavior of PdAg-ceramic composite membranes between 823 K and 923 K.
• Analyze the H2 flux, activation energy, and surface changes.
• Determine the underlying mechanism for observed phenomena.

Main Methods:

• Operated PdAg-ceramic composite membranes under H2 at 873 K and above.
• Monitored H2 flux and activation energy over time.
• Analyzed membrane surface morphology and composition.
• Performed annealing under N2 at 923 K for reversal studies.

Main Results:

• H2 flux decreased continuously for membranes with <10 μm alloy layers at ≥873 K.
• Activation energy for H2 permeation steadily increased.
• Observed growth of Ag-depleted crystallites on the membrane surface.
• Reversal of all phenomena achieved through N2 annealing at 923 K.

Conclusions:

• Observed changes are consistent with a segregation mechanism involving metal sublimation and resublimation.
• Suggests enhanced metal activities in PdAg hydride phases over metallic alloys.
• Implications for thin-layered, supported PdAg membranes for high-temperature H2 separation discussed.