Changes in hydrogen permeability and surface state of Pd-Ag/ceramic composite membranes after thermal treatment

Changes in hydrogen permeability and surface state of a Pd-Ag/ceramic membrane have been investigated as a function of thermal treatment. The surface state is determined by X-ray photoelectron spectroscopy (XPS). The XPS data show a significant enrichment of the Ag alloying element and an accumulation of impurity atoms (C, S, Na, Cl, O) in the surface layer of an as-prepared membrane. The thermal treatment, conducted at 400 °C in air for 1 h, results in a decrease of the Pd/Ag ratio in the surface (from 1:3.4 to 1:2.9), and an obvious chemical shift (ca. 1.4 eV) of Pd 3d core level but no change in the Ag 3d level, indicating the Pd2+ state of PdO is formed. The thermal treatment does not change the surface states of impurities of Na, Cl and O. On the other hand, XPS signal of the oxidized sulfur, SO42− species, is found. It is noted that the carbon concentration is decreased dramatically (from 67 to 32 at.%) by decomposition of carbon, a new XPS peak (BE = 292.5 eV) of CO2 is clearly identified from a treated surface. After the thermal treatment, the hydrogen permeability of the membrane is increased by a factor of 2.5, such behavior can be mainly attributed to the removing of the carbon contaminate initially present in the surface.