Hydrogen adsorption and absorption on a Pd-Ag alloy surface studied using in-situ X-ray photoelectron spectroscopy under ultrahigh vacuum and ambient pressure

The hydrogen permeation process is crucial for hydrogen purification through a metal alloy. At the initial stage, it is connected with the dissociation and adsorption of hydrogen on the surface; subsequently, the process involves hydrogen absorption in the bulk. On the PdAg ₂₃ alloy, we investigated the mechanisms of adsorption and absorption by in-situ ultrahigh vacuum and ambient pressure X-ray photoelectron spectroscopy (AP-XPS) measurements, respectively. During the adsorption under a H ₂ pressure of 5 × 10 ⁻⁸ Torr at 300–620 K, the surface Ag atoms act as adsorption sites for the hydrogen atoms. The AP-XPS results show that hydrogen absorption is significantly enhanced above 473 K under H ₂ exposure at 1.5 Torr, which is likely correlated to the α-β hydride phase transition occurring around 473 K. Hydrogen preferentially bonds with the Pd atoms rather than the Ag atoms in the case of hydrogen absorption into the PdAg ₂₃ alloy. The atomic fractions of Pd on the surface were enhanced after hydrogen absorption, and hydrogen adsorption at a hydrogen pressure of 5 × 10 ⁻⁸ Torr at 620 K. This surface segregation of Pd atoms provides more reactive sites for hydrogen absorption, which may promote the hydrogen permeability of a PdAg ₂₃ alloy.