H₂ reduction of surface oxides on Pd-based membrane model systems – The case of Pd(1 0 0) and Pd₇₅Ag₂₅(1 0 0)

Highlights: • H₂ reduction of the surface oxide is significantly slower for Pd₇₅Ag₂₅(1 0 0) compared to Pd(1 0 0). • The reduction behavior shows complex temperature dependence, not well described by Avrami kinetics. • Oxygen spillover effects during the surface oxide reduction are observed for Pd(1 0 0). • For Pd(1 0 0) the observed reduction rate is rather independent of temperature. • For Pd₇₅Ag₂₅(1 0 0) the reduction rate displays a non-monotonic variation with temperature. - Abstract: Reduction of the (√(5)×√(5))R27° surface oxide on Pd(1 0 0) and Pd₇₅Ag₂₅(1 0 0) surfaces by H₂ has been studied using high-resolution photoelectron spectroscopy in situ at H₂ pressures 5 × 10⁻⁹ mbar and 5 × 10⁻⁸ mbar and selected temperatures in the range 30 °C to 170 °C. The reduction is slower on Pd₇₅Ag₂₅(1 0 0) compared to Pd(1 0 0) for all temperatures and pressures investigated. For Pd(1 0 0), the surface oxide reduction rate is rather independent of temperature, while for Pd₇₅Ag₂₅(1 0 0) a non-monotonic variation is observed. As indicated by kinetic analysis, the complex reduction behavior is not well described by Avrami kinetics. Oxygen spillover effects contribute to this picture for Pd(1 0 0), while surface compositional effects appear to dominate the performance for Pd₇₅Ag₂₅(1 0 0). These findings may have implications for understanding the oxidation, reduction and hydrogen transport properties of Pd–Ag membranes