H2 reduction of surface oxides on Pd-based membrane model systems – The case of Pd(100) and Pd75Ag25(100)

Abstract

Reduction of the (root 5 x root 5)R27 degrees surface oxide on Pd(1 0 0) and Pd75Ag25(1 0 0) surfaces by H-2 has been studied using high-resolution photoelectron spectroscopy in situ at H-2 pressures 5 x 10(-9) mbar and 5 x 10(-8) mbar and selected temperatures in the range 30 degrees C to 170 degrees 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 nonmonotonic 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. (C) 2014 Elsevier B.V. All rights reserved. (Less)