Segregation dynamics of a Pd-Ag surface during CO oxidation investigated by NAP-XPS

• CO oxidation over Pd 75% Ag 25% (100) studied by NAP-XPS. • Reversed hysteresis in CO 2 formation during heating and cooling ramps observed. • Hysteresis behaviour depends on exposure, heating rate, maximum temperature, and sample history. • Dynamic changes in Pd/Ag surface composition during cycling to 450 °C or 600 °C observed. • Relatively facile kinetics of segregation that proceeds even at ambient temperature. The dynamic changes in composition in the near-surface region of a Pd 75% Ag 25% (100) single crystal were monitored using near-ambient pressure x-ray photoelectron spectroscopy (NAP-XPS) during CO oxidation under oxygen rich conditions at a total pressure of 1.1 mbar. Six CO oxidation temperature cycles were investigated at different heating rates and maximum temperatures of 450 °C or 600 °C. It was found that the history of the bimetallic sample plays an important role, as the CO 2 formation profile varies depending on initial conditions, and previous heating rates and maximum temperatures. In terms of CO coverage effects, normal, reversed and no hysteresis behaviour were all observed. In agreement with previous modelling predictions, the NAP-XPS data confirm a dynamic segregation behaviour upon heating/cooling where the amount of Pd in the surface region decreases with increasing temperature. Nevertheless, the Pd 3d 5/2 core level relative area assessment is not fully capable of capturing all the surface dynamics inferred from the temperature dependent CO 2 formation profiles, due to the probing depth. While residing at ambient temperature in the reaction mixture, however, there is a build-up of adsorbed CO at the surface showing that CO induces segregation of Pd to the topmost surface layer under these conditions. In total, this suggests that the segregation is kinetically relatively facile during temperature cycling, and that adsorbate coverage is the main controlling factor for the surface termination.