Temperature-induced modifications of PdZn layers on Pd(111)

Ultrathin PdZn surface alloys on Pd(111) are model systems well-suited for obtaining a microscopic understanding of the mechanisms of Pd/Zn-based catalysis for methanol steam reforming. The temperature-induced compositional and structural changes of these alloy films are investigated in the catalytically relevant temperature range. Heating of multilayer Zn films to 500 K results in the formation of multilayer PdZn alloy films with surface and near-surface composition close to 1:1. In the temperature regime above 550 K the subsurface layers deplete quickly in Zn due to diffusion of Zn atoms into the Pd bulk. In contrast, the composition of the surface layer changes only slightly, indicating formation of a PdZn film with strong monolayer character. This change in subsurface composition triggers a change of the original Zn-out/Pd-in surface corrugation, leading ultimately to a Pd-out/Zn-in situation for annealing temperatures beyond 700 K. The altered corrugation pattern is also obtained when submonolayer amounts of Zn are heated to ~ 500 K. The observed structural changes are in qualitative agreement with predictions by DFT calculations