More than skin deep: In spite of their identical 1:1 surface composition, the geometric and electronic structures of a multilayer and monolayer PdZn surface alloy are different, as are their catalytic selectivities. The CO2 selective multilayer alloy features surface ensembles of PdZn exhibiting a “Zn‐up/Pd‐down” corrugation (see picture). These act as “bifunctional” active sites both for water activation and for the conversion of methanol into CO2. On the monolayer alloy CO and not CO2 is produced.

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.

A Zn-in-Cu near-surface alloy covered by a thin wetting layer of interfacial Zn(ox) is the most active state of an inverse CuZn catalyst. The bifunctional action of the mixed Cu(Zn)0/Zn(ox) surface allows for selective dehydrogenation of methanol to formaldehyde and for optimized water activation, thus providing the required source of oxygen for the total oxidation of HCHO to CO2