Steam reforming of methanol on PdZn near-surface alloys on Pd(1 1 1) and Pd foil studied by in-situ XPS, LEIS and PM-IRAS

The bifunctional sites for methanol reforming on a multi- and monolayer Pd–Zn surface are structurally different, despite identical surface composition. The multilayer alloy activates water for reaction to CO 2 and H 2. The CO 2 selectivity in methanol steam reforming was investigated for a “multilayer” PdZn 1:1 surface alloy (thickness of ∼1.3 nm) and for a subsurface-Zn diluted “monolayer” Pd–Zn surface alloy, both exhibiting a 1:1 composition in the surface layer. Despite having almost the same surface layer stoichiometry, these two types of near-surface alloys exhibit different corrugations and electronic structures. The CO 2-selective multilayer alloy features a lowered density of states close to the Fermi edge and surface ensembles of PdZn exhibiting a “Zn-up/Pd-down” corrugation, acting as bifunctional active sites both for reversible water activation as ZnOH and for reaction of methanol (via formaldehyde + ZnOH) toward CO 2. The thermochemical stability limit of the multilayer alloy at around 573 K was determined in-situ at elevated pressures of water, methanol and CO, applying in-situ XPS, PM-IRAS spectroscopy, LEIS and AES. Above 573 K, the coordination of the surface 1:1 PdZn layer with subsurface-Zn gradually decreased by bulk diffusion of Zn “escaping” from the second and deeper layers, resulting in a transition from the CO 2-selective PdZn “multilayer” state to the unselective “monolayer” state, which only catalyzes methanol dehydrogenation to CO.