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Abstract

The CO2 selectivity in methanol steam reforming was investigated for a "multilayer" PdZn 1:1 surface alloy (thickness of not, vert, similar1.3 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 CO2-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 CO2.

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 CO2-selective PdZn "multilayer" state to the unselective "monolayer" state, which only catalyzes methanol dehydrogenation to CO.