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

Abstract

The CO2-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" PdZn 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 subsurfaceZn 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 catalyses methanol dehydrogenation to CO.

In Situ Spectroscopy of Complex Surface Reactions on Supported Pd–Zn, Pd–Ga, and Pd(Pt)–Cu Nanoparticles

“…11,26 Most importantly, the density of states (DOS) near the Fermi level changes, evident from the valence band ( Figure 5C), which suggests a "Cu-like" electronic structure with lower DOS close to the Fermi level. 14,20,24 The similarity of effects detected by FTIR and XPS confirms the good comparability of model and applied PdZn bimetallic catalysts and is corroborated by DFT. 23 The different electronic structure of PdZn and Pd clearly affects the adsorption of reactants and intermediates, thus modifying catalysis.…”

High CO2 Selectivity in Methanol Steam Reforming through ZnPd/ZnO Teamwork

“…Ultimately, these results confirm previous studies on related model systems wherein oxidized Zn species have been clearly identified by in situ Xray photoelectron spectroscopy as being present in the CO 2selective state of the catalyst, and as being capable of efficient water activation, a prerequisite for a CO 2 -selective catalyst. [12,[14][15][16] Nevertheless, XPS is often not suitable for exploring supported catalysts such as ZnPd/ZnO, because of the induced charging on insulating samples and the excess of ZnO that makes it impossible to identify ZnO patches formed in situ. This is the first time that the selective state of a ZnPd/ZnO catalyst is made clearly visible and is shown to contain special interfacial sites, where uncovered intermetallic ZnPd particles and ZnO coexist in close proximity.…”

“…[12] The presence of oxidized Zn in near-surface regions has been shown to be inevitably linked to a high CO 2 -selectivity in inverse model catalyst studies of near-surface intermetallic phases. [14][15][16] The knowledge transfer from these model systems to high-performance catalysts represents a great step towards understanding MSR.…”

Molecular catalysis science: Nanoparticle synthesis and instrument development for studies under reaction conditions

“…A variety of vibrational spectroscopy methods were utilized to follow the reactant to product transformation [35,36]. Among these methods are surface sensitive techniques such as SFG (sum frequency generation) vibrational spectroscopy [32,37] and PM-IRAS (polarization modulation infrared reflection absorption spectroscopy) [38,39] that detect active intermediates during catalytic reactions. Time resolved IR spectroscopy measurements analyze the kinetic properties of catalytic reactions [40,41] while IR microspectroscopy can track the spatial distribution of reactants and products within the catalyst [42][43][44].…”