Geometric Arrangement of Components in Bimetallic PdZn/Pd(111) Surfaces Modified by CO Adsorption: A Combined Study by Density Functional Calculations, Polarization-Modulated Infrared Reflection Absorption Spectroscopy, and Temperature-Programmed Desorption

Abstract

Combining density functional theory (DFT) calculations with experimental data from temperature-programmed desorption (TPD) and polarization-modulated infrared reflection absorption spectroscopy (PM-IRAS), we have provided evidence for the rearrangement of Pd and Zn atoms in PdZn surface alloys on Pd(111) in the presence of CO. Such systems represent valuable models for novel methanol steam reforming catalysts, with CO being an undesired byproduct of this reaction. The reconstructed surface was calculated to be more stable in the presence of CO and capable of adsorbing CO on top of Pd atoms up to a saturation coverage of 1/2 monolayer (ML). To the contrary, on a surface with conventional, bulklike geometric arrangement of Pd and Zn atoms in rows, CO adsorption on bridge sites with a saturation coverage of ∼1/3 ML was predicted. Adsorption of CO on top sites was confirmed with PM-IRAS and the saturation coverage of 1/2 ML was derived from TPD experiments, supporting the suggested reconstruction of PdZn/Pd(111). The rearrangement of the atomic components on the surface was calculated to affect structural and adsorptive properties but kept essentially unchanged the electronic structure, reflecting the intermetallic character of PdZn on Pd(111). This study sheds light on a new degree of complexity of bimetallic systems, that is, changes of component arrangement on the surface in the presence of certain adsorbates, which is not accompanied by surface segregation. Such rearrangement is expected to notably affect the properties of a bimetallic catalyst.