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.

Intermetallic compounds in heterogeneous catalysis—a quickly developing field

“…In the case of ZnPd, even exposure to carbon monoxide results in structural modifications [83, 84], preventing the use of this frequently used test molecule in heterogeneous catalysis to determine the nature of the surface of ZnPd-based catalysts. Up to now, the behavior of ZnPd under reaction conditions is not at all explored in a systematic way (for a comprehensive review on ZnPd see [85]), but potentially holds the key for a full understanding of the requirements for MSR catalysts and, thus, the development of innovative materials combining higher activity and selectivity.…”

The influence of CO adsorption on the surface composition of cobalt/palladium alloys

“…[19] In this study we used reflection absorption infrared spectroscopy (RAIRS) and medium energy ion scattering (MEIS) to probe the effect of CO adsorption on the surface composition of bimetallic CoPd surfaces as a function of initial Pd composition. We recently reported a MEIS study of the growth and alloying behaviour of Co thin films on Pd{111} at 300 K. [20] …”

“…[19] It is possible that CO has a similar effect on the PdCo surface such that while, under vacuum conditions, the p(2x1) PdCo structure is favoured, after exposure to CO, the surface reconstructs in order to maximise the favoured Pd 3 and Co 1 sites at the surface.…”

Insight into the mechanism and possibility of ethanol formation from syngas on Cu(100) surface

“…On the other hand, to validate the reliability of the selected method, the geometrical parameters of CO molecule in gas phase are calculated using the different functionals (PW91, PBE and LDA), [48], and 2143 cm −1 [49][50][51], respectively, as well as other DFT calculated values 2121 cm −1 [48], and 2120 cm −1 [50]. As a result, GGA-PW91 functional is used throughout the present work.…”