Surface structure and stability of PdZn and PtZn alloys: Density-functional slab model studies

Abstract

Geometric parameters of binary ͑1:1͒ PdZn and PtZn alloys with CuAu-L1 0 structure were calculated with a density functional method. Based on the total energies, the alloys are predicted to feature equal formation energies. Calculated surface energies of PdZn and PtZn alloys show that ͑111͒ and ͑100͒ surfaces exposing stoichiometric layers are more stable than ͑001͒ and ͑110͒ surfaces comprising alternating Pd ͑Pt͒ and Zn layers. The surface energy values of alloys lie between the surface energies of the individual components, but they differ from their composition weighted averages. Compared with the pure metals, the valence d-band widths and the Pd or Pt partial densities of states at the Fermi level are dramatically reduced in PdZn and PtZn alloys. The local valence d-band density of states of Pd and Pt in the alloys resemble that of metallic Cu, suggesting that a similar catalytic performance of these systems can be related to this similarity in the local electronic structures.

In situ study of the formation and stability of supported Pd2Ga methanol steam reforming catalysts

“…Several explanations were proposed for the remarkable alteration in the catalytic properties on certain supports, such as the formation of Pd-Zn, Pd-Ga, and PdIn alloys. By XPS and UPS valence band studies, it was found that the electronic properties of the 1:1 PdZn alloy resemble those of metallic Cu [16,17], which was confirmed by DFT density of states calculations [18,19]. Using DFT, Neyman et al [18] calculated reaction barriers for methoxy dehydrogenation to CO and H 2 on Pd, PdZn, and Cu surfaces and found similar barriers for the dehydrogenation of intermediate formaldehyde to CO on Cu and PdZn, which are both notably higher than on Pd.…”

Stress release mechanisms for Cu on Pd(111) in the submonolayer and monolayer regimes

“…17 The energy of a Cu impurity in a Pd host is close to the experimental value 16 and the surface energies are consistent with the measurements 16 and first-principles studies. 18 The potential correctly predicts the system to be in the wetting regime although for a bulk alloy it favors fcc instead of the observed B2 bcc phase. 19 To identify the low-energy configurations, we employ two complementary strategies, namely, basin-hopping global optimization ͑GO͒ and activation-minimization ͑AM͒ techniques.…”

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

“…Reduction of Pd supported on ZnO in H 2 at T~523 K leads to the formation of particularly stable 1:1 PdZn nanoparticles crystallizing in the tetragonal AuCu-type lattice [1,2,4]. Both experimental and theoretical studies have already addressed the important issue of surface composition and segregation trends of the Pd-Zn intermetallic system (near-surface alloy on a single crystal and 1:1 PdZn nanoparticles on ZnO [5][6][7][8][9][10][11]) as a function of the Zn:Pd ratio (Zn coverage) and the temperature. For the specific 1:1 stoichiometry, DFT calculations identified a 1:1 ratio of Pd and Zn as the most stable configuration also in the surface layer, at least from the viewpoint of surface/bulk energetics that were not affected by high gas pressures or elevated temperature [8,10].…”

“…Both experimental and theoretical studies have already addressed the important issue of surface composition and segregation trends of the Pd-Zn intermetallic system (near-surface alloy on a single crystal and 1:1 PdZn nanoparticles on ZnO [5][6][7][8][9][10][11]) as a function of the Zn:Pd ratio (Zn coverage) and the temperature. For the specific 1:1 stoichiometry, DFT calculations identified a 1:1 ratio of Pd and Zn as the most stable configuration also in the surface layer, at least from the viewpoint of surface/bulk energetics that were not affected by high gas pressures or elevated temperature [8,10]. Related experimental studies of the Zn/Pd(111) system using XPS reported different stages of the alloying process as a function of temperature, among them the formation of the theoretically predicted 1:1 surface composition [5,11,12].…”