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.

No abstract

No abstract

No abstract

PtNi thin film catalysts provide both higher activity and enhanced Pt efficiency in the oxygen reduction reaction (ORR) in comparison to pure Pt catalysts. In order to explore the structural transformations and degradation mechanisms in such films, we combine studies by cyclic voltammetry (CV), electrochemical atomic force microscopy (EC-AFM), and electrochemical infrared reflection absorption spectroscopy (EC-IRRAS) using CO as a probe molecule. The PtNi model thin film catalysts were prepared by magnetron sputtering on carbon coated Au targets or freshly cleaved highly ordered pyrolytic graphite (HOPG) and characterized by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS). Subsequently, structural changes of the film and changes of the CO adsorption properties were followed by EC-IRRAS as a function of the applied potential. All results were compared to reference experiments on Pt(111) performed under identical conditions. For Pt(111), well-ordered (111) facets are stable upon potential cycling up to 1.2 of the voltage of the reversible hydrogen electrode (VRHE). At higher potential, surface roughening initially leads to the formation of [110] and [100] steps whereas [110] steps are the most dominant defect structure at potentials above 1.3 VRHE. The roughening transition gives rise to characteristic changes in IR spectrum of adsorbed CO. The sputtered PtNi catalyst film shows a weak decrease in grain size upon potential cycling up to 1.1 VRHE. Freshly prepared PtNi catalysts show two characteristic IR bands in the on-top CO region. The signal at lower wavenumbers is assigned to isolated CO on Pt sites. Based on calculations using density functional theory (DFT) modeling we suggest that another peculiar blue-shifted CO band can be attributed to dicarbonyls on low-coordinated Pt centers, which are generated by the leaching of surface Ni. The blue-shifted band decreases upon cycling to higher potential and vanishes at 1.1 VRHE as a result of the increasing Pt mobility. A dramatic change of the film structure is observed upon potential cycling to 1.2 VRHE. CV indicates the formation of [110] and [100] steps and AFM points out a strong decrease in particle size. EC-IRRAS shows the appearance of a new CO band that is broadened and red-shifted by more than 20 cm-1. Based on calculated DFT data, we assign these changes to a transient enrichment of Ni in the surface or subsurface region upon dissolution of Pt. Upon cycling to even higher potential (up to 1.5 VRHE), Ni is completely leached from the film, and large Pt particles are formed by ripening and/or agglomeration, which again show the characteristic CV and CO IR spectra of rough polycrystalline Pt.

No abstract

No abstract

Heterogeneous catalysis is commonly governed by surface active sites. Yet, areas just below the surface can also influence catalytic activity, for instance, when fragmentation products of catalytic feeds penetrate into catalysts. In particular, H absorbed below the surface is required for certain hydrogenation reactions on metals. Herein, we show that a sufficient concentration of subsurface hydrogen, H^sub, may either significantly increase or decrease the bond energy and the reactivity of the adsorbed hydrogen, H^ad, depending on the metal. We predict a representative reaction, ethyl hydrogenation, to speed up on Pd and Pt, but to slow down on Ni and Rh in the presence of H^sub, especially on metal nanoparticles. The identified effects of subsurface H on surface reactivity are indispensable for an atomistic understanding of hydrogenation processes on transition metals and interactions of hydrogen with metals in general