Surface structure and composition of Pt50Rh50(): room temperature analysis of the (1×3) missing-row reconstruction (original) (raw)
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Chemical ordering and reconstruction of Pt25Co75(100): an LEED/STM study
Surface Science, 1998
The surface of a disordered Pt25Co75(100) alloy has been investigated using quantitative LEED, AES and UHV-STM at room temperature. Atomic-resolution images reveal that it reconstructs with close-packed rows shifted by half the interatomic distance, from hollow to bridge sites. The density of shifted rows increases with the surface Pt concentration, leading to (1 × 5), (1 × 6) and (1
1984
Describing the binding energy of both d and s valence electrons within the LCAO formalism, and by including repulsive Born-Mayer type interactions, we study the structural stability of the reconstructed and unreconstructed Pt(ll0) surfaces. Our main result is that amongst the various models for the (1 x2) reconstruction the "Bonzel-Ferrer" model is unfavoured, while the "missing-row" model seems to be energetically degenerate with the unreconstructed surface. Our calculation predicts also a small surface concentration, which, however, has only a minor effect on the total energy of the system.
Surface electronic structure of Pt(110): comparison with Ni and Pd
Progress in Surface Science, 2003
The unoccupied electronic structure of Pt(1 1 0) was investigated by inverse photoemission. The results were compared with the data for Ni(1 1 0) and Pd(1 1 0) with particular emphasis on surface states. The surface states in the Y gap of Pt(1 1 0) are shifted upwards relative to Ni and Pd, as a consequence of the (1 · 2) missing-row reconstruction. In contrast, the surface state at X is only weakly affected, which indicates that it is essentially a one-dimensional state, localized on the densely-packed atomic chains on the Pt(1 1 0) surface.
Thermodynamics of the Atomic Distribution in Pt3Pd2, Pt2Pd3 and their Corresponding (111) Surfaces
2021
In this study, we have developed solid-state models of platinum and palladium bimetallic catalysts, Pt 3 Pd 2 and Pt 2 Pd 3 , which are rapidly thermally annealed at 800°C. These models were constructed by determining all the unique atomic configurations in a 2 × 2 × 1 supercell, using the program Site-Occupation Disorder (SOD), and optimized with the General Utility Lattice Program (GULP) using Sutton-Chen interatomic potentials. Each catalyst had 101 unique bulk models that were developed into surface models, which were constructed using the two-region surface technique before the surface energies were determined. The planes and compositions with lowest surface energies were chosen as the representative models for the surface structure of the bimetallic catalysts. These representative models will now be used in a computational study of the HyS process for the production of hydrogen.
Unexpected Behavior of the Surface Composition of PtRh Alloys during Chemical Reaction
Journal of The American Chemical Society, 2005
The changes in the surface composition of a Pt50Rh50(100) alloy due to an ongoing 2H2 + O2f2H2O chemical reaction have been studied in situ. Exploiting the high-energy resolution and surface sensitivity of synchrotron radiation core-level spectroscopy it was possible to monitor the population of the two transitions metals atoms at the gas-surface interface. Sequences of fast high-resolution core-level spectra of the Rh3d 5/2, Pt4f7/2, and O1s core levels showed a continuous exchange of atoms between the first and subsurface layers. An unexpected Pt surface enrichement was found under slightly oxidizing conditions, opposite to what found in a highly oxidizing atmosphere.
Atomic engineering of platinum alloy surfaces
Ultramicroscopy, 2013
A major practical challenge in heterogeneous catalysis is to minimize the loading of expensive platinum group metals (PGMs) without degrading the overall catalytic efficiency. Gaining a thorough atomicscale understanding of the chemical/structural changes occurring during catalyst manufacture/operation could potentially enable the design and production of ''nano-engineered'' catalysts, optimized for cost, stability and performance. In the present study, the oxidation behavior of a Pt-31 at% Pd alloy between 673-1073 K is investigated using atom probe tomography (APT). Over this range of temperatures, three markedly different chemical structures are observed near the surface of the alloy. At 673 K, the surface oxide formed is enriched with Pd, the concentration of which rises further following oxidation at 773 K. During oxidation at 873 K, a thick, stable oxide layer is formed on the surface with a stoichiometry of PdO, beneath which a Pd-depleted (Pt-rich) layer exists. Above 873 K, the surface composition switches to enrichment in Pt, with the Pt content increasing further with increasing oxidation temperature. This treatment suggests a route for tuning the surfaces of Pt-Pd nanoparticles to be either Pd-rich or Pt-rich, simply by adjusting the oxidation temperatures in order to form two different types of core-shell structures. In addition, comparison of the oxidation behavior of Pt-Pd with Pt-Rh and Pd-Rh alloys demonstrates markedly different trends under the same conditions for these three binary alloys.
A structure investigation of Pt-Co bimetallic catalysts fabricated by mechanical alloying
Materials Chemistry and Physics, 2009
Three Pt-Co mixtures of composition Pt25Co75, Pt50Co50 and Pt75Co25 respectively, were synthesized using the high-energy ball milling technique of the elemental powders with a view to prepare catalysts for fuel cells. The kinetics of phase evolution, their structure and average microstructure properties were quantitatively investigated by x-ray powder diffraction with the Rietveld method. The results show that the ball milling technique is able to produce Pt-Co solid solutions soon after few minutes of mechanical treatment. Of the two polymorphs of cobalt the fcc allotrope appears to be involved preferentially in the early stage of alloying reaction with fcc platimum. For the three compositions, a sigmoidal equation based on a interdiffusion-controlled mechanism satisfactorily accounts for the evolution of the solid solution as a function of mechanical treatment time. A characteristic reaction time of 3-6 h is observed for the solid state transformation reaction with the milling conditions adopted in our reactor. In the case of the Pt25Co75 composition, a competitive-consecutive reaction is observed. Lattice parameters of the solid solutions after extended times of milling and related atomic volumes turn out to be slightly above the values ideally predicted on the basis of the Vegard law. For the Pt75Co25 composition the average crystallite size is reduced down to ca 150 Å after 12 h when the lattice microstrain is also at a maximum, but further mechanical treatment tends to increase the average crystal size value and to decrease the strain. Similar results are found for equiatomic and Co-rich compositions. Annealing of the alloyed equiatomic powders effects a cubic-to-tetragonal transformation which is already operative at 600°C. In facts, after this treatment two tetragonal phases are observed. Further thermal treatment and annealing at 700 °C induces peak sharpening of the diffraction patterns, on account of ordering phenomena still in progress.
Pt Gd alloy formation on Pt(111): Preparation and structural characterization
Surface Science, 2016
Pt x Gd single crystals have been prepared in ultra high vacuum (UHV). This alloy shows promising catalytic properties for the oxygen reduction reaction. The samples were prepared by using vacuum deposition of a thick layer of Gd on a sputter cleaned Pt(111) single crystal, resulting in a ∼63 nm thick alloy layer. Subsequently the surfaces were characterized using X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED), ion scattering spectroscopy (ISS) and temperature programmed desorption (TPD) of CO. A Pt terminated alloy was observed upon annealing the sample to 600 • C. The LEED and synchrotron XRD experiments have shown that a slightly compressed (2×2) alloy appear. The alloy film followed the orientation of the Pt(111) substrate half the time, otherwise it was rotated by 30 •. The TPD spectra show a well-defined peak shifted down 200 • C in temperature. The crystal structure of the alloy was investigated using ex-situ X-ray diffraction experiments, which revealed an in-plane compression and a complicated stacking sequence. The crystallites in the crystal are very small, and a high degree of twinning by merohedry was observed.