Pt Atoms

A survey of SCF calculations of electron binding energies in neutral atoms can be found in [1], with a list of published computer codes for atomic structure calculations and a chronological list of selected publications. Electron binding energies have been calculated in various models for Os, Ir, and Pt. 

Effective potentials make the atomic structure calculations easier. Tables of optimized and ab initio effective central potentials are given in [15] and [18], respectively. Pseudopotentials are discussed in [16]. Fairly accurate ground state energies for any atom can be obtained in a very simple atomic shell model [17]. 

Ionization potentials for atoms and ions have been calculated by Carlson etal. [14] with estimated accuracies of 5% for multicharged ions recent spectroscopic measurements of Pt °+ [18] indicate a deviation of 8% from the predictions of [14]. 

The importance of using a relativistic model for high-Z elements is made obvious by Table 2/71, where total energies and electron binding energies from [6] and [11] are com- 

Total Energies (in a.u.) for the Average of the Configurations 56 6s (or 5d 6s in the Relativistic Calculation of Pt), and Electron Binding Energies (in a.u.) from Numerical Hartree-Fock [6] and Numerical Dirac-Fock [11] Calculations (respectively NR and R). For p, d, and f electrons, an asterisk indicates p /g. cl3/2 md fg/g, respectively.  

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This potential will lead to a single water molecule adsorbing at the PZC on Pt with the dipole pointmg axi ay from the surface and the oxygen atom pointing directly at a Pt-atom site (on-top configuration). 

Figure Bl.23.2. (a) Shadow cone of a stationary Pt atom in a 4 keV Ne ion beam, appearing with the overlapping of ion trajectories as a fiinction of the impact parameter. The initial position of the target atom that recoils in the collision is indicated by a solid circle, (b) Plot of the nonnalized ion flux distribution density across the shadow cone in (a). The flux density changes from 0 inside the shadow cone, to much greater than l in the focusing region, converging to 1 away from the shadow cone edge, (c) Blocking cones...

Figure Bl.23.11. Above selected time-resolved SARIS images of 4 keV Ar scadering from Pt l 11 ] along (I 12). Below view of Pt 111 ] surface along (112) showing Ar scadering from a first-lay er Pt atom (1) and spliding into two focused beams by an atomic lens fonned by neighbouring first-layer Pt atoms (2, 3, 4).

The platinum-group metals (PGMs), which consist of six elements in Groups 8— 10 (VIII) of the Periodic Table, are often found collectively in nature. They are mthenium, Ru rhodium, Rh and palladium, Pd, atomic numbers 44 to 46, and osmium. Os indium, Ir and platinum, Pt, atomic numbers 76 to 78. Corresponding members of each triad have similar properties, eg, palladium and platinum are both ductile metals and form active catalysts. Rhodium and iridium are both characterized by resistance to oxidation and chemical attack (see Platinum-GROUP metals, compounds). 

In order to check the consistency and mutual relations of ECIs calculated by various methods, as well as to compare them with experimental data, we have performed calculations for several alloy systems, as diverse as Cu-Nl, Al-Li, Al-Ni, Ni-Pt and Pt-Rh. Here we present the results for Al-Ni, Pt-Rh and Ni-Pt alloys in some detail, because the pair interactions between the first neighbors are dominant in these alloys which makes the interpretation relatively simple. On the other hand, the pair interactions between more distant neighbors and also triplet interactions are important for Al-Li and Cu-Ni. The equilibrium atomic radii, bulk moduli and electronegativities of A1 and Ni are rather different, while Pt and Rh are quite similar in this respect. The Ni and Pt atoms differ mainly by their size. 

The effect of the presence of alkali promoters on ethylene adsorption on single crystal metal surfaces has been studied in the case ofPt (111).74 77 The same effect has been also studied for C6H6 and C4H8 on K-covered Pt(l 11).78,79 As ethylene and other unsaturated hydrocarbon molecules show net n- or o-donor behavior it is expected that alkalis will inhibit their adsorption on metal surfaces. The requirement of two free neighboring Pt atoms for adsorption of ethylene in the di-o state is also expected to allow for geometric (steric) hindrance of ethylene adsorption at high alkali coverages. 

As expected, the Pt(l 11) surface is covered under ambient conditions by the well-known Pt(lll)-(2x2)-0 adlattice which corresponds to Oq -0.25 where the superscript Pt denotes that the coverage is based on the total surface Pt atoms. The measured interatomic distance of 5.61 A (Fig. 5.49a) is in excellent agreement with literature for the Pt(lll)-(2x2)-0 adlatice. As manifest by the Fourier transform spectmm (Fig. 5.49b) of the surface image of Fig. 5.49a there exists on the surface a second hexagonally ordered adlattice,... 

Assuming that the reactive oxygen corresponds to the oxygen which forms the well-known Pt(lll)-(2x2)-0 structure,82 one can define the second Na coverage scale, Q a shown in Figure 5.54, which is based on the number of surface Pt atoms, and equals... 

Figure 5.54. Effect of sodium coverage on the change AUWR of polycrystalline Pt catalyst potential UWr and on the catalytic rates of CO oxidation (solid lines37) and C2H4 oxidation (dashed lines36). Comparison with the theoretical Na coverage required to form the Pt(l 11)-(12xl2)-Na adlayer 0 is based on the number of surface Pt atoms 09a is based on the number of surface O atoms corresponding to the Pt(l 1 l)-(2x2)-0 adlattice. Reprinted from ref. 78 with permission from Elsevier Science,...

The optical spectra of the nickel-triad metals have been reinvestigated (111) in Ar, Kr, and Xe matrices, and, although the data for Ni and Pt atoms correlated well both with previous studies and with the... 

It is now assumed that each active site consists of four Pt atoms and the reactivity of 1 g of catalyst is tested under conditions where the rate is first order in oxygen concentration. The flow over the reactor is set to 100 mL min with 21% oxygen, the temperature 500 K, the pressure to 1 bar, and the TOE (turnover frequency per site) per Pt site under the chosen conditions is known from surface science experiments to be 0.001 s . The amount of oxygen converted is considered negligible. 

As mentioned above, when a Pt-atom Is surrounded by Ag, Au, Sn, etc., that is, by atoms considerably less active than Cu, very well Isolated Ft atoms or small Ft ensembles can be created, which, even without any assistance of the neighbouring Ag, Au, Sn, etc., will tend to Isomerlze hexane rather than to split It. However, when the alloys are not well homogenized, as Is frequently the case with alloys on some supports, sufficient larger ensembles of Ft, but now more difficult to be selfpoisoned (like the smallest Ft particles), may coexist next to Au, Ag, Sn or their alloys and be responsible for higher hydrogenolytlc selectivity (J ). Such effects may be then easily misinterpreted as a consequence of an alloy - support (non specified. ) electronic Interaction. 

Steady state and non steady state kinetic measurements suggest that methane carbon dioxide reforming proceeds in sequential steps combining dissociation and surface reaction of methane and CO2 During admission of pulses of methane on the supported Pt catalysts and on the oxide supports, methane decomposes into hydrogen and surface carbon The amount of CH, converted per pulse decreases drastically after the third pulse (this corresponds to about 2-3 molecules of CH< converted per Pt atom) indicating that the reaction stops when Pt is covered with (reactive) carbon CO2 is also concluded to dissociate under reaction conditions generating CO and adsorbed... 

The first step of the grafting process is probably the physisorption of Bu4Sn on the surface of the support (higher surface area), which then migrates from the support to the metal surface. Then, when the physisorbed complex interacts with the surface Pt atoms covered by hydrogen atoms (Pts-H ), there is... 

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