H-Au/Pt systems

FTM and atom-probe studies of thin films of Ni, Au, Pt, a-Ge H, a-Si H and WO3, etc., on various substrates were reported by Krishna-swamy et a/.81 First, field ion tips each with a field evaporated surface were prepared. They are placed in an MRC model 8502 r.f. sputtering system. Tips were mounted on a recessed and shielded structure behind the sputtering surface which is bored with small holes about 1 to 2 mm in diameter. The very end of the tips came out of the holes to approximately the same level of the sputtering surface. Films were sputtered at about 20 mTorr Ar at an r.f. power of about 50 W. Thickness of a deposited thin film was controlled by both the r.f. power and the deposition time. Film thickness in the range of a few hundred to a few thousand A were studied. These tips were then imaged with Ne in the field ion microscope, or analyzed in the flight-time-focused ToF atom-probe. 

Stability at lower temperatures is easier to achieve. Some examples are 350 h for an Au/ZnO catalyst and 500 h for an Au-Pt/ZnO catalyst, at 353 K, for selective catal dic removal of CO by the oxidation method in a hydrogen-rich system [191]. 

Pd-h Ru/Ru(0001), Ag-h Pt/Pt(lll), and Au + Pt/Au(lll) shown in Figure 12.17 are typical examples of this class of Type I surface alloys. It should be noted that this direction of size mismatch seems to be a prerequisite for the stabilization of Type I surface alloys that is, we are not aware of any documented Type I surface alloy where the guest atom has a smaller atomic radius than the host atom. Some experimentally observable systems where smaller guest atoms are blended into... 

The metal catalysed hydroboration and diboration of alkenes and alkynes (addition of H-B and B-B bonds, respectively) gives rise to alkyl- or alkenyl-boronate or diboronate esters, which are important intermediates for further catalytic transformations, or can be converted to useful organic compounds by established stoichiometric methodologies. The iyn-diboration of alkynes catalysed by Pt phosphine complexes is well-established [58]. However, in alkene diborations, challenging problems of chemo- and stereo-selectivity control stiU need to be solved, with the most successful current systems being based on Pt, Rh and An complexes [59-61]. There have been some recent advances in the area by using NHC complexes of Ir, Pd, Pt, Cu, Ag and Au as catalysts under mild conditions, which present important advantages in terms of activity and selectivity over the established catalysts. 

Diboration of terminal alkenes has also been studied with other d " metals (Fig. 2.12) including the Ag and Au complexes 75-77 and the Pt" complexes 78-79. Styrene is diborylated with 100% selectivity and good conversions in THF (46% for 75 and 94% for 77 at 5 mol%, 60 h) using equimolecular amounts of (Bcat)j. The difference in activity between the Ag and Au complexes has been ascribed to the increased lability of the Ag-NHC bond, which may lead to catalyst decomposition under the reaction conditions, hi both catalytic systems it is believed that the active species involves only one coordinated NHC ligand. Complex 77 is less active than 74 and 75, possibly due to steric reasons. The enantioselectivity of 77 in the diboration of prochiral alkenes is very low [63]. 

Palladium hydride is not a stoichiometric chemical compound but simply a metal in which hydrogen is dissolved and stored in solid state, in space between Pd atoms of crystal lattice of the host metal. Relatively high solubility and mobility of H in the FCC (face-centered-cubic) Pd lattice made the Pd H system one of the most transparent, and hence most studied from microstructural, thermodynamic, and kinetic points of view. Over the century that followed many metal-hydrogen systems were investigated while those studies were driven mostly by scientific curiosity. Researchers were interested in the interaction of hydrogen molecule with metal surfaces adsorption and diffusion into metals. Many reports on absorption of in Ni, Fe, Ni, Co, Cu, Pd, Pt, Rh, Pd-Pt, Pd-Rh, Mo-Fe, Ag-Cu, Au-Cu, Cu-Ni, Cu-Pt, Cu-Sn, and lack of absorption in Ag, Au, Cd, Pb, Sn, Zn came from Sieverts et al. [30-33]. 

Fig. 1.8. Dalton s atoms and the electronic states in an atom. A, a chart in Dalton s A New System of Chemical Philosophy, published in 1808. In modern symbols, these atoms are 1, H 2, N 3, C 4, O 5, P 6, S 7, Mg 8, Ca 9, Na 10, K 11, Sr 12, Ba 13, Fe 14, Zn 15, Cu 16, Pb 17, Ag 18, Pt 19, Au 20, Hg. The major modem modification to Dalton s theory is that the atoms are divisible. The contour maps in B represent typical electronic states in atoms. The outermost contour on each map represents a density of 10 A The successive contours rcpre.sent an increase of a factor of 2. The regions with dashed-curve contours have opposite phases in the wavefunction from those with solid-curve contours.

As for a third system in the early stages of dissociative H2 chemisorption, it is thought that H2 approaches perpendicular to the surface, as in 34. Consider Ni(lll), related to the Pt(lll) surface discussed earlier. Figure 18 shows a series of three snapshots of the total DOS and its ou (H2) projection.30 These are computed at separations of 3-0, 2.5, and 2.0 A from the nearest H of H2 to the Ni atom directly below it. The [Pg.101]

The deposition of polonium on metal wires gives rise to a useful a-source. Tips of metal wires having a length 10 mm and a diameter of 0.2 mm were utilized. They were made of Al, Ni, Pd, Pt or Au. Each was immersed in 100 j,l of a solution containing Po (300 Bq ml ) for 15 h at 27°. Alpha particle emission was measured using a liquid scintillation system. There was an observed diminution in the a-pulse spectra for all of the wires except Al. This was attributed to the mutual diffusion between the wire metal and °Po. The °Po deposited on the Al wire had a tendency to be eluted with the liquid scintillator. This was attributed to physical absorption on the porous metal oxide layer on the Al wire and °Po. The Po deposited by the Al wire had a tendency to be eluted with the liquid scintillator. It was possible to prepare a Po -Al wire as a useful a-source by heating at 120° for 30 minutes. 

Sum and difference frequency generation at electrode suifaces [130] The investigation of vibrational and electronic properties of the electrochemical interface by using nonlinear optical techniques of visible-infrared sum (SFG) and difference (DFG) frequency generation constitutes an interesting aspect of the study of adsorbed species. The vibrational behavior of H-Pt(hkl) system, both in the underpotential and overpotential regime, adsorption of CN on Au(hkl) electrode (using DFG), electrode surface electronic properties were studied and measurements of the PZC were carried out [131]. 

Figure P.2 shows the relative abundance of the elements in the universe. It is seen that 99.9% of the elements in the universe are H and He the abundance scale being a quasi-logarithmic one indicates H to be 10 times more abundant than He. Figure P.2 (right) shows planets differentiation in our solar system in relation to abundance of atmosphere, silicates (products with Si) and other metals. Planets are made up of rare materials that include elements with affinity for oxygen (litophile) Si, Al, Ti, Cr, Mn, Fe alkaline elements alkaline metals (crystals) rare elements those with affinity for sulfur and oxygen (calcophile) Cu, Co, Ni, Zn, Pb, Sb, Mo, Fe and metal alloy of iron sidemphile) Fe, Ni, Pt, Ir, Os, Re, Au, Rh.

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