Platinum crystal

The majority of crystallites observed were 3 or 4 nm In size. In Figure 3, a bar graph Illustrates the size range distribution and a comparison of mass variation for the 3 and 4 nm crystallite sizes. Although only thirty analyses were oiade, overall visual analysis confirmed the presence of hundreds of 3 to 4 nm platinum crystals with negligible numbers less or greater than these dimensions. It appears that slight variations In crystallite diameter and thickness have resulted In a fairly uniform number of platinum atoms per crystallite for the majority of the crystallites analyzed. In order to normalize count rates, the decrease In the field emission Intensity was taken Into account. 

Gdowski GE, Farr JA, Madix RJ. 1983. Reactive scattering of small molecules from platinum crystal surfaces D2CO, CH3OH, HCOOH and the nonanomalous kinetics of hydrogen atom recombination. Surf Sci 127 541. 

Ellis, J.W., Harrison, K.N., Hoye, P.A.T., Orpen, A.G., Pringle, P.G., and Smith, M.B., Water-soluble tris(hydroxymethyl)phosphine complexes with nickel, palladium, and platinum. Crystal structure of Pd P(CH2OH)3 4].cntdot.CH3, Inorg. Chem., 31, 3026, 1992. 

The surface lattice plane of Pt (lOO)-(l x 1), created by cleavage along the close-packed cubic lattice plane (100) of platinum crystals, transforms into the... 

Fig. 6-1. TVo-dimensional atomic structure on the (100) plane of platinum crystals (1x1) = cubic close-packed surface plane identical with the (100) plane (5 x 20) = hexagonal dose-packed surface plane reconstructed finm the original (100) plane. [From Kolb, 1993.]...

The reconstructed surface (5 x 20) of platinum crystals contains as many atoms as 1.2 times the original surface (1 x 1) atoms, and hence the transformation of surface lattice in the reverse direction from (5 x 20) to (1 x 1) forces the excess siuface atoms to cohere in a striped pattern on the un-reconstructed (1 x 1) surface. 

Fig. S-2. Activation energy both ibr reconstruction of the surface (100) plane of platinum crystals in vacuum and for im-reconstruction of the reconstructed surface due to adsorption of C 0 (1x1) (5x20) is surface lattice transformation (reconstruction and un-reconstruc-tion). 6 = adsorption coverage. [From Ertl, 1985.]...

The zeolite provides the environment for shape selective chemistry and is also a high surface area support on which to disperse platinum in a relatively confined environment. The small platinum crystals within the zeolite channels and the orientation effect of the channel window are responsible for the high efficiency of the Pt-KL catalyst to convert linear paraffin to aromatics. Zeolite KL also provides an electron rich environment to enhance stronger platinum-substrate interaction via stronger platinum-support interaction. A review on the subject can be found in the article written by Meriasdeau and Naccache [85]. 

We believe that the problems connected to the metastability/instability of the networks of steps deserve further experimental investigation, as well. If e is positive, as we think should be the case for gold and platinum crystals, one should be able, starting from the metastable network of steps, to observe a nucleation of arrays of parallel steps connected under a ridge. Probably this could be observed in practice, e.g. in STM experiments, only for sufficiently high temperatures and in very pure samples. 

Cyclohexane forms a (9 X 9) surface structure on the Ag(l 11) crystal face and a (j j) surface structure on the Pt(l 11) crystal face at around 200 K. This latter surface structure corresponds to the (001) surface orientation of the monoclinic bulk crystal structure of the molecule. On heating the platinum crystal face to 450 K a ( ) surface structure forms that is identical to the surface structure formed by cyclohexene monolayers at the same temperature. It appears that cyclohexane dehydrogenates at elevated temperatures on platinum to form the same species or that of cyclohexene. 

A. The Atomic Surface Structure of the Clean (111) Platinum Crystal Face. 8... 

D. Hydrocarbon Reactions on Platinum Crystal Surfaces at High Pressures... 

VI, Active Sites for C H, H-H, and C-C Bond Breaking on Platinum Crystal... 

Platinum crystal surfaces that were prepared in the zones indicated by the arrows at the sides of the triangle are thermally unstable. These surfaces, on heating, will rearrange to yield the two surfaces that appear at the end of the arrows. There is reason to believe that the thermal stability exhibited by various low and high Miller index platinum surfaces is the same for other fee metals. There are, of course, differences expected for surfaces of bcc solids or for surfaces of solids with other crystal structures. 

Fig. 6. A stereographic triangle of a platinum crystal depicting the various high Miller index surfaces of platinum that were studied.

This and similar instruments (3,4) that allow one to study reaction rates and product distributions on small-area crystal and catalyst surfaces have been used in our studies of the mechanism of heterogeneous catalysis and the nature of active sites. These studies, which concentrated primarily on hydrocarbon reaction as catalyzed by platinum crystal surfaces, will be reviewed in the next section. 

B. Dehydrogenation and Hydrogenolysis of Cyclohexane on Platinum Crystal Surfaces at Low Pressures (< 10-4 Torr)... 

Initial Turnover Numbers on Platinum Crystal Surfaces for Various Hytlrocarbon Reactions at Hit/h and Low Pressures ... 

Studies to correlate the reactivity and the surface structure and composition of platinum surfaces indicate that the active platinum crystal surface must be heterogeneous. The heterogeneity involves the presence of various atomic sites that are distinguishable by their number of nearest neighbors (atoms in terraces, steps, and kinks), and also variation in surface chemical composition. A model that depicts the active platinum surface is shown schematically in Fig. 28. Part of the surface is covered with a partially de-... 

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