Platinum reconstruction

The experimental investigation was performed by depositing copper films on the (100) -surface of a platinum single crystal. It was found that the reconstruction of the Pt surface was lifted upon Cu adsorption. The system was then heated to different temperatures and the formation of different ordered surface alloys was evidenced by... 

Figure 2.14. Reaction mechanisms for the oxidation of CO on the (110) surface of platinum (left) and on the reconstructed surface (right). See text for explanation.

We have also discussed two applications of the extended ab initio atomistic thermodynamics approach. The first example is the potential-induced lifting of Au(lOO) surface reconstmction, where we have focused on the electronic effects arising from the potential-dependent surface excess charge. We have found that these are already sufficient to cause lifting of the Au(lOO) surface reconstruction, but contributions from specific electrolyte ion adsorption might also play a role. With the second example, the electro-oxidation of a platinum electrode, we have discussed a system where specific adsorption on the surface changes the surface structure and composition as the electrode potential is varied. 

Van Hove MA, Koestner RJ, Stair PC, Biberian IP Kesmodel LL, Bartos I, Somorjai GA. 1981. The surface reconstructions of the (100) crystal faces of iridium, platinum and gold, 1. Experimental-observations and possible structural models. Surf Sci 103 189-217. 

Figure 1.3 The arrangement of atoms on the reconstructed (100) crystal faces of gold, iridium and platinum. Side and top views are illustrated. From G.A. Somorjai. Chemistry in Two Dimensions, Cornell University Press, London, 1981, p. 145. Used by permission of Cornell...

Tlie low index svirfaces of platinum single crystals were examined with cyclic voltammetry. As mentioned in Chapter 2, single crystal surfaces are reconstructed by Pt-OH formation and reduction at a higher potential. Therefore, it was impossible to clean the surface thoroughly by potential steps without disturbing the surface structure. 

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.]...

Some metal surfaces reconstruct either in the clean state or in the presence of adsorbed gases. Platinum, iridium, and gold (100) surfaces, which have square symmetry, all reconstruct to hexagonal close-packed (111) surfaces... 

Antitumor drugs cisplatin as, history, 37 175-179 platinum compounds future studies, 37 206-208 resistance to, 37 192-193 second-generation, 37 178 Antiviral agents, 36 37-38 AOR, see Aldehyde oxidoreductase Aphanothece sacrum, ferredoxins, amino acid sequence, 38 225-227 Apo-calcylin, 46 455 Apo-caldodulin, 46 449-450 Apoenzyme, 22 424 Apoferritin biosynthesis, 36 457 cystalline iron core, 36 423 Fe(III)distribution, 36 458-459 Fe(II) sequestration, 36 463-464 ferroxidase centers, 36 457-458 iron core reconstruction in shell, 36 457 mineralization, 36 25 Mdssbauer spectra, 36 459-460 optical absorbance spectra, 36 418-419 subunit conformation and quaternary structure, 36 470-471... 

Benzene forms a rotationally disordered structure on the reconstructed (100) platinum surface. However, the work function changes with increasing surface coverage are similar to that of benzene on the (111) crystal face. 

Numerous studies with low-energy electron diffraction (LEED) revealed that most of the clean surfaces of the platinum group metals exhibit an atomic arrangement that is identical to that expected from an undistorted termination of the bulk. Variations of the vertical lattice spacings between the topmost atomic layers are very small, if present at all (66). Exceptions are, however, found with the (100) and (110) planes of Ir and Pt. The clean and thermodynamically stable structures of the Pt(100) (67-69) and Ir(100) (70, 71) surfaces were found to reconstruct and to exhibit 5 x 1-LEED patterns. A plausible explanation (72) is that in these cases the topmost atomic layer forms a hexagonal arrangement, similar to that within the (111)... 

In summary, a variety of LEED patterns have been observed for oxygen adsorption on the platinum metals. However, their interpretation is complicated by the uncertainty in the oxygen coverage. A second complication is the uncertainty introduced by oxygen-induced surface reconstruction. The stability of 22 high index Miller planes upon exposure to oxygen has recently been investigated by Blakely and Somorjai (160). 

Schmidt et al. (139) postulated that in the presence of excess oxygen, platinum was transported as volatile oxides through the gas phase and boundary layer. This mechanism could not adequately explain the reconstruction observed far into the excess ammonia regime. It was suggested that under these and other conditions, other volatile platinum species formed. Moreover, these species might decompose by reaction in the boundary layer, leading eventually to the platinum replating itself. 

Now possibilities of the MC simulation allow to consider complex surface processes that include various stages with adsorption and desorption, surface reaction and diffusion, surface reconstruction, and new phase formation, etc. Such investigations become today as natural analysis of the experimental studying. The following papers [282-285] can be referred to as corresponding examples. Authors consider the application of the lattice models to the analysis of oscillatory and autowave processes in the reaction of carbon monoxide oxidation over platinum and palladium surfaces, the turbulent and stripes wave patterns caused by limited COads diffusion during CO oxidation over Pd(110) surface, catalytic processes over supported nanoparticles as well as crystallization during catalytic processes. 

This unusual reaction has not been reported previously, however, a number of studies have demonstrated the formation of analogous structures when platinum/alumina specimens were heated to temperatures in excess of 8009C. Baker and co-workers [23] using the CAEM technique to study the sintering characteristics of platinum on alumina in oxygen observed spectacular transitions in the appearance of the specimens at temperatures in excess of 800°C. The metal particles initially spread on the alumina to from diffuse islands and then quite suddenly reconstructed to produce well defined dense shapes. Sprys and Mencik [24] found the same effect when platinum/alumina specimens were subjected to intense electron beams within the electron microscope and characterized the structures as the intermetallic compound PtaAl from electron diffraction analysis. 

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