Platinum atomic surface structure

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

While low coordination number sites, steps, and kinks, are the active sites for bond breaking in platinum, the atomic terrace sites with larger coordination numbers may also become active sites with unique chemistry for other elements. It will perhaps become possible to identify the bond-breaking ability of various coordination number sites of a given metal in breaking H—H, C—H, C —C, 0=0, N=N, etc., chemical bonds. By varying the atomic surface structure, which would change the relative concentrations of the different coordination number surface sites, the product distribution in surface chemical reactions may be markedly varied. 

The correlation between the coverage of surface platinum atoms by bismuth adatoms (Ggi) and the measured rate of 1-phenylethanol oxidation was studied on unsupported platinum catalysts. An electrochemical method (cyclic voltammetry) was applied to determine G i and a good electric conductivity of the sample was necessary for the measurements. The usual chemisorption measurements have the disadvantage of possible surface restructuring of the bimetallic system at the pretreatment temperature. Another advantage of the electrochemical polarization method is that the same aqueous alkaline solution may be applied for the study of the surface structure of the catalyst and for the liquid phase oxidation of the alcohol substrate. 

Data was taken in the electron energy range of 10-200 eV, but little sensitivity to the organic adsorbate is found above 100 eV. The observed diffraction pattern arises from three equivalent 120° — rotated domains of (2 X 2) unit cells. The optimum agree "ent between calculated and experimental intensity data for the metastable acetylene structure is achieved for an atop site coordination. The molecule is located at a z-distance of 2.5 A from the underlying surface platinum atom. However, the best agreement is obtained if the molecule is moved toward a triangular site, where there is a platinum atom in the second layer, by 0.25 A, as shown in Fig. 7.2. 

In a recent study of the adsorption of acetylene on platinum single crystals by low energy electron diffraction [160], it has been shown that acetylene adsorbs on the (111) planes. These results show that, on a clean Pt (111) surface, acetylene adsorbs at a distance of 1.95 A above the topmost plane of platinum atoms, either in the C2 or, less likely, the Bl mode shown in Fig. 23. No evidence was found for adsorption in the A or A2 modes, which corresponds to a 7r-complex structure or for the B2 mode corresponding to a di-o-complex, although it was stated that such structures may be possible with a less stable overlayer which had been observed. 

LEED studies have revealed that the atoms in this platinum surface are in the positions expected from the projection of the X-ray unit cell to the surface (5). The diffraction pattern that is exhibited (Fig. 4) clearly indicates a sixfold rotational symmetry that is expected in such a surface. Calculations of surface structure from LEED beam intensities indicate that atoms are in those positions in the surface layer (with respect to the second layer) as indicated by the X-ray unit cell within 5% of the interlayer distance (6,7). 

We have been able to identify two types of structural features of platinum surfaces that influence the catalytic surface reactions (a) atomic steps and kinks, i.e., sites of low metal coordination number, and (b) carbonaceous overlayers, ordered or disordered. The surface reaction may be sensitive to both or just one of these structural features or it may be totally insensitive to the surface structure, The dehydrogenation of cyclohexane to cyclohexene appears to be a structure-insensitive reaction. It takes place even on the Pt(l 11) crystal face, which has a very low density of steps, and proceeds even in the presence of a disordered overlayer. The dehydrogenation of cyclohexene to benzene is very structure sensitive. It requires the presence of atomic steps [i.e., does not occur on the Pt(l 11) crystal face] and an ordered overlayer (it is poisoned by disorder). Others have found the dehydrogenation of cyclohexane to benzene to be structure insensitive (42, 43) on dispersed-metal catalysts. On our catalyst, surfaces that contain steps, this is also true, but on the Pt(lll) catalyst surface, benzene formation is much slower. Dispersed particles of any size will always contain many steplike atoms of low coordination, and therefore the reaction will display structure insensitivity. Based on our findings, we may write a mechanism for these reactions by identifying the sequence of reaction steps ... 

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-... 

As discussed earlier, it is now possible to make and study deposits of monosized, highly dispersed, transition metal clusters.(S) In this section we summarize results from the first measurements of the valence and core level photoemission spectra of mass selected, monodispersed platinum clusters. The samples are prepared by depositing single size clusters either on amorphous carbon or upon the natural silica layer of a silicon wafer. We allow the deposition to proceed until about 10 per cent of the surface in a 0.25 cm2 area is covered. For samples consisting of the platinum atom through the six atom duster, we have measured the evolution of the individual valence band electronic structure and the Pt 4f... 

---