Platinum group metals surface structure

CO adsorption on electrochemically facetted (Clavilier), 135 Hamm etal, 134 surfaces (Hamm etal), 134 Platinum group metals in aqueous solutions, 132 and Frumkin s work on the potential of zero charge thereon, 129 Iwasita and Xia, 133 and non-aqueous solutions, 137 potentials of zero charge, 132, 137 preparation of platinum single crystals (Iwasita and Xia), 133 Platinum-DMSO interfaces, double layer structure, 141 Polarization time, 328 Polarons, 310... 

A very important characteristic of surface constitution for any metal is the position of its PZC. Table 10.1 reports values for the PZC for a number of metals. We can see that these values vary within rather wide limits. An important difference between platinum group metals and most other metals is the ability of the latter upon anodic polarization to form relatively thick superhcial oxide or salt layers. Owing to their great practical value, these layers are considered in more detail in Section 16.3. For investigations of the structure and of properties of platinum and other electrodes, many nonelectrochemical methods are also widely applied, which is discussed in more detail in Chapter 27. 

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)... 

The next question is Where do supported metal catalysts fit into this pattern of co-ordination numbers Most platinum group metal catalysts can be prepared in supported forms in which the dispersion (defined as the % of metal atoms exposed at the surface of the particles) approaches 100%. While there may be good grounds for doubting the accuracy of calculations of dispersions, depending as they do on arbitrary assumptions about particle shapes,14 adsorption ratios, etc., it is certain that dispersions greater than, say, 50% are frequently obtained. Table 1 shows how the dispersion relates to particle diameter and to number of atoms for a simple octahedral structure. From this we see that 50% dispersion corresponds to a particle diameter of... 

Earlier, it was difficult to produce a clean surface and to characterize its surface structure. However, with the development of electronic industry, techniques have been developed to produce clean surface with well-defined properties. It has been possible to investigate catalytic oxidation on metal surface in depth. Example of dynamic instability at gas-liquid interface is provided by such studies. Studies on chemical oscillations during oxidation of CO over surface of platinum group metals have attracted considerable interest [62-68]. 

Attempts have been made to mimic proposed steps in catalysis at a platinum metal surface using well-characterized binuclear platinum complexes. A series of such complexes, stabilized by bridging bis(diphenyl-phosphino)methane ligands, has been prepared and structurally characterized. Included are diplati-num(I) complexes with Pt-Pt bonds, complexes with bridging hydride, carbonyl or methylene groups, and binuclear methylplatinum complexes. Reactions of these complexes have been studied and new binuclear oxidative addition and reductive elimination reactions, and a new catalyst for the water gas shift reaction have been discovered. 

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