Platinum crystal defects

Fig. VIII-2. Scanning tunneling microscopy images illustrating the capabilities of the technique (a) a 10-nm-square scan of a silicon(lll) crystal showing defects and terraces from Ref. 21 (b) the surface of an Ag-Au alloy electrode being electrochemically roughened at 0.2 V and 2 and 42 min after reaching 0.70 V (from Ref. 22) (c) an island of CO molecules on a platinum surface formed by sliding the molecules along the surface with the STM tip (from Ref. 41).

Table 2. Crystal Structures and Boron Coordination of Platinum Metal Borides WITH Isolated B Atoms (Owing to Defect Boron Sublattice)...

From the results of Pt(lll). water can be used as an oxs gen source for COad oxidation even on a smooth single crystal surface. This path is available in a dilute add as infrared measurements show that CO2 buildup starts at very low potential like 300 mV. Probably because the niunber of the sites for this reaction is limited, so is the reaction. When the add is concentrated, this path seems only available when CO is adsorbed on spedal sites (probably defects, kinks or edges) at a low potential such as 50 mV. On high area platinum, this path is more likely to occur because the density of those sites is higher. 

Fig. 3. LEED patterns and schematic representations of the surface configurations of platinum single-crystal surfaces, (a) Pt(Ill) containing less than 1012 defects/cm2, (b) Pt(557) face containing 2.5 x 1014 step atoms/cm2 with an average spacing between steps of 6 atoms, and (c) Pt(679) containing 2.3 x 10 4 step atoms/cm2 and 7 x 1014 kink atoms/cm2 with an average spacing between steps of 7 atoms and between kinks of 3 atoms.

Glusker JP, Traeblood KN (1985) Crystal Stmctnre Analysis. 2nd edition. Oxford Univ Press, Oxford, UK Gnutzmaim V, Vogel W (1990) Surface oxidation and reduction of small platinum particles observed by in situ X-ray diffraction. Z PhysikD (Atoms, Molecules, Clusters) 12 597-600 Greenwood NN (1970) Ionic Crystals Lattice Defects and Norrstoichiometry. Butterworths, London Guinier A (1963) X-ray Diffraction in Crystals, Imperfect Crystals, and Amorphous Bodies. W H Freeman, San Francisco... 

Most of the work in the previous sections of this chapter has dealt with mercury electrodes for the reasons discussed in Section 13.2.1. However, electrochemists are also interested in studying the interfacial structure of solids, because most electrochemical studies are carried out with solid electrodes (e.g., platinum or carbon). Such studies are difficult, because there are problems in reproducing a surface and in keeping it clean. Impurities in solution can diffuse to the electrode surface and adsorb, thereby significantly changing the interfacial properties. Moreover, the surfaces of solids, unlike those of mercury, are not atomically smooth, but have defects, such as dislocation lines, with a density of at least 10 to 10 cm. In comparison, a typical metal surface density has about 10 atoms cm. Especially important to the understanding of solid electrodes has been the use of so-called well-defined metal electrodes, that is, single crystal metals with very carefully prepared surfaces of known orientation (35). 

Table XIII summarizes the results of Chaudhri and Field [133]. The observations were made on freshly prepared crystals, 10 mm long and chosen to be free of macroscopic defects. Initiation was by heated platinum wires.

Fig. 4.9 X-ray topography to allow the visualisation of dislocations here, defects in a benzile crystal which was grown from solution. In the centre, one can recognise the loop of a platinum wire which suspended the seed crystal platelet in the solution. Many linear dislocations start at inclusions in the surface of...

Caution must be exercised in interpretation of the physical data for the tetracyanoplatinate complexes (as well as all other one-dimensional systems) because purity and morphology are extremely critical for one-dimensional systems. For example, a 1.00 x 0.01 x 0.01 mm perfect needle crystal of K2Pt(CN)4Xo.3 would contain — lx 10 parallel strands each of 3.5 x 10 collinear platinum atoms. Thus, purity (foreign impurities, end groups, and/or crystalline defects) levels of one part per million indicate that each strand averages more than three defects, which may drastically alter some (and in particular transport) measurements. Besides the intrinsic purity problem of one-dimensional systems, the physical properties of K2Pt(CN)4-Xo.3(H20)a are a strong function of hydration. Dehydration alters the crystal structure and thus properties of the complexes (78). Care must be maintained to ensure that dehydration is not caused by the measurement technique. For... 

For many years it has been well known that CO electrooxidation on platinum is a structure-sensitive reaction. Studies with singlecrystal electrodes have shown that the kinetic parameters depend not only on the surface composition of the catalyst but also on the symmetry of the surface and that the presence of steps and defects alters significandy the reaction rate. As a consequence, the surface structure of the nanoparticles should also affect the performance for the oxidation of CO. Understanding how the different variables affect CO oxidation on Pt nanoparticles dispersed on carbon requires the control of the platinum surface in a similar way as has been achieved for single-crystal electrodes. In this sense, the influence of the surface site distribution on CO oxidation using nanoparticles of well-defined... 

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