Platinum facets

MacDonald on the adsorption of chloride ions in passivation, 237 of CO on electrochemically facetted platinum, 135 of diols on mercury, 188 of neutral compounds on electrodes, 185 of perchlorate ions, copper and, 94 specific adsorption, anodic dissolution and, 256... 

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... 

Cyclic voltammetry is perhaps the most important and widely used technique within the field of analytical electrochemistry. With a theoretical standard hydrogen electrode at hand, one of the first interesting and challenging applications may be to try to use it to make theoretical cyclic voltammograms (CVs). In following, we set out to do this by attempting to calculate the CV for hydrogen adsorption on two different facets of platinum the (111) and the (100) facets. 

Figure 7.11 Field ion microscopy image of a platinum tip. As Pt has the fee structure, the fourfold symmetry of the picture implies that the center corresponds to a (100) facet. Dark areas near the four comers are (111) facets (from Muller [26]). 

The shape of the single crystal obtained by the method described above is a sphere with several flat facets as drawn in Fig. 2-6. Usually seven large facets, which are assigned to seven of possible eight (111) surfaces, are seen on the apex positions of a cube. Five small facets, which are assigned to five of possible six (100) surfaces, are also seen on the center of the faces of a cube. The missing (111) and (100) facets are supposed to be located where the shaft is attached. Figure 2—7 shows the relative positions of the three low index surfaces of platinum, which is a face-centered cubic lattice. 

The [111] plane the most closely packed (six-coordinated) surface of the fee structure and is very stable. The five-coordinated B5 structures may have [110] or [311] configuration (137). The four-coordinated [100] plane rearranges reversibly to a six-coordinated structure (138). Carbon-platinum interactions cause faceting of the [100] plane, giving rise to the formation of [211] or [311] structures (139). 

We have noted however that neither the interplanar spacing or packing density of the planes is a sufficient criterion of stability, for, in rock salt, copper and silver the more open 100 is the most stable facet whilst in gold and platinum the more closely packed 111 planes are the most stable. 

Faceted platinized platinum surfaces with preferred crystallographic orientation were prepared by square-wave potential-modulated DC electrodeposition [216]. 

A detailed comparison of the voltammet-ric behavior of platinum single-crystal and faceted electrodes was also given [218]. 

Lee et al. (61) measured the equilibrium shape of clean platinum by monitoring the changes in the shape of a series of micrometer-sized platinum droplets during annealing at I200°C in 10 7 Torr of oxygen. Consistent with the work of Schmidt, their results showed that the equilibrium particle shape is influenced by what gas is present. As shown in Fig. 4, at equilibrium, the clean particle shape is nearly spherical, with distinct (100) and (111) facets. The facets occupy only 16% of the surface. However, if the particle is contaminated with carbon, the particle is a cubooctahedron with large (111), (100), and (110) facets. 

Fig. 4. Scanning electron micrographs of the equilibrium shapes of platinum particles show that both the gas phase and impurity in the metal can influence equilibrium shape, (a) A clean Pt particle is nearly spherical with distinct (100) and (111) facets after treatment in IO 7Torr of oxygen at 1200°C. (b) A carbon-covered Pt particle is cubo-octahedral (61).

Other workers (165) used X-ray photoelectron spectroscopy (XPS) to examine the influence of ammonia oxidation on the surface composition of alloy gauzes. After several months on stream, the surface was covered by the same types of highly faceted structures noted by others. As illustrated in Fig. 14, XPS analysis provides evidence that the top microns, and in particular the top 100 A of the surface, were enriched in rhodium. This enrichment was attributed to the preferential volatilization of platinum oxide. The rhodium in the surface layers was present in the oxide form. Other probes confirm the enrichment of the surface in rhodium after ammonia oxidation (166). Rhodium enrichment has been noted by others (164, 167), and it has been postulated that in some cases it leads to catalyst deactivation (168). 

Fig. 26. Surface morphology of a platinum sample after 4 h of oxygen plasma treatment. The platinum surface is partially covered by faceted particles 5 pm in diameter (245).

Chromicyanides, manganicyanides, cobalticyanides, ruthenocyanides, and osmocyanides, are also known, similar in formulas to the ferro- and ferri-cyanides. On the other hand, nickel and platinum form double cyanides similar in formula to K2Pt(CN)4. The platinum salts are very beautiful, possessing the property of dichroism, i.e. of transmitting light different in colour from that which the crystals reflect moreover, only some of the facets of the crystals have this property. 

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