Platinum particle shape

The reduction of transition metal salts in solution is the most widely practiced method for synthesis of metal colloidal suspensions [7]. In the preparation process, polymer is often used in order to prevent the agglomeration of metal particles as well as to control their size. Ahmadi et al. [5] reported that the concentration of the capping polymer affects the shape of platinum particles obtained by salt reduction. This means that the addition of a... 

The study of shape and crystal structure of small metallic particles is of prime importance in modern catalysis science. The relation between reactivity and structure is still not well known. The main problem in studying small metallic particles is that conventional techniques fail in the manometer diameter range. However it is possible to overcome these difficulties by the application of non-conventional methods. It is the purpose of this paper to review some of these methods and to present some results on the characterization of gold and platinum particles. 

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).

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... 

The influence of catalyst preparation on the surface properties of fine carbon black-supported platinum particles of similar size (4nm) was investigated. Different adsorption behavior was indicated by varying shapes and fine structures of the vibrational modes of the dissociatively adsorbed atomic hydrogen on these nanoparticles (58b). 

EBL was used to fabricate uniform platinum nanoparticle arrays on Si02 (mean platinum particle diameter 30-1000 nm 52,53,106,107,398)), and evaporation techniques were used to prepare smaller particles and a continuous platinum film. The EBL microfabrication technique allows the production of model catalysts consisting of supported metal nanoparticles of uniform size, shape, and interparticle distance. Apart from allowing investigations of the effects of particle size, morphology, and surface structure (roughness) on catalytic activity and selectivity, these model catalysts are particularly well suited to examination of diffusion effects by systematic variations of the particle separation (interparticle distance) or particle size. The preparation process (see Fig. 1 in Reference 106)) is described only briefly here, and detailed descriptions can be found in References 53,106,399). 

The Pt NMR of small platinum particles on classic oxide supports show s that the clean-surface LDOS is largely independent of the support (sihca, alumina, and titania) and of the method of preparation (impregnation, ion exchange, and deposition of colloids). At a given resonance position, one always finds the same relaxation rate, independent of particle size or support. The shape of the spectrum is related to the sample dispersion. The same is true lor particles protected in fihiis of PVP. [However, samples prepared under conditions giving strong SMSIs behave differently 171)]... 

Experiments were conducted to measure the equilibrium shape of clean platinum particle and platinum exposed to various adgasses [4,9]. Some of the data are displayed in Figures 2 to 5. Figure 2 shows a scries of micrographs of a single platinum particle annealed in vacuum at 1473... 

In summary, wc conclude tluu particle shape control in supported metal catalysis is feasible. When a supported platinum catalyst is annealed under conditions where the platinum particles stay dean, the platinum particles assume a spherical shape with flat facets in the (100) and (111) directions. All of the particles in the catalyst assume the same general shape. In contrast, different shapes are observed when the catalyst is annealed in various udgasscs. Shi[ll] showed theoretically that only a few planes can be produced by eciuilibratiug the particles in simple adsorbates. However, w e speculate that one may be able to produce a wide distribution of panicle siiapes. with appropriate adsorbates,... 

Several metals have been used to catalyze redox reactions the most commonly studied are platinum and gold. There is debate concerning the exact catalytic nature - homogeneous or heterogeneous - of these catalysts. We summarize the results on the dependence of the reaction rate and TOP for redox reactions catalyzed by colloidal nanoparticles on (1) particle size, (2) local particle environment, (3) particle concentration, and (4) particle shape. 

Narayanan and El-Sayed investigated the effect of the electron-transfer reaction between ferricyanide and thiosulfate on the stability of particle shape [39,40]. The change in shape of the nanoparticle was time-dependent this change was in the form of a thermodynamic rounding of the particle into a sphere due to the dissolution of platinum atoms from the comers and edges of the tetrahedral and cubic platinum nanoparticles. Figure 18.3 demonstrates that the tetrahedral particle evolves into a distorted tetrahedral particle after one reaction cycle (Fig. 18.3a and b). For the cubic platinum nanoparticles (Fig. 18.3c), the rate of dissolution of platinum atoms was slower, and distorted cubic platinum nanoparticles (Fig. 18.3d) were dominant after two reaction cycles. 

Figure 5.66 (a) HR-TEM images of the cobalt-platinum particles used. The larger particles have a cubic shape (top right). The lower image shows a monolayer of cobalt-platinum particles between two gold electrodes (the inset shows the obtained Fourier transformed pattern) (b) /(V) characteristics of... 

Lee, L, Morales, R., Albiter, M. A., and Zaera, F. 2008. Synthesis of heterogeneous catalysts with well shaped platinum particles to control reaction selectivity. 

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