Alloys metal particle size effects

It has been already mentioned in passing that indications exist in the literature showing that the 3C isomerization can take place by formation of at least two different 3C complexes, having different activation energies of isomerization, different particle size effects, different responses to alloying, etc. (157, 195-198). The suggestions presented above offer a choice of different complexes for further speculations. However, a definitive description of isomerization mechanisms under different conditions (H2 pressure, temperature, etc.) and with different catalysts (pure metals, alloys, etc.) is not yet possible. 

Is it known that the rate of hydrogenolysis reactions are extremely sensitive to effects of alloying, surface contamination, poisoning, etc. Consequently, in all cases where supported metals are used there must be concern as to whether apparent particle size effects are due to structure sensitivity or to some minor contamination effect. In the few cases where clean single crystal surfaces have been used there is evidence of a structure effect.338 However, the maximum change in activity between different crystal faces seems to be about a factor of 10. For Ni single crystals the (100) surface is more active than the (111) surface. A similar conclusion has been reached for oriented Ni powder samples.339... 

Part V addresses the critical issues of particle size effects, alloying, spillover, classic and electrochemical promotion, and metal-support interactions on the catalytic and electrocatalytic performance of nanoparticles. Recent experimental evidence is presented on the functional similarities and operational differences of promotion, electrochemical promotion, and metal-support interactions. 

High surface area since a noble metal is used in a reaction at low temperature, a very high dispersion of the active metal is required, with a support that allows the stabilization of small metal particles. Nevertheless, there are also some structure-sensitive half-reactions (e.g. ORR) involved, that implies poorer kinetics at lower metal particles size, due to geometric effects. Additionally, the morphology of the metal particle also plays an important role in determining catalytic activity, as well as the formation of alloys between metals, and this is not always obtained by increasing the surface area. Practical applications will use a support of 150-250 m g" ... 

Continued efforts have heen carried out to determine the effect of heat treatment on the individual components of the fuel ceU catalyst. For a platinum-based catalyst, heat treatment has been known to have a significant impact on the metal particle size, particle size distribution, particle morphology, and dispersion on the support. A heat-treatment process could also be used to remove impurities present in the metaUic catalyst (Bezerra et al., 2007). These effects help to increase the ECSA of the catalytic metal and generally improve the performance of these types of catalysts. Figure 3.10 illustrates the results obtained by Chen et al. who plotted the heat-treatment temperature of Vulcan XC-72R carbon black with specific corrosion rate seen in platinum alloy cathode catalysts at 1 V in H3PO4 at 180°C. 

Surface heterogeneity may merely be a reflection of different types of chemisorption and chemisorption sites, as in the examples of Figs. XVIII-9 and XVIII-10. The presence of various crystal planes, as in powders, leads to heterogeneous adsorption behavior the effect may vary with particle size, as in the case of O2 on Pd [107]. Heterogeneity may be deliberate many catalysts consist of combinations of active surfaces, such as bimetallic alloys. In this last case, the surface properties may be intermediate between those of the pure metals (but one component may be in surface excess as with any solution) or they may be distinctly different. In this last case, one speaks of various effects ensemble, dilution, ligand, and kinetic (see Ref. 108 for details). 

Some of the reports are as follows. Mizukoshi et al. [31] reported ultrasound assisted reduction processes of Pt(IV) ions in the presence of anionic, cationic and non-ionic surfactant. They found that radicals formed from the reaction of the surfactants with primary radicals sonolysis of water and direct thermal decomposition of surfactants during collapsing of cavities contribute to reduction of metal ions. Fujimoto et al. [32] reported metal and alloy nanoparticles of Au, Pd and ft, and Mn02 prepared by reduction method in presence of surfactant and sonication environment. They found that surfactant shows stabilization of metal particles and has impact on narrow particle size distribution during sonication process. Abbas et al. [33] carried out the effects of different operational parameters in sodium chloride sonocrystallisation, namely temperature, ultrasonic power and concentration sodium. They found that the sonocrystallization is effective method for preparation of small NaCl crystals for pharmaceutical aerosol preparation. The crystal growth then occurs in supersaturated solution. Mersmann et al. (2001) [21] and Guo et al. [34] reported that the relative supersaturation in reactive crystallization is decisive for the crystal size and depends on the following factors. 

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