Reduction particle size effects

Kinoshita K. 1990. Particle size effects for oxygen reduction on highly dispersed platinum in acid electrolytes. J Electrochem Soc 137 845-848. 

Yano H, Inukai J, Uchida H, Watanabe M, Babu PK, Kobayashi T, Chung JH, Oldfield E, Wieckowski A. 2006b. Particle-size effect of nanoscale platinum catalysts in oxygen reduction reaction an electrochemical and Pt EC-NMR study. Phys Chem Chem Phys 8 4932-4939. 

Guerin S, Hayden BE, Pletcher D, RendaU ME, Suchsland J-P. 2006a. A combinatorial approach to the study of particle size effects on supported electrocatalysts oxygen reduction on gold. J Comb Chem 8 679-686. 

Kabbabi A, Gloaguen F, Andolfatto F, Durand R. 1994. Particle-size effect for oxygen reduction and methanol oxidation on Pt/C inside a proton-exchange membrane. J Electroanal Chem 373 251-254. 

There is evidence to show that the particle size of the filler also plays a significant role in flammability resistance. For example, below a certain particle size (about 1-2 pm), in many tests, including oxygen index, aluminum hydroxide shows enhanced fire-retarding performance,34 which may be associated with the rate of filler decomposition and/or with the formation of a more stable ash. However, it has been found that the particle size effect is absent, or less evident, in the cone calorimeter test.35 Similarly, particle size reduction has been shown to enhance fire retardancy in magnesium hydroxide-filled PP in this case, samples were characterized by the UL94 test.36 This raises the question as to whether further reductions in particle size to the nanoscale will lead to an additional increase in flammability performance, and perhaps enable filler overall levels to be significantly reduced. This aspect is considered in a later section. 

Electron microscopic examination of these catalysts showed that the powders that exhibited Type A behavior had many particles less than 10 nm in diameter, whereas the powders showing Type B characteristics had relatively few of these small particles. 2 similar size effect was reported for Ni(P) catalysts prepared by the reduction of nickel orthophosphate.93 When the catalyst was reduced at temperatures near 400°C, Type A behavior was observed, whereas Type B behavior occurred with catalysts prepared at higher reduction temperatures. Here, too, a particle size effect was proposed to explain these results. Lower temperature reduction gives fine clusters of Ni interspersed with the Ni(P), and it was considered that these small particles were more effective for... 

Correlation of the effect of particle size on the oxygen reduction reaction has been reviewed extensively by Kinoshita [4,5], Stonehart [8], and Mukerjee [9]. The general consensus, based on a large number of steady-state polarization measurements in several electrolytes, is that ORR exhibits a strong particle size effect in the... 

The improvements in the activation polarization defined as either mass-specific activity or site-specific activity (activity/number of specific crystal planes on the surface) were reported, especially for the kinetically difficult ORR. Wealth of prior data on both ORR as well as direct methanol oxidation (both multielectron reduction and oxidation processes) showed clear particle-size effects. Bulk of these... 

Designing alloy electrocatalysts by the so-called ad-atom method, and by alloy sputtering for oxidation of CH3OH and CO, and for CO tolerance in H2 oxidation, respectively, as well as for O2 reduction are discussed. Many years of experience are summarized and collaborations with other groups are highlighted. The particle size effect in electrocatalysis by small particle electrodes, and the effect of corrosion of carbon-black supported nanoparticles on the electrocatalytic activity are also discussed. All these factors, as well as catalyst lifetimes, are very important in fuel cell performance and in the final cost estimates for the practical fuel cell applications. 

Another interesting aspect of particle size effect is related to the density of photons absorbed by semiconductor particles, in comparison with the density of particles in a solution. Considering two solutions containing colloids of different sizes, in one case for instance 3-nm and in the other 4-pm particles, then many more particles are present in the solution of 3-nm colloid than in that of 4-pm colloid, provided that the total concentration of the semiconductor material is identical in both solutions. The two solutions differ only insofar as the same material is distributed over a small density of large particles (4 pm) or over a high density of small particles (3 nm). As can be easily calculated, a time interval of 5.4 ms exists between the absorption events of two photons in one individual 3-nm particle for a photon flux of 4 x lO cm s , assuming that all photons are absorbed in the colloidal solution [62], In the case of the 4-pm particles, the time interval between two absorption events is only about 20 ps for the same photon flux, i.e. it is shorter by a factor of 10, compared with the time interval estimated for the 3 nm particle. This difference can be important for reactions where two or more electrons are involved, typically in many oxidation and reduction reactions with organic molecules. 

However, if the Ir particle size is increased by increasing the calcination temperature but keeping the reduction temperature constant then the optical yield and reaction rate fall. The particle size effects have been confirmed by TEM. The best optical yields over Ir/SiOh (66.5% R, 33.5% S) are nearly as good as the best yields over Ir/CaCOj (69% R, M% S). 

With regard to particle size effects contradictory results have been published (11,12). Assuming that the filler particles change the properties of the resin In the Immediate neighbourhood (13), an Influence of particle size should be expected. A reduction of particle radius from 25 to 1 means a 1600 fold number of particles and a 300 fold surface area. Even if actually Important aspects like agglomeration of filler particles and filler/matrix adhesion are neglected it seems almost impossible to deduce effective mechanical properties from constituent properties without knowing more about bondary layers. 

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