Particle-size effect

Magnetic silica aerogels generated from both pure ferric nitrate and its mixture with an iron complex exhibit low saturation magnetization values of 14 and 8.5 emu g , respectively [6], in comparison with bulk maghemite values of 74-76 emu g . The low magnetization values suggest lack of crystal order. These materials may still be useful in catalysis and other applications. 

Saturation magnetization has also been observed to vary with the coating. Ms of ultrasmall iron oxide nanoparticles increases with silica coating [29], while very large coercivity fields at 5K were measured for nanoparticles obtained in a gel due to a larger effective anisotropy coming from the crystal disorder. 

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As on previous occasions, the reader is reminded that no very extensive coverage of the literature is possible in a textbook such as this one and that the emphasis is primarily on principles and their illustration. Several monographs are available for more detailed information (see General References). Useful reviews are on future directions and anunonia synthesis [2], surface analysis [3], surface mechanisms [4], dynamics of surface reactions [5], single-crystal versus actual catalysts [6], oscillatory kinetics [7], fractals [8], surface electrochemistry [9], particle size effects [10], and supported metals [11, 12]. 

Figure 30. Particle size effects on minimum fluidization and bubbling velocities.

P-10 gas, 45, 219 Pair production, 290 Palladium, determination by x-ray emission spectrography, 328 Particle size, effect of variations of, in mineral analysis, 200 Philips Autrometer, 252-256, 280 Philips Electronics gas analyzer, 135 Philips Electronics improved Coolidge tubes, 248, 252, 253... 

Particle Size Effects in Detonation. The raison d etre for particle size effects in detonations was clearly stated many years ago in the following quotation from Eyring (Ref 1) ... 

For the more sensitive expls like PETN, the influence of particle size effects on D are much less pronounced that for TNT (unpublished work by the writer and Ref 2)... 

A particle size effect has been detected by Chou and Olson [486] in the isothermal decomposition of isothiocyanatopentammine cobalt(III) perchlorate. Below a = 0.09, the larger crystals decompose relatively more rapidly than the smaller, whereas for a > 0.09, the reverse is true. This behaviour was attributed to enhanced nucleation in the larger particles due to strain, but this favourable factor was later offset by the inhibiting influence of the product ammonia which accumulated in the larger crystals. 

Gerischer H, Liibke M (1986) A particle size effect in the sensitization of TiOa electrodes by a CdS deposit. J Electroanal Chem 204 225-227... 

Co step. Previously it had been observed for the C0/AI2O3 O)/ Ni/Si02 (10), and Fe/Si02 (11, 12) systems that highly dispersed oxide species (small particles) were more difficult to reduce than their corresponding bulk or bulk-like oxides. Nucleation, interaction with the support, and reaction with the support were given as possible explanations for these differences. Further experiments are needed to determine the reasons for the observed particle size effect on the Co/Si02 system. 

Structure sensitivity is not a single phenomenon but can often be regarded as a set of independent, but interlinked mechanisms. At the very beginning it is important to realize that the term particle size effect (PSE) not only refers to the size of active component particles but moreover comprises effects deriving from peculiarities in their morphology, that is, their shape and structure. 

The liquid-phase oxidation of glycerol was carried out by using carbon-supported gold particles of different sizes (2.7 2 nm) which were prepared by a colloidal route [120]. Indeed, a particle-size effect was observed because the selectivity to glyceric acid was increased to 75% with smaller particle sizes (4)ptmimn = 3.7 nm). 

As was demonstrated in the preceding sections, structure-sensitivity phenomena are mostly confined to particle size regimes smaller than 3-4 nm. A process of industrial relevance was investigated by de Jong et al. [127] in their study on cobalt particle size effects in the Fischer-Tropsch reaction. Earlier works noted distinct drop in activity for Co particles smaller than lOnm and ascribed this phenomenon to either a partial oxide or carbide formation which should be enhanced for particles in this size regime [128-139]. In order to avoid similar effects, de Jong used... 

An apparent particle size effect for the hydrodechlorination of 2-chlorophenol and 2,4-dichlorophenol was observed by Keane et al. [147], Investigating silica supported Ni catalysts (derived from either nickel nitrate or nickel ethane-diamine) with particles in the size range between 1.4 and 16.8 nm, enhanced rates for both reactions were observed with increased size over the full range (Figure 13). As electronic factors can be ruled out in this dimension, the observed behavior is traced back to some sort of ensemble effect, known from CFC transformations over Pd/Al203... 

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. 

Cherstiouk OV, Simonov PA, Savinova ER. 2003a. Model approach to evaluate particle size effects in electrocatalysis Preparation and properties of Pt nanoparticles supported on GC and HOPG. Electrochim Acta 48 3851-3860. 

Frelink T, Visscher W, vanVeen JAR. 1995. Particle-size effect of carbon-supported platinum catalysts for the electrooxidation of methanol. J Electroanal Chem 382 65-72. 

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. 

Maillard F, Martin M, Gloaguen F, Leger JM. 2002. Oxygen electroreduction on carbon-sup-ported platinum catalysts. Particle-size effect on the tolerance to methanol competition. Electrochim Acta 47 3431-3440. 

Maillard F, Savinova ER, Stimming U. 2007b. CO monolayer oxidation on Pt nanoparticles Further insights into the particle size effects. J Electroanal Chem 599 221-232. 

Mayrhofer KJJ, Blizanac BB, Arenz M, Stamenkovic VR, Ross PN, Markovic NM. 2005b. The impact of geometric and surface electronic properties of Pt-catalysts on the particle size effect in electrocatalysis. J Phys Chem B 109 14433-14440. 

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