Sphere Formation Mechanism

The following outer-sphere oxidation mechanism is favoured over the radical-cation formation which is a feature of similar oxidations by Mn(III) acetate (p. 375). With toluene... 

Cao, S.W. and Zhu, Y.J. (2009) Iron oxide hollow spheres microwave-hydrothermal ionic liquid preparation, formation mechanism, crystal phase and morphology control and properties. Acta Materialia, 57 (7), 2154-2165. 

It is no wonder that the particles are spherical but crystalline, if one considers the formation mechanism. The rather smooth surface of the spherical magnetite may be due to the rapid contact recrystallization of the constituent primary particles (5), forming the rigid polycrystalline structure. Flowever, it must be noted that polycrystalline spheres are also prepared by normal deposition of monomeric solute, as shown in the formation of the uniform spherical polycrystalline particles of metal sulfides in Chapters 3.1-3.3. Thus, while we may be able to predict the final particle shape and structure from the formation mechanism, it is risky to conclude the formation mechanism only from characterization of the product. As a rule, scrupulous analyses are needed for concluding the growth mechanism in a particle system. 

The rate-controlling step in reductive dissolution of oxides is surface chemical reaction control. The dissolution process involves a series of ligand-substitution and electron-transfer reactions. Two general mechanisms for electron transfer between metal ion complexes and organic compounds have been proposed (Stone, 1986) inner-sphere and outer-sphere. Both mechanisms involve the formation of a precursor complex, electron transfer with the complex, and subsequent breakdown of the successor complex (Stone, 1986). In the inner-sphere mechanism, the reductant... 

For non-electrophilic strong oxidants, the reaction with an alkane typically follows an outer-sphere ET mechanism. Photoexcited aromatic compounds are among the most powerful outer-sphere oxidants (e.g., the oxidation potential of the excited singlet state of 1,2,4,5-tetracyanobenzene (TCB) is 3.44 V relative to the SCE) [14, 15]. Photoexcited TCB (TCB ) can generate radical cations even from straight-chain alkanes through an SET oxidation. The reaction involves formation of ion-radical pairs between the alkane radical cation and the reduced oxidant (Eq. 5). Proton loss from the radical cation to the solvent (Eq. 6) is followed by aromatic substitution (Eq. 7) to form alkylaromatic compounds. 

Figure 16.8 Schematic illustration of the formation mechanisms for mesoporous Ti02 hollow and solid spheres (from [73]).

This raised the question as to why the first step (Eq. (154)) is ratedetermining for [MoiCNig] , whereas for metal ions such as Fe, , and NpOj the second step, Cr(V) Cr(IV), is rate-determining. In the case of cyanometalates this may be explained by way of an inner-sphere transfer mechanism in which formation of Cr(IV) (Eq. (158)) is rate-determining, the reaction sequence being... 

Synthetic polyelectrolytes (PEs) play a very important role in science, technique and medicine at present. At the same time constant broadening of these polymers application spheres is observed. As a consequence we observe the growth of requirements for creation of novel polyelectrolytes - polymers and copolymers of prescribed chemical and stereochemical structure and molecular mass. This fact stimulates investigations of synthesis and formation mechanism of this class of polymer compounds. It is also obvious that the most simple and convenient methods of PEs preparation are the reactions of ionizing monomers radical polymerization and copolymerization. 

Hollow silicon carbide (SiC) spheres have been synthesized by a microwave heating and carbothermal reduction method with carbon spheres as template and fly ash (a solid waste from coal-fired power plant) as silica source. X-ray diffraction and scanning electron microscope were employed to characterize the morphology, structure of the products. The results show that hollow spheres prepared at 1300 "C under argon atmosphere have a hollow core and SiC shell structure. The shell of a hollow SiC sphere is composed of a lot of irregular SiC nanowires with 5-20 pm in length and 50-500 nm in diameter which belongs to the p-SiC. Moreover, the formation mechanism of the hollow SiC spheres is also discussed. 

The formation and size of the colloidal spheres depend on the molecular structures and preparation conditions such as the initial concentration of the polymer in THF, the water content in the medium, and the water-dropping rate. As the polymers studied are of different types and have polydispersity in the molecular weight and DF, it is difficult to give a quantitative relationship to correlate the polymer structures with the colloid size and formation details. As a general tendency, CWC decreases as the molecular weight and hydrophobicity increase, and the colloid size increases as the hydrophobicity increases. The influence of the polymer structure can be better understood after discussing the colloid formation mechanism in Section 5.3.3. The effect of the preparation conditions on the colloid formation and size is discussed herein by using PEAPE (DF = 49.9%) as a typical example. 

Replacement of chloride or bromide in tra 5 -[Pt(oxalate)2X2] by iodide proceeds by reduction to [Pt"(oxalate)2] , followed by oxidative addition to give the [Pt (oxalate)2la] product. Reaction of [Pt(SCN)g] with bases similarly has as its first step formation of [Pt"(SCN)4] . The replacement of one bromide by chloride in /ra/z5 -[Pt(CN)4Br2] and /m 5-[Pt(N02)4Br2] is catalysed by [Pt(CN)4] or [Pt(N02)4] . The reactions follow a third-order rate law the mechanism is the usual inner-sphere redox mechanism of substitution. In the cyanide case there is an additional term independent of chloride concentration in the rate law, indicating a solvent-assisted path. Closely related to these systems is the oxidative addition of bromine to [Pt(CN)4] to give rm/2j-[Pt(CN)4Br2] , which has been studied in acid solution by stopped-flow techniques. Here initial fast production of tranj -[Pt(CN)4(OH2)Br] is followed by slow displacement of water by bromide, which explains the marked catalytic effect of added bromide on the overall reaction. ... 

Munaiah, Y., B. G. Sundara Raj, T. Prem Kumar, and P. Ragupathy. 2013. Facile synthesis of hollow sphere amorphous MnOj The formation mechanism, morphology and effect of a bivalent cation-containing electrolyte on its supercapacitive behavior. Journal of Materials Chemistry A 1 4300-4306. 

---