Topochemically reduced

Particle-Particle Interactions. Loss of strong acid content of aerosol particles can also occur because of reactions between co-collected acidic and basic particles impacted together on the collection surface. This phenomenon most frequently occurs as the result of interaction of coarse (>2.5 xm diameter), alkaline, soil-derived particles with fine (<2.5 xm diameter) acidic sulfate particles (66). Particle-particle interactions with net neutralization can be reduced in many cases by sampling with a virtual impactor or a cyclone to remove coarse particles, although this procedure does not prevent the effect if external mixtures of fine particles of different acid contents are sampled. In situ methods with shorter sampling times can be used such that these topochemical reactions are less likely to occur. 

The topochemical differences in the physical environment of dissolved and crystallized active centers explain easily the difference in reactivity ratios between the two phases. The gain in free energy arising from immediate crystallization of growing chain ends enhances the incorporation of trioxane into the crystalline copolymer. Simultaneous crystallization is considered an important driving force in copolymerization as well as in the homopolymerization of trioxane. On the other hand, dioxolane units do not fit the crystal lattice of polyoxymethylene and reduce the crystallinity of the polymer. This impedes the incorporation of dioxolane units into the crystalline copolymer. 

Both types of reactions involving the spillover of either H2 or 02 have been termed topochemical heterogeneous catalysis (62). Besides the catalyzed reduction of metal oxides either to metals or suboxides, the formation of new and specific reduced oxides, such as the well-known hydrogen bronzes of W, Mo, and V, have attracted considerable attention (7,66-68). In many cases the reduction of the corresponding oxides by spillover of H2 led to reduced compounds not otherwise obtainable (69). 

Jezierska et al. [67] applied the Kohonen neural network to select the most relevant descriptors. Here a Kohonen network is built with the transposed matrix, i.e., with the matrix where the roles of descriptors and molecules are exchanged. From the map of descriptors, 36 descriptors were selected. This number of descriptors was further reduced to six, five, four, or three descriptors. Statistical parameters of compared models are reported in Table 3 of [67]. It is evident from the table that the model built with four selected descriptors show comparable parameters to the model built with 36 descriptors. The selected descriptors belong to topostructural and topochemical classes. 

The essence of this phenomenon is that properties of defect clustered centers and kinetic features of the oxidation reactions depend upon stoichiometry of the surface layer. For oxides studied, surface reduction is a topochemical type process and proceeds via spreading of the reduced zone from the extended surface defects accompanied by a cations redistribution between the regular and the interstitial positions. Reoxidation as well as hydroxylation/carbonization causes shrinkage of this zone. 

Figure 2 Ca3Fc2MnOg (I) is reduced topochemically to Ca2Fe4y3M2/305 (II) (II) transforms to the biownmillerite (BM) structure on annealing in vacuum.

The ability of the system to undergo a topochemical reaction depends on several prerequisites the solid should not dissolve too rapidly (otherwise the reaction does not occur at the interface but in solution). This prerequisite is often fulfilled by sulfates of trivalent metals. If necessary, the rate of solution may be reduced by adding a sufficient quantity of an organic substance to the aqueous solution. The organic substance may be acetone, glycol, glycerol, dioxane and so forth, or in the case of hydroxide precipitations, it may be pyridine, mono-, di- or triethanolamine, morpholine and so forth (see [6]). 

Graphite reacts with alkali metals - potassium, cesium and rubidium - to form lamellar compounds with different stoichiometries. The most widely known intercalate is the potassium-graphite which has the stoichiometry of CgK. In this intercalate the space between the graphite layers is occupied by K atoms. CgK functions as a reducing agent in various reactions such as reduction of double bonds in a,fl-unsaturated ketones [19], carboxylic acids and Schiff bases alkylation of nitriles [20], esters and imines [21] reductive cleavage of carbon-sulfur bonds in vinylic and allylic sulfones [22]. The detailed reaction mechanism of CgK is not known, and the special properties which are ascribed to the intercalate come either from the equilibrium between K+/K [23], or topochemical observations (the layer structure) [24]. 

CH4-TPR. Typical curves of product evolution in CH4 temperature-programmed reduction of LSFNo 3-GDC composite calcined at 1200°C are shown in Fig. 107. First CO2 appears at 700 °C, then CO and H2 simultaneously evolve. Isothermal experiments (Fig. 108) revealed that CO and H2 formation is described by typical topochemical kinetics even at 700 °C, i.e. reduction of nanocomposite by methane proceeds via nucleation and growth of reduced phases (Ni or Ni alloy particles). CO2 appears immediately after CH4 contact with the catalyst surface evidencing a high reactivity of the surface oxygen of these composites. 

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