Heterogeneity secondary surface

However, the use of the heterogeneous catalysts in applicative enantioselective syntheses has a limited success. Several factors contribute to this situation (1) a long time is required to achieve an effective heterogeneous enantioselective catalyst compared with the homogeneous ones, (2) a more complex structure of the heterogeneous catalyst surface on which centers coexist with different catalytic activity and selectivity, which can lead to undesired secondary reactions, and (3) an increased difficulty to create an effective asymmetric environment and to accommodate it with the multitude of reactions that are interesting to be carried out under enantioselective restrictions. 

Soil particles differ widely in chemical composition, which introduces a degree of heterogeneity into the spatial distribution of minor components. Chemically reactive species become concentrated within smaller sized particles or associated with secondary surface coatings and must be considered with respect to the collection and preparation of truly representative soil samples. 

Finally, atmospheric chemical transformations are classified in terms of whether they occur as a gas (homogeneous), on a surface, or in a liquid droplet (heterogeneous). An example of the last is the oxidation of dissolved sulfur dioxide in a liquid droplet. Thus, chemical transformations can occur in the gas phase, forming secondary products such as NO2 and O3 in the liquid phase, such as SO2 oxidation in liquid droplets or water films and as gas-to-particle conversion, in which the oxidized product condenses to form an aerosol. 

Processes associated with two opposing electrode processes of a different nature, where the anodic process is the oxidation of a metal, are termed electrochemical corrosion processes. In the two above-mentioned cases, the surface of the metal phase is formed of a single metal, i.e. corrosion occurs on a chemically homogeneous surface. The fact that, for example, the surface of zinc is physically heterogeneous and that dissolution occurs according to the mechanism described in Section 5.8.3 is of secondary importance. 

Note Nucleation may be classified as primary or secondary. Primary nucleation can be homogeneous or heterogeneous if heterogeneous nucleation is initiated by entities having the same composition as the crystallizing polymer, it is called self-nucleation. Secondary nucleation is also known as surface nucleation. 

The IR study performed in static controlled atmospheres in the IR cell allowed us to identify a number of adsorbed intermediate and secondary products, together with the main reaction products in oxidation and ammoxidation of toluene and the three xylene isomers. Surface reactions schemes are proposed that account for most of the mechanistic features of the heterogeneously-catalyzed industrial reactions. Our data support the following conclusions ... 

Because of the gaseous nature of many of the important primary and secondary pollutants, the emphasis in kinetic studies of atmospheric reactions historically has been on gas-phase systems. However, it is now clear that reactions that occur in the liquid phase and on the surfaces of solids and liquids play important roles in such problems as stratospheric ozone depletion (Chapters 12 and 13), acid rain, and fogs (Chapters 7 and 8) and in the growth and properties of aerosol particles (Chapter 9). We therefore briefly discuss reaction kinetics in solution in this section and heterogeneous kinetics in Section E. 

The principal difference between homogeneous and heterogeneous reaction rates is that the latter is based on mass, volume, or more rarely, on the area of the solid and not on the fluid-phase volume or reactor volume. The reactor volume or liquid-phase volume is of secondary significance in heterogeneous reactions since the reaction takes place on the solid rather than throughout the reactor volume. Moreover, the mass of the solid is usually used instead of the solid volume or surface, because it is the most easily measured property. 

Metal bioavailability is generally increased with decreasing pH. This is due to the presence of phosphoric, sulfuric and carbonic acids, which increasingly solubilize organic- and particulate-bound metals. Particulate-bound metals are considered those bound to secondary minerals, for example, clays, iron and aluminum oxides, carbonates and sulfidic and phosphoric minerals. Due to the heterogeneous nature of soils and sediments, wide fluctuations in pH can exist in a given environment. For instance, metals may be more soluble in surface layers where plant exudates, microbial activity, moisture and leaching lower pH. 

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