Physical and Chemical Activation

Ultrasonic waves represent an easy, powerful activation method, even with low-intensity cleaning baths. Topological and physical effects are involved during this process. Transformation of the metal piece to a slurry or a fine powder sometimes occurs, and the increase in the reaction rate parallels that of the surface area. However, it was stressed in Ch. 4 that the phenomena occurring on a sonicated solid surface can be complex, with both physical and chemical components. This statement obviously applies to the activation of metals. 

The more simple and popular access to many organometallics starts from the metal itself, as schematized in Eq. 1. 

This highly versatile process allows further reactions with an organic partner or, if desired, a metal exchange (transmetallation) step. It will be helpful to keep this reaction in mind, as a model, for the following discussion. 

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The mutual chemistry of plastic containers and food products must be considered for any proposed application. There is continuous physical and chemical activity at the interface between the food product and the container. The type and extent of this activity determines whether or not the plastic container can successfully hold and protect the food product. However, the U.S. Food and Drug Administration and the American public are increasingly suspicious of all plastics, particularly the halo-genated compounds. The recent ban (April 1973) on poly(vinyl chloride)... 

Soil is a mixture of solid materials, air and, usually, water and organic matter. The radium content of soil often reflects that of the rocks from which the solid materials are derived by physical and chemical activity. The observed ranges are from 0 to 20 Bq kg for ultrabasic rocks (dunite) to 1 to 1835 Bq kg for igneous metamorphic rocks (gneiss) (Wollenberg, 1984). While these ranges are broader than those for measured for soils, the mean values for rocks, excluding alkali rocks, is consistent with the means observed for soils. 

The physical and chemical activation processes have been generally employed to prepare the porous carbons.18"35 However, the pore structures are not easily controlled by the activation processes and the size of the pores generated by the activation processes is limited to the micropore range only. Recently, much attention has been paid to the synthesis of meso/macroporous carbons with various pore structures and pore size distributions (PSD) by using various types of such inorganic templates as silica materials and zeolites.17,36 55... 

The activated carbons have been prepared by carbonization of precursors, i.e., raw materials with subsequent activation by physical treatment and/or chemical treatment. The activation procedure is conducted in order to enlarge the volume and size of the pores which were already created during the carbonization process and to create some new porosity. Most commonly employed activation methods are divided into physical and chemical activations.14"16,18"35... 

In general, two main methods for the preparation of activated carbons are used physical and chemical activation methods. Physical activation consists of a two-step process carried out at high... 

The ability to perform even the simplest of muscle movement requires complex coordination of the physical and chemical activities of the tissue. In recent years, nutritionists and exercise physiologists have described how the primary energy sources in food carbohydrates, fats, and proteins are transformed into the universal "currency" of biological energy, ATP. Oxidative metabolism processes the substrates through a cascade of enzymatic events to Insure maximal efficiency in energy conversion. At every level of this conversion, one or more metal ions serve as a cofactor to facilitate these biochemical reactions. The requirement of metals in the production of... 

Williams PT, Reed AR (2006) Development of activated carbon pore structure via physical and chemical activation of biomass fibre waste. Biomass Bioenergy 30(2) 144-152... 

Namasivayam, C. and Kadirvelu, K. (1997). Activated carbons prepared from coir pith by physical and chemical activation methods. Bioresour. Technol. 62,123-127. 

Figures 17.1 and 17.2 contain N2 adsorption isotherms of ACFs prepared by physical and chemical activations, as examples of the results that can be...

In industry, activated carbons are essentially produced by carbonization (pyrolysis at temperatures up to 900°C under neutral atmosphere) of various precursors (lignocellulosic, polymers, anthracites,. ..), followed by physical and chemical activation [1], Physical activation is generally realized around 900°C through partial gasification of carbon, using CO2 or steam, according to Eqs. 12.1 and 12.2 ... 

The development of ACs with tailored porosity is necessary to improve their performance in classical applications and to prepare better adsorbents to satisfy new and emerging applications [4-7,17,18]. The preparation of such ACs can be carried out by two different methods the so-called physical and chemical activations [1,19-28], The differences between them lie mainly in the procedure and the activating agents used. 

We should emphasize that in some cases (especially for graphite) these two processes (up to 40 to 50% reaction and accompanied by metal intercalation) do not necessarily result in porosity development (see Figures 1.4a and 1.4b). Additionally, if we recall the comparison between physical and chemical activation (see Section III), we can readily conclude that the BO of the carbon atoms (physical activation) or carbon atoms reacted (chemical activation) alone cannot explain the development of porosity, because much higher BO is necessary by gasification with CO2 to get similar porosity than by the liquid-phase reaction with NaOH or KOH. Consequently, much more work is needed before a full understanding of the chemical activation process can be achieved. 

Silicon carbide is a semiconductor material that is why it is a potential catalyst of thermal oxidation and pyrolysis processes. The silicon carbide partieles have sharp comers and it allows to expect the appearance of physical and chemical activity in the processes of adsorption and chemical reactions (due to Ihe presence of unpaired electrons and excess surfece energy). Silicon carbide also can be used as so ealled microbarrier because of its plastic forms on the surface layers of the material. But the usage of silicone carbide in elastomeric materials is poorly understood. 

The processes of physical and chemical activation of carbons, Chapters 5 and 6, extract carbon atoms from the interior of the network of porosity in carbons. It would be an advantage to have some idea of the structure of this network, in three dimensions, in order to understand the extraction (gasification) process. This three-dimensional network or porosity can be described as a labyrinth and this automatically takes us into children s maze games, some of which are complex (computer generated) as illustrated in Figure 3.28(b, c), while others are quite simple, as shown in Figure 3.28(d), designed for children under five. 

Fig. 2.3 Synthesis of activated carbon both by physical and chemical activation (Gupta and...

Jenner, G. (1996) Comparative study of physical and chemical activation modes. The case of the synthesis of p-amino derivatives. Tetrahedron, 52,13557-68. 

Petersen, T., Peterson, J., Karlsson, H.T. and Bjerle, I. (1988) " Physical and Chemical Activation of Fly Ash to Produce Reagent for Dry FGD Processes", First Combined FGD and Dry SO2 Control Symp., St. LOUIS, Missouri. 

We distinguish between physically and chemically active media according to the effect of aggressive substances. 

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