Chemical activation process

Subsequent studies (63,64) suggested that the nature of the chemical activation process was a one-electron oxidation of the fluorescer by (27) followed by decomposition of the dioxetanedione radical anion to a carbon dioxide radical anion. Back electron transfer to the radical cation of the fluorescer produced the excited state which emitted the luminescence characteristic of the fluorescent state of the emitter. The chemical activation mechanism was patterned after the CIEEL mechanism proposed for dioxetanones and dioxetanes discussed earher (65). Additional support for the CIEEL mechanism, was furnished by demonstration (66) that a linear correlation existed between the singlet excitation energy of the fluorescer and the chemiluminescence intensity which had been shown earher with dimethyl dioxetanone (67). 

In chemical activation processes, the precursor is first treated with a chemical activation agent, often phosphoric acid, and then heated to a temperature of 450 -700 °C in an activation kiln. The char is then washed with water to remove the acid from the carbon. The filtrate is passed to a chemical recovery unit for recycling. The carbon is dried, and the product is often screened to obtain a specific particle size range. A diagram of a process for the chemical activation of a wood precursor is shown in Fig. 3. 

Fig. 3. Chemical activation process for production of activated carbon...

An abrasive-free CMP is an enhanced chemically active process, which provides lower dishing, erosion, and less or no mechanical damage of low-k materials compared to conventional abrasive CMP processes [52]. 

Novel Manufacturing Processes. Different chemical activation processes have been used to produce carbons with enhanced adsorption characteristics. Activated carbons of exceptionally high surface area (>3000 m2/g) have been produced by the chemical activation of carbonaceous materials with potassium hydroxide (28,29). Activated carbons are also produced commercially in the form of cloths (30), fibers (31), and foams (32) generally by chemical activation of the precursor with a Lewis acid such as aluminum chloride, ferric chloride, or zinc chloride. 

Reaction 10.178 is a chemical activation process. Note that this reaction does not involve a third body M for creation of the excited intermediate species, which differs from the unimolecular initiation event in Eq. 10.99. 

Since the nature of chemical combination is electrical it is natural to inquire whether there is any essential connexion between chemical activation processes and ionization. From the early days of the electron theory experiments have been made with the object of establishing such a relationship, but most of the evidence seems to indicate that any ionization accompanying ordinary chemical reactions is very small and probably of a purely secondary character. ... 

With the chemical activation process, the precursor is oxidized by silver ion. An electrochemical process using a carbon-carbon pair of electrodes, has been shown to promote the formation of the ylid which is reacted with electron-poor olefins 452... 

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... 

First, the original mechanism of the CIEEL process involves, as a key tenet, the chemically activated transfer of the electron from the donor (activator) to the acceptor (peroxide) functionality. Indeed, as estimated from electrochemical data, the electron transfer from an activator is endothermic at the equilibrium geometry of the peroxide bond. Thus, the chemical activation process involves stretching of the oxygen-oxygen bond to accommodate the transferred electron. Consistent with the semiempirical computations, the 0-0 bond in the radical anion is markedly elongated relative to the neutral dioxetane (cf. Fig. 1). 

This section will describe the chemical activation process with the most commonly used reagents zinc chloride, plnsphoric acid and potassium hydroxkle. Other low-volume processes utilize various Lewis acids such as aluminium chloride and ferric chloride with fibrous materials such as rayon (a product of cellulose) to jModuce activated cloths or fibrK. 

Unlike other chemical activation processes, the most approplate raw materials are those with a low volatile content and a rich carbon content such as high rank coals, previously devolatilized chars or petroleum coke. Thus, most of the examples cited in the work of Wennerberg and O Grady [47] referred to petroleum coke as a raw material. 

Collins and Kimball [4] suggested that the chemically activated process which leads to the formation of products from the encounter pair occurs at a rate proportional to the probability that the encounter pair exists. Defining an encounter pair as a pair of reactants which lie within a distance of R to (R + 8R) of one another, and since the probability that B is within this range of distances about A is p R), then the rate of reaction of encounter pairs is feactP( )- act is the second-order rate coefficient for the reaction of A and B when they are almost in contact and close enough to react with each other. It is the rate coefficient for reaction between A and B if the rate of diffusion were infinitely rapid. It has unit of dm mol" s" . From eqn. (7), the rate at which B diffuses towards A to form encounter pairs is 47r(f + 5/ ) D(3p/3r) R+5jj. For sufficiently small 5R (e.g. <0.01 nm), the term in Si is unimportant and this becomes the diffusive flux to the encounter separation 4irR D dp/dr) ji from Fick s first law. Providing the probability of A and B existing as an encounter pair rapidly reaches a steady-state value, the rate of formation and rate of reaction of the encounter pairs may be equated, i.e. 

To apply the results of the chemical activation and the thermal dissociation analysis for comparison to literature or experimental data (when available) it is necessary to construct an elementary chemical reaction mechanism. The mechanism includes all the reactions involved in the chemical activation process, including stabilizations and reactions for thermal dissociation of the stabilized species. The reactions are reversible, so that we implicitly take into account some of the thermal dissociation reactions as the reverse of the forwards (chemically activated) reactions. For example, the dissociation of DBFOO to DBF + O2 is included as being the reverse reaction of DBF + O2 o DBFOO. ThermKin is used to determine the elementary reaction rate coefficients and express the rate coefficients in Arrhenius forms. 

Finally, we should also note an experiment by Rabinovitch and his students [71.0] in which the thermal decomposition of the sec-butyl radical (formed in a chemical activation process) showed no variation in rate from 0.01 to 200 atmospheres of added hydrogen. ... 

Third, and already well recognised, is the problem of decay versus stabilisation of a molecule initially prepared in a highly excited state, either photochemically or by a chemical activation process. The solution to this problem requires more than a knowledge of just the smallest eigenvalue [81.V2], and the obvious approach is to use the separable approximation. 

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