Dissolution reaction scheme

Direct-Liquefaction Kinetics All direct-liquefac tion processes consist of three basic steps (1) coal slurrying in a vehicle solvent, (2) coal dissolution under high pressure and temperature, and (3) transfer of hydrogen to the dissolved coal. However, the specific reac tion pathways and associated kinetics are not known in detail. Overall reaction schemes and semiempirical relationships have been generated by the individual process developers, but apphcations are process specific and limited to the range of the specific data bases. More extensive research into liquefaction kinetics has been conducted on the laboratory scale, and these results are discussed below. 

The lower part of the reaction scheme describes three possibilities for further dissolution reactions of the radical (Eq. 31 and 32). Only the first step in this sequence is essential. It is important to note here that in one path (case A) the first step is a pure... 

On polar semiconductors the dissolution may also involve electrons from the conduction band, leading to the production of soluble anions. For example, under accumulation conditions the dissolution of n-type CdS takes place according to the reaction scheme ... 

FIA has also found wide application in pharmaceutical analysis.214,215 Direct UV detection of active ingredients is the most popular pharmaceutical analysis application of FIA. For single component analysis of samples with little matrix interference such as dissolution and content uniformity of conventional dosage forms, many pharmaceutical chemists simply replace a column with suitable tubing between the injector and the detector to run FIA on standard HPLC instrumentation. When direct UV detection offers inadequate selectivity, simple online reaction schemes with more specific reagents including chemical, photochemical, and enzymatic reactions of derivatization are applied for flow injection determination of pharmaceuticals.216... 

The scheme in Fig. 5.5 indicates that the ligand, for example, oxalate, is adsorbed very fast in comparison to the dissolution reaction thus, adsorption equilibrium may be assumed. The surface chelate formed is able to weaken the original Al-oxygen bonds on the surface of the crystal lattice. The detachment of the oxalato-aluminum species is the slow and rate-determining step the initial sites are completely regenerated subsequent to the detachment step provided that the concentrations of the reactants are kept constant, steady state conditions with regard to the oxide surface species are established (Table 5.1). If, furthermore, the system is far from dissolution equilibrium, the back reaction can be neglected, and constant dissolution rates occur. 

A scheme for the dissolution reaction of a trivalent oxide is given in Fig. 5.8. Al-... 

Fig. 4.1 Reaction scheme proposed for the chemical dissolution of (100) oriented silicon surfaces in alkaline solutions.

This fast removal of Si-F species can be ascribed to the weakening of the Si backbonds induced by the strong polarizing effect of F [Ubl], The weak back-bonds are then attacked by HF or H20. This reaction scheme for the dissolution process is supported by quantum-chemical calculations [Trl]. The observed dissolution valence of two for Jelectron injection current and Si-F bond density [Be22] are experimental findings that are in support of the divalent dissolution mechanism, as shown in Fig. 4.3 [Lei, Ge7, Ho6]. 

Fig. 4.3 Reaction scheme proposed for the anodic, divalent dissolution of silicon electrodes in HF.

Fig. 4.4 Reaction scheme proposed for the anodic, tetravalent dissolution of silicon electrodes in aqueous HF. The reaction can be separated into two parts first an oxide is...

By following the reaction scheme proposed by dos Santos Afonso and Stumm (22) for the reductive dissolution of hematite surface sites (Scheme 1), we were able to explain perfectly the observed pH pattern of the oxidation rate of H2S. The rate is proportional to the concentration of inner-sphere surface complexes of HS" formed with either the neutral (>FeOH) or the protonated (>FeOH2+) ferric oxide surface sites. 

Reductive Dissolution of Oxides by Organic Reductants 164 Reaction Scheme and Mechanism 164 Specific Studies 166... 

Two thermochemical reaction schemes are generally used to derive the enthalpies of formation of these compounds. The first is based on the dissolution of the lanthanide metal as well... 

EtOH - AcOH 1 3) were chosen to ensure the complete dissolution of the starting components. Workup and isolation of the products became a critical issue in the context of the high speed of the methodology. The proper choice of the above-mentioned solvents made the workup simple and efficient as the formed products many times precipitated directly from the reaction mixture after completion of the reaction (Scheme 44). 

Fig. 12.23. General reaction scheme for iron dissolution (Redrawn from D. M. Drazic, in Modem Aspects of Electrochemistry, No. 19, Plenum, 1989).

Proton-promoted dissolution reactions are exemplified for carbonates, silicates, and metal oxyhydroxides by Eqs. 3.15, 3.18-3.20, 3.25, 3.39, 3.46, 3.53, 3.56, and 3.59c. The typical response of the rate of dissolution to varying pH is illustrated in Fig. 3.2, and this response is often hypothesized to be a result of the proton adsorption-bond-weakening structural detachment sequence described in connection with Eq. 3.60.36 This sequence can be represented by the following generic reaction scheme ... 

Fig. 8.16. Reaction scheme for photoanodic dissolution of silicon in low intensity limit illustrating the competition between hole capture steps (rate constants k to k ) and electron injection steps (rate constants k to k,). The nominal valence states of the silicon intermediates are indicated. The final product Si(IV) is the soluble hexafluorosilicate species.

Ferritin induced nanoparticle synthesis was adapted from a number of different synthetic strategies reliant upon the physical nature of ferritin. For instance, ferritin can readily exist in two stable forms (native ferritin with an intact iron oxide core or apo-ferritin lacking a mineral core) owing to the enhanced structural integrity of the protein shell. As a result, two general reaction schemes were adopted. The first route utilized the iron oxide core of native or reconstituted ferritin as a precursor to different mineral phases and types of iron nanoparticles, while the second invokes mineralization within the empty cavity of apo-ferritin. In the latter approach, the native protein must be demetallated by reductive dissolution with thioglycolic acid to yield apo-ferritin. Ultimately, apo-ferritin provides a widely applicable means to the synthesis of various nanoparticle compositions under many conditions. 

The lower part of the reaction scheme describes three possible dissolution pathways (Eqs. (55)-(56)). Only the first step in each sequence is essential. It is important to note that in one path (case A) the first step is a pure chemical reaction, characterized by the rate v whereas in the others (case B) a hole transfer is involved. According to the kinetics of this reaction scheme, an intensity dependence of the stability factor s (defined in Eq. (47)) or of the ratio joi/jcorr occurs only for case B, as shown in a previous review article [114]. There are various examples published in the literature which demonstrate this effect... 

For the dissolution of kaolinite and sepiolite, the following reaction schemes were employed ... 

The rates of dissolution on the one hand and in vitro hydrolysis of the solid by the enzyme pancreatic lipase on the other hand are given in Fig. 7.4. If dissolution is the first step in the total hydrolysis process, the reaction scheme may be written as... 

Figure 13.10 gives a general scheme explaining in a simplified way the various mechanisms of the dissolution reaction of an oxide. Figure 13.11 shows a set of representative experimental data and how these data are interpreted. A... 

An explanation for the transition of the dissolution reaction from valence 4 to valence 2 with increasing light intensity was given in a model proposed by Kooij and Vamnaekelbergh. They assume the presence of an intermediate Si(l), which is an electron-deficient surface silicon atom and has a catalytic effect on the hydrogen reduction reaction. The essential steps in the reaction scheme are described by the following equations ... 

The results presented in the previous sections show that the anodic reactions on a silicon electrode may proceed via different paths depending on the conditions and that those in HF solutions and those in KOH solutions are rather different. They also show that the mechanistic models proposed for the reactions in HF and KOH solutions from the many studies in the literature are largely separated. However, in both HF and KOH solutions, the silicon/electrolyte interface is fundamentally similar differing only in the concentrations of hydroxyl and fluoride ions. Thus, a reaction scheme must be coherent with respect to the experimental observations in both FIF and KOH solutions. For comparison. Table 5.8 summarizes the characteristic features of the reactions occurring on silicon in FIF and KOH, in terms of nature of the reaction, rate, effective dissolution valence, photoeffect, and uniformity of the surface. 

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