Thermal decomposition dehydroxylation

The chemistry and structure of the hydrogen form of zeolite Y have been thoroughly investigated 82) and are not considered further. The structure of the dehydroxylated zeolite proposed by Uytterhoeven, Christ-ner, and Hall 15) remains unchanged. Recently Ward, on the basis of infrared studies, suggested that this form may be amorphous 27). The extreme instability of dehydroxylated zeolite Y to moisture complicates detailed study 19). The elucidation of the detailed nature of this material lies in the future. At present, completely dehydroxylated Y is little understood and presents a challenging void in our knowledge of the nature of ammonium zeolite Y thermal decomposition products. 

Hydrated clay surfaces are acidic. When isomorphic substitution occurs in the tetrahedral layer, acid leaching or NH thermal decomposition may generate acidic surface OH. For clays whose negative charges are produced by isomorphic substitutions in the octahedral layer, mild dehydration removes the source of acidity, because of the reversibility of reaction (3). Deamination of the ammonium exchanged clay with octahedral substitution drives protons into the octahedral layer, as evidenced by the lowered temperature at structural dehydroxylation. 

Figure 4.9. Changes in the Si MAS NMR spectra during the thermal decomposition of chrysotile (white asbestos). Note the evidence for the two dehydroxylated phases, that at — 72 ppm forming forsterite directly, that at — 97 ppm forming enstatite by the thermal decomposition sequence shown schematically at the right. From MacKenzie and Meinhold, (1994a), by ptermission of copyright owner.

On a partially dehydroxylated support both species will be present. Thermal decomposition of Fe3(C0)l2 supported on alumina has also been studied by Brenner, A., J.C.S. Chem. Comm., 251 (1979) Brenner, A., and Hucul, D.A., Inorg. Chem., 18, 2836 (1979). 

The Ahrrenius plot., figure 6, which is preliminary, shows time constants at three anneal temperatures. It suggests that, as with loss of crystallinity (ref. 5), there are two regimes of dehydroxylation with activation energies of 0.7 eV above and J3.4 eV below about 750°C. The latter value is not inconsistent with the activation energy of 4.3 eV for diffusion of framework aluminiums deduced from thermal decomposition studies (ref. 5). Further measurements are in progress. 

The extremely high temperature in a plasma jet leads, even during the very short residence time (hundreds of microseconds to few milliseconds, depending on particle density and size) of the hydroxyapatite particles, to dehydroxylation and finally thermal decomposition by incongruent melting. This thermal decomposition of hydroxyapatite in the hot plasma jet occurs in four consecutive steps as shown in Table 6.7. 

Quantitative evaluation of the 2D-HETCOR spectra are presented in Table 7.3. As expected incubation in r-SBF causes over time dissolution of phases characterised by the distorted states 1 and 2 associated with oxyhydroxyapatite, that is partially dehydroxylated SRO-structured hydroxyapatite as well as dissolution of the thermal decomposition products TTCP and TCP, but to a lesser extent. Concurrently the relative proportion of crystalline well-ordered hydroxyapatite increases from 46 mass% in an as-sprayed coating to 74 mass% in a coating incubated under physiological conditions for 12 weeks. 

MgO ex-hydroxide (MgO-h) was prepared by thermal decomposition of the parent Mg(OH)2 under vacuum conditions directly inside the IR chamber. The hydroxide was slowly decomposed in vacuo at ca. 523 K and finally outgassed at 1123 K. This procedure gives MgO with high specific surface area (SSAbet= 200 m -g ) which is assumed to be completely dehydroxylated, as no OH stretching vibration bands were observed in the background IR spectrum. IR spectra of the adsorption of H2 at room temperature were obtained by a Bruker IPS 48 instrument the resolution was 4 cm . The IR chamber, linked to a vacuum pump, allowed both the thermal pretreatment and the adsorption-desorption experiments to be performed in situ . The spectra are reported in absorbance, the background spectrum of the MgO san le before H2 absorption being subtracted. 

The thermal decomposition of argentojarosite (AgFe3(S04)2(0H)e) has been studied by TG, spectroscopic and infrared emission techniques [165]. Frost and coworkers proved that the dehydroxylation occurs in three stages at 228, 383, 463 with the loss of 2, 3 and 1 hydroxyl units. The loss of sulfate occurs at... 

Thermal decomposition of layered double hydroxides typically involves endothermic processes of dehydration, dehydroxylation, and the removal of anions. These treatment processes are of great importance in the production of oxide and oxide-supported catalysts (see Sec. VI.C). For this reason, because of the wide range of metal and anion combinations of interest in this context and because of various preparative methods available, this remains an active area of research. It is, however, a difficult area because of the problems of observing metastable intermediate phases and the poor crystallinity of the thermolyzed material for this reason it employs a range of old and new techniques (1,4,13). 

For better comprehension of thermal decomposition process, the TG-DTA curves for MgAl-LDH are presented in Figure 20.5. It can be observed that the typical LDH decomposition is characterized by two endothermic transitions involving the following consecutive or overlapping steps dehydration, dehydroxylation, decomposition of anions, and segregation of oxides. 

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