Dehydroxylation by thermal

During the past decade, the main advances in the spectroscopic characterization of silica powders have come from 29Si NMR, Raman diffusion, and Fourier transform infrared (FTTR) studies. NMR studies have given two types of results. First, single and geminal silanols have been quantitatively differentiated as a function of dehydroxylation by thermal treatment and subsequent rehydroxylation by liquid water (7). The fraction of geminal silanols... [Pg.198]

A summary is given for spectral variations as a function of dehydration in vacuum, dehydroxylation by thermal pretreatment up to 700 °C, rehy-droxylation, and grafting. On the basis of recent NMR, X-ray, and Raman results, the infrared spectra are further assigned in terms of surface sites related to the local structure of the underlying SiO network. Emphasis is given to a detailed structure analysis of a fumed silica. Additional features are necessary to describe other types of silicas. [Pg.200]

G.J.C. Carpenter, Z.S. Wronski, NanocrystaUine NiO and NiO-Ni(OH) composite powders prepared by thermal and mechanical dehydroxylation of nickel hydroxide, Nanostructured Mater. 11(1) (1999) 67-80. [Pg.80]

Girard JP, Savin S (1996) IntercrystaUine fractionation of oxygen isotopes between hydroxyl and non-hydroxyl sites in kaoUnite measured by thermal dehydroxylation and partial fluorination. [Pg.245]

Mainly Fe aggregates form when highly dehydrated magnesia is impregnated with Fe3(CO)i2 or Fe(CO)5 and the resulting surface species are thermally decomposed conversely, when MgO is not dehydroxylated the thermal treatment generates Fe and H2 because of the electrophihc attack by surface protons on the carbonyhc surface species [73]. [Pg.325]

Furthermore, the removal of these groups by thermal treatment (dehydroxyl-ation) yields coordinatively unsaturated oxygens and anions in which coordina-tively unsaturated aluminum atoms are exposed (Lewis acid sites). In general, the total concentration of OH groups on a alumina support ranges between 10 and... [Pg.349]

G. J. C. Carpenter, Z. S. Wronski, Nanocrystalline NiO and NiO-Ni(OH)2 Composite Powders prepared by Thermal and Mechanical Dehydroxylation of Nickel Hydroxide, Nanostructured Mater., 1999,... [Pg.434]

Aluminas. Again the active exchange sites are the surface hydroxyl groups, which now have a more basic character and will also exchange or react with anions. These surface hydroxyl groups can again be removed by thermal dehydroxylation or be deactivated by anionic replacement by anions of both minerals and organic acids. The dehydroxylated surface is more readily rehydrated and is fairly readily hydrolysed by mineral acids. [Pg.13]

In MgO thin films, O vacancies can be generated by electron bombardment or sputtering with Ar. In polycrystalline MgO samples, O vacancies can be created by thermal treatment of hydroxylated surfaces. The dehydroxylation occurs at the expense of a lattice O anion with consequent formation of a surface vacancy or F center. Anion vacancies on the MgO surface, the centers corresponding to the removal of O, are difficult to observe, as these centers do not have specific spectral properties. However, they can act as electron traps and their existence can be deduced by doping the material with excess electrons to form the corresponding Fg and Fg centers which can be detected either by EPR (Fg" ") [130-133] or by optical (Fg+ and Fg) [134-137] spectroscopies. In the F+ or F centers one or two electrons are associated with the defect they are localized in the vacancy [39-43,50] by the MP. [Pg.213]

Information on the degree of disorder introduced by thermal treatment of hydroxyapatite during plasma spraying could be obtained by NMR spectroscopy. The positions and shift of 1H-MAS and 31P-MAS NMR peaks are indicative of the environment of the P043- tetrahedra in calcium orthophosphates and allow distinguishing between dehydroxylation (oxyhydroxyapatite) and decomposition (tricalcium phosphate, tetracalcium phosphate) products of hydroxyapatite. [Pg.326]

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. [Pg.132]

Au/y-Al203 deactivates in CO oxidation, but it exhibits stable activity in selective CO oxidation (SCO) in the presence of H2. The activity for CO oxidation could be suppressed significantly by thermal treatment at 100°C, and the lost activity could be recovered by exposing the catalyst to water vapor at room temperature. A catalyst deactivated in CO oxidation could be regenerated by exposing it to H2 at room temperature or by running the SCO reaction over it. The results can be explained with a reaction mechanism for CO oxidation involving an active site that consists of an ensemble of metallic Au atoms and Au-hydroxyl. Deactivation in CO oxidation is due to the formation of an inactive carbonate, and deactivation by thermal treatment is due to dehydroxylation of the Au-hydroxyl. [Pg.375]

Soled [26] proposed the description of y-form, considering OH species as structural elements, and proposed formula with the stoichiometry -Al2.5[ ]o.503.5(OH)o.5. The changes of y-form to 5- and 0- by thermal treatment are than depicted as a particle growth which results from condensation of hydroxyl species and formation of water. This model accounts for a more dehydroxylated character of 8- and 0- aluminas compared to y-. However, in this formulation the ratio between trivalent and divalent cationic sites does not meet the requirement of the spinel structure that must be equal to 2. To make it correct, Burtin et al [27] introduced into the formula the new structural element, which corresponds to oxygen vacancies, noted < >. Oxygen vacancies were believed to a result from removing of the water molecules. The common features of the of y-, 8- and 0-forms of transition aluminas which were found in their structural and chemical properties allow to describe them by the general formula ... [Pg.601]

It has been proposed that the evolution of transition aluminas (pure and doped) during the thermal treatment can be described as a single phase in which the stoichiometric concentrations of the quasi-chemical species vary. This evolution is consists of dehydroxylation by removal of water followed by the exothermic structural transformation to a-phase, which can be described in terms of an annihilation reaction between anionic and cationic vacancies ... [Pg.605]

The most intensively investigated dehydroxylation is probably the reaction of Mg(OH)2, though detailed results are also available for the hydroxides of certain other divalent cations. Several summaries of the mechanistic deductions obtained from such work, including literature sources, were presented at a conference at Dijon in 1974 [87]. The extensive literature concerned with the thermal analysis of hydroxides has been reviewed by Dollimore [79] who has also included the behaviour of oxides. Water elimination can be regarded as the first in a sequence of structurally related steps through which the hydroxide is converted into the thermally most stable oxide. [Pg.137]

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