Hydroxyl coverage

Boehm et al.2 29 utilized this reaction to evaluate the hydroxyl group concentration at the surface of Aerosil. They found a maximal hydroxyl coverage of 3.1 OH/nm2 for a pretreatment temperature of 450 K. In the same period, also Uytterhoeven,30 Elmer31 and Folman14 have investigated the reaction of silica with SOCl2. 

Gel Compositions, Specific Area and Hydroxyl Coverage Ratio for Some Heat-Treated Gels... 

The thermal stabilities of hydrogen faujasites and mordenites with different Si/Al ratios are reported. The temperature fields are outlined which characterize the thermal resistance of the lattice, framework Al, hydroxyl coverage, and the active sites. By choosing the proper conditions for activation of hydrogen zeolites, it is possible to induce the release of Al from the framework and in this way to promote the formation of strong add sites and enhance the catalytic activity. 

To elucidate the nature of the hydroxyl coverage and to quantitatively determine the concentration of OH groups on the silica surface, such groups must be differentiated from the molecularly adsorbed H2O and from bound water inside the silica particles. The separation of the two processes — dehydration and dehydroxylation — is a difficult task. 

Role of hydroxyl groups in catalysis and hydroxyl coverage of catalytic... 

Inelastic neutron scattering (INS) is another technique that has been used for the characterization of surface hydroxyls, but the number ofpubfications is hmited (69—72). Information on the hydroxyl coverage can also be... 

Anatase is another interesting phase of Ti02- The natural crystallographic faces are the (101) and (100) orientations. The acid-base adsorption mechanism is very similar to the one of the metal/oxide already discussed. On the (101) surface, molecular adsorption is exothermic (16.6 kcal/mol for full coverage) [99]. In contrast, water dissociates on the (001) plane, which is more reactive [100]. It is worth noticing that the surface morphology has a very important effect on the hydroxyl coverage [100]. 

Thus, investigations of a variety of mixed nanooxides using several methods reveal that the surface properties of SA, ST, and AST materials (such as content of SiOH, Si(OH)2, (-0)4Si, A1(V1), A1(V), and Al(IV), intensity of the FTIR bands of surface hydroxyls, heat of immersion in water, desorption temperature, and content of desorbed water) are complex functions of the specific surface area, total and surface content of all oxide components, composition of the surface (both oxide phase distributions and distributions of the Ti and A1 atoms in the silica phase and vice versa), and treatment temperature (differently affecting SA, ST, and AST samples because ternary oxides can lose hydroxyl coverage at lower temperatures than binary oxides). Many of the mentioned properties very clearly correlate with the surface content of alumina and/or titania in mixed oxides. Despite these differences, nanooxides are morphologically similar even at different specific surface areas because of features of HT synthesis of these oxides. 

Fry, R., Pantano, C.G., and Mueller, K.T. 2003a. Effect of boronoxide on surface hydroxyl coverage of alumi-noborosUicate glass fibers A F solid-state NMR study. Phys. Chem. Glasses 44 64—68. 

Figure 5.2. Relaxed configurations of 7-AI2O3 (100) surface for different hydroxyl coverages (9 in OH nm . The most relevant surface sites are quoted. Ain stands for aluminum atoms surrounded by n oxygen atoms, and HO-jUm for OH groups linked to m aluminum atoms. Oxygen atoms are black, aluminum atoms are shown in gray whereas hydrogen atoms are whitel l.

Effect of boron oxide on surface hydroxyl coverage of aluminoborosilicate glass fibres by solid-state NMR has been reported. ... 

Figure 14 Hydroxyl coverage as a function of initial oxygen coverage, Qq, on Cu(l 10). Maximum hydroxyl formation occurs for = //8. (From Ref. 109.)...

Hydroxyl Coverage of a Variety of High-Surface-Area Silicates Determined by Different methods. 

Hydroxyl coverage of the silica surface versus dehydroxylalion temperature (I). Shaded area range of data on a variety of silicas investigated by Davydov et at. (36). Dehydration of annealed (700°C) and rehydrated silica during heating in air A and in vacuum B (47). Broken line, C data on unannealed silica (48-51). 

More recently, Zhuravlev [37] determined the OH coverage of amorphous silicates using a deuterio-exchange method [38,39] that distinguished between surface and bulk OH. He found that the average hydroxyl coverage... 

Thermal dehydroxylation relies on condensation reactions (Eq. 8) occurring on the gel surface. Thermal dehydroxylation of silicates has been investigated extensively, and summaries of numerous studies are reported by Her [1]. Hydroxyl coverage versus temperature is shown in Fig. 9 for a variety of... 

Variation of BET surface area (circles) and hydroxyl coverage (boxes) for multi-component borosiiicate samples during healing at 2°C/min (filled symbols no isothermal hold open symbols l8-hour isothermal hold) (57). 

Microprobe elemental mapping of a cross section of the fully densified gel (Fig. 19) shows both that considerable Cl is retained in the densified gel and that chlorine removal causes essentially complete dealkalization of the exterior gel surface. Quantitative elemental analysis (Fig. 20) indicates that the interior of the sample contains 1.5 wt Vo Cl and 0.9 wt Vb Na20 (compared to 1.2 wt% Na20 in the original gel), whereas the exterior surface (—300 m thick) contains 0.6 wt% Cl and 0wt% Na20. A Cl content of 1.5 wt% corresponds to replacement of 38% of the surface hydroxyls with Cl, based on a hydroxyl coverage at 450°C of 1.2 0H/nm [56] and a BET specific... 

The rehydration of dehydroxylated surfaces has been investigated extensively. (See, e.g. [1,75,76,86,87].) The rehydration process involves water adsorption followed by dissociative chemisorption (Eq. 15). Either the adsorption step or the subsequent dissociative chemisorption step could be rate-determining [30]. Hair [22] states that rehydration is sluggish for surfaces dehydroxylated above about 450°C due to surface hydrophobicity (physisorption rate determining). Below this temperature there is sufficient hydroxyl coverage that the adsorption step is facile. 

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