Alumina surface, reactivity

Prediction of interaction between metal clusters with oxide surface The HSAB principle classifies the interaction between acids and bases in terms of global softness. In the last few years, the reactivity index methodology was well established and had found its application in a wide variety of systems. This study deals with the viability of the reactivity index to monitor metal cluster interaction with oxide. Pure gold cluster of a size between 2 and 12 was chosen to interact with clean alumina (100) surface. A scale was derived in terms of intra- and intermolecular interactions of gold cluster with alumina surface to rationalize the role of reactivity index in material designing [43]. 

On an alumina support, independently of the cobalt carbonyl precursor used, complex cobalt sub-carbonyls compounds, [Co(CO)4] and hydrogencarbonate species formed [143, 149]. However, the reactivity of the alumina surface depends on the degree of hydroxylation highly hydroxylated alumina is more reactive against Co2(CO)g and facilitates decarbonylation, whereas dehydroxylated alumina favors the formation of high nuclearity species like [Co6(CO),5] , which would need higher temperatures than the initial Co2(CO)8 to be decarbonylated [149]. 

The model of Knozinger and Ratnasamy [60] is widely accepted as the most comprehensive way to rationalize the reactivity of the Al-OH groups on the alumina surface. This empirical model proposes that y-Al203 has a defective spinel structure, whose (111), (110) and (100) faces are covered by hydroxyl groups. Five configurations for the hydroxyl groups can be present (Scheme 11.3). 

Fig. 3 Surface-reactivity of the molybdenum hexacarbonyl complex as a function of the area density of - OH groups on an alumina surface...

Silica, alumina, and silica-alumina surfaces are of great importance for catalysis and chromatography. Reactivity of these materials is determined by the structure of the surface and its relative acidity, and considerable effort is being expended to characterize it. Of particular interest are the surface hydroxyl groups. Among the methods used for their study the most powerful are IR spectroscopy and titration with acid-base indicators. Conventional NMR can cope with the observation of adsorbed species, where a considerable amount of motional averaging is present MAS NMR must be used to study the surface directly. 

In this review, the relationships between structure, morphology, and surface reactivity of microcrystals of oxides and halides are assessed. The investigated systems we discuss include alkali halides, alkaline earth oxides, NiO, CoO, NiO-MgO, CoO-MgO solid solutions, ZnO, spinels, cuprous oxide, chromia, ferric oxide, alumina, lanthana, perovskites, anatase, rutile, and chromia/silica. A combination of high-resolution transmission electron microscopy with vibrational spectroscopy of adsorbed probes and of reaction intermediates and calorimetric methods was used to characterize the surface properties. A few examples of reactions catalyzed by oxides are also reported. 2001... 

The pyridone surface species has a C=0 stretching band at 1634 cm-1,3 Hydrogen gas has been detected by mass spectrometry (210), and the formation of this surface compound has been established by chemical methods by Boehm (215). This surface reaction points to the existence of strongly basic OH" ions held to certain sites on alumina surfaces, their number being of the order of magnitude of 1013/cm2 (121). Additional evidence for the existence of these reactive and strongly basic OH" ions on aluminas comes from surface reactions observed on adsorption of nitriles and ketones (see Section IV.F) and of carbon dioxide (see Section IV.G). These reactions may, thus, be valuable for the detection of the corresponding sites that most probably have to be considered as acid-base pair sites. 

Another level of complexity is caused by the chemical reactivity of the alumina surface, which can be altered in an acidic medium. 

We believe the effect of alumina additives on catalyst slurry viscosity is associated with the surface reactivity of the additive. OH" is a catalyst for polymerization and Si-O-Si bonding of uncondensed silanols higher pH promotes conversion to a solid phase consisting of discrete silica particles (19). Ostermaier and Elliott (15) suggest that pH be carefully controlled at a value less than 3.5, or thickening occurs in the alumina-free reference formula. 

Studies of organoactinide surface chemistry and catalysis have been carried out by the Marks group under rigorously anhydrous/anaerobic conditions on either partially dehy-droxylated alumina (PDA) or DA at surface coverages of 0.25 0.50 molecules nm. The PDA and DA surfaces are represented schematically in equation (68). Product yields, isotopic labeling, and surface reactivity indicate that the irreversible y-alumina adsorption chemistry of Cp 2AnMe2 can be described by three methane-evolving pathways (equations 69-71). Equation (69) dominates on PDA. On DA,... 

The more demanding research topic will be the description of the radiolytic species surface chemistry. Owing to the very high specific surface of nanostructured materials (up to 1000 m g ), even moderate reaction rates between radiolytic species and surface may have a profound impact on the radiolytic schemes. The few studies available deal only with the surface reactivity of hydroxyl radical in gas phase and suggest a HO capture by silica and alumina. This shows that surfaces that are usually considered as inert may become active under irradiation, once more demonstrating the exceptional reactivity of radiolytic species. 

The use of a synthetic model system has provided valuable mechanistic insights into the molecular catalytic mechanism of P-450. Groves et al. [34]. were the first to report cytochrome P-450-type activity in a model system comprising iron meso-tetraphenylporphyrin chloride [(TPP)FeCl] and iodosylbenzene (PhIO) as an oxidant which can oxidize the Fe porphyrin directly to [(TPP)Fe =0] + in a shunt pathway. Thus, (TPP)FeCl and other metalloporphyrins can catalyze the monooxygenation of a variety of substrates by PhIO [35-40], hypochlorite salts [41, 42], p-cyano-A, A -dimethylanihne A -oxide [43-46], percarboxylic acids [47-50] and hydroperoxides [51, 52]. Catalytic activity was, however, rapidly reduced because of the destruction of the metalloporphyrin during the catalytic cycle [34-52]. When (TPP)FeCl was immobilized on the surface of silica or silica-alumina, catalytic reactivity and catalytic lifetime both increased significantly [53]. There have been several reports of supported catalysts based on such metalloporphyrins adsorbed or covalently bound to polymers [54-56]. Catalyst lifetime was also significantly improved by use of iron porphyrins such as mew-tetramesitylporphyrin chloride [(TMP)FeCl] and iron mcA o-tetrakis(2,3,4,5,6-pentafluorophenyl)por-phyrin chloride [(TPFPP)FeCl], which resist oxidative destruction, because of steric and electronic effects and thereby act as efficient catalysts of P-450 type reactions [57-65]. 

The silylation of hydroxylated substrates has been discussed in numerous studies [2] and proceeds via condensation with reactive surface OH groups. Two examples are given in Fig.2 In case of the silica substrate, the absorption band due to isolated Si-OH groups (3745 cm" ) [3] disappears after application of the epoxysilylester, whereas on the alumina surface isolated Al-OH centers (3695 cm ) [4] are involved in the reaction. 

AH = -5 3 kJ/mol (written on the basis of one mole of cations). Thus this phase is energetically only marginally more stable than a mixture of end-members. Its entropy of formation is unknown. These data suggest that the formation of the surface precipitate, relative to a mixture of cobalt hydroxides and carbonates adsorbed on a fully hydrated alumina surface, probably does not lower the free energy of the system by more than a small amount. Nevertheless, such a surface precipitate may alter further reactivity. 

C) completely refresh and reactivate the alumina surface. As a result, the sensor has the maximum sensitivity to a low humidity level every time humidity is measured. With the integrated microheater, the sensitivity limit was shown to be less than 1 ppm.69,74... 

A layer with a high specific surface area could be developed on woven glass fiber supports by leaching the nonsilica components out of commercial fabrics in acidic solution [54,62], This treatment created mesoporosity and specific surface areas between 5 and 275 m2 g, depending on the temperature and the contact time with HCI solution. In some cases, the surface of porous glass fibers was modified by titania, zirconia, or alumina to increase the thermomechanical stability and to vary the surface reactivity. The modification was made by impregnation of the porous glass fibers with aqueous solutions of the appropriate salts and subsequent calcinations in air. 

Modifeation of alumina surfaee to enhance selective adsorption of particular compounds is an area of rapid development. The activated alumina surface contains a range of surface sites differing in their chemical structure and reactivity. Modification of the surface to contain a greater proportions of surface fuctionalities that enhance the desired separtion or reaction which reducing undesired sites, is a powerful tool in the design of selective adsorption process. In the present study the modification of alumina surface is effected by treatment with acid and base to enhance the adsorption of an antioxidant (tert-butyl catechol) from aromatic hydrocarbon (styrene). 

It is the varied surface chemistry associated with the y-form that is the most important property of y-alumina. The termination of the various planes with hydroxyl groups and Lewis acid sites (anion vacancies) is responsible for the reactivity of y-alumina. The reactivities of Bronsted and Lewis acid sites are dependent upon the pretreatment of the surface. The surface area of a typical y-form ranges between 200 and 220 m /g. Further, the y-form is highly reactive compared to the a-form, which is considered inert and has a typical surface area of 2 m /g or lower. 

Ceramics used in fabricating implants can be classified as nonabsorbable (relatively inert), bioactive or surface reactive (semi-inert) [Hench, 1991,1993] and biodegradable or resorbable (non-inert) [Hentrich et al., 1971 Graves et al., 1972]. Alumina, zirconia, silicone nitrides, and carbons are inert bioceramics. Certain glass ceramics and dense hydroxyapatites are semi-inert (bioreactive) and calcium phosphates and calcium aluminates are resorbable ceramics [Park and Lakes, 1992]. 

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