Alumina surface structure

As in the case of alumina, surface structures have been characterized mainly by the hydroxylation/dehydroxylation behavior of the two modifications. The first infrared spectra of surface OH groups were reported by Yates (132), by Lewis and Parfitt (133), and later by Criado et al. (134). 

Crystal structures and morphologies of aluminas Surface Structures and Chemistry of Aluminas... 

Surface heterogeneity may be inferred from emission studies such as those studies by de Schrijver and co-workers on P and on R adsorbed on clay minerals [197,198]. In the case of adsorbed pyrene and its derivatives, there is considerable evidence for surface mobility (on clays, metal oxides, sulfides), as from the work of Thomas [199], de Mayo and co-workers [200], Singer [201] and Stahlberg et al. [202]. There has also been evidence for ground-state bimolecular association of adsorbed pyrene [66,203]. The sensitivity of pyrene to the polarity of its environment allows its use as a probe of surface polarity [204,205]. Pyrene or ofter emitters may be used as probes to study the structure of an adsorbate film, as in the case of Triton X-100 on silica [206], sodium dodecyl sulfate at the alumina surface [207] and hexadecyltrimethylammonium chloride adsorbed onto silver electrodes from water and dimethylformamide [208]. In all cases progressive structural changes were concluded to occur with increasing surfactant adsorption. 

Many solids have foreign atoms or molecular groupings on their surfaces that are so tightly held that they do not really enter into adsorption-desorption equilibrium and so can be regarded as part of the surface structure. The partial surface oxidation of carbon blacks has been mentioned as having an important influence on their adsorptive behavior (Section X-3A) depending on conditions, the oxidized surface may be acidic or basic (see Ref. 61), and the surface pattern of the carbon rings may be affected [62]. As one other example, the chemical nature of the acidic sites of silica-alumina catalysts has been a subject of much discussion. The main question has been whether the sites represented Brpnsted (proton donor) or Lewis (electron-acceptor) acids. Hall... 

Alumina is known to have more ionic character and its surface has a more complex structure than that of silica. Reaction of Bu3SnH with the surface of partially dehydroxylated aluminas was followed and it was found that the extreme sensitivity of tin chemical shifts to the molecular environment constitutes a method whereby surface organometallic complexes of tin can be used as molecular probes for determining surface structures of oxides.248... 

Park PW, Ledford JS (1998) The influence of surface structure on the catalytic activity of cerium promoted copper oxide catalysts on alumina oxidation of carbon monoxide and methane. Catal Lett 50(1—2) 41 48... 

In situ dynamic surface structural changes of catalyst particles in response to variations in gas environments were examined by ETEM by Gai et al. (78,97). In studies of copper catalysts on alumina, which are of interest for the water gas shift reaction, bulk diffusion of metal particles through the support in oxygen atmospheres was shown (78). The discovery of this new catalyst diffusion process required a radical revision of the understanding of regeneration processes in catalysis. 

The dehydration of cis,alumina surfaces, could readily explain the observed data ... 

Ruthenium catalysts, supported on a commercial alumina (surface area 155 m have been prepared using two different precursors RUCI3 and Ru(acac)3 [172,173]. Ultrasound is used during the reduction step performed with hydrazine or formaldehyde at 70 °C. The ultrasonic power (30 W cm ) was chosen to minimise the destructive effects on the support (loss of morphological structure, change of phase). Palladium catalysts have been supported both on alumina and on active carbon [174,175]. Tab. 3.6 lists the dispersion data provided by hydrogen chemisorption measurements of a series of Pd catalysts supported on alumina. is the ratio between the surface atoms accessible to the chemisorbed probe gas (Hj) and the total number of catalytic atoms on the support. An increase in the dispersion value is observed in all the sonicated samples but the effect is more pronounced for low metal loading. 

Membranes have been developed that possess improved characteristics with respect to their lateral wicking and spot resolution. The best example of this type of material is anodically oxidized alumina whose structure consists of pores, with very little material forming the walls, possessing a surface area ratio of approximately 500 f. The benefits of this material are its higher sensitivity (more immobilized capture probe) and higher probe densities. 

The carbonyl cluster Rh,5(CO)i,5 was initially stable as such on the completely dehydroxylated alumina surface. But as soon as hydroxyl groups were generated (e.g., by adding traces of water) it decomposed to give various surface transformations. First, the cluster structure was dismpted, with breakage of the core cluster frame, into (Al-0-)(Al-0H)Rh (C0)2, Rh > monoatomic species sigma and n-bonded to the oxygens atoms of the alumina surface, with formation of molecular... 

Figure 11.1 DFT-calculated reaction energies and structures for two models of the interaction between a siloxane-capped aluminosilsesquioxanemonosilanol cube (representing the silica-alumina surface) and CHsReOs. Color scheme Re (yellow), Al (purple), Si (blue), O (red), C (green), H (white). (Reprinted from Reference [39].)...

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). 

Supported Cu-Pd catalysts have the potential to provide new alternatives to conventional commercial methanol synthesis catalysts (based on the Cu-ZnO-alumina system). Cu-Pd catalysts are also of industrial interest in hydrogenolysis and CO oxidation (Bulatov 1995). To interpret the catalyst behaviour and selectivity, including CO hydrogenation, a fundamental understanding of the structure, surface structure and stability of the phases in this system is required. The Cu-Pd phase diagram indicates that at temperatures greater than 600 °C, Cu... 

The literature of the vibrational spectra of adsorbed alkynes (acetylene and alkyl-substituted acetylenes) is very much in favor of single-crystal studies, with fewer reported investigations of adsorption on oxide-supported metal catalysts. Fewer studies still have been made of the particulate metals under the more advantageous experimental conditions for spectral interpretation, namely, at low temperatures and on alumina as the support. (The latter has a wide transmittance range down to ca. 1100 cm-1.) A similar number of different single-crystal metal surfaces have been studied for ethyne as for ethene adsorption. We shall review in more detail the low-temperature work which usually leads to HCCH nondissociatively adsorbed surface structures. Only salient features will be discussed for higher temperature ethyne adsorption that often leads to dissociative chemisorption. Many of the latter species are those already identified in Part I from the decomposition of adsorbed ethene. 

The surface of alumina is highly reactive, not only to water, ammonia or acetic acid, but also to a number of other substances. Surface alcoho-lates are products of the interaction with alcohols [31] and carboxylate surface structures are formed from a fraction of adsorbed alcohol molecules [32]. The action of hydrofluoric acid [33—35], as well as impregnation by BF3 [31,34], increases the acidity of alumina. 

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. 

The surface structure and acid sites of alumina-supported molybdenum nitride catalysts have been studied using temperature-programed desorption (TPD), and reduction (TPR), diffuse reflectance infrared spectroscopy, and X-ray diffraction (XRD) analysis. The nitride catalysts were prepared by the temperature-programmed reaction of alumina-supported molybdenum oxide (12.5% and 97.1%) with NH3 at temperatures of 773, 973, and 1173 K. TPR and XRD analyses showed that y-Mo2N was already formed at 973 K. On the basis of NH3-TPD measurements and IR spectroscopy, it was found that Lewis acid sites were predominant over Bronsted acid sites on the surface of Mo2N/A1203. 

Although the synthesis and catalytic activity of unsupported and alumina-supported molybdenum nitride have been studied extensively, much less attention has been given to examining the surface structure and... 

Figure 4. Schematic of proposed surface structure of a low (1 1) Pt/Sn-alumina catalyst.

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