Bulk Structure Alumina

No dependence of F relaxation time on adsorbed water was observed for concentrated fluorine-doped aluminas (- 12 wt. % F). Since the bulk structure of the solids is known to be unaffected by adsorbed water, the effect of physically adsorbed water on the F relaxation time is a surface phase effect. There are several mechanisms of relaxation by which it is possible to explain the influence of adsorbed water in a qualitative manner ... 

On the other hand, lead poisoning deactivation curves have non-selective characteristics (Type A). These result when the poison is less strongly chemisorbed, and it tends to suffer many collisions with the alumina washcoat structure before chemisorption. Consequently, lead is found deep inside the washcoat structure, as is demonstrated by electron probe microanalysis, and the more accessible metal sites are left active to gaseous reactants by the faster bulk transport processes. 

This chapter does not cover the most common aspects of the solid-state NMR techniques employed in the study of heterogeneous catalysts such techniques are described in Chapter 4. Since this chapter emphasizes the surface characterization of silica and alumina systems and silica aluminas by NMR methods, only those technical aspects highly relevant to surface characterization and not otherwise emphasized in this volume are explicitly discussed here. NMR studies of zeolites and clays are treated in separate chapters, and the bulk structures of silica and alumina systems are covered by Eckert. Unavoidably this chapter is also concerned with dynamics at the surface, although the amount of detailed work on that subject to date is limited. With the increasing availability of variable-temperature solid-state NMR equipment, however, one can expect that attention devoted to dynamics at surfaces will increase markedly during the next few years. 

To understand the varied chemistry of the aluminas, techniques need to be developed for studying the surface independent of the bulk. Structural and dynamical aspects of the surface do have their origins in the bulk, but the specific details delineating the surface will be different. Clearly it would be advantageous to apply the same surface selective CP methodology developed for the silicas [12-16] to the surface of the aluminas. Before addressing this particular point, however, we need to consider the feasibility of the experiment. Are the aluminum atoms at the surface indeed observable by NMR methods If surface aluminum atoms are observable, we must then recognize that the spin of interest, Al, is not a spin-1/2 nuclide (/ for Al is 5/2) hence Al has a nonzero nuclear electric quadrupole moment. Cross-polarization from protons to a quadrupolar nucleus presents the experimenter with another layer of complication in compari-... 

Even though the structure of active sites has been specified, it seems essential that the bulk structure of silica-alumina composites be described in a manner which is consistent with the data. The high surface area (500-600 ... 

As to the transition aluminas, the literature up to 1977 has been reviewed by Knozinger and Ratnasamy. In this contribution, the authors also discuss the possible structures of the surface starting from the bulk structure of defective spinels typical of transition aluminas. It is assumed that the T7-phase preferentially exposes the (HI) face and the 7-phase the (110) face. By this, both octahedral and tetrahedral ions are at the surface, in contrast with earlier models, which only envisaged octahedral cationic sites. The situation is further complicated by the fact that both in the case... 

In Chapter 3, Busca summarizes the current state of knowledge of aluminas, the various polymorphs of which constitute some of the most commonly used catalyst components. The author starts with a discussion of the bulk structures of transition aluminas, which are the intermediate phases formed in the thermal transformation of aluminum oxyhydroxides into the thermodynamically most stable modification, a-alumina. Crucial are the definitions of the various phases, which are based on the methods of preparation rather than on the structural properties. The understanding of many alumina structures is incomplete, and progress, even with modem analytical methods and theory, is hampered by the defective and disordered nature of these materials. The stabilities of the various phases are governed by both thermodynamics and kinetics, either of which can be affected by impurities. The uncertainties in the surface stmctures are even greater than those of the bulk stmctures. Numerous models of alumina surface stmctures have been formulated over decades, but the tme stmctures seem to become even more elusive. Busca concludes his chapter with a list of research needs. 

The variations in the thermodynamic stability of the bulk structure are not necessarily significant enough to determine which of the polymorphs exists under particular conditions. It recendy became evident that the contribution of the surface to the overall energy and thus stability is larger than that of the bulk, provided the crystaUites are small (i.e., in the case of fine powders) (160). Moreover, as will be discussed in Section 2.2.10, the phase transformations may proceed so slowly that kinetics can have a more prominent role than thermodynamic stability in determining which of the alumina polymorphs actually exist. 

Co(II) sorption complexes on rutile surfaces appear to form extensions of the bulk structure, with EXAFS-derived Co-Ti distances very similar to Ti-Ti distances in rutile. The small number of Ti second neighbors detected by EXAFS spectroscopy indicates that Co occurs dominantly in Ti-equivalent sites at the rutile surface (21). A bond-valence analysis of Co(II) sorption on rutile also indicates that Co(II) can bond stably to a variety of surface oxygen sites (26). When the sorption density of Co is high on rutile, O Day et al (21) found Co-Ti distances characteristic of anatase. The observed differences in the style of Co sorption on quartz, rutile, and alumina under similar conditions and sorption densities are consistent with structural differences among the three sorbents that result in different reactivities of available surface sites. 

Scientists from Politecnico di Milano and Ineos Vinyls UK developed a tubular fixed-bed reactor comprising a metallic monolith [30]. The walls were coated with catalytically active material and the monolith pieces were loaded lengthwise. Corning, the world leader in ceramic structured supports, developed metallic supports with straight channels, zig-zag channels, and wall-flow channels. They were produced by extrusion of metal powders, for example, copper, fin, zinc, aluminum, iron, silver, nickel, and mixtures and alloys [31]. An alternative method is extrusion of softened bulk metal feed, for example, aluminum, copper, and their alloys. The metal surface can be covered with carbon, carbides, and alumina, using a CVD technique [32]. For metal monoliths, it is to be expected that the main resistance lies at the interface between reactor wall and monolith. Corning... 

The application of infrared photoacoustic spectroscopy to characterize silica and alumina samples is reported. High quality infrared photoacoustic spectra illuminate structural changes between different forms of silica and alumina, as well as permit adsorbate structure to be probed. Adsorption studies on aerosil suggest adsorbed species shield the electric fields due to particle-particle interactions and induce changes in the vibrational spectra of the adsorbates as well as in the bulk phonon band. It is shown that different forms of aluminum oxides and hydroxides could be distinguished by the infrared spectra. 

The structure of alumina on NiAl(l 1 0) was the subject of a surface X-ray diffraction study by Stierle et al. [46]. The model derived by Stierle et al. from the analysis of the X-ray diffraction data was based on a strongly distorted double layer of hexagonal oxygen ions, where the Al ions are hosted with equal probability on octahedral- and tetrahedral-coordinated sites the resulting film structure was closely related to bulk k-A1203. An attractive feature of Stierle s model was that it provided a natural explanation of the domain structure of the alumina overlayer, which is induced by a periodic row matching between film and substrate lattices. However, as pointed out recently by Kresse et al. [47], this structure model has two bonds with... 

In summary, the alumina nanolayers formed by the high-temperature oxidation on NiAl alloy surfaces are structurally and chemically very different from the bulk-terminated surfaces of the various A1203 phases, and they thus provide very prototypical examples of oxide phases with novel emergent properties because of interfacial bonding and thickness confinement effects. 

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