Ceramics surface sites

It is worthwhile, however, emphasizing the extensive work that has been carried out with ISS on ceramic surface sites [I3.I4[. The review by Brongersma et al. 114 summarizes studies of composition, structure, diffusion, segregation, growth, adsorption, desorption, depth profiling, spunering. neutralization, an-... 

Aluminum is produced commercially by the electrolysis of cryolite, Na3AlF6, but bauxite, A1203, is the usual naturally occurring source of the metal. The oxide is a widely used catalyst which has surface sites that function as a Lewis acid. A form of the oxide known as activated alumina has the ability to adsorb gases and effectively remove them. Other uses of the oxide include ceramics, catalysts, polishing compounds, abrasives, and electrical insulators. 

Experimental studies of the polyphase ceramics noted above demonstrate that hollandite is one of the most durable titanate phases in aqueous solutions. Pham et al. (1994) carried out experimental work on synthetic Ba-hollandite doped with Cs and containing Al on the B-site for charge balance. These authors suggested that, following the initial release of Cs and Ba from reactive surface sites the first few monolayers of the structure rapidly dissolved due to the release of Al and consequent precipitation of Al-OH species, driving solution pH to lower values. However, the alteration process was mediated via the formation of a continuous Al- and Ti-rich surface layer. Further evidence for selective removal of Ba and enrichment of Al and Ti on the surface of hollandite tested at 250-300 °C was presented by Myhra et al. (19886). These conclusions were largely based on the different release rates of Ba (10 g/m2/d), Al (7 x 10 3 g/m2/d), and Ti (<8 x 10 4 g/ m2/d) after 14 days of dissolution testing, combined with XPS analyses of the altered surfaces. 

To account for the apparent strong adsorption of electrolyte ions at the ceramic powder/water interface, Yates et al. [10,21] proposed the formation of ion pairs at charged surface sites (the site-binding model for the metal oxide/aqueous solution interface). This model and its modifications have been successfully applied to many oxide/aqueous solution interfaces in the presence of simple monovalent inorganic ions [21-23]. For a ceramic powder surface in a simple electrolyte solution (e.g., KNO3 and NaCl), the formation of ion pairs can be represented as... 

Incorporation of HAp into wet PA-1 ITIO significantly modified its viscoelastic properties (Figure 13.4). It is interesting to note that in the presence of HAp, a new secondary relaxation is evident at T hap -10°C. A concomitant decrease in the high-temperature side of the tan 5 peak of the a- and j8-relaxation associated with amide sites indicates that these sites interact with themselves or with water molecules rather than directly with the ceramic surface. This results in a 15°C shift to a lower T of the primary a -loss peak as early as for 2.5 wt% HAp. The related increases mobility results from a plasticization Nanoparticles break intermolecular bonds by establishing new ones and decrease the overall interaction strength between macromolecules. The vitreous and rubbery moduli increase linearly with HAp content... 

Krilov OV, KisUev VF (1981) Adsorption and catalysis on the transition metals and their oxides. Chemistry, Moscow Kroger FA, Vink HJ (1956) Relations between concentrations of imperfections in crystaUine solids. In Seitz F, TumbuU D (eds) Solid state physics, vol 3. Academic, New York, pp 307-435 Kulkami D, Wachs IE (2002) Isopropanol oxidation by pure metal oxide catalysts number of active surface sites and turnover frequencies. Appl Catal A 237 121-137 Kulwicki BM (1991) Humidity sensors. J Am Ceram Soc 74 697-708... 

The application of nuclear methods to studies of ceramic surfaces and interfaces is a well-developed methodology, although not as yet applied extensively to mineral surfaces. Relevant applications of RBS, PIXE, NRA, ERDA and high-resolution a-spectroscopy have been described in the review by Matzke [9.1]. Analyses by RBS, used in the channeling mode, and NRA have revealed structural defects in U Ov-y RBS is sensitive mainly to the metal atom sublattice. whereas NRA can be used selectively to study the nonmeial sublattice. It was shown that the U sublattice is nearly identical to that of UO2, whereas significant cluster formation occurs in the O sublattice these observations have implications for surface sites in the material. Investigations of surface reactions (e.g., leaching and dis.soIution) of titanates and UO2 by nuclear techniques have... 

The examples discussed in Section 4.3 in connection with minerals are al.so illustrative of the infomiation required about surface sites in ceramic materials. The techniques applied on a routine basis in order to obtain this information are essentially the same. 

In this brief review we illustrated on selected examples how combinatorial computational chemistry based on first principles quantum theory has made tremendous impact on the development of a variety of new materials including catalysts, semiconductors, ceramics, polymers, functional materials, etc. Since the advent of modem computing resources, first principles calculations were employed to clarify the properties of homogeneous catalysts, bulk solids and surfaces, molecular, cluster or periodic models of active sites. Via dynamic mutual interplay between theory and advanced applications both areas profit and develop towards industrial innovations. Thus combinatorial chemistry and modem technology are inevitably intercoimected in the new era opened by entering 21 century and new millennium. 

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