Structurally Ordered Oxides

Nuclear Magnetic Resonance Studies of Interfacial Phenomena 

FIGURE 2.80 MAS NMR spectrum of an aluminum-containing hydrated MCM-41 with signals at 53 and 0 ppm due to Aljy and Alyi species, respectively. (Taken from Micropor. Mesopor. Mater., 125, Meynen, V., Cool, P., and Vansant, E.F., Verified syntheses of mesoporons materials, 170-223, 2009, Copyright 2009, with permission from Elsevier, and decomposed here.) 

Ganapathy et al. (2004) studied molecular sieve ETAS-10 and showed that the silicon sites tetrahedrally connected to aluminum in framework positions of a molecular sieve can be identified by a selective reintroduction of the heteronuclear Al- Si dipolar interaction through Rotational Echo Adiabatic Passage DOuble Resonance (REAPDOR) NMR. They used an effective dipolar dephasing of the Si-O-Al, over Si-O-Si, environments to identily silicon sites in the immediate vicinity of aluminum. The direct NMR estimation of Al-Si distance gave =0.323 nm. REAPDOR 

FIGURE 2.81 Changes in the content of Si states 04 (-115 ppm), 0j (-103 ppm), and 0j (-92 ppm) in WSZ materials calcnlated here on the basis of the Si MAS NMR spectra. (Published by J. Solid-State Chem., 180, Chen, L.F., Wang, J.A., Norena, L.E. et al.. Synthesis and physicochemical properties of Zr-MCM-41 mesoporous molecular sieves and Pt/H3PW,j04o/Zr-MCM-41 catalysts, 2958-2972, 2007, Copyright 2007, with permission from Elsevier.) 

Shouro et al. (2000) modified mesoporous silica FSM-16 with various oxides. The BET areas of oxides (8 wt%) supported on FSM-16 were smaller than that of silica alone (1065 m /g) Fc203/FSM-16, CdO/FSM-16, and AI2O3/FSM-I6 have Sbet=979, 796, and 546 m /g, respectively. Intensities of all the XRD peaks of the catalysts were less than those of FSM-16. Thus, the pore structure of the catalysts was slightly destroyed by the impregnation process. 

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Vladimir M. Gun ko, DSc, is a professor and head of the Department of Amorphous and Structurally Ordered Oxides, Chuiko Institute of Surface Chemistry of the National Academy of Sciences of Ukraine. He graduated in theoretical physics from Dnipropetrovsk State University (DSU) in 1973 and received his PhD in chemistry from the Institute of Physical-Organic Chemistry (IPOC), Kiev, in 1983 and his DSc in physics, chemistry, and technology of surface from the Institute of Surface Chemistry (ISC) in 1995. He currently serves as professor of physics and chemistry of surface at ISC, where he has been since 1985. He has been elected Marie Curie Fellow at the University of Brighton (United Kingdom), 2010-2011. He has coauthored three books, edited one book, published about 400 papers, and made about 200 presentations at conferences. He also serves on the editorial board of four journals and Surface, a periodic book published one to two times per year. He is a member of the American Nano Society (the United States) and an electronic member of The Royal Society of Chemistry (the United Kingdom). 

The physical and chemical properties of any material are closely related to the type of its chemical bonds. Oxygen atoms form partially covalent bonds with metals that account for the unique thermal stability of oxide compounds and for typically high temperatures of electric and magnetic structure ordering, high refractive indexes, but also for relatively narrow spectral ranges of transparency. 

These three structures are the predominant structures of metals, the exceptions being found mainly in such heavy metals as plutonium. Table 6.1 shows the structure in a sequence of the Periodic Groups, and gives a value of the distance of closest approach of two atoms in the metal. This latter may be viewed as representing the atomic size if the atoms are treated as hard spheres. Alternatively it may be treated as an inter-nuclear distance which is determined by the electronic structure of the metal atoms. In the free-electron model of metals, the structure is described as an ordered array of metallic ions immersed in a continuum of free or unbound electrons. A comparison of the ionic radius with the inter-nuclear distance shows that some metals, such as the alkali metals are empty i.e. the ions are small compared with the hard sphere model, while some such as copper are full with the ionic radius being close to the inter-nuclear distance in the metal. A consideration of ionic radii will be made later in the ionic structures of oxides. 

While XAS techniques focus on direct characterizations of the host electrode structure, nuclear magnetic resonance (NMR) spectroscopy is used to probe local chemical environments via the interactions of insertion cations that are NMR-active nuclei, for example lithium-6 or -7, within the insertion electrode. As with XAS, NMR techniques are element specific (and nuclear specific) and do not require any long-range structural order in the host material for analysis. Solid-state NMR methods are now routinely employed to characterize the various chemical components of Li ion batteries metal oxide cathodes, Li ion-conducting electrolytes, and carbonaceous anodes.Coupled to controlled electrochemical in-sertion/deinsertion of the NMR-active cations, the... 

Soil A reversible equilibrium is quickly established when aniline covalently bonds with humates in soils forming imine linkages. These quinoidal structures may oxidize to give nitrogen-substituted quinoid rings. The average second-order rate constant for this reaction in a pH 7 buffer at 30 °C is 9.47 x 10 L/g-h (Parris, 1980). In sterile soil, aniline partially degraded to azobenzene, phenazine, formanilide, and acetanilide and the tentatively identified compounds nitrobenzene and jD-benzoquinone (Pillai et ah, 1982). 

Several other types of monomers are capable of yielding stereoisomeric polymer structures. Ordered structures are possible in the polymerization of carbonyl monomers (RCHO and RCOR ) and the ring-opening polymerizations of certain monomers. Thus, for example, the polymers from acetaldehyde and propylene oxide can have isotactic and syndiotactic structures as shown in Figs. 8-3 and 8-4. 

Almost all the iron oxides, hydroxides and oxide hydroxides are crystalline. The degree of structural order and the crystal size are, however, variable and depend on the conditions under which the crystals were formed. All Fe oxides display a range of crystallinities except for ferrihydrite and schwertmannite which are poorly crystalline. 

The emphasis of the present chapter is on the correlation of the physical properties and structures of oxide melts. Since long-range order is destroyed in the process of fusion, the meaning of structure is necessarily different for the crystalline solid and its melt. For the latter, structural information is often only obtainable at the present by indirect means such as the comparison of certain properties at a particular temperature. Here, a meaningful interpretation may become doubtful because of the lack of a corresponding temperature. For instance, if the melting points of two oxides differ by 1000°C, on what basis can a property of their respective melts be compared For such reasons, some of the conclusions regarding structure discussed below must be considered as qualitative and treated with reservations. 

Macroscopic experiments allow determination of the capacitances, potentials, and binding constants by fitting titration data to a particular model of the surface complexation reaction [105,106,110-121] however, this approach does not allow direct microscopic determination of the inter-layer spacing or the dielectric constant in the inter-layer region. While discrimination between inner-sphere and outer-sphere sorption complexes may be presumed from macroscopic experiments [122,123], direct determination of the structure and nature of surface complexes and the structure of the diffuse layer is not possible by these methods alone [40,124]. Nor is it clear that ideas from the chemistry of isolated species in solution (e.g., outer-vs. inner-sphere complexes) are directly transferable to the surface layer or if additional short- to mid-range structural ordering is important. Instead, in situ (in the presence of bulk water) molecular-scale probes such as X-ray absorption fine structure spectroscopy (XAFS) and X-ray standing wave (XSW) methods are needed to provide this information (see Section 3.4). To date, however, there have been very few molecular-scale experimental studies of the EDL at the metal oxide-aqueous solution interface (see, e.g., [125,126]). 

St. Pierre, T. G. Chan, R Bauchspiess, K. R. Webb, J. Betteridge, S. Walton, S. Dickson, D. P. E. Synthesis, structure and magnetic properties of ferritin cores with varying composition and degrees of structural order models for iron oxide deposits in iron-overload diseases. Coord. Chem. Rev. 1996, 151, 125-143. 

Calas, G. Petiau, J. (1983) Structure of oxide glasses Spectroscopic studies of local order and crystallochemistry. Geochemical implications. Bull. Mineral., 106,33-55. 

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