Spectroscopic methods, characterization silica surface

Mesoporous silica materials with hierarchical tubule-within-tubule structures have been prepared and characterized by spectroscopic methods.97 Strong photoluminescence of these materials is explained as due to the presence of Si-OH complexes located on the nanotube surface, which also explains the persistence of the signal for some time after the pumping laser is turned off. 

The adsorption of surfactants onto ODS is more straightforward and well understood. Here, the formation of the monolayer of the surfactant on the ODS surface proceeds through the adsorption of the surfactant through its alkyl chain with the polar head group pointing into the bulk aqueous phase. Such an adsorption only results in a monolayer coverage, and the chromatographic behavior of these adsorbed monolayers can be discerned with established principles. The absorbed monolayers of surfactants on reversed-phase silica have been characterized by several spectroscopic methods [2] and will not be discussed here. 

Although numerous experiments and spectroscopic characterizations have been conducted on the Phillips catalyst, the precise structure of the active site on the silica surface, reduction of the surface chromate species during the induction period, the formation of the first chromium-carbon bond, and the mechanism for ethylene polymerization still need to be further clarified [11]. In order to achieve more specific information, molecular modeling approaches could provide a useful complement to the experiments and enable us to study these obscure mechanistic problems directly at the atomic and molecular level. In the last decade, very precise mechanistic pictures of the Cr-based polymerization catalysts have been obtained using different theoretical methods, especially through a combination of the experimental findings with theoretical calculations. 

Evidenced has been obtained previously that the reinforcing effect of different grades of fumed silicas on silicone elastomers is influenced by the surface fractality [1] and that the surface roughness increases with the specific surface energy. The aim of the present work is to demonstrate variations by calling on NMR and infrared spectroscopic methods, which are applied to fumed silica samples that have been carefully characterized through adsorption methods including IGC analysis. 

Chemically modified oxide surfaces are found in many industrial processes and in environmental analysis and remediation. Among the most frequently encountered of these uses are chemically modified separation materials and catalysts. Whatever the ultimate application of the modified oxide material is, it is the surface that determines the properties which are exploited for the intended use. Therefore a survey of spectroscopic techniques which can study the nature of surfaces and chemically modified surfaces will be applicable to most materials regardless of their specific use. This paper presents a selection of the most important and most frequently used spectroscopic methods for the characterization of chemically modified surfaces. In most cases, the examples are related to materials used in separation processes. The most commonly used separation support material is silica which is available in a wide range of physical formats, including porous and nonporous substances. When a porous matrix is required, silica can be obtained in a broad range of particle sizes with a well-defined pore structure and specific surface areas from a few square meters to hundreds of square meters per gram. 

Catalysis by Supported Metal-cluster Compounds. Further work has been reported recently on methods of chemically binding cluster compounds to supports and on the characterization of the resulting materials by various spectroscopic techniques. For example, the reaction of Rh6(CO)i6 with amine- and phosphine-modified silicas has been examined by infrared spectroscopy and has shown that cluster breakdown occurs giving L Rh(CO)2 and Lfn I (CO), where L comprizes the surface attached ligands. This behaviour is similar to that observed with Rh4(CO)i2 on unmodified silica where cluster breakdown occurs readily, particularly in the presence of traces of water and/or oxygen. ... 

Substantial progress in the elucidation of the surface structure of crystalline and amorphous silicas has been achieved by means of high-resolution spectroscopic techniques, for example, Si cross-polarization magic-angle spinning NMR spectroscopy and Fourier transform IR spectroscopy. The results lead to a better understanding of the acidity, dehydration properties, and adsorption behavior of the surface. These properties are key features in the design of novel advanced silica materials. The current methods of characterization are briefly reviewed and summarized. 

While silica is probably the most frequently encountered oxide surface, other materials particularly alumina, titania and zirconia also have considerable use and spectroscopic characterization is beneficial. One study mentioned above explored the potential for modifying alumina, zirconia, titania and thoria surfaces in a marmer similar to silica [6]. While bonding of the moiety is usually the method of choice, in some cases adsorption is sufficient. For example, polyacrylates adsorbed on alumina are a useful dispersent in the production of certain ceramic products. The successful adsorption of these compounds on aluminia has been monitored by FTIR using the carbonyl stretching frequency for the acrylate species which appears between 1602-1606 cm [15]. Polybutadiene which has been adsorbed on alumina for use as a chromatographic phase can be detected by FTIR [16]. A similar adsorption process has also been tested on zirconia [17,18]. It has also been shown that FTIR can be used to detect Langmuir-Blodgett layers on the metal surfaces of thin-film devices [19]. 

The use of IR and Raman spectroscopy as complementary analytical techniques (to other physical and theoretical methods) is unlimited, especially in the case of characterizations. Also, a major use of theoretical calculations is the prediction and validation of experimental (including IR and Raman) spectroscopic data, and some applications have been reported earlier. Thus, in the surface binding of DMMP, DIMP, DFP, and Sarin to silica, a DFT comparison with the experimental IR shifts showed that the theoretically-modelled adsorption sites are similar to those found by experiment, and in the case of Cyclophosphamide, the IR and Raman spectra were assigned from DFT calculations, also a homologous series of aminobisphosphonates were studied and characterized using IR and NMR spectroscopy. ... 

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