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Dehydration surface spectroscopy

In the past few years, in situ Raman spectroscopy studies of supported metal oxide catalysts have focused on the state of the surface metal oxide species during catalytic oxidation reactions (see Table 2). As mentioned earlier, there has been a growing application of supported metal oxide catalysts for oxidation reactions. The influence of different reaction environments upon the surface molybdena species on Si02 was nicely demonstrated in two comparative oxidation reaction studies (see Fig. 4). The dehydrated surface molybdena on silica is composed of isolated species (no Raman bands due to bridging Mo—O—Mo bonds at —250 cm ) with one terminal Mo=0 bond that vibrates at —980 cm" The additional Raman bands present at —800, —600, and 500-300 cm in the dehydrated sample are due to the silica support. During methane oxidation, the surface... [Pg.820]

The participation of siloxane groups in the reaction increases with the temperature of dehydration of Si02 and quantity of organometallic compound introduced in the reaction. According to the data of infrared spectroscopy (139), the allyl ligands formed in the surface organometallic complexes of Ni and Cr keep the 7r-allyl character of the metal-ligand bond. [Pg.190]

Surface spectroscopic techniques must be separated carefully into those which require dehydration for sample presentation and those which do not. Among the former are electron microscopy and microprobe analysis, X-ray photoelectron spectroscopy, and infrared spectroscopy. These methods have been applied fruitfully to show the existence of either inner-sphere surface complexes or surface precipitates on minerals found in soils and sediments (13b,30,31-37), but the applicability of the results to natural systems is not without some ambiguity because of the dessication pretreatment involved. If independent experimental evidence for inner-sphere complexation or surface precipitation exists, these methods provide a powerful means of corroboration. [Pg.225]

Even though the vacuum-oriented surface techniques yield much useful information about the chemistry of a surface, their use is not totally without problems. Hydrated surfaces, for example, are susceptible to dehydration due to the vacuum and localized sample heating induced by x-ray and electron beams. Still, successful studies have been conducted on aquated inorganic salts (3), water on metals (3), and hydrated iron oxide minerals (4). Even aqueous solutions themselves have been studied by x-ray photoelectron spectroscopy (j>). The reader should also remember that even dry samples can sometimes undergo deterioration under the proper circumstances. In most cases, however, alterations in the sample surface can be detected by monitoring the spectra as a function of time of x-ray or electron beam exposure and by a careful, visual inspection of the sample. [Pg.390]

Structural characterization of the surface metal oxide species was obtained by laser Raman spectroscopy under ambient and dehydrated conditions. The laser Raman spectroscope consists of a Spectra Physics Ar" " laser producing 1-100 mW of power measured at the sample. The scattered radiation was focused into a Spex Triplemate spectrometer coupled to a Princeton Applied Research DMA III optical multichannel analyzer. About 100-200 mg of... [Pg.32]

In case of non-ionic surfactants in water, the behaviour of the water structure outlines three main concentration regions, which closely coincide with the three phases intersected by the experimental isotherms. In the micellar solution phase, no significant changes in the water structure are indicated, while, in the lamellar phase, rapid destruction of the tetrahedral hydrogen bond network occurs due to the confinement of the water between the hydrophilic surfaces of the lamellae. The dehydration of the surfactant head groups was found to start near the border between the lamellar and the reverse micellar solution phases. At higher concentrations, water demonstrates its trend to form clusters of tetrahedrally bonded molecules even at the very low content in the system. The results with surfactant solutions have been obtained by Raman spectroscopy (Marinov el al., 2001). [Pg.75]

In short, the reaction mechanism consists of a dehydration, a flip, and a hydration. The first and the last steps appear to be well defined on the basis of spectroscopy, crystallography, and chemical common sense. The details of the flip, perhaps the key feature of the mechanism, are less clear. There are no crystals of the cis-aconitate complex (in fact, there should be two different complexes, 5 and 7). The free-enzyme intermediate 6 has not been isolated. Displacement of one cis-aconitate by another one is expected to require significant conformational changes in the cleft from the catalytic center to the protein surface. It has not been established that the leaving and entering cA-aconitases are not, in fact, one and the... [Pg.217]

The interaction of acetylene with Mg(001) was investigated by LEED (198-200). At 88 K, C2H2 molecules lie almost parallel to the surface, and neither molecule nor substrate distortions have been observed, indicating that only a weak physisorption occurs. Calculations with semiempirical potentials confirm the structure determined by LEED (199). The isomerization of d.v-2-butene on MgO has been reported (201). The dissociation of cyclopen-tadiene on a few active acid-base pair sites of totally dehydrated MgO was followed by IR spectroscopy, and the formation of surface hydroxyl groups and C5H5 species was proposed (202). Methanol decomposition (203) and ethanol decomposition (204, 205) have been reported. [Pg.298]


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