Supported Raman spectroscopy

In addition to the many applications of SERS, Raman spectroscopy is, in general, a usefiil analytical tool having many applications in surface science. One interesting example is that of carbon surfaces which do not support SERS. Raman spectroscopy of carbon surfaces provides insight into two important aspects. First, Raman spectral features correlate with the electrochemical reactivity of carbon surfaces this allows one to study surface oxidation [155]. Second, Raman spectroscopy can probe species at carbon surfaces which may account for the highly variable behaviour of carbon materials [155]. Another application to surfaces is the use... 

The relative abilities of nitromethane, sulpholan, and acetic acid to support the ionisation of nitric acid to nitronium ions are closely similar to their efficiencies as solvents in nitration. Raman spectroscopy showed that for a given concentration of mixed acid (i i nitric and sulphuric acids) the concentration of nitronium ions in these three solvents varied in the order nitromethane > sulpholan > acetic acid. The concentration of mixed acid needed to permit the spectroscopic detection of nitronimn ions was 25 %, 50 % and 60 % in the three solvents, respectively (see 4.4.3). 

Raman spectroscopy has provided information on catalytically active transition metal oxide species (e. g. V, Nb, Cr, Mo, W, and Re) present on the surface of different oxide supports (e.g. alumina, titania, zirconia, niobia, and silica). The structures of the surface metal oxide species were reflected in the terminal M=0 and bridging M-O-M vibrations. The location of the surface metal oxide species on the oxide supports was determined by monitoring the specific surface hydroxyls of the support that were being titrated. The surface coverage of the metal oxide species on the oxide supports could be quantitatively obtained, because at monolayer coverage all the reactive surface hydroxyls were titrated and additional metal oxide resulted in the formation of crystalline metal oxide particles. The nature of surface Lewis and Bronsted acid sites in supported metal oxide catalysts has been determined by adsorbing probe mole-... 

The global minimum of a neutral Li molecule with AN valence electrons N= 1) does not adopt a square structure (D ) but a rhombus structure (D ) [43]. The Raman spectroscopy supported the rhombic structure for the Li [44], Na and clusters [45,46]. 

However, a recent study of the lithium ion complexation with N-labelled polyphosphazenes, including N-MEEP, was performed by Luther [600]. The data obtained for the MEEP/LiSOjCFj system by NMR, IR and Raman spectroscopies do not support that assumption, and show that the coordination of the lithium ion also occurs with the nitrogen nuclei. 

In reality, several factors were mentioned as being responsible for this behavior, such as variations in bond angle distortion, in the internal stress or in the hydrogen content [40, 76], but all of them are also strongly correlated with the variation of optical gap width in amorphous carbon films. Theoretical work on Raman spectroscopy on DLC materials gave additional support for Dillon s interpretation [77]. 

It is important to stress that nitrogen incorporation in a-C H films always result, at least above a certain level, in a strong decrease in the tetrahedrally bonded carbon atom fraction. Raman spectroscopy also gives support to this observation, because the increase in the size of graphitic clusters only can proceed with also increasing sp fraction. 

Raman spectroscopy has been used for a long time in order to study supported and promoted metal catalysts and oxide catalysts [84] since many information can be obtained (1) identification of different metal oxide phases (2) structural transformations of metal oxide phases (3) location of the supported oxide on the oxide substrate and... 

Hess, C. and Lunsford J.H. (2002) Mechanism for N02 Storage in Barium Oxide Supported on Magnesium Oxide Studied by in Situ Raman Spectroscopy,./. Phys. Chem. B, 106, 6358. 

Vanduyne R.P., Hulteen J.C., Treichel D.A., Atomic-force microscopy and surface-enhanced Raman-spectroscopy. 1. Ag island films and Ag film over polymer nanosphere surfaces supported on glass, J. Chem. Phys. 1993 99 2101-211. 

Raman spectroscopy is emerging as a powerful analytical tool in the pharmaceutical industry, both in PAT and in qualitative and quantitative analyses of pharmaceuticals. Reviews of analyses of pharmaceuticals by Raman spectroscopy have been published.158 159 Applications include identification of raw materials, quantification of APIs in different formulations, polymorphic screening, and support of chemical development process scale-up. Recently published applications of Raman spectroscopy in high-throughput pharmaceutical analyses include determination of APIs in pharmaceutical liquids,160,161 suspensions,162 163 ointments,164 gel and patch formulations,165 and tablets and capsules.166-172... 

Note that in all the examples discussed so far, infrared spectroscopy gives its information on the catalyst in an indirect way, via hydroxyl groups on the support, or via the adsorption of probe molecules such as CO and NO. The reason why it is often difficult to measure the metal-oxide or metal-sulfide vibrations of the catalytically active phase in transmission infrared spectroscopy is that the frequencies are well below 1000 cm-1, where measurements are difficult because of absorption by the support. Infrared emission and Raman spectroscopy, discussed later on in this chapter, offer better opportunities in this respect. 

A strong point of Raman spectroscopy for research in catalysis is that the technique is highly suitable for in situ studies. The spectra of adsorbed species interfere weakly with signals from the gas phase, enabling studies under reaction conditions to be performed. A second advantage is that typical supports such as silica and alumina are weak Raman scatterers, with the consequence that adsorbed species can be measured at frequencies as low as 50 cm-1. This makes Raman... 

Figure 1.13 Raman spectra for a number of transition metal oxides supported on y-AI203 [75,102], Three distinct regions can be differentiated in these spectra, namely, the peaks around 1000 cm-1 assigned to the stretching frequency of terminal metal-oxygen double bonds, the features about 900 cm 1 corresponding to metal-oxygen stretches in tetrahedral coordination sites, and the low-frequency (<400 cm-1) range associated with oxygen-metal-oxygen deformation modes. Raman spectroscopy can clearly complement IR data for the characterization of solid catalysts. (Reproduced with permission from The American Chemical Society.)...

In the preceeding section mention was made of ion association (ion-pairing) which, for the purposes of this paper, will refer to coulombic entities with or without cosphere overlap. Experimental support for ion-pairing has come from sound attenuation (2). Raman spectroscopy (2) and potentiometry (2, 2). Credibility has resulted from the model of Fuoss (2) applied by Kester and Pytkowicz (2). 

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