Raman alumina

Vuurman M A and Waohs I E 1992 In situ Raman speotrosoopy of alumina-supported metal oxide oatalysts J. Rhys. Chem. 96 5008-16... 

Fig. 4.56. Schematic diagram of a SERS-active substrate and the measurement arrangement. Alumina nanoparticles are deposited on a glass surface and produce the required roughness. A thin silver layer is evaporated on to the nanoparticles and serves for the enhancement. Organic molecules adsorbed on the silver surface can be detected by irradiation with a laser and collecting the Raman scattered light.

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-... 

This review will endeavor to outline some of the advantages of Raman Spectroscopy and so stimulate interest among workers in the field of surface chemistry to utilize Raman Spectroscopy in the study of surface phenomena. Up to the present time, most of the work has been directed to adsorption on oxide surfaces such as silicas and aluminas. An examination of the spectrum of a molecule adsorbed on such a surface may reveal information as to whether the molecule is physically or chemically adsorbed and whether the adsorption site is a Lewis acid site (an electron deficient site which can accept electrons from the adsorbate molecule) or a Bronsted acid site (a site which can donate a proton to an adsorbate molecule). A specific example of a surface having both Lewis and Bronsted acid sites is provided by silica-aluminas which are used as cracking catalysts. 

UV diffuse reflectance spectra of the titanates were obtained with a JASCO UVIDEC-660 spectrophotometer using a sintered alumina disc as a reference. Raman spectra were recorded at room temperature on a JASCO NR-1100 spectrometer. 

Bello J.M., Stokes D.L., Vo-Dinh T., Silver-coated alumina as a new medium for surfaced-enhanced Raman-scattering analysis, Appl. Spectrosc. 1989 43 1325-1330. 

Li Y.S., Wang Y., Chemically prepared silver alumina substrate for surface- enhanced Raman-scattering, 4/ /)/. Spectrosc 1992 46 142-146. 

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... 

Even more remarkable differences arise after calcination at 775 K. The sharp Raman spectra of the calcined Mo03/Si02 catalysts are those of crystalline Mo03. No crystalline Mo03 is present in the calcined Mo03/Al203 catalysts, however. Here the Raman spectra show similarities with those of the polymolybdates, which indicates that these species have a sufficiently strong interaction with the alumina support to withstand calcination at 775 K. 

Figure 8.12 Raman spectra of alumina- and silica-supported molybdena catalysts after impregnation of the supports with solutions of ammonium heptamolybdate, (NH4)6Mo7024 4 H20 of different pH values, and after calcination in air at 775 K. See Table 8.3 for a list of characteristic Raman frequencies of molybdate species. The sharp peaks in the spectra of the calcined MoOySiOj catalyst are those of crystalline Mo03 (from Kim el at. [43J).

Fig. 3 Raman spectra of Co/y-Al203 and CoRu/y-Al203 after different pretreatments, CoO, C03O4 (spinel), and CoA1204 (spinel).26 Reprinted from Journal of Catalysis, Vol. 204, J. Jongsomjit, J. Panpranot and J. G. Goodwin, Jr, Co-Support compound formation in alumina-supported cobalt catalysts, pp. 98-109. Copyright (2001), with permission from Elsevier.

The high sensitivity of tunneling spectroscopy and absence of strong selection rules allows infrared and Raman active modes to be observed for a monolayer or less of adsorbed molecules on metal supported alumina. Because tunneling spectroscopy includes problems with the top metal electrode, cryogenic temperatures and low intensity of some vibrations, model catalysts of evaporated metals have been studied with CO and acetylene as the reactive small molecules. Reactions of these molecules on rhodium and palladium have been studied and illustrate the potential of tunneling spectroscopy for modeling reactions on catalyst surfaces,... 

Surface-enhanced Raman scattering using a silver-coated alumina support selectively enhanced the spectrum of p-aminobenzoic acid. This allowed the determination of this compound at low ppm levels in vitamin B complex354,355. 

The use of surface-enhanced resonance Raman spectroscopy (SERRS) as an identification tool in TLC and HPLC has been investigated in detail. The chemical structures and common names of anionic dyes employed as model compounds are depicted in Fig. 3.88. RP-HPLC separations were performed in an ODS column (100 X 3 mm i.d. particla size 5 pm). The flow rate was 0.7 ml/min and dyes were detected at 500 nm. A heated nitrogen flow (200°C, 3 bar) was employed for spraying the effluent and for evaporating the solvent. Silica and alumina TLC plates were applied as deposition substrates they were moved at a speed of 2 mm/min. Solvents A and B were ammonium acetate-acetic acid buffer (pH = 4.7) containing 25 mM tributylammonium nitrate (TBAN03) and methanol, respectively. The baseline separation of anionic dyes is illustrated in Fig. 3.89. It was established that the limits of identification of the deposited dyes were 10 - 20 ng corresponding to the injected concentrations of 5 - 10 /ig/ml. It was further stated that the combined HPLC-(TLC)-SERRS technique makes possible the safe identification of anionic dyes [150],... 

A. Christodoulakis, E. Heracleous, A.A. Lemonidou and S. Boghosian, An operando Raman study of structure and reactivity of alumina-supported molybdenum oxide catalysts for the oxidative dehydrogenation of ethane, J. 

Alumina, chlorided alumina, Laser Raman spectra of chemisorbed 60... 

Brown and Makovsky (77) have studied the Raman spectra of CoMo/Al commercial catalyst containing 5% Si02. The oxidized catalyst showed bridged Mo—O—Mo and terminal Mo=0 structures with no evidence for bulk Mo03. The sulfided catalyst showed spectra similar to MoS2 (also detected by XRD). There is reason to believe that Mo interaction with silica-alumina is weaker than with A1203 (28) thus, sulfiding may more easily destroy the surface interaction complex present in the oxidized catalyst. 

The vibrational spectrum of 4-pyridine-carboxylic acid on alumina in Fig. 4d is equivalent to an infrared or Raman spectrum and can provide a great deal of information about the structure and bonding characteristics of the molecular layer on the oxide surface. For example, the absence of the characteristic > q mode at 1680 cm 1 and the presence of the symmetric and anti-symmetric O-C-O stretching frequencies at 1380 and 1550 cm indicate that 4-pyridine-carboxylic acid loses a proton and bonds to the aluminum oxide as a carboxylate ion. 

Cortez, G.G. Banares, M.A. A Raman Spectroscopy Study of Alumina-Supported Vanadium Oxide Catalyst During Propane Oxidative Dehydrogenation with Online Activity Measurement /. Catal. 2002, 209, 197-201. 

Figure 7. Peak areas of Raman bands for the adsorption of pyridine on alumina. (Reproduced from Ref. 16. Copyright 1974, American Chemical Society.)...

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