Alumina Raman studies

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. 

FIGURE 16 Raman spectra recorded during propane ammoxidation on alumina-supported nanocrystalline V-Sb-O system and simultaneous activity data determined by online gas chromatography (Guerrero-Perez M.O., and Banares, M.A. Chem. Commun. 1292 (2002), Operando Raman study of alumina-supported Sb-V-O catalyst during propane ammoxidation to acrylonitrile with on line activity measurement, reproduced with permission of the Royal Society of Chemistry) (Guerrero and Banares, 2002). 

The aqueous preparation oT supported niobium oxide catalysts was developed by using niobium oxalate as a precursor. The molecular states oT aqueous niobium oxalate solutions were investigated by Raman spectroscopy as a -function o-f pH. The results show that two kinds o-f niobium ionic species exist in solution and their relative concentrations depend on the solution pH and the oxalic acid concentration. The supported niobium oxide catalysts were prepared by the incipient wetness impregnation technique and characterized by Raman, XRD, XPS, and FTIR as a -function o-f niobium oxide coverage and calcination temperature. The Raman studies reveal that two types o-f sur-face niobium oxide species exist on the alumina support and their relative concentrations depend on niobium oxide coverage. Raman, XRD, XPS, and FTIR results indicate that a monolayer oT sur-face niobium oxide corresponds to 19%... 

More detailed studies demonstrate that crystalline Al2(W04)3 is produced from the direct solid-state reaction between crystalline WO3 and the alumina support and occurs by the migration of alumina to the WO3 particles at these elevated temperatures. The initial transformation of the surface tungsten-oxide phase to crystalline WO3 particles at elevated temperatures, when the alumina surface area decreases, reveals that the tungsten-oxide species are not miscible in the alumina support and are actually diffusing away from the alumina support. Thus, it appears that the surface tungsten oxide-alumina mixed oxide forms an "immiscible oxide system" because of the inability of the alumina support to accommodate the large charge and size of the cation. More recent Raman studies... 

A resonance Raman study of HOO-Com(bleomycin) shows that vCo-OOH is at 545 cm-1 (518 cm-1 for lsO).329 The Raman spectra of alumina-supported Co and Ag/Co catalysts showed the presence of C03O4. 30... 

Chiistodoulakis, A., Heracleous, E., Lemonidou, A., etal (2006). An Operando Raman Study of Structure and Reactivity of Alumina-supported Molybdenum Oxide Catalysts for the Oxidative Dehydrogenation of Ethane, J. Catal., 242, pp. 16-25. 

Guerrero-Perez, M. and Banares M. (2002). Operando Raman Study of Alumina-supported Sb-V-O Catalyst during Propane Ammoxidation to Acrylonitrile with On-line Activity Measurement, Chem. Commun., 12, pp. 1292-1293. 

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. 

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... 

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,... 

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. 

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. 

Application of Raman spectroscopy to a study of catalyst surfaces is increasing. Until recently, this technique had been limited to observing distortions in adsorbed organic molecules by the appearance of forbidden Raman bands and giant Raman effects of silver surfaces with chemisorbed species. However, the development of laser Raman instrumentation and modern computerization techniques for control and data reduction have expanded these applications to studies of acid sites and oxide structures. For example The oxidation-reduction cycle occurring in bismuth molybdate catalysts for oxidation of ammonia and propylene to acrylonitrile has been studied in situ by this technique. And new and valuable information on the interaction of oxides, such as tungsten oxide and cerium oxide, with the surface of an alumina support, has been obtained. 

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