Raman spectroscopy, alumina-supported

The molecular state ot vanadium oxide supported on ditterent alumina phases (7, S-0, and a) was investigated with Raman spectroscopy. The supported vanadium oxide was "found to "form a molecularly dispersed overlayer on the di-f-ferent alumina phases. The molecular state oT the surTace vanadium oxide phase, however, was dependent on the nature oT the alumina support. This variation was primarily due to the presence oT surTace impurities, in particular sodium oxide. The surface sodium oxide content was found to increase with the calcination temperature required to form the different transitional alumina phases (a, 6-0, 7). The... 

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-... 

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... 

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. 

Surface vanadium appears to be most stable (to reduction) at low (<1%) V concentration when present as monomeric vanadyl units. Its stability decreases with increasing V levels. It is least stable (to reduction) at high (5%) V levels when present as a supported Vanadia phase. This difference in reactivity with V concentrations is believed responsible for the rapid decline in cracking activity observed in dual function cracking catalysts containing alumina when V start to exceed the 1.0-1.25 wt.% level (4). Further details of the mechanism of catalyst deactivation by V age the subject of continuing investigations 1n our laboratories by 31V solid state NMR, XPS, and Raman spectroscopy. 

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. 

A similar comparison of results of TPR/TPO-Raman spectroscopy with those of quantitative TPR has also been made for alumina-supported vanadia (Kanervo et al., 2003). However, the Raman signal of V205 crystals is at least ten times more intense than that of surface VOx species for excitation in the wavelength range of 514—532 nm, because of resonance enhancement (Xie et al., 2000). Thus, only a minor fraction of the surface VO species on alumina aggregated to form microcrystals during reduction and oxidation cycles. 

Similar reduction experiments were performed with alumina-supported chromia (Kanervo and Krause, 2001, 2002), and several complementary techniques were employed, including DRIFT, Raman, and EXAFS spectroscopies (Airaksinen et al., 2003). 

These results led to questioning of the presumed similar performance of polymeric and isolated surface species. Raman spectroscopy and UV-vis DRS investigations of zirconia- and alumina-supported vanadia showed that the oxidation state of the surface vanadium oxide species was determined by the propane-to-C>2 ratio in the feed (Garcra-Cortez and Banares, 2002). The combination of two spectroscopic techniques provided more detail about the structural state of the supported species during moderate reduction under reaction conditions (Gao et al., 2002). 

These observations are consistent with those of other UV-vis experiments (Puurunen and Weckhuysen, 2002 Puurunen et al., 2001). Raman spectroscopy of a working alumina-supported vanadia catalyst, showed that the surface population ratio of polymeric-to-isolated vanadia species decreased during reduction (as indicated by a relative loss in intensity of the 1009-cm 1 band relative to that of the 1017-cm 1 band), whereas the total activity and selectivity in propane ODF1 essentially remained unaffected (Garcia-Cortez and Banares, 2002). This result suggested that the active sites for ODH of propane and of ethane on alumina-supported vanadia should be isolated surface vanadia sites, whereas other arrangements of vanadium sites such as polymeric species did not seem to be crucial. 

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%... 

A series of supported niobium oxide on alumina catalysts, 0-45% Nb205/Al203, were further characterized by XRD, XPS, CO2 chemisorption, as well as Raman spectroscopy in order to determine the monolayer content of the Nb205/Al203 system. The transition from a two-dimensional metal oxide overlayer to three-dimensional metal oxide particles can be detected by monitoring the... 

Raman Spectroscopy of Vanadium Oxide Supported on Alumina... 

In the present investigation, the interaction ot vanadium oxide with dif-ferent alumina phases (7, 6-6, and a) is examined with Raman spectroscopy. Comparison ot the Raman spectra ot the supported vanadium oxide catalysts with those obtained -from vanadium oxide reference compounds allows for the structural assignment of these supported species. The present Raman data demonstrate that the molecular structures of the surface vanadium oxide phases are significantly influenced by the presence of surface impurities on the alumina supports and this overshadows the influence, if any, of the alumina substrate phase. 

In this case study, we have carried out further investigation into the structure of VO, supported on alumina under both oxidized (dehydrated) and reduced environments using both UV- (244 nm) and visible- (488 nm) excited Raman spectroscopy. Special attention has been directed towards the structure of supported VO at extremely low surface density (down to 0.01 Vnm ). 

In sim Raman spectroscopy of alumina-supported metal oxide catalysts. Journal of Physical Chemistry, 96 (12), 5008-16. 

S. and Jackson, S.D. (2005) On the structure of vanadium oxide supported on aluminas UV and visible Raman spectroscopy, UV-visible diffuse reflectance spectroscopy, and temperature-programmed reduction studies. Journal of Physical Chemistry, 109, 2793-800. 

Rhenium oxide supported on alumina is present on the surface as a single rhenium species. Laser-Raman spectroscopy studies by Kerkhof, MouUjn, and Thomas indicate that the species consists of tetrahedral Re04 ions, which are dynamically distorted by the carrier or surface hydroxyl groups, as illustrated by the broadening of the band at 916 cm". Neither laser-Raman spectroscopy nor X-ray diffraction indicates that other rhenium species, e.g., octahedrally co-ordinated rhenium or Al-Re-0 compounds, are formed. 

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