Franklin D. Hardcastle

Raman spectroscopy of chromium oxide supported on Al2O3, TiO2 and SiO2: a comparative study

The interaction of chromium oxide with Al2O3, TiO2 and SiO2 supports is investigated with Raman spectroscopy. The influence of the nature of the oxide support, calcination temperature and chromium oxide loading upon the molecular state of the supported chromium oxide is determined. The Raman studies reveal that the oxide supports stabilize the chromium oxide as Cr(VI) in tetrahedral coordination at moderate chromium oxide coverages. The surface chromium oxide is present as monomers and dimers on alumina, monomers and possibly dimers on titania, and monomers and polymers (dimers, trimers and tetramers) on silica. On the alumina support, the ratio of dimers/monomers increases with the chromium oxide coverage. On the silica support, the ratios of trimers/dimers and tetramers/dimers also increase with chromium oxide coverage. The surface chromium(VI) oxide species on titania, however, are not stable to elevated calcination temperatures and appear to be converted to a lower chromium oxide oxidation state. The silica support stabilizes the surface chromium(VI) oxide state at elevated calcination temperatures, but the surface chromium oxide polymers are not stable and convert to isolated surface chromium(VI) oxide monomers. Most of these differences are thought to be related to the differing surface-hydroxyl chemistries of Al2O3, TiO2 and SiO2 supports.

The molecular structure of bismuth oxide by Raman spectroscopy

A new method is presented for interpreting the Raman spectra of bismuth oxides. The method relies on empirical relations between bismuth-oxygen (BiO) bond lengths, bond strengths, and Raman stretching frequencies. A least-squares exponential fit of crystallographically determined BiO bond lengths and Raman stretching frequencies is presented along with a relation between BiO bond strengths, in valence units, and Raman stretching frequencies. The empirical bond length/bond strength/Raman stretching frequency relationships lead to a unique and effective method of interpreting Raman spectra of bismuth oxide species. This method allows the systematic determination of the BiO bond lengths and oxygen coordination of a BiOx polyhedron from its Raman spectrum. The utility of the method is illustrated by estimating the Raman stretching frequencies for ideally symmetric bismuth oxide structures BiO4, BiO5, BiO6, BiO7, and BiO8. As a final, practical example the method is used to determine the bond lengths and coordinations of the bismuth oxide species in the β- and δ-phases of Bi2O3. This new approach for evaluating the Raman spectra of bismuth oxide species is expected to be generally applicable to all bismuth oxides, regardless of environment, physical state, or oxidation state.

Physicochemical properties of MoO3TiO2 prepared by an equilibrium adsorption method

The adsorption phenomena of molybdena species onto titania surfaces and the surface properties of the catalysts have been studied by using an equilibrium adsorption method. 95Mo NMR and UV spectroscopic studies show that the aqueous molybdena species vary as a function of the pH of the impregnating solution. For acidic pH values, polymeric species, Mo7O246 ions, are present, while in the basic solutions it is the monomeric MoO42− ions that are present. The adsorbed amounts of molybdate anion are strongly dependent on the pH of the impregnating solution and increase as an inverse function of the pH. XRD, Raman, and XPS data of the calcined samples show that monolayer coverage of molybdenum oxide is established at pH 3.98 (6.6 wt%). The Raman studies reveal that the molybdenum oxide monolayer is composed of distorted octahedra. At more acidic pH regions, pH < 3.98, crystalline MoO3 is formed above monolayer coverage. The results of catalytic oxidation of methanol show that the catalysts up to monolayer coverage of surface molybdate species possess higher turnover numbers than the catalysts possessing more than monolayer coverage (presence of crystalline MoO3. The primary methanol oxidation product is dimethoxymethane at low conversions; methyl formate is next in abundance. The selectivity for dimethyl ether, which occurred as a side reaction on the acidic sites of catalysts, increases as the Mo loading increases.

The structure of surface rhenium oxide on alumina from laser raman spectroscopy and x-ray absorption near-edge spectroscopy

Abstract The interaction of rhenium oxide with an alumina support is examined over a wide range of conditions (rhenium oxide loading, calcination temperature and environment) using laser Raman spectroscopy (LRS) and X-ray absorption near-edge spectroscopy (XANES). These structural probes reveal that supported rhenium oxide on alumina is present as an atomically dispersed surface [ReO4]ads species coordinated to the alumina support. The [ReO4]ads species possesses C3v symmetry which is consistent with the presence of three equivalent terminal Re-O bonds and one inequivalent Re-O bond as part of the Re-O-Al linkage to the alumina support. The [ReO4]ads species are solvated by water molecules at ambient conditions, and the solvating water molecules readily desorb on heating in dry environments. The surface coverage of the [ReO4]ads species declines with increasing calcination temperature, because the surface rhenium oxide species can apparently recombine in dry environments and elevated temperatures to yield gaseous dimeric Re2O7. Additional studies are required to determine if a [ReO4]ads surface species can be isolated on the alumina support in dry environments and at high rhenium oxide surface coverages. The supported rhenium oxide on alumina system is dynamic, and the state of rhenium oxide depends on temperature, moisture/water content and rhenium oxide loading.

Determination of niobium-oxygen bond distances and bond orders by Raman spectroscopy

Abstract An empirical correlation is established for relating Raman stretching frequencies of niobium-oxygen (NbO) bonds to their respective bond distances in niobium oxide compounds. The correlation is an exponential least-squares fit between measured Raman frequencies and reported crystallographic bond lengths. Niobium-oxygen bond strengths (in valence units) are also related to Raman stretching frequencies by incorporating the present result with that of a previously derived bond strength/bond length relation. The NbO correlation established in the present study is expected to offer invaluable insight into the structures of niobate species in chemical systems which are not amenable to analysis by diffraction or other spectroscopic techniques. In the present study, applications of the correlation are illustrated by predicting Raman stretching frequencies for perfect NbO4 and NbO6 structures, determining the NbO bond lengths in bismuth niobate, BiNbO4, as well as the bond length and bond strength of the terminal NbO bond for dehydrated surface niobate species in the Nb2O5/Al2O3 system.

Characterization of CrO3/Al2O3 catalysts under ambient conditions: Influence of coverage and calcination temperature

Abstract The CrO 3 /Al 2 O 3 catalyst system was investigated by Raman spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction, and BET surface area measurements in order to determine the molecular structures and monolayer coverage of the surface chromium oxide phase under ambient conditions. Up to a surface coverage of 9% CrO 3 /Al 2 O 3 , the chromium oxide is stabilized by the alumina support in the +6 oxidation state after calcination at 120–1050°C. The molecular structures of the chromium(VI) oxide surface species are a function of the surface coverage and calcination temperature because under ambient conditions the surface structures depend on the net surface pH at point of zero charge of the hydrated oxide surface. Increasing the surface coverage results in a decrease of the net surface pH and formation of more polymerized chromium oxide species. High calcination temperatures (⩾950°C) cause a reduction of the BET surface area, as well as a phase transformation of γ-Al 2 O 3 into θ,δ-Al 2 O 3 , and also result in an increase in the surface density of the chromium oxide overlayer (more polymerized surface chromium oxide species). Monolayer coverage is reached at ca . 12% CrO 3 /Al 2 O 3 and crystalline Cr 2 O 3 particles are found on the alumina surface together with surface chromium oxide species at higher loadings. At high calcination temperatures (⩾800°C), the Cr 2 O 3 crystalline particles react with the alumina support to form Cr(III) in solid solution with α-Al 2 O 3 (corundum). The α-Al 2 O 3 lattice is slightly expanded due to the incorporation of chromia.

Vanadium(V) Environments in Bismuth Vanadates: A Structural Investigation Using Raman Spectroscopy and Solid State 51V NMR

The Bi2O3V2O5 system was examined using Raman spectroscopy and solid state 51V wideline, magic-angle spinning (MAS), and nutation NMR spectroscopy. The methods are shown to be complementary in the identification of the various phases and in the characterization of their vanadium site symmetries. Most of the compositions examined (1:1 ≤ Bi:V ≤ 60:1) are multiphasic. Depending on the Bi:V ratio, the following phases have been identified: BiVO4, Bi4V2O11, a triclinic type-II phase, a cubic type-I phase, γ-Bi2O3 doped with V(V) (sillenite), and β-Bi2O3. Detailed spectroscopic characterization reveals that vanadium is tetrahedrally coordinated in all these compounds, and that the degree of symmetry increases with increasing Bi:V ratio. At the highest Bi:V ratios, the combined interpretation of the Raman and NMR data provides strong evidence for the presence of Bi5+O4 tetrahedra.

The interaction of V2O5 and Nb2O5 with oxide surfaces

Abstract The interaction of vanadium oxide and niobium oxide with Al 2 O 3 and TiO 2 supports is examined with Raman spectroscopy. The Raman spectra of the supported metal oxides reveal that the strong interaction of the metal oxides with Al 2 O 3 and TiO 2 supports results in the formation of two-dimensional surface metal oxide overlayers. The surface vanadium oxide overlayers are unstable to high calcination temperatures and readily undergo solid state transformations. The surface niobium oxide overlayers are much more stable to high-temperature calcination treatments and retard solid state transformations of the mixed oxide systems.

Raman Spectroscopy of Chromium Oxide Supported on Alumina, Titania, and Silica: A Comparative Study

The interaction of chromium oxide with alumina, titania, and silica supports is investigated with Raman spectroscopy. The influence of the nature of the oxide support, calcination temperature and chromium oxide loading upon the molecular state of the supported chromium oxide is determined. The Raman studies reveal that the oxide supports stabilize the chromium oxide as Cr(VI) in tetrahedral coordination at moderate chromium oxide coverages. The surface chromium oxide is present as monomers and dimers on alumina, monomers and possibly dimers on titania, and monomers and polymers (dimers, trimers, and tetramers) on silica. On the alumina support, the ratios of dimers/monomers increases with the chromium oxide coverage. On the silica support, the ratios of trimers/dimers and tetramers/dimers also increase with chromium oxide coverage. The surface chromium(VI) oxide species on titania, however, are not stable to elevated calcination temperatures and appear to be converted to a lower chromium oxide oxidation state. The silica support stabilizes the surface chromium(VI) oxide state at elevated calcination temperatures, but the surface chromium oxide polymers are not stable and convert to isolated surface chromium(VI) oxide monomers. Most of these differences are thought to be related to the differing surface-hydroxyl chemistries of alumina, titania, and silica supports.

Raman Spectroscopy of Surface Rhenium Oxide on Alumina, Silica, Titania, Niobia, and Magnesia

The interaction of rhenium oxide with a variety of support materials is examined as a function of rhenium oxide loading, calcination temperature, and moisture content. Raman spectroscopy is used to monitor the rhenium oxide/support interaction for alumina, silica, titania, niobia, and magnesia supports, and to determine the structure of the rhenium oxide surface species. The surface rhenium oxide is found to be fully oxidized, tetrahedrally coordinated, and molecularly isolated as [ReO4] ads species on the five support materials. Each support, however, possesses a unique [ReO4] ads species characteristic of that support because each support exhibits a unique degree of interaction with the surface species. Metathesis reactivity data are presented and correlated with each [ReO4] ads species.