In Situ Raman Spectroscopy of Supported Chromium Oxide Catalysts: ^18 O2- ^16 O2 Isotopic Labeling Studies

The isothermal isotopic exchange reaction of 18 O2 with 16 O of chromium(VI) oxide supported on zirconia, alumina, and titania has been investigated with in situ laser Raman spectroscopy. The isotopic exchange reaction is dependent on the support type, the Cr loading, and the reaction temperature. Complete isotopic exchange of chromium(VI) oxide with 18 O2 is difficult to achieve and requires several successive butane reduction- 18 O2 oxidation cycles at relatively high temperatures. The efficiency of the isothermal isotopic exchange reaction increases from alumina over titania to zirconia and with increasing Cr loading and reduction temperature. The observed Raman shifts upon isotopic labeling are consistent with a mono-oxo surface chromium oxide(VI) species

In Situ Raman Spectroscopy of Supported Chromium Oxide Catalysts: Reactivity Studies with Methanol and Butane

The interactions of methanol and butane with supported chromium oxide catalysts under oxidizing and reducing conditions were studied by in situ Raman spectroscopy as a function of the specific oxide support (Al2O3, ZrO2, TiO2, SiO2, Nb2O5, 3% SiO2/TiO2, 3% TiO2/SiO2, and a physical mixture of SiO2 and TiO2) and chromium oxide loading (1-6 wt% CrO3). Two surface chromium oxide species were observed on the TiO2, ZrO2, Al2O3, 3% SiO2/TiO2, and a physical mixture of SiO2 and TiO2 surfaces: a monomeric species (characterized by a CrdO stretching frequency at ? 1030 cm -1 ) and a polymeric species (possessing a CrdO stretching frequency at ~ 1005-1010 cm -1 and a Cr-O-Cr bending mode at ~880 cm -1 ). The SiO2 and 3% TiO2/SiO2 surfaces possess only the monomeric species. The extent of reduction of the surface chromium oxide species, reflected by the decrease in the Raman intensity of the CrdO bonds, demonstrates that the polymeric surface chromium oxide species is more easily reducible than the monomeric chromium oxide species on the same support. The extent of reduction of the surface chromium oxide species strongly depends on the specific oxide support (3% TiO2/SiO2 = 3% SiO2/TiO2 = (SiO2 + TiO2) = TiO2 > ZrO2)

In Situ Spectroscopic Investigation of Molecular Structures of Highly Dispersed Vanadium Oxide on Silica under Various Conditions

The molecularly dispersed V2O5/SiO2 supported oxides were prepared by the incipient wetness impregnation of 2-propanol solutions of V-isopropoxide. The experimental maximum dispersion of surface vanadium oxide species on SiO2 was achieved at ~12 wt % V2O5 ( ~2.6 V atoms/nm^2 ). The surface structures of the molecularly dispersed V2O5/SiO2 samples under various conditions were extensively investigated by in situ Raman, UV- vis-NIR DRS and XANES spectroscopies. The combined characterization techniques revealed that in the dehydrated state only isolated VO4 species are present on the silica surface up to monolayer coverage. Interestingly, the three-member siloxane rings on the silica surface appear to be the most favorable sites for anchoring the isolated, three-legged (SiO)3 V=O species. Hydration dramatically changes the molecular structure of the surface vanadium oxide species. The specific structure of the hydrated surface vanadium oxide species is dependent on the degree of hydration. The molecular structure of the fully hydrated vanadium oxide species closely resembles V2O5·nH2O gels, rather than V2O5 crystallites. The fully hydrated surface vanadium oxide species are proposed to be chain and/or two-dimensional polymers with highly distorted square-pyramidal VO5 connected by V-OH-V bridges, which are stabilized on the silica surface by the sixth neighbor of Si-OH hydroxyls via Si-OH···V hydrogen bonds. In analogy to the hydration process, alcoholysis occurs during methanol chemisorption, and similar molecular structures are proposed to interpret the interaction between methanol molecules and the surface vanadium oxide species on silica

Oxidative dehydrogenation of propane over niobia supported vanadium oxide catalysts

Oxidative dehydrogenation (ODH) of propane is examined over a series of catalysts, which include Nb2O5 supported monolayer V2O5 catalysts, bulk vanadia-niobia with different vanadium oxide loadings and prepared by four different methods, V2O5and Nb2O5. The intrinsic activity (TOF) of the samples studied indicates that vanadium containing active sites are indispensable for catalytic oxidative dehydrogenation of propane. Variations in the chemical environment of the vanadium ion do not cause significant changes in activity per site and, hence, all samples show similar TOF when the rates are normalised to the concentration of V on the surface. Selectivity to propene on the other hand strongly depends on the nature of the catalyst because readsorption and interaction of propene with the acid sites leads to total oxidation. Optimization of the weak sorption of propene is, therefore, concluded to be the key factor for the design of selective oxidative dehydrogenation catalysts

Raman spectroscopy of supported chromium oxide catalysts : determination of chromium-oxygen bond distances and bond orders

An empirical correlation is described for relating Raman stretching frequencies of chromium—oxygen (Cr—O) bonds to their bond lengths in chromium oxide reference compounds. An exponential fit of crystallographically determined Cr—O bond lengths to Cr—O Raman symmetric stretching frequencies (800–1300 cm–1) is presented along with a relation between Cr—O bond strengths and Raman stretching frequencies. These empirical correlations have led to a systematic method for determining the coordination and bond lengths of chromates. The developed method is illustrated for chromates with unknown local structure in Bi2O3· K2CrO4 sillenite and for different supported chromium oxide catalysts. The obtained results are compared with those previously obtained by X-ray absorption spectroscopy

Structure and reactivity of surface vanadium oxide species on oxide supports

Supported vanadium oxide catalysts, containing surface vanadia species on oxide supports, are extensively employed as catalysts for many hydrocarbon oxidation reactions. This paper discusses the current fundamental information available about the structure and reactivity of surface vanadia species on oxide supports: monolayer surface coverage, stability of the surface vanadia monolayer, oxidation state of the surface vanadia species, molecular structures of the surface vanadia species (as a function of environment and catalyst composition), acidity of the surface vanadia species and reactivity of the surface vanadia species. Comparison of the molecular structure and reactivity information provides new fundamental insights into the catalytic properties of surface vanadia species during hydrocarbon oxidation reactions: (1) the role of terminal V=O, bridging V-O-V and bridging V-O-support bonds, (2) the number of surface vanadia sites required, (3) the influence of metal oxide additives, (4) the influence of surface acidic and basic sites, (5) the influence of preperation methods and (6) the influence of the specific oxide support phase. The unique physical and chemical characteristics of supported vanadia catalysts, comparedto other supported metal oxide catalysts, for hydrocarbon oxidation reactions are so discussed