Goutam Deo

Characterization of titania silicalites

Abstract Titanium-substituted silicalites, prepared from two different silicalite precursors, are characterized using Raman spectroscopy, X-ray absorption spectroscopy, pyridine adsorption, and methanol oxidation. The silicalite precursors used are tetraethyl orthosilicate and Ludox AS 40. Two different types of titania species are observed in the titania silicalites. Below 1.6 mole% Ti (Ti + Si) , a dispersed TiOx species is present that does not possess a terminal TiO bond or Bronsted acidity. The (average) structure of the TiOx species changes with the titania substitution into the silicalites, but this structural change, apparently, does not affect the reactivity of the catalytic active center during reactions involving liquid water. This TiOx species in silicalites is active toward redox reactions, and its reactivity is similar to the titania species present on the surface of amorphous SiO2 (Cab-O-Sil). Above 1.6 mole% Ti (Ti + Si) , TiO2 (anatase) particles are present in addition to the dispersed TiOx species. The TiO2 (anatase) particles are not effective in redox reactions and form weak Lewis acid sites. The titania silicalites produced from the different precursors were structurally similar, but differed chemically due to acidic impurities present in Ludox AS 40. The critical factor in determining the reactivities of Ti silicalite and surface titania supported on amorphous silica is the stability of the TiOSi bond in the presence of liquid water.

In situ IR, Raman, and UV-Vis DRS spectroscopy of supported vanadium oxide catalysts during methanol oxidation

The application of in situ Raman, IR, and UV-Vis DRS spectroscopies during steady-state methanol oxidation has demonstrated that the molecular structures of surface vanadium oxide species supported on metal oxides are very sensitive to the coordination and H-bonding effects of adsorbed methoxy surface species. Specifically, a decrease in the intensity of spectral bands associated with the fully oxidized surface (V5+) vanadia active phase occurred in all three studied spectroscopies during methanol oxidation. The terminal V = O (∼1030 cm−1) and bridging V–O–V (∼900–940 cm−1) vibrational bands also shifted toward lower frequency, while the in situ UV-Vis DRS spectra exhibited shifts in the surface V5+ LMCT band (>25,000 cm−1) to higher edge energies. The magnitude of these distortions correlates with the concentration of adsorbed methoxy intermediates and is most severe at lower temperatures and higher methanol partial pressures, where the surface methoxy concentrations are greatest. Conversely, spectral changes caused by actual reductions in surface vanadia (V5+) species to reduced phases (V3+/V4+) would have been more severe at higher temperatures. Moreover, the catalyst (vanadia/silica) exhibiting the greatest shift in UV-Vis DRS edge energy did not exhibit any bands from reduced V3+/V4+ phases in the d–d transition region (10,000–30,000 cm−1), even though d–d transitions were detected in vanadia/alumina and vanadia/zirconia catalysts. Therefore, V5+ spectral signals are generally not representative of the percent vanadia reduction during the methanol oxidation redox cycle, although estimates made from the high temperature, low methoxy surface coverage IR spectra suggest that the catalyst surfaces remain mostly oxidized during steady-state methanol oxidation (15–25% vanadia reduction). Finally, adsorbed surface methoxy intermediate species were easily detected with in situ IR spectroscopy during methanol oxidation in the C–H stretching region (2800–3000 cm−1) for all studied catalysts, the vibrations occurring at different frequencies depending on the specific metal oxide upon which they chemisorb. However, methoxy bands were only found in a few cases using in situ Raman spectroscopy due to the sensitivity of the Raman scattering cross-sections to the specific substrate onto which the surface methoxy species are adsorbed.

Physical and chemical characterization of surface vanadium oxide supported on titania: influence of the titania phase (anatase, rutile, brookite and B)

Abstract Different phases of titania were prepared and used to support ca. 1 wt.-% V 2 O 5 . The different titania phases prepared were: anatase (A22), rutile (R28), brookite (BT110) and B-phase (B18). Physical characterization of the various vanadia-titania catalysts was performed using X-ray photoelectron spectroscopy (XPS), in situ Raman and 51 V solid state nuclear magnetic resonance (NMR) spectroscopy. The XPS results reveal that the all the catalysts contain various levels of impurities. In situ dehydration Raman shows, for all the samples, the stretching vibration of the terminal VzO bond at ca. 1030 cm −1 . Solid state 51 V NMR spectra of all the samples in the dehydrated state show basically the same powder pattern with a peak maximum around −660 to −670 ppm. The combined Raman and NMR results indicate that the same surface vanadium oxide species is present on all the titania supports irrespective of the crystal structure of the bulk titania phase. Partial oxidation of methanol show similar activity and selectivity for the various vanadia-titania catalysts. The reaction selectivity was primarily to formaldehyde and methyl formate (92–96%). The turnover number for methanol oxidation was essentially the same for all the vanadia-titania catalysts and ranged from 1.4 to 2.8 s −1 . These results indicate that the type of titania phase used as the support is not critical for partial oxidation over vanadia-titania catalysts as long as other parameters (e.g. surface impurities ) are similar. Thus, the structure-reactivity studies of the different vanadia-titania catalysts suggest that the specific titania phase is not a critical parameter in determining the physical or chemical nature of the surface vanadia phase.

Effect of water vapor on the molecular structures of supported vanadium oxide catalysts at elevated temperatures

The effect of water vapor on the molecular structures of V2O5-supported catalysts (SiO2, Al2O3, TiO2, and CeO2) was investigated by in situ Raman spectroscopy as a function of temperature (from 500°;C to 120°;C). Under dry conditions, only isolated surface VO4 species are present on the dehydrated SiO2 surface, and multiple surface vanadium oxide species (isolated VO4 species and polymeric vanadate species) are present on the dehydrated Al2O3, TiO2, and CeO2 surfaces. The Raman features of the surface vanadium oxide species on the SiO2 support are not affected by the introduction of water vapor due to the hydrophobic nature of the SiO2 support employed in this investigation. However, the presence of water has a pronounced effect on the molecular structures of the surface vanadium oxide species on the Al2O3, TiO2, and CeO2 supports. The Raman band of the terminal V = O bond of the surface vanadia species on these oxide supports shifts to lower wavenumbers by 5–30 cm−1 and becomes broad upon exposure to moisture. Above 230°C, the small Raman shift of the surface vanadium oxide species in the presence of water suggests that the dehydrated surface VOx species form a hydrogen bond with some of the adsorbed moisture. Upon further decreasing the temperature below 230°C, the hydrogen-bonded surface VOx species are extensively solvated by water molecules and form a hydrated surface vanadate structure (e.g. decavanadate). The broad Raman band at ≈ 900 cm−1, which is characteristic of the polymeric VOV functionality, appears to be minimally influenced by the presence of water vapor and is a consequence of the broadness of this band. Oxygen-18 isotopic labeling studies revealed that both the terminal V=O and bridging VOV bonds readily undergo oxygen exchange with water vapor. The current observations account for the inhibiting effect of moisture upon oxidation reactions over supported metal oxide catalysts and are critical for interpreting in situ Raman data during hydrocarbon oxidation reactions where H2O is a reaction product.

Combined Raman and IR study of MOx–V2O5/Al2O3(MOx= MoO3, WO3, NiO, CoO) catalysts under dehydrated conditions

The influence of a second metal oxide (tungsten oxide, molybdenum oxide, nickel oxide, cobalt oxide) upon a V2O5/Al2O3 catalyst has been investigated by a combined Raman and IR study under dehydrated conditions. The presence of tungsten or molybdenum oxide was found to increase the concentration of polymerized surface vanadium oxide species, and this reflects the higher surface coverages of the metal oxides on the alumina support. This is probably caused by competition between vanadium and molybdenum oxide (or vanadium and tungsten oxide) species for reaction with the alumina hydroxy groups, since the IR spectra showed that on addition of these metal oxides the same type of alumina hydroxy groups are consumed. The presence of nickel or cobalt oxide on alumina increases the concentration of polymerized vanadium oxide species dramatically, which indicates that the presence of these oxides is also experienced by the surface vanadium oxide. However, the hydroxy groups are not affected as they are for the molybdenum and tungsten oxide systems.

In situ Raman spectroscopy studies of bulk and surface metal oxide phases during oxidation reactions

Bulk V-P-O and model supported vanadia catalysts were investigated with in situ Raman spectroscopy during n-butane oxidation to maleic anhydride in order to determine the fundamental molecular structure-reactivity/selectivity insights that can be obtained from such experiments. The in situ Raman studies of the bulk V-P-O catalysts provided information about the bulk crystalline phases, the hemihydrate precursor and its transformation to vanadyl pyrophosphate. However, the Raman experiments could not provide any molecular structural information about the amorphous and surface phases also present in this bulk metal oxide catalyst because of the strong Raman scattering from the crystalline phases. In contrast, in situ Raman studies of the model supported vanadia catalysts, where the active phase is present as a two-dimensional surface metal oxide overlayer, provided new insights into this important hydrocarbon oxidation reaction. In addition, the surface properties of the supported vanadia catalysts could be molecularly engineered to probe the role of various functionalities upon the structure-reactivity/selectivity relationship of n-butane oxidation to maleic anhydride. These fundamental studies revealed that the oxidation of n-butane required only one surface vanadia site and that the critical rate determining step involved the bridging V-O-P or V-O-support bonds. The selective oxidation of n-butane to maleic anhydride could occur over one surface vanadia site as well as multiple adjacent surface vanadia sites, but the reaction is more efficient with multiple sites. The n-butane oxidation TOF increased with the introduction of both surface Briinsted and Lewis acid sites, but only the surface Lewis acid sites increased the maleic anhydride selectivity.

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.

The formation of titanium oxide monolayer coatings on silica surfaces

The formation of a dispersed titanium oxide layer on Cabosil-fumed silica and on nonporous silica spheres was studied by infrared and Raman spectroscopies and by transmission electron microscopy (TEM). The procedure for obtaining the titania coatings involved reacting the silanol groups on the silica surface with titanium alkoxides under a N2 atmosphere. This self-limiting reaction led to a coating of dispersed titania on the silica spheres with a weight loading between 0.5 and 1.4 × 10−3 g/m2. The dispersed titanium oxide on the silica spheres was visible as a surface texturing of the silica in TEM images, and led to over two orders of magnitude increase in the reactivity of the silica spheres for 1-propanol dehydration. Raman spectroscopy and TEM confirmed that the dispersed titania was stable to calcination in dry air at 973 K or to heating under a vacuum of 2 × 10−7 Torr up to 1058 K. However, under alcohol dehydration reaction conditions, the dispersed titania transformed into crystals of anatase, 3 nm in diameter. On Cabosil-fumed silica, on the other hand, a similar preparation resulted in a titania loading (per square meter) that was only 7% of that seen on the silica spheres. Higher loadings caused the appearance of bands due to crystalline TiO2 (anatase) in the Raman spectra. The lower monolayer capacity on Cabosil silica can be correlated with the presence of singly bound hydroxyls as seen by IR. The Stober spheres on the other hand show hydroxyl bands that show significant hydrogen bonding.

Oxidative dehydrogenation of propane over Cr2O3/Al2O3 and Cr2O3 catalysts: effects of loading, precursor and surface area

Several alumina supported chromium oxide catalysts were prepared by varying the chromium oxide loading, precursors and surface areas of the support. The prepared catalysts were characterized using BET, XRD, XPS and UV–VIS spectroscopic techniques. The monolayer limit was observed to be ∼9mol Cr/m 2 . Below monolayer limits, surface chromium oxide species were present irrespective of precursors and surface area of the support. The activity of the prepared samples was tested for ODH of propane. It was observed that the supported chromium oxide samples were active for the ODH reaction and that propene was the major product. The activity and selectivity increased with loading up to monolayer limits and decreased for higher loadings. Bulk Cr2O3 was also studied for the ODH reaction and found to behave differently than the Cr2O3/Al2O3 catalysts. The efficiency of each surface chromium oxide species on alumina to carry out the ODH of propane, the turn over frequency (TOF), was relatively independent of chromium oxide loading for the monolayer catalysts. However, it appears that a constant fraction of the surface chromium oxide species is active for the ODH of propane. This constant fraction of the surface chromium oxide phase is given by the polymeric chromium oxide species. The inactive fraction is given by the monomeric chromium oxide species that is not reducible.

Redox properties of niobium oxide catalysts

Niobia catalysts can contain niobium oxide as a two-dimensional niobium oxide overlayer (surface niobia species), an oxide support (niobia supported surface redox sites) or a mixed oxide (solid solution or compound of niobium oxide). The molecular structures and redox properties of these different niobia-containing catalysts are investigated for several different oxidation reactions. These studies reveal that niobia present as surface metal oxide species, an oxide support and mixed oxides can directly as well as indirectly influence the redox properties of oxide catalysts.