Jih-Mirn Jehng

Acidic properties of supported niobium oxide catalysts: An infrared spectroscopy investigation

Abstract Chemisorption of pyridine was applied as a method for studying the acidic properties of niobium pentoxide supported on silica, magnesia, alumina, titania, and zirconia. The infrared spectra of adsorbed pyridine were used to evaluate the concentration and the relative strength of Bronsted and Lewis acid sites. Lewis acidity was found in all the supported niobium oxide systems, while Bronsted acid sites were only detected for niobia supported on the al umina and silica supports. The origin and characteristics of the surface acid sites present in supported niobium oxide catalysts are discussed in the present study.

The effect of the phase composition of model VPO catalysts for partial oxidation of n-butane

X-ray diffraction, Raman spectroscopy, 3’P MAS-NMR and spin-echo NMR indicated that model vanadium phosphorus oxide (VPO) precursors and catalysts contained various minor phases depending oxboth the synthetic approach and P/V ratios used. Raman spectroscopy revealed the presence of a number of micro-crystalline and amorphous V(W) and V(V) phases not evident by XRD. The presence of VOPO, phases was detrimental to the performance of the VP0 catalysts for KN-butane oxidation. The best model organic VP0 catalyst contained only vanadyl pyrophosphate with the highest degree of stacking order and virtually no VOPO, phase impurity. Raman spectroscopy detected vanadyl metaphosphate. VO(PO,),, in the catalysts derived from aqueous precursors possessing P/V ratios greater than I. Pure vanadyl metaphosphate catalyst was inactive in n-butane oxidation. s’P NMR demonstrated the absence of vanadyl metaphosphate and other impurity phases in the best catalyst derived from organic precursors at P/V = 1.18. The experimental data strongly indicate that the best VP0 catalysts for n-butane oxidation contain only vanadyl pyrophosphate with well-ordered stacking of the (200) planes.

Combined DRS-RS-EXAFS-XANES-TPR study of supported chromium catalysts

The surface chemistry of supported chromium catalysts (Cr/SiO2· Al2O3) has been systematically investigated as a function of the support composition (Si:Al ratio) and the Cr oxide loading by a combination of diffuse reflectance spectroscopy (DRS), Raman spectroscopy (RS), X-ray Absorption spectroscopy (EXAFS–XANES) and temperature-programmed reduction (TPR). Combination of all the obtained results by these characterization techniques leads to a uniform interpretation and general picture of surface Cr. On hydrated surfaces, the molecular structure of the Cr oxide species depends on the isoelectric point of the oxide support and the Cr loading: more polymerized Cr oxide species correspond to higher Cr loading and silica content of the support. After calcination, the Cr oxide species are anchored onto the surface by reaction with surface hydroxy groups of the supports. On alumina the reaction starts with the most basic OH groups on alumina. This suggests that the anchoring process is an acid–base reaction. On calcined surfaces, the polymerization of the anchored Cr oxide species and the amount of Cr2O3 clusters increases with silica content and Cr loading. Reduced Cr samples possess both Cr2+ and Cr3+, the relative concentrations of which are support and loading dependent: lower Cr2+:Cr3+ ratios correspond to higher Cr loading and alumina content of the support.

The molecular structures and reactivity of supported niobium oxide catalysts

The molecular structure-reactivity relationships for supported niobium oxide catalysts were achieved by combining Raman spectroscopy structural studies with chemical probes that measured the acidity and reactivity of the surface niobium oxide sites. The Raman spectra of niobium oxide compounds are related to the specific niobium oxide molecular structures. The molecular structures of the surface niobium oxide phases present in supported niobium oxide catalysts under ambient conditions, where adsorbed moisture is present, are controlled by the surface pH of the system. Basic surfaces result in the formation of highly distorted NbO6 groups and acidic surfaces result in the formation of slightly distorted NbO6, NbO7, and NbO8 groups. Under in situ conditions the adsorbed moisture desorbs upon heating and the surface niobium oxide overlayer on oxide supports become dehydrated. The dehydration process further distorts the highly distorted NbO6 octahedra due to removal of the coordinated water, but does not perturb the slightly distorted NbO6 octahedra. The highly distorted NbO6 octahedra possess NbO bonds and are associated with Lewis acid sites. The slightly distorted NbO6 octahedra as well as NbO7 and NbO8 groups only possess NbO bonds and are associated with Bronsted acid sites. The Lewis acid surface sites are present on all the supported niobium oxide systems, but the Bronsted acid surface sites are limited to the Nb2O5/Al2O3 and Nb2O5/SiO2 systems. The surface niobium oxide Lewis acid sites are significantly more active when coordinated to the Al2O3 and SiO2 surfaces than to the TiO2, ZrO2, and MgO surfaces (surface oxide-support interaction). Furthermore, these surface niobium oxide sites on SiO2 behaves as redox sites and the surface niobium oxide on Al2O3 are acid sites during partial oxidation reactions.

Identification of molybdenum oxide nanostructures on zeolites for natural gas conversion

Waste not, want not Natural gas often escapes or is deliberately burned at remote exploration sites because there is no economical way to transport it to markets. One route proposed for converting its main component, methane, into a more readily transported liquid is conversion to benzene over zeolites containing molybdenum (Mo). However, this method suffers from low conversion efficiency. Gao et al. identified the active Mo nanostructures as well as deactivated carbide species that form during this reaction. They were also able to recover and even enhance the zeolite catalytic activity after oxygen treatments. Science, this issue p. 686 Catalytically active isolated molybdenum nanostructures on a zeolite can be recovered after reaction through oxygen treatment. Direct methane conversion into aromatic hydrocarbons over catalysts with molybdenum (Mo) nanostructures supported on shape-selective zeolites is a promising technology for natural gas liquefaction. We determined the identity and anchoring sites of the initial Mo structures in such catalysts as isolated oxide species with a single Mo atom on aluminum sites in the zeolite framework and on silicon sites on the zeolite external surface. During the reaction, the initial isolated Mo oxide species agglomerate and convert into carbided Mo nanoparticles. This process is reversible, and the initial isolated Mo oxide species can be restored by a treatment with gas-phase oxygen. Furthermore, the distribution of the Mo nanostructures can be controlled and catalytic performance can be fully restored, even enhanced, by adjusting the oxygen treatment.

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.

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.

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.