Surface Molybdate Species Research Articles

Raman spectroscopy and XRD study on molybdenum oxide supported titania

The effect of pre-calcination of titania on the nature of interaction that occurs between titania and a dopant such as metal oxide (molybdenum oxide) was examined using Raman spectroscopy and X-ray diffraction. The presence of diffraction peaks at 2θ ​= ​9.7° and 13.6° for the catalytic materials could be associated with surface molybdate species which will be useful for many catalytic reactions. The presence of these surface molybdate species were also confirmed by Raman spectroscopy by the presence of lines at 932, and 883 ​cm−1 for the catalytic materials in which the support was not previously calcined. However, in addition to the surface molybdate such as MoO, Mo–O–Mo, bulk MoO3 was present in the materials in which the support was calcined before wet impregnation of molybdenum oxide. Therefore, the surface of MoO3/TiO2 can be tuned by simple calcination of the titania before the addition of the molybdenum oxide.

Acid sites on silica-supported molybdenum oxides probed by ammonia adsorption: Experiment and theory

The origin of Brønsted acidity in a series of silica-supported molybdenum oxide catalysts with Mo loadings of 2.1–13.3 wt%, and apparent Mo surface densities of 0.2–2.5 nm−2, respectively, was analyzed by ammonia adsorption investigated by temperature-programmed desorption, infrared spectroscopy, and DFT calculations. Every surface molybdenum atom in the molybdenum oxide (sub-)monolayer is involved in the interaction with ammonia, either as Lewis or as Brønsted acid site. A model is proposed that ascribes Brønsted acidity to the interaction between silanol groups and adjacent surface molybdate species under formation of pseudo-bridging Si—O(H)---Mo(=O)2 species with a Mo---O(Si) distance of 2.1 Å and a N-H(OSi) distance of <1.1 Å in the formed adsorption complex of the ammonia molecule. The combined experimental and computational study contributes to an improved fundamental understanding of acidity in amorphous mixed metal oxides.

Catalysts for the thermo-catalytic cracking (TCC) process: Interactions between the yttria in yttria-doped alumina aerogel and the mono-oxide MoO3, CeO2, and bi-oxide MoO3–CeO2 species

The influence of yttria used as a doping agent for catalyst support, on the dispersion and the resistance to sintering of MoO3, CeO2 and bi-oxide MoO3–CeO2 species in the TCC catalysts has been investigated. Characterization techniques used included N2 adsorption, X-ray diffraction, Raman, thermogravimetric analysis, and iso-electric point (IEP). It was explicitly found that yttria-doped alumina aerogel is by far more capable of homogenously dispersing the active molybdenum species and significantly retarding their sintering at quite high-calcination temperature than conventional alumina and alumina aerogel. Characterization results of supported mono-oxide MoO3 catalysts have clearly indicated that the fraction of (surface) tetrahedral monomolybdate species increases in the detriment of crystalline MoO3 as we varied the support from conventional alumina to yttria-doped alumina aerogel with increasing amounts of the yttria loading. This can be attributed to the incorporation of yttria into alumina aerogel network, which led to a change in the support surface charge (IEP) and subsequently the stability of surface molybdate species. The dispersion degree of ceria (substituted ceria) on supported mono-oxide CeO2 was improved as well, upon using yttria-doped alumina aerogel. Such improvement was attributed to the formation of stronger Ce–Y–O bond, and/or the formation of CeO2–Y2O3 solid solution. In the bi-oxide MoO3–CeO2, characterization results have indicated the presence of surface interaction between Mo and Ce, probably through the formation of surface “Mo–O–Ce” type phase between the dispersed ceria and the molybdate monolayer. This interaction, which was highly favourable over yttria-doped alumina surface, contributed significantly to the overall surface stability of Mo–Ce catalysts.

Structure and catalytic properties of molybdenum oxide catalysts supported on zirconia

MoO 3/ZrO 2 catalysts with different MoO 3 loadings (2–12 wt%) were prepared by the wet impregnation method. These catalysts were characterized by various techniques, such as X-ray diffraction (XRD), temperature-programmed reduction (TPR), laser Raman spectroscopy (LRS), X-ray photoelectron spectroscopy (XPS), and temperature-programmed desorption of NH 3 and the catalytic properties were evaluated for vapor-phase ammoxidation of toluene to benzonitrile. XRD patterns show the presence of crystalline MoO 3 peaks above 6.6 wt% MoO 3, which corresponds to the theoretical monolayer loading of MoO 3 on the zirconia used in the present study. The TPR suggests that reduction of the catalysts occurs in two stages and indicates that the reducibility of the catalysts increases with increase in MoO 3 loading up to 6.6 wt%. The acidity of the catalysts was also found to increase up to 6.6 wt% of molybdena loading and it does not increase much at higher loadings. Raman results show that the surface molybdate species are present in low-loading samples, while crystalline MoO 3 bands are observed from 9 wt% of MoO 3 and above loadings. XPS spectra showed that molybdenum was present at Mo 6− on all fresh samples. The Mo/Zr atomic ratio shows that the dispersion of molybdena is high below 6.6 wt% MoO 3 and dispersion decreases at higher molybdena loadings. The catalytic activity of the catalysts during ammoxidation of toluene was found to increase with loading up to 6.6 wt% and did not change appreciably beyond this loading.

UV Resonance Raman Spectroscopic Studies on the Genesis of Highly Dispersed Surface Molybdate Species on γ-Alumina

The molybdate species and their coordination structures on gamma-alumina are characterized by UV resonance Raman spectroscopy and UV-vis diffuse reflectance spectroscopy. The tetrahedral and octahedral molybdate species can be definitely identified on the basis of the resonance Raman effect. The evolution of species has been studied during the catalyst preparation under various conditions including the pH of the impregnating solution and the pretreatment temperature. It is shown that the coordination structures of the supported molybdate in the wet state are dependent on the structures of molybdate in the impregnation solution as well as the property of gamma-alumina. When the impregnating solution is at pH = 10.0, only tetrahedral molybdate species are formed on gamma-alumina, while both tetrahedral and octahedral species are formed on gamma-alumina at pH = 3.0, although the molybdate is in only the octahedral form in the impregnation solution at pH = 3.0. For the samples prepared by the impregnating solution of (NH4)(6)Mo7O24, some tetrahedral molybdate species polymerize to the octahedral species during calcination even when the loading of molybdate is extremely low. However, the molybdate species hardly aggregate into the octahedral species when the sample prepared by the impregnating solution of Na2MoO4. It turns out that the surface pH value (i.e., the pH of region) is a key factor governing the coordination structure of the molybdate species.

ESR, FT-Raman spectroscopic and ethanol partial oxidation studies on MoO 3 /SnO 2 catalysts made by metal oxide vapor synthesis

A series of SnO2-supported MoO3 catalysts were prepared by the metal oxide vapor synthesis (MOVS) technique. ESR studies indicated the presence of highly dispersed Mo5+ species in both octahedral and tetrahedral coordination environments at all the loadings studied. At the highest MoO3 loading of 12 wt%, the formation of MoO3 microcrystallites was indicated from the lower intensity of the ESR signal. Raman studies also showed the presence of well dispersed surface molybdate species up to 4.4 wt% MoO3 loading, and the peaks corresponding to microcrystallites of molybdena were observed at 12 wt% MoO3 loading. The ethanol partial oxidation activities of the catalysts increased with increase in MoO3 loading and the catalyst with 4.4 wt% molybdena content showed the highest activity; all the MOVS catalysts showed 100% selectivity to acetaldehyde at low conversions.

Surface Structure of Highly Dispersed MoO3 on MgO Using in Situ Mo L3-Edge XANES

The evolution of the dispersed surface molybdate phase on high-surface-area magnesium oxide is studied as a function of weight loading of MoO3 and calcination temperature using in situ Mo L3-edge X-ray absorption near edge spectroscopy, in situ laser Raman spectroscopy, and thermogravimetry−mass spectroscopy. Under ambient, hydrated conditions, the magnesium support is present as magnesium hydroxide, rather than MgO, and the molybdate species is present on the surface as an isolated tetrahedral species for weight loadings of 5−20 wt % MoO3. Under these hydrated conditions the form of the molybdate species is controlled by the net surface pH at the point of zero charge and is the same as that observed in aqueous solution. As the catalyst is calcined in air, the molybdate species transforms from a tetrahedral species, MoO4, to an octahedral one, MoO6. This transformation occurs at the same temperature, ∼450 °C, as that at which the support changes from hydroxide to oxide. This transformation temperature is independent of the MoO3 loading. It is suggested that it is this support-induced structural change that drives the change in the form of the surface molybdate species and not the desorption of adsorbed water.

Supported molybdena catalysts: Characterization of oxidized, reduced, and sulfided MoO3 prepared by an adsorption method

Highly dispersed molybdena-titania catalyst can be prepared by an equilibrium adsorption method. In this method, molybdate anions adsorb onto the positively charged titania surfaces via electrostatic attraction by controlling the pH of the impregnating solution and they increase as an inverse function of the pH. 95Mo-NMR and UV spectroscopic studies of impregnating solution show that the polymeric species like Mo7O24 6-ions are adsorbed on titania in the acidic impregnating solution. XRD, Raman, and XPS data of the calcined samples show that mono-layer coverage of molybdenum oxide over-layer possesses a highly distorted MoO6 group with a molecular geometry resembling the distorted square pyramid. The catalytic oxidation of methanol over the surface molybdate species on titania possesses higher turnover numbers and higher selectivities of partial oxidation products than the catalysts supported on alumina, silica, zirconia, or magnesia. Changes of the surface properties either after reduction and sulfiding treatment over monolayer catalyst on titania have also been investigated. The NO chemisorption and XPS studies show that two types of active sites appeared after reduction treatment: one site is active for hydrogenation of 1,3-butadiene and the other site is active for metathesis of propene. A higher degree coordinative unsaturations of MO is required for hydrogenation than metathesis. After sulfiding treatments of the catalyst, hydrogenation of 1,3-butadiene also requires triply coordinative unsaturation, and hydrogenolysis of thiophene requires the ensemble of doubly or triply coordinative unsaturations.

Determination of molybdenum–oxygen bond distances and bond orders by Raman spectroscopy

AbstractA correlation is developed for relating Raman stretching frequencies of molybdenum–oxygen (MoO) bonds to their respective bond distances in molybdenum oxide compounds. MoO bond orders are also related to stretching frequencies. The MoO correlation is expected to offer invaluable insight into the structures of molybdate species in chemical systems which are not amenable to analysis by diffraction or other spectroscopic techniques. In the present study, the correlation is used to predict stretching frequencies for perfect MoO4 and MoO6 structures, and to determine MoO bond distances of the MoO6 octahedron in Ba2CaMoO6 as well as the dehydrated surface molybdate species in MoO3/Al2O3.

ChemInform Abstract: Spectroscopic Studies of Molybdate Species Deposited on a Nb2O5 Support

Abstract(XRD, Raman, FTIR, diffuse reflectance, ESCA, FABMS‐SIMS) show that the nature and structure of surface molybdate species mainly depend on the Mo loading and to a lesser extent on the preparation method (impregnation at pH 0.5 and 5.5, equilibrium adsorption methods).

Supported molybdenum oxide catalyst. Structure and chemistry of oxidized, reduced, and sulfided surfaces.

Highly dispersed molybdena-titania catalyst can be prepared by an equilibrium adsorption method. In this method, molybdate anions adsorb onto positively charged titania surfaces via electrostatic attraction by controlling pH of the impregnating solution. The amounts of absorbed molybdate anion are strongly dependent on pH of the impregnating solution and they increase as an inverse function of pH. XRD, Raman, and XPS data of the calcined samples show that monolayer coverage of molybdenum oxide over-layer possesses a highly distorted MoO6 group with a molecular geometry resembling a distorted square pyramid. The Mo species are fixed by an oxygen bridge with Ti, as Mo-O-Ti, or with neighboring Mo, as Mo-O-Mo.The catalytic oxidation of methanol over the surface molybdate species on titania possess higher turnover numbers and higher selectivities of partial oxidation products than do the catalysts exceeding monolayer coverage which possess the crystalline MoO3 phase. Changes of the surface properties either, after reduction and sulfiding treatment over monolayer catalyst, have also been investigated. The NO chemisorption and XPS studies show that two types of active sites appeared after reduction treatment: one site is active for hydrogenation of 1, 3-butadiene and the other site is active for metathesis of propene. A higher degree of coordinative unsaturations of Mo is required for hydrogenation than for metathesis. After subjecting sulfiding treatments to the catalyst, hydrogenolysis of thiophene requires the ensemble of doubly or triply coordinative unsaturations.

Physicochemical properties of MoO 3TiO 2 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, Mo 7O 24 6 ions, are present, while in the basic solutions it is the monomeric MoO 4 2− 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 MoO 3 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 MoO 3. 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.

Spectroscopic studies of molybdate species deposited on a Nb2O5 support

Three series of molybdate–Nb2O5 catalyst samples were prepared by impregnating Nb2O5 either at pH 0.5 (series A) or at pH 5.5 (series B) and at pH of equilibrium adsorption (0.5)(series C). X.r.d., Raman, F.t.i.r., diffuse reflectance, ESCA and FABMS-SIMS techniques have been used to characterize structural properties of the samples. The nature and structure of surface molybdate species were observed to depend on Mo loading and to a lesser extent on the method of preparation involving in particular the pH value of the impregnation solution. It is shown that polymeric and not monomeric molybdate species are formed first up to ca. 5 wt % MoO3. At higher Mo loadings MoO3 crystallites are also formed. SIMS data showed that for low Mo loading the top surface layer is much richer in oxygen ions than for MoO3 crystallites. This is presumably related to its poor catalytic partial oxidation properties and to its acidic features. This behaviour is very different from that observed for molybdate species deposited on an SiO2 support.

Interactions of molybdenum oxide with various oxide supports: Calcination of mechanical mixtures

Laser Raman and Fourier transform infrared spectroscopies as well as X-ray diffraction were used to study the interactions of MoO 3 with various metal oxide supports. Intimate mechanical mixtures containing an amount of MoO 3 corresponding to less than a monolayer were calcined at 720 K. In the cases where the supports were TiO 2, SnO 2, Al 2O 3, and MgO surface molybdate species were formed on calcination via thermally induced spreading of the bulk MoO 3 phase. Interactions between the support and the MoO 3 were sufficiently strong in these cases that no bulk MoO 3 could be detected in the calcined mixture. A weaker interaction occurs in the case of SiO 2. Raman results from the calcined MoO 3 TiO 2 mixture agree with spectra obtained from catalysts prepared by equilibrium adsorption and incipient wetness impregnation; the nature of the surface molybdate species is relatively independent of the preparation method.

Photo-oxidation of methanol using [formula omitted]: Catalyst structure and reaction selectivity

Laser Raman spectra, UV-visible spectra, and photo-catalytic activity measurements are described which show that supported MoO3TiO2 catalysts prepared by impregnation possess a layer of surface molybdate species coordinated to the titania support. The saturation coverage of this surface molybdate monolayer corresponds to a surface density of ca. 4 × 1014 Mo atoms per cm2 of TiO2. The properties of this surface monolayer are markedly different from bulk MoO3, as reflected in different vibrational frequencies for the surface molybdate species and for bulk MoO3, and in the higher photo-catalytic activity of the surface molybdate relative to bulk MoO3. The photo-efficiency of the TiO2 catalyst containing the surface molybdate monolayer is about one-fifth of the photo-efficiency of pure TiO2, but the selectivity for suppressing secondary oxidation reactions is significantly greater. The major photo-assisted oxidation product on TiO2 is methyl formate, independent of methanol conversion. In contrast, the molybdate monolayer catalyst has nearly 100% selectivity for dimethoxymethane (the reversible condensation product of CH2O + 2 CH3OH) at low conversions. For both catalysts the selectivity of the primary oxidation step appears to be insensitive to the mode of excitation (i.e., thermal vs photo-assisted). Photo-induced changes in selectivity at a given conversion are due primarily to differences in adsorbate coverage at the different operating temperatures required to give equal conversion for dark vs light reactions.

Raman spectroscopic investigation of the effect of H 2O on the molybdenum surface species in [formula omitted] catalysts

The Raman spectroscopic characterization of a set of MoO 3 Al 2 O 3 catalysts containing 3 to 25 wt% MoO 3 is presented. Controlled atmosphere measurements show that the Raman spectra of these catalysts are influenced drastically by the water content of the samples. Catalysts calcined in O 2 at 500 °C have spectra very different from previously reported spectra. The MoO stretching vibration is at about 1000 cm −1 in calcined samples, whereas in H 2O-exposed samples the frequency is near 950 cm −1. The position of the MoO stretching vibration is shown by repeated H 2O exposure, O 2 calcination cycles to be reversible between 950 and 1000 cm −1. Models for the interaction of H 2O with the surface molybdate species are presented and discussed with respect to the Raman data.