Surface Molybdenum Oxide Species Research Articles

Multiwavelength Raman Spectroscopic Characterization of Alumina-Supported Molybdenum Oxide Prepared by Vapor Deposition

Molybdenum oxide species on alumina have been prepared via gas phase deposition of bis(ethylbenzene) molybdenum with loadings in the range of 0.05–3.1 Mo atoms/nm2. These materials have been characterized relative to analogous impregnated samples using UV–Visible diffuse reflectance spectroscopy, UV Raman spectroscopy, and visible Raman spectroscopy. The presence and distribution of monomolybdates, two- and three-dimensional polymolybdates, and Al2(MoO4)3 depends on the Mo loading and synthesis technique employed, especially at low loadings. Alternately dosing bis(ethylbenzene) molybdenum and water generates uniform polymolybdates at all loadings. In contrast, pulsing the precursor into the reactor without intervening water leads to monomolybdates at low loadings and polymolybdates at high loadings. The ability to influence the distribution of surface molybdenum oxide species can facilitate the rational design of molybdenum based catalysts.

Structure, activity and kinetics of supported molybdenum oxide and mixed molybdenum–vanadium oxide catalysts prepared by flame spray pyrolysis for propane OHD

A series of molybdenum oxide (2 to 15wt% Mo) and mixed molybdenum–vanadium oxide (4 to 15wt% Mo and 2wt% V) on alumina catalysts have been synthesized by flame spray pyrolysis (FSP). The materials were structurally characterized by BET surface area, X-ray diffraction (XRD), Raman and diffuse reflectance UV–vis spectroscopy and evaluated as catalysts for the oxidative dehydrogenation (ODH) of propane. The results show that samples with high specific surface areas between 122 and 182m2/g were obtained, resulting in apparent MoOx and VOx surface densities from 0.7 to 7.7nm−2 and 1.5 to 1.9nm−2, respectively. Raman spectroscopy, UV–vis spectroscopy and XRD confirmed the high dispersion of molybdenum and vanadia species on γ-Al2O3 as the main crystalline phase. Only at the highest loading of 15wt% Mo, with theoretically more than monolayer coverage, some crystalline molybdenum oxide was observed. For the mixed molybdenum–vanadium oxide catalysts the surface species were separate molybdenum oxide and vanadium oxide monomers at low loadings of molybdenum, but with increasing molybdenum loading interactions between surface molybdenum and vanadium oxide species were observed with Raman spectroscopy.The catalytic experiments showed that the most selective molybdenum oxide catalysts for the ODH reaction were those with high Mo loadings of 7 to 15wt% Mo, while the most selective mixed molybdenum–vanadium oxide catalyst were at 4wt% Mo, where separate surface species of molybdenum and vanadium oxide were observed by Raman spectroscopy. A simple kinetic model based on the propane ODH reaction, parallel combustion of propane and sequential combustion of propene described the experimental results well and could be used to determine the optimal reaction conditions.

Molybdenum role: Mo-Sm-V-O catalytic system for propane oxidative dehydrogenation

SmVO 4-impregnated molybdenum oxide catalysts have been prepared with different molybdenum amounts to obtain surface coverages below and above the theoretical monolayer. The physicochemical properties of the catalysts were characterized by BET specific surface area measurements, X-ray diffraction (XRD), temperature-programmed reduction (TPR), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). At low molybdenum loading (surface coverage below the theoretical monolayer), surface molybdenum oxide species were detected by Raman. TPR and XPS techniques were used to investigate the reducibility of this species. The results suggest that a strong molybdenum–SmVO 4 interaction exist. At higher molybdenum loadings (exceeding the monolayer coverage), crystalline MoO 3 is formed. The catalytic behavior was evaluated in the propane oxidative dehydrogenation (ODH). The incorporation of low molybdenum loading results in an increase of the selectivity to propene. The highest yield to propene was obtained on SmVO 4-impregnated with 3 mol% Mo. The performance of SmVO 4-impregnated molybdenum oxide catalysts was compared with that of SmVO 4-impregnated vanadium oxide catalysts.

Effect of modifiers on the reactivity of Cr2O3/Al2O3 and Cr2O3/TiO2 catalysts for the oxidative dehydrogenation of propane

Chromium oxide supported on alumina and titania supports was modified with oxides of sodium, vanadium and molybdenum. The modified and unmodified chromium oxide catalysts were characterized by several techniques. The presence of surface chromium oxide and surface molybdenum and vanadium oxide species was detected in the unmodified and molybdenum and vanadium oxide modified supported chromium oxide catalysts. The reducibility (Tmax and H/Cr ratio) of the surface chromium species was not affected for the vanadium and molybdenum oxide modified catalysts; however, the reducibility changed noticeably for sodium modified supported chromium oxide catalysts. Studies of the reactivity of the ODH of propane revealed the effect of modifiers on the reactivity properties of the surface chromium oxide species. The activity and propene selectivity decreased for sodium modified supported chromium oxide catalysts. However, the activity increased for vanadium oxide modified catalysts and was similar for molybdenum oxide modified catalysts irrespective of the support. The propene selectivity was higher for molybdenum oxide modified chromium oxide catalysts. However, the propene selectivity for vanadium oxide modified catalysts depends on the support since it appears that the inherent selectivity of the surface vanadium oxide species is reflected.

Quantitative Determination of the Number of Surface Active Sites and the Turnover Frequencies for Methanol Oxidation over Metal Oxide Catalysts: Application to Bulk Metal Molybdates and Pure Metal Oxide Catalysts

The present work investigates the number and nature of the surface active sites and the catalytic activity of bulk metal molybdates and bulk metal oxides on methanol selective oxidation. Bulk metal molybdates were synthesized by coprecipitation and characterized by laser Raman spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction, and specific surface area analysis. The number of surface active sites (Ns) was determined by measuring the amount of methoxy species produced by methanol chemisorption on the catalysts at 100°C. The specific activity values (TOFs) were calculated by normalizing the reaction rate by the number of surface active sites. The significant differences in catalytic behavior of bulk metal molybdates and bulk metal oxides, and the surface molybdenum enrichment observed on metal molybdates, gave evidences that the surface of bulk metal molybdates may be composed only by molybdenum oxide species in a two-dimensional overlayer. It was not possible to establish the structure of surface molybdenum oxide species; however, it can be concluded that the structure of surface active sites species of bulk metal molybdates and monolayer supported oxide catalysts are different according to the dispersion of the Ns values of these systems. This work shows, for the first time in the literature, that bulk metal molybdates and monolayer supported molybdenum oxide catalysts possess similar activity and TOF in methanol selective oxidation. The number of active surface sites and the specific activity toward selective oxidation products (TOF redox), CO2 (TOF basic), and dimethyl ether (TOF acid) of bulk metal oxides were also determined. In contrast to bulk metal molybdates, most bulk metal oxides catalyze the total combustion of methanol even at low methanol conversion.

UV-Raman Characterizations of MoO3/ZrO2 Catalysts with Extremely Low MoO3 Loadings

Abstract UV-Raman spectroscopy was first used and proved to be a powerful tool for the characterization of the surface molybdenum oxide species supported on ZrO2, especially for samples with extremely low MoO3 loadings due to the Raman bands of the surface molybdenum oxide species are greatly enhanced by this method. Only one kind of molybdenum oxide species was detected in samples with very low to near a mono-layer coverage of MoO3 loading, and it is suggested that the structure of the highly dispersed molybdenum species is inevitably related to the structure of the support.

The dynamic states of silica-supported metal oxide catalysts during methanol oxidation

The in situ Raman spectra of silica-supported metal oxide catalysts (containing surface metal oxide species of V, Nb, Cr, Mo, W and Re) were measured during methanol oxidation. Stable surface methoxy species were found to form via reaction with the surface SiOH groups for all the catalysts investigated. The surface SiOH groups were also titrated by the surface metal oxide species and the surface concentration of SiOH groups decreases with increasing metal oxide loading. The stable surface MOCH 3 were only found for the V 2O 5/SiO 2 catalysts. The surface metal oxide species were all influenced by the methanol oxidation reaction. The surface rhenium oxide species were removed from the silica surface and the surface molybdenum oxide species were partially agglomerated to crystalline β-MoO 3 particles by the formation of Re-methoxy and Mo-methoxy complexes. The surface niobium oxide and tungsten oxide species were partially reducing by the net reducing methanol oxidation environment. In situ Raman spectra for the CrO 3/SiO 2 catalysts could not be obtained due to the formation of reduced chromium oxide species during methanol oxidation which gave rise to sample fluorescence. The in situ Raman observations provided a fundamental basis for understanding the selectivity patterns of the silica-supported metal oxide catalysts during methanol oxidation. However, the mechanism by which the silica support ligands activate the redox properties of the surface metal oxide species is not completely understood.

Genesis and Stability of Silicomolybdic Acid on Silica-Supported Molybdenum Oxide Catalysts: In-Situ Structural-Selectivity Study on Selective Oxidation Reactions

The formation of silicomolybdic acid (SMA, H4SiMo12O40) on silica-supported molybdenum oxide catalysts has been studied by in situ Raman spectroscopy, by TGA measurements, and for the selective oxidation of methane to formaldehyde. The formation of silico-molybdic acid requires exposing the MoO3/SiO2 to air saturated with water for several hours at room temperature. The large amount of water deposited on the silica support allows solubilization of part of the silica support in the presence of solvated heptamolybdate species, which leads to the formation of silicomolybdic acid. Desorption of water via thermal treatments breaks the silicomolybdic acid into dehydrated or partial hydrated species which are stable up to ca. 573 K. Above 573 K, only an isolated and distorted mono-oxe surface molybdenum oxide species is observed by in situ Raman spectroscopy. Consequently, the silicomolybdic acid species on SiO2 should not result in catalytic behavior different from that of conventional MoO3/SiO2 catalysts for reactions taking place above 573 K. Unlike surface molybdenum oxide species, the surface SMA species on SiO2 are stable during methanol oxidation at 503 K and do not transform into crystalline β-MoO3 phase. The selective oxidation of methane to formaldehyde (843-883 K) shows no difference between conventional silica-supported molybdenum oxide and silica-supported silicomolybdic acid catalysts. In situ Raman spectroscopy studies during methanol oxidation at temperatures above 573 K reveal that the surface silicomolybdic acid species are not stable and transform into crystalline β-MoO3.

Catalytic Properties of Supported Molybdenum Oxide Catalysts: In Situ Raman and Methanol Oxidation Studies

The oxidation of methanol was studied over supported molybdenum oxide catalysts as a function of the specific oxide support (TiO2,ZrO2, Nb2O5, and A1203) and molybdenum oxide loading (surface coverage). The surface molybdenum oxide species were selective for the production of formaldehyde, and the oxide support sites yielded dimethyl ether (alumina and niobia) and methyl formate (zirconia) or were relatively inactive (titania). The turnover frequency (TOF) for the selective oxidation of methanol to formaldehyde varied by a factor of 2-4 with surface molybdenum oxide coverage and a factor of approximately 10 with the specific oxide support at monolayer coverage. The molecular structures of the surface molybdenum oxide species (isolated, tetrahedral at low coverages and polymerized, octahedralketrahedral at high coverages) did not affect the reaction selectivity but did appear to influence the slight increase in TOF with surface coverage. The order of magnitude variation in TOF with the specific oxide support correlated with the reducibility of the support and suggests that the Mo-0-support bond is critical in controlling the TOF. In situ Raman studies during methanol oxidation revealed that the supported molybdenum oxide species were 100% dispersed up to monolayer coverage. The percent reduction of the surface molybdenum oxide species, reflected by the decrease in the Raman intensity of the Mo-0 bond, during methanol oxidation was not a strong function of surface coverage and the specific oxide support. This suggests that the order of magnitude variation in the TOF with the specific oxide support is primarily related to the activity per site of the surface molybdenum oxide species rather than variation in the number of participating sites.

Molybdena on Silica Catalysts: Role of Preparation Methods on the Structure-Selectivity Properties for the Oxidation of Methanol

A series of MoO 3/SiO 2 samples prepared by several research groups, employing different silicas and preparation methods, have been studied by in situ Raman spectroscopy, X-ray photoelectron spectroscopy, and methanol oxidation as a probe reaction. The in situ Raman spectroscopy studies show that under dehydrated conditions the molecular structure of the silica-supported molybdenum oxide surface species is independent of the preparation methods used in the present study. The surface molybdenum oxide species is assigned to an isolated, highly distorted octahedral mono-oxo Mo structure. The appearance of new structures in some samples is due to the presence of calcium impurities in the silica supports which result in the formation of calcium molybdate. Neither the preparation method nor the specific silica used affected the methanol oxidation activity of the MoO 3/SiO 2 catalysts. The surface molybdenum oxide coverage on silica is the only relevant factor that determines the catalytic properties during methanol oxidation. In situ Raman spectroscopy during methanol oxidation shows aggregation of surface molybdenum oxide species to crystalline beta-MoO 3. The extent of aggregation increases with the surface molybdenum coverage, even at very low coverages, and accounts for the decrease in the methanol oxidation catalytic activity with increasing surface molybdenum coverage.

Molecular Structures and Reactivity of Supported Molybdenum Oxide Catalysts

Supported molybdenum oxide catalysts were prepared by an equilibrium adsorption method. The molecular structures of the molybdenum oxide overlayers on different oxide supports (Al 2O 3, TiO 2, ZrO 2, SiO 2, and MgO), under in situ conditions, were investigated by Raman spectroscopy. The molybdenum oxide species on TiO 2, ZrO 2, and Al 2O 3 possess a highly distorted, octahedrally coordinated surface molybdenum oxide species with one short MoO bond regardless of the molybdenum oxide content. The MoO 3/SiO 2, catalysts primarily contain crystalline MoO 3 because of the lower density and reactivity of the silica surface OH groups. The MoO 3/MgO catalysts possess MgMoO 4 and CaMoO 4 compounds due to the high aqueous solubility of MgO and CaO (an impurity in the MgO support) and the strong acid-base interaction between molybdenum oxide and MgO/CaO. The methanol oxidation reaction studies revealed that the MoO 3/TiO 2 (anatase and rutile) and MoO 3/ZrO 2 catalysts are the most active catalysts and that their activities (TOFs) are at least 1-2 orders of magnitude higher than those of the MoO 3/Al 2O 3, MoO 3SiO 2, and MoO 3/MgO catalysts. It was also found that the catalytic activities correlate with the reducibility of the surface molybdenum oxide species on the oxide supports as well as their surface morphology (e.g., molybdenum oxide dispersion and molybdate compound formation). These studies demonstrate that the specific oxide support controls the reactivity of the supported molybdenum oxide phases.

Effect of alkali metal cations on the structure of [formula omitted] catalysts and its relevance to the selective oxidation of methane and methanol

The effect of alkali metal additives on both the structure of MoO 3 SiO 2 and its behavior as a catalyst for selective oxidation of methane and methanol to formaldehyde was investigated. The structure of the silica-supported molybdenum oxide catalysts was determined by in situ Raman spectroscopy and the amount of reducible oxygen in the catalyst was determined by temperature-programmed reduction. In the absence of alkali metal, only an isolated surface molybdenum oxide species was present on the silica support. Addition of alkali metals decreased the number of isolated surface molybdenum oxide species and formed new alkali-molybdate compounds due to the weak interaction of the Mo species with the silica surface. The oxygen associated with the alkali-molybdate compounds was generally not available for oxidation reactions. Consequently, the addition of alkali metal decreased the catalytic activity for the oxidation of both methane and methanol. The activity for methane oxidation was found to correlate with the number of remaining isolated Mo species and the activity for methanol oxidation was found to correlate with the amount of reducible oxygen present in the catalyst.

Raman characterization of alumina supported MoVFe catalysts: Influence of calcination temperature

Alumina supported V 2O 5, MoO 3, Fe 2O 3, Fe 2O 3V 2O 5, and MoO 3V 2O 5 catalysts were investigated by Raman spectroscopy as a function of calcination temperature under ambient conditions where the surfaces are hydrated. The single metal oxide systems (V 2O 5, MoO 3, or Fe 2O 3) and the mixed metal oxide systems (Fe 2O 3V 2O 5 and MoO 3V 2O 5) are present as surface metal oxide species on the alumina support at intermediate calcination temperatures. For the single metal oxide species, the surface vanadium oxide species consists of metavanadate species ([VO 3] x− n ), the surface molybdenum oxide species consists of heptamolybdate (Mo 7O 6− 24) and octamolybdate (Mo 8O 4− 26) species, and the surface iron oxide species is primarily present as a surface spinel containing Fe 3+ ions in the surface region of the alumina support. Addition of iron oxide and molybdenum oxide to the V 2O 5/Al 2O 3 transforms the surface metavanadate species to surface pyrovanadate species (V 2O 2− 7) and surface decavanadate species (V 10O 6− 28), respectively. The thermal stability of the Supported metal oxide on the alumina support depends on the specific nature of the surface metal oxide species. The supported iron oxide on alumina has a high thermal stability, similar to the unpromoted alumina, which is probably related to Fe 3+ absorption into the surface region of the alumina lattice. In contrast, the supported molybdenum oxide on alumina system is not stable above 800°C due to significant loss in the alumina surface area and volatilization of molybdenum oxide. The surface vanadium oxide species on alumina is stable up to a calcination temperature of 800°C and transforms to crystalline V 2O 5 at higher calcination temperatures due to significant loss in surface area of the alumina support. The mixed metal oxide species (Fe 2O 3V 2O 5 and MoO 3V 2O 5) on alumina could not stabilize the surface vanadium oxide on alumina above 800°, and accelerated the rate of surface area loss and phase transformations. No FeVO 4, AlVO 4, or VMoO compounds were formed in these mixed metal oxide species on alumina at elevated temperatures.

Effect of cobalt oxide on surface structure of alumina supported molybdena catalysts studied by in situ Raman spectroscopy

A set of Co promoted 10% Mo/Al2O3 samples have been characterized by means of Raman spectroscopy under ambient as well as in situ dehydrated conditions. Under ambient conditions, the degree of the polymerization of surface molybdenum oxide species decreases with increasing Co loading. Under dehydrated conditions, the polymeric molybdenum oxide species is absent with the addition of only 0.2% Co. At low Co loadings (⩽2%), before the formation of CoMoO4 compound, the spectral features are very similar under ambient conditions. Dehydration causes the upward shift of the Mo=O symmetric stretching mode. A broad band around 920–930 cm−1 was thus observed. This band has been suggested to be associated with the Co-Mo interaction species. In contrast to crystalline CoMoO4, this species shows a reversibility on H2 reduction-O2 reoxidation treatments. From the results obtained, it is proposed that cobalt oxide interacts with the most polymerized molybdenum oxide species to form Co-Mo interaction species and/or crystalline CoMoO4; therefore, the amount of the surface molybdenum oxide species decreases with a change in the molecular structure as a function of the Co concentration.

Surface chemistry of magnesium oxide-supported molybdenum oxide: an in situ Raman spectroscopic study

Suggested molybdenum oxide on magnesium oxide was investigated by in situ Raman spectroscopy as a function of molybdenum loading and calcination temperature in order to determine the molecular structures of the surface molybdenum oxide species. Molybdenum ladings from 1.0 to 26.7 wt % were studied after calcination at temperatures from 80 to 700 o C. For samples calcined at 600 o C in dry air the observed structure of the molybdenum oxide species was dependent upon weight loading. At low molybdenum loadings (<3.3% Mo), an isolated, highly distorted octahedral molybdena species is present on the MgO surface

Surface structures of supported molybdenum oxide catalysts under ambient conditions

Two sets of supported molybdenum oxide catalysts, wet (dried at room temperature) and calcined (calcined at 773 K) samples, were prepared by an equilibrium adsorption method at different pH values of the impregnating solution. The adsorbed amounts of molybdenum oxide species onto the oxide support are strongly dependent on the pH of the impregnating solution and increase with decreasing pH. The Raman spectroscopic studies reveal that the surface molybdenum oxide species under ambient conditions, wet and calcined, are hydrated and essentially in an aqueous medium. Furthermore, the surface structures of molybdenum oxide species on the oxide support were found to depend on the net surface pH at point of zero charge (PZC) under ambient conditions. The net surface pH at PZC under ambient conditions is determined by the specific oxide support and surface molybdenum oxide coverage. The surface molybdenum oxide structures in the wet, uncalcined, samples are not only dependent on the net surface pH at PZC but also on the number of NH 4 + cations which coordinate to the surface molybdenum oxide species for compensation of net charge: Mo 7O 24 6− species in NH 4 +-rich concentrations (high pH region) favor formation of (NH 4) 6Mo 7O 24 · 4H 2O. Upon calcination, the NH 4 + ions are removed and the surface molybdenum oxide species become rehydrated upon exposure to air by adsorbing moisture. Consequently, the structures of surface molybdenum oxide species in the calcined samples which have been exposed to ambient are also dependent on the net surface pH at PZC.