MoOx Species Research Articles

Oxidative dehydrogenation of propane over nanodiamond modified by molybdenum oxide

MoOx-modified nanodiamond (ND) catalysts were investigated for oxidative dehydrogenation (ODH) of propane. The catalysts were prepared by impregnation method and characterized by TEM, XRD, XPS, Raman and FTIR spectroscopies. ND was found a better carrier than carbon nanotubes (CNTS) for MoOx dispersion. The MoOx species are two-dimensional polymerized MoOx which get covalently bonded onto the ND surface via MoOC bonds. A major portion of Mo6+ species was found, together with a small amount of Mo4+ species. Adding 1wt% MoOx on ND (1Mo) can increase both propane conversion and propene selectivity, whereas excess MoOx leads to a drastically decrease in propene selectivity due to the aggregation of the MoOx species and the coverage of the active sites of ND. Both ND and 1MoOx/ND catalysts show good stability after a few hours of testing. A sp3–sp2 hybrid structure is formed in situ and the ketonic/diketonic carbonyl groups are generated during the reaction, which are believed to be favourable to the reaction. Our results indicate the modification of ND by low-loading MoOx can improve the selectivity at the same conversion rate, thus enhance the propene yield.

Molybdenum oxide film with stable pseudocapacitive property for aqueous micro-scale electrochemical capacitor

Conductive MoO2+x film deposited at 150°C by magnetron sputtering is investigated as negative electrode material for micro-scale electrochemical capacitors. It is found that the film 860nm thick exhibits high volumetric capacitance (385 F cm−3), good rate capability (47% capacitance retained at 500mV s−1 relative to that at 20mV s−1) and excellent cycling stability (100% retained after 5000 cycles). An asymmetric micro-device of MoO2+x(-)//2M Li2SO4//MnO2(+) is obtained, showing a high energy density of 2.8μWhcm−2 at a real power density of 0.35 mW cm−2, and good stability with no capacitance loss for 10000 cycles. It is revealed that the amorphous MoOx species with oxidation state of +5 to +6 provides pseudocapacitance through reversible insertion/extraction of H+ and Li+. More importantly, extra capacitance is contributed by catalytic decomposition of hydrated water on MoO2 grain surfaces, and the presence of large amounts of MoO2 grains ensures fast electronic transfer and structural stability. Thus the superior capacitive property of the MoO2+x film can be attributed to its multivalent and multi-phase structure with integrated functions.

The effect of supported MoO(X) structures on the reaction pathways of propene formation in the metathesis of ethylene and 2-butene.

The kind of surface MoOX structures on Al2O3-SiO2 was found to determine propene selectivity in the metathesis of ethylene and 2-butene. Compared to isolated tetrahedral MoOX species, their polymerized octahedral counterparts show significantly lower activity for isomerisation of 2- to 1-butene thus hindering non-selective metathesis of these butenes. In addition, they reveal higher ability to engage ethylene in propene formation.

The Enhancing Effect of Brønsted Acidity of Supported MoOx Species on their Activity and Selectivity in Ethylene/trans‐2‐Butene Metathesis

AbstractSupported catalysts with a nominal Mo surface density of 0.15 and 1.5 Mo atoms nm−2 were synthesized by impregnation of alumina, silica, and alumina–silica supports with silica content between 1 and 70 wt %. They were tested for their activity and selectivity in the metathesis of ethylene and trans‐2‐butene to propene between 343 and 603 K at 125 kPa. The catalysts were characterized by UV/Vis, Raman, and IR spectroscopy, XRD and H2 temperature‐programmed reduction for elucidating the distribution, degree of polymerization, reducibility, and acidity of MoOx species. We established that Brønsted acidity of highly dispersed tetrahedral and polymerized octahedral MoOx species is required to ensure high metathesis activity. The acidic character of these species is influenced by their structure and support. Tetrahedral MoOx species with Brønsted acidic character are only formed on supports possessing such acidity, whereas Brønsted acidic octahedral MoOx is also created on supports without such acidic sites.

Investigation of the Deactivation Phenomena Occurring in the Cyclohexane Photocatalytic Oxidative Dehydrogenation on MoOx/TiO2 through Gas Phase and in situ DRIFTS Analyses

In this work, the results of gas phase cyclohexane photocatalytic oxidative dehydrogenation on MoOx/SO4/TiO2 catalysts with DRIFTS analysis are presented. Analysis of products in the gas-phase discharge of a fixed bed photoreactor was coupled with in situ monitoring of the photocatalyst surface during irradiation with an IR probe. An interaction between cyclohexane and surface sulfates was found by DRIFTS analysis in the absence of UV irradiation, showing evidence of the formation of an organo-sulfur compound. In particular, in the absence of irradiation, sulfate species initiate a redox reaction through hydrogen abstraction of cyclohexane and formation of sulfate (IV) species. In previous studies, it was concluded that reduction of the sulfate (IV) species via hydrogen abstraction during UV irradiation may produce gas phase SO2 and thereby loss of surface sulfur species. Gas phase analysis showed that the presence of MoOx species, at same sulfate loading, changes the selectivity of the photoreaction, promoting the formation of benzene. The amount of surface sulfate influenced benzene yield, which decreases when the sulfate coverage is lower. During irradiation, a strong deactivation was observed due to the poisoning of the surface by carbon deposits strongly adsorbed on catalyst surface.

Glycerol steam reforming on supported Ru-based catalysts for hydrogen production for fuel cells

Glycerol steam reforming on Ru and Ru–Me (Me = Fe, Co, Ni, and Mo) catalysts supported on yttria, ceria–zirconia, and γ-alumina is studied at high temperatures for the production of hydrogen for fuel cell applications. The nature of the support notably affects the catalytic properties of these catalysts resulting in significant enhancements of the H2 production turnover rate and product selectivity on the reducible yttria and ceria–zirconia supported Ru-based catalysts via facilitation of the water–gas shift reaction. The acidic γ-alumina supported Ru-based catalysts demonstrate a low H2 production turnover rate with a high CO product selectivity and also favor the formation of C1–C2 hydrocarbons. Differently, the promotion effects due to Fe, Co, Ni, and Mo on the bimetallic Ru–Me catalysts are limited with only small increases in the glycerol conversion turnover rate for the Ru–Ni, Ru–Mo, and Ru–Co catalysts. The alumina supported Ru-based catalysts are deactivated by a significant extent with increasing on-stream time due to coking. The carbon deposition is insignificant on the yttria and ceria-zirconia supported catalysts, but moderate deactivation occurs due to sintering of the dispersed metal clusters. Influenced by the surface MoOx species that hamper sintering of the surface metal clusters and by the Y2O3 support that prevents coking on the catalyst, the Ru–Mo/Y2O3 catalysts exhibit superior catalytic stability against deactivation.

Photocatalytic Degradation of Acetone over Sulfated MoOx/MgF2 Composite: Effect of Molybdenum Concentration and Calcination Temperature

This paper presents a novel visible-light-driven catalyst, a SO42–/MoOx/MgF2 composite, which was synthesized by a simple solution method. Multiple techniques, including Brunauer-Emmett-Teller (BET), scanning electron microscopy (SEM), X-ray diffraction (XRD), Raman spectroscopy, Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), and diffuse reflectance spectroscopy (DRS) were applied to investigate the physical and photophysical properties of the catalysts. The photocatalytic activities were evaluated in the degradation of acetone in gas phase. In the photodegradation of acetone, the highest conversion was obtained over a catalyst containing 5 mol % molybdenum. The XRD and Raman characterizations indicate that the molybdenum oxide was highly dispersed in the MgF2 matrix. These MoOx species might be the active sites of the catalysts, which is the reason for the visible-light response of the composite catalyst. The MgF2 matrix acts to isolate the MoOx species and retard the electron–hole pair recombination. When the molybdenum concentration is >5 mol %, crystalline MoO3 phase was observed. The large MoO3 particle would decrease the separation efficiency. Thus, the photocatalytic activity was reduced. Besides the molybdenum concentration, the calcination temperature also shows a great effect on the activity. A sulfated 5 mol % MoOx/MgF2 catalyst that was calcined at 350 °C showed the highest photocatalytic activity. Based on the results of the characaterization, the origin of the high activity was discussed. The light absorption ability and the MoOx size effect are considered as the key factors.

Catalytically Active Interface between Noble Metal and Low-valence Transition Metal for C-O Hydrogenolysis

Two-components catalysts Rh-ReOx/SiO2, Rh-MoOx/SiO2 and Ir-ReOx/SiO2 were active in the C-O hydrogenolysis of glycerol, 1,2-propanediol and tetrahydrofurfuryl alcohol. The activities were much higher than those of monometallic catalysts. The selectivities to terminal alcohols decreased in the following order: Ir-ReOx/SiO2 > Rh-ReOx/SiO2 > Rh-MoOx/SiO2 » Rh/SiO2. Characterization with XRD, TEM, CO adsorption, temperature-programmed reduction (TPR), XANES and EXAFS revealed that noble-metal particles were partially covered with ReOx or MoOx species. The MoOx species in Rh-MoOx/SiO2 formed a monomeric structure. The ReOx species in Rh-ReOx/SiO2 and Ir-ReOx/SiO2 formed monolayer clusters and multilayer clusters, respectively. A reaction mechanism was proposed where the alkoxide species formed on ReOx (or MoOx) was attacked by the active hydrogen species on Rh (or Ir) atoms bonded with the Re (or Mo) atom. The difference of the size of the modifier may affect the selectivity.

Anomalous Surface Compositions of Stoichiometric Mixed Oxide Compounds

Coated: Surface analytical studies (including low-energy ion scattering (LEIS)) show that the outer surface of bulk, stoichiometric mixed vanadates and molybdates can be strongly enriched with VOx or MoOx species. Such surface reconstruction even in the calcined initial state has implications for the discussion of catalytic and other materials properties. Detailed facts of importance to specialist readers are published as ”Supporting Information”. Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Systematic synthesis of mixed-metal oxides in NiO–Co3O4, NiO–MoO3, and NiO–CuO systems via liquid-feed flame spray pyrolysis

We report here on the systematic synthesis of three nanopowder series along the NiO–Co3O4, NiO–MoO3, and NiO–CuO tie lines. Sixteen individual samples were produced via liquid-feed flame spray pyrolysis (LF-FSP) and analyzed by SSA, SEM, EDX, FTIR, TGA-DTA, and XRD. The LF-FSP process is a general aerosol combustion synthesis route to a wide range of lightly agglomerated oxide nanopowders. The materials reported here were produced by aerosolizing ethanol solutions of propionate and ammonium molybdate precursors synthesized by reacting the metal nitrate with propionic acid. Particular ratios of the precursors were selected to control the compositions of the samples produced. The powders typically consist of single crystal particles <35 nm in diameter and with specific surface areas (SSAs) of 20–50 m2 g−1. X-Ray powder diffraction (XRD) studies show a gradual change in their patterns from pure NiO to pure MOx (M = Co, Mo, Cu). Most compositions yielded single phase materials but mixed phase materials were also detected. We believe that higher vapor pressures of the ions produced for the transition metals studied resulted in SSAs about a third lower than those measured typically in nanopowders containing NiO. The partial pressure of O2 in LF-FSP affects the formation of particular oxide phases and controls to a certain degree the morphologies of the as-produced materials. We observe in the NiO–MoO3 system preferential growth of certain crystallographic planes in MoO3, due to the relatively high vapor pressure of MoOx species. Unusual particle morphologies seen in the NiO–Co3O4 system are attributed to some phase separation in the as-produced materials. TGA studies combined with diffuse reflectance infrared Fourier transform (DRIFT) spectroscopic studies indicate that both physi- and chemi-sorbed H2O are the principal surface species present in the as-processed nanopowders.

Highly Dispersed Molybdenum Oxide Supported on HZSM-5 for Methane Dehydroaromatization

Mo/HZSM-5 catalyst with highly dispersed MoOx species was prepared by adding ammonia solution to the ammonium heptamolybdate aqueous solution during the impregnation process. Compared with the large \( {\text{Mo}}_{{\text{7}}} {\text{O}}_{{{\text{24}}}} ^{{6 - }} \) species which is predominantly presented in the conventional impregnation solution, the monomer \( {\text{MoO}}_{{\text{4}}} ^{{2 - }} \) formed in ammonia solution could efficiently diffuse into the micropores and/or channels of the HZSM-5, resulting in higher dispersion of Mo species as well as enhanced interaction with the surface –OH groups of HZSM-5. Consequently, the obtained Mo/HSZM-5 catalyst showed rather high catalytic stability and greatly enhanced selectivity towards benzene for methane dehydroaromatization reaction by effectively inhibiting the coke formation.

Dispersion and site-blocking effect of molybdenum oxide for CO chemisorption on the Pt(1 1 0) substrate

The Pt(1 1 0) model surfaces modified by the metallic molybdenum and by the MoOx (molybdenum oxide species) were fabricated via thermal decomposition of Mo(CO) 6 and subsequent oxidation on a clean Pt(1 1 0) substrate, by means of Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS), as well as high-resolution electron-energy-loss spectroscopy (HREELS). CO chemisorption on these model surfaces was investigated using thermal desorption spectroscopy (TDS) and HREELS. The presence of the metallic molybdenum and MoOx species suppresses CO chemisorption on the Pt(1 1 0) substrate, indicating a physically site-blocking effect. The electron-deficiency of the Pt(1 1 0), due to the modification of metallic molybdenum, causes the low-temperature peak of CO desorption to shift downward in temperature by ∼30 K. The interaction between the MoOx and the underlying Pt(1 1 0) substrate has no influence on CO desorption temperature.

A Study of the Redox Properties of MoOx/SiO2

A sample of MoOx/SiO2, in which all of the Mo cations are present as isolated mono-oxo molybdate moieties, was prepared and investigated to understand the redox chemistry of such molybdate species and their ability to exchange oxygen with O2 and H2O. Raman spectroscopy was used to monitor the exchange of 18O for 16O in the Mo=O bond of isolated molybdate species, whereas mass spectrometry was used to follow the isotopic composition of the gaseous species, i.e., O2 and H2O. Reduction in H2 at 920 K results in the loss of one O atom per Mo atom, and consistent with this, all of the Mo(VI) cations are reduced to Mo(IV) cations. Raman spectroscopy shows that virtually all Mo=O bonds of the original molybdate species are lost upon reduction. While reoxidation of Mo(IV) cations by O2 is quantitative, studies using 18O2 reveal that only a small part of the newly formed Mo=O bonds are 18O labeled, and that the balance are 16O labeled, indicating that O-atom exchange between the support, SiO2, and the supported MoOx species occurs during reoxidation. Rapid exchange of O atoms was observed upon exposure of both bare SiO2 and MoOx/SiO2 to H2(18)O at 920 K, and the presence of MoOx species was found to enhance the rate of exchange. By contrast, very slow exchange of O atoms was observed when the oxidized catalyst was exposed to 18O2 at 920 K. In situ observations of the catalyst during exposure to a mixture of H2 and 18O2 at 920 K showed that all of the Mo cations remained in the VI oxidation state and that O atom exchange occurred at a rate comparable to that observed upon exposure to H2(18)O. The results of this investigation suggest that reoxidation of Mo(IV) cations following H2 reduction involves the formation of a Mo-peroxide species and subsequent O atom migration from such a species to the SiO2 support. It is proposed that the steady-state oxidation of H2 also involves the formation of Mo-peroxide species by interaction of O2 with a small number of Mo(IV) centers. The Mo-peroxide species are then rapidly reduced by H2 to form H2O and a Mo=O bond. The rapid exchange of O atoms between the gas phase and the catalyst observed during steady-state oxidation of H2 is attributed to interactions of the product H2O with the catalyst, rather than to O atom migration originating from the Mo-peroxide species formed on the catalyst surface.

Spectroscopic and Structural Characterization of Chlorine Loading Effects on Mo/Si:Ti Catalysts in Oxidative Dehydrogenation of Ethane

The structural changes induced in a silica-titania mixed-oxide support (1:1 molar ratio) by chlorine addition at different loading levels, their relation to the structural characteristics of supported MoOx species over the support, and their correlation with ethane oxidative dehydrogenation (ODH) activity have been examined. The molybdenum and chlorine precursors are incorporated into the Si/Ti support network as it forms during gelation by using a "one-pot" modified sol-gel/coprecipitation technique. In situ X-ray diffraction during calcination shows the Si/Ti 1:1 mixed-oxide support is in a state of nanodispersed anatase titania over amorphous silica. With the addition of molybdenum and chlorine modifier, this anatase feature becomes more pronounced, indicating a decreased dispersion of titania. The effective titania surface area on the chlorine-doped Si:Ti support obtained from 2-propanol temperature-programmed reaction supports this observation. Raman spectra of dehydrated samples point to an enhanced interaction of MoOx species with silica at the expense of titania. X-ray photoelectron spectroscopic results show that, without forming a molybdenum chloride, the presence of chlorine significantly alters the relative surface concentration of Si vs Ti, the electronic structure of the surface MoOx species, and the oxygen environment around supported MoOx species in the Si/Ti network. Secondary ion mass spectrometry detected the existence of SiCl fragments from the mass spectra, which provides molecular insight into the location of chlorine in Mo/Si:Ti catalysts. The observed increase in ethane ODH selectivity with chlorine modification may be ascribed to the MoOx species sharing more complex ligands with silica and titania with the indirect participation of chlorine. Steady-state isotopic transient kinetic analysis (SSITKA) is used to to examine the oxygen insertion and exchange mechanisms. The catalysts show very little oxygen exchange with the gas phase in the absence of a reaction medium. During the steady-state ODH reaction, lattice oxygen appears to be the primary source of oxygen in the formation of water and CO2.

Characterization of MoOx species on γ-Al2O3, Y and ZSM-5 zeolites during thermally activated solid–solid synthesis

We characterized the Mo surface species present during thermally activated solid–solid synthesis ofMo/γ-Al2O3,Mo/H-ZY andMo/H-ZSM-5 starting frommixtures of MoO3 with γ-Al2O3 or zeolites H-Y and H-ZSM-5. We identified different surface molybdate speciesby using diffuse reflectance measurements in the UV–vis region and a bandgapanalysis of the deconvoluted spectra. Species assignments were confirmedvia theoretical DFT calculations of simplified zeolite clusters containingMoOx species as well as by analysis of the spectra of pure standards.Migration of MoOx onto H-Y and H-ZSM-5 zeolites resulted in an important fraction of tetrahedralMo species (monomers and dimers). On the other hand, migration onγ-Al2O3 led to the presence of a larger fraction of octahedral Mo oligomericspecies. The variation in surface structures seems to be a function of theSi/Al ratio and of the pore structure of supports. Our results are consistent with amechanism involving formation and migration of monomers and dimers (equivalent to[MoO4]2− and [Mo2O7]2−) leading to the formation of oligomers (resembling[Mo7O24]6−). We present a simplified kinetic scheme for the formation of the different Mo compoundsconsistent with the characterization results.

Zirconia-Supported MoOx Catalysts for the Selective Oxidation of Dimethyl Ether to Formaldehyde: Structure, Redox Properties, and Reaction Pathways

Dimethyl ether (DME) reacts to form formaldehyde with high selectivity at 500−600 K on MoOx−ZrO2 catalysts with a wide range of MoOx surface density (0.5−50.1 Mo/nm2) and local structure (monomers, oligomers, MoO3 crystallites, and ZrMo2O8). Reaction rates (per Mo-atom) increased markedly as MoOx surface density increased from 2.2 to 6.4 Mo/nm2 and two-dimensional polymolybdates and MoO3 clusters became the prevalent active species. The rate of incipient stoichiometric reduction of MoOx−ZrO2 samples in H2 also increased with increasing MoOx surface density, suggesting that catalytic turnovers involve redox cycles that become faster as the size and dimensionality of MoOx domains increase. DME reaction rates (per Mo-atom) decreased as MoOx surface densities increased above 6.4 Mo/nm2, as MoO3 and ZrMo2O8 clusters with increasingly inaccessible MoOx species form. On MoOx and ZrMo2O8, areal reaction rates reach a constant value at MoOx surface densities above 10 Mo/nm2, as the exposed surfaces become covered ...

Molecular selection of MoOx species during migration on Al2O3 and zeolites Y and ZSM-5

Surface migration is the basis of the solid state preparation of various materials. In the case of heterogeneous catalysts, it involves the spreading of metal oxides with low Tammann temperature, such as MoO3, on various refractory supports like γ-Al2O3 and zeolites. In spite of its importance, the speciation of oxide surface migration has not been extensively studied. In this work Mo/γ-Al2O3, Mo/ZY and Mo/ZSM-5 were prepared from mixtures of MoO3 and γ-Al2O3, or zeolites Y and ZSM-5, subjected to thermal activation. X-Ray diffraction (XRD), Raman and UV-vis diffuse reflectance spectroscopies were used to study the spread of MoOx species on refractory metal oxide supports. It was found that during thermal activation the formation and migration of different MoOx species depends strongly on the structure of the support.

Spectroscopic and Structural Characterization of Low-Level Alkali Doping Effects on Mo/Silica−Titania Catalysts

The structural characteristics of supported MoOx species inherent over a high TiO2 content silica−titania mixed-oxide support (1:1 molar ratio), combined with the structural effects that are induce...

Mo Loading Effects over Mo/Si : Ti Catalysts in the Oxidative Dehydrogenation of Ethane

A series of molybdena catalysts (0–20 wt% loadings) were studied with regard to their activity for the oxidative dehydrogenation (ODH) of ethane. Catalysts with single oxide supports of silica and titania were also examined. X-ray diffraction, Raman spectroscopy (ambient and dehydrated conditions), and X-ray photoelectron spectroscopy were used to determine the nature of the molybdena phases supported over the mixed oxide support. Initially, MoOx preferentially supports on titania. However, interaction with silica is present even at the lowest wt% loading. With increased wt% loading, MoOx increasingly interacts with the silica phase of the support as titania increasingly segregates into larger anatase domains. Since MoOx species may share several types of support ligands with differing electronegativity, one cannot adequately describe MoOx species as being supported exclusively on SiO2 or TiO2 domains but rather, as indicated by the Raman results, as being supported on a mix of both species. The structural differences are reflected in ODH reaction performance. The ethylene yield increases with Mo loading over Si : Ti 1 : 1 to a maximum at 10% Mo/Si : Ti 1 : 1, where there may exist an optimum coverage of MoOx that shares several types of support ligands from both silica and titania domains of the support. The desorption behavior of products formed following ethane adsorption was studied with Mo wt% loading using temperature-programmed desorption.

The Effects of Silanation of External Acid Sites on the Structure and Catalytic Behavior of Mo/H–ZSM5

The selective silanation of external acid sites in H–ZSM5 using large organosilane reagents was used to decrease the density of such sites and the number of MoOx species retained at external surfaces during cation exchange using MoO3/H–ZSM5 physical mixtures. On samples prepared using silica-modified H–ZSM5, acid sites, MoOx precursors, and the active MoCx species formed during CH4 reactions at 950 K reside predominately within zeolite channels, where spatial constraints inhibit bimolecular chain-growth pathways. As a result, these samples show higher selectivities to desired C2–C6 aromatics and lower deactivation rates than those prepared from untreated H–ZSM5. CH4 conversion rates are also higher on Mo/H–ZSM5 samples prepared from silanated H–ZSM5, because of the more complete exchange and higher dispersion of MoOx precursors. In situ X-ray absorption measurements showed that silanation of external OH groups does not influence the structure of the MoOx precursors formed during synthesis or of active MoCx species formed during CH4 reactions from exchanged (Mo2O5)2+ dimers. The substantial absence of acidic OH groups and of MoCx species sites at unconstrained external surfaces leads to significant improvements in activity, selectivity, and stability for Mo/H–ZSM5 catalysts for CH4 pyrolysis reactions.