MoO3 TiO2 Research Articles

Dispersion and catalytic activity of MoO3on TiO2‐SiO2binary oxide support

AbstractThe authors present results of an investigation into the dispersion of MoO3on TiO2‐SiO2composite and its reactivity in transesterification of dimethyl oxalate (DMO) with phenol to produce methyl phenyl oxalate (MPO) and diphenyl oxalate (DPO). They examined the effects of support composition, MoO3content, and reaction time on activities of the transesterification. The results show that MoO3/TiO2‐SiO2is more active and selective than MoO3/TiO2, MoO3/SiO2, and TiO2/SiO2catalysts in the transesterification reaction. The best catalytic performance is obtained over a 12%MoO3/8%TiO2‐SiO2, providing a 71.8% DMO conversion and 59.0%, 40.1% selectivities for MPO and DPO, respectively. Through employing X‐ray diffraction and inductively coupled plasma‐optic emission spectrometry, they show evidence strongly suggesting that improvement of catalytic reactivity over the TiO2‐modified SiO2support can be ascribed to the enhanced metal oxide support interactions and the increased dispersion capacity of MoO3phase. They also present evidence showing that incorporation of TiO2in SiO2can elevate the monolayer dispersion capacity of MoO3on SiO2. Thus, they conclude that increased DPO selectivity from MoO3/TiO2‐SiO2catalysts can be primarily attributed to the improvement of MoO3dispersion and the synergistic effect between amorphous MoO3and amorphous TiO2. © 2008 American Institute of Chemical Engineers AIChE J, 2008

Application of ion beam analysis to the selective sublimation processing of thin films for gas sensing

Ion beam analysis was successfully applied to a novel technique, named selective sublimation process (SSP), for deposition of nanostructured gas-sensing films through reactive sputtering. The method consists of the co-deposition of a mixed oxide, one of which has a relatively low sublimation temperature. Annealing at suitable temperature causes the sublimation of the most volatile compound, leaving a layer with adjustable composition. The appropriate choice of thermal treatments and the consequent tailoring of the composition play a crucial role in the determination of the microstructural properties.We developed a model based on diffusion equations that provides a useful guide to control the deposition and processing parameters and we applied the model on the systems TiO2–WO3 and TiO2–MoO3. Rutherford backscattering (RBS) was demonstrated to be effective for the characterization of the diffusion and sublimation processes during SSP. Experimental results fully agree with theoretical prediction, and allowed the calculation of all the parameters involved in SSP.

Comparison of single and binary oxide MoO3, TiO2 and WO3 sol–gel gas sensors

A systematic comparison of sol–gel prepared titanium dioxide (TiO2), WO3, and MoO3 single metal oxide based gas sensors was conducted. Process variables such as solution concentration, deposition parameters, gelling time, annealing time and temperature, remained constant. Sensors based on binary compound MoO3–TiO2 and MoO3–WO3 were also investigated to determine if the performance is superior to their single oxide constituents. The sensors were systematically exposed to O2, O3, CO and NO2 gases and ethanol vapor at concentration levels of particular interest. MoO3 binary compound based sensors showed promising O3, CO and NO2 gas response. Their use as a sensing film for gas is limited due to the materials low evaporating temperature, limiting its operating temperature below 350°C. However, the binary oxide of MoO3–WO3 showed a high response to ethanol vapor and a highly selective response to NO2.

Microstructure characterization of sol-gel prepared MoO3–TiO2 thin films for oxygen gas sensors

Binary metal–oxide MoO3–TiO2 films have been prepared using the sol-gel technique. The thin films were annealed at several temperatures including 400, 450, 500, 550, and 600 °C for 1 h. The morphology, crystalline structure, and chemical composition of the films have been analyzed using scanning electron microscopy (SEM) and atomic force microscopy, x-ray diffraction (XRD), Rutherford backscattering spectroscopy (RBS), and x-ray photoelectron spectroscopy (XPS) techniques. The SEM analysis showed that there are two different sizes of grains in the films annealed at temperatures of 400, 450, 500, and 550 °C. One grain type is small with 20–100 nm; the other is a large grain type several micrometers in length. The XRD analysis revealed that the films annealed at 400 °C were a mixture of orthorhombic and hexagonal MoO3 phases. The films annealed at 450 °C showed an increase in the hexagonal phase. A preferential orientation growth along the (100) plane of the hexagonal phase and the (010) plane of the orthorhombic phase has been found in both samples. RBS and XPS analysis showed that the films were stoichiometric. When the annealing temperature was increased beyond 500 °C, the concentration ratio of MoO3–TiO2 decreased due to the evaporation of MoO3. For the study of the electrical and gas sensing properties, the films were deposited on sapphire substrates with interdigital electrodes on the frontside and a Pt heater on the backside. The MoO3–TiO2 thin films are sensitive to oxygen gas. The film has exhibited the O2 response (S=Rg/Rb) of 2.1, 8.1, and 80 for 120, 1000, and 10 000 ppm concentration of O2, respectively.

Surface structure and reactivity of molybdena–titania catalysts prepared by different methods

MoO3–TiO2 samples have been prepared from aqueous solutions of ammonium heptamolybdate and titania (by conventional impregnation and equilibrium adsorption) and by mechanically mixing both oxides (with and without further hydrothermal treatment). The samples have been characterized by X-ray diffraction, specific surface area assessment, Fourier-transform infrared (FTIR) and ultraviolet–visible diffuse refectance (UV–VIS DR) spectroscopies and temperature-programmed reduction (TPR). Surface properties have been studied following the adsorption of probe molecules by FTIR spectroscopy. Dispersed phases are formed preferentially in samples prepared by equilibrium adsorption and in physical mixtures submitted to hydrothermal treatment; in all other cases crystalline MoO3 is mainly formed. Dispersed phases are more easily reduced. They have been identified as surface molybdenyl species bonded to Ti cations through bridging oxygens, and their overall coordination depends on the degree of surface hydration. These species predominate quantitatively on the catalyst surfaces and have been found to be strong Lewis and Bronsted acids. They are also more easily reduced than bulk MoO3 under TRP conditions, and very active as oxidizing agents for methanol.

Untersuchungen zur Kinetik der Anatas-Rutil-Umwandlung an DeNOx-Katalysatoren für V2O5-WO3-TiO2 (Anatas) und V2O5-MoO3-TiO2 (Anatas)

The effect of preparation — specific parameters such as energy input, thermal treatment, V2O5-content — on the kinetics of the anatase-rutile transformation, were investigated and their relative contributions analysed. The various degrees of transformation were determined by quantitative X-ray diffraction measurements and checked for various rate laws. The rate constants evaluated according to Avrami for the V2O5—WO3—TiO2 (anatase)-system were extrapolated from Arrhenius plots for temperature values corresponding to conditions in power plants. The experimental results yield information on the stabilities (exposition periods) of the anatase-modification which react sensitively towards changes in phase composition at the anatase phase boundary, thus reflecting activation and deactivation processes which have an effect on the kinetics of the anatase-rutile phase transformation. The kinetic behaviour determined according to Avrami for the V2O5—WO3—TiO2 (anatase)-system has not been found for the V2O5—MoO3—TiO2 (anatase)-system. A unique feature of MoO3-systems consists in the following: after passing through a kinetically controlled region they form stable phases which can no longer be influenced kinetically. The anatase content of such phases can be adjusted by maintaining specific preparation parameters.

Studies of architecture of MoO3–TiO2 catalysts

X-Ray diffraction, F.t.i.r., scanning electron microscopy (SEM) and ion-scattering spectroscopy (ISS) have been applied to characterize a series of MoO3–TiO2(anatase) samples prepared by the grafting and impregnation methods. The sample prepared with the grafting method exhibits good dispersion of MoO3, whereas the impregnation technique leads to a poorer dispersion of molybdena on the surface of titania. The poorer dispersion of MoO3 is due to the clustering of molybdena when the MoO3 loading does not exceed the theoretical monolayer coverage (6.3 wt% MoO3). At higher MoO3 loading, crystals of MoO3 of morphology different from pure MoO3 crystals, appear. A possible explanation of this phenomenon is discussed.

The Oxidation Activity and Acid-base Properties of Mixed Oxide Catalysts Containing Titania. I. The TiO2–MoO3 and TiO2–V2O5 Systems

Abstract The amounts of both the acidic and basic sites of two series of catalysts, TiO2–MoO3 and TiO2–V2O5, with different compositions were measured by studying the adsorption of the basic and acidic molecules in the gas phase, using both the static and pulse methods. The acidity of TiO2–MoO3 catalysts is very high at Mo=40–60 atom%, though those of the TiO2-rich (Mo<30 atom%) and MoO3-rich (Mo>80 atom%) catalysts are fairly low. The acidity of the TiO2–V2O5 increases steadily with the V2O5 content. The basicity of the catalysts is remarkably enhanced by the introduction of a small amount of MoO3 or V2O5. It can be said that the catalysts are basic at Mo<20 atom% or V<10 atom%, while the MoO3-rich or V2O5-rich catalysts are acidic. The vapor-phase oxidation of 1-butene, butadiene,and acetic acid, the dehydration and dehydrogenation of isopropyl alcohol (IRA), and the isomerization of 1-butene were carried out in the presence of an excess of air, and the relationship between the catalytic behavior and the acid-base properties was investigated. It was concluded that the activities and selectivities can be relatively well explained by the acid-base properties between the catalyst and the reactant.

Pronounced catalytic activity of specially prepared MoO3–TiO2for reduction of nitrous oxide with hydrogen

Although an MoO3–TiO2(α) catalyst prepared from orthotitanic acid showed only 2·5% conversion in the reduction of nitrous oxide with hydrogen at 250 °C, an MoO3–TiO2(β) catalyst prepared from metatitanic acid showed 100% conversion, and its activity was markedly higher than that of MoO3–SiO2, MoO3–Al2O3, MoO3–MgO, or MoO3–active carbon.