Iron Molybdate Research Articles

Impurity band conductance through oxygen vacancy donor states in bismuth iron molybdate

Under chemical reduction at 620 K in an atmosphere of methanol vapor in a nitrogen carrier gas, bismuth iron molybdate [Bi3FeO4(MoO4)2] will give up lattice oxygen and form highly mobile vacancies. These vacancies act as donors and sit about 0.4 eV below the conduction band edge: The band gap is 2.7 eV. Because of the ability to remove up to 1% of the lattice oxygen and still maintain crystallographic stability, it is relatively easy to produce high donor carrier densities (up to 1020 cm−3). Under these conditions, and taking into account that this is a wide gap semiconductor with well compensated deep donor levels, it is possible to measure an electrical conductivity dominated by carrier movement through the impurity band. Both phonon assisted hopping and the transition to metallic impurity conduction have been observed. The crossover temperature from conduction dominated by the impurity band to conduction dominated by the conduction band was seen to increase from 125 to 380 K with increasing oxygen depletion.

Methanol and Formaldehyde Oxidation Study over Molybdenum Oxide

Formaldehyde oxidation to carbon oxides in conjunction with methanol oxidation was studied to improve understanding about the selectivity of the partial oxidation of methanol to formaldehyde. The good selectivity to formaldehyde over bulk molybdenum trioxide and iron molybdate is attributed to water, a reaction product, and methanol, which retards the further oxidation of formaldehyde. Competitive adsorption experiments monitored by IR indicate that CH2O, CH3OH, and H2O compete for the same site. The loss of the formaldehyde selectivity over the silica-supported catalyst in methanol oxidation can not be attributed to the further oxidation of gaseous formaldehyde.

In-situ Mössbauer spectroscopic study of iron site evolution in iron and cobalt molybdates catalysts in propene oxidation reaction conditions

In-situ Mössbauer spectroscopy was used to study Fe x Co 1− x MoO 4 molybdates and their mixtures with Bi 2(MoO 4) 3 in the 360–415°C range under a flow of a mixture C 3H 6/O 2/N 2 = 100/100/560 Torr. Ferrous molybdate FeMoO 4 was progressively oxidized to ferric molybdate and ferric oxide. Mixed cobalt and iron molybdates Fe x Co 1− x MoO 4 exhibited the following behaviour: (1) for all iron contents, the α phase spectrum diminished to the benefit of the β phase. The Fe 3+ sites of the solid solution underwent reduction. At high iron content, the Fe 3+ in Fe 2(MoO 4) 3 was detected and diminished by half under catalytic conditions; (2) when Bi 2(MoO 4) 3 was added to Fe x Co 1− x MoO 4, there was no reduction of Fe 3+ sites in the solid solution. The α → β phase transition was not observed in the case of the low and medium iron contents. In the high iron content samples the α-phase spectrum decreased to the expense of the β phase but did not disappear totally after catalytic reaction. It is concluded that cobalt stabilizes Fe 2+ sites in the solid solution while Bi 2(MoO 4) 3 stabilizes Fe 3+ in the solid solution. It is proposed that Fe 2+-Fe 3+ pairs take an important place in the mechanism of propene mild oxidation.

Influence of the preparation method on the behaviour of Fe-Mo catalysts for the oxidation of methanol

A series of Fe-Mo catalysts with different properties was prepared by modification of the synthesis variables. Their catalytic behaviour in the oxidation reaction of methanol to formaldehyde was determined. The differences in their textural, morphological, chemical and structural properties were determined using nitrogen adsorption, SEM-energy dispersive X-ray analysis, X-ray photoelectron spectroscopy, X-ray diffraction and Fourier transform-infrared analyses. We have attempted to establish relationships between these properties and their catalytic activity in the above reaction. We have found that their catalytic behaviour principally correlated with the atomic ratio of Mo∶ Fe. This ratio was sensibly higher than that required stoichiometrically for iron(III) molybdate.

Electron Paramagnetic Resonance and Mössbauer Studies of Fe3+Ions in the Fe2O3[dbnd] MoO3System

Abstract The electron paramagnetic resonance (EPR) and Mossbauer spectra of ironmolybdenum mixed oxides system have been investigated. Both EPR and Mossbauer measurements revealed the formation of ferric molybdate, Fe2 (MoO4)3 and the existence of two different Fe3+ species. The EPR spectra of Fe2 (MoO4)3, show two resonance lines of Fe3+ ions indicating that a strong exchange interaction is dominates in pure iron molybdate. Mosbauer measurements revealed that the isomer shift (I. S) increases with increasing the iron content. A maximum of quadrupole splitting (Q. S) is observed at the stoichiometric concentration corresponding to the formation of Fe2 (MoO4)3. The effect of a catalytaic dehydration process on the redox behaviour of Fe3+ located in iron molybdate frame work is investigated. Results indicate that the catalytaic dehydration of 2-proppanol over this catalyst reduces Fe3+ ions to Fe2+ leading to the formation of the inactive phase FeMoO4. the essential role of oxygen, in the alcohol gas feed, ...

Study of Multiphasic Molybdate-Based Catalysts

Results are reported concerning the synergy effect observed in the oxidation of propene to acrolein over bismuth and mixed iron and cobalt molybdates. The pure bismuth, iron, and cobalt molybdates and mixed cobalt and iron molybdates (solid solutions) have been prepared and individually tested as catalysts. Mechanical mixtures of these phases have been prepared and tested as catalysts. All the catalysts have been characterized before and after the catalytic reaction by several techniques such as ESR, XPS, EDX-STEM, TEM, XRD, and Mossbauer and UV spectroscopies. The synergy effect observed is tentatively explained as due to the deposition on the large bismuth molybdate particles of smaller mixed iron and cobalt molybdate particles with spreading of the bismuth molybdate over the latter particles. It is proposed that the FexCo1−xMoO4 phase plays the role of the fast electron conducting material which enhances the electron mobility and the efficiency of the redox mechanism, the active and selective phase being the overlying bismuth molybdate compounds.

Study of Multiphasic Molybdate-Based Catalysts: I. Electrical Conductivity Study of Valence States and Solubility Limits in Mixed Iron and Cobalt Molybdates

The electrical conductivity of a series of iron-cobalt molybdates has been measured at 370°C, as a function of the iron content. An important increase of the electrical conductivity occurred when small amounts of iron were substituted for cobalt. This takes place because iron is partially in the 3+ oxidation state. This species, which is stabilized into the CoMoO 4 structure, creates a valence induction effect which is responsible for the drastic variation of the electrical conductivity with compounds whose iron content ranges from 0 to 20%. UV spectroscopy allows us to propose that the ferric species involved in this valence induction phenomenon are those giving rise to the intervalence charge transfer between iron cations (Fe 2+-O-Fe 3+). By contrast, ESR spectroscopy is silent for such a phenomenon (too short relaxation time) and shows two typical Fe 3+ ( d 5 ion) spectra for Fe 3+ engaged either in a different manner in the solid solution (strongly distorted octahedral site) or a given compound such as Fe 2(MoO 4) 3 observed at high iron content.

Vacancy donors and the electrical properties of bismuth iron molybdate pellets

Conductance changes of sintered pellets of bismuth iron molybdate can be used to detect low concentrations of atmospheric reducing vapours such as ethanol. It is known that the conductance can be greatly affected by the stoichiometry (number of oxygen vacancies) of the compound, but what is not as well known is the relative contributions of the bulk interior, the near surface, and chemisorbed gases. To determine these, temperature-dependent conductance measurements are performed on pellets cooled in either oxidizing or inert atmospheres, for a variety of initial conditions (temperature and degree of chemical reduction). It is determined that the sensitivity of the surface to chemisorbed oxygen is history dependent and that long-term oxidation at about 350 °C is needed to remove this sensitivity. At room temperature the conductance is dominated by physisorption of water and organic vapours. A model, asserting the dominance of near-surface conductance, is presented that explains the results.

Optical Measurements on Heavily Reduced Bismuth Iron Molybdate: Evidence for Vacancy-Induced Phase Transitions

It is known that the conductance of a bismuth iron molybdate pellet is greatly affected by the stoichiometry (number of oxygen vacancies) of the compound. The number of these vacancies is determined by the amount of reducing vapor (such as ethanol), which tends to increase their number and the amount of oxygen, which tends to decrease it, in the atmosphere. For this reason the compound can be used as a chemical sensor. Earlier measurements of the conductance in a highly reducing methanol vapor atmosphere, determined by measuring the current passing between two gold wires embedded in a pressed pellet, have shown three well-defined stages in the conductance, which are related to solid-state phase transitions occurring on the surface of the pellet granules. In this paper we further examine the nature of these transitions using infrared, Raman, and photoluminescence spectroscopy to follow the structural changes that occur as greater amounts of oxygen are removed. These changes are seen as shifts in relative scattering intensity as well as frequency.

Vacancy-induced transitions in the conductance of bismuth iron molybdate

Changes in the conductivity of bismuth iron molybdate can be used to detect low concentrations of atmospheric reducing vapours such as ethanol. It is known that the conductance is greatly affected by the stoichiometry (number of oxygen vacancies) of the compound. Since the conductance is determined by measuring the current passing between two gold wires embedded in a pressed pellet, geometric effects are also important. This involves the interconnectivity of the granules making up the pellet, the fact that chemical reduction occurs at the surface, and the speed of diffusion of oxygen vacancies into the bulk. The conductance is measured as a function of time spent in a highly reducing atmosphere of methanol vapour. These measurements show three well defined stages in the conductance, which are characterized by a change in the sensitivity to further chemical reduction. These stages are also characterized by a lowering of the electronic activation energy which is interpreted as the onset of impurity band conduction.

Solid-solid phase equilibria in the binary system cobalt molybdate (CoMoO4)-iron molybdate (FeMoO4) and effect of iron on the phase equilibria

Solid-solid phase equilibria in the binary system CoMo 4 -FeMoO 4 have been studied by temporature programmed electrical conductivity measurements (TPEC) and confirmed, in some cases, by differential thermal analysis (DTA), X-ray diffraction, and IR spectroscopy. The results show that two continuous solid solutions exist between the two polymorphic forms of CoMoO 4 and FoMoO 4 (α and β)

Comparative study of polymorphic phase transition by differential thermal analysis, high temperature x-ray diffraction and temperature programmed electrical conductivity measurements: case study of mixed iron and cobalt molybdate

A comparative analysis of the results, obtained by differential thermal analysis (DTA), high temperature X-ray diffraction (XRD), and temperature programmed electrical conductivity measurements (TPEC), in the study of the polymorphic transition (a to B form) of a mixed ferrous and cobaltous molybdate is reported. In order to prom in the interpretation of the curves obtained using TPEC, the results obtained with these techniques, used at different heating rates, have been compared. This comparison enables one to evidence the occurrence of the phase transition on the surface and in the bulk of the sample particles.

In situ Mössbauer spectroscopic study on cobalt and iron molybdates and their mixtures with Bi2(MoO4)3 during propene mild oxidation

Several Co and Fe and mixed Co, Fe molydates have been studied by Mossbauer spectroscopy at 360–415°C in a flow C3H6+O2+N2 100/100/560 Torr, with and without adding Bi2 (MoO4)3. It is concluded that cobalt stabilizes Fe2+ sites and Bi2(MoO4)3 stabilizes Fe3+ in solid solution and it is proposed that Fe2+−Fe3+ pairs act as active sites in propene mild oxidation.

A study of the cobalt sites in cobalt and iron molybdate catalysts by emission Mössbauer spectroscopy

Molybdates samples containing 74MBq (2mCi) of cobalt 57 have been studied by emission Mossbauer spectroscopy. Spectra of57CoMoO4 exhibited the three usual Fe2+ doublets corresponding to α and β ferrous phases, and two Fe3+ doublets accounting for about one third of the absorption area. The Co2+ sites of β57CoMoO4 phase exhibit higher quadrupole splittings, i.e. are more dissymmetrical than Fe2+ sites of βFeMoO4 phase. Cobalt and iron molybdates Fex57Co1−xMoO4 provided spectra where only one Fe3+ doublet appeared and occupied less than 7% of the spectral area. It is concluded that a part of cobalt was present in57CoMoO4 as Co3+.

Raman spectroscopy of iron molybdate catalyst systems: Part II. Preparation of supported catalysts

Iron molybdates (mixed oxides of iron and molybdenum) are used to effect the partial oxidation of methanol to formaldehyde. In the present work, Raman spectroscopy was used to characterize supported mixed iron/molybdenum oxide catalysts. These supported catalysts were prepared by impregnation of silica and alumina with solutions of ferric nitrate and ammonium molybdate. The Raman spectroscopic studies were employed to obtain information about the influence of variations in the preparation protocol on the properties of the final catalysts. The spectra were indicative of surface enrichment of one species [(MoO 3 or Fe 2(MoO 4) 3)] and also provided evidence for catalyst-support interactions.

Raman spectroscopy of iron molybdate catalyst systems:: Part I. Preparation of unsupported catalysts

Raman spectroscopy was used to characterize unsupported iron/molybdenum oxide catalysts of the type used for the partial oxidation of methanol to formaldehyde. Bulk catalysts were prepared by coprecipitation from solutions of ferric nitrate and ammonium molybdate. The Raman spectra of the catalysts contained bands characteristic of Fe 2(MoO 4) 3 and MoO 3. For unsupported catalysts with Mo/Fe ratios greater than 1.5, the relative intensities of the MoO 3 and Fe 2(MoO 4) 3 peaks in the Raman spectra were approximately proportional to the corresponding atomic ratio. These catalysts are comprised of Fe 2(MoO 4) 3 and MoO 3 which are formed in the calcination step of the preparation protocol. Raman spectra indicate that a solid phase reaction begins as the precipitates are heated at 105 °C. At 125 °C Raman bands characteristic of Fe 2(MoO 4)a were observed. At 250 °C bands corresponding to MoO 3 appeared. In situ and ex situ spectroscopic studies were used to obtain further information about the influence of variations in the preparation protocol on the properties of the final catalysts.

Raman spectroscopy of iron molybdate catalyst systems: Part III. IN SITU studies of supported and bulk catalysts under reaction and redox conditions

Raman spectra of both supported and unsupported Fe/Mo/O catalysts of the type used for the partial oxidation of methanol to formaldehyde were obtained while this reaction was proceeding over the catalyst surface. No changes in band locations or relative peak intensities associated with these mixed metal oxides were detected in the presence of reactant and product species. Moreover, no spectral features which could be attributed to adsorbed species could be detected under reaction conditions. Band intensities in the Raman spectra were related to the extent to which the catalysts were reduced; fully oxidized catalysts were stronger Raman scatterers than partially reduced catalysts. Although differences in color were noted between fresh and used catalysts, the Raman and Mössbauer spectra of these catalysts were identical. Calcination of used catalysts in an oxygen environment at 500 °C was sufficient to restore partially reduced catalysts to their original fully oxidized states.

NGR study of iron molybdates and iron-cobalt molybdates catalysts

Iron molybdates FM and cobalt-iron molybdates CFxM (which means Co1−yFeyMoO4 withy=x/100 6≤x≤67), used as matrix of catalysts for propene mild oxidation, were studied by Mossbauer spectroscopy at 295 K and 4 K. All the spectra exhibit three Fe2+ doublets, two of them correspond to β-FeMoO4 and the third one to α-FeMoO4. Enhancement of quadrupole splittings of Fe2+ in the spectra of the CFxM catalysts is ascribed to the occurrence of a solid solution of either iron in CoMoO4 or cobalt in FeMoO4. In each spectrum at 295 K one or several Fe3+ doublets were present, which low temperature spectra allow to be assigned to Fe2(MoO4)3, small particles of Fe2O3, solid solution of Fe3+ in ferrous molybdates and even Fe2MoO4 in some cases. The behaviour of all the iron species of these solids during reduction by hydrogen is described.

Adsorption of selenium by amorphous iron oxyhydroxide and manganese dioxide

This work compares and models the adsorption of selenium and other anions on a neutral to alkaline surface (amorphous iron oxyhydroxide) and an acidic surface (manganese dioxide). Selenium adsorption on these oxides is examined as a function of pH, particle concentration, oxidation state, and competing anion concentration in order to assess how these factors might influence the mobility of selenium in the environment. The data indicate that 1. 1) amorphous iron oxyhydroxide has a greater affinity for selenium than manganese dioxide, 2. 2) selenite [Se(IV)] adsorption increases with decreasing pH and increasing particle concentration and is stronger than selenate [Se(VI)] adsorption on both oxides, and 3. 3) selenate does not adsorb on manganese dioxide. The relative affinity of selenate and selenite for the oxides and the lack of adsorption of selenate on a strongly acidic surface suggests that selenate forms outer-sphere complexes while selenite forms inner-sphere complexes with the surfaces. The data also indicate that the competition sequence of other anions with respect to selenite adsorption at pH 7.0 is phosphate > silicate > molybdate > fluoride > sulfate on amorphous iron oxyhydroxide and molybdate ≥ phosphate > silicate > fluoride > sulfate on manganese dioxide. The adsorption of phosphate, molybdate, and silicate on these oxides as a function of pH indicates that the competition sequences reflect the relative affinities of these anions for the surfaces. The Triple Layer surface complexation model is used to provide a quantitative description of these observations and to assess the importance of surface site heterogeneity on anion adsorption. The modeling results suggest that selenite forms binuclear, innersphere complexes with amorphous iron oxyhydroxide and monodentate, inner-sphere complexes with manganese dioxide and that selenate forms outer-sphere, monodentate complexes with amorphous iron oxyhydroxide. The heterogeneity of the oxide surface sites is reflected in decreasing equilibrium constants for selenite with increasing adsorption density and both experimental observations and modeling results suggest that manganese dioxide has fewer sites of higher energy for selenite adsorption than amorphous iron oxyhydroxide. Modeling and interpreting the adsorption of phosphate, molybdate, and silicate on the oxides are made difficult by the lack of constraint in choosing surface species and the fact that equally good fits can be obtained with different surface species. Finally, predictions of anion competition using the model results from single adsorbate systems are not very successful because the model does not account for surface site heterogeneity. Selenite adsorption data from a multi-adsorbate system could be fit if the equilibrium constant for selenite is decreased with increasing anion adsorption density.

Evolution during calcination of Mo-Fe oxidation catalysts doped with chromium

Iron and molybdenum oxides doped with chromium, useful as catalysts for methanol oxidation to formaldehyde, have been prepared by coprecipitation from aqueous solutions and further calcination at increasing temperatures. The solids have been studied by chemical analysis, TG/DTG and DTA, XRD, scanning electron microscopy, visible-ultraviolet spectroscopy, and nitrogen adsorption at 77 K for surface area and porosity. From the X-ray diffraction studies, it is concluded that molybdena and iron(III) molybdate are the major components, while chromium-containing species are hardly detected due to their low content and because iron(III) and chromium(III) molybdates are isostructural. Differential thermal analysis and thermogravimetric analysis of the precursor solids showed that singular solids are obtained after performing calcination at 533, 673, and 873 K. Specific surface areas increased with the calcination temperature and then decreased, due to sintering, in agreement with scanning electron microscopy results. The presence of chromium changed the topology of the molybdena crystallites from stick- to plate-like. If calcination was carried out well above the operating temperature of the catalyst ( ca. 680 K), sintering of the crystallites took place.