Bismuth Molybdate Phases Research Articles

One-step microwave-assisted hydrothermal synthesis of α-Bi2Mo3O12 without surfactants at low temperature for their application in tetracycline photodegradation

This research reports obtaining the bismuth molybdate phase α-Bi 2 Mo 3 O 12 by a microwave-assisted hydrothermal method using sustainable environmental conditions compared to the typical material synthesis by solid-state reaction. The α-Bi 2 Mo 3 O 12 sample obtained by microwave assistance (BMO) was evaluated for the photocatalytic degradation of tetracycline (TC) using simulated sunlight and compared against the same phase obtained by the typical solid-state reaction (BSS). BMO presented a higher TC degradation efficiency (56%) than BSS (35%). This was attributed to greater surface area and lower electron-hole pair recombination according to the photoluminescence spectrum than BSS. The scavenger tests determined that H 2 O 2 is the main specie involved in the TC degradation, which was corroborated by the construction of the bands position diagram carried out by X-ray Photoelectron Spectroscopy (XPS) determination. The conduction band of BMO was calculated at -1.62 eV, which allows it to generate H 2 O 2 at a potential of 0.70 V. • The BMO sample presented a higher surface area than the BSS sample • The VB and CB positions were calculated to be +1.28 eV and -1.62 eV by XPS analysis • The BMO sample showed the higher photocatalytic efficiency in the TC degradation • The scavengers test determined that H 2 O 2 is the main species of TC degradation • The α-Bi 2 Mo 3 O 12 presented high stability and caused a partial mineralization of TC

Construction of a Bi2MoO6:Bi2Mo3O12 heterojunction for efficient photocatalytic oxygen evolution

In this study, two different crystallographic phases of bismuth molybdate, α-Bi2Mo3O12 and γ-Bi2MoO6 were synthesized hydrothermally. A series of Bi2Mo3O12/Bi2MoO6 heterostructures with different phase ratios were readily prepared by tuning the concentration of the molybdenum precursor while retaining the same bismuth precursor concentration. The Bi2Mo3O12/Bi2MoO6 heterojunction was found to substantially enhance the photocatalytic water oxidation capability relative to neat Bi2Mo3O12 and Bi2MoO6. The Bi2MoO6 and Bi2Mo3O12 were found to possess a complementary band alignment between which enabled the charge transfer process across the heterojunction interface. Ag and MnOx photodeposition was conducted on the heterostructures to affirm the charge transfer direction between the two crystal phases. Under visible light irradiation, photogenerated holes transfer from the active Bi2MoO6 phase to the inactive Bi2Mo3O12, suggesting efficient charge separation occurs between the two components, explaining the better photocatalytic performance.

Reactivity of Bismuth Molybdates for Selective Oxidation of Propylene Probed by Correlative Operando Spectroscopies

α-Bi2Mo3O12, β-Bi2Mo2O9 , and γ-Bi2MoO6 as target bismuth molybdate phases were prepared by hydrothermal synthesis and flame spray pyrolysis and tested for their catalytic performance in the selective oxidation of propylene. Their structure and reactivity during temperature-programmed reaction (TPR) and under reaction conditions were investigated by in situ and operando X-ray absorption spectroscopy (XAS), X-ray diffraction (XRD), and Raman spectroscopy. To gain insight into amorphous and crystalline structures at the same time, XAS and XRD as well as XAS and Raman spectroscopy were combined in one experiment. TPR in propylene revealed that the reduction of Mo6+ to Mo4+ occurred at lower temperatures than from Bi3+ to Bi0 in scheelite-structured systems. In a reaction cycle, mainly reduction of molybdenum was observed and EXAFS fitting confirmed the removal of oxygen from MoO42– entities. Minor structural transformations were detected by XRD and Raman spectroscopy. The catalytic performance of aurivillius...

Ammoxidation of acrolein to acrylonitrile over bismuth molybdate catalysts

The present work deals with the potentially significant process converting acrolein of green origin to acrylonitrile using mesoporous bismuth molybdate catalysts. The ammoxidation catalysts were characterized by N2 physisorption, X-ray diffraction, and catalytic tests under various conditions at different temperatures, contact times, and reactant molar ratios. The results indicated a catalytic activity proportional to specific surface area, which depends on bismuth molybdate phases, and concentration of oxygen in the gas feed. The selectivity of the catalysts only depends on reaction temperature. ACN selectivity obtained at 350–400°C was 100% and reduced to 97% at 450°C.

Bismuth Molybdate Catalysts Prepared by Mild Hydrothermal Synthesis: Influence of pH on the Selective Oxidation of Propylene

A series of bismuth molybdate catalysts with relatively high surface area was prepared via mild hydrothermal synthesis. Variation of the pH value and Bi/Mo ratio during the synthesis allowed tuning of the crystalline Bi-Mo oxide phases, as determined by X-ray diffraction (XRD) and Raman spectroscopy. The pH value during synthesis had a strong influence on the catalytic performance. Synthesis using a Bi/Mo ratio of 1/1 at pH ≥ 6 resulted in γ-Bi2MoO6, which exhibited a better catalytic performance than phase mixtures obtained at lower pH values. However, a significantly lower catalytic activity was observed at pH = 9 due to the low specific surface area. γ-Bi2MoO6 synthesized with Bi/Mo = 1/1 at pH = 6 and 7 exhibited relatively high surface areas and the best catalytic performance. All samples prepared with Bi/Mo = 1/1, except samples synthesized at pH = 1 and 9, showed better catalytic performance than samples synthesized with Bi/Mo = 2/3 at pH = 4 and 9 and γ-Bi2MoO6 synthesized by co-precipitation at pH = 7. At temperatures above 440 °C, the catalytic activity of the hydrothermally synthesized bismuth molybdates started to decrease due to sintering and loss of surface area. These results support that a combination of the required bismuth molybdate phase and a high specific surface area is crucial for a good performance in the selective oxidation of propylene.

One-step synthesis of bismuth molybdate catalysts via flame spray pyrolysis for the selective oxidation of propylene to acrolein.

Flame spray pyrolysis (FSP) of Bi(III)- and Mo(VI)-2-ethylhexanoate dissolved in xylene resulted in various nanocrystalline bismuth molybdate phases depending on the Bi/Mo ratio. Besides α-Bi2Mo3O12 and γ-Bi2MoO6, FSP gave direct access to the metastable β-Bi2Mo2O9 phase with high surface area (19 m(2) g(-1)). This phase is normally only obtained at high calcination temperatures (>560 °C) resulting in lower surface areas. The β-phase was stable up to 400 °C and showed superior catalytic performance compared to α- and γ-phases in selective oxidation of propylene to acrolein at temperatures relevant for industrial applications (360 °C).

Mesostructural Bi-Mo-O catalyst: correct structure leading to high performance

Structure-activity relationship has been one of the main topics of research on catalysts all the time. Component and structure are the two moieties governing the performance of solid materials as catalysts. Multicomponent bismuth molybdates are well known catalysts for propene oxidation but pure crystalline phases of bismuth molybdate are inactive for the reaction. We have designed mesostructural Bi-Mo-O catalyst with pure bismuth molybdate nanocrystals attached to molybdenum oxide nanobelts and found it is a high performance catalyst for the reaction, though the two domains themselves are inactive. The strongly expitaxial interaction between the two domains causes the lattice shrinkage and distortion of the bismuth molybdate nanocrystals and extremely promotes their catalytic activity toward propene oxidation while keeping high selectivity at the same time. The results are instructive for design of nano oxide catalysts with mesostructures leading to high performance.

Influence of Graphite as a Shaping Agent of Bi Molybdate Powders on Their Mechanical, Physicochemical, and Catalytic Properties

The influence of using graphite (G) as a shaping agent for bismuth molybdate (BiMo) catalysts was analyzed. Shaping was done by tableting, with addition of different loadings of graphite (0, 0.5, 1, 3, 5, 7, and 10 wt %). The use of graphite during the pressing of bismuth molybdate powders eases tableting because of the lubricating properties of the former. Furthermore, the compressive strength of BiMo-G tablets is higher than that of pure BiMo. Concerning the physicochemical properties of BiMo-G, XRD and XPS showed that graphite changes neither the relative distribution of the crystallographic phases of bismuth molybdate nor the oxidation state of bismuth and molybdenum in the tableted powders. Consequently, the shaped BiMo-G catalysts displayed similar, or slightly better, performances in the selective oxidation of propylene to acrolein. TGA analysis of samples of BiMo-G confirmed the thermal stability of the catalysts under oxidative conditions. Graphite was observed to experience crystallization into the hexagonal 2H phase during the catalytic tests. The results reported herein demonstrate that graphite is an effective shaping agent for bismuth molybdate powders.

The synergy effect between gamma and beta phase of bismuth molybdate catalysts: Is there any relation between conductivity and catalytic activity?

This work aims to clarify the postulate that the catalytic activity of bismuth molybdates (β-Bi 2Mo 2O 9 and γ-Bi 2MoO 6) and their mixtures is related to their conductivity. Mixtures of these pure bismuth molybdate phases have been recognized to possess high catalytic activities owing to the synergy effect. Pure bismuth molybdate phases and their mixtures were synthesized using sol–gel method. This method has been considered as one of the best methods to result in stoichiometric bismuth molybdate products. Conductivities of these samples were recorded at the catalytic reaction temperature (300–450 °C). The comparison of the measured conductivities allows us to conclude that the high number of active sites to abstract the α-hydrogen is more important factor than the influence of conductivity in the mechanism of the selective oxidation of propylene.

Molecular Molybdenum/Bismuth Compounds

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Watching the crystallisation of complex oxides by in situ X-ray techniques

Abstract We report here the results from recent studies where in situ X-ray techniques have been employed to follow the structural evolution of complex oxide materials from amorphous precursors. Specifically we detail how combined X-ray absorption spectroscopy/X-ray diffraction (XAS/XRD) allowed us to obtain new insight into the heat-induced crystallization of industrially important bismuth molybdate catalysts (Bi2O3·nMoO3 where n = 3, 2 or 1 corresponding to the α, β and γ-phases) and nano-sized spinel (CoAl2O4) particles from amorphous zeolites. In addition we illustrate the power of the energy dispersive diffraction technique (EDXRD) to follow the evolution of bismuth molybdate phases in a real autoclave via low temperature (

Phase Composition and Charge Transport in Bismuth Molybdates

The study of structural and electrical properties of three pure bismuth molybdate phases, α-Bi2Mo3O12, β-Bi2Mo2O9 and γ-Bi2MoO6, prepared by the spray drying technique, is described and discussed. The structure of polycrystalline, layered samples investigated by means of XRD and Raman spectroscopy was found to be monoclinic for the α- and β-phases and orthorhombic for the γ-phase. The microstructure of the as-prepared samples was not sufficiently developed under the given conditions of preparation, however, the thermal treatment can improve it. The high polarizability of Bi3+ cations with their lone-pair electrons influences the stability of the disordered oxygen sublattice. All as-prepared phases undergo a slight structural change in the temperature region of 280–430°C resulting in a decrease of the electrical conductivity probably due to an order ⇄ disorder transition in the oxygen arrangement during the sample heating. The change of electrical conductivity observed was found to be reversible in the high-temperature region and irreversible in the low-temperature one. The blocking of oxygen transport by the bismuth lone-pair electrons results in an increase of the activation energy and a decrease of the electrical conductivity in the high-temperature region. Relatively high relative dielectric permittivities ɛr, 36–52, were observed in dependence on the investigated phase.

In situ Study of the Formation of Crystalline Bismuth Molybdate Materials under Hydrothermal Conditions.

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In Situ Study of the Formation of Crystalline Bismuth Molybdate Materials under Hydrothermal Conditions

The three well-known and catalytically important phases of bismuth molybdate have been prepared, in a phase pure form, employing hydrothermal methods at temperatures below 200 °C. An in situ study of the formation of these phases using the time-resolved energy-dispersive X-ray diffraction (EDXRD) technique revealed that the α and γ crystalline phases are formed directly from the amorphous gel mixture without forming any intermediate phases. Kinetic data analysis using the Avrami-Erofe'ev nucleation-growth model suggests that the mechanisms for formation are different for each phase. This hydrothermal method of preparation yielded materials with a higher surface area as compared to the solids prepared by conventional techniques.

Reactions of cyclopentadienylmolybdenum compounds with bismuth alkoxides

The unique ability of bismuthmolybdate phases to act as heterogeneous catalysts for the oxidation of olefins stimulates research focussing on molecular Bi/Mo complexes, and in this context the behaviour of cyclopentadienyl molybdenum compounds in the presence of bismuth alkoxides has been investigated. While the reaction of [CpMoCl 4] with [Bi(OEt) 3] x did not yield a tractable product, its reaction with [Bi{OCH(CF 3) 2} 3(THF)] 2 ( 6), led to the isolation of [CpMoCl{OCH(CF 3) 2} 3] ( 7), formed in a halide/alkoxide exchange reaction. The crystal structure of 7 is discussed. A heterometallic Mo/Bi complex could be obtained via treatment of [Cp 2MoH 2] with 6. Elimination of HOCH(CF 3) 2 provided [Cp 2Mo(Bi{OCH(CF 3) 2} 2) 2] ( 8), containing two MoBi bonds as revealed by a single-crystal X-ray structure analysis. The oxidation of 8 with t BuOOH gave rise to the formation of the molybdocene alkoxide [Cp 2Mo{OCH(CF 3) 2} 2] in good yields, i.e. the MoBi bonds were cleaved in a formal four-electron oxidation process — possibly via the intermediate formation of MoOBi moieties. To test for any intermediate and its in situ reactivity, the same reaction was also performed in the presence of an excess of cis-stilbene. It turned out that 8 behaved as an efficient catalyst for the epoxidation of this olefin.

Following the Formation of γ-Phase Bi2MoO6 Catalyst by in Situ XRD/XAS and Thermogravimetric Techniques

We report a detailed in situ study of the preparation of the γ-phase Bi2MoO6 catalyst from a gel. In particular, the local coordination around molybdenum remained tetrahedral, although the poorly crystalline bismuth molybdate phase started to appear below 400°C; above this temperature there was a distinct change in the coordination around molybdenum to a distorted octahedron. Combining the information obtained from TGA, DTA, combined XRD/XAS and DRIFT techniques, we were able to track clearly the changes in the local coordination around the molybdenum as well as the loss of water and ammonia molecules during the course of the formation of the crystalline γ-Bi2MoO6 phase from a poorly crystalline material.

Activity Control by Structural Design of Multicomponent Scheelite-Type Molybdate Catalysts for the Selective Oxidation of Propene

Control of the catalytic selective oxidation of propene to acrolein was achieved by designing multicomponent metal oxide catalysts to have the multifunctions required for the reaction, such as activation of allylic hydrogen of propene, oxygen insertion, reduction–oxidation coupling, lattice oxygen mobility, and activation of molecular oxygen. Scheelite-type Na0.5−3xLa0.5+xMoO4 oxides having lattice oxide ion mobility were chosen as a basic catalytic material and used as a support for bismuth molybdate and cerium oxide loadings. The catalytic activity was primarily controlled by the introduction of lanthanum into the Na0.5−3xLa0.5+xMoO4 lattice, and the selectivity to acrolein was attained separately by loading bismuth molybdates on the surface of the support. It was found that the catalytic activity of Bi2Mo3O12/Na0.5−3xLa0.5+xMoO4 is strongly governed by the value of x irrespective of the loading of Bi2Mo3O12. It was also demonstrated that high activities exceeding the primary activity were achieved by designedly introducing cerium both into the surface bismuth molybdate phase and in the vicinity of the surface of the scheelite support.

Bismuth Carboxylates as Precursors for the Incorporation of Bismuth in Oxide-based Materials

The use of bismuth(III) carboxylates (acetate, oxoacetate, lactate, oxalate) as precursors for the incorporation of this element on oxide supports is evaluated as a new tool to generate bismuth-based oxide phases at the surface of MoO3or WO3supports. These insoluble precursors are deposited as small particles from a slurry in a liquid hydrocarbon under appropriate experimental conditions. Bismuth molybdate and tungstate phases are produced by solid-state reactions betweenin situgenerated Bi2O3and the supporting oxide at 673 K. The samples are characterized by specific surface area measurements, X-ray diffractometry, and X-ray photoelectron spectroscopy. The selective oxidation of isobutene to methacrolein is employed as a reaction test to demonstrate the applicability of this approach. The use of precursors containing lactate or acetate-type ligands is found to generate materials displaying enhanced specific surface area and high bismuth-to-molybdenum surface atomic ratios. This bismuth enrichment of the surface is indicative of the presence of dispersed crystallites of either Bi2O3itself or ternary Bi–Mo–O or Bi–W–O phases, which are known to promote partial oxidation of alkenes. The performances of these materials in the isobutene to methacrolein conversion are indeed in most cases definitely better than those of the so-called reference materials obtained from the simple mixtures of the same oxides prepared separately.

Time-of-flight SIMS study of BiMo selective oxidation catalysts

The potential applicability of time-of-flight secondary ion mass spectrometry (TOF SIMS) to control the various stages of preparation of silica-supported BiMo selective oxidation catalysts has been studied. These catalysts were prepared from coordination compounds (bismuth(III) and molybdenum(II) acetates or benzoates) as precursors. It has been shown that: (1) through the observation of the molecular ions and most characteristic fragments of the coordination compounds, TOF SIMS can provide direct information about the preservation of these precursors on the silica surface before calcination. (2) as the three pure bismuth molybdate phases (α-Bi 2Mo 3O 12, β-Bi 2Mo 2O 9 and γ-Bi 2MoO 6) show the same fragmentation patterns in TOF SIMS, they can only be distinguished in a indirect way, namely by comparing some SIMS intensity ratios from their positive spectra. These SIMS intensity ratios have been successfully used to determine the nature of bismuth molybdate phases formed on silica surface. And (3) TOF SIMS imaging provides valuable information about the dispersion of bismuth molybdate phase on silica surface.

Alkene oxidation over the Mo–Bi mixed oxides/Au | yttria-stabilized zirconia | Ag system

Oxidation of propene and but-1-ene has been carried out in an MoO3–Bi2O3/Au | yttria-stabilized zirconia (YSZ)| Ag electrochemical reactor at 723 K and compared with the result obtained by means of a mixed-gas flow reaction over the MoO3–Bi2O3 catalyst. The MoO3–Bi2O3 catalyst was prepared from MoO3, three phases of bismuth molybdate [Bi2Mo3O12(α), Bi2Mo2O9(β), Bi2MoO6(γ)] and Bi2O3 and each catalyst was successfully deposited by vacuum evaporation as a compact thin film on the Au anode. For propene oxidation, MoO3 showed the highest activity among the catalysts used on the Au anode and its activity was up to 600 times higher than that obtained with the same catalyst using a mixed-gas flow reaction. These results suggest that oxygen species bound to molybdenum metal possess a definite role for both α-H abstraction from propene and oxygen insertion into the allylic intermediate under a sufficient oxygen supply. On the other hand, oxygen can permeate through a Bi2O3 film, suggesting that the bismuth species is effective for O2 incorporation into the catalyst and also for transportation of oxide ions through the catalyst bulk. Each phenomenon, i.e. the insertion of lattice oxide ions into the allylic intermediate and O2 incorporation into the lattice was separately observed. The electrochemical-reactor system showed some advantages for propene oxidation, e.g. high selectivity to arylaldehyde under high reaction temperatures and higher activity, compared with the mixed-gas flow reaction. In but-1-ene oxidation the main product was buta-1,3-diene by oxidative dehydrogenation and Bi2Mo2O9(β) showed the highest activity. The critical effect of molybdenum species was not observed in this reaction but high activity was still obtained over the molybdenum-rich catalysts.