High dispersion of heteropolyacid nanoparticles on hydrothermally Cs-modified three-dimensionally ordered macroporous SiO2 with excellent selectivity in methacrolein oxidation

Heteropolyacid nanoparticles (NPs) were supported on Cs-modified three-dimensionally ordered macroporous (3DOM) SiO₂ and used as the catalyst for the oxidation of methacrolein (MAL) to methacrylic acid (MAA). Hydrothermal treatment and incipient wetness impregnation were employed respectively for the Cs-modification. It was found that hydrothermal Cs-modification of 3DOM SiO₂ promoted the dispersion of the supported heteropolyacid, which showed an average particle size of 5.2 nm, much smaller than that (17.6 nm) on the Csmodified 3DOM SiO₂ prepared by incipient wetness impregnation. The effects of hydrothermal treatment on the structure and catalytic performance of the catalyst were investigated. Results showed that the ion exchange between Cs⁺ and the surface silanol groups on 3DOM SiO₂ was promoted with the increase of the hydrothermal temperature. Meanwhile, Cs-modification helped the heteropolyacid to retain intact Keggin structure, inhibiting the formation of MoO₃. Highly dispersed heteropolyacid NPs with enhanced structural stability exhibited excellent selectivity to MAA in the oxidation of MAL.     

Vanadium-substituted Keggin heteropolycompounds as catalysts for ecofriendly liquid phase oxidation of 2,6-dimethylphenol to 2,6-dimethyl-1,4-benzoquinone

Molybdovanadophosphate heteropolycompounds, H₄PMo₁₁VO₄₀ and H₉PMo₆V₆O₄₀ and the corresponding Cs salts, were synthesized. The influence of thermal treatment on their Keggin primary structures and their acidic properties were studied. The presence of Keggin units and the V incorporation into these units were observed by TGA, DTA, DRS and FT-IR. H₄PMo₁₁VO₄₀ possesses an acid strength higher than that of the V-free sample and H₉PMo₆V₆O₄₀, at room temperature. Heating above 200 °C leads to considerable structural changes and acidity decrease. The Cs salts from H₄PMo₁₁VO₄₀ possesses stronger acidic sites than those from H₉PMo₆V₆O₄₀. Moreover when the Cs number increases, the acidic properties decrease. The salts with one, two, and three Cs are more unstable with temperature than the salt with 2.5 Cs. The specific surface area for the Cs salts depends on the temperature and Cs number. The prepared heteropolycompounds were used as catalysts for the liquid phase reaction of synthesis of 2,6-dimethyl-p-benzoquinone from 2,6-dimethylphenol, using hydrogen peroxide, with high yields. A selectivity to the p-benzoquinone of 79–59% was obtained when the reaction was catalyzed by Cs salts from the H₄PMo₁₁VO₄₀ acid.     

Characterization and reactivity of MoO3/SiO2 catalysts in the selective catalytic oxidation of ammonia to N2

Silica-supported MoO₃ catalysts with MoO₃ loadings up to 21% w/w were prepared, characterized and tested in the selective catalytic oxidation (SCO) of ammonia to N₂ under dilute conditions. It is found that the catalysts are active and selective in the reaction, and that the catalytic performance increases on increasing the Mo loading. Crystalline MoO₃, detected over the silica support, is supposed to be the active species in the reaction. The reactivity of the catalysts is depressed by water addition to the feed at low temperatures and is enhanced by the presence of selected promoters, like Bi and Pb.     

甲醇氧化制甲醛铁钼催化剂表面结构与活性

利用共沉淀法在不同搅拌速度下制备了相同Mo/Fe原子比的甲醇氧化制甲醛催化剂,采用SEM、XRD和拉曼光谱等对催化剂进行表征,在固定床微反上评价催化剂活性和选择性。结果表明,搅拌速度增大,催化剂比表面积增大,催化活性增强,甲醛收率由600 r·min^-1时的73.8%增加到10000 r·min^-1时的95.7%（280℃）。此外,催化剂由片状的MoO₃和颗粒状的Fe₂（MoO₄）₃两部分组成,游离的片状MoO₃无明显催化活性,只有与Fe₂（MoO₄）₃结合时才具有催化活性。     

Direct conversion of methane into methanol over MoO3/SiO2 catalyst in an excess amount of water vapor

Well-dispersed MoO₃ on SiO₂ showed a high activity for partial oxidation of methane (mixed with oxygen in a molar ratio of 9:1) into methanol and formaldehyde at 873 K in an excess amount of water vapor, which is attributed to the formation of silicomolybdic acid (SMA) on the catalyst surface during reaction. One of the roles of SMA for the partial oxidation of methane is proved to depress the successive oxidation of methanol and formaldehyde into carbon oxides.     

Photocatalytic degradation of 4-nitrophenol in aqueous suspension by using polycrystalline TiO2 samples impregnated with Cu(II)-phthalocyanine

A number of supported metal oxide catalysts were screened for their catalytic performance for the oxidation of carbon black (CB; a model diesel soot) using NO₂ as the main oxidant. It was found that contact between the carbon and catalyst was a key factor in determining the rate of oxidation by NO₂. Oxides with low melting points, such as Re₂O₇, MoO₃ and V₂O₅ showed higher activities than did Fe₃O₄ and Co₃O₄. The activities of MoO₃ and V₂O₅ on various supporting materials were also examined. MoO₃/SiO₂ was the most active catalyst among the supported MoO₃ examined, whereas, V₂O₅/MCM-41 showed the highest activity among the supported V₂O₅. Different performances of the supported MoO₃ catalysts were explained by the interaction of MoO₃ with the supports: a strong MoO₃/support interaction may result in a poor mobility of MoO₃ and a poor activity for oxidation of carbon by NO₂. The high activity of V₂O₅/MCM-41 was associated with its catalysis of the oxidation of SO₂ by NO₂ to form SO₃, which substantially promotes oxidation of carbon by NO₂. Addition of transition metal oxides or sulfates to supported MoO₃ and V₂O₅ was also investigated. Combining MoO₃ or V₂O₅ with CuO on SiO₂, adding VOSO₄ to MoO₃/SiO₂ or MoO₃/Al₂O₃ and adding TiOSO₄ or CuSO₄ to V₂O₅/Al₂O₃ improved the catalytic performance.     

New insights in the understanding of the behaviour and performances of bismuth molybdate catalysts in the oxygen-assisted dehydration of 2-butanol

Pure Bi₂Mo₃O₁₂, Bi₂Mo₂O₉, Bi₂MoO₆, MoO₃ and -Sb₂O₄ and their mechanical mixtures were investigated in the oxygen-assisted dehydration of 2-butanol at atmospheric pressure and at low temperature (220 and 250°C). All catalysts were characterized before and after the catalytic reaction by BET surface area measurement, Raman spectroscopy, XRD and XPS. A strong parallelism is confirmed with the results obtained in the selective oxidation of olefins. In the frame of the remote control concept, , β and γ-bismuth molybdates are able to play a dual role: donor of spillover oxygen (Oso) with respect to MoO₃, and acceptor of Oso with respect to -Sb₂O₄. On one hand, this duality leads to mutual increase of activity when the bismuth molybdates are mixed together. In the presence of MoO₃, the phase seems to be a stronger Oso donor than β and γ, and β has a donor strength between and γ. On the other hand, when the Bi molybdates are reacted in the presence of a big quantity of spillover oxygen, like in a mixture with -Sb₂O₄, they undergo a dramatic decrease of activity. The phenomenon originates from the full oxidation of the reduced Mo species to Mo⁶⁺ induced by Oso. In parallel with other reactions involving oxygen, this confirms that the real active and selective state of molybdenum-containing oxides is that slightly reduced possessing Mo⁵⁺.     

A comparative study of the partial oxidation of methane to formaldehyde on bulk and silica supported MoO3 and V2O5 catalysts

The mechanism of the partial oxidation of methane to formaldehyde with O₂ has been investigated on bulk and differently loaded silica supported (4–7 wt%) MoO₃ and (5–50 wt%) V₂O₅ catalysts at 600–650°C in a pulse reactor connected to a quadrupole mass spectrometer. The reaction rate and product distribution in the presence and in the absence of gas-phase O₂ have been evaluated. On bare SiO₂, low and medium loaded silica supported MoO₃ and V₂O₅ catalysts the reaction proceeds via a concerted mechanism involving the activation of gas-phase oxygen on the reduced sites of the catalyst surface as proved by the direct correlation between catalytic activity and density of reduced sites evaluated in steady-state conditions, while on highly loaded catalysts as well as on bulk MoO₃ and V₂O₅ the reaction rate drops dramatically and the reaction pathway via redox mechanism becomes predominant. The results indicate that the surface mechanism is essentially more effective than the redox mechanism enabling also a higher selectivity to HCHO.     

Propene oxidation over MoO3 film deposited on an Au|YSZ|Ag system

Propene oxidation was carried out with an electrochemical reactor, MoO₃/Au|YSZ|Ag (YSZ: 8 mol.-% yttria-doped ZrO₂), at 475°C under oxygen pumping, and the catalytic activity of the thin MoO₃ film related to its crystal morphologies was discussed. The thin MoO₃ film was deposited on an Au anode by means of vacuum deposition or sputtering method at room temperature or 300°C. Each method showed a characteristic texture as well as crystal morphology of the MoO₃ film, resulting in variations in the catalytic activity. Sputtering at 300°C gave porous films composed of leaf-like crystals with preferential orientation of (010) plane parallel to the pore channel and perpendicular to the Au surface, resulting in the highest activity for acrylaldehyde production. A relatively high step density was observed on the oriented (010) plane of the leaf-like crystal. The high activity of this MoO₃ film is probably due to the high density of active sites for the partial oxidation of propene and also to their highly porous structure, which is favorable to the surface migration of oxygen to the reduced active sites.     

Impact of surface mobility in selective oxidation. Isotopic exchange of O2 with O2 on various oxides: MoO3, SnO2 and Sb2O4. Effect of a reducer gas

Isotopic oxygen exchange experiments were carried out on simple oxides (Sb₂O₄, MoO₃ and SnO₂) and mechanical mixtures of MoO₃ or SnO₂(acceptor phase) with Rh//Al₂O₃ or Sb₂O₄ (donor phase). With Rh//Al₂O₃ a synergy effect is found: the amount of oxygen exchanged on the mixture is higher than the sum of oxygen exchanged on the two separate phases. This is a prove of oxygen mobility between donor and acceptor phases. With Sb₂O₄ this effect is less marked suggesting that the presence of a reducer gas is required. In presence of hydrocarbon, no isotopic exchange is observed: only total oxidation occurs. The presence of Sb₂O₄ decreases the rate of total oxidation by blocking, by oxygen spillover, the most active sites for oxidation.     

Synergistic catalysis of carbon black oxidation by Pt with MoO3 or V2O5

A series of SiO₂-supported MoO₃, V₂O₅, and Pt catalysts were prepared for the study of model soot oxidation with simulated diesel exhaust gas. Composite samples of Pt with the metal oxides demonstrated higher oxidation activities than the single-component SiO₂-supported MoO₃, V₂O₅ or Pt catalysts in the absence of SO₂ in the reactant gas. Based on the effects of NO₂ on carbon oxidation, a synergistic reaction mechanism was suggested to explain the effects of combining Pt with the oxides: Pt catalyzes the oxidation of NO with gas phase O₂ to NO₂, while MoO₃ and V₂O₅ catalyze the oxidation of carbon with NO₂. Finally, the effects of SO₂ on the carbon oxidation reaction were examined and discussed.     

Mechanistic evidence of the partial oxidation of methane to formaldehdye over silica based oxide catalysts by temperature programmed reaction studies

The catalytic behaviour of SiO₂ supported MoO₂ and V₂O₅ catalysts in the partial oxidation of methane to formaldehyde with O₂ (MPO) in the range 400–800°C has been investigated by temperature programmed reaction (TPR) tests. Both the sequence of the onset temperature of product formation and the product distribution patterns signal that MPO on silica based oxide catalysts occurs mainly via a consecutive reaction path: CH₄ → HCHO → CO → CO₂. At T >/ 700°C a parallel surface assisted gas-phase reaction pathway leads to the formation of minor amounts of C₂ products both on SiO₂ and MoO₃/SiO₂ catalysts. The redox properties of MoO₃/SiO₂ and V₂O₅SiO₂ catalysts have been systematically evaluated by H₂ and CH₄ temperature programmed reduction (H₂-TPR, CH₄-TPR) measurements. H₂-TPR results do not account for the reactivity scale of oxide catalysts in the MPO. CH₄-TPR measurements indicate that the enhancement in the specific activity of the silica is controlled by the capability of MoO₃ and V₂O₅ promoters in providing ‘active’ lattice oxygen species.     

High temperature calcined K–MoO3/γ-Al2O3 catalysts for mixed alcohols synthesis from syngas: Effects of Mo loadings

One series of oxidized K–MoO₃/γ-Al₂O₃ samples with different Mo loadings (MoO₃/Al₂O₃ (wt ratio)=0.05–0.45) was prepared by impregnating K and Mo compounds and successive calcination in air at 800°C. The oxidized samples were sulfided and then utilized for mixed alcohols synthesis from syngas. The structural information from laser Raman spectroscopy (LRS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), ammonia saturation, temperature programmed desorption (TPD) and ethanol decomposition were studied to elucidate the reaction properties.

The results indicated that with Mo loading increased from MoO₃/Al₂O₃=0.05 to 0.25, the total yields of mixed alcohols and hydrocarbons decreased, but the selectivity to mixed alcohols was enhanced sharply from 3% to 50%. With Mo loading increased from MoO₃/Al₂O₃=0.25 to 0.45, the CO conversion was enhanced, but the selectivity to mixed alcohols leveled off. On these catalysts, Fischer–Tropsch (FT) synthesis to linear alcohols and the condensation reaction of low alcohols to form branched i-C₄OH occurred at the same time. With increased Mo loading, activity of the alcohols condensation became high.

Structural studies demonstrated that on oxidized samples with increased Mo loading the same K–Mo–O species was formed, but the dispersion of these K–Mo species decreased. The catalyst's acidity decreased remarkably with Mo loading up to MoO₃/Al₂O₃=0.25, and stayed unchanged as Mo loading was further increased to MoO₃/Al₂O₃=0.45. With increased Mo loading, the activity for ethanol dehydration changed parallel to the acidity. Results of the activity experiments for mixed alcohols' synthesis and the structural measurements indicated that the dispersion state of Mo species and the content of unreduced Mo species influenced the total CO conversion, and that the acidity of the catalyst controlled the selectivity to mixed alcohols.     

Structural evolution of H4PVMo11O40⋅xH2O during calcination and isobutane oxidation: New insights into vanadium sites by a comprehensive in situ approach

H₄PVMo₁₁O₄₀⋅8H₂O was studied during thermal activation as well as during isobutane oxidation by simultaneous in situ-EPR/UV–vis/Raman spectroscopy, in situ-FTIR spectroscopy, and quasi-in situ-¹H and -⁵¹V-MAS-NMR. In as-synthesized form, most V sites are pentavalent, octahedrally coordinated, and located within the intact Keggin anions. Stepwise dehydration in N₂ up to 350 °C leads to partial reduction and disintegration of the V sites from the Keggin units, followed by their condensation on the outer surface of the latter in square-pyramidal form. In water-free H₄PVMo₁₁O₄₀, only V⁵⁺ (not V⁴⁺) is stable inside the Keggin unit. Thus, disintegration of V from the latter is favored by its reduction to the tetravalent state and thus depends on the redox properties of the atmosphere. Active sites in isobutane oxidation are most likely composed of single O₄V^4+/5+O species connected to Mo⁶⁺ via oxygen bridges. Partial deactivation occurs by formation of carbon-containing deposits.     

In situ Raman spectroscopy. A powerful tool for studies in selective catalytic oxidation

In this review, some of advantages and problems of in situ Raman spectroscopy are briefly mentioned. The applications of laser Raman spectroscopy LRS to the study of catalysts used for selective oxidation is then reviewed based on published work on catalysis by molybdate and VPO catalysts. Investigations of synergy effects have also been carried and results on ¹⁸O-exchange and reoxidation of SbO_x/MoO₃ physical mixtures are reported. Spillover oxygen — if present — could not be detected.

Peroxide species have been detected on working methane coupling catalysts at temperatures up to 1070 K and they have been identified as centers for the activation of methane.

Model studies were performed of the methanol oxidation to formaldehyde on Ag single crystal surface at temperatures up to 1170 K. Surface enhancement permitted the detection of a strongly chemisorbed surface oxygen species O_γ, which was in equilibrium with a bulk species O_β and which was identified as the reactive species involved in the methanol oxidation.     

Quantitative determination of the number of surface active sites and the turnover frequency for methanol oxidation over bulk metal vanadates

The present work investigates the number and nature of the surface active sites, selectivity and turnover frequency towards methanol selective oxidation of a series of bulk metal vanadates. The catalysts were synthesized through an organic route and characterized by laser Raman spectroscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and specific surface area analysis (BET). The number of surface active sites (N_s) was determined by measuring the concentration of surface methoxy species adsorbed on the catalysts exposed to an atmosphere of 2000 ppm of methanol in helium at 100 °C. The specific activity values (TOFs) were calculated by normalizing the methanol oxidation reaction rate by the number of surface active sites probed by methanol chemisorption. The comparison of the methanol oxidation products distribution from bulk metal vanadates, pure V₂O₅ and corresponding metal oxides (NiO, MnO, etc.) strongly suggests that the metal vanadate catalysts consist of only surface vanadium oxide sites. The comparison of the TOF values demonstrated that bulk metal vanadates possess similar activity to monolayer vanadium oxide supported catalysts and are more active than bulk metal molybdates for methanol selective oxidation. Moreover, bulk metal vanadates are as active and selective as the commercial MoO₃/Fe₂(MoO₄)₃ catalysts at high methanol conversion.     

Structural and optical properties of MoO3 and V2O5 thin films prepared by Spray Pyrolysis

MoO₃ and V₂O₅ thin films were prepared on glass substrates by Spray Pyrolysis technique at a substrate temperature of 423 K. The precursor solutions were obtained by varying the concentrations of MoCl₅ and VCl₃ in bi-distilled water. The structural investigation conducted by X-ray diffraction showed that MoO₃ and V₂O₅ thin films were polycrystalline with orthorhombic structure. The optical properties studied in the 300–2500 nm range suggest that the thin film behaviours are related to bound electronic states. The optical band gaps have been estimated from slopes of ln(hν) versus hν plots of MoO₃ and V₂O₅ films were 3.35 and 2.44 eV, respectively. The electrical conductivity was measured using four probes method.     

Support effects on structure and activity of molybdenum oxide catalysts for the oxidative dehydrogenation of ethane

The structural and catalytic properties of MoO₃ catalysts supported on ZrO₂, Al₂O₃, TiO₂ and SiO₂ with Mo surface densities, n_s, in the range of 0.5–18.5 Mo/nm² were studied for the oxidative dehydrogenation (ODH) of ethane by in situ Raman spectroscopy and catalytic activity measurements at temperatures of 400–540 °C. The molecular structure of the dispersed surface species evolves from isolated monomolybdates (MoO₄ and MoO₅, depending on the support) at low loadings to associated MoO_x units in polymolybdate chains at high loadings and ultimately to bulk crystalline phases for loadings exceeding the monolayer coverage of the supports used. The nature of the oxide support material and of the Mo–O–support bond has a significant influence on the catalytic behaviour of the molybdena catalysts with monolayer coverage. The dependence of reactivity on the support follows the order ZrO₂ > Al₂O₃ > TiO₂ > SiO₂. The oxygen site involved in the anchoring Mo–O–support is of relevance for the catalytic activity.     

Effect of adsorbed water vapor on Mg intercalation in electrochromic a-MoO3 films

The nature of interaction between the water vapor and MoO₃ thin films has been investigated using infrared spectroscopy technique. On prolonged exposure to high humidity conditions, presence of free water vapor and formation of Mo–OH bonds was detected in MoO₃ films. Films exposed to high humidity for various durations and freshly deposited were intercalated with Mg ions under identical experimental conditions. Free water vapor and Mo–OH bonds did not prevent/hinder the intercalation process. The diffusivity (D) of Mg ions in MoO₃ films has been estimated using Galvanostatic Intermittent Titration Technique (GITT) as a function Mg concentration. It is observed that D values are lesser in case of films exposed to humidity. Cyclic voltammetry studies carried out on films revealed reduced cyclability in exposed films compared to that of fresh films.