Mo–V–Nb mixed oxides as catalysts in the selective oxidation of ethane

Mo–V–Nb–O mixed metal oxides, obtained by heat-treatment in N₂ at 425 °C, have been studied as catalysts in the oxidative dehydrogenation of ethane. They present higher catalytic activity, while maintaining the same selectivity to ethylene, than the corresponding metal oxides calcined under air. Both amorphous and crystalline phases are present on active and selective catalysts. The implications of the presence of these phases as well as their physicochemical characteristics on the nature of active and selective sites are discussed.     

The role of molybdenum in Mo-doped V–Mg–O catalysts during the oxidative dehydrogenation of n-butane

A detailed study on the influence of the addition of molybdenum ions on the catalytic behaviour of a selective vanadium–magnesium mixed oxide catalyst in the oxidation of n-butane has been performed. The catalysts have been prepared by impregnation of a calcined V–Mg–O mixed oxides (23.8 wt% of V₂O₅) with an aqueous solution of ammonium heptamolybdate, and then calcined, and further characterised by several physico-chemical techniques, i.e. S_BET, XRD, FTIR, FT-Raman, XPS, H₂-TPR. MgMoO₄, in addition to Mg₃V₂O₈ and MgO, have been detected in all the Mo-doped samples. The incorporation of molybdenum modifies not only the number of V⁵⁺-species on the catalyst surface and the reducibility of selective sites but also the catalytic performance of V–Mg–O catalysts. The incorporation of MoO₃ favours a selectivity and a yield to oxydehydrogenation products (especially butadiene) higher than undoped sample. In this way, the best catalyst was obtained with a Mo-loading of 17.3 wt% of MoO₃ and a bulk Mo/V atomic ratio of 0.6. From the comparison between the catalytic properties and the catalyst characterisation of undoped and Mo-doped V–Mg–O catalysts, the nature of selective sites in the oxidative dehydrogenation of n-butane is also discussed.     

A novel catalyst fabricated from Al–Cu–Fe quasicrystal for steam reforming of methanol

The optimum preparation condition of Al–Cu–Fe quasicrystalline (QC) catalyst with excellent catalytic performance for steam reforming of methanol (SRM) has been investigated. The QC alloy is superior to the other crystalline Al–Cu–(Fe) alloys (i.e., beta and theta phase) as a catalyst material because of the brittle nature of QC. The wet milling process (in ethanol) for the QC powders is much better than the dry milling process to obtain fine particles with high surface area. The QC powder prepared by the wet process followed by leaching in Na₂CO₃ aq. at 323 K exhibited the highest catalytic performance (activity and stability) in the present study. From these findings, it is clear that the QC catalyst with the excellent catalytic performance could be obtained by controlling the initial grain size of the QC powder and the leaching temperature.     

Alumina-supported V–Mo–O mixed oxide catalysts, the formation of phases involving aluminum: AlVMoO7

The structure and reactive properties of alumina-supported molybdena, vanadia and molybdena–vanadia above monolayer coverage are studied by XPS, XRD, Raman spectroscopy and methanol temperature-programmed surface reaction (TPSR). Alumina-supported series are prepared by impregnation. Reference bulk Mo–V–Al oxide systems are prepared. The bulk Mo–V–Al oxide system provides structural references to characterize the alumina-supported series. The Raman bands of AlVMoO₇ are reported here for the first time, to the best of our knowledge. It is shown that the chemistry of the bulk Al–V–Mo system is also present in the alumina-supported Mo–V oxide catalysts. Methanol TPSR data show that the systems possess essentially redox activity.     

Effect of preparation and reaction condition on the catalytic performance of Mo–V–Te–Nb catalysts for selective oxidation of propane to acrylic acid by high-throughput methodology

Combinatorial screening technique has been applied to investigate the catalytic activity and selectivity of quaternary Mo–V–Te–Nb mixed oxide catalysts treated with various chemicals during preparation for selective oxidation of propane to acrylic acid. The catalyst libraries were prepared by the slurry method and catalytic activities were examined in 32-channel high-throughput screening reactor system coupled with a mass spectrometer and/or gas chromatograph.The obtained results provided substantial evidence that the sample preparation condition would have strong effect on the catalytic performance for propane selective oxidation. Among screened samples, Mo–V–Te–Nb treated with HIO₃ solution presented a better performance. The reaction results of promising catalysts selected from the libraries were applied to further scale-up of the system and confirmed catalytic performance. Quantification of the result of Mo–V–Te–Nb treated with HIO₃ solution was realized by combination of GC and MS and relationship between the MS data and the GC results can be established.     

Competition between NO reduction and NO decomposition over reduced V–W–O catalysts

The SCR of NO and NO decomposition were investigated over a V–W–O/Ti(Sn)O₂ catalyst on a Cr–Al steel monolith. The conversions of NO and NH₃ over the reduced and oxidised catalysts were determined. The higher conversion of NO than of NH₃ was observed in SCR over the reduced catalyst and very close conversions of both substrates were found over the oxidised one. The increase of the pre-reduction temperature was found to cause an increase in catalyst activity and its stability in direct NO decomposition. The surface tungsten cations substituted for vanadium ones in vanadia-like active species are considered to be responsible for the direct NO decomposition. The results of DFT calculations for the 10-pyramidal clusters: V₁₀O₃₁H₁₂ (V–V) and V₉WO₃₁H₁₂ (V–W) modelling (0 0 1) surfaces of vanadia and WO₃–V₂O₅ solid solution (s.s.) active species, respectively, show that preferable conditions for NO adsorption exist on W sites of s.s. species and that reduction causes an increase in their ability for electron back donation to the adsorbed molecule. Electron back donation is believed to be responsible for the electron structure reorganisation in the adsorbed NO molecule resulting in its decomposition. The high selectivity of NO decomposition to dinitrogen was considered to be connected with the formation of the tungsten nitrosyl complexes solely via the W–N bond.     

Copper-impregnated Al–Ce-pillared clay for selective catalytic reduction of NO by C3H6

The selective catalytic reduction (SCR) of NO by hydrocarbon is an efficient way to remove NO emission from lean-burn gasoline and diesel exhaust. In this paper, a thermally and hydrothermally stable Al–Ce-pillared clay (Al–Ce-PILC) was synthesized and then modified by SO₄²⁻, whose surface area and average pore diameter calcined at 773 K were 161 m²/g and 12.15 nm, respectively. Copper-impregnated Al–Ce-pillared clay catalyst (Cu/SO₄²⁻/Al–Ce-PILC) was applied for the SCR of NO by C₃H₆ in the presence of oxygen. The catalyst 2 wt% Cu/SO₄²⁻/Al–Ce-PILC showed good performance over a broad range of temperature, its maximum conversion of NO was 56% at 623 K and remained as high as 22% at 973 K. Furthermore, the presence of 10% water slightly decreased its activity, and this effect was reversible following the removal of water from the feed. Py-IR results showed SO₄²⁻ modification greatly enhanced the number and strength of Brönsted acidity on the surface of Cu/SO₄²⁻/Al–Ce-PILC, which played a vital role in the improvement of NO conversion. TPR and XPS results indicated that both Cu⁺ and isolated Cu²⁺ species existed on the optimal catalyst, mainly Cu⁺, as Cu content increased to 5 wt%, another species CuO aggregates which facilitated the combustion of C₃H₆ were formed.     

Selective oxidation of methanol to formaldehyde over V–Mg–O catalysts

The results of a complex investigation of V–Mg–O catalysts for oxidative dehydrogenation (ODH) of methanol are presented. The efficiency of vanadium–magnesium oxide catalysts in production of formaldehyde has been evaluated. Strong dependence of the formaldehyde yield and selectivity upon vanadium oxide loading and the conditions of heat treatment of the catalyst were observed. The parameters of the preparation mode for the efficient catalyst were identified. In optimised reaction conditions the V–Mg–O catalysts at the temperature approximate 450 °C ensured the formation of formaldehyde with the yield of 94% at the selectivity of 97%.

No visible changes in the performance of the catalyst (methanol conversion, formaldehyde yield and selectivity) were detected during the 60 h of operation in prolonged runs. Characterization of the catalyst by XRD, IR, and UV methods suggests the formation of species of the pyrovanadate type (Mg₂V₂O₇) with irregular structure on the surface of a V–Mg–O catalyst. These species make the catalyst efficient for methanol ODH.     

Selective oxidation and ammoxidation of propane on a Mo–V–Te–Nb–O mixed metal oxide catalyst: a comparative study

A comparative study on the selective oxidation and the ammoxidation of propane on a Mo–V–Te–Nb–O mixed oxide catalyst is presented. The catalyst has been prepared hydrothermally at 175 °C and heat-treated in N₂ at 600 °C for 2 h. Catalyst characterization results suggest the presence mainly of the orthorhombic Te₂M₂₀O₅₇ (M = Mo, V and Nb) bronze in samples before and after use in oxidation and ammoxidation, although some little modifications have been observed after its use in ammoxidation reaction. Propane has been selectively oxidized to acrylic acid (AA) in the 340–380 °C temperature range while the ammoxidation of propane to acrylonitrile (ACN) has been carried out in the 360–420 °C temperature interval. The steam/propane and the ammonia/propane molar ratios have an important influence on the activity and the selectivity to acrylic acid and acrylonitrile, respectively. The reaction network in both oxidation and ammoxidation reactions as well as the nature of active and selective sites is also discussed. The catalytic results presented here show that the formation of both ACN and AA goes through the intermediate formation of propene.     

High surface area Mo–V–Te–Nb–O catalysts: Preparation, characterization and catalytic behaviour in ammoxidation of propane

Mo–V–Te–Nb mixed oxides with a molar ratio of 1:0.30:0.20:0.15 were prepared by citrate and dry-up method, both associated with hydrothermal treatments in the presence of a cationic surfactant (cetyl trimethylammonium bromide, CTAB), and tested in the ammoxidation of propane. The catalysts were characterized by adsorption–desorption isotherms of nitrogen at 77 K, particle size measurements, XRD, and XPS. By using the surfactant, the surface area increased significantly, and samples with surface area between 110 and 239 m²/g were obtained. These catalysts exhibited a propane conversion near 48% with selectivity to acrylonitrile of about 32% for a space velocity 30 times higher than generally reported.     

Selective oxidation of hydrogen sulfide over mixture catalysts of V–Sb–O and Bi2O3

The selective oxidation of hydrogen sulfide containing excess water and ammonia was studied over vanadium–antimony mixed oxide catalysts. The investigation was focused on the phase cooperation between V–Sb–O and Bi₂O₃ in this reaction. Strong synergistic phenomenon in catalytic activity was observed for the mechanically mixed catalysts of V–Sb–O and Bi₂O₃. Temperature-programmed reduction (TPR) and oxidation (TPO), two separated bed reaction tests, and XPS analyses were carried out to explain this synergistic effect by the reoxidation ability of Bi₂O₃.     

Cathodic breakdown of anodic oxide film on Al and Al–Sn alloys in NaCl solution

The effect of cathodic polarisation on stability of defined oxide films on Al and Al–Sn alloys (with up to 0.40% Sn) has been investigated in a 0.5 M NaCl solution using the potentiostatic pulse method. The dependence of the cathodic current on time (in the period of 1, 10 and 100 s) was recorded on Al and Al–Sn alloys when subjected to a potential pulse from E_OCP to different negative values (up to −2.0 V). Anodic current responses to the return to the E_OCP were also recorded at three different time scales (1, 10 and 100 s). It has been established that the cathodic polarisation of passivated Al and Al–Sn alloys in a chloride solution is characterized by two regions of potentials with distinctly different phenomena: the range of low and high cathodic potentials (LCP and HCP). In the LCP range, the oxide film retains its properties, while in the HCP range cathodic breakdown and hydration of the oxide take place. The boundary between these two potential ranges shifts towards more negative potential values when the percentage of Sn in the alloy increases. The longer the duration of the cathodic pulse, the more positive the potentials at which the oxide film breakdown takes place. This shift is more marked with alloys containing higher percentage of Sn. Cathodic polarisation (duration of 100 s) activates alloys with 0.20% and 0.40% Sn for anodic dissolution.     

A study of the abatement of VOC over V2O5–WO3–TiO2 and alternative SCR catalysts

The conversion of C3 organic compounds (propane, propene, 1- and 2-propanol, allyl alcohol, propanal, acrolein, acetone and 1- and 2-chloropropane) in the presence of excess oxygen has been investigated over two V–W–TiO₂ commercial SCR catalysts differing in the V content and over Mn–TiO₂ alternative SCR catalysts. V–W–Ti catalysts show poor activity in the oxidation of hydrocarbons and oxygenates and give significant amounts of partial oxidation products. Moreover they give rise to CO in excess of CO₂. The sample higher in V is more active. Mn–TiO₂ is definitely more active in oxidation of hydrocarbons and oxygenates, and produces, at total conversion, CO₂ as the only detectable product.

V–W–Ti catalysts are very active in dehydrochlorination of the two 2-chloropropane isomers and retain the same oxidation activity also in the presence of HCl. On the contrary, Mn-based catalysts in the presence of chlorocarbons convert into dehydrochlorination catalysts but lose their catalytic activity in oxidation. V–W–Ti catalysts can be used in Cl-containing atmospheres while Mn–TiO₂ can be proposed for DeNO_x and VOC abatement in Cl-free atmospheres such as for diesel engine exhaust gas purification.     

Fischer–Tropsch synthesis over Co–SiO2 catalysts prepared by the sol–gel method

Fischer–Tropsch synthesis was carried out in slurry phase over uniformly dispersed Co–SiO₂ catalysts prepared by the sol–gel method. When 0.01–1 wt.% of noble metals were added to the Co–SiO₂ catalysts, a high and stable catalytic activity was obtained over 60 h of the reaction at 503 K and 1 MPa. The addition of noble metals increased the reducibility of surface Co on the catalysts, without changing the particle size of Co metal significantly. High dispersion of metallic Co species stabilized on SiO₂ was responsible for stable activity. The uniform pore size of the catalysts was enlarged by varying the preparation conditions and by adding organic compounds such as N,N-dimethylformamide and formamide. Increased pore size resulted in decrease in CO conversion and selectivity for CO₂, a byproduct, and an increase in the olefin/paraffin ratio of the products. By modifying the surface of wide pore silica with Co–SiO₂ prepared by the sol–gel method, a bimodal pore structured catalyst was prepared. The bimodal catalyst showed high catalytic performance with reducing the amount of the expensive sol–gel Co–SiO₂.     

Catalytic wet peroxide oxidation over mixed (Al–Fe) pillared clays

Mixed (Al–Fe) pillared clays are very efficient solid catalysts for oxidation of organic compounds in water by hydrogen peroxide. We have shown that in rather mild experimental conditions (atmospheric pressure, T≤70°C) and with a low excess (20%) of hydrogen peroxide, phenol was rapidly converted, mainly to CO₂, without significant catalyst leaching. The (Al–Fe) pillared clay catalyst (called FAZA) can be used several times without any change of its catalytic properties. According to the low leaching observed and a previous Mössbauer spectroscopy study, the iron species appear to be strongly bonded to the aluminium pillars.     

Alumina supported Mo–V–Te–O catalysts for the ammoxidation of propane to acrylonitrile

The effect of Te addition over Mo–V–O catalysts supported on alumina is discussed for the ammoxidation of propane to acrylonitrile. Catalyst composition and atmosphere of activation are evaluated. Catalysts are characterized before and after catalytic reaction by XPS, XRD and in situ Raman spectroscopies. The absence of Te in catalysts formulation and the presence of a high amount of vanadium induce the presence of V⁵⁺ species and the formation of V₂O₅ oxide; associated with a decrease in acrylonitrile selectivity. The presence of Mo-based polyacids structures decreases the selectivity to acrylonitrile. V⁵⁺ sites are responsible of propane activation and of the subsequent -H abstraction to form the intermediate propylene. Then, a Mo–V rutile-like structure in which vanadium species are reduced as V⁴⁺, is responsible for nitrogen insertion and acrylonitrile formation. The formation of such structure is favoured when Te is added to catalysts and is promoted during propane ammoxidation.     

Isomerization of n-hexane over mono- and bimetallic Pd–Pt catalysts supported on ZrO2–Al2O3–WOx prepared by sol–gel

The catalytic performance of mono- and bimetallic Pd (0.6, 1.0 wt.%)–Pt (0.3 wt.%) catalysts supported on ZrO₂ (70, 85 wt.%)–Al₂O₃ (15, 0 wt.%)–WO_x (15 wt.%) prepared by sol–gel was studied in the hydroisomerization of n-hexane. The catalysts were characterized by N₂ physisorption, XRD, TPR, XPS, Raman, NMR, and FT-IR of adsorbed pyridine. The preparation of ZrW and ZrAlW mixed oxides by sol–gel favored the high dispersion of WO_x and the stabilization of zirconia in the tetragonal phase. The Al incorporation avoided the formation of monoclinic-WO₃ bulk phase. The catalysts increased their S_BET for about 15% promoted by Al₂O₃ addition. Various oxidation states of WO_x species coexist on the surface of the catalysts after calcination. The structure of the highly dispersed surface WO_x species is constituted mainly of isolated monotungstate and two-dimensional mono-oxotungstate species in tetrahedral coordination. The activity of Pd/ZrW catalysts in the hydroisomerization of n-hexane is promoted both with the addition of Al to the ZrW mixed oxide and the addition of Pt to Pd/ZrAlW catalysts. The improvement in the activity of Pd/ZrAlW catalysts is ascribed to a moderated acid strength and acidity, which can be correlated to the coexistence of W⁶⁺ and reduced-state WO_x species (either W⁴⁺ or W⁰). The addition of Pt to the Pd/ZrAlW catalyst does not modify significantly its acidic character. Selectivity results showed that the catalyst produced 2MP, 3MP and the high octane 2,3-dimethylbutane (2,3-DMB) and 2,2-dimethylbutane (2,2-DMB) isomers.     

High performance of V–Ga–O catalysts for oxidehydrogenation of propane

The catalytic performance in the oxidehydrogenation (ODH) of propane of vanadium oxide catalysts supported on gallium oxide, VO_x/Ga₂O₃, with vanadium coverages lower or near the theoretical monolayer has been studied as a function of the vanadium content and compared with those of other known effective V–M–O (M=Mg, Ca) catalysts. Catalyst activity was very high and increased with the increase of vanadium loading in the range studied, while the selectivity trend was similar for the studied catalysts, excepting that with the lower V content. FT-Raman and ⁵¹V solid state NMR spectroscopies show that for coverages below the theoretical monolayer vanadium atoms are in tetrahedral co-ordination either in isolated or polymeric species, while the onset of vanadia formation is detected above that coverage. Interestingly, these catalysts show an one order of magnitude higher area-specific rate, similar initial olefin selectivity and slightly higher selectivity decrease with the increase of conversion than the best VMgO catalyst. This is due to the high intrinsic activity of isolated tetrahedral vanadium species. The combination of these factors produces an enhanced olefin productivity of V–Ga–O catalysts.     

Ammoxidation of methylpyrazine over V–Ti oxide system

The catalytic properties of vanadium–titanium oxide system in ammoxidation of methylpyrazine have been studied. Catalytic activity increases monotonically and yield of selective products passes a wide maximum in the range of V₂O₅ content from 10 to 75 wt.% with increase in the V/Ti relation. The active centers of binary catalysts include V⁵⁺ cations with distorted octahedral coordination strongly bounded with titania apparently owing to formation of V–O–Ti bonds.     

Characterizations of platinum catalysts supported on Ce, Zr, Pr-oxides and formation of carbonate species in catalytic wet air oxidation of acetic acid

Catalytic wet air oxidation (CWAO) of aqueous solution of acetic acid (78 mmol L⁻¹) was carried out with pure oxygen (2 MPa) at 200 °C in a stirred batch reactor on platinum supported oxide catalysts (Pt/oxide, oxide = CeO₂, Zr_0.1Ce_0.9O₂, Zr_0.1(Ce_0.75Pr_0.25)_0.9O₂ and ZrO₂). Platinum was loaded on oxides by impregnation (5 wt%), and then the catalysts were reduced under H₂. Homogenous dispersions of 2–3 nm metal crystallites were obtained. The catalytic activity depended on the ability of the support to resist to the formation of carbonates. Ce(CO₃)OH species, determined by FT-IR and XRD, were rapidly formed during the CWAO reaction especially on mixed oxides. These carbonates were responsible to a drastic drop in catalytic performances. Amounts of carbonate species increase with the ability of the catalyst to transfer oxygen.