Showing papers on "Mixed oxide published in 2011"

Catalytic oxidation of toluene over binary mixtures of copper, manganese and cerium oxides supported on γ-Al2O3

Abstract: The catalytic oxidation of toluene has been investigated over binary mixtures of copper, manganese and cerium oxides supported on high surface area γ-Al 2 O 3 in comparison with the corresponding single metal oxide components. Catalysts were synthesized with the impregnation method and were characterized with respect to their specific surface area (BET method), phase composition and mean crystallite size (XRD technique), reducibility (H 2 -TPR) and adsorption/desorption characteristics toward toluene (TPD followed by TPO). Results obtained using a feed composition consisting of 1000 ppm toluene in air showed that the catalytic performance of mixed oxide catalysts can be improved significantly by proper selection of metal oxide (M x O y ) loading and composition. The intrinsic activity of optimized catalysts, measured under differential reaction conditions, was found to be significantly higher compared to that of their single-component counterparts. This has been attributed to the better dispersion of the active M x O y phases, their increased reducibility (reactivity of surface oxygen), and their lower tendency to form coke deposits under reaction conditions. Addition of a second VOC (propane) or water vapor in the feed affected differently the activity of optimized-mixed oxide catalysts. As a general trend, inhibition by the presence of propane was more pronounced for CeO 2 -containing samples whereas the presence of H 2 O affected mainly the CuO x -containing catalysts. It is concluded that the VOC oxidation activity of Al 2 O 3 -supported mixed oxide catalysts is determined by the reducibility of the dispersed active phases, which may be controlled by proper selection of M x O y nature, loading and composition.

Effect of Ce/Zr composition and noble metal promotion on nickel based CexZr1−xO2 catalysts for carbon dioxide methanation

Abstract: Carbon dioxide methanation was carried out over a series of Ni-CexZr1-xO2 catalysts prepared by a pseudo sol-gel method. The influence of CeO2/ZrO2 mass ratio and noble metal addition was investigated. The catalysts were subsequently characterized by means of XRD,TPR, BET, H-2-TPD and SEM-EDX. The modification of structural and redox properties of these materials was evaluated in relation with their catalytic performances. All catalysts gave impressive CO2 conversion and extremely high selectivity to methane (superior to 98%). Ni2+ incorporation into the CZ structure was proved to enhance catalysts specific activity. The global performance of the studied systems depended not only on the surface of available metallic nickel but also on the composition of the support and on its modification by Ni2+ doping. As a result of these two phenomena, the Ni-based mixed oxide having a CeO2/ZrO2 = 60/40 exhibited the highest catalytic activity, owing to an optimal Ni2+/Ni-0 ratio. Noble metal addition led to higher Ni dispersion, resulting in a raise of both activity and catalyst life-time. It did not modify the support intrinsic activity. The deactivation was shown not to be due to carbon deposits but rather to nickel particles sintering. The investigated parameters thus allowed an improvement of the previously studied 5 wt% Ni-Ce0.72Zr0.28O2 system. (C) 2010 Elsevier B.V. All rights reserved.

Calcium-based mixed oxide catalysts for methanolysis of Jatropha curcas oil to biodiesel.

Abstract: Calcium-based mixed oxides catalysts (CaMgO and CaZnO) have been investigated for the transesterification of Jatropha curcas oil (JCO) with methanol, in order to evaluate their potential as heterogeneous catalysts for biodiesel production Both CaMgO and CaZnO catalysts were prepared by coprecipitation method of the corresponding mixed metal nitrate solution in the presence of a soluble carbonate salt at similar to pH 8-9 The catalysts were characterized by X-ray diffraction (XRD), temperature programmed desorption of CO(2) (CO(2)-TPD), scanning electron microscopy (SEM) and N(2) adsorption (BET) The conversion of JCO by CaMgO and CaZnO were studied and compared with calcium oxide (CaO), magnesium oxide (MgO) and zinc oxide (ZnO) catalysts Both CaMgO and CaZnO catalysts showed high activity as CaO and were easily separated from the product CaMgO was found more active than CaZnO in the transesterification of JCO with methanol Under the suitable transesterification conditions at 338 K (catalyst amount = 4 wt %, methanol/oil molar ratio = 15, reaction time = 6 h), the JCO conversion of more than 80% can be achieved over CaMgO and CaZnO catalysts Even though CaO gave the highest activity, the conversion of JCO decreased significantly after reused for forth run whereas the conversion was only slightly lowered for CaMgO and CaZnO after sixth run (C) 2010 Elsevier Ltd All rights reserved

Direct conversion of bio-ethanol to isobutene on nanosized Zn(x)Zr(y)O(z) mixed oxides with balanced acid-base sites.

Abstract: We report the design and synthesis of nanosized ZnxZryOz mixed oxides for direct and high-yield conversion of bio-ethanol to isobutene (∼83%). ZnO is addded to ZrO2 to selectively passivate zirconia’s strong Lewis acidic sites and weaken Bronsted acidic sites, while simultaneously introducing basicity. As a result, the undesired reactions of bio-ethanol dehydration and acetone polymerization/coking are suppressed. Instead, a surface basic site-catalyzed ethanol dehydrogenation to acetaldehyde, acetaldehyde to acetone conversion via a complex pathway including aldol-condensation/dehydrogenation, and a Bronsted acidic site-catalyzed acetone-to-isobutene reaction pathway dominates on the nanosized ZnxZryOz mixed oxide catalyst, leading to a highly selective process for direct conversion of bio-ethanol to isobutene.

CeO2–WO3 Mixed Oxides for the Selective Catalytic Reduction of NOx by NH3 Over a Wide Temperature Range

Abstract: A series of cerium-tungsten oxide catalysts was prepared by the co-precipitation method and was evaluated for the selective catalytic reduction of NO x by ammonia (NH3-SCR) over a wide temperature range. These catalysts were characterized by BET, XRD, XPS and H2-TPR analyses. The experimental studies demonstrated that, among cerium-tungsten oxides, CeO2–WO3 with a Ce/W molar ratio of 3/2 exhibited the best activity toward NH3-SCR reactions, N2 selectivity and SO2 durability over a broad temperature range of 175–500 °C at a space velocity of 47,000 h−1. The strong interaction between Ce and W could be the main factor leading to the high activity of the CeO2–WO3 mixed oxide catalyst. A series of cerium-tungsten oxide catalysts was prepared by the co-precipitation method and was evaluated for the selective catalytic reduction of NO x by ammonia (NH3-SCR) over a wide temperature range. These catalysts were characterized by BET, XRD, XPS and H2-TPR analyses. The experimental studies demonstrated that, among cerium-tungsten oxides, CeO2–WO3 with a Ce/W molar ratio of 3/2 exhibited the best activity toward NH3-SCR reactions, N2 selectivity and SO2 durability over a broad temperature range of 175–500 °C at a space velocity of 47,000 h−1. The strong interaction between Ce and W could be the main factor leading to the high activity of the CeO2–WO3 mixed oxide catalyst.

Titanium-embedded layered double hydroxides as highly efficient water oxidation photocatalysts under visible light

Abstract: Here, we have synthesized the new titanium-embedded layered double hydroxides (LDHs), such as (Ni/Ti)LDH and (Cu/Ti)LDH. First of all, the formation of LDH structures and the bonding nature for a mixed oxide structure of LDHs are explored in this work. Also, it is determined that our LDHs show two absorption bands in the red and blue regions under visible light, thus different from those of a pure titanium oxide with absorption bands in only the UV region. We find that the (Ni/Ti)LDH with the high surface area showed a higher reaction rate, producing 49 μmol O2 in water oxidation by using 200 mg of the photocatalyst and 1 mmol of AgNO3 as a sacrificial agent. Also, the (Cu/Ti)LDH showed a good reaction rate and produced 31 μmol of O2 under the same condition. On the other hand, conventional TiO2 nanoparticles generated a very small amount of oxygen within the error range under this visible light irradiation. Consequently, these results imply that absorption bands in the visible range and the large surface area of an LDH could result in the high water oxidation photocatalytic activity under visible light.

Influence of the Catalyst Preparation Method, Surfactant Amount, and Steam on CO2 Reforming of CH4 over 5Ni/Ce0.6Zr0.4O2 Catalysts

Abstract: A series of ceria−zirconia mixed oxide supports with nominal composition “Ce0.6Zr0.4O2” were synthesized by two different routes, namely, a surfactant-assisted route and a coprecipitation route. Am...

Photocatalytic Activity of Combustion Synthesized ZrO2 and ZrO2–TiO2 Mixed Oxides

Abstract: Tetragonal ZrO2, synthesized by solution combustion technique, was found to be photocatalytically active for the degradation of anionic dyes. The compound was characterized by FT-Raman spectroscopy, X-ray photoelectron spectroscopy, FT-infrared spectroscopy, UV–vis spectroscopy, BET surface area analysis, and zero point charge pH measurement. A high concentration of surface hydroxyl groups was observed over the catalyst, as confirmed by XPS and FTIR. The photocatalytic degradation of orange G, amido black, remazol brilliant blue R, and alizarin cyanine green (ACG) was carried out with this material. The effect of pH, inorganic salts, and H2O2 on the activity of the catalyst was also studied, and it was found that the catalyst maintained its activity at a wide range of pH and in the presence of inorganic salts. Having established that ZrO2 was photocatalytically active, mixed oxide catalysts of TiO2–ZrO2 were also tested for the photocatalytic degradation of ACG, and the 50% ZrO2–TiO2 mixed oxides showed a...

Low-temperature oxidation of ethanol over a Mn0.6Ce0.4O2 mixed oxide

Abstract: A Mn0.6Ce0.4O2 mixed oxide exhibited rather high activity and stability for ethanol oxidation. Complete conversion of ethanol to CO2 was obtained at a temperature as low as 463 K and the activity maintained for 120 h on-stream without obvious deactivation. The Mn0.6Ce0.4O2 catalyst showed higher ethanol oxidation rate and better selectivity to CO2 which was even superior to a Pt/Al2O3 catalyst. Temperature-programmed surface reaction and spectroscopic studies have revealed that ethoxy species were formed immediately upon ethanol adsorption on the catalyst at room temperature. These intermediates were further oxidized to acetate and carbonate species, and finally converted to CO2 at elevated temperatures. The effective activation of molecule oxygen over the Mn0.6Ce0.4O2 solid solution plays an essential role in determining the catalytic performance. Oxygen transfer from molecular oxygen to MnO2 active sites through CeO2 in the solid solution realized the effective activation of molecular oxygen. At lower temperatures, the oxidation of ethanol mainly produced acetaldehyde, and the direct oxidation of ethanol to CO2 became the major route at higher temperatures, depending on the activation of molecular oxygen.

Consequences of the iron–aluminium exchange on the performance of hydrotalcite-derived mixed oxides for ethanol condensation

Abstract: The effect of the partial and total substitution of aluminium by iron on the performance of different hydrotalcite-derived mixed oxides for the condensation of ethanol for obtaining valuable C4 products (mainly butanol and 1,3,-butadiene) has been studied in this work. Ethanol condensation reactions have been performed in a fixed bed reactor at 0.1 MPa, WHSV = 0.215 h−1 and 473–723 K. Three different hydrotalcite-derived mixed oxide with an atomic Mg2+/M3+ ratio of 3 (namely Mg6Al2O9, Mg6AlFeO9 and Mg6Fe2O9) have been prepared by a previously optimized procedure, tested as catalyst for the above mentioned reaction and characterized by TG–DTG, NH3-TPD, CO2 adsorption (calorimetric, TG–DTG and FTIR), Mossbauer Spectroscopy and TPR. The substitution of Al3+ by Fe3+ in the structure of the resulting mixed oxide leads to a slight decrease of the basic sites and a more marked decrease of the concentration of acid sites, being the concentration of these last sites negligible when the Al3+ cation is completely replaced by Fe3+. Accordingly, Mg–Fe mixed oxide is the most selective catalyst for the formation of C4 compounds, especially butanol. The almost total abatement of the acid sites of this material largely decrease the selectivity for ethanol dehydration, resulting in an increase of the formation of the dehydrogenation product (acetaldehyde), key reactant for condensation reactions.

Acid-base properties of niobium-zirconium mixed oxide catalysts for glycerol dehydration by calorimetric and catalytic investigation

Abstract: The acid–base properties of fresh and used niobium-zirconium mixed oxide catalysts, used for the dehydration of glycerol to acrolein, have been characterized by various techniques. These techniques include ammonia thermo-programmed desorption (TPD), infrared spectroscopy of absorbed pyridine, and adsorption microcalorimetry of ammonia and sulfur dioxide. Relationships between the catalytic properties and the acid–base properties of fresh catalysts have been investigated. The most efficient catalysts were shown to be those for which the zirconia support had a better niobium oxide species covering, but no specific relationship could be established between the acid–base, and the selectivity to acrolein. Characterization of the used catalysts showed that the acidic properties of the catalysts had changed considerably with the time spent on stream. No further strong acid sites, and only weak or very weak acid sites were detected. A linear relationship between the total quantity of remaining acidic sites and the rate of glycerol conversion was determined, taking into account an intrinsic activity of the stronger sites, which is more than ten times that of the weaker ones. The deactivation of the catalysts as a function of time on stream has been related to the formation of cyclic molecules, produced by the reaction of acrolein with by-products resulting from the decomposition of hydroxyacetone and also possibly acrolein and glycerol.

Flame Aerosol Synthesis of Metal Oxide Catalysts with Unprecedented Structural and Catalytic Properties

Abstract: In the past two decades flame aerosol synthesis of novel materials has experienced significant growth in both industry and academia. Recent research is focused on the development of new materials in the nanosized range to be used in various applications, such as catalysts, gas sensors, pigments, and batteries. Several studies indicate that this scalable synthesis method can result in novel and metastable phases of mixed metal oxides of high purity, which may not be easy accessible by conventional wet- or solid-state processes. Especially for catalytic applications this synthesis method is emerging as an attractive fast and single-step production route for high surface area materials, often with unprecedented structural and catalytic properties. The large variety of possible organometallic precursors especially for the liquid-fed aerosol flame synthesis makes this technique very versatile for catalyst synthesis. Using the example of the widely used vanadia-based mixed oxide catalysts, we analyze the structural and catalytic properties of flame-derived catalysts and compare them to corresponding catalysts prepared by classical wet-chemistry methods. The often unique structural properties along with their control at proper synthesis conditions and their influence on catalyst performance in selected reactions are discussed. Subsequently, we give an overview of other recent flame-made mixed metal oxide based catalysts and make an attempt to assess the potential and limitations of flame synthesis for the preparation of catalytic mixed metal oxide materials, and finally we identify future challenges in research.

The generality of surface vanadium oxide phases in mixed oxide catalysts

Abstract: The nature of VO x sites in mixed oxides of supported VO x (on both pure oxide and mixed oxide supports), molecular sieves, zeolites, clays, hydrotalcites, stochiometric bulk oxides and bulk solid solutions were investigated. For supported metal oxides, zeolites and molecular sieves, the VO x species are exclusively present as surface VO x phases below monolayer coverage or the maximum dispersion limits. For layered clays and hydrotalcites, the VO x is present in the hydroxide layers at modest temperatures and react with the clays and hydrotalcites at higher temperatures (>350 °C) when their layered structures decompose. Surface VO x species are always also present for bulk oxides and bulk solid solutions. The rapid diffusion kinetics of VO x , due to its low Tammann temperature, coupled with the lower surface free-energy of vanadium oxide are responsible for the universal presence of surface VO x sites on all mixed oxide materials. Furthermore, surface reactivity studies demonstrate that the surface VO x sites are the catalytic active sites for all V-containing mixed oxide catalytic materials.

Effect of Co-content on the structure and activity of Co–Al hydrotalcite-like materials as catalyst precursors for CO oxidation

Abstract: The present investigation was undertaken in an endeavor to study the effect of the cobalt content on the structure and activity of Co–Al hydrotalcite-like materials as catalyst precursors for CO oxidation by varying the Co 2+ /Al 3+ atomic ratio, thermal treatment of the samples and the reaction temperature. The samples (Co 2+ /Al 3+ = 0.5, 1.5, 3.0) have been synthesized by the co-precipitation method. The unsupported Co 3 O 4 has been prepared according to the same procedure as the reference compound in order to reveal the role of Al 3+ ions presence. The physicochemical characterization of the uncalcined, hydrothermally treated, calcined and tested samples has been accomplished appropriately by ICP-AES, N 2 adsorption, Powder X-ray diffraction technique and Diffuse Reflectance Spectroscopy and H 2 -TPR measurements. The samples were examined by a number of heating–cooling cycles during the activity tests as a procedure to screen the most active catalyst precursor. It was established that the hydrotalcite-like structure of all uncalcined samples had been completely destroyed during the CO oxidation reaction. A concomitant phase transformation into poorly crystallized spinel-type Co 2+ (Co 3+ ,Al 3+ ) 2 O 4 mixed oxide occurred. This spinel-like mixed oxide phase is better organized in all samples after their calcination at 500 °C. The TPR examinations reveal concomitant presence of high-temperature reduced non-stoichiometric CoAl 2 O 4 . It was found out that the Co–Al mixed oxide, derived from the sample with the highest cobalt loading (Co 2+ /Al 3+ = 3.0) preserves a complete and prolonged CO oxidation ability even after cooling down to ambient temperature. On the contrary, the samples with ratios Co 2+ /Al 3+ = 0.5 and 1.5 as well as the Co 3 O 4 oxide deactivate more rapidly during the cycles. A hypothetic scheme is proposed for activation/deactivation of the catalysts. It is related to the oxygen ion-radicals O 2 x – stabilization by Al 3+ cation association with the Co 2+ /Co 3+ redox couple via anionic vacancy.

Phase Stability and Permeation Behavior of a Dead-End Ba(0.5)Sr(0.5)Co(0.8)Fe(0.2)O(3-delta) Tube Membrane in High-Purity Oxygen Production

Abstract: Phase stability and oxygen permeation behavior of Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) dead-end tube membranes were investigated in long-term oxygen production at 950 and 750 °C. At 950 °C, the BSCF tube membranes exhibit good long-term phase stability and a stable oxygen permeation flux. However, at the intermediate temperature of 750 °C, both the oxygen permeation flux and the oxygen purity decrease continuously. This behavior is related to the formation of two secondary phases that are a hexagonal perovskite, Ba0.5±xSr0.5±xCoO3−δ, and a trigonal mixed oxide, Ba1–xSrxCo2–yFeyO5, that evolved in the ceramic membrane made of cubic BSCF perovskite during the dynamic flow of oxygen through it. Tensile stress as a result of phase formation causes the development of cracks in the membrane, which spoil the purity of the permeated oxygen. The partial degradation of cubic BSCF perovskite in the intermediate temperature range (750 °C) was more pronounced under the strongly oxidizing conditions on the oxygen supply (fe...

Transition metal-containing mixed oxides catalysts derived from LDH precursors for short-chain hydrocarbons oxidation

Abstract: MMgAlO mixed oxide catalysts (M = Mn, Fe, Co, Ni, Cu, Zn, Ag and Pd) obtained from layered double hydroxide (LDH) precursors calcined at 1023 K were used in the complete oxidation of methane and the oxidative dehydrogenation of propane. The catalysts were characterized by XRD, N 2 adsorption, EDX and H 2 -TPR experiments. In the catalytic complete oxidation of methane, the Pd-based catalyst has been proved the most active, but the less stable catalyst. Other mixed oxides containing transition metals have been tested and their catalytic activity followed the order: MgAlO ≈ FeMgAlO In the oxidative dehydrogenation of propane in the temperature range from 723 K to 873 K the propene selectivity passed through a maximum for the Mn- and Fe-containing catalysts and decreased continuously for the other transition metal-containing catalysts to the benefit of CO x for CuMgAlO and of cracking products for CoMgAlO, NiMgAlO and ZnMgAlO. Considering the propene yield, the catalysts can be ranked in the following order: CoMgAlO > MnMgAlO > NiMgAlO > ZnMgAlO > FeMgAlO > CuMgAlO. No straight correlation between the H 2 -TPR reducibility and the catalytic performances of the samples was found in the oxidative dehydrogenation of propane into propene. The effects of the contact time and of the propane-to-oxygen molar ratio on the catalytic performances of the most active CoMgAlO, MnMgAlO and NiMgAlO mixed oxides have been investigated.

Fabrication of iron-cerium mixed oxide: an efficient photocatalyst for dye degradation

Abstract: We report herein the fabrication of nanostructured and mesoporous iron-cerium mixed oxides for photocatalytic application. Phase, electronic structure and other properties of the products were characterized by both low-angle and wide-angle X-ray diffraction, diffuse reflectance spectroscopy, transmission electron microscopy, raman spectroscopy, X-ray photoelectron spectroscopy, and N 2 adsorption-desorption isotherm methods. Analytical results demonstrate that the catalyst is in the nano order and mesoporous in nature. These samples were screened for photocatalytic degradation of phenol, methylene blue (MB) and congo red (CR). About 13 % (phenol) and 93 % (MB) photodegradation were observed where as complete mineralization was obtained in case of CR. The reason for higher catalytic activity of 1:1 (Fe/Ce) sample is ascribed to their higher surface area, surface acidity which determines the active sites of the catalyst and accelerates the photocatalytic reaction. Keywords : Mixed oxide, photocatalyst, photodegradation International Journal of Engineering, Science and Technology , Vol. 2, No. 9, 2010, pp. 53-65

Facile preparation of highly-dispersed cobalt-silicon mixed oxide nanosphere and its catalytic application in cyclohexane selective oxidation.

Abstract: Highly dispersed cobalt-silicon mixed oxide [Co-SiO2] nanosphere was successfully prepared with a modified reverse-phase microemulsion method. This material was characterized in detail by X-ray diffraction, transmission electron microscopy, Fourier transform infrared, ultraviolet-visible diffuse reflectance spectra, X-ray absorption spectroscopy near-edge structure, and N2 adsorption-desorption measurements. High valence state cobalt could be easily obtained without calcination, which is fascinating for the catalytic application for its strong oxidation ability. In the selective oxidation of cyclohexane, Co-SiO2 acted as an efficient catalyst, and good activity could be obtained under mild conditions.

Heterogeneously catalyzed efficient hydration of alkynes to ketones by tin-tungsten mixed oxides.

Abstract: The Sn-W mixed oxide prepared by calcination of the Sn-W mixed hydroxide precursor with a Sn/W molar ratio of 2:1 at 800 °C (SnW2-800) acts as an efficient heterogeneous catalyst for the hydration of alkynes. Structurally diverse terminal and internal alkynes, including aromatic, aliphatic, and double-bond-containing ones, can be converted into the corresponding ketones in moderate to high yields. The catalytic activity of SnW2-800 is much higher than those of previously reported heterogeneous catalysts and commonly utilized acid catalysts. The observed catalysis was truly heterogeneous, and the retrieved catalyst can be reused at least three times with retention of its high catalytic performance. The reaction rate for the SnW2-800-catalyzed hydration was decreased by addition of 2,6-lutidine and the hydration hardly proceeded in the presence of an equimolar amount of this compound with respect to that of the Bronsted acid sites in SnW2-800. Therefore, the present hydration is mainly promoted by the Bronsted acid sites in SnW2-800.

Electronic Structure of Magnesia−Ceria Model Catalysts, CO2 Adsorption, and CO2 Activation: A Synchrotron Radiation Photoelectron Spectroscopy Study

Abstract: We have studied the electronic properties of single crystal based ceria and magnesia−ceria model catalysts, the CO2 adsorption, and the CO2-induced reoxidation of these systems by synchrotron radiation photoelectron spectroscopy (SR-PES). All model systems were prepared starting from a fully stoichiometric and well-ordered CeO2(111) film grown on Cu(111). Different magnesia−ceria mixed oxide films were prepared by physical vapor deposition (PVD) of magnesium, oxygen treatment, and subsequent annealing. The preparation procedure was varied to obtain samples with different oxidation state, structure, and surface composition. Different carbon-containing species were identified, including surface carbonates formed in the vicinity of Mg2+ and Ce3+/4+ and surface carboxylates. The presence of Mg2+ was observed to strongly enhance carbonate formation but suppress the formation of carboxylates. Changes in the oxidation state of ceria upon CO2 exposure were monitored with highest sensitivity by resonant photoelect...