Showing papers on "Mixed oxide published in 2010"

Monitoring surface metal oxide catalytic active sites with Raman spectroscopy

Abstract: The molecular aspect of the Raman vibrational selection rules allows for the molecular structural and reactivity determinations of metal oxide catalytic active sites in all types of oxide catalyst systems (supported metal oxides, zeolites, layered hydroxides, polyoxometalates (POMs), bulk pure metal oxides, bulk mixed oxides and mixed oxide solid solutions). The molecular structural and reactivity determinations of metal oxide catalytic active sites are greatly facilitated by the use of isotopically labeled molecules. The ability of Raman spectroscopy to (1) operate in all phases (liquid, solid, gas and their mixtures), (2) operate over a very wide temperature (−273 to >1000 °C) and pressure (UHV to ≫100 atm) range, and (3) provide molecular level information about metal oxides makes Raman spectroscopy the most informative characterization technique for understanding the molecular structure and surface chemistry of the catalytic active sites present in metal oxide heterogeneous catalysts. The recent use of hyphenated Raman spectroscopy instrumentation (e.g., Raman–IR, Raman–UV-vis, Raman–EPR) and the operando Raman spectroscopy methodology (e.g., Raman–MS and Raman–GC) is allowing for the establishment of direct structure–activity/selectivity relationships that will have a significant impact on catalysis science in this decade. Consequently, this critical review will show the growth in the use of Raman spectroscopy in heterogeneous catalysis research, for metal oxides as well as metals, is poised to continue to exponentially grow in the coming years (173 references).

Investigation of reactive cerium-based oxides for H2 production by thermochemical two-step water-splitting

Abstract: This study focuses on the use of cerium-based mixed oxides for hydrogen production by solar-driven thermochemical two-step water-splitting. Mixed cerium oxides are proposed in order to decrease the reduction temperature of ceria and to avoid material sublimation occurring above 2,000 °C during the high-temperature solar step. Ceria-based nanopowders were synthesized by soft chemistry methods including the modified Pechini method. The influence of the synthesis method, the type of cationic element mixed with cerium, and the content of this added element was investigated by comparing the reduction temperatures of the derived materials. The synthesized powders were characterized by X-ray diffraction, thermogravimetric analysis, SEM, and Raman spectroscopy. Results showed that the synthesized pure cerium oxide is more reactive toward reduction than a commercial powder. Among the different elements added to ceria that were screened, the addition of zirconium significantly improved the reduction of ceria at temperatures below 1,500 °C. Increasing zirconium content further favored cerium reduction yield up to 70%. Water-splitting tests were performed to demonstrate the reactivity of the developed materials for H2 production. The amount of H2 evolved was enhanced with a temperature increase, the maximum H2 production from Ce0.75Zr0.25O2−δ was 0.24 mmol/g at 1,045 °C, and the powder reactivity upon cycling was demonstrated via thermogravimetry through two successive reduction–hydrolysis reactions.

Phase-pure Cu,Zn,Al Hydrotalcite-like Materials as Precursors for Copper rich Cu/ZnO/Al2O3 Catalysts

Abstract: A series of hydrotalcite-like (htl) compounds of the general composition (Cu,Zn)1−xAlx(OH)2(CO3)x/2·m H2O was prepared with a fixed Cu:Zn ratio of 70:30. Phase pure samples could be obtained for 0.3 ≤ x ≤ 0.4. The htl precursors thermally decompose in multiple steps. After dehydration and dehydroxylation amorphous materials were obtained at 330 °C. Phase segregation during this mild calcination was only observed for samples with a Zn:Al ratio deviating strongly from 1:2. A mechanism for this low-temperature segregation process basing on the preformation of the ZnAl2O4 phase within the amorphous material is proposed. Samples with Zn:Al ratios near 1:2 form an amorphous carbonate-modified mixed oxide “(CuO)x(ZnAl2O4)y” of homogeneous microstructure. Crystallization occurs upon carbonate decomposition at temperatures higher than 500 °C. Despite the small size of the Cu nanoparticles (around 7 nm) formed upon reduction, the accessible Cu surface area is below 5 m2g−1. This can be explained by the unfavorable ...

Molecular Structural Determination of Molybdena in Different Environments: Aqueous Solutions, Bulk Mixed Oxides, and Supported MoO3 Catalysts

Abstract: UV−vis diffuse reflectance spectroscopy (DRS) and Raman spectroscopy were used to examine the electronic and molecular structures, respectively, of well-defined Mo(VI) bulk mixed oxide reference compounds ((i) isolated MoO4 or MoO6 monomers, (ii) dimeric O3Mo−O−MoO3, (iii) chains of alternating MoO4 and MoO6 units, (iv) MoO6-coordinated Mo7−Mo12 clusters, and (v) infinite layered sheets of MoO5 units), aqueous molybdate anions as a function of solution pH, and supported MoO3 catalysts (MoO3/SiO2, MoO3/Al2O3, and MoO3/ZrO2). Raman spectroscopy confirmed the identity and phase purity of the different bulk and solution molybdenum oxide structures. UV−vis DRS provided the corresponding electronic edge energy (Eg) of the ligand-to-metal charge transfer (LMCT) transitions of the Mo(VI) cations. A linear inverse correlation was found between Eg and the number of bridging Mo−O−Mo covalent bonds around the central Mo(VI) cation. A relationship between Eg and the domain size (NMo) for finite MoOx clusters, however,...

Effects of adsorbed and gaseous NOx species on catalytic oxidation of diesel soot with MnOx–CeO2 mixed oxides

Abstract: MnOx–CeO2, MnOx and CeO2 catalysts were synthesized by the sol–gel method. The structural, redox and adsorption properties of the individual and mixed oxides were investigated by X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET), CO temperature-programmed reduction (TPR), NO temperature-programmed oxidation (TPO), NOx temperature-programmed desorption (TPD) and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The TPO tests were performed with the soot–catalyst mixture under loose contact conditions to evaluate the catalytic activity of the oxide catalysts for soot oxidation. The MnOx–CeO2 mixed oxide catalyst presents the lowest soot oxidation temperature among the catalysts investigated in the presence of NO and O2. The synergetic effect between manganese oxide and ceria restrains the growth of oxide crystallites, increases the specific surface area and improves the low-temperature redox property. Especially, the activity of MnOx–CeO2 mixed oxides for NO oxidation and its capacity for NO2 storage in the form of surface nitrates are greatly enhanced. Not only the NO2 released from decomposition of surface nitrates but also that formed by catalytic oxidation of gaseous or adsorbed NO on the catalyst is confirmed important for soot oxidation. It is found by the in situ DRIFTS tests with the soot–catalyst mixture that the generation of surface oxygen complexes (SOCs), such as carboxylic anhydrides, lactones, quinine, ceto-enol groups, ethers and phenols, occurs at about 100 °C lower temperature with exposure to NO than in the absence of NO, which is an important step for soot oxidation.

Sol−Gel Based Chemical Synthesis of Nd2Fe14B Hard Magnetic Nanoparticles

Abstract: Nd2Fe14B hard magnetic nanoparticles were synthesized by chemical synthesis techniques. Nd−Fe−B gel was prepared using NdCl3·6H2O, FeCl3·6H2O, H3BO3, citric acid, and ethylene glycol (EG) by a Pechini type sol−gel method. This gel was subsequently annealed to produce mixed oxide powders. Nd2Fe14B nanoparticles were prepared from these oxides by a reduction−diffusion process. The phase analysis, structure, and magnetic properties were determined by Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance (NMR), X-ray diffraction (XRD), field emission scanning electron microscope (FESEM), transmission electron microscope (TEM), and vibrating sample magnetometer (VSM) techniques. The mechanism of Nd2Fe14B formation was investigated by differential scanning calorimetry (DSC), XRD, and thermodynamic free energy change data. Our experimental and modeling results showed that the reduction−diffusion of the Nd−Fe−B mixed oxide was a three step process. The reduction of Fe2O3 to Fe and B2O3 to B o...

Nanostructured TiO2–CeO2 mixed oxides by an aqueous sol–gel process: Effect of Ce:Ti molar ratio on physical and sensing properties

Abstract: Nanostructured TiO 2 –CeO 2 thin films and powders were prepared by a straightforward aqueous particulate sol–gel route. Titanium (IV) isopropoxide and cerium chloride were used as precursors, and hydroxypropyl cellulose was used as a polymeric fugitive agent in order to increase the specific surface area. The effect of Ce:Ti molar ratio was studied on the crystallisation behaviour of the products. X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) revealed that the powders crystallised at the low temperature of 500 °C, containing anatase–TiO 2 , rutile–TiO 2 and cubic-CeO 2 phases, as well as Ti 8 O 15 , Ti 3 O 5 and Ce 11 O 20 depending on annealing temperature and Ce:Ti molar ratio. Furthermore, it was found that CeO 2 retarded the anatase to rutile transformation up to 700 °C. The activation energy of crystallite growth was calculated in the range 1.92–8.79 kJ/mol. Transmission electron microscope (TEM) image showed that one of the smallest crystallite sizes was obtained for TiO 2 –CeO 2 binary mixed oxide, being 3 nm at 500 °C. Field emission scanning electron microscope (FE-SEM) analysis revealed that the deposited thin films had nanostructured morphology with the average grain size in the range 17–28 nm at 500 °C. Thin films produced under optimised conditions showed excellent microstructural properties for gas sensing applications. They exhibited a remarkable response towards low concentrations of CO gas at low operating temperature of 200 °C, resulting in increased thermal stability of sensing films as well as a decrease in their power consumption. Furthermore, calibration curves revealed that TiO 2 –CeO 2 sensors follow the power law, S = A [gas] B (where S is sensor response, coefficients A and B are constants and [gas] is the gas concentration) for the two types of gases, and they have excellent capability for the detection of low gas concentrations.

Enhanced photoassisted water electrolysis using vertically oriented anodically fabricated Ti-Nb-Zr-O mixed oxide nanotube arrays.

Abstract: Self-ordered, highly oriented arrays of titanium-niobium-zirconium mixed oxide nanotube films were fabricated by the anodization of Ti(35)Nb(5)Zr alloy in aqueous and formamide electrolytes containing NH(4)F at room temperature. The nanostructure topology was found to depend on the nature of the electrolyte and the applied voltage. Our results demonstrate the possibility to grow mixed oxide nanotube array films possessing several-micrometer-thick layers by a simple and straightforward electrochemical route. The fabricated Ti-Nb-Zr-O nanotubes showed a ∼17.5% increase in the photoelectrochemical water oxidation efficiency as compared to that measured for pure TiO(2) nanotubes under UV illumination (100 mW/cm(2), 320-400 nm, 1 M KOH). This enhancement could be related to a combination of the effect of the thin wall of the fabricated Ti-Nb-Zr-O nanotubes (10 ± 2 nm) and the formation of Zr oxide and Nb oxide layers on the nanotube surface, which seems to slow down the electron-hole recombination in a way similar to that reported for Gratzel solar cells.

Syngas production from CO2 reforming of methane using Ce-doped Ni-catalysts obtained from hydrotalcites by reconstruction method

Abstract: Ce-doped Ni, Mg and Al hydrotalcites were synthesized through the reconstruction method in the presence of the [Ce(EDTA)] − complex The hydrotalcites and the mixed oxides derived after their thermal decomposition at 500 °C (16 h) were characterized by XRD, TGA-DTA, ICPS, IR, Raman, SEM-EDX, TEM, TPR-H 2 , TPD-CO 2 , XPS and N 2 sorptometry The reconstruction of the mixed oxide is not total in [Ce(EDTA)] − solutions with Ce between 1 and 10%wt The hydrotalcite is reconstructed as hydroxyl-type phase; the reconstruction takes place in the external edges of the oxide granules The complex is deposited, with high distribution, on the external surface of the reconstructed layered structure as species of Ce 4+ and after the calcination generates surface CeO 2 -doped periclase oxides Solids were used in the production of syngas through CO 2 reforming of methane (CH 4 :CO 2 :Ar 5/5/40) using a total fixed flow of 50 mL min −1 and WHSV = 30 L g −1 h −1 between 500 and 800 °C with prior reduction The increase in the Ce load had a promoter effect in catalytic conversions H 2 /CO ratios between 08 and 12 were obtained Ce had a positive effect in the elimination of coke, since a nominal optimal value of 3%wt was obtained

One-step non-hydrolytic sol-gel preparation of efficient V2O5-TiO2 catalysts for VOC total oxidation

Abstract: Vanadia-titania catalysts are highly regarded for the control of industrial and domestic gas effluents. They are particularly used for the abatement of volatile organic compounds and for the removal of aromatic, polyaromatic and polychlorinated air pollutants like furans and dioxins. Here, mixed oxide catalysts are formed by non-hydrolytic condensation of chloride precursors with diisopropyl ether in non-aqueous medium. Calcination was applied to provoke the migration of the vanadium oxide toward the surface of the anatase particles, leading to well-spread vanadia species. The materials are made of mesoporous spherical particles of a few microns, themselves constituted by the aggregation of 10-25 nm sized particles. Their chemical composition is precisely controlled. The samples are characterized by ICP-AES, N-2-physisorption, XRD, XPS, SEM and TOF-SIMS. Their performances as total oxidation catalyst are evaluated in the deep oxidation of benzene, chosen as a model atmospheric pollutant. The activity of the materials compares well with that of catalysts prepared by classical wet impregnation. Along with the V surface concentration, the environment in which V surface species are located is identified as a key parameter. The superior activity of polymeric species as compared to isolated ones is shown. (C) 2009 Elsevier B.V. All rights reserved.

Selective methanation of CO in hydrogen-rich gases involving large amounts of CO2 over Ru-modified Ni-Al mixed oxide catalysts

Abstract: Selective methanation of CO was investigated in hydrogen-rich gas streams involving 20% CO2 over a Ru-modified Ni-Al mixed oxide catalyst. The Ni-Al mixed oxide was prepared by the solution-spray plasma technique, by which fine powders of homogeneous mixed oxides can be easily synthesized. By the addition of a small amount of Ru, the catalytic activity and selectivity of the Ni-Al mixed oxide catalyst for CO methanation was improved dramatically. The added Ru enhanced the formation of nano-sized Ni metal particles via reduction of the mixed oxide by spill-over-hydrogen during reduction treatment, even at low temperature. Kinetic measurements revealed that the methanation of CO took place predominantly over the Ni metal sites. Selectivity for the CO methanation was also improved significantly by the addition of Ru due to suppression of CO2 dissociation over the Ni metal sites.

Spinel‐Type Cobalt–Manganese‐Based Mixed Oxide as Sacrificial Catalyst for the High‐Yield Production of Homogeneous Carbon Nanotubes

Abstract: Potential applications of carbon nanotubes (CNTs) in very different fields such as electronics, medicine, or catalysis have been widely demonstrated on a laboratory scale. The development of CNT-based commercial products now mainly relies on CNTs’ ability to also fulfill the expectations on an industrial scale and on our ability to produce them at a reasonable cost. In the past, several attractive materials, such as fullerenes or nanodiamonds, lost some of their appeal because their synthesis is difficult to scale up for technical and economical reasons. For multiwalled carbon nanotubes (MWCNTs), their integration in commercial products depends on the availability of highpurity nanotubes with high homogeneity in size and in structure at a relatively low price. To date, high-quality MWCNTs are produced by catalytic chemical vapor deposition (CCVD) using transition metal heterogeneous catalysts. Despite many efforts, the yields remain relatively low, at best a few tens of grams MWCNTs per gram of catalyst. Purification using strong acids is therefore necessary to remove the remaining catalyst, which significantly increases the complexity of the production process. Herein, we demonstrate that spinel-type cobalt– manganese-based mixed oxide is a unique catalyst to produce MWCNTs. The yield is typically 2–9 times higher than that obtained with conventional catalysts, so that the purification step is no longer needed. In addition, the MWCNTs are exceptionally homogeneous in diameter, the standard deviation being only 4 nm. The high quality and high yield obtained allow for, to our knowledge, the first time MWCNTs to become a commodity chemical. Co, Mn, Al, and Mg in their nitrate forms were coprecipitated at pH 10 by using sodium hydroxide (see the Supporting Information). The precipitate was then filtered and thoroughly washed with distilled water to eliminate sodium and nitrate ions until the conductivity of the washing solution matched that of the freshly distilled water. The filtrate was subsequently dried in air at 180 8C for 5 h and calcined at 400 8C for 4 h to decompose it into the corresponding spinel-type mixed oxide. Finally, its efficiency in growing MWCNTs was tested by reducing it in diluted hydrogen (50 %) and exposing it to an ethylene/hydrogen feed at 650 8C for 2 h (see the Supporting Information for additional details). The yield was 179 gCNT g 1 catalyst (17 900 wt %), which is 2–9 times higher than the best yields previously obtained under similar conditions (Figure 1). Furthermore, residual catalyst impurities in the synthesized material were scarce and the purity was close to 99.5 % carbon, which is sufficient for most applications. Therefore, the traditional purification step with strong mineral acids is no longer required, thus rendering the overall process safer, greener and cheaper. In addition, the diameter distribution calculated from high-resolution scanning electron microscopy (HRSEM) and transmission electron microscopy (HRTEM) images was centered at 14 nm with a standard deviation of 4 nm, which is remarkably narrow. The produced MWCNTs were fully character-

Novel CeO2 promoted TiO2–ZrO2 nano-oxide catalysts for oxidative dehydrogenation of p-diethylbenzene utilizing CO2 as soft oxidant

Abstract: The influence of ceria on structural and catalytic properties of TiO2–ZrO2 mixed oxide was investigated for oxidative dehydrogenation (ODH) of p-diethylbenzene (DEB) to p-divinylbenzene (DVB) utilizing CO2 as soft oxidant. The investigated catalysts were synthesized by coprecipitation and impregnation methods involving microwave treatment. Catalyst characterization was achieved using X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), BET surface area and temperature programmed desorption/oxidation (TPD/TPO) methods. The XRD and TEM results revealed that the prepared samples are in nanocrystalline form with 10–20 nm size. The presence of acid–base sites and their distribution was examined by NH3 and CO2 TPD measurements. The ceria promoted sample exhibited more number of acid sites. The TPO measurements revealed that CO2 dissociates over the reduced surface of the catalysts. Among various catalysts investigated, the microwave treated CeO2/TiO2–ZrO2 sample exhibited better conversion and a high product selectivity. The time-on-stream studies revealed that ceria promoted TiO2–ZrO2 catalysts exhibit higher stability for ODH of DEB.

Naphthalene steam reforming over Mn-doped CeO2–ZrO2 supported nickel catalysts

Abstract: In this study, the catalytic activity of nickel supported on Ce 0.75 Zr 0.25− x Mn x O 2 ( x = 0, 0.10, 0.20 and 0.25) mixed oxide catalysts prepared by urea hydrolysis and incipient wetness impregnation was investigated for steam reforming of naphthalene selected as a model compound for biomass-derived tar. The results showed that nickel supported on Ce 0.75 Zr 0.25− x Mn x O 2 mixed oxide catalysts exhibit good activity for naphthalene steam reforming. It was found that the amount of carbon deposition reduced substantially when Mn was introduced to the CeO 2 –ZrO 2 mixed oxide. This might be due to the fact that the incorporation of manganese ions into the ceria lattice would improve the oxygen storage capacity and the oxygen mobility on the surface of mixed oxides, resulting in enhancing the gasification of the deposited carbon.

Copper–manganese oxide catalysts supported on alumina: Physicochemical features and catalytic performances in the aerobic oxidation of benzyl alcohol

Abstract: We report the facile synthesis of alumina supported copper–manganese mixed oxide catalysts by a homogeneous deposition–precipitation method and the catalytic applications of these catalysts in the oxidation of benzyl alcohol using molecular oxygen. Benzyl alcohol conversion of ca. 90.9% was achieved with benzaldehyde as the main product. The catalyst can be recycled consecutively up to seven runs without appreciable loss of its activity and selectivity. Characterizations with X-ray diffraction, transmission electron microscopy, Raman, X-ray absorption spectroscopy, and hydrogen temperature-programmed reduction suggested the formation of a mixed oxide (Mn 1.5 Cu 1.5 O 4 ) microcrystalline phase after adding copper to the manganese supported on alumina. Such formation would account for the high catalytic performance. Calcination under high temperature decomposed this mixed oxide phase, resulting in a poor catalytic activity.

Vanadium-promoted tin oxide semiconductor carbon monoxide gas sensors

Abstract: The semiconductor gas sensors investigated are constituted by vanadium-tin oxide nanoparticles, prepared by a co-precipitation method. They were evaluated for carbon monoxide detection at low temperatures. The sensing characteristics obtained are associated with the composition, structure and surface state of oxide semiconductor. XPS analysis revealed an electronic interaction between Sn and V atoms in the mixed oxide structure and the formation of vanadium cations with multiple valences. Combined XRD and FTIR analyses showed that the tin oxide crystallinity decreased with increasing vanadium loadings. Lower levels of vanadium dopants in tin oxide enhanced response to CO gas in air because they facilitated oxygen adsorption by scavenging electrons to form reactive oxygen ions on the surface, as indicated by electrical and EPR measurements. The response to CO gas in nitrogen atmosphere was observed, and increased with increasing CO concentrations. Sensing mechanisms that discuss the roles of the vanadium redox pairs and oxygen vacancies in the CO detection process are proposed.