Showing papers on "Mixed oxide published in 2008"

Ce 3d XPS investigation of cerium oxides and mixed cerium oxide (CexTiyOz)

Abstract: This article presents an XPS study of Ce 3d emission spectra dominated by atomic multiplet effects in core level spectroscopy of rare earth compounds (Ce oxides). Core level spectroscopy has been used to study the electronic states of Ce 3d5/2 and Ce 3d3/2 levels in Ce4+ and Ce3+ states. The well-resolved components of Ce 3d5/2 and Ce 3d3/2 spin-orbit components, due to various final states (4f0, 4f1, 4f2 configurations), were determined on 3d XPS spectra from commercial powders (CeO2, CePO4). These results were used to study the 3d spin-orbit component of mixed cerium-titanium oxide. This compound was prepared by co-melting commercial powders of CeO2 and TiO2 at 1800 K under air using a solar furnace with a flux density of 16 MW.m−2 at the focal point of the parabolic concentrator. The mixed oxide Ce2Ti2O7 was produced and contained Ce(III) species which may be reactive with water to give back the initial metal oxides and generate hydrogen, a valuable product considered as a promising energy carrier in the future in replacement of oil. The 3d photoemission spectra revealed the presence of mixed components attributed to mainly Ce(III) and Ce(IV) species. Copyright © 2008 John Wiley & Sons, Ltd.

Pt/MnOx-CeO2 catalysts for the complete oxidation of formaldehyde at ambient temperature

Abstract: MnO x –CeO 2 mixed oxides with a Mn/(Mn + Ce) molar ratios of 0–1 were prepared by a modified coprecipitation method and investigated for the complete oxidation of formaldehyde. The MnO x –CeO 2 with Mn/(Mn + Ce) molar ratio of 0.5 exhibited the highest catalytic activity among the MnO x –CeO 2 mixed oxides. Structure analysis by X-ray powder diffraction and temperature-programmed reduction of hydrogen revealed that the formation of MnO x –CeO 2 solid solution greatly improved the low-temperature reducibility, resulting in a higher catalytic activity for the oxidation of formaldehyde. Promoting effect of Pt on the MnO x –CeO 2 mixed oxide indicated that both the Pt precursors and the reduction temperature greatly affected the catalytic performance. Pt/MnO x –CeO 2 catalyst prepared from chlorine-free precursor showed extremely high activity and stability after pretreatment with hydrogen at 473 K. 100% conversion of formaldehyde was achieved at ambient temperature and no deactivation was observed for 120 h time-on-stream. The promoting effect of Pt was ascribed to enhance the effective activation of oxygen molecule on the MnO x –CeO 2 support.

Ca and Zn mixed oxide as a heterogeneous base catalyst for transesterification of palm kernel oil

Abstract: Transesterification of palm kernel oil with methanol over mixed oxides of Ca and Zn has been investigated batchwise at 60 °C and 1 atm. CaO·ZnO catalysts were prepared via a conventional co-precipitation of the corresponding mixed metal nitrate solution in the presence of a soluble carbonate salt at near neutral conditions. The catalysts were characterized by using techniques of X-ray diffraction (XRD), scanning electron microscope (SEM), and thermogravimetric analysis (TGA). The results indicated that the mixed oxides possess relatively small particle sizes and high surface areas, compared to pure CaO and ZnO. Moreover, the combination of Ca and Zn reduced the calcination temperature required for decomposition of metal carbonate precipitates to active oxides. Influences of Ca/Zn atomic ratio in the mixed oxide catalyst, catalyst amount, methanol/oil molar ratio, reaction time, and water amount on the methyl ester (ME) content were studied. Under the suitable transesterification conditions at 60 °C (catalyst amount = 10 wt.%, methanol/oil molar ratio = 30, reaction time = 1 h), the ME content of >94% can be achieved over CaO·ZnO catalyst with the Ca/Zn ratio of 0.25. The mixed oxide can be also applied to transesterification of palm olein, soybean, and sunflower oils. Furthermore, the effects of different regeneration methods on the reusability of CaO·ZnO catalyst were investigated.

Ga–Al Mixed‐Oxide‐Supported Gold Nanoparticles with Enhanced Activity for Aerobic Alcohol Oxidation

Abstract: The selective oxidation of alcohols is one of the most challenging reactions in green chemistry. Although a number of methods have been developed, the search for new, facile, cost-effective, and environmentally benign procedures that avoid the use of a large excess of toxic and expensive stoichiometric metal oxidants has attracted substantial interest. An attractive method is the direct oxidation of alcohols—promoted by reusable heterogeneous catalysts—using air or molecular oxygen (O2) under solventfree conditions or (in the case of solid alcohols) in green organic solvents. Ideally, the reaction should also be performed under mild conditions (preferably at room temperature) for the synthesis of complex, thermolabile compounds, which are typical in fine chemistry. Satisfactory results were attained in only very few cases, in which a large excess of base additives was required, and this was usually achieved at the expense of selectivity. Therefore, the development of excellent reusable catalysts for liquid-phase aerobic oxidation of alcohols under mild conditions would constitute a breakthrough in both green chemistry and organic synthesis. Recently, supported gold nanoparticles have attracted considerable attention because of their extraordinarily high activity and selectivity. The outstanding catalytic ability of gold is related to the size and shape of the nanoparticles, the degree of coordinative unsaturation of the gold atoms, and the interactions between gold and the oxide support. Although several gold systems have been reported for the catalysis of alcohol oxidation reactions, in most cases they have been applied at temperatures above 100 8C. Dehydrogenation is known to be the rate-limiting step in the oxidation of alcohols on various noble metals. Therefore, the combination of gold nanoparticles with a suitable support (characterized by an exceptional alcohol-dehydrogenation activity) may allow the fabrication of new, versatile gold catalysts that could be used for liquid-phase organic synthesis under mild conditions. Herein, we demonstrate for the first time that mesostructured Ga–Al mixed-oxide solid solutions are highly promising supports for the fabrication of exceptionally effective gold catalysts for aerobic alcohol oxidation under mild conditions. A series of binary mesostructured Ga–Al mixed-oxide supports (denoted as GaxAl6 xO9; x= 2, 3, 4), along with unitary oxides of g-Ga2O3 and g-Al2O3, was prepared through an alcoholic sol–gel pathway. The X-ray diffraction (XRD) patterns of all as-synthesized binary substrates are characteristic of g-Ga2O3/Al2O3 solid solutions with a spinel-type structure. When gold nanoparticles were deposited onto these high-surface-area materials, no gold diffraction line was detected, and the pattern showed no significant differences relative to that of the support, thus indicating that the structure of the catalyst was maintained. A representative transmission electron microscopy (TEM) image of the Au/ GaxAl6 xO9 sample confirms that the gold particles were evenly deposited on the Ga–Al mixed-oxide support, with most particles being smaller than about 6 nm (see the Supporting Information for TEM and XRD data). To check the possible alcohol-dehydrogenation capability of the Au/GaxAl6 xO9 materials, we adsorbed 2-propanol on their surface and performed temperature-programmed surface reaction (TPSR) measurements of the desorbed H2 molecules (see the Supporting Information). Ga-containing mixed-oxide supports were found to be indispensable for attaining highly active alcohol-dehydrogenation materials (Figure 1A). Furthermore, the dehydrogenation activity of the catalysts was observed to be strongly dependent on the composition of these supports. A strongly enhanced hydrogen signal was identified in the case of a Ga3Al3O9 solid solution containing a Ga/Al molar ratio of 1:1—in sharp contrast to what was observed for the reference gold catalysts Au/TiO2 and Au/Fe2O3 (provided by the World Gold Council), where no H2 species were detected. These results can be rationalized by assuming that the formation of Ga–Al mixed-oxide solid solutions may favor the creation of specific dehydrogenation sites as a consequence of the presence of Ga atoms at the surface atomic sites (Td and Oh) of Al2O3 and highly dispersed GaO4 tetrahedra in the surface spinels. [16] These sites are responsible for the considerably enhanced dehydrogenation activity observed for the Ga–Al mixed-oxide-supported Au catalysts. Our initial aerobic-oxidation studies focused on the case of benzyl alcohol (Figure 1B), with the aim to understand the effect of the composition of the support on the catalytic performance of the gold catalysts. The reactions were performed in a magnetically stirred glass batch reactor in the presence of a solvent (at 90 8C) under O2 and at [*] F. Z. Su, Dr. Y. M. Liu, L. C. Wang, Prof. Y. Cao, Prof. H. Y. He, Prof. K. N. Fan Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials Fudan University, Shanghai 200433 (P. R. China) Fax: (+86)21-6564-2978 E-mail: yongcao@fudan.edu.cn

Thermal ageing of Pt on low-surface-area CeO2-ZrO2-La2O3 mixed oxides : Effect on the OSC performance

Abstract: This work aims at exploring the thermal ageing mechanism of Pt on ceria-based mixed oxides and the corresponding effect on the oxygen storage capacity (OSC) performance of the support material. Pt was supported on low-surface-area CeO2–ZrO2–La2O3 mixed oxides (CK) by impregnation method and subsequently calcined in static air at 500, 700 and 900 °C, respectively. The evolutions of textural, microstructural and redox properties of catalysts after the thermal treatments were identified by means of X-ray diffraction (XRD), Raman, X-ray photoelectron spectroscopy (XPS), temperature programmed reduction (TPR) and high-resolution transmission electron microscope (HRTEM). The results reveal that, besides the sintering of Pt, encapsulation of metal by the mixed oxides occurs at the calcination temperature of 700 °C and above. The burial of Pt crystallites by support particles is proposed as a potential mechanism for the encapsulation. Further, the HRTEM images show that the distortion of the mixed oxides lattice and other crystal defects are distributed at the metal/oxides interface, probably indicating the interdiffusion/interaction between the metal and mixed oxide. In this way, encapsulation of Pt is capable to promote the formation of Ce3+ or oxygen vacancy on the surface and in the bulk of support. The OSC results show that the reducibility and oxygen release behavior of catalysts are related to both the metal dispersion and metal/oxides interface, and the latter seems to be more crucial for those supported on low-surface-area mixed oxides. Judging by the dynamic oxygen storage capacity (DOSC), oxygen storage capacity complete (OSCC) and oxygen releasing rate, the catalyst calcined at 700 °C shows the best OSC performance. This evident promotion of OSC performance is believed to benefit from the partial encapsulation of Pt species, which leads to the increment of Ce3+ or oxygen vacancies both on the surface and in the bulk of oxides despite a loss of chemisorption sites on the surface of metal particles.

Co3O4 based catalysts for NO oxidation and NOx reduction in fast SCR process

Abstract: Reaction activities of several developed catalysts for NO oxidation and NO x (NO + NO 2 ) reduction have been determined in a fixed bed differential reactor. Among all the catalysts tested, Co 3 O 4 based catalysts are the most active ones for both NO oxidation and NO x reduction reactions even at high space velocity (SV) and low temperature in the fast selective catalytic reduction (SCR) process. Over Co 3 O 4 catalyst, the effects of calcination temperatures, SO 2 concentration, optimum SV for 50% conversion of NO to NO 2 were determined. Also, Co 3 O 4 based catalysts (Co 3 O 4 -WO 3 ) exhibit significantly higher conversion than all the developed DeNO x catalysts (supported/unsupported) having maximum conversion of NO x even at lower temperature and higher SV since the mixed oxide Co-W nanocomposite is formed. In case of the fast SCR, N 2 O formation over Co 3 O 4 -WO 3 catalyst is far less than that over the other catalysts but the standard SCR produces high concentration of N 2 O over all the catalysts. The effect of SO 2 concentration on NO x reduction is found to be almost negligible may be due to the presence of WO 3 that resists SO 2 oxidation.

Combinatorial Discovery and Optimization of a Complex Oxide with Water Photoelectrolysis Activity

Abstract: A ternary oxide containing cobalt, aluminum, and iron and not previously known to be active for the photoelectrolysis of water was identified using a high throughput combinatorial technique. The technique involves ink jet printing overlapping patterns of oxide precursors onto fluorine-doped tin oxide conductive glass substrates. Subsequent pyrolysis yields patterns of mixed oxide compositions that were screened for photoelectrolysis activity by scanning a laser over the material while it was immersed in an electrolyte and mapping the photocurrent response. The composition and optimum thickness for photoelectrochemical response of the newly identified material was further refined using quantitative ink jet printing. Chemical analysis of bulk and thin film samples revealed that the material contains cobalt, aluminum, and iron in a Co3O4 spinel structure with Fe and Al substituted into Co sites with a nominal stoichiometry of Co3−x−yAlxFeyO4 where x and y are about 0.18 and 0.30, respectively. The material i...

Impact of Heat Treatment and Composition of ZnO−TiO2 Nanoparticles for Photocatalytic Oxidation of an Azo Dye

Abstract: TiO2−ZnO mixed oxide nanoparticles were synthesized by precipitation of zinc nitrate and titanium tetrachloride as precursors and then heat treated using two techniques: hydrothermal and calcination. The particles were characterized using XRD, BET, UV diffuse reflectance, and ζ potential techniques. Additionally, the particles were tested for their ability to induce the photocatalytic oxidation and adsorption of Procion Red MX-5B dye. The highest photocatalytic oxidation was observed for nanoparticulate mixed oxides that were near 100% ZnO or TiO2. These materials did not have the highest specific surface areas, yet it is believed that their near-neutral ζ potentials and ideal band gap energies contributed to their higher photocatalytic activities. XRD patterns indicated that sintering and increased crystallinity occurred for the calcined TiO2 samples.

Comparative Study of Ni-based Mixed Oxide Catalyst for Carbon Dioxide Reforming of Methane

Abstract: Carbon dioxide reforming of methane (CDRM) to synthesis gas was studied over various Ni-based catalysts. It is shown that the mixed oxide supports CeO2−ZrO2, CeO2−Al2O3, and La2O3−Al2O3, prepared using surfactant, exhibit a high catalytic activity and stability for CDRM. Temperature program reduction (TPR) results demonstrate that the presence of CeO2, ZrO2, or La2O3 leads to the enhancement of the Ni reducibility compared to Al2O3, which is an important indicator of high activity and stability of these Ni catalysts for CDRM. Our thermodynamic calculations indicate that CeO2 could react with CH4 to produce synthesis gas, and then CO2 might reoxidize CeO2−x to its oxidation state. Furthermore, CeO2 might help in gasification of deposited carbon to inhibit the carbon formation and therefore improve catalyst stability. The presence of alumina tends not to affect the stability of the catalyst as well.

The effect of yttrium incorporation on the oxidation resistance of Cr–Al–N coatings

Abstract: Cr–Al–N hard coatings exhibit considerable oxidation resistance due to the formation of stable, dense α-(Al,Cr)2O3 mixed oxide scales, which makes them promising candidates for advanced machining and other high temperature applications. Here, the effect of Y on the oxidation resistance of magnetron sputtered Cr–Al–N coatings with Al/Cr ratios close to 1.2 and Y-contents of 0, 1, 2, and 4 at.%, corresponding to YN mole fractions of 0, 2, 4, and 8% is studied. The oxidation resistance is investigated under isothermal and dynamic conditions up to 1400 °C. Structure and morphology of the oxidized films are studied by X-ray diffraction and electron microscopy. Based on our results we can conclude that Y incorporation has a considerable impact on the oxidation performance of the (Cr1 − xAlx)1 − yYyN films studied. Coatings with 2 and 4 mol% YN show significantly lower weight gains during isothermal oxidation and hence exhibit a higher oxidation resistance than Y-free Cr–Al–N films. However, YN-mole fractions > 4% are found to be detrimental to the oxidation resistance, as fast growing, porous oxide scales are formed.

Gel-combustion synthesis of nanocrystalline spinel catalysts for VOCs elimination

Abstract: Gel-combustion synthesis (GCS) in self-sustaining mode was used for preparation of mixed oxide with spinel structure AB2O4 (where A = Co, Cu and B = Cr, Co). To support these mixed oxides on CeO2 and ZrO2 powders, the GCS technique in self-propagating mode was developed. The synthesized materials were characterized by IR-spectroscopy, X-ray diffraction and transmission electron microscopy. The GCS preparation leads to the formation of nanocrystalline, single-phase spinel catalysts, which showed high activity in VOCs (hexane) elimination. The best catalytic performance was obtained over copper cobaltite catalyst prepared by GCS from glycerin-chelated precursor.

Synthesis, characterisation and catalytic evaluation of iron–manganese mixed oxide pillared clay for VOC decomposition reaction

Abstract: A series of iron and manganese mixed oxide pillared clay with varying manganese to iron ratio has been successfully synthesized. Prepared materials were characterized by low angle XRD, TG/DTA, hydrogen TPR and nitrogen adsorption desorption experiment. All the materials were found to be thermally stable up to 500 °C having basal spacing ≥16.5 Ǻ. Surface area and pore volume increases with the increase in the metal uptake irrespective of manganese to iron ratio. Catalytic decomposition of acetone and trichloroethylene was studied to find out the suitability of the material for VOC decomposition reaction. Catalyst (S-16/4) having high manganese content acts as better catalyst for the acetone decomposition reaction. Time on stream experiment justifies the stability of the prepared catalyst at least for 20 h with minor change in the activity. However, in case of trichloroethylene (TCE) decomposition catalyst (S-8/12) with higher iron content shows better activity. Quick deactivation of the catalyst for TCE decomposition is noticed for all the prepared materials.

Supported and mixed oxide catalysts based on iron and titanium for the oxidative decomposition of chlorobenzene

Abstract: Iron oxide supported on titanium dioxide (Fe2O3/TiO2) and iron–titanium mixed oxide (Fe–Ti-oxide) catalysts were prepared via wetness impregnation and sol–gel methods, respectively. The catalytic activity of the two materials for the oxidation of chlorobenzene was studied and compared with the activity of pure titanium and iron oxides as well as MgO-supported iron oxide. Fe2O3/TiO2 and Fe–Ti-oxide have shown higher catalytic activities for the oxidation of chlorobenzene than the corresponding pure iron and titanium oxides at a reaction temperature of 325 °C, and this enhanced activity was more pronounce at higher temperatures. The Fe–Ti-oxide, in particular, exhibited a unique activity for the complete oxidation at relatively low temperature, 325 °C, without the formation of other chlorinated organics. Chlorine, measured by iodometric titration was the only Cl-containing product. The absence of HCl as a product and the negative effect of water suggest that the surface active sites are more likely to be Lewis acid–base sites on which chlorobenzene molecules dissociatively adsorb forming metal–Cl bonds and surface phenolate intermediates. Desorption of Cl2 from the surface and possible interaction of the aromatic ring with metal sites result in the activation of the ring forming partially oxidized intermediates involving lattice oxygen ions. Finally, reactions with molecular oxygen result in the complete oxidation to CO2 and regenerate the surface.

Structural Characterization and Catalytic Activity of Nanosized Ceria−Terbia Solid Solutions

Abstract: Structural characteristics and catalytic activity of nanosized ceria−terbia mixed oxides have been investigated using X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), ion scattering spectroscopy, temperature-programmed reduction/oxidation, and Brunauer−Emmett−Teller surface area techniques. The catalytic usefulness has been evaluated for oxygen storage-release capacity (OSC) and CO oxidation activity. The XRD and TEM results suggest that the crystallite sizes of these nano-oxides are in the range 5−12 nm within the investigated temperature range of 773−1073 K. The mixed oxide solid solutions adopted a fluorite-type structure and exhibited cell parameters with respect to Vegard’s rule. The XPS measurements revealed that both cerium and terbium are in 3+ and 4+ oxidation states, 4+ being dominant in both cases. The reduction temperature of the Ce−Tb oxide is observed to be lower than that of the pure ceria and exhibited better redox properties due to t...

A resonant photoelectron spectroscopy study of Sn(Ox) doped CeO2 catalysts

Abstract: The interaction of tin with cerium oxide thin film and cerium oxide nanosized powder was studied using XPS and resonant photoelectron spectroscopy (RPES) in the Ce 4d–4f photoabsorption region. A strong tin–ceria interaction led to partial Ce4+→ Ce3+ transition and Sn+ ion formation. A corresponding increase of the Ce 4f state occupancy was observed as a giant 4f resonance enhancement. In the case of powder catalyst a strong CeO2Sn interaction lead to formation of CeSnO mixed oxide system characterized by Ce3+ ion concentration enhancement and increase of Ce 4f occupation. It explains generally higher catalytic activity of the CeSnO mixed oxide catalysts when comparing with the individual oxides. Copyright © 2008 John Wiley & Sons, Ltd.

Cerium–manganese mixed oxides for high temperature H2S removal and activity comparisons with V–Mn, Zn–Mn, Fe–Mn sorbents

Abstract: High temperature H2S sorption activities of cerium–manganese mixed oxide sorbents containing different molar ratios of Ce/Mn and pure CeO2, prepared by the complexation technique, were investigated in a fixed bed flow reactor. Sulfur retention capacity and the H2S sorption rate of CeO2 were significantly enhanced by the incorporation of Mn into the sorbent. Cerium–manganese mixed oxide having a Ce/Mn ratio of 1/3 showed very high sorption rate. Another advantageous feature of this sorbent was its excellent regeneration properties. No deactivation of this sorbent was observed after 10 successive sulfidation–regeneration cycles. Also about 90% of the retained sulfur was converted to elemental sulfur instead of SO2, during regeneration step. The H2S sorption rate constant observed at 600 °C with the Ce1–Mn3 mixed oxide sorbent was also found to be much higher than the corresponding values for Zn–Mn, V–Mn and Fe–Mn mixed oxide sorbents prepared in this study.

Composite electrode comprising a carbon structure coated with a thin film of mixed metal oxides for electrochemical energy storage

Abstract: A composite electrode is created by forming a thin conformal coating of mixed metal oxides on a highly porous carbon structure. The highly porous carbon structure performs a role in the synthesis of the mixed oxide coating and in providing a three-dimensional, electronically conductive substrate supporting the thin coating of mixed metal oxides. The metal oxide mixture shall include two or more metal oxides. The composite electrode, a process for producing said composite electrode, an electrochemical capacitor and an electrochemical secondary (rechargeable) battery using said composite electrode are disclosed.

The Production of Ti–Mo Alloys from Mixed Oxide Precursors via the FFC Cambridge Process

Abstract: Ti-10 wt % W alloys were produced via the electrochemical deoxidation of mixed TiO2+WO3 sintered precursors in a molten CaCl2 electrolyte at 1173 K. Fully metallic samples were retrieved after 15 h of reduction. This reduction time was longer than that observed for metallization of (Ti,Mo)O-2 sintered precursors. This was believed to occur as a result of significant differences in the reduction pathway, despite tungsten and molybdenum possessing similar interactions with titanium. It was found that the reduction initiated with the rapid reduction of WO3 to a fine W-Ti particulate. TiO2 then proceeded to reduce sequentially through the lower oxides, with concurrent formation of Ca(Ti,W)O-3. Between 1 and 3 h of reduction the sample is believed to be composed of Ca(Ti,W)O-3 and TiO. A comproportionation reaction between these two phases is then observed, resulting in the formation of W-Ti and CaTi2O4. However homogenization between the product titanium and W-Ti does not take place until the titanium is sufficiently deoxidized; thus, beta-Ti forms late in the reduction process. It is believed that the late formation of beta-Ti in the reduction process, coupled with the lack of a conductive metal oxide network, accounts for the relatively slow reduction time.

Slurry photodegradation of 2,4-dichlorophenoxyacetic acid: A comparative study of impregnated and sol–gel In2O3–TiO2 mixed oxide catalysts

Abstract: The synthesis, characterization and photocatalytic properties of impregnated In 2 O 3 /TiO 2 and In 2 O 3 –TiO 2 sol–gel catalysts were reported. A specific surface area was obtained on the In 2 O 3 –TiO 2 catalysts prepared by the sol–gel method (153 m 2 /g) which was three times higher than that on the In 2 O 3 /TiO 2 impregnated catalysts (58 m 2 /g). The XRD spectra of the samples showed the anatase phase as the only crystalline phase present in both catalysts. The determination of the E g band gap by UV–vis spectroscopy showed a band gap of 3.5 eV for the In 2 O 3 –TiO 2 photocatalysts while for the In 2 O 3 /TiO 2 samples it was of 3.1 eV. The obtained photoactivity for the 2,4-dichlorophenoxyacetic acid degradation by the In 2 O 3 –TiO 2 sol–gel catalysts ( t 1/2 = 35 min) was higher than the obtained by the In 2 O 3 /TiO 2 impregnated catalysts ( t 1/2 = 88). The higher activity showed by the sol–gel photocatalysts can be related with specific surface area effects combined with those produced by the formation of highly dispersed In 2 O 3 particles; and by the insertion of some In 3+ cations in the titania framework.