Showing papers on "Mixed oxide published in 2002"

Catalytic oxidation of methane over CeO2-ZrO2 mixed oxide solid solution catalysts prepared via urea hydrolysis

Abstract: In this study, CeO 2 -ZrO 2 mixed oxide catalysts were prepared via urea hydrolysis and tested for methane oxidation. Highly uniform solid solution particles of ceria-zirconia were obtained under the conditions of this study. The incorporation of Zr into the CeO 2 lattice was found to promote the redox properties. The methane oxidation activity of the mixed oxides was found to be dependent on the Ce:Zr ratio, which relates to the degree of reducibility. It was postulated that the cubic phase, fluorite structure, which is mainly found in Ce 1 − x Zr x O 2 (where x 1 − x Zr x O 2 (where x >0.5). The catalytic activity decreased with an increasing Zr content. The mixed oxide catalyst, Ce 0.75 Zr 0.25 O 2 solid solution, was reported to exhibit the highest activity for methane oxidation. Kinetic studies of methane oxidation over such a mixed oxide catalyst (Ce 0.75 Zr 0.25 O 2 ) showed that the methane oxidation rates strongly depend on methane concentration, but only slightly on the oxygen concentrations. The Langmuir–Hinshelwood mechanism (oxygen dissociative chemisorption on the active sites and non-dissociative chemisorption of methane) can satisfactorily fit the experimental results obtained from the kinetic studies for this catalyst. The activation energy of methane oxidation is calculated based on this surface reaction mechanism as being 100.8 kJ/mol.

XPS, XANES and EXAFS investigations of CuO/ZnO/Al2O3/ZrO2 mixed oxide catalysts

Abstract: Systematic X-ray photoelectron spectroscopy (XPS), X-ray induced Auger electron spectroscopy (AES), X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) studies were undertaken to investigate the electronic structure, chemical states and local geometry of the active species in the CuO/ZnO/Al2O3/ZrO2 multicomponent mixed oxide catalysts employed in the oxidative steam reforming of methanol (OSRM) reaction for H2 production. The core level XPS and AES indicated the existence of CuO and ZnO-like species. Two kinds of zirconium species, one similar to that of ZrO2 and another with relatively higher electron density were noticed from the Zr 3d core level XPS of Zr- containing catalysts. The valence band (VB) XPS studies revealed that for Zr-containing catalysts, the Cu 3d anti-bonding orbital splits from the main VB and shifts toward lower binding energy (BE). The surface Cu/(Al + Zr) ratios were found to be close to those in the bulk while segregation of Zn at the surface was evidenced in all samples. The XANES and EXAFS results also indicated the existence of CuO and ZnO-like species, whose local environments are modified with respect to the chemical composition. The EXAFS study of the Zr-containing catalysts indicated the existence of a “Cu–O–Zr” bonding with a Cu–Zr distance in the range 3.5 to 3.9 A. The results indicated the existence of a Cu–Zr synergistic interaction in these catalysts which improved the catalytic performance in the OSRM reaction

Surface characterization of sulfate, molybdate, and tungstate promoted TiO2-ZrO2 solid acid catalysts by XPS and other techniques

Abstract: Titania–zirconia binary oxide supported sulfate, molybdate, and tungstate promoted solid acid catalysts were prepared by suspending the hydrous mixed oxide support in aqueous solutions of sulfuric acid, ammonium heptamolybdate, and ammonium metatungstate. The suspensions were refluxed at 383 K followed by evaporation of the water, drying and calcination at 1073 K. The resulting materials were characterized by means of X-ray powder diffraction, BET surface area, FT-Raman, and X-ray photoelectron spectroscopy methods. The calcined TiO 2 -ZrO 2 binary oxide, X-ray amorphous at 773 K, forms a definite crystalline ZrTiO 4 compound at 1073 K. The XRD measurements further reveal the presence of Ti 2 (SO 4 ) 3 and Zr(SO 4 ) 2 compounds in the case of SO 4 2− /TiO 2 -ZrO 2 sample and Zr(MoO 4 ) 2 and Zr(WO 4 ) 2 , respectively, in the case of molybdenum- and tungsten-oxide promoted catalysts. The FT-Raman results also support these observations. The XPS measurements show peak broadening and shift in the binding energies of O 1s, Ti 2p, and Zr 3d lines in the case of sulfated catalyst. The characterization results indicate that the impregnated sulfate ion shows a relatively strong influence on the physicochemical properties of the TiO 2 -ZrO 2 mixed oxide, which is followed by molybdate.

Surface Characterization of La2O3−TiO2 and V2O5/La2O3−TiO2 Catalysts

Abstract: The techniques of X-ray diffraction, O2 chemisorption, FT-Raman, and X-ray photoelectron spectroscopy were utilized to characterize La2O3−TiO2 composite oxide and V2O5/La2O3−TiO2 catalyst calcined at 773 and 1073 K temperatures. The investigated La2O3−TiO2 (1:5 mole ratio) mixed oxide was obtained by a homogeneous coprecipitation method with in situ generated ammonium hydroxide. A nominal 12 wt % V2O5 was impregnated over the calcined (773 K) composite oxide by adopting a wet impregnation method from ammonium metavanadate dissolved in aqueous oxalic acid solution. The characterization results suggest that the calcined La2O3−TiO2 mixed oxide primarily consists of a mixture of TiO2 anatase and La−Ti oxides. The La2O3−TiO2 also accommodates a monolayer equivalent of vanadia in a highly dispersed state. The O 1s, Ti 2p, La 3d, and V 2p photoelectron peaks of the V2O5/La2O3−TiO2 sample are highly sensitive to the calcination temperature. The XPS line shapes and the corresponding binding energies indicate that ...

Partial oxidation of methane over Ni/Mg-Al oxide catalysts prepared by solid phase crystallization method from Mg-Al hydrotalcite-like precursors

Abstract: Ni-supported catalyst was prepared by the solid phase crystallization (spc) method starting from Mg-Al hydrotalcite (HT) anionic clay as the precursor; it was tested for the partial oxidation of CH4 to synthesis gas. The precursor based on [Mg6Al2(OH)16CO32−]·4H2O was prepared by co-precipitation method from nitrates of the metal components. The precursor was then thermally decomposed and reduced to form Ni-supported catalyst (spc-Ni/Mg-Al). Ni2+ can well occupy the Mg2+ sites in the hydrotalcite resulting in the formation of highly dispersed Ni metal particles on spc-Ni/Mg-Al. The spc-Ni/Mg-Al thus prepared, showed high activity and selectivity to synthesis gas even at high space velocity. When Ni was supported by impregnating Mg-Al mixed oxide prepared from Mg-Al HT, the activity of imp-Ni/Mg-Al was higher than those of Ni/α-Al2O3 and Ni/MgO while it was close to that of spc-Ni/Mg-Al. The relatively high activity of the imp-Ni/Mg-Al may be due to the regeneration of Mg-Al HT phase from the mixed oxide during the preparation resulting in incorporation of Ni2+ on the Mg2+ sites in the HT as seen in the spc method.

Preparation, physico-chemical characterization and catalytic activity of sulphated ZrO2–TiO2 mixed oxides

Abstract: A series of ZrO 2 –TiO 2 mixed oxides was prepared varying mole fraction of titanium from 0 to 1, and characterized by XRD, FT-IR, TG/DTA, UV–VIS–DRS, surface area, surface acidity, and sulphur content. The UV–VIS–DRS and pH of point of zero charge (pH ZPC ) data for the mixed oxide samples determined by acid–base titration method revealed that a different surface was formed when zirconium and titanium were co-precipitated. XRD pattern of 700 °C calcined samples showed the formation of ZrTiO 4 phase. The optimum concentration of zirconium in the zirconia–titania mixed oxide in the precipitation solution was 60 mol% for obtaining high surface area and highly acidic mixed oxide system. Sulphur analysis of the sulphate promoted samples showed that the sample containing Ti mole fraction 0.4 retained the highest amount of sulphur. The surface acidity determined by irreversible adsorption of organic bases, such as pyridine, piperidine and 2,6-dimethyl pyridine showed the highest acidity for the above sample. This catalyst also showed the highest catalytic activity towards isopropanol dehydration.

Photoemission and in situ XRD investigations on CuCoZnAl-mixed metal oxide catalysts for the oxidative steam reforming of methanol

Abstract: A series of CuCoZnAl-multicomponent mixed oxide catalysts with various Cu/Co atomic ratios were prepared by the thermal decomposition of CuCoZnAl-hydrotalcite (HT)-like precursors at 450 °C in stat...

Preparation, characterisation and catalytic behaviour of a new TeVMoO crystalline phase

Abstract: TeMxMo1.7O mixed oxides (M = V and/or Nb; x = 0-1.7) have been prepared by calcination of the corresponding salts at 600 °C in an atmosphere of N2. A new crystalline phase, with a Te/V/Mo atomic ratio of 1/0.2-1.5/1.7, has been isolated and characterised by XRD and IR spectroscopy. This phase is observed in the TeVMo or TeVNbMo mixed oxide but not in the TeNbMo mixed oxide. The new crystalline phase shows an XRD pattern similar to Sb4Mo10O31 and probably corresponds to the M1 phase recently proposed by Aouine et al. (Chem. Commun. 1180, 2001) to be present in the active and selective MoVTeNbO catalysts. Although these catalysts present a very low activity in the propane oxidation, they are active and selective in the oxidation of propene to acrolein and/or acrylic acid. However, the product distribution depends on the catalyst composition. Acrolein or acrylic acid can be selectively obtained from propene on Nb-free or Nb-containing TeVMo catalysts, respectively. The presence of both V and Nb, in addition to Mo and Te, appears to be important in the formation of acrylic acid from propene.

Single-Crystal Particles of Mesoporous Niobium−Tantalum Mixed Oxide

Abstract: Single-crystal particles of mesoporous niobium−tantalum mixed oxide with a wormhole-like structure were successfully prepared and characterized. The mixed oxide was prepared by a neutral templating method, and the samples provided have thick walls and an unordered pore structure. Neutral block copolymer, HO(CH2CH2O)20(CH2CH(CH3)O)70(CH2CH2O)20H, and metal chlorides were used as the template and inorganic source in propanol. The crystallized mesoporous single-crystal structures were obtained by a two-step calcination process; the first calcination process removes the template, forming a mesoporous structure with amorphous wall material, and the second calcination process crystallizes the pore walls while maintaining the mesoporosity. The submicrometer mesoporous particles forming the walls were confirmed by transmission electron microscopy to be single crystals. The pore diameter was found to increase upon crystallization, while retaining the open-pore system. N2-gas adsorption−desorption analysis indicate...

In situ Raman spectroscopy for the characterization of MoVW mixed oxide catalysts

Abstract: In situ Raman spectroscopy was used to characterize the synthesis and activation steps of mixed metal oxide catalysts starting from the mixed solutions, through subsequent drying and activation procedures, to the catalytic propene partial oxidation reaction. Comparison with Raman spectra recorded for defined, well-crystallized reference oxides allowed the assignment of the spectra obtained during the catalyst preparation to certain oxides, such as Mo5O14. Especially the latter oxide phase is relevant for selective partial oxidation catalysis. The selectivity for partial oxidation products could be improved between two- and three-fold when the amount of Mo5O14-type oxide increased. This phase forms from mixtures of molybdenum and tungsten and from mixtures of molybdenum, tungsten and vanadium, but not from binary mixtures of molybdenum and vanadium with high vanadium concentrations. Tungsten and vanadium play an important role as structural promoters in the formation and stabilization of this oxide and for high catalytic activity. A resonance Raman effect was proved for reduced molybdenum oxides due to resonant coupling of the exciting laser to the IVCT transitions between fivefold-coordinated Mo5+ and sixfold-coordinated Mo6+. This resonance enhancement renders possible the in situ characterization of operating, reduced molybdenum oxide catalysts. In contrast to the Mo5O14-type phase, MoO3−x exhibits a high selectivity to total oxidation, and it even becomes re-oxidized during propene partial oxidation. These different catalytic properties of MoO3−x and the Mo5O14-type oxide led to the development of a structure–activity relationship which explains the behavior of industrial catalysts. A model is proposed on the basis of a bond order–Raman wavenumber relationship, which explains the different selectivities of these two oxides in terms of metal–oxygen bond strengths, i.e. oxygen basicity and oxygen lability, respectively. Copyright © 2002 John Wiley & Sons, Ltd.

Mesostructured silica-titania mixed oxide thin films

Abstract: Poly(ethylene glycol) containing surfactant molecules were modified with titanium alkoxides. These surfactants were used in the synthesis of mesostructured silica−titania mixed metal oxide thin films serving two different functions: (a) as a structure-directing agent and (b) as a moderator of the hydrolysis rate of the titanium alkoxide. The resulting amorphous material showed a well-ordered hexagonal mesostructure with titanium species present in a tetrahedral environment. Upon heat treatment above 500 °C, the amorphous structure converted into anatase and finally rutile.

On the Nature and Structure of a New MoVTeO Crystalline Phase

Abstract: A new MoVTeO mixed oxide has been obtained during the calcination of the corresponding catalyst precursor mixture at 600 °C in N2. The crystal structure has been studied by means of X-ray diffracti...

Processing of (La,Sr) (Ga,Mg)O3 solid electrolyte

Abstract: The preparation of La0.8Sr0.2Ga0.8Mg0.2O3 (LSGM) powders by the mixed oxide route requires significantly longer annealing times of about 60 h compared to the combustion synthesis and Pechini method which requires less than 6 h. In comparison to the mixed oxide route the soft chemical synthesis methods produce significantly smaller grain sizes after solid-state reactive sintering, which is due to the significantly shorter annealing times in order to achieve well crystallized ceramics. It is emphasized that at 1400°C in air single phase LSGM samples with the composition La0.8Sr0.2Ga0.8Mg0.2O3 could be prepared neither by the mixed oxide route nor the combustion synthesis or the Pechini method, but only at higher temperatures, for example 1500°C. Taking the results of the dilatometric studies for processing of LSGM ceramics into account, it is obviously that sinter temperatures of above 1300°C in air are required in order to prepare dense LSGM ceramics.

Vanadium-antimony mixed oxide catalysts for the selective oxidation of H2S containing excess water and ammonia

Abstract: The selective oxidation of hydrogen sulfide in the presence of excess water and ammonia was studied over V-Sb-O/TiO 2 catalyst. They showed very high conversion of H 2 S without any harmful emission of SO 2 . The investigation was focused to understand the roles of several oxides. Therefore, V 2 O 5 /SiO 2 , V 2 O 5 /TiO 2 , the mechanical mixture catalyst (V 2 O 5 +Sb 2 O 4 ) and V-Sb-O/TiO 2 are studied separately. TiO 2 support increased the activity and stability of V 2 O 5 . Sb 2 O 4 increased the reoxidation of reduced V 2 O 5 by supplying spillover oxygen through the boundaries of the metal oxides. VSbO 4 was one of the main active phase in V-Sb-O/TiO 2 mixed oxides.

Molybdenum oxide based partial oxidation catalyst: Part 3. Structural changes of a MoVW mixed oxide catalyst during activation and relation to catalytic performance in acrolein oxidation

Abstract: The activation of a Mo9V3W1.2Ox catalyst was investigated in the partial oxidation of acrolein as function of reaction temperature and atmosphere. The activity and selectivity to acrylic acid considerably increased during activation in the acrolein oxidation reaction comparable to the recently reported activation after thermal pretreatment in inert gas. The activation during the catalytic acrolein oxidation, however, proceeds at about 200 K lower temperatures as compared to the inert gas pretreatment. The initial nanocrystalline catalyst structure changed during operation in the acrolein oxidation, as shown by X -ray diffraction (XRD), scanning electron microscopy (SEM), transition electron microscopy and energy dis- persive X-ray (EDX) analysis. A (MoVW)5O14-type mixed oxide was found to be the main crystalline phase in the active and selective catalysts. Hence, this (MoVW)5O14 phase crystallizes already during catalysis at the low acrolein oxidation reaction temperatures. The evolution of the catalytic performance is directly related to this low temperature formation of the (MoVW)5O14 phase. It is suggested that this (MoVW)5O14 phase has to be present in a detected, specific ordering state which is vital for optimum selective oxidation properties. In addition, other minority phases were identified by transmission electron microscopy (TEM) in the operating catalyst. The so-called bundle-type, and a new corona-type texture was detected, which show ordering in only one or two dimensions, respectively. These disordered structures are also relevant candidates for active catalyst phases as they are detected during the activation period of the MoVW mixed oxide.

Catalyst for lowering the amount of nitrogen oxides in the exhaust gas from lean burn engines

Abstract: The invention relates to a catalyst for lowering the amount of NO x in the lean exhaust gas from lean burn engines, comprising active aluminum oxide, magnesium oxide and at least one noble metal of the platinum group of the periodic table of the elements, as well as at least one nitrogen oxide storage material. The catalyst is characterized in that the magnesium oxide forms a homogeneous mixed oxide with aluminum oxide and is present in a concentration of about 1 to about 40 wt- %, based on the total weight of the mixed oxide.

Nanoparticles having a rutile-like crystalline phase and method of preparing the same

Abstract: Nanometer-sized particles comprise a mixed oxide of titanium and antimony and are characterized by rutile-like crystal phases The particles are easily prepared by hydrothemal processing, and may be used as colloids, or in various compositions and articles

Non-hydrolytic sol–gel routes based on alkyl halide elimination: toward better mixed oxide catalysts and new supports: Application to the preparation of a SiO2–TiO2 epoxidation catalyst

Abstract: The first part of this article is a concise review on non-hydrolytic sol-gel routes based on alkyl halide elimination. The molecular chemistry founding these routes is presented, as well as examples of applications to the synthesis of oxides and mixed oxides that may be of interest as catalysts or supports. In the second part, the straightforward non-hydrolytic preparation of a SiO2 –TiO2catalyst (not involving supercritical drying) is reported. The resulting SiO2–TiO2 calcined xerogel was highly homogeneous and displayed a high BET surface area located mainly in the mesopore region. Preliminary tests in the epoxidation of cyclohexene by cumene by hydroperoxide indicated good selectivity and activity.

Degradation of thermal barrier coatings due to thermal cycling up to 1150°C

Abstract: The degradation of thermal barrier coatings (TBCs) due to thermal cycling up to 1150°C has been studied. During thermal cycling, the bond coat in the TBCs was oxidised to form an alumina and a mixed oxide layer between the top coat of yttria stabilised zirconia (YSZ) and the bond coat of MCrAlY alloy. The mixed oxide layer mainly consists of α-Cr2O3 and (Ni,Co)(Cr,Al)2O4 spinel phases, which are formed above the α-alumina layer. Interestingly, the alumina layer gradually disappeared during the oxidation while the content of chromium in the mixed oxide increased with increasing oxidation time. As the oxidation accelerated after the disappearance of the alumina layer, cracks initiated and propagated in the mixed oxide layer near the YSZ. Eventually, the crack propagation induced the spallation of some YSZ top coatings after the 2000 h oxidation.