Showing papers on "Mixed oxide published in 1998"

SnO2–TiO2 solid solutions for gas sensors

Abstract: Mixed oxide gas sensors of SnO 2 –TiO 2 have been prepared in the form of polycrystalline ceramics and rf-sputtered thin films. The influence of the chemical and phase composition on the crystallographic structure, microstructure and sensor performance is discussed. The measurements of the electrical resistance as a function of hydrogen partial pressure have been performed with air or argon as a reference gas. The results are analyzed in terms of the surface and bulk interaction models. It is proposed that hydrogen detection in air involves the preadsorbed O − species. Hydrogen interaction with the SnO 2 –TiO 2 system in Ar atmosphere is governed by bulk diffusion of oxygen vacancies.

Chemistry of a Chromate Conversion Coating on Aluminum Alloy AA2024‐T3 Probed by Vibrational Spectroscopy

Abstract: Infrared and Raman spectroscopy were used to characterize a chromate conversion coating (CCC) on 2024-T3 aluminum aircraft alloy with a long range objective of determining the anticorrosion mechanism of the CCC. Spectra were compared to those from synthetic mixed oxides of aluminum, Cr(III), and Cr(VI) made by treating pure reagents with NaOH. The Fourier transform infrared (FTIR) and Raman spectra of the CCC showed similar behavior to the chromium III/VI mixed oxide for both the initial materials and after heat-treatment. Analysis of the CCC and chromium mixed oxide by UV-vis spectroscopy indicated that both have a 3:1 ratio of Cr(III) to Cr(VI). When the chromium mixed oxide was immersed in pH 4 HNO 3 , the ratio of released H + to released Cr(VI) ranged from 0.98 to 1.07. In addition, a compound with a Raman spectrum very similar to that of a CCC was formed by a reaction of Cr(III) hydroxide with Cr 2 O 7 -2 , CrO 4 -2 , or the Alodine chromating bath. The results indicate a strong structural similarity between the Cr-mixed oxide and a major component of the CCC. A likely structure for this common material involves covalent bonding between polymeric Cr(III) hydroxide and Cr 2 O 7 -2 or CrO 4 2 , This mixed oxide structure may hydrolyze to release H + and soluble chromate.

Catalytic combustion of diesel soot particles. Activity and characterization of Co/MgO and Co,K/MgO catalysts

Abstract: The catalytic combustion of diesel soot particles was studied on Co/MgO (12 wt% Co) and potassium-promoted Co/MgO (1.5 wt% K) that were calcined at different temperatures in the 300 to 700°C range. Catalyst samples were characterized by various techniques including nitrogen adsorption (BET), temperature programmed reduction (TPR),X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), electron spin resonance (ESR),X-ray photoelectron spectroscopy (XPS) and temperature programmed oxidation (TPO). As observed by TPO experiments, the catalyst activity depends strongly on the calcination temperature: calcination at 300 and 400°C produced samples that were much more active than those calcined at higher temperatures, on which an inactive Mg Co mixed oxide is formed, as suggested by TPR, ESR and XRD results. FTIR shows carbonate species on the surface. Unpromoted samples seem to correlate their activity with the amount of reducible Co species present. Potassium not only increased the sample activity, probably due to the improvement in surface mobility, but also enhanced stability at high temperatures. Experiments with different soot to catalyst ratios showed no significant variation in combustion temperature. TheK-promoted catalyst burns off soot at a temperature lower than the one needed for calcination, thus proving to be a promising catalyst.

Transition metal-aluminate catalysts for NO reduction by C3H6

Abstract: Transition metal-aluminum mixed oxide catalysts (Cu, Ni and Co–Al 2 O 3 ), with spinel-type structure, were prepared by coprecipitation method. These catalysts showed high activity and selectivity for the selective reduction of NO by C 3 H 6 in excess oxygen, and showed moderate activity even in the presence of 10% water. Among the catalysts tested, Cu–Al 2 O 3 with 16 wt.% Cu content exhibited the highest activity at low temperature, though a bend over of NO conversion was significant at high level of O 2 concentration. At low level of O 2 concentration, Cu–Al 2 O 3 exhibited higher activity than Cu-ZSM-5 below 623 K. In addition, Cu–Al 2 O 3 showed higher hydrothermal stability than Cu-ZSM-5. The high activity and selectivity of M-Al 2 O 3 were attributed to the surface aluminate phase, containing highly dispersed transition metal cations in Al 2 O 3 matrix.

Spectroscopic Characterization of Sol−Gel-Derived Mixed Oxides

Abstract: Al2O3-, TiO2-, SiO2-, and ZrO2-based mixed-oxide supports were synthesized by sol−gel techniques using 2,4-pentanedione as the complexing/templating agent. These materials have been investigated by employing nitrogen adsorption, 27Al, 29Si, and 1H solid-state magic-angle spinning nuclear magnetic resonance (MAS NMR), diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS), and Fourier transform laser Raman spectroscopy (FT-Raman). Nitrogen adsorption isotherms of the mixed oxides Al2O3−SiO2 and TiO2−SiO2 were of type IV, with hysteresis loops of type I, indicating well-defined pore structure, while other mixed oxides Al2O3−TiO2, Al2O3−ZrO2, TiO2−ZrO2, and ZrO2−SiO2 exhibited type II hysteresis loops, indicating pores of undefined nature. 27Al MAS NMR studies on the alumina-based mixed oxides showed the presence of four-, five-, and six-coordinated aluminum species, the number of four-coordinated framework aluminum sites being highest in the Al2O3−SiO2 mixed oxide. 29Si CP MAS NMR spectra of t...

Reaction kinetics of benzylation of benzene with benzyl chloride on sulfate-treated metal oxide catalysts

Abstract: Sulfate-promoted metal oxides of ZrO2 (SZ) and Fe2O3 (SF) and the mixed oxide ZrO2−Fe2O3 (SZF) have been prepared by coprecipitation and wet impregnation techniques. The synergistic effect of the dopant metal oxide in the matrix of the parent sulfated oxide, SO42-/ZrO2 (SZ), has been studied for the alkylation reaction of benzene with benzyl chloride. An attempt has been made to decrease the polyalkylation observed on the highly reactive catalyst, SO42-/Fe2O3(SF). Detailed kinetic study of the reaction has been carried out to evaluate the kinetic parameters.

Highly dispersed phosphate supported in a binary silica-titania matrix : Preparation and characterization

Abstract: Titanium phosphate supported in a mixed oxide matrix was prepared by reacting SiO2/TiO2, obtained from the sol−gel process, with phosphoric acid. HPO42- is the species present in the matrix, and it is immobilized by the Ti−O−P bond on the surface. The titanium phosphate is highly and homogeneously dispersed in the SiO2/TiO2 matrix, with the domain of titanium oxide particles about 1 nm, estimated from the UV absorption band threshold. The specific surface areas, determined by the BET multipoint method, changed between 397 and 755 m2 g-1 and the average pore volumes between 0.38 and 0.63 mL g-1 for samples prepared with different contents of titanium phosphate. The titanium hydrogenphosphate is thermally very stable and showed a high proton exchange capacity between 0.50 and 1.10 mmol g-1.

NO reduction by H2 over perovskite-like mixed oxides

Abstract: Perovskite-like mixed oxides La0.9A0.1A′BO3+δ (A′=Ce or Eu; B=Mn or Co) and La0.8Sr0.2BO3+δ (B=Mn, Fe, Co or Ni) prepared by the amorphous citrate method were used as catalysts for NO reduction by hydrogen. XRD patterns showed a fully crystalline perovskitic structure only in the case of Ce- and Eu-substituted samples. The results suggest that the presence of structural defects is important for the activity of these catalysts, as shown by pretreatment under different atmospheres (He and O2). La0.9Ce0.1CoO3+δ was the most active of the mixed oxide catalysts investigated and its activity was influenced by the presence of both anion vacancies and Co2+ species. La0.8Sr0.2NiO3+δ showed a particular catalytic behaviour, attributed to surface Ni reduction.

Limitations of V-Sb-W and Mo-V-Nb-Te mixed oxides in catalyzing propane ammoxidation

Abstract: Testing of several catalysts proposed in the literature revealed that the activity and selectivity of Mo–V–Nb–Te mixed oxides for the ammoxidation of propane compares favourably with the activity and selectivity of V–Sb, Bi–Mo, and VPO mixed oxides under oxidizing reaction conditions. In addition to a higher acrylonitrile yield, the required reaction temperature is also lower. Reactor temperature, feed composition, and catalyst composition were optimized. Acrylonitrile yields of >50% were obtained with Mo–V–Nb–Te mixed oxide catalysts using a feed consisting of propane, ammonia, and oxygen in a ratio of 1 : 1.2 : 3. Data on the decomposition of ammonia and the combustion of propane over Mo–V–Nb–Te, V–Sb, and Bi–Mo mixed oxides revealed that the ammoxidation of propane is limited by propane activation for Bi–Mo mixed oxide and by ammonia activation for V–Sb mixed oxide. The high activity and selectivity of Mo–V–Nb–Te mixed oxides was attributed to its ability to activate both, propane and ammonia at low temperatures.

Perovskite oxides in catalysis: Past, present and future

Abstract: The application of perovskite oxides in catalysis is reviewed. Selective and total oxidation, nitrogen oxides decomposition and reduction as well as other miscellaneous uses of mixed oxide catalysts are discussed. A final section describes the most promising areas of research to increase the number of commercial uses of perovskite oxides.

The influence of surface structure on the catalytic activity of cerium promoted copper oxide catalysts on alumina: oxidation of carbon monoxide and methane

Abstract: X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) have been used to characterize a series of Cu/Ce/Al2O3 catalysts. Catalysts were prepared by incipient wetness impregnation using metal nitrate and alkoxide precursors. Catalyst loadings were held constant at 12 wt% CuO and 5.1 wt% CeO2. Mixed oxide catalysts were prepared by impregnation of cerium first, followed by copper. The information obtained from surface and bulk characterization has been correlated with CO and CH4 oxidation activity of the catalysts. Cu/Al2O3 catalysts prepared using Cu(II) nitrate (CuN) and Cu(II) ethoxide (CuA) precursors consist of a mixture of copper surface phase and crystalline CuO. The CuA catalyst shows higher dispersion, less crystalline CuO phase, and lower oxidation activity for CO and CH4 than the CuN catalyst. For Cu/Ce/Al2O3 catalysts, Ce has little effect on the dispersion and crystallinity of the copper species. However, Cu impregnation decreases the Ce dispersion and increases the amount of crystalline CeO2 present in the catalysts, particularly in Ce modified alumina prepared using cerium alkoxide precursor (CeA). Cerium addition dramatically increases the CO oxidation activity, however, it has little effect on CH4 oxidation.

Catalyst composition containing an intimately mixed oxide of cerium and praseodymium

Abstract: A catalyst suitable for the treatment of automotive engine exhaust exhibiting enhanced oxygen storage capacity The catalyst composition contains, in addition to a catalytic material such as one or more of platinum, rhodium and palladium dispersed on an activated alumina support, an oxygen storage component ("OSC") which is an intimately mixed oxide of ceria and praseodymia having a PrCe atomic ratio in the range of about 2:100 to 100:100 and optionally containing one or more other rare earth metal oxides The OSC is segregated from the catalytic material and may optionally have dispersed thereon a limited amount of a second catalytic metal component, such as platinum, rhodium and/or palladium The intimately mixed oxide may be formed by co-precipitation, by impregnation of bulk ceria particles with a praseodymium precursor, or any other method to form an intimately mixed oxide The OSC may comprise at least about 5 % by weight of the combined weight of the OSC and the catalytic component

Dynamic States of V2O5 Supported on SnO2/SiO2 and CeO2/SiO2 Mixed-Oxide Catalysts during Methanol Oxidation

Abstract: A series of V2O5 supported on the SnO2/SiO2 and CeO2/SiO2 mixed oxides were investigated during methanol oxidation by in situ Raman spectroscopy, and the catalytic properties of these catalysts were probed by methanol oxidation kinetic studies. The Raman studies revealed that tin oxide forms a surface SnOx overlayer on the silica surface owing to the absence of Raman features of the SnO2 crystallite, but cerium oxide forms bulk CeO2 particles on the silica surface. The impregnated vanadium oxide formed a surface vanadia overlayer on all the catalysts owing to the absence of V2O5 crystallite Raman features. In situ Raman studies of the V2O5/SnO2/SiO2 and V2O5/CeO2/SiO2 catalysts during methanol oxidation indicate that the formation of the VOx−SnOx and VOx−CeO2 interactions totally blocks the formation of surface V−OCH3 groups, which are observed in the V2O5/SiO2 catalysts. The interaction between the surface VOx and the surface SnOx overlayer on silica increases the methanol oxidation reactivity by 1−2 ord...

Transition metal oxide pillared clay: 5. Synthesis, characterisation and catalytic activity of iron–chromium mixed oxide pillared montmorillonite

Abstract: A mixed iron–chromium pillared montmorillonite was prepared using the trinuclear acetato complexes of iron and chromium as the pillaring agents. By changing the iron to chromium ratio in the complex solution it was possible to change the metal ratio in the pillared material. The samples were characterised by low angle XRD, TG-DSC, and nitrogen adsorption–desorption experiments. All the materials are stable up to 773 K with the basal spacing of ≥16.8 A. Catalytic activity values of the materials are evaluated using methanol and cumene conversion reaction as probe reactions. For methanol conversion reaction, with increasing iron content in the samples, hydrocarbon selectivity decreases, while the selectivity of decomposition product (CO+CH4) increases. Samples calcined at 773 K can be used as catalysts for hydrocarbon dehydrogenation reaction with high selectivity. The iron to chromium ratio seems to controls the dehydrogenation selectivity.

Methane Dry Reforming over Well-dispersed Ni Catalyst Prepared from Perovskite-type Mixed Oxides

Abstract: Methane dry reforming with carbon dioxide was performed using perovskite-type mixed oxide catalysts, La 1-x Sr x NiO 3 (x = 0, 0.1) and La 2-x Sr x NiO 4 (x = 0-1) at 600-900°C and atmospheric pressure. Catalytic activity and resistance against coke formation were varied with the type of perovskite and the amount of Sr-substitution. LaNiO 3 showed high activity without coke formation while La 2 NiO 4 did not show any activity. Among Sr-substituted catalysts, La 0.9 Sr 0.1 NiO 3 and La 1.8 Sr 0.2 NiO 4 had the highest catalytic activity without coke formation. For these catalysts, initially the catalytic activity increased with time and then reached a steady state. The XRD spectra of used catalysts showed that catalysts were transformed to mixed phases of La 2 O 2 CO 3 and SrCO 3 with highly dispersed Ni metal during the reaction. This transformation could be caused by the removal of lattice oxygen by Sr-substitution, which was promoted by the reducing atmosphere of the reaction (CH 4 /CO 2 = 1). The enhanced catalytic activity of Sr-substituted perovskite could come from dual active sites, La 2 O 3 for CO 2 adsorption and Ni for CH 4 activation.