Al2O3 Mixed Oxides Research Articles

High-Temperature NO x Storage and Sulfur-Resistance of the Lithium-Based Lean-Burn NO x Trap Catalyst Pt/Li/TiO2–Al2O3

A series of lithium-based lean-burn NO x trap catalysts Pt/Li/TiO2–Al2O3 were prepared by sequential impregnation. The doping of TiO2 into the support Al2O3 significantly enhances the sulfur-resistance performance of the catalyst Pt/Li/Al2O3. On TiO2–Al2O3 mixed oxides, the Pt and lithium species are more highly dispersed, giving higher NO x storage capacity, as compared with those on Al2O3 and TiO2. In situ DRIFTS reveals that the NO x storage on Pt/Li/TiO2–Al2O3 mainly proceeds on –OLi sites forming bidentate nitrate species at 500 °C. The catalyst Pt/Li/TiO2–Al2O3 with 40% TiO2 in the support is the most promising one applicable to the lean-burn NO x abatement. The sulfated NSR catalyst Pt/Li/TA(40) is easier to reduce than the sulfated Pt/Ba/TA(40), showing better regeneration ability.

Electronic theory study on high temperature oxidation mechanism of Nb-Ti-Al alloy

The electronic structure parameters of density of electronic states, atoms embedding energy, affinity energy and cluster energy of Nb alloy have been calculated using the recursive method. The high-temperature oxidation mechanism of Nb alloy was investigated. Studies show that the oxygen adsorption on Nb alloy surface can lower the adsorption energy, so oxygen is easily adsorbed on the alloy surface, and gradually diffuses into the Nb alloy matrix. Oxygen has a high solubility in the Nb alloy matrix because of the negative atom embedding energy, and it has similar density of states to Nb. Because the atom embedding energies of Ti and Al in the alloy matrix are higher than that on the alloy surface, Ti and Al atoms diffuse from the alloy matrix to the alloy surface and segregate on the alloy surface, ultimately making the Nb surface rich in Ti and Al. The clustering energy calculation shows that Ti and Al atom tend to gather on their own area, forming Ti and Al atom clusters respectively. Oxygen can interact with Nb, Ti and Al because of the negative affinity energy with Nb, Ti and Al on Nb alloy surface, to generate the Nb2O5, TiO2 and Al2O3 mixed oxide film which has protective effect on Nb alloy.

A Rapid Microwave-Induced Solution Combustion Synthesis of Ceria-Based Mixed Oxides for Catalytic Applications

We have been exploring the utilization of a simple and fast microwave-induced solution combustion synthesis technique for the preparation of various ceria-based mixed oxides for different catalytic applications. In our comprehensive investigation, CeO2–SiO2 (MWCS), CeO2–TiO2 (MWCT), CeO2–ZrO2 (MWCZ) and CeO2–Al2O3 (MWCA) mixed oxides were synthesized by solution combustion synthesis method using microwave dielectric heating and employed for CO and soot oxidation applications. The intricate relationship between ceria and other supporting oxides has been explored with the help of various analytical techniques namely, X-ray diffraction (XRD), temperature programmed reduction/oxidation (TPR/TPO), temperature programmed desorption (TPD) of ammonia and CO2, Raman spectroscopy (RS), UV–vis diffuse reflectance spectroscopy (UV–vis DRS), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), BET surface area and thermogravimetry analysis (TGA) methods. XRD results revealed amorphous nature of the material in case of ceria-silica mixed oxide and formation of a specific cubic fluorite type Ce0.5Zr0.5O2 solid solution in the case of ceria-zirconia mixed oxide. Ceria-titania and ceria-alumina mixed oxides exhibited diffraction lines only due to crystalline ceria. Zirconia-based mixed oxide exhibited a lower reduction temperature and better redox properties compared to other samples. TPD of ammonia and CO2 results revealed superior acid–base properties for MWCS mixed oxide. TGA measurements indicated a complete combustion in all preparations. RS results suggested defective structure of mixed oxides resulting in the formation of oxygen vacancies. XPS results revealed that ceria-zirconia mixed oxide contained more Ce3+ compared to other oxides. Among all the mixed oxides, the MWCZ sample exhibited a higher oxygen storage capacity, and better CO and soot oxidation activities. All these interesting findings have been elaborated in this publication.

MgO–Al2O3 Mixed Oxides-Supported Co–Mo-Based Catalysts for High-Temperature Water–Gas Shift Reaction

MgO–Al2O3 mixed oxides were prepared and used as the support of Co–Mo-based water–gas shift reaction (WGSR) catalysts. X-ray diffraction (XRD) characterization showed that the MgO–Al2O3 mixed oxides support is composed of MgO, γ-Al2O3, and magnesia–alumina spinel. The MgO–Al2O3 mixed oxides-supported Co–Mo-based catalysts exhibited high shift activity at high temperature (360–450 °C) and high stability. The addition of potassium enhanced the activities but affected adversely the stabilities of Co–Mo-based catalysts. ESR characterization shows that the Mo5+ species are not connected with the WGSR activity. Magnesium in the support may be closely related with the formation of formate species intermediate for the WGSR.

Synthesis of macroporous ZrO2–Al2O3 mixed oxides with mesoporous walls, using polystyrene spheres as template

Macroporous ZrO2–Al2O3 mixed oxides with mesoporous walls were synthesized. The three-dimensional interconnected macroporous structures, of inorganic zirconia–alumina mixed oxides containing different alumina compositions (25, 50, 100 wt%), were prepared by sol–gel method from inorganic precursors and using polystyrene microspheres with diameters of 685 and 1520 nm as templates. The final porous arrays with controllable pore size in the submicrometer range could be obtained by calcination of the organic template. The structural characteristics are discussed. The physicochemical characterization of the samples was carried out by N2 physisorption (SBET), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The shrinkage of pore diameter was approximately 35%, and the wall thickness of inorganic framework varied between 135 and 154 nm. The specific surface areas, of the samples, were between 123 and 287 m2/g, obtaining the largest surface area with the highest alumina contents and the smallest templates.

Study of the citral/acetone reaction on MgyAlOx oxides: Effect of the chemical composition on catalyst activity, selectivity and stability

The liquid-phase citral/acetone reaction was studied on MgO, Al2O3 and MgyAlOx mixed oxides with Mg/Al molar ratios of 0.11–3. The density, strength and nature of surface acid sites were determined by NH3 thermodesorption and FTIR of pyridine. The surface basic properties were probed by temperature-programmed desorption of CO2. The rate of the citral/acetone cross-aldolization to pseudoionones increased linearly with the density of strong O2− base sites. Thus, the pseudoionone synthesis was efficiently promoted on MgO and Mg-rich MgyAlOx samples. In contrast, the initial rate of the parallel acetone self-condensation reaction increased with the density of Lewis acid sites, and diacetone alcohol was more rapidly formed on Al2O3 and Al-rich MgyAlOx samples. Differences in the reactant adsorption strength on surface Al3+ sites may explain the observed differences between the acid-catalyzed mechanisms of cross- and self-aldolization reactions. In situ catalyst deactivation was determined by performing two consecutive catalytic runs. The activity decay of MgyAlOx samples during the citral/acetone reaction was between 15 and 20%, irrespective of the catalyst composition.

Enhanced Activity of Spinel-type Ga2O3–Al2O3 Mixed Oxide for the Dehydrogenation of Propane in the Presence of CO2

Catalytic performance of a series of Ga2O3–Al2O3 mixed oxides prepared by alcoholic-coprecipitation method for the dehydrogenation of propane in the presence of CO2 was investigated. It is shown that the combination of Ga and Al oxides greatly improved the performance of the Ga2O3-based materials for catalytic dehydrogenation of propane, with the highest performance attainable at a Ga2O3–Al2O3 catalyst with a 20 mol% aluminum content. While the same tendency was observed for the specific activity normalized by BET surface area, significantly enhanced stability was achieved for Ga2O3–Al2O3 with higher aluminum content. X-ray diffraction (XRD) revealed that a homogeneous spinel-type Ga2O3–Al2O3 solid solution is uniformly formed by substitution of Ga3+ for Al3+ in the Al2O3 lattice. The enhanced activity of Ga2O3–Al2O3 mixed oxides was accounted for by the abundance of surface weak acid sites due to the synergetic interaction between Ga2O3 and Al2O3 in the solid solution systems.

Highly active NiMo/TiO2–Al2O3 catalysts: Influence of the preparation and the activation conditions on the catalytic activity

NiMo/TiO2–Al2O3 catalysts were prepared over a TiO2–Al2O3 mixed oxide (5% of Al2O3) by impregnation with nickel and molybdenum salts precursors at different pH solution and the catalysts have been activated by several calcination–sulfidation procedures. Evaluation of the catalysts in tetraline hydrogenation under pressure indicated that the sample obtained after impregnation under basic condition and sulfided without any previous calcination presents the highest activity. Characterizations of the samples by Raman and UV–vis diffuse reflectance spectroscopies, XPS and HRTEM allowed to follow the evolution of the Ni and Mo species along the different steps of the preparation and to understand how to get a high promoting effect of Ni over such a TiO2 rich support.

Development of a Tar Reforming Catalyst for Integration in a Ceramic Filter Element and Use in Hot Gas Cleaning

The development of a suitable tar reforming catalyst for integration in a ceramic hot gas filter element is presented. Calcined dolomite, CaO−Al2O3 mixed oxide, and MgO were used as catalyst suppor...

Mixed silica and alumina hosted carboxylate oxide for removal of chromium species from wastewater

Increasing in the use of various chromium species over the past few decades has inevitably resulted in an increased flux of their metallic substances in aquatic life. Therefore, experimental work was developed to improve the physical and chemical characteristics of the prepared mesoporous SiO2–Al2O3 mixed oxides via a sol–gel process. The effective key parameter on the properties of the materials was achieved by the addition of carboxylate functional group such as Alph-hydroxyl isobuteric acid (AHIBA) during the gelation process and prior the calcinations step. Better performance of these carboxylate resins are attributed to the complexing ability of the carboxylate groups toward polyvalent chromium species which is added to the normal sorption properties of the oxides. Urea has been used as a means of producing an aluminum precipitate that is easier to filter than that produced by rapid addition of base. Introduction of Al and Si salts together to form mixed oxide modified the way of their substitution in the hosting resin than when they were separately added. The data revealed that mesoporous materials with very narrow pore diameter distribution (micron-sized) and high surface area were obtained. The surface areas and pore size distributions were mainly depend on SiO2 content. Systematic investigations were carried out on the set of the prepared mixed oxides with different porosity, cross-linking degree and exchange loading to find out the best sorbent for chromium removal. The final objective in all cases was aimed at determining feasible and economically reliable solution to the environmental problems related to the tannery industry.

Effect of modification by alkali on theγ-Ga2O3–Al2O3 mixed oxides prepared by the solvothermal method

The solvothermal reaction of the mixtures of aluminium triisopropoxide (AIP) and gallium triacetylacetonateat 300 °C directlyyielded γ-Ga2O3–Al2O3 mixed oxides(solid solutions, but Ga3+ ions preferentially occupied the tetrahedral sites of the defect spinelstructure). The reaction of AIP alone in 2-(methylamino)ethanol yieldedγ-Al2O3, and the reaction of the mixture of AIP andGa(acac)3 yielded γ-Ga2O3–Al2O3 mixedoxides having an Al ratio identical to the charged ratio. Although the reaction of AIP alone in tolueneat 315 °C yielded χ-Al2O3, thereaction at 300 °C gave an amorphous product, while the reaction ofGa(acac)3 aloneafforded γ-Ga2O3 at 300 °C. The reaction of the mixtures of the two starting materials yielded theγ-type mixed oxide even with extremely high charged ratio ofAl. It was concluded that, in toluene, the initial formation ofγ-Ga2O3 nuclei controlled the crystal structure of the mixed oxide. The mixed oxidecatalysts with lower acid density showed lower methane consumption bycombustion in selective catalytic reduction of NO with methane. By alkalimodification of the catalyst, the acid density of the mixed oxide decreased, and theCH4 combustion rate was diminished. Although NO conversion activity was also decreased bythe modification, the efficiency of methane used for NO reduction was improved.

Metal oxide catalysts for DME steam reforming: Ga2O3 and Ga2O3?Al2O3 catalysts

The steam reforming of dimethyl ether (DME) was performed on Ga2O3–Al2O3 mixed oxides prepared by sol–gel method. Ga2O3 significantly affects the catalytic performance with respect to the DME conversion and H2 yield. The catalytic activity increases with the Ga concentration in Ga2O3–Al2O3 mixed oxides. It is very interesting that without the aid of an additional transition metal component, Ga2O3 and Ga2O3 containing Al2O3 mixed oxide system exhibit good activity in the reforming reaction. To the best of our knowledge, this is the first report that reveals the reforming ability of Ga2O3 for the production of H2 from DME and/or methanol.

Temperature-programmed reduction and acidic properties of molybdenum supported on MgO–Al2O3 and their correlation with catalytic activity

A series of catalyst with Mo loading from 2 to 14 wt.% on MgO–Al2O3 mixed oxide support was prepared by incipient wetness impregnation method. Pure MgO and Al2O3 were prepared by using Mg(NO3)2·6H2O and Al(NO3)3·9H2O salt solutions and urea as hydrolyzing agent. MgO–Al2O3 (1:1) mixed oxide was prepared by co-precipitation of appropriate quantities of magnesium nitrate and aluminum nitrate salts. BET surface area, pore volume, pore size distribution, TPR, acidity and acid strength distributions of catalysts and supports were measured. MgO–Al2O3 (1:1) mixed oxide showed a biomodal pore size distribution. LTOC and TPR results show that an optimum 8% Mo on MgO–Al2O3 is sufficient to form MoO3 monolayer. TPR technique can also be used as a vital tool to determine the monolayer coverage. HDS activity has been correlated with LTOC. Introduction of basic MgO in the lattice of Al2O3 moderates the strong acidity in MgO–Al2O3. Impregnation of acidic MoO3 species further enhances the acidity favorable for HYC.

SUPPORT AND CoPcTS EFFECTS ON THE CATALYTIC ACTIVITY AND PROPERTIES OF MOLYBDENUM SULFIDE CATALYSTS

The effects of the support and Cobalt tetrasulfophthalocyanine (CoPcTS) on the catalytic activity of CoMo and NiMo catalysts were investigated. The TiO2–Al2O3, SiO2–Al2O3, ZrO2–Al2O3 mixed oxide carriers were prepared by impregnating Al2O3 with an ethanol solution of tetrabutyl titanate, tetraethyl orthosilicate, zirconium oxychloride, respectively. BET measurements showed that no large decrease of surface area and average pore size for the mixed oxide carrier. TPR data revealed that support affects the reduction temp. of molybdenum oxide, the reduction temp. was in the following order, TiO2–Al2O3<Al2O3<ZrO2–Al2O3<SiO2–Al2O3. The catalytic activity measurements showed the activities for thiophene were: TiO2–Al2O3> Al2O3>ZrO2–Al2O3>SiO2–Al2O3. CoPcTS can increase the HDS and HYD catalytic activities when supported on CoMo/Al2O3, while on CoMo/TiO2–Al2O3 and NiMo/TiO2–Al2O3 decrease the HDS and HYD activities.

Preparation of xTiO2.(1-x)Al2O3 catalytic supports by the sol-gel method : physical and structural characterisation

A series of xTiO2·(1 − x)Al2O3 mixed oxides has been prepared by the sol-gel method with variable amounts of TiO2, from pure alumina to pure titania. Textural, bulk and surface characterisation of the samples has been carried out by nitrogen physisorption (SBET, porosity), thermal analysis (TGA, DTA), X-ray diffraction (XRD), zero point charge (ZPC) and surface acidity. The textural results show that at low titania contents, higher surface areas than those of pure alumina are obtained, and in the titania-rich samples, higher surface areas than for pure titania are stabilised. The titanium content also affects the crystallization process. Furthermore, the strength and distribution of the acid sites may be varied by changing the composition of the mixed oxide.

Solid catalysts for the production of fine chemicals: the use of ALPON and hydrotalcite base catalysts for the synthesis of arylsulfones

α#x2010;phenylsulfonylcinnamononitrile and derivatives as well as α-phenylsulfonylchalcone have been obtained by condensation of phenylsulfonylacetonitrile and phenylsulfonylacetophenone with benzaldehyde and 4-substituted benzaldehydes, using solid base catalysts with different basic strengths. Cs-exchanged X zeolite showed low activity for the above reactions, while high-surface-area MgO and MgO–Al2O3 mixed oxides derived from hydrotalcite were active and selective catalysts. An optimum basicity was observed with MgO–Al2O3 with the atomic ratio Al/(Al+Mg) between 0 and 0.20, showing the increase in basicity when introducing Al in the MgO structure. Nitridation of a high-surface-area aluminophosphate gives active and selective catalysts for the above reactions. The activity per unit surface area for the less demanding reaction between sulfonylacetonitrile and benzaldehyde is directly proportional to the level of nitridation. However, formation of α-phenylsulfonylchalcone, which is a more demanding reaction, does not increase linearly but goes through a maximum with the nitrogen content of the solid, showing the heterogeneity of active sites on ALPONs and the decrease in the activity coefficient of the basic sites when the density of N atoms increases.

Organic polymers as pore-regulating agents in TiO2–Al2O3 mixed oxide catalytic supports

The synthesis of TiO2–Al2O3 mixed oxide catalytic supports with a TiO2/(TiO2+Al2O3) molar ratio equal to 0.5, was made by the co-precipitation of the corresponding metallic isopropoxides using different organic polymers as pore-regulating agents. The influence of the preparation parameters (type of polymeric additive, its amount, molecular weight and method of additive incorporation into the hydroxide precipitate) on the surface area and pore structure of the final solid were studied. It was found that organic polymers added during the hydrolysis step, in general, resulted in a significant increase in surface area and total pore volume. In all cases the pore-size distribution becomes clearly bimodal with two maxima in the pore-size distribution curve around 3 and 30–50 nm. In order to investigate the action mechanism of the polymeric additives, characterizations by Fourier transform-infrared, thermogravimetric/differential thermogravimetry and differential thermal analysis, as well as a TEM study of the hydroxide precipitates before calcination, were carried out. The results from these characterizations indicate that the textural changes induced by the polymeric additives can be rationalized in terms of a “filler” effect in the case where the additive is incorporated during the filtration step, and that when the additive is added during the hydrolysis step it is the chemistry of the polymer functional groups which determines the final texture of the solid. © 1998 Chapman & Hall

On the influence of the acid-base character of catalysts on the oxidative dehydrogenation of alkanes

Vanadium oxides supported on metal oxide, i.e. Al2O3, MgO and Mg-Al mixed oxide, and V-containing microporous materials (VAPO-5 and MgVAPO-5) have been tested in the oxidative dehydrogenation of C2-C4 alkanes. In all cases, tetrahedral vanadium species (isolated and/or associated) were mainly observed from51V-NMR and diffuse reflectance spectroscopies. The reducibility of V5+-species, determined from the onset-reduction temperature, decreases as follows: VOx/AL > VAPO-5 > MgVAPO-5 =VOx/MG > VOx/MG + AL. The acid character of catalysts, determined from the FTIR spectra of pyridine adsorbed, decreases as: MgVAPO-5 > VOx/AL > VAPO-5 > VOx/MG + AL > VOx/MG. A similar trend between V-reducibility of the catalyst and its catalytic activity for the alkane conversion was observed. However, the selectivity to olefins depends on the acid-base character of catalyst and the alkane fed. In the ODH ofn-butane, the higher the acid character of the catalyst the lower the selectivity to C4-olefins, while in the ODH of ethane an opposite trend between the catalyst acidity and the selectivity to ethene was observed.

Acidic and basic properties of titania–alumina mixed oxides; active sites for propan-2-ol dehydration

TiO2–Al2O3 mixed oxides have been prepared by cohydrolysis of Ti4+ and Al3+ isopropoxides. Textural and acid–base properties were investigated using nitrogen physisorption, adsorption of probe molecules followed by FTIR spectroscopy, and test-reactions. Catalysts were found to exhibit high surface areas and pore volumes. Results obtained from adsorbed CO2 probe molecules and COS hydrolysis activity agreed well and showed that the hydroxy basicity of mixed oxides was lower than that of the two oxide components. Pyridine and 2,6-dimethylpyridine were used to titrate Lewis and Bronsted acidic sites, respectively. A linear increase of the number of strong Lewis acid sites with alumina content was observed, whereas the number of Bronsted acid sites reached a maximum for 30% TiO2–70% Al2O3 compared with that expected from the composition and the activity of pure oxides. Comparison between probe-molecule adsorption and test reaction, points out that both types of site are involved in propan-2-ol dehydration. A transformation of Lewis acid sites into Bronsted sites is proposed when propan-2-ol is chemisorbed on Lewis acid sites.

Formation of V2O5-based mixed oxides in flames

V2O5–TiO2 and V2O5–Al2O3 mixed oxide powders were synthesized in a hydrogen-oxygen flame using VOCl3, TiCl4, and Al(CH3)3 as precursors. The particle formation processes were investigated as a function of VOCl3 concentration by laser light-scattering and by collecting particles directly onto transmission electron microscopy grids. In the V2O5–TiO2 system, the oxides condense as an intimate mixture at all three VOCl3 concentrations. Spherical particles, 40 to 70 nm in diameter, are obtained. In the V2O5–Al2O3 system, chain-like particles composed of an intimate mixture of V2O5 and Al2O3 form at the lowest VOCl3 concentration. At high VOCl3 concentrations, the chain-like particles have a core-mantle structure (a core mainly of Al2O3 and a mantle mainly of V2O5). The crystalline form and the surface area of these mixed oxides were determined by x-ray diffractometry, FT-IR spectroscopy, and BET analysis by nitrogen desorption. These measurements indicate that amorphous vanadium oxide forms at low VOCl3 concentrations, and V2O5 is obtained at the higher VOCl3 concentrations. The structure of the amorphous vanadium oxide matches that published for vanadium oxide “supported” catalysts.