TiO2 Al2O3 Mixed Oxides Research Articles

Catalytic behavior of gold nanoparticles supported on a TiO2-Al2O3 mixed oxide for CO oxidation at low temperature.

The present work highlights the versatility of a TiO2-Al2O3 mixed oxide bearing highly dispersed gold nanoparticles that was applied in the CO oxidation reaction at room temperature. The TiO2, Al2O3, and TiO2-Al2O3 supports were synthesized by the sol-gel method, while gold nanoparticles were added by the deposition-precipitation with urea method using a theoretical Au loading of 2 wt.%. A promotional effect of the TiO2-Al2O3 support on the activity of gold catalysts with respect to TiO2 and Al2O3 was observed; Au/TiO2-Al2O3 showed outstanding CO oxidation, being active from 0°C and stable throughout a 24-h test. As for the alumina content (5, 10, and 15 wt.%) in TiO2, it improved the textural properties by retarding the crystal growth and anatase-rutile phase transformation of TiO2, suppressing the deposition of carbon on the catalyst surface and stabilizing the Au nanoparticles even at high temperatures. Gold was highly dispersed with nanoparticle sizes ranging from 1 to 2nm when H2 was used to treat thermally the Au/TiO2-Al2O3, Au/TiO2, and Au/Al2O3 materials. In addition, the XPS technique helped elicit that Au0 and Au1+ boosted their interaction with the TiO2, Al2O3, and TiO2-Al2O3 supports by means of charge transfer, which resulted in outstanding CO oxidation activity from 0°C. Likewise, the key factors that control the peculiar catalytic performance in the CO oxidation reaction are discussed, which represents a step forward in the versatility behavior of gold catalysts supported on mixed oxide catalysts.

Thermal growth of exclusive alumina scale on a TiAl based alloy: Shot peening effect

Shot peening, although generally accepted as a useful surface treatment method to improve oxidation resistance of Ni-, Co- and Fe-based alloys, has been less applied to TiAl based alloys. This work shows that a shot peening treatment converts the Ti-45Al-2Nb-2Mn-1B alloy from thermally growing TiO2-Al2O3 mixed oxides to an exclusive Al2O3 in air at 900 °C. The basic reason for this is that the shot peening catalyzes formation of an α-Al2O3 thin film, which prevents Ti of the alloy from oxidation at the high temperature. The slow oxidation of the shot-peened alloy mitigates the degradation of typical γ/α2 lamellar microstructure.

High temperature oxidation behaviour of TiAl–Cr–Nb–Mo alloys

The oxidation behaviour of six different TiAl–Cr–Nb–Mo alloys was investigated by thermogravimetric method. Isothermal experiments were carried out at 900 °C and 1000 °C in air.The aim of this work was to evaluate the effect of aluminium and of other alloying elements on the oxidation behaviour of TiAl intermetallic alloys. The alloy's microstructure and composition as well as the composition distribution of the oxide scales were analysed by Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS) and X-Ray Diffractometry (XRD). The study highlighted that the oxidation resistance of the considered TiAl-based alloys is strongly affected by its aluminium content, while it seems that an increase of Cr, Nb and Mo concentrations does not affect the oxidation behaviour of the alloy. Yttrium addition, at least in the tested quantity, has a detrimental effect since it promotes the formation of a TiO2–Al2O3 mixed oxide instead of a compact and thin alumina layer.

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.

Effect of TiO2 loading on the activity of V/TiO2-Al2O3 in the catalytic oxidehydrogenation of ethylbenzene with carbon dioxide

TiO2-Al2O3 mixed oxides with different compositions ranging from 40wt-% to 95wt-% of TiO2 were prepared by sol-gel method and impregnated with different amounts of VO x . Supports and catalysts were characterized by X-ray diffraction (XRD), physisorption, temperature preprogrammed reduction (H2-TPR), and ammonia temperature programmed desorption (NH3-TPD). TiO2 content in the support had obvious effect on the crystal structure, texture characteristic, acid property, and catalytic activity in dehydrogenation of ethylbenzene (EB) with carbon dioxide. The highest catalytic activity was acquired when the TiO2 content was 50 wt-%.

Effect of TiO2–Al2O3 Sol–Gel Supports on the Superficial Ni and Mo Species in Oxidized and Sulfided NiMo/TiO2–Al2O3 Catalysts: Influence on Dibenzothiophene Hydrodesulfurization

The present work presents a comparative study of NiMo catalysts supported on sol-gel TiO2-Al2O3 mixed oxides with 5 and 95 mol% content of Al2O3. The DRX and N2 physisorption results showed that the sol-gel method allows preparation of TiO2-Al2O3 mixed oxides possessing high superficial area and an amorphous TiO2 structure. Results of f-potential showed that the net surface pH of the supports depends on their structure and compo- sition. According to UV-Vis and Raman spectra obtained from the solids after impregnation, catalysts with high content of Al2O3 showed Mo7O24 2- and Mo8O26 4- species displaying Mo-O-Mo stretching vibration modes. On the other hand, catalysts with high content of TiO2 showed Mo7O24 2- and Mo8O26 4- species with vibration modes corresponding to terminal Mo=Ot bonds. Therefore, it appears that impregnation of catalysts with a pH 9 solution allows a polymerization process of MoO4 2- and (Ni 2? 4O 2- ) solution species to Mo8O26 4- and Mo7O24 2- species with a close interaction with (Ni 2? 6O 2- ) species. However, these species have low interaction with the support. Thus, composition of the support appears to be more important than net surface pH in order to obtain a better distribution of superficial Mo species. XPS results suggest a higher proportion of ''NiMoS'' phase on the TiO2 rich support. The most active catalyst in the dibenzothiophene hydrodesulfurization was NiMo/TiO2- Al2O3 with 5 mol% Al2O3. This suggests that Mo7O24 2- and Mo8O26 4- in combination with (Ni 2? 6O 2- ) species produce a better Ni/(Ni ? Mo) ratio and NiMoS phase.

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.

Improvement of the Mo/TiO2-Al2O3Catalyst by the Control of the Sol-Gel Synthesis

Traditional hydrotreating catalysts are constituted by molybdenum deposited on Al2O3 promoted by nickel and phosphorous. Several studies have shown that TiO2-Al2O3 mixed oxides are excellent supports for the active phases. Results concerning the preparation, characterization and testing of molybdenum catalyst supported on titania-alumina are presented. The support was prepared by sol-gel route using titanium and aluminum isopropoxides, the titanium one chelated with acetylacetone (acac) to promote similar hydrolysis ratio for both the alcoxides. The effect of nominal molar ratio [Ti]/[Ti+Al] on the microstructural features of nanometric particles was analyzed by X-Ray Diffraction, N2 Adsorption Isotherms and Transmission Electron Microscopy. The catalytic activity of Mo impregnated supports was evaluated using the thiophene hydrodesulfurization at different temperatures and atmospheric pressure. The pores size distribution curve moves from the micropores to the mesopores by increasing the Ti contents, allowing the fine tuning of average size from 2.5 to 6 nm. Maximal (367 m2·g−1) and minimal (127 m2·g−1) surface area were found for support containing [Ti]/[Ti+Al] ratio equal to 0.1 and 1, respectively. The good mesopore texture of alumina-titania support with [Ti]/[Ti+Al] molar ratio between 0.3 and 0.5 was found particularly valuable for the preparation of well dispersed MoS2 active phase, leading to HDS catalyst with somewhat higher activity than that prepared using a commercial alumina support.

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

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.

Properties of Acid Sites on TiO2–SiO2 and TiO2–Al2O3 Mixed Oxides Measured by Infrared Spectroscopy

Abstract Both Lewis and Brönsted acid sites were found on the surface of TiO2–SiO2 mixed oxide, thouth no Brönsted acid sites were detected on the surface of pure TiO2 and SiO2. While on the surface of TiO2–Al2O3 mixed oxide, no Brönsted acid sites were generated. The rate of isomerization of 1-butene was found to be enhanced on the Brönsted acid sites formed on the TiO2–SiO2 mixed oxide.

Shielding effect of aluminium on sulphur dioxide deactivation of vanadium oxide on titania-alumina DeNOx catalysts

Vanadium oxide supported on a TiO2–Al2O3 mixed oxide prepared by gel-supported precipitation shows enhanced properties of resistance to deactivation by SO2 during the selective removal of NOX with ammonia in flue gases owing to the transfer of sulphate groups to Al sites.