José Luiz Zotin

Multistep structural transition of hydrogen trititanate nanotubes into TiO2-B nanotubes : a comparison study between nanostructured and bulk materials

H-trititanate nanotubes obtained by alkali hydrothermal treatment of TiO(2) followed by proton exchange were compared to their bulk H(2)Ti(3)O(7) counterpart with respect to their thermally induced structural transformation paths. As-synthesized and heat-treated samples were characterized by XRD, TEM/SAED, DSC and spectroscopy techniques, indicating that H(2)Ti(3)O(7) nanotubes showed the same sequence of structural transformations as their bulk counterpart obtained by conventional solid state reaction. Nanostructured H(2)Ti(3)O(7) converts into TiO(2)(B) via multistep transformation without losing its nanotubular morphology. The transformation occurs between 120 and 400 degrees C through topotactic mechanisms with the intermediate formation of nanostructured H(2)Ti(6)O(13) and H(2)Ti(12)O(25), which are more condensed layered titanates eventually rearranging to TiO(2)(B). Our results suggest that the intermediate tunnel structure H(2)Ti(12)O(25) is the final layered intermediate phase, on which TiO(2)(B) nucleates and grows. The conversion of nanostructured TiO(2)(B) into anatase is completed at a much lower temperature than its bulk counterpart and is accompanied by loss of the nanotubular morphology.

Surface acidic properties of alumina-supported niobia prepared by chemical vapour deposition and hydrolysis of niobium pentachloride

Abstract Niobia–aluminas were prepared by chemical vapour deposition at 150°C of niobium pentachloride on the surface of γ-aluminas calcined at different temperatures and with controlled degrees of hydration, followed by hydrolysis with water vapour at 150°C and a thermal treatment with steam at 440°C aimed at removing surface chloride contamination. The samples were characterised with respect to chemical composition, surface area, acidity by temperature-programmed desorption of ammonia, nature of acid sites by infrared spectroscopy of adsorbed pyridine and catalytic activity at 370°C in the dealkylation of cumene. The results showed that, for each alumina calcination temperature, the catalysts with the lowest niobium content have a higher density of acid sites than the alumina support, but the acidity decreased, within each series with an increase in the niobium content. Comparatively to the TPD results, catalytic activity in cumene dealkylation was much more sensitive to the history and composition of the samples. The niobia-alumina samples were much less active than the alumina support, but this was most likely due to the severe hydrothermal treatment for chlorine removal, since their activity was close to that of an alumina submitted to the same treatment. A strong decrease in the acidic activity was observed with increase in the niobium content. A sample of pure niobium oxide had a much higher activity than the niobia-alumina samples. Bronsted acidic sites could only be observed by the IR spectra of adsorbed pyridine on the surface of the pure niobium oxide sample.

Effect of Sulfur or Nitrogen Poisoning on the Activity and Selectivity of Y-Zeolite-Supported Pt–Pd Catalysts in the Hydrogenation of Tetralin

The effects of dibenzothiophene or quinoline poisoning on the activity and selectivity of Y-zeolite-supported Pt–Pd catalysts, in the hydrogenation of tetralin at 523 K and 70 bar initial pressure, were studied in a high pressure batch reactor as a function of composition. It was found that both poisons influenced, in very similar ways, the patterns of tolerance to poisons and of cis/trans selectivity vs. catalyst composition.

Thiotolerant Ir/SiO2–Al2O3 bifunctional catalysts: effect of support acidity on tetralin hydroconversion

Tetralin hydroconversion over supported iridium catalysts has been investigated under a pressure of 4 MPa in the presence of H2S in a continuous high-pressure gas-phase microreactor. Decalin, naphthalene, ring-contraction bicyclic products and one-ring-opening products are formed. A screening of silica, alumina and amorphous silica–alumina (ASA) supports demonstrates that only ASA provides thiotolerance and ring opening/contraction selectivity to iridium nanoparticles. By testing Ir/ASA catalysts with various silica–alumina ratios but similar Ir particle size (1.5 nm), it is shown that the intermediate concentration of silica (40 wt%) leads to the highest activity and selectivity, in correlation to the Bronsted acidity measured by infrared spectroscopy of adsorbed pyridine.

Enhancement of the catalytic properties of NiMo and CoMo alumina-supported sulfide catalysts by addition of ruthenium sulfide dispersed in Y zeolites

Abstract Y zeolite- or alumina-supported ruthenium sulfide catalysts were investigated for the hydrogenation of pyridine to piperidine. Their activity was compared with that of commercial NiMo or CoMo alumina-supported catalysts and they were found to be more active than the latter. RuKY exhibited higher intrinsic activities than RuHY and Ru Al 2 O 3 catalysts. Mechanical mixtures consisting of RuKY and commercial alumina-supported NiMo- or CoMo-based catalysts exhibited synergy for the hydrogenation of pyridine. This phenomenon was not observed for the other supports. This observation is related to the particular ability of highly dispersed ruthenium sulfide for hydrogen activation.

Effect of basicity and pore size distribution of transition aluminas on their performance in the hydrogen sulphide-sulphur dioxide reaction

Abstract Aluminas prepared from different aluminium hydroxides (boehmite, bayerite and gibbsite) and containing controlled amounts of impurities (sodium or sulphate) introduced in different ways (by impregnation or residual ions from the preparation) were characterized with respect to their acidic properties (n-butylamine adsorption at 373 K, gravimetric method), basic properties (sulphur dioxide chemisorption at 373 K, volumetric method) and textural properties (surface area and pore size distribution). Their activity in the hydrogen sulphide plus sulphur dioxide reaction was measured in a tubular flow microreactor at 523 K. The effect of the nature of and mode of introduction of the impurities on the acid-base properties of the aluminas is discussed. It was established that, besides the basicity, the pore size distribution of the alumina strongly influences its catalytic performance, probably due to capillary condensation of sulphur in small pores.

Oxide surface modification: Synthesis and characterization of zirconia-coated alumina

Four aluminas were used as supports for impregnation with a zirconium oxide with the aim to achieve a coating, without phase separation, between support and modifier. The supports were impregnated with different concentrations of zirconium aqueous resin, obtained through the polymeric precursor method. After impregnation the samples were calcined and then characterized by XRD, which led to identification of crystalline zirconia in different concentrations from each support used. Using a simple geometric model the maximum amount of surface modifier oxide required for the complete coating of a support with a layer of unit cells was estimated. According to this estimate, only the support should be identified below the limit proposed and crystalline zirconium oxide should be identified above this limit when a complete coating is reached. The results obtained from XRD agree with the estimated values and to confirm the coating, the samples were also characterized by EDS/STEM, HRTEM, XPS, and XAS. The results showed that the zirconium oxide on the surface of alumina support reached the coating in the limit of 15 Zr nm(-2), without the formation of the ZrO(2) phase.

Deep hydrodenitrogenation on Pt supported catalysts in the presence of H2S, comparison with NiMo sulfide catalyst.

A comparison of the catalytic activities in 1-4decahydroquinoline hydrodenitrogenation between conventional NiMo sulfide and Pt supported catalysts in the presence of low partial pressure of H 2 S has been made. The results suggested that supported Pt (1 wt-%) are good candidates for a second stage process for deep denitrogenation in the presence of 100 ppm of H 2 S (total pressure 40 bars), Pt on silica-alumina exhibited a better activity than NiMo sulfide catalyst. The variation of H 2 S partial pressure (in the range 0-500 ppm) had no effect on the sulfide catalyst whereas platinum catalysts are deeply affected.

Influence of the basicity of alumina catalysts on their activity in the H2S+SO2 reaction

Abstract Several aluminas were prepared with different crystal structures (gamma, eta and chi) and controlled amounts of impurities (sodium and sulphate). The aluminas were characterized with respect to basicity in a volumetric apparatus, using the adsorption of SO2 at 373 K. Activity measurements for the reaction H2S+SO2 were carried out in a tubular flow microreactor at 523 K. A linear correlation was found between the intrinsic activity and basic site density of the aluminas in the range of 0.3 – 2×1014 sites/cm2. However, a gamma-alumina containing 3.9% weight sodium and ca. 2.8×1014 sites/cm2 displayed a much lower activity than expected from the correlation, possibly due to the presence of strongly basic sites which lead to the formation of non-reactive adsorbed SO2 species.

Preparo de óxido de nióbio suportado em alumina por deposição química em fase vapor: caracterização por espectroscopia vibracional e termogravimetria

Alumina supported niobium oxide was prepared by chemical vapor deposition (CVD) of NbCl5. The alumina was calcined and pretreated at differents temperatures in order to vary the density of OH groups on the surface which was determined by thermogravimetric analysis. A good correlation was found between the amount of anchored niobium and the total number of anionic sites (oxide and hydroxyl groups) on the surface of the alumina. The infrared spectra on the OH stretching region indicate that OH groups coordinated to at least one tetrahedral aluminum were more reactive towards NbCl5.