Juan Navarrete - Academia.edu (original) (raw)

Papers by Juan Navarrete

Research paper thumbnail of Dependence of Boehmite Thermal Evolution on Its Atom Bond Lengths and Crystallite Size

Journal of Solid State Chemistry, 2001

The thermal properties of boehmite with a crystallite size between 1 and 27 nm were analyzed by t... more The thermal properties of boehmite with a crystallite size between 1 and 27 nm were analyzed by thermogravimetry and di4erential thermal analysis, and correlated with its crystallography, crystallite morphology, and the atom bond lengths determined by re5ning its crystalline structure with the Rietveld method. Boehmite:s thermal evolution depended on atom bond lengths: the dehydratation temperature was determined by the interaction between an aluminum atom and the oxygen atom of hydroxyl. After dehydratation, the number of hydroxyls in crystals was larger than expected from stoichiometry, because oxygen atoms on crystal surfaces perpendicular to the (020) plane reacted with hydrogen and hydroxyls in environment. The transition temperature from boehmite into -alumina was determined by the hydrogen bond in boehmite. The transformation temperature from transitional aluminas into -Al 2 O 3 also varied with boehmite:s crystallite size; however, it was not determined by the amount of hydroxyls in transitional aluminas, but probably by the aluminum+oxygen bonds in boehmite.

Research paper thumbnail of Free Radical Formation in ZrO 2–SiO 2Sol–Gel Derived Catalysts

Journal of Catalysis, 1999

Zirconia-silica mixed oxides have been prepared by the sol-gel method using zirconium butoxide an... more Zirconia-silica mixed oxides have been prepared by the sol-gel method using zirconium butoxide and tetraethoxysilane as starting materials. The samples were sulfated using sulfuric acid as alkoxide hydrolysis catalyst or by impregnation with sulfuric acid of a sample previously prepared using HCl as alkoxide hydrolysis catalyst. In thermally treated samples, an intense EPR signal was observed for the sample prepared using HCl, while the EPR signal was lower in the sulfated samples. FTIR-pyridine adsorption spectra showed that Brønsted and Lewis acidity is developed in the sulfated samples. The formation of two types of Brønsted sites depending upon the sulfating method used is proposed. The catalytic activity of the samples was evaluated in isopropanol and n-butanol dehydration. The mechanism by which sulfation is achieved and the Brønstedtype site formed are discussed.

Research paper thumbnail of Effects of structural defects and acid–basic properties on the activity and selectivity of isopropanol decomposition on nanocrystallite sol–gel alumina catalyst

Journal of Molecular Catalysis A-chemical, 1999

. The surface acid-basic properties of sol-gel alumina catalysts were studied by Fourier transfor... more . The surface acid-basic properties of sol-gel alumina catalysts were studied by Fourier transform infrared FTIR spectroscopy of pyridine adsorption and temperature-programmed desorption of CO and NH . The number of acid and 2 3 basic sites on the samples varied with the calcination temperatures of the samples. The populations of the three different aluminum ions-tetrahedral, pentacoordinated and octahedral, which were identified by the 27 Al MAS NMR, were strongly affected by the sample calcination temperature and the crystalline composition. In the reaction temperature range between 100 and 2508C, isopropanol decomposition on sol-gel alumina catalysts was carried out. It was found that isopropanol decomposition on alumina catalyst was a structural-defect sensitive reaction. The dehydrogenation selectivity to acetone depended on the surface basic sites and the concentration of aluminum vacancies in the crystalline structure of g-Al O .

Research paper thumbnail of Structural Defects and Acidic and Basic Sites in Sol−Gel MgO

Journal of Physical Chemistry B, 1997

MgO prepared with the sol-gel technique had crystalline structure defects, obtained from X-ray po... more MgO prepared with the sol-gel technique had crystalline structure defects, obtained from X-ray powder diffraction and crystalline structure refinement, that correlated with its acidic and basic sites. The acidic sites, measured by adsorbing pyridine and NH 3 , were found when brucite and MgO coexisted, which only occurred after annealing the sample at 673 K. Two different basic sites, measured by adsorbing CO 2 , were present in a total concentration of about 10 21 sites per gram. These sites correlated to the presence of magnesium vacancies in MgO crystal. Both basic site and magnesium vacancy concentrations diminished when the sample annealing temperature was increased.

Research paper thumbnail of Effect of sulfation methods on TiO 2–SiO 2 sol–gel catalyst acidity

Applied Catalysis A-general, 2000

A series of x TiO 2 -SiO 2 (x=1.7, 4.0 and 6.7 wt.% TiO 2 ) binary mixed oxides was prepared by a... more A series of x TiO 2 -SiO 2 (x=1.7, 4.0 and 6.7 wt.% TiO 2 ) binary mixed oxides was prepared by a sol-gel method in acidic and basic conditions. In order to upgrade the acidic properties of the obtained solids, their sulfation was carried out in three different ways: (1) by in situ (H 2 SO 4 was admixed with the gel), (2) impregnating the dried solids with aqueous solutions of H 2 SO 4 , and (3) by impregnating as in but with (NH 4 ) 2 SO 4 . FTIR-pyridine adsorption and benzene deuteroexchange showed that the acid site strength depends on the sulfation method. The highest values of total acidity, determined by ammonia thermodesorption, correspond to the in situ sulfated samples. The obtained pore size distribution (4.0-5.0 nm) was narrow and the specific BET areas (250-350 m 2 /g) were large. X-Ray diffraction, RDF, XPS spectroscopy and catalytic activity in the 2-butanol dehydration show that one step sulfated in situ TiO 2 -SiO 2 sol-gel preparations resulted in catalysts with homogeneous and strong acidity.

Research paper thumbnail of Preparation of TiO 2–ZrO 2 mixed oxides with controlled acid–basic properties

Journal of Molecular Catalysis A-chemical, 2004

Titania-zirconia mixed oxides with various ZrO 2 content in TiO 2 (10, 50 and 90 wt.%) were prepa... more Titania-zirconia mixed oxides with various ZrO 2 content in TiO 2 (10, 50 and 90 wt.%) were prepared by the sol-gel method. High specific surface areas (77-244 m 2 /g) were obtained. Acidity determined by NH 3 -TPD and FTIR-pyridine adsorption showed that in mixed oxides the number of acid sites is dramatically increased; it varies from 173 mol NH 3 /g for TiO 2 to 1226-1456 mol NH 3 /g for the mixed oxides.

Research paper thumbnail of New Insights into the Defective Structure and Catalytic Activity of Pd/Ceria

Chemistry of Materials, 2002

A surfactant-assisted synthesis method was applied to prepare nanosized ceria particles used as t... more A surfactant-assisted synthesis method was applied to prepare nanosized ceria particles used as the three-way catalyst supports. The Rietveld refinements of crystalline structures of ceria calcined at various temperatures, together with XRD, FTIR, and TGA techniques, confirm that the surfactant cations are incorporated into the structural network of hydrous cerium oxide during the preparation stage, thus reducing the surface tension and diminishing the shrinkage of the mesoporous channels in the dried materials. The surface area is therefore enhanced but it provokes a significant distortion effect on the structure. It is evidenced by XRD analyses that a well-ordered cubic structure of ceria was formed at a temperature as low as 80°C. The average crystallite size of ceria is determined by XRD within nanoscale range (i.e., 9-31 nm) when the calcination temperature increases from 80 to 800°C, which is in good agreement with the data determined by TEM and electron diffraction methods. In addition, cationic defects are present in the crystalline structure of the solids, and the defect concentration in the unit lattice cell decreases as calcination temperature increases. CO chemisorption analysis by FTIR shows that CO may adsorb on Pd/ceria catalyst in different modes: linear Pd-CO and Ce 4+ -CO bonds, bridged Pd(CO) 2 bonds, and multibonded Pd 2 (CO) 2 bonds. CO adsorption might lead to CO 2 production from CO reacting with lattice oxygen and/or CO disproportionation catalyzed by palladium crystals. The CO oxidzed over the Pd supported catalysts makes it that T 50 and T 90 for the ceria prepared in the presence of surfactant diminish at 80°C and 110°C respectively, which are much lower than the ones prepared without surfactant, mainly due to its defective structure, enhanced surface area, and different CO adsorption behavior.

Research paper thumbnail of Preparation of Ni/ZrO 2–SO 4 2− catalysts by incipient wetness method: effect of nickel on the isomerization of n-butane

Journal of Molecular Catalysis A-chemical, 1999

A series of Ni-promoted sulfated zirconia catalysts with different nickel concentration (from 1 t... more A series of Ni-promoted sulfated zirconia catalysts with different nickel concentration (from 1 to 9.6 wt.%) were prepared by incipient wetness method. Ni and SO 4 2− promoters were co-impregnated to a parent zirconium hydroxide by a solution of Ni(NO 3 ) 2 ·6H 2 O in H ...

Research paper thumbnail of Sulfate Ions in Titania Polymorphs

Journal of Sol-gel Science and Technology, 2004

Sol-gel titania was sulfated by using sulfuric acid as hydrolysis catalyst, or by impregnating wi... more Sol-gel titania was sulfated by using sulfuric acid as hydrolysis catalyst, or by impregnating with ammonium sulfate fresh samples prepared with nitric acid or ammonium hydroxide as hydrolysis catalyst. Samples were characterized with X-ray powder diffraction, infrared spectroscopy, thermogravimetry and atomic absorption spectroscopy. Sulfate ions were found anchored to brookite and anatase phases, because they have short O—O atomic bond lengths slightly larger than the largest O—O bond length of sulfate ion. Since the shortest O—O atomic bond in anatase is smaller than the one in brookite, the sulfate ions are then less deformed when they are anchored to anatase, and consequently more stable. Therefore when the sample temperature is raised, the brookite with sulfate ions was transformed mainly to anatase and not into rutile, which is the most probably transformation when these ions are not involved. Sulfate ions also hindered anatase and brookite crystallite growing and stabilized the crystalline structure of anatase. When the sulfate ions are lost the crystalline anatase phase is transformed into rutile, leaving a large number of vacancies that favored atom diffusion and consequently the growing of rutile crystallites. The crystalline evolution of the samples as a function of the annealing temperature is almost independent of the sulfating method.

Research paper thumbnail of Preparation of sol-gel sulfated ZrO 2 SiO 2 and characterization of its surface acidity

Applied Catalysis A-general, 1995

Zr-Si mixed oxides containing 1.3 to 6.7 wt.-% Zr have been prepared by a sol-gel technique, usin... more Zr-Si mixed oxides containing 1.3 to 6.7 wt.-% Zr have been prepared by a sol-gel technique, using tetraethoxysilane and zirconium acetate as precursors. Zirconium is incorporated into the lattice of silica, but can easily be extracted either by self steaming or by acid treatment. The solids obtained in basic medium show a low surface area and low acidity. The samples prepared in acidic medium show large surface areas between 410 and 740 m2/g and can be converted to strong acids by contact with a solution of sulfuric acid. They retain pyridine up to 773 K and contain a total number of acid sites comparable to that of HY zeolites i.e. 1.7 meq/g. The catalytic activity for the bifunctional isomerization of n-hexane expressed per gram of zirconium is comparable for the sample containing 4wt.-% Zr and sulfated zirconia.

Research paper thumbnail of Molecular vibrational analysis and MAS-NMR spectroscopy study of epilepsy drugs encapsulated in TiO2-sol–gel reservoirs

Journal of Biomedical Materials Research Part A, 2006

A nanostructured matrix, consisting of titania, was designed in such a way that an antiepileptic ... more A nanostructured matrix, consisting of titania, was designed in such a way that an antiepileptic drug could be encapsulated and released according to a well-defined time release schedule. The titania was synthesized by a sol–gel method in which titanium n-butoxide was used as the precursor for the formation of the sol. The synthesis was optimized to yield a homogeneous particle size with a high porosity and an anatase crystal structure. The antiepilectic drugs, phenytoine or valproic acid, were added during the gelation stage in order to obtain a homogeneous gel phase. The resulting nanostructured matrix including the drug showed only weak attractive forces, such as London forces, dipole–dipole coupling, and in some cases hydrogen bonds. The resulting assembly, referred to as a reservoir, was characterized using conventional FTIR and NMR spectroscopic techniques. Theoretical simulation studies were performed so as to obtain an understanding of the equilibrium electrostatic potential distribution and the relative charges on the titania and the anticonvulsants. © 2006 Wiley Periodicals, Inc. J Biomed Mater Res, 2006

Research paper thumbnail of Characterizations of the thermal decomposition of brucite prepared by sol–gel technique for synthesis of nanocrystalline MgO

Materials Letters, 1998

An alternative method to diminish the nitrogen pollutant levels in waste waters is to encapsulate... more An alternative method to diminish the nitrogen pollutant levels in waste waters is to encapsulate microorganisms whitin ceramic materials. The sol-gel method has been extensively used for the preparation of such kind of materials permitting a higher stability and viability of useful organisms. In this work, the thermal characterization during dehydration of nitrifying and denitrifying aqueous emulsions of mud encapsulated in sol-gel silica is presented during the process of dehydration in ambient conditions and as a function of temperature. The characterization was performed by a photopyroelectric (PPE) technique, whose detector was made with a 110 mm polyvinyldifluoride (PVDF). The cell was constructed in such a way that the sample was inside the cell, and the bottom of the cell was closed by the PVDF foil. Thermal effusivity as a function of temperature was obtained illuminating the PVDF directly by a modulated 1W tungsten lamp. The sample is enclosed inside a chamber, using a Peltier cell that controls temperature in a range from 40 to 27 • C. The sample is on top of the PVDF, which is illuminated by the modulated tungsten lamp.

Research paper thumbnail of Thermal stability of 12-tungstophosphoric acid supported on zirconia

Applied Catalysis A-general, 2000

A freshly precipitated zirconia was impregnated with an ethanol solution of H 3 [W 12 PO 40 ]·6H ... more A freshly precipitated zirconia was impregnated with an ethanol solution of H 3 [W 12 PO 40 ]·6H 2 O (TPA) in order to obtain 0-25 wt.% TPA/ZrO 2 . The solids, heat-treated from 373 to 1073 K, were examined by means of differential thermal analysis, X-ray diffraction, surface area, infrared, Raman, 31 P nuclear magnetic resonance analyses, and used as catalysts in the decomposition of isopropanol. Below 673 K, TPA on ZrO 2 exist as distorted intact Keggin species interacting with ≡Zr-OH or ≡Zr + groups. However, at 773 K, bulk-like intact TPA species were detected. Above 773 K, the Keggin structure of TPA collapse and transforms into WO 3 and phosphorous oxides. The characteristic diffraction peaks of TPA cannot be observed even with 20 wt.% TPA loading, indicating a high dispersion or a very small crystal size of TPA, while 20 wt.% TPA loaded on SiO 2 clearly shows the presence of TPA particles.The heat-treatment temperature affects the catalytic activity of TPA on Zirconia. The reaction rate was 3.3 times higher in a TPA/ZrO 2 calcined at 773 than that at 473 K.

Research paper thumbnail of Effect of tin content on silica mixed oxides: Sulfated and unsulfated catalysts

Journal of Molecular Catalysis A-chemical, 1997

SnO,-SiO, binary oxides were prepared by the sol-gel method, with different contents of tin oxide... more SnO,-SiO, binary oxides were prepared by the sol-gel method, with different contents of tin oxide. and later sulfated. The resulting solids were characterized by Fourier transform infrared spectroscopy (IT-IR), X-ray diffraction (XRD), surface area measurements and also titrated using Hammett indicators. To correlate the acidic properties with catalytic activity, solids were tested in the conversion of 2-propanol. Tin addition to SiO, produces two important effects: an increase in the surface area in most of the resulting mixed oxides and high catalytic activity for the dehydration reaction in the case of the sulfated samples.

Research paper thumbnail of Dependence of Boehmite Thermal Evolution on Its Atom Bond Lengths and Crystallite Size

Journal of Solid State Chemistry, 2001

The thermal properties of boehmite with a crystallite size between 1 and 27 nm were analyzed by t... more The thermal properties of boehmite with a crystallite size between 1 and 27 nm were analyzed by thermogravimetry and di4erential thermal analysis, and correlated with its crystallography, crystallite morphology, and the atom bond lengths determined by re5ning its crystalline structure with the Rietveld method. Boehmite:s thermal evolution depended on atom bond lengths: the dehydratation temperature was determined by the interaction between an aluminum atom and the oxygen atom of hydroxyl. After dehydratation, the number of hydroxyls in crystals was larger than expected from stoichiometry, because oxygen atoms on crystal surfaces perpendicular to the (020) plane reacted with hydrogen and hydroxyls in environment. The transition temperature from boehmite into -alumina was determined by the hydrogen bond in boehmite. The transformation temperature from transitional aluminas into -Al 2 O 3 also varied with boehmite:s crystallite size; however, it was not determined by the amount of hydroxyls in transitional aluminas, but probably by the aluminum+oxygen bonds in boehmite.

Research paper thumbnail of Free Radical Formation in ZrO 2–SiO 2Sol–Gel Derived Catalysts

Journal of Catalysis, 1999

Zirconia-silica mixed oxides have been prepared by the sol-gel method using zirconium butoxide an... more Zirconia-silica mixed oxides have been prepared by the sol-gel method using zirconium butoxide and tetraethoxysilane as starting materials. The samples were sulfated using sulfuric acid as alkoxide hydrolysis catalyst or by impregnation with sulfuric acid of a sample previously prepared using HCl as alkoxide hydrolysis catalyst. In thermally treated samples, an intense EPR signal was observed for the sample prepared using HCl, while the EPR signal was lower in the sulfated samples. FTIR-pyridine adsorption spectra showed that Brønsted and Lewis acidity is developed in the sulfated samples. The formation of two types of Brønsted sites depending upon the sulfating method used is proposed. The catalytic activity of the samples was evaluated in isopropanol and n-butanol dehydration. The mechanism by which sulfation is achieved and the Brønstedtype site formed are discussed.

Research paper thumbnail of Effects of structural defects and acid–basic properties on the activity and selectivity of isopropanol decomposition on nanocrystallite sol–gel alumina catalyst

Journal of Molecular Catalysis A-chemical, 1999

. The surface acid-basic properties of sol-gel alumina catalysts were studied by Fourier transfor... more . The surface acid-basic properties of sol-gel alumina catalysts were studied by Fourier transform infrared FTIR spectroscopy of pyridine adsorption and temperature-programmed desorption of CO and NH . The number of acid and 2 3 basic sites on the samples varied with the calcination temperatures of the samples. The populations of the three different aluminum ions-tetrahedral, pentacoordinated and octahedral, which were identified by the 27 Al MAS NMR, were strongly affected by the sample calcination temperature and the crystalline composition. In the reaction temperature range between 100 and 2508C, isopropanol decomposition on sol-gel alumina catalysts was carried out. It was found that isopropanol decomposition on alumina catalyst was a structural-defect sensitive reaction. The dehydrogenation selectivity to acetone depended on the surface basic sites and the concentration of aluminum vacancies in the crystalline structure of g-Al O .

Research paper thumbnail of Structural Defects and Acidic and Basic Sites in Sol−Gel MgO

Journal of Physical Chemistry B, 1997

MgO prepared with the sol-gel technique had crystalline structure defects, obtained from X-ray po... more MgO prepared with the sol-gel technique had crystalline structure defects, obtained from X-ray powder diffraction and crystalline structure refinement, that correlated with its acidic and basic sites. The acidic sites, measured by adsorbing pyridine and NH 3 , were found when brucite and MgO coexisted, which only occurred after annealing the sample at 673 K. Two different basic sites, measured by adsorbing CO 2 , were present in a total concentration of about 10 21 sites per gram. These sites correlated to the presence of magnesium vacancies in MgO crystal. Both basic site and magnesium vacancy concentrations diminished when the sample annealing temperature was increased.

Research paper thumbnail of Effect of sulfation methods on TiO 2–SiO 2 sol–gel catalyst acidity

Applied Catalysis A-general, 2000

A series of x TiO 2 -SiO 2 (x=1.7, 4.0 and 6.7 wt.% TiO 2 ) binary mixed oxides was prepared by a... more A series of x TiO 2 -SiO 2 (x=1.7, 4.0 and 6.7 wt.% TiO 2 ) binary mixed oxides was prepared by a sol-gel method in acidic and basic conditions. In order to upgrade the acidic properties of the obtained solids, their sulfation was carried out in three different ways: (1) by in situ (H 2 SO 4 was admixed with the gel), (2) impregnating the dried solids with aqueous solutions of H 2 SO 4 , and (3) by impregnating as in but with (NH 4 ) 2 SO 4 . FTIR-pyridine adsorption and benzene deuteroexchange showed that the acid site strength depends on the sulfation method. The highest values of total acidity, determined by ammonia thermodesorption, correspond to the in situ sulfated samples. The obtained pore size distribution (4.0-5.0 nm) was narrow and the specific BET areas (250-350 m 2 /g) were large. X-Ray diffraction, RDF, XPS spectroscopy and catalytic activity in the 2-butanol dehydration show that one step sulfated in situ TiO 2 -SiO 2 sol-gel preparations resulted in catalysts with homogeneous and strong acidity.

Research paper thumbnail of Preparation of TiO 2–ZrO 2 mixed oxides with controlled acid–basic properties

Journal of Molecular Catalysis A-chemical, 2004

Titania-zirconia mixed oxides with various ZrO 2 content in TiO 2 (10, 50 and 90 wt.%) were prepa... more Titania-zirconia mixed oxides with various ZrO 2 content in TiO 2 (10, 50 and 90 wt.%) were prepared by the sol-gel method. High specific surface areas (77-244 m 2 /g) were obtained. Acidity determined by NH 3 -TPD and FTIR-pyridine adsorption showed that in mixed oxides the number of acid sites is dramatically increased; it varies from 173 mol NH 3 /g for TiO 2 to 1226-1456 mol NH 3 /g for the mixed oxides.

Research paper thumbnail of New Insights into the Defective Structure and Catalytic Activity of Pd/Ceria

Chemistry of Materials, 2002

A surfactant-assisted synthesis method was applied to prepare nanosized ceria particles used as t... more A surfactant-assisted synthesis method was applied to prepare nanosized ceria particles used as the three-way catalyst supports. The Rietveld refinements of crystalline structures of ceria calcined at various temperatures, together with XRD, FTIR, and TGA techniques, confirm that the surfactant cations are incorporated into the structural network of hydrous cerium oxide during the preparation stage, thus reducing the surface tension and diminishing the shrinkage of the mesoporous channels in the dried materials. The surface area is therefore enhanced but it provokes a significant distortion effect on the structure. It is evidenced by XRD analyses that a well-ordered cubic structure of ceria was formed at a temperature as low as 80°C. The average crystallite size of ceria is determined by XRD within nanoscale range (i.e., 9-31 nm) when the calcination temperature increases from 80 to 800°C, which is in good agreement with the data determined by TEM and electron diffraction methods. In addition, cationic defects are present in the crystalline structure of the solids, and the defect concentration in the unit lattice cell decreases as calcination temperature increases. CO chemisorption analysis by FTIR shows that CO may adsorb on Pd/ceria catalyst in different modes: linear Pd-CO and Ce 4+ -CO bonds, bridged Pd(CO) 2 bonds, and multibonded Pd 2 (CO) 2 bonds. CO adsorption might lead to CO 2 production from CO reacting with lattice oxygen and/or CO disproportionation catalyzed by palladium crystals. The CO oxidzed over the Pd supported catalysts makes it that T 50 and T 90 for the ceria prepared in the presence of surfactant diminish at 80°C and 110°C respectively, which are much lower than the ones prepared without surfactant, mainly due to its defective structure, enhanced surface area, and different CO adsorption behavior.

Research paper thumbnail of Preparation of Ni/ZrO 2–SO 4 2− catalysts by incipient wetness method: effect of nickel on the isomerization of n-butane

Journal of Molecular Catalysis A-chemical, 1999

A series of Ni-promoted sulfated zirconia catalysts with different nickel concentration (from 1 t... more A series of Ni-promoted sulfated zirconia catalysts with different nickel concentration (from 1 to 9.6 wt.%) were prepared by incipient wetness method. Ni and SO 4 2− promoters were co-impregnated to a parent zirconium hydroxide by a solution of Ni(NO 3 ) 2 ·6H 2 O in H ...

Research paper thumbnail of Sulfate Ions in Titania Polymorphs

Journal of Sol-gel Science and Technology, 2004

Sol-gel titania was sulfated by using sulfuric acid as hydrolysis catalyst, or by impregnating wi... more Sol-gel titania was sulfated by using sulfuric acid as hydrolysis catalyst, or by impregnating with ammonium sulfate fresh samples prepared with nitric acid or ammonium hydroxide as hydrolysis catalyst. Samples were characterized with X-ray powder diffraction, infrared spectroscopy, thermogravimetry and atomic absorption spectroscopy. Sulfate ions were found anchored to brookite and anatase phases, because they have short O—O atomic bond lengths slightly larger than the largest O—O bond length of sulfate ion. Since the shortest O—O atomic bond in anatase is smaller than the one in brookite, the sulfate ions are then less deformed when they are anchored to anatase, and consequently more stable. Therefore when the sample temperature is raised, the brookite with sulfate ions was transformed mainly to anatase and not into rutile, which is the most probably transformation when these ions are not involved. Sulfate ions also hindered anatase and brookite crystallite growing and stabilized the crystalline structure of anatase. When the sulfate ions are lost the crystalline anatase phase is transformed into rutile, leaving a large number of vacancies that favored atom diffusion and consequently the growing of rutile crystallites. The crystalline evolution of the samples as a function of the annealing temperature is almost independent of the sulfating method.

Research paper thumbnail of Preparation of sol-gel sulfated ZrO 2 SiO 2 and characterization of its surface acidity

Applied Catalysis A-general, 1995

Zr-Si mixed oxides containing 1.3 to 6.7 wt.-% Zr have been prepared by a sol-gel technique, usin... more Zr-Si mixed oxides containing 1.3 to 6.7 wt.-% Zr have been prepared by a sol-gel technique, using tetraethoxysilane and zirconium acetate as precursors. Zirconium is incorporated into the lattice of silica, but can easily be extracted either by self steaming or by acid treatment. The solids obtained in basic medium show a low surface area and low acidity. The samples prepared in acidic medium show large surface areas between 410 and 740 m2/g and can be converted to strong acids by contact with a solution of sulfuric acid. They retain pyridine up to 773 K and contain a total number of acid sites comparable to that of HY zeolites i.e. 1.7 meq/g. The catalytic activity for the bifunctional isomerization of n-hexane expressed per gram of zirconium is comparable for the sample containing 4wt.-% Zr and sulfated zirconia.

Research paper thumbnail of Molecular vibrational analysis and MAS-NMR spectroscopy study of epilepsy drugs encapsulated in TiO2-sol–gel reservoirs

Journal of Biomedical Materials Research Part A, 2006

A nanostructured matrix, consisting of titania, was designed in such a way that an antiepileptic ... more A nanostructured matrix, consisting of titania, was designed in such a way that an antiepileptic drug could be encapsulated and released according to a well-defined time release schedule. The titania was synthesized by a sol–gel method in which titanium n-butoxide was used as the precursor for the formation of the sol. The synthesis was optimized to yield a homogeneous particle size with a high porosity and an anatase crystal structure. The antiepilectic drugs, phenytoine or valproic acid, were added during the gelation stage in order to obtain a homogeneous gel phase. The resulting nanostructured matrix including the drug showed only weak attractive forces, such as London forces, dipole–dipole coupling, and in some cases hydrogen bonds. The resulting assembly, referred to as a reservoir, was characterized using conventional FTIR and NMR spectroscopic techniques. Theoretical simulation studies were performed so as to obtain an understanding of the equilibrium electrostatic potential distribution and the relative charges on the titania and the anticonvulsants. © 2006 Wiley Periodicals, Inc. J Biomed Mater Res, 2006

Research paper thumbnail of Characterizations of the thermal decomposition of brucite prepared by sol–gel technique for synthesis of nanocrystalline MgO

Materials Letters, 1998

An alternative method to diminish the nitrogen pollutant levels in waste waters is to encapsulate... more An alternative method to diminish the nitrogen pollutant levels in waste waters is to encapsulate microorganisms whitin ceramic materials. The sol-gel method has been extensively used for the preparation of such kind of materials permitting a higher stability and viability of useful organisms. In this work, the thermal characterization during dehydration of nitrifying and denitrifying aqueous emulsions of mud encapsulated in sol-gel silica is presented during the process of dehydration in ambient conditions and as a function of temperature. The characterization was performed by a photopyroelectric (PPE) technique, whose detector was made with a 110 mm polyvinyldifluoride (PVDF). The cell was constructed in such a way that the sample was inside the cell, and the bottom of the cell was closed by the PVDF foil. Thermal effusivity as a function of temperature was obtained illuminating the PVDF directly by a modulated 1W tungsten lamp. The sample is enclosed inside a chamber, using a Peltier cell that controls temperature in a range from 40 to 27 • C. The sample is on top of the PVDF, which is illuminated by the modulated tungsten lamp.

Research paper thumbnail of Thermal stability of 12-tungstophosphoric acid supported on zirconia

Applied Catalysis A-general, 2000

A freshly precipitated zirconia was impregnated with an ethanol solution of H 3 [W 12 PO 40 ]·6H ... more A freshly precipitated zirconia was impregnated with an ethanol solution of H 3 [W 12 PO 40 ]·6H 2 O (TPA) in order to obtain 0-25 wt.% TPA/ZrO 2 . The solids, heat-treated from 373 to 1073 K, were examined by means of differential thermal analysis, X-ray diffraction, surface area, infrared, Raman, 31 P nuclear magnetic resonance analyses, and used as catalysts in the decomposition of isopropanol. Below 673 K, TPA on ZrO 2 exist as distorted intact Keggin species interacting with ≡Zr-OH or ≡Zr + groups. However, at 773 K, bulk-like intact TPA species were detected. Above 773 K, the Keggin structure of TPA collapse and transforms into WO 3 and phosphorous oxides. The characteristic diffraction peaks of TPA cannot be observed even with 20 wt.% TPA loading, indicating a high dispersion or a very small crystal size of TPA, while 20 wt.% TPA loaded on SiO 2 clearly shows the presence of TPA particles.The heat-treatment temperature affects the catalytic activity of TPA on Zirconia. The reaction rate was 3.3 times higher in a TPA/ZrO 2 calcined at 773 than that at 473 K.

Research paper thumbnail of Effect of tin content on silica mixed oxides: Sulfated and unsulfated catalysts

Journal of Molecular Catalysis A-chemical, 1997

SnO,-SiO, binary oxides were prepared by the sol-gel method, with different contents of tin oxide... more SnO,-SiO, binary oxides were prepared by the sol-gel method, with different contents of tin oxide. and later sulfated. The resulting solids were characterized by Fourier transform infrared spectroscopy (IT-IR), X-ray diffraction (XRD), surface area measurements and also titrated using Hammett indicators. To correlate the acidic properties with catalytic activity, solids were tested in the conversion of 2-propanol. Tin addition to SiO, produces two important effects: an increase in the surface area in most of the resulting mixed oxides and high catalytic activity for the dehydration reaction in the case of the sulfated samples.