R. Gómez

Sol—Gel preparation of supported metal catalysts

Variables important in the synthesis of supported metal catalysts using the sol-gel method are reviewed. Potential applications of these materials are presented with emphasis on: thermally resistant materials, catalysts with low deactivation rates, high surface area materials, catalytic membrane reactors and high surface area superacid materials. Emphasis is on synthesis conditions including the control of variables which enable the preparation of well defined pore size distributions. The reforming reactions of n-hexane are used as a basis for comparison between catalysts.

Effect of hydrolysis catalyst on the Ti deficiency and crystallite size of sol-gel-TiO 2 crystalline phases

We prepared sol-gel titania by using different hydrolysis catalysts, and characterized it by x-ray powder diffraction. The structure of the crystalline phases—brookite, anatase, and rutile—in the samples annealed between 70 and 900 °C was refined by using the Rietveld technique. From the refinement we obtained the structure parameters, the concentration of each phase, and their average crystallite size. These quantities and their evolution with temperature depended on the hydrolysis catalyst. Anatase and rutile were deficient in Ti, suggesting that their crystalline structure contained hydrogen atoms, forming OH − ions inside. In anatase this deficiency depended on its crystallite size, but it was constant in rutile. When anatase was annealed, it dehydroxylized, producing either crystallitc growing up or its conversion into rutile. From the analysis we also found the conditions for obtaining single-phase samples that could be used as precursors for making up titania single-phase thin films.

FTIR and UV-Vis (diffuse reflectance) spectroscopic characterization of TiO2 sol-gel

Abstract The semiconductive properties of titania are determined by the density of point defects in the crystalline structure. The synthesis method (sol-gel) provides pure and reproducible solids. This technique is very sensitive to hydrolysis catalysts, pH and calcination temperature. In this work, each one of these parameters is systematically varied and correlations with the structures of the obtained solids are established. It is shown that very acidic or basic catalysts are selective to rutile (900 °C). Acid catalysts favour an hydrolysis reaction, but pH does not change the shape or the size of the particles. The reaction mechanism is a nucleophilic substitution SN 2 in the basic medium and most intermediates in the sol-gel reaction are Ti-OH. The characterization techniques are FTIR, UV-Vis (diffuse reflectance), X-ray diffraction, DTA-TGA and scanning electron microscopy.

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

Abstract Brucite was synthesized by the sol–gel technique using magnesium diethoxide in a homogeneous reaction medium. Its decomposition, including dehydroxylation, crystallization and phase transformation was studied by XRD, TGA–DTG, CTEM, HRTEM, SAED and FTIR techniques. It has been proposed that during the dehydroxylation and decomposition of the magnesium diethoxide gel, some vacancies are generated. The decomposition of brucite with hexagonal structure to form periclase with the cubic structure occurs in the temperature range from 473 to 773 K. In this case, nanocrystalline MgO is formed from the break down of the brucite crystalline structure.

Preparation of sol-gel sulfated ZrO2SiO2 and characterization of its surface acidity

Abstract 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 m 2 /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.

Effects of structural defects and acid-basic properties on the activity and selectivity of isopropanol decomposition on nanocrystallite sol-gel alumina catalyst

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 23 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. 23 Bimolecular reaction to isopropylether was largely governed by the pentacoordinated aluminum ions which were related to the coordinately unsaturated aluminum ions. A mechanism for the formation of isopropylether was proposed: oxygen . q vacancies were suggested to involve the adsorption step of isopropanol, an intermediate species, CH HC , reacted with 32

Ru/SiO2-impregnated and sol-gel-prepared catalysts: synthesis, characterization, and catalytic properties

Impregnation and sol-gel preparation methods are compared. Only when prepared by sol-gel synthesis, ruthenium is incorporated in the silica network; RuO2 particles are observed, but most Ru is occluded in the silica network. The ruthenium migrates to the surface with thermal treatments and small particles are formed. In the case of impregnated catalysts only RuO2 particles are observed. Their diameter is larger when the catalyst is treated at high temperatures, ortho-Xylene hydrogenation was performed on these two kinds of catalysts. The deactivation and selectivity values are attributed to the structural differences of the catalysts.

Photocatalytic Activity in the 2,4-Dinitroaniline Decomposition Over TiO2 Sol-Gel Derived Catalysts

Titania was prepared by the sol-gel method from titanium alkoxide. Depending on the pH of the gelling solution, specific surface areas between 88 and 10 m2/g were obtained. The band gap (Eg) of the samples was found between 3.05 and 3.32 eV. In samples gelled at pH5 and 9 and calcined at 400°C only anatase phase is observed, while for pH3 and pH7 brookite, anatase and rutile or anatase-rutile phases coexist. It was found that the photoactivity in the 2,4-dinitroanailine decomposition depends on the Eg and on the crystalline phases. The highest activity corresponds to the catalysts having the lowest Eg and more than one crystalline phases co-existing.

DTA-TGA and FTIR spectroscopies of sol-gel hydrotalcites: aluminum source effect on physicochemical properties

Hydrotalcites (crystalline mixed oxides of magnesium and aluminum) were synthesized by the sol-gel method at pH 10. The magnesium source was magnesium ethoxide and the aluminum precursors were aluminum nitrate, aluminum chlorate or aluminum sulfate. The resulting solids were characterized by infrared spectroscopy (FTIR) and thermal analysis (DTA-TGA). It was found that thermal stability depended strongly on aluminum precursor in the following sequence: aluminum chloride > aluminum nitrate > aluminum sulfate.

Synthesis of magnesium oxide by the sol-gel method: Effect of the pH on the surface hydroxylation

Magnesium oxide was prepared by the sol-gel method. The method involves the hydrolysis of magnesium alkoxide in the presence of acid or basic catalysts followed by a polycondensation reaction. The synthesized solids were characterized by IR spectroscopy, X-ray diffraction, DTA, TGA, and electron microscopy. It is shown that the hydrolysis process depends on the catalysts pH (HCI > CH3COOH > H2C204 > H20 > NH4OH). This effect can be explained by the reaction mechanism. The reaction is controlled by the hydrolysis at acid pH and by the polycondensation at a basic one. Solids prepared by this method are chemically homogeneous, very pure, and specifically hydroxylated. They can be widely used in the study of magnesium support effects in different catalytic reactions, c~ 1991 Academic Press. Inc.