Tomás Viveros

A new cobalt oxide electrodeposit bath for solar absorbers

Abstract A study was carried out in a Hull cell in order to optimize the deposition conditions of cobalt xide (black cobalt) in an electrolytic bath, which uses cobalt nitrate for direct obtention of black cobalt. Thermal stability of the material was surveyed on several samples of black cobalt prepared on stainless-steel with a thickness of approximately of 2.5 μm. It was found that the optical properties change, in respect to the initial values, with time of treatment until an equlibrium is reached. This equilibrium depends on the substrate and the temperature of the treatment used.

Cobalt oxide films grown by a dipping sol-gel process

Cobalt oxide thin films were prepared by the dipping sol-gel process, using two different inorganic precursors: cobalt chloride and cobalt nitrate salts. Also, samples with a different number of dipping-annealing cycles (3, 5, and 7) where prepared. Composition, structure, surface morphology and optical properties of such films have been characterised by means of X-ray diffraction, differential thermogravimetric analysis (DTA), transmittance spectra and atomic force microscopy (AFM). The results show that starting from distinct precursors leads to different properties: film water contents, surface roughness, crystallite size, total film transmittance, absorption coefficient and refractive index. Absorption coefficients higher than 104 cm−1 where found for all the samples. Refractive indices vary from n ~ 1.9–2.8 in the near infrared region. Our study shows that using a relatively simple preparation method like the sol-gel process, cobalt oxide films with specific properties, can be made.

Synthesis and characterization of MTiO3(M = Mg, Ca, Sr, Ba) sol-gel

Multiple oxide titanates have been prepared via the sol-gel method by the reaction of an alcoholic solution of titanium n-butoxide, Ti(OC4H9)4, with nitrate solutions, M(NO3)2(M = Mg, Ca, Sr, Ba), at room temperature. The amorphous gels so obtained under acidic conditions were calcined at temperatures between 700 and 900 °C to obtain high-purity, crystalline MTiO3 powders. The samples were characterized by powder X-ray diffraction (XRD), scanning (SEM) and energy-dispersive electron microscopy, thermogravimetric (TG) and differential thermal analysis (DTA) and BET surface area measurements. The stability and large surface area obtained for the Sr and Ba compounds make them suitable for use as catalyst supports.

Physical characterization of TiO2 and Al2O3 prepared by precipitation and sol-gel methods

Abstract Alumina and titania were prepared by precipitation and sol-gel methods. The effects of water/alkoxide and acid/alkoxide mole ratios and calcination temperature were studied. Samples were characterized by TGA,DTA, X-ray diffraction and N2 physisorpdon. Alumina was amorphous at room temperature and at 500 °C for sol-gel samples, but the γ-phase appears at 500 °C in the precipitation sample. Anatase titania was obtained at room temperature in the sol-gel preparations but the titania was amorphous in the precipitated sample. Surface areas for alumina were larger for sol-gel than for precipitation preparations. Titania precipitation samples treated at 500 °C show larger surface areas than those of sol-gel samples.

Alumina-titania oxides: Synthesis and characterization

Alumina-titania mixed oxides with nominal atomic ratios Al/Ti=25 and 2 have been synthesized by cohydrolysis of Al(OsecBu)3 and Ti(OBu)4. The oxides were amorphous at 500°C, showing only short range order. Ti was incorporated in alumina in a well dispersed way creating a true Al/Ti mixed oxide. The strength of surface acid sites and specific surface areas were increased at the higher Ti content. The TiO2 anatase phase was avoided and rutile was formed directly at 900°C.

Structural studies of supported tin catalysts

Tin oxide was supported on aluminium oxide, titanium oxide, magnesium oxide and silicon oxide, and the resulting interactions between the components in the prepared samples and after reduction were characterized by Mössbauer spectroscopy. It was observed that in the oxide state, tin is present as SnO2 on alumina, magnesia and silica, but on titania tin occupies Ti sites in the structure. After hydrogen treatment at high temperatures, tin is reduced from Sn(4) to Sn(2) on alumina and titania; it is reduced from Sn(4) to Sn(0) on silica, and is practically not reduced on magnesia. These results reveal the degree of interaction between tin and the different supports studied.

Structural Behavior of Pt–Sn Supported on MgO

Pt–Sn supported on magnesia and alumina were characterized, before and after treatment with hydrogen, by Mössbauer spectroscopy and X-ray diffraction. For the calcined samples on both supports tin is present as SnO2 and platinum as metal. After reduction with hydrogen, platinum and tin diffuse into the magnesia lattice to form a solid solution. On alumina Sn(IV), Sn(II), Sn(0), Pt, Pt3Sn, PtSn and PtSn2 alloys are formed. The SnO interacts strongly with the alumina support. The catalytic activity of both Pt–Sn catalysts is strongly affected by the support. On alumina the dehydrogenation of cyclohexane is very high, whereas that on magnesia is almost non-active.

Mössbauer study of supported Pt—Sn

Pt—Sn supported on alumina was characterized before and after treatment with hydrogen by Mössbauer spectroscopy and X-ray diffraction. For the calcined sample tin is present as SnO2 and platinum as metal. After reduction with hydrogen, Sn(IV), Sn(II), Sn(0), Pt, Pt3Sn, PtSn alloys are formed. SnO interacts strongly with the support.

Alumina support modified by Zr and Ti. Synthesis and characterization

Abstract Alumina-zirconia and alumina-titania mixed oxides were prepared by coprecipitation of their corresponding chlorides. Samples were synthezised at several concentrations, dried and calcined at temperatures between 400°C and 800°C. All dried samples showed the bayerite structure, which upon calcination became first amorphous and later Al 2 O 3 . Anatase was observed only on the high content, high calcination temperature sample, but no crystalline phase of zirconia was detected. Surface areas of mixed oxides were higher than for alumina single oxide, and decreased with temperature. Carbon tetrachloride adsorption increased with TiO 2 content, but the opposite effect was observed on alumina-zirconia samples.

The Effect of Solvent on the Kinetics and Mass Transfer Resistances for 4,6 DMDBT HDS

The effect of different solvents for model hydrodesulfurization (HDS) reactions on both the kinetics and the mass transfer resistances on a wide range of hydrogen partial pressures has been investigated. Special attention was directed to study the effect of low hydrogen partial pressures. While some researchers associate solvent effects to competitive adsorption of the solvent molecules on the catalytic surface, there is evidence that other effects may also play an important role in the modification of the kinetics. In this work, HDS experiments were performed in a slurry stirred tank reactor using the model molecule 4,6-dimethyldibenzothiophene (4,6-DMDBT) over a commercial NiMoP/alumina catalyst. The effect of hydrogen pressure on the transformation of 4,6 DMDBT in different solvents was studied. Thermodynamic calculations for hydrogen solubility under the whole set of conditions was considered. From our experiments, modification of the catalyst activity was observed to occur only at low hydrogen pressures (less than 34.7 bar) when n-dodecane was the used solvent. On the other hand, when using decaline there seems to be practically no effect in the catalytic activity of the hydrogen pressure over the entire range of pressures that were studied (18.0-55.5 bar). Additionally, the selectivity to direct desulfurization (DDS) products increased, while simultaneously decreasing the selectivity to the hydrogenation (HYD) pathway products, as the hydrogen partial pressure diminished below 34.7 bar when both solvents were used. It was concluded that hydrogen solubility effects become important and modify the activity at pressures below 34.7 bar.