Miguel A. Baltanás

Supported titanium oxide as photocatalyst in water decontamination: State of the art

Abstract Over the last two decades the interest on systems based on supported titanium oxide as a photocatalyst for water decontamination has grown significantly. A variety of supporting materials, coating methods, and reactor arrangements have been investigated from engineering as well as from more fundamental points of view. A thorough search of the published reports of these investigationes was carried out and they are analyzed in this paper. An overview of the state of the art in the subject is given.

Lubricants from chemically modified vegetable oils.

This work reports laboratory results obtained from the production of polyols with branched ether and ester compounds from epoxidized vegetable oils pertaining to annual, temperate climate crops (soybean, sunflower and high-oleic sunflower oils), focusing on their possible use as components of lubricant base stocks. To this end, two different opening reactions of the epoxide ring were studied. The first caused by the attack with glacial acetic acid (exclusively in a single organic phase) and the second using short-chain aliphatic alcohols, methanol and ethanol, in acid media. Both reactions proceed under mild conditions: low synthesis temperature and short reaction times and with conversions above 99%. Spectroscopic (NMR), thermal (DSC) and rheological techniques were used to characterize the oils, their epoxides and polyols, to assess the impact of the nature of the vegetable oil and the chemical modifications introduced, including long-term storage conditions. Several correlations were employed to predict the viscosity of the vegetable oils with temperature, and good agreement with the experimental data was obtained.

Soybean oil epoxidation with hydrogen peroxide using an amorphous Ti/SiO2 catalyst

We report a study of the epoxidation of soybean oil and soybean methyl esters with hydrogen peroxide in dilute solution (6 wt%) using an amorphous heterogeneous Ti/SiO2 catalyst in the presence of tert-butyl alcohol. The influence of some relevant process variables such as temperature and the hydrogen peroxide-to-double bond molar ratio on performance is examined. The highest yields of epoxidized olefins were obtained upon using a H2O2 n ∶ substrate molar ratio of 1.1 ∶ 1. Higher ratios than this were not effective for speeding up the reaction. Under the experimental conditions employed in this work, no degradation of the oxirane ring was observed.

Preparation of PdSiO2 cfor methanol synthesis III. Exposed metal fraction and hydrogen solubility

Tetramminepalladium complex was ion exchanged (IE) on purified, well characterized macro- and microporous Davison G-59 and G-03 silica gels (Pd loadings: 1.1–7.7% Pd w/w). These catalytic materials were reduced in hydrogen (473 to 723 K) after calcining them under both oxidating and inert atmospheres, in a wide range of experimental conditions. Two different regions were identified, each one showing its own dependence for the exposed metal fraction (FE) vs the maximum calcining temperature (Tc): (i) at Tc 423 K the FE is a function of the metal loading and the support structure, but not of the complex species mentioned. Hydrogen solubility on these Pd/SiO2 systems diminishes with the mean diameter of the metal crystallites, reaching a constant value for d < 20 A. The fraction of soluble hydrogen can be quantitatively decomposed into that of β-HPd and another one corresponding to hydrogen sorbed onto subsurface sites. It is shown that a suppression of H2 solubility exists for the metal crystallites occluded by the microporous support.

Stability of formate species on β-Ga2O3

Gallia (gallium oxide) has been proved to enhance the performance of metal catalysts in a variety of catalytic reactions involving methanol, CO and H(2). The presence of formate species as key intermediates in some of these reactions has been reported, although their role is still a matter of debate. In this work, a combined theoretical and experimental approach has been carried out in order to characterize the formation of such formate species over the gallium oxide surface. Infrared spectroscopy experiments of CO adsorption over H(2) (or D(2)) pretreated beta-Ga(2)O(3) revealed the formation of several formate species. The beta-Ga(2)O(3) (100) surface was modelled by means of periodic DFT calculations. The stability of said species and their vibrational mode assignments are discussed together with the formate interconversion barriers. A possible mechanism is proposed based on the experimental and theoretical results: first CO inserts into surface (monocoordinate) hydroxyl groups leading to monocoordinate formate; this species might evolve to the thermodynamically most stable dicoordinate formate, or might transfer hydrogen to the surface oxidizing to CO(2) creating an oxygen vacancy and a hydride group. The barrier for the first step, CO insertion, is calculated to be significantly higher than that of the monocoordinate formate conversion steps. Monocoordinate formates are thus short-lived intermediates playing a key role in the CO oxidation reaction, while bidentate formates are mainly spectators.

Photocatalytic reactors I. Optical behavior of titanium oxide particulate suspensions

Abstract Photocatalytic reactions employing finely divided titanium dioxide have become increasingly attractive, particularly for their potential applications to pollution abatement in water streams. Kinetic and reactor design studies require the knowledge of the existing radiation field in order to evaluate the volumetric rate of energy absorption inside the reaction vessel. To describe this photon distribution, the radiation absorption and transport properties of the solid suspensions must be known inside the reaction space. From the fundamental principles of radiative transport, an apparent property has been defined, the apparent Napierian extinctance, which was used to investigate the optical properties of photocatalytic suspensions employing simple spectrophotometric measurements. The effects of particle size, particle concentration, stirring, recycling, environmental pH, oxygen bubbling and strong irradiation on the apparent Napierian extinctance of titanium oxide suspensions as a function of wavelength are reported. Data from different titanium oxides or titanium oxides having different nominal particle sizes, as well as from agglomerates formed after the stirring or recycling of neutral reacting solutions, cannot be interpreted in terms of a Lambert-type equation. These findings constitute a confirmation of predictions from radiation theory which indicate that at least two parameters, or more precisely, two parameters and one distribution function are required to describe fully the radiation transport even in non-reactive solid suspensions.

Preparation of Pd/SiO2 catalysts for methanol synthesis I. Ion exchange of [Pd(NH3)4)(AcO)2

Abstract An in-depth analysis of the adsorption via ion exchange (IE) of aminepalladium complexes prepared from palladium acetate on well-characterized commercial gels of silica (Davison, G-59 and G-03) in aqueous alkaline solutions is carried out. The nature of the exchanged species after different IE and washing treatments is scrutinized with diffuse reflectance spectroscopy. It is shown that (i) there is a maximum of IE at pH 10.3; (ii) IE occurs without any exchange of ligands; (iii) adsorption equilibrium is reached in about 1 h and (iv) IE follows a common Frumkin isotherm on both supports.

CO, CO2 and H2 adsorption on ZnO, CeO2, and ZnO/CeO2 surfaces: DFT simulations

The adsorption of the molecules CO, CO2, and H2 on several ceria and zinc oxide surfaces was studied by means of periodical DFT calculations and compared with infrared frequency data. The stable CeO2(111), CeO2(331), and ZnO(0001) perfect faces were the first substrates considered. Afterwards, the same surfaces with oxygen vacancies and a ZnO monolayer grown on Ceria(111) were also studied in order to compare the behaviors and reactivities of the molecules at those surfaces. The ceria surfaces were substantially more reactive than the ZnO surface towards the CO2 molecule. The highest adsorption energy for this molecule was obtained on the CeO2(111) surface with oxygen vacancies. The molecules CO and H2 both presented low or very low reactivities on all of the surfaces studied, although some reactivity was observed for the adsorption of CO onto the surfaces with oxygen vacancies, whereas H2 exhibited reactivity towards the CeO2(111) surface with oxygen vacancies. This work was performed to provide a firm foundation for novel process development in methanol synthesis from carbon oxides, steam reforming of methanol for hydrogen production, and/or the water-gas shift reaction.

In-Situ DRIFT Study of Au–Ir/Ceria Catalysts: Activity and Stability for CO Oxidation

AbstractnMonometallic gold and iridium, and bimetallic gold–iridium on ceria catalysts were synthesized by deposition–precipitation with urea. The activity of these ceria supported catalysts for CO oxidation was: Au–Irxa0≫xa0Auxa0>xa0Ir. The bimetallic catalyst was highly active and stable during the carbon monoxide oxidation reaction, showing synergism between both metals. The catalysts were thoroughly characterized chemical and structurally. High-resolution electron microscopy coupled with energy dispersive spectroscopy and X-ray photoelectron spectroscopy showed evidences of iridium–gold intimacy. In situ infrared spectroscopy in the diffuse reflectance mode was used to investigate the reactivity of the active sites. Concentration-modulation excitation spectroscopy allowed selective identification of intermediates and ‘spectator’ species. CO chemisorbed onto iridium sites (2096, 2060 and 2012xa0cm−1) was found inactive, whereas carbon monoxide on metallic gold sites (Au0–CO, 2110xa0cm−1) exhibited reactivity for CO oxidation. In the bimetallic catalyst, new active sites (Auδ−–CO, 2095 and 2026xa0cm−1) together with Au0–CO species were observed, most likely produced by the isolation and stabilization of gold atoms by iridium.

Preparation of Pd/SiO2 catalysts for methanoi synthesis: Part II. Thermal decomposition of [Pd(NH3)4]2+/SiO2

Abstract As a part of an experimental program aimed at the systematic analysis of the controlled deposition of Pd/SiO2 on well characterized macroporous and microporous silica gels (Davison G-59, 254 m2 g−1 and G-03, 558 m2 g−1) the nature of the surface species upon drying and thermal decomposition of the tetramminepalladium complex (TPSiO) obtained by ion exchange (IE) of palladium acetate in aqueous ammonium hydroxide (Pd loading: 0.5–11%Pd w w ), has been systematically followed by the combined use of diffuse reflectance spectroscopy (DRS), differential scanning calorimetry (DSC), thermogravimetric analysis (TG), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). DRS indicates that drying in air (T ≈ 393 K) leaves a stable diammmepalladium complex (DPSiO) on the surfaces. Decomposition of DPSiO upon calcination in inert and oxidizing atmospheres was studied from 308 to 773 K, with heating rates of 4–64 K min−1 (N2) and 2–32 K min−1 (air). Two decomposition zones are identified with DSC and TG. (1) In the low-temperature region (308–473 K) endothennal signals which correspond to a liberation of NH3 around 359 K (G-59) or 371 K (G-03) were observed; (2) the high-temperature region (473–773 K) only shows endothennal peaks when N2 is used, but in air several signals indicate that a sequence of transformations of the Pd occurs. Ultradispersed Pdo is the final product on both catalyst types when N2 is the decomposing atmosphere, whereas either a mixture of Pd°+[(SiO)2]2−Pd2+ (on the microporous G-03) or pure [(SiO)2]2− Pd2+ (on the macroporous G-59) are the final products when air is employed.