Carolina Belver

Palladium enhanced resistance to deactivation of titanium dioxide during the photocatalytic oxidation of toluene vapors

Abstract Different Pd/TiO2 samples were prepared with metal loading ranging from 0.1 to 1% (w/w) and using two different titanium dioxide powders samples as support. The XRD data reveal that the incorporation of palladium can increment slightly the rutile to anatase ratio, but it does not change the textural parameters of TiO2 extensively. The H2 chemisorption measurements indicate that the dispersion of palladium decreases on increasing the metal loading. The Pd particles remain in a metallic state after reduction in hydrogen and the subsequent exposure to atmospheric air, as revealed by UV-Vis DRS. Most of the Pd/TiO2 catalysts show a considerable increment of the conversion of toluene vapor after prolonged UV-irradiation, meanwhile the benzaldehyde production decreases slightly with respect to bare TiO2. However, this enhancement of the photoactivity is not observed initially and it is independent of the palladium content and dispersion. Therefore, the main effect of palladium is to hinder the deactivation of the photocatalysts. On the other hand, the improvement of the photocatalytic performance also depends on the characteristics of TiO2, since for pure anatase of low surface area the addition of palladium is slightly detrimental for the photoactivity. The presence of water vapor in the feed stream is beneficial for the photoactivity for both Pd-loaded and neat TiO2. In contrast, a moderate increment in the operation temperature is not beneficial for the photoactivity, despite Pd/TiO2 samples present a significant thermal activity in the dark. FTIR analyses of the used Pd/TiO2 catalysts indicate that the surface of both the metallic particles and the support are partly covered with by-products of the photo-oxidation of toluene. A limited oxidation of the palladium particles also takes place during photoreaction.

EPR study on oxygen handling properties of ceria, zirconia and Zr–Ce (1 : 1) mixed oxide samples

The characteristics of ceria, zirconia and Zr–Ce (1 : 1 molar ratio) samples prepared by using inverse microemulsions have been studied to obtain information on the modifications produced in the oxygen handling properties of the single oxides by the formation of the mixed oxide. The structural characteristics of the samples are compared on the basis of XRD and Raman spectroscopy data, which show that the Zr–Ce sample forms a homogeneous mixed oxide structure corresponding to the (pseudocubic) tetragonal phase t″ while CeO2 presents, as expected, a cubic fluorite structure and ZrO2 is formed mainly by a tetragonal phase with only traces of the thermodynamically most stable monoclinic phase. Evaluation by EPR of the type and intensity of superoxide radicals generated upon oxygen adsorption on the outgassed samples reveals a higher surface reducibility in the mixed oxide sample, associated vacancies defects being generated on it in higher amounts and at lower temperatures than on pure ceria. It is also shown that both Ce and Zr ions participate in the oxygen chemisorption process on the mixed oxide sample, actually giving rise to much larger amounts of O2-–Zr4+ species than the pure ZrO2 sample; the latter species show a higher thermal stability than O2-–Ce4+ species.

The structural and electronic properties of nanostructured Ce1-x-yZrxTbyO2 ternary oxides: unusual concentration of Tb3+ and metal oxygen metal interactions.

Ceria-based ternary oxides are widely used in many areas of chemistry, physics, and materials science. Synchrotron-based time-resolved x-ray diffraction, x-ray absorption near-edge spectroscopy (XANES), Raman spectroscopy, and density-functional calculations were used to study the structural and electronic properties of Ce-Zr-Tb oxide nanoparticles. The nanoparticles were synthesized following a novel microemulsion method and had sizes in the range of 4-7 nm. The Ce1-x-yZrxTbyO2 ternary systems exhibit a complex behavior that cannot be predicted as a simple extrapolation of the properties of Ce1-xZrxO2, Ce1-xTbxO2, or the individual oxides (CeO2, ZrO2, and TbO2). The doping of ceria with Zr and Tb induces a decrease in the unit cell, but there are large positive deviations with respect to the cell parameters predicted by Vegards rule for ideal solid solutions. The presence of Zr and Tb generates strain in the ceria lattice through the creation of crystal imperfections and O vacancies. The O K-edge and Tb LIII-edge XANES spectra for the Ce1-x-yZrxTbyO2 nanoparticles point to the existence of distinctive electronic properties. In Ce1-x-yZrxTbyO2 there is an unexpected high concentration of Tb3+, which is not seen in TbO2 or Ce1-xTbxO2 and enhances the chemical reactivity of the ternary oxide. Tb<-->O<-->Zr interactions produce a stabilization of the Tb(4f,5d) states that is responsible for the high concentration of Tb(3+) cations. The behavior of Ce1-x-yZrxTbyO2 illustrates how important can be metal<-->oxygen<-->metal interactions for determining the structural, electronic, and chemical properties of a ternary oxide.

Silica–alumina/sepiolite nanoarchitectures

Novel silica–alumina/sepiolite nanoarchitectures have been prepared by an original approach based on sol–gel procedures. A commercial organo-sepiolite is used as a starting material whose organocations are provided by alkylchains that contribute to the controlled hydrolysis and polycondensation of silicon and aluminium alkoxides mixtures added to a non-aqueous suspension of that sepiolite. This procedure yields the coagulation of the alkoxides–organosepiolite suspension, resulting in the formation of a homogenous gel. After thermal treatment of the dried system, the organic matter is removed and the silica–alumina network consolidated. The characterization of the resulting materials reveals the formation of silica–alumina nanoparticles that remain anchored to the surface of sepiolite fibers through the silanol groups of the core silicate. These novel nanoarchitectures show specific surface areas in the range of 250–300 m2 g−1. The porosity of the resulting materials can be controlled depending on the nature of the oxide network generated. Thus, high silica contents generated microporous systems, whereas the incorporation of alumina enhances the mesoporous formation. The incorporation of a silica–alumina network on the surface sepiolite also permits the tuning of its surface acidity, typical values increasing from 0.1 to 1.4 mmol of acid sites per gram, according to the aluminium content. The acid–base properties of the new nanoarchitectures were evaluated as acid catalysts from the 2-propanol dehydration test. The catalytic behaviour is discussed in terms of the acidic properties of the involved solids.

Zr-doped TiO2 supported on delaminated clay materials for solar photocatalytic treatment of emerging pollutants.

Solar light-active Zr-doped TiO2 nanoparticles were successfully immobilized on delaminated clay materials by a one-step sol-gel route. Fixing the amount of TiO2 at 65wt.%, this work studies the influence of Zr loading (up to 2%) on the photocatalytic activity of the resulting Zr-doped TiO2/clay materials. The structural characterization demonstrates that all samples were formed by a delaminated clay with nanostructured anatase assembled on its surface. The Zr dopant was successfully incorporated into the anatase lattice, resulting in a slight deformation of the anatase crystal and the reduction of the band gap. These materials exhibit high surface area with a disordered mesoporous structure formed by TiO2 particles (15-20nm) supported on a delaminated clay. They were tested in the solar photodegradation of antipyrine, usually used as an analgesic drug and selected as an example of emerging pollutant. High degradation rates have been obtained at low antipyrine concentrations and high solar irradiation intensities with the Zr-doped TiO2/clay catalyst, more effective than the undoped one. This work demonstrates the potential application of the synthesis method for preparing novel and efficient solar-light photocatalysts based on metal-doped anatase and a delaminated clay.

Preparation of Porous Silica by Acid Activation of Metakaolins

Different metakaolins were prepared from a Spanish natural kaolin, by calcination at various temperatures. These solids were submitted to acid activation under different conditions. The behaviour of these samples was studied by different techniques paying special attention to the study of their porosity. This treatment produces the modification of the starting materials and the result was the synthesis of porous silica. Optimal conditions of activation were found.

Ag-Coated Heterostructures of ZnO-TiO2/Delaminated Montmorillonite as Solar Photocatalysts

Heterostructures based on ZnO-TiO2/delaminated montmorillonite coated with Ag have been prepared by sol–gel and photoreduction procedures, varying the Ag and ZnO contents. They have been thoroughly characterized by XRD, WDXRF, UV–Vis, and XPS spectroscopies, and N2 adsorption, SEM, and TEM. In all cases, the montmorillonite was effectively delaminated with the formation of TiO2 anatase particles anchored on the clay layer’s surface, yielding porous materials with high surface areas. The structural and textural properties of the heterostructures synthesized were unaffected by the ZnO incorporated. The photoreduction led to solids with Ag nanoparticles decorating the surface. These materials were tested as photocatalysts for the degradation of several emerging contaminants with different nitrogen-bearing chemical structures under solar light. The catalysts yielded high rates of disappearance of the starting pollutants and showed quite stable performance upon successive applications.

Porous Silica Gel by Acid Leaching of Metakaolin

Solids, mainly composed of silica gel, were prepared by acid leaching of metakaolins. The metakaolins were prepared by calcination of a natural kaolin at different temperatures (600, 700, 800 and 900°C). The metakaolins thus prepared were treated with 6 M aqueous HCl at room temperature and at 90°C, varying the duration of the treatments. All metakaolins prepared by calcination at 600, 700 or 800°C showed a very similar reactivity, while that prepared by calcination at 900°C showed lower reactivity, due to a beginning of sintering. Treatments at room temperature did not alter either the structure or the properties of the metakaolins. Treatment under reflux conditions led to the leaching of most of the octahedral Al3+ cations. Silica-based solids were thus obtained reaching, under optimal activation conditions, high BET surface areas up to 219 m2/g and total pore volumes up to 0.065 cm3 g−1. Harsher treatments conditions (for longer periods of time) resulted in a drastic amorphisation of the final products. The solids obtained are promising adsorbents or catalyst supports.

Effects of pH value and calcium hardness on the removal of 1,1,1-trichloroethane by immobilized nanoscale zero-valent iron on silica based supports

Immobilizing nanoscale zero-valent iron (NZVI) particles on silica-based supports is an effective way to overcome the NZVI aggregation. The pH value and calcium hardness can change the aggregation kinetics and alter the stability of the suspensions of NZVI-silica based materials, thus change the reactivity of these NZVI-silica based materials to remove chlorinated aliphatic hydrocarbons (CAHs). The removal of CAHs by these NZVI-silica based materials includes adsorption by silica based supports and degradation by NZVI particles. Using 1,1,1-TCA and mesoporous hydrated silica (mHS) as model chlorinated aliphatic hydrocarbon (CAH) and silica based support, the effects of pH value and Ca2+ concentration on both the adsorption and adsorption-degradation processes of CAHs by NZVI-silica based materials were studied. The structural and textural features, suspension stability, particle size distribution, and Zeta potential of the materials under various conditions were characterized by different techniques. Both decreasing initial pH value and increasing Ca2+ concentration can reduce the Zeta potential of mHS and lead to the aggregation of mHS particles, thus inhibiting the removal of 1,1,1-TCA via adsorption by mHS through decreasing the number of sites for adsorption. Low initial pH value can accelerate the corrosion of NZVI core and remove the passivation layer, thus promoting the removal of 1,1,1-TCA via adsorption-degradation by NZVI@mHS. Ca2+ can decrease the sites for adsorption and form precipitates which can block mesoporous channels, thus hinder the 1,1,1-TCA removal via adsorption-degradation by NZVI@mHS.

Clay Materials for Selective Catalytic Reduction of NO x

Interest in pillared interlayered clays (PILCs) has over the past decades been centered on their prospective industrial utilization in catalysis, sorption, and separations (air – gas mixtures, small hydrocarbons, multicomponent hydrocarbon mixtures, large organic molecules). In this chapter, the application of PILCs materials and other related ones (layered clays, LDHs, PCHs) in selective catalytic reduction of NO x will be tackled in detail.