María José López-Muñoz

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.

Nanofiltration removal of pharmaceutically active compounds

Abstract Capability of nanofiltration membranes (NF) to remove pharmaceutical active compounds from wastewater streams was investigated. Sulfamethoxazole, diclofenac sodium, hydrochlorothiazide, 4-acetamidoantipyrine, nicotine and ranitinide hydrochloride were selected as model compounds since they are widely produced as pharmaceutical agents. Two commercially available polyamide nanofiltration membranes (NF-90 and NF-270 from Dow FilmTec) were tested. Solute retention by NF-90 membrane was very high in all cases (over 95%), whereas NF-270 retention systematically appeared lower ranging from 75% (nicotine) to 95% (ranitidine hydrochloride). Temporal evolution of flux decline was also investigated. The influence of physicochemical properties of both membrane and solutes on membrane performance was analyzed to explore the main solute-membrane interactions that determine the solute transfer across the membrane. The influence of operation pressure on NF-90 and NF-270 rejection was also studied.

CHAPTER 4:Solar Photocatalysis: Fundamentals, Reactors and Applications

The use of solar light to drive photocatalytic processes has huge potential from the environmental and the economic viewpoints. However, the size and therefore the cost of solar collectors are strongly dependent on the availability of solar resources. This chapter describes the main aspects to be considered in the design of solar photocatalytic reactors. Compound parabolic collectors (CPCs) tilted towards the equator at an angle equivalent to the local latitude are considered nowadays the state of the art for solar photocatalytic applications, as they maximize the use of the available ultraviolet radiation, both direct and diffuse. Regarding the photocatalytic materials, most large-scale solar photocatalytic applications reported in the literature have been based on the use of titanium dioxide slurries. However, many efforts have been devoted to the modification of TiO2 by doping, coupling with another semiconductor or dye sensitization with the aim of enhancing absorption of the visible light to improve the use of the photons available in solar light. Applications of solar photocatalysis have been reviewed, especially focusing on water decontamination and disinfection. Finally, synergistic effects of the combination of photocatalytic processes with biological treatments or membrane processes have been also evaluated.

Effect of thermal treatment on the photocatalytic behavior of TiO2 supported on zeolites

Photocatalysts based on TiO2 supported on two different synthetic zeolites (faujasite CBV 760: SiO2/Al2O3 = 60 and mordenite CBV 21A, SiO2/Al2O3 = 20), containing 10 wt% and 40 wt% TiO2 loading, have been synthesized using a sol–gel method without any further calcination. The structural, textural and optical properties of the samples have been characterized using X-ray diffraction (XRD), elemental analysis (ICP-OES), N2 adsorption–desorption isotherms, aberration corrected Scanning Transmission Electron Microscopy with High Angle Annular Dark Field imaging combined with Electron Energy Loss Spectroscopy (STEM/HAADF/EELS), and UV-Vis Diffuse Reflectance Spectroscopy (UV-Vis DRS). The characterization data confirmed that an amorphous layer containing polycrystalline anatase TiO2 was formed efficiently on the surface of the zeolites, showing 2–4 nm crystalline domains. The photocatalytic activities of different samples were tested for the photocatalytic degradation of methyl orange, which showed increased adsorption of the dye molecule, and a faster degradation reaction than the calcined counterparts. Interestingly, although the supported TiO2 catalysts were not calcined, the best catalysts, 10TZY60nc and 40TZY60nc, showed negligible Ti4+ leaching and their reusability was verified for three cycles without noticeable photocatalytic activity decrement.

Sol-Gel Titania and Titania-Silica Mixed Oxides Photocatalysts

This paper presents a review of the work published by the authors on the synthesis, characterization and evaluation of the photocatalytic activity of TiO2/SiO2 materials. The use of titania-silica mixed oxides photocatalysts is proposed basically as a process improvement to overcome the difficulties of recovering titania from the slurries after the photocatalytic treatment of contaminated waters. To understand the mechanism governing the photocatalytic activity of these materials, several titania-silica photocatalysts have been prepared through a sol-gel method that allows controlling the main variables to obtain materials with different textural properties, degree of titania incorporation, dispersion of the photoactive phase and crystallinity of titanium dioxide. The samples have been characterized in depth, looking for correlations between the main physicochemical properties (TiO2 crystallite size, band gap energy and titania surface area) and the activity shown in the photocatalytic oxidation of cyanide, selected as model pollutant. The results suggest that the photocatalytic activity is strongly influenced by the quality of the titania crystal network, which in turn is improved by the use of a hydrothermal crystallization procedure. Additionally, the evaluation of the fraction of the total surface area corresponding to titania is mandatory for comparing the catalytic activity of different materials in processes in which titanium dioxide is the only phase catalytically active and silica behaves as an inert support.