Size-activity relationship of iridium particles supported on silica for the total oxidation of volatile organic compounds (VOCs)

12 Figures, 2 Tables.-- Datos suplementarios disponibles en línea en la página web del editor.-- © 2019. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/This systematic catalytic study reveals for the first time a strong size-activity relationship of iridium-based catalysts for the total oxidation of short chain alkanes reaction. Silica has been selected as support for its inertness and weak silica-iridium interaction to avoid its contribution to the catalytic activity. The size of the iridium particles can be increased from ∼5 to 27 nm by increasing the calcination temperature from 350 to 750 °C. Unlike other precious metals such as palladium or platinum, in the case of iridium catalysts, the oxidation activity increases when the size decreases. This effect is also maintained when the activity is normalized per metallic surface area revealing a higher intrinsic activity as the iridium size decreases beyond its simple increase in metallic surface area. Indeed, as the particle size decreases, a higher proportion of highly reducible iridium species as well as an increase in defective Ir3+ species on the surface is observed by XPS, directly related to the enhanced activity. The highly reducible species are oxidized under the reaction conditions, leading to an initial decrease in activity before reaching a stable rate of oxidation reaction. This knowledge provides useful guidelines for the design of iridium-based systems for the total oxidation of volatile organic compounds at low temperatures.Authors from UV thank the University of Valencia (UV-INV-AE16-484416 project) and MINECO (MAT2017-84118-C2-1-R project) for funding. Mathias Van de Vyver is acknowledged for the Raman study. T.G. would also like to thank the Regional Government of Aragón (DGA) for the support provided under the research groups support programme.Peer reviewe

Supported iridium catalysts for the total oxidation of short chain alkanes and their mixtures: influence of the support

Catalytic total oxidation of noxious volatile organic compounds (VOCs) is an important process to remove these compounds from the atmosphere. This is the first systematic study of the influence of the support on the activity of iridium oxide supported catalysts for the total oxidation of VOCs. Iridium catalysts supported on titania, γ-alumina, silica and zeolites have been prepared using different calcination temperatures. The activity for the total oxidation of short chain alkanes and their mixtures has been evaluated and the physicochemical properties characterized by N2 adsorption, XRD, (HR)TEM, EDX, CO-Chemisorption, TPR, XPS and Raman spectroscopy. Both the calcination temperature and the nature of the support of iridium catalysts play an important role for the catalytic performance. Silica, ZSM-5 zeolites and titania are suitable supports for IrOx, in contrast with γ-alumina. A strong influence of the Lewis acidity of the support on the turnover frequency of the iridium oxide is found. Additionally, for a given support, the calcination temperature has an effect on the catalytic activity. A possible size effect is discussed. However, the major controlling factor is the nature of the support. Therefore, our results provide a guideline towards a rational design of more active IrOx catalysts for the total oxidation of VOCs

TiO2 nanostructures synthesized by electrochemical anodization in green protic ionic liquids for photoelectrochemical applications

This work studies the influence of the 2-hydroxyethylammonium acetate (2-HEAA) ionic liquid (IL) as an electrolyte in the electrochemical anodization of titanium for the synthesis of nanostructures for photoelectrochemical water splitting. Different 2-HEAA IL concentrations were used ranging from 0 to 4% v/v (IL-0 to IL-4) in electrolytes containing NH4F, water and ethylene glycol. Morphological, structural and electrochemical characterization of the nanostructures was carried out by means of field emission scanning electron microscopy (FE-SEM), high resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), electrochemical impedance spectroscopy (EIS), and Mott-Schottky (MS) analysis. Additionally, photoelectrochemical tests were carried out in order to evaluate the efficiency of these materials as catalysts for water splitting applications. According to the obtained results, the electrolyte used for electrochemical anodization should contain little amount of NH4F (0.05 M) in order to obtain efficient nanostructures for photoelectrochemical purposes. However, small concentrations of IL (IL-0.25) resulted in nanostructures with higher photocurrents than doubling the NH4F concentration to 0.1 M. Therefore, the IL addition contributes to a more sustainable electrolyte formulation. The best photoelectrochemical response for water splitting processes was obtained for the nanostructures anodized with 1% v/v of 2-HEAA IL (IL-1) due to their high surface/area (higher pore diameters, smaller nanotubes wall thickness and higher nanotubes lengths), better crystallinity and electrochemical response, showing photocurrents more than 100% higher than the ones obtained for the nanotubes anodized without IL