Au/TiO 2 Lyogels for Hydrogen Production
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Au/TiO2 Lyogels for Hydrogen Production Elies Molins1, Mónica Benito1, Ignasi Mata1, Lester Martínez2, Lluís Soler2, Jordi Llorca2 1 Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), c/ Til·lers, 1, Campus UAB, 08193 Bellaterra, Spain 2 Institute of Energy Technologies and Barcelona Research Center in Multiscale Science and Engineering, Universitat Politècnica de Catalunya, EEBE, 08019 Barcelona, Spain ABSTRACT Au/TiO2 lyogels have been prepared by sol-gel process followed by freeze-drying and calcination in order to induce crystallization. In the synthesis, a dispersion of AuNPs was added either during the sol-gel process or mixed with the lyogels in a ball mill after the freeze-drying. The lyogels are formed of TiO2 crystallites decorated with Au nanoparticles. The size of the TiO2 crystallites and the proportion of anatase and rutile depend on the parameters of the calcinations. The efficiency in photocatalysis diminishes with the size of the crystallites and is optimal when the lyogels include rutile together with the photoactive anatase. INTRODUCTION One of the main issues in replacing fossil fuels by hydrogen is the development of efficient technologies for hydrogen production. A very promising technology for this purpose is water splitting by photocatalysis. As one of the most well-known photocatalysts for this reaction is titania doped with gold nanoparticles (Au/TiO2) [1], there is an interest in the development of synthetic routes that allow the preparation of Au/TiO2 with high specificity and low cost. Gold nanoparticles suppress electron–hole pair recombination in TiO2 following UV excitation and create cathodic sites on for H2 evolution. The addition of sacrificial agents (typically, electron donors such as methanol or ethanol), with oxidation potentials lower than that of water, further suppresses electron–hole pair recombination in TiO2. The effect of the TiO2 support composition on the activity of Au/TiO2 photocatalysts for H2 production in alcohol–water systems has been the subject of a number of investigations [2], but interpreting data from existing literature is not straightforward, as additional factors including TiO2 crystallite size, crystallite shape, surface area and photocatalytic test conditions also strongly impact H2 production rates in Au/TiO2 systems. Highly porous titania with very large surface areas, known as aerogels, can be synthesized by preparing first a titania alcogel, then removing the alcohol by supercritical drying [3]. As this last step is difficult to implement for large scale production, freeze drying has been proposed as an alternative to supercritical drying much suited for industrial applications. Porous materials prepared by freeze drying, known as lyogels, have been already prepared, obtaining results comparable in some cases to aerogels [4]. However, examples of titania lyogels are scarce [5], and, to our knowledge, this methodology has not been applied yet to the synthesis of Au/TiO2. Au/TiO2 lyogels have been synthesized by preparing an alcogel using te
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