Hydrogen Production from Ethanol. Comparing Thermal Catalytic Reactions to Photo-catalytic Reactions
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Hydrogen Production from Ethanol. Comparing Thermal Catalytic Reactions to Photo-catalytic Reactions. M. Scott1, A.M. Nadeem1, G.I.W. Waterhouse1, H. Idriss*2 1 Department of Chemistry, University of Auckland, Auckland, New Zealand 1 Department of Chemistry, University of Aberdeen, Aberdeen, UK *Corresponding author: [email protected] ABSTRACT Hydrogen production from renewables such as bio-ethanol is one of the most promising processes for energy carriers in a sustainable way. In this work we review and compare two catalytic systems: one based on thermal activation over bimetallic catalysts (Rh-Pd/CeO2) and the other over photo-excited semiconductor catalysts (Au/TiO2 anatine, rutile and anatase/rutile). It is found that the hydrogen yield is far higher on the thermally activated catalysts (at 773K) when compared to that of the photo-exited catalysts (at room temperature); about 60 times. However, the photo-excited catalysts are a promising way to create a fully sustainable system for future applications if the complete removal of hydrogen atoms from water and ethanol are obtained at room temperature. INTRODUCTION The catalytic reaction of ethanol to hydrogen has been studied for over a decade now and many active catalysts have been found [1-10]. The reaction requires a few key points including the breaking of the carbon-carbon bond and the efficient oxidation of CO to CO2 (direct and/or via water gas shift reaction, WGSR) [9, 10]. One of the main challenges is the development of sustainable catalysts requiring not only renewable feedstock but also renewable energy needed for the reaction to occur. This has motivated the development of photo-catalytic materials that can use energy from the sun to conduct the reaction. In this paper we are discussing two prototype catalysts for the hydrogen generating reaction: one, the most common, needing thermal activation and the other needing photoexcitation. The thermally excited catalysts are composed of a noble metal such as Rh or Ru together with another noble metal such as Pt or Pd; both are deposited on a reducible support such as CeO2. The role of Rh or Ru is to break the carbon-carbon bond while that of Pt or Pd is to enhance the hydrogen production via hydrogen-hydrogen bond association either direct or through WGSR. While the exact reasons for the role of the transition metals needed to break the carbon-carbon bond are not fully understood, transition metals such as Rh or Ru have low activation energy for breaking the carbon-hydrogen bonds and may therefore brings the carbon-carbon bond closer to the surface initiating its dissociation. The photo-catalysts are composed of a semiconductor such as TiO2 on which a noble metal is deposited such as gold in nano-particle size. We have recently found that Au/TiO2 anatase catalysts are active for the production of hydrogen from ethanol [11]. However to date no catalysts have been found to break the carbon-carbon bond efficiently at room temperature under photo-excitation. The following two reactions describe hydrogen produc
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