Ethanol Photoreaction on RuO x /Ru-Modified TiO 2 (110) S. Kundu, † A. B. Vidal, †,‡ M. A. Nadeem, § S. D. Senanayake, † H. Idriss, § P. Liu, † J. A. Rodriguez, † and D. Stacchiola* ,† † Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973, United States ‡ Centro de Química, Instituto Venezolano de Investigaciones Cientificas (IVIC), Apartado 21827, Caracas 1020-A, Venezuela § Department of Chemistry, University of Aberdeen and School of Engineering, Robert Gordon University, Aberdeen, United Kingdom ABSTRACT: During the photochemical reaction of organic molecules on oxide surfaces, radicals are formed and participate in heterogeneous photocatalytic processes; how- ever, understanding the mechanistic origins and the fate of such species under reaction conditions is difficult. In this work we carry out a combined experimental and theoretical study on the thermal and photochemical interaction of ethanol with RuO x /TiO 2 (110) surfaces. Ethanol dissociatively adsorbs on both TiO 2 and RuO x /TiO 2 surfaces forming ethoxide. Our DFT calculations indicate that the ethoxide formation is more exothermic on RuO x /TiO 2 (110) surfaces (ΔE= -1.61 eV) than on the clean rutile TiO 2 (110) surface (ΔE= -0.95 eV). Defect sites present on RuO x /TiO 2 surfaces can dissociate part of the ethoxide to acetaldehyde even below 300 K, which can be further oxidized to acetate resulting in the reduction of the RuO x nanoparticles. Exposure to UV irradiation of the ethoxide covered surfaces in the presence of oxygen at 300 K resulted in considerable decrease in ethoxide species by conversion to acetate. It is found that the Ru/TiO 2 system is more active for the photo-oxidation of ethanol to acetaldehyde than TiO 2 . A linear trend of the rate of acetaldehyde and carbon dioxide production from exposure to ethanol of Ru/TiO 2 surfaces in the presence of O 2 indicates that more surface sites are available for the adsorption of O 2 than on bare TiO 2 surfaces, possibly at the interface of the Ru metal nanoparticles and TiO 2 surfaces, which facilitates the photo-oxidation. ■ INTRODUCTION Ethanol has been proposed as a renewable hydrogen carrier for energy purposes. 1,2 Hence, there has been growing interest in studying the interaction of ethanol with surfaces of model catalysts, which can liberate the hydrogen contained in ethanol. Among different catalytic processes, ethanol photoreactions on metal oxides are receiving major attention due to their potential as materials for clean hydrogen production. 3,4 The conversion of ethanol to hydrogen in the presence of steam or O 2 has been reported at relatively high temperatures. 1 In comparison with thermal processes, photoreactions could operate at lower temperatures and the energy required for the process could be fully renewable. Recently, it has been reported that Au/TiO 2 can be a fast and efficient catalyst for hydrogen production from ethanol opening up the possibility of using metal/metal- oxide systems as promising catalysts for the ethanol photo- reaction. 3 There have been several studies on the photocatalytic reactions of organic compounds using Ru-doped TiO 2 as a catalyst. In 1989, Sobczynski and coworkers reported the hydrogen production from a methanol and water solution on polycrystalline Ru/TiO 2 photocatalysts. 5 It was reported that a very low concentration of ruthenium (ca. 0.75 wt.%) was most active for hydrogen production. Wetchakun and coworkers prepared a series of photocatalyst with 0.1, 0.2, 0.5, 1.0, and 2.0 wt % Ru loading on polycrystalline TiO 2 to study the effect of ruthenium loading on the photocatalytic activity. 6 TiO 2 doped with 0.1% Ru presented the highest photocatalytic activity for photomineralization of formic acid. 7 In 2006, Sasirekha and coworkers studied the photocatalytic reduction of CO 2 on Ru/ TiO 2 anatase in the presence of water. 8 They monitored the photocatalytic activity by analyzing the possible reduction products of the reaction including formic acid, formaldehyde, methanol, and methane. The study revealed that Ru-doped TiO 2 particles have higher photocatalytic activity than pure TiO 2 , and it was proposed that the driving force for the observed enhanced activity was charge separation due to the formation of a Schottky barrier at the metal-oxide interface. There has been no detailed study on the interaction of ethanol with ruthenium metal/metal-oxide-modified TiO 2 surfaces and its photocatalytic activity toward ethanol decomposition. Model catalysts prepared on single crystal surfaces can offer a more methodical approach to understand the reaction mechanism and accurately monitor the effect of metal oxide surfaces on the reaction products of ethanol. Moreover, our previous study of RuO x 1D nanostripes deposited on TiO 2 shows special chemical properties for CO oxidation and water dissociation in comparison with bulk ruthenium oxide. 9,10 It is important to reveal the changes imposed on the electronic structure of TiO 2 Received: February 12, 2013 Revised: April 26, 2013 Published: May 1, 2013 Article pubs.acs.org/JPCC © 2013 American Chemical Society 11149 dx.doi.org/10.1021/jp4015367 | J. Phys. Chem. C 2013, 117, 11149-11158