ARTICLE pubs.acs.org/JPCC
Active Sites for Hydrogen Dissociation over TiOx/Au(111) Surfaces I. Nakamura, H. Mantoku, T. Furukawa, and T. Fujitani* Research Institute for Innovation in Sustainable Chemistry, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8569, Japan ABSTRACT: We carried out H2 D2 exchange reactions over TiOx/Au(111) (x = 0 2) model surfaces to clarify the active sites for hydrogen dissociation over the Au/TiO2 catalyst. Ti oxides with controlled oxidation states were prepared on the Au(111) surface by regulation of O2 exposure and temperature. The HD formation rate depended on the concentration of TiO2 over Au(111), indicating that hydrogen dissociation sites were created between the stoichiometric TiO2 and Au. Atomic force microscopy observations indicated that the number of monolayer TiO2 islands with diameters of about 20 nm increased with increasing Ti coverage below ΘTi = 0.65 and that as ΘTi was increased further, a TiO2 multilayer began to form over Au(111). The turnover frequencies for HD formation over the TiO2/Au(111) surface, which were calculated from the length of the perimeter interface between the TiO2 islands and the Au substrate, were consistent with the frequencies reported for Au/TiO2(110) surfaces. Furthermore, the activation energies for HD formation over TiO2/Au(111) agreed well with those for Au/TiO2(110). We thus speculated that the active sites for hydrogen dissociation over Au/TiO2 were the Auδ+ Oδ- Ti sites, which were formed at the perimeter interface between the stoichiometric TiO2 and Au.
1. INTRODUCTION Au is chemically inert, but Au nanoparticles supported on metal oxides exhibit high catalytic activity for various oxidation reactions such as CO oxidation and propylene epoxidation.1 Numerous investigations have been carried out to elucidate the source of the high catalytic activity for CO oxidation. Previous reports have identified quantum size effects that depend on the thickness of the Au nanoparticles layer2,3 and the presence of low-coordinated Au atoms on the surface of the nanoparticles4 6 as possible contributors to the high activity. The formation of active sites by the interaction of Au nanoparticles with oxide supports has also been reported.7 In particular, charge transfer from an oxide support to Au nanoparticles is reported to form negatively charged Au particles8,9 and a reactive Au metal oxide interface.1,10,11 The Au metal oxide interface is currently considered to be the most likely active site for CO oxidation over Au catalysts. Au nanoparticles supported on metal oxides are also highly reactive for hydrogenation reactions such as the conversions of propylene to propane,12 acetylene to ethylene,13 and acrolein to allyl alcohol.14 Hydrogen dissociation is one of the rate limiting steps in such hydrogenation reactions, and the nature of the active sites for hydrogen dissociation over Au catalysts has been the subject of various studies.15 For example, hydrogen is reported to dissociate at low-coordinated edge or corner atoms of Au nanoparticles at 298 373 K.16 Both the shape and the size of Au particles have been suggested as being important for hydrogen dissociation.17 In contrast, in our recent investigation of the active sites for hydrogen dissociation over Au deposited on a TiO2(110) surface, we demonstrated that hydrogen dissociates at the perimeter interface between the Au nanoparticles and the TiO2 support.18 r 2011 American Chemical Society
Thus, we believe that this interface plays an important role in hydrogenation reactions. Therefore, elucidation of the structure and electronic states of Au and Ti at the interface is important for understanding the nature of the active sites on the Au/TiO2 catalyst. The structure and electronic state of the perimeter interface of Au/TiO2 have been studied.19,20 Akita et al. examined the crystal orientations between Au particles and TiO2 (anatase and rutile) by high-resolution transmission electron microscopy.19 Au particles were supported on the anatase TiO2 with the Au(111) plane parallel to the TiO2(112) plane. In contrast, for rutile TiO2, Au particles were supported with the Au(111) plane parallel to the TiO2(110) plane. Besenbacher et al. used density functional theory calculations to demonstrate that covalent bonds form between Au clusters and TiO2 on a reduced-TiO2(110) surface, whereas the bonding of Au clusters on an oxidized-TiO2(110) surface is partially ionic, which results in the Au cluster becoming cationic.20 These results indicate that the oxidation states of Ti strongly affect the electronic state of Au at the perimeter interface. However, the oxidation state of Ti and the structure of Ti oxide at the perimeter interface that acts as the active site for hydrogen dissociation are not entirely clear. In this study, we performed H2 D2 exchange reactions on TiOx/Au(111) model surfaces, that is, on Au(111) surfaces bearing Ti oxides in various oxidation states, to clarify the oxidation state of Ti and the structure of Ti oxide at the perimeter interface of the Au/TiO2 catalyst. Received: May 6, 2011 Revised: July 6, 2011 Published: July 07, 2011 16074
dx.doi.org/10.1021/jp2042087 | J. Phys. Chem. C 2011, 115, 16074–16080
The Journal of Physical Chemistry C
ARTICLE
Figure 1. Ti 2p XP spectra before and after oxidation of Ti-bearing Au(111) surfaces at ΘTi = 0.25, 0.75, and 1.25. Before oxidation (a), after oxidation, 7 L O2 exposure at 500 K (b), after oxidation, 35 L O2 exposure at 500 K (c), and after oxidation, 300 700 K in 3 10 7 Torr O2, followed by 200 L O2 exposure at 700 K, and then cooling to 400 K in O2 (d).
2. EXPERIMENTAL METHODS The experiments were performed in an ultrahigh vacuum apparatus composed of four chambers: a load-lock chamber (
