ARTICLE pubs.acs.org/JPCC
Preparation, Characterization, and Catalytic Properties of Tungsten Trioxide Cyclic Trimers on FeO(111)/Pt(111) Shao-Chun Li,†,§ Zhenjun Li,§ Zhenrong Zhang,‡ Bruce D. Kay, Roger Rousseau,* and Zdenek Dohnalek* Chemical and Materials Sciences Division, Fundamental and Computational Sciences Directorate, and Institute for Integrated Catalysis, Pacific Northwest National Laboratory, Post Office Box 999, Mail Stop K8-88, Richland, Washington 99352, United States ABSTRACT: The structure and catalytic activity of tungsten oxide clusters formed via sublimation of monodispersed cyclic (WO3)3 onto FeO(111)/ Pt(111) has been studied by a combination of scanning tunneling microscopy (STM), X-ray photoemission spectroscopy (XPS), infrared reflection absorption spectroscopy (IRAS), temperature-programmed desorption (TPD), and density functional theory (DFT). After (WO3)3 deposition, STM images reveal new features composed of three bright maxima arranged in a equilateral triangular configuration with an edge length of ∼10 Å. This length is significantly larger than the size of (WO3)3, indicating that the clusters dissociated. This conclusion is corroborated by DFT calculations showing that cluster dissociation into surface-bound WO3 monomers is exothermic and kinetically feasible at 300 K. The dissociation is accompanied by significant FeO(111) rearrangements with the Fe ions being pulled on top of the surface and bonded to the WO3 fragments. Both surface spectroscopies (XPS and IRAS) and calculations indicate that the W ions in the WO3 monomers remain in their original oxidation state (6+) and possess a single terminal WdO group. TPD studies show that this system does not efficiently catalyze alcohol dehydration. This inactivity is explained on the basis of the reaction mechanism calculated by DFT.
overlayer grown on Pt(111).26 In contrast with TiO2(110), which has important catalytic properties on its own, we expected the oxygen-terminated FeO(111) surface to be significantly less reactive and therefore ideal as an inert support for our (WO3)3 clusters. Surprisingly, our combined scanning tunneling microscopy (STM), X-ray photoemission spectroscopy (XPS), infrared reflection absorption spectroscopy (IRAS), and density functional theory (DFT) study shows that the FeO(111) interacts strongly with (WO3)3 clusters via a Lewis acid/base chemistry. The clusters are shown to dissociate into three WO3 monomers already at 300 K, forming additional W O and O Fe bonds with the surface FeO(111) layer. Moreover, the resulting adsorbed tungsten oxide species are shown to be inert toward reactions with alcohols. The inert nature of this system is understood via a reaction mechanism developed by use of DFT.
1. INTRODUCTION Early transition metal oxides represent an important class of catalytically active materials and, as such, they have found numerous applications, for example, in partial oxidation of alcohols, oxidative dehydrogenation of hydrocarbons, and selective reduction of NOx.1 5 Supported WO3 clusters that are the subject of this work, have been widely studied.6 17 It has been shown that the oxide support can substantially influence their catalytic activity.1,9 Tailored well-characterized model systems are thus essential to understand how the oxide support affects the cluster binding and catalytic properties. Such model systems further allow for a close coupling with theory, creating ideal platforms for developing a detailed molecular-level understanding of the cluster structures, unambiguous identification of the catalytically active sites, and elucidation of the reaction mechanisms. Recently, our group has focused on the structural characterization and catalytic properties of model WO3 catalysts (thin films and supported clusters) prepared via direct sublimation of WO3.13,14,18 22 This technique yields a source of predominantly monodispersed (WO3)3 clusters that can be deposited on any substrate. Prior gas-phase studies show that the (WO3)3 clusters are cyclic, with the ring composed of three (-W-O-) units and two terminal tungstyl groups (WdO) and all W ions in (6+) oxidation state.23 25 Our catalytic studies of (WO3)3 embedded in an alcohol matrix and/or supported on TiO2(110) and Pt(111) demonstrated that the Lewis acid/base chemistry of WdO groups plays a prominent role in alcohol chemistry, leading to formation of alkenes, aldehydes, ketones, and ethers,13,14 and in the polymerization of formaldehyde.20,21 To further understand the interactions of (WO3)3 with oxide supports, we focus on (WO3)3 cluster deposition on a FeO(111) r 2011 American Chemical Society
2. METHODS 2.1. Experimental Details. The experiments were performed in two ultrahigh-vacuum (UHV) systems: a molecular beam scattering apparatus, devoted to ensemble averaged studies, and a scanning probe microscopy apparatus, used for atomically resolved studies. The experimental setup and procedures used in both systems are described below. The molecular-beam scattering apparatus (base pressure
