Dfsfs

G Model CATTOD-7603; No. of Pages 4

ARTICLE IN PRESS
Catalysis Today xxx (2011) xxx–xxx

Contents lists available at SciVerse ScienceDirect

Catalysis Today
journal homepage: www.elsevier.com/locate/cattod

Scanning tunneling microscopy evidence for the Mars-van Krevelen type mechanism of low temperature CO oxidation on an FeO(1 1 1) film on Pt(1 1 1)
M. Lewandowski, I.M.N. Groot, S.Shaikhutdinov ∗ , H.-J. Freund
Abteilung Chemische Physik, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany

a r t i c l e

i n f o

a b s t r a c t
We provide experimental evidence that the low temperature CO oxidation reaction on an ultrathin FeO(1 1 1) film on Pt(1 1 1) proceeds through the Mars-van Krevelen type mechanism which includes thefollowing steps: the FeO film is oxidized into an FeO2−x film with trilayer O–Fe–O structure; CO reacts with the weakly bound, topmost oxygen atoms, effectively reducing the FeO2−x surface to FeO-like; and the subsequent oxidation by O2 recovers the FeO2−x structure. © 2011 Elsevier B.V. All rights reserved.

Article history: Received 14 February 2011 Received in revised form 4 May 2011 Accepted 17 August2011 Available online xxx Keywords: Thin oxide films CO oxidation Scanning tunneling microscopy

1. Introduction The catalytic oxidation of CO over platinum group metals is one of the most widely studied reactions in heterogeneous catalysis (e.g., see reviews [1–3] and references therein). Due to its technological importance in automotive catalysts [4] and fuel cells [5,6], detailed studies havebeen performed on model systems, primarily on Pt(1 1 1) single crystals, to unravel the reaction mechanisms. It is now well established that the oxidation reaction follows the Langmuir–Hinshelwood mechanism, in which CO2 is formed through the associative reaction of chemisorbed CO and O atoms dissociatively adsorbed on the surface [1]. Note that the CO oxidation reaction on platinum surfaces mayeven show oscillatory behavior, which was observed on Pt(1 1 1) only at relatively high pressures as the reaction proceeds most likely through surface oxidation–reduction cycles [7]. Recently, it has been found that thin oxide films grown on metals can promote this reaction at low temperatures [8,9]. Indeed, a monolayer film of FeO(1 1 1) grown on Pt(1 1 1) showed activity for CO oxidation alreadyat 450 K, i.e., at a temperature where bare Pt(1 1 1) is almost inactive [8]. The concept of using thin films as catalysts was further demonstrated on supported systems where metal nanoparticles were encapsulated by a thin oxide film as a result of strong metal-support interaction with a reducible support [10–12]. Also, for a three monolayer thick MgO film on Ag(1 0 0), a density functional theory(DFT) study predicted CO oxidation with a

∗ Corresponding author. E-mail address: shaikhutdinov@fhi-berlin.mpg.de (S. Shaikhutdinov). 0920-5861/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cattod.2011.08.033

much lower activation barrier than on Pt(1 1 1) [9] (experimentally not proven yet). The atomic structure of ultrathin FeO(1 1 1) films on Pt(1 1 1) is welldocumented in the literature [13]. It consists of close-packed layers of Fe and O, stacked as O–Fe–Pt, and exhibits a Moiré pattern due to the lattice mismatch between the FeO(1 1 1) and Pt(1 1 1) lattices. However, it has been found that, at elevated oxygen pressures, the bilayer O–Fe film transforms into a trilayer O–Fe–O film [8,14]. This reaction is computed to be site specific within the large(ca. 2.6 nm) Moiré unit cell [15], that may explain scanning tunneling microscopy (STM) results showing the formation of close-packed FeO2 islands rather than a continuous FeO2 film [14]. In addition, it √ √ has been shown that the FeO2 islands exhibit ( 3 × 3)R30◦ reconstruction which has tentatively been assigned to the relaxation of the Fe layer between the O-layers. The reaction mechanism of CO...