Priority Communication Surface science approach to catalyst preparation – Pd deposition onto thin Fe 3 O 4 (1 1 1) films from PdCl 2 precursor Hui-Feng Wang, Hiroko Ariga 1 , Rhys Dowler, Martin Sterrer ⇑ , Hans-Joachim Freund Department of Chemical Physics, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany article info Article history: Received 23 August 2011 Revised 21 September 2011 Accepted 22 September 2011 Available online 22 October 2011 Keywords: Catalyst preparation Surface science approach Thin film Impregnation Palladium Iron oxide Scanning tunneling microscopy X-ray photoelectron spectroscopy abstract In this work, we introduce a surface science approach to supported metal catalyst preparation utilizing thin, single-crystalline oxide films as substrates. The use of thin oxide films allows for combined morpho- logical and chemical characterization of the sample at various steps of a typical catalyst preparation pro- cedure. A Fe 3 O 4 -supported Pd model catalyst was prepared by impregnation of Fe 3 O 4 (1 1 1) films with acidic PdCl 2 solution, and the transformation of the adsorbed precursor into supported Pd nanoparticles by stepwise heating in vacuum was followed with scanning tunneling microscopy (STM) and X-ray pho- toelectron spectroscopy (XPS). The results provide evidence for homogeneous nucleation of Pd particles out of a monolayer of adsorbed precursor and an enhancement of the particle-support interaction with increasing annealing temperature. Chlorine, which remains on the model catalyst surface after vacuum annealing, could be removed by oxidation/reduction. This treatment also leads to particle sintering with an increase in the average particle diameter from 2 nm to 4 nm after oxidation/reduction. Ó 2011 Elsevier Inc. All rights reserved. 1. Introduction The preparation of supported metal catalysts is a complex mul- ti-step procedure [1,2]. In the most commonly applied preparation route, impregnation, the first step involves the interaction of aque- ous solutions containing metal precursors with the support, fol- lowed by drying, calcination, and reduction to transform the metal precursor into the catalytically active compound. Clearly, catalytic activity in a given test reaction is the ultimate criterion for the usefulness of a specific preparation procedure and is often combined with pre, in situ or post-reaction characterization to determine the physical (particle size and shape) and chemical (oxi- dation state, chemisorption) properties of the active catalyst and to reveal details about the reaction mechanism. Maximizing the abundance and stability of active sites requires, in turn, molecular level control and an understanding of the important steps during catalyst preparation and activation [3–7]. Surface science experiments using morphologically well-de- fined samples contributed to a great extent to the atomic level understanding of catalytically active metals. It is highly desirable to extend such studies and therefore the benefit of ultimate mor- phological control to the important field of supported metal cata- lyst preparation. Previous attempts related to metal deposition from liquid phase precursors onto flat oxide substrates (single crystal surfaces and single crystal fragments, or noncrystalline oxide thin films) provided some relevant insights; however, they lack of combined morphological and chemical characterization [8–13]. In the approach presented herein, we take advantage of the ability to reproducibly grow under ultra-high vacuum (UHV) conditions thin, single-crystalline oxide films with well-defined morphology, which allow typical surface science analysis tools to be applied and use them as substrates for studying the formation of supported metal nanoparticles following wet chemical catalyst preparation procedures. As a prototype system, we have chosen to use Fe 3 O 4 (1 1 1) films grown on Pt(1 1 1) as substrate for Pd nanoparticles. UHV-depos- ited Pd nanoparticles on Fe 3 O 4 have been extensively investigated in our laboratory in the past and served as a model system for de- tailed studies of Pd nanoparticle reactivity in oxidation and hydro- genation reactions [14,15]. The UHV preparation route is exceptionally clean and does not include possible chemical inter- ference from the precursor used in realistic preparations. In partic- ular, catalysts prepared via impregnation with PdCl 2 , the most frequently used precursor for supported Pd catalysts [16], retain chlorine, which has been shown to act as a poison in certain cata- lytic applications, supposedly by blocking the active sites [17–19]. Here, we report on the preparation of a Fe 3 O 4 (1 1 1)-supported Pd 0021-9517/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.jcat.2011.09.026 ⇑ Corresponding author. E-mail address: sterrer@fhi-berlin.mpg.de (M. Sterrer). 1 Present address: Catalysis Research Center, Hokkaido University, Kita-ku, North 21 West 10, Sapporo 001-0021, Japan. Journal of Catalysis 286 (2012) 1–5 Contents lists available at SciVerse ScienceDirect Journal of Catalysis journal homepage: www.elsevier.com/locate/jcat