Oxygen-induced Restructuring of a Pd/Fe 3 O 4 Model Catalyst
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Catalysis Letters Vol. 107, Nos. 3–4, March 2006 (Ó 2006) DOI: 10.1007/s10562-005-0007-5
Oxygen-induced restructuring of a Pd/Fe3O4 model catalyst T. Schalowa, B. Brandta, D. E. Starra, M. Laurina, S. Schauermanna, Sh. K. Shaikhutdinova, J. Libudaa,b,*, and H. -J. Freunda a Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-14195 Berlin, Germany Lehrstuhl fu¨r Physikalische Chemie II, Universita¨t Erlangen-Nu¨rnberg, Egerlandstraße 3, D-91058 Erlangen, Germany
b
Received 1 November 2005; accepted 16 November 2005
We have studied the influence of oxygen on the structure and morphology of a Pd/Fe3O4 model catalyst using molecular beam (MB) methods, IR reflection absorption spectroscopy (IRAS) and scanning tunneling microcopy (STM). The model catalyst was prepared under ultrahigh vacuum (UHV) conditions by physical vapor deposition (PVD) and growth of Pd nanoparticles on an ordered Fe3O4 thin film on Pt(111). It is found that surface oxides are formed on the Pd nanoparticles even under mild oxidation conditions (temperatures of 500 K and effective oxygen partial pressures of around 10)6 mbar). These surface oxides are initially generated at the Pd/Fe3O4 interface and, subsequently, are formed at the Pd/gas interface. The process of formation and reduction of surface and interface oxides on the Pd particles is fully reversible in that all oxides formed can be fully reduced. As a result, the oxide phase acts like a storage medium for oxygen during oxidation reactions, as probed via CO oxidation. The process of surface and interface oxidation is directly connected with the onset of a non-reversible sintering process of the Pd particles. It is suggested that this sintering process occurs via a mobile Pd oxide species, which is stabilized by interaction with the Fe3O4 support. The restructuring is monitored via STM and IRAS using CO as a probe molecule. In addition to a decrease in particle density and Pd surface area, a reshaping of the particles occurs, which is characterized by the formation of well-ordered crystallites and with a relatively large fraction of (100) facets. After a few oxidation/reduction cycles at 500 K, the sintering process becomes very slow and the system shows a stable behavior under conditions of CO oxidation. KEY WORDS: palladium; iron-oxide; oxidation; CO; model catalysts; STM; morphology; molecular beams; IR reflection absorption spectroscopy.
1. Introduction In many cases, the activity of supported metal catalysts critically depends on the size, dispersion and morphology of the active metal particles as well as on the oxide support itself. At the microscopic level, the origin of these effects is poorly understood in many cases. Often it is assumed, that the size and support dependent properties arise from the presence of specific sites at the particle support interface or on the active particles themselves. However, related experimental studies of these properties are seriously complicated by the fact that the active surface may restructure or reconstruct in the presence of react
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