Silicon Oxide Surface Segregation in CO Oxidation on Pd: An in situ PEEM, MS and XPS Study
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Silicon Oxide Surface Segregation in CO Oxidation on Pd: An in situ PEEM, MS and XPS Study D. Vogel • Z. Budinska • C. Spiel • R. Schlo¨gl Y. Suchorski • G. Rupprechter
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Received: 30 October 2012 / Accepted: 12 December 2012 / Published online: 8 January 2013 Ó The Author(s) 2013. This article is published with open access at Springerlink.com
Abstract The effect of silicon oxide surface segregation on the locally-resolved kinetics of the CO oxidation reaction on individual grains of a polycrystalline Pd foil was studied in situ by PEEM, MS and XPS. The silicon oxide formation induced by Si-impurity segregation at oxidizing conditions, was monitored by XPS and its impact on the global and local (spatially resolved) kinetics of the CO oxidation was determined by MS and PEEM. The results reveal a drastic inhibiting effect of silicon oxide on the Pd reactivity towards CO oxidation, manifested both in the collapse of the global CO2 formation rate and in the modified local reactive properties of individual Pd micrograins. The presence of adsorbed oxygen on the Pd surface effectively enhances the silicon segregation to the Pd surface. Keywords CO oxidation Polycrystalline Pd foil Si segregation Si oxide formation Photoemission electron microscopy X-ray photoelectron spectroscopy
1 Introduction Catalytic CO oxidation on Pt-group metal surfaces is often considered as a simple model reaction of significant practical meaning with respect to pollution emission control. D. Vogel Z. Budinska C. Spiel Y. Suchorski (&) G. Rupprechter Institute of Materials Chemistry, Vienna University of Technology, Getreidemarkt 9, 1060 Vienna, Austria e-mail: [email protected] D. Vogel R. Schlo¨gl Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany
More than three decades ago, pioneering molecular beam experiments of CO oxidation on Pd(111) surfaces performed by Engel and Ertl provided first evidence that the reaction follows the Langmuir–Hinshelwood mechanism [1]. Further applications of the surface science approach to single crystal surfaces revealed the elementary steps of this reaction such as molecular adsorption of CO, dissociative adsorption of oxygen, surface reaction and desorption of carbon dioxide, and culminated in the Nobel Prize awarded to Gerhard Ertl in 2007 [2]. However, at the same time this seemingly simple model reaction was found to be quite complex in case of real catalysts consisting of oxide supported precious metal nanoparticles, and bridging the ‘‘materials and pressure gap’’ between the single crystal surfaces in UHV and real catalysts under ambient conditions is still a challenging task. To overcome the ‘‘material gap’’ between single crystal surfaces and real catalysts, many approaches have been applied, e.g. the investigation of field emitter tips [3], cylindrical single crystals [4] or polycrystalline foils [5–7]. A polycrystalline foil is a particularly suitable model system since it exhibits lm-sized domains of different orientations which
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