Subsurface oxygen formation on Pt(1 0 0): Experiments and modeling Noah McMillan a , Tanmay Lele c,1 , Christopher Snively a,b , Jochen Lauterbach a, * a Department of Chemical Engineering, University of Delaware, Newark, DE 19716, USA b Department of Materials Science and Engineering, University of Delaware, Newark, DE 19716, USA c School of Chemical Engineering, Purdue University, West Lafayette, IN 47907, USA Available online 13 June 2005 Abstract Spatially resolved techniques, such as photoemission electron microscopy (PEEM) and ellipsomicroscopy for surface imaging (EMSI), have been particularly useful in the investigation of pattern formation during CO oxidation on platinum catalysts. One surprising result of these studies has been the discovery of subsurface oxygen on Pt(1 0 0). The formation of subsurface oxygen has been reported previously on Pt(1 0 0) during CO oxidation at low pressures (<1  10 4 Torr). This communication reports the formation of subsurface oxygen at intermediate pressures (0.1 Torr). These observations show that subsurface oxygen plays a role in catalytic CO oxidation and pattern formation, which has implications for catalysts operated at higher pressures. New microkinetic models of CO oxidation that incorporate subsurface oxygen are discussed. Results of these models are qualitatively similar to experimental observations of subsurface oxygen, confirming the importance of this species in the reaction dynamics. # 2005 Elsevier B.V. All rights reserved. Keywords: Subsurface oxygen; Pattern formation; Pt(100); PEEM; EMSI; CO oxidation 1. Introduction CO oxidation over noble metal catalysts is one of the most widely studied surface reactions. This system has been found to exhibit complex nonlinear behavior, including reaction rate oscillations and spatiotemporal pattern forma- tion over a wide range of conditions from high vacuum to atmospheric pressures [1,2]. The Langmuir–Hinshelwood model alone cannot explain these phenomena, and detailed experimental investigations are required for the develop- ment of more appropriate models. The surface reconstruction model is a generally accepted explanation for the complex behavior of CO oxidation on Pt(1 0 0) at low pressures (<1  10 4 Torr) [3]. The clean Pt(1 0 0) surface spontaneously rearranges from its bulk-like (1  1) termination into a quasi-hexagonal (hex) recon- structed surface. This reconstruction is favored because it minimizes the surface energy in the absence of adsorbates. Lifting of the reconstruction (i.e. recovery of the 1  1 surface) is induced by adsorption of a critical coverage of CO or O 2 . The adsorption probability of O 2 on the 1  1 surface is 0.1, compared to 10 4 on the hex surface, therefore the rate of O 2 adsorption increases with the lifting of the hex reconstruction [4–6]. The increasing rate of O 2 adsorption leads to a higher reaction rate with CO to form CO 2 , which immediately desorbs at typical reaction temperatures. As a result, the surface coverage decreases and the surface reverts to the hex reconstruction. Repetition of this cycle accounts for reaction rate oscillations and the appearance of spatiotemporal patterns. One limitation of the surface reconstruction model is that, as in the Langmuir–Hinshelwood model, adsorbates are restricted to the catalyst surface. However, it has been known for some time that adsorbates can penetrate the surface under reaction conditions, in some cases causing significant changes in reaction dynamics and catalytic activity. Such oxygen species have been referred to in the literature as ‘‘bulk oxygen,’’ ‘‘dissolved oxygen,’’ or ‘‘surface oxide.’’ For example, Turner et al. reported a surface oxide on platinum wire that formed during CO oxidation between 100 and 800 Torr and temperatures as low as 520 K [7]. www.elsevier.com/locate/cattod Catalysis Today 105 (2005) 244–253 * Corresponding author. Tel.: +1 302 831 6327; fax: +1 302 831 1048. E-mail address: lauterba@che.udel.edu (J. Lauterbach). 1 Present address: Departments of Pathology and Surgery, Children’s Hospital and Harvard Medical School, Boston, MA 02138, USA. 0920-5861/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cattod.2005.02.042