One-Dimensional PtO 2 at Pt Steps: Formation and Reaction with CO J. G. Wang, 1 W. X. Li, 1, * M. Borg, 2 J. Gustafson, 2 A. Mikkelsen, 2 T. M. Pedersen, 1 E. Lundgren, 2 J. Weissenrieder, 2 J. Klikovits, 3 M. Schmid, 3 B. Hammer, 1 and J. N. Andersen 2 1 Interdisciplinary Nanoscience Center (iNANO) and Department of Physics and Astronomy, University of Aarhus, DK-8000 Aarhus C, Denmark 2 Department of Synchrotron Radiation Research, Lund University, Box 118, S-221 00, Sweden 3 Institut fu ¨r Allgemeine Physik, Technische Universita ¨t Wien, A-1040 Wien, Austria (Received 5 August 2005; published 15 December 2005) Using core-level spectroscopy and density functional theory we show that a one-dimensional (1D) PtO 2 oxide structure forms at the steps of the Pt(332) surface after O 2 exposure. The 1D oxide is found to be stable in an oxygen pressure range, where bulk oxides are only metastable, and is therefore argued to be a precursor to the Pt oxidation. As an example of the consequences of such a precursor exclusively present at the steps, we investigate the reaction of CO with oxygen covered Pt(332). Albeit more strongly bound, the oxidic oxygen is found to react more easily with CO than oxygen chemisorbed on the Pt terraces. DOI: 10.1103/PhysRevLett.95.256102 PACS numbers: 68.47.De, 68.43.Bc, 68.43.Fg, 82.65.+r Steps are inevitably present on the surface of nanoscale particles commonly used in heterogeneous catalysis. The adsorption of, and reactions amongst, molecules are often very different at steps than on the flat parts of the particle surfaces. The steps may therefore profoundly influence the catalytic properties of the surfaces, which has indeed been demonstrated for, e.g., NO=Ru [1], N 2 =Ru [2], and O 2 =Pt [3]. The altered adsorption and reaction behavior is often ascribed to the fact that the lower coordination number at a step modifies the electronic structure in its vicinity and that the molecules may adsorb in new geometrical configura- tions at steps [4]. However, in addition to these effects, the steps may also be significantly modified by one or more of the reactant molecules through local compound formation occurring as an integral part of the reaction. Such com- pound formation may be expected to occur more easily at steps because of the larger freedom for geometrical rear- rangements there than on flat parts of the surface. The possibility of compound formation is of particular relevance in oxidation reactions, since typical catalyst materials all have a high propensity for oxide formation [5]. It was recently shown that the active phase of Ru in oxidation reactions is that of a thin bulk oxide [6], and for late-transition and noble metal elements such as Rh [7], Pd [8], and Ag [9], surface oxides have been proposed as the active phases. In the present Letter we give experimental and theoreti- cal evidence that for the stepped Pt(332) surface, oxide formation also takes place even at relatively moderate oxygen pressures. The oxide found, however, only occurs in the form of one-dimensional (1D) Pt-oxide stripes along the steps. These oxide stripes turn out to be a key ingredient in understanding CO oxidation on this stepped surface. We demonstrate, using core-level spectroscopy (CLS), that adsorbates at steps and on the terraces of such stepped surfaces may lead to different core-level binding energies. Detailed information on the adsorption sites is extracted from these core-level binding energy fingerprints via com- parison to density functional theory (DFT) calculations. The Letter is organized as follows. First, the O adsorption site is identified comparing measured Pt 4f 7=2 and O 1s CLS and DFT results. Then, the C 1s CLS is introduced as a probe to (step vs terrace) adsorption sites of CO and, finally, the C 1s CLS is used to monitor the sequence of oxygen removal over Pt(332) during annealing after low- temperature CO exposure. The experiments were performed at beam line I311 at MAX-lab, Lund, Sweden [10]. The Pt crystals were cleaned by Ar  sputtering, annealing in O 2 , and final annealing in vacuum at 1200 K in order to remove residual oxygen. The sample cleanliness was checked by monitor- ing the Pt 4f 7=2 ,C 1s, and O 1s core-level regions. The sample temperature was measured by Chromel-Alumel thermocouples spot welded to the crystals. Surface order- ing was checked by low-energy electron diffraction show- ing well-ordered surfaces. The slab based DFT calculations were done using the DACAPO package [11] with ultrasoft pseudopotentials, plane waves (E cut  25 Ry) k-point mesh, the revised Perdew, Burke, Ernzerhof (RPBE) exchange-correlation functional [12], and the calculated Pt lattice constant of 4.02 A ˚ . For Pt(332) we used a (2  1) super cell [cf. Figs. 1(b) and 1(c)] with four (111) Pt layers, two of which were relaxed together with the adsorbates. Core-level shifts were calculated using pseudopotentials with core holes [13]. The Pt(332) surface is composed of six atomic rows wide (111) terraces and a (111) type step. Hence core-level photoemission spectra for Pt(332) and Pt(111) are ex- pected to be very similar. In Fig. 1(a) we present the Pt 4f 7=2 core spectra obtained after saturation of the two surfaces with O as achieved by a p  10 6 Torr O 2 expo- sure for 500 s at 310 K. Indeed, both spectra contain a bulk component (dashed line) at 70.90 eV, a component at 70:5 eV (‘‘clean’’) from surface Pt atoms not coordinat- PRL 95, 256102 (2005) PHYSICAL REVIEW LETTERS week ending 16 DECEMBER 2005 0031-9007= 05=95(25)=256102(4)$23.00 256102-1 © 2005 The American Physical Society