NO and CO Adsorption on Nonhomogeneous Pt(100) Surfaces D. Y. Zemlyanov,* M. Yu. Smirnov, and E. I. Vovk Boreskov Institute of Catalysis, Pr. Akademika Lavrentieva 5, 630090 Novosibirsk, Russia Received July 1, 1998. In Final Form: October 1, 1998 The adsorption of NO and CO on a nonhomogeneous Pt(100) surface made up of both the (hex) and (1 × 1) structural phases was studied at 300 K by means of high-resolution electron energy-loss spectroscopy (HREELS). The nonhomogeneous surface was prepared by titrating NOads/(1 × 1) islands formed on the original (hex) surface with deuterium and subsequent heating to desorb residual Dads. The as-prepared surface was assumed to consist of patches of the (1 × 1) structure surrounded by the (hex) surface. The ratio between the (1 × 1) patches and the (hex) area was controlled by the initial coverage of NOads before titration. The exposure of the nonhomogeneous surface to NO or CO at 300 K led to saturation of the (1 × 1) patches first. It was supposed that the NO and CO molecules adsorbing on the (hex) areas quickly diffuse along the surface, meet the (1 × 1) patches, and are “trapped” by them. NOads (COads) spreads on the surface of the (1 × 1) patches. As soon as the (1 × 1) patches are saturated, the adsorption-induced (hex) f (1 × 1) back-reconstruction takes place. The details of the NO and CO adsorption on the nonhomogeneous surface were compared with the process on the reconstructed and unreconstructed Pt- (100) surfaces. 1. Introduction The Pt(100) surface shows two surface structures, which exhibit dramatically different adsorption and catalytic properties, 1 making it an attractive subject for model studies. The clean Pt(100) surface undergoes a recon- struction under ultrahigh vacuum (UHV) conditions and exhibits a hexagonal (hex) surface structure, characterized by a (5 × 20) low-energy electron diffraction (LEED) pattern. 2-5 The unreconstructed (1 × 1) surface can be prepared by the procedure described by Bonzel et al. 5 This procedure includes NO adsorption on the reconstructed Pt(100) surface up to saturation at 300 K, subsequent heating of the NO ads layer, reduction with hydrogen of the O ads species formed by NO dissociation, and heating to remove residual H ads . The clean (1 × 1) surface is metastable, converting back to the (hex) structure at temperatures above 400 K. The adsorption of both CO and NO on the Pt(100) surface has been studied in detail by a number of surface science techniques. The following scheme of NO and CO adsorption on the Pt(100)-(hex) surface at 300 K could be formulated on the basis of data from the literature. Adsorption of NO and CO induces locally the transition of the (hex) phase to the (1 × 1) one, leading to the formation of NO ads / (1 × 1) or CO ads /(1 × 1) islands. 6-12 The local coverage on the (1 × 1) islands is approximately 0.5 ML, 13 whereas the (hex) phase surrounding the islands is nearly free of adsorbates. 6,9 According to LEED and scanning tunneling microscopy (STM) data, 14-17 the formation of the CO ads /(1 × 1) and NO ads /(1 × 1) islands proceeds through a nucleation and trapping mechanism that includes two stages. The first stage is assumed to be the generation of the centers of nucleation presumably on the (hex) phase on the basis of surface structural defects. 17 During the second stage the growth of the adsorption islands proceeds without the appearance of new centers of nucleation. The atomic density of the (1 × 1) surface is 1.28 × 10 15 cm -2 . 4 This value is approximately 20% lower than the one of the (hex) surface (1.61 × 10 15 cm -2 ). 15 This difference results in a significant mass-transport of platinum atoms during the NO ads /(1 × 1) and CO ads /(1 × 1) island formation. STM data demonstrate 15-17 that platinum atoms expelled during the (hex) f (1 × 1) transition form clusters with a size of ∼15-25 Å. The clusters are distributed randomly within the (1 × 1) patches. Two molecular adsorption states of NO were detected in the NO ads /(1 × 1) islands by means of infrared absorption spectroscopy (IRAS) 6 and high- resolution electron energy-loss spectroscopy (HREELS), 18-20 namely on the surface of the (1 × 1) patches and on structural defects lifted by the (hex) f (1 × 1) reconstruc- * Corresponding author. Fax: +7 (3832* 343056. E-mail: dzem@ catalysis.nsk.su. (1) Helms, C. R.; Bonzel, H. P.; Kelemen, S. J. Chem. Phys. 1976, 65, 1773. (2) McCarroll, J. J. Surf. Sci. 1975, 53, 297. (3) Heilmann, P.; Heinz, K.; Mu ¨ ller, K. Surf. Sci. 1979, 83, 487. (4) Norton, P. R.; Davies, J. A.; Creber, D. K.; Sitter, C. W.; Jackman, T. E. Surf. Sci. 1981, 108, 205. (5) Bonzel, H. P.; Broden, G.; Pirug, G. J. Catal. 1978, 53, 96. (6) Gardner, P.; Tushaus, M.; Martin, R.; Bradshaw, A. M. Surf. Sci. 1990, 240, 112. (7) Gardner, P.; Martin, R.; Tushaus, M.; Bradshaw, A. M. J. Electron Spectrosc. Relat. Phenom. 1990, 54/55, 619. (8) Martin, R.; Gardner, P.; Bradshaw, A. M. Surf. Sci. 1995, 342, 69. (9) Behm, R. J.; Thiel, P. A.; Norton, P. R.; Ertl, G. J. Chem. Phys. 1983, 78, 7437. (10) Thiel, P. A.; Behm, R. J.; Norton, P. R.; Ertl, G. J. Chem. Phys. 1983, 78, 7448. (11) Yeo, Y. Y.; Vattuone, L.; King, D. A. J. Chem. Phys. 1996, 104, 3810. (12) Jackman, T. E.; Griffiths, K.; Davies, J. A.; Norton, P. R. J. Chem. Phys. 1983, 79, 3529. (13) One monolayer (ML) is equal to the number of platinum atoms of the topmost layer of the unreconstructed (1 × 1) surface, 1.28 × 10 15 cm -2 . (14) Thiel, P. A.; Behm, R. J.; Norton, P. R.; Ertl, G. Surf. Sci. 1982, 121, L553. (15) Ritter, E.; Behm, R. J.; Potschke, G.; Wintterlin, J. Surf. Sci. 1987, 181, 403. (16) Hosler, W.; Ritter, E.; Behm, R. J. Ber. Bunsen-Ges. Phys. Chem. 1986, 90, 205. (17) Borg, A.; Hilmen, A. M.; Bergene, E. Surf. Sci. 1994, 306, 10. (18) Pirug, G.; Bonzel, H. P.; Hopster, H.; Ibach, H. J. Chem. Phys. 1979, 71, 593. (19) Zemlyanov, D. Yu.; Smirnov, M. Yu. React. Kinet. Catal. Lett. 1994, 53, 97. (20) Zemlyanov, D. Yu.; Smirnov, M. Yu.; Gorodetskii, V. V.; Block, J. H.; Surf. Sci. 1995, 329, 61. 135 Langmuir 1999, 15, 135-140 10.1021/la980793r CCC: $18.00 © 1999 American Chemical Society Published on Web 01/05/1999