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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 reconstruction 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 NOads layer, reduction with hydrogen of the Oads species formed by NO dissociation, and heating to remove residual Hads. 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 NOads/ (1 × 1) or COads/(1 × 1) islands.6-12 The local coverage on * 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.
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 COads/(1 × 1) and NOads/(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 × 1015 cm-2.4 This value is approximately 20% lower than the one of the (hex) surface (1.61 × 1015 cm-2).15 This difference results in a significant mass-transport of platinum atoms during the NOads/(1 × 1) and COads/(1 × 1) island formation. STM data demonstrate15-17 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 NOads/(1 × 1) islands by means of infrared absorption spectroscopy (IRAS)6 and highresolution 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(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 × 1015 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.
10.1021/la980793r CCC: $18.00 © 1999 American Chemical Society Published on Web 01/05/1999
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tion both on the boundary between the (1 × 1) patches and the surrounding (hex) phase and in the vicinity of the expelled Pt clusters.6,18 These two adsorption states denoted as NO1×1 and NOdef are characterized by N-O stretching bands at 1620 and 1780 cm-1, respectively. Carbon monoxide in the COads/(1 × 1) islands adsorbs into bridge (CObr) and on-top (COtop) states characterized by C-O stretching bands at 1880 and 2100 cm-1, respectively.7-10 NO dissociates during the early stage of adsorption on the (1 × 1) surface at 300 K.18,19 Further adsorption leads to the formation of a single molecularly adsorbed state, identical to the NO1×1 state observed on the (hex) surface.6,18,19 The ν(NO) stretching band gradually shifts with increasing NO coverage from 1570 to 1620 cm-1,19 due to the increase in the dipole-dipole interaction between identical molecular oscillators.6,21 CO adsorption on the (1 × 1) surface proceeds in a similar way as on the (hex) surface, yielding both the bridge and on-top coordinated species. The ν(CO) stretching band of the COtop state shifts gradually toward high frequencies by 20 cm-1 with increasing CO coverage, due to an increment of the dipole-dipole interaction.8 Such significant shifts of the ν(NO) and ν(CO) bands are not observed during adsorption on the (hex) surface because the local coverage and, therefore, the dipole-dipole interaction are practically unchanged in the adsorbed islands.22 It has been established that the surface structure becomes nonuniform and can consist of mixed (hex) and (1 × 1) phases in the course of a number of catalytic reactions on the Pt(100) surface, including the reaction between NO and CO.23-26 Despite this, even the simple adsorption of individual reactants on the nonhomogeneous Pt(100) surfaces did not receive much attention in the literature. Thiel et al. reported a LEED study of CO adsorption on a nonhomogeneous surface containing the (hex) area and the (1 × 1) patches.14 The nonhomogeneous surface was prepared by flashing the Pt(100)-(1 × 1) surface slightly above the temperature of the (1 × 1) f (hex) transition. Unfortunately, this method cannot be used for the preparation of a surface with a specified ratio between the (hex) and (1 × 1) phases. Nevertheless, the authors showed that CO adsorbing on the (hex) phase diffuses on the surface until reaching the (1 × 1) area where it is accommodated. Attempts at mathematical simulation of adsorption and reactions on the nonhomogeneous Pt surface were undertaken.27 A method of preparation of the nonhomogeneous Pt(100) surfaces with a definite phase composition is proposed in the present work. The adsorption of both NO and CO on the nonhomogeneous Pt(100) surfaces was studied by HREELS, and the results were compared with the ones on adsorption on the (hex) and (1 × 1) surfaces. 2. Experimental Section The experiments were performed in the stainless steel chamber of a VG ADES 400 electron spectrometer (base pressure e 5 × 10-11 mbar). The HREEL spectra were obtained in the specular direction at an angle of approximately 35° with respect to the surface normal, using an EMU 50 monochromatic electron gun (21) Scheffler, M. Surf. Sci. 1979, 76, 562. Hollins, P.; Pritchard, J. Prog. Surf. Sci. 1985, 19, 275. (22) Crossley, A.; King, D. A. Surf. Sci. 1980, 95, 131. (23) Schwartz, S. B.; Schmidt, L. D. Surf. Sci. 1988, 206, 169. (24) Fink, Th.; Dath, J.-P.; Imbihl, R.; Ertl, G. J. Chem. Phys. 1991, 95, 6236. (25) Veser, G.; Imbihl, R. J. Chem. Phys. 1994, 100, 8483. (26) Imbihl, R. Prog. Surf. Sci. 1993, 44, 185. (27) Savchenko, V. I. Kinet. Katal. 1994, 35, 349 (in Russian).
Zemlyanov et al. and a hemispheric deflector type electron energy analyzer. The HREELS resolution, measured as the full width at the halfmaximum (fwhm) of the elastic peak, was approximately 80 cm-1 (10 meV) with a kinetic energy of electrons of ∼2.5 eV. Thermal desorption (TD) spectra were recorded at a heating rate of 10 K/s by means of a VG QXK 400 quadrupole mass spectrometer supplied with a twin-cathode assembly and a channeltron detector. A self-designed processor-controlled device was interfaced to the spectrometer for the acquisition of the HREELS and mass-spectrometer data and for controlling temperature. A Pt single-crystal oriented along the (100) direction within
