406
Langmuir 1986,2, 406-411
Vibrational Spectroscopy of Sulfur Dioxide on the Ag( 110) Surface: Comparison to Inorganic Complexes Duane A. Outka and Robert J. Madix* Departments of Chemical Engineering and Chemistry, Stanford University, Stanford, California 94305
Galen B. Fisher and Craig L. DiMaggio Physical Chemistry Department, General Motors Research Laboratories, Warren, Michigan 48090 Received August 1, 1985. I n Final Form: March 13, 1986 The adsorption of SO2on the Ag(ll0) surface has been characterized with vibrational spectra obtained by using high-resolution electron energy loas spectroscopy and with supporting surface science spectroscopies. A t 100 K multilayers of SO2are adsorbed which desorb at 120 K. Adsorption or annealing a high coverage to temperatures near 120 K forms a two-layer coverage of SO2. A single-layer coverage is then formed by annealing above 175 K. Sulfur dioxide in the first layer is molecularly adsorbed on the clean Ag(ll0) surface via a Ag-S bond. The plane of the SOz molecule is tilted toward the surface with the oxygens parallel to the surface. This configuration is directly analogous to a common confiiation for SO2ligands in metal complexes. Sulfur dioxide desorbs completely and molecularly from the clean Ag(ll0) surface at temperatures above 275 K. In the presence of oxygen adatoms on Ag(llO), SOz also adsorbs molecularly below 200 K in a configuration similar to the clean surface but with an increased tilt with respect to the surface normal.
Introduction Sulfur dioxide is an important chemical intermediate in industry and a major component of air pollution. Because of this, sulfur dioxide reactions on metal surfaces are important under a variety of circumstances, including automobile emissions control, corrosion of metals, poisoning of catalysts, and the manufacture of sulfuric acid. The adsorption and reactions of sulfur dioxide on several metal surfaces have been studied in order to gain insight into these processes. Sulfur dioxide is observed to dissociatively adsorb on most metal surfaces at ambient temperatures. The range of metals that have been studied includes W(po1ycrystalline),' Ni(poly),2 Rh(l10),3 Pt(l10),3 F e ( p ~ l y ) and ,~ Pt(111).5fjThe products of the dissociative adsorption of SOz on these surfaces are adsorbed sulfur and oxygen atoms. An SO4 species has also been identified after SO2 adsorption on Ni(polyI2and Fe(po1~)~ surfaces. Exceptions to this pattern of dissociative adsorption occur on AuAg(111): and Ag(lO0)' on which SO2has not been observed to adsorb under low-pressure conditions ( torr). Molecular adsorption of SO2 has been previously observed only on the Ag(ll0) This system has been characterized by temperature-programmed desorption (TPD), low-energy electron diffraction (LEED), u1traviolet photoelectron spectroscopy (UPS), and X-ray photoelectron spectroscopy (XPS).These studies showed that SO2 reversibly adsorbs on Ag(ll0) below room temperature and suggested that SO2 acts as an electron acceptor forming a Ag-S bond with a strength of about 64 kJ Lateral interactions among the adsorbed SOz molecules were evident in LEED which indicated the (1) Golub, S.; Fedak, D. G. Surf. Sci. 1974, 45, 213. (2) Brundle, C. R.; Carley, A. F. Faraday Discuss.,Chem. SOC.1975,
60,51. (3) Ku. R. C.: Wvnblatt. P. A D D /Surf. . Sci. 1981. 8. 250. (4) F&y&, M:; Kishi; K.; Ikeda, S. 2. Electron hpectrosc. Related Phenom. 1978. 13. 59. (5) Kohler, 'U.; 'Wassmuth, H.-W. Surf. Sci. 1982, 11 7, 668. (6) Astegger, St.; Bechtold, E. Surf. Sci. 1982,122, 491. (7) Rovida, R.; Pratesi, F. Surf. Sci. 1981, 104, 609. (8) Outka, D. A.; Madix, R. J. Surf. Sci. 1984, 137, 242.
formation of incommensurate overlayers similar to those formed by noble gases on this surface and sodium atoms on Ni(ll0). Adsorption studies below room temperatures have not been performed on the other crystallographic faces of silver or on gold. In this study we examine further the bonding of sulfur dioxide with the Ag(110) surface using high-resoIution electron energy loss spectroscopy (HREELS). With this technique the vibrational spectra of SO2 were examined for the first time on a well-defined surface. Analysis of the vibrational spectra indicates that SO2in the first layer bonds to the Ag(ll0) surface via sulfur in a manner analogous to that of certain SO2ligands in inorganic metal complexes. We also find strong similarities between the molecular adsorption of SO2on Ag(ll0) and the molecular adsorption of CO and NO on surfaces. In addition, the effect of oxygen adatoms on the bonding of SOz to the Ag(ll0) surface was examined at temperatures below 200 K, where SO2 adsorption is molecular. Above 200 K, a reaction occurs between SO2 and oxygen adatoms on Ag(110) which will be the subject of a separate paper.g
Experimental Section The experiments were performed at General Motors Research Laboratories in an UHV (ultrahighvacuum) chamber with a base pressure of 2 X torr. The sulfur dioxide and oxygen were dosed through separate, multichannel array dosers positioned =5 mm in front of the crystal. The exposures from these dosers for oxygen were calibrated by comparing TPD peak areas to the peak areas from background exposurea at a known pressure and referred to uncorrected ion gauge readings. The oxygen doser calibration fador was also used for SO2,but this may lead to an underestimate of SO2exposures. Sulfur dioxide exposures were performed with a crystal temperature of 100 K or in some cases near 120 K to approximate the adsorption conditions in ref 8. Oxygen doses were performed with a crystal temperature of 275 K followed by momentarily annealing to 475 K to ensure the absence of molecular oxygen and to order the atomic oxygen overlayer. The preoxidized surface was prepared by dosing 100 langmuir (1.0 langmuir is equivalent to an exposure of 1 x lo+ torr for 1 s) of 02. This coverage corresponded to approximately 2.1 X 1014atoms
(9) Outka, D. A.; Madix, R. J.; Fisher, G. B.; DiMaggio, C. L., J. Chem. Phys., in press.
0743-7463/86/2402-0406$O1.5O/O0 1986 American Chemical Society
Langmuir, Vol. 2, No. 4, 1986 407
Vibrational Spectroscopy of Sulfur Dioxide
system
Table I. Vibrational Frequencies and Assignments for S02/Ag( 110) second layer or multilayer first layer OS0 phonon' va v, va v, wag bend, 6 related
SOz/Ag(llO), 100 K SOZ/Ag(llO), 135 K SOZ/Ag(llO),175 and 220 K SOz/Ag(llO)-0, 100 K SOz/Ag(llO)-O, 170 K SOz solid SOz gas
1320 1305
1145 1135
1315
1145
1313,1327 1360.5
1147 1151.4
b
1005' 1000 985
685
b
1010
670
530 500 470 535 565 525 517.8
330'
v(Ag-SO2) 200 210 220
ref this work this work this work this work this work 26 11
'Uncertain assignment, see text. Mode absent, see text. cm-z or 0.25 monolayer of atomic oxygen (1.0 monolayer is defined as the number of surface silver atoms).I0 The energy losses were usually measured in the specular direction at 100 K after annealing momentarilyto various temperatures. The incident beam energy was 3.5-4.0 eV. Losses were measured in the off specular direction by rotating the crystal.
Results As previously shown: SO2 adsorbs molecularly at 130 K on clean Ag(ll0) and desorbs in three previously characterized SO2adsorption states labeled a', a2,and as which desorb near 175,225, and 275 K, respectively. These states were associated with the second layer (al)and two coverages in the first layer (a2,as)of SO2 adsorbed on Ag(ll0). In this study where adsorptions were done
