J. Phys. Chem. 1987, 91, 5705-5709 Below are summarized the main results and conclusions of adsorption on the clean, sulfur-covered, and carbon-covered Re(OOO1) surfaces. (1) D2chemisorption on clean Re(0001) showed similarities to Pt( 111). A low-temperature state was observed for D2 (200-250 K) at high coverage. (a) Preadsorbed sulfur was observed to block adsorption sites and lower the desorption temperature slightly (30 K). (b) Preadsorbed carbon also acts to block sites for adsorption, but it also lowers the lowest temperature for desorption to 290 from 380 K (at the same exposure). The periphery of the carbon patches is suggested to be the site for the weaker D2 adsorption. (2) Unsaturated hydrocarbons sequentially dehydrogenate during thermal desorption. The H2 desorption spectra for these compounds exhibit peaks in the same temperature range: 300 K, 350-380 K, 420 K, and a tail from 500 to 700 K. It is concluded that the similarity in H2 desorption temperatures is caused by similar dehydrogenation energies for the unsaturated C4 hydrocarbons. (3) Preadsorbed sulfur acts to block sites for dehydrogenation and enhances weak (9-1 1 kcal/m01)~~ reversible molecular adsorption. (4) Preadsorbed carbon also blocks sites for dehydrogenation and enhances weak (9-1 1 kcal/m01)~~ reversible molecular ad-
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sorption. However, preadsorbed carbon also alters bonding to the surface as exemplified by the enhanced hydrogenation of butadiene. The nature of chemisorbed carbon varies greatly with the position of the transition metal in the periodic table. The formation of carbidic or graphitic carbon on these surfaces greatly influences the chemistry of the carbon overlayer and reinforces the concept of “active” versus “inactive” carbon. In general, we cannot make a definitive conclusion regarding the nature of the carbon overlayer formed in these studies on Re(OOO1). The site-blocking ability and the carbon AES both are typical for “inert” carbon. However, the lowering of the D2 binding energy and the hydrogenation of butadiene suggest that the carbon overlayer is slightly active.
Acknowledgment. We thank the Division of Materials Sciences and Office of Basic Energy Sciences of the U.S. Department of Energy under Contract DE-AC03-76SF00098. In addition, one of us (J.A.O.) gratefully acknowledges the support of Junta de andalucia for foreign study. Finally, we thank M. Salmeron for many useful discussions. Registry No. Dz,7782-39-0; Re, 7440-15-5; S , 7704-34-9; C , 744044-0; thiophene, 110-02-1;1,3-butadiene,106-99-0; 1-butene, 106-98-9; cis-2-butene, 590-18-1; trans-2-butene, 624-64-6.
Adsorption and Reaction of Chlorine with the Ti(0001) Surface Philip R. Watson* and Scott M. Mokler Department of Chemistry and Center for Advanced Materials Research, Oregon State University, Corvallis, Oregon 97331 (Received: April 20, 1987)
We have studied the adsorption of chlorine on the Ti(0001) surface using Auger electron spectroscopy (AES), low-energy electron diffraction (LEED), and thermal desorption spectroscopy (TDS). Chlorine overlayers were generated by using an electrochemical source of molecular chlorine at temperatures ranging from 30 to 600 OC. Adsorption is dissociative with an approximately constant sticking coefficient at low coverages. The adsorbed chlorine is sensitive to electron beam stimulated desorption (ESD) but does not thermally desorb below 750 OC. We did not observe any ordered LEED patterns unless a high-coverage surface was annealed to a temperature of 620 “C.The resulting complex LEED pattern suggests that the adsorbed chlorine forms a coincidence lattice on the titanium surface. The data is consistent with the formation of a pseudehalide surface layer related to TiC1,.
Introduction The purpose of this investigation is to understand the nature of the interaction of chlorine with a well-characterized singlecrystal plane of titanium. Such knowledge may be important in the understanding of corrosion and as a prelude to studies of Ziegler-Natta olefin polymerization.’ This catalytic reaction is thought to occur at surface defects on titanium chloride crystallitesZ and so a chlorided titanium surface can serve as a simple model for this type of catalyst. Previous studies of the Ti/C12 system are rather few in number. Smith3 measured the surface segregation of chlorine to the Ti(0001) surface using AES and ellipsometry. Anderson and Gani4 performed volumetric adsorption experiments on a Ti film, without the benefit of any analytical facilities, and suggested the formation of a TiC12 layer. Anderson and Thompson5 reached a similar conclusion from field emission microscopy studies on a Ti film. Khan6 investigated the reaction of chlorine with a Ti(lOi0) surface using RHEED and concluded that epitaxial growth of TiC13 occurred. Unfortunately, that paper does not make the evidence for this assignment very clear. A more recent TDS study on polycrystalline material by Cox et al.’ found that, at high coverages of chlorine, TiC13 desorbed at high temperatures, while high-
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0022-3654/87/2091-5705$01.50/0
temperature desorption of unidentified halide(s) occurred at lower coverages.
Experimental Section For these experiments we used a standard ion-pumped ultrahigh vacuum chamber equipped with four-grid LEED optics, which also functioned as an Auger spectrometer, a quadrupole mass spectrometer (Dycor M200), and an ion-sputter gun. The Ti single crystal (Metal Crystal Inc., UK) was polished to 0.05-pm alumina and the orientation checked to be within 1O of (0001) by using a laser and Laut camera. The sample was secured to the manipulator via tantalum heating wires. Bulk Ti has a bulk phase transition at 885 OC9 and studies of the Ti(lOT0) surfacelo revealed a lower value for the transition temperature (1) Boor,J. Jr., Zeigler-Natta Catalysts and Polymerizations; Academic: New York, 1979. (2) Arlman, E. J.; Cossee, P.J . Catal. 1964, 3, 99. (3) Smith, T. J . Electrochem. SOC.1972, 119, 1398. (4) Anderson, J. R.; Gani, M. S . J. J. Phys. Chem. Solids 1962,23, 1087. (5) Anderson, J. R.; Thompson, N. Surf.Sci. 1971, 28, 84. (6) Khan, I. H. Surf. Sci. 1975, 48, 537. (7) Cox, M. P.; Foord, J. S.;Lambert, R. M.; Prince, R. H. Surf. Sci. 1983, 129, 375. (8) Spencer, N. D.; Goddard, P. J.; Davies, P. W.; Kitson, M.; Lambert,
R. M. J . Vac. Sci. Technol. 1983, A l , 1554. (9) Samsonov, G. V. Handbook of the Physicochemical Properties of the Elements; Plenum: New York, 1968.
0 1987 American Chemical Society
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Watson and Mokler
of 800 "C at this surface. Accordingly, the sample was never heated above 750 OC to avoid damage. The chamber also contained an in-situ molecular chlorine source based on the design of Spencer et aL8 Operating on the basis of the solid-state electrochemical cell Ag/AgCl/Pt, C12, it allows for high dosing levels without a corresponding increase in background pressure. For atomic radii of 1.47 and 1.81 A for Ti and C1, respectively, we can calculate from Faraday's law that passage of about 160 pC of charge through the cell per cm2 of sample surface will result in monolayer coverage (1.5 X 10l5atoms/cm2) by chlorine atoms assuming a constant unit sticking probability and no losses in or external to the gun. Calculations assuming Clausing flow14 suggest that less than 5% of the chlorine flux from the cell is actually incident upon the sample. Experimental tests have revealed this estimate to be rea~onable.'J~*'~ The source used in this work is very similar to that used in these calculations with a similar sample geometry. Hence, for our sample of roughly 0.5 cm2 area we might expect a chlorine dose equivalent to the passage of roughly 1600-2000 pC (4-5% efficiency) of charge through the cell to produce a monolayer coverage of C1, given a constant unit sticking probability.
C and 0 built up on the surface within 1 h of the last flash. However, once the bulk had been depleted of S, we found that a short (1 5 min) hot bombardment cycle followed by a flash to 720 OC would recover the clean surface. To avoid contamination problems all experiments were carried out as quickly as possible after a cleaning cycle. Samples containing substantial amounts of surface chlorine were much less reactive. The surface of the sample was cleaned and immediately rotated 150" from the LEED/Auger optics and pointed at the chlorine gun. The exposure is expressed as microcoulombs of charged passed through the cell and was determined by the time the sample spent directly in front of the chlorine gun at a certain cell current. Figure I C shows a typical Auger spectrum for a saturation chlorine dose of 3000 pC. This spectrum shows that the sample picked up a small amount of carbon, estimated at
