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Langmuir 1992,8, 1950-1954
Interactional Effects in Corrosive Chemisorption of C 4 and 0 2 on Fe(ll0) A. L. Linsebigler, V. S. Smentkowski, and J. T. Yates, Jr.' Surface Science Center, Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260 Received February 12,1992. I n Final Form: April 29, 1992 The effect on chlorine surface chemistry of the preadsorption of oxygen on Fe(ll0) has been measured using temperature programmed desorption and Auger spectroscopy. Preadsorbed oxygen, at a surface temperature of 90 K,influences the binding of molecular Clz(a), causing early formation of a multilayer C12ice over the oxide layer. Submonolayercoverages of oxygen produce surface sites capable of promoting FeC12formation and nucleation, just as is seen for artificial defect sites produced by Ar+ bombardment of the clean Fe(ll0) surface. Oxygen coverages above one monolayer attenuate FeCl2 nucleation and shift high-temperature iron chloride desorption (T> 900 K) into a lower temperature (T 750 K) FeClz desorption process(es). 1. Introduction
Chlorine and oxygen are two very active molecules, producing corrosive chemisorption on metal surfaces. Recently, independent studies of each of these molecules interacting with the close-packed Fe(ll0) surface have been carried out.lI2 The adsorption of C12 by Fe(ll0) occurs dissociatively at 90 K, resulting in the oxidation of the iron surface as seen by changes in the Auger spectrum of iron.' Following dissociative adsorption in the first monolayer, a second layer of molecular CMa) is produced and is characterized by a desorption temperature near 116 K. Multilayer8 of Cl2 (ice) are deposited on top of the second layer and are characterized by a desorption temperature of about 110
K. Upon heating the chlorine-covered Fe(ll0) surface above the desorption temperature of Clz(g), iron chloride gas evolution is observed as a result of corrosive chemisorption. It is believed that the lowest temperature FeCl2 desorption process (560 K, zero-order kinetics) results from FeC12 nucleation at defect sites on the Fe(ll0) surface, and the activation energy for FeCla(g) desorption is almost identical to maublimation for FeCla(s). Additional iron chloride desorption processes also occur above 560 K.l Oxygen also adsorbs dissociatively and irreversibly on the Fe(110)surface and forms several different iron oxides as a function of the temperature of adsorption and the coverage.2 Obviously, an understanding of the fundamental interaction of chlorine together with oxygen on iron surfaces is quite important to the area of corrosion chemistry. Recent reports estimate that the natural process of corrosion costa the United States tens to hundreds of billions of dollars a year.3 This work is concerned with the interactions which occur when the two active oxidizing agents, Cl2 and 0 2 , interact together with Fe(ll0). 2. Experimental Section The stainlesssteel ultrahigh vacuum system used in this work contains a Perkin-Elmer single pass CMA digital Auger spectrometer, a UTI l00C quadrupole mass spectrometer (QMS), and a 60' off-axis Leybold-Heraeus IQE 10/35 ion source for (1) Linsebigler, A. L.; Smentkowski, V. S.; Ellison, M. D.; Yates, J. T., Jr. J. Am. Chem. SOC. 1992,114, 465. (2) Smentkowski, V. S.; Yates, J. T., Jr. Surf. Sci. 1990,232,113; see also references therein to many studies of the oxygen-iron interaction. (3) Ricker, R. E. Science 1991,252, 1232.
0743-7463/92/2408-1950$03.00/0
sputter cleaningand for producing defects on the clean Fe(ll0) crystal surface. The mass spectra data are digitized and multiplexed using a Teknivent/Vector One interface package. Accurate gas exposureswere delivered to the crystal surface via a microcapillaryarray collimated beam doser calibrated with an accuracy of delivery of *9% with Nz(g), using standard volumetric methods.','+' The doser is positioned at a reproducible crystal-to-doserdistance of 0.80 cm. Conductance through the doser was controlled by a 6 - ~ minternal pinhole aperture. The details and schematics of the ultrahigh vacuum apparatus are presented e1sewhere.l0J1 A differentially pumped shieldlOJ1surrounds the quadrupole mass spectrometerused to performthe temperature programmed desorption (TPD) measurements. The center of the crystal surface can be accurately positioned 1.0 mm away from the sampling aperture (2 mm in diameter) of the QMS shield. This crystal-to-aperturegeometry allows for reproducibleline-of-sight detection of desorptionspecies(includinghighly reactivespecies) as well as for eliminationof extraneous gas molecules desorbing from the edgesof the crystal,the heating leads,the thermocouple, and the copper support assembly. The temperatureprogrammed desorption (TPD)experiments were carried out using a digitaltemperatureprogra"er12 which resistively heated the crystal at a rate of 2.2 K/s. The crystal temperature was measured using a 0.006 cm diameter chromelalumel thermocouple spot welded to the back of the crystal. The initial preparation and cleaning of the 5 mm diameter Fe(ll0) single crystal surface has been described in detail elsewhere.ll The experimentsdescribed here involve a different crystal from that of ref 11, containing a lower level of natural defects. Prior to the s t a r t of eachexperiment,the Fe(ll0)crystal was cleaned by Ar+ sputtering using a 2.5-kV beam at a current of 1.9 X 10" A for -15 min, followed by annealing the crystal at 700 K for -5 min. Auger spectra confirm the cleanliness of the Fe(ll0) crystal surface prior to each experiment. Chlorine (99.99%minimum purity) was obtained in a break seal flask from Matheson and ' 8 0 2 (98%minimum isotopic purity)
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(4) Bozack, M. J.; Muehlhoff, L.; Russell, J. N., Jr.; Choyke, W. J.; Yates, J. T., Jr. J. Vac. Sci. Technol. 1987, A5, 1. (5) Winkler, A.; Yates, J. T., Jr. J. Vac. Sci. Technol. 1988, A6 (5), 2929. (6) Campbell, C. T.; Valone, S. M. J. Vac. Sci. Technol. 1986, A3 (2), 408. (7) Madey, T. E. Surf. Sci. 1972, 33, 355. (8) Yates, J. T., Jr. Methods Exp. Phys. 1986,22,425. (9) Dresser, M. J.; Taylor, P. A.; Wallace, R. M.; Choyke, W. J.; Yates, J. T., Jr. Surf. Sci. 1989, 218, 75. (10) Smentkowski, V. S.; Yates, J. T., Jr. J. Vac. Sci. Technol. 1989, A7 (6), 3325. (11) Smentkowski, V. S.; Cheng, C. C.; Yates, J. T., Jr. Langmuir 1990, 6, 147. (12) Muha, R. J.; Gates, S. M.; Basu, P.; Yates, J. T., Jr. Rev. Sci. Instrum. 1986,56,613.
0 1992 American Chemical Society
Corrosive Chemisorption of
Cl2
and
02
was obtained from Cambridge Isotopes. Both gases were used without further purification. The procedure for chlorine passivation of the gas line and beam doser was described elsewhere.' Auger spectroscopy was used in this work to characterize the adsorption of chlorine on the Fe(ll0) surface pre-exposed to various coverages of oxygen. These studies were conducted at temperaturesgreaterthan 300 K. Typical Auger data acquisition conditions utilize a 3-kV electron beam, a current of 1.5 X 10" A, a measured electron beam area of 7.2 x 10-3 cm2, an acquisition time of 70 s, and translation of the crystal for sequential measurements of Auger intensityon regions of the crystal surface not previously exposed to the electron beam. Controlled electron bombardment experimentshave been performed previously for C12/Fe(llO)land Oz/Fe(110).2 Essentially no modifications to the Auger spectra from electron beam effects occur for chemisorbed C1 at surface temperatures near 300 K, using the Auger acquisition procedures described above.lV2
3. Results and Discussion For the series of experiments that follow, adsorption of oxygen and chlorine was conducted in the following manner: (1)the Fe(ll0) crystal surface was pre-exposed to produce various coveragesof oxygen measured in monolayers (ML) using previous calibrations;2 (2) this was followed by a fixed chlorine exposure of 3.0 X 1015 CW cm2, The 02(g) exposures used in this work were converted to absolute surface coverages of 0 2 (02/cm2), calculated using the reaction probability for 02 on the Fe(ll0) surface and the flux of 02 molecules to the crystal surface.2 Since molecular oxygen adsorbs dissociatively at 90 K producing oxide layers at higher exposures, the absolute surface coverages of oxygen (0)were then reported as monolayers of atomic oxygen coverage defined as 1ML = 1.72 X 1015 O/cm2 = number of Fe atoms/cm2. 3.1. T h e r m a l Desorption Measurements. 3.1.1. Temperat ure-Programmed Desorption of Molecular Chlorine for Various Atomic Oxygen Preadsorbed Coverages. In the absence of preadsorbed atomic oxygen, chlorine initially adsorbs dissociatively and irreversible on the Fe(ll0) surface a t 90 K. As the exposure of Cl2 is increased, an overlayer of weakly-chemisorbed molecular Cl2(a) (having an upper limit to its coverage) is then witnessed through the formation of a desorption state at 116 K. This is followed by the formation of a molecular Cl2 ice that desorbs near 110 K;I the surface coverage of the molecular Cl2 ice is unlimited in magnitude. A chlorine Cl2/cm2 to the Fe(ll0) surface at exposure of 3.0 X 90 K produces a saturated weakly-chemisorbed monolayer desorption state at -116 K and a small multilayer desorption state at 105 K.' This desorption spectrum is shown at the bottom of Figure 1(0.0 ML of 0 coverage). As the coverage of preadsorbed atomic oxygen is increased, the yield of the weakly-chemisorbed Clz(a)monolayer desorption state at 116 K decreases until it is depleted at an oxygen coverage greater than 1.0 ML. For the same Cl2 exposure, the yield of the Cl2 multilayer desorption state a t 105 K increases and the peak maximum shifts to slightly higher temperatures as the coverage of preadsorbed atomic oxygen increases. The multilayer Cl2 (ice) desorption state becomes broader and less intense as the coverage of preadsorbed atomic oxygen approaches a monolayer. The behavior shown in Figure 1indicates that adsorbed oxygen below 1monolayer coverage forms surface species which block the production of the 116 K Cl2 desorption process. Instead, Cl2 molecules populate the multilayer to a higher degree as indicated by the increase of desorption yield in the -105 K state. It should be noted that a shoulder appears on the low temperature side of the peak maximum for the Cl2 mul-
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Langmuir, Vol. 8, No. 8,1992 1951
on Fe(ll0)
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I ' ' ' ' I ' ' ' ' I ' i5 ' l i 2 ' ' 21 EXPOSURE = 3.0 x 10 cm -2 1.5~ 10 A I
0 COVERAGE(ML) 3.5 3.0 2.4
11.
I
JAh0.S
90
110
130
150
170
190
TEMPERATURE(K)
Figure 1. Temperature-programmeddesorption of undissociated Clz from the Fe(ll0) surface as a function of preadsorbed oxygen coverage, followed by a Clz exposure of 3.0 X 1016 C12/cm2 at 90 K.
tilayer (ice) desorption state at an oxygen coverage greater than 2.0 ML. This effect is not understood; it is known that for increasing coveragethe iron oxide layer transforms from FeO to Fe304.2 This new low temperature Cl2 desorption state probably originates from the FeaOr layer. 3.1.2. Temperature-Programmed Desorption of FeCl2 following Chlorine Adsorption on the Fe(110) Surface. At Cl2 exposures greater than 2.0 X 10l6 Cld cm2 (no oxygen present) three desorption states are observed from the Fe(ll0) surface while monitoring the 126 amu signal in the QMS. These desorption states include an FeC12 desorption state at 560 K, an FeClz desorption state which is evolved over a wide temperature range (580-750 K), and an iron chloride species desorption state a t T > 900 K (attributed to something other than pure F e c l ~ ) .The ~ desorption state at T > 900 K, is the only desorption state observed at low Clz exposures (
1015,-2
0.0 ML
0.8
0.4
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