J. Phys. Chem. 1990, 94, 6028-6033
6028
wo = 10
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extract
high
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Figure 9. Process for dense fluid reverse. micelle extraction from aqueous
media. solute(s) of interest is mixed at high pressure, perhaps 125 bar, with the dense-gas micellar solution. After the two phases are separated, the micellar solution is expanded to precipitate the desired solute in concentrated form with little water present. In a conventional process, the distribution of a solute is manipulated by a chemical change such as pH. In some cases, this may not lead to recovery or may cause denaturation, and the product is recovered in dilute aqueous solution. Although the S C F process may overcome these limitations, it is limited by the need for high-pressure equipment and losses of surfactant in the aqueous phase. Further study is needed to evaluate the phase behavior at high pressure. Conclusions The new microsampling variable-volume view cell provides for rapid mixing, complete observation of the phases, and accurate analysis. The solubilities of proline and tryptophan change little with pressure in the solid-fluid region, where Woand the AOT concentration are constant. At lower pressures in the solid-liquid-fluid region, pressure may be used to adjust the number and size of the reverse micelles and thus the amino acid concentration. These conclusions are consistent with a previous study of pressure effects on Womt and on the nature of water in (solute-free) reverse micelles in the two-phase liquid-fluid and one-phase fluid regions.15 The surface monolayer model, which treats the partitioning of an amino acid between the water pool and the interfacial region, explains the experimental results. This partitioning may be ad-
justed with pressure by changing Womt and hence the curvature and rigidity of the interface. The high selectivity for solid proline versus tryptophan may be predicted with the surface monolayer model based on each amino acid's K," and water solubility. In the solid-solid-fluid region at higher pressures these properties are relatively constant so the selectivity does not change with pressure. At the lower pressures, the selectivity increases rapidly with pressure as the size and number of the micelles increase. Unlike previous studies of AOT reverse micelles in SCFs, it was possible to determine the sign of the natural curvature and the distribution coefficient of AOT, since the composition of the condensed phase was known. As expected on the basis of studies in liquid solvents, salt has a profound effect on the phase behavior. It decreases Womt for the AOT-water-NaC1-propanetryptophan system due to screening of the head-group repulsion, but it increases the concentration of AOT in the fluid phase. Above 125 bar, this system exhibits Winsor type I1 behavior, in which the AOT is distributed predominantly into the fluid phase. The natural tendency of this system is to curve about water. The measured distribution coefficients of tryptophan, AOT, and water suggest that it is possible to extract an amino acid from dilute aqueous solution into a propane-rich fluid phase, and to reduce pressure to recover the solute in concentrated form. A more detailed investigation of the effect of salt on the phase behavior will be reported in the next paper in this series. Acknowledgment. This material is based on work supported by the National Science Foundation under Grant No. CTS89008 19. Any opinions, findings, and conclusions or recommendations expressed in this publication do not necessarily reflect the views of the National Science Foundation. Acknowledgment is also made to the State of Texas Energy Research in Applications Program, the Camille and Henry Dreyfus Foundation for a Teacher-Scholar Grant (K.P.J.), and the Separations Research Program of the University of Texas. We extend our thanks to Greg McFann and Doug Peck for useful comments, to Gerald (Rusty) Cantrell, Ram Gupta, and Parvin Yazdi for their assistance in the laboratory, to Sandy Smith of the UT Protein Sequencing Center for the proline analyses, and to Alan Hatton and Richard Smith for providing manuscripts prior to publication. Registry No. TRP, 73-22-3; PRO, 147-85-3; AOT, 577-1 1-7; ethane, 74-84-0; propane, 74-98-6.
Effects of Postdosed Species on Preadsorbed CO on Fe(100): Adsorption Site Conversion Caused by Site Competition Jiong-Ping Lu, M. R. Albert, and S. L. Bernasek* Department of Chemistry, Princeton University, Princeton, New Jersey 08544 (Received: January 9, 1990; In Final Form: March 20, 1990)
Carbon monoxide (CO) adsorption on the clean Fe( 100) surface and the c(2x2)CO-Fe( 100) surface has been studied. The effects of postdosed oxygen (0,) and methanethiol (CH,SH) on preadsorbed CO were examined. These studies were performed under ultrahigh-vacuum (UHV) conditions, using high-resolution electron energy loss spectroscopy (HREELS), temperature-programmed desorption spectroscopy (TPD), Auger electron spectroscopy (AES), and low-energy electron diffraction (LEED).For CO, four associative adsorption states and one dissociative adsorption state were identified. Postdosed species were found to cause 4-fold hollow site CO molecules to migrate into lower coordination sites. This type of adsorption site conversion is discussed in terms of adsorption site competition between preadsorbed species and postdosed species. 1. Introduction
Many surface processes, such as heterogeneous catalysis, involve more than one adsorbed species on the surface. Studying toadsorption is an important step in elucidating the mechanisms of these complex surface processes on a molecular scale. Furthermore, coadsorption itself is a subject of fundamental importance, 0022-3654/90/2094-6028$02.50/0
since coadsorption systems contain multipair interactions and there exist many Phenomena unique to these systems.' In studying coadsorption, systems containing co as one Of the adsorbates are (1) White, J. M.; Akhter, S. CRC Crit. Reo. Solid State Muter. Sci. 1988, 14, 131.
0 1990 American Chemical Society
Postdosed Species on Preadsorbed CO on Fe( 100)
The Journal of Physical Chemistry, Vol. 94, No. 15, 1990 6029
most attractive, due to the following reasons. (1) Adsorption of CO alone on transition-metal surfaces has been ektensively studied in the past2 so that many valuable references are available for comparison with CO coadsorbed systems. (2) CO molecules have relatively simple geometric and electronic structures, and the properties of adsorbed CO can be more easily studied both experimentally and theoretically than more complex adsorbates. (3) CO molecules are involved in many technologically important processes such as Fischer-Tropsch synthe~is,’,~ CO ~xidation,~ and water-gas shift reactiow6 have revealed that there is rich chemistry Several in the CO/Fe(100) system, as illustrated by the following observations: ( I ) both associative and dissociative adsorption of CO take place on the Fe( 100) surface: (2) there are multiple associative adsorption states for adsorbed CO; (3) the most tightly bonded CO, known as C O ( O ~ ) ,has ~ , ~an unusual bonding configuration and reduced vibrational frequency.’*’s Effects of preadsorbed species on the adsorption of CO on the Fe( 100) surface have been reported.8-9.’4.’6 However, the effects of postdosed species on preadsorbed C O on the surface have not been so well investigated. In this paper, experimental studies of the effects of postdosed O2 and CH3SH on preadsorbed C O on the Fe( 100) surface are reported. Adsorption of O2and CH3SH alone on the Fe( 100) surface have been studied, and the results of these investigations have been p ~ b l i s h e d . ” ~ ~ ~
2. Experimental Section Experiments reported here were performed in a stainless steel UHV chamber, equipped with multiple surface techniques, including a Perkin-Elmer four-grid LEED optics, a Perkin-Elmer single-pass cylindrical mirror analyzer (CMA) for A m , an Inficon quadrupole mass spectrometer for TPD, and a McAllister/RHK HREELS spectrometer. The chamber was pumped by a 400 L/s ion pump (Varian), a 170 L/s turbo-molecular pump (Balzer), and a liquid nitrogen cold trap combined with a Ti sublimator. Typical base pressure of the chamber was 2 X Torr. The sample crystal was mounted on a manipulator with liquid nitrogen cooling, resistive heating, X-Y-Z motion, and rotation capabilities. A chromel-alumel (K type, Omega) thermocouple was spotwelded on the back of the crystal to monitor the temperature of the sample. The crystal can be cooled to 103 K and heated to 1000 K. The sample surface was cleaned by Ar+ sputteringannealing cycles, followed by 5-min annealing at 723 K and a flashing to 923 K. The cleanliness and ordering of the cleaned surface were checked by AES, HREELS, and LEED. All chemicals used were from Matheson. Argon (Ar) is a research purity (99.99%) gas, C O is a research purity gas with a purity of 99.99%, and O2 is also a research purity gas, with a purity of 99.998%. CH3SH has a purity of 99.5%. These (2)Yates, Jr., J. T.; Madey, T. E.; Campuzano, J. C. In The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis, Vol. 3A; King, D. A., Woodruff, D. P., Eds.; Elsevier: Amsterdam, in press. (3)Anderson, R. 9. The Fischer-Tropsch Synthesis; Academic: Orlando, FL, 1984. (4)Dry, M. E. In Catalysis: Science and Technology, Vol. 1; Anderson, J. R.,Boudart, M., Eds.; Springer-Verlag: Berlin, 1981. (5) Taylor, K. C. In Cafalysis: Science and Technology, Vol. 5; Anderson, J. R.,Boundart, M., Eds.; Springer-Verlag: Berlin, 1984. (6)Newsome. D. S. Cafal.Reo.-Sci. Eng. 1980, 21, 275. (7) Bundle, C. R. IBM J. Res. Deo. 1978, 22, 235. (8)Benziger, J. 9.; Madix, R. J. Surf. Sci. 1980, 94, 119. (9) Moon, D. W.; Dwyer, D. J.; Bernasek, S.L.Surf. Sci. 1985, 163, 215. (IO) Moon. D. W.; Bernasek, S. L.;Dwyer, D. J.; Gland, J. L. J. Am. Chem. Soc. 1985, 107,4363. ( I 1) Benndorf, C.; Krueger, 9.; Thieme, F. Surf. Sci. 1985, 163, L675. (12)Benndorf, C.; Nieber, B.; Krueger, B. Surf. Sci. 1986, 177,L907. (13) Moon, D. W.;Cameron, S.;Zaera, F.; Eberhardt, W.; Carr, R.; Bernasek, S. L.;Gland, J. L. Surf. Sci. 1987, 180, L123. (14) Moon, D. W.; Bernasek, S.L.;Lu, J.-P.; Gland, J. L.;Dwyer, D. J. SurJ Sci. 1987, 184, 90. (15) Mehandru, S.P.; Anderson, A. B. Surf. Sci. 1988. 201, 345. (16)Cameron, S. D.; Dwyer, D. J. J. Vac. Sci. Technol. 1988, A6, 797. (17) Lu,J.-P.; Albert, M. R.;Bemasek, S. L.;Dwyer, D. J. Surf. Sei. 1989, 215, 348. (18)Albert, M. R.;Lu, J.-P.; Bernasek, S.L.;Cameron, S.D.; Gland, J. L. Surf.Sci. 1988, 206, 398.
I
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Figure 1. TPD spectra of CO on the Fe(100) surface as a function of CO exposure.
chemicals were contained in lecture bottles, and they were used directly without further purification. Ar, CO, and O2 were introduced into the UHV chamber through a Varian adjustable leak valve. CH’SH was dosed onto the sample surface through a separate doser, which consists of a 1/4-in. stainless steel tubing attached to a Varian leak valve. The exposure (1 langmuir = 1 X 10” Torr s) of CO and O2 was controlled by exposing the sample surface to 2 X lo+’ Torr dynamic flow of the corresponding gas for a certain period of time. Relative exposure of CH3SH was indicated by AES signals of elemental sulfur left on the surface after the decomposition of CH’SH is completed. 3. Results 3.I . Adsorption of CO on Clean and CO Precovered Fe( 100) Surfaces. Figure 1 shows TPD results for C O on the Fe( 100) surface as a function of C O exposure. The results shown here are qualitatively consistent with previously reported dataeg However, more detailed exposure-dependent studies were performed in the present work, and a new desorption peak at 340 K was resolved, which overlapped the peak at 306 K.9 To be consistent with previous notation^,^.^ the three associative adsorption states previously reported will still be referred to as CO(al),CO(a2),and CO(a3). The new adsorption state observed here will be referred to as C O ( a i ) . At very low exposure (0.1 langmuir), only one desorption peak above 800 K can be detected. As CO exposure is increased, this peak grows in intensity and its peak position shifts to lower temperature. This high-temperature peak originates from the recombination of atomic C and 0 (Le., dissociated CO) and is designated as CO(p), to be consistent with previous n o t a t i ~ n . ~While * ~ the CO(p) peak grows, the CO(a3) peak at 440 K also develops. Saturation of the CO(p) occurs at an exposure of around 0.3 langmuir, while the CO(a3) peak increases in intensity up to 0.5-langmuir exposure. A further increase in C O exposure results in an increase in the intensity of C O ( a i ) . As C O exposure increases further, C 0 ( a 2 )a t 305 K also appears and subsequently C 0 ( a I ) develops. The C O ( a i ) peak appears as a shoulder on C 0 ( a 2 ) at these exposures. C O saturation occurs at an exposure near 2 langmuirs. A further increase in C O exposure does not cause a further increase in the intensities of any of the C O desorption peaks. Results shown in Figure 1 clearly indicate that CO(a2) and CO(ai) originate from two different adsorption states. A clean Fe( 100) surface shows a well-ordered p( 1x1) structure in LEED observations. When the clean Fe( 100) surface is exposed
6030 The Journal of Physical Chemistry, Vol. 94, No. 15, 1990 I
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Figure 2. Combination of HREEL spectra and TPD spectra illustrating the preparation of a c(2X2)CO-Fe(100) surface: left panel, HREELS for 5 langmuirs of CO adsorbed on Fe( 100) at 103 K and subsequently flashed to 383 K; right panel, TPD of 5 langmuirs of CO on Fe(100).
mo
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Figure 3. Comparison of TPD results for CO on the clean Fe(100) surface (left panel) and CO on c(2X2)CO-Fe(100) (right panel). HREELS o f Oxygen/CO/FellOo~
to 5 langmuirs of CO at 103 K, no extra LEED spots can be observed, but the background intensity increases. When this surface is flashed to 383 K, a well-ordered ~ ( 2 x 2 LEED ) pattern can be observed. It can be seen from Figure 1 that the aboveprepared surface contains no CO(al), C0(a2), and CO(a2/), since all these states desorb upon heating to 383 K. Previous XPS9 and HREELS'O results showed that no CO dissociation occurs upon heating to 383 K. Therefore, the ~ ( 2 x 2 overlayer ) contains the pure CO(a3) state and will be referred to as c(2X2)CO-Fe(lOO). After a c(2X2)CO-Fe(lOO) overlayer is heated above 573 K, the ~ ( 2 x 2 LEED ) pattern is still observed. HREELS measurements indicate that only dissociated CO (i.e., atomic C and 0, corresponding to vibrational peaks at 520 and 400 cm-I) remains on this surface. This surface is designated as c(2X2)C,O-Fe( 100). Figure 2 further illustrates the preparation of a pure CO(a3) covered surface. Adsorption of 5 langmuirs of CO at 103 K results in four vibrational bands (Figure 2). From coverage-dependent HREELS data,I0 it can be seen that the 1245-cm-' band results exclusively from CO(a3)adsorbed molecules. The 2065-cm-' peak can be assigned as C-0 stretching from CO(al),C0(a2), and CO(a,l). Due to the resolution limitation of the electron spectroscopy (typical fwhm of the elastic peaks in these experiments is 60 cm-I), the different peaks cannot be resolved. The two vibrational bands at 400 and 530 cm-I result from metal-CO vibrations. After the above surface is heated to 383 K, CO(al), C0(a2), and CO(a,') desorb, and only two vibrational bands can be detected (Figure 2). The 1175-cm-' peak is due to C-0 stretching of the adsorbed CO(a3) molecules. The peak at 370 cm-l is associated with the metal-(CO) vibration. As seen in Figure 2, the C-0 stretching frequency of the CO(a3) state is far removed from that of the other adsorption states. This provides an experimental advantage in monitoring site conversion of the adsorbed CO(a3) molecules, as will be seen in the coadsorption experiments presented in sections 2.2 and 2.3. TPD results for C O adsorption on the c(2X2)CO-Fe(100) surface are shown in Figure 3 (right panel), along with TPD for CO adsorbed on the clean Fe( 100) surface for comparison. As can be seen from the right panel, desorption of CO from the single c(2X2)CO-Fe( 100) overlayer (Le., without postdosed CO) results in two desorption peaks; CO(a3) at 440 K and CO(p) at 820 K. Since the c(2X2)CO-Fe( 100) overlayer contains only CO(a3) adsorbed molecules, both desorption signals originate from CO(a3) associatively adsorbed molecules. The CO@) peak is also observed here, due to the partial dissociation which takes place upon heating the c(2X2)CO-Fe( 100) overlayer. Exposing a c(2X2)CO-Fe(100) surface to C O gas results in two additional desorption signals: CO(a2) at 305 K and CO(al) at 225 K (as seen in the right panel of Figure 3). Compared with TPD data for CO on the clean Fe( 100) surface, the C0(a2') peak is absent for C O adsorbed onto a well-ordered c(2X2)CO-Fe( 100) overlayer. The results in Figure 3 confirm that C O ( a i ) originates from a distinct
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Postdosed Species on Preadsorbed CO on Fe( 100)
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Figure 6. Left panel: HREEL spectra for a saturation overlayer of CH3SH on c(2XZ)CO-Fe(100) at 103 K and subsequently flashed to higher temperatures. Right panel: TPD of CO taken after a ~ ( 2 x 2 ) CO-Fe(100) overlayer is saturated with CH3SH at 103 K. TPD uf CO f r o m W ~ t h ~ ~ ~ t h i u l / C O / F e l l O O ~ l
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CO(a3) into different adsorption states has been observed. Figure 4 also indicates that the population of CO(a3) decreases while the population of the new state increases monotonically as O2 exposure is increased, implying that the conversion is caused by postdosed 02. TPD results confirm these HREELS observations. Figure 5 shows the TPD spectra of CO from 02/c(2X2)CO-Fe(100) overlayers as a function of postdosed O2exposures. As more O2 is added to the c(2X2)C@Fe( 100) surface, the intensities of the CO(a3) and CO(8) desorption peaks decrease, while desorption peaks at lower temperatures appear. The increase in intensity of the lower temperature desorption peaks occurs concurrently with the reduction of CO(a3) and CO(/3) intensity. Since both CO(a3) and CO(8) desorption originates from the adsorbed CO(a3) molecules, the TPD results shown in Figure 5 confirm that CO(a3) is converted to.other adsorption states in the presence of postdosed 02. The res& in Figure 5 also show that CO(8) disappears at a lower O2exposure than does the CO(a3) peak. When the clean Fe( 100) surface is saturated with CO molecules by adsorption of 5 langmuirs of CO at 103 K, and then exposed to 02, no change in the CO TPD spectrum is observed. Both TPD and AES experiments indicate that no O2 adsorbs on a CO saturated Fe( 100) surface. These results suggest that adsorption of 0 2 is completely blocked by a saturation overlayer of CO on the surface and that postdosed O2has no effect on this saturation overlayer. 3.3. Coadsorption of CH3SH with Preadsorbed CO. Figure 6 shows HREELS and TPD results for a c(2X2)CO-Fe(100) overlayer saturated with postdosed CH3SH. As shown in the right panel of the figure, C O desorption at 440 K is greatly reduced by postdosed CH3SH, while a new peak at 340 K appears. The TPD result indicates that most of the preadsorbed CO(a3) molecules are converted to a different adsorption state induced by the postdosed CH3SH. The TPD result is confirmed by the HREELS data shown in the left panel of Figure 6, which also indicates that the conversion occurs at a temperature higher than 103 K, after the c(2X2)CGFe(100) overlayer is saturated with CH3SH.18 This suggests that the coadsorbed overlayer is covered with multilayer CH3SH. After this overlayer is heated to 148 K, a band at 1855 cm-I is seen in the HREELS. Since there is no loss observed in this frequency region for pure CH3SH/Fe( nor for c(2X2)CO-Fe( 100) (Figure 2), the new band is assigned as the C - O stretching of a new adsorption state with desorption temperature of 340 K. After heating to a higher
340 440
TEMPERATURE I K e l v l n I
Figure 7. TPD of CO from CH,SH/CO/Fe(lOO) overlayers as a function of relative CH,SH coverage. A CH3SH/CO/Fe(100) overlayer was prepared by postdosing CH,SH onto c(2X2)CO-Fe(100) at 103 K.
temperature (198 K), the intensity of the new vibrational band increases further and then remains constant at even higher temperature (223 K). After the overlayer is heated to 383 K, this band disappears, consistent with desorption of the 340 K CO peak. The small loss observed around 1165 cm-I is due to residual CO(a3) on the surface. Both the TPD and HREELS results in Figure 6 indicate that conversion of CO(a3) to another adsorption state takes place due to postdosed CH3SH. Comparison of TPD and HREELS results show that the 1895cm-' vibration can be assigned unambiguously to the C - O stretch of the adsorption state with a desorption temperature of 340 K. Since the 500-cm-' loss appears and disappears together with the 1895-cm-' loss, it is likely to be due to m e t a l 4 0 stretching of the new state. The assignments of the other vibrational bands in Figure 6 are discussed in detail in previous ~ t u d i e s . ' ~ * ~ ~ Figure 7 shows TPD spectra for CO from the CH3SH/c(2X 2)CO-Fe( 100) overlayer as a function of relative CH3SH cov(19) Sheppard, N.; Ngugen, T. T.Adu. Infrared Raman Spectrosc. 1978, 5, 61.
The Journal of Physical Chemistry, Vol. 94, No. 15, 19‘90
6032
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Figure 8. HREEL spectra CH3SH/CO/Fe( 100) overlayers (left panel) and subsequently flashed to 198 K (right panel).
erage. The relative coverage is indicated by AES peak to peak height (APPH) ratio, R, = APPH(S 150 eV)APPH(Fe 650 eV), measured after flashing the coadsorption overlayers to 923 K. The R, value is a monotonic function of CH3SH exposure and therefore provides a relative measurement of the amount of CH3SH postdosed on the surface. As is shown in Figure 7, a ~ ( 2 x 2 ) CO-Fe( 100) overlayer without postdosed CH3SH results in one single molecular C O desorption peak at 440 K. After a c(2X 2)CO-Fe( 100) overlayer is exposed to CH3SH molecules, the desorption peak at 440 K decreases in intensity, and a peak at 340 K appears. The desorption intensity at 340 K increases as more CH3SH is dosed onto the C O preadsorbed surface, while the intensity of the CO(a3) desorption concurrently decreases. However, the conversion of CO(a3) into the new state is not complete even with a saturation dose of CH3SH. In contrast to the 02/CO/Fe( 100) system, the CO(a3) desorption temperature in the CH3SH/Fe( 100) system does not shift in the presence of postdosed CH3SH. Figure 8 shows HREELS spectra of CH3SH/c(2X2)CO-Fe(100) overlayers as a function of relative CH3SH coverage. The spectra in the left panel were taken immediately after postdosing CH3SH on the c(2X2)CO-Fe(100) surface at 103 K, while the spectra in the right panel were taken after the coadsorbed overlayers were heated to 198 K. The spectra in the left panel indicate that no conversion of CO(a3) into a new adsorption state occurs at 103 K, since no new vibrational band appears near 1900 cm-I. After heating to 198 K, a vibration near 1900 cm-I does appear, whose intensity grows as more CHJSH is dosed onto the surface. This is consistent with the TPD results shown in Figure 7 . HREELS results in Figure 8 also indicate that the conversion process is a thermally activated one. 4. Discussion Saturation of a clean Fe(100) surface with CO at 103 K results in four associative adsorption states (Figure 1). After the C O saturated overlayer is heated to 383 K, only adsorbed CO(a3) molecules exist on the surface (Figure 2). This pure CO(a3) overlayer shows a ~ ( 2 x 2 LEED ) pattern. It has been proposed that the adsorbed CO molecules in the a3 state occupy 4-fold hollow sites.14 This site assignment has been confirmed by a recent theoretical study using an Fe,, cluster m0de1.l~ Based on these results and combined with LEED observations, a geometric site model can be proposed to describe the structure of the ~ ( 2 x 2 ) CO-Fe( 100) overlayer, as shown in Figure 9a. After the 4-fold hollow sites are saturated with C O in the c(2X2)CO-Fe(l00) surface, all 2-fold bridge sites are adjacent to 4-fold hollow sites occupied by the adsorbed CO molecule, as can be seen from Figure 9a. However, for CO adsorption at 103 K, the first adsorbed 4-fold hollow site CO molecules do not arrange in a perfect ~ ( 2 x 2 ) configuration, and there may exist defect regions in the lowtemperature ordered CO(a3) overlayer, as schematically shown in Figure 9b. For this nonideal overlayer, there are two different types of 2-fold bridge sites, one adjacent to 4-fold hollow site CO
b) Figure 9. (a) A geometric model for a c(2X2)CO-Fe(100) overlayer, where x indicates that the site is occupied by CO molecules. (b) Adsorption sites for CO(a2)and CO(a2/), where x is the site occupied by CO(a,), a2 is the site for CO(a2).and a i is the site for CO(a2’).
molecules and the other not. Increased CO exposure at low temperature on this surface results in sequential occupation of these two different bridge sites, designated as azand ai. This results in the observation of C0(a2) and CO(ai) desorption peaks as shown in the left panel of Figure 3. In contrast, when C O is exposed to the well-ordered c(2x2)CO-Fe( 100) surface, no CO(ai)desorption results, as shown in the right panel of Figure 3. An additional adsorption site available on the Fe( 100) surface is the on-top site. Although the C-0 stretching frequency of CO(aI) cannot be resolved from that of CO(az) and CO(ai) in the HREELS spectra, the desorption peak temperature for CO(aI) is quite different from that of CO(a2) and CO(ai). This large difference in desorption temperature is attributed to CO binding in the on-top site for a , . When a c(2X2)CO-Fe(lOO) overlayer is exposed to O2at 103 K, the preadsorbed CO(a3), Le., 4-fold hollow site CO, is converted to other adsorption states, as shown in Figures 4 and 5. The new vibrations (Figure 4) can be assigned to C-O stretching of on-top CO (around 2010 cm-I) and 2-fold bridge site CO (around 1910 ~ m - l ) .The ~ ~multiple desorption peaks in the TPD spectra (Figure 5 ) confirm that the 4-fold hollow site C O has been converted to lower coordination sites. Previous experimental studies!’ have shown that Oz dissociatively adsorbs on the Fe(100) surface a t 103 K. HREELS,” LEEDZo and cluster calculationz2 results indicate that 0 atoms on the Fe( 100) surface preferentially occupy 4-fold hollow sites. Therefore, on the Fe( 100) surface 0 and CO will compete to occupy these sites. Since 0 atoms bind to the 4-fold hollow sites more strongly than C O molecules (no desorption of oxygen can be detected in the temperature range up to IO00 K), postdosed oxygen replaces the 4-fold hollow site CO molecules. The replaced CO can either go to lower coordination sites or desorb into the gas phase. Since desorption requires a higher activation energy, site conversion would be a favorable pathway. Therefore, the observed CO site conversion can be explained in terms of site competition. On a CO saturated surface, there are no unoccupied sites for C O site conversion, so this process is not observed. In addition to site conversion, postdosed O2has other effects on the properties of coadsorbed CO. As shown in Figure 5 , the desorption peak temperature of CO(a3)decreases as more 0 is coadsorbed on the surface. This indicates that coadsorbed 0 atoms (20) Leg& K. 0.;Jona, F.; Jepson, D. W.; Marcus, P. M. Phys. Reu. 1977, 816, 5271. (21) Van Zoest, J. M.; Fluit, J. M.; Vink, T. J.; Van Hassel, B.A. Surj. Sci. 1987, 182, 179. (22) Huang, H.; Hermanson, J. Phys. Reu. 198k,832, 6312.
Postdosed Species on Preadsorbed CO on Fe( 100) reduce the binding energy between adsorbed CO molecules and the Fe substrate. This is very different from the effect of coadsorbed S atoms on C O adsorption? in which case the adsorbed S does not cause a change in the CO(a3) desorption peak temperature. Since both CO(a3) and CO(/3) desorption originates from species adsorbed in 4-fold hollow sites, site conversion of CO caused by postdosed O2results in the reduction of both the CO(a3) and CO(8) desorption peak intensities. However, CO(/3) decreases in intensity faster than does CO(a3). This is due to the fact that dissociation of one CO molecule requires a pair of adjacent 4-fold hollow sites. Occupation of a 4-fold hollow site by 0 reduces the number of hollow site pairs more quickly than the single hollow sites. Since only the CO(a3) adsorption state is active in dissociation, this site conversion process decreases the probability of CO dissociation on the Fe(100) surface. This phenomenon is conceptually important and may contribute to understanding the mechanism of complex catalyst poisoning processes. CO site conversion has also been observed for the CH3SH/ CO/Fe( 100) system. However, the site conversion in this system does not take place readily at 103 K (Figures 6 and 8) as in the case of 02/c(2X2)CO-Fe( 100). Previous HREELS studies'* showed that, in the temperature range of 123-273 K, surface thiomethoxy (-SCH3) exists on the Fe( 100) surface. Therefore, the site conversion of C O may be caused by site competition between CO and -SCH3. A LEED intensity has indicated that S atoms on the Fe( 100) surface are located on 4-fold hollow sites. Site conversion observed in this work implies that thiomethoxy also preferentially occupies the 4-fold hollow sites. Another feature of the CH3SH/CO/Fe( 100) system, which is different from the 02/CO/Fe(100) system, is that in this case the CO(a3) only converts to one new state. The new state is characterized by a desorption peak at 340 K and C-O stretching frequency of 1900 cm-'. The desorption peak temperature of this new state is very close to that of C O ( a i ) , suggesting that they have a similar binding environment. Both can be attributed to C O adsorbed on bridge sites without an adjacent CO occupied hollow site (Figure 9). (23) Legg, K. 0.; Jona, F.; Jepson, D. W.; Marcus, P. M. Surf Sci. 1977, 66, 25.
The Journal of Physical Chemistry, Vol. 94, No. 15, 1990 6033 Adsorption site conversion caused by postdosed species has also
been observed for the following systems: *3C0/12CO/Fe( H20/CO/Pt( 111),2s02/CO/Cr(l and CO/NO/RU(OO~).*~ These site conversions result from site competition between preadsorbed species and postdosed species. It appears that site competition can occur between atoms (e.g., 0),molecules (e.g., CO) or even a group in a molecule (e.g., -SCH3). One of the most important factors governing this site conversion process is the difference in binding energies of the competing species with the substrate surface. Stronger binding between one species and the surface can be a major driving force for the site conversion process. If the enthalpy difference is small, then the entropy may play an important role, as was observed in the '3C0/'2CO/Fe( 100) system.24 5. Conclusion Four molecular adsorption states, referred to as CO(a,), CO( a 2 )CO(a,l), , and CO(a3), have been observed on the Fe(100) surface. After the desorption of CO(al), C0(a2), and C O ( a i ) from a CO-saturated surface, an overlayer consisting of pure CO(a3) molecules can be prepared. Adsorbed C O molecules in this overlayer occupy 4-fold hollow sites in a ~ ( 2 x 2 configuration. ) When the c(2X2)CO-Fe( 100) overlayer is exposed to 02,the preadsorbed 4-fold hollow site CO is forced to migrate to lower coordination sites. This type of adsorption site conversion is caused by site competition between 0 and C O driven by the difference in binding energy of the 0 and CO adspecies. Coadsorbed 0 atoms also weaken the metal C O bonding of remaining hollow site CO molecules. Postdosed CH3SH also caused CO adsorption site conversion. Since the postdosed species converts the dissociation precursor adsorption state, CO(a3), into nonactive adsorption states, the postdosed species serves to poison the dissociation reaction of CO on the Fe( 100) surface.
Acknowledgment. This research was supported by the National Science Foundation, Division of Materials Research. (24) Lu, J.-P.;Albert, M. R.; Bernasek, S. L.; Dwyer, D. J. Surf Sci. 1988, 199, L406. (25) Tornquist, W. J.; Griffin, G . L. J . Vac. Sci. Techno/. 1986, A4, 1437. (26) Shinn, N. D.; Madey, T. E. J . Vac. Sci. Technol. 1985, A3, 1673. (27) Thiel, P. A.; Weinberg, W. H.; Yates, Jr., J. T. J . Chem. Phys. 1979, 71, 1643.
