Chemisorption of Carbon Monoxide on the Iridium(111) Surface

Aug 15, 1996 - temperature and coverage dependence of the adsorption probability is consistent with an extrinsic precursor model. Isobars were obtaine...
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Ind. Eng. Chem. Res. 1996, 35, 2986-2992

Chemisorption of Carbon Monoxide on the Iridium(111) Surface: In Situ Studies of Adsorption and Desorption Kinetics via Vibrational Spectroscopy Robert W. Boyle, Jochen Lauterbach,* Matthias Schick, William J. Mitchell, and W. Henry Weinberg Department of Chemical Engineering, University of California at Santa Barbara, Santa Barbara, California 93106

The IR absorption peak position of the CO intramolecular stretching vibration, measured with Fourier transform infrared reflection-absorption spectroscopy, has been employed for the first time as a nonintrusive coverage probe to study the adsorption of CO on Ir(111). The observed temperature and coverage dependence of the adsorption probability is consistent with an extrinsic precursor model. Isobars were obtained under steady-state conditions for pressures ranging from 10-9 to 10-5 Torr. Isosteres were constructed from these data and used to determine both isosteric heats of adsorption and the adsorbate entropy as functions of coverage. The isosteric heat of adsorption decreased from 44 ( 2 kcal/mol at 0.05 ML to 37 ( 1 kcal/mol at 0.6 ML. A pronounced decrease in the binding energy at 0.35 ML coincides with the weakening of the (x3 × x3)R30° LEED pattern and is attributed to near-neighbor CO repulsions. The entropy of the adsorbate layer indicates two ordering regions which correspond to the formation of the (x3 × x3)R30° and (2x3 × 2x3)R30° overlayer structures. 1. Introduction Vibrational spectroscopy has proven to be one of the most powerful tools available to the surface scientist. In particular, Fourier transform infrared reflectionabsorption spectroscopy (FT-IRAS) is a nonintrusive surface probe which allows high-resolution measurements over a large pressure range [Hoffmann (1983); Weinberg (1985); Chabal (1988); Bradshaw and Schweizer (1988)]. In the present study FT-IRAS was used to examine the adsorption of carbon monoxide on the (111) surface of iridium under both steady-state and transient conditions. These results contribute to the wealth of data that currently exists for model CO/ transition-metal systems and will aid in the elucidation of the geometric and chemical effects relevant to the numerous catalytic reactions involving CO. The adsorption of CO on iridium surfaces has previously been studied using low-energy electron diffraction (LEED) [Christmann and Ertl (1973); Comrie and Weinberg (1976a,b); Ku¨ppers and Plagge (1976); Hagen et al. (1976); Taylor et al. (1978); Kisters et al. (1991)], temperature-programmed desorption (TPD) [Ageev and Ionov (1973); Christmann and Ertl (1973); Comrie and Weinberg (1976a,b); Ku¨ppers and Plagge (1976); Hagen et al. (1976); Taylor et al. (1978)], UV-photoelectron spectroscopy (UPS) [Ku¨ppers and Plagge (1976); Zhdan et al. (1976); Broden and Rhodin (1976); Seabury et al. (1980, 1981)], X-ray-photoelectron spectroscopy (XPS) [Zhdan et al. (1976)], high-resolution electron energy loss spectroscopy (HREELS) [Marinova and Chakarov (1989); Kisters et al. (1991)], and, most recently, FTIRAS [Lyons et al. (1995); Lauterbach et al. (1996)]. These investigations provide useful information concerning CO adsorption on the (111), (110), and (100) surfaces of iridium and on polycrystalline iridium films. In all cases CO was found to adsorb molecularly, with * Author to whom correspondence is addressed. Phone: (805) 893-8596. Fax: (805) 893-4731. E-mail: jochen@ engineering.squid.ucsb.edu.

S0888-5885(95)00747-0 CCC: $12.00

bonding through the carbon atom at terminal sites. In the case of the hexagonally close-packed Ir(111) surface, a LEED pattern corresponding to a (x3 × x3)R30° ordered overlayer structure appears and reaches maximum intensity at CO coverages (ΘCO) of 1/3 ML. A further increase in ΘCO results in a weakening of the (x3 × x3)R30° superstructure and eventually leads to a diffuse (2x3 × 2x3)R30° pattern at ΘCO ) 7/12 ML. A TPD analysis indicates the existence of three sequentially populated CO bonding states on the Ir(111) crystal, the third one of which has only recently been identified [Lauterbach et al. (1996)]. The first peak with a desorption peak temperature of ∼550 K fills in until the maximum intensity of the (x3 × x3)R30° LEED pattern is observed at ΘCO ) 1/3 ML. At this point a second state, desorbing at lower temperatures, begins to fill in. A third, low-temperature shoulder, which corresponds to a CO coverage greater than 0.67 ML, is only observed for very high exposures and for sample temperatures around 300 K, indicating that a certain mobility of the CO admolecules is required for its formation. In the present study FT-IRAS is used to determine absolute CO coverages on the surface. Using this approach, adsorption probabilities below the threshold temperature of desorption, steady-state isobars, and isosteric heats of adsorption for CO on Ir(111) could be measured. 2. Experimental Details All experiments were carried out under ultrahighvacuum (UHV) conditions. The stainless steel chamber was pumped to a base pressure below 1 × 10-10 Torr by a 240 L/s turbomolecular pump (Balzers), a 220 L/s ion pump (Varian), and a titanium sublimation pump. The chamber was equipped with a differentially pumped quadrupole mass spectrometer (UTI 100C), reverse-view LEED optics (Fisions), and an Auger electron spectroscope (Varian). A separate UHV chamber was mounted on top of the chamber via a 6 in. gate valve (allowing © 1996 American Chemical Society

Ind. Eng. Chem. Res., Vol. 35, No. 9, 1996 2987

Figure 1. Schematic drawing of the optical setup used for FTIR.

high-pressure experiments while maintaining UHV in the chamber below). In order to perform surface-sensitive IRAS experiments, it was necessary to couple a commercial Fourier transform infrared spectrometer (Perkin-Elmer 1725X) with the top chamber by installing two evacuated (ionpumped to 10-6 Torr) “optical boxes” which contain the necessary mirrors, polarizers, and detectors for beam transfer, focusing, and processing. A schematic of the beam path through the boxes and top chamber is shown in Figure 1. After exiting from the spectrometer, the IR beam (parallel, 2 in. diameter) passes through a KBr window, W1 (0.25 in. thick, 2.25 in. diameter), into the input optical box where it is reflected from a flat mirror, F1, to another flat mirror, F2. At this point the beam can be split into two paths through the UHV chamber, namely, a reference beam that passes directly through for gas-phase spectroscopy at higher pressures (via KBr windows W2 and W3) and a sample beam that is reflected from the sample for surface-sensitive spectroscopy (via KBr windows W4 and W5). The reference-tosample beam intensity ratio is adjusted by moving the flat mirrors F2 and F5 which are mounted on singleaxis translation stages. The motion of these stages is driven by stepping motors (external to the vacuum) which are computer controlled. In Figure 1, the reference-to-sample beam ratio shown is 50:50. The sample beam fraction is reflected from F2 to an off-axis parabolic mirror, P1, with an effective focal length of 4.7 in. which, when in the 100% sample beam configuration, focuses the radiation at an incident angle of 80 ( 10° with respect to the surface normal (i.e., the beam numerical aperture is 0.34, implying a f/1.45 beam at the sample) to a 5 mm spot size on the 10 mm crystal located inside the top UHV chamber. As the sample beam fraction is reduced below 100%, the near-normal rays are lost, and the incident angle becomes more grazing. The reflected beam from the crystal is recollimated by a second parabolic mirror, P2, and is directed to the detector focusing mirror, P3, which is located underneath the detector, via the flat mirror, F4. A wiregrid polarizer located between F4 and P3 polarizes the reflected beam parallel to the plane of incidence at the crystal, i.e., p-polarization. The focusing mirror, P3, is off-axis parabolic with an effective focal length of 2 in. and focuses the beam onto the entrance window at the bottom of the detector (with 3× demagnification with respect to the sample). The reference beam path is much simpler. The beam bypasses F2 and is reflected from the flat mirrors F3 and F5 to the detector mirror, P3. All the mirrors are mounted on precise, lockable kinematic mounts. The flat mirrors consist of a Pyrex substrate coated with aluminum, and the parabolic mirrors are electroformed from Ni and are also alumi-

num coated. The wire-grid polarizer is a KRS-5 substrate with aluminum wires holographically etched onto the surface with a density of 1800 wires/mm. The IR detector is a narrow-band mercury-cadmium-telluride (MCT) photoconductive detector. The KBr windows connected to the top UHV chamber, i.e., W2, W3, W4, and W5, are Viton O-ring sealed and are mounted in custom-designed Conflat flanges. They are clamped in place by aluminum clamps. The differential pumping provided by the boxes means that these Viton O-rings can be used to seal the windows and still maintain 10-11 Torr within the top chamber. Both the sample compartment and external parallel beam accessory of the spectrometer are continuously purged by dry, CO2-free air which is supplied by a purge-gas generator, whereas the interferometer area is separately sealed and desiccated (using molecular sieves). The remainder of the IR beam path, i.e., external to the spectrometer, is completely in vacuum. Special software permitting time-resolved FT-IR (Perkin Elmer) was used to obtain data at a rate of one averaged spectrum/2 s with 4 scans per averaged spectrum and 4 cm-1 resolution. Although adequate signal-to-noise ratios were achieved under these conditions, some of the IR data were recorded with 2 cm-1 resolution and a minimum of 100 scans per spectrum, ensuring an improved signal-to-noise ratio greater than 25 000. The Ir(111) crystal was mounted on a cryostat with two tungsten ribbons and could be heated resistively and cooled with liquid nitrogen. A type-C thermocouple (W/5% Re vs W/26% Re) was spot welded to the back of the sample. In order to access the many surface analysis instruments located on both the top and bottom UHV chambers, we designed and installed a UHV precision manipulator for sample positioning. The design allows 1.75 in. x-y travel, 16 in. z travel, and 270° rotation about the z axis. For ease of operation, motion along each axis is driven by stepping motors which are each controlled by a stepping motor controller that is interfaced with an IBM-PC. This controller also manipulates the moveable flat mirrors within the IR optical boxes. The crystal was frequently sputtered in argon, cleaned in oxygen at 1000 K, and then annealed to 1400 K. The cleanliness of the surface was verified regularly using Auger electron spectroscopy. Research-grade CO, which was of 99.99% purity (Matheson), was used in the experiments. 3. Results and Discussion 3.1. Calibration of the Coverage. As reported previously, a single IR absorption peak is observed upon CO adsorption on the Ir(111) surface for all surface temperatures and CO exposures [Lauterbach et al. (1996)]. This peak corresponds to CO molecules adsorbed in a linear configuration. The relationship between the IRAS absorption peak position and the CO coverage is shown in Figure 2. These data resulted from a sequence of experiments at various CO exposures in which an IR spectrum was obtained prior to a TPD measurement. The thermal desorption areas were integrated, and the peak area was converted into absolute coverages using the intensity maximum of the (x3 × x3)R30° LEED pattern as a calibration point for ΘCO ) 1/3. The appearance of the second, low-temperature peak in the TPD spectra provides additional confirmation of the coverage calibration. The position of the IR absorption peak shifts with increasing CO

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Figure 2. FT-IRAS absorption peak position, corresponding to the CO stretching frequency, as a function of CO coverage on Ir(111) at 90 K (circles) and at 300 K (triangles).

coverage to higher wavenumbers. This shift, shown in Figure 2, is mainly attributed to dipole-dipole interactions between the admolecules and is examined and modeled in detail elsewhere [Lauterbach et al. (1996)]. Due to its pronounced and very reproducible increase with coverage, the IR absorption peak position is in this case very well suited to act as a nonintrusive CO coverage probe. Therefore, the CO coverage can be monitored under both reversible and irreversible conditions without perturbation of the overlayer. The integrated IR absorption intensity has been used previously to serve a similar purpose [Truong et al. (1992); Takagi et al. (1994); Kuhn et al. (1994)]. However, for the CO/ Ir(111) system it has been shown that the IR peak area increases with coverage up to 0.25 ML and then plateaus to a constant value [Lauterbach et al. (1996)]. This limits the utility of this technique as a surface coverage probe to the low-coverage regime, as was also the case for the other investigations [Truong et al. (1992); Takagi et al. (1994); Kuhn et al. (1994)]. Furthermore, the IR peak area was found to lack the superior day-to-day reproducibility of the peak position calibration. Further experiments were carried out at sample temperatures between 90 and 350 K in order to resolve any temperature-dependent peak shift [Lauterbach et al. (1996)]. Data measured at 90 K depart from the 300 K curve slightly at coverages greater than 0.2 ML, as can be seen in Figure 2. This small shift to higher frequencies reaches a maximum of 5 cm-1 at 0.55 ML and is explained in terms of a restricted mobility of the CO molecules on the surface at low temperatures. At these low surface temperatures, CO is not sufficiently mobile to form energetically ideal overlayers, the structure of which is governed by repulsive nearest-neighbor and next-nearest-neighbor interactions [Meng and Weinberg (1994)]. This assertion is demonstrated by LEED, where the order of the (x3 × x3)R30° and the (2x3 × 2x3)R30° superstructures becomes less pronounced at lower surface temperatures. At approximately 250 K the data for the IR peak position converge to the data measured at 300 K. Therefore, the 300 K curve is consistent with all data collected from 250 to 350 K. In addition, no difference between experiments at any temperature could be observed for CO coverages below ∼0.2 ML, i.e., in the range where a disordered CO overlayer is present on the surface. This confirms that

Figure 3. Ratio of the CO adsorption probability s on Ir(111) to the initial adsorption probability s0 as a function of CO coverage for different surface temperatures.

the shift observed at 90 K was solely due to limited mobility of the CO admolecules and not, for example, due to a temperature-dependent dephasing process. Therefore, it can be assumed that the peak position is independent of temperature for all temperatures above 250 K. 3.2. Adsorption Probabilities. Using the calibration plot in Figure 2, adsorption probabilities of CO on Ir(111) were determined as a function of coverage and temperature. The adsorption probability is defined by the relationship

ra ) dΘCO/dt ) Pa(ΘCO,T) F

(1)

where ra is the rate of adsorption, F is the gas impingement flux, and Pa is the adsorption probability which is a function of surface coverage and surface temperature. The molecular flux can be rewritten in terms of gas pressure, temperature, and mass using a relationship from the kinetic theory of gases [Atkins (1990)]. The adsorption probabilities were determined by exposing the Ir(111) crystal to a constant pressure of CO by backfilling the UHV chamber to 5 × 10-9 Torr. The CO coverage was monitored in time using the timeresolved FT-IRAS software. The IR peak position was then converted to coverage using the calibration curve shown in Figure 2. The slight frequency shift observed at low temperatures (vide supra) was accounted for in these conversions. The measured change in the CO coverage as a function of time could then be converted into adsorption probabilities using eq 1 by measuring the slopes (dΘCO/dt) of these curves as a function of coverage. The adsorption probabilities obtained in this way are plotted as a function of coverage parametric in temperature in Figure 3. The initial (zero coverage) adsorption probability was calculated to be 0.90 ( 0.10 at all temperatures investigated (90-300 K). Previously reported initial adsorption probabilities for CO on Ir surfaces range from 0.6 [Hagen et al. (1976)] to 1 [Comrie and Weinberg (1976a,b)]. However, the accuracy of absolute adsorption probabilities is limited mainly by the calibration of the ion gauge since the adsorption probability is directly proportional to the absolute gas pressure. For this reason, adsorption probabilities are reported in Figure 3 as the ratio of the adsorption probability at a given coverage ΘCO to the initial adsorption probability, Pa(0). The adsorption probabilities remain nearly constant at all temperatures

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Figure 4. Isobars measured at pressures ranging from 1 × 10-9 to 1 × 10-5 Torr for CO adsorption on Ir(111).

up to a CO coverage of ∼0.1 ML. When the coverage exceeds a certain temperature-dependent threshold, the adsorption probability decreases from its initial value of approximately unity to a final value close to zero. This type of behavior is frequently observed for the adsorption of CO on various transition metals (Weinberg et al. (1976); Ertl et al. (1977); Ibach et al. (1980); Behm et al. (1980); Lauth et al. (1989)] and is attributed to precursor-mediated chemisorption. If the adsorption were occurring directly, one would expect the adsorption probability to be a first-order function of the fraction of vacant surface sites, i.e., the so-called “direct” or “Langmuirian” chemisorption. In this case the adsorption probability should scale as

Pa(ΘCO) ) Pa(0) (1 - ΘCO)

(2)

However, Figure 3 clearly shows that this is not the case. According to an extrinsic precursor model, the CO molecules are first adsorbed into weakly bound precursor states and allowed to migrate over the occupied sites on the surface. Therefore, measured adsorption probabilities can be close to unity even at higher adsorbate coverages, as is obviously observed here. The strong temperature dependence of the adsorption probability that is apparent in Figure 3 can also be described in terms of the precursor model. Since the precursor state is weakly bound, its lifetime at room temperature is much less than that at lower temperatures, due to the differing desorption rates. This explains why the adsorption probability near CO coverages of 0.3 ML is an order of magnitude larger at 140 K than at 305 K, for example. Due to the greater lifetime of the precursor state at lower temperatures, the CO molecule has simply more time to “find” a vacancy for chemisorption. Another important factor to consider is the mobility of the chemisorbed CO molecules on the surface. At room temperature the adlayer is sufficiently mobile to form ordered structures on the surface, whereas at lower temperatures a lack of mobility results in a mostly disordered adlayer. Figure 3 indicates that the decrease in adsorption probabilities measured at 300 K coincides with the initial formation of the (x3 × x3)R30° overlayer structure, and the probability has become very low for this optimally ordered superstructure at ΘCO ) 0.33. This suggests that it is more difficult to adsorb into an ordered adlayer than into a disordered layer. Therefore, it is reasonable to assume that the mobility of the adlayer has some additional influence on the behavior observed in Figure 3. 3.3. Isosteric Heats of Adsorption. In Figure 4

adsorption isobars for CO on Ir(111) are shown for CO pressures between 1 × 10-9 and 1 × 10-5 Torr. The isobars are qualitatively similar to those obtained via FT-IRAS [(Kuhn et al. (1994)] and work function measurements [Tracy and Palmberg (1969); Tracy (1972); Behm et al. (1980); Ibach et al. (1980)] for CO chemisorption on other transition-metal surfaces. The data were obtained by setting a constant CO pressure at a given crystal temperature. The IR absorption peak position was then continuously monitored until steady state was reached. The calibration shown in Figure 2 enabled the conversion of this steady-state peak position into the steady-state CO coverage for the corresponding temperature and pressure. The data were acquired in a series of experiments, each with a constant pressure of CO, from low to high temperatures. However, as expected, the data were reproducible regardless of whether steady state was approached from higher temperature or lower temperature. At pressures above 1 × 10-6 Torr and at temperatures above 550 K, a slow downward shift in the IRAS absorption peak was observed after the system reached steady state. This shift continued until the CO absorption band could no longer be detected. This behavior is the result of dissociation of the CO molecules, since after the experiments considerable amounts of residual carbon were detected on the surface by Auger electron spectroscopy. Therefore, it was necessary to clean the crystal frequently when working under these temperature and pressure conditions. In spite of the carbon contamination, vigilant experimental procedures have enabled accurate data acquisition for pressures up to 1 × 10-5 Torr. Unfortunately, precise data were unobtainable at higher pressures. The experiments were also limited at lower temperatures (higher coverages) due to the extreme length of time required for the low-pressure system to achieve steady state. The time scale required to reach steady state was not significantly shorter than the time scale inherent for surface contamination for the 1 × 10-8 and 1 × 10-9 Torr isobars at 320 K. Therefore, data were also difficult to obtain in this region. Horizontal slices (isosteres) in Figure 4 illustrate the CO pressure/adsorbate temperature relationship necessary to maintain a constant surface coverage. Isosteric heats of adsorption (which are equal to the binding energy to an accuracy on the order of RT [Ehrlich (1962)] may be calculated using these isosteres and the Clausius-Clapeyron equation

ln p [∂ ∂T ]

hg - h hs

sg - sjs ) θ

RT

) RT

2

-qst )

RT2

(3)

where sg and hg are the molar entropy and enthalpy of the gas phase, and sjs and h h s are the partial molar entropy and partial molar enthalpy of the adsorbate. This relationship was developed by assuming that the gas phase and the adsorbed phase are in equilibrium. Although the experimental steady-state conditions do not represent true equilibrium because the temperature of the gas phase is not equal to the temperature of the adsorbed phase, it has been shown that the error introduced by this deviation is on the order of RT for this system [Ehrlich (1962)]. Figure 5 includes a selection of isosteres plotted in accordance with the Clausius-Clapeyron equation for CO coverages between 0.05 and 0.57 monolayer. The slope of each isostere (slope ) -qst/R) was used to

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Figure 5. Selection of isosteres plotted in accordance with the Clausius-Clapyeron relationship for CO adsorption on Ir(111). Coverages range from 0.05 to 0.57 ML.

Figure 6. Isosteric heat of adsorption for CO on Ir(111) as a function of CO coverage.

determine the isosteric heat of adsorption as a function of coverage, as shown in Figure 6. The isosteric heat of adsorption for CO on Ir(111) varies from 44 ( 2 kcal/ mol at a CO coverage of 0.05 monolayer to 37 ( 1 kcal/ mol at 0.6 monolayer. A similar desorption energy difference (∼6 kcal/mol) between high and low coverages can be predicted from the positions of the first and second CO TPD peaks using a Redhead analysis [Redhead (1962)]. The decrease in binding energy observed in Figure 6 for ΘCO g 0.35 ML coincides with a weakening of the (x3 × x3)R30° ordered overlayer structure and the initial filling of the second TPD peak. This decrease is caused by near-neighbor repulsions between the adsorbed CO molecules. In the perfectly ordered (x3 × x3)R30° superstructure, each CO admolecule has zero nearest neighbors. However, the next adsorbing CO molecule must occupy a position with three nearest neighbors. The repulsion introduced as nearest-neighbor sites are occupied causes a decrease in the binding energy. This behavior is common and has also been reported for CO adsorption on other hexagonally closepacked transition-metal surfaces [Christmann et al. (1974); Pfnu¨r et al. (1983); Pfnu¨r and Menzel (1983)]. At low coverages the heat of adsorption increases from 40.5 ( 1 kcal/mol at Θ ) 0.15 ML to 44 ( 2 kcal/mol at Θ ) 0.05 ML. A similar increase has been reported for CO on Ni(111) [Christmann et al. (1974)] and Cu(100) [Tracy (1972); Truong et al. (1992)]. No satisfactory explanation can be made to account for this increase which has previously been attributed to long-range

repulsions between the adsorbed CO molecules. However, this seems unlikely. According to Figure 6, the decrease in binding energy observed at low coverages is as pronounced or even more pronounced than the one seen at 0.35 ML. This would imply that the long-range repulsions at coverages less than 0.15 ML are as strong as the nearest-neighbor repulsions observed above 1/3 ML. There clearly must be some other explanation for this behavior. It has previously been suggested that this low-coverage increase in binding energy is caused by the presence of defects and inhomogeneities on the surface which preferentially bind CO with higher energies. Although this may occur at coverages of less than ∼0.02 ML for the Ir(111) crystal which was used, it is extremely unlikely that defects and inhomogeneities could cause such pronounced changes in the binding energy at coverages of up to 0.15 ML. At CO coverages from 0.15 to 0.35 ML, the isosteric heat of adsorption has the relatively constant value of 40.5 ( 1 kcal/mol. This region coincides with the filling of the highest temperature TPD peak. Although the fluctuations seen in Figure 6 near coverages of 0.23 and 0.35 monolayers were reproducible in our experiments, they lie within the experimental uncertainty defined by the error bars and may not be significant. It is worth noting that a similar, yet far more pronounced, increase in binding energy (prior to an abrupt decrease at 1/3 ML) has been reported for CO adsorption on the geometrically similar Ru(001) surface [Pfnu¨r et al. (1983); Pfnu¨r and Menzel (1983)]. In that case the behavior was explained in terms of a next-nearest-neighbor attractive CO interaction. The sharp decrease which follows coincides with the onset of nearest-neighbor repulsions. Isosteric heats of adsorption and desorption energies have been reported previously for CO adsorption on various iridium surfaces. For CO on Ir(111), isosteric heat of adsorption of 39 ( 3 kcal/mol at 1/3 ML was measured using LEED [(Hagen et al. (1976)]. This result is consistent with the data in Figure 6. However, other LEED experiments on Ir(111) have given isosteric heats of adsorption of 35 ( 1 [Comrie and Weinberg (1976a,b)] and 33 kcal/mol [Ku¨ppers and Plagge (1976)] at 1/3 ML. On the Ir(110) surface, which consists of (111) microfacets, an isosteric heat of CO adsorption of 37 kcal/mol has been reported [Christmann and Ertl (1973)]. The differences in these values suggest that LEED measurements are less well suited for isosteric heat of adsorption measurements since this method is not entirely nonintrusive and is limited to certain specific coverages. Ageev and Ionov reported a desorption energy of 44 kcal/mol in the limit of zero coverage for CO on Ir(111) [Ageev and Ionov (1973)], which is consistent with the present results. However, other thermal desorption studies have given widely varying desorption energies for this system [Ku¨ppers and Plagge (1976); Marinova and Chakarov (1989)]. The isosteric heats of adsorption in Figure 6 were used to calculate the change in entropy of CO upon adsorption using eq (3), where

sg ) sg° - R ln(p/p°)

(4)

∆s ) sjs - sg° ) -R ln(p/p°) - qst/T

(5)

Therefore,

and sg° ) 47.3 cal/mol K for CO at p ) p° ) 760 Torr. The difference between the partial molar entropy of the

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caused by short-range nearest-neighbor CO repulsions. The isosteric heats of adsorption were then converted into adsorbate entropies. These data indicate that two ordering regions exist which correspond to the formation of the (x3 × x3)R30° and (2x3 × 2x3)R30° adlayer structures. Acknowledgment This research was supported by the National Science Foundation (Grant CHE-930020). J.L. and M.S. are Feodor Lynen Fellows of the Alexander von Humboldt Foundation. Figure 7. Partial molar entropy of adsorbed CO on Ir(111) with respect to the entropy of CO in the gas phase at 1 atm plotted as a function of CO coverage.

adsorbed CO and the entropy of gas-phase CO at 1 atm is shown in Figure 7 as a function of CO coverage. These data indicate a maximum disorder in the adsorbed layer at coverages less than 0.1 ML. This is consistent with the inhomogeneous broadening of the IR peak reported previously for low CO coverages [Lauterbach et al. (1996)]. From ΘCO ) 0.2 ML to ΘCO ) 0.33 ML, a significant decrease in the entropy of the adsorbed layer is observed, which coincides with the formation of the ordered (x3 × x3)R30° superstructure. The entropy of the adlayer than remains essentially constant for coverges from 0.33 to 0.5 ML. At coverages greater than 0.5 ML, a further decrease in entropy is observed which corresponds to the formation of the (2x3 × 2x3)R30° superstructure at 7/12 ML. 4. Summary The adsorption of CO on Ir(111) has been studied using the FT-IRAS absorption peak position of CO as a coverage probe. Two sets of experiments were conducted in this study. First, the CO surface coverage was monitored with time as a clean Ir(111) crystal was exposed to a constant flux of CO at various temperatures. This direct measurement of the rate of adsorption allowed an evaluation of adsorption probabilities for various temperatures and coverages. The adsorption probability for rather low coverages is constant and nearly unity at temperatures from 90 to 300 K. The temperature and coverage dependence of the adsorption probability indicates that adsorption is likely to proceed via an extrinsic precursor pathway. The second set of experiments was directed toward determining the isosteric heat of adsorption for CO on Ir(111) as a function of coverage. The steady-state coverages of CO at pressures from 1 × 10-9 to 1 × 10-5 Torr were determined at various temperatures using FT-IRAS. These isobars were than converted to isosteres using the Clausius-Clapeyron relationship which were then used to determine isosteric heats of adsorption. The isosteric heat of adsorption was found to decrease from 44 ( 2 kcal/mol at 0.05 ML to 37 ( 1 kcal/mol at 0.6 ML. An initial decrease in the binding energy for coverages from 0.05 ML to 0.15 ML remains unexplained. A second decrease in the binding energy coincides with the weakening of th(x3 × x3)R30° overlayer structure on the surface and the filling of the second, lower energy TPD peak. This energy drop is

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Received for review December 15, 1995 Revised manuscript received March 1, 1996 Accepted March 4, 1996X IE950747B

X Abstract published in Advance ACS Abstracts, August 15, 1996.