J. Phys. Chem. 1992, 96, 1826-1835
1826
be explained on the basis of the energy level scheme. It was deduced that Eu3+ cannot act as a recombination center. The time and potential dependence of the Tb3+PL and EL reflect the defect properties and the charge carrier recombination kinetics of the Ta205film. Experiments involving the Tb3+ SD4decay time in H 2 0 and D2Q and the EL of adsorbed ions show that the characteristic EL of the terbium-doped tantalum oxide layer
originates at least partially from the surface. Acknowledgment. The work described here was supported by the Netherlands Foundation for Chemical Research (SON), with financial aid from the Netherlands Organization for Scientific Research (NWO). Registry No. Ta205, 1314-61-0; Tb3+,22541-20-4; H202,7722-84-1.
Kinetics of CO Adsorption on Ni(ll0) Studied in Real Time by Ion-Stimulated Desorption of Neutral Molecules J. H. Campbell, Department of Chemistry, Stanford University, Stanford, California 94305
J. J. Vajo,+ and C. H. Becker* Molecular Physics Laboratory, SRI International, Menlo Park, California 94025 (Received: June 24, 1991; In Final Form: September 27, 1991)
The adsorption of CO on Ni( 110) has been studied in situ using a low-current pulsed Ar+ beam, followed by laser ionization of the desorbed neutral CO species and reflecting time-of-flight mass spectrometry of the resulting CO+ photoions. Since the fractional surface coverage of CO is monitored in real time, adsorption kinetics are conveniently studied even at temperatures where adsorption and desorption occur concurrently and equilibrium coverages are below saturation. We have measured the adsorption kinetics with a constant CO pressure of 3 X IO-* Torr for temperatures between 196 and 429 K. At 196 K the probability of adsorption of CO is independent of CO coverage, ec0, until Oc0 -0.75 of saturation coverage; this behavior clearly suggests precursor-mediated adsorption kinetics. The initial adsorption probability at low CO coverages is constant for surface temperatures between 196 and 429 K. The implications of this observation on the energy barrier separating the precursor and the chemically adsorbed states are discussed within the context of several kinetic adsorption models. In addition, for temperatures between 357 and 403 K, where the equilibrium CO coverage is 0.7-0.3 monolayer (ML), the adsorption kinetics are unusual. Specifically, the adsorption probability under these conditions is constant up to nearly 95% of the equilibrium coverage. No single set of kinetic parameters, whether coverage independent or linearly dependent on coverage, can explain the CO adsorption data over the entire temperature range studied.
1. Introduction The adsorption of CQ on the group 10 (formerly group VIIIA) metals has been well studied because of its relevance to commercially important catalytic processes.'+ Typical experimental observations for CQ adsorption on the group 10 metals are that the adsorption or sticking probability, s, remains constant up to high CO coverages and that s is often close to one.'O This type of adsorption behavior is generally attributed to the existence of a weakly bound mobile state on the surface which acts as an intermediate or precursor state to chemisorption.1° The concept of a mobile precursor state originated with Langmuir,11*12who noted that the adsorption probability of cesium on tungsten did not scale with the number of open sites on the surface but instead remained constant up to nearly saturation ~ o v e r a g e . ' ~ - 'Two ~ types of precursors may be defined: an "intrinsic" precursor which exists over an empty site on the surface and an "extrinsic" precursor which exists over a filled site.1° The precursor molecule need not be the same molecule as that found in the final adsorption state. For example, the precursor to the dissociative chemisorption of CO on Ni is the molecularly adsorbed state, which will not dissociate until a surface temperature of T, 600 K is reached." Due to the stability of chemisorbed molecules, their isolation and detection are fairly straightf0rward;'O thus, many dissociatively chemisorbed systems with precursor-mediated kinetics have been ob~erved.~*-~~ Direct observations of precursors to molecular chemisorption, however, are more difficult because these precursors are considered to be in a physisorbed state, which are more weakly bound than
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'Current address: Hughes Research Laboratories, Malibu, CA 90265.
0022-3654/92/2096-1826$03.00/0
molecular precursors to dissociative chemisorption. Typical values for physisorption well depths are -2-3 kcal/mol and are almost (1) Vannice, M. A. In Catalysis Science and Technology; Anderson, J. K., Boudart, M., Eds.; Springer-Verlag: Berlin, 1982; Vol. 3. (2) Ertl, G. In Catalysis Science and Technology; Anderson, J. R., Boudart, M., Eds.; Springer-Verlag: Berlin, 1983; Vol. 4. (3) Christmann, K.; Schober, 0.;Ertl, G. J . Chem. Phys. 1974,60,4719. (4) Campuzano, J. C.; Dus, R.; Greenler, R. G. Surf. Sci. 1981, 102, 172. (5) Klier, K.; Zettlemoyer, A. C.; Leidheiser, Jr., A. J. Chem. Phys. 1970, 52, 589. (6) Pfniir, H.;Menzel, D. J. Chem. Phys. 1983, 79, 2400. (7) Thiel, P. A,; Williams, E. D.; Yates, Jr., J. T.; Weinberg, W. H. Surf. Sci. 1979, 84, 54. (8) (a) Madden, H. H.;Kiippers, J.; Ertl, G. J . Chem. Phys. 1973, 58, 3401. (b) Taylor, T. N.; Estrup, P. J. J . Vac. Sci. Technol. 1973, 10, 26. (9) Banhofer, J.; Hock, M.; Kiippers, J. Surf. Sci. 1987, 191, 395. (IO) Weinberg, W. H. In Kinerics of Interface Reactions; Grunze, M., Kreuzer, H.J., Eds.; Springer-Verlag: Berlin, 1987; p 94. ( I 1) Langmuir, I. Chem. Rev. 1929, 6, 451. (12) Taylor, J. B.; Langmuir, I. Phys. Reu. 1933, 44, 423. (1 3) Kisliuk, P. J. Phys. Chem. Solids 1957, 3, 95. (14) Kisliuk, P. J . Phys. Chem. Solids 1958, 5 , 78. ( I 5 ) Becker, J. A. In Structure and Properties at Solid Surfaces; Gomer, R., Smith, C., Eds.; University of Chicago: Chicago, 1953; p 459. (16) Ehrlich, G. J . Phys. Chem. 1955, 59, 473. (17) (a) Steinriick, H. P.; D'Evelyn, M. P.; Madix, R. J. Surf. Sci. 1986, 172, L561. (b) DEvelyn, M. P.; Madix, R. J. Surf. Sci. Rep. 1984, 3, 413. (18) Gland, J. L. Surf. Sci. 1980, 93, 487. (19) Bock, C.; DeGroot, C. P. M.; Biloen, P. Surf. Sri. 1981, 104, 300. (20) Hsu,Y.-P.; Jacobi, K.; Rotermund, H. H. Surf. Sci. 1982, 117, 581. (21) Shayegan, M.; Cavallo, J. M.; Glover 111, R. E.; Park, R. L. Phys. Rev. Lett. 1984, 53, 1578. (22) Poelsema, B.; Verheij, L. K.; Comsa, G . Surj. Sci. 1985, 152/153, 486.
0 1992 American Chemical Society
Kinetics of CO Adsorption on Ni( 110) always less than 10 kcal/moLZ4 Hence, lower temperatures are required to thermally stabilize precursors to molecular chemisorption. Molecular adsorption of CO on the (1 1l), (loo), and (1 10) faces of nickel, as well as low-index faces of other transition metals, exhibits behavior strongly indicative of precursor-mediated kinetics, e.g., CO/Ru(001),6 COdRh(l1 1),7 CO/Ni(ll lL3v4 CO/Ni(100),5 and CO/Ni( 1 Attempts at isolation and detection of precursor CO on Ni(l1 1)27,28 and Ni(1 utilizing low temperatures, however, have produced conflicting results concerning the existence of thermalized intrinsic precursors to CO chemisorption. Scattering studies utilizing neutral molecular beams can give information about energy transfer at a surface29 and the existence of precursor states. Accordingly, several neutral CO molecular beam studies have been reported, e.g., on Ni( 111)28and Ni( but the results have not yet produced a consensus on the existence of intrinsic precursors to CO adsorption on these surfaces, although an extrinsic precursor to CO chemisorption on Ni( 100) has been identified.’7v30 In this paper, we report the adsorption kinetics of CO on Ni(ll0) from -200 to 430 K at constant CO background pressure. We address questions concerning the nature and existence of precursor CO on Ni(ll0) and attempt to ascertain the extent to which precursors effect the adsorption behavior of CO on clean and partially covered Ni( 110). If no precursor state is involved in the chemisorption process, that is, if chemisorption occurs directly from the gas phase, then the adsorption kinetics are expected to be independent of the surface temperature, T,, for the temperature range investigated here.3’ For an equilibrated precursor, an increase in surface temperature may change the ratio of desorption to chemisorption rates in favor of the desorption channel.31 Additionally, the trapping probability into the precursor state is expected to decrease as the surface temperature in~reases.~’ Thus, precursor-mediated kinetics are expected to be dependent on surface temperatures3’ In this work, adsorption is monitored in real time using the surface analysis by laser ionization (SALI) t e c h n i q ~ e , ) ~which - ~ ~ involves stimulated desorption of neutral atoms or molecules followed by photoionization and then reflecting time-of-flight mass spectrometry (TOF-MS) of the photoions. The SALI method has two notable advantages over the traditional method of temperature-programmed desorption (TPD) for the determination of adsorption kinetics. The SALI technique allows a complete adsorption curve to be obtained for each adsorbate dose, while the TPD method requires that the surface be cleaned and then dosed with adsorbate prior to the collection of each data point on the adsorption curve. Additionally, the SALI technique enables measurement of adsorption curves at high temperatures, while methods employing TPD are unable to obtain data at temperatures where desorption is significant. Our results consist of time- and temperature-dependent adsorption data for C O on Ni(ll0) at Pco = 3 X lo-* Torr. We find that, for all but the highest temperatures, the rate of CO adsorption does not scale linearly with the number of open sites 25926
(23) Muller, J. E. Phys. Rev. Letr. 1987, 59, 2943. (24) Piper, J.; Morrison, J. A.; Peters, C. Mol. Phys. 1984, 53, 1463. (25) Alvey, M. D.; Dresser, M. J.; Yates, Jr., J. T. Surf Sci. 1986, 165, 447. (26) Behm, R. J.; Ertl, G.; Penka, V. Surf. Sci. 1985, 160, 387. (27) Shayegan, M.; Williams, V.; Glover 111, R. E.; Park, R. L. Surf. Sci. 1985, 154, L239. (28) Beckerle, J. D.; Yang, Q. Y.; Johnson, A. D.; Ceyer, S. T.Surf Sci. 1988, 195, 77. (29) Rettner, C. T.; Bethune, D. S.; Auerbach, D. J. J. Chem. Phys. 1989, 91, 1942. (30) DEvelyn, M. P.; Steinriick, H.-P.; Madix, R. J. Surf. Sci. 1987, 180, 41.
(31) Rettner, C. T.; Schweizer, E. K.; Stein, H.; Auerbach, D. J. Phys. Rev. Letr. 1988, 61,986. (32) (a) Becker, C. H.; Gillen, K. T. Anal. Chem. 1984, 56, 1671. (b) Mamyrin, B. A.; Karataev, V. I.; Shmikk, D. V.; Zagulin, V. A. Sou. Phys. JETP (Engl. Traml.) 1973, 37, 45. (33) Pallix, J . B.; Gillen, K. T.; Becker, C. H. Nucl. Insrrum. Methods Phys. Res., in press. (34) Pallix, J. B.; Becker, C. H.; Newman, N. J . VUC.Sci. Technol. A 1988, 6, 1049. (35) Vajo, J. J.; Campbell, J. H.; Becker, C. H. J . VUC.Sci. Technol. A 1989, 7 , 1949.
The Journal of Physical Chemistry, Vol. 96, No. 4, 1992 1827
on the surface, which suggests that precursor-mediated kinetics are operative.*O We also report unusual, and previously unobserved, adsorption behavior for relative CO coverages between -0.3 and 0.7 ML (1 M L = 1.14 X 1015 C O molecules cm-2), which correspond to surface temperatures 403 1 T, 2 357 K. These experimental results are compared with previous results for CO adsorption on Ni( 1lo), Ni(l1 l), and Ni(100). Additionally, several kinetic models are applied, and their ability to describe CO adsorption is discussed. 2. Experimental Procedures Our experimental apparatus consists of a two-tier stainless steel ultrahigh-vacuum chamber designed specifically for studies using electron- and ion-induced desorption. The upper level contains a quadrupole mass spectrometer with an electron-impact ionizer, an Auger electron spectrometer, and a low-energy electron diffraction (LEED) optics system. The lower level contains a differentially pumped ion gun, used for both sputter cleaning and stimulated desorption, an electron gun for electron-stimulated desorption, and a reflecting time-of-flight mass spectrometer used for SALI measurements. Laser entrance and exit windows are located on an axis perpendicular to the TOF axis, and the laser beam is aligned to allow photoionization to occur -3 mm in front of the TOF ion optics and 1 mm above the surface. A doser assembly, also located on the lower level, is used both to expose the crystal directly to O2for cleaning by placing the crystal 1 mm in front of the doser tube and to backfill the chamber with CO at low pressures for adsorption experiments. The vacuum chamber is pumped by a 200 L/s ion pump and a titanium sublimation pump (TSP). Additionally, the ion source is differentially pumped by a turbomolecular pump and the drift region of the TOF-MS is pumped by another TSP. Differentially pumping the ion source limits the pressure rise at the sample to 0.7 ML) was obtained with AE = 0 kcal/mol and v 2 / v 3= 0.12, as seen for adsorption at T, = 196 and 292 K in Figure 1.
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(46) CRC Handbook at Chemistry and Physics; Weast, R. C., Ed.; CRC Press: Boca Raton, FL, 1981; p C694. (47) Chen, J. R.; Gomer, R. Sur/. Sci. 1980, 94, 456.
1834 The Journal of Physical Chemistry, Vol. 96, No. 4, 1992
Campbell et al.
presence of physisorbed CO on clean Ni(100).30 Neither neutral D’Evelyn et al. obtained similar results for ~ 2 / ~and 3 AI3 using CO molecular beam scattering experiments* nor low-temperature a model similar to model A to describe their neutral CO molecular beam scattering results for the CO/Ni(100) adsorption system.)O s t ~ d i e sof~ CO ~ , ~interactions ~ with Ni( 111) conclusively demonstrate the presence of weakly bound, thermalized CO molecules These workers pointed out that, for physically reasonable values on the clean Ni( 1 11) surface. From these results it can be conof u2/v3, equilibrium of CO into the precursor state would be unlikely because the activation barrier to chemisorption from the cluded that intrinsic precursor CO, defined as a localized, weakly bound species in equilibrium with the surface, does not exist. We precursor state would be too small to allow CO to thermalize in note, however, that intrinsic precursor CO as we have defined it the precursor state.30 D’Evelyn et al. postulated that precursor corresponds to CO thermalized in a shallow well on a one-diCO might exist as a dynamical or “hot” precursor in an attempt mensional potential energy surface and that the desorption rate to reconcile the facts that while no thermalization occurs, preis not necessarily determined by this simplistic description but cursor-mediated kinetics are observed. For surface temperatures rather is determined by the complete N-dimensional hypersurface. up to approximately 403 K, even the maximum limiting k 2 / k 3 values give a smaller than expected value for the ratio of preexDoren and Tullyso have shown that multidimensional interactions can be reduced to an effective one-dimensional potential energy ponentials (uZ/u3 = 0.84) from the intercept of the Arrhenius plot for model A (see Figure 3b). D’Evelyn et al. also observed that curve using the ‘potential of mean force” method. The resulting so was constant as a function of surface temperature for C O potential energy surface depends on entropic effects because it adsorption on Ni( 110). These observations indicate that CO is obtained from the equilibrium average and can therefore be different from the minimum-energy pathway. The potential of adsorption on the Ni(100) and Ni(ll0) surfaces is nonactivated mean force method also contains provisions for the “recrossing” and that AE = 0 for model A. If adsorption only occurs via a or “transmission” factor; hence, dynamical effects are included precursor state, as postulated in model A, the unphysically small as well. In particular, it has been shown that entropic and dyvalues of vZ/u3obtained with AE 2: 0 can be reconciled by connamical effects can impart significant temperature dependence sideration of a “hot” precursor, because thermalization of CO into an intrinsic precursor state potential energy well is not required. to the potential of mean force, such that a secondary minimum Several other attempts to directly or indirectly detect the may be present and effect the adsorption/desorption kinetics at presence of intrinsic precursor CO on Ni surfaces have been made. high temperatures, while being absent at low temperature^.^^ Hsu et al. attempted to cryogenically ( T , = 20 K) isolate and Hence, the results from model A do not rule out the possibility detect intrinsic precursor CO on Ni( 110) but were unable to detect that a dynamical precursor may influence the adsorption kinetics, nor do they.preclude entropic effects. physisorbed CO by UPS during uptake of the first monolayer of It is also apparent from the discussion above that if dynamical C0.20 For a surface temperature as low as 20 K, it should have been possible to isolate detectable amounts of weakly bound, precursors play no role in the adsorption kinetics and entropic thermalized, intrinsic precursor CO. In spite of the low temeffects are as expected (see above), then model B provides the peratures used, detectable amounts of C O were not isolated, more physically reasonable description of CO adsorption on Ni(l10). This conclusion is based on the ratio of preexponentials indicating that CO does not thermalize into an intrinsic precursor state. Attempts to isolate and detect intrinsic precursor C O on obtained from the intercept of the Arrhenius plot of log ( k z / k 3 ) Ni(ll1) have not yet produced a consensus in the l i t e r a t ~ r e . ~ ~ * versus ~ ~ lOOO/T,, Le., v2/u3 = 20. Hsu et al. did, however, detect physisorbed CO on top of a The adsorption curves obtained for the temperature range 357 IT, I403 K exhibit a sharp ‘knee” in comparison with the monolayer of CO, and D’Evelyn et al. have reported CO transadsorption curves obtained for surface temperatures above and lational energy accommodation at high coverages, as deduced by below this temperature range. The coverage range over which observing cosine scattering behavior for C O incident upon a this behavior is observed is 0.3 5 8, I0.7 ML and can be modeled monolayer of CO on Ni(100).17 These results indicate that CO thermalizes into extrinsic precursor states on both Ni( 110) and by incorporating a linear dependence on relative C O coverage, 8, into the activation barrier to desorption, i.e., E,(O) for model Ni( 100) and suggest that CO adsorption may occur via an extrinsic precursor state. A and E4(8)for model B. The rate of change in the desorption Tang et al.48utilized a monoenergetic neutral CO molecular energy required to describe the data at T, = 386 K is found to be the same for both models, -10 kcal/(mol.ML). The data at beam to investigate the adsorption process for CO on Ni( 111). Using a well-defined molecular beam spot size, they investigated T, = 386 K,together with the fits obtained from both models using the spot profie using AES and found that for incident CO energies a coverage-dependent desorption energy decreasing at a rate of below 4 kcal/mol the resulting spot profile on the surface was 10 kcal/(mol.ML), are shown in Figure 5, where the fit from approximately 500 pm larger than that obtained for incident CO model B is displaced upward by 0.15 ML for clarity. energies above this much larger than the lo-A diffusion Although the models fit the data well with the incorporation length we estimated earlier for our Ni( 110) surface. From this of a desorption energy linearly dependent on 8, the rate of change required to make the kinetic models fit the data at T , = 386 K data, Tang et al. concluded that CO impinging on Ni( 111) with an incident energy below 4 kcal/mol accessed a reaction channel is not observed experimentally. Gurney et al. used electron energy with a lower activation barrier to migration, which they referred loss spectroscopy (EELS) to determine the desorption energy for to as the precursor reaction channel.48 Their analysis of the data CO from Ni(ll0) and found that the desorption energy decreased required an activation barrier to chemisorption and desorption by 10 kcal/mol by the time the surface had adsorbed a monolayer, from the precursor of 6-12 and 9-15 kcal/mol, re~pectively.~~ A although the decrease in desorption energy did not begin until 8 precursor potential energy well this deep seems unlikely, since a 0.75-0.8 ML.@ Feigerlie et al. found that the activation barrier CO precursor molecule bound this strongly should have exhibited to desorption actually increased by 1 kcal/mol before beginning a TPD peak between -125 and 200 K, which has not been to decrease at B 2: 0.4 ML.42 Once a monolayer had adsorbed, observed.49 Tang et al. also considered the possibility of a dyFeigerlie et al. found that the desorption energy had decreased namical precursor, for which translational energy converts to by only -3-4 kcal/m01.~~They also observed that the preexvibrational energy over the course of several collisions with the ponential factor for desorption increased by a factor of 10 up to surface.48 8 2: 0.6 ML, whereupon it began to decrease until, at 8 i* 1.0 ML, Physisorbed CO is not seen on clean Ni( 110) a t low temperit had returned to its original value of 2:lOI4 The observed atures?O nor have CO molecular beam experiments indicated the decrease in desorption energy is most likely due to depletion of metal d electrons which strengthen the surface-adsorbate bond by donating electron density into the ?r* orbitals of the C O (48) (a) Tang, S. L.; Beckerle, J. D.; Lee, M. B.; Ceyer, S. T. J . Chem. molecule. Additionally, short-range repulsive interactions have Phys. 1986, 84, 6488. (b) Tang, S . L.; Lee, M. B.; Beckerle, J. D.; Hines, M. A.; Ceyer, S. T. J . Chem. Phys. 1985,82,2826. (c) Tang, S. L.; Lee, M. been postulated to be responsible for decreases in the desorption
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8.; Beckerle, J. D.; H i m , M. A.; Ceyer, S. T. J . Vac. Sci. Technol. A 1985, 3, 1665. (49) Sumer, L.; Xu, 2.; Yates, Jr., J. T. Surf. Sci. 1988, 201, 1.
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(50) Doren, D. J.; Tully, J. C. Lnngmuir 1988, 4, 257.
Kinetics of CO Adsorption on Ni( 1 10) energy on Ni( 1 10) for relative coverages above 0.8 MLaN Inclusion of desorption parameters having the functional form of the data of Feigerlie et a1.42or of Gurney et alaa does not, however, fit our experimental data. If the desorption rate constant is assumed to have the coverage dependence described by either Feigerlie et a1.42or Gurney et al.,* additional coveragedependent kinetic parameters are then required to fit the observed adsorption data, e.g., k2(e) or k,(8). Unfortunately, we cannot uniquely determine the coverage dependence of the kinetic parameters from our data. 5.3. Ordered Overlayer Effects. The adsorption behavior observed at intermediate surface temperatures (357 IT, I403 K), as manifested by a constant adsorption probability up to 0.95 of the saturation (equilibrium) coverage, may be related to ordered CO overlayer formation. Indeed, ordered CO overlayers have been reported for the same coverage regime for which we observe this unusual adsorption behavior. LEED observations of C O on Ni(ll0) at 130 K by Behm et indicate the initiation of a ~ ( 8 x 2 ordered ) CO overlayer beginning at coverages as low as 8 = 0.3 ML. The ordered ~ ( 8 x 2 islands ) continue to grow up to 8 = 0.6 ML, whereupon islands having a ~ ( 4 x 2 structure ) begin to appear.26 Unfortunately, our attempts to investigate surface order at 130 K with LEED using an incident electron energy below the 40 eV damage threshold were largely unsuccessful, as we were only able to observe the clean Ni(lX1) structure and the (2X l)p2mg ordered structure associated with a monolayer of CO on Ni(ll0). The ~ ( 8 x 2 and ) ~ ( 4 x 2 ordered ) overlayers are known to have poor long-range order, and are therefore difficult to see. Our inability to observe these ordered overlayers at intermediate coverages may have been due to a phosphor screen incapable of producing a visible image from the weak spot intensities resulting from these ordered overlayers. We note also that although Behm et a1.26observe these ordered overlayers at 130 K, the adsorption kinetics at intermediate coverages are occurring at temperatures at least 200 K higher. The presence of ordered overlayers of CO on Ni( 110) at elevated temperatures has not been confirmed. Assuming that ordered overlayers can form at 357 IT, I403 K, however, it is intriguing that the unusual adsorption behavior we ohserve occursover the same merage regime where the ~ ( 8 x 2 ) ordered overlayer is seen to form. Perhaps CO adsorbed in the ~ ( 8 x 2 )structure is more stable than the ordered ~ ( 4 x 2 )CO overlayer, leading to a higher desorption energy and the observed adsorption kinetics. Island edge effects may also play a role along the borders of the ~ ( 8 x 2 ordered ) domains. At low coverages (8 a 0.3 ML) the small nucleation sites for ~ ( 8 x 2 island ) growth may act as particularly reactive adsorption sites, while at high coverages (8 = 0.7 ML), CO chemisorbed at the phase boundary between islands of ~ ( 8 x 2 and ) ~ ( 4 x 2 ordered ) overlayer domains may be destabilized, leading to a reduction in the CO desorption energy.
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The Journal of Physical Chemistry, Vol. 96, No. 4, 1992 1835 In general, it is reasonable to expect that the formation of ordered overlayers by an adsorbate will effect the adsorption kinetics. The propensity of CO to form ordered overlayers on Ni( 110) and to undergo phase changes, particularly at coverages where we see changes in the adsorption behavior, suggests that the relationship between CO adsorption kinetics and ordered CO overlayers on Ni( 110) is not yet well established. 6. Conclusions
We have found that CO adsorption on Ni(ll0) in 3 X lo4 Torr of CO exhibits three distinct types of adsorption behavior over the temperature range 196 IT, I429 K. For adsorption at T, I292 K,the adsorption probability is independent of CO coverage up to 8 = 0.758, and is described well by precursor-mediated adsorption kinetics. At intermediate temperatures, 357 IT, I 403 K, the adsorption probability is independent of coverage up to 0 a 0.950,, which suggests that precursor-mediated adsorption kinetics are still operative. However, of the two kinetic models including precursor states (models A and B), neither was capable of adequately describing the observed adsorption data over the entire temperature range studied, regardless of whether coverage-independent kinetic parameters or kinetic parameters with a simple linear coverage dependence were used. Adsorption at a given temperature in the intermediate temperature range could be described by a desorption rate increasing exponentially as a function of coverage, although independent measurements do not support this type of coverage dependence. At the highest temperatures studied (for which 8, < 0.1 ML), Le., T, > 403 K, the rate of CO adsorption on Ni( 110) is linearly dependent on CO surface coverage. The unusual adsorption kinetics observed for intermediate temperatures and coverages appear to be correlated with the appearance and disappearance of the ~ ( 8 x 2 ordered ) CO overlayer observed by others, although the temperatures at which we observe the unusual adsorption kinetics are -200 K higher than the temperature at which the ordered overlayers were observed. Kinetic modeling of the results suggests that a thermally equilibrated intrinsic precursor CO species is unlikely on the Ni( 110) surface, although the existence of a precursor state strongly affected by dynamical and/or entropic effects cannot be ruled out. Modeling of the results using kinetic model B (see section 4.3), for which adsorption occurs by direct chemisorption as well as via an extrinsic precursor state, suggests that the data can be described by physically reasonable parameters with this model. Although the CO/Ni( 110) adsorption system has been extensively studied, many details of the adsorption process are still not understood. Acknowledgment. We thank the National Science Foundation for financial support. Registry No. CO, 630-08-0;Ni, 7440-02-0.
