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Langmuir 1998, 14, 1384-1391
Ensemble Effects in the Oxygen/Chlorine/Pd(100) System Kamil Klier,* Gary W. Simmons, Kenneth T. Park, Yarw-Nan Wang,† James S. Hess, and Richard G. Herman Zettlemoyer Center for Surface Studies and Department of Chemistry, Sinclair Laboratory, 7 Asa Drive, Lehigh University, Bethlehem, Pennsylvania 18015 Received April 28, 1997. In Final Form: October 30, 1997 Chlorohydrocarbons chemisorb dissociatively on Pd surfaces at g200 K and g10-8 Torr, as shown by high-resolution electron energy loss spectroscopy (HREELS) and C 1s, Cl 2p, and Pd 3d surface core level shifts (SCLS). From CH2Cl2-generated overlayers on Pd(100), hydrogen is removed thermally and carbon is removed by oxidation as CO and CO2, leaving voids between the chlorine (Cl) ensembles that are accessible to other adsorbates. The resulting Cl overlayers are partially ordered depending on initial conditions. The concomitant low-energy electron diffraction (LEED) patterns show that the Cl ensembles are stable to high-temperature reaction cycles. The order-disorder phenomena observed in the temperature range 300-900 K include the generation of domains consisting of Cl only that surround reactive sites of the metal. With CH2Cl2/Pd(100), these domains are formed by lateral packing of 16 Pd/CCl2 units that restrict the supply of O(a) for oxidation of C. Selectivity is switched from CO2 to CO with increasing Cl concentration. Lateral interactions are of two types: mobile O-immobile Cl and mobile O-mobile O. This is reflected in a lowering of the O2 temperature programmed desorption (TPD) maxima with increasing Cl concentration. A statistical-mechanical model is presented for the effects of Cl(a) with phase-equilibration between a dense and a rare phase of O(a).
Introduction Ensemble effects are known as an important mechanism for controlling selectivity in catalytic reactions. Among the processes in which the selectivity is affected in a crucial way by atoms or molecules that block a portion of the catalyst surface are the silver-catalyzed epoxidation of ethylene controlled by the presence of chlorinated hydrocarbons,1-10 steam reforming over nickel catalysts partially poisoned by sulfur,11-14 and partial oxidations of hydrocarbons.15-17 The strong metal-support interaction (SMSI) effect18-22 also has been identified as one in which the active metal area is restricted by an overlayer of an † Current address: Millennium Inorganic Chemicals, 3901 Fort Armistead Rd., Baltimore, Maryland 21226.
(1) Law, G. H. and Chitwood, H. C. U. S. Patent 2,279,469, April 14, 1942; assigned to Carbide and Carbon Chem. Corp. (2) McBee, E. T.; Hass, H. B.; Wiseman, P. A. Ind. Eng. Chem. 1945, 37, 432. (3) Voge, H. H.; Adams, C. R. Adv. Catal. 1967, 17, 151. (4) Kilty, P. A.; Rol, N. C.; Sachtler, W. M. H. In Proceedings of the 5th International Congress on Catalysis; Hightower, J. W., Ed.; North Holland: Amsterdam, 1973; p 929. (5) Verykios, X. E.; Stein, F. P.; Coughlin, R. W. Catal. Rev.-Sci. Eng. 1980, 22, 197. (6) Bhasin, M. M. U. S. Patent 4,908,343, March 13, 1990; assigned to Union Carbide Chemicals and Plastics Company, Inc. (7) Bhasin, M. M.; Ellgen, P. C.; Hendrix, C. D. U. S. Patent 4,916,243, April 10, 1990; assigned to Union Carbide Chemicals and Plastics Company, Inc. (8) Michell, S. F.; Minahan, D. M. A.; Bhasin, M. M. U. S. Patent 5,051,395, September 24, 1991; assigned to Union Carbide Chemicals and Plastics Company, Inc. (9) Bhasin, M. M. U. S. Patent 5,057,481, October 15, 1991; assigned to Union Carbide Chemicals and Plastics Company, Inc. (10) Chou, P. Y.; Bhasin, M. M.; Soo, H.; Thorsteinson, E. M. U. S. Patent 5,504,053, April 2, 1996; assigned to Union Carbide Chemicals and Plastics Company, Inc. (11) Rostrup-Nielsen, J. R. J. Catal. 1984, 85, 31. (12) Dibbern, H. C.; Olesen, P.; Rostrup-Nielsen, J. R.; Tøttrup, P, B.; Udengaard, N. R. Hydrocarbon Process. 1986, 65(1), 31. (13) Alstrup, I.; Andersen, N. T. J. Catal. 1987, 104, 466. (14) Rostrup-Nielsen, J. R.; Alstrup, I. In Catalysis 1987: Studies in Surface Science and Catalysis; Ward, J. W., Ed.; Elsevier: Amsterdam, 1988; Vol. 38, p 725. (15) Knox, W. R.; Taylor, K. M.; Tullman, G. M. U. S. Patent 3,833,638, Sept. 3, 1974; assigned to Monsanto.
oxide originating from the support under high-temperature reduction conditions.22-26 In partial oxidation of methane, addition of chloromethanes resulted in switching the product selectivity from full oxidation to CO2 and water to formaldehyde.27,28 In addition, the magnitude of the selectivity switch was in the order CH2Cl2 > CHCl3 > CH3Cl > CCl4 when the chloromethane was added in concentrations of 0.25-4.6% in a continuous flow to the gaseous reactant mixture of methane and oxygen.28 The effects of surface chlorine (Cl) originating from dissociative chemisorption of dichloromethane on the reactivity of oxygen and oxidation of the carbon fragments was investigated on the single-crystal Pd(100) surface29 with the result that surface Cl determined the outcome of the carbon oxidation; that is, at Cl coverages of 1-5%, the product was CO2, at 10% Cl coverage, the product consisted of equal amounts of CO2 and CO, and at 20% Cl coverage, the dominant product was CO. At the same (16) Grasselli, R. K.; Brazdil, J. F.; Burrington, J. D. In Proceedings of the 8th International Congress on Catalysis; Verlag Chemie: Weinheim, 1984; Vol. V, p 369. (17) Giordano, N.; Bart, J. C. J.; Vitarelli, P.; Cavallaro, S. Oxid. Commun. 1984, 7, 99. (18) Tauster, S. J.; Fung, S. C.; Garten, R. L. J. Am. Chem. Soc. 1978, 100, 170. (19) Baker, R. T. K.; Prestridge, E. B.; Garten, R. L. J. Catal. 1979, 56, 390; ibid. 59, 293. (20) Tauster, S. J.; Fung, S. C. J. Catal. 1981, 55, 28. (21) Tauster, S. J.; Fung, S. C.; Baker, R. T. K.; Horsley, J. A. Science 1981, 211, 1121. (22) Tauster, S. J. Acc. Chem. Res. 1987, 20, 389. (23) Santos, J.; Phillips, J.; Dumesic, J. A. J. Catal. 1983, 81, 147. (24) Simoens, A. J.; Baker, R. T. K.; Dwyer, D. J.; Lund, C. R. F.; Madon, R. J. J. Catal. 1984, 86, 359. (25) Dumesic, J. A.; Stevenson, S. A.; Sherwood, R. D.; Baker, R. T. K. J. Catal. 1986, 99, 79. (26) Fleisch, T. H.; Bell, A. T.; Regalbuto, J. R.; Thomson, R. T.; Lane, G. S.; Wolf, E. E.; Hicks, R. F. In Catalysis 1987: Studies in Surface Science and Catalysis; Ward, J. W., Ed.; Elsevier: Amsterdam, 1988; Vol. 38, p 791. (27) Cullis, C. F.; Keene, D. E.; Trimm, D. L. J. Catal. 1970, 19, 378. (28) Mann, R. S.; Dosi, M. K. J. Chem. Tech. Biotechnol. 1979, 29, 467. (29) Wang, Y.-N.; Marcos, J. A.; Simmons, G. W.; Klier, K. J. Phys. Chem. 1990, 94, 7597.
S0743-7463(97)00442-3 CCC: $15.00 © 1998 American Chemical Society Published on Web 01/22/1998
The O2/Cl/Pd(100) System: Ensemble Effects
time, the Cl coverage and the low-energy electron diffraction (LEED) patterns remained unchanged after repeated exposures to oxygen at temperatures as high as 1000 K, and the presence of partial Cl overlayers accelerated oxygen desorption and lowered its effective activation energy. The selectivity control by halogen dopants is likely to be more complex than a simple site blocking effect. When chemisorbed, the halogens may exert lateral interactions of a longer range than the size of sites blocked, as was previously demonstrated for sulfur and Cl on nickel and ruthenium in methanation.13,30-33 Such effects have been considered to be of an electronic, through-metal nature or of a “through-space”, primarily electrostatic nature, which is analogous but opposite to the electron-donating effects of alkali on metals in influencing the adsorption and activation of CO (e.g., see the discussion in ref 34).34 In addition, dopants may participate in gas phase reactions with the reactants, leading to free radical chemistry that is especially important at high temperatures.35-37 The requirement for continuous feeding of the halogen or sulfur dopant prompts a broad question of whether the selectivity promotion effect depends on the presence of the dopant on the surface, from which it is gradually removed by side reactions, whether the added molecules enter the reaction cycle, or whether both mechanisms occur simultaneously. The fundamental issues involved in the selectivity control by gaseous additives call for an examination of the chemical nature and location of the dopant on the catalyst surface, the accessibility of the “doped” surface to reactants, the influence of the dopant on the reactant sorption energy and reactivity, and the stability of the dopant on the surface under the reaction conditions. Because single-crystal surfaces offer the opportunity to study such issues in a greater detail than the more complex “real” catalysts, it was expected that the fundamental questions of ensemble control in catalysis could be resolved by step-by-step investigation of the partial processes involved. However, even on single crystals, unexpected complexities were encountered; that is, surface reconstructions,38 phase transformations and phase splitting,39 questions of mobility40 of both the dopant and reactants, and the nature of lateral interactions.40,41 Although limited in scope against the large background of all the issues encountered with surface dopant effects, the specific goal of the present work was to analyze the influence of Cl surface dopants, introduced by dissociative chemisorption of chlorohydrocarbons, on the behavior of adsorbed oxygen in the O/Cl/Pd(100) system. In previous work with Pd(100), chemisorption of dichloromethane29 and tetrachloroethylene42 was found to be (30) Kishinova, M.; Goodman, D. W. Surf. Sci. 1981, 108, 64. (31) Goodman, D. W.; Kishinova, M. Surf. Sci. 1981, 105, 265. (32) Peden, C. H. F.; Goodman, D. W. ACS Symp. Ser. 1985, 288,185. (33) Andersen, N. T.; Topsøe, F.; Alstrup, I.; Rostrup-Nielsen, J. R. J. Catal. 1987, 104, 454. (34) Ponec, V. In New Trends in CO Activation: Studies in Surface Science and Catalysis; Guczi, L., Ed.; Elsevier: Amsterdam, 1991; Vol. 64, p 117. (35) Lunsford, J. H. In Methane Conversion: Studies in Surface Science and Catalysis; Bibby, D. M.; Chang, C. D.; Howe, R. F.; Yurchak, S., Eds.; Elsevier: Amsterdam, 1988; Vol. 36, p 359. (36) Lunsford, J. H. Catal. Today 1990, 6, 235. (37) Lunsford, J. H. In Natural Gas Conversion II: Studies in Surface Science and Catalysis; Curry-Hyde, H. E.; Howe, R. F., Eds.; Elsevier: Amsterdam, 1994; Vol. 81, p 1. (38) Somorjai, G. A. Introduction to Surface Chemistry and Catalysis, Wiley & Sons: New York, 1994; pp 55-58. (39) Simmons, G. W.; Wang, Y.-N.; Marcos, J.; Klier, K. J. Phys. Chem. 1991, 95, 4522. (40) Klier, K.; Wang, Y.-N.; Simmons, G. W. J. Phys. Chem. 1993, 97, 633. (41) Zhdanov, V. P. Surf. Sci. 1981, 111, 63 and L662.
Langmuir, Vol. 14, No. 6, 1998 1385
dissociative at room temperature and higher, forming very stable Cl overlayers. In the case of CH2Cl2, the dissociated chlorohydrocarbon overlayer was partially disordered, leaving enough space on the metal surface for oxygen chemisorption, and the surface carbon could be removed by reaction with the surface oxygen.29 In contrast, the C2Cl4 molecules dissociated on Pd(100) into an ordered, fully saturated p(2 × 2) surface structure that contained Cl atoms and two-carbon residues, with no metal sites accessible to oxygen and from which the carbon fragments could not be removed by oxidation.42 The presence of small surface concentrations of Cl from CH2Cl2 caused the oxygen to desorb at lower temperatures, indicating a weakening of the OsPd bond by lateral repulsion between the Cl and the oxygen adatoms, and the range of this repulsion was over several PdsPd interatomic distances.29 In separate experiments, photoelectron diffraction of surface core-level shifted Pd 3d emission indicated that the OsPd interaction extended over several PdsPd distances along the surface inward normal direction.43,44 These experiments suggested that the range of adsobate interactions involving oxygen on Pd extends both laterally and into the metal beyond the nearest metal neighbors. In this report, we examine the lateral interactions due to the presence of Cl, and the effect of the mobility or the lack thereof of both the oxygen and the Cl adsorbates on the behavior of the system. A Statistical Model for Ensemble Effects with Phase Equilibration: the O/Cl/Pd(100) System Ensembles of “spectator” blocking agents on heterogeneous catalyst surfaces have so far been assumed to be statistically distributed, either by chemisorption of the dopant from the gas phase33 or by mixing unreactive components into an active catalyst to reduce activity and increase selectivity to partial oxidation products.45 It is conceivable that more organized ensembles may be devised on heterogeneous catalysts such that confinement of reactants will be due to shape as well as to site blocking. To date, there is no evidence of, for example, recognition of reactant molecules by two-dimensional ensemble shapes except for surfaces derivatized by organic molecules, an example of which was recently demonstrated by Mirkin et al.46 for a DNA-based method for assembling gold nanoparticles. There are indications, however, that the distribution of the dopant ensembles is subject to restrictions that originate from the source molecule, and that this restricted distribution is “remembered” by the system over many reaction cycles (i.e., the distribution is not entirely random even on solid surfaces). Specifically in the O/Cl/P system, the Cl distribution is different depending on whether the Cl adatoms originate from gaseous Cl2,29 C2Cl 4,42 or CH2Cl2.29 In the first case of elemental Cl adsorption, the Cl adsorbate forms the well-known c(2 × 2)Cl/Pd(100) structure where the Cl adatoms are located in alternate fourfold holes of the Pd(100) surface29; in the case of tetrachloroethylene adsorption, the C2Cl4 molecules dissociate, as was clearly demonstrated by large X-ray photoelectron spectroscopy (XPS) C 1s and Cl 2p core level binding energy shifts,42 into a well-defined p(2 × 2)Cl/C/ Pd(100) densely packed structure from which the carbon cannot be removed by oxidation; and in the case of (42) Park, K. T.; Klier, K.; Wang, C. B.; Zhang, W. X. J. Phys. Chem. 1997, 101, 5420. (43) Gu¨rer, E.; Klier, K. Phys. Rev. B 1992, 46, 4884. (44) Park, K. T.; Simmons, G. W.; Klier, K. Surf. Sci. 1996, 367, 307. (45) Grasselli, R. K. Adv. Catal. 1981, 30, 131. (46) Mirkin, C. A.; Letsinger, R. L.; Mucic, R. C. Nature 1996, 382, 607.
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and Pd(679) (Wang et al.56 ). The system was treated as non-adiabatic, and the desorption rates vd, were calculated from absolute activities, λ ) exp(µ/kBT), as follows:
νd ) (kBT/h) (λ2s /λq0)
(1)
where λs is the absolute activity of surface oxygen atoms in the rare phase and λ0‡ is that of the activated moleculelike complex in its standard state. The chemical potentials µs and µ0‡ were determined from the partition functions of the adatoms and the activated complex that involve a wanderer atom cell model and interact laterally via the ensemble average of the pairwise potentials
φ(r) ) (A/r12) - (B/r6) + (d2/r3)
Figure 1. TPD curve obtained after adsorption of oxygen at ambient temperature to form a half monolayer coverage of oxygen on the surface of Pd(100).
dichloromethane, the CH2Cl2 molecules also dissociate to form a partially disordered structure with the Cl adatoms located in fourfold holes characterized by vibrational frequency of 225 cm-1, a structure from which the carbon residue can be removed by oxidation and that forms a new partially ordered structure after a complete carbon removal.29 An interesting feature of the O/Pd(100) system is that the room-temperature-saturated c(2 × 2) structure (θo ) 0.5, where the surface coverage is defined as the number of adsorbate atoms per surface Pd atom) splits into two or more phases upon increase of temperature, before any oxygen desorption takes place.39 This phenomenon is illustrated in Figure 1 in terms of ranges of existence and coexistence of the different surface structures identified by LEED. Oxygen desorption occurs at higher temperatures from a mixture of a rare phase that makes the p(2 × 2) structure (Θo ) 0.25) upon quenching to room temperature, and a dense (x5 × x5)R27 structure (Θo ) 0.8).39,47,48 After the dense structure is removed by rapid desorption as a sharp temperature-programmed desorption (TPD) peak,39,40,49 a slower desorption process from the rare “(2 × 2)” phase50,51 takes over until all oxygen is removed at 1000 K. The properties of the rare phase have been analyzed on kinetic models that quantitatively accounted for the oxygen TPD curves from a range of coverages.40 Both the King-Adams model52,53 and the Klier et al. model40 led to the conclusion that the rare phase is mobile, highly entropic, and suffers a lateral self-repulsion that was attributed by Klier et al.40 to a short-range Lennard-Jones type repulsion combined with a longerrange dipole repulsion. Oxygen penetration into Pd has been carefully considered, and no evidence has been found for oxygen penetration into Pd(100) (cf. Vu et al.48 and Kolthoff et al.54 ), in contrast to Pd(111) (Conrad et al.55) (47) Orent, T. W.; Bader, S. D. Surf. Sci. 1982, 115, 323. (48) Vu, D. T.; Mitchell, K. A. R.; Warren, O. L.; Thiel, P. A. Surf. Sci. 1994, 318, 129. (49) Stuve, E. M.; Maddix, R. J.; Brundle, C. R. Surf. Sci. 1984, 146, 144. (50) Chang, S. L.; Thiel, P. A. J. Chem. Phys. 1988, 88, 2071. (51) Chang, S. L.; Thiel, P. A.; Evans, J. W. Surf. Sci. 1988, 205, 117. (52) King, D. A. Surf. Sci. 1975, 47, 384. (53) Adams, D. L. Surf. Sci. 1974, 42, 12. (54) Kolthoff, D.; Ju¨rgens, D.; Schwennicke, C.; Pfnu¨r, H. Surf. Sci. 1996, 365, 374.
(2)
where A and B are the Lennard-Jones potential constants and d is the dipole associated with the adatom. Using the same partition functions for the adatoms and the activated complex as in ref 40, the desorption rate vd from the rare phase is given by:
νd )
g0q e2mqp231/2 kBT (θrFs0*)2 2 h g (s *)2m2k
{ [( ) 0
0
)
B
2Λs gl exp 3.5 + 6 (θrFs0*)6 kBT g0 gm 3d2 4.24 + 6 (θrFs0*)3 + 2Md g0 k Ta3
(
{
]}
B
}
∆f(i) Λq [0.5(θqr Fs0*q)6 - 1.06(θqr Fs0*q)3] + kBT kBT
(3)
Here, functions g0, gl, and gm are given by integrals over the reduced area of the wanderer adatom y ) (r/a)2, where a is the distance between the centers of the neighboring cells determined by the governing adatom concentration40,57 :
gi )
∫0y
max
[
Pi(y) exp
]
Ψ(y) - Ψ(0) dy kBT
(4)
and P0(y) ) 1, Pl(y) ) (1 + y)(1 + 24y + 76y2 + 24y3 + y4)/(1 - y)11 - 1, Pm(y) ) (1 + 4y + y2)/(1 - y)11 - 1, and Pn(y) ∞ ∞ y2k [Γ(k + 3/2)/Γ(k + 1/2)]2 ) (1/π) ∑k)1 y2k (k ) (1/π) ∑k)1 + 1/2)2. The ensemble-averaged interaction potential Ψ is given as a function of the reduced area y by the following expression:
Ψ(y) - Ψ(0) ) Λ{(θr F s0*)6 Pl(y) 2(θr F s0*)3 Pm(y)} + 6(d2/a3)Pn(y) (5) Here Λ is the Lennard-Jones interaction energy, Λ ) (3/2)B2/A, θr is the fractional coverage of the adatoms in the rare phase, F is the number of Pd atoms on 1 cm2, and s0* ) [A(27)1/2/4B]1/3 is the combined interaction constant according to Devonshire.58 An analysis of the model shows that the ensemble properties, such as the adsorbate entropy and interaction energy, the activation energy and activation entropy for desorption, and overall desorption rates are strongly influenced by a restriction of the “free (55) Conrad, H.; Ertl, G., Ku¨ppers, J.; Latta, E. E. Surf. Sci. 1977, 65, 245. (56) Wang, Y.-N.; Herman, R. G.; Klier, K. Surf. Sci. 1992, 279, 33. (57) Klier, K.; Zettlemoyer, A. C.; Leidheiser, H. J. Chem. Phys. 1970, 52, 589. (58) Devonshire, A. F. Proc. Roy. Soc. London 1937, A163, 132.
The O2/Cl/Pd(100) System: Ensemble Effects
Langmuir, Vol. 14, No. 6, 1998 1387
Table 1. Initial Surface Coverages of Oxygen (Θ0) and Chlorine (ΘCl) Atomic Adsorbatesa and Values of Critical Coverage Θc (eq 6) Below Which Only the Rare Phase Exists on Pd(100)b Θ0
ΘCl
Θ0 + ΘCl
Θc
0.50 0.45 0.33 0.25 0.23 0.20
0.00 0.05 0.08 0.15 0.17 0.22
0.50 0.50 0.41 0.40 0.40 0.42
0.40 0.36 0.26 0.20 0.18 0.16
a Reference 29. b All coverages are in units of atoms of adsorbate per Pd atom.
area” Ψ for the wanderer atom. It is this restriction that will be used in the present analysis for modeling the effects of an additional surface dopant such as the Cl adatoms. The property of the oxygen overlayer that at least two surface phases coexist at desorption temperatures was considered and incorporated into the kinetic model for pure oxygen desorption,40 and will now be extended to the treatment of restrictions imposed on the oxygen atom ensembles due to the presence of Cl adatoms. The consequence of compression of the oxygen rare phase will be to force more of the oxygen into the dense phase, if the phase equilibration documented earlier for pure oxygen overlayers is in effect, and to shift the desorption to lower temperatures because of the reduction of entropy stabilization of the rare phase and the increase of the surface concentration of the dense phase compared with the Clfree surface. We will retain the oxygen phase equilibration concept40 while redefining the densities of the rare and the dense phase in the presence of the Cl dopant as follows: we denote the surface coverages of the rare oxygen phase as Θr ) nr/F, that of the dense oxygen phase as Θd ) nd/F, and that of the Cl adsorbate as ΘCl ) nCl/F, where ni (i ) r, d, or Cl) is the number of adatoms on 1 cm2 containing F ) 1.3223 × 1015 Pd atoms. The initial oxygen coverage is Θ0 ) n0/F, and the critical coverage below which only the rare phase exists is:
Θc ) 0.4Θ0/0.5
(6)
where 0.5 stands for the half-coverage by the c(2 × 2) structure on a Cl-free Pd surface and Θc ≡ Θoc ) 0.4 for the Cl-free surface has been taken from previous work.40 In the presence of adsorbed Cl, Θ0 is Θc when both phases coexist, and
nd0/F ) Θd0 ) 0
(8a)
nr0/F ) Θ0
(8b)
and
when only the rare phase exists below the critical concentration. The effect of the ensemble blocking dopant is treated in the simplest way as one of compression of the free area available to the wanderer adatoms, as schematically represented in Figure 2. The relations between the coverages of the dense phase (Θd), the rare phase (Θr), and the Cl dopant (ΘCl) are:
nd/F ) Θd ) 0.8(n/F - Θc)/(0.8 - Θc)
(9a)
nr/F ) Θr ) Θc (0.8 - n/F)/(0.8 - Θc)
(9b)
nsd/F ) (n/F - Θc)/(0.8 - Θc)
(10a)
nsr /F ) (0.8 - n/F)/(0.8 - Θc)
(10b)
where nsd is the number of palladium atoms occupied by nd atoms of the dense phase, nsr is the number of palladium atoms occupied by nr atoms of the rare phase, n ) (nr + nd) is the total number of adsorbed (oxygen) atoms on F palladium atoms, and Θc is related to Θ0 through eq 6. Further, experimental ΘCl is related to Θ0 via entries given in Table 1. Equations 9 and 10 hold if n/F > Θc, and eq 11 holds if n/F e Θc:
Θr ) n/F;
nsd ) 0;
nsr ) F
(11)
The problem of how to account for the exclusion of a part of the surface by the Cl dopant [cross-hatched areas
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in Figure 2 (top)], which according to the Monte Carlo simulation29 has variable patch size and shape, is circumvented here by examining the ensemble-average effect of the Cl adatom blocking on the effective size of the cell available to the wanderer oxygen atom in the rare phase. In Figure 2 (bottom), such an exclusion area per cell is represented as a shaded area of size B ) π(r02 - rB2), where r0 ) a/2, half the distance between the centers of neighboring cells. The exclusion area B will comprise both the “site blocking” and longer range repulsion such as that due to dipoles. The maximum value of the reduced cell area variable y ) (r/a)2 is ymax ) 1/4 for the free surface and yB ) (rB/a)2 ) ymax [1 - B/(πa2)] ) ymax (1 - BFΘr) for the Cl dopant-containing surface as the number of atoms in the rare phase is nr ) FΘr ) 1/(πa2/4). We now estimate the excluded area B per cell from the surface Cl concentration ΘCl. The excluded area on 1 cm2 is nr B ) FΘr B, and this area is related, at low Cl coverages, to the Clcovered area as FΘr B ) 2 R2 ΘCl, where R is a linear range of interaction between the Cl blocker and the oxygen atoms in the rare phase. The factor 2 stands for each Cl atom blocking 2 Pd atoms in the alternate fourfold holes and R2 for the area effect. We finally have the following relationship:
yB ) (1/4) (1 - 2 R2 ΘCl)
(12)
At higher ΘCl (g0.1), the number of ClsCl neighbors is increasing as is evident from the Monte Carlo simulation described in ref 29, and the repulsive effect of each Cl adatom on the oxygen overlayer is reduced to that across the ClsO “interface”. Such an interface is complex because of the statistical distribution of Cl adatoms, and its shape and size is not in a simple relation to ΘCl. Thus, at higher ΘCl, an effective yB also is not simply related to ΘCl. We estimate a minimum value of yB at high ΘCl from the condition that the critical density Θoc in the rare phase is given as the ratio of the maximum number of the (O) wanderer atoms in the rare phase, nr,max, to the number of Pd atoms available to this phase, nr,Pd, yielding Θoc ) (nr,max/nr,Pd). The area Ar covered by the rare phase is Ar ) nr,maxπr2B,min, where rB,min is the minimum cell radius in the compressed rare phase. Further, since nr,Pd ) FAr, πr2B,min ) nr,Pd/(F nr,max) ) 1/(F Θoc ) and yB,min ) (πr2B,min/ πa2) ) Θro/(4Θoc ) are obtained. For the critical coverage Θoc ) 0.4, yB,min ) Θro/1.6, where θro can be obtained from eq 7b or 8b. For the lowest coverage Θ0 ) 0.20 (corresponding to ΘCl ) 0.22, cf. Table 1), Θro ) 0.15 by eqs 7b and 6, yielding yB,min ) 0.09. The same value of yB,min is obtained for Θ0 ) 0.33 (ΘCl ) 0.08 in Table 1) using eq 12 with R ≈ 2, and therefore the value of yB,min ) 0.09 was used in modeling all of the oxygen TPD curves for ΘCl g 0.08, whereas eq 12 was used for ΘCl e 0.08. The reduced area limit yB for the wanderer oxygen atoms was used to simulate the effects of the Cl blockers, and the experimental TPD curves of ref 29 were reasonably accounted for by R ≈ 2 for ΘCl < 0.1 and yB ) 0.08 for ΘCl > 0.1. The TPD curves constructed from this model in Figure 3 show the characteristics of the experimental TPD patterns as dependent on the Cl blocker surface concentration: the TPD tails for the rare phase are shifted to lower temperatures with increasing Cl coverage, and the sharp TPD maxima for desorption from the coexisting dense and rare phases are also somewhat shifted to lower temperatures, in agreement with experimental results. Although the general behavior of the desorption is well accounted for, the agreement between the present model and experimental results is not quantitative; but it isn’t
Figure 3. Oxygen desorption curves based on the model presented in the text, with phase equilibration and blocking by Cl adsorbate. The dashed line passing through the tail inflexions indicates the shifts to lower temperatures due to the compression of the oxygen rare phase by increasing surface concentrations of the Cl. The dotted vertical indicates where the inflexion points are expected if the Cl adsorbate were mobile.
Figure 4. Apparent activation energy for oxygen desorption from the rare phase as a function of coverage (ML ) monolayer) and the concentration of the Cl dopant (ΘCl ) 0, 0.05, 0.08, 0.15, 0.18, and 0.22).
expected to be quantitative given the averaging of what appears to be a wide distribution of ensembles based on the Monte Carlo simulation.29 Perhaps more importantly, the model permits the apparent activation energies and activation entropies to be extracted as a function of oxygen coverage. The results are shown in Figure 4 for the activation energies and in Figure 5 for the free energies of activation as a function of oxygen coverage for different fixed Cl coverages. In a non-adiabatic system considered here, the activation energy does not have the meaning of a molecular or an atomic-surface bonding, but nevertheless can be determined both experimentally and from the present model as the temperature coefficient of the logarithmic rate dependence on reciprocal absolute temperature at
The O2/Cl/Pd(100) System: Ensemble Effects
Figure 5. Free energy of activation for oxygen desorption from the rare phase at low coverages and from the dense phase at higher coverages. The breaking points are apparent between the regime when both phases coexist at higher coverages and where only the rare phase exists.
each coverage. The oxygen coverage dependence of the apparent activation energy for the Cl-free surface (Figure 4) has a course very similar to that determined from experiment by trailing edge analysis (see Figure 3 in ref 40). The model also shows a decrease of the apparent activation energy and the free energy of activation at the low-coverage end of the oxygen concentration with increasing Cl precoverage, further exemplifying the repulsive interaction introduced by the Cl blocker whose action range R is greater than the Pd area covered, indicating longer range interactions to be in effect. Consistent with the model, these longer range interactions may be identified with the dipole repulsion betwen the Cl and the oxygen adatoms, as well as with neighboring site exclusion. In previous work, the activation energies for oxygen desorption from Cl-precovered Pd(100) have been estimated by Wang et al.29 by Redhead analysis, yielding Ea values of 29, 20, 18, and 14 kcal/mol for ΘCl ) 0, 0.05, 0.08, and 0.15 from postsaturated oxygen layers. These values fall within the range of the apparent activation energies determined from the present model and their decrease with increasing ΘCl is in qualitative agreement with the trends shown in Figure 4. Thus, Redhead analysis gave a correct qualitative picture of the effects of ΘCl on oxygen desorption, but the distribution of Eas values that are the property of the present model has not been revealed by this analysis. Likewise, the present model gives activation entropy as a natural consequence of lateral interactions in the pool of the rare phase oxygen. The values are large and negative because of the mobility of the oxygen adsorbate, but become less negative in the presence of the Cl adatoms because of the restriction of oxygen mobility. The activation entropies can be gleaned from the differences between the Ea and ∆Aa values in Figures 4 and 5. It is noted that the model of the Cl blocking action assumes that the Cl atoms are immobile, because otherwise the wanderer cell area of oxygen would not be reduced by the amount commensurate with Cl coverage. Instead, the oxygen and the Cl adatoms would wander in cells of approximately the same size if the dipole moments of these two adatoms with the palladium metal were comparable. The free energy of activation for oxygen desorption has been calculated as ∆Aa ) Ea - T∆Sa for each desorption temperature and converted to a ∆Aa versus oxygen coverage graph in Figure 5. When the two surface phases coexist, ∆Aa is nearly independent of coverage. This behavior is caused by the two-dimensional phase equi-
Langmuir, Vol. 14, No. 6, 1998 1389
librium between the rare and the dense phases, which buffers the system through the nearly constant chemical potential of the dense phase of constant concentration, like in equilibration of three-dimensional solid phases with gases and liquids. However, for the rare phase alone, ∆Aa decreases with increasing oxygen coverage because of the changes of interactions in its changing density and also decreases with increasing amount of the Cl adsorbate because of the ClsO repulsion and compression of the oxygen rare phase. It is noted that the sum of the Cl and oxygen coverages is nearly constant (cf., Table 1). Therefore, the repulsion effect of Cl toward oxygen in the rare phase is stronger than that of oxygen itself, which is attributed here to immobility of the Cl adatoms. Other effects to be considered are dipole interactions, where the dipoles associated with mobile Cl adatoms would have to be larger than those associated with the oxygen adatoms, an unlikely occurrence because of the lower electronegativity of Cl (3.0 eV) than that of oxygen (3.5 eV), or through-metal quantum mechanical long-range repulsion. Chlorine Ensembles and their Stability under High-Temperature Cycling with Oxygen The three Cl-containing structures studied, the c(2 × 2)Cl/Pd(100) from chemisorption of molecular chlorine,29 the p(2 × 2)Cl/C/Pd(100) from adsorption of C2Cl4,42 and the ordered-disordered Cl/C/Pd(100) from dichloromethane,29 are all dissociated at room temperature. Evidence for this is provided by LEED,29 XPS binding energy shifts,42 and high resolution electron energy loss spectroscopy (HREELS)29 that shows a vibrational band at 225 cm-1 characteristic of Cl atoms in fourfold holes. The Cl atoms in the p(2 × 2)/Pd(100) structure formed from C2Cl4 have also been suggested to reside in the fourfold holes.42 The c(2 × 2)Cl/Pd(100) structure is inaccessible to oxygen,29 showing exclusion of 50% of the alternate empty fourfold holes for oxygen chemisorption. The p(2 × 2)Cl/C/ Pd(100) overlayer from C2Cl4, in which the Cl atoms make a 0.25 monolayer coverage and the carbon is likely in C2 pairs surrounded by four Cl atoms, leaves 62.5% of the fourfold holes empty, and its inaccessibility to oxygen indicates a larger exclusion than that in the c(2 × 2) structure. Furthermore, the carbon residues are not oxidized by a direct reaction with gaseous oxygen at room temperature.42 The first structure that is accessible to oxygen is formed by dissociation of CH2Cl2, where the maximum Cl coverage attained at room temperature is 22% and the Cl + C combined coverage is 33%, leaving 67% of the Pd surface free after desorption of hydrogen.29 Oxygen does adsorb into this Cl/C/Pd(100) overlayer and oxidizes the C to CO and CO2 at elevated temperatures, leaving Cl on the surface at temperatures up to 1000 K.29 Between 1000 K and 1200 K, Cl was desorbed at identical temperatures as those from the c(2 × 2)Cl/ Pd(100) structure, and only atomic Cl was observed by mass spectrometry.29 Combined with the observed EELS vibrational frequency of 225 cm-1, these results indicate that Cl from CH2Cl2 was atomic and located in the fourfold holes of the Pd(100) surface. Coverages between 0 and 22% have been obtained by different exposures of Pd(100) to CH2Cl2, and the behavior of the Cl overlayer in this range of coverages was identical except for the quantities adsorbed. The ordered-disordered Cl/C/Pd(100), Cl/C/O/Pd(100), Cl/Pd(100), and Cl/O/Pd(100) overlayers from CH2Cl2 followed by oxygen treatment have been monitored by LEED, HREELS, TPD, and Auger analysis, with the principal result that the Cl adsorbate retained all its properties after oxidation and oxygen desorption cycles
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Klier et al.
Figure 6. Comparison of the oxygen TPD curves obtained from Pd(100) surfaces containing low Cl coverages after two and three cycles of oxygen dosing (90 langmuirs of O2) at room temperature and heating.
carried out up to 1000 K.29 This result is demonstrated by the repeated oxygen TPD curves shown in Figure 6 for Pd(100) initially containing low CCl2 coverages, where all of the carbon was removed during the first and second TPD cycles. Of particular interest is the interconversion of the LEED patterns that takes place concurrently with the dissociative chemisorption of dichloromethane followed by oxygen treatments, which is summarized in Figure 7. The dissociation of CH2Cl2, followed by removal of hydrogen by thermal desorption at 300-400 K, gives rise to the LEED pattern shown in Figure 7a. The fuzzy spots around (h,k) ) ((1/2,(1/2) have been previously accounted for by Monte Carlo simulation,29 wherein the carbon and the Cl adatoms originating from a CH2Cl2 molecule were placed in diagonally opposed fourfold holes with nearest neighbor site exclusion for the next dichloromethane molecule. In the next step, carbon was removed by several oxidation cycles involving adsorption of oxygen into the CCl2 overlayer at room temperature and flash desorption of CO and CO2 at 300-1000 K. The resulting LEED pattern of the Cl overlayer, shown in Figure 7b, consists of streaks at (h,k) ) (0,(1/2) and ((1/2, 0), with continuous extensions in the ((x,(1/2) and the ((1/2,(y) directions. This pattern can be understood as originating from different population densities of Cl adatoms in alternate rows of fourfold holes of the Pd(100) structure: odd rows are populated by Cl atoms originating from the CCl2 fragments, whereas even rows are depopulated by the removal of the C adatoms by oxidation. The periodicity along the rows is broken, however, by the aforementioned exclusion of the nearest neighbor sites, which gives rise to the streaks along the ((x,(1/2) and ((1/2,(y) directions. Rotation by 90° forms domains that generate the aforementioned streaky pattern with a fourfold symmetry. These features are also apparent from Monte Carlo simulation of the Cl/Pd(100) ensembles reported earlier.29 The LEED pattern changes reversibly upon chemisorption of oxygen to give rise to the c(2 × 2) pattern of the combined Cl and oxygen overlayer shown in Figure 7c. This c(2 × 2) pattern is that of an adsorbate containing complementary Cl and oxygen coverages presented in Table 1, totalling a fraction
Figure 7. Interconversion of LEED patterns of Pd(100) observed (a-top) after dissociative chemisorption of CH2Cl2 at 300 K and thermal desorption of hydrogen at 300-400 K, (bmiddle) after oxidative removal of carbon by oxygen chemisorption at 300 K and subsequent desorption of CO and CO2 at 300-1000 K, and (c-bottom) after chemisorption of oxygen at 300 K into the Cl overlayer in (b). Desorption of oxygen from the O/Cl/Pd(100) overlayer (c) gives rise to a LEED pattern identical to that shown in (b).
0.4-0.5 of the available surface Pd atoms. The atomic scattering factors of Cl and oxygen have different angular variations, although their magnitudes are similar,60 and it is remarkable that the combined layer of these two elements makes a relatively sharp c(2 × 2) pattern. This (60) Fink, M.; Ingram, J. Atomic Data 1972, 4, 129.
The O2/Cl/Pd(100) System: Ensemble Effects
effect has been observed repeatedly and is also characteristic of coadsorbed Cl and CO. The most interesting observation is that the O/Cl/ Pd(100) c(2 × 2) pattern returns to the streaky (1/2,0) Cl/Pd(100) pattern of Figure 7b upon temperature programmed desorption of oxygen at 500-1000 K, and the streaky Cl/Pd(100) pattern returns to the O/Cl/Pd(100) c(2 × 2) pattern upon oxygen chemisorption at room temperature. The oxygen can be selectively removed again, giving rise to TPD curves identical to those in the previous cycles, as demonstrated in Figure 6, whereas Cl remains on the surface below 1000 K. At the same time, the oxygen TPD curves are shifted to lower temperatures with increasing ΘCl, consistent with the kinetic model already presented. The streaky patterns at ((1/2, 0) positions are not the property of overlayers originating by chemisorption of elemental Cl that builds LEED spots around the (1/2,1/2) position,29 or by chemisorption of other chlorocarbons [e.g., tetrachloroethylene that forms a p(2 × 2) structure42]. Thus, the Cl overlayer from CH2Cl2 precursor has a “memory” of its molecular origin even after removal of the carbon and repeated oxygen exposures and desorptions. Likewise, the oxygen adsorbate responds to the presence of the Cl overlayer in a repeatable fashion, both in coverage, in combined LEED patterns, and the position and shape of the TPD curves. It is concluded that all the observed phenomena corroborate the model used in the described kinetic treatment, which is based on immobility of the Cl adatoms and strong lateral interactions thereof with the oxygen adsorbate that becomes mobile just below and at desorption temperatures. The consequence of this type of interaction is an ensemble
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effect due to chemisorbed Cl that not only restricts the extent of oxygen chemisorption but also significantly affects its bond strength to the metal surface. Conclusions The Cl surface dopant exerts significant effects on the behavior of oxygen on the Pd metal surface at coverages as low as ΘCl ) 0.05. The range of lateral ClsO interactions is estimated to be some two PdsPd distances beyond and above the blocking of the fourfold holes and its nearest diagonal neighbors. This effect is attributed to the compression of the wanderer oxygen atom rare phase by restricting the free area for the oxygen atom mobility. The oxygen rare phase is in rapid equilibrium with the dense phase (Θ0 ) 0.8), and compression of the rare phase by Cl(ads) forces an adjustment of the relative concentrations of the two oxygen phases, thereby also indirectly affecting the low-temperature sharp desorption from the dense phase. The surface analyses for Cl and oxygen, the oxygen TPD, and the LEED patterns demonstrate that the Cl overlayer retains its coverage, distribution, and structure after cycling the system with oxygen up to 1000 K. Acknowledgment. This work was supported by U. S. Department of Energy Division of Basic Energy Sciences Grant No. DE-FG02-86ER13580. JSH also appreciates partial support by a Sherman Fairchild Fellowship in Solid State Studies and a direct grant from Lehigh University. LA970442X
