Chemisorption of Trichloroethene on the PdCu Alloy (110) Surface: A

France, and Laboratoire de Chimie Théorique et des Matériaux Hybrides, Ecole Normale Supérieure de Lyon, 46, Allée d'Italie, Lyon 69364 Cedex ...
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Chemisorption of Trichloroethene on the PdCu Alloy (110) Surface: A Periodical Density Functional Study L. A. M. M. Barbosa,*,†,‡ D. Loffreda,†,‡ and P. Sautet†,‡ Institut de Recherches sur la Catalyse, Centre National de la Recherche Scientifique, 2 Avenue Albert Einstein, Villeurbanne Cedex 69626, France, and Laboratoire de Chimie The´ orique et des Mate´ riaux Hybrides, Ecole Normale Supe´ rieure de Lyon, 46, Alle´ e d’Italie, Lyon 69364 Cedex 07, France Received July 17, 2001. In Final Form: December 21, 2001 In the present work different adsorption modes of trichloroethene on the PdCu alloy have been investigated systematically. With application of ab initio periodic density functional theory, some insights about these adsorption modes have been revealed. The two different (110) terminations of the Cu3Pd alloy have been employed as models for the surface region of the Cu50Pd50 alloy because of Cu segregation. They are based on the regular phase of the face-centered cubic (fcc) structure. The first model shows a mixed (Pd/Cu )1) top layer, whereas the other one is Cu-terminated. The analysis of the position of the center of the d-band projected on the Pd and Cu atoms of both surfaces indicated that the Pd atom in the alloy has similar reactivity to the pure metal surface, whereas the opposite trend has been found for the Cu atoms. This has also been confirmed by the adsorption energy, calculated for the distinct modes. Trichlorethene prefers to interact with the Pd atoms on the mixed PdCu surface. Both di-σ and π modes are the most stable configurations among all studied. On the other hand, the adsorption via the chlorine atoms is the preferred on the Cu-terminated surface. The interaction of the CdC bond of trichloroethene on the Pd atoms is similar to the one of the ethene molecule. The main difference between the adsorption of these two molecules is due to the extra chlorine interactions with the Cu atoms. This allows one to suggest that the di-σ configurations are the precursors of the dissociation reaction on the mixed surface.

1. Introduction The catalytic cleavage of the C-Cl bond and its replacement with the C-H bond in chlorohydrocarbons and chlorofluorohydrocarbons has become an important environmental demand not only because of the problem of the ozone layer destruction but also in relation to the groundwater contamination. Metals such as Pt, Pd, and Cu have been suggested to be excellent catalysts for the dechlorination reaction in terms of activity, selectivity, and stability.1-18 The Cu catalysts have been shown to be extremely effective for C-Cl cleavage reaction; however, their * To whom correspondence should be addressed. Present address: ICI Strategic Group, Wilton Centre, P.O. Box 90, Wilton Teesside TS 90 8 JE, United Kingdom. E-mail: [email protected]. † Centre National de la Recherche Scientifique. ‡ Ecole Normale Supe ´ rieure de Lyon. (1) Bloxham, L. H.; Haq, S.; Mitchell, C.; Raval, R. Surf. Sci. 2001, 489, 1. (2) Jenks, C. J.; Bent, B. E.; Bernstein, N.; Zaera, F. J. Phys. Chem. B 2000, 104, 3008. (3) Lowry, G. V.; Reinhard, M. Environ. Sci. Technol. 2000, 34, 3217. (4) Benitez, J. L.; del Angel, G. React. Kinet. Catal. Lett. 2000, 70, 67. (5) Cabibil, H.; Ihm, H.; White, J. M. Surf. Sci. 2000, 447, 91. (6) Zhou, G.; Chan, C.; Gellman, A. J. J. Phys. Chem. B 1999, 103, 1134. (7) Chan, A. S. Y.; Turton, S.; Jones, R. G. Surf. Sci. 1999, 433-435, 234. (8) Laroze, S. C.; Haq, S.; Raval, R.; Jugnet, Y.; Bertolini, J. C. Surf. Sci. 1999, 433-435, 193. (9) Chan, C.; Gellman, A. J. Catal. Lett. 1998, 53, 139. (10) Rochefort, A.; Martel, R.; McBreen, P. H. Surf. Sci. 1998, 441, 38. (11) Jugnet, Y.; Prakash, N. S.; Bertolini, J. C.; Laroze, S. C.; Raval, R. Catal. Lett. 1998, 56, 17. (12) Yang, M. X.; Sarkar, S.; Bent, B. E.; Bare, S. R.; Holbrook, M. T. Langmuir 1997, 13, 229. (13) Yang, M. X.; Kash, P. W.; Sun, D.-H.; Flynn, G. W.; Bent, B.; Holbrook, M. T.; Bare, S. R.; Fischer, D. A.; Gland, J. L. Surf. Sci. 1997, 380, 151. (14) Karpinski, Z.; Early, K.; d’Itri, J. L. J. Catal. 1996, 164, 378.

application as a dechlorination catalyst is limited by the strong interaction between Cu and Cl atoms, which poisons the catalyst.11-13 Pd-based catalysts have also been suggested, in combination with H2, to be effective for the destruction of halogenated compounds;1,3,4 however, the production of HCl during the process strongly inhibits the reaction. If Pd is associated with other metals, for instance, PdRh and PdSn,19 this inhibiting effect decreases. In the case of the PdRh catalysts, a dilution of the Pd surface was suggested to be the reason for such reduction, whereas an electronic effect was indicated as a cause for the PdSn catalyst. Ichikawa and co-workers20 have done an extensive study modifying Pd catalysts for the hydrochlorination reaction of 1,1,2-trichlorotrifluorethane. One of the combinations that shows enhanced selectivity for the dechlorination reaction was PdCu with 1/2 atomic ratio. More recently, Pd50Cu50 alloy has been studied for hydrodechlorination of chloroethenes.21,22 These studies have indicated that the surface has a dynamical response to the adsorption of different chlorinated molecules, changing its composition upon contact with different species. A detailed understanding of the effect of the Pd/ (15) Yang, M. X.; Eng, J., Jr.; Kash, P. W.; Flynn, G. W.; Bent, B.; Holbrook, M. T.; Bare, S. R.; Gland, J. L.; Fischer, D. A. J. Phys. Chem. 1996, 100, 12431. (16) Cassuto, A.; Hugenschmidt, M. B.; Parent, Ph.; Laffon, C.; Tourillon, H. G. Surf. Sci. 1994, 310, 390. (17) Lin, J.-L.; Bent, B. E. J. Phys. Chem. 1993, 97, 9713. (18) Lin, J.-L.; Bent, B. E. J. Phys. Chem. 1992, 96, 8529. (19) Bodnariuk, P.; Coq, B.; Ferrat, G.; Figueras, F. J. Catal. 1989, 116, 469. (20) Ohnishi, R.; Wang, W.-L.; Ichikawa, M. Appl. Catal. A 1994, 113, 29. (21) Baddeley, C. J.; Bloxham, L. H.; Laroze, S. C.; Raval, R.; Noakes, T. C. Q.; Bailey, P. Surf. Sci. 1999, 433-435, 827. (22) Noakes, T. C. Q.; Bailey, P.; Laroze, S. C.; Bloxham, L. H.; Raval, R.; Baddeley, C. J. Surf. Interface Anal. 2000, 30, 81.

10.1021/la011113e CCC: $22.00 © 2002 American Chemical Society Published on Web 03/01/2002

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Cu composition in this dechlorination reaction, however, remains to be formulated. Here, the adsorption of trichloroethene (TCE) on the two different (110) surfaces of the Cu3Pd alloy, which model the Cu segregation of the PdCu (110) surface, has been studied by applying first-principles quantum chemical calculations. The atomic details of the adsorption process have been presented to understand the catalytic effect of these surfaces. 2. Methods In the work reported here all calculations were performed using the Vienna Ab Initio Simulation Package (VASP).23,24 This code carries out periodic density functional calculations (DFT) using pseudopotentials and a plane wave basis set. The DFT was parametrized in the local-density approximation (LDA), with the exchange-correlation functional proposed by Perdew and Zunger25 and corrected for nonlocality in the generalized gradient approximations (GGA) using the Perdew-Wang 91 functional.26 The interaction between the core and electrons is described using the ultrasoft pseudopotentials introduced by Vanderbilt27 and provided by Kresse and Hafner.28 Forces, which are used to relax atoms into their equilibrium positions, can be easily calculated. The VASP package has been used in diverse studies, related to catalysis on metal surfaces, with great success.29-37 The Cu3Pd alloy is modeled by a periodic five-layer-slab with TCE adsorbed on one side of the slab. One slab is separated from its periodic image in the z direction by a vacuum space, equivalent to five metallic layers. In some specific cases, this vacuum space has been increased to eight metallic layers to prevent interaction between the adsorbate and the periodic image of the substrate (slab). Each layer is composed of eight metallic atoms and only the bottom layer has been maintained frozen in all optimizations. In the calculations TCE is ordered over the surface in the following structure: (4 × 2) 0.125 monolayer (ML). Different types of adsorption modes were studied here: top, bridge, and hollow, which will be discussed in more detail. Two different surface terminations have been employed here. They are based on the regular phase of the face-centered cubic (fcc) structure and for the (110) orientation. The first model, surface A, has an equally mixed Pd-Cu top layer. The second model is a Cu-terminated surface, surface B (see Figure 1a,b). More details about these surfaces will be provided later in the following sections. To minimize the effect of the stress that occurs due to the constraints in the slab model, the optimal bulk metal-metal distance was calculated. The calculated bulk nearest Pd-Cu distance is 2.70 Å, which is in good agreement with the measured value of 2.62 Å for the Cu3Pd alloy.38 The Brillouin-zone integrations have been performed on a 2 × 3 × 1 Monkhorst-Pack grid of k-points for all structures, which allows one to reach convergence for the calculated energy. A spin-restricted approach has been used because spin polarization (23) Kresse, G.; Furthmu¨ller, J. Comput. Mater. Sci. 1996, 6, 15. (24) Kresse, G.; Furthmu¨ller, J. Phys. Rev. B 1996, 54, 169. (25) Perdew, J.; Zunger, A. Phys. Rev. B 1981, 23, 8054. (26) Perdew, J.; Wang, Y. Phys. Rev. B 1986, 33, 8800. (27) Vanderbilt, D. Phys. Rev. B 1990, 41, 7892. (28) Kresse, G.; Hafner, J. J. Phys. Condens. Matter 1994, 6, 8245. (29) Loffreda, D.; Simon, D.; Sautet, P. Chem. Phys. Lett. 1998, 291, 15. (30) Loffreda, D.; Simon, D.; Sautet, P. Surf. Sci. 1999, 425, 68. (31) Delbecq, F.; Sautet, P. Surf. Sci. 1999, 442, 338. (32) Eicher, A.; Mittendorfer, F.; Hafner, J. Phys. Rev. B 2000, 62, 4744. (33) Eicher, A.; Hafner, J.; Groβ, A.; Scheffler, M. Phys. Rev. B 1999, 59, 13297. (34) Ciobıˆcaˇ, I. M.; Frechard, F.; van Santen, R. A.; Kleyn, A. W.; Hafner, J. Chem. Phys. Lett. 1999, 311, 185. (35) Ciobıˆcaˇ, I. M.; Frechard, F.; van Santen, R. A.; Kleyn, A. W.; Hafner, J. J. Phys. Chem. B 2000, 104, 3364. (36) Raybaud, P.; Hafner, J.; Kresse, G.; Kasztelan, S.; Toulhoat, H. J. Catal. 2000, 190, 128. (37) Mittendorfer, F.; Hafner, J. Surf. Sci. 2001,472, 133. (38) Villar, A. P.; Calvet, L. D. Pearson’s Handbook of Crystallography Data for Intermetallic Phases; ASM International: Materials Park, OH, 1991.

Barbosa et al. effects have been found to be negligible in other works using Pd surfaces.29-31

3. Results and Discussion 3.1. Different Surface Composition for the Cu3Pd Surface Model. The Pd50Cu50 alloy has been extensively studied experimentally by different approaches: Low electron diffraction (LEED),39 auger electron spectroscopy (AES),39 and medium energy ion scattering (MEIS).21,22 The PdCu (110) surface is usually cleaned and stabilized by annealing at high temperature. After this procedure a well-defined Cu-rich surface is formed with a mean copper concentration in the top layer at 70%, instead of 50% in the bulk. Furthermore, low-energy electron diffraction (LEED) and scanning tunneling microscope (STM) show a well-ordered (2 × 1) structure, all the results being characteristic of the formation of a R′-Cu3Pd phase, with alternating of pure Cu and mixed PdCu planes.40 Under contact with adsorbed molecules (trichloroethene, ethene, and 1,2- and 1,1-dichloroethene), this alloy surface suffers an enrichment in either component. The adsorption of ethene at 460 K causes Pd segregation on the topmost layer, whereas chlorinated molecules cause Cu segregation.21,22 A slab with an ordered Cu3Pd structure is hence a good model to represent the Cu segregation in the surface region of the clean Pd50Cu50. Such Cu3Pd has two terminations, previously described as surface A and B. Surface A (see Figure 1a) has a mixed Pd-Cu topmost layer (Pd/Cu ratio equal to 1). The atomic rows in the direction [1h ,1,0] show alternating Pd and Cu atoms. The second layer is purely composed of Cu atoms, which is followed by another mixed layer of Pd/Cu. By repeating this pattern up to the fifth layer, the total Cu content of this slab is about 70%. The metal distribution per layer has similar trends to the one studied by Noakes et al.;22 see Table 1. The other surface model, called B (see Figure 1b), has in contrast a top layer composed purely of Cu atoms and a mixed Pd/Cu second layer. Therefore, with this new arrangement being repeated up to the fifth layer, the total Cu content of the slab is about 80%. This surface composition has also been observed by other studies.41-45 For instance, Anderson et al.,41 who investigated Pd dissolution and deposition into the Cu substrate at 300353 K by re-emitted-positron spectroscopy, AES and LEED, concluded that there is a formation of a partially ordered second-layer Cu-Pd alloy covered by a pure Cu plane. The total density of states (DOS) diagrams projected onto the d orbitals of these two different alloy surfaces and the related pure Cu and Pd ones are shown in Figure 2. The DOS of surface A and B have great similarity to the DOS of the Cu surfaces because of the large Cu content in both surfaces. This result is in good agreement with the X-ray photoelectron spectroscopy (XPS),46 photoemis(39) Loboba-Cackovic, J. Vacuum 1997, 48, 578. (40) Lianos, L.; Debauge, Y.; Massardier, J.; Jugnet, Y.; Bertolini, J-C. Catal. Lett. 1997, 44, 211. (41) Anderson, G. W.; Pope, T. D.; Jensen, K. O.; Griffiths, K.; Norton, P. R.; Schultz, P. J. Phys. Rev. B 1993, 48, 15283. (42) Newton, M. A.; Francis, S. M.; Li, Y.; Law, D.; Bowker, M. Surf. Sci. 1991, 259, 45. (43) Bergmans, R. H.; van de Grift, M.; van der Gon, A. W. D.; Brongersma, H. H. Surf. Sci. 1996, 345, 303. (44) Goapper, S.; Barbier, L.; Salanon, B. Surf. Sci. 1998, 409, 81. (45) Goapper, S.; Barbier, L.; Salanon, B.; Loiseau, A.; Torrelles, X. Phys. Rev. B 1998, 57, 12497. (46) Matersson, N.; Nyholm, R.; Cale´n, H.; Hedman, J.; Johansson, B. Phys. Rev. B 1981, 24, 1725.

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Langmuir, Vol. 18, No. 7, 2002 2627 Table 1. Composition Data for the PdCu Surface (Model A) first layer second layer third layer

surface A

Noakes et al.22

50% Pd 50% Cu 0% Pd 100% Cu 50% Pd 50% Cu

53% Pd 47% Cu 35% Pd 65% Cu 55% Pd 45% Cu

Figure 3a,b. This is due to the small number of Pd neighbors and the fact that the interactions between atoms of different metals are weaker than the ones from the same metal.49 One important electronic criterion is the position of the center of the d band with respect to the Fermi level, as noted d. For the Pd atom on surface A (Figure 3a,b, position of the arrows) this value is not significantly shifted in comparison to the one for the pure Pd surface. This may be due to the fact that the metalmetal distance in the alloy is similar to the one found in the pure Pd surface, being 2.70 Å in the alloy (calculated here) compared to 2.80 Å (theoretical value)29 and 2.75 Å (experimental value),50 respectively. Taking into account that the change of the d position can be used as a possible measure of the reactivity of the transition metal,51-54 one may conclude that the Pd atoms should behave similarly in both pure and alloy surfaces. This interesting result has also been observed for the Pt atom in Cu3Pt alloy for hydrogen dissociation55 and for the Pd atom in the Pd-Ni bimetallic system for butadiene adsorption.56 In the case of the Cu atoms the DOS projected onto the d orbitals for these surfaces shows a different trend; see Figure 3c-e. The diagrams look similar to the one for pure Cu. This may be due to the similar number of Cu neighbors in these three surfaces for a given Cu atom: 4 (surface A and B) and 6 for the pure Cu surface. Analyzing the d shift of the d band of the Cu atoms from both surfaces A and B, one may note that they have moved slightly toward the Fermi level. When a surface undergoes a compressive or tensile strain, the d-band center moves downward or upward in relation to the Fermi level. Large lattice constants (larger metal-metal distance) lead to a higher d position.54,57 The Cu-Cu distance for both surfaces is around 2.70 Å, whereas it is 2.56 Å for the pure Cu surface.50 Moreover, if one considers that the Pd atom is an impurity of this Cu surface, both d shifts follow the trend indicated in ref 51. In summary, the Cu atoms in both surfaces seem to be slightly more active than the ones in the pure Cu surface. These results are in agreement with a recent theoretical study of CO adsorption on CuPd (111) surfaces.58 They have noticed that the presence of Pd atoms induce destabilization of the 3d Cu band, shifting the d toward the Fermi level. Figure 1. Cu3Pd (110) surface models. (a) Surface A: mixed PdCu top layer and pure Cu second layer. (b) Surface B: pure Cu top layer and mixed PdCu second layer.

sion,47 and theoretical studies48 of PdCu alloys, which have already indicated the existence of Cu-like character at the Fermi level region for copper-rich samples. The DOS diagram for the Pd atoms of surface A looks very different (narrower with a much reduced contribution at the Fermi level) compared to the one for pure Pd; see (47) Rochefort, A.; Abon, M.; Deliche`re, P.; Bertolini, J. C. Surf. Sci. 1993, 294, 43. (48) Fernadez-Garcia, M.; Conesa, J. C.; Clotet, A.; Ricart, J. M.; Lopez, N.; Illas, F. J. Phys. Chem. B 1998, 102, 141.

(49) Delbecq, F.; Sautet, P. Phys. Rev. B 1999, 59, 5142. (50) Kittel, C. Introduction to Solid State Physics, 4th ed.; John Wiley & Sons: Chichester, 1996; Chapter 1. (51) Ruban, A.; Hammer, B.; Stoltze, P.; Skriver, H. L.; Nørskov, J. K. J. Mol. Catal. A 1997, 115, 421. (52) Hammer, B.; Morikawa, Y.; Nørskov, J. K. Phys. Rev. Lett. 1996, 76, 2141. (53) Hammer, B.; Nielsen, O. H.; Nørskov, J. K. Catal. Lett. 1997, 46, 31. (54) Hammer, B.; Nørskov, J. K. Adv. Catal. 2000, 45, 71. (55) Hammer, B.; Nørskov, J. K. Surf. Sci. 1995, 343, 211. (56) Hermann, P.; Simon, D.; Sautet, P.; Bigot, B. J. Catal 1997, 167, 33. (57) Mavrikakis, M.; Hammer, B.; Nørskov, J. K. Phys. Rev. Lett. 1998, 81, 2819. (58) Lopez, N.; Nørskov, J. K. Surf. Sci. 2001, 477, 59.

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Figure 2. DOS projected on d orbitals for the pure Pd, Cu, and the two alloy surface models.

Figure 3. DOS projected on d orbitals of the surface Pd and Cu atoms for the pure and alloy surfaces.

3.2. Adsorption of Trichloroethene (C2Cl3H) on Different Surfaces. In this section different adsorption modes have been analyzed for TCE on the Cu3Pd (110) surfaces. There are three different types of adsorption modes (families): bridge, top, and hollow. In the first mode mainly the carbon atoms have an interaction with the surface, whereas in the last one the chlorine atoms are playing the major role. At the top position, both carbon and chlorine atoms can interact preferentially with the surface. Each of these families has a number of distinct geometrical positions, which depend on the orientation and also the position of the molecule on the surface. Thus, a nomenclature has been employed to give a systematic way of comparison among all these families and different surfaces. This nomenclature has been proposed in the following manner: Xyz, where X describes the type of adsorption site: “B” bridge, “H” hollow, and “T” top. The backbone orientation on the surface is identified by the superscript index z: “||” denotes that the CdC bond is parallel to the 〈1 h 10〉 direction, “⊥” this bond is parallel to the 〈001〉, and “×” this bond is parallel to the 〈1 h 11〉 direction. The index y corresponds to the atom of the surface with which the molecule interacts. In some cases, the functional group CCl2 of the molecule is used as a

reference to indicate which is the surface atom corresponding to the y index. For instance, the code BPd|| indicates a bridge position, where the Pd atoms interacts with the CCl2 group and the molecule sits parallel to the 〈1 h 10〉 direction. 3.2.1. Screening the Chemisorption Modes on the Mixed Pd/Cu Surface (Surface A). The Bridge Family. Four different positions have been tested, as shown in Figure 4a. The strongest interaction corresponds to the long bridge on Pd: BPd⊥. This structure can also be identified as a di-σ adsorption. Such a long-bridge di-σ geometry is specific to this alloy surface, and it is not obtained on pure Pd, where a short-bridge configuration along the 〈1 h 10〉 direction has been shown to be the most stable situation for ethene adsorption.59 Interestingly, there are two different possibilities for the di-σ geometry, depending on the bending of the molecule toward the surface; see Figure 4a. The configuration BPd⊥(1) has two chlorine atoms interacting with the Cu atoms of the surface. The other one, BPd⊥ (2), has just a single chlorine atom interacting with the surface. The former, however, is the most stable position among all on surface A. (59) Filhol, J. S., private communication.

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Figure 4. Configurations of the trichloroethene on the surface model, adsorption energy, and local geometry of the chemisorption structure. (a) Bridge site, surface A. (b) Top C site (π-mode), surface A. (c) Top Cl (interaction with CCl2 group), surface A. (d) Top Cl (interaction with different Cl atoms), surfaces A and B. (e) Top Cl (interaction with Cl2 group), surface B. (f) Hollow site, surface B.

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Table 2. Bond Lengths for the Most Stable Configurations of Trichloroethene on the Studied Surfaces (Values in Å and Degrees) CdC

C-Cl

C-Pd

Cl-Cu

Cl-Pd

angle between the molecular plane and the [110] direction

2.54, 2.55

0

Surface A bridge position (di-σ) BPd⊥(1) BPd⊥(2) top C position (π-mode) TPd⊥ top Cl position TPd⊥

1.47 1.46

1.82, 1.83 (A), 1.79 1.76, 1.77, 1.89 (A)

2.09, 2.07 2.11, 2.07

2.44, 2.47, 4.02 2.40, 4.10, 4.10

1.41

1.75, 1.77, 1.75

2.16, 2.17

3.48, 3.67, 3.49

1.34

1.74, 1.75, 1.72 Surface B

hollow position HCu|| HPd|| top Cl position T|| gas phase

1.34 1.34

1.72, 1.72, 1.73 1.72, 1.72, 1.72

1.34

1.73, 1.73, 1.72

1.34

1.72

3.95, 3.98

It is also important to note that the C-Cl bond is strongly perturbed in these two configurations; see Table 2. The stretching found is about 0.10 Å for BPd⊥(1) and 0.17 for BPd⊥(2) by comparison with the gas phase. Similarly, one may observe the Cu-Cl distance in both di-σ configurations. In the BPd⊥(1) this distance has the following values: 2.44 and 2.47 Å, whereas in the other configuration this distance is only 2.40 Å; see Table 2. Because the value of the Cu-Cl bond is 2.34 Å in the CuCl crystal,50 one may conclude that Cl has a true bond with the surface Cu atoms, resulting in a strong perturbation in the C-Cl bond. The same trend is observed for the CdC bond, which becomes elongated by 0.13 Å for both cases. This perturbation of the CdC bond is very similar to the one calculated for ethene, adsorbed on Pd(111)60 and Pd(ML)/Ag(111),61 where it becomes elongated by 0.12 Å. The value for the Cl-C-C-Cl dihedral angle gives an indication of the extent of sp2-sp3 hybridization of the carbon atoms. This value for the BPd⊥(1) mode is 129° and for the BPd⊥(2) is 133 °, which are much more closer to the value of 120° for alkanes than is 180° (the original value). These results indicate that the molecule is strongly activated and that such bridge structures may be the precursors of the dissociation reaction on this surface. The other bridge configurations are less stable; see Figure 4a. For instance, the position BCu⊥ has not been found as a stable minimum on the potential energy surface. The Top Family. The top position of TCE is now considered (Figure 4b). As in the previous case, the most stable modes correspond to the adsorption on a Pd atom of the surface: TPd|| and TPd⊥. These configurations can also be identified as a π-adsorption of the molecule. Interesting to note that adsorption energy, calculated for these configurations, seems to be independent of the orientation of the CdC bond on the surface. This value is also only slightly lower than the ones found for the di-σ cases, which indicates that both structures can be found simultaneously on the surface. A similar observation was obtained for the ethylene on Pd(100) at 80 K.62 By reflection-absorption infrared spectroscopy (RAIRS) trans-dichloroethene has been noticed to prefer to adsorb without C-Cl activation and to keep its molecular plane parallel to the Pd(110) surface at 80-130 K.1 The TPd⊥ (60) Neurock, M.; van Santen, R. A. J. Phys. Chem. B 2000, 104, 11127. (61) Pallassana, V.; Neurock, M. J. Catal. 2000, 191, 301. (62) Stuve, E. M.; Madix, R. J. J. Phys. Chem. 1985, 89, 3183.

2.86, 2.83, 2.93 2.84, 2.87, 2.86 2.84, 2.75

26.8

configuration geometry is consistent with the previous experimental observation. There is also no interaction between the chlorine atom and the surface, which is also noticed by a quasi negligible C-Cl bond stretching (0.05 Å) and a large Cu-Cl distance (3.5 Å); see Table 2. The CdC bond becomes slightly elongated by 0.08 Å, which shows that bridge (di-σ) configurations are more activated than the top (π) one. A similar bond elongation was calculated for the ethene on Pd(111) for the same adsorption mode: 0.095 Å,60 which indicates that the Pd atoms of surface A present the same chemisorption properties compared to the pure Pd surface. Other top positions have been studied in which the chlorine atoms interact directly with the surface, as seen in Figure 4c,d. In the first case, the CCl2 group is interacting directly with the surface with both Cl atoms (Figure 4c), whereas the other case has one Cl atom from each different group (CCl2 and CClH); see Figure 4d (for surface A). In both calculations, the separation of one slab from its periodic image was increased to eight metallic layers. In most of the cases, the interaction is very weak, except when the molecule interacts with the Pd atom: TPd⊥. The adsorption energy for this structure is, however, about 20 kJ mol-1 lower than the values found for the bridge sites. This suggests that TPd⊥ mode may occur at high loading of TCE. The Hollow Family. Four different positions have been tested and shown to be very unstable. The complete optimization of such structures ends on the bridge ones. 3.2.2. Screening the chemisorption modes on surface B. The Bridge Family. On surface B the stability of the bridge position is very weak. For the only two possible positions, BCu⊥ and BCu|| (not shown here), the adsorption energies found were -14 and +19 kJ mol-1, respectively. Experimentally this di-σ configuration has not been found when this molecule is adsorbed on the pure Cu(110) surface.8 On the other hand, trans-dichloroethene adsorption on the same surface has been described as di-σ species.11 This different behavior was attributed to steric difficulty because of the presence of three Cl atoms in the former molecule. In the case of surface B this difficulty seems, however, to be overcome by the increase of reactivity that occurs in the Cu atoms, compared to the pure Cu surface. The Top Family. The top position of the molecule in which the carbon atoms interact with the Cu atoms could not be found. Together with the previous results for the

Chemisorption of Trichloroethene on the PdCu Alloy

bridge site, one can note that the carbon atoms interact weakly with the Cu surface. This is in good agreement with the experimental observation that copper surfaces are usually not effective in the coupling or total dissociation of hydrocarbon fragments.12,18 However, the adsorption at the top position via the chlorine atom is stable; see Figure 4d,e. The preferred position of the molecule is the T||, with a slightly higher stability when the molecule interacts with the chlorine atoms from different groups (CHCl or CCl2). In this case again, the vacuum space between the periodic images of the substrate has been increased. The adsorption energy is very similar to the one shown in the previous case (the molecule adsorption on surface A); see the configurations T⊥ and TCu⊥ in Figure 4d. This is in agreement with the reactivity analysis for the Cu surfaces; see section 3.1. The adsorption of alkyl halides on the Cu surfaces is shown to be dominated by the interaction between Cl and Cu atoms,2,7 which is in agreement with the results found here. An interesting point is that the molecule in the T|| configuration is tilted with respect to the [110] direction by 26.8°; see Figure 4e and Table 2. The tilting of the molecular plane in relation to the surface normal has been observed experimentally by Bent and co-workers15 for vinyl bromide on Cu(100), by Yang et al.13 for chloroethene molecules on Cu(100), and Raval and co-workers8 for multilayers of trichloroethene on Cu(110). Furthermore, the molecule does not suffer any perturbation in this adsorption mode. The CdC and C-Cl bond have the same length as the molecule in the gas phase; see Table 2. The Hollow Family. In this case all configurations have the same stability, with a small preference to the HPd|| position. The expansion of the Cu-Cu distance due to the presence of Pd in the Cu neighborhood is responsible for this slight effect in the molecular interaction. One may note that the molecule interacts with the surface via the chlorine atoms, despite the fact that the C atoms are positioned above the second-layer Pd atom; see Figure 4f. The adsorption energies for the hollow positions are the largest values among all the different modes on surface B. They are, however, less than half of the previous ones calculated for surface A. The molecule has not been activated by the surface, as one can note from the values of the CdC and C-Cl bond lengths; see Table 2. In a way similar to the top configuration, these bonds have the same length as those found for the molecule in the gas phase. Experimentally, the TCE adsorption has been shown to be weak on the pure Cu surfaces.8,13 In ref 8, the RAIRS spectrum of the molecule monolayer at 150 K indicates that TCE prefers a parallel orientation, in contrast to the multilayer (inclined orientation). By using the near-edge X-ray adsorption fine structure (NEXAFS) of a 0.80 ML at 95 K, Yang et al.13 showed that there is no significant increase in the CdC bond distances for trichloroethene on Cu(100). They also suggested that the molecule lays approximately parallel to the surface via a π-coordination. On the basis of the calculations, this adsorption configuration may be better assigned as a hollow-type configuration for which there is an interaction between all three chlorines with the Cu atoms of the surface. Taking into account the values of the calculated adsorption energies for all configurations on surface B, it is possible to conclude that the molecule might desorb upon surface heating. This is in good agreement with the experimental observation that the reactivity of alkyl halides on the Cu surfaces reflects the competition between

Langmuir, Vol. 18, No. 7, 2002 2633

the rate of carbon-halogen bond dissociation and the rate of desorption alkyl halides desorption.18 For instance, alkyl chlorides do not dissociate on Cu(100) until the chain length reaches several carbons.18 3.2.3. Understanding the Stability of the Configurations. To understand stabilization of the molecule, one has to investigate the effect of the various interactions on the chemical bonds within and between the adsorbate and the surface. The metal-adsorbate bonding is accomplished at the expense of bonding within the metal and the adsorbed molecule.63 The DOS projected onto the Pd atoms of the surface, which interacts with TCE, is shown in Figure 5a. One may notice that the d band for the top Cl chemisorption mode has an almost identical shape to the one for the bare surface. This indicates that there is a weak interaction between adsorbate and substrate. Two small peaks can be seen between -5 and -10 eV, which are related to a small mixing of the p orbitals of Cl atoms and the d band of the Pd. A strong interaction occurs between the molecular orbitals of TCE and the d band of the Pd atoms in the case of the di-σ and π configurations. The Pd d orbitals acquire contributions in the electronic states below the d band (in the region [-5, -8 eV]), and above the Fermi level in the region [0, 2 eV]. The distribution of the states of the d band is also markedly modified with a decrease of the contributions in its upper part; see Figure 5a. On the other side, the pz states on the carbon atoms are strongly broadened on energy, especially the π* and π orbitals. In the lowest part of the DOS projected onto the pz orbital of carbon (see Figure 5b) appear states mainly localized on the Cl atoms, which interacts with the carbon atoms. The DOS for the two di-σ configurations are very similar, with a good mixing between the pz orbitals of the carbon atoms and the surface d band. This confirms the quasiidentical C-Pd interaction of both di-σ configurations (Figure 4a). There are, however, only small changes below -5 eV, where the states localized on the Cl atoms are. The interaction between the molecule and the surface weakens the metal-metal bond. This can be seen by the increase of the Cu-Pd bond distance by 0.04 to 0.06 Å, as shown in Table 3. A much larger surface relaxation is observed for the Pd atoms, with a strong decrease of the distance between these atoms upon interaction with the adsorbate. The new Pd-Pd distance (2.93 Å) becomes similar to the metal-metal bond of the Pd surface (2.80 Å); see Table 3. One may note that these Pd atoms were not linked before the adsorption, the original distance between these atoms being equal to 3.8 Å. This surprising result for the adsorption of trichloroethene is similar to the case of the ethene chemisorption on the Pd(110) surface for which the short bridge is the preferred site, where the Pd-Pd distance is 2.80 Å.59 On the Cu3Pd alloy surface, such a Pd-Pd short bridge does not exist because of the Pd-Cu atomic distribution on the surface. Hence, the molecule strongly distorts the Pd-Pd long bridge to create a favorable situation for adsorption. This adsorbate-substrate interaction also weakens the C-C bond, mainly from the π* contributions that are moved below the Fermi level and are populated; see Table 2 for the C-C bond length and Figure 5b. In the case of the top C (π-chemisorption mode) the DOS curves show globally the same features compared to the di-σ ones. Here, the DOS diagram is related to only (63) Hoffman, R. Rev. Mod. Phys. 1988, 60, 601.

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Barbosa et al.

Figure 5. The C-Pd interaction on surface A: the di-σ configurations correspond to the BPd⊥ (1) and (2): the top C configuration to TPd⊥ (the π mode), and the top Cl to TPd⊥ (the adsorption by the Cl atoms). (a) DOS projected on d orbitals of the surface Pd atoms involved in the chemisorption: di-σ and top Cl (2 atoms), top C (1 atom). (b) DOS projected on pz orbitals of the C atoms. Table 3. Pd-Pd Bond Length, All Values in Å, for Reference Surfaces and in the Case of the Chemisorption Modes Shown in Figure 4a,b Pd surface surface A BPd⊥(1) BPd⊥(2) TPd⊥(top C) Pd-Pd (long bridge) Pd-Pd (short bridge) Cu-Pd (short bridge)

3.96

3.82

2.93

2.94

3.80

2.72

2.76

2.76

2.74

2.72

2.76

2.78

2.80

one Pd atom, which the molecule sits on, whereas the other curves are related to two Pd atoms. The adsorbate-substrate interaction is not strong, as seen from the more localized character of the π orbital; see the peak at -5 eV in Figure 5b. One may also observe the smaller mixing of the Pd states just about the Fermi level; see Figure 5a. This agrees with the smaller elongation of the C-C and the Pd-Cu bond; see Tables 2 and 3, respectively. The Pd-C interactions and the comparison between the di-σ/π configurations are very similar to the ones studied for other alkenes molecules.64,65 The major difference in the present case is the formation of Cu-Cl bonds. The projected DOS onto the d band of the Cu atom of the surface, which the chlorine atoms of the di-σ species are interacting with, are shown in Figure 6a. The d band becomes more broad and strongly modified with a decrease of the contributions in its upper region compared to the bare surface. This indicates a strong interaction with the Cl atom of the molecule. (64) Paul, J.-F.; Sautet, P. Catal. Lett. 1991, 9, 245. (65) Delbecq, F.; Sautet, P. Catal. Lett. 1994, 28, 89.

Figure 6. The Cu-Cl interaction on surface A: the di-σ configurations correspond to the BPd⊥ (1) and (2): (a) DOS projected on d orbitals of the surface Cu atoms involved in the chemisorption. (b) DOS projected on pz orbitals of the Cl atoms interacting with the surface.

The DOS projected onto the pz orbital of the Cl atoms are shown in Figure 6b. These chlorine atoms are identified

Chemisorption of Trichloroethene on the PdCu Alloy

Langmuir, Vol. 18, No. 7, 2002 2635

of the chlorine atoms compared to the broad di-σ projected DOS; see Figures 7b and 6b, respectively. In summary, the interaction in the BPd⊥(di-σ) (1) configuration is the strongest one because of the contribution of the Cl-Cu bond, which is formed upon adsorption of the molecule. This contribution can be clearly seen in the adsorption energy trend found on surface A: BPd⊥ (1) > BPd⊥ (2) ≈ top C (π). 4. Conclusion

Figure 7. The Cu-Cl interaction on surface B: (a) DOS projected on d orbitals of the surface Cu atoms involved in the chemisorption. (b) DOS projected on pz orbitals of the Cl atoms interacting with the surface.

by label A in Figure 4a. The pz orbital is also broadened and delocalized in energy. In the region [-5,-6 eV] there is a clear indication of a strong mixing between this orbital and the d states of the Cu; see Figure 6a,b. Therefore, it is not surprising that the C-Cl bonds of the di-σ modes are the most activated ones; see Table 2. A different picture is found for the trichloroethene adsorption on surface B. For instance, the most stable situation encountered is the hollow configuration (HPd||), which has all chlorine atoms interacting with the surface; see Figure 4f. The Cu d bands of the bare surface B and after adsorption of the molecule are very similar. There is a subtle contribution in the d states in the region [-5,-6 eV], which indicates a weak interaction between adsorbate and substrate. This has also been confirmed by the presence of welllocalized states in the projected DOS onto the pz orbital

In the present work the interaction of different adsorption modes of trichloroethene on the PdCu alloy (110) surface has been investigated systematically. With application of ab initio periodic density functional theory, some insights about the most stable adsorption mode have been revealed. With analysis of the position of the center of the d band of the Pd and Cu atoms of both surfaces, it was noticed that the Pd atoms in surface A have a similar reactivity to that in the pure metal surface, whereas the Cu atoms of both surface models are slightly more reactive than those on the pure copper surface. Trichlorethene prefers to interact with the Pd atoms on surface A. Both di-σ and π modes on the Pd atom have been shown to be the most stable configurations among all studied. On the other hand, adsorption via chlorine atoms has been verified to be more stable on surface B. The interaction of the CdC bond of TCE on the Pd atoms is similar to the one for the ethene molecule. The main difference between the adsorption of these two molecules originates from the extra chlorine interactions with the Cu atoms. These results suggest a cooperative effect of the Pd and Cu atoms in the adsorption and dissociation of the C-Cl bonds. The two di-σ configurations are proposed to be the precursors of the molecule dissociation on this surface. Moreover, the strong chemisorption of the molecule on the Pd atoms, associated with the Cl interaction with the Cu atom, induces a surface reconstruction. This may be suggested as the possible cause for the changes in the composition profile of the surface upon the dissociation reaction, which have been observed experimentally. Acknowledgment. The authors thank Dr. Rasmita Raval, Dr. Yvette Jugnet, and Prof. Jean-Claude Bertolini for useful discussions and IDRIS at CNRS for the attribution of CPU time under Project No. 609. This project was possible due to the European Associated Laboratory between Leverhulme Centre for Innovative Catalysis and Institut de Recherches sur la Catalyse. LA011113E