Interaction of Chlorine and Oxygen with the Cu (100) Surface

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Interaction of Chlorine and Oxygen with the Cu(100) Surface Ibrahim A. Suleiman,† Marian W. Radny,*,‡ Michael J. Gladys,‡ Phillip V. Smith,‡ John C. Mackie,†,§ Eric M. Kennedy,† and Bogdan Z. Dlugogorski† School of Engineering, The UniVersity of Newcastle, Callaghan, NSW 2308, Australia, and School of Mathematical and Physical Sciences, The UniVersity of Newcastle, Callaghan, NSW 2308, Australia ReceiVed: August 20, 2010; ReVised Manuscript ReceiVed: September 29, 2010

Density functional theory calculations have been performed to study the combined interaction of oxygen and chlorine with the Cu(100) surface. We found the presence of atomic chlorine increases the stability of molecular oxygen adsorption, and that the barrier required to dissociate the oxygen molecule in the presence of chlorine is three times larger than the dissociation barrier of molecular oxygen on the clean Cu(100) surface. In addition, chlorine monoxide was generated on the surface when molecular oxygen was adsorbed horizontally into a hollow site immediately adjacent to atomic chlorine. Our calculations indicate that while chlorine is easily adsorbed dissociatively on the clean Cu(100) surface, it is stable in the molecular form in the presence of atomic oxygen. The presence of chlorine leads to the production of subsurface atomic oxygen and enables an oxygen atom to go into the Cu bulk with a small activation energy barrier. 1. Introduction This study is part of a larger study of the industrial Deacon process in which gaseous chlorine is generated by the reaction of HCl with O2 in the presence of a catalyst according to the equation

2HCl(g) + 1/2O2(g) f H2O(g) + Cl2(g)

(1)

Previous investigations1 have shown that copper and/or copper compounds are the preferred catalysts for this reaction. In this paper we investigate the combined interaction of oxygen, chlorine, and a mixture of both, with the Cu(100) surface. The Cu(100) surface is of particular importance because it is a relatively simple surface, more reactive than the (111) surface, and does not reconstruct spontaneously like other metallic surfaces.2 Chlorine and oxygen adsorption on copper surfaces have been the subject of extensive studies,3-17 whereas studies combining both species are less frequent.18-22 With respect to oxygen reacting with Cu(100), theoretical studies predict that the most stable configuration for molecular oxygen adsorption is the bridge-bridge site with the molecule oriented parallel to the surface.9,23 However, SEXAFS measurements suggest that oxygen molecules adsorbed on the Cu(100) surface reside at the hollow-bridge sites with a 27° tilt.24 This is supported by a Hartree-Fock cluster study by Torras et al.,25 which predicted that an oxygen molecule would be stable when adsorbed on the hollow-bridge site with a 31° tilt. However, their density functional theory (DFT) calculations also predicted that the bridge-bridge site is the most stable site for molecular oxygen adsorption. In the case of the adsorption of Cl on the Cu(100) surface, it has been shown in many studies utilizing different techniques5,26-28 * To whom correspondence should be addressed. Tel: (+61 2) 4921 5447. Fax: (+61 2) 4921 6907. E-mail: [email protected]. † School of Engineering. ‡ School of Mathematical and Physical Sciences. § Also at School of Chemistry, The University of Sydney.

that a c(2 × 2) periodic layer of Cl is formed on Cu(100) at the saturation coverage of 1/2 monolayer (ML). In this structure the Cl adatoms occupy the 4-fold hollow sites. Only a few studies have treated surface reactions that contain both chlorine and oxygen. These include copper,18-22 silver,29,30 gold,31 and platinum32 surfaces. Auger electron spectroscopy (AES), ultraviolet photoemission spectroscopy (UPS), X-ray photoemission (XPS), and work function measurements of Fang et al.20 have shown that the adsorption of chlorine on an oxygen preadsorbed Cu(100) surface leads to the subsurface adsorption of oxygen atoms. Davies et al.18,22 have shown that the adsorption of chlorine in the presence of oxygen on Cu(110) at low coverage leads to the formation of CuCl2, indicating that oxygen plays a significant role in the phase transition from copper to copper chloride. On the other hand, Kamath et al.19 in their XPS and UPS studies have shown that the presence of preadsorbed chlorine on Cu surfaces increases the stability of molecular oxygen adsorption. Prabhakaran21 studied the effect of HCl on the adsorption of oxygen on the Cu(110) surface using electron energy loss spectroscopy (EELS). His results show that the presence of HCl increases the stability of molecular oxygen adsorption on Cu(110). They also show that the following reactions are occurring on the surface

HCl(g) f HCl(ads) O2(g) f O2(ads) O2(ads) + HCl(ads) f O(ads) + OH(ads) + Cl(ads) where (g) denotes the gas phase and (ads) denotes adsorbed species on the surface. In this study, we discuss the adsorption of atomic and molecular oxygen and chlorine on the Cu(100) surface, as well as the coadsorption of oxygen and chlorine. Our results show that the presence of chlorine on Cu(100) increases the stability of molecular oxygen on the surface, and promotes the subsurface adsorption of oxygen atoms. Our study also shows that while

10.1021/jp1078983  2010 American Chemical Society Published on Web 10/20/2010

Interaction of Cl and O with the Cu(100) Surface

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Figure 1. Schematic diagram of the top layer Cu atoms showing the different possible adsorption sites on the Cu(100) surface.

chlorine is adsorbed dissociatively on clean Cu(100), it can be adsorbed molecularly on the oxygen preadsorbed Cu(100) surface. 2. Computational Details We have performed periodic density functional calculations using the DMol3 program.33,34 The PW91 gradient corrected GGA functional of Perdew and Wang35,36 has been employed in conjunction with a double numeric quality basis set with polarization functions (DNP). All electrons have been included in these calculations for all atoms, together with a Fermi smearing of 0.136 eV and a real space cutoff of 4.4 Å. The clean Cu(100) surface, as well as the adsorption systems, were modeled using a 5 layer slab and a p(2 × 2) supercell. The slab was repeated periodically in the x-y directions with a 10 Å vacuum region between the slabs in the z-direction. The Brillouin zone integrations were performed using the 4 × 4 × 1 Monkhorst-Pack k-point sampling set.37 In all of the calculations the bottom two layers of the slab were kept fixed at their bulk-like positions (calculated lattice constant ) 3.67 Å), while the remaining atoms in the top three layers, and the adsorbed O and Cl atoms and/or molecules were allowed to move. The tolerances on the energy, gradient, and displacement convergence were set to 2 × 10-6 eV, 5 × 10-4 eV/Å, and 5 × 10-3 Å, respectively. The average binding energies of the adsorption have been calculated by using the formula

1 Eb ) [Ex/slab - (Eslab + nEx)] n

(2)

where Eslab, Ex, and Ex/slab are the total energies of a clean slab, an isolated x atom/molecule, and the x/Cu (100) adsorption system, respectively, with x being either oxygen or chlorine, and n the number of adsorbed oxygen/chlorine atoms/molecules

within each supercell. The vertical distance between a molecule and the surface is defined as the distance between the surface topmost layer and the atom of the molecule nearest to the surface. Transition state searches have been performed using the synchronous transit methods.38 These methods rely strongly on possessing reasonable initial and final structures for the reaction system. Starting from optimized initial and final structures, the synchronous transit methods interpolate a reaction pathway to find a transition state. This is first done by performing a linear synchronous transit (LST) followed by an energy minimization in directions conjugate to the reaction pathway. Quadratic synchronous transit (QST) maximization is then performed to find the maximum energy structure along the reaction path to provide an upper limit to the barrier height for the reaction. 3. Results and Discussion 3.1. Oxygen Adsorption on Clean Cu(100). We have investigated atomic oxygen adsorption on Cu(100) for three symmetric sites: hollow, bridge, and top (see Figure 1). The most stable site for atomic oxygen has been found to be the hollow site with a binding energy of -5.02 eV. For this site, the vertical distance between the oxygen atom and the surface (⊥(Cu-O)) was found to be 0.80 Å. For molecular oxygen (O2) adsorption we have investigated horizontal, vertical, and tilted configurations. For vertical adsorption we considered the hollow, bridge, and top sites, but we found only the hollow site is stable. The O2 bond length (d(O-O)) for the oxygen molecule sitting vertically above the hollow site was 1.36 Å (an 11.5% elongation with respect to our calculated value of 1.22 Å for the gas phase molecule), while the distance between the oxygen molecule and the substrate’s topmost layer (⊥(Cu-O2)) was 1.04 Å. For the bridge and top sites, we found that the oxygen molecule moves from its initial vertical configuration to a horizontal one above the bridge-bridge site (see Figure 2). In our earlier work using a p(2 × 3) unit cell,39 however, we found that the bridge site can provide a stable site for vertical molecular oxygen adsorption at lower coverage. With respect to horizontal configurations, we have tested three sites; these are the bridge-bridge, hollow-hollow, and top-top sites (see Figure 2). The calculations show that the oxygen molecule is not stable in the horizontal configuration at either the hollow-hollow or top-top sites, but moves from these positions to the bridge-bridge site, in agreement with the DFT calculations of Liem et al.15 Briner et al.40 using scanning tunnelling microscopy (STM) also found a horizontal configuration for molecular oxygen adsorption on Cu(100) but could not determine the location of the oxygen molecule due to the limitation of the STM resolution. The structural parameters for molecular oxygen occupying the bridge-bridge site are d(O-O)

Figure 2. Schematic diagrams of possible O2 (shown in blue) adsorption configurations on the Cu(100) surface. Only the top layer Cu atoms are shown.

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Figure 3. Top and side views for an oxygen molecule (shown in blue) at a hollow-bridge site of the Cu(100) surface for two different tilt configurations. Only the top layer Cu atoms are shown.

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Figure 5. Schematic diagram of different possible sites, P1, P2, and P3, for a single oxygen atom with respect to an oxygen molecule (shown in blue) adsorbed horizontally above a hollow site of the Cu(100) surface. Only the top layer Cu atoms are shown.

TABLE 1: Relative Energies and Geometric Parameters for a Horizontal Oxygen Molecule Combined with Atomic Oxygen on Cu(100) (Horizontal O2-O/Cu(100) Structures) with Different Positions for the Atomic Oxygena oxygen atom position

d(O-O) (Å)

⊥(Cu-O2) (Å)

⊥(Cu-O) (Å)

rel energy (eV)

P1 P2 P3

1.43 1.44 1.42

1.44 1.41 1.48

-0.27 1.12 0.34

0.91 0.00 0.26

d(O-O) is the O2 bond length, ⊥(Cu-O2) is the vertical distance between the topmost layer and the O2 molecule, and ⊥(Cu-O) is the vertical distance between the topmost layer and the single O atom. The energies are relative to the lowest energy structure. a

Figure 4. Dissociation pathways for an oxygen molecule (shown in blue) on the clean Cu(100) surface: (A) starting from the horizontal configuration and (B) starting from the vertical configuration. TS denotes the transition state.

) 1.51 Å (which is 23.8% longer than the bond length of the isolated oxygen molecule in the gas phase), and ⊥(Cu-O2) ) 1.34 Å. Energetically, the horizontal orientation for molecular oxygen adsorption on Cu(100) is more favorable than the vertical orientation by 1.26 eV. In an earlier work,39 we found that molecular oxygen adsorption in a horizontal orientation can also occur at a top-top site at lower coverage (one molecule per p(2 × 3) unit cell). To test the possibility of an oxygen molecule forming a tilted configuration at a hollow-bridge site, as suggested by previous works,24,25 we have considered two different orientations: one with the oxygen molecule occupying the hollow-bridge site with a +30° tilt, and the other with a -30° tilt (see Figure 3). We have found that both configurations are unstable, with the oxygen molecule returning to the horizontal configuration at the bridge-bridge site. This is in agreement with DFT cluster calculations but in contrast with the HF cluster calculations.25 Two pathways for oxygen dissociation have been investigated. The first pathway starts with the oxygen molecule occupying the bridge-bridge site horizontally (Figure 4A), while the second pathway starts with molecular oxygen occupying the hollow site vertically (Figure 4B). The final configuration in each case corresponds to the two dissociated oxygen atoms occupying hollow sites (see Figure 4, parts A and B). The barriers have been calculated to be 0.30 and 0.12 eV for the first and second dissociation pathways, respectively. The 0.30 eV barrier for the first dissociation pathway is in excellent agreement with the value of 0.33 eV obtained by Yata et al.41 using a supersonic molecular beam technique.

3.2. Oxygen Adsorption on Oxygen Preadsorbed Cu(100). To investigate the geometries and corresponding energetics resulting from molecular oxygen adsorption on an oxygen preadsorbed Cu(100) surface, we have included both atomic and molecular oxygen in the same unit cell using the most stable adsorption site for each component. Starting with molecular oxygen in the horizontal bridge-bridge site configuration (see Figure 2), the atomic oxygen can be chemisorbed at any of the three remaining hollow sites P1, P2, and P3 (see Figure 5). Table 1 summarizes the geometries and energies of the three different final configurations. In each case, the oxygen molecule remains in a horizontal configuration at its original bridge-bridge site and hence the geometric parameters of the oxygen molecule, ⊥(Cu-O2) and d(O-O), are similar for all three structures. The lowest energy structure occurs when the oxygen atom occupies the position P2. The vertical distance between the topmost Cu layer and the oxygen atom, ⊥(Cu-O), varies significantly from one case to another with the atomic oxygen being below the surface when it occupies the position P1. By contrast, for the lowest energy P2 configuration, the oxygen atom is predicted to sit 1.12 Å above the Cu(100) surface, 0.32 Å higher than for the clean surface. When the oxygen molecule is vertically oriented above the hollow site, positions P1 and P3 are identical by symmetry. We have, therefore, only investigated the sites P1 and P2. The geometries and energies for these configurations are summarized in Table 2. The oxygen molecule remains at its initial location with vertical orientation. The lowest energy configuration corresponds to the oxygen atom being at the P2 site with the oxygen molecule lying 1.18 Å above the Cu(100) surface. In contrast to the case of horizontal oxygen molecule adsorption,

Interaction of Cl and O with the Cu(100) Surface

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TABLE 2: Relative Energies and Geometric Parameters for Vertical O2-O/Cu(100) Structures for Different Positions of the Atomic Oxygena

TABLE 4: Relative Energies and Geometric Parameters for Horizontal O2-Cl/Cu(100) Structures and Different Positions of the Atomic Chlorinea

oxygen atom position

d(O-O) (Å)

⊥(Cu-O2) (Å)

⊥(Cu-O) (Å)

rel energy (eV)

chlorine atom position

d(O-O) (Å)

⊥(Cu-O2) (Å)

⊥(Cu-Cl) (Å)

rel energy (eV)

P1 P2

1.30 1.34

1.43 1.18

0.54 0.81

0.28 0.00

P2 P3

1.45 1.45

1.39 1.32

1.66 1.75

0.00 0.98

a d(O-O) is the O2 bond length, ⊥(Cu-O2) is the vertical distance between the topmost layer and the O2 molecule, and ⊥(Cu-O) is the vertical distance between the topmost layer and the single O atom. The energies are relative to the lowest energy structure.

TABLE 3: Comparison of the Calculated Structural Parameters and Binding Energies for the On-Surface Adsorption of a Cl Atom at a Hollow and Bridge Site of the Cu(100) Surfacea system

Eb (eV)

⊥(Cu-Cl) (Å)

hollow site bridge site exptl5

-3.40 -3.30

1.68 1.83 1.58

a Eb is the binding energy calculated with eq 2 and ⊥(Cu-Cl) is the vertical distance between the topmost layer and the chlorine atom.

Figure 6. c(2 × 2) structure for Cl atoms (shown in black) on the Cu(100) surface at 1/2 ML coverage. Only the top layer Cu atoms are shown.

the vertical distance between the topmost layer and the oxygen atom, ⊥(Cu-O), is now almost identical to the value of 0.80 Å for an isolated oxygen atom on the Cu(100) surface. 3.3. Chlorine Adsorption on Clean Cu(100). Our calculations show that the chlorine molecule is adsorbed dissociatively on Cu(100) in agreement with the experimental observations.5,26-28 For the adsorption of atomic chlorine on Cu(100), we have examined the hollow, bridge, and top sites. Our results are summarized in Table 3 and show that for 1/2 ML coverage, the hollow site is the most stable site and results in a c(2 × 2) structure (see Figure 6) in agreement with experiment.4,5,26,27,42-47 The calculated value for the vertical distance between the chlorine atom and the topmost layer ⊥(Cu-Cl) of 1.68 Å is in good agreement with the XRD experimental value of 1.58 Å.5 The top site has been found to be unstable with the chlorine atom moving from this site to the hollow site. 3.4. Chlorine-Oxygen Coadsorption on the Cu(100) Surface. To study the influence of chlorine on molecular oxygen adsorption on the Cu(100) surface we have combined an oxygen molecule with a chlorine atom in the p(2 × 2) unit cell. Similar to the calculations that we have carried out for molecular-atomic

a d(O-O) is the O2 bond length, ⊥(Cu-O2) is the vertical distance between the topmost layer and the O2 molecule, and ⊥(Cu-Cl) is the vertical distance between the topmost layer and the single Cl atom. The energies are relative to the lowest energy structure.

oxygen coadsorption, the oxygen molecule was initially placed in its horizontal hollow-site configuration, and the chlorine atom at one of the three hollow sites P1, P2, and P3 (see Figure 5). The energetics and geometric parameters for the positions P2 and P3 are summarized in Table 4. In both cases, the oxygen molecule remains at its original location with horizontal orientation, and the vertical distance between the chlorine atom and the topmost layer is close to that for an isolated chlorine atom on clean Cu(100) (see Table 3). The total energy of each structure clearly depends on the position of the chlorine atom, with P2 being the preferred site. We have also studied the dissociation barrier for the O2 molecule in the presence of a Cl atom at the position P2. We have found that the barrier required to dissociate the O2 molecule is 0.95 eV. This barrier is three times larger than our calculated barrier of 0.30 eV for dissociation of an O2 molecule in the horizontal configuration on a clean Cu(100) surface. This is consistent with the experimental observations of Prabhakaran21 which showed that the molecular oxygen species is more stable on the HCl-covered Cu(110) surface than on the clean Cu(110) surface. Interestingly, the presence of atomic chlorine at the P1 hollow site leads to very different behavior to the molecular oxygen adsorption that occurs for the P2 and P3 sites. We have found that the presence of atomic chlorine at position P1 leads to oxygen dissociation, with one of the oxygen atoms occupying the original hollow site, and the other moving below the chlorine atom to form a chlorine monoxide (ClO) molecule adsorbed on the Cu(100) surface at the P1 hollow site (see Figure 7). ClO radicals play an important role in stratospheric ozone depletion by providing a series of catalytic cycle reactions. ClO can react with a free oxygen atom releasing a free chlorine radical that can then react with an ozone molecule to form a new ClO molecule and O2.48 To the best of our knowledge, there is no previous study predicting the formation of chlorine monoxide on metal surfaces. The parameters of the final configuration in this case are d(O-Cl) ) 1.78 Å, ⊥(Cu-(O-Cl)) ) 1.21 Å, and ⊥(Cu-O) ) 0.33 Å. The calculated O-Cl bond length of 1.78 Å is greater than the gas phase O-Cl bond length of 1.57 Å.49 The vertical distance between the ClO molecule and the copper surface of 1.21 Å is close to that between the O2 molecule in the vertical configuration and the copper surface of 1.18 Å (see Table 2) indicating that the ClO molecule has been actually formed on the surface. Furthermore, the experimental50 ClO bond length in the HOCl molecule (1.69 Å) is close to the bond length of the ClO molecule (1.78 Å) formed on the surface. The stability of vertical molecular oxygen adsorption at a hollow site in the presence of atomic chlorine has also been investigated. Two neighboring hollow site positions for the atomic chlorine have been considered. These are P1 and P2,

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Figure 7. Final optimized structure resulting from molecular oxygen adsorption in the presence of atomic chlorine at the position P1. In the top view, the second O atom is lying directly underneath the Cl atom. Oxygen atoms are shown in blue and chlorine atoms in black. Only the top layer Cu atoms are shown.

since positions P3 and P1 are equivalent by symmetry. When the chlorine atom occupies position P1, the oxygen molecule moves from the vertical to the horizontal configuration, and from its original position to the position P3. The geometric parameters are the same as the parameters for the oxygen molecule in the horizontal hollow site configuration combined with atomic chlorine occupying the position P2 (see Table 4). This, together with the above discussion regarding molecular dissociation, indicates that an oxygen molecule is unlikely to become adsorbed at its initial site either vertically or horizontally when a chlorine atom occupies its neighboring P1 site. When atomic chlorine occupies the position P2 the oxygen molecule remains in its vertical configuration. The oxygen bond length d(O-O) has been found to be 1.34 Å with the vertical distance between the oxygen molecule and the Cu(100) surface ⊥(Cu-O2) being 1.21 Å, while the vertical distance between the chlorine atom and the substrate ⊥(Cu-Cl) is 1.69 Å. The first two of these values are very similar to those obtained for the corresponding case where an oxygen atom replaces the chlorine at the P2 site (see Table 2). The oxygen dissociation barrier with atomic chlorine at the P2 site has been calculated to be 0.33 eV, which is significantly higher than the 0.12 eV barrier required to dissociate an oxygen molecule in the vertical configuration in the absence of atomic chlorine (see Figure 4). For all of the different atomic chlorine/molecular oxygen chemisorption starting configurations, we have observed only two final configurations for the oxygen molecule: the horizontal and vertical configurations at a hollow site. We have also combined the c(2 × 2)-Cl structure (see Figure 6), which is the most stable structure for chlorine on Cu(100), with atomic oxygen at a neighboring hollow site. The optimized structure for this combined configuration is shown in Figure 8. We have found that the presence of the chlorine atoms leads to subsurface adsorption of the oxygen atom without any associated energy barrier. This is in agreement with the experimental observations for oxygen adsorption on a preadsorbed chlorine Cu(100) surface.20 The calculated structural parameters and binding energies are summarized in Table 5. We observe that the presence of oxygen lowers the binding energy of the chlorine (compare with the values in Table 3). The binding energy of the oxygen is also reduced from its value of -5.02 eV in the absence of chlorine to -3.80 eV. We have also studied the diffusion of a surface oxygen atom on the clean Cu(100) and Cl-modified Cu(100) surfaces. We have found that the barrier for surface oxygen diffusion to the

Figure 8. Optimized structure for coadsorbed atomic Cl (shown in black) and atomic O (shown in blue) on Cu(100). Only the top layer Cu atoms are shown.

TABLE 5: Parameters of the Optimized Cl, O/Cu(100) Structurea parameter

value

⊥(Cu-Cl) (Å) ⊥(Cu-O) (Å) chlorine binding energy (eV) oxygen binding energy (eV)

1.71 -0.14 -2.60 -3.80

⊥(Cu-Cl) is the vertical distance between the topmost layer and the chlorine atom, and ⊥(Cu-O) is the vertical distance between the topmost layer and the oxygen atom. a

subsurface octahedral site on the clean Cu(100) surface is 2.38 eV (see Figure 9). The presence of chlorine, however, reduces this value to 0.14 eV (see Figure 10). This shows that the presence of chlorine greatly enhances the diffusion of oxygen into the bulk of Cu(100). Similar behavior has been observed by Jia et al.29 for oxygen on the Cl-Ag(111) modified surface. 3.5. Cl2 on Cu(100) in the Presence of Atomic Oxygen. We now turn to consider the adsorption of a chlorine molecule on the preadsorbed oxygen surface. For the starting configurations we have assumed the chlorine molecule to be adsorbed horizontally at a bridge-bridge site (i.e. above a hollow site) of the Cu(100) surface, and the oxygen atom to be adsorbed at one of the remaining P1, P2, or P3 hollow sites (see Figure 5). We have found that chlorine can be adsorbed molecularly on the oxygen preadsorbed surface, in contrast to the clean surface. This shows that the presence of oxygen increases the stability of molecular chlorine on the Cu(100) surface. The parameters of the resulting optimized structures are summarized in Table 6. We observe that the lowest energy configuration occurs when the oxygen atom occupies the position P2, while chemisorption

Interaction of Cl and O with the Cu(100) Surface

J. Phys. Chem. C, Vol. 114, No. 44, 2010 19053 4. Conclusions In this paper we have investigated the adsorption of oxygen and chlorine, and the coadsorption of oxygen and chlorine on the Cu(100) surface using DFT. The results show that the presence of chlorine on the Cu(100) surface increases the stability of molecular oxygen on the surface and increases the barrier required to dissociate an oxygen molecule by a factor of 3. Our investigations also show that a chlorine molecule can be adsorbed molecularly on an oxygen preadsorbed Cu(100) surface rather than dissociatively as on the clean Cu(100) substrate. In addition, the presence of chlorine on the Cu(100) surface has been found to produce subsurface adsorption of atomic oxygen and oxygen diffusion into the Cu bulk with a small barrier energy.

Figure 9. Energy diagram and side views for the diffusion of an oxygen atom (shown in blue) on the clean Cu(100) surface. Red atoms with a symbol X are the front copper atoms while the unmarked red (copper) atoms lie behind. TS denotes the transition state.

Acknowledgment. This research was undertaken on the NCI National Facility in Canberra, Australia, which is supported by the Australian Commonwealth Government. M.W.R., J.C.M., E.M.K. and B.Z.D. acknowledge the Australian Research Council (ARC) for support (Project no. DP0988907). I.A.S. acknowledges the award of a UNIPRS and UNRSC by the University of Newcastle. References and Notes

Figure 10. Energy diagram and side views for the diffusion of an oxygen atom on the Cl-modified Cu(100) surface. The oxygen atom is shown in blue and the chlorine atoms in black. Red atoms with a symbol X are the front copper atoms while the unmarked red (copper) atoms lie behind. TS denotes the transition state.

TABLE 6: Relative Energies and Geometric Parameters for Horizontal Cl2-O/Cu(100) Structures and Different Positions of the Atomic Oxygena oxygen atom position

d(Cl-Cl) (Å)

⊥(Cu-Cl2) (Å)

⊥(Cu-O) (Å)

rel energy (eV)

P1 P2 P3

2.57 2.60 2.60

2.21 2.15 2.14

-0.40 0.54 0.53

0.99 0.00 0.02

a d(Cl-Cl) is the Cl2 bond length, ⊥(Cu-Cl2) is the vertical distance between the topmost layer and the Cl2 molecule, and ⊥(Cu-O) is the vertical distance between the topmost layer and the single O atom. The energies are relative to the lowest energy structure.

at position P1 leads to subsurface adsorption for the oxygen atom. As stated earlier, subsurface oxygen adsorption has been observed experimentally when chlorine is present on Cu(100).20 However, to our knowledge, there is no experimental evidence confirming that the adsorption of Cl2 produces subsurface adsorption of atomic oxygen.

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