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2008, 112, 17768–17771 Published on Web 10/29/2008
Active Sites for N2 Dissociation on Ruthenium S. Shetty,* A. P. J. Jansen, and R. A. van Santen Schuit Institute of Catalysis, EindhoVen UniVersity of Technology, P.O. Box 513, 5600 MB EindhoVen, The Netherlands ReceiVed: September 26, 2008; ReVised Manuscript ReceiVed: October 09, 2008
In this letter, we investigate the reaction paths for the N2 dissociation on the Ru(112j1) surface. Our results show that the N2 molecule has two preactivated states where it is in a 4-fold coordination. The reaction path which involves the N2 in a B5 coordination in the transition state has an apparent activation energy of 19 kJ/mol. This is about 16 kJ/mol lower than the path in which N2 has a B4 coordination. Our results confirm that it is a B5 coordination of the N2 in the transition state that leads to a low dissociation barrier and that such a coordination may be found on stepped, double stepped, and open surfaces as well as nanoparticles. It is a well-accepted proposition that the N2 dissociation is the rate-limiting step in the ammonia synthesis, mostly on the Ru surface.1-4 This implies that a low N2 activation barrier would enhance the rate of ammonia synthesis. Dissociation of N2 is dependent on the structure and reactivity of the local active sites involved in this process. Recent experimental and theoretical studies have proved that the steps are more reactive than the terraces for the activation of diatomic molecules such as N2, CO, and NO.1-12 Van Hardeveld and Van Montfoort in an experimental study proposed that the activity of N2 adsorption on Ni metal particles depends on the B5 (5-fold) sites present on these particles.13 They further showed with the help of mathematical modeling that these B5 sites can consist of two different geometrical arrangements present along certain directions such as (110) and (113) planes of the FCC crystal, for example, on an incomplete cub-octahedron. However, these special facets are not present on a perfect cub-octahedron with (111) and (100) planes where only B3 (3-fold) and B4 (4-fold) sites exist. Consequently, the reactivity of particles consisting of B3 and B4 sites is less compared to the particles consisting of B5 sites. Recently, Dahl et al. proposed that the active sites present on the Ru steps reduce the N2 dissociation barrier by at least 1.5 eV compared to the flat Ru surfaces.1 Honkala et al. used ammonia synthesis as an example to show that density functional approach can be applied to calculate the rate of reaction to be compared to the experimental studies.2 Interestingly, they carried out the calculations on Ru particles supported on magnesium aluminum spinel. These studies have also shown that the minimum number of five Ru atoms (B5 sites) are involved in the N2 dissociation process. This site was similar to the one proposed by van Hardeveld and van Montfoort along the (113) plane. Recently, we have shown that the CO molecule which is isoelectronic to N2 requires 6 Ru atoms, i.e., 4-fold hollow and bridge site, for optimum dissociation on the Ru(112j1) surface.14 Kim et al. in an experimental study showed that the N2 dissociation barrier on a double step of Ru(109) is only 0.26 ( 0.05 eV.5 This barrier was shown to be about 0.14 eV lower than the one observed on steps of Ru(0001) by Dahl * Corresponding author. E-mail:
[email protected].
10.1021/jp8085478 CCC: $40.75
et al.1 This indicates that the N2 dissociation is sensitive to the configuration of step topology. Most of the recent work has been focused on understanding the reactivity of the steps compared to the terrace sites on the Ru surfaces toward ammonia synthesis. Considerably less is known about the reactivity of the single-crystal open Ru surfaces for the N2 adsorption and dissociation. This raises several questions such as: (1) how reactive are the open surfaces for N2 dissociation compared to the stepped surfaces?, (2) do the open surfaces also have similar active sites for N2 dissociation as observed on the stepped or double stepped surfaces?, and (3) is the activation barrier and reaction path on the open Ru surfaces different than on the stepped and flat surfaces? Dieterich et al. experimentally studied the N2 dissociation on flat Ru(0001) and more open Ru(101j0) and Ru(112j1) single-crystal surfaces.15 Interestingly, the Ru(112j1) consists of the two geometrically different active B5 sites which are similar to the one proposed on the Ni particles by Hardeveld and Montfoort.13 These aspects of the open Ru(112j1) surface prompt us to study the reaction path for the N2 dissociation on this surface. In the present theoretical study, we explore two reaction paths for N2 dissociation where the N2 molecule is in a preactivated state on the Ru(112j1) surface. We have performed periodic DFT calculations using plane waves on a 2 × 2 supercell consisting of a 12 layered slab and 16 layers of vacuum between the slabs. The calculations have been carried out using the VASP code.17 The thickness of the slab and the vacuum are 7.09 and 9.66 Å, respectively. The dipole-dipole interaction between the supercells has been avoided by adsorption on both sides of the surface retaining a center of inversion. Initially, the optimization has been carried out using the Vanderbilt’s ultrasoft pseudopotential for the ion core.18 The exchange-correlation functional is expressed by the generalized-gradientapproximation(GGA)withthePerdew-Wang 91 functional (USPP-PW91).19 The optimized geometries obtained from the above calculations were refined using PAW potentials with PBE functionals (PAW-PBE).20,21 The differences in the energetics obtained from the above two calculations will be discussed later. It should be noted that all the atoms in the unit cell have been completely relaxed during the optimiza 2008 American Chemical Society
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Figure 1. (a) Ru(112j1) surface. Blue, green, yellow, and gray spheres correspond to the first-, second-, third-, and fourth-layer atoms, respectively. (b) Different active sites possible on the Ru(112j1) surface: 4-fold hollow (4F), 3-fold hollow (3F), top (T), bridge (B).14 The different possibilities for each site are represented in the parentheses by A, B, etc.14 The B5 sites exisiting on the Ru(112j1) surface are shown in red and orange circles as (I) and (II), respectively.
tion. The optimized model of the Ru(112j1) surface is shown in Figure 1. Frequencies have been calculated from the geometries generated in the geometry optimization using the vibrationsfrom-geometry-optimization (VFGO) method recently developed by Jansen et al.22 The reaction paths have been generated by the nudged elastic band (NEB) as implemented in VASP.23 The transition states have been confirmed by the saddle points obtained from the frequency calculations. The open nature of the Ru(112j1) surface exposes fourth layer atoms and hence creates several possibilities for the adsorption sites in the subsurface region. These sites are similar to the active sites found on the stepped and double stepped Ru surfaces or on Ni particles. For example, the B5 site indicated by (I) in Figure 1b which is a combination of 4-fold (4F) and 3-fold (3F(A)) sites is similar to the one found on Ru steps or along the (113) plane on Ni particles.1,13 The other B5 site found along the (100) plane on the Ni particles is shown by (II) in Figure 1b and is the combination of 3F(E) and 3F(F) sites on the Ru(112j1) surface (Figure 1). In the present work, we denote the coordination of N atoms using (a,b) notation, where a and b correspond to the coordination of the N1 and N2 atoms, rescpectively, of the N2 molecule in the initial, transition, and final states. In the present work, we discuss two dissociation paths (path I and path II) where the N2 molecule is in a preactivated state on the Ru(112j1) surface. We would like to mention that the difference in the bond length of N-N and Ru-N between the USPP-PW91 and PAW-PBE calculations is ( 0.01 Å. The adsorption energies of N2 also increase by 5-10 kJ/mol when the calculations are changed from USPP-PW91 to PAW-PBE. From now onward, we will only mention the energies and the geometrical parameters obtained from PAW-PBE calculations instead of USPP-PW91, unless mentioned explicitly. The first reaction path (path I) (Figures 2 and 3) is when the N2 molecule is in a 4-fold coordination as shown in Figure 2a. This state is obtained when the N2 molecule is placed in the 4-fold (4F) or 3-fold (3F(A)) hollow sites and optimized, as discussed in the above procedure. The adsorption energy of the
J. Phys. Chem. C, Vol. 112, No. 46, 2008 17769 N2 in this state is -57 kJ/mol (Figure 3). This kind of 4-fold coordination of the N2 molecule has also been observed on the double step sites of Ru(109) by Morgan et al.24 They suggested this kind of 4-fold coordinated state to be involved in the activated dissociation of N2 on the Ru(109) double stepped surface. The N-N bond length in this adsorbed state is 1.25 Å. Both the N atoms (N1 and N2) are attached to the bridge sites (B(A) and B(C)) in a (2, 2) coordination as shown in Figure 2a. The average Ru-N distances are 2.1 Å. The final state is the one in which the two N atoms are stable in the 3-fold hollow site (3, 3) as shown in Figure 2c. This state is about 39 kJ/mol more stable than the adsorbed state (Figure 3). This shows that the N2 dissociation along this path is an exothermic process. The 2N atoms in the final state are separated by about 4.0 Å. The calculated stretching frequencies of the Ru-N bond in the final state are 74 (596 cm-1) meV and 69 (556 cm-1) meV. These frequencies are in good agreement with the frequencies reported by Jacobi et al. for 3-fold N adsorption on the Ru(112j1) surface.16 In the transition state, one of the N atoms (N1) is located in the 2-fold hollow site, and the other N atom (N2) is attached to the bridge site with a (2, 3) configuration (Figure 2b). This confirms that there are five Ru atoms (B5 site) involved in the transition state. The N-N bond length in the transition state is 1.82 Å. This transition state observed on the Ru(112j1) surface is similar to the one found on the Ru step sites by Dahl et al.1 The apparent activation energies calculated from USPP-PW91 and PAW-PBE are 9 and 19 kJ/mol (Figure 3), respectively. However, the barrier with respect to the adsorbed state is 75 and 76 kJ/mol using USPP-PW91 and PAW-PBE, respectively (Figure 3). The next dissociation path (path II (Figures 3 and 4)) is the one in which the N2 molecule is adsorbed in the 3F(F) hollow site (Figure 4a). The adsorption energy of N2 in this configuration is -70 kJ/mol (Figure 3). The N-N bond length in this adsorbed state is 1.25 Å. One of the N atoms (N1) is adsorbed in the 3F(F) site, and the other N (N2) is attached to the top (T(A)) site with a (3, 1) coordination (Figure 4a). In the final state, N1 is in the 3F(F) site, and the other N2 attached to the T(A) (3, 1) site moves to the 3F(B) site, i.e., to (3, 3) configuration (Figure 4d). The final state is 29 kJ/mol more stable than the adsorbed state and hence indicates an exothermic reaction path as the previous reaction path. The Ru-N stretching frequencies are 72 meV (580 cm- 1) and 69 meV (556 cm- 1). One can notice that the frequencies obtained for this final state and the final state discussed earlier are almost the same which justifies the similar adsorption energies between these two final states (Figure 3). The direct dissociation of the N2 from the 3F(F) state to the 3F(F) + 3F(B) (3, 3) state gives several imaginary frequencies indicating a second-order saddle point. So, we choose a different path in which the two N atoms (N1 and N2) in the final states are located in the 3F(F) and T(A) site (3, 1) (Figure 4c). The two N atoms in this state are 3.0 Å apart. We identify this state as a prefinal state (3, 1) to differentiate from the more stable final state (3, 3) (Figure 4d). This state is 37 kJ/mol less stable than the adsorbed state. The dissociation along this path gives an apparent activation barrier of about 35 kJ/mol and with respect to the adsorbed state 105 kJ/mol (Figure 4). In the transition state, the two N atoms, i.e., N1 and N2, are attached to the 3F(F) and the T(A) sites, respectively, in a (3, 1) coordination as shown in Figure 4c. The N-N bond length in the transition state corresponds to 2.0 Å. In the more stable final state, the N atom attached to the T(A) site moves into the 3F(B) site. The N-N distance in the final state is 3.0 Å. We can see that, although the apparent
17770 J. Phys. Chem. C, Vol. 112, No. 46, 2008
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Figure 2. (a), (b), and (c) correspond to the initial, transition, and final state structures, respectively, of N2 dissociation along path (I) (blue line in Figure 3). The two N atoms are indicated by N1 and N2. Blue, green, yellow, and gray spheres correspond to the first-, second-, third-, and fourth-layer Ru atoms of the Ru(112j1) surface, respectively. The Ru atoms coordinated to the N atoms are shown with black spots and lines.
Figure 4. (a), (b), (c), and (d) correspond to the initial, transition, prefinal state, and the most stable final state of N2 dissociation, respectively, along the reaction path (II) (red line in Figure 3). The two N atoms are indicated by N1 and N2. Blue, green, yellow, and gray spheres correspond to the first-, second-, third-, and fourth-layer Ru atoms of the Ru(112j1) surface, respectively. The Ru atoms coordinated to the N atoms are shown with black spots and black lines.
Figure 3. Schematic diagram of the reaction path of the N2 dissociation along reaction path I (blue line) and reaction path II (red line) on the Ru(112j1) surface. The energies are with respect to the gas-phase N2 molecule and are given in kJ/mol. The values in the parentheses are from USPP-PW91 calculations.
activation energy changes by 10 kJ/mol using different levels of calculations, the activation barrier with respect to the adsorbed state does not change much. From the above discussion, we can deduce that reaction path I is more favorable for N2 dissociation than path II. This is due to the stability of the N2 molecule in the transition state of path I where it is situated in a B5 site or a (3, 2) coordination. Contrary to this, in the transition state of reaction path II the N2 molecule is located in a B4 site, i.e., (3, 1) coordination. We recall from our recent results that the reaction path for the CO dissociation is completely different from that of N2 on the Ru(112j1) surface.14 In the case of CO dissociation, the transition state has C and O in their most stable state and hence very close to the final state configuration. This was one of the significant factors for a low-barrier CO dissociation on the Ru(112j1) surface.14 We can see that although CO and N2 are isoelectronic they have different configurations in the adsorbed, transition, and final states leading to different reaction paths on the Ru(112j1) surface. Interestingly, the maximum number of
atoms in the final state for CO and N2 is six, i.e., C and O having four and two coordination, respectively, and N2 with (3, 3). We believe that this coordination in the final state depends on the valence states of C, O, and N. In analogy to CO dissociation on the Ru(112j1) surface, one can infer that the N2 molecule would have still a low dissociation barrier if it is involved in a 6-fold coordination in the transition state, where both the N atoms are located in 3-fold sites (3, 3) without sharing the Ru atoms. This configuration would be very close to the final state (two N atoms in 3-fold sites) and result in a low dissociation barrier for N2. Despite several 3-fold sites on the Ru(112j1) surface, we could not find such a reaction path. In conclusion, we have explored two reaction paths for the N2 dissociation on the Ru(112j1) surface. Our interpretation reveals that the N2 apparent activation barrier is decreased by 16 kJ/mol when it is involved in a 5-fold coordination compared to a 4-fold coordination in the transition state. We propose that due to the active B5 sites the ammonia synthesis can be efficiently carried out on the single-crystal open Ru surface as it is on the stepped Ru surface. We also show that the activation energy with respect to the adsorbed state is independent of the level of functional used. Moreover, in conjunction with our recent results, we prove that, although N2 and CO are isoelectronic, they have different reaction paths on the Ru(112j1) surface. Acknowledgment. We would like to thank National Computing Facilities (NCF) for providing computational resources (grant: SH-067-07).
Letters References and Notes (1) Dahl, S.; Logadottir, A.; Egeberg, R. C.; Larsen, J. H.; Chorkendorff, I.; To¨rnqvist, E.; Norskov, J. K. Phys. ReV. Lett. 1999, 83, 1814. (2) Honkala, K.; Hellman, A.; Remediakis, I. N.; Logadottir, A.; Carlsson, A.; Dahl, S.; Christensen, C. H.; Norskov, J. K. Science 2005, 307, 555. (3) Logadottir, A.; Norskov, J. K. J. Catal. 2003, 220, 273. (4) Hellman, A.; Baerends, E. J.; Biczysko, M.; Bligaard, T.; Christensen, C. H.; Clary, D. C.; Dahl, S.; van Harrevelt, R.; Honkala, K.; Jonsson, H.; Kroes, G. J.; Luppi, M.; Manthe, U.; Norskov, J. K.; Olsen, R. A.; Rossmeisl, J.; Skulason, E.; Tautermann, C. S.; Varandas, A. J. C.; Vincent, J. K. J. Phys. Chem B 2006, 110, 17719. (5) Kim, Y. K.; Morgan, G. A.; Yates, J. T. Surf. Sci. 2005, 598, 14. (6) Ciobica, I. M.; van Santen, R. A. J. Phys. Chem. B 2003, 107, 3808. (7) Zubkov, T.; Morgan, G. A., Jr.; Yates, J. T., Jr.; Ku¨hlert, O.; Lisowski, M.; Schillinger, R.; Fick, D.; Ja¨nsch, H. J. Surf. Sci. 2003, 57, 526. (8) Ge, Q.; Neurock, M. J. Phys. Chem. C 2006, 110, 15368. (9) Zambelli, T.; Wintterlin, J.; Trost, J.; Ertl, G. Science 1996, 273, 1688. (10) Hammer, B. Phys. ReV. Lett. 1999, 83, 3681. (11) Hammer, B. Surf. Sci. 2000, 459, 323. (12) Loffreda, D.; Simon, D.; Sautet, P. J. Catal. 2003, 213, 211.
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JP8085478
