Mechanism of N2O Formation During NO Reduction on the Au(111

Key Laboratory of Colloid and Interface Chemistry, Ministry of Education, Institute of Theoretical Chemistry, Shandong University, Jinan, 250100, P. R...
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J. Phys. Chem. C 2010, 114, 2711–2716

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Mechanism of N2O Formation During NO Reduction on the Au(111) Surface Yingying Wang,† Dongju Zhang,*,† Zhangyu Yu,†,‡ and Chengbu Liu*,† Key Laboratory of Colloid and Interface Chemistry, Ministry of Education, Institute of Theoretical Chemistry, Shandong UniVersity, Jinan, 250100, P. R. China, and Heze UniVersity, Heze 274015, Shangdong, P. R. China ReceiVed: October 29, 2009; ReVised Manuscript ReceiVed: December 28, 2009

Density functional theory calculations have been performed to elucidate the mechanism of N2O formation over the Au(111) surface during NO reduction. It is shown that the dissociation of NO into an N atom and an O atom involves a barrier as high as 3.9 eV, implying that the formation of N2O does not occur via the direct dissociation mechanism of NO. Alternatively, we find that the reaction may occur via a dimer mechanism; i.e., two NO molecules initially associate into a dimeric (NO)2, which then dissociates into a N2O molecule and a N atom. We have scanned the potential energy surface forming N2O along different pathways, which involve a trapezoid OadNNOad dimer, an inverted trapezoid ONadNadO dimer, a zigzag ONadNOad dimer, or a rhombus ONadOadN dimer. The trapezoid dimer, OadNNOad, is found to be a necessary intermediate for the formation of N2O, and the calculated barrier for the rate-determining step along the energetically most favorable pathway is only 0.34 eV. The present results rationalize the early experimental findings well and enrich our understanding of the reduction of NO on the Au surface. 1. Introduction The NO reduction on supported noble metal catalysts has attracted remarkable attention largely because of its importance in many applied fields of research, such as surface science1,2 and environmental catalysis.3-5 To better understand this important process, a number of experimental works have studied the structure and reactivity of NO on various metal surfaces.1,3-5 It is observed that N2O is an intermediate product during the NO reduction.1,3-5 Two possible mechanisms for its formation have been proposed in the literature,6-12 which are referred to asthedirectdissociationmechanism6-8 andthedimermechanism9-12 in the present work, respectively. In the former, a NO molecule first dissociates into N and O atoms, and then another NO molecule associates the N atom to give a N2O molecule. In the latter, two NO molecules initially associate into a dimer molecule (NO)2, which then dissociate into a N2O molecule and a N atom. However, up to now, the validity of these two mechanisms is still unknown.6-12 The experiments by So6,7 and Hayden8 on the adsorption of NO on Ag(111) and Pt(111) supported the direct dissociation mechanism since adsorbed N and O atoms were observed, and no dimeric (NO)2 was identified on the surface. In contrast, Brundle,9,10 Ludviksson,11 and Gland12 believed that N2O formation on metal surfaces follows the dimeric mechanism because they did not find adsorbed N atoms on Ag(111) and Pt(111) surfaces. These contradictory results indicate that the mechanistic aspect of catalytic NO reduction on the noble metal surface is still not understood well, although the practical chemistry of catalytic NO reduction has been extensively researched.13-16 In recent years, supported gold nanoparticles17-21 and singlecrystal gold surfaces22-24 like other noble metal catalysts such as Ru, Rh, and Pt have been discovered to exhibit high catalytic activity for selective NO reduction. For example, using pulsed field desorption mass spectrometry (PFDMS), Chau et al.22 * To whom correspondence should be addressed. E-mail: zhangdj@ sdu.edu.cn (D.Z.); [email protected] (C.L.). † Shandong University. ‡ Heze University.

claimed that N2O was observed when NO adsorbed on Au(111) at 300 K, and using X-ray photoelectron spectroscopy (XPS) and temperature programmed desorption (TPD) techniques, Vinod et al.23 found that NO can be decomposed on the Au(310) surface at temperatures as low as 80 K. These experimental studies have provided useful information for understanding NO reduction catalyzed by Au-based catalyst, but the knowledge about the detailed reaction mechanism is only indirect. In this paper, we present a comprehensive theoretical study of the N2O formation mechanism during NO reduction on the Au(111) surface. By performing density functional theory (DFT) calculations, we show the potential energy surface details of N2O formation according to the two possible mechanisms. It is found that the dimeric mechanism is energetically much more favorable than the direct dissociation mechanism. 2. Computational Details Our calculations are based on the DFT-slab approach25,26 at the GGA-PBE27,28 approximation level, as implemented in the CASTEP code.29 Ionic cores are described by ultrasoft pseudopotentials,30 and the Kohn-Sham one-electron states are expanded in a plane wave basis set up to a cutoff energy of 340 eV. Monkhorst-Pack meshes with 3 × 3 × 1 k-grid sampling in the surface Brillouin zone are used. The criteria for energy and maximum force convergence used are 2.0 × 10-5 eV/atom and 0.05 eV/Å. With this setting we get the lattice constant of bulk Au as 4.046 Å, which agrees well with the experimental finding, 4.078 Å.31 The Au(111) surface is modeled using a periodic three-layer slab with a p(2 × 2) unit cell. The vacuum separation between periodically repeated slabs is 10 Å, which is large enough to avoid interactions between slabs. In the slab calculations, the top layer is relaxed, whereas the other two layers are fixed at their bulk-truncated structure. The transition states (TSs) are searched for with the linear and quadratic synchronous transit (LST/QST) complete search.32 To support our choice of setting, we provide benchmark calculations of bond lengths (R) and angles (∠) for NO, N2O, and (NO)2. As shown in Table 1, all calculated data are in fairly

10.1021/jp9103596  2010 American Chemical Society Published on Web 01/26/2010

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TABLE 1: Calculated and Experimental Geometrical Parametersa for NO, N2O, and cis-(NO)2 R(N-O)b NO N2 O cis-(NO)2

R(N-N)b

∠ONN

1.180 (1.129)d 1.963 (2.18)e

103.8 (101)e

c

1.189 (1.151) 1.215 (1.188)d 1.199 (1.12)e

a Bond lengths are in angstroms and angles are in degree. b The values in parentheses are experimental data. c Ref 33. d Ref 34. e Ref 35.

good agreement with the corresponding experimental results,33-35 indicating the acceptable accuracy and reliability of the computational method employed. It should be noted that our main concern in the present work is the accuracy of the relative energies for several adsorption and dissociation processes considered. It is well-known that the energy errors for the initial state and final state of a chemical process are almost the same and can counteract each other. Thus, we consider that the conclusion to be drawn in the present work based on the calculated relative energies is reliable. In addition, the DFT method used in the present work has been proven by Liu et al.36 to be accurate enough for the calculations of reaction barrier in heterogeneous Au catalysis. 3. Results and Discussion 3.1. NO Adsorption over the Au(111) Surface. NO adsorption over the surfaces of noble metal including Au has been investigatedexperimentallyandtheoreticallyinearlyliterature.24,37-43 It is well established that NO could adsorb and react on these noble metal surfaces, where NO either donates its 2π* electron to the surface or accepts electron density from the surface into its half-filled 2π* orbital. We note that in all previous theoretical reports, it is always proposed that NO adsorption occurs with the N atom close to the surface, although no direct evidence supports such a supposition.24,37,40-43 The initial adsorption manner of a molecule on a metal surface is expected to affect the following surface reaction. To better understand the reactivity of NO on the Au(111) surface and provide evidence for the validity of previous supposition, we here reexamine the NO adsorption behavior on the Au(111) surface by considering three possible adsorption manners, the N atom close to the surface, the O atom close to the surface, and both the N and O atoms close the surface, and the resulting geometries will be referred to as Nad-, Oad-, and NadOad-geometries, respectively. Four possible adsorption sites, including top, bridge, fcc, and hcp sites, are illustrated in Figure 1. The adsorption energy of NO on the Au(111) surface in this study is calculated as Ead ) ENO + Esurf - Etotal, where ENO, Esurf, and Etotal refer to the energies of a isolated NO molecule, the clean surface, and the adsorbed system, respectively. The optimized geometries and calculated Ead are summarized in Figure S1 and Table S1 in the Supporting Information. We find that, although the NO molecule is initially set with lying horizontally on the surface, the optimization calculations always lead the system to the Nadgeometry. Thus NadOad-geometry is believed to be unstable from present calculations. It is noted that in all situations the NO molecular axis is tilted with respect to the surface normal, and the most stable adsorptions for both Nad- and Oad-geometries occur at the top site. This can be understood by analyzing the frontier orbital interaction between the NO molecule and the Au(111) surface. As shown in panel III of Figure S1 (Supporting Information), in a tilted configuration, the interaction between the dz2 orbital of the Au atom and the 2π* orbital of NO is favored by symmetry, and the largest overlay occurs as the N

Figure 1. Top view of the p(2 × 2) surface unit cell of the Au(111) surface with possible adsorption sites of NO. For clarity, only two layers are illustrated. The yellow balls and gray balls denote the top-layer and the second-layer Au atoms, respectively.

or O atom approaching the top site. From Table S1 (Supporting Information), it is clear that the Nad-geometries are energetically more favorable than the Oad-geometries. In addition, NO binds weakly to the Au(111) surface with the N atom close to the top site of the surface; i.e., the covalent bonding between the NO and Au(111) surface is absent. This can be understood according to the model of Hammer,44 who suggested that in an adsorption process the valence states of small molecules and electronegative atoms (e.g., N and O atoms) covalently interact with the localized metal d states. The d band of Au is nearly fully occupied, and so its covalent interaction with small molecules, such as NO in the present work, is very weak. 3.2. Direct Dissociation Mechanism for N2O Formation. It is well-known that the dissociation of molecules on the metal surface is the first step in many catalytic processes. During the catalytic NO reduction on metal surfaces, the formation of N2O via the direct dissociation mechanism is considered as the most straightforward pathway.45 As such, we first investigate the possibility of the direct dissociation mechanism. The calculated potential energy surface (PES) profiles along the reaction coordinate with the optimized intermediates and transition states are shown in Figure 2, where the sum of the energies of the isolated NO molecule and clean Au surface is taken as zero energy. Starting from the Nad- and Oad-geometries, two possible pathways (a) and (b) for NO dissociation have been modeled. The separation of N and O atoms goes through two structurally very similar late transition states TSa and TSb, which converge to the final state where the N and O atoms coadsorbed onto the surface. The final states are more unstable than the initial states by 3.03 and 2.91 eV, respectively. This instability is mainly due to the low adsorption energy of the N atom on the Au(111) surface. As shown in Table S1 (Supporting Information), the binding energy of a isolated N atom on Au(111) is much lower than that of a free O atom. The analysis of the projected density of states (PDOS) also supports this proposal. Figure 3 compares the PDOS of the N atom adsorbed on the fcc site of the Au(111) surface with that of the O atom. The highest PDOS peak of the N 2p orbital is nearer to the Fermi energy level than that of the O atom, indicating the N atom adsorbed on the surface is less stable than the corresponding O atom. Our calculated barriers for the NO dissociation on Au(111) are 3.95 and 3.87 eV along the two pathways, respectively. These extremely high barriers and the strong endothermicity of the reaction indicate that the NO dissociation over the Au(111) surface is both kinetically and thermodynamically very

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Figure 2. Potential energy surface (PES) profiles with the optimized geometries of intermediates (side views), transition states (top views), and product (top views) for the NO dissociation. Distances are in angstroms and the relative energies are in electronvolts.

Figure 3. Calculated p-projected density of states onto N 2p and O 2p orbitals for the systems where the N and O atoms are adsorbed at the fcc site of the Au(111) surface, respectively. The Fermi level (Ef) has been set to be zero.

unfavorable. We thus rule out the possibility of direct NO dissociation on Au(111) and hence the direct dissociation mechanism for N2O formation during NO reduction on the Au(111) surface. 3.3. Dimeric Mechanism for N2O Formation. Figure 4 shows optimized minima (I-VIII) and transition states (TS1-TS9) according to the dimeric mechanism. The relevant potential energy surface profiles along the reaction coordinate are depicted in Figure 5, where the sum of the energies of the isolated reactants (2NO and the clean Au surface) is taken as zero energy. It should be noted that the gas-phase (NO)2 dimer was first structurally characterized in 1970 by Ewing et al.46 using infrared spectroscopy. Recently, demeric (NO)2 was also found on the Au(111) surface during NO reduction.22 Here, we have considered three possible situations for forming the dimer on the Au(111) surface, where two NO molecules are preadsorbed with two O atoms, two N atoms, or the O atom of a molecule and the N atom of the other molecule close to the surface, as indicated by structures I, IV, and VI in Figure 4. Starting from these initial structures, we have searched detailed mechanisms for forming N2O.

3.3.1. N2O Formation Wia the OadNNOad Dimer. As can be seen in Figure 5, the path starting from structure I, where two NO molecules are preadsorbed with two O atoms close to the surface, is referred to as path I, which involves two elementary steps: the formation of a trapezoid dimer OadNNOad and the sequent rupture of the N-Oad bond to form a N2O molecule and an Oad atom. Along this path, structure I, which is energetically more stable by 0.46 eV than the separated reactants, evolves into structure II, where the trapezoid OadNNOad dimer has been formed, via TS1 with a minimal barrier of 0.11 eV. Structure II is found to lie below the reaction entrance by 0.38 eV. As shown in Figure 4, the N-N distance in structure II shortens to 1.471 Å, which is much shorter than that in our calculated isolated trapezoid dimer (1.963 Å). This is attributed to the effective electron transfer from the surface to the dimer, resulting in accumulation of net charge on adsorbed (NO)2. The extra electron added into (NO)2 enters into the N-N π bond of the dimer, which modifies the structure of the dimer dramatically; namely, the N-N distance is shortened to 1.471 Å. Along the reaction coordinate, structure II is converted into structure III, where a N2O molecule has been formed and a remaining O atom is in a nearby fcc site of the Au (111) surface, via TS2, where the N-Oad bond (2.183 Å) is almost ruptured. From Figure 5, we can see that this step is the rate-determining step in path I, and the corresponding barrier is found to be only 0.34 eV, indicating this process is energetically very favorable. The overall reaction is calculated to be exothermic by 1.06 eV. 3.3.2. N2O Formation Wia the ONadNadO Species. Path II involves an inverted trapezoid dimer ONadNadO, which starts from structure IV with two N atoms close to the surface. Structure IV is calculated to be more stable by 0.91 eV. Along the reaction coordinate, two NO monomers in structure IV approach each other to result in the formation of the inverted trapezoid dimer ONadNadO, as indicated by structure V in Figure 4. Structures IV and V are connected by transition state TS3, where the forming Nad-Nad bond distance is 2.041 Å. The barrier from structure IV to TS3 is 0.34 eV. Structure V is energetically more stable by 0.98 eV than the separated reactants. After structure V, pathway II splits into two branches, IIa and IIb. Branch IIa is a straightforward pathway for forming N2O via the Nad-O bond breaking. However, our calculations show that this process involves a barrier as high as 1.59 eV. Branch IIb involves a transition from the inverted trapezoid

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Figure 4. Optimized geometries for the intermediates and the transition states involved during N2O formation along different paths. Distances and angles are in angstroms and degrees, respectively.

dimer ONadNadO to the trapezoid OadNNOad. Thus, path IIb crosses into path I via TS5. From the calculated PES profile shown in Figure 5, we conjecture that the contribution to the observed N2O formation from the inverted trapezoid dimer ONadNadO is expected to be small since the barrier involved in the rate-determining step along path IIb is 0.80 eV, which is much higher than that in path I, 0.34 eV. 3.3.3. N2O Formation Wia OadNNadO and ONadOadN Species. We now discuss the third possible situation for N2O formation, i.e., path III, which starts from structure VI where the O atom of a NO molecule and the N atom of another NO molecule initially interact with the surface Au atoms. Structure VI lies below the reaction entrance by 0.20 eV, where the NadO molecule more effectively interacts with the surface than the NOad molecule and the two molecules are away from each other. Along the reaction coordinate, structure VI can evolve into two different intermediates, structures VII and VIII; i.e., path III splits into two branches, IIIa and IIIb. In structures VII and VIII,

Figure 5. Calculated potential energy surface profiles for forming N2O along different reaction paths.

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Figure 6. Calculated d-projected density of states (d-PDOS) of the Au atoms in NadO/Au(111) and OadNNOad/Au(111) systems. The Au atoms are those that bond with NadO or OadNNOad. The Fermi level (Ef) has been set to be zero.

the zigzag dimer ONadNOad and the rhombus dimer ONadOadN have been formed, via transition states TS6 and TS8, respectively. These two processes have small barriers (0.17 and 0.29 eV), indicating both the zigzag and rhombus dimers easily form and thus are possible isomers of (NO)2 dimers over Au(111). In particular, structure VII with the zigzag dimer is found to be the globe minimum on the potential energy surface from the present calculations, which is more stable by 1.07 eV than the reactants. However, for the sequent N-O bond rupture steps along branches IIIa and IIIb, where structure VII and VIII convert into structure III via transition states TS7 and TS9, the calculated barriers are 1.34 and 0.72 eV, which are larger by 1.00 and 0.38 eV than those of the rate-determining steps in paths I and IIb, respectively, indicating these two branches are also energetically unfavorable for N2O formation. It should be noted that the surface seems to be poisoned after some time due to the high stability of structure VII and to the large energy barrier calculated to go from VII to III via TS7. However, there is no experimental evidence to support the surface poison proposition.22 Thus, we conjecture the structure VII would be converted into other minimum such as structures II and V, from which the observed N2O is easily formed. It is clearly seen from the above discussions that N2O formation on the Au(111) surface intrinsically favors the dimer mechanism rather than the direct dissociation mechanism. In other words, dimer (NO)2 is a necessary intermediate for N2O formation on the Au(111) surface, which is more active toward dimeric (NO)2 than toward monomer NO. The formation of dimer (NO)2 over the Au(111) surface is a thermodynamically favorable process, whose driving force comes from the cancellation of the spin density on 2π* orbitals of two NO molecules and the effective electron transfer from the surface to the dimer, which greatly stabilizes the system by the extra electrons on the dimer entering into the N-N bonding orbital. To better understand the different reactivities of Au(111) toward the NO monomer and (NO)2 dimer, we here compare the d-states’ projected density of states (d-PDOS) of Au atoms for NadO/ Au(111) and OadNNOad/Au(111) systems. It is known that the position of the d band center of surface metal atoms for a molecule-surface system has been widely used as a reactivity measure for metals, and it has successfully explained many experimental findings.47 On the basis of the Newns model,48

Density functional theory is employed to clarify whether N2O formation during NO reduction over the Au(111) surface proceeds according to the direct dissociation mechanism or the dimer mechanism. Our calculations show that the direct dissociation mechanism is impossible due to the extremely high barriers for NadO or NOad dissociating into Nad and Oad atoms. In contrast, the present theoretical results support the dimer mechanism, where two NO molecules initially associate into a dimeric (NO)2 molecule, which then dissociates to form the observed N2O molecule by rupturing one of two N-O bonds in the dimer. It is found that the trapezoid dimer OadNNOad is a necessary intermediate for the formation of N2O, and the calculated barrier for the rate-determining step along the energetically most favorable pathway is only 0.34 eV. From the present calculations, we suggest that contribution to the observed N2O formation from the inverted trapezoid dimer ONadNadO is small since the transition from ONadNadO to OadNNOad involves a barrier of 0.80 eV. Acknowledgment. This work was supported by the National Natural Science Foundation of China (Nos. 20873076 and 20873075). Supporting Information Available: Figure S1 and Table S1. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Meunier, F. C.; Breen, J. P.; Zuzaniuk, V.; Olsson, M.; Ross, J. R. H. J. Catal. 1999, 187, 493. (2) Taniike, T.; Tada, M.; Coquet, R.; Morikawa, Y.; Sasaki, T.; Iwasawa, Y. Chem. Phys. Lett. 2007, 443, 66. (3) Burch, R.; Breen, J. P.; Meunier, F. C. Appl. Catal., B 2002, 39, 283. (4) Bogdanchikova, N.; Menuier, F. C.; Avalos-Borja, M.; Breen, J. P.; Pestryakov, A. Appl. Catal., B 2002, 36, 287. (5) Gluhoi, A. C.; Lin, S. D.; Nieuwenhuys, B. E. Catal. Today 2004, 90, 175. (6) So, S. K.; Franchy, R.; Ho, W. J. Chem. Phys. 1989, 91, 5701. (7) So, S. K.; Franchy, R.; Ho, W. J. Chem. Phys. 1991, 95, 1385. (8) Hayden, B. E. Surf. Sci. 1983, 131, 419. (9) Nelin, C. J.; Bagus, P. S.; Behm, R. J.; Brundle, C. R. Chem. Phys. Lett. 1984, 105, 58. (10) Behm, R. J.; Brundle, C. R. J. Vac. Sci. Technol. A 1984, 2, 1040. (11) Ludviksson, A.; Huang, C.; Jansch, H. J.; Martin, R. M. Surf. Sci. 1993, 284, 328. (12) Gland, J. L.; Sexton, B. A. Surf. Sci. 1980, 94, 355. (13) Okumura, K.; Motohiro, T.; Sakamoto, Y.; Shinjoh, H. Surf. Sci. 2009, 603, 2544. (14) Miller, D. D.; Chuang, S. S. C. J. Phys. Chem. C 2009, 113, 14963. (15) Kumar, A.; Medhekar, V.; Harold, M. P.; Balakotaiah, V. Appl. Catal., B 2009, 90, 642. (16) Hu, Y. H.; Griffiths, K.; Norton, P. R. Surf. Sci. 2009, 603, 1740. (17) Salama, T. M.; Ohnishi, R.; Shido, T.; Ichikawa, M. J. Catal. 1996, 162, 169. (18) Ueta, A.; Oshima, T.; Haruta, M. Appl. Catal., B 1997, 12, 81. (19) Dekkers, M. A. P.; Lippits, M. J.; Nieuwenhuys, B. E. Catal. Today 1999, 54, 381. (20) Debeila, M. A.; Coville, N. J.; Scurrell, M. S.; Hearne, J. R.; Witcomb, M. J. J. Phys. Chem. B 2004, 108, 18254. (21) Ilieva, L.; Pantaleo, G.; Ivanov, I.; Nedyalkova, R.; Venezia, A. M.; Andreeva, D. Catal. Today 2008, 139, 168. (22) Chau, T. D.; Bocarme, T. V.; Kruse, N. Catal. Lett. 2004, 98, 85.

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