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Cooperative Adsorption of O2 and C2H4 on Small Gold Clusters Andrey Lyalin*,† and Tetsuya Taketsugu DiVision of Chemistry, Graduate School of Science, Hokkaido UniVersity, Sapporo 060-0810, Japan ReceiVed: April 14, 2009; ReVised Manuscript ReceiVed: May 29, 2009
The adsorption of O2 and C2H4 molecules on small gold clusters consisting of up to 10 atoms has been investigated using theoretical methods based on density-functional theory. It is shown that, in addition to a conventional mechanism of the catalytic activation of O2 adsorbed on a gold cluster, the interaction of C2H4 with small gold clusters results in considerable weakening of the carbon-carbon double bond. Moreover, coadsorption of O2 and C2H4 on small gold clusters with an odd number of atoms leads to a cooperative effect which further stabilizes the O2-AuN-C2H4 system. Hence, simultaneous adsorption of the O2 and C2H4 molecules on free gold clusters can considerably promote the oxidation process via the Langmuir-Hinshelwood mechanism. The unique catalytic activity of gold nanoparticles supported on metal oxides in the process of CO oxidation was discovered experimentally more than 20 years ago.1 Since then, a large body of experimental and theoretical works have been devoted to catalytic processes involving gold clusters. A comprehensive survey of the field can be found in review papers; see, e.g., refs 2-5. It is well-known that gold, in its bulk form, does not possess any catalytic properties. However, gold at the nanoscale manifests extraordinary catalytic activity which increases with a decrease in the cluster size of up to 1-5 nm.6-8 Moreover, recent studies demonstrate that catalytic activity of gold clusters adsorbed on an iron oxide support correlates with the presence of very small clusters of ∼10 atoms.9 The origin of such sizedependent catalytic activity of gold remains highly debated and yet to be fully understood. It was demonstrated both theoretically and experimentally that the catalytic activity of gold clusters may depend upon the relative number of low-coordinated atoms; see, e.g., refs 5, 10, and 11. However, no correlation between the geometrical structures of AuN- anion clusters and activities toward O2 adsorption was found in ref 12. The remarkable catalytic behavior of small gold clusters might arise from strong electronic interaction between the gold nanoparticle and the support material, the environment or special additives; see, e.g., ref 3. However, recent experiments indicate that small gold clusters supported on inert materials are an efficient catalyst for oxidation of alkenes by dioxygen.8 Hence, even free gold clusters can be effective catalysts. Currently, a large variety of catalytic reactions on gold clusters has been studied experimentally. This includes the processes of catalytic oxidation of carbon monoxide, as well as more complex reactions such as alcohol oxidation, the direct synthesis of hydrogen peroxide, and alkene epoxidation, a reaction in which an oxygen atom is inserted into the * To whom correspondence should be addressed. E-mail: lyalin@ mail.sci.hokudai.ac.jp. † On leave from: Institute of Physics, St Petersburg State University, 198504 St Petersburg, Petrodvorez, Russia.
10.1021/jp903423j CCC: $40.75
carbon-carbon double bond of the alkene.7,8,13,14 These processes, and in particular the selective epoxidation of alkenes, are crucial in a wide range of industrial applications.15 However, theoretical studies of the gold nanocatalysis have mainly focused on the investigation of adsorption and reaction of O2 and CO. Nonetheless, the theoretical study of catalytic oxidation of alkenes on gold clusters has been relatively unexplored despite the tremendous importance of these processes in practical applications. In this Letter, we present results of a theoretical investigation of the adsorption of O2 and C2H4 on free neutral gold clusters consisting of up to 10 atoms. The choice of C2H4 is stipulated by the fact that ethylene is the simplest alkene containing an isolated carbon-carbon double bond. Hence, it can be treated as a simple model molecule to study the epoxidation process of different alkenes. We find that C2H4 readily adsorbs on small gold clusters, resulting in a weakening of the carbon-carbon double bond. This process concurs with the conventional activation of the adsorbed O2. Hence, activated dioxygen (O2) can readily attack the loosened double bond in C2H4, thereby oxidizing the ethylene molecule. We also demonstrate that small gold clusters can mediate the interaction between the O2 and C2H4 molecules, resulting in cooperative adsorption of dioxygen and ethylene. This effect can be particularly important for understanding the mechanism of catalytic oxidation of hydrocarbons by dioxygen. The calculations in this work are based on theoretical methods of density-functional theory (DFT). The hybrid Becke-type three-parameter exchange density functional paired with the gradient-corrected Perdew-Wang 91 correlation functional (B3PW91)16 is used. The standard LANL2DZ basis set of primitive Gaussians is used to expand the gold cluster orbitals formed by the 5s25p65d106s1 outer electrons of Au (19 electrons per atom). The remaining 60 core electrons of the Au atom are represented by the Hay-Wadt effective core potential accounting for relativistic effects.17 The molecular orbitals of O2 and C2H4 are treated with the use of the aug-cc-pVTZ basis set. The B3PW91 functional is selected among the B3LYP, PW91, 2009 American Chemical Society
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Figure 1. Molecular adsorption energy, Eads (upper row); O-O and C-C bond distances in adsorbed molecules, r (middle row); and charge transfer to the adsorbate, ∆q (lower row), calculated for O2 (left column) and C2H4 (right column) adsorbed on the neutral AuN clusters with 1 e N e 10. For the C2H4-Au5 cluster, both di-σ and π isomers are shown. Open stars in the right column correspond to the π mode of C2H4 adsorption on Au5. Optimized geometries of O2-AuN and C2H4-AuN clusters are shown in the upper left and upper right figures, respectively. Dashed lines in the middle left and middle right figures denote the equilibrium bond distances calculated for free O2 and C2H4, respectively.
PBE, and TPSS functionals on the basis of careful comparison of the obtained theoretical data on dissociation energies and bonding in Au2 (1.904 eV, 2.546 Å), O2 (5.406 eV, 1.199 Å), and C2H4 (7.596 eV, 1.324 Å) with those of earlier experimental studies, Au2 (2.30 eV, 2.472 Å),18 O2 (5.12 eV, 1.207 Å),18 and C2H4 (7.76 eV, 1.339 Å).19 Calculations have been carried out with the use of the Gaussian 03 software package.20 The structural properties of gold clusters have been a subject of numerous theoretical investigations; see, e.g., refs 2-4 for a review. In the present work, the cluster geometries have been determined with the use of the cluster fusion algorithm.21 We have successfully used a similar approach to find the optimized geometries of various types of atomic clusters.22-24 The optimized structures of the neutral gold clusters AuN with the number of atoms N e 10 are in good agreement with those reported in previous theoretical studies; see, e.g., refs 25-27. In order to obtain the most stable structures of O2-AuN, C2H4-AuN, and O2-AuN-C2H4 clusters, we have created a large number of starting geometries by adding O2 and C2H4 molecules in different positions on the surface of the most stable and up to seven isomer structures (including 3D structures) of the corresponding AuN, O2-AuN, and C2H4-AuN clusters. The starting structures have been optimized without any geometry constraints. We first investigated the independent adsorption of O2 and C2H4 molecules on neutral gold clusters AuN with 1 e N e 10.
Figure 1 shows the evolution of molecular adsorption energy, Eads; the O-O and C-C bond distances in adsorbed molecules, r; and the charge transfer to the adsorbate (obtained from the natural bond orbitals population analysis),28 ∆q, as a function of cluster size N. Adsorption of O2 on small gold clusters has been intensively investigated in previous works; see, e.g., refs 3, 25, and 29 and references therein. We found the ground state of O2-AuN clusters to be a doublet for odd N and a triplet in the case of even N. The optimized geometries of O2-AuN for 1 e N e 10 are shown in Figure 1a. The structures optimized for small O2-AuN clusters with 1 e N e 5 are in good agreement with those reported in refs 25 and 29. However, for clusters with 6 e N e 10, we found that the lowest-energy isomers optimized with the B3PW91 method differ from those obtained with the B3LYP (O2-Au6)25 and PBE (O2-Au6-10)29 methods. We found that O2 adsorbs in a bridge position for AuN clusters with an odd number of atomssN ) 3, 5, 7, and 9sand perpendicularly to the cluster plane for AuN clusters with an even number of atomssN ) 6, 8, and 10 (see Figure 1a). We also found that the dissociative adsorption of O2 on Au6 and Au7 clusters is not energetically favorable within the B3PW91 method, as it was reported for the PBE DFT functional.29 The difference in structure arises due to the fact that the PBE functional considerably overestimates the oxygen binding.3,30
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It is seen from Figure 1a that the adsorption energy of O2 on O2 , has an odd-even oscillatory behavior as a AuN clusters, Eads function of N with local maxima for an odd number of Au atomssN ) 3, 5, 7, and 9. For clusters with an even number O2 is small. Hence, O2 readily of N, the adsorption energy Eads adsorbs only on the neutral gold clusters with an odd number of Au atoms. The N-evolution of the bond distance, rO-O, in adsorbed O2 (Figure 1b), and charge transfer to the adsorbate, ∆qO2 (Figure 1c) show a very similar behavior. Parts b and c of Figure 1 demonstrate that, in the case of odd N, the O-O bond length is enlarged similar to the superoxide state (the calculated O-O bond length in O2- is 1.332 Å). Additionally, there is also considerable charge transfer from the gold cluster to the adsorbed O2. Thus, O2 adsorbed on small gold clusters with an odd number of atoms is catalytically activated. In contrast, the O-O bond length in O2 adsorbed on gold clusters with an even number of atoms remains unchanged. Similar odd-even variations in O2 adsorption have been reported earlier; see, e.g., refs 3, 25, and 29. The appearance of the odd-even oscillations in O2 , rO-O, and ∆qO2 is a result of the electronic shell effects in Eads the gold clusters, in accordance with the jellium model.4,31 Adsorption of O2 on small gold clusters with an odd number of atoms leads to the transfer of an unpaired valence electron from the gold cluster to the oxygen antibonding 2π* orbital, causing considerable weakening of the O-O bond. Note that the electron transfer from Au7 and Au9 clusters to the adsorbed O2 leads to a dramatic change in cluster structure (that resembles the structure of the positively charged Au7+ and Au9+) in comparison with that for corresponding free clusters. Such a rearrangement in cluster structure arises due to the interplay of electronic and geometry shell effects; see, e.g., ref 24. To the best of our knowledge, adsorption and oxidation of ethylene on small gold clusters have not been studied theoretically. The mechanism of C2H4 interaction with gold clusters is very different from that for O2-AuN interaction. Thus, the bonding of ethylene with gold involves electron transfer from the filled bonding π orbital of the C2H4 to the metal, alongside a back-donation from the d orbital of gold to the empty π* antibonding C2H4 orbital in accordance with the DewarChatt-Duncanson model.32 The processes of donation and backdonation of the electrons result in a weakening of the C-C bond in C2H4. Hence, adsorption of C2H4 on gold clusters leads to its activation which, in turn, promotes the oxidation process. The results of the geometry optimization for C2H4 adsorbed on AuN clusters are shown in Figure 1d. The spin states of the opimized C2H4-AuN structures are doublet and singlet for odd and even N, respectively. It is seen that ethylene adsorbs on small gold clusters preferably as π-bonded species where only one Au atom is involved in the adsorption of the molecule. An exception occurs in the case of a single Au atom which interacts with one carbon atom in C2H4, forming a σ bond, and the Au5 cluster, where the di-σ configuration (i.e., two Au atoms are involved in adsorption of the molecule via a σ bonding) is favorable energetically. However, the difference in adsorption energies of di-σ and π modes of C2H4 on the Au5 cluster is small, ∼0.05 eV, as is seen from Figure 1d. It is interesting to note that, in the case of C2H4 adsorption on the Au(111) surface, di-σ ) 0.6502 eV) is energetically favorable in the di-σ mode (Eads π ) 0.1545 eV).33 comparison to the π mode (Eads It is known that the type of C2H4 bonding depends upon the surface structure. It has been found that the π-bonded mode dominates in the adsorption on low-coordinated atoms and step sites.34 As the number of low-coordinated atoms decreases with an increase in cluster size, we suppose that the di-σ mode will
Letters become dominant. The transition from the π- to the di-σ mode of adsorption with an increase in cluster size can be observed experimentally, for example, by various methods of vibrational spectroscopy.35 Parts d-f of Figure 1 demonstrate that the N-evolution of C2 H 4 , the bond distance, rC-C, and the the adsorption energy, Eads C 2H4 charge transfer, ∆q , calculated for C2H4 adsorbed on small AuN clusters is considerably different from that of O2. Thus, there is no pronounced odd-even oscillations in the adsorption C2 H 4 (however, some weak oscillatory behavior in the energy Eads C 2H4 can be seen for 6 e N e 10). Hence, N-dependence of Eads the shell effects do not play a dominant role in the case of C2H4 adsorption. Note, however, that we found pronounced oddeven oscillations in the adsorption energy of di-σ bonded C2H4 on AuN clusters. Figure 1e shows that the C-C bond in the π-bonded C2H4 is enlarged up to 1.37-1.39 Å in comparison with the case of a free C2H4 (1.32 Å). The rC-C value for the π mode of ethylene adsorption depends slightly on the cluster size, evolving with C2H4 . Our calculations demonstrate that for the N similar to Eads di-σ-bonded ethylene in the C2H4-Au5 cluster the C-C bond length is 1.47 Å. Such an increase in the C-C bond length, as well as the noticeable change of the bending of the H atoms for di-σ-bonded ethylene, can be explained by the increasingly important role of the sp3 hybridization due to the electron donation from the gold cluster to the antibonding π* orbital of C2H4. Hence, the di-σ-bonded ethylene is activated more strongly in comparison with the π-bonded one; thus, it is more reactive toward oxidation. The total charge of the adsorbed C2H4 depends on the balance between the donation and back-donation processes (see Figure 1f) and evolves with N similar to rC-C. The excess of electrons in the gold cluster should promote the charge transfer to the antibonding π* orbital of C2H4; hence, it should stabilize the di-σ mode of adsorption. We have demonstrated that both O2 and C2H4 molecules can be activated as a result of independent adsorption on neutral gold clusters consisting of up to 10 atoms. Hence, simultaneous adsorption of O2 and C2H4 on small gold clusters can promote the oxidation process via the Langmuir-Hinshelwood mechanism. However, the initial adsorption of the first O2 (C2H4) molecule on a gold cluster can considerably change the cluster geometry and/or its charge state. Such an alteration in the cluster structure can be destructive for coadsorption of the second molecule. To elucidate the mechanism of possible destructive interaction of adsorbed molecules, we have studied simultaneous adsorption of O2 and C2H4 on neutral gold clusters AuN with N ) 1,..., 10. We found the spin states of O2-AuN-C2H4 to be doublet and triplet for an odd and even number of gold atoms N, respectively. Optimized structures of O2-AuN-C2H4 are presented in Figure 2. The curve with open dots in Figure 2 demonstrates the N-dependence of the sum of adsorption energies of O2 and C2H4 sum O2 C2H4 on AuN: Eads ) Eads + Eads . It can be suggested that values of sum Eads should give an upper limit for the energy of simultaneous O2 and C2H4 adsorption of two molecules on the gold cluster Εads ) Εtot(AuN) + Εtot(O2) + Εtot(C2H4) - Εtot(O2 - AuN - C2H4) (here, Etot(M) denotes the total energy of the most stable structure of the molecule (cluster) “M”). Indeed, the adsorption energy O2 and C2H4 Εads presented in Figure 2 by the curve with filled dots is sum lower or equal to Eads when N is even. As has been discussed above, the oxygen molecule bonds weakly to gold clusters with an even number of atoms. In that case, coadsorption of the ethylene molecule disturbs the system, further weakening the oxygengold bonding.
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O2 C2H4 Figure 2. Sum of adsorption energies of Eads and Eads as a function of cluster size N (open dots), the energy of simultaneous adsorption of O2 and C2H4 O2 and C2H4 on AuN, Εads (filled dots). Optimized geometries of O2-AuN-C2H4 clusters are shown for each N.
Surprisingly, the energy of simultaneous adsorption of O2 and C2H4 on small gold clusters with an odd number of N is larger sum by 0.7 eV for N ) 1 and 0.1-0.05 eV for the rest of than Eads the cases. That means that coadsorption of O2 and C2H4 on gold clusters with an odd number of atoms is energetically favorable in comparison with independent adsorption. It is interesting that, if we fix the position and orientation of adsorbed O2 and C2H4 molecules as shown in Figure 2 and remove the corresponding gold cluster, the direct interaction between O2 and C2H4 becomes strongly repulsive. Hence, small gold clusters with an odd number of atoms mediate attractive interaction between O2 and C2H4 molecules. The cooperative effect in simultaneous adsorption of O2 and C2H4 on neutral clusters of gold with an odd number of atoms can be explained by the strong charge transfer effects in the system. Indeed, adsorption of O2 on AuN with an odd number of atoms results in electron transfer from the gold cluster to the antibonding 2π* orbital of O2. In this case, C2H4 effectively adsorbs on the positively charged gold cluster. The excess of the positive charge on the cluster stabilizes the π mode of adsorption, strengthening the Au-C bond. A similar effect has been predicted for adsorption of propene on O2-AuN clusters.36 It was supposed that the binding of propene to O2-AuN should be stronger than the binding to the AuN cluster due to the effective charge transfer from the gold cluster to the oxygen molecule.36 Such an effect has also been found for adsorption of CO on cationic gold clusters AuN+ with N e 6.37 Our calculations demonstrate that coadsorption of O2 and C2H4 on the Au5 cluster favors the π mode of ethylene adsorption in comparison with the di-σ mode favorable energetically for the Au5-C2H4 system. It is interesting that in the case of the Au5 cluster the cooperative effect results in dissociative adsorption of O2. The dissociative structure of O2-Au5-C2H4 has been obtained spontaneously during the optimization process. Thus, coadsorption of C2H4 on O2-AuN can stimulate dissociation of O2. It is important to note that we have considered only those configurations where O2 does not interact directly with C2H4. The direct interaction between adsorbates can lead to the formation of the more bounded dissociative states. However, transition to such states will require overcoming an activation barrier. Finally, we note that the most dramatic enhancement of interaction occurs for the coadsorption of O2 and C2H4 on a single gold atom, as seen from Figure 2. In that case, ΕOads2 and C2H4 O2 ) 1.17 eV, which exceeds the sum of adsorption energies Eads C 2H4 and Eads by 0.7 eV. Formation of the O2-Au-C2H4 complex,
J. Phys. Chem. C, Vol. 113, No. 30, 2009 12933 where both O2 and C2H4 molecules are activated, can be an important step for catalytic reactions with gold. Thus, we suggest that O2 can selectively attack and break the strong double bonds in alkenes via an intermediate interaction with the gold atom. Such an effect, however, requires additional and further theoretical investigation. In summary, we demonstrated that, in addition to a conventional mechanism of the catalytic activation of O2 adsorbed on a gold cluster, the interaction of C2H4 with small gold clusters results in considerable weakening of the carbon-carbon double bond. Catalytic activation of both O2 and C2H4 molecules strongly depends on the size of the gold cluster. We also found a cooperative effect in simultaneous adsorption of O2 and C2H4 on small neutral gold clusters with an odd number of atoms. This effect can play an important role in the mechanism of catalytic oxidation of alkenes by dioxygen on a surface of gold clusters. In the present work, we have considered a coadsorption of two molecular species onto small gold clusters consisting of up to 10 atoms. Many interesting problems beyond the scope of the present work arise when considering coadsorption and multiple molecular adsorption on gold nanoparticles of larger sizes up to 1-5 nm, where the strong dependence of the catalytic activity of gold nanoparticles has been observed experimentally. Another important direction for future development is to estimate the reaction barriers for catalytic oxidation of C2H4 by O2. This will allow favorable reaction channels and reaction dynamics to be identified. Acknowledgment. This work was supported by the Global COE Program (Project No. B01: Catalysis as the Basis for Innovation in Materials Science) from the Ministry of Education, Culture, Sports, Science and Technology, Japan. Supporting Information Available: Figure of the optimized structures of the most stable and several isomer states of the Au6-O2 system and table presenting results of the calculations of the adsorption (dissociation) energies of O2 on the Au6 cluster obtained within different levels of approximation. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Haruta, M.; Kobayashi, T.; Sano, H.; Yamada, N. Chem. Lett. 1987, 16, 405. (2) Pyykko¨, P. Chem. Soc. ReV. 2008, 37, 1967. (3) Coquet, R.; Howard, K. L.; Willock, D. J. Chem. Soc. ReV. 2008, 37, 2046. (4) Ha¨kkinen, H. Chem. Soc. ReV. 2008, 37, 2046. (5) Hvolbæk, B.; Janssens, T. V. W.; Clausen, B. S.; Falsig, H.; Christensen, C. H.; Nørskov, J. K. Nanotoday 2007, 2, 14. (6) Haruta, M. Catal. Today 1997, 36, 2153. (7) Tsunoyama, H.; Sakurai, H.; Negishi, Y.; Tsukuda, T. J. Am. Chem. Soc. 2005, 127, 9374. (8) Turner, M.; Golovko, V. B.; Vaughan, O. P. H.; Abdulkin, P.; Berenguer-Murcia, A.; Tikhov, M. S.; Johnson, B. F. G.; Lambert, R. M. Nature 2008, 454, 981. (9) Herzing, A. A.; Kiely, Ch. J.; Carley, A. F.; Landon, P.; Hutchings, G. J. Science 2008, 321, 1331. (10) Janssens, T. V. W.; Clausen, B. S.; Hvolbæk, B.; Falsig, H.; Christensen, C. H.; Bligaard, T.; Nørskov, J. K. Top. Catal. 2007, 44, 15. (11) McKenna, K. P.; Shluger, A. L. J. Phys. Chem. C 2007, 111, 18848. (12) Kim, Y. D.; Fischer, M.; Gantefo¨r, G. Chem. Phys. Lett. 2003, 377, 170. (13) Hughes, M.; Xu, Y.-J.; Jenkins, P.; et al. Nature 2005, 437, 1132. (14) Landon, P.; Collier, P. J.; Papworth, A. J.; Kiely, C. J.; Hutchings, G. J. Chem. Commun. 2002, 2058. (15) Haruta, M. Nature 2005, 437, 1098.
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