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Density functional study of propylene oxidation on Ag and Au surfaces. Comparison to ethylene oxidation
Journal of Molecular Structure: THEOCHEM 762 (2006) 57–67 www.elsevier.com/locate/theochem
Density functional study of propylene oxidation on Ag and Au surfaces. Comparison to ethylene oxidation Hisayoshi Kobayashi a,*, Yoshiki Shimodaira b b
a Department of Chemistry and Materials Technology, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan Department of Applied Chemistry, Faculty of Science, Science University of Tokyo, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan
Received 4 July 2005; accepted 23 August 2005 Available online 4 January 2006
Abstract Density functional theory (DFT) calculations were carried out to investigate the reaction mechanisms for propylene oxide formation on Ag and Au catalysts. The energy profile and structural change along reactions were mutually compared between ethylene and propylene, and between Ag and Au catalysts using 5 and 7 atom cluster models. Ag is known as unique catalyst for ethylene epoxidation, but Au is not a good catalyst. For propylene oxidation, the influences of doping co-catalysts and supporting metal-oxides were investigated by varying the charge of clusters. The reaction profiles were calculated repeatedly using neutral, cationic and anionic clusters. In almost all cases considered on the Ag and Au clusters, propylene oxide formation is less favorable than hydrogen elimination which leads to p-allyl intermediates. The reaction with O atom as oxidizing agent results in both lower reaction barrier and lower selectivity in the products. For the interaction with O2 molecule, cationic Ag cluster favors propylene oxide formation and disadvantages p-allyl formation. This unique result is also obtained for cationic Au cluster. As for propylene oxidation, the difference between Ag and Au metals is not so distinct compared to that for ethylene oxidation. q 2005 Elsevier B.V. All rights reserved. Keywords: Propylene oxide formation; Difference in Ag and Au; Density functional theory
1. Introduction There is a long history of research for ethylene oxide (epoxide) formation, because the epoxidation is known to be one of the most important industrial processes. The technique has been established whereas the improvement of catalysts still continues. On the other hand, partial oxidation of propylene leading to propylene oxide is more difficult reaction. Propylene oxide is a useful raw material from which resin such as polyurethane is synthesized by polymerization. This technology is under development. Haruta et al. reported that propylene oxide is produced on very small Au particles supported on (anatase) TiO2 with hydrogen as well as propylene and oxygen in the gas phase [1,2]. We investigated the reaction mechanisms between propylene and oxidizing species (O2 molecule or O atom) using the DFT calculation and MX (MZ Ag or Au, and XZ5 or 7) clusters. In our previous works on ethylene oxidation on Ag surfaces [3,4], the energetics was
* Corresponding author. Tel.: C81 75 724 7561; fax: C81 75 724 7580. E-mail address: [email protected] (H. Kobayashi).
0166-1280/$ - see front matter q 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.theochem.2005.08.040
compared between ethylene oxide formation and acetaldehyde formation which is thought to be an intermediate to complete oxidation, i.e. combustion. In this paper, oxidation of ethylene on Au5 cluster is investigated, and the obtained results are compared with those on Ag5 cluster. The oxidation of propylene is investigated using Ag5, Ag7 and Au7 cluster models. The energetics is compared between propylene oxide formation and p-allyl formation. Differences in the mechanisms are discussed in terms of differences in molecules, metals, and clusters. Real catalysts used in chemical industry include various cocatalysts such as halides and alkaline metal ions, and are supported on metal-oxide supports. These materials influence the catalysts in a complicated way, and should be reflected in the models. However the location of co-catalyst ions and the structures of catalyst-support interface are generally unknown. The clusters including metal-oxide supports rapidly increase the size of calculation. Among complicated roles of cocatalysts and supports, the first effect is electronic interactions such as electron releasing and attracting if we confine to the theoretically accessible factors. (One of the roles of supports is to prevent from aggregation and keep high surface area, but it is not the subject of present calculation). We deal with this electronic interaction in an indirect way. The energy profiles
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for reactions between propylene and oxygen species are examined using the neutral, cationic, and anionic M7 clusters. 2. Cluster models and method of calculation The diamond shaped M5 cluster consists of four atoms in the (111) surface and one atom in the second layer. This M5 cluster is the one used in our previous works [3,4], and represents a model for flat surfaces. (Fig. 1) For reactions
with ethylene, M5 cluster is adequate in size, whereas larger reaction area seems to be necessary for reactions with propylene. M7 cluster is a wedge like shape as shown in Fig. 8. It is a model of step site facetted by (111) and (100) faces. For propylene, both M5 and M7 clusters are used, and the results are compared. Influences by the co-catalysts and supports are partially taken into consideration using charged M7 cluster models. The hybrid Hartree–Fock (HF)-DF method is used in this work [5,6]. This method is implemented in Gaussian 98 program [7]. Parametrization is the one suggested by Pople et al. [6], i.e. 0.2, 0.8, and 0.72 for the HF, Slater [8], and Becke [9] exchange functionals, and 0.19 and 0.81 for the Vosko–Wilk–Nusair [10] and Lee–Yang–Parr [11] correlation functionals. Two types of the Los Alamos effective core potentials (ECP) are used for Ag and Au atoms along with the corresponding valence basis sets [12]. For the small core-large valence ECP (LanL2DZ), the 4s, 4p, 4d, 5s(5p) electrons for Ag and the 5s, 5p, 5d, 6s(6p) electrons for Au are treated explicitly. For the large core-small valence ECP (LanL1DZ), which was employed in our earlier works on Ag, the valence is reduced to the 4d, 5s(5p) electrons for Ag and the 5d, 6s(6p) electrons for Au. For H, C, and O atoms, the geometry optimization is carried out with the Dunning– Huzinaga full double zeta (D95) basis set, in which the contraction is (10s5p)/[3s2p] and (4s)/[2s] for C, O, and H atoms, respectively [13]. After the local minima and transition states (TS’s) are obtained, the total energy is reevaluated with larger basis sets, i.e. 6-311CG(2d) for O atom, and 6-311CG for C and H atoms. Together with the ECP for metal atoms, the latter larger basis set is abbreviated as LanL2DZCspdd or LanL1DZCspdd. Throughout this work, the structures of Ag and Au clusters are fixed, and the structure and relative orientation of ethylene or propylene and oxygen species with respect to the clusters are optimized. The stabilization energy was defined as follows: DE Z fEðcombined systemÞKEðmetal clusterÞ C EðC3 H6 or C2 H4 Þ C xEðO2 Þg
Fig. 1. Optimize structures for local minima and TS’s along ethylene oxide formation with O2 molecule on Au5 cluster.
where x is 1 or 1/2 depending on oxygen molecule or atom, respectively. Negative values in DE mean stabilization, i.e. thermodynamically easy step to proceed. Similar to our previous works [3,4] and those by Nakatsuji and co-workers [14,15], the Eley–Rideal mechanism was assumed. Partly this assumption is requested by the calculation model. The reaction with an O2 molecule will leave an active O atom on the surface, and furthermore in the reaction with an O atom, it has to be bound on the surface at the beginning. Experimentally, the adsorption and dissociation of O2 molecule is very fast, and the adsorption of O species is much stronger than that of ethylene. So the Eley–Rideal mechanism in which the O species is adsorbed first and then ethylene or propylene is co-adsorbed on it, is a reasonable reaction profile.
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3. Results and discussion 3.1. Reaction of ethylene and oxygen species adsorbed on Au5 Cluster. The energy profile for reaction between ethylene and oxygen species on the Au5 cluster is shown in Figs. 3 and 4 with the LanL1DZ basis set. In these figures, the energies of corresponding reactant, stable intermediates, TS’s, and product for the reaction on Ag5 cluster are also shown, which are reproduced from Ref. [3]. The structure change along the reaction on Au5 cluster is shown in Figs. 1 and 2.For the reaction with O2 molecule, the energy profile is in parallel between Ag and Au, except for the relatively higher energies for Au5 cluster. There are two TS’s in Figs. 1 and 3.The first TS corresponds to activated adsorption of ethylene on Au5/O2, and the second TS corresponds to the O–O bond fission and rotation of O atom to form the C–C–O triangle. Figs. 2 and 4 show the energy profile and structure change for the reaction with O atom, in which a single TS is found for forming the C–C–O triangle. A characteristic difference between Au and Ag is a high energy for Au5–O intermediate, which means that dissociated O atom is unstable on Au5 cluster. Compared to the reaction on Ag5 cluster, Au5 cluster does not effectively stabilize and catalyze ethylene oxidation, which is in agreement with the work by Nakatsuji et al. [15]. Fig. 5 shows the energy profiles for the reaction on the Au5 cluster with the LanL1DZ and LanL2DZ basis. In the latter that is more precise, a less attractive energy surface is obtained with O2.For the reaction with O, destabilization in O atom adsorption (Au5–O) is increased with the LanL2DZ basis set although the product (Au5–C2H4O) is more stabilized. Thus Au metal is not a good catalyst for ethylene oxide formation. 3.2. Reaction of propylene oxidation on Ag5 cluster The energetics of propylene oxidation with O2 and O on the Ag5 cluster is shown in Fig. 6. On the reaction with O2, three TS’s (TS1, TS2, and TS3) are characterized for propylene oxide formation. The TS1 corresponds to activated adsorption of propylene onto the Ag5/O2, the TS2 to elongation of the O–O bond, and the TS3 to rotation of O atom to form the C–C–O triangle. The energy of TS2 is the highest, and it corresponds to the reaction barrier. There is one TS for p-allyl formation, and its energy is lower than the TS2 for propylene oxide formation. At the product formation, the energy of p-allyl is higher than the propylene oxide. Our analysis suggests that the Ag5–OOH structure is left, and the OOH fragment is not stabilized with small Ag5 cluster. We will examine this point later again using a larger cluster model. On reaction with O atom, a single TS is found for both the propylene oxide and p-allyl formation, and the TS energy is a little lower for the latter. Comparison of TS energies indicates that p-allyl formation is more facile reaction on Ag5 cluster with either O2 or O species as oxidizing agent.
Fig. 2. Optimize structures for local minima and TS’s along ethylene oxide formation with O atom on Au5 cluster.
3.3. Reaction of propylene oxidation on neutral Ag7 cluster Reactions between propylene and oxygen species is examined using Ag7 cluster again. The equilibrium structures considered are the local minima for O2 and O adsorption, coadsorption of oxygen and propylene, and the final product, and the TS’s between the last two local minima. The energy profiles with O2 oxidizing agent are shown in Fig. 7. The optimized structures are shown in Figs. 8 and 9, for propylene oxide and p-allyl formation, respectively. For propylene oxide
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Fig. 3. Energy profile along reaction coordinate of ethylene oxide formation with O2 molecule on Au5 cluster (gray line). Energy profiles for ethylene oxide (black solid line) and acetaldehyde formation (black dashed line) on Ag5 cluster are also plotted for comparison. Basis set used is LanL1DZCspdd.
Fig. 4. Energy profile along reaction coordinate of ethylene oxide formation with O atom on Au5 cluster (gray line). Energy profiles for ethylene oxide (black solid line) and acetaldehyde formation (black dashed line) on Ag5 cluster are also plotted for comparison. Basis set used is LanL1DZCspdd.
Fig. 5. Comparison of energy profiles for ethylene oxide formation on Au5 cluster between LanL1DZCspdd (black line) and LanL2DZCspdd (gray line) basis sets. Top: with O2 molecule, and bottom: with O atom. Basis set used is LanL1DZCspdd.
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Fig. 6. Energy profiles along reaction coordinate for propylene oxide formation (black line) and p-allyl formation (gray line) on Ag5 cluster. Top: with O2 molecule, and bottom: with O atom. Basis set used is LanL1DZCspdd.
formation, stabilization energy for the co-adsorbed state is smaller than that for O2 adsorption. This result also appears in Figs. 3, 5 and 6, and indicates that propylene adsorption onto O2 is an activated process. From the co-adsorbed state to the product, a considerably large (more than 32 kcal/mol) activation energy is encountered, and the reaction proceeds endothermic. The energy surface for p-allyl formation lies always lower than that for propylene oxide formation. The TS structure is not characterized since it locates close to the product in the sense of energy and structure. In either case of no TS or small TS, p-allyl formation is more favorable than propylene oxide formation. There are some differences in the energetics obtained with Ag5 and Ag7 clusters. Except for the TS1 for activated adsorption of propylene, two TS’s are obtained with Ag5, but only one TS with Ag7. More complicated energy surface is ascribed to smallness of Ag5 cluster and the flat surface model. Since Ag7 cluster represents the step sites on surfaces, the
interactions seem to be more strong. The LanL1DZ and LanL2DZ basis sets are employed in the calculations with Ag5 and Ag7, respectively. The latter is more accurate but gives more repulsive energy surfaces, as compared in Fig. 5. Fig. 10 shows the energy profile for reactions of propylene with O atom, and the optimized structures for local minima and TS’s are shown in Figs. 11 and 12. Among the energies along the reaction, those for co-adsorption states are most different, and it is lower for the propylene oxide formation. The energies are almost the same for the product and TS. It is difficult to say which reaction proceeds more favorably. If the stabilization energy at the co-adsorbed state is dispersed, it is like the thermal sink, and propylene oxide formation will encounter the large activation energy. However, the major part of stabilization energy will be maintained in terms of the internal vibrational energies, and it could be used to climb up the activation barrier. This result suggests that both propylene oxide and p-allyl formation proceed without selectivity.
Fig. 7. Energy profiles along reaction coordinate for propylene oxide formation (black line) and p-allyl formation (gray line) with O2 molecule on Ag7 cluster. Basis set used is LanL2DZCspdd.
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Fig. 9. Optimize structures for local minima along p-allyl formation with O atom on Ag7 cluster. The TS structure is not obtained.
Fig. 8. Optimize structures for local minima and TS along propylene oxide formation with O2 molecule on Ag7 cluster.
3.4. Reaction of propylene oxidation on charged Ag7 cluster Fig. 13 shows the energetics for propylene oxide and p-allyl formation with O2 for the anionic and cationic Ag7 clusters as well as the neutral one. (The energies for the neutral cluster are the same as those shown in Fig. 7.) The energy surface for anionic clusters lies higher than the neutral ones, but that for cationic clusters locates in lower energies except for the initial O2 adsorption. For the energy profile for p-allyl formation, the energies increase in the order of neutral, anionic and cationic clusters for the O2 adsorption and co-adsorption states. However, for the product, the order is inverted, and the cationic cluster is the most stable. Thus there is no merit on
anionic clusters for both reactions. The reactions on cationic clusters are disadvantageous by unstable O2 adsorption. Fig. 14 shows the energy profiles with O atom for the neutral, anionic and cationic Ag7 clusters. Generally, neutral and anionic clusters show a similar trend in both reactions. The energetics for cationic cluster is different from others, and is more attractive except for the initial O atom adsorption. For propylene oxide formation with cationic cluster, the TS structure can not be characterized, and the reaction proceeds without barriers (down-hill reaction). For p-allyl formation, the energetics of cationic cluster lies in the lowest energy region. Thus the cationic charge states make the both reactions easy to proceed, but do not contribute to enhance the selectivity. 3.5. Reaction of propylene oxidation on neutral and cationic Au7 cluster Reactions of propylene and oxygen species are examined using Au7 cluster. Fig. 15 shows the energy profiles for propylene oxide and p-allyl formation with O2 species. Two
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Fig. 10. Energy profiles along reaction coordinate for propylene oxide formation (black line) and p-allyl formation (gray line) with O atom on Ag7 cluster. Basis set used is LanL2DZCspdd.
Fig. 11. Optimize structures for local minima and TS along propylene oxide formation with O atom on Ag7 cluster.
Fig. 12. Optimize structures for local minima and TS along p-allyl formation with O atom on Ag7 cluster.
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Fig. 13. Energy profiles along reaction coordinates for propylene oxidation on neutral (black solid line), cationic (black dashed line), and anionic (gray line) Ag7 clusters by O2 molecule. Top: propylene oxide formation, bottom: p-allyl formation. Basis set used is LanL2DZCspdd.
sets of energies for intermediates shift rather parallel, and lower activation energy is found for p-allyl formation. The energy profiles with O atom are shown in Fig. 16. For propylene oxide formation, the bond formation between the O
and C atoms proceeds with very small activation energy (below 1 kcal/mol), and the resulting intermediate is considerably stable (DEZK20.4 kcal/mol). However in this intermediate, another bond is formed between the central C atom of
Fig. 14. Energy profiles along reaction coordinates for propylene oxidation on neutral (black solid line), cationic (black dashed line), and anionic (gray line) Ag7 clusters by O atom. Top: propylene oxide formation, bottom: p-allyl formation. Basis set used is LanL2DZCspdd.
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Fig. 15. Energy profiles along reaction coordinate for propylene oxide formation (black line) and p-allyl (gray line) with O2 molecular on neutral Au7 cluster. Basis set used is LanL2DZCspdd.
Fig. 16. Energy profiles along reaction coordinate for propylene oxide formation (black line) and p-allyl (gray line) with O atom on neutral Au7 cluster. Basis set used is LanL2DZCspdd.
propylene and the Au atom neighbor to the O atom. To produce propylene oxide, this C–Au bond must be broken and the C–C– O angle is decreased to ca. 60 degree. The activation energy for this deformation is larger than the amount of stabilization of the intermediate. The resulting TS is the highest along the course of reaction, and it is higher than the co-adsorbed state by 5.6 kcal/mol. For p-allyl formation, the amount of activation energy measured from the co-adsorbed state is 7 kcal/mol. Eventually both the activation energies for propylene oxide and
p-allyl formation are almost the same, and both reactions proceed without selectivity. We investigated the energy profiles using the cationic AuC 7 cluster. This partly reflects the metal-support interactions, since the lattice O atoms in TiO2 withdraw electrons from Au atoms. Fig. 17 shows the energy profiles with O2 species. In this case, propylene oxide formation uniquely proceeds with lower activation energy than p-allyl formation. The former reaction is expected to proceed selectively. Figs. 18 and 19 shows
Fig. 17. Energy profiles along reaction coordinate for propylene oxide formation (black line) and p-allyl (gray line) with O2 molecular on cationic Au7 cluster. Basis set used is LanL2DZCspdd.
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Fig. 18. Optimize structures for local minima and TS along propylene oxide formation with O2 molecule on cationic Au7 cluster.
optimized structures for the local minima and TS for propylene oxide and p-allyl formation, respectively. The energy profiles with O species are shown in Fig. 20. The energetics for the two reactions is almost the same except for the products, and both the reactions are down hill, and expected to proceed without selectivity. Thus among the four types of reaction with Au7 cluster, the reactions with O species lead to very low selectivity judging from the similar energies of their TS’s. The selectivity between the two reactions is apparent with O2 species. p-allyl formation is favorable for neutral cluster, but propylene oxide formation is favorable for cationic cluster. Another interesting character is that the O–O bond is not completely broken even in the product structures. This may be ascribed to the wedge shape of Au7 cluster. After the O2 species forms the oxidation product (either
Fig. 19. Optimize structures for local minima and TS along p-allyl formation with O2 molecule on cationic Au7 cluster.
propylene oxide or p-allyl), atomic oxygen is left on the cluster. This O species will react with other propylene molecule with lower selectivity. Experimentally, considerable amount of hydrogen gas as well as oxygen is introduced to the system. The role of hydrogen is expected to remove O species as water. Recently, Zwijnenburg et al. examined the Au/TiO2 system in detail by Mo¨ssbauer spectroscopy and other techniques [16]. They reported that metallic gold is the active phase, and no evidence for charge transfer between support and Au particle. However this result does not contradict to our result that cationic cluster is favorable for propylene oxide formation, since slightly electron deficient Au cluster is brought by O2 molecules adsorbed
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Fig. 20. Energy profiles along reaction coordinate for propylene oxide formation (black line) and p-allyl (gray line) with O atom on cationic Au7 cluster. Basis set used is LanL2DZCspdd.
around the reaction sites as the spectators. In a recent work, Haruta et al. stressed the roles of TiO2 support [17]. Stangland et al., reported that isolated Ti atoms are favorable for high selectivity and reactivity by suppressing cracking of propylene [18]. Thus more attention is paid to TiO2 supports rather than Au catalysts, and more sophisticated models including the TiO2 as well as Au clusters become inevitable for the next step.
Culture. The authors are grateful for helpful discussion to Dr. Katsumi Nakashiro and Dr. Tomoatsu Iwakura of Mitsubishi Chemical Co. The authors express their thanks to the undergraduate course students, Toshitaka Tanaka and Hiroaki Katsuyama for their intensive works on drawing figures.
4. Concluding remark
[1] M. Haruta, Catal. Today 36 (1997) 153. [2] T. Hayashi, K. Tanaka, M. Haruta, J. Catal. 178 (1998) 566. [3] H. Kobayashi, K. Nakashiro, T. Iwakuwa, Theor. Chem. Acc. 102 (1999) 237. [4] H. Kobayashi, K. Nakashiro, T. Iwakura, Internet Electron. J. Mol. Des. 1 (2002) 620. [5] A.D. Becke, J. Chem. Phys. 98 (1993) 1372. [6] P.M.W. Gill, B.G. Johnson, J.A. Pople, Int. J. Quantum Chem. Symp. 26 (1992) 319. [7] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, V.G. Zakrzewski, J.A. Montgomery Jr., R.E. Stratmann, J.C. Burant, S. Dapprich, J.M. Millam, A.D. Daniels, K.N. Kudin, M.C. Strain, O. Farkas, J. Tomasi, V. Barone, M. Cossi, R. Cammi, B. Mennucci, C. Pomelli, C. Adamo, S. Clifford, J. Ochterski, G.A. Petersson, P.Y. Ayala, Q. Cui, K. Morokuma, D.K. Malick, A.D. Rabuck, K. Raghavachari, J.B. Foresman, J. Cioslowski, J.V. Ortiz, B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. Gomperts, R.L. Martin, D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A. Nanayakkara, C. Gonzalez, M. Challacombe, P.M.W. Gill, B. Johnson, W. Chen, M.W. Wong, J.L. Andres, C. Gonzalez, M. Head-Gordon, E.S. Replogle, J.A. Pople, GAUSSIAN 98, Revision A.5, Gaussian Inc., Pittsburgh, PA, 1998. [8] J.C. Slater, Phys. Rev. 81 (1951) 385. [9] A.D. Becke, Phys. Rev. A38 (1988) 3098. [10] S.H. Vosko, L. Wilk, M. Nusair, Can. J. Phys. 58 (1980) 12001; L. Wilk, S.H. Vosko, J. Phys. C: Solid State Phys. 15 (1982) 2139. [11] C. Lee, W. Yang, R.G. Parr, Phys. Rev. B37 (1988) 785. [12] P.J. Hay, W.R. Wadt, J. Chem. Phys. 82 (1985) 270; W.R. Wadt, P.J. Hay, J. Chem. Phys. 82 (1985) 284; P.J. Hay, W.R. Wadt, J. Chem. Phys. 82 (1985) 299. [13] T.H. Dunning Jr., in: H.F. Schaefer III (Ed.), Modern Theoretical Chemistry, Plenum Press, New York, 1976, p. 1. [14] H. Nakatsuji, H. Nakai, K. Ikeda, Y. Yamamoto, Surf. Sci. 384 (1997) 315. [15] H. Nakatsuji, Z.-M. Hu, H. Nakai, K. Ikeda, Surf. Sci. 387 (1997) 328. [16] A. Zwijnenburg, A. Goossens, W.G. Sloof, M.W.J. Craje, A.M. van der Kraan, L.J. de Jongh, M. Makkee, J.A. Moulijn, J. Phys. Chem. B106 (2002) 9853. [17] B.S. Uphade, T. Akita, T. Nakamura, M. Haruta, J. Catal. 209 (2002) 331. [18] E.E. Stanglang, B. Taylor, R.P. Andres, W.N. Delgass, J. Phys. Chem. B109 (2005) 2321.
The authors’ previous works on ethylene oxidation on Ag5 cluster were extended to investigate propylene oxidation. The oxidation mechanisms were compared between ethylene and propylene and between Ag and Au metals. Au metal was not a good catalyst for ethylene oxide formation since adsorption of the dissociated O atom is not stable on Au5 cluster. The energetics of propylene and oxygen species on Ag5 cluster reveals that the H atom abstraction leading to p-allyl occurs more easily than the O atom insertion leading to propylene oxide with either O2 or O species as oxidizing agent. The same reactions were reexamined with larger Ag7 cluster modeling the step site. The p-allyl formation was favored with O2 molecule, and both the two reactions proceed without selectivity with O atom. The energetics was also evaluated using cationic and anionic clusters. In both reactions, cationic clusters most destabilized the initial O species adsorption but most stabilized the product. We could not find favorable charge states for propylene oxide formation. Propylene oxidation was examined with Au7 cluster. For the neutral cluster, the p-allyl formation was favored with O2 molecule, and both the reactions proceeded equally with O atom, which was the same trend as that obtained for Ag7 cluster. However, propylene oxide formation was found to be advantageous to p-allyl formation for the cationic Au7 cluster with O2 molecule. This unique result was obtained only on this condition. With O atom, the oxidation again proceeded without selectivity. Acknowledgements This research was supported by the Grant-in Aid for Scientific Research from the Ministry of Education, Science, Sports, and
References
Density functional study of propylene oxidation on Ag and Au surfaces. Comparison to ethylene oxidation Hisayoshi Kobayashi a,*, Yoshiki Shimodaira b b
a Department of Chemistry and Materials Technology, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan Department of Applied Chemistry, Faculty of Science, Science University of Tokyo, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan
Received 4 July 2005; accepted 23 August 2005 Available online 4 January 2006
Abstract Density functional theory (DFT) calculations were carried out to investigate the reaction mechanisms for propylene oxide formation on Ag and Au catalysts. The energy profile and structural change along reactions were mutually compared between ethylene and propylene, and between Ag and Au catalysts using 5 and 7 atom cluster models. Ag is known as unique catalyst for ethylene epoxidation, but Au is not a good catalyst. For propylene oxidation, the influences of doping co-catalysts and supporting metal-oxides were investigated by varying the charge of clusters. The reaction profiles were calculated repeatedly using neutral, cationic and anionic clusters. In almost all cases considered on the Ag and Au clusters, propylene oxide formation is less favorable than hydrogen elimination which leads to p-allyl intermediates. The reaction with O atom as oxidizing agent results in both lower reaction barrier and lower selectivity in the products. For the interaction with O2 molecule, cationic Ag cluster favors propylene oxide formation and disadvantages p-allyl formation. This unique result is also obtained for cationic Au cluster. As for propylene oxidation, the difference between Ag and Au metals is not so distinct compared to that for ethylene oxidation. q 2005 Elsevier B.V. All rights reserved. Keywords: Propylene oxide formation; Difference in Ag and Au; Density functional theory
1. Introduction There is a long history of research for ethylene oxide (epoxide) formation, because the epoxidation is known to be one of the most important industrial processes. The technique has been established whereas the improvement of catalysts still continues. On the other hand, partial oxidation of propylene leading to propylene oxide is more difficult reaction. Propylene oxide is a useful raw material from which resin such as polyurethane is synthesized by polymerization. This technology is under development. Haruta et al. reported that propylene oxide is produced on very small Au particles supported on (anatase) TiO2 with hydrogen as well as propylene and oxygen in the gas phase [1,2]. We investigated the reaction mechanisms between propylene and oxidizing species (O2 molecule or O atom) using the DFT calculation and MX (MZ Ag or Au, and XZ5 or 7) clusters. In our previous works on ethylene oxidation on Ag surfaces [3,4], the energetics was
* Corresponding author. Tel.: C81 75 724 7561; fax: C81 75 724 7580. E-mail address: [email protected] (H. Kobayashi).
0166-1280/$ - see front matter q 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.theochem.2005.08.040
compared between ethylene oxide formation and acetaldehyde formation which is thought to be an intermediate to complete oxidation, i.e. combustion. In this paper, oxidation of ethylene on Au5 cluster is investigated, and the obtained results are compared with those on Ag5 cluster. The oxidation of propylene is investigated using Ag5, Ag7 and Au7 cluster models. The energetics is compared between propylene oxide formation and p-allyl formation. Differences in the mechanisms are discussed in terms of differences in molecules, metals, and clusters. Real catalysts used in chemical industry include various cocatalysts such as halides and alkaline metal ions, and are supported on metal-oxide supports. These materials influence the catalysts in a complicated way, and should be reflected in the models. However the location of co-catalyst ions and the structures of catalyst-support interface are generally unknown. The clusters including metal-oxide supports rapidly increase the size of calculation. Among complicated roles of cocatalysts and supports, the first effect is electronic interactions such as electron releasing and attracting if we confine to the theoretically accessible factors. (One of the roles of supports is to prevent from aggregation and keep high surface area, but it is not the subject of present calculation). We deal with this electronic interaction in an indirect way. The energy profiles
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for reactions between propylene and oxygen species are examined using the neutral, cationic, and anionic M7 clusters. 2. Cluster models and method of calculation The diamond shaped M5 cluster consists of four atoms in the (111) surface and one atom in the second layer. This M5 cluster is the one used in our previous works [3,4], and represents a model for flat surfaces. (Fig. 1) For reactions
with ethylene, M5 cluster is adequate in size, whereas larger reaction area seems to be necessary for reactions with propylene. M7 cluster is a wedge like shape as shown in Fig. 8. It is a model of step site facetted by (111) and (100) faces. For propylene, both M5 and M7 clusters are used, and the results are compared. Influences by the co-catalysts and supports are partially taken into consideration using charged M7 cluster models. The hybrid Hartree–Fock (HF)-DF method is used in this work [5,6]. This method is implemented in Gaussian 98 program [7]. Parametrization is the one suggested by Pople et al. [6], i.e. 0.2, 0.8, and 0.72 for the HF, Slater [8], and Becke [9] exchange functionals, and 0.19 and 0.81 for the Vosko–Wilk–Nusair [10] and Lee–Yang–Parr [11] correlation functionals. Two types of the Los Alamos effective core potentials (ECP) are used for Ag and Au atoms along with the corresponding valence basis sets [12]. For the small core-large valence ECP (LanL2DZ), the 4s, 4p, 4d, 5s(5p) electrons for Ag and the 5s, 5p, 5d, 6s(6p) electrons for Au are treated explicitly. For the large core-small valence ECP (LanL1DZ), which was employed in our earlier works on Ag, the valence is reduced to the 4d, 5s(5p) electrons for Ag and the 5d, 6s(6p) electrons for Au. For H, C, and O atoms, the geometry optimization is carried out with the Dunning– Huzinaga full double zeta (D95) basis set, in which the contraction is (10s5p)/[3s2p] and (4s)/[2s] for C, O, and H atoms, respectively [13]. After the local minima and transition states (TS’s) are obtained, the total energy is reevaluated with larger basis sets, i.e. 6-311CG(2d) for O atom, and 6-311CG for C and H atoms. Together with the ECP for metal atoms, the latter larger basis set is abbreviated as LanL2DZCspdd or LanL1DZCspdd. Throughout this work, the structures of Ag and Au clusters are fixed, and the structure and relative orientation of ethylene or propylene and oxygen species with respect to the clusters are optimized. The stabilization energy was defined as follows: DE Z fEðcombined systemÞKEðmetal clusterÞ C EðC3 H6 or C2 H4 Þ C xEðO2 Þg
Fig. 1. Optimize structures for local minima and TS’s along ethylene oxide formation with O2 molecule on Au5 cluster.
where x is 1 or 1/2 depending on oxygen molecule or atom, respectively. Negative values in DE mean stabilization, i.e. thermodynamically easy step to proceed. Similar to our previous works [3,4] and those by Nakatsuji and co-workers [14,15], the Eley–Rideal mechanism was assumed. Partly this assumption is requested by the calculation model. The reaction with an O2 molecule will leave an active O atom on the surface, and furthermore in the reaction with an O atom, it has to be bound on the surface at the beginning. Experimentally, the adsorption and dissociation of O2 molecule is very fast, and the adsorption of O species is much stronger than that of ethylene. So the Eley–Rideal mechanism in which the O species is adsorbed first and then ethylene or propylene is co-adsorbed on it, is a reasonable reaction profile.
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3. Results and discussion 3.1. Reaction of ethylene and oxygen species adsorbed on Au5 Cluster. The energy profile for reaction between ethylene and oxygen species on the Au5 cluster is shown in Figs. 3 and 4 with the LanL1DZ basis set. In these figures, the energies of corresponding reactant, stable intermediates, TS’s, and product for the reaction on Ag5 cluster are also shown, which are reproduced from Ref. [3]. The structure change along the reaction on Au5 cluster is shown in Figs. 1 and 2.For the reaction with O2 molecule, the energy profile is in parallel between Ag and Au, except for the relatively higher energies for Au5 cluster. There are two TS’s in Figs. 1 and 3.The first TS corresponds to activated adsorption of ethylene on Au5/O2, and the second TS corresponds to the O–O bond fission and rotation of O atom to form the C–C–O triangle. Figs. 2 and 4 show the energy profile and structure change for the reaction with O atom, in which a single TS is found for forming the C–C–O triangle. A characteristic difference between Au and Ag is a high energy for Au5–O intermediate, which means that dissociated O atom is unstable on Au5 cluster. Compared to the reaction on Ag5 cluster, Au5 cluster does not effectively stabilize and catalyze ethylene oxidation, which is in agreement with the work by Nakatsuji et al. [15]. Fig. 5 shows the energy profiles for the reaction on the Au5 cluster with the LanL1DZ and LanL2DZ basis. In the latter that is more precise, a less attractive energy surface is obtained with O2.For the reaction with O, destabilization in O atom adsorption (Au5–O) is increased with the LanL2DZ basis set although the product (Au5–C2H4O) is more stabilized. Thus Au metal is not a good catalyst for ethylene oxide formation. 3.2. Reaction of propylene oxidation on Ag5 cluster The energetics of propylene oxidation with O2 and O on the Ag5 cluster is shown in Fig. 6. On the reaction with O2, three TS’s (TS1, TS2, and TS3) are characterized for propylene oxide formation. The TS1 corresponds to activated adsorption of propylene onto the Ag5/O2, the TS2 to elongation of the O–O bond, and the TS3 to rotation of O atom to form the C–C–O triangle. The energy of TS2 is the highest, and it corresponds to the reaction barrier. There is one TS for p-allyl formation, and its energy is lower than the TS2 for propylene oxide formation. At the product formation, the energy of p-allyl is higher than the propylene oxide. Our analysis suggests that the Ag5–OOH structure is left, and the OOH fragment is not stabilized with small Ag5 cluster. We will examine this point later again using a larger cluster model. On reaction with O atom, a single TS is found for both the propylene oxide and p-allyl formation, and the TS energy is a little lower for the latter. Comparison of TS energies indicates that p-allyl formation is more facile reaction on Ag5 cluster with either O2 or O species as oxidizing agent.
Fig. 2. Optimize structures for local minima and TS’s along ethylene oxide formation with O atom on Au5 cluster.
3.3. Reaction of propylene oxidation on neutral Ag7 cluster Reactions between propylene and oxygen species is examined using Ag7 cluster again. The equilibrium structures considered are the local minima for O2 and O adsorption, coadsorption of oxygen and propylene, and the final product, and the TS’s between the last two local minima. The energy profiles with O2 oxidizing agent are shown in Fig. 7. The optimized structures are shown in Figs. 8 and 9, for propylene oxide and p-allyl formation, respectively. For propylene oxide
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Fig. 3. Energy profile along reaction coordinate of ethylene oxide formation with O2 molecule on Au5 cluster (gray line). Energy profiles for ethylene oxide (black solid line) and acetaldehyde formation (black dashed line) on Ag5 cluster are also plotted for comparison. Basis set used is LanL1DZCspdd.
Fig. 4. Energy profile along reaction coordinate of ethylene oxide formation with O atom on Au5 cluster (gray line). Energy profiles for ethylene oxide (black solid line) and acetaldehyde formation (black dashed line) on Ag5 cluster are also plotted for comparison. Basis set used is LanL1DZCspdd.
Fig. 5. Comparison of energy profiles for ethylene oxide formation on Au5 cluster between LanL1DZCspdd (black line) and LanL2DZCspdd (gray line) basis sets. Top: with O2 molecule, and bottom: with O atom. Basis set used is LanL1DZCspdd.
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Fig. 6. Energy profiles along reaction coordinate for propylene oxide formation (black line) and p-allyl formation (gray line) on Ag5 cluster. Top: with O2 molecule, and bottom: with O atom. Basis set used is LanL1DZCspdd.
formation, stabilization energy for the co-adsorbed state is smaller than that for O2 adsorption. This result also appears in Figs. 3, 5 and 6, and indicates that propylene adsorption onto O2 is an activated process. From the co-adsorbed state to the product, a considerably large (more than 32 kcal/mol) activation energy is encountered, and the reaction proceeds endothermic. The energy surface for p-allyl formation lies always lower than that for propylene oxide formation. The TS structure is not characterized since it locates close to the product in the sense of energy and structure. In either case of no TS or small TS, p-allyl formation is more favorable than propylene oxide formation. There are some differences in the energetics obtained with Ag5 and Ag7 clusters. Except for the TS1 for activated adsorption of propylene, two TS’s are obtained with Ag5, but only one TS with Ag7. More complicated energy surface is ascribed to smallness of Ag5 cluster and the flat surface model. Since Ag7 cluster represents the step sites on surfaces, the
interactions seem to be more strong. The LanL1DZ and LanL2DZ basis sets are employed in the calculations with Ag5 and Ag7, respectively. The latter is more accurate but gives more repulsive energy surfaces, as compared in Fig. 5. Fig. 10 shows the energy profile for reactions of propylene with O atom, and the optimized structures for local minima and TS’s are shown in Figs. 11 and 12. Among the energies along the reaction, those for co-adsorption states are most different, and it is lower for the propylene oxide formation. The energies are almost the same for the product and TS. It is difficult to say which reaction proceeds more favorably. If the stabilization energy at the co-adsorbed state is dispersed, it is like the thermal sink, and propylene oxide formation will encounter the large activation energy. However, the major part of stabilization energy will be maintained in terms of the internal vibrational energies, and it could be used to climb up the activation barrier. This result suggests that both propylene oxide and p-allyl formation proceed without selectivity.
Fig. 7. Energy profiles along reaction coordinate for propylene oxide formation (black line) and p-allyl formation (gray line) with O2 molecule on Ag7 cluster. Basis set used is LanL2DZCspdd.
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Fig. 9. Optimize structures for local minima along p-allyl formation with O atom on Ag7 cluster. The TS structure is not obtained.
Fig. 8. Optimize structures for local minima and TS along propylene oxide formation with O2 molecule on Ag7 cluster.
3.4. Reaction of propylene oxidation on charged Ag7 cluster Fig. 13 shows the energetics for propylene oxide and p-allyl formation with O2 for the anionic and cationic Ag7 clusters as well as the neutral one. (The energies for the neutral cluster are the same as those shown in Fig. 7.) The energy surface for anionic clusters lies higher than the neutral ones, but that for cationic clusters locates in lower energies except for the initial O2 adsorption. For the energy profile for p-allyl formation, the energies increase in the order of neutral, anionic and cationic clusters for the O2 adsorption and co-adsorption states. However, for the product, the order is inverted, and the cationic cluster is the most stable. Thus there is no merit on
anionic clusters for both reactions. The reactions on cationic clusters are disadvantageous by unstable O2 adsorption. Fig. 14 shows the energy profiles with O atom for the neutral, anionic and cationic Ag7 clusters. Generally, neutral and anionic clusters show a similar trend in both reactions. The energetics for cationic cluster is different from others, and is more attractive except for the initial O atom adsorption. For propylene oxide formation with cationic cluster, the TS structure can not be characterized, and the reaction proceeds without barriers (down-hill reaction). For p-allyl formation, the energetics of cationic cluster lies in the lowest energy region. Thus the cationic charge states make the both reactions easy to proceed, but do not contribute to enhance the selectivity. 3.5. Reaction of propylene oxidation on neutral and cationic Au7 cluster Reactions of propylene and oxygen species are examined using Au7 cluster. Fig. 15 shows the energy profiles for propylene oxide and p-allyl formation with O2 species. Two
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Fig. 10. Energy profiles along reaction coordinate for propylene oxide formation (black line) and p-allyl formation (gray line) with O atom on Ag7 cluster. Basis set used is LanL2DZCspdd.
Fig. 11. Optimize structures for local minima and TS along propylene oxide formation with O atom on Ag7 cluster.
Fig. 12. Optimize structures for local minima and TS along p-allyl formation with O atom on Ag7 cluster.
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Fig. 13. Energy profiles along reaction coordinates for propylene oxidation on neutral (black solid line), cationic (black dashed line), and anionic (gray line) Ag7 clusters by O2 molecule. Top: propylene oxide formation, bottom: p-allyl formation. Basis set used is LanL2DZCspdd.
sets of energies for intermediates shift rather parallel, and lower activation energy is found for p-allyl formation. The energy profiles with O atom are shown in Fig. 16. For propylene oxide formation, the bond formation between the O
and C atoms proceeds with very small activation energy (below 1 kcal/mol), and the resulting intermediate is considerably stable (DEZK20.4 kcal/mol). However in this intermediate, another bond is formed between the central C atom of
Fig. 14. Energy profiles along reaction coordinates for propylene oxidation on neutral (black solid line), cationic (black dashed line), and anionic (gray line) Ag7 clusters by O atom. Top: propylene oxide formation, bottom: p-allyl formation. Basis set used is LanL2DZCspdd.
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Fig. 15. Energy profiles along reaction coordinate for propylene oxide formation (black line) and p-allyl (gray line) with O2 molecular on neutral Au7 cluster. Basis set used is LanL2DZCspdd.
Fig. 16. Energy profiles along reaction coordinate for propylene oxide formation (black line) and p-allyl (gray line) with O atom on neutral Au7 cluster. Basis set used is LanL2DZCspdd.
propylene and the Au atom neighbor to the O atom. To produce propylene oxide, this C–Au bond must be broken and the C–C– O angle is decreased to ca. 60 degree. The activation energy for this deformation is larger than the amount of stabilization of the intermediate. The resulting TS is the highest along the course of reaction, and it is higher than the co-adsorbed state by 5.6 kcal/mol. For p-allyl formation, the amount of activation energy measured from the co-adsorbed state is 7 kcal/mol. Eventually both the activation energies for propylene oxide and
p-allyl formation are almost the same, and both reactions proceed without selectivity. We investigated the energy profiles using the cationic AuC 7 cluster. This partly reflects the metal-support interactions, since the lattice O atoms in TiO2 withdraw electrons from Au atoms. Fig. 17 shows the energy profiles with O2 species. In this case, propylene oxide formation uniquely proceeds with lower activation energy than p-allyl formation. The former reaction is expected to proceed selectively. Figs. 18 and 19 shows
Fig. 17. Energy profiles along reaction coordinate for propylene oxide formation (black line) and p-allyl (gray line) with O2 molecular on cationic Au7 cluster. Basis set used is LanL2DZCspdd.
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Fig. 18. Optimize structures for local minima and TS along propylene oxide formation with O2 molecule on cationic Au7 cluster.
optimized structures for the local minima and TS for propylene oxide and p-allyl formation, respectively. The energy profiles with O species are shown in Fig. 20. The energetics for the two reactions is almost the same except for the products, and both the reactions are down hill, and expected to proceed without selectivity. Thus among the four types of reaction with Au7 cluster, the reactions with O species lead to very low selectivity judging from the similar energies of their TS’s. The selectivity between the two reactions is apparent with O2 species. p-allyl formation is favorable for neutral cluster, but propylene oxide formation is favorable for cationic cluster. Another interesting character is that the O–O bond is not completely broken even in the product structures. This may be ascribed to the wedge shape of Au7 cluster. After the O2 species forms the oxidation product (either
Fig. 19. Optimize structures for local minima and TS along p-allyl formation with O2 molecule on cationic Au7 cluster.
propylene oxide or p-allyl), atomic oxygen is left on the cluster. This O species will react with other propylene molecule with lower selectivity. Experimentally, considerable amount of hydrogen gas as well as oxygen is introduced to the system. The role of hydrogen is expected to remove O species as water. Recently, Zwijnenburg et al. examined the Au/TiO2 system in detail by Mo¨ssbauer spectroscopy and other techniques [16]. They reported that metallic gold is the active phase, and no evidence for charge transfer between support and Au particle. However this result does not contradict to our result that cationic cluster is favorable for propylene oxide formation, since slightly electron deficient Au cluster is brought by O2 molecules adsorbed
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Fig. 20. Energy profiles along reaction coordinate for propylene oxide formation (black line) and p-allyl (gray line) with O atom on cationic Au7 cluster. Basis set used is LanL2DZCspdd.
around the reaction sites as the spectators. In a recent work, Haruta et al. stressed the roles of TiO2 support [17]. Stangland et al., reported that isolated Ti atoms are favorable for high selectivity and reactivity by suppressing cracking of propylene [18]. Thus more attention is paid to TiO2 supports rather than Au catalysts, and more sophisticated models including the TiO2 as well as Au clusters become inevitable for the next step.
Culture. The authors are grateful for helpful discussion to Dr. Katsumi Nakashiro and Dr. Tomoatsu Iwakura of Mitsubishi Chemical Co. The authors express their thanks to the undergraduate course students, Toshitaka Tanaka and Hiroaki Katsuyama for their intensive works on drawing figures.
4. Concluding remark
[1] M. Haruta, Catal. Today 36 (1997) 153. [2] T. Hayashi, K. Tanaka, M. Haruta, J. Catal. 178 (1998) 566. [3] H. Kobayashi, K. Nakashiro, T. Iwakuwa, Theor. Chem. Acc. 102 (1999) 237. [4] H. Kobayashi, K. Nakashiro, T. Iwakura, Internet Electron. J. Mol. Des. 1 (2002) 620. [5] A.D. Becke, J. Chem. Phys. 98 (1993) 1372. [6] P.M.W. Gill, B.G. Johnson, J.A. Pople, Int. J. Quantum Chem. Symp. 26 (1992) 319. [7] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, V.G. Zakrzewski, J.A. Montgomery Jr., R.E. Stratmann, J.C. Burant, S. Dapprich, J.M. Millam, A.D. Daniels, K.N. Kudin, M.C. Strain, O. Farkas, J. Tomasi, V. Barone, M. Cossi, R. Cammi, B. Mennucci, C. Pomelli, C. Adamo, S. Clifford, J. Ochterski, G.A. Petersson, P.Y. Ayala, Q. Cui, K. Morokuma, D.K. Malick, A.D. Rabuck, K. Raghavachari, J.B. Foresman, J. Cioslowski, J.V. Ortiz, B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. Gomperts, R.L. Martin, D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A. Nanayakkara, C. Gonzalez, M. Challacombe, P.M.W. Gill, B. Johnson, W. Chen, M.W. Wong, J.L. Andres, C. Gonzalez, M. Head-Gordon, E.S. Replogle, J.A. Pople, GAUSSIAN 98, Revision A.5, Gaussian Inc., Pittsburgh, PA, 1998. [8] J.C. Slater, Phys. Rev. 81 (1951) 385. [9] A.D. Becke, Phys. Rev. A38 (1988) 3098. [10] S.H. Vosko, L. Wilk, M. Nusair, Can. J. Phys. 58 (1980) 12001; L. Wilk, S.H. Vosko, J. Phys. C: Solid State Phys. 15 (1982) 2139. [11] C. Lee, W. Yang, R.G. Parr, Phys. Rev. B37 (1988) 785. [12] P.J. Hay, W.R. Wadt, J. Chem. Phys. 82 (1985) 270; W.R. Wadt, P.J. Hay, J. Chem. Phys. 82 (1985) 284; P.J. Hay, W.R. Wadt, J. Chem. Phys. 82 (1985) 299. [13] T.H. Dunning Jr., in: H.F. Schaefer III (Ed.), Modern Theoretical Chemistry, Plenum Press, New York, 1976, p. 1. [14] H. Nakatsuji, H. Nakai, K. Ikeda, Y. Yamamoto, Surf. Sci. 384 (1997) 315. [15] H. Nakatsuji, Z.-M. Hu, H. Nakai, K. Ikeda, Surf. Sci. 387 (1997) 328. [16] A. Zwijnenburg, A. Goossens, W.G. Sloof, M.W.J. Craje, A.M. van der Kraan, L.J. de Jongh, M. Makkee, J.A. Moulijn, J. Phys. Chem. B106 (2002) 9853. [17] B.S. Uphade, T. Akita, T. Nakamura, M. Haruta, J. Catal. 209 (2002) 331. [18] E.E. Stanglang, B. Taylor, R.P. Andres, W.N. Delgass, J. Phys. Chem. B109 (2005) 2321.
The authors’ previous works on ethylene oxidation on Ag5 cluster were extended to investigate propylene oxidation. The oxidation mechanisms were compared between ethylene and propylene and between Ag and Au metals. Au metal was not a good catalyst for ethylene oxide formation since adsorption of the dissociated O atom is not stable on Au5 cluster. The energetics of propylene and oxygen species on Ag5 cluster reveals that the H atom abstraction leading to p-allyl occurs more easily than the O atom insertion leading to propylene oxide with either O2 or O species as oxidizing agent. The same reactions were reexamined with larger Ag7 cluster modeling the step site. The p-allyl formation was favored with O2 molecule, and both the two reactions proceed without selectivity with O atom. The energetics was also evaluated using cationic and anionic clusters. In both reactions, cationic clusters most destabilized the initial O species adsorption but most stabilized the product. We could not find favorable charge states for propylene oxide formation. Propylene oxidation was examined with Au7 cluster. For the neutral cluster, the p-allyl formation was favored with O2 molecule, and both the reactions proceeded equally with O atom, which was the same trend as that obtained for Ag7 cluster. However, propylene oxide formation was found to be advantageous to p-allyl formation for the cationic Au7 cluster with O2 molecule. This unique result was obtained only on this condition. With O atom, the oxidation again proceeded without selectivity. Acknowledgements This research was supported by the Grant-in Aid for Scientific Research from the Ministry of Education, Science, Sports, and
References