ARTICLE pubs.acs.org/JPCA
Comparative ab Initio Study of CO Adsorption on Scn and ScnO (n = 2 13) Clusters Yanbiao Wang,† Guangfen Wu,† Jinli Du,† Mingli Yang,‡ and Jinlan Wang*,†,§ †
Department of Physics and §School of Chemistry & Chemical Engineering, Southeast University, Nanjing 211189, P. R. China ‡ Institute of Atomic and Molecular Physics, Sichuan University, Chengdu 610065, China
bS Supporting Information ABSTRACT: Using a cluster model, we investigated the similarities and differences in chemical activity and the magnetic properties of Scn clusters (n = 2 13) and their oxides, ScnO, toward CO molecule adsorption via a spin-polarized density functional theory approach. The Scn and ScnO clusters have similar chemical activity at small sizes of n = 2 10, whereas remarkable differences are observed at large sizes of n = 11 13. More interestingly, different magnetic responses are found in the Scn and ScnO clusters with the presence of CO molecule: The magnetic moment is attenuated significantly for Scn with n = 2, 4, 12, and 13, whereas for ScnO, it is enhanced at n = 4 and 13 and is reduced for n = 7, 8, 10, and 11. In particular, the magnetic moment remarkably increases from 7 μB of Sc13O to 13 μB of Sc13OCO, whereas it reduces from 19 μB of Sc13 to 5 μB of Sc13CO.
I. INTRODUCTION Small clusters exhibit intriguing physical and chemical properties different from their bulk phase because of their large surface volume ratio, significant size, and quantum effects. Transition metal (TM) clusters are of particular interest resulting from their great diversity in geometrical structure, electronic configuration, and spin state.1 For example, Cr and Mn are both antiferromagnetic in the bulk phase; however, net magnetic moments have been found in chromium (Cr8 156) and manganese (Mn5 22) clusters.2,3 Remarkable magnetic moments were also detected in the clusters of Rh9 60 and Sc5 20, even though their corresponding bulks are paramagnetic.4,5 Another wellknown example is gold, whose clusters show exceptional chemical activity toward CO at low temperature while their bulk is inert.6 Transition metal oxide clusters with strong coupling between O and TM atoms usually exhibit quite different structural, electronic, magnetic, and catalytic properties from their bulks and their corresponding TM clusters. For instance, (V2O5)n (n = 2 5, 8, 10, 12) clusters prefer to form polyhedral cages, whereas vanadium oxide in solid phase takes layered structure and the Vn clusters at the same sizes tend to adopt compact structures.7 Although Fe clusters are ferromagnetic, the magnetic moments of their oxide clusters decline markedly with increasing O concentration, until quenching in O-rich environment.8 The Cr2 cluster possesses an antiferromagnetic ground state, while the incoming of O atom makes the ferromagnetic state more energetically favorable.9 Moreover, iron oxide in bulk phase is a poor catalyst; however, its clusters can activate N O and C O bonds effectively.10,11 Similarly, vanadium bulk or particles are scarcely used as catalysts, but their corresponding oxide particles are widely exploited in a variety of industrially relevant processes.12 18 r 2011 American Chemical Society
Meanwhile, the adsorption of small molecules on TM clusters has complicated influences on the magnetic properties of the clusters.19 25 For instance, molecular NO (or CO) adsorption on rhodium (or cobalt) clusters quenches the magnetic moment of the adsorbate clusters;19,20 atomic O adsorption has negligible impact on the magnetization of cobalt clusters21 but remarkably enhances the magnetic moments of iron and chromium clusters;22 24 the hydrogen chemisorption also increases the magnetic moment of iron clusters.25 Compared to the extensive studies on adsorption behaviors of metal clusters, however, there is still a lack of knowledge about how the adsorption of small molecules affects the geometrical, electronic, and magnetic properties of metal oxide clusters. Recently, the structures and magnetic properties of bare Scn (n = 2 14)26,27 and ScnO clusters have been systematically studied; the additional O atom in ScnO can induce either magnetic enhancement or magnetic reduction for not only the O adsorption on Scn surfaces28 but also the O doping into them.29 Some further interesting questions may be raised: what are the differences in their chemical activity between TM and their oxide clusters? How does their interaction with adsorbates affect the electronic and magnetic properties of the TM oxide clusters? Starting from our earlier work of CO molecule adsorption on pure Scn clusters,30 we further explore in this work the interaction of CO with ScnO clusters within the framework of a spinpolarized density functional theory (DFT) approach, aiming to compare the similarities and differences in the chemical activity and magnetic response between Scn and ScnO clusters toward CO. Our calculations reveal that ScnO clusters of n = 11 and Received: August 28, 2011 Revised: December 6, 2011 Published: December 06, 2011 93
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Figure 1. Low lying isomers of ScnOCO complexes (n = 2 7): purple, isolated O; red, O in CO; black, C; gray, Sc.
12 show better chemical activity than the pure Scn clusters. Moreover, the incoming CO molecule may alter the magnetic moments of ScnO, which are structure- and size-dependent, but the variations could be different from those of corresponding pure Scn clusters. Figure 2. Low lying isomers of ScnOCO complexes (n = 8 12): purple, isolated O; red, O in CO; black, C; gray, Sc.
II. COMPUTATIONAL METHOD We used the Perdew Burke Ernzerhof (PBE)31 functional and the double numerical basis set including d-polarization functions (DND) for the O and C atoms and a DFT based relativistic semicore pseudopotential (DSPP)32 for the Sc atom, as implemented in the DMol3 package,33 which were employed in our previous works on the bare Scn clusters and their interaction with atomic O and CO molecule.26,28,30 This method well reproduced the measured magnetic moments of the Scn clusters, the Sc O bond length (1.674 vs 1.668 Å), and the vibration frequency (984 vs 965 cm 1).28,34 Moreover, the calculated CO bond length and vibration frequency are 1.117 Å and 2334 cm 1, respectively, in good accordance with the experimental values of 1.128 Å and 2143.2 cm 1.35 More details are accessible in our previous works.26,28,30 In addition to the most stable ScnO clusters, a number of lowlying isomers which were obtained in our previous work28 must be taken into account in looking for the energetically favorable ScnO CO complexes since the interaction may change significantly the energy orders of ScnO clusters, and the ambient conditions like temperature may obscure the energy differences between the low-lying isomers. In this study, a large number of initial ScnO CO conformers were built starting from all the lowlying ScnO isomers we previously reported.28 We examined all independent adsorption sites for every conformer, and at every site we considered three different adsorption patterns (atop site adsorption (ASA), bridge site adsorption (BSA), and hollow site adsorption (HSA)) with CO initially about 1.5 Å away from the sites. Meanwhile, various spin multiplicities were examined to identify the energetically preferred spin state. All the geometries were optimized without symmetry constraint, and all the most stable structures were further confirmed to be local minima via harmonic vibration frequency computations.
Figure 3. Low lying isomers of Sc13OCO complexes: purple, isolated O; red, O in CO; black, C; gray, Sc.
Figures 1, 2, and 3. Some other geometrical and electronic properties, such as the HOMO LUMO gaps and C O bond lengths, are summarized in Table S1 in the Supporting Information. Both C and O atoms may bind with the Sc atoms in ScnO clusters. The CO bond is elongated compared to free CO molecule, but the C O distances still remain in the range of 1.27 1.50 Å. Both hollow and bridge site adsorptions (HAS and BSA) are noted among the low-lying isomers. CO is at hollow site for n = 3, 4, and 6 and at bridge site for n = 2, 5, and 7 13 in their ground states. Moreover, the C atom generally forms more coordinates to Sc atoms than O does, due to the sp3 hybridization of C. For Sc2OCO, the most stable structure is a triplet planar conformer (1a), in which the CO molecule and the isolated O atom sit on the opposite sides of the Sc Sc bond. A singlet threedimensional (3D) isomer (1a-i) is less stable than the most stable one by 0.108 eV in energy, in which CO is perpendicular to the
III. RESULTS AND DISCUSSION A. Isomers and Corresponding Spin States of ScnOCO (n = 2 13). The low-lying isomers of ScnOCO, n = 2 13, with
their relative energies smaller than 0.20 eV are displayed in 94
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Sc Sc bond. The lowest energy structure of Sc3OCO is a quartet with CO HSA to the Sc3 cluster (1b). In the case of Sc4OCO, the energetically preferred structure is again a HSA configuration in triplet state (1c). For Sc5OCO, two BSA conformers are found with their energy difference of 0.043 eV and the O of CO locating in different sites, as shown in Figures 1d and 1d-i. For Sc6OCO, five isomers with close energy, two in singlet state and three in triplet state, are identified with CO binding at different sites of a singly capped octahedron conformer of Sc6O; the lowest energy structure is an HSA singlet (1e), followed by a triplet (1e-i) with 0.022 eV higher energy. Another singlet (see Figure 1e-ii) with CO and atomic O on two adjacent surfaces is 0.029 eV higher in energy than the ground state. Two other triplet states (1e-iii and 1e-iv) are 0.035 and 0.037 eV higher in energy than the ground state, respectively. In addition, a BSA configuration (1e-v) is also identified, which is 0.189 eV higher in energy and has high spin (quintet). In contrast, CO in the best three Sc7OCO structures adopts BSA. The second (1f-i) and third (1f-ii) lie 0.066 and 0.154 eV above the lowest-energy structure, respectively. Similar to the case of Sc7OCO, CO is also found to be BSA for n = 8 12, as displayed in Figure 2. The lowest energy structure (2a) of Sc8OCO is a singlet state. For Sc9OCO, the ground state is a quartet (2b), 0.136 eV more stable than the other quartet (2b-i) and 0.149 eV more stable than the doublet (2b-ii), respectively. In Sc10OCO, a singlet structure (2c) with CO far away from the isolated O atom has the lowest energy. The second best structure (2c-i) is a quintet state and 0.181 eV higher in energy. For Sc11OCO, five different isomers are identified with CO adsorbed on different sites of the O-capped-square-antiprism parent conformer. The energetically most preferred one is in a quartet state, and the other four (2d-i 2d-iv) are sextet and 0.130, 0.138, 0.156, and 0.173 eV higher in energy, respectively. For Sc12OCO, five O-capped interpenetrating pentagonalbipyramid-octahedron-based isomers are addressed. Two energetically nearly degenerate (ΔE = 0.004 eV) quintet states (2e and 2e-i) were located; a triplet (2e-ii) and two other quintet states (2e-iii and 2e-iv) are found to be less stable by 0.054, 0.132, and 0.138 eV in energy, respectively. Nine low-energy isomers based on O-capped icosahedron structures are identified for Sc13OCO, as displayed in Figure 3. All these structures have remarkably high magnetic moment of either 9 μB or 13 μB. The most stable structure is a BSA with the CO molecule and isolated O atom landing on two neighboring facets of the cluster (3a), 0.017 eV more stable than the second structure (3b) in which CO is slightly away from the isolated O atom. Four other low-lying structures in HSA pattern (3c 3f) are 0.041, 0.047, 0.083, and 0.091 eV higher in energy, respectively. Another BSA structure (3g) and two HSA conformers (3 h, 3i) are less stable than the ground state by 0.136, 0.155, and 0.170 eV in energy, respectively. B. Size-Dependent C O Bond Lengths, Adsorption Energies, and Charges. The bond length variations and the absorption energy (Ead) of the adsorbates are two important indicators of chemical activity of the clusters. The adsorption energy of CO on ScnO is calculated by Ead(n) = E[ScnO] + E[CO] E[ScnOCO], where E[*] is the total energy of relaxed ScnO, CO, and ScnOCO, respectively. The C O bond lengths and adsorption energies of ScnOCO, are presented in Figure 4, together with those of ScnCO for comparison. The C O bond lengths in ScnOCO complexes range from 1.27 to 1.50 Å, lengthened by 13.7 34.3%, with respect to that of isolated CO (RC O = 1.117 Å). The shortest C O bond is
Figure 4. CO bond lengths (a), adsorption energies (b), and net charges on CO (c) in the lowest-energy Scn CO (black) and ScnO CO (red) clusters of n = 2 13.
found in Sc2OCO, but it has an evident increase from n = 2 to n = 3. Afterward, odd even oscillation is observed in the range of n = 4 10, with bottoms at even n and peaks at odd n. At these sizes, the C O bond lengths of ScnOCO and ScnCO show very similar size-dependent variations, indicating that the incoming of O atom does not change the chemical activity of the Scn (n = 2 10) clusters toward CO. The C O bond lengths in ScnOCO are 1.47, 1.48, and 1.41 Å for n = 11, 12, and 13, respectively. The corresponding values in ScnCO are 1.37, 1.37, and 1.48 Å, respectively, implying that the chemical activity of ScnO is more or less enhanced at n = 11, 12 and is reduced at n = 13, as compared with the corresponding bare Scn clusters. The reduction at n = 13 can be partly attributed to its different magnetic response upon CO adsorption, which will be discussed in the magnetic section. Adsorption energies (Ead) of ScnOCO clusters also show clear size-dependent variations. The minimum is identified at n = 2, and small oscillations are observed in the range of n = 3 12. The Ead drops considerably at n = 13. Almost the same size dependence in Ead is observed for ScnCO and ScnOCO clusters (see Figure 4b), with the exceptions at n = 2, 8, 12, and 13 for which the differences are comparatively large. The Ead of CO on Sc2O is about 1.0 eV lower than on Sc2, while the Ead values on ScnO of n = 8, 12, and 13 are higher than those on the corresponding Scn by about 0.5 eV. Net charges of CO in ScnOCO are presented in Figure 4c. The atomic charges of CO in ScnCO and isolated CO are also provided for comparison in Table S2. Large charge transfer is observed for n = 5, 7, 9, 11, and 12, while relatively small charge transfer is found for n = 2 4, 6, 8, 10, and 13. Interestingly, the charges and adsorption energies of the ScnCO and ScnOCO show similar size-dependent variations, suggesting that the adsorption of CO on ScnO or Scn clusters is mainly governed by a charge-transfer process, in which both the C and O atoms accept electrons from the nearby Sc atoms. More precisely, large charge transfer corresponds to large adsorption energy, and vice versa. C. Size-Dependent Magnetic Moment and Magnetic Response upon CO Adsorption. The total magnetic moments of the lowest energy structures of ScnOCO clusters are presented in Figure 5a together with those of Scn, ScnCO, and ScnO for comparison. Two types of odd even oscillations are noted in the magnetic moments of ScnOCO ranging from n = 2 to 11: the magnetic moment peaks at even n and bottoms at odd n for n = 2 5, but peaks at odd n and bottoms at even n for n = 5 11. 95
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Figure 5. (a) Total magnetic moments of the lowest energy structures of Scn, ScnCO, ScnO, and ScnOCO (n = 2 13). (b) Magnetic moment difference relative to Scn and ScnO are plotted. Cyan, Scn; black, ScnCO; blue, ScnO; red, ScnOCO.
In addition, large magnetic moments are noted on Sc12OCO (4 μB) and Sc13OCO (13 μB). To illustrate the influence of incoming CO on the magnetism of ScnO clusters, the magnetic moment changes of ScnOCO with respect to ScnO are displayed in Figure 5b. The corresponding values of ScnCO and Scn clusters are also given for comparison. Clearly, the magnetic response of ScnO to CO is more complicated than that of Scn to CO. After the adsorption of CO, the magnetic moment of ScnO retains for n = 2, 3, 5, 6, 9, and 12, decreases for n = 7, 8, 10, and 11, and increases for n = 4 and 13. In contrast, the magnetism of Scn is either kept (n = 3, 5 11) or weakened (n = 4, 12, 13) on the loading of CO. In particular, the magnetic moment of Sc13O, 7 μB, increases to 13 μB in Sc13OCO, while that of Sc13 (19 μB) decreases to 5 μB in Sc13CO. Some other intriguing behaviors of the magnetic response in Figure 5 are worthy of mentioning: for example, the loading of both CO and O does not change the total magnetic moment of Sc8 (4 μB), which is, however, quenched in Sc8OCO. To understand the mechanism of the different magnetic responses toward CO adsorption, we investigated the s-, p-, and d-projected density of states (PDOS) for Sc8,13 and Sc8,13O, and the local density of states (LDOS) for Sc8,13 in Sc8,13CO and Sc8,13O in Sc8,13OCO as a case study (Figure 6). The corresponding total PDOS of Sc8,13CO and Sc8,13OCO, as well as the LDOS of CO in Sc8,13CO and Sc8,13OCO, are given in Figure S1 for comparison. The magnetism of the adsorbed complex ScnOCO mainly stems from the contribution of the transition metal cluster Scn and negligible quantity from the nonmetal adsorbate. Near the Fermi level (EF), as shown in the figures, the d-projected LDOS of Sc8,13O is dominant in Sc8,13OCO accompanied by a small amount of s- and p-projected DOS, whereas much less magnitude and more symmetric distribution of the LDOS are found on CO (see Figure S1). Therefore, the magnetism mainly comes from the asymmetric distribution of the majority (spin-up) and minority (spin-down) spins to the EF in the d-projected LDOS of Sc8,13O domain in Sc8,13OCO. The charge transfer between the cluster and adsorbate may affect the magnetism of both parts, reflected by the DOS plotted in Figure 6. The occupied (unoccupied) LDOS of Sc8,13O (Sc8,13) cluster near EF are pushed upward (downward), while those of the adsorbates are both pushed downward. Molecular CO, which is paramagnetic in isolated status, shifts downward indiscriminately when it attaches to the cluster, thus retaining its paramagnetic nature in the adsorbed complexes. Furthermore, the position of the majority- and minority-spin near EF with respect
Figure 6. Local s-, p-, and d-projected density of states (LDOS) of Scn in ScnCO, and ScnOCO together with PDOS of pure Scn and ScnO are displayed (n = 8 and 13). The dashed line refers to the Fermi level which is shifted to zero. The Fermi level corresponds to HOMO here. Gaussion broadening of half-width 0.002 eV is used. Magnetic moment is given in the box.
to EF plays a significant role in determining the magnetism. Some predictions can be made by inspecting the PDOS and LDOS of these systems. For Sc8, the nodes of both highest occupied majority-spin and minority-spin states locate at the EF. As a result, both spin states have equal preference for donating electrons, making its magnetism retained after CO adsorption. For Sc8O, the node of majority-spin state is at the EF, while the minority-spin state transverses the EF; therefore, the magnetism of Sc8O is quenched because of electron transfer from majorityspin state to CO. This phenomenon is also evident for n = 13. There is a remarkable translation of the occupied majority-spin and minority-spin states near EF in Sc13. The peak of the spin-up state traverses the EF, while that of the spin-down states is below the EF. Such band structures would facilitate electron donation from the spin-up state to CO and spin-down state. Therefore, the magnetic moment of Sc13 is reduced upon CO adsorption. However, for Sc13O, both majority-spin and minority-spin states traverse the EF. According to Hund’s rule of maximum spin multiplicity, the minority-spin states are more likely to donate electrons to keep the parallel spin distribution on degenerate energy levels and as a consequence enhance the magnetism of Sc13OCO.
IV. CONCLUSION We have systematically studied the structural, electronic, magnetic, and adsorption properties of CO on ScnO (n = 2 13) clusters by using gradient-corrected DFT calculations and compared their chemical activity and magnetic responses with those on pure Scn clusters. CO molecule prefers hollow site adsorption on ScnO for n = 3, 4, 6 and bridge site adsorption for 96
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n = 2, 5 and n g 7. The Scn and ScnO clusters have similar chemical activity toward CO molecule in the range of n = 2 10, while relatively better activity is found for ScnO with n = 11 and 12. The well-matched size-dependent variations of C O bond lengths, adsorption energies, and net charges on CO suggest that charge transfer plays an important role in the interaction between CO and TM oxide clusters. The incoming CO molecule leads to distinctly different magnetic responses in the Scn and ScnO clusters. The magnetic moments of the adsorbed complexes are attenuated for Scn of n = 2, 4, 12, and 13 but are enhanced at n = 4 and 13 and are reduced at n = 7, 8, 10, and 11 for ScnO. Particularly, the magnetic moment increases remarkably from 7 μB of Sc13O to 13 μB of Sc13OCO, while it reduces from 19 μB of Sc13 to 5 μB of Sc13CO.
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’ ASSOCIATED CONTENT
bS
Supporting Information. Magnetic moments, HOMO LUMO gaps, and C O bond lengths of isomers (Table S1); atomic charges of CO in ScnCO and isolated CO (Table S2); total PDOS and LDOS (Figure S1); HOMO LUMO gaps as a function of cluster size (Figure S2). This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT This work is supported by the NBRP (Contract Nos. 2009CB623200, 2010CB923401, 2011CB302004, 2011CB606200), NSF (Grant Nos. 20873019, 20873088, 11074035, 21173040), SRFDP (Contract No. 20090092110025), the Outstanding Young Faculty Grant, and Peiyu Foundations of SEU in China. The authors acknowledge the computational resource at Department of Physics, SEU. ’ REFERENCES (1) Alonso, J. A. Chem. Rev. 2000, 100, 637. (2) Bloomfield, L. A.; Deng, J.; Zhang, H.; Emmert, J. W. In Proceedings of the International Symposium on Cluster and Nanostructure Interfaces; Jena, P., Khanna, S. N., Rao, B. K., Eds.; World Publishers: Singapore, 2000; p 131. (3) Knickelbein, M. B. Phys. Rev. B 2004, 70, 014424. (4) Cox, A. J.; Louderback, J. G.; Bloomfield, L. A. Phys. Rev. Lett. 1993, 71, 923. (5) Knickelbein, M. B. Phys. Rev. B 2004, 71, 184442. (6) (a) Haruta, M.; Kobayashi, T.; Sano, H.; Yamada, N. Chem. Lett. 1987, 34, 405. (b) Haruta, M. Catal. Today 1997, 36, 153. (7) (a) Vyboishchikov, S. F.; Sauer, J. J. Phys. Chem. A 2001, 105, 8588. (b) Wu, X.; Ray, A. K. J. Chem. Phys. 1999, 110, 2437. (8) Shiroishi1, H.; Oda1, T.; Hamada, I.; Fujima, N. Eur. Phys. J. D 2003, 24, 85. (9) Tono, K.; Terasaki, A.; Ohta, T.; Kondow, T. Phys. Rev. Lett. 2003, 90, 133402. (10) Reddy, B. V.; Khanna, S. N. Phys. Rev. Lett. 2004, 93, 068301. (11) Reilly, N. M.; Reveles, J. U.; Johnson, G. E.; Khanna, S. N.; Castleman, A. W., Jr. J. Phys. Chem. A 2007, 111, 4158. (12) Dong, F.; Heinbuch, S.; Xie, Y.; Bernstein, E. R.; Rocca, J. J.; Wang, Z. C.; Ding, X. L.; He, S. G. J. Am. Chem. Soc. 2009, 131, 1057. (13) Dong, F.; Heinbuch, S.; Xie, Y.; Rocca, J. J.; Bernstein, E. R.; Wang, Z. C.; Deng, K.; He, S. G. J. Am. Chem. Soc. 2008, 130, 1932. 97
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