Density Functional Theory Study of the Interaction of Carbon

Sep 10, 2010 - Using spin-polarized density functional calculations, we have studied the interaction of carbon monoxide (CO) with bimetallic ConMn (n ...
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Density Functional Theory Study of the Interaction of Carbon Monoxide with Bimetallic Co-Mn Clusters Jinli Du,†,‡ Guangfen Wu,† and Jinlan Wang*,† Department of Physics, Southeast UniVersity, Nanjing, 211189, P. R. China, and Department of Physics, China Pharmaceutical UniVersity, Nanjing, 210009, P. R. China ReceiVed: July 8, 2010; ReVised Manuscript ReceiVed: August 15, 2010

Using spin-polarized density functional calculations, we have studied the interaction of carbon monoxide (CO) with bimetallic ConMn (n ) 1-6) and ConMn6-n (n ) 0-6) clusters. Various adsorption sites including atop, hollow, and bridge adsorption patterns and different possible spin states are considered. The CO molecule prefers to adsorb at the Co site rather than at the Mn site. Atop adsorption structure is energetically more favored over the hollow and bridge adsorption ones for the bimetallic clusters with an exception of Co5Mn. Large adsorption energy is found at Co3Mn, Co2Mn4, and Co3Mn3, associating with the relative stability of the bare Co-Mn clusters and the electrostatic interactions as well as adsorption patterns. The activation of the C-O bond and the red shift of the C-O stretching frequency are sensitive to the adsorption sites and high chemical activity is identified for Co6, Co5Mn, and Mn6 clusters. More interestingly, the adsorption of CO has different influence on the magnetism of the clusters: the magnetic moment remains unchanged for CoMn and Co2Mn, while it is reduced by 2 µB for ConMn (n ) 3-6) and ConMn6-n (n ) 0-5) and is enhanced by 2 µB for Mn6 when a CO molecule is loaded to the cluster. I. Introduction Transition metal (TM) clusters have attracted great attention due to their potential applications in heterogeneous catalysis as well as in nanostructured electronic and magnetic devices.1–8 For example, the magnetic clusters of iron and cobalt are good catalysts for the Fischer-Tropsch process and the growth of single-walled and multiwalled carbon nanotubes.5–8 Small molecules such as CO, NO, and O2 adsorb on the cluster as the first step for catalytic process, which is of particular importance to understand the catalytic mechanism and to design improved catalysts with high selectivity or resistance to poisons.8 On the other hand, loading a small molecule or atom can modify the electronic and magnetic properties of TM clusters.9–24 The magnetism of Fen (n ) 2-6) clusters is attenuated upon the adsorption of CO,9 while it is enhanced upon hydrogen10 or oxygen11 atomic chemisorption. Magnetic reduction is also found in CO adsorption on isolated12–14 or deposited15 cobalt clusters; however, the O atomic adsorption has little influence on the magnetism.16 Both CO and H adsorption on Nin clusters reduce the magnetic moments for all the sizes studied, while O atomic adsorption results in the magnetic enhancement or reduction.17 In our recent study, CO adsorption on Scn clusters generally attenuates the magnetic moment, and especially for Sc13, whose magnetic moment is reduced significantly from 19 to 7 µB upon adsorption,18 while O atomic adsorption reduces the magnetism of Sc2,4,6,12,13,14 clusters partly or totally and enhances the magnetic moment of Sc7,9,11.19 A reduced magnetic moment is also found when rhodium clusters adsorb a NO molecule.20 Despite these great progresses, most studies have been focused on the interaction of a small molecule or atom with monocomponent TM clusters rather than with bimetallic clusters. * Corresponding author. E-mail: [email protected]. † Southeast University. ‡ China Pharmaceutical University.

Song et al.25 have carried out density functional theory (DFT) calculations on Pt-Au clusters and their interaction with CO. They found that CO favors adsorption to Pt instead of Au and about one-third of the clusters studied change the magnetic moment when a CO molecule is absorbed. Explorations of CO adsorption on binary gold clusters26,27 have shown that the adsorption affinity trend for dimers is Au2 > TMAu > TM2 (TM ) Ag/Cu), TM2 > TMAu > Au2 (TM ) Pd/Pt), while Au3 > TMAu2 > TM2Au > TM3 (TM ) Ag/Cu), Pd3 > Pd2Au > PdAu2 > Au3, and PtAu2 > Pt2Au > Pt3 > Au3 for the trimers. In fact, bimetallic clusters whose electronic structure, magnetism, and chemical reactivity can be tuned by carefully choosing chemical components with appropriate percentage, may offer more opportunity for fabricating good catalyst or even magnetocatalysts. E.g., bimetallic Pt-Au clusters can catalyze important reactions such as low-temperature CO oxidation28 and NO reduction.29 TMFe5 and TMCo5 (TM ) Mn, Mo, and Zr) clusters are found to be good magnetic catalysts for the Fischer-Tropsch process with improved resistance to sulfur poison and without degradation of adsorption properties of the active sites.8,30,31 In our recent work, we have studied pure Con and Mn-doped Con-1Mn clusters and found that the replacement of Co with Mn enhances the magnetic moments by about 2 µB,32 which is consistent with the experimental observation for larger sizes.33 How will the magnetic properties of the bimetallic Co-Mn clusters change when a small molecule such as CO is loaded? What is the chemical reactivity of Co-Mn clusters to activate a CO molecule? What is the influence of cluster size and chemical component on the reactivity of Co-Mn clusters to adsorb CO? To our knowledge, the study on the interaction of the Co-Mn bimetallic clusters with CO molecule is very limited either experimentally or theoretically so far,8 and further detailed studies are needed. In this article, we have performed spinpolarized DFT cacluations to investigate CO molecule adsorption on ConMn (n ) 1-6) and ConMn6-n (n ) 0-6) clusters.

10.1021/jp106321s  2010 American Chemical Society Published on Web 09/10/2010

Interaction of CO with Bimetallic Co-Mn Clusters Our results show that the CO molecule tends to adsorb to the Co site instead of the Mn site. The Co6, Co5Mn, and Mn6 clusters are found to have high chemical activity to activate the CO molecule. Most interestingly, the magnetic moments of ConMn (n ) 3-6) and ConMn6-n (n ) 0-5) are reduced by 2 µB while it is enhanced by 2 µB for Mn6 upon adsorption of the CO molecule. II. Computational Method The calculations were carried out using the spin-polarized DFT method as implemented in the Vienna Ab initio Simulation Package (VASP).34 To compare with the bare Co-Mn cluster, the same computational approach as our previous work32 was employed in this work. That is, we exploited the Perdew-BurkeErnzerhof (PBE) exchange-correlation functional34 and projector augmented wave method (PAW).35 The accuracy of this combination of PBE/PAW was verified on Co2, Mn2, and the mixed CoMn clusters in our previous work32 and further assessed on the free CO molecule. The calculated C-O bond length and vibrational frequency of the CO molecule are 1.143 Å and 2123 cm-1, respectively, which are in good agreement with the experimental values of 1.128 Å and 2143 cm-1.36 In the optimization, we placed the cluster in a cubic supercell with an edge length of 15 Å, which is sufficiently large to neglect the interaction between the supercells. The integration over the Brillouin zone was carried out at the Γ-point only. The energy cutoff was set to 350 eV. All the structures were optimized without symmetry constraint and with conjugate gradient methods until the Hellmann-Feynman forces acting on each atom was less than 0.005 eV/Å. In our previous work,32,37 considering various possible configurations as initial structures, we have located low-lying energy structures of the bare Co-Mn clusters. In this work, those low-lying bimetallic isomers were considered as the initial radials to adsorb a CO molecule. For each conformer, various adsorption sites including atop, bridge, and hollow sites are considered with CO binding to Co or Mn or the cluster surface at a distance of about 2.0 Å. For each adsorption pattern, possible adsorption sites were taken into account. We used a loose symmetry criterion for the bare clusters, as the CO adsorption will influence the structure of the Co-Mn cluster and the adsorbed structure will then be relaxed with no symmetry constraint. Such treatment is reasonable and is useful to reduce the number of independent adsorption sites. Moreover, we considered various magnetic states by fixing the spin states to optimize them. The lowest-energy structures were further verified to be true minima via harmonic frequency computations. III. Results and Discussion A. Low-Lying Isomers of ConMnCO (n ) 1-6) and ConMn6-nCO (n ) 0-6) Clusters. 1. ConMnCO (n ) 1-6). The equilibrium structures, relative energies with respect to the most stable configuration, and their corresponding magnetic moments of ConMnCO (n ) 1-6) together with the lowest energy structure of ConMn are displayed in Figure 1. As seen from the figure, CO adsorbs on the ConMn cluster in a molecular form with C rather than O bonding to metal atoms. The most stable structures of ConMnCO are all based on the ground state structures of the bare ConMn counterparts with certain distortion and lowered symmetry. The ground state of CoMnCO is a planar structure with CO atop adsorbing to Co and the Mn-Co-C angle is 93.9°. The Co-Mn bond length is elongated by 0.131 Å upon adsorption and the C-Co bond length is 1.702 Å. The C-O bond length

J. Phys. Chem. A, Vol. 114, No. 39, 2010 10509 in CoMnCO is 1.179 Å, which is slightly longer than that of the free CO molecule (1.143 Å). The magnetic moment of CoMnCO is 6 µB, which is the same as the bare CoMn cluster. The structures with the same spin state but different Mn-Co-C angles (132.7° and 180°) are 0.132 and 0.165 eV higher in energy, respectively. This indicates that the orientation of CO plays an important role in the determination of the stability of the cluster-CO complexes. The lowest energy configuration with CO adsorption at Mn site possesses much higher energy (∆E ) 1.215 eV), suggesting that CO favors to adsorb on the Co site instead of Mn. For Co2MnCO, both atop and bridge adsorption structures of CO binding to Co atom or Co-Co bond are identified with nearly equal energy (∆E ) 0.0003 eV) and they have a high magnetic moment of 9 µB. Another atop site adsorption structure with magnetic moment of 7 µB lies 0.085 eV higher in energy above the ground state. Two planar isomers with CO adsorption at the bridge site of Co-Mn and Co-Co bond are less stable by 0.111 and 0.133 eV higher in energy than the most stable atop site structure, respectively. In the case of Co3MnCO, a tetrahedron-based structure with CO atop adsorption to the Co atom is the most stable and it has a magnetic moment of 10 µB. We also identified two bridge site structures with different C-O orientations and different magnetic states: the one with the 12 µB moment is 0.032 eV and the other with 10 µB is 0.146 eV higher in energy than the most stable configuration, respectively. Three kinds of adsorption patterns, atop, bridge, and hollow site, were identified for CO adsorbing on the Co4Mn cluster. Similarly, CO atop site adsorption on the cluster is found as the most energetically favorable; the bridge and hollow site adsorption structures locate 0.079 and 0.118 eV above, respectively. All these structures possess a quite high magnetic moment of 13 µB. For Co5MnCO, an octahedron-based structure with 14 µB moment and with CO adsorption at a hollow site is the energetically most favored, which is followed by an atop adsorption structure with the same moment and with only 0.008 eV higher energy. The isomer with a 16 µB moment and with CO adsorption on another atop Co atom possesses much higher energy (∆E ) 0.475 eV). It should be pointed out that the structure of Co5MnCO obtained in ref 8 has the same moment (14 µB) but a different CO adsorption pattern (atop site) with our ground state structure. That is because the authors only considered atop site adsorption of CO to the cluster, while we found that the hollow site adsorption structure is energetically more favored than the atop and bridge site adsorption ones for Co5MnCO. The lowest energy structure of Co6MnCO is a capped tetragonal bipyramid based structure with CO adsorption on the atop Co site, which is energetically preferred over the hollow or bridge site adsorption ones by 0.016 or 0.038 eV, respectively. 2. ConMn6-nCO (n ) 0-6). To explore the influence of Mn concentration on the chemical reactivity and magnetic properties of Co-based catalyst, we have also performed DFT calculations on ConMn6-nCO (n ) 0-6). The equilibrium structures together with their energetic and magnetic information are presented in Figure 2. The most stable configuration of Co6CO is an octahedron-based hollow site adsorption structure with a magnetic moment of 12 µB. The C-O bond length (1.211 Å) is considerably larger than that of free CO molecule (1.143 Å). The atop and bridge adsorption structures are less stable than the ground state structure by 0.024 and 0.104 eV higher in energy, respectively. Similarly, the structure of Co6CO obtained in ref 8 has the same moment (12 µB) as our ground state structure but a different CO adsorption pattern (atop site). The

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Figure 1. Low-energy structures of ConMnCO and their corresponding bare clusters together with their magnetic moments and the energy relative to the most stable structure. The blue and cyan balls represent Co and Mn, and the yellow and red balls represent C and O atom.

case of Co5MnCO is discussed above and does not repeat here. For Co4Mn2CO, two atop site adsorption structures with different CO orientations are energetically preferred over the hollow site adsorption one by 0.171 or 0.009 eV, respectively. The most stable structure of Co3Mn3CO is also atop site adsorption, and it possesses a quite high magnetic moment of 18 µB. A similar structure with a magnetic moment of 8 µB is less stable by 0.128 eV higher in energy. Another atop site adsorption structure with an 18 µB moment has a much higher energy (∆E ) 0.243 eV) than the ground state structure. In the case of Co2Mn4CO, we identified three low-lying isomers with CO atop adsorption to Co atom in different CO orientations. The most stable structure is the CO molecule locating on the principal axis of the cluster with a magnetic moment of 20 µB. The structure with CO tilted bonding to the cluster with the same moment lies 0.047 eV above the most stable one. The similar motif of the most stable structure but in a different magnetic state (10 µB) is less stable by 0.129 eV higher in energy. The lowest energy structure of CoMn5CO is also CO atop site adsorption to the Co atom, and its favorable magnetic state is 12 µB. A high magnetic state of 22 µB is

identified as the low-lying isomer with 0.189 eV higher in energy than the ground state. Another similar motif with different CO orientation and with 14 µB moment lies 0.213 eV higher in energy. As for Mn6CO, a bridge site adsorption structure with C bonding to two Mn atoms and O bonding to the C atom and one of the Mn atoms is the most energetically favored and the favorable magnetic state is 10 µB. The CO atop site adsorption distorts the octahedron significantly and this structure locates about 0.03 eV higher in energy than the bridge site adsorption structure. The structure with CO bridge adsorption to the rhombus of the octahedron is 0.13 eV higher in energy with respect to the ground state. These two metastable structures are both of 14 µB magnetic moment. As discussed above, there are three characteristics for CO adsorption on the bimetallic Co-Mn clusters. First, the CO molecule prefers to adsorb on Co atoms rather than on Mn atoms. That is because Co atoms in Co-Mn clusters exhibit negative charges due to more electronegativity than Mn, which is favorable for electron transfer to the CO molecule. Second, the atop site adsorption structure is energetically favored over

Interaction of CO with Bimetallic Co-Mn Clusters

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Figure 2. Low-energy structures of Co6-nMnnCO and their corresponding pure clusters. The figure caption is the same as in Figure 1.

the bridge or hollow site adsorption ones with an exception of Co5MnCO. Third, for the same adsorption pattern, the orientation of CO greatly influences the relative stability of the cluster-CO complexes. B. Adsorption Energy. To evaluate the adsorption strength of bimetallic Co-Mn clusters to CO molecule, we computed adsorption energies of CO on the Co-Mn clusters and displayed them in Figure 3. The adsorption energies are defined as

Eads ) E[Co-Mn] + E[CO] - E[Co-Mn-CO] where E[*] is the total energy of CO molecule, Co-Mn clusters, and Co-Mn-CO complexes. As showed in Figure 3a, the adsorption energies of ConMnCO (n ) 1-6) decrease monotonically with n except for Co3MnCO, which has the maximum value. This indicates that Co3Mn interacts more strongly with CO than the other ConMn clusters. This can be understood in terms of the relative stability of the

bare ConMn clusters. Generally speaking, the more stable the bare cluster, the lower the reactivity of the cluster to adsorb CO. In our previous work,32 the second difference in energy of the bare ConMn clusters, ∆2E(n), shows the Co3Mn cluster is the least stable and thus it has the greatest adsorption energy to CO molecule. Moreover, the curve of adsorption energy shows that the adsorption ability is Co5MnCO < Co4MnCO < Co2MnCO < Co3MnCO, consistent with the relative stability of the bare clusters [Co5Mn > Co4Mn > Co2Mn > Co3Mn]. However, although exceptional stability is found for the Co5Mn cluster, its adsorption energy to the CO molecule is larger than that of Co6Mn. This is due to its different adsorption pattern where the CO molecule is hollow site adsorbed on the surface of the Co5Mn cluster and the CO atop site adsorbs to the Co atom in the Co6Mn cluster, the former having more metal-carbon bonds than the latter and thus inducing stronger binding to the CO molecule. To explore how the concentration of Co (or Mn) affects the adsorption strength of the bimetallic clusters, we also present

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Figure 3. (a) Adsorption energies of ConMnCO (n ) 1-6), Ead, as well as the second difference in energy of bare ConMn, ∆2E(n), versus cluster size obtained in ref 32. (b) Adsorption energies of ConMn6-nCO (n ) 0-6) and the strength of electrostatic interaction, qQ, vs the composition of Mn. Where the qQ represents the product of the charges on C atom and on Co or Mn atoms near adsorption site.

the adsorption energies of ConMn6-nCO (n ) 0-6) clusters in Figure 3b. Clearly, the adsorption energies decrease with the increase of Co atoms except for Mn6CO and CoMn5CO. The extremely small adsorption energy of CoMn5CO is attributed to the high stability of the bare cluster, as discussed above.38 The decreased adsorption energies with Co concentration can be quantitatively understood from the electrostatic interaction between the cluster and CO molecule. From charge population analysis, the electrons donated from the cluster to the CO molecule are mostly located on the O atom, and the C atom is positively charged. Moreover, the Co or Mn atoms at or near the adsorption site have a relatively large negative charge. Such charge distribution leads to strong electrostatic interaction between the C and Co/Mn atoms, which plays a major role in the determination of adsorption strength. Generally, the stronger the electrostatic interaction, the larger the adsorption strength. The strength of electrostatic interaction can be qualitatively reflected by the product qQ of the charges of C atom (q) and Co/Mn (Q) on the adsorption site, as shown in Figure 3b; the charges are obtained from natural orbital analysis (the computational details will be given below). The qQ curve shows a similar dependence to the number of Co atoms as the adsorption energy except for Mn6CO. The exception of Mn6CO may be associated with its different adsorption pattern, where CO is bridge site adsorbed to the cluster with more metal-C bonds and the formation of the Mn-O bond. Moreover, although the qQ is relatively small for Mn and C atoms, the electrostatic interaction between Mn and O is significant (+0.11e for Mn and -0.57e for O). Thus, both the stronger electrostatic interactions between the cluster and CO molecule and the more bonds of Mn-C and Mn-O enhance the adsorption strength of this cluster. This indicates that the adsorption strength of ConMn6-n clusters to CO depends heavily on the electrostatic interaction, adsorption pattern, and relative stability of the corresponding bare clusters. The adsorption energy reaching a maximum at n ) 2 implies that the adsorption ability can be enhanced by doping the appropriate percentage of Mn- to Cobased clusters.

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Figure 4. Stretching frequency and bond length of CO in (a) ConMnCO (n ) 1-6) and (b) ConMn6-nCO (n ) 0-6) complexes are plotted as a function of cluster size and the Co (or Mn) composition.

C. CO Stretching Frequency and Bond Length. To further evaluate the activation of CO, we presented the stretching frequency and bond length of CO in Co-Mn-CO complexes in Figure 4. Clearly, the frequencies show a totally opposite trend with the bond lengths as a function of cluster size or composition. Compared with the values of free CO (2123 cm-1 and 1.143 Å), the frequency in the Co-Mn-CO complexes is significantly reduced (red shift) while the bond length is remarkably elongated, suggesting that Co-Mn clusters can be good candidates to activate the CO molecule. Particularly larger C-O bond length and smaller frequency are found for Co6, Co5Mn, and Mn6, indicating their relatively high chemical activity. Moreover, the frequency and bond length of CO show clear site-dependent variations. The CO frequencies and bond lengths are in the range 1896.0-1953.2 cm-1 and 1.70-1.79 Å for the atop site adsorption (ConMnCO, n ) 1-4, 6, and ConMn6-nCO, n ) 1-4), while for hollow site adsorption (Co5MnCO and Co6CO) the values ranges from 1687.5 to 1691.7 cm-1 and from 1.20 to 1.21 Å, respectively, and for bridge site adsorption (Mn6CO), the value are 1549.5 cm-1 and 1.239 Å. This indicates that the CO frequency shift and bond activation are more sensitive to the adsorption site and are less sensitive to the cluster size and composition, which is similar to the interaction of CO with Pt-Au bimetallic clusters.39 D. Magnetic Moment. The total magnetic moments of the ground state structures of ConMn and ConMnCO (n ) 1-6) as well as ConMn6-n and ConMn6-nCO (n ) 0-6) are displayed in Figure 5a,b, respectively. For CoMn and Co2Mn, the magnetic moment remains unchanged when a CO molecule is loaded; however, that of ConMnCO (n ) 3-6) clusters is reduced by 2 µB as compared with the bare counterparts. Similar magnetic reduction is also observed for ConMn6-nCO (n ) 0-5). However, the magnetic moment of Mn6CO is enhanced by 2 µB upon CO adsorption. This indicates that the CO adsorption has a great influence on the magnetic properties of the clusters. To gain more insightful understanding of different magnetic response of the clusters upon the adsorption of CO, we performed natural population analysis (NPA) on the ground state

Interaction of CO with Bimetallic Co-Mn Clusters

Figure 5. Total magnetic moments of the ground states of (a) ConMn (n ) 1-6) and (b) ConMn6-n (n ) 0-6) clusters with and without CO are plotted as a function of cluster size and the Co (or Mn) composition.

structures of the bare and adsorbed clusters. The calculations were carried out using PBE functional and LANL2DZ basis set,40 implemented in the Gaussian 03 package.41 The NPA atomic charge and spin density are presented in Figure 6. As can be seen from the figure, Mn atoms possess quite large magnetic moments around 3.29-4.5 µB, and Co atoms have relatively small moments varying in the range 0.72-1.99 µB, while the C and O atoms have negligible magnetic moments (smaller than 0.2 µB). The CoMn cluster has a total magnetic moment of 6 µB with spins of 1.71 and 4.29 µB paralleled on the Co and Mn atom, respectively. The CO adsorption does not change the total magnetic moment of the cluster, while it reduces the local moment on the neighboring Co atom and increases the moment

J. Phys. Chem. A, Vol. 114, No. 39, 2010 10513 on Mn atom in distance. This is correlated with the electron transfer. For CoMnCO, on one hand, some spin up (majority) electrons of Co atom transfer to CO and Mn, on the other hand, some spin down (minority) electrons of Mn transfer to the Co atom. These lead to the reduction of the majority electrons on Co and the increase on Mn, while verse vice for the minority electrons of Co and Mn. Thus, the magnetic moment on Co is reduced, while that on Mn is increased as compared with the bare CoMn cluster. For the case of Co2Mn4, the CO adsorption breaks the high symmetry of the bare cluster and significantly attenuates the magnetic moment of two Co atoms from 1.99 to 0.72 µB (especially for the one at the adsorption site), resulting in the decreased total magnetic moment. These can also be understood from the aforementioned electron transfer mechanism. Some majority Co electrons transfer to CO while some minority CO electrons transfer to Co, which reduces the majority electrons and increases the minority ones on Co atom, thus the net atomic magnetic moment on Co atoms is eventually decreased. Moreover, the electron transfer at the adsorption site is more pronounced than at the other site; thus, the magnetic reduction on the former is larger than on the latter. In addition, few electrons transfer between four Mn and two Co atoms; therefore, the magnetic change on four Mn is rather small (∆M ) 0.06 µB) upon adsorption. For the bare Mn6 cluster, two Mn atoms are antiferromagnetically coupled with the other four. The CO molecule is a bridge site adsorbing over the two Mn atoms having negative spins. Thus, the electrons transfer from the Mn atom to CO and the spin-up electrons transfer from CO to the Mn atom, which decreases the net magnetic moment on these two Mn atoms. In addition, a small amount of spin-down electrons transfer from the other four Mn atoms to the two spin-down Mn atoms, which also leads to a small increase (less than 10%) of magnetic moment on the four Mn atoms. Thus, the CO adsorption enhances the total magnetic moment of the cluster. IV. Conclusion In summary, we have performed spin-polarized density functional theory calculations to study the interaction of the bimetallic ConMn (n ) 1-6) and ConMn6-n (n ) 0-6) clusters with CO molecule. The optimal adsorption site and adsorption

Figure 6. NPA magnetic moment and charge (in parentheses) on each atom of the most stable structure of CoMn, Co2Mn4, and Mn6 with and without the CO molecule.

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pattern of CO to the clusters are located. The size/component dependent structures, adsorption energies, and magnetic properties are discussed. The CO molecule is favorable to adsorb on Co rather than on the Mn atom and the atop site adsorption structure is energetically preferred than the bridge or hollow site adsorption ones for the bimetallic Co-Mn clusters with an exception of Co5MnCO. Large adsorption energy is observed at Co3Mn, Co2Mn4, and Co3Mn3 and the adsorption strengths are correlated with the relative stability of the bare clusters, the strength of electrostatic interactions, and the adsorption patterns. The Co6, Co5Mn, and Mn6 clusters have high chemical reactivity to activate the CO molecule. Furthermore, the CO adsorption has some influences on the magnetic moment of the Co-Mn clusters by either altering their atomic spin density or enhancing or attenuating the total magnetism. More precisely, although the total magnetic moment of CoMn and Co2Mn remains unchanged upon CO adsorption, the Co atoms lose certain spins while Mn in distance to CO gains some spins. However, the electron transfer between CO and the cluster reduces the total magnetic moment by 2 µB for ConMn (n ) 3-6) and ConMn6-n (n ) 0-5) while it increases the magnetic moment by 2 µB for Mn6. Acknowledgment. This work is supported by the NSF (Grant Nos 20873019, 11074035), NBRP (Contract Nos. 2009CB623200 and 2010CB923401), SRFDP (20090092110025), and Peiyu Foundations of SEU in China. We thank the computational resource at Department of Physics, SEU. References and Notes (1) Alonso, J. A. Chem. ReV. 2000, 100, 637. (2) Bansmann, J.; Baker, S. H.; Binns, C.; et al. Surf. Sci. Rep. 2005, 56, 189. (3) Zacarias, A. G.; Castro, M.; Tour, J. M.; Seminario, J. M. J. Phys. Chem. A 1999, 103, 7692. (4) Bobadova-Parvanova, P.; Jackson, K. A.; Srinivas, S.; Horoi, M. Phys. ReV. B 2002, 66, 195402. (5) Gavillet, J.; Loiseau, A.; Journet, C.; Willaime, F.; Ducastlle, F.; Charlier, J. C. Phys. ReV. Lett. 2001, 87, 275504. (6) Sen, R.; Suzuki, S.; Kataura, H.; Achiba, Y. Chem. Phys. Lett. 2001, 349, 383. (7) Kukovitsky, E. F.; L’vov, S. G.; Sainov, N. A.; Shustov, V. A.; Chernozatonskii, L. A. Chem. Phys. Lett. 2002, 355, 497. (8) Belosludov, R. V.; Sakahara, S.; Yajima, K.; Takami, S.; Kubo, M.; Miyamoto, A. Appl. Surf. Sci. 2002, 189, 245. (9) Gennady, L. G.; Charles, W. B. J. Chem. Phys. 2003, 119, 3681. (10) Knickelbein, M. B. Chem. Phys. Lett. 2002, 353, 221.

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