X (X = Ni, Pd, Pt, Ti, and Zr) Clusters - American Chemical Society

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Al12X (X= Ni, Pd, Pt, Ti and Zr) Clusters: Promising Low-Cost and High-Activity Catalysts for CO Oxidation Ling Guo, and Xiao Zhang J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 10 Dec 2013 Downloaded from http://pubs.acs.org on December 19, 2013

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The Journal of Physical Chemistry

Al12X (X= Ni, Pd, Pt, Ti and Zr) Clusters: Promising Low-Cost and High-Activity Catalysts for CO Oxidation Ling Guo*, Xiao Zhang School of Chemistry and Material Science, Shanxi Normal University, Linfen 041004, China

ABSTRACT: CO oxidation on the surface of various Al12X (X= Ni, Pd, Pt, Ti and Zr) clusters are investigated by density functional theory (DFT) calculations. The molecular structures and surface-adsorbate interaction energies of CO and O2 on the Al12X (X= Ni, Pd, Pt, Ti and Zr) clusters surfaces are predicted. The Frontier Orbital Picture (FOP) successfully predicted the cluster-CO or cluster-O2 ground-state configurations for all Al12X clusters. The calculated results indicate that CO oxidation reaction is sensitive to the properties of different transition metals, and the order of activation barrier for CO reaction with molecular O2 and CO reaction with the adsorbed oxygen atom is different for Al12X (X= Ni, Pd, Pt) clusters. A trimolecular Langmuir-Hinshelwood (LH) mechanism with coadsorbed two CO molecules and one O2 molecule is proposed, which will lead to the spontaneous formation (due to extremely low energy barrier) of two CO2 molecules as product. The calculated results show that the order of activation barrier for two CO molecules oxidation on the Al12X clusters is Al12Ti (0.08 eV) < Al12Zr (0.13 eV) < Al12Pt (0.19 eV) < Al12Ni (0.22 eV) < Al12Pd (0.24 eV). The lower calculated barrier energies of two CO oxidation on Al12X indicate that CO could be efficiently oxidized at low temperature on Al12X clusters. These findings enrich the applications of Al-based materials to the high-activity catalytic field. Keywords: Al-based materials; Langmuir-Hinshelwood (LH) mechanism; Density functional calculations; two CO molecules oxidation * Corresponding author. Telephone number: +863572398380.E-mail address: [email protected] 1

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1. Introduction The oxidation of carbon monoxide (CO) to carbon dioxide (CO2) has attracted great interest in recent years because the reaction has many industrial applications such as in catalytic conversion of automobile exhaust and in fuel cells.

1 2

Many studies have been performed for CO oxidation

on different transition metals by experimental and theoretical studies. oxidation, platinum (Pt) metal and Pt-group metals (Pd, Ni)

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9-11

Among catalysts for CO

are the commonly used

formulations due to their high catalytic activity. However, Pt metal and Pt-group metals above are both rare and very expensive. Moreover, it is easy for Pt catalysts to become

contaminated by CO strong adsorption. So their catalytic effectivity is reduced. Therefore, it is necessary to find alternative materials to replace Pt-group metals in catalytic and electrocatalytic applications. Aluminum (Al) and Al-based metals have been demonstrated that possess good catalytic activities for molecules, such as CO, H2, HI, I2 and CH3I,

12-16

and are applied in different areas.

Among them, Al-based transition metals especially attracted much more attention for the free electrons of the unfilled 3d shell of transition metals which carried a finite magnetic moment caused by Hund correlation.17 Tarakeshwar et al.

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carried out ab initio calculations of hydrogen

saturation of bimetallic titanium-aluminum nanoclusters. Geometric transition and electronic properties of titanium-doped aluminum clusters: AlnTi (n =2−24) has also been investigated by Hua et al. Other transition metals (such as Pt-group metals Ni, Pd, Pt) were also added as the dopants in the Al clusters.

19-22

Among them, Al12Ti cluster with some particular number of

valence electrons (40) shows enhanced stability against dissociation. So it could be termed “magic” clusters and act as candidates for novel cluster-assembled materials. Furthermore, Al12Ti

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has a high-symmetry icosahedral structure, providing both electronic and geometric stability.

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Though Al12Zr still does not have relevant report, the spherical jellium model predicts that it with 40 valence electron have higher stability because of the closure of electronic shells. And evolution of small Zr clusters and dissociative chemisorption of H2 on Zr clusters have been investigated by Sheng et al.23 Motivated by the above and the potential technological application of Al12X (X= Ni, Pd, Pt, Ti and Zr) alloy as catalysts, in this article, the Ni, Pd, Pt, Ti and Zr atom was taken as the impurity to probe into (1) its influences on the structural and electronic properties of Al12 clusters and (2) them using as catalysts. There are no study that concern the difference of CO oxidation on Al12X (X= Ni, Pd, Pt, Ti and Zr). In this work, the detailed reaction process of CO oxidation on Al12X (X= Ni, Pd, Pt, Ti and Zr) is investigated by performing DFT calculations. First, the active sites of the Al12X (X=Ni, Pd, Pt, Ti, Zr) cluster is taken into account and identified. Then, adsorption of CO and O2 and coadsorption of CO+O2, CO2+O, and CO+O on Al12X (X= Ni, Pd, Pt, Ti and Zr) clusters are analyzed. Finally, a systematic exploration of the single and double CO molecules oxidation reactions on the Al12X (X=Ni, Pd, Pt, Ti, Zr) is conducted via LH mechanism

(where the reaction occurs through interactions among adsorbed molecules, radicals, or fragments of the reactant molecules). 2 Computation Details Kohn-Sham DFT calculations are performed with a Perdew-Burke-Ernzerhof hybrid functional (PBE),24 which is known to give accurate results for aluminum cluster systems.

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The standard

6-31G(d, p) basis set is used for aluminum, oxygen and carbon. Considering the strong relativistic effect of X (X= Ni, Pd, Pt, Ti and Zr), the LANL2DZ pseudopotential is adopted for the valence electrons, and its core electrons are represented by the LANL2DZ effective core potential (ECP).26,

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This scheme is a good compromise between accuracy and computational effort. And its

application has been shown to be effective for many species including transition atom such as PtmAun,28 (ZrO2)n29 and (TiO2)n.30 No symmetric constraints are imposed during geometrical optimizations. The synchronous transit-guided quasi-newton method 31 is adopted for locating the transition states. The nature (minima or first-order saddle points) of optimized structures is identified by the subsequent frequency calculations that also provide zero-point vibrational energy (ZEP) corrections. The quadratic synchronoustransit (QST) method is used in determining the transition state (TS) geometries. All the structures are fully optimized without any constraint. For reaction pathways, minima are connected to each transition state (TS) by tracing the intrinsic reaction coordinate (IRC) 32, which have been performed to verify that each saddle point links two desired minima. All of the calculations are carried out with the Gaussian03 program.33 In this work, we calculated the zero-point energy (ZPE) corrected binding energy (BE) of adsorbate A with an Al12X cluster, which is defined as BE=(Etotal-EAl12X-EA)+( EtotalZPE-EAl12XZPE-EAZPE) In the aforementioned equations, the Etotal, EAl12X, EA correspond to the energies of adsorbed species on the Al12X clusters, the bare Al12X clusters, a gas-phase adsorbate, respectively. A more negative BE corresponds to stronger adsorption. 3 Results and Discussion 3.1. Identification of active sites on Al12X (X= Ni, Pd, Pt, Ti and Zr) clusters. A distorted icosahedron (Figure 1a) is the lowest energy isomer of Al12X (X= Ni, Pd, Pt), and an icosahedral structure with the X atom in the cage apex (Figure 1d) is the ground state of Al12X

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(X=Ti and Zr). Low energy structures may be missed if the starting configurations of the search are not set appropriately. In order to avoid this, the ab initio simulations starting with several initial two-dimensional and three-dimensional structures for each Al12X cluster are performed. The initial input structures are taken either from published results for Al13 by adding X atoms in different positions, or from references published before for Al12X (X=Ti and Zr),

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or from the

results reported for other Al12X (X= Cr, Mn, and Fe) 34 clusters, or arbitrarily constructed and fully optimized via the Berny algorithm. Other three low energy isomers of Al12X are also shown in Figure 1b, 1c, 1e. In fact, the five configurations of Al12X (X= Ni, Pd, Pt, Ti and Zr) are similar, we only show the geometries of Al12Ni to avoid redundancy. The lowest energy isomers (Figure 1a and 1d) of Al12X are chosen as the model configuration for CO and O2 adsorption. For the lowest energy isomer of Al12X (X= Ni, Pd, Pt, Ti and Zr), label of the binding sites are shown in Figure 2. The lowest energy structure of Al12X (X= Ni, Pd, Pt) have a mirror-symmetry with respect to the plane given by the atoms in the ring (no. 12, 3, 11, 8), which cuts the structure in half. Hence the atoms in the two sides, i. e. no. 1, 2, 7, 6, and 5, 4, 9, 10, are equivalent, respectively. Here, our initial focus is placed on CO and O2 adsorption on the Al12X clusters. Because the surfaces of Al12X (X= Ni, Pd, Pt) clusters have no well-defined planes, we have predicted the most stable binding sites for CO and O2 by the orbital roughness Metiu et al.

35

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of Frontier Orbital Picture (FOP).

described the orbital roughness where the strongest binding of the electron donor

occurs to be at the site where the lowest unoccupied molecular orbital (LUMO) protrudes farthest; similarly, the strongest binding site of the electron acceptor is where the highest occupied molecular orbital (HOMO) protrudes farthest. When O2 and CO adsorb on the Al12X (X= Ni, Pd,

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Pt) cluster surface, they are all the electron acceptors. Therefore, we calculated the HOMO for the Al12Ni cluster, as shown in Figure 2a. The adsorbates are placed at atoms with large protruding HOMO orbital (Figure 2a) because those atoms could have good activity. So the adsorption site 9 should have good activity. The binding orientation of adsorbed CO on site 8 of Al12Ni can also be successfully explained using the matching of orbital symmetries 36 in FOP. The binding orientation is that there is maximum favorable overlap between the HOMO of Al12Ni and the LUMO of CO. So the adsorption site 8 should also have good activity. Actually, by our calculation, we found these two Al12Ni-CO complexes formed by CO binding with sites 9 and 8 of Al12Ni are almost degenerate configurations (Figure 2a). In order to prove the prediction by FOP, we also investigate the adsorption configurations for all possible species on different sites. For example, for the CO adsorb on the Al12Ni, the cluster has eight distinct adsorption sites (according to the symmetry of the cluster), which are present with arabic numerals in Figure 2a. The BE of CO on sites 9, 8, 4, 3, 12, 11, 1 and 6 are -1.51, -1.50, 1.40, -1.09, -0.73, -0.23, -0.21, and -0.20, respectively. So the calculated results are consistent with the FOP. However, it must be pointed out that we would not use the structures of sites 9, 8, and 4 in the subsequent coadsorption for the large deformation of their structure (Figure 2). And CO adsorption in the sites 3 and 12 (Figure 2a) are chosen as the preferred structures. Similarly, the BE of O2 on different adsorption sites have the same change trend as those of CO. The ground state and second-most stable isomers of O2 adsorbed Al12Ni complex is also shown in the Figure 2b, which become deformed. And O2 adsorption in the sites 3 and 12 (Figure 2b) are also chosen as the preferred geometries. The calculated adsorption properties of CO and O2 on Al12X (X= Ni,

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Pd, Pt, Ti and Zr) including the adsorption site, spin multiplicity (M), binding energy (BE) in eV, bond distances (R) in Å, NBO charge and vibrational frequency in cm-1 are listed in Table 1. To avoid redundancy we do not include the isosurfaces and ground-state geometries of Al12Pt and Al12Pd in this paper as the shape/symmetry of Al12Pt and Al12Pd is exactly the same as that of Al12Ni. For Al12X (X= Ti and Zr) clusters, we have predicted the most stable binding sites for CO and O2 according to the FOP, too. The LUMO of CO or O2 and the HOMO of Al12Ti when they are at infinite distance from each other and the lowest energy and substable isomers of the Al12Ti-CO or Al12Ti-O2 complexes are shown in Figure 2c and 2d, respectively. So the adsorption sites 13 and 3 should have good activity. And the calculation results prove its validity. Furthermore, to avoid redundancy we do not include the isosurfaces and ground-state geometry of Al12Zr in this paper as the shape/symmetry of Al12Zr is exactly the same as that of Al12Ti. 3.2. Adsorption of CO on Al12X (X= Ni, Pd, Pt, Ti and Zr) clusters It is observed that in the site 3 structures of Al12XCO (X= Ni, Pd, Pt) complexes (Figure 2), CO binds to Al atoms through the C atom and CO adsorption takes place in a slightly titled on-top position (Al12ZrCO>Al12NiCO>Al12PtCO>Al12PdCO. NBO charge analysis shows that charge transfer from Al12X to adsorbed CO takes place in the order: Al12PtCO> Al12PdCO> Al12NiCO> Al12ZrCO > Al12TiCO. CO binding energies of all Al12X clusters have no simple correction with NBO charges on adsorbed CO (Table 1). This is attributed to the complicated mechanism of CO binding with Al12X clusters. For Al12X (X= Ni, Pd, Pt, Ti) clusters, the trend of C-O bond distances and NBO charges are similar, which are due to the increased metal→∏* back-donation leading to weakening of C-O bonds and thereby these bonds get lengthen. Specially, Al12X (X=Ti, Zr) are an electronic magic-number cluster (due to shell-closing) with a relatively large HOMO-LUMO gap compared to Al12X (X= Ni, Pd, Pt) clusters. Remarkably, DFT calculations show that these clusters are also very special due to its relatively strong CO and O2 adsorption capability than Al12X (X= Ni, Pd, Pt) clusters (Table 1). 3.3. Adsorption of O2 on Al12X (X= Ni, Pd, Pt, Ti and Zr) clusters As shown in part b of Figure 2, we considered four strong adsorption sites (sites 9, 8, 3 and 12)

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for the O2 molecule on Al12X (X= Ni, Pd, Pt). The site 9 and 8 structures take place large deformation (Figure 2b). So O2 molecule adsorption in the sites 3 and 12 (Figure 2b) are chosen as the preferred geometries. Molecular adsorption with the O2 occupying top sites 3 of Al12X (X= Ni, Pd, Pt) clusters appears to be energetically most favorable for all the complexes. We have observed that O-O bond distances (Table 1) are larger in case of adsorbed O2 in molecular adsorption mode than in the free molecule (1.21 Å), being maximal (1.32 Å) for adsorption of O2 on Al12Pd cluster. Four adsorption models of O2 on Al12X (X= Ti, Zr) clusters are also considered. The adsorption energies for configurations with O2 adsorbed on Ti13, Al3, Al1, and Al2 sites are calculated to be -1.60, -1.15, -0.90, -0.60 eV, respectively. And The BE of O2 on sites Zr13, Al3, Al1 and Al2 of Al12Zr are -2.12, -1.08, -1.12, -0.98 eV, respectively. So both the calculated results and FOP analysis (Figure 2d) indicate that the O2 molecule is adsorbed at Ti (Zr) site 13 preferentially. This relatively strong O2 adsorption capability on surface sites of Al12X (X= Ti, Zr) is crucial for higher catalytic activities (see below). When O2 is adsorbed on the cluster surface, charge transfer for O2 in various Al12X (X= Ni, Pd, Pt, Ti and Zr) clusters have negative values. NBO analysis shows that charge transfer from Al12X to adsorbed O2 takes place in the order: Al12Pd> Al12Ni=Al12Pt=Al12Zr> Al12Ti. And the trend of O-O bond distance and NBO charge is similar, which is due to the fact that metal to oxygen back-donation increase the population of ∏* orbital leading to weakening of O-O bonds and thereby this bond gets lengthen. 3.4. Coadsorption of CO+O2, CO2+O, and CO+O on Al12X (X= Ni, Pd, Pt, Ti and Zr) clusters To map out the mechanism of CO oxidation process, the coadsorption of CO+O2, CO2+O, and

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CO+O are also studied; see Table 2 and Figure 3. Previous studies have shown that coadsorption of CO and O2 on the catalyst is the crucial initial step for CO oxidation.37-39 A few systematic studies have been studied. Different combinations of adsorption configurations for CO and O2 are chosen to determine the most stable initial structure. We find that, after geometric optimization, coadsorbed CO and O2 both end up the top configuration of neighbor sites. The calculated coadsorption energy results show that the most stable system is CO(12)+O2(3) on the Al12X (X= Ni, Pd, Pt) (Figure 3a) and CO(13)+O2(13) on the Al12X (X= Ti and Zr) (Figure 3b). Al12X (X= Ti and Zr) are special cases as they can adsorb CO more strongly than the Al12X (X= Ni, Pd, Pt). This unusual case may be attributed to the ultrahigh stability of the “magic-number” cluster Al12X (X= Ti and Zr) with a larger HOMO-LUMO gap of 0.67 eV (Al12Ti) and 0.84 eV (Al12Zr). The coadsorptions of CO2+O and CO+O on Al12X surfaces are also investigated. Different combinations of adsorption configurations for CO2, CO and oxygen atoms are chosen to determine the most stable structures for the reaction. The calculated coadsorption energy resultsshow that the most stable system is CO2+O(3,12), and CO (12) + O(2, 3, 12) on the Al12X (X= Ni, Pd, Pt) (Figure 3a), CO2+O(9,13), and CO (8) + O(9, 13) on the Al12X (X= Ti and Zr) (Figure 3b). In addition, the different adsorption sites of CO2 on Al12X (X= Ni, Pd, Pt, Ti and Zr) surfaces are studied in order to determine the most stable final state. The most stable adsorption site of CO2 is bridge site on different surfaces. The calculated adsorption energies are very small, and the energy difference between different adsorption sites is only a few meV, and so the physisorption of CO2 is very evident. 3.5 CO oxidation on Al12X (X=Ni, Pd, Pt) clusters For CO oxidation on the Al12Ni cluster, the LH mechanism is considered, which is proposed to

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be most likely mechanism for CO oxidation by many studies. 40-42 As shown in Figure 4a, the LH mechanism starts from the CO and the O2 molecule both adsorb at the top and neighboring Al sites to form intermediate (IM1) with the corresponding coadsorption energy 1.52 eV. At the transition states (TS1/2), CO molecule moves to the off-top site from its initial site, whereas the O2 molecule is at the bridge site. The activation barrier of CO oxidation is calculated to be 0.30 eV; the distance between two oxygen atoms 1.32 Å; and the distance between the C atom and one oxygen atom is 2.37 Å at the TS1. Similarly, for the TS1/2 configurations of CO oxidation on Al12Pd and Al12Pt clusters, the CO molecule adsorbs at the off-top site and the O2 adsorbs at the bridge site compared with their initial coadsorption site. The activation barrier of CO oxidation on Al12Pd and Al12Pt is calculated to be 0.18 and 0.15 eV; the corresponding distance between two oxygen atoms is 1.37 and 1.36 Å at the TS1/2; and the distance between the C atom and one Oxygen atom is 2.11 and 2.11 Å at the TS1/2. After that, the reaction involves a four-center OC-O-O intermediate state (labeled as IM2). At the IM2, the corresponding coadsorption energies are -2.92, -2.77 and -2.87 eV for Al12Ni, Al12Pd, and Al12Pt, respectively. The shortest distances between two oxygen atoms are 1.474, 1.475, 1.473 Å, and the distance between one oxygen atom and one C atom are 1.386, 1.397, 1.395 Å, respectively. The reaction energy of CO reacting with molecular O2 is -1.40, -1.61, -1.62 eV from the IM1 to the IM2 on Al12X (X=Ni, Pd, Pt) clusters. The configuration of the TS2/3 is similar to that of the IM2, and the activation barrier of CO oxidation is calculated to be 0.53, 0.08, 0.26 eV. The shortest distance between two oxygen atoms is 1.475, 1.485, 1.475 Å at the TS2/3. After the TS2/3, the CO2 molecule on the final state IM3 is formed, which then desorbs from the cluster surfaces. And it leaves an adsorbed oxygen atom on different clusters. From the IM2 to the IM3 on Al12X (X=Ni, Pd, Pt) clusters, the reaction energy is -4.18, -3.85 and -3.66 eV. The

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calculated results indicate that CO oxidation reaction is sensitive to the properties of different transition metals and the order of activation barrier on different Al12X clusters for CO oxidation is Al12Pd< Al12Pt< Al12Ni. In the case of the remainder oxygen atom, which can also react with CO to form molecular CO2, a reaction pathway is proposed: It is the reaction between adsorbed CO and adsorbed oxygen atom, which is known as the LH mechanism. As shown in Figure 4b, along the path, the configuration for coadsorption of CO and O has been described as the IM4, which corresponds to the oxygen atom and the CO molecule adsorb at the hollow and neighbor top sites with a coadsorption energy of -7.39, -7.43, -7.43 eV for Al12Ni, Al12Pd, and Al12Pt, respectively. After coadsorption, CO oxidation proceeds to form IM5 (OCO) by an association of the O atom and CO molecule via transition state TS4/5. The calculated reaction barriers are 0.45, 1.23, 1.28 eV for Al12Ni, Al12Pd, and Al12Pt, respectively, because the bonding of O on cluster surface is too strong to react with CO. This result also indicates that a much higher energy is required for the formation of the CO2 in the absence of a neighboring O atom when compared to that in the presence of a neighboring O atom. And then the oxygen leans toward the CO fragment with the CO2 molecule is formed at the TS5. After the TS 5, the CO2 molecule then desorbs from the final state (FS). Finally, we found that the energy gain in the CO2 formation via this mechanism is about 1.23, 1.13, 1.12 eV, implying that it is difficult for the CO2 molecule to desorb from the cluster surfaces because of their large adsorption energies. The calculated results show that order of activation barrier for CO reacting with the adsorbed oxygen atom on clusters surfaces is Al12Ni< Al12Pd< Al12Pt. Naturally, an interesting question arises: Why is there such a large difference in activity? And why is the order of activation barrier for CO reaction with molecular O2 and CO reaction with the

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adsorbed oxygen atom different? Liu and Hu 43 found that the activation barrier is almost a linear function of the total chemisorption energy of reactants on metals (Ecoads) at the initial state for CO oxidation on TM surfaces; that is, the larger the total chemisorption energy of reactants at the initial state, the more difficult they are to activate. And this rule holds for our calculation. The same result is obtained; namely, Al12Pd shows the smallest energy barrier and the Al12Ni shows the largest energy barrier for CO reaction with molecular O2. So the order is Al12Pd< Al12Pt< Al12Ni. Whereas for the reaction of CO with adsorbed oxygen atom, Al12Ni shows the smallest energy barrier and the Al12Pt shows the largest energy barrier, the order is Al12Ni< Al12Pd< Al12Pt. The calculated results also indicate that the reaction mechanism of CO reaction with molecular O2 is more favorable than that of CO reaction with the adsorbed oxygen atom on Al12X (X=Ni, Pd, Pt) clusters surfaces. Although CO oxidation with the oxygen atom is facile, the significantly high barriers on Al12X (X=Ni, Pd, Pt) clusters surfaces (0.89, 1.23, 1.28 eV) make them less active for CO oxidation. So a trimolecular LH mechanism with coadsorbed two CO molecules and one O2 molecule at a unique triangular Al3 active site is proposed in section 3.7, which will lead to the spontaneous formation (due to extremely low energy barrier) of two CO2 molecules as product. 3.6 CO oxidation on Al12X (X=Ti, Zr) clusters For Al12X (X=Ti, Zr) clusters, a coadsorption of CO and O2 on Ti/Zr atom has been described as the intermediate IM1, with coadsorption energy of -3.41 (Al12Ti) and -3.13 (Al12Zr) eV, respectively. (see Figure 5a and Table 2). Al12X (X=Ti, Zr) are both electronic magic-number cluster with a relatively large HOMO-LUMO gap compared to Al12X (X=Ni, Pd, Pt) clusters. Remarkly, DFT calculations show that these clusters are very special due to their relatively strong CO and O2 adsorption capability than Al12X (X=Ni, Pd, Pt) clusters (see Table 1). This relative

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strong CO and O2 adsorption capability is crucial for higher catalytic activities (see below). Then, the intermediate IM2, which corresponds to the configuration of O2-side-on-Ti/Zr, CO-top-on-Ti/Zr with coadsorption energy of -4.61 (Al12Ti) and -4.25 (Al12Zr) eV is formed. The reaction barriers of this step are 0.07 and 0.05 eV, respectively, for Al12Ti and Al12Zr. Subsequently, the dissociation reaction of O2 (O2 O-O) occurs. Then the O atom and CO molecule approaches each other passing over the TS2/3 with energy barriers of 0.02 and 0.58 eV to produce an adsorbed complex (IM3), and this step is the rate-determining step of Al12Zr. Proceeding to the next step, the adsorbed OCO complex (IM3) desorbs from surface to gas phase (IM4) via barriers of 0.45 and 0.47 eV and this step is the rate-determining step of Al12Ti. They are exothermic by -7.81 and -6.91 eV. The configuration for coadsorption of CO and O has been described as the IM5 (Figure 5b), which corresponds to the O-bridge-Al-Ti/Zr, CO-top-Al (neighboring site) configure with coadsorption energy of -7.52 and -7.48 eV. After coadsorption, CO oxidation proceeds to form OCO (IM6) by an association of the O atom and CO molecule via TS5/6 (with an activation barrier of 0.38 and 0.55 eV). Finally, the CO2 molecule is formed and desorbed from the Al12Ti and Al12Zr surfaces via TS6 (with an activation barrier of only 0.03 and 0.24 eV). The overall LH reaction is thus calculated to be exothermic by -6.29 and -6.24 eV. We find that (1) an opposing conclusion to Al12X (X=Ni, Pd, Pt) clusters is obtained by using the Al12Ti and Al12Zr clusters. Namely, the calculated results indicate that the activation barrier of the reaction mechanism of CO reacting with molecular O2 is higher than that of CO reacting with the oxygen atom. Therefore, the oxygen atom mechanism is more favorable than the molecular O2 mechanism on Al12X (X=Ti, Zr) clusters surfaces. (2) The above Liu and Hu’s rule fails for the

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comparing between Al12Ti and Al12Zr clusters. Al12Ti show the smaller energy barrier although the order for Ecoads is Al12Ti>Al12Zr (Table 2). So it appears that coadsorption energies do not correlate with the activation barrier. (3) As a comparison, the reaction barriers associated with the magic-number cluster Al12X(X=Ti, Zr) (Al12Ti, 0.45 eV; Al12Zr, 0.58 eV) are notably lower than those of Al12X (X=Ni, Pd, Pt) clusters. 3.7 Two CO oxidation on Al12X (X=Ti, Zr) clusters As mentioned by Ref.,

44-47

it is known that coadsorbed water molecule, CO molecule can

promote CO, propene oxidation or alcohol electrochemical oxidation and its own electrochemical oxidation on gold surface. We have come to the same conclusion. We found that multiply adsorbed CO molecules can significantly promote the CO oxidation. And we will take into account the role of coadsorbed CO molecules when discussing the CO oxidation mechanism. Here, we consider two CO molecules occupying two corners of a triangular Al3 active site of Al12X (X=Ni, Pd, Pt) while an O2 molecule is adsorbed on a neighboring corner as shown in Figure 6a. The energy diagrams in Figure 6b give the coadsorption energies of the two neighboring CO molecules, which demonstrate that the coadsorption of two CO molecules at the triangular active site is energetically favorable. As shown in Figure 6a, a major effect due to the coadsorbed neighboring CO molecule on the CO oxidation is that the significantly high barriers (Ea(TS5) for Al12Ni, Ea(TS4/5) for Al12Pd and Al12Pt) on Al12X (X=Ni, Pd, Pt) clusters surfaces in the single CO oxidation are notably reduced. Comparing Figure 4 and 6a, the first step of two CO oxidation is the same as that under the conventional bimolecular LH mechanism in that the O2 molecule attacks nucleophilically a neighboring CO molecule to form the OCOO* intermediate. In this step, the additional neighboring CO molecule has little effect on the energy barrier. The second reaction

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step proceeds in the same way: direct O-O bond scission from the OCOO* intermediate. But the activation energy of O-O scission due to the electrophilic attack of the additional CO molecule at the OCOO* intermediate is as low as 0.22 eV for Al12Ni, 0.24 eV for Al12Pd and 0.19 eV Al12Pt, respectively, significaptly lower than those (0.53, 0.08, 0.26 eV) through a direct dissociation of the O-O bond from the OCOO* intermediate. The third reaction step proceeds in two ways: CO2 molecule is formed and desorbs from the Al12X (X=Ni, Pd, Pt) clusters surfaces for one CO molecule oxidation Figure 4a or two CO2 molecules are spontaneous sequential formation to complete the catalytic cycle for two CO molecules oxidation Figure 6a. In the latter way, the energy barriers are as low as 0.02 eV for Al12Ni, 0.06 eV for Al12Pd and 0.07 eV Al12Pt, respectively. As for Al12Ti and Al12Zr, two CO molecules are involved in the CO oxidation. The reaction proceeds with four steps. The reaction starts firstly with the formation of IM1, an intermediate state of adsorbed CO coupled with Al12Ti (Al12Zr)–CO–O2 complex, which is energetically favored by -4.00 and-3.55 eV than the free reactants, CO and O2. The reaction involves the transition state TS1/2, where the O–O bond length is changed to 1.298 (1.296) Å from 1.285 (1.295) Å in IM1. In the second step, the O–O bond is broken and new C–O bond is formed in IM2. In TS2/3, C atom of the adsorbed CO is just 1.504 (1.463) Å away from an O atom of O2 molecule, indicating that a new C–O bond forms. The only imaginary frequency in the TS2/3 is -391.6 (-359.5) cm-1. Just like Al12X (X=Ni, Pd, Pt) clusters, the O-O bond-breaking barrier, caused by the attack of the additional CO molecule, is typically less than 0.1eV, which is much lower than those from direct O-O dissociation. The third step involves the formation of another new C–O bond. With the aid of the other CO molecule, the reactions from the intermediate IM3 state, to the

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product-like, IM4, via a transition state of TS3/4. In TS3/4, C atom of the adsorbed CO is just 1.744 (1.677) Å away from an O atom of O2 molecule, indicating that a new C–O bond forms. The only imaginary frequency in the TS3/4 is -359.5 (-400.8) cm-1. The barrier along this pathway are 0.09(0.11), 0.08(0.13), 0.07(0.02) eV, respectively, which are all lower than those of one CO adsorption on Al12Ti and Al12Zr, demonstrating that the two-CO-adsorption can prompt CO oxidation. The lower calculated barrier energies of two CO oxidation on Al12Ti and Al12Zr indicate that CO could be efficiently oxidized at low temperature on Al12Ti and Al12Zr clusters. The promotion may result from the charge transfers from CO to the cluster. On the CO adsorbed Al, there are 0.136 e (0.144 e) charges donated from CO to Al, possibly leading more charges transferred from cluster to the anti-bonding p⁄orbital of O2 to activate O2. 3.8 Analysis of charge and electronic state during CO oxidation The electronic local density of states (LDOS) of the system is projected onto the orbitals for the adsorbed constructs of O2 (left panel) and CO (right panel) species, as well as the d-projected of the bound Ti atoms, which are depicted in Figure 7. Figure 7a shows the LDOS before the O2, CO, and Al12Ti interactions; Figure 7b-7e corresponding to the LDOS of IM1-IM4, respectively, for pathway of LH mechanism of Al12Ti (Figure 5a). For the O2 adsorbate on the Al12Ti (see Figure 7b1), there is a broadening of 2П* overlap with the Ti d state, indicating a strong interaction between the adsorbed O2 and Ti atom. And there is also some overlap between the 1П orbital of O2 and the Ti d state. The results imply that charge shifts from the positive charged Ti atom to the more electronegative oxygen. As seen in Figure 7b2, there is a visible overlap between the unoccupied orbital 2П* of CO and the Ti d states. Thus, the substantial interaction between the d band of the Ti atom and the 2П* of CO results in charge transfer from CO species to Ti in the IM1.

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As these interactions proceed (from Figs. 6c to 6e), the antibonding orbitals 2П* of O2 and CO species on the Ti spread back and overlap with the d state of Ti. Therefore, the slight increase in O-O bond of O2 and C-O bond of CO is due to back-donation from antibonding orbitals 2П* of O2 species and CO to Ti and the electronic resonance between the antibonding orbitals 2П* of O2 species and CO and the d state of the Ti atom. Fianlly, as shown in Figure 7e, the LDOS for the products is similar to the separated CO2 gas phase and adsorbate O on Ti nanoparticle, indicating that the CO2 molecule is only physisorbed on the Al12Ti cluster. 4 Conclusions CO oxidation on the Al12X (X= Ni, Pd, Pt, Ti and Zr) clusters have been studied by means of density functional theory calculations with the aim to shed light on the reaction mechanism and catalytic activity of Al12X clustes. Several conclusions drawn from these calculations include the following: (1) Our results show a distorted icosahedron (Figure 1a) is the lowest energy isomer of Al12X (X= Ni, Pd, Pt) and an icosahedral structure the X atom in the cage apex (Figure 1d) is the ground state of Al12X (X=Ti and Zr). (2) We have predicted the most stable binding sites for CO and O2 by the orbital roughness of Frontier Orbital Picture (FOP) (Figure 2). The calculated results indicate that the most stable adsorption site of CO molecule and the most stable adsorption site of O2 molecule are Al3 site on Al12X (X= Ni, Pd, Pt) and Ti13 site on Al12X (X= Ti and Zr), respectively. (3) The calculated results indicate that CO oxidation reaction is sensitive to the properties of different transition metals, and the order of activation barrier for CO reaction with molecular O2 and CO reaction with the adsorbed oxygen atom is different for Al12X (X= Ni, Pd, Pt) clusters.

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The order of activation barrier for CO reaction with molecular O2 is Al12Pd< Al12Pt< Al12Ni. And order of activation barrier for CO reacting with the adsorbed oxygen atom is Al12Ni< Al12Pd< Al12Pt. The reaction mechanism of CO reaction with molecular O2 is more favorable than that of CO reaction with the adsorbed oxygen atom on Al12X (X=Ni, Pd, Pt) clusters surfaces. (4)An opposing conclusion to Al12X (X=Ni, Pd, Pt) clusters is obtained by using the Al12Ti and Al12Zr clusters. Namely, the activation barrier of the reaction mechanism of CO reacting with molecular O2 is higher than that of CO reacting with the oxygen atom. The above Liu and Hu’s rule fails for the comparing between Al12Ti and Al12Zr clusters. Al12Ti show the smaller energy barrier although the order for Ecoads is Al12Ti>Al12Zr (Table 2). So it appears that coadsorption energies do not correlate with the activation barrier. As a comparison, the reaction barriers associated with the magic-number cluster Al12X(X=Ti, Zr) are notably lower than those of Al12X (X=Ni, Pd, Pt) clusters. (5) A trimolecular LH mechanism with coadsorbed two CO molecules and one O2 molecule is proposed, which will lead to the spontaneous formation (due to extremely low energy barrier) of two CO2 molecules as product. The major effect due to the coadsorbed neighboring CO molecule on the CO oxidation is that the O−O dissociation energy barrier is notably reduced, in comparison to that in the single CO oxidation path or that in the direct breaking of a O−O bond without the promotion by the additional CO. (6) The calculated results show that the order of activation barrier for two CO molecules oxidation on the Al12X clusters is Al12Ti (0.08 eV) < Al12Zr (0.13 eV) < Al12Pt (0.19 eV) < Al12Ni (0.22 eV) < Al12Pd (0.24 eV). The lower calculated barrier energies of two CO oxidation on Al12X (X=Ni, Pd, Pt, Ti, Zr) indicate that CO could be efficiently oxidized at low temperature on Al12X (X=Ni,

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Pd, Pt, Ti, Zr) clusters. Acknowledgment. This work was financially supported by the Natural Science Foundation of Shanxi (Grant No. 2013011009-6), the High School 131 Leading Talent Project of Shanxi, Undergraduate Training Programs for Innovation and Entrepreneurship of Shanxi Province (Grant No. 2013145) and Shanxi Normal University (SD2013CXCY-65) and Teaching Reform Project of Shanxi Normal University (SD2013JGXM-51). References (1) Kim H. Y.; Lee, H. M.; Henkelman, G. CO Oxidation Mechanism on CeO2-Supported Au Nanoparticles. J. Am. Chem. Soc. 2012, 134, 1560−1570 (2) Allian A. D.; Takanabe, K.; Fujdala, K. L.; Hao, X; Truex, T. J.; Cai, J.; Buda, C.; Neurock, M.; Iglesia, E. Chemisorption of CO and Mechanism of CO Oxidation on Supported Platinum Nanoclusters. J. Am. Chem. Soc. 2011, 133, 4498−4517 (3) Nagasaka, M.; Kondoh, H.; Nakai, I.; Ohta, T. CO Oxidation Reaction on Pt (111) Studied by the Dynamic Monte Carlo Method including Lateral Interactions of Adsorbates. J. Chem. Phys. 2007, 126, 044704−044707 (4) Gustafson, J.; Westerstrom, R.; Balmes, O.; Resta, A.; Van Rijn, R.; Torrelles, X.; Herbschleb, C. T.; Frenken, J. W. M.; Lundgren, E. Catalytic Activity of the Rh Surface Oxide: CO Oxidation over Rh (111) under Reaction Conditions. J. Phys. Chem. C. 2010, 114, 4580−4583 (5) Erikat, I. A.; Hamad, B. A.; Khalifeh, J. M. Catalytic Oxidation of CO on Ir (100). Phys. Status. Solidi. B. 2011, 248, 1425−1430 (6) Chen, H. T.; Chang, J. G.; Ju, S. P.; Chen, H. L. First-Principle Calculation on CO Oxidation

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Page 21 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Catalyzed by a Gold Nanoparticle. J. Comput. Chem. 2010, 31, 258−265 (7) Fajín, J. L. C.; Cordeiro, M. N. D. S.; Gomes, J. R. B. DFT Study of the CO Oxidation on the Au(321) Surface. J. Phys. Chem. C. 2008, 112, 17291−17302 (8) Kim, H. Y.; Kim, D. H.; Ryu, J. H.; Lee, H. M. Design of Robust and Reactive Nanoparticles with Atomic Precision:13Ag-Ih and 12Ag-1X (X=Pd, Pt, Au, Ni, or Cu) Core-Shell Nanoparticles. J. Phys. Chem. C. 2009, 113, 15559−15564 (9) Peng, G. W.; Merte, L. R.; Knudsen, J.; Vang, R. T.; Lægsgaard, E.; Besenbacher, F.; Mavrikakis, M. On the Mechanism of Low-Temperature CO Oxidation on Ni(111) and NiO(111) Surfaces. J. Phys. Chem. C. 2010, 114, 21579–21584 (10) Kaden, W. E.; Kunkel, W. A.; Roberts, F. S.; Kane, M.; Anderson, S. L. CO Adsorption and Desorption on Size-Selected Pdn/TiO2 (110) Model Catalysts: Size Dependence of Binding Sites and Energies, and Support-Mediated Adsorption. J. Chem. Phys. 2012, 136, 204705-204711 (11) Inukai, J.; Tryk, D. A.; Abe, T.; Wakisaka, M.; Uchida, H.; Watanabe, M. Direct STM Elucidation of the Effects of Atomic-Level Structure on Pt (111) Electrodes for Dissolved CO Oxidation. J. Am. Chem. Soc. 2013, 135, 1476−1490 (12) Wang, L.; Zhao, J.; Zhou, Z.; Zhang, S. B.; Chen, Z. First-Principles Study of Molecular Hydrogen Dissociation on Doped Al12X (X=B, Al, C, Si, P, Mg, and Ca) Clusters. J. Comput. Chem. 2009, 30, 2509-2514 (13) Zhang, F.; Wang, Y.; Chou, M. Y. Hydrogen Interaction with the Al Surface Promoted by Subsurface Alloying with Transition Metals. J. Phys. Chem. C. 2012, 116, 18663−18668 (14) Bergeron, D. E.; Castleman, A. W.; Morisato, T.; Khanna, S. N. Formation of Al13I-: Evidence for the Superhalogen Character of Al13. Science. 2004, 304, 84-87 (15) Bergeron, D. E.; Roach, P. J.; Castleman Jr, A. W.; Jones, N. O.; Khanna, S. N. Al Cluster Superatoms as Halogens in Polyhalides and as Alkaline Earths in Iodide Salts. Science. 2005, 307, 231-235 (16) Bergeron, D. E.; Castleman Jr, A. W.; Chemical Formation of Neutral Complexes from Charged Metal Clusters: Reaction of Pre-formed Aluminum Cluster Anions with Methyl Iodide. Chem. Phys. lett. 2003, 371, 189-193 21



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Page 22 of 44

(17) Chen, X. S.; Zhao, J. J.; Wang, G. H. The Local Electronic and Magnetic Properties of Fe Impurity in Al Clusters. Zeitschrift für Physik D Atoms, Molecules and clusters. 1995, 35, 149−153 (18) Tarakeshwar, P.; Dhilip Kumar, T. J.; Balakrishnan, N. Hydrogen Multicenter Bonds and Reversible Hydrogen Storage. J. Chem. Phys. 2009, 130, 114301 (19) Hua, Y. W,; Liu, Y. L.; Jiang, G.; Du, J. G.; Chen. J. Geometric Transition and Electronic Properties of Titanium-Doped Aluminum Clusters: AlnTi (n=2-24). J. Phys. Chem. A. 2013, 117, 2590−2597 (20) Giuranno, D.; Tuissi, A.; Novakovic, R.; Ricci, E. Surface Tension and Density of Al-Ni Alloys. J. Chem. Eng. Data. 2010, 55, 3024–3028 (21) Krajcí, M.; Hafner, J. Intermetallic Compound AlPd as a Selective Hydrogenation Catalyst: A DFT Study. J. Phys. Chem. C. 2012, 116, 6307−6319 (22) Javorský, P.; Diviš, M.; Givord, F.; Boucherle, J.-X.; Rusz, J.; Lelièvre-Berna, E.; Sechovsky, V.; Andreev, A. V.; Bourdarot, F. Magnetization Densities in UPtAl: Experimental and Theoretical Study. Phys. Rev. B. 2003, 67, 224429-224437 (23) Sheng, X. F.; Zhao, G. F.; Zhi, L. L. Evolution of Small Zr Clusters and Dissociative Chemisorption of H2 on Zr Clusters. J. Phys. Chem. C. 2008, 112, 17828-17834 (24) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865-3868 (25) Schultz, N. E.; Staszewska, G.; Staszewski, P.; Truhlar, D. G. Validation of Theoretical Methods for the Structure and Energy of Aluminum Clusters. J. Phys. Chem. B. 2004, 108, 4850-4861 (26) Hay, P. J.; Wadt, W. R. Ab Initio Effective Core Potentials for Molecular Calculations. Potentials for the Transition Metal Atoms Sc to Hg. J. Chem. Phys. 1985, 82, 270-283 (27) Hay, P. J.; Wadt, W. R. Ab Initio Effective Core Potentials for Molecular Calculations. Potentials for K to Au Including the Outermost Core Orbital. J. Chem. Phys. 1985, 82, 299-310 (28) Wang, F.; Zhang, D.; Ding, Y. DFT Study on CO Oxidation Catalyzed by PtmAun (m+n=4) Clusters: Catalytic Mechanism, Active Component, and the Configuration of Ideal Catalysts. 22


Page 23 of 44

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J. Phys. Chem. C. 2010, 114, 14076 (29) Jin, R.; Zhang, S.; Zhang, Y.; Huang, S.; Wang, P.; Tian, H. Theoretical Investigation of Adsorption and Dissociation of H2 on (ZrO2)n (n=1-6) clusters. Int. J. Hydrogen. Energy. 2011, 36, 9069-9078 (30) Syzgantseva, O. A.; Gonzalez-Navarrete, P.; Calatayud, M.; Bromley, S.; Minot, C. Theoretical Investigation of the Hydrogenation of (TiO2)N Clusters (N=1-10). J. Phys. Chem. C. 2011, 115, 15890-15899 (31) Peng, C.; Schlegel, H. B. Combining Synchronous Transit and Quasi-Newton Methods to Find Transition States. Israel. J. Chem. 1993, 33, 449-454 (32) Fukui, K. Formulation of the Reaction Coordinate. J. Phys. Chem. 1970, 74, 4161 (33) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakasuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, R.; Ishida, M. J.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Knox, X. Li. J. E. R.; Hratchian, Cross, H. P. J. B.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K. G.; Voth, A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S. S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, J. A.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B. G.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03 (Revision C02), Gaussian, Inc.: Pittsburgh, PA. 2003. (34) Wang, M.; Huang, X. W.; Du, Z. L.; Li, Y. C. Structural, Electronic, and Magnetic Properties of a Series of Aluminum Clusters Doped with Various Transition Metals. Chem. Phys. Lett. 2009, 480, 258−264 (35) Chrétien, S.; Buratto, S. K.; Metiu, H. Catalysis by Very Small Au Clusters. Curr. Opin. Solid. State. Mater. Sci. 2007, 11, 62-75 (36) Joshi, A. M.; Delgass, W. N.; Thomson, K. T. Analysis of O2 Adsorption on Binary-Alloy 23



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Clusters of Gold: Energetic and Correlations. J. Phys. Chem. B. 2006, 110, 23373-23387 (37) Popolan, D. M.; Bernhardt, T. M. Communication: CO Oxidation by Silver and Gold Cluster Cations: Identification of Different Active Oxygen Species. J. Chem. Phys. 2011, 134, 091102-091104 (38) Li, H.; Pei, Y.; Zeng, X. C. Two-dimensional to Three-Dimensional Structural Transition of Gold Cluster Au10 during Soft Landing on TiO2 Surface and its Effect on CO Oxidation. J. Chem. Phys. 2010, 133, 134707-134713 (39) Davran-Candan, T.; Gőnay, M. E.; Yildirim, R. Structure and Activity Relationship for CO and O2 Adsorption over Gold Nanoparticles using Density Functional Theory and Artificial Neural Networks. J. Chem. Phys. 2010, 132, 174113 (40) Gao, Y.; Shao, N.; Pei, Y.; Chen, Z. F.; Zeng, X. C. Catalytic Activities of Subnanometer Gold Clusters (Au16-Au18, Au20, and Au27-Au35) for CO Oxidation. Acs Nano. 2011, 5, 7818-7829 (41) Weng, M. H.; Ju, S. P. CO Oxidation Mechanism on Tungsten Nanoparticle. J. Phys. Chem. C. 2012, 116, 18803-18815 (42) Ham, H. C.; Stephens, J. A.; Hwang, G. S.; Han, J.; Nam, S. W.; Lim T. H. Role of Small Pd Ensembles in Boosting CO Oxidation in AuPd Alloys. J. Phys. Chem. Lett. 2012, 3, 566-570 (43) Liu, Z. P.; Hu, P. General Trends in the Barriers of Catalytic Reactions on Transition Metal Surfaces. J. Chem. Phys. 2001, 115, 4977-4980 (44) Chang, C. R.; Wang, Y. G.; Li, J. Theoretical Investigations of the Catalytic Role of Water in Propene Epoxidation on Gold Nanoclusters: A Hydroperoxyl-Mediated Pathway. Nano. Res. 2011, 4, 131−142 (45) Rodriguez, P.; Koverga, A. A.; Koper, M. Carbon Monoxide as a Promoter for its Own Oxidation on a Gold Electrode. Angew. Chem. Int. Ed. 2010, 49, 1241−1243 (46) Rodriguez, P.; Kwon, Y.; Koper, M. T. M. The Promoting Effect of Adsorbed Carbon Monoxide on the Oxidation of Alcohols on a Gold Catalyst. Nature. Chem. 2012, 4, 177−182 (47) Liu, C. Y.; Tan, Y. Z.; Lin, S. S.; Li, H.; Wu, X. J.; Li, L.; Pei, Y.; Zeng, X. C. CO Self-Promoting Oxidation on Nanosized Gold Clusters: Triangular Au Active Site and CO Induced O−O Scission. J. Am. Chem. Soc. 2013, 135, 2583−2595

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Figure captions Figure 1. Low energy isomers of Al12X (X=Ni, Pd, Pt, Ti and Zr). The numbers under the structures are the relative total energies (in eV) with respect to those of the lowest energy isomers. For (a) and (d), label of the binding sites are shown. The structure (a) has a mirror-symmetry with respect to the plane given by the atoms in the ring (no. 12, 3, 11, 8), which cuts the structure in half. Hence the atoms in the two sides, i. e. no. 1, 2, 7, 6, and 5, 4, 9, 10, are equivalent, respectively. Figure 2. (a) The LUMO of CO and the HOMO of Al12Ni when they are at infinite distance from each other and the lowest energy and substable isomers of the Al12Ni-CO complexes. (b) The LUMO of O2 and the HOMO of Al12Ni when they are at infinite distance from each other and the lowest energy and substable isomers of the Al12Ni-O2 complexes. (c) The LUMO of CO and the HOMO of Al12Ti when they are at infinite distance from each other and the lowest energy and substable isomers of the Al12Ti-CO complexes. (d) The LUMO of O2 and the HOMO of Al12Ti when they are at infinite distance from each other and the lowest energy and substable isomers of the Al12Ti-O2 complexes. Figure 3. Optimized geometries of CO+O2, CO2+O, and CO+O coadsorption on Al12X(X=Ni, Pd, Pt) (a) and Al12X (X=Ti, Zr) clusters (b). Figure 4. (a) Potential energy surfaces for CO oxidation promoted by Al12X (X=Ni, Pd, Pt). The corresponding intermediates and transition states are also presented. The sum of energies of free Al12X, O2, and CO is taken as the zero-point energy, which is in eV. The blue, pink, gray, and red balls denote X, Al, C and O atoms, respectively. (b) Potential energy surfaces of the LH reaction for CO+O reactions on the Al12X (X=Ni, Pd, Pt). The corresponding intermediates and transition

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states are also presented. The sum of energies of free Al12X, O, and CO is taken as the zero-point energy, which is in eV. The blue, pink, gray, and red balls denote X, Al, C and O atoms, respectively. Figure 5. (a) Potential energy surfaces for CO oxidation promoted by Al12X (X=Ti, Zr). The corresponding intermediates and transition states are also presented. The sum of energies of free Al12X, O2, and CO is taken as the zero-point energy, which is in eV. The blue, pink, gray, and red balls denote X, Al, C and O atoms, respectively. (b) Potential energy surfaces of the LH reaction for CO+O reactions on the Al12X (X=Ti, Zr). The corresponding intermediates and transition states are also presented. The sum of energies of free Al12X, O, and CO is taken as the zero-point energy, which is in eV. The blue, pink, gray, and red balls denote X, Al, C and O atoms, respectively. Figure 6. (a) Potential energy surfaces of the reaction for double CO molecules oxidation reactions on the Al12X (X=Ni, Pd, Pt). The corresponding intermediates and transition states are also presented. The sum of energies of free Al12X, O2, and 2CO is taken as the zero-point energy, which is in eV. The blue, pink, gray, and red balls denote X, Al, C and O atoms, respectively. (b)Potential energy surfaces of the reaction for double CO molecules oxidation reactions on the Al12X (X=Ti, Zr). The corresponding intermediates and transition states are also presented. The sum of energies of free Al12X, O2, and 2CO is taken as the zero-point energy, which is in eV. The blue, pink, gray, and red balls denote X, Al, C and O atoms, respectively. Figure 7. Local density of states (LDOS) projected onto O-O and C-O for CO oxidation on A112Ti via the proposed minimum-energy pathway. (a) Before interaction, (b) IM1, (c) IM2, (d) IM3, (e) IM4. The black, blue, and red lines represent the LDOS of O-O, C-O, and Ti (d band), respectively.

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The dashed line represents the Fermi level.

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Page 28 of 44

Table 1. Adsorption Properties of CO and O2 on Al12X (X= Ni, Pd, Pt, Ti and Zr) including the Adsorption site, Spin Multiplicity (M), Binding Energy (BE) in eV, Bond Distances (R) in Å, NBO Charge (q) and Vibrational Frequency (v) in cm-1

Cluster

Species

Site

M

BE (eV)

RC-AlnX (À)

RO-AlnX (À)

RC- O/O-O (À)

q

v

Al12Ni

CO

3 12 3 12 3 12 3 12 3 12 3 12 13 3 13 3 13 3 13 3

1 1 3 3 1 1 3 3 1 1 3 3 3 3 3 3 3 3 3 3

-1.09 -0.73 -0.90 -0.78 -0.84 -0.63 -0.83 -0.80 -0.92 -0.55 -0.82 -0.72 -1.60 -0.92 -2.17 -1.15 -1.47 -0.73 -2.12 -1.08

1.97 1.97 1.98 2.01 2.00 2.03 2.034 2.026 2.20 2.03 -

3.13 3.13 1.84 1.88 3.14 3.17 1.91 1.87 3.13 3.18 1.89 1.88 3.20 3.08 1.79 1.84 3.37 3.07 1.98 1.85

1.17 1.17 1.30 1.29 1.18 1.16 1.32 1.30 1.19 1.16 1.30 1.30 1.17 1.19 1.29 1.37 1.17 1.19 1.30 1.35

-0.19 -0.04 -0.47 -0.38 -0.31 -0.06 -0.49 -0.41 -0.35 -0.08 -0.47 -0.44 -0.14 -0.48 -0.39 -0.62 -0.15 -0.52 -0.47 -0.60

1929 1975 1169 1188 1881 2007 1089 1185 1844 2008 1163 1185 1866 1803 1251 1057 1879 1788 1212 1070

O2 Al12Pd

CO O2

Al12Pt

CO O2

Al12Ti

CO O2

Al12Zr

CO O2

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 2. Coadsorption Energies (eV) and Geometrical Parameters for CO+O2, CO2+O, and CO+O Coadsorption on Al12X (X= Ni, Pd, Pt, Ti and Zr) Clusters

Cluster

Species

Al12Ni

CO+O2 O2 (3)+CO (12) O2 (8)+CO (9) O2 (4)+CO (5) O2 (12)+CO (3) CO+O2 O2 (3)+CO (12) O2 (8)+CO (9) O2 (4)+CO (5) O2 (12)+CO (3) CO+O2 O2 (3)+CO (12) O2 (8)+CO (9) O2 (4)+CO (5) O2 (12)+CO (3) CO+O2 O2 (13)+CO(13) O2 (13)+CO (1) CO+O2 O2 (13)+CO (13) O2 (13)+CO (1) CO2+O (3, 12) CO2+O (3, 12) CO2+O (3, 12) CO2+O (1,8,13) CO2+O (1,8,13) CO (12)+O (2,3,12) CO (12)+O (2,3,12) CO (12)+O (2,3,12) CO (8)+O (1,13) CO (8)+O (1,13)

Al12Pd

Al12Pt

Al12Ti

Al12Zr

Al12Ni Al12Pd Al12Pt Al12Ti Al12Zr Al12Ni Al12Pd Al12Pt Al12Ti Al12Zr

Ecoads

RC-AlnX (Å)

RO-AlnX (Å)

RO-O (Å)

RC-O (Å)

-1.523 -1.447 -1.368 -1.230

1.332 1.326 1.312 1.335

1.181 1.183 1.190 1.163

1.983 2.010 2.054 2.013

1.837 1.827 1.818 1.849

-1.165 -0.900 -0.866 -1.189

1.315 1.326 1.312 1.335

1.189 1.183 1.190 1.163

1.999 2.010 2.054 2.013

1.834 1.827 1.818 1.850

-1.250 -0.861 -0.784 -1.212

1.316 1.326 1.312 1.335

1.190 1.183 1.190 1.163

2.020 2.010 2.054 2.013

1.817 1.827 1.818 1.850

-3.414 -2.753

1.287 1.291

1.185 1.162

2.053 1.983

1.794 1.776

-3.131 -2.548 -7.121 -6.545 -6.559 -7.807 -7.459 -7.393 -7.428 -7.430 -7.518 -7.475

1.292 1.301 3.278 3.277 3.306 5.694 4.112 -

1.175 1.161 1.200 1.194 1.194 1.195 1.201 1.161 1.160 1.159 1.159 1.163

2.247 1.998 2.906 3.053 3.076 3.097 3.143 2.031 2.041 2.042 2.063 2.019

1.994 1.946 1.804 1.759 1.761 1.833 1.958 1.866 1.869 1.872 1.873 1.824

29



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Al12Ni Al12Pd Al12Pt Al12Ti Al12Zr

(a) Cs 0.00eV 0.00eV 0.00eV 1.37eV 1.88eV

(b) D5h 0.51eV 0.50eV 0.64eV 5.52eV 4.18eV

(c) Ih 0.58eV 0.81eV 1.01eV 4.76eV 5.09eV

(d) Ih 2.05eV 1.66eV 2.17eV 0.00eV 0.00eV

Page 30 of 44

(e) D5h 3.69eV 3.09eV 3.11eV 0.28eV 0.33eV

Figure 1. Low energy isomers of Al12X (X=Ni, Pd, Pt, Ti and Zr). The numbers under the structures are the relative total energies (in eV) with respect to those of the lowest energy isomers.

30


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Site 9

Site 8

Site 3

Site 12 (a)

31



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Page 32 of 44

Site 9

Site 8

Site 3

Site 12 (b) 32


Page 33 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Site 13

Site 3 (c)

Site 13 (c)

Site 3 (d) 33



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 44

Figure 2. (a) The LUMO of CO and the HOMO of Al12Ni when they are at infinite distance from each other and the lowest energy and substable isomers of the Al12Ni-CO complexes. (b) The LUMO of O2 and the HOMO of Al12Ni when they are at infinite distance from each other and the lowest energy and substable isomers of the Al12Ni-O2 complexes. (c) The LUMO of CO and the HOMO of Al12Ti when they are at infinite distance from each other and the lowest energy and substable isomers of the Al12Ti-CO complexes. (d) The LUMO of O2 and the HOMO of Al12Ti when they are at infinite distance from each other and the lowest energy and substable isomers of the Al12Ti-O2 complexes.

34


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


CO(12)+O2(3)

CO(3)+O2(12)

CO(9)+O2(8)

CO(5)+O2(4)

CO2+O (3, 12)

CO(12)+O (2, 3, 12) (a)

CO(13)+O2(13)

CO(1)+O2(13)

35



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CO2+O (9, 13)

Page 36 of 44

CO(8)+O (9,13) (b)

Figure 3. Optimized geometries of CO+O2, CO2+O, and CO+O coadsorption on Al12X(X=Ni, Pd, Pt) (a) and Al12X (X=Ti, Zr) clusters (b).

36


Page 37 of 44

(a) 1 0 -1 -2

Al12X+CO+O 2 0

IM1

-1.16 -1.25 -1.52

Relative Energy (eV)

-3 -4

TS1/2

-0.98 -1.10 -1.22

-5

TS 2/3

IM2

-2.39 -2.61 -2.69

-2.77 -2.87 -2.92

-6 -7 -8

Al12Ni

IM3

Al12Pd

-9

Al12Pt

-6.54 -6.56 -7.12

-10 -11

Reaction Coordinates

(b) 1 0 -1

Al12Pt

Al12X+CO+O 0

Al12Pd Al12Ni

-2


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-3 -4 -5 -6 -7 -8 -9

-10

IM4

-7.39 -7.43 -7.43

TS4/5 -6.15 -6.20 -6.94

IM5 -6.66 -6.76 -6.80

TS5

-5.91 -6.03 -6.06

FS

-6.16 -6.30 -6.31

-11

Reaction Coordinates 37



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 44

Figure 4. (a) Potential energy surfaces for CO oxidation promoted by Al12X (X=Ni, Pd, Pt). The corresponding intermediates and transition states are also presented. The sum of energies of free Al12X, O2, and CO is taken as the zero-point energy, which is in eV. The blue, pink, gray, and red balls denote X, Al, C and O atoms, respectively. (b) Potential energy surfaces of the LH reaction for CO+O reactions on the Al12X (X=Ni, Pd, Pt). The corresponding intermediates and transition states are also presented. The sum of energies of free Al12X, O, and CO is taken as the zero-point energy, which is in eV. The blue, pink, gray, and red balls denote X, Al, C and O atoms, respectively.

38


Page 39 of 44

(a) 1 0 -1


-2

Al12X+CO+O2 0

-3 -4

IM1

-5

-3.13 -3.41

T1/2

-3.08 -3.34

IM2

-4.25 -4.61

-6

T2/3 -3.67 -4.59

-7

Al12Ti

-8

Al12Zr

IM3

-9

-7.72 -8.13

-10

T3/4

-7.25 -7.68

IM4 -7.46 -7.81

-11


(b) 1

Al12Ti

0 -1 -2


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Al12Zr

Al12X+CO+O 0

-3 -4 -5 -6

FS

-7 -8 -9

IM5

-7.48 -7.52

TS 5/6 -6.93 -7.14

IM6

-7.40 -7.44

TS 6

-6.24 -6.29

-7.16 -7.41

-10 -11


39



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Page 40 of 44

Figure 5. (a) Potential energy surfaces for CO oxidation promoted by Al12X (X=Ti, Zr). The corresponding intermediates and transition states are also presented. The sum of energies of free Al12X, O2, and CO is taken as the zero-point energy, which is in eV. The blue, pink, gray, and red balls denote X, Al, C and O atoms, respectively. (b) Potential energy surfaces of the LH reaction for CO+O reactions on the Al12X (X=Ti, Zr). The corresponding intermediates and transition states are also presented. The sum of energies of free Al12X, O, and CO is taken as the zero-point energy, which is in eV. The white, pink, gray, and red balls denote X, Al, C and O atoms, respectively.

40


Page 41 of 44

(a)

1 0 -1 -2

Al12X+2CO+O2 0


-3

IM1

-2.16 -2.22 -2.61

-4 -5

TS1/2

-2.00 -2.05 -2.42

IM2 TS2/3

-3.88 -3.64 -3.95 -3.76 -4.33 -4.11

-6 -7

Al12Ni

-8

Al12Pd

FS

TS3/4

IM3

-6.47 -6.66 -6.68

Al12Pt

-9

-6.41 -6.59 -6.66

-10

IM4

-7.41 -7.56 -7.71

-6.21 -6.37 -6.38

-11


(b) 1 0 -1

Al12Ti

Al12X+2CO+O2

Al12Zr

0

-2


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-3 -4 -5 -6 -7

IM1

-3.55 -3.99

T1/2

-3.44 -3.91

IM2

T2/3

-4.91 -4.78 -5.22 -5.14

FS

-8

IM3

-9

-7.43 -7.78

T3/4

-7.41 -7.71

-10

IM4

-6.33 -6.95

-7.72 -7.89

-11

Reaction Coordinates 41



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 42 of 44

Figure 6. (a) Potential energy surfaces of the reaction for double CO molecules oxidation reactions on the Al12X (X=Ni, Pd, Pt). The corresponding intermediates and transition states are also presented. The sum of energies of free Al12X, O2, and 2CO is taken as the zero-point energy, which is in eV. The blue, pink, gray, and red balls denote X, Al, C and O atoms, respectively. (b)Potential energy surfaces of the reaction for double CO molecules oxidation reactions on the Al12X (X=Ti, Zr). The corresponding intermediates and transition states are also presented. The sum of energies of free Al12X, O2, and 2CO is taken as the zero-point energy, which is in eV. The blue, pink, gray, and red balls denote X, Al, C and O atoms, respectively.

42


Page 43 of 44

1 0

1 0

( a 1 )

8

σ

6

( a 2 )

8

u *

π

σ

4

π

g *

u

1 π

2 π *

6

2 σ

4

2 σ *

3 σ

2

2

0

0

1 π

( b 1 )

6

2 σ

4

2 σ ∗

2 π ∗

3 σ

1 π

( b 2 )

6

2

2 σ

4

2 σ *

0

8

4

2 σ *

2 σ

6

1 π

( c 1 )

6

2 π * 3 σ

4

1 π

( c 2 ) 2 σ 3 σ

2

2

0

2 π ∗

3 σ

2

0

LDOS(states/eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2 π *

2 σ *

0

8 6

( d 1 )

6

4

4

2

2

0

0

6

( e 1 )

(d 2 )

8 6

4

( e 2 )

4

2

2 0

0

-25

-20

-15

-10

-5

0

5-25

-20

-15

-10

-5

0

5

Energy (eV) Figure 7. Local density of states (LDOS) projected onto O-O and C-O for CO oxidation on A112Ti via the proposed minimum-energy pathway. (a) Before interaction, (b) IM1, (c) IM2, (d) IM3, (e) IM4. The black, blue, and red lines represent the LDOS of O-O, C-O, and Ti (d band), respectively. The dashed line represents the Fermi level.

43



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86x74mm (72 x 72 DPI)


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X (X = Ni, Pd, Pt, Ti, and Zr) Clusters - American Chemical Society

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