Single Layer of Polymeric Metal–Phthalocyanine: Promising Substrate

Jan 10, 2014 - Single atom catalysts (SAC) are highly desirable to maximize atom ... We found that Ti–Pc is the most appropriate compound by virtue ...
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Single Layer of Polymeric Metal−Phthalocyanine: Promising Substrate to Realize Single Pt Atom Catalyst with Uniform Distribution X. F. Chen, J. M. Yan, and Q. Jiang* Key Laboratory of Automobile Materials, Ministry of Education, and Department of Materials Science and Engineering, Jilin University, Changchun 130022, China S Supporting Information *

ABSTRACT: Single atom catalysts (SAC) are highly desirable to maximize atom efficiency, while fabricating them is largely dependent on choosing a suitable substrate to effectually fasten them. Here, in terms of strong metal−metal interaction, first-principle calculations have been performed to study the possibility of monolayer polymeric transition metal− phthalocyanines (TM−Pcs, M from Sc to Zn) as the substrate in an effect to obtain evenly distributed single Pt atom catalyst. We found that Ti−Pc is the most appropriate compound by virtue of high binding strength of Pt/Ti−Pc system and high diffusion energy barrier of Pt catalyst atom, which jointly prevent the formation of Pt clusters. CO oxidation on Pt/Ti−Pc presents some unexpected behaviors, where a presorbed CO (pCO) molecule could activate the catalyst and reduce the reaction barrier. Examining the electronic structure evolution in the reaction process demonstrates that the presence of a pCO promotes O2 adsorption and activation. Our results demonstrate that two-dimensional (2D) TM− Pcs provide a unique platform to fabricate regularly and separately distributed SAC with high activity.

1. INTRODUCTION Currently, supported noble-metal catalysts are widely used in industry by virtue of their high activity and/or selectivity for many important chemical reactions, while their efficiency is extremely low on a per metal atom basis because only the surface active-sites are used.1 Given the high cost and limited supply of noble-metal resource, improving their service efficiency and yet preserving or even increasing their catalytic activity becomes extremely important and desirable.2−4 Existing methods to improve the service efficiency of noble-metal-based catalysts can be categorized into two groups: alloying with lowcost transition metal (TM) elements to form the so-called bimetallic catalysts5−9 or preparation of core−shell structures where only the outer surface consists of noble-metal atoms.10−14 Besides, an intuitive route is to reduce the size of noble-metal clusters to nanometer scale or even smaller.3,15−17 With the reduced size, many new properties would appear due to quantum confinement effect, and synchronously, it would provide more atoms with low coordination which generally act as active sites for catalysis.18−21 In this respect, single atom catalysts (SAC) should be the limit case in the field of nanocatalysts, which would provide the highest service efficiency if they render comparable/better catalytic activity compared with bulk. However, preparation of such SAC in experiments has been largely hampered by the high mobility of noble-metal atoms on substrate and their easiness to sinter under realistic reaction conditions.1,22 Thus, many attentions in this area have been paid to find appropriate substrates that can effectually anchor the catalyst atoms without degrading their activities.17 Because of the huge surface-to-volume ratio, graphene has been © XXXX American Chemical Society

extensively studied as a substrate material for heterogeneous catalysts since it was discovered.23−28 On graphene, the SAC can be fabricated through a two-step process including local creation of monovacancies by high-energy atom/ion bombardment followed by filling these vacancies with desired catalyst atoms.29 The resulting metal-embedded SAC have shown good activities and thermal stabilities due to the strong bonding between catalyst atoms and graphene substrate.23−25 In addition, metals30 and metal oxides, such as Fe3O4, TiO2, alumina,31,32 MgO2, CeO233, etc., have also been considered as support materials for SAC. Recently, by using a chemical coprecipitation method with finely tuned temperature and PH value, Qiao et al. have successfully demonstrated the SAC of Pt on the FeOx support,1 which provides a grand step toward the development of SAC. Despite considerable efforts, up to now, the creation of well-ordered and uniform distributed SAC remains practically challenging because fabricating them on these substrate materials generally requires the creation of monovacancies on substrates first for anchoring the catalyst atoms. However, atom-scale control of the monovacancy formation is experimentally quite difficult, and the resulting distribution of the catalyst atoms is rather disordered. To this end, exploiting any new alternative material that could anchor the single catalyst atoms uniformly and yet avoiding the difficulty of creating the evenly distributed vacancies would be desirable. Received: November 13, 2013 Revised: January 7, 2014

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Recently, Abel et al. have synthesized a single layer of polymeric Fe−phthalocyanine (Pc) successfully through a metal-directed surface reaction and pointed out that the synthesis procedure was flexible for other central metal atoms.34 In terms of the fact that the germ of the agglomeration effect or the sinter problem of metal catalyst atoms lies in the stronger metal−metal interaction than metal−substrate interaction, the TM−Pcs should be a suitable substrate material for anchoring single Pt catalyst atom by using the strong Pt−metal interaction. Motivated by the above considerations, using firstprinciples method, we systematically investigated the anchoring effect of the free-standing two-dimensional (2D) TM−Pc sheets on single Pt atoms with possible TM atoms ranging from Sr to Zn in the periodic table. It is found that Ti−Pc is the most appropriate compound with high bonding strength toward Pt and catalytic activity for CO oxidation. The reaction of CO with O2 above Pt/Ti−Pc has an overall energy barrier of 0.55 eV.

Figure 1. Top (left) and side (right) view of Pt atom adsorbed on different sites of TM−Pc. The gray, white, blue, and steelblue represent carbon, hydrogen, nitride, and platinum atoms, respectively. X denotes the TM atoms ranging from Sc to Zn.

(Ead) for all these positions are defined with respect to single Pt atom and TM−Pc sheet at equilibrium state, that is, Ead = Etotal − E TM−Pc − E Pt

2. COMPUTATIONAL METHODS Calculations have been performed by density functional theory as implemented in Dmol3 code with numerical functions on an atom-centered grid as its atomic basis.35 Generalized gradient approximation (GGA) and Perdew−Burke−Ernzerhof (PBE) are employed as the exchange correlation potential.36,37 Allelectron core treatment including some relativistic effects and double numeric plus polarization (DNP) is invoked for all of the calculations. A vacuum space lager than 20 Å is used to prevent the interaction effect from neighboring cells. The minimum energy paths (MEPs) for CO oxidation are obtained by LST/QST tools in Dmol3 code, which has been well validated to find a transition state (TS) structure.38,39 We use 3 × 3 × 1 Monkhorst−Pack k-point meshes to sample the Brillouin zones. The root-mean-square (rms) convergence for TS search is set to be 0.005 eV/Å and convergence tolerance of energy, maximum force, and maximum displacement for geometry optimization are 1.0 × 10−5 Ha, 0.002 Ha/Å, and 0.005 Å, respectively. The adsorption energy E for gas molecule is computed by E(X) = EX/S − EX − ES where the X denotes O2 and (or) CO molecule, and S means the Pt/Ti−Pc. The interaction energy Eint, indicating the bonding strength between the adsorbed molecule(s) with the Pt/Ti−Pc substrate in its deformed geometry, is calculated by Eint(X) = EX/S − EX − ES′ where EX and ES′ denote the energies of the gas molecule(s) and Pt/Ti− Pc in its deformed geometry, respectively. Accordingly, the deformation energy Edef can be obtained in light of the difference between E and Eint, namely, Edef = E − Eint, which demonstrates the energy cost to maintain the structure deformation in Pt/Ti−Pc.

where Etotal is the energy of the optimized structure of Pt/TM− Pc and ETM−Pc, and EPt are the energies of the isolated TM−Pc sheet and single Pt atom, respectively. Accordingly, a negative Ead value suggests the adsorption of single Pt atom on the surface of TM−Pc is energetically favorable. To avoid Pt atoms congregating, the TM−Pc should catch and bind it strongly as it gets close to the TM−Pc surface. As shown in Table 1, for 2D Table 1. Calculated Adsorption Energies for Pt Atoms Adsorbed on the TM, 3N, Bridge, and 1N Sites of TM−Pcs Ead (eV)/TM−Pc

TM

3N

bridge

1N

6C

Sc Ti V Cr Mn Fe Co Ni Cu Zn

−3.29 −4.60 −3.95 −2.28 −2.36 −2.15 −1.79 −0.84 −0.68 −0.99

TM TM TM TM TM TM TM −1.55 −0.68 bridge

−2.95 TM TM TM TM TM −1.78 −1.50 −1.56 −1.78

−1.94 −1.92 −1.90 −1.85 −1.84 −1.79 −1.80 −1.79 −1.84 −1.88

−1.25 −1.44 −1.27 −1.14 −1.19 −1.13 −0.87 −1.10 −1.10 −1.09

Sc−Pc, Ti−Pc, and V−Pc, the most favorable adsorption site lies in the TM site with adsorption energies of −3.29, −4.60, and −3.95 eV, respectively. In contrast, for Cr−Pc, Mn−Pc, and Fe−Pc, despite the TM site remains the most energetically one, their Ead values are comparable with other sites. Also, these Ead values (from −2.15 to −2.28 eV), become much smaller than those of Sc−Pc, Ti−Pc, and V−Pc. As for Co−Pc, Ni−Pc, Cu− Pc, and Zn−Pc, the most favorable adsorption site is no longer the TM site but turning into the 1N site. Generally, there is a decline of Ead from Sc−Pc to Zn−Pc except for Sc, Cr, and Zn, while the differences of Ead in both 1N and 6C sites for all the TM−Pcs considered are considerably small, indicating that the effects of TM atoms on the electronic properties of Pc are very localized. In addition, except for Ni−Pc and Cu−Pc, Pt atom adsorbed on 3N site will migrate to the TM site spontaneously without any barrier. Therefore, strong interaction between TMs and Pt makes them an effective potential well to trap and fasten the Pt atom. According to the above results, Ti−Pc satisfies our requirement with the most preferable Ead value (−4.6 eV). We note that this value is larger than the bonding energies of Cu in Cu-embedded graphene (−3.4 eV),25 Au in Au-embedded

3. RESULTS AND DISCUSSION Single layer of Fe−Pc has recently been grown successfully on Au(111) and Ag(111) substrates.34 Our calculated result for the lattice constant of Fe−Pc is 1.07 nm, being in good agreement with the experimental value of 1.15 ± 0.1 nm and other theoretical values.34,39 Figure 1 shows schematically the structures of 2D TM−Pc with X representing the TM atoms from Sc to Zn. When single Pt atom is introduced into the TM−Pc surface, there are five most possible positions, namely, TM, 3N, bridge, 1N, and 6C as shown in Figure 1. To assess the preferred adsorption configuration, the adsorption energies B

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graphene (−2.2 eV) and boron nitride (−3.03 eV),23,40,41 and silicene (−2.46 eV),42 while smaller than Pt-embedded graphene (−7.2 eV),43 all of which have demonstrated good stability and reactivity. Consequently, in the following, we will concentrate our discussion on the Pt/Ti−Pc system. Given the possible clustering problem of Pt atoms on substrates, we calculate the diffusion barrier of Pt atom from the TM sites to other possible adsorption sites. As mentioned above, the Pt atom placed on 3N and bridge sites will spontaneously move to the TM site after optimization. Thus, in the case of Ti−Pc, there are only two other possible adsorption sites, namely, 1N and 6C, except for the TM site. We present the structures of initial state, transition state, and final state in Figure 2a−d and the corresponding diffusion reaction energy

strong Ti−Pt interaction. Moreover, Pt atom anteriorly adsorbed on 6C or 1N site can easily diffuse to its thermodynamic stable TM site. The barrier needed to diffuse from 6C to 1N is 0.02 eV (not shown in Figure 2e) and that from 1N to TM site is 0.45 eV. Previous studies have demonstrated that reactions with energy barrier below 0.70 eV can occur spontaneously at low temperature.44 Thus, Pt atoms dropped on the Ti−Pc surface will be anchored by the TM site easily, making convenience for experimental deposition of Pt catalyst atoms with controlled sites. To further check the thermodynamic stability of Pt/Ti−Pc with monodisperse Pt atoms, we have calculated the energies of two and four Pt atom clusters at their most stable sites. The corresponding structures and energies with respect to those of Pt/Ti−Pc in their monodisperse configurations are shown in Figure 2f,g. One can find that Pt atoms on Ti−Pc are inclined to form uniform distribution compared with the Pt cluster. Formation of Pt/Ti−Pc system changes the electronic properties of each other, which in turn affects the catalytic properties of Pt. Figure 3a,b presents the geometric and

Figure 3. Side views of the geometry (a) of Pt atom fastened by the Ti−Pc. (b) Spin-polarized local density of states projected onto Ti and Pt before and after adsorption. Ef is set to zero.

electronic structures of Pt/Ti−Pc. The bond length between the Pt atom and Ti atom is 2.18 Å, and the Ti atom shows a little heave of 0.77 Å above the Pc plane. By comparison, the distances between Pt and each N atom of Pc base plane are all around 3.46 Å, indicating weak interaction between them. Thus, the anchoring effect to Pt is solely determined by the Pt− metal interaction. To give deeper insight into the Pt−Ti interaction, the spin-polarized local density of states (LDOS) before and after anchoring Pt are plotted, as shown in Figure 3b. Projected densities of states of the metal Ti ions in free Ti− Pc are spin-polarized, indicating the existence of local magnetic moment. The calculated magnetic moment on Ti is 1.43 μB, which is mainly concentrated on its d orbitals, being in agreement with the previous result.45 The electronic structure of Ti−Pc is changed after anchoring Pt atom. The most significant variation lies at the quenching of spin splitting in Ti−Pc. Specifically, spin-up d orbital of Ti atom shifts toward a higher level of 1.52 eV above the Fermi level (Ef) and becomes unoccupied, suggesting that electrons transfer out from its d orbital (around 2e− obtained by integrated DOS). However, in the case of the s orbital of Ti, electrons transfer in and the states above Ef are occupied. After these electron rearrangements, the occupations of spin-up and spin down of Ti-d orbitals become symmetrical and the magnetism of the system disappears (all the geometrical parameters and magnetic moments for Pt on other TM-Pcs can be found in Table S1, Supporting Information). The interaction between Ti−Pc and Pt atom also changes the electronic states of Pt atom. In Figure 3b, we

Figure 2. Atomic configurations of (a) TM site, (b) transition state, (c) 1N site, and (d) 6C site for Pt atom migration on Ti−Pc and (e) the corresponding energy profile. The barrier for migration from the 6C site to 1N site is only 0.02 eV, which is omitted in the profile. Relative energies Δε of (f) two and (g) four Pt atom clusters with respect to the monodisperse configurations are also given.

profiles from the TM site to 1N and 6C sites in Figure 2e. One can find that the diffusion of Pt from the TM site to its neighboring 1N site is endothermic by 2.84 eV, and the computed energy barrier is 3.29 eV, being larger than that on Cu-embedded graphene (2.34 eV)25 and comparable with Feanchored graphene oxide (3.27 eV).42 These results demonstrate that Ti−Pc could bind Pt atoms tightly in terms of the C

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difference as shown in the middle of Figure 4b where electrons are piled up on O2 molecule and depleted on the Pt atom. Similar to the case of CO adsorption, the occupation of antibonding orbital activates O2 and leads to the elongation of lO−O mentioned above. To this end, because of the bigger adsorption energy of CO compared with O2, Pt/Ti−Pc should be first covered by a CO molecule when it is immerged in the mixture of CO and O2 gas. Interestingly, we find that preadsorption of a CO (pCO) would enhance the ability of Pt atom to adsorb O2. The most favorable configuration of O2 adsorbed on CO precovered Pt/ Ti−Pc system is shown in Figure 4c, where O2 tilts down and the adsorption configuration changes from the end on (without CO preadsorption) to side on. lO−O (1.38 Å) is much larger than the case of its separate adsorption (1.29 Å), indicating that O2 is more activated. Mulliken charge analysis demonstrates that O2 obtains 0.48e− from the Pt/Ti−Pc system. The coadsorption of CO and O2 on Pt/Ti−Pc has an Ead(CO + O2) value of −3.10 eV, considerably larger than the sum of Ead(CO) and Ead(O2). Such enhanced adsorption can be rationalized by analyzing the electronic structures. Generally, the adsorption of CO plays a significant role to modulate the electronic properties of Pt, making it more suitable to adsorb O2. As shown in Figure 4b, without pCO, electron resonances and hybridizations between O2 and Pt only localize at high energy range of −2.5 eV to approximately −0.6 eV, which are close to Ef. In contrast, with the existence of pCO, new peaks appear at relative low energy states in the range of −8.3 eV to approximately −4.8 eV and −4.1 eV to approximately −1.2 eV, providing more electronic resonance sites for O2 adsorption. Consequently, stronger electron hybridizations between O2 and Pt can be observed in Figure 4c compared with those in Figure 4b. In addition, pCO also enhances the electron donation ability of Pt to O2, which can be corroborated by the mulliken charge analysis (0.48e− vs 0.27e−). This is in agreement with the electron density differences presented in the middle of Figure 4b,c, where more electrons accumulate on O2 molecule in Figure 4c. To give a deeper insight into the origin of the enhanced adsorption energy, we separate it into three parts, namely, the contributions from O2, CO, and geometry deformation, respectively. In light of Table 2, for coadsorption

present the DOS of single Pt atom before adsorption, where the spin down component is half occupied due to its d9 electron configuration. After interacting with Ti, its d orbitals with energies close to Ef shift toward deep levels and become broader. Because of the strong d−d hybridization with Ti, d orbitals of Pt begin to split and new peaks appear in the range of −3.1 to −0.8 eV as shown in Figure 3b. Also, the formation of the Ti−Pt bond leads to the partial occupation of Pt-5d and Pt-6s with high density of states localized around Ef, which benefits for activating the reactants.23−25,39,43 To examine the catalytic performance of single Pt atoms supported on Ti−Pc, we choose CO oxidation as probe reaction. Before investigating the CO oxidation, adsorption properties of O2 and CO on the catalyst are studied first. Various adsorption sites have been considered in order to find out the most stable configuration for each adsorbate. Figure 4a

Figure 4. Side views of the geometry and electron density differences of CO (a), O2 (b), and their coadsorption (c) on Pt/Ti−Pc, respectively. The red and blue regions demonstrate the electron accumulation and depletion, respectively. Spin-polarized local density of states projected onto O2 and CO is also presented, where Ef is set to zero.

Table 2. Eint, E, and Edef Values Calculated for CO and (or) O2 Adsorption in eV

shows the most stable configuration of CO adsorbed on Pt/ Ti−Pc with E(CO) = −1.34 eV and Eint(CO) = −1.67 eV. The well-known mechanism of CO−metal interactions are applicable for CO adsorbed on Pt/Ti−Pc where electrons donate from CO-5σ to Pt-5d and back-donate from Pt-5d to CO2π*.46 Occupation of CO-2π* orbital destabilizes the CO molecule and results in the elongation of lC−O from 1.14 Å (gas) to 1.17 Å. The hybridizations between Pt-5d and CO-2π* can be observed near Ef from the LDOS, and those between Pt5d and CO-5σ are in the range of −3.7 to −0.9 eV as shown in Figure 4a. Compared with CO, O2 is absorbed weakly on Pt/ Ti−Pc with E(O2) = −0.34 eV and Eint(O2) = −0.54 eV as shown in Figure 4b. The most favorable adsorption configuration is characterized by O2 perpendicular to the Ti− Pc plane with an end-on configuration. The distance of Pt and O atom is about 2.01 Å and lO−O of the adsorbed O2 is 1.29 Å, being 4.9% larger than that of the free O2 (1.23 Å). Then, about 0.27e− transfer from Pt/Ti−Pc to O2, residing in the O2-2π* antibonding orbital. This is consistent with the charge density

CO + O2 Eint E Edef

O2

CO

O2

CO

−0.54 −0.34 0.20

−1.67 −1.34 0.33

−2.17

−3.31 −3.10 2.38

of CO and O2, Eint(O2) changes from −0.54 to −2.17 eV and Eint(CO) from −1.67 to −3.31 eV. To this end, the total energy gain after coadsorption of CO and O2 should be −5.48 eV without considering the contribution of geometry deformation in Pt/Ti−Pc substrate. However, upon the coadsorption of CO and O2, there exists deformation in Pt/Ti−Pc as evidenced by the observation of lPt−Ti elongation to be 2.47 Å. This is an endothermic process and costs energy with Edef = 2.38 eV. Meanwhile, with the elongation of lPt−Ti, the bonding strength of Pt/Ti−Pc decreases, which makes Pt atom more active to adsorb O2. Additionally, we have also studied the configurations of coadsorption of O2 with other common radicals in D

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heterogeneous catalysis including −OH, −H, −CH3, and −SH. However, no promote effects on the adsorption of O2 are observed. Direct reaction of the coadsorbed CO and O2 molecules via the well-known LH mechanism is rather difficult due to high energy barrier. The emergence of such high energy barrier can be attributed to be their adsorption configuration, where O2 resides in the opposite direction of CO and arranges in a straight line. Consequently, in order to approach and react with the CO molecule, O2 has to dissociate or circumrotate first while these processes need to conquer a high energy barrier. As mentioned above, the adsorption of CO molecule activates the O2 molecule since more electrons occupy its 2π* antibonding orbital. Thus, we investigate another reaction path where gasphase CO molecule directly reacts with the activated O2. The reaction paths can be characterized as CO + O2 → OOCO → O + CO2 followed by the reaction CO + O → CO2. To search the minimum energy pathway (MEP), we choose the most stable configuration as an initial state (IS), where CO physisorbed above the activated O2 molecule (Figure 5). The

investigated, in which the CO physisorbed on the Oβ atom with a nearest distance of 3.52 Å is selected as the IS. The FS state is selected as the configuration of CO2 adsorbed on Pt. To reach the TS, CO molecule gets close to the adsorbed Oβ atom and pushes it away from the Pt atom. The distance between the C atom and Oβ atom is decreased to 2.08 Å, and lOβ−Pt is elongated about 0.07 Å. The energy barrier is about 0.27 eV, which is nearly half of the barrier for CO + O2 reaction in the previous step. As the adsorption of CO2 is only 0.23 eV, it can be desorbed from the catalyst easily. We have also checked out the reaction of CO directly reacting with adsorbed O2 without pCO (seeing Figure S1, Supporting Information) where the energy barrier calculated is 1.13 eV, which is two times larger than that with pCO. Thus, the presence of the pCO is also important to reduce the reaction barrier. This promoting role of the pCO is unexpected due to its notorious poison effect in heterogeneous catalysis and electrocatalysis. To give a deeper understanding, we investigate the electronic structure evolution in the reaction process. Figure 6 presents the spin-polarized electronic LDOS

Figure 6. Spin-polarized LDOS projected onto O−O, C−O (a), and C−O (b) for IS1 (left) and TS1 (right). Black curves represent the LDOS of presorbed CO (a) and nearly no changes are observed in the reaction progress. Ef is set to zero.

projected onto C−O and O−O bonds along with the dprojected LDOS of Pt in IS1 and TS1. To differentiate the two CO molecules, we define the pCO molecule as CO (a) and the other as CO (b), that reacts with the O2 molecule directly. Here, we ignore the s- and p-projected LDOS of Pt atom as they have negligible change during the reaction progresses. The CO (b) molecule is only physisorbed above O2 molecule in the IS1 state, and thus, electronic state hybridizations between them are considerably small. In contrast, the antibonding orbital of O2−2π* is partially occupied with two peaks near Ef due to remarkable hybridization with the Pt-5d orbital, providing the electronic requirement to be attacked by the CO (b) molecule. From IS1 to TS1 state, with the formation of unstable peroxo-type O−O−C−O composite, there is an obvious redistribution of the LDOS and orbital shift for both CO (b) and O2. O2-2π* becomes more occupied, and its peak at Ef is slightly lowered. The peaks of O2-1π and O2-5σ orbitals are decreased and broadened significantly due to the breaking of O−O bond and interacting with CO (b). However, in this whole reaction process, the electron states of CO (a) are scarcely involved. Thus, the promoting role of pCO in the reaction is mainly through enhancing the ability of Pt/Ti−Pc to adsorb and activate O2 molecule.

Figure 5. Proposed reaction pathways for CO oxidation on Pt/Ti−Pc. The configurations of each step are presented. The inset in the cycle shows the calculated energy profile.

final state (FS) consists of a CO2 molecule physisorbed on Pt/ Ti−Pc with a chemisorbed atomic O nearby. Detailed configurations of the adsorbates on catalyst at each state are displayed in Figure 5. With a CO adsorbed above the activated O2 molecule, one of the O atoms (Oα) in the O2 molecule begins to approach the C atom of CO, and at the same time, the CO follows down to get close to the Oα, which leads to the TS (seeing Figure 5). In this state, the lOα‑Oβ is elongated to be 1.42 Å, and the lC−Oα is shortened to be 1.80 Å. This is an endothermic process with an energy barrier of 0.55 eV. Meanwhile, a peroxo-type O−C− Oα−Oβ complex is formed over the Pt atom. This complex is still maintained until a metastable configuration (MS) is reached, where lOα−Oβ is further elongated from 1.42 to 1.59 Å, while lC−Oα is shortened to be 1.28 Å. Passing over the MS without an energy barrier, CO2 is formed with a single O atom bonded with the Pt atom, similar to the case in the Au/ graphene system.23 Then, the CO2 molecule can be easily desorbed from the Pt/Ti−Pc at low temperature as its adsorption energy is only 0.11 eV. Subsequently, reaction of the remained Oβ atom with another CO molecule is

4. CONCLUSIONS In summary, the possibility of monolayer TM−Pcs (M from Sc to Zn) as potential substrate for single Pt atom catalyst is studied by using first-principles calculations. Pt atoms bond E

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strongly to Ti−Pc and have a relatively high diffusion barrier, which prevents the formation of Pt clusters. CO oxidation has been selected as a probe reaction to evaluate its catalytic performance. A pCO molecule is important to activate the catalyst and reduce the reaction barrier. Subsequent CO oxidation occurs with quite low catalytic energy barriers of 0.55 and 0.27 eV. As the electron states of pCO are scarcely involved in this whole reaction process, its promoting effect on the single atom Pt catalyst is attributed to be the ligand effect where the pCO modulates the electronic properties of Pt, making it more suitable for subsequent O2 adsorption and activation. Our results indicate that two-dimensional TM−Pcs provide a unique platform to fabricate SAC with regular distribution.

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ASSOCIATED CONTENT

S Supporting Information *

Geometrical parameters (Å) for Pt/TM−Pcs. Mulliken analysis of the charges and magnetic moments for Pt atoms in Pt/TM− Pcs. Schematic energy profile and corresponding local configurations of CO molecule directly reacting with adsorbed O2 molecule. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*(Q.J.) Tel/Fax: 86-431-85095876. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge supports by National Key Basic Research, Development Program (Grant No. 2010CB631001), and High Performance Computing Center (Jilin University).

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REFERENCES

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