A First-Principles Study of O2 Dissociation on Platinum Modified

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A First Principles Study of O2 Dissociation on Platinum Modified Titanium Carbide –A Possible Efficient Catalyst for the Oxygen Reduction Reaction Shiyan Wang, Xingli Chu, Xilin Zhang, Yanxing Zhang, Jianjun Mao, and Zongxian Yang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b05348 • Publication Date (Web): 18 Sep 2017 Downloaded from http://pubs.acs.org on September 22, 2017

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A First Principles Study of O2 Dissociation on Platinum Modified Titanium Carbide –A Possible Efficient Catalyst for the Oxygen Reduction Reaction Shiyan Wang1, Xingli Chu1, Xilin Zhang1, Yanxing Zhang1, Jianjun Mao1, and Zongxian Yang1†,2 1

College of Physics and Materials Science, Henan Normal University, Xinxiang, Henan 453007, China 2 Collaborative Innovation Center of Nano Functional Materials and Applications, Henan Province, China

Abstract: The adsorption and dissociation of O2 on the Pt modified TiC(001) surfaces with different Pt coverages of 1/4, 1/2, 3/4 and 1 ML are comparatively investigated using ab initio density functional theory calculations. The geometric and electronic structures are analyzed in detail. The strong interaction of Pt atoms with the TiC(001) is beneficial to improving the stability and activity of Pt catalyst. Compared with Pt(111), the MLPt/TiC(001) (3 × 3) have a positive impact on promoting the scission of the O-O bond (leading to a dissociation barrier comparable to that on Pt(111)) and weakening the adsorption of atomic O (the dissociation product of O2), which shed meaningful light on the important role of TiC(001) as support to improve the efficiency of Pt for oxygen reduction reaction.

†

Corresponding author: College of Physics and Materials Science, Henan Normal University, Xinxiang, Henan 453007, China E-mail address: [email protected] (Z. Yang) 1

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1.

INTRODUCTION

The development of new materials that can solve challenging problems in the areas of clean energy production, conversion and storage is of paramount importance in the quest to find an alternative to environmentally unfriendly fossil-fuel use. One promising alternative is the proton exchange membrane fuel cell (PEMFC)1-6, a device in which hydrogen, through its reaction with oxygen, produces water as its only product. Although the traditional Pt/C electrocatalyst for PEMFC is active for the oxygen reduction reaction (ORR), it also encounters some issues when regarding the catalyst particle dissolution, high cost and low stability in the electrochemical environment.7-9 Therefore, the development of catalysts with less Pt content without compromising the performance of the ORR is very essential. Titanium carbide (TiC) has been the subject of many investigations in surface science and the fields of catalysis, which has been proposed as optional candidate choices to replace of the more expensive Pt-group metals catalysts due to its lower cost and equal or even better catalytic properties.10-11 Using as substrate, TiC plays an important role in the promotion of electrocatalytic performance and the selectivity to reduce Pt loading, which provides a good opportunity to reduce the cost in electrocatalytic applications by modifying the geometric and electronic structures. Some previous experiments provided verification of low loading of Pt on TiC surface as low-cost and stable electrocatalysts in heterogeneous catalysis.12-14 Chiwata et al.15 verified that TiC can be used as an efficient support for Pt electrocatalysts, which exhibits higher electrochemically active surface area, much improved electrocatalytic activity and long-term durability toward an ORR when compared with the traditional Pt/C catalysts. Regmi et al.16 and Jalan et al.17 also found that Pt/TiC has as much as four times higher ORR activity compared to commercially available Pt/C catalyst, and show high stability under PEMFC conditions at 200◦C in 100% H3PO4. These experimental researches forcefully recommend that the combination of Pt clusters or monolayer with TiC substrate may result in ideal catalysts for ORR. Theoretical study can provide a large number of insights and help to interpretation of experimental mechanism of ORR. However, the ORR happens in a complex electrochemical environment, and the predictive simulation of ORR in fuel cells consumes a large amount of computer resources using density functional theory (DFT) calculations. The dissociation process of O2 is vital for understanding the mechanism of ORR that happens in the cathode of PEMFCs. Moreover, the dissociation of O2 on metal surfaces is a key reaction step in many chemical reactions, e.g., oxidation of CO.18-20 For clean TiC(001), the adsorption energy of O2 molecule is relatively small (Eads~0.45 eV).21 But after the scission of the O-O bond, the O atoms bind quite strongly (Eads~5.8 eV) to the TiC(001), which even oxidizes it.22-24 The similar phenomenon was seen for the adsorption of O on the surfaces of the ZrC(001) and HfC(001).24-26 Photoemission results also showed that the O atom adsorbs quite strongly on the surfaces of TiC after dissociation of O2 at room temperature.23 The over strong interaction of atomic O with the TiC(001) may poison the surface by forming oxycarbides. The addition of Pt on the TiC surfaces may serve as a skin protecting them from forming oxycarbides, which would drastically modify its chemical properties. Due to the essential 2


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inaccessibility of the electrode/electrolyte interface in the conditions of low temperature and ultravacuum, it is practically difficult to directly obtain the molecular information on the adsorption and dissociation of O2 during the electrochemical reduction. Accordingly, more DFT calculations become a valuable alternative to understand the admetal-carbide interaction by modifying the geometric and electronic structures of carbides for efficiently promoting O2 dissociation. For example, Rodríguez et al.27 using DFT and high-resolution photoemission found that the addition of two-dimensional (2D) Au clusters to TiC(001) greatly improves the rate of O2 dissociation at room temperature. Mao et al.28 found that MLPd/TiC(001) could be a good candidate for substituting the current Pt/C catalysts for ORR. Zhang et al.29 showed that dissociation of O2 on MLPt/ZrC(001) is easy to take place as compared with that on the bare ZrC(001). Our previous theoretical study also found that the 2D M4 (M=Au, Pd, Pt) clusters supported on HfC(001) may be possible substitutes with lower cost for the current Pt/C catalyst for O2 dissociation.30 Therefore, it is expected that the low loading of 2D Pt clusters or monolayer supported on TiC(001) would not only reduce the use of Pt but also has high catalytic activity for O2 dissociation. In this work, we provide a detailed, systematic theoretical investigation on the adsorption and dissociation of O2 on the Ptn/TiC(001) with different coverages of Pt as a simple descriptor of ORR activity using DFT calculations. Besides, we calculate and analyze the dissociation barrier of O2 on Ptn/TiC(001) surfaces with various number of Pt atoms to gain further insight into the structure-property relationship of the Pt modified systems. Meanwhile, the adsorption strength of O atom on the MLPt/TiC(001) (MLPt = platinum monolayer) is investigated and compared with that on the surfaces of TiC(001) and Pt(111). Our results show that MLPt/TiC(001) (3 × 3) is efficient for decomposing O2 and releasing the produced O atoms, which is more active than the traditional Pt/C catalysts for O2 dissociation and may be a possible substitute with lower cost for the current Pt/C catalyst for ORR. The results are consistent with the previous experiments of Chiwata et al.15 which verified that Pt/TiC surfaces exhibits a high stability and electrochemically active toward an ORR. In Section 2, a brief description of the model construction and the computational method is given. In Section 3, the results and discussion for the adsorption and dissociation of O2 on Ptn/TiC(001) are presented. Conclusions are drawn in the final section.

2.

THE MODELS AND CALCULATION DETAILS

All the spin polarized calculations are performed by using the first-principles method based on DFT as embedded in the Vienna Ab Initio Simulation Package (VASP).31-32 The Pt-5d6s, C-2s2p, Ti-3d4s, and O-2s2p are treated as valence electrons while the ionic cores are represented by the projector augmented wave (PAW) method.33 The exchange–correlation potential is described by using the generalized gradient approximation of Perdew–Burke–Ernzerhof (GGA-PBE).34 The Kohn–Sham orbitals are expanded using plane waves with the well converged cutoff energy of 415 eV.27-28 Brillouin zone integrations are performed with the k-points sampled using Monkhorst-Pack (MP) grids35 of 9 × 9× 9 and 5 × 5× 1 for bulk TiC and slab (p(2 × 2) and (3 × 3) Pt/TiC(001)) calculations. For slabs, our test calculations with MP grids28, 36 of 5 × 5× 1 gave almost the same adsorption energies and geometries. The convergence criteria are set to 10−5 3



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eV and 0.02 eV/Å for the electronic self-consistent iteration and the forces on each atom, respectively. Previously, the stability of three low-index Miller surfaces has been studied in detail for 37 TiC , which is consistent with previous theoretical studies for the TiC38 surfaces that the (001) surface is the most stable one. These calculations have provided a large number of insights and helped to answer the long standing questions about the interpretation of experimental measurements. We, therefore, choose (001) surface as the substrate to study the catalytic activity of TiC. TiC adopts a NaCl lattice structure with a calculated equilibrium lattice constant of 4.33 Å, which is in good agreement with the previous theoretical39 and experimental results40. The TiC(001) surface is built based on the calculated lattice constant of bulk TiC and represented by a supercell composed of a 4-layer slab (which has been tested and used in other articles widely24, 41) with separation of vacuum regions of 15 Å in the direction perpendicular to the substrate to wipe out the interaction between periodic images. The bottom two atomic layers are fixed, whereas the remaining layers and adsorbates are fully optimized. The supercell models with the (2 × 2) and (3 × 3) TiC(001) surfaces are used to support Ptn with one, two, three, four Pt atoms on the former and nine Pt atoms on the latter, respectively. Various reasonable initial adsorption structures are considered before structure optimization as labeled in Figure 1a. It is interesting that they all prefer to move to the top of the C atoms (see Figure 1), which is consistent with previous theoretical studies for TiC(001)36, 41 that the Pt atoms prefer to sit at the top-C sites. In the present study, the average binding energy ( ) per atom for Ptn deposited on the TiC(001) surface is defined as = −{ − ( + )}/ (1) where , , and represent the energies of a single Pt atom, the bare TiC(001), and the metal/substrate complex, respectively. For the Ptn deposited on the substrate, the overall binding energy is divided into two contributions: the metal–substrate adhesive and the metal– metal cohesive interactions. The metal–metal cohesive energy ( , in eV per Pt atom) is defined as = −{ − )}/ (2) The metal–substrate adhesive energy ( , in eV per Pt atom) can be evaluated by = − = −{ − ( + )}/ (3) where the is the energy of isolated Ptn of n atoms. The similar definition is used by Mao et al.28 and Zhang et al.29 to study the interactions of MLPd on TiC(001) and Pt clusters on ZrC(001) surface, respectively. The adsorption of O2 and atomic O on the optimized Ptn/TiC(001) is then studied and the corresponding adsorption energies, ( ) for O2 and () for O, are calculated by means of the following equations: ( ) = / − ( + ) (4) 1 () = / − ( + ) (5) 2 4


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The dissociative adsorption energy (" ), i.e., the adsorption energy of dissociated O2 is defined as " = {/ − ( + )}/2 (6) where 2()/$%&' and are the total energies of the Ptn/TiC(001) surface with and without the adsorbate O2 (O), respectively; is the total energy of an O2 molecule in the gas phase; / is the total energy of the surface with two separate dissociated O atoms. Bader charge analysis42 is used to evaluate the atomic charge in the systems and the electrons transfer in the reactions. The climbing image nudged elastic band method (CI-NEB)43 is employed to investigate the transition state (TSs) structure and the minimum energy paths (MEPs) for the dissociation of O2. Vibrational frequencies for the initial, transition and final states of the reactions are calculated and the transition states are confirmed by the presence of one imaginary frequency. Once a MEP is determined, the TSs is located and the activation energy barriers (( ) for O2 dissociation is calculated as ( = ) − *) (7) where ) and *) are energies of the transition state and initial state, respectively.

3. RESULTS AND DISCUSSION 3.1.

The Stability and Electronic Structures of Ptn/TiC(001)

The most stable geometric structures of Ptn on TiC(001) and the corresponding PDOS plots are shown in Figures 1 and 2, respectively. The key geometric parameters (including the Eb, EPt–Pt, EPt-C, Bader charges of the Pt atoms gained from the substrate, and the bond length) are summarized in Table 1. It is found that the single Pt atom prefers to be bound at the C top site (see Figure 1a) with a binding energy of 1.01 eV. The Bader charge analysis results show that the adsorbed Pt atom gets about 0.41 e from the substrate on Pt1/TiC(001), mainly from the C atom beneath the adsorbed Pt. This result can be found also from the DOS analysis (see Figure 2a), which shows the strong coupling between d-orbitals of Pt and p-orbitals of C from −6.25 to −3.75 eV with respect to the Fermi level. From Figure 1b, the C top site adjacent to the first Pt atom is the best adsorption site for the second Pt atom. The calculated average binding energy of Pt2 on TiC(001) is 3.18 eV, including an EPt–Pt of 1.52 eV and an EPt-C of 1.66 eV. The interaction can be further proved by the significant resonance of Pt-5d states with the C-2p as observed from −6.25 to −3.00 eV with respect to the Fermi level (see Figure 2b), indicating a strong covalent interaction. Mao et al.44 using DFT calculations also verified that the large migration barrier (1.24 eV) of Pt2 on TiC(001) means a strong interaction too. The similar adsorption behavior is also found for Pt3 deposited on TiC(001) (see Figure 1c) that the Pt atoms prefer to sit on the top of the C atoms with a large binding energy of 3.89 eV. From the DOS analysis (see Figure 2c), the strong resonances of the Pt-5d states with C-2p states are noticed ranging from −6.25 to −3.00 eV and −2.00 to 1.25 eV. Likewise, when the fourth Pt atom is added to the Pt3/TiC(001) (2 × 2), the Pt monolayer is formed on TiC(001). We further put the gas phase Pt4 with a square configuration on the TiC(001) and finally find that in the optimized structure all of Pt atoms migrate to the C top sites (see 5



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Figure 1d) with a significantly large binding energy of 4.39 eV, reflecting the strong interaction between MLPt and substrate. This is in agreement with the experiment that the Pt monolayer deposited on TiC surface had been realized by Kimmel et al.13 and verified that it is electrochemically stable and can be synthesized with high catalytic activity as bulk Pt. The Pt–Pt distance (DPt-Pt) is 3.06 Å, which is larger than the calculated bond distances of the highly stable planar and three-dimensional Pt clusters (2.3–2.9 Å) by Xiao et al.45 In this structure, the Pt–C (DPt-C) distance is 1.99 Å, which is consistent with the theoretical studies of Chu et al.36 for the Pt–C bonds on MLPt/TiC(001). Moreover, the relationship between energies (E, representing Eb, EPt-Pt or EPt-C) and the number of Pt atoms deposited on the TiC(001) is given in Figure 3. It is found that the average binding energies increases gradually from Pt1 to MLPt. The strong interaction between MLPt and substrate results in the higher stability of the MLPt/TiC(001). The similar relationship is found from the Pt-Pt cohesive energy and Pt-C adhesive energy, which is helpful to maintain the configuration avoiding clustering and beneficial to enhancing the stability between the MLPt and the TiC(001) substrate. Here, the EPt−C is bigger than the EPt−Pt on both (2 × 2) and (3 × 3) surfaces, which is in line with the previous theoretical results that the Pt-substrate interaction is stronger than the Pt-Pt interaction29, resulting in the formation of Pt monolayer on TiC(001). In addition, the Bader charge analysis show that a very small amount of net charge (about 0.24 e per atom) was transferred from the TiC(001) substrate to the Pt adatoms on MLPt/TiC(001), which is probably responsible for the catalytic activity of O2 dissociation. The DOS of MLPt/TiC(001) shows a higher peak at the Fermi energy, which is attributed to the change in the electronic structure of the outermost MLPt modified by the TiC(001), indicating again a high activity of the supported MLPt on TiC(001). Table 1. The adsorption properties of Ptn/TiC(001), including the binding energy (Eb, in eV), metal-metal cohesive energy (EPt−Pt, in eV), the metal-substrate adhesive energy (EPt−C, in eV), the nearest-neighbor metal-metal distances (DPt-Pt in Å), the distance between Pt and the underneath C (DPt-C in Å), and Bader charges (Q in e) of the adsorbed Pt atoms gained from the substrate. (2 × 2) (3 × 3) Pt1-TiC Pt2-TiC Pt3-TiC MLPt-TiC MLPt-TiC Eb 1.01 3.18 3.89 4.39 4.38 EPt-C 1.01 1.66 2.00 2.25 2.24 EPt-Pt ... 1.52 1.89 2.14 2.14 DPt-C 1.93 1.95 1.97 1.99 1.99 DPt-Pt ... 3.06 3.06 3.06 3.06 Q -0.41 -0.28 -0.25 -0.24 -0.24

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Figure 1. The geometric structures of Ptn/TiC(001) (top panel) and the corresponding O2 adsorption structures on Ptn/TiC(001) (bottom panel), respectively. Color code: the black, light gray, blue and red spheres denote the C, Ti, Pt and O, respectively.

Figure 2. The PDOS plots of Pt1/TiC(001) (a), Pt2/TiC(001) (b), Pt3/TiC(001) (c), MLPt/TiC(001) (2 × 2) (d), and MLPt/TiC(001) (3 × 3) (e). The vertical broken line represents Fermi energy.

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Figure 3. The correlation of the energies (Eb, EPt-Pt and EPt-C) with the number of Pt atoms deposited on TiC(001) surface.

3.2.

The Adsorption of O2 on Ptn/TiC(001)

The adsorption properties of O2 on Ptn/TiC(001) are studied in detail. All possible adsorption sites (Hollow, Bridge and Top sites shown in Figure 1e) with various adsorption patterns of the orientation of O2 parallel or vertical to the surfaces are examined to reveal the most stable adsorption configuration characterized with the largest adsorption energy. The optimized structures and the PDOS plots for the systems of O2 on Ptn/TiC(001) are shown in Figures 1 and 4, respectively. The corresponding key geometric parameters (including the adsorption energy, Bader charges of the O2 gained from the substrate, and the bond length) are summarized in Table 2.

3.2.1. On the Ptn/TiC(001) (2 × 2) Surface On Pt1/TiC(001), it is found that O2 prefers to be adsorbed at the Pt-Top site (see Figure 1f) with the O-O bond parallel to the surface. The calculated adsorption energy is 1.18 eV, which is much bigger than that on the pure TiC(001)21 (0.45 eV), and the O-O distance is more elongated on Pt1/TiC(001) (1.37 Å) than on pure TiC(001)21 (1.35 Å). The adsorption of O2 on Pt1/TiC(001) is accompanied with more electrons (0.58 e) transferred from the substrate compared with that on pure TiC(001)21 (0.36 e). It is obvious from the DOS (see Figure 4a) that there exists strong 8


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resonance of O2-1π with the Pt-5d states, manifesting the significant charges transfer from the substrate to the 1π states of O2. The results show that the adsorbed Pt atom plays an important role in enhancing the interaction between O2 molecules and substrate. It is worth mentioning that the main peak of 2π* states of O2 is still forceful, which means that the scission of O-O bond may be difficult. Different with the adsorption behaviors of O2 on Pt1/TiC(001), the O2 tends to be adsorbed at the Bridge site of the Pt2/TiC(001) (see Figure 1g). The calculated adsorption energy is 2.30 eV accompanied with about 0.70 e transferred to the adsorbed O2 from the substrate, which enforce the O-O bond elongated to 1.40 Å. The adsorption behaviors are similar to that of O2 on pure Pt(111)46 where O2 adsorbed at the Pt-Pt bridge site with an O-O bond of 1.40 Å. From the DOS analysis (see Figure 4b), it is found that there exists strong resonance of O2-1π states with the Pt-5d states, implying a majority of charges transferred from the substrate to the 1π states of O2. Compared with that on Pt1/TiC(001), the 2π* states of O2 are fully filled on the Pt2/TiC(001), which facilities the cleavage of O-O bonds. Similarly, the most stable adsorption sites (Bridge) of O2 on the Pt3/TiC(001) and MLPt/TiC (001) are shown in Figures 1h and i with the adsorption energy of 2.29 and 2.25 eV, respectively. The O-O distances are more elongated on Pt3/TiC(001) and MLPt/TiC(001) (1.41 Å) as compared with those on the Pt(111)46 (1.36-1.40 Å) and TiC(001)21 (1.35 Å). It can be further understood by analyzing the DOS (see Figures 4c and d) that the electrons are mainly transferred to the 1π states of O2 because of the serious resonance of the O2-1π states with the Pt-5d states. Compared with that on Pt2/TiC(001), the intense resonance between the 2π* states of O2 and the 5d states of Pt occurred from about −3.0 eV to the Fermi energy, which would correspond to higher activity toward O2 dissociation. Therefore, the scission of O2 on the Pt3/TiC (001) and MLPt/TiC(001) would be easier to take place than that on Pt2/TiC(001).

3.2.2. On MLPt/TiC(001) (3 × 3) Surface The similar adsorption configuration is obtained on the (3 × 3) supercell (see Figure 1j), where O2 tends to be strongly bound at the Pt-Pt Bridge site. The interaction between O2 and MLPt/TiC(001) is rather stronger with a significantly larger adsorption energy of 2.35 eV as compared with those on the Pt(111)46 (0.45-0.65 eV), the pure TiC(001)21 (0.45 eV), and Pt/TiC(001) (2 × 2) (1.18-2.30 eV) and also bigger than that on Au supported on TiC(001)27 (0.99-1.80 eV with different Au coverages), MLPd/TiC(001)28 (2.08 eV). The adsorbed O2 gets more electrons on the MLPt/TiC(001) (3 × 3) than on the Pt(111)46, TiC(001)21, MLPd/TiC(001)28 and Ptn/TiC(001) (2 × 2) for the preferred configurations (0.79 vs 0.51-0.65, 0.36, 0.71 and 0.58-0.78 e). The O-O bond is more elongated to 1.42 Å, reaching a maximum among the systems studied. The larger (1.42 Å) bond lengths of adsorbed O2 on MLPt/TiC(001), as compared with those on the Pt(111)46 (1.36-1.40 Å), TiC(001)21 (1.35 Å) and MLPd/TiC(001)28 (1.39 Å), indicates an obvious bond weakening of the adsorbed O2. The charge-transfer effects and the intense interaction of O2 with the MLPt/TiC(001) can be further understood by analyzing the partial DOS curves (see Figure 4e), where the serious hybridization of O2-1π states with the 5d states of Pt exists clearly, implying that a majority of charges transfer from the substrate to the 9



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O2-1π states, which are accordant with the Bader charge analysis results that more electrons flow to the 1π states of O2. Moreover, the intense resonance between the 2π* states of O2 and the 5d states of Pt is found from about −3.5 eV to the Fermi energy, resulting in easy dissociation of O2.

Figure 4. The PDOS plots of O2 adsorbed on Pt1/TiC(001) (a), Pt2/TiC(001) (b), Pt3/TiC(001) (c), MLPt/TiC(001) (2 × 2) (d), MLPt/TiC(001) (3 × 3) (e). The vertical broken line represents Fermi energy. Table 2. The adsorption properties of O2 on Ptn/TiC(001), Pt(111), Pt3Ti(111) and the bare TiC(001), including the adsorption energy (Eads in eV), Bader charges (Q in e) of the adsorbed O2 gained from the substrate, O–O distances (DO-O in Å). System Site Eads DO-O Q 2 × 2 Pt1 Pt–top −1.18 1.37 0.58 Pt2 Pt–Pt–bridge −2.30 1.40 0.70 Pt3 −2.29 1.41 0.74 MLPt −2.25 1.41 0.78 3 × 3 MLPt −2.35 1.42 0.79 28 MLPd Pd–Pd–bridge −2.08 1.39 0.71 46 Pt(111) t–h–b −0.45 1.39 0.62 t–b–t −0.63 1.36 0.51 t–f–b −0.65 1.40 0.65 21 TiC(001) Ti–Ti–bridge −0.45 1.35 0.36 47 Pt3Ti(111) Pt–Pt–bridge −0.41 … …

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3.3.

Dissociation of O2 on Ptn/TiC(001)

The dissociation processes of O2 on Ptn/TiC(001) and the corresponding reaction barriers are shown in Figures 5 and 6. The corresponding key properties (including the activation energy, the heat of reaction, and the dissociation adsorption energy) are summarized in Table 3.

3.3.1. On Ptn/TiC(001) (2 × 2) Surface The MEP for the dissociation of O2 on Pt1/TiC(001) is shown in Figure 5a, in which the structure with the O2 adsorbed at the Pt-top site is taken as the initial state (IS) and that with two oxygen atoms at the TiTiC site as the final state (FS). The calculated energy barrier for the dissociation of O2 is as large as 1.35 eV, indicating that adsorbed O2 is not easy to be dissociated on Pt1/TiC(001) at room temperature. On Pt1/TiC(001), the adsorption energy of O2 is smaller than the barrier for dissociation, meaning that desorption will take place before dissociations with increasing the temperature. From the Figures 5b and c, we select the structure with O2 at the Bridge site as the IS and two oxygen atoms adsorbed at the Top-Top site as the FS for dissociation of O2. The calculated activation barriers for the dissociation of O2 on Pt2/TiC(001) (0.91 eV) and Pt3/TiC(001) (0.86 eV) are significantly lower than that on Pt1/TiC(001) (1.35 eV). This is consistent with the DOS analysis that the 2π* states of O2 are fully filled on both the Pt2/TiC(001) and Pt3/TiC(001) surfaces, which facilities the cleavage of O-O bonds as compared with that on Pt1/TiC(001). On MLPt/TiC(001) (see Figure 5d), different from that on the Pt2/TiC(001) and Pt3/TiC(001), the structure with two oxygen atoms adsorbed at the two Bridge sites is taken as the FS. As shown in Figure 5d, the calculated dissociation barrier for the first reaction path (TS1) is smaller (0.81 eV) than those for the others (TS2, 0.99 eV and TS3, 1.23 eV). Furthermore, the FS1 can transform to FS3 with a diffusion barrier of 0.65 eV via the same transition states with TS3, which is displayed in Figure 5d. Moving from Pt1 to Pt4, as expected, has a clear effect on the energy profile. The activation energy to break the O-O bond (see Figure 5) is lowered with the increase of the Pt coverage.

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Figure 5. The energy pathway calculated for the dissociation process of O2 (O2∗→2O∗) on (2 × 2) surfaces of the Pt1/TiC(001) (a), Pt2/TiC(001) (b), Pt3/TiC(001) (c) and MLPt/TiC(001) (d). The values represent the energies (in eV) relative to the initial state (IS).

3.3.2. On MLPt/TiC(001) (3 × 3) Surface Using the in situ x-ray photoelectron spectroscopy experiments, Getman et al. showed that O2 dissociation rate is sensitive to the different coverage on Pt(111).48 Comparing the previous available theoretical values of O2 dissociation energy barriers on the Pt(111) with different coverage,49-50 it is found that the effects are to a certain extent path dependent. In order to test the effects on the dissociation behavior, the dissociation of O2 on (3 × 3) surface is also studied as compared with (2 × 2) surface. In Figure 6, the possible reaction paths (TS1, TS2 and TS3) for the cleavage of O-O bonds are shown. Compared with the O2 dissociation on the MLPt/TiC(001) (2 × 2), the dissociation state is similar on the (3 × 3) supercell after the geometry optimization, i.e., the most stable dissociation state is the one with two O atoms bound at Pt-Pt bridge sites. Three dissociation pathways are shown in Figure 6 with the energy barriers of 0.36, 0.51 and 0.61 eV, respectively, which are smaller than those on the Ptn/TiC(001) (2 × 2) (0.81-1.35 eV), and are close to the minimum energy barrier (0.37 eV) of O2 on Pt(111)49. The results indicate that dissociation of O2 on MLPt/TiC(001) is probable because of the smaller energy barrier, which is consistent with the previous DOS results that the electrons transfer mainly to the 1π and 2π* states of O2 due to the serious resonance of the O2-1π and 2π* states with the 5d states of the supported Pt layers. In addition, the reactions are more exothermic, which suggest at least the dissociation of O2 is easier as compared with those on the Pt(111) and MLPd/TiC(001) (3 × 3). These results indicate again that the MLPt modified TiC(001) is efficient for breaking the O-O bond. 12


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Figure 6. The energy pathway calculated for the dissociation process of O2 (O2∗→2O∗) on the MLPt/TiC(001) (3 × 3). The values represent the energies (in eV) relative to the initial state (IS). The dissociation activation energies (Eact) of O2 on the surfaces of Ptn/TiC(001), MLPd/TiC (001)28 and Pt(111)49 are depicted in Figure 7. It is clear from the above calculations that the MLPt/ TiC(001) (3 × 3) have excellent catalytic activity with very low activation energies for O2 dissociation (0.36 eV), which is even lower (more efficient) than on the MLPd/TiC(001)28 (0.48 eV), and is similar to the minimum energy barrier (0.37 eV) of O2 on Pt(111)49. Instead, on Pt/TiC(001) (2 × 2), O2 has quite large dissociation activation energies, suggesting that O2 dissociation on Pt/TiC(001) (2 × 2) is difficult. The Eact of O2 is slowly decreasing with the increase of the number of the Pt atoms in the Ptn/TiC(001) systems. A minimum appears at MLPt/TiC(001) (3 × 3) system, which indicates that the MLPt/TiC(001) (3 × 3) system would be a good catalyst with higher activity for the dissociation of O2.

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Figure 7. The correlation between Eact and the number of the Pt atoms in the different systems (2 × 2) and (3 × 3) Pt/TiC(001), MLPd/TiC(001)28 and Pt(111)49 for O2 dissociation. To do a simple summary, compared with the previous experimental48, 51-52 and theoretical studies28, 49 on the O2 dissociation on the Pt(111) with the barriers of around 0.37–0.9 eV as shown in Table 3, our results show that the dissociation barriers are not large on the MLPt/TiC(001) (3 × 3). From the Figure 8, it is found that the Eads′ and Eact are correlated with the lower of dissociation adsorption energy (more negative) corresponding to the lower dissociation barrier of O2, which coincides with the universal Brønsted–Evans–Polanyi (BEP)53-56 relationship. Previously, Fajín et al.54, 56 provided compelling evidence that BEP relationships derived from PW91, PBE and TPSS functional are essentially coincidental in heterogeneous catalysis. The BEP relationship shown in Figure 8 provides a way to estimate the kinetic behavior of a chemical reaction from simple data in our calculation. The Eads′ and Eact values of O2 on (3 × 3) MLPt/TiC(001) are close to those on Pt(111), which is in agreement with the universal criterion for the optimal catalysts (giving the largest catalytic activity is in the range from −1 to −2 eV of Eads′ )30, 57-58 for a number of diatomic molecules reactions, indicating that (3 × 3) MLPt/TiC(001) is expected to have high ORR catalytic activities. What is more, Stamenkovic et al.59 reported that the Pt(111) surface is 9-fold more active for the ORR than the current state-of-the-art Pt/C catalysts for PEMFCs. Our theoretical study is consistent with the previous experiments of Chiwata et al.15 which verified activity of Pt/TiC toward an ORR under PEMFC conditions. Thus, we speculate that the MLPt/TiC(001) (3 × 3) could be a good candidate for substituting the current Pt/C catalysts for PEMFCs. 14


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Figure 8. Linear relationships between Eact and Eads′ of the O2 dissociation processes on Pt/TiC(001) MLPd/TiC(001)28 and Pt(111)49. Table 3. The activation energy (Eact in eV, O2 dissociation barrier) on the Ptn/TiC(001), MLPd/TiC (001), Pt(111) and Pt3Ti(111). Er (eV) represents the heat of reaction (energy difference between the reactants and products) and Eads′ (eV) is the dissociation adsorption energy. System Path Eact Er Eads′ 2 × 2 Pt1 Top—2 × fcc 1.35 −3.41 −2.29 Pt2 Bridge—2 × top 0.91 0.77 −0.78 Pt3 0.86 0.60 −0.85 MLPt Bridge—2 × bridge 0.81/0.65 0.52/−0.64 −0.86/−1.44 0.99 −0.58 −1.41 1.23 −0.64 −1.44 3 × 3 MLPt 0.36 −0.79 −1.57 0.51 −1.17 −1.76 0.61 −1.17 −1.76 28 MLPd Hollow—2 × hollow 0.48 −0.84 −1.41 Hollow—hollow + bridge 0.52 −0.79 −1.38 28 Pt(111) t-h-b—2 × fcc 0.38 −0.40 … 15



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49

Pt(111) Pt(111)51-52 Pt3Ti(111)47

3.4.

t-f-b—2 × hcp Bridge—2 × hcp … Bridge—2 × fcc

0.56 0.52/0.37 0.8−0.9 0.53

0.10 … … −0.92

… −1.21/−1.49 … …

The Adsorption of O on MLPt/TiC(001)

As known, the high efficiency of the fuel cells depends on the balance of the reactions on the cathode and anode. The catalysts on the cathode that possess the two known beneficial effects (weak atomic oxygen bound and small dissociation barriers of O2) may play an important role in serving as good catalysts for ORR. All possible high symmetric adsorption sites (Top, Bridge and Hollow) are tested for O adsorption to reveal the most stable adsorption configuration. The optimized structures and detailed adsorption properties are shown in Figure 9 and Table 4. It is found that the O atom prefers to be adsorbed at the Bridge site on MLPt/TiC(001), which is similar to that on Pt4/HfC(001)30. Meanwhile, the adsorption energies of O atom on MLPt/TiC(001) for the (2 × 2) and (3 × 3) surfaces are 1.69 and 1.86 eV by forming two O-Pt bonds of 2.019, 2.016 Å, respectively. Based on the Bader charge analysis, O gets more electrons (0.03 e as shown in Table 4) from the substrate on the (3 × 3) surface. Compared with the MLPt/TiC(001), the adsorption energies of O atom on the pure surfaces of TiC(001)24 (5.80 eV) and Pt(111)46 (4.76 eV) are significantly larger, which may form inactive site, resulting in the deactivation of the catalyst. The lower adsorption energy of O on MLPt/TiC(001) means the weakened binding of O on the surface. These results indicate that the activity of Pt modified TiC for ORR may be better than the Pt/C catalysts from the O2 decomposition and O removal points of view, which is in line with the experimental results that the Pt/TiC exhibited higher activity toward an ORR15. Moreover, the complete reaction process and the reaction mechanism, as well as the rate-limiting step of ORR on MLPt/TiC(001) would be considered in future studies. These investigations would be helpful for designing new catalyst with lower Pt consumption, as well as higher efficiency and stability for ORR.

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Figure 9. Optimized structures for O adsorption at the Bridge site (Top view and Side view) on the surfaces of 2 × 2 (a) and 3 × 3 (b) MLPt/TiC(001). The atoms in the bottom two atomic layers (in the dashed rectangular box) are fixed at their bulk positions. Color code: the black, light gray, blue and red spheres denote the C, Ti, Pt and O, respectively. Table 4. The adsorption energies (Eads in eV), the nearest Pt–O distance (DPt−O in Å) for an O atom, and Bader charges (Q in e) of the adsorbed O gained on the MLPt/TiC(001) surfaces. The data available for O on the surfaces of TiC(001), Pt(111) and Pt3Ti(111) are also shown for comparison. System Site Eads DPt−O Q 2×2 Bridge −1.69 2.019 0.85 3×3 Bridge −1.86 2.016 0.88 24 TiC(001) fcc −5.80 ... ... 46 Pt(111) fcc −4.76 2.04 ... 47 Pt3Ti(111) fcc −3.84 … … The discussion in the previous sections showed that the Pt atoms prefers to be adsorbed at the C-top sites. Considering that the Ti atoms in the substrate may move to the adsorbed Pt 17



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particles/clusters forming the PtTi alloy species, it would be interesting to have a comparison with previous studies for O2 adsorption and dissociation on the PtTi systems. Previous experiments60-62 had verified that the catalytic performance of the PtTi alloy catalysts is very sensitive to the Pt:Ti atomic ratio. The activity of the PtTi alloy drops as the composition shifts away from the optimum Pt:Ti atomic ratio of 3:1, displaying a twofold increase for ORR as compared to a benchmark Pt/C catalyst. The previous theoretical study47 had shown that O2 is possible to dissociate on Pt3Ti(111), which may be used as a cathode electrocatalyst for promoting ORR in PEMFC. In comparison, the energy barrier for the dissociation of O2 on Pt3Ti(111)47 (0.53 eV) is larger than that on MLPt/TiC (001) (3 × 3) (0.36 eV), and the adsorption energy of atomic O on Pt3Ti(111)47 (3.84 eV) is larger than that those on MLPt/TiC(001) (1.69 and 1.86 eV). The smaller dissociation barriers of O2 and the weaker binding of atomic O on MLPt/TiC(001) (3 × 3) would play an important role in serving as better catalysts for ORR than the Pt3Ti(111) alloy.

4. SUMMARY AND CONCLUSIONS To shed light on searching effective nanocatalysts for ORR in PEMFC, the adsorption and dissociation of O2 on the Pt modified TiC(001) surfaces with different Pt coverages of 1/4, 1/2, 3/4 and 1 ML are comparatively investigated using ab initio DFT calculations. It is found that: (a) There exists intense interaction between Pt atoms and TiC(001), which is large enough to maintain the configuration avoiding clustering and beneficial to enhancing the stability between the Pt and TiC(001) substrate. In addition, previous experiments12-15 have verified that TiC(001) is electrochemically stable and an ideal support for low loading of Pt as a low-cost and stable electrocatalyst under electrochemical conditions. (b) The presence of TiC(001) substrate will strongly modify the electronic structure of MLPt. The combination of MLPt with the TiC(001) substrate can improve the efficiency for the dissociation of O2 and weaken the O binding strength simultaneously. (c) Compared with the results for O2 adsorption and dissociation on Pt(111), Pt3Ti(111) and Pd/TiC systems, the lower dissociation barriers of O2 and weaker binding of atomic O on MLPt/TiC(001) (3 × 3) would play an important role in serving as good catalysts for ORR. The MLPt/TiC(001) (3 × 3) can serve as a possible substitute with high stability, lower cost and high efficiency for the current Pt/C catalyst for ORR. Our results provides a clue to design highly efficient ORR catalyst based on transition carbides.

AUTHOR INFORMATION Corresponding Author † E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS

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This work was supported by the National Natural Science Foundation of China (No. 11174070 and 11474086). Parts of the simulations are performed on resources provided by the High Performance Computing Center of Henan Normal University.

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Table 1. The adsorption properties of Ptn/TiC(001), including the binding energy (Eb, in eV), metal-metal cohesive energy (EPt−Pt, in eV), the metal-substrate adhesive energy (EPt−C, in eV), the nearest-neighbor metal-metal distances (DPt-Pt in Å), the distance between Pt and the underneath C (DPt-C in Å), and Bader charges (Q in e) of the adsorbed Pt atoms gained from the substrate. (2 × 2) (3 × 3) Pt1-TiC Pt2-TiC Pt3-TiC MLPt-TiC MLPt-TiC Eb 1.01 3.18 3.89 4.39 4.38 EPt-C 1.01 1.66 2.00 2.25 2.24 EPt-Pt ... 1.52 1.89 2.14 2.14 DPt-C 1.93 1.95 1.97 1.99 1.99 DPt-Pt ... 3.06 3.06 3.06 3.06 Q -0.24 -0.41 -0.28 -0.25 -0.24

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Table 2. The adsorption properties of O2 on Ptn/TiC(001), Pt(111), Pt3Ti(111) and the bare TiC(001), including the adsorption energy (Eads in eV), Bader charges (Q in e) of the adsorbed O2 gained from the substrate, O–O distances (DO-O in Å). DO-O Q System Site Eads 2 × 2 Pt1 Pt–top −1.18 1.37 0.58 Pt2 Pt–Pt–bridge −2.30 1.40 0.70 Pt3 −2.29 1.41 0.74 MLPt −2.25 1.41 0.78 3 × 3 MLPt −2.35 1.42 0.79 28 MLPd Pd–Pd–bridge −2.08 1.39 0.71 46 Pt(111) t–h–b −0.45 1.39 0.62 t–b–t −0.63 1.36 0.51 t–f–b −0.65 1.40 0.65 21 Ti–Ti–bridge −0.45 1.35 0.36 TiC(001) 47 Pt3Ti(111) Pt–Pt–bridge −0.41 … …

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Table 3. The activation energy (Eact in eV, O2 dissociation barrier) on the Ptn/TiC(001), MLPd/TiC (001), Pt(111) and Pt3Ti(111). Er (eV) represents the heat of reaction (energy difference between the reactants and products) and Eads′ (eV) is the dissociation adsorption energy. Er Eads′ System Path Eact 2 × 2 Pt1 Top—2 × fcc 1.35 −3.41 −2.29 Pt2 Bridge—2 × top 0.91 0.77 −0.78 Pt3 0.86 0.60 −0.85 MLPt Bridge—2 × bridge 0.81/0.65 0.52/−0.64 −0.86/−1.44 0.99 −0.58 −1.41 1.23 −0.64 −1.44 3 × 3 MLPt 0.36 −0.79 −1.57 0.51 −1.17 −1.76 0.61 −1.17 −1.76 28 MLPd Hollow—2 × hollow 0.48 −0.84 −1.41 Hollow—hollow + bridge 0.52 −0.79 −1.38 28 Pt(111) t-h-b—2 × fcc 0.38 −0.40 … t-f-b—2 × hcp 0.56 0.10 … Pt(111)49 Bridge—2 × hcp 0.52/0.37 … −1.21/−1.49 … 0.8−0.9 … … Pt(111)51-52 47 Pt3Ti(111) Bridge—2 × fcc 0.53 −0.92 …

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Table 4. The adsorption energies (Eads in eV), the nearest Pt–O distance (DPt−O in Å) for an O atom, and Bader charges (Q in e) of the adsorbed O gained on the MLPt/TiC(001) surfaces. The data available for O on the surfaces of TiC(001), Pt(111) and Pt3Ti(111) are also shown for comparison. System Site Eads DPt−O Q 2×2 Bridge −1.69 2.019 0.85 3×3 Bridge −1.86 2.016 0.88 fcc −5.80 ... ... TiC(001)24 46 Pt(111) fcc −4.76 2.04 ... 47 Pt3Ti(111) fcc −3.84 … …

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Figure Captions: Figure 1. The geometric structures of Ptn/TiC(001) (top panel) and the corresponding O2 adsorption structures on Ptn/TiC(001) (bottom panel), respectively. Color code: the black, light gray, blue and red spheres denote the C, Ti, Pt and O, respectively. Figure 2. The PDOS plots of Pt1/TiC(001) (a), Pt2/TiC(001) (b), Pt3/TiC(001) (c), MLPt/TiC(001) (2 × 2) (d), and MLPt/TiC(001) (3 × 3) (e). The vertical broken line represents Fermi energy. Figure 3. The correlation of the energies (Eb, EPt-Pt and EPt-C) with the number of Pt atoms deposited on TiC(001) surface. Figure 4. The PDOS plots of O2 adsorbed on Pt1/TiC(001) (a), Pt2/TiC(001) (b), Pt3/TiC(001) (c), MLPt/TiC(001) (2 × 2) (d), MLPt/TiC(001) (3 × 3) (e). The vertical broken line represents Fermi energy. Figure 5. The energy pathway calculated for the dissociation process of O2 (O2∗→2O∗) on (2 × 2) surfaces of the Pt1/TiC(001) (a), Pt2/TiC(001) (b), Pt3/TiC(001) (c) and MLPt/TiC(001) (d). The values represent the energies (in eV) relative to the initial state (IS). Figure 6. The energy pathway calculated for the dissociation process of O2 (O2∗→2O∗) on the MLPt/TiC(001) (3 × 3). The values represent the energies (in eV) relative to the initial state (IS). Figure 7. The correlation between Eact and the number of the Pt atoms in the different systems (2 × 2) and (3 × 3) Pt/TiC(001), MLPd/TiC(001)28 and Pt(111)49 for O2 dissociation. Figure 8. Linear relationships between Eact and Eads′ of the O2 dissociation processes on Pt/TiC(001) MLPd/TiC(001)28 and Pt(111)49. Figure 9. Optimized structures for O adsorption at the Bridge site (Top view and Side view) on the surfaces of 2 × 2 (a) and 3 × 3 (b) MLPt/TiC(001). The atoms in the bottom two atomic layers (in the dashed rectangular box) are fixed at their bulk positions. Color code: the black, light gray, blue and red spheres denote the C, Ti, Pt and O, respectively.

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3×3 2×2

TTi Bridge HTiTiC TC HCCTi

Top Hollow

Hollow

Bridge

(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

(i)

(j)

C

Ti


Pt

O

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(a)

(b)

(d)


(c)

(e)


5 E b

E

4

P t-C

E

P t-P t

3

E (e V )

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2 1 0

P t1

P t2

P t3 ACS Paragon Plus Environment

(2 × 2 )M L P t (3 × 3 )M L P t

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(a)

(c)

(b)

(d)


(e)


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(a)

(c)

(b)

(d)


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TS3

FS3

IS

(0.00)

(0.61) (0.51) (0.36) TS2

TS1


FS2

(-0.79) (-1.17) FS1


1 .6

C u rv e F ittin g lin e

1 .4 1 .2

E a c t(e V )

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1 .0 0 .8 0 .6 0 .4 0 .2 P t1

P t2

P t3 (2 × 2 )M L P t (3 × 3 )M L P d P t(1 1 1 ) (3 × 3 )M L P t ACS Paragon Plus Environment

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P t2

0 .9

P t3 0 .8

E a c t(e V )

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


(2 × 2 )M L P t

0 .7 0 .6 0 .5 0 .4 0 .3

(3 × 3 )M L P d P t(1 1 1 ) (3 × 3 )M L P t -1 .6

-1 .5

-1 .4

-1 .3

-1 .2

-1 .1

E a d s '( e V )


-1 .0

-0 .9

-0 .8

-0 .7


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Top view

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Side view

C

(a)

Ti

Pt O

(b)


A First-Principles Study of O2 Dissociation on Platinum Modified

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