Design of Efficient Catalysts for CO Oxidation on Titanium Carbide

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Design of Efficient Catalysts for CO Oxidation on Titanium Carbide–Supported Platinum via Computational Study Shiyan Wang, Zongxian Yang, Xingli Chu, and Weichao Wang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b07741 • Publication Date (Web): 19 Oct 2018 Downloaded from http://pubs.acs.org on October 19, 2018

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The Journal of Physical Chemistry 1

Design of Efficient Catalysts for CO Oxidation on Titanium Carbide–Supported Platinum via Computational Study Shiyan Wang1, Zongxian Yang1†, Xingli Chu1, Weichao Wang1†,2 1College 2School

of Physics and Materials Science, Henan Normal University, Xinxiang 453007, China

of Electronics and Optical Information, and Tianjin Key Laboratory of Photo-Electronic Thin Film Device and Technology, Nankai University, Tianjin, 300071, China

ABSTRACT: The formation, geometries, electronic structures and catalytic properties of monovacancy and divacancy of the defective TiC(001) with single Pt atom and Pt dimer are systematically studied based on the first-principles calculations. Compared with the diffusion barrier of Pt1 on VCTiC(001), Pt2 on VC-TiC(001) has a larger diffusion barrier, indicating that the VC-TiC(001) substrate can stabilize the Pt dimer and inhibit its diffusion efficiently. Compared with the Pt1/VCTiC(001), the DOS plot of Pt2/VC-TiC(001) presents a peak at the Fermi energy, causing by variations in the electronic structure of the VC-TiC(001)-modified outermost Pt2, which indicates that the supported Pt2 has a high activity on VC-TiC(001). The steady Pt2/VC-TiC(001) catalyst exhibits outstanding activity for CO oxidation in the LH and TER mechanisms for the rate-limiting step of the OOCO and OCOOCO dissociation, having the energy barriers of 0.32 and 0.52 eV, respectively, which are both more preferable than the ER mechanism. Therefore, the Pt2/VCTiC(001) is quite efficient for CO oxidation. The existing results are expected to help us to develop efficient catalysts that are highly tolerant to CO poisoning.

†

Corresponding authors: E-mail addresses: [email protected] (Z. Yang), [email protected] (W Wang).

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1. INTRODUCTION PEMFCs (Proton exchange membrane fuel cells) are promising to solve the challenges in the fields of production, conversion and storage of clean energy.1-3 However, even a small amount of CO in fuels (typically 10 ppm) can significantly decrease the performance of PEMFC, so the most outstanding matter that we face now is CO poisoning. For CO, its catalytic oxidation has been widely studied in recent years4-8. There are more and more environmental problems due to CO emissions of vehicles9-10 and industries11, so the study will play a critical role in this field. Poisoning can be avoided through removing CO contaminations for PEMFC12, as for design of heterogeneous catalysts, this is an important prototype reaction.13 Actually, CO poisoning is common in traditional Pt/C catalysis, because Pt electrodes may have a stronger interaction with CO than other reaction gases14 (e.g., O2 in cathode or H2 in anode). Therefore, under the premise of ensuring the catalytic performance, we should find new catalysts to avoid such a problem.15-16 In the fields surface science and catalysis, TiC (Titanium carbide) has been taken as an important subject in several studies, and with the advantages of superior catalytic properties and low cost, it has also been selected as optional candidate for replacing catalysts of precious Pt-group metals.17-18 TiC, as substrate, may greatly promote the selectivity and electrocatalytic performance in reducing Pt loading, thus decreasing the cost of electrocatalytic applications through modifying the electronic and geometric structures of supported catalysts. Theoretical studies and tests have verified that low loading of Pt on TiC (when it was used as a steady and low-cost catalyst) for ORR (abbreviation of oxygen reduction reaction)19-21, OER (abbreviation of oxygen evolution reaction)20-22, and HER (abbreviation of hydrogen evolution reaction)23 in fields of PEMFC and other relevant electrocatalytic applications. Compared with the catalyst supported by carbon in PEMFC, Pt/TiC catalyst could greatly enhance the electrooxidation of CO and methanol, and also alleviate the effect of CO poisoning24-26. The high activity of CO oxidation for single Pt atom (embedded on the BN27, graphene28, metal oxide29-30 and MXene31) has been widely reported recently. Yan et al.32 showed that in a lowtemperature and inert environment, the atomic layer deposition can be used to fabricate Pt2 dimers 2


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on graphene. Hu et al.33 found that the graphene sheet-supported metal dimers could create new phenomena of interesting, such as magnetic and electronic properties. In catalytic reactions, CeO2 substrate-supported metal dimer catalysts and bimetallic nanoclusters may have a high activity for CO oxidation.34-35 Rodriguez et al.36-37 reported the higher stability of Au2 supported on TiC(001) toward CO oxidation compared with the Au4. Very recently, studies have shown that compared with single-atom catalyst, metal dimer on porous C2N monolayer could have superior performance and high stability regarding CO oxidation38 and ORR39. In addition, WC(0001) (PtML/WC(0001)) supported Pt monolayer, with a large barrier for CO oxidation, showed that PtML/WC(0001) had a low resistance to CO poisoning.40 The results showed that the active sites of supported metal dimers and single metal atom could greatly enhance the interaction between catalysts and reactants, and create a high activity through increasing the number of transferred electrons. However, no enough systematic analyses have been conducted on catalytic properties and geometric stability of TiCsupported single and double Pt atoms. Inspired by the exciting theoretical predictions and experimental discoveries, the catalytic activities of Pt dimer and single Pt atom anchored on defective TiC(001) have been systemically examined on the basis of DFT, and the CO oxidation reaction mechanisms on the most stable Pt/TiC have also been investigated. The reaction processes (with energy barriers) have been investigated with the method of climbing image nudged elastic band, which showed that, from the energy barrier perspective, the LH (Langmuir-Hinshelwood) and TER (termolecular Eley–Rideal) mechanisms are more preferable to occur than the ER (Eley-Rideal) mechanism.

2. THE MODELS AND CALCULATION DETAILS Based on DFT that has been embedded in the VASP (Vienna Ab Initio Simulation Package), all spin polarized calculations have been conducted with the first-principles method.41-42 While the projector augmented wave (PAW) method has been used to represent the ionic cores, the Pt-5d6s, Ti-3d4s, O-2s2p and C-2s2p can be deemed as valence electrons.43 The exchange–correlation potential has been described by the generalized gradient approximation of Perdew-BurkeErnzerhof (GGA-PBE).44 Kohn–Sham orbitals have been expanded using plane waves with a well

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converged cutoff energy (415 eV).45-46 Brillouin zone integrations have been carried out, based on sampled k-points, through MP (Monkhorst-Pack) grids46-49 (5 × 5× 1 and 9 × 9× 9) for slab Pt/TiC (001) and bulk TiC calculations. For slabs, the calculations based on MP grids (5 × 5× 1) obtained nearly the same geometries and energies of adsorption as those obtained in previous studies46-48. The convergence criteria have been set as 0.02 eV/Å and 10−5 eV for the forces on each atom and electronic self-consistent iteration, respectively. TiC adopts a NaCl lattice structure, which has constant of calculated equilibrium lattice of 4.33 Å, being consistent with the of previous theoretical50 and test51 results. The explorations on stability showed that of the three low-index Miller surfaces, (001) surface is the steadiest one, which is consistent with what stated in the reports52-53. Therefore, the TiC(001) surface was selected as the substrate, for studying its catalytic activity against CO oxidations. As shown in Figure 1a, TiC(001) surface is formed on the basis of bulk TiC with a calculated lattice constant, and expressed by a supercell of 4-layer slab, which has been widely used and tested in other articles54-55; perpendicular to the substrate, there is separation of 15 Å in vacuum regions for wiping out the false interaction among periodic images. The two atomic layers are fixed at the bottom, and adsorbates and other layers are optimized fully.

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Figure 1. Schematic models (top view & side view) of the 4-layer slab model, (a) TiC(001), (b) Pt1/VC-TiC(001), (c) Pt2/VC-TiC(001). The DOS (density of states) plots of (d) Pt1/VC-TiC(001) and (e) Pt2/VC-TiC(001). For the two atomic layers at the bottom, as shown in the dashed rectangular box, the atoms are fixed at bulk positions. The broken line (vertical) refers to Fermi energy. Color code: the black, light gray and blue spheres refer to C, Ti and Pt, respectively.

Usually, there may be defects on the TiC(001) surface, which would affect the catalytic performance. Previous theoretical studies53, 56-58 showed that compared with those with titanium vacancies (VTi-TiC(001)), the defective surfaces with carbon vacancies (VC-TiC(001)) could be more steady. The tests52 also verified that the surface properties of both the carburized surfaces and the carbides greatly depend on carbon content of the topmost layer; with the increase of the carbon content, the surface would become inactive to gas reaction. Yang et al.59 found that TiC could be potentially taken as the materials for supporting single-atom catalysts, Pt atoms of transition-metal could be mixed at the TiC defect site (Pt1/VC-TiC(001)). They also found that such composite could significantly express product selectivity and catalytic activity for electrochemical oxygen reduction 5



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reaction. To simulate the Pt1/VC-TiC(001) catalyst, a Pt atom was used to substitute a surface C atom. The reaction and adsorption of various species would occur on the Ptn/VC-TiC(001) (n=1, 2) surface. In the present study, the adsorption energies are calculated in the following manner: 𝐸𝑎𝑑𝑠 = ― {𝐸adsorbate/𝑠𝑙𝑎𝑏 ― 𝐸𝑎𝑑𝑠𝑜𝑟𝑏𝑎𝑡𝑒 ― 𝐸𝑠𝑙𝑎𝑏}

(1)

Where, 𝐸𝑎𝑑𝑠𝑜𝑟𝑏𝑎𝑡𝑒/𝑠𝑙𝑎𝑏 and 𝐸𝑠𝑙𝑎𝑏 refer to total energies of Ptn/VC-TiC(001) with and without the presence of adsorbate, respectively, and 𝐸𝑎𝑑𝑠𝑜𝑟𝑏𝑎𝑡𝑒 refers to the energy of free adsorbates (e.g. CO, O2). In this definition, positive adsorption energy refers to a steady adsorption structure. Bader charge analysis60 is used to assess the reaction systems and atomic charge during transfer of electrons. MEPs (minimum energy paths) for CO oxidation and transition state (TSs) structure were investigated with CI-NEB method (climbing image nudged elastic band)61. Vibrational frequencies calculation was performed to confirm the transition states. The activation energy barriers for CO oxidation are calculated by the following formula: 𝐸𝑎𝑐𝑡 = 𝐸𝑇𝑆 ― 𝐸𝐼𝑆

(2)

Where, 𝐸𝑇𝑆 and 𝐸𝐼𝑆 refer to energies at the transition and initial states, respectively.

3. RESULTS AND DISCUSSION 3.1.

The Stability and Electronic Structures of Ptn/VC-TiC(001) It was reported that the vacancies or defects usually appear on TiC(001), which may play

important roles in substrate functions. Therefore, it could be expected that carbon vacancies in TiC(001) could change its interaction properties with the Ptn adatoms. To this end, the stability of Pt dimer and a single Pt atom on the defective TiC(001) with divacancy and monovacancy, as well as the catalytic features of CO oxidation, was investigated in this study. Initially, the Pt1/VC-TiC(001) configurations with a single Pt atom were optimized in and over the VC-TiC(001) plane. The optimized configurations are shown in Figure 1b, which showed that single Pt atom would be easily bound at the monovacancy in VC-TiC(001), capping on four neighboring identical Ti atoms. The lengths of calculated Pt–C bond are identical (2.63 Å). The configuration of the Pt dimer embedded in two adjacent monovacancies is shown in Figure 1c, 6


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where two C atoms are replaced by two Pt atoms. The lengths of the Pt–Ti and Pt–Pt bonds are 2.59 and 2.89 Å. Pt dimer is lying out of the plane, to acquire more space (1.19 Å), for it has larger atomic radius than carbon atoms. To study the stability of Pt dimers and a single Pt atom on VC-TiC(001), the binding energy of the adatoms were computed by 𝐸𝑏 = ―{𝐸𝑃𝑡𝑛 + (𝑉𝐶 ― 𝑇𝑖𝐶) ― (𝐸𝑃𝑡𝑛 + 𝐸𝑉𝐶 ― 𝑇𝑖𝐶)}

(3)

Where, 𝐸𝑃𝑡𝑛 + (𝑉𝐶 ― 𝑇𝑖𝐶) and 𝐸𝑉𝐶 ― 𝑇𝑖𝐶 refer to the total energies of the VC-TiC(001) with and without presence of adatoms, respectively. 𝐸𝑃𝑡𝑛 refers to the energy of free adatoms Ptn (n=1, 2). The binding energies of Pt dimer and a single Pt atom at the divacancy and monovacancies in TiC(001) are 5.65eV and 4.58, respectively, which are larger than those on the perfect TiC(001)62 (4.73 and 4.48 eV). The results showed that both the single Pt atom and Pt dimer interact strongly with the VC-TiC(001) substrate, and may be easily synthesized based on the thermodynamics. In Figure S1a and b, one Pt is trapped by VC and the other moves around it, indicating the possible moving direction and locations of Pt. However, the optimized structure is shown in Figure S1c, which indicates that the VC-TiC(001) substrate could effectively stabilize the Pt dimer that has been adsorbed and inhibit its dissociation. The bonding features and electronic structures are analyzed by density of states (DOS), which could provide a fundamental understanding on the interaction between substrate and Ptn, and on the activity. As shown in Figure 1d, the overlapped peaks occur at about −3.00 eV, and the strong coupling between d-orbitals of Ti and Pt is observed in the range of energy from −5.00 eV to the Fermi level of the Pt1/VC-TiC(001) system, showing the strong interaction between VC-TiC(001) substrate and Pt1. The DOS plots for the Pt2/VC-TiC(001) system is shown in Figure 1e, in which the overlapped peaks occur at about −3.00 eV and the greatly hybridized d-orbitals of Ti and Pt are observed from around −5.00 to 2.00 eV, consistent with the Bader charge analysis. Compared with the Pt1/VC-TiC(001) (0.58 e/Pt atom), more electrons are transferred to Pt atom from Ti atom (0.70 e/Pt atom) in the Pt2/VC-TiC(001), suggesting a stronger interaction between VC-TiC(001) substrate and Pt2. Compared with Pt1/VC-TiC(001), DOS of Pt2/VC-TiC(001) presents a small peak above the Fermi level, which is caused by the variations in the electronic structure of the VC-TiC(001)7



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modified outermost Pt2, suggesting that the supported Pt2 has a high activity on VC-TiC(001). From thermodynamics and kinetics, to study the stability of Ptn/VC-TiC(001), the diffusion properties of Pt dimer and the single Pt atom on VC-TiC(001) are investigated. In fact, all (steady and possible) adsorption sites of Pt should be examined to find out the diffusion pathway conveniently. The steadiest configuration of Pt adsorbed on vacancy was taken as the initial state (IS) and one of other metastable configurations of adsorption of Pt adsorbed on CTiTi site was taken as the final states (FS). The most possible diffusion paths of Ptn on VC-TiC(001) are shown in Figure 2. Compared with the perfect TiC(001)62, for single Pt atom and Pt dimer on VC-TiC(001), the diffusion barriers are significantly larger (0.35 and 0.50 vs 1.31 and 3.76eV), suggesting that the Ptn on VC-TiC(001) is hard to migrate. Compared with Pt1 on VC-TiC(001) in Figure 2a, the larger diffusion barrier of Pt2 on VC-TiC(001) in Figure 2b showed that adsorbed Pt dimer could be effectively stabilized by substrate of VC-TiC(001), and its diffusion could be inhibited.

Figure 2. The most probable diffusion paths of (a) Pt1 on VC-TiC(001) and (b) Pt2 on VC-TiC(001).

3.2.

Adsorption of Reaction Species on Ptn/VC-TiC(001) 8


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It is important to survey the adsorption properties of the reaction species (e.g. CO, O2) to CO oxidation. For revealing the steadiest adsorption configuration with the highest adsorption energy, all possible adsorption sites should be examined. The adsorption energy and the steadiest geometry of adsorbate on Ptn/VC-TiC(001) are shown in Figures S2, S3, 3 and 4.

3.2.1. Adsorption of Reaction Species on Pt1/VC-TiC(001) The adsorption orientations and probable adsorption sites of the gas molecules on Pt1/VCTiC(001) were examined. The adsorption energy (Eads) values and the steadiest configurations are shown in Figure S2. As shown in Figure S2a, at Pt-Top site, O2 would be easily adsorbed, with a rather small adsorption energy (0.34 eV) and a large Pt–O bond length (2.01 Å). It is interesting that, due to the charge transfer of 0.15 electrons to the O2 molecule from the substrate, the bond length of O2 has been increased from 1.23 Å of free O2 to 1.31 Å. CO molecules that are adsorbed vertically at the Pt-Top site could form a Pt–C bond of 1.92 Å (see Figure S2b). The adsorption energy is calculated as 1.13 eV, with about 0.25 electrons transferred to Pt1/VC-TiC(001) from the CO, and the C–O distance is consequently elongated from 1.14 Å to 1.16 Å in the free molecule. The calculated adsorption energies showed that there might be a competition adsorption between O2 and CO at the Pt-Top site. The interaction of Pt1/VC-TiC (001) with CO is stronger than that with O2, so the traditional ER mechanism would not be favorable, and the catalytic reaction may be suppressed, thus poisoning the catalyst. As shown in Figure S2c, the co-adsorption energy of O2 and CO is 0.75 eV, being smaller than the individual Eads of CO. Therefore, the LH mechanism would not occur. In addition, the study considered the co-adsorption at active sites of more CO molecules, and found that only two CO molecules were co-adsorbed on the Pt1/VC-TiC(001), which could avoid the problem of CO poisoning31, 63. For co-adsorption configuration of the two molecules of CO on Pt1/VC-TiC(001) (see Figure S2d), the corresponding adsorption energy is 1.75 eV, being much higher than that of the isolated CO or O2 adsorption, showing that the TER mechanism would probably occur.

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3.2.2. Adsorption of Reaction Species on Pt2/VC-TiC(001) Figure 3a showed that the O2 molecule is adsorbed at the Top site of Pt atom, and there was a O–O bond that is parallel to the surface, thus forming two Pt–O bonds of 2.02 Å, and the O–O bond was elongated to 1.43 Å. For O2, the adsorption energy was 1.29 eV, which was stronger than that on Pt1/VC-TiC(001), based on Bader charge analysis, it would result from intensive charge transfer (0.32 electrons): from the Pt2/VC-TiC(001) to the adsorbed of O2. Moreover, in the energy region from−8.0 to −2.5 eV, the 1π and 5σ orbitals of O2 would well overlap with the O2-2π* peaks and Pt-5d states emerging around the Fermi level (see Figure 3b), indicating that the adsorbed Pt atom would play an important role in improving the interaction between substrate and O2 molecules. In general, the charge transfer to the O2 from the substrate and the intense hybridization between the O2 (2p states) and Pt (5d states) would lead to an elongated O-O bond, suggesting that the oxygen molecule would be activated on Pt2/VC-TiC(001), and carbon monoxide molecule would be oxidized. The steadiest adsorption configuration and corresponding DOS plots for CO on Pt2/VCTiC(001) are shown in Figure 3c and d, respectively. Unlike the behaviors of adsorption of CO on Pt1/VC-TiC(001) (see Figure S2c), the CO molecule would be vertically adsorbed at Pt-Pt Bridge site through forming Pt-C bonds of 2.10 Å with an elongated C-O bond (1.18 Å) on Pt2/VCTiC(001). The corresponding energy of adsorption is 1.18 eV, slightly smaller than that of the adsorption of O2, showing that compared with CO, the adsorption of O2 is slightly preferable. The Bader charge analysis showed that the adsorbed CO would transfer 0.28 electrons to Pt2/VCTiC(001), which is consistent with the following result: from −25 to −5 eV, the 4σ, 4σ*, 5σ and 1π orbitals of CO were hybridized with the Pt-5d states. For CO adsorption, some electrons would be transferred from Pt to CO-2π*, CO-2π* orbitals would also be partially occupied. The intense hybridization between Pt atom and CO molecule, and the charge-transfer effects to the CO from the substrate, which showed that, as compared with the free CO molecule, the C-O bond distances are more elongated due to the adsorption (1.18 Å vs 1.14 Å).

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Figure 3. The steadiest configurations of adsorption of (a) O2, (c) CO on Pt2/VC-TiC(001) and the DOS plots of (b) and (d). Color code: the blue and red spheres, black and light gray respectively refer to the C, Ti, Pt and O.

CO2, as the CO oxidation’s final product, is only physisorbed on Pt2/VC-TiC(001) (see Figure 4a), which had a small adsorption energy (0.02 eV). Considering the tiny charge transfer between CO2 and the support, the weak interaction showed that the species of CO2 would be released at room temperature, spontaneously. The steadiest adsorption structure of the atomic O on Pt2/VC-TiC(001) is shown in Figure 4b, where atomic O would be adsorbed on Pt atom’s top site. As compared with the adsorption of O2, the Pt–O bond is lengthened to 2.04 Å, and the adsorption energy of the atomic O is reduced to 0.85 eV (see Figure 3a). The adsorbed atom O is negatively charged by 0.30 electrons, showing that the O– species would be more reactive than oxygen for oxidizing CO. 11



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The O2 molecule would be easily adsorbed on one of Pt sites, then CO molecule would be adsorbed on the other Pt site more strongly (see Figure 4c). The co-adsorption energy of CO and O2 is significantly larger than that of the isolated CO or O2 molecule adsorption on Pt2/VC-TiC(001) (1.55 vs 1.18 or 1.29 eV), which indicated there would be probably CO oxidation reaction on Pt2/VC-TiC(001) through the LH mechanism. Upon the first CO adsorbed on one of Pt sites, the second CO would be more strongly adsorbed on the other Pt site (see Figure 4d), the reason was that the co-adsorption energy of two CO is significantly larger than that of a single CO (1.98 vs 1.18 eV), where a O–C–Pt–Pt–C–O structure would be formed. The relatively high adsorption energies of isolated O2 and CO molecules and small difference in adsorption energy between CO and O2 may promote the catalytic reaction of CO oxidation, and also accordingly decrease the reaction barriers. The interaction between adsorbate and adsorbate would help to calculate the adsorption or coadsorption energy. For rough estimate, it would be necessary to present the variation of adsorption energy of CO (cell size: from 3 × 3 to 4 × 4). As presented in Figure S3, the adsorption of CO on the Pt/VC-TiC(001) (4 × 4) has a similar adsorption energy to that on the Pt/VC-TiC(001) (3 × 3).

Figure 4. Top (up) and side (down) views of the steadiest adsorption configurations of (a) CO2, (b) O, (c) CO + O2 and (d) CO + CO on Pt2/VC-TiC(001). The adsorption/co-adsorption energies and

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the selected geometric parameters are also shown in these figures.

3.3.

CO Oxidation on Pt2/VC-TiC(001) In general, there are two types of CO oxidation reaction mechanisms: LH and ER mechanisms,

which depend on the catalyst’s adsorption strength to mixed gases. LH mechanism involves the coadsorption of CO and O2 molecules before reaction, desorption of CO2, and formation of an intermediate state. ER mechanism is characterized by the reactant CO molecule approaching an O2 already activated. Recently, Mao et al.64 proposed a new TER mechanism, in which O2 can be simultaneously activated by two CO molecules. The details of CO oxidation mechanisms and the corresponding reaction barriers on Pt2/VC-TiC(001) are shown in Figure 5 to 7.

3.3.1. CO Oxidation on Pt2/VC-TiC(001) 3.3.1.1 ER mechanism

CO + O2*→ CO3*→ CO2 + O* For the ER mechanism, the optimized atomic configurations, including the transition state (TS), intermediate state (MS), initial state (IS) and final state (FS), are shown in Figure 5a. The configuration of CO that has been physisorbed (above O2 pre-adsorbed on Pt2/VC-TiC(001)) was selected as IS. The CO would approach the activated O2, thus reaching the transition state (TS1) with the elongating of the CO bond and scission of the O–O bond. Passing through TS1 that has an energy barrier of 0.48 eV, a intermediate state (CO3, MS) of carbonate-like would generate on the Pt atom. Moreover, the process would be exothermic by 3.81 eV, indicating that the reaction CO + O2→CO3 would be thermodynamically and kinetically favored. Passing through TS2 that has a relatively high energy barrier of 0.97 eV, a CO2 molecule would be released and atom O would be left on the Pt2/VC-TiC(001) (FS). However, the process of CO3→ CO2 + O would be endothermic by 0.68 eV, so this pathway would not be kinetically favorable. In Figure 5b, the atomic O at the Pt-Top site (IS1) would be easily diffused to Pt-Pt Bridge site (FS1), there would be an energy barrier of 0.06 eV on Pt2/VC-TiC(001), which is smaller than 0.71 eV on the Pt(111)65. 13



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CO + O*→ CO2 The IS2 for CO reaction with O starting from the configuration (with a CO molecule at 3.0 Å) away from the pre-adsorption O on Pt2/VC-TiC(001), as shown in Figure 5c. It is noted that CO would react with the O through the ER mechanism that had an energy barrier of 0.36 eV on Pt2/VCTiC(001), which is smaller than 0.59 eV on Pt/graphene28. In addition, it is found that the exothermic reaction energy of CO oxidation reaction CO + O → CO2 would be 2.39 eV, indicating that this process would be easier to take place.

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Figure 5. Different states' configuration and minimum energy profiles, involving IS, MS, FS and TS for CO oxidation on Pt2/VC-TiC(001). (a) CO + O2 → CO3 → CO2 + O, (b) O diffusion, and (c) CO + O → CO2 (ER mechanism).

3.3.1.2 LH mechanism 15



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The high barrier means that the CO oxidation on Pt2/VC-TiC(001) through the ER mechanism would be almost unpractical. Since the co-adsorption energy of O2 and CO was larger than either of the isolate O2 or CO, the paper focused on the study of LH mechanism.

O2* + CO* → OOCO* → CO2 + O* For various states along the reaction path, the energy barriers and atomic configurations are shown in Figure 6a. The O2 molecule prefers to be adsorbed on one of Pt sites, then CO molecule will be adsorbed on the other of Pt site more strongly. The configuration of CO and O2 that were co-adsorbed on the Pt atom (Pt–O and Pt–C bonds of 2.04 and 1.98 Å) was selected as the IS (see Figure 4c). After passing of TS1, the atom O would approach the atom C, thus generating OOCO (MS) with an extremely low energy barrier (0.06 eV); the distance of O-O bond would be elongated from 1.32 to 1.48 Å. Thermodynamically, this step would be exothermic by 0.83 eV, suggesting that this step (O2 + CO → OOCO) is feasible. Later, when TS2 was passed, the first CO2 molecule would be released from the catalyst, the cleavage of O–O bond would make an atomic O adsorbed on the Pt site (FS). This process would be exothermic by 1.10 eV, and should overcome a lowenergy barrier of 0.32 eV, which was significantly lower than that of the first ER route. The small reaction barriers and large exothermicity showed that the release of the first CO2 molecule and the decomposition of MS would readily take place at room temperature, indicating that this process is both thermodynamically and kinetically favored.

O* + CO* → OCO* → CO2 When the first CO2 was released, an atomic O would be left on the Pt site. Finally, the removal of O by CO through the ER mechanism would generate the second CO2. As shown in Figure 6b, on the other Pt atom, the second molecule of CO was adsorbed, as the IS1. Then the molecule of CO would approach the atom O through TS3, which had an extremely low energy barrier (0.18 eV), thus forming the intermediate OCO (MS1). From the aspect of thermodynamics, the reaction in this step would be exothermic by 1.28 eV, suggesting that this step is feasible in the field of experiment. Finally, after passing of TS4, with an extremely low energy barrier (0.10 eV), the catalyst would release the second molecule of CO2 through the LH mechanism, which would 16


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be exothermic by 0.16 eV. In addition, the adsorption energies calculated for the first and the second physisorbed CO2 molecules were 0.05 and 0.02 eV, showing the CO2 molecules could be easily removed. Interestingly, the corresponding energy barrier could be fully overcome by the energy released in each step of LH mechanism, indicating that through the LH mechanism, CO oxidation would be more flexible. The decomposition of CO3 and OOCO to a CO2 left an atomic O at the Pt site based on the ER (see Figure 5a) and LH (see Figure 6a) mechanisms. Then the CO could readily react with the atom O for releasing the last CO2 (CO + O → CO2) on Pt2/VC-TiC(001) based on ER (see Figure 5c) or LH (see Figure 6b) mechanisms. After releasing of CO2, the Pt2/VC-TiC(001) catalyst could be recovered for CO oxidation of a new cycle. The results showed that, for some reactions and species of adsorption, Pt site would be regarded as an active center, being consistent with the results of the previous theoretical studies27-31.

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Figure 6. Different states' configuration and minimum energy profiles, involving IS, MS, FS and TS for CO oxidation on Pt2/VC-TiC(001). (a) O2 + CO → OOCO → CO2 + O; (b) O + CO → OCO → CO2 (LH mechanism).

3.3.1.3 TER mechanism

O2+ 2CO* → OCOOCO* → 2CO2 Interestingly, for two molecules of CO on Pt2/VC-TiC(001), the stronger co-adsorption structure can be taken as a potential CO oxidation catalyst. The CO oxidation reaction profiles (TER mechanism) are shown in Figure 7. When the first CO was adsorbed on one of the Pt sites, the second CO would be adsorbed on the other Pt site, more strongly, the reason was that the coadsorption of 2CO could be more steady than that of the single CO (1.98 vs 1.18 eV). Subsequently, O2 was selected as the IS, for it is the third molecule that would be physisorption above the two coadsorbed molecules of CO, which have smaller adsorption energy of 0.08 eV. With O2 molecule approaching to CO, the O–O bond would gradually stretch and pass over the transition state (TS1), forming the intermediate OCOOCO (MS). The O2 + 2CO → OCOOCO reaction process would 18


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easily occur with a low-energy barrier of 0.38 eV, and would release 1.43 eV of heat. In this process, the O–O bond would be elongated from 1.25 to 1.59 Å, thus forming two new C–O bonds. After passing of TS2, the O−O bond would be broken with an energy barrier of 0.52 eV, and then the OCOOCO intermediate would dissociate into two CO2 molecules. The reaction process OCOOCO → 2CO2 would be exothermic of 2.59 eV. Two CO2 molecules, the final product of CO oxidation, could only be physisorbed on Pt2/VC-TiC(001) with a longer Pt–O distance, suggesting that molecules of CO2 formed would be spontaneously released. The reaction barriers of dissociation process and OCOOCO formation on Pt2/VC-TiC(001) were 0.52 and 0.38 eV, lower than CO oxidation reactions of 2CO + O2 → 2CO2 on Pt1/VC-TiC(001) (see Figure S4). With high CO oxidation energy barriers on Pt1/VC-TiC(001), the CO oxidation reaction could not be catalyzed. In this study, the CO oxidation on Pt2/TiC(001) was mainly studied. The discussion on Pt1/TiC(001) was to highlight the great catalytic properties of Pt2/TiC(001).

Figure 7. Different states' configuration and minimum energy profiles, involving IS, MS, FS and TS for CO oxidation on Pt2/VC-TiC(001). O2 + 2CO → OCOOCO → 2CO2 (TER mechanism).

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ER mechanisms have extremely-high energy barriers, so TER and LH mechanisms are better for CO oxidation on Pt2/VC-TiC(001) thermodynamically and underkinetically. For the preferable TER and LH mechanisms, the rate-limiting steps form the first molecule of CO2 and two molecules of CO2 by OCOOCO and OOCO intermediate dissociation; the corresponding reaction barriers are 0.52 and 0.32 eV, respectively. Considering the relatively small energy barriers of TER and LH, it is believed that both of the two mechanisms may occur at low temperature. In addition, for LH, the rate-limiting step’s energy barrier is significantly lower than that of the TER mechanism (0.32 vs 0.52 eV), showing that LH mechanism is more preferable than TER mechanism. Moreover, in each step, the released energy could fully cover the energy barrier, showing that the Pt2/VC-TiC(001) catalyzed CO oxidation is feasible in the aspect of experiment. The energy barriers of CO oxidation of the catalysts supported by metal Pt are compared with Pt2/VC-TiC(001), as shown in Table 1. In general, for the rate-limiting steps on Pt2/VC-TiC(001), the energy barriers are lower than those on other substrates, such as Pt(111)66, Pt/grapheme28, Pt/BN27, Pt/Mo2CO231, Pt/SnO229 and Pt/FeOx30, being lower than those on the PtML/WC(0001)40. Therefore, the study clearly demonstrated that Pt2/VC-TiC(001) had a high tolerance against CO poisoning due to its rapid CO oxidation. It is the objective for efficient catalyst design to compare the VC-TiC(001) structure with lower loading and higher activity of noble metal Pt catalyst with other mentioned catalysts.

Table 1. The rate-limiting-steps and the corresponding energy barriers of CO oxidation on various substrates via the corresponding mechanisms. Substrate

Barrier mechanism (eV)

Substrate

Barrier mechanism (eV)

Pt/TiC(001) PtML/WC(0001)40 Pt/Mo2CO231 Pt/BN27

0.32LH, 0.52TER 1.06LH 1.39LH, 0.49TER 1.31ER, 0.38LH

Pt/SnO229 Pt/FeOx30 Pt(111)66 Pt/graphene28

0.51ER 0.79LH 1.05LH 0.58LH, 0.59ER

4. SUMMARY AND CONCLUSIONS

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The highly-steady, highly-efficient and low-cost CO oxidation catalysts on Pt2/VC-TiC(001) were designed based on the first-principle calculations, which indicated that at low temperature, TER and LH mechanisms (with the overall energy barrier of 0.52 and 0.32 eV) were more preferred than ER mechanism for CO oxidation. In addition, the results also showed that the Pt2/VC-TiC(001) had a higher activity than pure Pt(111) surfaces, and Pt on other substrates including BN, Mo2CO2, grapheme, SnO2 and FeOx. The study is expected to attract interests in testing the performance of Pt2/VC-TiC(001) and similar composites, so as to search and design great CO oxidation catalysts in PEMFC.

ASSOCIATED CONTENT The dissociation of Pt2 with one Pt trapped by VC and the other moved around it (Figure S1); the steadiest configuration of adsorption of O2, CO, CO+O2 and CO+CO on Pt1/VC-TiC (001) (Figure S2); the steadiest configurations of adsorption of CO on Pt1/VC-TiC(001) (3 × 3), Pt1/VC-TiC(001) (4 × 4), Pt2/VC-TiC(001) (3 × 3), and Pt2/VC-TiC(001) (4 × 4) (Figure S3); different states' configuration and minimum energy profiles, involving TS, FS and IS for CO oxidation (O2 + 2CO →2CO2 by the TER mechanism) on Pt1/VC-TiC (001) (Figure S4).

AUTHOR INFORMATION Corresponding Author †E-mail:

[email protected] (Z. Yang) [email protected] (W Wang)

ORCID Zongxian Yang: 0000-0002-3015-3804 Weichao Wang: 0000-0001-5931-212X 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 (Nos. 11474086 and 11874141). Parts of the simulations are performed on resources provided by the High Performance Computing Center of Henan Normal University.

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TOC Graphic：

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The Journal of Physical Chemistry Side view

(d)

(a) VC

VTi

(b) (e)

(c)


C

Ti

Pt

The Journal of Physical Chemistry 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

IS

TS

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Ed = 1.31 eV, ∆E = 1.27 eV

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Ed = 3.76 eV, ∆E = 3.55 eV ACS Paragon Plus Environment

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FS

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The Journal of Physical Chemistry

(b)

(a)

Eads(O2) = 1.29 eV

(d)

(c)

Eads(CO) = 1.18 eV C

Ti

Pt

O



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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

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Eads(2CO) = 1.98 eV

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TS1


ER mechanism: CO + O2* → CO3* → CO2 + O*

IS

TS2 FS 0.48 eV MS 0.00 eV

0.97 eV ACS Paragon Plus Environment

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-3.13 eV

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TS4

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36 of*39 LH mechanism: O2* + CO* → OOCO* → CO2 Page +O


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0.06 eV

0.00 eV

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IS1 TS4 MS1 FS1

0.18 eV 0.00 eV

0.10 eV -1.28 eV ACS Paragon Plus Environment

-1.44 eV

TS1 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


Page 38 of 39

TER mechanism: O2 + 2CO* → OCOOCO* → 2CO2 TS2

IS

MS

0.38 eV FS

0.00 eV 0.52eV

-1.43 eV


-4.02 eV

Page 39 of 39

CO2 V. CO2 formation

I. Pt2-TiC(001) CO + O2

MS1. OCO

Energy (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


IV. CO adsorption CO

C

Ti

Pt

O

CO2 II. co-adsorbed CO and O2 ACS Paragon Plus Environment

MS. OOCO

III. CO2 formation

Design of Efficient Catalysts for CO Oxidation on Titanium Carbide

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