Strain Engineering of Defect-free Single-Layer MoS2 substrate for

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Energy, Environmental, and Catalysis Applications 2

Strain Engineering of Defect-free Single-Layer MoS substrate for Highly Efficient Single-Atom Catalysis of CO Oxidation Yandi Zhu, Ke Zhao, Jinlei Shi, Xiaoyan Ren, Xing-Ju Zhao, Yuan Shang, Xinlian Xue, Haizhong Guo, Xiangmei Duan, Hao He, Zhengxiao Guo, and Shun-fang Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b06435 • Publication Date (Web): 20 Aug 2019 Downloaded from pubs.acs.org on August 20, 2019

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ACS Applied Materials & Interfaces

Strain Engineering of Defect-free SingleLayer MoS2 substrate for Highly Efficient Single-Atom Catalysis of CO Oxidation Yandi Zhu,† Ke Zhao,† Jinlei Shi,‡ Xiaoyan Ren,† Xingju Zhao,‡ Yuan Shang,¶ Xinlian Xue,† Haizhong Guo,† Xiangmei Duan,§ Hao He,*† Zhengxiao Guo,⁑ and Shunfang Li*† †School

of Physics and Engineering, Zhengzhou University, Zhengzhou, Henan, 450001, China

‡Beijing

Computational Science Research Center, Beijing, 100193, China

¶Supercomputer

Center in Zhengzhou University, Zhengzhou University, Henan, 450001, China

§Department

of Physics, Faculty of Science, Ningbo University, Ningbo 315211, China

⁑Departments

of Chemistry and Mechanical Engineering, The University of

Hong Kong, Hong Kong SAR; and Zhejiang Institute of Research and Innovation, The University of Hong Kong, Dayuan Road, Hangzhou 311305, China 1

ACS Paragon Plus Environment

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KEYWORDS: single-atom catalysts, defect-free 2H-MoS2, strain engineering, electronic metal-substrate interactions, CO oxidation.

ABSTRACT: Single-atom catalysts (SACs) are of great scientific and technical importance due to their low cost, high site density and high specificity to enhance chemical reactions. Nevertheless, a major issue that severely limits the practical exploration of SACs is their instability, i.e., the preference of sintering and clustering over a defect-free substrate during operation. Here, we employ first-principles calculations to investigate how substrate engineering can stabilize SACs by strain-tuning of the electronic interactions between the metal and the substrate, using two Pd adatom on a defect-free single-layer MoS2 as a typical example. It is identified that, the Pd2 dimer is prone to dissociate and form highly efficient SACs for CO oxidation due to enhanced charge transfer and orbital hybridizations with the MoS2 substrate under a suitable tensile strain. The straining induces a semiconductive-to-metallic phase transition of the substrate. Moreover, low-cost elements, such as Ag, Ni, Cu and Cr, can also be stabilized into high performance SACs for CO oxidation with tunable reaction barriers by straining. The present findings offer a new avenue to inhibit the transition metal atoms from clustering into nanoclusters/particles, and provide clear guidance to the development of highly costefficient and stable SACs on defect-free substrates. 2


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1. INTRODUCTION Nanoparticles (NPs) or nanoclusters (NCs) usually exhibit excellent catalytic properties in many chemical processes, such as CO oxidation,1, shift interaction,3,

4

2

water-gas

and hydrogen generation.5 Such catalytic activities are

strongly size- and geometry- dependent,6-8 due to appropriate d-band centers9 and/or large fractions of the lowly-coordinated metal atoms serving as active sites.10-12 However, the complex geometric structure and flexible configuration of a given NC degrade product selectivity, probably due to their multiple active sites.13, 14 Single-atom catalysts (SACs),15, 16 with atomically deposited transition metal (TM) or noble metal (NM) atom monomers on a substrate, have been expected to maximize the efficiency, activity and selectivity, especially the NM catalysts.17 In 2011, Qiao et al. reported that Pt1-SACs on an iron oxide substrate exhibit excellent catalysis for CO oxidation.17 Subsequently, various experimental synthesis methods, including mass-selected soft-landing1, 15 and wet chemistry approaches,15, 16 have been developed to fabricate numerous exotic SACs on different substrates, such as metal oxides,17,

18

metal clusters19 and two-

dimensional (2D) materials.20, 21 Several of these SACs exhibit high catalysis selectivity - respectively for CO oxidation,17, 22 CO2 electroreduction,23, 24 water

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gas shift reaction,25 alcohol oxidation,26 hydrogen evolution reaction27, 28 and nitrogen reduction reaction.29 Nevertheless, due to their high activities, SACs are prone to clustering and sintering15, 30 during the chemical reactions. A strategy to resolve the issue is to enhance the electronic metal-substrate interactions (EMSI)31-33 and reduce the mobility of SACs. Usually, substrate defects34 and step edges35 are invoked to stabilize the SACs. However, the loading density of such exotic SACs was severely limited by the number of intrinsic defect sites of the substrate. Therefore, establishing a cost-effective way to stabilizing the SACs on defectfree substrates36 is of great importance to break this bottleneck and to achieve high density of stable SACs with much improved catalytic performance. Here, we propose a feasible method of applying strain to a two-dimensional (2D) single-layer defect-free MoS2 substrate34 to stabilize TM1-SACs for CO oxidation, taking into consideration of three indispensable conditions. First, the substrate should be sufficiently stable to support SACs; second, the substrate has a large specific surface area that facilitates a high loading density of SACs; and finally EMSI can be readily tuned to stabilize SACs and improve their performance. 2D materials can readily meet those three requirements, particularly with their extraordinary properties and promising applications.34, 3740

There are some efforts in enhancing catalysis by straining.34,

41-43

For

example, Zhou et al.44 report that suitable straining enhances the catalysis of 4


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Au clusters on graphene. Sayle et al.45 investigated the chemical reactivity of ceria nanorods subject to strain. In addition, we46 have also examined the role of substrate straining on defect effect in stabilizing SACs over a 2D WTe2. Nevertheless, to the best of our knowledge, there is still lack of investigation on straining for stabilization and improvement of SACs on defect-free 2D materials. Note also that, in 2011, Bertolazzi et al.47 reported that the in-plane strain of defect-free-like single-layer MoS2 can be experimentally applied up to ~ 11%, using an atomic force microscope. Therefore, in this work, taking defect-free single-layer MoS2 as a typical case, we carried out first-principles calculations to investigate the substrate straining effect on the stabilization of SACs by effective tuning of EMSI. It is found that on a pristine MoS2, the Pd adatoms prefer dimerization or clustering to separation. However, under tensile straining, the single-layer MoS2 exhibits semiconductive to metallic phase transition, and consequently, the deposited Pd adatoms tend to be stabilized due to the enhanced EMSI (due to charge transfer and orbital hybridization) and act as highly efficient SACs for CO oxidation. Importantly, we further reveal that several low-cost elements, such as Ag, Ni, Cu, and Cr, can also be rooted as SACs on defect-free MoS2, with tunable reaction rates and high catalytic performance for CO oxidation via substrate straining. The present findings provide a clear route to preventing deposited metal atoms from nucleation and clustering into NCs/NPs, at least at 5



the initial stage, and to guide new approaches to fabrication of more efficient and stable SACs with high selectivity. 2. COMPUTATIONAL METHODS Our spin-polarized DFT calculations48 are carried out using the Vienna ab-

initio simulation package (VASP)49 with projector-augmented wave (PAW) method50 and the Perdew Burke and Ernzerhof (PBE)51 as parametrized by Perdew and Zunger for the exchange-correlation functional. The 2H phase of 2D MoS2 is a typical single-layer TMD with Mo atomic chains sandwiched by adjacent S layers, which is simulated by a slab model. A vacuum region of 15 Å thickness is used to ensure the decoupling between the neighboring images. The optimized lattice constants are a = b = 3.167 Å of the monolayer MoS2, very close to the experimental value 3.160 Å.52 During the structural relaxation, all atoms are fully relaxed until the residual forces on each direction are smaller than 0.02 eV/Å. A 1 × 1 × 1 k-point mesh was used for the 8 × 8 × 1 supercell of the MoS2. The energy cutoff is set to 400 eV in all the calculations. In simulations of the adsorption of the TM adatoms, relativistic effect has been necessarily considered by including the spin-orbit coupling for heavy elements.53 The binding energies of per TM atoms on the substrate MoS2 is defined as Eb = − [E(TMn/MoS2) − E(MoS2) − nE(TMatom)]/n. In the formula, the first three terms represent the total energies of the optimized TMn/MoS2 complex, the optimized MoS2 substrate, the energy of a TM single atom in the 6


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gas phase, and n is the total number of the adsorbed TM atoms, respectively. According to this definition, a positive (negative) value of Eb indicates that the adsorption is exothermic (endothermic). Similarly, the adsorption energy (Eads) of O2 is defined as Eads(O2)= − [E(O2-TM/MoS2) − E(TM/MoS2) − E(O2)], of which a positive (negative) value indicates that the adsorption reaction is exothermic (endothermic). Biaxial strains ε are applied on mono-layer MoS2 by equally changing the in-plane lattices constants of a and b and relaxing the atomic positions, i.e., ε = εx = (a − a0)/a0  100% = εy = (b − b0)/b0  100%. Therefore, a positive ε means a tensile strain, vice versa. To investigate the kinetic processes of the diffusion energy barrier, O2 activation and CO oxidation, the improved climbing-image nudged elastic band (cNEB) method54, 55 is used to identify the transition states (TS) and minimum energy paths (MEP). 3. RESULT AND DISCUSSION As a starting point, we briefly describe the optimized geometric structure of the 2D MoS2. As reported previously, the most stable phase of 2D MoS2 consists of hexagonally arranged molybdenum atoms sandwiched between two planes of hexagonally arranged sulfur atoms.56 As shown in Figure 1a and 1b, the calculated Mo-S bond length is 2.41 Å, and the distance between the upper and lower sulfur atoms is 3.16 Å, in close agreement with previous investigations.57

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Figure 1. (a)-(c) Geometric structure, high symmetry adsorption sites, and the binding energy (Eb) of a Pd single atom on the pristine MoS2. (a) top view, and (b) side view. Eb, diffusion energy barrier (Ebar), and normalized diffusion rates (R) as a function of external tensile strain ε on single-layer MoS2 are shown in (d)-(f), respectively. Next, we investigate the preferred adsorption sites of a Pd single adatom on MoS2 upon geometric optimization. As illustrated in Figure 1a, four high 8


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symmetry surface adsorption sites were considered, i.e., on top of Mo site (TMo), hexagon-center or hollow site (H), top of S site (TS), and the bridge of two Mo sites (B). Meanwhile, we double-checked some other relatively low symmetry adsorption sites (not shown here) deviated from the aforementioned high symmetry sites. In all the cases, the Pd adatom is initially positioned about 3.50 Å above the surface S atoms. After the optimization, TMo is found to be the most favorable site with a binding energy (Eb) of 2.59 eV, and the less stable sites are TS and H sites, with Eb = 2.04 and 1.49 eV, respectively, see Figure 1c. Nevertheless, the Pd adatom cannot be stably adsorbed on the B or other low symmetry sites, and it will be automatically relaxed to the neighboring H site or the nearby high symmetry sites. Under external biaxial tensile strain ε on the 2D single-layer MoS2, Eb increases significantly with respect to ε, indicating that Pd1-SACs can be stabilized on MoS2 by such a simple method. Specifically, the Eb increases almost linearly from 2.85, 3.10, to 3.42 eV when the strain ε is increased from 5, 10, to 15%, respectively, see Figure 1d. Note that, recently, theoretical calculations show that the Mo-S bond breaks down at about 10% tensile strain on a phosphorus/MoS2 van der Waals heterojunction.58 However, our ab-initio molecular dynamics (AIMD) simulations show that both MoS2 and Pd/MoS2 complexes remain intact under a tensile strain over 10% at 500 K (see Figure

9



S1), directly supporting the experimental observation that a single crystal MoS2 monolayer can endure external strain up to ~ 11%.47 Importantly, the diffusion energy barrier (Ebar) of the Pd adatom on MoS2 substrate also increases almost linearly with respect to the applied strain, as shown in Figure 1e. Specifically, for a Pd single atom hoping from the most stable adsorption site TMo to the neighboring metastable TS site, the Ebar is 0.61 eV. However, it increases from 0.82, 1.00, to 1.42 eV, when the external tensile strain ε is applied from 5, 10, to 15% on the substrate, respectively. According to the Arrhenius form of R = A  exp[−Ebar/(kBT)], if we normalize the hopping rate of the Pd single atom from TMo site to the neighboring metastable TS site to be R = 1.00 for the case of ε = 0% at room temperature, R(ε) will be exponentially reduced as a function of the tensile strain (see Figure 1f). Therefore, one can expect that the mobility and collision/nucleation rate of the experimentally deposited Pd single adatoms on MoS2, for instance via a wet chemistry method, can be reduced by the introduction of applicable tensile strain on the substrate, which facilitates the stabilization of Pd1-SACs. We now continue to examine the direct interactions of two Pd adatoms on the substrate. As seen from Figure 2a, when one Pd adatom is located on the preferred TMo site, the other Pd adatom in formation of a Pd2 dimer can be located on the adjacent TMo, B1, or B2 sites. Moreover, we take into account the cases in which one Pd atom is located on the lowest metastable TS site and the 10


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other Pd atom on the nearby stable sites, such as TMo and H. Upon such initial adsorption configurations (with the Pd-Pd bond length R(Pd-Pd) < 3.66 Å), the optimizations lead to four low energy structures. Based on the locations of the two Pd atoms, these four optimized Pd2 dimer structures on MoS2 are labeled as TMo-TMo, TMo-B1, TMo-B2, and H-TS (see Figure 2b). Among these four cases, the most stable is the TMo-TMo configuration with an Eb of 2.65 eV/atom and R(Pd-Pd) = 3.09 Å (see Figure 2c).

Figure 2. (a) Four representative Pd2 dimer configurations based on the locations of two Pd adatoms (TMo-TMo, TMo-B1, TMo-B2 and H-TS) and (b) their average binding energy (Eb) per atom on MoS2. (c) Optimized most stable dimerization configuration, TMo-TMo. (d) Energy difference between the most 11



stable dimerization configuration and separation with the largest Pd-Pd distance as a function of ε applied on the 2D MoS2 substrate. For comparison, we also consider the separation case with each Pd atoms being located on the preferred TMo sites nevertheless keeping the largest separation distance (~16.46 Å), and the energy is 50 meV less stable than the dimerization case (see also Figure 2d). We define the energy difference between the dimerization and separation of the two Pd atoms, ΔE = E(separation) − E(dimerization), where the first term denotes the total energy of two Pd adatoms far apart from each other, and the second represents the most stable dimer configuration, respectively. As clearly seen from Figure 2d, the energy difference gradually declines with increasing tensile strain. Therefore, from both energetics and kinetics points of view, it can be concluded that such a simple, economical and applicable strain engineering approach can effectively stabilize and enhance the loading density of Pd1-SACs on MoS2. In addition, we have examined the stability of Pd1-SAC by comparing the Eb of the Pd atoms on the MoS2 with the average cohesive energies (ACE or average binding energies) of the Pd bulk and PdN clusters in sub-nanometer or nanometer regimes. In Figure S2, our calculations show that up to N=9, the Eb of the SAC on MoS2 is still larger than that of the ACE of the PdN clusters, which means that, at least from the energetics point of view, Pd9 cluster prefers to

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dissociate into 9 Pd1-SACs on the substrate in the present supercell, leading to a large loading density of the stable Pd1-SAC i.e., about 14% (9/64). To the best of our knowledge, such a high value is greater than the loading density reported recently.59 Moreover, we note that the proposed criterion for examining the stability of the Pd1-SAC can be well rationalized by an existing experiment. Recently, Lang et al.36 reported that high loading density of Pt1-SACs can be stabilized on defect-free Fe2O3 substrate. Our calculation show that, the Eb of a Pt single atom on the defect-free Fe2O3 is about 4.01 eV, in close agreement with experiment,36 which is also larger than the ACE of PtN clusters (N=2~9), nevertheless, smaller than the ACE of the Pt bulk (see Figure S3). Such findings convincingly demonstrate that the proposed approach has been verified by the available experiment. Moreover, based on such a criterion, we find that some other low cost TM elements (TM=Ag, Ni, Cu, and Cr) can also be stabilized on the MoS2 substrate under straining, as representatively addressed for the cases of Ni and Cu in Figure S4 (in S-4). In addition, we have further compared the stability of the TM1-SAC to the supported TMN clusters on MoS2 substrate, with ε = 0 and 5 %. Taking Pd as a prototypical example, we found that the Eb of a single Pd atom is significantly larger than the average adsorption energy (Eads) of the PdN clusters, implying that the dissociation of the PdN clusters into atomically dispersed Pd1-SAC on the substrate is energetically stable or metastable. Moreover, when N is larger than 4 (9), the 13



large values of the Ebar kinetically prevent the deposited TM atoms from clustering to larger TMN clusters and bulk, as show in Figure S5 (in S-5). Importantly, these TM1-SACs can exhibit high performance in CO oxidation, which will be discussed later.

Figure 3. (a) Electronic density of states (DOS) and (b) the charge transfer of a single Pd adatom on the preferred adsorption site of single-layer MoS2 as a function of applied external tensile strain ε. In (a), from top to the bottom, the four panels correspond to the applied ε of 0%, 5%, 10% and 15%, respectively. To reveal the underlying physical origin of the straining effect on the EMSI, i.e, the Pd-MoS2 interaction, we analyze the electronic structure evolution of the MoS2 as a function of ε in Figure 3a. Clearly, the intrinsic MoS2 exhibits semiconductive feature57 with an energy gap (Egap) of 1.97 eV, in close agreement with the previous investigations.37 When 5% strain is applied, the Egap is significantly reduced to be 0.90 eV; with the strain further increasing up to 10%, MoS2 exhibits metallic characteristics. Due to the smaller 14


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electronegativity (EN = 2.20) of the Pd atom than that of S atom (EN = 2.58), charge is transferred from the adsorbed Pd atoms to MoS2. Such an Egap reduction and the semiconductive to metallic phase transition significantly lower the energy gap between the highest occupied molecular orbital (HOMO) of the Pd and the conduction band minimum (CBM) of MoS2. Consequently, the charge transfer (ΔQ) from the Pd atoms to the substrate almost linearly increases as a function of the strain, as supported by the Bader charge analysis presented in Figure 3b, which strengthens the Eb. Moreover, the Eb can be enhanced because the overlapping matrix element between Pd and MoS2 is enlarged according to the Newns-Aderson model60, 61 due to the reduction of the HOMO(Pd)-CBM(MoS2) gap. Therefore, we confirm that straining on the MoS2 substrate tends to stabilize Pd1-SACs, i.e., the possibility of the deposited Pd single atom on MoS2 surviving as atomically dispersed Pd1SACs is considerably enhanced under tension.

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Figure 4. Energy difference [ΔE = E(separation) − E(dimerization)] between the most stable dimerization configuration and separation with the largest TM-TM distance of two TM adatoms (TM=Ag, Ni, Cu, and Cr) as a function of the applied ε on the single-layer MoS2 substrate. As briefly mentioned, motivated by the findings that the strain engineering on MoS2 facilitates in stabilizing Pd1-SACs, naturally, we perform additional extensive effort to establish that the present approach is also applicable to many of the 3d-, 4d-, and 5d-TM elements in the periodic table. Specifically, we considered both high and relatively low symmetry adsorption sites (not shown here) for a TM atom on MoS2 upon geometric optimizations and exhibit the Eb of Ag, Ni, Cu and Cr atom on the pristine MoS2 (see Figure S6). Particularly, in Figure 4, we display the relative stabilities (as reflected by ΔE) between nucleation and separation of those much cheaper TM elements (TM=Ag, Ni, Cu, and Cr). Significantly, the relative stabilities can be totally reversed by applying certain stain ε, i.e., in the range of 5% ~ 10%. For instance, on the perfect MoS2, two Ni adatoms prefer dimerization to separation by 0.40 eV, while the energy difference decreases to 0.11 eV under a 5% tensile strain. Importantly, when the ε is further increased up to about 10%, the two Ni adatoms favor separation over dimerization, by 0.20 eV.

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Figure 5. (a) Optimized adsorption configurations, adsorption energies (Eads). (b) Stretching vibrational frequency ( in cm-1) of an O2 molecule on pristine TM1-MoS2 (TM=Pd, Ag, Ni, Cu, and Cr). In the following sections, we show that those TM1-SACs (TM=Pd, Ag, Ni, Cu and Cr) stabilized on the MoS2 can exhibit good catalysis for CO oxidation. First, we examine the O2 adsorption and activation on the TM1/MoS2, which is a key step for CO oxidation. As shown in Figure 5a, the O2 molecule can be readily captured by the TM1-SACs active sites, leading to Eb = 0.61, 0.78, 0.91, 1.25 and 3.04 eV and an enlarged O-O bond length R(O-O) = 1.27, 1.29, 1.28, 1.31, and 1.44 Å, respectively. Moreover, we confirm that the adsorbed O2 is 17



significantly activated as reflected by the red-shifted O-O stretching vibration frequency () of 1296.8, 1239.9, 1268.3, 1165.1, and 931.9 cm-1 on Pd1-, Ag1, Ni1-, Cu1-, and Cr1-SACs, respectively, as compared to the value of 1560.8 of the ground state O2 (see Figure 5b). Moreover, we calculated the interaction of an O2 molecule with the MoS2 substrate under tensile strain of 5 and 10 %, respectively. The calculated results show that in both cases, the O2 molecule can only be very weakly (physically) adsorbed on the substrate, with an Eb of ~ 0.10 eV (see Figure S7), respectively, which suggests that the strained MoS2 is still fairly inert toward O2. Moreover, such an Eb of the O2 on the strained MoS2 substrate is significantly smaller than that (~ 0.61-3.04 eV) on the TM1 active sites of the TM1/MoS2 complex (TM=Pd, Ag, Ni, Cu and Cr), which further demonstrates that the incoming O2 molecules can be effectively captured and activated by the TM1SAC stabilized on the strained MoS2 substrate. Then, we further investigate the catalysis of these stable TM1-SACs for the CO oxidation, considering both Langmuir-Hinshelwood (L-H) and Eley-Rideal (E-R) mechanisms, taking Pd, Ni and Cr as the typical SACs in consideration of two factors. First, Pd and Ni (Cr) are representative expensive noble metal and economic metal elements, respectively; secondly, from Pd, through Ni, to Cr, the O2 molecule is more and more significantly activated, as reflected by Eads and the vibrational analysis presented (in Figure 5). 18


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Figure 6. Schematic view of the minimum energy path (MEP) of CO oxidation on Pd1-MoS2 system via the Langmuir-Hinshelwood (L-H) mechanism. Meanwhile, we also calculated the adsorption of a CO molecule on the same Pd active site, leading to an Eb of 0.49 eV, which means that O2 may compete with CO to pre-adsorb on the Pd1-SAC sites. We find that the CO oxidation prefers to proceed via the L-H process on Pd1-SAC. That is, CO first co-adsorbs with O2 on the single Pd atom and the co-adsorbed molecules CO/O2 undergo a bimolecular reaction resulting in a carbonate (CO3) species by overcoming the first Ebar = 0.57 eV. Then, the CO2 precursor can be readily released by 19



overcoming a relatively smaller Ebar = 0.39 eV, as detailed by the minimum energy path (MEP) and energetics in Figure 6. Specifically, we identify that the CO molecule adsorbs in the vicinity of the O2 molecule on the Pd1-SACs via the well-known back donation charge transfer mechanism,62 i.e., donation of CO 5σ electrons to the Pd1/MoS2 substrate and back-donation from the Pd1-SACs into the unoccupied 6π* orbital of CO, leading to an Eb = 1.04 eV of the CO molecule. Such a mechanism is also verified by the slightly enlarged C-O bond length of 1.16 Å and additional electronic charge density analysis (in Figure S8). It is noted that the effective activation of the O2 molecule and a modest activation of CO by the Pd monomer rendering it a good SAC candidate in avoiding the important issue of CO poisoning in heterogeneous catalysis. Furthermore, our calculations show that once a CO and an O2 are co-adsorbed, the second CO molecule is difficult to co-adsorb on the Pd site. Secondly, supposing that in the case of very high CO concentration, two CO molecules are first adsorbed on the Pd single atom site, the incoming O2 molecule can also simultaneously attack the two adsorbed CO molecules and generate two CO2 molecules via the tri-molecular E-R mechanism,63 however experiences a relatively high reaction barrier (~0.90 eV), see Figure S9. Moreover, our calculations show that, in the rate-limiting step, the Ebar is significantly increased up to 0.90 eV (see Figure S10), when the first round of 20


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CO oxidation proceeds via the two-molecular E-R mechanism. These results indicate that the CO oxidation on Pd/MoS2 may feasibly proceed via the twomolecular L-H process presented in Figure 6, rather than the E-R mechanism. When the substrate is subject to straining, the same mechanism is maintained, see Figure S10b. At the end of the above steps, an O atom dissociated from the adsorbed O2 molecule is still left on the Pd monomer, leading to an oxidized Pd1-SACs. Our extensive calculations show that a second incoming CO molecule can be readily oxidized into a CO2 precursor via the E-R reaction mechanism, and the calculated Ebar of this reaction is only 0.14 eV, as shown in Figure 6. After finishing the catalytic cycle by releasing the second CO2 upon overcoming a small energy barrier (around 0.24 eV), the Pd1-SAC is recovered to this original configuration.

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Figure 7 Reaction energy barriers (Ebar) calculated in the rate-limiting steps as a function of applied tensile strain (0%, 5%, and 10%) are also presented for three representative TM1/MoS2. (a) TM = Pd (L-H mechanism); (b) TM = Ni (E-R mechanism); (c) TM = Cr (E-R mechanism). Successively, we further examine the kinetic processes of the O2 activation and CO oxidation on the Pd1/MoS2 as a function of ε, as summarized in Figure 7a. Briefly, within the strain level currently considered, very similar CO oxidation processes and L-H mechanism are established in the MEP. However, due to the modulated EMSI under straining, the local projected d-type local projected density of states on the Pd single atom (d-LPDOS(Pd)) is changed by the Fermi level. Correspondingly, the catalytic properties of the Pd1/MoS2 can also be tuned upon the applied strain, as reflected by the modulated rate-limiting Ebar, varying within the range of 0.57 ~ 0.61 eV. Overall, the higher the d-LPDOS(Pd) by the Fermi level, the lower the Ebar in the rate-limiting steps for CO oxidation, and thus the better the catalysis of the Pd1-SAC. Such findings are of great importance in optimizing the selectivity of a given SAC via strain engineering. In addition, it is found that, when the first CO oxidation proceeds via the E-R process, the Ebar is significantly increased up to 0.94 and 0.89 eV for ε = 5 and 10 %, respectively, as shown in the Figure S10.

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Here we also emphasize that, experimentally, Kaden et al.1 have fabricated highly efficient atomic scale Pd catalysts on rutile TiO2(110) substrates and found that Pd2 dimer exhibits maximum catalytic activity for the CO oxidation in the small size regime. In 2015, Qiao et al.64 experimentally demonstrated that isolated Au single atoms dispersed on iron oxide (Au1/FeOx) exhibit extremely high reaction stability for CO oxidation in a wide temperature range, and activation barriers of 1.34, 0.84, and 0.80 eV are calculated in the multiple ratelimiting steps for the catalytic cycle of CO oxidation. Moreover, our previous work65

demonstrated

that

the

O2

can

be

effectively

activated

on

Pd2@TiO2(110), as supported by the low Ebar of 0.68 eV in the rate-limiting step for CO oxidation via the L-H mechanism. These results explain well the experimentally observed high performance of the Pd2 on TiO2(110) for CO oxidation.1 Note that, in general, 0.75 eV is regarded as a surmountable energy barrier for surface reactions occurring at room temperatures.66 Therefore, taking Au1/FeOx64 and Pd2@TiO2(110)65 single atomic scale catalysts as benchmarks for comparison, we believe that the present Pd single atom stabilized on MoS2 via the strain engineering approach leading to even smaller reaction barriers can be a very promising highly efficient SACs for CO oxidation. To this end, we briefly summarize the catalysis of more economical Ni1- and Cr1-SACs for CO oxidations, considering both L-H and E-R mechanisms. First, without introduction of straining, on both Ni1- and Cr1-SACs, CO oxidations can 23



readily proceed via the E-R mechanism, by overcoming Ebar of 0.29 and 0.44 eV (see Figures S11 and S12), respectively, rather than the L-H process (see Figures S13 and S14) therein significantly enlarged Ebar of 0.47 and 1.39 eV are obtained, respectively. Therefore, we continue to examine the strain effect on the reaction rates of the CO oxidation via the E-R mechanism. As expected, the external strain can further modulate the reaction Ebar, i.e., 0.46 (0.29) eV for the first CO oxidation on Ni1-SAC, and 0.53 (0.62) eV on Cr1-SAC, respectively, under ε = 5 (10) %; for the second CO oxidation via interacting with the left O atom, Ebar of 0.20 (0.22) eV and 0.76 (0.84) eV are calculated for the cases of Ni1-SAC and Cr1-SAC, respectively, under ε = 5 (10) % (Figure 7b,c). Interestingly, we found that when applying external strain on the substrate, the calculated Ebar for CO oxidation via the L-H mechanism may significantly lowered, especially on Cr1-SAC (Figure S14). Specifically, under ε = 5 (10) %, Ebar of 0.45 (0.18) eV are obtained in the rate-limiting step of the first CO oxidation. These findings suggest the preferred reaction mechanism may be changed from the E-R to L-H due to the straining. Again, by comparing their catalytic performances with Pd1-SACs on MoS2, Pd2@TiO2 (110),1,

65

and Au1/FeOx,64 we may conclude that much cheaper,

highly efficient, and stable TM1-SACs can be obtained via strain engineering on the single-layer MoS2 substrate. Consequently, we expect that elaborate experimental efforts can be stimulated to load applicable strain on the 2D 24


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material MoS2, for instance by fabricating the samples on other substrates with appropriate surface lattice mismatch. 4. CONCLUSIONS In summary, we performed first-principles calculations to investigate the substrate straining effect on stabilization of Pd single adatoms on defect-free MoS2 by tuning EMSI. It is found that on the pristine MoS2, the Pd adatoms prefer dimerization or clustering, rather than separation. However, formation of highly efficient SACs for the CO oxidation due to the enhanced EMSI can be realized under appropriate tensile strain of the MoS2 substrate. This is accompanied by a semiconductive-to-metallic phase transition. Such a costeffective strain engineering approach is also found to be applicable to much cheaper elements of Ag, Ni, Cu and Cr, which can be stabilized to SACs with high catalysis for CO oxidation. Moreover, the reaction barriers in the ratelimiting steps can also be modulated by the external strain, indicating that the catalytic selectivity of the present TM1-SACs/MoS2 can be optimized via the strain engineering approach. The present findings offer a new avenue to inhibit the transition metal atoms from clustering and sintering into NCs/NPs over twodimensional materials, at least at the initial stage, and provide clear guidance to the development of high efficiency and high loading density of SACs stabilized on defect-free substrates. ASSOCIATED CONTENT 25



Supporting Information The following files are available free of charge. AIMD simulations of MoS2, Average cohesive energy of TMN clusters (TM=Pd, Pt, Au, Ni, and Cr), Adsorption sites of TM atoms, Binding energy of O2 molecule, Electronic charge density analysis, MEP of CO oxidation. AUTHOR INFORMATION Corresponding Authors *E-mail:

[email protected]

*E-mail:

[email protected]

Author Contributions The manuscript was written through contributions of all authors. Shunfang Li, Hao He, Xiaoyan Ren, and Zhengxiao Guo conceived the central ideas and directed the project. Yandi Zhu and Ke Zhao carried out the calculations and drafted the manuscript. Haizhong Guo, Xiangmei Duan, and all the other authors discussed the results and polished the paper. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. 26


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ACKNOWLEDGMENT This work was supported by the NSF of China (Grants No. 11674289, 11804306, 11574167). The calculations were performed on the Supercomputer Center in Zhengzhou University, Zhengzhou University, Henan. REFERENCES (1)

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Strain Engineering of Defect-free Single-Layer MoS2 substrate for

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