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C: Surfaces, Interfaces, Porous Materials, and Catalysis m

n

Ni Mo (m+n=5) Clusters for Hydrogen Electric Reduction: Synergistic Effect of Ni and Mo on the Adsorption and OH Breaking of HO 2

Xue-Lian Zheng, Yuan-Yuan He, Jiu Chen, Di Gao, Yunhuai Zhang, Peng Xiao, and Wei Quan Tian J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b01281 • Publication Date (Web): 18 Mar 2019 Downloaded from http://pubs.acs.org on March 21, 2019

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The Journal of Physical Chemistry

NimMon (m+n=5) Clusters for Hydrogen Electric Reduction: Synergistic Effect of Ni and Mo on the Adsorption and OH Breaking of H2O Xue-Lian Zheng a§, Yuan-Yuan He a§, Jiu Chen a, Di Gao a, Yun-Huai Zhang a, Peng Xiao b and Wei Quan Tian *a a

Chongqing Key Laboratory of Theoretical and Computational Chemistry, College of Chemistry and Chemical Engineering,

Chongqing University, Huxi Campus, Chongqing, 401331, P. R. China.. b

College of Physics, Chongqing University, Huxi Campus, Chongqing, 401331, P. R. China.

§

The first two authors contribute equally to this work.

*Corresponding author, E-mail address: [email protected]

Abstract High efficient catalyst for hydrogen evolution reaction are crucial for hydrogen production. As potential catalysts for hydrogen evolution reaction, NimMon (m+n=5) clusters have been studied in the present work on their structures, stabilities, and chemical reactivity toward H2O adsorption, OH bond breaking, and H adsorption with density functional theory based method. Mo is the active site for H2O adsorption and the adsorption strengths vary slightly with NiMo ratio. On the other hand, the split H prefers to form a triangle with Ni and Mo stabilized by the synergistic effect of Ni and Mo, i.e. adsorption on Mo and activation (and stabilization) by Ni. Electro-reduction makes the OH bond breaking barrierless on Ni4Mo and this rationalizes the high catalytic efficiency of Ni4Mo in hydrogen evolution reaction (in agreement with experiment), and other H2O adsorption related reactions.

NiMo related alloy or clusters are such potential efficient catalysts for HER according to experimental investigations,2,7 as either the relatively weaker Ni-H and stronger Mo-H bond strength than that of Pt-H as indicated on the volcano chart8 deteriorates the catalytic activity of Ni or Mo toward HER. In a previous study,9 a series of Ni-based binarycomposites were studied on their electro-catalytic activities for the HER in alkaline-water electrolytic cells. The activities of those binary alloys rank in this order: NiMo > Ni-Zn > Ni-Co > Ni-W > Ni-Fe > Ni-Cr. The physical properties (e.g. size and morphology) have a strong influence on the HER catalytic activity of NiMo binary materials. A large number of NiMo bimetallic composites with various physical properties, such as NiMo alloy nanosheet array,10 nanowires,11 porous film,12 nanorods13, and nanopowders,14 and so on, have been studied, which exhibiting diverse and significant catalytic activities. Notably, in the study of NiMo nanopowders,14

1. Introduction The quest of high efficient catalyst for hydrogen evolution reactions (HER) gets urgent as the demand for clean and renewable energy rises high. As potential catalysts for HER, atomic and nano-metallocluster have been investigated experimentally and theoretically.1,2 The Pt-group metals probably are the most effective catalysts for HER.2 However, the high cost and rarity of Pt-group metals leads to extensive search for low-cost and efficient new catalysts for HER.3-6 According to the volcano chart (correlation of HER exchange current density with metal-H bond strrngth,2 proper metal-H bond strength facilitates the adsorption and desorption of H on metal which in turn results in high catalytic activity of metal catalyst.), alloys consisting metals from the two sides of the volcano could have comparable catalytic activity to that of Pt in HER because of synergistic effect, and 1

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pure nickel nanopowders synthesized using the same process revealed lower catalytic activity than NiMo nanopowders, which indicates that HER activity is improved by alloying with the surface of Mo atoms, as atomic and nano-size catalysts could have significantly improved catalytic efficiency.15,16 However, the mechanism of HER on NiMo related catalysts is far from being disclosed17,18 and the essential causes of activity improvement need to be explored, as only simple energy profile of H2O adsorption on NiMo alloy surface17 was outlined and only simple H2O adsorption on NiMo clusters was simulated18. Under neutral and alkaline conditions, the generally accepted mechanism of electrocatalytic hydrogen evolution—consisting of a Volmer-Tafel or Volmer-Heyrovsky mechanism—is as follows,19 H2O + e → Hads + OH Volmer Hads + Hads → H2 Tafel  Hads + H2O + e → H2 + OH Heyrovsky As mentioned above, the Volmer reaction serves as the first step in both reaction mechanisms and plays an extremely important role in the study of the HER mechanism, which involves the reduction of adsorbed H2O on the surface of the catalyst, and forms an adsorbed hydrogen intermediate and a hydroxide ion after reduction. To further understand the Volmer reaction in the HER mechanism on NiMo surface or clusters, in the present work, NimMon (m+n=5) clusters are used as model clusters to study the reactivity of NiMo nanoclusters toward H2O adsorption, O-H bond breaking, and adsorption of the split H in neutral or alkaline solution. The model selection is inspired by experiment that Ni4Mo alloy has high HER activity,10,17 where the Ni4Mo alloy affords ultralow η of –10 mV at –10 mA cm–2,20 and this HER activity is well comparable with the most of the promising earth-abundant electrocatalysts.21 The structure and morphology of nanocluster and the effects of pH are important on the catalytic activities of HER catalysts,22 and those effects and corresponding mechanisms will be studied in a

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separate work and could also be inferred from similar systems.23 Bigger NiMo clusters should have similar activity to that of NimMon (m+n=5). As HER is usually facilitated by photo-physical means or electro-chemical reduction, the H2O adsorption, O-H bond breaking, and adsorption of the split H are studied in reduced state for NimMon (m+n=5) clusters as well. For comparison, Mo5 and Ni5 are also investigated. NimMon (m+n=5) clusters are small enough to be encapsulated in a cage like metalorganic framework18 and large enough for H2O adsorption and OH bond breaking. The present work hopes to provide information on the mechanism of H2O initial adsorption and subsequent evolution on NiMo clusters for HER and also for reactions involving H2O adsorption on NiMo clusters in gas phase. Solvent effect is very important to the activity of metal clusters. After the initial reaction, the NiMo cluster could also be oxidized by O forming metaloxide bonds on clusters, which in turn result in new reaction mechanism for H2O adsorption and OH bond breaking. Such effect and the subsequent reactions would be studied in future work. 2. Computational details The partially occupied valence d orbitals in transition metal lead to multi-determinant wavefunction and complicated bonding in transition metal clusters, and this requires reliable methods for structure and property prediction. Wavefucntion based single determinant ab initio methods might not be suitable for transition metal cluster simulation. For this purpose, several density functional theory based (DFT) methods, including hybrid and general gradient approximations (GGA), are employed to predict the ground state of Mo2 and Ni2 in comparison with experiment (Table S1). The binding energy of metal dimers was calculated by the following formula: Eb = E(M2)  2E(M) 2


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Where Eb is the binding energy of M2 dimer (M represents Ni or Mo atom), E(M) is the energy of metal atom M and E(M2) is the energy of M2. Based on the availability and performance of basis sets, the LanL2DZ24 basis sets are used for Ni and Mo in all predictions. Mo2 has singlet ground state25 and Ni2 has mixed ground state with singlet and triplet.26 The GGA methods (BPW91,27,28 PW91PW91,28 and PBEPBE29) have better predictions on the bond length and binding energies for Mo2 and Ni2 than the hybrid methods (B3LYP,30,31 B3PW91,28,30 and PBE1PBE32). Other GGA method has similar performance on prediction of Ni2.33 The inferior performance of hybrid methods might come from the mixing of exact exchange. All those DFT (GGA and hybrid) methods predict triplet as the ground state for Ni2. The hybrid DFT methods predict the quintet as ground state for Mo2 [The stability of triplet of Mo2 lies between singlet and quintet and the binding energies of those three states are listed in Table S2]. PBEPBE has the overall best performance and it is used to predict NimMon clusters and HER of H2O on those clusters. Basis sets 6-31++G(d,p)34,35 are used for H2O. The binding energy of NimMon and adsorption energy of H2O on NimMon were computed using the following expressions: Eb = E(NimMon)  nE(Mo)  mE(Ni) Eads = E(NimMon-H2O)  E(NimMon)  E(H2O) where Eb is the binding energy of NimMon cluster, E(Mo) is the energy of Mo atom, E(Ni) is the energy of Ni atom and E(NimMon) is the energy of NimMon cluster. Eads is the adsorption energy of H2O on NimMon, E(H2O) is the energy of H2O and E(NimMonH2O) is the energy of NimMon with H2O adsorbed. The search for the most stable structure of all metal clusters starts with all possible geometries (except for linear conformation, as shown in Figure S1) of 5membered clusters from singlet to nonet to warrant that the most stable structure can be located. Vibrational frequency calculations are carried out to verify the nature of stationary points on potential

energy surface. Intrinsic reaction coordinate (IRC) simulations are performed for transition states to ensure that the transition state connects the correct reactant and product. The relative energies of stationary points on reaction pathway are evaluated with zero-point energy (calcualted through vibrational frequency calculations) and Gibbs free energy corrections with PBEPBE/6-31++G(d,p). All calculations are carried out with the Gaussian09 package. 36 3. Results and discussion 3.1. Relative stability and conformations The most stable structures of metal clusters are shown in Figure 1. Their binding energies and vertical electron affinities are listed in Table 1. The distorted trigonal bipyramid (DTB) conformation of Mo5 in singlet is the most stable structure and this is consistent with the previous predictions and the application of small core pseudopotential in predicting small Mo clusters was proposed in those works.37,38 The energy of triplet DTB Mo5 is only 3.70 kcal/mol higher than that of the singlet state. The most stable structure of Ni5 [square pyramid (SP)] is different from that predicted by B3LYP39 while similar to the one with GGA functionals.40,41 With the same method (PBE), the trigonal bipyramid (TB) conformation of Ni5 was predicted as the most stable structure.41 In the present work, the energy of SP Ni5 conformation is (0.18 kcal/mol) slightly lower than that of the TB conformation as predicted by PBEPBE/LanL2DZ, i.e. they have similar stabilities. For all clusters, only the most stable conformations are shown in Figure 1 and the other conformations with close energies are in Figure S2. As the Mo-Mo bond has stronger bonding strength than the Ni-Ni bond, the binding energies decrease with the number of Mo in those NimMon clusters (Table 1). 3



Mo5-singlet

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r1-3: 3.26 Å

r1-4: 3.34 Å

r1-4: 3.31 Å

r4-5: 3.27 Å

r2-3: 3.37 Å

r2-3: 3.31 Å

NiMo4-singlet

Ni2Mo3-singlet

Ni3Mo2-triplet

Ni4Mo-heptet

Ni5-heptet

Figure 1. The most stable structures of bimetallic NimMon (m+n=5) clusters. The number on the structure in the upper row is bond length in Å. The number in parentheses on the structure in the lower row is natural atomic charge and the number outside parentheses is atomic net spin. The aquamarine and deep purple balls represent Mo and Ni atoms, respectively. Table 1. The multiplicity, binding energy, symmetry and vertical electron affinities (VEAs) of the most stable conformations of NimMon (m+n=5) clusters. Clusters

Multiplicity

Eb(kcal/mol)

Mo5(DTBa)

1

NiMo4(DSPb)

1

Ni2Mo3(TBc)

1

Ni3Mo2(DTB) Mo(SPd)

Ni4

Ni5(SP) a

Symmetry

VEAs(eV)

302.20

C2

1.05

295.60

CS

1.02

281.60

CS

1.17

3

276.92

C2

1.23

7

258.01

C2V

1.22

7

259.39

1.51

C4V

DTB: distorted trigonal bipyramid. DSP: distorted square pyramid. TB: trigonal bipyramid. SP: square pyramid. b

c

d

3.2. Bonding pattern and bonding nature in NimMon distortion from regular square pyramid and the Ni-Mo bonds are around 2.50 Å. In those NiMo clusters, the Mo-Mo and Ni-Ni bonds essentially shorten as the number of Ni atoms increases. Ni-Mo bond varies in the range from 2.40 to 2.70 Å. The bonding pattern is reflected by the overall bond lengths as shown in Figure 1. The binding energies of NimMon clusters decrease with the number of Mo atoms, i.e. Mo-Mo bonding is stronger than Ni-Ni bonding. Mo-

Starting from Mo5 to Ni5, the geometries of NimMon clusters change from DTB to SP, varying with NiMo ratio. In Mo5, each Mo atom essentially has two short Mo-Mo bonds (around 2.30 to 2.40 Å) and two long Mo-Mo bonds (c.a. 2.70-2.90 Å). In Ni5, all Ni-Ni bonds are around 2.34 Å. The Ni-Ni bond distances in Ni2Mo3, Ni3Mo2, and Ni4Mo are similar. The most stable conformation of NiMo4 has conspicuous 4


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dominant NimMon clusters prefer low multiplicity while Ni-dominant clusters prefer high multiplicity as indicated by the multiplicity of NimMon clusters (Table 1). On the other hand, the electron affinity of Ni-dominant NimMon clusters is larger than that of Mo-dominant NimMon clusters, and it increases with the number of Ni atoms in those NimMon clusters (Table 1), i.e. electrons go to Ni upon reduction. To understand the bonding pattern in those NimMon clusters, diatomic clusters Mo2, Ni2, and NiMo were scrutinized to reveal the Mo-Mo, Ni-Ni, and Ni-Mo bonding nature (Table 2 and Table S3). In Mo2, the 4d55s1 configuration of Mo forms six bonds with singlet electronic states, and this bonding explains the large binding energy of Mo2. The six-bonding also explains the two-short (double bond) and two-long (single bond) Mo-Mo bonding pattern of Mo in Mo5. In Ni2, the 3d94s1 configuration (ds hybridization) of Ni forms overall double bond (dominant with σ bond), and leads to the essentially equivalent single Ni-Ni bonds in Ni5 with high multiplicity. In NiMo, the 4d55s1 configuration of Mo and the 3d94s1 configuration of Ni form essential Ni-Mo σ bond with high multiplicity. Quintet and heptet NiMo have similar stabilities with similar bonding (Table 2). Such bonding feature of Ni-Ni, Mo-Mo, and Ni-Mo leads to the bonding patterns in NimMon, i.e. in those Ni rich NimMon clusters, the Ni-moiety keeps the NiNi bonding feature, and the Mo-moiety in those Mo rich NimMon clusters keeps Mo-Mo bonding feature. Such bonding pattern is also revealed by the conformation and multiplicity of those clusters. According to the electronic configuration of Mo and Ni, a small amount of charge transfer from Mo to Ni occurs in NiMo. The partially occupied 4d orbitals in Mo can accept electrons and it would be active center toward electron rich reagent.

The most stable structures of adsorption of H2O on NimMon clusters are shown in Figure 2 (other isomers are shown in Figure S3). H2O adsorbs on Mo in all NimMon clusters because of the electron deficiency of Mo due to charge transfer from Mo to Ni, and the lone pair electons of H2O form bond with the available d orbial of Mo. In NiMo4, H2O adsorbs on one Mo atom bonded to Ni with O-Mo bond distance of about 2.30 Å. The adsorption of H2O on the end Mo atom (not connected to Ni) is 1.32 kcal/mol higher in energy than the most stable adsorption (in Figure S3). According to the O-H bond distances (shorter than 1.0 Å and the OH bond distance in H2O is 0.973 Å predicted at the same level of theory) in the adsorbed H2O and adsorption energies (Table 3), the adsorption of H2O on NimMon clusters is physical adsorption. The adsorption of H2O on Ni is also physical adsorption with O-Ni distance of about 2.10 Å. The adsorption energy does not change much with the number of Mo atoms (Table 3), i.e. the adsorption activity of NimMon does not change much with the composition of NimMon clusters. However, as Mo is the active adsorption center, the increase of coverage of Mo on alloy surface could enhance the reactivity of the catalyst. 3.4. OH bond breaking on NimMon In neutral state, the reaction barrier of OH bond breaking is larger than 3.00 kcal/mol on all NimMon clusters. The OH bond breaking on Ni4Mo has the smallest (3.45 kcal/mol) and that on Ni5 has the largest (15.38kcal/mol) activation barrier. Ni2Mo3 and Ni3Mo2 have similar activation barriers (about 10.0 kcal/mol). The distance from the split H to O ranges from c.a. 1.28 Å (Mo5) to 1.61 Å (Ni5), and the bonding pattern of the split H with metal atoms in

3.3. Adsorption of H2O on NimMon

5



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Table 2. The bond length, bonding nature, electronic configuration and vertical electron affinities (VEAs) of Mo2, Ni2, NiMo, and NiMo predicted at PBEPBE with the LanL2DZ basis Set. Clusters

Bonding Nature

Bond Length(Å)

α

Electron Configuration

β

VEAs (eV)

σ(Mo[5s]) δ(Mo[4dxy]) Mo2(singlet)

δ(Mo[4dx2-y2])

1.99

σ(Mo[4dz2])

4d5.025s1.005p0.01

0.65

3d8.954s1.004p0.04

0.74

π(Mo[4dyz]) π(Mo[4dxz]) σ(Ni[4s]) Ni2(triplet)

2.12

π(Ni[3dxz])

σ(Ni[4s])

δ+σ(Ni[3dx2-y2+3dz2]) σ+(δ)(Ni[4s+3dx2NiMo(quintet)

2.40

y2]+Mo[5s+4dx2-y2])

σ(Ni[4s)+3dz2]+Mo[5s+4dz2]

Ni[3d8.884s1.164p0.03]

σ+δ(Ni[3dx2-

)

Mo[4d5.015s0.875p0.03]

0.88

y2+3dz2+4s]+Mo[4dz2+4dx2-y2])

NiMo(heptet)

2.44

NiMo─(sextet)

2.24

σ(Ni[4s+3dz2]+Mo[5s+4dz2])

Ni[3d8.774s1.254p0.04]

σ(Ni[4s+3dz2]+Mo[5s+4dz2])

Mo[4d5.095s0.825p0.02]

0.88

Ni[3d9.084s1.414p0.095p0.01] Mo[4d5.105s1.145p0.18]

Table 3. The multiplicity, adsorption energy, adiabatic electron affinities (AEAs) and vertical electron affinities (VEAs) of the lowest energy structures of H2O adsorbed on bimetallic NimMon clusters. Clusters

Multiplicity

Eads(kcal/mol)

AEAs(eV)

VEAs(eV)

Mo5-H2O

1

18.53

0.89

0.80

NiMo4-H2O

1

19.90

1.13

1.04

Ni2Mo3-H2O

1

16.15

1.14

0.98

Ni3Mo2-H2O

3

15.95

1.31

1.10

Ni4Mo-H2O

7

17.21

1.26

1.10

Ni5-H2O

7

15.98

1.53

1.33

6


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a

b

d

c

e

f

Figure 2. The reaction pathway of adsorption of H2O and O-H bond breaking on NimMon (m+n=5) clusters in neutral (black line) and reductive states (blue line). The numbers on the reaction path are the relative energies to NimMon-H2O (NimMon-H2O), and ∆E is the reduction energy in kcal/mol. All relative energies are corrected with zero point energies (The relative energies corrected with Gibbs free energy are in Figure S5). The black number on the structure is bond length in Å, and the blue number on the transition state (TS) structure is the distance between the split H atom and the O atom after the OH bond is broken in H2O. The aquamarine, deep purple, red and black balls represent Mo, Ni, O and H atoms, respectively.

transition state structure varies with clusters. In the transition states of Mo5 and Ni2Mo3, the split H bonds to one Mo atom. While on the other four clusters, the

split H forms a triangle with two metal atoms in the transition state. After OH bond breaking, the adsorption of the split H on NimMon clusters varies. 7



On Mo5, the split H adsorbs on the same Mo atom as OH does (Mo3 in Figure 2a). On NiMo4, Ni2No3, and Ni4Mo, the split H forms a triangle with the Oadsorbed metal atom and one of its neighbour metal atom. Through forming a triangle with two metal atoms, the split H bonds to the other Mo atom (Mo2) and the top Ni atom (Ni3 in Figure 2d) in Ni3Mo2. The split H bonds to three Ni atoms in Ni5. All OH bond breakings are exothermal and the energy released upon OH bond breaking on the Ni5 cluster is the smallest (4.60 kcal/mol), and this released energy is not enough to overcome the bond-breaking activation barrier (15.38 kcal/mol). The energies released from OH bond breaking on the other clusters are much larger than the activation barrier. In all NimMon clusters, Mo serves as the activative center for H2O adsorption and OH bond breaking. Among all six clusters, the OH bond breaking on Ni4Mo (3.45kcal/mol) and NiMo4 (3.92kcal/mol) has the smallest reaction barrier. On Ni4Mo and NiMo4, the split H (electron deficient) bonds to Ni (electron rich) and Mo in both the transition state and the product. Such bonding stabilizes the transition state and the product, thus leading to small reaction barrier and stable product (The molecular orbitals involving bonding between H2O and Ni4Mo are in Figure S4). The adsorption of H2O on Mo and the OH bond breaking activation (and stabilization) on Ni (with the aid of charge transfer from Ni to Mo as shown in Figure 1) leads to the synergistic effect of NiMo alloy on the H-splitting of H2O in hydrogen evolution reaction. This might explain the synergistic effect of NiMo alloy in the Trasatti’s HER volcano plot2,8, and this synergistic effect might also be the effect of alloying.

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energy is calculated by the expression ∆E = E[(H2O-NimMon)] + E[Co(cp*)2+]  E(H2ONimMon)  E[Co(cp*)2] (the chemical equation is H2O-NimMon + Co(cp*)2 → (H2O-NimMon) + Co(cp*)2+). All strucutures in those calcualtions were optimzied separately. The reduction stabilizes all (H2O-NimMon) and the stabilization energy decreases with the number of Mo atoms (Figure 2). Upon reduction, the O-M and O-H bonds

Figure 3. The stabilization of reduction on H2O adsorption and OH bond breaking on Ni4Mo. The numbers on the reaction path are the relative energies to Ni4Mo-H2O (Ni4Mo-H2O), and ∆E is the reduction energy in kcal/mol.

3.5. Stabilization by reduction

The black number on the structure is bond length in Å, and the blue number on the transition state (TS) structure is the

To simulate the reduction of the adsorption of H2O on NimMon (m+n=5), decamethyl cobaltocene Co(cp*)2 is used as the reductant.42,43 The reduction

distance between the split H atom and the O atom after the OH bond is broken in H2O. The aquamarine, deep purple, red and black balls represent Mo, Ni, O and H atoms, 8


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slightly different from its counterpart in neutral state. The energy released from OH bond breaking is larger in the reductive state than that in neutral state on all metal clusters. In other words, reduction lowers the OH bond breaking barrier and stabilizes the dissociated product. If the systematic performance of the present DFT method is taken into account, all those reaction barriers are very low and such reaction barrier comparison is qualitative.

respectively.

get slightly longer than their counterparts in neutral state and the relative position of the adsorbed H2O changes slightly [e.g. in (Ni2Mo3-H2O), (Ni3Mo2H2O), and (Ni5-H2O)], and it is physical adsorption, i.e. the OH breaking needs to be activated. In reductive state, the OH bond breaking activation barrier and the adsorption of the split H on metal cluster are different from those in neutral state. The OH bond breaking activation barrier is lowered by reduction. The stabilization of reduction on the adsorption of H2O and the transition state of OH bond breaking can be rationalized by the destination of the electron upon reduction. In the adsorption structure of H2O on Ni4Mo and its transition state of OH bond breaking, the reducing electron goes to the spin-down lowest un-occupied molecular orbital (LUMO) [involving orbital overlap between two H and two Ni atoms in the adsorption structure of H2O on Ni4Mo, and overlap between the split H and two Ni atoms in the transition state as shown in Figure 3]. According to the activation energies, the stabilization on the transition state is stronger than that on the adsorption thus lowering the reaction barrier. The largest reaction barrier is 7.46 kcal/mol on Ni5 and the second largest reaction barrier is 5.91 kcal/mol on Ni3Mo2 in reductive state. The reaction barrier on Mo5, NiMo4, and Ni2Mo3 is about 2.07, 0.88, and 2.00 kcal/mol respectively. On the other hand, the reaction barrier on Ni4Mo is 0.27 kcal/mol, i.e. barrierless. Thus, upon reduction, the OH bond breaking on Ni4Mo is spontaneous. This is consistent with the experimental observation that Ni4Mo alloy is a good catalyst for hydrogen evolution reaction.10,17,20,21 The very low reaction barrier might be due to the strong bonding of the dissociated H2O with Ni4Mo thus stabilizing the transition state (Figure S4A) and the relatively small structure change from the reactant (Figure 3). The conformation of the transition states in reductive state change slightly and the adsorption of the split H on metal clusters (Mo5, NiMo4, Ni3Mo2, and Ni5) is also

4. Conclusions and perspectives The structure, bonding pattern and reactivity of Ni and Mo diatomic clusters and NimMon (m+n=5) have been studied with density functional theory based method. Mo tends to form strong multiple bonds with Mo in low multiplicity and bond with Ni in high multiplicity, and Ni tends to form single bond with Ni. Such bonding pattern dictates the bonding and conformation of NimMon (m+n=5) clusters. Mo is the active bonding site for the adsorption of H2O on NimMon clusters with moderate (about 20 kcal/mol) adsorption energy. Ni participates in the adsorption of the split H of H2O after O-H bond breaking with essential H-Mo-Ni triangular conformation. The adsorption on Mo and activation (and stabilization) by Ni, the synergistic effect, could be effect of NiMo alloying toward HER. Electroreduction stabilizes the H2O adsorption on NimMon clusters and lowers the O-H bond breaking barrier, thus facilitating the hydrogen evolution reactions. The synergistic bonding of Ni and Mo with H2O and H, along with the stabilization from electro-reduction make NiMo clusters efficient catalysts in electroreduction aided hydrogen evolution reactions. After the adsorption of H2O and the O-H bond breaking, NiMoO nano-local structure could form on NiMo clusters. The NiMoO nano-local structure could also serve as active center for hydrogen evolution reactions.2 NiMo nanoclusters could be hosted by metal-organic framework18 or substrates 9



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and the effects of substrates could also be important as the shape of nanocluster could change upon adsorption. The investigation on the H2 formation mechanism (including H2 formation and desorption), the activity of NiMoO nano-local structure, and effects of substrate in hydrogen evolution reactions deserve further investigations. Supporting Information The Supporting Information is available free of charge on the ACS Publications website http://pubs.acs.org/. Structures of Ni2, Mo2, NiMo, starting and optimized structures of NimMon (m+n=5), structures of H2O dasorption on NimMon (m+n=5), the reaction profiles and frontier molecular orbitals of the adsorption of H2O on NimMon (m+n=5) in neutral and reductive states. The full citation of reference 34. Acknowledgements This work is supported by the Open Project of Key Laboratory of Polyoxometalate Science of Ministry of Education (NENU) and the State Key Laboratory of Supramolecular Structure and Materials (JLU) (SKLSSM201818). References (1) Greeley J.; Jaramillo T. F.; Bonde J.; Chorkendorff I. B.; Nørskov J. K. Computational High-Throughput Screening of Electrocatalytic Materials for Hydrogen Evolution. Nat. Mater. 2006, 5, 909–913. (2) Roger I.; Shipman M. A.; Symes M. D. EarthAbundant Catalysts for Electrochemical and Photoelectrochemical Water Splitting. Nat. Rev. Chem. 2017, 1, 0003. (3) Li Y. G.; Wang H. L.; Xie L. M.; Liang Y. Y.; Hong G. S.; Dai H. J. MoS2 Nanoparticles Grown on Graphene: an Advanced Catalyst for the Hydrogen Evolution Reaction. J. Am. Chem. Soc. 2011, 133, 7296–7299. 10


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