New Insight into Mechanism of Hydrogen Evolution Reaction on MoP

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New Insight into Mechanism of Hydrogen Evolution Reaction on MoP (001) from First Principles Yayun Zhang, Hanwu Lei, Dengle Duan, Elmar Villota, Chao Liu, and Roger Ruan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b03976 • Publication Date (Web): 31 May 2018 Downloaded from http://pubs.acs.org on May 31, 2018

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ACS Applied Materials & Interfaces

New Insight into Mechanism of Hydrogen Evolution Reaction on MoP (001) from First Principles Yayun Zhang1, Hanwu Lei1*, Dengle Duan1, Elmar Villota1, Chao Liu2, Roger Ruan3. 1

Department of Biological Systems Engineering, Washington State University, Richland, WA 99354-1671, USA 2

Key Laboratory of Low-grade Energy Utilization Technologies and Systems, Ministry of

Education, College of Power Engineering, Chongqing University, Chongqing 400030, China 3

Center for Biorefining and Department of Bioproducts and Biosystems Engineering University of Minnesota, 1390 Eckles Ave., St. Paul, MN 55108, USA *Corresponding author. Tel.: +1 509 372 7628; fax: +1 509 372 7690. E-mail address: [email protected] (H. Lei).

KEYWORDS: MoP (001), hydrogen evolution reaction, water splitting reaction, catalytic reaction mechanism, surface defect, substitutional doping

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ABSTRACT: The molybdenum phosphide-based catalysts have recently exhibited excellent catalytic activities for hydrogen evolution reaction (HER) in wide pH range conditions, the intrinsic reaction mechanism, on the other hand, has not been well established. Herein, by employing the MoP as the prototypical molybdenum phosphide-based catalyst, HER activities in both acid and neutral conditions were studied by conducting periodic density functional theory (DFT) calculations.. Thermodynamic analysis of hydrogen atoms absorbed on both P- and Moterminated surfaces were compared, as well as all the reaction energy and activation energy barriers for reactions involved in the HER process. Calculation results revealed that, in an acid condition, the Volmer-Heyrovsky and Vomer-Tafel reaction mechanisms were dominated on the P-terminated and Mo-terminated catalyst surfaces, where Heyrovsky and Vomer reactions were the rate-determining step, respectively. Additionally, water splitting was introduced to current reaction mechanism and a small reaction activation energy barrier was revealed on the Pterminated surface. Besides, a relevant small activation energy was obtained in Tafel reaction on the defect of the P-terminated surface in a neutral solution. Theoretical results proved that HER could take place readily on both P- and Mo-terminated catalyst surfaces via different reaction mechanisms in the acid condition from the view of atom scale. More important, computational results uncovered that HER could also occur on the P-terminated surface with the assistance of surface defect in the neutral condition, which shed new light on HER mechanism on transition metal phosphite-based catalysts. The doping effect on HER activity was further investigated in theory and calculation results indicated that catalytic performance could be improved by substitutional doping of the Mo atom with metals like Mn and W.


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1.INTRODUCTION The increasing rate of consumption of traditional fossil fuel and the environment pollution induced by usage of carbon-emitting fuels have triggered people to seek renewable, abundant and eco-friendly energy resources in order to obtain a healthy, stable and sustainable development. Hydrogen, a pivotal chemical widely used in industrial processes, is regarded as a promising energy carrier, which, for example, can be consumed in the fuel cell technology that possesses a more green chemistry than other existing ways of hydrogen utilization.1-2 Current hydrogen supply is, however, mainly derived from fossil fuels. Therefore, developing methods to achieve the production of hydrogen economically from low-cost and renewable energy resources is of critical importance. Electrochemical processes, e.g. water splitting reactions derived from solar photoelectrochemical and electrolysis, have attracted continuous attentions as alternative routes to generate hydrogen from the earth-abundant resource water, where water splitting and hydrogen evolution reaction (HER) are usually involved.3-5 In order to obtain the hydrogen molecule, a catalyst is required to achieve high energy efficiency by lowering the overpotential that is necessary to drive the HER.6-7 Precious metal platinum is regarded as the best catalyst for HER, which requires very small overpotentials.8 Unfortunately, the scarcity and high cost of Pt limits its extensive usage and large-scale applications in industry,9 leaving the challenging of exploring earth-abundant catalysts for HER reaction to replace the current benchmark platinum. Over the past few decades, a great deal of effort has been devoted to finding the efficient catalyst in HER reactions. Non-noble-metal alternatives like nickel and nickel alloy, although widely used commercially as HER catalysts,10-11 are naturally not stable in acidic solutions, where proton exchange membrane (PEM)-based electrolyzers can be employed to dramatically increase the catalyst performance for HER.12-13 Hence, finding acid-stable and non-noble-metal


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HER catalysts turns to the hot research area of hydrogen generation. To date, several classes of candidate materials have been unveiled, such as sulfides,14 selenides,15 borides,16carbides17, and nitrides.18 These satisfied achievements, however, did not hinder steps of questing for more facile, efficiency and earth-abundant HER catalysts. As a consequence, transition metal phosphides (TMPs) with good electrical conductivity were awakened and have attracted a wide range of interests from worldwide researchers thereafter.19-20 Inspired by the previous experience that molybdenum phosphide (MoP) showed catalytic activities in the hydrodesulfurization (HDS) reaction,21 Jiang et al tested the MoP as an electrocatalyst in electrochemical hydrogen evolution reactions. Interestingly, their experimental results uncovered that MoP exhibited an excellent catalytic performance with smaller Tafel slope than other reported HER catalysts.22 In a similar work, Mo, MoP, and Mo3P were conducted and compared as HER electrocatalyst, where it was elucidated that phosphorization could potentially modify the molybdenum metal. Activities and stabilities of the modified catalysts could be altered by varying degrees of phosphrization.23 The simultaneous theoretical calculation completed by DFT studies confirmed their conclusion that the MoP was formed via phosphorization of molybdenum, on which the adsorption energy of H atom was near zero at a certain H coverage. At the same year, MoP was again reported as the promising HER catalyst, in which a closely interconnected network of MoP nanoparticles was developed.24 Later, the molybdenum phosphide three-dimensional (3D) structure (MoP2) and amorphous MoP were synthesized and also presented commendable HER catalytic activity.25-26 With continuous investigations participating in, combing MoP with other special materials and MoP doping with metal or non-metal elements also emerged and exhibited intriguing outcomes. Specifically, the MoP and MoP-graphite nanosheets were constructed through ball milling commercial compounds as efficient electrochemical materials, on which


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hydrogen was generated via hydrogen reduction mechanism.27-28 Anchoring MoP particles on reduced graphene oxide to form a highly uniform structure also presented superior catalytic activity for HER.29-30 Analogously, the hierarchical MoS2@MoP core-shell heterojunction electrocatalysts were synthesized to act as HER catalyst over a broad pH range.31 In the neutral medium, semimetallic MoP2, MoP nanosheet array, and cluster-like MoP anchored on reduced graphene oxide also exhibited HER activities, which revealed the potential of conducting HER reactions in acid and alkaline free solvents.28-29, 32 Besides, the most recent work conducted by Lee et.al. anchored the sulfur and nitrogen dual-doped molybdenum phosphide on graphene to obtain a hybrid electrocatalyst, which showed higher activity and stability for HER than most MoP-based catalysts in both acidic and basic electrolytes as a consequence of enhancing electron conductivity of MoP and stabilizing small MoP nanoparticles contributing to its activity and stability.33 The doped MoP with other elements also presented improved HER catalytic performance.34-36 This was in return stimulated the exploration of 2D metal sulphides (MS) being the active HER catalysts conducted by Lau et.al theoretically since sulfur has a similar chemical character to that of phosphorus, which pointed out a new direction towards to synthesis of efficient catalysts for HER.37-39 In addition, it is worth noting that MoP or P-containing materials also demonstrated the potential of being the active catalyst for photocatalytic hydrogen evolution, which further broadened the applications of TMPs in hydrogen generation processes.32, 40 The discovery of MoP-based HER catalysts opened a new window to achieve the research goal aforementioned and attracted more attention to excavating potentials of TMPs being the active HER catalysts in a wide pH range.41-42 In spite of the existing experimental advances, the inner reaction mechanism and pathways involved in HER processes on MoP-based catalysts


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remain unclear, especially in the neutral solvent, which impedes the further development of obtaining better MoP-based or TMPs catalysts for HER to a large extent. Density function theory (DFT) method, a quantum chemistry-based computational methodology, is a powerful tool to describe the atomistic reaction processes, which is extensively applied in various aspects of scientific research, especially in interfacial science.43 Nevertheless, few theoretical studies, however, have been reported hereunto to elucidate the mechanism of HER catalytic reaction on the MoP and MoP-based catalysts surface at the atomic scale and these studies mainly adopted the H adsorption free energy as the criterion merely.23, 25, 41 Besides, a complete HER pathway on MoP catalyst has not been reported, leaving the full picture of catalytic reactions being blank and mystical. More important, the molybdenum phosphide-based catalysts has emerged HER activities in the neutral solvent, thus it would be in high demand to explore whether HER can occur on untreated MoP catalyst surface and what catalytic processes will be involved in the neutral condition. DFT study was therefore used in the present work to build up a comprehensive and systematical understanding of HER processes on MoP surface. Additionally, the effect of defect on the surface was predicted by calculations as well as exploring tunable HER activity by doping with transition metals, which may helpful to promote the experimental synthesis of new high-efficient HER catalysts based on the MoP system. 2. COMPUTATIONAL MODEL AND METHODS In the present study, all DFT calculations were performed by employing the generalized gradient approximation (GGA), and the exchange-correlation energy of interacting electrons were determined by the Perdew-Burke-Ernzerhof (PBE) functional,44 which were implemented in the QUANTUM ESPRESSO package.45 The Gaussian smearing method was used to describe the total energy and the smearing width was set to 0.05. Electron-ion interactions were calculated


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based on ultrasoft pseudopotentials, and the kinetic energy cutoff was set to 400 eV, which is sufficient for the plane wave expansion. The Monkhorst-Pack method with the centered k-point grid (5×5×1) was used for integration in the Brillouin zone for surface calculations. The structural optimization was continued until the residual forces have converged to less than 0.01 eVÅ-1, and total energy to less than 1.0×10-6 eV. The climbing image nudged elastic band (CINEB) method was adopted for searching energy barrier of each elemental reaction on the surface. Five images were inserted between the initial and final states to search the reaction route. The MoP (001) surface was selected in present work since previous theoretical and experimental studies have supported assumptions of MoP (001) facet stability.23,

25

The

optimized unit cell parameter of MoP (a=b=3.229, c=3.213, r=120o) is highly consist with experimental data (a=b=3.231, c=3.207, r=120o).46 The MoP (001) surface was modeled using a six-atom-layer 2×2 super surface slab with two different (001) surface, namely, one with Mo as top surface called Mo-terminated (Mo-t) and the other with P as the top surface called Pterminated (P-t). The vacuum layer was set to ~15 Å thick perpendicular to the catalyst surface to avoid artificial interactions from its periodic images. The lowest one layer atoms were selected and fixed while other atoms were allowed to move during the geometry optimization in the whole calculations. In order to ensure the reliability of the computational model, adsorption energy of hydrogen on different layer number for the frozen part was calculated and results indicated that slight changes were obtained with variation in frozen part (Figure S1). Therefore, the model slab employed here was reasonable in elucidating catalytic surface reactions on MoP (001). The differential adsorption energy of hydrogen atom can be defined by the equation:


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∆E H = E (MoP + nH) − E (MoP + ( n − 1)H) −

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1 E (H 2 ) 2

(1)

where E (MoP + nH) , E (MoP + ( n − 1)H) , and E (H 2 ) are total energy of MoP with n hydrogen atoms adsorbed on the surface, the total energy of MoP with (n-1) hydrogen atoms adsorbed on the surface, and the total energy of a hydrogen molecule in the gas phase, respectively. The Gibbs free energy of the adsorbed hydrogen atom is calculated by:

∆GH = ∆EH + ∆EZPE − T ∆S H (2)

where ∆EH is adsorption energy of hydrogen atom described above, ∆EZPE is the

zero-point energy correction for hydrogen adsorption, which is determined by:

( n −1) H nH ∆EZPE = EZPE − EZPE −

1 H2 EZPE 2

(3)

( ) nH H2 where EZPE , EZPE and EZPE represents the zero-point energy of n adsorbed hydrogen atoms, n −1 H

zero-point energy of (n-1) adsorbed hydrogen atoms on MoP surface and gas phase hydrogen i 1 molecule, respectively. Each zero-point energy can be obtained by the equation ( ∑ hν i ), in 1 2

which h is Plank constant andν i is vibrational frequency obtained from frequency calculation. The calculated vibrational frequencies of adsorbed H on P-t surface are 2313, 601, and 600 cm-1, with negligible variation for different H coverage. Similarly, those of adsorbed H on Mo-t surface are 952, 947 and 833 cm-1, which have less difference with various H coverage. For an free hydrogen molecule, the calculated vibrational frequency is 2730 cm-1. Therefore, the ∆EZPE can be simplified as 0.13 eV and 0.08 eV on P-t and Mo-t surface, respectively. As for ∆S H , it 1 can be obtained by ∆S H ≅ − S H0 2 , where SH0 2 is the entropy of a hydrogen molecule in the gas 2


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phase at the standard condition (~130 J ⋅ mol −1 ⋅ K −1 ). Thus, ∆GH can be rewritten as ∆GH = ∆EH + 0.33 and ∆GH = ∆EH + 0.28 , where 0.33 eV and 0.28 eV are the contribution from

combination of ZPE and entropy at 298 K for P-t and Mo-t surface, respectively. The activation energy barrier of the elemental reaction on catalyst surface is based on the transition theory and defined as the equation: ∆Eb = ETS − ER

(4)

where ETS and E R presents the total energy of transition state and the reactant of a reaction, respectively.

3.RESULTS AND DISCUSSIONS Thermodynamic analysis of hydrogen atom adsorbed on MoP (001) surface. Previous studies preferred to use the criterion of describing HER activity by comparing the ∆GH of atomic hydrogen adsorbed on the catalyst.25, 41, 47-48 Specifically, ∆GH should be close to zero in an optimal HER activity, that is, the free energy of adsorbed H is approximated between the reactant and product. Large negative ∆GH will cause a strong binding between adsorbed hydrogen atom and the catalyst surface, result in the hard hydrogen generation and releasion. High positive ∆GH, on the other hand, will present a weak bonding between protons and the catalyst surface. Therefore, both of them will exhibit slow HER kinetics. In the present work, the hydrogen atom adsorption on MoP (001) was investigated first. The calculation results indicate that the most favorable adsorption sites are top and hollow2 on P-t and Mo-t surface (Figure S2 and Table S1), respectively. Besides, the top absorption site is the most activate site and up to four H atoms can be adsorbed on the studied P-t surface. So that the H coverage can be in the range of 1/4~4/4. On the contrary, up to 12 H atoms can be adsorbed onto top and hollow sites


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on Mo-t surface as shown in Figure S3. The corresponding H coverage hence varies from 1/12 to 12/12. The adsorption energy ∆EH and subsequently calculated ∆GH as a function of surface H coverage are shown in Figure 1. It can be seen that the adsorption energy is close to zero until the H coverage is 4/4. Taking ZPE and entropy into consideration, a free energy change near zero takes place with H coverage of 75 % (the ∆GH value is -0.06 eV) on P-t surface. Therefore, it is indicated that the HER is favored at the hydrogen coverage of around 75 %, which is in agreement with previous theoretical results.23, 25 Besides, the hydrogen atoms prefer to bind to the top site of P due to the high adsorption energy, regardless of the H coverage. On the other hand, the adsorption energy is around zero when the H coverage is no less than 0.4 on the Mo-t, indicating a weak interaction between H and surface. The free energy changes on Mo-t surface have a similar trend that ∆GH will be around zero with H coverage no less than 0.4, revealing a proper HER activity with initial H coverage. It should also be noted that a slight energy fluctuation occurs between 0.4 and 0.8 H coverage on Mo-t surface, which is caused by interaction among adsorbed hydrogen atoms. Therefore, based on the aforementioned ∆GH analysis, one can speculate that hydrogen revolution reaction will most likely occur on both P-t surface and Mo-t surface with the optimal activity at around 75 % and 40~90 % H coverage, respectively.


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0.3

(a)

0.9

0.0

-0.3

-0.6 ∆Ε Η ∆GH

-0.9 0.25

0.50

0.75

1.00

∆ΕΗ and ∆GΗ (ev)

∆ΕΗ and ∆GΗ (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 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(b)

0.6 0.3 0.0 -0.3 -0.6

∆ΕΗ

-0.9

∆GH

0.0

H coverage

0.2

0.4

0.6

0.8

1.0

H coverage

Figure 1. Adsorption energy (∆EH) and Gibbs free energy (∆GH) along with various H coverage on P-t surface (a) and Mo-t surface (b).


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Figure 2. Views of electron charge density differences for the adsorption of H atoms on P-t and Mo-t surfaces. P-t surface: (a)~(d) with 1/4~4/4 H coverage, respectively. Mo-t surface: (e)~(h) with 1/12, 2/12, 4/12, and 12/12 H coverage, respectively. Top and bottom panel represent top and side view, respectively. Color code: P: purple; Mo: dark green; H: grey. Charge accumulation and depletion are depicted by the yellow and light blue regions, respectively. The isosurface levels are set to ±0.0025 e bohr-3. In an attempt to obtain a deep understanding, herein, the charge density differences induced by adsorbed hydrogen atoms on MoP (001) surface were discussed, as shown in Figure 2. For H atoms adsorbed on P-t and Mo-t surfaces, the charge density difference, ∆ρ=ρsys −ρsur −ρH, where

ρsys, ρsur , and ρH are the charge densities of the catalytic surface/H slab system, the clean surface slab, and the H atom (s), respectively. Figure 2 (a)~(d) describe the charge density differences of P-t/H system with H coverage of 1/4~4/4, respectively. From these redistributions, it is found that charge transfer mainly takes place among the top two atom layers of the slab system and adsorbate (s), which is in agreement well with former calculation results about hydrogen adsorption energy on modeled slab with various frozen parts. Both of them reveal that atom interaction tends to occur between the adsorbed H atom and first two atom layers of the system during the surface catalytic reactions. When there is a 1/4 H coverage on P-t surface as shown in Figure 2(a), a accumulation (yellow) forms between adsorbed H and surface P atoms, indicating a bonding generated between them. A depletion (blue) of the electron density outside the H atom occurs due to the rearrangement of electron towards the bond of H and P. Besides, a significant electron depletion is observed around the upside of P atom because of net bonding contribution. It should be noted that the distance between the P (with H adsorbed ) and the second Mo layer shortens, inducing a electron density arrangement among the P and three Mo atoms nearby. With


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the H coverage increase on the P-t surface, the volume of electron depletion on both H and P atoms decrease gradually and that of electron accumulation also experiences a slight decrease on each adsorbed H atom, indicating a gradually decreased bonding strength between P and H, which consists with adsorption energy drop with increasing H coverage. As for the Mo-t surface (shown in Figure 2 (e)~(h)), the electron transfer mainly occurs between H and Mo atoms, indicating a strong interaction formed therein. Put it in detail, in a low H coverage, significant electron accumulation and depletion are presented around adsorbed H and connected three Mo atoms, respectively. Different from those on P-t surface, no electron depletion is found around H atoms on Mo-t surface with no more than 2/12 H coverage, revealing much electron is transferred from Mo to H atom and therefore giving a negative charge on adsorbed H atoms. When H coverage increases, the electron charge difference distribution becomes complex due to the various adsorption sites. As a consequence, the electron depletion emerges on some hydrogen atoms, especially for those adsorbed on the hollow sites. This is also in accordance with the fluctuations of adsorption energy for H adsorbed on Mo-t surface. These hydrogen atoms with the contrary electron distribution are proposed to react with each other and form hydrogen molecules more easily in the following HER process.

Generation of adsorbed hydrogen atoms on MoP (001) surface. Generally, there are two types of proposed pathways for HER in the acid media: the Volmer-Heyrovsky and the VolmerTafel mechanism.41 Both of them need the first step that is the Volmer reaction to generate adsorbed H atoms, where the protons transfer from the acid solution to the catalytic surface to generate the adsorbed H (H+ + e- → Had) with relevant small reaction activation energy. Subsequently, the solvated protons from the acid medium approaches to and reacts with the preadsorbed hydrogen atoms on the surface to form an H2 molecule (Had + H+ + e-→ H2) in the


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Volmer-Heyrovsky mechanism. Differently, two adjacent pre-adsorbed hydrogen atoms combine and react directly on the surface to generate an H2 molecule (Had + Had→ H2) in the VolmerTafel mechanism. In order to simulate the Volmer and Heyrovsky reactions, an approach that was proposed by Norskov et al.47 was employed in the current study to elucidate the HER mechanism on MoP (001), where a water-solid interface was constructed. The interface region usually contains the catalyst surface layer and one water layer above, forming about 3Å thick electrical double layer. And an electrode potential can be achieved by adding the proton to the water layer. Herein, the bilayer interface of the catalytic reaction system in the acid solution was, therefore, modeled with the catalyst surface slab and one proton-containing water lay as described in Figure 3. The 2×2 supercells of MoP and a four-water molecule layer with one proton were built to simulate the reaction system due to the limited model size.47 The initial configuration of water layer was obtained by conducting geometry optimization calculation.


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Figure 3. Model for the solvated MoP (001) bilayer system: P-t (a) and Mo-t surface (b). Top and bottom panel represent top and side view, respectively. Color code: P: purple; Mo: green; red and white sticks: water molecules. Volmer reaction was first studied in theory by using the bilayer system to generate adsorbed H atoms, where the surface adsorbed H comes from the proton in the acid solution. Since MoP (001) exists two types of surface, namely, P-t and Mo-t surface, we will discuss on both of them in following discussions. Figure 4(a). shows the minimum-energy reaction pathway of the Volmer reaction on P-t surface without H atoms pre-adsorbed, where the proton leaves the H3O+, moves toward to surface and finally bonds to P atom at the top site. At the transition state, the OH and P-H bond length are 1.324 and 1.667 Å, indicating the bond breaking of O-H in the H3O+ and bond formation of P-H on the surface occur, respectively. This is a thermal neutral process that reaction energy between initial and final state (∆E) is ~0 eV, and the activation energy barrier (∆Eb) is small (0.12 ev) indicating a fast reaction rate in Volmer reaction, which is also in accordance with previous theoretical studies.25 In order to understand the influence caused by pre-adsorbed H atom, the Volmer reaction was also studied with an initial 1/4 H coverage. Calculation results unveil that H atom is automatically transferred from reactant to product without activation energy barrier, indicating pre-adsorbed H atoms promote the subsequent H adsorption from the acid medium in the view of thermokinetic. This can be interpreted by the electron charge rearrangement introduced by pre-adsorbed H atom that lowers the H adsorption energy on P-t surface. On the contrary, the situation reverses on the Mo-t surface. Specifically, when there are no H atoms adsorbed on the surface, the proton can transfer from H3O+ to the surface smoothly without activation energy barrier and it is also a thermal neutral step. While the ∆Eb increases to 0.32 eV and the reaction become exothermic if H coverage of 3/12 is applied as


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shown in Figure 4(b). This can be ascribed to much electron charge has transferred from Mo to the pre-adsorbed H, with less electron giving to additional H that will be adsorbed or even gaining electron charge from H when H coverage is high on the Mo-t surface. As a consequence, the process becomes less favorable with increased activation energy barrier.

Figure 4. Minimum-energy reaction pathways of the Volmer reaction on MoP (001) surface: (a) P-t surface, with the pre-adsorbed H coverage of 0/4; (b) Mo-t surface, with the pre-adsorbed H coverage of 1/4; insets depict configurations of initial, transition, and final state. Green, purple, red and white ball represents Mo, P, O and H, respectively. Theoretically, water splitting reaction is another route to generate adsorbed H on catalyst surface though rare reports have been presented on MoP. Here, water-splitting reaction was theoretically investigated on MoP surface as an alternative way to form adsorbed H atoms for subsequent HER reactions. In this step, the water molecule is first chemically adsorbed on the catalyst surface, then the H-OH bond breaks to generate bonded H and OH to P/Mo atoms on the catalyst surface. Figure 5(a) shows the H2O splitting reaction process on the clean P-t surface, where the pre-adsorbed water molecule splits into H and OH bonding to two adjacent P atoms respectively. In the reaction process, the H-OH bond length first increases from 1.007 Å in the


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reactant to 1.194 Å in the transition state and then the H atom divides from water molecule to bind onto the P atom with a final distance between H and OH of 3.246 Å. Accordingly, the distance between H and P decreases from 2.444 Å in the reactant to 1.846 Å in the transition state. Meanwhile, the distance between P and O also drops from to 2.020 to 1.683 Å. By overcoming the transition state, new H-P and P-OH bonds on P-t surface are finally formed with a length of 1.421 and 1.627 Å, respectively. Interestingly, the reaction activation energy barrier (∆Eb) of this step is quite small (0.08 eV), indicating the adsorbed H atom can also be generated via facile H2O splitting reaction on the P-t surface in the view of kinetics analysis. The calculated ∆E of the reaction is -1.38 eV, indicating this is a exothermal step. Different from the Volmer reaction on P-t surface, water splitting reaction could also occur in the neutral solution according to the calculation results, which opens a new potential route of H generation on MoP surface. The effect of pre-adsorbed H atom on H2O splitting reaction was also calculated. In the Figure 5(b), it can be seen that the ∆Eb increases to 0.13 eV when the initial H coverage on the surface is 1/4, which is proposed to be caused by the repulsion between pre-adsorbed H atom and hydrogen of water molecule, agreeing well with the thermodynamic analysis of H adsorption on P-t surface without water layer. A similar reaction process is observed on Mo-t surface without pre-adsorbed H atoms, in which H2O molecule splits into H and OH bonded to two adjacent Mot hollow2 sites, respectively. Here, the bond length of H-OH in the adsorbed water molecule enlarges from 0.983 Å in the reactant to 1.362 Å in the transition state and the distance between H and the nearest three Mo atoms shortens to 3.2 Å. At the same time, the OH moves away from the top site towards to that of hollow2 with Mo-OH length decreasing from 2.304 to 1.984 Å in the reactant and transition state, respectively as shown in Figure 5(c). Finally, the divided H and OH are both bonded with three Mo atoms on two adjoined hollow2 adsorption sites with Mo-H


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and Mo-O bond length of 2.123 and 2.248 Å, respectively. The activation energy barrier here is higher (0.51 eV) than that on P-t surface and ∆E is -1.49 eV. It can be seen from Figure 5(d) that increasing H coverage to 1/6 results in a higher activation energy barrier of 0.76 eV. Therefore, calculation results indicate that water splitting reaction prefers to take place on the P-t surface regardless of slight side effect from hydrogen atoms initially adsorbed on the catalyst surface.

Figure 5. Minimum-energy pathways of H2O splitting reactions on MoP (001) surface: P-t surface, with the pre-adsorbed H coverage of 0/4 (a) and 1/4 (b); Mo-t surface, with the preadsorbed H coverage of 0/12 (c) and 2/12 (d); insets depict configurations of initial, transition, and final state. Green, purple, red and white ball represents Mo, P, O and H, respectively.


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Hydrogen evolution reactions on MoP (001) surface. Tafel and Heyrovsky reactions in Volmer-Tafel and Volmer-Heyrovsky mechanism were then adopted to theoretically investigate H2 generation, respectively. Figure 6(a) describes the Tafel reaction on the P-t surface, where two adjacent pre-bonded H atoms at top sites of P-t combine together to form an H2 molecule. In the reactant, the length of two P-H bonds are all 1.422 Å and they increase to around 2.095 Å in the transition state. The distance between two H shortens to 0.816 Å from the initial to transition state, after which the hydrogen molecule with H-H bond length being 0.751 Å is released with a distance of 3.3 Å above the surface. This step needs to overcome a relative high ∆Eb of 1.63 eV, indicating that H2 formation via Tafel reaction on P-t surface is extremely restrained in the view of thermokinetic analysis. In Figure 6(b), increasing H coverage (4/4) contributes to lowering the corresponding ∆Eb from 1.63 to 1.27 eV because adsorption energy between H and surface is close to zero. At the transition state, the distance between two H atoms drops to 0.988 Å and the nearest distance between H and P shortens to 1.734 Å. Nevertheless, these high activation energy barriers reveal that hydrogen generation through Tafel reaction is not favored on the P-t surface. On the contrary, the situation changes again on the Mo-t surface. Put it in detail, in Figure 6(c). the ∆Eb of H2 formation by Tafel reaction is 1.02 eV when there is no extra initial adsorbed H on the surface, where two adjacent H atoms adsorbed from the hollow sites move close to the nearest Mo atom and then bond to each other to generate the hydrogen molecule on the top of the Mo atom. At the transition state, the distance between two H atoms is 1.408 Å, and the two HMo bond length is equal (1.758 Å). In the final state, the H-H bond is formed with the length of 0.872 Å and they are still bonded to Mo with a length of 1.88 Å. We also investigated the effects of rising the initial H coverage to 6/12 (Figure 6(d)) and 12/12 (Figure S4) via the Tafel reaction on Mo-t surface. The obtained activation energy barrier decreases dramatically to 0.12 and 0.02


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eV with H coverage of 6/12 and 12/12, respectively, indicating an acceleration of forming the H2 via Tafel reaction with the surface partially covered with H atoms. This can be ascribed to the electron charge distribution rearrangement as we discussed in the H adsorption part, where the electron charge transferring between Mo and H becomes complex and electron depletion even occurs on H atoms with high H coverage resulting in adsorbed H atoms with contrary charge, and therefore favors the combination of hydrogen atoms to form the hydrogen molecule.

Figure 6. Minimum-energy pathways of the Tafel reactions on MoP (001) surface: P-t surface, with the pre-adsorbed H coverage of 2/4 (a) and 4/4 (b); Mo-t surface, with the pre-adsorbed H


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coverage of 2/12 (c) and 6/12 (d); insets depict configurations of initial, transition, and final state. Green, purple, and white ball represents Mo, P, and H, respectively. In the acid medium where H+ is adequate, the Heyrovsky reaction is considered as an important step to evolve hydrogen molecule on the catalyst surface. In our reaction system, the proton moves from water layer toward the surface and then combines with a pre-adsorbed H to generate a hydrogen molecule on the catalyst surface. Figure 7(a) shows the reaction path on the P-t surface with initial H coverage of 4/4. During the reaction step, the bond length of H-P increases from 1.433 Å in the reactant to 1.669 Å in the transition state and the proton (yellow) in H3O+ also departures toward to the initially adsorbed H atom; the distance between two hydrogen atoms is 1.180 Å. By overcoming the activation energy barrier of the transition state, the H2 molecule is formed and weakly adsorbed on the surface in the final state. The calculated ∆Eb is 0.31 eV, which is higher than that of Volmer reaction in the Volmer-Heyrovsky reaction mechanism on P-t surface. Therefore, it is indicated that the rate-determining step is the H desorption from P-t surface for the Volmer-Heyrovsky reaction pathway. In Figure 7(b), a similar hydrogen molecule evolvement can also take place on the Mo-t surface and the corresponding activation energy barrier is 0.35 eV with the pre-adsorbed H coverage of 4/12, also indicating that the rate-determining step is Heyrovsky reaction in the Volmer-Heyrovsky reaction pathway on Mo-t surface due to higher activation energy barrier.


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Figure 7. Minimum-energy reaction pathways of Heyrovsky reactions on MoP (001) surface: P-t surface, with the pre-adsorbed H coverage of 4/4 (a); Mo-t surface, with the pre-adsorbed H coverage of 4/12 (b); insets depict configurations of initial, transition, and final state. Green, purple, white and yellow ball represents Mo, P, H, and proton, respectively. According to the former discussion, we can divide HER activities on MoP catalyst surface into two parts based on different conditions, that is, in acid and neutral solution. The reaction activation energy barriers of steps involved are presented in Table 1. To be specific, in the acid solution, Volmer reaction is favored that the proton in the solution prefers to be adsorbed on both of P-t and Mo-t surface with low activation energy barrier, partially occupying the adsorption sites. Because of the initially adsorbed H atoms on the surface, the water-splitting reaction is prevented and will need to overcome a higher energy barrier to form adsorbed H atom on the surface. Therefore, the Volmer reaction will dominate the generation of adsorbed H on the MoP (001) surface. Subsequently, a relatively low energy barrier (~0.3 eV) is needed to form hydrogen molecule through Heyrovsky reaction on both P and Mo-t surface. A very high activation energy barrier exists in forming H2 on P-t surface via Tafel reaction, whereas, ∆Eb of this reaction process is much lower (~1.0 eV) on the Mo-t surface with assistance of initial H


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adsorbed. In the neutral solution, because of limited protons, rare H will be adsorbed to the catalyst surface coming from hydronium ions in the water phase. The water splitting reaction will be therefore awaked to generate adsorbed H atoms on the P-t surface due to relative low energy barrier. On the contrary, less adsorbed H atoms will be formed via H2O splitting reaction on Mo-t surface. Since H+ is scarce in neutral solution, the occurring of Heyrovsky reaction will also be limited. The Tafel reaction, on the other hand, will play a key role in generating hydrogen molecules on the Mo-t surface with a relatively low energy barrier. Therefore, water splitting reaction on P-t surface to generate adsorbed hydrogen atom and subsequent hydrogen molecule formation on Mo-t could constitute the whole HER on MoP (001) surface in a neutral solution.

Table 1.The reaction activation energy barrier of HER steps involved in acid and neutral conditions (unit: eV) Condition

Surface-type

Volmer

Water Split

Tafel

Heyrovsky

Acid

P-t

0.10

0.26

1.27

0.31

Mo-t

0.32

0.76

0.12

0.35

P-t

-a

0.07

1.63

-

Mo-t

-

0.51

1.02

-

Neutral

a. represents the reaction could not occur in this condition

HER reactions on defect P-t MoP (001) surface in neutral solution. Our calculation results have indicated that HER reaction can be achieved by water splitting reaction on a P-t surface and the subsequent Tafel reaction on Mo-t surface. It is worth noting that both P-t and Mo-t surface are necessarily constituent parts for HER processes. Best of our knowledge, mixed surface with both Mo-t and P-t surface have not been reported by relevant experiments. Thus combining two


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reactions on two different surface types to complete HER processes cannot be achieved theoretically on either type of surface merely. However, because the MoP (001) is constructed by commutative layers of Mo atoms and P atoms as shown in Figure 8(b), the defect that looses one P atom on the P-t surface will cause the Mo atoms exposure, which can be regarded as a part of Mo-t surface. Therefore, we proposed that the defects on one type of surface would introduce a partial "mixed type" catalyst surface. The defect P-t surface by moving three P atom was constructed here as the mixed surface to investigate the HER reactions from the view of theory. In order to minimize the change induced by defect, a larger surface was adopted, which was a six-atoms layer 4×4 supercell. In order to construct a local integrity Mo-t surface, three P atoms were removed as shown in Figure 8 (a)~(c). The optimized structure reveals that there were no changes to surface geometry after defect construction, indicating a strong self-stabilization on the defect surface.

Figure 8. The optimized 4×4 supercell P-t surface with defect after three P atoms was removed and H absorption energy on defect area: Top view (a); side view (b); Atoms in defect area were


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magnified (c); H adsorption energy (eV) on different sites of the defect (d). Green, purple ball represents Mo, P, respectively. In the neutral solution, adsorbed H atoms can be generated via water splitting reaction as studied in former part in theory. So that adsorbed H atoms can be formed on the intact P-t surface around the defect. Since the H2O splitting reaction is not favored on Mo-t surface, the generated adsorbed H should be transferred from the P-t surface onto the defect sites (with exposed Mo atoms) in order to achieve the following Tafel reaction. Calculation results indicate that negligible differences are observed in the adsorption energy of H atoms on both P and Mo atoms around and inside the defect area, respectively, results in a smoothly diffusion of H atoms from P to Mo active sites as shown in Figure 8(d). With the adsorbed hydrogen atoms on the Mo atoms in the defect, we need to explore the hydrogen evolution reaction on the defect. Here the Tafel reaction containing only two adsorbed H atoms on the defect to present minimum initial H coverage was investigated due to H+ shortage in the neutral condition. Figure 9 shows the relevant reaction steps. The reaction process is similar to that on the intact Mo-t surface, where two adsorbed H atoms move together and then react to form an adsorbed hydrogen molecule on the Mo atom. Interestingly, the calculated ∆Eb is only 0.71 eV, which is smaller than that on the normal Mo-t surface with same H coverage, indicating hydrogen evolving reaction is accelerated on the local Mo-t surface created by a defect on P-t surface. Thus, calculation results unveil that defects on P-t surface enable the catalysis surface to possess the characters of the "mixed catalyst surface" and water splitting and Tafel reactions can be achieved simultaneously to ultimately complete HER processes on the P-t surface.


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Figure 9. Minimum-energy reaction pathways of Tafel reaction on defect P-t surface, with the pre-adsorbed H coverage of 0/4; insets depict configurations of initial, transition, and final state. Green, purple, and white ball represents Mo, P, and H, respectively.

Overall reaction mechanism on MoP (001) surface. Previous studies mainly focused on the thermodynamic analysis of hydrogen adsorbed on the MoP surface to identify catalytic performance, leaving the whole process blank. In the present work, Volmer-Heyrovsky and Volmer-Tafel reaction mechanism were investigated and compared comprehensively in acid solution. In addition, exploratory water splitting reactions in neutral solution was studied to perfect our understanding of HER reactions on MoP (001) surface by DFT study. Figure 10 describes the diagram of HER reactions on MoP (001) surface. In the acid solution, Volmer reaction will dominate the adsorbed hydrogen generation on both P- and Mo-t surface from H+ in the liquid due to relatively lower activation energy barrier than that from water splitting reactions. Compared to Tafel reaction to form hydrogen molecule subsequently, a lower


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activation energy barrier will occur by Heyrovsky reaction on P-t surface. In this VolmerHeyrovsky reaction mechanism, Heyrovsky reaction is the rate-determining step because of higher activation energy barrier in hydrogen evolution step on P-t surface. Nevertheless, with the assistance from increased initial H coverage, hydrogen evolution will be favored on Mo-t surface via Tafel reaction, and the activation energy barrier will be lower by Tafel reaction when H coverage increases to 1/4. Therefore, Volmer-Heyrovsky mechanism and the Volmer-Tafel mechanism will dominate on Mo-t catalyst and the corresponding rate-determining steps are Volmer and Tafel reactions in weak and strong acid solution, respectively. In the neutral solution, water splitting reaction is introduced to the current mechanism and HER reactions will be promoted by forming defect on P-t surface based on simulation results. In the process, water molecules will split to generate adsorbed H atoms on P-t surface, the generated H atoms will diffuse onto Mo atoms in the defect area. Finally, the adsorbed H atoms on Mo surface will evolve to form hydrogen molecule with a relatively low energy barrier than that on the pristine surface. Generally, in the neutral and solution, the HER pathway involves the adsorption of H2O, electrochemical dissociation of adsorbed H2O into adsorbed OHads and Hads, the formation of H2 from Hads and desorption of OHads to refresh the surface of a catalyst. Theoretically, after water molecules split on the defect P-t surface area of MoP (001), the generated OHads may occupy the sites for H adsorption and prevent Hads diffusion from P to Mo sites, causing inefficient release of H2 and blocking the catalytic sites. This is in accordance with the relevant experiment results observed previously for the efficient HER in neutral and alkaline solutions,49 indicating the desorption of OHads could be an important factor in determining the efficiency of the HER for a catalyst in neutral solution. This is in return may provide an solution to release the potential of


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these efficient HER catalysts in neutral system by adding some chemical species to neutralize the excessive OHads with the aim of exposing more active sites for H2 evolution reactions.

Figure 10. The overall mechanism of HER on MoP (001) surface in acid and neutral conditions. Doping on MoP (001) surface. Doping has been extensively applied in tuning the physical and chemical properties of materials. Previous studies have also indicated that doping on transitionmetal would be an effective way to alter the chemical activity and understand the fundamental properties of a catalyst.50-51 Additionally, sulfur and nitrogen doping of MoP has been investigated in experiments and results revealed that HER activity will be enhanced with the assistance of doping.33, 52 A similar activity performance also existed in ion doping of MoP. Besides, recent DFT calculations were employed to study scaling reactions for adsorption energies on doped MoP surface.53 In order to further understand HER activity on MoP surface,


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the substitutional doping of the P site with N and S, and doping of the Mo site with Co, Cr, Fe, Mn, Ni and W transition metals were investigated via DFT calculations, respectively. For the P-t surface, 1/4 of top P atoms (doping atom1 in Figure 11(a)) will be replaced by N and S without any other changes, respectively. Besides, we also investigated doping of 1/4 Mo atoms below the top layer of P merely (doping atom2), as shown in Figure 10a. It was found that doping of P site by N, and S, and doping of Mo site by Co, Cr, Fe, Mn, Ni and W could not introduce visible structural changes to the P-t surface, whereas doping with Cu induces a slight larger lattice distortion that is about 0.5 % tensile strain, which is comparative to that in MoS2 structure.48 The calculated ∆GH of the doped system is presented in Figure 11(b) and according to the Gibbs free energy changes, it can be seen that doping with Mn and W at Mo site leads to the ∆GH value closer to zero compared to the pristine P-t surface. Therefore, we predict that doping with Mn or W could improve the HER catalytic activity of P-t MoP (001) surface.

Figure 11. (a) Front view of P-t surface with two doping atoms. (b) The calculated free energy diagram for HER on pristine and doped P-t MoP(001).


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SUMMARY AND CONCLUSIONS In the present work, DFT calculations were carried out to reveal the electrocatalytic HER mechanism on MoP (001) surface. Calculated ∆GH indicates that HER processes tend to take place with the hydrogen coverage of 75 % and 40~90 % on P- and Mo-t surface, respectively. Additionally, We studied systematically reaction pathways and associated activation energy barriers for elemental steps involved in HER processes. It was found that Volmer reaction was fast on both P- and Mo-t surface regardless of initial adsorbed H atoms in the acid solution; However, for the subsequent generation of hydrogen molecules, the Heyrovsky reaction had a relatively much lower activation energy barrier (~0.3 eV) than the Tafel reaction (>1.0 eV) on Pt surface. The activation energy barrier of Tafel decreased to comparative to that of Heyrovsky reaction with increasing initial H coverage on Mo-t surface. More important, water splitting reaction was also introduced to the current HER mechanism and calculation results indicated water splitting reaction could occur on P-t to produce adsorbed H atoms with a low activation energy barrier (0.07 eV) although this volume increased to ~0.3 eV when H coverage raised from 0/4 to 1/4. This result inspired us to further explore the potential of HER reaction in neutral solution. We proposed that the defect area on P-t surface enable the catalyst with properties of "mixed-surface" to support water splitting and Tafel reactions simultaneously. Last but not the least, calculations unveiled that the HER activity of P-t surface can be further enhanced by doping Mo atom with Mn and W. These results shed new light on understanding and improving the HER reactions catalyzed with MoP in acid and neutral condition as well as other molybdenum phosphide-based catalysts.

ASSOCIATED CONTENT


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Supporting Information. Adsorption energy on catalyst surface (Table S1), Adsorption sites on catalyst surface (Figure S1), Configuration H atoms adsorption on catalyst surface (Figure S2), and Minimum-energy pathways of the Tafel reactions on Mo-terminated surface (Figure S3).

AUTHOR INFORMATION Corresponding Author *Email: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors also appreciate the support of using Kamiak, a high performance computing cluster from Washington State University Center for Institutional Research Computing.

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New Insight into Mechanism of Hydrogen Evolution Reaction on MoP

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