Water Dissociation on Clean and Potassium Pre-Adsorbed Transition

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Water Dissociation on Clean and Potassium Pre-Adsorbed Transition Metals: A Systematic Theoretical Study Yan-Xin Wang, and Gui-Chang Wang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b04121 • Publication Date (Web): 07 Jun 2018 Downloaded from http://pubs.acs.org on June 7, 2018

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The Journal of Physical Chemistry

Water Dissociation on Clean and Potassium Pre-Adsorbed Transition Metals: A Systematic Theoretical Study

Yan-Xin Wang,1 and Gui-Chang Wang*,1,2 1

Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education) and the Tianjin Key Lab and Molecule-based Material Chemistry, College of Chemistry, Nankai University, Tianjin 300071, P. R. China; 2State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, 030001, P. R. China

*Corresponding author: Gui-Chang Wang. Telephone: +86-22-23503824 (O)

E-mail: [email protected]

Fax: +86-22-23502458

1

ACS Paragon Plus Environment

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Abstract: It is crucial to control the O-H bond cleavage on metal surfaces with pre-adsorbed potassium atoms in heterogeneous catalysis. Based on the density functional theory (DFT) calculations, the adsorption and dissociation of water on clean and potassium pre-adsorbed transition metal surfaces, including group 11 (Cu, Ag, and Au) and group 8-10 metal (Co, Ni, Ru, Rh, Pd, Ir, and Pt) surfaces have been investigated systematically. The calculation results show that presence of potassium atom enhances the binding strength of H2O but weaken the binding strength of OH. More importantly, it was found that the pre-adsorbed potassium can promote the catalytic activity of various metals for H2O dissociation to varying degrees and the more promoting effect of potassium on the water O-H bond scission occurs on the less chemical active transition metals. Based on electronic and geometric analysis, physical origin of the promotion effect can be attributed to the dipole-dipole interactions like ((Kδ+-OHδ-)-(Auδ+-OHδ-)), which stabilize OH group and weaken the interaction between OH and H at the TS and thus facilitate the dissociation of water. The functional mechanism illuminated in this paper could be applicable to other electropositive additives like Na, Cs in the activation of O-H bond involved in H2O or CH3OH.

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1. Introduction Introducing small amounts of additives to catalysts has been regarded as an effective means to alter the catalytic activity of metal catalysts by speeding up or slowing certain reaction step.1 Alkalimetal additives effects on heterogeneous catalysis have received continuous attention in the past few decades2-15 because alkali-metal, being good electron donors, can dramatically improve catalytic performance of many crucial reactions, such as water gas shift reactions,4-7 ammonia synthesis,8-11 Fischer Tropsch reactions.12-15 To date, much work has been carried out to explore the effect of the preadsorbed alkali metals over the transition metal surfaces on the activity of some basic reactions both experimentally and theoretically.16-21 DFT studies for CO dissociation on Rh(111) and K-covered Rh(111) by Hu et al.19 showed that K atom greatly facilitates the dissociation of CO when K was close to the dissociating adsorbate. By the experimental observation, Norskov et al.20 found that alkali metals promote the ammonia synthesis reaction over Ru catalysts and they also showed that the promotion effect is attributed to a direct electrostatic attraction between the adsorbed alkali atoms and the dissociating adsorbates. Bengaard et al.21 reported that the energy barrier for dissociation of less electronegative methane is increased by about 0.2 eV on potassium pre-covered nickel surfaces when compared to that on clean surfaces. Nevertheless, the wide application of potassium additives in chemical reactions does not indicate a thorough understanding of the mechanism of their promotional or inhibition abilities. Thus, we focus on here studying the effect of pre-covered potassium on the activity of the transition metals from water dissociation. Water dissociation is selected as a probe reaction for exploring the effects of promoters on catalytic performance due to its simplicity and necessity in surface reactions such as the water-gasshift reactions, Fischer-Tropsch synthesis, and reactions in fuel cells. Most of the previous theoretical studies focus on the effect of alkali-metal additives facilitating C-O bond, O-O bond and C-H bond cleavage on transition metal surfaces,22-26 however, experimental studies rather than theoretical reports have been devoted to potassium effecting on O-H bond scission of H2O in the past few years.27,29-33 The adsorption and dissociation of H2O on clean and K-covered Pt(111) were investigated by Bonzel et al.27 utilizing Auger, X-ray and ultra-violet photoemission spectroscopies. It was concluded by them that on one hand, no dissociation of the adsorbed H2O was noted on heating to higher temperatures, but on the other hand, adsorbed K induces the dissociation of 3



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H2O to OH species at T = 100-320 K with a lower activation barrier than an barrier of 43 kJ/mol reported by Creighton et al.28 for H2O dissociation in the temperature range 130-142 K on the Ocovered Pt(111) surface. This comparison confirms better effectiveness of potassium than oxygen in facilitating H2O dissociation. Other experimental investigations have also demonstrated that preadsorbed potassium atom has promoting effect on the cleavage of O-H bond on various metal surfaces.29-33 Although pre-adsorbed potassium promoter effect on the water dissociation has been studied experimentally for decades, the microscopic understanding of this effect remain be deficient. Therefore, it is necessary for us to investigate the effect of pre-adsorbed potassium on water dehydrogenation reactions on various transition metal surfaces at the atomic level. Staring from the adsorbed H2O, two sequential reactions may occur that H-O bond cleavage of H2O to produce OH and H (H2O  OH + H) and followed by the dissociation of OH to form H and O (OH  H + O), and herein, we focus on H2O partial dissociation to produce OH and H, because of the importance of OH in many catalytic processes. In this paper, we have carried out DFT calculations to examine water dissociation on clean and potassium pre-adsorbed group 11 metal surfaces (Cu(111), Ag(111), and Au(111)) and group 8-10 metal surfaces (Co(0001), Ni(111), Ru(0001), Rh(111), Pd(111), Ir(111), and Pt(111)) together with the corresponding adsorption properties, dissociation mechanism and the geometric and electronic analysis aiming to shed light on the origin of the potassium promotion effect on water dissociation.

2. Calculation Methods and Models All density functional theory (DFT) calculations were performed by using the Vienna ab initio simulation package (VASP).34,35 Potentials within the projector augmented wave method (PAW)36,37 and the generalized gradient approximation (GGA) with the Perdew-Wang-91 (PW91) functional38 were employed. The plane-wave cutoff energy was set at 400 eV to describe the electronic wave functions. The convergence criteria for electronic self-consistent iteration were set to 1.0×10-4 eV, and the atomic positions were optimized by means of a conjugate gradient algorithm until atomic forces were smaller than 0.03 eV/Å. Spin polarized calculations were only conducted for Ni and Co to account for the magnetic properties of them. The nudged elastic band (NEB) method39 was applied to locate the transition states (TS) by three steps: determine the initial state (IS) and the final state (FS) of the reaction; then interpolate a series of images between IS and FS and optimize these images until the 4


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force acting on the atom is smaller than 0.03 eV/Å; at last the frequency analysis was employed to confirm the TS. The van der Waals (vdW) correlation by the DFT-D3 method40 was also considered in calculating the adsorption energy of H2O on the clean and potassium pre-adsorbed metal surfaces due to the weak interaction between H2O and the substrate. Charge distributions were estimated using Bader's atoms in molecules (AIM) theory of using the algorithm developed by Henkelman.41,42 The metal surfaces were modeled by a periodic slab including four layers with full relaxation of the uppermost two layers, which was separated by ∼10 Å of vacuum region. A p(3 × 3) unit cell was used in this work, representing a monolayer of adsorbates with coverage of 1/9 ML. The surface Brillouin zone was sampled with a 3 × 3 × 1 Monkhorst–Pack mesh for better calculation accuracy.43 The adsorption energy (Eads) is defined as: Eads = Esubstrate + Eadsorbate - Eadsorbate/substrate, in which Esubstrate, Eadsorbate and Eadsorbate/substrate are the total energies of the substrate, the free adsorbate, and the adsorbate-substrate system in the equilibrium state, respectively. Moreover, the activation energy (Ea) and reaction energy (ΔE) are calculated using the formulae: Ea = ETS - EIS and ΔE = EFS - ETS. Here, EIS, ETS, and EFS represent the total energies of the initial state (IS), transition state (TS), and final state (FS), respectively. The charge density difference (Δρ) was calculated as: Δρ = ρ(adsorbate/substrate) ρ(substrate) - ρ(adsorbate), where ρ(adsorbate/substrate) is the total charge density of the adsorbate/substrate system, ρ(substrate) and ρ(adsorbate) are the charge density of the metal substrate and adsorbate, respectively, fixed at the adsorbed geometry.

3. Results and Discussion 3.1. Adsorption Properties of Possible Species To determine the adsorption and dissociation property of H2O on both clean and potassium preadsorbed surfaces of each transition metal, it’s essential for us to first examine the optimum adsorption sites for reactants (H2O) and its dissociation products (OH, H) on these surfaces. Due to the similar structures of initial states (ISs), transition states (TSs), and final states (FSs) for H2O dissociation on each clean and potassium pre-adsorbed metal surface, we present those on Cu metal as a representative case, as illustrated in Figure 1, respectively.

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Figure 1. Top view of the (a) initial (IS), (b) transition (TS) and (c) final state (FS) for H2O dissociation on clean Cu surface and (d) IS (e) TS and (f) FS for H2O dissociation on potassium pre-adsorbed Cu surface. The light red, purple, red and white balls represent the Cu, K, O and H atoms, respectively.

H2O is found to preferentially adsorb on the top site of all clean metal surfaces via O atom with the molecular plane being almost parallel to the surfaces (Figure 1a), in agreement with previous studies. The preferred adsorption sites for OH and H were searched in four high symmetric adsorption sites (top, bridge, fcc and hcp) of each metal surface and they were found all in favor of fcc sites on metals (Au, Ag, Cu, Pt, Ni, Rh, Ir and Pd) with (111) surface, while hcp sites on Ru(0001) and Co(0001) metal surfaces. Hence, as the FS of H2O dehydrogenation, the co-adsorption of OH and H on each clean metal surface favor the two closest fcc sites (hcp sites on Ru and Co) (Figure 1c). The K pre-adsorbed metal surfaces were modeled by adding one K atom on four high symmetric adsorption sites and the hcp site on each metal surface is found most stable for K adsorption, which coincides with the reports from Hu et al. that the K atom is favorably adsorbed on the hcp site of Rh(111) surface.19 In addition, a very slight hcp-fcc energy difference (maximum value of 0.03 eV) for K adsorption is found on the metals with (111) surfaces, hence we choose fcc sites on the metals with (111) surfaces and hcp sites on remaining metal surfaces as the adsorption sites of K. On each K pre-adsorbed metal surface, H2O still prefers the top site, while OH and H favor the fcc site (Au, Ag, Cu, Ni Rh, Ir and Pd) or hcp site (Ru and Co). Previous theoretical study by Hu and co-workers19 elucidated the issue that if the distance between K and the dissociating CO on Rh(111) is shorter than 3 Å, K can greatly promote CO dissociation, thus in our H2O + K co-adsorption system, the H2O molecule was placed at the specific top sites to ensure the distance between K and H2O kept in the range of 2-3 Å on various metal surfaces. The ISs on K pre-covered surfaces were determined and they 6


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are similar; H2O stays far away from the top site of each metal surface with H2O molecule plane lying nearly perpendicular to the substrate and K likewise kept away from the fcc or hcp sites of substrates with the almost same height as that of the O atom of H2O (Figure 1d). The distance between K and O atoms maintain in the range of 2.67-2.78 Å on various metal surfaces, indicating the strong interaction between K and O atoms. As the product of H2O dissociation, the position of OH transfers from the preferred fcc or hcp site to the metastable top site, while H favors the adjacent fcc site (Figure 1f), due to the strong effect of K.

Table 1. The Adsorption Energy of H2O (with and without vdW Corrections), OH and H on Clean and Potassium Pre-Adsorbed Metal Surfacesa Clean surface Metal

Potassium pre-adsorbed surface

Eads(H O) Eads(H2O) -0.15 (-0.36)b

Eads(OH)

Eads(H)

Eads(H O)

Eads(OH)

Eads(H)

-2.10

-2.21

-0.56 (-0.74)

-1.97

-2.16

-2.74

-2.08

-0.53 (-0.69)

-2.75

-2.14

Cu

-0.15 (-0.32) -0.36 -0.32 -0.20 (-0.39)b

-3.22

-2.48

-0.60 (-0.79)

-3.07

-2.53

Ni

-0.39 -0.32 (-0.53)b

-3.55

-2.84

-0.62 (-0.83)

-3.29

-2.83

Pt

-0.53 -0.27 (-0.48)b

-2.34

-2.73

-0.57 (-0.79)

-2.15

-2.74

Rh

-3.22

-2.83

-0.59 (-0.80)

-3.00

-2.82

Ir

-0.48 -0.41 (-0.63) -0.63 -0.30 (-0.54)

-2.68

-2.69

-0.58 (-0.79)

-2.46

-2.65

Pd

-0.54 -0.28 (-0.49)b

-2.81

-2.98

-0.60 (-0.83)

-2.55

-2.98

Ru

-0.49 -0.40 (-0.64) -0.64 -0.33 (-0.53)

-3.63

-2.84

-0.58 (-0.81)

-3.38

-2.83

Au Ag

Co

2

2

-3.82 -2.74 -0.62 (-0.80) -3.59 -2.81 -0.53 The data in the parentheses indicates the adsorption energy of H2O on each metal with vdW corrections; the unit for all adsorption energies is eV. b The calculated adsorption energies of H O on Au(111), Cu(111), Ni(111), Pd(111) and Pt(111) are in good agreement 2 with those of -0.14, -0.21, -0.29, -0.30 and -0.33 eV calculated by Schneider et al.44 a

The adsorption energies of H2O, OH, and H at their preferred adsorption sites on given clean and K-modified metals are presented in Table 1 as well as in Figure S1. As we know, the DFT method usually underestimates the adsorption energy of weakly bond molecule, e.g., water, as a result of its misestimating the long-range interaction (i.e., dispersion interaction). To examine the general rule for the effect of the dispersion interaction on activation energy of H2O on various metals, we calculated the adsorption energy of H2O on both clean and potassium pre-adsorbed metal surfaces with and without vdW corrections by the DFT-D3 method, as presented in Table 1. It is found that the discrepancies between the adsorption energies of H2O with vdW corrections and those without 7



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corrections for different clean metals keep in the range of 0.17-0.24 eV and for various K-adsorbed metals maintain in the range of 0.16-0.23 eV. This result indicates that the effect of dispersion interactions on binding strength of H2O remains basically unchanged for different metals with and without potassium adsorption. Additionally, since both OH and H strongly bind with the clean and Kmodified metal surfaces at the TS, the effect of dispersion interactions on TS can be ignored. Therefore, one can expect that the nearly same dispersion effect on activation energy of H2O occurs on different metal surfaces and we take no account of the vdW corrections effect on dissociation of H2O on each metal hereinafter. It can be found that the adsorption energies of H2O vary on different K-modified metal surfaces within a narrow range (from -0.53 to -0.62 eV) and the presence of potassium atom enhances the binding strength of H2O on metal surfaces with respect to that of the clean metal surfaces. This phenomenon is rationalized on the basis of strong electrostatic interaction between K and H2O. Besides, unlike the case of H2O, the pre-adsorbed potassium atom weakens the binding strength of OH on metal surfaces relative to that of the clean metal surfaces. The reason can be summarized that relatively weak electrostatic interaction between K and OH makes the interaction of OH and metals plays a leading role in which more electron density donation from K to the d-band of the transition metal, making the d-band more filled, thus increase the Pauli repulsion between electron-rich O and metal surfaces. To provide insight into the relationship between the adsorption strength of adsorbate on various transition metals and the electronic properties of these metals, we examined the adsorption energies of H2O, OH and H corresponding to H2O dissociation on both clean and potassium pre-adsorbed metal surfaces based on the d-band center analysis. The d-band center model proposed by Hammer and Nørskov45 is applied to detect the electronic structure and chemical reactivity of the metal surface, 

which can be calculated by the formula: 

c d

  

 

E  d ( E )dE



 d ( E )dE

, where  d represents the density of

states projected onto metal atom’s d band; E is the energy of d-band. The d-band center model is appropriate for transition metals which have not fully filled d shells and the values of d-band center energy for group 8-10 metals, that is, Co, Ni, Ru, Rh, Pd, Ir, and Pt have been calculated to be -1.17, 1.29, -1.41, -1.73, -1.83, -2.11,-2.25 eV46 respectively. For group 11 metals (Au, Ag and Cu), each of which has a fully filled d band, a different parameter, the adsorbate states-metal d-band coupling matrix 8


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element ( Vad2 ), is applied to represent their electronic properties and the values of Vad2 for Au, Ag and Cu are 3.35, 2.26 and 1.00.46 In this paper, we have explored the relationship between the adsorption energy of H2O, OH and H with and without K addition on metals and the d band center for group 810 metals, as well as Vad2 for group 11 metals. It should be noted that we use the pure metallic d-band center in the presence of K and do not consider the influence of K on the metallic d-band center. This might be not so exactly, but the general trends should be the same. It can be found from Figure 2a and Figure 2c that the adsorption energy of OH linearly correlate with the d-band center for both clean and K pre-covered metals; in addition, the closer to the Femi energy level of the d-band center, the stronger adsorption capacity of adsorbates. Figure 2b and Figure 2d show that there is a linear dependence between the adsorption energy of OH and the d-band coupling matrix element ( Vad2 ) for group 11 metals with and without K adsorption, which indicate that the stronger the overlap, namely, the large the repulsion, the weaker binding of the adsorbates. For the case of H2O and H, their adsorption energies on clean and K pre-covered surfaces cannot be well correlated with either the d-band center or the coupling matrix element of metals (see Figure S2 and Figure S3), due to the weak interaction between H2O (H) and metal surfaces which leads to an incompact relation with metal properties. As we can see, the d band center energy decreases from left to right across a row and from top to bottom down a column of the periodic table for group 8-10 metals; while the size of the coupling matrix element always increases down along group 11 metals in the periodic table. Due to the strong dependence of the calculated OH adsorption energies on either the d-band center for group 8-10 metals or the coupling matrix element for group 11 metals, the periodic trend applies equally to the binding strength of OH: The adsorption energy of OH becomes less negative as the metals shift from left to right across a row and from 3d to 5d down along a group across the periodic table. It is attributed to the gradually enhanced Pauli repulsion between the electron-rich O of OH and increasing filled d-band of the transition metal from left to right and from 3d to 5d of the periodic table.

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Figure 2. (a) Plot of adsorption energy of OH on clean Ni, Pt, Rh, Ir, Pd, Ru and Co metals versus the d band center; (b) on clean Au, Ag and Cu metals against the coupling matrix element squared ( Vad2 ); (c) Plot of adsorption energy of OH on K pre-adsorbed Ni, Pt, Rh, Ir, Pd, Ru and Co metals versus the d band center; (d) on K pre-adsorbed Au, Ag and Cu metals against the coupling matrix element squared (Vad2 ). R2 is the correlation coefficient.

3.2. Water Dissociation Properties In this section, we will focus on the reaction properties of water dissociation on pure metal surfaces. The calculated activation energy (Ea) and reaction energy (ΔE) for water dissociation as well as the important geometric parameters of TSs on clean and K-modified metal surfaces are summarized in Table 2. Our calculated results about water dissociation on clean metal surface are in broad accordance with experimental conclusions reported by Henderson.47 The geometry of the transition state is nearly identical for all the metals, thus we select the TS structures for water dissociation on clean and potassium pre-adsorbed Au(111) surfaces to describe the reaction mechanism.

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Table 2. Calculated Activation Energies (Ea) and Reaction Energies (ΔE) as well as the Geometric Parameters of TS Structures for H2O Dissociation on Clean and Potassium Pre-Adsorbed Metal Surfaces Clean surface

Potassium pre-adsorbed surface dO-Ha

dO-Mb (Å)

Ea (eV)

ΔE (eV)

dO-Ha (Å)

dO-Mb (Å)

1.90

2.24

1.76

1.36

1.85

2.52

1.17

1.90

2.22

1.57

0.89

1.78

2.50

1.46

0.39

1.62

1.99

1.12

0.38

1.58

2.20

Ni

0.94c

-0.15

1.54

1.93

0.81c

0.08

1.43

2.11

Pt

1.54

0.74

1.59

2.13

0.92

0.52

1.49

2.21

Rh

1.22

0.04

1.48

2.11

0.76

-0.01

1.46

2.20

Ir

1.44

0.46

1.56

2.14

0.89

0.37

1.43

2.22

Pd

1.52

0.27

1.63

2.12

0.99

0.30

1.56

2.23

Ru

0.91

-0.31

1.48

2.10

0.74

-0.11

1.46

2.25

Co

0.99

-0.29

1.52

1.97

0.75

-0.09

1.41

2.16

Metal

Ea (eV)

ΔE (eV)

Au

2.36

1.62

Ag

2.06

Cu

(Å)

a

dO-H is the distance between the dissociated H and O at TS of water. O-M means the distance between O and the metal atom which O interacts with at TS. c The activation energies of water dissociation on clean and potassium pre-adsorbed Ni(111) are in agreement with that of 0.86 eV and 0.81 eV, respectively, calculated by Liu et al.5 bd

3.2.1. Water Dissociation on Clean Metal Surfaces The TS for the activation of H2O on the clean Au(111) surface, as shown in Figure 3a, denotes a one-site-three-centered mechanism, in which one H atom moves away from the O atom along with the cleavage of O-H bond, whereas, both OH group and dissociating H atom still bond with the same Au atom, signifying the homolytic dissociation of H2O. The activation of the O-H bond proceeds via an oxidative addition process, that is, the surface Au atom adds into the O-H bond of H2O to form a three-centered σ-complex OH-Au-H, in which the Au atom and dissociating H atom loses a charge of 0.18e and 0.11e and OH group obtains a charge of 0.40e as illustrated in Figure 3a. By comparing the breaking O-H bond lengths at the TSs with those at the ISs, it can be concluded that O-H distances at the TSs are stretched by about 42% - 88%, respectively, compared to those at the ISs, as listed in Table S1. The much elongated O-H bonds at the TSs means that the TSs for both activation steps are all the “final TS type” on the clean metals, which implies that the properties of the TSs resemble those of FSs and quite differ from those of ISs.

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Figure 3. Transition states (TSs) for the dissociation of water on (a) Au(111) and (b) K pre-adsorbed Au(111) surfaces. The red, white and purple balls represent O, H and K atoms, respectively. The bond lengths of O-Au1 are labeled in green font and the distances between O and dissociating H are signified with dO-H. Bader charges on K, H and OH atoms are marked in red font and Bader charges on Au1, Au2, Au3 and Au4 atoms are indicated on the above the pictures. The units of bond lengths and Bader charges are angstroms and e, respectively.

Brönsted–Evans–Polanyi (BEP) correlation plays a significant role in forecasting activation energy based on the thermodynamic properties. A good linear relationship is presented between the activation energy (Ea) and reaction energy (ΔE) of H2O dissociation on pure metal surfaces, as depicted in Figure 4a. It can be seen that the more exothermic or less endothermic of H2O, the smaller of the corresponding activation barrier. The BEP correlations we obtained for dehydrogenation of H2O is Ea = (0.73 ± 0.05) ΔE + (1.15 ± 0.04), consistent with the conclusion drawn by Neurock et al.48 that the slope of the BEP plot for C-H bond cleavage of both ethylene and ethyl equal to 0.65. The analogous slopes for O-H bond activation of water and C-H bond activation of ethyl and ethylene on the BEP relationships may attribute to the very similar mechanism for both reactions (dehydrogenation reactions) and the identical geometrical properties of the transition state (final TS type) for all the metals.

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Figure 4. Relationship of activation energy barrier (Ea) and reaction energy (ΔE) for H2O dissociation (a) on clean metal surfaces and (b) potassium pre-adsorbed surfaces. R2 is the correlation coefficient.

3.2.2. Water Dissociation on Potassium Pre-Adsorbed Metal Surfaces In the case of the involvement of chemisorbed K in the decomposition of H2O to OH and H on metal surfaces, the TS still presents one active site (Au1), since the K promoter acts as a spectator which never gets involved in the reaction (see Figure 3b) which differs from the case of oxygenassisted H2O decomposition reaction in which the dissociated H atom is abstracted by the oxygen to form hydroxyl.49 Comparing the TS structure on clean Au(111) surface (Figure 3a) with that on K addition surface (Figure 3b), it is obviously found that on one hand, OH group stays far away from K pre-adsorbed Au(111) surface and the distance between O and Au1 atoms increase from 2.24 Å on the clean surface to 2.52 Å on K covered surface and on the other hand, the dissociating O-H bond shortens form 1.90 Å on the bare surface to 1.85 Å on K containing surface due to the electrostatic interaction between K and OH. As shown in Table 2, the distances between OH and H at TSs on K pre-adsorbed metal surfaces are always shorter than those on clean surfaces, while the distance between O of OH and surface metal atoms at TSs becomes longer on K covered surfaces. As good electron donors, K atom makes a redistribution of electrons on Au(111) surface in which the neighboring Au3 and Au4 atoms and OH become more negatively charged, the active Au1 site is slightly less positively charged and the dissociating H atom is more positive charged (see Figure 3b) . From Figure 4b, it is observed that the activation energies can linearly correlate with the reaction energies for H2O dissociation on potassium pre-adsorbed surfaces. The BEP relationship still holds for this case because the TSs on this occasion are all structurally more product-like with O-H distances at 13



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the TSs stretching by about 42% - 86% over that at the ISs (see Table S1). The energy barriers of H2O dissociation on both clean and K pre-covered surfaces as well as the d-band center energies over various transition metals are depicted in Figure 5a. It can be found that the barriers of H-O bond cleavage on these two kinds of surfaces show a similar trend, while the d-band center energies for different metals present an antipodal variation tendency. That is to say, the metal surface with a lower d-band center energy (i.e. the closer to the Fermi level) will generate a smaller bond breakage barrier and higher chemical activity, which is consistent with the standpoint proposed by Hammer and Nørskov.45 Thus the chemical activity of various transition metals for water dissociation also follows the periodic trend: Chemical activity increases as the metals shift from left to right and top to bottom across the periodic table and the per-adsorbed K atom has almost no effect on the chemical activity of transition metals. The values of reaction energy difference and activation barrier difference for H2O dissociation on various clean metal surfaces versus those on K-modified surfaces are shown in Figure 5b. It can be found that the variation trend of reaction energy generally keeps consistent with that of activation barrier. That is to say, the transition metals with K pre-adsorption which are thermodynamically beneficial for water dissociation are also kinetically conductive to water activation.

Figure 5. (a) The energy barriers of H2O dissociation on clean metal surfaces (energy barrier 1) and on K preadsorbed metal surfaces (energy barrier 2) as well as the d-band center energies over various transition metals. (b) The energy barrier difference and reaction barrier difference for H2O dissociation on clean surfaces versus that on K pre-adsorbed surfaces over various transition metals.

As displayed in Figure 5a, the energy barriers of H2O dissociation on potassium pre-adsorbed surfaces are all lower than those on clean metal surfaces and they are reduced in different degree, 14


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indicating that the appearance of K atom promotes H2O dissociation to various extents for different metal surfaces. It appears that the pre-adsorbed K atom presents a strong facilitating effect on H2O dehydrogenation on inert metals Au, Ag, Pt, Ir, Pd and Rh and a little promoting effect on active metals Cu, Co, Ni and Ru. It can be roughly estimated that the magnitude of K-induced promoting effect is associated with the chemical activity of different transition metals. Hence, we quantitatively analysis the relationship between the difference in the reaction barrier of H2O dissociation on a clean surface versus that on a K pre-adsorbed surface and the electronic properties of the surface, as illustrated in Figure 6. Figure 6 shows that the energy barrier difference linearly correlate with the d-band center for group 8-10 metals, or with the coupling matrix element ( Vad2 ) for group 11 metals; in addition, the closer to the Femi energy level of the d-band center or the smaller size of the coupling matrix element, the smaller the barrier difference and the weaker the promoting effect. From the paragraph above, we have found that the closer to the Femi energy level of the d-band center, the smaller barrier of O-H bond breakage, namely, the higher chemical activity of the metal. Thus, it can be concluded that the more promoting effect of potassium on the water O-H bond scission occurs on the less chemical active transition metals.

Figure 6. Plot of energy barrier difference for water dissociation on clean and potassium pre-adsorbed metal surface on clean Ni, Pt, Rh, Ir, Pd, Ru and Co metals versus the d band center; (b) on clean Au, Ag and Cu metals against the coupling matrix element squared ( Vad2 ).

Interestingly, this general rule is similar to the O-promotion behavior on the O-H or C-H bond activation of our previous work,49,50 and others,51 namely, the more chemical active of the metals, the less promotion effect will be. It should be pointed that the promotion mechanism of K on the H2O 15



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activation is different from the O promotion effect, because the K promoter can act as the spectator (indirect activation), while the O species get involved in the reaction, that is, a OH species is formed via the path of H2O + O  2OH (direct activation). Since the different mechanism of K and O in the activation of H2O (indirect vs. direct activation), the essential rule for O promotion effect, namely, the binding strength of O can be used a descriptor to measure the ability of oxygen promotion effect directly49-51 is not suitable for K promoter. It is expected that the K promotion properties would be more complex than that of O, which will be analyzed more detail in the latter sections.

3.2.3. Comparison with Experimental Results Based on above study, we know that the K promotion effect depends on the activity of transition metals, so it is worthy to compare those with experimental findings27,29-33,47 and confirm our point further. The experimental results about H2O activation over various metal catalysts under different experimental conditions are listed in Table S2 to be used for comparing with our calculated results. From our calculated activation energy barriers on the clean metal surfaces, it can be summarized that Ni, Ru, Co metals are the optimal catalysts for H2O dissociation; Cu, Rh and Ir show the moderate catalytic activity; H2O dissociation on Au, Ag, Pt and Pd metals is not so favorable. Previous experimental observations reported that the irreversible water dissociation occurs at higher temperatures under ultrahigh vacuum (UHV) conditions on Cu(111), Ni(111), Ru(111) and Co(111), whereas dissociation of isolated water is not observed on Au(111), Ag(111), Pd(111), and Pt(111) surfaces,47 which is in good agreement with our conclusion. The experimental results showed that dissociation of H2O is observed on clean Ni(111) surfaces when heating to higher temperatures, while adsorbed K induces the dissociation of H2O to OH species at lower temperatures.29 Previous studies found that dissociation of H2O on Cu(111) is strongly enhanced due to pre-adsorbed K30 and so does that on Pt(111) and OH coverage on Pt (111) increases linearly along with the increase of K coverage.27 Besides, the adsorbed H2O was found has a high reactivity toward the formation of KOH even at the low temperature on K-covered MgO/Ru(001) surface31 and Ag(111) surface.32 The surface science experimental results indicated that the water gas shift reaction (CO + H2O  CO2 + H2, WGS) catalyzed by Cu can be promoted by the addition of Cs, and the reason is the rate-determining step of water dissociation is increased in the presence of Cs.33 All these experimental reviews are consistent with our calculated results, that is pre-adsorbed atomic potassium promotes H2O dissociation to 16


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various extent for different transition metal surfaces.

3.3. Analysis of the Promoted Effect of Potassium on H2O Dissociation From the foregoing analysis, it can be concluded that the potassium additives can promote H2O dissociation on various metal surfaces. Now we focus on addressing the queries about why the preadsorbed potassium atom can promote the activities of the metals in this section. Aiming at answering this question, we firstly explore the nature of chemical bonding between potassium and the metal substrates, thus the charge density difference and Bader charge were analyzed. We have used K adsorption on Au(111) as a representative case to show the charge density difference maps for K adsorption on metal surfaces, as illustrated in Figure S4. Figure S4 shows that the abundant valence electrons of K are depleted and delocalized into the metal surface resulting in a significant increase of electronic density in the K/Au interface region, forming “an extra electronic layer” upon the surface. The feature of K 4s electron polarization toward the Au surface reveals that the K/Au bonding is rather ionic and metallic than covalent which is in good agreement with previous experimental and theoretical report by King and co-workers.52 Moreover, the Bader charges of potassium atom for various metal surfaces are all positive (see Table S3), suggesting charge transfer occurrence from the potassium atom to the metal surfaces.

Table 3. Energy Decomposition of the Calculated Activation Barrier for the First Dissociation Step of H2O on Au(111), Cu(111) and Ru(111) Surfaces, with and without Pre-Adsorbed Potassium Atom (Unit in eV) ΔEsub

EHdef2O

EHIS2O

Au(111)

0.00

4.33

-0.15

Cu(111)

0.00

3.15

Ru(111)

0.00

K/Au(111) K/Cu(111)

TS EOH

EHTS

EHintL OH

Ea

-1.67

-2.00

1.55

-0.20

-2.37

-2.40

2.88

2.36 1.46

2.41

-0.40

-3.06

-2.73

3.88

0.91

0.15 (0.15)

4.29 (-0.04)

-0.77 (-0.62)

-2.58 (-0.91)

-2.06 (-0.06)

1.19 (-0.36)

1.76

0.05 (0.05)

2.92 (-0.23)

-0.63 (-0.43)

-2.86 (-0.49)

-2.39 (0.01)

2.77 (-0.11)

1.12

-3.38 (-0.32) -2.70 (-0.03) 3.46 (-0.42) 0.74 0-2.49component to the barrier with respect to the clean Note: Values in parentheses are the contribution of the respective K/Ru(111)

0.04 (0.04)

2.30 (-0.11)

-1.02 (-0.62)

metal surface.

To provide further insight into the K promotion effect on the dissociation of H2O and the physical origin of the dissociation barrier, we use the following scheme53 to decompose the calculated barrier 17



Page 18 of 28

Ea . TS Ea = ΔEsub + EHdef2O - EHIS2O + EOH + EHTS + EHintL OH

(1)

where ΔEsub and EHdef2O represent the energy changes of the substrate and H2O, respectively, from IS TS to TS; EHIS2O is the adsorption energy of H2O at the IS; EOH and EHTS are adsorption energies of the

individual reactant (OH, H) at the TSs and Eint is the interaction energy between OH and H at the TS. Using eq 2, we have decomposed the activation barrier (Ea) for H2O dissociation on Au(111), Cu(111) and Ru(111) surfaces with and without pre-adsorbed K atom, and the results are listed in Table 3. The strong promoting effect of K on O-H bond cleavage on Au, moderate facilitating effect on Cu and little inducing effect on Ru make these three metals be chosen to represent all metals in order to clarify the main factors governing the barrier difference between clean and K pre-covered TS metal surfaces. It can be found that with the addition of K, the components of ΔEsub, EHIS2O , EOH , and

EHintL OH for each metal are changed obviously. ΔEsub values for K containing surfaces are positive since K atom tends to shift from the fcc hollow site at the IS towards the bridge site of the metal atoms at the TS to approach the dissociating H2O during the activation of H2O. The pre-adsorbed K atom strengthens the adsorption of H2O at the IS and thereby increases the barrier, which is not beneficial for the H2O dissociation. On the other hand, by stabilizing OH binding and weakening EHintL OH at the TS, K atom facilitates the reduction of the barrier and thus promotes the dissociation of H2O on various metals. These two effects together contribute to the reduction of the reaction barrier and will be discussed in terms of electronic factor and geometric factor below in details. (a) Electronic factor analysis. To make it clear that how can K atom affect the binding strength of OH at the TS, we analyzed the charge distribution by the charge density difference calculations between OH and other fragments (including K, dissociated H atom and surface metal atom beneath OH) which directly interact with OH on clean and K-covered Au(111) surface, as mapped in Figure 7. As displayed in Figure 7, strong electrostatic attraction has been formed between OH and dissociated H, OH and surface Au atoms (Figure 7a and Figure 7b), as well as OH and K atom (Figure 7b), in which the charge density outside K, H and Au atoms are depleted and electrons delocalize into O atom resulting in a significant increase of electronic density outside O atom. Comparing charge distribution in Figure 7a with that in Figure 7b, more electrons can be found accumulation around Au atom on 18


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clean surface than those on K added surface (marked with red frame), indicating that Au atom has donated more electrons to OH on K covered surface than those on clean surface, thereby a stronger interaction has formed between OH and Au atom on K covered surface. Besides, However, according to the Bader charge displayed in Figure 3, the Au atom at the TS is less positively charged on the K pre-adsorbed Au(111) surface than that on the clean surface, thus, it can be supposed that more lost charge on Au atom of K covered surface than that of clean surface resulting from the interaction between Au and OH is replenished duly by accepting charge from K atom. The illustration added in Figure 7b reflecting charge distribution between Au and other fragments confirms our guess in which charge transfer occurs from K atom to the surface Au atom (marked with red frame). The dipole-dipole interaction model can be used to interpret our results that the pre-adsorbed K atom on transition metal surface induces more charge transfer from K atom and the surface Au atom to the antibonding state of electronegative adsorbates and meanwhile, K atom timely complement the missing electrons on surface metal atom enabling dipole-dipole interactions like ((Kδ+-OHδ-)-(Auδ+-OHδ-)), which stabilize electronegative adsorbates (e.g. OH) and thus facilitates the dissociation of the adsorbates. It can be found that two crucial factors are indispensable in the whole interaction process, involving the highly ionic bonding nature of K adsorption as described earlier and the electronegative feature of the adsorbates. The OH binding at the TS on clean metal surfaces is not strengthened by such interactions and thereby has a much higher energy barrier. The dipole-dipole interaction mechanism has been reported to stabilize adsorbates and reduce the reaction barrier on the K-assisted dissociation of the electronegative CO and N2.54 (b) Geometric factor analysis. The interaction energy, EHintL OH , is believed to consist of three components:55 (i) the direct Pauli repulsion between the H and OH, which is strongly relative to the distance between them.56 (ii) the bonding competition effect, which is caused by the common metal atoms shared by OH and H.57 (iii) the electrostatic interaction between OH and H at the TS configuration. Apparently, these three components are all sensitive to the TS structure, thus, EHintL OH is a quantitative measure of the geometrical effect on H2O dissociation reaction. The shorter distances between OH and H at TSs on K pre-adsorbed metal surfaces (see Table 2) indicate that direct Pauli repulsion become stronger and it is not beneficial for reducing the barrier on K covered surfaces. Nevertheless, the fact that both OH and H, still bonding with the same metal atoms, keep far away 19



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from the substrates (see Table 2) results in a weak bonding competition, which helps lowering the barrier on K pre-adsorbed metal surfaces. In addition, the electrostatic interactions between OH and H fragments at TSs become more attractive on K pre-adsorbed metal surfaces as compared with those on clean surfaces, which can be roughly estimated from the Bader charge analysis for OH and H atoms at TSs, in which the charges of OH become much more negative and those of H are slightly more positive K pre-adsorbed metal surfaces (see Table 4). The more attractive interaction may also result in the shorter bond length of dissociating O-H at TSs on K covered surfaces, signifying less strain on the dissociating molecule and thus contribute much to a lower activation barrier than on the clean surfaces. As mentioned above, the weak bonding competition and strong attractive electrostatic interaction between OH and H at TSs led to the K adatom promotion on the O-H activation over the metal surfaces. In summary, we have explored the physical origin of K promotion effect on the dissociation of H2O electronically and geometrically. It can be found that two crucial factors involving the highly ionic bonding nature of K adsorption and the electronegative feature of the adsorbates enable dipoledipole interactions like ((Kδ+-OHδ-)-(Auδ+-OHδ-)), which stabilize electronegative OH and thus facilitate the dissociation of H2O. On the other hand, the dipole-dipole interactions result in more appropriate interactions between the dissociating fragments on K-covered surfaces compared with those on clean surfaces, which reduce the bonding competition between dissociating OH and H and reduces the strain on the dissociating H2O molecule, and thereby lowering the activation barrier.

Table 4. Bader Charge of OH and H Atoms at TSs for Various Metals with and without A Potassium Atom Au

Ag

Cu

Ni

Pt

Rh

Ir

Pd

Ru

Co

OH/e

-0.4

-0.50

-0.47

-0.45

-0.37

-0.4

-0.39

-0.38

-0.48

-0.45

H/e

0.11

-0.02

-0.01

0

0.18

0.14

0.14

0.13

0.04

-0.01

K/OH/e

-0.59

-0.63

-0.62

-0.59

-0.5

-0.55

-0.54

-0.53

-0.59

-0.60

K/H/e

0.12

0.08

0

0.08

0.23

0.15

0.20

0.13

0.06

0.04

20


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Figure 7. Charge density difference map for (a) OH adsorption on clean Au surface at the TS and (b) OH adsorption on K covered Au surface at the TS; the added illustration reflects charge distribution between Au and other fragments. Each plot is shown with a top view (left) and side view (right). The yellow and blue isosurface stand for the accumulation of electron density and the depletion of electron density, respectively.

4. Conclusions Water adsorption and dissociation on clean and potassium pre-adsorbed transition metal surfaces have been explored systematically using the DFT-GGA method. The calculation results show that presence of potassium atom enhances the binding strength of H2O due to the direct interaction between K and O of H2O, but weaken the binding strength of OH due to the repulsion between the electronrich O of OH and meal surfaces induced by the K atom. Further, the potassium adatom can be found to promote the catalytic activity of various metals for H2O dissociation to varying degrees and the magnitude of potassium-induced promoting effect is associated with the chemical activity of transition metals, that is, the more promoting effect of potassium on the water O-H bond scission occurs on the less chemical active transition metals. Moreover, the physical origin of potassium promotion effect on the dissociation of water was investigated electronically and geometrically. It can be found that two crucial factors involving the highly ionic bonding nature of K adsorption and the electronegative feature of the adsorbates enable dipole-dipole interactions like ((Kδ+-OHδ-)-(Auδ+-OHδ-)), which stabilizes OH group and weaken the interaction between OH and H at the TS and thus facilitate the dissociation of water. Our results suggest that the pre-adsorbed potassium additives play an important role in heterogeneous catalytic processes on metal surfaces, and it is hoped that the present results 21



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could be applicable to other electropositive additives like Na, Cs in the activation of O-H bond involved in H2O or CH3OH.

Supporting Information Structure properties of ISs and TSs for water dissociation on metal surfaces, experimental results of H2O dissociation on different metal surfaces, the adsorption energy and Bader charge of potassium atom on various bare metals, adsorption energies of H2O, OH, and H on various clean and K preadsorbed metals, the relationship between the adsorption strength of H2O and H on various metals with and without K and the electronic properties of these metals and charge density difference map for K atom adsorbed on Au(111) surface.

Acknowledgements This work was supported by the National Natural Science Foundation of China (Grants No. 21773123, 91545106), the 111 project(B12015), and the foundation of State Key Laboratory of Coal Conversion (Grant No. J17-18-908).

References 22


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Water Dissociation on Clean and Potassium Pre-Adsorbed Transition

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