Hydrogen-Bond Induced Nitric Oxide Dissociation on Cu(110) - The

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Hydrogen-Bond Induced Nitric Oxide Dissociation on Cu(110) Thanh Ngoc Pham, Masahiro Sugiyama, Fahdzi Muttaqien, Septia Eka Marsha Putra, Kouji Inagaki, Do Ngoc Son, Yuji Hamamoto, Ikutaro Hamada, and Yoshitada Morikawa J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01208 • Publication Date (Web): 16 May 2018 Downloaded from http://pubs.acs.org on May 16, 2018

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The Journal of Physical Chemistry

Hydrogen-Bond Induced Nitric Oxide Dissociation on Cu(110) Thanh Ngoc Pham,1,2 Masahiro Sugiyama,1 Fahdzi Muttaqien,1 Septia Eka Marsha Putra,1 Kouji Inagaki,1,3 Do Ngoc Son,2 Yuji Hamamoto,1,3 Ikutaro Hamada,1,3 and Yoshitada Morikawa1,3,4* 1

Department of Precision Science and Technology, Osaka University, 2-1 Yamada-oka, Suita,

Osaka 565-0871, Japan. 2

Faculty of Applied Science, Ho Chi Minh City University of Technology, VNU-HCM, 268 Ly

Thuong Kiet Street, District 10, Ho Chi Minh City, Vietnam. 3

Elements Strategy Initiative for Catalysts and Batteries (ESICB), Kyoto University, Katsura,

Kyoto 615-8520, Japan. 4

Research Center for Ultra-Precision Science and Technology, Graduate School of Engineering,

Osaka University, 2-1, Yamada-oka, Suita, Osaka 565-0871, Japan.

ABSTRACT. We have studied the dissociation process of nitric oxide (NO) on Cu(110) and the influence of the hydrogen bond with water by means of density functional theory calculations. We have found that an upright NO adsorbed at a short-bridge site and a side-on NO at a hollow site connecting two short-bridge sites are the two most stable molecularly adsorbed states, and the latter is the precursor for the dissociation process. Various NO dissociation pathways under

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the influences of the hydrogen bonds with water have been investigated. We have found that hydrogen bonds efficiently reduce the activation energy of NO dissociation by the introductions of a water dimer to O and water dimers to both sides of the side-on NO, respectively. More importantly, the promoting effect of water molecules on NO dissociation is dominant only when one of water molecules in a water dimer forms a hydrogen bond with O of the side-on NO. Our results provide a physical insight into the promoting effect of hydrogen bonds with water, which may be helpful for improving catalytic activity as well as designing novel catalysts for NO reduction.

Introduction Nitric oxide (NO) emission from the exhaustive gas of combustion process has caused negative impacts on the environment, e.g, acid rain, photochemical smog, and ozone depletion. Therefore, NO reduction to harmless substances is an important task to mitigate the environmental pollution .1–9 The catalysts of expensive precious metals1–27 such as Pt, Rh, and Pd are often employed to remove NO as well as CO and unburned hydrocarbon gases simultaneously from the exhaust gas. However, the practice requires that the catalysts must be abundant, low cost, and high catalytic activity. Among various alternative materials, copperbased catalysts are very promising owing to their excellent performances for the NO reduction.28–36 As essential steps for the NO reduction, the NO adsorption and dissociation to N and O on the copper surfaces have been intensively studied by both experiment 37-46 and theory.35,40,47,48 On low-index Cu surfaces, i.e., Cu(100), Cu(110), and Cu(111), NO is molecularly adsorbed at rather low temperature followed by formations of dimeric (NO)2 and N2O species with further gas exposure.

32,37-40,43-46

Recently, Shiotari and co-workers experimentally studied the NO


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adsorption and dissociation on the Cu(110) surface at low-coverage region using scanning tunneling microscopy (STM), reflection absorption infrared spectroscopy, and electron energy loss spectroscopy.38 They revealed the configuration changes of NO on the Cu(110) surface as a function of temperature, where NO initially adsorbs at a metastable short-bridge in an upright configuration, with its N atom binding to the surface, 38-40 then flips into a stable flat-lying one at 50 K and dissociates at 160 K. Moreover, the NO adsorption on this surface was studied by density functional theory (DFT) calculations by Brión-Ríos et al.47 and by Shiotari and coworkers.40 Both studies revealed that the electron back donation from the Cu surface to 2π* orbitals takes place upon adsorption. So far, catalytic activities for NO dissociation on the Cu surfaces have been recognized to be lower than that on Rh.8,9,18,19,21 This is attributed to the facts that Rh surfaces stabilize the molecular NO adsorptions as well as the final co-adsorptions of the N and O. 18,19,21 Moreover, a step effect of Rh induces the NO dissociation with lower energy barriers and higher exothermicities.12,13,17 Nevertheless, Shiotari et al. found that the hydrogen bond between NO and water monomers promotes the NO dissociation to take place almost barrierlessly on the Cu(110) surface at a very low temperature (~12 K),36 which is a promising way to reduce the use of the precious Rh catalyst. By utilizing the STM technique to image the reaction, a complex formed with an adsorbed NO and a water monomer (NO···HOH) was observed, in which hydrogen bond between adsorbates induces the back donation from the Cu surface, leading to the weakening of N-O bond. Then, another water monomer was introduced near the N side of NO···HOH and the adsorbed NO was dissociated spontaneously into a hydroxyl dimer 49 and (OH+NH) group along the [001] direction in the final product.


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Herein we present a DFT calculation to clarify the role of water molecules and the mechanism of the hydrogen bonding induced NO dissociation on the Cu(110) surface. First, we study the molecular adsorption and dissociation of single NO adsorption as well as stable NO-water complexes. Then, the promotion effect is assessed by examining energy barriers for the NO dissociation with and without the presence of water molecules. Finally, we clarify the electronic origin of the hydrogen bond induced NO dissociation by analyzing the electronic structure of adsorbed NO. II Computational details The calculations were carried out by using the simulation tool for atom technology (STATE) package.50-52 The Vanderbilt’s ultrasoft pseudopotentials were used to describe the electron-ion interactions.53 Wave functions and argumented charge density were expanded by a plane wave basis-set with cut-off energies of 36 and 400 Ry, respectively. In this study, we compared the NO adsorption and dissociation results obtained by the Perdew-Burke-Ernzerhof (PBE) 54 generalized gradient approximation (GGA) exchange-correlation functional to those obtained by van der Waals density functionals (vdW-DFs),55,56 which take into account the dispersion interaction, which cannot be described accurately by GGA. We used vdW-DF1, 55 optB86bvdW,57 and rev-vdW-DF258 functionals as implemented51,59,60 in the STATE code. To model the Cu(110) surface, we used a repeated slab model with four atomic layer thickness and a vacuum region of 12 Å. The slab was constructed based on the optimized Cu bulk lattice constant for each vdW-DF functional obtained in our previous work. 61 The bottom two layers were fixed at their respective bulk position, while the topmost two layers and adsorbates were allowed to relax until the force acting on them are less than 5 × 10

eV/Å. The surface

Brillouin zone was sampled with the Monkhorst-Pack62 k-point meshes of 8 × 4 × 1, 8 × 2 × 1,


4

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6 × 4 × 1, 4 × 6 × 1, and 4 × 3 × 1 for p(2 × 3), p(2 × 5), p(3 × 3), p(4 × 2), and p(4 × 4) unit cells, respectively. Minimum energy pathways (MEP) for the NO dissociations were investigated by using the nudged elastic band (NEB)63 and the climbing image nudged elastic band (CI-NEB) methods. 64 The saddle point of the minimum energy pathway (MEP) was confirmed by observing a single imaginary eigen frequency in the vibrational modes. We performed the vibrational mode analysis by applying the finite different method with fixed metal slab and evaluated the vibration modes of adsorbates only. The adsorption energy of molecule X (X= NO or H 2O) on Cu(110) is defined by 𝐸

=𝐸

where 𝐸

/

/

−𝐸

−𝐸

, 𝐸 , and 𝐸

,

(1)

are the total energies of X/Cu(110), Cu(110), and X molecule in the

gas phase, respectively. The spin polarization is taken into account for the open shell structure of gas phase NO. However, for the adsorbed NO, the spin polarization disappears, in agreement with the previous theoretical works of the NO adsorption on the Cu(110) surface. 40,47 For the NO-water complex with hydrogen bond on the surface, the co-adsorption energy is given by 𝐸

/

=𝐸

−𝐸

−𝐸

− 𝑛𝐸

.

(2)

Here, n is the number of water molecules co-adsorbed with NO. The hydrogen bond energy between the adsorbed NO and the water molecules is calculated as 𝐸

=𝐸

where 𝐸

− 𝐸 and 𝐸

+ 𝑛𝐸

,

(3)

are the adsorption energies for single NO and single water molecule at the

most favorable adsorption sites on the Cu(110) surface, respectively. With our definitions, the negative hydrogen bond energy means that the complex is more stable than the isolated fragments on the surface.


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III Results and discussion Dissociative adsorption of NO on the Cu(110) surface In this section, we discuss the single NO adsorption and MEP of the NO dissociation on the Cu(110) surface. After optimizing possible initial NO/Cu(110) configurations, we obtained two types of favorable adsorption configurations: (1) an upright NO in which N binds to the surface (N*O) and (2) a side-on NO connecting two short-bridge sites (N*O*) along [001] direction (Figure 1b). The asterisk denotes a binding site of NO to the surface. The possible N*O structures are on-top (T-N*O), long-bridge (LB-N*O), short-bridge (SB-N*O), and hollow (HN*O) (Figure 1a).

Figure 1. (a): Adsorption sites for the upright N*O on the Cu(110) p(2 × 3) surface: on-top (T), long-bridge (LB), short-bridge (SB), and hollow (H); (b) Schematic diagram of MEP for NO dissociation. The most stable short-bridge upright, metastable precursor, transition, and final product states are denoted by SB-N*O, N*O*, TS1, and N*+ O*, respectively. Color schemes: Cu: orange, O: red, N: blue.


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Table 1. Adsorption energy of single NO molecule (𝐸 of NO from the topmost Cu layer (ℎ normal (∠ (𝜈

), N-O bond length (𝑑

), height of N

), tilting angle of the adsorbed NO from the surface

). The 4-layer Cu(110) p(2 × 3) supercell is used. The stretching mode of NO

) is calculated with optB86b-vdW and zero-point energy corrected adsorption energies are

given in parentheses. Adsorption sites Functional

T-N*O

LB-N*O

PBE

-0.96

vdW-DF1

optB86b-vdW

-1.22

H-N*O

N*O*

-0.68

-1.06

𝐸

/eV

-0.90

𝑑

/Å

1.19

1.21

1.21

1.23

1.33

1.78

1.15

1.37

1.11

0.76

0

0

0

0

75

-1.19a

ℎ

/Å

∠

/˚

𝐸

/eV

-0.69

-0.77

-1.00

-0.49

-0.76

𝑑

/Å

1.19

1.22

1.21

1.22

1.32

1.81

1.18

1.38

1.17

0.76

0

0

0

0

80

-1.10

-1.21

-0.97

-1.42

(-1.06)

(-1.17)

(-0.97)

(-1.40)

1.19

1.22

1.21

1.23

1.34

1.78

1.14

1.38

1.00

0.75

0

0

0

0

72

1698

1442

1547

1341

946

ℎ

/Å

∠

/˚

𝐸

𝑑

/eV

/Å

ℎ

/Å

∠

/˚

𝜈

rev-vdW-DF2

SB-N*O

/cm-1

-1.46 (-1.42) -1.52b

𝐸

/eV

-1.03

-1.14

-1.39

-0.89

-1.32

𝑑

/Å

1.19

1.22

1.21

1.23

1.33


7


a

ℎ

/Å

∠

/˚

Page 8 of 38

1.73

1.17

1.37

1.03

0.73

0

0

0

0

76

Calculated with PBE in Ref. 40.

b

Calculated with optB86b-vdW in Ref. 47. The order of the adsorption strength of NO obtained with PBE, optB86-vdW and rev-vdW-

DF2 is SB-N*O > N*O* > LB-N*O > T-N*O > H-N*O, whereas vdW-DF1 gives a slightly different trend, in which LB-N*O is more favorable than N*O* (Table 1). The most favorable adsorption site is SB-N*O, regardless of the functional used, which is consistent with other theoretical works,40,47 and also experimental STM observations below 40 K. 38-40 The calculated adsorption energies of N*O* obtained with PBE, optB86b-vdW, and rev-vdW-DF2 are less stable than that of SB-N*O by ΔE of 0.16, 0.04, and 0.07 eV, respectively, as shown in Table 2. As will be discussed below, the relative stability of SB-N*O and N*O* depends on the unit cell size and slab thickness. Moreover, the activation energy (𝐸 ) for a flipping from SB-N*O to N*O* are 0.18, 0.10, and 0.14 eV with PBE, optB86b-vdW, and rev-vdW-DF2, respectively, in good agreement with the experimental estimate of 0.13 eV. The energy differences between SBN*O and N*O* (ΔE = 0.04 and 0.07 eV with optB86b-vdW and rev-vdW-DF2, respectively) as well as the energy barriers required for the conversion from SB-N*O to N*O* (𝐸

= 0.10 and

0.14 eV with optB86b-vdW and rev-vdW-DF2, respectively) are more comparable with the experimental values (ΔE = -0.05 eV and 𝐸 = 0.13 eV) than those calculated with PBE (ΔE = 0.16 eV and 𝐸 = 0.18 eV). Therefore, it is important to include the dispersion force to describe the stability of the NO chemisorption states on Cu(110). Table 2. Calculated energy difference between SB-N*O and N*O* (ΔE), energy barrier for the flipping process from SB-N*O to N*O* (𝐸 ), activation energy (𝐸

), and effective activation


8

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energy (𝐸

) of NO dissociation on the Cu(110) surface. The ZPE corrected energies are in

parentheses. The positive (negative) value of ΔE indicates SB-N*O is more (less) stable than N*O*.

Functional

Cell size Cu layers

ΔE /eV

/eV

𝐸

𝐸

/eV

𝐸

/eV

PBE

p(2 × 3)

4

0.16

0.18

0.82

0.99

vdW-DF1

p(2 × 3)

4

0.24

0.25

0.70

0.94

4

0.04 (0.02)

6

0.07 (0.04)

0.14

0.75

0.82

4

0.06 (0.04)

0.16

0.79

0.85

6

0.02 (-0.01)

4

0.04 (0.02)

6

0.01 (-0.01)

4

0.07

0.14

1.07

1.14

-0.05

0.13

0.10 (0.08) 0.76 (0.76) 0.80 (0.78)

p(2 × 3)

optB86b-vdW p(3 × 3)

p(4 × 2)

rev-vdW-DF2

p(2 × 3)

Exp.a a

0.39

Experimental value taken from Ref. 38. However, the experimental work by Shiorari et al.38 revealed the side-on configuration is more

stable than the upright one, which contradicts with our DFT results. To resolve the discrepancy, we performed the calculations with the zero-point energy correction using larger surface unit cells (lower NO coverage) and slab thickness. We used p(3 × 3) and p(4 × 2) surface supercells, which correspond to the NO coverage of 1/9 and 1/8, respectively. We find that the stabilities of SB-N*O and N*O* depend on the NO coverage and the slab thickness and N*O* are more


9


Page 10 of 38

stable than SB-N*O by both -0.01 eV for p(3 × 3) and p(4 × 2), respectively (Table 2). Thus, our results are consistent with the experimental data taken for the isolated NO on the surface. 38 We calculated the vibrational frequency of the adsorbed NO and find that the stretching frequency of NO decreases from 1855 cm-1 in gas-phase value to 1547 and 946 cm-1 for SB-N*O and N*O*, respectively (Table 1). The decrease of N-O stretching mode can be attributed to the electron back donation to 2* orbitals of adsorbed NO, leading to an increase of N-O bond lengths from 1.17 Å to 1.21 and 1.34 Å for SB-N*O and N*O*, respectively. Our calculated frequencies are in reasonable agreement with the previous experimental values 38 of 1517 and 855 cm-1. The schematic energy diagram for the NO dissociation is presented in Figure 1b and summarized in Table 2. An effective activation energy (𝐸

) is calculated from the most stable

SB-N*O to transition state TS1, while an activation energy ( 𝐸 metastable precursor N*O* to TS1. The calculated 𝐸

) is calculated from the

’s using the Cu(110) p(2 × 3) surface

are 0.99, 0.94, 0.80, and 1.14 eV obtained by PBE, vdW-DF1, optB86b-vdW, and rev-vdW-DF2, respectively. The calculated effective activation energies are lower than that of desorption energies (1.22, 1.00, 1.46, and 1.39 eV with PBE, vdW-DF1, optB86b-vdW, and rev-vdW-DF2, respectively), being consistent with the experimental observation, in which NO dissociation was observed rather than desorption when surface temperature is increased to 160K. 38 Moreover, the energy barriers are independent of the slab thickness and supercell size as the results from optB86b-vdW for the four-layer and six-layer Cu slabs as well as ones using p(2 × 3) and p(3 × 3) are almost analogous. From Table 2, the 𝐸 closest to the experimentally predicted 𝐸

of 0.80 eV calculated by optB86b-vdW is the of 0.39 eV.38

However, one should note the

optB86b-vdW based energy barrier is still higher than the estimated value in STM results. We


10

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attribute the discrepancy to the tip or the electric field induced dissociation effects by the experimental STM technique. Formation of N*O···HO*H complex

Figure 2. Top view of most favorable water monomer adsorption and the N*O···HO*H complexes on the Cu(110) surface. Color scheme: Orange: Cu, Blue: N, Red: O, White: H. Table 3. Calculated ZPE corrected co-adsorption energies (𝐸 (𝐸 ), N-O bond length (𝑑 ) of N*O···HO*H complexes. Cell size Cu layers

SB p(2 × 3)

p(3 × 3) p(4 × 4)

/eV

𝐸

T1

T2

T1

T2

/Å

𝑑

/eV

𝐸

SB

), hydrogen bond energy

SB

T1

T2

4

-1.97 -2.10 -2.09 -0.12 -0.25 -0.24 1.25 1.24 1.25

6

-1.98 -2.02 -2.00 -0.13 -0.17 -0.15 1.25 1.24 1.25

4

-2.08 -2.08 -2.06 -0.23 -0.23 -0.21 1.25 1.24 1.24

6

-1.98 -2.00 -1.97 -0.13 -0.15 -0.12 1.25 1.24 1.24

6

-2.04 -2.05 -2.04 -0.19 -0.20 -0.19 1.25 1.24 1.24

We obtained three types of N*O···HO*H complex in which water is adsorbed on top or shortbridge site of next topmost Cu [110] row as shown in Figure 2. The adsorbed NO is tilted due to an attractive interaction caused by the hydrogen bond between O of SB-N*O and H of H 2O*. The calculations only focus on optB86b-vdW since the stability of SB-N*O and N*O* is well reproduced. The calculated hydrogen bond energies are shown in Table 3. Here, 𝐸

is -0.51 (-


11


Page 12 of 38

0.43) eV without (with) ZPE correction, where the on-top site is the most favorable adsorption site for water monomer on Cu(110)65,66 (Figure 2). The obtained negative hydrogen bond energies indicate water molecules and NO tend to form a stable complex. The N-O bond lengths in the N*O···HO*H complexes are elongated by 0.04 Å due to the intermolecular hydrogen bonding. As shown in Table 3, T1 is more stable than SB regardless of the supercell size and slab thickness, meanwhile, T2 is less stable than SB calculated with the p(3 × 3) supercells. Therefore, the complex with H2O on the top site (T1) is the most stable. Experimentally, only the SB configuration was observed36 at a very low coverage regime of adsorbates while the N*O···HO*H complexes with water on top sites were not. Nevertheless, the relative energy differences of T1 with SB becomes smaller when the supercell (coverage of adsorbates) gets larger (lower), i.e. 0.04, 0.02, and 0.01 eV calculated with the 6-layer Cu(110) p(2 × 3), p(3 × 3), and p(4 × 4) supercells, respectively. Thus, the discrepancy might arise from other factors such as experimental conditions (e.g. the effects of the electric field from STM tip), or errors in prediction of the stable NO-H2O complexes using the vdW-DF method. Effect of water monomers on NO dissociation. We investigated the MEPs for NO dissociation in the presences of water monomers by using the CI-NEB method. We considered NO dissociation pathways starting with all three N*O···HO*H complexes (Figure 3), i.e., experimentally observed SB, T1, and T2. The influence of the second water monomer placed near N of N*O···HO*H is interpreted via a pathway starting with an H2O*+ N*O···HO*H structure (Figure 4). The p(2 × 3) unit cell was used to investigate the MEPs starting from N*O···HO*H complexes, while the sufficiently larger p(2 × 5) cell was adopted to investigate the effect of the second water monomer.


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Figure 3. The ZPE corrected MEP for NO dissociation starting with N*O···HO*H complexes on the Cu(110) p(2 × 3). Color schemes: Cu: orange, O: red, N: blue. The dissociation pathways starting with N*O···HO*H complexes are presented in Figure 3. Initially, NO molecule flips from the tilted configuration to side-on configuration, and the water molecule diffuses to the next neighboring short-bridge site. Those barriers of the pathways starting with SB, T1, and T2 structures are 0.65, 0.63, and 0.62 eV, respectively. From a precursor N*O*+ H2O*, the N-O bond is elongated and cleaved at TS3. The dissociated O diffuses over the topmost Cu row and reacts with the HO*H to form hydroxyl dimer. 49,67 The activation energy is 0.65 eV. The effective activation energies calculated from the N*O···HO*H states to TS3 are 0.80, 0.93, and 0.92 eV for SB, T1, and T2 pathways. It appears that the pathway starting with T1 is the most probable pathway for NO dissociation in the presence of one water monomer. Nevertheless, in the following, we use the SB pathway to discuss the


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Page 14 of 38

influence of water monomers on the NO dissociation, since the energy barriers of SB and T1 pathways differ only by 0.13 eV and our conclusion does not alter.

Figure 4. The ZPE corrected MEP for NO dissociation starting with H 2O*+N*O···HO*H complex on Cu(110). Color schemes: Cu: orange, O: red, N: blue. Table 4. Calculated ZPE corrected co-adsorption energy (𝐸 and ZPE corrected hydrogen bond energy (𝐸

), N-O bond length (𝑑

) at initial molecularly co-adsorbed states. The

activation energy for the flipping process of NO (𝐸 ), the activation energy (𝐸 effective activation energy (𝐸

𝐸

/ eV

),

∗

), and the

), along with the reaction energy (E), are also included.

𝑑

∗

/Å

𝐸

/eV 𝐸 /eV 𝐸

/eV 𝐸

/eV

E /eV

SB-N*O

-1.42

1.21

0

0.08

0.76

0.78

-0.56

N*O···HO*H

-1.97

1.25

-0.12

0.65

0.65

0.80

-1.19


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H2O*+ N*O···HO*H

-2.43

1.26

-0.15

0.71

0.66

0.95

-2.35

For the reaction path starting with H2O*+ N*O···HO*H shown in Figure 4, the first two elementary steps are analogous to that of the N*O···HO*H pathway. From Table 4, the hydrogen bond energy for H2O*+ N*O···HO*H (-0.15 eV) is slightly larger than that for N*O···HO*H (0.12 eV), indicating that a further supply of water monomer near N*O···HO*H does not significantly alter the hydrogen bond strengths of absorbates. The dissociation path starts with diffusion process toward a metastable precursor 2H2O*+ N*O*, followed by the N-O bond rupture and hydroxyl dimer formation. After these primary steps, the dissociated N diffuses over the Cu row with a barrier of 0.53 eV and reacts with water to reach the final state. The activation energy for the reaction path is 0.66 eV and effective activation energy is 0.95 eV. We find that the NO dissociation in the presence of water monomers is accompanied by significantly larger activation energies for transformation from the short-bridge to the side-on NO configurations (Table 4). Without the water monomers, the activation energy for this conversion is 0.08 eV, while these energies in the N*O···HO*H and H 2O*+ N*O···HO*H pathways are 0.65 and 0.71 eV, respectively. The larger barriers in the presence of the water monomers include the barriers for water diffusion and that for the dissociation of the hydrogen bond between N*O and HO*H. From Table 4, the activation energies of the dissociation paths starting with N*O···HO*H and H2O*+ N*O···HO*H are 0.65 and 0.66 eV, respectively, which are slightly lower than that of the MEP without the involvement of the water monomers (0.76 eV). However, the effective activation energies (0.80 eV, 0.95 eV for N*O···HO*H and H 2O*+ N*O···HO*H, respectively) are higher than that of NO dissociation (0.78 eV), indicating that these pathways are kinetically less favorable. One may argue that the hydrogen bonds are


15


Page 16 of 38

dissociated at the metastable precursors, N*O*+ H 2O* and N*O*+ 2H2O*, leading to higher effective energies for NO dissociation. We concluded these states are metastable precursors for the NO dissociation in the presence of the water monomers owing to our intensive investigations for the co-adsorption of the side-on NO and single water monomer that suggests the dissociation through N*O*+ H2O* is most probable pathway (see Supporting Information for details). In contrast, the reaction energies of the dissociation paths with the involvements of the water monomers are more exothermic than that without the presence of water (Table 4), implying these pathways are thermodynamically favorable. Nevertheless, our results indicate that the influence of the water monomers is ineffective in the induction of NO dissociation on the Cu(110) surface. Effect of water dimer in NO dissociation. We further investigated NO dissociation under the influence of a water dimer on Cu(110) surface. A single water dimer on the Cu(110) surface was experimentally observed by Kumagai and co-workers.68 We introduced a water dimer to O side (HO*H···OH2···O*N*) and N side (HO*H···OH2···N*O*) of N*O* and the p(2 × 3) unit cell of Cu(110) was used. The MEPs with the presence of one water dimer are presented in Figure 5.


16

Page 17 of 38 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


Figure 5. The ZPE corrected MEPs for NO dissociation on Cu(110) starting with HO*H···OH2···O*N* (red line) and HO*H···OH2···O*N* ( blue line). Color schemes: Cu: orange, O: red, N: blue, H: White. Table 5. The calculated ZPE corrected co-adsorption energy ( 𝐸 (𝑑

∗

∗

), the N*-O* bond length

), the hydrogen bond length between N*O* and water dimer (𝑑

hydrogen bond length of the water dimer (𝑑

∗ ∗

), and the internal

) at initial states. The activation energy (𝐸 )

∗

and the reaction energy (E) for the NO dissociation are presented. Site

Cell size

𝐸

HO*H···OH2···N*O*

p(2 × 3)

-2.50

1.37

1.95

p(2 × 3)

-2.49

1.39

p(2 × 5)

-2.42

1.39

*

*

HO H···OH2···O N

/eV

𝑑

∗

∗

/Å

/Å

/Å

𝐸 /eV

E /eV

1.59

0.50

-1.19

1.85

1.63

0.34

-0.96

1.86

1.62

0.34

-0.91

𝑑

∗ ( ∗)

𝑑

∗

*

Co-adsorption energies of HO*H···OH2···O*N* (-2.49 eV) and HO*H···OH2···N*O* (-2.50 eV) are only slightly more negative than that of isolated N*O···HO*H+ H 2O* (-2.40 eV) and


17


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N*O+ HO*H···O*H2 (-2.43 eV) (Figure 5 and Table 5). Thus, the initial states are slightly more stable than the complex of NO and the water monomer. The side-on N-O bond under the influence of a water dimer is elongated from 1.35Å in N*O*+ H2O* to 1.37, 1.39 Å in the HO*H···OH2···N*O* and HO*H···OH2···O*N* structure, respectively. For the pathways starting with HO*H···OH2···O*N*, the N-O bond is first ruptured, and then a hydroxyl dimer is formed through the proton transfer along the water dimer. The activation energy is 0.34 eV and the reaction energy is -0.96eV (Table 5). For the pathway starting with HO*H···OH2···N*O*, the formation of new O*N*H first takes place. This step is almost barrierless due to the hydronium ion formation at the transition state TS8 by the proton transfer along the water dimer. Then, the newly-formed O*N*H is dissociated with the activation energy of 0.50 eV and reaction energy of -1.19 eV (Table 5). Since reaction energies for both of the pathways are exothermic, the reverse reactions are not favorable, implying the water molecules at final states would desorb.


18

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Figure 6. The ZPE corrected MEP for NO dissociation on the Cu(110) p(2 × 5) surface starting with 2HO*H···OH2···N*O*. Red line (blue line) denotes dissociative (associative) pathway. Color schemes: Cu: orange, O: red, N: blue, H: White. We further explored the influence of water dimers on NO dissociation by introducing each water dimer near each side of the side-on N*O* (2HO*H···OH2···N*O*) on the Cu(110) p(2 × 5) surface. This reaction is proposed to proceed via two pathways, i.e., dissociative and associative ones, as shown in Figure 6. The NO bond length in 2HO*H···OH2···N*O* is 1.42 Å, which is more elongated than the one in HO*H···OH2···N*O* and HO*H···OH2···O*N*. For the dissociative pathway, the N-O bond ruptures first, followed by the formation of hydroxyl dimer from dissociated O*. The proton transfer to free N* is barrierless to reach the final state. The rate-limiting step in this pathway is the first elementary step (N-O bond rupture) with the activation energy of 0.27 eV. For the associative pathway, the barrierless formation of O*N*H is followed by the dissociation of this newly-formed intermediate, which needs to overcome the energy barrier of 0.36 eV. The rate-limiting step is the second step, namely, the O*N*H dissociation. Comparing the activation energies of both pathways, the formation of O*N*H makes an obstacle for the reaction occurs due to the stabilization of the intermediate. Moreover, the proton transfer process is spontaneous and barrierless. Once again, the obtained reaction energy is exothermic by -2.06 eV, suggesting that the reverse process is not favorable and two water molecules at final state will desorb. Formations of the ONH intermediates were discussed previously when NO gas is reduced by H2 on several precious metal surfaces, i.e., Rh,16 Pt,69,70 and Pd.71,72 By utilizing DFT calculations, Bai et al.69 found the formation of O*N*H on Pt(110) surface at a bridge site, in which N and O atoms of O*N*H simultaneously bind to the surface. However, an H-assisted NO


19


Page 20 of 38

reduction pathway on the Pt surface via such O*N*H intermediate is less favorable, similar to our results on Cu(110). On Pt(111),70 a combination of experimental kinetic and isotopic data with DFT based activation free energies indicates that the hydrogenated substance HNO is formed first, in which only N of HNO binds to the surface. Then, a further hydrogenation process occurs to form HNO-H, which limits NO-H2 reaction rates, followed by its decomposition into NH and OH intermediates that undergo next reactions to give final products H2O, N2O, NH3, and N2. Based on our calculated activation energies, the presence of the water dimers significantly lowers the activation energies of the NO dissociation. The hydrogen bonds with the water dimers are therefore effective in promoting the NO dissociation. Electronic structure analysis. We have studied several reaction pathways for the NO dissociation in the presence of the water monomers and dimers. Our calculated effective activation energies for NO dissociation with the involvements of the water monomers are slightly higher than the one without water, owing to the hydrogen bond dissociation at the metastable precursors, where NO is in the side-on configuration. By introducing the water dimers near N*O*, hydrogen bonds between NO and water are retained, and therefore the N-O bond is weakened as demonstrated by its longer bond length. The N-O bond is elongated from 1.34 Å in N*O* and 1.35 Å in both N*O*+ H 2O* and N*O*+ 2H2O* to 1.37, 1.39, and 1.42 Å in HO*H···OH2···N*O*, HO*H···OH2···O*N*, and 2HO*H···OH2···N*O*, respectively. Formation of the hydrogen bond with a water dimer activates the N-O bond in the side-on configuration, thereby lowering the activation barrier significantly.


20

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Figure 7. Projected densities of states (PDOSs) onto the molecular orbitals of the isolated NO molecule. The upper part demonstrates the valence states of NO (the occupied 1π, 5σ orbitals, and antibonding partially occupied 2π* orbitals) with four topmost Cu atoms. Doubly degenerate π[

],

π[

]

of π orbitals are dumbbell shapes along [001] and [110], respectively.

To further understand the role of the hydrogen bond in the promotion of the NO dissociation, we selected the N*O* (without the presence of water), N*O*+ H 2O* (with water but without hydrogen bond), and HO*H···OH2···O*N* (with water and hydrogen bond) structures to study electronic structure of the adsorbed NO in side-on configurations. Projected densities of states (PDOSs) onto molecular orbitals of isolated NO were calculated. As can be seen in Figure 7, the occupied orbitals of NO (1π and 5σ) in the presence of water dimer are destabilized as indicated by their PDOSs shift toward the Fermi level compared with the N*O* and N*O*+ H 2O*. In contrast, the PDOSs of antibonding 2π* orbitals in HO*H···OH2···O*N* are downshifted, and thus these orbitals are occupied further. Both suggest that the charge donation to 2π* orbitals is enhanced as confirmed by the elongations of the N-O bond.


21


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Charge rearrangement upon formation of the hydrogen bond between the adsorbates is shown in Figure 8, where the charge rearrangement of N*O*+ H2O* is calculated as 𝜌

∗ ∗

∗

= 𝜌

∗ ∗

∗

−𝜌

∗ ∗/

−𝜌

∗

.

(4)

Meanwhile, the charge density difference of HO*H···OH2···O*N* is defined by 𝜌

∗

⋯

⋯ ∗ ∗

= 𝜌

∗

⋯

⋯ ∗ ∗

−𝜌

∗

∗ ∗/

−𝜌

.

(5)

Figure 8. Charge density differences for N*O* + H2O* (a), HO*H···OH2···O*N* with Cu surface (b), and HO*H···OH2···O*N* without Cu surface (c). The atomic geometry in the HO*H···OH2···O*N* without Cu surface is kept fixed with that in the presence of substrate. Yellow and cyan indicate charge accumulation and depletion, respectively. The isosurface is plotted at 0.0067 e × Å . From Figure 8a, even though hydrogen bond between NO and water in N*O*+ H 2O* structure is dissociated, slight back donation from Cu substrate to 2π* orbitals is induced owing to the dipole field made by the water adsorbed at the next neighboring short-bridge site. The water adsorbed at the short-bridge site has an electric dipole directed towards the vacuum, which induces electric field at the NO site and charge transfer from N to O within the NO molecule.


22

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The electric field induced by the co-adsorbed water reduces the NO activation energy from 0.76 eV in the N*O* to 0.65 eV.73 The charge density differences for HO*H···OH2···O*N* structure with and without the Cu substrate are shown in Figure 8b and Figure 8c, respectively. In both cases, the charge density differences are quite similar to each other, indicating that the role of Cu substrate for the charge transfer is not so important and electron transfer from water dimer to NO is dominant. Therefore, the back donation to 2π* orbitals of NO is enhanced solely from the intermolecular hydrogen bonding, which significantly reduces the activation energy from 0.65 eV in N*O*+ H 2O* to 0.34 eV in HO*H···OH2···O*N*. Overall, the presence of water makes a contribution of 0.11 eV to reduce the activation energy, while one from the hydrogen bond is 0.31 eV, indicating the effect of the hydrogen bond is dominant in promoting the NO dissociation. Shiotari et al. revealed the hydrogen bond between water and NO in the N*O···HO*H complex promoted the NO dissociation by the STM technique.36

From our theoretical

investigations, we point out that the hydrogen bond between the water molecules and NO molecule in the side-on configuration is the important key to promote the dissociation. As discussed above, the back donation to 2π* orbitals is enhanced by the electron transfer from the water dimer to NO molecule via the hydrogen bond, being consistent with the experimental observation. Experimentally, two water monomers bonded to N and O of the upright NO adsorbed at the short-bridge site enhanced the NO dissociation. In our DFT studies, it turns out that the water dimer bonded to O of the side-on NO, not the monomer, enhanced the NO dissociation, which seems to be in contradiction with the experimental observations. Since the experimental phenomena were observed by STM, the electric field or tip effects might also contribute the enhancement of NO dissociation. Rather, our theoretical model mimics more


23


Page 24 of 38

realistic catalytic conditions of the NO dissociation in the presences of the water molecules on Cu. IV. Conclusions We have studied the influence of the hydrogen bond from water on NO dissociation on Cu(110) by using the vdW-DF methods. The van der Waals corrected functionals such as revvdW-DF2 and optB86b-vdW successfully reproduce the side-on configuration of adsorbed NO, which is an important precursor for the NO dissociation. The NO dissociation without water requires an activation energy of 0.76 eV. We find that hydrogen bonds efficiently reduce the activation energy of NO dissociation down to 0.34 eV and 0.27 eV with a water dimer and two water dimers near N*O*, respectively. Our study reveals the promotion effect of the water molecules is only dominant when one of the water molecules in a water dimer donates a hydrogen bond to O of side-on NO. The N-O bond is weakened as the results of the enhanced back donation by the hydrogen bond between the water dimer and side-on NO. Our present results provide a physical insight of the role of hydrogen bonds from water, which may be helpful to practical applications of copper surface in NO reduction as well as the design of novel catalysts for this purpose. ASSOCIATED CONTENT Supporting Information. Investigation of NO dissociation in N*O···H*OH AUTHOR INFORMATION Corresponding Author *Email: [email protected].


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ACKNOWLEDGMENT We thank Professors Hiroshi Okuyama and Akitoshi Shiotari for valuable discussions. T. N. P acknowledges the financial supports by Japan Student Services Organization (JASSO) and Toshiba scholarship for VNU-HCM. This work was supported by the Elements Strategy Initiative for Catalysts and Batteries (ESICB) from the Ministry of Education, Culture, Sports, Science, and Technology, Japan (MEXT) and the Advanced Catalytic Transformation Program for Carbon utilization (ACT-C) (Grant No. JPMJCR12YU) of the Japan Science and Technology Agency (JST), and partly supported by Grants-in Aid for Scientific Research on Innovative Areas 3D Active-Site Science (Grant Nos. 26105010 and 26105011) from the Japan Society for the Promotion of Science (JSPS). The authors acknowledge the usage of computer resources at the Institute for Solid State Physics, University of Tokyo, and the HPCI systems provided by Nagoya University, University of Tokyo, and Tohoku University through the HPCI System Research Project (Project ID: hp130112, hp140166, and hp150201). REFERENCES (1)

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Hydrogen-Bond Induced Nitric Oxide Dissociation on Cu(110) - The

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