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Cite this: DOI: 10.1039/c4cp05817h

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Oxygen-assisted water partial dissociation on copper: a model study† Ying-Qi Wang,a Li-Fen Yanb and Gui-Chang Wang*bc It is essential to understand and control the O–H bond cleavage on metal surfaces with pre-adsorbed oxygen atoms in heterogeneous catalytic processes. The adsorption and dissociation of water on clean and oxygen-pre-adsorbed copper surfaces, including Cu(111), Cu(110), Cu(100), Cu(210), Cu(211), Cu(310) and Cu(110)-(1  2), as well as Cu-ad-row and Cu-ad-atom, have been investigated by the density functional theory-generalized gradient approximation (DFT-GGA) method. The calculation results indicate that the presence of oxygen species significantly promotes the water dissociation. It is found that the promotion effect depends both on the adsorption energy of the pre-adsorbed oxygen

Received 12th December 2014, Accepted 11th February 2015

and the distance between the pre-adsorbed oxygen and the stripped hydrogen in water: the more

DOI: 10.1039/c4cp05817h

O–H bond cleavage; the shorter the distance between pre-adsorbed oxygen and hydrogen in water, the greater is the promotion effect. Based on electronic analysis, physical origin of the promotion effect can

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be attributed to the strong interaction of acid–base pair sites on oxygen–metal systems.

strongly the oxygen atom binds to the metal surface, the less the promotion effect it has on the water

1. Introduction It has long been known that the catalytic properties of metal catalysts can be significantly altered by alloying with other transition metals or by adding small amounts of promoters and/or poisons. In particular, extensive experimental studies have demonstrated that the pre-adsorbed oxygen atom has a significant effect on the cleavage of X–H (X = C, H, O, N, or S) bond, either a promotion effect or an inhibitive effect, depending on the nature of metals.1–6 Based on considerable investigation of theoretical calculations,7–12 it can be concluded that the oxygenbinding energy on metals may be the main reason for the promotion effect of oxygen atoms; i.e., lower binding energy of an oxygen atom gives rise to a higher promotion effect on the cleavage of X–H bond, whereas higher binding energy of an oxygen atom leads to a poor promotion effect. Despite numerous studies, the previous studies were limited to the situation in which the X–H bond cleavage occurred on same single crystal face such as fcc(111), and the effect of oxygen on other faces (i.e., the surface a

School of Chemistry and Chemical Engineering, ShanDong University, Jinan, 250100, P. R. China b Department of Chemistry, Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Nankai University, Tianjin 300071, P. R. China. E-mail: [email protected]; Fax: +86-22-23502458; Tel: +86-22-23503824 c State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, 030001, P. R. China † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c4cp05817h

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topology effect) is different and still unclear. For example, in the case of oxidation of ammonia on Pt, ammonia is considerably more readily activated by co-adsorbed oxygen atoms on Pt(100) than on Pt(111).13,14 Thus, it is important to investigate the effect of surface topology on the activation of X–H dissociation induced by pre-adsorbed oxygen atoms. Water is one of the most plentiful and essential compounds in nature, which makes its interaction with metal surfaces of particular interest to various fields of science. Because of its particular relevance to heterogeneous catalysis, electrochemistry and hydrogen production for fuel cells, it has prompted a large number of studies in both experimental and theoretical aspects.1–45 Water dissociation is an excellent probe due to its importance and simplicity, which needs detailed understanding in order to design, optimize and control some catalytic processes. Therefore, the elucidation of the water–surface interaction mechanism continues to be the focus of numerous investigations. Concerning the interaction between water and copper surfaces, various experimental techniques have been used.15–37 It is found that water weakly binds to Cu(111),15,16 Cu(100)17–20 and Cu(110)21,22 surfaces. Some experiments demonstrate that molecular water adsorbs on the copper surfaces;21,28,34 however, dissociative adsorption was concluded based on the detected hydroxyl species found on the clean Cu(110) at various temperatures from 90 to 190 K.26,27,29,36 Experimental36,38 and theoretical studies8,40–42 consistently indicate that the dissociation barriers range from 50 to 90 kJ mol1 on the clean Cu(110) surface. Some researchers suggested that the observed hydroxyl groups were attributed to surface impurity, defects, or electron

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beam damage.27,36,37 Water dissociation is facile on Cu(110) with surface oxygen.27,29,34 Thiel et al.1 and Henderson2 have provided comprehensive reviews on water interaction on clean, oxygen-pre-adsorbed single-crystal metal surfaces and real catalyst surfaces, whose analysis infers that the pre-adsorbed oxygen atoms exhibit varying degrees of induction activity towards water dissociation on different metals. To form a comprehensive analysis of the effect of oxygen on a catalyzed reaction, we extend our previous studies of water dissociation on a closed-packed metal surface9 to stepped and open metal surfaces containing Cu(111), Cu(100), Cu(110), Cu(110)-(1  2), Cu(210), Cu(211), Cu(310), ad-row [by the addition of one row on the Cu(111)] and ad-atom [by the addition of one atom on the Cu(111)] in this work using the density functional theory calculations. The calculation results show that both the binding energy of the chemisorbed oxygen atom and the interaction between the striped H atom in H2O and the pre-adsorbed oxygen atom affect the activity of O–H bond cleavage involved in water.

2. Calculation method and models To investigate the energy and structural details of water dissociation on metallic copper surfaces, we performed periodic, self-consistent DFT calculations with VASP (Vienna ab initio simulation package).46–48 The exchange–correlation effects have been described within the generalized gradient approximation (GGA), with the use of the Perdew–Wang (PW91) functional.49 The electron–core interaction is described by the projectoraugmented plane-wave (PAW) method.50,51 All the calculations were performed with a cutoff energy of 400 eV. The Brillouin zone was sampled with the Monkhorst–Pack grid.52 Electronic energies were calculated with (3  3  1) Monkhorst–Pack mesh k-points. When calculating the electronic properties, we used (11  11  1) k-points. The substrates were modeled by a slab that was separated by a 15 Å vacuum. The climbing-nudged elastic-band method (cNEB) was employed to locate the transition states (TSs).53–55 The general NEB method was employed to determine an approximated TS, and then the quasi-Newton algorithm was used to optimize the likely TS until the force acting on the atom was smaller than 0.03 eV Å1. Finally, the frequency analysis was carried out to confirm the TS. Vibrational frequencies were calculated by a second-order finite-difference approach with the step size at 0.015 Å. In this work, the adsorption (Ead) and activation energies (Ea) were calculated by the following two formulas: Ead = EA/M  EM  EA and Ea = ETS  EIS, where EA/M, EM, EA, ETS and EIS indicate the calculated energies of the adsorbate, substrate, adsorption system, and transition and initial states, respectively. For (111), (100), and (110), the p(2  2)-four-layer unit cell with a corresponding coverage of 0.25 monolayer was utilized (see Fig. 1). The experimental finding indicates that the strong adsorption of oxygen atoms would make Cu(110) reconstruct into an added-row structure, Cu(110)-(1  2).56 The stepped (110)-1  2 was modeled by adding two atoms onto a p(2  2)Cu(110) unit cell. The defect ad-row and ad-atom systems were modeled by adding two (one) additional Cu atoms per supercell

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Fig. 1

Adsorption sites of Cu(hkl) surfaces.

onto the fcc sites of the Cu(111) slab model, and the supercell of the perfect Cu(111), ad-atom, and ad-row defects consist of 16, 17, and 18 Cu atoms, respectively. The ad-atom model was usually used to simulate the single-atom site catalysts.57 For the aforementioned six models, adsorbates and the top two layers [for (111), (100), and (110)] or three layers [for (110)-(1  2), ad-row and ad-atom] were relaxed, whereas the bottom two layers were fixed at the bulk-truncated positions with the theoretical lattice constant being 3.64 Å. The Cu(211) slab was composed of 12 layers, including 24 Cu atoms in the p(2  1) supercell, and the top six layers were relaxed. Cu(210) [2(100)  (110)] was a stepped surface that had two-atom-wide (100) terraces and one-atom-high (110) steps. Cu(210) was modeled by p(2  2) containing eight layers, and the top four layers were relaxed. Cu(310)[3(100)  (110)] was stepped surface that had three-atom-wide (100) terraces and one-atom-high (110) steps. Cu(310) was modeled by p(2  2) containing 12 layers, and the top six layers were relaxed. The coordination numbers (CN) of the surface atom are 9, 8, 7, 7, 7, 6, 6, 5 and 3 on (111), (100), (110), (110)-(1  2), (211), (210), (310), ad-row and ad-atom, respectively.

3. Results and discussion 3.1.

Adsorption properties of possible species

The adsorption energies of H2O, OH, and H on various Cu model catalysts are presented in Tables 1 and 2. As expected, the heats of chemisorption demonstrate the well-known relative order i.e.,

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Paper Adsorption energies and geometry parameters of H2O on clean and oxygen-pre-adsorbed Cu surfacesa

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Clean

Cu(100) Cu(110) Cu(111) Cu(110)-(1  2) Cu(210) Cu(310) Cu(211) Cu-ad-row Cu-ad-atom

O/M

Site

dO–Cu (Å)

dO–H (Å)

Ead (eV)

Site

dO–Cu (Å)

dO–H (Å)

dH–O(ad) (Å)

Ead (eV)

Ed (eV)

Top Top Top Top Top Top Top Top Top

2.36 2.18 2.41 2.23 2.21 2.15 2.26 2.21 2.07

0.978 0.981 0.979 0.978 0.984 0.978 0.977 0.980 0.985

0.25 0.38 0.22 0.29 0.41 0.50 0.30 0.39 0.50

Top Top Top Top Top Top Top Top —

2.27 2.04 2.14 2.33 2.21 2.17 2.46 2.37 —

1.00 1.07 0.99 1.00 1.00 0.99 0.993 0.994 —

1.79 1.44 2.16 1.80 1.81 1.98 1.86 1.89 —

0.35 0.90 0.65 0.35 0.39 0.48 0.35 0.39 —

2.39 2.19 2.44 2.28 2.24 2.37 2.40 2.37 2.19

a

Note: dO–Cu is the distance between an oxygen atom involved in water and the copper atom, dO–H is the distance between O and H in molecular water, and dH–O(ad) is the distance between the stripped H atom and the adsorbed oxygen atom.

Table 2

Adsorption energies and geometrical parameters of O, H and OH on Cu surfacesa

O

Cu(100) Cu(110) Cu(111) Cu(110)-(1  2) Cu(210) Cu(310) Cu(211) Cu-ad-row Cu-ad-atom a

OH

H

Site

dO–Cu (Å)

Ead (eV)

Site

dO–Cu (Å)

Ead (eV)

Site

dH–Cu (Å)

Ead (eV)

4h 3h Fcc Fcc Edge-br Edge-br Edge-br Edge-br Edge-br

2.00 1.87 1.88 1.86 1.91 1.92 1.86 1.77 1.87

5.43 5.28 5.06 5.24 5.10 5.14 5.08 4.71 4.53

4h Sb Fcc Fcc Edge-br Edge-br Edge-br Edge-br Edge-br

2.14 1.95 2.04 2.01 2.05 2.06 1.94 1.93 2.07

3.51 3.62 3.26 3.52 3.57 3.67 3.74 3.99 3.17

4h Sb Fcc Fcc Edge-br Edge-br Edge-br Edge-br Edge-br

1.84 1.67 1.74 1.73 1.78 1.79 1.64 1.64 1.67

2.38 2.61 2.73 2.52 2.48 2.44 2.53 2.54 2.20

Note: dH–Cu is the distance between the H and copper atoms.

molecular adsorption o radical fragment adsorption o adatoms. Molecular H2O is physically adsorbed and prefers the top site. However, atoms and radical fragments, such as H, O and OH, bind considerably more strongly and prefer the higher threefold coordination sites. The adsorption energy of water increases in the order of (111) o (100) o (110)-(1  2) o (211) o (110) o ad-row o (210) o (310) = ad-atom is in general agreement with the coordinate number of surface Cu atoms, which proves that the adsorption of molecular water is relevant to surface atom density. Adsorption of oxygen on these surfaces are all very strong (see Table 2), and the electronegative atom (O) increases the acidity of neighboring substrate atoms. The electromagnetic induction effect induced by the modified atom increases the adsorption energy of water via through-space electronic interaction. The adsorption energy of water on O-modified surfaces are generally larger than corresponding clean surfaces (see Table 1), and one possible reason may be the formation of hydrogen bonds in the H2O–O co-adsorption configuration, which can be confirmed by the distance between Oad and H, dH–O(ad) (see Table 1). It is important to analyze the effect of oxygen species on the binding energy of water; herein, the energy decomposition scheme was employed. Usually, the adsorption energy for a given species can be decomposed into three parts: the deforma-

decomposition in Fig. 2, as well as in Table S1 in the ESI,† it is found that the deformation energy of both the substrate and the molecular water are generally larger on oxygen-pre-adsorbed Cu surfaces than on corresponding clean surfaces, particularly for (111) and (110) plane, which contributes to weaken the adsorption energy of molecular water. This may be attributed to the adsorbed configuration of molecular water induced by the chemisorbed oxygen atom. For example, the H–O bond length

H2 O tion energy of the molecule EDef the deformation energy of the substrate , and the interaction energy between these substrate EDef H2 O substrate two parts EInt :Ead ¼ EDef þ EDef þ EInt . Through the energy

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Fig. 2

Energy decomposition of water adsorption.

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is 0.97/0.98 Å on bare Cu(110), whereas it is 0.97/1.07 Å on oxygen-pre-adsorbed Cu(110). Namely, larger structural deformation occurred in the situation of O/Cu(110). However, the interaction between molecular water and the metal substrate is considerably stronger on the oxygen-pre-adsorbed surfaces, which increases the adsorption energy. The interaction energy, known as a dominant factor in determining the adsorption energy, has a clear relation with the adsorption energy, particularly in the presence of oxygen atom. Thus, the larger the interaction energy, the larger is the adsorption energy of molecular water. In general, the increase of the interaction energy brought by the oxygen atom is attributed to two effects originating from pre-adsorbed species: direct through adsorbate– adsorbate interaction and indirect through surface (or surfacemediated) interaction. The direct interaction can be measured by the distance between Oad and molecular water, i.e., the hydrogen bond strength, and the through-surface interaction can be measured by the distance between water and substrate. Because shorter distance indicates stronger interaction, through a careful examination of the adsorption configuration parameters of water in Table 1, we can determine that the hydrogen bond may play a dominant role on both (110) and (110)-1  2 because of the short distance between Oad and molecular water, whereas the through-surface interaction may play a key role on (111) due to the relatively short distance between water and copper in the presence of Oad. To measure the contribution of hydrogen bonds to the adsorption energy, the following formula was used to estimate the hydrogen bond involved in the H2O–O system: EH-Bond = (EH2O–O  EH2O  EO), where EH2O–O, EH2O and EO are the total energies of H2O–O complex, isolated water and oxygen atom at the fixed adsorption configuration without the metal substrate, respectively. The estimated strength of each hydrogen bond is 0.02 eV on (110), 0.06 eV on (111), and 0.18 eV on (110)-1  2. Compared with the water adsorption energy difference with and without the oxygen atom listed in Table 1, we know that the contribution of hydrogen bonds to water adsorption energy is very small on (110) and (111) (i.e. 0.52 eV and 0.43 eV), whereas it is large on (110)-1  2 (i.e., 0.06 eV). This result indicates that the interaction energy for (110) and (111) come primarily from the through-surface interaction (H2O–Cu), which can be further confirmed by the relatively short distance between water and the metal substrate in the presence of oxygen atoms (see dO–Cu in Table 1), and the interaction energy for (110)-1  2 primarily comes from the hydrogen bond (in fact, the magnitude of dO–Cu is relatively large in the presence of oxygen atoms, as can be seen in Table 1). Similar results obtained by the previous DFT study of dimer H2O–H2O system show that the hydrogen bond contributes less to water adsorption energy.58,59 3.2.

Water dissociation properties

3.2.1. Water dissociation on clean metal surfaces. In this section, we will focus on the characteristics of the minimum energy path and the reaction mechanism of water dissociation on pure metal surfaces. Water may proceed to partially dissociate to produce OH and H or completely dissociate to form 2H and O, and herein, we are interested in partial dissociation because of the importance of hydroxyl species in many catalytic processes. The calculated energy results include activation and

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reaction, as well as the important parameters of TSs, shown in Table 3, and the associated TSs are displayed in Fig. S1 in the ESI.† In general, the reaction mechanisms are similar on these Cu models, and thus only (110) and ad-atom models were chosen to describe the reaction process. On Cu(110), molecular water was adsorbed on the top site at a height of 2.18 Å. The bond of O–H was activated, as water approached the surface copper atom, and the activated H atom adsorbed on another Cu atom. The OH adsorbed on the nearby short bridge site through O. At TS, the bond distance between the stripped H and O was 1.59 Å. At FS, the dissociated H was adsorbed on the nearby short bridge (sb) site. The energy barrier and the heat energy were 1.05 eV and 0.20 eV, respectively. On the ad-atom Cu, the molecular water was adsorbed on a top site of the copper atom with an adsorption energy of 0.50 eV. At TS, the bond distance between the stripped H atom and O atom was 1.83 Å. The energy barrier and heat energy were 1.66 eV and 0.44 eV, respectively. The activation of water dissociation on the three flat surfaces decreases in the following order, (110) 4 (100) 4 (111), which is in accordance with the order of the coordinate number. Thus, the smaller the coordinate number, the stronger is the activation energy. In addition, a linear relation of d-band center (Ed as seen in Table 1) with activation energy is determined on flat surfaces; i.e., the closer the d-band center to the Fermi level, the lower the activation barrier would be. For reconstructed and stepped surfaces, except for (210), there are higher barriers, compared with flat surfaces such as (110), which might be induced by the complicated structure of these surfaces. Instead for the reconstructed surface (110)-(1  2), for example, although molecular water adsorbed on the top site is similar to (110), the dissociation occurred on the side face (111). This process is affected by the nearest neighboring atom; i.e., the side face is not as open as flat face of (111); thus, resulting in a higher activation energy. Similarly, water dissociation on (310) occurred at the terrace (110), but such a type of (110) face is not as open as the general (110) face, which also leads to the relatively high barrier. For stepped (211), because its chemical properties lie between (111) and (100), the kinetic behavior of water dissociation remains in the middle of (111) and (100), as shown in our present results. As a result, despite the stronger adsorption of water on the stepped surface, the activity of water dissociation is weaker, which proves the importance of surface topology in the reaction. It seems different from the general point that the defect site is highly active for the dissociative reaction. In fact, Offermans et al.60 investigated the stepped dehydrogenation of ammonia and found that the stepped surfaces do not efficiently promote the reaction compared with the flat one. Kokalj et al.61 studied methyl dissociation on Rh and found that the activation barrier on the stepped ad-atom model is considerably higher than that on a flat surface due to the limited available active sites during the reaction. For water dissociation on various Cu surfaces, the dissociation barrier is generally larger than the corresponding desorption barrier (estimated from its adsorption energy). Thus, it is expected that desorption of molecular water from the copper surface will occur before dissociation.

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3.2.2. Water dissociation on oxygen-pre-adsorbed copper surfaces. Interactions between water and other molecules adsorbed on surfaces are interesting from the point of view of catalysis, in which the influence of water in heterogeneous reactions has not been clearly understood. For water dissociation on the oxygen-pre-adsorbed metal surfaces, the dissociation mechanism is significantly different from the mechanism by which water is activated on clean metal surfaces because the pre-adsorbed oxygen atom can act either as the spectator or the reactant. Herein, we focused on the latter case, particularly H2O + Oad = 2OH; moreover, it is in fact a concerted process consisting of breaking of one O–H bond and the formation of another O–H bond. The calculated energetic results are shown in Table 3, and the corresponding TSs are shown in Fig. S1 (ESI†). Because the reaction features of water dissociation on oxygen-pre-adsorbed Cu models are similar, we only use (110) and ad-atom models to present reaction processes. On Cu(110), molecular water adsorbed on the top site at a height of 2.04 Å, and oxygen atom adsorbed on the 3h site with the distance of the hydrogen in water being 1.44 Å, which indicated a hydrogen bond interaction. Because of the hydrogen bond, the water adsorbed slightly away from the top site. The bond of O–H was activated when water approached the surface copper atom, and the activated H was between two oxygen atoms, with the distance of pre-adsorbed O and O in OH being 1.18 Å and 1.24 Å, respectively. Finally, the dissociated H bonded with adsorbed O, and the newly formed OH adsorbed on the short bridge site, whereas the remaining OH adsorbed on another sb site. The energy barrier and and heat energy were 0.03 eV and 0.21 eV, respectively. On the oxygen-pre-adsorbed Cu-adatom model, oxygen adsorbed at the edge-bridge site, and then molecular water approached the chemisorbed oxygen atom. Our calculations show that water dissociation is a spontaneous process in the presence of oxygen atoms on ad-atom surfaces, which indicates a strong promotion effect of chemisorbed oxygen atom on the activation of molecular water. This result may help to design a single-atom active site catalyst to strip hydrogen from water by the chemisorbed oxygen species. From Table 3, we find that the bond length of O–H at TS is generally smaller than that on clean Cu surfaces as the result of attractive interaction between Oad and H atoms in an H–OH Table 3 in eV.)

bond, which may lead to the reduction of the water dissociation barrier. In fact, it was found that there is a large difference in the activation energy barrier between clean and oxygen-preadsorbed surfaces, and that pre-adsorbed oxygen species can significantly promote water dissociation. Water dissociation is a structure-sensitive reaction, which is also sensitive to modified surfaces. Our result is in accordance with the former result of kinetic experimentation on WGS (water gas shift) reactions (water dissociation reaction on Cu(110) is faster than on Cu(111)),38 that is, water dissociation on Cu(110) has a lower activation energy barrier than that on Cu(111). Moreover, it was found that the activation energy barrier of water dissociation on oxygen-preadsorbed surfaces are all lower than corresponding clean surfaces. Table 3 lists the water dissociation barrier difference between clean and oxygen-pre-adsorbed surfaces (DEa). The difference decreases in the following order: ad-atom 4 Cu(310) 4 Cu(110)-(1  2) 4 Cu(210) 4 Cu(110) 4 Cu(211) 4 Cu(111) 4 Cu(100) 4 ad-row, which shows that the promotion effect of preadsorbed oxygen is stronger on stepped and reconstructed surfaces than that on flat surfaces. The order of activation of water on oxygen-pre-adsorbed surfaces is as follows: ad-atom 4 Cu(110) [Cu(210)] 4 Cu(310) 4 Cu(211) [Cu(110)-(1  2)] 4 ad-row 4 Cu(111) 4 Cu(100). The order of the adsorption energy of oxygen is reversed, except for (111). Therefore, the stronger the adsorption of pre-adsorbed oxygen, the weaker is the activity of the surface in the cleavage of H–OH bond, which is in general agreement with our previous studies.6,11,12 As for the exception on Cu(111), we find that there is a weak hydrogen bond between hydrogen in water and adsorbed oxygen in IS. From Table 1, we know that the distance between hydrogen in water and adsorbed oxygen dH–O(ad) on (111) is the longest, i.e. the weakest hydrogen bond. Thus, we confer that hydrogen bond may be another important factor that promotes the dissociation of water in addition to the binding energy of the oxygen species. After the comparison of the two series of data of dH–O(ad) and the adsorption energy of oxygen, we find that on (110), (210) and (111), the adsorption energy of oxygen are nearly the same, while on (110) dH–O(ads) is the shortest, (210) in the middle, (111) is the longest, and the activation energy increases in this trend; that on (100), (110)-(1  2) and (210), dH–O(ads) are nearly the same, the adsorption energy of oxygen on (100) is the strongest,

Energies and geometrical data of dissociation of water on clean and oxygen-pre-adsorbed Cu surfaces (all distances are in Å, and energies are

Clean

O/M DH

Ea Cu(100) Cu(110) Cu(111) Cu(110)-(1  2) Cu(210) Cu(310) Cu(211) Cu-ad-row Cu-ad-atom

a

1.23/1.26 1.05/1.14a 1.28/1.23a 1.44/1.38a 1.08/1.09a 1.38/1.34a 1.25/1.26a 1.08/1.10a 1.66/1.75a

0.12 0.20 0.24 0.15 0.06 0.02 0.15 0.33 0.44

d(O–H) 1.62 1.59 1.59 1.56 1.56 1.80 1.70 1.56 1.83

DH

Ea a

0.40/0.28 0.30 0.03/0.02a 0.21 0.36/0.35a 0.13 0.27/0.28a 0.06 0.03/0.02a 0.68 a 0.15/0.14 0.77 0.27/0.27a 0.29 0.23/0.25a 0.43 Spontaneously processed

d(O–H)

DEa

Ead(O)

1.35/1.12 1.24/1.18 1.35/1.12 1.30/1.16 1.32/1.15 1.27/1.18 1.24/1.24 1.28/1.18

0.85 1.02 0.92 1.18 1.05 1.23 0.98 0.85 1.66

5.43 5.28 5.06 5.24 5.10 5.14 5.08 4.71 4.53

Note: d(O–H) is the distance between the stripped H and oxygen atoms in water, as well as the chemisorbed oxygen species.a Dipole correction results.

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(110)-(1  2) in the middle, (210) the weakest, and the activation energy decreases in this turn; and that on (210) and (110), the activation energies are close to zero, which is mainly because of the weak adsorption of oxygen and the strong interaction of hydrogen in water with pre-adsorbed oxygen (exhibited by dH–O(ad)). From the aforementioned analysis it can be concluded that the main factor that promotes the dissociation of water on the oxygen-pre-adsorbed surface is the balance between the adsorption strength of oxygen species and the interaction of hydrogen in water with pre-adsorbed oxygen. 3.2.3. Comparison with previous calculation results. It is necessary to compare the present DFT results with previous calculation results of water adsorption on copper. In our previous work,8 the UBI-QEP (unity bond index-quadratic exponential potential) method was used to investigate the water dissociation on Cu(111), Cu(100) and Cu(110) faces, and the reaction energy barriers of water dissociation on clean and oxygen-pre-adsorbed Cu surfaces were found to be 1.02, 0.99 and 0.92 eV, and 0.80, 0.90 and 0.92 eV, respectively, whereas in the present DFT study the water dissociation barriers on oxygen-pre-adsorbed Cu surfaces are 0.36, 0.40 and 0.03 eV, respectively. The reason for the relatively high reaction barriers calculated by UBI-QEP is clear in that UBI-QEP method did not consider the hydrogen bond, whereas the general idea assumes that the hydrogen bond stabilizes TS more strongly than that of IS for water dissociation. On oxygen-pre-adsorbed Cu(110),42 the previous DFT-calculated water dissociation barrier is 0.05 eV, close to our present result. On Cu(111),43 with and without an oxygen atom, the water dissociation barriers were calculated to be 1.36 eV and 0.23 eV, which are in general agreement with our DFT data. On Cu(100),45 the reaction barrier of water dissociation is calculated to be 1.13 eV, similar to our results (1.23 eV). On Cu(321),44 the DFT calculation results show that the activation energy barriers for water dissociation were 0.93 and 0.71 eV on those of the clean and oxygen-pre-adsorbed one, respectively. The reaction barrier of water dissociation on bare Cu(321) is close to Cu(110) in our present work (1.05 eV, as seen in Table 3) due to the similar CN of these two models [i.e., six for (321) and seven for (110)], whereas the barrier on oxygen-pre-adsorbed Cu(321) is considerably higher than Cu(110) (0.71 vs. 0.03 eV). It should be pointed out that we put adsorbate on one side of the slab during the DFT calculation, especially for the defected/stepped structures, the electrostatic potentials on the two sides of the slab are different, where a dipole correction is necessary. We have tested all of the Cu models studied in this work, and it is found that dipole correction has a slight effect on the calculated barrier (less than 0.10 eV, as seen in Table 3), and it is independent of the surface structure. Thus, the active order of the copper model with different CN remains unchanged with the dipole correction in DFT periodic slab calculations, which differs significantly from the Cu2O(100) in our previous study,62 in which the dipole correction has a significant effect on water dissociation due to the strong polar effect of the Cu2O(100) surface. 3.2.4. The physical origination of Oad in the activation of water. It is worthy to analyze the reason behind the oxygen activation for water dissociation. Herein, two methods were applied in the following analysis.

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(a) Energy barrier decomposition scheme analysis. We also used the aforementioned energy decomposition method to analyze the original barrier. For dissociation reactions such as AB - A + B, the adsorption energy of the A + B activated TS TS complex at TS can be divided into three parts: ETS A+B = EA + EB + TS TS TS 63 TS TS EInt = S + EInt, where EA (EB ) is the adsorption energy of the reactant A(B) at TSs without B(A), and ETS Int is a measure of the interaction energy between A and B at TS. Thus, the dissociaIS (g) tion barrier can be expressed as Ea = STS + ETS Int  EAB  EAB, IS (g) where EAB is the adsorption energy of AB at IS, and EAB is the bond energy of AB at the gas phase. For molecular water, E(g) AB is calculated by the following equation EAB  EA  EB = 5.64 eV. Table 4 gives the calculated barrier decomposition results on (111), (110) and (110)-(1  2). Oxygen species stabilize the water adsorption at IS and weaken the binding energy of Ha/OHad at TS (STS), which increases the water dissociation barrier to some extent but also significantly increases the attractive interaction between Had and OHad at TS, which lowers the barrier. Therefore, all of them lead to a lower barrier on the oxygen-pre-adsorbed Cu surfaces. The magnitude of ETS Int can be considered as the contributions of direct Pauli repulsion measured by the distance between two fragments, bonding competition caused by sharing the same substrate atoms and electrostatic interaction between Had and OHad at the TS configuration. The fact that the distances between Had and OHad at TSs are usually shorter on the oxygen-pre-adsorbed Cu surfaces than those on pure Cu surfaces (see Table 3) indicates that the direct Pauli repulsion has a negative contribution to the ETS Int. In addition, the fact that at TSs both OHad and Had fragments are farther from the substrate (see Fig. S1, ESI†) and that they do not share the same substrate result in a weak bonding competition on oxygen-pre-adsorbed Cu surfaces, which contributes to the ETS Int in a favorable manner. Moreover, the electrostatic interaction is roughly estimated from the charge signs of the H and O atoms at TS, and the Bader charge analysis shows that the charges of O and H atoms become more negative and more positive, respectively, on the oxygen-pre-adsorbed Cu surface due to the strong electron withdrawal of Oad atom (see Table 5). Moreover, considering the relatively short bond length of O–H at TS on the O–Cu system, one might expect that the electrostatic interaction would become more attractive on the O–Cu system, and thus contribute much to ETS Int. Based on the aforementioned analysis, we may conclude that both the weak bonding competition and strong attractive electrostatic interaction between Had and OHad led to the oxygen promotion on the O–H activation over the copper surface. (b) Acid–base pair interaction model analysis. For the effect of the chemisorbed Oad on water dissociation on copper surfaces, Table 4 Energy decomposition of the calculated activation energy of water dissociation (eV)

Cu Ea

O/Cu SETS

ETS Int

EIS AB

Ea

SETS

ETS Int

EIS AB

(111) 1.28 4.98 0.42 0.22 0.36 3.94 2.00 0.65 (110) 1.05 4.80 0.17 0.38 0.03 4.34 2.16 0.90 (110)-(1  2) 1.44 3.26 1.23 0.29 0.27 1.14 4.56 0.35

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Table 5 Bader charge analysis of TS for water dissociation with and without an oxygen atom

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Cu

(111) (110) (110)-(1  2)

O–Cu

H–O/e

H/e

H–O/e

H/e

1.48 1.50 1.46

+0.04 +0.11 +0.08

1.55 1.61 1.57

+0.00 +0.67 +0.62

Oad. Fig. 4 shows the correlation between the water dissociation barrier and the acid–base strength. Clearly, the strong acid– base strength is related to the low barrier, except for (110) and (100), which is due to the very strong hydrogen bond in the coadsorption configurations of H2O–Oad, and the very strong basic of Oad (i.e., the very strong binding energy oxygen atom on (100)), respectively. Thus, it can be concluded that the surface geometry may also be an important factor controlling O–H bond breaking, in addition to the general acid–base interaction.

4. Conclusions

Fig. 3 Schematic for water dissociation process via the acid–base interaction model.

the Cu atoms may act as the Lewis acid site and the Oad atom may act as the Bronsted base site for proton abstraction from water, and the acid–base sites interacting with molecular H2O simultaneously led to a higher activity in water dissociation64,65 (see Fig. 3). The magnitude of acid and base was measured by the electron charge change based on the Bader charge analysis; i.e., the Lewis acid is an electron acceptor, and the Lewis base is an electron donor. Clearly, the more positively the Cu atoms charge on and the more negatively the O species charge in, the stronger the acid–base interaction will be, which results in strong promotion on the activation of the O–H bond. In general, a compensation effect exists on a given metal surface in the base–acid interaction: if the metal is highly active in the activation of H2O, which indicates that it strongly binds both OH and O, the metal atoms will go with strong acid and chemisorbed oxygen atoms will go with weak base, i.e., a compensation effect. The acid–base strength of the Cu–O pair was determined by the Bader charge difference between Cu and

Water dissociation on clean and oxygen-pre-adsorbed copper surfaces with different coordination numbers (CNs) have been investigated. The present calculation results show that both the basicity of an oxygen atom (or the binding energy of an oxygen atom on a metal surface) and the interaction between the stripped H atom and the pre-adsorbed oxygen (denoted by the dH–O(ad)) affect the process: the more strongly the oxygen atoms bond to the metal surface, the weaker is their promotion effect on the water O–H bond cleavage; the shorter the distance between pre-adsorbed oxygen and hydrogen in water, the greater the promotion effect will be. The present results show the bi-functional mechanism between metal atoms and adsorbed oxygen (Oad), where Oad as a base site strips the hydrogen atom of water via hydrogen bonding, which may help design an optimal catalyst for water dissociation by preparing the catalyst with suitable surface topology.

Acknowledgements This work was supported by the State Key Program of Natural Science of Tianjin (Grant No. 13JCZDJC26800), MOE Innovation Team (IRT13022) of China, the NSFC (21421001), and the State Key Program of National Natural Science Foundation of China (Grant No. 21433008), and the foundation of State Key Laboratory of Coal Conversion (Grant No. J15-16-908).

References

Fig. 4 Correlation between water dissociation barrier and the acid–base strength.

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Oxygen-assisted water partial dissociation on copper: a model study.

It is essential to understand and control the O-H bond cleavage on metal surfaces with pre-adsorbed oxygen atoms in heterogeneous catalytic processes...
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