First-principles Modeling of Direct versus Oxygen-assisted Water Dissociation on Fe(100) Surfaces

The O–H bond breaking in H 2 O molecules on metal surfaces covered with pre-adsorbed oxygen atoms is an important topic in heterogeneous catalysis. The adsorption configurations of H 2 O and relevant dissociation species on clean and O-pre-adsorbed Fe(100) surfaces were investigated by density functional theory (DFT). The preferential sites for H 2 O, HO, O, and H were investigated on both surfaces. Both the first H abstraction from adsorbed H 2 O and the subsequent OH dissociation are exothermic on the O-pre-adsorbed Fe(100) surface. However, the pre-adsorbed O significantly reduces the kinetics energy barriers for both reactions. Our results confirmed that the presence of pre-adsorbed oxygen species could significantly promote H 2 O dissociation.

Adsorbed hydroxyl, atomic oxygen and hydrogen are the simplest dissociation products of water on metal surfaces.Also, hydroxyl can further dissociate, giving rise to adsorbed atomic hydrogen and oxygen.Some experimental studies and theoretical calculations on adsorption/dissociation of water on the Fe(100) surface have been reported [28][29][30][31][32][33][34][35][36][37].Eder and Terakura [33] demonstrated that water preferred to adsorb at the bridge site via an upright molecular adsorption configuration.However, it was unstable leading to a spontaneous dissociation into H and OH species.The same conclusion was achieved by Jung and Kang [34] using density functional theory (DFT) calculations.The proposed upright H 2 O configuration at the bridge site by these authors, however, is not in line with the flat-lying configuration on the top site on other metals, such as Ru, Rh, Pd, Pt, Al, Cu, Ag, and Au [38][39][40][41].
The promotion effect of pre-adsorbed O atoms on the surfaces of the catalysts has been discovered and investigated by both experimental and theoretical approaches.The pre-adsorbed O atoms have various degrees of effect towards H 2 O dissociation on different metals.A comprehensive review on this topic has been provided by Thiel et al. [42] and Henderson [43].The positive effect of pre-adsorbed O atoms on water dissociation on Pd single crystals and thin films has been reported [44][45][46][47][48]. On the other hand, the water dissociation on the clean Pd surface has not been observed, in consistent with the DFT calculations [49].Shavorskiy et al. [50] also found the formation of OH when water reacted with co-adsorbed O on the missing-row reconstructed Pt{110}(1 ˆ2) surface.A similar promotion effect on the Fe(111) surface has been reported [51].Hung et al. [52] found that adsorbed oxygen facilitated water dissociation via the hydrogen transfer process on the pre-oxidized Fe(100) surface.Liu et al. [37]  Herein, we report a systematic DFT study of the adsorption of H 2 O and its dissociation fragments (OH, H and O) on clean and O-pre-adsorbed Fe(100) surfaces.The effect of two O atoms which came from the dissociation of O 2 molecule on the adsorption and dissociation of water molecule has not been reported previously.The dissociation pathways on both surfaces were also discussed based on the calculation results to understand the effect from the pre-adsorbed O atoms.

Adsorption of H 2 O, OH, O and H on the Clean Fe(100) Surface
The adsorption properties of H 2 O and the dissociation products H, O, and OH species on a clean Fe(100) surface will be discussed first.The most stable adsorption configurations of these species are presented in Figure 1.The corresponding adsorption energies and structural details are summarized in Tables 1 and 2. On the Fe(100) surface, H 2 O was found to be preferentially adsorbed on a top site via the oxygen atom with the H 2 O plane being almost parallel to the Fe surface (Figure 1a).The interaction between the Fe(100) surface and H 2 O molecule is weak, evidenced by a small adsorption energy (´0.65 eV).The O-H bond length of 0.98 Å and the =HOH bond angle of 105.4 ˝, as listed in Table 2, are almost identical to those of a free H 2 O molecule (0.98 Å and 104.4 ˝, respectively).Similar adsorption behaviors of H 2 O on Ni(111) [53][54][55], Ni(100) [55], Ni(110) [55], Cu(111) [56], Cu(100) [57] and Cu(110) [58,59] were observed.
The adsorption energies of the OH species at the bridge and hcp sites are ´3.93 eV and ´3.84 eV, respectively.The adsorption at the hollow hcp site is locally unstable.These results suggest that the most stable adsorption site for OH on Fe(100) is at the bridge, consistent with the previous study [28,33].The O-Fe bond length is1.99 Å, which is shorter than that of H 2 O adsorbed on clean Fe (100) surface (2.17 Å).Based on these results, one may conclude that the OH can be strongly adsorbed on the Fe(100) surface.
For the O species, Govender et al. [35] have found that the hcp and bridge sites were equally stable.On the other hand, Bło ński et al. [60] and Lu et al. [30] suggested that O atom only occupied the hollow hcp sites.We found that the adsorption energy of O atom at the hollow hcp is higher than that at the bridge site (´3.67 eV at the former vs. ´3.00eV at the latter).On contrast, the top site is not stable for O adsorption.Thus, our results echo that reported by Bło ński et al. [60] and Lu et al. [30].Finally, for the H atom, the adsorption energies at the bridge and hollow hcp sites are −3.99 eV and −3.86 eV, respectively.The top site is not stable for H adsorption.This result suggested that the bridge is the most stable adsorption site for H atoms, which is in good agreement with previous DFT calculation results [61].
Based on the calculated adsorption energies in Table 1, it can be concluded that the interaction with the Fe(100) surface increases in the order of H2O < O < OH <H.In the following sections, we will discuss the dissociation of H2O on the Fe(100) surface based on these results.

Adsorption of H2O and OH on the O-Pre-adsorbed Fe(100) Surface
As mentioned in the Introduction, the pre-adsorbed oxygen atoms could play an important role in the activation of O-H bond.Therefore, it is of interest to investigate the adsorption behavior of H2O on O-pre-adsorbed Fe(100) surface.We found that the H2O molecule could also be adsorbed on  Finally, for the H atom, the adsorption energies at the bridge and hollow hcp sites are ´3.99 eV and ´3.86 eV, respectively.The top site is not stable for H adsorption.This result suggested that the bridge is the most stable adsorption site for H atoms, which is in good agreement with previous DFT calculation results [61].
Based on the calculated adsorption energies in Table 1, it can be concluded that the interaction with the Fe(100) surface increases in the order of H 2 O < O < OH <H.In the following sections, we will discuss the dissociation of H 2 O on the Fe(100) surface based on these results.

Adsorption of H 2 O and OH on the O-Pre-adsorbed Fe(100) Surface
As mentioned in the Introduction, the pre-adsorbed oxygen atoms could play an important role in the activation of O-H bond.Therefore, it is of interest to investigate the adsorption behavior of H 2 O on O-pre-adsorbed Fe(100) surface.We found that the H 2 O molecule could also be adsorbed on the top site on the O-pre-adsorbed Fe surface, with a higher interaction energy (´1.13 eV) than that on a clean surface (´0.65 eV).The adsorption configuration was listed in Table 2.The pre-adsorbed electronegative O atom may increase the acidity of neighboring Fe atoms [62], and the adsorption energy of H 2 O via through-space electronic interaction.Another interesting finding is that the pre-adsorbed O atom and the H atom in the H 2 O may form a hydrogen bond with a bond distance of 2.04 Å in the co-adsorption configuration, as shown in Figure 2a.2. The pre-adsorbed electronegative O atom may increase the acidity of neighboring Fe atoms [62], and the adsorption energy of H2O via through-space electronic interaction.Another interesting finding is that the preadsorbed O atom and the H atom in the H2O may form a hydrogen bond with a bond distance of 2.04 Å in the co-adsorption configuration, as shown in Figure 2a.As for the adsorption of OH on O-pre-adsorbed Fe(100) surface, both OH and O are placed on their favorable bridge sites.The most stable structure of OH co-adsorbed with O atom is shown in Figure 2b.The distance between the O atom of hydroxyl group and the nearest Fe atom remains 1.98 Å.However, the adsorption energy changes from −3.93 eV to −4.02 eV, suggesting a stronger interaction with the O-pre-adsorbed Fe(100) surface than with a clean surface.The bond distance of O-H is 0.98 Å, identical to that on the clean surface.Similar effects from the pre-adsorbed-O atom were also found on an Au(100) surface [63].

H2O Dissociation on the Clean Fe(100) Surface
Water may partially dissociate to produce OHad and Had (Equation ( 1)), and follow the dissociation of OHad to form Had and Oad (Equation ( 2)).The calculated reaction energies and activation barriers for these two reaction steps are shown in Table 3.The associated transition states (TSs) are displayed in Figure 3.In the first step (Equation (1)), molecular water is adsorbed on the clean Fe(100) surface at the top site as discussed in Section 2.1 (Figure 3a).At TS1, one of the H atoms was stripped from the H2O molecule (Figure 3b).The bond distance between this H atom and O atom in the OHad is 1.57Å.The energy barrier and reaction energy for this step are 1.45 eV and −0.91 eV (Figure 4), respectively.At the equilibrium state, both the H and OH radicals are adsorbed at the bridge sites (Figure 3c).

H2Oad → Had + OHad
(1) As for the adsorption of OH on O-pre-adsorbed Fe(100) surface, both OH and O are placed on their favorable bridge sites.The most stable structure of OH co-adsorbed with O atom is shown in Figure 2b.The distance between the O atom of hydroxyl group and the nearest Fe atom remains 1.98 Å.However, the adsorption energy changes from ´3.93 eV to ´4.02 eV, suggesting a stronger interaction with the O-pre-adsorbed Fe(100) surface than with a clean surface.The bond distance of O-H is 0.98 Å, identical to that on the clean surface.Similar effects from the pre-adsorbed-O atom were also found on an Au(100) surface [63].

H 2 O Dissociation on the Clean Fe(100) Surface
Water may partially dissociate to produce OH ad and H ad (Equation ( 1)), and follow the dissociation of OH ad to form H ad and O ad (Equation ( 2)).The calculated reaction energies and activation barriers for these two reaction steps are shown in Table 3.The associated transition states (TSs) are displayed in Figure 3.In the first step (Equation (1)), molecular water is adsorbed on the clean Fe(100) surface at the top site as discussed in Section 2.1 (Figure 3a).At TS1, one of the H atoms was stripped from the H 2 O molecule (Figure 3b).The bond distance between this H atom and O atom in the OH ad is 1.57Å.The energy barrier and reaction energy for this step are 1.45 eV and ´0.91 eV (Figure 4), respectively.At the equilibrium state, both the H and OH radicals are adsorbed at the bridge sites (Figure 3c).
H     In the second step of dissociation (Equation ( 2)), the O atom remains at the bridge site while the H atom diffuses away and forms a new Fe-H bond (TS2 in Figure 3d).The distance between this H atom and the O atom is 1.70 Å, which is much longer than that of the initial value (0.98 Å).The Fe-H bond is not stable and H continues to move towards the nearby bridge site (Figure 3e).The energy barrier of the second step is 2.12 eV, which is almost 1.5 times of that of the first step.The reaction energy is −0.58 eV, indicating that the second step is an exothermic reaction.According to the activation energies for TS1 and TS2, both steps require significant energies to overcome the energy barriers.In the second step of dissociation (Equation ( 2)), the O atom remains at the bridge site while the H atom diffuses away and forms a new Fe-H bond (TS2 in Figure 3d).The distance between this H atom and the O atom is 1.70 Å, which is much longer than that of the initial value (0.98 Å).The Fe-H bond is not stable and H continues to move towards the nearby bridge site (Figure 3e).The energy barrier of the second step is 2.12 eV, which is almost 1.5 times of that of the first step.The reaction energy is −0.58 eV, indicating that the second step is an exothermic reaction.According to the activation energies for TS1 and TS2, both steps require significant energies to overcome the energy barriers.In the second step of dissociation (Equation ( 2)), the O atom remains at the bridge site while the H atom diffuses away and forms a new Fe-H bond (TS2 in Figure 3d).The distance between this H atom and the O atom is 1.70 Å, which is much longer than that of the initial value (0.98 Å).The Fe-H bond is not stable and H continues to move towards the nearby bridge site (Figure 3e).The energy barrier of the second step is 2.12 eV, which is almost 1.5 times of that of the first step.The reaction energy is ´0.58 eV, indicating that the second step is an exothermic reaction.According to the activation energies for TS1 and TS2, both steps require significant energies to overcome the energy barriers.

H 2 O Dissociation on the O-Preadsorbed Fe(100) Surface
The dissociation mechanisms of H 2 O on the O-pre-adsorbed metal surfaces are significantly different from those on a clean surface.The process of H 2 O dissociation on an iron surface, promoted by adsorbed oxygen, can be expressed as Equations ( 3) and ( 4).
2OH ad `Oad Ñ OH ad `2O ad `Had (4) The pre-adsorbed O atom may act as a reactant.The reaction energies and activation barriers for Equation ( 3) and the consequent Equation ( 4) are also calculated and shown in Table 3.The corresponding reaction steps are illustrated in Figure 4. On the O-pre-adsorbed Fe(100) surface, molecular water is adsorbed at the top site, similar to that on the clean surface (2.20 Å on the O-covered surface vs. 2.17 Å on the clean surface).The pre-adsorbed O atom forms a hydrogen bond with one of the H atom in the adsorbed H 2 O with a bond distance of 2.04 Å (Figure 2a).Due to the interaction with the pre-adsorbed O atom, the adsorption of H 2 O molecule is slightly away from the top site (Figure 5a).Next, the pre-adsorbed O pulls away the hydrogen atom to form a new OH ad species adsorbed at the short bridge site, leaving the remaining OH at another bridge site (Figure 5b,c).The energy barrier and heat energy for this step were 0.92 eV and ´0.53 eV, respectively.Therefore, the energy barrier of H 2 O splitting in the presence of O atom on the Fe(100) surface is much lower than that on the clean surface.The strong promotion effect of pre-adsorbed-O atoms on the Fe(100) surface may help design more active catalyst by considering partially oxidizing the metal surfaces.The decrease of the bond length of O-H from 1.57 Å in TS1 to 1.42 Å in TS1' leads to the reduction of the water dissociation barrier from 1.45 eV on a clean surface to 0.92 eV on an O-pre-adsorbed one.We may conclude that the pre-adsorbed O significantly promote H 2 O dissociation.
The energy barrier for Equation (4) reported in our work (0.92 eV) is significantly higher than that reported by Liu et al. (0.18 eV) [37].One of the reasons is the different computation methods used.In the current study, GGA-PBE functional in CASTEP code was used, rather than GGA-PBE-D2 functional in VASP code that includes the long-range dispersion correction for van der Waals (vdW) interactions [37].The dissociation mechanisms of H2O on the O-pre-adsorbed metal surfaces are significantly different from those on a clean surface.The process of H2O dissociation on an iron surface, promoted by adsorbed oxygen, can be expressed as Equations ( 3) and (4).
The pre-adsorbed O atom may act as a reactant.The reaction energies and activation barriers for Equation (3) and the consequent Equation ( 4) are also calculated and shown in Table 3.The corresponding reaction steps are illustrated in Figure 4. On the O-pre-adsorbed Fe(100) surface, molecular water is adsorbed at the top site, similar to that on the clean surface (2.20 Å on the Ocovered surface vs. 2.17 Å on the clean surface).The pre-adsorbed O atom forms a hydrogen bond with one of the H atom in the adsorbed H2O with a bond distance of 2.04 Å (Figure 2a).Due to the interaction with the pre-adsorbed O atom, the adsorption of H2O molecule is slightly away from the top site (Figure 5a).Next, the pre-adsorbed O pulls away the hydrogen atom to form a new OHad species adsorbed at the short bridge site, leaving the remaining OH at another bridge site (Figure 5b,c).The energy barrier and heat energy for this step were 0.92 eV and −0.53 eV, respectively.Therefore, the energy barrier of H2O splitting in the presence of O atom on the Fe(100) surface is much lower than that on the clean surface.The strong promotion effect of pre-adsorbed-O atoms on the Fe(100) surface may help design more active catalyst by considering partially oxidizing the metal surfaces.The decrease of the bond length of O-H from 1.57 Å in TS1 to 1.42 Å in TS1' leads to the reduction of the water dissociation barrier from 1.45 eV on a clean surface to 0.92 eV on an O-preadsorbed one.We may conclude that the pre-adsorbed O can significantly promote H2O dissociation.
The energy barrier for Equation (4) reported in our work (0.92 eV) is significantly higher than that reported by Liu et al. (0.18 eV) [37].One of the reasons is the different computation methods used.In the current study, GGA-PBE functional in CASTEP code was used, rather than GGA-PBE-D2 functional in VASP code that includes the long-range dispersion correction for van der Waals (vdW) interactions [37].OH is also difficult to dissociate on clean Fe surfaces due to the high energy barrier (2.12 eV).In the presence of a pre-adsorbed O atom, the energy barrier is reduced to 2.02 eV.According to Figure 5d, the bond length of O-H at TS2' (1.56 Å) is shorter than that on clean Fe surfaces (1.70 Å), suggesting that the O atom promotes the dissociation of hydroxyl.These results are also elucidated on Au(100) [61] and Pd (111) [64] surfaces.The high energy barriers of OH dissociation on the clean and O-pre-adsorbed Fe(100) surfaces make the occurrence of this reaction kinetically difficult.Consequently, the decomposition of the O-H bond in the OH ad (Equations ( 2) and ( 4)) are the rate-determining steps of the whole dissociation reaction of H 2 O.

Computational Methods
Spin-polarized periodic DFT calculations were performed using the Cambridge Sequential Total Energy Package (CASTEP) program in the Materials Studio 6.0 package (Accelrys Software Inc., San Diego, CA, USA).All calculations were performed using ultrasoft pseudopotentials, with kinetic energy cutoff of 400 eV.Electronic exchange and correlation effects were described within the generalized gradient approximation (GGA) [65] using Perdew-Burke-Ernzerhof (PBE) [66] functionals.
The optimized lattice parameter of bulk Fe was calculated using a bcc unit cell sampled with a 15 ˆ15 ˆ15 Monkhorst-Pack k-point grid.The calculated value was 2.816 Å, which was comparable to the most accepted experimental value of 2.87 Å [67], with a difference of less than 2.0%.
A (3 ˆ3) surface unit cell with a slab of four layers' thickness was selected as the model.This slab was repeated periodically with a 15 Å of vacuum region between the slabs.Only one H 2 O molecule per super cell was adsorbed on one side of the slab to reduce lateral interactions between adsorbates.The geometry optimization including all degrees of freedom of the adsorbates and the two topmost metal layers were considered.The total energy calculation and the surface structural relaxation were performed by sampling the Brillouin zone with a 6 ˆ6 ˆ1 Monkhorst-Pack grid.All of the considered geometries were fully relaxed so that the forces became smaller than 0.01 eV¨Å ´1.A 0.1 eV Fermi smearing was used and the convergence criteria for the geometry optimizations were 10 ´5 eV for the total energy.The computed lattice constant (2.643 Å) and magnetic moment (2.22 µB) are close to the experimental values (2.866 Å [68] and 2.22 µB [69]).
As usually defined [70], the adsorption energy (E ad ) was calculated by Equation ( 5): where E T , E Fe and E S are the total energy of the system, the energy associated to the isolated surface, and the energy of the isolated species (H, O, OH and H 2 O in the present study), respectively.A negative value of E ad indicates an exothermic chemisorption process.The linear synchronous transit (LST) calculation [71] combined with a quadratic synchronous transit (QST) calculation and conjugate gradient refinements [72] were used to obtain the transition state (TS).A LST optimization was performed in the calculations of LST/QST, while QST maximization was used to obtain the TS approximation.The same conjugate gradient minimization was repeated until a stationary point was obtained.We carried the Hubbard correction for H 2 O adsorption on the Fe(110) surface using the DFT+U method.DFT + U implementation in the code was based on the formalism summarized in Ref. [73].Only on-site Coulomb repulsion was used and all higher-order multipolar terms were neglected.The effect of the Hubbard correction was found to be negligible for adsorption energies of H 2 O, O, OH and H on the Fe(100) surface (Table 4).

Conclusions
The adsorption configurations of H 2 O and relevant dissociation species on clean and O-pre-adsorbed Fe(100) surfaces were investigated by DFT.It was found that H 2 O is preferably adsorbed on the top site, O is absorbed on the bridge and hollow hcp site, while OH and H are adsorbed on the bridge site.The calculated adsorption energies revealed that the interactions between the adsorbates and the Fe surface increase in the order of H 2 O < OH < H < O.In addition, we found that both the first H abstraction from adsorbed H 2 O and the subsequent OH dissociation are exothermic on both clean Fe(100) and O-pre-adsorbed Fe(100) surfaces.However, the pre-adsorbed O significantly reduces the kinetics energy barriers for both reactions.Our results confirmed that the presence of pre-adsorbed oxygen species significantly promotes H 2 O dissociation and will help in the design of better catalysts for water dissociation.
studied the effect of the O atom which came from the dissociation of the first H 2 O molecule on the consequent dissolution of the second H 2 O molecule.The O atom assisted H 2 O dissociation (O + H 2 O = 2OH) was kinetically favorable, and further OH dissociation was roughly thermo-neutral.In that case, the co-adsorption of H coming from the dissociation of the first H 2 O molecule on the Fe(100) might have an effect on the reaction pathway.In our manuscript, we studied the effect of two O atoms which came from the dissociation of O 2 molecule on the adsorption and dissociation of water molecule.

Figure 1 .
Figure 1.Top views of the adsorption configurations of (a) H2O; (b) OH; (c) O; and (d) Hon clean Fe(100) surfaces.The slate blue, red and white balls stand for Fe, O and H atoms, respectively.

Figure 1 .
Figure 1.Top views of the adsorption configurations of (a) H 2 O; (b) OH; (c) O; and (d) Hon clean Fe(100) surfaces.The slate blue, red and white balls stand for Fe, O and H atoms, respectively.

Figure 2 .
Figure 2. Top views of the adsorption configurations of (a) H 2 O + O; and (b) OH+ O on O-pre-adsorbed Fe(100) surfaces.

Figure 4 .
Figure 4. Schematic energy diagram for the dissociation of H2O on clean (black lines) and O-preadsorbed (red lines) Fe(100) surfaces.

Figure 4 .
Figure 4. Schematic energy diagram for the dissociation of H2O on clean (black lines) and O-preadsorbed (red lines) Fe(100) surfaces.

Figure 4 .
Figure 4. Schematic energy diagram for the dissociation of H 2 O on clean (black lines) and O-pre-adsorbed (red lines) Fe(100) surfaces.

Table 1 .
Adsorption energies (Eads, in eV) of H2O, OH, O and H on clean and O-pre-adsorbed Fe(100) surfaces.

Table 2 .
Geometrical parameters for H2O, OH, O, and H adsorbed on clean and O-pre-adsorbed Fe(100) surfaces.

Table 2 .
Geometrical parameters for H 2 O, OH, O, and H adsorbed on clean and O-pre-adsorbed Fe(100) surfaces.Species Clean Fe(100) O-pre-adsorbed Fe(100) d O-H (Å) A H-O-H ( ˝) d O(/H)-Fe (Å) a d O-H (Å) A H-O-H ( ˝) d O-Fe (Å) a a d O(/H)-Fe (Å) is the distance to the first Fe neighbor; b Values in parentheses correspond to gas-phase species or free radicals.

Table 3 .
Energy of reaction (∆E) and energy of barrier (E a ) for the dissociation of H 2 O on clean and O-pre-adsorbed Fe(100) surfaces.

Table 3 .
Energy of reaction (ΔE) and energy of barrier (Ea) for the dissociation of H2O on clean and Opre-adsorbed Fe(100) surfaces.

Table 4 .
Comparison of adsorption energies of H 2 O, OH, O and H on Fe(110) with and without Hubbard correction.